Control of Excited-State Proton-Coupled Electron Transfer by Ultrafast Pump-Push-Probe Spectroscopy in Heptazine-Phenol Complexes: Implications for Photochemical Water Oxidation
Control of Excited-State Proton-Coupled Electron Transfer by Ultrafast Pump-Push-Probe Spectroscopy in Heptazine-Phenol Complexes: Implications for Photochemical Water Oxidation
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DOI：
10.1021/acs.jpcc.0c00415
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发表时间：
2020-04-30
影响因子：
3.7
通讯作者：
Schlenker, Cody W.
中科院分区：
文献类型：
--
作者：
Corp, Kathryn L.;Rabe, Emily J.;Schlenker, Cody W.
我们展示了化学调谐和激光驱动控制分子间H原子提取质子溶剂分子。使用多脉冲超快泵浦-推-探测瞬态吸收（TA）光谱，我们监测氢提取由官能化庚嗪（Hz）从取代苯酚在凝聚相氢键复合物。Hz是普遍存在的有机聚合物光催化剂石墨碳氮化物（g-C3 N4）的单体单元。在此之前，我们报道了Hz衍生物2，5，8-三（4-甲氧基苯基）-1，3，5，6，7，9，9 b-七氮杂非那烯（TAHz）可以光化学地从水中提取H原子，此外还表现出与g-C3 N4在水悬浮液中相匹配的光催化放氢活性。本文将联合收割机超快多脉冲TA光谱与基于预测波函数的从头算电子结构计算相结合，探讨了混合n π */pi π * 上激发态在指导羟基化合物中H原子的夺取中的作用.我们使用紫外线（365 nm）激光脉冲光激发TAHz到一个明亮的上激发态，并且，在大约6 ps的弛豫期后，我们使用近红外（NIR）（1150 nm）脉冲将发色团从长寿命的S-1态“推”到一个更高的激发态。当苯酚存在时，NIR推动诱导S-1 TA信号幅度的持续降低（Δ Δ OD），表明光化学分支比的脉冲驱动变化。在存在的取代苯酚与供电子部分，Δ Δ OD的幅度显着减少，由于这些配合物的激发态反应性的增加，伴随着在苯酚氧化电位的阴极移位。在后一种情况下，H原子的提取在没有来自推动脉冲的额外能量的帮助下进行。这些结果揭示了新的见解之间的非反应性局部激发态和反应性分子间电荷转移状态的分支机制。他们还提出了分子设计策略，用于官能化氮杂芳族化合物以驱动重要的光反应，例如从水中提取H原子。更一般地说，这项研究证明了在光化学领域的一个热切期望的成就，合理地重新定向激发态与光的反应。
We demonstrate chemical tuning and laser-driven control of intermolecular H atom abstraction from protic solvent molecules. Using multipulse ultrafast pump-push-probe transient absorption (TA) spectroscopy, we monitor hydrogen abstraction by a functionalized heptazine (Hz) from substituted phenols in condensed-phase hydrogen-bonded complexes. Hz is the monomer unit of the ubiquitous organic polymeric photocatalyst graphitic carbon nitride (g-C3N4). Previously, we reported that the Hz derivative 2,5,8-tris(4-methoxyphenyl)-1,3,5,6,7,9,9b-heptaazaphenalene (TAHz) can photochemically abstract H atoms from water, in addition to exhibiting photocatalytic activity for H-2 evolution matching that of g-C3N4 in aqueous suspensions. In the present work, we combine ultrafast multipulse TA spectroscopy with predictive wave function-based ab initio electronic-structure calculations to explore the role of mixed n pi*/pi pi* upper excited states in directing H atom abstraction from hydroxylic compounds. We use an ultraviolet (365 nm) laser pulse to photoexcite TAHz to a bright upper excited state, and, after a relaxation period of roughly 6 ps, we use a near-infrared (NIR) (1150 nm) pulse to "push" the chromophore from the long-lived S-1 state to a higher-lying excited state. When phenol is present, the NIR push induces a persistent decrease (Delta Delta OD) in the S-1 TA signal magnitude, indicating an impulsively driven change in photochemical branching ratios. In the presence of substituted phenols with electron-donating moieties, the magnitude of Delta Delta OD diminishes markedly due to the increased excited-state reactivity of these complexes that accompanies the cathodic shift in phenol oxidation potential. In the latter case, H atom abstraction proceeds unaided by additional energy from the push pulse. These results reveal new insight into branching mechanisms among unreactive locally excited states and reactive intermolecular charge-transfer states. They also suggest molecular design strategies for functionalizing aza-aromatics to drive important photoreactions, such as H atom abstraction from water. More generally, this study demonstrates an avidly desired achievement in the field of photochemistry, rationally redirecting excited-state reactivity with light.