First Author: Xuan Yang
Corresponding Authors: Daming Zhao, Liejin Guo
Affiliations: Sun Yat-sen University; Xi’an Jiaotong University
DOI: 10.1002/anie.3903637
Recently, the research team led by Academician Liejin Guo and Associate Professor Daming Zhao from the School of Advanced Energy, Sun Yat-sen University, achieved an important breakthrough in the field of photocatalytic oxygen evolution reaction, or OER, based on organic polymer photocatalysts.
The team proposed a molecular design strategy based on bridge-bond geometric isomerization and successfully synthesized three perylene diimide, or PDI, based linear conjugated polymers, also known as LCPs. Among them, cis-maleamide-PDI, referred to as MA-PDI, exhibited outstanding photocatalytic oxygen evolution activity, achieving an apparent quantum yield, AQY, of up to 13.4% under 380 nm irradiation. This performance surpasses that of most previously reported organic polymer photocatalysts.
By combining theoretical calculations with in situ infrared spectroscopy, the study revealed that isomerization-induced high crystallinity, compact π-stacking, and enhanced built-in electric fields synergistically accelerate exciton dynamics and activate carbonyl active sites on the bridge chains.
This work provides a new isomerization-based design concept for developing high-performance organic photocatalysts. The related results were published in Angewandte Chemie International Edition under the title “Isomerization-Accelerated Exciton Kinetics in PDI Linear Conjugated Polymers for High-Efficiency Photocatalytic Oxygen Evolution.”
Photocatalytic clean energy production is one of the key technologies for addressing the growing global demand for sustainable energy. At present, considerable progress has been made in reductive photocatalytic processes such as photocatalytic water splitting for hydrogen production and CO₂ reduction. However, the oxygen evolution reaction, OER, which is coupled with these reduction reactions, remains relatively underdeveloped.
OER involves a four-electron transfer process and has a high kinetic energy barrier, making it the rate-determining step in overall water splitting. Therefore, more attention should be paid to the development of efficient OER photocatalysts.
Perylene diimide, PDI, based linear conjugated polymers, LCPs, have shown great potential in OER due to their low cost, tunable optoelectronic properties, strong light absorption, and deep valence band positions, which provide strong oxidative capability.
However, similar to other organic semiconductor polymers, PDI-based polymers suffer from strong Coulombic interactions, resulting in high Frenkel exciton binding energies. This leads to inefficient exciton dissociation and charge transport, severely limiting their photocatalytic performance.
Therefore, accelerating exciton dynamics and promoting charge separation at the molecular level remain key challenges in this field.
The equipment used in this study was the Perfectlight GAS1000 Micro Gas Reaction Evaluation System.
This system was developed by Beijing Perfectlight Technology Co., Ltd. based on years of research and development experience in airtight glass reaction systems. It is designed to meet the experimental requirements of photocatalytic reactions and is suitable for catalytic processes involving trace gas generation, such as photocatalytic water splitting and photocatalytic CO₂ reduction.
By regulating the cis-trans isomerism of bridge bonds, namely cis-maleamide and trans-fumaramide, together with saturated succinamide as a control structure, the researchers synthesized several PDI-based LCPs.
In the cis-structured MA-PDI, the head-to-head carbonyl configuration generates a strong dipole moment, forcing the polymer chains to form compact π-stacking and high crystallinity. This further enhances the built-in electric field and significantly improves charge separation and migration efficiency.
MA-PDI achieved an AQY of 13.4% under 380 nm irradiation, which is much higher than those of the trans-isomer FU-PDI and the saturated side-chain control polymer SU-PDI. Its performance also exceeds that of most reported organic photocatalysts.
In addition, the catalyst exhibited excellent stability during continuous photocatalytic operation.
Through the combination of DFT/TD-DFT calculations, femtosecond transient absorption spectroscopy, and in situ infrared spectroscopy, the study directly demonstrated that isomerization-induced compact π-stacking and enhanced built-in electric fields accelerate exciton dissociation.
Meanwhile, the carbonyl groups, C=O, on the bridge chains were identified as the true active sites for oxygen evolution. This mechanism provides an important reference for the design of other polymeric photocatalytic systems.
Figure 1 |
(a) XRD patterns of MA-PDI, FU-PDI, and SU-PDI.
(b, e) Unit cells of MA-PDI and FU-PDI, showing the intramolecular chain arrangement.
(c, f) Experimental diffraction patterns of MA-PDI and FU-PDI and the refined profiles calculated by Pawley refinement.
(d) SAED intensity distribution and XRD pattern of MA-PDI.
(g) Low-magnification TEM image and
(h) high-resolution TEM image of MA-PDI. Inset: SAED pattern.
(i) Enlarged image of the red-boxed region in panel h. Inset: corresponding filtered inverse FFT image.
Key Points:
XRD and Crystal Structure Analysis
MA-PDI exhibits high crystallinity, and its diffraction peaks are highly consistent with the selected-area electron diffraction, SAED, pattern. Pawley refinement further confirms the unit-cell parameters of MA-PDI.
High-Resolution Electron Microscopy Observation
The HR-TEM image of MA-PDI shows clear lattice fringes with an interplanar spacing of 8.77 Å. The sharp SAED spots directly confirm its highly ordered π-π stacking structure.
Figure 2
Figure 2 |
Left: Dynamic behavior of excitons and charge carriers during the photocatalytic reaction.
Right: π-electron density distribution, electron-hole overlap, and charge-density difference between the ground state and the S₁ excited state for MA-PDI_DFT, FU-PDI_DFT, and SU-PDI_DFT polymer fragments.
D represents the distance between the centroids of regions with increased and decreased electron density after excitation.
Eᶜ represents the Coulombic attraction energy calculated by DFT.
Sᵣ represents the hole-electron overlap index.
Green indicates electron distribution, and blue indicates hole distribution.
Key Points:
Five-Stage Model of the Exciton Process
The photocatalytic process includes photon absorption, exciton generation, exciton dissociation into free charge carriers, bulk/surface recombination, and surface chemical reactions. The isomerization strategy exerts a positive influence on each stage.
DFT Electronic Structure Analysis
In MA-PDI, the HOMO and LUMO are located on the bridge bond and PDI core, respectively, forming a donor-acceptor, D-A, structure. In contrast, SU-PDI shows severe HOMO/LUMO overlap.
The π orbitals of MA-PDI extend across the entire planar structure through the C=C bond, giving it the highest degree of conjugation, as shown in the first column on the right side of Figure 2.
Excited-State Charge Separation
MA-PDI exhibits the largest charge-transfer distance, D = 7.79 Å, the lowest electron-hole overlap index, Sᵣ = 0.18, and the lowest Coulombic attraction energy, Eᶜ = 2.14 eV. These results indicate that excitons in MA-PDI are the easiest to dissociate.
By comparison, the Eᶜ values of FU-PDI and SU-PDI are as high as 4.19 eV and 4.21 eV, respectively.
Figure 3
Figure 3 |
(a–c) Surface potentials of the prepared samples measured by Kelvin probe force microscopy, KPFM.
(d–f) Exciton binding energies, Eᵦ, of the samples.
(g–l) Transient absorption spectra and decay kinetic curves.
Key Points:
KPFM Surface Potential
MA-PDI shows a surface potential as high as 36.2 mV, much higher than those of FU-PDI and SU-PDI, demonstrating that MA-PDI possesses the strongest built-in electric field.
Exciton Binding Energy, Eᵦ
Temperature-dependent photoluminescence spectroscopy reveals that MA-PDI has an exciton binding energy of only 40.6 meV, lower than those of FU-PDI, 43.7 meV, and SU-PDI, 43.8 meV.
Transient Absorption Spectroscopy
Under 490 nm excitation, MA-PDI exhibits a significantly prolonged exciton lifetime, τ = 629 ps, compared with the control samples.
Figure 4
Figure 4 |
(a–c) Photocatalytic oxygen evolution performance of the samples under simulated solar irradiation, AM 1.5G, 100 mW cm⁻².
(d) Photocatalytic oxygen evolution performance using different electron sacrificial agents.
(e) Determination of the oxygen source by ¹⁸O isotope labeling.
(f) Continuous photocatalytic oxygen evolution of MA-PDI using Na₂S₂O₈.
(g) Wavelength-dependent AQY and diffuse reflectance spectrum of MA-PDI.
(h) Comparison of MA-PDI with previously reported photocatalysts in terms of oxygen evolution performance and AQY.
Key Points:
Oxygen Evolution Rate
Under simulated solar irradiation, AM 1.5G, 100 mW cm⁻², MA-PDI achieves an oxygen evolution rate of up to 2119 μmol g⁻¹ h⁻¹.
Apparent Quantum Yield, AQY
MA-PDI delivers an AQY of 13.4% at 380 nm and maintains an AQY above 5% in the wavelength range of 400–500 nm, outperforming most reported photocatalysts.
Cycling Stability
During continuous testing for more than 30 h, MA-PDI shows no obvious decline in oxygen evolution activity, indicating excellent photocatalytic stability.
Figure 5
Figure 5 |
(a) Reaction sites on MA-PDI_DFT predicted by Fukui function analysis.
(b) Gibbs free energy changes, ΔG, for the oxygen evolution reaction pathway on MA-PDI_DFT.
(c–d) In situ XPS C 1s spectra and fitting curves of MA-PDI.
(e) In situ XPS O 1s spectra of MA-PDI.
(f) Proposed OER pathway on MA-PDI and in situ FT-IR spectra under illumination using H₂O and D₂O.
Key Points:
Active Site Prediction by Fukui Function Analysis
Theoretical calculations suggest that the bridge chain is the preferred active center for water oxidation.
Gibbs Free Energy Calculation
For the four-step OER process on MA-PDI, the Gibbs free energy changes at the carbonyl carbon sites of the bridge bonds are lower than those at the olefinic carbon sites on the bridge bonds.
In Situ FT-IR Spectroscopy
Under illumination, characteristic intermediate peaks corresponding to C=O*OH at approximately 1715 cm⁻¹ and COO⁻ at approximately 1540 cm⁻¹ appear on the surface of MA-PDI.
After replacing H₂O with D₂O, the related peaks show an isotope shift of approximately 3 cm⁻¹, providing solid evidence for the water oxidation pathway and the active centers involved in the reaction.
Summary and Outlook
In this study, three PDI-based linear conjugated polymers were successfully constructed for the first time through a bridge-bond geometric isomerization strategy. The work systematically reveals how isomerization simultaneously enables extended π-conjugation, enhanced crystallinity, strengthened built-in electric fields, and accelerated exciton dynamics.
Among the three polymers, cis-MA-PDI achieved an AQY of 13.4% at 380 nm and maintained stable oxygen evolution activity over long-term operation. In situ spectroscopic evidence combined with theoretical calculations confirmed that the carbonyl groups on the bridge chains serve as the genuine active centers.
This work not only provides deep insight into the structure-performance relationship of PDI-based photocatalysts, but also opens up a new avenue for designing isomerized organic semiconductor materials for efficient solar fuel conversion.
Reference Information
https://onlinelibrary.wiley.com/doi/10.1002/anie.3903637
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The company has led or participated in the formulation of multiple national and industry standards, undertaken national key R&D projects, and owns a number of core intellectual property rights and Beijing New Technology and New Product certifications. Perfectlight has also helped several enterprise users establish ton-scale and hundred-ton-scale photochemical production lines.
