Tuning the Thermal Stability and Photoisomerization of Azoheteroarenes through Macrocycle Strain

Azobenzene and its derivatives are one of the most-widespread molecular scaffolds in a range of modern applications, as well as in fundamental research. After photoexcitation, azo-based photoswitches revert back to the most stable isomer in a timescale ( ) that determines the range of potential applications. Attempts to bring to extreme values prompted to the development of azobenzene and azoheteroarene derivatives that either rebalance the E- and Z- isomer stabilities, or exploit unconventional thermal isomerization mechanisms. In the former case, one successful strategy has been the creation of macrocycle strain, which tends to impact the E/Z stability asymmetrically, and thus significantly modify . On the bright side, bridged derivatives have shown an improved optical switching owing to the higher quantum yields and absence of degradation. However, in most (if not all) cases, bridged derivatives display a <em>reversed</em> thermal stability (more stable Z-isomer), and smaller than the acyclic counterparts, which restricts their potential interest to applications requiring a fast forward and backwards switch. In this paper, we investigate the impact of alkyl bridges to the thermal stability of phenyl-azoheteroarenes using computational methods, and we reveal that is indeed possible to combine such improved photo-switching characteristics while preserving the <em>regular</em> thermal stability (more stable E-isomer), and increased values under the appropriate connectivity and bridge length.偶氮苯（Azobenzene）及其衍生物是一类在现代众多应用及基础研究中最为广泛使用的分子骨架之一。在光激发后，偶氮类光开关会在某一确定其潜在应用范围的时间尺度（）内恢复至最稳定的异构体。为将该时间尺度调控至极端值，研究人员开发出了偶氮苯及偶氮杂芳烃（azoheteroarene）衍生物，这类衍生物或可重新平衡E型与Z型异构体的稳定性，或采用非常规的热异构化机制。在前一种策略中，一种已获成功的方法是引入大环张力，这类张力往往会非对称地影响E/Z异构体的稳定性，进而显著改变该时间尺度。值得一提的是，桥联衍生物因具备更高的量子产率且无降解现象，展现出更优异的光开关性能。然而在绝大多数（即便不是全部）情况下，桥联衍生物表现出<em>反转</em>的热稳定性（Z型异构体更稳定），且其时间尺度小于非环类同类物，这将其潜在应用范围限制在了需要快速双向开关的场景中。本研究采用计算方法，探究了烷基桥联对苯基偶氮杂芳烃热稳定性的影响，结果表明，在合适的连接方式与桥联长度下，确实可以在保留<em>常规</em>热稳定性（E型异构体更稳定）的同时，兼具上述优化后的光开关特性，并提升该时间尺度。