Spectroscopic studies on monomers and dimers of thiaporphyrins

Abstract

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⁺, NH ⁺₄ ) and cation-crown ether complex. The induced red shifts upon dimerisation parallel findings reported for a variety of cofacial covalently linked porphyrin dimers.