Departamento de Química/Instituto de Tecnología Química UPV-CSIC, Universidad Politécnica de Valencia, Avenida de los Naranjos s/n, E-46022 Valencia, Spain.
J Phys Chem B. 2010 Sep 16;114(36):11920-6. doi: 10.1021/jp1053327.
This work has examined the photoreactivity of benzophenone (3), 2-benzoylthiophene (4), 4-methoxybenzophenone (5), 4,4'-dimethoxybenzophenone (6), and 4-carboxybenzophenone (7) with 2-aminobenzimidazole (1). Laser flash photolysis (LFP) revealed quenching of the aromatic ketone triplets by 1, leading to formation of ketyl radicals plus aminyl radical 1-H•. The quenching rate constants obtained for 3 (nπ* triplet) and 4 (ππ* triplet) were 6.2 × 10(9) and 3.9 × 10(9) M(-1) s(-1), respectively. The similarity between the two values suggests that the process is not a pure hydrogen abstraction but rather a charge transfer followed by proton transfer. This is in agreement with thermodynamic calculations, using the Rehm-Weller equation. In the case of 5 and 6, the transient absorption spectra showed distinct bands corresponding to both types of triplets (nπ* and ππ*); their ratio was found to depend on solvent polarity. In the presence of 1, spectral changes were also consistent with formation of the aminyl/ketyl radical pairs. The rate constants for quenching of both types of triplets were very high, in the range 10(9)-10(10) M(-1) s(-1). When an electron acceptor substituent was attached to the aromatic ring, as in 7 (nπ* triplet), the quenching rate constant was higher (8 × 10(9) M(-1) s(-1)), close to diffusion control. The reaction mechanism for hydrogen abstraction from 1 by triplet excited 3 or 4 was theoretically studied using density functional theory (DFT) methods. The results suggest formation of ground state molecular complexes, where one electron is transferred from the 2-aminobenzimidazole to the benzophenone or benzoylthiophene moieties upon excitation, giving radical ion pairs; subsequent proton transfer from the amino group to the carbonyl oxygen atoms leads to the neutral biradicals. A comparison between the relative energies and geometries of the species involved in the photochemical reactions indicates that all ketones follow a similar mechanism.
这项工作研究了二苯甲酮（3）、2-苯甲酰噻吩（4）、4-甲氧基二苯甲酮（5）、4,4'-二甲氧基二苯甲酮（6）和 4-羧基二苯甲酮（7）与 2-氨基苯并咪唑（1）的光反应性。激光闪光光解（LFP）显示 1 猝灭芳香酮三重态，导致酮基自由基加氨基自由基 1-H•的形成。对于 3（nπ三重态）和 4（ππ三重态），获得的猝灭速率常数分别为 6.2×10(9)和 3.9×10(9) M(-1) s(-1)。两个值的相似性表明该过程不是纯的氢提取，而是电荷转移随后是质子转移。这与使用 Rehm-Weller 方程的热力学计算一致。对于 5 和 6，瞬态吸收光谱显示出对应于两种三重态（nπ和 ππ）的明显谱带；发现它们的比例取决于溶剂极性。在 1 的存在下，光谱变化也与形成氨基/酮基自由基对一致。两种类型三重态的猝灭速率常数都非常高，范围在 10(9)-10(10) M(-1) s(-1)之间。当芳环上连接了一个电子受体取代基，如 7（nπ*三重态）时，猝灭速率常数更高（8×10(9) M(-1) s(-1)），接近扩散控制。使用密度泛函理论（DFT）方法对 1 由三重态激发的 3 或 4 从氢中提取的反应机制进行了理论研究。结果表明形成了基态分子配合物，其中在激发时，一个电子从 2-氨基苯并咪唑转移到二苯甲酮或苯甲酰噻吩部分，形成自由基离子对；随后氨基向羰基氧原子的质子转移导致中性双自由基。对光化学反应中涉及的物种的相对能量和几何形状的比较表明，所有酮都遵循相似的机制。
