Long-Lived Hole Stabilized at a Triphenylamine Core and Shielded by Rigid Phenylazomethine Dendrons: A Pulse Radiolysis Study

The optical properties and reactivity of one-electron oxidized states (radical cations) of dendrimers are investigated in benzonitrile solutions by nanosecond pulse radiolysis. The hole stabilized at the triphenylamine (TPA) core is effectively shielded by a rigid dendritic phenylazomethine (DPA) shell of four generations, leading to an extension of its lifetime by nearly 2 orders of magnitude in comparison with a core radical cation without dendrons. A continuous red shift of the peak in the photoabsorption spectrum in the visible region and a decrease in the extinction coefficients (oscillator strengths) are found with increasing dendrimer generation number. These experimental observations are compared to the results of time-dependent density functional theory. It is suggested that bulky, rigid, insulating DPA dendrons shield against outer reactants and stabilize hole at the TPA core. Correlating the dendrimer generation number with the optical properties and reactivities of the radical cations could shed light on fundamental aspects of structurally defined nanoenvironments having hyperbranched entities.