Synthesis, properties and Light-Emitting Electrochemical Cell (LEEC) device fabrication of cationic Ir(III) complexes bearing electron-withdrawing groups on the cyclometallating ligands
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The structure-property relationship study of a series of cationic Ir(III) complexes in the form of [Ir(C^N)2(dtBubpy)]PF6 [where dtBubpy = 4,4′-ditert-butyl-2,2′- bipyridine and C^N = cyclometallating ligand bearing an electron-withdrawing group (EWG) at C4 of the phenyl substituent, i.e. -CF3 ( 1 ), -OCF3 ( 2 ), -SCF3 ( 3 ), -SO2CF3 ( 4 )] have been investigated. The physical and optoelectronic properties of the four complexes were comprehensively characterized, including by X-ray diffraction analysis. All the complexes exhibit quasi-reversible dtBubpy-based reductions from -1.29 V to -1.34 V (vs. SCE). The oxidation processes are likewise quasi-reversible (metal+C^N ligand) and are between 1.54- 1.72 V (vs. SCE). The relative oxidation potentials follow a general trend associated with the Hammett parameter (σ) of the EWGs. Surprisingly, complex 4 bearing the strongest EWG does not adhere to the expected Hammett behavior and was found to exhibit red-shifted absorption and emission maxima. Nevertheless, the concept of introducing EWGs was found to be generally useful in blue-shifting the emission maxima of the complexes (λem = 484-545 nm) compared to that of the prototype complex [Ir(ppy)2(dtBubpy)]PF6 (where ppy = 2- phenylpyridinato) (λem = 591 nm). The complexes were found to be bright emitters in solution at room temperature (ΦPL = 45-66%) with long excited-state lifetimes (τe = 1.14-4.28 μs). The photophysical properties along with Density Functional Theory (DFT) calculations suggest that the emission of these complexes originates from mixed contributions from ligand-centered (LC) transitions and mixed metal-to-ligand and ligand-to-ligand charge transfer (LLCT/MLCT) transitions, depending on the EWG. In complexes 1 , 3 and 4 the 3LC character is prominent over the mixed 3CT character while in complex 2 , the mixed 3CT character is much more pronounced, as demonstrated by DFT calculations and the observed positive solvatochromism effect. Due to the quasi-reversible nature of the oxidation and reduction waves, fabrication of light emitting electrochemical cells (LEECs) using these complexes as emitters was possible with the LEECs showing moderate efficiencies.
Pal , A K , Cordes , D B , Slawin , A M Z , Momblona , C , Orti , E , Samuel , I D W , Bolink , H & Zysman-Colman , E 2016 , ' Synthesis, properties and Light-Emitting Electrochemical Cell (LEEC) device fabrication of cationic Ir(III) complexes bearing electron-withdrawing groups on the cyclometallating ligands ' Inorganic Chemistry , vol 55 , no. 20 , pp. 10361-10376 . DOI: 10.1021/acs.inorgchem.6b01602
Copyright 2016 American Chemical Society. This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited.
The authors are grateful to the European Research Council (grant 321305), the EPSRC (EP/M02105X/1) and the University of St Andrews for financial support. C.M. thanks Ministry of Economy and Competitiveness (MINECO, Spain) for her predoctoral contract.
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