title: Photochromic metal complexes of N-methyl-4,4'-bipyridinium: mechanism and influence of halogen atoms.
abstract: Photochromism of N-methyl-4,4'-bipyridinium (MQ(+)) salts and their metal complexes has never been reported. A series of MQ(+) coordinated halozinc complexes [(MQ)ZnX(3)] (X = Cl (1), Br (2), I (3)) and [(MQ)ZnCl(1.53)I(1.47)](2)(MQ)ZnCl(1.68)I(1.32) (4), with better physicochemical stability than halide salts of the MQ(+) cation, have been found to exhibit different photochromic behaviors. Compounds 1-3 are isostructural, but only 1 and 2 show photochromism. Introduction of partial Cl atoms to nonphotochromic compound 3 yields compound 4, which also displays photochromism. The photochromic response of 1, 2, and 4 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.
authors.affiliation: State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People's Republic of China.
authors.forename: Xiang-Ying
authors.initials: XY
authors.lastname: Lv
authors.forename: Ming-Sheng
authors.initials: MS
authors.lastname: Wang
authors.forename: Chen
authors.initials: C
authors.lastname: Yang
authors.forename: Guan-E
authors.initials: GE
authors.lastname: Wang
authors.forename: Shuai-Hua
authors.initials: SH
authors.lastname: Wang
authors.forename: Rong-Guang
authors.initials: RG
authors.lastname: Lin
authors.forename: Guo-Cong
authors.initials: GC
authors.lastname: Guo