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

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 1a−d. 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 1a−d systems ideal models for probing the reaction potential energy surface.