FAST DYNAMIC EXCITED-STATE PROTON-TRANSFER (ESPT) REACTION OF WEAK OXOAROMATIC BASES - AMINOCOUMARINS WITH FIXED AMINO GROUP

The excited state protonation of the carbonyl group of 7-aminocoumarin s, containing a 7-amino group fixed in the yulolidine fragment, is an irreversible diffusion-limited process in protic solvents and proceeds by a dynamic mechanism. The effect of the non-stationary diffusion of reagents contributes significantly to fluorescence quenching (25% in the presence of 0.1 M H2SO4 for IIb in EtOH). The limiting value (acid concentration --> infinity) of the fluorescence quantum yield of the product of the excited state proton transfer (ESPT) reaction (phi'(inf inity) ) reaches 0.48. The quenching of 7-aminocoumarins in protic sol vents is effected by the solvated proton. The dependence of k(q) on th e solvent is determined by the limiting proton conductance in the solv ent. The activation energy of the decay of the encounter complex betwe en the solvated base and proton is determined by two factors: (1) the energy of H-OR bond breakage in this complex; (2) the difference betwe en the stabilization energies of the encounter complex and the transit ion state in the polar solvent.