Carbazole-diazafluorene bipolar fluorophores: Synthesis, thermal stability, optical and electrochemical properties

Two bipolar fluorophores, namely CEDF and CPDF containing carbazole moiety as electron donor and 4,5-diazafluorene unit as electron acceptor were synthesized and characterized. Carbazole units attached to C-2, C-7 and C-9 positions of 4,5-dizafluorene posing an ideal model of donor-acceptor systems to investigate their photophysical properties for application in organic light emitting diode (OLED) devices. Both compounds possess high thermal stabilities (T-d > 450 degrees C) and high glass transition temperature (T-g), 182 degrees C and 180 degrees C for CEDF and CPDF respectively. These compounds revealed solvent polarity dependent emission spectra which confirm the excited state undergoes intramolecular charge-transfer (ICT) process in the molecule. Solid state emission is observed in the blue region at 447 nm for both the compounds. CPDF exhibit higher photoluminescence quantum yield (0.76) than CEDF (0.55) featuring its highly emissive ICT excited state. Using the oxidation and reduction potentials determined from cyclic voltammetry analysis, estimates of both the ionization potentials and electron affinity of the compounds are obtained under the same experimental conditions. HOMO and LUMO energy levels, singlet (E-S) and triplet (E-T) energy levels of the compounds were obtained through density functional theory (DFT) and time-dependent DFT calculations. A transient photoluminescence curve with a prompt fluorescent component gives an average lifetime of about 5.7 ns for both the compounds. These studies and the convenient synthetic methodologies starting from the 4,5-diazafluorene, make these molecules attractive opto-electronic materials for their application in OLED devices.