Photoinduced charge separation after excited energy transfer in snowflake-shaped Zn-porphyrin dendrimer with anthraquinone terminals: Enhancement of the electron-transfer rates by 'dendrimer effect'

摘要

Photoinduced electron transfer (ET) and energy transfer (EnT) of a snowflake-shaped Zn-porphyrin dendrimer with anthraquinone terminals (dendrimer-1), which has four rigid ethynylene-phenylene conjugated chains covered by soft benzyl ether (BE) branches, is investigated by means of steady-state and time-resolved fluorescence measurements. Dendrimer-2 without anthraquinone terminals undergoes a nearly quantitative EnT from the excited singlet states of BE branches to the Zn-porphyrin core, where the conjugated chain plays an important role as a mediator of EnT within this antenna type dendrimer (the dendrimer effect on EnT). A comparison of the rates of charge separation via the singlet excited state of Zn-porphyrin to the anthraquinone terminals through oligo(ethynylenephenylene)s in a series of model compounds reveals considerable acceleration of the rate constants by the dendrimer architecture (the dendrimer effect on ET). This acceleration is attributed to hydrophobic interactions between the flexible BE chains within dendrimer-1, resulting in a significant conformational change into coplanar forms between the Zn-porphyrin plane and the ethynylenephenylene-anthraquinone plane.