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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 d<sup>1</sup>âd<sup><i>x</i></sup> Oxido-Bridged Heterobimetallic Complexes Containing V<sup>IV</sup>: A New Direction for Metal-to-Metal Charge Transfer
Heterobimetallic
complexes composed only of first-row transition metals [(TMTAA)ÂV<sup>IV</sup>îťOâM<sup>II</sup>Py<sub>5</sub>Me<sub>2</sub>]Â(OTf)<sub>2</sub> (TMTAA = 7,16-dihydro-6,8,15,17-tetramethyldibenzoÂ[<i>b</i>,<i>i</i>]Â[1,4,8,11]ÂtetraazaÂcyclotetradecine;
Py<sub>5</sub>Me<sub>2</sub> = 2,6-bisÂ(1,1-bisÂ(2-pyridyl)Âethyl)Âpyridine;
M = Mn<sup>II</sup>, Fe<sup>II</sup>, Co<sup>II</sup>, Ni<sup>II</sup>, Cu<sup>II</sup>; OTf = trifluoromethanesulfonate) have been synthesized
through a dative interaction between a terminal oxido and M<sup>II</sup> metal centers. This is the first series of V<sup>IV</sup>îťOâM<sup>II</sup> heterobimetallic complexes containing an unsupported oxido
bridge. Among these five complexes, only V<sup>IV</sup>îťOâFe<sup>II</sup> (<b>3b</b>) has a clear new absorption band upon formation
of the dinuclear species (502 nm, Îľ = 1700 M<sup>â1</sup> cm<sup>â1</sup>). This feature is assigned to a metal-to-metal
charge transfer (MMCT) transition from V<sup>IV</sup> to Fe<sup>II</sup>, which forms a V<sup>V</sup>î¸Oî¸Fe<sup>I</sup> 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 d<sup>1</sup> metal center
Isolation of a (Dinitrogen)Tricopper(I) Complex
Reaction of a trisÂ(β-diketimine)
cyclophane, H<sub>3</sub><b>L</b>, with benzyl potassium followed
by [CuÂ(OTf)]<sub>2</sub>(C<sub>6</sub>H<sub>6</sub>) affords a tricopperÂ(I)
complex containing
a bridging dinitrogen ligand. rRaman (ν<sub>NâN</sub> = 1952 cm<sup>â1</sup>) and <sup>15</sup>N 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 N2Â(2pĎ*)
and CuÂ(3dĎ)/CuÂ(3dĎ) character (13.6% N, 70.9% Cu). â<sup>2</sup>Ď values for the CuâN<sub>2</sub> bond critical
points are analogous to those for polar closed-shell/closed-shell
interactions
Isolation of a (Dinitrogen)Tricopper(I) Complex
Reaction of a trisÂ(β-diketimine)
cyclophane, H<sub>3</sub><b>L</b>, with benzyl potassium followed
by [CuÂ(OTf)]<sub>2</sub>(C<sub>6</sub>H<sub>6</sub>) affords a tricopperÂ(I)
complex containing
a bridging dinitrogen ligand. rRaman (ν<sub>NâN</sub> = 1952 cm<sup>â1</sup>) and <sup>15</sup>N 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 N2Â(2pĎ*)
and CuÂ(3dĎ)/CuÂ(3dĎ) character (13.6% N, 70.9% Cu). â<sup>2</sup>Ď values for the CuâN<sub>2</sub> 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 BODIPYâcobaloxime 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 H<sub>2</sub> 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 d<sup>0</sup> Vanadium(V) Complex by Internal Conversion to a State of 3d<sub><i>xy</i></sub> Character
The excited state dynamics of a d<sup>0</sup> vanadiumÂ(V) oxido
ligand-to-metal charge transfer (LMCT) complex, VOL<sup>F</sup>, 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<sub><i>xy</i></sub> electron character, S<sub>1</sub>(d<sub><i>xy</i></sub>). 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 (âź420
ps) of S<sub>1</sub>(d<sub><i>xy</i></sub>) can be ascribed
to its poor optical and vibrational coupling to a distorted ground
state (S<sub>0</sub><sup>*</sup>)
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<sup>F</sup> 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, UVâvis 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 porphyrinâpaper conjugate, <b>Por</b><sup><b>(+)</b></sup>-paper, with inactivation upon
illumination (30 min, 65 Âą 5 mW/cm<sup>2</sup>, 400â700
nm) of all bacterial strains studied by 99.99+% (4 log units), regardless
of taxonomic classification. <b>Por</b><sup><b>(+)</b></sup>-paper also inactivated dengue-1 virus (>99.995%), influenza
A (âź99.5%), and human adenovirus-5 (âź99%). 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