New metallodendrimers containing an octakis(diphenylphosphino)-functionalized silsesquioxane core and ruthenium(II)-based chromophores

A new class of surface-modified dendrimers has been prepared by reactions of 8 equiv of the terpyridine-functionalized polyether monodendrons with a polyhedral oligomeric silsesquioxane (POSS) core. Subsequent reactions of these spherically shaped organic dendrimers with Ru(II)-based precursors afford photo- and redox-active metallodendrimers. These new dendrimers have been characterized using a combination of mass spectral analysis (MALDI-TOF/MS, ESI/MS, and FAB/MS), nuclear magnetic resonance (H-1, C-13, Si-29, and (31)p{H-1} NMR), photophysical analyses (electronic absorption, emission, excited-state lifetime, and quantum yield) and electrochemical measurement (cyclic voltammetry). Specifically, (31)p{H-1} NMR is used to monitor the completion of reactions and the purity of dendrimers and metallodendrimers. These new metallodendrimers exhibit large extinction coefficients that coincide with the number of peripheral Ru(II)-based chromophores. With the use of (-CH2-Ph-tpy)Ru-II(bpy)(2) type of chromophores, all metallodendrimers are found emissive at room temperature, with lifetimes in the range of 605-890 ns. Photophysical data also indicate similar steady-state emission maxima and single-exponential decay kinetics for all metallodendrimers, and the observed overall quantum yields of the G1, G2, and G3 metallodendrimers are found to be 14, 20, and 7 times higher than that of the monomeric model complex (CH3-Ph-tpy)Ru(bpy)(2)(PF6)(2). Electrochemical studies reveal the presence of surface-confined species, in addition to the ligand-centered and metal-centered redox processes.