Santos Willy G, Pina João, Burrows Douglas H, Forbes Malcolm D E, Cardoso Daniel R
Instituto de Química de São Carlos, Universidade de São Paulo, São Carlos, SP 13560-390, Brazil.
Universidade de Coimbra, Departamento de Química, Centro de Química, 3004-535 Coimbra, Portugal.
Photochem Photobiol Sci. 2016 Aug 31;15(9):1124-1137. doi: 10.1039/c6pp00169f. Epub 2016 Aug 16.
The photophysics and reactivity of two tetraphenylborate salts and triphenylborane have been studied using ultrafast transient absorption, steady-state fluorescence, electron paramagnetic resonance with spin trapping, and DFT calculations. The singlet excited state of tetraarylborates exhibit extended π-orbital coupling between two adjacent aryl groups. The maximum fluorescence band, as well as the transient absorption bands centered at 560 nm (τ = 1.05 ns) and 680 nm (τ = 4.35 ns) are influenced by solvent viscosity and polarity, indicative of a twisted intramolecular charge transfer (TICT) state. Orbital contour plots of the HOMO and LUMO orbitals of the tetraarylboron compounds support the existence of electron delocalization between two aryl groups in the LUMO. This TICT-state and aryl-aryl electron extension is not observed for the trigonal arylboron compound, in which excited π-orbital coupling only occurs between the boron atom and one aryl group, which restricts the twist motion of the aryl-boron bond. The excited triplet state is deactivated primarily through aryl-boron bond cleavage, yielding aryl and diphenylboryl radicals. In the presence of oxygen, this photochemistry results in phenoxyl and diphenylboroxyl radicals, as confirmed by EPR spectroscopy of spin trapped radical adducts. The TICT transition and radical generation is not expected for BoDIPY molecules where the rotational vibration of the B-aryl bond is rigid, restricting changes in the geometric structure. In this sense, this work contributes to the development of new BoDIPY derivatives where the TICT transition may be observed for aryl ligands with free rotational vibrations in the BoDIPY structure.
利用超快瞬态吸收、稳态荧光、自旋捕获电子顺磁共振以及密度泛函理论计算，研究了两种四苯基硼酸盐和三苯基硼烷的光物理性质和反应活性。四芳基硼酸盐的单重激发态在两个相邻芳基之间表现出扩展的π轨道耦合。最大荧光带以及以560 nm（τ = 1.05 ns）和680 nm（τ = 4.35 ns）为中心的瞬态吸收带受溶剂粘度和极性的影响，表明存在扭曲的分子内电荷转移（TICT）态。四芳基硼化合物的最高占据分子轨道（HOMO）和最低未占分子轨道（LUMO）的轨道轮廓图支持了LUMO中两个芳基之间存在电子离域。对于三角芳基硼化合物未观察到这种TICT态和芳基 - 芳基电子扩展，其中激发的π轨道耦合仅发生在硼原子和一个芳基之间，这限制了芳基 - 硼键的扭曲运动。激发三重态主要通过芳基 - 硼键断裂失活，产生芳基和二苯基硼基自由基。在有氧存在的情况下，这种光化学作用会产生苯氧基和二苯基硼氧基自由基，自旋捕获自由基加合物的电子顺磁共振光谱证实了这一点。对于B - 芳基键旋转振动刚性的BODIPY分子，预计不会发生TICT跃迁和自由基生成，这限制了几何结构的变化。从这个意义上讲，这项工作有助于开发新的BODIPY衍生物，在这种衍生物中，对于BODIPY结构中具有自由旋转振动的芳基配体，可能会观察到TICT跃迁。
