Fluorescence and photoinversion reactions in solutions of chiral diaryl sulfoxides with various sizes of aromatic rings

Chiral aryl 4-tolyl sulfoxides substituted with several sizes of aromatic rings, (S)-(-)-phenyl 4-tolyl sulfoxide (PHSO), (R)-(+)-1-naphthyl 4-tolyl sulfoxide (NASO), (R)-(+)-9-phenanthryl 4-tolyl sulfoxide (PNSO), and (R)-(+)-1-pyrenyl 4-tolyl sulfoxide (PYSO), were prepared. It has turned out for the first time that there are n-σ*, π-σ* and π-π* states in excited states of these molecules by MO calculation. Fluorescence spectra and quantum yields of fluorescence, Φ_fl, were measured in polar and non-polar solvents. Quantum yields of photoinversion, Φ_inv, and photolysis, Φ_dec, were also obtained with CD and HPLC measurements. Aromatic-ketone like characteristics have been noticed in the fluorescence measurements, i.e., fluorescence has been observed from sulfoxides substituted with large aromatic rings and their fluorescence becomes strong in a polar solvent. We attributed these characteristics to the decrease in π-π* transition energy as the degree of conjugation increases, while n-σ* and π-σ* transition energy does not decrease so much. The values of Φ_inv and Φ_dec for PHSO, NASO and PNSO were found to be 0.3-0.4 (Φ_inv) and 0.03-0.05 (Φ_dec). The quantum yields for PYSO were measured as Φ_inv = 0.02-0.03 in methanol and hexane, and Φ_dec = 0.002-0.003 in methanol and 0.012-0.016 in hexane. It was found that quantum yields of photoinversion and photolysis decrease as the π-π* transition energy decreases. We concluded that photoinversion occur during change in electronic states from n-σ* states to the ground state.