Tris‐Cyclometalated Iridium(III) Complexes of Carbazole(fluorenyl)pyridine Ligands: Synthesis, Redox and Photophysical Properties, and Electrophosphorescent Light‐Emitting Diodes

Abstract

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 lambda(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 (lambda(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.

A Tris‐Cyclometalated Iridium(III) Complex of 2‐(5,5‐Dioxido‐dibenzothiophen‐3‐yl)pyridine: Synthesis, Structural, Redox and Photophysical Properties

“…DFT calculations predict that dibenzothiophene S,S-dioxide has a LUMO energy level of 1.04 eV lower than that of 9,9-dialkylfluorene [11] and the substantial increase in oxidation potential of 3, compared to complexes 1 and 2 (E 1/2 ox = +0.40 V [7] and + 0.14 V, [9] respectively), indicates that the bridging SO 2 group has dramatically reduced the electron density on the iridium centre. On the contrary, the carbazole nitrogen of 2 donates electron density to the Ir centre and hence the ease of oxidation in the series is 2 Ͼ 1 Ͼ 3.…”

“…As evident from their relative extinction coefficients, the absorption bands seen at around 385 nm and 450 nm are assigned to spin-allowed 1 MLCT (λ = 385 nm, ε = 19500  -1 cm -1 ) and spin-orbit coupling enhanced mixed 3 MLCT-3 LC (λ = 455 sh nm, ε ≈ 4500  -1 cm -1 ) transitions, respectively ( Table 3). The whole spectrum of 3 is blue shifted compared to analogous fluorene and carbazole complexes 1 [7] and 2, [9] indicating that the bridging SO 2 group lowers the highest occupied molecular orbital (HOMO) energy level by withdrawing electrons from the system. These assignments were made on the basis of the literature data reported for other iridium complexes.…”

“…[2][3][4][5] Colour tuning in these complexes is achieved either by the peripheral functionalisation [2,3] of the parent 2-phenylpyridine with electron-withdrawing/donating substituents, or through variation of the cyclometalating ligands. [2,4] With regard to the latter, 2-(9,9-dialkylfluoren-2-yl)pyridine-based iridium(III) complexes 1 (alkyl = n-hexyl [6,7] or methyl [8] ) and 2-(Nalkylcarbazol-2-yl)pyridine-based iridium(III) complexes 2 (alkyl = hexyl [9] or ethyl [10] ) have been explored by us and others.…”

Recent Developments in the Application of Phosphorescent Iridium(III) Complex Systems

“…[43] A general protocol for the synthesis of Ir(ppy) 3 in high yields, starting from Ir(acac) 3 and Hppy, was described by the same group in 1991. [30] Since then, numerous variations of the basic Ir(ppy) 3 structure (e.g., introduction of electronwithdrawing or electron-donating substituents; [44][45][46][47][48][49] extension of the p-conjugated system; [50][51][52][53][54][55][56] replacement of the pyridine ring by other N-heteroaromatic rings; [26,57] or lateral functional groups for post-complexation modifications [58,59] ) have been reported (Scheme 1). The outstanding role of tris-cyclometallated Ir III complexes (both homoleptic and heteroleptic) based on phenylpyridine-type derivatives as ligands is underlined by the tremendous number of scientific publications and patents dealing with the synthesis and/or application of the respective complexes (June 2009: nearly 2000 hits in SciFinder).…”

New Iridium Complex Coordinated with Tetrathiafulvalene Substituted Triazole-pyridine Ligand: Synthesis, Photophysical and Electrochemical properties

“…Additionally, complex 1 show a irreversible oxidation peak (E p ox ) at 1.88 V, which is attributed to the metal-centered Ir 3+ /Ir 4+ oxidation couple. 21,22 In comparison with the ligand L, the two oxidation waves for complex 1 are shifted to more negative potentials. The observed results are different from the previous reported work, 23 it is possible that the triazole-pyridine unit is grafted on the TTF core through a non-conjugated spacer group, which is disadvantageous to intramolecular electron transfer and communications.…”