A tris-cyclometalated iridium(III) complex of 2-(5,5-dioxido-dibenzothiophen-3-yl)pyridine: Synthesis, structural, redox and photophysical properties

The first cyclometalated dibenzothiophene S,S-dioxide derivative, namely [fac-2-(5,5-dioxidodibenzothiophen-3-yl)pyridineliridium(III) (3) has been synthesised in high yield and characterised by X-ray crystallography, solution electrochemistry, absorption and emission spectroscopy. The Ir atom has a fac-octahedral coordination with three chelating ligands (A, B and C); each Ir-N bond is in trans position to an Ir-C bond. In cyclic voltammetry experiments in dichloromethane, complex 3 undergoes a reversible metal-centred Ir-III/Ir-IV oxidation at E-1/2(ox) = +1.04 V vs. Ag/Ag+ reference electrode. Complex 3 exhibits bright green photoluminescence lambda(max) = 525 nm in toluene) from mixed (MLCT)-M-3/(3)pi ->pi* states with quantum yield (Phi(PL)) of 0.26 in toluene solution. The phosphorescence emission decay follows first order kinetics, with a lifetime of 4.9 mu s. A comparison of complex 3 with analogues 1 and 2, where the dibenzothiophene S,S-dioxide unit is replaced by 9,9-dihexylfluorene and N-hexylcarbazole respectively, establish that the substituent para to the Ir metal atom, i.e. CR2 in 1, NR in 2 and SO2 in 3, has a major influence on the redox and emission properties in this series. Organic light-emitting diodes (OLEDs) were fabricated by spin-coating techniques using a polyspirobifluorene copolymer (PSBF) as a host and complex 3 as dopant. In a single-layer blend configuration ITO/PEDOT:PSS/ PSBF:3(8 wt.-%)/Ba/Al OLEDs showed pale blue/white light emission (CIE coordinates: x = 0.29, y = 0.31) arising from a combination of fluorescence from the host copolymer (lambda(max) = 450 nm) and phosphorescence from 3 (lambda(max) = 530 nm). ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)