Transport in Twisted Crystalline Charge Transfer Complexes

Abstract

Many crystals grow as banded spherulites from the melt with an optical rhythm indicative of helicoidal twisting. In this work, 23 of 41 charge transfer complexes (CTCs) are grown with twisted morphologies. As a group, CTCs more commonly twist (56%) than molecular crystals arbitrarily chosen in our previous research (31%). To analyze the effect of twisting on charge transport, three tetracyanoethylene-based CTCs with phenanthrene (PhT), pyrene (PyT), and perylene are characterized. PhT and PyT are subject to mobility measurements using organic field-effect transistors. The mobilities for twisted crystals are around three times higher than for crystals with no ostensible optical modulation, which are effectively straight. The differences in mobilities of straight and twisted crystals are considered computationally based on density functional theory. Straight crystal models built from crystallographic information files are calculated and present anisotropic hole and electron transport. For twisted crystal models, adjacent layers in the supercell are rotated by 0.01°around experimentally determined twisting directions. The modified transfer integrals lead to a slight increase (up to 25%) in the calculated mobilities of twisted crystals. Comparisons of model calculations on individual fibrils and measurements of ensembles of fibrils indicate that interfaces between single crystals are likely consequential.

Twisted tetrathiafulvalene crystals

“…This improvement of hole mobility for OFETs comprising twisted crystals is in line with our previous finding for twisted charge transfer complexes. 47…”

“…This improvement of hole mobility for OFETs comprising twisted crystals is in line with our previous finding for twisted charge transfer complexes. 47 The origin of this improvement in OFET mobility upon crystal twisting remains unknown. Crystal twisting pitches on the tens of microns length scale translate to a less than 0.01°r otation between adjacent molecular layers, so we do not expect twisting to significantly affect the charge transfer integral between adjacent molecules in crystals.…”

Charge Transport in Twisted Organic Semiconductor Crystals of Modulated Pitch

“…[23] An even higher percentage (56%) of twisted forms was observed among binary aromatic charge-transfer complexes (CTCs). [24] Here we aim to understand how growth-actuated crystal twisting affects electrical properties within the context of organic semiconductors that underlie the plastic electronic industry. [25][26][27] Previously, we studied the effect of twisting on the carrier mobilities of three tetracyanoethylene-based CTC films.…”

“…[25][26][27] Previously, we studied the effect of twisting on the carrier mobilities of three tetracyanoethylene-based CTC films. [24] Density functional theory (DFT) calculations predicted a small (several percent) mobility difference between straight and twisted single crystals with a typical twisting pitch of 10 µm. When used in OFETs, the electron mobilities were three times larger in the Many molecular crystals (approximately one third) grow as twisted, helicoidal ribbons from the melt, and this preponderance is even higher in restricted classes of materials, for instance, charge-transfer complexes.…”

Unravelling supramolecular features and opto-electronic properties of a binary charge transfer cocrystal of a blue fluorescent di-carbazole and TFT

“…22 Detailed studies on the crystal packing features and the nature of intermolecular interactions in D–A charge transfer cocrystals have been reported by a few research groups including us. 23 Such studies have been proved to be very helpful to get insights on the ‘crystal structure–opto-electronic property’ relationship in D–A cocrystals, especially the role of intermolecular interactions in crystal packing to dictate semiconductor properties. 23 a , d The nature of π⋯π stacking interaction reportedly plays a decisive role in determining fluorescence properties, viz.…”