Excited-State Modification of Phenylimidazole-Based Cyclometalated Ir(III) Complexes through Secondary Bulky Aryl Substitution and Inductive Modification Enhances the Blue Emission Efficiency in Phosphorescent OLEDs

To elucidate the key parameters governing the emission properties of phenylimidazole (pim)-based Ir(III) emitters, including their electronic structure and the bulky aryl substitution effect, a series of pim-based iridium(III) complexes (Ir(Rpim-X)3, Rpim-X = 1-R-2-(X-phenyl)-1H-imidazole) bearing secondary pendants of increasing bulkiness [R = methyl (Me), phenyl (Ph), terphenyl (TPh), or 4-isopropyl terphenyl (ITPh)] and three different primary pim ligands (X = F, F2, and CN) were designed and synthesized. Based on photophysical and electrochemical analyses, it was found that the excited state properties are highly dependent on the bulkiness of the secondary substituent and the inductive nature of the primary pim ligand. The incorporation of bulky TPh/ITPh substituents in the second coordination sphere significantly enhanced the emission efficiencies in the solid state (ΦPL = 72.1–84.9%) compared to those of the methyl- or phenyl-substituted Ir(III) complexes (ΦPL = 30.4% for Ir(Mepim)3 and 63.7% for Ir(Phpim)3). Further modification of the secondary aryl substituent (Ir(TPhpim)3 → Ir(ITPhpim)3) through the incorporation of an isopropyl group and F substitution on the primary pim ligand (Ir(TPh/ITPhpim)3 → Ir(TPh/ITPhpim-F/F2)3) resulted in a slight decrease in the LUMO and a significant decrease in the HOMO energy levels, respectively; these energy level adjustments consequently amplified emission blue shifts, thereby enabling efficient blue electroluminescence in phosphorescent organic light-emitting diodes. Theoretical calculations revealed that the excited-state properties of pim-based Ir(III) complexes can be modulated by the nature of the peripheral substituent and the presence of an EWG substituent. Among the fabricated blue-emitting TPh/ITPh-substituted Ir(III) complexes, Ir(ITPhpim-F)3, Ir(TPhpim-F2)3, and Ir(ITPhpim-F2)3 were tested as blue-emitting dopants for blue phosphorescent OLEDs owing to their high solid radiative quantum yields (ΦPL = 75.9–84.9%). The Ir(ITPhpim-F)3-doped multilayer device displayed the best performance with a maximum external quantum efficiency of 21.0%, a maximum current efficiency of 43.6 cd/A, and CIE coordinates of 0.18 and 0.31

Photoinduced Electron Transfer in a BODIPY-<i>ortho</i>-Carborane Dyad Investigated by Time-Resolved Transient Absorption Spectroscopy

We report the results of photoinduced electron transfer (PET) in a novel dyad, in which a boron dipyrromethene (BODIPY) dye is covalently linked to <i>o</i>-carborane (<i>o</i>-Cb). In this dyad, BODIPY and <i>o</i>-Cb act as electron donor and acceptor, respectively. PET dynamics were investigated using a femtosecond time-resolved transient absorption spectroscopic method. The free energy dependence of PET in the S₁ and S₂ states was examined on the basis of Marcus theory. PET in the S₁ state occurs in the Marcus normal region. Rates are strongly influenced by the driving force (−Δ<i>G</i>), which is controlled by solvent polarity; thus, PET in the S₁ state is faster in polar solvents than in nonpolar ones. However, PET does not occur from the higher energy S₂ state despite large endothermic Δ<i>G</i> values, because deactivation via internal conversion is much faster than PET

A Detailed Evaluation for the Nonradiative Processes in Highly Phosphorescent Iridium(III) Complexes

To understand the intrinsic nature of nonradiative processes in heteroleptic cyclometalated Ir­(III) complexes, highly phosphorescent Ir³⁺ complexes containing 2-(3-sulfonylfluorophenyl)­pyridine (ppySO₂F) as the cyclometalated ligand were newly synthesized. Three ancillary ligands, acetylacetonate (acac), picolinate (pic), and <i>tetrakis</i>-pyrazolyl borate (bor), were employed for the preparation of the Ir­(III) complexes [Ir­(ppySO₂F)₂(acac)] (<b>Ir-acac</b>), [Ir­(ppySO₂F)₂(pic)] (<b>Ir-pic</b>), and [Ir­(ppySO₂F)₂(bor)] (<b>Ir-bor</b>). The molecular structures were characterized by X-ray crystallography. Blue phosphorescence maxima were observed at 458, 467, and 478 nm for <b>Ir-bor</b>, <b>Ir-pic</b>, and <b>Ir-acac</b>, respectively, at 77 K, and the corresponding emission quantum yields were determined to be 0.79, 0.80, and 0.98 in anaerobic CH₂Cl₂ at 300 K. Additionally, the phosphorescence decay times were measured to be 3.58, 1.94, and 1.44<i>μ</i>s for <b>Ir-bor</b>, <b>Ir-pic</b>, and <b>Ir-acac</b>, respectively. No temperature dependence was observed for the emission lifetimes in 298–338 K. These results indicate that there is no activation barrier to crossing to a nonradiative state like metal-centered (MC, d–d) state. The radiative rate constants (<i>k</i>_r) are within a narrow range of 3.0–5.5 × 10^–5 s^–1. However, the nonradiative rate constants (<i>k</i>_nr) are within a wide range of 14.2–0.52 × 10^–4 s^–1. The <i>k</i>_nr values showed exponetial correlation with the energy gap. We carried out <i>ab</i> <i>initio</i> calculations to evaluate the energy states and their corresponding orbitals. The nonemissive MC states lie at higher energies than the emissive metal-to-ligand charge transfer (MLCT) state, and hence, the MC states can be excluded from the nonradiative pathway

Electronic Alteration on Oligothiophenes by <i>o</i>‑Carborane: Electron Acceptor Character of <i>o</i>‑Carborane in Oligothiophene Frameworks with Dicyano-Vinyl End-On Group

We studied electronic change in oligothiophenes by employing <i>o</i>-carborane into a molecular array in which one or both end(s) were substituted by electron-withdrawing dicyano-vinyl group(s). Depending on mono- or bis-substitution at the <i>o</i>-carborane, a series of linear A₁-D-A₂ (<b>1a</b>–<b>1c</b>) or V-shaped A₁-D-A₂-D-A₁ <b>(2a</b>–<b>2c</b>) oligothiophene chain structures of variable length were prepared; A₁, D, and A₂, represent dicyano-vinyl, oligothiophenyl, and <i>o</i>-carboranyl groups, respectively. Among this series, <b>2a</b> shows strong electron-acceptor capability of <i>o</i>-carborane comparable to that of the dicyano-vinyl substituent, which can be elaborated by a conformational effect driven by cage σ*−π* interaction. As a result, electronic communications between <i>o</i>-carborane and dicyano-vinyl groups are successfully achieved in <b>2a</b>

Photosensitization Behavior of Ir(III) Complexes in Selective Reduction of CO₂ by Re(I)-Complex-Anchored TiO₂ Hybrid Catalyst

A series of cationic Ir­(III) complexes ([Ir­(btp)₂(bpy-X₂)]⁺ (<b>Ir-X</b>^<b>+</b>: btp = (2-pyridyl)­benzo­[<i>b</i>]­thiophen-3-yl; bpy-X₂ = 4,4′-X₂-2,2′-bipyridine (X = OMe, ^<i>t</i>Bu, Me, H, and CN)) were applied as visible-light photosensitizer to the CO₂ reduction to CO using a hybrid catalyst (TiO₂/ReP) prepared by anchoring of Re­(4,4′-Y₂-bpy)­(CO)₃Cl (ReP; Y = CH₂PO­(OH)₂) on TiO₂ particles. Irradiation of a solution containing <b>Ir-X</b>^<b>+</b>, TiO₂/ReP particles, and an electron donor (1,3-dimethyl-2-phenyl-1,3-dihydrobenzimidazole) in <i>N</i>,<i>N</i>-dimethylformamide at greater than 400 nm resulted in the reduction of CO₂ to CO with efficiencies in the order X = OMe > ^<i>t</i>Bu ≈ Me > H; <b>Ir-CN</b>⁺ has no photosensitization effect. A notable observation is that <b>Ir-</b>^{<i><b>t</b></i>}<b>Bu</b>⁺ and <b>Ir-Me</b>⁺ are less efficient than <b>Ir-OMe</b>⁺ at an early stage of the reaction but reveal persistent photosensitization behavior for a much longer period of time unlike the latter. Comparable experiments showed that (1) the <b>Ir-X</b>^<b>+</b> sensitizers are commonly superior compared to Ru­(bpy)₃²⁺, a widely used transition-metal photosensitizer, and (2) the system comprising <b>Ir-OMe</b>⁺ and TiO₂/ReP is much more efficient than a homogeneous-solution system using <b>Ir-OMe</b>⁺ and Re­(4,4′-Y′₂-bpy)­(CO)₃Cl (Y′ = CH₂PO­(OEt)₂). Implications of the present observations involving reaction mechanisms associated with the different behavior of the photosensitizers are discussed in detail

Photophysics and Excited-State Properties of Cyclometalated Iridium(III)–Platinum(II) and Iridium(III)–Iridium(III) Bimetallic Complexes Bridged by Dipyridylpyrazine

We investigated the electrochemical and excited-state properties of 2,3-bis­(2-pyridyl)­pyrazine (dpp)-bridged bimetallic complexes, (L)₂Ir-dpp-PtCl [<b>1</b>, L = 2-(4′,6′-difluorophenyl)­pyridinato-<i>N</i>,<i>C</i>² (dfppy); <b>2</b>, L = 2-phenylpyridinato-<i>N</i>,<i>C</i>² (ppy)] and [(L)₂Ir]₂(dpp) [<b>3</b>, L = dfppy; <b>4</b>, L = ppy] compared to monometallic complexes, (L)₂Ir-dpp (<b>5</b>, L = dfppy; <b>6</b>, L = ppy) and dpp-PtCl (dpp-Pt^IICl₂; <b>7</b>). The single-crystal X-ray crystallographic structures of <b>1</b>, <b>3</b>, <b>5</b>, and <b>6</b> showed that <b>1</b> and <b>3</b> have approximately coplanar structures of the dpp unit, while the noncoordinated pyridine ring of dpp in <b>5</b> and <b>6</b> is largely twisted with respect to the pyrazine ring. We found that the properties of the bimetallic complex significantly depended on the electronic and geometrical modulations of each fragment: (1) electronic structure of the main L (C^N) ligand in an iridium chromophore (L = dfppy or ppy) and (2) planarity of the bridging ligand (dpp). Their electrochemical and photophysical properties revealed that efficient electron-transfer processes predominated in the bimetallic systems regardless of the second metal participation. The low efficiencies of photoluminescence of dpp-bridged Ir–Pt and Ir–Ir bimetallic complexes (<b>1</b>–<b>4</b>) could be explained by assuming the involvement of crossing to platinum- and iridium-based d–d states from the emissive state. Such stereochemical and electronic situations around dpp allowed thermally activated crossing to platinum- and iridium-based d–d states from the emissive triplet metal-to-ligand charge-transfer (³MLCT) state, followed by cleavage of the dpp-Pt and (L)₂Ir-dpp bonds. The transient absorption study further confirmed that the planarity of the dpp bridging ligand, which was defined as the magnitude of tilt between the pyridine ring and pyrazine, had a direct correlation with the degree of nonradiative decay from the emissive iridium-based ³MLCT to the Ir d–d or Pt d–d state, leading to photoinduced dissociation of bimetallic complexes. From the dissociation pattern of metal complexes analyzed after photoirradiation, we found that their dissociation pathways were directly related to the quenching direction (either Ir d–d or Pt d–d) with a significant dependency on the relative ³MLCT levels of the (L)₂Ir-dpp component

Electronic Alteration on Oligothiophenes by <i>o</i>‑Carborane: Electron Acceptor Character of <i>o</i>‑Carborane in Oligothiophene Frameworks with Dicyano-Vinyl End-On Group

We studied electronic change in oligothiophenes by employing <i>o</i>-carborane into a molecular array in which one or both end(s) were substituted by electron-withdrawing dicyano-vinyl group(s). Depending on mono- or bis-substitution at the <i>o</i>-carborane, a series of linear A₁-D-A₂ (<b>1a</b>–<b>1c</b>) or V-shaped A₁-D-A₂-D-A₁ <b>(2a</b>–<b>2c</b>) oligothiophene chain structures of variable length were prepared; A₁, D, and A₂, represent dicyano-vinyl, oligothiophenyl, and <i>o</i>-carboranyl groups, respectively. Among this series, <b>2a</b> shows strong electron-acceptor capability of <i>o</i>-carborane comparable to that of the dicyano-vinyl substituent, which can be elaborated by a conformational effect driven by cage σ*−π* interaction. As a result, electronic communications between <i>o</i>-carborane and dicyano-vinyl groups are successfully achieved in <b>2a</b>

Photophysics and Excited-State Properties of Cyclometalated Iridium(III)–Platinum(II) and Iridium(III)–Iridium(III) Bimetallic Complexes Bridged by Dipyridylpyrazine

We investigated the electrochemical and excited-state properties of 2,3-bis­(2-pyridyl)­pyrazine (dpp)-bridged bimetallic complexes, (L)₂Ir-dpp-PtCl [<b>1</b>, L = 2-(4′,6′-difluorophenyl)­pyridinato-<i>N</i>,<i>C</i>² (dfppy); <b>2</b>, L = 2-phenylpyridinato-<i>N</i>,<i>C</i>² (ppy)] and [(L)₂Ir]₂(dpp) [<b>3</b>, L = dfppy; <b>4</b>, L = ppy] compared to monometallic complexes, (L)₂Ir-dpp (<b>5</b>, L = dfppy; <b>6</b>, L = ppy) and dpp-PtCl (dpp-Pt^IICl₂; <b>7</b>). The single-crystal X-ray crystallographic structures of <b>1</b>, <b>3</b>, <b>5</b>, and <b>6</b> showed that <b>1</b> and <b>3</b> have approximately coplanar structures of the dpp unit, while the noncoordinated pyridine ring of dpp in <b>5</b> and <b>6</b> is largely twisted with respect to the pyrazine ring. We found that the properties of the bimetallic complex significantly depended on the electronic and geometrical modulations of each fragment: (1) electronic structure of the main L (C^N) ligand in an iridium chromophore (L = dfppy or ppy) and (2) planarity of the bridging ligand (dpp). Their electrochemical and photophysical properties revealed that efficient electron-transfer processes predominated in the bimetallic systems regardless of the second metal participation. The low efficiencies of photoluminescence of dpp-bridged Ir–Pt and Ir–Ir bimetallic complexes (<b>1</b>–<b>4</b>) could be explained by assuming the involvement of crossing to platinum- and iridium-based d–d states from the emissive state. Such stereochemical and electronic situations around dpp allowed thermally activated crossing to platinum- and iridium-based d–d states from the emissive triplet metal-to-ligand charge-transfer (³MLCT) state, followed by cleavage of the dpp-Pt and (L)₂Ir-dpp bonds. The transient absorption study further confirmed that the planarity of the dpp bridging ligand, which was defined as the magnitude of tilt between the pyridine ring and pyrazine, had a direct correlation with the degree of nonradiative decay from the emissive iridium-based ³MLCT to the Ir d–d or Pt d–d state, leading to photoinduced dissociation of bimetallic complexes. From the dissociation pattern of metal complexes analyzed after photoirradiation, we found that their dissociation pathways were directly related to the quenching direction (either Ir d–d or Pt d–d) with a significant dependency on the relative ³MLCT levels of the (L)₂Ir-dpp component

Photophysics and Excited-State Properties of Cyclometalated Iridium(III)–Platinum(II) and Iridium(III)–Iridium(III) Bimetallic Complexes Bridged by Dipyridylpyrazine

We investigated the electrochemical and excited-state properties of 2,3-bis­(2-pyridyl)­pyrazine (dpp)-bridged bimetallic complexes, (L)₂Ir-dpp-PtCl [<b>1</b>, L = 2-(4′,6′-difluorophenyl)­pyridinato-<i>N</i>,<i>C</i>² (dfppy); <b>2</b>, L = 2-phenylpyridinato-<i>N</i>,<i>C</i>² (ppy)] and [(L)₂Ir]₂(dpp) [<b>3</b>, L = dfppy; <b>4</b>, L = ppy] compared to monometallic complexes, (L)₂Ir-dpp (<b>5</b>, L = dfppy; <b>6</b>, L = ppy) and dpp-PtCl (dpp-Pt^IICl₂; <b>7</b>). The single-crystal X-ray crystallographic structures of <b>1</b>, <b>3</b>, <b>5</b>, and <b>6</b> showed that <b>1</b> and <b>3</b> have approximately coplanar structures of the dpp unit, while the noncoordinated pyridine ring of dpp in <b>5</b> and <b>6</b> is largely twisted with respect to the pyrazine ring. We found that the properties of the bimetallic complex significantly depended on the electronic and geometrical modulations of each fragment: (1) electronic structure of the main L (C^N) ligand in an iridium chromophore (L = dfppy or ppy) and (2) planarity of the bridging ligand (dpp). Their electrochemical and photophysical properties revealed that efficient electron-transfer processes predominated in the bimetallic systems regardless of the second metal participation. The low efficiencies of photoluminescence of dpp-bridged Ir–Pt and Ir–Ir bimetallic complexes (<b>1</b>–<b>4</b>) could be explained by assuming the involvement of crossing to platinum- and iridium-based d–d states from the emissive state. Such stereochemical and electronic situations around dpp allowed thermally activated crossing to platinum- and iridium-based d–d states from the emissive triplet metal-to-ligand charge-transfer (³MLCT) state, followed by cleavage of the dpp-Pt and (L)₂Ir-dpp bonds. The transient absorption study further confirmed that the planarity of the dpp bridging ligand, which was defined as the magnitude of tilt between the pyridine ring and pyrazine, had a direct correlation with the degree of nonradiative decay from the emissive iridium-based ³MLCT to the Ir d–d or Pt d–d state, leading to photoinduced dissociation of bimetallic complexes. From the dissociation pattern of metal complexes analyzed after photoirradiation, we found that their dissociation pathways were directly related to the quenching direction (either Ir d–d or Pt d–d) with a significant dependency on the relative ³MLCT levels of the (L)₂Ir-dpp component