Investigations on nanoparticle-chromophore and interchromophore interactions in pyrene-capped gold nanoparticles

Abstract

Three pyrene alkanethiol derivatives (P1, P2, and P3) possessing flexible alkyl groups of different lengths were attached to nanoparticles of gold (~2-3 nm in size) along with dodecanethiol (Au-P1, Au-P2, and Au-P3). The photophysical properties of these systems were investigated as a function of (i) distance of chromophore from gold core, (ii) concentration of pyrene on gold surface, and (iii) solvent polarity. The structured absorption bands of the pyrene chromophore were significantly perturbed near the surface of gold nanoparticles (Au-P1), indicating a strong ground state interaction between the plasmon electrons of Au nanoparticles and the π-electron cloud of the chromophore. Such effects were not observed in Au-P2 and Au-P3 systems, in which the linker groups are long enough to prevent any ground state interactions. A gradual increase in the peak intensity ratio of band III/I of the normal fluorescence of pyrene chromophore was found with an increase in length of the spacer group. These results indicate that the local environment close to the surface of the Au nanoparticle is more polar compared to the bulk medium. Interchromophoric interactions are limited in the Au-P1 system due to the restriction imposed by the curvature of spherical gold nanoparticle whereas the flexible alkyl chain tethering pyrene in Au-P2/Au-P3 allows free interaction between chromophores. Steady state and time-resolved emission studies indicate that the normal fluorescence and intermolecular excimer formation are the main deactivation channels of the singlet excited state of pyrene linked to Au nanoparticles, in nonpolar solvents. In contrast, the competitive electron transfer to the gold nanocore dominates in polar solvents.