Two-Dimensional Hetero- to Homochiral Phase Transition from Dynamic Adsorption of Barbituric Acid Derivatives

Barbituric acid derivative (TDPT) is an achiral molecule, and its adsorption on a surface results in two opposite enantiomerically oriented motifs, namely TDPT-S_p and R_p. Two types of building blocks can be formed; block I is enantiomer-pure and is built up of the same motifs (format S_pS_p or R_pR_p) whereas block II is enantiomer-mixed and composes both motifs (format S_pR_p), respectively. The organization of the building blocks determines the formation of different nanoarchitectures which are investigated using scanning tunneling microscopy at a liquid/HOPG interface. Sophisticated, highly symmetric "nanowaves" are first formed from both building blocks I and II and are heterochiral. The "nanowaves" are metastable and evolve stepwisely into more close-packed "nanowires" which are formed from enantiomer-pure building block I and are homochiral. A dynamic hetero- to homochiral transformation and simultaneous multi-scale phase transitions are demonstrated at the single-molecule level. Our work provides novel insights into the control and the origin of chiral assemblies and chiral transitions, revealing the various roles of enantiomeric selection and chiral competition, driving forces, stability and molecular coverage.