Designing Long-Range Charge Delocalization from First-Principles

Efficient electronic communication over long distances is a desirable property of molecular wires. Charge delocalization in mixed-valence (MV) compounds where two redox centers are linked by a molecular bridge is a particularly well-controlled instance of such electronic communication, thus lending itself to comparisons between theory and experiment. We study how to achieve and control long-range charge delocalization in cationic organic MV systems by means of Kohn–Sham density functional theory (DFT) and show that a captodative substitution approach recently suggested for molecular conductance (Stuyver et al. J. Phys. Chem. C 2018, 122, 3194) greatly enhances charge delocalization in p-phenylene-based wires. To ensure the adequacy of our DFT methods, we validate different protocols for organic MV systems of different lengths. The BLYP35 hybrid functional combined with a polarizable continuum model, established by Renz and Kaupp, is indeed capable of correctly describing experimentally observed length-dependent charge delocalization, in contrast to the long-range corrected functionals ω-B97X-D and ω-PBE. We also discuss the implications of these results for a first-principles description of the transition between coherent tunneling and incoherent hopping regimes in molecular conductance.