Symmetry and hole-burning properties of porphyrins

Reduction of D4h symmetry of the porphyrin dianion in such a way that the axes passing the pyrrolic nitrogens are made inequivalent leads to distinct structures of prototropic tautomers of the respective diprotonated (neutral) compound. The accompanying difference in S1 energies between the two states is a highly desirable property of a hole-burning (HB) system with minimum overlap between the educt and product absorption. Several porphyrins carrying electron-donating alkyl and electron-withdrawing Br substituents at pyrrolic β positions were doped in a poly(vinyl butyral) film and utilised as HB materials at 8 K. The band maxima, inhomogeneous widths, the Debye–Waller factors (DWF) and the HB efficiencies were determined for both tautomeric forms. The substitution at pyrrolic β positions is a delicate way of perturbing the π electron system as compared to breaking the conjugation by reduction of one of the pyrroles in chlorin. In chlorin the quantum yield, the DWF and the thermal stability of the photoproduct are unfavourable for HB as a result of too strong axial inequivalence. In the red shifted tautomers of 2,3,12,13-tetraethylporphine (TEP) and 7-Br-TEP the advantages of symmetric molecules such as octaethylporphine are retained.