Tuning Azoheteroarene Photoswitch Performance through Heteroaryl Design

Photoswitchable compounds, which can be reversibly switched between two isomers by light, continue to attract significant attention for a wide array of applications. Azoheteroarenes represent a relatively new but understudied type of photoswitch, where one of the aryl rings from the conventional azobenzene class has been replaced with a five-membered heteroaromatic ring. Initial studies have suggested the azoheteroarenesthe arylazopyrazoles in particularto have excellent photoswitching properties (quantitative switching and long Z isomer half-life). Here we present a systematic computational and experimental study to elucidate the origin of the long thermal half-lives and excellent addressability of the arylazopyrazoles, and apply this understanding to determine important structure–property relationships for a wide array of comparable azoheteroaryl photoswitches. We identify compounds with Z isomer half-lives ranging from seconds to hours, to days and to years, and variable absorption characteristics, all through tuning of the heteraromatic ring. Conformation perhaps plays the largest role in determining such properties: the compounds with the longest isomerization half-lives adopt a T-shaped ground state Z isomer conformation and proceed through a T-shaped isomerization pathway, whereas the most complete photoswitching is achieved for compounds that have a twisted (rather than T-shaped) Z isomer conformation. By balancing these factors, we report a new azopyrazole 3pzH, which can be quantitatively switched to its Z isomer (>98%) with 355 nm irradiation, near-quantitatively (97%) switched back to the E isomer with 532 nm irradiation, and has a very long half-life for thermal isomerization (t1/2 = 74 d at 25 °C). Given the large tunability of their properties, the predictive nature of their performance, and the other functional opportunities afforded by usage of a heteroaromatic system, we believe the azoheteroaryl photoswitches to have huge potential in a wide range of optically addressable applications.光响应化合物（Photoswitchable compounds）指可通过光在两种异构体间实现可逆切换的物质，目前在诸多应用领域中持续受到广泛关注。偶氮杂芳烃（azoheteroarenes）是一类相对新兴但研究尚不充分的光响应开关分子，其将传统偶氮苯（azobenzene）类分子中的一个芳环替换为五元杂芳环。早期研究表明，偶氮杂芳烃——尤其是芳基偶氮吡唑（arylazopyrazoles）——具备优异的光开关性能（可实现定量切换及较长的Z异构体热半衰期）。本研究通过系统性的计算与实验工作，阐明了芳基偶氮吡唑拥有长热半衰期与优异光学寻址性能的根源，并基于此理解，为一系列同类偶氮杂芳基光开关分子确立了关键的构效关系（structure–property relationships）。我们通过调控杂芳环结构，筛选出了Z异构体热半衰期从秒级、小时级、天级乃至年级，且吸收特性各异的多种化合物。构象或许是决定此类性能的最关键因素：拥有最长异构化半衰期的化合物，其基态Z异构体呈T型（T-shaped）构象，并通过T型异构化路径完成切换；而实现最完全光开关性能的分子，其Z异构体则为扭曲型（twisted）构象（而非T型构象）。通过平衡这些因素，我们开发出了新型偶氮吡唑衍生物3pzH：该分子可通过355 nm辐照定量转化为Z异构体（转化率>98%），经532 nm辐照可近乎定量（97%）变回E异构体（E isomer），且其热异构化半衰期极长（25 ℃下t1/2 = 74 d）。鉴于此类分子的性能可大幅调控、性能可预测，且杂芳环体系还可带来其他功能化潜力，我们认为偶氮杂芳基光开关分子在众多可光学寻址的应用领域中具备巨大应用潜力。