Two New Coordination Compounds with a Photoactive Pyridinium-Based Inner Salt: Influence of Coordination on Photochromism

The past years have evidenced the rapid development of photochromic coordination compounds; however, the impact of coordination on the photochromic behavior of organic dyes has never been explored in the pyridinium derivative photochromic system. In this work, two new coordination compounds with a photoactive pyridinium-based inner salt, [Zn(H2O)(6)](PTA)center dot(CEbpy)(2)center dot 2H(2)O (1, PTA = terephthalate, CEbpy = 1-carboxyethyl-4,4'-bipyridine) and [Zn(H2O)(2)(CEbpy)(2)](n)Br-2n center dot[Zn(H2O)(4)(PTA)(n)] (2), were selected as model compounds for this purpose. Compound 1 features an isolated structure, where uncoordinated photoactive CEbpy ligands connect to hexahydrated zinc ions through hydrogen bonds. Compound 2 features a 1-D chain structure with CEbpy ligands coordinating to zinc ions. Compound 1 shows faster coloration speed upon irradiation than 2, demonstrating that coordination of the electron donor in CEbpy is not in favor of photochromic behavior. Both compounds show significant photoluminescence quenching after coloration, and the intensity contrast before and after coloration for 1 is larger than that for 2. This finding will help to design and synthesize new photochromic compounds with high performance.