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Fabrizia Negri

Publications and source records attributed to Fabrizia Negri.

6 recordsLinked to original sources
PubMed2004

Graphitic molecules with partial "zig/zag" periphery.

A new synthetic protocol that affords novel hexa-peri-hexabenzocoronene (HBC) derivatives with partial "zig/zag" periphery and dramatically different electronic properties has been developed, which represents an important step toward understanding structure-property relationships of graphitic materials. The scope of this methodology was broadened by application to a much larger polycyclic aromatic hydrocarbon (PAH) with a more extended "zig/zag" character.

Journal Article↗
PubMed2004

Resonance Raman contribution to the D band of carbon materials: modeling defects with quantum chemistry.

Polycyclic aromatic hydrocarbons (PAHs) are employed to model the Raman features that are generally associated with sp(2) nanostructures in carbon materials or with disorder and defects in graphitic materials. To this end molecular parameters (geometry changes upon electronic excitation, vibrational normal modes, and displacement parameters) are computed with semiempirical quantum-chemical methods for a series of PAHs ranging from 6 to 384 carbon atoms, and Raman intensities are evaluated according to Albrecht's formalism restricted to the A term. The computed preresonance and resonance Raman intensities are compared with available experimental data for hexa-peri-hexabenzocoronene and for pyrene. For the latter compound, simulations carried out at semiempirical and ab initio levels of theory are shown to be of comparable quality. Finally, the collection of displacement parameters computed for the sample of conjugated molecules is used to model the effect of disorder and defects in the Raman response of a carbon material containing sp(2) islands. It is shown that the computed D-band frequency dispersion, with respect to excitation wavelength, reproduces closely the experimental data measured for sp(2) hybridized carbon materials.

Journal Article↗
PubMed2004

Origin, nature, and fate of the fluorescent state of the green fluorescent protein chromophore at the CASPT2//CASSCF resolution.

Ab initio CASPT2//CASSCF relaxation path computations are employed to determine the intrinsic (e.g., in vacuo) mechanism underlying the rise and decay of the luminescence of the anionic form of the green fluorescent protein (GFP) fluorophore. Production and decay of the fluorescent state occur via a two-mode reaction coordinate. Relaxation along the first (totally symmetric) mode leads to production of the fluorescent state that corresponds to a planar species. The second (out-of-plane) mode controls the fluorescent state decay and mainly corresponds to a barrierless twisting of the fluorophore phenyl moiety. While a "space-saving" hula-twist conical intersection decay channel is found to lie only 5 kcal mol(-1) above the fluorescent state, the direct involvement of a hula-twist deformation in the decay is not supported by our data. The above results indicate that the ultrafast fluorescence decay observed for the GFP chromophore in solution is likely to have an intrinsic origin. The possible effects of the GFP protein cavity on the fluorescence lifetime of the investigated chromophore model are discussed.

Anions↗
PubMed2004

Photochemical and electronic properties of conjugated bis(azo) compounds: an experimental and computational study.

We have investigated the photophysical, photochemical and electrochemical properties of two bis(azo) derivatives, (E,E)-m-1 and (E,E)-p-1. The two compounds, which can be viewed as being composed of a pair of azobenzene units sharing one of their phenyl rings, differ only for the relative position of the two azo groups on the central phenyl ring-meta and para for m-1 and p-1, respectively. The UV-visible absorption spectra and photoisomerisation properties are noticeably different for the two structural isomers; (E,E)-m-1 behaves similarly to (E)-azobenzene, while (E,E)-p-1 exhibits a substantial red shift in the absorption bands and a decreased photoreactivity. The three geometric isomers of m-1, namely the E,E, E,Z and Z,Z isomers, cannot be resolved in a mixture by absorption spectroscopy, while the presence of three distinct species can be revealed by analysis of the absorption changes observed upon photoisomerisation of (E,E)-p-1. Quantum chemical ZINDO/1 calculations of vertical excitation energies nicely reproduce the observed absorption changes and support the idea that, while the absorption spectra of the geometrical isomers of m-1 are approximately given by the sum of the spectra of the constituting azobenzene units in their relevant isomeric form, this is not the case for p-1. From a detailed study on the E-->Z photoisomerisation reaction it was observed that the photoreactivity of an azo unit in m-1 is influenced by the isomeric state of the other one. Such observations indicate a different degree of electronic coupling and communication between the two azo units in m-1 and p-1, as confirmed by electrochemical experiments and quantum chemical calculations. The decreased photoisomerisation efficiency of (E,E)-p-1 compared to (E,E)-m-1 is rationalised by modelling the geometry relaxation of the lowest pi-pi* state. These results are expected to be important for the design of novel oligomers and polymers, based on the azobenzene unit, with predetermined photoreactivity.

Journal Article↗
PubMed2002

Electronic states and transitions in C60 and C70 fullerenes.

A review of the most relevant aspects of fullerene electronic structure and spectroscopy is presented. Experimental data and their interpretation based on computational results are discussed both for fullerene C60 and C70, with particular attention to the properties of the isolated molecule. Concerning singlet state spectroscopy, it is shown that because of its high symmetry, only dipole-forbidden electronic states are found in the low excitation energy region of C60. Conversely, the lowering of symmetry in C70 leads to several complications in its electronic structure and spectroscopy, due to the presence of weakly allowed transitions in the low excitation energy region. A slightly less congested distribution of low lying excited states characterizes the triplet manifold of the fullerenes. It is concluded that while C60 is important in aiding understanding of the main features in electronic spectroscopy of fullerenes, such as the presence of strong absorptions in the high energy range, its spectra are deeply Influenced by its high symmetry and are very peculiar. On the other hand, C70, with its lower symmetry and more complex spectra, represents a more realistic model for the intricate details of the electronic structure and electronic spectroscopy of larger and smaller fullerenes and their derivatives, which are generally characterized by lower symmetry compared to C60.

Energy Transfer↗