Advancing Heteroaryl Azoswitches: From Molecular Design to Applications

Azobenzene (Ph-N = N-Ph) and its derivatives are widely studied in many fields owing to their efficient and reversible E⇄Z photoisomerization. However, the parent azobenzene suffers from performance limitations, including incomplete photoconversion, short thermal half-life, low quantum yield, and reliance on UV light excitation. While specific ortho-substitutions can substantially improve these properties, further performance enhancements for advanced applications remain challenging. In recent years, the emergence of heteroaryl azoswitches (Het-N = N-Het/Ph) has brought new development opportunities to this already mature field, benefiting from their tunable scaffolds and the distinct features arising from diverse heteroaryl motifs. Despite their great potential, heteroaryl azoswitches are still at an early stage, and their advancement faces multiple challenges, including the lack of systematic molecular design principles, challenging synthesis, poor solid-state isomerization efficiency, and insufficient focus on practical implementations. In this Account, we introduce our recent efforts toward the development of high-performance heteroaryl azoswitches, outlining constructive strategies from molecular design to functional materials. To begin with, systematic strategies are presented to overcome key performance trade-offs in solution: 1) Through synergistic optimization of spatial and electronic structures, (hetero)arylazopyrazoles overcome the inherent limitations of conventional azobenzenes, enabling (near-)quantitative bidirectional photoconversion and long thermal half-lives up to years. 2) By rationally combining heteroaryl scaffolds with substituent effects, visible/solar-light-driven heteroaryl azoswitches are developed to avoid using harmful UV light while offering high bidirectional photoconversion and prolonged thermal half-lives. Remarkably, leveraging the efficient isomerization in solution as an essential prerequisite, (near-)quantitative bidirectional photoconversion is realized in azobispyrazole crystals. Building on this foundation, the first reversible single-crystal-to-single-crystal photoisomerization of an azo photoswitch is established, challenging the long-held perception that azo isomerization inevitably compromises single crystallinity. Furthermore, the broad application prospects of heteroaryl azoswitches, ranging from energy storage to photoresponsive smart materials, are validated. Finally, we discuss the enduring challenges and future opportunities of this emerging field. We anticipate that these advancements will inspire subsequent exploration of heteroaryl azoswitches and pave the way for their implementation in next-generation functional materials.