The lowest singlet states of hexatriene revisited

The two lowest excited singlet states of trans-hexatriene and cis-hexatriene are studied by multireference perturbation theory approaches (NEVPT2 and CASPT2) in their quasi-degenerate version (QD-NEVPT2 and MS-CASPT2). For these states, we report spectroscopic properties such as the vertical and adiabatic excitation energies, some features of the topology of the potential energy surfaces (PES), and the emission energies. The theoretical vertical excitation energies for the 2 1 A − g and 1 1 B + u states of trans-hexatriene are found to be almost degenerate, with a value, ≃ 5.5–5.6 eV, higher than that normally accepted in the literature, 5.2 eV and 5.1 eV, respectively. The 2 1 A 1 and 1 1 B 2 states of cis-hexatriene are also almost degenerate and are estimated to be at ≃ 5.4–5.5 and ≃ 5.5 eV, respectively, again higher than the accepted values. The adiabatic excitation energies to the 2 1 A − g and 2 1 A 1 states can be observed experimentally (in particular for the cis isomer), and our results are in excellent agreement with the experimental values. On the contrary, the vertical excitation energies for these states are not directly observable in the experimental spectra and the “experimental” values are obtained by educated guesses. We show that the hypotheses underlying these guesses are not entirely grounded.