화학공학소재연구정보센터
Journal of Chemical Physics, Vol.111, No.8, 3599-3611, 1999
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(methylmethacrylate) 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 phonon sidebands at the lowest temperatures. As the temperature is raised the phonon bands broaden and gain relative intensity at the expense of the zero-phonon lines, and the spectra become qualitatively similar to the room-temperature 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 of azulene are modeled as simple undamped displaced harmonic oscillators while the intermolecular or matrix phonons are either modeled as a Brownian oscillator or treated as effective spectral densities extracted from published neutron scattering and/or low-frequency nonresonant Raman data in the same polymers. While the qualitative features of the spectra and their temperature dependence are reproduced, none of the spectral densities employed give a fully satisfactory fit to the experimental spectra. The results demonstrate the sensitivity of total emission spectra to the chromophore-matrix interactions, and suggest that the spectral densities describing these interactions are functions not only of the matrix but also of the chromophore involved.