Photoinduced Symmetry-Breaking Charge Separation: The Direction of the Charge Transfer

摘要

Photoinduced symmetry breaking (SB) charge separation (CS) occurs when an excited chromophore is surrounded by several identical electron donors or acceptors, but it can also happen between two identical molecular units when one of them is in an excited state. Such a process is observed in the reaction center of photosynthetic purple bacteria, where a bacteriochlorophyll dimer is surrounded by two almost identical branches of protein-bound cofactors. Despite this symmetry, electron transfer takes place almost exclusively along one branch. It has been shown that because the constituents of the dimer have slightly different geometries and local environments, CS along one branch is energetically more favorable and thus substantially faster than along the other. Photoinduced SB-CS has also been observed in a few multichromophoric systems containing two or more identical molecular units, such as anthracene, stilbene, naphthale-nediimides, and perylenediimides. Occurrence of SB-CS in these systems has been deduced from the transient absorption spectra recorded upon photoexcitation of one of the molecular units, M. However, the M~(·-) and M~(·+) ions could not be unambiguously recognized and distinguished, mainly because their absorption bands overlap or have too dissimilar intensities. Furthermore, the direction of the charge flow, that is, whether the electron is transferred from or toward the excited unit, could not be determined. The CS direction was supposed to be random and solely determined by solvent fluctuations, except in one case, where structural relaxation of the excited unit was proposed to be at the origin of SB.