Photochromic Metal Complexes
of <i>N</i>-Methyl-4,4′-Bipyridinium: Mechanism and
Influence of Halogen
Atoms
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Abstract
Photochromism of <i>N</i>-methyl-4,4′-bipyridinium
(MQ<sup>+</sup>) salts and their metal complexes has never been reported.
A series of MQ<sup>+</sup> coordinated halozinc complexes [(MQ)ZnX<sub>3</sub>] (X = Cl (<b>1</b>), Br (<b>2</b>), I (<b>3</b>)) and [(MQ)ZnCl<sub>1.53</sub>I<sub>1.47</sub>]<sub>2</sub>(MQ)ZnCl<sub>1.68</sub>I<sub>1.32</sub> (<b>4</b>), with better
physicochemical stability than halide salts of the MQ<sup>+</sup> 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<sup>+</sup> 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