PHOTOINDUCED INTRAMOLECULAR PROTON-TRANSFER AND CHARGE REDISTRIBUTIONIN IMIDAZOPYRIDINES

The ground-state conformation and fluorescence properties of two close ly related dyes 2-(2'-hydroxyphenyl)imidazo[1,2-a]pyridine (1,2-HPIP) and 3-(2'-hydroxyphenyl)imidazo[1,5-a]pyridi (1,5-HPIP) have been stud ied in various solvents at room temperature to determine the origin of the large red-shifted emission spectra. The two compounds have a grou nd-state planar conformation stabilized by an intramolecular ii bond ( MB) between the imino and phenol groups. On electronic excitation in c yclohexane solution a weak fluorescence is observed in the two dyes, w ith large Stokes shifts (1,2-HPIP: lambda(max) = 588 nm, Delta v = 11 000 cm(-1) 1,5-HPIP: lambda(max) 440 nm, Delta v = 9000 cm(-1)). For t he first dye it is proposed that the emission takes place from a zwitt erion produced by a proton-transfer reaction in the singlet manifold. A quinonoid, neutral phototautomer is excluded on the basis of the ele ctronic structure of the molecule. In the case of 1,5-HPIP, the IHB is broken on electronic excitation and neither a proton nor H atom trans fer occurs due to the reversal of the proton affinity of the imino gro up. The anomalous fluorescence is assigned to significant changes in t he geometry of the emitting state following excitation.