Conjugated Polymer Photocatalysts Research Articles

Enhancing photocatalytic hydrogen production in conjugated porous polymers through donor-π-donor fragment insertion

Recent progress in conjugated polymer photocatalysts has opened avenues for enhancing photocatalytic hydrogen production (PHP) activity through structural design. This study highlights the impact of the acceptor-to-donor feed ratio on PHP activity in copolymers, influencing their energy level structures. Five conjugated porous polymers (CPPs) were synthesized by adjusting the acceptor-to-donor molar ratio. Notably, at a 2:1 acceptor-to-donor ratio, the polymer demonstrated a high hydrogen evolution rate (HER) of 61.9 mmol g−1 h−1 under full arc irradiation without co-catalysts and an apparent quantum yield (AQY) of 7.77% at 475 nm. However, further increasing the proportion of the donor unit in the copolymers decreases their HER. Experiments and theoretical simulation characterized that the feed ratios effectively tune the polymer's molecular structure, hydrophilicity, band gap, and the generation and transport efficiency of photo-induced charge carriers, ultimately affecting the hydrogen production kinetics and efficiency. This research demonstrates that tuning the band structure and chemical compositions of CPPs through statistical copolymerization significantly impacts their PHP activity.

Polymer Photocatalysts with Side Chain Induced Planarity for Increased Activity for Sacrificial Hydrogen Production from Water

AbstractConjugated polymers are promising materials for photocatalytic hydrogen evolution. However, most reported materials are not solution‐processible, limiting their potential for large‐scale application, for example as solution cast films. Flexible side‐chains are commonly introduced to provide solubility, but these often impart unfavorable properties, such as hydrophobicity, which lowers photocatalytic activity. Here, computational predictions are employed to aid in the design of chloroform soluble polymer photocatalysts that show increased planarity through favorable intramolecular interactions. Using this approach, three conjugated polymer photocatalysts with identical poly(benzene‐dibenzo[b,d]thiophene sulfone) backbones but different solubilizing side‐chains on the benzene‐ring are explored, i.e., tri(ethylene glycol), n‐decyloxy, and n‐dodecyl. These side‐chain variations significantly alterr the properties of the polymers, specifically energy levels, optical gap, and wettability. The hydrophobic n‐decyloxy functionalized polymer has a sacrificial hydrogen evolution rate of 17.0 µmol h−1 in suspension, while the hydrophilic tri(ethylene glycol) functionalized polymer is almost three times more active (45.4 µmol h−1). Conversely, no hydrogen evolution is observed for the purely alkyl side‐chain (n‐dodecyl) containing polymer due to the side‐chain induced torsion of the backbone. A thin‐film of the most active polymer exhibits a promising area‐normalized sacrificial hydrogen evolution rate of 7.4 ± 0.3 mmol h−1 m−2 under visible light irradiation.

Green and sustainable synthesis of TPD-based donor–acceptor-type conjugated polymer photocatalysts for hydrogen production under visible light

The conjugated polymers are synthesized via greener direct C–H/C–H cross-coupling polycondensation for application in photocatalytic hydrogen production.

Furan-based conjugated polymer photocatalysts for highly active photocatalytic hydrogen evolution under visible light

Building donor–acceptor (D-A) structure in conjugated polymer photocatalysts has attracted intense attention for hydrogen evolution. Yet, it is still challenging for organic photocatalysts to realize high photocatalytic performance under visible light that contains almost half of the total solar energy. Herein, we employ furan (F) and dibenzofuran (BF) as electron donors to respectively couple with dibenzothiophene-S,S-dioxide (BTDO) acceptor to construct two furan-based D-A polymer photocatalysts (F-BTDO and BF-BTDO) for photocatalytic hydrogen production. Compared to BF-BTDO comprising BTDO and BF units, the polymer F-BTDO with furan donor shows a broader light adsorption range and more efficient charge separation, enabling F-BTDO to show a prominent photocatalytic activity with a hydrogen evolution rate of 120.68 mmol h−1 g−1 under visible light (λ > 420 nm) without adding Pt co-catalyst, which is much higher than that of BF-BTDO (4.83 mmol h−1 g−1) under the duplicate photocatalytic conditions. This work demonstrates that furan unit could be a promising donor for developing high-performance organic photocatalysts under visible light.

Effect of Controlling Thiophene Rings on D-A Polymer Photocatalysts Accessed via Direct Arylation for Hydrogen Production.

Conjugated polymer photocatalysts for hydrogen production have the advantages of an adjustable structure, strong response in the visible light region, adjustable energy levels, and easy functionalization. Using an atom- and step-economic direct C-H arylation method, dibromocyanostilbene was polymerized with thiophene, dithiophene, terthiophene, and fused thienothiophene and dithienothiophene, respectively, to produce donor-acceptor (D-A)-type linear conjugated polymers containing different thiophene derivatives with different conjugation lengths. Among them, the D-A polymer photocatalyst constructed from dithienothiophene could significantly broaden the spectral response, with a hydrogen evolution rate up to 12.15 mmol h-1 g-1. The results showed that the increase in the number of fused rings on thiophene building blocks was beneficial to the photocatalytic hydrogen production of cyanostyrylphene-based linear polymers. For the unfused dithiophene and terthiophene, the increase in the number of thiophene rings enabled more rotation freedom between the thiophene rings and reduced the intrinsic charge mobility, resulting in lower hydrogen production performance accordingly. This study provides a suitable process for the design of electron donors for D-A polymer photocatalysts.

Toward high-performance dibenzo[g,p]chrysene-based conjugated polymer photocatalysts for photocatalytic hydrogen production through donor-acceptor-acceptor structure design

The exploration on boosting the photocatalytic performance of donor-acceptor (D-A) type polymer photocatalysts is hindered by their fixed chemical compositions. Herein, a series of donor-acceptor-acceptor (D-A-A) type conjugated polymer photocatalysts with tunable components was synthesized through ternary statistical copolymerization by adopting dibenzo[g,p]chrysene and dibenzothiophene-S,S-dioxide as the donor and acceptor, respectively. It reveals that the hydrogen evolution rate (HER) of the resulting copolymers strongly depends on the molar ratio of donor to acceptor. The optimized photocatalyst DBC-BTDO-2 with a molar ratio of 1:3 for donor to acceptor shows an attractive HER of 214.43 mmol h−1 g−1 under UV–vis light without cocatalysts, which is twice as that of its counterpart DBC-BTDO with a D-A structure. Remarkably, DBC-BTDO-2 also realizes an ultra-high HER of 301.92 mmol h−1 g−1 under solar light. These findings highlight the great potential of D-A-A structural design with finely tuned component for boosting the photocatalytic activity of polymer photocatalysts.

Construction of Efficient D–A-Type Photocatalysts by B–N Bond Substitution for Water Splitting

Donor–acceptor (D–A)-type conjugated polymers are emerging as a promising platform for solar to chemical energy conversion, such as water splitting to H2 production. Disclosed herein is a new strategy that makes it possible to transform conventional conjugated polymers into D–A-type conjugated polymers by B–N bond substitution while retaining the original backbone. Using this method, we synthesize two conjugated polymers PCCP and PBNP, where PBNP containing the B–N bonds has a strong built-in electric field and a lower exciton binding energy (Eb) for solar-driven photocatalytic hydrogen evolution. PBNP without any co-catalyst addition exhibits a high hydrogen evolution rate (HER) of more than 9445 μmol g–1 h–1 for water splitting under AM 1.5G sunlight illumination (100 mW cm–2), a 28 times improvement relative to PCCP (333 μmol g–1 h–1). In addition, another contrasting BN-substituted D–A polymer PBNN also exhibits an excellent HER of 5740 μmol g–1 h–1. Theoretical and experimental studies confirm that the substitution of B–N bonds can completely change the electronic properties of the original polymer, allowing rapid charge separation and transfer. This work opens up new prospects for the preparation of efficient D–A conjugated polymer photocatalysts.

Impact of Interfaces, and Nanostructure on the Performance of Conjugated Polymer Photocatalysts for Hydrogen Production from Water

The direct conversion of sunlight into hydrogen through water splitting, and by converting carbon dioxide into useful chemical building blocks and fuels, has been an active area of research since early reports in the 1970s. Most of the semiconductors that drive these photocatalytic processes have been inorganic semiconductors, but since the first report of carbon nitride organic semiconductors have also been considered. Conjugated materials have been relatively extensively studied as photocatalysts for solar fuels generation over the last 5 years due to the synthetic control over composition and properties. The understanding of materials' properties, its impact on performance and underlying factors is still in its infancy. Here, we focus on the impact of interfaces, and nanostructure on fundamental processes which significantly contribute to performance in these organic photocatalysts. In particular, we focus on presenting explicit examples in understanding the interface of polymer photocatalysts with water and how it affects performance. Wetting has been shown to be a clear factor and we present strategies for increased wettability in conjugated polymer photocatalysts through modifications of the material. Furthermore, the limited exciton diffusion length in organic polymers has also been identified to affect the performance of these materials. Addressing this, we also discuss how increased internal and external surface areas increase the activity of organic polymer photocatalysts for hydrogen production from water.

Synergistic Contribution of Oligo(ethylene glycol) and Fluorine Substitution of Conjugated Polymer Photocatalysts toward Solar Driven Sacrificial Hydrogen Evolution.

To separately explore the importance of hydrophilicity and backbone planarity of polymer photocatalyst, a series of benzothiadiazole-based donor-acceptor alternating copolymers incorporating alkoxy, linear oligo(ethylene glycol) (OEG) side chain, and backbone fluorine substituents is presented. The OEG side chains in the polymer backbone increase the surface energy of the polymer nanoparticles, thereby improving the interaction with water and facilitating electron transfer to water. Moreover, the OEG-attached copolymers exhibit enhanced intermolecular packing compared to polymers with alkoxy side chains, which is possibly attributed to the self-assembly properties of the side chains. Fluorine substituents on the polymer backbone produce highly ordered lamellar stacks with distinct π-π stacking features; subsequently, the long-lived polarons toward hydrogen evolution are observed by transient absorption spectroscopy. In addition, a new nanoparticle synthesis strategy using a methanol/water mixed solvent is first adopted, thereby avoiding the screening effect of surfactants between the nanoparticles and water. Finally, hydrogen evolution rate of 26000 µmolg-1 h-1 is obtained for the copolymer incorporated with both OEG side chains and fluorine substituents under visible-light irradiation (λ> 420nm). This study demonstrates how the glycol side chain strategy can be further optimized for polymer photocatalysts by controlling the backbone planarity.

Tailoring the Exciton Binding Energy of 2D Conjugated Polymers for Powering Metal-Free CO2 Photoreduction

Metal-free conjugated polymer photocatalysts have provided promising environmentally friendly options in photoreduction of CO2 to high value-added chemicals. However, due to the strong Coulombic interaction of excited electron–hole pairs, most polymer photocatalysts show moderate photocatalytic performances compared with inorganic semiconductor photocatalysts. Herein, we prepared a series of 2D porphyrin-based conjugated polymers. By simply altering the aromatic acetylene linkers, we found that the exciton binding energy (Eb) of the 2D conjugated polymers could be fine-tuned. With an increasing number of acetylene units in linkers, their Eb values were reduced by 27% accompanied by promoted charge-carrier generation (about a 4-fold increase in photocurrent) and obviously enhanced catalytic activity of CO2 photoreduction. The optimized polymer, PTPP-DA, linked with diacetylene groups, affords CO and CH4 as the carbonaceous products with rates of 465 and 237 μmol g–1 h–1, respectively, without any metal or photosensitizer. This work reveals the important implications of conjugated linkers on mediating the Eb values of 2D conjugated polymers and provides new insight for designing efficient metal-free polymeric photocatalysts for CO2 reduction.

Main-chain engineering of polymer photocatalysts with hydrophilic non-conjugated segments for visible-light-driven hydrogen evolution

Photocatalytic water splitting is attracting considerable interest because it enables the conversion of solar energy into hydrogen for use as a zero-emission fuel or chemical feedstock. Herein, we present a universal approach for inserting hydrophilic non-conjugated segments into the main-chain of conjugated polymers to produce a series of discontinuously conjugated polymer photocatalysts. Water can effectively be brought into the interior through these hydrophilic non-conjugated segments, resulting in effective water/polymer interfaces inside the bulk discontinuously conjugated polymers in both thin-film and solution. Discontinuously conjugated polymer with 10 mol% hexaethylene glycol-based hydrophilic segments achieves an apparent quantum yield of 17.82% under 460 nm monochromatic light irradiation in solution and a hydrogen evolution rate of 16.8 mmol m−2 h−1 in thin-film. Molecular dynamics simulations show a trend similar to that in experiments, corroborating that main-chain engineering increases the possibility of a water/polymer interaction. By introducing non-conjugated hydrophilic segments, the effective conjugation length is not altered, allowing discontinuously conjugated polymers to remain efficient photocatalysis.

Tuning Acceptor Length in Photocatalytic Donor‐Acceptor Conjugated Polymers for Efficient Solar‐to‐Hydrogen Energy Conversion

Comprehensive SummaryThe development of conjugated polymer photocatalysts for efficient solar‐to‐hydrogen energy conversion is highly desirable for the sustainability of our society. Although the construction of donor‐acceptor (D‐A) structure in conjugated polymer photocatalysts for solar‐to‐hydrogen energy conversion has been well documented, less attention has been paid on how large D and how large A units combined together could achieve the best performance. Herein, a series of D‐A copolymers P(BDT‐DBTSOx) (x = 7, 19, 39, and 79) composed of a benzodithiophene (BDT) donor unit and an oligomeric dibenzo[b,d]thiophene sulfone (DBTSO) acceptor segment were synthesized and studied. It was found that the polymer photocatalytic stabilities under full‐arc irradiation improved upon shortening the length of the acceptor segment. Under visible light irradiation and in the presence of 3 wt% Pt cocatalyst, P(BDT‐DBTSO79) displayed the best performance with an optimal hydrogen evolution rate of 119.3 ± 5.8 mmol·g–1·h–1. This is 1.4‐fold as that of DBTSO homopolymer and 22.5‐fold as that of BDT/DBTSO alternative copolymer, highlighting the importance of acceptor length in D‐A structure for achieving high photocatalytic performance.

Adenine-functionalized conjugated polymer as an efficient photocatalyst for hydrogen evolution from water

Conjugated polymers have emerged as a promising class of organic photocatalysts for photocatalytic hydrogen evolution from water splitting due to their adjustable chemical structures and electronic properties. However, developing highly efficient organic polymer photocatalysts with high photocatalytic activity for hydrogen evolution remains a significant challenge. Herein, we present an efficient approach to enhance the photocatalytic performance of linear conjugated polymers by modifying the surface chemistry via introducing a hydrophilic adenine group into the side chain. The adenine unit with five nitrogen atoms could enhance the interaction between the surface of polymer photocatalyst and water molecules through the formation of hydrogen bonding, which improves the hydrophilicity and dispersity of the resulting polymer photocatalyst in the photocatalytic reaction solution. In addition, the strong electron-donating ability of adenine group with plentiful nitrogen atoms could promote the separation of light-induced electrons and holes. As a result, the adenine-functionalized conjugated polymer PF6A-DBTO2 shows a high photocatalytic activity with a hydrogen evolution rate (HER) of 25.21 mmol g−1 h−1 under UV-Vis light irradiation, which is much higher than that of its counterpart polymer PF6-DBTO2 without the adenine group (6.53 mmol g−1 h−1). More importantly, PF6A-DBTO2 without addition of a Pt co-catalyst also exhibits an impressive HER of 21.93 mmol g−1 h−1 under visible light (λ > 420 nm). This work highlights that it is an efficient strategy to improve the photocatalytic activity of conjugated polymer photocatalysts by the modification of surface chemistry.

Rapid chromium reduction by metal-free organic polymer photocatalysis via molecular engineering

The conversion of hexavalent chromium (Cr(VI)), a highly poisonous heavy metal found in natural environment, to less poisonous trivalent chromium (Cr(III)) has attracted a lot of interest. However, little interest has been paid to the development of metal-free catalysts. Here, we demonstrate for the first time a molecular engineering strategy to synthesize a range of donor-acceptor conjugated polymer photocatalysts, which can significantly increase the reduction efficiency of Cr(VI) by a factor of 5.2, corresponding to a significant change in the reduction reaction rate constant (from 0.0337 to 0.1740 min−1). In addition, the apparent quantum efficiency (AQE) of Cr(VI) removal was obtained, and the optimized photocatalyst (Py-SO1) could achieve the highest apparent quantum efficiency at wavelength of 420 nm in those samples. Despite the narrow light absorption of Py-SO1 polymer, its excellent exciton separation efficiency and efficient electron output enabled it to achieve excellent performance in photoreduction of Cr(VI), surpassing that of the reported metal-free photocatalysts. The results show that the present work provides a new perspective for designing suitable environmental remediation catalysts based on molecular engineering strategies.

Synergistic effect triggered by skeleton delocalization and edge induction of carbon nitride expedites photocatalytic hydrogen evolution

The multi-part functionalization of the carbon nitride (CN) molecular skeleton usually induces synergies to boost the photocatalytic activity, but it is a challenging undertaking owing to the uncontrollability in preparation process. Herein, the dual embellished CN is achieved by substituting bridged N with C atoms and grafting conjugated heterocycle on the skeleton edge through facile one-step thermal polymerization. The synergistic effect of skeleton delocalization and edge induction is triggered by the delocalized π bonds from incorporated C atoms and electron-withdrawing function of conjugated heterocycle, which importantly promotes the charge mobility and electron directional migration from center to edge of CN skeleton. The resulted photocatalytic hydrogen evolution (PHE) performance is propelled dramatically and the apparent quantum efficiency (AQE) reaches up to 22.4% at 420 nm. This work launches a typical exemplification for the high-efficiency photocatalytic application of conjugated polymer photocatalysts by triggering synergistic effect of multi-part functionalization.

Regulating donor-acceptor interactions in triazine-based conjugated polymers for boosted photocatalytic hydrogen production

The donor-acceptor (D-A) interactions between the triazine ring unit and adjacent substituents is one of the decisive factors that affect the performance of triazine-based conjugated polymer photocatalysts. Herein, we design and synthesize novel conjugated polymers by introducing electron-drawing 1,3,4-oxadiazole units into 1,3,5-triazine-based π-conjugated skeletons for photocatalytic hydrogen production. Compared with the bulk polymer, the modified photocatalysts show extended visible light harvesting and boosted charge separation. Notably, under the irradiation of full-spectrum solar light, as-synthesized polymer with bi-1,3,4-oxadiazole linkage (denoted as TCP-BOXD) shows a highly improved hydrogen-evolving rate up to 3000 μmol g−1 h−1. Furthermore, DFT calculation reveals that N atoms in the introduced 1,3,4-oxadiazole unit, coupled with those in the triazine ring, act as synergistic bi-active sites for superior photocatalytic hydrogen production. Our findings may help the rational design and controllable synthesis of novel triazine-based conjugated polymers for photocatalysis.

Effect of D/A Ratio on Photocatalytic Hydrogen Evolution Performance of Conjugated Polymer Photocatalysts

Effect of D/A Ratio on Photocatalytic Hydrogen Evolution Performance of Conjugated Polymer Photocatalysts

Molecular Design of Covalent Triazine Frameworks with Anisotropic Charge Migration for Photocatalytic Hydrogen Production

Covalent triazine frameworks (CTFs) represent promising polymeric photocatalysts for photocatalytic hydrogen production with visible light. However, the separation and transfer of charges in CTFs are isotropic because of the uniform distribution of donor-acceptor motifs in the skeleton. Herein, to achieve the anisotropic charge carrier separation and migration, thiophene (Th) or benzothiadiazole (BT) unit is selected as the dopant to modify the molecular structure of CTF-based photocatalysts. Both theoretical and experimental studies reveal that the incorporation of Th or BT units induces the anisotropic charge carrier separation and migration at the interface of CTFs. The optimized polymer manifests a much enhanced photocatalytic activity for photocatalytic hydrogen production with visible light, and thus this study provides a useful tool to design conjugated polymer photocatalysts at the molecular level for solar energy conversion.

A Universal Strategy for Boosting Hydrogen Evolution Activity of Polymer Photocatalysts under Visible Light by Inserting a Narrow‐Band‐Gap Spacer between Donor and Acceptor

AbstractIt is challenging for polymer photocatalysts to achieve high photocatalytic performance under visible light due to their weak light absorption in visible light region. Herein, a universal strategy for boosting the photocatalytic activity of donor–acceptor (D–A) conjugated polymer photocatalysts upon visible light irradiation by inserting a π‐spacer of thiophene unit between the electron donors and acceptors to form a D–π–A molecular structure is reported. The introduction of thiophene unit with narrow band gap can enhance the conjugation degree of the polymer chains and extend the light absorption range. Meanwhile, the introduction of thiophene spacer through ternary copolymerization also enables the controllability on the chemical structure of the resulting D–π–A polymers by altering the feed ratio between the electron donors and acceptors. The optimized D–π–A copolymer photocatalyst shows an impressive hydrogen evolution rate (HER) of 78.4 mmol h–1 g–1 under visible light irradiation, and the HER could be further improved to 127.9 mmol h–1 g–1 under UV–vis light irradiation by loading 1 wt% Pt co‐catalyst. More importantly, this strategy can also be extended to other polymeric photocatalysts with different donor and acceptor units, demonstrating the universality for enhancing the photocatalytic activity of polymeric photocatalysts.

Linear multiple-thiophene-containing conjugated polymer photocatalysts with narrow band gaps for achieving ultrahigh photocatalytic hydrogen evolution activity under visible light

A multiple-thiophene-containing polymer photocatalyst shows a remarkably high photocatalytic hydrogen generation rate of 108.59 mmol h−1 g−1 under visible light without adding Pt co-catalyst.