6-Amino-2,2 ': 6 ',2 ''-terpyridines as highly fluorescent compounds - effect of the number of pyridine rings on fluorescence properties

2,2': 6', 2-Terpyridine (tpy) was found to exhibit remarkably efficient fluorescence in organic solutions when substituted by 6-amino (2, lambda(fl) = 384 nm, Phi = 0.70, dichloromethane) and 6,6-diamino (3, lambda(fl) = 386 nm, Phi = 0.48, dichloromethane) groups. The fluorescence maxima of 6-amino tpys were shifted to longer wavelengths as the solvent polarity increased, whereas the absorption maxima were little affected. In protic solvents, the fluorescence was largely quenched. The absorption and fluorescence bands of 2 and 3 were observed in the same region as those of 6-amino-2,2'-bipyridine, but at much longer wavelengths compared to 2-aminopyridine. These results and detailed analysis of the absorption and fluorescence spectra reveal that the photophysical properties of tpys could be interpreted as resulting from a contribution of the two bipyridyl units within the tpy structures that share the center pyridyl ring; the 6-aminobipyridyl unit appeared to be the fluorescent chromophore of 2 and 3. Semi-empirical molecular orbital calculations supported the above conclusion.