Abstract:
The effect of structural changes on the photophysical properties of a series of boron porphyrin, porphyrazine and corrole complexes was investigated. Both in-plane and out-of-plane structural distortions are induced in the macrocycle by the coordination of two boron atoms within the macrocycle core. The implications of these effects for the application of these compounds in photodynamic therapy (PDT) is investigated by studying their singlet oxygen photosensitisation properties. The in-plane tetragonal elongation (TE) distortions are shown to significantly impact the vibrational frequencies of the boron porphyrins. The large down shifts in the frequency of many vibrations suggested a more flexible porphyrin ring with weakening of several bonds such as Cα-Cm or Cβ-Cβ . Analysis of the electronic structure showed that boron coordination breaks the symmetry of the macrocycle in such a way that it disrupts the degeneracy of the LUMOs. It is concluded that TE distortion could have a significant impact on the photophysical properties of the porphyrin. The in-plane and out-of-plane structural distortions exhibited by the flexible B2OF2 and B2OPhOH porphyrins significantly affect the ultrafast excited state dynamics of the systems and changes the balance of the processes involved in recovery to the ground state after excitation. Instead of undergoing the efficient intersystem crossing (ISC) exhibited by the free bases, boron porphyrins relax to the ground state mostly through internal conversion (IC) (≈ 60%). The increased flexibility and larger core size allows nuclear reorganization in the excited state which is responsible for the changes in photophysical properties and is driven by partial or complete bond inversion of the boron moiety. The ISC pathway is much less efficient which, in addition to poor photostability, results in very low singlet oxygen photosensitisation making boron porphyrins poor candidates for PDT applications. The smaller size of the corrole core does not allow the same extent of structural flexibility in boron corroles. As a result, despite large in-plane and out-of-plane distortions of the PhBHBPh and PhBH coordinated corrole cores, no major changes were observed in their photophysical properties compared upon boron coordination. Their 1O2 photosensitising properties remain unchanged relative to their free base equivalents. Boron porphyrazines are the most flexible macrocycle studied and their conformations can be manipulated by incorporating sterically bulky axial ligands. This allowed the effect of isomerisation to be investigated, with 1O2 quenching properties displayed by only one isomer. However their photosensitising properties are poor. In summary, the studies described in this thesis show distortion-dependent, potentially tunable photophysical behaviour exhibited by boron porphyrin, corrole and porphyrazine complexes.