Two Emissive Long-Lived Excited States of an Imidazole-Functionalized Ruthenium Dipyridophenazine Complex

100 μs Luminescence Lifetime Boosts the Excited State Reactivity of a Ruthenium(II)-Anthracene Complex in Photon Upconversion and Photocatalytic Polymerizations with Red Light

The triplet excited state lifetime of a photosensitizer is an essential parameter for diffusion-controlled energy- and electron-transfer, which occurs usually in a competitive manner to the intrinsic decay of a triplet excited state. Here we show the decisive role of luminescence lifetime in the triplet excited state reactivity toward energy- and electron transfer. Anchoring two phenyl anthracene chromophores to a ruthenium(II) polypyridyl complex (RuII ref) leads to a RuII triad with a luminescence lifetime above 100 μs, which is more than 40 times longer than that of the prototypical complex. The obtained RuII triad sensitizes energy transfer to anthracene-based annihilators more efficiently than RuII ref and enables red-to-blue photon upconversion with a pseudo anti-Stokes shift of 0.94 eV and a moderate upconversion efficiency near 1 % in aerated solution. Particularly, RuII triad allows rapid photoredox catalytic polymerizations of acrylate and acrylamide monomers under aerobic condition with red light, which are kinetically hindered for RuII ref. Our work shows that excited state lifetime of a photosensitizer governs the dynamics of the excited state reactions, which seems an overlooked but important aspect for photochemistry.

New Twist on the Light-Switch Effect: Controlling the Fate of Excited States with pH in a 4-Hydroxy-thiazol-extended Ruthenium(II) Dppz Complex

We present a pH-dependent study of the excited state dynamics of a novel Ru complex bearing a 4-hydroxy thiazol-substituted dppz (dipyridophenazine) ligand (RuTzOH) and its deprotonated form (RuTzO-). We combine steady-state and time-resolved absorption and emission spectroscopy with electrochemical investigations to characterize the excited state relaxation, which upon photoexcitation at 400 nm is determined by a multitude of initially populated MLCT states for both complexes. Subsequently, for RuTzOH, two long-lived excited states are populated, leading to dual emission from the complexes, a feature that vanishes upon deprotonation. Upon deprotonation, the electron density on the dppz moiety increases significantly, leading to rapid energy populating ligand-centered states and thus deactivating the initially excited MLCT states.

Full Solution Process of a Near-Infrared Light-Emitting Electrochemical Cell Based on Novel Emissive Ruthenium Complexes of 1,10-Phenanthroline-Derived Ligands and a Eutectic Alloy as the Top Electrode

Near-infrared luminescent materials have recently received considerable attention for a large number of applications, including in solid-state lighting, as bioimaging agents, as photovoltaic cells, and in the telecommunication industry. By adding diverse electron-donating or withdrawing groups on ancillary ligands based on benzenethiol-phenanthroline, we synthesized and optoelectronically characterized a series of novel ionic ruthenium complexes, namely RuS, RuSCl, RuSMe, and RuSNH₂, for using in a light-emitting electrochemical cell. The synthesized complexes are intense red emitters in the range of 584-605 nm in solution, which depends on the substitutions of electron donor/acceptor moieties on the ancillary ligands. To find a suitable quantum mechanical approach, benchmark calculations based on time-dependent density functional theory were carried out on these complexes. Our benchmark revealed that the M06-L method has results close to those of the experiment. Furthermore, to gain a deeper insight into electronic transitions, several excitation processes were investigated at the TD-DFT-SMD-MN12-L/gen level. The results showed that in the designed complexes, the dominant transition is between the 4d_Z² electron of Ru (particle) and the π* orbitals of the ancillary ligand (hole). The single-layer devices, including these complexes along with a Ga/In cathode by a facile deposition method without the addition of any electron or hole transport layers, were fabricated and displayed red (678 nm) to near-infrared (701 nm) emission as well as a decrease of turn-on voltage from 3.85 to 3.10 V. In particular, adding a methyl group to the ancillary ligand in the complex RuSNH₂ increases the external quantum efficiency to 0.55%, one of the highest observed values in the ruthenium phenanthroline family. This simple structure of the device lets us develop the practical applications of light-emitting electrochemical cells based on injection and screen-printing methods, which are very promising for the vacuum-free deposition of top electrodes.

A Combined Spectroscopic and Theoretical Study on a Ruthenium Complex Featuring a π-Extended dppz Ligand for Light-Driven Accumulation of Multiple Reducing Equivalents

The design of photoactive systems capable of storing and relaying multiple electrons is highly demanded in the field of artificial photosynthesis, where transformations of interest rely on multielectronic redox processes. The photophysical properties of the ruthenium photosensitizer [(bpy)2 Ru(oxim-dppqp)]2+ (Ru), storing two electrons coupled to two protons on the π-extended oxim-dppqp ligand under light-driven conditions, are investigated by means of excitation wavelength-dependent resonance Raman and transient absorption spectroscopies, in combination with time-dependent density functional theory; the results are discussed in comparison to the parent [(bpy)2 Ru(dppz)]2+ and [(bpy)2 Ru(oxo-dppqp)]2+ complexes. In addition, this study provides in-depth insights on the impact of protonation or of accumulation of multiple reducing equivalents on the reactive excited states.

Ruthenium Assemblies for CO2 Reduction and H2 Generation: Time Resolved Infrared Spectroscopy, Spectroelectrochemistry and a Photocatalysis Study in Solution and on NiO

Two novel supramolecular complexes RuRe ([Ru(dceb)₂(bpt)Re(CO)₃Cl](PF₆)) and RuPt ([Ru(dceb)₂(bpt)PtI(H₂O)](PF₆)₂) [dceb = diethyl(2,2′-bipyridine)-4,4′-dicarboxylate, bpt = 3,5-di(pyridine-2-yl)-1,2,4-triazolate] were synthesized as new catalysts for photocatalytic CO₂ reduction and H₂ evolution, respectively. The influence of the catalytic metal for successful catalysis in solution and on a NiO semiconductor was examined. IR-active handles in the form of carbonyl groups on the peripheral ligand on the photosensitiser were used to study the excited states populated, as well as the one-electron reduced intermediate species using infrared and UV-Vis spectroelectrochemistry, and time resolved infrared spectroscopy. Inclusion of ethyl-ester moieties led to a reduction in the LUMO energies on the peripheral bipyridine ligand, resulting in localization of the ³MLCT excited state on these peripheral ligands following excitation. RuPt generated hydrogen in solution and when immobilized on NiO in a photoelectrochemical (PEC) cell. RuRe was inactive as a CO₂ reduction catalyst in solution, and produced only trace amounts of CO when the photocatalyst was immobilized on NiO in a PEC cell saturated with CO₂.

Designing luminescent diimine-Cu (I)-phosphine complexes by tuning N-ligand and counteranions: correlation of weak interactions, luminescence and THz absorption spectra

Herein, six new [Cu(N^N)(P^P)]+/0 complexes with different N-ligand and counteranions [Cu2(dmp)2(bdppmapy)I2] (1), [Cu2(dmp)2(bdppmapy)(CN)2]·3CH3OH (2), [Cu(dmp)(bdppmapy)](BF4) (3), [Cu(dmp)(bdppmapy)](ClO4) (4), [Cu(phen)(bdppmapy)](BF4) (5), [Cu(phen)(bdppmapy)](ClO4) (6) have been synthesized and characterized (bdppmapy = N,N-bis[(diphenylphosphino)methyl]-2-pyridinamine,...

New Molecularly Engineered Binuclear Ruthenium (II) Complexes for Highly Efficient Near-Infrared Light-Emitting Electrochemical Cell (NIR-LEC)

Abstract: From practical point of view, the stability, response time and efficiency of near-infrared light-emitting electrochemical cell (NIR-LEC) are key factors. By using the high potential of chemical modification potential...

Prolonged Luminescence Lifetime of a Dual Emissive Ruthenium Dipyridophenazine-Type Complex in Aprotic and Protic Solvents

A recently reported ruthenium(II) complex bearing an extended dipyridophenazine ligand exhibits unusual long-lived dual emission at room temperature. In this study, the effect of the introduction of a methyl protecting group to the imidazole moiety of this ligand (L1, 11-methyl-11H-imidazo[4,5-i]dipyrido[3,2-a:2',3'-c]phenazine) on the photophysics of the respective ruthenium(II) complex [(tbbpy)₂Ru(L1)]²⁺ (C1) is demonstrated by means of electrochemistry, UV/vis absorption and emission spectroscopy, as well as emission lifetime measurements, and transient absorption spectroscopy on the nanosecond time scale. At room temperature, C1 shows dual emission both in aprotic and in protic solvents with time constants of 1.1/34.2 and 1.2/8.4 μs, respectively. These lifetimes are assigned to the emission from ³MLCT and ³LC states. The introduction of the methyl group increases the lifetime of the ³LC state in C1 almost by a factor of 2 in acetonitrile solution compared to the previously reported compound. Accordingly, the newly introduced methyl group is described as a protecting group for the imidazole moiety of the heterocyclic ligand, which enables prolonged lifetimes of the dual emissive complex in protic solvents. The stabilization of the electronic structure is further underlined by the enhanced stability toward electrochemical reduction as evidenced by cyclic voltammetry.

Influence of the Protonation State on the Excited-State Dynamics of Ruthenium(II) Complexes with Imidazole π-Extended Dipyridophenazine Ligands

Ruthenium(II) complexes, like [(tbbpy)₂Ru(dppz)]²⁺ (Ru-dppz; tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine, dppz = dipyrido-[3,2-a:2',3'-c]phenazine), have emerged as suitable photosensitizers in photoredox catalysis. Since then, there has been ongoing interest in the design of π-extended Ru-dppz systems with red-shifted visible absorption maxima and sufficiently long-lived excited states independent of the solvent or pH value. Herein, we explore the photophysical properties of protonation isomers of the linearly π-extended [(tbbpy)₂Ru(L)]²⁺-type complexes bearing a dppz ligand with directly fused imidazole (im) and methyl-imidazole units (mim) as L. Steady-state UV-vis absorption, resonance Raman, as well as time-resolved emission and transient absorption spectroscopy reveal that Ru-im and Ru-mim show desirable properties for the application in photocatalytic processes, i.e., strong visible absorbance and two long-lived excited states in the ³ILCT and ³MLCT manifold, at pH values between 3 and 12. However, protonation of the (methyl-)imidazole unit at pH ≤ 2 unit causes decreased excited-state lifetimes and an emission switch-off.

Photostable Ruthenium(II) Isocyanoborato Luminophores and Their Use in Energy Transfer and Photoredox Catalysis

Ruthenium(II) polypyridine complexes are among the most popular sensitizers in photocatalysis, but they face some severe limitations concerning accessible excited-state energies and photostability that could hamper future applications. In this study, the borylation of heteroleptic ruthenium(II) cyanide complexes with α-diimine ancillary ligands is identified as a useful concept to elevate the energies of photoactive metal-to-ligand charge-transfer (MLCT) states and to obtain unusually photorobust compounds suitable for thermodynamically challenging energy transfer catalysis as well as oxidative and reductive photoredox catalysis. B(C₆F₅)₃ groups attached to the CN ^- ligands stabilize the metal-based t_2g-like orbitals by ∼0.8 eV, leading to high ³MLCT energies (up to 2.50 eV) that are more typical for cyclometalated iridium(III) complexes. Through variation of their α-diimine ligands, nonradiative excited-state relaxation pathways involving higher-lying metal-centered states can be controlled, and their luminescence quantum yields and MLCT lifetimes can be optimized. These combined properties make the respective isocyanoborato complexes amenable to photochemical reactions for which common ruthenium(II)-based sensitizers are unsuited, due to a lack of sufficient triplet energy or excited-state redox power. Specifically, this includes photoisomerization reactions, sensitization of nickel-catalyzed cross-couplings, pinacol couplings, and oxidative decarboxylative C-C couplings. Our work is relevant in the greater context of tailoring photoactive coordination compounds to current challenges in synthetic photochemistry and solar energy conversion.

Photophysics of Ruthenium(II) Complexes with Thiazole π-Extended Dipyridophenazine Ligands

Transition-metal-based donor-acceptor systems can produce long-lived excited charge-transfer states by visible-light irradiation. The novel ruthenium(II) polypyridyl type complexes Ru1 and Ru2 based on the dipyridophenazine ligand (L0) directly linked to 4-hydroxythiazoles of different donor strengths were synthesized and photophysically characterized. The excited-state dynamics were investigated by femtosecond-to-nanosecond transient absorption and nanosecond emission spectroscopy complemented by time-dependent density functional theory calculations. These results indicate that photoexcitation in the visible region leads to the population of both metal-to-ligand charge-transfer (¹MLCT) and thiazole (tz)-induced intraligand charge-transfer (¹ILCT) states. Thus, the excited-state dynamics is described by two excited-state branches, namely, the population of (i) a comparably short-lived phenazine-centered ³MLCT state (τ ≈ 150-400 ps) and (ii) a long-lived ³ILCT state (τ ≈ 40-300 ns) with excess charge density localized on the phenazine and tz moieties. Notably, the ruthenium(II) complexes feature long-lived dual emission with lifetimes in the ranges τ_Em,1 ≈ 40-300 ns and τ_Em,2 ≈ 100-200 ns, which are attributed to emission from the ³ILCT and ³MLCT manifolds, respectively.

Electron Storage Capability and Singlet Oxygen Productivity of a RuII Photosensitizer Containing a Fused Naphthaloylenebenzene Moiety at the 1,10-Phenanthroline Ligand

As a novel rylene type dye a diimine ligand with a fully rigid and extended π-system in its backbone was prepared by directly fusing a 1,10-phenanthroline building block with 1,8-naphthalimide. The corresponding heteroleptic ruthenium photosensitizer bearing one biipo and two tbbpy ligands was synthesized and extensively analyzed by a combination of NMR, single crystal X-ray diffraction, steady-state absorption and emission, time-resolved spectroscopy and different electrochemical measurements supported by time-dependent density functional theory calculations. The cyclic and differential pulse voltammograms revealed, that the naphthaloylenebenzene moiety enables an additional second reduction of the ligand. Moreover, this ligand possesses a very broad absorption in the visible region. In the RuII complex this causes an overlap of ligand-centered and metal-to-ligand charge transfer transitions. The emission of the complex is clearly redshifted compared to the ligand emission with very long-lived excited states lifetimes of 1.7 and 24.7 μs in oxygen-free acetonitrile solution. This behavior is accompanied by a surprisingly high oxygen sensitivity. Finally, this photosensitizer was successfully applied for the effective evolution of singlet oxygen challenging some of the common RuII prototype complexes.