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

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 beta QA and gamma QA 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.