Equilibrium and Kinetic Studies of the Dimerization of Acridine Orange and Its 10-Alkyl Derivatives

The thermodynamics and kinetics of the dimerization of Acridine Orange (AO, 3,6-bis(dimethylamino)acridinium chloride) and its 10-alkyl derivatives in 0.1 M NaCl solution have been studied by spectrophotometry and temperature-jump method. Dimerization constants, which were determined spectrophotometrically, become large with an increase in the length of the introduced alkyl chain; this behavior is the same as that in water and a dilute HCl solution. The association and dissociation rate constants determined from the temperature-jump measurements also depend on the length of the alkyl chain. Especially, the rate constants for methyl–AO are unexpectedly small. Both the effect of added salt on the thermodynamic parameters and the kinetic results are discussed in view of the structure of hydration water surrounding dye molecules. 1H NMR data for AO and methyl–AO suggest that the planes of two component molecules in a dimer are parallel to each other. This result is consistent with the spectrophotometric and kinetic data.