Electron-Proton Transfer Mechanism Of Excited-State Hydrogen Transfer In Phenol-(Nh3)(N) (N=3 And 5)

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 pi pi* to pi sigma* states via conical intersection. This means that the electron is transferred to a diffuse Rydberg-like sigma* 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)(5) cluster, 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 pi pi* and pi sigma* states, and will discuss the ESHT mechanism and its cluster size-dependence between n = 3 and 5. It is suggested that the presence and absence of a barrier in the proton transfer coordinate cause the different dynamics.