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.

The Protective Nature of Dodecanethiol Self‐Assembled Monolayers Deposited on Au for the Electropolymerization of 3‐Methylthiophene

“…The current suppression experienced with the use of redox couples such as [Fe(CN) 6 ] 3À/4À in the voltammograms of the SAM-Au electrodes has provided evidence for the ion blocking characteristics in solution for several SAMs [19,20]. In the present study, we obtained small currents in the voltammograms of dodecanethiol-Au electrodes in an aqueous K 4 Fe(CN) 6 solution.…”

“…The current vs. reaction time variation can be represented by a combination of a two-and three-dimensional nucleation mechanism as has been proposed for the growth of polythiophene, and poly-3-methylthiophene and poly-3-hexylthiophene on a bare Pt electrode in an acetonitrile/ LiCF 3 SO 4 (or TBAPF 6 ) solution [18]. According to a proposed mechanism for polythiophenes [18,19], the polymer film is generated at a first stage by an instantaneous 2D growth, at a second stage, by a progressive 3D growth, and finally, by an instantaneous 3D growth, when the current reaches a plateau. Although the I-t curves obtained for the bare Au and SAM-Au electrodes indicates a higher polymerization rate for the PMThio films on the SAM-Au electrodes, they show similar shapes as expected for similar nucleation processes and morphologies.…”

Ultrathin Luminescent Films of Rigid Dinuclear Ruthenium(II) Trisbipyridine Complexes

“…In addition, the latter approach is synthetically very demanding and Au has also been shown to quench the emission of attached luminescent molecules. [48,49] Therefore, we decided to use the simple and highly reproducible LB technique for the deposition of the rigid molecules 1 and 2 on a substrate. The choice of dinuclear complexes for the realization of the thin films was dictated by the length of the molecules (3-5 nm), by their rod-type structure, which would favor close packing, and by the asymmetry of the systems (one site hydrophobic and the other hydrophilic), which can be easily obtained with our synthetic strategy.…”

Biomimetic Lipoglycopolymer Membranes: Photochemical Surface Attachment of Supramolecular Architectures with Defined Orientation

“…On the other hand, fluorescence is not fully quenched if the tether is longer than six methylene units, with a linear increase of fluorescence with increasing tether length. [26] We thus used a bis(aminododecyl) disulfide in combination with 1-mercaptohexane. Microphase separation of the functional amino termini is ensured by differences in alkyl chain length of five to six methylene units, [27] which is known to lead to a fine distribution of the longer chain [28] with no cluster formation.…”