Phonon Scattering of Frenkel Excitons in Molecular Microcrystallites Embedded in a Matrix

A matrix effect on the exciton-phonon coupling in microcrystallites embedded in a matrix is investigated theoretically. It is shown that a parameter σ defined as the square of the ratio of the phonon bandwidth of a matrix material to that of a microcrystallite material is a crucial one, affecting the exciton-phonon scattering. Phonons flow in the microcrystallite or out of it for either of σ > 1 or σ < 1, respectively; the probability for finding phonons in the microcrystallite increases with increasing σ. Thus the strength of the exciton-phonon coupling in the microcrystallite is influenced by phonons in the matrix. The flowing-in of phonons, which have energies higher than the maximum energy of phonons in the microcrystallite material, is of special importance and results in the broadening of the lines constituting an optical absorption spectrum.