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

Abstract

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.

1. Introduction

Organic triplet photosensitizers (PSs) have received tremendous attention in recent years due to their wide application in photocatalysis [1,2], photodynamic therapy [3,4,5], and triplet–triplet annihilation upconversion (TTA) [6,7,8]. Broadband visible light-absorbance, hypotoxicity, high efficiency of intersystem crossing (ISC) and long triplet excited state lifetimes are the desired characteristics for triplet PSs [9,10]. However, it is still a challenge to attain the overall properties in one triplet PS.

PSs with heavy atoms can produce the triplet state, whereas the narrow visible light-absorbance, 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 these compounds are highly demanding. Using fullerene as a spin converter is a useful method to construct PSs with predictable ISC ability, because it possesses an ISC efficiency of unity even after derivatization [24]. Furthermore, fullerene also shows unique properties in biological systems. For example, fullerene exhibits excellent antioxidant [25], antiamyloid [26] and antibacterial [27] properties, has an adhesive and membrane-like potential [28], possesses excellent abilities in penetrating cell membranes and modulating ion transport [29,30], etc. Thus, fullerene-based PSs are promising scaffolds for designing high-technology nanomaterials and drugs in biological field.

C₇₀, a higher molecular weight fullerene in the shape of a rugby ball [31], has a much more extended π system and higher absorption in the visible region compared to C₆₀ [32]. Like C₆₀, C₇₀ also possesses high ISC efficiency (near 1.0) and can produce a high quantum yield of ¹O₂ (0.81 ± 0.15) [32]. The photodynamic activity and TTA quantum yield of C₇₀ are higher than that of the C₆₀ counterparts [33,34,35]. In consequence, it is highly promising to synthesize heavy atom-free organic triplet sensitizers with better properties using C₇₀ as a spin converter. Similar to C₆₀, C₇₀ itself is not a good PS due to its low molar extinction coefficient in the visible region. Grafting suitable antennas onto a C₇₀ cage seems to be a useful strategy, because significantly greater photosensitization efficiency has been achieved in C₆₀-based triplet PSs due to improved visible light absorption [36,37,38,39,40]. Until now, most of the reported studies on the photosensitization of fullerenes have mainly been focused on C₆₀. C₇₀-antenna systems as triplet photosensitizers have rarely been reported [41].

Herein, we designed and synthesized a broadband visible light-absorbing [70]fullerene-BODIPY-triphenylamine triad (C₇₀-B-T) as a heavy atom-free organic triplet photosensitizer for photooxidation. BODIPY, owing to its high chemical stability, simple synthetic route and adjustable π-conjugation framework [42,43,44,45,46,47,48,49], was selected as the light-harvesting antenna in this triad. As we know, the absorption wavelength of unsubstituted BODIPY is at ~500 nm and the absorption range is relatively narrow. Therefore, the triphenylmethyl amine (TPA) units were grafted at the “2” and “6” positions of the BODIPY core to improve the absorption range. Direct carbon–carbon coupling at the “2” and “6” positions of the BODIPY will weaken the electronic conjugation and bend the molecule due to steric hindrance [50]. Thus, in this work, alkyne linkers were inserted between the TPA units and BODIPY core to release the steric congestion. In order to place C₇₀ and antenna at a fixed distance, TPA-fused BODIPY (B-T) and C₇₀ were connected by a rigid phenyl acetylene bridge via the Prato reaction. The structures of C₇₀-B-T, B-T and C₇₀-1 are shown in Figure 1. Both C₇₀-B-T and B-T show broadband visible light-absorbance from 270 to 700 nm, covering almost the entire UV–visible region.

2. Results and Discussion

2.1. 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 cross-coupling 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 CH₃CO₂H. A subsequent double Sonogashira coupling reaction of 6 with 3 afforded B-T in 68% yield. Finally, C₇₀-B-T was obtained in 45% yield by treating B-T with C₇₀ and sarcosine under nitrogen atmosphere in toluene. The reference compound C₇₀-1 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 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 ¹H-NMR spectrum of C₇₀-B-T in CDCl₃ is shown in Figure 2.

The ¹H-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 ¹³C-NMR spectrum of C₇₀-B-T also shows the expected signals. For example, the peaks from 122–151 ppm assign to the sp²-C of C₇₀ and benzene ring, peaks at ~80–117 ppm are carbons of alkyne, peaks from 58–71 ppm assign to the sp³-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.

2.2. 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 low-energy 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. Characteristic spectroscopic data of C₇₀-1, B-T and C₇₀-B-T in toluene and THF ^a.

Compound	Solvent	λabs (nm)	λem (nm)	ΦF b
				Ex. = 339 nm	Ex. = 605 nm
C70-1	toluene	284, 307, 398, 462, 537, 666	710	-	-
	THF	-	-	-	-
B-T	toluene	330, 443, 603	664	0.29	0.22
	THF	-	668	-	0.01
C70-B-T	toluene	284, 310, 344, 443, 600	664	0.03	0.04
	THF	-	666	-	0.005

^a c = 1.0 × 10⁻⁵ M. ^b Using (4-((trimethylsilyl)ethynyl)phenyl)-BODIPY as reference, Φ_F = 0.46 in CHCl₃, λ_exc 484 nm [53].

), suggesting no significant electronic communication between C₇₀ and the antenna at ground state.

The steady-state fluorescence spectra of C₇₀-B-T, C₇₀-1 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 C₇₀-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 C₇₀-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 C₇₀-B-T is relatively weak compared with that of B-T (the quenching efficiency is 88%). Direct excitation of the BODIPY moiety of C₇₀-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 C₇₀-B-T is largely quenched (82%, Φ_F = 0.04) due to intramolecular energy or electron transfer.

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 both C₇₀-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 C₇₀-B-T [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 3b, weak fluorescence emissions of C₇₀ at 687 nm, 702 nm and 710 nm are observed. However, these emission bands are not observed in C₇₀-B-T because of the fluorescence overlap.

2.3. 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.

When excited at 510 nm and recorded the fluorescence signal at 664 nm, B-T gives monoexponential decay with fluorescence lifetime of 0.86 ns. Similar results are obtained when C₇₀-B-T is excited, but the fluorescence lifetime of C₇₀-B-T is reduced to 0.19 ns, shorter than that of B-T. The much shorter fluorescence lifetime of BODIPY part detected in C₇₀-B-T indicates an effective excitation energy quenching of the BODIPY part by C₇₀, which is consistent with the result obtained in the steady-state fluorescence spectra.

The efficiency of energy transfer from BODIPY to C₇₀ in C₇₀-B-T can be calculated by using Equation (1).

$${\mathsf{\Phi }}_1=\frac{k_1}{\left({\mathrm{k}}_{0 }+{ \mathrm{k}}_1\right)}= \frac{1/{\mathsf{\tau }}_1-1/{\mathsf{\tau }}_0}{1/{\mathsf{\tau }}_1}$$

(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

2.4. 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.

Upon excitation at 532 nm, bleaching at about 474 and 538 nm was observed for both C₇₀-B-T and C₇₀-1 due to ground state absorption of C₇₀. C₇₀-1 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 C₇₀-B-T, no bleaching of the steady-state absorption of B-T part is observed. C₇₀-B-T also shows a characteristic absorption band of ³C₇₀* at 418 nm. However, the broad transient absorption band of C₇₀-B-T is blue-shifted and splits into three bands with peaks at ~585, 622 and 695 nm due to the derivatization of C₇₀ 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 ³C₇₀*, 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 ³C₇₀*. 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).

2.5. Photooxidation of 1,5-Dihydroxy Naphthalene Mediated by ¹O₂

The photooxidation ability of C₇₀-B-T was studied by using 1,5-dihydroxy naphthalene (DHN) as a ¹O₂ scavenger. In the presence of ¹O₂, 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 absorption 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 (Φ_Δ) of the photosensitizers were calculated [37,57], and the data are listed in

Table 2. The photooxidation rate constant and yield of singlet oxygen of the photosensitizers ^a.

Photosensitizers	kobs b/min−1	υi c	Φ∆ d
C70-B-T	67.5	6.75	0.78
C70-1	45.9	4.59	0.81 e
B-T	2.0	0.2	-
MB	44.8	4.48	0.57

^a In CH₂Cl₂/CH₃OH = 9:1 (v/v). c = 1.0 × 10⁻⁵ mol/L. ^b The rate constant k_obs was calculated by the rule: ln[(A − A′)/A₀] = −k_obst. In 10⁻³ min⁻¹. ^c Initial consumption rate of DHN, υ_i = k_obs[DHN]. In 10⁻⁶ M min⁻¹. ^d Quantum yield of singlet oxygen (¹O₂), with MB as standard (Φ_∆ = 0.57 in CH₂Cl₂). ^e Literature values [32].

.

Among all the investigated photosensitizers, C₇₀-B-T gives the highest photooxidation efficiency. The photooxidation rate constant (k_obs) of C₇₀-B-T is 67.5 × 10⁻³ min⁻¹, which is 1.47 times as that of C₇₀-1 (45.9 × 10⁻³ min⁻¹) and 1.51 times as that of MB (44.8 × 10⁻³ min⁻¹). B-T does not show obvious photooxidation ability. The more efficient photooxidation of C₇₀-B-T compared with that of C₇₀-1 demonstrates that the broadband visible light absorption antenna improves the photosensitizing ability. Here, the enhanced photosensitizing ability of C₇₀-B-T compared with that of C₇₀-1 and B-T should be attributed to the synergetic effect of C₇₀ and B-T. First, B-T harvested a broadband visible light and be excited to its excited singlet easily, then efficient intramolecular energy transfer from B-T to C₇₀ occurred and formed the ¹C₇₀*, finally the highly efficient ISC of C₇₀ would eventually lead to the population of ³C₇₀*. These photophysical processes can be supported by the steady-state and transient data mentioned above.

3. Materials and Methods

3.1. Materials

All reagents were obtained from commercial sources. C₇₀, ethyl 2-(methylamino)acetate hydrochloride, ethyl glyoxylate, 4-iodo-N,N-diphenylaniline, cuprous iodide (CuI), triphenylphosphine (PPh₃), trimethylsilylacetylene (TMSA), potassium carbonate (K₂CO₃), anhydrous sodium sulphate (Na₂SO₄), N-iodosuccinimide (NIS), sodium thiosulfate (Na₂S₂O₃) and DHN were purchased from Alfa Aesar. Trans-dichlorobis(triphenyl-phosphine)Palladium(II) (Pd(PPh₃)₂Cl₂) was purchased from Shanxi Kaida Chemical engineering Co., Ltd. Sarcosine, 4-iodobenzaldehyde and 1,2-dichlorobenzene (ODCB) were purchased from J&K Scientific LTD (Beijing, China).

Silica gel, carbon disulfide (CS₂), dichloromethane (DCM), petroleum ether (PE), methanol (MeOH), tetrahydrofuran (THF), triethylamine (Et₃N), acetic acid (CH₃COOH), trichloromethane (CHCl₃), and toluene were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). THF was distilled over sodium and benzophenone, other reagents used for the synthesis were used directly.

All synthesis compounds were characterized by ¹H and ¹³C-NMR spectroscopy on a BRUKER 400 MHz spectrometer. The mess analyses were performed using a Bruker ultrafleXtreme MALDI TOF/TOF (Bremen, Germany).

3.2. Synthesis

3.2.1. Synthesis of C₇₀-1

The solution of C₇₀ (140 mg, 0.16 mmol), ethyl 2-(methylamino)acetate hydrochloride (49.1 mg, 0.32 mmol) in ODCB (15 mL) was bubbled with N₂ for 30 min at room temperature. Ethyl glyoxylate (158 µL, 0.8 mmol, in a 50% toluene solution) was added and the reaction mixture was stirred at 130 °C for 90 min monitored by thin layer chromatography (TLC). After the solvent was removed by reduced pressure, the mixture was purified by silica gel column chromatography using CS₂/DCM (2/1, v/v) as the eluent to give C₇₀-1 as a brown black powder (70.0 mg, 42%). UV-vis (toluence) λ_max/nm 284 (77230 L mol⁻¹ cm⁻¹), 307 (51491 L mol⁻¹cm⁻¹), 398 (32516 L mol⁻¹ cm⁻¹), 462 (26537 L mol⁻¹ cm⁻¹), 537 (14119 L mol⁻¹cm⁻¹), 666 (2426 L mol⁻¹cm⁻¹). ¹H-NMR (400 MHz, CDCl₃) δ 5.83 (s, CHCO₂), 5.35 (s, CHCO₂), 5.30 (s, CHCO₂), 4.81(s, CHCO₂), 4.63–4.37(m, OCH₂CH₃), 4.32–4.17(m, OCH₂CH₃), 4.15-3.99 (m, OCH₂CH₃), 3.10 (s, NCH₃), 2.83 (s, NCH₃), 2.68 (s, NCH₃), 2.62 (s, NCH₃), 1.54 (t, J = 7.1 Hz, OCH₂CH₃), 1.42 (t, J = 7.2 Hz, OCH₂CH₃), 1.17 (t, J = 7.1 Hz, OCH₂CH₃), 1.04 (t, J = 7.2 Hz, OCH₂CH₃). ¹³C-NMR (100 MHz, CDCl₃) δ 170.18, 170.06, 157.73, 155.60, 155.32, 154.87, 151.56, 151.20, 151.14, 150.89, 150.83, 150.50, 150.46, 150.01, 149.78, 149.46, 149.37, 149.23, 149.20, 149.06, 148.87, 148.83, 147.60, 147.54, 147.29, 147.25, 147.18, 147.14, 147.12, 147.06, 146.98, 146.32, 146.10, 145.99, 145.91, 143.52, 143.50, 143.48, 143.40, 143.21, 141.42, 140.74, 140.46, 140.23, 138.52, 137.64, 133.90, 133.70, 133.27, 131.86, 131.73, 131.53, 131.49, 131.36, 75.93, 72.28, 65.54, 64.58, 62.07, 61.65, 35.18, 29.86, 14.84, 14.50, 14.70, 14.35, 14.28. FT-IR υ/cm⁻¹ (KBr) 2922, 2851, 1746, 1733, 1632, 1427, 1369, 1340, 1190, 1131, 1052, 795, 671, 637, 580, 534. HRMS (MALDI-TOF) m/z calcd for C₇₉H₁₅NO₄[M^−•]1041.1007, found 1041.1001.

3.2.2. Synthesis of Compound 2

4-Iodo-N,N-diphenylaniline 1 (500.0 mg, 1.35 mmol), CuI (128.3 mg, 0.67 mmol) and PPh₃ (176.6 mg, 0.67 mmol) were dissolved in 20 mL of THF/Et₃N (1/3, v/v) in an oven-dried, 50 mL one neck flask equipped with a gas inlet adaptor. The mixture was stirred at room temperature under N₂ atmosphere for 30 min. Then Pd(PPh₃)₂Cl₂ (189.0 mg, 0.27 mmol) and TMSA (945 µL, 6.74 mmol) were added. The mixture was stirred at 40 °C under N₂ atmosphere for 5 h until TLC monitoring indicated the disappearance of compound 1. The solvent was removed in vacuo, and the crude reaction mixture was purified by silica gel column chromatography using PE/CH₂Cl₂ (2/1, v/v) as eluent to give 2 [58] in 83% yield (391.0 mg). ¹H-NMR (400 MHz, CDCl₃) δ 7.34 (d, J = 8.7 Hz, 2H), 7.31-7.27 (m, 4H), 7.12–7.06 (m, 6H), 6.98 (d, J = 8.7 Hz, 2H), 0.27 (s, 9H).

3.2.3. Synthesis of Compound 3

Compound 2 (120.0 mg, 0.35 mmol) was dissolved in 7 mL of CH₂Cl₂/MeOH (1/1, v/v) in an oven-dried, 25 mL one neck flask equipped with a gas inlet adaptor. The mixture was stirred at room temperature under N₂ atmosphere for 30 min. Then, K₂CO₃ (145.5 mg, 1.05 mmol) was added. The mixture was stirred at room temperature under N₂ atmosphere for 1.5 h until TLC monitoring indicated the disappearance of compound 2. The crude mixture was extracted with CH₂Cl₂ and H₂O three times. The organic layer was dried by Na₂SO₄ and the solvent was then removed. The crude product was purified by silica gel column chromatography using PE/CH₂Cl₂ (2/1, v/v) as eluent to give 3 [58] in 90% yield (85.8 mg). ¹H-NMR (400 MHz, CDCl₃) δ 7.33 (d, J = 8.7 Hz, 2H), 7.29–7.25 (m, 4H), 7.11–7.04 (m, 6H), 6.97 (d, J = 8.6 Hz, 2H), 3.02 (s, 1H).

3.2.4. Synthesis of Compound 5

Compound 4 was synthesized by adapting the literature procedure and used directly [36]. 4 (394.4 mg, 1.13 mmol), 4-iodobenzaldehyde (313.2 mg, 1.35 mmol), CuI (107.6 mg, 0.57 mmol) and PPh₃ (148.2 mg, 0.57 mmol) were dissolved in 24 mL of THF/Et₃N (1/2, v/v) in an oven-dried, 50 mL one-neck flask equipped with a gas inlet adaptor. The mixture was stirred at room temperature under N₂ atmosphere for 30 min. Then, Pd(PPh₃)₂Cl₂ (158.6 mg, 0.23 mmol) was added. The mixture was stirred at 40 °C under N₂ atmosphere for 3.5 h until TLC monitoring indicated the disappearance of 4. The solvent was removed in vacuo, and the crude reaction mixture was purified by silica gel column chromatography using PE/CH₂Cl₂ (4/1, v/v) as eluent to give 5 in 83% yield (425.4 mg). ¹H-NMR (400 MHz, CDCl₃) δ 10.04 (s, 1H), 7.90 (d, J = 8.2 Hz, 2H), 7.72–7.69 (m, 4H), 7.33 (d, J = 8.2 Hz, 2H), 6.00 (s, 2H), 2.56 (s, 6H), 1.43 (s, 6H). ¹³C-NMR (100 MHz, CDCl₃) δ 191.52, 156.02, 143.05, 140.63, 135.85, 135.80, 132.64, 132.31, 131.26, 129.80, 129.19, 128.53, 123.47, 121.57, 92.61, 89.91, 14.75. FT-IR υ/cm⁻¹ (KBr) 2923, 2852, 1698, 1600, 1542, 1510, 1468, 1436, 1402, 1369, 1304, 1260, 1193, 1156, 1120, 1082, 1050, 1019, 979, 830, 766, 707, 477. HRMS (MALDI-TOF) m/z calcd for C₂₈H₂₃BF₂N₂O[M^+•]452.1871, found 452.1876.

3.2.5. 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 CHCl₃/CH₃COOH (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 CH₂Cl₂ and a saturated sodium thiosulfate aqueous solution three times. The combined organic layer was dried with anhydrous Na₂SO₄, filtered and concentrated. The crude reaction mixture was subjected to silica gel column chromatography using PE/CH₂Cl₂ (2/1, v/v) as eluent to give 6 in 89% yield (280.0 mg). ¹H-NMR (400 MHz, CDCl₃) δ 10.05 (s, 1H), 7.91 (d, J = 8.2 Hz, 2H), 7.73 (d, J = 8.2 Hz, 2H), 7.72 (d, J = 8.1 Hz, 2H), 7.30 (d, J = 8.2 Hz, 2H), 2.66 (s, 6H), 1.45 (s, 6H). ¹³C-NMR (100 MHz, CDCl₃) δ 191.50, 157.37, 145.26, 140.27, 135.90, 135.45, 132.91, 132.36, 131.15, 129.82, 129.03, 128.37, 124.15, 92.28, 90.37, 86.07, 17.36. FT-IR υ/cm⁻¹ (KBr) 2923, 1690, 1599, 1544, 1530, 1489, 1439, 1399, 1345, 1307, 1204, 1181, 1113, 1089, 995,918, 823, 765, 706, 588, 525. HRMS (MALDI-TOF) m/z calcd for C₂₈H₂₁BF₂I₂N₂O[M^+•]703.9804, found 703.9809.

3.2.6. Synthesis of Compound B-T

Compound 6 (100.0 mg, 0.14 mmol), CuI (13.5 mg, 0.07 mmol) and PPh₃ (18.6 mg, 0.07 mmol) were dissolved in 12 mL of toluene/Et₃N (1/3, v/v) in an oven-dried, 25 mL one neck flask equipped with a gas inlet adaptor. The mixture was stirred at room temperature under N₂ atmosphere for 30 min. Then Pd(PPh₃)₂Cl₂ (19.9 mg, 0.03 mmol) and compound 3 (152.8 mg, 0.57 mmol) were added. The mixture was stirred at 60 °C under N₂ atmosphere for 48 h until TLC monitoring indicated the disappearance of compound 6. The solvent was removed in vacuo, and the crude reaction mixture was purified by silica gel column chromatography using PE/CH₂Cl₂ (4/1, v/v) as eluent to give B-T in 68% yield (95.4 mg). UV–vis (toluence) λ_max/nm 330 (67,683 L mol⁻¹ cm⁻¹), 443 (13,215 L mol⁻¹ cm⁻¹), 603 (59,703 L mol⁻¹ cm⁻¹). ¹H-NMR (400 MHz, CDCl₃) δ 10.04 (s, 1H), 7.90 (d, J = 8.4 Hz, 2H), 7.74–7.71 (m, 4H), 7.34 (d, J = 8.3 Hz, 2H),7.31–7.28 (m, 6H), 7.27–7.24 (m,6H), 7.11–7.08 (m, 8H), 7.07–7.03 (m, 4H), 6.99–6.97 (m, 4H), 2.71 (s, 6H), 1.56 (s, 6H). ¹³C-NMR (100 MHz, CDCl₃) δ 191.53, 158.87, 148.03, 147.28, 143.25, 141.02, 135.91, 135.38, 132.79, 132.43, 132.35, 131.10, 129.82, 129.53, 129.14, 128.49, 125.06, 123.89, 123.69, 122.51, 116.89, 116.22, 97.05, 92.51, 90.19, 80.69, 13.90, 13.77. FT-IR υ/cm⁻¹ (KBr) 2923, 2853, 1635, 1590, 1525, 1492, 1400, 1318, 1264, 1186, 1173, 1078, 1011, 795, 753, 710, 695, 586, 533, 506. HRMS (MALDI-TOF) m/z calcd for C₆₈H₄₉BF₂N₄O[M^+•] 986.3973, found 986.3976.

3.2.7. Synthesis of Compound C₇₀-B-T

The mixture of C₇₀ (35.4 mg, 0.04 mmol), compound B-T (25.0 mg, 0.025 mmol) and sarcosine (18.8 mg, 0.21 mmol) was dissolved in 5 mL of toluene and stirred at 130 °C under N₂ atmosphere for 8 h, monitored by TLC. Then the solvent was removed under reduced pressure. The mixture was subjected to silica gel column chromatography using CS₂ as the eluent to afford unreacted C₇₀, then using CS₂/CH₂Cl₂ (2/1, v/v) as the eluent to give C₇₀-B-T as a mixture of isomers (21.3 mg, 45%). UV–vis (toluene) λ_max/nm 284 (111272 mol⁻¹ cm⁻¹), 310 (98534 mol⁻¹ cm⁻¹), 344 (83490 L mol⁻¹ cm⁻¹), 443 (31724 L mol⁻¹ cm⁻¹), 600 (58804 L mol⁻¹ cm⁻¹). ¹H-NMR (400 MHz, CDCl₃) δ 7.90–7.61 (m, phenyl ring H), 7.54–7.43 (m, phenyl ring H), 7.35–7.19 (m, phenyl ring H), 7.11–7.03 (m, phenyl ring H), 7.00–6.96 (m, phenyl ring H), 5.30 (s, NCHPh), 5.07 (s, NCHPh), 5.01 (s, NCHPh), 4.97 (s, NCHPh), 4.70 (d, J = 9.4 Hz, NCH₂), 4.32–4.21 (NCHPh, NCH₂), 4.08 (d, J = 9.7 Hz, NCH₂), 4.04 (s, NCHPh), 3.86 (s, NCHPh), 4.67–4.51 (NCHPh, NCH₂), 3.41 (d, J = 9.4 Hz, NCH₂), 3.22 (d, J = 9.5 Hz, NCH₂), 2.72 (s, NCH₃, pyrrole ring CH₃), 2.70 (s, NCH₃, pyrrole ring CH₃), 2.69 (s, NCH₃, pyrrole ring CH₃), 2.56 (s, NCH₃, pyrrole ring CH₃), 2.55 (s, NCH₃, pyrrole ring CH₃), 2.46 (s, NCH₃, pyrrole ring CH₃), 2.39 (s, NCH₃, pyrrole ring CH₃), 1.59 (s, pyrrole ring CH₃), 1.56 (s, pyrrole ring CH₃), 1.55 (s, pyrrole ring CH₃), 1.52 (s, pyrrole ring CH₃), 1.51 (s, pyrrole ring CH₃), 1.43 (s, pyrrole ring CH₃). ¹³C-NMR (100 MHz, CDCl₃) δ 167.85, 167.53, 158.95, 158.73, 158.50, 158.03, 156.54, 156.21, 155.52, 155.00, 154.96, 154.81, 154.77, 153.49, 152.54, 151.82, 151.66, 151.58, 151.56, 151.52, 151.47, 151.43, 151.21, 151.16, 151.02, 150.95, 150.83, 150.78, 150.76, 150.73, 150.64, 150.85, 150.55, 150.36, 150.32, 150.17, 150.07, 150.01, 149.96, 149.89, 149.83, 149.80, 149.77, 149.49, 149.44, 149.39, 149.34, 149.27, 149.21, 149.18, 149.10, 149.07, 149.05, 148.95, 148.91, 148.89, 148.79, 148.73, 148.43, 148.39, 148.37, 148.33, 148.25, 148.19, 148.15, 147.95, 147.89, 147.83, 147.60, 147.50, 147.24, 147.16, 147.02, 146.97, 146.95, 146.91, 146.75, 146.65, 146.57, 146.39, 146.32, 146.29, 146.23, 145.95, 145.89, 145.83, 145.78, 145.73, 145.65, 145.53, 145.44, 145.25, 145.17, 145.01, 144.87, 144.77, 144.64, 144.52, 144.48, 144.45, 144.39, 143.99, 143.90, 143.81, 143.73, 143.56, 143.51, 143.43, 143.36, 143.32, 143.28, 143.13, 142.99, 142.92, 142.85, 142.81, 142.40, 142.18, 142.08, 142.06, 142.02, 141.75, 141.28, 141.23, 141.15, 141.06, 140.95, 140.88, 140.81, 140.74, 140.67, 140.64, 140.54, 140.44, 140.29, 140.17, 138.75, 138.02, 137.97, 137.75, 137.58, 137.53, 137.42, 134.67, 134.61, 134.58, 133.92, 133.87, 133.75, 133.67, 133.60, 133.07, 132.67, 132.44, 132.11, 131.88, 131.82, 131.71, 131.68, 131.66, 131.60, 131.42, 131.35, 131.30, 131.19, 131.09, 131.05, 129.52, 128.97, 128.38, 128.28, 125.02, 124.57, 124.40, 123.66, 123.13, 122.86, 122.50, 116.87, 116.25, 97.12, 91.25, 91.14, 90.97, 89.61, 89.52, 89.44, 89.35, 88.09, 86.09, 83.15, 82.83, 82.18, 80.84, 79.93, 70.85, 70.27, 69.98, 68.86, 68.27, 66.68, 66.44, 66.28, 65.70, 62.18, 60.46, 58.87, 58.47, 39.69, 39.62, 32.06, 30.69, 30.44, 30.30, 30.18, 29.84, 29.50, 14.28, 13.89, 13.81. FT-IR υ/cm⁻¹ (KBr) 2959, 2923, 2852, 1708, 1673, 1592, 1502, 1402, 1339, 1317, 1283, 1174, 1090, 796, 546, 419. HRMS (MALDI-TOF) m/z calcd for C₂₁H₁₉BF₂N₂ [M^+•] 1857.4479, found 1854.4471.

3.3. Photooxidation Experiment

Compounds C₇₀-B-T, C₇₀-1, 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 CH₂Cl₂/MeOH (9:1, v/v), respectively. Then, the above solutions of sensitizers and DHN were mixed in a volume ratio of 1:1, and O₂ 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 NaNO₂ aqueous solution as a cutoff filter (0.17 mW/cm²). 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.

The singlet oxygen quantum yield (Φ_∆) was determined by using Equation (2).

$${\Phi }_{\Delta } = {\Phi }_{\Delta } \left(\mathrm{std}\right)\frac{{\mathrm{k}}_{\mathrm{obs}}\left(\mathrm{x}\right)•\mathrm{I}(\mathrm{std})}{{\mathrm{k}}_{\mathrm{obs}}\left(\mathrm{std}\right)•\mathrm{I}(\mathrm{x})}$$

(2)

In the equation, Φ_∆(std) is the singlet oxygen generation quantum yield of MB (0.57 in CH₂Cl₂), k_obs(x) and k_obs(std) were the absolute value of the slopes of ln[(A-A′)/A₀] 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.

3.4. Photostability Experiment

C₇₀-B-T in a concentration of 1.0 × 10⁻⁵ mol L⁻¹ in CH₂Cl₂/MeOH (9:1, v/v) was placed in a quartz cell and irradiated with a xenon lamp (0.17 mW/cm²) continuously for 1 h. The spectral response of C₇₀-B-T was recorded using a UV–vis spectrophotometer at 0 h and 1 h, respectively.

3.5. 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.

The corresponding fluorescence quantum yields were calculated by using Equation (3) [59] with (4-((trimethylsilyl)ethynyl)phenyl)-BODIPY as the standard, Φ_F = 0.46 in CHCl₃, λ_exc 484 nm [53].

$${\Phi }_{\mathrm{F}}{}_{\left(\mathrm{x}\right)} = {\Phi }_{\mathrm{F}}{}_{\left(\mathrm{std}\right)}\frac{{\mathrm{A}}_{\mathrm{std}}•{\mathrm{F}}_{{}_{\mathrm{x}}}}{{\mathrm{A}}_{\mathrm{x}}•{\mathrm{F}}_{{}_{\mathrm{std}}}} {\left(\frac{{\mathrm{n}}_{\mathrm{x}}}{{\mathrm{n}}_{\mathrm{std}}}\right)}^2$$

(3)

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.

4. Conclusions

In conclusion, a broadband visible light-absorbing [70]fullerene-BODIPY-triphenylamine triad (C₇₀-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 C₇₀-B-T covered virtually the entire UV–visible region. Upon the direct or indirect excitation of the BODIPY-part of C₇₀-B-T, the intramolecular singlet excited state energy transfer from BODIPY to C₇₀ unit occurs and produces ¹C₇₀^*. Then, the ISC of C₇₀ produces ³C₇₀^*. 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, C₇₀-B-T gives the best photooxidation efficiency. 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 C₇₀-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.