Low-lying singlet and triplet states of all- trans (10-s-cis)-2,4,6,8,10- undecapentaen-1-al: a theoretical determination

Abstract

Ab initio results for the electronic spectra of all-trans(10-s-cis)-2,4,6,8,10-undecapentaen-1-al are presented. Apart from its intrinsic interest, the system is a reasonable truncated model of all-trans-retinal. This paper includes geometry determination of the ground state and the low-lying valence singlet and triplet excited states. Vertical, emission, and non-vertical excitation energies have been computed using multiconfigurational second-order perturbation theory by means of the CASPT2 method. The most intense feature of the computed spectrum is due to the expected strongly dipole-allowed ππ* transition, placed in vacuo at 3.77eV. The singlet ππ* state is above the singlet nπ* and the Ag-like states by only 0.22eV. The lowest 0–0 singlet–singlet transition energy corresponds to the Ag-like state, at 2.48eV. The calculated fluorescence maxima from the nπ* and the Ag-like states are found in a similar energy range, at 1.87eV and 2.06eV, respectively. The lowest triplet state is of ππ* character, placed vertically at 1.74eV, adiabatically at 1.07eV, with a predicted phosphorescence maximum of 0.75eV. On the other hand, Rydberg states play a minor role in the description of the low-energy region of the spectra. The results are consistent with available experimental data of the system and related compounds in solution.