Light-Induced Conformational Variability Enabling [2+2] Cycloaddition Beyond Schmidt's Criteria: Unlocking New Horizons for Photomechanical Molecular Crystals

Abstract

Photoinduced [2+2] cycloaddition has been widely used for synthesizing natural products with strained four-membered rings. The solid-state photodimerization and the configuration of the cyclobutane are highly dependent on the pre-arrangement of the “olefin pair”. Topological [2+2] cycloaddition typically occurs when the geometrical parameters of the “olefin pair” satisfy Schmidt’s criteria. In this work, halogenated styrylpyrylium tetrafluoroborates are synthesized. For instance, 35FSPyL undergoes quantitative [2+2] cycloaddition within about 5 minutes, yielding a single photodimer. Notably, 35ClSPyL can also undergo quantitative photodimerization in stereospecific and regiospecific chemistry despite a vinyls separated by 5.224 Å. The single crystal structural analyses for the unirradiated and the in-situ irradiated crystals suggest that the light-induced conformational variability can bring the olefins closer together, enabling the formation of cyclobutane. The slower first-order kinetics for 35ClSPyL compared to that of 35FSPyL at the beginning of the cycloaddition may suggest the time consumed for the conformational variability. Additionally, the photomechanically responsive behaviors of bending, cracking, and salience for the molecular crystals based on halogenated styrylpyryliums are fueled by the topological photodimerization. This study provides new perspectives in understanding of [2+2] cycloaddition beyond Schmidt’s criteria. The breakthrough from the traditional spatial limit will broaden the applications in the green synthesis of cyclobutane skeletons and the new fuels for the development of the photomechanical materials employed in soft robotics and smart devices.