Theoretical study of the low-lying excited states of β-carotene isomers by a multireference configuration interaction method

The combined density functional theory and multireference configuration interaction method (DFT/MRCI) has been employed to explore the ground and low-lying electronically excited states of various beta-carotene monocis and dicis isomers. Although the excitation energies are generally somewhat underestimated by DFT/MRCI, the experimental trends are well reproduced and allow an interpretation of the main bands of the UV-Vis spectra. The optically bright signal is correctly assigned to S-0 -> S-2, corresponding to the HOMO -> LUMO transition, whereas the so-called cis-band originates mainly from the S-0 -> S-4 transition and arises from HOMO -> 1 -> LUMO and HOMO -> LUMO+1 excitations. The calculations reveal a correlation between the oscillator strengths of these transitions and the C6-C6' distance thus explaining the effect of the molecular configuration on the shape of the UV-Vis spectra. (C) 2010 Elsevier B. V. All rights reserved.