Raman spectroscopy of dipyrrins: nonresonant, resonant and surface-enhanced cross-sections and enhancement factors


M. R. Waterland, MacDiarmid Institute for Advanced Materials and Nanotechnology, Institute of Fundamental Sciences, Massey University, Private Bag 11 222, Palmerston North, New Zealand 4410. E-mail: M.Waterland@massey.ac.nz


Detailed studies of the mechanism of surface-enhanced (resonance) Raman spectroscopy (SE(R)RS), and its applications, place a number of demands on the properties of SERS scatterers. With large Raman cross-sections, versatile synthetic chemistry and complete lack of fluorescence, free dipyrrins meet these demands but the Raman and SE(R)RS spectroscopy of free dipyrrins is largely unknown. The first study of the Raman spectroscopy of free dipyrrins is therefore presented in this work. The nonresonant Raman, resonant Raman and surface-enhanced Raman spectra of a typical meso aryl-substituted-dipyrrin are reported. Absolute differential cross-sections are obtained for excitation wavelengths in the near infrared and visible region, in solution phase and for dipyrrin adsorbed on the surface of silver nanoparticles. Raman enhancement factors for SERRS and resonance Raman are calculated from the observed differential cross-sections. The magnitudes of the resonantly enhanced cross-sections are similar to those recently reported for strong SERS dyes such as Rhodamine 6G and Crystal Violet. Free dipyrrins offer the advantages of existing SERS dyes but without the drawback of strong fluorescence. Free dipyrrins should therefore find applications in all areas of Raman spectroscopy including fundamental studies of the mechanisms of SERS and bioanalytical and environmental applications. Copyright © 2011 John Wiley & Sons, Ltd.