Insights into the photoinduced anion translocation of donor–π–acceptor+ (ion)− molecules

Abstract

By strategic design and synthesis of a new series of phosphonium salts (compounds 1-7[OTf]), where [OTf]- stands for the trifluoromethanesulfonate anion, we performed comprehensive spectroscopic and dynamic studies on the photoinduced anion migration in toluene. Our aim is to probe if the anion migration is associated with an intrinsic barrier or is barrier-free. After the occurrence of excited-state intramolecular charge transfer (ESICT) in 1-7, the charge redistribution of the cation triggers the translocation of the counter anion [OTf]-, resulting in emission spectral temporal evolution. As a result, we describe the photoinduced anion migration by introducing spectral response function C(t), a concept adopted from the solvent diffusional relaxation. The experimental results indicate that the anion migration lacks an intrinsic barrier, i.e., the relaxation dynamics can be described by a biased Brownian motion along the charge transfer direction. The experimental findings are also qualitatively supported by theoretical calculations including restrained electrostatic potential (RESP) and hole-electron distribution analyses.