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

Excited state localisation cascades in inorganic semiconductor nanoparticles

Excited state relaxation in zinc sulfide (ZnS) nanoparticles is studied as a model for the fate of the excited state in inorganic nanoparticles in general. A series of time-dependent density functional theory optimisations on the S1 and T1 excited states predict the existence of not merely isolated minima, as found before, but rather a connected cascade of excited state minima ending up in a conical intersection between the excited state energy surface and the ground state. The localisation of the excited state in the different minima increases down the cascade, while the barriers separating these minima, studied here for the first time for nanoparticles, are predicted to be in some cases electronic (strongly avoided crossing) in origin. The cartoon picture of excited state relaxation in inorganic nanoparticles that involves relaxation to the bottom of only one approximately harmonic well followed by photoluminescence appears for the ZnS nanoparticles studied here to be at best rather simplistic. The localisation cascade is finally found to strongly affect the excited state properties of nanoparticles and predicted to lead to the formation of defected nanoparticles after de-excitation in selected cases

Exploration of the Photocatalytic Cycle for Sacrificial Hydrogen Evolution by Conjugated Polymers Containing Heteroatoms

We analyze the photocatalytic activity of heteroatom containing linear conjugated polymers for sacrificial hydrogen evolution using a recently proposed photocatalytic cycle. We find that the thermodynamic barrier to electron transfer, relevant both in the presence and absence of noble metal co-catalysts, changes with polymer composition, reducing upon going from electron-rich to electron-poor polymers, and disappearing completely for the most electron-poor polymers in a water rich environment. We discuss how the latter is probably the reason why electron-poor polymers are generally more active for sacrificial hydrogen evolution than their electron-rich counterparts. We also study the barrier to hydrogen-hydrogen bond formation on the polymer rather than the co-catalyst and find that it too changes with composition but is always, at least for the polymer studied here, much larger than that experimentally reported for platinum. Therefore, it is expected that in the presence of any noble metal particles these will act as the site of hydrogen evolution

Optical excitation of MgO nanoparticles:a computational perspective

The optical absorption spectra of magnesium oxide (MgO) nanoparticles, along with the atomic centres responsible, are studied using a combination of time-dependent density functional theory (TD-DFT) and coupled-cluster methods. We demonstrate that TD-DFT calculations on MgO nanoparticles require the use of range-separated exchange–correlation (XC-) functionals or hybrid XC-functionals with a high percentage of Hartree–Fock like exchange to circumvent problems related to the description of charge-transfer excitations. Furthermore, we show that the vertical excitations responsible for the experimentally studied range of the spectra of the MgO nanoparticles typically involve both 3-coordinated corner sites and 4-coordinated edge sites. We argue therefore that to label peaks in these absorption spectra exclusively as either corner or edge features does not provide insight into the full physical picture

Coupled cluster calculations on TiO2 nanoclusters

The excitation energies of the four lowest-lying singlet excited states of the TiO2, Ti2O4, and Ti3O6 clusters are calculated by a variety of different Equation-of-Motion Coupled Cluster (EOM-CC) approaches in order to obtain benchmark values for the optical excitations of titanium dioxide clusters. More specifically we investigate what the effect is of the inclusion of triple excitations “triples” in the (EOM-)CC scheme on the calculated excited states of the clusters. While for the monomer and dimer the inclusion of triples is found to only cause a rigid shift in the excitation energies, in the case of the trimer the crossing of the excited states is observed. Coupled cluster approaches where triples are treated perturbatively were found to offer no advantage over EOM-CCSD, whereas the active-space methods (EOM-CCSDt(II/I)) were demonstrated to yield results very close to full EOM-CCSDT, but at a much reduced computational cost

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

Polymer photocatalysts have shown potential as 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 its 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

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 rheology 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

Shining a Light on s-Triazine-Based Polymers

The strong interplay between the structure and optical properties of conjugated s-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

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

Hydrogen evolution from water using heteroatom substituted fluorene conjugated co-polymers

The photocatalytic performance of fluorene-type polymer photocatalysts for hydrogen production from water in the presence of a sacrificial hole scavenger is significantly improved by the incorporation of heteroatoms into the...</p