Energy Migration (energy + migration)

Distribution by Scientific Domains

Selected Abstracts

Molecular Origin of the Temperature-Dependent Energy Migration in a Rigid-Rod Ladder-Phenylene Molecular Host,

H. Wiesenhofer
Excitation diffusion is studied in a molecular host doped with a luminescent guest. An atomistic model based on the coupling of the electronic excitations to low-frequency intramolecular vibrations reproduces remarkably well the measured temperature-dependent host-to-guest energy transfer efficiency (see Figure). [source]

Formation of Highly Luminescent Supramolecular Architectures Possessing Columnar Order from Octupolar Oxadiazole Derivatives: Hierarchical Self-Assembly from Nanospheres to Fibrous Gels,

Shinto Varghese
Abstract The synthesis and study of the liquid crystalline, photophysical, and aggregation behavior of novel octupolar oxadiazole derivatives are reported. These molecules formed columnar mesophases at elevated temperatures which transformed into a glassy state at ambient temperatures wherein the columnar order was retained. Their spontaneous concentration dependent hierarchical self-assembly from spheres to fibrous gels has been investigated using TEM, SEM, and XRD. Retention of the hexagonal columnar (Colh) order was also observed in the fibrous aggregates. Concentration dependent luminescence spectral studies indicated that the change in morphology from spheres to fibrous aggregates was associated with a shift in chromophore packing from predominantly H -type to J -type aggregates. Time resolved anisotropic investigations revealed that the columnar stacking of molecules in the aggregated state provided a pathway for excitation energy migration to the lower energy J -traps. [source]

On the Origin of Green Emission in Polyfluorene Polymers: The Roles of Thermal Oxidation Degradation and Crosslinking,

W. Zhao
Abstract The green emission of poly(9,9,,-dioctylfluorenyl-2,7,,-diyl), end-capped by polyhedral oligomeric silsequioxanes, (PFO-POSS) has been investigated by photoluminescence (PL) and photoexcitation (PE), gel permeation chromatography (GPC), and transmission Fourier transform infrared (FTIR) spectroscopy. The green emission is closely correlated with thermal oxidation degradation and crosslinking of the polymer and is enhanced by annealing at elevated temperatures. The green-to-blue emission intensity ratio, used to assess the emission properties of thin (90,nm) films, was 3.70, 4.35, and 1.54 for an air-annealed film, its insoluble residue (crosslinked), and a film cast from its soluble portion, respectively. For thick (5,6,,m) film, the ratios are 13.33, 13.33, and 0.79, respectively. However, FTIR spectroscopy of thick films leads to the conclusion that the carbonyl-to-aromatic ring concentration ratio are 0.018, 0.015, and 0.032, respectively. Focusing on the recast films, the green emission is relatively low while the carbonyl concentration is relatively high. This suggests that the energy traps at crosslinked chains play an important role in green emission. It is likely that the crosslinking enhances the excitation energy migration and energy transfer to the defects by hindering chain segment twisting. [source]

RGB Emission through Controlled Donor Self-Assembly and Modulation of Excitation Energy Transfer: A Novel Strategy to White-Light-Emitting Organogels

Chakkooth Vijayakumar
A white-light-emitting organogel is designed by the controlled self-assembly of a bischolesterol-functionalized OPV donor that allows slow energy migration and partial energy transfer in the gel state to an encapsulated acceptor. The white-light emission occurs because of the combination of blue-light emission from the OPV monomers, green-light emission from the OPV self-assembly, and red-light emission from the acceptor. [source]

Unusual Photoinduced Response of mTHPC Liposomal Formulation (Foslip)

Dzmitry Kachatkou
Liposomal formulations of meso-tetra(hydroxyphenyl)chlorin (mTHPC) have already been proposed with the aim to optimize photodynamic therapy. Spectral modifications of these compounds upon irradiation have not yet been investigated. The objective of this study was to evaluate photobleaching properties of mTHPC encapsulated into dipalmitoylphosphatidylcholine (DPPC) liposomes, Foslip. Fluorescence measurements in DPPC liposomes with different DPPC:mTHPC ratios demonstrated a dramatic decrease in fluorescence anisotropy with increasing local mTHPC concentration, thus suggesting strong interactions between mTHPC molecules in lipid bulk medium. Exposure of Foslip suspensions to small light doses (<50 mJ/cm2) resulted in a substantial drop in fluorescence, which, however, was restored after addition to the sample of a non-ionic surfactant Triton X-100. We attributed this behavior to photoinduced fluorescence quenching. This effect depended strongly on the molar DPPC:mTHPC ratio and was revealed only for high local mTHPC concentrations. The results were interpreted supposing energy migration between closely located mTHPC molecules with its subsequent dissipation by the molecules of photoproduct acting as excitation energy traps. We further assessed the effect of photoinduced quenching in plasma protein solution. Relatively slow kinetics of photoinduced Foslip response during incubation in the presence of proteins was attributed to mTHPC redistribution from liposomal formulations to proteins. Therefore, changes in mTHPC distribution pattern in biological systems would be consistent with changes in photoinduced quenching and would provide valuable information on mTHPC interactions with a biological environment. [source]

Time-of-flight studies of secondary ions produced by 400,eV He+ ion impact on Ar, Kr, and Xe thin films at 8 K

Kenzo Hiraoka
Secondary ions produced by 400,eV He+ ion impact on Ar, Kr, and Xe thin films deposited on a silicon substrate at 8,K were measured as a function of film thickness using a time-of-flight secondary-ion mass spectrometer. For Ar, the cluster ions Arn+ up to n,=,4 as well as the monomer ion (n,=,1) were observed and they showed characteristic film-thickness dependence with increase of the film thickness up to ,200 monolayers. This is due to the momentum transfer in the elastic collision between the incident He+ ion and the matrix Ar atoms and also to the relaxation of electronic excitations (e.g., holes and excitons) to phonons resulting in the film erosion. In contrast, neither dimer nor cluster ions were detected for solid Kr and Xe films. This is due to the less efficient momentum transfer in the elastic collision between He+ and Rg's (Rg,=,Kr and Xe) and also to the efficient electronic energy migration in solid Kr and Xe. The ions originating from the silicon substrate such as Si+, SiCH3+, SiOH+, and C+ were found to be sensitized by the deposition of Xe film in the range of 0,40 monolayers. The penetration depths of the primary ion He+ through the rare gas films increase in the order Ar,<,Kr,<,Xe. Copyright 2002 John Wiley & Sons, Ltd. [source]