Optical dephasing of the electronic transitions of delocalized molecular dimer states

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Abstract
We present a theoretical study of the optical dephasing of the electronic transitions of delocalized molecular dimer states. The dephasing arises from the dynamical interactions of the dimer states with the phonon bath of the host crystal. We introduce a general dephasing model for dimers with strong intermolecular interactions and analyze the consequences of the model with optical Redfield theory. A quantum mechanical microscopic model is developed in which the interaction Hamiltonian of the dimer states and the phonon bath states is clearly delineated. We treat both linear and quadratic excitation-phonon coupling in which the two molecules comprising the dimer may be coupled differently to the phonons. Couplings to acoustic, optical, and pseudolocal phonons (librations) are considered explicitly. The temperature dependence of the dephasing rates are obtained for scattering between delocalized dimer states, scattering to dimer librations and phonon-induced pure dephasing. These results are used to analyze the photon echo studies on pentacene dimers and tetracene dimers in p-terphenyl and on naphthalene dimers in perdeuteronaphthalene. In the pentacene dimers and tetracene dimers, we conclude that the dephasing results from scattering to the dimer librations via a one-phonon process. In addition, a quantitative value for the libron–phonon coupling is determined. In the naphthalene dimer system, we find scattering between delocalized dimer states, induced by linear excitation-phonon coupling, to be the predominant dephasing mechanism. A quantitative value is determined for the phonon coupling matrix element responsible for scattering between the delocalized states.