Sequential energy transfer followed by electron transfer in a BODIPY–bisstyrylBODIPY bound to C₆₀ triad via a ‘two-point’ binding strategy

Abstract

Excitation transfer from ¹BODIPY* to bisstyrylBODIPY followed by electron transfer to C₆₀ leading to a charge separated state of appreciable lifetime in a supramolecularly assembled triad is demonstrated, as a mimic of the photosynthetic ‘antenna-reaction centre’.

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Solar energy harvesting is one of the most researched topics in modern science as it holds promise towards a sustainable society.¹ Mimicking the early photo-initiated events of natural photosynthesis (in plants and bacteria), and utilizing that information to build energy harvesting devices is considered to be one of the avenues to tap into the vast amount of solar energy.² Research performed on this topic by scientists across the globe has been highly promising and has strengthened our fundamental understanding of natural and artificial photosynthesis. In the process of mimicking the early events of photosynthesis, excitation transfer and electron transfer in a sequential fashion, similar to what happens in the antenna and reaction centre of natural photosynthesis,³ are deemed to be of paramount interest. Consequently, supramolecular donor–acceptor systems capable of performing these photoprocesses in a step-wise manner are considered to be very important. Although several donor–acceptor systems are known to undergo either energy transfer or electron transfer or simultaneous occurrence of these two photoprocesses in competition upon photoexcitation, there are not many systems where energy transfer and electron transfer in a sequential route have been reported.⁴ This becomes especially important for the maximum utilization of solar energy.

In the present study, we have accomplished this goal by designing a supramolecular triad whose structure is shown in Fig. 1. Here, BF₂-chelated dipyrromethene (BODIPY) is covalently linked to another functionalized BODIPY bearing two styryl benzo-18-crown-6 entities.⁵ As a consequence of extended conjugation offered by the styryl entities, the bisstyrylBODIPY absorption and emission experienced redshifts of about 150–160 nm. In addition, the emission of BODIPY overlaps with the absorption of bisstyrylBODIPY, satisfying the primary requirement for excitation energy transfer. Further, the two crown ether voids were able to bind fulleropyrrolidine functionalized with two alkyl ammonium cations via cation–dipole interactions.^6a As demonstrated using ultrafast transient spectroscopic techniques, selective excitation of BODIPY promoted singlet–singlet energy transfer to the neighboring bisstyrylBODIPY. Further, the intermediate photoproduct, ¹bisstyrylBODIPY*, promoted electron transfer to the bound C₆₀ entity to produce the BODIPY–bisstyrylBODIPY˙⁺–C₆₀˙⁻ charge separated state of appreciable lifetime.

Fig. 1 Structure of the newly synthesized BODIPY–bisstyrylBODIPY–C₆₀ supramolecular triad, 1:C₆₀, and the photoprocesses originated upon selective excitation of BODIPY within the triad.

The synthesis of BODIPY–bisstyrylBODIPY-biscrown, 1 is shown in Scheme S1 (ESI†) while the details are given in the ESI.† This involved first the synthesis of meso-(4-hydroxyphenyl)BODIPY, 1a, by reacting 4-hydroxybenzaldehyde and 2,4-dimethylpyrrole in the presence of trifluoroacetic acid (TFA) in THF for 12 h followed by the addition of dichlorodicyanobenzoquinone (DDQ). After 4 h, the reaction mixture was treated with boron trifluoride etherate (BF₃·OEt₂) in the presence of triethylamine (TEA). After column purification, 1a was treated with 4′-formylbenzo-18-crown-6 with catalytic amounts of piperidine and acetic acid in benzene using the Dean–Stark apparatus to remove water from the reaction mixture and to obtain bis(benzo-18-crown-6) functionalized BODIPY, 1b. In a separate experiment, meso-(4-carboxyphenyl)BODIPY, 1c, was synthesized following a procedure similar to that used for 1a. Finally, compounds 1b and 1c were treated with EDCI in the presence of catalytic amounts of DMAP to obtain the desired BODIPY–bisstyrylBODIPY-biscrown, 1. The synthesis of bisalkylammonium functionalized fullerene, C₆₀(NH₃)₂²⁺, is given elsewhere.^6c The newly synthesized compounds were characterized using ¹H and ¹³C NMR and HR-MALDI mass techniques (see the ESI† for spectral data).

Owing to the presence of two differently functionalized BODIPY entities, the absorption spectrum of 1 revealed good spectral coverage from 300 to 700 nm. The peak maxima were located at 385, 506, 606 and 658 nm (Fig. 2) and by comparison with the absorption spectra of control compounds, 1b and 1c, the 506 nm peak was assigned to BODIPY while the 606 and 658 nm peaks were assigned to the bisstyrylBODIPY entities, respectively. The excitation of the BODIPY entity in 1 at 500 nm revealed two peaks at 525 nm corresponding to BODIPY emission; however, its intensity was quenched over 90% with the appearance of a new peak at 684 nm, corresponding to bisstyrylBODIPY. These results suggest the occurrence of singlet–singlet energy transfer in 1.⁷ The excitation spectrum recorded for 1 also confirmed this assignment. Additionally, the excitation of 1 at 600 nm corresponding to bisstyrylBODIPY revealed an emission at 684 nm, however, without any appreciable quenching.

Fig. 2 Normalized absorption (dark line) and fluorescence (red – 505 nm excitation, blue – 600 nm excitation) of BODIPY–bisstyrylBODIPY-biscrown, 1 in benzonitrile.

Next, the supramolecular complexation of 1 with C₆₀(NH₃)₂²⁺ was performed. Fig. 3a shows the spectral changes during the increased addition of C₆₀(NH₃)₂²⁺ to a solution of 1 in benzonitrile. The complexation was accompanied by a diminished peak intensity of 1 with isosbestic points at 415, 456 and 535 nm. A 1 : 1 stoichiometry was also established from a plot of mole ratio (Fig. 3b). The binding constant evaluated by analyzing data (Fig. 3c) was found to be 8.9 × 10⁴ M⁻¹, revealing stable complexation.⁸ Further, HR-MALDI mass and ¹H NMR titrations were also performed to ascertain the 1 : 1 stoichiometric ratio of the 1:C₆₀(NH₃)₂²⁺ complex (see Fig. S11–S13 in ESI†). The complexation of C₆₀(NH₃)₂²⁺ also quenched the fluorescence of 1 at 684 nm (Fig. 3d), suggesting the occurrence of excited state events from the ¹BisstyrylBODIPY* to the bound C₆₀(NH₃)₂²⁺ in the supramolecular complex. The structure of the supramolecular complex was also optimized at the B3LYP/3-21G* level⁹ (see Fig. S1 in the ESI†). In the optimized structure, the two BODIPY entities, connected by a carboxy linker, were held at a dihedral angle of 31°, and had no steric overcrowding. The B–B, B(bisstyrylBODIPY)–C₆₀ and B(BODIPY)–C₆₀ distances were ∼18 Å, ∼12 Å and 30.1 Å, respectively. Hydrogen bonding between the alkyl–NH₃⁺ and crownether oxygens was primarily responsible for the supramolecular complexation. From electrochemical studies (see Fig. S2 in the ESI†), the first oxidation of BODIPY and bisstyrylBODIPY at 0.32 and 0.76 V vs. Fc/Fc⁺ and the first reduction of C₆₀(NH₃)₂²⁺ at −1.10 V vs. Fc/Fc⁺ were recorded. Free-energy calculations¹⁰ revealed that the energy transfer from ¹BODIPY* to the neighbouring bisstyrylBODIPY and the electron transfer from ¹bisstyrylBODIPY* to supramolecularly bound C₆₀(NH₃)₂²⁺ to produce the BODIPY–bisstyrylBODIPY˙⁺–[C₆₀(NH₃)₂²⁺]˙⁻ charge-separated state are both thermodynamically feasible with ΔG values of −0.56 and −0.42 eV, respectively.

Fig. 3 (a) Absorption and (d) fluorescence changes observed for 1 with increasing addition of C₆₀(NH₃)₂²⁺ (10 equivalents) in benzonitrile. The mole ratio and binding plots are shown in (b) and (c), respectively (monitored at 670 nm).

Next, the excitation transfer in 1 and the sequential energy and electron transfer in 1:C₆₀ were probed using a femtosecond transient spectroscopic technique. Fig. 4a shows the transient spectra at the indicated delay times of 1 at the excitation wavelength of 507 nm corresponding to BODIPY in benzonitrile. Immediately after excitation, a depleted transient peak at 512 nm corresponding to ground state bleaching of ¹BODIPY* was observed (see the spectrum at 1.5 ps). A rapid recovery of this peak was observed accompanied by new transient peaks corresponding to ¹bisstyrylBODIPY*, confirming the occurrence of singlet–singlet energy transfer in 1.¹¹ In the newly developed ¹bisstyrylBODIPY* spectra, depleted peaks at 670 and 753 nm, and a positive peak at 1044 nm were observed. The depleted peaks were assigned to the ground state bleaching and stimulated emission, while the positive peak was assigned to the singlet–singlet transition of ¹bisstyrylBODIPY*. This assignment was also confirmed by the direct excitation of 1 at 657 nm (see Fig. S3a in the ESI†). Fig. 4b shows the kinetic traces of the 512 nm peak of 1 and pristine BODIPY used as a control. The recovery lasted over 3 ns in the case of pristine BODIPY, consistent with its fluorescence lifetime of 1.9 ns in benzonitrile. However, owing to the occurrence of the energy transfer process, the recovery was rather rapid in the case of 1. By analyzing the kinetic profile of ¹BODIPY* at 512 nm, a time constant of 56 ps was obtained. Using these data and those of pristine BODIPY, the energy transfer rate constant, k_EnT, was calculated to be 1.73 × 10¹⁰ s⁻¹.

Fig. 4 Femtosecond transient spectra at the indicated delay times of (a) 1 and (c) 1:C₆₀ in benzonitrile (λ_ex = 507 nm). The time profiles of (b) pristine BODIPY (dark blue) and 1 (black) at 512 nm, (d) 1 (black) and (c) 1:C₆₀ (magenta) at 667 nm, and (d) C₆₀˙⁻ for the spectra shown in (e) at 1010 nm.

Fig. 4c shows the transient spectra of 1:C₆₀ at the excitation wavelength of 507 nm corresponding to BODIPY. Similar to the case of 1, a rapid recovery of the depleted peak of ¹BODIPY* accompanied by new transient peaks related to ¹bisstyrylBODIPY* was observed, indicating excitation transfer between these two entities. Additionally, the recovery/decay of the ¹bisstyrylBODIPY* peaks was also rapid (compared to that observed for 1) and was accompanied by another new set of transient peaks corresponding to the formation of the BODIPY–bisstyrylBODIPY˙⁺–[C₆₀(NH₃)₂²⁺]˙⁻ charge separated state.^6,12 That is, the peaks at 488 and 811 nm corresponding to bisstyrylBODIPY·⁺ and that at 1010 nm corresponding to the [C₆₀(NH₃)₂²⁺]·⁻ species were clear (see Fig. S4 (ESI†) for the spectrum recorded at 10 ps for 1 and 1:C₆₀). These results conclusively prove the occurrence of sequential energy transfer followed by electron transfer in 1:C₆₀. The k_EnT value calculated using a procedure similar to that explained in the preceding paragraph was found to be 3.12 × 10¹⁰ s⁻¹. The appreciable increase in energy transfer rate could be due to subtle changes in the geometry of 1 upon C₆₀(NH₃)₂²⁺ binding. Fig. 4d shows the kinetic traces of the 669 peak of 1 and 1:C₆₀. The rapid recovery in the case of 1:C₆₀ was evident (biexponential). From the fast decay component, a time constant for the charge separation process of 83 ps was arrived which yielded an electron transfer rate constant, k_ET, of 1.18 × 10¹⁰ s⁻¹. The decay of [C₆₀(NH₃)₂²⁺]˙⁻ shown in Fig. 4e lasted over 3 ns suggesting persistence of the charge separated state. The nanosecond transient spectra of 1:C₆₀ revealed weak signals corresponding to the radical ion-pair at the earliest monitoring time window of 20 ns, suggesting this to be the upper time limit for persistence of the charge separated state. As expected, the direct excitation of bisstyrylBODIPY at 657 nm in 1:C₆₀ also revealed charge separation (see Fig. S3b in the ESI†) with the time constants for charge separation and recombination close to those of the earlier discussed 1:C₆₀ complex.

A novel supramolecular triad has been newly synthesized and assembled to probe sequential energy transfer followed by electron transfer events. The covalently linked BODIPY and bisstyrylBODIPY entities in 1 functioned as an energy harvester by revealing ultrafast singlet–singlet energy transfer. The two crown ether voids on 1 comfortably accommodated C₆₀(NH₃)₂²⁺via a ‘two-point’ alkyl ammonium cation–crown ether binding motif to yield the stable supramolecular 1:C₆₀ complex. The presence of C₆₀ in 1:C₆₀ promoted electron transfer from the initial energy transfer product, ¹bisstyrylBODIPY*, to yield the BODIPY–bisstyrylBODIPY˙⁺–[C₆₀(NH₃)₂²⁺]˙⁻ charge separated state which persisted for about 20 ns. The present study brings out the importance of a BODIPY based multi-modular donor–acceptor conjugate to mimic the combined ‘antenna-reaction centre’ events of natural photosynthesis.

The authors are thankful to the National Science Foundation (Grant No. 1401188) and the National Research Foundation of Korea (NRF) funded by the Korea government (MEST) (2016R1E1A1A01943379) for support of this work. We thank Dr Guido Verbeck and the Laboratory for Imaging Mass Spectrometry at the University of North Texas for MALDI-Orbitrap Mass Spectrometry data.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures and synthetic details, additional femtosecond transient spectra, optimized geometry, ¹H, and ¹³C NMR spectra, and HR-MALDI mass data. See DOI: 10.1039/c7cc08063h

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