The lowest valence transitions of 1,1′-bicyclohexylidene and 1,1′;4′,1′-tercyclohexylidene. An ab initio MRDCI study

Ab initio MRDCI calculations were performed on the oligo(cyclohexylidenes) 1,1′-bicyclohexylidene (1) and 1,1′:4′,1″-tercyclohexylidene (2) using RHF/6-31G geometries and molecular orbitals for the assignment of their lowest valence transitions taking into account the four lowest states of each symmetry. The calculations unequivocally show that the valence transitions of 1 and 2 correspond to π→π* and π→σ* transitions. In the case of 1, the MRDCI results were verified by Direct-CI calculations. Mulliken population analyses suggest that the π→σ* transitions possess charge transfer character; upon excitation electron density shifts from the olefinic carbon atoms towards hydrogen atoms positioned in the cyclohexylidene rings. The transition energies and oscillator strengths of the π→π* and π→σ* transitions of 1 and 2 differ from those of the reference compounds tetramethylethene (3) and 1,4-diisopropylidene-cyclohexane (4), respectively. In going from 3 to 1 both the π→π* and the first σ→π* transitions of 1 are bathochromically shifted by ca. 0.24 eV. The first π→σ* transition of 1 is hypsochromically shifted (0.79 eV) with respect to that of 3. In the case of 2 both the π→π* and π→σ* transitions are ca. 0.9 eV bathochromically shifted with respect to those of 4