Through‐space electronic coupling in highly compressed <i>π</i> ‐stacked organic mixed‐valence systems: The role of charge type and redox‐center orientation

Abstract A quantitative understanding of through‐space electronic coupling in π ‐stacked organic mixed‐valence (MV) systems remains limited, particularly in regimes where severe geometric compression and conformational effects complicate conventional spectroscopic interpretations. Here, we present a combined experimental and theoretical investigation of highly compressed π ‐stacked MV systems derived from the cation‐radical 3˙ + and the anion‐radical 4˙ − , which possess closely comparable centroid‐to‐centroid separations well within the sub–van der Waals contact regime. Electrochemical and spectroscopic measurements, together with DFT and TD‐DFT analyses, reveal that the lowest‐energy optical transitions observed for these systems do not arise from equivalent electronic states in the cationic and anionic manifolds. In particular, the anion‐radical system exhibits a pronounced conformational dependence, in which the syn conformer displays fully delocalized electronic structure consistent with Robin–Day Class III behavior, whereas the anti conformer remains Class II. As a consequence, the experimentally observed strong low‐energy absorption of 4˙ − is dominated by the syn conformer and reflects π – π * excitation rather than a conventional intervalence charge‐transfer process. When analyzed within a consistent computational framework, the intrinsic electronic couplings of 3˙ + and 4˙ − converge to a coherent picture in which orientation‐dependent effects, rather than charge‐carrier polarity alone, govern the magnitude and character of through‐space electronic coupling in the highly compressed regime.