Materiais moleculares funcionais contendo n-heterociclos - capes

Liquid Crystals, Vol. 33, No. 4, April 2006, 381–390Synthesis and characterization of luminescent hockey stick-shapedliquid crystalline compoundsRODRIGO CRISTIANO, ANDRÉ ALEXANDRE VIEIRA, FERNANDO ELY and HUGO GALLARDO*Departamento de Química, Universidade Federal de Santa Catarina, CEP 88040-900, Florianópolis, SC, Brazil(Received 15 July 2005; accepted 13 October 2005 )A general synthetic strategy, based on a convergent approach, allowed us to prepare a seriesof luminescent unsymmetrical bent-core compounds (2,5-(disubstituted)-1,3,4-oxadiazolederivatives), via the Sonogashira crosscoupling reaction, all possessing a similar hockey stickshape. Their mesophases were characterized using polarizing optical microscopy anddifferential scanning calorimetry. The observed LC phases possess the classical textures ofcalamitic liquid crystals. Fluorescence in solution for these compounds exhibits strong blueemission (l max em. 5390–460 nm) with good quantum yields (50–85%).1. IntroductionThe design of novel thermotropic liquid crystals asadvanced functional materials involves suitable selectionof a core fragment, linking group, and terminalfunctionality. However, anisometric rodlike- ordisklike-shaped molecules used to be a fundamentalprerequisite for conventional thermotropic liquid crystalformation, because steric packing considerationsplay an important role in this interesting state of softmatter [1]. More recently, with the discovery of bananashapedliquid crystals, where bent molecules serve as acore, interest in the incorporation of nonlinear units hasgained importance. The study of the mesomorphicproperties of these compounds, which has gainedconsiderable importance since the discovery of polarswitching in such achiral compounds, and the subsequentdetailed analysis of the switching process [2], aremainly driven by their potential for application indisplay technology [3, 4]. A large number of bent-coreor banana-shaped mesogens have been synthesized toexamine their novel mesophase structures [5–7]. Most ofthe compounds reported in the literature are symmetricalaround a central phenyl ring and are derived from1,3-dihydroxybenzene or 2,7-dihydroxynaphthalene [8,9]. Usually these are five- or six-ring compounds andare substituted with two terminal alkyl/alkoxy chains.In recent years a wide range of emissive materials hasbeen reported for use in electroluminescent (EL)devices; these vary from low molecular mass moleculesto processable polymers [10]. In addition, highly*Corresponding author. Email: hugo@qmc.ufsc.brconjugated liquid crystals are desirable as the selforganizingproperties of these materials can be exploitedto improve device performance and to achieve linearlypolarized electroluminescence [11]. For example,Contoret et al. [12] have reported polarized electroluminescencefrom a nematic network, offering a possiblesubstitute for one of the polarizers and the back light ofTN-LCDs and STN-LCDs, with a lower power consumptionand/or a higher brightness. Among otherimportant aspects are their high charge carrier mobilityobserved in hexagonal discotic (Col h ), helical (H) andsmectic A, B phases and an unidentified smectic X phase[13], and their ability to develop defect-free layers [14].Functional LCs can be obtained by incorporating newproperties into the same molecule, besides its inherentself-organization, such as luminescence. This combinationhas led to intrinsically luminescent mesogens, whichare able to form ordered aggregates with large carriermobilities for EL devices. In this context, our interest hasbeen focused on luminescent liquid crystals containingthe heterocycle 1,3,4-oxadiazole [15, 16]. 1,3,4-Oxadiazole derivatives are well known for their highthermal and hydrolytic stability, resistance to oxidativedegradation and electron-accepting properties [17]. Theyusually exhibit a high photoluminescent quantum yield[18]. The introduction of an oxadiazole ring into amesogenic core can provide a lateral dipole from oxygenand nitrogen atoms and also a bent rigid core [19].In this paper, we report the synthesis, mesomorphicbehaviour and photophysical properties ofseveral unsymmetrical bent-core compounds basedon 2-(4-decyloxyphenyl)-5-(4-phenylethynylaryl)-1,3,4-oxadiazole (figure 1).Liquid CrystalsISSN 0267-8292 print/ISSN 1366-5855 online # 2006 Taylor & Francishttp://www.tandf.co.uk/journalsDOI: 10.1080/02678290500483304