Theoretical study of the pi ->pi states of linear polyenes: The energy gap between 1(1)B(u)(+) and 2(1)A(g)(-) states and their character

摘要

Multireference perturbation theory with complete active space self-consistent field (CASSCF) reference functions is applied to the study of the valence pi-->pi* excited states of 1,3-butadiene, 1,3,5-hexatriene, 1,3,5,7-octatetraene, and 1,3,5,7,9-decapentaene. Our focus was put on determining the nature of the two lowest-lying singlet excited states, 1(1)B(u)(+) and 2(1)A(g)(-), and their ordering. The 1(1)B(u)(+) state is a singly excited state with an ionic nature originating from the HOMO --> LUMO one-electron transition while the covalent 2(1)A(g)(-) state is the doubly excited state which comes mainly from the (HOMO)(2) --> (LUMO)(2) transition. The active-space and basis-set effects are taken into account to estimate the excitation energies of larger polyenes. For butadiene, the 1(1)B(u)(+) state is calculated to be slightly lower by 0.1 eV than the doubly excited 2(1)A(g)(-) state at the ground-state equilibrium geometry. For hexatriene, our calculations predict the two states to be virtually degenerate. Octatetraene is the first polyene for which we predict that the 2(1)A(g)(-) state is the lowest excited singlet state at the ground-state g geometry. The present theory also indicates that the 2(1)A(g)(-) state lies clearly below the 1(1)B(u)(+) state in decapentaene with the energy gap of 0.4 eV. The 0-0 transition and the emission energies are also calculated using the planar C-2h relaxed excited-state geometries. The covalent 2(1)A(g)(-) state is much more sensitive to the geometry variation than is the ionic 1(1)B(u)(+) state, which places the 2(1)A(g)(-) state significantly below the 1(1)B(u)(+) state at the relaxed geometry. (C) 1998 John Wiley & Sons, Inc. [References: 94]