Effect of donor-acceptor interaction strength on excitation energy migration and diffusion at high donor concentrations
ABSTRACT The migration and diffusion modulated excitation energy transfer has been studied in a new dye pair 7-diethylamino-4-methylcoumarin (donor) to 3,3'-dimethyloxacarbocyanine iodide (acceptor) by steady-state and picosecond time-resolved spectroscopy. To reduce the artifact of self-absorption, at high donor concentrations, the time-resolved studies have been carried out in thin films of polyvinyl alcohol (solid matrix) and in methanol (liquid phase) at front-face geometry of excitation. The Forster-type (nonradiative) energy transfer [Discuss. Faraday Soc. 27, 7 (1959)] takes place directly from donor to acceptor in case of solid matrix, while Yokota-Tanimoto model [J. Phys. Soc. Jpn. 22, 779 (1967)] for diffusion has been found to be operating in the liquid phase. It has been found here that the high interaction strength between donor and acceptor molecules as compared to that among donors masks the effect of energy migration and diffusion at high donor concentrations. The rate and efficiency of energy transfer increase with increasing the acceptor concentration. This has been confirmed by the study of acceptor kinetics.
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ABSTRACT: We present molecular simulation data for viscosity, self-diffusivity, and the local structural ordering of (i) a hard-sphere fluid and (ii) a square-well fluid with short-range attractions. The latter fluid exhibits a region of dynamic anomalies in its phase diagram, where its mobility increases upon isochoric cooling, which is found to be a subset of a larger region of structural anomalies, in which its pair correlations strengthen upon isochoric heating. This "cascade of anomalies" qualitatively resembles that found in recent simulations of liquid water. The results for the hard-sphere and square-well systems also show that the breakdown of the Stokes-Einstein relation upon supercooling occurs for conditions where viscosity and self-diffusivity develop different couplings to the degree of pairwise structural ordering of the liquid. We discuss how these couplings reflect dynamic heterogeneities. Finally, we note that the simulation data suggest how repulsive and attractive glasses may generally be characterized by two distinct levels of short-range structural order.The Journal of Chemical Physics 08/2007; 127(4):044502. DOI:10.1063/1.2753154 · 2.95 Impact Factor
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ABSTRACT: Two dye pairs suitable for energy transfer along with a photostable dye have been studied by using the closed-aperture (CA) Z-scan technique under pulsed and cw pumping. Here we use a theoretical model to elucidate the refractive and absorptive nonlinearity present simultaneously in the CA Z-scan profile. A separate open-aperture (OA) Z-scan study has been carried out to compare the nonlinear absorption parameters obtained from the CA Z-scan technique. At a fixed pumping wavelength, values of optical nonlinear parameters increase with the absorbance of dyes. It is found that the sign of refractive nonlinearity is dependent on the irradiance and the pulse width of the pump beam. In addition to the contribution of the third-order optical nonlinearity, various other mechanisms such as fifth-order nonlinearity, population relaxation to triplet states, and thermal effects are discussed here.Journal of the Optical Society of America B 09/2007; 24(9-Copyright (C) 2010 American Chemical Society (ACS). All Rights Reserved.):2147-2156. DOI:10.1364/josab.24.002147 · 1.97 Impact Factor
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ABSTRACT: We investigate by molecular dynamics simulations a continuous isotropic core-softened potential with attractive well in three dimensions, introduced by Franzese [J. Mol. Liq. 136, 267 (2007)], that displays liquid-liquid coexistence with a critical point and waterlike density anomaly. Besides the thermodynamic anomalies, here we find diffusion and structural anomalies. The anomalies, not observed in the discrete version of this model, occur with the same hierarchy that characterizes water. We discuss the differences in the anomalous behavior of the continuous and the discrete model in the framework of the excess entropy, calculated within the pair correlation approximation.The Journal of Chemical Physics 03/2008; 128(6):064901. DOI:10.1063/1.2830706 · 2.95 Impact Factor