Multireference perturbation theory with complete active space self-consistent field (CASSCF) reference functions is applied to the study of the valence π → π* 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, 11Bu+ and 21Ag-, and their ordering. The 11Bu+ state is a singly excited state with an ionic nature originating from the HOMO → LUMO one-electron transition while the covalent 21Ag- 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 11Bu+ state is calculated to be slightly lower by 0.1 eV than the doubly excited 21Ag- 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 21Ag- state is the lowest excited singlet state at the ground-state geometry. The present theory also indicates that the 21AG- state lies clearly below the 11Bu+ 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 C2h relaxed excited-state geometries. The covalent 21Ag- state is much more sensitive to the geometry variation than is the ionic 11Bu+ state, which places the 21Ag- state significantly below the 11Bu+ state at the relaxed geometry.
|Number of pages||19|
|Journal||International Journal of Quantum Chemistry|
|Publication status||Published - Jan 1 1998|
All Science Journal Classification (ASJC) codes
- Atomic and Molecular Physics, and Optics
- Condensed Matter Physics
- Physical and Theoretical Chemistry