Decoupling Electronic Coupling and Site-Energy Asymmetry in a Scaffold-Free π‑Assembly

Directional C–H···OC hydrogen bonding renders the core chromophores electronically inequivalent, producing static <i>Δε</i>_LE (∼12 meV) within a pair whose excitonic coupling, determined experimentally from a Spano vibronic analysis, is 93 meV. Electronic-structure calculations predict that this asymmetry is embedded in the lowest excited-state manifold at the Franck–Condon geometry, where the modest ground-state asymmetry yields a much larger <i>Δε</i>_CT (∼210 meV) splitting of opposing CT configurations.

Femtosecond transient absorption resolves a 1.2 ps evolution toward increased ionic spectral character, while nanosecond spectroscopy and photoluminescence show persistence over ∼20 ns as a radiatively active excited-state ensemble. These results establish hierarchical self-assembly as a strategy for decoupling electronic coupling from site-energy asymmetry and provide a structural basis for a directionally biased LE-CT manifold predicted by electronic-structure calculations and consistent with the observed excited-state dynamics.