Theoretical investigation of polymorph- and coformer-dependent photoluminescence in molecular crystals

Polymorph- and coformer-dependent photoluminescence (PL) are among the variety of novel solid-state PL phenomena recently observed in many molecular crystals. They are of particular research interest due to their direct connections to two heavily investigated topics in crystal engineering: polymorphism and cocrystallization. Herein, we apply a novel computational methodology, initially proposed and successfully applied in our previous investigation of piezochromism, to theoretical modeling of the polymorph- and coformer-dependent PL in the well-known ROY polymorphs and the recently synthesized 9-acetylanthracene (9-ACA) cocrystals, respectively. Our methodology offers satisfactory prediction of the experimentally observed color zoning for the ROY polymorphs and provides good qualitative and quantitative accuracy for the emission (fluorescence) energies of the 9-ACA cocrystals, although the results in both cases may be adversely affected by delocalization error in the density-functional methods employed. While the polymorph-dependent PL in ROY is found to be controlled by the intramolecular geometry, modeling of the periodic crystal environment is necessary for accurate prediction of the coformer-dependent PL in the 9-ACA cocrystals, which is driven by charge transfer.