Poly-p-phenylenes (PPs) are prototype systems for understanding the charge transport in π-conjugated polymers. In a combined computational and experimental study, we demonstrate that the smooth evolution of redox and optoelectronic properties of PP cation radicals toward the polymeric limit can be significantly altered by electron-donating iso-alkyl and iso-alkoxy end-capping groups. A multiparabolic model (MPM) developed and validated here rationalizes this unexpected effect by interplay of the two modes of hole stabilization: due to the framework of equivalent p-phenylene units and due to the electron-donating end-capping groups. A symmetric, bell-shaped hole in unsubstituted PPs becomes either slightly skewed and shifted toward an end of the molecule in iso-alkyl-capped PPs or highly deformed and concentrated on a terminal unit in PPs with strongly electron-donating iso-alkoxy capping groups. The MPM shows that the observed linear 1/n evolution of the PP cation radical properties toward the polymer limit originates from the hole stabilization due to the growing chain of p-phenylene units, while shifting of the hole toward electron-donating end-capping groups leads to early breakdown of these 1/n dependencies. These insights, along with the readily applicable and flexible multistate parabolic model, can guide studies of complex donor–spacer–acceptor systems and doped molecular wires to aid the design of the next generation materials for long-range charge transport and photovoltaic applications.

Magic blue (MB⁺˙ SbCl₆ ⁻ salt), i.e. tris-4-bromophenylamminium cation radical, is a routinely employed one-electron oxidant that slowly decomposes in the solid state upon storage to form so called ‘blues brothers’, which often complicate the quantitative analyses of the oxidation processes. Herein, we disclose the identity of the main ‘blues brother’ as the cation radical and dication of tetrakis-(4-bromophenyl)benzidine (TAB) by a combined DFT and experimental approach, including isolation of TAB⁺˙ SbCl₆ ⁻ and its X-ray crystallography characterization. The formation of TAB in aged magic blue samples occurs by a Scholl-type coupling of a pair of MB followed by a loss of molecular bromine. The recognition of this fact led us to the rational design and synthesis of tris(2-bromo-4-tert-butylphenyl)amine, referred to as ‘blues cousin’ (BC: E_ox1 = 0.78 V vs. Fc/Fc⁺, λ_max(BC⁺˙) = 805 nm, ε_max = 9930 cm⁻¹ M⁻¹), whose oxidative dimerization is significantly hampered by positioning the sterically demanding tert-butyl groups at the para-positions of the aryl rings. A ready two-step synthesis of BC from triphenylamine and the high stability of its cation radical (BC⁺˙) promise that BC will serve as a ready replacement for MB and an oxidant of choice for mechanistic investigations of one-electron transfer processes in organic, inorganic, and organometallic transformations.

A Glimpse of HOON: Bonds between two oxygen atoms are relatively weak, as manifested in the sometimes explosive reactivity of O ₂ and various peroxides. Thus, although nitrous acid (HONO) can be rearranged on paper to an isomer with an O-O rather than N-O bond, nitrosyl-O-hydroxide (HOON) has been considered too unstable to be observed. Crabtree et al. (p. 1354 ) used microwave spectroscopy to detect HOON formation in a dilute gaseous mixture of NO and OH in neon. Isotopic substitutions enabled determination of its structure, which included an unusually long O-O bond.