Reversible stimulus-responsive coordination polymers mainly involving conversion between the lone-pair–π and cation–π interactions

Abstract

A new 3-position substituted imidazo[1,2-a]pyridine ligand, 4-(2-chloroimidazo[1,2-a] pyridine-3-carboxamido)phthalic acid (H₂L), was conveniently synthesized and acts as a three-connecting T-shaped building block. Based on this ligand, two isostructural coordination polymers, [ML(H₂O)]_n (M = Zn (1) and Co (2)) with a rare one-dimensional (1D) ladder-like chain structure, were solvothermally synthesized and structurally characterized. There exist hydrogen bond and lone-pair–π interactions between two adjacent ladder chains. Compared to the H₂L ligand, coordination polymer 1 reveals a blue-shifted emission stemming from ligand-to-metal charge transfer, meanwhile, coordination polymer 2 presents a dominant antiferromagnetism. Interestingly, the carboxylate anion (COO⁻) in coordination polymers 1 and 2 could be protonated into carboxylic acid (–COOH), upon exposure to HCl vapor. As a result, coordination polymers 1 and 2 display reversible luminescent switching in the response to acid-base vapor stimuli, which mainly originates from the conversion between lone-pair–π and cation–π interactions mediated by the protonation-deprotonation processes. Such stimulus-responsive conversion between lone-pair–π and cation–π interactions is first reported in coordination polymers, and provides a promising approach to design and construct smart coordination polymer materials.

Graphical Abstract

Keywords:

• Imidazo[1,2-a]pyridine
• coordination polymers
• photoluminescence
• magnetic property
• stimulus-responsiveness

Disclosure statement

No potential conflict of interest was reported by the authors.

Acknowledgements

The authors are grateful to Dr. S.M. Zhou (HFNL, USTC) for the determination of the crystal structure.

Additional information

Funding

This work was supported by the Foundation of the USTC-Anhui Tobacco Joint Laboratory of Chemistry and Combustion.