One-Photon Excitation Followed by a Three-Step Sequential Energy-Energy-Electron Transfer Leading to a Charge-Separated State in a Supramolecular Tetrad Featuring Benzothiazole-Boron-Dipyrromethene-Zinc Porphyrin-C60.

ABSTRACT

A panchromatic triad, consisting of benzothiazole (BTZ) and BF2 -chelated boron-dipyrromethene (BODIPY) moieties covalently linked to a zinc porphyrin (ZnP) core, has been synthesized and systematically characterized by using 1 H NMR spectroscopy, ESI-MS, UV-visible, steady-state fluorescence, electrochemical, and femtosecond transient absorption techniques. The absorption band of the triad, BTZ-BODIPY-ZnP, and dyads, BTZ-BODIPY and BODIPY-ZnP, along with the reference compounds BTZ-OMe, BODIPY-OMe, and ZnP-OMe exhibited characteristic bands corresponding to individual chromophores. Electrochemical measurements on BTZ-BODIPY-ZnP exhibited redox behavior similar to that of the reference compounds. Upon selective excitation of BTZ (≈290 nm) in the BTZ-BODIPY-ZnP triad, the fluorescence of the BTZ moiety is quenched, due to photoinduced energy transfer (PEnT) from 1 BTZ* to the BODIPY moiety, followed by quenching of the BODIPY emission due to sequential PEnT from the 1 BODIPY* moiety to ZnP, resulting in the appearance of the ZnP emission, indicating the occurrence of a two-step singlet-singlet energy transfer. Further, a supramolecular tetrad, BTZ-BODIPY-ZnPImC60 , was formed by axially coordinating the triad with imidazole-appended fulleropyrrolidine (ImC60 ), and parallel steady-state measurements displayed the diminished emission of ZnP, which clearly indicated the occurrence of photoinduced electron transfer (PET) from 1 ZnP* to ImC60 . Finally, femtosecond transient absorption spectral studies provided evidence for the sequential occurrence of PEnT and PET events, namely, 1 BTZ* -BODIPY-ZnPImC60 →BTZ-1 BODIPY* -ZnPImC60 →BTZ-BODIPY-1 ZnP*ImC60 →BTZ-BODIPY-ZnP.+ImC60 .- in the supramolecular tetrad. The evaluated rate of energy transfer, kEnT , was found to be 3-5×1010  s-1 , which was slightly faster than that observed in the case of BODIPY-ZnP and BTZ-BODIPY-ZnP, lacking the coordinated ImC60 . The rate constants for charge separation and recombination, kCS and kCR , respectively, calculated by monitoring the rise and decay of C60 .- were found to be 5.5×1010 and 4.4×108  s-1 , respectively, for the BODIPY-ZnPImC60 triad, and 3.1×1010 and 4.9×108  s-1 , respectively, for the BTZ-BODIPY-ZnPImC60 tetrad. Initial excitation of the tetrad, promoting two-step energy transfer and a final electron-transfer event, has been successfully demonstrated in the present study.