THE EFFECT OF AMINO GROUP-STRUCTURE ON EXCITED-STATE PROTONATION OF 7-AMINOCOUMARINS

The reaction of proton transfer in the excited states of aminocoumarin s with primary (Ia and If), secondary (Ib and Ic), and tertiary (Id, I e, IIa; and IIb) 7-amino groups was studied. Unlike the case of 7-amin ocoumarins Ia - Ic and IIb, the limiting (acid concentration --> infin ity) fluorescence quantum yield phi(infinity)' of the photoprotonation product of 7-aminocoumarins Id and Ie with the unfixed tertiary amino group is abnormally low in polar (ethanol or dimethylsulfoxide) solve nts. A decrease in polarity of the medium leads to a considerable (by a factor of > 40 in cyclohexane) increase in phi(infinity)'. This incr ease in the quantum yield is due to a decrease in the formation rate o f the twisted intramolecular charge-transfer state that does not exhib it fluorescence.