The dual fluorescence (a andb) of 4-(dialkylamino)pyrimidines (4-DAAP) in solutions is explained in terms of the TICT state model. 4-DAAP form complexes with protic solvents: efficient radiationless depopulation is “switched on” by hydrogen bonding (HB). The photoinduced electron transfer is modified by HB or by coordination to the metal ion. In nitriles, two emitting states of 4-(N,N-dimethylamino)pyrimidine (I) and 4-(N,N-diethylamino)pyrimidine (II) reach equilibrium. In protic solvents, the fluorescencesa andb are not kinetically coupled: the different ground state complexes are responsible for each band. Jet-cooled 4-DAAP show only primary excited fluorescenceb. Microsolvation of pretwistedo-methylated compoundsIII andIV by small polar molecules, contrary to more planarI andII, gives rise to the long-wave (probably TICT) fluorescence.

Dual fluorescence of the lactone form of rhodamine B (LRB) in polar aprotic solvents has been ascribed to two forms of the dye: lactone and zwitterion, which is built up after the dissociation of the lactonic bond in the excited state. We have studied the kinetics of fluorescence in both bands in several aprotic solvents as a function of temperature by the SPC technique using global analysis of the decay curves. It has been found that under certain conditions (solvent, temperature) the decay kinetics of the zwitterion fluorescence cannot be described by monoexponential function. The two-exponential fits of zwitterionic fluorescence decays are discussed in terms of a quenching effect of the surroundings on the excited molecule. The possibility of diabatic (back-) closure of the lactone ring as a deactivation channel is also considered.