Photophysical and photochemical primary processes of diphenylacetylene derivatives and related compounds in liquid phase

Photophysical and photochemical primary processes of diphenylacetylene (DPA) derivatives and related compounds have been reviewed. Curious photophysical properties of the low lying excited singlet states of DPA, such as an exceptionally slow S-2-->-S-1 internal conversion, a distinct temperature effect of the S-2 lifetime, and a strong excitation energy dependence of the fluorescence yield, were correlated with the S-2-S-1 energy gap. Contradictions of the assignments of the lowest excited singlet states of DPA were resolved and the mechanism of the S-2-->S-1 internal conversion was proposed from the comparison of the dynamic behavior of DPA with that of diphenylpolyenes. The small S-2-S-1 energy gap as well as the large displacement between the potential curves of the upper and lower electronic states can be a reason why the S-2 state of the DPA derivatives is exceptionally long-lived. The DPA derivatives of which the intramolecular charge-separated states are formed in polar solvents were used as probe molecules of the solvent-solute interaction. Significant enhancement of the charge recombination of aminophenyl(phenyl)acetylene in protic solvents should be due to the interaction between amino nitrogen atom of the solute molecule and the hydrogen-bonded solvent oligomer.