Fluorescence microscopy and conductivity measurements reveal a remarkably strong effect of hydrophobic groups on the mobility of protons in water. The addition of 5 M of tetramethylurea (4 methyl groups per molecule) results in a reduction of the proton mobility by a factor of approximately 10: hydrophobic hydration strongly suppresses proton mobility. These observations demonstrate the collective nature of aqueous proton transport.

Essentially complete photoinduced electron transfer quenching of the fluorescence of a perylene-calixarene compound occurs in poly(methyl acrylate) and poly(vinyl acetate) above their glass transition temperatures (T(g)), but the fluorescence is completely recovered upon cooling the polymer matrix to a few degrees below the T(g). The switching can be observed in an on/off fashion at the level of individual molecules.

The photophysical properties are described of silicon nanoparticles protected with a shell of n-butyl chains, and with an asymmetric size distribution with a peak between 2 and 3 nm, and a tail extending up to 7 nm. The excited nanoparticles decay via multiexponential luminescence on a time scale of a few nanoseconds. Longer-lived nonluminescent dark states were observed by nanosecond transient absorption spectroscopy. The average lifetime of the luminescent excited states increased somewhat with increasing wavelengths of excitation and emission. The nanosecond transient absorption spectra shifted to longer wavelengths with time. Probably, these observations are related to the size distribution of the particles: larger particles are excited at longer wavelength, emit at longer wavelength, and have longer-lived and red-shifted nanosecond transient absorption spectra. Two-photon excited luminescence showed broader emission spectra than one-photon excited luminescence at the same excitation energies. The two-photon cross-sections were found to be surprisingly small.

Catalyst recovery is an important topic in homogeneous catalysis, since product/catalyst separation is one of the main obstacles towards application of this type of catalysis. So far, several strategies for catalyst recycling have been explored, but a general strategy remains elusive. [1][2][3] A widely studied approach to facilitate catalyst/product separation is attachment of homogeneous catalysts to soluble or insoluble supports, which can consist of organic polymers [4][5][6][7] or dendrimers, [8,9] inorganic materials, [10,11] or hybrids thereof. Inorganic materials have proved to be particularly suited as solid supports for homogeneous catalysts because of their physical strength and chemical inertness, and many such immobilized catalyst systems have been reported. Common drawbacks, however, are the generally lower activity and selectivity compared to the homogeneous counterpart. Surprisingly, however, an in-depth investigation of the effect of the immobilization process on the performance of the catalyst appears to be lacking. To gain more insight into immobilization of transition metal catalysts we set out to design a method for the detection of bis-phosphine ligands on surfaces. The ligand studied acts as a fluorescence probe, and detection is achieved by two-photon excitation fluorescence microscopy. In principle, this allows the immobilization product to be imaged with high spatial resolution (down to single molecules). Here we report the findings of the first study on the immobilization process employing an intrinsically fluorescent ligand imaged on a submicrometer level, which indicated that precondensation of ligands takes place prior to immobilization under the standard immobilization conditions, a conclusion supported by analysis of the liquid phase. These results directly translate to simple procedures that do not have these precondensation problems. The resulting immobilized catalysts show superior performance in hydroformylation catalysis, and open the way toward rational catalyst immobilization.Two-photon excitation (TPE) fluorescence microscopy has shown astonishing potential, but application has been mainly restricted to imaging biological samples.[12] Twophoton absorption (TPA) is a process in which two photons are absorbed simultaneously. Such a process only occurs at a very high flux of photons by focusing a pulsed near-infrared laser, thus restricting the excitation to a very small focal volume, with no appreciable off-focal fluorescence. Generally, the two-photon selection rule yields low background fluorescence, and hence high contrast in the images can be produced. [12,13] As part of a broader effort to study the immobilization of transition metal catalysts, [14][15][16][17] we were curious whether catalyst immobilization could be studied by fluorescence imaging. For this purpose we envisioned the class of xanthene-based phosphorus ligands, also known as Xantphos, suitable for various reasons. Metal complexes of these ligands have outstanding catalytic properties in various catalytic ...

In addition to the commonly observed single molecule fluorescence intensity fluctuations due to molecular reorientation dynamics, a perylene bisimide-calixarene compound (1) shows additional on-off fluctuations due to its ability to undergo intramolecular excited state electron transfer (PET). This quenching process is turned on rather sharply when a film of poly(vinylacetate) containing 1 is heated above its glass transition temperature (T g), which indicates that the electron transfer process depends on the availability of sufficient free volume. Spatial heterogeneities cause different individual molecules to reach the electron transfer regime at different temperatures, but these heterogeneities also fluctuate in time: in the matrix above T g molecules that are mostly nonfluorescent due to PET can become fluorescent again on timescales of seconds to minutes.The two different mechanisms for intensity fluctuation, rotation and PET, thus far only observed in compound 1, make it a unique probe for the dynamics of supercooled liquids.