Surface-confined energy transfer in mixed self-assembled monolayers

We have fabricated a set of self-assembled monolayers consisting of naphthalene and dansyl derivatives in a range of surface loading ratios for the purpose of examining excitation transport in mixed self-assembled monolayer systems. Both tethered chromophores were immobilized on an epoxide-terminated adlayer on silica via an identical spacer, where the linking chemistry produced an amide linkage. X-ray photoelectron spectroscopy (XPS), ellipsometry, and contact angle measurements were used to characterize these chromophore-containing layers. The excitation transfer behavior of these monolayers has been examined using steady-state and time-resolved fluorescence spectroscopy. Steady-state fluorescence measurements show that excitation transfer from the naphthalene to dansyl chromophores occurs, with the efficiency of excitation transport scaling with chromophore surface loading densities, as expected. The donor lifetimes decrease with increasing acceptor loading density, and the functional form of the acceptor decay was independent of the donor/acceptor ratio. Our findings are not consistent with a homogeneous adlayer, but do provide information on the structural heterogeneity that is characteristic of these interfaces.