Polyyne-Type Materials - Polyynes: Synthesis, Properties, and Applications - page 206

orientation in the crystalline state rather than the lamellar orientation.
Further, the longer separation between phenylene moieties results in much
weakening interaction between side chains, which causes considerable
lowering of phase transition temperatures; e.g. observation of the clear phase
transition from LC to isotropic liquid around 120 C. Some other PpPB
derivatives and LC-PABs have been synthesized [43,44]. In addition, it is
noteworthy that a high quality uniform molecular alignment of poly(2,5-
dialkoxy-p-phenylenebutadiynylene) has been achieved by a conventional
simple rubbing procedure [45].
Since PPBs show good fluorescence emission, as do PPEs, some attempts
have been made to prepare polymeric LEDs. The first electroluminescence
(EL) of PABs was achieved by Yoshino et al. from a single layer device
(ITO/polymer/Mg-In) using poly(2,5-dialkoxy-p-phenylenebutadiynylene)
as the emissive layer [46]. For the sake of efficient EL emissions, the
polymer must be improved in terms of good contact with the electrodes.
Problems included the high crystallinity in the solid state, low solubility in
solvents, and low molecular weight of the PpPB, which might result in low
efficiency of the LED devices. Most problems were dramatically solved by
simple modification by co-polymerization with a m-phenylene monomer.
Furthermore, multi-color emissions covering blue to orange-red are also
achieved by the co-polymerization (Chart 10.6) [37,47,48]. Nonetheless, the
CHART 10.6 PAB co-polymers for polymeric LED.