Benzoquinone, 1,4-naphthoquinone radical anions, and their didehydro derivatives: electroanalytical and predicted negative ion photoelectron spectroscopic studies

Abstract

An increased understanding of the relationship between the electrochemical properties and vibrational structures of polycyclic aromatic hydrocarbons (PAHs) containing quinone moieties is essential in many electron transfer reactions and molecular design of organic energy storage devices. In this work, a multi-method approach involving cyclic voltammetry (CV), high-level quantum-chemical calculations, and gas-phase negative ion photoelectron spectroscopy (NIPES), anchored on computed Franck–Condon (FC) factors, is implemented to gain insights into vibrational structures and electron transfer properties of 1,4-benzoquinone (1,4-BQ) and 1,4-naphthoquinone (1,4-NQ) radical anions together with their didehydro derivatives. The solution-phase reduction potentials have a direct correlation with the gas-phase adiabatic electron affinities (EAs) of the neutral species, whose vibrational structures are revealed by NIPES. The adiabatic EA for neutral 1,4-NQ in its singlet electronic ground state (S₀) is calculated to be 2.035 eV while the lowest triplet state (T₁) in 1,4-NQ is located 2.161 eV above the S₀ state, resulting in a singlet–triplet splitting (ΔE_ST) of 2.161 eV. The didehydro-1,4-NQ (DD-1,4-NQ) has an adiabatic EA of 2.210 eV and a ΔE_ST of 2.012 eV. Vibrational analysis reveals several FC vibrational progressions that include ring-distortion, C–C and CO symmetric stretching, consistent with the observed geometry changes between the anion and the final neutral states.