Vibronic spectra of mixed Frenkel and charge transfer excitons

Abstract

The excitonic and vibronic spectra of a molecular chain and of a crystal of NNЈ-dimetyl 3,4,9,10-perylene tetracarboxylic diimide ͑MePTCDI͒ are studied in the case when: ͑i͒ a Frenkel exciton ͑FE͒ and charge transfer excitons ͑CTEs͒ mix strongly; and ͑ii͒ 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: ͑a͒ in the exciton region taking into account the contribution of the transition dipoles of FE; and ͑b͒ 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: ͑i͒ a relative separation of vibronics of FE and CTEs; ͑ii͒ a stronger impact of the linear coupling on the intensity of the excitonic and vibronic spectra; and ͑iii͒ 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.

Donor–acceptor molecular aggregates: Vibronic spectra of mixed Frenkel and charge-transfer excitons

“…In the second case of vibration A only the CTE will couple directly to the vibration. Thus in the case of vibration D we have the As usual the linear exciton-phonon coupling can be eliminated using the canonical transformation [4]:…”

“…Furthermore it is assumed that both D and A each are coupled to a vibrational mode of frequencies o d and o ac . The Hamiltonian of the system [4][5][6][7] H ¼Ĥ F þĤ C þĤ FC þĤ P þĤ EP (1) is given by the contributions of the FEs, of the CTEs, of the coupling between FEs and CTEs, of the phonons and of the coupling between excitons and phonons. They arê…”

Vibronic Spectra of Mixed Frenkel and Charge-Transfer Excitons

“…In the following we apply the dynamic theory of vibronic spectra [4] and using the method of canonical transformation and Green functions (at zero temperature, T = 0) we calculate the linear optical susceptibility of the stack (for details, see [3]). …”

“…This paper contains the main results of the previous studies of the authors on the vibronic spectra of the MePTCDI and PTCDA organic crystals [1,2,3]. In molecular crystals, the vibronics consist of one exciton (electronic excitation) plus one or several quanta of internal phonons [4].…”

“…Moreover this makes the propagation of the CTEs faster. We wish to apply our theoretical study to vibronics of other molecular crystals and thus we include in [2,3] as well as in the present article the two mechanisms-linear + quadratic coupling. But the treatment of the vibronic problem also needs a hypothesis on the exciton-phonon coupling.…”