Real-time PMIRRAS studies of in situgrowth of C11Eg6OMe on gold and immersion effects

Abstract

We studied the growth of self-assembling monolayers of C₁₁Eg₆OMe on gold under aqueous conditions. With the help of polarisation modulation infrared reflection absorption spectroscopy (PMIRRAS) we monitored the evolution of characteristic absorption modes in the fingerprint region (1050–1500 cm⁻¹) during the later stages of the growth of the SAM. We observed a change from rather amorphous structure with mixed all-trans and helical conformation to an ordered structure with predominantly helical structure over time. Changes of mode positions, intensities and broadness can be described by a single exponential. In addition, we investigated the effect of hydration for SAMs with different degrees of crystallinity. To that end, we compared their spectra at certain ordering levels in contact with aqueous solution with the corresponding spectra in air. SAMs with a highly ordered crystalline structure in air show the same structure under aqueous conditions. However, SAMs which are still crystalline in air, but less perfect, show rather amorphous spectral features under aqueous conditions indicating a strong interaction with water. This implies that the ability of water to penetrate the EG moiety strongly depends on its structure which in turn is related inter alia to the surface coverage. Since the interaction with water plays an important role in the prevention of unspecific adsorption on oligo(ethylene glycols) this is important for its application. Our experiments also underline the importance of the in situ analysis of the film structure.