Ross Brown

Molecular dynamics in n-alkanes: Premelting phenomena and rotator phases

Molecular dynamics simulations of the n-alkanes C18H38, C19H40, and C20H42 are reported for temperatures just below the melting point. Besides thermodynamic and average structural data for the ordered phase, we discuss the molecular motions initiating the rotator phases observed in spontaneous phase transitions in isothermal, isostress simulations. The RI phase of C19H40 is initiated by particular cork-screw-like jumps combining a quarter turn about the long molecular axis and a half-chain-period translation along the axis. This motion occurs between the minimum-energy conformation of the ordered crystal and a secondary minimum. Transient analogs of the RI and RII phases of the odd alkanes are found on melting C18H38 and C20H42. Collective motions within lamellae of molecules are prominent in the dynamics.

Enhanced thermal stability of organic solar cells by using photolinkable end-capped polythiophenes

The use of poly(3-hexylthiophene) (P3HT) end-capped with anthracene (P3HT-A) in a blend with [6,6]-phenyl C61 butyric acid methyl ester (PCBM) is demonstrated to physically stabilize bulk-heterojunction photovoltaic solar cells. Bulk heterojunction-based devices are known to undergo phase separation of donor and acceptor materials during operation resulting in the formation of large (μm scale) PCBM crystals that dramatically decrease the photovoltaic characteristics of the cell. By way of a facile UV-curing step, the P3HT-A chain most likely reacts with PCBM fullerene via a [2+2] cyclo-addition to stabilize the blend. Photovoltaic devices based on P3HT-A and PCBM have been optimised in terms of thermal annealing to obtain initial devices to determine the UV-curing protocols. UV-exposure was found to improve device stability while simultaneously having a minimal effect on device efficiency. Optical microscopy demonstrates that the few reactions of P3HT-A with PCBM are efficient enough to prevent the formation of micro-sized PCBM crystals responsible for the failure of solar cells.

A new cyanoaromatic photosensitizer vs. 9,10-dicyanoanthracene: systematic comparison of the photophysical properties

The cyanoanthracene derivative, benzo[b]triphenylene-9,14-dicarbonitrile (1) can be prepared readily with a graftable function while maintaining (1)O(2) photosensitizing properties comparable to those of the standard compound 9,10-dicyanoanthracene (DCA). In view of the high potential of the derivatives of 1 for photooxidation reactions under heterogeneous conditions, we compared the photophysical properties of 1 in solution with those of DCA. In pursuing the comparison of 1 and DCA, we observed small but significant changes of the vibronic bands in the electronic absorption spectra of DCA in different solvents, which were well correlated with solvent polarity, similar to the pyrene polarity scale. The main difference between 1 and DCA is in the emission properties: we observed a much stronger sensitivity of the fluorescence emission spectrum to the electron-donating ability of the solvent than for DCA. The emission spectrum of 1 is in general structureless with a large Stokes shift. The ability of the singlet state of 1 to participate in charge transfer interactions with electron-donating solvents is proposed to account for these results. It makes 1 a highly sensitive probe to the surrounding medium. Reversible reduction was observed for both photosensitizers, with a small shift to more negative potentials for 1 compared to DCA. The reduction potential of the first singlet excited state is of the same order of magnitude in both cases. Several photo-oxidation reactions sensitized by 1 and DCA are compared in homogeneous solution and at the gas-solid interface by embedding 1 and DCA in silica monoliths. Our results confirmed the dual character of both cyanoanthracene derivatives as electron transfer and energy transfer sensitizers, highly efficient for singlet oxygen production.

Structure and molecular dynamics of crystalline and liquid anthracene and naphthalene: Possible transient rotator phase of naphthalene

We examine the structural and dynamical properties of the crystal and liquid states of anthracene and naphthalene, with special attention to melting. This molecular dynamics study is based on an all-atom force field, which we optimized for simulations of solid and liquid anthracene, over wide ranges of temperature and pressure. The force field is shown to be transferable to naphthalene. Local ordering of the simulated liquids is in fair agreement with structures deduced in the literature from X-ray scattering, while providing a much more detailed picture. In analogy with the rich polymorphism of substituted benzene and naphthalene complexes, we find for naphthalene (but not anthracene) a two-step melting process with a transient rotator phase in which rotational jumps precede the onset of full melting with translational diffusion.

Molecular simulation of solid–solid phase transitions

Applications of dl_poly to solid–solid phase transitions are reviewed, with particular attention to how details of the mechanisms of the transitions may be extracted from molecular dynamics simulations. Two examples in molecular crystals are discussed: the order–disorder transition of p-terphenyl initiated at around 200 K by the unlocking of ring flipping; and the rotator phases of n-alkanes with around 20 carbon atoms per chain, showing distinct molecular mechanisms in the dynamics just below the melting points of odd and even chains. Covalent-ionic materials are represented by an application to an aluminophophate molecular sieve, AlPO4-5.

Interpretation of spectral broadening and clustering of a pyrene derivative adsorbed on silica gels

This paper discusses a mechanism of broadening of the UV absorption spectra of an adsorbed species in a microporous material, with specific reference to a pyrene derivative adsorbed on silica gels [Lacombe et al. Micropororous Mesopororous Mater., 46 (2001) 311]. We show that broadening by resonance exchange interactions between clustered molecules explains the novel qualitative features of the data. The influences of pore size and adsorbate concentration are explored in a simple model of molecules adsorbed randomly on spherical pores. Absorption spectra are calculated from a tight binding Hamiltonian for the excited state, including inhomogeneous broadening and resonance exchange. Quantitative differences between the model and the experimental spectra show that spectral broadening is due to non-random clustering in the ground state of the adsorbate molecules.