Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong S.A.R., P. R. China.
J Am Chem Soc. 2012 Sep 12;134(36):14858-68. doi: 10.1021/ja304441n. Epub 2012 Sep 4.
The photophysics and photochemical reactions of 2-(1-hydroxyethyl) 9,10-anthroquinone (2-HEAQ) were studied using femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA), and nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopy techniques and density functional theory (DFT) calculations. In acetonitrile, 2-HEAQ underwent efficient intersystem crossing to the triplet excited state ((2-HEAQ)(3)). A typical photoreduction reaction for aromatic ketones took place via production of a ketyl radical intermediate for 2-HEAQ in isopropanol. In water-containing solutions with pH values between 2 and 10, an unusual photoredox reaction reported by Wan and co-workers was detected and characterized. Observation of the protonated species in neutral and acidic aqueous solutions by fs-TA spectra indicated the carbonyl oxygen of (2-HEAQ)(3) was protonated initially and acted as a precursor of the photoredox reaction. The preference of the photoredox reaction to occur under moderate acidic conditions compared to neutral condition observed using ns-TR(3) spectroscopy was consistent with results from DFT calculations, which suggested protonation of the carbonyl group was the rate-determining step. Under stronger acidic conditions (pH 0), although the protonated (2-HEAQ)(3) was formed, the predominant reaction was the photohydration reaction instead of the photoredox reaction. In stronger basic solutions (pH 12), (2-HEAQ)(3) decayed with no obvious photochemical reactions detected by time-resolved spectroscopic experiments. Reaction mechanisms and key reactive intermediates for the unusual photoredox reaction were elucidated from time-resolved spectroscopy and DFT results. A brief discussion is given of when photoredox reactions may likely take place in the photochemistry of aromatic carbonyl-containing compounds and possible implications for using BP and AQ scaffolds for phototrigger compounds.
2-(1-羟乙基)蒽醌(2-HEAQ)的光物理和光化学反应使用飞秒瞬态吸收(fs-TA)、纳秒瞬态吸收(ns-TA)和纳秒时间分辨共振拉曼(ns-TR(3))光谱技术和密度泛函理论(DFT)计算进行了研究。在乙腈中，2-HEAQ 经历了高效的系间窜越到三重激发态((2-HEAQ)(3))。在异丙醇中，2-HEAQ 通过生成酮基自由基中间体发生了典型的芳香酮光还原反应。在 pH 值在 2 到 10 之间的含水溶液中，检测到并表征了 Wan 及其同事报道的一种不寻常的光氧化还原反应。通过 fs-TA 光谱观察到中性和酸性水溶液中质子化物种的存在表明，(2-HEAQ)(3)的羰基氧最初被质子化，并作为光氧化还原反应的前体。与 ns-TR(3)光谱一致，DFT 计算表明羰基的质子化是速率决定步骤，与中性条件相比，在中等酸性条件下发生光氧化还原反应的偏好。在更强的酸性条件(pH 0)下，虽然形成了质子化的(2-HEAQ)(3)，但主要反应是光水合反应而不是光氧化还原反应。在更强的碱性溶液(pH 12)中，(2-HEAQ)(3)在时间分辨光谱实验中没有检测到明显的光化学反应而衰减。从时间分辨光谱和 DFT 结果阐明了不寻常的光氧化还原反应的反应机制和关键反应中间体。简要讨论了芳香羰基化合物光化学中可能发生光氧化还原反应的情况，以及使用 BP 和 AQ 支架作为光触发化合物的可能意义。
