Synthesis and optical properties of conjugated polymers containing polyoxometalate clusters as side-chain pendants

Hexamolybdate clusters have been covalently attached, for the first time, to the side chains of conjugated polymers. Two sets of such hybrid conjugated polymers have been prepared, one (Ia and Ib) with the clusters linked to the conjugated backbone through a rigid conjugated bridge, the other (IIa and IIb) through flexible alkyl chains. Within each set, polymers with different cluster loading ratios have been prepared. The covalent attachment of POM clusters has been confirmed by H-1 NMR, FTIR, and cyclic voltammetry measurements. These hybrid polymers are thermally stable up to 220 degrees C. Set I polymers (Ia and Ib) exhibit maximum absorption wavelengths (lambda(max)) around 410 nm, while set II polymers (IIa and IIb) show higher lambda(max) values, around 440 nm. Fluorescence studies show that side-chain POM pendants linked through conjugated bridges exhibit a much higher fluorescence quenching effect than those with flexible alkyl bridges, indicating that the through-bond photoinduced electron transfer may be the dominant mechanism for fluorescence quenching. With efficient fluorescence quenching that results in free charge carriers residing in different structural units (positively charged holes in the PPE backbone and negatively charged electrons in the POM clusters), these hybrid polymers may have great potential for applications in photovoltaic (PV) cells.