Syntheses, structural, photophysical and theoretical studies of heteroleptic cycloplatinated guanidinate(1-) complexes bearing acetylacetonate and picolinate ancillary ligands

Cycloplatination of symmetrical N,N',N''-triarylguanidines, (ArNH)2C[double bond, length as m-dash]NAr with cis-[Pt(TFA)2(S(O)Me2)2] in toluene afforded cis-[Pt(TAG)(TFA)(S(O)Me2)] (TAG = triarylguanidinate(1-)-κC,κN; TFA = OC(O)CF3; 6-9) in 75-82% yields. The reactions of 6-9 and the previously known cis-[Pt(TAG)X(S(O)Me2)] (X = Cl (1) and TFA (2-5)) with acetylacetone (acacH) or 2-picolinic acid (picH) in the presence of a base afforded [Pt(TAG)(acac)] (acac = acetylacetonate-κ2O,O'; 10-18) and [Pt(TAG)(pic)] (pic = 2-picolinate-κN,κO; 19) in high yields. The new complexes were characterised by analytical, IR and multinuclear NMR spectroscopies. Further, molecular structures of 11, 12, 13·0.5 toluene and 14-19 were determined by single crystal X-ray diffraction. Absorption spectra of 10-19 in solution and their emission spectra in crystalline form were measured. Platinacycles 10-19 are bluish green light emitter in the crystalline form, and emit in the λPL = 488-529 nm range (11 and 13-19) while 12 emits at λPL = 570 nm. Unlike other platinacycles, the emission band of 12 is broad, red shifted, and this pattern is ascribed to the presence of an intermolecular N-H⋯Pt interaction involving the endocyclic amino unit of the six-membered [Pt(TAG)] ring and the Pt(ii) atom in the adjacent molecule in an asymmetric unit of the crystal lattice. Lifetime measurements were carried out for all platinacycles in crystalline form, which revealed lifetime in the order of nanoseconds. The origin of absorption and emission properties of 11, 15, 18 and 19 were studied by TD-DFT calculations.

Organometallic Platinum(II) Photosensitisers that Demonstrate Ligand-Modulated Triplet-Triplet Annihilation Energy Upconversion Efficiencies

A series of 2-phenylquinoxaline ligands have been synthesised that introduce either CF₃ or OCF₃ electron-withdrawing groups at different positions of the phenyl ring. These ligands were investigated as cyclometalating reagents for platinum(II) to give neutral complexes of the form [Pt(C^N)(acac)] (in which C^N=cyclometalating ligand; acac=acetyl acetonate). X-ray crystallographic studies on three examples showed that the complexes adopt an approximate square planar geometry. All examples revealed strong Pt-Pt linear contacts of 3.2041(6), 3.2199(3) and 3.2586(2) Å. The highly coloured complexes display efficient visible absorption at 400-500 nm (ϵ ≈5000 M^-1  cm^-1 ) and orange red photoluminescent characteristics (λ_em =603-620 nm; Φ_em ≤37 %), which were subtly tuned by the ligand. Triplet emitting character was confirmed by microsecond luminescence lifetimes and the photogeneration of singlet oxygen with quantum efficiencies up to 57 %. Each complex was investigated as a photosensitiser for triplet-triplet annihilation energy upconversion using 9,10-diphenylanthracene as the annihilator species: a range of good upconversion efficiencies (Φ_UC 5.9-14.1 %) were observed and shown to be strongly influenced by the ligand structure in each case.

Pyrene-Decoration of a Chromium(0) Tris(diisocyanide) Enhances Excited State Delocalization: A Strategy to Improve the Photoluminescence of 3d6 Metal Complexes

There is a long-standing interest in iron(II) complexes that emit from metal-to-ligand charge transfer (MLCT) excited states, analogous to ruthenium(II) polypyridines. The 3d⁶ electrons of iron(II) are exposed to a relatively weak ligand field, rendering nonradiative relaxation of MLCT states via metal-centered excited states undesirably efficient. For isoelectronic chromium(0), chelating diisocyanide ligands recently provided access to very weak MLCT emission in solution at room temperature. Here, we present a concept that boosts the luminescence quantum yield of a chromium(0) isocyanide complex by nearly 2 orders of magnitude, accompanied by a significant increase of the MLCT lifetime. Pyrene units in the diisocyanide ligand backbone lead to an enlarged π-conjugation system and to a strongly delocalized MLCT state, from which nonradiative relaxation is less dominant despite a sizable redshift of the emission. While the pyrene moiety is electronically coupled to the core of the chromium(0) complex in the excited state, UV-vis absorption and 2D NMR spectroscopy show that this is not the case in the ground state. Luminescence lifetimes and quantum yields for our pyrenyl-decorated chromium(0) complex exhibit an unusual bell-shaped dependence on solvent polarity, indicative of two counteracting effects governing the MLCT deactivation. These two effects are identified as predominant deactivation either through an energetically nearby lying metal-centered state in the most apolar solvents, or alternatively via direct nonradiative relaxation to the ground state following the energy gap law in more polar solvents. This is the first example of a 3d⁶ MLCT emitter to benefit from an increased π-conjugation network.

Maloplatin-B, a Cisplatin-Based BODIPY-Tagged Mito-Specific "Chemo-PDT" Agent Active in Red Light

Maloplatin-B, a cisplatin-based complex, namely [Pt(A-BOD)(NH₃)₂](NO₃) (Pt-A-BOD) with a pendant boron-dipyrromethene (BODIPY) moiety, where HA-BOD is a methyl malonyl chloride derived monostyryl BODIPY ligand, was designed and developed as near-IR light (600-720 nm) organelle-targeting photodynamic therapy agent. The complex [Pt(acac)(NH₃)₂](NO₃) (Pt-Ac) was used as a control. Pt-A-BOD displayed an absorption band at 616 nm (ε = 2.9 × 10⁴ M^-1 cm^-1) in 10% dimethyl sulfoxide/Dulbecco's Modified Eagle's Medium (DMSO/DMEM, pH 7.2). This complex displayed a broad emission band within 650-850 nm with a λ_em value of 720 nm in 10% DMSO-DMEM (pH 7.2) upon excitation (λ_ex) at 615 nm with a large Stokes shift. The fluorescence quantum yield (Φ_F) value for Pt-A-BOD is 0.032 and for the ligand HA-BOD is 0.24. The BODIPY complex and ligand showed the formation of singlet oxygen as the ROS (reactive oxygen species) on irradiation with near-IR red light of 660 nm, as evidenced from a 1,3-diphenylisobenzofuran (DPBF) assay. The complex displayed remarkable apoptotic NIR light-induced PDT activity with half-maximum inhibitory concentration values (IC₅₀) of 1.6-2.4 μM in A549 lung and HeLa cervical cancer cells, while it was less active in the dark. The cellular ROS generation by the complex in red light was ascertained by a DCFDA (2',7'-dichlorofluorescein diacetate) assay. Cellular imaging showed its localization primarily in the mitochondria of A549 cancer cells. The JC1 and Annexin-V FITC/PI assays carried out for A549 cancer cells treated with the BODIPY complex showed the alteration of mitochondrial membrane potential and apoptotic cell death on near-IR red light (600-720 nm) irradiation, respectively.

Iridium(III) Sensitisers and Energy Upconversion: The Influence of Ligand Structure upon TTA-UC Performance

Six substituted ligands based upon 2-(naphthalen-1-yl)quinoline-4-carboxylate and 2-(naphthalen-2-yl)quinoline-4-carboxylate have been synthesised in two steps from a range of commercially available isatin derivatives. These species are shown to be effective cyclometallating ligands for Ir^III , yielding complexes of the form [Ir(C^N)₂ (bipy)]PF₆ (where C^N=cyclometallating ligand; bipy=2,2'-bipyridine). X-ray crystallographic studies on three examples demonstrate that the complexes adopt a distorted octahedral geometry wherein a cis-C,C and trans-N,N coordination mode is observed. Intraligand torsional distortions are evident in all cases. The Ir^III complexes display photoluminescence in the red part of the visible region (668-693 nm), which is modestly tuneable through the ligand structure. The triplet lifetimes of the complexes are clearly influenced by the precise structure of the ligand in each case. Supporting computational (DFT) studies suggest that the differences in observed triplet lifetime are likely due to differing admixtures of ligand-centred versus MLCT character instilled by the facets of the ligand structure. Triplet-triplet annihilation upconversion (TTA-UC) measurements demonstrate that the complexes based upon the 1-naphthyl derived ligands are viable photosensitisers with upconversion quantum efficiencies of 1.6-6.7 %.

Spectroscopic and Theoretical Investigation of Color Tuning in Deep-Red Luminescent Iridium(III) Complexes

A series of heteroleptic, neutral iridium(III) complexes of the form [Ir(L)2(N^O)] (where L = cyclometalated 2,3-disubstituted quinoxaline and N^O = ancillary picolinate or pyrazinoate) are described in terms of their synthesis and spectroscopic properties, with supporting computational analyses providing additional insight into the electronic properties. The 10 [Ir(L)2(N^O)] complexes were characterized using a range of analytical techniques (including 1H, 13C, and 19F NMR and IR spectroscopies and mass spectrometry). One of the examples was structurally characterized using X-ray diffraction. The redox properties were determined using cyclic voltammetry, and the electronic properties were investigated using UV-vis, time-resolved luminescence, and transient absorption spectroscopies. The complexes are phosphorescent in the red region of the visible spectrum (λem = 633-680 nm), with lifetimes typically of hundreds of nanoseconds and quantum yields ca. 5% in aerated chloroform. A combination of spectroscopic and computational analyses suggests that the long-wavelength absorption and emission properties of these complexes are strongly characterized by a combination of spin-forbidden metal-to-ligand charge-transfer and quinoxaline-centered transitions. The emission wavelength in these complexes can thus be controlled in two ways: first, substitution of the cyclometalating quinoxaline ligand can perturb both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital levels (LUMO, Cl atoms on the ligand induce the largest bathochromic shift), and second, the choice of the ancillary ligand can influence the HOMO energy (pyrazinoate stabilizes the HOMO, inducing hypsochromic shifts).

4,5-Substituted C^C* cyclometalated thiazol-2-ylidene platinum(ii) complexes - synthesis and photophysical properties

We report the synthesis of seven novel backbone functionalized N-phenyl-1,3-thiazol-2-ylidene platinum(ii) complexes and their photophysical properties. Electronically diverse N-phenyl-1,3-thiazol-2-thiones were prepared by a reaction of aniline with carbon disulfide and different α-haloketone compounds. Oxidative desulfuration and salt metathesis yielded the desired NHC-precursors with hexafluorophosphate counterions. In addition, a new route for the synthesis of N-phenyl-1,3-benzo[d]thiazole tetrafluoroborate via N-arylation using hypervalent iodine species is presented. All complexes were prepared from the corresponding NHC precursor in a one-pot process using silver(i)oxide, transmetalation to platinum and reaction with the β-diketone acetylacetone under basic conditions. These complexes exhibit strong phosphorescence with quantum yields up to 72% in 2 wt% PMMA films with decay lifetimes of 8.8-12.3 μs. The influence of methyl- and phenyl-groups, and an ester-substituent at the 4- and/or 5-position of the 1,3-thiazole moiety, as well as the N-phenyl-1,3-benzo[d]thiazole-derived motif is discussed. The 4,5-unsubstituted-N-phenyl-1,3-thiazol-2-ylidene platinum(ii) acetylacetonato complex served as a reference in this study to evaluate the electronic effects originating from the backbone substitution. All complexes emit in a narrow range of the bluish-green spectrum of the visible light (510 ± 10 nm).

Protein staining agents from low toxic platinum(ii) complexes with bidentate ligands

Low toxic platinum(ii) complexes with bidentate ligands have been developed as novel protein staining agents.

Changing the Emission Properties of Phosphorescent C^C*-Cyclometalated Thiazol-2-ylidene Platinum(II) Complexes by Variation of the β-Diketonate Ligands

Cyclometalated thiazol-2-ylidene platinum(II) complexes based on the N-phenyl-4,5-dimethyl-1,3-thiazol-2-ylidene N-heterocyclic carbene (NHC) ligand and seven different β-diketonate ligands have been synthesised and investigated for their structural and photophysical properties. The complexes were synthesised in a one-pot procedure starting with the in situ formation of the corresponding silver(I) carbene and transmetalation to platinum, followed by the reaction with the respective β-diketonate under basic conditions. All the compounds were fully characterised by standard techniques, including ¹⁹⁵ Pt NMR spectroscopy. Three solid-state structures revealed quite different aggregation behaviour depending on the β-diketonate architecture. The reported complexes showed strong phosphorescence at room temperature in amorphous poly(methyl methacrylate) films. The emission wavelengths (ca. 510 nm) were found to be independent of the β-diketonate ligand, but the electronically diverse β-diketonates strongly influence the observed quantum yields (QY) and decay lifetimes. The results of theoretical studies employing density functional theory (DFT) methods support the conclusion of a metal-to-ligand charge transfer (³ MLCT) as the main emission process, in accordance with the reported photophysical properties. Standard organic light-emitting diodes (OLEDs) prepared with unoptimised matrix materials using one of the complexes showed values of 12.3 % external quantum yield, 24.0 lm W^-1 luminous efficacy and 37.8 cd A^-1 current efficiency at 300 cd m^-2 .