Multifunctional polyconjugated molecules with carbazolyl and pyrazolyl moieties for optoelectronic applications

Diethylene derivative possessing carbazolyl and pyrazolyl moieties and its analogue containing additional cyano groups were synthesized and investigated as potential multifunctional materials for organic light emitting diodes. The influence of the electron affinitive cyano groups on the ionization potential (Ip) of the films as well as on the photoluminescence (PL) spectrum, PL quantum yield (η) and PL decay time of the dilute solutions and thin films of the diethylene derivatives was studied. PL measurements revealed that highly luminescent (η = 0.80) cyano-free pyrazole–carbazole derivative in solution became weakly emissive (η = 0.04) by attaching cyano groups as a result of these groups-induced torsional deactivation. However in the solid state, the steric and electrostatic effects of the bulky and polar cyano groups prevented close packing of the molecules, thus significantly reducing migration-induced quenching of the excitons at the defects. Incorporation of the cyano groups resulted in the 3-fold enhancement of the PL quantum yield in the neat film of the polyconjugated derivative as compared to that of the cyano-free film. The Ip of 5.50 eV estimated for the cyano groups-containing compound was found to be higher as compared to the Ip of 5.35 eV for the cyano-free analogue, which in conjunction with the PL data for the films indicated increased electron affinity (by 0.40 eV) in the cyano groups-containing diethylene derivative.