A Mechanistic Study of Halogen Addition and Photoelimination from π‑Conjugated Tellurophenes

The ability to drive reactivity using visible light is of importance for many disciplines of chemistry and has significant implications for sustainable chemistry. Identifying photochemically active compounds and understanding photochemical mechanisms is important for the development of useful materials for synthesis and catalysis. Here we report a series of photoactive diphenyltellurophene compounds bearing electron-withdrawing and electron-donating substituents synthesized by alkyne coupling/ring closing or palladium-catalyzed ipso-arylation chemistry. The redox chemistry of these compounds was studied with respect to oxidative addition and photoelimination of bromine, which is of importance for energy storage reactions involving X2. The oxidative addition reaction mechanism was studied using density functional theory, the results of which support a three-step mechanism involving the formation of an initial η1 association complex, a monobrominated intermediate, and finally the dibrominated product. All of the tellurophene derivatives undergo photoreduction using 430, 447, or 617 nm light depending on the absorption properties of the compound. Compounds bearing electron-withdrawing substituents have the highest photochemical quantum efficiencies in the presence of an alkene trap, with efficiencies of up to 42.4% for a pentafluorophenyl-functionalized tellurophene. The photoelimination reaction was studied in detail through bromine trapping experiments and laser flash photolysis, and a mechanism is proposed. The photoreaction, which occurs by release of bromine radicals, is competitive with intersystem crossing to the triplet state of the brominated species, as evidenced by the formation of singlet oxygen. These findings should be useful for the design of new photochemically active compounds supported by main-group elements.利用可见光驱动反应活性的能力对诸多化学分支学科具有重要意义，且对可持续化学领域有着深远影响。识别光活性化合物并阐明其光化学机制，对于开发可用于合成与催化的实用材料至关重要。本研究报道了一系列带有吸电子与给电子取代基的二苯碲吩类光活性化合物，其合成采用炔烃偶联/关环反应或钯催化ipso-芳基化(ipso-arylation)反应。针对这些化合物的氧化加成反应与溴的光消除反应，我们开展了氧化还原化学研究，该类反应对于涉及X₂的储能反应具有重要价值。本研究采用密度泛函理论对氧化加成反应机制进行了探究，结果表明该反应遵循三步机制：首先形成初始的η¹配位络合物，随后生成单溴代中间体，最终得到二溴代产物。所有碲吩衍生物均可根据自身的光吸收特性，在430 nm、447 nm或617 nm波长的可见光下发生光还原反应。带有吸电子取代基的化合物在烯烃捕获剂存在下展现出最高的光化学量子效率，其中五氟苯基功能化的碲吩类化合物的量子效率可达42.4%。本研究通过溴捕获实验与激光闪光光解(laser flash photolysis)技术，对该光消除反应进行了详细分析，并提出了相应的反应机制。该光反应通过释放溴自由基进行，其与卤代物种三重态的系间窜越过程存在竞争，这一点可通过单线态氧的生成得到证实。上述研究结果可为设计以主族元素为骨架的新型光活性化合物提供有益参考。