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
(hpc1) in the presence of 1,4-benzenedicarboxylate anions (BDC2–) and Zn2+ 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.