Syntheses and photochromic behavior of biquinolinium salts

Three isomers of N,N'-dibenzy-biquinolinium dibromide were synthesized and their photoinduced coloration properties were investigated in the thin poly (N-vinylpyrrolidone) films. Upon photo-irradiation, the colorless film of 4 ,4'-biquinolinium salt became intense purple and had a near IR-absorption a t 970 nm, whose color gradually bleached, reaching original ones. While pho to-induced absorption maxima of 6,6'-biquinolinium salt does not shift to l onger wavelength and 5,5'-one exhibits thermochromism.