# Crystallochromism: A Hybrid Model for the Spectral Properties of Quinacridone Polymorphs

## TL;DR

This paper introduces a hybrid model to explain how quinacridone's optical properties change in different crystal forms.

## Contribution

A new hybrid modeling strategy combining quantum chemical calculations and exciton model parameters is proposed.

## Key findings

- The hybrid model successfully describes the optical properties of βQA and γQA crystalline phases.
- Periodic DFT and TD-DFT were used to optimize geometry and extract parameters for the model.
- The approach was validated by measuring the missing optical spectra of the polymorphs.

## Abstract

The evolution of the optical properties of a molecule
from solution
to a crystalline phase is nontrivial, as it results from a complex
interplay of several interactions, including electrostatic and charge-transfer
intermolecular interactions and the coupling with molecular vibrations.
In order to address the crystallochromism observed in quinacridone
(QA), a hybrid modeling strategy is presented that successfully describes
the optical properties (absorption and emission) of the βQA
and γQA crystalline phases. The proposed protocol relies on
the parametrization of the Frenkel–Holstein Hamiltonian against
quantum chemical calculations. Periodic density functional theory
(DFT) is adopted to optimize the crystallographic geometry and to
extract effective atomic charges. Time-dependent DFT (TD-DFT) results
on the isolated molecule are exploited to parametrize the Holstein
coupling, while TD-DFT results on the embedded molecules and on embedded
clusters of increasing size are finally exploited to extract the exciton
model parameters. For safe validation, the missing optical spectra
of the two polymorphs were measured. The approach is general and paves
the way for the rationalization of crystallochromism of molecular
condensed phases.