Theoretical determination of the singlet → singlet and singlet → triplet electronic spectra, lowest ionization potentials, and electron affinity of cyclooctatetraene

The singlet --> singlet and singlet --> triplet electronic spectra of cycloocta-1,3,5,7-tetraene are studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbitals (ANOs) basis sets. The observed dipole-allowed features at 4.43, 6.02, and 6.42 eV and the spin-forbidden singlet triplet bands with maxima at 3.05, 4.05, and 4.84 eV (Frueholz, R. P.; Kuppermann, A. J. Chem. Phys. 1978, 69, 3614) are assigned as the transitions 1(1)A(1) --> 1(1)A(2), 1(1)A(1) --> 2(1)B(2)(3p(z)), 1(1)A(1) --> 3(1)E, and 1(1)A(1) --> 1(3)A(2), 1(1)A(1) --> 1(3)E, 1(1)A(1) --> 1(3)B(1), respectively. The lowest (3s) Rydberg singlet and triplet states are placed at 5.58 (2(1)A(1)) and 5.54 (1(3)A(1)) eV. New assignments are tentatively suggested for the recorded higher-energy peaks in the singlet manifold. The three lowest ionization potentials are also characterized. At the highest level of theory, the computed adiabatic electron affinity is 0.56 eV, in agreement with experimental determinations. As regards the role of cyclooctatetraene as an efficient triplet quencher of laser dye solutions, the computation supports it could act as acceptor with donors having a triplet excited-state energy equal to or higher than 0.8 eV.