Photochromism of Phenylazopyridines and Its Application to the Fluorescence Modulation of Zinc–Porphyrins

Abstract

Abstract Several 2- and 3-(phenylazo)pyridine derivatives were prepared. Most of their trans isomers have a large absorption band (∼2 × 104 M−1 cm−1 (1 M = 1 mol dm−3)) around 320 nm and a small absorption band (∼400 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.