Coherent Charge Transfer Exciton Formation in Regioregular P3HT: A Quantum Dynamical Study

Abstract

The ultrafast formation of charge transfer excitons (CTXs) in regioregular poly(3-hexyl thiophene) (rrP3HT) domains is elucidated by electronic structure and quantum dynamical studies of an aggregate model system comprising five stacked quaterthiophene units. Using a multistate vibronic coupling Hamiltonian parametrized by TDDFT calculations for 13 electronic states of Frenkel and CTX type, along with 78 vibrational modes, quantum dynamical simulations are carried out using the Multi-Layer Multi-Configuration Time-Dependent Hartree (ML-MCTDH) method. In line with time-resolved spectroscopic results et al. Nat. Commun. 2016, 7, 13742], it is found that CTX formation occurs immediately upon photoexcitation, accompanied by sustained regular oscillations with a ∼22 fs periodicity. These coherent features, whose presence may seem surprising in a high-dimensional aggregate or thin film material, can be traced back to a dominant vibronic signature of CC stretch-type high-frequency modes. These vibrational signatures are found to be enhanced due to a collective vibronic response that is prompted by the initial generation of a delocalized bright exciton and its subsequent relaxation, by internal conversion, to a polaronic local exciton ground state.

Vibronic coupling models for donor-acceptor aggregates using an effective-mode scheme: Application to mixed Frenkel and charge-transfer excitons in oligothiophene aggregates

“…In the present case, this observation underscores that a dynamical approximation which entirely relies on the most prominent high-frequency modes of the original spectral density tends to be inaccurate beyond the shortest time scale, as further detailed in our companion paper. 47 Effective mode schemes, which are closely related to time-dependent density matrix renormalization group (DMRG) approaches, 59 provide a flexible strategy to describe vibronic coupling in molecular aggregates. The case of correlated fluctuations of electronic subsystem variables induced by the vibrational modes is of particular interest in this context, and the effective-mode treatment is naturally suited to capture these correlations.…”

“…The modified parametrization permits to achieve better agreement with supermolecular calculations and a qualitatively correct representation of the absorption spectrum. 47 From Table I, one can see that the difference in on-site energies between the bright XT and CTX state is 0.41 eV.…”

“…16,44,45 The aggregate model Hamiltonian is parametrised based upon Time-Dependent Density Functional Theory (TDDFT) electronic structure calculations for an (OT4) 2 species, 46 and quantum dynamical calculations are subsequently performed for larger (OT4) n systems up to n = 5. 47 In the present paper, the performance of the effective mode transformation will be evaluated by comparing the resulting effective-mode SDs for the dimer (OT4) 2 and pentamer (OT4) 5 with the SDs obtained for the full normal mode space. In a companion study, we report on high-dimensional quantum dynamical simulations for an (OT4) 5 pentamer system using the ML-MCTDH method.…”

“…In a companion study, we report on high-dimensional quantum dynamical simulations for an (OT4) 5 pentamer system using the ML-MCTDH method. 47 The remainder of the paper is organised as follows. Sec.…”

A Fragment Diabatization Linear Vibronic Coupling Model for Quantum Dynamics of Multichromophoric Systems: Population of the Charge Transfer State in the Photoexcited Guanine Cytosine Pair

“…LVC models have attracted renewed interest recently, as a cost effective method to study nonadiabatic spectroscopy and intramolecular excited state dynamics. 8,9,[13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] On the other side, fragment-based models are popular approaches to study the excited state dynamics of multichromophore systems, 8,9,[22][23][24][25][26][27][28][29][30][31] but in most of their implementations they consider each single chromophore (site) as characterized by a single relevant excited state, or few states but without an internal (nonadiabatic) dynamics.…”