Photosensitive Self-Assembled Monolayers on Gold: Photochemistry of Surface-Confined Aryl Azide and Cyclopentadienylmanganese Tricarbonyl

Photosensitive self-assembled monolayers (SAMs) are prepared by the spontaneous reaction of di-11 -(4- azidobenzoate)-l-undecyl disulfide, I, or 11-mercaptoundecylcyclopentadienylmanganese tricarbonyl, II, with polycrystalline Au. SAMs of I are photosensitive by virtue of a pendant aryl azide moiety (λ_max = 270 nm, log ϵ = 4.2) which undergoes photoreaction with secondary amines to form Au-confined derivatives of 3-H azepine and hydrazine, while Au-II SAMs undergo photosubstitution of phosphine for CO. Au-I SAMs irradiated in the presence of various secondary amines were characterized by reflection absorption infrared spectroscopy (RAIRS), XPS, and cyclic voltammetry, and showed persistent attachment of approximately one monolayer of amine. Infrared spectroscopy was used to establish that primary photoproducts obtained upon irradiation of Au-I SAMs in diethylamine are nearly identical to primary photoproducts obtained upon irradiating an analog of I, methyl 4-azidobenzoate, in diethylamine solution. XPS analysis of Au-I SAMs before and after irradiation in the presence of secondary amines confirms loss of N₂ from the Au-I SAM and incorporation of one nitrogen atom per surface-confined molecule. Most definitively, cyclic voltammetry of Au-I after irradiation in the presence of 2-ferrocenylethyl-2′,2′,2′-trifluoroethylamine, III, showed 3.3 × 10^-10 mol cm^-2 of surface-confined ferrocene. Positive ion SIMS spectra of Au-II irradiated in the presence of PPh₂Et, PPh₂(n-Pr), PPh₂(CH₂)₁₁Fc, X, Fc = ferrocenyl, or PPh₂(CH₂)₂(CF₂)₅CF₃, XI, show that introduction of the phosphine onto the surface occurs upon near-UV irradiation. Importantly, the surface photochemistry of I and II allows the high lateral resolution patterning of Au surfaces and Au microstructures with a variety of molecular reagents. SIMS maps for vinyl ferrocenium (m/z 212) or F^- (m/z 19) of flat Au-II substrates irradiated through a Cr-on-glass mask in the presence of X or XI demonstrate photochemical patterning at a lateral resolution of <2 μm. SEM confirms the resolution of 2 μm features in the irradiated monolayer. Cyclic voltammetry and SIMS were used to demonstrate that Au microstructures derivatized with I can be patterned with molecular reagents, also at a lateral resolution of <2 μm by UV irradiation through a mask and a thin film of the desired amine. A condensation figure was used to demonstrate photopatterning of a flat Au-I substrate irradiated through a mask and a thin film of (C₂H₅OH)₂NH.