The influence of gold nanoparticles on the two photon absorption of photochromic molecular systems.

In this study, we investigate the influence of gold nanoparticles on the nonlinear optical properties of the dihydroazulene/vinylheptafulvene photo- and thermochromic system. The influence is studied using a combined quantum mechanical/molecular mechanical approach wherein the molecules are treated with quantum mechanics. The nanoparticle is modelled using molecular mechanics where the gold atoms are represented by their polarizabilities. The quantum mechanical part of the system, namely the molecules, is described using density functional theory with the long-range corrected functional CAM-B3LYP and the correlation consistent basis set aug-cc-pDVZ. The results of the investigation show that the two photon absorptions of the molecules are indeed affected by the gold nanoparticle. The influence of the nanoparticle is especially large for the vinylheptafulvene molecule when decreasing the molecule-cluster distance. From the results, we observe that the interactions with the gold nanoparticle are very dependent on molecular conformation, relative molecular orientation, and distance between the molecule and the cluster. We conclude that the three studied molecules are affected differently by the nanoparticle, and we suggest that experiments are carried out to further investigate how these molecules coordinate to and interact with gold nanoparticles. This could present a new tool for controlling molecular properties. Furthermore, we see that the results of this study are in good agreement with earlier studies of other properties on the same system.