posted on 2024-02-09, 03:29authored byKatherine
E. White, Erin M. Avery, Edison Cummings, Zixiang Hong, Jens Langecker, Aliaksei Vetushka, Michal Dušek, Jan Macháček, Jakub Višnák, Jan Endres, Zdeněk Bastl, Ersen Mete, Anastassia N. Alexandrova, Tomáš Baše, Paul S. Weiss
Carboranedithiol
isomers adsorbing with opposite orientations of
their dipoles on surfaces are self-assembled together to form mixed
monolayers where both lateral dipole–dipole and lateral thiol–thiolate
(S–H···S) interactions provide enhanced stability
over single-component monolayers. We demonstrate the first instance
of the ability to map individual isomers in a mixed monolayer using
the model system carboranedithiols on Au{111}. The addition of methyl
groups to one isomer provides both an enhanced dipole moment and extra
apparent height for differentiation via scanning tunneling microscopy
(STM). Associated computational investigations rationalize favorable
interactions of mixed pairs and the associated stability changes that
arise from these interactions. Both STM images and Monte Carlo simulations
yield similarly structured mixed monolayers, where approximately 10%
of the molecules have reversed dipole moment orientations but no direct
chemical attachment to the surface, leading to homogeneous monolayers
with no apparent phase separation. Deprotonating the thiols by depositing
the molecules under basic conditions eliminates the lateral S–H···S
interactions while accentuating the dipole–dipole forces. The
molecular system investigated is composed of isomeric molecules with
opposite orientations of dipoles and identical surface packing, which
enables the mapping of individual molecules within the mixed monolayers
and enables analyses of the contributions of the relatively weak lateral
interactions to the overall stability of the assemblies.