A discrete interaction model/quantum mechanical method to describe the interaction of metal nanoparticles and molecular absorption.

A frequency-dependent quantum mechanics/molecular mechanics method for the calculation of response properties of molecules adsorbed on metal nanoparticles is presented. This discrete interaction model/quantum mechanics (DIM/QM) method represents the nanoparticle atomistically, thus accounting for the local environment of the nanoparticle surface on the optical properties of the adsorbed molecule. Using the DIM/QM method, we investigate the coupling between the absorption of a silver nanoparticle and of a substituted naphthoquinone. This system is chosen since it shows strong coupling due to a molecular absorption peak that overlaps with the plasmon excitation in the metal nanoparticle. We show that there is a strong dependence not only on the distance of the molecule from the metal nanoparticle but also on its orientation relative to the nanoparticle. We find that when the transition dipole moment of an excitation is oriented towards the nanoparticle there is a significant increase in the molecular absorption as a result of coupling to the metal nanoparticle. In contrast, we find that the molecular absorption is decreased when the transition dipole moment is oriented parallel to the metal nanoparticle. The coupling between the molecule and the metal nanoparticle is found to be surprisingly long range and important on a length scale comparable to the size of the metal nanoparticle. A simple analytical model that describes the molecule and the metal nanoparticle as two interacting point objects is found to be in excellent agreement with the full DIM/QM calculations over the entire range studied. The results presented here are important for understanding plasmon-exciton hybridization, plasmon enhanced photochemistry, and single-molecule surface-enhanced Raman scattering.

A discrete interaction model/quantum mechanical method for describing response properties of molecules adsorbed on metal nanoparticles.

A new polarizable quantum mechanics/molecular mechanics method for the calculation of response properties of molecules adsorbed on metal nanoparticles is presented. This method, which we denote the discrete interaction model/quantum mechanics (DIM/QM) method, represents the nanoparticle atomistically which enables the modeling of the influence of the local environment of a nanoparticle surface on the optical properties of a molecule. Using DIM/QM, we investigate the excitation energies of rhodamine-6G (R6G) and crystal violet (CV) adsorbed on silver and gold nanoparticles of different quasispherical shapes and sizes. The metal nanoparticle is characterized by its static total polarizability, a reasonable approximation for frequencies far from the plasmon resonance. We observe that for both R6G and CV, the presence of the nanoparticle shifts the strongest excitation to the red approximately 40 nm and also increases the oscillator strength of that excitation. The shifts in excitation energies due to the nanoparticle surface are found to be comparable to those due to solvation. We find that these shifts decay quickly as the molecule is moved away from the surface. We also find that the wavelength shift is largest when the transition dipole moment is aligned with the edges of the nanoparticle surface where the electric field is expected to be the largest. These results show that the molecular excitations are sensitive to the local environment on the nanoparticle as well as the specific orientation of the molecule relative to the surface.

Controlling the non-resonant chemical mechanism of SERS using a molecular photoswitch.

In this work we present a detailed investigation of the Raman properties of a dithienylethene photoswitch interacting with a small gold cluster (Au(19)(+)) using time-dependent density functional theory (TD-DFT). The enhancement mechanism (CHEM) due to the molecule-surface chemical coupling in surface-enhanced Raman scattering (SERS) has been characterized for this system. We demonstrate that it is possible to control the CHEM enhancement by switching the photoswitch from its closed form to its open form. The open form of the photoswitch is found to be the strongest Raman scatterer when adsorbed on the surface whereas the opposite is found for the free molecule. This trend is explained using a simple two-state approximation. In this model the CHEM enhancement scales roughly as (omega(X)/omega(e)(4)), where omega(X) is the HOMO-LUMO gap of the free molecule and omega(e) is an average excitation between the HOMO of the photoswitch and the LUMO of the metal. We propose that the ability of this photoswitch to switch reversibly from open to closed will make it an excellent probe to control the CHEM enhancement of SERS.