Broadband and strong visible-light-absorbing cuprous sensitizers for boosting photosynthesis

Significance

It’s of great interest to replace the benchmark noble-metal photosensitizers (PSs) with those based on earth-abundant and low-cost metals for achieving large-scale implementation of solar energy. However, the short-lived excited state and poor visible-light absorption of current nonprecious metal-based complexes severely limit their practical applications. Here, we report cuprous PS with long excited-state lifetime and BSVLA ability for dramatically boosting photosynthesis. Owing to its earth-abundant nature, extremely high activity, and universal design concept, this type of strong sensitizing cuprous PSs exhibited a bright future for large-scale solar energy utilization.

Abstract

Rational construction of broadband and strong visible-light-absorbing (BSVLA) earth-abundant complexes is of great importance for efficient and sustainable solar energy utilization. Herein, we explore a universal Cu(I) center to couple with multiple strong visible-light-absorbing antennas to break the energy level limitations of the current noble-metal complexes, resulting in the BSVLA nonprecious complex (Cu-3). Systematic investigations demonstrate that double “ping-pong” energy-transfer processes in Cu-3 involving resonance energy transfer and Dexter mechanism enable a BSVLA between 430 and 620 nm and an antenna-localized long-lived triplet state for efficient intermolecular electron/energy transfer. Impressively, Cu-3 exhibited an outstanding performance for both energy- and electron-transfer reactions. Pseudo-first-order rate constant of photooxidation of 1,5-dihydroxynaphthalene with Cu-3 can achieve a record value of 190.8 × 10⁻³ min⁻¹ among the molecular catalytic systems, over 30 times higher than that with a noble-metal photosensitizer (PS) [Ru(bpy)₃]²⁺. These findings pave the way to develop BSVLA earth-abundant PSs for boosting photosynthesis.

In natural photosynthesis, Mg porphyrin complexes (e.g., chlorophyll a) represent a central component for light absorption and electron/energy transfer, which have a significant influence on the photosynthetic efficiency (1, 2). Inspired by natural photosynthesis, great efforts have been made to develop transition-metal complexes with fascinating photophysical and photochemical properties for efficient artificial photosynthesis (3–6). A basic design concept for transition-metal complexes is the coordination of an electron-rich metal center (e.g., with d⁶, d⁸, and d¹⁰ electron configurations) with strong π-acceptor ligands (5). Upon photoexcitation, metal-to-ligand charge transfer (¹MLCT) in the transition-metal complexes was triggered to reach the singlet charge transfer (CT) state, followed by intersystem crossing (ISC) to afford a long-lived ³MLCT state for intermolecular electron/energy transfer. In this field, typical photosensitizers (PSs), such as [Ru(bpy)₃]²⁺ (d⁶, bpy = 2,2-bipyridine, Ru-1), can store solar light with high excited-state energy or redox equivalent for photochemical reactions mediated by the energy or electron transfer (7–9). However, their molar extinction coefficient in the visible region was usually humble (ε < 20,000 M⁻¹ cm⁻¹) due to the character of CT transition, and their excited-state lifetime was shorter than 1 μs due to the strong spin−orbit coupling effect, leading to extremely low solar energy utilization. Recently, several attempts have been made to rigid couple the noble metal coordination complexes with strong visible-light-absorbing (SVLA) chromophores for improving their sensitizing ability (10–15). However, these PSs usually exhibited poor photostability and are hard to be modified due to their rigid coupling mode and rigid organic ligand. Therefore, it’s a promising approach to flexibly couple the coordination center with chromophores, yet it highly depends on the energy transfer process to utilize the excitation energy. However, owing to their high ¹MLCT energy level, the current Ru, Ir, Re, or Pt complexes were just compatible with ultraviolent absorbing chromophores via energy-transfer sensitization (4, 12–15). Otherwise, their large-scale applications for solar energy conversion were severely limited by high cost and low natural abundance of these precious metals. As a result, it’s highly desirable but remains a great challenge to replace precious metal PSs with strong sensitizing earth-abundant complexes (16–18).

Recently, increased attention has shifted to developing photosensitizing coordination complexes with earth-abundant metal elements (6, 19–21), particularly for the Fe(II)- and Cu(I)-based complexes, due to their similar MLCT electronic transition with Ru-1 and high abundance in Earth’s crust (22–30). However, the low-lying d–d states of Fe(II) complexes enabled rapid decay of the MLCT state back to the ground state via nonradiative deactivation, resulting in an ultra-short-lived excited state in picosecond timescales (31). Compared to Fe(II) complexes, the d–d transition for Cu(I) diimine complexes can be precluded due to their d¹⁰ configuration, which can extend their excited-state lifetimes to photocatalytically useful microsecond timescales (25, 32–35). However, the MLCT state of Cu(I) diimine complexes usually exhibited a “flattened” D₂ symmetry via Jahn−Teller (J-T) distortion (36), enabling the exposition of the Cu center to the solvent and susceptible to form the Cu(II)-solvent exciplexes (24). These structural features usually lead to a short-lived excited state and poor photochemical stability in Lewis basic solvents. The introduction of the steric functional group in the two and nine positions of the phenanthroline (Phen) ligands has been verified as efficient strategy to restrict J-T distortion and improve the excited-state property (37–40). In this field, it is of particular interest that [Cu(I)(dpp)₂]⁺ (dpp = 2,9-diphenylphenanthroline) exhibits a broadband visible absorption due to the π-stacking interactions between the phenyl groups and the opposite Phen ligands (27, 41–44). However, its poor light-absorbing intensity (ε = 3,800 M⁻¹ cm⁻¹ at 440 nm) severely impeded the utilization of solar energy. A strategy was proposed to decrease the energy level of the lowest unoccupied molecular orbital and increase the conjugated system by coupling the phenylethynyl group with Phen ligand for improving the visible-light absorption of [Cu(I)(dpp)₂]⁺ (ε = 10,000 M⁻¹ cm⁻¹ at 480 nm) (45–48). Despite the significant progress in this field, their visible light-absorbing ability and catalytic activity were scarcely comparable to that of noble-metal PSs (49). Based on the aforementioned challenges, ideal Cu(I) PSs should meet the following multiple requirements: SVLA ability (ε > 100,000 M⁻¹ cm⁻¹), broadband absorption, long-lived excited state, excellent photochemical stability, and suitable redox potential. To the best of our knowledge, such strong sensitizing Cu(I) PSs have never been explored for photosynthesis up to date.

In green plants, photosynthetic pigments with different absorption wavelengths, such as chlorophylls and carotenoids, can efficiently harvest solar light and then transfer the excitation energy to the reaction center (chlorophyll) for driving intercomponent electron transfer. To mimic the natural model, we coupled [Cu(I)(dpp)₂]⁺ with SVLA Bodipy derivatives via click reaction, resulting in a series of strong sensitizing Cu(I) PSs (Cu-1 to Cu-3). Especially, Cu-3 with multiple SVLA antennas can achieve double “ping-pong” energy-transfer processes in the nonprecious metal PS to break the energy level limitations of the current noble-metal complexes. As a result, Cu-3 presented dual strong absorbing peaks at 501 nm (ε = 140,400 M⁻¹ cm⁻¹) and 570 nm (ε = 174,000 M⁻¹ cm⁻¹) both with high molar extinction coefficients, representing the example of broadband and strong visible-light-absorbing (BSVLA) cuprous sensitizer. Systematic investigations reveal that the multiple energy transfer processes including forward resonance energy transfer (RET) and backward Dexter energy transfer in Cu-3 enabled a perfect synergism between multiple components, which contributed to significantly improving the visible-light utilization and extending the excited-state lifetime. Remarkably, Cu-3 can efficiently drive the photooxidation of 1,5-dihydroxynaphthalene (DHN) with a conversion rate (K_obs) of 190.78 × 10⁻³ min⁻¹, over 35 and 30 times higher than those of the typical PSs Cu-0 and noble-metal PSs Ru-1, respectively. Upon selective excitation of Bodipy and condensed Bodipy at 501 nm and 570 nm, respectively, Cu-3 with BSVLA ability exhibited much superior performance to that of the narrow band absorbing Cu-1 and Cu-2. In atom-transfer radical addition (ATRA) reactions, the photosynthesis yield of Cu-3 can reach as high as 97.7% within 10 h, which is over 10 and 14 times higher than that with Ru-1 (9.5%) and [Ir(ppy)₂(bpy)]⁺ (Ir-1) (6.8%), respectively. These results indicate a great advance in replacement of noble-metal PSs with strong sensitizing and earth-abundant PSs in the artificial photosynthesis. This work provided a different horizon to develop efficient noble-metal-free PSs by mimicking the biological processes for boosting photosynthesis.

Results

Synthesis and the Steady Spectra.

The synthetic detail of ligands (L-1 and L-2) and the organic intermediates are summarized in SI Appendix, Scheme S1. Alkynyl Bodipy and alkynyl condensed Bodipy were, respectively, coupled with Phen bearing azide group to afford the L-1 and L-2 ligands with flexible chain, which can increase collision probability between the Bodipy antenna and the Cu(I) coordination center for efficient intramolecular energy transfer. As shown in Scheme 1, three Cu(I) PSs (Cu-1 to Cu-3) can be facilely synthesized with high yield by controllable assembly of Cu(CH₃CN)₄PF₆ with L-1/L-2 and their mixture, respectively, under room temperature (SI Appendix, Scheme S2). All the intermediates, ligands (L-1 and L-2), and the Cu(I) complexes (Cu-0 to Cu-3) were systemically characterized by NMR spectroscopy and high-resolution mass spectrometry (SI Appendix, Figs. S1–S29).

Scheme 1.

Molecular design and structures. (Up) The molecular design by mimicking natural photosynthesis; (Down) the assembly process of Cu-1 to Cu-3 and the structure of ligands L-1 and L-2.

As shown in Fig. 1, the main absorption of the traditional cuprous sensitizer Cu-0 is mainly distributed in the ultraviolet region. By coupling with Bodipy and condensed Bodipy, the resulting PSs Cu-1 and Cu-2 exhibited strong absorption peaks at 501 nm (ε = 2.2 × 10⁶ M⁻¹ cm⁻¹) and 570 nm (ε = 2.7 × 10⁶ M⁻¹ cm⁻¹), respectively. Notably, Cu-3 with different chromophores presented two strong absorption bands between 400 nm and 600 nm, covering over 50% of the visible region with two high molar extinction coefficients of 140,400 M⁻¹ cm⁻¹ at 501 nm and 174,000 M⁻¹ cm⁻¹ at 570 nm, representing the example of BSVLA cuprous sensitizer. Such superior visible-light harvesting ability of these cuprous sensitizers greatly contributed to making full use of solar energy and boosting photocatalysis.

Fig. 1.

Photophysical properties of Cu(I) PSs. (A) Ground-state absorption of L-1/L-2 and Cu-0 to Cu-3. (B) Photoluminescence spectra of L-1,Cu-1, and Cu-3 with an excitation wavelength (λ_ex) of 500 nm. (C–F) Femtosecond transient absorption spectra of (C) the decay of Cu-1, λ_ex = 470 nm, (D) the decay of Cu-2, λ_ex = 550 nm, (E) the decay of Cu-3, λ_ex = 470 nm, and (F) the decay of Cu-3, λ_ex = 550 nm. Conditions: 5 μM in dichloromethane.

Emission spectra of the Cu(I) PSs (Cu-1 to Cu-3) and their ligands (L-1 and L-2) were investigated in order to evaluate energy transfer efficiency from Bodipy antenna to the Cu(I) coordination center (Fig. 1B and SI Appendix, Fig. S30). The L-1 and L-2 ligands exhibited strong emissive peaks at 516 nm and 590 nm, respectively, corresponding to the Bodipy part and condensed Bodipy part. After coordinating with Cu(I) centers, the steady emission and fluorescence lifetime of these ligands were both significantly quenched upon exciting the Bodipy antenna (SI Appendix, Figs. S30 and S31 and Table S1). The preliminary study indicates an efficient energy transfer from the Bodipy antenna to the Cu(I) coordination center with the estimated efficiencies of over 90%, which provides the possibility to fully utilize solar energy. This viewpoint was further supported by cyclic voltammetry and transient absorption spectra.

Cyclic Voltammetry.

Cyclic voltammetry of ligands (L-1 and L-2) and complexes (Cu-0 to Cu-3) was explored in order to evaluate the intra-/inter-molecular electron transfer ability of these Cu(I) PSs (SI Appendix, Fig. S32 and Table S2). The reduction potentials of Cu-1 and Cu-2 were determined to be -1.29 V and -1.17 V (vs. Saturated calomel electrode (SCE), respectively, well consistent with that of L-1 and L-2. The oxidation potentials of Cu-1 and Cu-2 were estimated as 0.66 V and 0.65 V (vs. SCE), respectively, which were well matched with that of Cu-0. Furthermore, the redox potentials of Cu-3 were estimated to be 0.64 V, −1.18 V, and −1.30 V (vs. SCE), respectively, close to the superimposition of that of Cu-1 and Cu-2. These results indicated that there is a weak electron interaction under the ground state between Bodipy units and the Cu(I) coordination center in the molecular array. As shown in SI Appendix, Table S2, the positive Gibbs free energy change ΔG_CS for PSs (Cu-1 to Cu-3) signified that the intramolecular electron transfer was thermodynamics infeasible from excited Bodipy units to the Cu(I) coordination center. Therefore, the fluorescence quenching of Bodipy antennas in PSs (Cu-1 to Cu-3) could be mainly attributed to the efficient energy transfer from the Bodipy antenna to the Cu(I) coordination center.

Femtosecond-/Nanosecond-Resolved Transient Absorption Spectra.

Femtosecond transient absorption was performed to unveil the intramolecular photophysical processes of Cu(I) complexes (Fig. 1 and SI Appendix, Figs. S33 and S34). As shown in SI Appendix, Fig. S33A, L-1 exhibited a positive absorption at 352 nm and a bleaching at 507 nm, and these two decays were determined to be approximately 3.8 ns and 3.5 ns, respectively, well matching with its fluorescence lifetime (4.9 ns at 516 nm). These results indicate that these two absorption peaks could be ascribed to the population of singlet excited state. Upon exciting the Bodipy part, the decays at 344 nm and 508 nm in Cu-1 were much faster than that in L-1, accompanied by a rise peak at around 380 nm (isosbestic point located at 360 nm), which could be attributed to the rapid singlet state energy transfer from the Bodipy part to the Cu(I) coordination center (SI Appendix, Fig. S33B). Intramolecular energy transfer rate constant in Cu-1 could be estimated to be 2.6 × 10¹⁰ s⁻¹ by detecting the evolution of the positive band at 380 nm. The photophysical processes of L-2/Cu-2 were similar to that of L-1/Cu-1. Two rapid decays at 484 nm and 570 nm and a rise process at 373 nm simultaneously occur (isosbestic point located at 412 nm), which indicate a fast energy transfer process from the condensed Bodipy to Cu(I) center in Cu-2. Compared to those of Cu-1 and Cu-2 bearing single chromophore, Cu-3 possesses two different chromophores with two strong absorption bands, which signifies a more complex intramolecular energy transfer process. Upon selective excitation of condensed Bodipy at 550 nm, the transient absorption spectrum of Cu-3 exhibited a similar evolution process to that of Cu-2, manifesting a rapid energy transfer from condensed Bodipy to the Cu(I) center. Upon photoexcitation at 470 nm, two bleaching bands were observed at around 500 nm and 570 nm, corresponding to the singlet excited state of Bodipy and condensed Bodipy, respectively. Notably, the excited-state absorption of Bodipy at 351 nm and 508 nm is significantly diminished with a concomitant increase of the bleaching peak at 570 nm and the positive peak at 373 nm. These results confirm the rapid singlet energy transfer from Bodipy to condensed Bodipy and the Cu(I) center. At longer times (20 to 800 ps, Fig. 1E), the bleach signals for condensed Bodipy decreased, accompanied by a concomitant increase of the positive absorption of the Cu(I) center at 515 nm. This result indicates an efficient energy transfer from condensed Bodipy to the Cu(I) center. Afterward, the absorption signal at 373 nm decreased with the increased absorption at 508 nm and 570 nm, signifying a redistribution of energy from the Cu(I) coordination center back to Bodipy and condensed Bodipy antennas in the form of triplet excited state. As shown in SI Appendix, Fig. S33G, upon excitation, the singlet state absorption of Cu-0 was observed at around 373 nm, further confirming the spectra assignment at around 373 nm for Cu-1 to Cu-3 PSs.

To further clarify the intramolecular energy transfer process and excited-state distribution of Cu(I) complexes, nanosecond-resolved transient absorption measurements were performed to elucidate the properties of the triplet excited states (Fig. 2). As shown in Fig. 2B and C, upon excitation of the Bodipy unit, Cu-1/Cu-2 exhibited a strong bleaching peak at around 500 nm/570 nm, well consistent with steady absorption of L-1/L-2, confirming that their triplet states were localized on the Bodipy/condensed Bodipy part. The triplet state lifetimes of Cu-1 and Cu-2 were, respectively, determined to be 78.2 μs and 122.0 μs, much longer than that of the typical Cu(I) complex Cu-0 (0.2 μs) (SI Appendix, Fig. S35). Therefore, it can be concluded that a “ping-pong” energy-transfer process occurs in Cu-1 and Cu-2, involving a forward singlet energy transfer from the Bodipy unit to the Cu(I) center and a backward triplet energy transfer from the Cu(I) center to the Bodipy unit. For Cu-3, it presented two strong absorption peaks at around 501 nm and 570 nm corresponding to Bodipy and condensed Bodipy, respectively. Upon selective excitation of condensed Bodipy with 580 nm, the transient spectrum well matched with that of Cu-2, indicating the triplet state population on the condensed Bodipy part. As a result, its excited-state lifetime was determined to be 169.8 μs. Upon excitation with 500 nm, two bleaching peaks at around 500 nm and 570 nm corresponding to the steady absorption of Bodipy and condensed Bodipy can be obviously observed (Fig. 2D). Accordingly, the triplet state of Cu-3 can be simultaneously populated on the Bodipy and condensed Bodipy parts, which is well consistent with that of femtosecond transient absorption (Fig. 1E). In addition, the triplet-state quantum yields for Cu-1 and Cu-3 were estimated to be 79% and 74.5%, respectively, which contributed to achieving the long-lived triplet state for efficient intermolecular electron/energy transfer.

Fig. 2.

Nanosecond transient absorption spectra. (A–E) Nanosecond transient absorption spectra of (A) Cu-0 after excitation at 440 nm, (B) Cu-1 after excitation at 500 nm, (C) Cu-2 after excitation at 560 nm, (D) Cu-3 after excitation at 500 nm, and (E) Cu-3 after excitation at 560 nm. (F) The molar extinction coefficients and triplet state lifetimes of Cu-0 to Cu-3. (G) Jablonski diagram of photophysics of the Cu-3. boron-dipyrromethene (BOD) and condensed BOD (cBOD) are, respectively, the Bodipy and condensed Bodipy in Cu-3. Cu is the Cu(I) coordination center in Cu-3. GS is ground state. c_PS = 5.0 μM in deaerated dichloromethane solution.

Based on the investigation of femtosecond/nanosecond-resolved transient absorption spectra, the intramolecular energy-transfer processes of Cu-3 were well clarified with well-defined triplet state properties. As shown in Fig. 2G, both Bodipy and condensed Bodipy in Cu-3 can efficiently harvest visible light to reach their singlet excited state, which can further transfer the excitation energy to the Cu(I) coordination center followed by an ISC process to attain the triplet state of the Cu(I) center. In addition, a RET process can occur from excited Bodipy to condensed Bodipy, which contributed to improving the utilization of excitation energy. Afterward, the backward energy transfer from the Cu(I) center to molecular antennas proceeded via Dexter mechanism to achieve the long-lived Bodipy/condensed Bodipy localized triplet states. Consequently, Cu-3 presented a BSVLA and long-lived excited state via delicate synergism among different components, highlighting its great potential for solar energy conversion.

Photooxidation and Photocatalysis.

Juglone possessed the high bioactivity, which can be used for irreversibly inhibiting peptidyl-prolyl cis/trans isomerases in the parvulin family (50). Here, we applied these earth-abundant Cu(I) PSs (Cu-0 to Cu-3) to drive photooxidation of 1,5-DHN to juglone for comparing their sensitizing ability (Fig. 3 and SI Appendix, Fig. S36 and Table S3). Upon irradiation with a 300-W xenon lamp (λ > 420 nm), Cu-3 exhibited an extremely high catalytic performance with a pseudo-first-order rate constant of 190.8 × 10⁻³ min⁻¹, over 35 and 30 times higher than those of the typical PSs Cu-0 and Ru-1, respectively. Remarkably, the Juglone yield with Cu-3 can reach 81.9% within 45 min, significantly higher than that with other PSs (Cu-0 to Cu-2 and Ru-1). Notably, the photooxidation ability of Cu-3 with broadband absorption was much higher than those of Cu-1 and Cu-2 with a single absorption band. As a result, the broadband absorbing PSs exhibited a higher efficiency for solar energy utilization than that with narrow absorbing PSs. Besides, the ligands (L-1 and L-2) and Bodipy unit presented a poor sensitizing ability, even lower than that of the typical PS Cu-0 (Fig. 3). Despite much stronger visible-light harvesting ability of the ligands compared to that of Cu-0, the short-lived excited state for L-1 and L-2 (< 5 ns) severely hindered the intermolecular energy transfer, leading to poor photooxidation ability. As a result, the advantages of Cu-3 in SVLA, broadband absorption, and long-lived excited state greatly contributed to improving the visible-light utilization and intermolecular energy transfer for boosting photooxidation.

Fig. 3.

Photooxidation of DHN. (A–C) Ultraviolet-Visible absorption spectral changes for DHN (0.15 mM) photooxidation with (A) Ru-1, (B) Cu-0, and (C) Cu-3 as singlet O₂ sensitizers. (D) Plot of ln(C_t/C₀) against irradiation time (t). (E) the juglone yields during 45 min irradiation. (F) The pseudo-first-order rate constant and the juglone yield with Cu-0, Cu-3, and Ru-1. c_PS = 5.0 μM in CH₂Cl₂/MeOH (9/1, v/v). Irradiated with a 300-W Xe lamp (20 mW cm⁻²) with a 420-nm filter.

In order to evaluate the influence of the intramolecular RET on the photooxidation efficiency, these Cu(I) PSs (Cu-0 to Cu-3) were selectively photoirradiated at the absorption wavelength of the Bodipy antenna (SI Appendix, Figs. S37 and S38). As shown in Fig. 4, the photooxidation ability of Cu-3/Cu-1 was much higher than that of Cu-2 upon excitation at 501 nm. This could be attributed to the strong absorption at around 501 nm for Cu-3/Cu-1. Upon irradiation with 570 nm, Cu-1 exhibited a poor sensitizing ability due to its negligible absorption at around 570 nm. Under this condition, the sensitizing ability of Cu-3/Cu-2 was more efficient than that of Cu-1. Remarkably, Cu-3 presented an excellent catalytic activity at both excitation wavelengths, confirming that the broadband absorbing made Cu-3 possess a much superior sensitizing ability than those of other Cu(I) PSs (Cu-1 and Cu-2) with a narrow absorbing band. In addition, the typical Cu(I) PS Cu-0 without the RET energy funneling effect exhibited very poor photooxidation ability at both excitation wavelength, revealing that the RET effect in the Cu(I) PSs (Cu-1 to Cu-3) is crucial to improve the photosensitizing ability. The cycling experiments of the photocatalytic reaction were performed with 1,3-diphenylisobenzofuran (DPBF) as substrate. As shown in SI Appendix, Fig. S39, the catalytic activity of Cu-3 was well maintained after five cycles, supporting Cu-3 as a stable PS during the photooxidation process.

Fig. 4.

Photooxidation of 1,3-DPBF. (A–C) Absorbance decrease of DPBF with illumination time in the presence of PSs: (A) Cu-2, (B) Cu-3, and (C) the comparison of the reaction rate with Cu-0 to Cu-3 as singlet O₂ sensitizers. (D–F) Absorbance decrease of DPBF with illumination time in the presence of PSs: (D) Cu-1, (E) Cu-3, and (F) the comparison of the reaction rate with Cu-0 to Cu-3 as singlet O₂ sensitizers. Upon excitation with (A–C) 501 nm, (D–F) 570 nm (0.25 mW cm⁻²), c_DPBF = 50.0 μM, and c_PS = 5.0 μM.

These strong sensitizing Cu(I) PSs can not only efficiently drive energy transfer reaction but also be compatible with electron transfer reaction. As a proof of concept, ATRA reaction was employed to investigate their sensitizing ability (Cu-0 to Cu-3) on electron transfer reaction (Fig. 5 and SI Appendix, Table S4) (51–53). Upon irradiation with >420 nm, the SVLA Cu(I) PSs (Cu-1 to Cu-3) can efficiently drive the addition of phenylethylene (1) to CBr₄ to afford the desired product 1a. Especially, the yield of 1a can reach as high as 97.7% in the presence of Cu-3, over 10 and 14 times higher than that with typical PSs of Ru-1 (9.5%) and Ir-1 (6.8%), respectively. Notably, Cu-3 with broadband absorption exhibited a much higher catalytic activities than single-band absorbing PSs (Cu-1 and Cu-2) for ATRA reaction.

Fig. 5.

Photocatalytic ATRA reactions. (A) ATRA reaction performance with different PSs. (B) Proposed mechanism for the ATRA reaction. (C) Yields of ATRA products in the presence of Ru-1, Ir-1, Cu-0, and Cu-3, upon irradiation with a 300-W Xenon lamp with >420 nm filter for 10 h.

Compared to the high activity of Cu-3, almost no product of 1a can be detected with Cu-0 as a photocatalyst, which could be ascribed to its poor visible-light absorption and short-lived excited state. Remarkably, Cu-3 presented an excellent tolerance to different substrates, much superior to that with PSs Cu-0, Ru-1, and Ir-1 (Fig. 5C). Furthermore, almost no product formed in the absence of PS or light, indicating that all the above factors were necessary for efficient photocatalysis. As shown in Fig. 5B, the excited Cu(I) PSs can transfer the electron to CBr₄ to produce an anion (Br⁻) and a radical (•CBr₃). The radical •CBr₃ can add to the alkene to afford the benzylic radical, which can subsequently donate the electron to [Cu-3]²⁺ to regenerate the PS. The resulting cationic compound can further recombine with Br⁻ to form the final ATRA product. The excited-state oxidation potentials of Cu(I) PSs were much more negative than the reduction potential of CBr₄ (−0.48 V vs. SCE), indicating the thermodynamically admissible for the electron transfer from excited Cu(I) PSs to CBr₄ (SI Appendix, Table S2). As a result, the advantages of Cu-3 in SVLA, broadband absorption, long-lived excited state, excellent photochemical stability, and suitable redox potential can facilitate the intermolecular electron transfer and solar energy utilization to further greatly boost photocatalysis.

Discussion

Inspired by natural photosynthesis, the BSVLA cuprous sensitizer (Cu-3) was developed by utilizing a universal Cu(I) center to couple with multiple SVLA antennas via the double “ping-pong” energy-transfer processes. Upon visible-light irradiation, Cu-3 exhibited dual strong absorbing peaks between 430 nm and 620 nm with two extremely high molar extinction coefficients of 140,400 M⁻¹ cm⁻¹ at 501 nm and 174,000 M⁻¹ cm⁻¹ at 570 nm, corresponding to the π–π* transition of Bodipy and condensed Bodipy, respectively. The excitation energy of Bodipy antennas can be efficiently transferred to the Cu(I) center via RET to further switch the singlet state to triplet state via the ISC process. Afterward, a backward triplet energy transfer from the Cu(I) center to Bodipy/condensed Bodipy antennas occurs to afford the long-lived triplet states. Consequently, the delicate synergism among different components in Cu-3 can greatly improve its solar light utilization and extend its excited-state lifetime for efficient photocatalysis. Remarkably, the catalytic performance of Cu-3 was much higher than that of the typical noble metal PSs (Ru-1 and Ir-1) for both energy- (over 30 times) and electron-transfer reactions (over 10 times). In addition, Cu-3 with broadband absorption presented a much superior sensitizing ability to those of Cu-1 and Cu-2 with narrow absorbing bands. This work opens up a different avenue to construct earth-abundant PSs with BSVLA ability by imitating the natural model for boosting artificial photosynthesis.

Materials and Methods

Full experimental details and procedures for the synthesis of the compounds used in the present study, molecular structures, and details of Cu-0 to Cu-3 are described in SI Appendix.

Supplementary Material

Appendix 01 (PDF)

Data, Materials, and Software Availability

All study data are included in the article and/or SI Appendix.