Low-Volume Reaction Monitoring of Carbon Dot Light Absorbers in Optofluidic Microreactors

Optical monitoring and screening of photocatalytic batch reactions using cuvettes ex situ is time-consuming, requires substantial amounts of samples, and does not allow the analysis of species with low extinction coefficients. Hollow-core photonic crystal fibers (HC-PCFs) provide an innovative approach for in situ reaction detection using ultraviolet-visible absorption spectroscopy, with the potential for high-throughput automation using extremely low sample volumes with high sensitivity for monitoring of the analyte. HC-PCFs use interference effects to guide light at the center of a microfluidic channel and use this to enhance detection sensitivity. They open the possibility of comprehensively studying photocatalysts to extract structure-activity relationships, which is unfeasible with similar reaction volume, time, and sensitivity in cuvettes. Here, we demonstrate the use of HC-PCF microreactors for the screening of the electron transfer properties of carbon dots (CDs), a nanometer-sized material that is emerging as a homogeneous light absorber in photocatalysis. The CD-driven photoreduction reaction of viologens (XV²⁺) to the corresponding radical monocation XV^•+ is monitored in situ as a model reaction, using a sample volume of 1 μL per measurement and with a detectability of <1 μM. A range of different reaction conditions have been systematically studied, including different types of CDs (i.e., amorphous, graphitic, and graphitic nitrogen-doped CDs), surface chemistry, viologens, and electron donors. Furthermore, the excitation irradiance was varied to study its effect on the photoreduction rate. The findings are correlated with the electron transfer properties of CDs based on their electronic structure characterized by soft X-ray absorption spectroscopy. Optofluidic microreactors with real-time optical detection provide unique insight into the reaction dynamics of photocatalytic systems and could form the basis of future automated catalyst screening platforms, where samples are only available on small scales or at a high cost.

In situ Detection of Cobaloxime Intermediates During Photocatalysis Using Hollow-Core Photonic Crystal Fiber Microreactors

Hollow-core photonic crystal fibers (HC-PCFs) provide a novel approach for in situ UV/Vis spectroscopy with enhanced detection sensitivity. Here, we demonstrate that longer optical path lengths than afforded by conventional cuvette-based UV/Vis spectroscopy can be used to detect and identify the CoI and CoII states in hydrogen-evolving cobaloxime catalysts, with spectral identification aided by comparison with DFT-simulated spectra. Our findings show that there are two types of signals observed for these molecular catalysts; a transient signal and a steady-state signal, with the former being assigned to the CoI state and the latter being assigned to the CoII state. These observations lend support to a unimolecular pathway, rather than a bimolecular pathway, for hydrogen evolution. This study highlights the utility of fiber-based microreactors for understanding these and a much wider range of homogeneous photocatalytic systems in the future.

An infrared study of CO2 activation by holmium ions, Ho+ and HoO. Phys Chem Chem Phys 2022;24:22716-22723. [PMID: 36106954 DOI: 10.1039/d2cp02862j]

We report a combined experimental and computational study of carbon dioxide activation at gas-phase Ho⁺ and HoO⁺ centres. Infrared action spectra of Ho(CO₂)_n⁺ and [HoO(CO₂)_n]⁺ ion-molecule complexes have been recorded in the spectral region 1700-2400 cm^-1 and assigned by comparison with simulated spectra of energetically low-lying structures determined by density functional theory. Little by way of activation is observed in Ho(CO₂)_n⁺ complexes with CO₂ binding end-on to the Ho⁺ ion. By contrast, all [HoO(CO₂)_n]⁺ complexes n ≥ 3 show unambiguous evidence for formation of a carbonate radical anion moiety, . The signature of this structure, a new vibrational band observed around 1840 cm^-1 for n = 3, continues to red-shift monotonically with each successive CO₂ ligand binding with net charge transfer from the ligand rather than the metal centre.

Bridging Plastic Recycling and Organic Catalysis: Photocatalytic Deconstruction of Polystyrene via a C–H Oxidation Pathway

Chemical recycling of synthetic polymers represents a promising strategy to deconstruct plastic waste and make valuable products. Inspired by small-molecule C–H bond activation, a visible-light-driven reaction is developed to deconstruct polystyrene (PS) into ∼40% benzoic acid as well as ∼20% other monomeric aromatic products at 50 °C and ambient pressure. The practicality of this strategy is demonstrated by deconstruction of real-world PS foam on a gram scale. The reaction is proposed to proceed via a C–H bond oxidation pathway, which is supported by theoretical calculations and experimental results. Fluorescence quenching experiments also support efficient electron transfer between the photocatalyst and the polymer substrate, providing further evidence for the proposed mechanism. This study introduces concepts from small-molecule catalysis to polymer deconstruction and provides a promising method to tackle the global crisis of plastic pollution.

Stern–Volmer analysis of photocatalyst fluorescence quenching within hollow-core photonic crystal fibre microreactors

Optofluidic microreactors enable Stern–Volmer analysis on nanolitre-scale photocatalyst–quencher mixtures. The method is used to measure bimolecular quenching coefficients for a photoredox-catalysed α-C–H alkylation reaction of primary alkylamines.

Atomic Cluster Au_{10}^{+} Is a Strong Broadband Midinfrared Chromophore

We report an intense broadband midinfrared absorption band in the Au_{10}^{+} cluster in a region in which only molecular vibrations would normally be expected. Observed in the infrared multiple photon dissociation spectra of Au_{10}Ar^{+}, Au_{10}(N_{2}O)^{+}, and Au_{10}(OCS)^{+}, the smooth feature stretches 700-3400  cm^{-1} (λ=14-2.9  μm). Calculations confirm unusually low-energy allowed electronic excitations consistent with the observed spectra. In Au_{10}(OCS)^{+}, IR absorption throughout the band drives OCS decomposition resulting in CO loss, providing an alternative method of bond activation or breaking.

Optofluidic Photonic Crystal Fiber Microreactors for In Situ Studies of Carbon Nanodot-Driven Photoreduction

Performing quantitative in situ spectroscopic analysis on minuscule sample volumes is a common difficulty in photochemistry. To address this challenge, we use a hollow-core photonic crystal fiber (HC-PCF) that guides light at the center of a microscale liquid channel and acts as an optofluidic microreactor with a reaction volume of less than 35 nL. The system was used to demonstrate in situ optical detection of photoreduction processes that are key components of many photocatalytic reaction schemes. The photoreduction of viologens (XV²⁺) to the radical XV^•+ in a homogeneous mixture with carbon nanodot (CND) light absorbers is studied for a range of different carbon dots and viologens. Time-resolved absorption spectra, measured over several UV irradiation cycles, are interpreted with a quantitative kinetic model to determine photoreduction and photobleaching rate constants. The powerful combination of time-resolved, low-volume absorption spectroscopy and kinetic modeling highlights the potential of optofluidic microreactors as a highly sensitive, quantitative, and rapid screening platform for novel photocatalysts and flow chemistry in general.

Infrared action spectroscopy of nitrous oxide on cationic gold and cobalt clusters

Understanding the catalytic decomposition of nitrous oxide on finely divided transition metals is an important environmental issue. In this study, we present the results of a combined infrared action spectroscopy and quantum chemical investigation of molecular N2O binding to isolated Aun+ (n ≤ 7) and Con+ (n ≤ 5) clusters. Infrared multiple-photon dissociation spectra have been recorded in the regions of both the N[double bond, length as m-dash]O (1000-1400 cm-1) and N[double bond, length as m-dash]N (2100-2450 cm-1) stretching modes of nitrous oxide. In the case of Aun+ clusters only the ground electronic state plays a role, while the involvement of energetically low-lying excited states in binding to the Con+ clusters cannot be ruled out. There is a clear preference for N-binding to clusters of both metals but some O-bound isomers are observed in the case of smaller Con(N2O)+ clusters.

Free electron laser infrared action spectroscopy of nitrous oxide binding to platinum clusters, Ptn(N2O)+

Infrared multiple-photon dissociation spectroscopy has been applied to study Pt_n(N₂O)⁺ (n = 1–8) clusters which represent entrance-channel complexes on the reactive potential energy surface for nitrous oxide decomposition on platinum.

Photodissociation dynamics and the dissociation energy of vanadium monoxide, VO, investigated using velocity map imaging

Velocity map imaging has been employed to study multi-photon fragmentation of vanadium monoxide (VO) via the C 4Σ- state. The fragmentation dynamics are interpreted in terms of dissociation at the three-photon level, with the first photon weakly resonant with transitions to vibrational energy levels of the C 4Σ- state. The dissociation channels accessed are shown to depend strongly on the vibrational level via which excitation takes place. Analysis of the evolution of the kinetic energy release spectrum with photon energy leads to a refined value for the dissociation energy of ground state VO of D0(VO) = 53 126 ± 263 cm-1.

Structural isomers and low-lying electronic states of gas-phase M+(N2O)n (M = Co, Rh, Ir) ion–molecule complexes

The structures of gas-phase group nine cation–nitrous oxide metal–ligand complexes, M⁺(N₂O)_n (M = Co, Rh, Ir; n = 2–7) have been determined by a combination of infrared photodissociation spectroscopy and density functional theory.

IR Signature of Size-Selective CO2 Activation on Small Platinum Cluster Anions, Ptn - (n=4-7)

Infrared multiple photon dissociation spectroscopy (IR-MPD) has been employed to determine the nature of CO₂ binding to size-selected platinum cluster anions, Pt_n ^- (n=4-7). Interpreted in conjunction with density functional theory simulations, the results illustrate that the degree of CO₂ activation can be controlled by the size of the metal cluster, with dissociative activation observed on all clusters n≥5. Of potential practical significance, in terms of the use of CO₂ as a useful C1 feedstock, CO₂ is observed molecularly-bound, but highly activated, on the Pt₄ ^- cluster. It is trapped behind a barrier on the reactive potential energy surface which prevents dissociation.

Infrared Spectroscopy of Au+(CH4) n Complexes and Vibrationally-Enhanced C-H Activation Reactions

A combined spectroscopic and computational study of gas-phase Au⁺(CH₄)_n (n = 3–8) complexes reveals a strongly-bound linear Au⁺(CH₄)₂ core structure to which up to four additional ligands bind in a secondary coordination shell. Infrared resonance-enhanced photodissociation spectroscopy in the region of the CH₄a₁ and t₂ fundamental transitions reveals essentially free internal rotation of the core ligands about the H₄C–Au⁺–CH₄ axis, with sharp spectral features assigned by comparison with spectral simulations based on density functional theory. In separate experiments, vibrationally-enhanced dehydrogenation is observed when the t₂ vibrational normal mode in methane is excited prior to complexation. Clear infrared-induced enhancement is observed in the mass spectrum for peaks corresponding 4u below the mass of the Au⁺(CH₄)_n=2,3 complexes corresponding, presumably, to the loss of two H₂ molecules.

A Density Functional Theory Investigation of the Bimetallic Clusters Nb2Rh and NbRh2 and the Complexes They Form with CO

The interaction of CO with the bimetallic clusters Nb2Rh and NbRh2 has been theoretically investigated using density functional theory. The lowest energy structure of Nb2Rh is found to be a doublet Cs scalene triangle and the global minimum of Nb2Rh–CO is a dissociative structure with C1 symmetry. The lowest energy minimum of NbRh2 is found to be a doublet C2v isosceles triangle and the global minimum of NbRh2–CO is a dissociative structure with Cs symmetry. In comparison with our previous work on Rh3 + CO (J. Comp. Chem., 2008, 29, 1497), these results show that substitution of a single Rh atom with Nb is sufficient to dissociate CO.