title: 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.
authors.affiliation: MOLTECH Anjou, UMR-CNRS 6200, Université d'Angers , 2 Bd Lavoisier, 49045 Angers, France.
authors.forename: Oksana
authors.initials: O
authors.lastname: Toma
authors.affiliation: MOLTECH Anjou, UMR-CNRS 6200, Université d'Angers , 2 Bd Lavoisier, 49045 Angers, France.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Mercier
authors.affiliation: MOLTECH Anjou, UMR-CNRS 6200, Université d'Angers , 2 Bd Lavoisier, 49045 Angers, France.
authors.forename: Magali
authors.initials: M
authors.lastname: Allain
authors.affiliation: IMMM, UMR-CNRS 6283, Université du Maine , Avenue O. Messiaen, 72085 Le Mans, France.
authors.forename: Abdel Adi
authors.initials: AA
authors.lastname: Kassiba
authors.affiliation: ICB, UMR-CNRS 6303, Université de Bourgogne , 9 A. Savary, 21078 Dijon, France.
authors.forename: Jean-Pierre
authors.initials: JP
authors.lastname: Bellat
authors.affiliation: ICB, UMR-CNRS 6303, Université de Bourgogne , 9 A. Savary, 21078 Dijon, France.
authors.forename: Guy
authors.initials: G
authors.lastname: Weber
authors.affiliation: ICB, UMR-CNRS 6303, Université de Bourgogne , 9 A. Savary, 21078 Dijon, France.
authors.forename: Igor
authors.initials: I
authors.lastname: Bezverkhyy