Low-Energy and Long-Lived Emission from Polypyridyl Ruthenium(II) Complexes Having A Stable-Radical Substituent

Novel polypyridyl ruthenium­(II) complexes having a 2,2′-bipyridine (bpy) derivative which possesses a 1,5-dimethyl-6-oxoverdazyl radical (OV) group as a stable-radical substituent were designed and synthesized. The radical–ruthenium­(II) complexes showed low-energy/intense MLCT absorption and low-energy/long-lived MLCT emission, and these characteristics of the complexes were explained by the electron-withdrawing nature of the OV group. Furthermore, the radical-substituent effects were enhanced by the presence of the electron-donating methyl groups at the 4- and 4′-positions of bpy in the ancillary ligands. The detailed electrochemical, spectroscopic, and photophysical properties of the complexes were discussed in terms of the systematic modification of the second coordination sphere in the main and ancillary ligands

Zero-Magnetic-Field Splitting in the Excited Triplet States of Octahedral Hexanuclear Molybdenum(II) Clusters: [{Mo₆X₈}(<i>n</i>‑C₃F₇COO)₆]^2– (X = Cl, Br, or I)

Temperature (<i>T</i>)-dependent emission from [{Mo₆X₈}­(<i>n</i>-C₃F₇COO)₆]^2– (X = Cl (<b>1</b>), Br (<b>2</b>), and I (<b>3</b>)) in optically transparent polyethylene glycol dimethacrylate matrices were studied in 3 K < <i>T</i> < 300 K to elucidate the spectroscopic and photophysical properties of the clusters, in special reference to zero-magnetic-field splitting (zfs) in the lowest-energy excited triplet states (T₁) of the clusters. The cluster complexes <b>1</b> and <b>2</b> showed the <i>T</i>-dependent emission characteristics similar to those of [{Mo₆Cl₈}­Cl₆]^2–, while <b>3</b> exhibited emission properties different completely from those of <b>1</b> and <b>2</b>. Such <i>T</i>-dependent emission characteristics of <b>1</b>, <b>2</b>, and <b>3</b> were explained successfully by the excited triplet state spin-sublevel (Φ_<i>n</i>, <i>n</i> = 1–4) model. The zfs energies between the lowest-energy (Φ₁) and highest-energy (Φ₄) spin sublevels, Δ<i>E</i>₁₄, resulted by the first-order spin–orbit coupling, were evaluated to be 650, 720, and 1000 cm^–1 for <b>1</b>, <b>2</b>, and <b>3</b>, respectively. The emission spectra of <b>1</b>, <b>2</b>, and <b>3</b> in CH₃CN at 298 K were reproduced very well by the Δ<i>E</i>₁₄ values and the population percentages of Φ_<i>n</i> at 300 K. We also report that the Δ<i>E</i>₁₄ values of the clusters correlate linearly with the fourth power of the atomic number (<i>Z</i>) of X: Δ<i>E</i>₁₄ ∝ {<i>Z</i>(X)}⁴

Emission Tuning of Heteroleptic Arylborane–Ruthenium(II) Complexes by Ancillary Ligands: Observation of Strickler–Berg-Type Relation

Novel heteroleptic arylborane–ruthenium­(II) complexes having a series of ancillary ligands L′ ([Ru­(B₂bpy)­L′₂]²⁺) in CH₃CN showed low-energy/intense metal-to-ligand charge transfer (MLCT)-type absorption and intense/long-lived emission compared to the reference complexes. The spectroscopic and photophysical properties of [Ru­(B₂bpy)­L′₂]²⁺ were shown to be manipulated synthetically by the electron-donating ability of the ancillary ligand(s). The intense and long-lived emission observed for [Ru­(B₂bpy)­L′₂]²⁺ in CH₃CN at 298 K is responsible for the accelerated radiative and decelerated nonradiative decay processes, which are controllable through the electronic structures of the ancillary ligand(s). On the basis of the present systematic study, furthermore, we succeeded in demonstrating the Strickler–Berg-type relation between the molar absorption coefficients of the MLCT bands and the radiative rate constants of the complexes

Photophysical and Photoredox Characteristics of a Novel Tricarbonyl Rhenium(I) Complex Having an Arylborane-Appended Aromatic Diimine Ligand

We report the synthesis and photophysical/photoredox characteristics of a novel tricarbonyl rhenium­(I) complex having a (dimesityl)­boryldurylethynyl (DBDE) group at the 4-position of a 1,10-phenanthroline (phen) ligand, [Re­(CO)₃(4-DBDE-phen)­Br] (<b>ReB</b>). <b>ReB</b> in tetrahydrofuran at 298 K showed the metal-to-ligand charge transfer (MLCT) emission at around 681 nm with the lifetime (τ^em) of 900 ns. The relatively long emission lifetime of <b>ReB</b> compared with that of [Re­(CO)₃(phen)­Br] (<b>RePhen</b>, τ^em = 390 ns) was discussed on the basis of the temperature dependent τ^em and Franck–Condon analysis of the emission spectra of the two complexes. Emission quenching studies of both <b>ReB</b> and <b>RePhen</b> by a series of electron donors revealed that the photoinduced electron transfer (PET) quenching rate constant of <b>ReB</b> was faster than that of <b>RePhen</b> at a given Gibbs free energy change of the PET reaction (Δ<i>G</i>_ET⁰ > −0.5 eV). All of the results on <b>ReB</b> were discussed in terms of the contribution of the CT interaction between the π-orbital(s) of the aryl group(s) and the vacant p-orbital on the boron atom in DBDE to the MLCT state of the complex

Rigid Medium Effects on Photophysical Properties of MLCT Excited States of Polypyridyl Os(II) Complexes in Polymerized Poly(ethylene glycol)dimethacrylate Monoliths

Higher-energy emissions from the metal-to-ligand charge-transfer (MLCT) excited states of a series of polypyridyl Os­(II) complexes were observed at the fluid-to-film transition in PEG-DMA550. The higher-energy excited states, caused by a “rigid medium effect” in the film, led to enhanced emission quantum yields and longer excited-state lifetimes. Detailed analyses of spectra and excited-state dynamics by Franck–Condon emission spectral analysis and application of the energy gap law for nonradiative excited-state decay reveal that the rigid medium effect arises from the inability of part of the local medium dielectric environment to respond to the change in charge distribution in the excited state during its lifetime. Enhanced excited-state lifetimes are consistent with qualitative and quantitative predictions of the energy gap law

Excited-State Characteristics of Tetracyanidonitridorhenium(V) and -technetium(V) Complexes with N‑Heteroaromatic Ligands

Six-coordinate tetracyanidonitridorhenium­(V) and -technetium­(V) with axial N-heteroaromatic ligands, (PPh₄)₂[MN­(CN)₄L] [M = Re, L = 4-(dimethylamino)­pyridine (dmap), 3,5-lutidine (lut), 4-picoline (pic), 4-phenylpyridine (ppy), pyridine (py), 3-benzoylpyridine (3bzpy), 4,4′-bipyridine (bpy), pyrazine (pz), 4-cyanopyridine (cpy), or 4-benzoylpyridine (4bzpy); M = Tc, L = dmap, lut, pic, py, pz, or cpy] were synthesized and characterized. The crystal structures of 11 complexes were determined by single-crystal X-ray analysis. All of the complexes showed photoluminescence in the crystalline phase at room temperature. The emission maximum wavelengths (λ_em) of the rhenium complexes with dmap, lut, pic, ppy, or py were similar to one another with a quite high emission quantum yield (Φ_em): λ_em = 539–545 nm, Φ_em = 0.39–0.93, and emission lifetime (τ_em) = 10–45 μs at 296 K. The emission spectra at 77 K exhibited vibronic progressions, and the emissive excited state is characterized as ³[(d_<i>xy</i>)¹(d_π*)¹] (d_π* = d_<i>xz</i>, d_<i>yz</i>). On the other hand, the emission maximum wavelength of the rhenium complex with 3bzpy, bpy, pz, cpy, or 4bzpy was significantly dependent on the nature of the axial ligand in the crystalline phase: λ_em = 564–669 nm, Φ_em ≤ 0.01–0.36, and τ_em = 0.03–13.3 μs at 296 K. The emission spectra at 77 K in the crystalline phase did not show vibronic progressions. The emissive excited state of the rhenium complex with bpy, pz, cpy, or 4bzpy is assignable to originate from the metal-to-N-heteroaromatic ligand charge-transfer (MLCT)-type emission with a spin-triplet type. The change in the excited-state characteristics of rhenium complexes by the N-heteroaromatic ligand is a result of stabilization of the π* orbital of the N- heteroaromatic ligand to a lower energy level than the d_π* orbitals. The emission spectral shapes of technetium complexes were almost independent of the nature of the N-heteroaromatic ligand with λ_em = 574–581 nm at room temperature. The different emission characteristics between the pz and cpy coordinate rhenium complexes and the technetium analogues would be due to stabilization of technetium-centered orbitals compared with the rhenium ones in energy

Photoluminescence Switching with Changes in the Coordination Number and Coordinating Volatile Organic Compounds in Tetracyanidonitridorhenium(V) and -technetium(V) Complexes

Six-coordinate distorted octahedral tetracyanidonitridorhenium­(V) and -technetium­(V) complexes with a volatile organic compound (VOC) coordinating at the trans position of a nitrido ligand, (PPh₄)₂[MN­(CN)₄L] (M = Re, L = MeOH, EtOH, acetone, or MeCN; M = Tc, L = MeOH), and five-coordinate square-pyramidal tetracyanidonitrido complexes without an axial ligand, (PPh₄)₂[MN­(CN)₄] (M = Re or Tc), were synthesized and characterized. Single-crystal X-ray structural analysis was carried out for (PPh₄)₂[MN­(CN)₄L] (M = Re, L = MeOH, EtOH, or acetone; M = Tc, L = MeOH) and (PPh₄)₂[ReN­(CN)₄]. All complexes studied showed photoluminescence in the solid state at room temperature. Reversible luminescence switching between six- and five-coordinate rhenium­(V) complexes and between the relevant six-coordinate rhenium­(V) complexes except that between the MeCN and acetone complexes was achieved by exposing them to VOC vapor in the solid state at room temperature. Luminescence changes were observed from the five-coordinate technetium­(V) complexes in a MeOH vapor atmosphere in the solid state. In contrast, no vapochromic luminescence was observed from the five- and six-coordinate complexes in an acetone vapor atmosphere

Excited-State Characteristics of Tetracyanidonitridorhenium(V) and -technetium(V) Complexes with N‑Heteroaromatic Ligands

Six-coordinate tetracyanidonitridorhenium­(V) and -technetium­(V) with axial N-heteroaromatic ligands, (PPh₄)₂[MN­(CN)₄L] [M = Re, L = 4-(dimethylamino)­pyridine (dmap), 3,5-lutidine (lut), 4-picoline (pic), 4-phenylpyridine (ppy), pyridine (py), 3-benzoylpyridine (3bzpy), 4,4′-bipyridine (bpy), pyrazine (pz), 4-cyanopyridine (cpy), or 4-benzoylpyridine (4bzpy); M = Tc, L = dmap, lut, pic, py, pz, or cpy] were synthesized and characterized. The crystal structures of 11 complexes were determined by single-crystal X-ray analysis. All of the complexes showed photoluminescence in the crystalline phase at room temperature. The emission maximum wavelengths (λ_em) of the rhenium complexes with dmap, lut, pic, ppy, or py were similar to one another with a quite high emission quantum yield (Φ_em): λ_em = 539–545 nm, Φ_em = 0.39–0.93, and emission lifetime (τ_em) = 10–45 μs at 296 K. The emission spectra at 77 K exhibited vibronic progressions, and the emissive excited state is characterized as ³[(d_<i>xy</i>)¹(d_π*)¹] (d_π* = d_<i>xz</i>, d_<i>yz</i>). On the other hand, the emission maximum wavelength of the rhenium complex with 3bzpy, bpy, pz, cpy, or 4bzpy was significantly dependent on the nature of the axial ligand in the crystalline phase: λ_em = 564–669 nm, Φ_em ≤ 0.01–0.36, and τ_em = 0.03–13.3 μs at 296 K. The emission spectra at 77 K in the crystalline phase did not show vibronic progressions. The emissive excited state of the rhenium complex with bpy, pz, cpy, or 4bzpy is assignable to originate from the metal-to-N-heteroaromatic ligand charge-transfer (MLCT)-type emission with a spin-triplet type. The change in the excited-state characteristics of rhenium complexes by the N-heteroaromatic ligand is a result of stabilization of the π* orbital of the N- heteroaromatic ligand to a lower energy level than the d_π* orbitals. The emission spectral shapes of technetium complexes were almost independent of the nature of the N-heteroaromatic ligand with λ_em = 574–581 nm at room temperature. The different emission characteristics between the pz and cpy coordinate rhenium complexes and the technetium analogues would be due to stabilization of technetium-centered orbitals compared with the rhenium ones in energy

Photoluminescence Switching with Changes in the Coordination Number and Coordinating Volatile Organic Compounds in Tetracyanidonitridorhenium(V) and -technetium(V) Complexes

Six-coordinate distorted octahedral tetracyanidonitridorhenium­(V) and -technetium­(V) complexes with a volatile organic compound (VOC) coordinating at the trans position of a nitrido ligand, (PPh₄)₂[MN­(CN)₄L] (M = Re, L = MeOH, EtOH, acetone, or MeCN; M = Tc, L = MeOH), and five-coordinate square-pyramidal tetracyanidonitrido complexes without an axial ligand, (PPh₄)₂[MN­(CN)₄] (M = Re or Tc), were synthesized and characterized. Single-crystal X-ray structural analysis was carried out for (PPh₄)₂[MN­(CN)₄L] (M = Re, L = MeOH, EtOH, or acetone; M = Tc, L = MeOH) and (PPh₄)₂[ReN­(CN)₄]. All complexes studied showed photoluminescence in the solid state at room temperature. Reversible luminescence switching between six- and five-coordinate rhenium­(V) complexes and between the relevant six-coordinate rhenium­(V) complexes except that between the MeCN and acetone complexes was achieved by exposing them to VOC vapor in the solid state at room temperature. Luminescence changes were observed from the five-coordinate technetium­(V) complexes in a MeOH vapor atmosphere in the solid state. In contrast, no vapochromic luminescence was observed from the five- and six-coordinate complexes in an acetone vapor atmosphere

General Synthesis of MOF Nanotubes via Hydrogen-Bonded Organic Frameworks toward Efficient Hydrogen Evolution Electrocatalysts

The application scope of metal–organic frameworks (MOFs) can be extended by rationally designing the architecture and components of MOFs, which can be achieved via a metal-containing solid templated strategy. However, this strategy suffers from low efficiency and provides only one specific MOF from one template. Herein, we present a versatile templated strategy in which organic ligands are weaved into hydrogen-bonded organic frameworks (HOFs) for the controllable and scalable synthesis of MOF nanotubes. HOF nanowires assembled from benzene-1,3,5-tricarboxylic acid and melamine via a simple sonochemical approach serve as both the template and precursor to produce MOF nanotubes with varied metal compositions. Hybrid nanotubes containing nanometal crystals and N-doped graphene prepared through a carbonization process show that the optimized NiRuIr alloy@NG nanotube exhibits excellent electrocatalytic HER activity and durability in alkaline media, outperforming most reported catalysts. The strategy proposed here demonstrates a pioneering study of combination of HOF and MOF, which shows great potential in the design of other nanosized MOFs with various architectures and compositions for potential applications