Two-Dimensional Supramolecular Arrangements of Enantiomers and Racemic Modification of 1,1‘-Binaphthyl-2,2‘-Dicarboxylic Acid

Formation of adlayers of the optically active compound 1,1‘-binaphthyl-2, 2‘-dicarboxylic acid (BINAC) on iodine-modified Au (111) surfaces in perchloric acid was investigated by in situ scanning tunneling microscopy (STM). Highly ordered arrays formed on the surfaces via simple spontaneous adsorption from a solution of enantiomers or the racemic BINAC, in spite of the fact that BINAC has a three-dimensionally complex stereochemical structure. Adlayers of both enantiomers essentially shared the same enantiomorphous structure. Observed parameters of the rectangular unit cell lattice for arrays of both enantiomers of BINAC were a = 2.3 ± 0.2 nm and b = 0.7 ± 0.2 nm. On the other hand, racemic modification formed an entirely different adlayer, which consisted of an alternate alignment of the two enantiomers, with an oblique unit cell lattice with parameters of a = 1.2 ± 0.2 nm, b = 0.8 ± 0.1 nm, and 74 ± 3°. No domain composed of a single enantiomer was observed. The stronger hetero-intermolecular interactions of enantiomer couples led to the formation of an alternate arrangement in the array prepared by racemic modification.