Adsorbed states of substituted α-aminophosphinic acids on a silver electrode surface: comparison with a colloidal silver substrate

We investigated the interfacial structures of various aromatic (each compound contains one or two phenyls) di-alpha-amino (L-1 - L-3) and alpha-amino-alpha-hydroxyphosphinic (L-4 - L-6) acids immobilized onto an electrochemically roughened silver electrode surface in an aqueous solution using surface-enhanced Raman scattering (SERS). These structures were compared to those on a colloidal silver surface to determine the relationship between adsorption strength and geometry. The presence of an enhanced nu(19a) ring band in the SERS spectra of L-6, L-2 and L-3 on the electrode indicated that the benzene rings of those molecules interact with the electrode surface through localized C=C bond(s). We observed significant band broadening of the benzene ring modes for all alpha-hydroxyphosphinic acids on both substrates, except for L-1 deposited onto the electrode surface. This suggests the possibility of direct interaction between the ring and Ag, although the benzene ring-surface pi overlap is weaker for the colloidal silver than for the Ag electrode. The downward shift in wavenumber and alternations in the enhancement of a nu(12) ring band indicate a general increase of tilt angle on both silver substrates in the order L-3 < L-4 < L-5 < L-6. The altered enhancement of the bands due to the vibrations of the -NH2 and O-P=O fragments, a finding observed on both silver substrates, strongly suggests that the groups interact with different strength and geometry with these substrates. Copyright (C) 2009 John Wiley & Sons, Ltd.