No abstract

Metal–organic frameworks (MOFs) have been a promising material for many applications, e.g., photocatalysis, luminescence-based sensing, optoelectronics, and electrochemical devices, due to their tunable electronic properties through linker functionalization. In this work, we investigate the effect of mixed organic linkers on the bandgap modulation of polymorphic zirconium-based MOFs, UiO-66 and MIL-140A using density functional theory (DFT) calculations. We show that the electronic properties of both MOFs are in contrast to Vegard’s law for semiconductors, that is, mixed-linker systems exhibit bandgaps not intermediate within the range of single-linker systems. Calculations of the total and partial density of states revealed the formation of mid-gap states in mixed-linker MOFs, causing the bandgap reduction. Interestingly, although both MOFs have similar composition, the effect is more significant in MIL-140A than in UiO-66. This is due to the presence of π–π stacking interactions in MIL-140A, which does not occur in UiO-66. The simulation results reveal a direct relationship between the strength of π–π interactions and the bandgap. This illustrates that distinct structural features, particularly the orientation of organic linkers can give rise to different consequences in bandgap modulation. Moreover, this computational work highlights the possibility to engineer the electronic properties of MOFs through a mixed-linker approach.

In spite of the many resolution techniques available to separate enantiomers, diastereomeric resolution still remains the most widely used technique in industry. However, drawbacks of this technique are the limited yield of the desired enantiomer and the expensive enantiopure resolving agent that is required. We show here for the first time that a combination of diastereomeric resolution with Viedma ripening using a racemic resolving agent can also provide a single stereoisomer when using an excess of the racemic resolving agent, without the need for the resolving agent to racemize. The requirements of this process are, like for an enantiomeric system, that the compound crystallizes as a racemic conglomerate and that at least one chiral center in the target molecule is racemizable. In addition, owing to the presence of the racemization reaction, substantial improvement in the yield can be obtained. We here demonstrate this approach using a metastable conglomerate salt of rac-2-phenylglycinamide with rac-N-acetyl tryptophan.

No abstract

The stereoisomeric system of rac -2-phenylglycinamide (PGA) and rac - N -acetyl tryptophan (NAT) is significant in the application of chiral resolution because it has been shown that this system can be used for enantioseparation of PGA and/or NAT using a novel deracemization route of the conglomerate salt formed. However, it was also found that the conglomerate salt eventually converted into different crystal forms that limited the time available for the separation. Herein, we try to understand the phase conversion occurring in this system using DSC, PXRD, and SC-XRD. The related structures of the salt (two polymorphs of the more stable homochiral ( dd - and ll -) salts and one polymorph of the less stable heterochiral ( dl - and ld -) monohydrate salts) are demonstrated and discussed relating to their relative stabilities. The successful deracemization was demonstrated using the heterochiral ( dl - or ld -) monohydrate salts. However, following Ostwald’s rule of stages, only limited time is available for the deracemization before the metastable compound converts into the more stable homochiral ( dd - and ll -) pair. Moreover, the occurrence of the ( dd - and ll -) phase always coincides with the formation of yet another phase of the racemic compound containing four components in a crystal. Ostwald’s rule of stages here thus involves three steps and phases and is highly significant during the deracemization of the homochiral species.