Abstract: N-(3,5-Ditert-butylsalicylidene)-2-aminopyridine (1) exhibits bulk second harmonic generation (SHG) activity (2 vs urea). X-ray diffraction measurements show a non-centrosymmetric structure (P3(2)). Experimental data of polarization-dependent SHG match the theoretical calculations on the basis of the crystallographic data. 1 is also thermochromic in the crystalline state, and SHG at 120 K is 4 times as high as at room temperature. Comparison is made with the isostructural isomer, N-(3,5-ditert-butylsalicylidene)-4-aminopyridine (2), which exhibits reversible photoinduced SHG variations.

Abstract: In photochromic materials, we demonstrated that non-linear optical (NLO) properties such as second harmonic generation (SHG) can be switched reversibly by alternate UV and visible irradiations. We report here on the design of materials fulfilling the prerequisites for SHG activity (push-pull and acentric space group) and photochromic properties. Photochromic and NLO properties of a series of anils are presented, along with the ability of some of them to switch SHG.

Abstract: 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.

Abstract: Highly fluorescent organic nanoparticles with size of about 300 nm were prepared by nanosecond laser ablation of micrometer-sized powder of dendronized perylenediimide dispersed in water. The nanoparticle colloidal solution provided a fluorescence quantum yield of 0.58. The absorption and emission spectral studies demonstrated that the bulky dendron groups at the side bays of perylenediimide chromophore efficiently suppress the interchromophoric interactions in the nanoparticles. Fluorescence measurement on several single nanoparticles underlines that the prepared nanoparticles are bright and photo-stable enough to be a useful probe for single particle fluorescence investigation.

Abstract: In situ observation of single oxidation/halogenation events by catalytically generated hypobromite species using single molecule fluorescence microscopy allows montioring 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.

Abstract: 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 solventscales 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 similar to 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+, M2+, Ca2+ Ba2+ Zn2+), producing large blue shifts in the excitation spectra and significant cation-induced fluorescence amplifications.

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Abstract: Polarization dependence of the second harmonic generation (SHG) was studied on a single crystal of the photochromic N-(3,5-di-tertbutylsalicylidene)-4-aminopyridine (4P). The optimal polarization condition for SHG is obtained when both the fundamental and second harmonic waves are parallel to the main axis of the crystal. This experimental result was correlated with calculations based on X-ray diffraction structure. Also, wavelength dependent studies showed that SHG-switching on near-UV irradiation could be tuned by utilizing the absorption and resonance effects of the photo-induced species. (c) 2007 Elsevier B.V. All rights reserved.

Abstract: The fast and reversible on/off switching of the fluorescence emission of the GFP-like fluorescent protein Dronpa has attracted considerable interest for applications in subdiffraction imaging. In this paper we study the use of a donut-mode beam in combination with two more overlapping laser beams to increase the imaging resolution through selective switching to the nonfluorescent photoswitched state. We devise and run a series of numerical simulations to determine suitable photophysical parameters of prospective, thermally stable photoswitchable molecules, in terms of photoswitching quantum yields, fatigue resistance, and possible presence of transient nonfluorescent states. Many of our findings are applicable to other measurements that make use of donut beams, and these guidelines can be used in the synthesis and screening of novel pholoswitchable compounds. We experimentally demonstrate the possibility of obtaining increased resolution by making use of the efficient and thermally stable Dronpa photoswitching, using equipment that is commonly available.

Abstract: In situ observation of the catalytic activity of individual M-chymotrypsin enzymes reveals a novel pathway for spontaneous deactivation. Rather than deactivating abruptly in a one-step process, the enzyme seems to struggle for life; the activity decreases stepwise with intermittent inactive periods before deactivating irreversibly. During the active periods, dynamic disorder and memory effects are observed, originating from conformational fluctuations within the enzyme's structure.

Abstract: Vapor deposition method was used to obtain nanocrystals of N-(3,5-di-tert-butylsalicylidene)-4-aminopyridine (1) and N-(3,5-di-tert-butylsalicylidene)-4-iodoaniline (2), and nanoscale thin layers (65 nm thick) of cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene (3). Compound 3 was also deposited on a substrate previously covered by gold nanoparticles (phi = 100 nm). Nanoscale crystals of 1 and 2 showed similarities with bulk crystals: they exhibit photochromic properties and they have similar shapes according to Atomic force microscopy (AFM) investigations. The thin layers of 3, with and without gold nanoparticles, showed light-induced absorption change in the visible. Copyright (c) 2007 John Wiley & Sons, Ltd.

Abstract: Shape persistent perylene diimide (PDI) multichromophores incorporating ethynylene bridges have been synthesized in high yield via palladium-catalyzed Hagihara coupling, which provides compounds with no rotational or constitutional isomerism in contrast to polyphenylene dendrimers. Their excited-state pathways have been studied at the ensemble and at the single-molecule level and compared to several model compounds. In an apolar solvent, energy hopping and/or energy transfer between the chromophoric units are the dominating processes. In a polar medium, energy hopping is still operative, but electron transfer from the phenyl ethynylene bridge to the chromophore occurs if the former is connected to the bay area of PDI. This effect should be considered when further developing this type of multichromophore, as this nonradiative deactivation process might be unwanted for applications such as optical and electronic devices. At the single-molecule level, the fluorescence intensity traces are characterized by rich on-off dynamics, which we attribute to oxygen-enhanced intersystem crossing leading to the formation of a long-lived dark charge-separated state.

Abstract: We have prepared two fluorescent dyes derived from 8-(4-tolyl)-4,4-difluoro-4-bora-3a, 4a-diaza-s-indacene with phenoxy and (o-bromo) phenoxy substituents at the 3,5- positions by a novel nucleophilic substitution reaction of the corresponding 3,5-dichloroBODIPY analogue. UV-vis absorption, steady-state and time-resolved fluorimetry have been used to investigate their solvent-dependent photophysical properties. The two BODIPY derivatives show narrow absorption and emission bands and display small Stokes shifts. The substituents at the 3,5- positions (phenoxy in 1 and o- bromophenoxy in 2) have a minor effect on the fluorescence quantum yields (0.16-0.40 for 1, 0.17-0.44 for 2) and lifetimes (1.09-2.51 ns for 1, 1.11-2.78 ns for 2). For both compounds, the fluorescence rate constant equals (1.5 +/- 0.1) x 10(8) s(-1).

Abstract: Several authors demonstrated that an oligonucleotide based pH-sensitive construct can act as a switch between an open and a closed state by changing the pH. To validate this process, specially designed fluorescence dye-quencher substituted oligonucleotide constructs were developed to probe the switching between these two states. This paper reports on bulk and single molecule fluorescence investigations of a duplex-triplex pH sensitive oligonucleotide switch. On the bulk level, only a partial quenching of the fluorescence is observed, similarly to what is observed for other published switches and is supposed to be due to intermolecular interactions between oligonucleotide strands. On the single molecule level, each DNA-based nanometric construct shows a complete switching. These observations suggest the tendency of the DNA construct to associate at high concentration.

Abstract: The single-molecule fluorescence blinking behavior of the organic dye Atto647N in various polymer matrixes such as Zeonex, PVK, and PVA as well as aqueous media was investigated. Fluorescence blinking with off-times in the millisecond to second time range is assigned to dye radical ions formed by photoinduced electron transfer reactions from or to the environment. In Zeonex and PVK, the measured off-time distributions show power law dependence, whereas, in PVA, no such dependence is observed. Rather, in this polymer, off-time distributions can be best fitted to monoexponential or stretched exponential functions. Furthermore, treatment of PVA samples to mild heating and low pressure greatly reduces the frequency of blinking events. We tentatively ascribe this to the removal of water pockets within the polymer film itself. Measurements of the dye immobilized in water in the presence of methylviologen, a strongly oxidizing agent, reveal simple exponential on- and off-time distributions. Thus, our data suggest that the blinking behavior of single organic molecules is sensitive to their immediate environment and, moreover, that fluorescence blinking on- and off-time distributions do not inherently and uniquely obey a power law.

Abstract: Covalently linked cyclic porphyrin arrays have been synthesized to mimic natural light-harvesting apparatuses and to investigate the highly efficient energy migration processes occurring in these systems for future applications in molecular photonics. To avoid an ensemble-averaged picture, we performed a single-molecule spectroscopic study on the energy migration processes of cyclic porphyrin arrays and a linear model compound embedded in a rigid polymer matrix by recording fluorescence intensity trajectories, by performing coincidence measurements, and by doing wide-field defocused imaging. Our study demonstrates efficient energy migration within the cyclic porphyrin arrays at the single-molecule level. By comparison with the data of the linear model compound, we could pinpoint the role of the dipole-dipole coupling between diporphyrin subunits and the rigidity of the cyclic structures on the energy transfer processes.

Abstract: N-(3,5-Di-tert-butylsalicylidene)-4-iodobenzene (1) is photochromic in the solid state, owing to its ability to yield enol-keto tautomerism. Despite its centrosymmetric structure (P2(1)/c), polycrystalline thin films of I show second harmonic generation (SHG). Moreover, this SHG can be reversibly switched by light. Investigations using confocal microscopy showed that a morphology change occurs when the fundamental beam of a Ti:sapphire (990 nm, 1.3 ps) is focused on the material, yielding an SHG-active solid. Copyright (c) 2007 John Wiley & Sons, Ltd.

Abstract: Single-molecule fluorescence spectroscopy was performed on a number of perylene diimide multichromophores with different dendritic geometries, with the particular goal of characterizing their performance as single-photon sources at room temperature. The quality of the different perylene diimide-containing dendrimers as single-photon sources was evaluated by determining the Mandel parameter. Values similar to ones reported previously for perylene monoimide dendrimers were found. The different arrangements of the chromophores in the different dendrimers do not noticeably affect their efficiency as single-photon emitters. Due to the formation of oxygen-enhanced long dark states, anaerobic conditions are found to be the best for optimizing their performance, which is in contrast with the case for perylene monoimide containing dendrimers.

Abstract: Two new anil molecules exhibiting photochromism in the crystalline state, N-(4-hydroxy)-salicylidene-amino-4-(methylbenzoate) (2) and N-(3,5-di-tert-butylsalicylidene)-4-aminopyridine (3), are obtained. Upon irradiation in the UV, the yellow crystals change color to red, owing to enol-keto intramolecular tautomerism. The red color disappears, when crystals are left in the dark or irradiated with visible light. 3 has the most stable keto form among all anil-type photochromic compounds (tau = 460 days at room temperature). Both exhibit nonlinear optical (NLO) properties and show powder second harmonic generation (SHG) of respectively 10 and 3 times vs urea. X-ray diffraction shows acentric structures where molecules line up "head-to-tail" through hydrogen bonds for 2 (space group Pc), or form a chiral helix 3 (space group P3(2)). Evidence of reversible structural change is given for 3, and we demonstrate the functionality of this crystal as an NLO switching material, as SHG can be photomodulated by about 30%.

Abstract: This paper reports the dynamics of the photochromic reaction of crystalline N-(3,5di-tert-butylsalicyliden)-4-aminopyridine which can be applied for nonlinear optical photoswitches. The transient absorption spectra of the microcrystalline powder were measured by femtosecond diffuse reflectance spectroscopy and compared with those in ethanol solution. Similar photochromic reaction occurs in solid state and in solution, however some intermediate species were observed before the long lived photoproduct in solid state probably due to steric hindrance in the crystalline state.

Abstract: Although being an efficient photochromic compound which absorbs in the blue in its stable form and in the orange in its photoactivated form, the mercury dithizonate complex is shown to be a poor optical limiter for nanosecond laser pulses at the wavelengths where both isomers absorb. Optical limiting effect, which is a consequence of reverse saturable absorption due to the photoactivated form, is demonstrated to be weak because of the back photobleaching of this form, which is important all the more as the laser intensity is high. Numerical integration of the spatio-temporal evolution of the laser beam intensity across the solution helps the understanding of the respective roles of the laser fluence and pulse duration. Finally, we draw the conclusion that photochromic compounds can only be used as optical limiters if the time constant for the back photochemical reaction is slow compared to the pulse duration.