The Intramolecular Charge Transfer State in Carbonyl-Containing Polyenes and Carotenoids

Numerous femtosecond time-resolved optical spectroscopic experiments have reported that the lifetime of the low-lying Si state of carbonyl-containing polyenes and carotenoids decreases with increasing solvent polarity. The effect becomes even more pronounced as the number of double bonds in the conjugated pi-electron system decreases. The effect has been attributed to an intramolecular charge transfer (ICT) state coupled to Si, but it is still not clear what the precise molecular nature of this state is, and how it is able to modulate the spectral and dynamic properties of polyencs and carotenoids. In this work, we examine the nature of the ICT state in three substituted polyenes: crocetindial, which contains two terminal, symmetrically substituted carbonyl groups in conjugation with the pi-electron system, 8,8'-diapocarotene-8'-ol-8-al, which has one terminal conjugated carbonyl group and one hydroxyl group, and 8,8'-diapocarotene-8,8'-diol, which has two terminal, symmetrically positioned, hydroxyl groups but no carbonyls. Femtosecond time-resolved optical spectroscopic experiments on these molecules reveal that only the asymmetrically substituted 8,8'-diapocarotene-8'-ol-8-al exhibits any substantial effect of solvent on the excited state spectra and dynamics. The data are interpreted using molecular orbital theory which shows that the ICT state develops via mixing of the low-lying Si (2'A(g)-like) and S-2 ((IBu)-B-1-like) excited singlet states to form a resultant state that preferentially evolves in polar solvent and exhibits a very large (similar to 25 D) dipole moment. Molecular dynamics calculations demonstrate that the features of the ICT state are present in similar to 20 fs.