Pyrenyl-Functionalized Fluorene and Carbazole Derivatives as Blue Light Emitters

New pyrenyl mono- and disubstituted fluorene and carbazole derivatives were synthesized and investigated as blue-emitting materials. Most of the synthesized compounds were capable of forming glasses with glass transition temperatures up to 105 degrees C. The mono- and disubstituted compounds exhibited efficient emission in a diluted form with the peak wavelengths of 416 and 422 nm and quantum yields of 0.72 and 0.82, respectively. Photophysical properties of the compounds in dilute solutions and solid state were investigated experimentally and rationalized by density functional theory calculations. The calculations revealed a low energy barrier for intramolecular twisting in the ground and excited states. The electronic spectra of the compounds were dominated by an allowed pyrene-like S-0 -> S-2 transition prevailing in nearly perpendicularly twisted molecular structures and competitive S-0 -> S-1 transition becoming increasingly allowed for more planar conformers. The disubstituted compounds demonstrated a 3-fold enhanced oscillator strength of S-0 -> S-1 transition, giving rise to the enhanced fluorescence quantum yield, significant shortening of the fluorescence lifetimes (from 2.5 ns down to 1.0 ns), and 3-fold reduced amplified spontaneous emission threshold as compared with those of the monosubstituted compounds. Introduction of the bulky dihexyl group into the pyrenyl-disubstituted fluorene enabled remarkable suppression of emission concentration quenching in a solid state and ensured high emission quantum yield (0.63) in the wet-casted neat films. Pyrenyl disubstitution of fluorene and a sterically hindered aliphatic group enabled observation of a low amplified spontaneous emission threshold (20 kW/cm(2)) for the compound dispersed in the inert polystyrene host at a rather large (5 wt %) chromophore concentration.