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.