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 <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

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 <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

6,6-Dicyanopentafulvenes: Electronic Structure and Regioselectivity in [2 + 2] Cycloaddition–Retroelectrocyclization Reactions

We present an investigation of the electronic properties and reactivity behavior of electron-accepting 6,6-dicyanopentafulvenes (DCFs). The electron paramagnetic resonance (EPR) spectra of the radical anion of a tetrakis­(silylalkynyl) DCF, generated by Na metal reduction, show delocalization of both the charge and unpaired electron to the nitrogens of the cyano moieties and also, notably, to the silicon atoms of the four alkynyl moieties. By contrast, in the radical anion of the previously reported tetraphenyl DCF, coupling to the four phenyl rings is strongly attenuated. The data provide physical evidence for the different conjugation between the DCF core and the substituents in both systems. We also report the preparation of new fulvene-based push–pull chromophores via formal [2 + 2] cycloaddition–retroelectrocyclization reaction of DCFs with electron-rich alkynes. Alkynylated and phenylated DCFs show opposite regioselectivity of the cycloaddition, which can be explained by the differences in electronic communication between substituents and the DCF core as revealed in the EPR spectra of the radical anions