Theoretical Studies of the Ground and Excited State Structures of Stilbene

Abstract

Optimized geometries are evaluated for the ground and low lying excited states of cis-stilbene, trans-stilbene, and 4a,4b-dihydrophenanthrene (DHP) from calculations performed with the improved virtual orbital, complete active space configuration interaction (IVO-CASCI) method. The calculations indicate that a nonplanar conformer of trans-stilbene is the most stable among the isomers. The calculated ground and low lying excited state geometries agree well with experiment and with prior theoretical estimates where available. Our IVO-CASCI based multireference Möller–Plesset (MRMP) computations place the 1Bu state of trans stilbene to be 4.0 eV above the ground X1Ag state, which is in accord with experiment and with earlier theoretical estimates. The 11Bu state of trans-stilbene can be represented by the highest occupied molecular orbital (HOMO) → lowest unoccupied molecular orbital (LUMO) transition (ionic type) from the ground state, whereas its 21Bu state is dominated by the HOMO → LUMO+1 and HOMO-1 → LUMO transitions (covalent type). Likewise, the 11B and 21B states of cis-stilbene and DHP are also found to be of ionic and covalent types, respectively.