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

Abstract

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 characterised by ¹H and ¹³C NMR spectroscopy, mass spectrometry, UV/Visible spectroscopy and cyclic voltammetry. In-situ UV/Visible and EPR measurements show that the site of the first electrochemical reduction is the benzylviologen component of the molecule. The second reduction wave from cyclic voltammetry was shown by in-situ EPR to comprise two unresolved one-electron processes, and this was confirmed by chronocoulometry. The first of these two reduction processes rendered the diad diamagnetic, as was shown by the disappearance of the signal due to the benzylviologen radical. The second gave 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 resolved spectrum of a monoreduced porphyrin radical.