How the Interplay among Conformational Disorder, Solvation, Local, and Charge-Transfer Excitations Affects the Absorption Spectrum and Photoinduced Dynamics of Perylene Diimide Dimers: A Molecular Dynamics/Quantum Vibronic Approach

Abstract

In this contribution we present a mixed quantum-classical dynamical approach for the computation of vibronic absorption spectra of molecular aggregates and their nonadiabatic dynamics, taking into account the coupling between local excitations (LE) and charge-transfer (CT) states. The approach is based on an adiabatic (Ad) separation between the soft degrees of freedom (DoFs) of the system and the stiff vibrations, which are described by the quantum dynamics (QD) of wave packets (WPs) moving on the coupled potential energy surfaces (PESs) of the LE and CT states. These PESs are described with a linear vibronic coupling (LVC) Hamiltonian, parameterized by an overlap-based diabatization on the grounds of time-dependent density functional theory computations. The WPs time evolution is computed with the multiconfiguration time-dependent Hartree method, using effective modes defined through a hierarchical representation of the LVC Hamiltonian. The soft DoFs are sampled with classical molecular dynamics (MD), and the coupling between the slow and fast DoFs is included by recomputing the key parameters of the LVC Hamiltonians, specifically for each MD configuration. This method, named Ad-MD|gLVC, is applied to a perylene diimide (PDI) dimer in acetonitrile and water solutions, and it is shown to accurately reproduce the change in the vibronic features of the absorption spectrum upon aggregation. Moreover, the microscopic insight offered by the MD trajectories allows for a detailed understanding of the role played by the fluctuation of the aggregate structure on the shape of the vibronic spectrum and on the population of LE and CT states. The nonadiabatic QD predicts an extremely fast (∼50 fs) energy transfer between the two LEs. CT states have only a moderate effect on the absorption spectrum, despite the fact that after photoexcitation they are shown to acquire a fast and non-negligible population, highlighting their relevance in dictating the charge separation and transport in PDI-based optical devices.

Solvent Effects on Ultrafast Charge Transfer Population: Insights from the Quantum Dynamics of Guanine‐Cytosine in Chloroform

“…We end this section by noting that single QD propagations performed using the averaged explicit solvent parameters shown in Table 1 produce slightly different results to the average over 100 simulations as shown in Figures S7–S12 in the SI, in particular for the Mov GC MM chl and Frz GC QM chl approaches. This is similar to the non‐linear behaviour of the averaged populations with respect to the Hamiltonian parameters some of us have observed for other systems previously [29,33,49] …”

“…We followed the numerical procedures used previously, [36] and further details may be found in the SI, Section S1.4. Absorption spectra of GC were calculated via the Fourier transform of the auto‐correlation function produced by the QD calculations, weighted by the diabatic transition dipoles [33,36,56] . Monomeric spectra of G and C were also calculated with auto‐correlation functions produced by QD propagations on LVC models parametrised by PCM(chloroform)/CAM‐B3LYP/6‐31G(d), in the same manner as we have previously done [36,57] .…”

“…We here make a first step, focusing in the ultrafast (∼100 fs) time‐scale on the population of the G→C CT state, without considering the subsequent PT process. This time‐regime is still challenging but more affordable, in particular if we assume that the nuclear degrees of freedom of the solvent remain frozen in the configuration they have when the photoexcitation takes place, the so‐called “static disorder“ limit [29–33] . However, also in this ultrafast time‐scale several chemical physical effects, related both to bulk solvation and ‘direct’ solute‐solvent interactions, can play a role in the population of a CT state, a reaction that leads to a substantial rearrangement of the solute electron density.…”

Nucleic Acids as a Playground for the Computational Study of the Photophysics and Photochemistry of Multichromophore Assemblies

“…As reported above, this methodology was recently applied to study the photodynamics of AT and GT base pairs, 39 and it promises to be of general applicability. 58 The integration between different techniques is also necessary to simulate the photoactivated dynamics, considering the interplay between elementary processes occurring on very different time scales. The population transfer between the nπ* and ππ* states in the FC region occurs in a few dozens of femtoseconds, but its simulation requires an accurate treatment of the vibronic interactions and makes the inclusion of quantum nuclear effects desirable.…”

“…We have shown that such an approach can be also exploited to parametrize generalized LVC Hamiltonians suitable to study at a fully QD level the competition between monomeric and inter-monomeric decay channels in MCAs. As reported above, this methodology was recently applied to study the photodynamics of AT and GT base pairs, and it promises to be of general applicability …”