Rhenium complex as emitting material in highly efficient phosphorescent organic light-emitting diodes

Interest in luminescent materials able to efficiently emit in the solid state is continuously growing, because in most applications the dyes are used as solid films. This is the case of the Organic Light Emitting Diodes (OLEDs), where electroluminescent metal complexes have been widely investigated as dopants, able to increase their performances [1]. We present here a dinuclear Re(I) complexes (1, see Scheme 1) whose emission is higher in solid state and in PMMA matrix than in solution and is used as dopant in OLEDs. This complex belong to the family of neutral Re(I) complexes with general formula [Re2(CO)6(μ-1,2-diazine)(μ-X)2], where X is halogen. Some of them have recently gained interest for their intense yellow/green emission, occurring from triplet metal-to-ligand charge transfer (3MLCT) states [2], showing a modulation effect of the diazine substituents on wavelengths, lifetimes and quantum yields of the emission. Φ up to 0.53 has been measured for the di-chloro complexes containing diazines bearing alkyl groups in both the β positions [3]. At variance with the chloro derivatives, the bromo analogue 1 is almost not-emitting in solution (Φem 0.002 in deareated toluene). We have now found that in the solid state 1 shows intense yellow-orange emission (Φem 0.50 in neat powder), blue shifted with respect to the solution (560 nm vs. 620 nm). The emission is intense also in PMMA matrix (Φem 0.19 in PMMA at 10% w/w, 556 nm). This very high aggregated emission here is discussed in terms of the restriction of the intramolecular roto-vibrational motions of the “Re2(CO)6(μ-Br)2” scaffold imposed by the rigid environment, as evidenced by the strong decrease of the knr. Moreover a parallel increase of kr is noticed, which clearly shows the influence of the halides in determining not only the energy of the excited state, but also the nature of the lowest (emitting) state. These properties allowed its successful use as a phosphorescent dopant in OLEDs and here we report the performances of OLED devices prepared both by solution- and vacuumprocessing. The values of the external quantum efficiency are 1.7% (4.3 cd/A and 1.7 lm/W) and 10% (29.1 cd/A and 22.1 lm/W) respectively and are the highest ever reported for rhenium-based devices, comparable to state-of-the-art devices employing Ir(III)- and Pt(II)- based metal complexes. The combination of synthetic strategy and encouraging results in potential OLEDs applications should make these dinuclear Re(I) complexes highly attractive to a broad spectrum of research fields. 1. “Highly efficient OLEDs with Phosphorescent Materials” H. Yersin Ed. Wiley-VCH, (2008). 2. D. Donghi, G. D’Alfonso, M. Mauro, M. Panigati, P. Mercandelli, A. Sironi, P. Mussini, L. D’Alfonso, Inorg. Chem. 28 (2008) 4243-4255. 3. M. Mauro, E. Quartapelle Procopio, Y. Sun, C. H. Chien, D. Donghi, M. Panigati, P. Mercandelli, P. Mussini, G. D’Alfonso, L. De Cola, Adv. Funct. Mater. 19 (2009) 2607-2614.