SOLVENT RELAXATION AROUND EXCITED 2-METHOXY-6-CHLORO-9-AMINOACRIDINE IN AQUEOUS SOLVENTS

Fluorescence emission of the monocationic 2-methoxy-6-chloro-9-aminoacridine (ACMA) in various organic solvent-water mixtures at different temperatures was investigated by both steady-state and nanosecond time-resolved fluorescence techniques. The fluorescence decay could be fit to a sum of two exponentials with lifetimes independent of the emission wavenumber. The preexponential factor of the short-lived component varied from positive values at high wavenumbers to negative values at low wavenumbers. The wavenumber dependence of the fluorescence decay is ascribed to the relaxation of polar molecules in the solvent cage surrounding the positively charged ACMA molecule subsequent to ACMA photoexcitation. Such a solvent relaxation depends on the interplay between the rate at which the excited-state population decays and the rate at which solvent molecules reorient around the excited molecule. The solvent relaxation rate constant for monocationic ACMA was determined to be around 4 × 108 s-1 in water at 20°C. This rate was unusually slow because of the supplementary monopole-dipole interaction between monocationic ACMA molecules and polar solvent molecules due to protonation on the acridinium nitrogen. It decreased in the presence of organic solvents (ethanol, dimethyl sulfoxide, glycerol). The effect of solvent viscosity was rather weak. The efficiency of dynamic quenching by iodide anion depended on the solvation state of the solvent-exposed ACMA. The microenvironment surrounding ACMA is demonstrated to play the dominant role in the excited-state process. © 1990 American Chemical Society.