Conjugated Polymer/Recombinant Escherichia coli Biohybrid Systems for Photobiocatalytic Hydrogen Production.

Biohybrid photocatalysts are composite materials that combine the efficient light-absorbing properties of synthetic materials with the highly evolved metabolic pathways and self-repair mechanisms of biological systems. Here, we show the potential of conjugated polymers as photosensitizers in biohybrid systems by combining a series of polymer nanoparticles with engineered Escherichia coli cells. Under simulated solar light irradiation, the biohybrid system consisting of fluorene/dibenzo [b,d]thiophene sulfone copolymer (LP41) and recombinant E. coli (i.e., a LP41/HydA BL21 biohybrid) shows a sacrificial hydrogen evolution rate of 3.442 mmol g-1 h-1 (normalized to polymer amount). It is over 30 times higher than the polymer photocatalyst alone (0.105 mmol g-1 h-1), while no detectable hydrogen was generated from the E. coli cells alone, demonstrating the strong synergy between the polymer nanoparticles and bacterial cells. The differences in the physical interactions between synthetic materials and microorganisms, as well as redox energy level alignment, elucidate the trends in photochemical activity. Our results suggest that organic semiconductors may offer advantages, such as solution processability, low toxicity, and more tunable surface interactions with the biological components over inorganic materials.

Potential Dependent Reorientation Controlling Activity of a Molecular Electrocatalyst.

The activity of molecular electrocatalysts depends on the interplay of electrolyte composition near the electrode surface, the composition and morphology of the electrode surface, and the electric field at the electrode-electrolyte interface. This interplay is challenging to study and often overlooked when assessing molecular catalyst activity. Here, we use surface specific vibrational sum frequency generation (VSFG) spectroscopy to study the solvent and potential dependent activation of Mo(bpy)(CO)4, a CO2 reduction catalyst, at a polycrystalline Au electrode. We find that the parent complex undergoes potential dependent reorientation at the electrode surface when a small amount of N-methyl-2-pyrrolidone (NMP) is present. This preactivates the complex, resulting in greater yields at less negative potentials, of the active electrocatalyst for CO2 reduction.

Studying the cation dependence of CO2 reduction intermediates at Cu by in situ VSFG spectroscopy.

The nature of the electrolyte cation is known to have a significant impact on electrochemical reduction of CO2 at catalyst|electrolyte interfaces. An understanding of the underlying mechanism responsible for catalytic enhancement as the alkali metal cation group is descended is key to guide catalyst development. Here, we use in situ vibrational sum frequency generation (VSFG) spectroscopy to monitor changes in the binding modes of the CO intermediate at the electrochemical interface of a polycrystalline Cu electrode during CO2 reduction as the electrolyte cation is varied. A CObridge mode is observed only when using Cs+, a cation that is known to facilitate CO2 reduction on Cu, supporting the proposed involvement of CObridge sites in CO coupling mechanisms during CO2 reduction. Ex situ measurements show that the cation dependent CObridge modes correlate with morphological changes of the Cu surface.

Unravelling the Roles of Integral Polypeptides in Excitation Energy Transfer of Photosynthetic RC-LH1 Supercomplexes.

Elucidating the photosynthetic processes that occur within the reaction center-light-harvesting 1 (RC-LH1) supercomplexes from purple bacteria is crucial for uncovering the assembly and functional mechanisms of natural photosynthetic systems and underpinning the development of artificial photosynthesis. Here, we examined excitation energy transfer of various RC-LH1 supercomplexes of Rhodobacter sphaeroides using transient absorption spectroscopy, coupled with lifetime density analysis, and studied the roles of the integral transmembrane polypeptides, PufX and PufY, in energy transfer within the RC-LH1 core complex. Our results show that the absence of PufX increases both the LH1 → RC excitation energy transfer lifetime and distribution due to the role of PufX in defining the interaction and orientation of the RC within the LH1 ring. While the absence of PufY leads to the conformational shift of several LH1 subunits toward the RC, it does not result in a marked change in the excitation energy transfer lifetime.

Monitoring interfacial electric fields at a hematite electrode during water oxidation.

To understand the mechanisms of water oxidation on materials such as hematite it is important that accurate measurements and models of the interfacial fields at the semiconductor liquid junction are developed. Here we demonstrate how electric field induced second harmonic generation (EFISHG) spectroscopy can be used to monitor the electric field across the space-charge and Helmholtz layers in a hematite electrode during water oxidation. We are able to identify the occurrence of Fermi level pinning at specific applied potentials which lead to a change in the Helmholtz potential. Through combined electrochemical and optical measurements we correlate these to the presence of surface trap states and the accumulation of holes (h+) during electrocatalysis. Despite the change in Helmholtz potential as h+ accumulate we find that a population model can be used to fit the electrocatalytic water oxidation kinetics with a transition between a first and third order regime with respect to hole concentration. Within these two regimes there are no changes in the rate constants for water oxidation, indicating that the rate determining step under these conditions does not involve electron/ion transfer, in-line with it being O-O bond formation.

Packing-induced selectivity switching in molecular nanoparticle photocatalysts for hydrogen and hydrogen peroxide production.

Molecular packing controls optoelectronic properties in organic molecular nanomaterials. Here we report a donor-acceptor organic molecule (2,6-bis(4-cyanophenyl)-4-(9-phenyl-9H-carbazol-3-yl)pyridine-3,5-dicarbonitrile) that exhibits two aggregate states in aqueous dispersions: amorphous nanospheres and ordered nanofibres with π-π molecular stacking. The nanofibres promote sacrificial photocatalytic H2 production (31.85 mmol g-1 h-1) while the nanospheres produce hydrogen peroxide (H2O2) (3.20 mmol g-1 h-1 in the presence of O2). This is the first example of an organic photocatalyst that can be directed to produce these two different solar fuels simply by changing the molecular packing. These different packings affect energy band levels, the extent of excited state delocalization, the excited state dynamics, charge transfer to O2 and the light absorption profile. We use a combination of structural and photophysical measurements to understand how this influences photocatalytic selectivity. This illustrates the potential to achieve multiple photocatalytic functionalities with a single organic molecule by engineering nanomorphology and solid-state packing.

Time-Resolved Raman Spectroscopy of Polaron Formation in a Polymer Photocatalyst.

Polymer photocatalysts are a synthetically diverse class of materials that can be used for the production of solar fuels such as H2, but the underlying mechanisms by which they operate are poorly understood. Time-resolved vibrational spectroscopy provides a powerful structure-specific probe of photogenerated species. Here we report the use of time-resolved resonance Raman (TR3) spectroscopy to study the formation of polaron pairs and electron polarons in one of the most active linear polymer photocatalysts for H2 production, poly(dibenzo[b,d]thiophene sulfone), P10. We identify that polaron-pair formation prior to thermalization of the initially generated excited states is an important pathway for the generation of long-lived photoelectrons.

Potential and pitfalls: On the use of transient absorption spectroscopy for in situ and operando studies of photoelectrodes.

Here, we discuss the application, advantages, and potential pitfalls of using transient UV/Vis (ultraviolet-visible) absorption spectroscopy to study photoelectrodes for water splitting. We revisit one of the most commonly studied water oxidation photoanodes (α-Fe2O3-x) to provide commentary and guidelines on experiment design and data analysis for transient absorption (TA) studies of photoelectrodes within a photoelectrochemical cell. We also assess the applicability of such in situ TA studies to understand photoelectrodes under operating conditions. A major limitation is that most, if not all, past in situ TA studies have been carried out using only pulsed light sources to generate carriers, with the electrode held in the dark at other times, which is shown to be a poor model for operating conditions. However, with a simple modification of existing TA experiments, a simple operando TA measurement is reported.

A stable covalent organic framework for photocatalytic carbon dioxide reduction.

Photocatalytic conversion of CO2 into fuels is an important challenge for clean energy research and has attracted considerable interest. Here we show that tethering molecular catalysts-a rhenium complex, [Re(bpy)(CO)3Cl]-together in the form of a crystalline covalent organic framework (COF) affords a heterogeneous photocatalyst with a strong visible light absorption, a high CO2 binding affinity, and ultimately an improved catalytic performance over its homogeneous Re counterpart. The COF incorporates bipyridine sites, allowing for ligation of the Re complex, into a fully π-conjugated backbone that is chemically robust and promotes light-harvesting. A maximum rate of 1040 µmol g-1 h-1 for CO production with 81% selectivity was measured. CO production rates were further increased up to 1400 µmol g-1 h-1, with an improved selectivity of 86%, when a photosensitizer was added. Addition of platinum resulted in production of syngas, hence, the co-formation of H2 and CO, the chemical composition of which could be adjusted by varying the ratio of COF to platinum. An amorphous analog of the COF showed significantly lower CO production rates, suggesting that crystallinity of the COF is beneficial to its photocatalytic performance in CO2 reduction.

Identification of separate isoenergetic routes for vibrational energy flow in p-fluorotoluene.

A deceptively simple feature in the S1 ← S0 spectrum of p-fluorotoluene (pFT), 1013 cm-1 above the origin, is studied using both zero-electron-kinetic-energy (ZEKE) and two-dimensional laser-induced fluorescence (2D-LIF) spectroscopy. It is found to consist of a cornucopia of overlapped transitions to eigenstates that arise from numerous interacting levels. A significant variation in the activity is seen employing both the ZEKE and 2D-LIF techniques. Detailed insight into the complicated spectra can be achieved, owing to the large number of vibrational wavenumbers that have been previously determined for the S0, S1, and D0 + states, summarized herein. It is found that the activity is dominated by two overtones, which are individually interacting with other levels, so providing largely independent routes for vibrational energy flow at the same internal energy. Additionally, other weak features located 900-1050 cm-1 above the origin are examined.

Vibrational sum-frequency generation spectroscopy of electrode surfaces: studying the mechanisms of sustainable fuel generation and utilisation.

The electrocatalytic oxidation of water coupled to the reduction of carbon dioxide, to make carbon based products, or the reduction of protons to provide hydrogen, offers a sustainable route to generating useful fuels. However new improved electrocatalysts and electrode materials are needed for these reactions. Similarly fuel cells for fuel utilisation rely on precious metal electrodes and new lower-cost materials are needed. Developing efficient catalysts for sustainable fuel generation can be accelerated with an improved understanding of the underlying mechanisms. Herein, we present a perspective on the use of vibrational sum-frequency generation (VSFG) spectroscopy to study such electrocatalytic mechanisms. We briefly outline the basic principles of VSFG spectroscopy pertinent to the study of electrochemical interfaces. We then review the use of VSFG to study water at charged and electrode interfaces, relevant to the mechanisms of water oxidation, the mechanisms of alcohol oxidation and also molecular electrocatalysts for carbon dioxide reduction.

Complexity surrounding an apparently simple Fermi resonance in p-fluorotoluene revealed using two-dimensional laser-induced fluorescence (2D-LIF) spectroscopy.

Two-dimensional laser-induced fluorescence (2D-LIF) spectroscopy is a powerful tool allowing overlapped features in an electronic spectrum to be separated, and interactions between vibrations and torsions to be identified. Here the technique is employed to assign the 790-825 cm-1 region above the origin of the S1 ← S0 transition in para-fluorotoluene, which provides insight into the unusual time-resolved results of Davies and Reid [Phys. Rev. Lett. 109, 193004 (2012)]. The region is dominated by a pair of bands that arise from a Fermi resonance; however, the assignment is complicated by contributions from a number of overtones and combinations, including vibration-torsion ("vibtor") levels. The activity in the 2D-LIF spectra is compared to the recently reported zero-electron-kinetic-energy spectra [Tuttle et al., J. Chem. Phys. 146, 244310 (2017)] to arrive at a consistent picture of the energy levels in this region of the spectrum.

Effects of symmetry, methyl groups and serendipity on intramolecular vibrational energy dispersal.

We consider two key parameters that have been proposed to be important for vibrational energy delocalization, closely related to intramolecular vibrational redistribution (IVR), in molecules. These parameters are the symmetry of the molecule, and the presence of torsional (internal rotor) modes of a methyl group. We consider four para-disubstituted benzene molecules and examine their vibrational character. The molecules selected are para-difluorobenzene, para-chlorofluorobenzene, para-fluorotoluene, and para-xylene. This set of molecules allows the above parameters to be assessed in a systematic way. The probe we use is zero-electron-kinetic-energy (ZEKE) spectroscopy, which is employed in a resonant scheme, where the intermediate levels are selected vibrational levels of the S1 excited electronic state, with wavenumbers up to 1300 cm-1. We conclude that symmetry, and the presence of a methyl groups, do indeed have a profound effect on "restricted" IVR at low energies. This is underpinned by serendipitous coincidences in the energies of the levels, owing to small shifts in vibrational wavenumbers between molecules, so bringing levels into resonance. Additionally, methyl groups play an important role in opening up new routes for coupling between vibrations of different symmetry, and this is critical in the transition to "statistical" IVR at lower energies for molecules that contain them. Further, the presence of two methyl groups in the symmetrically-substituted p-xylene causes more widespread IVR than does the single methyl group in the asymmetrically-substituted p-fluorotoluene.

Identifying complex Fermi resonances in p-difluorobenzene using zero-electron-kinetic-energy (ZEKE) spectroscopy.

The vibrations of the ground state cation ( X̃2B2g) of para-difluorobenzene (pDFB) have been investigated using zero-electron-kinetic-energy (ZEKE) spectroscopy. A comprehensive set of ZEKE spectra were recorded via different vibrational levels of the S1 state (<00 + 1300 cm-1). The adiabatic ionization energy for pDFB was measured as 73 869 ± 5 cm-1. Use of different intermediate levels allows different cationic vibrational activity to be obtained via the modification of the Franck-Condon factors for the ionization step, allowing the wavenumbers of different vibrational levels in the cation to be established. In addition, assignment of the vibrational structure in the ZEKE spectra allowed interrogation of the assignments of the S1 ← S0 transition put forward by Knight and Kable [J. Chem. Phys. 89, 7139 (1988)]. Assignment of the vibrational structure has been aided by quantum chemical calculations. In this way, it was possible to assign seventeen of the thirty vibrational modes of the ground state pDFB+ cation. Evidence for complex Fermi resonances in the S1 state, i.e., those that involve more than two vibrations, was established. One of these was investigated using picosecond time-resolved photoelectron spectroscopy. In addition, we discuss the appearance of several symmetry-forbidden bands in the ZEKE spectra, attributing their appearance to a Rydberg state variation of an intrachannel vibronic coupling mechanism.

Direct observation of vibrational energy dispersal via methyl torsions.

Explicit evidence for the role of methyl rotor levels in promoting energy dispersal is reported. A set of coupled zero-order vibration/vibration-torsion (vibtor) levels in the S1 state of para-fluorotoluene (pFT) are investigated. Two-dimensional laser-induced fluorescence (2D-LIF) and two-dimensional zero-kinetic-energy (2D-ZEKE) spectra are reported, and the assignment of the main features in both sets of spectra reveals that the methyl torsion is instrumental in providing a route for coupling between vibrational levels of different symmetry classes. We find that there is very localized, and selective, dissipation of energy via doorway states, and that, in addition to an increase in the density of states, a critical role of the methyl group is a relaxation of symmetry constraints compared to direct vibrational coupling.

Vibrations of the p-chlorofluorobenzene cation.

The vibrations of the ground state cation (X[combining tilde]2B1) of para-chlorofluorobenzene (pClFB) have been investigated using zero-electron-kinetic-energy (ZEKE) spectroscopy. ZEKE spectra were recorded using different vibrational levels of the S1 state as intermediate levels, for which assignments were put forward in an earlier paper [W. D. Tuttle, A. M. Gardner, and T. G. Wright, Chem. Phys. Lett., 2017, 684, 339]. These different intermediate levels dramatically modify the Franck-Condon factors for the ionization step. The adiabatic ionization energy (AIE) for pClFB was measured as 72 919 ± 5 cm-1, and analysis of the vibrational structure in the ZEKE spectra allowed further interrogation of the assignments of the REMPI spectrum. Assignment of the vibrational structure has been achieved by comparison with corresponding spectra of related molecules, via quantum chemical calculations, and via shifts in bands between the spectra of the 35Cl and 37Cl isotopologues. In this way it was possible to assign twenty out of the thirty vibrational modes of the ground state pClFB+ cation. Additionally, evidence for Fermi resonances between some vibrational levels was found in the S1 state, but no large-scale intramolecular vibrational redistribution (IVR) was seen in the spectra here. Finally, we discuss trends in AIE shifts for benzenes with one or two halogen atoms or methyl substituents.

Vibration and vibration-torsion levels of the S1 state of para-fluorotoluene in the 580-830 cm-1 range: Interactions and coincidences.

A study of the vibration and vibration-torsion levels of para-fluorotoluene in the 580-830 cm-1 region is presented, where a number of features are located whose identity is complicated by interactions and overlap. We examine this region with a view to ascertaining the assignments of the bands; in particular, identifying those that arise from interactions involving various zero-order states (ZOSs) involving both vibrations and torsions. Resonance-enhanced multiphoton ionization (REMPI) is employed to identify the wavenumbers of the relevant transitions, and subsequently zero-kinetic-energy (ZEKE) spectra are recorded to assign the various eigenstates. In some cases, a set of ZEKE spectra are recorded across the wavenumber range of a REMPI feature, and we construct what we term a two-dimensional ZEKE (2D-ZEKE) spectrum, which allows the changing ZOS contributions to the eigenstates to be ascertained. Assignment of the observed bands is aided by quantum chemical calculations and all b1 and a2 symmetry vibrational wavenumbers are now determined in the S1 state and cation, as well as those of the D10 vibration. We also compare to the activity seen in the corresponding S1 ← S0 spectrum of para-difluorobenzene.

Molecular symmetry group analysis of the low-wavenumber torsions and vibration-torsions in the S1 state and ground state cation of p-xylene: An investigation using resonance-enhanced multiphoton ionization (REMPI) and zero-kinetic-energy (ZEKE) spectroscopy.

For the first time, a molecular symmetry group (MSG) analysis has been undertaken in the investigation of the electronic spectroscopy of p-xylene (p-dimethylbenzene). Torsional and vibration-torsional (vibtor) levels in the S1 state and ground state of the cation of p-xylene are investigated using resonance-enhanced multiphoton ionization (REMPI) and zero-kinetic-energy (ZEKE) spectroscopy. In the present work, we concentrate on the 0-350 cm-1 region, where there are a number of torsional and vibtor bands and we discuss the assignment of this region. In Paper II [W. D. Tuttle et al., J. Chem. Phys. 146, 124309 (2017)], we examine the 350-600 cm-1 region where vibtor levels are observed as part of a Fermi resonance. The similarity of much of the observed spectral activity to that in the related substituted benzenes, toluene and para-fluorotoluene, is striking, despite the different symmetries. The discussion necessitates a consideration of the MSG of p-xylene, which has been designated G72, but we shall also designate [{3,3}]D2h and we include the symmetry operations, character table, and direct product table for this. We also discuss the symmetries of the internal rotor (torsional) levels and the selection rules for the particular electronic transition of p-xylene investigated here.

Vibrational and vibrational-torsional interactions in the 0-600 cm-1 region of the S1← S0 spectrum of p-xylene investigated with resonance-enhanced multiphoton ionization (REMPI) and zero-kinetic-energy (ZEKE) spectroscopy.

We assign the 0-600 cm-1 region of the S1← S0 transition in p-xylene (p-dimethylbenzene) using resonance-enhanced multiphoton ionization (REMPI) and zero-kinetic-energy (ZEKE) spectroscopy. In the 0-350 cm-1 range as well as the intense origin band, there are a number of torsional and vibration-torsion (vibtor) features. The latter are discussed in more detail in Paper I [A. M. Gardner et al., J. Chem. Phys. 146, 124308 (2017)]. Here we focus on the origin and the 300-600 cm-1 region, where vibrational bands and some vibtor activity are observed. From the origin ZEKE spectrum, we derive the ionization energy of p-xylene as 68200 ± 5 cm-1. The assignment of the REMPI spectrum is based on the activity observed in the ZEKE spectra coupled with knowledge of the vibrational wavenumbers obtained from quantum chemical calculations. We assign several isolated vibrations and a complex Fermi resonance that is found to comprise contributions from both vibrations and vibtor levels, and we examine this via a two-dimensional ZEKE spectrum. A number of the vibrational features in the REMPI and ZEKE spectra of p-xylene that have been reported previously are reassigned and now largely consist of totally symmetric contributions. We briefly discuss the appearance of non-Franck-Condon allowed transitions. Finally, we find remarkably similar spectral activity to that in the related disubstituted benzenes, para-difluorobenzene, and para-fluorotoluene.

Photodetachment spectroscopy of the beryllium oxide anion, BeO.

The X2Σ+→X1Σ+ anion to neutral ground state photodetachment of BeO- has been studied by means of photoelectron velocity-map imaging spectroscopy in a newly constructed apparatus. Vibrational intervals, rotational constants, and the electron detachment threshold of BeO- were determined for the first time. The small moment of inertia of beryllium oxide allowed for the observation of partially resolved rotational contours. Analyses of these contours provided evidence of several detachment channels resulting from changes in molecular rotational angular momenta of ΔN = 0, ±1, ±2, and ±3. The relative intensities of these detachment channels were found to be a function of the electron kinetic energy. Experimental results are compared to the predictions of high level ab initio calculations.