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 colored charge-transfer complexes. In MeCN, TCNPy is characterized by a quasireversible one-electron-reduction process at -0.51 V (versus SCE). The tetracyanopyridine radical anion 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-induced excitation of the intense charge-transfer bands of these compounds leads to a direct optical electron-transfer process for the formation of the corresponding radical-ion pairs. The presence of available electron donors that contain protic groups in close proximity to the TCNPy acceptor site opens up a new strategy for the photocontrolled generation of pyridinium radicals in a stepwise proton-coupled electron-transfer (PCET) sequence.