Hexamolybdate clusters have been embedded through covalent bonds into the main chain of poly(phenylene acetylene)s. These hybrid polymers were synthesized by palladium-catalyzed coupling reactions of a diiodo functionalized cluster with a diethynylbenzene derivative or a diethynyl functionalized cluster with a diiodobenzene derivative. These polymers are soluble in organic solvents such as N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), and free-standing films can be spin-coated or cast from solutions. While hybrid monomer 2a exhibits a sharp melting transition at 246 °C, polymers 5a and 5b show glass transitions at 125 and 102 °C, respectively. Cyclovoltammetry studies of the hybrid polymers revealed a reversible reduction wave at 1.19 V versus Ag/Ag+, comparable to those of bifunctionalized imido derivatives of hexamolybdates. These polymers show intense absorption in the visible range but with little fluorescence emissions, indicating efficient fluorescence quenching of the embedded polyoxometalate (POM) cluster on the organic phenylene acetylene units. Simple single-layer photovoltaic (PV) cells with a device configuration of indium−tin oxide (ITO)/polymer/Ca have been fabricated and a power conversion efficiency of 0.15% has been obtained, which is significantly higher than PV cells fabricated with other conjugated polymers in the same device configuration. These results convincingly demonstrate the potential applications of POM-based organic−inorganic hybrids in molecular electronics and photonics.

Two new charge-transfer hybrids with one or two ferrocenyl units covalently attached to a hexamolybdate cluster through an extended pi-conjugated bridge have been prepared using Pd-catalyzed coupling reactions on monoiodo- or diiodo-functionalized cluster substrates in over 60% yields. These hybrids have been characterized by (1)H NMR, FTIR, electrospray ionization mass spectrometry, and X-ray diffraction. The electronic spectra of these hybrids show a broad absorption tail extending beyond 550 nm, indicating the existence of charge-transfer transition from the ferrocenyl donor to the cluster acceptor. The observation of the clear charge-transfer transition indicates the contribution of charge-transfer resonance to the ground state in both 2a and 2b even though the donor-acceptor separation distance of 11.29 A is rather long, signaling a through-bond charge-transfer nature made possible by the organic pi-conjugated bridge. Cyclic voltammetry studies reveal a one-electron oxidation wave and a one-electron reduction wave for the hybrid with one ferrocenyl unit. For the one with two ferrocenyl units, a lower reduction potential and a two-electron oxidation wave are observed, indicating negligible electronic interactions between the two ferrocenyl units.

No abstract