Conspectus
Two-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-lasti...
