Electrochemical behaviors of nanoporous coordination polymer multilayers in hexacyanoferrate solution

Coordination polymers (CPs) can encapsulate small molecules within their nanosized coordination cages to form hybrid supramolecular materials. We reported here assembly of Pd(II)-mediated CP multilayers on the substrate surfaces by layer-by-layer method with six kinds of multidentate ligands as linkers, which included 4,4′-bipyridyl (BPy), binaphthylbis(amidopyridyl) (BNBAPy), tris(4-pyridyl)-1,3,5-triazine (TPyTa), tetra(4-pyridyl)porphyrin (TPyP), ligand-like complex of Fe(pyterpy)2 (pyterpy: 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine), and poly(4-vinylpyridine) (PVP). The multilayer assembly process was characterized by using UV–vis absorption spectra and X-ray photoelectron spectra. Electrochemical behaviors of the as-prepared CP-modified electrodes were investigated in hexacyanoferrate solution, which revealed well-reversible redox waves of Fe(CN)63−/4− ions for the electrode covered with the Pd–TPyTa or Pd–Fe(pyterpy)2, while irreversible ones for the electrode covered with the Pd–BPy or Pd–PVP. The results indicated that Fe(CN)63−/4− ions could reach electrode surface across the CP multilayers of TPyTa or Pd–Fe(pyterpy)2, which was attributed to the fact that their coordination cages were large enough for the Fe(CN)63−/4− ions to penetrate through. Charge-transfer diffusion coefficient (D) was calculated based on chronocoulomograms of the CP multilayer-modified electrodes. D was in the range from 0.34 × 10−10 to 1.3 × 10−10 cm2/s for the electrodes modified by four layers of Pd–TPyP, Pd–BNBAPy, Pd–TPyTa and Pd–Fe(pyterpy)2, respectively. The present methodology provides a facile way to construct three-dimensional supramolecular materials on the solid surfaces as well as to study the charge-transfer behaviors in artificial membranes.