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.

Porphyrins and metallopophyrins have attracted the attention of chemists for the past 100 years or more owing to their widespread involvement in biology. More recently, synthetic porphyrins and porphyrin-like macrocycles have attracted the attention of researchers due to their diverse applications as sensitizers for photodynamic therapy, MRI contrasting agents, and complexing agents for larger metal ions and also for their anion binding abilities. The number of π-electrons in the porphyrin ring can be increased either by increasing the numberof conjugated double bonds between the pyrrole rings or by increasing the number of heterocyclic rings. Thus, 22π sapphyrins, 26π rubyrins, 30π heptaphyrins, 34π octaphyrins and higher cyclic polypyrrole analogues containing 40π, 48π, 64π, 80π and 96π systems have recently been reported in the literature. These macrocycles show rich structural diversity where normal and different kinds of inverted structures have been identified. In this review, an attempt has been made to collect the literature of the inverted porphyrins and expanded porphyrins reported until December 2001. Since themeso aryl expanded porphyrins have tendency to form both inverted and non-inverted structures more emphasis has been given tomeso aryl expanded porphyrins.