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Excimer Formation (excimer + formation)

Distribution by Scientific Domains

Chemistry	64%
Polymers and Materials Science	21%

Selected Abstracts

Oligothiophenes Nano-organized on a Cyclotetrasiloxane Scaffold as a Model of a Silica-Bound Monolayer: Evidence for Intramolecular Excimer Formation

CHEMISTRY - A EUROPEAN JOURNAL, Issue 46 2009
Wojciech Mróz
Abstract Excimer formation in a new class of terthiophene-based fluorophores covalently bonded to a cyclotetrasiloxane scaffold has been demonstrated and the photophysical process ruling it has been investigated in detail and modeled theoretically. In contrast to the conventional systems in which long-living fluorophores such as pyrene are linked in the same molecule, an excimer is formed only when two terthiophene-based branches nano-organized on the same cyclotetrasiloxane scaffold are close enough together when excitation takes place. In such a case, excimer formation is extremely efficient, and the new bound excited states are quite stable. [source]

Complexation and Dynamic Switching Properties of Fluorophore-Appended Resorcin[4]arene Cavitands

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 5 2010
Laura D. Shirtcliff
Abstract Fluorophore-appended resorcin[4]arene-based cavitands having pyrene (2) and anthracene (3) moieties attached to the rims were prepared by short synthetic routes. Both undergo reversible temperature- and acid- (CF3COOD) induced vase,,,kite switching as evidenced by 1H NMR spectroscopy. The 1H NMR spectra also suggest that suitably sized solvents, such as [D8]toluene, efficiently solvate the cavity, reducing the conformational flexibility. In [D12]mesitylene, both cavitands undergo remarkably stable host-guest inclusion complexation with cycloalkanes. The larger cavity of 3 preferentially hosts cyclohexane, whereas the smaller cavity of 2 forms the most stable complex with cyclopentane. The propensity for the cavitands to facilitate ,,, stacking between the chromophores was confirmed by both 1H NMR and fluorescence spectroscopy. The interchromophoric interaction is strongly solvent-dependent: ,,, stacking between the pyrene moieties of 2 is not as efficient in [D8]toluene, as it solvates the inner cavity and prevents the two chromophores from approaching each other. Fluorescence studies revealed an unexpectedly large conformational flexibility of the cavitand structures both in the vase and kite forms, which was further confirmed by molecular dynamics simulations. Excimer formation is most preferred in [D12]mesitylene when the cavities are empty, whereas efficient solvation or guest binding in the interior spaces reduces the propensity for excimer formation. The observed high conformational flexibility of the cavitands in solution explains previous differences from the behavior of related systems in the solid state. This study shows that the rigid, perfect vase and kite geometries found for bridged resorcin[4]arene cavitands in the solid state are largely a result of crystal packing effects and that the conformational flexibility of the structures in solution is rather high. [source]

Oligothiophenes Nano-organized on a Cyclotetrasiloxane Scaffold as a Model of a Silica-Bound Monolayer: Evidence for Intramolecular Excimer Formation

Complexation and Dynamic Switching Properties of Fluorophore-Appended Resorcin[4]arene Cavitands

A Stimuli-Responsive, Photoluminescent, Anthracene-Based Liquid Crystal: Emission Color Determined by Thermal and Mechanical Processes

ADVANCED FUNCTIONAL MATERIALS, Issue 12 2009
Yoshimitsu Sagara
Abstract Here, a photoluminescent liquid crystal that exhibits a change of emission color on the metastable,stable phase transition induced by external stimuli is prepared. A 2,6-diethynylanthracene derivative with amide groups and dendritic side chains exhibits a columnar phase on slow cooling from the isotropic phase and shows blue emission in this columnar phase. In contrast, a cubic phase is obtained by rapid cooling from the isotropic phase. In the cubic phase, the 2,6-diethynylanthracene cores form excimers, resulting in yellow emission. While the columnar phase is a stable liquid-crystalline (LC) phase, the cubic phase is a metastable LC phase. It is found that a change of the photoluminescent color from yellow to blue is observed on the cubic-columnar phase transition induced by heating or mechanical shearing for this 2,6-diethynylanthracene derivative in the cubic phase. This change of photoluminescent color is ascribed to the inhibition of excimer formation on the metastable,stable LC phase transition. [source]

Effect of sequence distribution of PES/PEES random, block, and alternative copolymers on excimer formation in solution

JOURNAL OF APPLIED POLYMER SCIENCE, Issue 4 2007
Xiao-Ming Zhou
Abstract Three copolymers of poly(ether sulfone) and poly(ether ether sulfone) with the same composition but different sequence distribution were synthesized by three kinds of methods. Their molecular aggregation in dichloromethane was studied by fluorescence spectrophotometer and electron microscope. The experimental results revealed that the formation of intermolecular excimers in alternative copolymer (A50) dichloromethane solution were observed at a A50 concentration about 1.6 × 10,2 g/mL by the fluorescence analysis, but the formation of intermolecular excimers in dichloromethane were not found for random copolymer (R50) and block copolymer (B50). The electron micrograph of three copolymer films, heat-treated at 200°C for 7 days, presented a diffraction micrograph, which suggest that three copolymer molecular aggregation is changed from a randomly coiled amorphous phase to an ordered one, and the order structure of alternative copolymer (A50) was the most distinct in three copolymers. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007 [source]

Photoluminescence of Self-organized Perylene Bisimide Polymers

MACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 2 2004
Edda E. Neuteboom
Abstract Summary: Three polymers consisting of alternating perylene bisimide chromophores and flexible polytetrahydrofuran segments of different length have been studied using absorption and (time-resolved) photoluminescence spectroscopy. In o -dichlorobenzene, the chromophores self organize to form H-like aggregates. The photoluminescence spectra of the self-organized polymers consist of vibronically resolved monomeric perylene bisimde fluorescence (,max,=,538 nm, ,,=,3.9 ns) and unstructured excimer-type emission (,max,=,635 nm, ,,=,17 ns). An additional short-lived (,,,,2 ns) luminescence component is observed and ascribed to the dynamic deactivation of the monomeric photoexcited state via excimer formation or energy transfer. Structure of alternating perylene bisimide , polyTHF copolymers. [source]

Fusion,Fission Transport of Probes and Quenchers in Microdomains of an Amphiphilic Ionene Polyelectrolyte,

PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 3 2007
Celize M. Tcacenco
In aqueous solution, amphiphilic ionenes such as the [3,22]-ionene spontaneously adopt globular conformations and form microdomains that are highly micelle-like, i.e. are capable of solubilizing organic molecules, binding and exchanging counterions and accelerating or inhibiting the rates of bimolecular reactions. Time-resolved fluorescence decay of pyrene and pyrene derivatives solubilized in these microdomains at concentrations where excimer formation occurs show that even water-insoluble probes can migrate between the hydrophobic microdomains formed in aqueous solution by a [3,22]-ionene chloride (with the N-terminal groups quaternized with benzyl chloride). Time-resolved studies of the quenching of pyrene fluorescence by alkylpyridine derivatives revealed similar behavior. The observed quenching behavior requires that the migration be between microdomains on the same ionene chain or same group of associated ionene chains and is consistent with migration dominated by fusion/fission transport of the probe and quencher. [source]

Ultrafast Energy Delocalization and Electron Transfer Dynamics in 2-Aminopurine-Containing Trinucleotides,

PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 3 2007
Qiang Wang
The fate of electronically excited states in DNA base stacks is of tremendous importance for subsequent photochemical damage reactions in the genome. In this study we present a femtosecond broadband pump-probe study on the adenine isomer 2-aminopurine (Ap) incorporated into trinucleotides. After selective excitation of Ap we can monitor energy delocalization between neighboring Ap moieties as well as excited state electron transfer, depending on the sequence of the trinucleotide. Our results establish the time scale for intrastand excimer formation and reveal the lifetime of the excimer state. [source]

Time-resolved synchrotron diffraction and theoretical studies of very short-lived photo-induced molecular species

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 2 2010
Philip Coppens
Definitive experimental results on the geometry of fleeting species are at the time of writing still limited to monochromatic data collection, but methods for modifications of the polychromatic Laue data to increase their accuracy and their suitability for pump,probe experiments have been implemented and are reviewed. In the monochromatic experiments summarized, excited-state conversion percentages are small when neat crystals are used, but are higher when photoactive species are embedded in an inert framework in supramolecular crystals. With polychromatic techniques and increasing source brightness, smaller samples down to tenths of a micrometre or less can be used, increasing homogeneity of exposure and the fractional population of the excited species. Experiments described include a series of transition metal complexes and a fully organic example involving excimer formation. In the final section, experimental findings are compared with those from theoretical calculations on the isolated species. Qualitative agreement is generally obtained, but the theoretical results are strongly dependent on the details of the calculation, indicating the need for further systematic analysis. [source]

A Molecular Probe for the Detection of Homopurine Sequences

CHEMBIOCHEM, Issue 1 2007
Ivan Trkulja
The highly selective detection of homopurine target strands with a triplex-forming molecular probe is described. The binding of a clamp-type oligonucleotide containing two terminally attached pyrene molecules to the target sequence is easily monitored through excimer formation. The oligonucleotide probe allows the efficient discrimination of single nucleotide mismatches because of the high mismatch sensitivity of the triplex formation. [source]

Oligothiophenes Nano-organized on a Cyclotetrasiloxane Scaffold as a Model of a Silica-Bound Monolayer: Evidence for Intramolecular Excimer Formation

Modulating Charge Transfer through Cyclic D,L -,-Peptide Self-Assembly

CHEMISTRY - A EUROPEAN JOURNAL, Issue 4 2005
W. Seth Horne
Abstract We describe a concise, solid support-based synthetic method for the preparation of cyclic d,l -,-peptides bearing 1,4,5,8-naphthalenetetracarboxylic acid diimide (NDI) side chains. Studies of the structural and photoluminescence properties of these molecules in solution show that the hydrogen bond-directed self-assembly of the cyclic d,l -,-peptide backbone promotes intermolecular NDI excimer formation. The efficiency of NDI charge transfer in the resulting supramolecular assemblies is shown to depend on the length of the linker between the NDI and the peptide backbone, the distal NDI substituent, and the number of NDIs incorporated in a given structure. The design rationale and synthetic strategies described here should provide a basic blueprint for a series of self-assembling cyclic d,l -,-peptide nanotubes with interesting optical and electronic properties. [source]

Water Soluble Cruciforms: Effect of Surfactants on Fluorescence

CHEMISTRY - AN ASIAN JOURNAL, Issue 2 2009
Juan Tolosa Dr.
Abstract Brighten up! Adding surfactants to aqueous solutions of three different water-soluble cruciforms (XF) improves their fluorescence quantum yields. Additionally, changes are observed in the emission wavelength of the XF around the critical micelle concentration (cmc) of the surfactant. Three 1,4-bis-(aminostyryl)-2,5-bis(phenylethynyl) benzenes carrying four, six, or eight acetic acid units were investigated for their surfactochromicity. Anionic, cationic, and nonionic surfactants as well as a surfactant-like protein (bovine serum albumin, BSA) were added to buffered solutions of the XFs in water, causing,in most cases,the fluorescence quantum yield to increase significantly and a blue- or red-shifted emission to be observed. The addition of cationic (cetyltrimethylammonium bromide, CTAB) and neutral (Brij 35, TWEEN 20 and Triton X-100) surfactants to XFs causes a red shift in their emission at low or very low surfactant concentrations that we attribute to surfactant-induced excimer formation. The fluorescence quantum yield is in most cases a monotonous function of surfactant concentration. For the investigated ionic surfactants, the fluorescence quantum yield of the XFs does not change much after the critical micelle concentration (CMC) of the surface is reached. However, in the case of non-ionic surfactants, the fluorescence quantum yields of the XFs starts to increase after the CMC has been reached, suggesting that different effects are involved. [source]