Photochromic, Electrochemical, and Photoelectrochemical Properties of Novel Azopyridinium Derivatives

Organic photochrome azopyridiniums were systematically prepared by quaternizing azopyridines with either dimethyl sulfate or 1,3-propanesultone. Photochemical, electrochemical, and photoelectrochemical studies on the azopyridiniums as methyl sulfate, hexafluorophosphate, and zwitterionic compounds were carried out in aprotic solvents and in aqueous solutions. The azopyridiniums, which have 3-located quaternized nitrogens to the azo group, exhibited photochemical activity of trans ⇆ cis isomerization. Quaternization of the pyridine moiety decreased the photochemical conversion between the trans and cis forms. The photochromic properties were influenced by an electronic effect of quaternized nitrogens and an electrostatic interaction of counter anions. In electrochemical studies, new redox-active azopyridiniums were found by using cyclic voltammetry and an electrochemical technique coupled with UV-vis spectroscopy. By the quaternization of azopyridines, the formal reduction potentials were shifted to a positive region up to -0.27 V vs. Fc/Fc₊. The reversibility of the electrochemical reactions was strongly dependent on the location of quaternized nitrogen. The trans ⇆ cis photoisomerization of electroactive azopyridiniums influenced their electrochemical properties. The photochromic molecules exhibited a photoelectric effect, which was a change in the cathodic limiting currents of azopyridiniums, reversibly modulated by ultraviolet light of an external stimulation.