A series
of experiments and electronic structure calculations were
performed to identify metastable 1,1′-ferrocenedicarboxylic
acid supramolecular structures formed during solution deposition in
a vacuum on a Au(111) substrate, as well as to observe their evolution
into more stable species under mild annealing conditions. Electrospray
ionization mass spectrometry measurments were performed to determine
which species are likely to be present in the rapidly evaporating
droplet, and these experiments found that a hexamer can exist in solution
during deposition, albeit as a metastable species. The molecular clusters
present after solution deposition were observed and analyzed using
ultrahigh-vacuum scanning tunneling microscopy, and the initial monolayer
contains four basic classes of structures: ordered dimer domains,
tilted dimer rows, square tetramers, and rectangular chiral hexamers.
Electronic structure calculations indicate that the chiral hexamers
consist of a central dimer surrounded by four molecules oriented to
form birfurcated hydrogen bonds with other carboxylic acid groups
and weaker hydrogen bonds with hydrogens from the aromatic rings.
The calculations also indicated that the tetramers are clusters held
together by carboxylic acid dimer bonds on each ring oriented perpendicular
to each other, and that this conformation is slightly more stable
than two dimers for a cluster of four molecules. Annealing this surface
at 50 °C for 1 h results in the formation of both isolated tetramers
and ordered tetramer rows at the expense of the end-to-end dimer domains,
with few chiral hexamers remaining. Further annealing at 50 °C,
as well as annealing at 65 °C drives the system to form chiral
dimer domains, as well as several other minor structures. Annealing
at 75 °C resulted in a dramatic decrease in apparent surface
coverage, and most ordered structures existed as large tilted dimer
rows, whether isolated or in ordered domains. This drop in surface
coverage is likely due to some combination of decomposition of the
molecule, desorption, or the growth of three-dimensional crystal structures.
The observed coexistence of many forms of ordered dimer structures
after annealing indicates that the equilbrium conformation of 1,1′-ferrocenedicarboxylic
acid is some array of ordered dimers, and the variety of supramolecular
structures present after annealing is an indicator that this system
evolves under kinetically controlled growth conditions.