Tailoring the coordination microenvironment of Zn(ii) in a light-responsive coordination polymer system for molecular sensing and photodegradation performance

Abstract

The behaviour of photoexcited electrons in light-responsive coordination polymers (CPs) significantly determines their performance in fluorescence sensing, smart materials, photoelectric display and photocatalysis, and this behaviour can be skilfully manipulated by optimizing the geometric and electronic structures of the ligand field around the metal ion. To reveal the micro-environmental effect of the ligand field on the bandgap and photoexcited electrons, three semiconductive Cd(II)-based CPs have been achieved through coordination of π-conjugated 2,6-bis(2-pyrazin-2-yl)-4-(4-(tetrazol-5-yl)phenyl)pyridine (HL) and 1,3,5-benzenetricarboxylic acid (H₃BTC) with Cd(II) ion. Possessing N₃O₄ and O₆ donors, complex 1 assumed an extended triple-decker sandwich structure with a central {Cd₂(BTC)} sheet terminated up and down by planar L⁻ spacers. Two quadrangular {Cd₂(L)₂} dimers with N₄O₃ and N₄O₂ donors were propagated by doubly deprotonated HBTC²⁻ connectors to the cationic (4 4) layer of 2. The reproducible {Cd₂(L)₂} dimers with only N₄O₃ donors in 3 were extended by mirror-symmetric HBTC²⁻ linkers to a bent chain. The distinct donor combinations in the local ligand fields of Cd(II) ions dominated the bandgaps and movements of the charge-carriers. Emitting intense steel blue fluorescence, complex 1 served as a highly efficient bilirubin probe with a quenching constant up to 9.48 × 10⁴ M⁻¹ driven by photo-induced electron charge and Förster resonance energy transfer. In contrast, complex 2, with excellent separation efficiency of charge carriers, acted as a photocatalyst to completely degrade methylene blue up to 97% within 90 minutes of UV irradiation. Apparently, slight modifications of the surroundings of the ligand field by a heterodonor strategy achieved a remarkable effect on the photophysical properties of the semiconductive CPs, providing valuable hints for the construction of photosensitive CPs.