Effects of Matrix Temperature and Rigidity on the Electronic Properties of Solvatochromic Molecules: Electroabsorption of Coumarin 153

Abstract

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 polarizability ( ) that we measure depends critically on the rigidity of the matrix used. In rigid polymer and frozen organic glass matrices, the measured values are between 4 and 60 Å3. The smaller values in this range are similar to those obtained via semiempirical and ab initio calculations. In contrast, measurements made on polymer matrices that are above their glass-transition temperature or those containing trapped solvent are more than an order of magnitude higher. We postulate that large values of 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 excited states (|Δμ|) of C153 measured here ranges from 4.4 to 7.0 D, depending on the polarity and the rigidity of the environment. In addition, an apparent local enhancement of the polarity of the cavity containing C153 is observed in both the solvent and polymer glass matrices, as inferred by the absorption maximum of C153 in these environments.