S-1<-S-0 transition of phenylacetylene: Ab initio and resonant two-photon ionization studies

The S-1 <-- S-0 transition of phenylacetylene has been studied by ab initio molecular orbital calculations and resonant two-photon ionization time-of-flight mass spectrometry. The calculated results show that the molecule is planar with C-2v symmetry in both states. Upon excitation from S-0 to S-1, the phenyl ring expands while the acetylenic C drop C bond becomes longer and the substituent-sensitive C-C' bond becomes shorter. Our calculated results further predict an increased conjugation between the acetylenic and ring portions of phenylacetylene in the excited state. A comparison of the ground and excited state vibrational modes is presented and discussed in terms of the changes in reduced masses, force constants and mode mixing. All modes except the Kekule-type vibration showed a decrease in frequency in the excited state. With the help of calculated normal vibrational frequencies, we were able to assign not only the new bands observed by us but also several unassigned bands reported previously.