PICOSECOND TIME-RESOLVED ABSORPTION-SPECTRUM MEASUREMENTS OF THE HIGHER EXCITED SINGLET-STATE OF DIPHENYLACETYLENE IN THE SOLUTION PHASE

The photophysical properties of diphenylacetylene (DPA) in various solvents have been investigated by picosecond time-resolved absorption spectrum measurements. The absorption bands at 43 5 and 700 nm, assigned to the S(n) <-- S1 transition of DPA, decayed with a lifetime of about 200 ps, while the short-lived band at 500 nm was ascribed to the absorption from a second excited singlet state, S2. The fluorescence state of DPA was not the lowest excited singlet state but the short-lived S2 State. The change of the vibronic interaction between the excited singlet states from the intermediate case coupling to the statistical limit was deduced by the measurements of the excitation wavelength dependence of the S2 yield. The lifetime of the 500-nm band followed Arrhenius type temperature dependence, of which the activation energy was about 890 cm-1 in n-hexane, while the lifetime of S1 state did not depend on temperature significantly. The large temperature effect of the S2 state lifetime might be due to the turnover of the S1-S2 coupling.