Michel Sliwa

Ratiometric, Fluorescent BODIPY Dye with Aza Crown Ether Functionality: Synthesis, Solvatochromism, and Metal Ion Complex Formation

A new pH and metal ion-responsive BODIPY-based fluorescent probe with an aza crown ether subunit has been synthesized via condensation of 4-(1,4,7,10-tetraoxa-13-aza-cyclopentadec-13-yl)-benzaldehyde with the appropriate 1,3,5,7-tetramethyl substituted boron dipyrromethene moiety. Steady-state and time-resolved fluorometries have been used to study the spectroscopic and photophysical characteristics of this probe in various solvents. 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 Catalan solvent scales are found to be the most suitable for describing the solvatochromic shifts of the fluorescence emission. Fluorescence decay profiles of the dye can be described by a single-exponential fit in nonprotic solvents, whereas two decay times are found in alcohols. Protonation as well as complex formation with several metal ions are investigated in acetonitrile as solvent via fluorometric titrations. The aza crown ether dye undergoes a reversible (de)protonation reaction (pKa = 0.09) and shows a approximately 50 nm blue shift in the excitation spectra and a 10-fold fluorescence increase upon protonation. The compound also forms 1:1 complexes with several metal ions (Li(+), Na(+), Mg(2+), Ca(2+), Ba(2+), Zn(2+)), producing large blue shifts in the excitation spectra and significant cation-induced fluorescence amplifications.

Investigation of ultrafast photoinduced processes for salicylidene aniline in solution and gas phase: toward a general photo-dynamical scheme

Photodynamics of 2-hydroxybenzylideneaniline (photochromic salicylidene aniline SAOH) and N-(2-methoxybenzylidene)aniline (SAOMe) are studied by steady state and transient optical spectroscopy in solution and gas phase at different excitation wavelengths (266, 355 and 390 nm). Two competitive processes are observed from the enol* excited state: on one hand a rotation to get a twisted-enol, and on the other hand an excited state intramolecular proton transfer (ESIPT) followed by a cis-trans isomerisation to get the trans-keto photochromic product. For the first time both processes are characterized at an ultrashort time scale for salicylidene aniline. Resolution of the spectrokinetic data is achieved by multivariate curve resolution and attribution of the intermediate species recovered is performed in comparison with the results obtained for SAOMe, which can only undergo enol rotational isomerisation. It shows that ESIPT and rotation to the twisted-enol for SAOH occur within 100 fs, as predicted by recent quantum dynamical simulations, with an efficiency ratio dependent on the excitation wavelength. Therefore a general photoinduced mechanism for salicylidene aniline is drawn.

Photoactivation of Silver-Exchanged Zeolite A†

Clusters of silver atoms and ions have attracted the interest of scientists because of their pronounced catalytic and emissive properties. To prevent aggregation of the clusters into larger particles, stabilization in gas matrices at cryogenic temperatures, or in scaffolds such as polyphosphates, DNA, peptides or polymers at ambient temperature, has been suggested. Alternatively, zeolites have been proposed to stabilize small ionic silver clusters. The molecular dimensions of the zeolite cages and channels prevent aggregation into larger nanoparticles (because of steric reasons) while the net negative charge of the zeolite lattice, the coordinating properties of the lattice oxygen atoms, and the presence of additional cations play a crucial role in stabilizing cationic clusters and unstable intermediates during reduction. Reduction of silver in zeolites is usually a bulk process that requires reductants, such as hydrogen gas or sodium borohydride, but also g irradiation and visible light can cause reduction. One of the most studied systems is silver-exchanged zeolite A (LTA topology), and several models have been proposed to explain the nature and location of the silver clusters formed in this zeolite upon reduction. Aside from their catalytic properties, oligonuclear silver clusters show particularly bright and stable luminescent properties. However, the existing fluorescence studies about Ag clusters in zeolites are limited to the excitation and emission spectra of bulk powder samples. Here, we report on bright fluorescent (spots in) individual silverexchanged zeolite 3A crystals obtained upon photoactivation using a focused UV irradiation on a fluorescence microscope. Photoactivation of silver has been demonstrated for nanoscale Ag2O particles (and interpreted as a photoreduction process). In our study, the emissive silver material is confined within a zeolite framework, which results in a better control of the type and location of the emissive species formed upon UV irradiation. Silver-cluster-loaded crystals are technologically very attractive—for example, as secondary light sources in fluorescence lamps—because of their high emission intensity, their excellent photostability upon UV irradiation, and their large Stokes shift. Moreover, the space-resolved selective activation of the emission intensity may have important applications in data storage. The emissive zeolite particles used herein were prepared by exchanging zeolite 3Awith (8 1)wt%Ag (fromAgNO3 solutions), followed by heating for one day at 450 8C (see the Supporting Information). The enhanced emission exhibited by the silver-exchanged zeolites after the thermal treatment has been ascribed to two possible causes: 1) to the formation of charge-transfer complexes between the partially (de)hydrated silver ions and the oxygen atoms in the zeolite lattice or 2) to the emissive properties of autoreduced oligoatomic silver clusters that may be formed during the high-temperature treatment. Although this report focuses on the photoactivation of thermally treated Ag zeolites, control experiments performed on not thermally activated Ag zeolites (dried at 110 8C) show an analogous photoactivation behavior (see the supporting Information). Figure 1a(1),b(1) shows typical confocal scanning images of a heat-treated Ag-containing zeolite crystal (roughly 3 ? 3 mm in size) taken under a confocal microscope using a picosecond-pulsed 375 nm excitation source (doubled Ti:Sapphire, Spectra Physics; see the Supporting Information). Figure 1a and 1b were taken at excitation intensities of 10 and 20 Wcm , respectively. Zeolite crystals that were not treated thermally exhibited an emission ten-times-weaker than that of the thermally treated samples. The confocal approach in combination with the pinhole in the emission path (see the Supporting Information) allows the collection of photoemission data from selected parts inside the crystals. Diffraction-limited bright spots can be generated at specific domains inside an individual crystal by simply focusing a low-power UV laser at the target position; for instance, in the crystal shown in Figure 1a, three individual spots were selectively activated by irradiation during [*] Dr. Y. Antoku, Dr. M. Sliwa, S. Smout, Prof. Dr. J. Hofkens, Dr. T. Vosch Department of Chemistry Katholieke Universiteit Leuven Celestijnenlaan 200F, 3001 Heverlee (Belgium) Fax: (+32)1632-7989 E-mail: tom.vosch@chem.kuleuven.be

An efficient Ru(II) -Rh(III) -Ru(II) polypyridyl photocatalyst for visible-light-driven hydrogen production in aqueous solution.

The development of multicomponent molecular systems for the photocatalytic reduction of water to hydrogen has experienced considerable growth since the end of the 1970s. Recently, with the aim of improving the efficiency of the catalysis, single-component photocatalysts have been developed in which the photosensitizer is chemically coupled to the hydrogen-evolving catalyst in the same molecule through a bridging ligand. Until now, none of these photocatalysts has operated efficiently in pure aqueous solution: a highly desirable medium for energy-conversion applications. Herein, we introduce a new ruthenium-rhodium polypyridyl complex as the first efficient homogeneous photocatalyst for H2 production in water with turnover numbers of several hundred. This study also demonstrates unambiguously that the catalytic performance of such systems linked through a nonconjugated bridge is significantly improved as compared to that of a mixture of the separate components.

Mapping of Surface‐Enhanced Fluorescence on Metal Nanoparticles using Super‐Resolution Photoactivation Localization Microscopy

Photoactivation localization microscopy (PALM) was applied to study surface-enhanced fluorescence (SEF) on metal nanostructures (SEF-PALM). The detection of fluorescence from individual single molecules can be used to image the point-spread-function and spatial distribution of the fluorescence emitted in the vicinity of a metal surface. Due to the strong scattering effect, the angular distribution of the fluorescence is altered by metals, resulting in a spatial shift of fluorescence spots with respect to the metal nanostructures, and has to be taken into account in the analysis. SEF-PALM can be used to discriminate effects of labelling density when estimating the enhancement factor in SEF. Furthermore, nanostructures with sizes below the diffraction limit can be resolved using this technique. SEF-PALM is established as a powerful tool to study plasmon-mediated phenomena on metal nanostructures.

Synthesis and photophysical characterization of chalcogen substituted BODIPY dyes

Synthetic details and stationary and time-resolved photophysical properties of five BODIPY derivatives containing chalcogen atoms are presented. The photophysical data are compared to those of a chlorine atom containing BODIPY, acting as a reference. A strong impact in the HOMO–LUMO transition energy is achieved via nucleophilic substitution with chalcogen based units. Going from oxygen to tellurium a bathochromic shift in both absorption and emission spectra from the green to the near infrared region was observed. By employing fluorescence single photon timing experiments in two solvents of different polarity, the excited state dynamics and their solvent dependence indicate the presence of a mechanism involving a photoinduced charge transfer that dramatically affects the optical radiative processes of these derivatives.

Bridged photochromic diarylethenes investigated by ultrafast absorption spectroscopy: evidence for two distinct photocyclization pathways.

Two photochromic diarylethenes blocked by alkyl bridges in an ideal conformation for photocyclization are studied by stationary and femtosecond transient spectroscopy in order to depict the photocyclization processes: the bistable 1,2-dicyano[2.n]metacyclophan-1-ene with n = 2, abbreviated as [2.2], and its non-bistable analogue with n = 4, abbreviated as [2.4]. The data are interpreted in the light of AM1-CIS calculations and state correlation diagrams based on conclusive TD-DFT calculations. For [2.2], a solvent-sensitive excitation wavelength threshold governing the photocyclization yield is clearly evidenced between the S(1) and S(2) singlet states. Excitation above and beyond this threshold induces two distinct photochemical pathways. The S(1) vertical excitation induces direct efficient (phi approximately = 0.9-1), and ultrafast (approximately 120 fs) photocylization from S(1) open form that leads to a ground-state transition structure, probably through a conical intersection, then to a hot cyclized ground state that relaxes by vibrational cooling. Upon higher excitation energy, the system undergoes internal conversion to the hot S(1) state, then evolves toward the cyclized S(1) state and relaxes by ultrafast S(1)-S(0) internal conversion. Alternatively, the possibility for a second conical intersection near hot S(1) state is discussed. This second photoclosure reaction is less efficient and both the photocylization yield and overall kinetics depend on solvent polarity (phi = 0.49, tau = 2.5 ps in nonpolar solvent; phi = 0.7, tau = 1.5 ps in polar solvent). In the case of [2.4], for which the distance between the two reactive carbons is larger, a unique photoclosure mechanism is found and a structural effect is reported. Indeed, this mechanim is similar to the above second reaction of [2.2] but characterized by much slower kinetics ranging from 12 to 20 ps (depending on the excitation wavelength and solvent polarity). All polarity effects are rationalized in terms of stabilization of the transient states of charge-transfer character involved in the photocyclization process.

Exploration of Single Molecule Events in a Haloperoxidase and Its Biomimic: Localization of Halogenation Activity

In situ observation of single oxidation/halogenation events by catalytically generated hypobromite species using single molecule fluorescence microscopy allows monitoring of the diffusion behavior of these halonium species from the catalyst into the bulk solution. The fluororgenic probe specifically reacts with hypohalites, yielding fluorescein that can be detected with single molecule sensitivity. It was found for two investigated catalysts (Curvularia verruculosa enzymes and tungstate-exchanged LDH crystals) that in steady-state conditions hypobromite is able to diffuse over 800 nm in the bulk solution before it oxidizes organic substrates.

Excitation energy migration processes in cyclic porphyrin arrays probed by single molecule spectroscopy.

By using single molecule fluorescence spectroscopy we have investigated the excitation energy migration processes occurring in a series of cyclic porphyrin arrays bearing a close proximity in overall architectures to the LH2 complexes in purple bacterial photosynthetic systems. We have revealed that the conformational heterogeneity induced by the structural flexibility in large cyclic porphyrin arrays, which provides the nonradiative deactivation channels as an energy sink or trap, reduces significantly the energy migration efficiency. Our study provides detailed information on the energy migration efficiency of the artificial light-harvesting arrays at the single molecule level, which will be a guideline for future applications in single molecular photonic devices in the solid state.

Photochemistry of 2-Naphthoyl Azide. An Ultrafast Time-Resolved UV―Vis and IR Spectroscopic and Computational Study

The photochemistry of 2-naphthoyl azide was studied in various solvents by femtosecond time-resolved transient absorption spectroscopy with IR and UV-vis detection. The experimental findings were interpreted with the aid of computational studies. Using polar and nonpolar solvents, the formation and decay of the first singlet excited state (S(1)) was observed by both time-resolved techniques. Three processes are involved in the decay of the S(1) excited state of 2-naphthoyl azide: intersystem crossing, singlet nitrene formation, and isocyanate formation. The lifetime of the S(1) state decreases significantly as the solvent polarity increases. In all solvents studied, isocyanate formation correlates with the decay of the azide S(1) state. Nitrene formation correlates with the decay of the relaxed S(1) state only upon 350 nm excitation (S(0) → S(1) excitation). When S(n) (n ≥ 2) states are populated upon excitation (λ(ex) = 270 nm), most nitrene formation takes place within a few picoseconds through the hot S(1) and higher singlet excited states (S(n)) of 2-naphthoyl azide. The data correlate with the results of electron density difference calculations that predict nitrene formation from the higher-energy singlet excited states, in addition to the S(1) state. For all of these experiments, no recovery of the ground state was observed up to 3 ns after photolysis, which indicates that both internal conversion and fluorescence have very low efficiencies.