Resonance Raman Spectra of o-Safranin Dye, Free and Adsorbed on Silver Nanoparticles: Experiment and Density Functional Theory Calculation.

The properties of o-Safranin (SO) dye in the first electronic excited state were studied with combined experimental and theoretical methods. The electronic absorption spectra of SO molecules are measured in water solution and in the presence of silver nanoparticles. The normal Raman (NRS) and resonance Raman (RR) spectra of solid SO and the surface enhanced Raman (SERS) and surface enhanced resonance Raman (SE[R]RS) spectra of SO adsorbed on silver nanoparticles are measured at different excitation energies. The enhancement factors for selected vibrational bands of the RR, SERS, and SE[R]RS spectra of SO have been obtained with respect to the NRS spectra of the solid after a careful evaluation of the experimental conditions. The data furnished useful information on the excited electronic states and the interactions of SO with silver nanoparticles. The experimental results are discussed on the basis of DFT and TD-DFT calculations (B3LYP/6-311+G(d,p)) on the isolated SO molecule.