All-DFTB Approach to the Parametrization of the System-Bath Hamiltonian Describing Exciton-Vibrational Dynamics of Molecular Assemblies

Abstract

Spectral density functions are central to the simulation of complex many body systems. Their determination requires making approximations not only to the dynamics but also to the underlying electronic structure theory. Here, blending different methods bears the danger of an inconsistent description. To solve this issue we propose an all-DFTB approach to determine spectral densities for the description of Frenkel excitons in molecular assemblies. The protocol is illustrated for a model of a PTCDI crystal, which involves the calculation of monomeric excitation energies and Coulomb couplings between monomer transitions, as well as their spectral distributions due to thermal fluctuations of the nuclei. Using dynamically defined normal modes, a mapping onto the standard harmonic oscillator spectral densities is achieved.

Spectral Diffusion of Excitons in 3,4,9,10-Perylenetetracarboxylic-diimide (PTCDI) Thin Films

“…These peak energies are very close to those observed in absorption spectra of PTCDI thin films . In the crystal phase, PTCDI forms a monoclinic phase, in which the exciton interaction is mainly dominated by that between neighboring chromophores. ,, The vibronic-exciton bands at 2.0–2.5 eV have been well reproduced by a model calculation considering a mixing between Frenkel and CT excitons . In this work, we focus on the spectral diffusion in the 0–0 band, not in the 0–1 band.…”

“…Plötz and co-workers have studied on-site energy fluctuations in PTCDI crystal. , By using a time-dependent density-functional-theory-based tight-binding method, they proposed that several intramolecular vibrational modes with frequencies ranging from 400 to 1500 cm –1 contribute to the on-site energy fluctuations. The sum of the contributions of these intramolecular modes will act as a single overdamped mode with a spectral density covering a wide frequency region.…”

Redfield Propagation of Photoinduced Electron Transfer Reactions in Vacuum and Solution

“…Correspondingly, the terms containing δ­(ω ξ + ω) or δ­(ω k + ω) only contribute when ω = −ω ξ or ω = −ω k , respectively. The Fourier transformed bath time correlation function becomes The occupation number with a negative frequency argument can be rewritten, In addition, two different density of modes are defined, and The bath time correlation function simplifies to Two different spectral densities are defined, and with f k being the following coupling element, The bath time correlation function can thus be written in the compact form, , Meanwhile, the frequencies of the normal modes and harmonic oscillators are all positive. For that reason, only one of the spectral densities in each term will survive.…”

Excited-State Dynamics in Perylene-Based Organic Semiconductor Thin Films: Theory Meets Experiment

“…Approaches based on the Frenkel–Holstein model Hamiltonian − have very successfully been used to interpret the absorption spectra of perylene-based systems. − However, for emission spectra, difficulties exist because the parameters for the absorption may differ from the parameters needed for the emission. Improvements of this approach were suggested by Martínez and co-workers and Kühn and co-workers. , In the present work, we performed large scale quantum chemical calculations on the energetics and the characters of the involved states, using our dimer- or aggregate-based approach . This approach has been proven to be reliable for the investigations of the excited-state dynamics after optical excitations of DIP and other substituted perylene derivatives. − Further information about exciton diffusion processes in amorphous films is adopted from recent work on DIP films and DIP–C 60 interfaces. − Moreover, these data are combined with experimental information about the thin-film structures. − …”