Typical porphyrins are coloured compounds due to their characteristic wavelength-selective visible light absorptions. An exceptional "blackened" ZnII -porphyrin was recently obtained from fusing the β,β'-positions of the porphyrin core with four quinone units. Here, studies with metal-free and NiII -containing analogues are reported. These quinone-fused porphyrins were prepared from the corresponding β,β'-tetrasulfolenoporphyrins via thermally generated porphyrindienes, which were trapped by [4+2] cycloaddition as benzoquinone cycloadducts, which were subsequently oxidised. By using this strategy, metal-free and NiII -containing porphyrins with one, two, three or four conjugated naphthoquinone moieties were prepared efficiently. The presence of the π-conjugated naphthoquinone moieties changed the porphyrin chromophores profoundly, giving broadly absorbing "blackened" pigments. The influence of coordinated NiII ions, or of the absence of a metal ion in the modified porphyrin core, on their structural and spectroscopic properties was explored. Blackened quinone-conjugated porphyrins might be pigments suitable for solar energy conversion. With their unique peripheral functional groups they are also a set of porphyrins "programmed" for further covalent extension. Thus, they are building blocks for the preparation of supra-porphyrinoid assemblies that might be useful in optoelectronics and in the nanosciences.
Keywords: cycloaddition; naphthoquinones; porphyrinoids; solar energy; transition metals.
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