Relationship between electron–phonon coupling and intermolecular interaction parameters in dye-doped organic glasses

Abstract

Both the matrix-induced shift of the electronic transition frequency and the coupling strength of this transition to the low-energy vibronic modes are associated with the change in intermolecular interaction potentials on excitation. A review of the Debye–Waller factors and spectral hole widths for organic dyes imbedded in amorphous hosts is presented together with the solvent-shift data. Relative intensities of zero-phonon lines in centrosymmetric impurities depend on the solvent shifts or on the changes in molecular polarizabilities on excitation. In accordance with the small difference in intermolecular dispersion interaction energy in the ground and the excited states, free-base porphyrins show deep and narrow spectral holes at liquid-helium temperatures, which are persistent up to 100 K. In the case of polar molecules both the linear and the quadratic electron–phonon coupling are mediated mainly by the dipole–dipole interaction, since the Debye–Waller factors and hole widths are strongly correlated with the changes in molecular dipole moments (Δμ) between S1 and the ground state. To our knowledge, in glassy hosts no zero-phonon transitions have been reported for dopant molecules with Δμ values exceeding 2–3 D.