# Diabatization with Electrostatic Embedding for Studying Photophysics in Organic Molecular Crystals

## TL;DR

This paper introduces a computational method to study light-induced processes in organic molecular crystals, combining molecular and materials science techniques.

## Contribution

A new computational framework using diabatization and electrostatic embedding for simulating photophysics in organic molecular crystals is presented.

## Key findings

- The electrostatic embedding effect on photophysics was found to be modest (10–20%).
- Electronic interactions among fixed monomers explain the red shift in DBC crystals.
- Ultrafast population transfer from local excitations to charge transfer states was observed.

## Abstract

Highly emissive organic molecular crystals find applications
in
several areas, such as organic electronics, solar cells, and sensors.
Understanding the excited-state mechanisms underlying these applications
is essential for optimizing and controlling them effectively. Exciton
models coupled with nonadiabatic dynamics, particularly quantum dynamics,
provide crucial insights into photochemical and photophysical processes
in molecular crystals. Nevertheless, there remains a lack of general
tools and automated workflows to facilitate such simulations. In this
paper, we present a computational strategy to investigate the photoactivated
dynamics of organic molecular crystals, bridging methodologies traditionally
used for molecular systems and materials science, with a particular
focus on the interplay between local excitations and charge transfer
(CT) processes. We have implemented an interface between the fromage and Overdia programs,
enabling the construction of vibronic Hamiltonians for molecular crystals
within an excited-state ONIOM­(QM:QM′) framework, incorporating
long-range electrostatics through a RESP-based Ewald summation. Fragment-based
diabatization provides a route to quantum dynamics simulations in
weak-to-intermediate coupling regimes. The method was applied to the
photophysics of dibenzo­[g,p]­chrysene (DBC) crystals using time-dependent
DFT. The fromage/Overdia interface was employed to compute the couplings of local excitations
and CT states for 18 unique DBC dimers in the crystal and to quantify
the influence of electrostatic embedding, which was found to be modest
(10–20%). Simulations on π-stacked dimers reproduced
the small red shift observed experimentally from solution to crystal,
attributed to electronic interactions among fixed monomers rather
than crystal electrostatics. Quantum dynamics simulations revealed
ultrafast population transfer from bright local excitations to CT
states. This approach establishes a robust framework linking molecular
and solid-state excited-state dynamics, with potential applications
for studying excitations, defects, and impurities in molecular crystals.