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

Abstract

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.

The Tetracyanopyridinide Dimer Dianion, σ‐[TCNPy]22−

“…Hence, increasing the N heteroatom substitution to the ring increases it reduction potential. The TCNPy radical anion stabilizes in a secondary chemical process (EC mechanism), which was followed by spectroelectrochemistry. Its similar shape with respect to TCNP and TCNB suggests a similar relative orientation for the long, multicenter carbon–carbon bond in π‐[TCNP] 2 2− and in π‐[TCNB] 2 2− .…”

“…1 exhibits two υ C≡N absorptions at 2181 cm −1 (s, half‐width at half‐height, HWHH ≈25 cm −1 ) and a weaker peak at 2233 cm −1 (w, HWHH ≈15 cm −1 ), Figure . The major absorption is reduced by more than 69 cm −1 with respect to the sharp 2250 cm −1 υ C≡N absorption observed for TCNPy in accord with its reduction. The absorption at 1430 cm −1 is assigned as the ν C=C in [Cr(C 6 H 6 ) 2 ] + , whereas ν C=C from TCNPy moiety in 1 appear at 1485 cm −1 , that is, 62 cm −1 higher with respect to 1421 cm −1 observed for neutral TCNPy .…”

“…π‐[TCNB] 2 2− and π‐[TCNP] 2 2− each have a 2 e − /8c C−C bond and have the identical symmetry ( D 2 h ), size and shape; nonetheless, they have different orientational preferences . To identify new examples of compounds with long, multicenter C−C bonds as well as understand the bonding and conformation/orientational preferences we targeted the preparation of π‐[TCNPy] 2 2− (TCNPy=2,3,5,6‐tetracyanopyridine) that has lower C 2 v symmetry with respect to D 2 h TCNB and TCNP …”

“…TCNPy undergoes a quasi‐reversible one electron reduction at −0.51 V (vs. SCE in MeCN) that is ≈0.85 V harder to reduce than TCNE, but easier to reduce than TCNB (−0.64 V) albeit more difficult to reduce than TCNP (−0.31 V) . Hence, increasing the N heteroatom substitution to the ring increases it reduction potential.…”

Recent progress in homogeneous multielectron transfer photocatalysis and artificial photosynthetic solar energy conversion

“…One possible strategy to avoid rapid charge recombination after photoexcitation is to increase the distance between the donor and the acceptor sites in a series of vectorial, energetically downhill electron transfer steps (Fig 2a). Another approach [16,17] is to achieve instantaneous charge separation by a direct optical charge transfer (CT) transition between reducing (donor) and oxidizing (acceptor) subunits in supramolecular assemblies, ion pairs or coordination compounds (Fig. 2b).…”