  The realization of self-assembled molecular-electronic films, whose
room-temperature transport properties are controlled by quantum interference
(QI), is an essential step in the scale-up QI effects from single molecules to
parallel arrays of molecules. Recently, the effect of destructive QI (DQI) on
the electrical conductance of self-assembled monolayers (SAMs) has been
investigated. Here, through a combined experimental and theoretical
investigation, we demonstrate chemical control of different forms of
constructive QI (CQI) in cross-plane transport through SAMs and assess its
influence on cross-plane thermoelectricity in SAMs. It is known that the
electrical conductance of single molecules can be controlled in a deterministic
manner, by chemically varying their connectivity to external electrodes. Here,
by employing synthetic methodologies to vary the connectivity of terminal
anchor groups around aromatic anthracene cores, and by forming SAMs of the
resulting molecules, we clearly demonstrate that this signature of CQI can be
translated into SAM-on-gold molecular films. We show that the conductance of
vertical molecular junctions formed from anthracene-based molecules with two
different connectivities differ by a factor of approximately 16, in agreement
with theoretical predictions for their conductance ratio based on constructive
QI effects within the core. We also demonstrate that for molecules with thiol
anchor groups, the Seebeck coefficient of such films is connectivity dependent
and with an appropriate choice of connectivity can be boosted by ~50%. This
demonstration of QI and its influence on thermoelectricity in SAMs represents a
critical step towards functional ultra-thin-film devices for future
thermoelectric and molecular-scale electronics applications.
