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.

The effect of particle size on the optical and electronic properties of hydrogenated silicon nanoparticles.

We use a combination of many-body perturbation theory and time-dependent density functional theory to study the optical and electronic properties of hydrogen terminated silicon nanoparticles. We predict that the lowest excited states of these silicon nanoparticles are excitonic in character and that the corresponding excitons are completely delocalised over the volume of the particle. The size of the excitons is predicted to increase proportionally with the particle size. Conversely, we predict that the fundamental gap, the optical gap, and the exciton binding energy increase with decreasing particle size. The exciton binding energy is predicted to counter-act the variation in the fundamental gap and hence to reduce the variation of the optical gap with particle size. The variation in the exciton binding energy itself is probably caused by a reduction in the dielectric screening with decreasing particle size. The intensity of the excited state corresponding to the optical gap and other low energy excitations are predicted to increase with decreasing particle size. We explain this increase in terms of the 'band structure' becoming smeared out in reciprocal space with decreasing particle size, increasing the 'overlap' between the occupied and unoccupied quasiparticle states and thus, the oscillator strength. Fourier transforms of the lowest excitons show that they inherit the periodicity of the frontier quasiparticle states. This, combined with the delocalisation of the exciton and the large exciton binding energy, means that the excitons in silicon nanoparticles combine aspects of Wannier-Mott, delocalisation and effect of periodicity of the underlying structure, and Frenkel, large exciton binding energy, excitons.

Investigating the diastereoselective synthesis of a macrocycle under Curtin-Hammett control.

This work sheds new light on the stereoselective synthesis of chiral macrocycles containing twisted aromatic units, valuable π-conjugated materials for recognition, sensing, and optoelectronics. For the first time, we use the Curtin-Hammett principle to investigate a chiral macrocyclisation reaction, revealing the potential for supramolecular π-π interactions to direct the outcome of a dynamic kinetic resolution, favouring the opposite macrocyclic product to that expected under reversible, thermodynamically controlled conditions. Specifically, a dynamic, racemic perylene diimide dye (1 : 1 P : M) is strapped with an enantiopure (S)-1,1'-bi-2-naphthol group (P-BINOL) to form two diastereomeric macrocyclic products, the homochiral macrocycle (PP) and the heterochiral species (PM). We find there is notable selectivity for the PM macrocycle (dr = 4 : 1), which is rationalised by kinetic templation from intramolecular aromatic non-covalent interactions between the P-BINOL π-donor and the M-PDI π-acceptor during the macrocyclisation reaction.

A Chirally Locked Bis-perylene Diimide Macrocycle: Consequences for Chiral Self-Assembly and Circularly Polarized Luminescence.

Macrocycles containing chiral organic dyes are highly valuable for the development of supramolecular circularly polarized luminescent (CPL) materials, where a preorganized chiral framework is conducive to directing π-π self-assembly and delivering a strong and persistent CPL signal. Here, perylene diimides (PDIs) are an excellent choice for the organic dye component because, alongside their tunable photophysical and self-assembly properties, functionalization of the PDI's core yields a twisted, chiral π-system, capable of CPL. However, configurationally stable PDI-based macrocycles are rare, and those that are also capable of π-π self-assembly beyond dimers are unprecedented, both of which are advantageous for robust self-assembled chiroptical materials. In this work, we report the first bay-connected bis-PDI macrocycle that is configurationally stable (ΔG⧧ > 155 kJ mol-1). We use this chirally locked macrocycle to uncover new knowledge of chiral PDI self-assembly and to perform new quantitative CPL imaging of the resulting single-crystal materials. As such, we discover that the chirality of a 1,7-disubstituted PDI provides a rational route to designing H-, J- and concomitant H- and J-type self-assembled materials, important arrangements for optimizing (chir)optical and charge/energy transport properties. Indeed, we reveal that CPL is amplified in the single crystals of our chiral macrocycle by quantifying the degree of emitted light circular polarization from such materials for the first time using CPL-Laser Scanning Confocal Microscopy.

Controlling the thermoelectric properties of organo-metallic coordination polymers through backbone geometry.

Poly(nickel-benzene-1,2,4,5-tetrakis(thiolate)) (Ni-btt), an organometallic coordination polymer (OMCP) characterized by the coordination between benzene-1,2,4,5-tetrakis(thiolate) (btt) and Ni2+ ions, has been recognized as a promising p-type thermoelectric material. In this study, we employed a constitutional isomer based on benzene-1,2,3,4-tetrakis(thiolate) (ibtt) to generate the corresponding isomeric polymer, poly(nickel-benzene-1,2,3,4-tetrakis(thiolate)) (Ni-ibtt). Comparative analysis of Ni-ibtt and Ni-btt reveals several common infrared (IR) and Raman features attributed to their similar square-planar nickel-sulfur (Ni-S) coordination. Nevertheless, these two polymer isomers exhibit substantially different backbone geometries. Ni-btt possesses a linear backbone, whereas Ni-ibtt exhibits a more undulating, zig-zag-like structure. Consequently, Ni-ibtt demonstrates slightly higher solubility and an increased bandgap in comparison to Ni-btt. The most noteworthy dissimilarity, however, manifests in their thermoelectric properties. While Ni-btt exhibits p-type behavior, Ni-ibtt demonstrates n-type carrier characteristics. This intriguing divergence prompted further investigation into the influence of OMCP backbone geometry on the electronic structure and, particularly, the thermoelectric properties of these materials.

Linear conjugated polymer photocatalysts with various linker units for photocatalytic hydrogen evolution from water.

Polymer photocatalysts have shown potential for light-driven hydrogen evolution from water. Here we studied the relative importance of the linker type in two series of conjugated polymers based on dibenzo[b,d]thiophene sulfone and dimethyl-9H-fluorene. The alkenyl-linked polymers were found to be more active photocatalysts than their alkyl and alkyne-linked counterparts. The co-polymer of dibenzo[b,d]thiophene sulfone and 1,2-diphenylethene has a hydrogen evolution rate of 3334 µmol g-1 h-1 and an external quantum efficiency of 5.6% at 420 nm.

Solution-Processable Redox-Active Polymers of Intrinsic Microporosity for Electrochemical Energy Storage.

Redox-active organic materials have emerged as promising alternatives to conventional inorganic electrode materials in electrochemical devices for energy storage. However, the deployment of redox-active organic materials in practical lithium-ion battery devices is hindered by their undesired solubility in electrolyte solvents, sluggish charge transfer and mass transport, as well as processing complexity. Here, we report a new molecular engineering approach to prepare redox-active polymers of intrinsic microporosity (PIMs) that possess an open network of subnanometer pores and abundant accessible carbonyl-based redox sites for fast lithium-ion transport and storage. Redox-active PIMs can be solution-processed into thin films and polymer-carbon composites with a homogeneously dispersed microstructure while remaining insoluble in electrolyte solvents. Solution-processed redox-active PIM electrodes demonstrate improved cycling performance in lithium-ion batteries with no apparent capacity decay. Redox-active PIMs with combined properties of intrinsic microporosity, reversible redox activity, and solution processability may have broad utility in a variety of electrochemical devices for energy storage, sensors, and electronic applications.

The effect of particle size and composition on the optical and electronic properties of CdO and CdS rocksalt nanoparticles.

Quantum confinement like behaviour in CdO and CdS nanoparticles is demonstrated through explicit evGW-BSE many-body perturbation theory calculations on 0.6-1.4 nanometre particles of these materials. However, while the lowest optical excited-state, exciton, and the highest occupied and lowest unoccupied quasiparticle states in such nanoparticles are predicted to be delocalised, they are found to be delocalised over the surface of the particle only and not the whole particle volume. The electronic and optical properties of CdO and CdS rocksalt nanoparticles are predicted to differ dramatically from their structurally analogous MgO counterparts, where the lowest exciton and highest occupied and lowest unoccupied quasiparticle states are strongly localised, in contrast. This difference in behaviour between MgO and CdO/CdS is explained in terms of the more polarisable, less ionic, bonding in CdO and CdS. The effect on the optical and fundamental gaps of the particles due to the presence of amine capping agents on the particles' surface is explored and predicted to be relatively small. However, the highest occupied and lowest unoccupied quasiparticle states are found to consistently shift to more shallow values when increasing the surface density of capping agents. An explanation of this shift, finally, is proposed in terms of the dipole field induced by the aligned dipoles of the capping agents.

The Pink Box: Exclusive Homochiral Aromatic Stacking in a Bis-perylene Diimide Macrocycle.

This work showcases chiral complementarity in aromatic stacking interactions as an effective tool to optimize the chiroptical and electrochemical properties of perylene diimides (PDIs). PDIs are a notable class of robust dye molecules and their rich photo- and electrochemistry and potential chirality make them ideal organic building blocks for chiral optoelectronic materials. By exploiting the new bay connectivity of twisted PDIs, a dynamic bis-PDI macrocycle (the "Pink Box") is realized in which homochiral PDI-PDI π-π stacking interactions are switched on exclusively. Using a range of experimental and computational techniques, we uncover three important implications of the macrocycle's chiral complementarity for PDI optoelectronics. First, the homochiral intramolecular π-π interactions anchor the twisted PDI units, yielding enantiomers with half-lives extended over 400-fold, from minutes to days (in solution) or years (in the solid state). Second, homochiral H-type aggregation affords the macrocycle red-shifted circularly polarized luminescence and one of the highest dissymmetry factors of any small organic molecule in solution (glum = 10-2 at 675 nm). Finally, excellent through-space PDI-PDI π-orbital overlap stabilizes PDI reduced states, akin to covalent functionalization with electron-withdrawing groups.

Photocatalytic Overall Water Splitting Under Visible Light Enabled by a Particulate Conjugated Polymer Loaded with Palladium and Iridium.

Polymer photocatalysts have received growing attention in recent years for photocatalytic hydrogen production from water. Most studies report hydrogen production with sacrificial electron donors, which is unsuitable for large-scale hydrogen energy production. Here we show that the palladium/iridium oxide-loaded homopolymer of dibenzo[b,d]thiophene sulfone (P10) facilitates overall water splitting to produce stoichiometric amounts of H2 and O2 for an extended period (>60 hours) after the system stabilized. These results demonstrate that conjugated polymers can act as single component photocatalytic systems for overall water splitting when loaded with suitable co-catalysts, albeit currently with low activities. Transient spectroscopy shows that the IrO2 co-catalyst plays an important role in the generation of the charge separated state required for water splitting, with evidence for fast hole transfer to the co-catalyst.

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.

The effect of particle size on the optical and electronic properties of magnesium oxide nanoparticles.

The quasiparticle states, fundamental gaps, optical gaps, exciton binding energies and UV-vis spectra for a series of cuboidal nanoparticles of the prototypical oxide magnesium oxide (MgO), the largest of which has 216 atoms and edges of 1 nm, were predicted using many-body perturbation theory (evGW-BSE). The evolution of the properties with the particle size was explicitly studied. It was found that, while the highest occupied and lowest unoccupied quasiparticle states and fundamental gap change with the particle size, the optical gap remains essentially fixed for all but the smallest nanoparticles, in line with what was previously observed experimentally. The explanation for these observations is demonstrated to be that, while the optical gap is associated with an exciton that is highly localised around the particle's corner atoms, the highest occupied and lowest unoccupied quasiparticle states, while primarily localised on the oxygen corner atoms (hole) and magnesium corner atoms (electron), show significant delocalisation along the edges. The strong localisation of the exciton associated with the optical gap on the corner atoms is argued to also explain why the nanoparticles have much smaller optical gaps and red-shifted spectra compared to bulk MgO. Finally, it is discussed how this non-quantum confinement behaviour, where the properties of the nanoparticles arise from surface defects rather than differences in localisation of quasiparticle or exciton states, appears typical of alkaline earth oxide nanoparticles, and that the true optical gap of bulk crystals of such materials is also probably the result of surface defects, even if unobservable experimentally.

Water Oxidation with Cobalt-Loaded Linear Conjugated Polymer Photocatalysts.

The first examples of linear conjugated organic polymer photocatalysts that produce oxygen from water after loading with cobalt and in the presence of an electron scavenger are reported. The oxygen evolution rates, which are higher than for related organic materials, can be rationalized by a combination of the thermodynamic driving force for water oxidation, the light absorption of the polymer, and the aqueous dispersibility of the relatively hydrophilic polymer particles. We also used transient absorption spectroscopy to study the best performing system and we found that fast oxidative quenching of the exciton occurs (picoseconds) in the presence of an electron scavenger, minimizing recombination.

Molecular generation targeting desired electronic properties via deep generative models.

As we seek to discover new functional materials, we need ways to explore the vast chemical space of precursor building blocks, not only generating large numbers of possible building blocks to investigate, but trying to find non-obvious options, that we might not suggest by chemical experience alone. Artificial intelligence techniques provide a possible avenue to generate large numbers of organic building blocks for functional materials, and can even do so from very small initial libraries of known building blocks. Specifically, we demonstrate the application of deep recurrent neural networks for the exploration of the chemical space of building blocks for a test case of donor-acceptor oligomers with specific electronic properties. The recurrent neural network learned how to produce novel donor-acceptor oligomers by trading off between selected atomic substitutions, such as halogenation or methylation, and molecular features such as the oligomer's size. The electronic and structural properties of the generated oligomers can be tuned by sampling from different subsets of the training database, which enabled us to enrich the library of donor-acceptors towards desired properties. We generated approximately 1700 new donor-acceptor oligomers with a recurrent neural network tuned to target oligomers with a HOMO-LUMO gap <2 eV and a dipole moment <2 Debye, which could have potential application in organic photovoltaics.

Mapping the optoelectronic property space of small aromatic molecules.

Small aromatic molecules and their quinone derivatives find use in organic transistors, solar-cells, thermoelectrics, batteries and photocatalysts. These applications exploit the optoelectronic properties of these molecules and the ease by which such properties can be tuned by the introduction of heteroatoms and/or the addition of functional groups. We perform a high-throughput virtual screening using the xTB family of density functional tight-binding methods to map the optoelectronic property space of ~250,000 molecules. The large volume of data generated allows for a broad understanding of how the presence of heteroatoms and functional groups affect the ionisation potential, electron affinity and optical gap values of these molecular semiconductors, and how the structural features - on their own or in combination with one another - allow access to particular regions of the optoelectronic property space. Finally, we identify the apparent boundaries of the optoelectronic property space for these molecules: regions of property space that appear off limits for any small aromatic molecule.

Insight into the self-assembly of water-soluble perylene bisimide derivatives through a combined computational and experimental approach.

We use a combination of computational and experimental techniques to study the self-assembly and gelation of water-soluble perylene bisimides derivatised at the imide position with an amino acid. Specifically, we study the likely structure of self-assembled aggregates of the alanine-functionalised perylene bisimide (PBI-A) and the thermodynamics of their formation using density functional theory and predict the UV-vis spectra of such aggregates using time-dependent density functional theory. We compare these predictions to experiments in which we study the evolution of the UV-Vis and NMR spectra and the rheology and neutron scattering of alkaline PBI-A solutions when gradually decreasing the pH. Based on the combined computational and experimental results, we show that PBI-A self-assembles at all pH values but that aggregates grow in size upon protonation. Hydrogel formation is driven not by aggregate growth but reduction of the aggregation surface-charge and a decrease in the colloidal stability of the aggregation with respect to agglomeration.

Mapping binary copolymer property space with neural networks.

The extremely large number of unique polymer compositions that can be achieved through copolymerisation makes it an attractive strategy for tuning their optoelectronic properties. However, this same attribute also makes it challenging to explore the resulting property space and understand the range of properties that can be realised. In an effort to enable the rapid exploration of this space in the case of binary copolymers, we train a neural network using a tiered data generation strategy to accurately predict the optical and electronic properties of 350 000 binary copolymers that are, in principle, synthesizable from their dihalogen monomers via Yamamoto, or Suzuki-Miyaura and Stille coupling after one-step functionalisation. By extracting general features of this property space that would otherwise be obscured in smaller datasets, we identify simple models that effectively relate the properties of these copolymers to the homopolymers of their constituent monomers, and challenge common ideas behind copolymer design. We find that binary copolymerisation does not appear to allow access to regions of the optoelectronic property space that are not already sampled by the homopolymers, although it conceptually allows for more fine-grained property control. Using the large volume of data available, we test the hypothesis that copolymerisation of 'donor' and 'acceptor' monomers can result in copolymers with a lower optical gap than their related homopolymers. Overall, despite the prevalence of this concept in the literature, we observe that this phenomenon is relatively rare, and propose conditions that greatly enhance the likelihood of its experimental realisation. Finally, through a 'topographical' analysis of the co-polymer property space, we show how this large volume of data can be used to identify dominant monomers in specific regions of property space that may be amenable to a variety of applications, such as organic photovoltaics, light emitting diodes, and thermoelectrics.

Accelerated Discovery of Organic Polymer Photocatalysts for Hydrogen Evolution from Water through the Integration of Experiment and Theory.

Conjugated polymers are an emerging class of photocatalysts for hydrogen production where the large breadth of potential synthetic diversity presents both an opportunity and a challenge. Here, we integrate robotic experimentation with high-throughput computation to navigate the available structure-property space. A total of 6354 co-polymers was considered computationally, followed by the synthesis and photocatalytic characterization of a sub-library of more than 170 co-polymers. This led to the discovery of new polymers with sacrificial hydrogen evolution rates (HERs) of more than 6 mmol g-1 h-1. The variation in HER across the library does not correlate strongly with any single physical property, but a machine-learning model involving four separate properties can successfully describe up to 68% of the variation in the HER data between the different polymers. The four variables used in the model were the predicted electron affinity, the predicted ionization potential, the optical gap, and the dispersibility of the polymer particles in solution, as measured by optical transmittance.