Exciton coupled circular dichroism (ECCD), a sub-discipline of electronic circular dichroism, is a non-empirical, chiroptical method by which the absolute stereochemistry of chromophoric, chiral molecules can be determined. The sign of the ECCD signal can be directly correlated to the helical arrangement of the chromophores of the molecule thereby allowing for the assignment of configuration of the molecule. The end goal of our research is to develop ECCD host systems capable of accepting a wide range of chiral guest molecules and provide consistent and reliable ECCD results. The research disclosed, being multifaceted, explored several aspects of determination of absolute chemistry using porphyrin tweezer host systems as reporters of chirality. Specifically, the mechanism for chirality transfer from the chiral guest molecule to the porphyrin tweezer host molecule, development of ECCD protocols for different classes of guest molecules and evolution of new guest molecules were explored. The mechanism of chirality transfer was interrogated using a highly fluorinated alkyl substituted porphyrin tweezer. Results of the study indicate that alkyl substituents reduce interaction between the host and guest which in turn, reduce the efficiency of the transfer of chirality from the guest to the porphyrin tweezer. To determine if the methylene linker between the porphyrins of the tweezer played a role in the transfer of chirality from guest to host, a series of porphyrin tweezers with varied linker lengths were synthesized. The results of this study showed that the length of the linker could promote or inhibit formation of the host-guest complex. Using the porphyrin tweezer host systems, ECCD protocols for determination of absolute stereochemistry of chiral pyridyl alcohols, epoxy amides and aziridine amides were developed. These methodologies are capable of reliably reporting the stereochemistry of the guest molecule at micromolar concentrations. Finally, a benzoxazole tripodal system was synthesized as a host molecule for chiral ammonium salts. NMR analysis indicated that the host-guest complex did form although an ECCD signal was not observed. These encouraging results create a platform for future work into tripodal based host systems for ECCD.
Read
