A Novel Theoretical Approach to Model Electronic Excitations in Molecular Crystals
Photon-induced electronic excitation of organic chromophores in molecular crystals has received much attention due to potential applications in organic electronics, optics, and biomedicine. Theoretical modeling can aid our understanding of the excitation processes and becomes necessary for high-throughput screening of candidate materials for potential applications. However, predicting photoluminescence (PL) properties in the solid state is complex, as the excitation energies may shift drastically from their gas-phase counterparts. Also, these properties can be significantly influenced by subtle changes in the intermolecular interactions caused by different modes of crystal packing or molecular compositions within the solid-state material. This thesis aims to develop a novel computational methodology to achieve accurate and cost-efficient prediction of single-electron excitation energies in molecular crystals. Our methodology combines periodic-boundary and single-molecule density-functional theory (DFT) calculations. The periodic-boundary DFT method is paired with the exchange-hole dipole moment (XDM) dispersion model to accurately describe the intermolecular interactions within the crystal lattice. An efficient correction scheme, the virial exciton model, is then employed to obtain the singlet-triplet energy splitting in the first single-electron excited configuration from gas-phase molecular calculations, leading to the prediction of the singlet excitation energies. Herein, we detail the design, validation, and application of our novel computational methodology. Initial studies probe the effect of electronic excitations on London dispersion, and test the reliability of the virial exciton model for charge-transfer excitations. These studies provide validation for some key assumptions in the devised methodology. Our methodology is then applied to model a variety of solid-state PL properties in diverse sets of luminescent molecular crystals. These investigations encompass topics including piezochromism, polymorphism-dependent PL, and coformer-dependent PL. Our methodology proves highly successful in replicating the experimentally-observed PL behaviors of the investigated molecular crystals, demonstrating excellent reliability and transferability. Valuable insights into the underlying mechanisms of the investigated solid-state PL properties are also obtained through our results. We hope that the research presented herein could lead to accelerated theory-guided design and screening of industrially valuable solid-state luminescent materials. It may also uniquely contribute to the general understanding of the fundamental nature of electronic excitations in the solid state.