PHOTOPHYSICAL PROPERTIES OF DIPHENYLACETYLENE DERIVATIVES IN SOLUTION PHASE-III(1) - THERMAL REPOPULATION OF THE S2 STATE OF (AMINOPHENYL)PHENYLACETYLENE

By using picosecond transient absorption spectrum and fluorescence lifetime measurement techniques, we have investigated photophysical properties of (aminophenyl)phenylacetylene in n-hexane solution. Besides the strongly temperature dependent S2 --> S1 internal conversion which was reported for several diphenylacetylene derivatives previously, thermal repopulation of S2 from the longer lived S1 was observed. From the temperature dependence of the S2 fluorescence decay, the energy gap between S1 and S2 has been determined to be about 1270 cm-1. The frequency factor of the S2 --> S1 internal conversion is more than 1 order of magnitude smaller than that of the reverse process, which suggests that S1 has a more restricted structure than S2. The large frequency factor of the reverse process is responsible for the high yield of the delayed S2 --> S0 fluorescence and is characteristic to (aminophenyl)phenylacetylene.