Photoinduced CO Research Articles

Photochemical Formation and Electronic Structure of an Alkane σ-Complex from Time-Resolved Optical and X-ray Absorption Spectroscopy.

C-H bond activation reactions with transition metals typically proceed via the formation of alkane σ-complexes, where an alkane C-H σ-bond binds to the metal. Due to the weak nature of metal-alkane bonds, σ-complexes are challenging to characterize experimentally. Here, we establish the complete pathways of photochemical formation of the model σ-complex Cr(CO)5-alkane from Cr(CO)6 in octane solution and characterize the nature of its metal-ligand bonding interactions. Using femtosecond optical absorption spectroscopy, we find photoinduced CO dissociation from Cr(CO)6 to occur within the 100 fs time resolution of the experiment. Rapid geminate recombination by a fraction of molecules is found to occur with a time constant of 150 fs. The formation of bare Cr(CO)5 in its singlet ground state is followed by complexation of an octane molecule from solution with a time constant of 8.2 ps. Picosecond X-ray absorption spectroscopy at the Cr L-edge and O K-edge provides unique information on the electronic structure of the Cr(CO)5-alkane σ-complex from both the metal and ligand perspectives. Based on clear experimental observables, we find substantial destabilization of the lowest unoccupied molecular orbital upon coordination of the C-H bond to the undercoordinated Cr center in the Cr(CO)5-alkane σ-complex, and we define this as a general, orbital-based descriptor of the metal-alkane bond. Our study demonstrates the value of combining optical and X-ray spectroscopic methods as complementary tools to study the stability and reactivity of alkane σ-complexes in their role as the decisive intermediates in C-H bond activation reactions.

Insights into the Interfacial Charge Transfer Dynamics in Semiconductor–Molecular Catalyst Assemblies for Photo–Induced CO2 Reduction

AbstractThis mini–review summarises examples of photo and photoelectrochemical CO2 reduction reactions (CO2RR) using semiconductor–molecular catalyst–based hybrid assemblies in which studies of charge transfer dynamics were also performed. In these catalysts, the grand challenges are controlling ultra–fast one–electron charge separation and simultaneously governing its impact on the efficiency and product selectivity in the multielectron and multi–proton–assisted CO2RR. In practice, several unwanted electron recombination processes occur in the femtosecond to millisecond timescale range impacting the overall catalysis efficiency. Titanium di‐oxide (TiO2) nanoparticle, mesoporous TiO2 film, copper indium sulphide (CuInS2) quantum dots were used as semiconductor materials, in turn rhenium (I) bipyridine, cobalt (II) terpyridine, cobaloxamine complex, and iron (III) porphyrins were investigated as molecular catalysts. The results of the charge transfer dynamic studies operating in such hybrid systems shed light on their charge accumulation processes, their rate constants for the intramolecular charge transfer processes and the decay lifetimes of the different excited state species that are produced. Moreover, a comprehensive understanding of the catalysts’ charge transfer dynamics is crucial to identifying detrimental charge recombination pathways. These undesired pathways can be used as rational basis for improving the design of future multi–redox–driven hybrid catalysts.

Neutral and Cationic Re(I) Complexes with Pnictogen-Based Coligands and Tunable Functionality: From Phosphorescence to Photoinduced CO Release.

In this work, we have explored Re(I) complexes featuring triphenylpnictogen (PnPh3, Pn = P, As, or Sb)-based coligands and bidentate (neutral or monoanionic) luminophores derived from 1,10-phenantroline (phen), as well as from 2-(3-(tert-butyl)-1H-1,2,4-triazol-5-yl)pyridine (H(N-tBu)). The effect of the increasingly heavy elements on the structural parameters, photoexcited-state properties, and electrochemical behavior as well as the hybridization defects and polarization of the Pn atoms was related to the charges of the main luminophores (i.e., phen vs N-tBu) and explored in terms of photoluminescence spectroscopy, X-ray diffractometry, and quantum-chemical methods. Therefore, an in-depth analysis of the bonding, crystal packing, excited-state energies, and lifetimes was assessed in liquid solutions, frozen glassy matrices, and crystalline phases along with a semiquantitative photoactivation study. Notably, by changing the main ligand from phen to N-tBu, an increase in radiative and radiationless deactivation rates (kr and knr, respectively) at 77 K together with a faster photoinduced CO release and fragmentation at room temperature was detected. In addition, a progressively red-shifted phosphorescence was observed with the growing atomic number of the pnictogen atom, along with a boost in kr and knr at 77 K. Down the Vth main group and upon coordination of the Pn atom to the Re(I) center, an increasingly prominent jump of s-orbital participation on the binding sxp3.00-orbitals of the Pn atoms is evidenced. Based on these findings, the ability of these complexes to act as tunable photoluminescent labels able to perform as light-driven CO-releasing molecules is envisioned.

The slow photo-induced CO2 release of N-phthaloylglycine

As time-resolved infrared spectroscopy shows, the title compound undergoes photo-induced CO2 release within microseconds – orders of magnitudes longer than anticipated.

Primary and Secondary Processes in the Ultraviolet Photodissociation of CpCo(CO)2 (Cyclopentadienylcobalt Dicarbonyl).

We investigated the photodissociation dynamics of CpCo(CO)2 (cyclopentadienylcobalt dicarbonyl) in metal-to-ligand charge transfer (MLCT) bands. By employing DFT calculations, the absorption band (210-240 nm) was characterized as a charge transfer from the Co center to the Cp (cyclopentadienyl, C5H5) ligand. Ion imaging was utilized to analyze the CO fragments and coordinatively unsaturated complexes (CpCoCO, CpCo, and CoC3H3) across the entire MLCT band. Measuring the production yields of individual unsaturated complexes as a function of photolysis wavelength by considering wavelength dependence indicated the involvement of several photochemical pathways: the first photodissociation and sequential dissociation of CpCo(CO)2, and the second photodissociation of unsaturated intermediates within the pulse duration of the photolysis laser. The recoil velocity shifts of CpCo and CoC3H3 were attributed to the onset of the sequential dissociation of CpCoCO. Evidence for the second photodissociation of CpCoCO was obtained through the matching of linear momenta between the CO(v = 0, 1) and CpCo fragments. The DFT calculations performed to determine the electronic structures and potential energy curves for photoinduced CO loss in CpCo(CO)2 and CpCoCO supported our interpretation of the experimental results. This study presents a practical approach to selectively detecting specific processes among the mixture of products and intermediates when photolyzing transition-metal carbonyls, as their concurrent generation is unavoidable in laser-based experiments.

Disentangling the role of electrons and phonons in the photoinduced CO desorption and CO oxidation on (O,CO)-Ru(0001).

The role played by electronic and phononic excitations in the femtosecond laser induced desorption and oxidation of CO coadsorbed with O on Ru(0001) is investigated using ab initio molecular dynamics with electronic friction. To this aim, simulations that account for both kind of excitations and that only consider electronic excitations are performed. Results for three different surface coverages are obtained. We unequivocally demonstrate that CO desorption is governed by phononic excitations. In the case of oxidation the low statistics does not allow to give a categorical answer. However, the analysis of the adsorbates kinetic energy gain and displacements strongly suggest that phononic excitations and surface distortion also play an important role in the oxidation process.

Self-Promoted H2 Formation: The Feasibility of Photoinduced CO Removal for Lossless Hydrogen Purification

A photoinduced radical reaction operated at low temperature can be used to remove trace CO from a H2 stream by minimizing the reverse water-gas shift. However, H2 consumption resulting from nonselective oxidation by hydroxyl radicals becomes an obstacle to practical hydrogen purification. Inspired by hydrogen exchange transfer, we demonstrate here that molecular hydrogen can promote H2 formation from hydrogen radicals, which are generated from the reaction of CO and H2 with hydroxyl radicals. The slight increment in H2 along with the radical reaction encouraged us to configure a photocatalytic hydrogen purification fixed-bed reactor, which can reduce CO to ≤1 ppm in the H2 stream.

The Influence of a Changing Local Environment during Photoinduced CO2 Dissociation.

Though largely influencing the efficiency of a reaction, the molecular‐scale details of the local environment of the reactants are experimentally inaccessible hindering an in‐depth understanding of a catalyst's reactivity, a prerequisite to maximizing its efficiency. We introduce a method to follow individual molecules and their largely changing environment during a photochemical reaction. The method is illustrated for a rate‐limiting step in a photolytic reaction, the dissociation of CO2 on two catalytically relevant surfaces, Ag(100) and Cu(111). We reveal with a single‐molecule resolution how the reactant's surroundings evolve with progressing laser illumination and with it their propensity for dissociation. Counteracting processes lead to a volcano‐like reactivity. Our unprecedented local view during a photoinduced reaction opens the avenue for understanding the influence of the products on reaction yields on the nanoscale.

The Influence of a Changing Local Environment during Photoinduced CO2 Dissociation

AbstractThough largely influencing the efficiency of a reaction, the molecular‐scale details of the local environment of the reactants are experimentally inaccessible hindering an in‐depth understanding of a catalyst's reactivity, a prerequisite to maximizing its efficiency. We introduce a method to follow individual molecules and their largely changing environment during a photochemical reaction. The method is illustrated for a rate‐limiting step in a photolytic reaction, the dissociation of CO2 on two catalytically relevant surfaces, Ag(100) and Cu(111). We reveal with a single‐molecule resolution how the reactant's surroundings evolve with progressing laser illumination and with it their propensity for dissociation. Counteracting processes lead to a volcano‐like reactivity. Our unprecedented local view during a photoinduced reaction opens the avenue for understanding the influence of the products on reaction yields on the nanoscale.

Carbon Monoxide Controllable Targeted Gas Therapy for Synergistic Anti-inflammation.

SummaryCarbon monoxide (CO) plays an important role in the regulation of a variety of physiological processes and thus is regarded as a promising pharmaceutical agent. Nevertheless, therapeutic applications of CO are severely hampered by the difficulty of the delivery of controlled amounts of CO to biological targets. To address this deficiency, we present a spatiotemporally controllable CO-releasing platform (designated as Neu-MnO2/Fla) for synergistic anti-inflammation. With the assistance of neutrophil membrane coating, Neu-MnO2/Fla can target to inflammatory sites. Subsequently, excess H2O2 at the inflamed tissues can be decomposed into oxygen because of MnO2 as nanozymes possessing catalase (CAT) activity, which not only relieves oxidative stress but also achieves in situ rapid photo-induced CO release. The in vitro and in vivo results indicate our CO-releasing platform exhibits a strong synergistic anti-inflammatory effect. Our work will shed light on targeted CO release to avoid side effects of therapeutic applications of CO.

Spectroscopic and antimicrobial activity of photoactivatable tricarbonyl Mn(I) terpyridine compounds

The photoinduced fac-[MnBr(CO)3(L–k2N1,N2)] (L = 4′-(2-pyridyl)-2,2′:6′,2′'-terpyridine (LPy) (1) and 4′-(4-phenyl morpholine)-2,2′:6′,2′'-terpyridine (Lmorph) (2)) and fac-[Mn2Br2(CO)6(LDTP–k2N1,N2)] (LDTP = 1,4-bis(2,2′:6′,2′'-terpyridin-4′yl)benzene) (3) complexes, capable of release CO at 525 nm, were synthesized and fully characterized using different analytical and spectral tools. Compounds 1–3 had poor solubility or decomposed in most of the organic solvents, and thus they are unsuitable for the phototherapeutic CO applications. Reaction between azide complexes (4–6), prepared by exchange of the axial Br− ligand of 1–3, with ethyl 4,4,4-trifluoro-2-butynoate afforded triazolate complexes (7–9), which had good solubility in the organic solvents. The potential of 7–9 to act as photoinduced CO releasing molecules was studied at two excitation wavelengths, 468 and 525 nm. The ligand Lmorph and its complex (8) were strongly potent to Escherichia coli, Candida Albicans and Cryptococcus neoformans. All the ligands and complexes (except 8) exhibited cytotoxicity to the normal human cell line; human embryonic kidney cells (HEK293), but they had good compatibility with the human red blood cells (RBCs) at 32 μg/mL.

An enzyme-responsive and photoactivatable carbon-monoxide releasing molecule for bacterial infection theranostics.

Infections caused by pathogenic bacteria, especially the drug-resistant bacteria, are posing a devastating threat to public health, which underscores the urgent needs for advanced strategies to effectively prevent and treat these intractable issues. Here we report a feasible and effective theranostic platform based on an enzyme-sensitive and photoactivatable carbon monoxide releasing molecule (CORM-Ac) for the successive detection and elimination of bacterial infection. The extracellular bacterial lipase can trigger the excited state intramolecular proton transfer (ESIPT) via elimination of the ester group in CORM-Ac, thus providing a fluorescence switch for an early warning of infection. Subsequently, the potent bactericidal therapy against the model bacterial strains, Staphylococcus aureus (S. aureus) and notorious methicillin-resistant Staphylococcus aureus (MRSA), was readily realized via photoinduced release of CO. In addition, the CORM-Ac and CORM showed good biocompatibility within a wide range of concentrations. The results of an infected animal wound test also demonstrated that the CORM-Ac-loaded gauze was effective in indicating the wound infection and accelerating the wound healing via the photoinduced CO release. The simplicity, functional integration, good biocompatibility and broad adaptability make CORM-Ac very attractive for bacterial theranostic applications.

Synthesis of Tofisopam by Way of Photoinduced CO2 Fixation.

Herein reported is a unique synthetic route of Tofisopam, an anxiolytic drug containing a 2,3-benzodiazepine core structure. 3,4-Dimethoxypropylbenzene and 3,4-dimethoxybenzoic acid, which are both of plant origin, and CO2 constitute its carbon skeleton. These three renewable substances are united by two C-C bond forming reactions, i.e., a Friedel-Crafts acylation reaction and a photoinduced carboxylation reaction to construct the major carbon framework. Finally, a methyl group is introduced by a Kumada-type cross-coupling reaction to furnish Tofisopam. Various analogs of Tofisopam are readily synthesized by introducing other substituents than a methyl group at the last C-C bond forming step.

Plasma-Assisted Photocatalysis of CH4 and CO2 into Ethylene

Oxidative coupling of methane (OCM) using CO2 as the oxidant is a potential advancement for methane conversion and greenhouse gases reduction. However, the reaction is usually limited by both high energy consumption and strict reaction condition. Here, we report, for the first time, photoinduced efficient ethylene (C2H4) and CO production by OCM over TiO2-supported Ag nanoparticles at mild conditions. The success relies on a synergy coupling visible-light-induced strong surface plasma resonance (SPR) effect localized on Ag(0), ultraviolet-light-induced photoelectric effect on TiO2, and the separated adsorption of CO2 and CH4 on TiO2 and Ag. The yields of CO and C2H4 reach 1149 and 686 μmol·g–1·h–1, respectively, under simulated solar irradiation. The origination of carbon-based gas products from CO2 and CH4 is also certified by a 13C isotope-labeled experiment. This work presents a new and ideal route of photocatalytic OCM reaction for C2H4 and CO production by coupling both SPR and photoelectric effects.

Ultrafast Intersystem Crossing vs Internal Conversion in α-Diimine Transition Metal Complexes: Quantum Evidence.

Whereas third row transition metal carbonyl α-diimine complexes display luminescent properties and possess low-lying triplet metal-to-ligand charge transfer (MLCT) states efficiently accessible by a spin-vibronic mechanism, first row analogues hold low-lying metal-centered (MC) excited states that could quench these properties. Upon visible irradiation, different functions are potentially stimulated, namely, luminescence, electron transfer, or photoinduced CO release, the branching ratio of which is governed by the energetics, the character, and the early time dynamics of the photoactive excited states. Simulations of ultrafast nonadiabatic quantum dynamics, including spin-vibronic effects, of [M(imidazole)(CO)3(phenanthroline)]+ (M = Mn, Re) highlight the role of the metal atom. An ultrafast intersystem crossing process, driven by spin-orbit coupling, populates the low-lying triplet states of [Re(imidazole)(CO)3(phen)]+ within the first tens of fs. In contrast, efficient internal conversion between the two lowest 1MLCT states of [Mn(imidazole)(CO)3(phen)]+ is mediated within 50 fs by vibronic coupling with upper MC and MLCT states.

Carbon Dioxide Dimer Radical Anion as Surface Intermediate of Photoinduced CO2 Reduction at Aqueous Cu and CdSe Nanoparticle Catalysts by Rapid-Scan FT-IR Spectroscopy.

Monitoring of visible light sensitized reduction of CO2 at Cu nanoparticles in aqueous solution by rapid-scan ATR FT-IR spectroscopy on the time scale of seconds allowed structural identification of a one-electron intermediate and demonstrated its kinetic relevancy for the first time. Isotopic labeling (12C: 1632, 1358, 1346 cm-1; 13C: 1588, 1326, 1316 cm-1) revealed a species of carbon dioxide dimer radical anion structure, most likely bound to the catalyst surface through carbon. Intermediacy of Cu-C(═O)OCO2- surface species is in agreement with a recently proposed mechanism for electrocatalytic CO2 reduction at Cu metal nanoparticles based on Tafel slope analysis. Spontaneous decrease of the intermediate after termination of the photosensitization pulse (Sn porphyrin excited at 405 nm) was accompanied by the growth of HCO3-. CO was produced as well, but sensitive detection required photolysis for tens of minutes. A direct kinetic link between a C2O4- surface intermediate and the CO product was also demonstrated for photocatalyzed CO2 reduction at aqueous CdSe nanoparticles, where first order growth of a Cd-C(═O)OCO2- species was accompanied by rise of CO (monitored by a fast Ni complex trap) and HCO3- showing a distinct induction period. The detection of the one-electron surface intermediate and confirmation of its catalytic relevancy was enabled by the delivery of electrons one-by-one by the photosensitization method. The observation of carbon dioxide dimer radical anion points to approaches for rate enhancements of heterogeneous CO2 reduction by creating catalytic environments that favor formation of this intermediate.

Photo-induced CO2 reduction by CH4/H2O to fuels over Cu-modified g-C3N4 nanorods under simulated solar energy

Copper modified polymeric graphitic carbon nitride (Cu/g-C3N4) nanorods for photo-induced CO2 conversion with methane (CH4) and water (H2O) as reducing system under simulated solar energy has been investigated. The nanocatalysts, synthesized by pyrolysis and sonication, were characterized by XRD, FTIR, Raman analysis, XPS, SEM, N2 adsorption-desorption and PL spectroscopy. The presence of Cu2+ ions over the g-C3N4 structure inhibited charge carriers recombination process. The results indicated that photo-activity and selectivity of Cu/g-C3N4 photo-catalyst for CO2 reduction greatly dependent on the type of CO2-reduction system. CO2 was efficiently converted to CH4 and CH3OH with traces of C2H4 and C2H6 hydrocarbons in the CO2-water system. The yield of the main product, CH4 over 3wt.% Cu/g-C3N4 was 109μmoleg-cata.−1h−1 under visible light irradiation, significantly higher than the pure g-C3N4 catalyst (60μmole/g.cat). In photo-induced CO2-CH4 reaction, CO and H2 were detected as the main products with smaller amount of hydrocarbons. The highest efficiency was detected over 3wt.%Cu-loading of g-C3N4 and at optimal CH4/CO2 feed ratio of 1.0. The maximum yield of CO and H2 detected were 142 and 76μmoleg-catal.−1h−1, respectively at selectivity 66.6% and 32.5%, respectively. Significantly enhanced CO2/CH4 reduction over Cu/g-C3N4 was attributed to its polymeric structure with efficient charge transfer property and inhibited charges recombination rate. A proposed photo-induced reaction mechanism, corroborated with the experimental data, was also deliberated.

Luminescent Re(I) Carbonyl Complexes as Trackable PhotoCORMs for CO delivery to Cellular Targets.

A family of Re(I) carbonyl complexes of general formula [ReX(CO)3(phen)]0/1+ (where X = Cl-, CF3SO3-, MeCN, PPh3, and methylimidazole) derived from 1,10-phenanthroline (phen) exhibits variable emission characteristics depending on the presence of the sixth ancillary ligand/group (X). All complexes but with X = MeCN exhibit moderate CO release upon irradiation with low-power UV light and are indefinitely stable in anaerobic/aerobic environment in solution as well as in solid state when kept under dark condition. These CO donors liberate three, one, or no CO depending on the nature of sixth ligand upon illumination as studied with the aid of time-dependent IR spectroscopy. Results of excited-state density functional theory (DFT) and time-dependent DFT calculations provided insight into the origin of the emission characteristics of these complexes. The luminescent rheinum(I) photoCORMs uniformly displayed efficient cellular internalization by the human breast adenocarcinoma cells, MDA-MB-231, while the complex with PPh3 as ancillary ligand showed moderate nuclear localization in addition to the cytosolic distribution. These species hold significant promise as theranostic photoCORMs (photoinduced CO releasing molecules), where the entry of the pro-drug can be tracked within the cellular matrices.

Visible light photoactivatable CO releasing manganese (I) tricarbonyl complexes: Experimental and DFT studies

The reaction between [Mn(CO)5Br] and antibacterial drugs (Nifuroxazide (NIF) and Nitazoxanide (NTZ)) afforded fac-[MnBr(CO)3(NIF)] (1) and fac-[MnBr(CO)3(NTZ)2] (2) complexes, which were characterized by different analytical and spectral tools. The photo-induced CO releasing properties and the ability of the metal carbonyls to drive CO to the myoglobin solution were investigated. The green-light (525 nm) illumination of the DMSO solution of 1 for 20 min gave rise to the maximum amount of CO. Compared with the DMSO solution of 1, the rate of CO release from the 80% aqueous-DMSO mixed media is slower, and the plateau is reached upon the illumination for 140 min. On the contrast, a clear isosbestic point is developed during the storing of the DMSO solution of 2 for 6 h, which reflects its instability. To drive CO from 2 to myoglobin solution, the energy of the green light was not sufficient. However, the grown of the bands corresponding to the formation of carboxy-myoglobin species is stopped after 50 min of the irradiation at 468 nm and 36 min at 410 nm. The electronic structures and the related electronic transitions have been calculated by TD-DFT/B3LYP (or CAM-B3LYP)/LANL2DZ methods. The spectra calculated by the hybrid exchange-correlation functional CAM-B3LYP with a long range correction term are more closed to the experimental findings. The remarkable toxicity of 2 against the tested microbes, with respect to the free drug, may be attributed to the change of the lipophilicity of free NTZ upon the complex formation as well as the releasing of CO upon the incubation in dark.

Photoinduced Carbon Monoxide Release from Half-Sandwich Iron(II) Carbonyl Complexes by Visible Irradiation: Kinetic Analysis and Mechanistic Investigation.

Three half-sandwich iron(II) complexes, [Fe(η(5) -Cp)(cis-CO)2 X] (X(-) =Cl(-) , Br(-) , I(-) ), were synthesized and characterized. The kinetics of the CO-releasing behaviour of these complexes upon illumination by visible irradiation in various media was investigated. Our results indicated that the CO release was significantly affected by the auxiliary ligands. Of the three light sources used (blue, green, and red), blue light exhibited the highest efficiency. In the photoinduced CO release, the solvents and exogenous nucleophiles in the media were involved, which allowed their CO-releasing reaction to comply with pseudo first-order model rather than the characteristic zero-order model for a photochemical reaction. In aqueous media (D2 O), an intermediate bearing the core of {Fe(II) (cis-CO)2 } involving cleavage of cyclopentadiene was detected. Despite the non-absorption of the red light, its illumination combined with nucleophilic substitution did cause considerable CO release. Assessment of the cytotoxicity of the three complexes indicated that they showed good biocompatibility.