Experimental evidence of ultrafast quenching of the 3MLCT luminescence in ruthenium(II) tris-bipyridyl complexes via a 3dd state

Oxygen Concentration Effect in Photosensitized Generation of 1O2 from Normoxia to Hypoxia

Photodynamic therapy (PDT) has gained widespread acceptance as a clinical cancer treatment modality and has been attracting intensive attention on developing novel PDT strategies. However, the hypoxic environment in tumors is considered as a significant challenge for efficient type II PDT, based on the inference of the highly oxygen-concentration-related 1O2 generation. Contrary to this conventional understanding, our research demonstrates oxygen concentration independence in the photosensitized generation of 1O2, as evidenced through steady-state and transient spectroscopy for chlorin e6 and methylene blue from normoxic to hypoxic conditions. We propose an oxygen-concentration-independent kinetic model, suggesting that efficient 1O2 generation can take place as long as the triplet-state lifetime ratio of the photosensitizer (τh/τn) is in a similar range to pO2n/pO2h. Our findings provide insights into PDT mechanisms and indicate that the oxygen concentration reduction concerns may not be critical for effective PDT in hypoxic tumor environments.

Intrinsic 77 K Phosphorescence Characteristics and Computational Modeling of Ru(II)-(Bidentate Cyclometalated-Aromatic Ligand) Chromophores: Their Relatively Low Nonradiative Rate Constants Originating from Low Spin-Orbit Coupling Driven Vibronic Coupling Amplitudes between Emitting and Ground States

We investigated the photoinduced relaxation of Kasha-type emitting ruthenium-(bidentate cyclometalated aromatic ligand), Ru-CM, chromophores of [Ru(pzpy)2(CM)]+ ions (CM = 1-phenylisoquinoline, 2,3-diphenylpyrazine, and 1,4-diazatriphenylene and pzpy = 2-pyrazol-1-yl-pyridine). This is the first report of the phosphorescence behavior of pure Ru-(bidentate CM) chromophores. The 77 K photoinduced relaxation characteristics of phosphorescence chromophores showed emission quantum yields higher than those of reference Ru-bpy (bpy = 2,2'-bipyridine) chromophores in the emission region of 670-900 nm. This phenomenon of the Ru-CM chromophores could be attributed to their unusually low magnitudes for 77 K nonradiative rate constants (kNRD), although their radiative rate-constants (kRAD) are not remarkable. In order to examine the 77 K photoinduced behavioral relaxation difference between Ru-CM and Ru-bpy chromophores, we used computational simulation, applying the fundamental formalism of kRAD and temperature-independent kNRD equations, which included calculated spin-orbit coupling values.

Improving the Accuracy in the Prediction of Transition-Metal Spin-State Energetics Using a Robust Variation-Based Approach: Density Functional Theory, CASPT2 and MC-PDFT Applied to the Case Study of Tris-Diimine Fe(II) Complexes

Designing ligands for transition metal complexes with a specified low-spin, high-spin or spin-crossover behavior is challenging. A major advance was recently made by Phan et al. [J. Am. Chem. Soc. 2017, 139, 6437-6447] who showed that the spin state of a homoleptic tris-diimine Fe(II) complex can be predicted from the N-N distance in the free diimine. They could thus predict the change in magnetic behavior on passing from the complexes of 2,2'-bipyridine (bpy), 2,2'-biimidazole (bim) and 2,2'-bis-2-imidazoline (bimz) ligands to those obtained with the modified analogs 4,5-diazafluoren-9-one (dafo), 1,1'-(α,α'-o-xylyl)-2,2'-bisimidazole (xbim) and 2,3,5,6,8,9-hexahydrodiimidazo[1,2-a:2', 1'-c]pyrazine (etbimz), respectively. Theoretically, the challenge lies in the accurate determination of the HS-LS zero-point energy difference ΔEHL°. The issue can be circumvented by using a variation-based approach, wherein ΔEHL° is not directly evaluated but obtained from the estimate of its variation Δ(ΔEHL°) in series of related systems, which include one whose ΔEHL° is accurately known [Phys. Chem. Chem. Phys. 2013, 15, 3752-3763; J. Phys. Chem. A 2022, 126, 6221-6235]. In this study, density functional theory (DFT), second-order multireference perturbation theory in its CASPT2 formulation, multiconfigurational pair DFT (MC-PDFT) and its hybrid formulation (HMC-PDFT) have been applied to the determination of Δ(ΔEHL°) in the pairs of complexes ( [ F e ( b p y ) 3 ] 2 + , [ F e ( d a f o ) 3 ] 2 + ) , ( [ F e ( b i m ) 3 ] 2 + , [ F e ( x b i m ) 3 ] 2 + ) and ( [ F e ( b i m z ) 3 ] 2 + , [ F e ( e t b i m z ) 3 ] 2 + ) . In DFT, we used several semilocal functionals and their global hybrids, as well as their D2, D3, D3BJ and D4 dispersion-corrected forms; and in MC-PDFT, different translated and fully translated functionals. The results are consistent with one another and in very good agreement with experiments. They show small to vanishing dependence on key details of the methods used: namely, the exact-exchange contribution to global hybrids; the ionization potential-electron affinity shift and basis sets used in the CASPT2 calculations; and the active spaces employed for the CASSCF wave functions used in the MC-PDFT and HMC-PDFT calculations. Insights into the change in the spin-state energetics accompanying the ligand exchanges were gained through a complexation energy analysis. Using the accurate CCSD(T) estimate of the HS-LS adiabatic energy difference in [ F e ( N C H ) 6 ] 2 + [J. Chem. Theory Comput. 2012, 8, 4216-4231], the Δ(ΔEHL°)-approach has been applied to the determination of ΔEHL° in the diimine complexes. The CASPT2 and DFT-D2 methods only give results in agreement with experiments. This suggests for the other methods a limitation in their treatment of dispersion which prevents them from accurately describing the spin-state energetics change accompanying the passing from [ F e ( N C H ) 6 ] 2 + with the tetragonal arrangement of its nitrile ligands to the tris-diimine complexes with the trigonal packing of their bulkier ligands.

Dual Photoreactive Ternary Ruthenium(II) Terpyridyl Complexes: A Comparative Study on Visible-Light-Induced Single-Step Dissociation of Bidentate Ligands and Generation of Singlet Oxygen

The versatile and tunable ligand-exchange dynamics in ruthenium(II)-polypyridyl complexes imposed by the modulation of the steric and electronic effects of the coordinated ligands provide an unlimited scope for developing phototherapeutic agents. The photorelease of a bidentate ligand from the Ru-center is better suited for potent Ru(II)-based photocytotoxic agents with two available labile sites for cross-linking with biological targets augmented with possible phototriggered 1O2 generation. Herein, we introduced a phenyl-terpyridine (ptpy) ligand in the octahedral Ru(II) core of [Ru(ptpy)(L-L)Cl]+ to induce structural distortion for the possible photorelease of electronically distinct bidentate ligands (L-L). For a systematic study, we designed four Ru(II) polypyridyl complexes: [Ru(ptpy)(L-L)Cl](PF6), ([1]-[4]), where L-L = 1,2-bis(phenylthio)ethane (SPH) [1], N,N,N',N'-tetramethylethylenediamine (TMEN) [2], N1,N2-diphenylethane-1,2-diimine (BPEDI) [3], and bis[2-(diphenylphosphino)phenyl]ether (DPE-Phos) [4]. The detailed photochemical studies suggest a single-step dissociation of L-L from the bis-thioether (SPH) complex [1] and diamine (TMEN) complex [2], while no photosubstitution was observed for [3] and [4]. Complex [1] and [2] demonstrated a dual role, involving both photosubstitution and 1O2 generation, while [3] and [4] solely exhibited poor to moderate 1O2 production. The interplay of excited states leading to these behaviors was rationalized from the lifetimes of the 3MLCT excited states by using transient absorption spectroscopy, suggesting intricate relaxation dynamics and 1O2 generation upon excitation. Therefore, the photolabile complexes [1] and [2] could potentially act as dual photoreactive agents via the phototriggered release of L-L (PACT) and/or 1O2-mediated PDT mechanisms, while [4] primarily can be utilized as a PDT agent.

Synthesis and structural characterizations of three carbonyl(α-diimine)hydrido(triphenylphosphine)ruthenium(II) complexes with derivatives of 1,10-phenanthroline

Three new ruthenium(II) polypyridyl complexes containing α-diimine ligands, namely, carbonylhydrido(1,10-phenanthroline-κ2N,N)bis(triphenylphosphine-κP)ruthenium(II) hexafluorophosphate, [RuH(C12H8N2)(C18H15P)2(CO)]PF6, carbonylhydrido(2,9-dimethyl-1,10-phenanthroline-κ2N,N)bis(triphenylphosphine-κP)ruthenium(II) hexafluorophosphate, and carbonylhydrido(4,7-dimethyl-1,10-phenanthroline-κ2N,N)bis(triphenylphosphine-κP)ruthenium(II) hexafluorophosphate, both [RuH(C14H12N2)(C18H15P)2(CO)]PF6, were synthesized and characterized by spectroscopic and X-ray diffraction methods. In these complexes, the ruthenium(II) ion adopts a distorted octahedral geometry. There are no intermolecular hydrogen bonds in the crystal structures of the analysed complexes and Hirshfeld surface analysis showed that the H...H contacts constitute a high percentage, close to 50%, of the intermolecular interactions.

Photochemistry of Ru(II) Triazole Complexes with 6-Membered Chelate Ligands: Detection and Reactivity of Ligand-Loss Intermediates

Photochemical ligand release from metal complexes may be exploited in the development of novel photoactivated chemotherapy agents for the treatment of cancer and other diseases. Highly intriguing photochemical behavior is reported for two ruthenium(II) complexes bearing conformationally flexible 1,2,3-triazole-based ligands incorporating a methylene spacer to form 6-membered chelate rings. [Ru(bpy)2(pictz)]2+ (1) and [Ru(bpy)2(btzm)]2+ (2) (bpy = 2,2'-bipyridyl; pictz = 1-(picolyl)-4-phenyl-1,2,3-triazole; btzm = bis(4-phenyl-1,2,3-triazol-4-yl)methane) exhibit coordination by the triazole ring through the less basic N2 atom as a consequence of chelation and readily undergo photochemical release of the pictz and btzm ligands (ϕ = 0.079 and 0.091, respectively) in acetonitrile solution to form cis-[Ru(bpy)2(NCMe)2]2+ (3) in both cases. Ligand-loss intermediates of the form [Ru(bpy)2(κ1-pictz or κ1-btzm)(NCCD3)]2+ are detected by 1H NMR spectroscopy and mass spectrometry. Photolysis of 1 yields three ligand-loss intermediates with monodentate pictz ligands, two of which form through simple decoordination of either the pyridine or triazole donor with subsequent solvent coordination (4-tz(N2) and 4-py, respectively). The third intermediate, shown to be able to form photochemically directly from 1, arises through linkage isomerism in which the monodentate pictz ligand is coordinated by the triazole N3 atom (4-tz(N3)) with a comparable ligand-loss intermediate with an N3-bound κ1-btzm ligand also observed for 2.

Modulating Excited State Properties and Ligand Ejection Kinetics in Ruthenium Polypyridyl Complexes Designed to Mimic Photochemotherapeutics

Ruthenium(II) polypyridyl complexes have gained significant interest as photochemotherapeutics (PCTs) due to their synthetic viability, strong light absorption, well understood excited state properties, and high phototoxicity indexes. Herein, we report the synthesis, characterization, electrochemical, spectrochemical, and preliminary cytotoxicity analyses of three series of ruthenium(II) polypyridyl complexes designed to mimic PCTs. The three series have the general structure of [Ru(bpy)2(N-N)]2+ (Series 1), [Ru(bpy)(dmb)(N-N)]2+ (Series 2), and [Ru(dmb)2(N-N)]2+ (Series 3, where N-N is a bidentate polypyridyl ligand, bpy = 2,2'-bipyridine, and dmb = 6,6'-dimethyl-2,2'-bipyridine). In the three series, the N-N ligand was systematically modified to incorporate increased conjugation and/or electronegative heteroatoms to increase dπ-π* backbonding, red-shifting the lowest energy metal-to-ligand charge transfer (MLCT) absorptions from λmax = 454 to λmax = 580 nm, nearing the therapeutic window for PCTs (600-1100 nm). In addition, steric bulk was systematically introduced through the series, distorting the Ru(II) octahedra, making the dissociative 3dd* state thermally accessible at room and body temperatures. This resulted in a 4 orders of magnitude increase in photoinduced ligand ejection kinetics, and demonstrates the ability to modulate both the MLCT* and dd* manifolds in the complexes, which is critical in PCT drug design. Preliminary cell viability assays suggest that the increased steric bulk to lower the 3dd* states may interfere with the cytotoxicity mechanism, limiting photoinitiated toxicity of the complexes. This work demonstrates the importance of understanding both the MLCT* and dd* manifolds and how they impact the ability of a complex to act as a PCT agent.

Photophysical Studies of Helicate and Mesocate Double-Stranded Dinuclear Ru(II) Complexes

The metal-ligand charge transfer (3MLCT) and phosphorescence-quenching metal-centered (3MC) states of the helicate and mesocate diastereoisomers of a double-stranded dinuclear polypyridylruthenium(II) complex have been investigated using ultrafast transient absorption spectroscopy. At 294 K, transient signals of the helicate decayed significantly slower than those of the mesocate, whereas at 77 K, no clear contrast in kinetics was observed. Contributions to excited-state decay from high-lying 3MLCT states were identified at both temperatures. Spectroscopic data (294 K) suggest that the 3MC state of the helicate lies above the 3MLCT and that the reverse is true for the mesocate; this was further validated by density functional theory calculations. The stabilization of the 3MC state relative to the 3MLCT state in the mesocate was explained by a reduction in ligand field strength due to distortion near the ligand bridge, which causes further deviation from octahedral geometry compared to the helicate. This work illustrates how minor structural differences can significantly influence excited state dynamics.

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

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

Early bird or night owl? Controlling the ultrafast photodynamics of triphenylamine substituted 2,2':6',2''-terpyridine

Controlling the ultrafast photodynamics of metal-free organic molecules has great potential for technological applications. In this work, we use solvent polarity and viscosity as "external knobs" to govern the photodynamics of an electron-donating derivative of 2,2':6',2''-terpyridine (terpy), namely 4'-(4-(di(4-tert-butylphenyl)amine)phenyl)-2,2':6',2''-terpyridine (tBuTPAterpy). We combine femtosecond fluorescence upconversion (FlUC), transient absorption (TA) and quantum mechanical calculations to provide a comprehensive description of the tBuTPAterpy's photodynamics. Our results demonstrate that, by changing the solvent, the time scale of light-induced conformational changes of the system can be tuned over two orders of magnitude, controlling the tBuTPAterpy fluorescence spectral region and yield. As a result, depending on the local environment, tBuTPAterpy can act either as an "early bird" or a "night owl", with a tunability that makes it a promising candidate for metal-free sensors.

Photo-responsive hydrogels based on a ruthenium complex: synthesis and degradation

We report the synthesis of a photo responsive metallo-hydrogel based on a ruthenium(II) complex as a functional cross-linker. This metal complex contains reactive 4AAMP (= 4-(acrylamidomethyl)pyridine) ligands, which can be cleaved by light-induced ligand substitution. Ru[(bpy)2(4AAMP)2] cross-links 4-arm-PEG-SH macromonomers by thia-Michael-addition to the photocleavable 4AAMP ligand for the preparation of the hydrogel. Irradiation with green light at 529 nm leads to photodegradation of the metallo-hydrogel due to the ligand dissociation, which can be adjusted by adjusting the Ru[(bpy)2(4AAMP)2] concentration. The ligand substitution forming [Ru(bpy)2(L)2]2+ (L = H2O and CH3CN) can be monitored by 1H NMR spectroscopy and UV-visible absorption. The control of degradation by light irradiation plays a significant role in modulating the elasticity and stiffness of the light sensitive metallo-hydrogel network. The photo-responsive hydrogel is a viable substrate for cell cultures.

Multifaceted Deactivation Dynamics of Fe(II) N-Heterocyclic Carbene Photosensitizers

Excited state dynamics of three iron(II) carbene complexes that serve as prototype Earth-abundant photosensitizers were investigated by ultrafast optical spectroscopy. Significant differences in the dynamics between the investigated complexes down to femtosecond time scales are used to characterize fundamental differences in the depopulation of triplet metal-to-ligand charge-transfer (3MLCT) excited states in the presence of energetically accessible triplet metal-centered (3MC) states. Novel insights into the full deactivation cascades of the investigated complexes include evidence of the need to revise the deactivation model for a prominent iron carbene prototype complex, a refined understanding of complex 3MC dynamics, and a quantitative discrimination between activated and barrierless deactivation steps along the 3MLCT → 3MC → 1GS path. Overall, the study provides an improved understanding of photophysical limitations and opportunities for the use of iron(II)-based photosensitizers in photochemical applications.

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.

Low-Temperature Observation of the Excited-State Decay of Ruthenium-(Mono-2,2':6',2″-Terpyridine) Ions with Innocent Ligands: DFT Modeling of an 3MLCT-3MC Intersystem Crossing Pathway

The synthesis, electrochemistry, and photophysical characterization of five 2,2':6',2″-terpyridine ruthenium complexes (Ru-tpy complexes) is reported. The electrochemical and photophysical behavior varied depending on the ligands, i.e., amine (NH₃), acetonitrile (AN), and bis(pyrazolyl)methane (bpm), for this series of Ru-tpy complexes. The target [Ru(tpy)(AN)₃]²⁺ and [Ru(tpy)(bpm)(AN)]²⁺ complexes were found to have low-emission quantum yields in low-temperature observations. To better understand this phenomenon, density functional theory (DFT) calculations were performed to simulate the singlet ground state (S₀), T_e, and metal-centered excited states (³MC) of these complexes. The calculated energy barriers between T_e and the low-lying ³MC state for [Ru(tpy)(AN)₃]²⁺ and [Ru(tpy)(bpm)(AN)]²⁺ provided clear evidence in support of their emitting state decay behavior. Developing a knowledge of the underlying photophysics of these Ru-tpy complexes will allow new complexes to be designed for use in photophysical and photochemical applications in the future.

Ru Monoimines with Extended Excited-State Lifetimes and Geometrical Modulation of Photoinduced Mixed-Valence Interactions

The photophysical properties of monodentate-imine ruthenium complexes do not usually fulfil the requirements for supramolecular solar energy conversion schemes. Their short excited-state lifetimes, like the 5.2 ps metal-to-ligand charge transfer (MLCT) lifetime of [Ru(py)₄Cl(L)]⁺ with L = pz (pyrazine), preclude bimolecular or long-range photoinduced energy or electron transfer reactions. Here, we explore two strategies to extend the excited-state lifetime, based on the chemical modification of the distal N atom of pyrazine. On one hand, we used L = pzH⁺, where protonation stabilized MLCT states, rendering thermal population of MC states less favorable. On the other hand, we prepared a symmetric bimetallic arrangement in which L = {(μ-pz)Ru(py)₄Cl} to enable hole delocalization via photoinduced mixed-valence interactions. A lifetime extension of 2 orders of magnitude is accomplished, with charge transfer excited states living 580 ps and 1.6 ns, respectively, reaching compatibility with bimolecular or long-range photoinduced reactivity. These results are similar to those obtained with Ru pentaammine analogues, suggesting that the strategy employed is of general applicability. In this context, the photoinduced mixed-valence properties of the charge transfer excited states are analyzed and compared with those of different analogues of the Creutz-Taube ion, demonstrating a geometrical modulation of the photoinduced mixed-valence properties.

Ultrafast spectroscopy of RuII polypyridine complexes in the gas phase and the liquid phase: [Ru(2,2'-bipyridine)2(nicotinamide)2]2

[Ru(bipyridine)₂(nicotinamide)₂]²⁺ (1) and its monoaqua-complex [Ru(bipyridine)₂(nicotinamide)(H₂O)]²⁺ (2) were spectroscopically studied for the first time in the gas phase by static and time resolved UV photodissociation spectroscopy, observing nicotinamide and H₂O ligand dissociation for 1 and 2, respectively. Both processes and their ultrafast dynamics were investigated in parallel by transient absorption spectroscopy in aqueous solution. The latter data were newly acquired for the long-wavelength MLCT band excitation of 1 and provide novel ultrafast ligand dissociation results for 2, confirming the gas phase results, i.e., exclusive H₂O cleavage over nicotinamide loss. Similar apparent time constants in the sub-ps and few ps ranges were obtained for 1 in both phases, whereas a larger time constant of ca. two hundreds of ps for the ground state recovery was observed exclusively in the solution phase. Our reaction scheme accounts for faster dissociation dynamics in the gas phase by energetical lowering of the ³MC vs. the ³MLCT states by lack of solvent stabilization of the latter. Based on the apparent time constants, we favour, for the solution dynamics, a fast bimodal vibrational deactivation in the ³MLCT/³MC manifolds and a slow dissociation obfuscated by the ground state recovery. This is substantiated by a similar reaction scheme proposed for the ultrafast dynamics of 2, resulting in a new assignment for transient absorption features with λ > 550 nm to the ³MC manifold, and a common kinetic description for 1 and 2. Computations at the TD-DFT/cc-PVTZ/MDF28 level support our spectroscopic findings and the suggested deactivation pathways.

High Intrinsic Phosphorescence Efficiency and Density Functional Theory Modeling of Ru(II)-Bipyridine Complexes with π-Aromatic-Rich Cyclometalated Ligands: Attributions of Spin-Orbit Coupling Perturbation and Efficient Configurational Mixing of Singlet Excited States

A series of π-aromatic-rich cyclometalated ruthenium(II)-(2,2'-bipyridine) complexes ([Ru(bpy)2(πAr-CM)]+) in which πAr-CM is diphenylpyrazine or 1-phenylisoquinoline were prepared. The [Ru(bpy)2(πAr-CM)]+ complexes had remarkably high phosphorescence rate constants, k RAD(p), and the intrinsic phosphorescence efficiencies (ιem(p) = k RAD(p)/(νem(p))3) of these complexes were found to be twice the magnitudes of simply constructed cyclometalated ruthenium(II) complexes ([Ru(bpy)2(sc-CM)]+), where νem(p) is the phosphorescence frequency and sc-CM is 2-phenylpyridine, benzo[h]quinoline, or 2-phenylpyrimidine. Density functional theory (DFT) modeling of the [Ru(bpy)2(CM)]+ complexes indicated numerous singlet metal-to-ligand charge transfers for 1MLCT-(Ru-bpy) and 1MLCT-(Ru-CM), excited states in the low-energy absorption band and 1ππ*-(aromatic ligand) (1ππ*-LAr) excited states in the high-energy band. DFT modeling of these complexes also indicated phosphorescence-emitting state (Te) configurations with primary MLCT-(Ru-bpy) characteristics. The variation in ιem(p) for the spin-forbidden Te (3MLCT-(Ru-bpy)) excited state of the complex system that was examined in this study can be understood through the spin-orbit coupling (SOC)-mediated sum of intensity stealing (∑SOCM-IS) contribution from the primary intensity of the low-energy 1MLCT states and second-order intensity perturbation from the significant configuration between the low-energy 1MLCT and high-energy intense 1ππ*-LAr states. In addition, the observation of unusually high ιem(p) magnitudes for these [Ru(bpy)2(πAr-CM)]+ complexes can be attributed to the values for both intensity factors in the ∑SOCM-IS formalism being individually greater than those for [Ru(bpy)2(sc-CM)]+ ions.

Not All 3MC States Are the Same: The Role of 3MCcis States in the Photochemical N∧N Ligand Release from [Ru(bpy)2(N∧N)]2+ Complexes

Ruthenium(II) complexes feature prominently in the development of agents for photoactivated chemotherapy; however, the excited-state mechanisms by which photochemical ligand release operates remain unclear. We report here a systematic experimental and computational study of a series of complexes [Ru(bpy)₂(N^∧N)]²⁺ (bpy = 2,2'-bipyridyl; N^∧N = bpy (1), 6-methyl-2,2'-bipyridyl (2), 6,6'-dimethyl-2,2'-bipyridyl (3), 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (4), 1-benzyl-4-(6-methylpyrid-2-yl)-1,2,3-triazole (5), 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl (6)), in which we probe the contribution to the promotion of photochemical N^∧N ligand release of the introduction of sterically encumbering methyl substituents and the electronic effect of replacement of pyridine by 1,2,3-triazole donors in the N^∧N ligand. Complexes 2 to 6 all release the ligand N^∧N on irradiation in acetonitrile solution to yield cis-[Ru(bpy)₂(NCMe)₂]²⁺, with resultant photorelease quantum yields that at first seem counter-intuitive and span a broad range. The data show that incorporation of a single sterically encumbering methyl substituent on the N^∧N ligand (2 and 5) leads to a significantly enhanced rate of triplet metal-to-ligand charge-transfer (³MLCT) state deactivation but with little promotion of photoreactivity, whereas replacement of pyridine by triazole donors (4 and 6) leads to a similar rate of ³MLCT deactivation but with much greater photochemical reactivity. The data reported here, discussed in conjunction with previously reported data on related complexes, suggest that monomethylation in 2 and 5 sterically inhibits the formation of a ³MC_cis state but promotes the population of ³MC_trans states which rapidly deactivate ³MLCT states and are prone to mediating ground-state recovery. On the other hand, increased photochemical reactivity in 4 and 6 seems to stem from the accessibility of ³MC_cis states. The data provide important insights into the excited-state mechanism of photochemical ligand release by Ru(II) tris-bidentate complexes.

Balancing the interplay between ligand ejection and therapeutic window light absorption in ruthenium polypyridyl complexes

Ruthenium polypyridyl complexes have gained significant interest as photochemotherapies (PCTs) where their excited-state properties play a critical role in the photo-cytotoxicity mechanism and efficacy. Herein we report a systematic electrochemical, spectrochemical, and photophysical analysis of a series of ruthenium(II) polypyridyl complexes of the type [Ru(bpy)₂(N-N)]²⁺ (where bpy = 2,2'-bipyridine; N-N is a bidentate polypyridyl ligand) designed to mimic PCTs. In this series, the N-N ligand was modified through increased conjugation and/or incorporation of electronegative heteroatoms to shift the metal-to-ligand charge-transfer (MLCT) absorptions near the therapeutic window for PCTs (600-1100 nm) while incorporating steric bulk to trigger photoinduced ligand dissociation. The lowest energy MLCT absorptions were red-shifted from λ_max = 454 nm to 564 nm, with emission energies decreasing from λ_max = 620 nm to 850 nm. Photoinduced ligand ejection and temperature-dependent emission studies revealed an important interplay between red-shifting MLCT absorptions and accessing the dissociative ³dd* states, with energy barriers between the ³MLCT* and ³dd* states ranging from 850 cm^-1 to 2580 cm^-1 for the complexes measured. This work demonstrates the importance of understanding both the MLCT manifold and ³dd* state energy levels in the future design of ligands and complexes for PCT.

Intraligand Excited States Turn a Ruthenium Oligothiophene Complex into a Light-Triggered Ubertoxin with Anticancer Effects in Extreme Hypoxia

Ru(II) complexes that undergo photosubstitution reactions from triplet metal-centered (3MC) excited states are of interest in photochemotherapy (PCT) due to their potential to produce cytotoxic effects in hypoxia. Dual-action systems that incorporate this stoichiometric mode to complement the oxygen-dependent photosensitization pathways that define photodynamic therapy (PDT) are poised to maintain antitumor activity regardless of the oxygenation status. Herein, we examine the way in which these two pathways influence photocytotoxicity in normoxia and in hypoxia using the [Ru(dmp)2(IP-nT)]2+ series (where dmp = 2,9-dimethyl-1,10-phenanthroline and IP-nT = imidazo[4,5-f][1,10]phenanthroline tethered to n = 0-4 thiophene rings) to switch the dominant excited state from the metal-based 3MC state in the case of Ru-phen-Ru-1T to the ligand-based 3ILCT state for Ru-3T and Ru-4T. Ru-phen-Ru-1T, having dominant 3MC states and the largest photosubstitution quantum yields, are inactive in both normoxia and hypoxia. Ru-3T and Ru-4T, with dominant 3IL/3ILCT states and long triplet lifetimes (τTA = 20-25 μs), have the poorest photosubstitution quantum yields, yet are extremely active. In the best instances, Ru-4T exhibit attomolar phototoxicity toward SKMEL28 cells in normoxia and picomolar in hypoxia, with phototherapeutic index values in normoxia of 105-1012 and 103-106 in hypoxia. While maximizing excited-state deactivation through photodissociative 3MC states did not result in bonafide dual-action PDT/PCT agents, the study has produced the most potent photosensitizer we know of to date. The extraordinary photosensitizing capacity of Ru-3T and Ru-4T may stem from a combination of very efficient 1O2 production and possibly complementary type I pathways via 3ILCT excited states.

Improving the chemical stability of blue heteroleptic iridium emitter FIrpic in the lowest triplet state through ancillary ligand modification: a theoretical perspective

With the aim of providing a deeper understanding of the underlying degradation mechanisms associated with the lifetime of blue emitters during the decay process of blue PhOLEDs, quantum chemistry studies were performed to examine the chemical degradation mechanism of common sky blue emitter iridium(III)bis(4,6-di-fluorophenyl)-pyridinato-N,C^2')picolinate (FIrpic) and its derivatives with density functional theory (DFT) calculations. For these Ir(III) emitters, the Ir-N₁ bond between the ancillary ligand (picolinate) and central iridium will be broken by external light stimuli, which is followed by conversion from the initial emissive metal-to-ligand charge transfer (³MLCT) state to the non-emissive metal centered (³MC) state. The potential energy change for the photo-induced degradation path is then dominated by the energy levels of the ³MT and ³MC states, which are related to the triplet transition energy and the Ir-N₁ bond strength, respectively. Thereby, the Ir-N₁ bond dissociation in the lowest triplet state will be much harder to proceed if the S₀ → T₁ transition energy gets more energetically stable or the bond strength gets larger. It is believed that strategic modification of the ancillary ligand, especially by substitution of electron-donating groups at the para position of the pyridyl N atom or extension of the p-electron delocalization, is an effective and easy way to enhance the photochemical stability of the typical blue emitter FIrpic.

Light-responsive and Protic Ruthenium Compounds Bearing Bathophenanthroline and Dihydroxybipyridine Ligands Achieve Nanomolar Toxicity towards Breast Cancer Cells

We report new ruthenium complexes bearing the lipophilic bathophenanthroline (BPhen) ligand and dihydroxybipyridine (dhbp) ligands which differ in the placement of the OH groups ([(BPhen)2 Ru(n,n'-dhbp)]Cl2 with n = 6 and 4 in 1A and 2A , respectively). Full characterization data are reported for 1A and 2A and single crystal X-ray diffraction for 1A . Both 1A and 2A are diprotic acids. We have studied 1A , 1B , 2A , and 2B (B = deprotonated forms) by UV-vis spectroscopy and 1 photodissociates, but 2 is light stable. Luminescence studies reveal that the basic forms have lower energy 3 MLCT states relative to the acidic forms. Complexes 1A and 2A produce singlet oxygen with quantum yields of 0.05 and 0.68, respectively, in acetonitrile. Complexes 1 and 2 are both photocytotoxic toward breast cancer cells, with complex 2 showing EC50 light values as low as 0.50 μM with PI values as high as >200 vs. MCF7. Computational studies were used to predict the energies of the 3 MLCT and 3 MC states. An inaccessible 3 MC state for 2B suggests a rationale for why photodissociation does not occur with the 4,4'-dhbp ligand. Low dark toxicity combined with an accessible 3 MLCT state for 1 O2 generation explains the excellent photocytotoxicity of 2.

Ligand-Field Effects in a Ruthenium(II) Polypyridyl Complex Probed by Femtosecond X-ray Absorption Spectroscopy

We employ femtosecond X-ray absorption spectroscopy of [Ru(m-bpy)₃]²⁺ (m-bpy = 6-methyl-2,2'-bipyridine) to elucidate the time evolution of the spin and charge density upon metal-to-ligand charge-transfer (MLCT) excitation. The core-level transitions at the Ru L₃-edge reveal a very short MLCT lifetime of 0.9 ps and relaxation to the lowest triplet metal-centered state (³MC) which exhibits a lifetime of about 300 ps. Time-dependent density functional theory relates ligand methylation to a lower ligand field strength that stabilizes the ³MC state. A quarter of the ³MLCT population appears to be trapped which may be attributed to intramolecular vibrational relaxation or further electron transfer to the solvent. Our results demonstrate that small changes in the ligand field allow control of the photophysical properties. Moreover, this study underscores the high information content of femtosecond L-edge spectroscopy as a probe of valence charge density and spin-state in 4d transition metals.

Ruthenium Complexes Bearing Axially Chiral Bipyridyls: The Mismatched Diastereomer Showed Red Circularly Polarized Phosphorescence

Ruthenium^II complexes bearing three axially chiral bipyridyl ligands were synthesized as a new family of chiral complex dyes, and Δ-(S)- and Λ-(S)-diastereomers were obtained. The X-ray crystal structure analyses, spectroscopy, and DFT calculations suggested that all the bipyridyls maintained chirality in both the ground and excited states, and the Δ-(S)- and Λ-(S)-isomers are the matched (more relaxed) and mismatched (more constrained) pairs, respectively. The mismatched Λ-(S)-isomer exhibited red circularly polarized phosphorescence (CPP) both in solution and in the solid state. The solution state CPP is the most intense of ruthenium complexes, while the solid state CPP is the first example of them. It is supposed that, for the Λ-(S)-isomer, the six cumulative CH/π interactions suppress further distortion in the T₁ state.

Metal-to-Metal Distance Modulated Au(I)/Ru(II) Cyclophanyl Complexes: Cooperative Effects in Photoredox Catalysis

The modular synthesis of Au(I)/Ru(II) decorated mono- and heterobimetallic complexes with π-conjugated [2.2]paracyclophane is described. [2.2]Paracyclophane serves as a rigid spacer which holds the metal centers in precise spatial orientations and allows metal-to-metal distance modulation. A broad set of architectural arrangements of pseudo -geminal, -ortho, -meta, and -para substitution patterns were employed. Metal-to-metal distance modulation of Au(I)/Ru(II) heterobimetallic complexes and the innate transannular π-communication of the cyclophanyl scaffold provides a promising platform for the investigations of structure-activity relationship and cooperative effects. The Au(I)/Ru(II) heterobimetallic cyclophanyl complexes are stable, easily accessible, and exhibit promising catalytic activity in the visible-light promoted arylative Meyer-Schuster rearrangement.

Trifluoromethyl substitution enhances photoinduced activity against breast cancer cells but reduces ligand exchange in Ru(ii) complex

A series of five ruthenium complexes containing triphenyl phosphine groups known to enhance both cellular penetration and photoinduced ligand exchange, cis-[Ru(bpy)2(P(p-R-Ph)3)(CH3CN)]2+, where bpy = 2,2'-bipyridine and P(p-R-Ph)3 represent para-substituted triphenylphosphine ligands with R = -OCH3 (1), -CH3 (2) -H (3), -F (4), and -CF3 (5), were synthesized and characterized. The photolysis of 1-5 in water with visible light (λ irr ≥ 395 nm) results in the substitution of the coordinated acetonitrile with a solvent molecule, generating the corresponding aqua complex as the single photoproduct. A 3-fold variation in quantum yield was measured with 400 nm irradiation, Φ 400, where 1 is the most efficient with a Φ 400 = 0.076(2), and 5 the least photoactive complex, with Φ 400 = 0.026(2). This trend is unexpected based on the red-shifted metal-to-ligand charge transfer (MLCT) absorption of 1 as compared to that of 5, but can be correlated to the substituent Hammett para parameters and pK a values of the ancillary phosphine ligands. Complexes 1-5 are not toxic towards the triple negative breast cancer cell line MDA-MB-231 in the dark, but 3 and 5 are >4.2 and >19-fold more cytotoxic upon irradiation with blue light, respectively. A number of experiments point to apoptosis, and not to necrosis or necroptosis, as the mechanism of cell death by 5 upon irradiation. These findings provide a foundation for understanding the role of phosphine ligands on photoinduced ligand substitution and show the enhancement afforded by -CF3 groups on photochemotherapy, which will aid the future design of photocages for photochemotherapeutic drug delivery.

Evidence for a lowest energy 3MLCT excited state in [Fe(tpy)(CN)3]. Chem Commun (Camb) 2021;57:4658-4661. [PMID: 33977987 DOI: 10.1039/d1cc01090e]

Transient absorption data of [FeII(tpy)(CN)3]- reveals spectroscopic signatures indicative of 3MLCT with a ∼10 ps kinetic component. These data are supported by DFT and TD-DFT calculations, which show that excited state ordering is responsive to the number of cyanide ligands on the complex.

Simultaneous 3D surface profile and pressure measurement using phase-shift profilometry and pressure-sensitive paint

A simultaneous three-dimensional (3D) surface profile and pressure measurement method that integrates phase-shift profilometry and pressure-sensitive paint (PSP²) is proposed. The advantages of this novel technique over previous 3D pressure-sensitive paint (3D-PSP) techniques include a simplified system with low cost, no interference with PSP coatings, high spatial resolution, and high accuracy. A modified digital light-processing (DLP) projector-structured light generator is used to encode ultraviolet light and generate fringe projection to excite the pressure-sensitive paint. The 3D profile is reconstructed using four phase-shifting emission images. Meanwhile, the surface intensity ratio distribution is obtained. The PSP² method is applied to a nitrogen jet impingement experiment onto a spherical model. The intensity ratio results obtained using the PSP² method differ little from the conventional PSP results obtained using uniform excitation. The phase distortion due to the emission intensity fluctuation leads to errors in surface profile measurement, and the fringe projection with high contrast improves surface profile measurement accuracy. In most of the final results, the average total errors between the reconstructed 3D surface and the CAD geometry are less than 0.1 mm.

Near-IR Charge-Transfer Emission at 77 K and Density Functional Theory Modeling of Ruthenium(II)-Dipyrrinato Chromophores: High Phosphorescence Efficiency of the Emitting State Related to Spin-Orbit Coupling Mediation of Intensity from Numerous Low-Energy Singlet Excited States

Efficient charge-transfer (CT) phosphorescence in the near-IR (NIR) spectral region is reported for four substituted Ru-(R-dipyrrinato) complexes, [Ru(bpy)₂(R-dipy)](PF₆), where bpy is 2,2'-bipyridine and the substituent R is phenyl (ph), 2,4,6-trimethylphenyl, 4-carboxyphenyl (HOOC-ph), or 4-pyridinyl. The experimentally determined phosphorescence efficiency, ι_em(p) = k_RAD(p)/(ν_em(p))³ (where k_RAD(p) and ν_em(p) are the phosphorescence rate constant and the phosphorescence frequency, respectively), of the [Ru(bpy)₂(R-dipy)]⁺ complexes was approximately double that of [Ru(bpy)(Am)₄]²⁺ complexes (Am = ammine ligand) in the NIR region. Density functional theory (DFT) modeling indicated two strikingly different electronic configurations of the triplet emitting state (T_e) in the two types of complexes. The T_e of [Ru(bpy)₂(R-dipy)]⁺ complexes shows a CT-type corresponding to the metal-to-ligand charge transfer (MLCT)-(Ru-(R-dipy)) and the ππ*-(R-dipy) moiety configurations, and the T_e state in the [Ru(bpy)(Am)₄]²⁺ complexes corresponds to an approximately MLCT excited state consisting of mostly MLCT-(Ru-bpy) with a minimal ππ*(bpy) contribution. DFT modeling also indicated that the low-energy singlet excited states in the T_e geometry (S_n(T)) of the [Ru(bpy)₂(ph-dipy)]⁺ complex consist of numerous CT-S_n(T)-type states of the Ru-dipy and Ru-bpy moieties, whereas the [Ru(bpy)(Am)₄]²⁺ ions show quite simple MLCT-S_n(T)-type states of the Ru-bpy moiety. Based on experimental observations, DFT modeling, and the plain spin-orbit coupling (SOC) principle, we conclude that the remarkably high ι_em(p) amplitudes of the [Ru(bpy)₂(R-dipy)]⁺ complexes relative to those of [Ru(bpy)(Am)₄]²⁺ complexes can be attributed to the relatively substantial contribution of intrinsic SOC-mediated intensity stealing from the numerous low-energy CT-type S_n(T) states.

Singlet Oxygen Formation vs Photodissociation for Light-Responsive Protic Ruthenium Anticancer Compounds: The Oxygenated Substituent Determines Which Pathway Dominates

Ruthenium complexes bearing protic diimine ligands are cytotoxic to certain cancer cells upon irradiation with blue light. Previously reported complexes of the type [(N,N)₂Ru(6,6'-dhbp)]Cl₂ with 6,6'-dhbp = 6,6'-dihydroxybipyridine and N,N = 2,2'-bipyridine (bipy) (1_A), 1,10-phenanthroline (phen) (2_A), and 2,3-dihydro-[1,4]dioxino[2,3-f][1,10]phenanthroline (dop) (3_A) show EC₅₀ values as low as 4 μM (for 3_A) vs breast cancer cells upon blue light irradiation ( Inorg. Chem. 2017, 56, 7519). Herein, subscript A denotes the acidic form of the complex bearing OH groups, and B denotes the basic form bearing O^- groups. This photocytotoxicity was originally attributed to photodissociation, but recent results suggest that singlet oxygen formation is a more plausible cause of photocytotoxicity. In particular, bulky methoxy substituents enhance photodissociation but these complexes are nontoxic ( Dalton Trans 2018, 47, 15685). Cellular studies are presented herein that show the formation of reactive oxygen species (ROS) and apoptosis indicators upon treatment of cells with complex 3_A and blue light. Singlet oxygen sensor green (SOSG) shows the formation of ¹O₂ in cell culture for cells treated with 3_A and blue light. At physiological pH, complexes 1_A-3_A are deprotonated to form 1_B-3_B in situ. Quantum yields for ¹O₂ (ϕ_Δ) are 0.87 and 0.48 for 2_B and 3_B, respectively, and these are an order of magnitude higher than the quantum yields for 2_A and 3_A. The values for ϕ_Δ show an increase with 6,6'-dhbp derived substituents as follows: OMe < OH < O^-. TD-DFT studies show that the presence of a low lying triplet metal-centered (³MC) state favors photodissociation and disfavors ¹O₂ formation for 2_A and 3_A (OH groups). However, upon deprotonation (O^- groups), the ³MLCT state is accessible and can readily lead to ¹O₂ formation, but the dissociative ³MC state is energetically inaccessible. The changes to the energy of the ³MLCT state upon deprotonation have been confirmed by steady state luminescence experiments on 1_A-3_A and their basic analogs, 1_B-3_B. This energy landscape favors ¹O₂ formation for 2_B and 3_B and leads to enhanced toxicity for these complexes under physiological conditions. The ability to convert readily from OH to O^- groups allowed us to investigate an electronic change that is not accompanied by steric changes in this fundamental study.

Microsecond Photoluminescence and Photoreactivity of a Metal-Centered Excited State in a Hexacarbene-Co(III) Complex

The photofunctionality of the cobalt–hexacarbene complex [Co(III)(PhB(MeIm)₃)₂]⁺ (PhB(MeIm)₃ = tris(3-methylimidazolin-2-ylidene)(phenyl)borate) has been investigated by time-resolved optical spectroscopy. The complex displays a weak (Φ ∼ 10^–4) but remarkably long-lived (τ ∼ 1 μs) orange photoluminescence at 690 nm in solution at room temperature following excitation with wavelengths shorter than 350 nm. The strongly red-shifted emission is assigned from the spectroscopic evidence and quantum chemical calculations as a rare case of luminescence from a metal-centered state in a 3d⁶ complex. Singlet oxygen quenching supports the assignment of the emitting state as a triplet metal-centered state and underlines its capability of driving excitation energy transfer processes.

Proton-Coupled Electron Transfer for Photoinduced Generation of Two-Electron Reduced Species of Quinone

Purpose-built molecules that follow the fundamental process of photosynthesis have significance in developing better insight into the natural photosynthesis process. Quinones have a significant role as electron acceptors in natural photosynthesis, and their reduction is assisted through H-bond donation or protonation. The major challenge in such studies is to couple the multielectron and proton-transfer process and to achieve a reasonably stable charge-separated state for the elucidation of the mechanistic pathway. We have tried to address this issue through the design of a donor-acceptor-donor molecular triad (2RuAQ) derived from two equivalent [Ru(bpy)₃]²⁺ derivatives and a bridging anthraquinone moiety (AQ). Photoinduced proton-coupled electron transfer (PCET) for this molecular triad was systematically investigated in the absence and presence of hexafluoroisopropanol and p-toluenesulfonic acid (PTSA) using time-resolved absorption spectroscopy in the ultrafast time domain. Results reveal the generation of a relatively long-lived charge-separated state in this multi-electron transfer reaction, and we could confirm the generation of AQ^2- and Ru^III as the transient intermediates. We could rationalize the mechanistic pathway and the dynamics associated with photoinduced processes and the role of H-bonding in stabilizing charge-separated states. Transient absorption spectroscopic studies reveal that the rates of intramolecular electron transfer and the mechanistic pathways associated with the PCET process are significantly different in different solvent compositions having different polarities. In acetonitrile, a concerted PCET mechanism prevails, whereas the stepwise PCET reaction process is observed in the presence of PTSA. The results of the present study represent a unique model for the mechanistic diversity of PCET reactions.

Structural Characterization and Lifetimes of Triple-Stranded Helical Coinage Metal Complexes: Synthesis, Spectroscopy and Quantum Chemical Calculations

This work reports on a series of polynuclear complexes containing a trinuclear Cu, Ag, or Au core in combination with the fac-isomer of the metalloligand [Ru(pypzH)3 ](PF6 )2 (pypzH=3-(pyridin-2-yl)pyrazole). These (in case of the Ag and Au containing species) newly synthesized compounds of the general formula [{Ru(pypz)3 }2 M3 ](PF6 ) (2: M=Cu; 3: M=Ag; 4: M=Au) contain triple-stranded helical structures in which two ruthenium moieties are connected by three N-M-N (M=Cu, Ag, Au) bridges. In order to obtain a detailed description of the structure both in the electronic ground and excited states, extensive spectroscopic and quantum chemical calculations are applied. The equilateral coinage metal core triangle in the electronic ground state of 2-4 is distorted in the triplet state. Furthermore, the analyses offer a detailed description of electronic excitations. By using time-resolved IR spectroscopy from the microsecond down to the nanosecond regime, both the vibrational spectra and the lifetime of the lowest lying electronically excited triplet state can be determined. The lifetimes of these almost only non-radiative triplet states of 2-4 show an unusual effect in a way that the Au-containing complex 4 has a lifetime which is by more than a factor of five longer than in case of the Cu complex 2. Thus, the coinage metals have a significant effect on the electronically excited state, which is localized on a pypz ligand coordinated to the Ru atom indicating an unusual cooperative effect between two moieties of the complex.

An excited state dynamics driven reaction: wavelength-dependent photoisomerization quantum yields in [Ru(bpy)2(dmso)2]2

We report the excited-state behavior of a structurally simple bis-sulfoxide complex, cis-S,S-[Ru(bpy)2(dmso)2]2+, as investigated by femtosecond pump-probe spectroscopy. The results reveal that a single photon prompts phototriggered isomerization of one or both dmso ligands to yield a mixture of cis-S,O-[Ru(bpy)2(dmso)2]2+ and cis-O,O-[Ru(bpy)2(dmso)2]2+. The quantum yields of isomerization of each product and relative product distribution are dependent upon the excitation wavelength, with longer wavelengths favoring the double isomerization product, cis-O,O-[Ru(bpy)2(dmso)2]2+. Transient absorption measurements on cis-O,O-[Ru(bpy)2(dmso)2]2+ do not reveal an excited-state isomerization pathway to produce either the S,O or S,S isomers. Femtosecond pulse shaping experiments reveal no change in the product distribution. Pump-repump-probe transient absorption spectroscopy of cis-S,S-[Ru(bpy)2(dmso)2]2+ shows that a pump-repump time delay of 3 ps dramatically alters the S,O : O,O product ratio; pump-repump-probe transient absorption spectroscopy of cis-O,O-[Ru(bpy)2(dmso)2]2+ with a time delay of 3 ps uncovers an excited-state isomerization pathway to produce the S,O isomer. In conjunction with low-temperature steady-state emission spectroscopy, these results are interpreted in the context of an excited-state bifurcating pathway, in which the isomerization product distribution is determined not by thermodynamics, but rather as a dynamics driven 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.

NIR-Light-Driven Generation of Reactive Oxygen Species Using Ru(II)-Decorated Lipid-Encapsulated Upconverting Nanoparticles

The biological application of ruthenium anticancer prodrugs for photodynamic therapy (PDT) and photoactivated chemotherapy (PACT) is restricted by the need to use poorly penetrating high-energy photons for their activation, i.e., typically blue or green light. Upconverting nanoparticles (UCNPs), which produce high-energy light under near-infrared (NIR) excitation, may solve this issue, provided that the coupling between the UCNP surface and the Ru prodrug is optimized to produce stable nanoconjugates with efficient energy transfer from the UCNP to the ruthenium complex. Herein, we report on the synthesis and photochemistry of the two structurally related ruthenium(II) polypyridyl complexes [Ru(bpy)₂(5)](PF₆)₂ ([1](PF₆)₂) and [Ru(bpy)₂(6)](PF₆)₂ ([2](PF₆)₂), where bpy = 2,2-bipyridine, 5 is 5,6-bis(dodecyloxy)-2,9-dimethyl-1,10-phenanthroline, and 6 is 5,6-bis(dodecyloxy)-1,10-phenanthroline. [1](PF₆)₂ is photolabile as a result of the steric strain induced by ligand 5, but the irradiation of [1](PF₆)₂ in solution leads to the nonselective and slow photosubstitution of one of its three ligands, making it a poor PACT compound. On the other hand, [2](PF₆)₂ is an efficient and photostable PDT photosensitizer. The water-dispersible, negatively charged nanoconjugate UCNP@lipid/[2] was prepared by the encapsulation of 44 nm diameter NaYF₄:Yb³⁺,Tm³⁺ UCNPs in a mixture of 1,2-dioleoyl-sn-glycero-3-phosphate and 1,2-dioleoyl-sn-glycero-3-phosphocholine phospholipids, cholesterol, and the amphiphilic complex [2](PF₆)₂. A nonradiative energy transfer efficiency of 12% between the Tm³⁺ ions in the UCNP and the Ru²⁺ acceptor [2]²⁺ was found using time-resolved emission spectroscopy. Under irradiation with NIR light (969 nm), UCNP@lipid/[2] was found to produce reactive oxygen species (ROS), as judged by the oxidation of the nonspecific ROS probe 2',7'-dichlorodihydrofluorescein (DCFH^2-). Determination of the type of ROS produced was precluded by the negative surface charge of the nanoconjugate, which resulted in the electrostatic repulsion of the more specific but also negatively charged ¹O₂ probe tetrasodium 9,10-anthracenediyl-bis(methylene)dimalonate (Na₄(ADMBMA)).