Non-alternant polycyclic aromatic hydrocarbons versus closed carbon surfaces

Abstract

The investigations described in this thesis were initiated to study aspects of the chemistry of non-alternant PAH that are connected to the formation, the selective synthesis and the properties of closed carbon surfaces, i.e. fullerenes and carbon nanotubes.

Fundamental processes responsible for the build up and rearrangement of non-alternant PAH in the gas-phase using Flash Vacuum Thermolysis (FVT) are the subject of Part I of this thesis. Convincing evidence is given on the basis of FVT of selectively deuterated ethynyl-substituted (E-PAH) for the formation of cyclopenta-fused PAH (CP-PAH) from E-PAH via the previously proposed Brown mechanism. Unexpectedly, a hitherto unknown hydrogen migration process was identified. It is shown that the hydrogen atoms of alternant PAH and CP-PAH are mobile; they migrate under high temperature conditions along the perimeter of the molecule. The facile deuterium migration has important consequences for the interpretation of (earlier) mechanistic studies on the high temperature chemistry of (CP)-PAH.

In Part II of this thesis the use of unimolecular non-alternant PAH progenitors for the synthesis of C60, C70 and heterofullerenes is explored. The synthesis by design of C60 starting from the C60H30 PAH benzo[1,2-e:3,4-e':5,6-e'']tribenzo[l]acephenanthrylene is achieved via fifteen-fold consecutive H2 losses and ring closures induced by matrix-assisted-laser-desorption-ionization time-of-flight mass spectrometry (MALDI TOF-MS). This has led to the Schlegel-match proposition as a design criterion for the identification and preparation of (novel) unimolecular precursors for closed carbon surfaces.

Based on these results two different unimolecular precursors for C70 are proposed. One is a Schlegel-match precursor, viz. a corannulene derivative. The other C70 progenitor is a derivative of a fullerene pipe. The results of the calculations indicate that both progenitors can be converted into C70 by cyclodehydrogenations under similar conditions as applied to C60H30 rendering their syntheses worthwhile.

An ultimate objective of designing selective synthesis for closed carbon surfaces is to be able to modify their properties by for example the incorporation of hetero-atoms. A particular ensuing goal is the preparation of azafullerenes. Hence, based on the Schlegel-match proposition a C57H33N3 non-alternant PAH was identified as a precursor for a C60-derivative in which three carbon atoms are replaced by nitrogen atoms, i.e. the novel triazafullerane C57H3N3 and its ionic analogues C57H2N3+/C57H2N3-. It is shown that the non-alternant C57H33N3 hetero-PAH indeed converts into an (ionic) azafullerane via successive H2 losses and ring closures under MALDI TOF-MS conditions.

In Part III of this thesis the electronic and magnetic properties of fullerene and carbon nanotube sub-structures are explored. At first instance, the properties of the key sub-structures of classical fullerenes, viz. those entirely composed of hexagons and pentagons, i.e. cyclopenta-fused PAH were considered. The electronic effect of the externally-fused cyclopenta-moiety on a PAH perimeter in CP-PAH was studied by Cyclic Voltammetry (CV). Mono- and bis-CP-PAH with externally-fused cyclopenta-moieties were shown to undergo facile reduction and, hence, are expected to have high electron affinities (EA). To compare the electronic behavior of cyclopenta-fused PAH with that of their cyclohepta-fused counterparts, efficient syntheses of CH-PAH had to be developed since only a few representatives were known and only available via lengthy syntheses. The syntheses of the novel cyclohepta-fused PAH, cyclohepta[c,d]pyrene and cyclohepta[c,d]fluoranthene is achieved by FVT of bis-acetylated precursors at remarkably low temperatures. The electronic behavior of CH-PAH is anti-symmetric with that of the corresponding CP-PAH. Whereas CP-PAH have high electron affinities (n-type properties), CH-PAH have low first oxidation potentials (p-type properties). Finally, the magnetic properties of a series of CH-PAH are discussed. The presence of a heptagon leads in most cases to a strong paratropic current in this ring when a magnetic field is applied.