Photoexcitation dynamics in perylene diimide dimers

Abstract

We utilize first-principles theory to investigate photo-induced excited-state dynamics of functionalized perylene diimide. This class of materials is highly suitable for solar energy conversion because of the strong optical absorbance, efficient energy transfer, and chemical tunability. We couple time-dependent density functional theory to a recently developed time-resolved non-adiabatic dynamics approach based on a semi-empirical description. By studying the monomer and dimer, we focus on the role stacking plays on the time-scales associated with excited-state non-radiative relaxation from a high excitonic state to the lowest energy exciton. We predict that the time-scale for energy conversion in the dimer is significantly faster than that in the monomer when equivalent excited states are accounted for. Additionally, for the dimer, the decay from the second to the nearly degenerate lowest energy excited-state involves two time-scales: a rapid decay on the order of ∼10 fs followed by a slower decay of ∼100 fs. Analysis of the spatial localization of the electronic transition density during the internal conversion process points out the existence of localized states on individual monomers, indicating that the strength of thermal fluctuations exceeds electronic couplings between the states such that the exciton hops between localized states throughout the simulation.

Nonadiabatic Molecular Dynamics with Extended Density Functional Tight-Binding: Application to Nanocrystals and Periodic Solids

“…Nonadiabatic molecular dynamics (NA-MD) is a powerful computational technique that can model NA processes and describe the excited-state dynamics in various materials. NA-MD simulations have been successfully utilized in predicting the photochemistry and photophysics of molecular systems − and nanoscale materials. − These simulations have been widely used to gain insights into NA processes in solar cells, − light-emitting diodes, and chemical reactions. − However, due to the highly demanding computational costs, the NA-MD simulations of solid-state and nanoscale systems are often limited to a few hundred atoms. Simulation of realistic nanostructures, comparable to those studied experimentally is still computationally expensive, primarily due to the demanding costs of the underlying electronic structure calculations and their unfavorable scalability with the system size.…”

Nonadiabatic Molecular Dynamics Study of the Relaxation Pathways of Photoexcited Cyclooctatetraene

“…The excited-state molecular dynamics of COT is calculated using Tully’s fewest switches surface hopping (FSSH) approach combined with empirical decoherence corrections and trivial crossing tracking algorithms as implemented into the Nonadiabatic Excited-state Molecular Dynamics (NEXMD) package. − By applying the collective electron oscillator method with the semiempirical Austin Model 1 (AM1) Hamiltonian at the configuration interaction singles (CIS) level, the NEXMD can routinely handle the NAMD simulation up to 20 picoseconds (ps). This package has been successfully applied to the modeling of photoinduced processes in many other molecular systems. − …”

“…As with other systems, the semiempirical AM1/CIS results in red-shifted energies compared to time-dependent density functional theory (TD-DFT) and ab initio CASSCF/ CASPT2 methods. Importantly, the relaxation rates during the excited-state dynamics are closely related to the energy gaps between states 25 but not to the absolute values of the transition energies from the ground state. Therefore, the AM1/CIS approach is appropriate to conduct the COT NAMD simulation due to the good agreement of band gaps with other higher-level methods (e.g., TD-DFT and CASPT2).…”

Excitons: Energetics and spatio-temporal dynamics

“…In cases where multi-exciton states are crucial, as in singlet fission, approximate multireference methods like multireference configuration interaction DFT (DFT/MRCI) have been explored. 13 In addition to the chosen electronic structure method, the geometry dependence of the relevant electronic states is crucial as demonstrated by Deutsch et al 16 and Mukazhanova et al, 17 especially if complex excited-state relaxation and reorganization processes are involved, possibly leading to the formation of traps.…”