Ligand design aimed at arranging ligand entities and functional groups in order to generate non-covalent interactions, steric strain and ligand entanglement allowing to tune the electronic properties ...of transition metal complexes such as triplet emission energy and quantum yields.
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•Ligand design allows for turning-on of non-covalent intramolecular interactions.•H bridges, π-stacking, ligand entanglement and bulky groups promote rigidity.•Lewis acids, and bases further modify the electronic states.•Both effects allow tuning the electronic states and improve luminescence efficiency.•This review provides for the first time an overview of this approach.
Ligand design allows arranging ligand entities and functional groups in order to generate a plethora of non-covalent interactions such as π stacking, H bridging, and Lewis acid-base interactions in the second coordination sphere of transition metal complexes and thus modulate the photophysical properties. In addition to these forces, steric strain and ligand entanglement can lead to massive restrictions in the molecular geometry of complexes and in consequence to high rigidity and thus high geometrical similarity of the complexes electronic ground and excited states. This similarity is one of the prerequisites for efficient photoluminescence of such complexes since it hampers the unwanted radiationless decay from the excited states. Compared with other strategies to design efficient triplet emitting complexes, the approach of intramolecular non-covalent interaction has only recently entered the stage and has thus a lot of potential. In this review we will trace this approach presenting illustrative and recent examples for suitable ligand design.
The noncovalent intermolecular interactions (π–π stacking, metallophilic bonding) of the cyclometalated complexes Pt(NCN)L+X– (NCN = dipyridylbenzene, L = pyridine (1), acetonitrile (2)) are ...determined by the steric properties of the ancillary ligands L in the solid state and in solution, while the nature of the counterion X– (X– = PF6 –, ClO4 –, CF3SO3 –) affects the molecular arrangement of 2·X in the crystal medium. According to the variable-temperature X-ray diffraction measurements, the extensive Pt···Pt interactions and π-stacking in 2·X are significantly temperature-dependent. The variable concentration 1H and diffusion coefficients NMR measurements reveal that 2·X exists in the monomeric form in dilute solutions at 298 K, while upon increase in concentration Pt(NCN)(NCMe)+ cations undergo the formation of the ground-state oligomeric aggregates with an average aggregation number of ∼3. The photoluminescent characteristics of 1 and 2·X are largely determined by the intermolecular aggregation. For the discrete molecules the emission properties are assigned to metal perturbed IL charge transfer mixed with some MLCT contribution. In the case of oligomers 2·X the luminescence is significantly red-shifted with respect to 1 and originates mainly from the 3MMLCT excited states. The emission energies depend on the structural arrangement in the crystal and on the complex concentration in solution, variation of which allows for the modulation of the emission color from greenish to deep red. In the solid state the lability of the ligands L leads to vapor-induced reversible transformation 1 ↔ 2 that is accompanied by the molecular reorganization and, consequently, dramatic change of the photophysical properties. Time-dependent density functional theory calculations adequately support the models proposed for the rationalization of the experimental observations.
Copper subgroup metal ions in the +1 oxidation state are classical candidates for aggregation
via
non-covalent metal-metal interactions, which are supported by a number of bridging ligands. The ...bridging phosphines, soft donors with a relatively labile coordination to coinage metals, serve as convenient and essential components of the ligand environment that allow for efficient self-assembly of discrete polynuclear aggregates. Simultaneously, accessible and rich modification of the organic spacer of such P-donors has been used to generate many fascinating structures with attractive photoluminescent behavior. In this work we consider the development of di- and polynuclear complexes of M(
i
) (M = Cu, Ag, Au) and their photophysical properties, focusing on the effect of phosphine bridging ligands, their flexibility and denticity.
Stereochemical properties and flexibility of organic spacer in oligodentate phosphine ligand and spatial separation of phosphorus atoms one relative to other regulate the composition and architecture of multinuclear compounds of coinage metals.
The synthesis of π-extended pyrene-based luminescent compounds containing two six-membered phosphacycles has been realized through a two-step synthesis. It involves a Cu(II)-mediated double ...cyclization of tertiary diphosphane derivatives to afford dicationic molecules with quaternized phosphorus centers. Subsequent transformation of diphosphonium species into the corresponding P-oxide derivatives has been successfully achieved through Pd(0)-assisted cleavage of the P–Ph bonds, which opens a promising way for the functionalization of polyaromatic P-systems.
Tunable electron‐accepting properties of the cationic phosphorus center, its geometry and unique preparative chemistry that allows combining this unit with diversity of π‐conjugated motifs, define ...the appealing photophysical and electrochemical characteristics of organophosphorus ionic chromophores. This Minireview summarizes the achievements in the synthesis of the π‐extended molecules functionalized with P‐cationic fragments, modulation of their properties by means of structural modification, and emphasizes the important effect of cation‐anion interactions, which can drastically change physical behavior of these two‐component systems.
Ionic fifteenth: The diverse synthetic pathways to π‐conjugated systems with quaternary phosphorus, its unique connectivity and tunable electron deficient character, along with the non‐covalent cation‐anion interactions, introduce new photophysical and electrochemical features to ionic chromophores.
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 ( HP 3 O ) readily forms the disilver complex Ag2( P 3 O )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.
We report a homoleptic Au–Cu alkynyl cluster that represents an unexplored class of luminescent materials with stimuli-responsive photophysical properties. The bimetallic complex formulated as ...Au2Cu2(C2OHC5H8)4 n efficiently self-assembles from Au(SC4H8)Cl, Cu(NCMe)4PF6, and 1-ethynylcyclopentanol in the presence of NEt3. This compound shows remarkably diverse polymorphism arising from the modulation of metallophilic interactions by organic solvents. Four crystalline forms, obtained from methanol (1a); ethanol, acetone, or choloroform (1b); toluene (1c); and diethyl ether or ethyl acetate (1d), demonstrate different photoluminescent characteristics. The solid-state quantum yields of phosphorescence (Φ) vary from 0.1% (1a) to 25% (1d), depending on the character of intermetallic bonding. The structures of 1b–d were determined by single-crystal X-ray diffraction. The ethanol (1b, Φ = 2%) and toluene (1c, Φ = 10%) solvates of Au2Cu2(C2OHC5H8)4 n adopt octanuclear isomeric structures (n = 2), while 1d (Φ = 25%) is a solvent-free chain polymer built from two types of Au4Cu4 units. Electronic structure calculations show that the dramatic enhancement of the emission intensity is correlated with the increasing role of metal–metal bonding. The latter makes the emission progressively more metal-centered in the order 1b < 1c < 1d. The metallophilic contacts in 1a–d show high sensitivity to the vapors of certain solvents, which effectively induce unusual solid-state isomerization and switching of the absorption and luminescence properties via non-covalent interactions. The reported polymorphic material is the first example of a gold(I) alkynyl compound demonstrating vapochromic behavior.
The series of chelating phosphine ligands, which contain bidentate P 2 (bis(2-diphenylphosphino)phenyl ether, DPEphos; 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene, Xantphos; ...1,2-bis(diphenylphosphino)benzene, dppb), tridentate P 3 (bis(2-diphenylphosphinophenyl)phenylphosphine), and tetradentate P 4 (tris(2-diphenylphosphino)phenylphosphine) ligands, was used for the preparation of the corresponding dinuclear M(μ2-SCN)P 2 2 (M = Cu, 1, 3, 5; M = Ag, 2, 4, 6) and mononuclear CuNCS(P 3 /P 4 ) (7, 9) and AgSCN(P 3 /P 4 ) (8, 10) complexes. The reactions of P 4 with silver salts in a 1:2 molar ratio produce tetranuclear clusters Ag2(μ3-SCN)(t-SCN)(P 4 )2 (11) and Ag2(μ3-SCN)(P 4 )2 2+ (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 (λem varies from 448 nm for 1 to 630 nm for 10c). The emission of compounds 10 and 15, based on the P 4 ligand, strongly depends on the solid-state packing (λem = 505 and 625 nm for two crystalline forms of 15), which affects structural reorganizations accompanying the formation of electronically excited states.
A series of gold(I) iodide complexes 1–11 have been prepared from di‐, tri‐, and tetraphosphane ligands. Crystallographic studies reveal that the di‐ (1–7) and tetrametallic (11) compounds feature ...linearly coordinated gold(I) ions with short aurophilic contacts. Their luminescence behavior is determined by the combined influence of the phosphane properties, metal–metal interaction, and intermolecular lattice‐defined interactions. The proposed variable contribution of 3(X+M)‐centered (X=halogen; M=metal) and 3XLCT (halogen to ligand charge transfer) electronic transitions into the lowest lying excited state, which is influenced by supramolecular packing, is presumably responsible for the alteration of room‐temperature emission color from green (λ=545 nm, for 11) to near‐IR (λ=698 nm, for 2). Dinuclear compounds 6 and 7 exhibit distinct luminescence thermochromism with a blueshift up to 5750 cm−1 upon cooling. Such dramatic change of emission energy is assigned to the presence of two coupled triplet excited states of 3ππ* and 3(X+M)C/3XLCT nature, the presence of which depends on both molecular structure and the crystal lattice arrangement.
Control the glow: A delicate interplay of intramolecular features and subtle intermolecular interactions is capable of producing gold(I) complexes with distinct thermochromic luminescence (see figure), which results from reversible switching between two coupled excited states.
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.
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