Photophysical properties of three-coordinate heteroleptic Cu(I) β-diketiminate triarylphosphine complexes.

A series of heteroleptic copper(I) β-diketiminate triarylphosphine complexes is reported, having the general formula Cu(RNacNac)(PPh), where RNacNac is a substituted β-diketiminate and PPh is a triphenylphosphine derivative. A total of five different RNacNac ligands and three different triarylphosphines are used to assemble the nine complexes. The syntheses, X-ray crystal structures, cyclic voltammograms, and UV-vis absorption spectra of all compounds are described.

Ancillary Ligand Steric Effects and Cyclometalating Ligand Substituents Control Excited-State Decay Kinetics in Red-Phosphorescent Platinum Complexes.

Achieving high-efficiency red phosphorescence remains a significant challenge, especially in cyclometalated platinum complexes where radiative rates are inherently slower than their iridium counterparts. In this work, six red-emitting cyclometalated platinum complexes of the formula Pt(CN)[(Ar)acNac] (CN is the cyclometalating ligand, and (Ar)acNac is an aryl-substituted β-ketoiminate ancillary ligand) were synthesized and characterized. Two CN ligands were employed, 1-phenylisoquinoline (piq) and its cyano-substituted analogue 1-phenylisoquinoline-4-carbonitrile (piqCN), which both result in red phosphorescence in cyclometalated platinum complexes.

Dual-emitting cyclometalated Ir(III) complexes with salicylaldimine-bound fluorophores for ratiometric oxygen sensing.

A new class of oxygen-sensing dual-emitting cyclometalated Ir(III) complexes is described. They function as ratiometric sensors that combine the blue fluorescence from coumarin as a self-referenced internal standard with yellow to red phosphorescence from bis-cyclometalated iridium complexes. The compounds have phosphorescence quantum yields up to 15%, lifetimes ranging from 0.

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 (-) where the excited-state lifetime is extended by triplet-triplet energy transfer.

Sterically Encumbered Aryl Isocyanides Extend Excited-State Lifetimes and Improve the Photocatalytic Performance of Three-Coordinate Copper(I) β-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 β-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.

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.

Improved transition metal photosensitizers to drive advances in photocatalysis.

To function effectively in a photocatalytic application, a photosensitizer's light absorption, excited-state lifetime, and redox potentials, both in the ground state and excited state, are critically important. The absorption profile is particularly relevant to applications involving solar harvesting, whereas the redox potentials and excited-state lifetimes determine the thermodynamics, kinetics, and quantum yields of photoinduced redox processes. This perspective article focuses on synthetic inorganic and organometallic approaches to optimize these three characteristics of transition-metal based photosensitizers.

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 -[Pt(CNAr)(C≡CR)] and six acyclic diaminocarbene (ADC) complexes of the form -[Pt(ADC)(C≡CR)], all using the same five acetylide ligands, are described. The compounds are characterized by multinuclear NMR, FT-IR, and single-crystal X-ray diffraction.

Sterically Encumbered Heteroleptic Copper(I) β-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 β-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.

Reductive photoredox transformations of carbonyl derivatives enabled by strongly reducing photosensitizers.

Visible-light photoredox catalysis is well-established as a powerful and versatile organic synthesis strategy. However, some substrate classes, despite being attractive precursors, are recalcitrant to single-electron redox chemistry and thus not very amenable to photoredox approaches. Among these are carbonyl derivatives, ketones, aldehydes, and imines, which in most cases require Lewis or Brønsted acidic additives to activate photoinduced electron transfer.

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.

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.

Efficient blue-phosphorescent -bis(acyclic diaminocarbene) platinum(ii) acetylide complexes.

The lack of efficient and robust deep-blue phosphorescent metal complexes remains a significant challenge in the context of electroluminescent color displays. The emissive triplet states of blue phosphors are deactivated by low-lying metal-centered (MC) states, which can be ameliorated by increasing the σ-donating ability of the supporting ligands. Here we unveil a synthetic strategy to access blue-phosphorescent complexes with two supporting acyclic diaminocarbenes (ADCs), known to be even stronger σ-donors than N-heterocyclic carbenes (NHCs).

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

Compounds with good photoluminescence quantum yields (Φ) 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.

Ratiometric oxygen sensors of cyclometalated iridium(III) with enhanced quantum yields and variable dynamic ranges.

Four luminescent ratiometric oxygen sensors, pairing phosphorescent cyclometalated iridium with coumarin or BODIPY fluorophores, are presented here. These compounds realize three key improvements over our previous designs, namely higher phosphorescence quantum yields, the ability to access intermediate dynamic ranges better suited for typical atmospheric O levels, and the possibility of using visible excitation instead of UV. These ratiometric sensors are accessed very simple, 1-step syntheses involving direct reaction of the chloro-bridged cyclometalated iridium dimer with the pyridyl-substituted fluorophore.

Luminescent 1-1,3-benzazaphospholes.

2-R-1-1,3-Benzazaphospholes (R-BAPs) are an interesting class of σP heterocycles containing P[double bond, length as m-dash]C bonds. While closely related 2-R-1,3-benzoxaphospholes (R-BOPs) have been shown to be highly photoluminescent materials depending on specific R substituents, photoluminescence of R-BAPs has been previously limited to an example having a fused carbazole ring system. Here we detail the synthesis and structural characterization of a new R-BAP (3c, R = 2,2'-dithiophene), and compare its photoluminescence against two previously reported R-BAPs (3a, R, R' = Me and 3b, R = 2-thiophene).

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 () react with azide (N) to form novel luminescent structures.

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)(CNAr)], (C^N = cyclometalating ligand, CNAr = 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).

Heteroleptic copper(I) charge-transfer chromophores with panchromatic absorption.

Four new heteroleptic bis-chelate Cu(I) complexes showing panchromatic visible absorption are described here. With this heteroleptic design, we demonstrate that the energy levels of the spatially separated HOMO and LUMO can be independently and systematically controlled ligand modification, with charge-transfer absorption bands throughout the visible and NIR regions that cover a wider range than typical Cu(I) chromophores.

Cyclometalated iridium-coumarin ratiometric oxygen sensors: improved signal resolution and tunable dynamic ranges.

In this work we introduce a new series of ratiometric oxygen sensors based on phosphorescent cyclometalated iridium centers partnered with organic coumarin fluorophores. Three different cyclometalating ligands and two different pyridyl-containing coumarin types were used to prepare six target complexes with tunable excited-state energies. Three of the complexes display dual emission, with fluorescence arising from the coumarin ligand, and phosphorescence from either the cyclometalated iridium center or the coumarin.

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)] centers (C^N is the cyclometalating ligand).

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

We introduce phosphorescent platinum aryl acetylide complexes supported by -butyl-isocyanide and strongly σ-donating acyclic diaminocarbene (ADC) ligands. The precursor complexes -[Pt(CNBu)(C≡CAr)] () are treated with diethylamine, which undergoes nucleophilic addition with one of the isocyanides to form the -[Pt(CNBu)(ADC)(C≡CAr)] complexes (). The new compounds incorporate either electron-donating groups (4-OMe and 4-NMe) or electron-withdrawing groups [3,5-(OMe), 3,5-(CF), 4-CN, and 4-NO] on the aryl acetylide.

Effects of Ancillary Ligands on Deep Red to Near-Infrared Cyclometalated Iridium Complexes.

The design of organometallic compounds with efficient phosphorescence in the deep red to near-infrared portions of the spectrum is a long-standing fundamental challenge. Here we describe a series of heteroleptic bis-cyclometalated iridium complexes with phosphorescence in these low-energy regions of the spectrum. The cyclometalating ligands in this study feature a metalated benzothiophene aryl group substituted with a quinoline, isoquinoline, or phenanthridine heterocycle.

The diverse functions of isocyanides in phosphorescent metal complexes.

In this Perspective, we highlight many examples of photoluminescent metal complexes supported by isocyanides, with an emphasis on recent developments including several from our own group. Work in this field has shown that the isocyanide can play important structural roles, both as a terminal ligand and as a bridging ligand for polynuclear structures, and can influence the excited-state character and excited-state dynamics. In addition, there are many examples of isocyanide-supported complexes where the isocyanide serves as a chromophoric ligand, meaning the low-energy excited states that are important in the photochemistry are partially or completely localized on the isocyanide.