Synthesis and photochemistry of coumarin-based self-assembled monolayers on silicon oxide surfaces

In this study, we report on a system consisting of self-assembled monolayers (SAMs) formed by 7-(11-trichlorosilylundecyloxy)coumarin on mica and on quartz glass. For the first time, in the absence of an inert atmosphere or a stabilizing matrix, we demonstrate by means of absorption and fluorescence spectroscopy that the photochemical cycloaddition of coumarin head groups is completely reversible in SAMs. Photodimerization and photocleavage were monitored for five cycles of alternating irradiation with light of wavelengths 355 and 254 nm, respectively. SAM formation was analyzed using atomic force microscopy and contact angle measurements. The quantum yield of the single photon absorption induced photocleavage of coumarin dimers in a SAM was determined to be Phi = 0.29. Asymmetrical photocleavage after lactone ring-opening of the coumarin dimer SAM causes a change in contact angle from about 70 degrees to about 55 degrees. This may be observed, for example, as a significant change in surface wettability.