Microsecond 3MLCT excited state lifetimes in bis-tridentate Ru(ii)-complexes: significant reductions of non-radiative rate constants

Biodegradable Ruthenium-Rhenium Complexes Containing Nanoamplifiers: Triggering ROS-Induced CO Release for Synergistic Cancer Treatment

The constrained effectiveness of photodynamic therapy (PDT) has impeded its widespread use in clinical practice. Urgent efforts are needed to address the shortcomings faced in photodynamic therapy, such as photosensitizer toxicity, short half-life, and limited action range of reactive oxygen species (ROS). In this study, a biodegradable copolymer nanoamplifier is reported that contains ruthenium complex (Ru-complex) as photosensitizer (PS) and rhenium complex (Re-complex) as carbon monoxide (CO)-release molecule (CORM). The well-designed nanoamplifier brings PS and CORM into close spatial proximity, significantly promotes the utilization of light-stimulated reactive oxygen species (ROS), and cascaded amplifying CO release, thus enabling an enhanced synergistic effect of PDT and gas therapy for cancer treatment. Moreover, owing to its intrinsic photodegradable nature, the nanoamplifier exhibits good tumor accumulation and penetration ability, and excellent biocompatibility in vivo. These findings suggest that the biodegradable cascaded nanoamplifiers pave the way for a synergistic and clinically viable integration of photodynamic and gas therapy.

Energy transfer in metal-exchange binuclear complexes covalently linked by asymmetric ligands

Nitrogen complexation with π-conjugated ligands is an effective strategy for synthesizing luminescent molecules. The asymmetric bridging ligands L (L1 and L2) have been designed. The terminal chelating sites of the L1 and L2 bridging ligands consisted of 2,2'-bipyridine (bpy) and 1,10-phenanthroline moieties (where L = L1 and L2; L1 = 2-(3-((4-([2,2'-bipyridin]-6-yl)benzyl)oxy)phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline and L2 = 2-(3-((4-(6-phenyl-[2,2'-bipyridin]-4-yl)benzyl)oxy)phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline). The full use of the synthetic strategy of the "complexes as ligands and complexes as metals" was expected to successfully design and synthesize a series of conjugated metal-exchange complexes linked by the asymmetric bridging ligands L1 and L2. These compounds included monometallic complexes Ru(L) and (L)Ru (C1, C2, C7, and C8), homometallic complexes Ru(L)Ru (C3 and C4), and heterometallic complexes Os(L)Ru and Ru(L)Os (C5, C6, C9, and C10) with Ru- or Os-based units. C3-C10 complexes exhibited various degrees of octahedral distortion around the Ru(II) or Os(II) center, which was consistent with the optimized geometry of the coordination complexes based on density functional theory calculation. These complexes exhibited intense spin-allowed ligand-centered transitions with high absorbance at around 288 nm upon absorbing visible light. Notably, all complexes exhibited spin-allowed metal-to-ligand charge transfer absorption of the Ru-based units in the 440-450 nm range. In addition, the heterometallic C5, C6, C9, and C10 complexes showed absorption of the Os-based units in the range of 565-583 nm. The intramolecular energy transfer of C3 and C5 was briefly discussed by comparing the emission intensity of monometallic C1 and C2 to that of binuclear complexes C3 and C5, respectively. The results indicated that the intramolecular energy transfer of the Ru(II)/Os(II) polypyridine complexes proceeded from the Ru(II)- to the Os(II)-based units in the heterometallic C5 and C6 complexes.

New Tridentate Ligand Affords a Long-Lived 3MLCT Excited State in a Ru(II) Complex: DNA Photocleavage and 1O2 Production

Two new complexes, [Ru(tpy)(qdppz)](PF6)2 (1; qdppz = 2-(quinolin-8-yl)dipyrido[3,2-a:2',3'-c]phenazine, tpy = 2,2':6',2″-terpyridine) and [Ru(qdppz)2](PF6)2 (2), were investigated for their potential use as phototherapeutic agents through their ability to photosensitize the production of singlet oxygen, 1O2, upon irradiation with visible light. The complexes exhibit strong Ru(dπ) → qdppz(π*) metal-to-ligand charge transfer (MLCT) absorption with maxima at 485 and 495 nm for 1 and 2 in acetone, respectively, red-shifted from the Ru(dπ) → tpy(π*) absorption at 470 nm observed for [Ru(tpy)2]2+ (3) in the same solvent. Complexes 1 and 3 are not luminescent at room temperature, but 3MLCT emission is observed for 2 with maximum at 690 nm (λexc = 480 nm) in acetone. The lifetimes of the 3MLCT states of 1 and 2 were measured using transient absorption spectroscopy to be ∼9 and 310 ns in methanol, respectively, at room temperature (λexc = 490 nm). The bite angle of the qdppz ligand is closer to octahedral geometry than that of tpy, resulting in the longer lifetime of 2 as compared to those of 1 and 3. Arrhenius treatment of the temperature dependence of the luminescence results in similar activation energies, Ea, from the 3MLCT to the 3LF (ligand-field) state for the two complexes, 2520 cm-1 in 1 and 2400 cm-1 in 2. However, the pre-exponential factors differ by approximately two orders of magnitude, 2.3 × 1013 s-1 for 1 and 1.4 × 1011 s-1 for 2, which, together with differences in the Huang-Rhys factors, lead to markedly different 3MLCT lifetimes. Although both 1 and 2 intercalate between the DNA bases, only 2 is able to photocleave DNA owing to its 1O2 production upon irradiation with ΦΔ = 0.69. The present work highlights the profound effect of the ligand bite angle on the electronic structure, providing guidelines for extending the lifetime of 3MLCT Ru(II) complexes with tridentate ligands, a desired property for a number of applications.

Phototoxicity of Tridentate Ru(II) Polypyridyl Complex with Expanded Bite Angles toward Mammalian Cells and Multicellular Tumor Spheroids

Tridentate ligand-coordinated ruthenium (II) polypyridyl complexes with large N-Ru-N bite angles have been shown to promote ligand field splitting and reduce singlet-triplet state mixing leading to dramatically extended emission quantum yields and lifetimes under ambient conditions. These effects are anticipated to enhance their photoinduced singlet oxygen production, promoting prospects for such complexes as type II phototherapeutics. In this contribution, we examined this putative effect for [Ru(bqp)(bqpCOOEt)]2+, Ru-bqp-ester, a heteroleptic complex containing bqp = [2,6-bi(quinolin-8-yl)pyridine], a well-established large bite angle tridentate ligand, as well as its peptide conjugates [Ru(bqp)(bqpCONH-ahx-FrFKFrFK(Ac)-CONH2)]5+ (Ru-bqp-MPP) and [Ru(bqp) (bqp)(CONH-ahx-RRRRRRRR-CONH2)]10+ (Ru-bqp-R8) that were prepared in an effort to promote live cell/tissue permeability and targeting of the parent. Membrane permeability of both parent and peptide conjugates were compared across 2D cell monolayers; A549, Chinese hamster ovary, human pancreatic cancer (HPAC), and 3D HPAC multicellular tumor spheroids (MCTS) using confocal microscopy. Both the parent complex and peptide conjugates showed exceptional permeability with rapid uptake in both 2D and 3D cell models but with little distinction in permeability or distribution in cells between the parent or peptide conjugates. Unexpectedly, the uptake was temperature independent and so attributed to passive permeation. Both dark and photo-toxicity of the Ru(II) complexes were assessed across cell types, and the parent showed notably low dark toxicity. In contrast, the parent and conjugates were found to be highly phototoxic, with impressive phototoxic indices (PIs) toward HPAC cell monolayers in particular, with PI values ranging from ∼580 to 760. Overall, our data indicate that the Ru(II) parent complex and its peptide conjugates show promise at both cell monolayers and 3D MCTS as photosensitizers for photodynamic therapy.

Effect of electronic structure of energy transfer in bimetallic Ru(II)/Os(II) complexes

Novel monometallic (μ-LL')Ru, Ru(μ-LL'), homobimetallic Ru(μ-LL')Ru, and heterodimetallic Ru(μ-LL')Os and Os(μ-LL')Ru complexes based on two asymmetrical ligands LL' (where LL' = L¹L^1', L²L^2') have been synthesized and characterized. Spectroscopic analysis indicates that all complexes exhibit intense spin-allowed ligand-centered (LC) transitions at 288 nm and Ru-based moderate spin-allowed MLCT absorption between 440-450 nm. The Ru(μ-LL')Os and Os(μ-LL')Ru dinuclear complexes show Os-based unit absorption in the range of 565-583 nm. The Ru-based units of the complexes present different emission intensities due to differing steric hindrance at the coordination sites of the two bridging ligands. The Os(μ-LL')Ru dinuclear complexes present weaker emission intensity than their parent monometallic complexes (μ-LL')Ru. These results indicate that the emission of Os(μ-LL')Ru dinuclear complexes is quenched by the Os(II)-based units.

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.

MLCT Excited-State Behavior of Trinuclear Ruthenium(II) 2,2'-Bipyridine Complexes

Four trinuclear ruthenium(II) polypyridyl complexes were synthesized, and a detailed investigation of their excited-state properties was performed. The tritopic sexi-pyridine bridging ligands were obtained via para or meta substitution of a central 2,2'-bipyridine fragment. A para connection between the 2,2'-bipyridine chelating moieties of the bridging ligand led to a red-shifted MLCT absorption band in the visible part of the spectra, whereas the meta connection induced a broadening of the LC transitions in the UV region. A convergent energy transfer from the two peripheral metal centers to the central Ru(II) moiety was observed for all trinuclear complexes. These complexes were in thermal equilibrium with an upper-lying ³MLCT excited state over the investigated range of temperatures. For all complexes, deactivation via the ³MC excited state was absent at room temperature. Importantly, the connection in the para position for both central and peripheral 2,2'-bipyridines of the bridging ligand resulted in a trinuclear complex (T_pp) that absorbed more visible light, had a longer-lived excited state, and had a higher photoluminescence quantum yield than the parent [Ru(bpy)₃]²⁺, despite its red-shifted photoluminescence. This behavior was attributed to the presence of a highly delocalized excited state for T_pp.

Near-Infrared Electrochemiluminescence from Bistridentate Ruthenium(II) Di(quinoline-8-yl)pyridine Complexes in Aqueous Media

We report the synthesis, photophysics, electrochemistry and electrochemiluminescence (ECL) of two dqp (dqp=2,6-di(quinoline-8-yl)pyridine) based ruthenium(II) complexes, bearing either a n-butyl ester (1) or the corresponding carboxylic acid functionality (2). The complexes were prepared from [Ru(dqp)(MeCN)₃ ][PF₆ ]₂ by reaction with the dqp precursor using microwave irradiation. In both cases, photoluminescence spectra present strong ³ MLCT-based red/near-infrared (NIR) emissions centred at about 710 nm. The photoluminescence quantum yields were 6.1 % and 1.8 % for 1 and 2 respectively while the excited state lifetimes were 3.60 μs and 2.37 μs. Both complexes are ECL active, although ECL efficiency (Φ_ECL ) of 1 was substantially higher than 2, due to its more favourable electrochemical properties. Importantly, 1 also gave strong ECL in aqueous media, which is rare for near-infrared emitters. The results suggest the possibility of very interesting ECL sensing applications for this class of emitter in biological media.

Luminescent polypyridyl heteroleptic CrIII complexes with high quantum yields and long excited state lifetimes

Implementing high quantum yields and long-lived excited state lifetimes within heteroleptic luminescent CrIII complexes is a keystone for the design of supramolecular energy-converting devices exploiting this cheap metal. In this contribution, we discuss the stepwise and rational optimization of these two limiting factors within a series of heteroleptic CrIII complexes.

Tuning the excited-state deactivation pathways of dinuclear ruthenium(ii) 2,2′-bipyridine complexes through bridging ligand design

A detailed photophysical study of binuclear complexes was performed using steady-state and time-resolved photoluminescence measurements at variable temperature. The results were compared with the prototypical [Ru(bpy)₃]²⁺.

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.

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.

Superexchange in the fast lane - intramolecular electron transfer in a molecular triad occurs by conformationally gated superexchange

Photoinduced electron transfer via hopping is generally considered to have a stronger temperature dependence than electron transfer via superexchange. However, in this work, an opposite trend of the temperature dependence is observed. This unexpected result is rationalized by considering the specific geometrical and electronic structure of the Ru-bis(terpyridine) photosensitizer.

Light harvesting and energy transfer in a porphyrin-based metal organic framework

We present the synthesis and photophysical characterization of a water stable PCN-223(freebase) metal organic framework (MOF) constructed from meso-tetrakis(4-carboxyphenyl)porphyrin (TCPP). The photophysical properties of the synthesized crystalline material were studied using a wide range of steady-state and time-resolved spectroscopic techniques. Quenching experiments performed on TCPP and PCN-223 demonstrated that the extent and the rate of quenching in the MOF is significantly higher than the monomeric ligand. Based on these results, we propose that upon photo-excitation, the singlet excitation energy migrates across neutral TCPP linkers until it is quenched by a N-protonated TCPP linker. The N-protonated linkers act as trap states that deactivate the excited state to the ground state. Variable temperature measurements aided in understanding the mechanism of singlet-singlet energy transfer in the PCN-223 MOF. The rate of energy transfer and the total exciton hopping distance in PCN-223 were calculated in order to quantify the energy transfer characteristics of PCN-223. Nanosecond transient absorption spectroscopy was used to study the triplet excited state photophysics in both the free ligand and PCN-223 MOF. Furthermore, femtosecond transient absorption spectroscopy was employed to get a better understanding of the photophysical processes taking place in the ligand and MOF on ultrafast timescales. Efficient energy transfer (Förster radius = 54.5 Å) accompanied with long distance exciton hopping (173 Å) was obtained for the PCN-223 MOF.

Theoretical study on the excited state decay properties of iron(ii) polypyridine complexes substituted by bromine and chlorine

Transition metal iron(ii) polypyridyl complexes with quintet ground states were deeply investigated by density functional theory (DFT) and time-dependent density functional theory (TDDFT). Compared with the parent complex [Fe(tpy)₂]²⁺ (tpy = 2,2′:6′,2′′-terpyridine), the ground states of the complexes substituted by halogen atoms changed from singlet states to quintet states with rare high spin excited state lifetimes. The substituted complex [Fe(dbtpy)₂]²⁺ (1) results in a high spin metal–ligand charge transfer lifetime of 17.4 ps, which is 1.4 ps longer than that of [Fe(dctpy)₂]²⁺ (2) with the substitution of chlorine atoms. The reason for this is explored by a combination of electronic structures, absorption spectra, extended transition state coupled with natural orbitals for chemical valence (ETS-NOCV) studies and potential energy curves (PECs). The distortion of 1 in the angles and dihedrals of the ligands is slightly larger than that in 2, although the average metal–ligand bond lengths of the latter are larger. The twisted octahedron decreases the interactions between the d orbitals of iron(ii) and the n/π orbitals of the ligands. Compared with 2, the enlarged energy gaps among the different PECs of 1 and the increased energy crossing points caused by the larger distortion result in the increase of its excited state lifetime. The different pairwise orbital interaction contributions between the metal center and the ligands in their singlet states are qualitatively estimated by ETS-NOCV. The results show that the substitution of bromine atoms will decrease the electrostatic attraction between the metal and ligands but not significantly impact the orbital interactions.

Transition metal iron(ii) halogen substituted polypyridyl complexes with quintet ground states were deeply investigated by density functional theory (DFT) and time-dependent density functional theory (TDDFT).