Ben D. Allen

Selective Triplet‐State Formation during Charge Recombination in a Fullerene/Bodipy Molecular Dyad (Bodipy=Borondipyrromethene)

A conformationally restricted molecular dyad has been synthesized and subjected to detailed photophysical examination. The dyad comprises a borondipyrromethene (Bodipy) dye covalently linked to a buckminsterfullerene C60 residue, and is equipped with hexadecyne units at the boron centre in order to assist solubility. The linkage consists of a diphenyltolane, attached at the meso position of the Bodipy core and through an N-methylpyrrolidine ring at the C60 surface. Triplet states localised on the two terminals are essentially isoenergetic. Cyclic voltammetry indicates that light-induced electron transfer from Bodipy to C60 is thermodynamically favourable and could compete with intramolecular energy transfer in the same direction. The driving force for light-induced electron abstraction from Bodipy by the singlet excited state of C60 depends critically on the solvent polarity. Thus, in non-polar solvents, light-induced electron transfer is thermodynamically uphill, but fast excitation energy transfer occurs from Bodipy to C60 and is followed by intersystem crossing and subsequent equilibration of the two triplet excited states. Moving to a polar solvent switches on light-induced electron transfer. Now, in benzonitrile, the charge-transfer state (CTS) is positioned slightly below the triplet levels, such that charge recombination restores the ground state. However, in CH2Cl2 or methyltetrahydrofuran, the CTS is slightly higher in energy than the triplet levels, and decays, in part, to form the triplet state localized on the C60 residue. This step is highly specific and does not result in direct formation of the triplet excited state localized on the Bodipy unit. Subsequent equilibration of the two triplets takes place on a relatively slow timescale.

The photophysical properties of a julolidene-based molecular rotor

The photophysical properties of 9-dicyanovinyljulolidine are sensitive to solvent viscosity but are little affected by changes in polarity. In fluid solution, the lifetime of the first-excited singlet state is very short and triplet state formation cannot be detected by laser flash photolysis. Decay of the excited singlet state is strongly activated and weak phosphorescence can be observed in a glassy matrix at 77 K. Temperature dependent 1H NMR studies indicate that the molecule undergoes slow internal rotation in solution, for which the activation energy has a value of ca. 35 kJ mol(-1). This process is unlikely to account for the poor fluorescence quantum yield found in fluid solution. Instead, it is considered that the target compound undergoes rapid rotation around the dicyanovinyl double bond from the excited singlet state. The rate of rotation depends weakly on the viscosity of the solvent in a range of linear alcohols at room temperature. This might represent the fact that the rotor is relatively small and can pack into cavities in the solvent structure. In glycerol, the rate of rotation is more sensitive to viscosity effects but a quite complex temperature dependence is observed in ethanol. Here, the rate is almost activationless in a glassy matrix and in fluid solution at high temperature but strongly activated at intermediate temperatures.

Comparison of a cost-effective virtual cloud cluster with an existing campus cluster

The Cloud provides impartial access to computer services on a pay-per-use basis, a fact that has encouraged many researchers to adopt the Cloud for the processing of large computational jobs and data storage. It has been used in the past for single research endeavours or as a mechanism for coping with excessive load on conventional computational resources (clusters). In this paper we investigate, through the use of simulation, the applicability of running an entire computer cluster on the Cloud. We investigate a number of policy decisions which can be applied to such a virtual cluster to reduce the running cost and the effect these policies have on the users of the cluster. We go further to compare the cost of running the same workload both on the Cloud and on an existing campus cluster of non-dedicated resources. We simulate a Cluster computer running on the Cloud.We compare this to a Cluster running on Campus.We show that the cost of running a Cloud Cluster is inversely related to the make-span of work on the cluster.We compare the cost of using Cloud vs local clusters.

A mechanistic individual-based model of microbial communities

Accurate predictive modelling of the growth of microbial communities requires the credible representation of the interactions of biological, chemical and mechanical processes. However, although biological and chemical processes are represented in a number of Individual-based Models (IbMs) the interaction of growth and mechanics is limited. Conversely, there are mechanically sophisticated IbMs with only elementary biology and chemistry. This study focuses on addressing these limitations by developing a flexible IbM that can robustly combine the biological, chemical and physical processes that dictate the emergent properties of a wide range of bacterial communities. This IbM is developed by creating a microbiological adaptation of the open source Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). This innovation should provide the basis for “bottom up” prediction of the emergent behaviour of entire microbial systems. In the model presented here, bacterial growth, division, decay, mechanical contact among bacterial cells, and adhesion between the bacteria and extracellular polymeric substances are incorporated. In addition, fluid-bacteria interaction is implemented to simulate biofilm deformation and erosion. The model predicts that the surface morphology of biofilms becomes smoother with increased nutrient concentration, which agrees well with previous literature. In addition, the results show that increased shear rate results in smoother and more compact biofilms. The model can also predict shear rate dependent biofilm deformation, erosion, streamer formation and breakup.

Accessing molecular memory via a disulfide switch

Herein we describe the results of a combined theoretical and spectroscopic investigation into the design of a simple molecular system intended to act as a memory storage bank. The main operating principle revolves around the two-electron reduction of an aryl disulfide bond. Addition of the first electron leads to elongation of the S–S bond but it breaks only if there is accompanying protonation. Adding a second electron causes S–S bond cleavage, with or without protonation. The structural changes have been assessed by way of quantum chemical calculations and molecular dynamics simulations. Electrochemical studies show that the two-electron reduced product can be re-oxidised at mildly anodic potentials and the cycle can be repeated many times. Both theory and experiment point towards pronounced potential inversion whereby the second reduction potential lies at a significantly more positive potential than that for the first step. Computer simulations of the cyclic voltammograms give rise to numerical values for the reduction potentials that are in quite good agreement with the computed values and also allow determination of the electrochemical rate constants and transfer coefficients. Accurate simulation of the experimental data can be realised only if one proton accompanies the second reduction step. The possibility to design an effective molecular-scale memory device around this system is discussed briefly.