Synthesis of polyesters displaying high fluorescence intensities

A polycarbonate and a range of polyesters were prepared from a bisphenol containing a distyrylarylene moiety. The polymers were found to be highly fluorescent in solution and in the solid state. The nature of the ester linking group was found to influence the extent of fluorescence quenching due to aggregation of the distyrylarylene moieties both in solution and in the solid state. Aggregation was found to be greatest when a terephthalate linker was used and least when a sebacate linker was used. Whilst greater chromophore efficiency could be engineered by this approach, the dilution in overall chromophore concentration could not be offset. With one of the more luminescent polymers a simple electroluminescent (EL) device was assembled. This showed blue EL emission (λmax,EL = 453 nm), but the onset voltage was high (20 V) and brightness low (2 cd/m2). This is almost certainly due to the band gap of the partially conjugated material being relatively high, with the consequence that there is a significant probability that an energy level mismatch occurs at one of the electrodes. This means that injection efficiency of one or the other charge carrier is very low. HOMO levels measured by cyclic voltammetry were found to be deep (−5.78 eV) when compared with the comparatively low lying HOMO of the poly(3,4-ethylenedioxythiophen) hole injection contact layer (about −5.20 eV) used in our simple device. As a consequence it is likely that a problem of hole injection exists.