Surface-enhanced resonance Raman and hyper-Raman spectroscopy of water-soluble substituted stilbene and distyrylbenzene chromophores

Surface-enhanced Raman spectra (SERS) are presented for two water-soluble, donor - donor- or donor - acceptor-substituted phenylene vinylene derivatives adsorbed on the surfaces of colloidal silver nanoparticles. The normally water-insoluble, substituted stilbene and distyrylbenzene chromophores have been modified by attaching trimethylammoniumalkyl chains to the amino donor group, creating both aqueous solubility and a net positive charge that facilitates binding to citrate- or borohydride-reduced silver colloids. A symmetrically donor-substituted distyrylbenzene exhibits strong electronically pre-resonant SERS spectra that are not highly dependent on excitation wavelength and are qualitatively similar to the solution-phase resonance Raman (RR) spectra. A donor - acceptor-substituted stilbene exhibits weaker SERS enhancement despite being fully electronically resonant. The spectra vary considerably with excitation wavelength and show new lines not present in the solution-phase spectra, which are attributed to a photoproduct formed on the surface. Two-photon resonant hyper-Raman spectra are also presented for the donor - acceptor stilbene, both with and without surface enhancement.