TY - JOUR
T1 - Resonance Raman characterization of the different forms of ground-state 8-substituted 7-hydroxyquinoline caged acetate compounds in aqueous solutions
AU - An, Hui Ying
AU - Ma, Chensheng
AU - Li, Wen
AU - Harris, Kyle T.
AU - Dore, Timothy M.
AU - Phillips, David Lee
PY - 2010/2/25
Y1 - 2010/2/25
N2 - To investigate the substituent effect on the distribution of the forms of the ground-state species of 8-substituted 7-hydroxyquinolines, ultraviolet-absorption and resonance Raman experiments were performed for 8-chloro7-hydroxyquinoline (CHQ-OAc) and 8-cyano-7-hydroxyquinoline (CyHQ-OAc) in acetonitrile (MeCN), in NaOH-H2O/MeCN (60:40, v/v, pH 11-12), and in H2O/MeCN (60:40, v/v, pH 6-7) solutions, and these results were compared to those previously reported for the 8-bromo-7-hydroxyquinoline (BHQ-OAc) compound. Swapping a bromine atom in BHQ-OAc for a chlorine atom in CHQ-OAc causes the amount of the tautomeric species to become larger, although the neutral species is still the predominant species for both systems in water-rich solutions. The absorption spectra and the resonance Raman spectra of CyHQ-OAc suggest that, because of the strong electron-withdrawing nature of the cyano substituent, a measurable amount of the anionic species is present and the tautomeric species cannot be easily detected in water-rich solutions. The results reported here reveal large substituent effects on the distribution of the different forms of the XHQ-OAc compounds in largely aqueous solutions. The steric effect of the 8-substituted group and competitive hydrogen bonding between the 8-substituted group and water molecules hinders the formation of a cyclic BHQ-OAc-water complex, and the electron-withdrawing property of the 8-substituted group enhances the deprotonation of the phenol group while disfavoring the formation of the positively charged quinoline nitrogen. We briefly discuss the implications of the substituent effects for using these compounds as phototriggers.
AB - To investigate the substituent effect on the distribution of the forms of the ground-state species of 8-substituted 7-hydroxyquinolines, ultraviolet-absorption and resonance Raman experiments were performed for 8-chloro7-hydroxyquinoline (CHQ-OAc) and 8-cyano-7-hydroxyquinoline (CyHQ-OAc) in acetonitrile (MeCN), in NaOH-H2O/MeCN (60:40, v/v, pH 11-12), and in H2O/MeCN (60:40, v/v, pH 6-7) solutions, and these results were compared to those previously reported for the 8-bromo-7-hydroxyquinoline (BHQ-OAc) compound. Swapping a bromine atom in BHQ-OAc for a chlorine atom in CHQ-OAc causes the amount of the tautomeric species to become larger, although the neutral species is still the predominant species for both systems in water-rich solutions. The absorption spectra and the resonance Raman spectra of CyHQ-OAc suggest that, because of the strong electron-withdrawing nature of the cyano substituent, a measurable amount of the anionic species is present and the tautomeric species cannot be easily detected in water-rich solutions. The results reported here reveal large substituent effects on the distribution of the different forms of the XHQ-OAc compounds in largely aqueous solutions. The steric effect of the 8-substituted group and competitive hydrogen bonding between the 8-substituted group and water molecules hinders the formation of a cyclic BHQ-OAc-water complex, and the electron-withdrawing property of the 8-substituted group enhances the deprotonation of the phenol group while disfavoring the formation of the positively charged quinoline nitrogen. We briefly discuss the implications of the substituent effects for using these compounds as phototriggers.
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U2 - 10.1021/jp911143e
DO - 10.1021/jp911143e
M3 - Article
C2 - 20113003
AN - SCOPUS:77249166546
SN - 1089-5639
VL - 114
SP - 2498
EP - 2505
JO - Journal of Physical Chemistry A
JF - Journal of Physical Chemistry A
IS - 7
ER -