Effects of matrix temperature and rigidity on the electronic properties ofsolvatochromic molecules: Electroabsorption of coumarin 153

Using Stark effect (electroabsorption) spectroscopy to study the well-known solvatochromic probe molecule coumarin 153 (C153) in a variety of polymer matrices and organic glasses, we have found that the average change in pola rizability (<(Delta<(alpha)double under bar>)over bar>) that we measure dep ends critically on the rigidity of the matrix used. In rigid polymer and fr ozen organic glass matrices, the measured values <(Delta<(alpha)double unde r bar>)over bar> are between 4 and 60 Angstrom(3). The smaller values in th is range are similar to those obtained via semiempirical and ab initio calc ulations, In contrast, measurements made on polymer matrices that are above their glass-transition temperature or those containing trapped solvent are more than an ol-der of magnitude higher. We postulate that large values of acr result from field-induced orientation of the C153 molecule and/or the dipoles of the surrounding matrix in matrices that are not fully rigid. The absolute value of the change in dipole moment between the ground and excit ed states (\Delta<(mu)under bar>\) of C153 measured here:ranges from 4.4 to 7.0 D, depending on the polarity and the rigidity of the environment. In a ddition, an apparent local enhancement of the polarity of the cavity contai ning C153 is observed in both the solvent and polymer glass matrices, as in ferred by the absorption maximum of C153 in these environments.