Design, synthesis, and characterization of a -donor-bridge-acceptor-bridge-type block copolymer via alkoxy- and sulfone-derivatized poly(phenylenevinylenes)

A novel class of light harvesting conjugated block copolymers, with electron-donating conjugated blocks (D) connected to electron-accepting conjugated blocks ( A) via non conjugated and flexible bridge chains ( B), has been designed, synthesized, and characterized. Specifically, D is a decyloxy-substituted polyphenyl-enevinylene (C-10-PPV). A(1) and A(2) are PPVs with sulfone (SO2) acceptor moieties substituted on every other phenylene unit. A1 carries two decyloxy groups on every phenylene unit, while in A2, half of the phenylene units are unsubstituted. The optical energy gaps are 2.24 eV for the donor block (D), 2.33 and 2.45 eV for A(1) and A(2) acceptor blocks. LUMO level offsets are 0.24 and 0.16 eV for D/A(1) and D/A(2) pairs, respectively. Comparing the photoluminescence from both films and solutions, very large red shifts (71 and 74 nm for A(1) and A(2) respectively) were observed in the two acceptor polymers. These red shifts in the emission spectra were more than twice as much as that observed for D (31 nm). The (DBA(1)B)(n) and (DBA(2)B)(n) block copolymer films exhibited improved processability and optoelectronic properties when compared with the corresponding films composed of donor/ acceptor blends. Atomic force microscopic (AFM) studies of D, A(1), and A(2) films were also undertaken to observe the degree of aggregation in the films. The results indicate the tendency of intermolecular aggregation increases as A(2) > D > A(1). AFM topological images revealed that large aggregates of several hundreds of nanometers formed in donor/ acceptor blend films, while in block copolymer films, domain sizes were similar to individual block sizes which are 1 order of magnitude smaller than in the blend.