Visible and Red Light-Triggered Anticancer Profile of a Ferrocene-Re(I)-Tricarbonyl Conjugate: Experimental and Theoretical Studies.

We have red-shifted the light absorbance property of a Re(I)-tricarbonyl complex via distant conjugation of a ferrocene moiety and developed a novel complex ReFctp, [Re(Fctp)(CO)3Cl], where Fctp = 4'-ferrocenyl-2,2':6',2″-terpyridine. ReFctp showed green to red light absorption ability and blue emission, indicating its potential for photodynamic therapy (PDT) application. The conjugation of ferrocene introduced ferrocene-based transitions, which lie at a higher wavelength within the PDT therapeutic window. The time-dependent density functional theory and excited state calculations revealed an efficient intersystem crossing for ReFctp, which is helpful for PDT. ReFctp elicited both PDT type I and type II pathways for reactive oxygen species (ROS) generation and facilitated NADH (1,4-dihydro-nicotinamide adenine dinucleotide) oxidation upon exposure to visible light. Importantly, ReFctp showed effective penetration through the layers of clinically relevant 3D multicellular tumor spheroids and localized primarily in mitochondria (Pearson's correlation coefficient, PCC = 0.65) of A549 cancer cells. ReFctp produced more than 20 times higher phototoxicity (IC50 ∼1.5 µM) by inducing ROS generation and altering mitochondrial membrane potential in A549 cancer cells than the nonferrocene analogue Retp, [Re(CO)3(tp)Cl], where tp = 2,2':6',2″-terpyridine. ReFctp induced apoptotic mode of cell death with a notable photocytotoxicity index (PI, PI = IC50dark/IC50light) and selectivity index (SI, SI = normal cell's IC50dark/cancer cell's IC50light) in the range of 25-33.

Ru(II) Complexes with Absorption in the Photodynamic Therapy Window: 1O2 Sensitization, DNA Binding, and Plasmid DNA Photocleavage.

Two Ru(II) complexes, [Ru(pydppn)(bim)(py)]2+ [2; pydppn = 3-(pyrid-2'-yl)-4,5,9,16-tetraaza-dibenzo[a,c]naphthacene; bim = 2,2'-bisimidazole; py = pyridine] and [Ru(pydppn)(Me4bim)(py)]2+ [3; Me4bim = 2,2'-bis(4,5-dimethylimidazole)], were synthesized and characterized, and their photophysical properties, DNA binding, and photocleavage were evaluated and compared to [Ru(pydppn)(bpy)(py)]2+ (1; bpy = 2,2'-bipyridine). Complexes 2 and 3 exhibit broad 1MLCT (metal-to-ligand charge transfer) transitions with maxima at ∼470 nm and shoulders at ∼525 and ∼600 nm that extend to ∼800 nm. These bands are red-shifted relative to those of 1, attributed to the π-donating ability of the bim and Me4bim ligands. A strong signal at 550 nm is observed in the transient absorption spectra of 1-3, previously assigned as arising from a pydppn-centered 3ππ* state, with lifetimes of ∼19 µs for 1 and 2 and ∼270 ns for 3. A number of methods were used to characterize the mode of binding of 1-3 to DNA, including absorption titrations, thermal denaturation, relative viscosity changes, and circular dichroism, all of which point to the intercalation of the pydpppn ligand between the nucleobases. The photocleavage of plasmid pUC19 DNA was observed upon the irradiation of 1-3 with visible and red light, attributed to the sensitized generation of 1O2 by the complexes. These findings indicate that the bim ligand, together with pydppn, serves to shift the absorption of Ru(II) complexes to the photodynamic therapy window, 600-900 nm, and also extend the excited state lifetimes for the efficient production of cytotoxic singlet oxygen.

DFT Computational Analysis of the Mechanism of Action of Ru(II) Polypyridyl Complexes as Photoactivated Chemotherapy Agents: From Photoinduced Ligand Solvolysis to DNA Binding.

Photoactivated chemotherapy (PACT) is a form of target-oriented cancer therapy that exploits light of the proper wavelength to selectively activate the drug. Among the prodrugs used for this purpose, ruthenium-based complexes are particularly interesting, as when irradiated by light, they can release ligands by forming aquo-complexes able to bind DNA in both single and double strand fashions, causing its distortion. Using as model system a Ru(II) polypyridyl complex that has been demonstrated to be a promising photochemotherapeutic agent, all of the key aspects of the photoinduced solvolysis process and subsequent DNA interaction have been scrutinized using density functional theory (DFT) and time-dependent-DFT (TDDFT). Photoexcitation, intersystem crossing, internal conversion, mechanism by which photoinduced ligand release, and subsequent aquation steps occur have been examined. Pathways leading to the formation of both cis and trans biaquated photoproducts have been described, and the formation of the cis form of the biaquated photoproduct being the most favorable one, its reaction with a guanine base has also been reported in order to account for DNA binding.

Towards Selective Delivery of a Ruthenium(II) Polypyridyl Complex-Containing Bombesin Conjugate into Cancer Cells.

An increasing number of novel Ru(II) polypyridyl complexes have been successfully applied as photosensitizers (PSs) for photodynamic therapy (PDT). Despite recent advances in optimized PSs with refined photophysical properties, the lack of tumoral selectivity is often a major hurdle for their clinical development. Here, classical maleimide and versatile NHS-activated acrylamide strategies were employed to site-selectively conjugate a promising Ru(II) polypyridyl complex to the N-terminally Cys-modified Bombesin (BBN) targeting unit. Surprisingly, the decreased cell uptake of these novel Ru-BBN conjugates in cancer cells did not hamper the high phototoxic activity of the Ru-containing bioconjugates and even decreased the toxicity of the constructs in the absence of light irradiation. Overall, although deceiving in terms of selectivity, our new bioconjugates could still be useful for advanced cancer treatment due to their nontoxicity in the dark.

Formation of the N≡N Triple Bond from Reductive Coupling of a Paramagnetic Diruthenium Nitrido Compound.

Construction of nitrogen-nitrogen triple bonds via homocoupling of metal nitrides is an important fundamental reaction relevant to a potential Nitrogen Economy. Here, we report that room temperature photolysis of Ru2(chp)4N3 (chp- = 2-chloro-6-hydroxypyridinate) in CH2Cl2 produces N2 via reductive coupling of Ru2(chp)4N nitrido species. Computational analysis reveals that the nitride coupling transition state (TS) features an out-of-plane "zigzag" geometry instead of the anticipated planar zigzag TS. However, with intentional exclusion of dispersion correction, the planar zigzag TS geometry can also be found. Both the out-of-plane and planar zigzag TS geometries feature two important types of orbital interactions: (1) donor-acceptor interactions involving intermolecular donation of a nitride lone pair into an empty Ru-N π* orbital and (2) Ru-N π to Ru-N π* interactions derived from coupling of nitridyl radicals. The relative importance of these two interactions is quantified both at and after the TS. Our analysis shows that both interactions are important for the formation of the N-N σ bond, while radical coupling interactions dominate the formation of N-N π bonds. Comparison is made to isoelectronic Ru2-oxo compounds. Formation of an O-O bond via bimolecular oxo coupling is not observed experimentally and is calculated to have a much higher TS energy. The major difference between the nitrido and oxo systems stems from an extremely large driving force, ∼-500 kJ/mol, for N-N coupling vs a more modest driving force for O-O coupling, -40 to -140 kJ/mol.

Gated Proton Release during Radical Transfer at the Subunit Interface of Ribonucleotide Reductase.

The class Ia ribonucleotide reductase of Escherichia coli requires strict regulation of long-range radical transfer between two subunits, α and ß, through a series of redox-active amino acids (Y122•[ß] ↔ W48?[ß] ↔ Y356[ß] ↔ Y731[α] ↔ Y730[α] ↔ C439[α]). Nowhere is this more precarious than at the subunit interface. Here, we show that the oxidation of Y356 is regulated by proton release involving a specific residue, E52[ß], which is part of a water channel at the subunit interface for rapid proton transfer to the bulk solvent. An E52Q variant is incapable of Y356 oxidation via the native radical transfer pathway or non-native photochemical oxidation, following photosensitization by covalent attachment of a photo-oxidant at position 355[ß]. Substitution of Y356 for various FnY analogues in an E52Q-photoß2, where the side chain remains deprotonated, recovered photochemical enzymatic turnover. Transient absorption and emission data support the conclusion that Y356 oxidation requires E52 for proton management, suggesting its essential role in gating radical transport across the protein-protein interface.

An Activatable Near-Infrared Fluorescence Probe for in Vivo Imaging of Acute Kidney Injury by Targeting Phosphatidylserine and Caspase-3.

Renal-clearable and target-responsive near-infrared (NIR) fluorescent imaging probes have been promising for in vivo diagnosis of acute kidney injury (AKI). However, designing an imaging probe that is renal-clearable and concurrently responsive toward multiple molecular targets to facilitate early detection of AKI with improved sensitivity and specificity is challenging. Herein, by leveraging the receptor-mediated binding and retention effect along with enzyme-triggered fluorescence activation, we design and synthesize an activatable small-molecule NIR fluorescent probe (1-DPA2) using a "one-pot sequential click reaction" approach. 1-DPA2 can target both the externalized phosphatidylserine (PS) and active caspase-3 (Casp-3), two essential biomarkers of apoptosis, producing enhanced 808 nm NIR fluorescence and a high signal-to-background ratio (SBR) amenable to detecting the onset of cisplatin-induced AKI in mice as early as 24 h post-treatment with cisplatin. We not only monitor the gradual activation of Casp-3 in the kidney of mice upon AKI progression but also can report on the progressive recovery of kidney functions in AKI mice following N-acetyl-l-cysteine (NAC) therapy via real-time fluorescence imaging by 1-DPA2. This study demonstrates the ability of 1-DPA2 for longitudinal monitoring of renal cell apoptosis by concurrently targeting PS externalization and Casp-3 activation, which is efficient for early diagnosis of AKI and useful for prediction of potential drug nephrotoxicity as well as in vivo screening of anti-AKI drugs' efficacy.

Single-Cell Chemistry of Photoactivatable Platinum Anticancer Complexes.

The Pt(IV) prodrug trans, trans, trans-[Pt(pyridine)2(N3)2(OH)2] (Pt1) and its coumarin derivative trans, trans, trans-[Pt(pyridine)2(N3)2(OH)(coumarin-3-carboxylate)] (Pt2) are promising agents for photoactivated chemotherapy. These complexes are inert in the dark but release Pt(II) species and radicals upon visible light irradiation, resulting in photocytotoxicity toward cancer cells. Here, we have used synchrotron techniques to investigate the in-cell behavior of these prodrugs and visualize, for the first time, changes in cellular morphology and Pt localization upon treatment with and without light irradiation. We show that photoactivation of Pt2 induces remarkable cellular damage with extreme alterations to multiple cellular components, including formation of vacuoles, while also significantly increasing the cellular accumulation of Pt species compared to dark conditions. X-ray absorption near-edge structure (XANES) measurements in cells treated with Pt2 indicate only partial reduction of the prodrug upon irradiation, highlighting that phototoxicity in cancer cells may involve not only Pt(II) photoproducts but also photoexcited Pt(IV) species.

Phototriggered cytotoxic properties of tricarbonyl manganese(I) complexes bearing α-diimine ligands towards HepG2.

Reaction between bromo tricarbonyl manganese(I) and N,N'-bis(phenyl)-1,4-diaza-1,3-butadiene ligands, bearing different electron-donating and electron-withdrawing groups R = OCH3, Cl, and NO2 in the ortho- and para-positions on the phenyl substituent, afforded [MnBr(CO)3(N-N)] complexes. The influence of the character and position of the substituent on the dark stability and carbon monoxide releasing kinetics was systematically investigated and correlated with the data of the time-dependent density functional theory calculations. The combined UV/Vis and IR data clearly revealed that the aerated solutions of [MnBr(CO)3(N-N)] in either coordinating or noncoordinating solvents are dark stable and the fluctuations observed during the incubation period especially in the case of the nitro derivatives may be attributed to the exchange of the axial bromo ligand with the coordinating solvent molecules. The free ligands and nitro complexes were non-cytotoxic to HepG2 cells under both the dark and illumination conditions. In the dark, Mn(I) compounds, incorporating o-OCH3 and o-Cl, exhibited excellent cytotoxicity with IC50 values of 18.1 and 11.8 µM, while their para-substituted analogues were inactive in the dark and active upon the irradiation at 365 nm with IC50 values of 5.7 and 6.7 µM, respectively.

The anti-cancer effect of series of strained photoactivatable Ru(II) polypyridyl complexes on non-small-cell lung cancer and triple negative breast cancer cells.

Ruthenium complexes have been recently reported as potential chemotherapeutic agents that offer tumor selectivity and low tumor resistance. This study investigates the photochemistry and the effect of four strained photoactivatable polypyridyl ruthenium(II) complexes on non-small-cell lung cancer (A549) and triple negative breast cancer (MDA-MB-231) cells. All four ruthenium(II) complexes, [Ru(bpy)2dmbpy]Cl2 (C1) where (bpy = 2,2'-bipyridine and dmbpy = 6,6'-dimethyl-2,2'-bipyridine), [Ru(phen)2dmbpy]Cl2 (C2) where (phen = 1,10-phenanthroline), [Ru(dpphen)2dmbpy]Cl2 (C3) (where dpphen = 4,7-diphenyl-1,10-phenanthroline) and [Ru(BPS)2dmbpy]Na2 (C4) where (BPS = bathophenanthroline disulfonate) eject the dmbpy ligand upon activation by blue light. Determination of the octanol-water partition coefficient (log P) revealed that C3 was the only lipophilic complex (log P = 0.42). LC-MS/MS studies showed that C3 presented the highest cellular uptake. The cytotoxic effect of the complexes was evaluated with and without blue light activation using WST-1 kit. Data indicated that C3 exhibited the highest cytotoxicity after 72 h (MDA-MB-231, IC50 = 0.73 µM; A549, IC50 = 1.26 µM) of treatment. The phototoxicity indices of C3 were 6.56 and 4.64 for MDA-MB-230 and A549, respectively. Upon light activation, C3 caused significant ROS production and induced apoptosis in MDA-MB-231 cells as shown by flow cytometry. It also significantly increased Bax/Bcl2 ratio and PERK levels without affecting caspase-3 expression. C3 exhibited poor dark toxicity (IC50 = 74 µM) on rat mesenchymal stem cells (MSCs). In conclusion, the physical property of the complexes dictated by the variable ancillary ligands influenced cellular uptake and cytotoxicity. C3 may be considered a promising selective photoactivatable chemotherapeutic agent that induces ROS production and apoptosis.

Non-Catalytic Benefits of Ni(II) Binding to an Si(111)-PNP Construct for Photoelectrochemical Hydrogen Evolution Reaction: Metal Ion Induced Flat Band Potential Modulation.

We report here the remarkable and non-catalytic beneficial effects of a Ni(II) ion binding to a Si|PNP type surface as a result of significant thermodynamic band bending induced by ligand attachment and Ni(II) binding. We unambiguously deconvolute the thermodynamic flat band potentials (VFB) from the kinetic onset potentials (Von) by synthesizing a specialized bis-PNP macrochelate that enables one-step Ni(II) binding to a p-Si(111) substrate. XPS analysis and rigorous control experiments confirm covalent attachment of the designed ligand and its resulting Ni(II) complex. Illuminated J-V measurements under catalytic conditions show that the Si|BisPNP-Ni substrate exhibits the most positive onset potential for the hydrogen evolution reaction (HER) (-0.55 V vs Fc/Fc+) compared to other substrates herein. Thermodynamic flat band potential measurements in the dark reveal that Si|BisPNP-Ni also exhibits the most positive VFB value (-0.02 V vs Fc/Fc+) by a wide margin. Electrochemical impedance spectroscopy data generated under illuminated, catalytic conditions demonstrate a surprising lack of correlation evident between Von and equivalent circuit element parameters commonly associated with HER. Overall, the resulting paradigm comprises a system wherein the extent of band bending induced by metal ion binding is the primary driver of photoelectrochemical (PEC)-HER benefits, while the kinetic (catalytic) effects of the PNP-Ni(II) are minimal. This suggests that dipole and band-edge engineering must be a primary design consideration (not secondary to catalyst) in semiconductor|catalyst hybrids for PEC-HER.

Catalytic and Photochemical Strategies to Stabilized Radicals Based on Anomeric Nucleophiles.

Carbohydrates, one of the three primary macromolecules of living organisms, play significant roles in various biological processes such as intercellular communication, cell recognition, and immune activity. While the majority of established methods for the installation of carbohydrates through the anomeric carbon rely on nucleophilic displacement, anomeric radicals represent an attractive alternative because of their functional group compatibility and high anomeric selectivities. Herein, we demonstrate that anomeric nucleophiles such as C1 stannanes can be converted into anomeric radicals by merging Cu(I) catalysis with blue light irradiation to achieve highly stereoselective C(sp3)-S cross-coupling reactions. Mechanistic studies and DFT calculations revealed that the C-S bond-forming step occurs via the transfer of the anomeric radical directly to a sulfur electrophile bound to Cu(II) species. This pathway complements a radical chain observed for photochemical metal-free conditions where a disulfide initiator can be activated by a Lewis base additive. Both strategies utilize anomeric nucleophiles as efficient radical donors and achieve a switch from an ionic to a radical pathway. Taken together, the stability of glycosyl nucleophiles, a broad substrate scope, and high anomeric selectivities observed for the thermal and photochemical protocols make this novel C-S cross coupling a practical tool for late-stage glycodiversification of bioactive natural products and drug candidates.

A Dinuclear Ruthenium(II) Complex Excited by Near-Infrared Light through Two-Photon Absorption Induces Phototoxicity Deep within Hypoxic Regions of Melanoma Cancer Spheroids.

The dinuclear photo-oxidizing RuII complex [{Ru(TAP2)}2(tpphz)]4+ (TAP = 1,4,5,8- tetraazaphenanthrene, tpphz = tetrapyrido[3,2-a:2',3'-c:3″,2''-h:2‴,3'''-j]phenazine), 14+, is readily taken up by live cells localizing in mitochondria and nuclei. In this study, the two-photon absorption cross section of 14+ is quantified and its use as a two-photon absorbing phototherapeutic is reported. It was confirmed that the complex is readily photoexcited using near-infrared, NIR, and light through two-photon absorption, TPA. In 2-D cell cultures, irradiation with NIR light at low power results in precisely focused phototoxicity effects in which human melanoma cells were killed after 5 min of light exposure. Similar experiments were then carried out in human cancer spheroids that provide a realistic tumor model for the development of therapeutics and phototherapeutics. Using the characteristic emission of the complex as a probe, its uptake into 280 µm spheroids was investigated and confirmed that the spheroid takes up the complex. Notably TPA excitation results in more intense luminescence being observed throughout the depth of the spheroids, although emission intensity still drops off toward the necrotic core. As 14+ can directly photo-oxidize DNA without the mediation of singlet oxygen or other reactive oxygen species, phototoxicity within the deeper, hypoxic layers of the spheroids was also investigated. To quantify the penetration of these phototoxic effects, 14+ was photoexcited through TPA at a power of 60 mW, which was progressively focused in 10 µm steps throughout the entire z-axis of individual spheroids. These experiments revealed that, in irradiated spheroids treated with 14+, acute and rapid photoinduced cell death was observed throughout their depth, including the hypoxic region.

Fluorescent metal-based complexes as cancer probes.

The ability to track drugs inside of cells and tumours has been highly valuable in cancer research and diagnosis. Metal complexes add attractive features to fluorescent drugs, such as targeting and specificity, solubility and uptake or photophysical properties. This review focuses on the latest fluorescent metal-based complexes, their cellular targets, photophysical properties and possible anticancer effects.

Synthetic Strategies for Trapping the Elusive trans-Dirhodium(II,II) Formamidinate Isomer: Effects of Cis versus Trans Geometry on the Photophysical Properties.

The cis- and trans-dirhodium(II,II) complexes cis-[Rh2(µ-DTolF)2(µ-np)(MeCN)4][BF4]2 (1; DTolF = N,N'-di-p-tolylformamidinate and np = 1,8-naphthyridine), cis- and trans-[Rh2(µ-DTolF)2(µ-qxnp)(MeCN)3][BF4]2 [2 and 3, respectively, where qxnp = 2-(1,8-naphthyridin-2-yl)quinoxaline], and trans-[Rh2(µ-DTolF)2(µ-qxnp)2][BF4]2 (4) were synthesized and characterized. A new synthetic methodology was developed that consists of the sequential addition of π-accepting axially blocking ligands to favor formation of the first example of a bis-substituted formamidinate-bearing trans product. Isolation of the intermediates 2 and 3 provides insight into the mechanistic requirements for obtaining 4 and the cis analogue, cis-[Rh2(µ-DTolF)2(µ-qxnp)2][BF4]2 (5). Density functional theory calculations provide support for the synthetic mechanism and proposed intermediates. The metal/ligand-to-ligand charge-transfer (ML-LCT) absorption maximum of the trans complex 4 at 832 nm is red-shifted by 1173 cm-1 and exhibits shorter lifetimes of the 1ML-LCT and 3ML-LCT excited states, 3 ps and 0.40 ns, respectively, compared to those of the cis analogue 5. The shorter excited-state lifetimes of 4 are attributed to the longer Rh-Rh bond of 2.4942(8) Å relative to that in 5, 2.4498(2) Å. A longer metal-metal bond reflects a decreased overlap of the Rh atoms, which leads to more accessible metal-centered excited states for radiationless deactivation. The 3ML-LCT excited states of 4 and 5 undergo reversible bimolecular charge transfer with the electron donor p-phenylenediamine when irradiated with low-energy light. These results indicate that trans isomers are a source of unexplored tunability for potential p-type semiconductor applications and, given their distinct geometric arrangement, constitute useful building blocks for supramolecular architectures with potentially interesting photophysical properties.

Redox-Active Bis-Cyclometalated Iridium(III) Complex as a DNA Photo-Cleaving Agent.

The development of new photoactive metal complexes that can trigger oxidative damages to the genetic material is of great interest. In the present paper, we describe the detailed study of a highly photo-oxidant iridium(III) complex that triggers photoinduced electron transfer (PET) with purine DNA bases. The PET has been studied by luminescence and laser flash photolysis experiments. From plasmid DNA agarose gel electrophoresis experiments, we demonstrated the high ability of the iridium complex to induce strand breaks upon light irradiation. Reactive oxygen species (ROS)-specific scavengers and stabilizers were employed to identify that the photocleavage process, the results of which infer singlet oxygen and hydrogen peroxide as the predominant species. To the best of our knowledge, the present work represents one of the few study for highly photo-oxidant bis-cyclometalated iridium(III) complex toward DNA.

Bis-tridentate N-Heterocyclic Carbene Ru(II) Complexes are Promising New Agents for Photodynamic Therapy.

Ruthenium(II) complexes developed for photodynamic therapy (PDT) are almost exclusively tris-bidentate systems with C2 or D3 symmetry. This is due to the fact that this structural framework commonly produces long-lived excited states, which, in turn, allow for the generation of large amounts of singlet oxygen (1O2) and other reactive oxygen species. Complexes containing tridentate ligands would be advantageous for biological applications as they are generally achiral (D2d or C2v symmetry), which eliminates the possibility of multiple isomers which could exhibit potentially different interactions with chiral biological entities. However, Ru(II) complexes containing tridentate ligands are rarely studied as candidates for photobiological applications, such as PDT, since they almost exclusively exhibit low quantum yields and very short excited-state lifetimes and, thus, are not capable of generating sufficient 1O2 or engaging in electron transfer reactions. Here, we report a proof-of-concept approach to make bis-tridentate Ru(II) complexes useful for PDT applications by altering their photophysical properties through the inclusion of N-heterocyclic carbene (NHC) ligands. Three NHC and two terpyridine ligands were studied to evaluate the effects of structural and photophysical modulations of bis-substituted Ru(II) complexes. The NHC complexes were found to have superior excited-state lifetimes, 1O2 production, and photocytotoxicity. To the best of our knowledge, these complexes are the most potent light-activated bis-tridentate complexes reported.

Photofunctional Cyclometalated Iridium(III) Polypyridine Complexes Bearing a Perfluorobiphenyl Moiety for Bioconjugation, Bioimaging, and Phototherapeutic Applications.

In this article, we report the design, synthesis, and characterization of a series of cyclometalated iridium(III) polypyridine complexes containing a perfluorobiphenyl (PFBP) moiety [Ir(N^C)2(bpy-PFBP)](PF6) (bpy-PFBP = 4-(S-(perfluoro-(1,1'-biphenyl)-4-yl)-N-mercaptoethylaminocarbonyloxymethyl)-4'-methyl-2,2'-bipyridine; HN^C = 2-phenylpyridine (Hppy) (1a), 2-(4-hydroxymethylphenyl)pyridine (Hppy-CH2OH) (2a), 2-((1,1'-biphenyl)-4-yl)pyridine (Hpppy) (3a), 2-((4'-hydroxymethyl-1,1'-biphenyl)-4-yl)pyridine (Hpppy-CH2OH) (4a), 2-phenylquinoline (Hpq) (5a), 2-(4-hydroxymethylphenyl)quinoline (Hpq-CH2OH) (6a)). Their PFBP-free counterparts [Ir(N^C)2(bpy-C4)](PF6) (bpy-C4 = 4-(N-n-butylaminocarbonyloxymethyl)-4'-methyl-2,2'-bipyridine; HN^C = Hppy (1b), Hppy-CH2OH (2b), Hpppy (3b), Hpppy-CH2OH (4b), Hpq (5b), Hpq-CH2OH (6b)) were also prepared for comparison studies. Upon irradiation, all the complexes displayed intense and long-lived greenish-yellow to orange luminescence in solutions under ambient conditions and in low-temperature alcohol glass. Reactions of the PFBP complexes with peptides containing the FCPF sequence via the π-clamp-mediated cysteine conjugation afforded luminescent peptide conjugates that exhibited rich photophysical properties. Using complex 3a as an example, we demonstrated that the conjugation of complexes to organelle-targeting peptides is an effective means to modulate their intracellular localization behavior, which was further shown to be important to their performance in photodynamic therapy. The results of this work will contribute to the development of photofunctional transition metal complexes as theranostic agents.

Cyclometalated Iridium(III) Complexes as High-Sensitivity Two-Photon Excited Mitochondria Dyes and Near-Infrared Photodynamic Therapy Agents.

Photodynamic therapy (PDT) using two-photon near-infrared light excitation is a very effective way to avoid the use of short-wavelength ultraviolet or visible light which cannot efficiently penetrate into the biological tissues and is harmful to the healthy cells. Herein, a series of cyclometalated Ir(III) complexes with a structurally simple diimine ligand were designed and the synthetic route and preparation procedure were optimized, so that the complexes could be obtained in apparently higher yield, productivity, and efficiency in comparison to the traditional methods. Their ground state and excited singlet and triplet state properties were studied by spectroscopy and quantum chemistry theoretical calculations to investigate the effect of substituent groups on the photophysical properties of the complexes. The Ir(III) complexes, especially Ir1 and Ir3, showed very low dark toxicities and high phototoxicities under both one-photon and two-photon excitation, indicating their great potential as PDT agents. They were also found to be highly sensitive two-photon mitochondria dyes.

Luminescent Re(I)/Au(I) Species As Selective Anticancer Agents for HeLa Cells.

A series of neutral and cationic heterotrimetallic complexes of the type fac-[Re(CO)3(bipy(CC)2-(AuL)2)X]n, where bipy(CC)2 is 4,4'-alkynyl-2,2'-bipyridine; L is either triphenylphosphine (PPh3), [1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene] (IPr), or tert-butyl isocyanide (CNtBu); and X is a chloride (n = 0) or acetonitrile (n = 1), were synthesized and characterized together with their Re(I) precursors, i.e., fac-[Re(CO)3(bipy(CC)2)X]n. X-ray diffraction of complexes 1, 3, and 6 corroborated the expected octahedral and linear distribution of the ligands along the Re(I) and Au(I) centers, respectively. Luminescent studies showed that all the complexes displayed a broad emission band centered between 565 and 680 nm, corresponding to a 3MLCT from the Re(I) to the diimine derivative. The presence of the gold fragment coordinated to the diimine ligand shifted in all cases the emission maxima toward higher energies. Such an emission difference could be potentially used for assessing the precise moment of interaction of the probe with the biological target if the gold fragment is implicated. Antiproliferative studies in cancer cells, A549 (lung cancer) and HeLa (cervix cancer), showed a generalized selectivity toward HeLa cells for those heterotrimetallic species incubated at longer times (72 vs 24 h). ICP-MS spectrometry revealed the greater cell internalization of cationic vs neutral species. Preliminary fluorescence microscopy experiments showed a different behavior of the complexes in HeLa and A549 cell lines. Whereas the complexes in A549 were randomly distributed in the outside of the cell, those incubated with HeLa cells were located close to the cellular membrane, suggesting some type of interaction, and possibly explaining their cellular selectivity when it comes to the antiproliferative activity displayed in the different cell lines.