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-60

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 H-1 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 (approximate to 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 (BODIPY)-B-1* 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(60), was formed by axially coordinating the triad with imidazole-appended fulleropyrrolidine (ImC(60)), and parallel steady-state measurements displayed the diminished emission of ZnP, which clearly indicated the occurrence of photoinduced electron transfer (PET) from (ZnP)-Zn-1* to ImC(60). Finally, femtosecond transient absorption spectral studies provided evidence for the sequential occurrence of PEnT and PET events, namely, (1)BTZ*-BODIPY-ZnP:ImC(60)-> BTZ-(BODIPY)-B-1*-ZnP:ImC(60)-> BTZ-BODIPY-(ZnP)-Zn-1*:ImC(60)-> BTZ-BODIPY-ZnP.+:ImC(60)(.-) in the supramolecular tetrad. The evaluated rate of energy transfer, k(EnT), was found to be 3-5x10(10) s(-1), which was slightly faster than that observed in the case of BODIPY-ZnP and BTZ-BODIPY-ZnP, lacking the coordinated ImC(60). The rate constants for charge separation and recombination, k(CS) and k(CR), respectively, calculated by monitoring the rise and decay of C-60(.-) were found to be 5.5x10(10) and 4.4x10(8) s(-1), respectively, for the BODIPY-ZnP:ImC(60) triad, and 3.1x10(10) and 4.9x10(8) s(-1), respectively, for the BTZ-BODIPY-ZnP:ImC(60) 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.