Accounting for Vibronic Features through a Mixed Quantum-Classical Scheme: Structure, Dynamics, and Absorption Spectra of a Perylene Diimide Dye in Solution

Abstract

The optical absorption spectrum of a perylene diimide (PDI) dye in acetonitrile solution is simulated using the recently developed (J. Chem. Theory Comput. 2020, 16, 1215–1231) Ad-MD|gVH method. This mixed quantum-classical (MQC) approach is based on an adiabatic (Ad) separation of soft­(classical)/stiff­(quantum) nuclear degrees of freedom and expresses the spectrum as a conformational average (over the soft coordinates) of vibronic spectra (for the stiff coordinates) obtained through the generalized vertical Hessian (gVH) vibronic approach. The average is performed over snapshots extracted from classical molecular dynamics (MD) runs, performed with a specifically parameterized quantum-mechanically derived force field (QMD-FF). A comprehensive assessment of the reliability of different approaches, designed to reproduce spectral shapes of flexible molecules, is here presented. First, the differences in the sampled configurational space and their consequences on the prediction of the absorption spectra are evaluated by comparing the results obtained by means of the specific QMD-FF and of a general-purpose transferable FF with those of a reference ab initio MD (AIMD) in the gas phase, in both a purely classical scheme (ensemble average) and in the Ad-MD|gVH framework. Next, classical ensemble average and MQC predictions are also obtained for the PDI dynamics in solution and compared with the results of a ″static″ approach, based on vibronic calculations carried out on a single optimized perylene diimide structure. In the classical ensemble average approach, the remarkably different samplings obtained with the two FFs lead to sizeable changes in both position and intensity of the predicted spectra, with the one computed along the QMD-FF trajectory closely matching its AIMD counterpart. Conversely, at the Ad-MD|gVH level of theory, the different samplings deliver very similar vibronic spectra, indicating that the error found in the absorption spectra obtained with the general-purpose FF mainly concerns the stiff modes. In fact, it can be effectively corrected by the quadratic extrapolation performed by gVH to locate the minima of the ground- and excited-state potential energy surfaces along such coordinates. Furthermore, in the perspective of studying the self-assembling process of PDI dyes and the vibronic spectra of large-size aggregates, the use of a molecule-specific QMD-FF also appears mandatory, considering the significant errors found in the GAFF trajectory in the flexible lateral chain populations, which dictate the supramolecular aggregation properties.

Quantum and semiclassical dynamical studies of nonadiabatic processes in solution: achievements and perspectives

“…In this case this phenomenon occurs because solute/solvent mutual polarization stabilizes Spp* more than Snp*. This finding further highlights the importance of mutual solute/solvent polarization when studying excitedstates properties and dynamics, in line with the indications arising from the analysis of solvent shifts, 216,217 dynamical Stokes shifts, 41 and electronic spectra and dynamics. 136 A similar MQC approach is adopted in ref.…”

Sampling effects in quantum mechanical/molecular mechanics trajectory surface hopping non-adiabatic dynamics

“…At the same time, we are aware these sampling methods are not the only possibly way to generate initial condition for semi-classical non-adiabatic dynamics. Mixed classical-quantum methods have been highly promising to describe the shape of the UV absorption of dyes [ 40 , 41 ] or more complicated systems as transition metal complexes [ 39 ] or biological structures [ 42 ] in explicit solvent environments. It would be interesting to compare their applicability along non-adiabatic dynamics simulations.…”

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

“…The computational scheme presented in this work is a generalization to nonadiabatic cases of the MQC protocol named Ad-MD|gVH, which is briefly sketched in the left side of Figure 1 and was developed by some of the authors 50 and recently applied to compute the vibronic absorption spectrum of a solvated PDI monomer. 49 …”

“… Comparison between the Ad-MD|gVH scheme (left) 50 adopted in ref ( 49 ) to study the PDI monomer and the new MQC protocol (right) presented in this work for self-assembled dimers. In the boxes we highlight the main computational steps, while to the right of the brackets we indicate the main outputs.…”

“…For the former, we use a molecule-specific quantum mechanically derived force field (QMD-FF), built according to the JOYCE protocol, 53 − 55 that was recently benchmarked for the PDI monomer. 49 For the latter, we use the multiconfiguration time-dependent Hartree (MCTDH) QD wave-packet propagations, 56 , 57 and its multilayer (ML) generalization, 58 in combination with a linear vibronic coupling (LVC) Hamiltonian 59 parameterized with a fragment-diabatization approach recently developed by some of the authors. 60 The latter technique allows for defining the adiabatic electronic states of the PDI dimer as a combination of diabatic local excitations (LEs) of the PDI units and relative CT states.…”