Panchromatic Light-Absorbing [70]Fullerene-Perylene-BODIPY Triad with Cascade of Energy Transfer as an Efficient Singlet Oxygen Sensitizer

Abstract

A panchromatic light-absorbing [70]fullerene-perylene-BODIPY triad (C₇₀-P-B) was synthesized and applied as a heavy atom-free organic triplet photosensitizer for photooxidation. The photophysical processes were comprehensively investigated by the methods of steady-state spectroscopy, time-resolved spectroscopy, as well as theoretical calculations. C₇₀-P-B shows a strong absorption ability from 300–620 nm. Efficient cascading intramolecular singlet-singlet energy transfer in C₇₀-P-B was confirmed by the luminescence study. The backward triplet excited state energy transfer from C₇₀ moiety to perylene then occurs to populate ³perylene*. Thus, the triplet excited states of C₇₀-P-B are distributed on both C₇₀ and perylene moiety with lifetimes of 23 ± 1 μs and 175 ± 17 μs, respectively. C₇₀-P-B exhibits excellent photooxidation capacity, and its yield of singlet oxygen reaches 0.82. The photooxidation rate constant of C₇₀-P-B is 3.70 times that of C₇₀-Boc and 1.58 times that of MB, respectively. The results in this paper are useful for designing efficient heavy atom-free organic triplet photosensitizers for practical application in photovoltaics, photodynamic therapy, etc.

1. Introduction

For the wide application of singlet oxygen in the areas of photodynamic therapy (PDT) [1,2,3,4], in vivo oxygen sensing [5], bioimaging [3] and photocatalysis [6,7], researchers have been exploring ways to produce high yields of singlet oxygen in recent years. Singlet oxygen could be generated by photosensitizers (PSs) upon light irradiation in the presence of molecular oxygen. To achieve high photosensitizing efficiency, PSs should exhibit these characteristics, e.g., broadband absorption in the visible or NIR region, high efficiency of intersystem crossing (ISC), long triplet excited state lifetimes, and high photostability [8,9,10]. However, designing PSs with all of the overall properties is a challenge.

The most commonly used strategy for designing triplet PSs is the introduction of heavy atoms [8,11,12,13]. Although heavy-atoms can enhance the ISC to generate triplet states, these PSs still suffer from the weak absorption in the visible range, toxicity, and short triplet excited state lifetimes. A number of other strategies have also been developed to design PSs without heavy atoms, for instance, a twisted π-conjugation system [14,15], radical enhanced intersystem crossing [16], and spin-orbit charge transfer [17,18]. However, there are no clear rules regarding the relationship between the ISC and molecular structure in order to improve singlet oxygen generation efficiency. Moreover, most of the reported PSs are mono-chromophoric, resulting in their narrow absorption in the visible or NIR region. To overcome these shortcomings, connecting multi-chromophores with an electron spin convertor is a good strategy to construct heavy atom free PSs [8]. Benefiting from the high ISC efficiency, unique properties in biological systems and easy functionalization, fullerenes (mainly C₆₀) are frequently employed as efficient spin convertors [7,19,20,21,22]. Significantly greater photosensitization efficiency has been achieved in C₆₀-based triplet PSs. C₇₀, a higher molecular weight fullerene, also possesses high ISC efficiency (near 1.0) [23]. Compared with C₆₀, C₇₀ has a much more extended system, higher absorption in the visible region, better photodynamic activity, and higher TTA quantum yield [24,25,26]. As a consequence, it is highly promising to synthesize PSs with higher performance using C₇₀ as a spin converter. Until now, only a few C₇₀-based triplet photosensitizers have been reported [9,27].

In this study, we devised and synthesized a panchromatic light-absorbing [70]fullerene-perylene-BODIPY triad (C₇₀-P-B) as an efficient singlet oxygen sensitizer using amidation reactions. Considering the strong absorptions in the visible range (at about 505 nm), and energetically high lying excited states, BODIPY was selected as the light harvesting antenna [28]. Perylene, which shows strong absorptions from 525 nm to 620 nm, is complementary in terms of the absorptions to BODIPY. Thus, BODIPY and perylene were selected as light harvesting antennas, and C₇₀ was chosen as a spin converter to form a cascade of the energy transfer system. Moreover, the rigid molecular structure can increase ISC rates and quantum yields by suppressing the vibronic deactivation [29]. Thus, BODIPY, perylene and C₇₀ were connected through rigid phenyl and biphenyl linkers, respectively. The structure of C₇₀-P-B is shown in Figure 1. To determine the photophysical properties and photooxidation capacity of C₇₀-P-B, C₇₀-Boc, PDI [29], BOD [30,31] and P-B are also prepared as reference compounds. For good solubility, C₇₀-Boc is synthesized to replace C₇₀ as the reference monomer. More stable PDI is also synthesized as a reference monomer to replace perylene. The structures of all the reference compounds are also given in Figure 1. As expected, C₇₀-P-B can efficiently harvest broadband excitation light to generate triplet states. The photophysical processes of C₇₀-P-B were comprehensively investigated by steady-state UV-visible absorption and fluorescence spectroscopy, time-resolved fluorescence spectroscopy, nanosecond time-resolved transient absorption spectroscopy, as well as theoretical calculations. Thanks to the panchromatic light-absorbing and cascade of energy transfer, C₇₀-P-B can be used as an efficient photosensitizer to generate singlet oxygen with a high yield. Therefore, our results are very useful for the design and preparation of efficient singlet oxygen sensitizers.

2. Results

2.1. Synthesis

The synthetic routes of the triad C₇₀-P-B and the reference compounds are shown in Scheme 1, and the details are given in the Section 3.

Compound 1 was synthesized according to the reported procedures of our group [30]. Next, 1 was reacted with glyoxylic acid ethyl ester and C₇₀ by 1,3-dipolar cycloaddition in o-dichlorobenzene (ODCB) at 150 °C for 4 h to provide C₇₀-Boc in 27% yield. Compound 2 could be prepared by removing the Boc-group with TFA. BOD was also synthesized following the procedures of our group [31,32]. BOD then was reacted with 3,4,9,10-perylenetetracarboxylic dianhydride (3) using imidazole as a base to afford the monoanhydride derivative. Monoanhydride has strong adsorption on a silica gel column, and thus it is difficult to get the pure product. Without characterization, the monoanhydride derivative was reacted directly with 2 to synthesize C₇₀-P-B in 25% yield.

Following the similar procedures, the reference compounds PDI and P-B were prepared by the cross-condensation of 3, BOD and aniline in 41% and 31% yields, respectively.

The structures of all the new compounds were confirmed by NMR, MS and IR spectroscopy techniques. The NMR and MS spectra of all the compounds are given in the Supplementary Materials. For the sake of clarity, the expansions of the ¹H-NMR spectrum of C₇₀-P-B in CDCl₃ is shown in Figure 2.

The structural symmetry of C₇₀ is D_5h, lower than that of C₆₀, and, therefore, C₇₀ has four different [6,6] ring fusions with different reactivity [33]. As a result, C₇₀-P-B and C₇₀-Boc obtained by 1,3-dipolar cycloaddition are a mixture of isomers. From the ¹H NMR spectrum, it can be concluded that C₇₀-P-B contains at least three isomers. The peaks at ~8.25–8.19 ppm are assigned to the perylene skeleton protons; peaks at ~7.73–6.84 ppm are assigned to phenyl ring protons; peaks at ~6.01–5.32 ppm are assigned to pyrrolidine protons and pyrrole ring protons; peaks at ~4.48–3.52 ppm are assigned to OCH₂CH₃; peaks at ~2.64–2.06 ppm are assigned to protons of pyrrole ring CH₃, and peaks at ~1.52–0.87 ppm are assigned to OCH₂CH₃, protons of the pyrrole ring CH₃ and C(CH₃)₃ of perylene. The ¹³C NMR spectrum of C₇₀-P-B also shows the expected signals. For example, the peaks from 169.53–163.01 ppm are assigned to the carbons of C=O, the peaks from 157.42–118.97 ppm are assigned to the sp²-C of C₇₀ and the benzene ring, the peaks at ~74.23–72.86 ppm are carbons of CHCO₂, the peaks from 70.88–70.38 ppm are assigned the sp³-C of C₇₀, peaks from 65.40–60.33 ppm are assigned the carbons of OCH₂CH₃, peaks from 34.19–29.96 ppm are carbons of C(CH₃)₃ and C(CH₃)₃, and peaks from 14.62–13.93 ppm are carbons of CH₃. The mass spectrum of C₇₀-P-B gives a molecular peak at m/z 2482.7023, which is consistent with the calculated data.

2.2. UV-Vis Absorption and Steady-State Fluorescence

In order to investigate the photophysical properties of the C₇₀-P-B triad and each component, the UV-vis absorption and steady-state fluorescence spectra of C₇₀-P-B and the reference compounds were recorded and shown in Figure 3. C₇₀-Boc with only one C₇₀ moiety shows weak absorption in the visible range, and three absorption peaks are located at 398 nm (ε = 23,149 M⁻¹ cm⁻¹), 465 nm (ε = 17,772 M⁻¹ cm⁻¹) and 535 nm (ε = 9731 M⁻¹ cm⁻¹). P-B with BODIPY and perylene moieties gives the characteristic absorption peaks of BODIPY at 503 nm and perylene at 575 nm. Intense absorptions at 306 nm (ε = 74,056 M⁻¹ cm⁻¹) corresponding to the absorption of C₇₀, 503 nm (ε = 67,654 M⁻¹ cm⁻¹) corresponding to the absorption of BODIPY, and 575 nm (ε = 49,166 M⁻¹ cm⁻¹) corresponding to the absorption of perylene are found for C₇₀-P-B. Compared with the monomers, the maximum absorption peaks of C₇₀-P-B and P-B are redshifted by about 1 nm. The UV-vis spectrum of C₇₀-P-B is nearly a superimposition of the monomers, suggesting that electronic communication between P-B and C₇₀ is weak at the ground state.

The photoinduced intramolecular energy transfer of C₇₀-P-B and P-B was confirmed by the luminescence study. The results are presented in Figure 3b. With photoexcitation at 489 nm, the emission peaks belonging to the C₇₀ part in C₇₀-Boc and C₇₀-P-B were not detected due to the extremely low fluorescence quantum yield of C₇₀ [34,35,36], as well as the overlapping by the fluorescence emission of perylene. In the same conditions, BOD gives an intense fluorescence at 514 nm, whereas, due to the efficient intramolecular energy transfer from BODIPY to perylene, this emission is almost completely quenched in both C₇₀-P-B and P-B. C₇₀-P-B, P-B and PDI exhibit similar spectral characteristics with an emission peak at 607 nm corresponding to the perylene unit [37,38]. Compared with PDI, the luminescence intensity of P-B at 607 nm is largely enhanced (the luminescence intensity of P-B is 2.27 times that of PDI) due to the direct excitation of the perylene moiety and the energy transfer from the BODIPY moiety to perylene upon photoexcitation. Compared with the luminescence intensities of PDI and P-B, the emission corresponding to perylene moiety is largely quenched in C₇₀-P-B by the introduction of C₇₀. The quenching efficiency of C₇₀-P-B is about 97% compared to P-B. The emission quenching of perylene in C₇₀-P-B should be ascribed to the efficient excitation energy transfer from perylene to C₇₀ upon BODIPY part excitation. The intramolecular energy transfer from the perylene part to C₇₀ is thermodynamically allowed, because the energy of the S₁ state of perylene (2.16 eV) is higher than that of C₇₀ (1.85 eV) [9].

To further investigate the effect of solvent polarity on luminescence, the fluorescence emissions of C₇₀-P-B and P-B in THF, CHCl₃, DMF, PhCN and toluene at the same concentration were measured, as shown in Figure 3c,d. The fluorescence emission intensities are normalized. Clear shifts in emission peaks of C₇₀-P-B and P-B in different solvents could be observed due to the change in dipole moments. Both emission peaks of C₇₀-P-B and P-B are solvent sensitive [30,39].

2.3. Time-Resolved Fluorescence Spectroscopy

To further study the photo induced intramolecular energy transfer process, the transient fluorescence emission of C₇₀-P-B, P-B and PDI were investigated, and a single exponential fit was applied mathematically to all of these compounds. The results are shown in Figure 4.

With photoexcitation at 510 nm, the fluorescence signal at 600 nm was obtained for all three compounds. As shown in Figure 4, all fluorescence decay curves can be perfectly fitted by a single exponential process, and a fast-decay component (gray line, visible in Figure 4a with the full width at half maximum of 0.26 ns), is attributed to the instrument response profile [40]. As a result, the fluorescence lifetimes are determined to be 3.7 ± 0.5 ns for C₇₀-P-B, 5.7 ± 0.1 ns for P-B, and 6.1 ± 0.1 ns for PDI, respectively. The l258ifetime of C₇₀-P-B is apparently shorter than that of P-B and PDI, and close to the intrinsic lifetime of the C₇₀ monomer [36], providing extra evidence for the formation of ¹C₇₀*-P-B by intramolecular energy transfer. C₇₀-P-B undergoes significant attenuation relative to that of P-B, indicating that after the introduction of C₇₀, the excited state energy of perylene can be quenched via energy transfer. The results are consistent with that obtained in the steady-state fluorescence spectra.

2.4. Nanosecond Time-Resolved Transient Absorption Spectroscopy

To delve into the triplet state properties of C₇₀-P-B and C₇₀-Boc, nanosecond time-resolved transient absorption spectra were recorded with photoexcitation at 532 nm, and are displayed in Figure 5a,c, respectively.

For C₇₀-Boc, a negative peak at 476 nm and two positive absorption bands at 428 nm and ~700 nm were observed. The negative peak could be attributed to the ground-state bleaching (GSB) peak due to its similar position to the absorption of C₇₀-Boc, while the two positive bands originate from the triplet absorption of the C₇₀ moiety. For C₇₀-P-B, in addition to the three bands at 425 nm, 475 nm and ~700 nm, similar to C₇₀-Boc, a curiously negative peak at 590 nm was gradually generated. The global fit shows that this negative peak is generated with a characteristic time of 30 ± 2 μs, and is attenuated with a lifetime of 176 ± 11 μs (Figure S1). In comparison with the steady-state absorption spectrum, the negative peak is close to the absorption position of the perylene moiety, and can therefore be attributed to the GSB of the perylene. Considering that the GSB of C₇₀ at 475 nm and of perylene at 590 nm shows completely distinct evolution dynamics, it is inferred that there are two different triplet states of this photosensitizer. This deduction could also be verified by the dynamic decay behaviors of C₇₀-P-B and C₇₀-Boc at 700 nm. By fitting the kinetic curve of C₇₀-Boc, the decay of the C₇₀ triplet state was identified as mono-exponential, with a lifetime of 42 ± 2 μs. However, the decay process of C₇₀-P-B was fitted to a bi-exponential form, possessing a short lifetime of 23 ± 1 μs and a longer lifetime of 175 ± 17 μs (Figure 5b). Coincidentally, these two lifetimes matched up with the generation and decay lifetimes of the GSB for perylene. Combined with the result of transient absorption spectra, the shorter lifetime should be ascribed to the triplet state of C₇₀; the longer one is the triplet state of perylene. Thus, we propose that the backward triplet energy transfer from C₇₀ to the perylene moiety occurs, which also explains why the lifetime of the triplet state of the C₇₀ part in the triad is shorter than that in C₇₀-Boc.

Considering the results of steady-state fluorescence and time-resolved fluorescence spectroscopy parts, a “ping-pong” energy transfer mechanism is proposed for the decay dynamics of C₇₀-P-B upon photoexcitation. When the BODIPY part is excited, it can transfer energy to the perylene moiety to produce ¹perylene*. Singlet energy transfer from ¹perylene* to C₇₀ and efficient ISC processes leads to the formation of ³C₇₀*. According to the optimized geometry of C₇₀-P-B, as shown in Figure S2, the distance between the BODIPY and perylene units is around 15 Å, and that between the perylene and C₇₀ moieties is more than 20 Å. Therefore, Förster energy transfer should dominate for the intramolecular singlet-singlet energy transfer. The backward triplet excited state energy transfer from ³C₇₀* to perylene then occurs to populate the ³perylene*.

2.5. TD-DFT Calculations

To validate the energy transfer mechanism mentioned above, density functional theory (DFT) was adapted to calculate the optimized geometry, electronic configuration, vertical excitation energies, and frontier molecular orbitals of C₇₀-P-B. The optimized structure of C₇₀-P-B is shown in Figure S2, and the computational details are also available in the Supplementary Materials section.

Figure 6 shows the transition features of the two lowest triplet states, which are attributed to the HOMO→LUMO+2 and HOMO-3→LUMO transitions, respectively. Moreover, HOMO and LUMO+2 are primarily distributed on the perylene group, whereas HOMO-3 and LUMO are mostly located on the C₇₀ moiety. Consequently, these two transitions are the local excitation for perylene and C₇₀, respectively, hence the designations C₇₀-³P*-B and ³C₇₀*-P-B. Moreover, as shown in Figure 6, the excitation energies of ³C₇₀*-P-B and C₇₀-³P*-B are calculated to be 1.48 and 1.14 eV, respectively. The slightly high energy of ³C₇₀*-P-B confirms that the triplet energy transfer from the C₇₀ moiety to the perylene unit in the triad is thermodynamically feasible, which is in line with our experimental conclusions.

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

Using C₇₀-P-B, C₇₀-Boc and P-B as singlet oxygen sensitizers, their photooxidation properties were explored by using DHN as a chemical sensor and MB as a reference.

In the presence of ¹O₂, DHN can be easily oxidized to juglone. The photooxidation kinetics can 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-P-B, C₇₀-Boc, P-B and MB as the sensitizers upon excitation with a xenon lamp are presented in Figure 7 and Figure S3, respectively. For C₇₀-P-B, C₇₀-Boc 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 P-B as the photosensitizer. The photostability of C₇₀-P-B was also investigated by exposing it to light for 1 h, and no decrease was observed in the absorption (Figure S4). 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_∆), together with other photophysical properties of the photosensitizers, were calculated and are listed in

Table 1. The photophysical properties of the photosensitizers.

Photosensitizers	λabs a	λF b	τF c	τT d	kobs e/min−1	υi f	Φ∆ g
C70-P-B	306, 399, 503, 575	607	3.7	175, 23	83.3	8.33	0.82
C70-Boc	398, 465, 535	-	-	42	22.5	2.25	0.81 h
B-P	503, 575	607	5.7	-	3.7	0.37	-
MB	-	-	-	-	52.7	5.27	0.57

^a c = 1.0 × 10⁻⁵ M, in nm. ^b Fluorescence emission maximum, in nm. ^c Fluorescence lifetime, in ns. ^d Triplet state lifetime, in μs. ^e In DCM/MeOH (v/v) = 9/1. c = 10⁻⁵ M. The rate constant k_obs was calculated by the formula: ln[(A − A′)/A₀] = −k_obst. In 10⁻³ min⁻¹. ^f Initial consumption rate of DHN, υ_i = k_obs[DHN]. In 10⁻⁶ M min⁻¹. ^g Quantum yield of singlet oxygen (¹O₂), with MB as standard (Φ_∆ = 0.57 in CH₂Cl₂). ^h Literature values [23].

.

3. Materials and Methods

3.1. Materials

All reagents were obtained from commercial sources. The C₇₀, 1,6,7,12-tetra-tert-butylphenoxyperylene-3,4,9,10-tetracarboxylic dianhydride, ethyl glyoxylate, chloroform-d (CDCl₃), trifluoroacetic acid (TFA), anhydrous sodium sulphate (Na₂SO₄) and DHN were purchased from Alfa Aesar. The imidazole, aniline, and 1,2-dichlorobenzene (ODCB) were purchased from J&K Scientific Ltd. (Beijing, China). The silica gel, carbon disulfide (CS₂), dichloromethane (DCM), petroleum ether (PE), methanol (MeOH), tetrahydrofuran (THF), trichloromethane (CHCl₃), and toluene were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). THF was distilled over the sodium and benzophenone, while other reagents used for the synthesis were used directly.

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

3.2. Synthesis

3.2.1. Synthesis of C₇₀-Boc

C₇₀ (200.0 mg, 0.24 mmol), compound 1 [30] (180.0 mg, 0.48 mmol) and ODCB (8 mL) were added into a 25 mL, three-neck flask equipped with a gas inlet adaptor. The mixture was stirred at room temperature under an N₂ atmosphere for 1 h. The ethyl glyoxylate solution (550 µL, 2.8 mmol) was added immediately, and the mixture was stirred at 150 °C for 4 h. The solvent was then removed, and the mixture was subjected to column chromatography on silica gel with CS₂/CH₂Cl₂ (3:2) as eluent to give C₇₀-Boc as a brown orange powder (83.9 mg, 27%). ¹H NMR (400 MHz, CDCl₃) δ 7.49 (d, J = 8.7 Hz, phenyl ring H), 7.45–7.43 (m, phenyl ring H), 7.41–7.34 (m, phenyl ring H), 7.07 (d, J = 8.7 Hz, phenyl ring H), 6.91 (d, J = 8.8 Hz, phenyl ring H), 6.41 (bs, NHBoc), 6.39 (bs, NHBoc), 6.00 (s, CHCO₂), 5.78 (s, CHCO₂), 5.67 (s, CHCO₂), 5.50 (s, CHCO₂), 4.40 (q, J = 7.1 Hz, OCH₂CH₃), 4.31–4.23 (m, OCH₂CH₃), 4.10–4.02 (m, OCH₂CH₃), 3.88–3.78 (m, OCH₂CH₃), 1.52 (s, OC(CH₃)₃), 1.51 (s, OC(CH₃)₃), 1.40 (t, J = 7.1 Hz, OCH₂CH₃), 1.29 (t, J = 7.1 Hz, OCH₂CH₃), 1.04 (t, J = 7.1 Hz, OCH₂CH₃), 0.87 (t, J = 7.1 Hz, OCH₂CH₃). ¹³C NMR (100 MHz, CS₂/CDCl₃ with Cr(acac)₃ as relaxation reagent) δ 170.05, 169.99, 169.47, 157.63, 155.31, 155.11, 154.62, 152.76, 151.53, 151.46, 151.23, 151.18, 150.92, 150.83, 150.80, 150.61, 150.50, 150.38, 150.00, 149.97, 149.85, 149.43, 149.32, 149.22, 149.18, 148.94, 148.83, 148.80, 147.56, 147.53, 147.29, 147.25, 147.16, 147.06, 147.01, 146.92, 146.31, 146.26, 146.20, 146.11, 146.05, 145.95, 144.55, 143.72, 143.58, 143.55, 143.51, 143.40, 143.28, 143.24, 141.09, 140.45, 140.41, 140.17, 138.20, 137.48, 137.44, 135.27, 135.23, 134.65, 133.89, 133.75, 133.17, 132.50, 132.25, 131.93, 131.72, 131.52, 131.49, 131.33, 127.71, 127.58, 127.29, 127.23, 119.05, 118.90, 118.85, 80.68,74.48, 73.20, 72.86, 70.70, 64.82, 63.98, 62.30, 62.08, 61.90, 61.70, 61.68, 29.89, 28.48, 14.65, 14.52, 14.33, 14.13. FT-IR ν/cm⁻¹ (KBr) 2922, 2851, 1731, 1611, 1504, 1428, 1401, 1367, 1260, 1158, 1021, 818, 796, 534. HRMS (MALDI-TOF) m/z calcd for C₉₅H₃₀N₂O₆ [M⁻.] 1294.2109, found 1294.2104.

3.2.2. Synthesis of Compound 2

C₇₀-Boc (100.0 mg, 0.08 mmol) was dissolved in chloroform (10 mL). The mixture was stirred under an N₂ atmosphere at room temperature for 10 min. TFA (2 mL) was then added, and the mixture was stirred at room temperature for 1.5 h until C₇₀-Boc disappeared. The mixture was purified on a silica gel column using CS₂/CH₂Cl₂ (1:4) as eluent to give compound 2 as a brown orange powder (87.7 mg, 95%). ¹H NMR (400 MHz, CDCl₃) δ 7.46 (d, J = 8.6 Hz, phenyl ring H), 7.35 (d, J = 8.7 Hz, phenyl ring H), 7.31 (d, J = 8.4 Hz, phenyl ring H), 7.25 (d, J = 8.5 Hz, phenyl ring H), 7.05 (d, J = 8.6 Hz, phenyl ring H), 6.89 (d, J = 8.6 Hz, phenyl ring H), 6.66 (d, J = 8.4 Hz, phenyl ring H), 6.62 (d, J = 8.5 Hz, phenyl ring H), 5.99 (s, CHCO₂), 5.78 (s, CHCO₂), 5.66 (s, CHCO₂), 5.50 (s, CHCO₂), 4.40 (q, J = 7.1 Hz, OCH₂CH₃), 4.30–4.22 (m, OCH₂CH₃), 4.05 (q, J = 8.0 Hz, OCH₂CH₃), 3.89–3.78 (m, OCH₂CH₃), 3.64 (bs, NH₂), 1.39 (t, J = 7.1 Hz, OCH₂CH₃), 1.28 (t, J = 7.1 Hz, OCH₂CH₃), 1.04 (t, J = 7.1 Hz, OCH₂CH₃), 0.96 (t, J = 7.5 Hz, OCH₂CH₃), 0.86 (t, J = 7.1 Hz, OCH₂CH₃). ¹³C NMR (100 MHz, CS₂/CDCl₃ with Cr(acac)₃ as relaxation reagent) δ 169.72, 169.69, 169.66, 157.53, 154.97, 154.53, 151.40, 151.32, 151.10, 151.05, 150.82, 150.67, 150.50, 150.38, 150.28, 149.87, 149.84, 149.73, 149.72, 149.30, 149.21, 149.09, 149.05, 148.80, 148.72, 147.44, 147.16, 147.11, 146.94, 146.81, 146.20, 146.16, 146.00, 145.95, 145.87, 145.52, 145.47, 143.47, 143.44, 143.41, 143.39, 143.17, 143.12, 142.88, 140.98, 140.34, 140.21, 140.06, 138.09, 137.33, 135.30, 133.76, 133.75, 133.63, 133.04, 131.20, 130.82, 127.64, 127.58, 127.18, 127.05, 119.06, 118.86, 115.43, 74.31, 73.32, 73.01, 72.74, 70.55, 64.71, 63.83, 62.06, 61.85, 61.67, 61.57, 61.45, 14.61, 14.48, 14.28, 14.09. FT-IR ν/cm⁻¹ (KBr) 2963, 2922, 2834, 1731, 1622, 1501, 1428, 1400, 1262, 1180, 1095, 1022, 804, 534. HRMS (MALDI-TOF) m/z calcd for C₉₀H₂₂N₂O₄ [M⁻.] 1194.1585, found 1194.1589.

3.2.3. Synthesis of P-B

A mixture of BOD [31,32] (20.1 mg, 0.06 mmol), 1,6,7,12-tetra-tert-butylphenoxyperylene-3,4,9,10-tetracarboxylic dianhydride 3 (142.7 mg, 0.14 mmol), and imidazole (138.3 mg, 2.03 mmol) was stirred under an N₂ atmosphere at 80 °C in CHCl₃ (15 mL) for 13 h (monitored by TLC). Aniline (50 μL, 0.55 mmol) was then added, and the reaction mixture was allowed to react at 80 °C for 9 h. The mixture was cooled to room temperature and washed with water. The aqueous phase was extracted with CHCl₃, and the combined organic phase was dried over anhydrous Na₂SO₄. After the solvent was removed, the residue was purified by column chromatography on silica gel with toluene as the eluent to give PDI [30] (37.9 mg, 41%) and B-P (25.4 mg, 31%). B-P: ¹H NMR (300 MHz, CDCl₃) δ 8.29 (s, 2H), 8.24 (s, 2H), 7.53–7.38 (m, 7H), 7.23 (d, J = 8.7 Hz, 10H), 6.85 (dd, J = 8.7, 1.4 Hz, 8H), 6.00 (s, 2H), 2.55 (s, 6H), 1.50 (s, 6H), 1.28 (s, 18H), 1.26 (s, 18H); ¹³C NMR (75 MHz, CDCl₃ with Cr(acac)₃ as relaxation reagent) δ 163.67 (C=O), 163.64 (C=O), 156.47, 156.28, 155.94, 152.88, 152.87, 147.67, 143.37, 140.67, 136.16, 135.60, 135.29, 133.39, 133.27, 131.44, 129.82, 129.40, 129.14, 128.83, 128.63, 126.90, 126.84, 122.83, 122.39, 121.63, 121.16, 120.69, 120.23, 120.19, 119.93, 119.85, 119.59, 34.52, 31.56, 14.73, 14.65. FT-IR ν/cm⁻¹ (KBr) 2962, 1709, 1675, 1589, 1545, 1506, 1406, 1340, 1317, 1284, 1198, 1085, 982, 879, 834, 751. MALDI-TOF-MS m/z calcd for C₈₉H₈₀BF₂N₄O₈ [M⁻.] 1380.5959, found 1380.6159.

3.2.4. Synthesis of C₇₀-P-B

Compound 3 (167.7 mg, 0.17 mmol), BOD [31,32] (25.8 mg, 0.07 mmol) and imidazole (108.5 mg, 1.61 mmol) were dissolved in 15 mL of chloroform and stirred for 30 min under an N₂ atmosphere. The flask was equipped with an airproof stopper and stirred at 80 °C for 15 h monitored by TLC. Upon completion of the reaction, the reaction mixture was carefully quenched by water (50 mL). The mixture was transferred to a 250 mL separatory funnel. The aqueous layer was extracted with dichloromethane (2 × 50 mL). The combined organic layer was washed with saturated brine (200 mL), dried with anhydrous sodium sulfate, filtered, and concentrated. The residue was purified by TLC using CH₂Cl₂/PE (5:2) as an eluent to give monoanhydride as a crude product (43.6 mg, 50%). Without characterization, the crude monoanhydride was used directly. The crude monoanhydride, compound 2 (47.8 mg, 0.04 mmol), and imidazole (141.4 mg, 2.08 mmol) were dissolved in 6.5 mL of chloroform and stirred for 30 h under N₂ atmosphere at 80 °C. Upon completion of the reaction, the reaction mixture was carefully quenched by water (50 mL). The mixture was transferred to a 250 mL separatory funnel and extracted with dichloromethane (2 × 50 mL). The combined organic layer was washed with saturated brine (200 mL), dried with anhydrous sodium sulfate, filtered and concentrated. The residue was purified on a silica gel column using CH₂Cl₂/PE/CS₂ (5:1:1) as eluent to give C₇₀-P-B as a reddish brown solid (43.4 mg, 25%). ¹H NMR (400 MHz, CDCl₃) δ 8.25–8.198 (m, phenyl ring H), 7.73–7.52 (m, phenyl ring H), 7.46–7.38 (m, phenyl ring H), 7.26 (dd, J = 8.8, 3.0 Hz, phenyl ring H), 7.16 (d, J = 8.6 Hz, phenyl ring H), 7.01 (d, J = 8.7 Hz, phenyl ring H), 6.86 (dd, J = 8.6, 4.3 Hz, phenyl ring H), 6.06–5.32 (s, protons of CHCO₂ and pyrrole ring), 4.48–4.43 (m, OCH₂CH₃), 4.36–4.29 (m, OCH₂CH₃), 4.16–4.02 (m, OCH₂CH₃), 3.96–3.85 (m, OCH₂CH₃), 3.37–3.52 (m, OCH₂CH₃), 2.64 (s, pyrrole ring CH₃), 2.56 (s, pyrrole ring CH₃), 2.06 (s, pyrrole ring CH₃), 1.52 (s, pyrrole ring CH₃), 1.43 (t, J = 7.0 Hz, OCH₂CH₃), 1.329 (s, C(CH₃)₃), 1.321 (s, C(CH₃)₃), 1.313 (s, C(CH₃)₃), 1.07 (t, J = 7.1 Hz, OCH₂CH₃), 1.01 (t, J = 7.4 Hz, OCH₂CH₃), 0.94–0.88 (m, OCH₂CH₃); ¹³C NMR (100 MHz, CS₂/CDCl₃ with Cr(acac)₃ as relaxation reagent) δ 169.53, 169.44, 168.97, 163.15, 163.01, 157.42, 156.18, 156.08, 155.81, 155.66, 155.08, 154.87, 154.40, 152.77, 152.70, 151.33, 151.25, 151.01, 150.92, 150.74, 150.60, 150.39, 150.30, 150.18, 149.80, 149.75, 149.65, 149.61, 149.23, 149.10, 149.02, 148.98, 148.69, 148.64, 148.61, 147.36, 147.34, 147.30, 147.29, 147.09, 147.02, 146.97, 146.86, 146.72, 146.07, 146.04, 145.93, 145.82, 145.75, 144.15, 143.42, 143.39, 143.33, 143.21, 143.12, 143.07, 140.85, 140.33, 140.22, 140.18, 139.98, 137.98, 135.88, 135.34, 133.87, 133.63, 133.54, 133.23, 133.12, 132.94, 132.34, 132.31, 132.05, 131.67, 131.51, 131.29, 131.26, 131.13, 130.73, 129.68, 128.90, 128.83, 128.78, 128.13, 128.01, 127.45, 126.73, 126.69, 122.83, 122.32, 121.52, 120.97, 120.61, 120.12, 119.73, 119.43, 119.39, 118.97, 74.23, 74.19, 72.87, 70.88, 70.47, 70.39, 65.40, 64.64, 63.77, 62.10, 61.90, 61.69, 61.51, 60.94, 60.33, 34.20, 31.42, 30.74, 29.96, 14.62, 14.60, 14.46, 14.27, 14.07, 13.93. FT-IR ν/cm⁻¹ (KBr) 2959, 2924, 2852, 1708, 1673, 1592, 1504, 1402, 1339, 1283, 1174, 1090, 796, 546. HRMS (MALDI-TOF) m/z calcd for C₁₇₃H₉₄BF₂N₅O₁₂ [M⁻.] 2482.7001, found 2482.7023.

3.3. Photooxidation Experiment

The solutions of sensitizers C₇₀-P-B, C₇₀-Boc, P-B, 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 in CH₂Cl₂/MeOH (9:1, v/v) were mixed in a volume ratio of 1:1. O₂ was then bubbled through the mixtures for 10 min, and a xenon lamp (0.17 mW/cm², using 0.72 M NaNO₂ aqueous solution as a cutoff filter) was used as a broadband light source to irradiate the mixtures. The spectral responses of the mixtures were monitored by a UV-vis spectrophotometer at intervals of 5 min.

The singlet oxygen quantum yield (Φ_∆) was calculated by using Equation (1)

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

(1)

In this equation, Φ_∆(x) and Φ_∆(std) are the singlet oxygen generation quantum yield of the photosensitizers and MB. k_obs(x) and k_obs(std) were the absolute value of the slopes of ln[(A − A’)/A₀] versus the irradiation time for the photooxidation of DHN by photosensitizers and MB, respectively. I(x) and I(std) were the total light intensities absorbed by photosensitizers and MB, respectively.

3.4. Photostability Experiment

The photostability of C₇₀-P-B was measured in CH₂Cl₂/MeOH (9:1, v/v, 1.0 × 10⁻⁵ mol L⁻¹) and irradiated continuously for 1 h using a xenon lamp (0.20 mW/cm²). A UV-vis spectrophotometer was then used to record the spectral responses of C₇₀-P-B at 0 h and 1 h, respectively.

3.5. Measurement of Photophysical Properties

UV-Vis absorption and steady-state fluorescence were measured using an absorption spectrometer (UV-1800, Mapada, Shanghai, China) and a fluorescence spectrophotometer (FP8500, JASCO, Tokyo, Japan) at room-temperature. The fluorescence lifetimes of all the compounds were recorded via a time-correlated single photon counting (TCSPC) apparatus at room temperature, and a pulsed laser at 510 nm was used as the excitation source. The nanosecond transient absorption spectra were recorded with a home-built laser flash photolysis system. The pulsed excitation light was from the second harmonic 532 nm of a Q-Switched Nd:YAG laser (Dawa-100, Beamtech, Beijing, China) with a pulse duration of 8 ns and a 10-Hz repetition rate, and was intersected by a white light from a 500W Xenon lamp in a 10 mm × 10 mm quartz cuvette. In experiments, the pulse laser energy was set as ca.10 mJ/pulse to achieve a better signal-to-noise ratio. A monochromator equipped with a photomultiplier was used to record the transient absorption spectra within the wavelength range of 400–750 nm, with a spectral resolution of less than 1 nm. A kinetic curve of intermediate length was averaged by multi-shots and recorded with an oscilloscope (TDS3052B, Tektronix, Beaverton, OR, USA). All of the solutions were deoxygenated by purging with high purity argon (99.99%) for about 20 min prior to the measurements.

4. Conclusions

In conclusion, a panchromatic light-absorbing C₇₀-P-B triad with a cascade of energy transfer has been synthesized. The photophysical processes of the triad were investigated using steady-state UV-visible absorption and fluorescence spectroscopy, time-resolved fluorescence spectroscopy, nanosecond time-resolved transient absorption spectroscopy, and theoretical calculations. After the efficient cascade of singlet excited energy transfer from BODIPY to perylene and then to C₇₀ and the ISC process, the backward triplet excited state energy transfer from C₇₀ to perylene moiety occurred to populate the ³perylene*. The nanosecond time-resolved transient absorption results show that the triplet excited states of C₇₀-P-B are distributed on both C₇₀ and the perylene moiety. The bi-exponential decay processes are found in C₇₀-P-B. The shorter lifetime of the triplet state is 23 μs, and the longer one is 175 μs. For the synergistic effect of the antennas and C₇₀, C₇₀-P-B exhibits excellent photooxidation capacity. The photooxidation rate constant of C₇₀-P-B is 3.70 times as that of C₇₀-Boc and 1.58 times as that of MB, respectively. The yield of singlet oxygen of C₇₀-P-B is as high as 0.82. The results indicate that C₇₀-antennas that are panchromatic light-absorbing and have a cascade of energy transfer are useful structure motifs for generating high yields of singlet oxygen.