Potential strategy used for controlling the phosphorescent properties in tetradentate Pt(II) complexes: Effect of azole ligand

Designing deep‐blue phosphorescent materials is vital and essential in the construction of white organic light‐emitting diodes. Using density functional theory (DFT) and time‐dependent DFT, three tetradentate Pt(II) complexes were investigated in detail to reveal the influence of azole ligand with varying number of N atoms on the emission wavelengths and radiative and non‐radiative decay processes. The calculated results indicate that with an increase of N atoms in azole rings, the radiative decay process can be effectively facilitated. Moreover, an increase of N atoms in azole rings could lead to a distinct blue‐shift of emission wavelengths from 553 to 470 nm. Also, the non‐radiative decay processes, including temperature‐independent and temperature‐dependent ones, were taken into account. The results may provide some valuable and meaningful information for designing high‐performance phosphorescent Pt(II) complexes.     

Structures and fluorescent properties of picolinato zinc(II) and cadmium(II) complexes based on tridentate and tetradentate benzimidazole ligands

Five picolinato zinc(II) and cadmium(II) complexes, [Zn(ntb)(pic)]ClO₄·CH₃OH·2H₂O (1), [Zn(bbma)(pic)]NO₃·2CH₃OH (2), [Cd(ntb)(pic)]ClO₄·0.75CH₃OH·H₂O (3), [Cd₂(bbma)₂(pic)₂](ClO₄)₂ (4), and [Cd₂(bbp)(bbp_-H)(pic)₂(C₂H₅OH)]ClO₄ (5), have been synthesized, where pic is the anion of picolinic acid, ntb is tris(2-benzimidazolylmethyl)amine, bbma is bis(benzimidazol-2-yl-methyl)amine, and bbp is 2,6-bis(benzimidazol-2-yl)pyridine. All the complexes were characterized by X-ray single-crystal diffraction, elemental analysis, IR, fluorescence spectroscopy, and thermal gravimetric analysis. 1–3 are mononuclear complexes in which picolinate adopts a N,O-chelating mode. 4 is a symmetrical dinuclear complex bridged by two anti-parallel picolinates in a N,O,O-coordination mode. 5 is also a dinuclear complex in which only one picolinate is a bridge. A 1-D double chain is formed by extensive H-bonds and π–π stacking in 1, while single zigzag chains are formed in 5. Complexes 2–4 all exhibit 6³-hcb 2-D frameworks. They extend to form four-connected 6⁶-dia 3-D topological nets for 2 and 4 and five-connected 4⁶·6⁴-bnn 3-D topological nets for 3. The five complexes show emission maxima in the blue region in the solid state.     

TD-DFT Studies on Electronic and Spectral Properties of Platinum(II) Complexes with Phenol and Pyridine Groups

The molecular structures of the ground and the lowest triplet states for a series of Pt(II) complexes PtLCl(1)[L=6-(2-hydroxyphenyl)-2,2′-bipyridine], Pt(pp)₂[pp=2-(2-hydroxyphenyl)pyridine](2), PtbpyCl₂(bpy=2,2′- bipyridine)(3), and the free tridentate L ligand(4) were optimized by the density functional theory B3LYP and UB3LYP methods, respectively. On the basis of optimized geometries, the spectral properties were investigated with time-dependent density functional theory(TD-DFT). In comparison with those of complexes 2 and 3, the more rigid structure of complex 1 together with its low rate of the radiationless decay via nonemissive d-d state leads to higher photoluminescence quantum efficiency. And the phosphorescence quantum efficiency of complex 1 can be easily controlled by modifying auxiliary ligands. The introduction of fluorine ligand into complexes can effectively increase the radiation transition rate and decrease the radiationless d-d transition rate, and as a result, a novel complex PtLF(5) might be a good phosphorescent material suitable for organic electronic devices.     

Spectral and photophysical properties of mono and dinuclear Pt(II) and Pd(II) complexes: An unusual emission behaviour

Eight mononuclear complexes of the formula [M(N-N)(DHB)] and two binuclear complexes of the formula [M₂(BPY)₂(THB)] where M = Pd(II) or Pt(II), N-N = 2,2′-bipyridine (BPY), 2,2′-biquinoline (BIQ), 4,7-diphenyl-1,10-phenanthroline (DPP), 1,10-phenanthroline (PHEN); DHB = dianion of 3,4-dihydroxybenzaldehyde and THB = tetraanion of 3,3′,4,4′-tetrahydroxy benzaldazine were prepared and their electrochemical, spectral and photophysical properties were examined. These complexes were characterized by chemical analysis, IR and proton NMR spectroscopy. A detailed study on the absorption spectroscopy of these complexes was made. These complexes were found to show a low-energy solvatochromic ligand-to-ligand charge-transfer (LLCT) band. The electronic energies of these bands have been analyzed and compared with electrochemical data. Emission behaviour of the complexes of the series, [Pt(N-N)(DHB)], [Pt(N-N)(DHBA)] where DHBA is the dianion of 3,4-dihydroxybenzoic acid and [Pt₂(BPY)₂(THB)] was also investigated. These platinum complexes were found to emit from a low-energy state at low temperature and a high-energy state at room temperature. Photophysics of these complexes is also discussed.     

Synthesis,structures, and fluorescent properties of two oxalato-bridged dinuclear zinc(II) complexes with tridentate and tetradentate polybenzimidazole ligands

Two dinuclear oxalato-bridged zinc(II) complexes, [Zn₂(bbma)₂(μ₂-η⁴-ox)](ClO₄)₂ (1) and [Zn₂(ntb)₂(μ₂-η²-ox)](ClO₄)₂·4CH₃OH (2), containing tridentate and tetradentate polybenzimidazole ligands were synthesized, where bbma is bis(benzimidazol-2-yl-methyl)amine and ntb is tris(2-benzimidazolylmethyl)amine. They were characterized by single-crystal X-ray diffraction, elemental analysis, IR, and fluorescence spectroscopy. Zinc(II) complexes in 1 and 2 are five-coordinate in distorted trigonal bipyramidal geometry. Oxalate bridges two zinc(II) ions as a bis-bidentate chelate in 1, while bis-monodentate in 2. The Zn?Zn distances are 5.318(1)?Å for 1 and 7.1295(5)?Å for 2, respectively. 1-D chain structures are formed in 2 by intermolecular hydrogen bonds. The solid state fluorescence spectra have been studied for 1 and 2.     

Novel platinum(II) and (IV) complexes of naphthaldimine schiff bases

Naphthaldimines containing N₂O₂ donor centers react with platinum(II) and (IV) chlorides to give two types of complexes depending on the valence of the platinum ion. For [Pt(II)], the ligand is neutral, [(H₂L¹)PtCl₂]·3H₂O (1) and [(H₂L³)₂Pt₂Cl₄]·5H₂O (3), or monobasic [(HL²)₂Pt₂Cl₂]·2H₂O (2) and [(HL⁴)₂Pt]·2H₂O (4). These complexes are all diamagnetic having square-planar geometry. For [Pt(IV)], the ligand is dibasic, [(L¹)Pt₂Cl₄(OH)₂]·2H₂O (5), [(L²)Pt₃Cl₁₀]·3H₂O (6), [(L³)Pt₂Cl₄(OH)₂]·C₂H₅OH (7) and [(L⁴)Pt₂Cl₆]·H₂O (8). The Pt(IV) complexes are diamagnetic and exhibit octahedral configuration around the platinum ion. The complexes were characterized by elemental analysis, UV-Vis and IR spectra, electrical conductivity and thermal analyses (DTA and TGA). The molar conductances in DMF solutions indicate that the complexes are non-ionic. The complexes were tested for their catalytic activities towards cathodic reduction of oxygen.     

Synthesis and structure of highly substituted pyrazole ligands and their complexes with platinum(II) and palladium(II) metal ions

Reaction of 3-methoxycarbonyl-2-methyl- or 3-dimethoxyphosphoryl-2-methyl-substituted 4-oxo-4H-chromones 1 with N-methylhydrazine resulted in the formation of isomeric, highly substituted pyrazoles 4 (major products) and 5 (minor products). Intramolecular transesterification of 4 and 5 under basic conditions led, respectively, to tricyclic derivatives 7 and 8. The structures of pyrazoles 4a (dimethyl 2-methyl-4-oxo-4H-chromen-3-yl-phosphonate) and 4b (methyl 4-oxo-2-methyl-4H-chromene-3-carboxylate) were confirmed by X-ray crystallography. Pyrazoles 4a and 4b were used as ligands (L) in the formation of ML₂Cl₂ complexes with platinum(II) or palladium(II) metal ions (M). Potassium tetrachloroplatinate(II), used as the metal ion reagent, gave both trans-[Pt(4a)₂Cl₂] and cis-[Pt(4a)₂Cl₂], complexes with ligand 4a, and only cis-[Pt(4b)₂Cl₂] isomer with ligand 4b. Palladium complexes were obtained by the reaction of bis(benzonitrile)dichloropalladium(II) with the test ligands. trans-[Pd(4a)₂Cl₂] and trans-[Pd(4b)₂Cl₂] were the exclusive products of these reactions. The structures of all the complexes were confirmed by IR, ¹H NMR and FAB MS spectral analysis, elemental analysis and Kurnakov tests.     

Complexes of substituted dipyridylpyridazines with palladium(II) and platinum(II)

Reactions of 3,6-bis(2′-pyridyl)pyridazine derivatives (n-dppn)¶ ¶For the n-dppn ligands, n stands for the size of the cyclic aliphatic ring on positions 4 and 5 of the pyridazine ring, n?=?5, 6, 8, and 12. with MX₂(PhCN)₂ (M?=?Pd, Pt; X?=?Cl,?Br) have been investigated. The new complexes cis-[PdCl₂(n-dppn)] (n?=?5,?6,?8,?12), cis-[PtCl₂(n-dppn)]?·?H₂O (n?=?5,?6), cis-[PtCl₂(8-dppn)] and cis-[PtBr₂(5-dppn)] have been characterized by elemental analyses, conductivity measurements, infrared, electronic and ¹H-NMR spectra.     

Synthesis and characterization of picolinate-containing nickel(II) complexes with tridentate and tripodal tetradentate ligands

Two picolinate-containing nickel(II) complexes [Ni(bbma)(pic)(H₂O)]ClO₄ · CH₃OH (1) and [Ni(ntb)(pic)]Cl · CH₃OH · 3H₂O (2) were synthesized and characterized by infrared, elemental analysis, UV-Vis, and X-ray diffraction analyses, where bbma is bis(benzimidazol-2-yl-methyl)amine, ntb is tris(2-benzimidazolylmethyl)amine, pic is the anion of picolinic acid. X-ray analysis shows that both complexes are mononuclear with picolinate coordinated to Ni(II) in a μ₂-N,O chelating mode. Both complexes adopt distorted octahedral geometry. Intermolecular N–H ··· O and O–H ··· O hydrogen bonds and π–π interactions in 1 and 2 are important in stabilization of the crystal structures.     

Platinum(II) complexes with pi-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.     

Cytotoxic performances of new anionic cyclometalated Pt(II) complexes bearing chelated O^O ligands

The in vitro biological activity towards the MDA-MB-231 triple-negative breast cancer cell line of two different series of anionic Pt(II) organometallic complexes was tested. For the first time, cytotoxic activity of anionic Pt(II) complexes has been observed. The anionic compounds of general formula NBu₄[(C^N)Pt(O^O)], where (C^N) represents the cyclometalated form of 2-phenylpyridine (H(PhPy)), 2-thienylpyridine (H(Thpy)) or 2-benzo[h]quinoline (H(Bzq)), feature two different (O^O) chelated ligands: tetrabromocatechol [BrCat]²⁻ ( 1 – 3 ) or alizarine [Aliz]²⁻ ( 4 – 6 ). Complexes 1 – 6 displayed a significant cytotoxic effect against the studied cell line (IC₅₀ range of 1.9–52.8 μM). For BrCat-containing complexes 1 – 3 , the biological activity was independent of the nature of the coordinated (C^N) ligand. In contrast, in the case of 4 – 6 , the cytotoxicity (significantly high for 4 ) was concomitantly induced by the presence of either the PhPy or the [Aliz]²⁻ ligand. Since complexes 1–6 are emissive in solution, the potential use of 4 as a theranostic agent was investigated using confocal analysis. The fluorescence signal from MDA-MB-231 cells incubated with 4 indicated the localization of the compound into the cytosol region.     

New tridentate cyclometalated platinum(II) and palladium(II) complexes of N,2-diphenyl-8-quinolinamine: syntheses, crystal structures, and photophysical properties

A new tridentate cyclometalated platinum(II) complex derived from N,2-diphenyl-8-quinolinamine, which consists of two crystallographic independent molecules with two intermolecular N-H-Cl-Pt hydrogen bonds forming a dimer, exhibited a low-energy luminescence at ca. 740 nm in a 1 × 10⁻³ M dichloromethane solution and a strong emission centered at 670 nm in a solid state, but the analogous palladium(II) complex was nonemissive at room temperature.     

Mn (II) and Zn (II) complexes of a benzimidazole ligand having undecyl chains: Crystal structures,photophysical and thermal properties

A new tridentate benzimidazole ligand (L‐C¹¹) containing undecyl chains and its Mn (II) and Zn (II) complexes were synthesised and characterized by spectroscopic and analytical methods. Molecular structures of complexes [Mn(L‐C¹¹)Cl₂] and [Zn(L‐C¹¹)Cl₂] were evaluated by X‐ray diffraction studies. The X‐ray data showed metal ions in both complexes are five‐coordinate with distorted square pyramidal geometry around the metal centres. The undecyl chains in the structure of the complexes are aligned in an interdigitated manner (head to tail) forming a non‐polar domain. The aggregation properties of the ligand and its complexes were investigated by UV–Vis. absorption and emission spectroscopies in DMF‐water mixtures. The emission spectral data revealed that the compounds showed aggregation induced quenching (AIQ) in DMF‐water solutions. Moreover, thermal properties of the compounds were investigated by TG, DTG and DSC analysis.     

Theoretical insight into the photodeactivation pathway of the tetradentate Pt (II) complex with different inductive substituents

Density functional theory (DFT) and time‐dependent density functional theory (TD‐DFT) both were used to explore the impacts of different inductive substituents on the photophysical properties, radiative/nonradiative processes and photodeactivation mechanism for the Pt (II) complex with novel spiro‐arranged tetradentate ligand. Spectrum simulations show that the electron donor methoxyl (‐OCH₃) group can cause the emission wavelength to red‐shift but have little effect on the absorption spectrum. In the simulation of the radiative decay process for the tetradentate Pt (II) complex, the singlet‐triplet splitting energy is reduced by the introduction of substituents with strong electron‐releasing capability (i.e., from the original trifluoromethyl (‐CF₃) group to ‐OCH₃ group), accompanied with a lower radiative rate constant (k_r). The analyses of non‐radiative decay processes show that the substitution of ‐OCH₃ group on azole rings reduces the energy barriers of thermally activated non‐radiative photodeactivation pathway, which in turn increases the temperature‐dependent non‐radiative rate constants (k_nr(T)). In addition, the substitution of ‐CF₃ by ‐OCH₃ group slightly weakens molecular rigidity and enhances the Huang‐Rhys factor, but decreases the SOC between the triplex excited (T) state and the ground (S₀) state. Thereby, the two complexes may have the similar temperature‐independent non‐radiative rate constant (k_nr’). This work offers theoretical guidance for the design and optimization of the efficient organic light emitting diode (OLED) materials based on the structure of tetradentate Pt (II) complexes.     

Copper(II) complexes of tridentate SNO ligands: synthesis,characterization and crystal structure

A new series of binary copper(II) complexes, [Cu(L)₂] (2) [where L is a monobasic tridentate methylthioazophenolate having NSO donor sets], has been synthesized. The reddish brown colored complexes have been characterized by elemental analyses, spectroscopic and other physico-chemical tools. The detailed structure analysis of one of the complexes, [Cu(1a)₂] (2a), by single-crystal X-ray crystallography shows that thioether-S donor center participates in coordination with the copper(II) ion with a weak interaction with long Cu–S(thioether) bond distances [2.956(2) Å and 2.925(2) Å]. Electrochemical study of the complexes in methanol using TBAP as supporting electrolyte shows that heterogeneous electron-transfer rate is low at the applied potential.     

Advances on supramolecular assembly of cyclometalated platinum(Ⅱ) complexes

Organoplatinum(Ⅱ) compounds have received enormous attention over the past decades due to their square-planar geometry as well as intriguing photo-physical properties.Self-assembly has emerged as an excellent approach to create well-ordered supramolecular architectures with tunable properties,which underpin the role of solvent-directed approach for the design of functional materials.In this minireview,the recent advances on supramolecular self-assembly of cyclometalated platinum(Ⅱ) complexes have been discussed.During the self-assembly process,non-covalent Pt-Pt and π-π interactions play crucial roles in controlling the structures and functions of the resulting assemblies.     

Interactions of nitrogen-donor bio-molecules with dinuclear platinum(II) complexes

Substitution reactions of the dinuclear Pt(II) complexes, [{Pt(en)Cl}₂(μ-pz)]²⁺ (1), [{Pt(dach)Cl}₂(μ-pz)]²⁺ (2) and [{Pt(dach)Cl}₂(μ-4,4?-bipy)]²⁺ (3), and corresponding aqua analogs with selected biologically important ligands, viz. 1,2,4-triazole, L-histidine (L-His) and guanosine-5?-monophosphate (5?-GMP) were studied under pseudo-first-order conditions as a function of concentration and temperature using UV–vis spectrophotometry. The reactions of the chloride complexes were followed in aqueous 25 mmol L^?1 Hepes buffer in the presence of 40 mmol L^?1 NaCl at pH 7.2, whereas the reactions of the aqua complexes were studied at pH 2.5. Two consecutive reaction steps, which both depend on the nucleophile concentration, were observed in all cases. The second-order rate constants for both reaction steps indicate a decrease in the order 1 > 2 > 3 for all complexes. Also, the pK_a values of all three aqua complexes were determined. The order of the reactivity of the studied ligands is 1,2,4-triazole > L-His > 5?-GMP. ¹H NMR spectroscopy and HPLC were used to follow the substitution of chloride in the dichloride 1, 2, and 3 complexes by guanosine-5?-monophosphate (5?-GMP). This study shows that the inert and bridging ligands have an important influence on the reactivity of the studied complexes.     

Syntheses and crystal structures of a series of palladium(II) and platinum(II) complexes bearing new hydrazine-based bisphosphinite ligands equipped with pyridyl arms

This paper reports the synthesis of the hydrazine-based bisphosphinito ligands (RO)₂PN(Me)N(Me)P(OR)₂ (R = 2-hydroxy-6-methyl-pyridyl or 3-hydroxy-6-methyl-pyridyl) and their palladium and platinum complexes whose structures were unequivocally determined by X-ray crystallography.     

Copper(II), palladium(II) and platinum(II) chloride complexes with 5-amino-2-tert-butyltetrazole: Synthesis, characterization and cytotoxicity

Complexes CuL₃Cl₂, PdL₂Cl₂ and PtL₂Cl₂, where L is a novel ligand from the series of 2-substituted 5-aminotetrazoles, namely 5-amino-2-tert-butyltetrazole (1), have been synthesized by the reaction of metal(II) chlorides with 1 and characterized by IR spectroscopy, thermal and X-ray analyses. The crystallographic structural analysis of these complexes revealed that 1 acts as a monodentate ligand coordinated to the metal via endocyclic N4 atom. Platinum complex demonstrates promising cytotoxicity against human cervical carcinoma cells with IC₅₀ value average between those of cisplatin and carboplatin.     

Thermal and structural properties of coordination compounds of 2-mercapto-1-methylimidazole with palladium(II) and platinum(II)

Complexes of 2-mercapto-1-methylimidazole (TMZ) with Pd^II and Pt^II of the general formula M(TMZ)_nX₂ (whereM=Pd, Pt andX=Cl, Br, I or SO₄ andn=2 or 4) were obtained. The thermal stabilities of the compounds were estimated by derivatographic measurements and lattice constants were estimated from their X-ray powder diffraction patterns.