Two new polymorphs of piroxicam (Forms VI and VII) were discovered by melt crystallization and crystal structure prediction (CSP). High-quality single crystals were grown from melt microdroplets for structural solution by X-ray diffraction. Relative to the previously known polymorphs obtained mainly by solution crystallization, the new polymorphs are thermodynamically less stable, confirming melt crystallization as an important complement to solution crystallization in polymorph discovery. Although early workers reported that piroxicam is thermally unstable and unsuitable for melt crystallization, our use of melt microdroplets circumvented this problem because of the short exposure to a high temperature. Our CSP identified all known polymorphs of piroxicam including the two new forms with an energy ranking in reasonable agreement with the observed order, as well as low-energy structures not observed by experiment.
Perovskite solar cells (PSCs) have made unprecedented progress in improving power conversion efficiency in the past decade, and they are considered as one of the most promising photovoltaic technologies. However, the commercialization of PSCs still faces significant challenges, such as the stability issue and toxicity of lead. Recently, pursuing ways to alleviate the toxicity of lead has emerged as an attractive research direction in the community of PSCs. In this review, the discussion is on the toxicity of lead and the impact of lead leakage from perovskites to the environment, the recent progress made to reduce the leakage of lead is presented with an emphasis on the lead sequestration materials applied in encapsulation layers and functional layers of PSCs, and the recovery of lead from damaged or decommissioned PSC devices is concisely summarized. This review may serve as a guide for researchers interested in promoting PSCs from exploitation to application.
Polymorphism, the ability of the same substance to crystallize in more than one crystal structure, is a common phenomenon in organic crystals, influencing the physicochemical properties of solid materials in many important fields (foods, dyes and pigments, high energic materials, pharmaceuticals, etc.). The utilization of various polymorph discovery methods could increase the possibility of finding polymorphs with desired properties, achieving an optimal performance of the final product. Recently, there has been a steady development of polymorph discovery in both experimental and computational methods. To better guide the polymorph discovery, this paper reviews the recent advances in the polymorph screening methods of organic crystals, mainly including solution crystallization, melt crystallization, and crystal structure prediction. This paper also summarizes the nucleation theory in polymorphic systems to understand the formation of polymorphs and highlights the mechanisms of polymorph discovery by the kinds of methods. Finally, challenges of polymorph discovery are briefly discussed, aimed to shorten the screening time and make the polymorph discovery more effective.
Recently, favipiravir, as a broad-spectrum antiviral drug, has gain more attention because it might be a candidate to remedy the coronavirus disease 2019 (COVID-19). To improve its poor permeability and tabletability, four multicomponent crystals of favipiravir (FPV) were prepared by a slow evaporation or liquidassisted grinding method, including three cocrystals (FPVtheophylline, 1:1; FPV-saccharin, 1:1; FPV-5-fluorouracil, 1:1) and one salt (FPV-piperazine, 2:1). All of the crystal structures were solved by single-crystal X-ray diffraction. Interestingly, FPVtheophylline has a crystal structure similar to that of FPV, leading to similar properties, such as solubility, permeability, and tabletability. Except for FPV-theophylline, all of the other multicomponent crystals exhibit an enhanced permeability and tabletability. Our studies provide a new insight in overcoming the shortcomings of the important antiviral drug FPV.
Form II, Form III, and Form IV of sulfathiazole (STZ) crystallized simultaneously from water by a cooling crystallization method. Thus, how to obtain the pure form of STZ represents a major challenge during the sulfathiazole crystallization process. Here we report that the pure Form III or Form IV of STZ can be selectively grown in agarose matrix by only adjusting the concentration of agarose. Interestingly, at a specific concentration range of agarose, the cross-nucleation of Form III on Form IV was observed. Furthermore, the mechanisms of agarose that affect the process of cooling crystallization of STZ were investigated, which indicated that the crystalline outcomes correlate strongly with the state of agarose gel that existed in solution. The highly reproducible and controllable results will help to broaden the application of gel in controlling crystallization of polymorph in the pharmaceutical industry. It also can be used as a guide for polymorphic screening of STZ from cooling crystallization.
Three-dimensional perovskite AMX3 has great potential in photoelectric applications, but the poor stability is a major problem that restricts its practical application. The emergence of lower dimensional perovskite solves this problem. Here, we have synthesized a group of novel low-dimensional perovskites with diverse structures. Different amino acids were incorporated in the perovskite cage. The formulas of the compounds are (A′) m PbI m+2 (A′ = COOH(CH2) n NH2, n = 1, 3, 5, 7, 9). These families of materials demonstrate structure-related stability, tunable bandgap, and different photoluminescence. Single-crystal X-ray diffraction indicated that the five materials employ different structure types varying from edge-sharing structures to face- and corner-sharing Pb/I structures by adjusting the number of C atoms in organic cations, and the level of [PbI6]4– octahedral distortion was also identified. The film prepared using these materials with longer carbon chains (n = 5, 7, 9) showed better stability, and they did not decompose within one year at 75% RH, 40 °C. The bifunctional organic ions containing carboxyl groups as spacer cations will form additional hydrogen bonding between perovskite layers, resulting in higher stability of the material. The band gaps of these materials vary from 2.19 to 2.6 eV depending on the octahedral connection mode and [PbI6]4– octahedral distortion level, density functional theory calculations (DFT) are consistent with our experimental trends and suggest that the face-sharing structure has the maximum band gap due to its flatter electron band structure. Bright green fluorescence was observed in (COOH(CH2)7NH3)2PbI4 and (COOH(CH2)9NH3)2PbI4 when excited by 365 nm UV light. A thorough comprehension of the structure–property relationships is of great significance for further practical applications of perovskites.
PCE of PSCs has not yet reached its limit (30.5%), implying there is still much room for further improve the performance of PSCs.To obtain PSCs with higher photoelectric performance and higher stability, various strategies have been developed to optimize the perovskite layer and charge carrier transport layers. [10][11][12] Exploring appropriate materials as efficient electron transport layers (ETLs) is also vital to promote the rapid development of PSCs. Titanium dioxide (TiO 2 ) was initially used as ETL material in PSCs, owing to the advantages of low cost, high stability, and superior ability able to effectively transferring of electrons from the perovskite layer to the bottom electrode. However, the energy levels of TiO 2 and perovskite are not matched well. Meanwhile and the annealing temperature of TiO 2 is high (above 450 °C), which is energy consuming and incompatible with flexible devices. [13][14][15] Alternatively, tin dioxide (SnO 2 ), has become the most commonly used ETL material, due to its advantages of low-temperature manufacturing, good energy level matching, and high electron mobility. However, using SnO 2 as ETL is inevitable to encounter defects (oxygen vacancy defects) pinholes at the film surface. These defects have a significantly negative impact on the photovoltaic performance and stability performance of perovskite solar cells. [16,17] Based on this, interfacial modification and doping strategies were applied to PSCs. Ogomi et al. introduced a monolayer HOCO-R-NH 3
Solution-mediated concomitant phase transformation (SMCPT) of the Piroxicam (PCM) monohydrate was found. The PCM monohydrate could simultaneously transform to block Form I and needle Form II in acetone at 31 °C. However, the Form I and the Form II can be selectively grown in the presence of 0.04 mg/mL HPC and 1% H 2 O, respectively. In addition, the morphology of Form I is changed from block to rod-like or even needle with the increase of the concentration of HPC. Based on these phenomena, the roles of hydroxypropyl cellulose (HPC) and H 2 O in the solution-mediated concomitant phase transformation were studied. The mechanisms of HPC and H 2 O to affect the process of SMCPT of the PCM monohydrate are different. In the presence of 0.04 mg/mL HPC, the interactions between the HPC molecules and the PCM molecules was the main factor that governed the process of SMCPT of the PCM monohydrate. However, in the presence of 1% H 2 O, the driving force of SMCPT played the main role. In our work, the outcome of SMCPT could be tuned directly by additives for the first time, which provides a good guide to obtain the pure polymorph in the SMCPT for pharmaceutical industry.
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