In previous work with a longer chain analogue of 1a
(in which the thioacetyl group is connected to a 16-carbon chain instead of the 11 carbons in 1a
], we reported the photoconversion of a thioacetate-decorated SAM to a sulfonated surface by UV irradiation in air. In that case, the initially deposited thioacetate-functionalized SAM had more closely packed alkyl chains (FTIR: methylene stretching frequencies 2919 and 2850 cm−1
versus the 2922–2923 cm−1
and 2851–2852 cm−1
). Only 1i
displays some level of crystalline order (with methylene values of 2921/2851 cm−1
), and even that is not as ordered as the C16
The important conclusion from the oxidation of the acetylated thiols with the two different chain lengths is that both systems provide a sulfonated surface that is fully wetted. The acetyl-derived byproduct is easily removed by rinsing with water, and the resulting surface is hydrophilic. We note that while the oxidation of the longer chain thioesters (1b
) with OXONE takes longer (as indicated by reaction times in ), as would be expected for the more hydrophobic starting SAMs, the longer chain byproducts are successfully washed away and the resulting surface is also fully wetted. The slowing of the reaction with OXONE with increasing numbers of methylene units is reminiscent of what was seen by Sagiv et al. [21
] for permanganate oxidation in which an olefin at the monolayer surface was oxidized much faster than an olefin within the monolayer. The fact that the monolayers reported herein are somewhat less well-packed than those reported in the permanganate oxidation study may be responsible for the fact that the differences in reactivity observed herein are smaller than those reported for the permanganate oxidation.
A problem with the longer chain acyl units is seen in their photo-oxidation. In that case, there is no evidence for a slowing of the reaction based on the rate of disappearance of the carbonyl, but the high degree of hydrophilicity that is achieved with aqueous OXONE is not obtained in these longer chain systems. It seems that there is a buildup of longer chain byproducts that need organic solvents to effectively remove them. However, the exposure of the high-free-energy sulfonated surface to organic solvent leads to surface reorganization and loss of hydrophilicity. Thus, in order to take advantage of the enormous change in surface wetting achieved by the oxidation of a system such as 1g–i (from a water contact angle of over 100° to a fully wetted surface), oxidation in aqueous OXONE is most effective.
Beyond the impact of changing the chain length on the chemistry described above, we have also established a clear wavelength dependence on the photo-oxidation of the thioesters. The reactions of the benzene and thiophene derivatives are notable for a number of reasons. Firstly, the aromatic ring does not interfere with the chemistry described above. The reactivity of the thioester is not undermined (despite a small retardation of the OXONE reaction), either by the bulk of the aromatic rings or by the reduced electrophilicity, which is typical of conjugated carbonyl groups.
We also note the wavelength dependence of the photochemistry reported herein. The longer wavelength absorption of the conjugated chromophore is clearly a first step towards a system that could be photoreacted with longer wavelength light. This would provide a route to photopatterned sulfonate surfaces, in which the irradiation could be performed through regular glass or Pyrex, i.e., media that are not transparent to shorter wavelength UV radiation.