Crystal Twisting to Impart Chirality to Organic Semiconductor Crystals

Here we introduce crystal twisting about the fast growth direction as a general strategy to impart chirality to centrosymmetric organic semiconductor crystals. When crystallized from the melt, tetrathiafulvalene forms banded spherulites comprised of helicoidal crystalline fibrils that twist in concert as they grow radially outwards from the spherulite nucleation center. Similar behavior has been discovered for twenty-five different charge transfer complexes, triisopropylsilylethynyl anthradithiophene (TIPS ADT) and 2,5-bis(3-dodecyl-2-thienyl)-thiazolo[5,4-d]thiazole (BDT). Compared to films of melt-processed straight crystals, films of twisted organic semiconductor crystals exhibit higher charge mobilities and photocurrent. Crystal twisting further introduces chirality to these films, with helicoidal fibrils adopting either a clockwise or counterclockwise twisting direction. As such, films of twisted organic semiconductor crystals exhibit strong circular dichroism and birefringence, opening new avenues for chiroptical applications.