Maintaining high incident light absorption while minimizing luminescence reabsorption is a key challenge for organic luminescent solar concentrators (LSCs). Energy migration and trapping using light-harvesting donors and a low-energy highly emitting acceptor is one strategy to reduce the reabsorption issue. However, concentration quenching and the potential formation of quenching aggregates is a limiting factor in realizing efficient devices. We describe the synthesis of a novel molecularly insulated perylene diimide that can resist luminescence quenching at concentrations in excess of 50 mM. Photophysical measurements show the insulated perylene diimide has an excitation energy migration diffusion length of 230 ± 10 Å at 60 mM in poly(methyl methacrylate). LSC devices using a mixture of the insulated perylene diimide light absorber and perylene red (LR305) as the low-energy trap emitter exhibit reduced reabsorption and a better current output than LR305 only devices. The results demonstrate that appropriately designed organic molecule dyes can potentially meet the stringent requirements required for efficient LSCs. S tationary light concentration without external cooling can be realized using luminescent solar concentrators (LSCs), making these devices attractive for building integrated photovoltaic devices. A typical LSC consists of large sheets of plastic or glass containing luminescent molecules that absorb the solar spectrum and then re-emit the absorbed energy into a waveguide mode that directs the luminescence to the thin edges of the concentrator. Edge-mounted solar cells can then harvest the concentrated luminescence for photoelectric conversion. Reabsorption of the emitted light in the waveguide limits the achievable light concentration contributing to parasitic losses such as re-emission into the escape cone or nonunity fluorescence quantum efficiency due to competing nonradiative processes. Separation of both the energy of the peak maxima of absorption and emission (the Stokes shift) and tail absorption of the chromophores in an LSC is crucial in reducing reabsorption. 1,2 Organic solar concentrators that use diluted organic chromophores in inert host matrices are lagging behind their inorganic counterparts due to their small Stokes shifts. 3 A key advantage of organic chromophores over inorganic materials is their solubility in inexpensive host matrices or waveguides, such as poly(methyl methacrylate) (PMMA) or glass, without any complicated processing and the ability to fine-tune their properties by simple molecular engineering approaches. This is particularly important for thin-film LSCs where the concentration required to achieve total light absorption over the wavelength range of the chromophore can be high ( Figure 1).Modification of dyes to achieve a large Stokes shift has achieved only limited success for LSC applications. In some cases, increasing the Stokes shift of the dye has led to a decreased quantum yield and there often remains the long tail absorption of the dye. 4 Energy transfer ...

The entropy of hydrophobic solvation has been explained as the result of ordered solvation structures, of hydrogen bonds, of the small size of the water molecule, of dispersion forces, and of solvent density fluctuations. We report a new approach to the calculation of the entropy of hydrophobic solvation, along with tests of and comparisons to several other methods. The methods are assessed in the light of the available thermodynamic and spectroscopic information on the effects of temperature on hydrophobic solvation. Five model hydrophobes in SPC/E water give benchmark solvation entropies via Widom's test-particle insertion method, and other methods and models are tested against these particle-insertion results. Entropies associated with distributions of tetrahedral order, of electric field, and of solvent dipole orientations are examined. We find these contributions are small compared to the benchmark particle-insertion entropy. Competitive with or better than other theories in accuracy, but with no free parameters, is the new estimate of the entropy contributed by correlations between dipole moments. Dipole correlations account for most of the hydrophobic solvation entropy for all models studied and capture the distinctive temperature dependence seen in thermodynamic and spectroscopic experiments. Entropies based on pair and many-body correlations in number density approach the correct magnitudes but fail to describe temperature and size dependences, respectively. Hydrogen-bond definitions and free energies that best reproduce entropies from simulations are reported, but it is difficult to choose one hydrogen bond model that fits a variety of experiments. The use of information theory, scaled-particle theory, and related methods is discussed briefly. Our results provide a test of the Frank-Evans hypothesis that the negative solvation entropy is due to structured water near the solute, complement the spectroscopic detection of that solvation structure by identifying the structural feature responsible for the entropy change, and point to a possible explanation for the observed dependence on length scale. Our key results are that the hydrophobic effect, i.e. the signature, temperature-dependent, solvation entropy of nonpolar molecules in water, is largely due to a dispersion force arising from correlations between rotating permanent dipole

MgO sorbents doped with alkali-metal carbonates have been shown to exhibit improved sorption capacities at both low and moderate operating temperatures, compared to pure MgO sorbents. However, the mechanism of how alkali-metal carbonates enhance the CO2 sorption is not well understood. Using in situ X-ray diffraction, TEM, and thermogravimetric analysis, we have shown that the cesium dopants in the Cs2CO3-doped MgO nanoparticle system do not simply act as a promoter in sorption of CO2, but rather as a reagent alongside MgO. A new mixed magnesium–cesium carbonate phase has been found to be responsible for the improved sorption capacity of the doped-MgO-based sorbents. On the basis of our findings, it is suggested that a higher sorption capacity may be achieved if cesium is uniformly dispersed throughout the MgO particles.

We examine three possible explanations for the millisecond relaxation time of the dynamic surface tension of water: the diffusion of surfactant contaminants from the aqueous phase to the surface, the reorientation of surface water molecules' dipole moments, and the buildup of a charged surface layer of hydroxide ions. The relaxation time expected for hydroxide is by far the closest to the measured time. Our model for the surface layer agrees with static equilibrium experiments and, as we show here, predicts the relaxation time. The results strongly imply that the equilibrium surface of water is highly charged by a flow of hydroxide to the surface. The model predicts that neither diffusion nor autolysis dominates and shows that both processes are needed to describe the relaxation of the surface tension. We find a salt and pH dependence of the relaxation time and propose further experiments.