Matrix and Temperature Effects on the Electronic Properties of Conjugated Molecules: An Electroabsorption Study of<i>all</i>-<i>trans</i>-Retinal

Abstract

Using Stark effect (electroabsorption) spectroscopy to study all-trans-retinal (ATR) in a variety of polymer matrices and organic glasses, we have found that the average change in polarizability upon excitation ( ) that we measure is highly dependent on the rigidity of the matrix used. In rigid polymer and organic glass matrices, the measured values of are in the range of 20−50 Å3 and those calculated using semiempirical methods are in the range of 60−85 Å3. In contrast, 's that are up to an order of magnitude higher are measured when the ATR is entrained in nonrigid polymer matrices such as those that are above their glass-transition temperature, or those containing trapped solvent. We have postulated that large values of may be the result of field-induced orientation of ATR within polymer matrices that are not fully rigid. The |Δμ| of ATR ranges from roughly 3 to 11 D, depending on the polarity and the rigidity of the environment. In addition, vibrational structure is apparent in the electroabsorption spectra of ATR in methylcyclohexane and methyltetrahydrofuran glasses at 77 K that is assigned to a progression of the CC stretch.