Growth-Actuated Twisting of Photochromic Crystals

1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene (BpFCP) single crystals grown from the vapor phase were previously discovered to dynamically twist and coil when irradiated with UV light due to mechanical strain associated with a reversible, light-induced ring closure [1]. Here, we form BpFCP twisted crystals through growth-actuated twisting during crystallization from the melt. At high undercoolings, i.e. large crystallographic driving forces, BpFCP forms banded spherulites comprising helicoidal fibrils that twist in concert with one another as they grow radially from the spherulitic nucleation center. As-crystallized films appear transparent when viewed using unpolarized light. When imaged between crossed polarizers, on the other hand, concentric bands of interference colors are visible due to continuously rotating refractive indices as the crystals twist about the growth direction. Upon irradiation with UV light to induce the photochromic ring closure, alternating bands in BpFCP spherulites turn dark blue, creating a vibrant stripe pattern. Local Raman spectroscopy on adjacent bands revealed this patterning to be a result of orientation-dependent dichroism of photocyclized crystals, as opposed to orientation-dependent photoreactivity. Film cracking upon UV light exposure suggests that fibrils experience internal stress from photoisomerization-induced shifts in crystal lattice parameters. Mueller Matrix imaging further revealed strong circular dichroism (CD) and birefringence (CB) in BpFCP banded spherulites, with local CD and CB signals changing dynamically during photocyclization. Because banded spherulites expose different crystal orientations at the film surface, they provide a unique opportunity to investigate orientation-dependent chiroptoelectronic properties in these dynamic crystals.<br/><br/>[1] D. Kitagawa, H. Tsujioka, F. Tong, X. Dong, C.J. Bardeen, S. Kobatake. Control of Photomechanical Crystal Twisting by Illumination Direction<i>. J. Am. Chem. Soc.</i>, <i>140</i>, 4208 (2018).