Non-covalent Assembly of Reversible Photoswitchable Surfaces

Purohit, Nipa S

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Non-covalent Assembly of Reversible Photoswitchable Surfaces

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Previous studies carried out in our laboratory resulted in the development of noncovalently assembled multilayered thin films incorporating metal ions such as Cu(II) and organic ligands including dicarboxypyridine. In one study, a SAM consisting of 4-[(10- mercaptodecyl)oxy]pyridine-2,6-dicarboxylic acid was deposited on gold. The pyridine group was then used to complex a layer of Cu(II) ions which in turn were capped by cis- 2,2â€™-dipyridylethylene. This stilbene analog undergoes photoinduced cis-trans isomerization on the surface resulting in a substantial increase in the hydrophilicity of the surface leading to the possibility of creating virtual microfluidic valves and pumps. However, the photoswitchable wettability was irreversible. Stilbene-4,4-dicarboxylic acid was the ligand selected for generating a reversible system. The decision to use this stilbene moiety was based on molecular modeling and the commercial availability of both cis and trans forms. When 4-[(10-mercaptodecyl)oxy]pyridine-2,6-dicarboxylic acid was used as SAM, the stilbene-4,4â€™-dicarboxylic acid did not undergo photoisomerization. Prolonged irradiation leads to photodegradation of film. A mixed SAM of dodecanethiol and mercaptoundecanoic acid was used to create space on the surface and facilitate isomerization. But cis-trans isomerization of the stilbene moiety was not achieved by this system. When a mixed SAM of 4-[(10-mercaptodecyl)oxy]pyridine-2,6-dicarboxylic acid and 4-tert butylbenzenethiol was used, stilbene-4,4-dicarboxylic acid showed reversible photoinduced cis-trans isomerization for one complete cycle leading to a reversible change in wettability. After one cycle of isomerization the film photodegrades.

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