Highly Efficient Thermally Activated Delayed Fluorescence in Dinuclear Ag(I) Complexes with a Bis-Bidentate Tetraphosphane Bridging Ligand

Synthesis and Photophysical Properties of Silver(I) Coordination Polymers Bridged by Dimethylpyrazine: Comparison of Emissive Excited States between Silver(I) and Copper(I) Congeners

Highly luminescent silver(I) coordination polymers [Ag2X2(PPh3)2(Me2pyz)]n (X = I, Br, Cl; Me2pyz: 2,5-dimethylpyrazine) were prepared together with copper congeners [Cu2X2(PPh3)2(Me2pyz)]n (X = I, Br). All the complexes showed thermally activated delayed fluorescence from the charge-transfer states in the visible region, from blue to red. The isomorphous relationship among the complexes allowed a detailed discussion of the effect of halogenido ligands and crystal packing on their luminescence energy. The relaxation in the emissive excited states (ESs) was determined to be more remarkable in silver complexes than in copper complexes despite their isomorphous structures, and the electronic effect of halogenido ligands was comparable to the effect of relaxation in emissive ESs.

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.

A Multiple Stimuli-Responsive Ag/P/S Complex Showing Solvochromic and Mechanochromic Photoluminescence

The reaction of CF₃COOAg, 3-bdppmapy (N,N-bis(diphenylphosphanylmethyl)-3-aminopyridine) and HTZ (1,2,4-triazole-3-thiol) in CH₂Cl₂/MeOH resulted in a dinuclear Ag/P/S complex [Ag₂(TZ)₂(3-bdppmapy)₂]·xSol (1·xSol). Crystals of 1·xSol converted to 1·2MeOH in air at room temperature and further to 1 under vacuum upon heating. The solid-state, room-temperature photoluminescent emission of 1·xSol (510 nm) shifted to 494 nm (1·2MeOH) and 486 nm (1). Grinding solids of 1·2MeOH in air resulted in amorphous 1G characterized by solid-state emission at 468 nm, which converted to 1GR with 513 nm emission upon MeOH treatment. Grinding 1GR in air returned 1G, and this interconversion was reproducible over five cycles. The solid-state photoluminescence of 1G changed in response to vapors containing low-molecular weight alcohols but remained unchanged after exposure to other volatile organic compounds (VOCs) or to water vapor. Test papers impregnated with 1G could detect methanol in vapors from aqueous solutions at concentrations above 50%. Complex 1G is, therefore, an example of a stimuli-responsive molecular sensor for the detection of alcohols.

Effect of the dangling aromatic ring on neutral luminescent bis(phosphine) Cu(i)/Ag(i) complexes with the asymmetric pyridyl-tetrazolate ligands

A series of neutral luminescent bis(phosphine) Cu(i) complexes of pyridyl-tetrazolate ligands (L1-L3) with the general formula [CuI(L n )(P^P)] (1-6) were synthesized, which have been well characterized by IR, UV/vis, CV, 1H NMR and 31P NMR. For comparison, an Ag(i) complex [AgI(L2)(PPh3)2] (7) was also synthesized. The crystal structures of 2 and 7 have been further determined by X-ray crystallography. All these Cu(i) compounds show bright luminescence in the solid state with photoluminescence quantum yields (PLQYs) in the range of 25.8% to 85.0%. More interestingly, the Cu(i) complexes bearing an additional dangling aromatic ring on the diimine ligands exhibit enhanced luminescent performance in various solutions and their PLQYs are significantly higher than those of related Cu(i) complexes without steric protection. Compared with 1, the Cu(i) complexes with an additional dangling tetrazole moiety show a significant solvatochromic effect, which is uncommon for luminescent Cu(i) complexes. Moreover, [CuI(L2)(PPh3)2] (2) was further designed as an OLED material, which showed a high external quantum efficiency of 7.7%.

Photophysical properties of homobimetallic Cu(i)–Cu(i) and heterobimetallic Cu(i)–Ag(i) complexes of 2-(6-bromo-2-pyridyl)-1H-imidazo[4,5-f][1,10]phenanthroline

The heteronuclear Cu(i)–Ag(i) complexes show dual emission bands and enhanced luminescence compared with their isostructural homobinuclear Cu(i) complexes.

Ditopic Phosphide Oxide Group: A Rigidifying Lewis Base to Switch Luminescence and Reactivity of a Disilver Complex

Heterodentate phosphines containing anionic organophosphorus groups remain virtually unexplored ligands in the coordination chemistry of coinage metals. A hybrid phosphine-phosphine oxide (o-Ph₂PC₆H₄)₂P(O)H (HP³O) readily forms the disilver complex [Ag₂(P³O)₂] (1) upon deprotonation of the (O)P-H fragment. Due to the electron-rich nature, the anionic phosphide oxide unit in 1 takes part in efficient intermolecular hydrogen bonding, which has an unusual and remarkably strong impact on the photoluminescence of 1, changing the emission from red (644 nm) to green-yellow (539 nm) in the solid. The basicity of the R₂(O)P^- group and its affinity for both inter- and intramolecular donor-acceptor interactions allow converting 1 into hydrohalogenated (2, 3) and boronated (4) derivatives, which reveal a gradual hypsochromic shift of luminescence, reaching the wavelength of 489 nm. Systematic variable-temperature analysis of the excited state properties suggests that thermally activated delayed fluorescence is involved in the emission process. The long-lived excited states for 1-4, the energy of which is largely regulated by means of the phosphide oxide unit, are potentially suitable for triplet energy transfer photocatalysis. With the highest T₁ energy among 1-4, complex 4 demonstrates excellent photocatalytic activity in a [2+2] cycloaddition reaction, which has been realized for the first time for silver(I) compounds.

Room-Temperature Phosphorescence and Thermally Activated Delayed Fluorescence in the Pd Complex: Mechanism and Dual Upconversion Channels

The Pd complex PdN3N exhibits an unusual dual emission of room-temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF), but the mechanism is elusive. Herein, we employed both density functional theory (DFT) and time-dependent DFT (TD-DFT) methods to explore excited-state properties of this Pd complex, which shows that the S₀, S₁, T₁, and T₂ states are involved in the luminescence. Both the S₁ → T₁ and S₁ → T₂ intersystem crossing (ISC) processes are more efficient than the S₁ fluorescence and insensitive to temperature. However, the direct T₁ → S₁ and T₂-mediated T₁ → T₂ → S₁ reverse ISC (rISC) processes change remarkably with temperature. At 300 K, these two processes are more efficient than the T₁ phosphorescence and therefore enable TADF. Importantly, the T₁ → S₁ rISC and T₁ phosphorescence rates are comparable at 300 K, which leads to dual emissions of TADF and RTP, whereas these two channels become blocked at 100 K so that only the T₁ phosphorescence is recorded experimentally.

P∩N Bridged Cu(I) Dimers Featuring Both TADF and Phosphorescence. From Overview towards Detailed Case Study of the Excited Singlet and Triplet States

We present an overview over eight brightly luminescent Cu(I) dimers of the type Cu₂X₂(P∩N)₃ with X = Cl, Br, I and P∩N = 2-diphenylphosphino-pyridine (Ph₂Ppy), 2-diphenylphosphino-pyrimidine (Ph₂Ppym), 1-diphenylphosphino-isoquinoline (Ph₂Piqn) including three new crystal structures (Cu₂Br₂(Ph₂Ppy)₃ 1-Br, Cu₂I₂(Ph₂Ppym)₃ 2-I and Cu₂I₂(Ph₂Piqn)₃ 3-I). However, we mainly focus on their photo-luminescence properties. All compounds exhibit combined thermally activated delayed fluorescence (TADF) and phosphorescence at ambient temperature. Emission color, decay time and quantum yield vary over large ranges. For deeper characterization, we select Cu₂I₂(Ph₂Ppy)₃, 1-I, showing a quantum yield of 81%. DFT and SOC-TDDFT calculations provide insight into the electronic structures of the singlet S₁ and triplet T₁ states. Both stem from metal+iodide-to-ligand charge transfer transitions. Evaluation of the emission decay dynamics, measured from 1.2 ≤ T ≤ 300 K, gives ∆E(S₁-T₁) = 380 cm⁻¹ (47 meV), a transition rate of k(S₁→S₀) = 2.25 × 10⁶ s⁻¹ (445 ns), T₁ zero-field splittings, transition rates from the triplet substates and spin-lattice relaxation times. We also discuss the interplay of S₁-TADF and T₁-phosphorescence. The combined emission paths shorten the overall decay time. For OLED applications, utilization of both singlet and triplet harvesting can be highly favorable for improvement of the device performance.

Color-Tunable Aqueous Room-Temperature Phosphorescence Supramolecular Assembly

Developing room-temperature phosphorescence (RTP) materials with color-tunability performance in an aqueous environment is crucial for application in optoelectronic areas to a higher stage, such as multicolor display, visual detection of external stimulus, and high-level information anticounterfeiting, but still faces a formidable challenge. Herein, we propose an efficient design strategy to develop excitation wavelength-responsive RTP supramolecular co-assembly systems of a simple benzoic acid derivative and Laponite (Lap) clay nanoplates in aqueous solution, displaying an ultralong lifetime (0.632 s) and a high phosphorescence quantum efficiency (18.04%) simultaneously. Experimental and theoretical research studies suggest that this distinctive feature is due to the generation of more and efficient intersystem crossing pathways benefiting from the coexistence of isolated and J-aggregation states via controlling the doping of the benzoic acid derivative and the inhibition of phosphorescence quenching by water because of the synergistic effects of robust hydrogen-bonding interactions between Lap and the benzoic acid derivative, J-aggregations of the benzoic acid derivative, and good oxygen tolerance of the Lap clay. By virtue of their excellent RTP performances in aqueous solution, the visual colorimetric detection of Ag⁺ in a water environment was achieved for the first time, and visible and high-level information encryption was accomplished as well.

Synthesis and Photophysical Properties of Emissive Silver(I) Halogenido Coordination Polymers Composed of {Ag2X2} Units Bridged by Pyrazine, Methylpyrazine, and Aminopyrazine

Luminescent silver(I) coordination polymers having a {Ag₂(μ-X)₂} rhombic core (X = I, Br) were prepared using pyrazine (pyz), methylpyrazine (Mepyz), and aminopyrazine (ampyz) as bridging ligands. Photophysical measurements show that the complexes were strongly luminescent in the solid state at room temperature; further, the emissive excited state of the pyz and Mepyz complexes was a triplet charge-transfer (³CT) excited state, similar to that of their copper(I) congeners, whereas that of the ampyz complex was a intraligand (³IL) excited state. The energy of the ³CT excited state of a silver halogenido complex was revealed to be ca. 5000 cm^-1 higher than that of the corresponding copper complex.

Synthesis, Study, and Application of Pd(II) Hydrazone Complexes as the Emissive Components of Single-Layer Light-Emitting Electrochemical Cells

For the first time, square planar Pd(II) complexes of hydrazone ligands have been investigated as the emissive components of light-emitting electrochemical cells (LECs). The neutral transition metal complex, [Pd(L¹)₂]·2CH₃OH (1), (HL¹ = (E)-N'-(phenyl(pyridin-2-yl)methylene)isonicotinhydrazide), was prepared and structurally characterized. Complex 1 displays quasireversible redox properties and is emissive at room temperature in solution with a λ_max of 590 nm. As a result, it was subsequently employed as the emissive material of a single-layer LEC with configuration FTO/1/Ga/In, where studies reveal that it has a yellow color with CIE(x, y) = (0.33, 0.55), a luminance of 134 cd cm^-2, and a turn-on voltage of 3.5 V. Protonation of the pendant pyridine nitrogen atoms of L¹ afforded a second ionic complex [Pd(L¹H)₂](ClO₄)₂ (2) which is also emissive at room temperature with a λ_max of 611 nm, resulting in an orange LEC with CIE(x, y) = (0.43, 0.53). The presence of mobile anions and cations in the second inorganic transition metal complex resulted in more efficient charge injection and transport which significantly improved the luminance and turn-on voltage of the device to 188.6 cd cm^-2 and 3 V, respectively. This study establishes Pd(II) hydrazone complexes as a new class of materials whose emissive properties can be chemically tuned and provides proof-of-concept for their use in LECs, opening up exciting new avenues for potential applications in the field of solid state lighting.

Ag(i) and Cu(i) cyclic-triimidazole coordination polymers: revealing different deactivation channels for multiple room temperature phosphorescences

The photophysics of isostructural Ag(i) and Cu(i) 1D and 3D coordination polymers based on cyclic triimidazole reveals excitation wavelength-dependent multiple emissions with different radiative paths according to the coordinated metal.

Tunable from Blue to Red Emissive Composites and Solids of Silver Diphosphane Systems with Higher Quantum Yields than the Diphosphane Ligands

PMMA composites and solids of complexes of formulas [AgX(P-P)]_n [n = 1 and 2; X = Cl, NO₃, ClO₄, CF₃COO, and OTf; P-P = dppb, xantphos, (PR₂)₂C₂B₁₀H₁₀ (R = Ph and ⁱPr)] display the whole palette of colors from blue to red upon selection of the anionic ligand (X) and the diphosphane (P-P). The diphosphane seems to play the most important role in tuning the emission energy and thermally activated delayed fluorescence (TADF) behavior. The PMMA composites of the complexes exhibit higher quantum yields than that of the diphosphane ligands and those with dppb are between 28 and 53%. Remarkably, instead of blue-green emissions which dominate the luminescence of silver diphosphane complexes in rigid phases, those with carborane diphosphanes are yellow-orange or orange-red emitters. Theoretical studies have been carried out for complexes with P-P = dppb, X = Cl; P-P = dppic, X = NO₃; P-P = dppcc, X = Cl, NO₃, and OTf and the mononuclear complexes [AgX(xantphos)] (X = Cl, Br). Optimization of the first excited triplet state was only possible for [AgX(xantphos)] (X = Cl and Br). A mixed MLCT and MC nature could be attributed to the S₀ → T₁ transition in these three-coordinated complexes.

Synthesis and Thermochromic Luminescence of Ag(I) Complexes Based on 4,6-Bis(diphenylphosphino)-Pyrimidine

Two Ag(I)-based metal-organic compounds have been synthesized exploiting 4,6-bis(diphenylphosphino)pyrimidine (L). The reaction of this ligand with AgNO3 and AgBF4 in acetonitrile produces dinuclear complex, [Ag2L2(MeCN)2(NO3)2] (1) and 1D coordination polymer, [Ag2L(MeCN)3]n(BF4)2n (2), respectively. In complex 1, µ2-P,P′-bridging coordination pattern of the ligand L is observed, whereas its µ4-P,N,N′,P′-coordination mode appears in 2. Both compounds exhibit pronounced thermochromic luminescence expressed by reversible changing of the emission chromaticity from a yellow at 300 K to an orange at 77 K. At room temperature, the emission lifetimes of 1 and 2 are 15.5 and 9.4 µs, the quantum efficiency being 18 and 56%, respectively. On account of temperature-dependent experimental data, the phenomenon was tentatively ascribed to alteration of the emission nature from thermally activated delayed fluorescence at 300 K to phosphoresce at 77 K.

Lighting Silver(I) Complexes for Solution-Processed Organic Light-Emitting Diodes and Biological Applications via Thermally Activated Delayed Fluorescence

Luminescent coinage metal complexes have shown promising applications as electroluminescent emitters, photocatalysts/photosensitizers, and bioimaging/theranostic agents, rendering them attractive alternatives to transition metal complexes based on iridium, ruthenium, and platinum that have extremely low earth abundance. In comparison to the widely studied Au(I) and Cu(I) complexes, Ag(I) complexes have seldom been explored in this field because of their inferior emission properties. Herein, we report a novel series of [Ag(N^N)(P^P)]PF₆ complexes exhibiting highly efficient thermally activated delayed fluorescence by using easily accessible neutral diamine ligands and commercially available ancillary diphosphine chelates. The photoluminescence quantum yields (PLQYs) of the Ag(I) emitters are ≤0.62 in doped films. The high PLQY with a large delayed fluorescence ratio enabled the fabrication of solution-processed organic light-emitting diodes (OLEDs) with a high maximum external quantum efficiency of 8.76%, among the highest values for Ag(I) emitter-based OLEDs. With superior emission properties and an excited state lifetime in the microsecond regime, together with its potent cytotoxicity, the selected Ag(I) complex has been used for simultaneous cell imaging and anticancer treatment in human liver carcinoma HepG2 cells, revealing the potential of luminescent Ag(I) complexes for biological applications such as theranostics.

Recent Advances in Metal-TADF Emitters and Their Application in Organic Light-Emitting Diodes

In this contribution, recent advances in new classes of efficient metal-TADF complexes, especially those of Au(I), Au(III), and W(VI), and their application in OLEDs are reviewed. The high performance (EQE = 25%) and long device operational lifetime (LT₉₅ = 5,280 h) achieved in an OLED with tetradentate Au(III) TADF emitter reflect the competitiveness of this class of emitters for use in OLEDs with practical interest. The high EQE of 15.6% achieved in solution-processed OLED with W(VI) TADF emitter represents an alternative direction toward low-cost light-emitting materials. Finally, the design strategy of metal-TADF emitters and their next-stage development are discussed.

Photoluminescent properties and molecular structures of dinuclear gold(i) complexes with bridged diphosphine ligands: near-unity phosphorescence from 3XMMCT/3MC

Dinuclear gold(i) complexes with bridged diphosphine ligands display near-unity phosphorescence in the crystalline state at room-temperature.

Luminescent phosphine copper(i) complexes with various functionalized bipyridine ligands: synthesis, structures, photophysics and computational study

A new series of luminescent phosphine copper(i) complexes with cyano- and hydroxyl-substituted 2,2′-bipyridine ligands have been synthesized and structurally characterized. Their luminescent properties have also been investigated in detail.

Synthesis, structures and luminescence of multinuclear silver(i) pyrazolate adducts with 1,10-phenanthroline derivatives

A set of silver(i) 3,5-bis(trifluoromethyl) pyrazolate adducts with 1,10-phenanthroline (L²), 2,9-dimethyl-1,10-phenanthroline (neocuproine, L³) and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (bathocuproine, L⁴) was synthesized starting from trimeric silver pyrazolate Ag₃Pz₃. Reactions with sterically hindered L³ and L⁴ cause the destruction of the original trimeric core, yielding a dinuclear Ag₂Pz₂ cycle with an unprecedented chair configuration for L³, while bathocuproine L⁴ leads to the drastic rearrangement of the silver pyrazolate core into cationic Ag(L⁴)₂ and anionic Ag₅Pz₆ subunits. All complexes obtained exhibit phosphorescence in the solid state. Time-dependent density functional theory calculations demonstrate their different possible emission processes, explaining their emission behavior as well as their lifetimes.

Oligophosphine-thiocyanate Copper(I) and Silver(I) Complexes and Their Borane Derivatives Showing Delayed Fluorescence

The series of chelating phosphine ligands, which contain bidentate P² (bis[(2-diphenylphosphino)phenyl] ether, DPEphos; 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene, Xantphos; 1,2-bis(diphenylphosphino)benzene, dppb), tridentate P³ (bis(2-diphenylphosphinophenyl)phenylphosphine), and tetradentate P⁴ (tris(2-diphenylphosphino)phenylphosphine) ligands, was used for the preparation of the corresponding dinuclear [M(μ₂-SCN)P²]₂ (M = Cu, 1, 3, 5; M = Ag, 2, 4, 6) and mononuclear [CuNCS(P³/P⁴)] (7, 9) and [AgSCN(P³/P⁴)] (8, 10) complexes. The reactions of P⁴ with silver salts in a 1:2 molar ratio produce tetranuclear clusters [Ag₂(μ₃-SCN)(t-SCN)(P⁴)]₂ (11) and [Ag₂(μ₃-SCN)(P⁴)]₂²⁺ (12). Complexes 7–11 bearing terminally coordinated SCN ligands were efficiently converted into derivatives 13–17 with the weakly coordinating ^–SCN:B(C₆F₅)₃ isothiocyanatoborate ligand. Compounds 1 and 5–17 exhibit thermally activated delayed fluorescence (TADF) behavior in the solid state. The excited states of thiocyanate species are dominated by the ligand to ligand SCN → π(phosphine) charge transfer transitions mixed with a variable contribution of MLCT. The boronation of SCN groups changes the nature of both the S₁ and T₁ states to (L + M)LCT d,p(M, P) → π(phosphine). The localization of the excited states on the aromatic systems of the phosphine ligands determines a wide range of luminescence energies achieved for the title complexes (λ_em varies from 448 nm for 1 to 630 nm for 10c). The emission of compounds 10 and 15, based on the P⁴ ligand, strongly depends on the solid-state packing (λ_em = 505 and 625 nm for two crystalline forms of 15), which affects structural reorganizations accompanying the formation of electronically excited states.

Copper(I) and silver(I) thiocyanate complexes containing di-, tri-, and tetraphosphine ligands show efficient TADF in the solid state, dominated by the ligand to ligand SCN → π(phosphine) charge transfer, which is changed to d,p(M, P) → π(phosphine) transitions for the isothiocyanatoborate derivatives. The wide variation of the emission color from blue (448 nm) to red-orange (630 nm) is attributed to the nature of the P-donor ligands and the packing effects, influencing structural distortions in the excited state.

Polynuclear Cu(i) and Ag(i) phosphine complexes containing multi-dentate polytopic ligands: syntheses, crystal structures and photoluminescence properties

A series of polynuclear metal complexes, [Cu2(L1)(PPh3)4](ClO4)2 (1), [Cu3(L2)(PPh3)6](ClO4) (2), [Cu3(L3)(PPh3)6] (3), [Ag2(L1)(PPh3)4](BF4)2 (4), [Ag4(L2)2(PPh3)6] (5) and [Ag3(L3)(PPh3)5] (6), have been obtained from the reactions of the highly conjugated bridging ligands 2,3-bis(2-pyridyl)pyrazine (L1), 2,3-bis(2-tetrazoyl)pyrazine (H2L2) and 2,3-bis(2-tetrazoyl)imidazole (H3L3) with [Cu(MeCN)4]ClO4 and AgBF4, respectively. Their crystal structures have been determined by X-ray crystallography and their photophysical properties have been investigated in detail. Complexes 1 and 3 show photoluminescence in CH2Cl2 solution, while all the complexes exhibit obvious luminescence in the solid state; detailed photophysical studies and density functional theory calculations of these complexes have revealed that their lowest energy absorptions and emissions are predominantly derived from either metal-to-ligand charge-transfer (MLCT) or intraligand (IL) excited states.

Sky-blue thermally activated delayed fluorescence (TADF) based on Ag(i) complexes: strong solvation-induced emission enhancement

Remarkable solvation-induced emission enhancement is discovered on a new Ag(i) complex showing sky-blue thermally activated delayed fluorescence (TADF).

Ag(i) complex design affording intense phosphorescence with a landmark lifetime of over 100 milliseconds

A new Ag(i) complex was designed that shows an unprecedentedly long ambient temperature emission decay time of τ = 110 ms at an emission quantum yield of Φ_PL = 50%, as measured for a doped PMMA matrix.

Photoluminescence of Ag(i) complexes with a square-planar coordination geometry: the first observation

First examples of square-planar Ag(i) complexes showing MLCT emission are reported. They demonstrate an interesting thermochromic luminescence with the nano- and microsecond lifetime components.

Deep blue emitting Cu(i) tripod complexes. Design of high quantum yield materials showing TADF-assisted phosphorescence

In a previous investigation, it was shown that [Cu(tpym)(PPh₃)]PF₆1 with tpym = tris(2-pyridyl)methane represents a deep blue emitter (λ_max = 466 nm) though with a low emission quantum yield Φ_PL if doped in a polymer (7%) or dissolved in a fluid solvent (≪1%). In this study, we present new tripod compounds with sterically demanding ligands: [Cu(tpym)(P(o-tol)₃)]PF₆2 and [Cu(tpym)(P(o-butyl-ph)₃)]PF₆3 with P(o-tol)₃ = tris(ortho-tolyl)phosphine and P(o-butyl-ph)₃ = tris(ortho-n-butylphenyl)phosphine. These compounds show high emission quantum yields even in a fluid solution (dichloromethane) reaching a benchmark value for 3 of Φ_PL = 76%. This becomes possible due to the specific design of rigidifying the complexes. Importantly, the deep blue emission color is maintained or even further blue shifted to λ_max = 452 nm (compound 3 powder). Compound 2 is characterized photophysically in detail. In particular, it is shown that the lowest excited triplet state T₁ experiences very efficient spin-orbit coupling (SOC). Accordingly, the phosphorescence decay rate is as large as 5 × 10⁴ s^-1 (20 μs) belonging to the fastest T₁→ S₀ transition values (shortest decay times) reported so far. Investigations down to T = 1.5 K reveal a large total zero-field splitting (ZFS) of 7 cm^-1 (0.9 meV). Although thermally activated delayed fluorescence (TADF) grows in at T≥ 160 K, the phosphorescence of 2 still dominates (60%) over TADF (40%) at ambient temperature. Thus, the compound represents a singlet harvesting-plus-triplet harvesting material, if applied in an OLED.

A new modification of [Ag4Br4(PPh3)4]: synthesis, structure and properties

A new modification of the homometallic silver(I) cluster [Ag4Br4(PPh3)4] has been prepared and characterized by elemental analysis, UV/Vis and IR spectroscopy, and X-ray crystallography. The tetramer shows a polycyclic structure with a chair conformation. The bromine atoms adopt μ 2- and μ 3-bridging modes. The shortest Ag–Ag distance in the cluster is 3.159(2) Å, which indicates significant Ag–Ag interactions. A supramolecular structure is arranged by hydrogen bonds (C–H···Br). Cyclic voltammograms of the cluster indicate a quasi-reversible Ag+/Ag couple. The fluorescence properties of the ligand and the Ag(I) cluster were studied in the solid state. The emission peaks of the Ag(I) cluster are attributed to ligand-centered luminescence.

Design of Conformationally Distorted Donor-Acceptor Dyads Showing Efficient Thermally Activated Delayed Fluorescence

A highly potent donor-acceptor biaryl thermally activated delayed fluorescence (TADF) dye is accessible by a concise two-step sequence employing two-fold Ullmann arylation and a sequentially Pd-catalyzed Masuda borylation-Suzuki arylation (MBSA). Photophysical investigations show efficient TADF at ambient temperature due to the sterical hindrance between the donor and acceptor moieties. The photoluminescence quantum yield amounts to Φ_PL = 80% in toluene and 90% in PMMA arising from prompt and delayed fluorescence with decay times of 21 ns and 30 μs, respectively. From an Arrhenius plot, the energy gap Δ E(S₁ - T₁) between the lowest excited singlet S₁ and triplet T₁ state was determined to be 980 cm^-1 (120 meV). A new procedure is proposed that allows us to estimate the intersystem crossing time to ∼10² ns.

Dinuclear Cu(I) Complex with Combined Bright TADF and Phosphorescence. Zero-Field Splitting and Spin-Lattice Relaxation Effects of the Triplet State

The three-fold bridged dinuclear Cu(I) complex Cu₂(μ-I)₂(1 N- n-butyl-5-diphenyl-phosphino-1,2,4-triazole)₃, Cu₂I₂(P^N)₃, shows bright thermally activated delayed fluorescence (TADF) as well as phosphorescence at ambient temperature with a total quantum yield of 85% at an emission decay time of 7 μs. The singlet (S₁)-triplet (T₁) energy gap is as small as only 430 cm^-1 (53 meV). Spin-orbit coupling induces a short-lived phosphorescence with a decay time of 52 μs ( T = 77 K) and a distinct zero-field splitting (ZFS) of T₁ into substates by ∼2.5 cm^-1 (0.3 meV). Below T ≈ 10 K, effects of spin-lattice relaxation (SLR) are observed and agree with the size of ZFS. According to the combined phosphorescence and TADF, the overall emission decay time is reduced by ∼13% as compared to the TADF-only process. The compound may potentially be applied in solution-processed OLEDs, exploiting both the singlet and triplet harvesting mechanisms.

A unique tetranuclear Ag(i) complex emitting efficient thermally activated delayed fluorescence with a remarkably short decay time

A novel tetranuclear Ag(i) complex, [Ag4(μ-DMPTP)2(POP)3][BF4]2 (Ag4N2P3), has been designed to achieve highly efficient thermally activated delayed fluorescence (TADF). Photophysical investigations show that the compound exhibits highly efficient TADF (Φ = 76%) and has a very short ambient-temperature TADF decay time of only 0.65 μs, corresponding to a radiative decay rate of k = Φ/τ = 1.2 × 106 s-1, a value belonging to the fast radiative rates in TADF materials.

Dinuclear Ag(I) Complex Designed for Highly Efficient Thermally Activated Delayed Fluorescence

The dinuclear Ag(I) complex has been designed to show thermally activated delayed fluorescence (TADF) of high efficiency. Strongly electron-donating terminal ligands are introduced to destabilize the d orbitals of the Ag⁺ ions. Consequently, the orbitals distinctly contribute to the HOMO, whereas the LUMO is localized on the bridging ligand. This ensures charge transfer character of the lowest excited singlet S₁ and triplet T₁ states. Accordingly, a small energy gap ΔE(S₁-T₁) is obtained, being essential for TADF behavior. Photophysical investigations show that at ambient temperature the complex exhibits TADF reaching a quantum yield of Φ_PL = 70% with the decay time of only τ = 1.9 μs, manifesting one of the fastest TADF decays observed so far. Such an outstanding TADF efficiency is based on a small value of ΔE(S₁-T₁) = 480 cm^-1 combined with a large transition rate of k(S₁ → S₀) = 2.2 × 10⁷ s^-1.

TADF Material Design: Photophysical Background and Case Studies Focusing on CuI and AgI Complexes

The development of organic light emitting diodes (OLEDs) and the use of emitting molecules have strongly stimulated scientific research of emitting compounds. In particular, for OLEDs it is required to harvest all singlet and triplet excitons that are generated in the emission layer. This can be achieved using the so-called triplet harvesting mechanism. However, the materials to be applied are based on high-cost rare metals and therefore, it has been proposed already more than one decade ago by our group to use the effect of thermally activated delayed fluorescence (TADF) to harvest all generated excitons in the lowest excited singlet state S₁ . In this situation, the resulting emission is an S₁ →S₀ fluorescence, though a delayed one. Hence, this mechanism represents the singlet harvesting mechanism. Using this effect, high-cost and strong SOC-carrying rare metals are not required. This mechanism can very effectively be realized by use of Cu^I or Ag^I complexes and even by purely organic molecules. In this investigation, we focus on photoluminescence properties and on crucial requirements for designing Cu^I and Ag^I materials that exhibit short TADF decay times at high emission quantum yields. The decay times should be as short as possible to minimize non-radiative quenching and, in particular, chemical reactions that frequently occur in the excited state. Thus, a short TADF decay time can strongly increase the material's long-term stability. Here, we study crucial parameters and analyze their impact on the TADF decay time. For example, the energy separation ΔE(S₁ -T₁ ) between the lowest excited singlet state S₁ and the triplet state T₁ should be small. Accordingly, we present detailed photophysical properties of two case-study materials designed to exhibit a large ΔE(S₁ -T₁ ) value of 1000 cm^-1 (120 meV) and, for comparison, a small one of 370 cm^-1 (46 meV). From these studies-extended by investigations of many other Cu^I TADF compounds-we can conclude that just small ΔE(S₁ -T₁ ) is not a sufficient requirement for short TADF decay times. High allowedness of the transition from the emitting S₁ state to the electronic ground state S₀ , expressed by the radiative rate k^r (S₁ →S₀ ) or the oscillator strength f(S₁ →S₀ ), is also very important. However, mostly small ΔE(S₁ -T₁ ) is related to small k^r (S₁ →S₀ ). This relation results from an experimental investigation of a large number of Cu^I complexes and basic quantum mechanical considerations. As a consequence, a reduction of τ(TADF) to below a few μs might be problematic. However, new materials can be designed for which this disadvantage is not prevailing. A new TADF compound, Ag(dbp)(P₂ -nCB) (with dbp=2,9-di-n-butyl-1,10-phenanthroline and P₂ -nCB=bis-(diphenylphosphine)-nido-carborane) seems to represent such an example. Accordingly, this material shows TADF record properties, such as short TADF decay time at high emission quantum yield. These properties are based (i) on geometry optimizations of the Ag^I complex for a fast radiative S₁ →S₀ rate and (ii) on restricting the extent of geometry reorganizations after excitation for reducing non-radiative relaxation and emission quenching. Indeed, we could design a TADF material with breakthrough properties showing τ(TADF)=1.4 μs at 100 % emission quantum yield.

Observation of Contrary Thermo-responsive Trend for Single Crystal and Powder Samples in Mechano-, Thermo- and Solvato-responsive Luminescent Cubane [Ag4I4L4] Cluster

A new silver(I) iodide cluster [Ag₄I₄(TMP)₄] (TMP = tris(3-methylphenyl)-phosphine) 1 shows triply stimuli-responsive luminescent chromism, namely, mechano-, thermo- and solvent-responsive chromism, which is isostructural to our previously reported [Cu₄I₄(TMP)₄] 2 but shows quite different luminescence in response to the external stimuli. Especially, during the mechanical grinding, the relative intensities of HE and LE of 1 varied with a concomitant hypsochromic shift, and when the temperature was decreased from 300 to 5 K, unprecedented contrary thermo-responsive trend for single crystal and powered samples (blueshift of single crystals and redshift of powdered samples) was observed. These distinct characters of 1 should be due to the different molecular packing modes, metallic interactions and the unique character of Ag(I) ion.