Unusually Fast Phosphorescence from Ir(III) Complexes via Dinuclear Molecular Design

A series of blue phosphorescent iridium complexes with thermally activated delayed fluorescence and efficiency roll-off properties

Six iridium complexes were designed and studied using the DFT approach, (ppy)2Ir(pic) (1), (f4ppy)2Ir(pic) (2), (ppy)2Ir(tmd) (3), (f4ppy)2Ir(tmd) (4), (ppy)2Ir(tpip) (5) and (f4ppy)2Ir(tpip) (6). Here ppy denotes phenylpyridine, f4ppy denotes 2-(2,3,4,5-tetrafluorophenyl) pyridine, pic denotes benzoic acid, tmd denotes 5-hydroxy-2,2,6,6-tetramethylhept-4-en-3-one and tpip denotes tetraphenylimido-diphosphinate. The geometries, absorptions, emissions, frontier molecular orbitals, and spin-orbit coupling (SOC) constants of the 6 complexes were evaluated. An intriguing phenomenon was observed during the excitation process of these molecules. It was discovered that, in the ground state, the Frontier Molecular Orbitals (FMOs) of these molecules were loosely arranged throughout the molecule. However, in the Lowest Unoccupied Molecular Orbitals (LUMO) of the triplet excited state, the FMOs become concentrated around the metal core and a maximum of two ligands. Furthermore, the analysis of the energy difference between the lowest singlet excited state and the lowest triplet excited state (ΔE S1T1 ) of these complexes in conjunction with their spin-orbit coupling performance indicated that complex 1 exhibits characteristics consistent with Thermally Activated Delayed (TAD) fluorescence. We hope that this research can serve as a reference for practical experimental synthesis.

Visible to NIR-II Photoluminescence of Atomically Precise Gold Nanoclusters

Atomically precise gold nanoclusters (NCs) have emerged as a new class of precision materials and attracted wide interest in recent years. One of the unique properties of such nanoclusters pertains to their photoluminescence (PL), for it can widely span visible to near-infrared-I and -II wavelengths (NIR-I/II), and even beyond 1700 nm by manipulating the size, structure, and composition. The current research efforts focus on the structure-PL correlation and the development of strategies for raising the PL quantum yields, which is nontrivial when moving from the visible to the near-infrared wavelengths, especially in the NIR-II regions. This review summarizes the recent progress in the field, including i) the types of PL observed in gold NCs such as fluorescence, phosphorescence, and thermally activated delayed fluorescence, as well as dual emission; ii) some effective strategies that are devised to improve the PL quantum yield (QY) of gold NCs, such as heterometal doping, surface rigidification, and core phonon engineering, with double-digit QYs for the NIR PL on the horizons; and iii) the applications of luminescent gold NCs in bioimaging, photosensitization, and optoelectronics. Finally, the remaining challenges and opportunities for future research are highlighted.

Trinuclear Cyclometalated Iridium(III) Complex Exhibiting Intense Phosphorescence of an Unprecedented Rate

Herein, we present two novel cyclometalated Ir(III) complexes of dinuclear and trinuclear design, Ir2(dppm)3(acac)2 and Ir3(dppm)4(acac)3, respectively, where dppm is 4,6-di(4-tert-butylphenyl)pyrimidine ligand and acac is acetylacetonate ligand. In both cases, rac-diastereomers were isolated during the synthesis. The materials show intense phosphorescence of outstanding rates (kr = ΦPL/τ) with corresponding radiative decay times of only τr = 1/kr = 0.36 μs for dinuclear Ir2(dppm)3(acac)2 and still shorter τr = 0.30 μs for trinuclear Ir3(dppm)4(acac)3, as measured for doped polystyrene film samples under ambient temperature. Measured under cryogenic conditions, radiative decay times of the three T1 substates (I, III, and III) and substate energy separations are τI = 11.8 μs, τII = 7.1 μs, τIII = 0.06 μs, ΔE(II-I) = 7 cm-1, and ΔE(III-I) = 175 cm-1 for dinuclear Ir2(dppm)3(acac)2 and τI = 3.1 μs, τII = 3.5 μs, τIII = 0.03 μs, ΔE(II-I) ≈ 1 cm-1, and ΔE(III-I) = 180 cm-1 for trinuclear Ir3(dppm)4(acac)3. The determined T1 state ZFS values (ΔE(III-I)) are smaller compared to that of mononuclear analogue Ir(dppm)2(acac) (ZFS = 210-1 cm). Theoretical analysis suggests that the high phosphorescence rates in multinuclear materials can be associated with the increased number of singlet states lending oscillator strength to the T1 → S0 transition.

Tuning the Organelle-Specific Imaging and Photodynamic Therapeutic Efficacy of Theranostic Mono- and Trinuclear Organometallic Iridium(III) Complexes

The organelle-specific localization of mononuclear and trinuclear iridium(III) complexes and their photodynamic behavior within the cells are described herein, emphasizing their structure-activity relationship. Both the IrA2 and IrB2 complexes possess a pair of phenyl-benzothiazole derived from the -CHO moieties of mononuclear organometallic iridium(III) complexes IrA1 and IrB1, which chelates IrCp*Cl (Cp* = 1,2,3,4,5-pentamethylcyclopentadiene) to afford trinuclear complexes IrA3 and IrB3. Insights into the photophysical and electrochemical parameters of the complexes were obtained by a time-dependent density functional theory study. The synthesized complexes IrA2, IrA3, IrB2, and IrB3 were found to be nontoxic to human MCF7 breast carcinoma cells. However, the photoexcitation of complexes using LED light could effectively trigger intracellular reactive oxygen species (ROS) generation, leading to cell death. Furthermore, to check the organelle-specific localization of IrA2 and IrB2, we observed that both complexes could selectively localize in the endoplasmic reticulum. In contrast, trinuclear IrA3 and IrB3 accumulate in the nuclei. The photoexcitation of complexes using LED light could effectively trigger intracellular reactive oxygen species (ROS) generation, leading to cell death.

Elucidating the Near-Infrared Photoluminescence Mechanism of Homometal and Doped M25(SR)18 Nanoclusters

More than a decade of research on the photoluminescence (PL) of classic Au25(SR)18 and its doped nanoclusters (NCs) still leaves many fundamental questions unanswered due to the complex electron dynamics. Here, we revisit the homogold Au25 (ligands omitted hereafter) and doped NCs, as well as the Ag25 and doped ones, for a comparative study to disentangle the influencing factors and elucidate the PL mechanism. We find that the strong electron-vibration coupling in Au25 leads to weak PL in the near-infrared region (∼1000 nm, quantum yield QY = 1% in solution at room temperature). Heteroatom doping of Au25 with a single Cd or Hg atom strengthens the coupling of the exciton with staple vibrations but reduces the coupling with the core breathing and quadrupolar modes. The QYs of the three MAu24 NCs (M = Hg, Au, and Cd) follow a linear relation with their PL lifetimes, suggesting a mechanism of suppressed nonradiative decay in PL enhancement. In contrast, the weaker electron-vibration coupling in Ag25 leads to higher PL (QY = 3.5%), and single Au atom doping further leads to a 5× enhancement of the radiative rate and a suppression of nonradiative decay rate (i.e., twice the PL lifetime of Ag25) in AuAg24 (hence, QY 35%), but doping more Au atoms results in gold distribution to staple motifs and thus triggering of strong electron-vibration coupling as in the MAu24 NCs, hence, counteracting the radiative enhancement effect and giving rise to only 5% QY for AuxAg25-x (x = 3-10). The obtained insights will provide guidance for the design of metal NCs with high PL for lighting, sensing, and optoelectronic applications.

Aligning π-Extended π-Deficient Ligands to Afford Submicrosecond Phosphorescence Radiative Decay Time of Mononuclear Ir(III) Complexes

Herein, we report a profound investigation of the photophysical properties of three mononuclear Ir(III) complexes fac-Ir(dppm)₃ (Hdppm-4,6-bis(4-(tert-butyl)phenyl)pyrimidine), Ir(dppm)₂(acac) (acac-acetylacetonate), and Ir(ppy)₂(acac) (Hppy-phenylpyridine). The heteroleptic Ir(dppm)₂(acac) is found to emit with efficiency above 80% and feature a remarkably high rate of emission. As measured under ambient temperature, Ir(dppm)₂(acac) emits with the unusually short (sub-μs) radiative decay time of τ_r = τ_em/Φ_PL = 1/k_r = 0.91 μs in degassed toluene and τ_r = 0.73 μs in a doped polystyrene film under nitrogen. Investigations at cryogenic temperatures in glassy toluene showed that the emission stems from the T₁ state and thus represents T₁ → S₀ phosphorescence with individual decay times of the T₁ substates of T_1,I = 66 μs, T_1,II = 7.3 μs, T_1,III = 0.19 μs, and energy gaps between the substates of ΔE(T_1,II-T_1,I) = 14 cm^-1 and ΔE(T_1,III-T_1,I) = 210 cm^-1. Analysis of the electronic structure of Ir(dppm)₂(acac) showed that such a high rate of phosphorescence may stem from the two dppm ligands, with extended π-conjugation system and π-deficient character due to the pyrimidine ring, being serially aligned along one axis. Such alignment, along with the quasi-symmetric character of Jahn-Teller distortions in the T₁ state, affords a large chromophore, comprising four (het)aryl rings of the two dppm ligands. This affords an exceptionally large oscillator strength of the MLCT-character singlet state spin-orbit coupled with the T₁ state and thus brings about enhancement of the phosphorescence rate. These findings reveal molecular design principles paving the way to new phosphors of enhanced emission rates.

Fluorescence vs. Phosphorescence: Which Scenario Is Preferable in Au(I) Complexes with Benzothiadiazoles?

The photoluminescence of Au(I) complexes is generally characterized by long radiative lifetimes owing to the large spin-orbital coupling constant of the Au(I) ion. Herein, we report three brightly emissive Au(I) coordination compounds, 1, 2a, and 2b, that reveal unexpectedly short emission lifetimes of 10-20 ns. Polymorphs 2a and 2b exclusively exhibit fluorescence, which is quite rare for Au(I) compounds, while compound 1 reveals fluorescence as the major radiative pathway, and a minor contribution of a microsecond-scale component. The fluorescent behaviour for 1-2 is rationalized by means of quantum chemical (TD)-DFT calculations, which reveal the following: (1) S₀-S₁ and S₀-T₁ transitions mainly exhibit an intraligand nature. (2) The calculated spin-orbital coupling (SOC) between the states is small, which is a consequence of overall small metal contribution to the frontier orbitals. (3) The T₁ state features much lower energy than the S₁ state (by ca. 7000 cm^-1), which hinders the SOC between the states. Thus, the S₁ state decays in the form of fluorescence, rather than couples with T₁. In the specific case of complex 1, the potential energy surfaces for the S₁ and T₂ states intersect, while the vibrationally resolved S₁-S₀ and T₂-S₀ calculated radiative transitions show substantial overlap. Thus, the microsecond-scale component for complex 1 can stem from the coupling between the S₁ and T₂ states.

Optical Oxygen Sensors Show Reversible Cross-Talk and/or Degradation in the Presence of Nitrogen Dioxide

A variety of luminescent dyes including the most common indicators for optical oxygen sensors were investigated in regard to their stability and photophysical properties in the presence of nitrogen dioxide. The dyes were immobilized in polystyrene and subjected to NO₂ concentrations from 40 to 5500 ppm. The majority of dyes show fast degradation of optical properties due to the reaction with NO₂. The class of phosphorescent metalloporphyrins shows the highest resistance against nitrogen dioxide. Among them, palladium(II) and platinum(II) complexes of octasubstituted sulfonylated benzoporphyrins are identified as the most stable dyes with almost no decomposition in the presence of NO₂. The phosphorescence of these dyes is reversibly quenched by nitrogen dioxide. Immobilized in various polymeric matrices, the sulfonylated Pt(II) benzoporphyrin demonstrates about one order of magnitude more efficient quenching by NO₂ than by molecular oxygen. Our study demonstrates that virtually all commercially available and reported optical oxygen sensors are likely to show either irreversible decomposition in the presence of nitrogen dioxide or reversible luminescence quenching. They should be used with extreme caution if NO₂ is present in relatively high concentrations or it may be generated from other species such as nitric oxide. As an important consequence of nearly anoxic systems, production of nitrogen dioxide or nitric oxide may be therefore erroneously interpreted as an increase in oxygen concentration.

Synthesis, structures, and photophysical properties of two Cu(I) complexes supported by N-heterocyclic carbene and phosphine ligands

Two new cationic four-coordinate Cu(I) complexes supported by different chelating N-heterocyclic carbene ligands and the diphosphine ligand bis[2-(diphenylphosphino)phenyl]ether (POP) have been synthesized. The chemical structures of both complexes have been characterized by 1H NMR, 13C NMR, 31P NMR, and mass spectroscopy, and the crystal structure of one complex has been determined by single-crystal X-ray diffraction. Results of theoretical calculations indicate that the lowest energy electronic transitions of these complexes are mainly the metal-to-ligand charge transfer and ligand-to-ligand charge transfer transitions. The complexes in solid state show intense emissions with high photoluminescence quantum yields. The photophysical behavior at 298 and 77K shows that emissions of these complexes at room temperature are thermally activated delayed fluorescence mixed with phosphorescence.

Efficient Blue Electrophosphorescence and Hyperphosphorescence Generated by Bis-tridentate Iridium(III) Complexes

Four blue-emissive iridium(III) complexes bearing a 3,3'-(1,3-phenylene)bis[1-isopropyl-6-(trifluoromethyl)-3H-imidazo[4,5-b]pyridin-2-ylidene]-based pincer chelate, which are derived from PXn·H₃(PF₆)₂, where n = 1-4, and a cyclometalating chelate given from 9-[6-[5-(trifluoromethyl)-2λ²-pyrazol-3-yl]pyridin-2-yl]-9H-carbazole [(PzpyCz)H₂], were successfully synthesized and employed as both an emissive dopant and a sensitizer in the fabrication of organic light-emitting diode (OLED) devices. These functional chelates around a Ir^III atom occupied two mutually orthogonal coordination arrangements and adopted the so-called bis-tridentate architectures. Theoretical studies confirmed the dominance of the electronic transition by the pincer chelates, while the dianionic PzpyCz chelate was only acting as a spectator group. Phosphorescent OLED devices with [Ir(PX3)(PzpyCz)] (B3) as the dopant gave a maximum external quantum efficiency (EQE) of 21.93% and CIE_xy of (0.144, 0.157) and was subjected to only ∼10% of roll-off in efficiency at a high current density of 1000 cd m^-2. Blue-emissive narrow-band hyperphosphorescence was also obtained using B3 as an assistant sensitizer and ν-DABNA as a terminal emitter, giving both an improved EQE of 26.17% and CIE_xy of (0.116, 0.144), confirming efficient Förster resonance energy transfer in this hyperdevice.

Inherently dinuclear iridium(III) meso architectures accessed by cyclometalation of calix[4]arene-based bis(aryltriazoles)

Conventional cyclometalation of calix[4]arene bis(aryltriazoles) with iridium(III) chloride hydrate leads to unique meso architectures in which the Ir₂Cl₂ core is cross-bound by two (C^N)₂ ligands, which allows further replacement of the chloride bridges with ancillary ligands while maintaining the dinuclear structures of the complexes having independent or coupled iridium pairs.

Phosphorescent multinuclear complexes for optoelectronics: tuning of the excited-state dynamics

Luminescent transition metal complexes have attracted a great deal of attention in the last two decades from both fundamental and application points of view. The majority of the investigated and most efficient systems consist of monometallic compounds with judiciously selected ligand sphere, providing excellent triplet emitters for both lab-scale and real-market light-emitting devices for display technologies. More recently, chemical architectures comprising multimetallic compounds have appeared as an emerging and valuable alternative. Herein, the most recent trends in the field are showcased in a systematic approach, where the different examples are classified by metal center and ligand(s) scaffold. Their optical and electroluminescence properties are presented and compared as well. Indeed, the multimetallic strategy has proven to be highly suitable for compounds emitting efficiently in the challenging red to near-infrared region, yielding metal-based emitters with improved optical properties in terms of enhanced emission efficiency, shortened excited-state lifetime, and faster radiative rate constant. Finally, the advantages and drawbacks of the multimetallic approach will be discussed.

Rational Tuning of Bis-Tridentate Ir(III) Phosphors to Deep-Blue with High Efficiency and Sub-microsecond Lifetime

A new class of bis-tridentate Ir(III) complexes (Dap-1-4) was synthesized using carbene pincer pro-chelates PC1·H₃(PF₆)₂ or PC2·H₃(PF₆)₂ with either imidazolylidene or imidazo[4,5-b]pyridin-2-ylidene appendages, together with a second cyclometalating 2,6-diaryoxypyridine chelate, L1H₂ and L2H₂, differed by a NMe₂ donor at the central pyridinyl fragment. The respective emission tuning between the ultraviolet and blue region was rationalized using time-dependent density functional theory (TD-DFT) approaches. Next, a highly efficient blue emitter (Dap-5) was synthesized by concomitant addition of two methyl groups and a single CF₃ substituent at the central phenyl and peripheral imidazo[4,5-b]pyridin-2-ylidene entities of the carbene pincer chelate, respectively. The organic light-emitting diode (OLED) device with 15 wt % Dap-5 in DPEPO shows electroluminescence at 468 nm and with CIE (0.14, 0.15) and a max external quantum efficiency (max EQE) of 16.8% with low efficiency roll-off (EQE of 14.4% at 1000 cd m^-2); the latter is attributed to the relatively shortened triplet excited-state radiative lifetime. These results highlight the adequateness of bis-tridentate Ir(III) phosphors in fabrication of practical blue-emitting OLEDs.

Halide-Enhanced Spin-Orbit Coupling and the Phosphorescence Rate in Ir(III) Complexes

The spin-forbidden nature of phosphorescence in Ir(III) complexes is relaxed by the metal-induced effect of spin-orbit coupling (SOC). A further increase of the phosphorescence rate could potentially be achieved by introducing additional centers capable of further enhancing the SOC effect, such as metal-coordinated halides. Herein, we present a dinuclear Ir(III) complex Ir₂I₂ that contains two Ir(III)-iodide moieties. The complex shows intense phosphorescence with a quantum yield of Φ_PL(300 K) = 90% and a submicrosecond decay time of only τ(300 K) = 0.34 μs, as measured under ambient temperature for the degassed toluene solution. These values correspond to a top value T₁ → S₀ phosphorescence rate of k_r = 2.65 × 10⁶ s^-1. Investigations at cryogenic temperatures allowed us to determine the zero-field splitting (ZFS) of the emitting state T₁ ZFS(III-I) = 170 cm^-1 and unusually short individual decay times of T₁ substates: τ(I) = 6.4 μs, τ(II) = 7.6 μs, and τ(III) = 0.05 μs. This indicates a strong SOC of state T₁ with singlet states. Theoretical investigations suggest that the SOC of state T₁ with singlets is also contributed by halides. Strongly contributing to the higher occupied molecular orbitals of the complex (e.g., HOMO, HOMO - 1, and so forth), iodides work as important SOC centers that operate in tandem with metals. The examples of Ir₂I₂ and of earlier reported analogous complex Ir₂Cl₂ reveal that the metal-coordinated halides can enhance the SOC of state T₁ with singlets and, consequently, the phosphorescence rate. A comparative study of Ir₂I₂ and Ir₂Cl₂ shows that the share of halides in total contribution (halides plus metals) to the SOC of state T₁ with singlets increases strongly upon exchange of chlorides for iodides. The exchange also led to the decrease in values of ZFS of the T₁ state from ZFS(III-I) = 205 cm^-1 for Ir₂Cl₂ to T₁ ZFS(III-I) = 170 cm^-1 for Ir₂I₂. This results in a more efficient thermal population of the fastest emitting T₁ substate III, thus further enhancing the room-temperature phosphorescence rate.

Near Infrared Phosphorescent Dinuclear Ir(III) Complex Exhibiting Unusually Slow Intersystem Crossing and Dual Emissive Behavior

A dinuclear iridium(III) complex IrIr shows dual emission consisting of near infrared (NIR) phosphorescence (λ_max = 714 nm, CH₂Cl₂, T = 300 K) and green fluorescence (λ_max = 537 nm). The NIR emission stems from a triplet state (T₁) localized on the ditopic bridging ligand (³LC). Because of the dinuclear molecular structure, the phosphorescence efficiency (Φ_PL = 3.5%) is high compared to those of other known red/NIR-emitting iridium complexes. The weak fluorescence stems from the lowest excited singlet state (S₁) of ¹LC character. The occurrence of fluorescence is ascribed to relatively slow intersystem crossing (ISC) from state S₁ (¹LC) to the triplet manifold. The measured ISC rate corresponds to a time constant τ_ISC of 2.1 ps, which is an order of magnitude longer than those usually found for iridium complexes. This slow ISC rate can be explained in terms of the LC character and large energy separation (0.57 eV) of the respective singlet and triplet excited states. IrIr is internalized by live HeLa cells as evidenced by confocal luminescence microscopy.

Mono and dinuclear iridium(iii) complexes featuring bis-tridentate coordination and Schiff-base bridging ligands: the beneficial effect of a second metal ion on luminescence

Ditopic bis-N^N^O-coordinating ligands, prepared by Schiff base chemistry, lead to dinuclear iridium complexes that emit much more brightly than their mononuclear counterparts.