Photochromic properties of five complexes derived from 1, 3, 5-benzenetricarboxylic acid and 1-(2-carboxyethyl)-4,4′-bipyridinium bromide

Non-photochromic and photochromic complexes [Ni1.5(CBbpy)(H2O)7]·(BTC)·5·.5H2O (1), {[Co(CBbpy)2(H2O)2]·(HBTC)·8H2O}n (2), {[MnNa(CBbpy)(BTC)(H2O)3]·H2O}n (3), {[Zn3(CBbpy)2(BTC)2(H2O)]·H2O}n (4), {[Cd(CBbpy)(HBTC)(H2O)]·H2O}n (5) based on the bipyridinium-based ligand 1-(2-carboxyethyl)-4,4′-bipyridinium bromide (HCBbpyBr) and 1,3,5-benzenetricarboxylic acid (H3BTC) have been successfully obtained by tuning different metal ions. Although five complexes contain the same bipyridinium ligand, only 3–5 possess reversible photochromic behavior. Unfortunately, 1 and 2 exhibit non-photochromic action when exposed to UV light for approximately 2 h. EPR spectra verify that photochromic course of 3–5 is derived from the formation of CBbpy radicals by photo-induced electron transfer after irradiation. The effect of diverse metal ions on the structure and photochromic action of the five complexes has been discussed.