Bending, Twisting, and Propulsion of Photoreactive Crystals by Controlled Gas Release

Abstract

The rapid release of gas by a chemical reaction to generate momentum is one of the most fundamental ways to elicit motion that could be used to sustain and control the motility of objects. We report that hollow crystals of a three‐dimensional supramolecular metal complex that releases gas by photolysis can propel themselves or other objects and advance in space when suspended in liquid media. In needle‐like regular crystals, the reaction occurs mainly on the surface and results in the formation of cracks that evolve due to internal pressure; the expansion on the cracked surface of the crystal results in bending, twisting, or coiling of the crystal. In hollow crystals, gas accumulates inside their cavity and emanates preferentially from the recess at the crystal terminus, propelling the crystal to undergo directional photomechanical motion through the liquid. The motility of the object which can be controlled externally to perform work delineates the concept of “crystal microbots”, realized by photoreactive organic crystals where prolonged directional motion for actuation or delivery.

Crystal-Defect-Induced Longer Lifetime of Excited States in a Metal–Organic Framework Photocatalyst to Enhance Visible-Light-Mediated CO2 Reduction

“…Metal–organic frameworks (MOFs) are crystalline coordination polymers constructed by assembling metal ions/clusters and organic linkers. − MOFs possess large porous structures, high surface areas, synthetic feasibility, and rich diversity of inorganic/organic units. , Like semiconductors, MOFs excited by UV or visible light undergo electron–hole pair separation and transfer, leading to photochemical transformation. − However, the photocatalytic efficiency of MOFs still faces significant challenges, including a large band gap energy, low separation and transfer efficiency of photogenerated charges, and mismatch of the photoredox potential between MOFs and substrate molecules. Scientists have explored various strategies to improve the photocatalytic performance of MOFs, such as functionalizing organic linkers, creating structural defects, packaging transition metals, and constructing heterojunctions. − Among these strategies, defect engineering is an effective approach for enhancing the photocatalytic efficiency of MOFs by regulating the structure and composition of the framework. − Structural defects in MOFs refer to the absence of partial organic linkers or metal nodes from a perfect crystallographic structure . The general method for constructing structural defects in MOFs involves utilizing excess monocarboxylic acid ligands, such as formic acid, acetic acid (HAc), or trifluoroacetate acid, to coordinate with metal oxygen clusters instead of polycarboxylic acid ligands during the one-pot synthesis. , …”