Photochromic Metal Complexes of <i>N</i>-Methyl-4,4′-Bipyridinium: Mechanism and Influence of Halogen Atoms

Abstract

Photochromism of <i>N</i>-methyl-4,4′-bipyridinium (MQ⁺) salts and their metal complexes has never been reported. A series of MQ⁺ coordinated halozinc complexes [(MQ)­ZnX₃] (X = Cl (<b>1</b>), Br (<b>2</b>), I (<b>3</b>)) and [(MQ)­ZnCl_1.53I_1.47]₂(MQ)­ZnCl_1.68I_1.32 (<b>4</b>), with better physicochemical stability than halide salts of the MQ⁺ cation, have been found to exhibit different photochromic behaviors. Compounds <b>1</b>–<b>3</b> are isostructural, but only <b>1</b> and <b>2</b> show photochromism. Introduction of partial Cl atoms to nonphotochromic compound <b>3</b> yields compound <b>4</b>, which also displays photochromism. The photochromic response of <b>1</b>, <b>2</b>, and <b>4</b> indicates the presence of their long-lived charge separation states, which originate from X → MQ⁺ electron transfer according to ESR and XPS measurements. Studies on the influence of different coordinated halogen atoms demonstrate that the Cl atom may be a more suitable electron donor than Br and I atoms to design redox photochromic metal complexes