Beyond Energy-Level Tuning: Diffuse Bromine Orbitals of Naphthalenediimides in Electron Transport

Rational design of high-performance organic semiconductors requires a rigorous understanding of how functional group substitution modulate the electronic structure and charge transport. Although halogenation has traditionally been employed to modulate energy levels and engineer noncovalent interactions, the explicit role of halogen centric orbitals in charge transport remains insufficiently resolved. Here, we present a systematic investigation on the role of diffused p-orbitals of bromine actively enhance intermolecular electronic coupling in family of Naphthalenediimides (NDI). Corresponding nonbrominated (Br0) series was chosen as model compounds and core-dibrominated (Br2) analogues were preferred for detailed investigation. Using charge-transfer integral calculations within the DIPRO framework, density functional theory (DFT) band-structure analysis, projected density of states (PDOS), and energy decomposition analysis via Crystal Explorer all based on experimentally resolved single-crystal structures, we identify a distinct, orbital-driven charge transport mechanisms in the latter series. Br0 series exhibited largely dispersionless, carrier-localized electronic structures, whereas the Br2 series displayed pronounced band dispersion and the emergence of multidirectional transport pathways. To deconvolute the intrinsic electronic contribution from molecular packing-induced effects, controlled substitution analyses were performed while preserving the original molecular coordinates. These calculations unambiguously reveal that bromination intrinsically strengthens intermolecular interactions, increasing stabilization energies by ∼10–20 kJ mol–1 across the series. Among the investigated systems, EhBr2 exhibits the most favorable characteristics for efficient charge transport, combining optimal π–π overlap with efficient orbital hybridization and high charge transfer integral. Collectively, these results establish the diffuse valence orbital character of bromine as a decisive and previously underappreciated parameter in the molecular engineering of high-performance organic semiconductors.