Electron-Proton Transfer Mechanism of Excited-State Hydrogen Transfer in Phenol-(NH3)n (n=3 and 5)
Electron-Proton Transfer Mechanism of Excited-State Hydrogen Transfer in Phenol-(NH3)n (n=3 and 5)
复制标题
苯酚-(NH3)n（n=3和5）中激发态氢转移的电子-质子转移机制
DOI：
10.1002/chem.201704129
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发表时间：
2017
期刊：
影响因子：
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通讯作者：
Fujii Masaaki
中科院分区：
文献类型：
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作者：
Miyazaki Mitsuhiko;Ohara Ryuhei;Dedonder Claude;Jouvet Christophe;Fujii Masaaki
激发态氢转移（ESHT）负责芳烃的各种光化学过程，包括核基础的光保护。其机制可以通过圆锥相交从芳香族 ππ* 状态到 πσ* 状态的内部转换来解释。这意味着除了质子迁移之外，电子还转移到扩散的类里德堡 σ* 轨道。这张图意味着电子和质子不一起运动，动力学原理上是不同的。在这里，我们将皮秒时间分辨近红外（NIR）和红外（IR）光谱应用于ESHT的基准系统苯酚-（NH3）5团簇，并独立监测电子转移和质子运动。 NIR 跃迁监测到的电子转移在 3 ps 内上升，而 NH 振动的红外吸收检测到的整体 H 转移的寿命约为 20 ps。这清楚地证明了电子运动和质子迁移是解耦的。在具有三个氨分子的簇中尚未检测到近红外吸收和红外跃迁之间的时间演化差异。我们将报告我们的完整观察结果以及 ππ* 和 πσ* 态势能面的理论计算，并将讨论 ESHT 机制及其在 n=3 和 5 之间的簇尺寸依赖性。这表明质子转移坐标中势垒的存在和不存在会导致不同的动力学。
Excited‐state hydrogen transfer (ESHT) is responsible for various photochemical processes of aromatics, including photoprotection of nuclear basis. Its mechanism is explained by internal conversion from the aromatic ππ* to πσ* states via conical intersection. This means that the electron is transferred to a diffuse Rydberg‐like σ* orbital apart from proton migration. This picture means the electron and the proton do not move together and the dynamics are different in principle. Here, we have applied picosecond time‐resolved near‐infrared (NIR) and infrared (IR) spectroscopy to the phenol‐(NH3)5cluster, the benchmark system of ESHT, and monitored the electron transfer and proton motion independently. The electron transfer monitored by the NIR transition rises within 3 ps, while the overall H transfer detected by the IR absorption of NH vibration appears with a lifetime of about 20 ps. This clearly proves that the electron motion and proton migration are decoupled. Such a difference of the time‐evolutions between the NIR absorption and the IR transition has not been detected in a cluster with three ammonia molecules. We will report our full observation together with theoretical calculations of the potential energy surfaces of the ππ* and πσ* states, and will discuss the ESHT mechanism and its cluster size‐dependence betweenn=3 and 5. It is suggested that the presence and absence of a barrier in the proton transfer coordinate cause the different dynamics.