STATIONARY AND TIME-RESOLVED FLUORESCENCE INVESTIGATIONS COMBINED WITH PM3 CI STUDIES OF TAUTOMERIC PHENOMENA IN NEUTRAL AND PROTONATED 9-ACRIDINAMINE/

Stationary fluorescence excitation spectra of neutral, mono-and diprot onated 9-acridinamine in liquid phases of various polarity and ability to participate in specific interactions correlate with the relevant e lectronic absorption spectra only with respect to band (peak) position but not their relative intensities. The combination of this finding w ith the complex nature of stationary fluorescence spectra suggests tha t neutral and diprotonated 9-acridinamine co-exist in two tautomeric f orms whereas monoprotonated appears only in one such form-both in the ground and excited states. This is confirmed by the results of semiemp irical calculations carried out at the PM3/CI level. This method was a lso used to predict the geometries of molecules in the ground and firs t excited singlet states and subsequently the energies (wavelengths) o f adiabatic absorption and emission transitions, which correlate quali tatively with the experimental ones. The diexponential fluorescence de cay of neutral and diprotonated 9-acridinamine and the monoexponential fluorescence decay of monoprotonated 9-acridinamine are also explaine d by tautomeric phenomena. (C) 1997 Elsevier Science B.V.