Ab initio study on the electronic structures of styrene in the Franck‐Condon region

Abstract

The electronic structures of styrene in the Franck-Condon region have been theoretically examined by means of ab initio complete active space self-consistent field (CASSCF) and the second order multireference Møller-Plesset calculations. The optimized structure of styrene in S(0) is planar but the torsional motion of the phenyl group is very floppy. The S(1) state is assigned to the local pi-pi* excitation within the benzene ring. On the other hand, S(2), above S(1) by 0.561 eV, is assigned to a state that resembles the so-called V-state of ethylene. The transition intensity of S(0)-S(1) is weak, while that of S(0)-S(2) is strong. This is in good agreement with the experimental absorption spectrum where the S(0)-S(1) and S(0)-S(2) transitions are in the energy range of 290-220 nm. The optimized geometry of S(1), characterized by an enlarged benzene ring and its vibrational analyses, further justifies the assignment of the S(1) state.

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

“…Finally, we give our perspective of the common feature of the π-conjugated systems polyene and polyynes in the radiationless relaxation of photoisomerization or internal conversion. Up to now, we have examined the photochemistry of typical π-conjugated systems. ,− Thereby, it is found that the bond alternation of π-conjugated systems is essential for the radiationless relaxation of S 1 −S 0 or S n −S 1 ( n ≥ 2). We will further continue to check if the feature is commonly found in another π-conjugated system by means of ab initio methods.…”

Ab initio study on the photochemical behavior of styrene

“…As mentioned in the previous articles, eight electrons in eight orbitals complete active space self‐consistent field [denoted by (8,8)CASSCF] calculations are proper to discuss the photochemical behavior of styrene in the low‐lying excited states 1, 3. Therefore, we applied (8,8)CASSCF to the allocation of the S 2 /S 1 ‐CIX.…”

“…The basis set used in the present calculations is same as the previous one 1, 3. Except for the allocation of the S 2 /S 1 ‐CIX by GAUSSIAN98,4 we used GAMESS program in the present ab initio calculations 5…”

“…The photochemical behavior of styrene is a prototype of this process. In our preceding article,1 we gave a review on the photochemical behavior of styrene, but we again summarize the experimental findings for the later discussion on the photochemical behavior: In the Franck‐Condon (FC) region, the transition of S 0 ‐S 2 is mainly in the range of 290–220 nm where most of the photochemistry of styrene has been studied. The energy barrier for the photoisomerization is estimated to be greater than 3100 cm −1 . The photoisomerization rate is dependent on the temperature. Two types of fluorescences with a lifetime of 1.5 and 50 ns are observed. …”

“…Therefore, in the present article, we report the electronic structures at the S 2 /S 1 ‐CIX, which is another important ingredient of the photochemical behavior of styrene. Then, we try to properly integrate the information on the S 2 /S 1 ‐CIX with other important information such as the electronic structures in the FC region1 and those at the S 1 /S 0 ‐CIX where the radiationless relaxation into S 0 takes place 3. The present article is organized as follows.…”