Ruthenium-Decorated Lipid Vesicles: Light-Induced Release of [Ru(terpy)(bpy)(OH2)]2+ and Thermal Back Coordination

Reconfiguring hydrogel assemblies using a photocontrolled metallopolymer adhesive for multiple customized functions

Stimuli-responsive hydrogels with programmable shape changes are promising materials for soft robots, four-dimensional printing, biomedical devices and artificial intelligence systems. However, these applications require the fabrication of hydrogels with complex, heterogeneous and reconfigurable structures and customizable functions. Here we report the fabrication of hydrogel assemblies with these features by reversibly gluing hydrogel units using a photocontrolled metallopolymer adhesive. The metallopolymer adhesive firmly attached individual hydrogel units via metal-ligand coordination and polymer chain entanglement. Hydrogel assemblies containing temperature- and pH-responsive hydrogel units showed controllable shape changes and motions in response to these external stimuli. To reconfigure their structures, the hydrogel assemblies were disassembled by irradiating the metallopolymer adhesive with light; the disassembled hydrogel units were then reassembled using the metallopolymer adhesive with heating. The shape change and structure reconfiguration abilities allow us to reprogramme the functions of hydrogel assemblies. The development of reconfigurable hydrogel assemblies using reversible adhesives provides a strategy for designing intelligent materials and soft robots with user-defined functions.

Binding of Stimuli-Responsive Ruthenium Aqua Complexes with 9-Ethylguanine

Stimuli-responsive ruthenium complexes proximal- and distal-[Ru(C10tpy)(C10pyqu) OH2]2+ (proximal-1 and distal-1; C10tpy = 4'-decyloxy-2,2':6',2″-terpyridine and C10pyqu = 2-[2'-(6'-decyloxy)-pyridyl]quinoline) were experimentally studied for adduct formation with a model DNA base. At 303 K, proximal-1 exhibited 1:1 adduct formation with 9-ethylguanine (9-EtG) to yield proximal-[Ru(C10tpy)(C10pyqu)(9-EtG)]2+ (proximal-RuEtG). Rotation of the guanine ligand on the ruthenium center was sterically hindered by the presence of an adjacent quinoline moiety at 303 K. Results from 1H NMR measurements indicated that photoirradiation of a proximal-RuEtG solution caused photoisomerization to distal-RuEtG, whereas heating of proximal-RuEtG caused ligand substitution to proximal-1. The distal isomer of the aqua complex, distal-1, was observed to slowly revert to proximal-1 at 303 K. In the presence of 9-EtG, distal-1 underwent thermal back-isomerization to proximal-1 and adduct formation to distal-RuEtG. Kinetic analysis of 1H NMR measurements showed that adduct formation between proximal-1 and 9-EtG was 8-fold faster than that between distal-1 and 9-EtG. This difference may be attributed to intramolecular hydrogen bonding and steric repulsion between the aqua ligand and the pendant moiety of the bidentate ligand..

Theranostic imaging and multimodal photodynamic therapy and immunotherapy using the mTOR signaling pathway

Tumor metastases are considered the leading cause of cancer-associated deaths. While clinically applied drugs have demonstrated to efficiently remove the primary tumor, metastases remain poorly accessible. To overcome this limitation, herein, the development of a theranostic nanomaterial by incorporating a chromophore for imaging and a photosensitizer for treatment of metastatic tumor sites is presented. The mechanism of action reveals that the nanoparticles are able to intervene by local generation of cellular damage through photodynamic therapy as well as by systemic induction of an immune response by immunotherapy upon inhibition of the mTOR signaling pathway which is of crucial importance for tumor onset, progression and metastatic spreading. The nanomaterial is able to strongly reduce the volume of the primary tumor as well as eradicates tumor metastases in a metastatic breast cancer and a multi-drug resistant patient-derived hepatocellular carcinoma models in female mice.

Photoresponsive Diarylethene-Containing Polymers: Recent Advances and Future Challenges

Photoresponsive polymers have attracted increasing interest owing to their potential applications in anticounterfeiting, information encryption, adhesives, etc. Among them, diarylethene (DAE)-containing polymers are one of the most promising photoresponsive polymers and have unique thermal stability and fatigue resistance compared to azobenzene- and spiropyran-containing polymers. Herein, the design of DAE-containing polymers based on different types of structures, including main chain polymers, side-chain polymers, and crosslinked polymers, is introduced. The mechanism and applications of DAE-containing polymers in anti-counterfeiting, information encryption, light-controllable adhesives, and photoinduced healable materials are reviewed. In addition, the remaining challenges of DAE-containing polymers are also discussed.

Injectable 2D flexible hydrogel sheets for optoelectrical/biochemical dual stimulation of neurons

Major challenges in developing implanted neural stimulation devices are the invasiveness, complexity, and cost of the implantation procedure. Here, we report an injectable, nanofibrous 2D flexible hydrogel sheet-based neural stimulation device that can be non-invasively implanted via syringe injection for optoelectrical and biochemical dual stimulation of neuron. Specifically, methacrylated gelatin (GelMA)/alginate hydrogel nanofibers were mechanically reinforced with a poly(lactide-co-ε-caprolactone) (PLCL) core by coaxial electrospinning. The lubricant hydrogel shell enabled not only injectability, but also facile incorporation of functional nanomaterials and bioactives. The nanofibers loaded with photocatatlytic g-C₃N₄/GO nanoparticles were capable of stimulating neural cells via blue light, with a significant 36.3 % enhancement in neurite extension. Meanwhile, the nerve growth factor (NGF) loaded nanofibers supported a sustained release of NGF with well-maintained function to biochemically stimulate neural differentiation. We have demonstrated the capability of an injectable, hydrogel nanofibrous, neural stimulation system to support neural stimulation both optoelectrically and biochemically, which represents crucial early steps in a larger effort to create a minimally invasive system for neural stimulation.

Perturbation of Lipid Bilayers by Biomimetic Photoswitches Based on Cyclocurcumin

The use of photoswitches which may be activated by suitable electromagnetic radiation is an attractive alternative to conventional photodynamic therapy. Here, we report all-atom molecular dynamics simulation of a biomimetic photoswitch derived from cyclocurcumin and experiencing E/Z photoisomerization. In particular, we show that the two isomers interact persistently with a lipid bilayer modeling a cellular membrane. Furthermore, the interaction with the membrane is strongly dependent on the concentration, and a transition between ordered and disordered arrangements of the photoswitches is observed. We also confirm that the structural parameters of the bilayer are differently affected by the two isomers and hence can be modulated through photoswitching, offering interesting perspectives for future applications.

A Lock‐and‐Kill Anticancer Photoactivated Chemotherapy Agent †

Photosubstitutionally active ruthenium complexes show high potential as prodrugs for the photoactivated chemotherapy (PACT) treatment of tumors. One of the problems in PACT is that the localization of the ruthenium compound is hard to trace. Here, a ruthenium PACT prodrug, [Ru(3)(biq)(STF-31)](PF₆ )₂ (where 3 = 3-(([2,2':6',2″-ter- pyridin]-4'-yloxy)propyl-4-(pyren-1-yl)butanoate) and biq = 2,2'-biquinoline), has been prepared, in which a pyrene tracker is attached via an ester bond. The proximity between the fluorophore and the ruthenium center leads to fluorescence quenching. Upon intracellular hydrolysis of the ester linkage, however, the fluorescence of the pyrene moiety is recovered, thus demonstrating prodrug cellular uptake. Further light irradiation of this molecule liberates by photosubstitution STF-31, a known cytotoxic nicotinamide phosphoribosyltransferase (NAMPT) inhibitor, as well as singlet oxygen via excitation of the free pyrene chromophore. The dark and light cytotoxicity of the prodrug, embedded in liposomes, as well as the appearance of blue emission upon uptake, were evaluated in A375 human skin melanoma cells. The cytotoxicity of the liposome-embedded prodrug was indeed increased by light irradiation. This work realizes an in vitro proof-of-concept of the lock-and-kill principle, which may ultimately be used to design strategies aimed at knowing where and when light irradiation should be realized in vivo.

Luminescent Metal Complexes as Emerging Tools for Lipid Imaging

Fluorescence microscopy is a key tool in the biological sciences, which finds use as a routine laboratory technique (e.g., epifluorescence microscope) or more advanced confocal, two-photon, and super-resolution applications. Through continued developments in microscopy, and other analytical methods, the importance of lipids as constituents of subcellular organelles, signalling or regulating molecules continues to emerge. The increasing recognition of the importance of lipids to fundamental cell biology (in health and disease) has prompted the development of protocols and techniques to image the distribution of lipids in cells and tissues. A diverse suite of spectroscopic and microscopy tools are continuously being developed and explored to add to the "toolbox" to study lipid biology. A relatively recent breakthrough in this field has been the development and subsequent application of metal-based luminescent complexes for imaging lipids in biological systems. These metal-based compounds appear to offer advantages with respect to their tunability of the photophysical properties, in addition to capabilities centred around selectively targeting specific lipid structures or classes of lipids. The presence of the metal centre also opens the path to alternative imaging modalities that might not be applicable to traditional organic fluorophores. This review examines the current progress and developments in metal-based luminescent complexes to study lipids, in addition to exploring potential new avenues and challenges for the field to take.

Shorter Alkyl Chains Enhance Molecular Diffusion and Electron Transfer Kinetics between Photosensitisers and Catalysts in CO2 -Reducing Photocatalytic Liposomes

Covalent functionalisation with alkyl tails is a common method for supporting molecular catalysts and photosensitisers onto lipid bilayers, but the influence of the alkyl chain length on the photocatalytic performances of the resulting liposomes is not well understood. In this work, we first prepared a series of rhenium-based CO₂ -reduction catalysts [Re(4,4'-(C_n H_2n+1 )₂ -bpy)(CO)₃ Cl] (ReC_n ; 4,4'-(C_n H_2n+1 )₂ -bpy=4,4'-dialkyl-2,2'-bipyridine) and ruthenium-based photosensitisers [Ru(bpy)₂ (4,4'-(C_n H_2n+1 )₂ -bpy)](PF₆ )₂ (RuC_n ) with different alkyl chain lengths (n=0, 9, 12, 15, 17, and 19). We then prepared a series of PEGylated DPPC liposomes containing RuC_n and ReC_n , hereafter noted C_n , to perform photocatalytic CO₂ reduction in the presence of sodium ascorbate. The photocatalytic performance of the C_n liposomes was found to depend on the alkyl tail length, as the turnover number for CO (TON) was inversely correlated to the alkyl chain length, with a more than fivefold higher CO production (TON=14.5) for the C₉ liposomes, compared to C₁₉ (TON=2.8). Based on immobilisation efficiency quantification, diffusion kinetics, and time-resolved spectroscopy, we identified the main reason for this trend: two types of membrane-bound RuC_n species can be found in the membrane, either deeply buried in the bilayer and diffusing slowly, or less buried with much faster diffusion kinetics. Our data suggest that the higher photocatalytic performance of the C₉ system is due to the higher fraction of the more mobile and less buried molecular species, which leads to enhanced electron transfer kinetics between RuC₉ and ReC₉ .

Strategies to promote permeation and vectorization, and reduce cytotoxicity of metal complex luminophores for bioimaging and intracellular sensing

Transition metal luminophores are emerging as important tools for intracellular imaging and sensing. Their putative suitability for such applications has long been recognised but poor membrane permeability and cytotoxicity were significant barriers that impeded early progress. In recent years, numerous effective routes to overcoming these issues have been reported, inspired in part, by advances and insights from the pharmaceutical and drug delivery domains. In particular, the conjugation of biomolecules but also other less natural synthetic species, from a repertoire of functional motifs have granted membrane permeability and cellular targeting. Such motifs can also reduce cytotoxicity of transition metal complexes and offer a valuable avenue to circumvent such problems leading to promising metal complex candidates for application in bioimaging, sensing and diagnostics. The advances in metal complex probes permeability/targeting are timely, as, in parallel, over the past two decades significant technological advances in luminescence imaging have occurred. In particular, super-resolution imaging is enormously powerful but makes substantial demands of its imaging contrast agents and metal complex luminophores frequently possess the photophysical characteristics to meet these demands. Here, we review some of the key vectors that have been conjugated to transition metal complex luminophores to promote their use in intra-cellular imaging applications. We evaluate some of the most effective strategies in terms of membrane permeability, intracellular targeting and what impact these approaches have on toxicity and phototoxicity which are important considerations in a luminescent contrast or sensing agent.

Visible Light-Gated Organocatalysis Using a RuII -Photocage

The light-gated organocatalysis via the release of 4-N,N-dimethylaminopyridine (DMAP) by irradiation of the [Ru(bpy)₂ (DMAP)₂ ]²⁺ complex with visible light was investigated. As model reaction the acetylation of benzyl alcohols with acetic anhydride was chosen. The pre-catalyst releases one DMAP molecule on irradiation at wavelengths longer than 455 nm. The photochemical process was characterized by steady-state irradiation and ultrafast transient absorption spectroscopy. The latter enabled the observation of the ³ MLCT state and the spectral features of the penta-coordinated intermediate [Ru(bpy)₂ (DMAP)]²⁺ . The released DMAP catalyzes the acetylation of a wide range of benzyl alcohols with chemical yields of up to 99 %. Control experiments revealed unequivocally that it is the released DMAP which takes the role of the catalyst.

Incorporation of Ru(II) Polypyridyl Complexes into Nanomaterials for Cancer Therapy and Diagnosis

Ru(II) polypyridyl complexes are compounds of great interest in cancer therapy due to their unique photophysical, photochemical, and biological properties. For effective treatment, they must be able to penetrate tumor cells effectively and selectively. The development of nanoscale carriers capable of delivering Ru(II) polypyridyl complexes has the potential to passively or selectively enhance their cellular uptake in tumor cells. Many different strategies have been explored to incorporate Ru(II) polypyridyl complexes into a variety of nanosized constructs, ranging from organic to inorganic materials. Herein, recent developments in nanomaterials loaded with Ru(II) polypyridyl complexes are highlighted. Their rational design, preparation, and physicochemical properties are described, and their potential applications in cancer therapy are eventually discussed.

Intramolecular Hydrogen Bonding: A Key Factor Controlling the Photosubstitution of Ruthenium Complexes

Photosubstitution reactions of ruthenium complexes with pyrazole ligands, cis-[Ru(bpy)₂(pzH)₂]²⁺ (1a), cis-[Ru(bpy)₂(pz)(pzH)]⁺ (1b), and cis-[Ru(bpy)₂(pz)₂]⁰ (1c) (pzH = pyrazole, bpy = 2,2'-bipyridine), were investigated. Dicationic complex 1a was deprotonated to 1b using moderate base (pK_a = 15.2, MeCN), while the second deprotonation to give 1c required more severe conditions (pK_a = 26.9). Monocationic complex 1b possessed an N-H···N-type intramolecular hydrogen bond between the pyrazole and pyrazolate ligands, as corroborated by the solid-state crystal structure. The photosubstitution quantum yield of 1a (Φ = 0.26) was comparable to that of cis-[Ru(bpy)₂(pyridine)₂]²⁺ (Φ = 0.24) in acetonitrile solution. In contrast, the photodissociation of a pzH ligand was strongly suppressed by the deprotonation of a pyrazole ligand N-H group. In the presence of 10 000 equiv of 4,4'-dimethylaminopyridine, the quantum yield dropped to ∼2 × 10^-6 in acetonitrile. The photosubstitution quantum yield of 1b was even smaller than that of neutral complex 1c, although 1c had a smaller HOMO-LUMO energy gap than monocationic complex 1b. The small quantum yield of 1b was attributed to intramolecular hydrogen bonding between pyrazole and pyrazolate ligands. The apparent rate constants for the photosubstitution of 1b were highly solvent-dependent. The photosubstitution of 1b was suppressed in aprotic solvents, while the reaction was accelerated by 2 orders of magnitude in protic solvents with strong proton donor abilities.

Optical properties of photodynamic therapy drugs in different environments: the paradigmatic case of temoporfin

Computational tools have been used to study the photophysical and photochemical features of photosensitizers in photodynamic therapy (PDT) - a minimally invasive, less aggressive alternative for cancer treatment. PDT is mainly based on the activation of molecular oxygen through the action of a photoexcited sensitizer (photosensitizer). Temoporfin, widely known as mTHPC, is a second-generation photosensitizer, which produces the cytotoxic singlet oxygen when irradiated with visible light and hence destroys tumor cells. However, the bioavailability of the mostly hydrophobic photosensitizer, and hence its incorporation into cells, is fundamental to achieve the desired effect on malignant tissues via PDT. In this study, we focus on the optical properties of the temoporfin chromophore in different environments -in vacuo, in solution, encapsulated in drug delivery agents, namely cyclodextrin, and interacting with a lipid bilayer.

Design and synthesis of lipid-mimetic cationic iridium complexes and their liposomal formulation for in vitro and in vivo application in luminescent bioimaging

Two iridium [Ir(N^C)₂(N^N)]⁺ complexes with the diimine N^N ligand containing a long polymethylene hydrophobic chain were synthesized and characterized by using NMR and ESI mass-spectrometry: N^N – 2-(1-hexadecyl-1H-imidazol-2-yl)pyridine, N^C – methyl-2-phenylquinoline-4-carboxylate (Ir1) and 2-phenylquinoline-4-carboxylic acid (Ir2). These complexes were used to prepare the luminescent PEGylated DPPC liposomes (DPPC/DSPE-PEG2000/Ir-complex = 95/4.5/1 mol%) using a thin film hydration method. The narrowly dispersed liposomes had diameters of about 110 nm. The photophysics of the complexes and labeled liposomes were carefully studied. Ir1 and Ir2 give red emission (λ_em = 667 and 605 nm) with a lifetime in the microsecond domain and quantum yields of 4.8% and 10.0% in degassed solution. Incorporation of the complexes into the liposome lipid bilayer results in shielding of the emitters from interaction with molecular oxygen and partial suppression of excited state nonradiative relaxation due to the effect of the relatively rigid bilayer matrix. Delivery of labeled liposomes to the cultured ARPE-19 cells demonstrated the usefulness of Ir1 and Ir2 in cellular imaging. Labeled liposomes were then injected intravitreally into rat eyes and imaged successfully with optical coherence tomography and funduscopy. In conclusion, iridium complexes enabled the successful labeling and imaging of liposomes in cells and animals.

Novel lipoidal Ir(iii) phosphorescent labels were embedded into liposomes and used for imaging in cells and animals.

Metallopolymer Organohydrogels with Photo-Controlled Coordination Crosslinks Work Properly Below 0 °C

Controlling the structures and functions of gels is important for both fundamental research and technological applications. Introducing photoresponsive units into gels enables remote control of their properties with light. However, existing gels show photoresponsiveness only at room temperature or elevated temperatures. The development of photoresponsive gels that work below 0 °C can expand their usage in cold environments. Here, photoresponsive metallopolymer organohydrogels that function even at -20 °C are reported. The organohydrogels are prepared using photoresponsive Ru-thioether coordination bonds as reversible crosslinks to form polymer networks. A water/glycerol mixture is used as an anti-freezing solvent. At -20 °C, the Ru-thioether coordination bonds are dissociated under light irradiation and reformed reversibly in the dark, which result in alternating crosslinking densities in the polymer networks. This process enables inducing reversible gel-to-sol transitions, healing damaged gels, controlling the mechanical properties and volumes of the gels, and rewriting microstructures on the gels below 0 °C.

Interaction of Hydrophobic Tungsten Cluster Complexes with a Phospholipid Bilayer

Nanoconstructions composed of lipid vesicles and inorganic units (nanoparticles, metal complexes) arouse much interest across materials science and nanotechnology as hybrid materials combining useful functionalities from both parts. Ideally, these units are to be embedded into the bilayer to keep the biophysical performance of lipid vesicles having inorganic moieties screened from the environment. This can be achieved by doping a lipid bilayer with cluster complexes of transition metals. In this work, we report the preparation of nanoparticles from trinuclear W₃S₄ cluster complexes and egg phosphatidylcholine. A systematic study of their properties was performed by the differential scanning calorimetry, NMR spectroscopy, dynamic light scattering, and transmission electron microscopy. Phospholipids and clusters have been found to spontaneously self-assemble into novel cluster-lipid hybrid materials. The behavior of clusters in the hydrophobic lipid environment is determined by the structure of the ligands and cluster-to-lipid ratio. Intact cluster complexes bearing compact hydrophobic ligands are embedded into the hydrophobic midplane of a lipid bilayer, whereas cluster complexes bearing larger ligands drive the aggregation of lipids and cluster complexes. Considering these differences, it could be possible to obtain different self-assembled associates such as cluster-doped liposomes or lipid-covered crystals. These cluster-lipid hybrids can be a platform for the design of new materials for nanotechnology.

Photo- and Redox-Driven Artificial Molecular Motors

Directed motion at the nanoscale is a central attribute of life, and chemically driven motor proteins are nature's choice to accomplish it. Motivated and inspired by such bionanodevices, in the past few decades chemists have developed artificial prototypes of molecular motors, namely, multicomponent synthetic species that exhibit directionally controlled, stimuli-induced movements of their parts. In this context, photonic and redox stimuli represent highly appealing modes of activation, particularly from a technological viewpoint. Here we describe the evolution of the field of photo- and redox-driven artificial molecular motors, and we provide a comprehensive review of the work published in the past 5 years. After an analysis of the general principles that govern controlled and directed movement at the molecular scale, we describe the fundamental photochemical and redox processes that can enable its realization. The main classes of light- and redox-driven molecular motors are illustrated, with a particular focus on recent designs, and a thorough description of the functions performed by these kinds of devices according to literature reports is presented. Limitations, challenges, and future perspectives of the field are critically discussed.

Incorporation of 7-dehydrocholesterol into liposomes as a simple, universal and efficient way to enhance anticancer activity by combining PDT and photoactivated chemotherapy

Incorporation of 7-dehydrocholesterol instead of cholesterol can efficiently enhance the anticancer activity of photosensitizer-encapsulated liposomes upon irradiation.

Photoisomerization and thermal isomerization of ruthenium aqua complexes with chloro-substituted asymmetric bidentate ligands

Introduction of a chloro substituent to the bidentate ligand of ruthenium aqua complexes enhanced photoisomerization and thermal back-isomerization.

Reconfiguring surface functions using visible-light-controlled metal-ligand coordination

Most surfaces are either static or switchable only between “on” and “off” states for a specific application. It is a challenge to develop reconfigurable surfaces that can adapt to rapidly changing environments or applications. Here, we demonstrate fabrication of surfaces that can be reconfigured for user-defined functions using visible-light-controlled Ru–thioether coordination chemistry. We modify substrates with Ru complex Ru-H₂O. To endow a Ru-H₂O-modified substrate with a certain function, a functional thioether ligand is immobilized on the substrate via Ru–thioether coordination. To change the surface function, the immobilized thioether ligand is cleaved from the substrate by visible-light-induced ligand dissociation, and then another thioether ligand with a distinct function is immobilized on the substrate. Different thioethers endow the surface with different functions. Based on this strategy, we rewrite surface patterns, manipulate protein adsorption, and control surface wettability. This strategy enables the fabrication of reconfigurable surfaces with customizable functions on demand.

Configuring surfaces on-demand for desired functionalities is an ongoing challenge. Here, diverse and tailorable modifications of quartz and porous silica surfaces that are rapidly and reversibly switchable by the use of visible light are achieved via ruthenium-thioether coordination.

Synthesis and Avidin Binding of Ruthenium Complexes Functionalized with a Light-Cleavable Free Biotin Moiety

In this work the synthesis, photochemistry, and streptavidin interaction of new [Ru(tpy)(bpy)(SRR′)](PF₆)₂ complexes where the R′ group contains a free biotin ligand, are described. Two different ligands SRR′ were investigated: An asymmetric ligand 1 where the Ru‐bound thioether is a N‐acetylmethionine moiety linked to the free biotin fragment via a triethylene glycol spacer and a symmetrical ligand 2 containing two identical biotin moieties. The coordination of these two ligands to the precursor [Ru(tpy)(bpy)Cl]Cl was studied in water at 80 °C. In such conditions the coordination of the asymmetric ligand 1 occurred under thermodynamic control. After the reaction, a mononuclear and a binuclear complex were isolated. In the mononuclear complex, the ratio of methionine‐ {[6](PF₆)₂} vs. biotin‐bound {[7](PF₆)₂} regioisomer was 5.3 and the free biotin fragment of [6](PF₆)₂ allowed to purify it from its isomer [7](PF₆)₂ at small scales using avidin affinity chromatography. Coordination of the symmetrical ligand 2 afforded [Ru(tpy)(bpy)(2)](PF₆)₂ {[8](PF₆)₂} in synthetically useful scales (100 mg), good yield (82 %), and without traces of the binuclear impurity. In this complex, one of the biotin remains free whereas the second one is coordinated to ruthenium. Photochemical release of ligand 2 from [8](PF₆)₂ occurred upon blue light irradiation (465 nm) with a photosubstitution quantum yield of 0.011 that was independent of the binding of streptavidin to the free biotin ligand.

Ru(II) Polypyridyl Complexes Derived from Tetradentate Ancillary Ligands for Effective Photocaging

Metal complexes have many proven applications in the caging and photochemical release of biologically active compounds. Photocaging groups derived from Ru(II) traditionally have been composed of ancillary ligands that are planar and bi- or tridentate, such as 2,2'-bipyridine (bpy), 2,2':6',2″-terpyridine (tpy), and 1,10-phenanthroline (phen). Complexes bearing ancillary ligands with denticities higher than three represent a new class of Ru(II)-based photocaging groups that are grossly underdeveloped. Because high-denticity ancillary ligands provide the ability to increase the structural rigidity and control the stereochemistry, our groups initiated a research program to explore the applications of such ligands in Ru(II)-based photocaging. Ru(TPA), bearing the tetradentate ancillary ligand tris(2-pyridylmethyl)amine (TPA), has been successfully utilized to effectively cage nitriles and aromatic heterocycles. Nitriles and aromatic heterocycles caged by the Ru(TPA) group show excellent stability in aqueous solutions in the dark, and the complexes can selectively release the caged molecules upon irradiation with light. Ru(TPA) is applicable as a photochemical agent to offer precise spatiotemporal control over biological activity without undesired toxicity. In addition, Ru(II) polypyridyl complexes with desired photochemical properties can be synthesized and identified by solid-phase synthesis, and the resulting complexes show properties to similar to those of complexes obtained by solution-phase synthesis. Density functional theory (DFT) calculations reveal that orbital mixing between the π* orbitals of the ancillary ligand and the Ru-N dσ* orbital is essential for ligand photodissociation in these complexes. Furthermore, the introduction of steric bulk enhances the photoliability of the caged molecules, validating that steric effects can largely influence the quantum efficiency of photoinduced ligand exchange in Ru(II) polypyridyl complexes. Recently, two new photocaging groups, Ru(cyTPA) and Ru(1-isocyTPQA), have been designed and synthesized for caging of nitriles and aromatic heterocycles, and these complexes exhibit unique photochemical properties distinct from those derived from Ru(TPA). Notably, the unusually greater quantum efficiency for the ligand exchange in [Ru(1-isocyTPQA)(MeCN)₂](PF₆)₂, Φ₄₀₀ = 0.033(3), uncovers a trans-type effect in the triplet metal-to-ligand charge transfer (³MLCT) state that enhances photoinduced ligand exchange in a new manner. DFT calculations and ultrafast transient spectroscopy reveal that the lowest-energy triplet state in [Ru(1-isocyTPQA)(MeCN)₂](PF₆)₂ is a highly mixed ³MLCT/³ππ* excited state rather than a triplet metal-centered ligand-field (³LF) excited state; the latter is generally accepted for ligand photodissociation. In addition, Mulliken spin density calculations indicate that a majority of the spin density in [Ru(1-isocyTPQA)(MeCN)₂](PF₆)₂ is localized on the isoquinoline arm, which is opposite to the cis MeCN, rather than on the ruthenium center. This significantly weakens the Ru-N6 ( cis MeCN) bond, which then promotes the ligand photodissociation. This newly discovered effect gives a clearer perception of the interplay between the ³MLCT and ³LF excited states of Ru(II) polypyridyl complexes, which may be useful in the design and applications of ruthenium complexes in the areas of photoactivated drug delivery and photosensitizers.

Ru(ii) polypyridyl complexes as photocages for bioactive compounds containing nitriles and aromatic heterocycles

Photocaging allows for precise spatiotemporal control over the release of biologically active compounds with light. Most photocaged molecules employ organic photolabile protecting groups; however, biologically active compounds often contain functionalities such as nitriles and aromatic heterocycles that cannot be caged with organic groups. Despite their prevalence, only a few studies have reported successful caging of nitriles and aromatic heterocycles. Recently, Ru(ii)-based photocaging has emerged as a powerful method for the release of bioactive molecules containing these functional groups, in many cases providing high levels of spatial and temporal control over biological activity. This Feature Article discusses recent developments in applying Ru(ii)-based photocaging towards biological problems. Our groups designed and synthesized Ru(ii)-based platforms for the photoinduced delivery of cysteine protease and cytochrome P450 inhibitors in order to achieve selective control over enzyme inhibition. We also reported Ru(ii) photocaging groups derived from higher-denticity ancillary ligands that possess photophysical and photochemical properties distinct from more traditional Ru(ii)-based caging groups. In addition, for the first time, we are able to rapidly synthesize and screen Ru(ii) polypyridyl complexes that elicit desired properties by solid-phase synthesis. Finally, our work also defined steric and orbital mixing effects that are important factors in controlling photoinduced ligand exchange.

Upconversion Nanocarriers Encapsulated with Photoactivatable Ru Complexes for Near-Infrared Light-Regulated Enzyme Activity

Enzyme activity is important for metabolism, cell functions, and treating diseases. However, remote control of enzyme activity in deep tissue remains a challenge. This study demonstrates near-infrared (NIR) light-regulated enzyme activity in living cells based on upconverting nanoparticles (UCNPs) and a photoactivatable Ru complex. The Ru complex is a caged enzyme inhibitor that can be activated by blue light. To prepare a nanocarrier for NIR photoinhibition of enzyme activity, a UCNP and the caged enzyme inhibitors are encapsulated in a hollow mesoporous silica nanoparticle. In such a nanocarrier, the UCNP can harvest NIR light and convert it into blue light, which can activate the caged enzyme inhibitors. This photoactivation process is feasible in deep tissue because of the tissue penetration ability of NIR light. The nanocarrier is compatible to LNCaP, PC3, and SAOS-2 cells, which show high enzyme expression. NIR irradiation induces release of the inhibitors and inhibition of enzyme activity in living cells. NIR light provides high spatiotemporal resolution to regulate enzyme activity in deep tissue.

Photochemical and Photobiological Activity of Ru(II) Homoleptic and Heteroleptic Complexes Containing Methylated Bipyridyl-type Ligands

Light-activated compounds are powerful tools and potential agents for medical applications, as biological effects can be controlled in space and time. Ruthenium polypyridyl complexes can induce cytotoxic effects through multiple mechanisms, including acting as photosensitizers for singlet oxygen (¹O₂) production, generating other reactive oxygen species (ROS), releasing biologically active ligands, and creating reactive intermediates that form covalent bonds to biological molecules. A structure-activity relationship (SAR) study was performed on a series of Ru(II) complexes containing isomeric tetramethyl-substituted bipyridyl-type ligands. Three of the ligand systems studied contained strain-inducing methyl groups and created photolabile metal complexes, which can form covalent bonds to biomolecules upon light activation, while the fourth was unstrained and resulted in photostable complexes, which can generate ¹O₂. The compounds studied included both bis-heteroleptic complexes containing two bipyridine ligands and a third, substituted ligand and tris-homoleptic complexes containing only the substituted ligand. The photophysics, electrochemistry, photochemistry, and photobiology were assessed. Strained heteroleptic complexes were found to be more photoactive and cytotoxic then tris-homoleptic complexes, and bipyridine ligands were superior to bipyrimidine. However, the homoleptic complexes exhibited an enhanced ability to inhibit protein production in live cells. Specific methylation patterns were associated with improved activation with red light, and photolabile complexes were generally more potent cytotoxic agents than the photostable ¹O₂-generating compounds.

An Amphiphilic Ruthenium Polymetallodrug for Combined Photodynamic Therapy and Photochemotherapy In Vivo

An amphiphilic Ru-containing block copolymer is used as a photoactivated polymetallodrug for anticancer phototherapy. The block copolymer self-assembles into nanoparticles, which can accumulate at tumor sites in a mouse model. Red light irradiation of the block copolymer nanoparticles releases anticancer Ru complexes and generates cytotoxic ¹ O₂ , both of which can inhibit tumor growth.

Synthesis, characterisation and self-assembly behaviour of emissive surfactant–ruthenium(ii) complexes

Bipyridine/phenanthroline based dodecylamine/hexadecylamine containing surfactant–ruthenium(ii) complexes form green fluorescent vesicles with an excellent stability in the aqueous medium.

Photoisomerization of ruthenium(ii) aquo complexes: mechanistic insights and application development

The perspective article highlights a new strategic synthesis of dinuclear ruthenium(ii) complexes acting as active water oxidation catalysts and also reports the development of unique visible-light-responsive giant vesicles, both of which are achieved based on photoisomerization.

Ruthenation of Non-stacked Guanines in DNA G-Quadruplex Structures: Enhancement of c-MYC Expression

Guanine quadruplexes (GQs) are compact four-stranded DNA structures that play a key role in the control of a variety of biological processes, including gene transcription. Bulky ruthenium complexes featuring a bipyridine, a terpyridine, and one exchangeable ligand ([Ru(terpy)(bpy)X]n+ ) are able to metalate exposed guanines present in the GQ of the c-MYC promoter region that are not involved in quadruplex base pairing. qRT-PCR and western-blot experiments indicated that the complexes promote a remarkable increase in the expression of this oncogene. We also show that exchangeable thioether ligands (X=RSR', Met) allow regulation of the metalating activity of the complex with visible light.

Near-Infrared-Triggered Azobenzene-Liposome/Upconversion Nanoparticle Hybrid Vesicles for Remotely Controlled Drug Delivery to Overcome Cancer Multidrug Resistance

Overcoming multidrug resistance is achieved by developing a novel drugdelivery-system paradigm based on azobenzene liposome and phosphatidylcholine-modified upconversion nanoparticle (UCNP) hybrid vesicles for controlled drug release using a nearinfrared (NIR) laser. Upon 980 nm light irradiation, the reversible photoisomerization of the azobenzene derivatives by simultaneous UV and visible light emitted from the UCNPs makes it possible to realize NIR-triggered release of the chemotherapeutic drug doxorubicin.

Multifunctional Nanoparticles Self-Assembled from Small Organic Building Blocks for Biomedicine

Supramolecular self-assembly shows significant potential to construct responsive materials. By tailoring the structural parameters of organic building blocks, nanosystems can be fabricated, whose performance in catalysis, energy storage and conversion, and biomedicine has been explored. Since small organic building blocks are structurally simple, easily modified, and reproducible, they are frequently employed in supramolecular self-assembly and materials science. The dynamic and adaptive nature of self-assembled nanoarchitectures affords an enhanced sensitivity to the changes in environmental conditions, favoring their applications in controllable drug release and bioimaging. Here, recent significant research advancements of small-organic-molecule self-assembled nanoarchitectures toward biomedical applications are highlighted. Functionalized assemblies, mainly including vesicles, nanoparticles, and micelles are categorized according to their topological morphologies and functions. These nanoarchitectures with different topologies possess distinguishing advantages in biological applications, well incarnating the structure-property relationship. By presenting some important discoveries, three domains of these nanoarchitectures in biomedical research are covered, including biosensors, bioimaging, and controlled release/therapy. The strategies regarding how to design and characterize organic assemblies to exhibit biomedical applications are also discussed. Up-to-date research developments in the field are provided and research challenges to be overcome in future studies are revealed.

Chemical Swarming: Depending on Concentration, an Amphiphilic Ruthenium Polypyridyl Complex Induces Cell Death via Two Different Mechanisms

The crystal structure and in vitro cytotoxicity of the amphiphilic ruthenium complex [3](PF6 )2 are reported. Complex [3](PF6 )2 contains a Ru-S bond that is stable in the dark in cell-growing medium, but is photosensitive. Upon blue-light irradiation, complex [3](PF6 )2 releases the cholesterol-thioether ligand 2 and an aqua ruthenium complex [1](PF6 )2 . Although ligand 2 and complex [1](PF6 )2 are by themselves not cytotoxic, complex [3](PF6 )2 was unexpectedly found to be as cytotoxic as cisplatin in the dark, that is, with micromolar effective concentrations (EC50 ), against six human cancer cell lines (A375, A431, A549, MCF-7, MDA-MB-231, and U87MG). Blue-light irradiation (λ=450 nm, 6.3 J cm(-2) ) had little influence on the cytotoxicity of [3](PF6 )2 after 6 h of incubation time, but it increased the cytotoxicity of the complex by a factor 2 after longer (24 h) incubation. Exploring the unexpected biological activity of [3](PF6 )2 in the dark elucidated an as-yet unknown bifaceted mode of action that depended on concentration, and thus, on the aggregation state of the compound. At low concentration, it acts as a monomer, inserts into the membrane, and can deliver [1](2+) inside the cell upon blue-light activation. At higher concentrations (>3-5 μm), complex [3](PF6 )2 forms supramolecular aggregates that induce non-apoptotic cell death by permeabilizing cell membranes and extracting lipids and membrane proteins.

Green light-induced apoptosis in cancer cells by a tetrapyridyl ruthenium prodrug offering two trans coordination sites

In this work, two new photopharmacological ruthenium prodrugs are described that can be activated by green light. Cell death occurs via apoptosis; it is not a consequence of singlet oxygen generation, but of light-induced photosubstitution reactions.

In this work, two new photopharmacological ruthenium prodrugs are described that can be activated by green light. They are based on the tetrapyridyl biqbpy ligand (6,6′-bis[N-(isoquinolyl)-1-amino]-2,2′-bipyridine), which coordinates to the basal plane of the metal centre and leaves two trans coordination sites for the binding of monodentate sulphur ligands. Due to the distortion of the coordination sphere these trans ligands are photosubstituted by water upon green light irradiation. In vitro cytotoxicity data on A431 and A549 cancer cell lines shows an up to 22-fold increase in cytotoxicity after green light irradiation (520 nm, 75 J cm^–2), compared to the dark control. Optical microscopy cell imaging and flow cytometry indicate that the cancer cells die via apoptosis. Meanwhile, very low singlet oxygen quantum yields (∼1–2%) and cell-free DNA binding studies conclude that light-induced cell death is not caused by a photodynamic effect, but instead by the changes induced in the coordination sphere of the metal by light, which modifies how the metal complexes bind to biomolecules.

An Artificial Molecular Transporter

The transport of substrates is one of the main tasks of biomolecular machines in living organisms. We report a synthetic small-molecule system designed to catch, displace, and release molecular cargo in solution under external control. The system consists of a bistable rotaxane that behaves as an acid-base controlled molecular shuttle, whose ring component bears a tether ending with a nitrile group. The latter can be coordinated to a ruthenium complex that acts as the load, and dissociated upon irradiation with visible light. The cargo loading/unloading and ring transfer/return processes are reversible and can be controlled independently. The robust coordination bond ensures that the cargo remains attached to the device while the transport takes place.

Ruthenium-Containing Block Copolymer Assemblies: Red-Light-Responsive Metallopolymers with Tunable Nanostructures for Enhanced Cellular Uptake and Anticancer Phototherapy

The use of self-assembled nanostructures consisting of red-light-responsive Ru(II)-containing block copolymers (BCPs) for anticancer phototherapy is demonstrated. Three Ru-containing BCPs with different molecular weights are synthesized. Each BCP contains a hydrophilic poly(ethylene glycol) block and an Ru-containing block. In the Ru-containing block, more than half of the side chains are coordinated with [Ru(2,2':6',2''-terpyridine)(2,2'-biquinoline)](2+) , resulting in more than 40 wt% Ru complex in the BCPs. The Ru complex acts as both a red-light-cleavable moiety and a photoactivated prodrug. Depending on their molecular weights, the BCPs assemble into micelles, vesicles, and large compound micelles. All of the BCP assemblies are taken up by cancer cells. Red-light irradiation releases the Ru complex and generates singlet oxygen ((1) O2 ) in cancer cells. The released Ru complex and (1) O2 inhibit the growth of cancer cells. Among the three BCP assemblies, the BCP micelle exhibits the most efficient cellular uptake and best anticancer performance.

Exploring the photochemosensitivity by novel cysteine-based mixed ligand complexes

A new series of cysteine-based metal(II) complexes with 2,2'-bipyridine or 1,10-phenanthroline as co-ligand have been prepared and characterized. Their DNA binding and cleavage properties have been studied. The analytical and spectroscopic data of complexes 1-18 reveal that the complexes adopt an octahedral geometry around the central metal ion in which the cysteine is coordinated through NS and NN atoms, respectively. Spectroscopic titration and viscosity measurements reveal that the complexes bind to DNA through an intercalative mode. Electrophoresis measurements exhibit that they cleave pBR322 DNA efficiently in the presence of 3-mercaptopropionic acid (MPA), probably via hydrolytic mechanism with the involvement of (•)OH. The in vitro anticancer activities indicate that the Cu(II) complexes are active against four selected human tumor cell lines. Furthermore, it is remarkable that all the complexes exhibit significant photocytotoxicity against human breast cancer cell lines (MCF-7) with a potency more than the widely used drugs photofrin and cisplatin indicating that they have the potential to act as effective anticancer drugs in a dose-dependent manner.

Manipulating pH using near-infrared light assisted by upconverting nanoparticles

Control of pH was achieved using near-infrared light assisted by upconverting nanoparticles and a ruthenium complex photobase.

Triplet-triplet annihilation upconversion followed by FRET for the red light activation of a photodissociative ruthenium complex in liposomes

Upconversion is a promising way to trigger high-energy photochemistry with low-energy photons. However, combining upconversion schemes with non-radiative energy transfer is challenging because bringing several photochemically active components in close proximity results in complex multi-component systems where quenching processes may deactivate the whole assembly. In this work, PEGylated liposomes were prepared that contained three photoactive components: a porphyrin dye absorbing red light, a perylene moiety emitting in the blue, and a light-activatable ruthenium prodrug sensitive to blue light. Time-dependent spectroscopic studies demonstrate that singlet perylene excited states are non-radiatively transferred to the nearby ruthenium complex by Förster resonance energy transfer (FRET). Under red-light irradiation of the three-component membranes, triplet-triplet annihilation upconversion (TTA-UC) occurs followed by FRET, which results in a more efficient activation of the ruthenium prodrug compared to a physical mixture of two-component upconverting liposomes and liposomes containing only the ruthenium complex. This work represents a rare example where TTA-UC and Förster resonance energy transfer are combined to achieve prodrug activation in the phototherapeutic window.