Synthesis, Properties, and Light-Emitting Electrochemical Cell (LEEC) Device Fabrication of Cationic Ir(III) Complexes Bearing Electron-Withdrawing Groups on the Cyclometallating Ligands.
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'-di-tert-butyl-2,2'-bipyridine and C^N = cyclometallating ligand bearing an electron-withdrawing group (EWG) at C4of the phenyl substituent, i.e., -CF3(1), -OCF3(2), -SCF3(3), -SO2CF3(4)] has been investigated. The physical and optoelectronic properties of the four complexes were comprehensively characterized, including by X-ray diffraction analysis. All the complexes exhibit quasireversible dtBubpy-based reductions from -1.29 to -1.34 V (vs SCE). The oxidation processes are likewise quasireversible (metal + C^N ligand) and are between 1.54 and 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 microsecond 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 the3LC character is prominent over the mixed3CT character, while in complex 2, the mixed3CT character is much more pronounced, as demonstrated by DFT calculations and the observed positive solvatochromism effect. Due to the quasireversible 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.
Organic Semiconductor Centre, EaStCHEM School of Chemistry, University of St Andrews , St Andrews, Fife KY16 9ST, United Kingdom.
Amlan K
AK
Pal
EaStCHEM School of Chemistry, University of St Andrews , St Andrews, Fife KY16 9ST, United Kingdom.
David B
DB
Cordes
EaStCHEM School of Chemistry, University of St Andrews , St Andrews, Fife KY16 9ST, United Kingdom.
Alexandra M Z
AM
Slawin
Instituto de Ciencia Molecular, Universidad de Valencia , C/J. Beltran 2, 46980 Paterna, Spain.
Cristina
C
Momblona
Instituto de Ciencia Molecular, Universidad de Valencia , C/J. Beltran 2, 46980 Paterna, Spain.
Enrique
E
Ortı
Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St Andrews , St Andrews, Fife KY16 9SS, United Kingdom.
Ifor D W
ID
Samuel
Instituto de Ciencia Molecular, Universidad de Valencia , C/J. Beltran 2, 46980 Paterna, Spain.
Henk J
HJ
Bolink
Organic Semiconductor Centre, EaStCHEM School of Chemistry, University of St Andrews , St Andrews, Fife KY16 9ST, United Kingdom.
Eli
E
Zysman-Colman
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