Solid-state emission of dicyanoplatinum(II) and -palladium(II) complexes of substituted 2,2'-bipyridines and 1,10-phenanthroline and x-ray crystal structures of isomorphous M(bpy)(CN)2 (bpy = 2,2'-bipyridine; M = Pt, Pd)

Noble Metal Complexes of a Bis-Caffeine Containing NHC Ligand

N-Heterocyclic carbenes (NHCs) have seen more and more use over the years. The go-to systems that are usually considered are derivatives of benzimidazole or imidazole. Caffeine possesses an imidazole unit and was already utilized as a carbene-type ligand; however, its use within a tridentate bis-NHC system has—to the best of our knowledge—not been reported so far. The synthesis of the ligand is straightforward and metal complexes are readily available via silver-salt metathesis. A platinum(II) and a palladium(II) complex were isolated and a crystal structure of the former was examined. For the Pt(II) complex, luminescence is observed in solid state as well as in solution.

Strategy for Achieving Long-Wavelength Near-Infrared Luminescence of Diimineplatinum(II) Complexes

Although many strategies have been used to help design effective near-infrared (NIR) luminescent materials, it is still a huge challenge to realize long-wavelength NIR luminescence of diimineplatinum(II) complexes in the solid state. Herein, we have successfully achieved long-wavelength NIR luminescence of a family of diimineplatinum(II) complexes based on a new strategy that combines a one-dimensional (1D) "Pt wire" structure with the electronic effect of the substituent. The structures of six solvated diimineplatinum(II) complexes based on 4,4-dichloro-2,2'-bipyridine or 4,4-dibromo-2,2'-bipyridine and 4-substituted phenylacetylene ligands have been determined, namely, 1·¹/₂toluene, 2·¹/₂THF, 3·¹/₈toluene, 4·¹/₂THF, 5·¹/₈CH₂Cl₂, and 6·¹/₄toluene. All of them crystallize in the monoclinic space group C2/c or C2/m and stack in the 1D "Pt wire" structure. In the solid state, six complexes exhibited unusual long-wavelength metal-metal-to-ligand charge-transfer luminescence that peaked at 984, 1044, 972, 990, 1022, and 935 nm, respectively. Interestingly, 2·¹/₂THF has the shortest Pt···Pt distance and the longest emission wavelength among the six complexes. As far as we know, the luminescence of 2·¹/₂THF at 1044 nm is the longest emission wavelength among known diimineplatinum(II) complexes. Systematic studies revealed that good molecular planarity, suitable substituent position, weak hydrogen-bond-forming ability of the substituents, appropriate molecular bending, and weakening of the interaction between solvated molecules and platinum molecules are conducive to the construction of a 1D "Pt wire" structure of the diimineplatinum(II) complex. Furthermore, the emission energy of the complex is mainly determined by the strength of the Pt-Pt interaction and electronic effect of the substituent.

Supramolecular Assembly of Phosphole Oxide Based Alkynylplatinum(II) 2,6-Bis(N-alkylbenzimidazol-2'-yl)pyridine Complexes-An Interplay of Hydrophobicity and Aromatic π-Surfaces

A new class of phosphole oxide based alkynylplatinum(II) 2,6-bis(N-alkylbenzimidazol-2'-yl)pyridine (bzimpy) complexes were synthesized and characterized. Their self-assembly was driven by hydrophobic-hydrophobic and π-π stacking interactions. The self-assembly properties were also investigated by UV/Vis absorption spectroscopy, which revealed that the alkyl-chain length of the bzimpy moiety and the π-surface area of the alkynyl ligand have significant influence on the overall self-assembly process. The alkyl-chain length also affected the morphological structures of the aggregates, which were studied by transmission electron microscopy and scanning electron microscopy.

Molecular Engineering of Platinum(II) Terpyridine Complexes with Tetraphenylethylene-Modified Alkynyl Ligands: Supramolecular Assembly via Pt···Pt and/or π-π Stacking Interactions and the Formation of Various Superstructures

A series of platinum(II) terpyridine complexes with tetraphenylethylene-modified alkynyl ligands has been designed and synthesized. The introduction of the tetraphenylethylene motif has led to aggregation-induced emission (AIE) properties, which upon self-assembly led to the formation of metal-metal-to-ligand charge transfer (MMLCT) behavior stabilized by Pt···Pt and/or π-π interactions. Tuning the steric bulk or hydrophilicity through molecular engineering of the platinum(II) complexes has been found to alter their spectroscopic properties and result in interesting superstructures (including nanorods, nanospheres, nanowires, and nanoleaves) in the self-assembly process. The eye-catching color and emission changes upon varying the solvent compositions may have potential applications in chemosensing materials for the detection of microenvironment changes. Furthermore, the importance of the directional Pt···Pt and/or π-π interactions on the construction of distinctive superstructures has also been examined by UV-vis absorption and emission spectroscopy and transmission electron microscopy. This work represents the interplay of both inter- and intramolecular interactions as well as the energies of the two different chromophoric/luminophoric systems that may open up a new route for the development of platinum(II)-AIE hybrids as functional materials.

Supramolecular assembly of a phosphole-based moiety into nanostructures dictated by alkynylplatinum(ii) terpyridine complexes through non-covalent Pt···Pt and π-π stacking interactions: synthesis, characterization, photophysics and self-assembly behaviors

A new class of platinum(ii) terpyridine complexes with a phosphole-derived bridging alkynyl ligand have been prepared. The X-ray crystal structure of complex 2 has been determined, and reveals a polymeric zig-zag chain structure with the existence of π-π stacking interactions. The photophysical properties have also been studied, with 3MLCT/3LLCT phosphorescence exhibited in degassed CH2Cl2; the energy of which is varied by the π-conjugation of the terpyridine ligands. The solvent-induced assembly of complex 1 has been studied. The incorporation of hydrophobic hydrocarbon chains has been shown to play an important role in assisting the formation of self-assembled nanostructures via Pt···Pt, π-π stacking and hydrophobic-hydrophobic interactions. It has been established that an isodesmic growth mechanism operates in polar media to give nanospheres, while fibrous networks originate from the self-assembly of the complexes in non-polar media, predominantly driven by π-π stacking interactions.

Triboluminescence and Metal Phosphor for Organic Light-Emitting Diodes: Functional Pt(II) Complexes with Both 2-Pyridylimidazol-2-ylidene and Bipyrazolate Chelates

We report the utilization of both pyrid-2-yl-imidazolylidene and dianionic bipz chelates as constituents in syntheses of a new series of charge-neutral Pt(II) complexes 1-4, among which complex 4 revealed remarkable triboluminescence, i.e., generation of photoemission upon grinding or cracking of the solid sample. The triboluminescence is found to be sensitive to the subtle changes of the associated substituents of pyrid-2-yl-imidazolylidene chelate, as verified by the disappearance of the triboluminescence for complexes 1-3. Alternatively, the well-ordered solid packing of 3, as indicated by the grazing incidence X-ray scattering experiment, serves as an ideal emitter for the fabrication of highly efficient OLEDs, rendering high external quantum efficienciy (25.9%) and luminesce efficiency (90 cd A^-1) at the practical brightness of 100 cd m^-2. The rather low roll-off in efficiency (24.4%, 85 cd A^-1 at high brightness of 1000 cd m^-2) is attributed to the short excited-state lifetime of 3 (∼800 ns) in the solid state, which in turn is associated with the MMLCT transition character.

Self-assembly of alkynylplatinum(II) terpyridine amphiphiles into nanostructures via steric control and metal-metal interactions

A series of mono- and dinuclear alkynylplatinum(II) terpyridine complexes containing the hydrophilic oligo(para-phenylene ethynylene) with two 3,6,9-trioxadec-1-yloxy chains was designed and synthesized. The mononuclear alkynylplatinum(II) terpyridine complex was found to display a very strong tendency toward the formation of supramolecular structures. Interestingly, additional end-capping with another platinum(II) terpyridine moiety of various steric bulk at the terminal alkyne would lead to the formation of nanotubes or helical ribbons. These desirable nanostructures were found to be governed by the steric bulk on the platinum(II) terpyridine moieties, which modulates the directional metal-metal interactions and controls the formation of nanotubes or helical ribbons. Detailed analysis of temperature-dependent UV-visible absorption spectra of the nanostructured tubular aggregates also provided insights into the assembly mechanism and showed the role of metal-metal interactions in the cooperative supramolecular polymerization of the amphiphilic platinum(II) complexes.

1D polymeric platinum cyanoximate: a strategy toward luminescence in the near-infrared region beyond 1000 nm

We report the synthesis and properties of the first representative of a new class of PtL2 complexes with ambidentate mixed-donor cyanoxime ligands [L = 2-cyano-2-oximino-N,N'-diethylaminoacetamide, DECO (1)]. Three differently colored polymorphs of "Pt(DECO)2" (3-5) were isolated, with the first two being crystallographically characterized. The dark-green complex [Pt(DECO)2]n (5) spontaneously forms in aqueous solution via aggregation of yellow monomeric complex 3 into the red dimer [Pt(DECO)2]2 (4), followed by further oligomerization into coordination polymer 5. A spectroscopic and light-scattering study revealed a "poker-chips"-type 1D polymeric structure of 5 in which units are held by noncovalent metallophilic interactions, forming a Pt---Pt wire. The polymer 5 shows a broad absorption at 400-900 nm and emission at unusually long wavelengths in the range of 1000-1100 nm in the solid state. The near-infrared (NIR) emission of polymer 5 is due to the formation of a small amount of nonstoichiometric mixed-valence Pt(II)/Pt(IV) species during synthesis. A featureless electron paramagnetic resonance spectrum of solid sample 5 recorded at +23 and -193 °C evidences the absence of Pt(III) states, and the compound represents a "solid solution" containing mixed-valence Pt(II)/Pt(IV) centers. Exposure of KBr pellets with 5% 5 to Br2 vapors leads to an immediate ∼30% increase in the intensity of photoluminescence at 1024 nm, which confirms the role and importance of mixed-valence species for the NIR emission. Thus, the emission is further enhanced upon additional oxidation of Pt(II) centers, which improves delocalization of electrons along the Pt---Pt vector. Other polymorph of the "Pt(DECO)2" complex--monomer--did not demonstrate luminescent properties in solutions and the solid state. An excitation scan of 5 embedded in KBr tablets revealed an emission only weakly dependent on the wavelength of excitation. The NIR emission of quasi-1D complex 5 was studied in the range of -193 to +67 °C. Data showed a blue shift of λmax and a simultaneous increase in the emission line intensity with a temperature rise, which is explained by analogy with similar behavior of known quasi-1D K2[Pt(CN)4]-based solids, quantum dots, and quantum wells with delocalized carriers. The presented finding opens a route to a new class of platinum cyanoxime based NIR emissive complexes that could be used in the design of novel NIR emitters and imaging agents.

Synthesis and characterization of luminescent cyclometalated platinum(II) complexes of 1,3-bis-hetero-azolylbenzenes with tunable color for applications in organic light-emitting devices through extension of π conjugation by variation of the heteroatom

A series of luminescent cyclometalated platinum(II) complexes of N^C^N ligands [N^C^N=2,6-bis(benzoxazol-2'-yl)benzene (bzoxb), 2,6-bis(benzothiazol-2'-yl)benzene (bzthb), and 2,6-bis(N-alkylnaphthoimidazol-2'-yl)benzene (naphimb)] has been synthesized and characterized. Two of the platinum(II) complexes have been structurally characterized by X-ray crystallography. Their electrochemical, electronic absorption, and luminescence properties have been investigated. In dichloromethane solution at room temperature, the cyclometalated N^C^N platinum(II) complexes exhibited rich luminescence with well-resolved vibronic-structured emission bands. The emission energies of the complexes are found to be closely related to the electronic properties of the N^C^N ligands. By varying the electronic properties of the cyclometalated ligands, a fine-tuning of the emission energies can be achieved, as supported by computational studies. Multilayer organic light-emitting devices have been prepared by utilizing two of these platinum(II) complexes as phosphorescent dopants, in which a saturated yellow emission with Commission International de I'Eclairage coordinates of (0.50, 0.49) was achieved.

Dynamic scaffold of chiral binaphthol derivatives with the alkynylplatinum(II) terpyridine moiety

Platinum(II)-containing complexes with inherently chiral binaphthol derivatives display a versatile scaffold between random coils and single-turn helical strands, in which the conformational transition is controlled by the Pt···Pt and π-π interactions of alkynylplatinum(II) terpyridine moiety upon solvent and temperature modulation. The bisignate Cotton effect in the circular dichroism spectra is indicative of the cooperative transformation from random coil state to a compact single-turn M- or P-helix. More importantly, as revealed by the appearance of new UV-vis absorption and emission bands during conformational change, the self-assembly of the platinum(II)-containing complex into a helical structure is assisted by the metal···metal and π-π interactions of the alkynylplatinum(II) terpyridine moieties. The folded structure with stabilization via metal···metal and π-π interactions has been supported by density functional theory calculations, which provide insights into the folded geometry of these kind of metallo-foldamers.

Emissive Pt(II) complexes bearing both cyclometalated ligand and 2-pyridyl hexafluoropropoxide ancillary chelate

A new series of mono-cyclometalated Pt(II) complexes 1-4 with chelating 2-pyridyl hexafluoropropoxide as the ancillary ligand were synthesized. Single crystal X-ray diffraction studies were examined, giving evidence for the occurrence of pi pi stacking between the cyclometalated ligands, but a lack of intermolecular Pt ... Pt interaction. Among these complexes, the benzo[h]quinoline analogue 2a shows the greatest degree of pi pi stacking, which is also confirmed by the observation of additional, large Stokes shifted emission attributed to the aggregated counterparts in solid thin film. All these Pt(II) complexes are highly emissive in solid state, whereas except for 4-phenylquinazoline analogues 4a and 4b, complexes 1-3 in solution are subject to dominant radiationless deactivation induced, in part, by the solvent collision to the square planar Pt(II) framework. Electroluminescent OLEDs employing 2-phenylpyridine analogue 1b as the dopants were fabricated, rendering satisfactory performance data suited for future improvement.

Luminescent Platinum(II) Complexes Containing Isoquinolinyl Indazolate Ligands: Synthetic Reaction Pathway and Photophysical Properties

New Pt(II) dichloride complexes [Pt(1-iqdzH)Cl2] (2a) and [Pt(3-iqdzH)Cl2] (2b), in which idqzH = 1- or 3-isoquinolinyl indazole, were prepared by treatment of the corresponding indazoles with K2PtCl4 in aqueous HCl solution. Despite their nonemissive nature, these complexes could react with excess indazole, sodium picolinate, and 3-trifluoromethyl-5-(2-pyridyl) pyrazole [(fppz)H] to afford the respective a and b series of luminescent complexes [Pt(1-iqdz)(L/\X)] and [Pt(3-iqdz)(L/\X)], where L/\X = 1-iqdz (1a), 3-iqdz (1b), pic (3a, 3b), and fppz (4a, 4b). Single-crystal X-ray diffraction studies of 1b, 2a, and 3b revealed a planar molecular geometry without notable intermolecular Pt...Pt contact in the solid crystal, a result of the steric repulsion imposed by the bulky indazole fragments. For coordination complexes 1, 3, and 4, photoluminescence in degassed CH2Cl2 revealed high quantum efficiency and short radiative lifetimes in the range of several microseconds. As supported by the spectral feature, the associated radiation lifetimes, and a computational approach based on time-dependent density function theory (TD-DFT), the origin of the emission is attributed to a mixed 3MLCT/3pipi transition. The TD-DFT approach further confirmed that, except for the series 1 complexes, the HOMO of 3-iqdz complexes 3b and 4b is much less located at the central Pt(II) atom than the HOMO orbitals of the respective 1-iqdz complexes 3a and 4a, leading to a smaller degree of MLCT contribution. Consequently, there are a blue-shifted emission signal and an inferior emission quantum yield for the 3-iqdz derivatives. OLED devices with a multilayer configuration of ITO/NPB/CBP:3a/BCP/Alq3/LiF/Al were fabricated using a CBP layer doped with various concentrations of 3a, ranging from 6% to 100%, within the emitting layer. The best device performance was realized using a 6% doping concentration, for which the external quantum yield of 4.93%, luminous efficiency of 12.19 cd/A, and power efficiency of 6.12 lm W-1 were observed at 20 mA/cm2, while a maximum luminescence as high as 20296 cd/m2 was also realized at 16 V, showing good prospect for the fabrication of Pt(II) based OLEDs.

Platinum(II) Complexes with π-Conjugated, Naphthyl-Substituted, Cyclometalated Ligands (RC^N^N): Structures and Photo- and Electroluminescence

The crystal structures and photophysical properties of mononuclear [(RC N N)PtX](ClO4)n ((RC N N)=3-(6'-(2''-naphthyl)-2'-pyridyl)isoquinolinyl and derivatives; X=Cl, n=0; X=PPh(3) or PCy(3), n=1), dinuclear [(RC N N)2Pt2(mu-dppm)](ClO4)2 (dppm=bis(diphenyphosphino)methyl) and trinuclear [(RC N N)3Pt3(mu-dpmp)](ClO4)3 (dpmp=bis(diphenylphosphinomethyl)phenylphosphine) complexes are presented. The crystal structures show extensive intra- and/or intermolecular pipi interactions; the two (RC N N) planes of [(RC N N)2Pt2(mu-dppm)](ClO4)2 (R=Ph, 3,5-tBu2Ph or 3,5-(CF3)2Ph) are in a nearly eclipsed configuration with torsion angles close to 0 degrees. [(RC N N)PtCl], [(RC N N)2Pt2(mu-dppm)](ClO4)2, and [(RC N N)3Pt3(mu-dpmp)](ClO4)3 are strongly emissive with quantum yields of up to 0.68 in CH2Cl2 or MeCN solution at room temperature. The [(RC N N)PtCl] complexes have a high thermal stability (T(d)=470-549 degrees C). High-performance light-emitting devices containing [(RC N N)PtCl] (R=H or 3,5-tBu2Ph) as a light-emitting material have been fabricated; they have a maximum luminance of 63,000 cd m(-2) and CIE 1931 coordinates at x=0.36, y=0.54.