Through-space electronic coupling in π-stacked organic mixed-valence systems: A quantitative comparison of cationic and anionic states

Abstract

This work investigates the structural characteristics, redox behavior, intervalence charge transfer (IVCT) transition, and electronic coupling of a mixed-valence (MV) system featuring quinoidal redox centers in 4′,4″′-(naphthalene-1,8-diyl)bis(4-methyl-[1,1′-biphenyl]-2,5-dione) (2). The three-dimensional structures of two conformers, [2_syn]^•− and [2_anti]^•−, were determined using density functional theory calculations. The centroid-to-centroid distances between the two redox centers were 3.69 Å for [2_syn]^•− and 3.41 Å for [2_anti]^•−, with a weighted average distance of 3.59 Å. Electrochemical methods that include cyclic voltammetry and differential pulse voltammetry revealed two closely spaced reduction potentials (−1.04 and − 1.13 V vs. Fc⁺/Fc). The electronic coupling between the redox centers was evaluated using two complementary approaches: the Mulliken–Hush analysis of the IVCT band obtained through the spectroelectrochemical method and partial charge resonance analysis derived from geometrical bond length variations. The results demonstrate a significant disparity in electronic coupling, with the anion radical MV system (2^•−, 413 cm⁻¹) exhibiting electronic coupling approximately one-third weaker than the previously reported cation radical MV system (1^•+, 2055 cm⁻¹). Charge transfer rate constants (k_ET) between two redox centers, calculated using parameters from spectroelectrochemical measurement, revealed that charge transfer in 2^•− (5.8 × 10¹³ s⁻¹) is 180 times slower than in 1^•+ (3.2 × 10¹¹ s⁻¹). These findings underscore the critical role of electronic coupling in determining charge transfer rates and highlight the pronounced advantage of hole-type charge carriers over electron-type carriers. The study provides valuable insights for the rational design of advanced materials in organic electronics.