Two crystalline Zn(II)-viologen compounds with photochromism, photomodulated luminescence and water-induced reversible crystal-to-amorphous transformation

In this work, two Zn(II)-viologen compounds, {[Zn(cpbpy)(L)](H2O)5}n (1) and [Zn(H2O)6](cpbpy)2(L)2(H2O)2 (2) (cpbpy = N-(3-carboxypyridin-5-yl)− 4,4′-bipyridinium, H2L = 1,4-benzenedicarboxylic acid) have been constructed via different synthetic routes. Structural analysis exhibits compound 1 features a two-dimension layered structure consisting of one-dimension [Zn(L)]n chains and bridging cpbpy ligands while compound 2 exists as a discrete monomeric structure. In a very short period of time, both compounds display electron-transfer photochromic behaviors. However, the photoproduct of compound 2 is more stable than compound 1 in air, which is attributed to better coplanarity of pyridyl rings of 4,4′-bipyridine moieties in compound 2. Also, both compounds show photomodulated luminescence properties. Strikingly, compound 2 also exhibits interesting water-induced reversible crystal-to-amorphous transformation accompanying with reversible luminescence switch behavior. To our knowledge, this is the first example of a crystalline viologen-based compound that displays water-induced reversible crystal-to-amorphous transformation.