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

详细信息    查看全文

• 作者：Miriam M. Enriquez ; Marcel Fuciman ; Amy M. LaFountain ; Nicole L. Wagner ; Robert R. Birge ; Harry A. Frank
• 刊名：Journal of Physical Chemistry B
• 出版年：2010
• 出版时间：September 30, 2010
• 年：2010
• 卷：114
• 期：38
• 页码：12416-12426
• 全文大小：777K
• 年卷期：v.114,no.38(September 30, 2010)
• ISSN：1520-5207

文摘

Numerous femtosecond time-resolved optical spectroscopic experiments have reported that the lifetime of the low-lying S₁ 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 π-electron system decreases. The effect has been attributed to an intramolecular charge transfer (ICT) state coupled to S₁, 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 polyenes 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 π-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 S₁ (2¹A_g-like) and S₂ (1¹B_u-like) excited singlet states to form a resultant state that preferentially evolves in polar solvent and exhibits a very large (25 D) dipole moment. Molecular dynamics calculations demonstrate that the features of the ICT state are present in 20 fs.