Twisted Hydrogen-Bonded Frameworks

Directing the assembly of molecules within hydrogen-bonded frameworks is being extensively explored as a modular crystal engineering strategy to access novel molecular packing arrangements of target molecules. Crystal twisting, a common but little-known phenomenon in molecular crystals, of these frameworks provides an additional handle to not only interrogate crystal orientation-dependent properties but also to impart chirality for chiroptoelectronics. Here, we demonstrate crystal twisting in solution-processed bis(guanidinium) naphthalene-1,5-disulfonate (G)₂(1,5-NDS) films in which molecular assembly is driven by hydrogen bonding between guanidinium and sulfonate moieties. When deposited from ethanol in conditions that favor low nucleation densities but high branching rates, (G)₂(1,5-NDS) crystallizes as banded spherulites of helicoidal fibrils emanating radially from a single nucleation center and twisting in concert with one another about the growth direction. Scanning electron micrographs (SEMs) reveal tightly packed fibrils that alternate between flat-on and edge-on orientations with respect to the substrate surface with a twisting pitch of ∼ 3 - 4 μm. 2D x-ray diffraction (XRD) patterns collected on (G)₂(1,5-NDS) banded spherulites confirm that the crystals adopt a bilayer packing motif with ethanol as guest molecules. These banded spherulites further exhibit band-dependent linear dichroism and birefringence, as well as circular dichroism and birefringence as a consequence of twist-induced loss of symmetry. Given the modular nature of these hydrogen-bonded frameworks to incorporate a broad range of molecular pillars and guests, combining hydrogen bond-directed assembly with crystal twisting will open unexplored avenues in the design of chiroptoelectronic materials.