Solvent effects on the sensitized photoxygenation of lidocaine

Detection of O₂(¹Δ_g) phosphorescence emission, λ_max=1270nm, following laser excitation and steady state methods were employed to determine both the total constant, k_T^LID, and the chemical reaction rate constants, k_R^LID, for reaction between the anaesthetic lidocaine and singlet oxygen in several solvents. Values of k_T^LID range from 0.20±0.09×10⁶M^-1s^-1 in trifluoroethanol to 45.8±2.40×10⁶M^-1s^-1 in N,N-dimethylacetamide. Values of k_R^LID are at least one order of magnitude lower than k_T^LID values in a given solvent. Solvent effect on quenching rates shows that reaction mechanism involves formation of a charge transfer exciplex. Correlation of k_T^LID values with solvent parameters does not follow that observed for a typical tertiary amine such as triethylamine. Although k_T^LID values are lower in hydrogen bond donor solvents, this solvent effect is significantly smaller than that for triethylamine, and no expected decrease in lidocaine reactivity with change from aprotic to protic solvents was found. This result is ascribed to weaker hydrogen bonding between the amino moiety in lidocaine and the solvent. Otherwise, hydrogen bond acceptor solvents increase k_T^LID to a greater extent than that triethylamine. This can be explained by intra-molecular hydrogen bonding or electrostatic interactions that stabilize lidocaine and hydrogen bond acceptor solvents disrupt these interactions.