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Exciton Migration (exciton + migration)
Selected AbstractsExciton Migration in Conjugated Dendrimers: A Joint Experimental and Theoretical StudyCHEMPHYSCHEM, Issue 18 2009Muhammet E. Köse Prof. Abstract We report a joint experimental and theoretical investigation of exciton diffusion in phenyl-cored thiophene dendrimers. Experimental exciton diffusion lengths of the dendrimers vary between 8 and 17 nm, increasing with the size of the dendrimer. A theoretical methodology is developed to estimate exciton diffusion lengths for conjugated small molecules in a simulated amorphous film. The theoretical approach exploits Fermi's Golden Rule to estimate the energy transfer rates for a large ensemble of bimolecular complexes in random relative orientations. Utilization of Poisson's equation in the evaluation of the Coulomb integral leads to very efficient calculation of excitonic couplings between the donor and the acceptor chromophores. Electronic coupling calculations with delocalized transition densities revealed efficient coupling pathways in the bulk of the material, but do not result in strong couplings between the chromophores which are calculated for more localized transition densities. The molecular structures of dendrimers seem to be playing a significant role in the magnitude of electronic coupling between chromophores. Simulated diffusion lengths correlate well with the experimental data. The chemical structure of the chromophore, the shape of the transition densities and the exciton lifetime are found to be the most important factors in determining the size of the exciton diffusion length in amorphous films of conjugated materials. [source] Ionic Iridium(III) Complexes with Bulky Side Groups for Use in Light Emitting Cells: Reduction of Concentration QuenchingADVANCED FUNCTIONAL MATERIALS, Issue 13 2009Carsten Rothe Abstract Here, the photophysics and performance of single-layer light emitting cells (LECs) based on a series of ionic cyclometalated Ir(III) complexes of formulae and where ppy, bpy, and phen are 2-phenylpyridine, substituted bipyridine and substituted phenanthroline ligands, respectively, are reported. Substitution at the N,N ligand has little effect on the emitting metal-ligand to ligand charge-transfer (MLLCT) states and functionalization at this site of the complex leads to only modest changes in emission color. For the more bulky complexes the increase in intermolecular separation leads to reduced exciton migration, which in turn, by suppressing concentration quenching, significantly increases the lifetime of the excited state. On the other hand, the larger intermolecular separation induced by bulky ligands reduces the charge carrier mobility of the materials, which means that higher bias fields are needed to drive the diodes. A brightness of ca. 1000,cd,m,2 at 3,V is obtained for complex 5, which demonstrates a beneficial effect of bulky substituents. [source] Competition between Host Aggregates and Isolated Guest Chromophores in Trapping Excitons in Polybenzazole Copolymers and BlendsMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 24 2006Shanfeng Wang Abstract Summary: Host,guest systems have been prepared using the blends and copolymers consisting of the host molecule poly(p -phenylene benzobisoxazole) (PBO) with a higher bandgap of 2.93 eV and the guest molecule poly(2,5-thienylbenzobisoxazole) (PBOT) with a lower bandgap of 2.57 eV. These systems have been investigated using photoluminescence (PL) spectra and time-resolved PL decay dynamics. Both PBOT-PBO copolymers and PBOT/PBO blends with the PBOT compositions less than 20% demonstrate higher intensities and narrower bandwidths in solid-state emission compared to that of PBOT, as well as larger fractions of the shorter lifetime component in PL decay dynamics. A general scheme on intrachain and interchain exciton migration and trapping mechanism has been proposed to interpret the phenomena in both solutions and thin films. Particularly, a competition in trapping exciton between PBO aggregates and isolated PBOT chromophores has been revealed. General scheme of exciton migration and trapping paths for host (D: donor),guest (A: acceptor) systems involving the isolated chromophores (A and D) and aggregates (AA and DD) in both ground state and excited state marked without or with an asterisk. [source] Effect of PCBM Concentration on Photoluminescence Properties of Composite MEH-PPV/PCBM Nanoparticles Investigated by a Franck,Condon Analysis of Single-Particle Emission SpectraCHEMPHYSCHEM, Issue 14 2009Daeri Tenery Dr. Abstract The emission of composite conjugated polymer (MEH-PPV)/fullerene (PCBM) nanoparticles is investigated by single particle spectroscopy (SPS), and changes in vibronic structure with nanoparticle composition are evaluated by means of a detailed Franck,Condon analysis. Consistent with previous reports we find that the emission spectra can be modeled as the superposition of two types of emitters, one with aggregate character and one with molecular character. Major findings from the fitting of the SPS data to a Franck,Condon model are that 1) the occurrence of each of the two types of emitters changes with nanoparticle composition to the point that no aggregate emitters are detected (at 50 wt,% PCBM), 2) at the highest PCBM doping levels (75 wt,% PCBM) aggregate emitters reappear due to nanoscale phase separation in the composite nanoparticles, 3) the molecular emitters show small Huang,Rhys factors that increase with PCBM doping, indicative of extensive delocalization and exciton migration that is reduced by the disorder introduced in the polymer material by PCBM doping and 4) the aggregate emitters show large Huang,Rhys factors, indicative of the localized nature of these energy trap sites, with a broad distribution of values of these Huang,Rhys factors. The latter observation suggests a broad heterogeneous distribution of aggregate morphologies in blended conducting polymer materials, which can be attributed to variations in polymer chain folding and stacking at the aggregate sites. The reported results obtained by the SPS approach show how blending conjugated polymers with fullerenes at various doping levels induces changes in interchain interactions and aggregate site density even at length scales below a few tens of nanometers that affect conjugated polymer material properties, an observation that has gone unnoticed in bulk studies of blended conjugated polymer films. [source] |