UV-Promoted exchange reaction as a tool for gradual tuning the composition of binary self-assembled monolayers and chemical lithography

We demonstrate that exchange reaction between a primary self-assembled monolayer (SAM) and a potential molecular substituent, capable to form SAM on the same substrate, can be promoted in a controlled way by preliminary exposure of the SAM to UV light with a suitable wavelength. Using a model system of a nonsubstituted alkanethiolate (AT) SAM as the primary film and COOH-substituted AT as the substituent, we showed that the composition of the binary mixed film formed by the UV-promoted exchange reaction (UVPER) can be precisely varied and tuned over the entire concentration range by the selection of a proper UV exposure time. The reason for the promotion effect of the UV light treatment is the appearance of chemical and structural defects in the primary film, induced by chemically active oxygen-derived species. Both the photooxidation of the thiolate tailgroups at the SAM-substrate interface and the structural and chemical defects in the aliphatic matrix promote the exchange reaction. Because both these defects are stochastically distributed over the target film, no phase separation occurs, and the resulting mixed SAM represents a true molecular mixture of both components. Apart from its usefulness for the preparation of mixed SAMs, the UVPER approach can be combined with lithography and applied for the fabrication of chemical patterns and molecular templates for which the chemical composition of the individual areas or lithographic features can be varied in any desirable gradual way.