Absorption profile and femtosecond intraband relaxation of the intense upper Davydov component in oligothiophenes

Abstract

AbstractThe diffuse shape of the high‐energy absorption band observed in oligothiophene crystals is interpreted in terms of Fano‐type mixing between the discrete upper Davydov component at k = 0 and the continuum of phonon‐accompanied exciton states at other values of crystal momentum. In temporal domain, this mixing is viewed as a crystal version of radiationless transition, and is followed by subsequent intra‐band exciton relaxation due to scattering processes with phonon release. The rates of energy dissipation in these latter processes, mediated by different intramolecular vibrational modes, are estimated from a simple expression based on the Fermi golden rule. Depopulation of long‐lived vibronic intermediates, acting as bottlenecks, is attributed to thermally activated processes with absorption of low‐frequency phonons. All essential input parameters are obtained from independent nonempirical calculations. The results are in excellent agreement both with the experimentally observed absorption band shapes and with energy‐dependent femtosecond dynamics afforded by measurements of sexithiophene (6T) fluorescence and photoinduced absorption.