Elucidating the Microscopic Origin of the Unique Optical Properties of Polypyrene

A combination of Time-Dependent Density Functional Theory (TD-DFT) and approximate coupled cluster theory (CC2) is used to elucidate the microscopic origin of the experimental observation that the absorption and fluorescence spectra of 1,3-polypyrene display a much smaller shift with chain length than other conjugated polymers. The optical absorption and fluorescence spectra of a large range of oligomers are calculated using TD-DFT and CC2 and successfully compared with available experimental data. The calculations show that the lowest singlet excitation is excitonic in nature and that this exciton becomes strongly localized upon excited state relaxation. This strong localization explains the negligible shift in fluorescence energy between the dimer/trimer and polymer, observed in experiment

Modeling the Water Splitting Activity of a TiO₂ Rutile Nanoparticle

We explore, from a theoretical perspective, the effect of particle size on the photocatalytic water splitting activity of TiO₂ rutile (nano)­particles by a combination of explicit quantum chemistry calculations on a hydroxylated rutile nanoparticle in a realistic solvation environment and a comparison with the calculated properties of bulk rutile (surfaces) from the literature. Specifically, we use density functional theory (DFT) and time-dependent DFT to calculate the nanoparticle thermodynamic driving force for the water splitting half-reactions and identify in the process the crucial role of self-trapping of the free charge carriers responsible for proton reduction and water oxidation

Validating a Density Functional Theory Approach for Predicting the Redox Potentials Associated with Charge Carriers and Excitons in Polymeric Photocatalysts

We compare, for a range of conjugated polymers relevant to water-splitting photocatalysis, the predictions for the redox potentials associated with charge carriers and excitons by a total-energy ΔDFT approach to those measured experimentally. For solid-state potentials, of the different classes of potentials available experimentally for conjugated polymers, the class measured under conditions which are the most similar to those during water splitting, we find a good fit between the ionization potentials predicted using ΔB3LYP and those measured experimentally using photoemission spectroscopy (PES). We also observe a reasonable fit to the more limited data sets of excited-state ionization potentials, obtained from two-photon PES, and electron affinities, measured by inverse PES, respectively. Through a comparison of solid-state potentials with gas phase and solution potentials for a range of oligomers, we demonstrate how the quality of the fit to experimental solid-state data is probably the result of benign error cancellation. We discuss that the good fit for solid-state potentials in vacuum suggests that a similar accuracy can be expected for calculations on solid-state polymers interfaced with water. We also analyze the quality of approximating the ΔB3LYP potentials by orbital energies. Finally, we discuss what a comparison between experimental and predicted potentials teaches us about conjugated polymers as photocatalysts, focusing specifically on the large exciton-binding energy in these systems and the mechanism of free charge carrier generation

Isomorphism of Anhydrous Tetrahedral Halides and Silicon Chalcogenides: Energy Landscape of Crystalline BeF₂, BeCl₂, SiO₂, and SiS₂

We employ periodic density functional theory calculations to compare the structural chemistry of silicon chalcogenides (silica, silicon sulfide) and anhydrous tetrahedral halides (beryllium fluoride, beryllium chloride). Despite the different formal oxidation states of the elements involved, the divalent halides are known experimentally to form crystal structures similar to known SiX2 frameworks; the rich polymorphic chemistry of SiO2 is however not matched by divalent halides, for which a very limited number of polymorphs are currently known. The calculated energy landscapes yield a quantitative match between the relative polymorphic stability in the SiO2/BeF2 pair, and a semiquantitative match for the SiS2/BeCl2 pair. The experimentally observed polymorphs are found to lie lowest in energy for each composition studied. For the two BeX2 compounds studied, polymorphs not yet synthesized are predicted to lie very low in energy, either slightly above or even in between the energy of the experimentally observed polymorphs. The experimental lack of polymorphism for tetrahedral halide materials thus does not appear to stem from a lack of low-energy polymorphs but more likely is the result of a lack of experimental exploration. Our calculations further indicate that the rich polymorphic chemistry of SiO2 can be potentially matched, if not extended, by BeF2, provided that milder synthetic conditions similar to those employed in zeolite synthesis are developed for BeF2. Finally, our work demonstrates that both classes of materials show the same behavior upon replacement of the 2p anion with the heavier 3p anion from the same group; the thermodynamic preference shifts from structures with large rings to structures with larger fractions of small two and three membered rings

Carbon Nitride Photocatalysts for Water Splitting: A Computational Perspective

We study the thermodynamic ability of carbon nitride materials to act as water splitting photocatalysts using a computational approach that involves a combination of density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on cluster models of both triazine- and heptazine-based structures. We first use TD-DFT to calculate the absorption spectra of the different cluster models and compare these spectra to those measured experimentally and then calculate using DFT and TD-DFT the reduction potentials of the free electron, free hole, and exciton in these models. We predict that all classes of carbon nitride structures considered should thermodynamically be able to reduce protons and oxidize water. We further provide evidence for the hypothesis that the experimental lack of overall water splitting activity for pure carbon nitride arises from the fact that water oxidation is a four-hole reaction and hence very susceptible to competition with electron–hole recombination. Finally, we propose that the recently reported overall water splitting activity of carbon nitride loaded with polypyrrole nanoparticles arises from a junction formed at the interface of both materials, which assists in keeping electrons and holes apart

Dramatic Differences between the Energy Landscapes of SiO₂ and SiS₂ Zeotype Materials

Periodic density functional calculations have been carried out on a series of tetrahedral frameworks, which includeknown and hypothetical zeolite topologies. For each framework both SiO2 and SiS2 compositions were considered. We demonstrate that the energy landscape of tetrahedral sulfides is dramatically different from that of silica and that hypothetical frameworks are too distorted for silica, for example, supertetrahedral frameworks such as RWY are significantly stabilized as sulfides. We discuss this change in terms of the increased tolerance of sulfides for tetrahedral distortion and a more general increase in the material's flexibility. The latter is shown clearly from a comparison between the pseudofrequencies (the frequencies calculated with all atomic masses set to one for comparison) of the two compositions, being noticeably lower in the case of SiS2

Amine Molecular Cages as Supramolecular Fluorescent Explosive Sensors: A Computational Perspective

We investigate using a computational approach the physical and chemical processes underlying the application of organic (macro)­molecules as fluorescence quenching sensors for explosives sensing. We concentrate on the use of amine molecular cages to sense nitroaromatic analytes, such as picric acid and 2,4-dinitrophenol, through fluorescence quenching. Our observations for this model system hold for many related systems. We consider the different possible mechanisms of fluorescence quenching: Förster resonance energy transfer, Dexter energy transfer and photoinduced electron transfer, and show that in the case of our model system, the fluorescence quenching is driven by the latter and involves stable supramolecular sensor–analyte host–guest complexes. Furthermore, we demonstrate that the experimentally observed selectivity of amine molecular cages for different explosives can be explained by the stability of these host–guest complexes and discuss how this is related to the geometry of the binding site in the sensor. Finally, we discuss what our observations mean for explosive sensing by fluorescence quenching in general and how this can help in future rational design of new supramolecular detection systems

High-Throughput Screening Approach for the Optoelectronic Properties of Conjugated Polymers

We propose a general high-throughput virtual screening approach for the optical and electronic properties of conjugated polymers. This approach makes use of the recently developed xTB family of low-computational-cost density functional tight-binding methods from Grimme and co-workers, calibrated here to (Time-Dependent) Density Functional Theory ((TD)­DFT) data computed for a representative diverse set of (co)­polymers. Parameters drawn from the resulting calibration using a linear model can then be applied to the xTB derived results for new polymers, thus generating near DFT-quality data with orders of magnitude reduction in computational cost. As a result, after an initial computational investment for calibration, this approach can be used to quickly and accurately screen on the order of thousands of polymers for target applications. We also demonstrate that the (opto)­electronic properties of the conjugated polymers show only a very minor variation when considering different conformers and that the results of high-throughput screening are therefore expected to be relatively insensitive with respect to the conformer search methodology applied

High-Throughput Screening Approach for the Optoelectronic Properties of Conjugated Polymers

We propose a general high-throughput virtual screening approach for the optical and electronic properties of conjugated polymers. This approach makes use of the recently developed xTB family of low-computational-cost density functional tight-binding methods from Grimme and co-workers, calibrated here to (Time-Dependent) Density Functional Theory ((TD)­DFT) data computed for a representative diverse set of (co)­polymers. Parameters drawn from the resulting calibration using a linear model can then be applied to the xTB derived results for new polymers, thus generating near DFT-quality data with orders of magnitude reduction in computational cost. As a result, after an initial computational investment for calibration, this approach can be used to quickly and accurately screen on the order of thousands of polymers for target applications. We also demonstrate that the (opto)­electronic properties of the conjugated polymers show only a very minor variation when considering different conformers and that the results of high-throughput screening are therefore expected to be relatively insensitive with respect to the conformer search methodology applied

Shining a Light on <i>s</i>‑Triazine-Based Polymers

The strong interplay between the structure and optical properties of conjugated <i>s</i>-triazine-based framework (CTF) materials is explored in a combined experimental and computational study. The experimental absorption and fluorescence spectra of the CTF-1 material, a polymer obtained through the trimerization of 1,4-dicyanobenzene, are compared with the results of time-dependent density functional theory and approximate coupled cluster theory (CC2) calculations on cluster models of the polymer. To help explain the polymer data, we compare its optical properties with those measured and predicted for the 2,4,6-triphenyl-1,3,5-triazine model compound. Our analysis shows that CTFs, in line with experimental diffraction data, are likely to be layered materials based around flat hexagonal-like sheets and suggests that the long-wavelength part of the CTF-1 absorption spectrum displays a pronounced effect of stacking. Red-shifted peaks in the absorption spectrum appear that are absent for an isolated sheet. We also show that the experimentally observed strong fluorescence of CTF-1 and other CTF materials is further evidence of the presence of rings in the layers, as structures without rings are predicted to have extremely long excited state lifetimes and hence would display negligible fluorescence intensities. Finally, subtle differences between the experimental absorption spectra of CTF-1 samples prepared using different synthesis routes are shown to potentially arise from different relative arrangements of stacked layers