Fabrizio Messina

Real-time observation of the charge transfer to solvent dynamics

Intermolecular electron-transfer reactions have a crucial role in biology, solution chemistry and electrochemistry. The first step of such reactions is the expulsion of the electron to the solvent, whose mechanism is determined by the structure and dynamical response of the latter. Here we visualize the electron transfer to water using ultrafast fluorescence spectroscopy with polychromatic detection from the ultraviolet to the visible region, upon photo-excitation of the so-called charge transfer to solvent states of aqueous iodide. The initial emission is short lived (~60 fs) and it relaxes to a broad distribution of lower-energy charge transfer to solvent states upon rearrangement of the solvent cage. This distribution reflects the inhomogeneous character of the solvent cage around iodide. Electron ejection occurs from the relaxed charge transfer to solvent states with lifetimes of 100-400 fs that increase with decreasing emission energy.

Solvatochromism Unravels the Emission Mechanism of Carbon Nanodots

High quantum yield, photoluminescence tunability, and sensitivity to the environment are hallmarks that make carbon nanodots interesting for fundamental research and applications. Yet, the underlying electronic transitions behind their bright photoluminescence are strongly debated. Despite carbon-dot interactions with their environment should provide valuable insight into the emitting transitions, they have hardly been studied. Here, we investigate these interactions in a wide range of solvents to elucidate the nature of the electronic transitions. We find remarkable and systematic dependence of the emission energy and kinetics on the characteristics of the solvent, with strong response of the photoexcited dots to hydrogen bonding. These findings suggest that the fluorescence originates from the radiative recombination of a photoexcited electron migrated to surface groups with holes left in the valence band of the crystalline core. Furthermore, the results demonstrate the fluorescence tunability to inherently derive from dot-to-dot polydispersity, independent of solvent interactions.

Fluorescent nitrogen-rich carbon nanodots with an unexpected β-C3N4 nanocrystalline structure

Carbon nanodots are a class of nanoparticles with variable structures and compositions which exhibit a range of useful optical and photochemical properties. Since nitrogen doping is commonly used to enhance the fluorescence properties of carbon nanodots, understanding how nitrogen affects their structure, electronic properties and fluorescence mechanism is important to fully unravel their potential. Here we use a multi-technique approach to study heavily nitrogen-doped carbon dots synthesized by a simple bottom-up approach and capable of bright and color-tunable fluorescence in the visible region. These experiments reveal a new variant of optically active carbonaceous dots, that is a nanocrystal of beta carbon nitride (β-C3N4) capped by a disordered surface shell hosting a variety of polar functional groups. Because β-C3N4 is a network of sp3 carbon and sp2 nitrogen atoms, such a structure markedly contrast with the prevailing view of carbon nanodots as sp2-carbon materials. The fluorescence mechanism of these nanoparticles is thoroughly analyzed and attributed to electronic transitions within a manifold of surface states associated with nitrogen-related groups. The sizeable bandgap of the β-C3N4 nanocrystalline core has an indirect, albeit important role in favoring an efficient emission. These results have deep implications on our current understanding of optically active carbon-based nanoparticles and reveal the role of nitrogen in controlling their properties.

Evidence of delocalized excitons in amorphous solids.

We studied the temperature dependence of the absorption coefficient of amorphous SiO2 in the range from 8 to 17.5 eV obtained by Kramers-Kronig dispersion analysis of reflectivity spectra. We demonstrate the main excitonic resonance at 10.4 eV to feature a close Lorentzian shape redshifting with increasing temperature. This provides a strong evidence of excitons being delocalized notwithstanding the structural disorder intrinsic to amorphous SiO2. Excitons turn out to be coupled to an average phonon mode of 83 meV energy.

Ligand-centred fluorescence and electronic relaxation cascade at vibrational time scales in transition-metal complexes.

Using femtosecond-resolved photoluminescence up-conversion, we report the observation of the fluorescence of the high-lying ligand-centered (LC) electronic state upon 266 nm excitation of an iridium complex, Ir(ppy)3, with a lifetime of 70 ± 10 fs. It is accompanied by a simultaneous emission of all lower-lying electronic states, except the lowest triplet metal-to-ligand charge-transfer ((3)MLCT) state that shows a rise on the same time scale. Thus, we observe the departure, the intermediate steps, and the arrival of the relaxation cascade spanning ∼1.6 eV from the (1)LC state to the lowest (3)MLCT state, which then yields the long-lived luminescence of the molecule. This represents the first measurement of the total relaxation time over an entire cascade of electronic states in a polyatomic molecule. We find that the relaxation cascade proceeds in ≤10 fs, which is faster than some of the highest-frequency modes of the system. We invoke the participation of the latter modes in conical intersections and their overdamping to low-frequency intramolecular modes. On the basis of literature, we also conclude that this behavior is not specific to transition-metal complexes but also applies to organic molecules.

Ultrafast Solvent-Assisted Electronic Level Crossing in 1-Naphthol†

Keywords: femtochemistry ; kinetics ; naphthol ; photoacids ; ultrafast dynamics Reference EPFL-ARTICLE-189622doi:10.1002/anie.201301931View record in Web of Science Record created on 2013-10-01, modified on 2017-05-12

Optical properties of phosphorus-related point defects in silica fiber preforms

We report an experimental study on phosphorus-related point defects in amorphous silica, based on photoluminescence, absorption, and electron spin resonance measurements carried out on P-doped SiO{sub 2} fiber preforms. By photoluminescence measurements excited by laser or synchrotron light we detect an emission band peaked at 3.0 eV with a lifetime in the range of ms. The excitation spectrum of the 3.0 eV emission consists of two transitions peaked at 4.8 and 6.4 eV, the former giving rise also to a measurable absorption band. We attribute this optical activity to a P-related point defect embedded in SiO{sub 2}, based on the spatial correlation between the emission intensity and the P doping level. A detailed spectroscopical investigation allows us to propose a scheme of the electronic levels of this P-related defect, in which the 4.8 and 6.4 eV excitation channels arise from transitions from the ground to two-excited singlet states, while the long-lived 3.0 eV emission is associated to a spin-forbidden transition from an excited triplet to the ground state. Finally, electron spin resonance measurements on X-irradiated samples lead us to propose a tentative microscopic model of the defect as a diamagnetic four-coordinated P impurity substitutional to a Si atom.

Hydrogen-related conversion processes of Ge-related point defects in silica triggered by ultraviolet laser irradiation

The conversion processes of Ge-related point defects triggered in amorphous SiO{sub 2} by 4.7 eV laser exposure were investigated. Our study has focused on the interplay between the (=Ge -H) H(II) center and the twofold coordinated Ge defect (=Ge ). The former is generated in the post-irradiation stage, while the latter decays both during and after exposure. The post-irradiation decay kinetics of =Ge is isolated and found to be anticorrelated to the growth of H(II), at least at short times. From this finding it is suggested that both processes are due to trapping of radiolytic H{sub 0} at the diamagnetic defect site. Furthermore, the anticorrelated behavior is preserved also under repeated irradiation, light at 4.7 eV destroys the already formed H(II) centers and restore their precursors =Ge . This process leads to repeatability of the post-irradiation kinetics of the two species after multiple laser exposures. A comprehensive scheme of chemical reactions explaining the observed post-irradiation processes is proposed and tested against experimental data.

Spectroscopic studies of the origin of radiation-induced degradation in phosphorus-doped optical fibers and preforms

In this paper, we study the radiation-induced point defects related to the phosphorus element that is commonly used to improve the optical properties of silica-based glasses but is responsible of a dramatic increase in their radiation sensitivity. To this aim, the influence of x-ray irradiation on prototype phosphorus-doped canonical fibers and their related preforms was investigated by in situ radiation induced attenuation (RIA), optical absorption, and electron spin resonance (ESR) spectroscopy. The RIA spectra in the (1.5–5 eV) range, can be explained by the presence of at least three absorption bands induced by radiation exposure. Additionally the X-dose dependence of such bands was studied. The main responsible defect for these absorption peaks was the phosphorus oxygen hole center (POHC) center, whose presence was also detected by ESR measurements both in irradiated fibers and preforms, together with the lineshape of the so called P2 defect. Correlations between the RIA bands and the ESR results all...

Growth of H(II) centers in natural silica after UV laser exposure

Abstract The post-irradiation increase in the amplitude of the electron spin resonance doublet split by 11.8 mT, associated with the H(II) centers (Ge H), was measured in dry and wet natural silica irradiated at room temperature with UV photons at 266 nm from a Nd:YAG laser. The concentration of these paramagnetic defects increases on increasing the delay time after the UV exposure, in a time scale of few hours and its final magnitude depends on the number of laser shots and on the OH content. The generation of H(II) centers is correlated with the bleaching of the 5.1 eV absorption band ascribed to the twofold coordinated Ge (Ge ) so supporting the occurrence of a conversion process between these defects whose kinetics is governed by the diffusion of molecular hydrogen in the silica matrix.