Water-soluble thiaporphyrins and metallothiaporphyrins; synthesis, characterization, ground- and excited-state properties

The synthesis and characterization of two anionic water-soluble thiaporphyrins 5,10,15,20-tetrakis(4-sulfonatophenyl)-21-thiaporphyrin (Htsptp4) and 5,10,15,20tetrakis(4-sulfonatophenyl)-21,23-dithiaporphyrin (tspdtp4) and derivatives of the former with Cu2+ and Ni2+{[Cu(tsptp)Cl]4 and [Ni(tsptp)Cl]4} have been carried out. Both Htsptp4 and tspdtp4 show aggregation in aqueous solution. Addition of cation and 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) to dilute monomeric solutions of either results in cofacial dimer formation similar to that observed for normal anionic water-soluble porphyrins. The electronic absorption spectra of both the paramagnetic metal derivatives of the monothiaporphyrin show a split Soret band and a complex pattern of Q bands due to reduced symmetry. The fluorescence of the two thiaporphyrins is quenched considerably relative to H2tspp4[5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin], the magnitude of quenching depending on the number of sulfur atoms in the porphyrin core. Electrochemical studies indicate harder oxidations and easier reductions for Htsptp4 and tspdtp4 relative to H2tspp4. Addition of a first electron to [Cu(tsptp)Cl]4 or [Ni(tsptp)Cl]4 results in the reduction of the metal centre suggesting that the metal dx2y2 orbital is lower in energy than the empty porphyrin eg(π∗) orbitals in contrast to those of [Cu(tspp)]4 and [Ni(tspp)]4