Methylisatin Structural Isomers Have Different Kinetic Pathways to Self-Assembly

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.