Harnessing Solar Energy, Extending Storage, and Accelerated Release in "Twisted" Negative Photochromic Azo Compounds

We present a series of twisted (hetero)arylazonaphthalene derivatives exhibiting negative photochromism, sunlight-induced E-Z photoisomerization, and stimuli-responsive accelerated Z-E isomerization, toward the storage and release of energy in molecular solar thermal (MOST) energy storage materials. Through structural evidence from XRD studies and structure-property relationships, we envisaged that the geometrical azo-twist governs the negative photochromism. The salient features of these derivatives include superior sunlight-driven photoswitching and tuneable thermal half-lives (t_1/2) of Z isomers (days to seconds) even under an acidic medium, and the Z-E isomerization under triplet photosensitization conditions, exemplifying multi-mode activation of catalytic Z-E relaxations on demand. Remarkably, a thin film of poly(methyl methacrylate) (PMMA) matrix encapsulating these targets not only retains the photoswitching behavior but also shows sensitivity toward acids and bases. More importantly, the combination of sunlight-driven forward E-Z photoisomerization (charging) in acidic medium, storage (ΔH_iso up to 35 kJ mol^-1), subsequent neutralization to extend the cis isomer's half-life, and the accelerated reverse Z-E isomerization (discharging) of these targets sheds new light on the development of MOST materials. Overall, the design of these azo compounds, their characteristics, and ease of functionalization, while retaining their features, render them suitable for applications in various domains.