Regioisomeric control of planarity enhances exciton dissociation in conjugated polymers for high-efficiency photocatalytic H₂ evolution

Abstract

Organic conjugated polymers (CPs) are promising photocatalysts for solar-to-hydrogen conversion, yet their efficiency is often limited by large exciton binding energies and poor charge separation. In this study, we report a molecular isomerization strategy to control polymer planarity and thereby enhance exciton dissociation and photocatalytic activity. Two isomeric CPs, Pyrene-Thiophene-Benzothiazole (Py-T-BT) and Pyrene-Thiophene-iso-Benzothiazole (Py-T-isoBT), were designed and synthesized, employing pyrene as the donor, thiophene as the π-bridge, and either benzothiadiazole (BT) or its structural isomer (isoBT) as the acceptor. The introduction of the isoBT unit induced steric hindrance, resulting in Py-T-isoBT exhibiting significantly reduced molecular planarity (dihedral angles of 30.3° and 9.2°) compared to the highly planar Py-T-BT (dihedral angle of 4.2°). Theoretical calculations and experimental characterization confirmed that the more planar Py-T-BT possessed a narrower band gap, a larger transition dipole moment, and consequently a significantly lower exciton binding energy. Time-resolved spectroscopy revealed that Py-T-BT enabled ultrafast formation of charge transfer excitons (τ_CT = 0.60 ps), in contrast to Py-T-isoBT (τ_CT = 3.95 ps). As a result, Py-T-BT demonstrated a photocatalytic hydrogen evolution rate of 7.95 mmol g⁻¹ h⁻¹ with a 3 wt% Pt cocatalyst under visible light irradiation (λ ≥ 420 nm), approximately 20 times higher than that of Py-T-isoBT. This work highlights the pivotal role of molecular planarity in modulating exciton dynamics and presents a generalizable strategy for designing high-efficiency polymer photocatalysts through isomeric engineering.

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Haowei Huang

Haowei Huang obtained his PhD from KU Leuven, Belgium, in 2020. After that, he conducted his postdoc research at KU Leuven and UC Berkeley. In 2024, he joined Southeast University, China, as a professor. His research has been primarily focused on solar energy conversion and utilization, with an emphasis on both fundamental and applied aspects of photocatalysis, photothermal catalysis and photoelectrocatalysis. Specifically, his work centres on the design and synthesis of highly efficient catalysts for resource utilization and the carbon cycle, the optimization of catalytic processes, and the exploration of microscopic molecular interaction mechanisms.

Introduction

Organic semiconductors have emerged as promising candidates for solar energy conversion, particularly in the field of photocatalysis.^1–6 Compared to traditional inorganic semiconductor photocatalysts, organic materials offer several distinct advantages, including tunable electronic structures, rich molecular diversity, and environmentally benign processing conditions.^7–10 However, their practical application is often limited by intrinsically high exciton binding energies, which hinder efficient exciton dissociation and charge transport, thereby reducing photocatalytic efficiency.^11–14

Rational molecular designs to reduce exciton binding energy have been widely explored to address this challenge.^15–18 Among these, the construction of donor–acceptor (D–A) architectures is one of the most effective approaches.^19–22 In D–A systems, an internal electric field is generated due to differences in electron affinities between the donor and acceptor units, which helps facilitate exciton dissociation.^23,24 Optimizing donor–acceptor strength,²⁵ tuning spatial configuration,^24,26 and engineering π-bridges^27,28 have proven to be effective means of enhancing the dipole moment in such systems. Beyond the D–A structure, recent studies have highlighted the importance of molecular planarity in promoting efficient exciton dissociation. Conformational distortion and torsional strain disrupt π-conjugation and hinder electron delocalization along the molecular backbone.^29–31 Enhancing molecular planarity, by minimizing dihedral angles, has therefore become a key strategy to boost charge separation and transport. For instance, Tan et al.³² developed highly planar covalent triazine frameworks (CTFs) by employing inherently rigid triazine linkers, achieving near-zero dihedral angles and significantly improved photocatalytic activity. These findings underscore the potential of structural planarity in organic photocatalysts. Besides introducing rigid molecular scaffolds to restrict the torsional angles, changing the arrangement of atoms in the molecule, specifically through geometric isomerism, could have the potential to change the torsional angles, leading to the efficient exciton dissociation. However, these traditional strategies often introduce simultaneous and complex changes to both the electronic properties and steric profile of the molecule. This makes it challenging to precisely decouple and study the influence of ‘molecular planarity’ as an isolated variable.

In this context, we designed and synthesized two donor–π–acceptor (D–π–A) conjugated polymers (CPs), Pyrene-Thiophene-Benzothiazole (Py-T-BT) and Pyrene-Thiophene-iso-Benzothiazole (Py-T-isoBT), using pyrene as the donor, thiophene as the π-bridge, and either benzothiadiazole (BT) or its isomer as the acceptor. In Py-T-isoBT, the conventional BT unit (=N–S–N=) was replaced with its geometric isomeric (–S–N=N–), introducing steric hindrance that increased the dihedral angles from 4.2° in Py-T-BT to 30.3° and 9.2° in Py-T-isoBT. Theoretical calculations and experimental characterization confirmed that Py-T-BT, being more planar, exhibited a larger transition dipole moment, resulting in a lower exciton binding energy. Time-resolved spectroscopy revealed that Py-T-BT enabled ultrafast formation of charge transfer excitons (τ_CT = 0.60 ps), compared to 3.95 ps for Py-T-isoBT. As a result, a remarkable enhancement in photocatalytic hydrogen evolution was achieved over Py-T-BT, which with Pt as cocatalyst, Py-T-BT exhibited a hydrogen evolution reaction (HER) rate of 7.95 mmol g⁻¹ h⁻¹, approximately 20 times higher than Py-T-isoBT (0.38 mmol g⁻¹ h⁻¹). In summary, this work highlights the crucial role of molecular planarity in promoting exciton dissociation and charge separation and introduces a new strategy to enhance molecular planarity, thereby enabling superior photocatalytic activity in organic polymer systems.

Results and discussion

Py-T-BT and Py-T-isoBT (Fig. 1) were synthesized through Pd-catalyzed Suzuki–Miyaura coupling reactions using 1,3,6,8-tetrakis(pinacolboronate)pyrene, 4,7-bis(2-bromo-5-thienyl)-2,1,3-benzothiadiazole and 4,7-bis(2-bromo-5-thienyl)-1,2,3-benzothiadiazole as precursors. To confirm the successful synthesis, Fourier-transform infrared spectroscopy (FT-IR) was first employed to examine structural changes before and after the reaction (Fig. 2a). After the coupling reaction, the characteristic C–Br stretching vibration at 460 cm⁻¹, which can be observed in the BT-S-Br and isoBT-S-Br precursors, disappeared in both Py-T-BT and Py-T-isoBT. This disappearance indicated the complete consumption of bromine groups during the cross-coupling process. Solid-state ¹³C CP/MAS NMR spectroscopy further confirmed the formation of the polymers (Fig. 2b). The carbon signal at 85 ppm, attributed to the boronic acid pinacol ester groups, was absent. Additionally, characteristic resonances were observed at 140–142 ppm, corresponding to C–S bonds in the thiophene units, and at 151–154 ppm, associated with –C=N– and –C–N= moieties in benzothiadiazole and its isomer. Furthermore, the broad and overlapping resonances observed in the 120–135 ppm range were attributed to the numerous sp²-hybridized carbons of the aromatic pyrene core and the thiophene rings. Due to the amorphous nature of the polymer and the resulting peak broadening in the solid-state spectrum, a precise assignment for each individual carbon is challenging. However, the presence of this signal cluster strongly confirmed the successful incorporation of the pyrene moiety into the polymer backbone. With X-ray photoelectron spectroscopy (XPS), four types of carbon bonds were observed in the C 1s spectrum of Py-T-BT and Py-T-isoBT (Fig. 2c). In detail, the peak at 284.8 eV corresponds to C=C/C–C bonds from the polymer backbone, while the peak at 285.5 eV can be attributed to C–S bonds in thiophene and isoBT units. Notably, Py-T-BT exhibited a peak at 286.1 eV corresponding to C=N in BT units, whereas Py-T-isoBT showed a distinct peak at 288.0 eV, attributed to C–N bonds, highlighting the structural differences between BT and isoBT. Moreover, the N 1s XPS spectra showed peaks at 399.4 eV (C=N–S) in BT units and 400.3 eV (N=N–S) in isoBT units, further confirming the successful incorporation of benzothiadiazole and its isomeric units in their respective structural forms (Fig. 2d). S 2p spectra provided further evidence for the distinct isomeric structures (Fig. S2). The spectra were a superposition of signals from sulfur in the thiophene rings and sulfur in the benzothiadiazole unit (an N–S–N environment for Py-T-BT and a C–S–N environment for Py-T-isoBT). In the spectrum of Py-T-BT, the overlapping signals from these two different types of sulfur (C–S–C and N–S–N) caused the overall envelope's shape and intensity ratio to deviate from the standard 2 : 1 expected for a single spin–orbit doublet. The component at higher binding energy was assigned to N–S bonds, while the component at lower binding energy was assigned to C–S bonds. In Py-T-isoBT, due to the different chemical environment of the sulfur in its benzothiadiazole ring, the overall peak shape was different from that of Py-T-BT. These discernible differences in the S 2p spectra further corroborated the successful synthesis of the two isomeric polymers. Elemental mapping analysis confirmed the uniform distribution of carbon (C), nitrogen (N), and sulfur (S) throughout both polymers, with no detectable signals from impurities. These results indicated the successful synthesis of the target polymers with regioisomeric structures.

Fig. 1 Polymer structures of Py-T-BT and Py-T-isoBT.

Fig. 2 (a) FT-IR spectra of Py-T-BT, Py-T-isoBT and related monomers; (b) solid-state ¹³C NMR spectra of Py-T-BT and Py-T-isoBT; XPS spectra of (c) C 1s and (d) N 1s in Py-T-BT and Py-T-isoBT; (e) PXRD patterns of Py-T-BT and Py-T-isoBT; (f) N₂ adsorption–desorption isotherms of Py-T-BT and Py-T-isoBT.

The crystalline properties of Py-T-BT and Py-T-isoBT were examined using powder X-ray diffraction (PXRD). Both materials exhibited a broad amorphous halo centered at 2θ ≈ 25°, corresponding to a d-spacing of approximately 3.5 Å, corresponding to a d-spacing of approximately 3.5 Å, which is characteristic of the chain-to-chain distance governed by short-range π–π stacking in amorphous conjugated polymers.^33,34 The absence of sharp diffraction peaks suggested that both polymers were predominantly amorphous (Fig. 2e).³⁵ Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that both materials exhibited irregularly stacked aggregates (Fig. S3). The lack of observable lattice fringes in the TEM images further supported the amorphous character of these materials, consistent with the PXRD findings. The porosity of Py-T-BT and Py-T-isoBT was evaluated using nitrogen adsorption–desorption isotherms. Both materials displayed typical type IV isotherms, indicative of mesoporous structures.³⁶ The Brunauer–Emmett–Teller (BET) surface areas were calculated to be 128.5 m² g⁻¹ for Py-T-BT and 122.8 m² g⁻¹ for Py-T-isoBT (Fig. 2f). Pore size distribution analysis revealed a hierarchical porous architecture dominated by mesopores with contributions from micropores (Fig. S4). These findings confirmed that both polymers possessed comparable specific surface areas and pore structures.

The geometry of Py-T-BT and Py-T-isoBT was investigated by density functional theory (DFT) calculations using Gaussian 09 software at the B3LYP/def2-TZVPD level. As shown in Fig. 3, Py-T-BT adopts a nearly planar configuration, with dihedral angles θ₁ and θ₂ between the thiophene bridge and the benzothiadiazole unit both measured at 4.2°. In contrast, Py-T-isoBT exhibited significant intramolecular twisting, with θ₁ and θ₂ measured at 30.3° and 9.2°, respectively. This deviation from planarity was primarily attributed to the significant steric hindrance introduced by the sulfur atom at position 3 in the isoBT unit. As confirmed by potential energy surface scan calculations (Fig. S5), this steric hindrance results in a twisted, non-planar structure being the lowest-energy conformation. Compared to the nitrogen atom, sulfur has a larger atomic radius, which led to distortion of the adjacent pentagonal ring and disrupted the coplanarity between the π-bridge and acceptor moieties. The Molecular Planarity Parameter (MPP) and Span of Deviation from Plane (SDP) were employed to quantitatively assess molecular planarity. In detail, the MPP could quantify the overall deviation of the molecular structure from planarity, and the SDP reflects the range of deviation relative to the fitted plane.^37–39 As presented in Fig. 3, Py-T-BT exhibited MPP and SDP values of 0.042 and 0.191 Å, respectively, which were significantly lower than those of Py-T-isoBT, 0.239 and 1.188 Å. These results solidly confirmed the better planarity of Py-T-BT relative to Py-T-isoBT. Furthermore, due to the sulfur atom at position 3, Py-T-isoBT presented a lower overall molecular symmetry. As a result, Py-T-isoBT exhibited a substantially larger ground-state dipole moment (∼2.72 Debye) in comparison to Py-T-BT, which possessed a relatively small dipole moment (∼0.48 Debye), owing to its structural symmetry (Fig. S6).

Fig. 3 (a) Dihedral angles of Py-T-BT and Py-T-isoBT; (b) geometrically optimized models along with MPP and SDP values of Py-T-BT and Py-T-isoBT.

As depicted in Fig. S7, differences in molecular planarity resulted in distinct electronic structures and properties for Py-T-BT and Py-T-isoBT. For both materials, density functional theory (DFT) calculations showed that the highest occupied molecular orbital (HOMO) is primarily localized on the pyrene donor and thiophene bridging units, while the lowest unoccupied molecular orbital (LUMO) is mainly distributed over the (iso)benzothiadiazole acceptor units. The calculated HOMO and LUMO energy levels were −1.45 eV and 0.81 eV for Py-T-BT, and −1.80 eV and 0.95 eV for Py-T-isoBT, respectively. Correspondingly, Py-T-BT exhibited a narrower HOMO–LUMO gap of 2.26 eV, compared to 2.75 eV for Py-T-isoBT. This is likely due to the enhanced planarity of the symmetric benzothiadiazole unit in Py-T-BT, which facilitates π-electron delocalization. To further explore these electronic structure differences, a series of experimental characterization studies was conducted. Solid-state UV-visible diffuse reflectance spectroscopy (UV-vis DRS) revealed that both materials possess intrinsic absorption bands arising from aromatic π–π* transitions, with an absorption range of 742 nm for Py-T-BT and 792 nm for Py-T-isoBT (Fig. S8), effectively covering the visible spectrum. Py-T-BT with an optical band gap of 1.72 eV appeared darker brown in color. Meanwhile, Py-T-isoBT displayed a lighter color and a wider band gap of 1.90 eV. The optical band gap of Py-T-BT is smaller than that in Py-T-isoBT, which is in agreement with the DFT-calculated HOMO–LUMO gaps. Electrochemical Mott–Schottky (M–S) analysis further revealed the band structures of both polymers (Fig. S9). The positive slopes of the M–S plots confirm their n-type semiconducting behavior.^40,41 The flat-band potentials were determined to be −0.90 V and −0.67 V vs. RHE for Py-T-BT and Py-T-isoBT, respectively, indicating their conduction band (CB) positions. By combining these values with the optical band gaps, the complete band structures were constructed (Fig. S10).

Next, time-dependent density functional theory (TD-DFT) calculations were employed to study the excited state of the two polymers. As shown in the smoothed maps of hole and electron distribution diagrams (Fig. 4a), in the excited state of Py-T-BT, electrons (green area) tend to localize in the benzothiadiazole segment, and holes (blue area) primarily reside on the pyrene units. However, in Py-T-isoBT, the blue and green areas exhibited a large overlap, indicating that the electrons and holes were located in the same region. These were confirmed by the charge transfer distance (D index) in both materials. The D index reached 4.7 Å for Py-T-BT but only 0.6 Å for Py-T-isoBT. This substantial disparity in the charge distribution indicated that Py-T-BT exhibited distinct charge transfer (CT) excitation characteristics, while Py-T-isoBT primarily demonstrated local excitation (LE) behavior. The charge transfer efficiency was further evaluated by calculating the amount of charge transferred during excitation (detailed calculation values in Tables S1 and S2).⁴³ The heat maps directly displayed the electron-rich and hole-rich regions in the excited states of both samples (Fig. 4b). Since we examined single-electron excitations in this analysis, the values directly reflected the degree of charge separation. Critically, the effective electron transfer from donor to acceptor units in Py-T-BT reached 0.49, far exceeding the calculated value of 0.09 for Py-T-isoBT. The excitation behavior difference can be attributed to the difference of exciton binding energy in both materials. Py-T-BT exhibited a larger dipole moment difference (Δμ_ge) between ground and excited states (17.6 D vs. 13.4 D for Py-T-isoBT), resulting in reduced coulomb attraction energy of the excited state excitons (3.1 eV vs. 4.0 eV). This lower coulombic attraction energy in Py-T-BT weakens the binding energy between electrons and holes. The exciton binding energies (E_b) of the materials were quantitatively evaluated using temperature-dependent steady-state photoluminescence (PL) spectroscopy. As shown in Fig. S11, PL intensity increases markedly as the temperature decreases from 298 K to 77 K. Fitting the PL data with the Arrhenius equation⁴² yielded E_b values of 59.7 meV for Py-T-BT and 102.9 meV for Py-T-isoBT. These results revealed that regioisomerization caused remarkable changes in the degree of electron–hole separation in the excited states. In Py-T-BT, the separation of electrons and holes in space was more pronounced than in the twisted Py-T-isoBT.

Fig. 4 (a) Smoothed maps of hole and electron distribution; (b) heat maps of hole and electron distribution in the excited states of polymer fragments.

The details on the dynamics of photogenerated excitons were revealed by time-resolved absorption and PL spectroscopies. Femtosecond transient absorption (fsTA) measurements of Py-T-BT revealed four features, three positive absorption bands at 496, 584, and 1024 nm, and one negative band at 682 nm. The bands at 496 and 584 nm can be assigned to charge transfer excitons (CTEs), while the 1024 nm feature corresponds to the S_n ← S₁ transition of Frenkel excitons (FEs). The transient decay dynamics of these bands are illustrated in Fig. 5a, b, d and e, with corresponding kinetic fitting data summarized in Table S3. The decay of FEs at 1024 nm was fitted using a tri-exponential model. The fastest component (τ_CT = 0.60 ± 0.05 ps) is attributed to the rapid transformation of FEs into CTEs, while the remaining components correspond to structural relaxation processes. For the CTE bands at 496 and 584 nm, bi-exponential decay fitting was employed. The short-lived component (τ ≈ 61 and 67 ps) was associated with structural relaxation of the CTEs, whereas the longer-lived component (τ ≈ 2.3 ns) reflected the exciton recombination process. The absorption spectrum of Py-T-isoBT exhibited distinct characteristics. The fsTA spectrum featured a negative band at 500 nm due to ground state bleaching (GSB), which decayed with a time constant of 5.59 ± 0.03 ps. A feature at 1029 nm corresponding to the S_n ← S₁ transition of FEs was also observed. Following structural relaxation, FEs transformed into CTEs with a time constant of τ_CT = 3.95 ± 0.36 ps, as evidenced by the emergent signal at 724 nm. The CTEs in Py-T-isoBT subsequently recombined to the ground state within 0.72 ± 0.09 ns. The comparison of exciton dynamics between the two polymers clearly revealed that CTE formation in Py-T-BT occurred significantly faster than in Py-T-isoBT. This can be attributed to the enhanced molecular planarity and electron delocalization facilitated by the symmetric structure of Py-T-BT, which promoted efficient exciton dissociation and charge separation. Furthermore, nanosecond time-resolved PL analysis provided additional insights into exciton recombination behavior. As shown in Fig. S12, Py-T-BT exhibited a longer average PL decay time of 2.36 ns compared to 0.69 ns for Py-T-isoBT, indicating that the improved planarity in Py-T-BT suppressed radiative recombination of excitons and enhanced the charge carrier lifetime.

Fig. 5 Femtosecond transient absorption spectra for Py-T-BT (a) and Py-T-isoBT aggregates (d); kinetic traces at selected probe wavelengths for Py-T-BT (b) and Py-T-isoBT aggregates (e); schematic illustration of photogenerated excited states and their photophysical processes for Py-T-BT (c) and Py-T-isoBT aggregates (f).

The ability of the materials to dissociate photogenerated excitons was further investigated via photoelectrical measurements. Under visible light irradiation, Py-T-BT exhibited a significantly higher surface photovoltage (SPV) of 38.2 mV compared to only 8.3 mV for Py-T-isoBT, as measured by in situ Kelvin probe force microscopy (KPFM) (Fig. 6a and b). For n-type semiconductors, a higher SPV indicates more efficient directional separation and transfer of photogenerated charge carriers.^44,45 Photocurrent density measurements and electrochemical impedance spectroscopy (EIS) further corroborated these findings (Fig. 6c and d). Py-T-BT showed a higher photocurrent response and lower charge transfer resistance relative to Py-T-isoBT, confirming superior charge separation and transport efficiency.

Fig. 6 KPFM images under darkness and under irradiation and the corresponding surface photovoltage profiles for (a) Py-T-BT and (b) Py-T-isoBT; (c) photocurrent responses for Py-T-BT and Py-T-isoBT; (d) electrochemical impedance spectra (EIS), fitting curve and equivalent circuit of polymers.

Based on the comprehensive characterization of photophysical properties and charge transfer behaviors, Py-T-BT was predicted to exhibit superior photocatalytic performance. The photocatalytic hydrogen evolution reaction (HER) activities of Py-T-BT and Py-T-isoBT were evaluated under visible light irradiation (λ ≥ 420 nm) using 0.1 M ascorbic acid (AA) as a sacrificial electron donor and 10 mg of polymer as the photocatalyst. As shown in Fig. S13, Py-T-BT exhibited hydrogen evolution activity with various sacrificial agents and performed the best under ascorbic acid. The optimization of the ascorbic acid concentration confirmed that 0.1 wt% was a suitable concentration for the reaction (Fig. S14). Furthermore, 10 mg was found to be the optimal amount of catalyst in this HER experiment (Fig. S15). As shown in Fig. 7a, hydrogen evolution over a 6 hour period demonstrated a marked contrast between the two materials: Py-T-BT achieved an HER rate of 2.88 mmol g⁻¹ h⁻¹, while Py-T-isoBT exhibited negligible hydrogen production under identical conditions. The loading of the Pt co-catalyst was optimized. The hydrogen evolution rate increased with Pt loading and reached its optimum at 3 wt% (Fig. S16). Upon deposition of 3 wt% platinum (Pt) as a co-catalyst, the HER performance of Py-T-BT was significantly enhanced, reaching 7.95 mmol g⁻¹ h⁻¹—approximately 20 times higher than that of Pt-loaded Py-T-isoBT (0.38 mmol g⁻¹ h⁻¹) (Fig. 7b). This substantial difference in photocatalytic activity was primarily attributed to the influence of steric hindrance from the isomeric acceptor units, which altered molecular planarity and consequently affected exciton dissociation and charge transport efficiency. Furthermore, the HER activity of Py-T-BT is highly competitive compared to many advanced systems (Table S4).

Fig. 7 (a) Time-dependent hydrogen evolution curves of Py-T-BT and Py-T-isoBT under visible light irradiation with/without the Pt co-catalyst; (b) the photocatalytic hydrogen evolution performance of the sample with 3 wt% Pt loading; (c) photocatalytic cycling stability of Py-T-BT (8 cycles, 5 hours each); (d) apparent quantum yield of Py-T-BT at different wavelengths.

Given the promising performance of Py-T-BT, its photocatalytic durability and reusability were further assessed. Long-term cycling tests over 40 hours (8 cycles) revealed no significant loss in activity, demonstrating excellent operational stability (Fig. 7c). Moreover, the apparent quantum efficiency (AQE) of Py-T-BT was measured under monochromatic illumination at various wavelengths. The highest AQE was recorded at 420 nm, reaching 3.9%. The AQE spectrum showed strong agreement with the UV-visible absorption profile, indicating that absorbed photons were effectively utilized in driving the photocatalytic HER process (Fig. 7d).

Based on these results, a possible photocatalytic hydrogen evolution mechanism has been proposed. Under visible light irradiation, Py-T-BT was excited, and the resulting photogenerated excitons can rapidly dissociate into free electrons (e⁻) and holes (h⁺) due to the polymer's excellent planarity. While the photogenerated holes were effectively captured and consumed by the sacrificial agent, the photogenerated electrons migrated to the surface of the platinum (Pt) co-catalyst, where they reduced protons (H⁺) to generate H₂. In this photocatalytic process, the superior exciton dissociation efficiency of Py-T-BT is the critical prerequisite for the overall catalytic activity.

Conclusions

In summary, we have successfully designed and synthesized two organic semiconductors, Py-T-BT and Py-T-isoBT, incorporating benzothiadiazole and its isomeric unit as electron acceptors. Owing to the steric hindrance introduced by the isomeric structure, Py-T-isoBT exhibits reduced molecular planarity compared to Py-T-BT. This structural difference results in a smaller transition dipole moment and a higher exciton binding energy in Py-T-isoBT, whereas Py-T-BT demonstrates enhanced π-conjugation and electronic delocalization. Time-resolved spectroscopic analysis revealed that Py-T-BT facilitates ultrafast formation of charge transfer excitons (τ_CT = 0.60 ps), contributing to its superior charge separation efficiency. As a result, when loaded with 3 wt% Pt as a co-catalyst, Py-T-BT achieved a hydrogen evolution rate of 7.95 mmol g⁻¹ h⁻¹ under visible light irradiation (λ ≥ 420 nm), approximately 20 times higher than that of Py-T-isoBT. Through a comprehensive investigation of the photoelectronic and photocatalytic properties of these isomeric polymers, our study provides fundamental insights into the critical role of molecular planarity in governing exciton dynamics and photocatalytic performance, offering valuable design guidelines for high-efficiency organic photocatalysts in solar energy conversion applications.

Conflicts of interest

There are no conflicts to declare.

Data availability

All raw data and analysis files supporting the findings of this study, including experimental procedures, characterization results, and computational details, are available from the corresponding author upon reasonable request. See DOI: https://doi.org/10.1039/d5ta04143k.

Acknowledgements

This work was financially supported by the NSFC (grants no. 22302038) and the Minjiang scholar award program.

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