Luminescent organic materials based on donor-acceptor-donor compounds containing carbazole donors and acceptors of varying strength: Spectroscopy, redox properties and application in organic light emitting diodes

Three D-pi-A-pi-D organic luminophores of tunable donor-acceptor interactions were synthesized by combining carbazole D-units with two different A units, namely 2,5-dithiophen-2-yl-[1,3,4]-thiadiazole and benzo[c][1,2,5] thiadiazole. In the third compound tert-butyl groups were introduced to the carbazole ring to increase its electron-donating properties. This compound, namely 4,7-bis(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)benzo [c][1,2,5]thiadiazole (CBTD2) turned out to be a very good photoluminophore showing the photoluminescence quantum yield (phi) of 62% in toluene solution and 45% when dispersed in a solid blend consisting of poly (vinylcarbazole) (PVK) and 2-([1,1 '-biphenyl]-4-yl)-5-(4'-(tert-butyl)-[1,1 '-phenyl]-4-yl)-1,3,4-oxadiazole (PBD). Moreover, its electrochemically determined ionization potential (IP = 5.55 eV) and electron affinity (EA =-2.73 eV) made it a very promising candidate for application as an active component in light emitting diodes of guest/host configuration with PVK-PBD matrix. The fabricated diodes of the following structure ITO/PEDOT: PSS/PVK-PBD + CBTD2/Ca/Ag showed the luminance reaching 6000 cd/m(2) with luminous efficiency of 1 cd/A.