Experimental and Theoretical Studies of the Photoreduction of Metal Ion−Dendrimer Complexes: Observation of a Delocalized Organic Radical

Abstract

Following complexation with certain dendrimer functionalities, metal ions can be reduced to zerovalent metal nanoparticles via UV irradiation and with the dendrimer oxidized to a radical cation. EPR-silent Zn(II) ions can serve as the oxidizing agent, enabling the nature of the dendrimer radical cation to be examined. Spectral simulations and quantum chemical calculations were carried out to elucidate the nature of the free radicals. Spectral simulations in conjunction with electronic structure calculations suggest that the electron spin density is localized on the central N−C−C−N core structure and delocalized over the N and C atoms in the core. The radical cations of model structures with the ethylenediamine (EDA) core and that of the G0-NH2 polyamidoamine (PAMAM) were found to have a weak central one-electron C−C bond. The description of the molecular structure of the cation falls between the limit of two iminium-type ions with a charge of +0.5 e on each (1/2+R2N = CH21/2•) interacting by a one-electron C−C bond and the other limit of a 1/2+1/2•NR2−CH2−CH2−NR21/2+1/2• structure with a spin of 1/2 and a charge of 1/2 on each N. For EDA, our calculated ionization energies and heats of formation at the coupled cluster (CCSD(T)) level are in good agreement with available experimental data. The ionization energy of the G0-NH2 PAMAM was found to be substantially lower than that of EDA. The reduction in the ionization energies for the dendrimers and other effects such as metal−ligand interaction and solvation contribute to the reduction of metal cations by dendrimers with UV irradiation. Similar experiments with the G0-NH2 poly(propylene imine) (PPI) did not produce metal nanoparticles, indicating these effects are not as favorable as those for G0-NH2 PAMAM.