Dipole moment differences of polar dyes in polymeric matrices: Stark effect and photochemical hole burning. II Original Research Article

As an extension of our first article of this series (J. Chem. Phys. 97 (1992) 5316), in which we reported data of the induced dipole moment differences Δμind of nonpolar dyes in polymeric matrices, we present results which were obtained with polar optical probe molecules, namely, chlorin and naphthazarin. We used the technique of persistent spectral hole burning to investigate both the frequency dependence of the permanent dipole moment differences Δμper of polar dyes, and the correlation of the dipole moment differences measured in the absorption maximum, 〈Δμabsmaxper〉 with the low-temperature dielectric constant ϵHe. In contrast to the difference Δμind of the induced dipole moments of centrosymmetric molecules the difference Δμper of the permanent dipole moments of polar dyes does not increase from higher to lower optical frequencies but is constant or decreases with decreasing frequency in matrices which are nonpolar or only moderately polar. The 〈Δμabsmaxper〉 values exhibit a linear dependence on the low-temperature dielectric constant, which is similar to the behavior found for centrosymmetric molecules. This linear dependence allows the separation of the matrix-induced contribution to the Δμper values and the “molecular” Δμmol values. Implications of the dependences of Δμ on the absorption frequency and on the dielectric constant of the matrix are discussed.