Performance of the Density Functional Theory/Multireference Configuration Interaction Method on Electronic Excitation of Extended π-Systems

The combined density functional theory/multireference configuration interaction (DFT/MRCI) method [Grimme and Waletzke. J. Chem. Phys. 1999, 111, 5645] has been employed to study the 1 L a and 1 L b states of linear polyacenes and the low-lying triplet and singlet states of linear polyenes and diphenyl-polyenes. We have systematically investigated the dependence of the electronic state properties on technical parameters of the calculations such as the atomic orbital basis set or the geometry optimization approach. The choice of basis set appears to be of minor importance whereas the excitation energies of the polyenes are quite sensitive to the ground-state geometry parameters. The DFT/MRCI energies at the B3-LYP optimized geometries systematically underestimate the experimental values, but we do not observe a bias toward one or the other type of state. The energy gaps between the electronically excited states are reproduced very well. In particular, this applies also to the first excited singlet 2 1 A g − and the optically bright 1 B u + state of the polyenes. The latter appears to be the S 3 or even S 4 state in longer polyenes where the multiconfigurational 1 B u − state represents S 2 . Frequencies and intensities of the excited-state absorption from the 2 1 A g − state are found to be strongly geometry dependent.