Chemical enhancement in the SERS spectra of indigo: DFT calculation of the Raman spectra of indigo-Ag14 complexes

The appearance of the Raman and the SERS spectra of indigo (IND) at 785 and 514 nm laser wavelength excitations has prompted the study of the interactions between the molecule and the silver substrate to which the molecule is adsorbed. In particular, no change of appearance between Raman and SERS spectra, apart some intensification, is observed at 785 nm excitation whereas some changes in the number and intensity of the bands are observed at 514 nm excitation. In a previous study a simplified model has been adopted where the molecule is simply bonded to an Ag-2 cluster. Structural models have now been devised with the molecule is sited either perpendicular (edge-on) or planar (surface-on) to the surface of a Ag-14 pyramidal cluster. The structure of the two complexes, the energy of the electronic excitations, the Raman spectra of the molecule/metal complexes have been studied through a TD-DFT simulation. The appearance of the experimental SERS spectra have been confronted with the prediction of the Herzberg-Teller surface selection rules of Lombardi et al., which have been formulated for the SERS spectra of molecules adsorbed on metal nanoparticles, in the frame of the Albrecht's theory for Raman scattering. The spectral predictions for the edge-on geometry fully meet the observation that the SERS spectra are dominated by totally symmetric A(g)(under C-2h) , symmetry) bands or, taking into account the lost of the inversion center after adsorption on the metal surface, in-plane A(g)(under C-s, symmetry) modes. This fully justifies the pop up of IR modes of the molecule in the SERS spectra, modes which belong to B u and A u species in the free molecule C-2h, symmetry group. A systematic assignment of the modes of the free molecule and of the molecule in the complex has been made through the inspection of the calculated vibrational coordinates of the single vibrations. Dynamic polarizability (pre-resonance) calculations have been performed at 475 and 690 nm wavelength excitations, corresponding to the experimental 514 and 785 nm wavelengths, respectively. A small spectral enhancement is predicted at 690 nm for both complex geometries which should depend on charge transfer (CT) states calculated, albeit with very weak oscillator strength, in that spectral range. A substantial enhancement is instead calculated for both the edge-on and the surface-on models at the excitation wavelength of about 475 nm. The calculated Raman spectra intensification originates from the proximity of electronic excitations of intra-cluster, intra-molecular and CT type. The plasmonic excitation cannot be accounted for by the present model calculation. Natural Transition Orbitals (NTO) have been generated for the electronic excitations of both geometries, by transforming the ordinary orbital representation into a more compact form in which each excited state is expressed, if possible, as single pair of orbitals.