Covalent Assembly of Stilbene-Based Monolayers:  Factors Controlling Molecular Interactions

A new series of stilbene-based chromophores have been used to prepare structurally related siloxane-based monolayers in order to determine which factors control the intermolecular chromophore−chromophore interactions in the solid state. The reaction of chromophore precursors 4-styrylpyridine (1), 4-2-(4-bromophenyl)-vinyl-pyridine (2), 4-(2-naphthalen-1-ylvinyl)-pyridine (3), 4-(2-anthracen-9-ylvinyl)-pyridine (4), and 4-(2-pyren-2-ylvinyl)-pyridine (5) with excess 3-iodo-n-propyl-1-trimethoxysilane resulted in the corresponding salts 6−10 in quantitative yield. The assembly of chromophores 6−10 on hydrophilic substrates from solution resulted in the formation of densely packed monolayers with a film thickness of ∼1 nm. The average chromophore density (∼1 chromophore/50 Å2) is well within the range that allows π−π stacking to occur. Transmission UV−vis spectroscopy of the siloxane-based films shows that the intermolecular interactions are a function of the aryl groups (e.g., phenyl, bromophenyl, naphthalene, anthracene, and pyrene). Relatively weak electronic interactions occur between the surface-bound chromophores 6, 7, and 10, whereas strong electronic interactions occur between surface-bound chromophores 8 and 9. The series of monolayers on sodium lime glass and polished silicon is characterized by a combination of physicochemical methods including X-ray photoelectron spectroscopy, advancing aqueous contact angle measurements, optical spectroscopy, atomic force microscopy, and synchrotron X-ray reflectivity.