Effects of non-planarity and beta-substitution on the singlet-excited-state properties of basket-handle porphyrins

The combined effects of non-planarity in the porphyrin macrocycle and beta-substitution on the photophysical properties of a series of basket-handle porphyrins are described. The introduction of a short chain across the periphery of the porphyrin ring induces deformation in the porphyrin ring and beta-substitution further enhances the degree of deformation. The perturbations in the porphyrin pi-system resulted in red-shifted emission bands, low quantum yields and reduction in singlet-excited-state lifetimes. A time-resolved fluorescence study invoked the possibility for an intramolecular electron transfer from the porphyrin ring to the bridging phenyl group which acts as an acceptor owing to the presence of electron-withdrawing substituents. The quenching of the fluorescence yields is interpreted in terms of enhancement in the non-radiative rates. The structure calculated using molecular mechanics reveals the enforced deformation caused by short bridging groups and beta-substitution, supporting the observed photophysical properties.