Efficient narrowband yellow organic light-emitting diodes based on iridium(III) complexes with the rigid indolo[3,2,1-jk]carbazole unit

Phosphorescent material with narrowband emission is crucial for advancing wide-color-gamut organic light-emitting diodes (OLEDs). In this work, two iridium(III) complexes, (PhthzICz)2Ir(tmd) and (thzICz)2Ir(tmd), using rigid 2-(benzothiazole-2-yl)indolo[3,2,1-jk]carbazole (PhthzICz) and 2-(thiazole-2-yl)indolo[3,2,1-jk]carbazole (thzICz) as cyclometalated ligands and 2,2,6,6-tetramethyl-3,5-heptanedione (tmd) as ancillary ligands, were synthesized. When these complexes were doped into the host material 3,3'-di(9H-carbazol-9-yl)-1,1'-biphenyl, the doped films exhibited yellow photoluminescence (PL) peaking at 537 and 531 nm, full width at half maximum (FWHM) bands of 35 and 60 nm, and PL quantum yields of 89.9% and 85.9%, respectively. OLEDs based on these two emitters display moderate performance characteristics with maximum external quantum efficiencies of 25.2% and 22.7%. Notably, the device based on (PhthzICz)2Ir(tmd) exhibits a narrow FWHM of 31 nm. Overall, the study highlights the practicality of incorporating rigid groups into the cyclometalated ligands of Ir(III) complexes as a viable strategy for achieving efficient Ir(III) complexes for OLEDs with narrow emission and high efficiency.

Luminescent cis-Iridium(III) Complex Based on a Bis(6,7-dimethoxy-3,4-dihydroisoquinoline) Platform Featuring an Unusual cis Orientation of the C∧N Ligands: From a Theoretical Approach to a Deep Red LEEC Device

By pursuing the strategy of manipulating natural compounds to obtain functional materials, in this work, we report on the synthesis and characterization of a luminescent cationic iridium complex (cis-1), designed starting from the catecholic neurotransmitter dopamine, exhibiting the unusual cis arrangement of the C∧N ligands. Through an integrated experimental and theoretical approach, it was possible to delineate the optoelectronic properties of cis-1. In detail, (a) a series of absorption maxima in the range 300-400 nm was assigned to metal-to-ligand charge transfer and weak and broad absorption maxima at longer wavelengths (400-500 nm) were ascribable to spin-forbidden transitions with a mixed character; (b) there was an intense red phosphorescence with emission set in the range 580-710 nm; and (c) a highest occupied molecular orbital was mainly localized on the metal and the 2-phenylpiridine ligand and a lowest unoccupied molecular orbital was localized on the N∧N ligand, with a ΔH-L set at 2.20 eV. This investigation allowed the design of light-emitting electrochemical cell (LEEC) devices endowed with good performance. The poor literature reporting on the use of cis-iridium(III) complexes in LEECs prompted us to investigate the role played by the selected cathode and the thickness of the emitting layer, as well as the doping effect exerted by ionic liquids on the performance of the devices. All the devices exhibited a deep red emission, in some cases, quite near the pure color (devices #1, #4, and #8), expanding the panorama of the iridium-based red-to-near-infrared LEEC devices. The characteristics of the devices, such as the brightness reaching values of 162 cd/m2 for device #7, suggested that the performances of cis-1 are comparable to those of trans isomers, opening new perspective toward designing a new set of luminescent materials for optoelectronic devices.

Blue-to-Green Emitting Neutral Ir(III) Complexes Bearing Pentafluorosulfanyl Groups: A Combined Experimental and Theoretical Study

A structure-property relationship study of neutral heteroleptic (1 and 2, [Ir(C^∧N)₂(L^∧X)]) and homoleptic (3 and 4, fac-[Ir(C^∧N)₃]) Ir(III) complexes (where L^∧X = anionic 2,2,6,6-tetramethylheptane-3,5-dionato-κO³,κO⁶ (thd) and C^∧N = a cyclometalating ligand bearing a pentafluorosulfanyl (-SF₅) electron-withdrawing group (EWG) at the C₄ (HL1) and C₃ (HL2) positions of the phenyl moiety) is presented. These complexes have been fully structurally characterized, including by single-crystal X-ray diffraction, and their electrochemical and optical properties have also been extensively studied. While complexes 1 ([Ir(L1)₂(thd)]), 3 (Ir(L1)₃), and 4 (Ir(L2)₃) exhibit irreversible first reduction waves based on the pentafluorosulfanyl substituent in the range of -1.71 to -1.88 V (vs SCE), complex 2 ([Ir(L2)₂(thd)]) exhibits a quasi-reversible pyridine_C^∧N-based first reduction wave that is anodically shifted at -1.38 V. The metal + C^∧N ligand oxidation waves are all quasi-reversible in the range of 1.08-1.54 V (vs SCE). The optical gap, determined from the lowest energy absorption maxima, decreases from 4 to 2 to 3 to 1, and this trend is consistent with the Hammett behavior (σ_m/σ_p with respect to the metal-carbon bond) of the -SF₅ EWG. In degassed acetonitrile, for complexes 2-4, introduction of the -SF₅ group produced a blue-shifted emission (λ_em 484-506 nm) in comparison to reference complexes [Ir(ppy)₂(acac)] (R1, where acac = acetylacetonato) (λ_em 528 nm in MeCN), [Ir(CF₃-ppy) (acac)] (R3, where CF₃-ppyH = 2-(4-(trifluoromethyl)phenyl)pyridine) (λ_em 522 nm in DCM), and [Ir(CF₃-ppy)₃] (R8) (λ_em 507 nm in MeCN). The emission of complex 1, in contrast, was modestly red shifted (λ_em 534 nm). Complexes 2 and 4, where the -SF₅ EWG is substituted para to the Ir-C_C^∧N bond, are efficient phosphorescent emitters, with high photoluminescence quantum yields (Φ_PL = 58-79% in degassed MeCN solution) and microsecond emission lifetimes (τ_ε = 1.35-3.02 μs). Theoretical and experimental observations point toward excited states that are principally ligand centered (³LC) in nature, but with a minor metal-to-ligand charge-transfer (³MLCT) transition component, as a function of the regiochemistry of the pentafluorosulfanyl group. The ³LC character is predominant over the mixed ³CT character for complexes 1, 2, and 4, while in complex 3, there is exclusive ³LC character as demonstrated by unrestricted density functional theory (DFT) calculations. The short emission lifetimes and reasonable Φ_PL values in doped thin film (5 wt % in PMMA), particularly for 4, suggest that these neutral complexes would be attractive candidate emitters in organic light-emitting diodes.

Synthetic control of spectroscopic and photophysical properties of triarylborane derivatives having peripheral electron-donating groups

The spectroscopic and photophysical properties of triarylborane derivatives were controlled by the nature of the triarylborane core (trixylyl- or trianthrylborane) and peripheral electron-donating groups (N,N-diphenylamino or 9H-carbazolyl groups). The triarylborane derivatives with and without the electron-donating groups showed intramolecular charge-transfer absorption/fluorescence transitions between the π orbital of the aryl group (π(aryl)) and the vacant p orbital on the boron atom (p(B), π(aryl)-p(B) CT), and the fluorescence color was tunable from blue to red by the combination of peripheral electron-donating groups and a triarylborane core. Detailed electrochemical, spectroscopic, and photophysical studies of the derivatives, including solvent dependences of the spectroscopic and photophysical properties, demonstrated that the HOMO and LUMO of each derivative were determined primarily by the nature of the peripheral electron-donating group and the triarylborane core, respectively. The effects of solvent polarity on the fluorescence quantum yield and lifetime of the derivatives were also tunable by the choice of the triarylborane core.

Singlet Generation from Triplet Excitons in Fluorescent Organic Light-Emitting Diodes

A potential major drawback with organic light-emitting devices, (OLEDs) is the limit of 25% singlet exciton production through spin-dependent charge recombination. Recent device results, however, show that this limit does not hold and far higher efficiencies can be achieved in purely fluorescent-based systems (Wohlgenannt et al. (2001), Dhoot et al. (2002), Lin et al. (2003), Wilson et al. (2001), Cao et al. (1999), Baldo et al. (1999), and Kim et al. (2000)). Thus, the question arises; is recombination spin dependent (Tandon et al. (2003)) or are singlet excitons generated in secondary processes? Direct measurement of the singlet generation rate in working devices of 44% has been shown (Rothe et al. (2006)), which have been verified as being part due to direct singlets formed on recombination and part from triplet fusion, singlets produced during triplet annihilation (Kondakov et al. (2009), King et al. (2011), and Zhang and Forrest (2012)). Here, the various routes by which triplet excitons can generate singlet states are discussed and their relative contributions to the overall electroluminescence yield are given. The materials requirements to obtain maximum singlet production from triplet states are discussed. These triplet contributions can give very high device yields for fluorescent emitters, which in the case of blue devices can be highly advantageous. Further, new devices architectures open up which are simple and have intrinsically low turn on voltages, ideal for large-area OLED lighting applications.

Improving the balance of carrier mobilities of host-guest solid-state light-emitting electrochemical cells

We report efficient host-guest solid-state light-emitting electrochemical cells (LECs) utilizing a cationic terfluorene derivative as the host and a red-emitting cationic transition metal complex as the guest. Carrier trapping induced by the energy offset in the lowest unoccupied molecular orbital (LUMO) levels between the host and the guest impedes electron transport in the host-guest films and thus improves the balance of carrier mobilities of the host films intrinsically exhibiting electron preferred transporting characteristics. Photoluminescence measurements show efficient energy transfer in this host-guest system and thus ensure predominant guest emission at low guest concentrations, rendering significantly reduced self-quenching of guest molecules. EL measurements show that the peak EQE (power efficiency) of the host-guest LECs reaches 3.62% (7.36 lm W(-1)), which approaches the upper limit that one would expect from the photoluminescence quantum yield of the emissive layer (∼0.2) and an optical out-coupling efficiency of ∼20% and consequently indicates superior balance of carrier mobilities in such a host-guest emissive layer. These results are among the highest reported for red-emitting LECs and thus confirm that in addition to reducing self-quenching of guest molecules, the strategy of utilizing a carrier transporting host doped with a proper carrier trapping guest would improve balance of carrier mobilities in the host-guest emissive layer, offering an effective approach for optimizing device efficiencies of LECs.