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Johan Hofkens

Publications and source records attributed to Johan Hofkens.

At least 19 recordsLinked to original sources
PubMed2007

CT-CT annihilation in rigid perylene end-capped pentaphenylenes.

The time-dependent spectral properties of a rigid, extended system consisting of three pentaphenylene units end-capped with perylene monoimide were investigated in detail by femtosecond transient absorption and single photon timing measurements. In polar solvents, the molecular system shows the occurrence of photoinduced charge transfer. Upon gradually increasing the excitation power, annihilation involving two states with charge-transfer character has been observed. Quantum-chemical calculations performed on the system consisting of two pentaphenylene units end-capped with perylene monoimide strongly support the experimental data.

Journal Article↗
PubMed2007

Formation of vesicles in block copolymer-fluorinated surfactant complexes.

Spherical micellar aggregates have been obtained in chloroform by mixing poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymers with perfluorinated surfactants (FS) bearing a carboxylic acid head. These micellar aggregates are resulting from the self-assembly of the insoluble P4VP/fluorinated complexes into a core surrounded by the soluble PS coronal chains. Their characteristic features have been studied as a function of various parameters including the composition of the PS-b-P4VP copolymer, the tail length of the fluorinated surfactant, the 4VP/FS molar ratio, the number of carboxylic acid group (1 or 2) on the surfactant, the presence of the PS block and of the fluorine atoms on the surfactant. Dilution of these initial micellar aggregates triggers a morphological reorganization resulting in the formation of more stable vesicles. The extent of this morphological transition is related to the solubility of the P4VP/fluorinated complexes during the dilution process. This transition is complete for short P4VP/FS complexes, incomplete for long P4VP/FS complexes, and not observed whenever an alpha,omega-difunctional FS is used in P4VP/FS complexes, leading to a cross-linked core. Finally, the spheres-to-vesicles transition has been advantageously used in order to encapsulate molecules, as demonstrated by confocal fluorescence microscopy.

Journal Article↗
PubMed2007

Fluorescent perylene diimide rotaxanes: spectroscopic signatures of wheel-chromophore interactions.

[2]- and [3]-rotaxanes with a tetraphenoxy perylene diimide core were synthesized. Hydrogen bonding between the wheel and the imide changes the optical properties of the perylene chromophore: the absorption and fluorescence spectra are red-shifted. The decay times of the rotaxanes are shorter in comparison with that of the axle. Single molecule fluorescence measurements reveal relatively narrow distributions of emission maxima and decay times. The averages are in agreement with ensemble measurements. The observed red shifts make the perylene diimide a suitable chromophore for sensing the position of the wheel on the axle.

Journal Article↗
PubMed2006

Inhomogeneity of electron injection rates in dye-sensitized TiO2: comparison of the mesoporous film and single nanoparticle behavior.

As it has been shown by pump-probe experiments electron injection at the interface between a dye molecule and mesoporous TiO2 proceeds with rates exceeding 1 x 10(13) s(-1). However, similar dye-TiO2 systems exhibit residual dye emission with lifetimes extending into the long nanosecond range. To address this inhomogeneity of injection rates time-correlated single photon counting microscopy was used to compare the emission behavior of dye-sensitized mesoporous films of TiO2 with that of individual anatase nanoparticles that had undergone extensive dialysis. The sensitized films produce intense residual emission with multiexponential decay components as long as 220 ns. The channels of mesoporous films contain physisorbed and trapped dye, which is the dominant source of the emission. It is likely that the wide range of lifetimes reflects the distribution of mean free paths experienced by the loose dye molecules diffusing within the film prior to undergoing oxidative quenching. In contrast, the intensity of emission from individual nanoparticles from which the loose dye was removed by dialysis is orders of magnitude lower. The lifetimes obtained from such particles are much shorter, with the primary component on a sub-nanosecond time scale. The presence of residual emission with a 230 ps lifetime shows that even on the surfaces of dialyzed nanoparticles there is a fraction of sensitizer molecules that do not inject electrons with the same high rate as is observed in ultrafast pump-probe experiments on films. Since the physisorbed dye was removed from these samples by dialysis, the residual emission is likely to originate from dye molecules bound to surface defects. Unusual collective emission bursts were observed in some of the measurements on sensitized nanoparticles. We attribute this behavior to stimulated emission from individual nanocrystallites.

2,2'-Dipyridyl↗
PubMed2006

Fast and reversible photoswitching of the fluorescent protein dronpa as evidenced by fluorescence correlation spectroscopy.

Controlling molecular properties through photoirradiation holds great promise for its potential for noninvasive and selective manipulation of matter. Photochromism has been observed for several different molecules, including green fluorescent proteins, and recently the discovery of a novel photoswitchable green fluorescent protein called Dronpa was reported. Dronpa displays reversible and highly efficient on/off photoswitching of its fluorescence emission, and reversible switching of immobilized single molecules of Dronpa with response times faster than 20 ms was demonstrated. In this Letter, we expand these observations to freely diffusing molecules by using fluorescence correlation spectroscopy with simultaneous excitation at 488 and 405 nm. By varying the intensity of irradiation at 405 nm, we demonstrate the reversible photoswitching of Dronpa under these conditions, and from the obtained autocorrelation functions we conclude that this photoswitching can occur within tens of microseconds.

Dose-Response Relationship, Radiation↗
PubMed2006

Photoconversion in the red fluorescent protein from the sea anemone Entacmaea quadricolor: is cis-trans isomerization involved?

Proteins from the family of the green fluorescent protein (GFP) are presently extensively used in molecular and cellular biology. Recent studies suggest that isomerization of the chromophore occurs upon excitation and is involved in nonradiative deactivation. Using Raman spectroscopy, we report on photoinduced cis-trans isomerization in the red fluorescent protein eqFP611 from the sea anemone Entacmaea quadricolor. The crystal structure of eqFP611 shows that the chemical structure of the chromophore, p-hydroxybenzylidene-imidazolinone with an extended -conjugated system, is nearly identical to the chromophore of other red fluorescent proteins such as DsRed and HcRed. However, the chromophore of eqFP611 has a trans configuration whereas the chromophore of DsRed has a cis configuration. Upon irradiation with 532-nm light, the absorption of eqFP611 peaking at 559 nm diminished, and concomitantly a drastic decrease in the quantum yield of fluorescence as well as more complex decay kinetics was observed. Upon irradiation, changes in the Raman spectrum of eqFP611 were observed, and the relative intensities and peak positions of the irradiated eqFP611 showed striking similarity with the peaks in the Raman spectrum of DsRed. These observations are tentatively interpreted as trans-to-cis isomerization of the chromophore taking place upon irradiation together with the opening of new, nonradiative pathways.

Animals↗
PubMed2006

Solvent and pH dependent fluorescent properties of a dimethylaminostyryl borondipyrromethene dye in solution.

Steady-state and time-resolved fluorescence techniques have been used to study the photophysical properties of the fluorescent BODIPY-derived dye 3-{2-[4-(dimethylamino)phenyl]ethenyl}-4,4-difluoro-8-(4-methoxyphenyl)-1,5,7-trimethyl-3a,4a-diaza-4-bora-s-indacene. This compound has been synthesized via a microwave-assisted condensation of p-N,N-dimethylaminobenzaldehyde with the appropriate 1,3,5,7-tetramethyl substituted borondipyrromethene unit. The fluorescence properties of the dye are strongly solvent dependent: increasing the solvent polarity leads to lower fluorescence quantum yields and lifetimes, and the wavelength of maximum fluorescence emission shifts to the red. The Catalán solvent scales are found to be the most suitable for describing the solvatochromic shifts of the fluorescence emission. These are dominated by polarity/polarizability effects, as confirmed by quantum-chemical calculations performed in the dielectric continuum approximation. Fluorescence decay profiles of the dye can be described by a single-exponential fit in most solvents investigated, while two decay times are found in alcohols. The dye undergoes a reversible protonation-deprotonation reaction in the acidic pH range with a pK(a) of 2.25 in acetonitrile solution. Fluorimetric titrations as a function of pH produce fluorescence emission enhancements at lower pH. The fluorescence excitation spectra show a hypsochromic shift from 600 nm for the neutral amine to 553 nm for the ammonium form, so that ratiometric measurements can be used to determine pK(a).

Journal Article↗
PubMed2006

Photo-induced protonation/deprotonation in the GFP-like fluorescent protein Dronpa: mechanism responsible for the reversible photoswitching.

Recently, reversible photoswitching in bulk samples or in individual molecules of Dronpa, a mutant of a green fluorescent protein (GFP)-like fluorescent protein, has been demonstrated. Intense irradiation at 488 nm changed Dronpa in a dim protonated form, and weak irradiation at 405 nm restored it to the bright deprotonated form. Here, we report on the mechanism of photoswitching of Dronpa by means of ensemble and single-molecule spectroscopy. Ensemble spectroscopy shows that the photoswitching can be described, in first approximation, by a three-state model including a deprotonated (B), a protonated (A1), and a photoswitched protonated (A2) forms of the chromophore. While the B and the A1 forms are in a ground state acid-base equilibrium, the B and the A2 forms are reversibly photoswitched upon irradiation with 488 and 405 nm light. At the single-molecule level, the on-times in fluorescence intensity trajectories excited at 488 nm decrease with increasing the excitation power, consistent with the photoswitching from the B to A2 form. The on-times agree well with expected values, which are calculated based on the ensemble spectroscopic properties of Dronpa. The fluorescence trajectory obtained with simultaneous dual-color excitation at 488 and 405 nm demonstrates reversible photoswitching between the B and the A2 forms at the single-molecule level. The efficiency of the photoswitching from the A2 to B form increased with increasing the excitation power of the 405 nm light. Our results demonstrate that Dronpa holds its outstanding photoswitching properties, based on a photo-induced protonation/deprotonation process, even at the single-molecule level.

Green Fluorescent Proteins↗
PubMed2006

Characterizing the fluorescence intermittency and photobleaching kinetics of dye molecules immobilized on a glass surface.

The blinking behavior of single Atto565 molecules on a glass surface is studied under air or nitrogen atmospheres using confocal microscopy. The broad distributions for both on- and off-time durations obey power law kinetics that are rationalized using a charge tunneling model. In this case, a charge is transferred from the Atto565 molecule to localized states found on the glass surface. Subsequent charge recombination by back charge tunneling from trap to Atto565 cation (i.e., dark state) restores the fluorescence. The off-time distribution is independent of excitation intensity (I), whereas the on-time distribution exhibits a power law exponent that varies with I. Two pathways have been identified to lead to the formation of the radical dark state. The first involves direct charge tunneling from the excited singlet S1 state to charge traps in the surrounding matrix, and the second requires charge ejection from the triplet T1 state after intersystem crossing from S1. Monte Carlo simulation studies complement the two-pathway model. Photobleaching curves of both single and ensemble molecules do not exhibit monoexponential decays suggesting complex bleaching dynamics arising from triplet and radical states.

Journal Article↗
PubMed2006

Spatially resolved observation of crystal-face-dependent catalysis by single turnover counting.

Catalytic processes on surfaces have long been studied by probing model reactions on single-crystal metal surfaces under high vacuum conditions. Yet the vast majority of industrial heterogeneous catalysis occurs at ambient or elevated pressures using complex materials with crystal faces, edges and defects differing in their catalytic activity. Clearly, if new or improved catalysts are to be rationally designed, we require quantitative correlations between surface features and catalytic activity--ideally obtained under realistic reaction conditions. Transmission electron microscopy and scanning tunnelling microscopy have allowed in situ characterization of catalyst surfaces with atomic resolution, but are limited by the need for low-pressure conditions and conductive surfaces, respectively. Sum frequency generation spectroscopy can identify vibrations of adsorbed reactants and products in both gaseous and condensed phases, but so far lacks sensitivity down to the single molecule level. Here we adapt real-time monitoring of the chemical transformation of individual organic molecules by fluorescence microscopy to monitor reactions catalysed by crystals of a layered double hydroxide immersed in reagent solution. By using a wide field microscope, we are able to map the spatial distribution of catalytic activity over the entire crystal by counting single turnover events. We find that ester hydrolysis proceeds on the lateral {1010} crystal faces, while transesterification occurs on the entire outer crystal surface. Because the method operates at ambient temperature and pressure and in a condensed phase, it can be applied to the growing number of liquid-phase industrial organic transformations to localize catalytic activity on and in inorganic solids. An exciting opportunity is the use of probe molecules with different size and functionality, which should provide insight into shape-selective or structure-sensitive catalysis and thus help with the rational design of new or more productive heterogeneous catalysts.

Journal Article↗
PubMed2006

Do enzymes sleep and work?

Single-enzyme studies suggest that dynamic disorder is a general characteristic of enzyme catalysis.

Catalysis↗
PubMed2005

Direct measurement of the end-to-end distance of individual polyfluorene polymer chains.

Wide-field imaging of individual multichromophoric molecules and successive photobleaching were used to determine, accurately, the relative position of the chromophores in such systems. First, a polyphenylene dendrimer with well-defined geometry was used to establish the accuracy in localization that can be obtained by this methodology. For a signal-to-noise ratio of 20, interchromophoric distances could be measured with 4 nm accuracy. Next, the method was used to determine the end-to-end distribution of an end-capped polyfluorene polymer. From comparison between the experimental and simulated distributions, information on the conformation of the polymer could be deduced. It was found that the polymer has a nonlinear conformation. A conjugation length of six monomer units gave the best fit of the experimental data to the proposed model.

Journal Article↗
PubMed2005

In situ space- and time-resolved sorption kinetics of anionic dyes on individual LDH crystals.

Fluorescence spectroscopy is employed to follow the ion exchange of an emissive dye--a carboxylated perylene imide--on a layered double hydroxide. The exchange of the carboxylate ions starts at the edges of the layered double hydroxide crystals and is followed by diffusion to the basal plane. Such space-resolved observations provide a solid basis for modelling and studying the mechanisms of exchange.

Journal Article↗
PubMed2005

Single molecule spectroscopy as a probe for dye-polymer interactions.

Experimental (Single Molecule Spectroscopy) and theoretical (quantum-chemical calculations and Monte Carlo and molecular dynamics simulations) techniques are combined to investigate the behavior and dynamics of a polymer-dye molecule system. It is shown that the dye molecule of interest (1,1'-dioctadecyl-3,3,3',3'-tetramethylindo-dicarbocyanine) adopts two classes of conformations, namely planar and nonplanar ones, when embedded in a poly(styrene) matrix. From an in-depth analysis of the fluorescence lifetime trajectories, the planar conformers can be further classified according to the way their alkyl side chains interact with the surrounding poly(styrene) chains.

Journal Article↗
PubMed2005

Probing intramolecular Förster resonance energy transfer in a naphthaleneimide-peryleneimide-terrylenediimide-based dendrimer by ensemble and single-molecule fluorescence spectroscopy.

We report on the ensemble and single-molecule (SM) dynamics of Förster resonance energy transfer (FRET) in a multichromophoric rigid polyphenylenic dendrimer (triad) with spectrally different rylene chromophores featuring distinct absorption and emission spectra which cover the whole visible spectral range: a terrylenediimide (TDI) core, four perylenemonoimides (PMIs) attached at the scaffold, and eight naphthalenemonoimides (NMIs) at the rim. For FRET from PMI to TDI taking place with an efficiency of 99.5%, single triad molecules optically excited at 490 nm show fluorescence exclusively from the TDI side in the beginning of their emission. On 360-nm excitation, NMI chromophores transfer their excitation energy either directly or in a stepwise fashion to the core TDI, the latter case involving scaffold-substituted PMIs as intermediate acceptors. Indeed, SM experiments on 360-nm excitation evidence highly efficient FRET from NMI chromophores to the TDI core since individual triad molecules show fluorescence exclusively either from TDI or from an intermediate (oxidized) species but never from PMI. Because PMI and TDI are chromophores with high fluorescence quantum yields and high resistance to photobleaching compared to NMI, 360-nm excitation of a single triad molecule leads to bleaching of NMI chromophores with no chance for PMI to be observed. The spatial positioning and the spectral properties of the chosen rylene chromophores make this multichromophoric system an efficient light collector, able to capture light over the whole visible spectral range and to transfer it finally to the core TDI, the latter releasing it as red fluorescence.

Journal Article↗