Spin manifests itself in a variety of ways in chemistry and physics through spectroscopy, magnetism, and chemical reactivity. Many of the common physical observables linked to spin are well documented such as UV-Vis selection rules and spin-orbit coupling. While the well-established field of spin chemistry has focused on how spin can affect a compound’s ground-state reactivity, much less has been reported pertaining to the effects of spin effects on excited-state reactivity. To gain a fundamental understanding and to definitively establish the role of spin conservation in excited-state quenching this report will focus on characterizing the photophysical processes of dinuclear donor-acceptor systems with the form of [M(tren)(pyacac)Re(bpyʹ)(CO)3]3+ (where, M = CoIII or CrIII. tren = tris(2-aminoethyl)amine, pyacac =3-(4-pyridyl)-2,4-pentanedione, and bpyʹ = 2,2ʹ-bipyridine). The excited-state reactivity of three dinuclear CrRe compounds was characterized and quenching of the MLCT excited state by Förster energy transfer was observed via a spin-allowed S = 3/2 pathway. Three dinuclear CoRe compounds were characterized, where the low-spin Co(III) provides a way to change the spin of the system and observe the changes in excited-state quenching. Despite Förster transfer being thermodynamically favorable for the CoRe compounds, coupling to a S = 0 excited state shuts down this pathway due to spin not being conserved, ΔS ≠ 0. By using time-resolved and steady-state emission spectroscopy in conjunction with transient absorption, photo-induced electron transfer from the excited state in the CoRe compounds is observed. This demonstrates the first time spin was used to select and control the quenching mechanism of a system.
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