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Quasi chiral phase separation in a two-dimensional orientationally disordered system: 6-nitrospiropyran on Au(111).

The adsorption and chiral expression of 6-nitrospiropyran (SP6) molecules on a Au(111) surface are studied by scanning tunneling microscopy (STM) in combination with density functional theory (DFT) calculations. Both the chirality and the adsorption orientation of each adsorbed SP6 molecule are determined. The racemic mixture of SP6 enantiomers forms two-dimensional (2D) domains with same close packed positional orders but different internal orientational structures due to the random distribution of two adsorption orientations in each domain. However, all these orientationally disordered 2D domains undergo spontaneous quasi chiral phase separation; the 2D SP6 domains separate into 1D homochiral chains in which the SP6 molecules adopt two orientations randomly. This novel phenomenon is attributed to the preferential formation of the energetic favorable configurations with both the C-H...O weak hydrogen bonds and the pi-stacking of the two moieties of each SP6 molecule.