Quantum-chemical studies on Porphyrins, Fullerenes and Carbon Nanostructures

All life on the Earth depends on photosynthesis which is a photochemical process in green plants and in some bacteria that has a very high quantum yield unusual for visible light energy transformation into organic chemical reactions. Important information about photosynthetic pigments has been obtained in studies of their luminescence, which is quite specific in comparison with typical organic dyes.

Fullerene-based materials have attracted considerable interest since the discovery of C₆₀. A promising area of research concerns metal–fullerene interactions and their application to advanced nano materials, with potential use in optical and switching devices, as photoconductors, and for hydrogen storage. Moreover, transition metal complexes of fullerenes show catalytic activity in homogeneous hydrogenation of acetylenic alcohols [1] and hydroformylation of alkenes [2]. In heterogeneous catalysis, exohedral metallofullerenes are found to promote hydrogenation of olefins and acetylenes [3, 4] as well as reduction of carbon monoxide to methane [5, 6].

During recent years, a significant progress has been observed in the field of synthesis and characterization of photoactive materials. In particular,tremendous effort have been invested in design of materials exhibiting high nonlinear optical response. A plethora of organic and organometallic systems have been studied in this context. One of the most common routes to design of new molecules with high values of first-order hyperpolarizability (\(\beta \)) has its roots in so called two-state model proposed in late seventies. Within this model, \(\beta \) is expressed in terms of dipole moment difference, transition intensity and energy difference between excited and ground electronic state. Various molecular systems of donor–acceptor (DA) type according to two-level model have been proposed in order to maximize \(\beta \). However, it has been reported recently that A–A systems containing fullerene may also exhibit high \(\beta \) values.

Since their discovery endohedral fullerenes have been extensively investigated because of their novel structure and properties. Electrical properties have been of major interest owing to a variety of possible applications ranging from qubits for quantum computation to organic photovoltaic devices. Our interest in this connection lies in the theoretical determination of the linear and nonlinear optical properties, i.e. the (hyper)polarizabilities, of these materials. For that purpose we have chosen initially to study the prototypical metal endohedral fullerene, Li@C\(_{60}\), and its cation \([\)Li@C\(_{60}]^+\).

A design of organic molecules for a wide range of nonlinear optical applications is still the subject of intense experimental and theoretical research [1, 2, 3]. The ease of processing and modification as well as relatively low cost of production make organic materials promising candidates for future photonic applications.

In 1991 the elongation method, an efficient method for quantum mechanical calculations of large systems, was originally proposed by Imamura [1] during one of his stays in Heidelberg, Germany. Although in the early 1990s the concept of and need for order-N [O(N)] methods didn’t exist, Prof. Akira Imamura was thinking about “how to avoid direct SCF calculations for large biological systems (biopolymers composed of hundreds if not thousands of residues of amino acids or nucleic acid base pairs in proteins and DNA or RNA) by treating only the local interactions between a few neighbor residues in large systems.” While contemplating how to perform such calculations, he got the idea of “theoretically simulating the synthesis of polymers so as to mimic the chemical reactions which occur in nature during polymerization reactions to form peptides, proteins and polynucleotides.