Electron–molecular vibration (e–mv) coupling in charge-transfer compounds and its consequences on the optical spectra: A theoretical framework

A general framework for the theory of electron–intramolecular vibration (e–mv) interaction in charge-transfer (CT) compounds is presented, particular attention being devoted to the effect of this interaction on the optical spectra of quasi-one-dimensional organic semiconductors. The common physical basis of different theoretical models of e–mv coupling is evidenced by taking into consideration an isolated CT pair. In particular, it is shown that a complete and substantially equivalent description of the phenomena is provided by the approaches based on the linear response theory or on the Herzberg–Teller coupling scheme, the latter method being developed here to its full extent. The comparison with experimental data shows that a collection of isolated CT pairs can satisfactorily model the consequences of e–mv interaction on the optical spectra of dimerized stack organic semiconductors, either of segregated or mixed type. In addition, we develop here for the first time a model also able to take into account the effect of a nonzero interdimer CT integral. Finally, the case of regular stack CT crystals is analyzed, in such a way, a complete picture of the e–mv (vibronic) perturbation on the optical spectra is obtained, putting in evidence how the spectra can be used to obtain detailed information on the structure and properties of quasi-one-dimensional organic semiconductors. The consequences of the coupling between molecular vibrations and lattice modes are also briefly investigated.