Self-Patterning Tetrathiafulvalene Crystalline Films

Abstract

Tetrathiafulvalene (TTF) crystals grown from the melt are organized as spherulites in which helicoidal fibrils growing radially from the nucleation center twist in concert with one another. Alternating bright and dark concentric bands are apparent when films are viewed between crossed polarizers, indicating an alternating pattern of crystallographic faces exposed at the film surface. Band-dependent reorganization of the TTF crystals was observed during exposure to methanol vapor. Crystalline growth appears on bright bands at the expense of the dark bands. After a 24 h period of exposure to methanol vapor, the original spherulites were completely restructured, and the films comprise isolated, concentric circles of crystallites whose orientations are determined by the initial TTF crystal fibril orientation. While the surface of these outgrowths appears faceted and smooth, cross-sectional SEM images revealed a semiporous inner structure, suggesting solventvapor-induced recrystallization. Collectively, these results show that crystal twisting can be used to rhythmically redistribute material. Crystal twisting is a common and often controllable phenomenon independent of molecular or crystal structure and therefore offers a generalizable path to spontaneous pattern formation in a wide range of materials.

Polymer-Assisted Polymorph Transition in Melt-Processed Molecular Semiconductor Crystals

“…The observed optical activity of banded spherulites arises not from the molecular or crystal structure, but rather from the splaying of anisotropic crystalline lamellae in thin films as they twist about the growth direction . We have recently observed similar behavior in banded spherulites of other achiral molecular semiconductors, such as tetrathiafulvalene , and 2,5-bis­(3-dodecyl-2-thienyl)-thiazolo­[5,4- d ] thiazole . The spherulite sends as few as 4 and as many as 10 angular domains that are alternately left- and right-handed.…”

Leveling up Organic Semiconductors with Crystal Twisting

“…As crystal fibers rotate about the growth direction, the narrow, high-surface-energy faces and wide, low-surface-energy faces of the ribbon-like fibrils are alternately exposed at the surface of the film. ,,,,− Crystal dissolution rates are face-dependent, , and the narrow, high-surface-energy bands in twisted crystals are expected to dissolve more quickly than the wide, low-surface-energy bands. TTF banded spherulites, for example, exhibit band-dependent dissolution in the presence of methanol Figure displays time-lapse SEM and optical micrographs of a TTF banded spherulite film exposed to methanol vapor for a period of 0–24 h. Selective dissolution and recrystallization occurred on dark and bright interference bands, respectively.…”

“…Bright-field optical micrographs are provided as insets. Adapted with permission from ref . Copyright 2023, American Chemical Society.…”