A Modular Approach to Tuning Emissive N-Quinolyl Through-Space Charge Transfer States Using sp3-Scaffolds

We have shown that palladium-catalyzed cascade processes provide modular access to rigid quinoline-containing tetracyclic amines. This modular approach enables fine-tuning of the through-space charge transfer (TSCT) state formation between the lone pair localized on the nitrogen atom in the cage moiety and the quinoline moiety by variation of both the intramolecular N-aryl distance and quinoline substitution. Decreasing this N-aryl distance enhances the formation of the TSCT species, giving control over the emission color and photoluminescence quantum yield. Methoxylation of the quinoline unit decreases the propensity of TSCT formation. The development of this structure-activity relationship provides great insight for TSCT formation with an impact on further understanding dimeric, excimeric, and exciplex species. This understanding is crucial for the work underpinning their use in biosensor applications, and the conclusions are of relevance to the broader field of photoluminescence.

Processing polymer photocatalysts for photocatalytic hydrogen evolution

Conjugated materials have emerged as competitive photocatalysts for the production of sustainable hydrogen from water over the last decade. Interest in these polymer photocatalysts stems from the relative ease to tune their electronic properties through molecular engineering, and their potentially low cost. However, most polymer photocatalysts have only been utilised in rudimentary suspension-based photocatalytic reactors, which are not scalable as these systems can suffer from significant optical losses and often require constant agitation to maintain the suspension. Here, we will explore research performed to utilise polymeric photocatalysts in more sophisticated systems, such as films or as nanoparticulate suspensions, which can enhance photocatalytic performance or act as a demonstration of how the polymer can be scaled for real-world applications. We will also discuss how the systems were prepared and consider both the benefits and drawbacks of each system before concluding with an outlook on the field of processable polymer photocatalysts.

Confined synthesis of conjugated microporous polymers for selective photocatalytic oxidation of amines

Photocatalytic oxidative coupling of amines is considered a mild, efficient, and sustainable strategy for the synthesis of imines. As a versatile organic semiconductor, conjugated microporous polymers (CMPs) are attractive in photocatalysis areas due to the diversity of their polymeric monomers. Herein, we report that in addition to the design of monomers, size-confined polymerization is also a feasible strategy to modulate the structure and photocatalysis properties of CMPs. We adopted dibromopyrazine as polymeric units to prepare pyrazine-involved hollow spherical CMPs (H-PyB) using a template method and successfully performed size-confined polymerization of hollow samples by resizing the templates. Interestingly, the small confinement space induced the formation of CMPs with better conjugate extensibility, resulting in enhanced conductivity, narrowed bandgaps, improved photoelectric performance, etc. As a result, small-sized H-PyB CMPs had superior activity for the photocatalytic oxidation of amines. Particularly, the smallest H-PyB CMPs that we designed in the present work exhibited excellent performance for the photocatalytic coupling oxidation of amines. When using benzylamine as a model substrate, the yield of the corresponding imine reached ∼ 113 mmol·g-1·h-1, accompanied by almost 100 % selectivity. Furthermore, the as-designed confined samples exhibited stable photocatalytic activity as well as good applicability for oxidative coupling of different amines. This work not merely reports a kind of CMP photocatalysts with excellent performance for the imine coupling oxidation but also proposes an alternative strategy for constructing high-performance organic photocatalysts by size-confined synthesis.

Rational design of triazine-based conjugated polymers with enhanced charge separation ability for photocatalytic hydrogen evolution

Triazine-based conjugated polymers (TCPs) are promising organic catalysts for green H2 production, since their photocatalytic performance can be easily regulated via appropriate molecular design. However, apart from weak absorption of visible light, weak charge separation and transport abilities also considerably restrict the photocatalytic performance of TCPs. Herein, we report two novel TCP photocatalysts with donor-acceptor (D-A) and donor-π-acceptor (D-π-A) structures using dibenzo[g,p]chrysene (Dc), thiophene (T), and 2,4,6-triphenyl-1,3,5-triazine (Tz) as the donor, π-spacer, and acceptor, respectively. Compared to Dc-Tz with a D-A structure, Dc-T-Tz exhibits a broader light absorption edge and more efficient charge separation and transmission due to its D-π-A structure and strong dipole effect. These properties enable Dc-T-Tz to display a prominent H2 production rate of 45.13 mmol h-1 g-1 under ultraviolet-visible (UV-Vis) light (λ > 300 nm). Therefore, Dc-T-Tz represents state-of-the-art TCP photocatalysts to date.

Organic Donor-Acceptor Systems for Photocatalysis

Organic semiconductor materials are considered to be promising photocatalysts due to their excellent light absorption by chromophores, easy molecular structure tuning, and solution-processable properties. In particular, donor-acceptor (D-A) type organic photocatalytic materials synthesized by introducing D and A units intra- or intermolecularly, have made great progress in photocatalytic studies. More and more studies have demonstrated that the D-A type organic photocatalytic materials combine effective carrier separation, tunable bandgap, and sensitive optoelectronic response, and are considered to be an effective strategy for enhancing light absorption, improving exciton dissociation, and optimizing carrier transport. This review provides a thorough overview of D-A strategies aimed at optimizing the photocatalytic performance of organic semiconductors. Initially, essential methods for modifying organic photocatalytic materials, such as interface engineering, crystal engineering, and interaction modulation, are briefly discussed. Subsequently, the review delves into various organic photocatalytic materials based on intramolecular and intermolecular D-A interactions, encompassing small molecules, conjugated polymers, crystalline polymers, supramolecules, and organic heterojunctions. Meanwhile, the energy band structures, exciton dynamics, and redox-active sites of D-A type organic photocatalytic materials under different bonding modes are discussed. Finally, the review highlights the advanced applications of organic photocatalystsand outlines prospective challenges and opportunities.

One Pot Photomediated Formation of Electrically Conductive Hydrogels

Electrically conductive hydrogels represent an innovative platform for the development of bioelectronic devices. While photolithography technologies have enabled the fabrication of complex architectures with high resolution, photoprinting conductive hydrogels is still a challenging task because the conductive polymer absorbs light which can outcompete photopolymerization of the insulating scaffold. In this study, we introduce an approach to synthesizing conductive hydrogels in one step. Our approach combines the simultaneous photo-cross-linking of a polymeric scaffold and the polymerization of 3,4-ethylene dioxythiophene (EDOT), without additional photocatalysts. This process involves the copolymerization of photo-cross-linkable coumarin-containing monomers with sodium styrenesulfonate to produce a water-soluble poly(styrenesulfonate-co-coumarin acrylate) (P(SS-co-CoumAc)) copolymer. Our findings reveal that optimizing the [SS]:[CoumAc] ratio at 100:5 results in hydrogels with the strain at break up to 16%. This mechanical resilience is coupled with an electronic conductivity of 9.2 S m-1 suitable for wearable electronics. Furthermore, the conductive hydrogels can be photopatterned to achieve micrometer-sized structures with high resolution. The photo-cross-linked hydrogels are used as electrodes to record stable and reliable surface electromyography (sEMG) signals. These novel photo-cross-linkable polymers combined with one-pot PEDOT (poly-EDOT) polymerization open possibilities for rapidly prototyping complex bioelectronic devices and creating custom-designed interfaces between electronics and biological systems.

Replacing CC Unit with B←N Unit in Isoelectronic Conjugated Polymers for Enhanced Photocatalytic Hydrogen Evolution

Three linear isoelectronic conjugated polymers PCC, PBC, and PBN are synthesized by Suzuki-Miyaura polycondensation for photocatalytic hydrogen (H2 ) production from water. PBN presented an excellent photocatalytic hydrogen evolution rate (HER) of 223.5 µmol h-1 (AQY420  = 23.3%) under visible light irradiation, which is 7 times that of PBC and 31 times that of PCC. The enhanced photocatalytic activity of PBN is due to the improved charge separation and transport of photo-induced electrons/holes originating from the lower exciton binding energy (Eb ), longer fluorescence lifetime, and stronger built-in electric field, caused by the introduction of the polar B←N unit into the polymer backbone. Moreover, the extension of the visible light absorption region and the enhancement of surface catalytic ability further increase the activity of PBN. This work reveals the potential of B←N fused structures as building blocks as well as proposes a rational design strategy for achieving high photocatalytic performance.

Synthesis of 3,3'-(Ethane-1,2-diylidene)bis(indolin-2-one) Promoted by Thermally-activated Electron Transfer and Photoreduction of CO2 to CH4 and CO

A Sonogashira coupling reaction leads to the formation of a serendipitous product C with the 3,3'-(ethane-1,2-diylidene)bis(indolin-2-one) unit. To our knowledge, our study provides the first example demonstrating that electron transfer between isoindigo and triethylamine can be thermally activated and can be employed in synthesis. The physical properties of C suggest that it possesses decent photo-induced electron-transfer capabilities. Under the illumination of 136 mW cm-2 intensity, C yields ≈2.4 mmol gcat -1 (per gram of catalyst) of CH4 and ≈0.5 mmol gcat -1 of CO in 20 h in the absence of additional metal, co-catalyst, and amine sacrificial agent. The primary kinetic isotope effect suggests that the bond cleavage of water is a rate-determining step in the reduction. Moreover, the CH4 and CO production can be boosted as the illuminance increases. This study demonstrates that organic donor-acceptor conjugated molecules are potential photocatalysts for CO2 reduction.

Molecular Engineering in D-π-A-A-Type Conjugated Microporous Polymers for Boosting Photocatalytic Hydrogen Evolution

Conjugated microporous polymer (CMP) photocatalysts with donor-π-acceptor (D-π-A) or donor-acceptor (D-A) structures have garnered great attention for solar-driven hydrogen generation because of their inherent charge separation nature and high surface area. Herein, we design a series of D-π-A-A-type CMP photocatalysts to uncover the influence of the content of the dibenzo[b,d]thiophene-S-S-dioxide (BTDO) acceptor on the photocatalytic activity. The results demonstrate that the acceptor content in the D-π-A-A-type CMP photocatalysts affects the electronic structure, the availability of reaction sites, and the separation between light-generated electrons and holes, which mainly determine the photocatalytic performance for H₂ release. Benefiting from the synergy of light absorption, hydrophilicity, and active sites, the bare polymer PyT-BTDO-2 with an optimized BTDO content exhibits a high H₂ production rate of 230.06 mmol h^-1 g^-1 under simulated sunlight, manifesting that the strategy of D-π-A-A structural design is efficacious for boosting the photocatalytic performance of CMP photocatalysts.

3D-Printed Organic Conjugated Trimer for Visible-Light-Driven Photocatalytic Applications

Small molecule organic semiconductors (SMOSs) have emerged as a new class of photocatalysts that exhibit visible light absorption, tunable bandgap, good dispersion, and solubility. However, the recovery and reusability of such SMOSs in consecutive photocatalytic reactions is challenging. This work concerns a 3D-printed hierarchical porous structure based on an organic conjugated trimer, named EBE. Upon manufacturing, the photophysical and chemical properties of the organic semiconductor are maintained. The 3D-printed EBE photocatalyst shows a longer lifetime (11.7 ns) compared to the powder-state EBE (1.4 ns). This result indicates a microenvironment effect of the solvent (acetone), a better dispersion of the catalyst in the sample, and reduced intermolecular π-π stacking, which results in improved separation of the photogenerated charge carriers. As a proof-of-concept, the photocatalytic activity of the 3D-printed EBE catalyst is evaluated for water treatment and hydrogen production under sun-like irradiation. The resulting degradation efficiencies and hydrogen generation rates are higher than those reported for the state-of-the-art 3D-printed photocatalytic structures based on inorganic semiconductors. The photocatalytic mechanism is further investigated, and the results suggest that hydroxyl radicals (HO⋅) are the main reactive radicals responsible for the degradation of organic pollutants. Moreover, the recyclability of the EBE-3D photocatalyst is demonstrated in up to 5 uses. Overall, these results indicate the great potential of this 3D-printed organic conjugated trimer for photocatalytic applications.

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.

Why Do Sulfone-Containing Polymer Photocatalysts Work So Well for Sacrificial Hydrogen Evolution from Water?

Many of the highest-performing polymer photocatalysts for sacrificial hydrogen evolution from water have contained dibenzo[b,d]thiophene sulfone units in their polymer backbones. However, the reasons behind the dominance of this building block are not well understood. We study films, dispersions, and solutions of a new set of solution-processable materials, where the sulfone content is systematically controlled, to understand how the sulfone unit affects the three key processes involved in photocatalytic hydrogen generation in this system: light absorption; transfer of the photogenerated hole to the hole scavenger triethylamine (TEA); and transfer of the photogenerated electron to the palladium metal co-catalyst that remains in the polymer from synthesis. Transient absorption spectroscopy and electrochemical measurements, combined with molecular dynamics and density functional theory simulations, show that the sulfone unit has two primary effects. On the picosecond timescale, it dictates the thermodynamics of hole transfer out of the polymer. The sulfone unit attracts water molecules such that the average permittivity experienced by the solvated polymer is increased. We show that TEA oxidation is only thermodynamically favorable above a certain permittivity threshold. On the microsecond timescale, we present experimental evidence that the sulfone unit acts as the electron transfer site out of the polymer, with the kinetics of electron extraction to palladium dictated by the ratio of photogenerated electrons to the number of sulfone units. For the highest-performing, sulfone-rich material, hydrogen evolution seems to be limited by the photogeneration rate of electrons rather than their extraction from the polymer.

Covalent organic frameworks with high quantum efficiency in sacrificial photocatalytic hydrogen evolution

Organic semiconductors offer a tunable platform for photocatalysis, yet the more difficult exciton dissociation, compared to that in inorganic semiconductors, lowers their photocatalytic activities. In this work, we report that the charge carrier lifetime is dramatically prolonged by incorporating a suitable donor-acceptor (β-ketene-cyano) pair into a covalent organic framework nanosheet. These nanosheets show an apparent quantum efficiency up to 82.6% at 450 nm using platinum as co-catalyst for photocatalytic H₂ evolution. Charge carrier kinetic analysis and femtosecond transient absorption spectroscopy characterizations verify that these modified covalent organic framework nanosheets have intrinsically lower exciton binding energies and longer-lived charge carriers than the corresponding nanosheets without the donor-acceptor unit. This work provides a model for gaining insight into the nature of short-lived active species in polymeric organic photocatalysts.

Water Oxidation and Hydrogen Evolution with Organic Photooxidants: A Theoretical Perspective

In this Perspective, we discuss a novel water-splitting scenario, namely the direct oxidation of water molecules by organic photooxidants in hydrogen-bonded chromophore-water complexes. In comparison with the established scenario of semiconductor-based water splitting, the distance of electron transfer processes is thereby reduced from mesoscopic scales to the Ångström scale, and the time scale is reduced from milliseconds to femtoseconds, which suppresses competing loss processes. The concept is illustrated by computational studies for the heptazine-H₂O complex. The excited-state landscape of this complex has been characterized with ab initio electronic-structure methods and the proton-coupled electron-transfer dynamics has been explored with nonadiabatic dynamics simulations. A unique feature of the heptazine chromophore is the existence of a low-lying and exceptionally long-lived ¹ππ* state in which a substantial part of the photon energy can be stored for hundreds of nanoseconds and is available for the oxidation of water molecules. The calculations reveal that the absorption spectra and the photochemical functionalities of heptazine chromophores can be systematically tailored by chemical substitution. The options of harvesting hydrogen and the problems posed by the high reactivity of OH radicals are discussed.

Conjugated porphyrin materials for solar fuel generation

Abstract:

Conjugated materials have emerged as a new class of photocatalysts for solar fuel generation, thus allowing for the Sun’s energy to be converted into a storable fuel that can be used without further emissions at the point of use. Many different building blocks have been used to make conjugated materials that act as photocatalysts allowing for efficient light absorption and tuing of photophysical properties. The porphyrin moiety is a very interesting building block for photocatalysts as the large π-conjugated system allows efficient light absorption. Metalation of porphyrins allows for further tuning of the materials’ properties, thus further expanding the property space that these materials can cover. This allows to design and better control over the properties of the materials, which is discussed in this review together with the state-of-the-art in porphyrin photocatalysts and hybrid systems.

EDOT-based conjugated polymers accessed via C-H direct arylation for efficient photocatalytic hydrogen production

3,4-Ethylene dioxythiophene (EDOT), as a monomer of commercial conductive poly(3,4-ethylene dioxythiophene) (PEDOT), has been facilely incorporated into a series of new π-conjugated polymer-based photocatalysts, i.e., BSO2-EDOT, DBT-EDOT, Py-EDOT and DFB-EDOT, through atom-economic C-H direct arylation polymerization (DArP). The photocatalytic hydrogen production (PHP) test shows that donor-acceptor (D-A)-type BSO2-EDOT renders the highest hydrogen evolution rate (HER) among the linear conjugated polymers (CPs) ever reported. A HER up to 0.95 mmol h-1/6 mg under visible light irradiation and an unprecedented apparent quantum yield of 13.6% at 550 nm are successfully achieved. Note that the photocatalytic activities of the C-H/C-Br coupling-derived EDOT-based CPs are superior to those of their counterparts derived from the classical C-Sn/C-Br Stille coupling, demonstrating that EDOT is a promising electron-rich building block which can be facilely integrated into CP-based photocatalysts. Systematic studies reveal that the enhanced water wettability by the integration of polar BSO2 with hydrophilic EDOT, the increased electron-donating ability by O-C p-π conjugation, the improved electron transfer by D-A architecture, broad light harvesting, and the nano-sized colloidal character in a H2O/NMP mixed solvent rendered BSO2-EDOT as one of the best CP photocatalysts toward PHP.

Linear conjugated polymer photocatalysts with varied linker units for photocatalytic hydrogen evolution from water

Polymer photocatalysts have shown potential as for light-driven hydrogen evolution from water. Here we studied the relative importance of the linker type in two series of conjugated polymers based on...

Nanostructured Lateral Boryl Substitution Conjugated Donor-Acceptor Oligomers for Visible-Light-Driven Hydrogen Production

Poor charge separation is the main factor that limits the photocatalytic hydrogen generation efficiency of organic conjugated polymers. In this work, a series of linear donor-acceptor (D-A) type oligomers are synthesized by a palladium-catalyzed Sonogashira-Hagihara coupling of electron-deficient diborane unit and different dihalide substitution sulfur functionalized monomers. Such diborane-based A unit exerts great impact on the resulting oligomers, including distinct semiconductor characters with isolated lowest unoccupied molecular orbital (LUMO) orbits locating in diborane-containing fragment, and elevated LUMO level higher than water reduction potential. Relative to A-A type counterpart, the enhanced dipole polarization effect in D-A oligomers facilitates separation of photogenerated charge carriers, as evidenced by notably prolonged electron lifetime. Owing to π-π stacking of rigid backbone, the oligomers can aggregate into an interesting 2D semicrystalline nanosheet (≈2.74 nm), which is rarely reported in linear polymeric photocatalysts prepared by similar carbon-carbon coupling reaction. Despite low surface area (30.3 m² g^-1 ), such ultrathin nanosheet D-A oligomer offers outstanding visible light (λ > 420 nm) hydrogen evolution rate of 833 µmol g^-1 h^-1 , 14 times greater than its A-A analogue (61 µmol g^-1 h^-1 ). The study highlights the great potential of using boron element to construct D-A type oligomers for efficient photocatalytic hydrogen generation.

Efficient non-metal based conducting polymers for photocatalytic hydrogen production: comparative study between polyaniline, polypyrrole and PEDOT

Incorporation of conducting polymers (CPs) with TiO2 is considered a promising pathway toward the fabrication of highly efficient non-metal based photocatalysts. Herein, we report the fabrication of TiO2@polyaniline, TiO2@polypyrrole, and TiO2@poly(3,4-ethylenedioxythiophene) photocatalyst heterostructures via the facile wet incipient impregnation method. The mass ratios of CPs in the composites were optimized. The structure, morphology, optical and surface texture of the samples were characterized by XRD, TEM, TGA, DRS, and N2-physisorption techniques. The TiO2@2PEDOT, TiO2@2PPy, and TiO2@5PAn composites were found to exhibit the highest H2 evolution rate (HER) of 1.37, 2.09, and 3.1 mmol h-1 g-1, respectively. Compared to bare TiO2, the HER was significantly enhanced by 16, 24, and 36-fold, respectively. Photoelectrochemical measurements (CV, CA and EIS) were conducted, to evaluate the photoelectric properties of the synthesized composites and assist in understanding the photocatalytic mechanism. The deposition method plays a key-role in forming the photocatalyst/CP interface. This simple impregnation route was found to provide an excellent interface for charge transfer between composite components compared to chemisorption and in situ polymerization methods. This study sheds light on the promising effect of CP incorporation with semiconductor photocatalysts, as a cheap and efficient matrix, on photocatalytic performance.

Polarization-induced charge separation in conjugated microporous polymers for efficient visible light-driven C-3 selenocyanation of indoles

Conjugated microporous polymers (CMPs) are cost-effective photocatalysts in organic transformations, while they are usually limited by the insufficient separation of photogenerated charges. Here we report a polarization strategy through molecular geometry optimization to promote the charge separation of CMPs. Three CMP photocatalysts with an alternative donor-acceptor skeleton and tunable symmetry were synthesized by the oxidative coupling of bis-carbazoles with electron-deficient bridges (benzene/pyridine/pyrimidine). Simply regulating the polarization of the starting monomers leads to tailorable porosity, photoelectric properties, and photocatalytic activity of the CMPs. They exhibited high efficiency in C-3 selenocyanation of indoles under visible-light and at room temperature, and pyridine-based CMPs with the largest dipole moment gave a yield of up to 94%, superior to their state-of-the-art photocatalyst counterparts. Photo-physical experiments combined with theoretical calculations further supported that the incorporation of the polarized linker introduced an internal electric field, benefitting efficient charge separation. This offered new insight into developing high-performance photocatalysts.

Conjugated nanomaterials for solar fuel production

Photocatalytic hydrogen production from water has the potential to fulfil future energy needs by producing a clean and storable fuel. In recent years polymer photocatalysts have attracted significant interest in an attempt to address these challenges. One reason organic photocatalysts have been considered an attractive target is their synthetic modularity, therefore, the ability to tune their opto-electronic properties by incorporating different building blocks. A wide range of factors has been investigated and in particular nano-sized particles have found to be highly efficient due to the size effect resulting from the ability of these to increase the number of charges reaching catalytic sites.

Promoting charge separation in donor–acceptor conjugated microporous polymers via cyanation for the photocatalytic reductive dehalogenation of chlorides

Conjugated microporous polymers (CMPs) have emerged as promising heterogeneous photocatalysts for organic transformations owing to their structural designability and functional versatility.

Tunable linear donor–π–acceptor conjugated polymers with a vinylene linkage for visible-light driven hydrogen evolution

Extending the in-plane conjugation or/and increasing the electron push–pull interaction of linear D–π–A polymers with a vinylene linkage could broaden the visible-light absorption band, promote the charge separation and transfer and the photocatalytic hydrogen production.

Efficient synthesis of vinylene-linked conjugated porous networks via the Horner-Wadsworth-Emmons reaction for photocatalytic hydrogen evolution

A simple, yet efficient synthetic approach for the construction of vinylene-linked conjugated porous networks was developed. Based on the Horner-Wadsworth-Emmons reaction, the condensation polymerization for the formation of an sp2 carbon-linkage can be achieved at room temperature. The resulting vinylene-linked frameworks exhibit a promising porous nature with the best surface area of up to 1373 m2 g-1. Their intrinsic conjugated architectures and semiconducting properties lead to photocatalytic evolution of hydrogen from water under visible light irradiation. This new synthesis method provides a facile means to prepare attractive sp2 carbon linked porous organic frameworks.