Luminescent Cyclometalated Platinum Research Articles

Luminescent cyclometalated platinum compounds with N, P, and O^O ligands: Density‐functional theory studies and analysis of the anticancer potential

Luminescent platinum cyclometalated complexes are species of interest mainly due to their applications in the optoelectronic and biological fields, especially with regard to their anticancer activity. Given this level of interest, a series of cyclometalated (2‐[2′‐thienyl]pyridinate, thpy and 2‐[2,4‐difluorophenyl]pyridinate, dfppy) platinum complexes with N‐donor, PTA (1,3,5‐triaza‐7‐phosphaadamantane) or chrysin‐derived ligands (incorporating piperidine, HL1, or morpholine, HL2, fragments) were synthesized. The complexes are luminescent with tunable emission wavelengths. Aggregation in solution was observed for [Pt(dfppy)L1], 5. Density‐functional theory (DFT) studies provided descriptions of the highest occupied molecular orbital (HOMO) and least unoccupied molecular orbital (LUMO) characteristics and their influence on the photophysical properties. The orbitals of 5–6 were different in nature to those of 1–4. Time‐dependent DFT (TD‐DFT) calculations showed that for 1–4 the excited states S1 and T1 reflect metal‐to‐ligand charge transfer (MLCT) and ligand‐centered (LC) (C^N) contributions while for 5–6 these states are an LC transition centered on L1 or L2. The speciation in DMSO and DMSO/H2O was evaluated. Biological studies showed that [Pt(thpy)Cl(Hthpy)], 1, [Pt(dfppy)Cl(Hdfppy)], 2, and 5 exert significant cytotoxic activity against human cervical (HeLa) and lung (A549) carcinoma cells. The cytotoxicity of 1 increased 2.84‐fold upon irradiation (blue). Microscopy assays on 5 showed that this compound accumulates in cytoplasmic organelles, preferentially in mitochondria. Mitochondrial metabolism was disrupted by the activity of the complexes, leading to a decline in the adenosine triphosphate (ATP) cellular content. Overall, the results show an alternative anticancer activity for complexes 1, 2, and 5, which could be of great interest for the treatment of tumors with acquired resistance to conventional DNA‐targeted anticancer drugs.

Efficiently red emitting cycloplatinated(II) complexes supported by N^O and N^P benzimidazole ancillary ligands

Eight neutral luminescent cyclometalated platinum(II) complexes with general formula (piq)Pt(N^O) and (piq)Pt(N^P), where piq = 1-phenylisoquinoline, N^O = deprotonated (2-(1H-benzimidazole)-phenyl)diphenylphosphine oxide derivatives and N^P = deprotonated (2-(1H-benzimidazole)-phenyl)diphenylphosphine derivatives, have been synthesized and characterized. These complexes display distorted square-planar molecular geometries around platinum(II) centers without obvious π-π interactions and Pt-Pt metallophilic interactions in the crystal lattice, due to the presence of sterically bulky N^O and N^P benzimidazole ancillary ligands. Hence, these complexes exhibit good monomeric emissions with the luminescent quantum yields of 15%-23%, which are far more than that of (piq)Pt(acac) (0.9%) in dichloromethane (DCM) at 298 K. The emission colors of platinum(II) complexes can be finely tuned in the range 600–645 nm by changing the chelating types of ancillary ligands. The structured emission combined with microsecond emission lifetimes and the support of TD-DFT calculations for these complexes indicate that their emissions mainly come from an admixture of intraligand (3ILCT) and metal-to-ligand charge transfer (3MLCT).

Luminescent cyclometalated platinum(ii) complexes with acyclic diaminocarbene ligands: structural, photophysical and biological properties.

Four new cyclometalated Pt(ii) complexes bearing acyclic diaminocarbene (ADC) ligands, [Pt(C^N)Cl{C(NHXyl)(NHR)}] [C^N = 2,6-difluorophenylpyridine (dfppy), phenylquinoline (pq); R = Pr 3a, 4a, CH2Ph 3b, 4b], were prepared by the nucleophilic attack on the isocyanide [Pt(C^N)Cl(CNXyl)] (C^N = dfppy 1, pq 2) by the corresponding amine RNH2 (R = Pr, CH2Ph). Complexes 3 show in their 1H NMR spectra in CDCl3 a notable concentration dependence, with a clear variation of the δH (NHXyl) signal, suggesting an assembling process implying donor-acceptor NHXylCl bonding, also supported by 1D-PGSE (Pulse Field Gradient Spin Echo) and 2D-DOSY (Diffusion Ordered Spectroscopy) NMR experiments in solution and X-ray diffraction studies. The intermolecular interactions in compounds 3a and 3b were studied by using Hirshfeld surface analysis and Non-Covalent Interaction (NCI) methods on their X-ray structures. Their photophysical properties were investigated by absorption and emission spectroscopies and also by TD-DFT calculations performed on 3a and 4b. These complexes show green (3) or orange (4) phosphorescence, attributed to a mixed 3IL/3MLCT excited state. The carbene ligand does not affect the emission maxima but it produces an increase of the quantum yields in relation to the isocyanide in the precursors. In fluid solutions, the emission is not concentration-dependent, but the complexes may show aggregation induced emission as detailed for complexes 3a and 4a. In addition, cytotoxicity studies in the human cell lines A549 (lung carcinoma) and HeLa (cervix carcinoma) showed good activity for these complexes and 3a, 3b and 4a exhibit a strong effect on DNA electrophoretic mobility. To the best of our knowledge, compounds 3 and 4 represent the first examples of cycloplatinated complexes bearing acyclic diamino carbenes with antiproliferative properties.

Tuning the Phosphorescence and Solid State Luminescence of Triarylborane-Functionalized Acetylacetonato Platinum Complexes.

A new series of luminescent cyclometalated platinum complexes with triarylborane-functionalized acetylacetonate ligands is reported. The complexes exhibit solid state luminescence and phosphorescence under ambient conditions. The luminescence color can be tuned from green to red by varying the cyclometalating ligand [2-phenylpyridine (for 1 and 2), 2-thiophenylpyridine (for 3 and 4), 2-thianapthenylpyridine (for 5 and 6)]. The luminescence originates from mixed 3MLCT/3IL [MLCT, metal to ligand charge transfer; IL, intraligand] states of square planar platinum and borane moieties. The π spacer (phenyl or duryl) which connects the boryl and platinum entities has a significant role in determining the photoluminescence efficiency. The bulky duryl spacer in 2, 4, and 6 significantly reduces π-π stacking of the square planar platinum moiety in the solid state and provides a rigid backbone, thereby increasing their quantum yield significantly. The role of Lewis-acidic borane on the photoluminescence features is evaluated by fluoride binding experiments.

Water soluble luminescent cyclometalated platinum(II) complex — A suitable probe for bio-imaging applications

A highly water soluble platinum(II) complex, [Pt(ppy)(en)]Cl, (where ppy=2-phenylpyridine and en=ethane-1, 2-diamine) with a strong green luminescence in solution was synthesized in two steps. Initially, an intermediate complex, [Pt(ppy)(ppyH)(Cl)], 1 was isolated which was transformed into the light-emitting mononuclear platinum(II) complex, [Pt(ppy)(en)]Cl, 2. The green emission of 2 was changed into yellow in their solid state. The strong π…π intermolecular interactions that interplays between the phenyl rings lying into the two consecutive molecules which led to the solid state emission. The low cytotoxicity of 2 ensures the feasibility for using these in bioimaging probe. The testing bioimaging results demonstrate that the compound penetrates the cell membrane in both cancer (HeLa, U87MG and Nuro2a) and normal cells (CHO-K1 and 3T3-L1). The cellular uptake was observed to much higher in cancerous cells only. Co-localization study confirmed the localization of it into the cell nucleus.

Impact of a carboxyl group on a cyclometalated ligand: hydrogen-bond- and coordination-driven self-assembly of a luminescent platinum(II) complex.

A new luminescent cyclometalated platinum(II) complex containing a carboxyl group, trans-[Pt(pcppy)(pic)][1-COOH; Hpcppy = 2-(p-carboxyphenyl)pyridine and Hpic = picolinic acid] has been synthesized and characterized. The luminescence behavior of 1-COOH in the solid and solution states is completely different despite the similarity of the luminescence in both states for the nonsubstituted complex, [Pt(ppy)(pic)] (1-H; Hppy = 2-phenylpyridine). Interestingly, 1-COOH exhibits concentration-dependent absorption and emission behavior based on its aggregation in a basic aqueous solution despite the absence of amphiphilic character.

Luminescent cyclometalated platinum(II) complex forms emissive intercalating adducts with double-stranded DNA and RNA: differential emissions and anticancer activities.

Luminescent metallo-intercalators are potent biosensors of nucleic acid structure and anticancer agents targeting DNAs. There are few examples of luminescent metallo-intercalators which can simultaneously act as emission probes of nucleic acid structure and display promising anticancer activities. Herein, we describe a luminescent platinum(II) complex, [Pt(C^N^N)(C≡NtBu)]ClO4 (1 a, HC^N^N= 6-phenyl-2,2'-bipyridyl), that intercalates between the nucleobases of nucleic acids, accompanied by an increase in emission intensity and/or a significant change in the maximum emission wavelength. The changes in emission properties measured with double-stranded RNA (dsRNA) are different from those with dsDNA used in the binding reactions. Complex 1 a exhibited potent anticancer activity towards cancer cells in vitro and inhibited tumor growth in a mouse model. The stabilization of the topoisomerase I-DNA complex with resulting DNA damage by 1 a is suggested to contribute to its anticancer activity.

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.

High Efficiency White Organic Light‐Emitting Devices Incorporating Yellow Phosphorescent Platinum(II) Complex and Composite Blue Host

AbstractA new class of charge neutral, strongly luminescent cyclometalated platinum(II) complexes supported by dianionic tetradentate ligand are synthesized. One of these platinum(II) complexes, Y‐Pt, displays a high photoluminescence quantum yield of 86% and electroluminescence efficacy (ηpower) of up to 52 lm W−1, and is utilized as a yellow phosphorescent dopant in the fabrication of white organic light‐emitting devices (WOLEDs). WOLEDs based on conventional structures with yellow emission from Y‐Pt in combination with blue emission from bis(4,6‐difluorophenyl‐pyridinato‐N,C2′) (picolinate) iridium(III) (FIrpic) show a total ηpower of up to 31 lm W−1. A two‐fold increase in ηpower by utilizing a modified WOLED structure comprising of a composite blue host is realized. With this modified device structure, the total ηpower and driving voltage at a luminance of 1000 cd m−2 can be improved to 61 lm W−1 and 7.5 V (i.e., 10 V for control devices). The performance improvement is attributed to an effectively broaden exciton formation‐recombination zone and alleviation of localized exciton accumulation within the FIrpic‐doped composite host for reduced triplet‐triplet annihilation, yielding blue light‐emission with enhanced intensity. The modified device structure can also adopt a higher concentration of Y‐Pt towards its optimal value, leading to WOLEDs with high efficiency.

Structure, photophysical properties and computational study of a highly luminescent mixed-metal platinum(ii)–silver(i) system. Potential building blocks for emissive supramolecular structures

A luminescent cyclometalated platinum(II) complex has been shown to sandwich a silver ion, which demonstrates intense luminescence with appreciable photoluminescence quantum yield. Computational studies have been performed to provide insights into the nature of the photophysical properties.

A luminescent cyclometalated platinum(ii) complex and its green organic light emitting device with high device performance

A luminescent cyclometalated platinum(II) complex has been demonstrated to show green electrophosphorescence in multilayer organic light-emitting devices (OLEDs) using a dual emissive layer with high current and external quantum efficiencies of 38.9 cd A(-1) and 11.5%, respectively.

Luminescent cyclometalated platinum(ii) complexes containing N-heterocyclic carbene ligands with potent in vitro and in vivo anti-cancer properties accumulate in cytoplasmic structures of cancer cells

Contrary to most platinum-based anti-cancer agents which target DNA, coordination of N-heterocyclic carbene (NHC) ligands to cyclometalated platinum(II) complexes confers these luminescent complexes to other cellular target(s). The strong Pt–Ccarbene bond(s) renders the platinum(II) complexes to display unique photophysical properties and enhanced stability against biological reduction and ligand exchange reactions. The platinum complexes described in this work are highly cytotoxic and display high specificity to cancerous cells. Among them, [(C^N^N)PtII(N,N′-nBu2NHC)]PF6 (1a, where HC^N^N = 6-phenyl-2,2′-bipyridine) with a lipophilic carbon chain on the carbene ligand induces apoptosis in cancer cells, demonstrates an enhancing synergistic effect with cisplatin in vitro, and displays potent in vivo activities using nude mice models. As this complex is strongly emissive, its cellular localization can be traced using emission microscopy. In contrast to common platinum-based anti-cancer agents, 1a does not accumulate in the vicinity of DNA but preferentially accumulates in cytoplasmic structures including sites where active survivin, an inhibitor of apoptosis (IAP), is located. In vitro, 1a significantly inhibits the expression of survivin, activates poly(ADP-ribose) polymerase (PARP) and induces apoptosis in cancer cells. Given the ease of structural modification of NHC ligand to alter the overall biological activities, these [(C^N^N)PtII(NHC)]+ complexes having unique photophysical properties provide an entry to a new class of potential anti-cancer drug leads.

Synthesis, Structure, and Photophysical Properties of Luminescent Platinum(II) Complexes Containing Cyclometalated 4-Styryl-Functionalized 2-Phenylpyridine Ligands

A series of new luminescent cyclometalated platinum(II) complexes functionalized with various substituted styryl groups on the cyclometallating ligand [Pt(C/\N-ppy-4-styryl-R)(O/\O-(O)CCR'CHCR'C(O))] (ppy-4-styryl-R = E-4(4-(R)styryl-2-phenylpyridine) (3, R' = Me (acac); 4, R' = (t)Bu (dpm); R = H, OMe, NEt2, NO2) have been prepared. All complexes undergo an E-Z photoisomerization process in CH2Cl2 solution under sunlight, as monitored by 1H NMR. The solid-state structures of 3-OMe, 3-NEt2, 3-NO2, and 4-OMe have been determined by X-ray diffraction studies and compare well with optimized geometries obtained by density functional theory (DFT) calculations. The orbital pictures of 3-H, 3-OMe, and 3-NO 2 are very similar, the highest occupied molecular orbital (HOMO) being highly Pt(5d) metal-based. For 3-NMe2, an additional contribution from the amino-styryl fragment leads to a decreased metal parentage of the HOMO, suggesting a predominantly ILCT character transition. Complexes 3-H, 3-OMe, and 3-NO2 show a low-energy band (350-400 nm) assigned to predominantly charge-transfer transitions. The amino derivative 3-NEt2 displays a very strong absorption band at 432 nm, tentatively assigned to a mixture of ILCT (Et2N --> CH=CH) and metal-to-ligand charge-transfer (MLCT) (dpi(Pt) --> pi) transitions. Complexes 3 are weakly luminescent in CH2Cl2 solution at room temperature; the low intensity may be due to a competitive quenching through the E-Z photoisomerization process. All complexes exhibit similar structured emission bands under these conditions (around 520 nm), independent of the nature of the styryl-R group. In a frozen EPA glass (77 K), the spectrum of the representative complex 3-H exhibits two sets of vibronically structured bands (460-560, 570-800 nm; lambda(max) = 596 nm), due to the presence of two emitting species, the E and Z isomers, which have significantly different triplet excited-state energies. The other three complexes show similar behavior to 3-H at 77 K, but the lower-energy emission bands are progressively red-shifted in the order H < OMe < NO2 < NEt2 (e.g., for 3-NEt2, lambda(max)(em) = 658 nm; tau = 26 micros). The very large red-shift compared to related unsubstituted complexes (e.g., to [Pt(C/\N-ppy)(O/\O-acac)]) is the result of the extension of the pi-conjugated system and the electronic effects of substituent R.

Luminescent cyclometalated platinum(ii) complexes with amino acid ligands for protein binding

Pt(C/N)(phe)(1, C/N = 2-(2'-thienyl)pyridine, phe = phenylalanine) shows a high binding affinity (ca. 10(6) dm(3) mol(-1)) and selectivity towards human serum albumin (HSA) and such binding is accompanied by an enhancement of photoluminescence at 562 nm; both the protein binding affinity and cytotoxicities of [Pt(C/N)(phe)(1), Pt(C/N)(trp)(2, trp = tryptophan) and Pt(C/N)(gly)(3, gly = glycine)] are affected by the amino acid ligand with having an IC(50) of up to 1 microM against a number of carcinoma cell lines.

Tuning the Excited‐state Properties of Cyclometalated Platinum (II) Complexes of 6‐Phenyl‐2,2′‐bipyridine by Ancillary Acetylide Ligand

AbstractA series of luminescent cyclometalated platinum (II) complexes, (CNN)Pt(CC̊) [HCNN = 4‐(4‐tolyl)‐6‐phenyl‐2,2′‐bipyridine; R = 4‐chlorophenyl (1), phenyl (2) and 4‐tolyl (3)], were synthesized, and their spectroscopic properties have been examined. These complexes are brightly emissive both in fluid solution and in the solid state, attributed to triplet metal‐to‐ligand charge transfer (3MLCT) state. The excited state energy can be tuned by ancillary acetylide ligands. The emission lifetimes in dichloro‐methane solution at room temperature were up to 1.64 μs and the emission quantum yields were in the range of 0.03–0.15.

Solvatochromic response imposed by environmental changes in matrix/chromophore entities: luminescent cyclometalated platinum (II) complex in Nafion and silica materials.

Luminescent cyclometalated complex [Pt(L)py]+ (1) immobilised in Nafion film exhibits a solvatochromic shift in emission maximum from 530 to 650 nm upon immersion in ethanol but no effect is detected with aprotic organic solvents, whereas the emission of the [Pt(L)]+ luminophore anchored in silica materials shows a blue shift from approximately 665 to 550 nm upon exposure to pentane vapour but no shift is observed for ethanol vapour.

Structure and spectroscopic properties of luminescent cyclometalated platinum(II) complexes with chiral phosphine substituted carbohydrate ligands

A series of new chiral cyclometalated platinum(II) complexes containing carbohydrate phosphine ligands have been prepared. The reaction of [Pt(ppy)Cl]2 (Hppy = 2-phenylpyridine) with the chiral phosphines n-Hmbpa (methyl 4,6-O-benzylidene-n-deoxy-n-(diphenylphosphino)-α-D-altropyranoside, n = 2 or 3) afforded cis-[Pt(ppy)(n-Hmbpa)Cl] (n = 2 2a or 3 2b) in high yields. Treatment of 2a or 2b with an excess of NaOCH3 gave the alkoxoplatinum(II) complexes trans-[Pt(ppy)(n-mbpa)] (n = 2 3a or 3 3b). The crystal structure of 3b shows that the phosphorus atom is located trans to the nitrogen atom of the ppy ligand and the pyranose ring is in a boat conformation. Moderately intense UV-vis absorption bands assigned to metal-to-ligand charge-transfer (MLCT) transitions are shifted from ca. 376–382 to ca. 414–416 nm when the chloride ligand is substituted by the pendant alkoxide group. In solid state and 77 K MeOH–EtOH (4∶1) glass solution, complexes 2 and 3 show a vibronic structured emission in the range 450–650 nm. Complex 2 is non-emissive in fluid solution at room temperature whereas 3 shows a long-lived 3MLCT emission in both CH3CN and CH2Cl2 at room temperature.

Carbene and Isocyanide Ligation at Luminescent Cyclometalated 6-Phenyl-2,2‘-bipyridyl Platinum(II) Complexes: Structural and Spectroscopic Studies

A series of luminescent cyclometalated platinum(II) diamino−carbene complexes, namely, [(CNN)Pt{C(NHR1)(NHR2)}]+ (HCNN = 6-phenyl-2,2‘-bipyridine; for R1 = tBu, R2 = Me (2), NH2 (3), CH2Ph (4); for R1 = 2,6-Me2C6H3 (Ar‘), R2 = Me (6), NH2 (7), CH2Ph (8)), are synthesized by nucleophilic attack of amines at the coordinated isocyanide ligands of [(CNN)PtC⋮NR1]+ (R1 = tBu (1), Ar‘ (5)). The binuclear bridging bis(carbene) derivative [{(CNN)Pt}2μ-{C(NHtBu)(NH(CH2)3)}2NMe]2+ (9) is prepared by treatment of 1 with excess 3,3‘-diamino-N-methyldipropylamine. The molecular structures of 2(ClO4)·0.5H2O, 3(ClO4)·0.5H2O, 4(ClO4), and 6(ClO4) reveal Pt−C(carbene) distances of 1.997(7), 1.992(4), 1.989(6), and 1.996(8) A, respectively. Short N−C(carbene) bond lengths (mean 1.332 A) imply substantial pπ−pπ interactions within the N−C(carbene)−N fragment, and minimal π bonding in the Pt−C(carbene) moiety is inferred. Weak π−π stacking interactions between the CNN ligands are observed in the crystal lattice of 2−4 and 6 (...