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Excited-State Intramolecular Proton Transfer in Five-Membered Hydrogen-Bonding Systems:  2-Pyridyl Pyrazoles

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posted on 14.08.2003, 00:00 by Wei-Shan Yu, Chung-Chih Cheng, Yi-Ming Cheng, Pei-Chi Wu, Yi-Hwa Song, Yun Chi, Pi-Tai Chou
The excited-state intramolecular proton transfer (ESIPT) reaction in five-membered N−H···N hydrogen-bonding systems has been explored through design and syntheses of a series of 5-(2-pyridyl) 1-H-pyrazoles 1ad. The ESIPT mechanism was confirmed through spectroscopy, relaxation dynamics, and corresponding methylated analogues. The results demonstrate for the first time a unique system among ESIPT molecules, in which ESIPT incorporates an appreciably large energy barrier fine-tuned by the skeletal reorganization. This makes 1ad systems ideal models for probing the reaction potential energy surface.