Pyrene-Functionalized Ruthenium Nanoparticles: Novel Fluorescence Characteristics from Intraparticle Extended Conjugation

Ruthenium nanoparticles functionalized with pyrene moieties were prepared by olefin metathesis reactions of carbene-stabilized nanoparticles with 1-vinylpyrene and 1-allylpyrene (referred to as RuVPy and RuAPy, respectively). 1H NMR spectroscopic measurements showed that the surface concentration of the pyrene moieties was 26.4 and 22.9%, respectively. In fluorescence measurements, RuVPy nanoparticles exhibited two prominent emission bands at 392 and 490 nm, very similar to those of (E)-1,2-di(pyren-1-yl)ethene, suggesting that the particle-bound pyrene moieties might behave equivalently to pyrene dimers with a conjugated linker by virtue of the Rucarbene π bonds. In sharp contrast, RuAPy nanoparticles displayed only a single emission peak at 392 nm, consistent with monomeric pyrene derivatives of 1-vinylpyrene and 1-allylpyrene. This implies that the intraparticle extended conjugation might be effectively turned off by the insertion of an sp3 carbon into the chemical linker that bound the pyrene moieties onto the nanoparticles. In addition, RuVPy nanoparticles exhibited a rather long lifetime of the emission at 490 nm, as compared to that of the conjugated pyrene dimer, suggesting that the nanoparticles might be less prone to photobleaching. This is attributed to the neighboring organic protecting ligands that may create a more rigid chemical environment for the nanoparticle-bound pyrene moieties and thus prolong the life of the excited state, as compared to the strong electrostatic perturbations which exist in free solution when surrounded by a polar solvent.