Electron Transport over 2D Molecular Materials and Assemblies

ConspectusTwo-dimensional (2D) molecular materials, in which the major interactions are confined in 2D planes with contrasted force fields acting in between the planes, have been key electronic functional materials since the past decade. Even without referring to the functionals of graphene-based systems, 2D electronic conjugated systems are expected to show extrawide dynamic ranges in electronic density of states (DOS) tuning, effective electron mass, electron mobility, and conductivity. A major advantage of 2D electronic systems is their compatibility with the ubiquitous electronic devices designed using planar structures, such as transistors and memories, which is associated with the utility of 2D active materials. The mobility of electrons in 2D systems is the key to their utility, and various conjugated molecular and 2D materials have been designed to optimize the mobility. This Account begins with an introduction for mobility assessment: using noncontact time-resolved microwave conductivity (TRMC) measurements as a technique to probe differential conductivity upon transient charge carrier injection into the materials. Electronic transport over 2D electronic materials such as graphenes, covalent organic frameworks (COFs), and metal–organic frameworks (MOFs) is discussed with a special emphasis on molecular building blocks, fine-tuning conducting species and linkages, topology of the framework, and controlling molecular doping. The superiority of β-ketoenamine-linked COF over imine-linked COF films in charge transport and dominant in-plane charge carrier mobility over out-of-plane mobility is also illustrated. Systematic molecular engineering of the building blocks of β-ketoenamine-linked COFs with varying degrees of donor–acceptor (D–A) conjugation, torsional angles, and reaction conditions resulted in the modulation of the efficiency of charge carrier generation/transport as well as exciton migration. The advantages of 2D systems are finally discussed in terms of the mobility interplaying with spatial arrangements of molecules as well as the substantial role of intermolecular interactions in stabilizing their condensed phases. The strong correlation between the dispersion of mobility and hierarchical intermolecular interactions sheds light on the way to overcome structural fluctuation on the optimization of charge transport in molecular electronic materials. The point of singularity in the dispersion at an intermolecular distance of d ∼ 0.3 nm is deduced from the overall mobility assessment in condensed phases of conjugated molecules, suggesting key roles of intermolecular electronic coupling: the new concept of electronic conjugation. Exceptional electronic coupling with relatively high charge carrier mobility was also observed, particularly in 2D spatial arrangements of chiral molecules in contrast to 3D analogues, where the reduction of gravitational density of the molecular condensates was impacting DOS: the Wallach's rule. 2D electronic systems are strong candidates for the violation of the long-lasting Wallach's rule in terms of DOS.