Snyder Joshua A, Grüninger Peter, Bettinger Holger F, Bragg Arthur E
Department of Chemistry, Johns Hopkins University , 3400 North Charles Street, Baltimore, Maryland 21218, United States.
Institut für Organische Chemie, Universität Tübingen , Auf der Morgenstelle 18, 72076 Tübingen, Germany.
J Phys Chem A. 2017 Nov 9;121(44):8359-8367. doi: 10.1021/acs.jpca.7b08190. Epub 2017 Oct 26.
Boron-nitrogen doping of polyaromatic hydrocarbon (PAH) materials can be used to tune their electronic properties while preserving the structural characteristics of pure hydrocarbons. Many multicycle PAHs can be synthesized photochemically; in contrast, very little is known about the photochemistry of their BN-doped counterparts. We present results of fs, ns, and μs time-resolved spectroscopic studies on the photoinduced dynamics of hexaphenyl benzene and hexaphenyl borazine in order to examine how BN doping alters photochemical C-C bond formation via 6π electrocyclization as well as the stability of resulting cyclized structures. Ultrafast measurements reveal photoinduced behaviors reflecting differences in excited-state decay pathways for the two molecules, with hexaphenyl borazine relaxing from its excited state with a rate that is 2 orders of magnitude faster than that of hexaphenyl benzene (3.0 vs 428 ps). Tetraphenyl dihydrotriphenylene generated from hexaphenyl benzene is observed to reopen with a ∼2 μs lifetime controlled by entropic stabilization of the cyclized structure; in contrast, photoinduced dynamics appear to be complete within 100 ps after excitation of hexaphenyl borazine. This significant difference in photochemical dynamics is reflected in the cyclodehydrogentation yields obtained for the two reactants (25 vs 0% for hexaphenyl benzene and borazine, respectively). Quantum-chemical computations predict that BN doping gives rise to energetic destabilization and increased singlet diradical character in cyclized structures. These findings indicate that the polarized BN bonds of the borazine core adversely impact photochemical bond formation relative to analogous hydrocarbons.
多环芳烃（PAH）材料的硼氮掺杂可用于调节其电子性质，同时保留纯烃的结构特征。许多多环PAH可以通过光化学方法合成；相比之下，对于它们的硼氮掺杂类似物的光化学却知之甚少。我们展示了对六苯基苯和六苯基硼嗪光诱导动力学的飞秒、纳秒和微秒时间分辨光谱研究结果，以研究硼氮掺杂如何通过6π电环化改变光化学C-C键的形成以及所得环化结构的稳定性。超快测量揭示了反映这两种分子激发态衰变途径差异的光诱导行为，六苯基硼嗪从激发态弛豫的速率比六苯基苯快2个数量级（3.0对428皮秒）。观察到由六苯基苯生成的四苯基二氢三亚苯以约2微秒的寿命重新打开，这由环化结构的熵稳定控制；相比之下，六苯基硼嗪激发后100皮秒内光诱导动力学似乎就已完成。这种光化学动力学的显著差异反映在两种反应物的环脱氢产率上（六苯基苯和硼嗪分别为25%和0%）。量子化学计算预测，硼氮掺杂会导致环化结构的能量不稳定和单重态双自由基特征增加。这些发现表明，相对于类似的碳氢化合物，硼嗪核心的极化硼氮键对光化学键的形成产生不利影响。
