Photophysics and stereomutation of aromatic sulfoxides

Abstract

Aromatic sulfoxides are photochemically active molecules. At 77 K in ether/isopentane/ethano! (EPA) glass, aromatic sulfoxides are shown to have weak phosphorescence. The triplet energies are a few kilocalories per mole higher than the corresponding ketones. The diaryl sulfoxides are about 3 kcal/mol lower than the corresponding ketones, and the diaryl sulfoxides are about 3 kcal/mol lower than the corresponding aryl methyl sulfoxides. The lifetimes of emission are generally under 100 ms. From the diffuse vibrational structure of the spectra, the lifetimes, and the effect of solvent on the triplet energy, it is concluded that the triplets are delocalized aromatic states that involve substantial charge transfer off the oxygen atom. The effect of a methanesulfinyl group on the photophysics of several aromatic chromophores has been investigated. Compared to the parent arenes, the spectroscopic singlet energies and the triplet energies are relatively unchanged by the substitution (±2 kcal/mol). The fluorescence quantum yields are reduced by at least one order of magnitude, whereas the phosphorescence quantum yields at 77 K are enhanced. Fluorescence lifetimes are greatly shortened, consistent with the reduced Op. The triplet yields at room temperature are slightly enhanced by the substitution. Unusual fluorescence quantum yield enhancements are observed on cooling the samples to 77 K. For instance, a 15-fold increase of fluorescence of 1-methanesuIfinylpyrene is observed upon freezing the sample whereas only a factor of 2 is seen in the unsubstituted pyrene. An attempt to relate the observed photophysics to photoinduced racemization is made, since the photoracemization is negligible at 173 K. Racemization yields as a function of temperature have also been obtained. For methanesulfmyl pyrene, the activation barrier of photoracemization is estimated to be 2-7 kcal/mol. Aromatic sulfoxides quench singlet excited states of sensitizers whose singlet energies render energy transfer unlikely as a mechanism. Well over 50 rate constants for singlet quenching of various sensitizers by a series sulfoxides have been obtained, as have estimates of the redox potentials for the series of sulfoxides. These data strongly suggest that the mechanism for quenching involves electron transfer and/or exciplex formation. Charge (electron) transfer is from the sensitizer to the sulfoxide.