Synthesis and chlorine atom position effect on the properties of unsymmetrical photochromic diarylethenes

Unsymmetrical photochromic diarylethenes 1o–4o have been synthesized, which diarylethenes 1o–3o have a chlorine atom at ortho-, meta- and para-position of the terminal phenyl ring, respectively. The substituent position effect of chlorine atom on their properties, including photochromism both in solution and in the single crystalline phase and their electrochemical properties were investigated in detail. The results elucidated that the chlorine atom and its substituted position had significantly affected on the absorption characteristics, photochromic reactivity as well as the electrochemical performances of these diarylethene compounds. When the chlorine atom was introduced into the phenyl ring of these diarylethene systems, the cycloreversion quantum yields were effectively depressed; but, the molar absorption coefficients of the open-ring isomer and the differences of oxidation potential onsets between the open- and closed-ring isomers of these diarylethenes were significantly increased. Moreover, the position of the chlorine atom had also influenced evidently the above optical and electrochemical features.