Spin-Vibronic Intersystem Crossing and Molecular Packing Effects in Heavy Atom Free Organic Phosphor

We present a detailed investigation into the excited state properties of a planar D3h symmetric azatriangulenetrione, HTANGO, which has received significant interest due to its high solid-state phosphorescence quantum yield and therefore potential as an organic room temperature phosphorescent (ORTP) dye. Using a model linear vibronic coupling Hamiltonian in combination with quantum dynamics simulations, we observe that intersystem crossing (ISC) in HTANGO occurs with a rate of ∼1010 s−1, comparable to benzophenone, an archetypal molecule for fast ISC in heavy metal free molecules. Our simulations demonstrate that the mechanism for fast ISC is associated with the high density of excited triplet states which lie in close proximity to the lowest singlet states, offering multiple channels into the triplet manifold facilitating rapid population transfer. Finally, to rationalize the solid-state emission properties, we use quantum chemistry to investigate the excited state surfaces of the HTANGO dimer, highlighting the influence and importance of the rotational alignment between the two HTANGO molecules in the solid state and how this contributes to high phosphorescence quantum yield

The kinetics of surfactant desorption at the air–solution interface

The kinetics of desorption of the anionic surfactant sodium dodecylbenzene sulfonate at the air–solution interface have been studied using neutron reflectivity (NR). The experimental arrangement incorporates a novel flow cell in which the subphase can be exchanged (diluted) using a laminar flow whilst the surface region remains unaltered. The kinetics of the desorption is relatively slow and occurs over many tens of minutes compared with the dilution timescale of approximately 10–30 minutes. A detailed mathematical model, in which the rate of the desorption is determined by transport through a near-surface diffusion layer into a diluted bulk solution below, is developed and provides a good description of the timedependent adsorption data.\ud \ud A key parameter of the model is the ratio of the depth of the diffusion layer, Hc , to the depth of the fluid, Hf, and we find that this is related to the reduced Péclet number, Pe*, for the system, via Hc/Hf, = C/Pe* 1/ 2 . Although from a highly idealised experimental arrangement, the results provide an important insight into the ‘rinse mechanism’, which is applicable to a wide variety of domestic and industrial circumstances

Probing Spin-Vibronic Dynamics Using Femtosecond X-ray Spectroscopy

Ultrafast pump-probe spectroscopy within the X-ray regime is now possible owing to the devel- opment of X-ray Free Electrons Lasers (X-FELs) and are opening new opportunities for direct probing the correlated evolution of the nuclei, the electronic and spin degrees of freedom on the femtosecond timescale. In this contribution we use excited state wavepacket dynamics of the photoexcited decay of a new Fe(II) complex, [Fe(bmip)2]2+ (bmip=2,6-bis(3-methyl-imidazole- 1-ylidine)pyridine), to simulate the experimental observables associated with femtosecond Fe K- edge X-ray absorption near-edge structure (XANES) and X-ray emission (XES) spectra. We show how the evolution of the nuclear wavepacket is translated into the experimental observable and the sensitivity of these approaches for following excited state dynamics

Evidence for a Peierls phase-transition in a three-dimensional multiple charge-density waves solid

The effect of dimensionality on materials properties has become strikingly evident with the recent discovery of graphene. Charge ordering phenomena can be induced in one dimension by periodic distortions of a material's crystal structure, termed Peierls ordering transition. Charge-density waves can also be induced in solids by strong Coulomb repulsion between carriers, and at the extreme limit, Wigner predicted that crystallization itself can be induced in an electrons gas in free space close to the absolute zero of temperature. Similar phenomena are observed also in higher dimensions, but the microscopic description of the corresponding phase transition is often controversial, and remains an open field of research for fundamental physics. Here, we photoinduce the melting of the charge ordering in a complex three-dimensional solid and monitor the consequent charge redistribution by probing the optical response over a broad spectral range with ultrashort laser pulses. Although the photoinduced electronic temperature far exceeds the critical value, the charge-density wave is preserved until the lattice is sufficiently distorted to induce the phase transition. Combining this result with it ab initio} electronic structure calculations, we identified the Peierls origin of multiple charge-density waves in a three-dimensional system for the first time.Comment: Accepted for publication in Proc. Natl. Acad. Sci. US

Inverting the handedness of circularly polarized luminescence from light-emitting polymers using film thickness

The emission of circularly polarized light is central to many applications, including data storage, optical quantum computation, biosensing, environmental monitoring, and display technologies. An emerging method to induce (chiral) circularly polarized (CP) electroluminescence from the active layer of polymer light-emitting diodes (polymer OLEDs; PLEDs) involves blending achiral polymers with chiral small-molecule additives, where the handedness/sign of the CP light is controlled by the absolute stereochemistry of the small molecule. Through the in-depth study of such a system we report an interesting chiroptical property: the ability to tune the sign of CP light as a function of active layer thickness for a fixed enantiomer of the chiral additive. We demonstrate that it is possible to achieve both efficient (4.0 cd/A) and bright (8000 cd/m2) CP-PLEDs, with high dissymmetry of emission of both left-handed (LH) and right-handed (RH) light, depending on thickness (thin films, 110 nm: gEL = 0.51, thick films, 160 nm: gEL = -1.05, with the terms "thick" and "thin" representing the upper and lower limits of the thickness regime studied), for the same additive enantiomer. We propose that this arises due to an interplay between localized CP emission originating from molecular chirality and CP light amplification or inversion through a chiral medium. We link morphological, spectroscopic, and electronic characterization in thin films and devices with theoretical studies in an effort to determine the factors that underpin these observations. Through the control of active layer thickness and device architecture, this study provides insights into the mechanisms that result in CP luminescence and high performance from CP-PLEDs, as well as demonstrating new opportunities in CP photonic device design

Convergence and Perturbation Resilience of Dynamic String-Averaging Projection Methods

We consider the convex feasibility problem (CFP) in Hilbert space and concentrate on the study of string-averaging projection (SAP) methods for the CFP, analyzing their convergence and their perturbation resilience. In the past, SAP methods were formulated with a single predetermined set of strings and a single predetermined set of weights. Here we extend the scope of the family of SAP methods to allow iteration-index-dependent variable strings and weights and term such methods dynamic string-averaging projection (DSAP) methods. The bounded perturbation resilience of DSAP methods is relevant and important for their possible use in the framework of the recently developed superiorization heuristic methodology for constrained minimization problems.Comment: Computational Optimization and Applications, accepted for publicatio

One size fits all? Mixed methods evaluation of the impact of 100% single room accommodation on staff and patient experience, safety and costs

BACKGROUND AND OBJECTIVES: There is little strong evidence relating to the impact of single-room accommodation on healthcare quality and safety. We explore the impact of all single rooms on staff and patient experience; safety outcomes; and costs.METHODS: Mixed methods pre/post 'move' comparison within four nested case study wards in a single acute hospital with 100% single rooms; quasi-experimental before-and-after study with two control hospitals; analysis of capital and operational costs associated with single rooms.RESULTS: Two-thirds of patients expressed a preference for single rooms with comfort and control outweighing any disadvantages (sense of isolation) felt by some. Patients appreciated privacy, confidentiality and flexibility for visitors afforded by single rooms. Staff perceived improvements (patient comfort and confidentiality), but single rooms were worse for visibility, surveillance, teamwork, monitoring and keeping patients safe. Staff walking distances increased significantly post move. A temporary increase of falls and medication errors in one ward was likely to be associated with the need to adjust work patterns rather than associated with single rooms per se. We found no evidence that single rooms reduced infection rates. Building an all single-room hospital can cost 5% more with higher housekeeping and cleaning costs but the difference is marginal over time.CONCLUSIONS: Staff needed to adapt their working practices significantly and felt unprepared for new ways of working with potentially significant implications for the nature of teamwork in the longer term. Staff preference remained for a mix of single rooms and bays. Patients preferred single rooms.<br/

Using digital and hand printing techniques to compensate for loss: re-establishing colour and texture in historic textiles

Conservators use a range of 'gap filling' techniques to improve the structural stability and presentation of objects. Textile conservators often use fabric supports to provide reinforcement for weak areas of a textile and to provide a visual infill in missing areas. The most common technique is to use dyed fabrics of a single colour but while a plain dyed support provides good reinforcement, it can be visually obtrusive when used with patterned or textured textiles. Two recent postgraduate dissertation projects at the Textile Conservation Centre (TCC) have experimented with hand printing and digital imaging techniques to alter the appearance of support fabrics so that they are less visually obtrusive and blend well with the colour and texture of the textile being supported. Case studies demonstrate the successful use of these techniques on a painted hessian rocking horse and a knitted glove from an archaeological context

A Quantum Dynamics Study of the Ultrafast Relaxation in a Prototypical Cu(I)-Phenanthroline

The ultrafast nonadiabatic dynamics of a prototypical Cu(I)phenanthroline complex, [Cu(dmp)(2)](+) (dmp = 2,9-dimethyl-1,10-phenanthroline), initiated after photoexcitation into the optically bright metal-to-ligand charge-transfer (MLCT) state (S-3) is investigated using quantum nuclear dynamics. In agreement with recent experimental conclusions, we find that the system undergoes rapid (similar to 100 fs) internal conversion from S-3 into the S-2 and S-1 states at or near the FranckCondon (FC) geometry. This is preceded by a dynamic component with a time constant of similar to 400 fs, which corresponds to the flattening of the ligands associated with the pseudo JahnTeller distortion. Importantly, our simulations demonstrate that this latter aspect is in competition with subpicosecond intersystem crossing (ISC). The mechanism for ISC is shown to be a dynamic effect, in the sense that it arises from the system traversing the pseudo JahnTeller coordinate where the singlet and triplet states become degenerate, leading to efficient crossing. These first-principles quantum dynamics simulations, in conjunction with recent experiments, allow us to clearly resolve the mechanistic details of the ultrafast dynamics within [Cu(dmp)(2)](+), which have been disputed in the literature