Synthesis and physical properties of biquinoxalinyl bridged bis-porphyrins: models for aspects of Photosynthetic Reaction Centres

The synthesis of biquinoxalinyl-bridged bis-porphyrin 4 and metallated derivatives 5 - 11 was achieved in high yields. UV-visible spectroscopy and electrochemical experiments indicated weak orbital coupling of the two quinoxalinyl units but minimal orbital coupling between the two porphyrins. The weak electronic communication is attributed to non-planarity, on average, of the molecule because of rotation about the inter-quinoxalinyl connection. This, combined with poor coupling across the fused junctions between the porphyrin and quinoxalinyl units, results in minimal inter-porphyrin communication. As a result, the biquinoxalinyl linkage is appropriate for inclusion in more elaborate synthetic compounds, such as the tris-porphyrin 1, that are designed to model the charge-separation apparatus of Photosynthetic Reaction Centres.