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

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 n-Bu4NPF6) and, in one case, a remarkably small HOMO–LUMO
gap (ΔE = 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.