Synthesis and Characterization of Luminescent Bis-Cyclometalated PlatinumIV Complexes

Synthesis and Optoelectronic Properties of a Novel Pt(IV)(C∧N∧C)(Cl)2(DMSO) Complex Cyclometalated by Tridentate Diphenylpyridine

A novel tridentate-cyclometalating Pt(IV) complex, namely, Pt(IV)(C∧N∧C)(Cl)2(DMSO), is initially synthesized through a series of reactions using Pt(II) complexes. The optoelectronic properties of Pt(IV)(C∧N∧C)(Cl)2(DMSO) are fully studied by adequate characterizations.

A new family of luminescent [Pt(pbt)2(C6F5)L]n+ (n = 1, 0) complexes: synthesis, optical and cytotoxic studies

Given the widely recognized bioactivity of 2-arylbenzothiazoles against tumor cells, we have designed a new family of luminescent heteroleptic pentafluorophenyl-bis(2-phenylbenzothiazolyl) PtIV derivatives, fac-[Pt(pbt)2(C6F5)L]n+ (n = 1, 0) [L = 4-Mepy 1, 4-pyridylbenzothiazole (pybt) 2, 4,4'-bipyridine (4,4'-bpy) 3, 1,2-bis-(4-pyridyl)ethylene (bpe) 4 (E/Z ratio: 90/10), 1,4-bis-(pyridyl)butadiyne (bpyb) 5, trifluoroacetate (-OCOCF3) 6] and a dinuclear complex [{Pt(pbt)2(C6F5)}2(μ-bpyb)](PF6)27, in which the trans ligand to the metalated C-(pbt) was varied to modify the optical properties and lipophilicity. Their photophysical properties were systematically studied through experimental and theoretical investigations, which were strongly dependent on the identity of the N-bonded ligand. Thus, complexes 1, 3 and 6 display, in different media, emission from the triplet excited states of primarily intraligand 3ILCT nature localized on the pbt ligand, while the emissions of 2, 5 and 7 were ascribed to a mixture of close 3IL'(N donor)/3ILCT(pbt) excited states, as supported by lifetime measurements and theoretical calculations. Irradiation of the initial E/Z mixture of 4 (15 min) led to a steady state composed of roughly 1 : 1.15 (E : Z) and this complex was not emissive at room temperature due to an enhanced intramolecular E to Z isomerization process of the 1,2-bis-(4-pyridyl)ethylene ligand. Complexes 1-3 and 6 showed excellent quantum yields for the generation of singlet oxygen in aerated MeCN solution with the values of ϕ(1O2) ranging from 0.66 to 0.86 using phenalenone as a reference. Cationic complexes 1-3 exhibited remarkable efficacy in the nanomolar range against A549 (lung carcinoma) and HeLa (cervix carcinoma) cell lines with notable selectivity relative to the non-tumorigenic BEAS-2B (bronchial epithelium) cells. In the A549 cell line, the neutral complex 6 showed low cytotoxicity (IC50: 29.40 μM) and high photocytotoxicity (IC50: 5.75) when cells were irradiated with blue light for 15 min. These complexes do not show evidence of DNA interaction.

Photoinduced Reductive C-C and C-Heteroatom Couplings from Bis-cyclometalated Pt(IV) Alkynyl Complexes

Unsymmetrical dicarboxylato complexes [Pt(tpy)2(O2CR)2] [tpy = cyclometalated 2-(p-tolyl)pyridine, R = Me, CF3] react with the terminal alkynes 4-methoxyphenylacetylene, phenylacetylene, 4-(trifluoromethyl)phenylacetylene or 3,5-difluorophenylacetylene in the presence of a base to produce complexes mer-[Pt(tpy)2(O2CR)(CCAr)], in which the metalated carbon atoms are in a meridional arrangement. Irradiation of the trifluoroacetato derivatives with a 365 nm LED source leads to isomerization to the facial complexes, which can be converted to chlorido derivatives upon reaction with NH4Cl. In contrast, irradiation of the acetato derivatives leads to four different processes, namely, reduction to cis-[Pt(tpy)2], annulations involving one of the tpy ligands and the Cα and Cβ atoms of the alkynyl to give benzoquinolizinium derivatives, isomerization to the facial geometry, or C-O couplings between the acetato ligand and one tpy. The first two processes are favored by the presence of electron-donating groups on the alkynyl, whereas electron-withdrawing groups favor the last two. Irradiation of complexes fac-[Pt(tpy)2(O2CCF3)(CCAr)] with a medium-pressure Hg UV lamp leads to a reductive C-C coupling involving the alkynyl Cα atom and one of the tpy ligands to give pyridoisoindolium derivatives, except for the methoxyphenylacetylide derivative, which is photostable. On the basis of TDDFT calculations, the photoreactivity of the mer complexes is attributed to 3LLCT [π(alkynyl) → π*(tpy)] excited states for annulations or 3LMCT [π(alkynyl) → dσ*] excited states for the rest of the processes, which are accessible through thermal population from 3LC(tpy) states. The C-C couplings from the fac complexes are attributed to photoreactive pentacoordinate intermediates.

Platinum(IV) Complexes with Tridentate, NNC-Coordinating Ligands: Synthesis, Structures, and Luminescence

Platinum(II) complexes of NNC-cyclometalating ligands based on 6-phenyl-2,2'-bipyridine (HL¹) have been widely investigated for their luminescence properties. We describe how PtL¹Cl and five analogues with differently substituted aryl rings, PtL^2-6Cl, can be oxidized with chlorine and/or iodobenzene dichloride to generate Pt(IV) compounds of the form Pt(NNC-Lⁿ)Cl₃ (n = 1-6). The molecular structures of several of them have been determined by X-ray diffraction. These PtLⁿCl₃ compounds react with 2-arylpyridines to give a new class of Pt(IV) complex of the form [Pt(NNC)(NC)Cl]⁺. Elevated temperatures are required, and the reaction is accompanied by competitive reduction processes and generation of side-products; however, four examples of such complexes have been isolated and their molecular structures determined. Reaction of PtL¹Cl₃ with HL¹ similarly generates [Pt(NNC-L¹)₂]²⁺, which we believe to be the first example of a bis-tridentate Pt(IV) complex. The lowest-energy bands in the UV-vis absorption spectra of all the PtLⁿCl₃ compounds are displaced to higher energy relative to the Pt(II) precursors, but they red-shift with the electron richness of the aryl ring, consistent with predominantly ¹[π_Ar → π*_NN] character to the pertinent excited state. A similar trend is observed for the [Pt(NNC)(NC)Cl]⁺ complexes. They display phosphorescence in solution at room temperature, centered around 500 nm for [PtL¹(ppy)Cl]⁺ and [Pt(L¹)₂]²⁺, and 550 nm for methoxy-substituted derivatives. The lifetimes are in the microsecond range, rising to hundreds of microseconds at 77 K, consistent with triplet excited states of primarily ³[π_Ar → π*_NN] character with relatively little participation of the metal.

Strongly Luminescent Pt(IV) Complexes with a Mesoionic N-Heterocyclic Carbene Ligand: Tuning Their Photophysical Properties

The synthesis, electrochemistry, and photophysical properties of a series of bis-cyclometalated Pt(IV) complexes that combine the mesoionic aryl-NHC ligand 4-butyl-3-methyl-1-phenyl-1H-1,2,3-triazol-5-ylidene (trz) with either 1-phenylpyrazole or 2-arylpyridine (C∧N) are reported. The complexes (OC-6-54)-[PtCl2(C∧N)(trz)] bearing cyclometalating 2-arylpyridines present phosphorescent emissions in the blue to yellow color range, which essentially arise from 3LC(C∧N) states, and reach quantum yields of ca. 0.3 in fluid solutions and almost unity in poly(methyl methacrylate) (PMMA) matrices at 298 K, thus representing a class of strong emitters with tunable properties. A systematic comparison with the homologous C2-symmetrical species (OC-6-33)-[PtCl2(C∧N)2], which contains two equal 2-arylpyridine ligands, shows that the introduction of a trz ligand leads to significantly lower nonradiative decay rates and higher quantum efficiencies. Computational calculations substantiate the effect of the carbene ligand, which raises the energy of dσ* orbitals in these derivatives and results in the higher energies of nonemissive deactivating 3LMCT states. In contrast, the isomers (OC-6-42)-[PtCl2(C∧N)(trz)] are not luminescent because they present a 3LMCT state as the lowest triplet.

Luminescent Platinum(IV) Complexes Bearing Cyclometalated 1,2,3-Triazolylidene and Bi- or Terdentate 2,6-Diarylpyridine Ligands

The synthesis, structure, and photophysical properties of luminescent Pt^IV complexes that combine cyclometalated 1,2,3-triazolylidene and bi- or terdentate 2,6-diarylpyridine ligands are reported. The targeted complexes represent the first examples of Pt^IV species with a cyclometalated mesoionic aryl-NHC ligand. They exhibit moderate or weak emissions in fluid solution at 298 K arising from ³ LC states, which become very intense in poly(methyl methacrylate) (PMMA) matrices at 298 K. DFT and TD-DFT calculations confirm that the chromophoric ligand is the cyclometalated 2,6-diarylpyridine and show that the aryl-NHC ligand exerts a beneficial effect on the emission efficiencies of these derivatives by increasing the energy of deactivating LMCT excited states with respect to comparable Pt^IV complexes with cyclometalated 2-arylpyridine ligands.

Selective synthesis, reactivity and luminescence of unsymmetrical bis-cyclometalated Pt(iv) complexes

The degree of MLCT admixture into the ³LC emitting state of unsymmetrical Pt(iv) complexes bearing two different cyclometalated ligands depends on the coordination position of the chromophoric ligand.

Synthesis, Mesomorphism, and Photophysics of 2,5-Bis(dodecyloxyphenyl)pyridine Complexes of Platinum(IV)

It has been shown for the first time that the Pt^IV complex cis-[Pt(N^C-tolpy)₂ Cl₂ ] (tolpy=2-(4-tolyl)pyridinyl) can be prepared in a one-pot reaction from K₂ [PtCl₄ ], although analogous complexes containing 2,5-bis(4-dodecyloxyphenyl)pyridine (=HL) could be prepared using existing routes. The resulting complexes cis-[Pt(N^C-L)₂ Cl₂ ] are liquid crystals and small-angle X-ray scattering suggests formation of a lamellar mesophase. Surprisingly, heating [Pt(κ² -N^C-L)₂ Cl(κ¹ -N^C-LH)] also leads to a mesomorphic compound, which results from thermally induced oxidation to cis-[Pt(N^C-L)₂ Cl₂ ] and what is presumed to be another geometric isomer of the same formula. The Pt^IV complexes are quite strongly luminescent in deoxygenated solution, with φ≈10 % and show vibrationally structured emission spectra, λ_max (0,0)=532 nm, strongly displaced to the red compared to cis-[Pt(N^C-tolpy)Cl₂ ]. Long luminescence lifetimes of 230 μs are attributed to a lower degree of metal character in the excited state accompanying the extension of conjugation in the ligand. There is no significant difference between the emission properties of the bromo- and chloro-complexes, in contrast with the known complexes cis-[Pt(N^C-ppy)X₂ ], where the quantum yield for X=Br is some 30 times lower than for X=Cl (ppyH=2-phenylpyridine). The lower energy of the excited state in the new complexes probably ensures that deactivating LLCT/LMCT states remain thermally inaccessible, even when X=Br.

Anionic cyclometalated Pt(ii) and Pt(iv) complexes respectively bearing one or two 1,2-benzenedithiolate ligands

Novel anionic cyclometalated Pt(ii) square-planar complexes NBu4[(C^N)PtII(S^S)], containing 2-phenylpyridine H(PhPy), 2-(2,4-difluorophenyl)-pyridine H(F2PhPy) and benzo[h]quinoline H(Bzq), respectively, as a cyclometalated ligand and the dianionic 1,2-benzenedithiolate (Thio)2- fragment as an (S^S) ligand, were synthesised. By the simple addition of an equivalent of (Thio)2- to the NBu4[(C^N)PtII(Thio)] complexes, octahedral anionic NBu4[(C^N)PtIV(Thio)2] analogues were obtained, representing, to the best of our knowledge, the first examples of Pt(iv) anionic cyclometalated complexes. The molecular structures of the obtained complexes in the case of the NBu4[(Bzq)PtII(Thio)] and the NBu4[(Bzq)PtIV(Thio)2] complexes were confirmed by single crystal X-ray diffraction analysis. Furthermore, the electrochemical and photophysical properties of the two series of Pt(ii) and Pt(iv) newly synthesised complexes were studied and DFT and TD-DFT calculations were performed in order to comprehensively investigate the displayed behaviour. All Pt(ii) and Pt(iv) complexes show intense luminescence in the solid state, with remarkable enhancement of the emission quantum yields, proving to be excellent examples of aggregation-induced emission systems.

Recent advances in phosphorescent platinum complexes for organic light-emitting diodes

Phosphorescent organometallic compounds based on heavy transition metal complexes (TMCs) are an appealing research topic of enormous current interest. Amongst all different fields in which they found valuable application, development of emitting materials based on TMCs have become crucial for electroluminescent devices such as phosphorescent organic light-emitting diodes (PhOLEDs) and light-emitting electrochemical cells (LEECs). This interest is driven by the fact that luminescent TMCs with long-lived excited state lifetimes are able to efficiently harvest both singlet and triplet electro-generated excitons, thus opening the possibility to achieve theoretically 100% internal quantum efficiency in such devices. In the recent past, various classes of compounds have been reported, possessing a beautiful structural variety that allowed to nicely obtain efficient photo- and electroluminescence with high colour purity in the red, green and blue (RGB) portions of the visible spectrum. In addition, achievement of efficient emission beyond such range towards ultraviolet (UV) and near infrared (NIR) regions was also challenged. By employing TMCs as triplet emitters in OLEDs, remarkably high device performances were demonstrated, with square planar platinum(II) complexes bearing π-conjugated chromophoric ligands playing a key role in such respect. In this contribution, the most recent and promising trends in the field of phosphorescent platinum complexes will be reviewed and discussed. In particular, the importance of proper molecular design that underpins the successful achievement of improved photophysical features and enhanced device performances will be highlighted. Special emphasis will be devoted to those recent systems that have been employed as triplet emitters in efficient PhOLEDs.

Influence of Different Zeolite Frameworks on the Geometry of Platinum(II)tetraammine Complex

The structural changes in the guest platinum(II)tetraammine complex due to the steric and electronic interactions with the host zeolite frameworks LTL, MWW, and Y have been investigated using density functional theory calculations. It is observed that the square planar geometry of platinum(II)tetraammine complex has been distorted to nonplanar geometry when encapsulated in supercages of zeolite framework. The distortion is found to be higher in LTL than that in Y and MWW frameworks, without affecting the nature of the zeolite framework. Geometrical parameters, highest occupied molecular orbital and lowest unoccupied molecular orbital energies, global hardness, and softness were calculated to understand the distortion in the pores of the zeolite matrix. The most plausible active site of the complex was identified using the Fukui functions.

Novel columnar metallomesogens based on cationic platinum(ii) complexes without long peripheral chains

Phosphorescent cationic platinum(ii) complexes without any peripheral flexible chains around the platinum(ii) center show thermotropic columnar liquid crystal properties.

Phosphorescent heterobimetallic complexes involving platinum(iv) and rhenium(vii) centers connected by an unsupported μ-oxido bridge

Heterobimetallic compounds containing an unsupported Pt(iv)–O–Re(vii) bridge are synthesized. The complexes exhibit rare phosphorescence emission in solid and frozen states.

Spotlight on the ligand: luminescent cyclometalated Pt(iv) complexes containing a fluorenyl moiety

A family of Pt(iv) complexes with 2-(9,9-dimethylfluoren-2-yl)pyridine is reported, including bis- and tris-cyclometalated derivatives, which exhibit intense ³LC phosphorescent emissions in fluid solution.

Cyano-decorated ligands: a powerful alternative to fluorination for tuning the photochemical properties of cyclometalated Ir(iii) complexes

Two bis-cyclometalated Ir(iii) photocatalysts were synthesized with nitrile moieties which imparted remarkable photophysical properties over conventional fluorinated ligands.

Developing strongly luminescent platinum(IV) complexes: facile synthesis of bis-cyclometalated neutral emitters

A straightforward, one-pot procedure has been developed for the synthesis of bis-cyclometalated chloro(methyl)platinum(IV) complexes with a wide variety of heteroaromatic ligands of the 2-arylpyridine type. The new compounds exhibit phosphorescent emissions in the blue to orange colour range and represent the most efficient Pt(IV) emitters reported to date, with quantum yields up to 0.81 in fluid solutions at room temperature.

Higher fluorescence in platinum(iv) orthometallated complexes of perylene imine compared with their platinum(ii) or palladium(ii) analogues

The reaction of 3-perylenylmethylen-4'-ethylaniline () with [Pt2Me4(μ-SMe2)2] (and subsequent addition of PPh3) or with [Pt2(η(3)-C4H7)2(μ-Cl)2] produced cyclometallated Pt(II) complexes [Pt(C^N)Me(PPh3)] () and, respectively, [Pt2(C^N)2(μ-Cl)2] () (HC^N = 3-C20H11CH[double bond, length as m-dash]NC6H4-p-C2H5), with Pt bound to the ortho site of the perylenyl fragment. From the mononuclear complexes [Pt(C^N)L2] (L2 = acac (); S2COMe (); S2CNEt2 () are easily formed. Oxidative addition of methyl iodide to the square-planar Pt(II) complexes , , and gave the corresponding cyclometallated Pt(IV) compounds [Pt(C^N)L2MeI] , and . The X-ray structures of , , and show that the perylenyl fragment remains essentially flat in and and slightly twisted in . Comparison of the optical properties of these Pt(II) complexes with those reported for similar Pd(II) derivatives reveals that the change of metal exerts a notable influence on the UV-vis spectra. In solution at room temperature, all the Pt complexes exhibit fluorescence associated with the perylene fragment with low emission quantum yields for the Pt(II) complexes (<1%) and remarkably higher emission values for the Pt(IV) complexes: up to 29%, with emission lifetimes of 1-5 ns. Time-dependent density functional theory (TD-DFT) calculations were performed on the perylene imine and on representative complexes [M(C^N)(acac)] (M = Pd, Pt) and [Pt(C^N)(acac)MeI] to analyse the absorption spectra. These calculations support a perylene-dominated intraligand π-π*emissive state based on the HOMO and LUMO orbitals of the perylene chromophore, and a ligand-to-ligand charge-transfer (more intense for the Pt(II) complex) that explains the observed influence of the metal on the absorption properties.

Exploring excited-state tunability in luminescent tris-cyclometalated platinum(IV) complexes: synthesis of heteroleptic derivatives and computational calculations

The synthesis, structure, electrochemistry, and photophysical properties of a series of heteroleptic tris- cyclometalated Pt(IV) complexes are reported. The complexes mer-[Pt(C^N)2 (C'^N')]OTf, with C^N=C-deprotonated 2-(2,4-difluorophenyl)pyridine (dfppy) or 2-phenylpyridine (ppy), and C'^N'=C-deprotonated 2-(2-thienyl)pyridine (thpy) or 1-phenylisoquinoline (piq), were obtained by reacting bis- cyclometalated precursors [Pt(C^N)2 Cl2] with AgOTf (2 equiv) and an excess of the N'^C'H pro-ligand. The complex mer-[Pt(dfppy)2 (ppy)]OTf was obtained analogously and photoisomerized to its fac counterpart. The new complexes display long-lived luminescence at room temperature in the blue to orange color range. The emitting states involve electronic transitions almost exclusively localized on the ligand with the lowest π-π* energy gap and have very little metal character. DFT and time-dependent DFT (TD-DFT) calculations on mer-[Pt(ppy)2 (C'^N')](+) (C'^N'=thpy, piq) and mer/fac-[Pt(ppy)3](+) support this assignment and provide a basis for the understanding of the luminescence of tris-cyclometalated Pt(IV) complexes. Excited states of LMCT character may become thermally accessible from the emitting state in the mer isomers containing dfppy or ppy as chromophoric ligands, leading to strong nonradiative deactivation. This effect does not operate in the fac isomers or the mer complexes containing thpy or piq, for which nonradiative deactivation originates mainly from vibrational coupling to the ground state.

[Ir(N^N^N)(C^N)L]+: a new family of luminophores combining tunability and enhanced photostability

The relatively unexplored luminophore architecture [Ir(N^N^N)(C^N)L](+) (N^N^N = tridentate polypyridyl ligand, C^N = 2-phenylpyridine derivative, and L = monodentate anionic ligand) offers the stability of tridentate polypyridyl coordination along with the tunability of three independently variable ligands. Here, a new family of these luminophores has been prepared based on the previously reported compound [Ir(tpy)(ppy)Cl](+) (tpy = 2,2':6',2″-terpyridine and ppy = 2-phenylpyridine). Complexes are obtained as single stereoisomers, and ligand geometry is unambiguously assigned via X-ray crystallography. Electrochemical analysis of the materials reveals facile HOMO modulation through ppy functionalization and alteration of the monodentate ligand's field strength. Emission reflects similar modulation shifting from orange to greenish-blue upon replacement of chloride with cyanide. Many of the new compounds exhibit impressive room temperature phosphorescence with lifetimes near 3 μs and quantum yields reaching 28.6%. Application of the new luminophores as photosensitizers for photocatalytic hydrogen generation reveals that their photostability in coordinating solvent is enhanced as compared to popular [Ir(ppy)2(bpy)](+) (bpy = 2,2'-bipyridine) photosensitizers. Yet, the binding of their monodentate ligand emerges as a source of instability during the redox processes of cyclic voltammetry and mass spectrometry. DFT modeling of electronic structure is provided for all compounds to elucidate experimental properties.

Rare observation of ‘aggregation induced emission’ in cyclometalated platinum(ii) complexes and their biological activities

Three strong solid state emissive cyclometalated platinum(ii) complexes showing AIE property were synthesized and the feasibility of one of the dyes for cell imaging was reported.

Homoleptic tris-cyclometalated platinum(iv) complexes: a new class of long-lived, highly efficient 3LC emitters

Tris-cyclometalated Pt(IV) complexes are reported for the first time. The facial isomers exhibit long-lived ³LC emissions with quantum yields up to 0.49, the highest ever found for Pt(IV) complexes, combined with a strong oxidizing character in the excited state.