Donor-acceptor Structures Research Articles

Construction of pyrrolo[3,2-b]pyrrolyl-linked covalent organic polymers to promote continuous overall H2O2 production

Sacrificial agent-free, solar-driven photocatalytic oxygen reduction holds promise for hydrogen peroxide production. However, the rapid recombination of electron–hole pairs in catalysts and the slow diffusion rate of oxygen on the catalyst surfaces significantly hinder the efficiency of hydrogen peroxide production. To address these issues, we developed a novel class of pyrrolo[3,2-b]pyrrolyl-linked covalent organic polymers (COPs) combined with microreactor technology to enhance hydrogen peroxide synthesis. These structurally well-defined polymers feature photoactive pyrrolo[3,2-b]pyrrolyl units were assembled from aldehyde, aniline, and 2,3-butanedione through a one-pot three-component reaction. Integrating pyrrolo[3,2-b]pyrrolyl moieties into the COPs creates donor–acceptor structures that facilitate the separation of photogenerated electrons and holes. Among them, COP-2 achieved the highest H2O2 yield of 5446 μmol g−1 h−1. Furthermore, we designed a coiled tube photomicroreactor to improve mass transfer efficiency in the gas–liquid–solid triphase system, boosting the H2O2 yield to 20285 μmol g−1h−1 without the need for a sacrificial agent. This study offers new insights into integrating polymer photocatalysts with microflow technology, underscoring the potential for future green and continuous H2O2 production.

Construction of Donor-Acceptor-Type Conjugated Microporous Polymers by Oxidative Coupling of Boranil-Carbazole Mixed Monomers for Enhanced Photocatalytic Oxidation.

Conjugated microporous polymers (CMPs), featuring photoactive structures, a high surface area, robust thermal stability, and facile modulation, provide a versatile platform for fabricating diverse heterogeneous photocatalysts. The incorporation of donor-acceptor (D-A) structures into CMPs to increase their charge separation potential and enhance the photocatalytic efficacy is a viable strategy. In this work, we designed and synthesized a unique set of D-A monomers, incorporating boranil dyes as electron-deficient moieties and carbazoles as electron-rich subunits. Subsequently, D-A CMPs were prepared via an economical and environmentally friendly oxidation coupling reaction, and their potential in photocatalytic oxidation reactions was investigated. Modulation of the polymer's photoelectronic properties and photocatalytic performance can be achieved by adjusting the boranil content in the monomer. The polymer pCZFB-3, with the highest content of boranil units, exhibited an optimal photocatalytic activity. This finding confirms that strengthening the D-A effect can significantly enhance a catalyst's photoelectronic properties and catalytic efficacy. This study presents insights into designing innovative heterogeneous photocatalysts based on boron-containing dyes.

Immobilization of uranyl ions in water by covalent organic frameworks: In-situ utilization of the photocatalytic produced H2O2

The photocatalytic extraction of uranyl (UO22+) ions from aqueous solutions using covalent organic frameworks (COFs) represents a promising approach for mitigating environmental concerns and recovering uranium resources. In this study, benzotrithiophene (BTT)-based COFs with electron donor–acceptor structures were synthesized to harness photocatalytic capabilities. Under visible light, the synergistic effect of COF-BTT-TAPB and methanol achieved UO22+ removal efficiencies of 99 % and 92 % (at an initial U concentration of 50 ppm) with and without N2 pretreatment. Notably, even in the absence of a sacrificial agent, COF-BTT-TAPB can remove 36 % of UO22+ from water. The photocatalytic mechanism for UO22+ immobilization on COF-BTT-TAPB does not involve the conventional reduction of U(VI). Instead, the formation of the solid-state (UO2)O2(H2O)2 results from an in-situ reaction between UO22+ and H2O2, the latter produced through visible light-induced catalysis on the COF-BTT-TAPB surface. Additionally, UO22+ can be photoexcited under visible light, undergoing a homogeneous reaction with methanol in the presence of O2 to produce (UO2)O2(H2O)2. This study presents a novel approach for extracting UO22+ from aqueous solutions by leveraging photocatalytically generated H2O2.

A Donor-Acceptor Molecular Octopus and the CLICK Procedure in Columnar Liquid Crystal Phases.

Two star-shaped mesogens with a (meso-tetraphenylporphinato) zinc (II) core and bithiophene conjugated arms with 3,4,5-trisdodecyloxyphenyl periphery were synthesized. One of these molecules was decorated with four fullerenes via an aliphatic spacer. This is the sterically overcrowded compound with an octapodal morphology. The other star lacks the fullerenes and provides free space between the conjugated arms. This mesogen does not aggregate in solution, but in solid state it forms a hexagonal columnar and a highly ordered oblique helical columnar phase, while the octopus molecule assembles in an amorphous solid. Photophysical studies of the octapodal compound in solution and the solid thin film reveal the formation of J-type aggregates, in which the interaction between donors (porphyrin) and acceptors (fullerene) dominates leading to absorption bands in the NIR region of the spectra. The mixture of both compounds results in a self-assembly which is called the Click procedure. Fullerenes of the octopus nanosegregate in the pockets of the star mesogens generating hexagonal columnar structures with a regular stacking along the columnar axis. Thus providing free space is a tool to control the competition between supramolecular interactions and nanosegregation. Such liquid-crystalline donor-acceptor structures may play a role in future LC photovoltaic applications.

Unveiling the photoexcitation mechanism of COF-2CN material: From the perspective of molecular structure

Covalent organic frameworks (D-A COFs) with electron donor–acceptor structures possess electron transfer channels between the donor and acceptor components, making them potential candidates for applications requiring high photocatalytic activity. In this paper, COF-2CN is selected as a representative material, and computational studies are conducted based on density functional theory (DFT) to gain further insight into its excitation mechanism. First, we used first-principles calculations to determine the molecular structure of COF-2CN. The results reveal that the material has a hexagonal pore shape with a pore size of 25.81 Å, and all atoms in the monolayer structure lie in the same plane. Next, we plotted the density of states (DOS) for COF-2CN and determined that the highest occupied molecular orbital (HOMO) energy level is −0.22 eV. Additionally, we calculated the oscillator strengths for the first 100 excited states and selected 8 states with values greater than 0.2 for further analysis. Subsequently, we analyzed the distribution of electron holes in these excited states and found that, in most cases, the electrons and holes are distributed along the edges of the six-membered rings, with the holes having a broader distribution than the electrons. Finally, we simulated the UV–visible absorption spectrum of COF-2CN, finding that maximum light absorption occurs at a wavelength of 466.1 nm, with an intensity of approximately 454,057 L mol−1 cm−1.

(Invited) Synthesis and Optical Properties of Chiral Spiro π-Conjugated Compounds with Donor–Acceptor Structure

Chiral spiro π-conjugated compounds with donor–acceptor structures were synthesized. Carbazol-9-yl, 9,9-dimethylacridin-10-yl, phenoxazin-10-yl donor units were introduced on the spiro-fused acceptor unit with sulfonyl groups. The donor units were found to have great impact on the longest absorption maximum and emission maximum. The chiral spiro π-conjugated compounds with 9,9-dimethylacridin-10-yl or phenoxazin-10-yl donor units showed delayed fluorescence. All of three compounds exhibited circularly polarized luminescence both in solution and solid states.

Construction and Electrochromic Properties of Two-Dimensional Covalent Organic Frameworks with Donor-Acceptor Structures of Triphenylamine and Bipyridine

The stacking between layers of a two-dimensional covalent organic framework (COF) leads to overlapping π orbitals, which enables charge carriers to be transported quickly through these pre-designed π orbitals. The two-dimensional COF featuring donor-acceptor interactions represents a straightforward approach for fabricating a high-performance organic electrochromic device. In this paper, N, N, N’, N’-tetrad(4-aminophenyl)−1,4-phenylenediamine (TPDA) with electron-rich structure and 2,2’-bipyridine-5,5’-dialdehyde (BPDA) with strong electron absorption ability were used as the construction unit. COFTPDA-BPDA electrochromic materials with donor-acceptor structure were synthesized by Schiff base reaction, which can achieve reversible switching from red to dark gray. The color/fade time of the film at 474 nm wavelength is 6.8 s/11.9 s. The contrast retention rate of the film can reach 97.6% after 20 potential cycles, the memory time is as long as 4278 s. The present study demonstrates that constructing a donor-acceptor (D-A) structural unit with conjugated triphenylamine as the electron donor linked to bipyridine electron-withdrawing groups enhances charge transfer and redox reactions. With the success of this design strategy, the construction of the D-A structure is an important methodology for improving the electrochromic properties of materials.

Rational design of g-C3N4-based photocatalysts with intramolecular donor-acceptor structures for deep oxidation of emerging organic micropollutants

An in-situ route for the fabrication of intramolecular electron donor-acceptor (D-A) structured g-C3N4 photocatalysts is developed by the preorganization of urea with the precursor of electron donor units, such as 4-iodobenzaldehyde, 5-bromo-2-thiophenaldehyde and 5-bromo-2-furaldehyde, followed by thermal polymerization. In the resulting benzene unit-, thiophene unit- and furan unit-based D-A structured g-C3N4 photocatalysts, namely, BHCNx, SCNx, and OCNx, the incorporated electron donor units (i.e., benzene units, thiophene units and furan units) link with the electron acceptors (i.e., heptazine units) through CN and CN covalent bonds. In comparison of bulk g-C3N4, all D-A structured g-C3N4 display notably enhanced visible-light photocatalytic oxidation activity in the degradation of emerging organic micropollutants, p-nitrophenol (PNP) and methylparaben (MPB), in which the apparent first-order kinetic constant of the optimized BHCN2, SCN2, and OCN2 is 1.6, 3.0 and 6.8 times higher than bulk g-C3N4 for PNP degradation and 1.9, 3.4 and 2.6 times higher than bulk g-C3N4 for MPB degradation. Importantly, the micropollutants can be not only degraded but also mineralized completely. The catalysts are also robust in the removal of mixed emerging organic micropollutants in pharmaceutical wastewater, exhibiting potential of dealing with organic micropollutants in wastewater. Mechanism studies reveal that the unique intramolecular charge transfer process in the catalysts enhances visible-light harvesting capacity, boosts directional transfer of the photogenerated charges and increases adsorption towards oxygen molecules. These collective improvements can maximum utilization the photogenerated electrons and holes to yield plentiful reactive oxygen species for deep oxidization of the pollutants.

Bis-Ortho-Donor-Modification of Boracyclic π-Electron Systems beyond Steric Protection to Produce Thermally Activated Delayed Fluorescence Materials.

Polycyclic π-conjugated compounds that contain tricoordinate boron atoms at their periphery represent an attractive class of materials with electron-accepting character. Their molecular design generally requires the introduction of a bulky aryl group onto the boron atom, where it provides predominantly kinetic stabilization. The addition of extra functionality to the aryl group on the boron atom can be expected to further expand the potential utility of this class of materials. Herein, we report the synthesis of a series of boracyclic π-conjugated molecules with firm ortho B⋅⋅⋅N nonbonding interactions by introducing N-containing electron-donors at the ortho-positions of the aryl group on the boron atom. X-ray crystallographic analysis revealed that the combination of a planar boracyclic π-skeleton with only sp2 carbons and a strong electron-donating phenothiazine moiety results in a particularly short B⋅⋅⋅N distance. Theoretical study provided insights into the inherent nature of the B⋅⋅⋅N interaction. Owing to their donor-acceptor (D-A) structures, these molecules exhibit substantially red-shifted fluorescence in solution, albeit that the fluorescence quantum yields (ΦF) are low. In contrast, when incorporated into films, these compounds exhibit thermally activated delayed fluorescence (TADF) with improved ΦF values. Organic light-emitting diodes (OLEDs) fabricated using the ortho-donor-substituted derivatives exhibit orange-red electroluminescence.

Programming Tetrathiafulvalene-Based Covalent Organic Frameworks for Promoted Photoinduced Molecular Oxygen Activation.

Despite the pivotal role of molecular oxygen (O2) activation in artificial photosynthesis, the activation efficiency is often restricted by sluggish exciton dissociation and charge transfer kinetics within polymer photocatalysts. Herein, we propose two tetrathiafulvalene (TTF)-based imine-linked covalent organic frameworks (COFs) with tailored donor-acceptor (D-A) structures, TTF-PDI-COF and TTF-TFPP-COF, to promote O2 activation. Because of enhanced electron push-pull interactions that facilitated charge separation and transfer behavior, TTF-PDI-COF exhibited superior photocatalytic activity in electron-induced O2 activation reactions over TTF-TFPP-COF under visible light irradiation, including the photosynthesis of (E)-3-amino-2-thiocyano-α,β-unsaturated compounds and H2O2. These findings highlight the significant potential of the rational design of COFs with D-A configurations as suitable candidates for advanced photocatalytic applications.

Piezocatalytic Performances of Conjugated Microporous Polymers with Donor–Acceptor Structures for Overall Water Splitting

Piezocatalytic Performances of Conjugated Microporous Polymers with Donor–Acceptor Structures for Overall Water Splitting

Multiple hydrogen-bonding induced nonconventional red fluorescence emission in hydrogels

The development of unconventional long-wavelength fluorescent polymer hydrogels without using polycyclic aromatic hydrocarbons or extended π-conjugation is a fundamental challenge in luminescent materials owing to a lack of understanding regarding the spatial interactions induced inherent clustering-triggered emission under water-rich conditions. Inspired by the color change of protein astaxanthin as a result of heat-induced denaturation, we propose a thermodynamically driven strategy to develop red fluorescence (~610 nm) by boiling multiple hydrogen-bonded poly(N-acryloylsemicarbazide) hydrogels in a water bath. We reveal that thermodynamically driven conformational changes of polymer chains from isolated hydrogen bonding donor-acceptor structures to through-space interaction structures induce intrinsic fluorescence shifts from blue to red during clustering-triggered emission. The proposed multiple hydrogen-bonding supramolecular hydrogel shows good fluorescence stability, mechanical robustness, and 3D printability for customizable shaping. We provide a viable method to prepare nonconventional long-wavelength fluorescent hydrogels towards soft fluorescent devices without initially introducing any fluorescent components.

Highly selective removal of U(VI) from aqueous solutions by porous nanomaterials

AbstractWith the fast development of nuclear energy peaceful utilization, large amounts of U(VI) are not only required to be extracted from solutions for sustainable nuclear fuel supply but also inevitably released into the environment to result in pollution, which is hazardous to human health. Thereby, the selective extraction of U(VI) from aqueous solutions is crucial to U(VI) pollution treatment and also to nuclear industry sustainable development. In this minireview, we summarized the selective extraction of U(VI) from solutions by porous nanomaterials (i.e., porous carbon nanomaterials, covalent organic frameworks, metal organic frameworks, and other nanomaterials) using different techniques, that is, sorption, electrocatalysis, photocatalysis, and other strategies. The efficient high extraction ability is dependent on the properties of porous nanomaterials and the used techniques. The high surface areas, abundant active sites, and functional groups are efficient for the high sorption of U(VI), but the special functional groups such as amidoxime groups are more critical for high selective extraction. The electrocatalytic extraction is related to the active sites, especially the single atom sites, of the porous nanomaterials as electrode. The special functional groups, bandgap, electron transfer pathway and electron donor–acceptor structures of photocatalysts contribute the high photocatalytic extraction of U(VI). The interaction mechanisms are discussed from spectroscopic analysis and computational simulation at molecular level. In the end, the challenges and prospectives for the efficient extraction of U(VI) are described.

Theoretical investigation on graphene induced self-alignment of donor-acceptor structures

Non-covalent functionalization of graphene (Gr) with conducting polymers (CPs) improves the electrical and mechanical properties of both materials owing to the noncovalent π-π stacking interaction between the surface and the aromatic rings in CPs. However, a comprehensive understanding of the self-organization and chain orientation of CPs on the Gr surface remains lacking, which may limit the engineering of CP-Gr complexes with optimized morphology for optoelectronic applications. This study focuses on the Density Functional Theory (DFT) calculations and homemade Python programming code to investigate the self-organization of donor (D)-acceptor (A) type molecules on the Gr surface, aiming to elucidate factors governing the direction of self-alignment. Specifically, basic structural parameters characterizing the aromatic D-A molecules, such as the intramolecular aromatic ring distances, are explored to determine the rules for the preferred alignment direction of the CP monomers on the Gr surface, either in armchair or zigzag paths. Additionally, the influence of Gr on the morphological properties of aromatic molecules, variations in interaction energies between Gr and D-A-D systems with changing A units, and the impact of Gr size on molecular fitting characteristics are examined. The accurate determination of chain alignment direction, which can be applied for all conjugated small molecules and polymers at 2D material interfaces is crucial for guiding experimental studies aimed at tailoring structures and enhanced properties, such as promoting the crystalline domains with high electrical conductivity.

Long-Lived Charge Separation in Single Crystals of an Electron Donor Covalently Linked to Four Acceptor Molecules.

Crystalline donor-acceptor (D-A) systems serve as an excellent platform for studying CT exciton creation, migration, and dissociation into free charge carriers for solar energy conversion. Donor-acceptor cocrystals have been utilized to develop an understanding of CT exciton formation in ordered organic solids; however, the strong electronic coupling of the D and A units can sometimes limit charge separation lifetimes due to their close proximity. Covalent D-A systems that preorganize specific donor-acceptor structures can assist in engineering crystal morphologies that promote long-lived charge separation to overcome this limitation. Here we investigate photogenerated CT exciton formation in a single crystal of a 2,5,8,11-tetraphenylperylene (PerPh4) donor to which four identical naphthalene-(1,4:5,8)-bis(dicarboximide) (NDI) electron acceptors are covalently attached at the para positions of the PerPh4 phenyl groups to yield PerPh4-NDI4. X-ray crystallography shows that the four NDIs pack pairwise into two distinct motifs. Two NDI acceptors of one PerPh4-NDI4 are positioned over the PerPh4 donors of adjacent PerPh4-NDI4 molecules with the donor and acceptor π-systems having a large dihedral angle between them, while the other two NDIs of PerPh4-NDI4 form xylene-NDI van der Waals π-stacks with the corresponding NDIs in adjacent PerPh4-NDI4 molecules. Upon selective photoexcitation of PerPh4 in the single crystal, CT exciton formation occurs in <300 fs yielding electron-hole pairs that live for more than ∼16 μs. This demonstrates the effectiveness of covalently linked D-A systems for engineering single crystal structures that promote efficient and long-lived charge separation for solar energy conversion.

Polyarene Oxides with Tunable Quinone Units for Photocatalytic CO2 Reduction: A Simple Strategy toward Effective and Selective Catalysts.

The photocatalytic transformation of carbon dioxide (CO2) into valuable chemicals is a challenging process that requires effective and selective catalysts. However, most polymer-based photocatalysts with electron donor-acceptor (D-A) structures are synthesized with a fixed D-A ratio by using expensive monomers. Herein, we report a simple strategy to prepare polyarene oxides (PAOs) with quinone structural units via oxidation treatment of polyarene (PA). The resultant PAOs show tunable D-A structures and electronic band positions depending on the degree of oxidation, which can catalyze the photoreduction of CO2 with water under visible light irradiation, generating CO as the sole carbonaceous product without H2 generation. Especially, the PAO with an oxygen content of 17.6% afforded the highest CO production rate of 161.9 μmol g-1 h-1. It is verified that the redox transformation between quinone and phenolic hydroxyl in PAOs achieves CO2 photoreduction coupled with water oxidation. This study provides a facile way to access conjugated polymers with a tunable D-A structure and demonstrates that the resultant PAOs are promising photocatalysts for CO2 reduction.

Enhancing Photosynthesis Efficiency of Hydrogen Peroxide by Modulating Side Chains to Facilitate Water Oxidation at Low-Energy Barrier Sites.

Hydrogen peroxide (H2O2) is a crucial oxidant in advanced oxidation processes. In situ, photosynthesis of it in natural water holds the promise of practical application for water remediation. However, current photosynthesis of H2O2 systems primarily relies on oxygen reduction, leading to limited performance in natural water with low dissolved oxygen or anaerobic conditions found in polluted water. Herein, a novel photocatalyst based on conjugated polymers with alternating electron donor-acceptor structures and electron-withdrawing side chains on electron donors is introduced. Specifically, carbazole functions as the electron donor, triazine serves as the electron acceptor, and cyano acts as the electron-withdrawing side chain. Notably, the photocatalyst exhibits a remarkable solar-to-chemical conversion of 0.64%, the highest reported in natural water. Furthermore, even in anaerobic conditions, it achieves an impressive H2O2 photosynthetic efficiency of 1365µmolg-1h-1, surpassing all the reported photosynthetic systems of H2O2. This remarkable improvement is attributed to the effective relocation of the water oxidation active site from a high-energy carbazole to a low-energy acetylene site mediated by the side chains, resulting in enhanced O2 or H2O2 generation from water. This breakthrough offers a new avenue for efficient water remediation using advanced oxidation technologies in oxygen-limited environments, holding significant implications for environmental restoration.

Deciphering the photophysical properties of naphthalimide derivatives using ultrafast spectroscopy.

Naphthalimide derivatives composed of donor-acceptor type structures hold significant promise across a wide range of applications. Here, the solvent polarity and viscosity controlled excited-state dynamics of a naphthalimide derivative with a donor-acceptor structure were studied using multiple spectroscopic techniques. From the stationary spectroscopic investigations, large Stokes shift and low fluorescence quantum yield were observed with increasing the solvent polarity, suggesting a more polar excited state relative to the ground state, which is evidenced by the Lippert-Mataga relationship. We also observe an enhanced fluorescence with a prolonged lifetime in a more viscous solution due to the restriction of excited-state molecular rearrangement. These observations result from the emerged twisted intramolecular charge transfer (TICT) state. The ultrafast spectroscopy studies further unravel a solvent polarity dependent excited state evolution from the intramolecular charge transfer state to the TICT state, revealing that the TICT state can be populated only in strong polar solvents. Control experiments by tuning the solvent viscosity in ultrafast experiments were employed to verify the excited state molecular rearrangement subsequently. These observations collectively emphasize how fine-tuning the photophysical properties of naphthalimide derivatives can be achieved through strategic manipulation of solvent polarity and viscosity.

Cross-linked conjugated polymers based on triazine and poly-branched polycyclic thiophene for improved photocatalytic hydrogen evolution

Conjugated porous polymers (CPPs) incorporating donor-acceptor (D-A) structures are gaining significant interest for their potential in photocatalytic hydrogen evolution reaction (HER). The microstructure and electronic properties of the polymer frameworks...

Promoting intersystem crossing by charge transfer state of dimers for persistent room temperature phosphorescence

Efficient intersystem crossing (ISC) is essential for phosphorescence emission. The charge transfer (CT) state is traditionally constructed by highly twisted donor-acceptor type structures to facilitate the ISC process. However, the highly twisted structural properties limit the diversity of room-temperature phosphorescence (RTP) molecules. By establishing the CT state in the intermolecular dimers structure, the dependence on the distorted donor-acceptor in the single-molecule design can be well eliminated. Based on this strategy, three novel pure organic luminescent molecules with long afterglow and mechanochromic luminescence (MCL) property were designed and synthesized. All of those exhibited triple emission, including prompt fluorescence (PF), delayed fluorescence (DF) and persistent RTP (pRTP) emission. In particular, CzC4-PhCN showed great potential for application in the field of anti-counterfeiting with its ultralong phosphorescent lifetime of 862 ms. Single crystal analysis and theoretical calculation revealed that the formation of CT states of intermolecular dimers effectively reduced ΔEST and promoted ISC process. This study would be highly instructive for realizing efficient ISC process and high-efficiency pRTP emission in pure organic systems.