Surface-guided polymorphism control of titanyl phthalocyanine single crystals†
Titanyl phthalocyanine (TiOPc) single crystals have received significant attention due to the remarkable mobility of charge carriers with excellent air stability and phase-dependent electrical properties. One of the biggest obstacles is their polymorphism behavior during crystal growth, which hinders the formation of desired crystals. Herein, we have attempted the selective polymorphism control of titanyl phthalocyanine (TiOPc) single crystals using the physical vapor transport (PVT) technique. Subsequently, the selective growth of the α-, β-, C-phase of TiOPc crystals can be successfully achieved depending on different types of substrates. The polymorphism control of the TiOPc crystals was found to be affected by interfacial energy variation through molecule–substrate interactions so that the unique molecular packing of TiOPc crystals is determined by the kind of substrate. Notably, it is reported for the first time that the C-phase of the TiOPc crystal has a rectangular tube-shaped morphology. Interestingly, it exhibits no PL emission as compared to that of the α- or β-phase crystals, indicating the dominant non-radiative relaxation of excitons. Accordingly, it shows much higher photoconductivity as compared to those of the α- and β-phases. These results provide an in-depth understanding of the crystal growth of phthalocyanines, particularly in view of polymorph control, and open up other possibilities for their practical applications in photoelectronic devices.