PHOTOINDUCED INTRAMOLECULAR PROTON-TRANSFER AND CHARGE REDISTRIBUTION IN IMIDAZOPYRIDINES

The ground-state conformation and fluorescence properties of two closely 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 studied in various solvents at room temperature to determine the origin of the large red-shifted emission spectra. The two compounds have a ground-state planar conformation stabilized by an intramolecular ii bond (MB) between the imino and phenol groups. On electronic excitation in cyclohexane solution a weak fluorescence is observed in the two dyes, with 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 the first dye it is proposed that the emission takes place from a zwitterion produced by a proton-transfer reaction in the singlet manifold. A quinonoid, neutral phototautomer is excluded on the basis of the electronic 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 transfer occurs due to the reversal of the proton affinity of the imino group. The anomalous fluorescence is assigned to significant changes in the geometry of the emitting state following excitation.