Crystal Engineering in Oligorylenes: The Quest for Optimized Crystal Packing and Enhanced Charge Transport

The crystal structures of organic semiconductors are critical when they are integrated into optoelectronic devices, such as organic field-effect transistors (OFETs). In this study, we introduce a crystal engineering approach that leverages weak, nondirectional dispersion forces and steric effects, working together to govern the molecular packing. We investigated how the substitution at the peri-position affects the crystal structure in a series of oligorylene molecules. Upon elucidation of the crystal structures, we found a distinct difference between symmetrical and unsymmetrical derivatives. The unsymmetrical derivatives are prone to forming a sandwich herringbone (SHB) motif, while symmetrical derivatives exhibit a typical herringbone (HB) motif. In most of the rylene derivatives, substitutions at the peri-position triggered an “end-to-face” orientation within the HB structure, rather than an “edge-to-face” orientation, which occurs more often. Results from the Hirschfeld surface analysis provide evidence that the “end-to-face” orientation promotes C–H–π interactions between terminal methyl groups and the π-core of the molecules. While these C–Hmethyl---π interactions contribute to the overall stability of the packing structure, they remain ineffective in enhancing the charge transport properties. In contrast, a particular derivative, tetramethyl perylene (TMP), exhibits a HB structure with an edge-to-face orientation, promoting both C–H---π and π---π interactions. These interactions are crucial for improving the charge carrier mobility, as evidenced by mobility values. For TMP, we could obtain the mobility value of 0.05 cm2 V–1 s–1 in OFETs, whereas a slightly higher mobility of 0.2 cm2 V–1 s–1 was observed with Field-Induced Time-Resolved Microwave conductivity (FI-TRMC) technique.