Transition metal complexes as photosensitisers in one- and two-photon photodynamic therapy

Photophysics and photochemistry of a prospective light-activated anticancer dirhodium complex

Dirhodium complexes Rh(II,II) with organic ligands are known to demonstrate light-induced cytotoxicity, combining both oxygen-dependent (like in the case of photodynamic therapy, PDT) and oxygen-independent (like in the case of photo-activated chemotherapy, PACT) anticancer activity. Photophysics and photochemistry of the previously reported light-activated anticancer dirhodium complex cis-[Rh2(μ-O2CCH3)2(dppn)2](O2CCH3)2 (dppn = benzo[i]dipyrido[3,2-a:2',3'-c]phenazine) were studied by means of several stationary (UV spectroscopy and capillary electrophoresis) and time-resolved (laser flash photolysis, ultrafast TA spectroscopy and direct singlet oxygen detection) methods. The only observed photochemical reaction is photoaquation with the release of an acetate ligand; its quantum yield is low (0.026%). The quantum yields of singlet oxygen formation in different solvents (D2O, CH3CN, and CD3OD) lie in the range (2-8)%, which is consistent with the presumably oxygen-independent pathway of light-induced cell-damage reported in the literature. Complicated behavior of intermediate absorption in the time interval from 100 fs to 20 μs was interpreted by "ladder" transitions between excited states of different nature, 1IL* → 3IL* → 3MLCT* → 3MC*. The lifetime of the longest-living excited state 3MC* is 6.2 μs. Estimations show that the oxygen-independent light-induced cytotoxic effect could be provided even without preliminary binding of the complex to the substrate.

Structural Features and Photophysical and Antiproliferative Properties of Me2N-pbt-Cycloplatinated Complexes with Picolinate Ligands

2-(4-dimethylaminophenyl)benzothiazolate (Me2N-pbt)-cyclometalated platinum complexes containing four different picolinate ligands, [Pt(Me2N-pbt)(R-pic-κN∧O)] (R = 3-H 1, 3-NH2 2, 3-OH 3, 4-COOH 4) were prepared and examined for their photophysical properties, singlet oxygen production, and bioactivity. X-ray studies of 1 and 3·CHCl3 revealed aggregation to give 1D infinite chains. They showed phosphorescent emissions essentially associated with metal-perturbed 3ILCT excited states in 1-3 and mixed 3LL'CT/3ILCT (L = Me2N-pbt, L' = pic) in 4, in agreement with theoretical calculations. Their tendency to self-assemble was demonstrated in films (1, 3) and DMSO/H2O (3). Complexes 1-3 showed singlet oxygen photosensitization quantum yields (ϕΔ 1O2) in the range of 13-17%. The in vitro biological activity toward selected cell lines in dark conditions and under 5 min irradiation at 450 nm was tested. Complex 3 showed the highest phototoxicity with up to 10 times improvement of the antiproliferative activity upon irradiation, with EC50 values in the nanomolar range, related to overproduction of ROS in dark conditions, further enhanced upon irradiation. Complexes 1-4 did not bind to DNA, while the most potent complex 3 demonstrated interaction with BSA and photooxidation of NADH. Finally, intracellular dose-dependent ROS production in MDA-MB-231 cells treated with 3 was observed in the dark and further stimulated upon blue light irradiation.

Molecular Rectangles Featuring Two Parallel NCN-Coordinated Platinum Units: Enhancing Near-Infrared Emission Through Excimer Formation

New macrocyclic molecules are described that incorporate Pt(NCN) units on opposite edges of a rectangular structure, with xanthene units constituting the other two sides. Here, NCN represents a cyclometallating tridentate ligand based on 2,6-di(2-pyridyl)benzene or its pyrimidine analog. The complexes display strong photoluminescence peaking in the near-infrared region of the spectrum in solution (λmax up to 761 nm). Photophysical data and DFT calculations indicate that the emission arises from "intramolecular excimers"-triplet excited states that form when the two Pt(NCN) units within the molecule are brought into close proximity to interact interfacially. In doped polymers, the necessary molecular distortion is inhibited, but related excited states that emit in a similar region can still form through intermolecular interactions.

Photodynamic Effectiveness of Copper-Iminopyridine Photosensitizers Coupled to Zinc Oxide Nanoparticles Against Klebsiella pneumoniae and the Bacterial Response to Oxidative Stress

One of the most urgent threats to public health worldwide is the ongoing rise of multidrug-resistant (MDR) bacterial strains. Among the most critical pathogens are MDR-Klebsiella pneumoniae strains. The lack of new antibiotics has led to an increased need for non-antibiotic antimicrobial therapies. Photodynamic therapy (PDT) has become increasingly significant in treating MDR bacteria. PDT uses photosensitizer compounds (PS) that generate reactive oxygen species (ROS) when activated by light. These ROS produce localized oxidative stress, damaging the bacterial envelope. A downside of PDT is the limited bioavailability of PSs in vivo, which can be enhanced by conjugating them with carriers like nanoparticles (NPs). Zinc nanoparticles possess antibacterial properties, decreasing the adherence and viability of microorganisms on surfaces. The additive or synergistic effect of the combined NP-PS could improve phototherapeutic action. Therefore, this study evaluated the effectiveness of the copper(I)-based PS CuC1 compound in combination with Zinc Oxide NP, ZnONP, to inhibit the growth of both MDR and sensitive K. pneumoniae strains. The reduction in bacterial viability after exposure to a PS/NP mixture activated by 61.2 J/cm2 of blue light photodynamic treatment was assessed. The optimal PS/NP ratio was determined at 2 µg/mL of CuC1 combined with 64 µg/mL of ZnONP as the minimum effective concentration (MEC). The bacterial gene response aligned with a mechanism of photooxidative stress induced by the treatment, which damages the bacterial cell envelope. Additionally, we found that the PS/NP mixture is not harmful to mammalian cells, such as Hep-G2 and HEK-293. In conclusion, the CuC1/ZnONP combination could effectively aid in enhancing the antimicrobial treatment of infections caused by MDR bacteria.

Monomer Versus Dimer of Cationic Ir(III) Complexes for Photodynamic Therapy by Two-Photon Activation: A Comparative Study

Iridium(III) complexes have been recognized as promising candidates for two-photon sensitized photodynamic therapy (PDT). In this context, we report on the study of two complexes: a monomer (IrL1) and a dimer (Ir2L2). Both complexes possess 2-phenylpyridine cyclometallating ligands and a pyridylbenzimidazole derivative as an ancillary ligand. In the dimer, the two Ir(III) centers are connected by a non-conjugated bridged bis(pyridylbenzimidazole). We compare the photophysical properties of these complexes. Both display phosphorescent emission in the orange-red part of the visible spectrum, with emissions centered at 610 nm for IrL1 and 625 nm for Ir2L2, both exhibiting quantum yields of ∼24%. However, Ir2L2 proves to be much brighter than the monomer, making the dimer four times brighter than IrL1. This trend is consistent under two-photon excitation (TPE), and the singlet oxygen generation quantum yields, with the dimer displaying a figure of merit (σTPA × ΦΔ) of 40, compared to only 5 for the monomer. Both complexes generate intracellular ROS and exhibit strong phototoxicity upon blue light activation (λ = 420 nm), achieving submicromolar IC50 values in HT29 and A549 cell lines after 24 h of incubation. Moreover, with TPE (λ = 800 nm), both complexes also generate intracellular ROS and induce cancer cell death.

Functionalization of a Ru(II) polypyridine complex with an aldehyde group as a synthetic precursor for photodynamic therapy

Photodynamic therapy has garnered significant attention over the past decades for its potential in treating various types of cancer, as well as bacterial, fungal, and viral infections. However, current clinically approved photosensitizers based on a tetrapyrrolic scaffold face notable limitations, including low water solubility, slow body clearance, and photobleaching. As a promising alternative, Ru(II) polypyridyl complexes have emerged due to their favorable photophysical and biological properties (i.e., reactive oxygen species generation, high water solubility, and biocompatibility). Despite these attractive properties, the vast majority of compounds are associated with poor tumor accumulation, representing a major hurdle for therapeutic applications. To overcome this limitation, herein, the chemical synthesis and photophysical evaluation of the functionalization of a Ru(II) polypyridyl complex with an aldehyde group, as a synthetic precursor for further conjugation, is reported. To ensure that the intrinsic chemical reactivity of the aldehyde group remains unaffected by the coordination environment to the metal center, a phenyl spacer was strategically introduced between the central ligand framework and the aldehyde functionality. Computational studies indicated that upon excitation of the metal complex, an excited state electron from the ruthenium t2g orbital is transferred to the π* ligand orbital in a metal-to-ligand charge transfer transition. The compound was found to be highly stable under physiological conditions as well as upon irradiation. Upon light exposure, the metal complex was found to efficiently convert molecular oxygen to singlet oxygen. These findings highlight the potential of the aldehyde functionalized Ru(II) polypyridyl complex as a versatile precursor for photodynamic therapy.

Luminescent Iridium-Peptide Nucleic Acid Bioconjugate as Photosensitizer for Singlet Oxygen Production toward a Potential Dual Therapeutic Agent

A novel bioorganometallic PNA conjugate (Ir-PNA) was synthesized by covalently bonding a model PNA tetramer to a luminescent bis-cyclometalated Ir(III) complex that acted as a photosensitizer under light irradiation to generate singlet oxygen (1O2). The conjugate was prepared using an Ir complex bearing the 1,10-phenanthroline ligand functionalized with either a free primary amine (Ir-NH2) or a carboxyl group (Ir-COOH) for the conjugation to PNA. The photophysical studies on the Ir-COOH and the Ir-PNA demonstrated that the luminescent properties were maintained after the conjugation of the Ir fragment to PNA. Furthermore, the abilities to produce 1O2 of Ir-COOH and Ir-PNA were confirmed in a cuvette under visible light irradiation employing 1,5-dihydroxynaphthalene as a reporter, and the measured singlet oxygen quantum yield (ΦΔ) supported the Ir-PNA conjugate efficacy as a photosensitizer (ΦΔ = 0.54). Two-photon absorption microscopy on HeLa cells revealed that Ir-PNA localized in both the cytosol and nucleus, suggesting its potential as an intracellular carrier for PNA. Cytotoxicity assays by MTT tests showed that Ir-PNA was nontoxic in the absence of light, but induced cell death (EC50 = 18 μM) after UV irradiation. Overall, the Ir-PNA conjugate represents a promising system for the intracellular delivery of the PNA and its application in PDT.

Advancing super-resolution microscopy with metal complexes: functional imaging agents for nanoscale visualization

Super-resolution microscopy (SRM) has transformed biological imaging by overcoming the diffraction limit, offering nanoscale visualization of cellular structures and processes. However, the widespread use of organic fluorescent probes is often hindered by limitations such as photobleaching, short photostability, and inadequate performance in deep-tissue imaging. Metal complexes, with their superior photophysical properties, including exceptional photostability, tuneable luminescence, and extended excited-state lifetimes, address these challenges, enabling precise subcellular targeting and long-term imaging. Beyond imaging, their theranostic potential unlocks real-time diagnostics and treatments for diseases such as cancer and bacterial infections. This review explores recent advancements in applying metal complexes for SRM, focusing on their utility in visualizing intricate subcellular structures, capturing temporal dynamics in live cells and elucidating in vivo spatial organization. We emphasize how rational design strategies optimize biocompatibility, organelle specificity, and deep-tissue penetration, expanding their applicability in multiplexed imaging. Furthermore, we discuss the design of various metal nanoparticles (NPs) for SRM, along with emerging hybrid nanoscale probes that integrate metal complexes with gold (Au) scaffolds, offering promising avenues for overcoming current limitations. By highlighting both established successes and potential frontiers, this review provides a roadmap for leveraging metal complexes as versatile tools in advancing SRM applications.

Effect of the Substitution of the Mesityl Group with Other Bulky Substituents on the Luminescence Performance of [Pt(1,3-bis(4-Mesityl-pyridin-2-yl)-4,6-difluoro-benzene)Cl]

The synthesis and characterization of two new complexes, namely [Pt(bis(4-(4-(tert-butyl)phenyl)-pyridin-2-yl)-4,6-difluorobenzene)Cl] and [Pt(bis(4-(3,5-di-tert-butylphenyl)-pyridin-2-yl)-4,6-difluorobenzene)Cl], are reported. Both are highly luminescent in the blue region (Φlum = 0.89-0.95 at 478-480 nm), like the parent complex [Pt(1,3-bis(4-mesityl-pyridin-2-yl)-4,6-difluoro-benzene)Cl] in degassed diluted dichloromethane solution. An increase in concentration leads to the formation of bi-molecular emissive excited states, as evidenced by a growing structureless band that peaked at 690-697 nm. This formation is more facile for the complex with one tert-butyl group in para of the phenyl group. It appears that the introduction of two tert-butyls in positions 3 and 5 of the phenyl group hampers the neighboring of the monomeric species, although less efficiently than the introduction of methyls in positions 2 and 6.

A red light-activable hetero-bimetallic [Fe(iii)-Ru(ii)] complex as a dual-modality PDT tool for anticancer therapy

We developed a novel red light activable hetero-bimetallic [Fe(III)-Ru(II)] complex by combining hydroxyl radical-generating Fe(III)-catecholate as a type I PDT agent and the singlet oxygen generating Ru(II)-paracymene complex as a type II PDT agent and it potentially functions as a dual-modality PDT tool for enhanced phototherapeutic applications. 2-Amino-3-(3,4-dihydroxyphenyl)-N-(1,10-phenanthrolin-5-yl)propenamide (L2) acted as a bridging linker. The single-pot synthesis of the hetero-bimetallic [Fe(III)-Ru(II)] complex was carried out through acid-amine coupling. Various photophysical assays confirmed the photo-activated production of (˙OH) radicals and (1O2) oxygen generation upon activation of the [Fe(III)-Ru(II)] complex with red light (600-720 nm, 30 J cm-2), which resulted in enhanced cytotoxicity with a photo-index of ∼45. The complex, [Fe(III)-Ru(II)], potentially bonded to the DNA through the ruthenium moiety was responsible for minimal dark toxicity. The cytotoxic potential of the complex under red light was a result of the photo-induced accumulation of reactive oxygen species through both type I and type II photodynamic therapy (PDT) mechanisms in A549 and HeLa cells, while non-cancerous HPL1D cells remained unaffected. We probed the caspase 3/7-dependent apoptosis of the complex, [Fe(III)-Ru(II)], in vitro. Overall, the hetero-bimetallic [Fe(III)-Ru(II)] complex is an ideal example of a red light activable dual-modality next-generation PDT tool for phototherapeutic anticancer therapy.

A comprehensive review on the development of chiral Cu, Ni, and Zn complexes as pharmaceutical agents over the past decades: Synthesis, molecular structure and biological activity

Chirality is a fundamental and widespread geometric structural property in living organisms that most biomacromolecules including nucleic acids, proteins and enzymes, possess. Consequently, the development of chiral drugs capable of binding specific targets have gradually gained wide attention in recent decades due to their selective effects on a broad spectrum of biological events ranging from cell metabolism to cell fate. In this context, the synthesis of chiral compounds as promising therapeutic candidates has assumed a major role in drug discovery. Among them, chiral metal complexes have attracted considerable interest due to their unique and intriguing structural features that could enable overcoming side effects and drug-resistance phenomena of metal-based drugs currently in the market such as cisplatin. In the current scenario, an in-depth overview of non-platinum chiral complexes needs to be presented and carried forward. Therefore, in this perspective article, an update of the scientific development of bioactive chiral copper, zinc and nickel complexes have been reported since they have not been thoroughly reviewed so far. Specifically, we focused the article mainly on metal complexes containing chiral ligands (type 2 chirality) as in literature they are more numerous than those with chirality at the metal center (type 1 chirality). Herein, not only their biological activity but also their mechanism of action is summarized. Furthermore, in the final section of the article we have highlighted copper-based complexes as those with a superior biological activity profile and greater prospects for development as a drug.

Advances in the Design of Photoactivatable Metallodrugs: Excited State Metallomics

Photoactivatable metal complexes offer the prospect of novel drugs with low side effects and new mechanisms of action to combat resistance to current therapy. We highlight recent progress in the design of platinum, ruthenium, iridium, gold and other transition metal complexes, especially for applications as anticancer and anti-infective agents. In particular, understanding excited state chemistry related to identification of the bioactive species (excited state metallomics/pharmacophores) is important. Photoactivatable metallodrugs are classified here as photocatalysts, photorelease agents and ligand-activated agents. Their activation wavelengths, cellular mechanisms of action, experimental and theoretical metallomics of excited states and photoproducts are discussed to explore new strategies for the design and investigation of photoactivatable metallodrugs. These photoactivatable metallodrugs have potential in clinical applications of Photodynamic Therapy (PDT), Photoactivated Chemotherapy (PACT) and Photothermal Therapy (PTT).

Push-pull effect - how to effectively control photoinduced intramolecular charge transfer processes in rhenium(I) chromophores with ligands of D-A or D-π-A structure

Over the last five decades, diimine rhenium(I) tricarbonyl complexes have been extensively investigated due to their remarkable and widely tuned photophysical properties. These systems are regarded as attractive targets for design functional luminescent materials and performing fundamental studies of photoinduced processes in transition metal complexes. This review summarizes the latest developments concerning Re(I) tricarbonyl complexes bearing donor-acceptor (D-A) and donor-π-acceptor (D-π-A) ligands. Such compounds can be treated as bichromophoric systems with two close-lying excited states, metal-to-ligand charge transfer (MLCT) and intraligand-charge-transfer (ILCT). A role of ILCT transitions in controlling photobehaviour was discussed for Re(I) tricarbonyls with six different diimine cores decorated by various electron-rich amine, sulphur-based and π-conjugated aryl groups. It was evidenced that this approach is an effective tool for enhancement of the visible absorptivity, bathochromic emission shift and significant prolongation of the excited-state, opening up new possibilities in the development of more efficient materials and expand the range of their applications.

Transition Metal Complex-Loaded Nanosystems: Advances in Stimuli-Responsive Cancer Therapies

Transition metal complex-loaded nanosystems (TMCNs) represent a cutting-edge platform for stimuli (light, ultrasound)-responsive cancer therapies. These nanosystems, incorporating metals such as manganese(II), zinc(II), ruthenium(II), rhenium(I), iridium(III), and platinum(IV), significantly enhance the efficacy of light-activated therapies, including photodynamic therapy (PDT) and photothermal therapy (PTT), as well as ultrasound-activated treatments like sonodynamic therapy (SDT). TMCNs based on ruthenium(II), rhenium(I), and iridium(III) improve PDT, while manganese(II) and iridium(III) demonstrate exceptional sonosensitizing properties. In PTT, ruthenium(II) and iridium(III)-based TMCNs efficiently absorb light and generate heat. Emerging synergistic approaches that combine SDT, PTT, PDT, chemotherapy, and immunotherapy are demonstrated to be powerful strategies for precision cancer treatment. Zinc(II), ruthenium(II), iridium(III), and platinum(IV)-based TMCNs play a critical role in optimizing these therapies, enhancing tumor targeting, and reducing side effects. Furthermore, TMCNs can amplify immunotherapy by inducing immunogenic cell death, thus strengthening the immune response. These advances address key challenges such as tumor hypoxia and therapeutic resistance, opening new possibilities for innovative photosensitizer-based cancer treatments. This review highlights the latest progress in TMCNs design and applications, demonstrating their potential to revolutionize stimuli-responsive cancer therapies.

Metal (M = Cr, Mo, W, Re) carbonyl complexes with porphyrin and carborane isocyanide ligands: light-induced oxidation and carbon oxide release for antitumor efficacy

The tetrapyrrolic macrocycle as a scaffold for various chemical modifications provides broad opportunities for the preparation of complex multifunctional conjugates suitable for binary antitumor therapies. Typically, illumination with monochromatic light triggers the photochemical generation of reactive oxygen species (ROS) (photodynamic effect). However, more therapeutically valuable effects can be achieved upon photoactivation of tetrapyrrole derivatives. Herein we report the novel porphyrin-based complexes of transition metals with isocyanide and carbonyl ligands. Synthesis of complexes presumed the use of 5-(p-isocyanophenyl)-10,15,20-triphenylporphyrin as a ligand in reactions with metal carbonyl complexes, M(CO)6 (M = Cr, Mo, W), Re2(CO)10 and Re(CO)5Cl. Based on these complexes and isocyanocarborane, the heteroleptic carbonyl complexes with porphyrin and carborane isocyanide ligands were prepared. In cell-free systems, the new compounds retained photochemical characteristics of the parental porphyrin derivative, such as triplet state formation and ROS generation, upon light-induced activation. In the cell culture, the carborane-containing derivatives demonstrated a more pronounced intracellular accumulation than their nonboronated counterparts. As expected, illumination at the Soret band (405 nm) of cells loaded with the new complexes caused photodynamic cell damage. In contrast, illumination at 530 nm instead initiated the release of carbon oxide (CO) followed by cell death independently of the photodynamic effect. Light-induced CO release was analyzed using second derivatives of UV-Vis spectra and our originally developed Spectrophotometric elimiNAtion of Photoinduced Side reactions (SNAPS) method. The yield of CO release decreased in the raw depending on metals in the carbonyl moiety: Mo ≥ Cr > W > Re ≥ Re2. Overall, our novel metal carbonyl complexes with porphyrin and carborane isocyanide ligands emerge as potent bi-functional conjugates for combined photodynamic and photoinducible CO-releasing antitumor agents.

Electrostatic Force-Enabled Microneedle Patches that Exploit Photoredox Catalysis for Transdermal Phototherapy

Microneedle patches for topical administration of photodynamic therapy (PDT) sensitizers are attractive owing to their safety, selectivity, and noninvasiveness. However, low-efficiency photosensitizer delivery coupled with the limitations of the hypoxic tumor microenvironment remains challenging. To overcome these issues, we developed an effective microneedle patch based on intermolecular electrostatic interactions within a photosensitizer matrix containing a zinc-containing porphyrin analogue, ZnBP (w). This design improved the mechanical strength of the microneedle patch and enhanced the photosensitizer loading efficiency in aqueous environments. A key feature of the system is efficient electron transfer between ZnBP (w) and NADH upon photoirradiation. Electrostatic interactions between ZnBP (w) and NADH were hypothesized to support initial binding and subsequent photoinduced electron transfer, disrupting NADH/NAD+ homeostasis and inducing tumor cell death. The developed microneedle patch demonstrated an antiangiogenesis effect in a vascular malformation model and an antitumor effect in a melanoma mouse model after transdermal administration. This study highlights the benefits of electrostatic interactions in designing microneedle PDT patches and their clinical potential, particularly in reducing systemic phototoxicity.

H2O2-activated mitochondria-targeting photosensitizer for fluorescence imaging-guided combination photodynamic and radiotherapy

Radiotherapy is a primary modality in cancer treatment but is accompanied by severe side effects to healthy tissues and radiation resistance to some extent. To overcome these limitations, we developed a H2O2-responsive photosensitizer, CyBT, which could be activated by the upregulated H2O2 induced by radiotherapy, enabling near-infrared fluorescence imaging-guided combination photodynamic and radiotherapy. The synthesis of CyBT began with the covalent linkage of hemicyanine and a free radical TEMPO through the click reaction, which demonstrated superior photodynamic properties. Shielding of fluorescence and photodynamic activity was achieved by incorporating phenylboronic acid pinacol ester. In X-ray irradiated tumor cells, the upregulation of H2O2 activated CyBT, thereby restoring its fluorescence and photodynamic activity. Additionally, the positive charge of CyBT facilitated its targeting to the mitochondria within tumor cells for more efficiently triggering cell apoptosis. CyBT was co-assembled with a polymer PEG-b-PDPA to form acid-responsive nanoparticles (NPs-CyBT). This formulation enhanced tumor targeting, improved water solubility of CyBT, and extended in vivo circulation time. Utilizing fluorescence imaging to guide photodynamic and radiotherapy, NPs-CyBT can accurately target solid tumors in mice, and lead to tumor elimination, suggesting that it is a potential strategy for the effective treatment of malignant tumors.

Tunability of triplet excited states and photophysical behaviour of bis-cyclometalated iridium(III) complexes with imidazo[4,5-f][1,10]phenanthroline

This is a comprehensive study of the photophysical behaviour of heteroleptic iridium(III) complexes with imidazo[4,5-f][1,10]phenanthroline (imphen) as an ancillary ligand, represented by the general formula [Ir(N∩C)2(imphen)]PF6. As cyclometalating ligands, 2-phenylpyridine (Hppy), 2-phenylquinoline (Hpquin), 2-phenylbenzothiazole (Hpbztz), and 2-(2-pyridyl)benzothiophene (pybzthH) were used. The impact of structural modifications of cyclometalating ligands was widely explored by a combination of steady-state and time-resolved optical techniques accompanied by theoretical calculations. We evidenced that the cyclometalating ligands induce essential changes in the nature of the emissive excited state and the emission characteristics of [Ir(N∩C)2(imphen)]PF6. While the complex [Ir(ppy)2(imphen)]PF6 (1) is a typical 3MLLCT emitter, the lowest triplet states of [Ir(pquin)2(imphen)]PF6 (2), [Ir(pbztz)2(imphen)]PF6 (3) and [Ir(pybzth)2(imphen)]PF6 (4) have a predominant 3LCN∩C character. The phosphorescence colour of the investigated Ir(III) complexes changes from greenish-yellow to red, their quantum yields vary from 56 to 2%, and their triplet excited-state lifetimes fall in the 743-3840 ns range. The highest photoluminescence quantum yield was revealed for 2 in CH2Cl2, while complex 3 in MeCN shows the most pronounced increase in the lifetime. Both complexes 2 and 3 show an increased efficiency of singlet oxygen generation. The herein discussed structure-property relationships are of high significance for controlling photoinduced processes in heteroleptic iridium(III) complexes with the imphen-based ancillary ligand, and making further progress in effectively tuning the emission energies, quantum yields and excited-state lifetimes of these systems by structural modifications of cyclometalating ligands, especially the π-conjugation, the position of the N-donor and the presence of sulfur heteroatoms.

Leveraging the Photofunctions of Transition Metal Complexes for the Design of Innovative Phototherapeutics

Despite the advent of various medical interventions for cancer treatment, the disease continues to pose a formidable global health challenge, necessitating the development of new therapeutic approaches for more effective treatment outcomes. Photodynamic therapy (PDT), which utilizes light to activate a photosensitizer to produce cytotoxic reactive oxygen species (ROS) for eradicating cancer cells, has emerged as a promising approach for cancer treatment due to its high spatiotemporal precision and minimal invasiveness. However, the widespread clinical use of PDT faces several challenges, including the inefficient production of ROS in the hypoxic tumor microenvironment, the limited penetration depth of light in biological tissues, and the inadequate accumulation of photosensitizers at the tumor site. Over the past decade, there has been increasing interest in the utilization of photofunctional transition metal complexes as photosensitizers for PDT applications due to their intriguing photophysical and photochemical properties. This review provides an overview of the current design strategies used in the development of transition metal complexes as innovative phototherapeutics, aiming to address the limitations associated with PDT and achieve more effective treatment outcomes. The current challenges and future perspectives on the clinical translation of transition metal complexes are also discussed.

Ru-terpyridine complexes containing clotrimazole as potent photoactivatable selective antifungal agents

The overuse of antimicrobial agents in medical and veterinary applications has led to the development of antimicrobial resistance in some microorganisms and this is now one of the major concerns in modern society. In this context, the use of transition metal complexes with photoactivatable properties, which can act as drug delivery systems triggered by light, could become a potent strategy to overcome the problem of resistance. In this work several Ru complexes with terpyridine ligands and the clotrimazole fragment, which is a potent antimycotic drug, were synthesized. The main goal was to explore the potential photoactivated activity of the complexes as antifungal agents and evaluate the effect of introducing different substituents on the terpyridine ligand. The complexes were capable of delivering the clotrimazole unit upon irradiation with visible light in a short period of time. The influence of the substituents on the photodissociation rate was explained by means of TD-DFT calculations. The complexes were tested against three different yeasts, which were selected based on their prevalence in fungal infections. The complex in which a carboxybenzene unit was attached to the terpyridine ligand showed the best activity against the three species under light, with minimal inhibitory concentration values of 0.88 μM and a phototoxicity index of 50 achieved. The activity of this complex was markedly higher than that of free clotrimazole, especially upon irradiation with visible light (141 times higher). The complexes were more active on yeast species than on cancer cell lines.

Light enhanced cytotoxicity and antitumoral effect of a ruthenium-based photosensitizer inspired from natural alkaloids

In this work, we report on the synthesis and properties of a new sensitizer for photodynamic therapy applications, constituted by a ruthenium(ii) complex (1) featuring a ligand inspired from natural isoquinoline alkaloids. The spectroscopic analysis revealed that 1 is characterized by an intense red emission (λ em = 620 nm, Φ = 0.17) when excited at 550 nm, a low energy radiation warranting for a safe therapeutic approach. The phototoxicity of 1 on human breast cancer (Hs578T) and melanoma (A375) cell lines was assessed after irradiation using a LED lamp (525 nm, total fluence 10 J cm-2). In vitro biological assays indicated that the cytotoxicity of 1 was significantly enhanced by light reaching IC50 values below the micromolar threshold. The cell damage induced by 1 proved to be strictly connected with the overproduction of reactive oxygen species (ROS) responsible for mitochondrial dysfunction leading to the activation of caspases and then to apoptosis, and for DNA photocleavage leading to cell cycle arrest.

Bombesin-Targeted Delivery of β-Carboline-Based Ir(III) and Ru(II) Photosensitizers for a Selective Photodynamic Therapy of Prostate Cancer

Despite advances in Ir(III) and Ru(II) photosensitizers (PSs), their lack of selectivity for cancer cells has hindered their use in photodynamic therapy (PDT). We disclose the synthesis and characterization of two pairs of Ir(III) and Ru(II) polypyridyl complexes bearing two β-carboline ligands (N^N') functionalized with -COOMe (L1) or -COOH (L2), resulting in PSs of formulas [Ir(C^N)2(N^N')]Cl (Ir-Me: C^N = ppy, N^N' = L1; Ir-H: C^N = ppy, N^N' = L2) and [Ru(N^N)2(N^N')](Cl)2 (Ru-Me: N^N = bpy, N^N' = L1; Ru-H: N^N = bpy, N^N' = L2). To enhance their selectivity toward cancer cells, Ir-H and Ru-H were coupled to a bombesin derivative (BN3), resulting in the metallopeptides Ir-BN and Ru-BN. Ir(III) complexes showed higher anticancer activity than their Ru(II) counterparts, particularly upon blue light irradiation, but lacked cancer cell selectivity. In contrast, Ir-BN and Ru-BN exhibited selective photocytoxicity against prostate cancer cells, with a lower effect against nonmalignant fibroblasts. All compounds generated ROS and induced severe mitochondrial toxicity upon photoactivation, leading to apoptosis. Additionally, the ability of Ir-Me to oxidize NADH was demonstrated, suggesting a mechanism for mitochondrial damage. Our findings indicated that the conjugation of metal PSs with BN3 creates efficient PDT agents, achieving selectivity through targeting bombesin receptors and local photoactivation.

Origin of Intersystem Crossing in Red-Light Absorbing Bodipy Derivatives: Time-Resolved Transient Optical and Electron Paramagnetic Resonance Spectral Studies with Twisted and Planar Compounds

We studied the intersystem crossing (ISC) property of red-light absorbing heavy atom-free dihydronaphtho[b]-fused Bodipy derivatives (with phenyl group attached at the lower rim via ethylene bridge, taking constrained geometry, i.e., BDP-1 and the half-oxidized product BDP-2) and dispiroflourene[b]-fused Bodipy (BDP-3) that have a twisted π-conjugated framework. BDP-1 and BDP-3 show strong and sharp absorption bands (i.e., ε = 2.0 × 105 M-1 cm-1 at 639 nm, fwhm ∼491 cm-1 for BDP-3). BDP-1 is significantly twisted (φ = 21.6°), while upon mono-oxidation, BDP-2 becomes nearly planar on the oxidized side (φ = 3.5°). Interestingly, BDP-2 showed efficient ISC (triplet state quantum yield, ΦT = 40%) due to S1/T2 state energy matching. Long-lived triplet excited state was observed (τT = 212 μs in solution and 2.4 ms in polymer matrix), and ISC takes 4.0 ns. Differently, twisted BDP-1 gives weak ISC only 5%, ISC takes 7.7 ns, and the triplet state is populated only with addition of ethyl iodide. Time-resolved electron paramagnetic resonance spectra of BDP-1 revealed the coexistence of two triplet states, with drastically different zero-field splitting D parameters of -2047 MHz and -1370 MHz, respectively, along with varying sublevel population ratios. We demonstrate that the ISC is not necessarily enhanced by torsion of the π-conjugation framework; instead, S1/Tn state energy matching is more efficient to induce ISC even in compounds that have planar molecular structures.

Exploiting the Potential of Iridium(III) bis-Nitrone Complexes as Phosphorogenic Bifunctional Reagents for Phototheranostics

Cross-linking strategies have found wide applications in chemical biology, enabling the labeling of biomolecules and monitoring of protein-protein interactions. Nitrone exhibits remarkable versatility and applicability in bioorthogonal labeling due to its high reactivity with strained alkynes via the strain-promoted alkyne-nitrone cycloaddition (SPANC) reaction. In this work, four cyclometalated iridium(III) polypyridine complexes functionalized with two nitrone units were designed as novel phosphorogenic bioorthogonal reagents for bioimaging and phototherapeutics. The complexes showed efficient emission quenching, which is attributed to an efficient nonradiative decay pathway via the low-lying T1/S0 minimum energy crossing point (MECP), as revealed by computational studies. However, the complexes displayed significant emission enhancement and lifetime extension upon reaction with (1R,8S,9s)-bicyclo[6.1.0]non-4-yne (BCN) derivatives. In particular, they showed a remarkably higher reaction rate toward a bis-cyclooctyne derivative (bis-BCN) compared with its monomeric counterpart (mono-BCN). Live-cell imaging and (photo)cytotoxicity studies revealed higher photocytotoxicity in bis-BCN-pretreated cells, which is ascribed to the enhanced singlet oxygen (1O2) photosensitization resulting from the elimination of the nitrone-associated quenching pathway. Importantly, the cross-linking properties and enhanced reactivity of the complexes make them highly promising candidates for the development of hydrogels and stapled/cyclized peptides, offering intriguing photophysical, photochemical, and biological properties. Notably, a nanosized hydrogel (2-gel) demonstrated potential as a drug delivery system, while a stapled peptide (2-bis-pDIKK) exhibited p53-Mdm2 inhibitory activity related to apoptosis and a cyclized peptide (2-bis-RGD) showed cancer selectivity.

Synthesis and Photodynamic Activities of Pyridine- or Pyridinium-Substituted Aza-BODIPY Photosensitizers

In this work, various novel pyridinyl- and pyridinium-modified Aza-BODIPY PSs were designed and constructed based on monoiodo Aza-BODIPY PSs (BDP-4 and BDP-15) in an attempt to construct "structure-inherent organelles-targeted" PSs to endow potential organelle-targeting ability. Pyridinyl PSs displayed potent photodynamic efficacy, and monorigidified PSs were very effective. The monorigidified PS 20 with meta-pyridinyl moiety displayed the most potent photoactivity and negligible dark toxicity with a favorable dark/phototoxicity ratio (>4800). To our surprise, monorigidified PS with meta-pyridinyl moiety (e.g., 20) was lipid droplet-targeted. 20 showed good cellular uptake and intracellular ROS generation compared with BDP-15. The preliminary cell death process exploration indicated that 20 resulted in lipid peroxidation and induced cell death through an iron-independent ferroptosis-like cell death pathway. In vivo antitumor efficacy experiments manifested that 20 significantly inhibited tumor growth and outperformed BDP-15 and Ce6 even under a single low-dose light irradiation (30 J/cm2).

Out-of-equilibrium dynamics of a grid-like Fe(ii) spin crossover dimer triggered by a two-photon excitation

The application of two-photon excitation (TPE) in the study of light-responsive materials holds immense potential due to its deeper penetration and reduced photodamage. Despite these benefits, TPE has been underutilised in the investigation of the photoinduced spin crossover (SCO) phenomenon. Here, we employ TPE to delve into the out-of-equilibrium dynamics of a SCO FeII dimer of the form [FeII(HL)2]2(BF4)4·2MeCN (HL = 3,5-bis{6-(2,2'-bipyridyl)}pyrazole). Optical transient absorption (OTA) spectroscopy in solution proves that the same dynamics take place under both one-photon excitation (OPE) and TPE. The results show the emergence of the photoinduced high spin state in less than 2 ps and with a lifetime of 147 ns. Time-resolved photocrystallography (TRXRD) reveals a single molecular reorganisation within the first 500 ps following TPE. Additionally, variable temperature single crystal X-ray diffraction (VTSCXRD) and magnetic susceptibility measurements confirm that the thermal transition is silenced by the solvent. While the results of the OTA and TRXRD utilising TPE are intriguing, the high pump fluencies required to excite enough metal centres to the high spin state may impair its practical application. Nonetheless, this study sheds light on the potential of TPE for the investigation of the out-of-equilibrium dynamics of SCO complexes.

Harnessing External Irradiation for Precise Activation of Metal-Based Agents in Cancer Therapy

Cancer is a significant global health issue. Platinum-based chemotherapy drugs, including cisplatin, are crucial in clinical anti-cancer treatment. However, these drugs have limitations such as drug resistance, non-specific distribution, and irreversible toxic and side effects. In recent years, the development of metal-based agents has led to the discovery of other anti-cancer effects beyond chemotherapy. Precise spatiotemporal controlled external irradiation can activate metal-based agents at specific sites and play a different role from traditional chemotherapy. These strategies can not only enhance the anti-cancer efficiency, but also show fewer side effects and non-cross-drug resistance, which are ideal approaches to solve the problems caused by traditional platinum-based chemotherapy drugs. In this review, we focus on various metal-based agent-mediated cancer therapies that are activated by three types of external irradiation: near-infrared (NIR) light, ultrasound (US), and X-ray, and give some prospects. We hope that this review will promote the generation of new kinds of metal-based anti-cancer agents.

Shining New Light on Biological Systems: Luminescent Transition Metal Complexes for Bioimaging and Biosensing Applications

Luminescence imaging is a powerful and versatile technique for investigating cell physiology and pathology in living systems, making significant contributions to life science research and clinical diagnosis. In recent years, luminescent transition metal complexes have gained significant attention for diagnostic and therapeutic applications due to their unique photophysical and photochemical properties. In this Review, we provide a comprehensive overview of the recent development of luminescent transition metal complexes for bioimaging and biosensing applications, with a focus on transition metal centers with a d6, d8, and d10 electronic configuration. We elucidate the structure-property relationships of luminescent transition metal complexes, exploring how their structural characteristics can be manipulated to control their biological behavior such as cellular uptake, localization, biocompatibility, pharmacokinetics, and biodistribution. Furthermore, we introduce the various design strategies that leverage the interesting photophysical properties of luminescent transition metal complexes for a wide variety of biological applications, including autofluorescence-free imaging, multimodal imaging, organelle imaging, biological sensing, microenvironment monitoring, bioorthogonal labeling, bacterial imaging, and cell viability assessment. Finally, we provide insights into the challenges and perspectives of luminescent transition metal complexes for bioimaging and biosensing applications, as well as their use in disease diagnosis and treatment evaluation.

Confined semiconducting polymers with boosted NIR light-triggered H2O2 production for hypoxia-tolerant persistent photodynamic therapy

Hypoxia featured in malignant tumors and the short lifespan of photo-induced reactive oxygen species (ROS) are two major issues that limit the efficiency of photodynamic therapy (PDT) in oncotherapy. Developing efficient type-I photosensitizers with long-term ˙OH generation ability provides a possible solution. Herein, a semiconducting polymer-based photosensitizer PCPDTBT was found to generate 1O2, ˙OH, and H2O2 through type-I/II PDT paths. After encapsulation within a mesoporous silica matrix, the NIR-II fluorescence and ROS generation are enhanced by 3-4 times compared with the traditional phase transfer method, which can be attributed to the excited-state lifetime being prolonged by one order of magnitude, resulting from restricted nonradiative decay channels, as confirmed by femtosecond spectroscopy. Notably, H2O2 production reaches 15.8 μM min-1 under a 730 nm laser (80 mW cm-2). Further adsorption of Fe2+ ions on mesoporous silica not only improves the loading capacity of the chemotherapy drug doxorubicin but also triggers a Fenton reaction with photo-generated H2O2 in situ to produce ˙OH continuously after the termination of laser irradiation. Thus, semiconducting polymer-based nanocomposites enables NIR-II fluorescence imaging guided persistent PDT under hypoxic conditions. This work provides a promising paradigm to fabricate persistent photodynamic therapy platforms for hypoxia-tolerant phototheranostics.

Lysosome-targeted cyclometalated Ir(III) complexes as photosensitizers/photoredox catalysts for cancer therapy

A novel lysosome-targeted photosensitizer/photoredox catalyst based on cyclometalated Ir(III) complex IrL has been designed and synthesized, which exhibited excellent phosphorescence properties and the ability to generate single oxygen (1O2) and photocatalytically oxidize 1,4-dihydronicotinamide adenine dinucleotide (NADH) under light irradiation. Most importantly, the aforementioned activities are significantly enhanced due to protonation under acidic conditions, which makes them highly attractive in light-activated tumor therapy, especially for acidic lysosomes and tumor microenvironments. The photocytotoxicity of IrL and the mechanism of cell death have been investigated. Additionally, the tumor-killing ability of IrL under light irradiation was evaluated using a 4T1 tumor-bearing mouse model. This work provides a strategy for the development of lysosome-targeted photosensitizers/photoredox catalysts to overcome hypoxic tumors.

Trinuclear ruthenium(II) polypyridyl complexes: Evaluation as photosensitizers for enhanced cervical cancer treatment

Trinuclear ruthenium(II) polypyridyl complexes anchored to benzimidazole-triazine / trisamine scaffolds were investigated as photosensitizers for photodynamic therapy. The trinuclear complexes were noted to produce a significant amount of singlet oxygen in both DMF and aqueous media, are photostable and show appreciable emission quantum yields (ɸem). In our experimental setting, despite the moderate phototoxic activity in the HeLa cervical cancer cell line, the phototoxic indices (PI) of the trinuclear complexes are superior relative to the PIs of a clinically approved photosensitizer, Photofrin®, and the pro-drug 5-aminolevulinic acid (PI: >7 relative to PI: >1 and PI: 4.4 for 5-aminolevulinic acid and Photofrin®, respectively). Furthermore, the ruthenium complexes were noted to show appreciable long-term cytotoxicity upon light irradiation in HeLa cells in a concentration-dependent manner. Consequently, this long-term activity of the ruthenium(II) polypyridyl complexes embodies their ability to reduce the probability of the recurrence of cervical cancer. Taken together, this presents a strong motivation for the development of polymetallic complexes as anticancer agents.

Photodynamic therapy for cancer: mechanisms, photosensitizers, nanocarriers, and clinical studies

Photodynamic therapy (PDT) is a temporally and spatially precisely controllable, noninvasive, and potentially highly efficient method of phototherapy. The three components of PDT primarily include photosensitizers, oxygen, and light. PDT employs specific wavelengths of light to active photosensitizers at the tumor site, generating reactive oxygen species that are fatal to tumor cells. Nevertheless, traditional photosensitizers have disadvantages such as poor water solubility, severe oxygen-dependency, and low targetability, and the light is difficult to penetrate the deep tumor tissue, which remains the toughest task in the application of PDT in the clinic. Here, we systematically summarize the development and the molecular mechanisms of photosensitizers, and the challenges of PDT in tumor management, highlighting the advantages of nanocarriers-based PDT against cancer. The development of third generation photosensitizers has opened up new horizons in PDT, and the cooperation between nanocarriers and PDT has attained satisfactory achievements. Finally, the clinical studies of PDT are discussed. Overall, we present an overview and our perspective of PDT in the field of tumor management, and we believe this work will provide a new insight into tumor-based PDT.

Two-Photon Photodegradation of E3 Ubiquitin Ligase Cereblon by a Ru(II) Complex: Inducing Ferroptosis in Cisplatin-Resistant Tumor Cells

Using photodynamic therapy (PDT) to trigger nonconventional cell death pathways has provided a new scheme for highly efficient and non-side effects to drug-resistant cancer therapies. Nonetheless, the unclear targets of available photosensitizers leave the manner of PDT-induced tumor cell death relatively unpredictable. Herein, we developed a novel Ru(II)-based photosensitizer, Ru-Poma. Possessing the E3 ubiquitin ligase CRBN-targeting moiety and high singlet oxygen yield of 0.96, Ru-Poma was demonstrated to specifically photodegrade endogenous CRBN, increase lipid peroxide, downregulate GPX4 and GAPDH expression, and consequently induce ferroptosis in cisplatin-resistant cancerous cells. Furthermore, with the deep penetration of two-photon excitation, Ru-Poma achieved drug-resistant circumvention in a 3D tumor cell model. Thus, we describe the first sample of the CRBN-targeting Ru(II) complex active in PDT.

A Semi-Automated, High-Throughput Approach for the Synthesis and Identification of Highly Photo-Cytotoxic Iridium Complexes

The discovery of new compounds with pharmacological properties is usually a lengthy, laborious and expensive process. Thus, there is increasing interest in developing workflows that allow for the rapid synthesis and evaluation of libraries of compounds with the aim of identifying leads for further drug development. Herein, we apply combinatorial synthesis to build a library of 90 iridium(III) complexes (81 of which are new) over two synthesise-and-test cycles, with the aim of identifying potential agents for photodynamic therapy. We demonstrate the power of this approach by identifying highly active complexes that are well-tolerated in the dark but display very low nM phototoxicity against cancer cells. To build a detailed structure-activity relationship for this class of compounds we have used density functional theory (DFT) calculations to determine some key electronic parameters and study correlations with the experimental data. Finally, we present an optimised semi-automated synthesise-and-test protocol to obtain multiplex data within 72 hours.

Reversible Photoinduced Ligand Substitution in a Luminescent Chromium(0) Complex

Light-triggered dissociation of ligands forms the basis for many compounds of interest for photoactivated chemotherapy (PACT), in which medicinally active substances are released or "uncaged" from metal complexes upon illumination. Photoinduced ligand dissociation is usually irreversible, and many recent studies performed in the context of PACT focused on ruthenium(II) polypyridines and related heavy metal complexes. Herein, we report a first-row transition metal complex, in which photoinduced dissociation and spontaneous recoordination of a ligand unit occurs. Two scorpionate-type tridentate chelates provide an overall six-coordinate arylisocyanide environment for chromium(0). Photoexcitation causes decoordination of one of these six ligating units and coordination of a solvent molecule, at least in tetrahydrofuran and 1,4-dioxane solvents, but far less in toluene, and below detection limit in cyclohexane. Transient UV-vis absorption spectroscopy and quantum chemical simulations point to photoinduced ligand dissociation directly from an excited metal-to-ligand charge-transfer state. Owing to the tridentate chelate design and the substitution lability of the first-row transition metal, recoordination of the photodissociated arylisocyanide ligand unit can occur spontaneously on a millisecond time scale. This work provides insight into possible self-healing mechanisms counteracting unwanted photodegradation processes and seems furthermore relevant in the contexts of photoswitching and (photo)chemical information storage.

Iridium(III) complexes decorated with silicane-modified rhodamine: near-infrared light-initiated photosensitizers for efficient deep-tissue penetration photodynamic therapy

Meeting the demand for efficient photosensitizers in photodynamic therapy (PDT), a series of iridium(III) complexes decorated with silicane-modified rhodamine (Si-rhodamine) was meticulously designed and synthesized. These complexes demonstrate exceptional PDT potential owing to their strong absorption in the near-infrared (NIR) spectrum, particularly responsive to 808 nm laser stimulation. This feature is pivotal, enabling deep-penetration laser excitation and overcoming depth-related challenges in clinical PDT applications. The molecular structures of these complexes allow for reliable tuning of singlet oxygen generation with NIR excitation, through modification of the cyclometalating ligand. Notably, one of the complexes (4) exhibits a remarkable ROS quantum yield of 0.69. In vivo results underscore the efficacy of 4, showcasing significant tumor regression at depths of up to 8.4 mm. This study introduces a promising paradigm for designing photosensitizers capable of harnessing NIR light effectively for deep PDT applications.

Structural and Photophysical Trends in Rhenium(I) Carbonyl Complexes with 2,2':6',2″-Terpyridines

This is the first comprehensive review of rhenium(I) carbonyl complexes with 2,2':6',2″-terpyridine-based ligands (R-terpy)-encompassing their synthesis, molecular features, photophysical behavior, and potential applications. Particular attention has been devoted to demonstrating how the coordination mode of 2,2':6',2″-terpyridine (terpy-κ2N and terpy-κ3N), structural modifications of terpy framework (R), and the nature of ancillary ligands (X-mono-negative anion, L-neutral ligand) may tune the photophysical behavior of Re(I) complexes [Re(X/L)(CO)3(R-terpy-κ2N)]0/+ and [Re(X/L)(CO)2(R-terpy-κ3N)]0/+. Our discussion also includes homo- and heteronuclear multicomponent systems with {Re(CO)3(R-terpy-κ2N)} and {Re(CO)2(R-terpy-κ3N)} motifs. The presented structure-property relationships are of high importance for controlling the photoinduced processes in these systems and making further progress in the development of more efficient Re-based luminophores, photosensitizers, and photocatalysts for modern technologies.

Design, Synthesis, and Characterization of Organometallic BODIPY-Ru(II) Dyads: Redox and Photophysical Properties with Singlet Oxygen Generation Capability†

A series of Ru(II)-acetylide complexes (Ru1, Ru2, and Ru1m) with alkynyl-functionalized borondipyrromethene (BODIPY) conjugates were designed by varying the position of the linker that connects the BODIPY unit to the Ru(II) metal center through acetylide linkage at either the 2-(Ru1) and 2,6-(Ru2) or the meso-phenyl (Ru1m) position of the BODIPY scaffold. The Ru(II) organometallic complexes were characterized by various spectroscopic methods, including nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, CHN, and high-resolution mass spectrometry (HRMS) analyses. The Ru(II)-BODIPY conjugates exhibit fascinating electrochemical and photophysical properties. All BODIPY-Ru(II) complexes exhibit strong absorption (εmax = 29,000-72,000 M-1 cm-1) in the visible region (λmax = 502-709 nm). Fluorescence is almost quenched for Ru1 and Ru2, whereas Ru1m shows the residual fluorescence of the corresponding BODIPY core at 517 nm. The application of the BODIPY-Ru(II) dyads as nonporphyrin-based triplet photosensitizers was explored by a method involving the singlet oxygen (1O2)-mediated photo-oxidation of diphenylisobenzofuran. Effective π-conjugation between the BODIPY chromophore and Ru(II) center in the case of Ru1 and Ru2 was found to be necessary to improve intersystem crossing (ISC) and hence the 1O2-sensitizing ability. In addition, electrochemical studies indicate electronic interplay between the metal center and the redox-active BODIPY in the BODIPY-Ru(II) dyads.

A hetero-bimetallic Ru(II)-Ir(III) photosensitizer for effective cancer photodynamic therapy under hypoxia

A hetero-bimetallic Ru(II)-Ir(III) photosensitizer was developed. Upon light exposure, contrary to the homogeneous Ru(II)-Ru(II) and Ir(III)-Ir(III) complexes that can only produce singlet oxygen, Ru(II)-Ir(III) can generate multiple reactive oxygen species and kill hypoxic tumors. This study presents the first example of a hetero-bimetallic type-I and type-II dual photosensitizer.

Controllable and Low-Loss Electrochemiluminescence Waveguide Supported by a Micropipette Electrode

One-dimensional molecular crystal waveguide (MCW) can transmit self-generated electrochemiluminescence (ECL), but heavy optical loss occurs because of the small difference in the refractive index between the crystal and its surroundings. Herein, we report a micropipette electrode-supported MCW (MPE/MCW) for precisely controlling the far-field transmission of ECL in air with a low optical loss. ECL is generated from one terminal of the MCW positioned inside the MPE, which is transmitted along the MCW to the other terminal in air. In comparison with conventional waveguides on solid substrates or in solutions, the MPE/MCW is propitious to the total internal reflection of light at the MCW/air interface, thus confining the ECL efficiently in MCW and improving the waveguide performance with an extremely low-loss coefficient of 4.49 × 10-3 dB μm-1. Moreover, by regulation of the gas atmosphere, active and passive waveguides can be resolved simultaneously inside MPE and in air. This MPE/MCW offers a unique advantage of spatially controlling and separating ECL signal readout from its generation, thus holding great promise in biosensing without or with less electrical/chemical disturbance.

Antimicrobial Potency of Fmoc-Phe-Phe Dipeptide Hydrogels with Encapsulated Porphyrin Chromophores Is a Promising Alternative in Antimicrobial Resistance

Antimicrobial resistance (AMR) poses a significant global health risk as a consequence of misuse of antibiotics. Owing to the increasing antimicrobial resistance, it became imperative to develop novel molecules and materials with antimicrobial properties. Porphyrins and metalloporphyrins are compounds which present antimicrobial properties especially after irradiation. As a consequence, porphyrinoids have recently been utilized as antimicrobial agents in antimicrobial photodynamic inactivation in bacteria and other microorganisms. Herein, we report the encapsulation of porphyrins into peptide hydrogels which serve as delivery vehicles. We selected the self-assembling Fmoc-Phe-Phe dipeptide, a potent gelator, as a scaffold due to its previously reported biocompatibility and three different water-soluble porphyrins as photosensitizers. We evaluated the structural, mechanical and in vitro degradation properties of these hydrogels, their interaction with NIH3T3 mouse skin fibroblasts, and we assessed their antimicrobial efficacy against Gram-positive Staphylococcus aureus (S. aureus) and Gram-negative Escherichia coli (E. coli) bacteria. We found out that the hydrogels are cytocompatible and display antimicrobial efficiency against both strains with the zinc porphyrins being more efficient. Therefore, these hydrogels present a promising alternative for combating bacterial infections in the face of growing AMR concerns.

Ir(III) Half-Sandwich Photosensitizers with a π-Expansive Ligand for Efficient Anticancer Photodynamic Therapy

One approach to reduce the side effects of chemotherapy in cancer treatment is photodynamic therapy (PDT), which allows spatiotemporal control of the cytotoxicity. We have used the strategy of coordinating π-expansive ligands to increase the excited state lifetimes of Ir(III) half-sandwich complexes in order to facilitate the generation of 1O2. We have obtained derivatives of formulas [Cp*Ir(C∧N)Cl] and [Cp*Ir(C∧N)L]BF4 with different degrees of π-expansion in the C∧N ligands. Complexes with the more π-expansive ligand are very effective photosensitizers with phototoxic indexes PI > 2000. Furthermore, PI values of 63 were achieved with red light. Time-dependent density functional theory (TD-DFT) calculations nicely explain the effect of the π-expansion. The complexes produce reactive oxygen species (ROS) at the cellular level, causing mitochondrial membrane depolarization, cleavage of DNA, nicotinamide adenine dinucleotide (NADH) oxidation, as well as lysosomal damage. Consequently, cell death by apoptosis and secondary necrosis is activated. Thus, we describe the first class of half-sandwich iridium cyclometalated complexes active in PDT.

Therapeutic and Diagnostic Agents based on Bioactive Endogenous and Exogenous Coordination Compounds

Metal-based coordination compounds have very special place in bioinorganic chemistry because of their different structural arrangements and significant application in medicine. Rapid progress in this field increasingly enables the targeted design and synthesis of metal-based pharmaceutical agents that fulfill valuable roles as diagnostic or therapeutic agents. Various coordination compounds have important biological functions, both those initially present in the body (endogenous) and those entering the organisms from the external environment (exogenous): vitamins, drugs, toxic substances, etc. In the therapeutic and diagnostic practice, both the essential for all living organisms and the trace metals are used in metal-containing coordination compounds. In the current review, the most important functional biologically active compounds were classified group by group according to the position of the elements in the periodic table.

Exploring a Mitochondria Targeting, Dinuclear Cyclometalated Iridium (III) Complex for Image-Guided Photodynamic Therapy in Triple-Negative Breast Cancer Cells

Photodynamic therapy (PDT) has emerged as an efficient and noninvasive treatment approach utilizing laser-triggered photosensitizers for combating cancer. Within this rapidly advancing field, iridium-based photosensitizers with their dual functionality as both imaging probes and PDT agents exhibit a potential for precise and targeted therapeutic interventions. However, most reported classes of Ir(III)-based photosensitizers comprise mononuclear iridium(III), with very few examples of dinuclear systems. Exploring the full potential of iridium-based dinuclear systems for PDT applications remains a challenge. Herein, we report a dinuclear Ir(III) complex (IRDI) along with a structurally similar monomer complex (IRMO) having 2-(2,4-difluorophenyl)pyridine and 4'-methyl-2,2'-bipyridine ligands. The comparative investigation of the mononuclear and dinuclear Ir(III) complexes showed similar absorption profiles, but the dinuclear derivative IRDI exhibited a higher photoluminescence quantum yield (Φp) of 0.70 compared to that of IRMO (Φp = 0.47). Further, IRDI showed a higher singlet oxygen generation quantum yield (Φs) of 0.49 compared to IRMO (Φs = 0.28), signifying the enhanced potential of the dinuclear derivative for image-guided photodynamic therapy. In vitro assessments indicate that IRDI shows efficient cellular uptake and significant photocytotoxicity in the triple-negative breast cancer cell line MDA-MB-231. In addition, the presence of a dual positive charge on the dinuclear system facilitates the inherent mitochondria-targeting ability without the need for a specific targeting group. Subcellular singlet oxygen generation by IRDI was confirmed using Si-DMA, and light-activated cellular apoptosis via ROS-mediated PDT was verified through various live-dead assays performed in the presence and absence of the singlet oxygen scavenger NaN3. Further, the mechanism of cell death was elucidated by an annexin V-FITC/PI flow cytometric assay and by investigating the cytochrome c release from mitochondria using Western blot analysis. Thus, the dinuclear complex designed to enhance spin-orbit coupling with minimal excitonic coupling represents a promising strategy for efficient image-guided PDT using iridium complexes.

A dipyridophenazine Ni(II) dithiolene complex as a dual-acting cancer phototherapy agent activatable within the phototherapeutic window

A combination of photodynamic therapy (PDT) and photothermal therapy (PTT) within the phototherapeutic window (600-900 nm) can lead to significantly enhanced therapeutic outcomes, surpassing the efficacy observed with PDT or PTT alone in cancer phototherapy. Herein, we report a novel small-molecule mixed-ligand Ni(II)-dithiolene complex (Ni-TDD) with a dipyridophenazine ligand, demonstrating potent red-light PDT and significant near-infrared (NIR) light mild-temperature PTT activity against cancer cells and 3D multicellular tumour spheroids (MCTSs). The four-coordinate square planar complex exhibited a moderately intense absorption band (ε ∼ 3700 M-1cm-1) centered around 900 nm and demonstrated excellent dark and photostability in an aqueous phase. Ni-TDD induced a potent red-light (600-720 nm) PDT effect on HeLa cancer cells (IC50 = 1.8 μM, photo irritation factor = 44), triggering apoptotic cell death through efficient singlet oxygen generation. Ni-TDD showed a significant intercalative binding affinity towards double-helical calf thymus DNA, resulting in a binding constant (Kb) ∼ 106 M-1. The complex induced mild hyperthermia and exerted a significant mild-temperature PTT effect on MDA-MB-231 cancer cells upon irradiation with 808 nm NIR light. Simultaneous irradiation of Ni-TDD-treated HeLa MCTSs with red and NIR light led to a remarkable synergistic inhibition of growth, exceeding the effects of individual irradiation, through the generation of singlet oxygen and mild hyperthermia. Ni-TDD displayed minimal toxicity towards non-cancerous HPL1D and L929 cells, even at high micromolar concentrations. This is the first report of a Ni(II) complex demonstrating red-light PDT activity and the first example of a first-row transition metal complex exhibiting combined PDT and PTT effects within the clinically relevant phototherapeutic window. Our findings pave the way for designing and developing metal-dithiolene complexes as dual-acting cancer phototherapy agents using long wavelength light for treating solid tumors.

Pyrenyl-Substituted Imidazo[4,5-f][1,10]phenanthroline Rhenium(I) Complexes with Record-High Triplet Excited-State Lifetimes at Room Temperature: Steric Control of Photoinduced Processes in Bichromophoric Systems

Photochemical applications based on intermolecular photoinduced energy triplet state transfer require photosensitizers with strong visible absorptivity and extended triplet excited-state lifetimes. Using a bichromophore approach, two Re(I) tricarbonyl complexes with 2-(1-pyrenyl)-1H-imidazo[4,5-f][1,10]phenanthroline (pyr-imphen) and 1-(4-(methyl)phenyl)-2-(1-pyrenyl)-imidazo[4,5-f][1,10]phenanthroline (pyr-tol-imphen) showing extraordinary long triplet excited states at room temperature (>1000 μs) were obtained, and their ground- and excited-state properties were thoroughly investigated by a wide range of spectroscopic methods, including femtosecond transient absorption (fs-TA). It is worth noting that the designed [ReCl(CO)3(pyr-imphen)] (1) and [ReCl(CO)3(pyr-tol-imphen)] (2) complexes form a unique pair differing in the mutual chromophore arrangement due to introduction of a 4-(methyl)phenyl substituent into the imidazole ring at the H1-position, imposing an increase in the dihedral angle between the pyrene and {ReCl(CO)3(imphen)} chromophores. The magnitude of the electronic coupling between the pyrene and {ReCl(CO)3(imphen)} chromophores was found to be an efficient tool to tune the photophysical properties of 1 and 2. The usefulness of designed Re(I) compounds as triplet photosensitizers was successfully verified by examination of their abilities for 1O2 generation and triplet-triplet annihilation upconversion. The phosphorescence lifetimes, ∼1800 μs for 1 and ∼1500 μs for 2, are the longest lifetimes reported for Re(I) diimine carbonyl complexes in solution at room temperature.

Electronic Tuning of Photoexcited Dynamics in Heteroleptic Cu(I) Complex Photosensitizers

Photoexcited dynamics of heteroleptic Cu(I) complexes as noble-metal-free photosensitizers are closely intertwined with the nature of their ligands. By utilizing ultrafast optical and X-ray transient absorption spectroscopies, we characterized a new set of heteroleptic Cu(I) complexes [Cu(PPh3)2(BPyR)]+ (R = CH3, H, Br to COOCH3), with an increase in the electron-withdrawing ability of the functional group (R). We found that after the transient photooxidation of Cu(I) to Cu(II), the increasing electron-withdrawing ability of R barely affects the internal conversion (IC) (e.g., Jahn-taller (JT) distortion) between singlet MLCT states. However, it does accelerate the dynamics of intersystem crossing (ISC) between singlet and triplet MLCT states and the subsequent decay from the triplet MLCT state to the ground state. The associated lifetime constants are reduced by up to 300%. Our understanding of the photoexcited dynamics in heteroleptic Cu(I) complexes through ligand electronic tuning provides valuable insight into the rational design of efficient Cu(I) complex photosensitizers.

Efficient Strategies to Use β-Cationic Porphyrin-Imidazolium Derivatives in the Photoinactivation of Methicillin-Resistant Staphylococcus aureus

Bacterial resistance to antibiotics is a critical global health issue and the development of alternatives to conventional antibiotics is of the upmost relevance. Antimicrobial photodynamic therapy (aPDT) is considered a promising and innovative approach for the photoinactivation of microorganisms, particularly in cases where traditional antibiotics may be less effective due to resistance or other limitations. In this study, two β-modified monocharged porphyrin-imidazolium derivatives were efficiently incorporated into polyvinylpyrrolidone (PVP) formulations and supported into graphitic carbon nitride materials. Both porphyrin-imidazolium derivatives displayed remarkable photostability and the ability to generate cytotoxic singlet oxygen. These properties, which have an important impact on achieving an efficient photodynamic effect, were not compromised after incorporation/immobilization. The prepared PVP-porphyrin formulations and the graphitic carbon nitride-based materials displayed excellent performance as photosensitizers to photoinactivate methicillin-resistant Staphylococcus aureus (MRSA) (99.9999% of bacteria) throughout the antimicrobial photodynamic therapy. In each matrix, the most rapid action against S. aureus was observed when using PS 2. The PVP-2 formulation needed 10 min of exposure to white light at 5.0 µm, while the graphitic carbon nitride hybrid GCNM-2 required 20 min at 25.0 µm to achieve a similar level of response. These findings suggest the potential of graphitic carbon nitride-porphyrinic hybrids to be used in the environmental or clinical fields, avoiding the use of organic solvents, and might allow for their recovery after treatment, improving their applicability for bacteria photoinactivation.

A biocompatible pure organic porous nanocage for enhanced photodynamic therapy

Porphyrin-based photosensitizers have been widely utilized in photodynamic therapy (PDT), but they suffer from deteriorating fluorescence and reactive oxygen species (ROS) due to their close π-π stacking. Herein, a biocompatible pure organic porphyrin nanocage (Py-Cage) with enhanced both type I and type II ROS generation is reported for PDT. The porphyrin skeleton within the Py-Cage is spatially separated by four biphenyls to avoid the close π-π stacking within the nanocage. The Py-Cage showed a large cavity and high porosity with a Brunauer-Emmett-Teller surface area of over 300 m2 g-1, facilitating a close contact between the Py-Cage and oxygen, as well as the fast release of ROS to the surrounding microenvironment. The Py-Cage shows superb ROS generation performance over its precursors and commercial ones such as Chlorin E6 and Rose Bengal. Intriguingly, the cationic π-conjugated Py-Cage also shows promising type I ROS (superoxide and hydroxyl radicals) generation that is more promising for hypoxic tumor treatment. Both in vitro cell and in vivo animal experiments further confirm the excellent antitumor activity of the Py-Cage. As compared to conventional metal coordination approaches to improve PDT efficacy of porphyrin derivatives, the pure organic porous Py-Cage demonstrates excellent biocompatibility, which is further verified in both mice and rats. This work of an organic porous nanocage shall provide a new paradigm for the design of novel, biocompatible and effective photosensitizers for PDT.

Interrogating bioinspired ESIPT/PCET-based Ir(iii)-complexes as organelle-targeted phototherapeutics: a redox-catalysis under hypoxia to evoke synergistic ferroptosis/apoptosis

Installing proton-coupled electron transfer (PCET) in Ir-complexes is indeed a newly explored phenomenon, offering high quantum efficiency and tunable photophysics; however, the prospects for its application in various fields, including interrogating biological systems, are quite open and exciting. Herein, we developed various organelle-targeted Ir(iii)-complexes by leveraging the photoinduced PCET process to see the opportunities in phototherapeutic application and investigate the underlying mechanisms of action (MOAs). We diversified the ligands' nature and also incorporated a H-bonded benzimidazole-phenol (BIP) moiety with π-conjugated ancillary ligands in Ir(iii) to study the excited-state intramolecular proton transfer (ESIPT) process for tuning dual emission bands and to tempt excited-state PCET. These visible or two-photon-NIR light activatable Ir-catalysts generate reactive hydroxyl radicals (˙OH) and simultaneously oxidize electron donating biomolecules (1,4-dihydronicotinamide adenine dinucleotide or glutathione) to disrupt redox homeostasis, downregulate the GPX4 enzyme, and amplify oxidative stress and lipid peroxide (LPO) accumulation. Our homogeneous photocatalytic platform efficiently triggers organelle dysfunction mediated by a Fenton-like pathway with spatiotemporal control upon illumination to evoke ferroptosis poised with the synergistic action of apoptosis in a hypoxic environment leading to cell death. Ir2 is the most efficient photochemotherapy agent among others, which provided profound cytophototoxicity to 4T1 and MCF-7 cancerous cells and inhibited solid hypoxic tumor growth in vitro and in vivo.