Photophysical properties of diphenylacetylene derivatives in solution phase. IV. N,N-dimethylanilino(phenyl)acetylene in n-hexane

By using picosecond transient absorption and fluorescence lifetime measurement techniques, we have investigated the photophysical properties of N,N-dimethylanilino(phenyl)-acetylene in n-hexane solution. The lifetime of the second excited singlet state was determined to be about 20 ps at room temperature and a strong delayed S2 fluorescence excited from S1 was observed. The characteristic features we observed seem to result from the accidental degeneracy of S2 and S1. Introduction of the dimethylamino group makes the vibrational level density high and results in a large rate constant of internal conversion from S2 to S1 without excess vibrational energy.