Charge Delocalization in Self-Assembled Mixed-Valence Aromatic Cation Radicals

The spontaneous assembly of aromatic cation radicals (D+•) with their neutral counterpart (D) affords dimer cation radicals (D2+•). The intermolecular dimeric cation radicals are readily characterized by the appearance of an intervalence charge-resonance transition in the NIR region of their electronic spectra and by ESR spectroscopy. The X-ray crystal structure analysis and DFT calculations of a representative dimer cation radical (i.e., the octamethylbiphenylene dimer cation radical) have established that a hole (or single positive charge) is completely delocalized over both aromatic moieties. The energetics and the geometrical considerations for the formation of dimer cation radicals is deliberated with the aid of a series of cyclophane-like bichromophoric donors with drastically varied interplanar angles between the cofacially arranged aryl moieties. X-ray crystallography of a number of mixed-valence cation radicals derived from monochromophoric benzenoid donors established that they generally assemble in 1D stacks in the solid state. However, the use of polychromophoric intervalence cation radicals, where a single charge is effectively delocalized among all of the chromophores, can lead to higher-order assemblies with potential applications in long-range charge transport. As a proof of concept, we show that a single charge in the cation radical of a triptycene derivative is evenly distributed on all three benzenoid rings and this triptycene cation radical forms a 2D electronically coupled assembly, as established by X-ray crystallography

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 (<b>5</b>) and dihydroanthracene (<b>DHA</b> or <b>11</b>), 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 <i>HOMO/HOMO–1 swapping in <b>11</b></i> (HOMO = highest occupied molecular orbital). Unlike the antisymmetric singly occupied molecular orbital (SOMO) that stabilizes a hole by charge resonance in <b>5</b>^•+ (0.57 V), the symmetric SOMO in <b>11</b>^•+ (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