Investigation of Morphology-Controlled Ultrafast Relaxation Processes of Aggregated Porphyrin

A bionic palladium metal-organic framework based on a fluorescence sensing enhancement mechanism for sensitive detection of phorate

We have developed a biomimetic fluorescent nanoprobe (Pd-MOF) that can accurately identify phorate at a fixed wavelength for rapid, sensitive and selective detection. Pd-MOF was a nanoparticle (260.00 ± 27.83 nm) based on the linkage of Pd metal and a TCPP organic framework. It could detect phorate according to the fluorescence principles similar to that of the bioluminescence of Chrysaora pacifica (substance interaction and chromophore fluorescence enhancement). When phorate molecules enter the pores of Pd-MOF and interact with each other, the energy transfer process is stimulated, and the fluorescence signal is significantly enhanced, thereby improving the detection sensitivity. According to shift of the white line in the XANES energy spectrum and the DFT results, phorate increased the energy gap of Pd-MOF from 0.025 eV to 0.046 eV, enhanced the stability of the system, and thus achieved fluorescence enhancement. The sensitivity of Pd-MOF was due to its much smaller energy gap (<80 times) than other metal MOFs and thus it was easier to get excited. The linear detection range for the phorate of the nanoprobe in the water system was 0.01-100 ppb, and the detection limit was 0.0017 ppb. The response time of the Pd-MOF nanoprobe to phorate was 45 seconds. The detection of phorate in tap water, pear and cabbage samples showed that the recovery rates were in the range of 87.69-106.12%, and the relative standard deviation (RSD) was less than 11.16%, which verified the possibility of Pd-MOF nanoprobe in practical application. The sensitive and specific recognition of phorate by Pd-MOF nanoprobe and the development of a phorate test strip (Pd-MOF@paper) confirmed its potential application in pesticide residue detection.

Porphyrin Aggregation under Homogeneous Conditions Inhibits Electrocatalysis: A Case Study on CO2 Reduction

Metalloporphyrins are widely used as homogeneous electrocatalysts for transformations relevant to clean energy and sustainable organic synthesis. Metalloporphyrins are well-known to aggregate due to π-π stacking, but surprisingly, the influence of aggregation on homogeneous electrocatalytic performance has not been investigated previously. Herein, we present three structurally related iron meso-phenylporphyrins whose aggregation properties are different in commonly used N,N-dimethylformamide (DMF) electrolyte. Both spectroscopy and light scattering provide evidence of extensive porphyrin aggregation under conventional electrocatalytic conditions. Using the electrocatalytic reduction of CO2 to CO as a test reaction, cyclic voltammetry reveals an inverse dependence of the kinetics on the catalyst concentration. The inhibition extends to bulk performance, where up to 75% of the catalyst at 1 mM is inactive compared to at 0.25 mM. We additionally report how aggregation is perturbed by organic additives, axial ligands, and redox state. Periodic boundary calculations provide additional insights into aggregate stability as a function of metalloporphyrin structure. Finally, we generalize the aggregation phenomenon by surveying metalloporphyrins with different metals and substituents. This study demonstrates that homogeneous metalloporphyrins can aggregate severely in well-solubilizing organic electrolytes, that aggregation can be easily modulated through experimental conditions, and that the extent of aggregation must be considered for accurate catalytic benchmarking.

2D Metal-Organic Framework Nanosheets based on Pd-TCPP as Photocatalysts for Highly Improved Hydrogen Evolution

In this report, a 2D MOF nanosheet derived Pd single-atom catalyst, denoted as Pd-MOF, was fabricated and examined for visible light photocatalytic hydrogen evolution reaction (HER). This Pd-MOF can provide a remarkable photocatalytic activity (a H2 production rate of 21.3 mmol/gh in the visible range), which outperforms recently reported Pt-MOFs (with a H2 production rate of 6.6 mmol/gh) with a similar noble metal loading. Notably, this high efficiency of Pd-MOF is not due to different chemical environment of the metal center, nor by changes in the spectral light absorption. The higher performance of the Pd-MOF in comparison to the analogue Pt-MOF is attributed to the longer lifetime of the photogenerated electron-hole pairs and higher charge transfer efficiency.

Biomimetic Approach toward Visible Light-Driven Hydrogen Generation Based on a Porphyrin-Based Coordination Polymer Gel

There has been a widespread interest in developing self-assembled porphyrin nanostructures to mimic nature's light-harvesting processes. Herein, porphyrin-based coordination polymer gel (CPG) has been developed as a "soft" photocatalyst material for hydrogen (H₂) production from water under visible light. The CPG offers a hierarchical nanofibrous network structure obtained through self-assembly of a terpyridine alkyl-amide appended porphyrin (TPY-POR)-based low molecular weight gelator with ruthenium ions (Ru^II) and produces H₂ with a rate of 5.7 mmol g^-1 h^-1 in the presence of triethylamine (TEA) as a sacrificial electron donor. Further, the [Fe₂(bdt)(CO)₆] (dbt = 1,2-benzenedithiol) cocatalyst, which can mimic the activity of iron hydrogenase, is coassembled in the CPG and shows remarkable improvement in H₂ evolution (catalytic activity; rate ∼10.6 mmol g^-1 h^-1 and turnover number ∼1287). The significant enhancement in catalytic activity was supported by several controlled experiments, including femtosecond transient absorption (TA) spectroscopy and also DFT calculation. The TA study supported the cascade electron transfer process from porphyrin core to [Ru(TPY)₂]²⁺ center, and subsequently, the electron transfers to the cocatalyst [Fe₂(bdt)(CO)₆] for H₂ production.

Visible-Light-Driven Photocatalytic CO2 Reduction to CO/CH4 Using a Metal-Organic "Soft" Coordination Polymer Gel

The self-assembly of a well-defined and astutely designed, low-molecular weight gelator (LMWG) based linker with a suitable metal ion is a promising method for preparing photocatalytically active coordination polymer gels. Here, we report the design, synthesis, and gelation behaviour of a tetrapodal LMWG based on a porphyrin core connected to four terpyridine units (TPY-POR) through amide linkages. The self-assembly of TPY-POR LMWG with Ru^II ions results in a Ru-TPY-POR coordination polymer gel (CPG), with a nanoscroll morphology. Ru-TPY-POR CPG exhibits efficient CO₂ photoreduction to CO (3.5 mmol g^-1  h^-1 ) with >99 % selectivity in the presence of triethylamine (TEA) as a sacrificial electron donor. Interestingly, in the presence of 1-benzyl-1,4-dihydronicotinamide (BNAH) with TEA as the sacrificial electron donor, the 8e^- /8H⁺ photoreduction of CO₂ to CH₄ is realized with >95 % selectivity (6.7 mmol g^-1  h^-1 ). In CPG, porphyrin acts as a photosensitizer and covalently attached [Ru(TPY)₂ ]²⁺ acts as a catalytic center as demonstrated by femtosecond transient absorption (TA) spectroscopy. Further, combining information from the in situ DRIFT spectroscopy and DFT calculation, a possible reaction mechanism for CO₂ reduction to CO and CH₄ was outlined.