Synthesis and Optical Properties of a Rod-Coil Diblock Copolymer with Polyoxometalate Clusters Covalently Attached to the Coil Block

A rod coil diblock copolymer (DCP) containing an oligo(phenylene vinylene) (OPV) rigid block and a polystyryl-type flexible block with polyoxometalate (POM) clusters as side chain pendants has been successfully synthesized. The coil block of 2,6-dimethyl-4-vinyl aniline protected by phthalic anhydride was prepared by atom transfer radical polymerization (ATRP). The terminal bromo-end group was then converted to an azide which subsequently coupled with an ethynyl terminated OPV rod block by "click" chemistry. After removing the phthalimide protection groups in the coil block to give free aryl amines, POM clusters were finally attached to the coil block covalently to yield the first POM-containing DCP. The hybrid DCP shows two absorption bands, one at 370 nm which is attributed to the ligand-to-metal charge transfer transition associated with imido-functionalized POM clusters, and the other at 450 nm which is due to the pi-pi* transition of the OPV backbone. Cyclic voltammetry measurements show a distinct reversible reduction process at -1.1 eV, significantly cathodically shifted compared to that of free hexamolybdate cluster. These results indicate that the POM clusters are indeed covalently linked to the DCP. Although little fluorescence quenching is observed in solution, the POM cluster is found to quench 74% of the OPV fluorescence in films. Such a significant fluorescence quenching is likely due to a photoinduced electron-transfer process from the OPV donor to the POM cluster, making it a potential candidate as an efficient photovoltaic material. Preliminary film morphology studies show that the hybrid DCP aggregates very differently from its precursor DCPs.