Sterically Encumbered Aryl Isocyanides Extend Excited-State Lifetimes and Improve the Photocatalytic Performance of Three-Coordinate Copper(I) ß-Diketiminate Charge-Transfer Chromophores.

Copper(I) complexes are prominent candidates to replace noble metal-based photosensitizers. We recently introduced a three-coordinate design for copper(I) charge-transfer chromophores that pair ß-diketiminate ligands with aryl isocyanides. The excited-state lifetime in these compounds can be extended using a bichromophoric "triplet reservoir" strategy, which comes at the expense of a decrease in excited-state energy and reducing power. In this work, we introduce a complementary, sterically driven strategy for increasing the excited-state lifetimes of these photosensitizers, which gives a higher-energy, more strongly reducing charge-transfer triplet state than does the bichromophore approach. The compounds presented (Cu1-Cu4) have the general formula Cu(CyNacNacMe)(CN-Ar), where CyNacNacMe is a cyclohexyl-substituted ß-diketiminate and CN-Ar is an aryl isocyanide with a variable steric profile. Their structural features and electrochemical and photophysical properties are described. The complexes with sterically encumbered 2,6-diisopropylphenyl or m-terphenyl isocyanide ligands (Cu2-Cu4) exhibit prolonged excited-state lifetimes relative to those of the parent 2,6-dimethylphenyl isocyanide compound Cu1. Specifically, one of the m-terphenyl isocyanide compounds, Cu3, displays an excited-state lifetime of 276 ns, approximately 30 times longer than that of Cu1 (9.3 ns). The photoluminescence quantum yield of Cu3 (0.09) also increases by two orders of magnitude compared to that of Cu1 (0.0008). The strong excited-state reducing power (*Eox = -2.4 V vs Fc+/0) and long lifetime of Cu3 lead to higher yields in photoredox and photocatalytic isomerization reactions, which include dehalogenation and/or hydrodgenation of benzophenone substrates, C-O bond activation of a lignin model substrate, and photocatalytic E/Z isomerization of stilbene.

Long Excited-State Lifetimes in Three-Coordinate Copper(I) Complexes via Triplet-Triplet Energy Transfer to Pyrene-Decorated Isocyanides.

There has been much effort to improve excited-state lifetimes in photosensitizers based on earth-abundant first-row transition metals. Copper(I) complexes have gained significant attention in this field, and in most cases, sterically driven approaches are used to optimize their lifetimes. This study presents a series of three-coordinate copper(I) complexes (Cu1-Cu3) where the excited-state lifetime is extended by triplet-triplet energy transfer. The heteroleptic compounds feature a cyclohexyl-substituted ß-diketiminate (CyNacNacMe) paired with aryl isocyanide ligands, giving the general formula Cu(CyNacNacMe)(CN-Ar) (CN-dmp = 2,6-dimethylphenyl isocyanide for Cu1; CN-pyr = 1-pyrenyl isocyanide for Cu2; CN-dmp-pyr = 2,6-dimethyl-4-(1-pyrenyl)phenyl isocyanide for Cu3). The nature, energies, and dynamics of the low-energy triplet excited states are assessed with a combination of photoluminescence measurements at room temperature and 77 K, ultrafast transient absorption (UFTA) spectroscopy, and DFT calculations. The complexes with the pyrene-decorated isocyanides (Cu2 and Cu3) exhibit extended excited-state lifetimes resulting from triplet-triplet energy transfer (TTET) between the short-lived charge-transfer excited state (3CT) and the long-lived pyrene-centered triplet state (3pyr). This TTET process is irreversible in Cu3, producing exclusively the 3pyr state, and in Cu2, the 3CT and 3pyr states are nearly isoenergetic, enabling reversible TTET and long-lived 3CT luminescence. The improved photophysical properties in Cu2 and Cu3 result in improvements in activity for both photocatalytic stilbene E/Z isomerization via triplet energy transfer and photoredox transformations involving hydrodebromination and C-O bond activation. These results illustrate that the extended excited-state lifetimes achieved through TTET result in newly conceived photosynthetically relevant earth-abundant transition metal complexes.

Combined Nucleophilic and Electrophilic Functionalization to Optimize Blue Phosphorescence in Cyclometalated Platinum Complexes.

Ligand-based functionalization strategies have emerged as powerful approaches to tune and optimize blue phosphorescence, which can involve nucleophilic addition to coordinated ligands or electrophilic functionalization via the coordination of exogenous Lewis acids. Whereas both have been used separately to enhance the photophysical properties of organometallic compounds with high-energy triplet states, in this work, we show that these two strategies can be used together on the same platform. Isocyanide-supported cyclometalated platinum compounds undergo nucleophilic addition with diethylamine to form a strong σ-donor acyclic diaminocarbene-supporting ligand. In a subsequent step, a cyanide ancillary ligand is converted into a more strongly π-acidic isocyanoborate via the coordination of a borane Lewis acid. Importantly, both of these ligand-based functionalization steps improve the quantum yields and lifetimes of the blue-phosphorescent complexes. This synergy results in complexes with photoluminescence quantum yields up to 0.40 for deep blue and 0.75 for sky blue regions and PL lifetimes on the order of 10-5 s.

Substituent-Dependent Azide Addition to Isocyanides Generates Strongly Luminescent Iridium Complexes.

Ligand-centered functionalization reactions offer diverse strategies to prepare luminescent organometallic compounds. These compounds can have unique structures that are not accessible via traditional coordination chemistry and can possess enhanced or unusual photophysical properties. Here we show that bis-cyclometalated iridium bis-isocyanide complexes (1) react with azide (N3-) to form novel luminescent structures. The fate of the reaction with azide is determined primarily by the substituent on the aryl isocyanide. Those with electron-withdrawing substituents (CF3 or NO2) react with 1 equiv of azide followed by N2 extrusion, forming aryl cyanamido products (2). With electron-donating groups on the aryl isocyanide the reactivity is more diverse, and three outcomes are possible. In two cases, the isocyanide and azide undergo a [3 + 2] cycloaddition to form a C-bound tetrazolato structure (3). In three other cases, 2 equiv of azide are involved in the formation of a previously unobserved structure, where a tetrazolato and aryl cyanamido couple and rearrange to form a chelating ligand comprised of an N-bound tetrazolato and an acyclic diaminocarbene (4). Finally, a bimetallic aryl cyanamido complex (5) is isolated in one case. All compounds are luminescent, some with exceptional photoluminescence quantum yields as high as 0.81 in solution for sky-blue emission, and 0.87 for yellow emission and 0.65 for orange-red emission in polymer films.

Enhanced Deep-Red Phosphorescence in Cyclometalated Iridium Complexes with Quinoline-Based Ancillary Ligands.

Compounds with good photoluminescence quantum yields (ΦPL) in the deep-red to near-infrared parts of the spectrum are desired for a variety of applications in optoelectronics, imaging, and sensing. However, in this region of the spectrum, quantum yields are usually modest, which is explained by the energy gap law and the inherently slower radiative decay rates for low-energy emitters according to the second-order perturbation theory. In this work, we outline a new direction in deep-red luminescence, introducing a new suite of bis-cyclometalated iridium complexes with efficient luminescence beyond 650 nm. Seven new complexes are prepared using two different cyclometalating (C^N) ligands with four quinoline-derived ancillary ligands (L^X). The chosen cyclometalating ligands are well-established to produce deep-red phosphorescence and include a metalated phenyl ring appended to a conjugated heterocycle. The ancillary ligands combine a rigid quinoline or benzoquinoline "L" donor with a variable anionic "X" donor comprised of an O-donor aryloxy or carboxylate or an N-donor amidate. These complexes phosphoresce in the deep-red region with wavelengths between 650 and 700 nm and solution quantum yields between 0.018 and 0.42.

Triplet-Triplet Energy Transfer in Bis-Cyclometalated Iridium Complexes with Pyrene-Substituted Isocyanides.

Nonlinear optical (NLO) materials are able to modulate responses of electromagnetic radiation, leading to phenomena critical to modern telecommunications technologies. The last two decades have seen significant advances in the area of molecular nonlinear chromophores, particularly with respect to reverse-saturable absorption (RSA). Here, we introduce a strategy for intense excited-state absorption (ESA) that involves bis-cyclometalated iridium complexes with isocyanide ancillary ligands decorated with pyrene triplet acceptors. Upon excitation, the complexes undergo rapid triplet-triplet energy transfer (TTET) to the acceptor excited states. This report describes five bis-cyclometalated iridium complexes using two different pyrene-substituted isocyanides with the general formula [Ir(C^N)2(CNAr)2]PF6 (C^N = cyclometalating ligand, CNAr = isocyanide ancillary ligand: CNArpyr = 2,6-dimethyl-4-(1-pyrenyl)phenyl isocyanide, CNpyr = 1-pyrenyl isocyanide). The synthesized complexes were thoroughly characterized via 1H and 13C{1H} NMR spectroscopy, Fourier-transform Infrared spectroscopy, and electrospray ionization mass spectrometry. The excited states were evaluated with UV-vis absorption, steady-state and time-resolved photoluminescence, and transient absorption spectroscopy. Phosphorescence is completely quenched at room temperature, but in the solvent glass matrix at 77 K, there is luminescence originating from a π → π* triplet state on the pyrene moiety, abbreviated herein as 3pyrene. All five complexes display intense and long-lived ESA originating from the 3pyrene state. The localization of the ground-state absorption on the cyclometalating ligands and the excited-state absorption on the pyrene moiety allows for independent tuning of ground-state absorption (GSA) and ESA to optimize RSA and other NLO attributes.

Modular Imine Chelates with Variable Anionic Donors Promote Red Phosphorescence in Cyclometalated Iridium Complexes.

The lack of red and deep-red emitting molecular phosphors with high photoluminescence quantum yields remains a significant fundamental challenge and has implications in optoelectronic technologies for color displays and other consumer products. In this work, we introduce a series of seven new red or deep-red emitting heteroleptic bis-cyclometalated iridium(III) complexes, supported by five different ancillary ligands (L^X) from the salicylaldimine and 2-picolinamide families. Previous work had shown that electron-rich anionic chelating "L^X" ligands can be effective in supporting efficient red phosphorescence, and the complementary approach described here, in addition to being synthetically simpler, offers two key advantages over the previous designs. First, the "L" and "X" functionalities can be independently tuned, providing excellent control over the electronic energy levels and excited-state dynamics. Second, these classes of L^X ligands can have beneficial impacts on the excited-state dynamics but do not significantly perturb the emission color profile. Cyclic voltammetry experiments show that the substituents on the L^X ligand impact the HOMO energy but have a minimal effect on the LUMO energy. Photoluminescence measurements reveal that all the compounds luminesce in the red or deep-red region as a function of the cyclometalating ligand and exhibit exceptionally high photoluminescence quantum yields (ΦPL), comparable or superior to the best-performing red-emitting iridium complexes.

Luminescent Platinum Complexes with π-Extended Aryl Acetylide Ligands Supported by Isocyanides or Acyclic Diaminocarbenes.

In this work, we present a series of luminescent platinum acetylide complexes with acetylides that are electronically modified and/or π-extended. Six isocyanide-supported complexes with the general formula cis-[Pt(CNAr)2(C≡CR)2] and six acyclic diaminocarbene (ADC) complexes of the form trans-[Pt(ADC)2(C≡CR)2], all using the same five acetylide ligands, are described. The compounds are characterized by multinuclear NMR, FT-IR, and single-crystal X-ray diffraction. In most cases, the phosphorescence arises from an acetylide-centered 3(π → π*) excited state, although in one of the isocyanide compounds there is evidence for a charge-transfer excited state. The photoluminescence wavelength depends strongly on the substitution pattern and extent of the π conjugation on the acetylide, with maxima spanning the range of ca. 460-540 nm. Most photoluminescence lifetimes are long, beyond 50 µs, and quantum yields are low to moderate, 0.043-0.27. The photoluminescence quantum yields and lifetimes in these compounds do not systematically improve in the ADC complexes compared to the isocyanide versions, suggesting the neutral ligand σ-donor character does not play a large role in the excited-state dynamics when the triplet excited state is delocalized over a large π system.

Sterically Encumbered Heteroleptic Copper(I) ß-Diketiminate Complexes with Extended Excited-State Lifetimes.

One of the main challenges in developing effective copper(I) photosensitizers is their short excited-state lifetimes, usually attributed to structural distortion upon light excitation. We have previously introduced copper(I) charge-transfer chromophores of the general formula Cu(N^N)(ArNacNac), where N^N is a conjugated diimine ligand and ArNacNac is a substituted ß-diketiminate ligand. These chromophores were promising regarding their tunable redox potentials and intense visible absorption but were ineffective as photosensitizers, presumably due to short excited-state lifetimes. Here, we introduce sterically crowded analogues of these heteroleptic chromophores with bulky alkyl substituents on the N^N and/or ArNacNac ligand. Structural analysis was combined with electrochemical and photophysical characterization, including ultrafast transient absorption (UFTA) spectroscopy to investigate the effects of the alkyl groups on the excited-state lifetimes of the complexes. The molecular structures determined by single-crystal X-ray diffraction display more distortion in the ground state as alkyl substituents are introduced into the phenanthroline or the NacNac ligand, showing smaller τ4 values due to the steric hindrance. UFTA measurements were carried out to determine the excited-state dynamics. Sterically encumbered Cu5 and Cu6 display excited-state lifetimes 15-20 times longer than unsubstituted complex Cu1, likely indicating that the incorporation of bulky alkyl substituents inhibits the pseudo-Jahn-Teller (PJT) flattening distortion in the excited state. This work suggests that the steric properties of these heteroleptic copper(I) charge-transfer chromophores can be readily modified and that the excited-state dynamics are strongly responsive to these modifications.

Trimetallic Iridium-Nickel-Iridium Bis(formazanate) Assemblies.

Formazans have attracted a lot of attention in coordination chemistry since the early 1940s because of their unique properties engendered by the nitrogen-rich conjugated backbone. Although many studies have been done using formazanates to chelate transition metals, research using formazanates as building blocks for polynuclear compounds and supramolecular chemistry remains rare. In this paper, we describe a synthetic strategy that uses a pyridyl-substituted bis(formazanato)nickel complex as a metalloligand to further assemble with two [Ir(C^N)2]+ centers (C^N is the cyclometalating ligand). The trimetallic complexes represent a new binding mode for flexidentate pyridyl-substituted formazanates and a new structural class of polynuclear formazanate complexes. This work expands the chemistry of polynuclear formazanate complexes, for the first time pairing 3d and 5d metals in the same assembly. The redox chemistry of these trimetallic complexes, evaluated via cyclic voltammetry, is described. The compounds described in this work are luminescent, and studies of bis-cyclometalated iridium model complexes lacking the formazanate bridge confirm that the phosphorescence arises from the iridium center.

Panchromatic Excited-State Absorption in Bis-Cyclometalated Iridium Isocyanide Complexes.

In this work, we introduce a series of cyclometalated iridium complexes and evaluate the suitability of this class of compounds in nonlinear optical (NLO) applications, with an emphasis on long-lived, panchromatic reverse-saturable absorption (RSA). The investigated complexes are represented by the general formula [Ir(C^N)2(CNArdmp)2]+, (C^N = cyclometalating ligand, CNArdmp = 2,6-dimethylphenyl isocyanide). Seven such complexes were synthesized and characterized, including in-depth analysis of their photophysical properties (UV-vis absorption, photoluminescence, and transient absorption). This series of compounds contains seven different cyclometalating ligands (2-phenylbenzothiazole (bt) (Ir1), 5-nitro-2-phenylpyridine (ppyNO2) (Ir2), 5-nitro-2-(9-phenanthryl)pyridine (phenNO2) (Ir3), 2-(benzo[b]thiophen-2-yl)quinoline (btq) (Ir4), 6-(benzo[b]thiophen-2'-yl)phenanthridine (btph) (Ir5), 2,4-diphenylquinoline (dpq) (Ir6), and 6-nitro-2,4-diphenylquinoline (NO2dpq) (Ir7)), which have profound effects on their ground-state and excited-state absorption spectra. To evaluate the effects of the isocyanide ancillary ligands, some heteroleptic bis-cyclometalated iridium(III) acetylacetonate (acac) analogue complexes are included as points of comparison. In the ground state, the bis-isocyanide complexes display UV-vis absorption with the characteristic 1LC (π → π*) band at λ < 350 nm and 1MLCT bands at 350-500 nm. Five of the complexes (Ir1, Ir2, Ir4, Ir5, and Ir6) display broad, intense triplet excited-state absorption with no ground-state bleach (GSB) over the spectral window of 400-900 nm, with excited-state lifetimes spanning three orders of magnitude from ∼32 ns to 12 µs. The photophysical data suggests that the isocyanide ancillary ligand blue-shifts the GSB transient into the UV, which is normally found in the middle of the visible region for cyclometalated iridium complexes. This study demonstrates the applicability of cationic cyclometalated iridium(III) bis-isocyanide complexes as candidate RSA materials.

Platinum(II)-Substituted Phenylacetylide Complexes Supported by Acyclic Diaminocarbene Ligands.

We introduce phosphorescent platinum aryl acetylide complexes supported by tert-butyl-isocyanide and strongly σ-donating acyclic diaminocarbene (ADC) ligands. The precursor complexes cis-[Pt(CNtBu)2(C≡CAr)2] (4a-4f) are treated with diethylamine, which undergoes nucleophilic addition with one of the isocyanides to form the cis-[Pt(CNtBu)(ADC)(C≡CAr)2] complexes (5a-5f). The new compounds incorporate either electron-donating groups (4-OMe and 4-NMe2) or electron-withdrawing groups [3,5-(OMe)2, 3,5-(CF3)2, 4-CN, and 4-NO2] on the aryl acetylide. Experimental HOMO-LUMO gaps, estimated from cyclic voltammetry, span the range of 2.68-3.61 eV and are in most cases smaller than the unsubstituted parent complex, as corroborated by DFT. In the ADC complexes, peak photoluminescence wavelengths span the range of 428 nm (2a, unsubstituted phenylacetylide) to 525 nm (5f, 4-NO2-substituted), with the substituents inducing a red shift in all cases. The phosphorescence E0,0 values and electrochemical HOMO-LUMO gaps are loosely correlated, showing that both can be reduced by either electron-donating or electron-withdrawing substituents on the aryl acetylides. The photoluminescence quantum yields in the ADC complexes are between 0.044 and 0.31 and the lifetimes are between 4.8 and 14 µs, a factor of 1.8-10× higher (for ΦPL) and 1.2-3.6× longer (for τ) than the respective isocyanide precursor (ΦPL = 0.014-0.12, τ = 2.8-8.2 µs).

Coordination-Driven Self-Assembly of Cyclometalated Iridium Squares Using Linear Aromatic Diisocyanides.

Here, we demonstrate facile [4 + 4] coordination-driven self-assembly of cyclometalated iridium(III) using linear aryldiisocyanide bridging ligands (BLs). A family of nine new [Ir(C^N)2(µ-BL)]44+ coordination cages is described, where C^N is the cyclometalating ligand-2-phenylpyridine (ppy), 2-phenylbenzothiazole (bt), or 1-phenylisoquinoline (piq)-and BL is the diisocyanide BL, with varying spacer lengths between the isocyanide binding sites. These supramolecular coordination compounds are prepared via a one-pot synthesis, with isolated yields of 40-83%. 1H NMR spectroscopy confirms the selective isolation of a single product, which is affirmed to be the M4L4 square by high-resolution mass spectrometry. Detailed photophysical studies were carried out to reveal the nature of the luminescent triplet states in these complexes. In most cases, phosphorescence arises from the [Ir(C^N)2]+ nodes, with the emission color determined by the cyclometalating ligand. However, in two cases, the lowest-energy triplet state resides on the aromatic core of the BL, and weak phosphorescence from that state is observed. This work shows that aromatic diisocyanide ligands enable coordination-driven assembly of inert iridium(III) nodes under mild conditions, producing supramolecular coordination complexes with desirable photophysical properties.

Cyano-Isocyanide Iridium(III) Complexes with Pure Blue Phosphorescence.

In this paper, we report a series of six neutral, blue-phosphorescent cyclometalated iridium complexes of the type Ir(C^Y)2(CNAr)(CN). The cyclometalating ligands in these compounds (C^Y) are either aryl-substituted 1,2,4-triazole or NHC ligands, known to produce complexes with blue phosphorescence. These cyclometalating ligands are paired with π-acidic, strongly σ-donating cyano and aryl isocyanide (CNAr) ancillary ligands, the hypothesis being that these ancillary ligands would destabilize the higher-lying ligand-field (d-d) excited states, allowing efficient blue photoluminescence. The compounds are prepared by substituting the cyanide ancillary ligand onto a chloride precursor and are characterized by NMR, mass spectrometry, infrared spectroscopy, and, for five of the compounds, by X-ray crystallography. Cyclic voltammetry establishes that these compounds have large HOMO-LUMO gaps. The mixed cyano-isocyanide compounds are weakly luminescent in solution, but they phosphoresce with moderate to good efficiency when doped into poly(methyl methacrylate) films, with Commission Internationale de L'Eclairage coordinates that indicate deep blue emission for five of the six compounds. The photophysical studies show that the photoluminescence quantum yields are greatly enhanced in the cyano complexes relative to the chloride precursors, affirming the benefit of strong-field ancillary ligands in the design of blue-phosphorescent complexes. Density functional theory calculations confirm that this enhancement arises from a significant destabilization of the higher-energy ligand-field states in the cyanide complexes relative to the chloride precursors.

Dinuclear Complexes of Flexidentate Pyridine-Substituted Formazanate Ligands.

The utility of flexidentate pyridyl-substituted formazanate ligands for assembling dinuclear coordination complexes with iridium(III) and/or platinum(II) building blocks is demonstrated herein. The dinuclear complexes are prepared either via a stepwise strategy, adding one metal unit at a time, or via one-pot self-assembly. Eight of the new complexes, including both mononuclear precursors and dinuclear products, are structurally characterized by single-crystal X-ray diffraction and NMR spectroscopy, revealing several distinct binding modes of the formazanates. All complexes are characterized by UV/Vis absorption spectroscopy and cyclic voltammetry. The frontier orbitals are primarily localized on the formazanate ligand, and a characteristic, intense formazanate-centered π→π* absorption band is observed in the absorption spectra.

Efficient Deep Blue Platinum Acetylide Phosphors with Acyclic Diaminocarbene Ligands.

Here we report five blue-phosphorescent platinum bis-phenylacetylide complexes with an investigation of their photophysical and electrochemical attributes. Three of the complexes (1-3) are of the general formula cis-Pt(CNR)2 (C≡CPh)2 , in which CNR is a variably substituted isocyanide and C≡CPh is phenylacetylide. These isocyanide complexes serve as precursors for complexes of the general formula cis-Pt(CNR)(ADC)(C≡CPh)2 (4 and 5), in which ADC is an acyclic diaminocarbene installed by amine nucleophilic addition to one of the isocyanides. All of the complexes exhibit deep blue phosphorescence with λmax ∼430 nm in poly(methyl methacrylate) (PMMA) thin films. Whereas isocyanide complexes 1-3 exhibit modest photoluminescence quantum yields (ΦPL ), incorporation of one acyclic diaminocarbene ligand results in a three-fold to 16-fold increase in ΦPL while still maintaining an identical deep blue color profile.

Room-Temperature Phosphorescent Platinum(II) Alkynyls with Microsecond Lifetimes Bearing a Strong-Field Pincer Ligand.

The use of organometallic triplet emitters in organic light emitting diodes (OLEDs) is motivated by the premise of efficient intersystem crossing leading to unit internal quantum efficiencies. However, since most devices are based on solid-state components, an inherent limitation to square-planar platinum(II) phosphors is their tendency toward aggregation-based quenching. Here, a new class of emissive, four-coordinate PtII species based on the bisimidazolyl carbazolide (BIMCA) ligand is introduced, which displays highly efficient, long-lived solid-state phosphorescence at room temperature. A set of four BIMCAPt phenyl acetylides were synthesized that emit in the green (λmax =507-540 nm) with >60 % quantum yield and millisecond lifetimes. The structures of the resulting species reveal a non-planar structure imposed by steric clashes between BIMCA and the iodo or alkynyl co-ligand. Ground-state and photophysical characterization are presented. Density functional theory calculations indicate that the BIMCA ligand dominates the frontier orbitals along with the first Franck-Condon singlet and triplet excited states.

Facile Synthesis of Luminescent Ir-Pt-Ir Trimetallic Complexes.

A new class of luminescent, heterotrimetallic supramolecular constructs partnering two bis-cyclometalated iridium centers with a diimine platinum acetylide center is introduced. Whereas most supramolecular constructs featuring cyclometalated iridium involve elaborate bridging ligands and are prepared under forcing conditions with low to moderate yields, the three Ir-Pt-Ir complexes described here are prepared at room temperature from simple precursors and isolated in near-quantitative yields. ESI-MS, NMR spectroscopy, and diffusion ordered spectroscopy confirm the identity and homogeneity of the trimetallic products. In comparison with monometallic model complexes, analysis of UV/Vis absorption, steady-state photoluminescence and time-resolved emission reveals the impacts of supramolecular assembly on the photophysical properties. UV/Vis absorption and cyclic voltammetry suggest perturbation of some frontier orbital energies as a result of assembly, and the emission spectra and lifetimes reveal efficient excited-state energy transfer via a Dexter mechanism, and show that the site of luminescence (platinum or iridium) depends on the identity of the cyclometalating ligand bound to iridium.

Ancillary Ligand Effects on Red-Emitting Cyclometalated Iridium Complexes.

In this work, a series of ten new red-emitting heteroleptic iridium(III) complexes of the type Ir(C^N)2 (L^X) (C^N=cyclometalating ligand, L^X=monoanionic chelating ancillary ligand) is introduced. The suite of new complexes includes two different cyclometalating ligands and five different ancillary ligands, with the primary goal of investigating the effect of the ancillary ligand structure on the excited-state dynamics. The structural variety of the ancillary ligands permitted investigations of the effects of donor atom identity, chelate ring size, and substituents on the electronic structure and excited state properties. Electrochemical analysis showed that the ancillary ligand has a substantial effect on the energy of the HOMO, whereas the LUMO is left unperturbed. Photoluminescence spectra showed that the ancillary ligand can sometimes strongly influence the emission wavelength, but in all cases is an important determinant of the excited-state dynamics.

Formazanate Complexes of Bis-Cyclometalated Iridium.

In this work we describe a series of bis-cyclometalated iridium(III) formazanate complexes, expanding the coordination chemistry of the redox-active formazanate class to iridium. A total of 18 new complexes are described, varying the substituent pattern on the formazanate and the identity of the cyclometalating ligand on iridium. Eight of the new compounds are structurally characterized by single-crystal X-ray diffraction, which along with NMR spectroscopy evinces two binding modes of the formazanate. Two of the compounds are isolated in a C2-symmetric geometry where the formazanate is bound in a six-member chelate "closed" conformation, involving the 1- and 5-positions of the 1,2,4,5-tetraazapentadienyl formazanate core. In most of the examples, the major isomer that forms and is exclusively isolated involves the formazanate bound in a five-member chelate "open" form, coordinating through the 1- and 4-positions of the formazanate core and resulting in C1 point-group symmetry. All complexes are characterized by UV-vis absorption spectroscopy and cyclic voltammetry, with these features depending primarily on the substitution pattern on the formazanate, and to a lesser extent on the identity of the cyclometalating ligand and formazanate binding mode.