Generation Dependent Ultrafast Charge Separation and Recombination in a Pyrene-Viologen Family of Dendrons.

The ability of a dendritic network to intercept electrons and extend the lifetime of a short-lived photoinduced charge separated (CS) state was investigated in a homologous family of methyl viologen (MV2+) dendrons spanning four generations, G0 through G3. The CS state in the parent pyrene methylene viologen G0 system with a single acceptor exhibits an extremely short lifetime of tau = 0.72 ps. The expansion of the viologen network introduces slower components to the recombination kinetics by allowing the injected electron to migrate further away from the donor. The long-lived fraction of the population increases monotonically in the order G3 > G2 > G1 > G0, while the respective recombination rates decrease. In the highest generation of the dendron similar to 14% of the CS state population experiences a 10-fold or greater lifetime extension. Long range tunneling across multiple viologen units and sequential site-to-site hopping both contribute to the overall effect. The large excess energy deposited in the apical viologen upon charge separation and the presence of an extended network of low lying pi-orbitals likely facilitate shuttling the electron further down the dendron.