Enhanced Raman spectrum of lawsone on Ag surface: Vibrational analyses, frequency shifts, and molecular geometry

The surface-enhanced Raman spectrum of lawsone, a well known natural dye, has been investigated. Activation with KNO3 or Na2SO4 solution was necessary to enhance the Raman signal, whereas addition of NaCl solution depletes the effects. In the enhanced Raman spectrum, the strong double-bond stretching bands are most distinctive and show large red shifts from those in the infrared and FT-Raman spectra. The observed strong double-bond stretching bands reflect lawsone coordinated perpendicular to the Ag surface. DFT computations have been carried out for the plausible configurations of lawsone coordinated to an adatom on the Ag surface. Lawsone coordinated to an Ag+ adatom with H+ released best reproduces the observed vibrational characteristics.

Graphical abstractLawsone coordinated to an Ag+ adatom. Infrared, powder FT-Raman, and surface enhanced Raman spectra of lawsone have been investigated, and its configuration that explains the large red shifts in the double-bond stretching region has been searched for. Lawsone with release of H+ and coordinated to an Ag+ adatom best reproduces the observed SERS frequencies and the shifts from the infrared values. The strong double-bond stretching bands and Ag+⋯O stretching bands suggest that the molecular plane is perpendicular to the Ag surface.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The SERS spectrum of lawsone has been investigated with a micro-Raman set-up. ► The distinctive double-bond stretching bands show considerable red shifts. ► The double-bond and Ag+⋯O stretching bands suggest a perpendicular molecular orientation. ► Lawsone coordinated to an Ag+ adatom reproduces the observed SERS spectrum.