Alkyne-stabilized ruthenium nanoparticles: manipulation of intraparticle charge delocalization by nanoparticle charge States.

Monolayer-protected transition metal nanoparticles are a unique family of functional nanomaterials in which the properties of the materials can be readily manipulated not only by the chemical nature of the metal cores and the organic protecting ligands, but also the metal–ligand interfacial bonding interactions. The latter is largely motivated by recent progress in nanoparticle passivation by metal–carbon covalent bonds, where intraparticle charge delocalization may occur as a result of the strong metal–carbon interfacial bonding interactions, in sharp contrast to nanoparticles that are functionalized by mercapto derivatives. For instance, when ferrocene moieties are bound onto a ruthenium nanoparticle surface by ruthenium–carbene p bonds, effective intervalence transfer occurs between the ferrocenyl metal centers at mixed valence, as manifested in electrochemical and near-infrared (NIR) spectroscopic measurements and density functional calculations. Furthermore, when fluorophores are attached onto the nanoparticle surface by the same conjugated linkage, novel emission characteristics emerge that are consistent with those of dimeric derivatives with a conjugated spacer. In a more recent study, effective intraparticle charge delocalization was also observed with ruthenium nanoparticles passivated by alkynyl fragments. This result was ascribed to the unique interfacial bonding interactions (Ru C ) formed by ruthenium and sphybridized carbon atoms of the ligands. In these studies, the nanoparticle metal cores serve as the conducting media to facilitate charge transfer between the functional moieties covalently bound onto the nanoparticle surface. Therefore it is anticipated that the extent of intraparticle conjugation may be readily controlled by the nanoparticle charge state, which is the primary motivation of the present study. Experimentally, by exploiting the molecular capacitor characters of Ru C nanoparticles, the charge states of the nanoparticles were varied by simple chemical reduction or oxidation. The impacts of the nanoparticle charge states on the particle optical and electronic properties were then carefully examined by FTIR spectroscopy, X-ray photoelectron spectroscopy (XPS), and photoluminescence measurements, and compared to those of the as-prepared nanoparticles. The synthetic procedure for the preparation of ruthenium nanoparticles passivated by 1-octynyl fragments (Ru-OC) has been detailed previously. TEM measurements showed that the nanoparticles exhibited an average core diameter of (2.55 0.15) nm. The nanoparticle charge states were then varied by chemical redox reactions. Specifically, to render the nanoparticles negatively charged, in a typical reaction, 5 mg of Ru-OC nanoparticles were dissolved in dichloromethane (1 mL); a freshly prepared water solution of NaBH4 (1 mL, 5 mgmL ) was then added. The mixture was stirred for 30 min and then water was removed. The resulting nanoparticles exhibited negative net charges and were denoted as Ru-OCRed. Positively charged nanoparticles were prepared in a similar fashion by mixing the nanoparticle solution with an aqueous solution of saturated Ce(SO4)2 for 30 min. The resulting nanoparticles were denoted as Ru-OCOx. Transition metal nanoparticles passivated with a lowdielectric organic protecting layer have long been known to act as nanoscale molecular capacitors. In fact, based on a concentric structural model, the nanoparticle capacitance (CMPC) can be estimated by CMPC= 4pe0e(r+d) r d, where e0 is the vacuum permittivity, e is the effective dielectric constant of the organic protecting layer, r is the radius of the metal core, and d is the length of the organic protecting ligand. For the octyne-passivated ruthenium (Ru-OC) nanoparticles, r= 1.275 nm, d= 0.848 nm (estimated by Hyperchem), and e= 2.6. Thus, the nanoparticle capacitance can be estimated to be about 0.92 aF. To quantify the change of the nanoparticle charge state after reduction or oxidation, we measured the open circuit potentials of the nanoparticles electrochemically. It was found that the as-prepared Ru-OC nanoparticles exhibited an open circuit potential of + 0.140 V (versus Ag/AgCl). After reduction by NaBH4, it decreased to + 0.024 V, whereas after oxidation by ceric sulfate, it increased to + 0.250 V. This result indicated that the reduced nanoparticles (Ru-OCRed) exhibited an average charging of 0.67 electrons per nanoparticle, whereas the oxidized nanoparticles (Ru-OCOx) were formed by an average discharging of 0.63 electrons per nanoparticle. Interestingly, despite these subtle changes of nanoparticle charge states, rather drastic impacts were observed on the nanoparticle optoelectronic properties. Figure 1 depicts the FTIR spectra of the nanoparticles before and after reduction or oxidation. For the as-prepared Ru-OC nanoparticles, the C C stretching band appeared at 1965 cm 1 (inset). In comparison to octyne monomers, for which the C C stretch[*] X. W. Kang, N. B. Zuckerman, Prof. J. P. Konopelski, Prof. S. W. Chen Department of Chemistry and Biochemistry, University of California 1156 High Street, Santa Cruz, CA 95064 (USA) Fax: (+1)831-459-2935 E-mail: [email protected] Homepage: http://chemistry.ucsc.edu/~ schen