First hyperpolarizability of para-aminoaniline induced by a variety of gold nano particles

Molecules exposed to an electromagnetic field due to the presence of nearby nanoparticles are known to be affected. The nonlinear response of the molecular system, the first hyperpolarizability, is investigated for 4-nitroaniline (PNA) in a variety of different nanoparticle environments. In order to introduce different molecular environments the quantum mechanical–molecular mechanics DFT/MM response method was utilized for calculating the electromagnetic properties of the molecule interacting with the nanoparticles and the frequency dependent electromagnetic fields. We clearly show that the metal nanoparticles are able to influence the first hyperpolarizability of the molecule, and that the influence strongly depends on the chosen molecular environment. It is found that the first hyperpolarizability of the molecule strongly depends on the distance and orientation to the nanoparticles, whereas the size of the nanoparticles is of little importance when a sufficiently sized nanoparticle is used for representing a bulk metal cluster. Especially, the different orientations of the nanoparticles with respect to the molecule show the chemical effect of different functional groups, since the nonlinear response is significantly larger for the electron withdrawing group pointing towards the nanoparticle.