Broadband Visible Light-Absorbing [70]Fullerene-BODIPY-Triphenylamine Triad: Synthesis and Application as Heavy Atom-Free Organic Triplet Photosensitizer for Photooxidation

A broadband visible light-absorbing [70]fullerene-BODIPY-triphenylamine triad (C70-B-T) has been synthesized and applied as a heavy atom-free organic triplet photosensitizer for photooxidation. By attaching two triphenylmethyl amine units (TPAs) to the π-core of BODIPY via ethynyl linkers, the absorption range of the antenna is extended to 700 nm with a peak at 600 nm. Thus, the absorption spectrum of C70-B-T almost covers the entire UV–visible region (270–700 nm). The photophysical processes are investigated by means of steady-state and transient spectroscopies. Upon photoexcitation at 339 nm, an efficient energy transfer (ET) from TPA to BODIPY occurs both in C70-B-T and B-T, resulting in the appearance of the BODIPY emission at 664 nm. Direct or indirect (via ET) excitation of the BODIPY-part of C70-B-T is followed by photoinduced ET from the antenna to C70, thus the singlet excited state of C70 (1C70*) is populated. Subsequently, the triplet excited state of C70 (3C70*) is produced via the intrinsic intersystem crossing of C70. The photooxidation ability of C70-B-T was studied using 1,5-dihydroxy naphthalene (DHN) as a chemical sensor. The photooxidation efficiency of C70-B-T is higher than that of the individual components of C70-1 and B-T, and even higher than that of methylene blue (MB). The photooxidation rate constant of C70-B-T is 1.47 and 1.51 times as that of C70-1 and MB, respectively. The results indicate that the C70-antenna systems can be used as another structure motif for a heavy atom-free organic triplet photosensitizer.

PSs with heavy atoms can produce the triplet state, whereas the narrow visible lightabsorbance, toxicity and high cost limit their applications [11][12][13][14][15]. PSs without heavy atoms usually suffer from the unpredictable ISC ability. In order to address the aforementioned problems, strategies such as using a twisted π-conjugation system [16,17], singlet fission [18,19], spin-orbit charge transfer [20,21], and radicals [22,23] have been developed to enhance the ISC of heavy atom-free PSs. However, the synthetic methods of

Synthesis
The synthetic procedures of C70-B-T and B-T are outlined in Scheme 1, and the details are given in the Materials and Methods section. By coupling with aromatic compounds at the "2" and "6" positions, the π-conjugation framework of BODIPY could be extended. In order to construct TPA-fused BODIPY, the preparation of 4-ethynyl-N,N-diphenylaniline (3) and 2,6-diiodo-BODIPY (6) was required. Compound 3 was synthesized by a standard Sonogashira reaction between trimethylsilylacetylene (TMSA) and 4-iodo-N,N-diphenylaniline, followed by deprotection of the trimethylsilyl group.
BODIPY 4 was synthesized according to the reported procedures [36]. The crosscoupling reaction of 4 with 4-iodobenzaldehyde afforded 5 in 83% yield. BODIPY 6 was prepared in 89% yield by treating 5 with N-iodosuccinimide (NIS) in the presence of CH 3 CO 2 H. A subsequent double Sonogashira coupling reaction of 6 with 3 afforded B-T in 68% yield. Finally, C 70 -B-T was obtained in 45% yield by treating B-T with C 70 and sarcosine under nitrogen atmosphere in toluene. The reference compound C 70 -1 was prepared according to the Prato procedure [51] as described in the Materials and Methods section.
The structures of all the intermediates and final compounds were fully confirmed by NMR, mass and IR techniques. Due to the lower structural symmetry of C 70 , C 70 -B-T and C 70 -1 are mixture of isomers. The NMR spectra of all the compounds are given in the Supplementary Materials. For the sake of clarity, the expansion of the 1 H-NMR spectrum of C 70 -B-T in CDCl 3 is shown in Figure 2. pared according to the Prato procedure [51] as described in the Materials and Methods section.
The structures of all the intermediates and final compounds were fully confirmed by NMR, mass and IR techniques. Due to the lower structural symmetry of C70, C70-B-T and C70-1 are mixture of isomers. The NMR spectra of all the compounds are given in the Supplementary Materials. For the sake of clarity, the expansion of the 1 H-NMR spectrum of C70-B-T in CDCl3 is shown in Figure 2. The 1 H-NMR spectrum shows that there are more than three isomers in C70-B-T and gives all the expected proton signals. For instance, the peaks at ~7.90-6.96 ppm assign to the phenyl ring protons; peaks at ~5.30-2.39 ppm assign to the pyrrolidine protons, protons of N-CH3 and pyrrole ring CH3 and peaks at ~1.59-1.43 ppm assign to the protons of pyrrole ring CH3. The 13 C-NMR spectrum of C70-B-T also shows the expected signals. For example, the peaks from 122-151 ppm assign to the sp 2 -C of C70 and benzene ring, peaks at ~80-117 ppm are carbons of alkyne, peaks from 58-71 ppm assign to the sp 3 -C of C70 and the carbons of pyrrolidine. The mass spectrum of C70-B-T gives a molecular peak at m/z 1854.4471, which is consistent with the calculated data. The 1 H-NMR spectrum shows that there are more than three isomers in C 70 -B-T and gives all the expected proton signals. For instance, the peaks at~7.90-6.96 ppm assign to the phenyl ring protons; peaks at~5.30-2.39 ppm assign to the pyrrolidine protons, protons of N-CH 3 and pyrrole ring CH 3 and peaks at~1.59-1.43 ppm assign to the protons of pyrrole ring CH 3 . The 13 C-NMR spectrum of C 70 -B-T also shows the expected signals. For example, the peaks from 122-151 ppm assign to the sp 2 -C of C 70 and benzene ring, peaks at~80-117 ppm are carbons of alkyne, peaks from 58-71 ppm assign to the sp 3 -C of C 70 and the carbons of pyrrolidine. The mass spectrum of C 70 -B-T gives a molecular peak at m/z 1854.4471, which is consistent with the calculated data.

UV-Vis Absorption and Steady-State Fluorescence
The UV-vis absorption and steady-state fluorescence of C 70 -B-T, and the reference compounds C 70 -1 and B-T were recorded in toluene in 1.0 × 10 −5 M and are shown in Figure 3. Triad C 70 -B-T and dyad B-T show broadband absorption in the entire UV-visible region (270-700 nm) with a strong absorption peak at about 600 nm (ε = 58,804 L mol −1 cm −1 ). Compared with traditional BODIPY, a bathochromic shift of about 100 nm of the low-energy absorption peak is found in B-T, indicating that the TPA units extend the conjugation length effectively. The UV-vis spectrum of C 70 -B-T is the sum of C 70 -1 and B-T (the data are given in Table 1), suggesting no significant electronic communication between C 70 and the antenna at ground state.
The steady-state fluorescence spectra of C 70 -B-T, C 70 -1 and B-T in toluene upon excitation at 339 and 605 nm are presented in Figure 3b and 3c, respectively. The emission maxima and fluorescence quantum yields of C 70 -B-T and B-T are listed in Table 1. When the TPA part was excited at 339 nm, only emissions of BODIPY moiety are observed both in B-T and C 70 -B-T. The fluorescence of TPA (447 nm) part is largely quenched, showing that efficient excitation energy transfer from TPA to BODIPY occurs [52]. The emission peaks of both the triad and dyad are located at 664 nm, but the emission intensity of C 70 -B-T is relatively weak compared with that of B-T (the quenching efficiency is 88%). Direct excitation of the BODIPY moiety of C 70 -B-T and B-T at 605 nm results in fluorescence spectra resemble those ones obtained upon TPA-part excitation. B-T still gives intense fluorescence with an emission peak at 664 nm (Φ F = 0.22), whereas the emission of C 70 -B-T is largely quenched (82%, Φ F = 0.04) due to intramolecular energy or electron transfer.

UV-Vis Absorption and Steady-State Fluorescence
The UV-vis absorption and steady-state fluorescence of C70-B-T, and the reference compounds C70-1 and B-T were recorded in toluene in 1.0 × 10 −5 M and are shown in Figure  3. Triad C70-B-T and dyad B-T show broadband absorption in the entire UV-visible region (270-700 nm) with a strong absorption peak at about 600 nm (ε = 58,804 L mol −1 cm −1 ). Compared with traditional BODIPY, a bathochromic shift of about 100 nm of the lowenergy absorption peak is found in B-T, indicating that the TPA units extend the conjugation length effectively. The UV-vis spectrum of C70-B-T is the sum of C70-1 and B-T (the data are given in Table 1), suggesting no significant electronic communication between C70 and the antenna at ground state.  The emissions of C70-B-T and B-T in THF were also measured to investigate the effect of solvent polarity on the emission behavior. The results are shown in Figure 4. The emission intensities of C70-B-T and B-T are quite sensitive to solvent polarity due to the presence of dipole moments inside the molecules. For instance, the emission intensities drop largely in THF in comparison to that in toluene. Whereas the emission peak positions of  The emissions of C 70 -B-T and B-T in THF were also measured to investigate the effect of solvent polarity on the emission behavior. The results are shown in Figure 4. The emission intensities of C 70 -B-T and B-T are quite sensitive to solvent polarity due to the presence of dipole moments inside the molecules. For instance, the emission intensities drop largely in THF in comparison to that in toluene. Whereas the emission peak positions of both C 70 -B-T and B-T do not show an obvious shift. The low solvent polarity effect suggests the formation of a neutral excited state in C 70 -B-T [37,54]. Thus, the emission quenching of the BODIPY part observed in C 70 -B-T should mainly be ascribed to the intramolecular energy transfer from B-T to C 70 , and the electron transfer from the antenna to C 70 is not significant [41,55]. The intramolecular energy transfer from B-T to C 70 is possible, because the energy of the S 1 state of B-T is higher than that of C 70 [32,41]. In the magnified spectrum of Figure [37,54]. Thus, the emission quenching of the BODIPY part observed in C70-B-T should mainly be ascribed to the intramolecular energy transfer from B-T to C70, and the electron transfer from the antenna to C70 is not significant [41,55]. The intramolecular energy transfer from B-T to C70 is possible, because the energy of the S1 state of B-T is higher than that of C70 [32,41]. In the magnified spectrum of Figure

Time-Resolved Fluorescence Spectroscopy
For deeper insight into the photoinduced intramolecular transfer processes, the fluorescence decays of C70-B-T and B-T were investigated using time-resolved fluorescence

Time-Resolved Fluorescence Spectroscopy
For deeper insight into the photoinduced intramolecular transfer processes, the fluorescence decays of C 70 -B-T and B-T were investigated using time-resolved fluorescence spectroscopy techniques. The results are shown in Figure 5.

Time-Resolved Fluorescence Spectroscopy
For deeper insight into the photoinduced intramolecular transfer processes, the fluorescence decays of C70-B-T and B-T were investigated using time-resolved fluorescence spectroscopy techniques. The results are shown in Figure 5. The efficiency of energy transfer from BODIPY to C70 in C70-B-T can be calculated by using Equation (1).
where τ0 and τ1 are the fluorescence lifetimes of B-T and C70-B-T, respectively. Thus, the energy transfer efficiency Φ1 is determined as 0.78 for C70-B-T, which is in consistence with the result of the steady-state fluorescence spectra The efficiency of energy transfer from BODIPY to C 70 in C 70 -B-T can be calculated by using Equation (1).
where τ 0 and τ 1 are the fluorescence lifetimes of B-T and C 70 -B-T, respectively. Thus, the energy transfer efficiency Φ 1 is determined as 0.78 for C 70 -B-T, which is in consistence with the result of the steady-state fluorescence spectra

Nanosecond Time-Resolved Transient Absorption Spectroscopy
Nanosecond time-resolved transient absorption spectroscopy was used to investigate the triplet excited states of the triad and C 70 -1. The nanosecond transient absorption spectra were recorded by a nanosecond flash photolysis system (LP980 ENDINBURGH) with a pulse laser (7 ns, 1 Hz) from a Neodymium-doped Yttrium Aluminium Garnet (Nd:YAG) laser at a wavelength of 532 nm. The results are shown in Figure 6.
Upon excitation at 532 nm, bleaching at about 474 and 538 nm was observed for both C 70 -B-T and C 70 -1 due to ground state absorption of C 70 . C 70 -1 shows a sharp transient absorption band at 420 nm and a broad transient absorption band from 644 to 735 nm ( Figure 6c). In the time-resolved transient absorption spectrum of C 70 -B-T, no bleaching of the steady-state absorption of B-T part is observed. C 70 -B-T also shows a characteristic absorption band of 3 C 70 * at 418 nm. However, the broad transient absorption band of C 70 -B-T is blue-shifted and splits into three bands with peaks at~585, 622 and 695 nm due to the derivatization of C 70 by the antenna [41]. The dynamic decay behaviors of the three bands are similar to that of the peak at 418 nm, suggesting that the three bands also belong to the absorptions of 3 C 70 *, agreed well with the reported studies [41]. Therefore, the peaks observed in the transient absorption spectrum of C 70 -B-T should be ascribed to the absorptions of the 3 C 70 *. When excited, the triplet state of C 70 -B-T is exclusively localized on the C 70 unit. The lifetime of the triplet state of C 70 -B-T is 13.9 µs, slightly longer than that of C 70 -1 (12.5 µs).

Nanosecond Time-Resolved Transient Absorption Spectroscopy
Nanosecond time-resolved transient absorption spectroscopy was used to investigate the triplet excited states of the triad and C70-1. The nanosecond transient absorption spectra were recorded by a nanosecond flash photolysis system (LP980 ENDINBURGH) with a pulse laser (7 ns, 1 Hz) from a Neodymium-doped Yttrium Aluminium Garnet (Nd:YAG) laser at a wavelength of 532 nm. The results are shown in Figure 6.  [41]. The dynamic decay behaviors of the three bands are similar to that of the peak at 418 nm, suggesting that the three bands also belong to the absorptions of 3 C70*, agreed well with the reported studies [41]. Therefore, the peaks observed in the transient absorption spectrum of C70-B-T should be ascribed to the absorptions of the 3 C70*. When excited, the triplet state of C70-B-T is exclusively localized on the C70 unit. The lifetime of the triplet state of C70-B-T is 13.9 µs, slightly longer than that of C70-1 (12.5 µs).

Photooxidation of 1,5-Dihydroxy Naphthalene Mediated by 1 O2
The photooxidation ability of C70-B-T was studied by using 1,5-dihydroxy naphthalene (DHN) as a 1 O2 scavenger. In the presence of 1 O2, DHN can be easily oxidized to juglone [56]. The kinetics of the photooxidation could be measured by following the decrease in the absorption of DHN at 301 nm or the increase in the absorption of juglone at 427 nm with time. The spectral responses of DHN using C70-B-T, B-T, C70-1, and methylene blue (MB) as the sensitizers upon broadband excitation with a xenon lamp are presented in Figure 7 and Figure S1, respectively. For C70-B-T, C70-1, and MB, the change of the absorption at 301 nm is obvious, indicating the significant consumption of DHN and the efficient photosensitization ability of the triplet PSs, whereas, nearly no UV-vis ab- The photooxidation ability of C 70 -B-T was studied by using 1,5-dihydroxy naphthalene (DHN) as a 1 O 2 scavenger. In the presence of 1 O 2 , DHN can be easily oxidized to juglone [56]. The kinetics of the photooxidation could be measured by following the decrease in the absorption of DHN at 301 nm or the increase in the absorption of juglone at 427 nm with time. The spectral responses of DHN using C 70 -B-T, B-T, C 70 -1, and methylene blue (MB) as the sensitizers upon broadband excitation with a xenon lamp are presented in Figure 7 and Figure S1, respectively. For C 70 -B-T, C 70 -1, and MB, the change of the absorption at 301 nm is obvious, indicating the significant consumption of DHN and the efficient photosensitization ability of the triplet PSs, whereas, nearly no UV-vis absorption change is observed in the spectral responses of DHN with B-T as the photosensitizer. The photostability of C 70 -B-T was also investigated by exposing to light for 1 h and no decrease is observed in the absorption ( Figure S2). This further proves that the decrease in the absorption at 301 nm is caused by photooxidation instead of the decomposition of the photosensitizers.
The photooxidation ability of the triplet photosensitizers was quantitatively compared by plotting the ln[(A − A )/A 0 ] against the irradiation time. The photooxidation rate constant and the yield of singlet oxygen (Φ ∆ ) of the photosensitizers were calculated [37,57], and the data are listed in Table 2. sorption change is observed in the spectral responses of DHN with B-T as the photosensitizer. The photostability of C70-B-T was also investigated by exposing to light for 1 h and no decrease is observed in the absorption ( Figure S2). This further proves that the decrease in the absorption at 301 nm is caused by photooxidation instead of the decomposition of the photosensitizers. The photooxidation ability of the triplet photosensitizers was quantitatively compared by plotting the ln[(A − A′)/A0] against the irradiation time. The photooxidation rate constant and the yield of singlet oxygen (ΦΔ) of the photosensitizers were calculated [37,57], and the data are listed in Table 2.  and be excited to its excited singlet easily, then efficient intramolecular energy transfer from B-T to C 70 occurred and formed the 1 C 70 *, finally the highly efficient ISC of C 70 would eventually lead to the population of 3 C 70 *. These photophysical processes can be supported by the steady-state and transient data mentioned above.
All synthesis compounds were characterized by 1 H and 13 C-NMR spectroscopy on a BRUKER 400 MHz spectrometer. The mess analyses were performed using a Bruker ultrafleXtreme MALDI TOF/TOF (Bremen, Germany).

Synthesis of Compound 6
Compound 5 (200.1 mg, 0.44 mmol) and NIS (217.7 mg, 0.97 mmol) were dissolved in 12 mL of CHCl 3 /CH 3 COOH (2/1, v/v) in a 25 mL one-neck flask and the mixture was stirred at 60 • C for 12 h, monitored by TLC. Then the mixture was extracted with CH 2 Cl 2 and a saturated sodium thiosulfate aqueous solution three times. The combined organic layer was dried with anhydrous Na 2 SO 4 , filtered and concentrated. The crude reaction mixture was subjected to silica gel column chromatography using PE/CH 2 Cl 2 (2/1, v/v) as eluent to give 6 in 89% yield (280.0 mg). 1

Photooxidation Experiment
Compounds C 70 -B-T, C 70 -1, B-T and MB in a concentration of 2.0 × 10 −5 mol L −1 and DHN in a concentration of 2.0 × 10 −4 mol L −1 were dissolved in CH 2 Cl 2 /MeOH (9:1, v/v), respectively. Then, the above solutions of sensitizers and DHN were mixed in a volume ratio of 1:1, and O 2 was bubbled through the mixture for 10 min. The mixture was then placed in a quartz cell and irradiated with a broadband light source-xenon lamp using 0.72 M NaNO 2 aqueous solution as a cutoff filter (0.17 mW/cm 2 ). The consumption of DHN was monitored by a decrease in the absorption at 301 nm using a UV-vis spectrophotometer (UV-1800, Mapada, Shanghai, China) at intervals of 5 min.
In the equation, Φ ∆ (std) is the singlet oxygen generation quantum yield of MB (0.57 in CH 2 Cl 2 ), k obs (x) and k obs (std) were the absolute value of the slopes of ln[(A-A )/A 0 ] versus irradiation time for the photooxidation of DHN by sensitizers and MB, respectively. I(x) and I(std) were the total light intensities absorbed by sensitizers and MB, respectively.

Photostability Experiment
C 70 -B-T in a concentration of 1.0 × 10 −5 mol L −1 in CH 2 Cl 2 /MeOH (9:1, v/v) was placed in a quartz cell and irradiated with a xenon lamp (0.17 mW/cm 2 ) continuously for 1 h. The spectral response of C 70 -B-T was recorded using a UV-vis spectrophotometer at 0 h and 1 h, respectively.

Measurement of Photophysical Properties
UV-vis absorption and fluorescence spectra were recorded via an absorption spectrometer (UV-1800, Mapada) and a fluorescence spectrophotometer (FP8500, JASCO, Tokyo, Japan) at room-temperature, respectively. The fluorescence lifetime measurements were conducted using a time-correlated single photon counting (TCSPC) apparatus at room temperature and a pulsed laser at a wavelength of 510 nm was used as the excitation source. The nanosecond transient absorption spectra were recorded by a nanosecond flash photolysis system (LP980, Edinburgh instruments, UK) with a pulse laser (7 ns, 1 Hz) from a Nd:YAG laser at a wavelength of 532 nm. The samples in 10 mm path length quartz cuvettes were freshly prepared and deoxygenated by bubbling nitrogen for over 20 min before measurement. The analyzing light was a 450 W pulsed xenon lamp. A monochromator equipped with a photomultiplier for collecting the spectral range from 350 to 850 nm was used to analyze transient absorption spectra. The decay curves were fitted by least-squares regression using a custom-written algorithm in the Matlab.
where Φ F(x) and Φ F(std) are the fluorescence quantum yields of the sensitizers and standard, respectively. A x and A std are the absorbance of the sensitizers and standard, F x and F std are the area under the emission curve of the sensitizers and standard, and n is the refractive index of the solvents used in measurement.

Conclusions
In conclusion, a broadband visible light-absorbing [70]fullerene-BODIPY-triphenylamine triad (C 70 -B-T) has been synthesized and used as a heavy atom-free organic triplet photosensitizer for photooxidation. Two TPA units were introduced to the π-core of BODIPY and the absorption spectrum of C 70 -B-T covered virtually the entire UV-visible region. Upon the direct or indirect excitation of the BODIPY-part of C 70 -B-T, the intramolecular singlet excited state energy transfer from BODIPY to C 70 unit occurs and produces 1 C 70 * . Then, the ISC of C 70 produces 3 C 70 * . The photophysical processes were confirmed by steady-state and transient spectroscopies. The photooxidation ability of the photosensitizers was investigated using DHN as a chemical sensor. Among all the investigated compounds, C 70 -B-T gives the best photooxidation efficiency. The photooxidation rate constant of C 70 -B-T is 1.47 and 1.51 times as that of C 70 -1 and MB, respectively. The results indicate that C 70 -antenna could be used as another heavy atom-free organic triplet photosensitizer structure motif, with potential applications in photodynamic therapy, photocatalysis, photovoltaics and TTA upconversion.

Data Availability Statement:
The data presented in this study are available in Supplementary Materials.