Dalton Transactions Dynamic Article Links Cite this: Dalton Trans., 2012, 41, 8502 www.rsc.org/dalton COMMUNICATION Syntheses and photophysical properties of new iminopyrrolyl boron complexes and their application in efficient single-layer non-doped OLEDs prepared by spin coating† D. Suresh, a Clara S. B. Gomes, a Pedro T. Gomes,* a Roberto E. Di Paolo, a António L. Maçanita, a Maria José Calhorda, b Ana Charas, c Jorge Morgado c,d and M. Teresa Duarte a Received 16th February 2012, Accepted 16th April 2012 DOI: 10.1039/c2dt30487b Efficient non-doped OLEDs have been achieved using new binuclear tetracoordinate organoboron complexes containing 2-(N-aryl)formiminopyrrolyl ligands. Tetracoordinate mononuclear boron compounds containing che- lating N,O-, N,N- and N,C-chromophores are a family of efficient emitters, and several complexes have been designed and synthesised. 1 Variations on the chromophore part of the molecule influence the HOMO (highest occupied molecular orbital)– LUMO (lowest unoccupied molecular orbital) energies and thereby the colour of emission. 1a,c In addition, some of these compounds exhibit electron-transport properties and, conse- quently, can be employed in making photoluminescent (PL) and electroluminescent (EL) devices, including organic light-emit- ting diodes (OLEDs) and sensors. 1a–c,2 Nevertheless, their per- formances have been under par. Recently, a rapid progress has been observed in the production of low-cost, high efficiency materials for making OLEDs, mainly for application in flat-panel displays. 3 For high performance OLEDs, the intense lumines- cence and high carrier mobility are the two most important par- ameters, which are accomplished with molecules having planar geometry along with extended π-conjugated systems. 4 Recent reports suggested that compounds containing π-conjugated ladder-type skeletons coordinated to multi Lewis acidic boryl groups constrain the π-conjugated framework to intensify the emission, and enhance the electron-transport properties. 5 This indicates that the incorporation of multiboron centres into rigid conjugated π-systems may be an ideal synthetic strategy to achieve high performance OLEDs. Only few rigid multiboron- containing π-systems have been obtained to date owing to the lack of efficient synthons, whose preparation involves several reaction stages. The 2-(N-aryl)formiminopyrrole ligand precursors are an example of such synthons where the extended π-conjugation moiety can be readily attained by incorporating various aromatic spacers via condensation reactions. 6 The π-conjugation can also be extended through fusing aromatic groups on the edges of pyrrole ring, as reported by our group with blue/green light emit- ting 2-(N-aryl)formiminophenanthro[9,10-c]pyrrolyl zinc com- plexes. 7 However, their potential application as OLEDs was not successful. This prompted us to synthesise and characterise new boron complexes of 2-(N-aryl)formiminopyrrolyl ligands and study their remarkable photoluminescent and electroluminescent properties. The 2-(N-aryl)formiminopyrrolyl ligand precursors 1–3 herein reported were prepared and characterised according to the reported literature methods. 6,7 Refluxing the 2-(N-aryl)formimi- nopyrrole ligand precursors 1–3 and triphenylborane in toluene, followed by crystallisation, afforded the target compounds 4–6 in good yields (Scheme 1). These complexes were completely characterised by multinuclear NMR spectroscopy, elemental ana- lyses and/or single crystal X-ray diffraction. 8 All the complexes are bright yellow solids, which show fluorescence in solution and in solid state, and are sensitive to air and moisture. The molecular structure of 5 is depicted in Fig. 1 (4 is depicted in ESI†), along with the selected metric parameters. It is notable that, in both compounds, the boron atom has a charac- teristic pseudo-tetrahedral geometry, in which the chelating 2-(N- aryl)formiminopyrrolyl ligands show bite angles (N–B–N) of 94.95(10) and 94.87(15)°, and C–B–C angles of 115.56(11) (C13–B1–C19) and 115.79(17)° (C10–B–C16), for 4 and 5, respectively. The average B–C bond distances are 1.612 Å for both complexes. The two B–N distances (1.6327(19) and † Electronic supplementary information (ESI) available: Experimental details: syntheses and characterisation of compounds 4–6; X-ray crystal data and structure refinements for complexes 4 and 5 (Table S1), mol- ecular structure of 4 (Fig. S1) and crystal packing of 4 and 5 (Fig. S2 and S3); spectroscopic measurements and fluorescence decays (Fig. S4– S6); computational studies including calculated absorption spectra of and molecular orbitals of 4 (Fig. S7 and S8); voltammograms of com- plexes 4–6 (Fig. S9); OLED fabrication and characterisation (Fig. S10 and S11); energy level diagram of the components involved in the “single-layer” devices based on complexes 4–6 (Fig. S12). CCDC 867381 and 867382. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2dt30487b a Centro de Química Estrutural, Departamento de Engenharia Química, Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal. E-mail: pedro.t.gomes@ist.utl.pt; Tel: +351 218419612 b Departamento de Química e Bioquímica, CQB, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, Ed. C8, 1749-016 Lisboa, Portugal c Instituto de Telecomunicações, Av. Rovisco Pais, 1049-001 Lisboa, Portugal d Departamento de Bioengenharia, Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal 8502 | Dalton Trans., 2012, 41, 8502–8505 This journal is © The Royal Society of Chemistry 2012 Downloaded by Universidade Tecnica de Lisboa (UTL) on 27 June 2012 Published on 17 April 2012 on http://pubs.rsc.org | doi:10.1039/C2DT30487B View Online / Journal Homepage / Table of Contents for this issue