Multicolor Zinc(II)-Coordinated Hydrazone-Based Bistable Photoswitches for Rewritable Transparent Luminescent Labels.

Repeatable printing of invisible multicolor luminescence patterns with long retention times on transparent substrates is of significant importance, but it remains a formidable challenge. Here, two novel hydrazone-based on/off fluorescent photoswitches with decent emission quantum efficiencies, good reversible photoisomerization properties, and extremely long thermal half-lives, were designed and synthesized. Excitingly, X-ray crystallography data of both Z and E isomers of one hydrazone-based photoswitch were obtained. Multiple emission colors of blue, cyan, green, yellow, and orange can be obtained readily for these two photoswitches upon coordination with various zinc salts. Moreover, the photo-controlled rewritable printing of invisible multicolor images with high resolution was achieved by using these photoswitches. Importantly, the legibility of the printed patterns can last at least over 3 months without detectable emission intensity loss under ambient conditions.

Dynamic Metal-ligand Coordination for Fluorescence Color Regulation of Hydrazone-based Bistable Photoswitches.

Achieving effective manipulation of emission color in photoresponsive materials is crucial for various advanced photonic applications. In this study, we designed and synthesized a hydrazone compound 1, ethyl (Z)-2-(2-([2,2':6',2''-terpyridin]-4'-yl)hydrazineylidene)-2-(4-(diphenylamino)phenyl)acetate, which possesses a push-pull structure incorporating triphenylamine and terpyridine. The emission intensity of compound 1 can be repeatedly switched "off" and "on" by irradiation with visible light and UV light, which induces the isomerization transition between the Z and E forms. In addition, compound 1 is capable of changing its emission wavelength from 540 nm to 607 nm through coordination with different zinc salts in toluene/CH2 Cl2 mixture (v : v=1 : 1). Importantly, we have successfully achieved dynamic manipulation of fluorescence color and intensity by altering the counterions of zinc complexes and switching the isomer from Z to E. Moreover, both compound 1 and its zinc complexes demonstrate remarkable photoswitchable properties with different fluorescence colors in the thin films. Finally, these films with various fluorescence colors were used for the production of luminescent tags.