Two-Coordinate Thermally Activated Delayed Fluorescence Coinage Metal Complexes: Molecular Design, Photophysical Characters, and Device Application.

Since the emergence of the first green light emission from a fluorescent thin-film organic light emitting diode (OLED) in the mid-1980s, a global consumer market for OLED displays has flourished over the past few decades. This growth can primarily be attributed to the development of noble metal phosphorescent emitters that facilitated remarkable gains in electrical conversion efficiency, a broadened color gamut, and vibrant image quality for OLED displays. Despite these achievements, the limited abundance of noble metals in the Earth's crust has spurred ongoing efforts to discover cost-effective electroluminescent materials. One particularly promising avenue is the exploration of thermally activated delayed fluorescence (TADF), a mechanism with the potential to fully harness excitons in OLEDs. Recently, investigations have unveiled TADF in a series of two-coordinate coinage metal (Cu, Ag, and Au) complexes. These organometallic TADF materials exhibit distinctive behavior in comparison to their organic counterparts. They offer benefits such as tunable emissive colors, short TADF emission lifetimes, high luminescent quantum yields, and reasonable stability. Impressively, both vacuum-deposited and solution-processed OLEDs incorporating these materials have achieved outstanding performance. This review encompasses various facets on two-coordinate TADF coinage metal complexes, including molecular design, photophysical characterizations, elucidation of structure-property relationships, and OLED applications.

Interactions between nonfullerene acceptors lead to unstable ternary organic photovoltaic cells.

For organic photovoltaic (OPV) devices to achieve consistent performance and long operational lifetimes, organic semiconductors must be processed with precise control over their purity, composition, and structure. This is particularly important for high volume solar cell manufacturing where control of materials quality has a direct impact on yield and cost. Ternary-blend OPVs containing two acceptor-donor-acceptor (A-D-A)-type nonfullerene acceptors (NFAs) and a donor have proven to be an effective strategy to improve solar spectral coverage and reduce energy losses beyond that of binary-blend OPVs. Here, we show that the purity of such a ternary is compromised during blending to form a homogeneously mixed bulk heterojunction thin film. We find that the impurities originate from end-capping C=C/C=C exchange reactions of A-D-A-type NFAs, and that their presence influences both device reproducibility and long-term reliability. The end-capping exchange results in generation of up to four impurity constituents with strong dipolar character that interfere with the photoinduced charge transfer process, leading to reduced charge generation efficiency, morphological instabilities, and an increased vulnerability to photodegradation. As a consequence, the OPV efficiency falls to less than 65% of its initial value within 265 h when exposed to up to 10 suns intensity illumination. We propose potential molecular design strategies critical to enhancing the reproducibility as well as reliability of ternary OPVs by avoiding end-capping reactions.

Simultaneous Thermally Stimulated Delayed Phosphorescence (TSDP) and Thermally Activated Delayed Fluorescence (TADF) in a Two-Coordinated Au(I) Bimetallic Complex Featuring a Tandem Carbene Structure.

A bimetallic, two-coordinated carbene-metal-amine (cMa) Au(I) complex featuring a twisted tandem carbene structure (NHC1-Au-NHC2-Au-carbazolyl) was synthesized. The molecular structure in single crystals revealed a large dihedral angle between the two carbene ligands, while the bridged carbene NHC2 and carbazolyl (Cz) ligands were coplanar. A bluish green thermally stimulated delayed phosphorescence (TSDP) was observed in crystals with an emission lifetime over 70 µs, which can be attributed to the spin allowed diabatic population of a high-lying emissive triplet state from the 3LE characterized low-lying ones. The small rotation energy barrier of Cz along the coordination bond allowed conformers with large dihedral angles between NHC2 and Cz. The ICT characterized S1 state was consequently stabilized to achieve a thermally accessible energy gap to facilitate ISC between triplets and the S1, leading to the thermally activated delayed fluorescence (TADF). Simultaneous TSDP and TADF dual emission can be recorded in its doped polymer film owing to the coexistence of these different conformers.

Structural Diversity in 2-(2-Aminoethyl)pyridine-Based Lead Iodide Hybrids.

While 2D metal-organic hybrids have emerged as promising solar absorbers due to their improved moisture stability, their inferior transport properties limit their potential translation into devices. We report a new hybrid containing 2-(2-ammonioethyl)pyridine [(2-AEP)+], forming a 2D hybrid with the composition (2-AEP)2PbI4. The organic bilayer comprises of (2-AEP)+, which is arranged in a face-to-face stacking that promotes π-π interactions between neighboring pyridyl rings. We also demonstrate the structural diversity of 2-(2-aminoethyl)pyridine-based lead iodide hybrids in solution-processed films. This report highlights the importance of solution-processing conditions in trying to obtain single-phase films of hybrids containing dibasic organic species.

Bis-oxazoline derivatives as ancillary ligands for bis-cyclometalated iridium complexes.

Organometallic iridium complexes with two cyclometalated ligands (CN) and one bis-oxazoline derived ancillary ligand (L^X), i.e. (CN)2Ir(L^X), are reported. The CN ligands are 1-phenylpyrazoline (ppz), 2-(4,6-difluorophenyl)pyridine (F2ppy), 2-phenylpyridine (ppy), 1-phenylisoquinoline (piq). The box ligand is (4S)-(+)-phenyl-α-[(4S)-phenyloxazolidin-2-ylidene]-2-oxazoline-2-acetonitrile. The emission of these complexes span across the visible and into the near-ultraviolet region of the electromagnetic spectrum with moderate to high photoluminescence quantum yields (ΦPL = 0.45-1.0). These complexes were found to emit from a metal-ligand to ligand charge transfer (ML'LCT) state and have lifetimes (1.3-2.1 µs), radiative rates (105 s-1), and nonradiative rates (104-105 s-1) comparable to state-of-the-art iridium emitters. The (ppy)2Ir(BOX-CN) complexes were resolved into the Δ- and Λ- diastereomers using differences in their solubility and additionally characterized by x-ray crystallography, stability, and chiroptic studies. The high ΦPL of these isomers results in the best to date brightness for circularly polarized luminescence (CPL) from iridium complexes (7.0 M-1 cm-1), with dissymmetry factors of -0.57 × 10-3 and +1.9 × 10-3 for 3Δ and 3Λ, respectively. The significant difference in CPL magnitude between 3Δ and 3Λ likely arises from interligand interactions (edge-to-face arrangement versus strong π-π interaction) for the pendant phenyl ring of the BOX-CN ligand which differ for the two isomers.

Luminescent Bimetallic Two-Coordinate Gold(I) Complexes Utilizing Janus Carbenes.

A series of bimetallic carbene-metal-amide (cMa) complexes have been prepared with bridging biscarbene ligands to serve as a model for the design of luminescent materials with large oscillator strengths and small energy differences between the singlet and triplet states (ΔEST). The complexes have a general structure (R2N)Au(:carbene─carbene:)Au(NR2). The bimetallic complexes show solvation-dependent absorption and emission that is analyzed in detail. It is found that the molar absorptivity of the bimetallic complexes is correlated with the energy barrier to rotation of the metal-ligand bond. The bimetallic cMa complexes also exhibit short emission lifetimes (τ = 200-300 ns) with high photoluminescence efficiencies (ΦPL > 95%). The radiative rates of bimetallic cMa complexes are 3-4 times faster than that of the corresponding monometallic complexes. Analysis of temperature-dependent luminescence data indicates that the lifetime for the singlet state (τS1) of bimetallic cMa complexes is near 12 ns with a ΔEST of 40-50 meV. The presented compounds provide a general design for cMa complexes to achieve small values for ΔEST while retaining high radiative rates. Solution-processed organic light-emitting devices (OLEDs) made using two of the complexes as luminescent dopants show high efficiency and low roll-off at high luminance.

Two-Coordinate Coinage Metal Complexes as Solar Photosensitizers.

Generating sustainable fuel from sunlight plays an important role in meeting the energy demands of the modern age. Herein, we report two-coordinate carbene-metal-amide (cMa, M = Cu(I) and Au(I)) complexes that can be used as sensitizers to promote the light-driven reduction of water to hydrogen. The cMa complexes studied here absorb visible photons (εvis > 103 M-1 cm-1), maintain long excited-state lifetimes (τ ∼ 0.2-1 µs), and perform stable photoinduced charge transfer to a target substrate with high photoreducing potential (E+/* up to -2.33 V vs Fc+/0 based on a Rehm-Weller analysis). We pair these coinage metal complexes with a cobalt-glyoxime electrocatalyst to photocatalytically generate hydrogen and compare the performance of the copper- and gold-based cMa complexes. We also find that the two-coordinate complexes herein can perform photodriven hydrogen production from water without the addition of the cobalt-glyoxime electrocatalyst. In this "catalyst-free" system, the cMa sensitizer partially decomposes to give metal nanoparticles that catalyze water reduction. This work identifies two-coordinate coinage metal complexes as promising abundant metal, solar fuel photosensitizers that offer exceptional tunability and photoredox properties.

Controlling Symmetry Breaking Charge Transfer in BODIPY Pairs.

Symmetry breaking charge transfer (SBCT) is a process in which a pair of identical chromophores absorb a photon and use its energy to transfer an electron from one chromophore to the other, breaking the symmetry of the chromophore pair. This excited state phenomenon is observed in photosynthetic organisms where it enables efficient formation of separated charges that ultimately catalyze biosynthesis. SBCT has also been proposed as a means for developing photovoltaics and photocatalytic systems that operate with minimal energy loss. It is known that SBCT in both biological and artificial systems is in part made possible by the local environment in which it occurs, which can move to stabilize the asymmetric SBCT state. However, how environmental degrees of freedom act in concert with steric and structural constraints placed on a chromophore pair to dictate its ability to generate long-lived charge pairs via SBCT remain open topics of investigation.In this Account, we compare a broad series of dipyrrin dimers that are linked by distinct bridging groups to discern how the spatial separation and mutual orientation of linked chromophores and the structural flexibility of their linker each impact SBCT efficiency. Across this material set, we observe a general trend that SBCT is accelerated as the spatial separation between dimer chromophores decreases, consistent with the expectation that the electronic coupling between these units varies exponentially with their separation. However, one key observation is that the rate of charge recombination following SBCT was found to slow with decreasing interchromophore separation, rather than speed up. This stems from an enhancement of the dimer's structural rigidity due to increasing steric repulsion as the length of their linker shrinks. This rigidity further inhibits charge recombination in systems where symmetry has already enforced zero HOMO-LUMO overlap. Additionally, for the forward transfer, the active torsion is shown to increase LUMO-LUMO coupling, allowing for faster SBCT within bridging groups.By understanding trends for how rates of SBCT and charge recombination depend on a dimer's internal structure and its environment, we identify design guidelines for creating artificial systems for driving sustained light-induced charge separation. Such systems can find application in solar energy technologies and photocatalytic applications and can serve as a model for light-induced charge separation in biological systems.

Sparkling Organic Phosphorescence from Fluorinated Tetrathia[7]helicenes: Synthesis and Photophysical, Electrochemical and Computational Studies.

Structure-property correlations in the thiahelicene family are often not trivial beacuse most of the functional groups present on the helical scaffold modify the conjugation size of the π-system. Selecting fluorine-containing groups to provide strong inductive effects without interacting with low-lying orbitals of the system could be the way to overcome the issue. Here we report a study on three fluorine-functionalized tetrathia[7]helicenes, highlighting interesting correlations between the position of the functional groups and the conjugated skeleton properties. Helicenes Heli-F2 and Heli-CF-F2 were prepared by photoinduced isomerization-electrocyclization (the Mallory photocyclization) of the corresponding fluorinated benzodithienyl-ethenes Alk-F2 and Alk-CF-F2, which were prepared in high yields through stereo-conservative Stille reaction. Notably these helicenes were found to display green phosphorescence around 530-550 nm, and the studies suggest an efficient spin-orbit coupling mechanism in these high-energy triplet nonplanar conjugated molecules. Both helicenes and their precursors were thoroughly characterized by means of optical and electrochemical measurements, while DFT calculations enable a rationale on their structure-property correlations to be defined.

Temperature dependence of radiative and non-radiative decay in the luminescence of one-dimensional pyridinium lead halide hybrids.

The photoluminescence properties of organic-inorganic pyridinium lead bromide [(pyH)PbBr3] and iodide [(pyH)PbI3] compounds were investigated as a function of temperature. The inorganic substructure consists of face-sharing chains of PbX6 octahedra. Diffuse reflectance spectra of the compounds show low energy absorption features consistent with charge transfer transitions from the PbX3 chains to the pyridinium cations. Both compounds display extremely weak luminescence at room temperature that becomes strongly enhanced upon cooling to 77 K. Broad, featureless low energy emission (λem > 600 nm) in both compounds have large Stokes shifts [1.1 eV for (pyH)PbBr3 and 0.46 eV for (pyH)PbI3] and are assigned to transitions from self-trapped excitons on the inorganic chains whereas emission at higher energy in (pyH)PbBr3 (λem = 450 nm) is assigned to luminescence from a free exciton state. Analysis of data from temperature-dependent luminescence intensity measurements gives activation energies (Ea) for non-radiative decay of the self-trapped excitons in (pyH)PbBr3 and (pyH)PbI3, (Ea = 0.077 eV and 0.103 eV, respectively) and for the free exciton in (pyH)PbBr3 (Ea = 0.010 eV). Analysis of temperature dependent luminescence lifetime data indicates another non-radiative decay process in (pyH)PbI3 at higher temperatures (Ea = 0.17 eV). A large increase in the luminescence lifetime of (pyH)PbI3 below 80 K is consistent with thermalization between triplet sublevels. Analysis of the luminescence power dependence for (pyH)PbI3 shows superlinear response suggestive of quenching by static traps.

A Comparison between Triphenylmethyl and Triphenylsilyl Spirobifluorenyl Hosts: Synthesis, Photophysics and Performance in Phosphorescent Organic Light-Emitting Diodes.

This study presents the synthesis and characterization of two spirobifluorenyl derivatives substituted with either triphenylmethyl (SB-C) or triphenylsilyl (SB-Si) moieties for use as host materials in phosphorescent organic light-emitting diodes (PHOLED). Both molecules have similar high triplet energies and large energy gaps. Blue Ir(tpz)3 and green Ir(ppy)3 phosphorescent devices were fabricated using these materials as hosts. Surprisingly, SB-Si demonstrated superior charge-transporting ability compared to SB-C, despite having similar energies for their valence orbitals. In particular, SB-Si proved to be a highly effective host for both blue and green devices, resulting in maximum efficiencies of 12.6% for the Ir(tpz)3 device and 9.6% for the Ir(ppy)3 device. These results highlight the benefits of appending the triphenylsilyl moiety onto host materials and underscore the importance of considering the morphology of hosts in the design of efficient PHOLEDs.

π-Extended Ligands in Two-Coordinate Coinage Metal Complexes.

Two-coordinate carbene-MI-amide (cMa, MI = Cu, Ag, Au) complexes have emerged as highly efficient luminescent materials for use in a variety of photonic applications due to their extremely fast radiative rates through thermally activated delayed fluorescence (TADF) from an interligand charge transfer (ICT) process. A series of cMa derivatives was prepared to examine the variables that affect the radiative rate, with the goal of understanding the parameters that control the radiative TADF process in these materials. We find that blue-emissive complexes with high photoluminescence efficiencies (ΦPL > 0.95) and fast radiative rates (kr = 4 × 106 s-1) can be achieved by selectively extending the π-system of the carbene and amide ligands. Of note is the role played by the increased separation between the hole and electron in the ICT excited state. Analysis of temperature-dependent luminescence data and theoretical calculations indicate that the hole-electron separation exerts a primary effect on the energy gap between the lowest-energy singlet and triplet states (ΔEST) while keeping the radiative rate for the singlet state relatively unchanged. This interpretation provides guidelines for the design of new cMa derivatives with even faster radiative rates in addition to those with slower radiative rates and thus extended excited state lifetimes.

A Luminescent Two-Coordinate AuI Bimetallic Complex with a Tandem-Carbene Structure: A Molecular Design for the Enhancement of TADF Radiative Decay Rate.

A luminescent bimetallic AuI complex comprised of N-heterocyclic carbene (NHC) and carbazole (Cz) ligands, that is, (NHC')Au(NHC)AuCz has been synthesized and studied. Both carbene ligands in the bimetallic complex act as electron acceptors in tandem to increase the energy separation between the ground and excited state, which is higher than those found in either monometallic analogue, (NHC)AuCz and (NHC')AuCz. A coplanar geometry designed into the tandem complex ensures sufficient electronic coupling between the π-orbitals of the ligands to impart a strong oscillator strength to the singlet intra-ligand charge-transfer (1 ICT) transition. Theoretical modelling indicates that the emissive ICT excited state involves both NHC ligands. The tandem complex gives blue luminescence (λmax =480 nm) with a high photoluminescent quantum yield (ΦPL =0.80) with a short decay lifetime (τ=0.52 µs). Temperature-dependent photophysical studies indicate that emission is via thermally assisted delayed fluorescence (TADF) and give a small singlet-triplet energy difference (ΔEST =50 meV, 400 cm-1 ) consistent with the short TADF lifetime.

Synthesis and Characterization of Zinc(II) Complexes Bearing 4-Acridinol and 1-Phenazinol Ligands.

The synthesis and characterization of zinc(II) chelates bearing acridin-4-ol (A), phenazin-1-ol (P), and benzo[b]phenazin-1-ol (bP) are presented. The formation of homoleptic (ZnX2) or heteroleptic (ZnX1) products can be controlled by stochiometric or excess amounts of zinc(II) acetylacetonate monohydrate, Zn(acac)2, respectively. Electrochemical and photophysical studies show that the homoleptic complexes (ZnA2, ZnP2, and ZnbP2) have ligand-centered properties inherited from the corresponding free ligands. Calculations using density functional theory (DFT) agree with the observed experimental ligand-centered photophysical and electrochemical behavior.

In Vivo Experimental and Analytical Studies for Bevacizumab Diffusion Coefficient Measurement in the Rabbit Vitreous Humor.

In order to measure the effective diffusion coefficient D of Bevacizumab (Avastin, Genentech) in the vitreous humor, a new technique is developed based on the "contour method" and in vivo optical coherence tomography measurements. After injection of Bevacizumab-fluorescein conjugated compound solution into the rabbit eye, the contours of drug concentration distribution at the subsurface of injection were tracked over time. The 2D contours were extrapolated to 3D contours using reasonable assumptions and a numerically integrated analytical model was developed for the theoretical contours for the irregularly shaped drug distribution in the experimental result. By floating the diffusion coefficient, different theoretical contours were constructed and the least-squares best fit to the experimental contours was performed at each time point to get the best fit solution. The approach generated consistent diffusion coefficient values based on the experiments on four rabbit eyes over a period of 3 h each, which gave D = 1.2 ± 0.6 × 10 - 6 cm 2 / s , and the corresponding theoretical contours matched well with the experimental contours. The quantitative measurement of concentration using optical coherence tomography and fluorescein labeling gives a new approach for the "noncontact" in vivo drug distribution measurement within vitreous.

Enhancement of the Luminescent Efficiency in Carbene-Au(I)-Aryl Complexes by the Restriction of Renner-Teller Distortion and Bond Rotation.

A series of (carbene)Au(I)(aryl) complexes are reported. The nature of the lowest excited state in these complexes changes character from metal-to-ligand charge transfer (MLCT) to interligand charge transfer (ICT) with increasing electron-donating strength of the aryl ligand. Complexes that have the MLCT lowest excited state undergo a Renner-Teller bending distortion upon excitation. Such a distortion leads to a large rate of nonradiative decay, on the order of 108 s-1. Renner-Teller-based nonradiative decay does not occur in chromophores with an ICT emissive state. Introducing a julolidine moiety and ortho-methyl substituents to the aryl group makes the molecule rigid and hinders the rotation along the Au-Caryl-coordinate bond. Consequently, the nonradiative decay rates of these ICT emitters are decreased and become lower than the radiative decay rate constants (kr = 105 s-1). Thus, high-luminescent efficiencies (ΦPL = 0.61 and 0.77) along with short lifetimes (τ < 2 µs) are obtained for yellow and green emitters, respectively. Thermally assisted delayed fluorescence behavior is observed, owing to the small exchange energy (ΔEST < 1600 cm-1) in these emitters.

Influence of Dimethyl Sulfoxide on the Structural Topology during Crystallization of PbI2.

Hybrid metal-organic halides are an exciting class of materials that offer the opportunity to examine how fundamental aspects of chemical bonding can influence the structural topology. In this work, we describe how solvent adducts of lead halides can influence the crystallization and subsequent annealing of these hybrid phases. While the size and shape of organic molecules are known to govern the final topology of the hybrid, we show that the affinity of solvent molecules for Pb ions may also play a previously underappreciated role.

Improving Photocatalysis for the Reduction of CO2 through Non-covalent Supramolecular Assembly.

We report the enhancement of photocatalytic performance by introduction of hydrogen-bonding interactions to a Re bipyridine catalyst and Ru photosensitizer system (ReDAC/RuDAC) by the addition of amide substituents, with carbon monoxide (CO) and carbonate/bicarbonate as products. This system demonstrates a more-than-3-fold increase in turnover number (TONCO = 100 ± 4) and quantum yield (ΦCO = 23.3 ± 0.8%) for CO formation compared to the control system using unsubstituted Ru photosensitizer (RuBPY) and ReDAC (TONCO = 28 ± 4 and ΦCO = 7 ± 1%) in acetonitrile (MeCN) with 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as sacrificial reductant. In dimethylformamide (DMF), a solvent that disrupts hydrogen bonds, the ReDAC/RuDAC system showed a decrease in catalytic performance while the control system exhibited an increase, indicating the role of hydrogen bonding in enhancing the photocatalysis for CO2 reduction through supramolecular assembly. The similar properties of RuDAC and RuBPY demonstrated in lifetime measurements, spectroscopic analysis, and electrochemical and spectroelectrochemical studies revealed that the enhancement in photocatalysis is due not to differences in intrinsic properties of the catalyst or photosensitizer, but to hydrogen-bonding interactions between them.

Highly Efficient Photo- and Electroluminescence from Two-Coordinate Cu(I) Complexes Featuring Nonconventional N-Heterocyclic Carbenes.

A series of six luminescent two-coordinate Cu(I) complexes were investigated bearing nonconventional N-heterocyclic carbene ligands, monoamido-aminocarbene (MAC*) and diamidocarbene (DAC*), along with carbazolyl (Cz) as well as mono- and dicyano-substituted Cz derivatives. The emission color can be systematically varied over 270 nm, from violet to red, through proper choice of the acceptor (carbene) and donor (carbazolyl) groups. The compounds exhibit photoluminescent quantum efficiencies up to 100% in fluid solution and polystyrene films with short decay lifetimes (τ ≈ 1 µs). The radiative rate constants for the Cu(I) complexes ( kr = 105-106 s-1) are comparable to state of the art phosphorescent emitters with noble metals such as Ir and Pt. All complexes show strong solvatochromism due to the large dipole moment of the ground states and the transition dipole moment that is in the opposite direction. Temperature-dependent studies of (MAC*)Cu(Cz) reveal a small energy separation between the lowest singlet and triplet states (Δ ES1-T1 = 500 cm-1) and an exceptionally large zero-field splitting (ZFS = 85 cm-1). Organic light-emitting diodes (OLEDs) fabricated with (MAC*)Cu(Cz) as a green emissive dopant have high external quantum efficiencies (EQE = 19.4%) and brightness of 54 000 cd/m2 with modest roll-off at high currents. The complex can also serve as a neat emissive layer to make highly efficient OLEDs (EQE = 16.3%).