Effect of alkyl chain length on the fluorescence of 9-alkylfluorenyl thiols as self-assembled monolayers on gold

Spectral characterization indicates that SAMs of 9-alkylfluorenyl thiols 1 form well-ordered monolayers on polycrystalline gold. Sessile drop contact angle measurements demonstrate the hydrophobic nature of these self-assembl ed thin films. The thicknesses of the films as measured by optical ellipsom etry correspond well to the calculated thickness of monolayer films in whic h the extended alkyl chains are oriented at a 30 degrees angle to the An su rface. The films are densely packed and are passivating to electron transfe r between the Au surface and K4Fe(CN)(6) in a contacting aqueous KCI soluti on. Grazing angle reflectance FTIR spectra show that the aromatic fluorenyl groups are fixed at a defined distance from the Au surface as determined b y alkyl chain length disposed in an all-trans conformation. Surface fluores cence spectra for the SAMs are red-shifted and broadened compared with the corresponding molecule's fluorescence when measured in dilute solution. Lif etimes for the terminal fluorenyl groups when bound as a SAM to the Au Surf ace are bimodal, with a long-lived species corresponding to that observed i n solution (similar to3.2 ns), and a second shorter lifetime species (simil ar to 260 ps) reflecting a metal-mediated quenching process. The fluorescen ce intensity increases with increasing alkyl spacer length in monotonic fas hion, This order is as expected, with the rate of fluorescence quenching by the metal surface being correlated inversely to the distance separating a covalently bound fluorescent probe molecule from the gold surface.