Vibronic spectra of mixed Frenkel and charge transfer excitons

The excitonic and vibronic spectra of a molecular chain and of a crystal of NN'-dimetyl 3,4,9,10-perylene tetracarboxylic diimide (MePTCDI) are studied in the case when: (i) a Frenkel exciton (FE) and charge transfer excitons (CTEs) mix strongly; and (ii) two mechanisms of coupling between these mixed excitons and intramolecular vibrations, notably linear and quadratic coupling, are acting. Using a convenient canonical transformation and the Green function method, we calculate the linear optical susceptibility: (a) in the exciton region taking into account the contribution of the transition dipoles of FE; and (b) in one-phonon vibronic regime. The spectra of linear absorption in the excitonic and vibronic regions have been calculated introducing the exciton parameters of MePTCDI. These spectra exhibit: (i) a relative separation of vibronics of FE and CTEs; (ii) a stronger impact of the linear coupling on the intensity of the excitonic and vibronic spectra; and (iii) the appearance of a spectral doublet of vibronics of the CTEs-its splitting depends on the parameters of linear and quadratic exciton-phonon coupling in the neutral excited molecule and ions. Moreover, in the case of weak linear exciton-phonon coupling the vibronic line of FE, being wide and flat, lies in many-particle continuum while in the case of intermediate and strong linear coupling the linear absorption is dominated by the bound exciton-phonon states and their narrow Lorentzian maxima depend strongly on the quadratic coupling.