Polymorph-Selective Preparation and Structural Characterization of Perylene Single Crystals

Abstract

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 towards a polymorph-selective perylene single-crystal growth. Though crystals of both polymorphs are obtained from toluene solution (either by cooling of saturated solution or by evaporation of solvents) they exhibit numerous defects and their size cannot be precisely controlled. Vapor deposition and re-sublimation favors the formation of α-crystals which can be rationalized by a newly identified thin-film phase 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 re-sublimation while the liquid mediated crystals exhibit exceptionally flat surfaces and enable detailed fluorescence studies without defect-related emission signals.

Low-lying excited states in crystalline perylene

“…Tuning of the growth parameters (substrate temperature and molecular flux) enables further a polymorphselective growth of individual perylene crystals of both phases. As demonstrated in previous work (19), the respective phases as well as the azimuthal directions can be easily identified due to the characteristic crystalline shapes (so-called tracht; Fig. 1).…”

“…Both polymorphs are easily distinguishable based on their appearance: the (100)-oriented α-and β-phase crystals reveal an almost quadratic or rhombic base area, respectively (19), as depicted in Fig. 1A.…”

“…Additional crystals were prepared by repeated resublimation of perylene powder onto oxidized Si wafers as described in ref. 19, yielding thicker samples which are used for the reflection measurements. All optical measurements are performed using a microscope setup with a spatial resolution below 5 μm, featuring all-reflective optics to avoid chromatic aberrations and ensure a high throughput of UV light wherever possible.…”

Ambient Pressure Sublimation Technique Provides Polymorph‐Selective Perylene Nonlinear Optical Microcavities

“…In a recent study, though, air‐sublimation of perylene could yield only the α‐phase, no systematic optimization of temperature was performed to achieve the vital β perylene phases . In another report, organic molecular beam deposition of perylene onto silicone‐oil‐covered substrates allows the fabrication of both polymorphs . On the other hand, physical vapor transport is an effective technique to grow ultrapure large‐size organic semiconductor crystals; it requires the use of inert gas, and sometimes needs very low pressure .…”

Controllable Synthesis of Organic Microcrystals with Tunable Emission Color and Morphology Based on Molecular Packing Mode

“…What is more, the cocrystal approach can also significantly influence the molecular staking modes and finally lead to the disparity in the performance of the miniature organic devices . Up to today, the approach toward modulating the molecular staking mode of organic crystals of single molecules is seldom reported …”