Pyrazolyl-substituted polyconjugated molecules for optoelectronic applications

Pyrazolyl-substituted diethylene derivative 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 (I-p) of the films as well as on the photoluminescence (PL) spectrum, PL quantum yield (eta) and PL decay time of the dilute solutions and thin films of the pyrazole derivatives was studied. PL measurements revealed that highly luminescent (eta = 0.88) cyano-free pyrazole derivative in solution became non-emissive (eta = 0.01) 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 tight packing of the pyrazole derivative molecules, thus significantly reducing migration-induced quenching of the excitons at the defects. Incorporation of the cyano groups resulted in the two-fold enhancement of the PL quantum yield in the film of the pyrazole derivative as compared to that of the cyano-free film. The I-p of 5.90 eV estimated for the cyano groups-containing compound was found to be higher as compared to the I-p of 5.46 eV for cyano-free analogue, what in conjunction with the PL data for the films indicates increased electron affinity in the pyrazole derivative with additional cyano groups. (C) 2009 Elsevier Ltd. All rights reserved.