In this study, the pharmaceutical carbamazepine (CBZ) and the related molecule dibenzazepine (DBZ) are prepared on the Au(111) surface and visualized using scanning tunneling microscopy. Monolayers of CBZ consist of tetrameric clusters that are highly ordered, while a monolayer of the molecular analog in which the amide group is removed, DBZ, consists of disordered clustering. Electrospray ionization mass spectra of CBZ show a relatively intense tetramer peak that is roughly six times more intense than the trimer peak, indicating the anomalous stability of a four-molecule cluster. The mass spectrum of DBZ shows relatively little clustering and does not show preference for tetramers. Our molecular modeling suggests that the CBZ tetramer is a NH···O hydrogen-bonded cluster, with reinforcing π···π interactions between benzene rings of neighboring molecules, which are responsible for the long-range order of the tetramers.

Monolayers of methylisatin structural isomers are prepared on the Au(111) surface and imaged using scanning tunneling microscopy in ultrahigh vacuum. Monolayer preparation via solution deposition results in different monolayer structures for these two isomers. 5-Methylisatin forms rectangular hexamer clusters that are closely packed, while 6-methylisatin forms cyclic pentamers, dimers, and a close-packed phase. Monolayer preparation via vapor deposition results in the formation of cyclic pentamers for both 5- and 6-methylisatin. These results are compared to isatin monolayers, which also form cyclic pentamers upon both solution and vapor deposition. DFT binding energy calculations corroborate a stable pentamer, as there is a calculated binding energy well for 5-molecule clusters for isatin, 5-methylisatin, 6-methylisatin. The formation of the unusual 5-methylisatin hexamer structure is likely due to nonequilibrium adsorption kinetics present in the solution (pulse) deposition preparation technique.