Probing the Effects of Conformation on the Photophysics of Conjugated Porphyrin Oligomers

Licentiate thesis, 2013

Controlling the conformational heterogeneity of molecular systems is fundamental to envisage future applications of these systems, such as in solar cells or molecular devices to name a few. The work presented in this Thesis investigates the effects of conformational flexibility on the photophysical properties of a series of conjugated porphyrin oligomers Pn (n = 1-4, 6, 8). First the influence of temperature on the conformational distribution displayed by these systems has been explored. At low temperature, these porphyrins spontaneously form highly-ordered planar aggregates resulting in characteristic changes in their spectral properties. The observed spectral changes could be explained by a planarization of the oligomers accompanying the aggregate formation. Further the influence of conformation on the electron transfer reactions in long fullerene-appended porphyrin oligomers PnC60 (n = 4, 6) was studied thoroughly. The rate of charge separation could be tuned by selective excitation of different populations of conformations. Twisted conformations showed faster charge separation rates and higher efficiencies than planar conformations. The disparity in charge separation rates was ascribed to the differences in driving force for charge separation between twisted and planar conformations. The conformational distribution of PnC60 systems could also be restrained and controlled by means of coordination to an octadentate ligand T8, thus providing simpler model systems to investigate the influence of conformation on the charge separation. The semi-circular complexes PnC60-T8 showed significant differences in their spectral properties compared to their linear counterparts that were attributed to their planarity and higher rigidity. Charge separation was also investigated in these semi-circular complexes. These complexes displayed an extremely slow rate of charge separation and low efficiency of charge separation. The reason for this slow charge separation was the lack of driving force for charge separation to occur, as expected for more planar conformations.