Self-assembled monolayers of fluorene- and nitrofluorene-terminated thiols on polycrystalline gold electrode: Electrochemical and optical properties

In this paper, two new thiols, [4-(9H-fluoren-9-ylmethyl)-phenyl]-methanethiol (6a) and [4-(2-nitro-9H-fluoren-9-ylmethyl)-phenyl]-methanethiol (6b), were synthesized, and self-assembled monolayers (SAMs) of these thiols were formed on gold electrodes. The structure and surface properties of molecular films were investigated by contact angle measurements and attenuated total reflectance infrared spectroscopy (ATR-FTIR). The blocking behavior of Au-6aand Au-6b SAMs was examined with cyclic voltammetry in the presence of redox probes such as K₃Fe(CN)₆, Ru(NH₃)₆Cl₃ and ferrocene. Electrochemical measurements revealed that the voltammetric behavior of the redox probes was dependent on the nature of the probe molecules, the electrolytic solution composition and the monolayer structure. The optical properties of the SAMs were studied by steady-state and time-resolved fluorescence spectroscopy. It was obtained that the fluorescence emission bands of Au-6a and Au-6b monolayers were red-shifted and broadened compared to those of free thiols in solution as well as significant reduction at their emission intensities. The fluorescence decay profiles of 6a and 6b monolayers were described by a monoexponential function. In order to determine the possible deactivation mechanism between the metal support and photoexcited molecules, spectroelectrochemical steady-state fluorescence and lifetime measurements on the Au-fluorophore electrodes were also performed as a function of the applied potential. These results indicate that the fluorescence quenching occurs via an energy transfer mechanism from the excited molecules to the gold substrate.