The pH-dependent orientation of N3 dye on a gold substrate is revealed using heterodyne-detected vibrational sum frequency generation spectroscopy Available to Purchase

We report on systematic changes to the adsorption geometry of the dye N3 {[cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato ruthenium(II)]} on a gold substrate as the pH of the deposition environment is altered. The protonation states of the four —COOH groups of the N3 dye change according to the modified pH conditions, thus affecting the number of —COOH and —NCS functional groups that participate in the adsorption to gold. Here, we use heterodyne detected vibrational sum frequency generation (HD-VSFG) spectroscopy to obtain surface specific vibrational information on both —COOH and —NCS groups as a function of pH of the deposition conditions. Polarization-dependent HD-VSFG yields sets of complex χ⁽²⁾ spectra, enabling us to perform a simultaneous fitting procedure to the polarization-dependent real and imaginary components and thus extract detailed structural information of the N3/gold interface. Our results show that N3 preferentially adsorbs to gold either with two —COOH groups and one —NCS group in more acidic conditions or with one —COOH group and two —NCS groups in more basic conditions.