The synthesis and photolysis mechanisms of 8-nitroquinoline-based photolabile caging groups for carboxylic acid

• Published in: Journal of physical organic chemistry 2014-12, Vol.27 (12), p.981-985
• Main Authors: Sun, Fude, Zhang, Lei, Yan, Jianhua, Xu, Lida, Fang, Decai, Luo, Shi-Zhong
• Format: Article
• Language: English
• Subjects: caged compounds; photolysis; quinolone; transition state
• ISSN: 0894-3230; 1099-1395
• DOI: 10.1002/poc.3385

Photolabile protecting groups have been extensively studied and applied for protection of small biological molecules, which make it convenient to detect the biological processes of the caged compounds. In this study, a series of 8‐nitroquinoline‐based photolabile caging groups for carboxylic acid were synthesized with improved photolysis efficiency. Among them, 6‐bromo‐8‐nitro‐1, 2‐dihydroquinolinyl chromophore was proven the best derivative on account of its longest absorption wavelength (345 nm), highest caging ability, and quantum yield (Φ = 0.003). Moreover, density functional theory calculations were performed in order to study the photolysis mechanisms. Theoretical calculations revealed that the reaction was kinetically inert under general mild condition with the high barrier height of 34.3 kcal/mol at carbonyl migration step, while under the photolysis condition, because of the large energy gap (64.5 kcal/mol) between S0 and S1 states, the reaction should be accessible in the triplet ground state (T1) through successive excitation of S0 → S1 states, subsequent intersystem crossing of S1 → T1 states, and finally returned to the stable S0 state for product via potential energy surface crossing between T1 and S0 states. Copyright © 2014 John Wiley & Sons, Ltd. Theoretical calculations support a mechanistic pathway involving the micro‐processes of excitation of S0 → S1 states, intersystem crossing between S1 and T1 states, reacting on the surface of T1 state, and finally, return to the S0 state through potential energy surface.