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About Supramolecular Assemblies of π-Conjugated Systems

Publisher Summary This chapter discusses a model that relies on the knowledge of the molecular electrostatic potential, which is derived from a molecular wavefunction by using the usual methods for calculating the mean expectation value of an operator. In its basic premises the model employs quantum mechanics, with only the approximations necessary in molecular quantal calculations. The model is also discussed regarding its relationships with the Hellmann–Feynman theorem. The electrostatic potential V itself is examined in order to show how the electrostatic potential reflects the characteristics of the electronic distribution of a molecule and then the reliability of V is discussed as a reactivity index. The shape of the electrostatic potential and its relationship to the electronic molecular structure is discussed with the aid of various examples. One of them includes the glycine tautomers and the corresponding anion example. The chapter also discusses the electrostatic molecular potential in terms of local group contributions.

Electronic Molecular Structure, Reactivity and Intermolecular Forces: An Euristic Interpretation by Means of Electrostatic Molecular Potentials

A survey is provided of recent progress in the understanding of singlet fission, a spin-allowed process in which a singlet excited molecule shares its energy with a ground-state neighbor to produce two triplet excited molecules. It has been observed to occur in single-crystal, polycrystalline, and amorphous solids, on timescales from 80 fs to 25 ps, producing triplet yields as high as 200%. Photovoltaic devices using the effect have shown external quantum efficiencies in excess of 100%. Almost all the efficient materials are alternant hydrocarbons of the acene series or their simple derivatives, and it is argued that a wider structural variety would be desirable. The current state of the development of molecular structure design rules, based on first-principles theoretical considerations, is described along with initial examples of implementation.

Recent Advances in Singlet Fission

Fulleropyrrolidines: A Family of Full-Fledged Fullerene Derivatives

Photoprocesses associated with the complexation of a pyridine-functionalized C60 fullerene derivative to ruthenium- and zinc-tetraphenylporphyrins (tpp) have been studied by time-resolved optical and transient EPR spectroscopies. It has been found that upon irradiation in toluene, a highly efficient triplet–triplet energy transfer governs the deactivation of the photoexcited [Ru(tpp)], while electron transfer (ET) from the porphyrin to the fullerene prevails in polar solvents. Complexation of [Zn(tpp)] by the fullerene derivative is reversible and, following excitation of the [Zn(tpp)], gives rise to very efficient charge separation. In fluid polar solvents such as THF and benzonitrile, radical-ion pairs (RPs) are generated both by intramolecular ET inside the complex and by intermolecular ET in the uncomplexed form. Charge-separated states have lifetimes of about 10 μs in THF and several hundred of microseconds in benzonitrile at room temperature.

Efficient charge separation in porphyrin-fullerene-ligand complexes.

Synthesis of N-acylated fulleropyrrolidines: New materials for the preparation of Langmuir-Blodgett films containing fullerenes

A noncovalently linked, dynamic fullerene porphyrin dyad. Efficient formation of long-lived charge separated states through complex dissociation

The properties of Langmuir layers made up of a series of derivatized fulleropyrrolidines featuring hydrogenated and fluorinated acyl chains have been investigated, focusing on their ability to form a monomolecular layer. We have found that compounds with hydrocarbon chains form floating films on the subphase whose thickness depends on the concentration of the spreading solution as observed in pure C[sub 60]. On the contrary, the fluorinated derivative gives rise to monomolecular layers irrespective of the concentration values. 14 refs., 6 figs., 1 tab.

Incorporation of an Acyl Group in Fulleropyrrolidines: Effects on Langmuir Monolayers

Transient EPR spectroscopy has been applied to measure the spectra of excited triplet states in two series of fullerene bisadducts. Eight different bisadducts of C60 with bis(ethoxycarbonyl)methylene and five bis-N-methylpyrrolidines have been analyzed. The D and E parameters of electron dipolar interaction and the populations of the zero field triplet sublevels, accounting for spin polarization, have been determined by spectrum simulation. They appear peculiar for each bisadduct. It is shown that triplet EPR technique represents a useful spectroscopic tool that allows structural assignments to bisadducts of C60.

Luigi Pasimeni

Papers

Efficient charge separation in porphyrin-fullerene-ligand complexes.

Synthesis of N-acylated fulleropyrrolidines: New materials for the preparation of Langmuir-Blodgett films containing fullerenes

A noncovalently linked, dynamic fullerene porphyrin dyad. Efficient formation of long-lived charge separated states through complex dissociation

Incorporation of an Acyl Group in Fulleropyrrolidines: Effects on Langmuir Monolayers

Use of transient epr spectroscopy of excited triplet state for the structural assignment of bisadducts of fullerene c60