High-contrast fluorescence switching using a photoresponsive dithienylethene coordination compound

A novel hybrid system (Zn4) containing a zinc bis(acylamidine) fluorescent dye covalently decorated with two photochromic dithienylethene (DTE) structures was prepared using a concise synthetic route. The photoregulated quenching of the fluorescence from the metal complex was achieved by reversibly switching the DTE component between its two isomers using UV and visible light. The fluorescence quenching can be attributed to intramolecular energy transfer, electron transfer or a combination of the two. Either process is only possible when the DTE is in its ring-closed form (Zn4b). The ring-open counterpart (Zn4a) lacks an absorption band in the visible region of the spectrum to accommodate energy transfer from the metallobis(acylamidine) to the photochromic DTE, and has an oxidation potential that is too large to match the reduction potential of the metal complex making electron transfer thermodynamically unfavourable. UV–vis absorption and emission spectroscopy demonstrated that the photoregulation of fluorescence has little effect on the photochromic process and the fluorescence could be turned “on” and “off” several times without the appearance of notable side products. The high contrast ratio (∼100:1) between the bright and dark states combined with the excellent thermal bistability make the hybrid an excellent candidate for potential use in non-destructive read-out and biological labeling applications, although several limitations need to be overcome (speed of photoswitching, and stability and solubility in water).