Donor–donor’–acceptor triads based on [3.3]paracyclophane with a 1,4-dithiafulvene donor and a cyanomethylene acceptor: Synthesis, structure, and electrochemical and photophysical properties

Donor–donor’–acceptor triads (1, 2), based on [3.3]paracyclophane ([3.3]PCP) as a bridge, with electron-donating properties (D’) using 1,4-dithiafulvene (DTF; TTF half unit) as a donor and dicyanomethylene (DCM; TCNE half unit) or an ethoxycarbonyl-cyanomethylene (ECM) as an acceptor were designed and synthesized. The pulse radiolysis study of 1a in 1,2-dichloroethane allowed the clear assignment of the absorption bands of the DTF radical cation (1a^·+), whereas the absorption bands due to the DCM radical anion could not be observed by γ-ray radiolysis in 2- methyltetrahydrofuran rigid glass at 77 K. Electrochemical oxidation of 1a first generates the DTF radical cation (1a^·+), the absorption bands of which are in agreement with those observed by a pulse radiolysis study, followed by dication (1a²⁺). The ESR spectrum of 1a^·+ showed a symmetrical signal with fine structure and an ESR simulation predicted that the spin of 1a^·+ is delocalized over S and C atoms of the DTF moiety and the central C atom of the trimethylene bridge bearing the DTF moiety. Pulse radiolysis, ESR, and electrochemical studies indicate that the DTF radical cation of 1a^·+ is more stable than that of 6^·+, and the latter shows a strong tendency to dimerize. This result indicates that the [3.3]PCP moiety as a bridge can stabilize the DTF radical cation more than the 1,3-diphenylpropane moiety because of kinetic stability due to its rigid structure and the weak electronic interaction of DTF and DCM moieties through [3.3]PCP.