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

Abstract

In conventional π-stacked systems, electronic coupling (J) and site-energy asymmetry (Δε) are governed by the same structural coordinate. Co-facial packing maximizes J but enforces Δε ≈ 0, yielding symmetry-equivalent local-excitation-charge-transfer (LE-CT) manifolds near the charge-resonance/excimer limit, whereas chemically differentiated donor–acceptor systems impose Δε ≫ J and favor localized CT states. The intermediate regime, with finite static Δε while preserving strong coupling, thus remains difficult to access. Nature achieves this in photosynthetic special pairs by separating the structural origins of J (cofactor stacking) and Δε (protein electrostatics). Inspired by this principle, we construct a scaffold-free hierarchical π-assembly of four C3-symmetric perylene diimide triads, which comprises strongly coupled dimeric cores and monomer-like peripheral units. Directional C–H···O═C hydrogen bonding renders the core chromophores electronically inequivalent, producing static ΔεLE (∼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 ΔεCT (∼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.