Synthesis and spectroscopic properties of porphyrin macrocycles with sulphur as the heteroatom in the porphyrin core have been studied. Electronic absorption spectra of these macrocycles show porphyrin-like behaviour with a strong Soret band and weakQ-bands. Substitution of the -NH groups of tetraphenylporphyrin (TPPH₂) by sulphur causes a red shift of all the absorption bands and the magnitude of the red shift depends on the number of sulphur atoms substituted. Both the mono and dications of dithiaporphyrins (S₂TPP) show larger bathochromic shifts ofQ-bands relative to TPPH₂ indicating a stronger resonance interaction with the phenyl groups. A positive shift for both oxidation and reduction potentials is observed upon substitution of sulphur atoms.¹H NMR spectra of symmetrically substituted dithiaporphyrins show two sharp singlets for pyrrole protons and thiophene protons confirming the presence of a two-fold axis of symmetry. Only monothia derivatives (STPPH) form metal complexes [Ni(II), Cu(II)] and these metal complexes are five-coordinate with an apical chloride ligand.

The water-soluble S₂TPPS, prepared from sulphonating the para positions of phenyl rings shows extensive aggregation at high concentrations (> 10⁻⁴M). At low concentrations (≈ 10⁻⁶–10⁻⁷M), dimerisation can be induced by the addition of cations (K⁺, NH₄⁺ ) and cation-crown ether complex. The induced red shifts upon dimerisation parallel findings reported for a variety of cofacial covalently linked porphyrin dimers.