Polymorph-Selective Preparation and Structural Characterization of Perylene Single Crystals

Organic semiconductors occurring in polymorphic structures represent excellent model systems for fundamental studies of optoelectronic excitations in different crystalline configurations. Perylene is an archetypal polycyclic aromatic hydrocarbon appearing in two polymorphs known as α- and β-phases which adopt different molecular packing motifs. However, the growth of high quality single crystals with appropriate sizes and polymorph selectivity remains challenging. In this study, we compare various approaches toward polymorph-selective perylene single-crystal growth. Though crystals of both polymorphs are obtained from toluene solution (either by cooling of a saturated solution or by evaporation of the solvents), they exhibit numerous defects and their size cannot be precisely controlled. Vapor deposition and resublimation favor the formation of α-crystals which can be rationalized by a newly identified thin-film phase that forms initially. Further, we demonstrate that organic molecular beam deposition onto silicone-oil-covered substrates enables the fabrication of high-quality crystals of both phases. The relative occurrence of the individual polymorphs is controlled by the actual deposition parameters. Combining the results of X-ray diffraction, atomic force microscopy, and fluorescence analysis enables an unambiguous polymorph identification solely based on the characteristic crystal shape. The morphological characterization reveals characteristic screw dislocations at crystals grown from solution or by resublimation while the liquid-mediated crystals exhibit exceptionally flat surfaces and enable detailed fluorescence studies without defect-related emission signals.