Monolayer-protected clusters with fluorescent dansyl ligands

Monolayer-protected An cluster (MPC) molecules with mixed monolayers of alkanethiolate and omega-carboxylalkanethiolate ligands were functionalized with the fluorescent label dansyl cadaverine. The emission intensity per MPC-attached dansyl group varies with (a) the number of dansyl sites per MPC (i.e., loading, 4 to 24), (b) with the linker chainlength between Au core and dansyl site, and (c) with chainlengths of alkanethiolates in the surrounding MPC monolayer coating. Emission intensity per MPC-attached dansyl group increases with loading and with the length of core-dansyl linker chains when the surrounding alkanethiolate monolayers have chain lengths equal to or greater than the linker. These results are interpreted in terms of distance-dependent energy transfer from the excited dansyl label to the metal-like MPC core and various steric constraints on thermal fluctuations of linker chains that influence the distance of approach of the label to the core. Correction for self-absorption by the highly colored MPC solution was necessary. Emission intensities of MPC-attached dansyl labels are lower than those from equivalent concentrations of dansyl units and MPCs as unattached cosolutes.