PAPER www.rsc.org/dalton | Dalton Transactions Heteroleptic platinum(II) complexes of 8-quinolinolates bearing electron withdrawing groups in 5-position† Fabian Niedermair, a Ohyun Kwon, b Karin Zojer, c Stefan Kappaun, d Gregor Trimmel, a Kurt Mereiter e and Christian Slugovc* a Received 20th March 2008, Accepted 12th May 2008 First published as an Advance Article on the web 19th June 2008 DOI: 10.1039/b804832k A series of novel luminescent platinum(II) complexes bearing orthometalated 2-phenylpyridine ligands (C∧N), namely 2-phenylpyridine (4) and 3-hexyloxy-2-phenylpyridine (5), and several 5-substituted quinolinolate ligands (5-X-Q), where X = NO 2 (a), X = CHO (b), X = Cl (bearing another Cl in 7-position of the Q-ligand) (c) and X = H(d) have been synthesized, characterized and their photophysical properties were studied. All complexes were obtained as a single isomer with N atoms of the C∧N and Q ligands trans-coordinated to the platinum center as evidenced using single-crystal X-ray crystallography and NMR spectroscopy. Absorbance, luminescence as well as lifetime measurements in solution and in the solid state have been performed to establish a qualitative relationship between structure and luminescence properties. The compounds under investigation absorb intensively via an intraligand charge transfer (ILCT) in the visible range (460–480 nm) and emit from fluid solution and in the solid state at room temperature at 600–630 nm. The complexes show quantum yields up to 25% and lifetimes in the range of 20–30 ls in deoxygenated organic solvents at room temperature. The emitting state can be best described as a triplet intraligand charge-transfer state localized mainly on the quinolinolate ligand. In these complexes the phenylpyridine ligand can be essentially regarded as an ancillary ligand. Density functional theory (DFT) calculations were carried out on both the ground (singlet) and excited (triplet) states of these complexes and revealed the influence of the substitution of the quinolinolate ligand on the HOMO/LUMO energies and the oscillator strengths. Substitution on 3-position of the phenylpyridine ligand does not impact on the transition energies, and is thus suited to introduce other functional moieties, such as a solubilizing hexyloxy group. Introduction In the last couple of years research on luminescent platinum(II) complexes has been focused on their potential applications in different fields such as chemical sensors, 1 organic light emitting devices (OLEDs), 2 or photovoltaics. 3 Emission from Pt(II) com- plexes is typically assigned to either ligand centered (LC) or metal- to-ligand charge transfer (MLCT) states. Luminescence quantum yields of the complexes are greatly affected by the accessibility a Institute for Chemistry and Technology of Organic Materials (ICTOS), Graz University of Technology, Stremayrgasse 16, A 8010, Graz, Austria. E-mail: slugovc@tugraz.at; Fax: +43 316 873 8951; Tel: +43 316 873 8454 b School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, 30332-0400, USA c Institute of Theoretical and Computational Physics, Graz University of Technology, Petersgasse 16, A 8010, Graz, Austria d NanoTecCenter Weiz Forschungsgesellschaft mbH, Franz-Pichler Strasse 32, A 8160, Weiz, Austria e Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9/164, A 1060, Vienna, Austria †Electronic supplementary information (ESI) available: X-Ray crystal- lographic data file for 5a; synthesis and characterization of 4b, 5b, 4c and 4d, absorption and emission data for all compounds, life-time measurements in films, calculated excitation energies, dominant single electron excitations, and oscillator strengths from TDDFT calculations and calculated triplet state energies. CCDC reference number 682647. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b804832k of the platinum centered excited state through population of the d x 2 −y 2 as well as the d z 2 orbitals, which opens efficient canals of nonradiative decay. 2c,4–8 A strategy to overcome this situation is employing ligands with a very strong ligand field, to raise the energy of the d–d state, which is realized by using cyclometalated ligands. A second strategy to enable efficient emission of Pt(II) complexes tempts to provide a low-lying ligand centered emitting excited state, which would not be quenched by the d–d state. 9 When following the first strategy, using cyclometalating ligands, typically 2-arylpyridine or 2-(2 ′ -thienyl)pyridine derivatives are used. The resulting homoleptic 10 and heteroleptic complexes bearing either two cyclometalated ligands 11 or a single cyclomet- alating ligand and a non-cyclometalating ancillary ligand 12 have been reported to be emissive in solution and in the solid state with luminescence lifetimes in the microsecond range indicating emission from the triplet excited states. Although the homoleptic Pt(II) complexes exhibit desirable photophysical properties for different applications, heteroleptic Pt(II) complexes offer several advantages such as facilitated preparation and an easy tuning of solubility as well as photophysical properties. However, commonly used luminescent materials such as b-diketonate Pt complexes with e.g. acetylacetonate (acac) as ancillary ligand have been shown to be unstable in the presence of PEDOT:PSS in elec- troluminescent devices. 13 Another general disadvantage of acac complexes is that efficiency can significantly decrease at higher current densities. 14 Secondly, acac complexes tend to decompose at 4006 | Dalton Trans., 2008, 4006–4014 This journal is © The Royal Society of Chemistry 2008