Flexible Organic Crystals toward Functional Devices: From Serendipitous Discovery to Rational Design

Abstract

Organic small-molecule single crystals, owing to their long-range ordered packing structures and anisotropic physical properties, have long been regarded as important material platforms for the construction of high-performance optoelectronic devices. However, because of the densely packed molecular arrangements and the relatively fragile networks of noncovalent interactions, these rigid lattices are prone to local stress concentrations under external mechanical perturbations, which can further lead to macroscopic fracture. Over the past decade, the continuous emergence of flexible organic crystals has expanded the current understanding of the mechanical behaviors of organic crystals and contributed to the development of “crystal adaptronics”, an emerging research direction centered on mechanically adaptive molecular crystals and their functional applications. Under appropriate molecular conformations and lattice symmetry conditions, some organic crystals can accommodate pronounced deformation, including bending, through reversible and cooperative lattice expansion-contraction or through interlayer slippage, thereby dissipating stress while preserving structural integrity and intrinsic optoelectronic functionality. In this review, we move beyond a simple categorization of recent works to critically summarize the mechanical mechanisms of flexible organic crystals, the corresponding molecular design strategies, and recent advances in their applications in functional devices, and further discuss the major challenges and future directions in this field.