Iodine Doping of Tetrathiafulvalene Twisted Crystals to Enhance Conductivity

ABSTRACT Optically active tetrathiafulvalene (TTF) banded spherulite films comprising helicoidal fibrils exhibit an order of magnitude higher hole mobility compared to their unbanded counterparts. These mobilities of ∼10 −5 cm 2 V −1 s −1 are, however, low among organic semiconductors. We enhance the electrical conductivity of TTF banded spherulite films over seven orders of magnitude by iodine doping. Pale yellow films became dark red as TTF + formed in the presence of iodine vapor, with the local doping rate dependent on the crystal face exposed at the film surface. A six‐order‐of‐magnitude increase in conductivity is observed during the first two min of doping, reaching a maximum value of 0.7 ± 0.6 S cm −1 after 10 min. TTF 7 I 5 needles nucleated at the film surface and grew downwards during iodine vapor exposure, consuming β‐TTF helicoidal fibrils. The four principal linear optical anisotropies, linear and circular birefringence and dichroism, are mapped, and all four properties decreased over 10 min of iodine vapor exposure due to conversion to TTF 7 I 5 . For short doping times, however, circular birefringence and dichroism remained unchanged. These results demonstrate that the conductivity of TTF banded spherulite films can be increased without disrupting the microstructure that gives rise to optical activity, expanding their utility in chiroptoelectronics.