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

By using picosecond transient absorption spectrum and fluorescence lif etime measurement techniques, we have investigated photophysical prope rties of (aminophenyl)phenylacetylene in n-hexane solution. Besides th e strongly temperature dependent S2 --> S1 internal conversion which w as reported for several diphenylacetylene derivatives previously, ther mal repopulation of S2 from the longer lived S1 was observed. From the temperature dependence of the S2 fluorescence decay, the energy gap b etween S1 and S2 has been determined to be about 1270 cm-1. The freque ncy factor of the S2 --> S1 internal conversion is more than 1 order o f magnitude smaller than that of the reverse process, which suggests t hat S1 has a more restricted structure than S2. The large frequency fa ctor of the reverse process is responsible for the high yield of the d elayed S2 --> S0 fluorescence and is characteristic to (aminophenyl)ph enylacetylene.