SERS Spectra of Alizarin Anion-Agn (n=2, 4, 14) Systems: TDDFT Calculation and Comparison with Experiment

Using density functional theory (DFT) and various cluster models for the simulation of the interaction between alizarin and silver nanoparticles, we calculated the SERS spectra of the AZ anion-Ag-n (n = 2, 4, 14) complexes and compared the results with experiment. The analysis of the calculated SERS spectra helped distinguishing the contribution of the chemical and electromagnetic mechanisms to the spectral enhancement, under the assumption that the excitation energies of the clusters are comparable with the local plasmon energies of nanoparticles. The results show a certain dependence of the relative Raman intensities and peak positions on the silver cluster size. Calculation of UV-vis transition energies and Raman spectra of the complexes have been performed under the assumption that the AZ anion is bounded to the silver clusters through the oxygen atoms of the C=O groups in 1,9 positions, in a edge-on perpendicular orientation. The calculated SERS spectra show an acceptable similarity with the experimental SERS spectra carried out with excitation at 632 nm. The results of the calculation under preresonance condition with respect to chromophore located and cluster located excitations are compatible with mechanisms of enhancement acting on different parts of the AZ anion molecule. The calculations were performed using the B3LYP hybrid density functional. A 6-31g(d) basis set for H, C, O, and LANL2DZ basis set for Ag were used. Vertical excitation energies and the corresponding oscillator strengths were calculated by means of time dependent density functional theory (TDDFT).