Many molecular crystals (approximately one third) grow as twisted helicoidal ribbons from the melt, and this preponderance is even higher in restricted classes of materials, for instance charge transfer complexes. Previously, twisted crystallites of such complexes present an increase in carrier mobilities. Here, the effect of twisting on charge mobility is better analyzed for a mono-component organic semiconductor 2 2,5-bis(3-dodecyl-2-thienyl)-thiazolo [5,4-d]thiazole (BDT) that forms twisted crystals with varied helicoidal pitches and makes a possible correlation with carrier mobility. These films were analyzed by Xray scattering and Mueller matrix polarimetry to characterize the microscale organization of the polycrystalline ensembles. Carrier mobilities of organic field effect transistors were five times higher when the crystals were grown with the smallest pitches (most twisted) compared to those with the largest pitches along the fiber elongation direction. A ten-fold increase was observed along the perpendicular direction.Simulation of electrical potential based on scanning electron micrographs and density functional theory suggests that the twisting-enhanced mobility is mainly controlled by the fiber organization in the film. A greater number of tightly packed twisted fibers separated by numerous smaller gaps permits better charge transport over the film surface compared to fewer big crystallites separated by larger gaps.

Optically-active optoelectronic materials are of great interest for many applications, including chiral sensing and circularly polarized light emission. Traditionally, such applications have been enabled by synthetic strategies to design chiral...

Tailor-made additives (TMAs) have found a role in crystal morphology engineering and control through specific binding to crystal surfaces through stereochemical recognition. The utility of TMAs, however, has been largely limited to crystal growth from solutions. In this review, we illustrate examples where TMAs have been used to influence the growth of crystals during cooling of their melts. In solution, the crystal growth driving force is governed by solute supersaturation, which corresponds to the deviation from equilibrium. In growth from melts, however, undercooling is the important thermodynamic parameter responsible for crystallization outcomes, a key difference that can influence the manner in which TMAs affect growth kinetics, crystal morphology, nucleation, enantioselective surface recognition, and the determination of the absolute sense of polar axes. When the crystallization driving force in a melt is small and diffusion is comparatively high, TMAs can exert their influence on well-faceted single crystals with the stereochemical richness observed in solution growth. Under high supercooling, where the driving force is large, ensembles of crystals can grow radially, masking stereochemical information and requiring new optical tools for understanding the influence of TMAs on emerging crystals. Expected final online publication date for the Annual Review of Materials Research, Volume 53 is July 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

A great proportion of molecular crystals can be made to grow as twisted fibrils. Typically, this requires high crystallization driving forces that lead to spherulitic textures. Here, it is shown how micron size channels fabricated from poly(dimethylsiloxane) (PDMS) serve to collimate the circular polycrystalline growth fronts of optically banded spherulites of twisted crystals of three compounds, coumarin, 2,5-bis(3-dodecyl-2-thienyl)-thiazolo [5,4-d]thiazole, and tetrathiafulvalene. The relationships between helicoidal pitch, growth front coherence, and channel width are measured. As channels spill into open spaces, collimated crystals "diffract" via small angle branching. On the other hand, crystals grown together from separate channels whose bands are out of phase ultimately become a single in-phase bundle of fibrils by a cooperative mechanism yet unknown. The isolation of a single twist sense in individual channels is described. We forecast that such chiral molecular crystalline channels may function as chiral optical wave guides.

The concept of Eshelby untwisting, the effect of an axial screw dislocation driving an intrinsically twisted nanocrystal towards a straighter configuration more consistent with long-range translational symmetry, is introduced here....