Synthesis and luminescent studies of poly(phenylenevinylene)s containing a biphenyl moiety

The polymers discussed in this contribution consist of phenylenevinylene chromophores linked together across flexible biphenyl 'hinges to shorten the effective conjugation length and to give sufficiently twisted structures that interchain aggregation is limited or prevented. They are poly[(2,5dihexyloxy-p-phenylenevinylene)-alt-(4,4'-dihexyloxy-3,3'-biphenylenevinylene)] (1), poly [(2,5-dihexyloxy-p-phenylenevinylene)-alt-(2,2'-dihexyloxy-3,3'-biphenylenevinylene)I (2), and poly[(2,5-dihexyloxy-p-phenylenevinylene)-alt-(2,2'-biphenylenevinylene)I (3). Absorption spectra in dilute solution and solid states are very similar for 1-3, consistent with the absence of aggregation effects in their ground electronic states. Photoluminesence emission spectra showed substantial red shifts in the solid state relative to dilute solution phase spectra. Solution emission quantum yields ranged from 0.26 to 0.42. LEDs based on 1-3 gave blue-green emission with maxima in the 480-510 nm range. The similarity of the photoluminescence and electroluminescence spectra for the polymers is consistent with emission from the same or very similar excited-state species. For LEDs based upon the highest quantum efficiency photoluminescent emitter, 1, better luminance was achieved using PEDOT-PSS hole injection layers in double-layer LEDs than using PPV.