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

The zwitterionic bipyridinium carboxylate ligand 1-(4-carboxyphenyl)4,4'-bipyridinium (hpcl) in the presence of 1,4-benzenedicarboxylate anions (BDC2-) and Zn2+ ions affords three porous coordination polymers (PCPs): [Zn-5(hpcl)(2)(BDC)(4)(HCO2)(2)]center dot 2DMF center dot EtOH center dot H2O (1), [Zn-3(hpcl)(BDC)(2)(HCO2)(OH)(H2O)]center dot DMF center dot EtOH center dot H2O (2), and [Zn-10(hpcl)(4)(BDC)(7)(HCO2)(2)(OH)(4)(EtOH)(2)]center dot 3DMF center dot 3H(2)O (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 O-2 (reoxidation of organic radicals) due to the diffusion of O-2 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', [Zn-10(hpcl)(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 degrees 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 degrees C under vacuum points to strong host-guest interactions.