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 × 10 13 s −1 ) is 180 times slower than in 1 •+ (3.2 × 10 11 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.