BN-Heterocycles Bearing Two BN Units: Influence of the Linker and the Location of BN Units on Electronic Properties and Photoreactivity

Four diboron BN-heterocycles bearing two BN units have been synthesized via double lithiation/borylation with the aim to examine their thermal and photoelimination properties. In two of the BN-heterocycles, (BN)2-1 and (BN)2-4, the BN units share the central linker unit, while the other two BN-heterocycles (BN)2-2 and (BN)2-3 are isomers with a benzene ring in the middle of the backbone separating the two chelate units. Only compound (BN)2-3 can thermally and photochemically undergo complete double elimination producing green fluorescent (BN)2-3a due to the highly crowded boron centers. (BN)2-2 can partially convert (∼56%) to (BN)2-2a under 350 nm UV irradiation. (BN)2-1 and (BN)2-4 are either inactive or unstable toward photolysis. The molecular structures and electronic properties of these (BN)2-heterocycles have been investigated experimentally as well as computationally using TD-DFT to further elucidate the origin of differences in optical and electronic properties.

本研究通过双重锂化/硼化反应合成了四种含双BN单元的二硼掺杂BN杂环化合物，旨在考察其热消除与光消除性能。其中两种BN杂环(BN)2-1与(BN)2-4的BN单元共享一个中心连接基，而另外两种(BN)2-2与(BN)2-3为同分异构体，其分子主链中部以苯环分隔两个螯合单元。由于硼中心空间位阻极高，仅化合物(BN)2-3可通过热反应与光化学反应实现完全双消除，生成绿色荧光产物(BN)2-3a。(BN)2-2在350 nm紫外光照射下可部分转化为(BN)2-2a，转化率约为56%。(BN)2-1与(BN)2-4对光解反应则无活性或稳定性欠佳。本研究通过实验与含时密度泛函理论（Time-Dependent Density Functional Theory，TD-DFT）计算相结合的手段，对上述二硼掺杂BN杂环化合物的分子结构与电子性质展开了系统研究，以进一步阐明其光学与电子性质存在差异的根源。