Communication: Functionalized poly(propylene imine) dendrimers are efficient electron donors in photoinduced electron transfer reactions to Buckminsterfullerene in solution. The photoinduced electron transfer reaction produces persistent fullerene radical anions at room temperature even with light switched off, as evidenced from ESR spectroscopy. The reaction exhibits a remarkable size-effect: persistent electron transfer is observed only for dendrimer generations G greater than or equal to 2.