Growth and characterization of melem hydrate crystals with a hydrogen-bonded heptazine framework

Role of Hydrogen Bonding in Crystal Structure and Luminescence Properties of Melem Hydrates

In recent years, carbon nitride (CN) compounds, such as g-C3N4 and melem, have attracted attention as new visible light-driven photocatalysts with a variety of functions, including water splitting, organic decomposition, and dark photocatalysis. The building unit of these materials is the heptazine ring, and molecules with this structure have attracted considerable attention as luminescent materials. Melem is an organic molecule with amino groups at the three termini of its heptazine ring. Melem exhibits near-UV (NUV) emission with high quantum yield via thermally activated delayed fluorescence (TADF). Materials exhibiting TADF can achieve highly efficient luminescence without the use of heavy metals, generating interest in their potential as luminescent materials for organic electroluminescent devices. Compared to materials that emit in the visible-light region, there are few reports on TADF materials such as melem that exhibit NUV emissions. Melem hydrate is easily obtained by hydrothermal treatment of melem. Unlike melem crystals, melem hydrate (Mh) has a porous structure because of a hydrogen-bond network formed between melem and water molecules. To date, only one type of Mh has been well-investigated. Mhs are expected to exhibit novel properties, such as photocatalysis, molecular adsorption, and highly efficient NUV emission. Mh also provides an opportunity to investigate how hydrogen bonds between the melem molecule and crystal water affect the TADF NUV emissions. This provides clues to the mechanism of the TADF action exhibited by other melem compounds. In this study, we focus on a new melem hydrate with a parallelogram shape, Mhp, first reported by Dai et al. in 2022. The crystal structure of Mhp reportedly differs from that of Mh; however, the Mhp crystal structure has not been determined to date, and its physical properties have not been investigated. Therefore, in this study, we reexamined the conditions for growing single crystals of Mhp and succeeded in growing samples that could be used to measure physical properties. We also determined its crystal structure and investigated the role in crystal formation of the hydrogen bonds between melem and water molecules. We evaluated the thermal behavior and optical properties and discussed their correlation with the crystal structure. Similar to melem, Mhp displayed NUV luminescence in its photoluminescence (PL) spectrum. This luminescence was found to have high quantum yield and delayed fluorescence. At low temperatures, the PL of Mhp dramatically increased at a wavelength of approximately 350 nm. This behavior was attributed to a significant change in the hydrogen-bond network between melem and water molecules in the Mhp crystal at low temperatures. We found that distortion of the melem molecule in the excited state at low temperatures was suppressed by its strong hydrogen bonds with water molecules. As a result, the displacement of the atomic nuclei of the atoms that make up the melem molecules in the excited state produced by light absorption is small, and in the de-excitation process, radiative transitions to low-energy vibrational levels are promoted. At the same time, nonradiative deactivation was suppressed, resulting in high fluorescence quantum efficiency. The results of this research provide deep insight into the role of hydrogen bonds in the optical properties of hydrate crystals that exhibit highly efficient luminescence, including TADF.

Hydrogen-Bond-Induced Melem Assemblies to Resist Aggregation-Caused Quenching for Ultrasensitive ECL Detection of COVID-19 Antigen

Nowadays, aggregation-caused quenching (ACQ) of organic molecules in aqueous media seriously restricts their analytical and biomedical applications. In this work, hydrogen bond (H-bond) was utilized to resist the ACQ effect of 2,5,8-triamino-1,3,4,6,7,9,9b-heptaazaphenalene (Melem) as an advanced electrochemiluminescence (ECL) luminophore, whose ECL process was carefully studied in an aqueous K2S2O8 system coupled with electron paramagnetic resonance (EPR) measurements. Notably, the H-bond-induced Melem assemblies (Melem-H) showed 16.6-fold enhancement in the ECL signals as compared to the Melem aggregates (Melem-A), combined by elaborating the enhanced mechanism. On such basis, the effective ECL signal transduction was in situ achieved through the specific recognition of the double-stranded DNA embedded in Melem-H assemblies (Me-dsDNA) with spike protein (SP) of coronavirus disease 2019 (COVID-19). For that, such an ECL biosensor showed a wider linear range (1.0-125.0 pg mL-1) with a lower limit of detection (LOD) down to 0.45 pg mL-1, which also displayed acceptable results in analysis of human nasal swab samples. Therefore, the work provides a distinctive insight on addressing the ACQ effect and broadening the application scope of the organic emitter and offers a simple platform for biomedical detection.

Hydration/Dehydration Induced Reversible Transformation between a Porous Hydrogen-Bonded Organic Framework and a Nonporous Molecular Crystal for Highly Efficient Gas Dehydration

Gas dehydration is a critical process in gas transportation and chemical reactions, yet traditional drying agents require an energy-intensive dehydration and regeneration step. Here, we present a nonporous molecular crystal called Melem that can be synthesized and scaled up through solid-state synthesis methods. Melem exhibits exceptional water selectivity in gas dehydration and can be reactivated under moderate conditions. According to the single-crystal structure and powder X-ray diffraction studies, a reversible structural transformation between Melem and its hydrated form, Melem-H2O, induced by hydration/dehydration processes has been observed. Melem displays water adsorption properties with a maximum uptake of 11 mmol·g-1 at p/p 0 = 0.92 and 298 K. Additionally, Melem retained consistent water capture capacities after 5 adsorption-desorption cycles. The remarkable gas dehydration performance of Melem was confirmed by column breakthrough experiments, which achieved a separation factor of up to 654.

A Study on Byproducts in the High-Pressure Melamine Production Process

The industrial production of melamine is carried out by the thermal decomposition of urea in two technological processes, using high or low pressure. The reaction may be accompanied by the formation of undesirable byproducts, oxoaminotriazines, and so-called polycondensates, mainly melam, melem, and melon, as well as their hydrates and adducts. Their presence leads to the deterioration of the quality of the final product and may lead to the release of troublesome deposits inside the apparatus of the product's separation node. With the limited possibility of controlling the crystallization of the byproducts of the process, improving the technological process requires the precise determination of the composition of the separated insoluble reaction byproducts, which is the main objective of this work. This work presents the results of qualitative and quantitative analyses of the composition of deposits sampled in the technological process of melamine production. The full characterization of the deposits was performed using inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) techniques. The elemental analysis (EA) of carbon, hydrogen, and nitrogen allowed us to obtain characteristic C/H, C/N, and H/N ratios. X-ray diffraction (XRD) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy were also performed to confirm the obtained data. In addition, the morphology of the solid byproducts of the reaction was investigated, and the characteristics of the structures were determined using a scanning electron microscope. The elemental composition was investigated using scanning electron microscopy and the energy-dispersive X-ray spectroscopy (SEM-EDS) technique. The key finding of this research is that about 95% of the deposits are a mixture of melem and melem hydrate. The soluble part of the deposits contains melamine, urea, and oxyaminotriazines, as well as trace inorganic impurities.

Mechanism of high photoluminescence quantum yield of melem

To produce high-efficiency organic light-emitting diodes, materials that exhibit thermally activated delayed fluorescence (TADF) are attracting attention as alternatives to phosphorescent materials containing heavy metallic elements. Melem, a small molecule with a heptazine backbone composed only of nitrogen, carbon, and hydrogen, is known to emit light in the near-ultraviolet region and exhibit high photoluminescence (PL) quantum yield and delayed fluorescence. However, the mechanism underlying the high PL quantum yield remains unclear. This study aimed to elucidate the mechanism of the high PL quantum yield of melem by examining its optical properties in detail. When the amount of dissolved oxygen in the melem solution was increased by bubbling oxygen through it, the PL quantum yield and emission lifetime decreased significantly, indicating that the triplet state was involved in the light-emission mechanism. Furthermore, the temperature dependence of the PL intensity of melem was investigated; the PL intensity decreased with decreasing temperature, indicating that it increases thermally. The experimental results show that melem is a TADF material that produces an extremely high PL quantum yield by upconversion from the triplet to the singlet excited state.