Structural design of thiadiazole-based donor-acceptor COF/Fe-doped N vacancy g-C3Nx nanosheets for photocatalytic nitrogen fixation under visible light

The rational design of efficient photocatalysts to achieve artificial nitrogen fixation is an urgent challenge. Herein, we combined donor-acceptor covalent organic framework with iron-doped nitrogen vacancy graphitized carbon nitride (D-A COF/Fe-g-C3Nx) for photocatalytic nitrogen fixation. The photocatalyst exhibited good crystallinity, high porosity, and a large specific surface area. Without a sacrificial agent, the optimal 40 % D-A COF/Fe-g-C3Nx exhibited an excellent rate of ammonia production (646 μmol h-1 g-1) at 420 nm, and durable stability after successive cycling. Exhaustive experimental research and theory calculations verified that the D-A unit and Fe doping redistributed the distribution of the charge, which enhanced the visible light utilization and provided chemisorption sites for further polarization. Besides N-vacancies can serve as electron-trapping active sites to promote the directional migration of carriers. The reaction mechanism demonstrated that superoxide radical and hydrogen peroxide were formed by electron and hole, respectively, which promote the reduction of nitrogen to ammonia. This work provides a new idea for the rationalizing design of efficient catalysts for photocatalytic nitrogen fixation under mild conditions.

Anthracene-based dual channel donor-acceptor triazine-containing covalent organic frameworks for superior photoelectrochemical sensing

Covalent organic frameworks (COFs) exhibit excellent photoelectrically active structures and serve as channels for photon capture and charge carrier transport. However, their relatively high charge-carrier recombination rates and lack of specific recognition sites limit their application in photoelectrochemical sensing. This paper reports a functionalized donor-acceptor (D-A) COF comprising electron-rich polycyclic aromatic moieties and electron-deficient triazines (Tz) incorporating boronic acid through ligand exchange. The number of aromatic rings in the polycyclic aromatic moiety is crucial for establishing an efficient D-A system within COF. In the absence of an external electron donor, the anthracene-based COF exhibited a five-fold enhancement in photocurrent compared to the naphthalene-based COF. The resulting anthracene-based D-A COF exhibited enhanced orbital overlap and electron push-pull interactions, facilitating more effective charge separation. Furthermore, introducing boronic acid enabled the selective enrichment of low-concentration external electron donors, such as dopamine, in the inner Helmholtz plane. This ingenious approach establishes a unique dual-channel D-A system that allows direct measurement of dopamine in serum. Under optimized conditions, the test platform achieves good correspondence for dopamine at 1 to 100 nM and 0.5 to 100 μM with a detecting limit of 0.36 nM (3σ/S, n = 11). This strategy introduces a novel dimension to photoelectrochemical sensing, focusing on the effect of spatial separation between the external electron donor and the photoelectrode interface that intricately shapes the behavior and enhances the performance of the photoelectric system.

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.

Theoretical insights into the excited-state properties of room-temperature phosphorescence-emitting N-substituted naphthalimides

The optical absorption, fluorescence, and phosphorescence properties along with the intersystem crossing (ISC) rate constants of a series of N-substituted naphthalimides (NNIs), which can be used as organic emitters, were calculated using density functional theory (DFT) and time-dependent DFT (TDDFT). The calculated absorption, fluorescence, and phosphorescence energies as well as the fluorescence and ISC rate constants were found to be in good agreement with available experimental data. According to the calculations, the first excited singlet and first excited triplet states are π-π* in nature, and ¹π-π^* → ³n-π* is the dominant ISC channel for 1,8-naphthalimide (NI) and N-methyl-1,8-naphthalimide (Me-NI). Intermolecular charge-transfer (CT) states are observed in the energy region between the ¹π-π* and ³π-π* states for NNIs with an electron-donating group. The presence of these CT states leads to a reduction in the S₁-state and T₁-state energy splitting in NNIs with an electron-donating group compared to the energy splitting observed in NI and Me-NI. The singlet CT state (¹CT) has very weak emission oscillator strength, so it is nonemissive. For NNIs with an electron-donating group, the ¹CT → ³π-π* transition was found to be the dominant ISC channel. Our results indicate that for NNIs with an electron-donating group, the nonemissive S₁ state (¹CT) transits to the T₁ state (³π-π*) via ISC. The ¹CT → ³π-π* transition increases the population of the T₁ state and thus the phosphorescence quantum yield at room temperature, even in the absence of a heavy atom. Graphical Abstract The S₁ state (¹CT), which is nonemissive, transits to the T₁ state (³π-π*) via intersystem crossing and increases the population of the T₁ state in N-substituted naphthalimides with an electron-donating group.

BODIPY dyads and triads: synthesis, optical, electrochemical and transistor properties

A series of D–A dyads and D–A–D triads molecular systems based on triphenylamine and 9-ethyl-carbarzole as donor (D) and BODIPY as acceptor (A) has been designed and synthesized. The optoelectronic properties including optical, electrochemical, and charge carrier mobility of these molecules have been investigated. We found that the D–A–D triads exhibited broader absorption, raising the HOMO energy levels and increase hole carrier mobilities. Analysis surface morphology revealed that BODIPY containing carbazole demonstrated smooth film and no macro phase aggregation was observed upon thermal annealing.