Matrix and temperature effects on the electronic properties of conjugated molecules: An electroabsorption study of all-trans-retinal

Using Stark effect (electroabsorption) spectroscopy to study all-trans-reti nal (ATR) in a variety of polymer matrices and organic glasses, we have fou nd that the average change in polarizability upon excitation (<(Delta alpha )over bar>) that we measure is highly dependent on the rigidity of the matr ix used. In rigid polymer and organic glass matrices, the measured values o f <(Delta alpha)over bar> are in the range of 20-50 Angstrom(3) and those c alculated using semiempirical methods are in the range of 60-85 Angstrom(3) . I, contrast, <(Delta alpha)over bar>'s that are up to an order of magnitu de higher are measured when the ATR is entrained in nonrigid polymer matric es such as those that are above their glass-transition temperature, or thos e containing trapped solvent. We have postulated that large values of <(Del ta alpha)over bar> may be the result of field-induced orientation of ATR wi thin polymer matrices that are not fully rigid. The /Delta mu/ of ATR range s from roughly 3 to 11 D, depending on the polarity and the rigidity of the environment. In addition, vibrational structure is apparent in the electro absorption spectra of ATR in methylcyclohexane and methyltetrahydrofuran gl asses at 77 K that is assigned to a progression of the C=C stretch.