Novel carbazole-based ladder-type polymers for electronic applications

New carbazole-based ladder-polymers have been prepared utilising a 3,6-diacyl-2,7-dibromocarbazole building block 4. Suzuki polycondensation of 4 with carbazole-2,7-diboronic acid followed by addition of methyl lithium and ring closure with boron trifluoride gave ladder-polymers 9 with all methine bridges, analogous to ladder-type poly(para-phenylene). In very dilute solution, these polymers show a blue-green fluorescence similar to that from the corresponding LPPP. At higher concentrations, a broader red-shifted emission is seen which suggests that the chains are aggregating in solution. Homopolymerisation of 4 followed by condensation of the carbonyls with boron sulfide produced a novel ladder-type structure 13 with alternating five- and six-membered rings. This ladder-polymer displays bright yellow-green fluorescence. Cyclic voltammetry indicated that these materials have HOMO energy levels comparable to the work function of ITO, making them good candidates for use as hole accepting emissive materials LEDs.