The hybrid film consisting of alternating layers of EuIII substituted Keggin polyoxometalate anion [Eu(BW11O39)2]15− (EuBW) and RuII complex cation of [Ru(L)2]2+ {L = 4′-(4-(imidazol-1-yl)phenyl)-2,2′:6′,2″-terpyridine}, abbreviated as RuL1, was prepared on quartz and indium-tin oxide (ITO)-coated glass substrates by electrostatically self-assembled method, and studied by UV–vis absorption spectroscopy, cyclic voltammetry, permeability of the film to the redox probe [Fe(CN)6]3−, electrochemical impedance spectroscopy and photoelectrochemical measurements. The uniform depositions of the two components into the films were evidenced by uniform growths of the UV–vis absorption peaks. The ITO electrodes modified with the hybrid film showed stable electrochemical property in relation to EuBW anion and RuL1. The significant cathodic photocurrent generation was observed upon visible light irradiation with a maximum photocurrent density of 8.47 μA/cm2 being achieved for a 5-layer film of (EuBW/RuL1)5 at an applied potential of −0.3 V versus saturated calomel electrode. The effects of substitution atoms in MBW (M = EuIII vs. ZnII) and experimental conditions including applied bias potentials, layer numbers of the films and irradiated light intensities on photocurrent generation were discussed.

•Four new composite films consisting of polyoxometalates and hemicyanines are prepared.•Composite films have synergistically enhanced photovoltaic performance.•Structures and compositions of polyoxometalates and alkyl linker lengths of hemicyanines have significant effects on photoelectric properties.•Effects of applied potentials and electron acceptor in electrolyte solution on photocurrent generation are examined.Four composite films consisting of Zn(II) and Cu(II) substituted tungstoborates K7[BW11O39M(H2O)] (BW11M, M = Zn, Cu) and two bipolar hemicyanines of (E)-1,1′-(propane-1,3-diyl)bis(4-(4-(dimethylamino)styryl)pyridinium) bromide (H3Br2) and (E)-1,1′-(decane-1,10-diyl)bis(4-(4-(dimethylamino)styryl)pyridinium) bromide (H10Br2), were prepared by electrostatic layer-by-layer self-assembly technique. These films of (BW11M/Ha)n (M = Zn, Cu; a = 3, 10) exhibited ultraviolet-visible characteristic absorption bands of BW11M and HaBr2 with their absorption intensities increasing linearly with increasing the layer number n, indicating that the composite films were successfully and uniformly deposited. With respect to the H3Br2 and H10Br2 aqueous solutions, the intramolecular charge transfer absorption bands of H3Br2 and H10Br2 in the films exhibited bathochromic shifts to different extents depending on the hemicyanines and BW11M in the films. These films could generate stable, cathodic photocurrents with photocurrent action spectra being consistent with corresponding absorption spectra of the hemicyanines in the films, when illuminated with 100 mW/cm2 polychromatic light (730 nm > λ > 325 nm). The effects of the alkyl chain length, BW11M, layer number of the (BW11M/Ha)n films, and applied bias potential and electron acceptor in the electrolyte solution on the photocurrent densities are discussed.Four composite films consisting of zinc(II) and copper(II) substitued tungstoborates and two bipolar hemicyanine derivatives with different alkyl chain length, were prepared by electrostatic layer-by-layer self-assembly technique and evidenced by UV–vis absorption spectroscopy. These films could generate stable, cathodic photocurrents with a maximum photocurrent density of 12.4 μA/cm2. The structures and compositions of POMs and the lengths of alkyl linker of dipolar hemicyanines in the hybride films had significant effects on the photoelectric conversion properties.Download high-res image (195KB)Download full-size image

•Thin film of Ru(II) complex with europium tungstoborate anion is prepared.•Surface-confined redox behaviors is observed.•Photoelectric conversion of Ru(II) complex is enhanced by the anionA new electrostatically self-assembled hybrid film consisting of alternating layers of europium-substituted tungstoborate anion of [Eu(BW11O39)2]15− (EuBW) and dinuclear ruthenium(II) polypyridyl complex cation of [(bpy)2Ru(BPBH)Ru(bpy)2]4+ {in which bpy = 2,2′-bipyridine and BPBH = 1,6-bis-(2-(2-phenyl)benzimidazoyl)hexane, RuBPBH} was successfully prepared. UV–vis absorption spectroscopy provided the evidences for the layer-by-layer deposition of EuBW anion and RuBPBH cation on quartz substrate. The (EuBW/RuBPBH)n films showed stable electrochemical and photoelectrochemical properties with a photocurrent density of 2.66 μA/cm2 being found for a 4-layer film of (EuBW/RuBPBH)4, while irradiated with polychromatic light (325 nm < λ < 730 nm) at an applied potential of −0.3 V versus saturated calomel electrode. The enhanced photocurrent generation by presence of the EuBW layer in this hybrid film has also been demonstrated as compared to the (PSS-c-MS/RuBPBH)1 film by using a photoelectrochemical inactive polyanion of PSS-c-MS instead of the EuBW anion.A electrostatic thin film of an europium-substituted tungstoborate anion with a dinuclear ruthenium(II) complex cation is prepared layer-by-layer as evidenced by ultraviolet–visible absorption spectroscopy. The surface confined good redox behaviors of the cation and anion in the films were observed. The films showed stable and large cathodic photocurrent with a maximum photocurrent density of 2.66 μA cm−2 was achieved for 4 layer film at an applied potential of −0.3 V versus SCE, which is 2.28 folds as great as that of similar film with an inert polymer layer instead of the anion layer.

•Three electrostatic thin films of hemicyanine with three polyoxometalates (POMs).•Visible absorption maxima were strongly affected by POMs.•Stable photocurrents that were differed by over 7 folds depending on POMs.•Photocurrent difference is related with LUMOs of POMs.A series of layer-by-layer electrostatically self-assembled films consisting of one of three Keggin-type polyoxometalates (POMs) of [BW12O40]5−, [PW12O40]3− and [PMo12O40]3−, and a hemicyanine of (E)-1,1′-methylenebis(4-(4-(dimethylamino)styryl)pyridinium)bromide (H1Br2) are prepared. The UV–vis absorption spectroscopy showed that the films were uniformly deposited with the visible absorption bands for the hemicyanine being bathochromically shifted with respect to the H1Br2 aqueous solution to different extents depending on POMs. The (POMs/H1)n films exhibited stable photocurrents that were differed by over 7 folds depending on POMs as irradiated with white light. The effects of applied bias voltages, electron acceptor added in the electrolyte, and layer numbers of the films, irradiated light intensities, and the compositions of POMs on photocurrent generation of the films were discussed.Three electrostatic thin films based on a hemicyanine and [BW12O40]5−, [PW12O40]3− and [PMo12O40]3− have been reported. Variations of POMs in these films were found to strongly affect film-forming, visible absorption optical, and photoelectric conversion properties. [BW12O40]5− and [PW12O40]3−, based monolayer films exhibited significant cathodic photocurrent densities of 4.58-4.90 A/cm2, while other two films gave weak photocurrent densities. Relationship between the POM compositions and photoelectric performance is expolored.

•Uniform growth of static films of a Ru(II) complex and a polyoxometalate is observed.•Surface concentration and metal-based redox properties are improved compared to the Ru complex-based covalent film.•Large cathodic photocurrent is observed for only one bilayer.•The photocurrents increased with increasing layer number up to 5 bilayers.An electrostatic film composed of [Ru(IPTP)2]2+ {IPTP=4′-(4-(imidazol-1-yl)phenyl)-2,2′:6′,2″-terpyridine} and [BW11Zn(H2O)O39]7− was prepared. The film showed uniform and compact deposition and reversible redox responses of the two components. A 5-layer film exhibited significant cathodic photocurrent density of 6.9 μA cm−2 in 0.1 M Na2SO4.

A series of photoelectric conversion films consisting of sandwich-type polyoxometalates [P2W18M4(H2O)2O68]10− (P2W18M4, M = Ni, Cu, Zn) and a bichromophore hemicyanine of (E)-1,1′-(hexane-1,6-diyl)bis(4-(4-(dimethylamino)styryl)pyridinium) bromide (H6) were prepared on quartz and indium–tin–oxide (ITO)-coated glass substrates by electrostatic self-assembly. The UV–visible absorption spectroscopy showed that the films were uniformly deposited with the visible absorption bands for H6 in the films exhibiting different extents of bathochromic shifts with respect to H6 aqueous solution. The (P2W18M4/H6)n films exhibited cathodic photocurrents as irradiated with white light. The photocurrent action spectra were found to match with the absorption spectra of the films, indicating that the cathodic photocurrents were generated based on intramolecular charge transfer excitation of the H6 in the films. The photocurrents were found to strongly depend on the transition metal ions M in (P2W18M4/H6)n films. The effects of applied bias voltages, electron acceptor added in the electrolyte, and layer numbers of the films on photocurrent generation of (P2W18M4/H6)n films were also examined, and photoelectric mechanism discussed.Graphical abstractHighlights► Electrostatically self-assembled films from polyoxometalates and hemicyanine. ► Transition metals in polyoxometalates strongly affected large cathodic photocurrents. ► Bathochromic shift in the visible absorption band of the films.

Three inorganic/organic hybrid films consisting of sandwich-type tetrazinc(II) tungstophosphorate [P2W18Zn4(H2O)2O68]10− (P2W18Zn4) and three hemicyanines of (E)-1,1′-(propane-1,3-diyl) bis (4-((E)-2-(4-(dimethylamino) naphthalen-1-yl) vinyl)-pyridinium) bromide (N3), (E)-1,1′-(hexane-1,6-diyl) bis (4-((E)-2-(4-(dimethylamino) naphthalen-1-yl) vinyl) pyridinium) bromide (N6), and (E)-1,1′-(decane-1,10-diyl) bis (4-((E)-2-4-(dimethylamino) naphthalen-1-yl) vinyl) pyridinium) bromide (N10), were prepared by electrostatic self-assembly technique. The UV–visible spectra showed that three inorganic–organic hybrid films of P2W18Zn4/Na (a = 3, 6, 10) were uniformly deposited and the hemicyanine molecules formed J-aggregations in the films. The monolayer (P2W18Zn4/N3,6,10)1 films exhibited stable, large cathodic photocurrent densities of 1.41–4.62 μA/cm2 that strongly depended on the alkyl chain lengths, while irradiated with 100 mW/cm2 polychromatic light of 730 nm > λ > 325 nm at an applied potential of −0.3 V vs. saturated calomel electrode. The photocurrent action spectra indicated that the cathodic photocurrents were generated by the charge transfer excitation of the hemicyanines in the films. The effects of applied bias voltages, electron acceptor added into the electrolyte solution, and layer numbers of the (P2W18Zn4/Na)n films on photocurrent generation of (P2W18Zn4/Na)n film were also examined.Three electrostatically self-assembled films made of [P2W18Zn4(H2O)2O68]10− (P2W18Zn4) and a hemicyanine dimer linked by varied alkyl chain lengths, were uniformly prepared with large bathochromic shifts compared to the hemicyanine aqueous solutions. A 3-layer film gave stable, large cathodic photocurrent densities of 13.7 μA/cm2 at an applied potential of −0.3 V vs. saturated calomel electrode.Highlights► Electrostatically self-assembled films from tetrazinc tungstophosphorate and hemicyanines were prepared. ► Alkyl chain lengths in hemicyanines strongly affected cathodic photocurrents. ► A maximum photocurrent density of 1.33 μA/cm2 was achieved at zero bias voltage.