Electron Transport\nover 2D Molecular Materials and\nAssemblies

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