Theoretical Studies of the Ground and Excited State Structures of Stilbene

Optimized geometries are evaluated for the ground and low lying excited states of <i>cis-</i>stilbene, <i>trans-</i>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 <i>trans-</i>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 ¹B_u state of <i>trans</i> stilbene to be ∼4.0 eV above the ground X¹A_g state, which is in accord with experiment and with earlier theoretical estimates. The 1¹B_u state of <i>trans-</i>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 2¹B_<i>u</i> state is dominated by the HOMO → LUMO+1 and HOMO-1 → LUMO transitions (covalent type). Likewise, the 1¹B and 2¹B states of <i>cis-</i>stilbene and DHP are also found to be of ionic and covalent types, respectively