Photo- and Thermochromic and Adsorption Properties of Porous Coordination Polymers Based on Bipyridinium Carboxylate Ligands

Abstract

The zwitterionic bipyridinium carboxylate ligand 1-(4-carboxyphenyl)-4,4'-bipyridinium (hpc1) in the presence of 1,4-benzenedicarboxylate anions (BDC(2-)) and Zn(2+) ions affords three porous coordination polymers (PCPs): [Zn5(hpc1)2(BDC)4(HCO2)2]·2DMF·EtOH·H2O (1), [Zn3(hpc1)(BDC)2(HCO2)(OH)(H2O)]·DMF·EtOH·H2O (2), and [Zn10(hpc1)4(BDC)7(HCO2)2(OH)4(EtOH)2]·3DMF·3H2O (3), with the formate anions resulting from the in situ decomposition of dimethylformamide (DMF) solvent molecules. 1 and 3 are photo- and thermochromic, turning dark green as a result of the formation of bipyridinium radicals, as shown by electron paramagnetic resonance measurements. Particularly, crystals of 3 are very photosensitive, giving an eye-detectable color change upon exposure to the light of the microscope in air within 1-2 min. A very nice and interesting feature is the regular discoloration of crystals from the "edge" to the "core" upon exposition to O2 (reoxidation of organic radicals) due to the diffusion of O2 inside the pores, with this discoloration being slower in an oxygen-poor atmosphere. The formation of organic radicals is explained by an electron transfer from the oxygen atoms of the carboxylate groups to pyridinium cycles. In the structure of 3', [Zn10(hpc1)4(BDC)7(OH)6(H2O)2], resulting from the heating of sample 3 (desolvation and loss of CO molecules due to the decomposition of formate anions), no suitable donor-acceptor interaction is present, and as a consequence, this compound does not exhibit any chromic properties. The presence of permanent porosity in desolvated 1, 2, and 3' is confirmed by methanol adsorption at 25 °C with the adsorbed amount reaching 5 wt % for 1, 10 wt % for 3', and 13 wt % for 2. The incomplete desorption of methanol at 25 °C under vacuum points to strong host-guest interactions.

A Two-Dimensional Metal-Organic-Framework Formed From a Cobalt(II) Ion and a Bifunctional Ligand Exhibiting Thermochromic Behavior

“…Depending on the external stimuli, which causes the color change in these materials, such as, light, temperature, pressure, and solvent molecules, they are categorized as photochromic (Pardo et al, 2011;Tandekar et al, 2018), thermochromic (Lim et al, 2018;Liu and Li, 2019), piezochromic , and solvatochromic materials (Lu et al, 2011;Mehlana et al, 2013), respectively. The investigation of the underlying mechanism for such sort of chromic behavior reveals that the color change is associated to the one of the following reasons: (i) charge transfer (CT)/electron transfer between the organic ligands [such as, 4,4-bipyridinium (Toma et al, 2015;Zhang et al, 2016), vialogen based ligands (Wan et al, 2015;Hu et al, 2017)] and the metal which changes their absorption properties, (ii) disruption of the interaction between the solvent molecules and the compounds which leads to the alterations in the crystal packing/supramolecular interactions (such as, hydrogen bonding, and π···π interactions), and (iii) coordination geometry transformations (Kundu et al, 2014;Burneo et al, 2015;Thapa et al, 2018) due to the loss of metal bound solvent molecules/rearrangement of the coordinating atoms around the metal centers.…”

Ionic metallo‐Schiff base polymers of VO2+, Zn2+ and Cu2+: Synthesis, characterization and solid‐state conductivity

“…Thus, the design of new ligands with special and different characteristics and combining them with metal ions open new horizons in materials architecture. The alkylation of N , N ′‐bipyridine gives N , N ′‐bipyridinium dications, known as viologens, which show unique photoactivity and have been investigated in biology, electrochemistry, photochemistry and conductivity . Thus new Schiff base compounds can be synthesized by reacting an appropriate N , N ′‐alkylbipyridinium dication dialdehyde and an amine.…”

Supramolecular Open-Framework of a Bipyridinium-Carboxylate Based Copper Complex with High and Reversible Water Uptake

“…Bipyridinium units (V 2+ ) are known to undergo a reversible one‐electron reduction, which is accompanied with a color change as well as V •+ radicals are created due to the different absorption domains of V 2+ (UV) and V •+ (visible) . In the solid state, the color change process, which can be activated by light or by heating, results from an electron transfer from electron donors such as chlorides or caboxylates, towards the electron acceptor bipyridinium units. Thus, the bipyridinium‐carboxylate ligands appeared as very suitable to achieve photo‐ and/or thermochromic materials because of the presence of both the electron donor and acceptor parts on the same molecular unit.…”

“…However, the interest of incorporating such linkers in PCPs is double. Because of their cationic nature, they can have a similar role than metal open sites for efficiency H 2 or CO 2 uptake, and because of their electron‐deficient character, they can strongly interact with polar and/or electron‐rich guest molecules for chemical sensor or gas storage applications , , . These materials are mainly based on zwitterionic bipyridinium carboxylate ligands, metal ions, and an anionic ligand, whereas recently it has been proposed to only use an anionic bipyridinium carbolylate linker .…”

“…These materials are mainly based on zwitterionic bipyridinium carboxylate ligands, metal ions, and an anionic ligand, whereas recently it has been proposed to only use an anionic bipyridinium carbolylate linker . However the use of rigid linkers instead of flexible ones extensively adopted in the field of photochromic materials seems to be the better strategy to achieve porous compounds , …”