S-1[-S-0 TRANSITION OF PHENYLACETYLENE - AB-INITIO AND RESONANT 2-PHOTON IONIZATION STUDIES

The S-1 <-- S-0 transition of phenylacetylene has been studied by ab i nitio molecular orbital calculations and resonant two-photon ionizatio n time-of-flight mass spectrometry. The calculated results show that t he molecule is planar with C-2v symmetry in both states. Upon excitati on from S-0 to S-1, the phenyl ring expands while the acetylenic C dro p C bond becomes longer and the substituent-sensitive C-C' bond become s shorter. Our calculated results further predict an increased conjuga tion between the acetylenic and ring portions of phenylacetylene in th e excited state. A comparison of the ground and excited state vibratio nal modes is presented and discussed in terms of the changes in reduce d masses, force constants and mode mixing. All modes except the Kekule -type vibration showed a decrease in frequency in the excited state. W ith the help of calculated normal vibrational frequencies, we were abl e to assign not only the new bands observed by us but also several una ssigned bands reported previously.