Donor-Substituted Octacyano[4]dendralenes: Investigation of π‑Electron Delocalization in Their Radical Ions

Abstract

Symmetrically and unsymmetrically electron-donor-substituted octacyano[4]­dendralenes were synthesized and their opto-electronic properties investigated by UV/vis spectroscopy, electrochemical measurements (cyclic voltammetry (CV) and rotating disk voltammetry (RDV)), and electron paramagnetic resonance (EPR) spectroscopy. These nonplanar push–pull chromophores are potent electron acceptors, featuring potentials for first reversible electron uptake around at −0.1 V (vs Fc⁺/Fc, in CH₂Cl₂ + 0.1 M <i>n</i>-Bu₄NPF₆) and, in one case, a remarkably small HOMO–LUMO gap (Δ<i>E</i> = 0.68 V). EPR measurements gave well-resolved spectra after one-electron reduction of the octacyano[4]­dendralenes, whereas the one-electron oxidized species could not be detected in all cases. Investigations of the radical anions of related donor-substituted 1,1,4,4-tetracyanobuta-1,3-diene derivatives revealed electron localization at one 1,1-dicyanovinyl (DCV) moiety, in contrast to predictions by density functional theory (DFT) calculations. The particular factors leading to the charge distribution in the electron-accepting domains of the tetracyano and octacyano chromophores are discussed