New amphiphilic rodlike polymers with pendent hemicyanine groups. 2. Monolayer behavior

Self-assembled monolayers (SAMs) of 2,5-[(E)-N-methyl-4-[2-[4-[(L)-prolinoxy]phenyl]ethenyl]-pyridinium p-toluensulfonate] (POST)(4)-ester and POST4-amide polymers were prepared on negatively charged solid substrates by the electrostatic interaction between the chromophore and the substrate and characterized using W-vis spectroscopy, second harmonic generation intensity (SHG) measurement, ellipsometry, and surface-enhanced Raman scattering (SERS) spectroscopy. Simultaneous measurements of W-vis absorption, SHG intensity, and ellipsometry for the various reaction times indicated that the most densely packed monolayers were found for the two samples. For POST4-ester: reaction time, 30 min; thickness, 12 Angstrom; tilt angle, 34.6 degrees; chromophore density, 3.43/nm(2). For POST4-amide: reaction time, 60 min; thickness, 6 Angstrom; tilt angle, 41.7 degrees; chromophore density, 3.60/nm(2). POST4-amide SAMs showed higher chromophore density and SHG intensity than POST4-ester SAMs due to the intermolecular hydrogen bonding between amide backbones. The order of the second-order susceptibility coefficient, d(33) was measured to be ca. 400 pm/V for POST4-amide. In situ thermal relaxation studies also revealed high thermal stability. For example, at a high temperature of 120 degreesC, it was demonstrated that the chromophores inside SAMs were not disorganized but changed in their chemical structure due to the chemical shift of the resonance balance between benzenoid and quinoid forms of the chromophore.