Dinuclear Cyclometalated Pincer Nickel(II) Complexes with Metal-Metal-to-Ligand Charge Transfer Excited States and Near-Infrared Emission

Facile non-radiative decay of low-lying metal-centered (MC) dd excited states has been well documented to pose a significant obstacle to the development of phosphorescent NiII complexes due to substantial structural distortions between the dd excited state and the ground state. Herein, we prepared a series of dinuclear Ni2 II,II complexes by using strong σ-donating carbene-phenyl-carbene (CNHC Cphenyl CNHC) pincer ligands, and prepared their dinuclear Pt2 II,II and Pd2 II,II analogues. Dinuclear Ni2 II,II complexes bridged by formamidinate/α-carbolinato ligand exhibit short Ni-Ni distances of 2.947-3.054 Å and singlet metal-metal-to-ligand charge transfer (1MMLCT) transitions at 500-550 nm. Their 1MMLCT absorption energies are red-shifted relative to the Pt2 II,II and Pd2 II,II analogues at ~450 nm and ≤420 nm respectively. One-electron oxidation of these Ni2 II,II complexes produces valence-trapped dinuclear Ni2 II,III species, which are characterized by EPR spectroscopy. Upon photoexcitation, these Ni2 II,II complexes display phosphorescence (τ=2.6-8.6 μs) in the NIR (800-1400 nm) spectral region in 2-MeTHF and in the solid state at 77 K, which is insensitive to π-conjugation of the coordinated [CNHC Cphenyl CNHC] ligand. Combined with DFT calculations, the NIR emission is assigned to originate from the 3dd excited state. Studies have found that the dinuclear Ni2 II,II complex can sensitize the formation of singlet oxygen and catalyze the oxidation of cyclo-dienes under light irradiation.

Phosphorescent Cyclometalated Palladium(II) and Platinum(II) Complexes Derived from Diaminocarbene Precursors

Metal-mediated self-assembly of isocyanides and methyl 4-aminopyrimidine-5-carboxylate leads to luminescent PdII and PtII complexes featuring C,N-cyclometalated acyclic diaminocarbene (ADC) ligands. The solid-state luminescent properties of these diaminocarbene derivatives are attributed to their triplet-state metal/metal-to-ligand charge-transfer (3MMLCT) nature, which is driven by attractive intermolecular M···M interactions further reinforced by the intramolecular π-π interactions even in the structure of the Pd compound, which is the first Pd-ADC phosphor reported.

Highly efficient and stable thermally activated delayed fluorescent palladium(II) complexes for organic light-emitting diodes

Transition metal complexes exhibiting thermally activated delayed fluorescence (TADF) remain underdeveloped for organic light-emitting diodes (OLEDs). Here, we describe a design of TADF Pd(II) complexes featuring metal-perturbed intraligand charge-transfer excited states. Two orange- and red-emitting complexes with efficiencies of 82 and 89% and lifetimes of 2.19 and 0.97 μs have been developed. Combined transient spectroscopic and theoretical studies on one complex reveal a metal-perturbed fast intersystem crossing process. OLEDs using the Pd(II) complexes show maximum external quantum efficiencies of 27.5 to 31.4% and small roll-offs down to 1% at 1000 cd m^-2. Moreover, the Pd(II) complexes show exceptional operational stability with LT₉₅ values over 220 hours at 1000 cd m^-2, benefiting from the use of strong σ-donating ligands and the presence of multiple intramolecular noncovalent interactions beside their short emission lifetimes. This study demonstrates a promising approach for developing efficient and robust luminescent complexes without using the third-row transition metals.

Luminescence and Palladium: The Odd Couple

The synthesis, photophysical properties, and applications of highly fluorescent and phosphorescent palladium complexes are reviewed, covering the period 2018–2022. Despite the fact that the Pd atom appears closely related with an efficient quenching of the fluorescence of different molecules, different synthetic strategies have been recently optimized to achieve the preservation and even the amplification of the luminescent properties of several fluorophores after Pd incorporation. Beyond classical methodologies such as orthopalladation or the use of highly emissive ligands as porphyrins and related systems (for instance, biladiene), new concepts such as AIE (Aggregation Induced Emission) in metallacages or in coordination-driven supramolecular compounds (CDS) by restriction of intramolecular motions (RIM), or complexes showing TADF (Thermally Activated Delayed Fluorescence), are here described and analysed. Without pretending to be comprehensive, selected examples of applications in areas such as the fabrication of lighting devices, biological markers, photodynamic therapy, or oxygen sensing are also here reported.

Room-temperature luminescence from Pd(II) and Pt(II) complexes: from mechanochromic crystals to flexible polymer matrices

A series of Pd(II) (PdLOMe, PdLOHex) and Pt(II) (PtLOMe, PtLOHex) complexes bearing tetradentate ligands as dianionic luminophores were synthesized. Hence, the cyclometallating chelators were alternatively decorated with two n-hexyloxy (LOHex) or two methoxy (LOMe) moieties to promote crystallization and processability. The new compounds were unambiguously characterized by means of multiple NMR spectroscopies and mass spectrometry as well as by single crystal X-ray diffractometric analysis (PtLOMe and PdLOMe). Steady state and time-resolved photoluminescence spectroscopic studies were carried out in crystalline phases, in fluid solutions at room temperature, in frozen glassy matrices at 77 K and in a flexible polymeric matrix (PMMA). PtLOMe presents an intriguing mechanochromism resulting from the responsive metal-metal interactions involving adjacent monomeric units. Incorporation of the Pd(II) complexes into the polymeric matrix boosts their photophysical properties by stiffening of the coordination environment while reducing non-radiative deactivation pathways mediated by dissociative metal-centred states, which also become thermally inaccessible at 77 K.

Role of the Trifluoropropynyl Ligand in Blue-Shifting Charge-Transfer States in Emissive Pt Diimine Complexes and an Investigation into the PMMA-Imposed Rigidoluminescence and Rigidochromism

Square-planar Pt^II complexes are of interest as dopants for the emissive layer of organic light-emitting diodes. Herein, the photophysics of three Pt bipyridyl complexes with the strongly e^- withdrawing, high-field, 3,3,3-trifluoropropynyl ligand has been investigated. One complex, (phbpy)PtC₂CF₃ (phbpy = 6-phenyl-2,2'-dipyridyl), has also been characterized by single-crystal X-ray diffraction. All complexes reported are emissive in both RT CH₂Cl₂ solution (Φ_PL = 0.007 to 0.027) and PMMA film (Φ_PL = 0.25 to 0.42). The trifluoropropynyl ligand elevates the energy of the MLCT and LL'CT states above that of the IL π-π* state, resulting in IL emission in all cases. The emission energies of the trifluoropropynyl compounds are also blue-shifted relative to the analogous pentafluorophenylethynyl compounds, suggesting that the trifluoropropynyl ligand is one of the most electron-withdrawing alkynyl ligands. Rate constants for radiative and nonradiative deactivation were determined from experimentally determined values of Φ_PL and excited-state lifetimes in both solution and PMMA films. The increase in Φ_PL upon incorporation into PMMA film (rigidoluminescence) results from a decrease in the rate constant for non-radiative relaxation. Experimental activation energies for excited-state decay in combination with TDDFT are consistent with the rigidoluminescence resulting from an increase in the energy of the non-emissive triplet metal-centered state. Two of the complexes investigated, (^Ph2bpy)Pt(C₂CF₃)₂ and (^t-Bu2bpy)Pt(C₂CF₃)₂, where ^t-Bu2bpy = 4,4'-di-tert-butyl-2,2'-dipyridyl and ^Ph2bpy = 4,4'-diphenyl-2,2'-dipyridyl, exhibit concentration-dependent excimer emission (orange) along with monomer emission (blue), enabling fine-tuning of the emission color. However, excimer emission was absent in cured PMMA films up to the solubility limit for solution processing of (^Ph2bpy)Pt(C₂CF₃)₂ in CH₂Cl₂, demonstrating the diffusional nature of excimer formation.

Phosphorescent NIR emitters for biomedicine: applications, advances and challenges

Application of NIR (near-infrared) emitting transition metal complexes in biomedicine is a rapidly developing area of research. Emission of this class of compounds in the "optical transparency windows" of biological tissues and the intrinsic sensitivity of their phosphorescence to oxygen resulted in the preparation of several commercial oxygen sensors capable of deep (up to whole-body) and quantitative mapping of oxygen gradients suitable for in vivo experimental studies. In addition to this achievement, the last decade has also witnessed the increased growth of successful alternative applications of NIR phosphors that include (i) site-specific in vitro and in vivo visualization of sophisticated biological models ranging from 3D cell cultures to intact animals; (ii) sensing of various biologically relevant analytes, such as pH, reactive oxygen and nitrogen species, RedOx agents, etc.; (iii) and several therapeutic applications such as photodynamic (PDT), photothermal (PTT), and photoactivated cancer (PACT) therapies as well as their combinations with other therapeutic and imaging modalities to yield new variants of combined therapies and theranostics. Nevertheless, emerging applications of these compounds in experimental biomedicine and their implementation as therapeutic agents practically applicable in PDT, PTT, and PACT face challenges related to a critically important improvement of their photophysical and physico-chemical characteristics. This review outlines the current state of the art and achievements of the last decade and stresses the most promising trends, major development prospects, and challenges in the design of NIR phosphors suitable for biomedical applications.

Photoluminescence of Ni(II), Pd(II), and Pt(II) Complexes [M(Me2dpb)Cl] Obtained from C‒H Activation of 1,5-Di(2-pyridyl)-2,4-dimethylbenzene (Me2dpbH)

The three complexes [M(Me₂dpb)Cl] (M = Ni, Pd, Pt) containing the tridentate N,C,N-cyclometalating 3,5-dimethyl-1,5-dipyridyl-phenide ligand (Me₂dpb^-) were synthesised using a base-assisted C‒H activation method. Oxidation potentials from cyclic voltammetry increased along the series Pt < Ni < Pd from 0.15 to 0.74 V. DFT calculations confirmed the essentially ligand-centred π*-type character of the lowest unoccupied molecular orbital (LUMO) for all three complexes in agreement with the invariant reduction processes. For the highest occupied molecular orbitals (HOMO), contributions from metal d_yz, phenyl C4, C2, C1, and C6, and Cl p_z orbitals were found. As expected, the d_z² (HOMO-1 for Ni) is stabilised for the Pd and Pt derivatives, while the antibonding d_x²_-y² orbital is de-stabilised for Pt and Pd compared with Ni. The long-wavelength UV-vis absorption band energies increase along the series Ni < Pt < Pd. The lowest-energy TD-DFT-calculated state for the Ni complex has a pronounced d_z²-type contribution to the overall metal-to-ligand charge transfer (MLCT) character. For Pt and Pd, the d_z² orbital is energetically not available and a strongly mixed Cl-to-π*/phenyl-to-π*/M(d_yz)-to-π* (XLCT/ILCT/MLCT) character is found. The complex [Pd(Me₂dpb)Cl] showed a structured emission band in a frozen glassy matrix at 77 K, peaking at 468 nm with a quantum yield of almost unity as observed for the previously reported Pt derivative. No emission was observed from the Ni complex at 77 or 298 K. The TD-DFT-calculated states using the TPSSh functional were in excellent agreement with the observed absorption energies and also clearly assessed the nature of the so-called "dark", i.e., d‒d*, excited configurations to lie low for the Ni complex (≥3.18 eV), promoting rapid radiationless relaxation. For the Pd(II) and Pt(II) derivatives, the "dark" states are markedly higher in energy with ≥4.41 eV (Pd) and ≥4.86 eV (Pt), which is in perfect agreement with the similar photophysical behaviour of the two complexes at low temperatures.

Luminescent Platinum(II) Complexes with Bidentate Diacetylide Ligands: Structures, Photophysical Properties and Application Studies

A series of platinum(II) complexes supported by terphenyl diacetylide as well as diimine or bis-N-heterocyclic carbene (NHC) ligands have been prepared. The diacetylide ligands adopt a cis coordination mode featuring non-planar terphenyl moieties as revealed by X-ray crystallographic analyses. The electrochemical, photophysical and photochemical properties of these platinum(II) complexes have been investigated. These platinum(II) diimine complexes show broad emission with peak maxima from 566 nm to 706 nm, with two of them having emission quantum yields >60% and lifetimes <2 μs in solutions at room temperature, whereas the platinum(II) diacetylide complexes having bis-N-heterocyclic carbene instead of diimine ligand display photoluminescence with quantum yields of up to 28% in solutions and excited state lifetimes of up to 62 μs at room temperature. Application studies revealed that one of the complexes can catalyze photoinduced aerobic dehydrogenation of alcohols and alkenes, and a relatively non-toxic water-soluble Pt(II) complex displays anti-angiogenic activity.

Photophysics and Excited State Dynamics of Cyclometalated [M(Phbpy)(CN)] (M = Ni, Pd, Pt) Complexes: A Theoretical and Experimental Study

Cyclometalated complexes [M(Phbpy)(CN)] (HPhbpy = 6-phenyl-2,2'-bipyridine) of the group 10 metals (Ni, Pd, and Pt) bearing a carbanionic ^-C^∧N^∧N pincer ligand were synthesized and studied in a combined experimental and computational DFT approach. All three complexes were crystallographically characterized showing closely packed dimers with head-to-tail stacking and short metal-metal contacts in the solid state. The computational models for geometries, excited states, and electronic transitions addressed both monomeric (Ni-mono, Pd-mono, and Pt-mono) and dimeric (Ni-dim, Pd-dim, and Pt-dim) entities. Photophysical properties and excited state dynamics of all title complexes were investigated in solution and in the solid at 298 and 77 K. [Ni(Phbpy)(CN)] and [Pd(Phbpy)(CN)] are virtually nonemissive in solution at 298 K, whereas [Pt(Phbpy)(CN)] shows phosphorescence in CH₂Cl₂ (DCM) solution (λ_em = 562 nm) stemming from a mixed ³MLCT/ILCT (metal-to-ligand charge transfer/intraligand charge transfer) state. At 77 K in a glassy frozen DCM:MeOH matrix, [Pd(Phbpy)(CN)] shows a remarkable emission (λ_em = 571 nm) with a photoluminescence quantum yield reaching almost unity, whereas [Ni(Phbpy)(CN)] is again nonemissive. Calculations on the monomeric models M-mono show that low-lying metal-centered states (MC, i.e., d-d* configuration) with dissociative character quench the photoluminescence. In the solid state, the complexes [M(Phbpy)(CN)] show defined photoluminescence bands (λ_em = 561 nm for Pd and 701 nm for Pt). Calculations on the dimeric models M-dim shows that the axial M···M interactions alter the photophysical properties of Pd-dim and Pt-dim toward MMLCT (metal-metal-to-ligand charge transfer) excited states with Pd-dim showing temperature-dependent emission lifetimes, suggesting thermally activated delayed fluorescence, whereas Pt-dim displayed phosphorescence with excimeric character. The metal-metal interactions were analyzed in detail with the quantum theory of atoms in molecules approach.

Helically chiral Pd(ii) complexes containing intramolecular PdPd metallophilicity as circularly polarized molecular phosphors

Carbon-centred chirality of the pincer-type cyclometalated ligands is transferred to the helical chirality of dinuclear and tetranuclear Pd(ii) arylacetylide complexes, and hence phosphorescence with quantum yields up to 50% and dissymmetry factors in the 10^-3 scale from the metal-metal-to-ligand charge-transfer excited states has been recorded in diluted solutions.

Modulation of Metallophilic and π-π Interactions in Platinum Cyclometalated Luminophores with Halogen Bonding

Luminescent cyclometalated complexes [M(C^N^N)CN] (M=Pt, Pd; HC^N^N=pyridinyl- (M=Pt 1, Pd 5), benzyltriazolyl- (M=Pt 2), indazolyl- (M=Pt 3, Pd 6), pyrazolyl-phenylpyridine (M=Pt 4)) decorated with cyanide ligand, have been explored as nucleophilic building blocks for the construction of halogen-bonded (XB) adducts using IC₆ F₅ as an XB donor. The negative electrostatic potential of the CN group afforded CN⋅⋅⋅I noncovalent interactions for platinum complexes 1-3; the energies of XB contacts are comparable to those of metallophilic bonding according to QTAIM analysis. Embedding the chromophore units into XB adducts 1-3⋅⋅⋅IC₆ F₅ has little effect on the charge distribution, but strongly affects Pt⋅⋅⋅Pt bonding and π-stacking, which lead to excited states of MMLCT (metal-metal-to-ligand charge transfer) origin. The energies of these states and the photoemissive properties of the crystalline materials are primarily determined by the degree of aggregation of the luminophores via metal-metal interactions. The adduct formation depends on the nature of the metal and the structure of the metalated ligand, the variation of which can yield dynamic XB-supported systems, exemplified by thermally regulated transition 3↔3⋅⋅⋅IC₆ F₅ .