Visible Light-Induced Electron Transfer from Di-mu-oxo Bridged Dinuclear Mn Complexes to Cr Centers in Silica Nanopores

The compound (bpy)2MnIII(mu-O)2MnIV(bpy)2, a structural model relevant for the photosynthetic water oxidation complex, was coupled to single CrVI charge-transfer chromophores in the channels of the nanoporous oxide AlMCM-41. Mn K-edge EXAFS spectroscopy confirmed that the di-mu-oxo dinuclear Mn core of the complex is unaffected when loaded into the nanoscale pores. Observation of the 16-line EPR signal characteristic of MnIII(mu-O)2MnIV demonstrates that the majority of the loaded complexes retained their nascent oxidation state in the presence or absence of CrVI centers. The FT-Raman spectrum upon visible light excitation of the CrVI-OII --> CrV-OI ligand-to-metal charge-transfer reveals electron transfer from MnIII(mu-O)2MnIV (Mn-O stretch at 700 cm-1) to CrVI, resulting in the formation of CrV and MnIV(mu-O)2MnIV (Mn-O stretch at 645 cm-1). All initial and final states are directly observed by FT-Raman or EPR spectroscopy, and the assignments corroborated by X-ray absorption spectroscopy measurements. The endoergic charge separation products (DELTA Eo = -0.6 V) remain after several minutes, which points to spatial separation of CrV and MnIV(mu-O)2MnIV as a consequence of hole (OI) hopping as a major contributing mechanism. This is the first observation of visible light-induced oxidation of a potential water oxidation complex by a metal charge-transfer pump in a nanoporous environment. These findings will allow for the assembly and photochemical characterization of well defined transition metal molecular units, with the ultimate goal of performing endothermic, multi-electron transformations that are coupled to visible light electron pumps in nanostructured scaffolds

Synthesis of Unsupported d¹–d^<i>x</i> Oxido-Bridged Heterobimetallic Complexes Containing V^IV: A New Direction for Metal-to-Metal Charge Transfer

Heterobimetallic complexes composed only of first-row transition metals [(TMTAA)­V^IVO→M^IIPy₅Me₂]­(OTf)₂ (TMTAA = 7,16-dihydro-6,8,15,17-tetramethyldibenzo­[<i>b</i>,<i>i</i>]­[1,4,8,11]­tetraaza­cyclotetradecine; Py₅Me₂ = 2,6-bis­(1,1-bis­(2-pyridyl)­ethyl)­pyridine; M = Mn^II, Fe^II, Co^II, Ni^II, Cu^II; OTf = trifluoromethanesulfonate) have been synthesized through a dative interaction between a terminal oxido and M^II metal centers. This is the first series of V^IVO→M^II heterobimetallic complexes containing an unsupported oxido bridge. Among these five complexes, only V^IVO→Fe^II (<b>3b</b>) has a clear new absorption band upon formation of the dinuclear species (502 nm, ε = 1700 M^–1 cm^–1). This feature is assigned to a metal-to-metal charge transfer (MMCT) transition from V^IV to Fe^II, which forms a V^VOFe^I excited state. This assignment is supported by electrochemical data, electronic absorption profiles, and resonance Raman spectroscopy and represents the first report of visible-light induced MMCT in a heterobimetallic oxido-bridged molecule where the electron originates on a d¹ metal center

Isolation of a (Dinitrogen)Tricopper(I) Complex

Reaction of a tris­(β-diketimine) cyclophane, H₃<b>L</b>, with benzyl potassium followed by [Cu­(OTf)]₂(C₆H₆) affords a tricopper­(I) complex containing a bridging dinitrogen ligand. rRaman (ν_N–N = 1952 cm^–1) and ¹⁵N NMR (δ = 303.8 ppm) spectroscopy confirm the presence of the dinitrogen ligand. DFT calculations and QTAIM analysis indicate minimal metal-dinitrogen back-bonding with only one molecular orbital of significant N2­(2pπ*) and Cu­(3dπ)/Cu­(3dσ) character (13.6% N, 70.9% Cu). ∇²ρ values for the Cu–N₂ bond critical points are analogous to those for polar closed-shell/closed-shell interactions

Isolation of a (Dinitrogen)Tricopper(I) Complex

Reaction of a tris­(β-diketimine) cyclophane, H₃<b>L</b>, with benzyl potassium followed by [Cu­(OTf)]₂(C₆H₆) affords a tricopper­(I) complex containing a bridging dinitrogen ligand. rRaman (ν_N–N = 1952 cm^–1) and ¹⁵N NMR (δ = 303.8 ppm) spectroscopy confirm the presence of the dinitrogen ligand. DFT calculations and QTAIM analysis indicate minimal metal-dinitrogen back-bonding with only one molecular orbital of significant N2­(2pπ*) and Cu­(3dπ)/Cu­(3dσ) character (13.6% N, 70.9% Cu). ∇²ρ values for the Cu–N₂ bond critical points are analogous to those for polar closed-shell/closed-shell interactions

Light-Driven Hydrogen Evolution by BODIPY-Sensitized Cobaloxime Catalysts

We report four photocatalytically active cobaloxime complexes for light-driven hydrogen evolution. The cobaloxime catalysts are sensitized by different <i>meso</i>-pyridyl boron dipyrromethene (BODIPY) chromophores, bearing either two bromo- or iodo-substituents on the BODIPY core. The pyridine linker between the BODIPY and cobaloxime is further modified by a methyl substituent on the pyridine, influencing the stability and electronic properties of the cobaloxime catalyst and thus the photocatalytic efficiency of each system. Four cobaloxime catalyst complexes and three novel BODIPY chromophores are synthesized and characterized by absorption, fluorescence, infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry, and electrochemistry. Crystal structures for the BODIPY–cobaloxime complexes <b>2</b> and <b>3</b> are presented. In contrast to the photocatalytically inactive, nonhalogenated reference complex <b>1</b>, the four newly reported molecules are active for photocatalytic hydrogen evolution, with a maximum turnover number (TON) of 30.9 mol equiv of H₂ per catalyst for the <i>meso</i>-methylpyridyl 2,6-diiodo BODIPY-sensitized cobaloxime complex <b>5</b>. We conclude that accessing the photoexcited triplet state of the BODIPY chromophore by introducing heavy atoms (i.e., bromine or iodine) is necessary for efficient electron transfer in this system, enabling catalytic hydrogen generation. In addition, relatively electron-donating pyridyl linkers improve the stability of the complex, increasing the overall TON for hydrogen production

Long-Lived LMCT in a d⁰ Vanadium(V) Complex by Internal Conversion to a State of 3d_<i>xy</i> Character

The excited state dynamics of a d⁰ vanadium­(V) oxido ligand-to-metal charge transfer (LMCT) complex, VOL^F, were investigated via a combination of static optical and X-ray absorption (XAS) spectroscopy, transient optical absorption spectroscopy, and time-dependent density functional theory (TD-DFT). Upon excitation of the LMCT in the visible region, transient absorption data reveal that internal conversion traps the excited carrier population into a long-lived charge transfer state of 3d_<i>xy</i> electron character, S₁(d_<i>xy</i>). The internal conversion is substantiated by an isosbestic point in the transient absorption data, two nearby charge transfer states that couple well by TD-DFT, multiple rates in the ground state recovery, and the decay kinetics of an excited state absorption with the energy of a d-d transition in O K-edge XAS spectra. The long lifetime (∼420 ps) of S₁(d_<i>xy</i>) can be ascribed to its poor optical and vibrational coupling to a distorted ground state (S₀^*) via a negligible electronic dipole transition in TD-DFT. The lack of luminescence or an identifiable triplet state also suggests attributing the lifetime to electronic contributions. In conjunction with its strong visible absorption and reduction potential, the long-lived LMCT suggests that molecules such as VOL^F could have potential utility for energy conversion applications. Moreover, the results show that internal conversion between two nearby charge transfer states, differentiated by their 3d character, can form a long-lived charge transfer excitation, broadly informing the discovery of 3d metal-centered optical absorbers with long-lived charge transfer lifetimes

Synthesis, Characterization, and Antimicrobial Efficacy of Photomicrobicidal Cellulose Paper

Toward our goal of scalable, antimicrobial materials based on photodynamic inactivation, paper sheets comprised of photosensitizer-conjugated cellulose fibers were prepared using porphyrin and BODIPY photosensitizers, and characterized by spectroscopic (infrared, UV–vis diffuse reflectance, inductively coupled plasma optical emission) and physical (gel permeation chromatography, elemental, and thermal gravimetric analyses) methods. Antibacterial efficacy was evaluated against Staphylococcus aureus (ATCC-2913), vancomycin-resistant Enterococcus faecium (ATCC-2320), Acinetobacter baumannii (ATCC-19606), Pseudomonas aeruginosa (ATCC-9027), and Klebsiella pneumoniae (ATCC-2146). Our best results were achieved with a cationic porphyrin–paper conjugate, <b>Por</b>^<b>(+)</b>-paper, with inactivation upon illumination (30 min, 65 ± 5 mW/cm², 400–700 nm) of all bacterial strains studied by 99.99+% (4 log units), regardless of taxonomic classification. <b>Por</b>^<b>(+)</b>-paper also inactivated dengue-1 virus (>99.995%), influenza A (∼99.5%), and human adenovirus-5 (∼99%). These results demonstrate the potential of cellulose materials to serve as scalable scaffolds for anti-infective or self-sterilizing materials against both bacteria and viruses when employing a photodynamic inactivation mode of action