ACID-BASE INTERACTION OF ,3-DIMETHYL-6-PHENYLIMIDAZO-[1,2-B]-1,2,4-TRIAZINE IN THE GROUND AND EXCITED-STATES

The acid-base interaction of 2,3-dimethyl-6-phenylimidazo-[1,2-b]-1,2 (DMPIT) in the ground and excited states was studied. The electronic s tructures of 6-phenylimidazo-[1,2-b]-1,2,4-triazine and its cations we re calculated by the CNDO/S and MNDO methods. The ground state protona tion of the DMPIT molecule proceeds at the N-5 atom of the imidazole r ing. In hydrocarbon solvents, a hydrogen-bonded 1:1 complex and 1:2 io nic pair of DMPIT with trifluoroacetic acid are formed. In polar and b asic solvents, a 1:1 ionic pair is formed (pK(a) = 2.76). On excitatio n of DMPIT, the basicity of the N-5 atom decreases (pK(a)= -0.88) and that of the N-4 atom increases. In the excited state, the dissociatio n of N-5-protonated DMPIT takes place. The rate of dissociation increa ses as the solvent basicity increases. This reaction induces a radiati onless deactivation (quantum yield of adiabatic reaction phi(r) = 0.46 in water). In dioxan, ethanol and water, excited DMPIT is protonated by an acid at the N-4 atom by a dynamic mechanism. Tn hydrocarbon solv ents, the phototautomerization of the N-5 cation to the N-4 cation tak es place in the hydrogen-bonded 1:2 ionic pair. This photoreaction is similar to that in the 7-azaindole-solvent complex.