Excited-state intramolecular proton transfer (ESIPT) fine tuned by quinoline-pyrazole isomerism: pi-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.
Chung
Min-Wen
MW
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan.
Lin
Tsung-Yi
TY
Hsieh
Cheng-Chih
CC
Tang
Kuo-Chun
KC
Fu
Hungshin
H
Chou
Pi-Tai
PT
Yang
Shen-Han
SH
Chi
Yun
Y
D003198:Computer Simulation; D003510:Cyclohexanes / Q000737:chemistry; D007536:Isomerism; D015394:Molecular Structure; D011522:Protons; D011720:Pyrazoles / Q000737:chemistry*; D011804:Quinolines / Q000737:chemistry*
D003510:Cyclohexanes; D011522:Protons; D011720:Pyrazoles; D011804:Quinolines; C031280:pyrazole; C506365:Cyclohexane; C037219:quinoline