Effect of Alkyl Chain Length on the Fluorescence of 9-Alkylfluorenyl Thiols as Self-Assembled Monolayers on Gold

Abstract

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-assembled thin films. The thicknesses of the films as measured by optical ellipsometry correspond well to the calculated thickness of monolayer films in which the extended alkyl chains are oriented at a 30° angle to the Au surface. The films are densely packed and are passivating to electron transfer between the Au surface and K 4 Fe(CN) 6 in a contacting aqueous KCl solution. Grazing angle reflectance FTIR spectra show that the aromatic fluorenyl groups are fixed at a defined distance from the Au surface as determined by alkyl chain length disposed in an all-trans conformation. Surface fluorescence spectra for the SAMs are red-shifted and broadened compared with the corresponding molecule's fluorescence when measured in dilute solution. Lifetimes for the terminal fluorenyl groups when bound as a SAM to the Au surface are bimodal, with a long-lived species corresponding to that observed in solution (∼3.2 ns), and a second shorter lifetime species (∼260 ps) reflecting a metal-mediated quenching process. The fluorescence intensity increases with increasing alkyl spacer length in monotonic fashion. 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.