Five generations of nitroxyl-functionalized dendrimers

The preparation and electron paramagnetic resonance (EPR) spectroscopy of a series of fully functionalized poly(propylene imine) dendrimers (DAB-dendr-(NH2)(n); n = 2, 4, 8, 16, 32, 64) with 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxy radical end groups is described. The pendant nitroxyl end groups exhibit a strong exchange interaction. As a consequence, the EPR spectrum directly reveals the number of end groups from the splitting between the hyperfine transitions for the lower generations. For the higher generations an increasingly exchanged-narrowed EPR spectrum is observed. The temperature and solvent dependency of the exchange interaction is used to assess the dynamic behavior of the dendritic branches. FT-IR spectroscopy shows that a hydrogen-bonded network between the amide functionalities is present, consistent with the observed solvent effect on the EPR spectra.