Two covalently linked tetrads comprised of charge stabilizing triphenylamine (TPA)-oligothiophene(bi- or terthiophene, BT or TT) linked at the meso-position of a well-known photosensitizer, BF2-chelated boron dipyrromethene (BODIPY), was further functionalized to carry an electron acceptor,fulleropyrrolidine (C60) by modification of BODIPY at the boron center. Both tetrads were characterized by arange of spectroscopic techniques including single-crystal X-ray analysis of some of the intermediates.Electrochemical studies were performed to evaluate the redox potentials of different entities of the tetrads.Electronic structures deduced from computational studies revealed the HOMO to be on the triphenylamineoligothiophenesegment while the HOMO-1 to be on the BODIPY entity while the LUMO to be fullylocalized on the fullerene entity. These results coupled with electrochemical redox potentials indicated theformation of (TPA-TT).+-BODIPY-C60 .- and (TPA-BT).+-BODIPY-C60.- as the final charge-separated statesupon photoexcitation of BODIPY in these tetrads. Expectedly, steady-state emission studies revealed fluorescencequenching in the tetrads indicating the occurrence of excited state events. Further, femtosecondtransient absorption studies coupled with data analysis using Glotaran, performed in polar benzonitrile andnonpolar toluene, confirmed the occurrence of photoinduced charge separation in both dyads. A long-livedcharge-separated state (~20 ns) was possible to observe in the case of TPA-TT-BODIPY-C60 as a result ofhole shift mechanism while for TPA-BT-BODIPY-C60 lifetime of the charge separated state was ~ 1 ns.
Sequential photoinduced electron transfer followed by hole transfer leading to long-lived charge-separated states in newly synthesized covalently linked tetrads is demonstrated.
Volume 134, 2022
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