Pyrazolyl-substituted polyconjugated molecules for optoelectronic applications E. Arba  ciauskien _ e a , K. Kazlauskas b, * , A. Miasojedovas b , S. Jur  s _ enas b , V. Jankauskas c , W. Holzer d , V. Getautis a , A.  Sa  ckus a, ** a Department of Organic Chemistry, Kaunas University of Technology, Radvil _ enų pl. 19, LT-50270, Kaunas, Lithuania b Institute of Applied Research, Vilnius University, Saul _ etekio 9-III, LT-10222, Vilnius, Lithuania c Department of Solid State Electronics, Vilnius University, Saul _ etekio 9-III, LT-10222, Vilnius, Lithuania d Department of Drug and Natural Product Synthesis, Faculty of Life Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria article info Article history: Received 17 June 2009 Received in revised form 13 October 2009 Accepted 17 October 2009 Available online 28 October 2009 Keywords: Pyrazole Cyano groups Fluorescence Ionization potential Charge carrier mobility abstract 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 (h) and PL decay time of the dilute solutions and thin films of the pyrazole derivatives was studied. PL measurements revealed that highly luminescent (h ¼ 0.88) cyano-free pyrazole derivative in solution became non-emissive (h ¼ 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. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Low-molecular-weight functional dyes exhibiting both emission and charge transport are of interest for the application in opto- electronic devices such as organic light emitting diodes (OLEDs) [1]. Although much progress has been achieved on OLEDs, there are still some challenges of improving the performance and durability of OLEDs for displays and lighting applications. One of the key issues for developing the next generation high-performance OLEDs is the design and synthesis of multifunctional materials with both light emission and charge transport properties. The molecules consisting of polyconjugated structure end-capped with heteroaromatic or polycyclic aromatic chromophores are known to be highly fluo- rescent [2e4]. Vinylene linkage that is often employed in the design of the polyconjugated molecules serve not only as a “conjugated bridge”, it also improves electroluminescent properties by reducing steric interactions between aromatic rings, and thus, decreasing the free rotation between units [5]. Pyrazoline derivatives are known to exhibit efficient blue emission and hole-transport performance, and therefore, might serve as the multifunctional materials for light emitting devices [6]. As distinct from pyrazoline, aromatic pyrazole is much less studied and exploited as electroactive moiety. Introduction of cyano groups into the vinylene linkages of the polyconjugated materials has an advantage of increasing electron affinity, and therefore, greatly improving electron injection from the metal contact of an OLED [7]. Incorporation of cyano moieties into fluorescent compounds also facilitates formation of highly fluorescent organic nanoparticles, which have potential applica- tions in on/off fluorescence sensors [8]. To this end, pyrazolyl-substituted diethylene derivative 1,4-bis {(E)-2-[(1-biphenylyl-3-hexyloxy)pyrazol-4-yl]ethenyl}benzene (P1) and its analogue with cyano moieties 1,4-bis{(E)-1-nitrile-2- [(1-biphenylyl-3-hexyloxy)pyrazol-4-yl]ethenyl}benzene(P2) have been synthesized and characterized. Here, the photophysical and * Corresponding author. Tel.: þ370 52 366032; fax: þ370 52 366059. ** Corresponding author. Tel.: þ370 37 300194; fax: þ370 37 451432. E-mail addresses: karolis.kazlauskas@ff.vu.lt (K. Kazlauskas), algirdas.sackus@ ktu.lt (A.  Sa ckus). Contents lists available at ScienceDirect Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig 0143-7208/$ e see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.dyepig.2009.10.007 Dyes and Pigments 85 (2010) 79e85