Synthesis and Photophysical Characterization of sub-Stoichiometric Ruthenium-Functionalized PAMAM Dendrimers
Authors:Elizabeth Grossman, Michelle Haas, Garrett Johnson, Jeff Kuwahara, Jonas Marks, Anneka Rienstra, Tamara Sparks
Mentor:Steven Contakes, Professor of Chemistry, Westmont College
We are interested in preparing Ruthenium modified PAMAM dendrimers that can bind and photoreduce ions in aqueous solution. In the present work, we investigated the use HOBt/EDC coupling to attach [Ru(bpy)2(dcbpy)]2+ (dcbpy = 4,4’-dicarboxybipyridine) to G2.0 PAMAM dendrimers containing amine terminal groups and [Ru(bpy)2(5-aminophen)]2+ (5-aminophen = 5’-amino-1,10-phenanthroline) to G1.5 PAMAM dendrimers containing carboxy terminal groups. The yields of the former reaction increase from 41 to 96% as the ratio of [Ru(bpy)2(dcbpy)]2+ to G2.0 PAMAM increases from 1:1 top 8:1 and slowly decrease thereafter. The average Ru-G2.0 PAMAM ratio in the isolated products also increases across this range, approaching the theoretical maximum of 16:1 for [Ru(bpy)2(dcbpy)]2+ to G2.0 PAMAM reactant ratios of 32 and higher. The yields for the conjugation of Ru(bpy)2(5-aminophen)]2+ to G1.5 PAMAM are much lower (7 - 22%) and do not exhibit any consistent trends with reactant ratio, although the Ru:dendrimer ratio in the isolated products increases from 0.1 to 3.1 as the [Ru(bpy)2(dcbpy)]2+ to PAMAM reactant ratio increases from 1:1 to 1:64. To test whether the low coupling yield between Ru(bpy)2(5-aminophen)]2+ to G1.5 PAMAM was due to the low nucleophilicity of the former, we prepared [Ru(bpy)2(4-NH2(CH2)2phen)]2+ and [Ru(bpy)2(5-NH2(CH2)2phen)]2+ via three-step syntheses in poor overall yields of 1 and 2%, respectively. These aliphatic amine derivatives could be attached to G1.5 PAMAM in near-stoichiometric yields, consistent with our hypothesis. The photophysical properties of the Ru-dendrimer conjugates are consistent with interior bound Ru-diimine groups that experience increasingly more hydrophilic environments at higher loadings. In particular, their MLCT bands are broadened and their emission spectra exhibit maxima at unusually long wavelengths, although the Ru(bpy)2(dcbpy)-G2.0 conjugates’ spectra approach those of [Ru(bpy)2(phen)]2+ at higher Ru:PAMAM ratios. The low loading G2.0 conjugates also exhibit anomalously large emission quantum yields (~12%) that decrease to ~2% as the Ru:dendrimer ratio increases increases from 3 to10.