REVERSAL OF EXCITATION BEHAVIOR OF PROTON-TRANSFER VS CHARGE-TRANSFER BY DIELECTRIC PERTURBATION OF ELECTRONIC MANIFOLDS

Reversal of excitation behavior of proton-transfer vs. charge-transfer by dielectric perturbation is reported for 4'-(diethylamino)-3-hydroxyflavone (1). At room temperature I exhibits a dominant proton transfer emission and a nonnegligible normal emission with maxima at 555 and 440 nm, respectively in alkane solvents, in contrast to 3-hydroxyflavone in which only tautomer emission is observed. The result maybe rationalized by a mechanism incorporating fast proton tunneling between normal and tautomer forms which are in equilibrium in the excited singlet state. The normal emission maximum exhibits a drastic solvent dependence with a red shift of > 3000 cm-1 from n-heptane to methanol. In ethanol a unique normal emission with an unusually high yield (PHI is similar to 0.52 +/- 0.02) is observed. It is proposed that the S(l) state of I may be ascribed to a zwitterionic configuration induced by the charge transfer from the 4'-diethylamino group to the carbonyl oxygen, for which the energy is even lower than that of the tautomer in strong polar, protic solvents, precluding the proton-transfer reaction.