Iodine Doping in Tetrathiafulvalene Twisted Crystals

Tetrathiafulvalene (TTF), a workhorse of the organic electronics community since it’s superconductor tetracyano-p-quinodimethane co-crystal was discovered in 1973,[1] is an optically inactive centrosymmetric molecule that crystallizes in an optically inactive centrosymmetric space group <i>P</i>-1 when grown from the melt. We recently discovered that growth-actuated crystal twisting breaks this symmetry to render TTF twisted crystals optically active and thus promising candidates for chiroptoelectronics.[2] In this work, tetrathiafulvalene (TTF) banded spherulites comprising helicoidal fibrils are doped with iodine to improve their conductivity for (chir)optoelectronics. Within five minutes of exposure to iodine vapor, TTF film conductivity increased by six orders of magnitude, from 100 pA to 100 µA, accompanied by a color change from pale yellow to dark red. Grazing incidence x-ray diffraction patterns also revealed a change in crystal structure upon iodine doping. The rate of iodine doping was found to depend on the local crystal face presented at the film surface, which alternates between faces as the crystalline fibrils twist about the growth direction, forming concentric "bands" surrounding the spherulite center. Crystal face-dependent iodine intercalation was observed by optical microscopy, light absorbance spectroscopy, and electron dispersive x-ray spectroscopy. Faces corresponding to fast iodine intercalation were previously identified as the 1-11 face.[3] Mueller Matrix imaging revealed that crystal twisting is not disrupted by iodine intercalation, with circular dichroism and birefringence retained after five minutes of film exposure to iodine vapor. Combined with the orders of magnitude improvement in electrical conductivity, optically active doped TTF banded spherulites will provide a platform to examine chiral-induced anisotropies in electron and photon interactions in molecular semiconductor twisted crystals.<br/>[1] J Am Chem Soc (1973) 95. 948-949<br/>[2] MSDE (2022) 7. 569-576<br/>[3] In preparation