Synthesis, photophysics, electrochemical and electroluminescent properties of divinylene compounds with phthalimide moieties

Three new divinylene compounds F, C and P which contained fluorene, carbazole and phenylene as central unit, respectively, and terminal phthalimide groups were synthesized by Heck coupling. The alkyl or alkoxy chains attached to the central unit as well as the cyclohexyl ring attached to the terminal phthalimides render the samples very soluble in common organic solvents. They had relatively low glass transition temperatures (Tgʹs), being in the range of 54–81 °C. P was stable up to about 200 °C while F and C were stable up to 300 °C. The absorption maximum was located at 387–440 nm with optical band gap of 2.48–2.56 eV. They emitted blue-green light with emission maximum at 484–562 nm. P showed the most red-shifted absorption and emission maximum. The HOMO and LUMO levels of the compounds, estimated from their cyclic voltammograms, were −6.42 to −5.58 eV and −3.06 to −2.99 eV, respectively, from which their band gaps were estimated to be 3.36 eV, 2.68 eV, and 2.52 eV. The emission spectra and performance of the EL devices, using blends of the compounds and PVK as emitting layer, also depended strongly on the compounds. The P device emitted green light (530 nm) with maximal luminance and current efficiency being at 1358 cd/m2 and 0.57 cd/A, while those of the F device were green-blue (490 nm), 593 cd/m2 and 0.22 cd/A.