Theoretical Study on the Photochemical Behavior of Diphenylacetylene in the Low-Lying Excited States

Ab initio complete active space self-consistent field (CASSCF) and second-order multireference Möller−Plesset (MRMP2) calculations have been performed to examine the photochemical behavior of diphenylacetylene (DPA) theoretically. The stable structure of DPA in S 0 (S 0 -geometry) is optimized to be D 2 h . DPA at S 0 -geometry is mainly excited into the S 3 (B 1u ) state and then relaxes into the stable geometry in the B 1u state (B 1u -geometry) which is characterized as a quinoid structure. The B 1u -geometry further relaxes into the globally stable geometry in S 1 ( t S 1 -geometry) which takes a trans-bent form. Around t S 1 -geometry, DPA moves into the lowest triplet state through intersystem crossing and finally relaxes into the stable geometry in T 1 with D 2 h . The vibrational analyses at the important conformations mentioned above are in good agreement with the experimental findings of time-resolved transient spectroscopy.