Hierarchical bending microstructures in π-conjugated molecular cocrystals

Flexible organic crystals, endowed with the characteristic of ordered molecular arrangement, exhibit tremendous application potential in flexible (opto)electronics, micro/nanoscale integrated photonics, and other related fields. While notable progress has been made in achieving self-bending behavior1 and controllable curvature in crystals,2 many current bending strategies still rely on manual or external stimuli, which are often time-consuming and limit precise manipulation.3 Meanwhile, mechanical deformation frequently induces irreversible structural damage to crystals and optoelectronic performance degradation. These challenges severely limit the practical application and industrialization of flexible organic crystals in high-performance optoelectronic devices and integrated systems.4 Therefore, it is highly urgent to develop a universal strategy to realize spontaneous crystal bending without external forces, precise control of bending angles, crack-free bending interfaces, and excellent optoelectronic functions, which is crucial for promoting the development of this field.

In a recent study published in Nature Communications, Wang et al.5 proposed a universal molecular cocrystal strategy that introduces directional charge-transfer noncovalent interactions into molecular systems, weakening the intrinsic intermolecular forces of π-conjugated molecular crystals and triggering the spontaneous deformation of crystals from slip to bending. By virtue of this strategy, a series of flexible organic crystals with precisely controlled bending angles (61.8°–85.0°) and devoid of fractured bending interfaces were successfully fabricated (Figure 1A). This strategy features remarkable expandability and universality, being applicable to a variety of donor-acceptor (D-A) π-conjugated molecular systems; accordingly, bent crystals covering the full red, green, and blue (RGB) spectral range were obtained (Figure 1B). For the first time, the controllable fabrication of 2- to 6-level hierarchical bending microstructures was achieved by stepwise regulation of the substrate temperature (Figure 1C), which greatly expands the design space for complex architectures of π-conjugated molecular crystals.