The synthesis and study of polycyclic aromatic hydrocarbons

The work in this thesis can readily be divided into two parts, the former concerned with the syntheses and absorption spectra of polycyclic aromatic hydrocarbons, and the latter dealing with their NMR spectra. Naphtho-(2'.3’:1.2)-pentacene (I) has been synthesised to complete the annellation series, anthracene, 1.2-benzotetracene, naphtho-(2'.3':1.2)-pentacene. This compound exhibits the typical properties of a pentacene, being readily photo-oxidised and undergoing maleic anhydride addition very readily. 2-Phenyltetracene (II), 2.8-diphenyltetracene (III), 2-phenylpentacene (IV), 2.9-diphenylpentacene (V), and 9-phenyltetraphene (VI) have all been characterised with the exception of (III) which defied synthesis, although a number of possible routes have been investigated. Since β-phenyl and β,β'-diphenyl substituted naphthalenes and anthracenes exhibit an exocyclic quartet of electrons, (II) - (V), the appropriate compounds in the tetracene and pentacene series might also be expected to display this phenomenon. However, a careful examination of the positions of the absorption maxima suggests that the quartet of electrons has not been formed. (VI) has been synthesised so that a molecule, capable of forming both an induced sextet and an exocyclic quartet, could be investigated. The shifts of the B-bands indicate that both types of interaction exist. 2.3,4.5,8.9,10.11-Tetrabenzoperylene (VII) has been made as an intermediate in the synthesis of 1.2,3.4,7.8,9.10-tetrabenzocoronene (VIII). (VII) is an important hydrocarbon in its own right, and shows some interesting and instructive differences in its reactivity with maleic anhydride when compared with other benzo- and naphtho-perylenes. Like its parent compound, perylene, (VII) exists in monomer and dimer crystals, whose colour is slightly different. By twin-fold addition of maleic anhydride, followed by decarboxylation, the coronene homologue (VIII) can be synthesised from (VII); this represents the first unambiguous synthesis of 1.2,3.4,7.8,9.10-tetrabenzocoronene (VIII). A photochemical route to this hydrocarbon has also been investigated: it failed to give positive results. In studying the UV-spectra of a large number of higher annellated pyrenes, certain anomolous peaks appeared in some of the spectra, apparently belonging to the 3.4-benzophenanthrene skeleton. To try and gain further information on this problem, 5.6-benzopicene (IX) has been synthesised. In this compound, either the benzophenanthrene or picene moieties could cause the principal absorption maxima; a study of the UV-spectrum of (IX) however, failed to give any unambiguous results. Naphtho-(3'.1':1.14); (3''.1'':4.5); (1'''.3'":7.8); (3"".1"":11,12)-pentacene (X), a kind of twin tribenzopyrene, has been synthesised and its UV-spectrum studied with regard to the problems outlined in this paragraph. The recent work of Buu-Hoi in proving that 1.2,3.4-dibenzopyrene (XI) is, in actual fact, a dibenzofluoranthene has led us to attempt a new synthesis of dibenzopyrene (XI). The desired compound has not been made, and instead a yellow hydrocarbon has been obtained, to which has been assigned an isomeric dibenzofluoranthene structure on UV and mass spectral evidence. Turning now to the second half of the thesis, the NMR spectra of a large number of polycyclic aromatic hydrocarbons and related compounds have been studied. Methyl derivatives of phenanthrene and pyrene were considered first; methyl substitution in the 9-position in phenanthrene and in the 1- and 6-positions in pyrene caused doublets indicating the high double bond character of these positions. Elsewhere the methyl signal appeared as a singlet, so confirming the benzenoid character of these rings. Further work on methyl derivatives of chrysene, picene, benzophenanthrene and related hydrocarbons has led to more complicated results which for the most part can be explained by the strict application of Robinson’s Sextet. A comprehensive study of the position of particular protons in the NMR spectra of higher annellated pyrenes has yielded results which cannot be correlated with simple ring current theory. To explain these results, a limitation of the conjugation present in these hydrocarbons has been proposed, a concept which receives considerable support from absorption spectroscopy studies. Finally, the NMR spectra of some twenty compounds related to pyrene have been analysed by 1st order techniques. From the experience thus gained, a new interpretation of the NMR spectrum of pyrene itself has been made; a number of suitably alkylated pyrenes have been examined in an attempt to gain support for this interpretation. In addition, a considerable amount of relatively straightforward synthetic chemistry has been carried out to obtain the necessary hydrocarbons for spectroscopic examination