Tu Ting-Husn, Chen Yi-Ting, Shen Jiun-Yi, Lin Ta-Chun, Chou Pi-Tai
Department of Chemistry , National Taiwan University , Taipei 10617 , Taiwan.
J Phys Chem A. 2018 Mar 8;122(9):2479-2484. doi: 10.1021/acs.jpca.8b00379. Epub 2018 Feb 26.
We investigated the excited-state proton transfer (ESPT) reaction for 3-cyano-7-azaindole (3CAI) in aqueous solution and in ice. 3CAI undergoes water-catalyzed ESPT in the aqueous solution, giving normal (355 nm) and proton transfer tautomer (∼472 nm) emission bands. Detailed temperature-dependent studies showed that the values of activation free energy (Δ G) were similar between N-H and N-D isotopes. Therefore, water-catalyzed ESPT involves a stepwise mechanism incorporating solvation equilibrium ( K) to form a 1:1 (molar ratio) water:3CAI cyclic hydrogen-bonded complex as an intermediate, followed by perhaps proton tunneling reaction. In sharp contrast, 3CAI in ice undergoes entirely different photophysical properties, in which 3CAI self-organizes to form a double-hydrogen-bonded dimers at the grain boundary of the polycrystalline. Upon excitation, the dimer proceeds with a fast excited-state double proton transfer reaction, giving rise to solely a tautomer emission (∼450 nm). The distinct difference in ESPT properties between water and ice makes azaindoles feasible for the investigation of water-ice interface property.
我们研究了3-氰基-7-氮杂吲哚（3CAI）在水溶液和冰中的激发态质子转移（ESPT）反应。3CAI在水溶液中发生水催化的ESPT反应，产生正常发射带（355 nm）和质子转移互变异构体发射带（约472 nm）。详细的温度依赖性研究表明，N-H和N-D同位素之间的活化自由能（ΔG）值相似。因此，水催化的ESPT涉及一种逐步机制，该机制包含溶剂化平衡（K）以形成1:1（摩尔比）的水:3CAI环状氢键复合物作为中间体，随后可能发生质子隧穿反应。与之形成鲜明对比的是，冰中的3CAI具有完全不同的光物理性质，其中3CAI在多晶颗粒边界处自组装形成双氢键二聚体。激发后，二聚体发生快速的激发态双质子转移反应，仅产生互变异构体发射（约450 nm）。水和冰之间ESPT性质的显著差异使得氮杂吲哚成为研究水-冰界面性质的可行对象。
