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-C.

A panchromatic triad, consisting of benzothiazole (BTZ) and BF -chelated boron-dipyrromethene (BODIPY) moieties covalently linked to a zinc porphyrin (ZnP) core, has been synthesized and systematically characterized by using 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 BTZ to the BODIPY moiety, followed by quenching of the BODIPY emission due to sequential PEnT from the 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-ZnP:ImC , was formed by axially coordinating the triad with imidazole-appended fulleropyrrolidine (ImC ), and parallel steady-state measurements displayed the diminished emission of ZnP, which clearly indicated the occurrence of photoinduced electron transfer (PET) from ZnP* to ImC . Finally, femtosecond transient absorption spectral studies provided evidence for the sequential occurrence of PEnT and PET events, namely, BTZ -BODIPY-ZnP:ImC →BTZ- BODIPY -ZnP:ImC →BTZ-BODIPY- ZnP :ImC →BTZ-BODIPY-ZnP :ImC in the supramolecular tetrad. The evaluated rate of energy transfer, k , was found to be 3-5×10  s , which was slightly faster than that observed in the case of BODIPY-ZnP and BTZ-BODIPY-ZnP, lacking the coordinated ImC . The rate constants for charge separation and recombination, k and k , respectively, calculated by monitoring the rise and decay of C were found to be 5.5×10 and 4.4×10  s , respectively, for the BODIPY-ZnP:ImC triad, and 3.1×10 and 4.9×10  s , respectively, for the BTZ-BODIPY-ZnP:ImC 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.