Synthesis and Characterization of Redox-Active Charge-Transfer Complexes with 2,3,5,6-Tetracyanopyridine (TCNPy) for the Photogeneration of Pyridinium Radicals

The heteroaromatic polynitrile compound tetracyanopyridine (TCNPy) is introduced as a new electron acceptor for the formation of deeply coloured charge-transfer complexes. In acetonitrile solution, TCNPy is characterized by a quasireversible one-electron reduction process at -0.51 V vs. SCE. The tetracyanopyridine radical anion formed undergoes a secondary chemical reaction, which is assigned to a protonation step. TCNPy has been demonstrated to generate 1:1 complexes with various electron donors including tetrathiafulvalene (TTF) and dihydroxybenzene derivatives such as p-hydroquinone and catechol. Visible- or NIR-light excitation of the intense charge-transfer bands of these compounds leads to a direct optical electron transfer process forming the corresponding radical ion pairs. When electron donors containing protic groups are available in close proximity to the TCNPy acceptor site, this opens a novel strategy for the photo-controlled generation of pyridinium radicals following a stepwise proton-coupled electron transfer (PCET) sequence.