Synthesis, electrochemistry and spectroelectrochemistry of a porphyrin-viologen donor-acceptor diad

A new donor-acceptor (D-A) molecule, 5,10,15,20-[N-benzyl-N'-(4-benzyl-4,4' -bipyridinium-4-pyridyl)]- triphenylporphyrin tris(hexafluorophosphate), 4, has been synthesised. The diad, 4, and its precursors, have been fully cha racterised by H-1 and C-13 NMR spectroscopy, mass spectrometry, UV/Visible spectroscopy and cyclic voltammetry. In-situ UV/Visible and EPR measurement s show that the site of the first electrochemical reduction is the benzylvi ologen component of the molecule. The second reduction wave from cyclic vol tammetry was shown by in-situ EPR to comprise two unresolved one-electron p rocesses, and this was confirmed by chronocoulometry. The first of these tw o reduction processes rendered the diad diamagnetic, as was shown by the di sappearance of the signal due to the benzylviologen radical. The second gav e rise to the appearance of a new EPR signal, which was found to correspond to the porphyrin radical. We believe this to be the first reported resolve d spectrum of a monoreduced porphyrin radical.