Bd. Bursulaya et al., FAST DYNAMIC EXCITED-STATE PROTON-TRANSFER (ESPT) REACTION OF WEAK OXOAROMATIC BASES - AMINOCOUMARINS WITH FIXED AMINO GROUP, Journal of photochemistry and photobiology. A, Chemistry, 92(3), 1995, pp. 163-172
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.