Fabrizia Negri

Quantum-chemical investigation of Franck-Condon and Jahn-Teller activity in the electronic spectra of Buckminsterfullerene

Abstract Quantum-chemical results are reported which indicate that the absorption, fluorescence and phosphorescence spectra of Buckminsterfullerene are governed by a 1 T 1u ← 1 A g , 1 T 2g → 1 A g and 3 T 2g → 1 A g transition, respectively. Normal modes are calculated and their Franck-Condon and Jahn-Teller activity in these spectra are evaluated, along with the Jahn-Teller distortion of the radical anion. Vibrational progressions are expected to be short. A high phosphorescence quantum yield is predicted.

Interpretation of the vibrational structure of the emission and absorption spectra of C60

The vibronic intensity borrowing activity of the lowest electronically excited singlet states of C60 has been obtained through quantum chemical calculations. The vibrational structure of the UV–visible spectra is found to be dominated by false origins. The calculated intensities of the false origins of the T1g state agree with the vibrational structure observed in the fluorescence spectrum. The same false origins are recognized to be responsible for the vibrational structure of the red edge portion of the absorption spectrum. Only two bands in the spectra are assigned as combination bands involving an ag or a Jahn–Teller active mode. Absorption bands that may be associated with false origins of the states T2g and Gg which are quasidegenerate with S1 are tentatively assigned.

Fully Conjugated Tri(perylene bisimides): An Approach to the Construction of n-Type Graphene Nanoribbons

We present an experimental study encompassing synthesis and characterization of fully conjugated tri(perylene bisimides) (triPBIs), having 19 six-membered carbon rings in the core and six imide groups at the edges. Two structural isomers of triPBIs resulting from the two probable coupling positions were successfully separated by HPLC. To assist the identification of the two structural isomers, quantum-chemical calculations of electronic structure, NMR, and optical spectra were carried out. Calculations predict stable helical and nonhelical configurations for both triPBIs isomers and allow the assignment of triPBIs 6 unequivocally to the most bathochromically shifted absorption spectrum. Increasing the number of PBI units in oligo-PBIs leads to an expansion of the pi system, in turn associated with a reduction of the transport and optical band gaps, and a remarkable increase in electron affinities, which make oligo-PBIs promising n-type functional components in optoelectronic devices.

Hybrid rylene arrays via combination of Stille coupling and C-H transformation as high-performance electron transport materials.

Hybrid rylene arrays have been prepared via a combination of Stille coupling and C-H transformation. The ability to extend the π system along the equatorial axis of rylenes not only leads to broadened light absorption but also increases the electron affinity, which can facilitate electron injection and transport with ambient stability.

Origin of the D line in the Raman spectrum of graphite: A study based on Raman frequencies and intensities of polycyclic aromatic hydrocarbon molecules

A common valence force field for polycyclic aromatic hydrocarbons (PAHs) and graphite has been used in order to explain their Raman spectra from a unified viewpoint. On this basis the correlation observed between experimental spectra has been explained and rationalized. Quantum chemical density functional theory calculations of Raman intensities of small PAHs have been also performed, supporting the conclusions obtained from the dynamical analysis. The results obtained are useful for the characterization of materials containing graphitic domains and provide some new insight on the nature of the D peak in disordered graphite.

Theoretical study of the force fields of the three lowest singlet electronic states of linear polyenes

The potential energy surfaces of all trans hexatriene and octatetraene are investigated within the harmonic approximation in the diabatic and adiabatic representations for the 1A−g, 2A−g, and 1B+u electronic states by an extended Pople–Pariser–Parr (PPP/CI) model. The effect of excitation and of vibronic coupling on the molecular force fields of the three states is examined. While electronic excitation affects only diagonal force constants of local oscillators, vibronic coupling changes drastically the couplings between local oscillators. The calculations reproduce well the observed increase of the frequency of the in‐phase ag C=C stretch upon excitation to the 2A−g state.

Raman activation in disordered graphites of the A1′ symmetry forbidden k≠0 phonon: The origin of the D line

The analysis of the tensors of polarizability derivatives, computed with quantum-chemical methods for several large polycyclic aromatic hydrocarbon molecules, allows to rationalize the origin of the Raman D line observed in disordered graphites as due to a relaxation of the phonon selection rules. It is shown that a minimal rearrangement of the atomic and electronic structure in a lattice different from that of a perfect graphite crystal causes a selective activation of a specific totally symmetric phonon whose frequency falls in the region of the D band and whose motion is dominated by the breathing vibration of aromatic rings. We numerically proof this conjecture on the basis of polarizability derivatives obtained with Density Functional Theory calculations.

Tri-N-annulated Hexarylene: An Approach to Well-Defined Graphene Nanoribbons with Large Dipoles

We report a highly efficient synthetic methodology toward tri-N-annulated hexarylenes from easily available N-annulated perylenes, which is favored by the oxidative ring fusion driven by DDQ/Sc(OTf)(3). To assist the characterization of the new compounds, quantum-chemical calculations of structural and spectroscopic properties have been carried out for three oligomers of N-annulated rylenes. It is shown that tri-N-annulated hexarylene dyes display remarkably large dipole moments likely associated with the formation of H aggregates, as suggested by the marked concentration dependence of the measured UV-vis spectra. It is suggested that the combination of pi-pi stacking interactions and dipole-dipole interactions may favor the formation of highly ordered supramolecular structures, resulting in enhanced charge carrier mobilities.

The vibronic structure of the S0↔S1 and S0↔S2 transitions in simple oligomers of thiophene

We present a quantum‐chemical study of the electronic spectra of the oligomers of thiophene. Geometries and vibrational force fields of the ground and excited electronic states are obtained by an updated version of the semiempirical quantum consistent force field/π electron method implemented to describe sulphur atoms and by ab initio Hartree–Fock and configuration interaction singles methods. The displacement parameters of totally symmetric modes are then obtained and used to model the vibrational structure of the electronic spectra. The contribution of sulphur atoms to the description of the excited state is predicted to be negligible both by ab initio and semiempirical methods which, conversely, indicate a close similarity of thiophene oligomers and polyenes. Based on the results of the simulated spectra a reassignment of some of the bands is proposed. It is shown that mode mixing upon excitation, and not large frequency changes, are responsible for the different Franck–Condon structure of the absorpti...

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.