Porphyrins and porphyrin derivatives are naturally occurring molecules, whereas carbaporphyrinoids are synthesized porphyrin derivatives. They have received much attention in recent years by the scientific community due to their diverse potential applications in technological developments such as molecular electronic devices and conversion of solar energy. However, the full utilization of this class of compounds can not be realized without an in-depth understanding of their chemical and physical properties. Two of such properties are aromaticity and optical properties. In this thesis, the aromatic properties and the light absorption spectra in the ultraviolet and visible (UV/Vis) range have been studied computationally for some recently synthesized carbaporphyrins and carbachlorins using first-principle computational approaches.
In the first part of the thesis, the background of carbaporphyrinoids and some examples of naturally occurring porphyrins and porphyrin derivatives are delineated. The second and third part review theoretical and computational methods that are employed in studies of the molecular aromaticity and electronic excitation spectra of molecules. The computational studies of magnetically induced current densities and electronic excitation energies are discussed in the fourth chapter. The obtained results are also presented in chapter four and the main conclusions are summarized in the last chapter.
The study shows that all the carbaporphyrinoids studied sustain a magnetically induced ring current in the porphyrin macro ring. This indicates that they are aromatic according to the ring-current criterion. However, the calculated ring-current pathways differ from those predicted from the nucleus independent chemical shift (NICS) calculations and the current pathways deduced from H NMR spectroscopy studies.
The vertical excitation energies which is akin to the ultraviolet-visible spectrum obtained experimentally for some of the selected carbaporphyrinoids also showed deviations from those of the experimental values. These deviations can be ascribed to solvent effects as in the calculation of the vertical excitation energies, solvent effects were not accounted for.
