STM Observation of Open- and Closed-Ring Forms of Functionalized Diarylethene Molecules Self-Assembled on a Au(111) Surface

Ordered self-assembled monolayers (SAMs) of thiolated diarylethene photochromic molecules have been built on a Au(111) surface and characterized by mean of scanning tunneling microscopy (STM) for the first time. It has been revealed that molecules deposited from UV-illuminated (closed-ring isomer) and initial solutions (open-ring isomer) both occupy 4 × 2√3 adsorption sites on Au(111) and present a conformation where conjugated parts are parallel to the surface, forming “umbrella-like” structure. On the basis of the distances measured from STM images with intramolecular resolution and on quantum chemical DFT calculations of electronic structure for single molecules, an identical model of molecular packing with a 4 × 4√3 unit cell is proposed for both isomers. The variation of internal orbitals between open- and closed-ring isomers leads to significant change of STM contrast, allowing direct identification of each diarylethene isomer using STM images.