Photochromism of phenylazopyridines and its application to the fluorescence modulation of zinc-porphyrins

Several 2- and 3-(phenylazo)pyridine derivatives were prepared. Most of their trans isomers have a large absorption band (similar to2 x 10(4) M-1 cm(-1) (1 M = 1 mol dm(-3))) around 320 nm and a small absorption band (similar to400 M-1 cm(-1)) around 450 nm. All of these compounds photoisomerize upon irradiation of the shorter (trans-to-cis) and longer (cis-to-trans) wavelength absorption bands. trans Isomers of 3-(phenylazo)pyridines axially coordinate more strongly to Zn-porphyrins than cis isomers, while the opposite is the case for 2-(phenylazo)pyridines, due to steric reasons. These phenylazopyridines quench the fluorescence of Zn-porphyrin upon coordination. These properties were exploited for the light-triggered fluorescence modulation of Zn-porphyrins. Light was irradiated to a mixed solution of phenylazopyridine and Zn-porphyrin to induce photoisomerization of the phenylazopyridine, which underwent association with, or dissociation from, the Zn-porphyrin, resulting in a decrease or increase of Zn-porphyrin fluorescence. For example, the fluorescence intensity of Zn-tetraphenylporphyrin reversibly changed by up to 50%, when 4-methoxy-2-(phenylazo)pyridine was employed.