Designing Long-Range Charge Delocalization from First-Principles

Abstract

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 longrange 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.

Mixed‐Valent Compounds and their Properties – Recent Developments

“…The BLYP35 functional is a compromise hybrid, “35” meaning that it incorporates 35 % of exact exchange, this percentage being adjusted to describe properly the state of localization/delocalization . Recent uses of this protocol appear promising, in particular to design molecular wires with long‐range charge delocalization …”

Towards a First-Principles Evaluation of Transport Mechanisms in Molecular Wires

“…In our previous studies on organic mixed-valence systems, the ratio between the local charges on the donor and acceptor moieties connected by a bridge served as a measure for the degree of charge localization, where complete charge delocalization would be expressed by a ratio of one, whereas a ratio of zero would indicate a fully localized charge on one redox centre. 48 Since here, the redox centers are replaced by non-redox-active anchoring groups, we define the degree of charge delocalization r deloc by relating the smallest possible subregion on which a suitably chosen large percentage of spin density is located to that percentage,…”

“…58 In a previous study on first-principles approaches, we validated different computational protocols based on Kohn-Sham density-functional theory (KS-DFT) regarding their capability of describing length-dependent charge localization in comparison with experiments for organic mixed-valence systems. 48 This type of donor-bridge-acceptor systems has the advantage of representing atomistically better-defined model systems than molecular junctions, while still being closely related to them as the electron transfer properties as expressed in their Robin-Day class are also length-dependent. 59,60 We found that, in contrast to other DFT protocols, a combination of the B1LYP hybrid functional with 35% Hartree-Fock exchange (BLYP35) and a polarizable continuum model (PCM), previously proposed by the group of Kaupp, [44][45][46] works well in this case.…”

“…59,60 We found that, in contrast to other DFT protocols, a combination of the B1LYP hybrid functional with 35% Hartree-Fock exchange (BLYP35) and a polarizable continuum model (PCM), previously proposed by the group of Kaupp, [44][45][46] works well in this case. 48 This was even though it can be assumed that entropic effects are playing an increasingly important role for the degree of charge localization in molecular wires as they get more flexible with increasing length, thus leading to a greater variety of possible structures with potentially different charge localization properties.…”

“…2 We aim at the prediction of the tunneling to hopping crossover observed in molecularconductance experiments on conjugated organic wires [15][16][17][18][19] on the basis of our static DFT approach. 46,48 In Section 2, we describe our procedure of assessing and quantifying the degree of charge delocalization in more detail. In Section 3, we present our results from DFT calculations on charge localization properties for a variety of molecular wires and compare them with respect to their agreement with transition lengths from experiments.…”