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Tris‐Cyclometalated Iridium(III) Complexes of Carbazole(fluorenyl)pyridine Ligands: Synthesis, Redox and Photophysical Properties, and Electrophosphorescent Light‐Emitting Diodes
Author(s) -
Bettington Sylvia,
Tavasli Mustafa,
Bryce Martin R.,
Beeby Andrew,
AlAttar Hameed,
Monkman Andrew P.
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200600888
Subject(s) - homoleptic , phosphorescence , iridium , pyridine , carbazole , chemistry , ligand (biochemistry) , moiety , crystallography , redox , oled , stereochemistry , metal , photochemistry , medicinal chemistry , fluorescence , inorganic chemistry , physics , catalysis , organic chemistry , biochemistry , receptor , layer (electronics) , quantum mechanics
Using ligands synthesized by Suzuki cross‐coupling methodology, new phosphorescent homoleptic tris‐cyclometalated complexes have been obtained, namely fac ‐[Ir(Cz‐2‐Fl n Py) 3 ] ( 1 d – f ) and fac ‐[Ir(Cz‐3‐Fl n Py) 3 ] ( 2 d – f ), which are solution‐processible triplet emitters (Cz denotes N ‐hexylcarbazole, n is the number of 9,9′‐dihexylfluorene (Fl) units ( n =0,1,2) and Py is pyridine). In all cases, Py and Fl are substituted at the 2‐ and 2,7‐positions, respectively, and Cz moieties are substituted by either Py or Fl at the 2‐ or 3‐positions, in series 1 and 2 , respectively. The oxidation potential of 1 d studied by cyclic voltammetry ( ${E{{{\rm ox}\hfill \atop 1/2\hfill}}}$ =0.14 V, versus Ag/AgNO 3 , CH 2 Cl 2 ) is less positive (i.e. raised HOMO level) compared to that of the isomer 2 d ( ${E{{{\rm ox}\hfill \atop 1/2\hfill}}}$ =0.30 V), where the Cz‐nitrogen is meta to the Ir center. Ligand‐centered oxidations occur at more positive potentials, leading to 7+ oxidation states with good chemical reversibility and electrochemical quasi‐reversibility, for example, for 2 f ${E{{{\rm ox}\hfill \atop {\rm pa}\hfill}}}$ =0.45 (1e), 0.95 (3e), 1.24 V (3e). Striking differences are seen in the solution‐state photophysical data between complexes [Ir(Cz‐2‐Py) 3 ] ( 1 d ) and [Ir(Cz‐3‐Py) 3 ] ( 2 d ), in which the Cz moiety is bonded directly to the metal center: for the latter there is an 85 nm blue‐shift in emission, a decrease in the luminescence lifetime and an increase in the PLQY value. Organic light emitting devices were made by spin‐coating using polyspirobifluorene:bis(triphenyl)diamine (PSBF:TAD) copolymer as host and the complexes 1 d or 2 d as dopants. Turn‐on voltages are low (3–4 V). With 1 d orange light is emitted at λ max =590 nm with an EQE of 1.3 % (at 7.5 mA cm −2 ) and an emission intensity (luminance) of 4354 cd m −2 (at 267 mA m −2 ). The green emission from 2 d devices ( λ max =500 nm) is due to the reduced electron‐donating ability of the carbazole unit in 2 d . Recording the EL spectra of the 1 d device at 6 V (current density, 100 mA cm −2 ) established that the time to half brightness was about 9 h under continuous operation with no change in the spectral profile, confirming the high chemical stability of the complex.