Metal-dependent photochromic performance in two isostructural supramolecular chains

The combination of a conjugated coplanar dipyridine moiety 1,10-phenanthroline (1,10-phen) with a metal carboxylate system produces two isostructural supramolecular chains [M(HBTA)(1,10-phen)₂] (M = Zn for 1, M = Cd for 2) (H₃BTA = benzene-1,2,3-tricarboxylic acid). Both 1 and 2 feature monomeric units as molecular building blocks (MBBs), which further connect with each other to form a supramolecular chain via forming hydrogen bonds with adjacent units. The coordinate linkage of 1,10-phen as π-electron acceptors (π-EAs) and tricarboxylate as electron donors (EDs) results in the electron transfer (ET)-induced photochromic functionality of 1 and 2 in response to Xe-lamp irradiation under ambient conditions. Distinct from the plenty of previous photochromic compounds derived from photosensitive moieties such as pyridinium-derivatives and photodeformable molecules, the photochromism in 1 and 2 is driven by the photoinduced ET between tricarboxylate and non-photochromic 1,10-phen units. Because of the coplanar characteristics of 1,10-phen, the photoactivated samples feature good stability under ambient conditions. More importantly, the resulting photochromism of isostructural 1 and 2 could be modulated by the category of metal ions, which is totally different from the previous works with focus on the design of organic ligands. Considering the great varieties of carboxylate ligands, this work offers a general method for the construction of photochromic complexes via integrating coplanar 1,10-phen units with metal-carboxylate systems under the guidance of the ET mechanism and MBB assembly strategy and modulating the photochromism of the resultant isostructural products via tuning the category of metal ions.