Long Excited-State Lifetimes in Three-Coordinate Copper(I) Complexes via Triplet-Triplet Energy Transfer to Pyrene-Decorated Isocyanides

There has been much effort to improve excited-state lifetimes in photosensitizers based on earth-abundant first-row transition metals. Copper(I) complexes have gained significant attention in this field, and in most cases, sterically driven approaches are used to optimize their lifetimes. This study presents a series of three-coordinate copper(I) complexes (Cu1-Cu3) where the excited-state lifetime is extended by triplet-triplet energy transfer. The heteroleptic compounds feature a cyclohexyl-substituted β-diketiminate (CyNacNacMe) paired with aryl isocyanide ligands, giving the general formula Cu(CyNacNacMe)(CN-Ar) (CN-dmp = 2,6-dimethylphenyl isocyanide for Cu1; CN-pyr = 1-pyrenyl isocyanide for Cu2; CN-dmp-pyr = 2,6-dimethyl-4-(1-pyrenyl)phenyl isocyanide for Cu3). The nature, energies, and dynamics of the low-energy triplet excited states are assessed with a combination of photoluminescence measurements at room temperature and 77 K, ultrafast transient absorption (UFTA) spectroscopy, and DFT calculations. The complexes with the pyrene-decorated isocyanides (Cu2 and Cu3) exhibit extended excited-state lifetimes resulting from triplet-triplet energy transfer (TTET) between the short-lived charge-transfer excited state (3CT) and the long-lived pyrene-centered triplet state (3pyr). This TTET process is irreversible in Cu3, producing exclusively the 3pyr state, and in Cu2, the 3CT and 3pyr states are nearly isoenergetic, enabling reversible TTET and long-lived 3CT luminescence. The improved photophysical properties in Cu2 and Cu3 result in improvements in activity for both photocatalytic stilbene E/Z isomerization via triplet energy transfer and photoredox transformations involving hydrodebromination and C-O bond activation. These results illustrate that the extended excited-state lifetimes achieved through TTET result in newly conceived photosynthetically relevant earth-abundant transition metal complexes.

Fifty Shades of Phenanthroline: Synthesis Strategies to Functionalize 1,10-Phenanthroline in All Positions

1,10-Phenanthroline (phen) is one of the most popular ligands ever used in coordination chemistry due to its strong affinity for a wide range of metals with various oxidation states. Its polyaromatic structure provides robustness and rigidity, leading to intriguing features in numerous fields (luminescent coordination scaffolds, catalysis, supramolecular chemistry, sensors, theranostics, etc.). Importantly, phen offers eight distinct positions for functional groups to be attached, showcasing remarkable versatility for such a simple ligand. As a result, phen has become a landmark molecule for coordination chemists, serving as a must-use ligand and a versatile platform for designing polyfunctional arrays. The extensive use of substituted phenanthroline ligands with different metal ions has resulted in a diverse array of complexes tailored for numerous applications. For instance, these complexes have been utilized as sensitizers in dye-sensitized solar cells, as luminescent probes modified with antibodies for biomaterials, and in the creation of elegant supramolecular architectures like rotaxanes and catenanes, exemplified by Sauvage's Nobel Prize-winning work in 2016. In summary, phen has found applications in almost every facet of chemistry. An intriguing aspect of phen is the specific reactivity of each pair of carbon atoms ([2,9], [3,8], [4,7], and [5,6]), enabling the functionalization of each pair with different groups and leading to polyfunctional arrays. Furthermore, it is possible to differentiate each position in these pairs, resulting in non-symmetrical systems with tremendous versatility. In this Review, the authors aim to compile and categorize existing synthetic strategies for the stepwise polyfunctionalization of phen in various positions. This comprehensive toolbox will aid coordination chemists in designing virtually any polyfunctional ligand. The survey will encompass seminal work from the 1950s to the present day. The scope of the Review will be limited to 1,10-phenanthroline, excluding ligands with more intracyclic heteroatoms or fused aromatic cycles. Overall, the primary goal of this Review is to highlight both old and recent synthetic strategies that find applicability in the mentioned applications. By doing so, the authors hope to establish a first reference for phenanthroline synthesis, covering all possible positions on the backbone, and hope to inspire all concerned chemists to devise new strategies that have not yet been explored.

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.

Characterization of Single-Spheroid Oxygen Consumption Using a Microfluidic Platform and Fluorescence Lifetime Imaging Microscopy

Oxygen consumption has been used to evaluate various cellular activities. In addition, three-dimensional (3D) spheroids have been broadly exploited as advanced in vitro cell models for various biomedical studies due to their capability of mimicking 3D in vivo microenvironments and cell arrangements. However, monitoring the oxygen consumption of live 3D spheroids poses challenges because existing invasive methods cause structural and cell damage. In contrast, optical methods using fluorescence labeling and microscopy are non-invasive, but they suffer from technical limitations like high cost, tedious procedures, and poor signal-to-noise ratios. To address these challenges, we developed a microfluidic platform for uniform-sized spheroid formation, handling, and culture. The platform is further integrated with widefield frequency domain fluorescence lifetime imaging microscopy (FD-FLIM) to efficiently characterize the lifetime of an oxygen-sensitive dye filling the platform for oxygen consumption characterization. In the experiments, osteosarcoma (MG-63) cells are exploited as the spheroid model and for the oxygen consumption analysis. The results demonstrate the functionality of the developed approach and show the accurate characterization of the oxygen consumption of the spheroids in response to drug treatments. The developed approach possesses great potential to advance spheroid metabolism studies with single-spheroid resolution and high sensitivity.

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.

Coulomb interactions for mediator-enhanced sensitized triplet-triplet annihilation upconversion in solution

Sensitized triplet-triplet annihilation upconversion offers an attractive possibility to replace a high-energy photon by two photons with lower energy through the combination of a light-harvesting triplet sensitizer and an annihilator for the formation of a fluorescent singlet state. Typically, high annihilator concentrations are required to achieve an efficient initial energy transfer and as a direct consequence the most highly energetic emission is often not detectable due to intrinsic reabsorption by the annihilator itself. Herein, we demonstrate that the addition of a charge-adapted mediator drastically improves the energy transfer efficiency at low annihilator concentrations via an energy transfer cascade. Inspired by molecular dyads and recent developments in nanocrystal-sensitized upconversion, our system exploits a concept to minimize intrinsic filter effects, while boosting the upconversion quantum yield in solution. A sensitizer-annihilator combination consisting of a ruthenium-based complex and 9,10-diphenylanthracene (DPA) is explored as model system and a sulfonated pyrene serves as mediator. The impact of opposite charges between sensitizer and mediator - to induce coulombic attraction and subsequently result in accelerated energy transfer rate constants - is analyzed in detail by different spectroscopic methods. Ion pairing and the resulting static energy transfer in both directions is a minor process, resulting in an improved overall performance. Finally, the more intense upconverted emission in the presence of the mediator is used to drive two catalytic photoreactions in a two-chamber setup, illustrating the advantages of our approach, in particular for photoreactions requiring oxygen that would interfere with the upconversion system.

Improved transition metal photosensitizers to drive advances in photocatalysis

To function effectively in a photocatalytic application, a photosensitizer's light absorption, excited-state lifetime, and redox potentials, both in the ground state and excited state, are critically important. The absorption profile is particularly relevant to applications involving solar harvesting, whereas the redox potentials and excited-state lifetimes determine the thermodynamics, kinetics, and quantum yields of photoinduced redox processes. This perspective article focuses on synthetic inorganic and organometallic approaches to optimize these three characteristics of transition-metal based photosensitizers. We include our own work in these areas, which has focused extensively on exceptionally strong cyclometalated iridium photoreductants that enable challenging reductive photoredox transformations on organic substrates, and more recent work which has led to improved solar harvesting in charge-transfer copper(i) chromophores, an emerging class of earth-abundant compounds particularly relevant to solar-energy applications. We also extensively highlight many other complementary strategies for optimizing these parameters and highlight representative examples from the recent literature. It remains a significant challenge to simultaneously optimize all three of these parameters at once, since improvements in one often come at the detriment of the others. These inherent trade-offs and approaches to obviate or circumvent them are discussed throughout.

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.

Recent Advances in Mitochondria-Localized Luminescent Ruthenium(II) Metallodrugs as Anticancer Agents

Presently, the most effective way to transport drugs specifically to mitochondria inside the cells is of pharmacophoric interest, as mitochondria are recognized as one of the most important targets for new drug design in cancer diagnosis. To date, there are many reviews covering the photophysical, photochemical, and anticancer properties of ruthenium(II) based metallodrugs owing to their high interest in biological applications. There are, however, no reviews specifically covering the mitochondria-localized luminescent Ru(II) complexes and their subsequent mitochondria-mediated anticancer activities. Therefore, this review describes the physicochemical basis for the mitochondrial accumulation of ruthenium complexes, their synthetic strategies to localize and monitor the mitochondria in living cells, and their related underlying anticancer results. Finally, we review the related areas from previous works describing the mitochondria-localized ruthenium complexes for the treatment of cancer-related diseases. Along with this, we also deliberate the perspectives and future directions for emerging more bifunctional Ru(II) complexes that can target, image, and kill tumors more efficiently in comparison with the existing mitochondria-targeted cancer therapeutics.

Heavy Atom-Free Triplet Photosensitizers: Molecular Structure Design, Photophysical Properties and Application in Photodynamic Therapy

Photodynamic therapy (PDT) is a promising method for the treatment of cancer, because of its advantages including a low toxicity, non-drug-resistant character, and targeting capability. From a photochemical aspect, a critical property of triplet photosensitizers (PSs) used for PDT reagents is the intersystem crossing (ISC) efficiency. Conventional PDT reagents are limited to porphyrin compounds. However, these compounds are difficult to prepare, purify, and derivatize. Thus, new molecular structure paradigms are desired to develop novel, efficient, and versatile PDT reagents, especially those contain no heavy atoms, such as Pt or I, etc. Unfortunately, the ISC ability of heavy atom-free organic compounds is usually elusive, and it is difficult to predict the ISC capability of these compounds and design novel heavy atom-free PDT reagents. Herein, from a photophysical perspective, we summarize the recent developments of heavy atom-free triplet PSs, including methods based on radical-enhanced ISC (REISC, facilitated by electron spin-spin interaction), twisted π-conjugation system-induced ISC, the use of fullerene C₆₀ as an electron spin converter in antenna-C₆₀ dyads, energetically matched S₁/T_n states-enhanced ISC, etc. The application of these compounds in PDT is also briefly introduced. Most of the presented examples are the works of our research group.

Effects of NiO, SnO2, and Ni-doped SnO2 semiconductor metal oxides on the oxygen sensing capacity of H2TPP

Improving the performance of optical oxygen sensors can be accomplished by adding metal oxide semiconductors (MOSs) additives to the composition comprising an oxygen-sensing agent immobilized in a polymeric thin film. For several decades, MOSs have attracted great interest in gas sensors due to their high sensitivity to many target gasses. Herein, meso-tetraphenylporphyrin (H₂TPP) dye was immobilized into the poly(1-trimethylsilyl-1-propyne) (poly(TMSP)) silicone rubber in the presence of NiO, SnO₂, Ni:SnO₂ metal oxide particles as additives, and their thin films were prepared to investigate oxygen-sensitive optical chemical sensor properties. The characterizations of the synthesized metal oxide powders were carried out through XPS, XRD, FT-IR, PL spectroscopy and SEM methods. Intensity-based spectra and decay kinetics of H₂TPP-based thin films were investigated for the concentration range of 0%-100% [O₂]. The oxygen sensitivity (I₀/I₁₀₀) of the porphyrin was calculated as 70%. Whereas the relative signal intensity values of H₂TPP-based sensor slides were measured as 75%, 80%, and 88% in the presence of NiO, SnO₂, Ni:SnO₂ additives, respectively. The H₂TPP in combination with Ni:SnO₂ semiconductor provided a higher I₀/I₁₀₀ value, larger response range, higher Stern-Volmer constant (K_SV) value, and faster response time compared to the undoped form, and also NiO and SnO₂ additive-doped forms of H₂TPP. The response and the recovery times of the porphyrin-based sensing slide along with Ni:SnO₂ additives have been measured as 12 and 50 s. These results make the H₂TPP along with the MOSs promising candidates as oxygen probes.

Co-facial π-π Interaction Expedites Sensitizer-to-Catalyst Electron Transfer for High-Performance CO2 Photoreduction

The sunlight-driven reduction of CO₂ into carbonaceous fuels can lower the atmospheric CO₂ concentration and provide renewable energy simultaneously, attracting scientists to design photocatalytic systems for facilitating this process. Significant progress has been made in designing high-performance photosensitizers and catalysts in this regard, and further improvement can be realized by installing additional interactions between the abovementioned two components, however, the design strategies and mechanistic investigations on such interactions remain challenging. Here, we present the construction of molecular models for intermolecular π-π interactions between the photosensitizer and the catalyst, via the introduction of pyrene groups into both molecular components. The presence, types, and strengths of diverse π-π interactions, as well as their roles in the photocatalytic mechanism, have been examined by ¹H NMR titration, fluorescence quenching measurements, transient absorption spectroscopy, and quantum chemical simulations. We have also explored the rare dual emission behavior of the pyrene-appended iridium photosensitizer, of which the excited state can deliver the photo-excited electron to the pyrene-decorated cobalt catalyst at a fast rate of 2.60 × 10⁶ s^-1 via co-facial π-π interaction, enabling a remarkable apparent quantum efficiency of 14.3 ± 0.8% at 425 nm and a high selectivity of 98% for the photocatalytic CO₂-to-CO conversion. This research demonstrates non-covalent interaction construction as an effective strategy to achieve rapid CO₂ photoreduction besides a conventional photosensitizer/catalyst design.

Naphthalimide-Carbazole Compact Electron Donor-Acceptor Dyads: Effect of Molecular Geometry and Electron-Donating Capacity on the Spin-Orbit Charge Transfer Intersystem Crossing

We prepared a series of naphthalimide (NI)-carbazole (Cz) compact electron donor-acceptor dyads showing different substitution positions, C-N/C-C linkers, and conformation restriction magnitudes to study the spin-orbit charge transfer intersystem crossing (SOCT-ISC). The varied conformation restrictions lead to different dihedral angles between the donor and acceptor (37°-81°) and electronic coupling magnitude (matrix elements V: 1290-3070 cm^-1). Based on the comparison between the dyads containing C-N and C-C linkers, we found that a large dihedral angle between the donor and acceptor is favorable to efficient SOCT-ISC. For one dyad, the singlet oxygen quantum yield (Φ_Δ) is up to 84.4% (in dichloromethane), which is much higher than that of the previously reported NI-phenothiazine (PTZ) analogue dyad (Φ_Δ = 16.0% in n-hexane). The intrinsic triplet state lifetime (τ_T) is 270 μs, longer than that accessed by the heavy atom effect (75.2 μs). As compared with the NI-PTZ analogue dyad, the Cz unit in the current dyads is a weaker electron donor than PTZ. Thus, a higher CT state energy in NI-Cz dyads was observed, which makes the SOCT-ISC efficient in solvents with a wide range of polarities. Meanwhile, the localized triplet state (³LE) becomes the lowest-lying state in the NI-Cz dyads, which is different from the triplet charge transfer (³CT) state observed in the analogue NI-PTZ dyad. Moreover, the large energy gap between the CT and ³LE states inhibits the reverse ISC; as a result, no thermally activated delayed fluorescence was observed for the current NI-Cz dyads.

Determination and Imaging of Small Biomolecules and Ions Using Ruthenium(II) Complex-Based Chemosensors

Luminescence chemosensors are one of the most useful tools for the determination and imaging of small biomolecules and ions in situ in real time. Based on the unique photo-physical/-chemical properties of ruthenium(II) (Ru(II)) complexes, the development of Ru(II) complex-based chemosensors has attracted increasing attention in recent years, and thus many Ru(II) complexes have been designed and synthesized for the detection of ions and small biomolecules in biological and environmental samples. In this work, we summarize the research advances in the development of Ru(II) complex-based chemosensors for the determination of ions and small biomolecules, including anions, metal ions, reactive biomolecules and amino acids, with a particular focus on binding/reaction-based chemosensors for the investigation of intracellular analytes' evolution through luminescence analysis and imaging. The advances, challenges and future research directions in the development of Ru(II) complex-based chemosensors are also discussed.

Relevance of the electron transfer pathway in photodynamic activity of Ru(II) polypyridyl complexes containing 4,7-diphenyl-1,10-phenanthroline ligands under normoxic and hypoxic conditions

The purpose of this study was to investigate the correlation between the spectroscopic and photophysical properties of Ru(II) polypyridyl complexes and their photodynamic activity in vitro. A series of Ru(II) polypyridyl complexes with 4,7-diphenyl-1,10-phenanthroline (dip) and 2,3-bis(2-pyridyl)quinoxaline (dpq) and its derivatives were synthesized and characterized regarding their photophysical, biological, and photodynamic properties. The complexes were evaluated not only in the context of ¹O₂ generation but also regarding other types of reactive oxygen species (ROS) to assess the possibility of Ru(II) complexes to induce phototoxicity via various ROS using fluorescence and EPR spectroscopy. The compounds were found to be moderately cytotoxic with IC₅₀ values ranging from 1 to 35 μM and retained their cytotoxic activity under hypoxic conditions. The unraveled phototoxic activity is based mainly on the generation of H₂O₂ and ¹O₂, highlighting the importance of electron-transfer processes in the observed photodynamic activity of Ru polypyridyl complexes. A combination of photodynamic activity with cytotoxicity under decreased dioxygen concentrations may help overcome the current photodynamic therapy (PDT) limitation. The findings highlight the need for broadening the scope of tested Ru-based photosensitizers.

Harnessing Bismuth Coordination Chemistry to Achieve Bright, Long-Lived Organic Phosphorescence

A new bismuth(III)-organic compound, Hphen[Bi₂(HPDC)₂(PDC)₂(NO₃)]·4H₂O (Bi-1; PDC = 2,6-pyridinedicarboxylate and phen = 1,10-phenanthroline), was synthesized, and the structure was determined by single-crystal X-ray diffraction. The compound was found to display bright-blue-green phosphorescence in the solid state under UV irradiation, with a luminescent lifetime of 1.776 ms at room temperature. The room temperature and low-temperature (77 K) emission spectra exhibited the vibronic structure characteristic of Hphen phosphorescence. Time-dependent density functional theory studies showed that the excitation pathway arises from an energy transfer from the dimeric structural unit to Hphen, with participation from a nine-coordinate Bi center. The triplet state of Hphen is believed to be stabilized via supramolecular interactions, which, when coupled with the heavy-atom effect induced by Bi, leads to the observed long-lived luminescence. The compound displayed a solid-state quantum yield of over 27%. To the best of our knowledge, this is the first such compound to exhibit phenanthrolinium phosphorescence with such long-lived, room temperature lifetimes in the solid state. To further elucidate the energy-transfer mechanism, Ln³⁺ (Ln = Eu, Tb, Sm) ions were successfully doped into the parent compound, and the resulting materials exhibited dual emission from Hphen and Ln, promoting tunability of the emission color.

Two Luminescent Iridium Complexes with Phosphorous Ligands and Their Photophysical Comparison in Solution, Solid and Electrospun Fibers: Decreased Aggregation-Caused Emission Quenching by Steric Hindrance

In this paper, we prepared two phosphorescent Ir complexes with ligands of 2-phenyl pyridine (ppy), and two phosphorous ligands with large steric hindrance, hoping to allow enough time for the transformation of the highly phosphorescent ³MLLCT (metal-to-ligand-ligand-charge-transfer) excited state. Their large steric hindrance minimized the π-π interaction between complex molecules, so that the aggregation-induced phosphorescence emission (AIPE) influence could be minimized. Their single crystals indicated that they took a distorted octahedral coordination mode. Photophysical comparison between these Ir complexes in solution, in the solid state and in electrospun fibers was performed to confirm the realization of limited aggregation-caused quenching (ACQ). The potential surface crossing and energy transfer from ³ML_BPECT/³ML_BPEL_ppyCT to ³ML_ppyCT in these Ir complexes were revealed by density functional theory calculation and temperature-dependent emission. It was confirmed that these two phosphorous ligands offered large steric hindrance, which decreased the ACQ effect, allowing the efficient emissive decay of the ³ML_ppyCT excited state. This is one of the several luminescent Ir complexes with a high emission yield (Φ = 0.27) and long emission lifetime (0.43 μs) in the solid state.

Core-Shell Pluronic-Organosilica Nanoparticles with Controlled Polarity and Oxygen Permeability

Nanostructured systems constitute versatile carriers with multiple functions engineered in a nanometric space. Yet, such multimodality often requires adapting the chemistry of the nanostructure to the properties of the hosted functional molecules. Here, we show the preparation of core-shell Pluronic-organosilica "PluOS" nanoparticles with the use of a library of organosilane precursors. The precursors are obtained via a fast and quantitative click reaction, starting from cost-effective reagents such as diamines and an isocyanate silane derivative, and they condensate in building blocks characterized by a balance between hydrophobic and H-bond-rich domains. As nanoscopic probes for local polarity, oxygen permeability, and solvating properties, we use, respectively, solvatochromic, phosphorescent, and excimer-forming dyes covalently linked to the organosilica matrix during synthesis. The results obtained here clearly show that the use of these organosilane precursors allows for finely tuning polarity, oxygen permeability, and solvating properties of the resulting organosilica core, expanding the toolbox for precise engineering of the particle properties.

Twisted BODIPY derivative: intersystem crossing, electron spin polarization and application as a novel photodynamic therapy reagent

The photophysical properties of a heavy atom-free BODIPY derivative with a twisted π-conjugated framework were studied. Efficient intersystem crossing (ISC quantum yield: 56%) and an exceptionally long-lived triplet state were observed (4.5 ms in solid polymer film matrix and 197.5 μs in solution). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy and DFT computations confirmed the delocalization of the triplet state on the whole twisted π-conjugated framework and the zero-field-splitting (ZFS) D parameter of D = -69.5 mT, which is smaller than that of 2,6-diiodoBODIPY (D = -104.6 mT). The electron spin polarization (ESP) phase pattern of the triplet state TREPR spectrum of the twisted BODIPY is (a, a, e, a, e, e), which is different from that of 2,6-diiodo BODIPY (e, e, e, a, a, a), indicating that the electron spin selectivity of the ISC of the twisted structure is different from that of the spin orbital coupling effect. According to the computed spin-orbit coupling matrix elements (0.154-1.964 cm^-1), together with the matched energy of the S₁/T_n states, ISC was proposed to occur via S₁→T₂/T₃. The computational results were consistent with TREPR results on the electron spin selectivity (the overpopulation of the T_Y sublevel of the T₁ state). The advantage of the long-lived triplet state of the twisted BODIPY was demonstrated by its efficient singlet oxygen (¹O₂) photosensitizing (Φ_Δ = 50.0%) even under a severe hypoxia atmosphere (pO₂ = 0.2%, v/v). A high light toxicity (EC₅₀ = 1.0 μM) and low dark toxicity (EC₅₀ = 78.5 μM) were observed for the twisted BODIPY, and thus the cellular studies demonstrate its potential as a novel potent heavy atom-free photodynamic therapy (PDT) agent.

Charge separation, charge recombination and intersystem crossing in orthogonal naphthalimide-perylene electron donor/acceptor dyad

We prepared an orthogonal electron donor/acceptor dyad (NI-Py) with perylene (Py) as electron donor and 4-aminonaphthalimide (NI) as an electron acceptor. The molecule adopts orthogonal geometry due to the steric hindrance exerted by the 4-amino substituents on the NI moiety. The photophysical properties of dyad were studied by steady-state UV-Vis absorption and fluorescence spectroscopies, femtosecond/nanosecond transient absorption spectroscopies and DFT computations. Ground state interaction between the NI and Py units is negligible; however, charge separation occurs upon photoexcitation, indicated by the quenching of the fluorescence of the dyad in polar solvents, i.e. fluorescence quantum yield (Φ_F) is 61.9% in toluene and Φ_F = 0.2% in methanol. Spin-orbit-coupled charge transfer-induced intersystem crossing (SOCT-ISC) was confirmed by femtosecond transient absorption spectroscopy (charge separation takes 1.7 ps and charge recombination takes 6.9 ns, in CH₂Cl₂). Nanosecond transient absorption spectra indicated the formation of perylene-localized triplet state, and the triplet state lifetime (175 μs) is much longer than that accessed with the heavy atom effect (3-bromoperylene; 16 μs). The singlet oxygen quantum (Φ_Δ) yield of the dyad is 2.2% in hexane and 9.5% in dichloromethane. The low SOCT-ISC efficiency as compared to the previously reported analogue (Φ_Δ = 80%) is attributed to the mismatch of the ¹CT/T_n state energies, and/or the orientation of the NI and Py units, i.e. orthogonal geometry is not sufficient for achieving efficient SOCT-ISC in compact electron donor/acceptor dyads.

Long-Lived Local Triplet Excited State and Charge Transfer State of 4,4'-Dimethoxy Triphenylamine-BODIPY Compact Electron Donor/Acceptor Dyads

The spin-orbit charge transfer intersystem crossing (SOCT-ISC) and the formation of a long-lived charge transfer (CT) state were studied with a series of 4,4'-dimethoxy triphenylamine-BODIPY compact electron donor/acceptor dyads. Different torsion freedoms were applied in the dyads to tune the electronic coupling between the donor and acceptor, and a red-shifted CT absorption band was observed for one dyad. The dyads show solvent polarity-dependent singlet oxygen photosensitizing ability (quantum yields 3%-79%). Nanosecond transient absorption spectra of the dyad in nonpolar solvent confirm the formation of triplet states. The intrinsic triplet state lifetime is up to 383 μs (in fluid solution), which is much longer than that accessed with the heavy atom effect (276 μs). Intermolecular triplet photosensitizing of the dyads in a polar solvent produces a long-lived ³CT state (lifetime, τ_CT = 8.0 μs supported by the electron spin density surface analysis). The triplet state lifetime of the dyads doped in a Clear Flex 50 polymer film is exceptionally long (7.6-11.4 ms), and formation of a long-lived CT state (37 μs) was observed. Triplet-triplet annihilation upconversion was performed with the electron donor/acceptor dyads used as the triplet photosensitizer and perylene used as the triplet acceptor; the upconversion quantum yield is up to 15.8%.

Elucidation of the Intersystem Crossing Mechanism in a Helical BODIPY for Low-Dose Photodynamic Therapy

Intersystem crossing (ISC) of triplet photosensitizers is a vital process for fundamental photochemistry and photodynamic therapy (PDT). Herein, we report the co-existence of efficient ISC and long triplet excited lifetime in a heavy atom-free bodipy helicene molecule. Via theoretical computation and time-resolved EPR spectroscopy, we confirmed that the ISC of the bodipy results from its twisted molecular structure and reduced symmetry. The twisted bodipy shows intense long wavelength absorption (ϵ=1.76×105  m-1  cm-1 at 630 nm), satisfactory triplet quantum yield (ΦT =52 %), and long-lived triplet state (τT =492 μs), leading to unprecedented performance as a triplet photosensitizer for PDT. Moreover, nanoparticles constructed with such helical bodipy show efficient PDT-mediated antitumor immunity amplification with an ultra-low dose (0.25 μg kg-1 ), which is several hundred times lower than that of the existing PDT reagents.

Improving photosensitization for photochemical CO2-to-CO conversion

Inspired by nature, improving photosensitization represents a vital direction for the development of artificial photosynthesis. The sensitization ability of photosensitizers (PSs) reflects in their electron-transfer ability, which highly depends on their excited-state lifetime and redox potential. Herein, for the first time, we put forward a facile strategy to improve sensitizing ability via finely tuning the excited state of Ru(II)-PSs (Ru-1–Ru-4) for efficient CO₂ reduction. Remarkably, [Ru(Phen)₂(3-pyrenylPhen)]²⁺ (Ru-3) exhibits the best sensitizing ability among Ru-1–Ru-4, over 17 times higher than that of typical Ru(Phen)₃²⁺. It can efficiently sensitize a dinuclear cobalt catalyst for CO₂-to-CO conversion with a maximum turnover number of 66 480. Systematic investigations demonstrate that its long-lived excited state and suitable redox driving force greatly contributed to this superior sensitizing ability. This work provides a new insight into dramatically boosting photocatalytic CO₂ reduction via improving photosensitization.

Tremblay A, Maris T, Leal do Santos S, Venancio EC, Ferreira Santos S, Duong A. Programmed Molecular Construction: Driving the Self-Assembly by Coordination and Hydrogen Bonds Using 6-(Pyridin-2-yl)-1,3,5-triazine-2,4-diamine with M(NO3)2 Salts

A new series of hydrogen-bonded metallotecton networks 6-9 of the general formula [M(2)₂(NO₃)₂] were obtained from the reaction of 6-pyridin-2-yl-[1,3,5]-triazine-2,4-diamine 2 with transition-metal ions [M: Co(II), Ni(II), Cu(II), and Zn(II)]. Their supramolecular networks and associated properties were characterized by single-crystal and powder X-ray diffraction, IR, solid-state UV-vis spectroscopy, and thermogravimetric analysis associated with differential scanning calorimetry. On the basis of standard patterns of coordination involving 2,2'-bipyridine and simple derivatives, compound 2 binds transition-metal ions with predictable constitution and the diaminotriazinyl (DAT) groups serve orthogonally to ensure the intermetallotecton interactions by hydrogen bonding according to well-established motifs I-III. As expected, compound 2 formed octahedral 2:1 metallotectons with M(NO₃)₂, and further self-assembled by hydrogen bonding of the DAT groups to produce pure, crystalline, homogeneous, and thermally stable materials. In these structures, nitrate counterions also play an important role in the cohesion of intermetallotectons to form two-dimensional and three-dimensional networks. These results illustrated the effectiveness of the synthetic approach to create a wide range of novel ordered materials with controllable architectures and tunable properties achieved by varying the central metal ion. Crystal morphologies of 6-9 were also investigated by scanning electron microscopy and calculation using Bravais-Friedel-Donnay-Harker method from their single-crystal structure.

Excited-State Dynamics of [Ru(bpy)3 ]2+ Thin Films on Sensitized TiO2 and ZrO2

The excited state dynamics of Tris(2,2'-bipyridine)ruthenium(II) hexafluorophosphate, [Ru(bpy)3 (PF6 )2 ], was investigated on the surface of bare and sensitized TiO2 and ZrO2 films. The organic dyes LEG4 and MKA253 were selected as sensitizers. A Stern-Volmer plot of LEG4-sensitized TiO2 substrates with a spin-coated [Ru(bpy)3 (PF6 )2 ] layer on top shows considerable quenching of the emission of the latter. Interestingly, time-resolved emission spectroscopy reveals the presence of a fast-decay time component (25±5 ns), which is absent when the anatase TiO2 semiconductor is replaced by ZrO2 . It should be specified that the positive redox potential of the ruthenium complex prevents electron transfer from the [Ru(bpy)3 (PF6 )2 ] ground state into the oxidized sensitizer. Therefore, we speculate that the fast-decay time component observed stems from excited-state electron transfer from [Ru(bpy)3 (PF6 )2 ] to the oxidized sensitizer. Solid-state dye sensitized solar cells (ssDSSCs) employing MKA253 and LEG4 dyes, with [Ru(bpy)3 (PF6 )2 ] as a hole-transporting material (HTM), exhibit 1.2 % and 1.1 % power conversion efficiency, respectively. This result illustrates the possibility of the hypothesized excited-state electron transfer.

Heteroleptic Ir(III)N6 Complexes with Long-Lived Triplet Excited States and in Vitro Photobiological Activities

A series of cationic heteroleptic iridium(III) complexes bearing tris-diimine ligands [Ir(phen)2(R-phen)]3+ (R-phen = phenanthroline (1), 3,8-diphenylphenanthroline (2), 3,8-dipyrenylphenanthroline (3), 3-phenylphenanthroline (4), 3-pyrenylphenanthroline (5), and 3,8-diphenylethynylphenanthroline (6)) were synthesized and characterized. These complexes possessed phen ligand-localized 1π,π* transitions below 300 nm, and charge transfer (1CT) and/or 1π,π* transitions between 300 and 520 nm. In 1, 2, 4, and 6, the low-energy bands were mixed 1CT/1π,π*. However, the increased π-donating ability of the pyrenyl substituent(s) in 3 and 5 split the low-energy bands into a pyrene-based 1π,π* transition at 300-380 nm and an intraligand charge transfer (1ILCT) transition at 380-520 nm. All complexes were emissive at room temperature in CH3CN, but the parentage of the emitting state varied depending on the R substituent(s). Complex 1 exhibited predominantly phen ligand-localized 3π,π* emission mixed with metal-to-ligand charge transfer (3MLCT) character, whereas the emission of 2, 4, and 6 was predominantly from the excited-state with 3π,π*/3ILCT/3MLCT character. The emission from 3 and 5 was dominated by pyrene-based 3π,π* states mixed with 3ILCT character. The different natures of the lowest triplet excited states were also reflected by the different spectral features and lifetimes of the triplet transient absorption of these complexes. Complexes 3 and 5 had singlet oxygen quantum yields as high as 81 and 72%, respectively. Both gave submicromolar phototoxicities toward cancer cells (SK-MEL-28 human melanoma) and bacteria ( S. aureus and S. mutans) with visible-light activation (and marginal to no photobiological activity with red light). Their visible-light phototherapeutic indices (PIs) toward SK-MEL-28 cells were 248 for 3 and >435 for 5; PIs were lower in bacteria (≤62) because of their inherent antimicrobial activities. Both complexes were shown to produce substantial amounts of intracellular reactive oxygen species (ROS), which may account for their photobiological activities.

Transition Metal Complexes and Photodynamic Therapy from a Tumor-Centered Approach: Challenges, Opportunities, and Highlights from the Development of TLD1433

Transition metal complexes are of increasing interest as photosensitizers in photodynamic therapy (PDT) and, more recently, for photochemotherapy (PCT). In recent years, Ru(II) polypyridyl complexes have emerged as promising systems for both PDT and PCT. Their rich photochemical and photophysical properties derive from a variety of excited-state electronic configurations accessible with visible and near-infrared light, and these properties can be exploited for both energy- and electron-transfer processes that can yield highly potent oxygen-dependent and/or oxygen-independent photobiological activity. Selected examples highlight the use of rational design in coordination chemistry to control the lowest-energy triplet excited-state configurations for eliciting a particular type of photoreactivity for PDT and/or PCT effects. These principles are also discussed in the context of the development of TLD1433, the first Ru(II)-based photosensitizer for PDT to enter a human clinical trial. The design of TLD1433 arose from a tumor-centered approach, as part of a complete PDT package that includes the light component and the protocol for treating non-muscle invasive bladder cancer. Briefly, this review summarizes the challenges to bringing PDT into mainstream cancer therapy. It considers the chemical and photophysical solutions that transition metal complexes offer, and it puts into context the multidisciplinary effort needed to bring a new drug to clinical trial.

Synthesis, Characterization and Photobiological Studies of Ru(II) Dyads Derived from α-Oligothiophene Derivatives of 1,10-Phenanthroline

Three new bis(2,2'-bipyridine)-heteroleptic Ru(II) dyads incorporating thienyl groups (n = 1-3, compounds 1, 2 and 3, respectively) appended to 1,10-phenanthroline were synthesized and characterized to investigate the impact of n on the photophysical and photobiological properties within the series. All three complexes showed unstructured emission near 618 nm from a triplet metal-to-ligand charge transfer (³ MLCT) state with a lifetime (τ_em ) of approximately 1 μs. Transient absorption measurements revealed an additional excited state that was nonemissive and long-lived (τ_TA  = 43 μs for 2 and 27 μs for 3), assigned as a triplet intraligand (³ IL) state that was accessible only in 2 and 3. All three complexes were strong singlet oxygen (¹ O₂ ) sensitizers, with quantum yields (Φ_∆ ) for 2 and 3 being the largest (74-78%), and all three were photocytotoxic to cancer cells with visible light activation in the order: 3 > 2 > 1. Cell-free DNA photodamage followed the same trend, where potency increased with decreasing ³ IL energy. Compounds 2 and 3 also showed in vitro photobiological effects with red light (625 nm), where their molar absorptivities were <100 m^-1  cm^-1 . These findings highlight that Ru(II) dyads derived from α-oligothiophenes directly appended to 1,10-phenanthroline-namely 2 and 3-possess low-lying ³ IL states that are highly photosensitizing, and they may therefore be of interest for photobiological applications such as photodynamic therapy (PDT).

"Chemistry-on-the-complex": functional RuII polypyridyl-type sensitizers as divergent building blocks

Ruthenium polypyridyl type complexes are potent photoactive compounds, and have found - among others - a broad range of important applications in the fields of biomedical diagnosis and phototherapy, energy conversion schemes such as dye-sensitized solar cells (DSSCs) and molecular assemblies for tailored photo-initiated processes. In this regard, the linkage of RuII polypyridyl-type complexes with specific functional moieties is highly desirable to enhance their inherent photophysical properties, e.g., with a targeting function to achieve cell selectivity, or with a dye or redox-active subunits for energy- and electron-transfer. However, the classical approach of performing ligand syntheses first and the formation of Ru complexes in the last steps imposes synthetic limitations with regard to tolerating functional groups or moieties as well as requiring lengthy convergent routes. Alternatively, the diversification of Ru complexes after coordination (termed "chemistry-on-the-complex") provides an elegant complementary approach. In addition to the Click chemistry concept, the rapidly developing synthesis and purification methodologies permit the preparation of Ru conjugates via amidation, alkylation and cross-coupling reactions. In this regard, recent developments in chromatography shifted the limits of purification, e.g., by using new commercialized surface-modified silica gels and automated instrumentation. This review provides detailed insights into applying the "chemistry-on-the-complex" concept, which is believed to stimulate the modular preparation of unpreceded molecular assemblies as well as functional materials based on Ru-based building blocks, including combinatorial approaches.

Modular Construction of Photoanodes with Covalently Bonded Ru- and Ir-Polypyridyl Visible Light Chromophores

1,10-phenanthroline is grafted to indium tin oxide (ITO) and titanium dioxide nanoparticle (TiO₂) semiconductors by electroreduction of 5-diazo-1,10-phenanthroline in 0.1 M H₂SO₄. The lower and upper potential limits (-0.20 and 0.15 V_SCE, respectively) were set to avoid reduction and oxidation of the 1,10-phenanthroline (phen) covalently grafted at C5 to the semiconductor. The resulting semiconductor-phen ligand (ITO-phen or TiO₂-phen) was air stable, and was bonded to Ru- or Ir- by reaction with cis-[Ru(bpy)₂(CH₃CN)₂]²⁺ (bpy = 2,2'-bipyridine) or cis-[Ir(ppy)₂(CH₃CN)₂]⁺ (ppy = ortho-C_phenyl metalated 2-phenylpyridine) in CH₂Cl₂ and THF solvent at 50 °C. Cyclic voltammetry, X-ray photoelectron spectroscopy, solid-state UV-vis, and inductively coupled plasma-mass spectrometry all confirmed that the chromophores SC-[(phen)Ru(bpy)₂]²⁺ and SC-[(phen)Ir(ppy)₂]⁺ (SC = ITO or TiO₂) formed in near quantitative yields by these reactions. The resulting photoanodes were active and relatively stable to photoelectrochemical oxidation of hydroquinone and triethylamine under neutral and basic conditions.

The development of ruthenium(ii) polypyridyl complexes and conjugates for in vitro cellular and in vivo applications

Ruthenium(ii) [Ru(ii)] polypyridyl complexes have been the focus of intense investigations since work began exploring their supramolecular interactions with DNA. In recent years, there have been considerable efforts to translate this solution-based research into a biological environment with the intention of developing new classes of probes, luminescent imaging agents, therapeutics and theranostics. In only 10 years the field has expanded with diverse applications for these complexes as imaging agents and promising candidates for therapeutics. In light of these efforts this review exclusively focuses on the developments of these complexes in biological systems, both in cells and in vivo, and hopes to communicate to readers the diversity of applications within which these complexes have found use, as well as new insights gained along the way and challenges that researchers in this field still face.

Synthesis and photophysical properties of ruthenium(ii) polyimine complexes decorated with flavin

Phosphorescent emission from a flavin localized triplet excited state (³IL) is observed for the first time in a flavin decorated tris(dipyridine) Ru(ii) complex with strong visible light absorption.

Synthesis, characterisation and ion-binding properties of oxathiacrown ethers appended to [Ru(bpy)2]2+. Selectivity towards Hg2+, Cd2+and Pb2+

Oxathiacrown ether modified ruthenium complex2facilitates a selective naked-eye detection of Hg²⁺with an instrumental detection limit of 68 ppm.