Through-Space or Through-Bond? The Important Role of Cofaciality in Orbital Reordering and Its Implications for Hole (De)stabilization in Polychromophoric Assemblies

Developing predictive tools for the elucidation of redox and optical properties of polychromophoric assemblies is crucial for the rational design of efficient charge-transfer materials. Here, such tools are introduced to explain the curious observation that a pair of bichromophoric electron donors based on ethanoanthracene (5) and dihydroanthracene (DHA or 11), having similar interchromophoric separation between the carbons in the region of orbital overlap, show dramatically different (>300 mV) stabilization of their cation radicals. Analysis of molecular orbital diagrams reveals the important interplay between through-space and through-bond electronic couplings, which results in HOMO/HOMO–1 swapping in 11 (HOMO = highest occupied molecular orbital). Unlike the antisymmetric singly occupied molecular orbital (SOMO) that stabilizes a hole by charge resonance in 5•+ (0.57 V), the symmetric SOMO in 11•+ (0.88 V) does not afford hole stabilization by charge resonance; rather, the hole localizes onto a single benzenoid unit. This important finding is expected to aid in the design of polychromophoric assemblies with chromophores of graded redox potentials for optimization of long-range charge transfer.