Electrically induced fluorescence depolarisation of macromolecules

Quantifiable changes have been recorded in the polarised fluorescence components as macromolecular solutions were subjected to pulsed electric fields. The advantages of this new molecular characterisation method are discussed. Illustrative data are presented for DNA tagged with ethidium bromide.     

Optical excitations in star-shaped fluorene molecules

A detailed study of the low-energy optical transitions in two families of star-shaped molecules is presented. Both families have 3-fold rotational symmetry with oligofluorene arms attached to a central core. In one family, the core of the molecule is a rigid meta-linked truxene, while the other is a meta-linked benzene moiety. The low-energy transitions were studied both experimentally and using time-dependent density functional theory (TD-DFT). The optical transitions of these new star-shaped molecules were compared with corresponding linear oligofluorenes. Both families of star-shaped molecules showed higher absorption and fluorescence dipoles and photoluminescence quantum yields than straight chain oligofluorenes. TD-DFT calculations show that absorption takes place across the entire molecule, and after excited state relaxation, the emission results from a single arm. In both theory and experiment the transition dipole moments show an approximate n(0.5) dependence on the number of fluorene units in each arm.     

Energy harvesting star-shaped molecules for electroluminescence applications

Novel energy harvesting molecules featuring hexaarylbenzene based triarylamine donors and a dithienyl benzothiadiazole acceptor, and that emit red light in electroluminescent devices, have been prepared for the first time.     

Solvent molecules in an epitaxially grown scaffold of star-shaped nanographenes

A scanning tunneling microscopy (STM) study of a star-shaped hexa-peri-hexabenzocoronene (HBC-star) derivative at solid-solution interfaces is presented. The star-shape of the molecules provides voids at their periphery which can be filled by smaller molecules. The use of solvents with different affinities to fill the voids allows for the fine-tuning of the structure of self-assembled architectures of HBC-stars. This concept is demonstrated by the use of solvents of different polarity and size, which leads to the formation of complex, epitaxial architectures at the interface. For small polar solvent molecules, a surprising decrease of the tunneling barrier is observed. The self-assembled architecture may serve as a useful model system for studying the dependence of electron tunneling on order, mobility, and polarity of adsorbates.     

Flow-cytometric monitoring of mitochondrial depolarisation: from fluorescence intensities to millivolts

Redistribution potentiometric dyes represent a powerful tool for monitoring membrane potential of mitochondria, especially when these dyes are used with flow cytometry. In particular, tetramethylrhodamine methyl ester proved to be suitable for the screening of mitochondrial membrane potential in cultured human skin fibroblasts from patients suffering from different defects of oxidative phosphorylation. We have developed a method that makes it possible to measure the changes in mitochondrial membrane potential, or to assess the differences between respective mitochondrial membrane potentials in investigated cells and controls in the absolute scale of millivolts. Our approach employs the fact that a logarithmic transformation of Nernst equation-controlled intensity of fluorescence from potentiometric dyes accumulated in mitochondria leads to a linear scale for mitochondrial membrane potentials.     

Chiral close-packing of achiral star-shaped molecules on solid surfaces

From the interplay of scanning tunneling microscopy and theoretical calculations, we study the chiral self-assembly of achiral HtB-HBC molecules upon adsorption on the Cu(110) surface. We find that chirality is expressed at two different levels: a +/-5 degrees rotation of the molecular axis with respect to the close-packed direction of the Cu(110) substrate and a chiral close-packed arrangement expected for star-shaped molecules in 2D. Out of the four possible chiral expressions, only two are found to exist due the effect of van der Waals (vdW) interactions forcing the molecules to simultaneously adjust to the atomic template of the substrate geometry and self-assemble in a close-packed geometry.     

Two-photon absorption properties of star-shaped molecules containing peripheral diarylthienylamines

Two-photon absorption (TPA) properties of several novel donor–acceptor–donor (D–A–D) quadrupolar and octupolar star-shaped molecules containing peripheral diarylthienylamines were investigated and are discussed in this Letter. The effects of donor–acceptor strength, conjugation length, and molecular symmetry on the effective TPA cross-section of these novel molecules were studied. It was found that incorporation of a triazine acceptor center in octupolar molecules can effectively enhance TPA response. This provides a strategy for designing novel octupolar molecules with a large TPA response but without red-shifting their linear absorption wavelengths.     

Rigid star-shaped multipodes: molecules designed for molecular reinforcement

Star-like rigid multipodes consisting of an adamantane core and rigid oligophenylenes arms with various lengths were synthesized with the aim to induce molecular reinforcement effects in mixtures of such multipodes with polymer matrices. The expectations based on theoretical considerations were that multipodes display an enhanced solubility both in low molar mass solvents and in polymer matrices. We investigated the shape of such multipodes, their structure formation and dynamics in the condensed state as well as their miscibility. The finding is that the peculiar shape of such multipodes does not prevent a dense packing and crystallization in the condensed state. The solubility in low molar mass solvents can be enhanced by up to three orders of magnitude just by increasing the arm length by one phenyl group. Finally, we consider the properties of mixtures of such multipodes with polymer matrices.     

Theoretical study on photophysical properties of 2,1,3-benzothiadiazole-based star-shaped molecules

Star-shaped molecules with tailoring functional groups in the core and the arms have great potential application in organic light-emitting devices, because it can be designed to realize low band gap, broad absorption, and excellent solubility for low-cost solution process. To gain an insight into the structure?Cproperty relationships, a set of four-arm star-shaped molecules with 2,1,3-benzothiadiazole as the core, different ??-conjugated groups as the arm, and triphenylamine or 2-(pyridin-2-yl) pyridine as the end-group were designed. In this study, a systematic investigation into them was carried out using the density functional theory and time-dependent density functional theory methods. The calculated ionization potentials, electron affinities, and reorganization energies (??) show that the properties of the ??-conjugated bridge and the end-group significantly affect the carrier injection and transport characteristics of these molecules, especially for S-BTDP and S-EBTD. Among these molecules, S-BTDP exhibits better electron injection ability due to the introduction of 2-(pyridin-2-yl) pyridine as the end-group. However, S-EBTD, with ethylene as ??-conjugated bridge, has excellent hole injection and carrier transport behaviors. We also calculated the singlet-to-triplet exciton-formation cross-section ratio (??_S/??_T), the exciton-formation fractions (??_S), and the absorption and emission spectra of these molecules. We calculated that ??_S/??_T ranges from 1.78 to 2.76 and that ??_S is ca. 0.37?C0.48. These molecules have two absorption bands in the range of 340?C410?nm and 500?C613?nm, respectively. The calculated emission spectra range from 619 to 706?nm. It can be deduced that the studied 2,1,3-benzothiadiazole-based star-shaped molecules can serve as efficient red light-emitting electroluminescent materials.     

Glycosylated star-shaped conjugated oligomers for targeted two-photon fluorescence imaging

A glucopyranose functionalized star-shaped oligomer, N-tris{4,4',4'-[(1E)-2-(2-{(E)-2-[4-(benzo[d]thiazol-2-yl)phenyl]vinyl}-9,9-bis(6-2-amido-2-deoxy-1-thio-β-D-glucopyranose-hexyl)-9H-fluoren-7-yl)vinyl]phenyl}phenylamine (TVFVBN-S-NH(2)), is synthesized for two-photon fluorescence imaging. In water, TVFVBN-S-NH(2) self-assembles into nanoparticles with an average diameter of ～49?nm and shows a fluorescence quantum yield of 0.21. Two-photon fluorescence measurements reveal that TVFVBN-S-NH(2) has a two-photon absorption cross-section of ～1100 GM at 780?nm in water. The active amine group on the glucopyranose moiety allows further functionalization of TVFVBN-S-NH(2) with folic acid to yield TVFVBN-S-NH(2) FA with similar optical and physical properties as those for TVFVBN-S-NH(2). Cellular imaging studies reveal that TVFVBN-S-NH(2) FA has increased uptake by MCF-7 cells relative to that for TVFVBN-S-NH(2), due to specific interactions between folic acid and folate receptors on the MCF-7 cell membrane. This study demonstrates the effectiveness of glycosylation as a molecular engineering strategy to yield water-soluble materials with a large two-photon absorption (TPA) cross-section for targeted cancer-cell imaging.     

Structural features of fullerene-containing star-shaped polystyrene molecules with Oligomer arms in solutions

The structure and conformational properties of star-shaped oligostyrenes containing fullerene C₆₀ as a branching center and short arms with lengths at the level of the persistent length or a segment of a polystyrene chain are studied by small-angle neutron scattering in deuterotoluene. The gyration radii of linear precursor oligomers (～0.4 and 0.6 nm) and corresponding star-shaped molecules (～1.1 and 1.4 nm) are calculated under the Guinier approximation. The linear oligomer (4–5 units) is found to be a rodlike molecule; arms of star-shaped molecules based on it assume the straightened conformations as well. Linear oligomer chains composed of 6–7 units deviate from the rodlike shape and acquire a certain flexibility in solution, but oligomer chains grafted onto the C₆₀ center preserve the extended conformations. There is no marked tendency toward screening of fullerene by radially extended arms. The number of branches in the star-shaped oligostyrenes corresponds to a functionality of f = 6 preset by the conditions of synthesis.     

Ultrafast fluorescence depolarisation in the yellow fluorescent protein due to its dimerisation.

Transient absorption spectroscopy with sub-100 fs time resolution was performed to investigate the oligomerisation behaviour of eYFP in solution. A single time constant tau(AD)=2.2+/-0.15 ps is sufficient to describe the time-resolved anisotropy decay up to at least 200 ps. The close contact of two protein barrels is deduced as the exclusive aggregation state in solution. From the final anisotropy r(infinity)=0.28+/-0.02, the underlying quaternary structure can be traced back to the somewhat distorted structure of the dimers of wt-GFP. The use of autofluorescent proteins as rulers in F?rster resonance energy transfer (FRET) measurements may demand polarisation-sensitive detection of the fluorescence with high time resolution.     

Surface grafting of PEO-based star-shaped molecules for bioanalytical and biomedical applications

This article reviews surface grafting of star-shaped PEO. The use of star-shaped polymers is compared to linear PEO chains regarding the layer preparation and the ability of the resulting surfaces to resist protein adsorption. We then focus on the use of end-functionalized, star-shaped, PEO-based prepolymers that are able to form covalent crosslinks and functional polymer networks on the substrate. Examples are given for specific protein adsorption as well as for cell adhesion on such layers by covalent embedding of biofunctional molecules. The possibility of coating biomedically relevant polymer substrates in three-dimensional geometries is discussed and examples are shown for poly(ethylene terephthalate) monofilament constructs.     

Dynamics of trehalose molecules in confined solutions

The dynamics of trehalose molecules in aqueous solutions confined in silica gel have been studied by quasielastic neutron scattering (QENS). Small-angle neutron scattering measurements confirmed the absence of both sugar clustering and matrix deformation of the gels, indicating that the results obtained are representative of homogeneous trehalose solutions confined in a uniform matrix. The pore size in the gel is estimated to be 18 nm, comparable to the distances in cell membranes. For the QENS measurements, the gel was prepared from D2O in order to accentuate the scattering from the trehalose. Values for the translational diffusion constant and effective jump distance were derived from model fits to the scattering function. Comparison with QENS and NMR results in the literature for bulk trehalose shows that confinement on a length scale of 18 nm has no significant effect on the translational diffusion of trehalose molecules.     

Absorption and fluorescence of single molecules

Simultaneous detection of single molecules by absorption and fluorescence is demonstrated using confocal microscopy at cryogenic temperature. Dynamical processes such as blinking and spectral jumping of single emitters are observed in both detection channels. The relative magnitude of fluorescence and absorption varies between molecules. In particular, we observe molecules that do not emit detectable Stokes-shifted fluorescence but show a strong absorption signal. The fact that coherent resonant scattering underlies the absorption process is demonstrated by a correlation between small linewidth and large absorption amplitude.     

Dynamics of branched polymer molecules in dilute solution

Theoretical formulas for the intrinsic viscosity and viscoelastic properties of some model branched molecules in dilute solution are calculated by means of the normal coordinate method of Rouse modified to include hydrodynamic interactions. The calculations are exact except for the usual approximation of the hydrodynamic interactions by the Kirkwood-Riseman formula. The ratio of the intrinsic viscosity of a branched molecule to that of a linear molecule of the same weight is found to vary almost as the square root of the ratio of the mean square radii, instead of as the latter ratio to three-halves power, as has been postulated before. It is proposed that this square root relation is applicable in general to branched molecules of all types. Several sets of experimental data in the literature are shown to agree well with this hypothesis.     

Dynamics of benzene molecules situated in metal-organic frameworks

In this paper, we investigate the gyroscopic motion of a benzene molecule C₆H₆, which comprises an inner carbon ring and an outer hydrogen ring, and is suspended rigidly inside a metal-organic framework. The metal-organic framework provides a sterically unhindered environment and an electronic barrier for the benzene molecule. We model such gyroscopic motion from the inter-molecular interactions between the benzene ring and the metal-organic framework by both the Columbic force and the van der Waals force. We also capture additional molecular interactions, for example due to sterical compensations arising from the carboxylate ligands between the benzene molecule and the framework, by incorporating an extra empirical energy into the total molecular energy. To obtain a continuous approximation to the total energy of such a complicated atomic system, we assume that the atoms of the metal-organic framework can be smeared over the surface of a cylinder, while those for the benzene molecule are smeared over the contour line of the molecule. We then approximate the pairwise molecular energy between the molecules by performing line and surface integrals. We firstly investigate the freely suspended benzene molecule inside the framework and find that our theoretical results admit a two-fold flipping, with the possible maximum rotational frequency reaching the terahertz regime, and gigahertz frequencies at room temperature. We also show that the electrostatic interaction and the thermal energy dominate the gyroscopic motion of the benzene molecule, and we deduce that the extra energy term could possibly reduce the rotational frequency of the rigidly suspended benzene molecule from gigahertz to megahertz frequencies at room temperature, and even lower frequencies might be obtained when the strength of these interactions increases.     

The Stokes-shifted fluorescence risetime of dye molecules in solution

We have measured the rise of Stokes-shifted spontaneous fluorescence intensity during a 10 ps 530 nm actinic light pulse for rhodamine B, rhodamine 6G and erythrosin B dissolved in water, ethanol and methanol. The best numerical fit to our data corresponds to a time delay of less than 1 ps between excitation and fluorescence emission.