Luminescent Iridium(III) Complexes with N^∧C^∧N-Coordinated Terdentate Ligands: Dual Tuning of the Emission Energy and Application to Organic Light-Emitting Devices

A family of complexes (<b>1a</b>-<b>3a</b> and <b>1b</b>-<b>3b</b>) was prepared, having the structure Ir­(N^∧C^∧N)­(N^∧C)­Cl. Here, N^∧C^∧N represents a terdentate, cyclometallating ligand derived from 1,3-di­(2-pyridyl)­benzene incorporating CH₃ (<b>1a</b>,<b>b</b>), F (<b>2a</b>,<b>b</b>), or CF₃ (<b>3a</b>,<b>b</b>) substituents at the 4 and 6 positions of the benzene ring, and N^∧C is 2-phenylpyridine (series <b>a</b>) or 2-(2,4-difluorophenyl)­pyridine (series <b>b</b>). The complexes are formed using a stepwise procedure that relies on the initial introduction of the terdentate ligand to form a dichloro-bridged iridium dimer, followed by cleavage with the N^∧C ligand. ¹H NMR spectroscopy reveals that the isomer that is exclusively formed in each case is that in which the pyridyl ring of the N^∧C ligand is trans to the cyclometallating aryl ring of the N^∧C^∧N ligand. This conclusion is unequivocally confirmed by X-ray diffraction analysis for two of the complexes (<b>1b</b> and <b>3a</b>). All of the complexes are highly luminescent in degassed solution at room temperature, emitting in the green (<b>1a</b>,<b>b</b>), blue-green (<b>2a</b>,<b>b</b>), and orange-red (<b>3a</b>,<b>b</b>) regions. The bidentate ligand offers independent fine-tuning of the emission energy: for each pair, the “<b>b</b>” complex is blue-shifted relative to the analogous “<b>a</b>” complex. These trends in the excited-state energies are rationalized in terms of the relative magnitudes of the effects of the substituents on the highest occupied and lowest unoccupied orbitals, convincingly supported by time-dependent density functional theory (TD-DFT) calculations. Luminescence quantum yields are high, up to 0.7 in solution and close to unity in a PMMA matrix for the green-emitting complexes. Organic light emitting devices (OLEDs) employing this family of complexes as phosphorescent emitters have been prepared. They display high efficiencies, at least comparable, and in some cases superior, to similar devices using the well-known tris-bidentate complexes such as <i>fac</i>-Ir­(ppy)₃. The combination of terdentate and bidentate ligands is seen to offer a versatile approach to tuning of the photophysical properties of iridium-based emitters for such applications

Site-Selective Benzannulation of <i>N</i>‑Heterocycles in Bidentate Ligands Leads to Blue-Shifted Emission from [(<i>P^N</i>)Cu]₂(μ-X)₂ Dimers

Benzannulated bidentate pyridine/phosphine (<i>P^N</i>) ligands bearing quinoline or phenanthridine (3,4-benzoquinoline) units have been prepared, along with their halide-bridged, dimeric Cu­(I) complexes of the form [(<i>P^N</i>)­Cu]₂(μ-X)₂. The copper complexes are phosphorescent in the orange-red region of the spectrum in the solid-state under ambient conditions. Structural characterization in solution and the solid-state reveals a flexible conformational landscape, with both diamond-like and butterfly motifs available to the Cu₂X₂ cores. Comparing the photophysical properties of complexes of (quinolinyl)­phosphine ligands with those of π-extended (phenanthridinyl)­phosphines has revealed a counterintuitive impact of site-selective benzannulation. Contrary to conventional assumptions regarding π-extension and a bathochromic shift in the lowest energy absorption maxima, a blue shift of nearly 40 nm in the emission wavelength is observed for the complexes with larger ligand π-systems, which is assigned as phosphorescence on the basis of emission energies and lifetimes. Comparison of the ground-state and triplet excited state structures optimized from DFT and TD-DFT calculations allows attribution of this effect to a greater rigidity for the benzannulated complexes resulting in a higher energy emissive triplet state, rather than significant perturbation of orbital energies. This study reveals that ligand structure can impact photophysical properties for emissive molecules by influencing their structural rigidity, in addition to their electronic structure

Luminescent Iridium(III) Complexes with N^∧C^∧N-Coordinated Terdentate Ligands: Dual Tuning of the Emission Energy and Application to Organic Light-Emitting Devices

A family of complexes (<b>1a</b>-<b>3a</b> and <b>1b</b>-<b>3b</b>) was prepared, having the structure Ir­(N^∧C^∧N)­(N^∧C)­Cl. Here, N^∧C^∧N represents a terdentate, cyclometallating ligand derived from 1,3-di­(2-pyridyl)­benzene incorporating CH₃ (<b>1a</b>,<b>b</b>), F (<b>2a</b>,<b>b</b>), or CF₃ (<b>3a</b>,<b>b</b>) substituents at the 4 and 6 positions of the benzene ring, and N^∧C is 2-phenylpyridine (series <b>a</b>) or 2-(2,4-difluorophenyl)­pyridine (series <b>b</b>). The complexes are formed using a stepwise procedure that relies on the initial introduction of the terdentate ligand to form a dichloro-bridged iridium dimer, followed by cleavage with the N^∧C ligand. ¹H NMR spectroscopy reveals that the isomer that is exclusively formed in each case is that in which the pyridyl ring of the N^∧C ligand is trans to the cyclometallating aryl ring of the N^∧C^∧N ligand. This conclusion is unequivocally confirmed by X-ray diffraction analysis for two of the complexes (<b>1b</b> and <b>3a</b>). All of the complexes are highly luminescent in degassed solution at room temperature, emitting in the green (<b>1a</b>,<b>b</b>), blue-green (<b>2a</b>,<b>b</b>), and orange-red (<b>3a</b>,<b>b</b>) regions. The bidentate ligand offers independent fine-tuning of the emission energy: for each pair, the “<b>b</b>” complex is blue-shifted relative to the analogous “<b>a</b>” complex. These trends in the excited-state energies are rationalized in terms of the relative magnitudes of the effects of the substituents on the highest occupied and lowest unoccupied orbitals, convincingly supported by time-dependent density functional theory (TD-DFT) calculations. Luminescence quantum yields are high, up to 0.7 in solution and close to unity in a PMMA matrix for the green-emitting complexes. Organic light emitting devices (OLEDs) employing this family of complexes as phosphorescent emitters have been prepared. They display high efficiencies, at least comparable, and in some cases superior, to similar devices using the well-known tris-bidentate complexes such as <i>fac</i>-Ir­(ppy)₃. The combination of terdentate and bidentate ligands is seen to offer a versatile approach to tuning of the photophysical properties of iridium-based emitters for such applications

Platinum(II) Complexes of N^∧C^∧N‑Coordinating 1,3-Bis(2-pyridyl)benzene Ligands: Thiolate Coligands Lead to Strong Red Luminescence from Charge-Transfer States

A new family of platinum­(II) complexes of the form PtL^<i>n</i>SR have been prepared, where L^<i>n</i> represents a cyclometalating, N^∧C^∧N-bound tridentate ligand and SR is a monodentate thiolate ligand. The complexes fall into two groups, those of PtL¹SR where HL¹ = 1,3-bis­(2-pyridyl)­benzene, and those of PtL²SR, where HL² = methyl 3,5-bis­(2-pyridyl)­benzoate. Each group consists of five complexes, where R = CH₃, C₆H₅, <i>p</i>-C₆H₄-CH₃, <i>p</i>-C₆H₄-OMe, <i>p</i>-C₆H₄-NO₂. These compounds, which are bright red, orange, or yellow solids, are formed readily upon treatment of PtL^<i>n</i>Cl with the corresponding potassium thiolate KSR in solution at room temperature. The replacement of the chloride by the thiolate ligand is accompanied by profound changes in the photophysical properties. A broad, structureless, low-energy band appears in the absorption spectra, not present in the spectra of PtL^<i>n</i>Cl. In the photoluminescence spectra, the characteristic, highly structured phosphorescence bands of PtL^<i>n</i>Cl in the green region are replaced by a broad, structureless emission band in the red region. These new bands are assigned to a π_S/d_Pt → π*_N^∧C^∧N charge-transfer transition from the thiolate/platinum to the N^∧C^∧N ligand. This assignment is supported by electrochemical data and TD-DFT calculations and by the observation that the decreasing energies of the bands correlate with the electron-donating ability of the substituent, as do the increasing nonradiative decay rate constants, in line with the energy-gap law. However, the pair of nitro-substituted complexes do not fit the trends. Their properties, including much longer luminescence lifetimes, indicate that the lowest-energy excited state is localized predominantly on the arenethiolate ligand for these two complexes. Red-emitting thiolate adducts may be relevant to the use of PtL^<i>n</i>Cl complexes in bioimaging, as revealed by the different distributions of emission intensity within live fibroplast cells doped with the parent complex, according to the region of the spectrum examined

Highly Luminescent Dinuclear Platinum(II) Complexes Incorporating Bis-Cyclometallating Pyrazine-Based Ligands: A Versatile Approach to Efficient Red Phosphors

A series of luminescent dinuclear platinum­(II) complexes incorporating diphenylpyrazine-based bridging ligands (L^<i>n</i>H₂) has been prepared. Both 2,5-diphenylpyrazine (L²H₂) and 2,3-diphenylpyrazine (L³H₂) are able to undergo cyclometalation of the two phenyl rings, with each metal ion binding to the two nitrogen atoms of the central heterocycle, giving, after treatment with the anion of dipivaloyl methane (dpm), complexes of formula {Pt­(dpm)}₂L^<i>n</i>. These compounds are isomers of the analogous complex of 4,6-diphenylpyrimidine (L¹H₂). Related complexes of dibenzo­(f,h)­quinoxaline (L⁴H₂), 2,3-diphenyl-quinoxaline (L⁵H₂), and dibenzo­[3,2-a:2′,3′-c]­phenazine (L⁶H₂) have also been prepared, allowing the effects of strapping together the phenyl rings (L⁴H₂ and L⁶H₂) and/or extension of the conjugation from pyrazine to quinoxaline (L⁵H₂ and L⁶H₂) to be investigated. In all cases, the corresponding mononuclear complexes, Pt­(dpm)­L^<i>n</i>H, have been isolated too. All 12 complexes are phosphorescent in solution at ambient temperature. Emission spectra of the dinuclear complexes are consistently red shifted compared to their mononuclear analogues, as are the lowest energy absorption bands. Electrochemical data and TD-DFT calculations suggest that this effect arises primarily from stabilization of the LUMO. Introduction of the second metal ion also has the effect of substantially increasing the molar absorptivity and, in most cases, the radiative rate constants. Meanwhile, extension of conjugation in the heterocycle of L⁵H₂ and L⁶H₂ and planarization of the aromatic system favored by interannular bond formation in L⁴H₂ and L⁶H₂ leads to further red shifts of the absorption and emission spectra to wavelengths that are unusually long for cyclometalated platinum­(II) complexes. The results may offer a versatile design strategy for tuning and optimizing the optical properties of d-block metal complexes for contemporary applications

Palladium-Catalyzed Direct Arylation of Luminescent Bis-Cyclometalated Iridium(III) Complexes Incorporating <i>C</i>^<i>N-</i> or <i>O</i>^<i>O</i>‑Coordinating Thiophene-Based Ligands: an Efficient Method for Color Tuning

We report the palladium-catalyzed direct 5-arylation of both metalated and nonmetalated thiophene moieties of iridium complexes <b>2</b>, <b>3</b>, and <b>4</b> with aryl halides via C–H bond functionalization. This method opens new routes to varieties of Ir complexes in only one step, allowing easy modification of the nature of the ligand. The photophysical properties of the new functionalized complexes have been studied by means of absorption and emission spectroscopy. The extension of the π-conjugated system induces a bathochromic and hyperchromic shift of the absorption spectra, an effect reproduced by first principle calculations. Indeed, the bathochromic shifts are related to a more delocalized nature of the excited-states. All complexes are photoluminescent in the red region of the spectrum. This study establishes that arylation of the thienyl ring affects strongly the electronic properties of the resulting complexes, even when the thienyl ring is remote and not directly metalated to the iridium center, as in the thienyltrifluoroacetonate complex <b>4</b>

Palladium-Catalyzed Direct Arylation of Luminescent Bis-Cyclometalated Iridium(III) Complexes Incorporating <i>C</i>^<i>N-</i> or <i>O</i>^<i>O</i>‑Coordinating Thiophene-Based Ligands: an Efficient Method for Color Tuning

We report the palladium-catalyzed direct 5-arylation of both metalated and nonmetalated thiophene moieties of iridium complexes <b>2</b>, <b>3</b>, and <b>4</b> with aryl halides via C–H bond functionalization. This method opens new routes to varieties of Ir complexes in only one step, allowing easy modification of the nature of the ligand. The photophysical properties of the new functionalized complexes have been studied by means of absorption and emission spectroscopy. The extension of the π-conjugated system induces a bathochromic and hyperchromic shift of the absorption spectra, an effect reproduced by first principle calculations. Indeed, the bathochromic shifts are related to a more delocalized nature of the excited-states. All complexes are photoluminescent in the red region of the spectrum. This study establishes that arylation of the thienyl ring affects strongly the electronic properties of the resulting complexes, even when the thienyl ring is remote and not directly metalated to the iridium center, as in the thienyltrifluoroacetonate complex <b>4</b>

Palladium-Catalyzed Direct Arylation of Luminescent Bis-Cyclometalated Iridium(III) Complexes Incorporating <i>C</i>^<i>N-</i> or <i>O</i>^<i>O</i>‑Coordinating Thiophene-Based Ligands: an Efficient Method for Color Tuning

We report the palladium-catalyzed direct 5-arylation of both metalated and nonmetalated thiophene moieties of iridium complexes <b>2</b>, <b>3</b>, and <b>4</b> with aryl halides via C–H bond functionalization. This method opens new routes to varieties of Ir complexes in only one step, allowing easy modification of the nature of the ligand. The photophysical properties of the new functionalized complexes have been studied by means of absorption and emission spectroscopy. The extension of the π-conjugated system induces a bathochromic and hyperchromic shift of the absorption spectra, an effect reproduced by first principle calculations. Indeed, the bathochromic shifts are related to a more delocalized nature of the excited-states. All complexes are photoluminescent in the red region of the spectrum. This study establishes that arylation of the thienyl ring affects strongly the electronic properties of the resulting complexes, even when the thienyl ring is remote and not directly metalated to the iridium center, as in the thienyltrifluoroacetonate complex <b>4</b>