Exciton-phonon coupling for the lowest Davydov component of quaterthiophene

The temperature dependence of the 0-0 absorption transition to the lower Davydov exciton band in quaterthiophene single crystals is reported and examined in terms of exciton-phonon interactions. Below an activation temperature, a Lorentzian lineshape is found as a consequence of exciton coherence in a bath of both acoustic and optical phonons, the latter giving only a phonon-absorption broadening. At higher temperatures, a modification of position, intensity, shape, and width of the exciton band is observed due to exciton scattering. From the experimental data, the activation temperature, the average phonon energy, the exciton-phonon stabilization energy are deduced.