ABSTRACT

PART I

THE EFFICIENCY OF TRIPLET ENERGY TRANSFER FROM KETONES
TO RARE EARTH IONS

By
Herbert Nick Schott

The Norrish Type II reaction of p-methoxyvalerophenone, which is
known to react only from its triplet state, has been used as a
monitoring tool to determine whether the rate of energy transfer from
ketone triplets to rare earth ions in solution is really diffusion

controlled as had been assumed previously.

Samples, made from methanol solutions containing p-methoxy-
valerophenone and various concentrations of the following rare

3 3 3 +3, and Tb+3, were irradiated at

earth ions, Eu+ , Sm+ , Dy+ , Er
3l30 A. The rate constants, kq, for quenching of product formation,
as determined by Stern-Volmer kinetics, for the lanthanide ions,
were compared to the rate constant for quenching by naphthalene.
Since naphthalene is known to quench triplet state reactions at

near the diffusion controlled rate, the comparison between the
quenching constant for each individual rare earth ion and the
quenching constant for naphthalene was a good indicator of the

efficiency of ketone-rare earth ion energy transfer. The results

indicated that energy transfer from ketone triplets to rare earth
ions is approximately two orders of magnitude slower than the

diffusion controlled rate.

PART II
THE NATURE OF THE PHOTOREACTIVE STATE IN
P-METHOXYPHENYL ALKYL KETONES

Ketones possessing lowest n-n* triplet states are generally
unreactive in hydrogen abstraction reactions. However, the p-methoxy-
phenyl alkyl ketones, whose two triplet states are very close
together, with the n-n* state just below the n-n* one, do undergo the
Norrish Type II reaction, although the reactivity of these ketones
is less than that of the unsubstituted phenyl alkyl ketones which
possess a lowest n-n* triplet state.

There are two possibilities as to what the nature of the reactive
state in the p-methoxy ketones is. It can be either the lowest n-n*
state with enough n-n* character mixed in it due to vibronic coupling
to make it reactive, or it can be the upper n-n* triplet, which is
known to be the reactive state in the phenyl alkyl ketones.

In order to distinguish between these two possibilities, several
anisyl ketones with electron-withdrawing groups near the y carbon
atom were synthesized. It was expected that the behaviOr of the
two types of triplets towards the hydrogens on the y carbon of these

ketones should be quite different.

The n-n* triplet of ketones was known to be an electrophilic
species and therefore its reactivity in abstracting a hydrogen from
a carbon near an electron-withdrawing group should be subject to
quite strong inductive effects. The n-n* triplet, on the other
hand, was expected to be quite nucleophilic, and should be subject
to opposite inductive effects. A comparison of the data for the anisyl
ketones, as obtained from Stern-Volmer kinetics and quantum yield
determinations, with the data for the analogous phenyl alkyl ketones,
indicated that the reactive state in these two types of ketones was
subject to identical inductive effects. This led to the conclusion
that hydrogen abstraction in the p-methoxy ketones was occurring from
the upper n-n* triplet state, which is in thermal equilibrium with the

lower n-n* state.

If this conclusion is true, the only difference between the observed

rate constant, kobs

, for hydrogen abstraction by the anisyl ketones, and
k2, the rate constant for hydrogen abstraction by benzoyl triplets, is
due to the lower concentration of reactive n-n* triplets in the p-methoxy
ketones. From the magnitude of Xn’ the mole fraction of reactive n-n*
triplets present in the anisyl ketones, it was calculated that the energy
separation between the reactive upper n-n* state and the lower n-n*

state is about three kilocalories per mole. This energy separation

was found to be dependent on the nature of the solvent, since it

increases in going from a non-polar to a polar solvent.

[ill A ll l!4ll '4‘

PART I
THE EFFICIENCY OF TRIPLET ENERGY TRANSFER FROM KETONES
T0 RARE EARTH IONS
PART II

THE NATURE OF THE PHOTOREACTIVE STATE IN
P-METHOXYPHENYL ALKYL KETONES

By

Herbert Nick Schott

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
DOCTOR OF PHILOSOPHY

Department of Chemistry

1970

 

(J: {'5 7 31/ if?!

This thesis is dedicated to my wife, Marie, for her
inspiration and encouragement, and to my parents, for

their assistance and guidance, during my studies.

ii

ACKNOWLEDGMENTS

I would like to extend my deepest appreciation to
Professor Peter J. Wagner for his patience, interest, and
encouragement during the course of this investigation. I
am also grateful to my fellow students for their aid and
advice during the past years.

Special thanks is extended to the United States Department
of Health, Education, and Welfare for a NDEA Title IV Fellowship
from January, l969 to September, 1970.

I.
II.

III.

II.

III.

IV.

TABLE OF CONTENTS

PART I

THE EFFICIENCY OF TRIPLET ENERGY TRANSFER FROM KETONES
TO RARE EARTH IONS

INTRODUCTION ......................
RESULTS AND DISCUSSION .................
A. Quenching Studies .................
LITERATURE CITED ....................

PART II

THE NATURE OF THE PHOTOREACTIVE STATE IN
P-METHOXYPHENYL ALKYL KETONES

INTRODUCTION ......................
RESULTS ........................
A. Solvent Effects ..................
B. Quenching Studies .................
DISCUSSION .......................
A. Mechanistic Interpretation .............
B. Solvent Effects ..................
C. Summary ......................
D. Further Experiments ................
EXPERIMENTAL ......................

A. Chemicals .....................

iv

Page

2
9
l4
l7

2l

44
49
49

57
57
59

TABLE OF CONTENTS (Continued)

Page

l. Solvents .................... 59
a. Benzene ................... 59

b. Methanol .................. 59

c. Pyridine .................. 59

d. t-Butanol .................. 59

2. Ketones .................... 59
a. p-Methoxyacetophenone (PMACP) ........ 59

b. p-Methoxyvalerophenone (PMVP) ........ 60

c. y-Methyl—p-methoxyvalerophenone (GMPMVP) . . 60

d. p-Methoxybutyrophenone (PMBP) ........ 60

e. p-Chlorovalerophenone (PCVP) ........ 61

f. p-Methylvalerophenone (PMeVP) ........ 6l

9. m-Trifluoromethylvalerophenone (MTFVP) . . . 61
h. Acetophenone (ACP) ............. 6l

i. 6-Carboethoxy-p-methoxyvalerophenone
(DCEPMVP) .................. 61

j. 6-Cyano-p-methoxyvalerophenone (DCPMVP). . . 63
k. Valerophenone (VP) ............. 63

l. y-Carbomethoxy-p-methoxybuterphenone. . . . 64

3. Internal Standards ............... 64
a. Tetradecane ................. 64
b. Pentadecane ................. 64
c. Hexadecane ................. 64

TABLE OF CONTENTS (Continued)

Page

d. Octadecane ................. 64

4. Quenchers ................... 64
a. Lanthanide Chlorides ............ 64

b. Naphthalene ................ 64

c. 2,4-Dimethyl-2,5-hexadiene ......... 64

d. gisfPiperylene ............... 65

B. General Procedures ..... ‘ ........... 65
T. Stern-Volmer Quenching Studies ........ .65
2. Polar Solvent Effect Studies ......... 66
3. Photolysis .................. 67
4. Vapor Phase Chromatography .......... 67
5. Valerophenone Actinometry ........... 68

6. Cisypiperylene/Acetophenone Actinometry. . . . 69
III. LITERATURE CITED ................... 70
IV. APPENDIX ....................... 75

vi

LIST OF TABLES

TABLE Page

l. Quenching of triplet p-Methoxyacetophenone by
Naphthalene and Lanthanide Chlorides ......... 15

2. Quantum Yields for Substituted n-Butyl Phenyl
Ketones ....................... 44

3. Photoelimination of Select p-Anisyl Ketones

CH3O-@-8CH2CH2R .................. 47

4. Photoelimination of Select Phenyl Ketones

PhCOCHZCHZR ..................... 48
5. Photoelimination of p-Methoxyvalerophenone in
Methanol ....................... 48

6. Stern-Volmer Quenching Study of 0.10 M p-Methoxy-
valerophenone by Naphthalene in Methanol ....... 75

7. Stern-Volmer Quenching Study of 0.10 M p-Methoxy-
valerophenone by Naphthalene in Methanol ....... 75

8. Stern-Volmer Quenching Study of 0.1 M p-Methoxy-
valerophenone by EuCl3 in Methanol .......... 77

9. Stern-Volmer Quenching Study of 0.1 M p-Methoxy-
valerophenone by EuCl3 in Methanol .......... 77

10. Stern-Volmer Quenching Study of 0.10 M p-Methoxy-
valerophenone by SmC13-6H20 in Methanol ....... 78

ll. Stern-Volmer Quenching Study of 0.10 M p-Methoxy-
valerophenone by TbC13-6H20 in Methanol ....... 78

12. Stern-Volmer Quenching Study of 0.10 M p-Methoxy-
valerophenone by TbC13°6H20 in Methanol ....... 79

13. Stern-Volmer Quenching Study of 0.10 M p-Methoxy-
valerophenone by DyC13°6H20 in Methanol ....... 79

vii

TABLE
14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

Stern-Volmer Quenching Study of 0.10 M p-Methoxy-
valerophenone by DyCl3'6H20 in Methanol .......

Stern-Volmer Quenching Study of 0.10 M p-Methoxy—
valerophenone by ErC13-6H20 in Methanol .......

Stern-Volmer Quenching Study of 0.10 M p-Methoxy-
valerophenone by ErC13-6H20 in Methanol .......

Type II Quantum Yield Determinations for p-Methoxy-
phenyl Alkyl Ketones ................

Acetophenone/gj§;Pipery1ene Actinometry for
p-Methoxyvalerophenone and y-Methyl-p-methoxy-
valerophenone ....................

Acetophenone/gi§;Piperylene Actinometry for
p-Methoxybutyrophenone ...............

Type 11 Quantum Yield Determinations for Ring
Substituted Valerophenones .............

TYPE II Quantum Yield Determination for
5-Substituted p-Methoxyvalerophenones ........

Valerophenone Actinometry for 6-Carboethoxy-p-
methoxyvalerophenone and y-Carbomethoxy-p-methoxy-
butyrophenone ....................

Valerophenone Actinometry for 6-Cyano-p-
methoxyvalerophenone ................

Valerophenone Actinometry for p-Methoxyvalero-
phenone in Methanol .................

Determination of 63¢ for ortho, meta, and para-
Methoxyvalerophenone in Benzene by Isomerization
of gj§:Piperylene ...................

 

The Photolysis of 0.10 M p-Methoxyvalerophenone
in Benzene and added Methanol ............

The Photolysis of 0.10 M p-Methoxyvalerophenone
in Benzene and added t-Butanol ...........

The Photolysis of 0.10 M p-Methoxyvalerophenone
in Benzene and added Pyridine ............

viii

Page

80

8O

81

82

82

83

83

84

84

85

85

86

86

87

TABLE
29.

30.

31.

32.

33.

34.

35.

36.

37.

38.

39.

40.

41.

42.

The Photolysis of 0.10 M y-Methyl-p-methoxy-

valerophenone in Benzene and Added t-Butanol .....

Photolysis of 0.10 M p-Methoxybutyrophenone

in Benzene and Added t-Butanol ............

Photolysis of 0.06 M m-Trifluoromethylvalerophenone

in Benzene and Added t-Butanol ............

Photolysis of 0.06 M p-Methylvalerophenone

in Benzene and Added t-Butanol ............

Photolysis of 0.06 M p-Chlorovalerophenone

in Benzene and Added t-Butanol ............

Photolysis of 0.05 M 5-Carboethoxy-p-methoxy—

valerophenone in Benzene and Added t-Butanol .....

Photolysis of 6-Cyano-p-methoxyva1erophenone

in Benzene and Added t-Butanol ............

Values of Maximum Quantum Yields of Selected

Phenyl n-Butyl Ketones .............. -. .

Stern-Volmer Quenching Study of 0.10 M p-Methoxy-
valerophenone by 2,4-Dimethyl-2,5-hexadiene in

Benzene .......................

Stern-Volmer Quenching Study of 0.10 M y-Methyl-p-
methoxyvalerophenone by 2,4-Dimethyl-2,5-hexadiene

in Benzene ......................

Stern-Volmer Quenching Study of 0.10 M p-Methoxy-
butyrophenone by 2,4-Dimethy1-2,S-hexadiene in

Benzene .......................

Stern-Volmer Quenching Study of p-Methoxybutyro-

phenone by 2,4-Dimethyl-2,5-hexadiene in Benzene. . .

Stern-Volmer Quenching Study of 0.05 M y-Carbo-
methoxy-p-methoxybuterphenone by 2,4-Dimethyl-

2,5-hexadiene in Benzene ...............

Stern-Volmer Quenching Study of 0.05 M 6-Carbo-
ethoxy-p-methoxyvalerophenone by 2,4-Dimethy1-2,5-

hexadiene in Benzene .................

ix

Page

88

88

89

89

90

9O

91

91

92

94

94

TABLE Page

43. Stern-Volmer Quenching Study of 0.05 M 6-Cyano-p-
methoxyvalerophenone by 2,4-Dimethy1—2,5-hexadiene
in Benzene ...................... 95

44. Stern-Volmer Quenching Study of 0.05 M 6-Cyano-p-
methoxyvalerophenone by 2,4-Dimethyl-2,5-hexadiene
in Benzene ...................... g5

LIST OF FIGURES
FIGURE Page
1. Schematic Energy Level Diagram for a Rare Earth
Chelate Possessing Low-lying 4f Electron States. . . 4

2. Modified Jablonski Diagram for Phenyl Alkyl Ketones. 11

3. Modified Jablonski Diagram for p-Methoxyphenyl
Alkyl Ketones .................... 39

4. Stern-Volmer Plot for the Quenching of p-Methoxy-
valerophenone by Naphthalene in Methanol ....... 76

xi

PART I

THE EFFICIENCY OF TRIPLET ENERGY TRANSFER FROM KETONES
T0 RARE EARTH IONS

I. INTRODUCTION

In 1942 S. I. Weissman1 published his classic paper on the transfer
of excitation energy from the ligands of several europium chelates to
the metal ion with subsequent emission from the ion. At that time it
was known that the solvated lanthanide ions in solution usually exist
in the tripositive state and absorb and emit light only weakly. However,
since the ions readily coordinate with many chelating agents whose
ligands strongly absorb light above 3000 A, this provided a convenient
means of introducing energy into the ion.

Weissman's initial work and Sevchenko2 et al.'s later investigation
of these systems did not cause any real waves in the sea of chemistry for
about twenty years until the renaissance of rare earth ion photochemistry
occurred. This period of rediscovery was stimulated by a growing
interest in the use of rare earth ions and chelates as possible laser
materials and the subsequent willingness of several government agencies
and private corporations to fund research in this area.

The rare earth ions3 owe their possible lasing action to their
position in the periodic table. They comprise a group of 15 elements
that form a f-type inner transition series in which the 4f level falls

below that of the 5d level in energy and is consequently filled first.

The 4f7 and 4fM electron arrangement in this series is reached
as soon as possible due to the stability associated with the complete
single (4f7) or complete double (4f14) occupancy of any set of orbitals.

This entails the transfer of the single 5d1 electron that is

0

present at the beginning (Lanthanum, Z = 58, 4f , 5d1), middle

(Gadolinium, z = 64, 4f7, 5d1

1

, 652) and end (Lutetium, Z = 71, 4f14,

5d , 652) of the series into a lower f—orbital during the buildup
of the series. According to Hund's Rule then, a maximum of seven

7, 652) which in

unpaired electrons is reached in europium (Z = 63, 4f
the tripositive state will have 6 unpaired e1ectrons3.

As expected, the ions which have the closed f-orbital configurations,

+3 3 14

La and Lu+ (4f0 and 4f respectivelyL and Gd+3 with its 4f7 complete
single occupancy configuration,have no electronic transition in the
visible or infrared and on excitation of their chelates, show
predominantly ligand phosphorescence only“’5. However, the ions which
have unfilled 4f orbitals can have a lower lying electron excited to

a higher level, and if this happens to be a resonance level, line
emission corresponding to a transition from that level to a lower 4f
level occurs. This process is represented schematically below, where
solid lines represent radiative transitions and wavy lines represent

radiationless transitions.

FIGURE 1. Schematic Energy Level Diagram for a
Rare Earth Chelate Possessing Low-lying
4f Electronic States.

Ligand
Singlet Ligand
/ L1* Triplet
S1 ‘3 T2 Vm
T1 ’f’

 

 

 

 

a Q
\ 3

Low Ground State

 

. .

 

 

 

 

 

 

-—-——Reaction Progress——-§>

Early work by Crosby and Whan6 and Bhaumik and El-Sayed7 demonstrated
clearly that triplet energy transfer occurs from the excited ligand to the
metal ion. Crosby, Whan and Alire8 found that the lowest triplet state
energy level of the complex must equal or lie above the resonance energy
level of the rare earth ion for emission to occur. In particular, for

+3

Eu ions, which fluoresce strongly from two well-established resonance

levels7, selective excitation of the lower level occurs in chelates whose

3 resonance levels. El-Sayed

ligand triplet states lie between the two Eu+
and Bhaumik10 showed that the ligand-induced rare earth ion fluorescence
could be sensitized by such triplet sensitizers as benzophenone, and

quenched by piperylene or naphthalenell. They assumed that such energy

transfer occurred at the diffusion controlled rate. Furthermore,
Matsuda and coworkers'7 have shown that the phosphorescence of lanthanum
dibenzoylmethide can be quenched by europium dibenzoylmethide but not

by europium ions. In this case the phosphorescence of the lanthanum
Chelate is due solely to the ligand triplet to ligand ground state
transition and thus involves only triplet energy.

Despite various attempts to enhance the fluorescence yields of
chelated rare earth ions by the addition of Lewis basesl3, change of
ligandsl“a15 use of substituted ligand§”:15, having two different ions
bound to the same ligand17, and addition of extraneous inorganic ionsla,
energy transfer from ligand to ion has several disadvantages. Notable
among these are photodisassociation of the Chelate to produaaligand
negative ions which can phosphoresce19, the inability to choose the
most efficient solvent and sensitizer combination,and the inherent
inefficiency of energy transfer between ligand and ion,as manifested
in ligand phosphorescence in many instanceszo.

This inefficiency, and the possible utility of energy transfer
to rare earth ions as an analytical tool21 has led to the investigation
of intermolecular energy transfer to unchelated rare earth ions.
Matovich and Suzuki22 reported the initial triplet energy transfer from
excited ketones to rare earth ions. Irradiation of EuCl3 in neat aceto-
phenone, propiophenone and molten benzophenone at 3660 R where only the
ketones absorb produced the characteristic red emission of europium.
Heller and Nasserman23 have measured energy transfer from simple aromatic
aldehydes and ketones to europium and terbium ions by monitoring the

intensity of emission of the ions. They noted a decrease in fluorescence

intensity in going from acetic to decanoic acid and attributed this

to a lower rate of diffusion in the more viscous solvent. Ballard

and Edwardsz“,in measuring triplet energy transfer from acetophenone

to various lanthanide nitrates as a function of lanthanide concentration

assumed a diffusion limited value as the rate constant for energy

3 3

transfer. Filipescu and Mushrush25 tried to sensitize EuJr and Tb+
emission with a large series of organic compounds in dimethylformamide.
This series included compounds having triplet energies both above and
below the rare earth ion acceptors. However, sensitization occurred
only with donors having both a higher triplet energy than the ions and
a large 5* + T* intersystem crossing yield. From viscosity measurements
they assumed that diffusion controlled energy transfer occurred and
suggested that oxygen impurity quenching was responsible for the short
ketone lifetimes obtained by using these diffusion rate constants.

The transfer of energy between ketones and rare earth ions can
also be realized by utilizing other lanthanide ions as intermediates
in the transferring process. Terbium is particularly useful and has
3

been used to transfer energy from 4,4'-dimethoxybenzophenone to Eu+

in acetic anhydride26.
rt

9
ROWOR -—“—“—-—> R Q I 6 0R (1)

I
0*

I
RUMOR + Tb+3 —'—>*Tb+3+ R0_@ 9 OR (2)

+3 +3

+3 + Eu + *Eu

*Tb + Tb+3 (3)

*Eu+3 + Eu+3 + hp (4)

Since different ketone triplets show remarkable selectivity in
energy transfer efficiency to lanthanide ions, even though such transfer
may be highly exothermic, this can be used as a means of sensitizing
ions which are not sensitized directly by certain ketones.

In all the above cases of ketone - rare earth ion energy transfer,
it was assumed that the rate constant for energy transfer was diffusion
controlled. This assumption is not altogether unreasonable since most
reported cases of exothermic energy transfer between two organic
molecules proceed at nearly the diffusion controlled rate in solution27.
However, in the studies with the lanthanide ions, the competing chemical
reactions of donor triplets, the effect of changing solvents, and the
inability to measure emission intensities accurately have been largely
ignored. Consequently the factors influencing energy transfer rates
are not well known and it still remains to be determined whether energy
transfer does actually occur at a diffusion controlled rate.

The method chosen to determine the actual efficiency of the energy
transfer process was to simply monitor the quantum yields of some
photochemical reaction of triplet ketones as a function of rare earth
ion concentration. Standard Stern-Volmer plots of quantum yield ratios
versus rare earth ion concentration will yield the ratio of rates for
energy transfer to chemical reaction. By comparing the slopes, kq 1,

()btained by the addition of the various lanthanide salts, to the slope

k'q I, obtained by using a known diffusion controlled quencher such

as a 2,4-hexadiene or naphthalene, it is possible to obtain a direct
comparison of the energy transfer efficiency of ketones to rare earth
ions versus ketones to other organic triplet quenchers. This method

has been used successfully by Hammond and Foss to measure quenching

rates of triplet benzophenone by transition metal chelatesZB.

The reaction chosen in this case was the Norrish Type II elimination

of phenyl alkyl ketones which is known to proceed strictly from the

triplet state29.

However, other photochemical monitors such as photoreduction or photo-

cycloaddition to olefins could also have been used.

II. RESULTS AND DISCUSSION

The Type II reaction30 of a phenyl alkyl ketone, K, is initiated
by the absorption of light of suitable wavelength resulting in excitation
to higher singlet levels. After rapid internal conversion to the lowest
excited singlet state (1012 sec.'1 in solution), intersystem crossing
to the triplet manifold occurs with unit efficiency. The triplet ketone
that is formed has three possible modes of reaction available to it:
a) It can abstract a hydrogen from the gamma carbon to form a
biradical intermediate, BR.
b) It may undergo radiationless decay to the ground state, and
c) A transfer of triplet energy to an acceptor molecule may
occur. This yields ground state ketone and an excited triplet
quencher molecule.
The reaction sequence under consideration and the rate for each of the
various processes is the following:
Rate
K + hv ————{;> 1K* Ia (7)
1K* —Ei§5L—{;> 3K* k. (]K*) (8)
15C

3K* 7L9 BR kr (3w) (9)
3K* -——£L——j%> K kd (3K*) (10)
3K* + o —‘*——> K+3o* kq (Q)(3K*) (n)

10

The biradical intermediate that is formed has three reaction pathways
available to it also:
a) It can disproportionate back to ground state ketone by

retransferring the hydrogen on the oxygen to the gamma carbon.
H O R
o I kd. R
——> (12)

b) Coupling can occur between the two radical sites to form a

cyclobutanol.
OH
. k H0 R
° --c-—>
c) The biradical can cleave to yield the enol form of an

acetophenone and an olefin. H
OH ‘
k ___J/R
W -»S-—> \ + — (14)

Schematically, the whole process may represented as follows:

11

FIGURE 2. Modified Jablonski Diagram
for Phenyl Alkyl Ketones

 

 

 

 

S ._ K*
A '" “m
3K*
I k
E I +
a Q ‘*—BR
k
d C + ks
kq kd'
Products
V/ f \/ Ko

 

 

 

 

 

Reaction Progress ———{3>

The quantum yield of acetophenone formation, 4A, is defined31 as

¢ _ The number of molecules of acetophenone formed
A The number of quanta absorbed by the reacting ketone

 

This quantum yield is the mathematical product of the probabilities
of each of the individual steps in the reaction sequence occuring32.

Hence for the Type II reaction,

¢A : ¢isc¢BR¢P (16)

Where ¢.

15c is the probability of intersystem crossing from singlet

‘to triplet ketone, (which equals unity in these ketoneng), 4 R is the

B

12

probability of biradical formation from triplet ketone, and 4P is the
probability that the biradical once formed will go on to desired product.
The quantum yield can also be expressed in terms of the various rate
constants of the overall process by assuming that steady state conditions
exist during the course of the photolysis. The steady-state assumption
simply implies that the triplets and the biradical intermediates that

are formed are highly transient species and do not remain in solution

long enough to build up to any appreciable concentration. The expressions
obtained for 4A either in the presencean absence of quencher are:

k k ¢.

 

 

r 5 15C .

¢ = . (with quencher) (17)

A de + kq(Q) + kr)(kC + kS + kd T‘
and

k k a.
o r 5 15C .
¢ = . (without quencher) (18)
A (kd + kr)(kS + kc + kd )

If the ratio of the quantum yield without quencher to the quantum yield

with quencher is taken, the familiar Stern-Volmer expression is obtained.

 

q) + k (Q) + k
"A“ = d k 21 r (‘9)
¢A d r
k (0)
= l + TL— (20)

+ k
r.

13

which can be rewritten as

0
(PA
(PA q

 

T ‘ k—’+_k—’ (22)

A plot of ez/eA versus quencher concentration will have an intercept
of one and a slope of kq r. The numerical value of r is obtained by
substituting the values of the diffusion controlled rate constant and
the slope of the plot (using naphthalene or diene as a quencher) into

the expression

= slope
T k diff (23)
q
Once the value of T is known and the quenching slopes for the various

lanthanide ions are obtained, it will be simple to calculate the

bimolecular quenching constants for each ion from the expression

kq = ——E——S“T’e (24)

which will be a direct indication of the efficiency of the energy
transfer process.

The ketone chosen for this study was p-methoxyvalerophenone (ET =
73 kcal/mole)32. Initial work with p-methoxybutyrophenone in methanol

showed that several side reactions occurred with the solvent during the

l4

photolysis which prevented quantitative analysis of the p-methoxy-
acetophenone that is formed. It was desirable to utilize a methoxy
ketone since it is known that their triplet states have long lifetimes
compared to unsubstituted ketone529. Less error should occur in the
quenching slopes of long lived ketone triplets since they are more
sensitive to a relatively inefficient quencher than ketones whose
triplets are short-lived and quenched only with difficulty. The
diffusion controlled quencher that was used for comparison with the

9M'] sec-1 for

lanthanide ions was naphthalene,whose kq = 4.7 t .4 x 10
quenching ketone triplets in benzene33,since it was found that 2,4-

dimethy1-2,5-hexadiene is somewhat photolabile in alcoholic solution.

A. Quenching Studies

 

3M

Degassed methanol solutions containing .1 M ketone, 4 x 10'
octadecane and various concentrations of quenchers were photolyzed at
3130 A or at 3660 A and the amount of p-methoxyacetophenone formed was
determined by vapor phase chromatography.

Table I gives the kq r values obtained from quenching runs with
naphthalene and several representative rare earth ions. The r and k
values were calculated from equations (23) and (24) respectively.

The kq value assumed for the bimolecular quenching rate constant
of a triplet ketone by naphthalene in methanol is 7.5 x 109M'1 sec-1
which is near the diffusion controlled limit27. The rare earth ions,
however, are only 1/30 to 1/200 as effective as energy acceptors and
their k values are two orders of magnitude smaller than those for organic

q
quenchers. Similar results have been obtained in quenching the

15

TABLE 1. Quenching of Triplet p-Methoxyacetophenone
by Naphthalene and Lanthanide Chlorides.

 

 

Quencher qu, W]a r (X 106) sec kq (M‘l sec-1)
Naphthalene 13,050 . 50 1.73 7.5 x 109
Euc13 325 i 75 1.73 1.9 x 108
SmCl3-6H20b 48 z 3 1.73 2.9 x 107
TbCl3~6H20 56 t 6 1.73 3.2 x 107
DyCl3°6H20 53 . 4 1.73 3.1 x 107
ErCl3'6H20 90 t 20 1.73 5.2 x 107

 

aThe qu values given are the average of two runs, the precision shown
is that between the two runs; bOnly one run, precision shown is that for
drawing slope.

photoreduction of benzophenone (ET = 69 kcal/mole) in aqueous acetic
acid - dimethoxyethane solution3”.

These results indicate that other factors, aside from the require-
ment that it be exothermic, influence the rate of energy transfer from
ketone triplets to the lanthanide ions. Nuclear magnetic resonance
studies by Heller and Wasserman, and the fact that no shift in ion
emission occurs while donors are being varied, have shown that no
long-lived ground state complex forms between the ion and the ketone23.
However, this does not exclude the possibility that the ion and the
carbonyl group of the ketone must exist in a special configuration
for energy transfer to take place. If this is the case, the actual

process of energy transfer may have a significant activation energy

16

or entrOpy22. The bare ions are strongly solvated in protic solvents,
and it is very possible that the solvation shell of the ion prevents

a close enough approach of the ion and the triplet carbonyl so that

the orbital overlap which is necessary for effective exchange interaction
to occur never develops sufficiently35. It is also possible that the
poor or varying spectral overlap between ketone and ions may cause
energy transfer to be less than maximum efficient. Heller and

Wasserman found that high energy carbonyl compounds such as acetophenone
and benzaldehyde, which have their most intense peak below 4500 A
sensitize europium ions much more efficiently than terbium ions,
possibly because the high 502 (4640 A) level of europium is more

easily populated than the 504 level of terbium (5460 A) due to better
spectral overlap23.

Although this work shows that energy transfer between ketone
triplets and lanthanide ions is definitely slower thanthe diffusion
controlled rate, the factors responsible for the inefficieny have not
been determined. A systematic study using the anhydrous lanthanide
salts in various aprotic solvents with different carbonyl compounds
may further elucidate the chemistry of the energy transfer process.

Also of interest is the fact that some of the lanthanide ions
undergo chemical reactions during the course of the photolysis.
Solutions with relatively high (greater than 10'2 M) concentrations
of EuCl3 turned yellow and the solutions of erbium and dysprosium

deposited pink mirrors on the wall of the photolysis tubes.

III. LITERATURE CITED

10.

ll.

12.

l3.

I4.
15.

16.

S. I. Weissman, J. Chem. Phys., 44, 214 (1942).

LITERATURE CITED

 

A. N. Sevchenko and A. K. Trofimov, J. Exp. Theor. Phys,,

g4, 220 (1951).

 

Therald Moeller,"The Chemistry of the Lanthanidesg Reinhold

Publishing Corporation, New York, 1963, Chapter 2.

G. A. Crosby and M. Kasha, Spectrochim. Acta, 44, 377 (1958).

 

 

R. E. Whan and G. A. Crosby, J. Mol. Spectry., 4, 315 (1962).

G. A. Crosby and R. E. Whan, J. Chem. Phys., 44, 863 (1962).

 

M. L. Bhaumik and M. A. El-Sayed, J. Chem. Phys., 4%, 787 (1965).

G. A. Crosby, R.

743 (1961).

 

E; Whan and R. M. Alire, J. Chem. Phys., 44,

 

E. V. Sayre and S. Freed, J. Chem. Phys., 44, 1231 (1956).

M. A. El-Sayed and M. L. Bhaumik, J. Chem. Phys.
M. L. Bhaumik and M. A. El-Sayed, J. Phys. Chem.
Y. Matsuda, S. Makishima and S. Shionoya, Bull.
41, 1513 (1968).

'b’b

M. Kleinerman, R. J. Hovey and D. 0. Hoffman, J.

4009 (1964).

E. P. Riedel and R. G. Charles, J. Chem. Phys.,
N. Filipescu. S. Bjorklund, N. McAvoy and C. R.
45, 1714 (1967).

’V'b

N. Filipescu, W. F. Sager and F. A. Serafin, J.

3324

(1964).

 

 

 

, 32, 2391 (1963).
. Q9, 275 (1965).

Chem. Soc. Japan,

 

Chem. Phys., 44,

 

 

17

44, 1908 (1966).

Hurt, Can. J. Chem.,

 

Phys. Chem., 48,

 

17.
18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

30.

31.

32.

18

F. S. Quiring, J. Chem. Phys., 42, 2448 (1968).

 

H. Samelson, V. A. Brophy, C. Brecher and A. Lempicki, J. Chem.
Phys., 44, 3998 (1964).
M. L. Bhaumik, L. Ferder and M. A. El-Sayed, J. Chem. Phys.,

 

4g, 1843 (1965).

F. Halverson, J. S. Brinen and J. R. Leto, J. Chem. Phys,,

 

4g, 2790 (1964).
W. J. McCarthy and J. D. Winefordner, Anal. Chem., 44, 848 (1966).

 

E. Matovich and C. K. Suzuki, J. Chem. Phys., 44, 1442 (1963).
A. Heller and E. Wasserman, J. Chem. Phys., 44, 949 (1965).

 

R. E. Ballard and J. W. Edwards, Spectrochim. Acta, 44, 1353 (1965).

 

N. Filipescu and G. W. Mushrush, J. Phys. Chem., 14, 3517 (1968).

 

P. K. Gallagher, A. Heller and E. Wasserman, J. Chem. Phys., 44,

 

3921 (1964).
P. J. Wagner and I. Kochevar, J. Amer. Chem. Soc., 44, 2232 (1968).

 

 

G. S. Hammond and R. P. Foss, J. Phys. Chem., 48, 3739 (1964).
E. J. Baum, J. K. S. Wan and J. N. Pitts, Jr., J. Amer. Chem. Soc.,

 

84, 2652 (1966).

For a brief review, see, P. J. Wagner and G. S. Hammond, Agyan,
Photochem., 4, 94,(1968).

W. A. Noyes, Jr. and P. A. Leighton, "The Photochemistry of Gases",

Dover Publications, Inc., New York, 1966, p. 151.

 

P. J. Wagner and A. E. Kemppainen, J. Amer. Chem. Soc., 44, 5896
(1968).

l9

 

33. G. Porter and R.T. Topp, Proc. Roy. Soc. (London),4, 444, 163 (1970).
34. P. J. Wagner, unpublished results.
35. D. L. Dexter, J. Chem. Phys., 21, 836 (1953).

’\;’b

 

PART II

THE NATURE OF THE PHOTOREACTIVE STATE IN
P-METHOXYPHENYL ALKYL KETONES

20

I. INTRODUCTION

The ability of a photoexcited ketone or aldehyde to abstract a
hydrogen atom from good hydrogen donors in the solution phase was
first discovered by Ciamician and Silberl in 1900 when they found that
the action of sunlight on a solution of benzophenone in ethanol produced

a good yield of benzpinacol.

@
©

Ciamician and Silberza3 found that both aromatic carbonyls and

hv

 

 

HO

dialkyl ones, such as acetone, could be photoreduced in the presence
of alcohol. Cohen“ extended the work of Ciamician and Silber and
found that certain ketones would not photoreduce. These included
phenyl-a-naphthyl ketone, Michler's ketone (p,p'-dimethylaminobenzo-
phenone) and p-phenylbenzophenone. These findings were corroberated
by the results of Hirshberg and coworkersSa6 who studied both
naphthyl and substituted phenyl ketones and found that neither the
naphthyl ketones nor p-methoxypropiophenone or p-phenylbenzophenone

yielded pinacols. Pinacol formation is also inhibited if the reaction

21

22

is carried out in the presence of alkoxides, and hydrols are formed

instead7.

OH

-+ . o o
0 >—OH
<::) ,:>-*0Na

Early mechanistic work on photoreduction dealt mostly with the

 

elucidation of the chemical processes following light absorption
rather than the nature of the excited state. Hirshberg5 found that

if acetophenone is photolyzed in the presence of optically active
a-phenylethanol, the pinacols formed are optically inactive, but the
residual carbinol essentially retains its optical activity. This
result implied that way little disproportionation occurred between two
donor molecules after initial hydrogen abstraction. PittsB, in more
quantitative studies twenty years later, confirmed these results by
photolyzing ben20phenone in the presence of optically active 2-butanol
and also found no loss of optical activity in the remaining alcohol.
Thus the photoreduction of acetophenone by a-phenylethanol may be

represented as:

@‘j'i “V > @jL (3)
H OH 0 OH

@1 +63} ——> 2 @4960 +@(
(4)

 

23

OH

©J: +@/I\__>@<'\+<@j\ (5)

 

/\

0H ¢ H
2 (By.k > \ K) <6)
H0 9

Credit for the proposal that the excited state of benzophenone
can be represented as a biradical in the hydrOgen abstraction process
should be given to Backstr6m9 who suggested this in 1934, and later
indicated that these biradical states could be considered equivalent
to triplet stateslo. However, it was not until the late fifties and
early sixties that work in several laboratories indicated that the
photoreduction of ben20phenone did occur exclusively from the triplet
state. Backster and Sandros11 were the first to study the transfer
of energy from the excited state of benzophenone to various acceptors
in solution. Of particular importance was the quenching of excited
benzophenone by biacetyl with the concommitant formation of biacetyl
phosphorescencelz. Hammond and coworkers13 studied the photoreduction
of benzophenone by benzhydrol in detail. Analysis of their data
indicated that the lifetime of the chemically active state must be so
long as to preclude the possibility that it is the singlet state.
Quenching by added paramagnetic species supported the assignment of
the triplet state as the reactive species. Finally, several groups
have quenched the photoreduction of benzophenone with naphthalene which
has a higher singlet and a lower triplet energy than benzophenonel“,15a16

and shortly thereafter, the triplets of benzophenone17 and 2-aceto-

naphthoneufi anong others, were observed and their lifetimes measured

24

directly by flash photolysis.

Since it has been shown that the quantum yield of intersystem
crossing for most aromatic carbonyls is unity19:2°, and that the
rate of intersystem crossing is extremely rapid (>1010 sew)“,22
it can be safely assumed that in general aryl ketones will yield
only products that are derived from their excited triplet states.

One of the najor problems, which has stymied research workers for
almost a decade now, arises from the fact that,a priori, there are
two triplets that can react in aryl ketones. These are the n,n* triplet
of the carbonyl group, which is known to be the reactive state in
dialkyl ketones, and the n,n* triplet state that arises from the n
electrons of the aryl ring, and which is generally believed to be
unreactive. Which triplet state is lowest can be determined from the
phosphorescence lifetime at 77°K. If the lifetime is long (on the
order of 10'1 to one second) the lowest state is n-n* in nature, and

if the lifetime is short (less than 10'2

second), the lowest triplet

is of the n~n* type23. Most of the early work dealt with the relation—
ship between photochemical reactivity and the nature of the lowest
triplet.

As mentioned above, reports in the early literature5,6,7 indicated
that naphthyl ketones and ketones possessing electron donating ring
substituents usually fail to photopinacolize. Pitts and coworkersz‘H25
reinvestigated the photoreduction of several p-amino benzophenones in
isopropanol and found that no product formation occurred after twenty-
four hours of irradiation, while p-phenylbenzophenone reacted only one

tenth as efficiently as benzophenone. Similar results were reported

by Beckett and Porter26 in their study of p-hydroxybenzophenones.

25

Hammond and Leermakers27 reinvestigated the photoreduction of l-
naphthaldehyde and 2-acetonaphthone and found that these compounds,
unlike benzophenone, were not photoreduced by secondary alcohols.
Photoreduction occurred only in the presence of strong hydrogen donors
like tributylstannane. The above investigators, citing spectroscopic
data as supporting evidence, suggested that the loss of photoreactivity
in each case was due to a reversal of the order of the two triplet
states as compared to benzophenone,with the n,n* level being the lowest
state in these ketones. Porter later changed his mind concerning this
explanation28, and along with Suppan29, has become strong advocate for
the existence of a charge transfer state which is responsible for the
lack of reactivity in ketones substituted with electron donating groups.

This CT state, rather than a reversal of the two triplet levels,
is used by them to explain the fact that the quantum yields for photo-
reduction of p-hydroxy- and p-aminobenzophenones is quite substantial
in cyclohexane, but dr0ps to less than 10'4 in alcohol. Suppan3O has
reinvestigated the photoreduction of 2-acetonaphthone and has found that
it does photoreduce somewhat in ethanol, the quantum yield of ketone

3 3 at 90°C on

disappearance going from 2.8 X 10- at 20°C to 12 X 10'
irradiation at 3650 A.

The n-n* triplets in the above mentioned unreactive compounds
are quite a bit lower in energy than the n-h* triplets. Yang and
coworkers31’32, in an attempt to make the transition from a n,n* state
to a n,n* state a gradual one, introduced successive methyl and methoxy
groups onto the ring of acetophenone and then studied the ability of

these ketones to be photoreduced. They found that a direct relationship

26

existed between the decrease in reactivity, measured as the lowering
of the quantum yield, and the increase in phosphorescence lifetime in
going from parent ketone to ketones with several methoxy groups.

In an analogous investigation of the photoreduction of benzophenone,
however, Yang and Dusenberry33 found that the introduction of methyl
substituents had a much smaller effect on the reactivity of the benzo-
phenones as compared to the acetophenones. This difference in the
effect of substituents on the two types of ketones was attributed to
a smaller interaction between the two states in the diaryl ketones as
compared to the aryl alkyl ketones. Wagner and Capen31+ have carried
out a detailed investigation into the photochemistry of pyridyl ketones
and have correlated the photochemical reactivity of these compounds
with the position of the aza substituent in the ring. Their results
indicated that the triplet states of pyridyl ketones are 2-3 times
more photoreactive than the analogous phenyl ketones. This increased
reactivity is attributed to an inductive effect on the lowest n,n*
triplet. Finally, both Wagner35 and Yang32 have attempted the
disentanglement of the various factors that affect the quantum yields
of ring substituted aceto- and valerophenones by performing quenching
studies on these compounds. Although these experiments show that
electron-donating substituents, which produce n,n* lowest triplets,
also decrease triplet state reactivity, they did not tell unequivocally
what the nature of the reactive state is.

The aryl alkyl ketones are an excellent example where divergent

tools of chemistry can be brought to bear on the same problem. Thus

27

the study of the spectroscopy of these ketones has been closely related
to the elucidation of their photochemical behavior and there has been a
determined effort to interpret photochemical reactivity on the basis of
spectroscopic findings.

Yang and Murov36, in one of the earlier papers on this topic,
reported that l-indanone and other phenyl alkyl ketones exhibit two
groups of emission in their phosphorescence spectra at low temperatures.
These two groups could be resolved and the individual lifetime of each
Imfisured. They assigned the short*1ived component to a n,n* state,
while the longer lived emission displayed mixed n,n* and n,n* properties.
Kearns and Case23, using the technique of phosphorescence excitation,
found that all acetophenones substituted with electron donating
substituents seemed to possess lowest lying n,n* triplet states, with
the n,n* state lying slightly above it. Similar results were found
for various butyrophenones. The two triplets in these ketones are
extremely close together, so that it has been very difficult to establish
which state is actually the lowest one.

It is well known that the energetic disposition of the two triplet
levels, aside from being substituent sensitive, is also strongly solvent
dependent37. In general, n,n* transitions are shifted towards higher
energy (shorter wavelength) in going from nonpolar to polar solvents
whereas the h,n* transitions are shifted to lower energy (red shift).
The blue shift of the n,n* transitions is attributed, at least in part,
to solvation (or hydrogen bonding in the case of protic solvents) of
the lone pair of electrons on the oxygen in the ground state. In the

n,h* transition a n electron is removed from the oxygen, and the

28

remaining n electron is not adequate to sustain the hydrogen bond in
the excited state. Thus the breaking of the hydrogen bond in going
from ground to excited state increases the energy required for the
promotion of an electron from a n to a W* orbital. The shift to lower
energy for the n,h* states, on the other hand, in going from non-polar
to polar solvents is due to the fact the h,n* excited states are quite
polarizable and can become strongly solvated in the excited state.

Lamola38 has been able to invert the lowest triplet states in
acetophenone by going to very polar solvents. In hydrocarbon solvents
the n,n* state is lowest, while in phosphoric acid the 3La h,n* state
(vide infra) predominates.

The observation of two components in the emission spectra of
butyrophenones39 and other aryl ketones”:1+1 seems to be a general
phenomena. These two components are observed when the ketones are in
polar glasses39 or absorbed on silica“2. The general interpretation
of the nature of the two components in aryl alkyl ketone phosphorescence
spectra is that the shorter lived one is due to the n,n* state and the
longer lived one predominantly to the n,n* state. Both Wagner‘+3 and
Lim“” disagree with this interpretation since it implies that the two
states are not in equilibrium and that the rate of internal conversion
between the two states is slower than phOSphorescence. Lim postulates
that the long lived component may be due to emission from an excited
enolate ion since 2,2-dimethyl-l-indanone, which has no a hydrogens,
has only a short lived component in its spectrum.

Since neither quantum yield and quenching studies, nor spectro-

scopic work have so far given a clear-cut indication as to the nature

29

of the reactive excited state in these ketones, it was decided to
exploit the inherent difference in reactivity of the 3n,n* and 3n,n*
states towards hydrogen abstraction in order to solve this problem.
The n,n* excitation of ketones involves the promotion of a non-bonding
electron belonging to the carbonyl oxygen to an antibonding orbital of
the carbon oxygen bond. Thus a n,n* triplet can be visualized as a
system that contains two n electrons, one n electron and one n* electron.
Both molecular orbital (a) and valence bond (b) representation indicate

that the excited carbonyl group should be a partially bipolar species.

)9 (7a . (,7 (79

$7‘ 4’0-----09 (a)

/000 \ 7/000

 

 

0,! 4i," ‘ 06°
-—— <e——+> ”C ———-9 ée—4> c:::o (b)
l l_ . l_ .
6 6 6 6

The oxygen with its seven electrons around it becomes a radical-
like electrophilic species whose electron deficiency is present as
a half vacant orbital in the plane of the carbonyl function. Walling
and Gibian“5 as well as Padwa‘+6 found that the behavior of benzophenone,
whose lowest triplet possesses essentially pure n,n* character”7,
paralleled very closely that of t-butoxy radicals in the abstraction
of hydrogen from various donors. The excited benzophenone triplet

did however show a somewhat greater selectivity and also a somewhat

30

greater sensitivity toward electron availability, which is in accord
with the notion that it is an electrophilic species.

On the other hand, promotion of an electron of a n bonding orbital
to a n* antibonding orbital results in a n,n* state. As mentioned, the

3n,n* state with respect to the 3n,n* state is strongly

position of the
solvent and substituent dependent”8. Lamola38, in an elegant piece of
work, demonstrated that the so called 3n,n* state of phenyl alkyl
ketones corresponds to the 3La’ or lowest triplet state, of benzene.
One of the proposed canonical structures for this state is a 1,4

biradical“9 in which the inductive effect of the electron donating ring

<77 <7 ~ <7
lLa 3

La

 

substituents can be transmitted to the carbonyl group. Porter28 and
Suppan29 have proposed that a CT state exists in ketones where strong
electron donating substituents are present. Whether or not this is
actually the case, it is fairly certain that in ketones having lowest
3n,n* states, the carbonyl oxygen of the excited molecule should be

at least as electron-rich as it is in the ground state. This leads

to the expectation that, in the n,n* triplets of ketones having electron
donating ring substituents, the carbonyl oxygen should be a nucleophilic
species possessing a minimum of alkoxy radical-like properties. Thus

3

the behavior of ketones reacting from a n,n* state can be expected to

3

be radically different from those that react from a n,n* state.

31

We have focused our attention on the photochemistry of the p-
methoxyphenyl alkyl ketones. The two triplets in these ketones are
close together”:31 with the n-n* triplet only a few kilocalories lower
than the n-n* triplet. Furthermore, Wagner35 and Yang32 have also
shown that the reactivity of these compounds is less than that of
ketones possessing lowest n-n* states such as simple phenyl alkyl
ketones. The work of Wagner and Kemppainen35 indicated that the lowering
of the reactivity in these ketones, although significant, was not of
such great magnitude as to preclude the use of these compounds in
hydrogen abstraction studies.

The reaction system chosen as the most suitable one for the
determination of the nature of the reactive states in the p-methoxy
ketonesnms the Norrish Type II eliminationso. This process has been
one of the most actively studied photochemical reactions of the last
few yearsSI. The major reaction products are an olefin and the enol
of a smaller carbonyl compound52, although in most instances cyclo-
butanols are also formed in substantial amounts53. YangS3, who was
the first to observe the cyclobutanol formation, hypothesized that
a 1,4 biradical intermediate is formed during the course of the

reaction.

 

32

 

 

0* 0H
76.72 '
1 R, > Ri?\_/ԤR2 (9)
3
I)” / \l/
R1C=CH2 H 2
R1 R3 (10)
+ R2R3C—-CH2

 

 

 

The quantum yield of the Type II photoelimination is usually
less than unity for most ketones. This is somewhat suprising since
in most cases the reactivity of the triplets is high enough to compete
very efficiently with radiationless decays”. Wagner and Hammond have
postulated that this inefficiency may be due to the reversal of 'the

hydrogen transfer step5”.

 

\/

1 P1

Work by Yang and CoulsonSS supports this suggestion since they
showed that during the photolysis of 2-hexanone-5,5-d2 the y hydrogen
is directly involved in whatever process returns the majority of the
excited ketone molecules back to the ground state. Further proof for
the existence of the biradical has come from recent solvent and
optical activity studies by Wagner and his group. They found that the
quantum yield for valerophenone disappearance rises from 0.45 in hydro-
carbon solvents to unity in alcohols and acetonitrile55. The
explanation for this effect, which has been observed with other aryl

ketones3“, is that the polar solvent will hydrogen bond with the

33

hydroxyl group of the biradical and in this manner prevent the dispro-
portionation of the hydroxyl hydrogen back to the y carbon, which
allows all the biradicals to proceed on to product. The dis-
proportionation of the biradical back to ground state ketone is the
only logical explanation for the fact that the reactivity of ketones
may be increased by certain y carbon substituents, but yet the overall
quantum yield will be lower in these compounds than it is for the
unsubstituted ketone557. Since ring substituents affect the nature
and the reactivity of the triplet state, it is not unreasonable to
expect that they also influence the behavior of the biradical
intermediate. This was shown to be the case for several substituted
butyrophenonesSB.

The recent studies of Wagner and Kelso59 involving the photolysis
of optically active y-methylcaprophenone are the best evidence for a
biradical intermediate. Since the rates for the various photoprocesses
for this type of ketone are known, they were able to predict the
amount of racemization that should occur due to reverse hydrogen
transfer at various percent conversion. The results obtained were
in agreement with the predicted values. Furthermore, no loss of
optical activity occurred when the ketone was photolyzed in the
presence of an alcohol.

Early work by several groups indicated that in the case of
dialkyl ketones the Type II reaction may proceed from both the
singlet and triplet states“,61 since product formation is not
completely quenched by high concentrations of dienes. Elimination

from phenyl alkyl ketones, on the other hand, can be quenched

34

completely5”,which is taken as evidence that the reaction proceeds
completely from the triplet state.

There are several reasons why the Type II process is the reaction
system of choice in the study of the excited states of ketones. The
photoreduction of benzophenone or acetophenone is a bimolecular
abstraction process based on the donation of a hydrogen atom from
either solvent or added donor to the carbonyl of the ketone. Thus
the act of varying the hydrogen donor in order to obtain data on the
reaction of a ketone with various molecules having different C-H
bond strengths at the same time alters the environment of the carbonyl
group and introduces additional solvent and viscosity effects.
Furthermore, intermolecular photoreduction is not really a I'clean
reaction" in the sense that the products formed are fairly independent
of reaction conditions. Pitts8 and other workers62 have found that
there are several labile intermediates formed in the photoreduction
of benzophenone (and presumable acetophenone also) in isopropyl
alcohol which affect both the kinetics and quantum yield of the
reaction. Another disadvantage of intermolecular photoreduction is
that the products usually formed are high melting, making analysis of
product formation by vpc techniques difficult. Instead, starting
material disappearance is usually measured by spectroscopic means in
the analysis of the reaction.

0n the other hand, since in the Type II reaction hydrogen
abstraction is a unimolecular process, substituents can be introduced
on or near the y carbon atom which can alter the y C-H bond in a

predictable manner without affecting the physical environment of the

35

whole molecule. This prevents any extraneous solvent and viscosity
effects from interferring in the study of the ketone triplet reactivity,
since the solvent can be kept the same for a whole series of experiments.
The products formed in the Type II process are also fairly well
characterized and can be analyzed conveniently on a gas chromatograph,
which makes life easier from an experimental standpoint.

With the above considerations in mind, it was decided that a

comparison of the effect on the rates of hydrogen abstraction between

a series of p-methoxybutyrophenones and their unsubstituted counter-
parts where successive substitution by electron withdrawing or electron
donating groups drastically alters the electronic environment of the
hydrogens on the y carbon, should be a good monitoring procedure
capable of indicating what the nature of the excited state is in the
photoreduction of these ketones.

There are two mechanistic possibilities for the hydrogen

abstraction step by aryl ketone triplets. They are:

a. Only the n-n* type is the reactive species, even when it
is not the lowest.

b. The two close lying triplets interact to form mixed states
which possess reactivity characteristic of each individual
state, such that the lowest triplet can react even when it
is mostly n,n* in character.

The reactivity of the electrophillic n-n* triplet of the benzoyl

group in the phenyl alkyl ketones should show a different type of

selectivity towards C-H bonds having different electronic environments,

36

than the n-n* triplet of the p-methoxy ketones, if it is the reactive
state. Thus inductive effects by electron-withdrawing or donating
groups on hydrogen abstraction should give an indication as to what the
nature of the reactive state in ketones having lowest n-n* triplet

level is.

The ability to use the Type II reaction as a monitoring tool is
predicamfl upon an understanding of the various processes that occur
following the initial absorption of a quantum of light by the ketone.

A general mechanism which takes into account equilibration between

the two triplet states of p-methoxyphenyl alkyl ketones is given

below. Absorption of a quantum of light by the ketone in solution
results in the formation of the first excited singlet state, *K],
(rate = Ia) which can intersystem cross to the upper n-n* triplet,

Tn (rate constant = kisc)' The triplet Tn can decay to the lower

n-n* triplet, Tn, (rate constant equals k21), react to form a biradical
intermediate BR (rate constant equals k2), decay to ground state ketone
(rate constant = ki) or transfer its energy to a quencher Q (rate
constant = k").

q
Process Rate

K fl; *K] Ia (12)
k.

]K* —‘S—C—> Tn kisc (*K‘) (13)

Tn Ta BR k: (Tn) (14)

Th ——>i KO k,- (Tn) (15)

37

Process Rate
T ———>k2‘ T k (T) (16)
n n 21 n
kn
Tn + o —‘i—> K" + 0* kg (an0) (m

The T" triplet may thermally equilibrate back to the upper
triplet, Tn’ (rate constant = k12), react to form the biradical
intermediate, BR, (rate constant = k1), decay to ground state ketone
(rate constant = kd), or be quenched by a quencher (rate constant =

k" .
q)

 

Process Rate

k12
Tn :> Tn k12 (Tn) (18)

kTT
T11 —L—> BR k: (Tn) (19)
T ————>kd K0 k (T) (20)
n d n

kTI

T.” + Q 7‘1—7 K° + 0* kg (37(0) (21)

Finally, the biradical BR may decay to ground state ketone
(rate constant = kd), cleave to yield an enol and an olefin (rate
constant = ks),or cyclize to yield a cyclobutanol (rate constant =

kc).

Process Rate
BR XL? K0 k. (BR) (22)
d
k
s .
BR —-———7;> Enol and Olef1n kS (BR) (23)

38

Process Rate

k
c
BR ———~——-{3> Cyclobutanol kC (BR) (24)

These processes can be depicted in terms of an energy level
diagram as shown in Figure 3.

The quantum yield for product formation, which is the only
quantity that can be measured directly under steady state photolytic

conditions, may be defined as:

Number of molecules of final product formed (25)
Number of quanta of light absorbed by starting material

 

This quantum yield is the product ofihe probabilities that each

step in the overall process will occur. For the Type II reaction,

¢isc¢8R¢P (25)

where ¢- is the probabiiity that the absorption of a quantum of light

lSC
will produce the requisite triplet excited states, ¢BR is the probability
of biradical formation from the excited states, and 6p is the probability
that the biradicals will proceed on to products.

By using steady state kinetics, expressions for the quantum yield
in terms of the rate constants for the various processes may be
derived63. The expression for the three kinetic possibilities for

product formation for the p-methoxyphenyl alkyl ketones are as follows:

a. Assuming that only the lowest n-n* triplet reacts, then,

- TI = , 11
¢ - ¢isc¢pkrT ¢iscd’p kr (27)

 

m¢c0pm¥ PxxF< Facmcnxx05pmzia com Ememmwo wxmcopnco umw+weoz

 

 

mmmemoee cowpummm

.m mmsoHn

 

 

 

 

 

 

 

 

 

 

 

 

 

 

\
/
> \ \
mpusuoea
ex
.8 a
x + a F
. m
moHex x H
_aeweacwm a
flagex
N_x Fm
\ J]
.x .m

 

 

 

 

 

40

where l, the lifetime of the triplet, is defined as

i = 1 (28)

b. If only the upper n-n* triplet reacts, then the expression for

the quantum yield becomes:

- n = . n
¢ _ XnkrTeq¢isc¢p ¢isc‘I’p Xnkr (29)

n
Xn(kr + kl) + Xnkd

where Xn and XTI are defined as the mole fractions of the n-n* and

n-n* triplet, respectively.

xn = k21 and xn = k12 (30)
12 k12 T k21

and Teq, the equilibrium lifetime of the two triplets in this case,

is defined as:

1

1
eq x (k" + k.) + x k
n r l n

 

d

c. Finally, if both triplets react, the quantum yield in the absence
of quencher is given by equation (32)

n n
' _ ¢isc¢p(xnkr + Xnkr)
n r i n r d

O _ n n
4 — (Xnkr + Xnkr) T

41

where Teq , the equilibrium lifetime of the two states, equals the
denominator of the right equation. Since the rate constant for decay
from the upper triplet, ki’ is known to be negligibly small compared

to k2, it may be ignored35. Equation (32) then becomes:

 

0 _ n n .
¢ T (Xnkr + Xnkr) Teq ¢p¢isc (33)
where 1
T ' =
eq n n
Xnkr + Xn(kr + kd) (34)

Assuming that k3 = kg== kq, since ketone triplets are known to
be quenched by dienes at the diffusion controlled ratee”, one can write
the expression for the quantum yield in the presence of quencher in the

following way:

 

 

 

 

n n
~ , Xnkr + Xnkr
¢ T ¢isc¢p n n (35)
Xn(kr + kqto])+ x"(kr + ka + quQI)
Division of equation (35) into equation (33) yields the following
expression:
n n
¢isc¢p (Xnkr T Xnkr)
n n
.9: z wk") + x"(k‘” + kd) (36)
¢ n n
¢isc¢p (Xnkr + xnkr)
n n
xn(k, +kq[o])+ xn(kr + ka + quQ])
which reduces to:
n n
.23. z Xn (kr T kJD])+TXn(kr + kd T kq[Q]) (37)
9

 

x k"+X(k"
11

n r r + kd)

42

and on multiplying out, equals:

 

 

n 11
_23_ = Xnkr + Xnkr + Xnkd + (Xn + X“) kg[Q] (38)
n TT
xnkr + Xnkr + Xnkd
which can be rewritten as:
,o __ (xn + x") quQ]
TET' T 1 T n n (39)
Xnkr + XNIkr + kd)

Since (Xn + X") equals unity, substitution of the value for T ' as

eq
given in equation (34) into equation (39) yields the familiar Stern-

Volmer expression.

(1’0 _ I
T - 1 + kq[Q]Teq (40)

Equation (40)allows one to determine the observed lifetime of
the ketone triplet. By choosing a quencher, such as 2,4-dimethyl-

2,5-hexadiene, which is known to quench triplet reactions at close

91-1

to diffusion controlled ratesP“ with kq = 5 x 10 MT sec in benzene

at room temperature65, plots of 90/9 versus quencher concentration

should give a straight line with an intercept of one and a lepe of
kq Tobs.
lifetime can be obtained from expression (41).

Since the value of kq is known, the value for the observed

Tobs = l = slope of S-V plot
obs diff
kr + Xflkd kq

 

II. RESULTS

A. Solvent Effects

 

The effect of polar solvents on the quantum yield for a series
of ketones was determined in order to evaluate how ring substituents
affect the behavior of the biradical intermediate. Solutions 0.10 M
in ketone in benzene or benzene and alcohol as solvent were
irradiated at 3130 A to approximately 20 - 25% conversion. The
samples were analyzed by vpc and quantum yields were determined
relative to gj§;piperylene/acetophenone actinometersl9. Addition
of polar solvents caused the quantum yield to rise until it reached
a maximum value at which point it usually leveled off or slowly
decreased. Once the quantum yield reached a maximum, it was assumed
that enough polar solvent was present to insure that all the biradicals
formed proceeded on to products and any remaining inefficiency was due
to triplet decay. Solvent effect studies provide a convenient means
of determining 6p, the probability of product formation from the
biradical intermediate,since 6p equals the ratio of the quantum yield
in benzene to the maximum quantum yield in benzene plus alcohol.
Table 2 lists the quantum efficiencies for a series of substituted
phenyl n-butyl ketnes in benzene and benzene plus alcohol, along

with the corresponding values of 6p.

43

44

TABLE 2. Quantum Yields for Substituted n-Butyl Phenyl Ketones

 

 

__.-_——-..._—...._...._ - -.._ ’._ _ __ _._..
‘ .— .-._ -‘r... .. .__- .- _-..'_ _. ___._‘-__.-._.___ “-w. . - __ - - - - ..-...~ -—

 

Substituent ¢ benzenea 4 Alc.b 0p
p-MeO 0.18 0.26: 0.69
p-Me 0.39 0.80 0.50
None (H)d 0.40 1.00 0.40
p-Cl 0.30 0.80 0.37
3-(N)d 0.29 1.00 0.29
2-(N) 0.18 1.00 0.18

 

aTotal quantum yield for reaction of ketone. bMaximum quagtum yield
values as obtained from plot of 4 versus [t-butanol]. Values for
4 max are 0.26 and 0.25 asing methanol and pyridine as added polar
solvents, respectively. Values for valerophenone and the aza
substituents were determined by Wagner and Capen3”.

B. Quenching Studies

Linear Stern-Volmer plots were obtained in the quenching of the
anisyl ketones by low concentrations of 2,5-dimethyl-2,4-hexadiene.
As mentioned, this diene is known to quench triplet reactions in
benzene at room temperature at close to the diffusion controlled

9 1

ratee“ with kq equalling 5 x 10 MT secT155. Solutions 0.10 M in

ketone, with benzene as solvent and octadecane as internal standard,
were irradiated at 3130 A to less than 25% conversion, and quantum

yields were determined by either gj_s_-piperylene/acetophenone19 or

valerophenone actinometry.

45

The quantum yield as given in equation (26) is:

¢ T ¢isc¢BR¢p (26)

which can be rewritten as:

_ obs obs
¢ - kr T ¢isc¢p (42)
where kObs TObS has been substituted for ¢BR since
obs
¢BR T kr
obs (43)
kr + Xnkd

Rearrangement of equation (42) yields the following equation for

obtaining kgbs:
b 4
k? S = W“ (44)
¢p¢isc

Thus in order to calculate the values for kobs, it was necessary

to determine the four variables of equation (44). The quantum yield,
as mentioned above, was determined directly by actinometry. The
observed lifetime, 10b5, was determined from the Stern-Volmer plot
using equation (41), and the values for 6p, the probability of the
biradical going on to product, were measured in the solvent effect

studies. Finally, ¢ the probability of forming the triplet states

isc’
can be determined by comparing the amount of gi§7piperylene
isomerized to trans by a ketone such as benzophenone, whose quantum

yield of intersystem crossing is known to be unity19 to that isomerized

46

by the methoxy ketones. In this manner, it was found that the quantum
yield of intersystem crossing for o-, p-, and m-methoxyvalerophenone

was unity.

obs

Once kr was known, it was also possible to calculate the values

for kgbs, the amount of decay from the n-n* state in the p-anisyl

ketones, by rearranging equation (41) which is:

 

Tobs : obs l (4])
k? + Xnkd
to
obs _ obs obs
kd - I/T - kr (45)
where kgbs equals Xnkd'

A summary of the kinetic parameters obtained for the p-anisyl
ketones is given in Table 3, and Table 4 lists the data for the
analogous phenyl ketones. Table 5 contains the data for p-methoxy-
valer0phenone using methanol as a solvent and naphthalene as the

diffusion controlled quencher66.

47

.Auxmp mmmv cowmgm>cou 0:00 000 cc 0200:0000 00010000 000 mmzpm>

.0000Lm>coo 70 co ummmm .poFa mcwcucmzc LmEFo>-cempm we maopmo .Fogoupm Fxpaa-u 0000 0:00:00 :0

 

 

 

0
100005001 0000000 mo upmwx Easwxmzn .wcmNcmn :0 000005107 0030000 HH 0001 $0 00000 50010000
0.0 000.0 00.0 000 0_0.0 000.0 001000010 0-1
0.0 00.0 00.0 0000 0_0.0 000.0 10010010 0-1
0.0 00.0 00.0 0000 10.0 10_.0 00000010010 0-1
0.0 00.0 00.0 000_ 00.0 00.0 01010010 0-1
0._ 00.0 01.0 0000 00.0 010.0 010010 0-1
0._ 00.0 00.0 0000 10.0 000.0 010 _-1
.0 . L . . a x05 0:00
FM0000 mop x F.0000 oo_ 0000 000 @101 e 01-: e x no 00 1
10100100 0010 0000001 000001-0 000000 00 000000000_000010 .0 00010

=
0

 

48

TABLE 4. Photoelimination of Select Phenyl Ketones PhCOCHZCHZRa

 

 

d
b -1C n 7 -l
R 0“ qu, M kr’ 10 sec

Pl CH3 0.36 660 0.8

P2 CH2CH3 0.33 40 12.5

P3 CH(CH3)2 0.25 11 45.0

P4 CHZCHZCOZCH3 0.64 125 4.0

P5 CHZCHZCN 0.46 500 1.0

P6 CHZCOZCH3 0.46 440 1.1

 

aData obtained by Wagner and Kemppainen57. b Quantum yields of
acetophenone formation in benzene. CSlope of Stern-Volmer quenching

plot, averages of duplicate runs. dk: = 1/T.

TABLE 5. Photoelimination of p-Methoxyvalerophenone in Methanol

 

 

 

—la obs -1 obs -l
Ketone 011 qu, M 1/T , sec kr , sec
PMVP 0.09 13000 1.15 x 106 1.0 x 105

 

aQuencher is naphthalene, kq equals 1.5 x 1010 secT1 M

III. DISCUSSION

A. Mechanistic Interpretation

 

Yang3‘a37, as a result of his work on the photoreduction of
substituted acetophenones, has suggested that the hydrogen
abstraction reactions of phenyl alkyl ketones substituted with
electron-donating groups occur from a mixed lowest triplet state.
This state is mostly n-n* in character, but has sufficient n-n*
character mixed in due to vibronic coupling to impart it with a
slight reactivity. Wagner35, on the other hand,lns suggested that
the alternate possibility of thermal equilibration between the
lower, unreactive n-n* triplet and the upper, reactive n-n* state
cannot be ignored.

This suggestion is not unreasonable since participation by
upper reactive triplet states has been invoked in other photochemical
reactions. Among these are the suggestion by Yang68 that an upper
triplet may be reacting in the photoaddition of 9-anthraldehyde to

2,3-dimethyl-2-butene, and the proposal by Liu and Edman69 that

 

 

energy transfer from anthracene to bis(perfluoromethyl) bicyclo[2.2.2]-

octatriene occurs from an upper triplet level.

49

50

Our data strongly support the suggestion that hydrogen
abstraction in the p-methoxyphenyl alkyl ketones occurs almost
exclusively from the upper reactive n-n* triplet rather than the
lower h-n* state. Comparison of the data for the p-methoxy ketones
A-l, A-2, and A-3 from Table 3 and the phenyl ketones P-1, P-2,
and P-3 from Table 4 indicates that the selectivity of the anisoyl
and benzoyl triplets toward primary, secondary, and tertiary C-H
bonds is almost identical. The selectivity shown towards abstraction
of a primary hydrogen versus a tertiary one is manifested as an
approximately fifty-fold increase in the relative kr values for each
ketone series, despite the fact that the triplet of the anisoyl
group is only about l/200th as reactive as the triplet of the benzoyl
group. Even more significant are the results obtained by comparing
the data for ketones A-2, A-4, and A-5 with the data for ketones P-2,
P-4, and P-5. All three ketones in each series possess secondary
C-H bonds in the y position. Ketones A-4 and P-4 have an ester group
in the 6 position and ketones A-5 and P-5 are substituted with 6-
cyano groups. The inductive effect of the two electron-withdrawing
groups was eXpected to strongly deactivate the electrophilic n-n*
triplet of the benzoyl group in the abstraction of a hydrogen from
the y carbon. Indeed, the decrease in relative reactivity is greater
than an order of magnitude in going from valerophenone to 6-cyano-
valerophenone. An identical effect is exerted on the triplet state
reactivity of the anisoyl group. The relative reactivity of its
triplet drops from 1.00 to 0.06 in going from p-methoxyvalerophenone

to 6-cyano-p-methoxyvalerophenone.

51

The observed inductive effect of the two electron-withdrawing
groups on the photoreactivity of the p-methoxy ketones is opposite
to the one expected if the lower n-n* triplet was the reactive
state in these compounds. It is extremely unlikely that the nucleo-
philic n-n* triplet of the methoxy ketones, which must have a
relatively electron-rich oxygen atom (even if some n-n* character
is present due to vibronic coupling),should be subject to identical
inductive effects as the electrophilic n-n* triplet of the benzoyl
group. This leads to the conclusion that, rather than having
hydrogen abstraction from the lower n-n* state, almost all reaction
takes place from the upper n-n* triplet.

If hydrogen abstraction is occurring from the upper triplet
state, then the only difference between the observed rate constants
for abstraction by the anisoyl triplets as compared to the rate
constants for the benzoyl triplets, which equal k2, should be due
solely to the lower number of reactive n-n* triplets present in the

p-methoxy ketones. In that case,

obs _ n
kr - Xnkr (47)

Equation (47) can be rearranged to equation (48),

kobs
r

 

71
kr

which allows calculation of the mole fraction of reactive n-n*

triplets present in the p-methoxy ketones. Substitution of

obs

appropriate values for kr

from Table 3 and values for k: from

Table 4 into equation (48) yields 0.005 as the average value of

52

X", the mole fraction of reactive n-n* triplets present in these
ketones.

Knowledge of the numerical value of Xn permits calculation
of the approximate energy difference, A E, that separates the two
triplet states in these ketones at room temperature in benzene.
Equation (49) is the general expression for determining the value

of K, the thermodynamic equilibrium constant based on concentrations.

e-AE/RT (49)

where R = the gas constant and T is the temperature in degrees
Kelvin. The equilibrium constant, K, in this case, represents

the ratio of Xn to X“. That is,

K T Xn T k12

k
" 21

(50)

 

If 0.005 is used as the value for Xn’ then X1T = 1.00 - 0.005 T
1.00 and K will also equal 0.005. Rewriting equation(49) into

equation (51) and substituting the values for K, R, and T into
AE = -RT In K (51)

it yields a value of about 3 kilocalories per mole as the energy
separation between the two triplet levels.

Thus, rather than hydrogen abstraction occurring from the
lowest n-n* state, all reaction takes place from a level that is

about three kilocalories above the lowest vibrational level of

53

the triplet n-n* state. This upper level should be mostly n-n* in
character regardless whether or not it is discrete or mixed with
the upper vibrational levels of the n-n* triplet70. Even if
vibronic mixing of the states occurs, it apparently is too weak to
be the primary factor that determines the behavior of the anisoyl
triplets towards different types of hydrogens71. Although the
exact rate at which the lower n-n* triplet returns to the reactive

9 secT1

upper n-n* triplet is not known, it should be at least 10
if only vibrational excitation is required, since decay from a
higher vibrational level to a lower one proceeds at a rate greater

than 1012

secT122. Saltiel has shown that the reasonably short
lived triplet of benzophenone apparently has enough time to
equilibrate thermally with the excited singlet state, which is
believed to lie five kilocalories above the triplet72.

The data in Table 3 and Table 4 deserve one or two additional
comments. It should be pointed out that ketone A-6 and P—6 have
their electron-withdrawing substituent attached directly to the
y carbon atom. The low quantum yield and high T value for the
anisyl ketone indicate that its behavior is similar to that of
ketones A-4 and A-5. However, determination of the kinetic rate
constants for ketones A—6 and P-6 are complicated by the fact that
the olefin formed in the Type II reaction, methyl acrylate, is
also a quencher. In ketones where a product is a quencher, the
quantum yield will be inversely proportional to the per cent

conversion that the reaction is carried out to. The data given

in Table 3 and Table 4 for ketones A-6 and P-6 are those obtained

54

under conditions where quenching by product occurred. The value
for kgbs for ketone A-6 may be correct, but the larger kd value,
and consequently the shorter lifetime, are attributable to quenching
by product. Thus compounds of this type belong to a separate class
and deserve to be investigated further.

The other additional point of interest concerns the rate of
decay of the n-n* triplets. The values obtained for the ketones
A-l through A-5 are fairly uniform, but the puzzling feature is
the fact that the rate of decay is extremely rapid. The rate of
decay for many n-n* triplets at room temperature in benzene of
comparable purity to that used here is 103 secT173, which is some
three orders of magnitude slower than that for the p-methoxy
ketones. This rapid rate of decay most likely is an inherent

property of the n-n* triplets of the p-methoxyphenyl alkyl ketones.

B. Solvent Effects

 

Our result indicating that the energy separation between the
upper n-n* triplet and the lower n-n* state is only 3 kilocalories
per mole in benzene is somewhat in disagreement with the value of
5 to 6 kilocalories obtained both by Kearns and Case23 and by Yang31
using phosphorescence excitation methods. However, their results
are for ketones irradiated in polar glasses at 77°K. Analysis of
the data in Table 5 for the photolysis of p-methoxyvalerophenone
in methanol, as given below, indicates that the energy difference

between the two triplet levels increases in going from a non-polar

55

(benzene) to a highly polar (methanol) medium and approaches the
value reported by Yang and Kearns.

The Stern-Volmer slope for the quenching of p-methoxyvalero-

l

phenone triplets in methanol by naphthalene is 13,000 MT , which is

almost six times as large as the corresponding slope in benzene.
This dramatic increase in the slope must be due to either an
increase in the rate constant for quenching the triplet state, kq,

or an increase in the observed lifetime of the triplet state,

10b5, or both,since the slope equals the product of these two

10 -1 -1

terms. Using a high value of 1.5 x 10 sec M as the quenching rate

constant, Equation (41) gives a value of 8.7 x lOT7 seconds for
10b5, and 1/1Obs equals 1.15 x 106 sech. Equation (44) gives
a value of l x 105 for kObS, which in turn gives a value of about

r
0.0008 for Xn’ the number of reactive n-n* triplets present in

methanol. The energy separation corresponding to this value of
Xn is about 4.3 kilocalories per mole, which approaches the

reported value. The kd value of 1.05 x 106 secT1

is, as expected,
close to that found in benzene.

The effect of polar solvents on the two triplet levels of
valerophenone (VP) and p-methoxyvalerophenone (PMVP) is shown in

the diagram below, (where HC is hydrocarbon).

 

 

 

 

. ' , PMVP in
PMVP 1" HC ,1 0 -— T 3n--n»* p01ar

3 VP in HC / ’ solvent

TT-TT —-———- ,
\\/
/ ‘\\
3n-n* ’1 '\
3 *

56

It is readily conceivable that polar solvents may increase
the magnitude of the energy separation between the 3n-n* state and
the 3n-n* state until a point is reached where any increase in
quantum yield due to solvation of the biradical is outweighed by
the decrease in the probability of biradical formation due to the
lowering of the equilibrium concentration of reactive n-n* triplets.
If the energy gap between the two states becomes sufficiently large,
the concentration of reactive states will be so small as to render
the ketone unreactive from the n-n* triplet, and the only
reactivity remaining under these circumstances would be that which
is imparted to the n-n* state from vibronic interaction with the
n-n* state.

Several other facts can be obtained from the information in
Table 2. The trend of substituent effects on 6p, the probability
of biradical going on to product, indicates quite clearly that
disproportionation of the biradical is favored by electron-withdrawing
groups on the ring, suggesting strongly that the disproportionation
reaction is sensitive to the acidity of the hydroxyl group of the
biradical. It is also interesting to note that the chloro-, methyl-,
and methoxy substituted valerophenones, all of which possess lowest
n-n* triplets23, undergo some direct radiationless decay which
competes with the chemical reactions of the triplet state. This is
not the case with valerophenone itself or the pyridyl ketones, all
of which possess lowest n-n* triplets3”, since their maximum quantum

yield in alcohol rises to unity.

57

C. Summary
Our results indicate that hydrogen abstraction in the Type II

reaction of p—methoxyphenyl alkyl ketones occurs from a small

number of upper reactive n-n* triplets rather than from the lower
n-n* triplets. The inductive effects exerted by electron-withdrawing
groups in the vicinity of the y carbon on benzoyl triplets of phenyl
alkyl ketones during hydrogen abstraction are identical to those
experienced by the anisoyl triplets in the p-methoxyphenyl alkyl
ketones. Since it is unlikely that the nucleophilic n-n* triplet
state of the methoxy ketones would be subject to the identical
inductive effect as the n-n* triplet state of the phenyl ketones,

it can be concluded that all reaction in the p-methoxy ketones

takes place from an equilibrium concentration of upper n-n*
triplets. The energy separation between the two states is
approximately three kilocalories in benzene, but increases in

going from a non-polar to a polar solvent.

0. Further Experiments

 

Presumably equilibration between a lower n-n* triplet and an
upper n-n* triplet can occur in all ketones where the two states
are sufficiently close together for this equilibration to occur.
Further experiments or work that can be performed on systems like
the one studied in this thesis include the following:

a. The number of ketones studied with electron withdrawing

substituents near the y carbon should be increased so that the total

58

range of electron-withdrawing substituents, from weak ones like
chlorine, to strong ones like trifluoromethyl, is covered.

b. The effect of varying the position of ring substituents
on the behavior of the two triplet states should be investigated.
For example, Wagner and Kemppainen35 have found that the rate of

radiationless decay for meta substituted ketones is an order of

magnitude greater than that for the para_substituted ones, although
the apparent rate of hydrogen abstraction is approximately the same
for both.

c. Temperature effect studies need to be performed on the
Type II reaction. Since the equilibrium concentration of each
state is temperature dependent, it should be possible to increase
or decrease the mole fraction of reactive n-n* triplets by
correspondingly increasing or decreasing the temperature.

d. The effect of solvents on the relative position
of the two triplets should be investigated further in order to
disentangle the various factors that influence the quantum yields
of ketones in polar solvents.

Finally, these studies need not be confined to the Type II
reaction alone, but should be extended to include other triplet state
reactims such as oxetane formation or photorearrangements to see
whether these reactions also take place from equilibrium concentrations

of upper reactive triplet states.

IV. EXPERIMENTAL

A. Chemicals

1. Solvents

a. Benzene. Analytical grade benzene was stirred with
concentrated sulfuric acid until the acid no longer turned yellow.
It was then washed with dilute sodium hydroxide solution, water,
and sodium chloride solution, respectively, and dried over anhydrous
magnesium sulfate. After drying, the benzene was distilled from
phosphorous pentoxide, and only the middle fraction retained.

b. Methanol. Reagent grade methanol was distilled from
magnesium turnings and only the middle fraction was retained.

c. Pyridine. Reagent grade pyridine was distilled from
barium oxide and only the middle fraction was retained.

d. t-Butanol. Reagent grade t-butanol was distilled from
sodium shavings and only the middle fraction was retained.

2. Ketones

The ketones used in the photolysis were either purchased or
prepared via common organic reactions. All melting points were
taken on a Thomas Hoover capillary melting point apparatus and
are uncorrected.

a. p-Methoxyacetophenone (PMACP), p-Methoxyacetophenone

 

(Aldrich Chemical Company, Milwaukee, Wisconsin) was recrystallized

twice from petroleum ether (bp 60-90°C.). Vpc analysis revealed no

significant impurities (M.P. 39°C).

59

60

b. p-Methoxyvalerophenone (PMVP). Method A: PMVP was prepared

 

from the addition of n-butylcadmium to p-anisoyl chloride according

to the general method of Cason7”. After hydrolysis and ether
extraction, the ketone was vacuum distilled (bp l35-40°C. at 3-5 torn)
and recrystallized several times from a pentane-methanol mixture.

Method B: Valeryl chloride was used in the Friedel-Crafts
acylation of anisole. The catalyst employed was aluminum chloride
and benzene was used as a solvent. The ketone was vacuum distilled
(bp as above) and recrystallized several times from methanol-pentane
solution (total per cent yield equals m50%).

c. y-Methyl1p-methoxyvalerophenone (GMPMVP). To the Grignard
reagent prepared from l-bromo-3-methy1butane (Eastman Organic Chemicals,
Rochester, N.Y.) in ether was added an ether solution of p-anisonitrile
according to the general method of Hauser75. After hydrolysis of the
imine salt, the ketone was vacuum distilled (bp 137-38°C. at 3 torr.)
and recrystallized several times from a pentane-methanol solution.

Vpc analysis showed no appreciable impurities. (Total per cent yield
equals m25% after purification.)

d. p-Methoxybutyrophenone (PMBP). Butyryl chloride was used

 

in the Friedel-Crafts acylation of anisole with aluminum chloride

as catalyst and carbon disulfide as solvent. The crude ketone was
vacuum distilled (bp 113-115°C at 3 torr.) and recrystallized several
times from a pentane-ethanol mixture. The recrystallized product
was vacuum sublimed using a dry ice-acetone cooled sublimer. Vpc
analysis showed no appreciable impurities. (Total per cent yield

equals ~60%).

61

e- p-Chlorovalerophenone (PCVP). Pfaltz and Bauer p-chloro-

 

valerophenone was recrystallized several times from hexane. Vpc
analysis showed no appreciable impurities (mp 31-32°).

f. p-Methylvalerophenone (PMeVP). Pfaltz and Bauer (Flushing,
N.Y.) p-methylvalerophenone was recrystallized several times from
hexane. Vpc analysis showed no appreciable impurities (mp l9-20°C.).

g. m-Trifluoromethylvalerophenone (MTFVP). m-Trifluoro-
methylvalerophenone was prepared by Professor Peter J. Wagner and
purified by vacuum distillation (bp 116-ll7°C at 11 torr).

h. Acetophenone (ACP). Matheson, Coleman and Bell (Norwood,

 

Ohio) acetophenone was distilled under reduced pressure (bp 55°C
at 3 torr). Vpc analysis showed no appreciable impurities.

i. 6-Carboethoxy;p-methoxyvalerophenone (DCEPMVP). The
general procedure of Diaper was used76. To the Grignard reagent
prepared from 0.5 moles of p-bromoanisole in ether was added an
ether solution of cyclohexanone. After hydrolysis of the Grignard
salt by dilute hydrochloric acid, the anisyl cyclohexanol was
obtained by ether extraction. The anisyl cyclohexanol was dehydrated
to the corresponding anisyl cyclohexene by refluxing it in benzene
with a catalytic amount of p-toluenesulfonic acid and collecting
the water in a Dean-Stark trap. After the dehydration was complete,
the solution was washed with dilute sodium bicarbonate and water to
remove any acid and then dried over magnesium sulfate. Vacuum
distillation of the mixture yields relatively pure olefin, (bp 147-
148°C at 6 torr, literature77: bp 155°C at 14 torr, mp 35°C.)

in 63% of theoretical yield. The anisyl cyclohexene was dissolved

62

in excess ethyl acetate, and ozonized at dry ice-acetone temperatures
by passing a stream of 1% ozone in oxygen through the solution from

a Welsbach ozone generator. After ozone absorption ceased (measured
by a potassium iodide indicator solution), the ozonide was decomposed
by adding the ethyl acetate solution dropwise to 3 grams of zinc in
300 ml water and a little acetic acid. After stirring for several
hours the mixture was filtered to remove the zinc, and the crude
ketoaldehyde, which was not isolated, was oxidized to the ketoacid
directly by addition of excess 30% hydrogen peroxide and formic acid
to the filtrate. The mixture was allowed to stir for at least 12
hours and then extracted with ether. The combined ether extracts
were washed several times with aqueous ferrous sulfate to destroy

any peroxides present and then extracted with sodium carbonate
solution until the sodium carbonate solution remained basic to litmus.
The combined sodium carbonate extracts were acidified cautiously and
the precipitated acid was collected by suction filtration. The crude
acid was converted to the ethyl ester by refluxing it with excess
ethanol and a trace of concentrated sulfuric acid in benzene and
removing the water with a Dean-Stark trap. The dark mixture was
washed with sodium bicarbonate solution to remove any acid, dried
with magnesium sulfate, and the solvent removed by a rotary
evaporator. The dark oil was dissolved in hot ethanol and treated
with decolorizing carbon several times, The fairly white solid
obtained was recrystallized from pentane-benzene solution at least
twelve times until vpc analysis showed that no appreciable impurities

were present (mp 54°, overall percent yield equals ~20%).

63

j. 6-Cyanoapfmethoxyvaler0phenone (DCPMVP). The general

 

procedure of Cason7“ was used. To the Grignard reagent prepared

from 0.25 moles of p-bromoanisole in ether was added an ether
solution of 4-chlorobutyronitrile. After the addition was completed,
the mixture was allowed to stir for 30 minutes while being heated

on a steambath and then poured into a cold dilute hydrochloric

acid solution. The organic layer that formed was separated, and

the aqueous layer was heated for several hours on the steam bath

to hydrolyze the imine salt. 0n cooling of the mixture, a dark oil
separated out, which on standing solidified. The collected solid

was dissolved in the ether extract of the aqueous filtrate, and

the ether solution was treated with decolorizing carbon and magnesium
sulfate to dry it. On dilution with pentane or hexane and cooling,
crystals formed which were collected and recrystallized several

times from hexane-ether mixtures. The crude chloroketone obtained

in this manner was dissolved in dry dimethylsulfoxide and excess
sodium cyanide was added. The mixture was heated to 90° for two
hours and then poured into water. The solid that formed was collected
and recrystallized several times from ether-hexane solution after
treatment with decolorizing carbon. The white solid obtained (mp
61°C.) showed no appreciable impurities on vpc analysis (overall
percent yield equals m 30%).

k. Valerophenone(VP). Eastman Chemical Company (Rochester,

 

N.Y.) valerophenone purified by A. E. Kemppainen was used.

64

l. y-Carbomethoxy-p-methoxybutyrophenone. The procedure

 

of Johnson78 was used in the preparation of this ketone. The
compound was purified by repeated crystallization from methanol
(mp 54-55°, overall percent yield equals 50%).

3. Internal Standards

 

a. Tetradecane. Columbia Chemical Company (Columbia, S.C.)

 

tetradecane was treated in the same manner as benzene and vacuum
distilled (bp 92.5° at 4 torr).

b. Pentadecane. Pentadecane (Columbia Chemical Company,

 

Columbia, S.C.) was treated in the same manner as benzene and
vacuum distilled (bp 132° at 10 torr).

c. Hexadecane. Aldrich hexadecane was treated in the same

 

manner as benzene and vacuum distilled (bp 146° at 10 torr).

d. Octadecane. Octadecane (Chemical Samples Company,

 

Columbus, Ohio) was treated in the same manner as benzene and then

recrystallized from ethanol (mp 30°).

4. Quenchers

a. Lanthanide Chlorides. The lanthanide chlorides were

 

purchased from Alfa Inorganics (Beverly, Mass.) and used as
obtained. All were of greater than 99% purity, and all, except
the europium chloride, were the hexahydrates.

b. Naphthalene. Aldrich Chemical Company (Milwaukee,

 

Wisconsin) naphthalene was recrystallized twice from ethanol.

c. 2,4-Dimethyl-2,5-hexadiene. Aldrich Chemical Company

 

(Milwaukee, Wisconsin) 2,4-dimethyl-2,5-hexadiene was recrystallized

from itself and allowed to sublime on standing in the refrigerator.

65

d. gig—Piperylene. Chemical Samples Company (Columbus,

 

Ohio) gig-piperylene (greater than 99% pure) was used as purchased.

8. General Procedures

 

l. Stern-Volmer Quenching Studies

 

Two stock solutions were prepared for each run. One solution
contained the ketone and the appropriate internal standard, and
the other containing the triplet quencher. Both stock solutions
were prepared by weighing the appropriate amount of compound into
a volumetric flask followed by dilution with benzene up to the
volume line. The concentration and volume of ketone and quencher
stock solutions depended upon the number of samples to be prepared
for that individual run. Equal aliquots of the ketone-standard
solution were pipetted into 10 ml volumetric flasks. To these were
added varying aliquots of the quencher solution, followed by benzene
addition to dilute the sample up to the volumetric mark. The
concentrations of ketone and standard generally, were about 0.10 M
and 0.004 M, respectively, unless specified differently for a specific
run. The concentration of the quencher stock solution and the amount
of final quencher in the samples that were to be photolyzed depended
on the lifetime of the ketone triplet which was to be quenched. In
general quencher concentrations greater than 10T2 M were not used in
the samples, if the quencher was an organic compound. Usually each
run contained two samples without quencher which were photolyzed, and
one sample without quencher which was unphotolyzed and retained as

a blank.

66

In the runs where the lanthanide chlorides were used as quenchers,
the ketone stock solution was prepared as above, except that methanol
was used as solvent. Due to solubility limitations, stock solutions
of the lanthanide chlorides could not be prepared. Instead, varying
amounts of the lanthanide chloride were weighed into 10 ml volumetric
flasks to which equal aliquots of the stock ketone solutions were
added, followed by dilution with methanol to the volumetric mark.

2. Polar Solvent Effect Studies

 

Two stock solutions were again prepared for each run. One
solution contained the ketone and the internal standard, just as in
the quenching studies, dissolved in benzene. The other was a 10
molar solution of the polar solvent in benzene. From this 10 molar
stock solution, a substock solution 1.0 molar in polar solvent was
prepared, which was to be used in the preparation of samples con-
taining less than 1.0 molar polar solvent. Both stock solutions
were prepared by weighing the appropriate amount of compound into
a volumetric flask followed by dilution with benzene up to the volume
mark. Equal aliquots of the ketone-standard solution were pipetted
into 10 m1 volumetric flasks. To these were added varying aliquots
of the polar solvent master solution, and the volume was then brought
up to the volumetric mark by addition of benzene. The concentration
of polar solvent in the samples generally ranged from 0.10 M to 8.0
M. Usually each run contained two samples without any polar solvent
which were photolyzed, and one sample without any polar solvent that

was kept as a blank.

67

3. Photolysis

 

After the solutions containing proper concentrations of
ketone, internal standard, and quencher or added polar solvent
were prepared, 2.8 ml aliquots of each were placed in 13 x 10 mm
Pyrex tubes using a 5 ml syringe with a six inch needle. The
tubes had previously been constricted about 2 cm from the top, so
that they could be easily sealed after being degassed by four
freeze-thaw cycles (P < 0.005 torr), using liquid nitrogen. The
samples were irradiated in parallel using a water bath immersed
merry-go-round apparatus79 to insure that all the samples received
the same amount of incident light and that the temperature remained
constant. A 450 watt Hanovia medium pressure mercury lamp was
used as light source, and the 3130 A region was isolated by a 1 cm
path of a 0.002 M potassium chromate - 1% potassium carbonate
aqueous filter solution. The 3660 A line, used in quenching studies

with naphthalene, was isolated by a set of Corning No. 7083 filters.

4. Vapor Phase Chromatography_

 

Analysis for acetophenones, cyclobutanols, and alkane internal
standards were conducted on either an Aerograph Hy-Fi, Model 600-0,
or a Varian Aerograph Hy-Fi III, Series 1200, chromatograph. The
instruments used a flame ionization detector and were equipped with
a 6' x 1/8" column containing 4% QF-l and 1% Carbowax 20 M on
Chromosorb G (Column A). A column temperature of 150-160° effectively
separated p-methoxyacetOphenone and octadecane. Lower temperatures

used for analysis of other acetophenones and alkanes will be specified

68

for that individual run. §j§;piperylene/acetophenone actinometers

were analyzed on a Aerograph Hy-Fi Model 600-D equipped with a 20'

x 1/8" aluminum column packed with 25% l,2,3-tris(2-cyanoethoxy)propane
on 60/80 Chromosorb P (Column B). A column temperature of 50 degrees
allowed good separation of the tran§_isomer from the gig, The
recorders used with these chromatographs were equipped with Disc
integrators, from which the relative peak areas are given in

integrator counts.

5. Valerophenone Actinometry

 

Samples containing 0.10 M valerophenone and 0.004 M tetradecane
as internal standard were irradiated concurrently, for less than
2.0 hour periods, with ketone samples whose quantum yield was to
be determined. Analysis for acetophenone and tetradecane were
performed using column A at a temperature of 106-110°. The Type II
quantum yield for valerophenone was taken to equal 0.3335. Since
the vpc detector will respond differently to each compound, it is
necessary to include a standardization factor, (S.F.), in the
analysis of the amount of product formed. This S.F. equals the ratio
of area/mole of standard to area/mole of product. For acetophenone,
the standardization factor equals 2.0 (Note: all S.F. used in
this thesis were determined by A. E. KemppainenBW and the concentration
of acetophenone formed will equal:

[acetophenone] = [tetradecane] x S.F. x Counts of ACP (52)
Counts of C14

 

69

The number of incident light quanta on the samples during the

period of irradiation of the actinometer tube will then equal

 

 

quanta absorbed = [Acetophenone]
0.33

2. (53)

6. Ci§;piperylene/Acetophenone Actinometry

 

Samples containing 0.10 M acetophenone and 0.20 M gj§:piperylene
were irradiated concurrently with ketone samples whose quantum yield
was to be determined. All the incident light is absorbed by the
acetophenone, which intersystem crosses to the triplet state
quantitatively19. The triplet acetophenone is completely quenched
by gisfpiperylene, whose triplet state then decays to a mixture of
the gi§_and tran§_isomers of the diene. Irradiation of actinometer
tubes were usually carried out to less than 15 percent tragg
formation, and column B was used in the vpc analysis to determine
the percent tran§:piperylene formed. The following relationship
gives the number of incident quanta of light on the actinometer
sample during the photolysis time19.

[gig-piperylene]o x ln .555 (54)
.555 - 7 trans

= [excited triplet piperylene]

where the concentration of excited piperylene equals the intensity

of incident light on the sample.

111, LITERATLRE CITED

10.
11.

12.
13.

14.
15.
16.
17.
18.
19.
20.

DOOM

LITERATURE CITED

. Ciamician and P. Silber, Chem. Ber., 44, 2911 (1900).
Ciamician and P. Silber, 161g , 44, 1541 (1901).
Ciamician and P. Silber, 161g , 44, 1280 (1911).

D. Cohen, Rec. Trav. Chim., 44, 243 (1920).

 

026306)

. Weizmann, E. Bergman and Y. Hirshberg, J. Amer. Chem. Soc.,

 

44, 1530 (1938).

E. Bergmann and Y. Hirshberg, 161g,, 44, 1429 (1943).

W. E. Bachman, 161g , 44, 391 (1933).

J. N. Pitts, Jr., R. Letsinger, R. Taylor, J. Patterson, G.
Recktenwald and R. Martin, 191g , 44, 1068 (1959).

H. L. J. Backstram, Z. physik. Chem., 444, 99 (1934).

 

 

H. L. J. Backstram and K. Sandros, J. Chem. Phys., 44, 2197 (1955).
H. L. J. Backstrfim and K. Sandros, Acta Chem. Scand., 44, 822
(1958).

 

H. L. J. Backstrom and K. Sandros, ibid., 44 , 48 (1960).

 

 

W. Moore, G. Hammond and R. P. Foss, J. Amer. Chem. Soc., 44,
2789 (1961).

G. Porter and R. Wilkinson, Trans. Faraday Soc., 44, 1686 (1961).
G. Hammond and P. Leermakers, J. Phys. Chem., 44, 1148 (1962).

W. Moore and M. Ketchum, J. Amer. Chem. Soc., 44, 1368 (1962).

 

J. A. Bell and H. Linschitz, ibid., 44, 528 (1963).
. Bryce and C. H. J. Wells, Can. J. Chem., 44, 2722 (1963).

 

W A
A. A. Lamola and G. S. Hammond, J. Chem. Phys., 44, 2129 (1965).
R F. Borkman and D. R. Kearns, Chem. Commun., 44, 446 (1966).

 

7D

21.
22.
23.

24.

25.

26.
27.
28.
29.
30.
.31.

32.
33.
34.
35.

36.
37.

38.
39.

71

M. A. El-Sayed, J. Chem. Phys., 44, 2834 (1963).

 

M. A. El-Sayed, Accts. Chem. Res., 4, 8 (1968).

 

D. R. Kearns and W. A. Case, Jr., J. Amer. Chem. Soc., 44,

 

5067 (1966).
J. N. Pitts, Jr., W. H. Johnson and T. Kuwana, J. Phys. Chem.,

 

44, 2456 (1962).
L. Piette, J. Sharp, T. Kuwana and J. N. Pitts, Jr., J. Chem.

Phys., 44, 3094 (1962).
A. Beckett and G. Porter, Trans. Faraday. Soc., 44, 2051 (1963).

 

 

. Hammond and P. Leermakers, J. Amer. Chem. Soc., 44, 206 (1964).
. Porter and P. Suppan, Pure and Appl. Chem., 4, 499 (1964).

 

 

G

G

P. Suppan, J. Mol. Spec., 44, 17 (1969).

P. Suppan, Ber. Bunsenges. Phys, Chem., 44, 321 (1968).
N. C. Yang, 0. S. McClure, S. L. Murov, J. J. Houser, and R.

Dusenberry, J. Amer. Chem. Soc., 44, 5466 (1967).
N. C. Yang and R. L. Dusenberry, ibid., 44, 5899 (1968).

 

N. C. Yang and R. L. Dusenberry, M01. Photochem., 4, 159 (1969).
P. J. Wagner and G. Capen, ibid., 4, 173 (1969).

 

P. J. Wagner and A. E. Kemppainen, J. Amer. Chem. Soc., 44,
5898 (1968).
N. C. Yang and S. Murov, J. Chem. Phys., 44, 4358 (1966).

 

H. Jaffe and M. Orchin, "Theory and Application of Ultraviolet
Spectroscopy", John Wiley and Sons, New York, N.Y., 1962, p. 186.

 

A. A. Lamola, J. Chem. Phys., 44, 4810 (1967).
R. D. Rauh and P. A. Leermakers, J. Amer. Chem. Soc., 44, 2246

 

(1968).

40.
41.
42.

43.

44.
45.
46.
47.
48.

49.
50.
51.

52.

53.
54.
55.
56.
57.
58.
59.

72

R. N. Griffin, Photochem. and Photobiol., 4, 159 (1968).

 

 

R. Hochstrasser and C. Marzzacoo,J. Chem. Phys., 44, 971 (1968).
P. Leermakers and H. T. Thomas, J. Amer. Chem. Soc., 44, 1621

 

(1965).

P. J. Wagner, M. May, R. Haug, and D. Graber, J. Amer. Chem.

 

Sgg,, in press (1970).

Y. Kanda, J. Stanislaus and E. C. Lim, 191g , 44, 5085 (1969).
C. Walling and M. J. Gibian, 191g,,441, 3361 (1965).

A. Padwa, Tetrahedron Lett., 3465 (1964).

 

H. Tsubomura and S. Tanaka, Chem. Phys. Letters, 4, 309 (1967).

 

J. N. Murrell, "The Theory of the Electronic Spectra of Organic
Molecules", J. Wiley and Sons, New York, N.Y., 1963, p. 172.
D. Bryce-Smith, Pure Appl. Chem., 44, 47 (1968).

 

C. H. Banford and R. G. W. Norrish, J. Chem. Soc., 1504 (1935).

 

For a brief review, see, P. J. Wagner and G. S. Hammond, flgggg,
Photochem., 4, 94 (1968).

G. R. McMillan, J. G. Calvert and J. N. Pitts, Jr., J. Amer.
Chem. Soc., 44, 3602 (1964).

. Yang and D. D. H. Yang, 191g , 44, 2913 (1958).

. Wagner and G. S. Hammond, 1219,, 44, 1245 (1966).

. Yang and D. R. Coulson, 191g , 44, 4511 (1966).

. Wagner, 191g , 44, 5898 (1967).

. Wagner and A. E. Kemppainen, jgjg,, 44, 5896 (1968).

. Wagner and H. N. Schott, 1gjg,, 44, 5383 (1969).

'UV'U'UZ'U
LCAQQOGC

. Wagner and P. A. Kelso, Michigan State University,

personal communication, 1970.

60.
61.
62.

63.
64.

65.

66.

67.

68.

69.
70.

71.
72.

73.

74.

75.

76.

73

P. J. Wagner and G. S. Hammond, J. Amer. Chem. Soc., 44, 4010 (1965).

 

T. J. Dougherty, jgjg,, 44, 4011 (1965).

N. Filipescu and F. L. Minn, igi_., 44, 1544

(1968).

P. J. Wagner, "Energy Transfer Kinetics in Solution" in press.

H. J. L. Backstrfim and K. Sandros, Acta. Chem. Scand., 44,

 

958 (1962).
W. D. Clark, A. D. Litt, and C. Steel, J. Amer. Chem. Soc.,

 

44, 5413 (1969).

G. Porter and R. M. Topp, Proc. Roy. Chem. Soc. (London) 4,
444, 163 (1970).

P. J. Wagner, A. E. Kemppainen and H. N. Schott, J. Amer. Chem.

 

 

§gg,, 44, (1970), in press.
N. C. Yang, R. Loeschen, and 0. Mitchell, ibid.,

 

42. 5455 (1957).
R. s. H. Liu and a. R. Edman, ibid., 24, 213 (1968).

 

E. C. Lim, Y. Kanda, and J. Stanislaus in "Molecular Luminescence",
E. C. Lim, Ed., W. A. Benjamin, Inc., N.Y., 1968, p. 111.

E. C. Lim, ibid., p. 469.

J. Saltiel, H. C. Curtis, L. Metts, J. W. Miley, J. Winterle,

 

and M. Wrighton, J. Amer. Chem. Soc., 44, 410 (1970).

W. G. Herkstroeter and G. 5. Hammond, 1919,, 44, 4769 (1966).
J. Cason, 191g , 44, 2078 (1946).

C. R. Hauser, W. S. Humphlett, and M. J. Weiss, jgig,, 44, 426
(1948).

D. G. M. Diaper, Can. J. Chem., 44, 1721 (1955).

 

77.
78.

79.

80.

74

J. v. Braun, Ann., 472 (1929).
—- ’V'Vb

W. S. Johnson, A. R. Jones, and W. P. Schneider, J. Amer.

Chem. Soc., 44, 2395 (1950).

F. G. Moses, R. S. H. Liu, and B. M. Monroe, M01. Photochem.,

 

4, 245 (1969).
A. E. Kemppainen, Michigan State University, personal

communication, 1967.

IV. APPENDIX

APPENDIX

TABLE 6. Stern-Volmer Quenching Study of 0.10 M
p-Methoxyvalerophenone by Naphthalene
in Methanol.

 

 

 

Sample [Naphthalene]a Egflggi EMAfiPb ¢°/¢d
18 38
0c None 1.15 1.00
1 2.0 x 10’5 M 1.00 1.15
2 4.0 x 10‘5 M 0.715 1.61
3 6.0 x 10'5 M 0.620 1.85
4 8.0 x 10'5 M 0.580 1.98

 

aAll concentrations in moles per liter.b0.004 M C18H38 used as

internal standard. CAverage of two samples, samples irradiated

for one week at 3660 R. qu 1 equals 13,100 M".

TABLE 7. Stern-Volmer Quenching Study of 0.10 M
p-Methoxyvalerophenone by Naphthalene
in Methanola.

 

 

 

Sample [Naphthalene]b Counts PMACPC ¢o/¢d
Counts C H
18 38
0e None 0.80 1.00
1 2 x 10‘5 M 0.70 1.15
2 4 x 10’5 M 0.53 1.52
3 6 x 10'5 M 0.44 1.82
4 8 x 10'5 M 0.38 2.09
b

 

aPlotted in Figure 4. All concentrations in moles per liter.
c0.004 M C18H38 used as internal standard. dkdrequa1s 13,000 M".
eAverage of two samples, samples irradiated for one week at 3660 a.

75

76

m

o_ x HmcwpecpeanH

m
1. _ _

 

Focmgpwz cw acmpongqmz ha mcocmsaosmrm>>xogp021a
mo m:?;ucm=o mgp so» “or; ems—o>-csmum

.w umszu

filloo.F
illoF.F
.llcN.P
.llém.—
lluov.F
rllOm.~
illoo.~
Illon.P
nllom.F

Ilnom.P

.rlloo.m

Illo_.N

“<09

 

77

TABLE 8. Stern-Volmer Quenching Study of 0.1 M a
p-Methoxyvalerophenone by EuCl3 in Methanol.

 

 

 

Sampleb [Eu+3] gaunts PMACPc ¢°/¢
ounts C18H38

0 None 1.85 1.00

1 0.001 M 1.42 1.30

2 0.002 M ---- ----

3 0.003 M 0.79 2.34

4 0.004 M 0.74 2.50

 

akq T equals 400 M'T. USolutions contain 0.004 M C18H38 as internal
standard. CSamples irridiated for 14 hours at 3130 R. Tubes had

turned yellow.

TABLE 9. Stern-Volmer Quenching Study of 0.1 M a
p-Methoxyvalerophenone by EuCl3 in Methanol.

 

 

 

 

+3 Counts PMACPC o
Sampleb [Eu ] <I> /¢
Counts c18H38

0 None 1.87 1.00

1 0.001 M 1.50 1.25

2 0.002 M 1.26 1.47

3 0.003 M 1.00 1.87

4 0.004 M 0.935 1.98
akq T equals 250 M']. bSolutions contain 0.004 M C18H38 as internal

standard. CSamples irradiated for 15 hours at 3130 a.

78

TABLE 10. Stern-Volmer Quenching Study of 0.10 M
p-Methoxyvalerophenone by SmCl3-6H20
in Methanol?

 

 

 

Sampleb [Sm+3] ESSEE: gT:fi::: ¢Ol¢
0 None 1.98 1.00
1 0.010 M 1.18 1.68
2 0.013 M 1.25 1.58
3 0.016 M 1.09 1.81
4 0.020 M 1.02 1.94

 

akq T equals 48 M']. bSolutions contain 0.004 M C18 as internal

standard. CSamples irradiated for 14 hours at 3130 3.

TABLE 11. Stern-Volmer Quenching Study of 0.10 M
p-Methoxyvalerophenone by TbC13-6H20 in

 

 

 

Methanola.
b +3 Counts PMACPC 0
Sample [Tb ] ¢ /¢
Counts C18H38
0 None 1.95 1.00
1 0.01 1.00 1.95
2 0.02 0.84 2.32
3 0.03 0.68 2.87
4 0.04 0.65 3.00

 

ak T equals 62.5 M’T. 5Solutions contain 0.004 M C18 as internal

standard. cSamples irradiated for 11 hours at 3130 R.

79

TABLE 12. Stern-Volmer Quenching Study of 0.10 M
p-Methox§va1erophenone by TbCl3-6H20 in

 

 

 

Methanol .
c
Sampleb [Tb+3] £3321: 212%:8 ¢°/¢
0 None 1.68 1.00
1 0.010 1.05 1.60
2 0.013 0.98 1.71
3 0.016 0.96 1.75
4 0.020 0.84 2.00

 

akq T equals 50 M']. bSolutions contain 0.004 M c18H38 as internal

standard. cSamples irradiated for 12 hours at 3130 3.

TABLE 13. Stern-Volmer Quenching Study of 0.10 M
p-Methox¥va1erophenone by DyC13-6H20 in

 

 

 

Methano .
Sampleb [Dy+3] EgggfiggMAfiPc ¢°/¢
18 38
0 None 3.11 1.00
1 0.01 1.63 1.90
2 0.02 1.63 1.90
3 0.03 1.27 2.44

 

akq T equals 50 M'T. bSolutions contain 0.004 M C18H38 as internal

standard. CSamples irradiated for 12 hours at 3130 R.

80

TABLE 14. Stern-Volmer Quenching Study of 0.10 M
p-Methoxyvalerophenone by DyC13-6H20 in

 

 

 

 

 

 

 

Methanola.
c
10831 23:12:: 2:34;, 4°»
0 None 2.08 1.00
1 0.01 0.99 2.10
2 0.02 0.86 2.42
3 0.03 0.83 2.50
4 0.04 0.69 3.02
akq T equals 57 M']. bSolutions contain 0. 004 M C18H38as sinternal
standard. CSamples irradiated for 12 hours at 3130 R.
TABLE 15. Stern- Volmer Quenching Study of 0.10 M
p-Methoxyvalerophenone by ErCl3 6H20 in
Methanola
Sampleb [Er+3] ggflgfig ETQEZB ¢°/¢
0 None 0.97 1.00
l 0.01 0.52 1.87
2 0.02 0.34 2.75
3d 0.03 0.39 2.49
4 0.04 0.17 5.70

 

-1 b

akq T equals 111 M Solutions contain 0. 004 M c181138.as internal
standard. cSamples irradiated for 21 hours at 31303
in samples with higher quencher concentration.

Windows formed

81

TABLE 16. Stern-Volmer Quenching Study of 0.10 M
p-Methox%va1erophenone by ErC13'6H20 in

 

 

 

Methanol .
c
Sampleb [Er+3] 2332:: grgfigg ¢°/¢
None 2.26 1.00
0.010 1.38 1.64
0.013 1.33 1.70
0.016 1.12 2.02
0.020 0.95 2.39

 

akq T equals 70 M']. bSolutions contain 0.004 M C18H38 as internal

standards. cSamples irradiated at 3130 R for 21 hours. dWindows
formed in samples with higher quencher concentrations.

82

TABLE 17. Type 11 Quantum Yield Determigations for
p-Methoxyphenyl Alkyl Ketones

 

Counts PMACP

 

 

Ketone Counts C H [PMACP] [C18H38] I/1. 411
18 38

PMBPb 0.71 0.006 M 0.004 M 0.540 0.012

PMVPC 2.82 0.026 M 0.004 M 0.178 0.145

GMPMVPd 2.50 0.023 M 0.004 M 0.104 0.22

PMVPe’f 1.97 0.018 M 0.004 M 0.200 0.09

 

aValues determined by acetophenone/gj§:piperylene actinometry.
bSamples photolyzed for 20 hours at 3130 a. cSamples photolyzed for
10 hours at 3130 A. dSamples photolyzed for 6 hours at 3130 R.
eSamples photolyzed for 8 hours at 3130 R in methanol. fValerophenone
actinometry used.

TABLE 18. Acetophenone/gj§;Piperylene Actinometry for
p-Methoxyvalerophenone and y-Methyl-p-

 

 

 

methoxyvalerophenone
Photolysis Light Quanta Absorbed Cumulative
Sample Time % 33995— 2.5 hour 1. light quanta
A1 2.5 hours 10.5 0.0414 0.0414
A2 2.5 hours 11.0 0.0446 0.860
A3 2.5 hours 11.2 0.0454 0.1314
A4 2.5 hours 11.6 0.0470 0.1784

 

a0.10 M Acetophenone and 0.20 M gjgfpiperylene in benzene.

83

TABLE 19. Acetophenone/cis-Piperylene Actinometrya
for p-MethoxyEfifyrophenone

 

Light Quanta Absorbed

 

 

Sample Photolysis Time % trans hour 1'
A1 1.28 hours 9.5 0.030
A2 2.00 hours 14.1 0.029
A3 1.60 hours 9.2 0.025

 

a0.10 M Acetophenone and 0.20 M gjgfpiperylene in benzene.

TABLE 20. Type 11 Quantum Yield Determinations
for Ring Substituted Valerophenones

 

 

 

. Product Counts
Subst1tuent StandardfCounts SF [CnHZn + 2] ¢II
p-c1b 0.925 2.3 6.0 x 10'3M 0.253
m-cr3c 0.79 1.8 " 0.175
p-CH3d 1.18 2.2 " 0.31

 

3Based on actinometer (0.10 M acetophenone and 0.205 M gj§;pipery1ene)
isomerization to 12.30% trans. Photolysis time is 1.5 hours. bVPC
column #A, Column Temperature = 150°C. CVPC column #A, Column

d

Temperature = 115°C. VPC column #A, Column Temperature = 125°C.

84

TABLE 21. Type II Quantum Yield Determination for
o-Substituted p-Methoxyvalerophenones

 

 

 

. Counts PMACP
Subst1tuents Counts7C H [PMACP] [C18H38] 411
18 38
a -C02Eta 2.10 0.0193 M 4 x 10'3 M 0.104
6 -CNb 0.45 0.0041 M 4 x 10'3 M 0.009

 

2.3, photolyzed for 12 hours at 3130 A.
2.3, photolyzed for 27 hours at 3130 R.

a0.05 M ketone, S.F.
b0.05 M ketone, S.F.

TABLE 22. Valer0phenone Actinometry for o-Carboethoxy-
p-methoxyvalerophenone and y-Carbomethoxy-p-

 

 

 

 

methoxybutyrophenone

. b

a Counts ACP L1ght Quanta Absorbed
Sample Counts C‘ H [ACP] hour 1.

14 30

A1 0.65 5.2 x 10‘3M 0.0155
A2 0.63 5.0 x 10‘3M 0.0151
A3 0.66 5.3 x 10'3M 0.0158
A4 0.64 5.1 x 10’3M 0.0153
A5 0.65 5.2 x 10'3M 0.0155
A6 0.64 5.1 x 10'3M 0.0153

 

a0.10 M ketone, 0.004 M C14H30 as internal standard, S.F. = 2.0.
bAverage lamp output = 0.0155 quanta (of incident light) per hour
per liter.

85

TABLE 23. Valerophenone Actinometry for
o-Cyano-p-methoxyvalerophenone

 

 

 

 

4343421330 no] A Q4331115b5°rbed
A1 0.69 5.5 x 10'3M 0.0165
A2 0.70 5.6 x 10‘3M 0.0168
A3 0.73 5.8 x 10‘3M 0.0170
A4 0.68 5.4 x 10‘3M 0.0160
A5 0.68 5.4 x 10‘3M 0.0160

 

a0.10 M valerophenone, 0.004 M C14H38 as internal standard, S.F. =

2.0. bAverage lamp output = 0.0165 quanta (of incident light) per
hour per liter.

TABLE 24. Valerophenone Actinometry for
p-Methoxyvalerophenone in Methanol

 

 

 

 

a Counts ACP Light Quanta Absorbed
Sample Counts C H [ACP] hour 1.
14 30
A1 2.35 0.019 0.026
A2 2.02 0.016 0.025
A3 1.07 0.0085 0.026

 

aSolutions contain 0.10 M valerophenone and 0.004 M CMH30 as
internal standard. bAverage lamp output equals 0.0255 quanta per
hour per liter.

86

TABLE 25. Determination of ¢isc

Methoxyvalerophenone in Benzene by Isomerization
a

for ortho, meta, and para-

 

of gi§;Piperylene

 

 

b Sample 1 Sample 2 Averagec
Ketone % trans % trans % trans
Benzophenone 16.4 16.6 16.5
0MVP 15.9 15.8 15.8
MMVP 16.4 16.0 16.2
PMVP 16.5 16.1 16.3

 

aSolutions contained 0.10 M ketone and 0.20 M gj§;pipery1ene.
bSamples irradiated for 2.5 hours at 3130 3. cVPC analysis performed
using column 8.

TABLE 26. The Photolysis of 0.10 M p-Methoxyvalerophenone
in Benzene and added Methanol

 

 

 

 

b
Samplea [Methanol] ESEEE: EMAfiP ¢AI¢B 411
18 38
0 None 1.51 1.00 0.145
1 0.10M 1.78 1.18 0.171
2 0.20M 1.99 1.32 0.191
3 0.40M 1.94 1.28 0.186
4 0.60M 1.63 1.08 0.157
5 0.80M 1.88 1.24 0.180
6 1.00M 1.91 1.26 0.183
7 2.00M 1.32 0.87 0.126
8 3.00M 1.12 0.74 0.108
9 4.00M 0.39 0.26 0.038
10 5.00M 0.37 0.25 0.036
aSolutions contain 0.0056M c18H38 as internal standard. bSamples

photolyzed for 10 hours at 3130 R.

87

TABLE 27. The Photolysis of 0.10 M p-Methoxyvalerophenone
in Benzene and added t-Butanol

 

 

 

b
Counts PMACP
Samplea [t-butanol] 6 /¢ 4
Counts C18H38 A B 11

0 None 1.86 1.00 0.145
1 0.10M 2.25 1.21 0.175
2 0.20M 2.32 1.25 0.181
3 0.40M 2.72 1.46 0.212
4 0.60M 2.72 1.46 0.212
5 0.80M 2.79 1.50 0.218
6 1.00M 2.80 1.51 0.218
7 1.60M 2.95 1.59 0.230
8 2.40M 2.87 1.54 0.224
9 3.20M 2.75 1.48 0.216
10 4.00M 2.35 1.26 0.183

 

aSolutions contain 0.0050 M ClBH38 as internal standard. bSamples
photolyzed for 9 hours at 3130 R.

TABLE 28. The Photolysis of 0.10 M p-Methoxyvalerophenone
in Benzene and added Pyridine.

 

 

 

b
a . . Counts PMACP
Sample [Pyr1d1ne] Counts C18H38 ¢Pl¢P ¢II
0 None 1.56 1.00 0.145
1 0.10M 1.70 1.09 0.158
2 0.20M 1.93 1.24 0.180
3 0.40M 2.06 1.32 0.192
4 0.60M 2.13 1.37 0.199
5 0.80M 2.13 1.37 0.199
6 1.00M 2.14 1.37 0.199
7 2.00M 2.10 1.35 0.196
8 3.00M 1.98 1.27 0.184
9 4.00M 1.89 1.21 0.176
10 5.00M 1.72 1.10 0.160

 

aSolutions contain 0.005 M c18H38 as internal standard. bSamples
photolyzed for 9.5 hours at 3130 A.

88

 

 

 

TABLE 29. The Photolysis of 0.10 M y-Methyl—p-
methoxyvalerophenone in Benzene and
Added t-Butanol
7" a -- Counts PMACPb
Sample [t-butanol] Counts C H ¢A/¢B . ¢II
18 38
0 None 2.48 1.00 0.22
l 0.50M 5.73 2.32 0.29
2 1.00M 6.83 2.76 0.39
3 2.00M 6.89 2.80 0.40
4 4.00M 6.25 2.53 0.33
5 6.00M 2.48 1.00 0.22
6 8.00M 1.82 0.74 0.16

 

aSolutions contain 0.004 M C

18H38 as internal standard. bSamples

were photolyzed for 10 hours 3130 R.

TABLE 30.

Photolysis of 0.10 M p-Methoxybutyrophenone

in Benzene and Added t-Butanol

 

b

 

 

Samplea [t-Butanol] Egggiz ngfiga ¢AI¢B 411
0 None 0.415 1.00 0.0121
1 0.10M 0.430 1.04 0.0125
2 0.20M 0.42 1.03 0.0124
3 0.50M 0.80 1.93 0.0232
4 1.00M 1.08 2.60 0.0312
5 2.00M 1.24 2.98 0.0358
6 4.00M 1.24 2.98 0.0358

 

aSolutions contain 0.004 M C
bSamples photolyzed for 24 hours at 3130 A.

18”38

as internal standard.

89

TABLE 31. Photolysis of 0.06 M m-Trifluoromethyl-
valerophenone in Benzene and t-Butanol

 

 

 

Samplea [t-Butanol] Egflgfig gIZSEEb ¢A/¢B ¢II
0 None 1.23 1.00 0.175
1 2.0M 2.53 2.02 0.35
2 4.0M 2.72 2.17 0.39
3 6.0M 2.80 2.28 0.40
4 8.0M 3.15 2.56 0.45

 

aSolution contains 0.004 M CMH30 as internal standard.
bSamples photolyzed for 1.5 hours at 3130 R.

TABLE 32. Photolysis of 0.06 M p-Methylvalerophenone
in Benzene and t-Butanol

 

 

 

Samplea [t-Butanol] E3322: grgfigzb ¢AI¢B 411
0 None 2.12 1.00 0.31
1 2.0M 4.51 2.13 0.66
2 4.0M 4.62 2.18 0.68
3 6.0M 4.64 2.19 0.68
4 8.0M 2.25 1.06 0.32

 

aSolutions contain 0.004 M C15H32 as internal standard.
b5amp1es photolyzed at 3130 A for 1.5 hours.

 

 

 

 

 

 

 

TABLE 33. Photolysis of 0.06 M p-Chlorovalerophenone
in Benzene and Added t-Butanol
Sam 1ea [t-Butanol] Counts PCACPb 4 /¢ 4
p Counts C H A 8 II
16 34
0 None 1.59 1.00 0.25
1 2.0M 4.08 2.57 0.64
2 4.0M 4.44 2.78 0.70
3 6.0M 4.59 2.88 0.72
4 8.0M 4.35 2.74 0.68
aSolutions contain 0.004 M C16H34 as internal standard.
bSamples photolyzed for 1.5 hours at 3130 3.
TABLE 34. Photolysis of 0.05 M o-Carboethoxy-p-
methoxyvalerophenone in Benzene and
Added t-Butanol
Sam 1ea [t-Butanol] Counts PMACPb 4 /¢ 4
p Counts C H A B II
18 38
(4 None 1.79 1.00 0.104
1 None 1.78 1.00 0.104
2 0.3M 1.96 1.10 0.114
3 0.5M 1.89 1.06 0.111
4 1.0M 2.34 1.32 0.137
5 2.0M 1.97 1.12 0.116
6 4.0M 1.85 1.05 0.108
7 6.0M 1.00 0.56 0.058
8 8.0M 0.78 0.44 0.046

 

aSolutions contain 0.004 M C18H38 as internal standard. BSamples
photolyzed for 9 hours at 3130 R. CSample contains 0.10 M ketone.

TABLE 35. Photo1ysis of 6-Cyano-p-methoxyva1erophenone

91

in Benzene and Added t-Butano1

 

 

 

 

 

 

 

b
a Counts PMACP
Samp1e [t-Butano1] Counts C18H38 4A/4B II
0 None 0.45 1.00 0.009
1 0.1M 0.31 0.69 0.006
2 0.3M 0.35 0.78 0.007
3 0.5M 0.41 0.92 0.008
4 1.0M 0.59 1.51 0.014
5 2.0M 0.62 1.37 0.012
6 4.0M 0.59 1.31 0.012
7 6.0M 0.48 1.06 0.010
aSo1utions contain 0.004 M C18H38 as interna1 standard.
bSamp1es photo1yzed for 26 hours at 3130 A.
TABLE 36. Va1ues of Maximum Quantum Yie1ds of
Se1ected Pheny1 n-Buty1 Ketones
Counts ch1obutano1 a,b
Ketone [t-Butano1] Counts Standard ¢ cyCI‘ ¢II ¢tota1
p-Meo None 0.50 0.03 0.15 0.18
p-Meo 1.6M 0.64 0.04 0.23 0.27
p-Me None 0.78 0.07 0.31 0.38
p-Me 6.0M 1.05 0.10 0.68 0.78
p-C1 None 0.40 0.05 0.25 0.30
p-C1 6.0M 0.49 0.08 0.72 0.80

 

aQuantum yie1d va1ues in a1coho1 are maximum quantum yie1ds reached

on addition of po1ar so1vents.

bVa1ues are t 10%.

92

TABLE 37. Stern-Vo1mer Quenching Study of 0.10 M
p-Methoxyva1erophenone by 2,4-Dimethy1-
2,5-hexadiene in Benzenea

 

Counts PMACPC

 

 

samp‘eb [Diene] Counts C18H38 ¢OII/¢II
0 None 2.87 1.00
1 0.5 x 10'4M 1.45 1.98
2 1.0 x " " 0.91 3.18
3 1.5 x " " 0.66 4.35
4 2.0 x " " 0.51 5.66

 

a -1 b . .
kq T equa1s 2300 M . So1ut1on conta1n 0.004 M C18H38 as

interna1 standard. CSamp1es photo1yzed for 9.5 hours at 3130 A.

.TABLE 38. Stern-Vo1mer Quenching Study of 0.10 M
y-Methy1-p-methoxyva1erophenone by
2,4-Dimethy1-2,5-hexadiene in Benzene

 

Counts PMACPc

 

 

 

18 38 as
interna1 standard. cSamp1es photo1yzed for 6 hours at 3130 A.

Samp1eb [Diene] Counts C18H38 4011/4II
0 None 2.50 1.00
1 1.0 x 10’3 M 1.32 1.90
2 2.0 x " " 0.77 3.24
3 3.0 x " " 0.57 4.38
4 4.0 x " " 0.47 5.32
akq t equa1s 1100 M:1. bSo1utions contain 0.004 M C H

93

TABLE 39. Stern—Vo1mer Quenching Study of 0.10 M
p-Methoxybutyrophenone by 2,4-Dimethy1-
2.5-hexadiene in Benzene

 

 

 

 

samp‘eb [Diene] ESEEE: EMAEPC ®OII/¢II
18 38
0 None 0.415 1.00
1 4.0 x 1o'4M 0.166 2.44
2 6.0 x " " 0.158 2.58
3 8.0 x " " 0.113 3.78
akq T equa1s 3300 M-1. bSo1utions contain 0.044 M C18H38 as

interna1 standard. CSamp1es photo1yzed for 24 hours at 3130 A.

TABLE 40. Stern-Vo1mer Quenching Study of p-
Methoxybutyrophenone by 2,4-Dimethy1-
2.5-hexadiene in Benzene

 

Counts PMACPC

 

 

Samp1eb [Diene] Counts C18H38 4011/4II
0 None 0.90 1.00
1 1.0 x 10‘4M 0.66 1.36
2 2.0 x " " 0.57 1.58
3 3.0 x " " 0.45 2.02
4 4.0 x " " 0.39 2.31

 

akq T equa1s 3300 M']. bSo1utions contain 0.004 M C18H38 as interna1

standard. cTubes photo1yzed at 3130° for 24 hours.

94

TABLE 41. Stern-Vo1mer Quenching Study of 0.05 M
y-Carbomethoxy-p-methoxybutyrophenone by
2,4-Dimethy1-2,5-hexadiene in Benzenea

-_- -. .. _. - - -7 -v- _ - _ -__.._.. _._———r-fi-

 

 

 

 

 

b . Counts PMACPC A Io
Samp1e [D1ene] Counts C H 4 II/4II
18 38
0 None 0.32 1.00
1 0.001 M 0.215 1.49
2 0.002 M 0.112 2.87
3 0.003 M 0.101 3.18
4 0.004 M 0.074 4.34
1 b

akq T equa1s 830 M' . So1utions contain 0.004 M C1 H38 as interna1
standard. CSamp1es photo1yzed for 29 hours at 3130 g. dBased on 6%
conversion; va1ues are strong1y dependent on per cent conversion.

TABLE 42. Stern-Vo1mer Quenching Study of 0.05 M
6-Carboethoxy-p-methoxyva1erophenone by
2,4-Dimethy1-2,5-hexadiene in Benzenea

 

 

 

Samp1eb [Diene] 2332:: ngfig: 40/4II
0 None 2.10 1.00
1 2.0 x 10‘3M 0.45 4.67
2 4.0 X " " 0.21 10.00
3 6.0 X " " 0.14 11.20
4 8.0 X " " 0.10 20.80

 

akq T equa1s 2500 M'1. bSo1utions contain 0.004 M C18H38 as

interna1 standard. CSamp1es photo1yzed for 12 hours at 3130 A.

95

TABLE 43. Stern-Vo1mer Quenching Study of 0.05 M
o-Cyano-p-methoxyva1erophenone by
2,4-Dimethy1-2,5-hexadiene in Benzene

 

 

Counts PMACPC

 

 

 

samDIEb [Diene] Counts C18H38 ¢OII/¢II
0 None 0.45 1.00
1 1.0 x 10‘4M 0.38 1.17
2 2.0 x " " 0.35 1.26
3 3.0 x " " 0.25 1.80
4 4.0 x " " 0.22 2.04
akq T equa1s 2500 M-]. bSo1utions contain 0.004 M C18H38 as

interna1 standard. CSamp1es photo1yzed for 27 hours at 3130 A.

TABLE 44. Stern-Vo1mer Quenching Study of 0.05 M
o-Cyano-p-methoxyva1erophenone by
2,4-Dimethy1-2,5-hexadiene in Benzene

 

 

 

[Diene] $2324: 2133:; m
0 None 0.55 1.00
1 2.0 x 10'4M 0.39 1.40
2 4.0 x " " 0.26 2.12
3 6.0 x " " 0.22 2.56
4 8.0 x " " 0.18 3.04

 

akq T equa1s 2500 M']. bSo1utions contain 0.004 M C18H38 as

interna1 standard. cTubes photo1yzed for 25 hours at 3130 A.