Excited-State Intramolecular Proton Transfer (ESIPT) Fine Tuned by Quinoline−Pyrazole Isomerism: π-Conjugation Effect on ESIPT

A series of quinoline/isoquinoline-pyrazole isomers (I-III), in which the pyrazole moiety is in a different substitution position, was strategically designed and synthesized, showing a system with five-membered intramolecular hydrogen bonding. Despite the similarity in molecular structure, however, only I undergoes excited-state intramolecular proton transfer, as evidenced by the distinct 560 nm proton-transfer emission and its associated relaxation dynamics. The experimental results support a recent theoretical approach regarding the conjugation effect on a proton (or hydrogen atom) transfer reaction (J. Phys. Chem. A 2009, 113, 4862-4867). The concept simply predicts that more extended pi conjugation, i.e., resonance, for proton-transfer tautomer species could allow for efficient delocalization of excess charge in the reaction center, resulting in a larger thermodynamic driving force for proton transfer.