Size effect on the fluorescence properties of dansyl-doped silica nanoparticles.

We present here the study of the photophysical properties of new dye-doped silica nanoparticles (DDNs) bearing dansyl fluorescent derivatives covalently linked to the silica matrix. The described experimental evidences show how the different location of the chromophores induces great changes in their photophysical behavior, suggesting that fluorophores located near the surface of the nanoparticles have a very different behavior with respect to the internal molecules. These latter ones, in fact, are shielded from the solvent and have a strong blue emission, while those at the periphery interact with the solvent and show a weaker red-shifted emission. As a consequence, the fluorescence properties of these nanoparticles are an average between the characteristics of the two different families of dyes. The relative amount of fluorophores located in the two compartments can be controlled simply by changing the size since, from our results, the thickness of the solvent permeable layer is not relevantly affected by the diameter of the nanoparticles. It is noteworthy that the fluorophores located in the outer shell exhibit very peculiar features: they are sensitive and interact with small molecules such as solvent molecules but, at the same time, they are not accessible to big receptor species such as beta-cyclodextrins. Such results indicate that most of the solvent-sensitive dansyl moieties are located within pores large enough to only accommodate solvent but not big molecules as cyclodextrins, giving precious insight on the morphology of the nanoparticles.

Turning fluorescent dyes into Cu(II) nanosensors.

There is great interest in the self-organization of the proper subunits as a new strategy for the realization of fluorescent chemosensors. In this article, it is shown that commercially available fluorescent dyes, functionalized with triethoxysilane moieties, can be converted into fluorescent chemosensors by simple inclusion into silica nanostructures. Dye-doped silica nanoparticles and thin films detect Cu(II) ions in the micromolar range by the quenching of fluorescence emission. The different response toward Zn(II), Ni(II), and Co(II) metal ions was also investigated and is reported. The self-organization of the silica structures leads, at the same time, to the formation of metal ion binding sites as well as to the linking of a fluorescent reporter in their proximity. Structural features of the materials, particularly particle size and network porosity, strongly affect their ability to act as fluorescent sensors.

Synthesis, photophysics, and photoresponse of fullerene-based azoaromatic dyads.

The synthesis and photophysical characterization of a series of fullerene-based, donor-acceptor dyads is presented, along with a description of their behavior as single molecular components in photovoltaic cells. The spectroscopic and photophysical properties of the dyads, investigated by steady-state fluorescence spectroscopy, pico- and nanosecond transient optical spectroscopy and time-resolved electron paramagnetic resonance (EPR) spectroscopy, revealed that the dyads undergo multiple-step energy transfer from the donor singlet excited state to the fullerene triplet excited state, which in turn decays to the donor triplet state. The inefficient formation of a charge-separated state, both in solution and in the solid state, translates into a poor photovoltaic performance of dyads 2 b-4 b if compared to that of dyad 1 b, in which photoinduced electron transfer is operative in the solid state. In addition, the results of the photophysical investigation suggested that the performance of the solar cells was also limited by the low-lying donor triplet excited state that acts as a photoexcitation energy sink.