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 1H 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 1BTZ* to the BODIPY moiety, followed by quenching of the BODIPY emission due to sequential PEnT from the 1BODIPY* 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:ImC60, 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 1ZnP* to ImC60. Finally, femtosecond transient absorption spectral studies provided evidence for the sequential occurrence of PEnT and PET events, namely, 1BTZ*‐BODIPY‐ZnP:ImC60→BTZ‐1BODIPY*‐ZnP:ImC60→BTZ‐BODIPY‐1ZnP*: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‐ZnP:ImC60 triad, and 3.1×1010 and 4.9×108 s−1, respectively, for the BTZ‐BODIPY‐ZnP:ImC60 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.

Recent advances in supramolecular fullerene chemistry

“…81–84 The non-covalent assembly between multiple fullerenes or chromophores is another promising approach for preparing organic photosensitizers. 85–87…”

“…[81][82][83][84] The non-covalent assembly between multiple fullerenes or chromophores is another promising approach for preparing organic photosensitizers. [85][86][87] The process of triplet-triplet annihilation upconversion (TTA-UC) is of interest for efficiently harvesting diffuse visible/ near-IR light and achieving high upconversion quantum yields. 89 To improve TTA-UC efficiency, Yang and coworkers developed a new strategy using host-guest complexation between an alkyl nitrile chain functionalized C 60 -BODIPY sensitizer ( 14) and a tetraperylene-based pillar [5]arene emitter (15) (Fig.…”

Does Location of BF2-Chelated Dipyrromethene (BODIPY) Ring Functionalization Affect Spectral and Electron Transfer Properties? Studies on α-, β-, and Meso-Functionalized BODIPY-Derived Donor–Acceptor Dyads and Triads

“…In recent years, our groups have explored various antenna−donor−acceptor model compounds possessing BODIPY as one of the active components. [18][19][20][21][22][23][24][25][26]37,38 In the majority of the studies where BODIPY formed part of the donor−acceptor system, the functionalization was carried out at the meso-position. 18−26,29−36 Only a handful of studies utilized the pyrrole α and β positions.…”

“…Establishing structure–function relations in light-induced charge separation processes in synthetic donor–acceptor conjugates is not only important to further our understanding of natural photosynthesis , but also vital for solar energy harvesting and building optoelectronic devices. − In this context, harvesting sunlight into chemical energy by an artificial photosynthetic approach is considered one of the possible solutions to fulfill the ever increasing global energy needs without significantly affecting the environment. , For this, a wide variety of artificial photosynthetic donor–acceptor conjugates have been designed and synthesized; some of these systems have shown to be capable of both wide-band capture of solar photons and the ability to use the absorbed energy to generate charge-separated states of appreciable energy. − Using elegant synthetic chemistry, it has been possible to fine-tune the light capture and conversion efficiency to a desired value by altering the D/A units, their relative distance, and the π-linker connecting them. − …”

“…BF 2 -chelated dipyrromethenes (BODIPYs) (see Figure for the structure) have gained much attention in recent years as building blocks of artificial photosynthetic systems, sensors, and probes. − BODIPYs exhibit high absorption coefficients, high fluorescence quantum yields, and tunable redox potentials. − Their electronic and photonic properties can be tuned by a proper choice of electron rich/deficient addends and by performing substitution at different positions of the BODIPY macrocycle. − These features have made BODIPY an attractive fluorophore as its absorption can be extended over a wide spectral range. In recent years, our groups have explored various antenna–donor–acceptor model compounds possessing BODIPY as one of the active components. − ,, In the majority of the studies where BODIPY formed part of the donor–acceptor system, the functionalization was carried out at the meso-position. − ,− Only a handful of studies utilized the pyrrole α and β positions. − Importantly, although substitution effect on absorption and fluorescence properties as a function of different locations of the BODIPY ring has been studied, no systematic study has been reported on the substitution effects of different locations on the BODIPY ring on their electron transfer properties. In the present study, we have undertaken a task where BODIPY has been functionalized at the meso-, α-, and β-pyrrole positions to visualize its effects on the spectral and electrochemical properties with acetylene ( 1–3 in Figure ) and bis-acetylene ( 4–6 ) π-extenders.…”