Stabilizing Excited-State of Organic Phosphorescent Photosensitizers via Self-Assembly for Boosting Photoreduction of CO2-to-CO

doi:10.21203/rs.3.rs-5438801/v1

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Stabilizing Excited-State of Organic Phosphorescent Photosensitizers via Self-Assembly for Boosting Photoreduction of CO₂-to-CO

https://doi.org/10.21203/rs.3.rs-5438801/v1

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The development of pure organic photosensitizers remains challenging due to low intersystem crossing (ISC) efficiency and the instability of triplet excitons. Herein, a series of organic phosphorescent molecules are prepared as the photosensitizers. The extensive conjugation of fused-ring cores enhances visible-light absorption, while heteroatom-rich structures promote ISC process, effectively generating triplet excitons. 2,3,5,6,9,10-Hexabutoxy-8-phenyldithieno-triphenyleno-pyridine (TPy) exhibits high ISC efficiency and can sensitize Fe-catalyst efficiently for photoreduction CO₂-to-CO. A novel strategy of doping phosphorescent molecules into the core of amphiphilic nanoparticles (NPs) formed by polystyrene-block-poly(ethylene glycol) (PS-b-PEG) was further developed. PS, the core of the NPs, can stabilize the triplet excitons generated by TPy due to its rigid polymer backbone, resulting in enhanced photocatalytic efficiency. Meanwhile, PEG serves as the corona of the NPs, improving stability and dispersibility. Notably, this self-assembled strategy increases the sensitization efficiency of TPy (TON_CAT = 2041, 51.02 µmol CO) by nearly 50%, and even after three catalytic cycles, 93.5% of the original sensitization ability is retained. This result opens a new avenue for developing novel molecular organic photosensitizers and stabilizing their excited-states for efficient application in photoredox reactions.

Physical sciences/Chemistry/Materials chemistry/Optical materials

Physical sciences/Chemistry/Photochemistry

Artificial photosynthesis has attracted much academic and industrial interest for utilizing solar energy, avoiding harsh reaction conditions, and reducing the formation of side reactions.^1-4 As a key mediator in photocatalytic systems, the development of photosensitizer is a hot research topic. The use of organometallic complexes containing noble metals (such as Ru(bpy)₃²⁺ and Ir(ppy)₂(bpy)⁺) has been critical to photocatalysis advancements.^5-9 These complexes are capable of absorbing visible-light to initiate electron or energy transfer reactions from their reactive excited states.^5,6 The impressive performance of these photosensitizers arises from their photophysical properties, including excellent visible-light absorption and long-lived excited states that can engage in electron transfers.^6,10,11 However, the development of organometallic complexes is limited by their high cost and complex preparation.^12-15 As a result, the development of pure organic photosensitizers would be highly desirable to drive various photocatalytic reactions.^16,17 Generally, spin-forbidden relaxation to the ground state prolongs the lifetime of a triplet state compared to the spin-allowed singlet state.^18-20 This provides enough time to mediate intermolecular electron transfer processes. However, most organic molecules only exhibit the short lifetime transitions from the lowest singlet state to the ground state due to the forbidden intersystem crossing (ISC) process.^20,21 Rational functionalization of boron dipyrromethene and aminoanthraquinone chromophores to break the short-lived excited state limitation has been achieved for photocatalytic reactions.^22,23 However, it is still a significant challenge to develop new, efficient, and stable organic photosensitizers with long-lived triplet excitons.

Organic room temperature phosphorescence (RTP) is a recently reported fascinating phenomenon in which an afterglow emission lasts for several seconds or even minutes after removal of the irradiation owing to the presence of an ultralong-lived excited state.^20,24-26 The similarity in design between phosphorescent molecules and organic photosensitizers demonstrates that phosphorescent molecules are potential and strong candidates for organic photosensitizers. The abundance and longevity of the triplet excitons in these RTP materials allow pure organic compounds to serve as metal-free redox organic photosensitizers for photoreactions. This is because the photoreaction rate constant may be faster than the phosphorescence decay constant after sufficient interaction between the reaction substance and RTP materials.^27-29 However, the utilization of RTP materials for photoredox reactions (e.g., CO₂ to CO conversion) has not been achieved yet. In addition, understanding the relationship between RTP characteristics and photosensitizing properties remains a challenge. Studying this relationship may provide a great opportunity for the high-throughput pre-estimation of photosensitizing properties in organic systems using spectroscopy. Our work in the field of organocatalytic photoreactions began with our interest in the modulation of triplet exciton decay in organic RTP materials.^30,31 Host-guest doping strategies achieve high-performance RTP by effectively facilitating the ISC processes and stabilizing triplet excitons.^24,32-34 In combination with nanonization preparation, the stable triplet excitons in the formed doped-RTP nanoparticles (NPs) are not dissipated through radiative decay, but instead can transfer electrons or energy to surrounding medium, such as ³O₂, for photodynamic antibacterial treatment.³⁵ Therefore, we proposed a strategy of doping the phosphorescent molecules into the core of NPs to construct advanced photosensitizers. The phosphorescent molecules can generate abundant triplet excitons, which can be stabilized by the rigid core of NPs. Under irradiation conditions, electron transfer can occur in NPs, thus improving photocatalytic efficiency.

In this work, we designed and prepared host-guest doped NPs for visible-light-driven reduction of CO₂to CO (Scheme 1). A series of polycyclic aromatic hydrocarbons (PAHs) with buckybowl-based structure was selected as guest candidates. Heteroatoms with abundant long-pair electrons, including oxygen, nitrogen, and sulfur were introduced into the PAHs, leading to the S₁ (n, π*) to T_n (π, π*) spin flip transitions (EI-Sayed rules) and facilitating the ISC process to generate triplet excitons.^20,26,36 The dissymmetry electrostatic potential distribution on the concave and convex surfaces of the buckybowl structures enables π-π interaction with the aromatic-containing polymer matrix.^31,37 Therefore, polystyrene (PS) was used as a host to stabilize the triplet excitons, achieving RTP emission (Scheme 1a). Among the selected PAHs, 2,3,5,6,9,10-Hexabutoxy-8-phenyldithieno-triphenyleno-pyridine (TPy) showed the highest triplet exciton generation and the best RTP performance in the PS matrix. Further, to utilize and manipulate the triplet excitons stabilized by PS, self-assembled TPy NPs were prepared by using PS-block-poly(ethylene glycol) (PS-b-PEG) polymer (Scheme 1b). TPy was dispersed into the hydrophobic PS segment to avoid molecular aggregation and motion, which further stabilized the triplet excitons and prolonged their lifetime. The hydrophilic PEG corona made NPs disperse uniformly in the catalytic system, avoiding triplet annihilation caused by aggregation. Thus, the catalytic efficiency of NPs-containing catalytic system was improved by 50% compared to TPy. An impressive turnover number (TON vs. catalyst) of the optimized photocatalytic CO₂-to-CO system was 2041 with 51.02 μmol CO yield under 18 h visible-light illumination. Remarkably, after three catalytic cycles, the sensitization capacity was effectively restored to 93.5%, realizing a highly sustainable CO₂ photoreduction process via a reductive route. Our research clarifies the relationship between the characteristic of RTP and the sensitization ability of photosensitizers, and provides a new idea for the development of high-efficiency and stable organic photosensitizers.

Photophysical properties and theoretical analysis. A series of PAHs with buckybowl-based structure was prepared in accordance with our prior research^38–41 and denoted TP, TPy, TH, and TQ (Fig. 1a). All the PAHs demonstrate visible-light absorption (1×10^− 5 M in THF, Table S1), indicating their ability to perform photochemical reactions under natural light. The absorption and fluorescence characteristics of these PAHs exhibit a bathochromic shift from TP to TQ, which is attributed to the extension of π-conjugation (Fig. 1b, Table S2).⁴². This indicates the tunable energy level of the designed phosphors. Delayed emission spectra were recorded in 2-MeTHF (1 × 10^− 5 M) at 77 K (Fig. 1c). Notably, TP, TPy, and TH display phosphorescence emission at 77 K, while TQ shows no phosphorescence emission. Thus, TQ is not capable of generating triplet excitons. In this context, assessing the triplet exciton yield of PAHs is pivotal because this parameter is related to the sensitization efficiency of photosensitizers.

Theoretical calculations facilitate an understanding the photophysical properties of the PAHs. Highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of PAHs are distributed throughout the molecule, indicating no obvious intermolecular charge transfer (Figure S1). With the increase of conjugation, the energy gap (E_g) between HOMO and LUMO decreases continuously, which is consistent with the UV-vis absorption spectra (Fig. 1b). To gain further insight into electronic transition process of the molecule, the excited state PAHs were calculated based on time-dependent density functional theory (TD-DFT). The hole-electron population of each state was evaluated using Multiwfn 3.8 package.⁴³

The evaluation of the ISC ability of a molecule can be conducted through a joint analysis of the spin-orbit coupling (SOC) values and the energy difference (ΔE_ST) between single-triplet states.^44–46 The lowest singlet state (S₁) and the lowest triplet state (T₁) of PAHs have the same orbital configuration with a large proportion, suggesting an efficient ISC channel (Table S3-6). The SOC constants < S₁|H_SO|T₁ > of TP, TPy, TH, and TQ are 0.16, 1.38, 0.71, and 0.74, respectively. Additionally, the electron-hole distribution of S₁ and T₁ of PAHs is highly similar (Figure S2), which further ensures that the singlet excitons are easily spin-flipped into triplet excitons. TP, TPy, and TH exhibit small ΔE_ST values of 0.15 eV, 0.20 eV, and 0.17 eV, respectively (Fig. 1d), implying that the presence of additional ISC channels. In contrast, TQ demonstrates a large energy difference (0.78 eV), significantly impeding the ISC process, despite the comparable S₁ and T₁ electron-hole distributions in TQ molecules. Thus, TQ do not emit phosphorescence, which aligns with the delayed spectrum observed for TQ at 77 K. Consequently, TPy exhibits the highest SOC constants and generates the most triplet excitons, suggesting that TPy possesses the best photosensitizing ability among the four PAHs.

PAHs photosensitizers for photocatalytic CO ₂ reduction. To reveal the effects of the photophysical processes in the four PAHs on CO₂ photoreduction, photocatalytic activities of these molecules were investigated in 5 mL CO₂-saturated CH₃CN/H₂O (v/v = 4/1) solution containing [Fe(qpy)(OH₂)₂]²⁺ catalyst (referred to as CAT) and electron donors (EDs): triethylamine (TEA) and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH).^47,48, TEA is used as a deprotonated Brønsted base to ensure the irreversibility of the more potent sacrificial BIH.⁴⁹ The TON_CAT of TPy-containing system is as high as 896 (22.41 µmol CO), followed by TH, while the TP and TQ molecules are almost inactive (Fig. 2a). These results reveal that the efficient ISC processes of TPy results in active photosensitizing properties for CO₂ photoreduction. Importantly, the organic fused-ring molecule with the high structural stability and a large π-conjugation for charge delocalization³⁸ after the photoredox showed great advantage for CO₂ photoreduction.

Further, nanosecond transient absorption spectra of kinetic decay were employed to understand the photosensitizing process of the organic phosphors in CO₂ photoreduction system. The TP, TPy, and TH molecules show long-lived triplet excitons capable of sufficient intermolecular electron transfer between the photosensitizers and photoreaction substances, with triplet exciton lifetimes of 74.56 µs, 48.74 µs and 102.58 µs, respectively (Fig. 2b). These findings establish the close relationship between the capacity of PAHs to generate triplet excitons and photosensitization efficiency, particularly in the presence of long-lived triplet excitons. Moreover, the stability of the triplet excitons is a vital factor in ensuring sufficient time for intermolecular electron transfer to boost photosensitization. TPy with the highest ISC efficiency also leads to a relatively short triplet excited state lifetime, potentially impairing the sensitization ability. In addition, TPy shows poor solubility in the photocatalysis solution and gradually aggregates (Figure S3), leading to a decline in photocatalytic efficiency. Therefore, stabilizing both TPy molecules in solution and its triplet excitons may further promote photosensitization efficiency.

RTP phenomenone of the doped system. A polymer host can effectively inhibit molecular motion and stabilize triplet excitons.^{24,25,50–52} Herein, inspired by the preparation of organic RTP materials in our previous work,⁵¹ PS was chosen as a polymer matrix, and a series of TPy@PS doped films with different mass ratios were prepared. In prompt spectra, the highest luminescence peak is observed at a mass ratio of 1:75 (Figure S4), attributed to the phosphorescence peak of molecularly dispersed TPy at 620 nm (Fig. 1c). Of note, TPy molecule shows no phosphorescence at room temperature (Figure S5). This indicates that PS is capable of acting as a rigid matrix to inhibit the non-radiative transition of TPy, thereby stabilizing the triplet excitons and facilitating RTP. The robust interaction between the phenyl ring in PS and PAHs can avoid strong π-π stacking interaction between photosensitizers, enhancing triplet exciton stability.³¹ Therefore, a guest@host doping mass ratio of 1:75 was used for subsequent experiments. In vacuum delayed spectra, TPy@PS shows a strong phosphorescence emission (Fig. 2c, Figure S6), with a phosphorescence quantum yield (QY) is up to 14.84% (Fig. 2d). In comparison, TP@PS and TH@PS have very weak phosphorescence (Figure S7), which further indicates their weak triplet exciton generation ability. TPy@PS has the highest phosphorescence efficiency, followed by TH@PS and TP@PS (Fig. 2d), which is consistent with theoretical calculations of PAHs (Fig. 1d). These results suggest that utilizing PS as a polymer matrix can stabilize the triplet excitons, leading to improved RTP emission. Importantly, this straightforward doping strategy enables the rapid screening and evaluation of RTP materials as efficient photosensitizers using spectroscopy.

Photophysical properties of self-assembled NPs. To further utilize the triplet excitons stabilized by the PS matrix for CO₂ photoreduction, TPy and the amphiphilic block copolymer PS-b-PEG were assembled via microfluidic technology (Fig. 3a).^35,53 The properties of NPs are highly related to their size and morphology.^17,54,55 Large NPs may prevent the transfer of triplet excitons to the reaction substance, while triplet excitons may not be stable enough in small NPs.³⁵ It is vital for the preparation of monodisperse NPs with controllable aggregates and high uniformity by microfluidic technology. During the self-assembly process, the hydrophobic PS segment within PS-b-PEG acts as the host, wrapping TPy molecules and stabilizing the triplet excitons. Meanwhile, the hydrophilic PEG segment enables the stable dispersion of the NPs in water. TPy do not emit phosphorescence in either oxygenated or deoxygenated water (Fig. 3b). Notably, the NPs also do not emit phosphorescence in oxygenated water, but emit in argon-deoxygenated water. This phenomenon shows that the hydrophobic PS segment is capable of stabilizing the triplet excitons without hindering the interaction of TPy with various components in the solution, thus manipulating energy decay of triplet excitons. Consequently, the capacity of NPs to generate triplet excitons is evaluated by their phosphorescence emission intensity in deoxygenated water.

Modifying the hydrophilic-hydrophobic ratio of the amphiphilic polymer can tune the particle size of the self-assembled NPs.^55,56 Therefore, a series of diblock polymer was prepared with different PS_15k-b-PEG_xk hydrophilic-hydrophobic ratio (x = 2, 5, 10, 23, 35). With increasing hydrophilic ratio, the size of the NPs becomes smaller (Figure S8). However, when x = 2, insufficient hydrophilic segments result in the formation of unstable NPs that are prone to agglomeration and precipitation in the solution and during the catalytic process. Changing the hydrophilic PEG segment from 2k to 23k in deoxygenated water leads to a significant enhancement of phosphorescence intensity and a prolonged phosphorescence lifetime (Fig. 3b, Figure S9). However, an excessive hydrophilic ratio diminishes the hydrophobicity of the PS segment, causing loose wrapping and a subsequent decrease in phosphorescence intensity and lifetime. This trend is further validated by total and phosphorescence QYs, confirming that NPs-23k generate the highest triplet excitons (Fig. 3c). In addition, the the UV-vis absorption test shows that the structural stability of NPs-23k is significantly higher than that of TPy under long-term irradiation (Figure S10), which clearly indicates the protection and stability of PS for the TPy structure. Finally, the hydrodynamic diameter of NPs-23k (~ 160 nm, polydispersity index: 0.19) is determined by dynamic light scattering (DLS). The insert transmission electron microscope (TEM) image shows the spherical morphology of NPs-23k (~ 110 nm) in dry state (Fig. 3d, Figure S11). These experiments confirm that NPs-23k exhibit excellent photobleaching resistance, uniform particle size, good dispersion, large surface area, and the highest yield of triplet excitons. Consequently, it can be reasonably inferred that NPs-23k may exhibit a superior sensitization effect on the reactants in the catalytic system.

Self-assembled NPs for photocatalytic CO ₂ reduction. Finally, the performance of NPs-23k for photocatalytic CO₂ reduction was investigated. NPs-23k generates long-lived triplet excitons with a lifetime of 96.19 µs, which is twice of TPy molecule (Fig. 2b, 4d). Under the same conditions, the catalytic efficiency of NPs-23k-containing system is increased by nearly 50% compared to that of TPy, the CO yield can reach 33.40 µmol with a TON_CAT value of 1336 (Figure S12). However, the catalytic effect of the system without a filter is inferior. The CO yield of the NPs-23k-containing system is only 7.72 µmol (Figure S13), a value comparable to that observed under irradiation with a 395 nm monochromatic light source (Figure S14). This observation was attributed to the disruption of the organic structure caused by prolonged exposure to high-energy ultraviolet radiation. The above findings demonstrates that utilizing PS-b-PEG with host function effectively stabilizes the triplet excitons of TPy. Moreover, after self-assembly, spherical NPs-23k are uniformly dispersed in the catalytic system (Figure S15), which substantially increases the surface area for electron transfer to the surrounding medium. Additionally, in contrast to visible TPy aggregates (Figure S3), monodisperse NPs-23k can reduce exciton losses caused by triplet exciton annihilation in aggregates⁵⁷, thus improving the sensitization efficiency. Next, the concentrations of the photosensitizer and EDs in the photocatalytic system were optimized (Table 1). The quantification of the gaseous products shows that the C/H selectivity of photoreduction CO₂ to CO is about 95% under all conditions. The TON_CAT of NPs-23k-containing system reaches 2041 (51.02 µmol CO) within 18 h illumination (100 mw∙cm^− 2, λ > 400 nm) under the optimal conditions (Table 1, Group 2). Clearly, the simple self-assembly method employed in this work endows TPy with more stable and longer-lived triplet excitons. This enables the facile transfer of electrons to surrounding compounds, thus favoring the photosensitization process.

Table 1

The results of photocatalytic reduction of CO₂ to CO.^[a]
Group		Photo-sensitizers (µM)	BIH (mM) + TEA (mM)	CO (µmol)	H₂ (µmol)	TON_CO	TOF_CO^[b] (h^− 1)	Sel_CO (%)
1	NPs-23k	10	18 + 0.3	26.50	1.51	1060	132.7	94.6
2	NPs-23k	20	18 + 0.3	51.02	2.77	2041	255.5	94.9
3	NPs-23k	20	36 + 0.6	33.13	1.94	1325	166.0	94.5
4	TPy	10	18 + 0.3	17.54	0.91	702	89.2	95.1
5	TPy	20	18 + 0.3	33.77	1.63	1351	171.7	95.4
^[a]In 5 mL CO₂-saturated CH₃CN/H₂O (v/v = 4/1) solution with 5 µmol CAT under irradiation with a 300 W Xenon lamp (100 mw·cm^− 2, λ > 400 nm, irradiation time: 18 h). ^[b]6 h

To investigate the stability of catalytic system under irradiation, recycle experiments were performed by re-adding NPs-23k, CAT, and EDs after 6 h irradiation (Figure S16). Following the addition of CAT and EDs in the 2nd cycle, the photocatalytic activity of the reaction system is almost entirely restored. This indicates that the inactivation of the photocatalytic system is primarily caused by the decomposition or consumption of CAT and EDs. To assess the stability of TPy and NPs-23k, consumed CAT and EDs were re-added after each 6 h cycle. As shown in Fig. 4a, after three cycles (18 h), the sensitization ability of NPs-23k remains at 93.5%, while that of TPy is only 74.9%. Remarkably, the total CO yield of the NPs-23k-containing system reaches 113.6 µmol. This indicates that the stability of NPs-23k photosensitizers is higher than that of TPy, because the vibrational relaxation of TPy is not suppressed in the absence of the PS coating, which leads to declining stability under long-term irradiation. These results further emphasize the significance of assembling TPy with PS-b-PEG. The self-assembly strategy prolongs the triplet excitons lifetime, enlarges the area of electron transfer with the medium, prevents the loss of triplet excitons, thus effectively improving the photosensitization cyclic ability of the catalytic system.

Mechanism analysis of photocatalytic system. In the absence of photosensitizer (TPy/NPs-23k), CAT, EDs, light, or CO₂, trace or no CO was detected (Table S7), indicating all of these factors are indispensable for CO₂ photoreduction. Moreover, to confirm the source of CO, isotope-labeling experiments with ¹³CO₂ as the carbon source were performed, and ¹³CO was detected as the main product (Figure S17). These results prove that the CO product is indeed generated from CO₂ instead of the decomposition of organic photosensitizers. To elucidate the photocatalytic mechanism, the thermodynamic feasibility of intermolecular electron transfer was evaluated, and cyclic voltammetry (CV) was conducted to study the key components of the reaction system (TPy, CAT, and BIH) in a degassed acetonitrile solution (Figure S18, Table S8). The oxidation potential of BIH is far more negative than the excited state reduction potential of TPy (*Red) (0.52 V < < 1.95 V vs. SCE), and the excited state oxidation potential of TPy (*Ox) is more negative than the reduction potential of CAT (-1.21 V < -1.05 V vs. SCE) (Fig. 4b). The results indicate that both the reduction and oxidation quenching mechanisms are thermodynamically feasible in the studied photocatalytic system. However, the potential difference for the reduction quenching mechanism is greater than that for the oxidation mechanism (-1.43 V vs. -0.16 V). In addition, the absorption spectra of TPy remained unchanged before and after adding the BIH or CAT (Figure S19), revealing the absence of intermolecular electronic interaction between TPy and CAT or BIH under ground state. Phosphorescence quenching experiments were performed with NPs-23k using CAT or BIH as quenchers. At the concentration scale of CAT and BIH in the photocatalytic system, the phosphorescence intensity of NPs-23k is more efficiently quenched by BIH compared to CAT (Fig. 4c). These findings suggest that the photocatalytic process is primarily governed by the reduction mechanism, which was further supported by nanosecond transient absorption studies (Figure S20, Fig. 4d and 4e).

Nanosecond transient absorption spectra were utilized to probe the excited state properties and intermolecular electron transfer of NPs-23k and TPy. As shown in Figure S20a and b, a strong bleaching peak below 350 nm is observed upon pulsed laser excitation, while positive peaks at 360 nm and beyond 450 nm correspond to the excited states of NPs-23k and TPy.^6,22 The excited state map of NPs-23k is similar to that of TPy, indicating that the excited state type of TPy remains unchanged after self-assembly. When BIH is added to the NPs-23k solution, the peak at 360 nm shows a lower intensity and narrower peak width (Figure S20b and c). In addition, the positive signal above 530 nm vanishes and a new short negative peak appears at 410 nm. This new peak is assigned to the reduced NPs-23k with a long lifetime of 191.48 µs (Figure S20c, Fig. 4e). The subsequent introduction of CAT reduces the lifetime of NPs-23k to 13.27 µs, indicating rapid electron transfer from the reduced NPs-23k to CAT, and the excited state map of the entire system is restored (Fig. 4e, Figure S20b and d). Analogous observations are made in TPy-containing systems (Figure S21 and 22). Consequently, the electron transfer route of the NPs-23k-containing and TPy-containing photocatalytic systems are all classified as reduction mechanism (Fig. 4f).

We propose a novel strategy for self-assembly of phosphorescent molecules with the amphiphilic block copolymer PS-b-PEG to construct stable and efficient photosensitizers. Specifically, at the core of doped NPs-23k, TPy has the highest ISC efficiency owing to extended conjugation structure and abundance of heteroatoms, as proven by systematic experiments and theoretical calculations, and can sensitize the photoreduction CO₂-to-CO catalytic system. Meanwhile, the hydrophobic PS segment with host function stabilizes the triplet excitons of TPy, doubling their lifetime to 96.19 µs and improving the catalytic efficiency by 50%. Additionally, the hydrophilic PEG segment as corona of self-assembled NPs-23k stably disperses the NPs to prevent aggregation-induced triplet annihilation, ensuring the maximum utilization of triplet excitons of TPy. This self-assembly method endows NPs-23k with better photosensitivity, achieving a TON_CAT of 2041 (51.02 µmol) with 18 h illumination, and improves the stability of NPs-23k, maintaining 93.5% photosensitivity after three cycles, and obtaining a CO yield of 113.6 µmol. Finally, NP-23k/TPy-containing photocatalytic systems are determined as reduction mechanism. The strategy employed in this work provides novel insights for transforming RTP materials with high-phosphorescence QY into efficient photosensitizers, greatly expands the number of organic photosensitizers in photocatalytic systems, and provides the possibility for organic photosensitizers to be used in homogeneous pure water catalysis

Photocatalytic CO ₂ reduction. Photocatalytic CO₂ reduction was conducted under the illumination of a 300 W Xenon lamp (50 mw·cm^− 2, λ > 400 nm) in the presence of PAHs, CAT, BIH, and TEA in 5 mL CO₂-saturated CH₃CN/H₂O (v/v = 4/1) solution. The following components (in the order indicated), including BIH, photosensitizers, H₂O, CAT, CH₃CN, and TEA were added to the catalytic system. The photocatalytic system was bubbled with CO₂ for 20 min.

Preparation of doped films. Preparation of PS matrix: PS powder (1 g) was dissolved in THF (10 mL) at room temperature with magnetic stirring for 24 h to obtain a homogeneous solution (100 g·L^− 1), followed by filtering with 0.45 µm syringe filter. Preparation of PAH@PS doped films: According to the different mass ratio of host to guest, PAH (guest) was dispersed in 1 mL PS (100 g·L^− 1 THF solution). A homogeneous solution was obtained after 10 h magnetic stirring. Then, the films were fabricated with 500 µL solution on each quartz plate (1.5 × 1.5 cm) by drop-coasting method. Finally, solid films with different mass ratio were obtained after heating at 40°C for 5 h.

Preparation of nanoparticles. Amphiphilic block copolymers (PS_15k-b-PEG_xk, x = 2, 5, 10, 23, 35) were purchased from Xi’an Ruixi Biological Technology Co., Ltd.. TPy and PS_15k-b-PEG_xk (x = 2, 5, 10, 23, 35) were dissolved in THF solution. The mass ratio of TPy to hydrophobic PS_15k segment is 1: 5. Self-assembled NPs-xk were prepared in microfluidic chips. Basically, the organic solution and aqueous solution (DI water) were introduced separately into the microfluidic chip through two inlets, and the syringe pump drove two solutions to mix at the microscale with a controlled velocity, leading to the formation of nanoparticles. The microfluidic chip is composed of a glass substrate and a polydimethylsiloxane (PDMS) membrane, and the structure of the microchannel was designed with two inlets (width: 150 mm, height: 150 mm, length: 1.5 cm and 1.0 cm) and one outlet (width: 300 mm, height: 300 mm and linear length: 3 cm).

Competing interests

We declare no conflict of interest.

Author contributions

Z.X.C, Z.M.Z and X.F.S conceived and designed this project. C.C.X, P.W. and Y. M. performed the experiments, C.C.X. and Y.F.Z carried out the DFT calculation, C.C.X, P.W., C.C., X.C, L.L.K. and G.C.L analyzed the data, C.C.X and W.P wrote the article, P.S, J.B.S, X.F.S, Z.M.Z, Z.X.C and Y.P.D. revised the article.

Acknowledgements

The authors acknowledge the National Natural Scientific Foundation of China (Grant Number: 52473290, 22222501, 22175023 and 22401017), the Natural Science Foundation of Beijing Municipality (2232022 and 2242060). This work was also supported by BIT Research and Innovation Promoting Project (2023YCXZ016) and China Postdoctoral Science Foundation (2023M740242).

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image1.png
Scheme 1. (a) Chemical structures of PS (host) and TPy (guest) and design strategy of PS-based organic RTP materials. (b) Schematic diagram of photophysical process of self-assembled NPs for CO₂ photoreduction.
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Stabilizing Excited-State of Organic Phosphorescent Photosensitizers via Self-Assembly for Boosting Photoreduction of CO₂-to-CO

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