HIGHER EXCITED SINGLET-STATE OF DIPHENYLACETYLENE IN SOLUTION-PHASE

By using picosecond transient absorption spectral measurement techniqu es, we have investigated photophysical properties of several mono-subs tituted diphenylacetylenes (DPA). The strongly temperature dependent S 2-->S1 internal conversion which was reported for DPA previously was o bserved for methoxy-DPA, hydroxy-DPA, and chloro-DPA. In these compoun ds the S2 and S1 states are fluorescent B-1(1u) and nonfluorescent 1A( g) or B-1(2u), respectively, while the S1 state of cyano-DPA and CH3CO O-DPA should be fluorescent B-1(1u) The probable mechanism of the S2-- >S1 internal conversion, where the promoting vibrational mode was exci ted in the S2 state, was proposed on the basis of the results that the former group has a similar activation energy of the internal conversi on.