Structural Control of Highly Efficient Thermally Activated Delayed Fluorescence in Carbene Zinc(II) Dithiolates.

Luminescent metal complexes based on earth abundant elements are a valuable target to substitute 4d/5d transition metal complexes as triplet emitters in advanced photonic applications. Whereas CuI complexes have been thoroughly investigated in the last two decades for this purpose, no structure-property-relationships for efficient luminescence involving triplet excited states from ZnII complexes are established. Herein, we report on the design of monomeric carbene zinc(II) dithiolates (CZT) featuring a donor-acceptor-motif that leads to highly efficient thermally activated delayed fluorescence (TADF) with for ZnII compounds unprecedented radiative rate constants kTADF =1.2×106  s-1 at 297 K. Our high-level DFT/MRCI calculations revealed that the relative orientation of the ligands involved in the ligand-to-ligand charge transfer (1/3 LLCT) states is paramount to control the TADF process. Specifically, a dihedral angle of 36-40° leads to very efficient reverse intersystem-crossing (rISC) on the order of 109  s-1 due to spin-orbit coupling (SOC) mediated by the sulfur atoms in combination with a small ΔES1-T1 of ca. 56 meV.

Towards Fast Circularly Polarized Luminescence in 2-Coordinate Chiral Mechanochromic Copper(I) Carbene Complexes.

A series of chiral mechanochromic copper(I) cAAC (cAAC=cyclic (alkyl)(amino)carbene) complexes with a variety of amide ligands have been studied with regard to their photophysical and chiroptical properties to elucidate structure-property relationships for the design of efficient triplet exciton emitters exhibiting circularly polarized luminescence. Depending on the environment, which determines the excited state energies, either thermally activated delayed fluorescence (TADF) from 1/3 LLCT states or phosphorescence from 3 LLCT/LC states occurs. However, neither chiral moieties at the carbene nor at the carbazolate ligands provide detectable luminescence dissymmetries glum . An exception is [Cu(phenoxazinyl)(cAAC)], showing orange to deep red TADF with λmax =601-715 nm in solution, powders and in PMMA. In this case, the amide ligand can undergo distortions in the excited state. This design motif leads to the first linear, non-aggregated CPL-active copper(I) complex with glum of -3.4 ⋅ 10-3 combined with a high radiative rate constant of 6.7 ⋅ 105  s-1 .

Synthesis and Photophysical Studies of Copper(I) CAAC Half-Sandwich Complexes as a Highly Modifiable Class of Emitters.

Cyclopentadienyls are well-known strong donor ligands and have been successfully employed in catalysis as they tolerate a variety of substituents to adjust their steric and electronic properties. Although such highly modifiable ligands are of great interest for luminescence and photocatalytic applications, studies of CpR-containing photoactive transition-metal complexes are quite rare. In this work, we present a structural, electrochemical, and first elaborated photophysical investigation of a series of copper(I) half-sandwich complexes bearing cyclic alkyl(amino)carbenes (CAACs) as chromophore ligands and compare them with [Cu(Cp)(IDipp)] and [Cu(Cp*)(IDipp)] bearing a traditional N-heterocyclic carbene. Furthermore, we present the first molecular structure derived from single-crystal X-ray diffraction of a copper(I) indenyl complex, which can be described as an η2 (σ, π)-coordination. The CuI half-sandwich complexes show blue-green to orange phosphorescence with a photoluminescence quantum yield of up to 59% and radiative rate constants kr of up to 4 × 104 s-1 in the solid state, depending on the substitution pattern of the CpR ligand. Our TD/DFT calculations suggest that the emitting excited states are of 3MLCT/LLCT character. We determined the excited-state lifetime of the CuI half-sandwich complexes in solution to be as long as 600 ns, which in combination with the large π-surface of the CpR ligands allows for Dexter energy transfer for photocatalytic applications. In addition, the chiroptical properties of chiral [Cu(Cp/Cp*)(CAACMenthone)] were studied and compared to [CuCl(CAACMenthone)], of which we demonstrate that its circular polarized luminescence is the result of excimer formation and not, as previously reported, attributed to the monomeric C1-symmetric structure.

Synthesis and Fluorescence Studies of Diborene Coinage Metal Complexes.

Nine different coinage metal (Cu, Ag) π complexes of diborenes with various anionic diborene (aryl, heteroaryl) and metal substituents (Cl, Br, C6F5, C2SiMe3), stabilizing neutral donors (N-heterocyclic carbene = NHC, phosphine), configurations (cis/trans, acyclic/cyclic diborene), and charges (neutral, cationic) were synthesized and characterized by multinuclear NMR spectroscopy and X-ray crystallographic analyses. Their optical properties were investigated by UV-vis absorption and steady-state as well as time-resolved luminescence spectroscopy in solution and the solid state to gain insights into the excited-state behavior of this unusual class of photoactive compounds and to provide structure-property relationships. The structural and electronic modification of the (B═B)···M motif greatly influences not only the visible light absorption but also the photostability and quantum yields, which can reach high values of up to f = 0.42. The lifetimes are found in the nanosecond regime, providing estimated radiative rate constants over a wide range of kr = 1.3-14 × 107 s-1, indicative of fluorescence. Intersystem crossing (ISC) is sufficiently slow for prompt emission from the S1 state to be observed, while the spin-orbit coupling in the T1 state is too weak for phosphorescence to occur at room temperature. ISC can be accelerated, however, by modifying diborene ligand substitution and the coinage metal center, hinting at the potential for exploiting the properties of long-lived triplet excited states of metal diborene complexes in the future.

Simple Synthetic Routes to Carbene-M-Amido (M=Cu, Ag, Au) Complexes for Luminescence and Photocatalysis Applications.

The development of novel and operationally simple synthetic routes to carbene-metal-amido (CMA) complexes of copper, silver and gold relevant for photonic applications are reported. A mild base and sustainable solvents allow all reactions to be conducted in air and at room temperature, leading to high yields of the targeted compounds even on multigram scales. The effect of various mild bases on the N-H metallation was studied in silico and experimentally, while a mechanochemical, solvent-free synthetic approach was also developed. Our photophysical studies on [M(NHC)(Cbz)] (Cbz=carbazolyl) indicate that the occurrence of fluorescent or phosphorescent states is determined primarily by the metal, providing control over the excited state properties. Consequently, we demonstrate the potential of the new CMAs beyond luminescence applications by employing a selected CMA as a photocatalyst. The exemplified synthetic ease is expected to accelerate the applications of CMAs in photocatalysis and materials chemistry.

Carba-closo-dodecaboranylethynyl ligands facilitating luminescent reversed charge-transfer excited states in gold/silver complexes.

A set of mono- and dinuclear AuI and AgI alkynyl complexes bearing the carba-closo-dodecaboranylethynyl ligand show intense room temperature phosphorescence. The {closo-1-CB11} cage participates in an unprecedented way as an electron donating moiety, changing the direction of the charge-transfer excited state.

Cu-F Interactions between Cationic Linear N-Heterocyclic Carbene Copper(I) Pyridine Complexes and Their Counterions Greatly Enhance Blue Luminescence Efficiency.

A series of easily accessible linear N-heterocyclic carbene (NHC) copper(I) complexes, bearing pyridine (py) and its derivatives as chromophore ligands, are barely emissive in the single-crystalline solid state. However, their powders, neat films, and dilute doped films of poly(methyl methacrylate) (PMMA; 1-10%) show very intense blue-to-blue-green photoluminescence with remarkable quantum yields φ of up to 87% and microsecond lifetimes, indicative of triplet states being involved. These luminescence properties are similar to trigonal coordinated NHC copper(I) bis(pyridine) complexes, which we have also isolated and characterized with respect to their structures and photophysics. Our spectroscopic and theoretical studies provide detailed insight into the nature of the luminescence enhancing effect of the linear two-coordinated copper(I) compounds, which is based on the formation of Cu-F interactions between the BF4- anions and [Cu(NHC)(2-R-py)]+ (R = H, Me, Ph) cations. These interactions are absent in the single crystals but lead to a distorted ground-state structure in the precipitated powders or in PMMA films, giving rise to high kr. In addition, we found that our linear copper(I) complexes exhibit mechanochromic luminescence because grinding of the single crystals leads to enhanced emission intensity. In light of the recently reported cation-anion contact-induced mechanochromic luminescence of two-coordinated copper(I) complexes, this study supports the generality of this new mechanism for the design of mechanoresponsive phosphorescent materials.

Stimulus-Triggered Formation of an Anion-Cation Exciplex in Copper(I) Complexes as a Mechanism for Mechanochromic Phosphorescence.

The investigation of the mechanisms of mechanochromic luminescence is of fundamental importance for the development of materials for photonic sensors, data storage, and luminescence switches. The structural origin of this phenomenon in phosphorescent molecular systems is rarely known and thus the formulation of structure-property relationships remains challenging. Changes in the M-M interactions have been proposed as the main mechanism with d10 coinage metal compounds. Herein, we describe a new mechanism-a mechanically induced reversible formation of a cation-anion exciplex based on Cu-F interactions-that leads to highly efficient mechanochromic phosphorescence and unusual large emission shifts from UV-blue to yellow for CuI complexes. The low-energy luminescence is thermo- and vaporesponsive, thus allowing the generation of white light as well as for recovering the original UV-blue emission.

Synthesis, Structures, and Photophysical Properties of a Series of Rare Near-IR Emitting Copper(I) Complexes.

Herein, we report on the synthesis and structural characterization of a series of trigonal and tetrahedral cationic copper(I) complexes, bearing phosphine or N-heterocyclic carbene ligands as donors, with benzthiazol-2-pyridine (pybt) and benzthiazol-2-quinoline (qybt) acting as π-chromophores. The compounds are highly colored due to their 1MLCT (MLCT = metal-to-ligand charge transfer) states absorbing between ca. λabs = 400-500 nm, with 1ILCT (ILCT = intraligand charge transfer) states in the UV region. The relative shifts of the S0→S1 absorption correlate with the computed highest occupied molecular orbital-lowest unoccupied molecular orbital gaps, the qybt complexes generally being lower in energy than the pybt ones due to the larger conjugation of the quinoline-based ligand. The compounds exhibit, for CuI complexes, rare intense long-lived near-IR emission with λmax ranging from 593 to 757 nm, quantum yields of up to Φ = 0.11, and lifetimes τ of several microseconds in the solid state as well as in poly(methyl methacrylate) films. Although a bathochromic shift of the emission is observed with λmax ranging from 639 to 812 nm and the lifetimes are greatly increased at 77 K, no clear indication for thermally activated delayed fluorescence was found, leaving us to assign the emission to originate from a 3(Cu→pybt/qybt)MLCT state. The red to near-IR emission is a result of incorporation of the sulfur into the chromophore ligand, as related nitrogen analogues emit in the green to orange region of the electromagnetic spectrum. The photophysical results and conclusions have further been corroborated with density functional theory (DFT)/time-dependent DFT calculations, confirming the nature of the excited states and also the trends of the redox potentials.

Silver(I) Clusters with Carba-closo-dodecaboranylethynyl Ligands: Synthesis, Structure, and Phosphorescence.

Salts of anionic silver(I) clusters with the carba-closo-dodecaboranylethynyl ligand were obtained from {Ag2 (12-C≡C-closo-1-CB11 H11 )}n , selected pyridines, and [Et4 N]Cl or [Ph4 P]Br. Salts of octahedral silver(I) clusters [Et4 N]2 [Ag6 (12-C≡C-closo-1-CB11 H11 )4 (4-X-C5 H5 N)x ] were formed with pyridine (X=H, x=8), 4-methylpyridine (X=Me, x=8), and 4-cyanopyridine (X=CN, x=10). In contrast, 3,5-lutidine (3,5-Me2 Py) did not result in salts of dianionic clusters, even in the presence of excess of [Et4 N]Cl or [Ph4 P]Br; instead salts of monoanionic AgI7 clusters, [Et4 N][Ag7 (12-C≡C-closo-1-CB11 H11 )4 (3,5-Me2 Py)9 ] and [Ph4 P][Ag7 (12-C≡C-closo-1-CB11 H11 )4 (3,5-Me2 Py)13 ] were obtained. The AgI7 cluster is pentagonal bipyramidal in the former, but is an edge-capped octahedron in the latter. The 4-methylpyridine and 3,5-lutidine complexes show green phosphorescence at room temperature. Although argentophilic interactions give rise to sufficient spin-orbit coupling for intersystem crossing S1 →Tn and moderate-to-high radiative rate constants, time-resolved measurements indicate that the quantum yields are greatly influenced by the pyridine ligands, which mainly determine the non-radiative rate constants. In addition, the crystal structures of [Ag16 (12-C≡C-closo-1-CB11 H11 )8 (Py)9.25 (CH3 CN)2 (CH2 Cl2 )0.75 ]⋅CH2 Cl2 , [Ag8 (12-C≡C-closo-1-CB11 H11 )4 (Py)12 ], [Ag10 (12-C≡C-closo-1-CB11 H11 )4 (4-MePy)10 Br2 ], [Ag7 (12-C≡C-closo-1-CB11 H11 )3 (4-tBuPy)11 Cl]⋅(4-tBuPy), and [Ag9 (12-C≡C-closo-1-CB11 H11 )4 (3,5-Me2 Py)11 Cl] were elucidated.

Strongly Phosphorescent Transition Metal π-Complexes of Boron-Boron Triple Bonds.

Herein are reported the first π-complexes of compounds with boron-boron triple bonds with transition metals, in this case CuI. Three different compounds were isolated that differ in the number of copper atoms bound to the BB unit. Metalation of the B-B triple bonds causes lengthening of the B-B and B-CNHC bonds, as well as large upfield shifts of the 11B NMR signals, suggesting greater orbital interactions between the boron and transition metal atoms than those observed with recently published diboryne/alkali metal cation complexes. In contrast to previously reported fluorescent copper(I) π-complexes of boron-boron double bonds, the Cun-π-diboryne compounds (n = 2, 3) show intense phosphorescence in the red to near-IR region from their triplet excited states, according to their microsecond lifetimes, with quantum yields of up to 58%. While the Cu diborene bond is dominated by electrostatic interactions, giving rise to S1 and T1 states of pure IL(π-π*) nature, DFT studies show that the CuI π-complexes of diborynes reported herein exhibit enhanced metal d orbital contributions to HOMO and HOMO-1, which results in S1 and T1 having significant MLCT character, enabling strong spin-orbit coupling for highly efficient intersystem-crossing S1 → Tn and phosphorescence T1 → S0.

Highly efficient cold-white light emission in a [Au2CuCl2(P∩N)2]PF6 type salt.

In the trinuclear, heterometallic cluster compound [Au2CuCl2(P∩N)2]PF6 metallophilic interactions give rise to very efficient cold-white light emission as a result of at least two thermally non-equilibrated emissive triplet states (one of mainly Cu → py and the other of Au → py character, respectively) with exceptional spin-orbit coupling and short emission lifetimes, which are competitive to PtII- and IrIII-based emitters.

Unprecedented Efficient Structure Controlled Phosphorescence of Silver(I) Clusters Stabilized by Carba-closo-dodecaboranylethynyl Ligands.

{Ag2 (12-C≡C-closo-1-CB11 H11 )}n and selected pyridine ligands have been used for the synthesis of photostable Ag(I) clusters that, with one exception, exhibit for Ag(I) compounds unusual room-temperature phosphorescence. Extraordinarily intense phosphorescence was observed for a distorted pentagonal bipyramidal Ag(I) 7 cluster that shows an unprecedented quantum yield of Φ=0.76 for Ag(I) clusters. The luminescence properties correlate with the structures of the central Ag(I) n motifs as shown by comparison of the emission properties of the clusters with different numbers of Ag(I) ions, different charges, and electronically different pyridine ligands.