Photochromic Properties of a Series of Zinc(II)-Viologen Complexes with Structural Regulation by Anions

The solvothermal reaction of 1-(3,5-dicarboxybenzyl)-4,4'-bipyridinium chloride (H2L+Cl-) with different anionic Zn(II) salts leads to three diverse metal viologen complexes, formulated as {[Zn(HL)Cl-2](2)center dot 2H(2)O} (1), {[Zn-3(L)(2)(H2O)(8)]center dot 2(SO4)center dot 2(H2O)}(n) (2), and {[Zn(L)H2O]center dot NO3 center dot H2O}(n) (3). Single-crystal X-ray analyses revealed that complex 1 displays dinuclear structure. Complex 2 features a 2D layered structure, which consists of four-fold interpenetrating stacked layers with sulfate ions locating between the sheets. Complex 3 shows a 3D two-fold interpenetrating framework with nitrate anions interspersing the void space of the framework, and it features an uninodal three-connected ThSi2 (ths) topology. The structural disparities reveal that the different geometries of counterions, coordination modes, and conformations of ligand L-, together with the deprotonation degree of aromatic carboxylate, have great influence on the formation of various structures. Interestingly, complexes 1-3 are all photoactive to UV-visible light and undergo obvious and reversible color transformations from light yellow to blue/green. Structural analyses indicated that not only the carboxylate-O donor and N acceptor of the bipyridinium moiety can provide an effective electron transfer pathway for photochromic behavior, but the Cl -> L electron transfer in complex 1 also has a crucial effect on the photochromic behavior of it.