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

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

Harnessing synergistic effects in GQD@Pt(II) nanocomposites for enhanced photovoltaic performance: a computational study

CONTEXT

The development of efficient solar energy conversion technologies is crucial for addressing global energy challenges and reducing reliance on fossil fuels. Platinum(II) complexes are promising materials for photovoltaic applications due to their strong light absorption and long-lived excited states. However, their narrow absorption in the visible spectrum and stability issues limit their performance. Combining platinum(II) complexes with graphene quantum dots (GQDs) can enhance photovoltaic performance by leveraging the complementary light harvesting and charge transfer characteristics of the two components. This study utilizes density functional theory (DFT) calculations to explore their electronic structures, charge transfer dynamics, and photoelectric performance. Specifically, it investigates the effects of incorporating different substituents, either electron-donating or electron-withdrawing, onto the fluorene motif of the Pt(II) complex. The findings reveal that combining GQDs with Pt(II) complexes extends light absorption into the UV range, enabling comprehensive solar utilization. Upon photoexcitation, electrons migrate between the GQD conduction band and the Pt(II) complex, stabilizing charges and enhancing extraction. Substituents significantly influence charge transfer dynamics: electron-withdrawing groups promote transfer to the GQD, while electron-donating groups encourage charge separation and delocalization. Nanocomposites featuring electron-donating substituents achieve the highest energy conversion efficiencies, with GQD@Pt(II)-NPh2 reaching 24.6%. This is attributed to improved light harvesting, efficient charge injection, and reduced recombination. These insights guide the rational design of GQD-Pt(II) nanocomposites, optimizing charge separation and transfer processes for enhanced photovoltaic performance. The computational approach employed here provides a robust tool for developing advanced materials in renewable energy technologies.

METHODS

The computational studies reported in this work were performed using the DFT approach, specifically employing the hybrid functional PBE0. The PBE0 functional's accuracy in describing electronic structures and excited-state properties is essential for understanding charge transfer processes, photoabsorption, and emission characteristics in metal-organic complexes. Geometry optimizations and time-dependent DFT (TD-DFT) calculations were carried out to investigate the properties of the nanocomposites. The effects of solvents were replicated using the conductor-like polarizable continuum model (CPCM). The charge transfer length (ΔL) and interfragment charge transfer (ΔQ) were calculated using the Multiwfn software package, and all calculations were performed using the BDF software package.

A detailed density functional theory exploration of the photodissociation mechanism of ruthenium complexes for photoactivated chemotherapy

Polypyridyl Ru(II) complexes have attracted much attention due to their potential as light-activatable anticancer agents in photoactivated chemotherapy (PACT). The action of ruthenium-based PACT compounds relies on the breaking of a coordination bond between the metal center and an organic ligand via a photosubstitution reaction. Here, a detailed computational investigation of the photophysical properties of a novel trisheteroleptic ruthenium complex, [Ru(dpp)(bpy)(mtmp)]2+ (dpp = 4,7-diphenyl-1,10-phenanthroline, bpy = 2,2'-bipyridine and mtmp = 2-methylthiomethylpyridine), has been carried out by means of DFT and its time-dependent extension. All the aspects of the mechanism by which, upon light irradiation, the mtmp protecting group is released and the corresponding aquated complex, able to bind to DNA inducing cell death, is formed have been explored in detail. All the involved singlet and triplet states have been fully described, providing the calculation of the corresponding energy barriers. The involvement of solvent molecules in photosubstitution and the role played by pyridyl-thioether chelates as caging groups have been elucidated.

Mehrere Triplett-Metall-zentrierte Jahn-Teller-Isomere bestimmen die temperaturabhängigen Lumineszenzlebensdauern in [Ru(bpy)3]2

Ein genaues Verständnis der Faktoren, welche die Lumineszenzlebensdauer von Übergangsmetallverbindungen bestimmen, ist für Anwendungen in der Photokatalyse und der photodynamischen Therapie von entscheidender Bedeutung. Die im Falle von [Ru(bpy)3]2+ (bpy=2,2’‐Bipyridin) allgemein akzeptierte Theorie besagt, dass die Emissionslebensdauer durch Optimierung der Energiebarriere zwischen dem emittierenden Triplett‐Zustand des Metall‐Liganden‐Ladungstransfers (3MLCT) und dem thermisch aktivierten Triplett‐Zustand des Metall‐Zentrums (3MC), oder der Energielücke zwischen beiden Zuständen gesteuert werden kann. Hier zeigen wir, dass dies nicht allgemeingültig ist. Darüber hinaus demonstrieren wir, dass die Betrachtung eines einzelnen Relaxationspfades, der vom energetisch niedrigsten Minimum aus bestimmt wird, zu falschen Vorhersagen der temperaturabhängigen Emissionslebensdauer führt. Stattdessen erhalten wir eine ausgezeichnete Übereinstimmung mit den experimentellen temperaturabhängigen Lebensdauern, wenn ein erweitertes kinetisches Modell herangezogen wird, welches alle Pfade im Zusammenhang mit mehreren Jahn–Teller‐Isomeren und ihren effektiven Reaktionsbarrieren beinhaltet. Diese Konzepte sind für das Design weiterer lumineszierender Übergangsmetallkomplexe mit individuell angepassten Emissionslebensdauern auf der Grundlage theoretischer Vorhersagen unerlässlich.

Multiple Triplet Metal-Centered Jahn-Teller Isomers Determine Temperature-Dependent Luminescence Lifetimes in [Ru(bpy)3 ]2

Understanding the factors that determine the luminescence lifetime of transition metal compounds is key for applications in photocatalysis and photodynamic therapy. Here we show that for[ Ru ( bpy ) 3 ] 2 + ${[{\rm{Ru}}({\rm{bpy}})_{\rm{3}} ]^{{\rm{2 + }}} }$ (bpy = 2,2'-bipyridine), the generally accepted idea that emission lifetimes can be controlled optimizing the energy barrier from the emissive triplet metal-to-ligand charge-transfer (3 MLCT) state to the thermally-activated triplet metal-centered (3 MC) state or the energy gap between both states is a misconception. Further, we demonstrate that considering a single relaxation pathway determined from the minimum that is lowest in energy leads to wrong temperature-dependent emission lifetimes predictions. Instead, we obtain excellent agreement with experimental temperature-dependent lifetimes when an extended kinetic model that includes all the pathways related to multiple Jahn-Teller isomers and their effective reaction barriers is employed. These concepts are essential to correctly design other luminescent transition metal complexes with tailored emission lifetimes based on theoretical predictions.

Unraveling the Marked Differences of the Excited-State Properties of Arylgold(III) Complexes with C∧N∧C Tridentate Ligands

Three structurally similar gold(III) complexes with C∧N∧C tridentate ligands, [1; C∧N∧C = 2,6-diphenylpyridine], [2; C∧N∧C = 2,6-diphenylpyrazine], and [3; C∧N∧C = 2,6-diphenyltriazine], have been investigated theoretically to rationalize the marked difference in emission behaviors. The geometrical and electronic structures, spectra properties, radiative and nonradiative decay processes, as well as reverse intersystem crossing and reverse internal conversion (RIC) processes were thoroughly analyzed using density functional theory (DFT) and time-dependent DFT calculations. The computed results indicate that there is a small energy difference Δ E T 1 - T 1 ' between the lowest-energy triplet state (T1) and the second lowest-energy triplet state (T1') of complexes 2 and 3, suggesting that the excitons in the T1 state can reach the emissive higher-energy T1' through the RIC process. In addition, the non-emissive T1 states of gold(III) complexes in solution can be ascribed to the easily accessible metal-centered (3MC) state or possibly tunneling into high-energy vibrationally excited singlet states for nonradiative decay. The low efficiency of 3 is attributed to the deactivation pathway via the 3MC state. The present study elucidates the relationship between structure and property of gold(III) complexes featuring C∧N∧C ligands and providing a comprehensive understanding of the significant differences in their luminescence behaviors.

Influences of Both N,N,N- and N,N,C-Coordination Modes of Tolyl-terpyridine on the Photophysical Properties of Cyclometalated Ru(II) Complexes: Combined Experimental and Theoretical Investigations on Acid/Base-Dependent Reversible Cyclometalation

With the goal of developing a new strategy for the synthesis of luminescent Ru(II) complexes, we have prepared herein a new set of bis-tridentate complexes of the type [(py-bpy-Ph-X)Ru(tpy-PhCH3)]ClO4 (X = -CH3, -CH2Br, and -CHO) incorporating both non-cyclometalated and cyclometalated coordination motifs of two isomeric forms of methylphenyl-terpyridine (tpy-PhCH3). Thorough characterization of the synthesized complexes is carried out using standard analytical tools and single crystal X-ray diffraction. Detailed investigations on their photophysical and electrochemical behaviors are carried out in MeCN. The presence of a carbanionic center in the cyclometalating unit increases the absorption spectral window of the complexes into a longer-wavelength region. The complexes also exhibit room-temperature luminescence in the NIR domain with enhanced excited-state lifetimes (up to 20.1 ns) compared to their non-cyclometalated counterpart, [Ru(tpy-PhCH3)2]2+. In the presence of acid, the non-coordinated nitrogen atom in the secondary coordination sphere of the complexes allows fine-tuning of the absorption and emission spectral properties. Excess acid induces de-coordination of the Ru-C bond, which is signaled by a remarkable alteration of their spectral profiles. Cleavage of the Ru-C bond is also possible upon treating the acidified solution of the complexes with visible light. Restoration of the Ru-C bond is again feasible upon treating the solution with base at an elevated temperature (∼70 °C). In essence, "on-off" and "off-on" switching of emission is facilitated upon alternating treatment of the Ru(II) complexes with acid, base, and temperature. DFT and TD-DFT calculations are also performed for assignments of the spectral bands as well as to understand structural changes associated with the switching behaviors of the complexes.

The overlooked NIR luminescence of Cr(ppy)3

Cr(ppy)₃, a structural analog of the green phosphorescent Ir(ppy)₃, emits even in solution at room temperature from a weakly distorted spin-flip state at 910 nm (Hppy = 2-phenylpyridine). The low energy arises from an enhanced covalence of the Cr-C bonds as compared to Cr-N bonds. Lower temperature reduces thermally activated decay increasing the emission intensity.

Tuning of photo-redox behaviours and thermodynamic and kinetic aspects of intercomponent energy transfer in trimetallic complexes of Ru(II) and Os(II) by exploiting their second coordination sphere

This paper deals with a thorough investigation of pH-induced tuning of the ground and excited state photophysical as well as electrochemical behaviours of two series of our recently reported homo- and heterotrimetallic complexes of the type [(bpy)₂Ru(d-HIm-t)M(t-HIm-d)Ru(bpy)₂]⁶⁺ and [(bpy)₂Os(d-HIm-t)M(t-HIm-d)Os(bpy)₂]⁶⁺ (M = Ru^II and Os^II) derived from a heteroditopic bpy-tpy (d-HIm-t) type bridging ligand through the exploitation of their second coordination sphere. A small bathochromic shift of the absorption and emission spectral band along with substantial alteration of emission intensity and lifetime of the triads is noted upon deprotonation of the NH motifs at elevated pH values. The lowering of the half wave potential of a M³⁺/M²⁺ couple is also observed upon removal of the NH protons. Both ground and excited state pK_a values of the triads are estimated from their absorption/emission versus pH spectral profiles. In addition, the variation of the free energy change (ΔG) and the rate of intercomponent energy transfer (k_en) in the triads upon stepwise deprotonation of the NH motifs are also addressed in the present study.

On the Importance of Ligand-Centered Excited States in the Emission of Cyclometalated Ir(III) Complexes

The photophysical behavior of the cyclometalating Ir(III) complexes [Ir(ppy)₂(bpy)]⁺, where Hppy is 2-phenylpyridine and bpy is 2,2′-bipyridine (complex 1), and [Ir(diFppy)₂(dtb-bpy)]⁺, where diFppy is 2-(2,4-difluorophenyl)pyridine and dtb-bpy is 4,4′-di-tert-butyl-2,2′-bipyridine (complex 2), has been theoretically investigated by performing density functional theory calculations. The two complexes share the same molecular skeleton, complex 2 being derived from complex 1 through the addition of fluoro and tert-butyl substituents, but present notable differences in their photophysical properties. The remarkable difference in their emission quantum yields (0.196 for complex 1 in dichloromethane and 0.71 for complex 2 in acetonitrile) has been evaluated by characterizing both radiative and nonradiative decay paths. It has emerged that the probability of decaying through the nonradiative triplet metal-centered state, normally associated with the loss of the emission quantum yield, does not appear to be the reason behind the reported substantially different emission efficiency. A more critical factor appears to be the ability of complex 2 to emit from both the usual metal-to-ligand charge-transfer state and from two additional ligand-centered states, as supported by the fact that the respective minima belong to the potential energy surface of the lowest triplet T₁ state and that their phosphorescence lifetimes are in the same order of magnitude. In contrast, the emission of complex 1 can be originated only from the metal-to-ligand charge-transfer state, being the only emissive T₁ minimum. The results constitute a significant case in which the emission from ligand-centered states is the key for determining the high emission quantum yield of a complex.

The reasons behind the significant increase in the emission quantum yield of the [Ir(diFppy)₂(dtb-bpy)]⁺ complex with respect to [Ir(ppy)₂(bpy)]⁺ are rationalized on the basis of DFT, TDDFT, and TDDFT-SOC calculations, revealing the key role that low-lying LC states can play in the emission properties of the complex.

Substituted 2,4-Di(pyridin-2-yl)pyrimidine-Based Ruthenium Photosensitizers for Hydrogen Photoevolution under Red Light

The photocatalytic reduction of water to form hydrogen gas (H₂) is a promising approach to collect, convert, and store solar energy. Typically, ruthenium tris(bipyridine) and its many derivatives are used as photosensitizers (PSs) in a variety of photocatalytic conditions. The bis(terpyridine) analogues, however, have only recently gained attention for this application because of their poor photophysical properties. Yet, by the introduction of electron-donating or -withdrawing groups on the terpyridine ligands, the photophysical and electrochemical properties can be considerably improved. In this study, we report a series of nonsymmetric 2,6-di(pyridin-2-yl)pyrimidine ligands with peripheral pyridine substituents in different positions and their corresponding ruthenium(II) complexes. The presence of the pyrimidine ring stabilizes the lowest unoccupied molecular orbital, leading to a red-shifted emission and prolonged excited-state lifetimes as well as higher luminescence quantum yields compared to analogous terpyridine complexes. Furthermore, all complexes are easier to reduce than the previously reported bis(terpyridine) complexes used as PSs. Interestingly, the pyridine substituent in the 4-pyrimidine position has a greater impact on both the photophysical and electrochemical properties. This correlation between the substitution pattern and properties of the complexes is further investigated by using time-dependent density functional theory. In hydrogen evolution experiments under blue- and red-light irradiation, all investigated complexes exhibit much higher activity compared to the previously reported ruthenium(II) bis(terpyridine) complexes, but none of the complexes are as stable as the literature compounds, presumably because of an additional decomposition pathway of the reduced PS competing with electron transfer from the reduced PS to the catalyst.

Green-Light Activation of Push-Pull Ruthenium(II) Complexes

Synthesis, characterization, electrochemistry, and photophysics of homo- and heteroleptic ruthenium(II) complexes [Ru(cpmp)₂ ]²⁺ (2²⁺ ) and [Ru(cpmp)(ddpd)]²⁺ (3²⁺ ) bearing the tridentate ligands 6,2''-carboxypyridyl-2,2'-methylamine-pyridyl-pyridine (cpmp) and N,N'-dimethyl-N,N'-dipyridin-2-ylpyridine-2,6-diamine (ddpd) are reported. The complexes possess one (3²⁺ ) or two (2²⁺ ) electron-deficient dipyridyl ketone fragments as electron-accepting sites enabling intraligand charge transfer (ILCT), ligand-to-ligand charge transfer (LL'CT) and low-energy metal-to-ligand charge transfer (MLCT) absorptions. The latter peak around 544 nm (green light). Complex 2²⁺ shows ³ MLCT phosphorescence in the red to near-infrared spectral region at room temperature in deaerated acetonitrile solution with an emission quantum yield of 1.3 % and a ³ MLCT lifetime of 477 ns, whereas 3²⁺ is much less luminescent. This different behavior is ascribed to the energy gap law and the shape of the parasitic excited ³ MC state potential energy surface. This study highlights the importance of the excited-state energies and geometries for the actual excited-state dynamics. Aromatic and aliphatic amines reductively quench the excited state of 2²⁺ paving the way to photocatalytic applications using low-energy green light as exemplified with the green-light-sensitized thiol-ene click reaction.

On the Possible Coordination on a 3MC State Itself? Mechanistic Investigation Using DFT-Based Methods

Understanding light-induced ligand exchange processes is key to the design of efficient light-releasing prodrugs or photochemically driven functional molecules. Previous mechanistic investigations had highlighted the pivotal role of metal-centered (MC) excited states in the initial ligand loss step. The question remains whether they are equally important in the subsequent ligand capture step. This article reports the mechanistic study of direct acetonitrile coordination onto a 3MC state of [Ru(bpy)3]2+, leading to [Ru(bpy)2(κ1-bpy)(NCMe)]2+ in a 3MLCT (metal-to-ligand charge transfer) state. Coordination of MeCN is indeed accompanied by the decoordination of one pyridine ring of a bpy ligand. As estimated from Nudged Elastic Band calculations, the energy barrier along the minimum energy path is 20 kcal/mol. Interestingly, the orbital analysis conducted along the reaction path has shown that creation of the metallic vacancy can be achieved by reverting the energetic ordering of key dσ* and bpy-based π* orbitals, resulting in the change of electronic configuration from 3MC to 3MLCT. The approach of the NCMe lone pair contributes to destabilizing the dσ* orbital by electrostatic repulsion.

H/D solvent isotope effects on the photoracemization reaction of enantiomeric the tris(2,2′-bipyridine)ruthenium(ii) complex and its analogues

This work assessed solvent isotope effects on the photoracemization rate and emission lifetime for [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) in water.

Excited State Dynamics of Bistridentate and Trisbidentate RuII Complexes of Quinoline-Pyrazole Ligands

Three homoleptic ruthenium(II) complexes, [Ru(Q3PzH)₃]²⁺, [Ru(Q1Pz)₃]²⁺, and [Ru(DQPz)₂]²⁺, based on the quinoline-pyrazole ligands, Q3PzH (8-(3-pyrazole)-quinoline), Q1Pz (8-(1-pyrazole)-quinoline), and DQPz (bis(quinolinyl)-1,3-pyrazole), have been spectroscopically and theoretically investigated. Spectral component analysis, transient absorption spectroscopy, density functional theory calculations, and ligand exchange reactions with different chlorination agents reveal that the excited state dynamics for Ru(II) complexes with these biheteroaromatic ligands differ significantly from that of traditional polypyridyl complexes. Despite the high energy and low reorganization energy of the excited state, nonradiative decay dominates even at liquid nitrogen temperatures, where triplet metal-to-ligand-charge-transfer emission quantum yields range from 0.7 to 3.8%, and microsecond excited state lifetimes are observed. In contrast to traditional polypyridyl complexes where ligand exchange is facilitated by expansion of the metal-ligand bonds to stabilize a metal-centered state, photoinduced ligand exchange occurs in the bidentate complexes despite no substantial MC state population, while the tridentate complex is extremely photostable despite an activated decay route, highlighting the versatile photochemistry of nonpolypyridine ligands.

Photophysical Properties and Photobiological Activities of Ruthenium(II) Complexes Bearing π-Expansive Cyclometalating Ligands with Thienyl Groups

A new family of cyclometalated ruthenium(II) complexes [Ru(N^N)₂(C^N)]⁺ derived from the π-extended benzo[h]imidazo[4,5-f]quinolone ligand appended with thienyl groups (n = 1-4, compounds 1-4) was prepared and its members were characterized for their chemical, photophysical, and photobiological properties. The lipophilicities of 1-4, determined as octanol-water partition coefficients (log P_o/w), were positive and increased with the number of thienyl units. The absorption and emission bands of the C^N compounds were red-shifted by up to 200 nm relative to the analogous Ru(II) diimine systems. All of the complexes exhibited dual emission with the intraligand fluorescence (¹IL, C^N-based) shifting to lower energies with increasing n and the metal-to-ligand charge transfer phosphorescence (³MLCT, N^N-based) remaining unchanged. Compounds 1-3 exhibited excited state absorption (ESA) profiles consistent with lowest-lying ³MLCT states when probed by nanosecond transient absorption (TA) spectroscopy with 532 nm excitation and had contributions from ¹IL(C^N) states with 355 nm excitation. These assignments were supported by the lifetimes observed (<10 ns for the ¹IL states and around 20 ns for the ³MLCT states) as well as a noticeable ESA for 3 with 355 nm excitation that did not occur with 532 nm excitation. Compound 4 was the only member of the family with two ³MLCT emissive lifetimes (15, 110 ns), and the TA spectra collected with both 355 and 532 nm excitation was assigned to the ³IL state, which was corroborated by its 4-6 μs lifetime. The ESA for 4 had a rise time of approximately 10 ns and an initial decay of 110 ns, which suggests a possible ³MLCT-³IL excited state equilibrium that results in delayed emission from the ³MLCT state. Compound 4 was nontoxic toward human skin melanoma cells (SKMEL28) in the dark (EC₅₀ = >300 μM); 1-3 were cytotoxic and yielded EC₅₀ values between 1 and 20 μM. The photocytotoxicites with visible light ranged from 87 nM with a phototherapeutic index (PI) of 13 for 1 to approximately 1 μM (PI = >267) for 4. With red light, EC₅₀ values varied from 270 nM (PI = 21) for 3 to 12 μM for 4 (PI = >25). The larger PIs for 4, especially with visible light, were attributed to the much lower dark cytotoxicity for this compound. Because the dark cytotoxicity contributes substantially to the observed photocytotoxicity for 1-3, it was not possible to assess whether the ³IL state of 4 led to a much more potent phototoxic mechanism in the absence of dark toxicity. There was no stark contrast in cellular uptake and accumulation by laser scanning confocal and differential interference contrast microscopy to explain the large differences in dark toxicities between 1-3 and 4. Nevertheless, the study highlights a new family of Ru(II) C^N complexes where π-conjugation beyond a certain point results in low dark cytotoxicity with high photocytotoxicity, opposing the notion that cyclometalated Ru(II) systems are too toxic to be phototherapeutic agents.

High-Yielding Syntheses of Multifunctionalized RuII Polypyridyl-Type Sensitizer: Experimental and Computational Insights into Coordination

Ru^II complexes based on functionalized 2,6-di(quinolin-8-yl)pyridine (dqp) ligands feature excellent photophysical and geometrical properties, thus suggesting dqp ligands as ideal surrogates for 2,2'-bipyridine (bpy) or 2,2':6',2″-terpyridine (tpy). However, the synthesis of multifunctionalized [Ru(dqp)₂]²⁺-based complexes is often low-yielding, which has hampered their practical value to date. In this study, a universal high-yielding route was explored and corroborated by a mechanistic investigation based on ¹H NMR, MS, and density functional theory. With application of high-boiling but less-coordinating solvents (i.e., DMF) during the coordination of dqp by the precursor [Ru(dqp)(MeCN)₃]²⁺, the required reaction temperature is lowered considerably (by 30 °C). In comparison to tpy, the reaction rate for dqp is further reduced which is assigned to the higher steric demand upon the coordination process. Namely, the onset of coordination of a tpy derivative at 60 °C and of dqp at 90 °C is significantly milder than in previous protocols. The versatility of the procedure is demonstrated by the high-yielding syntheses of multifunctionalized Ru^II complexes reaching up to 90%, whereby the presence of hydroxyl groups and losses during purification may lower the isolated yields substantially. In addition, the same strategy of high-boiling but less-coordinating solvents enabled a milder one-pot protocol to prepare [Ru(dqp)₂]²⁺ from a [Ru(MeCN)₆]²⁺ source, i.e., without the need for in situ reduction or halide abstraction as typical for Ru^IIICl₃ hydrate. Hence, the developed protocol benefits from an improved thermal tolerance of sensitive functional groups, which may be applicable also to related polypyridyl-type ligands.

Unraveling the marked differences of the phosphorescence efficiencies of blue-emitting iridium complexes with isomerized phenyltriazole ligands

Quantum chemical insights into the marked quantum efficiencies of blue-emitting iridium complexes with isomerized ptz ligands.

Photochemical Resolution of a Thermally Inert Cyclometalated Ru(phbpy)(N-N)(Sulfoxide)+ Complex

In this work a photosubstitution strategy is presented that can be used for the isolation of chiral organometallic complexes. A series of five cyclometalated complexes Ru(phbpy)(N−N)(DMSO-κS)](PF₆) ([1]PF₆-[5]PF₆) were synthesized and characterized, where Hphbpy = 6′-phenyl-2,2′-bipyridyl, and N–N = bpy (2,2′-bipyridine), phen (1,10-phenanthroline), dpq (pyrazino[2,3-f][1,10]phenanthroline), dppz (dipyrido[3,2-a:2′,3′-c]phenazine, or dppn (benzo[i]dipyrido[3,2-a,2′,3′-c]phenazine), respectively. Due to the asymmetry of the cyclometalated phbpy^– ligand, the corresponding [Ru(phbpy)(N–N)(DMSO-κS)]⁺complexes are chiral. The exceptional thermal inertness of the Ru–S bond made chiral resolution of these complexes by thermal ligand exchange impossible. However, photosubstitution by visible light irradiation in acetonitrile was possible for three of the five complexes ([1]PF₆-[3]PF₆). Further thermal coordination of the chiral sulfoxide (R)-methyl p-tolylsulfoxide to the photoproduct [Ru(phbpy)(phen)(NCMe)]PF₆, followed by reverse phase HPLC, led to the separation and characterization of the two diastereoisomers of [Ru(phbpy)(phen)(MeSO(C₇H₇))]PF₆, thus providing a new photochemical approach toward the synthesis of chiral cyclometalated ruthenium(II) complexes. Full photochemical, electrochemical, and frontier orbital characterization of the cyclometalated complexes [1]PF₆-[5]PF₆ was performed to explain why [4]PF₆ and [5]PF₆ are photochemically inert while [1]PF₆-[3]PF₆ perform selective photosubstitution.

Selective Preparation of a Heteroleptic Cyclometallated Ruthenium Complex Capable of Undergoing Photosubstitution of a Bidentate Ligand

Cyclometallated ruthenium complexes typically exhibit red‐shifted absorption bands and lower photolability compared to their polypyridyl analogues. They also have lower symmetry, which sometimes makes their synthesis challenging. In this work, the coordination of four N,S bidentate ligands, 3‐(methylthio)propylamine (mtpa), 2‐(methylthio)ethylamine (mtea), 2‐(methylthio)ethyl‐2‐pyridine (mtep), and 2‐(methylthio)methylpyridine (mtmp), to the cyclometallated precursor [Ru(bpy)(phpy)(CH₃CN)₂]⁺ (bpy=2,2′‐bipyridine, Hphpy=2‐phenylpyridine) has been investigated, furnishing the corresponding heteroleptic complexes [Ru(bpy)(phpy)(N,S)]PF₆ ([2]PF₆–[5]PF₆, respectively). The stereoselectivity of the synthesis strongly depended on the size of the ring formed by the Ru‐coordinated N,S ligand, with [2]PF₆ and [4]PF₆ being formed stereoselectively, but [3]PF₆ and [5]PF₆ being obtained as mixtures of inseparable isomers. The exact stereochemistry of the air‐stable complex [4]PF₆ was established by a combination of DFT, 2D NMR, and single‐crystal X‐ray crystallographic studies. Finally, [4]PF₆ was found to be photosubstitutionally active under irradiation with green light in acetonitrile, which makes it the first cyclometallated ruthenium complex capable of undergoing selective photosubstitution of a bidentate ligand.

Exploring the potential of iron to replace ruthenium in photosensitizers: a computational study

In an effort to replace the widely used ruthenium metal complexes with low-cost, earth abundant iron complexes as photosensitizers for dye-sensitized solar cell (DSSC) applications, herein we report the computational design of heteroleptic iron complexes (FC1-3) coordinated with benzimidazole-phenylcarbene (C^N) ligands. DFT and TDDFT calculations predicted the stronger σ-donating and π-accepting nature of phenyl carbene ligands substituted with electron-withdrawing CF₃, donating -N(CH₃)₂, and benzothiazine annulation than the imidazole carbene ligands (FC4); consequently, the metal-ligand bond distances and interactions that influence the ordering of charge transfer states with respect to metal centered states are altered in FC1-3 complexes. Detailed analysis based on energy decomposition analysis, spin density distribution analysis, and ab initio ligand field theory parameters were enabled to understand the nature of heteroleptic ligand interactions with the rest of the metal complex. The results from the study shed light on the judicious choice of ligands, as the same non-innocent ligand that is experimentally proven as favorable for Ru-dyes (TC1) is found to be detrimental for Fe-dyes (FC1). Among the complexes studied, the FC3 complex is a promising sensitizer for DSSC with ^1,3MLCT energy level well separated from ^3,5MC, thereby preventing the deactivation of MLCT. The outcome of the study is therefore an important step toward the development of photosensitizers based on iron metal. Graphical abstract Potential photosensitzers based on earth-abundant, low cost iron metal have been designed for dye sensitized solar cell applications.

Cyclometalated Ruthenium(II) Complexes Derived from α-Oligothiophenes as Highly Selective Cytotoxic or Photocytotoxic Agents

The photophysical and photobiological properties of a new class of cyclometalated ruthenium(II) compounds incorporating π-extended benzo[ h]imidazo[4,5- f]quinoline (IBQ) cyclometalating ligands (C^N) bearing thienyl rings ( n = 1-4, compounds 1-4) were investigated. Their octanol-water partition coefficients (log P_o/w) were positive and increased with n. Their absorption and emission energies were red-shifted substantially compared to the analogous Ru(II) diimine (N^N) complexes. They displayed C^N-based intraligand (IL) fluorescence and triplet excited-state absorption that shifted to longer wavelengths with increasing n and N^N-based metal-to-ligand charge transfer (MLCT) phosphorescence that was independent of n. Their photoluminescence lifetimes (τ_em) ranged from 4-10 ns for ¹IL states and 12-18 ns for ³MLCT states. Transient absorption lifetimes (τ_TA) were 5-8 μs with 355 nm excitation, ascribed to ³IL states that became inaccessible for 1-3 with 532 nm excitation (1-3, τ_TA = 16-17 ns); the ³IL of 4 only was accessible by lower energy excitation, τ_TA = 3.8 μs. Complex 4 was nontoxic (EC₅₀ > 300 μM) to SK-MEL-28 melanoma cells and CCD1064-Sk normal skin fibroblasts in the dark, while 3 was selectively cytotoxic to melanoma (EC₅₀= 5.1 μM) only. Compounds 1 and 2 were selective for melanoma cells in the dark, with submicromolar potencies (EC₅₀ = 350-500 nM) and selectivity factors (SFs) around 50. The photocytotoxicities of compounds 1-4 toward melanoma cells were similar, but only compounds 3 and 4 displayed significant phototherapeutic indices (PIs; 3, 43; 4, >1100). The larger cytotoxicities for compounds 1 and 2 were attributed to increased cellular uptake and nuclear accumulation, and possibly related to the DNA-aggregating properties of all four compounds as demonstrated by cell-free gel mobility-shift assays. Together, these results demonstrate a new class of thiophene-containing Ru(II) cyclometalated compounds that contain both highly selective chemotherapeutic agents and extremely potent photocytotoxic agents.

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.

Excited-State Decay Pathways of Tris(bidentate) Cyclometalated Ruthenium(II) Compounds

The synthesis, electrochemistry, and photophysical characterization are reported for 11 tris(bidentate) cyclometalated ruthenium(II) compounds, [Ru(N^N)₂(C^N)]⁺. The electrochemical and photophysical properties were varied by the addition of substituents on the 2,2'-bipyridine, N^N, and 2-phenylpyridine, C^N, ligands with different electron-donating and -withdrawing groups. The systematic tuning of these properties offered a tremendous opportunity to investigate the origin of the rapid excited-state decay for these cyclometalated compounds and to probe the accessibility of the dissociative, ligand-field (LF) states from the metal-to-ligand charge-transfer (MLCT) excited state. The photoluminescence quantum yield for [Ru(N^N)₂(C^N)]⁺ increased from 0.0001 to 0.002 as more electron-withdrawing substituents were added to C^N. An analogous substituent dependence was observed for the excited-state lifetimes, τ_obs, which ranged from 3 to 40 ns in neat acetonitrile, significantly shorter than those for their [Ru(N^N)₃]²⁺ analogues. The excited-state decay for [Ru(N^N)₂(C^N)]⁺ was accelerated because of an increased vibronic overlap between the ground- and excited-state wavefunctions rather than an increased electronic coupling as revealed by a comparison of the Franck-Condon factors. The radiative (k_r) and non-radiative (k_nr) rate constants of excited-state decay were determined to be on the order of 10⁴ and 10⁷-10⁸ s^-1, respectively. For sets of [Ru(N^N)₂(C^N)]⁺ compounds functionalized with the same N^N ligand, k_nr scaled with excited-state energy in accordance with the energy gap law. Furthermore, an Arrhenius analysis of τ_obs for all of the compounds between 273 and 343 K was consistent with activated crossing into a single, fourth ³MLCT state under the conditions studied with preexponential factors on the order of 10⁸-10⁹ s^-1 and activation energies between 300 and 1000 cm^-1. This result provides compelling evidence that LF states are not significantly populated near room temperature unlike many ruthenium(II) polypyridyl compounds. On the basis of the underlying photophysics presented here for [Ru(N^N)₂(C^N)]⁺, molecules of this type represent a robust class of compounds with built-in design features that should greatly enhance the molecular photostability necessary for photochemical and photoelectrochemical applications.

Bridged cyclometalated diruthenium complexes for fundamental electron transfer studies and multi-stage redox switching

Four bridged cyclometalated diruthenium systems are highlighted in this Frontier article, including strongly-coupled diruthenium complexes with a short phen-1,4-diyl or a planar pyren-2,7-diyl bridge, redox asymmetric diruthenium complexes characterized by different terminal ligands on the two ends, diruthenium complexes with a urea bridge that allows modulating the degree of electronic coupling, and those with a redox-active amine bridge with varying electronic structures. These complexes posess redox couples with low potentials and intense intervalence charge transfer absorptions in the near-infrared region in the one-electron-oxidized mixed-valent state. They are appealing not only for providing a platform for fundamental electron transfer studies but also as molecular materials with multi-stage redox switching properties.

Asymmetric Cyclometalated RuII Polypyridyl-Type Complexes with π-Extended Carbanionic Donor Sets

A series of novel cyclometalated Ru^II complexes were investigated featuring the tridentate dqp ligand platform (dqp is 2,6-di(quinolin-8-yl)pyridine), in order to utilize the octahedral coordination mode around the Ru center to modulate the electrochemical and photophysical properties. The heteroleptic complexes feature C₁ symmetry due to symmetry breaking by the peripheral five- or six-membered carbanionic chelate (phenyl, naphthyl, or anthracenyl units). The chelation mode is controlled by the steric effects and C-H activation selectivity of the ligand, which prompted the development of a general synthesis protocol. The optimized conditions to achieve high overall yields (55-75%) involve NaHCO₃ as the base and an simplified purification protocol: i.e., facile chromatographic separation using commercially available amino-functionalized silica applying nonaqueous salt-free conditions to omit the necessity of counterion exchange. The structural, photophysical, and electrochemical properties were studied in depth, and the results were corroborated by density functional theory (DFT) calculations. Steady state and time-resolved spectroscopy revealed red-shifted absorption (up to 750 nm) and weak IR emission (800-1000 nm) combined with prolonged emission lifetimes (up to 20 ns) in comparison to classical tpy-based (tpy is 2,2':6',2″-terpyridine) complexes. An enhanced stability was observed by blocking the reactive positions of the carbanionic ligand framework, while the reactive positions may be exploited for further functionalization.

Luminescent Dinuclear Ruthenium Terpyridine Complexes with a Bis-Phenylbenzimidazole Spacer

A conjugated bis-terpyridine bridging ligand, 2-(4-(2,6-di(pyridin-2-yl)pyridin-4-yl)phenyl)-6-(2-(4-(2,6-di(pyridin-2-yl)pyridin-4-yl)phenyl)-1H-benzo[d]imidazol-6-yl)-1H-benzo[d] imidazole (tpy-BPhBzimH₂-tpy), was designed in this work by covalent coupling of 3,3'-diaminobenzidine and two 4'-(p-formylphenyl)-2,2':6',2″-terpyridine units to synthesize a new series of bimetallic Ru(II)-terpyridine light-harvesting complexes. Photophysical and electrochemical properties were modulated by the variation of the terminal ligands in the complexes. The new compounds were thoroughly characterized by ¹H NMR spectroscopy, high-resolution mass spectrometry, and elemental analysis. Absorption spectra of the complexes consist of very strong ligand-centered π-π* and n-π* transitions in the UV, metal-to-ligand, and intraligand charge transfer bands in the visible regions. Steady-state and time-resolved emission spectral measurements indicate that the complexes exhibit moderately intense luminescence at room temperature within the spectral domain of 653-687 nm having luminescence lifetimes in the range between 6.3 and 55.2 ns, depending upon terminal tridentate ligand and solvent. Variable-temperature luminescence measurements suggest substantial increase of the energy gap between luminescent ³metal-to-ligand charge transfer state and nonluminescent ³metal centered in the complexes compared to the parent [Ru(tpy)₂]²⁺. Each of the three bimetallic complexes exhibits only one reversible couple in the positive potential window with almost no detectable splitting corresponding to simultaneous oxidation of the two remote Ru centers. All the complexes possess a number of imidazole NH protons, which became sufficiently acidic upon metal ion coordination. By utilizing these NH protons, we thoroughly studied anion recognition properties of the complexes in pure organic as well as predominantly aqueous media through multiple optical channels and spectroscopic methods. Finally computation investigations employing density functional theory (DFT) and time-dependent DFT were done to examine the electronic structures of the complexes and accurate assignment of experimentally observed optical spectral bands.

Boosting Vis/NIR Charge-Transfer Absorptions of Iron(II) Complexes by N-Alkylation and N-Deprotonation in the Ligand Backbone

Reversing the metal-to-ligand charge transfer (³ MLCT)/metal-centered (³ MC) excited state order in iron(II) complexes is a challenging objective, yet would finally result in long-sought luminescent transition-metal complexes with an earth-abundant central ion. One approach to achieve this goal is based on low-energy charge-transfer absorptions in combination with a strong ligand field. Coordinating electron-rich and electron-poor tridentate oligopyridine ligands with large bite angles at iron(II) enables both low-energy MLCT absorption bands around 590 nm and a strong ligand field. Variations of the electron-rich ligand by introducing longer alkyl substituents destabilizes the iron(II) complex towards ligand substitution reactions while hardly affecting the optical properties. On the other hand, N-deprotonation of the ligand backbone is feasible and reversible, yielding deep-green complexes with charge-transfer bands extending into the near-IR region. Time-dependent density functional theory calculations assign these absorption bands to transitions with dipole-allowed ligand-to-ligand charge transfer character. This unique geometric and electronic situation establishes a further regulating screw to increase the energy gap between potentially emitting charge-transfer states and the non-radiative ligand field states of iron(II) dyes.