Spectroscopic Study of Platinum and Rhodium Dendrimer (PAMAM G4OH) Compounds: Structure and Stability

Hydroxyl-terminated, poly(amidoamine) (PAMAM)\uddendrimer-capped platinum and rhodium nanoparticles were\udstudied with UV-vis, Fourier transform IR, and UV-Raman\udspectroscopy. The adsorption bands in the region of 190-\ud900 nm were shown to be sensitive to the electronic structure of\udPt compounds in solution and in the solid state. The electron\udcharge-transfer Pt2þ r N band at 260 nm was attributed to\udmononuclear Pt2þ compounds, and resonance Raman (244 nm)\udestablished that the electron donors are amide nitrogen atoms.\udMultidentate coordination bonding with Pt2þ causes a strong\udstabilizing chelate effect, and this entropic factor hinders reduction of Pt2þ by BH4\ud-. UV diffuse-reflectance spectra showed the\udappearance of new bands at 300 and 342 nm in PtG4OH deposited on mesoporous silica (SBA-15) induced by heating under\udreducing conditions. The new bands can be assigned to platinum clusters like binuclear Pt2þ-Pt2þ and higher, dendrimer-capped\udanalogs of platinum blue compounds. In the temperature range of 30 to 350 C, the decomposition of G4OH begins above 150 C\udwith dehydrogenation of backbone. The presence of platinum and rhodium particles decreased the initial temperature of decay. In\udreducing conditions, the PtG4OH undergoes rearrangements of its secondary structure due to the formation of metallic platinum\udclusters. UV- photochemical degradation of G4OH, Pt- and Rh-dendrimer systems in ozone-rich atmosphere as well in H2 and Ar\udshowed that effect of UV light is greater than the of ozone. The ratio of initial rates for photolysis and “dark” ozonolysis was Whv/WO3\ud∼ 40