Construction of efficient solid emitters with conventional and AIE luminogens for blue organic light-emitting diodes

9,9-Bis(9-heptyl-3-carbazolyl) fluorenes (BPyBCF, BAnBCF, BTPABCF, and BTPEBCF, where B = Bis, Py = pyrene, C = carbazole, F = fluorene, An = anthracene, TPA = triphenylamine, and TPE = tetraphenylethene) with different chromophoric units at the 2,7-positions are synthesized in moderate to high yields (52-89%) by Suzuki coupling reactions of 9,9-bis(9-heptyl-3-carbazolyl)-2,7-dibromofluorene with the corresponding arylboronic acid and utilized as active layers for the construction of blue organic light-emitting diodes (OLEDs). BPyBCF, BAnBCF and BTPABCF emit intense blue light with high fluorescence quantum yields (Phi(F) = 75-94%) in solution. However, they exhibit much lower Phi(F) values (30-61%) in the film state, revealing that aggregate formation has quenched their light emission. On the contrary, BTPEBCF is weakly emissive in solution (Phi(F) = 0.3%) but becomes a strong emitter (Phi(F) = 100%) when fabricated into solid film, demonstrating a phenomenon of aggregation-induced emission (AIE). Restriction of intramolecular rotation and suppression of intermolecular interactions due to the propeller-like tetraphenylethene unit are responsible for the AIE phenomenon. All the luminogens are thermally and morphologically stable, showing high glass-transition (T-g = 109-147 degrees C) and thermal-degradation temperatures (T-d = 396-478 degrees C). Non-doped OLEDs using BPyBCF, BAnBCF, and BTPABCF as light-emitting layers are constructed, which give blue electroluminescence with maximum current (eta(C,max)) and external quantum (eta(ext,max)) efficiencies of 4.8 cd A(-1) and 2.3%. With the same device configuration, BTPEBCF shows higher eta(C,max) and eta(ext,max) values of 7.9 cd A(-1) and 2.9%, respectively, thanks to its AIE feature.