Symmetry-breaking charge transfer is one of the key process in photosynthetic reaction centers and specific artificial optoelectronic devices such as dye-sensitized solar cells. Here we report the synthesis of aromatic-bridged BF2-smaragdyrin dimers, meso-free BF2-smaragdyrin, and its meso-meso-linked BF2-smaragdyrin dimer. The decays of S1-states of these dimers are accelerated with an increase in solvent polarity and a decrease in the distance between the two BF2-smaragdyrin units, suggesting symmetry-breaking charge transfer. The fluorescence lifetimes of the dimers become shortened in polar solvents. However, ultrafast transient absorption spectroscopy do not detect charge-separated ion pairs. On the basis of these results, we conclude that the decays of the excited states of the BF2-smaragdyrin dimers are accelerated by solvation-induced symmetry-breaking charge transfer, depending on the degree of the electronic interaction between the smaragdryin units as a rare case for porphyrinoids. The degree of charge transfer is larger for dimers with larger electronic interactions.
In recent years, expanded porphyrins have emerged as attractive π-functional molecules in light of versatile electronic properties, rich coordination chemistry, and charming structures1,2,3,4,5. Among these, smaragdyrin had been possessing a legendary position, since its presence was reported along with sapphyrin by Woodward but had been long elusive until our first synthesis of 5,10,19-triarylsmaragdyrins in 20186. Instead, Chandrashekar et al have extensively explored the chemistry of core-modified smaragdyrins to demonstrate that these porphyrinoids are promising in many fields including catalysts, molecular hosts, nonlinear optical materials, and coordinating ligands for large metal ions7,8,9,10,11. Our synthesis of 5,10,19-triarylsmaragdyrins 1 was based on a double nucleophilic aromatic substitution (SNAr) reaction of α,α’-dibrominated boron-dipyrromethene’s (BODIPY’s) with a 5,10-diaryltripyrrane6. Later, this synthetic protocol was used for the synthesis of singly and doubly Neo-confused smaragdyrins 2 and 3 (Fig. 1)12.
In this paper, we report the synthesis of aromatic-bridged BF2–smaragdyrin complex dimers 6a–6d by similar SNAr reactions of aromatic-bridged α,α’-dibrominated BODIPY dimers 4a–4d with 5,10-diaryltripyrrane 513,14. Further, we synthesized meso-unsubstituted BF2-smaragdyrin 11 and succeeded in its oxidative dimerization with air to give meso–meso-linked BF2–smaragdyrin dimer 12. While several meso–meso-linked dimers of core-modified smaragdyrins and analogs were reported15,16, this is the first example of meso–meso-linked smaragdyrin dimer.
Symmetry breaking charge separations (SB-CS) have been recognized to be important in relation to the charge transfer character of the bacteriochlorophyll special pair in the photosynthetic reaction centers, which is considered to trigger the sequential electron-transfer cascade17,18. But photoinduced SB-CS of artificial dimers composed of the same chromophores is rare17,18,19,20,21,22,23,24,25, mainly because the energy gaps of the charge separation between the same chromophores are seldom negative enough to allow SB-CS. SB-CS has been never observed for porphyrins, while so many porphyrin dimers and oligomers were synthesized so far. Recently, an interesting example was reported for SB-CS in excitonically coupled subphthalocyanine dimer26. In this respect, smaragdyrin is an intriguing porphyrinoid that is active both in reduction and oxidation due to its expanded conjugated π-network. In this paper, we conclude that these smaragdyrin dimers revealed that the excited-state decays are clearly accelerated with an increase of the solvent polarity and a decrease of the distance between the two smaragdyrin units, suggesting the key role of symmetry-breaking charge transfer in their decays as a rare case for porphyrinoids.
As the first targets, we synthesized aromatic-spacer-bridged BF2–smaragdyrin dimers 6a–6d. Building blocks 4a–4d were obtained in moderate yields via a set of transformations including α-bromination of the corresponding spacer-bridged dipyrromethene dimers, and oxidation and complexation with BF2 unit. Since the solubilities of 4a–4d were very poor in common solvents and only acceptable in o-dichlorobenzene (o-DCB), the coupling reactions of 4a–4d with tripyrromethane 5 were conducted in refluxing o-DCB in the presence of an excess amount of cesium carbonate for 48 h. Dimers 6a–6d were isolated in low but reproducible yields of 2.8, 2.2, 2.7, and 2.4% yield, respectively (Fig. 2). These dimers were characterized by 1H and 13C NMR spectra and high-resolution matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra (Supplementary Figs. 9, 11, 12, 14, 15, 17, 18, 20, 46–49). The structures of 6b and 6c were confirmed by X-ray diffraction analysis (Fig. 3). The crystal structure of 6b shows that a dihedral angle between the BF2–smaragdyrin units and the central 1,4-phenylene bridge is 47.3(2)°. This suggests that the 1,4-phenylene unit may serve as a moderate π-conjugation bridge for the BF2-smaragdyrins units. The crystal structure of 6c shows a dihedral angle of 51.7(2)° for the BF2–smaragdyrin units and 4,4-biphenylene unit and a dihedral angle of 31.8(3)° for the two phenylene units in the 4,4’-biphenylene bridge. These structural features suggest that the 4,4’-biphenyl bridge serves as a weaker π-conjugation mediator.
The 1H NMR spectra of 6a–6d in CDCl3 are simple, reflecting the symmetric structures. Characteristically, signals due to the inner pyrrolic protons are downfield shifted as compared with the reference monomer 11. On the other hand, signals of the β-protons (labeled as H1, H2, H4, and H5) are observed at nearly the same chemical shifts. Signals due to the bridging groups are down-field shifted, reflecting the diatropic ring current of the BF2–smaragdyrin ring.
As the second target, we synthesized meso–meso-linked BF2-smaragdyrin dimer 12. Initially, preparation of α,α’-dibrominated and dichlorinated meso-free BODIPY’s 8-Br and 8-Cl were attempted by halogenations of di(2-pyrroyl)methane 7 with NBS and NCS. But these attempts failed due to the instabilities of 8-Br and 8-Cl. We chose regioselective chlorination of BODIPY’s with CuCl2 developed by Jiao and Hall27. BODIPY 9 was prepared by oxidation of 7 with p-chloranil in CH2Cl2 at −40 °C under nitrogen followed by reaction with BF3•OEt2 in 38% yield. Then, the reaction of 9 with CuCl2 and Cu(OTf)2 under refluxing acetonitrile gave 10 in 55% yield (Fig. 4). The structure of 10 was fully confirmed by 1H NMR, 13C NMR, HR-MS, and single crystal X-ray diffraction analysis (Fig. 4 and Supplementary Figs. 2, 3, 38, 51).
With 10 in hand, we tried to synthesize meso-diaryl-substituted BF2–smaragdyrins by our method6. Namely, the reaction of 10 with 5,10-dimesityl-tripyrromethene 5 in the presence of cesium carbonate in refluxing p-xylene overnight afforded 5,10-dimesityl-smaragdyrin BF2-complex 11 in 5.2% yield after usual work up (Fig. 5). The 1H NMR spectrum of 11 in CDCl3 displays a singlet due to the meso-proton at δ = 10.71 and five signals due to the outer pyrrolic β-protons between δ = 10.46 and 8.94 ppm, a singlet due to the inner NH proton at −7.76 ppm, and a triplet due to the inner NH proton at −4.90 ppm that is coupled with the two fluorine atoms through-space coupling with J = 11.2 Hz. The 1H NMR spectrum of 11 is similar to that of the triaryl-substituted BF2–smaragdyrin reported before6, indicating its aromatic character arising from its 22π-circuit. The meso-free smaragdyrin BF2-complex 11 was isolated in a pure form.
In the meanwhile, however, we found that meso-free BF2-smaragdyrin complex 11 was not stable and slowly oxidized to meso–meso-linked BF2-smaragdyrin dimer 12 in the air. Actually, this oxidative coupling was accelerated at high temperature. After some experimentations, we found that the best yield of 12 (34%) was reproducibly accomplished by heating a p-xylene solution of 11 at 135 °C in the air for 48 h. The 1H NMR spectrum of 12 in CDCl3 is simple, displaying five signals due to the outer pyrrolic protons between 10.44 and 9.06 ppm, a singlet due to the inner NH proton at −7.29 ppm, and a triplet due to the inner NH proton at −4.37 ppm. The disappearance of the meso-proton signal of 11 was consistent with the formation of the meso-meso-linked dimer. The parent ion peak of 12 was observed at m/z = 1292.5397 (calcd for [C82H70B2F4N10]+; 1292.5925 ([M]+)) in its high-resolution MALDI-TOF mass spectrum. The structure of 12 was unambiguously confirmed by X-ray diffraction analysis (Fig. 5). The C(meso)–C(meso) bond length is 1.497(4) Å, which is within the range of typical C–C bond. The dihedral angle of the BF2–smaragdyrin rings is 62.5(1)°. These structural features indicate that conjugative interaction between the two BF2–smaragdyrin units is weak.
UV/Vis/NIR absorption and fluorescence spectra
The UV/Vis/NIR spectra of these BF2-smaragdyrins in toluene, CH2Cl2, and benzonitrile are shown in Fig. 6 and the Supplementary Fig. 21–26). The absorption spectrum of 11 in toluene shows weak Q-bands at 596, 649, and 702 nm, which are similar to that of 5,10,19-triaryl-substituted BF2-smaragdyrin. As compared with the absorption spectra of monomer 11, those of dimers 6a–6d are all broader both in the Soret and Q-like band regions, probably owing to exciton coupling between the two BF2–amaragdyrin units. Consistent with this, the most red-shifted Q-like bands are red-shifted in the order of 6d < 6c < 6a < 6b < 12 with decreasing distance of the two BF2–amaragdyrin units. Namely, meso–meso-linked dimer 12 exhibits the most red-shifted and broadest Q-like band at 762 nm. The UV/Vis/NIR absorption spectra of these BF2–smaragdyrins are rather insensitive to solvent polarity.
The normalized fluorescence spectra of these BF2–smaragdyrin dimers are shown in Fig. 7 and Supplementary Figs. 21–26. Their maximum emission wavelengths are all in the near-infrared region. Gradual redshifts are observed in the fluorescence spectra in going from 11, 6d, 6c, and 6a, and a substantial shift are found for 6b, and a bigger shift is observed for 12. The fluorescence quantum yields of the dimers are quite characteristic, decreasing with a decrease in the distance between the BF2–smaragdyrin (Table 1), suggesting that the electronic interaction between the two BF2–smaragdyrins causes additional decaying channel. The fluorescence quantum yields in toluene decrease in the order of 11 (0.138) > 6c (0.079) > 6d (0.054) > 6b (0.032) > 6a (0.028) > 12 (<0.001). The observed practically nonfluorescent behavior of 12 in nonpolar toluene is noteworthy. It is also important that an increase in solvent polarity decreases the fluorescence quantum yield. For example, in the case of 6a, 0.032 (toluene) > 0.019 (CHCl3) > 0.018 (THF) > 0.005 (CH2Cl2) > < 0.001 (acetonitrile). Similar trends were observed for 6b–6d. These data have suggested that BF2–smaragdyrin dimers, despite the identical chromophores in the molecule, undergo intramolecular charge transfer (CT), depending on the solvent polarity and the distance between the two BF2–smaragdyrin units. It is probable that the required asymmetric situation is provided by a fluctuation of polar solvents17,18,19,20,21,22,23,24,25. Stokes shifts became increasingly larger from toluene, dichloromethane, and benzonitrile, suggesting that the relaxations of Franck–Condon states to emitting states involve CT. In particular, the photoexcited-state dynamics of 12 is quite curious, since it scarcely emits fluorescence and its Stokes shifts are large, being 868 cm−1 in toluene, 811 cm−1 in CH2Cl2 and 1094 cm−1 in benzonitrile. These large Stokes shifts suggested that emitting state might have large CT properties. It is notable that dimer 6b showed relatively large Stokes shifts of 615 cm−1 in benzonitrile. It was thought that the extent of the CT in the emitting states depended on the electronic interactions between the two smaragdyrins. Thus, the emitting state of dimer 12 might have the largest CT character in the series.
We measured the fluorescence lifetime of BF2–smaragdyrin dimers using the time-correlated single-photon counting (TCSPC) method and the results are summarized in Supplementary Note 1, Supplementary Figs. S57 and S58. We representatively selected three solvents, toluene (ε = 2.38, TOL), CH2Cl2 (ε = 8.93, DCM), and benzonitrile (ε = 25.93, BCN). Compared with the fluorescence lifetime of BF2–smaragdyrin dimers in toluene, those in CH2Cl2 and benzonitrile are clearly shortened, showing the favorable formation of CT state in polar solvents. In the cases of 6a–6d, the fluorescence lifetimes in polar solvents become shorter in 6a and 6b compared with that of 6d, intermediate in 6c. These results indicate stronger CT characters in spatially closer dimers with stronger electronic interactions. The fluorescence lifetimes in benzonitrile are 0.98 ns for 6d > 0.37 ns for 6c > 0.19 ns for 6a > 0.15 ns for 6b, while that of 12 is 0.37 ns, being longer than those of 6a–6c, suggesting that the emitting state of 12 is considerably different from those of 6a–d.
Furthermore, to explore the excited-state population dynamics, femtosecond transient absorption (fs-TA) measurements were carried out in the region of 450–780 nm using an excitation pulse at 710 nm (Fig. 8, Supplementary Note 2, Supplementary Figs. 59–61). The TA spectra of the BF2–smaragdyrin dimers display ground-state bleaching (GSB) features in 450–500 nm and above 725 nm regions, which correspond to the steady-state absorption spectra, and broad excited-state absorption (ESA) features in 500–700 nm region.
The TA spectral signatures of the BF2–smaragdyrin dimers are similar to those reported by the previous studies on the monomer6. No significant spectral changes in the TA spectra were observed except for the decay time constants. Namely, we did not detect charge-separated states. The TA kinetic profiles are well-fitted with triple exponential decay functions. We attributed the initial decay component (τ1) to the conformational dynamics. In polar solvents, contribution by the τ1 component increases in the order of 12 < 6a < 6b < 6c which is in accordance with the increased conformational flexibility of the smaragdyrin dimers. Based on the solvation time constants of CH2Cl2 and benzonitrile, we assigned the structural change as the symmetry-breaking charge transfer process. Especially, in 12, this process is favorable in benzonitrile, which is consistent with the steady-state fluorescence spectra.
Additionally, we found that the τ2 component corresponded to the fluorescence lifetimes measured by TCSPC. Thus, we attributed the τ2 component to the singlet state fluorescence or CT emission decay process, depending on the electronic structures of dimers. These time constants of a series of BF2–smaragdyrin dimers become gradually shortened with an increase of solvent polarity. In the cases of 6a and 6b, which have one phenylene bridge, meta- and para-phenylene linker, respectively, while the spatial separation between the monomer units of 6a is shorter than 6b, 6a exhibits a longer lifetime because the electronic interaction is weaker than 6b. The major decaying components (τ2) of 12 were 1.4 ns in toluene, 0.98 ns in CH2Cl2, and 0.37 ns in benzonitrile. These decaying time constants cannot explain the observed very low fluorescent quantum yields of 12. This means that the emitting state of 12 is not a usual S1-excited state but probably a considerable CT state with a small radiative decaying channel to the ground state. Most probably, the CT state of 12 is formed at a very early time assisted by asymmetric solvation of polar solvent, and decays with a slow time constant to regenerate the ground state. While not a symmetric dimer, Similar accelerated decay of the excited state via charge transfer interaction was reported for a directly linked Zn(II) porphyrin–Zn(II) pheophorbide dyad28.
Finally, we attributed the τ3 component to the long-lived triplet state formation through the intersystem crossing. Consequently, the TA spectral features and excited state dynamics unveil that the charge transfer processes, corroborated with the structural evolution according to the structural flexibility and the distance between the monomer units, occur in the BF2–smaragdyrin dimers.
Estimation of the free energy gap for the symmetry-breaking charge separation is important to understand the photo-excited dynamics of the smaragdyrin dimers. Thus, we examined their electrochemical properties. The electrochemical properties of 6a–6d, and 12 are examined by cyclic voltammetry and differential pulse voltammetry in CH2Cl2 and benzonitrile containing 0.1 M n-Bu4NPF6 as an electrolyte and the results are summarized in Table 2. (For detailed electrochemical data see Supplementary Fig. 27–37, Supplementary Tables 1 and 2) In both solvents, the energy levels of the S1 states and the charge-separated states are almost the same. Considering that the reorganization would be necessary for the charge separation, the symmetry-breaking full charge separation would be difficult. Instead, these data would support the feasibility of partial CT.
All calculations were carried out using the Gaussian 09 program. All structures were fully optimized without any symmetry restriction. All geometries were optimized with the crystal structures as the starting structure at the density functional theory (DFT) method with restricted B3LYP (Becke’s three-parameter hybrid exchange functionals and the Lee–Yang–Parr correlation functional) level19,20,21,22,23,24,25. We calculated the absorption spectra of important compounds, 12, 6b, and 6c (For detailed theoretical calculations data see Supplementary Fig. 54–56, Supplementary Data 9). The calculation results of time-dependent density functional theory (TD-DFT) are in good agreement with the actual absorption spectral data of the compound. The maximum absorption peaks of the Q-like band of these compounds mainly come from electron transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).
In summary, we have synthesized meso-diaryl-substituted BF2–smaragdyrin dimers in low but acceptable yields via the twofold SNAr reaction and subsequent oxidation reaction with DDQ. We also synthesized meso-free and meso–meso directly linked BF2–smaragdyrin dimer. These molecules were investigated by UV/Vis/NIR absorption, fluorescence measurements, TCSPC method, fs-TA measurements, cyclic voltammetry and theoretical quantum calculations. The excited-state decays of the BF2–smaragdyrin dimers were accelerated with increase of the solvent polarity and a decrease in the distance between the smaragdyrin units. It has been concluded that symmetry-breaking charge transfer plays an important role in the excited-state decays of the smaragdyrin dimers in polar solvents.
Materials and characterization
1H NMR (500 MHz) and 13C NMR (126 MHz) spectra were taken on a Bruker ADVANCE-500 spectrometer, and chemical shifts were reported as the delta scale in ppm relative to CHCl3 as an internal reference for 1H NMR (δ = 7.260 ppm) and 13C NMR (δ = 77.000 ppm) (For 1H NMR and 13C NMR spectra see Supplementary Figs. 1–20). UV/Vis absorption spectra were recorded on a Shimadzu UV-3600 spectrometer. Fluorescence emission spectra were recorded on a HITACHI F-4500 spectrometer (For UV/Vis and fluorescence emission spectra see Supplementary Fig. 21–26). MALDI-TOF mass spectra were obtained with a Bruker Ultrafle Xtreme MALDI-TOF/TOF Mass spectrometer and Thermo Scientific Q Exactive ESI Mass spectrometer (For HRMS spectra see Supplementary Figs. 42–53). X-ray crystallographic data were taken on an Agilent SuperNova X-ray diffractometer equipped with a large-area CCD detector. Using Olex2, structures of compounds 6b, 6c, 10, and 12 were solved with the ShelXS structure solution program using Direct Methods and refined with the ShelXL refinement package using least squares minimization. Disordered solvent molecules were treated by the SQUEEZE program of Platon (For single crystal data see Supplementary Figs. 38–41, Supplementary Tables 3–6). Redox potentials were measured by the cyclic voltammetry and differential pulse voltammetry method on an ALS660 electrochemical analyzed model (Solvent: PhCN or CH2Cl2, electrolyte: 0.1 M n-Bu4NPF6, working electrode: glassy carbon, reference electrode: Ag/AgNO3, Counter electrode: Pt wire, scan rate: 0.05 V/s, external reference: ferrocene/ferrocenium cation). Benzonitrile passed through the alumina column was used for electrochemical analysis. Unless otherwise noted, materials obtained from commercial suppliers were used without further purification.
Synthesis of 4a–4d
To a solution of corresponding dipyrromethane (1 mmol) in THF (70 mL) at −78 °C, NBS (712 mg, 4 mmol) was added in three portions within 1 h. The mixture was stirred at a low temperature for 3 h. Then DDQ (499 mg, 2.2 mmol) was added to the solution. The mixture was stirred at −78 °C, for 0.5 h and then at room temperature for 3 h. The residue was purified through a long Al2O3 column using THF as an eluent. The solvent was removed in vacuo. Then, CH2Cl2 (100 mL) was added to the remaining solids. TEA (4.0 mL, 28 mmol) and BF3•OEt2 (5.0 mL, 45 mmol) were added dropwise to the solution. The mixture was stirred at room temperature for 24 h. The solvent was removed in vacuo. Products were simply purified by recrystallization with MeOH/H2O. 4a (280 mg), 4b (274 mg), 4c (340 mg) and 4d (452 mg). 4a: HR-MS (MALDI-TOF-MS): m/z = 773.7518, calcd for (C24H12B2Br4F4N4)+ = 773.7883 ([M]+). 4b: HR-MS (MALDI-TOF-MS): m/z = 773.7731, calcd for (C24H12B2Br4F4N4)+ = 773.7883 ([M]+). 4c: HR-MS (MALDI-TOF-MS): m/z = 849.7946, calcd for (C30H16B2Br4F4N4)+ = 849.8198 ([M]+). 4d: HR-MS (MALDI-TOF-MS): m/z = 925.8197, calcd for (C36H20B2Br4F4N4)+ = 925.8514 ([M]+).
Synthesis of 6a–6d
A solution of brominated BODIPY compound (0.051 mmol), 5 (50 mg, 0.108 mmol), and Cs2CO3 (180 mg, 0.60 mmol) were purged with argon and then charged with dry o-DCB (16 mL). The mixture was stirred at 135 °C for 48 h. The reaction mixture was passed through a short silica gel column using CHCl3 as an eluent and the solvent was evaporated in vacuo. The product was purified by column chromatography on silica gel (CH2Cl2/hexanes, 1:2 V/V) and recrystallization with CH2Cl2/MeOH. 6a (1.9 mg, 0.0014 mmol, 2.8% yield), 6b (1.5 mg, 0.0011 mmol, 2.2% yield), 6c (2.0 mg, 0.0014 mmol, 2.7% yield), and 6d (1.9 mg, 0.0012 mmol, 2.4% yield) were obtained all as green solids. 6a: 1H NMR (500 MHz, CDCl3): δ = 10.49 (d, 4H, J = 4.5 Hz, β-H), 10.13–10.11 (m, 8H, β-H), 9.95 (s, 1H, Ar-H), 9.17 (dd, 2H, J = 7.5, 1.5 Hz, Ar-H), 9.08 (d, 4H, J = 1.0 Hz, β-H), 8.88–8.87 (m, 4H, β-H), 8.59 (t, 1H, J = 7.5 Hz, β-H), 7.42 (s, 8H, Ar-H), 2.72 (s, 12H, Me-H), 1.92 (s, 24H, Me-H), −4.44 (t, 4H, J = 11.5 Hz, N-H), and −7.37 (s, 2H, N-H) ppm. 13C NMR (126 MHz, CDCl3) δ = 140.1, 138.4, 138.0, 134.9, 134.4, 132.4, 131.4, 129.8, 128.3, 125.7, 124.6, 123.7, 122.0, 119.6, 118.3, 116.6, 105.8, 21.6, and 21.2 ppm. λmax (ε [M−1cm−1] in toluene) =348 (74000), 373 (60000), 467(441000), 604(38000), 658(37000), 728(129000) nm. HR-MS (MALDI-TOF-MS): m/z = 1368.5771, calcd for (C88H74B2F4N10)+ = 1368.6239 ([M]+). 6b: 1H NMR (500 MHz, CDCl3): δ= 10.59 (d, 4H, J = 4.5 Hz, β-H), 10.21–10.20 (m, 4H, β-H), 10.17 (d, 4H, J = 4.0 Hz, β-H), 9.30 (s, 4H, Ar-H), 9.13 (s, 4H, β-H), 8.95 (br, 4H, β-H), 7.47 (s, 8H, Ar-H), 2.75 (s, 12H, Me-H), 1.98 (s, 24H, Me-H), −4.34 (t, J = 10.5 Hz, 4H, N-H), and −7.27 (s, 2H, N-H) ppm. 13C NMR (126 MHz, CDCl3) δ = 140.1, 139.2, 138.4, 138.0, 135.0, 134.2, 132.4, 131.4, 129.8, 128.3, 125.7, 124.7, 123.8, 122.0, 119.7, 118.4, 100.0, 21.6, and 21.2 ppm. λmax (ε [M−1cm−1] in toluene) = 347 (58000), 371 (47000), 471 (428000), 600(32000), 647(28000), 738(105000) nm. HR-MS (MALDI-TOF-MS): m/z = 1368.5771, calcd for (C88H74B2F4N10)+ = 1368.6239 ([M]+). 6c: 1H NMR (500 MHz, CDCl3): δ = 10.48 (d, 4H, J = 4.0 Hz, β-H), 10.14–10.13 (m, 4H, β-H), 9.88 (d, 4H, J = 4.5 Hz, β-H), 9.09 (s, 4H, β-H), 8.95 (d, 4H, J = 7.5 Hz, Ar-H), 8.90 (dd, 4H, J = 4.0, 1.5 Hz, β-H), 8.64 (d, 4H, J = 7.5 Hz, Ar-H), 7.45 (s, 8H, Ar-H), 2.74 (s, 12H, Me-H), 1.95 (s, 24H, Me-H), −4.39 (t, J = 11.5 Hz, 4H, N-H), and −7.31 (s, 2H, N-H) ppm. 13C NMR (126 MHz, CDCl3) δ = 140.1, 138.4, 138.0, 135.7, 134.9, 132.2, 131.4, 129.8, 128.3, 127.4, 125.7, 124.6, 123.6, 121.9, 119.5, 118.3, 116.6, 105.8, 21.6, and 21.2 ppm. λmax (ε [M−1cm−1] in toluene) = 346 (45000), 371 (36000), 466 (422000), 604(24000), 657(23000), 726(80000) nm. HR-MS (MALDI-TOF-MS): m/z = 1444.6035, calcd for (C94H78B2F4N10)+ = 1444.6554 ([M]+). 6d: 1H NMR (500 MHz, CDCl3): δ = 10.44 (d, 4H, J = 4.4 Hz, β-H), 10.12–10.11 (m, 4H, β-H), 9.80 (d, 4H, J = 4.0 Hz, β-H), 9.08 (s, 4H, β-H), 8.88 (dd, 4H, J = 4.0, 1.5 Hz, β-H), 8.84 (d, 4H, J = 8.0 Hz, Ar-H), 8.42 (d, 4H, J = 7.5 Hz, Ar-H), 8.32 (s, 4H, Ar-H), 7.44 (s, 8H, Ar-H), 2.73 (s, 12H, Me-H), 1.94 (s, 24H, Me-H), −4.40 (t, J = 11.5 Hz, 4H, N-H), and −7.32 (s, 2H, N-H) ppm. 13C NMR (126 MHz, CDCl3) δ = 140.1, 138.4, 138.0, 135.5, 134.9, 132.1, 131.4, 129.8, 128.3, 128.1, 127.0, 125.7, 124.6, 123.6, 121.8, 119.5, 118.2, 116.61, 116.58, 105.7, 21.6, and 21.2 ppm. λmax (ε [M−1cm−1] in toluene) = 344 (47000), 371 (34000), 464 (408000), 603(20000), 657(20000), 721(74000) nm. MS (MALDI-TOF-MS): m/z = 1520.6715, calcd for (C100H82B2F4N10)+ = 1520.6868 ([M]+).
Synthesis of 9
To a slurry of p-chloranil (1850 mg, 7.4 mmol) in CH2Cl2 (140 mL) at −40 °C under nitrogen, a solution of 7 (1000 mg, 6.8 mmol) in CH2Cl2 (200 mL) under N2 was added dropwise over several minutes. The reaction mixture was stirred for 3 h. The color of the mixture changed from brown to bright yellow. After DIPEA (7.0 mL, 41 mmol) was added, the solution was stirred for 30 min. BF3•OEt2 (6.8 mL, 61.2 mmol) was then added slowly over several min, and the mixture was stirred for 18 h, during which time the temperature was allowed to rise to 22 °C. The fluorescent solution was sonicated for 30 min and then filtered through a Celite pad to remove insoluble materials. Then, the reaction mixture was washed with saturated aq. NH4Cl and the organic extract was dried over MgSO4. The solvent was removed in vacuo. Purification using silica gel (CH2Cl2/hexanes, 1:1 V/V) gave 9 as dark red solids (488 mg, 38 %). 9: 1H NMR (500 MHz, CDCl3) δ = 7.90 (s, 2H, α-H), 7.42 (s, 1H, meso-H), 7.15 (d, J = 4.0 Hz, 2H, β-H), and 6.55 (d, J = 4.0 Hz, 2H, β-H) ppm. MS (ESI-MS): m/z = 193.0739, calcd for (C9H8BF2N2)+ = 193.0743 ([M + H]+).
Synthesis of 10
A mixture of BODIPY 9 (30 mg, 0.156 mmol), CuCl2•2H2O (132.9 mg, 0.78 mmol), and Cu(OTf)2 (282.1 mg, 0.78 mmol) in CH3CN (6 mL) was stirred under air at 80 °C for 20 min. After cooling down to room temperature, the reaction mixture was poured in CH2Cl2, and washed with sat aq. NH4Cl four times, dried over anhydrous Na2SO4, and filtered and evaporated under vacuum. Purification using a silica gel column with an eluent of CH2Cl2/hexanes, 1:1 V/V and recrystallization with n-hexane gave 10 as red solids (17 mg, 55 %). 10: 1H NMR (500 MHz, CDCl3) δ = 7.14 (s, 1H, meso-H), 7.09 (d, 2H, J = 4.0 Hz, β-H), and 6.44 (d, 2H, J = 4.0 Hz, β-H) ppm. 13C NMR (126 MHz, CDCl3) δ = 145.8, 134.1, 131.4, 127.8, and 119.2 ppm. HR-MS (MALDI-TOF-MS): m/z = 259.9694, calcd for (C9H5BCl2F2N2)+ = 259.9887 ([M]+).
Synthesis of 11
A solution of 10 (30 mg, 0.108 mmol), 5 (54.1 mg, 0.118 mmol), and Cs2CO3 (141.3 mg, 0.432 mmol) was purged with argon, and then charged with p-xylene (10 mL). The mixture was stirred at reflux for 48 h. The reaction mixture was passed through a short silica-gel column using CHCl3 as an eluent and the solvent was evaporated in vacuo. The product was purified by column chromatography on silica gel (CH2Cl2/hexanes, 1:4 V/V) and recrystallization with CH2Cl2/MeOH, 11 (3.6 mg, 0.0056 mmol, 5.2% yield) was obtained as green solids. Samples can be stored under inert atmosphere environment at low temperature. 11: 1H NMR (500 MHz, CDCl3): δ = 10.71 (s, 1H, meso-H), 10.46 (d, 2H, J = 4.0 Hz, β-H), 10.16–10.15(m, 2H, β-H), 9.84 (d, 2H, J = 4.0 Hz, β-H), 9.14 (s, 2H, β-H), 8.94 (dd, 2H, J = 4.0, 1.5 Hz, β-H), 7.44 (s, 4H, Ar-H), 2.73 (s, 6H, Me-H), 1.91 (s, 12H, Me-H), −4.90 (t, 2H, J = 11.0 Hz, N-H), and −7.76 (s, 1H, N-H) ppm. 13C NMR (126 MHz, CDCl3) δ = 140.1, 138.5, 138.0, 134.6, 132.6, 131.1, 129.2, 128.3, 125.5, 124.4, 123.2, 121.8, 119.6, 118.5, 105.5, 100.6, 21.6, and 21.2 ppm; HR-MS (MALDI-TOF-MS): m/z = 647.2793, calcd for (C41H36BF2N5)+ = 647.3033 ([M]+).
Synthesis of 12
A solution of 11 (9 mg, 0.014 mmol) in p-xylene (1 mL) was stirred in the air at 135 °C for 48 h. The solvent was evaporated in a vacuo. The product was purified by column chromatography on silica gel (CH2Cl2/hexanes, 1:2 V/V) and recrystallization with CH2Cl2/MeOH gave 12 (3.0 mg, 0.0024 mmol, 34% yield) as yellow-green solids. 12: 1H NMR (500 MHz, CDCl3): δ = 10.41 (d, 4H, J = 4.5 Hz, β-H), 10.22 (dd, 4H, J = 4.5, 1.5 Hz, β-H), 9.33 (d, 4H, J = 4.5 Hz, β-H), 9.23 (s, 4H, β-H), 9.02 (dd, 4H, J = 4.0, 1.5 Hz, β-H), 7.49 (s, 8H, Ar-H), 2.76 (s, 12H, Me-H), 2.02 (s, 24H, Me-H), −4.41 (t, 4H, J = 11.5 Hz, N-H), and −7.32 (s, 2H, N-H) ppm. 13C NMR (126 MHz, CDCl3) δ = 140.2, 138.5, 138.1, 136.8, 134.9, 131.5, 129.0, 128.3, 125.7, 125.2, 125.1, 122.2, 120.0, 118.9, 112.1, 106.1, 21.6, and 21.3 ppm. λmax (ε [M−1cm−1] in toluene) =351 (36000), 373 (33000), 480 (301000), 604(20000), 649(19000), 762(52000) nm. HR-MS (MALDI-TOF-MS): m/z = 1292.5397, calcd for (C82H70B2F4N10)+ = 1292.5925 ([M]+).
The X-ray crystallographic coordinates for structures reported in this study have been deposited at the Cambridge Crystallographic Data Center (CCDC), under deposition numbers 2183007, 2191271, 2183009, 2183010 (6b, 6c, 10, and 12). These data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif. The data obtained in this study are available within this article and its Supplementary information and are also from the corresponding authors upon reasonable request. Original 1H and 13C spectra, UV/vis absorption spectra and fluorescence spectra, X-ray crystal data, electrochemical data, HR-MS Spectra, TD-DFT calculation results, time-resolved fluorescence decay data, femtosecond transient absorption spectra and decay profiles of the compounds obtained in this manuscript are available in Supplementary information, the cif file of 6b is available in Supplementary Data 1, and check cif file of 6b is available in Supplementary Data 2, the cif file of 6c is available in Supplementary Data 3, and check cif file of 6c is available in Supplementary Data 4, the cif file of 10 is available in Supplementary Data 5, and check cif file of 10 is available in Supplementary Data 6, the cif file of 12 is available in Supplementary Data 7, and check cif file of 12 is available in Supplementary Data 8, optimized atomic coordinates are available in Supplementary Data 9. The authors declare that all other data supporting the findings of this study are available within the paper and its Supplementary information files.
Sessler, J. L. & Seidel, D. Synthetic expanded porphyrin chemistry. Angew. Chem. Int. Ed. 42, 5134–5175 (2003).
Saito, S. & Osuka, A. Expanded porphyrins: intriguing structures, electronic properties, and reactivities. Angew. Chem. Int. Ed. 50, 4342–4373 (2011).
Stepien, M., Sprutta, N. & Latos-Grazynski, L. Figure eights, mobius bands, and more. conformations and aromaticity of porphyrins. Angew. Chem. Int. Ed. 50, 4288–4340 (2011).
Tanaka, T. & Osuka, A. Chemistry of meso-aryl expanded porphyrins: aromaticity and molecular twist. Chem. Rev. 117, 2584–2640 (2017).
Szyszko, B., Bialek, M. J., Pacholska-Dudziak, E. & Latos-Grazynsli, L. Flexible porphyrinoids. Chem. Rev. 117, 2839–2909 (2017).
Xie, D. et al. meso-Triaryl-substituted smaragdyrins: facile aromaticity switching. J. Am. Chem. Soc. 140, 16553–16559 (2018).
Chatterjee, T., Srinivasan, A., Ravikanth, M. & Chandrashekar, T. L. Smaragdyrins and sapphyrins analogues. Chem. Rev. 117, 3329–3376 (2017).
Pareek, Y., Ravikanth, M. & Chandrashekar, T. K. Smaragdyrins: emeralds of expanded porphyrin family. Acc. Chem. Res. 45, 1801–1816 (2012).
Broadhurst, M. J., Grigg, R. & Johnson, A. W. The synthesis of 22π-electron macrocycles. sapphyrins and related compounds. J. Chem. Soc. Perkin Trans. 1, 2111–2116 (1972).
Narayanan, S. J., Sridevi, B., Chandrashekar, T. K., Englich, U. & Ruhland-Senge, K. Core-modified smaragdyrins: first examples of stable meso-substituted expanded corrole. Org. Lett. 1, 587–590 (1999).
Narayanan, S. J., Sridevi, B., Chandrashekar, T. K., Vij, A. & Roy, R. Novel core-modified expanded porphyrins with meso-aryl substituents: synthesis, spectral and structural characterization. J. Am. Chem. Soc. 121, 9053–9068 (1999).
Rao, Y. et al. Singly and doubly neo-confused smaragdyrins. J. Am. Chem. Soc. 141, 18836–18844 (2019).
Crawford, S. M. & Thompson, A. Investigations into the nucleophilic meso-substitution of F-BODIPYs and improvements to the synthesis of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene. Heterocycles 83, 311–322 (2011).
Groves, B. R., Crawford, S. M., Lundrigan, T. & Thompson, A. Synthesis and characterisation of the unsubstituted dipyrrin and 4,4-dichloro-4-bora-3a,4a-diaza-s-indacene: improved synthesis and functionalisation of the simplest BODIPY framework. Chem. Commun. 49, 816–818 (2013).
Misra, R., Kumar, R., Chandrashekar, T. K. & Suresh, C. H. Meso-meso linked core modified 22π smaragdyrins with unusual absorption properties. Chem. Commun. 46, 4584–4586 (2006).
Kalita, H. K., Lee, W. & Ravikanth, M. Synthesis and functionalization of BF2-complexes of meso-free 25-oxasmaragdyrin. J. Org. Chem. 78, 6285–6290 (2013).
Grabowski, Z. R., Rotkiewicz, K. & Rettig, W. Structural changes accompanying intramolecular electron transfer; focus on twisted intramolecular charge-transfer states and structures. Chem. Rev. 103, 3899–4032 (2003).
Vauthey, E. Photoinduced symmetry-breaking separation. ChemPhysChem 13, 2001–2011 (2012).
Giaimo, J. M., Gusev, A. V. & Wasielewski, M. R. Excited state symmetry breaking in cofacial and linear dimers of a green perylenediimide chlorophyll analogue leading to ultrafast charge separation. J. Am. Chem. Soc. 124, 8530–8531 (2002).
Zijlstra, R. W., Grozema, F. C., Swart, M., Feringa, B. L. & van Duijnen, P. T. Solvent induced charge separation in the excited states of symmetrical ethylene: a direct reaction field study. J. Phys. Chem. A 105, 3583–3590 (2001).
Holman, M. W., Yan, P., Adams, D. M., Westenhoff, S. & Silva, C. Ultrafast spectroscopy of the solvent dependence of electron transfer in a perylenebisimide dimer. J. Phys. Chem. A 109, 8548–8552 (2005).
Rybtchinski, B., Sinks, L. E. & Wasielewski, M. R. Combining light-harvesting and charge separation in a self-assembled artificial photosynthetic system based on perylenediimide chromophores. J. Am. Chem. Soc. 126, 12268–12269 (2004).
Bhpsale, S. et al. Photoproduction of proton gradients with π-stacked fluorophore scaffold in liquid bilayer. Science 313, 84–86 (2006).
Sisson, A. L. et al. Zipper assembly of vectorial rigid-rod π-stack architechtures with red and blue naphthalenedimides: toward supramolecular caskade n/p-heterojunctions. Angew. Chem. Int. Ed. 47, 3727–3729 (2008).
Wu, Y. et al. Untrafast photoinduced symmetry-breaking charge-separation and electron sharing in perylenedimide molecular triangles. J. Am. Chem. Soc. 137, 13236–13239 (2015).
Roy, P., Bressan, G., Gretton, J., Cammidge, A. N. & Meech, S. R. Untrafast excimer formation and solvent controlled symmetry breaking charge separation.in the excitonically coupled subphthalocyanine dimer. Angew. Chem. Int. Ed. 60, 10568–10572 (2021).
Zhou, X., Yu, C., Feng, Z., Yu, Y. & Jiao, L. Highly regioselective α-chlorination of the bodipy chromophore with copper(II) chloride. Org. Lett. 17, 4632–4635 (2015).
Wasielewski, M. R. et al. Chlorophyll-porphyrin heterodimers with orthogonal Pi-systems; solvent polarity dependent photophysic. J. Am. Chem. Soc. 112, 6482–6488 (1990).
The work is supported by the National Natural Science Foundation of China (Grant nos. 22071052 and 21772036), Science and Technology Innovation Program of Hunan Province (2021RC4059).
The authors declare no competing interests.
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Wen, L., Cao, X., Lee, S. et al. Aromatic-bridged and meso-meso-linked BF2-smaragdyrin dimers exhibit fast decays in polar solvents by symmetry-breaking charge transfer. Commun Chem 6, 25 (2023). https://doi.org/10.1038/s42004-023-00822-8
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