LOW-LYING SINGLET AND TRIPLET-STATES OF ALL-TRANS(10-S-CIS)-2,4,6,8,10-UNDECAPENTAEN-1-AL - A THEORETICAL DETERMINATION

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 in terest, the system is a reasonable truncated model of all-trans-retina l. This paper includes geometry determination of the ground state and the low-lying valence singlet and triplet excited states. Vertical, em ission, 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 du e to the expected strongly dipole-allowed pi pi transition, placed in vacuo at 3.77 eV. The singlet pi pi state is above the singlet n pi* and the A(g)-like states by only 0.22 eV. The lowest 0-0 singlet-sing let transition energy corresponds to the A(g)-like state, at 2.48 eV. The calculated fluorescence maxima from the n pi and the A(g)-like st ates are found in a similar energy range, at 1.87 eV and 2.06 eV, resp ectively. The lowest triplet state is of pi pi character, placed vert ically at 1.74eV, adiabatically at 1.07 eV, with a predicted phosphore scence maximum of 0.75 eV. On the other hand, Rydberg states play a mi nor role in the description of the low-energy region of the spectra. T he results are consistent with available experimental data of the syst em and related compounds in solution.