title: Unraveling the mechanism of the photodeprotection reaction of 8-bromo- and 8-chloro-7-hydroxyquinoline caged acetates.
abstract: Photoremovable protecting groups (PPGs) when conjugated to biological effectors forming "caged compounds" are a powerful means to regulate the action of physiologically active messengers in vivo through 1-photon excitation (1PE) and 2-photon excitation (2PE). Understanding the photodeprotection mechanism is important for their physiological use. We compared the quantum efficiencies and product outcomes in different solvent and pH conditions for the photolysis reactions of (8-chloro-7-hydroxyquinolin-2-yl)methyl acetate (CHQ-OAc) and (8-bromo-7-hydroxyquinolin-2-yl)methyl acetate (BHQ-OAc), representatives of the quinoline class of phototriggers for biological use, and conducted nanosecond time-resolved spectroscopic studies using transient emission (ns-EM), transient absorption (ns-TA), transient resonance Raman (ns-TR(2)), and time-resolved resonance Raman (ns-TR(3)) spectroscopies. The results indicate differences in the photochemical mechanisms and product outcomes, and reveal that the triplet excited state is most likely on the pathway to the product and that dehalogenation competes with release of acetate from BHQ-OAc, but not CHQ-OAc. A high fluorescence quantum yield and a more efficient excited-state proton transfer (ESPT) in CHQ-OAc compared to BHQ-OAc explain the lower quantum efficiency of CHQ-OAc relative to BHQ-OAc.
authors.affiliation: Department of Chemistry, The University of Hong Kong, Hong Kong S.A.R., P.R. China.
authors.forename: Jiani
authors.initials: J
authors.lastname: Ma
authors.forename: Adam C
authors.initials: AC
authors.lastname: Rea
authors.forename: Huiying
authors.initials: H
authors.lastname: An
authors.forename: Chensheng
authors.initials: C
authors.lastname: Ma
authors.forename: Xiangguo
authors.initials: X
authors.lastname: Guan
authors.forename: Ming-De
authors.initials: MD
authors.lastname: Li
authors.forename: Tao
authors.initials: T
authors.lastname: Su
authors.forename: Chi Shun
authors.initials: CS
authors.lastname: Yeung
authors.forename: Kyle T
authors.initials: KT
authors.lastname: Harris
authors.forename: Yue
authors.initials: Y
authors.lastname: Zhu
authors.forename: Jameil L
authors.initials: JL
authors.lastname: Nganga
authors.forename: Olesya D
authors.initials: OD
authors.lastname: Fedoryak
authors.forename: Timothy M
authors.initials: TM
authors.lastname: Dore
authors.forename: David Lee
authors.initials: DL
authors.lastname: Phillips
mesh_terms: D000085:Acetates / Q000737:chemistry*; D006912:Hydroxyquinolines / Q000737:chemistry*; D007700:Kinetics; D015394:Molecular Structure; D010777:Photochemistry; D011522:Protons; D011789:Quantum Theory; D011804:Quinolines / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry; D013059:Spectrum Analysis, Raman / Q000379:methods
chemical_list: C575767:8-bromo-7-hydroxyquinoline; C575766:8-chloro-7-hydroxyquinoline; D000085:Acetates; D006912:Hydroxyquinolines; D011522:Protons; D011804:Quinolines; D012997:Solvents