Temperature-dependent total emission spectra of azulene in polymers: Modeling using spectral densities

Monochromatically excited total emission spectra have been measured for the S-1--> S-0 transition of azulene in polyethylene, polystyrene, and poly(me thylmethacrylate) matrices over a temperature range from 1.4 to 100 K. The spectra in all three polymers exhibit strong zero-phonon lines (excitation of azulene vibrations only) accompanied by well-defined Stokes-shifted phon on sidebands at the lowest temperatures. As the temperature is raised the p honon bands broaden and gain relative intensity at the expense of the zero- phonon lines, and the spectra become qualitatively similar to the room-temp erature liquid-phase spectra with sharp Raman lines on a broad fluorescence background. The near-origin-excited data are simulated by calculating the complete emission spectrum as a chi((3)) process that assumes no artificial partitioning between "Raman" and "fluorescence." The internal vibrations o f azulene are modeled as simple undamped displaced harmonic oscillators whi le the intermolecular or matrix phonons are either modeled as a Brownian os cillator or treated as effective spectral densities extracted from publishe d neutron scattering and/or low-frequency nonresonant Raman data in the sam e polymers. While the qualitative features of the spectra and their tempera ture dependence are reproduced, none of the spectral densities employed giv e a fully satisfactory fit to the experimental spectra. The results demonst rate the sensitivity of total emission spectra to the chromophore-matrix in teractions, and suggest that the spectral densities describing these intera ctions are functions not only of the matrix but also of the chromophore inv olved. (C) 1999 American Institute of Physics. [S0021-9606(99)50332-2].