Photoluminescence and conductivity studies of anthracene-functionalized ruthenium nanoparticles

Carbene-stabilized ruthenium nanoparticles were functionalized with anthryl moieties by olefin metathesis reactions with 9-vinylanthracene, at a surface concentration of about 19.7%, as estimated by H-1 NMR spectroscopic measurements. Because of the conjugated metal-ligand interfacial bonding interactions, UV-vis measurements of the resulting nanoparticles showed a new broad absorption band centered at 612 nm, in addition to the peaks observed with monomeric vinylanthracene. FTIR measurements depicted apparent red-shifts of the aromatic vibrational stretches as compared to those of the monomeric vinylanthracene, suggestive of decreasing bonding order of the aromatic moieties as a result of extended conjugation between the particle-bound anthracene groups. Photoluminescence measurements confirmed the notion that effective intraparticle charge delocalization occurred by virtue of the conjugated metal-ligand interfacial bonding interactions, with apparent red-shifts of the excitation peaks and blue-shifts of the emission features, as compared to those of the monomeric vinylanthracene. The diminishment of the Stokes shift was, at least in part, attributed to the different chemical environments surrounding the anthryl moieties on the nanoparticle surface. Electronic conductivity measurements showed that because of the conjugated Ru=C pi bonds, the activation energy for interparticle charge transport was about one order of magnitude lower than that observed with particles passivated by alkanethiolates. Additionally, whereas the original carbene-stabilized nanoparticles exhibited a semiconductor-metal transition within the temperature range of 100 to 320 K, anthracene-functionalized nanoparticles displayed apparent semiconducting behaviors with the ensemble conductivity increasing monotonically with temperature, most likely due to the disordering within the nanoparticle solids that arose from the different structures of the carbene ligands and anthryl moieties. These studies indicate that anthracene functionalization may be exploited as an effective route towards the manipulation of nanoparticle optoelectronic properties.