Predicting Crystalline Packing Arrangements of Molecules That Form Hydrogen-Bonded Tapes

Exploring the excited-state charge transfer fluorescence profile of 7-hydroxycoumarin and 2-methylimidazole - a combined X-ray diffraction and theoretical approach

This study investigated the effect of 2-methylimidazole (2-MIM) addition on the fluorescence of ethyl-7-hydroxy-2-oxo-2H-chromene-3-carboxylate using low-cost density functional theory (DFT) and Time-Dependent DFT calculations on single crystal X-ray geometries of ethyl-7-hydroxy-2-oxo-2H-chromene-3-carboxylate hydrate (1), 2-MIM (2), and the 1 : 1 co-crystal of (1) and (2), (3). At low concentrations (1 : 1-1 : 10) of 2-MIM, the fluorophore shows a decrease in the fluorescence intensity, but at higher concentrations (above 1 : 10) the fluorescence excitation maximum shifted from 354 nm to 405 nm, with a significant emission intensity increase. The changed excitation and emission profile at high concentrations is due to the deprotonation of the coumarin's phenolic group, which was confirmed by the increased shielding of the aromatic protons in the titration ¹H NMR spectra. The experimental fluorescence data between the 1 : 1 and 1 : 10 ratios agreed with the theoretical fluorescence data, with a redshift and decreased intensity when comparing (1) and (3). The data indicated that combining the fluorophore with 2-MIM increased levels of vibronic coupling between 2-MIM and the fluorophore decreasing de-excitation efficiency. These increased vibronic changes were due to charge transfer between the fluorophore and 2-MIM in (3). The subtle movement of the proton, H(5) toward N(2') (0.07 Å) caused a significant decrease in fluorescence due to electron density distribution (EDD) changes. This was identified by comparison of the EDD in the excited (S₁) and ground (S₀) states plotted as an isosurface of EDD difference. For the higher concentrations, an alternative excitation pathway was explored by modifying the crystal geometry of (3) based on ¹H NMR spectroscopy data to resemble excitoplexes. Theses excitoplex geometries reflected the fluorescence profile of the fluorophore with high concentrations of 2-MIM; there were dramatic changes in the theoretical fluorescence pathway, which was 100% vibronic coupling compared to 15.31% in the free fluorophore. At this concentration, the de-excitation pathway causes remodelling of the lactone ring via stretching/breaking the CO bond in the S₁ causing increased fluorescence by movement of the transition dipole moment. These results reflect previous studies, but the methods used are less experimentally and computationally expensive. This study is among the first to explain charge transfer fluorescence using crystalline geometries. This study will be of interest to the fields of crystal engineering and fluorescence spectroscopy.

Chiral 1,2,3-Triazolium Salt Catalyzed Asymmetric Mono- and Dialkylation of 2,5-Diketopiperazines with the Construction of Tetrasubstituted Carbon Centers

Novel asymmetric mono- and dialkylation reactions of α-substituted 2,5-diketopiperazines catalyzed by new chiral spirocyclic-amide-derived triazolium organocatalysts have been developed, resulting in a range of enantioenriched 2,5-diketopiperazine derivatives containing one or two tetrasubstituted carbon stereocenters. The reactions feature high yields (up to 98%), and excellent cis-diastereo- and enantioselectivities (up to >20:1 dr, >99 % ee), and they provide a new asymmetric synthetic approach to important functionalized 2,5-diketopiperazine skeletons. Furthermore, a possible reaction mechanism was proposed based on both control experiments and extensive DFT calculations.

Unravelling conformational and crystal packing preferences of cyclohexane-5-spirohydantoin derivatives incorporating a halogenated benzoyl group

The conformational and supramolecular diversity of spirohydantoins have been investigated.

Various Stacking Patterns of Two-Dimensional Molecular Assemblies in Hydrogen-Bonded Cocrystals: Insight on Competitive Intermolecular Interactions and Control of Stacking Patterns

We first demonstrate the rational control over the stacking patterns in two-dimensional (2D) molecular assemblies using chemical modification. A target system is a hydrogen-bonded cocrystal composed of 2-pyrrolidone (Py) and chloranilic acid (CA) with 2:1 composition ( PyCA ). X-ray crystallography revealed that various weak intersheet interactions give rise to a variety of metastable overlapping patterns comprised of the 2D assemblies mainly formed via hydrogen bonds, affording reversible and irreversible structural phase transitions. We successfully prepared cocrystals of Py and anilic acids bearing different halogens, in which 2D assemblies isostructural with those observed in PyCA exhibit various overlapping patterns. The order of stability for each overlapping pattern estimated using theoretical calculations of the intermolecular interactions did not completely coincide with those indicated by our experimental results, which can be explained by considering the entropic effect, i.e., the molecular motion of Py as detected using nuclear quadrupole resonance spectroscopy.

Crystal structure and prediction

The notion of structure is central to the subject of chemistry. This review traces the development of the idea of crystal structure since the time when a crystal structure could be determined from a three-dimensional diffraction pattern and assesses the feasibility of computationally predicting an unknown crystal structure of a given molecule. Crystal structure prediction is of considerable fundamental and applied importance, and its successful execution is by no means a solved problem. The ease of crystal structure determination today has resulted in the availability of large numbers of crystal structures of higher-energy polymorphs and pseudopolymorphs. These structural libraries lead to the concept of a crystal structure landscape. A crystal structure of a compound may accordingly be taken as a data point in such a landscape.

Crystal engineering with 1,4-piperazine-2,5-diones

Piperazinediones1that incorporate three geometrically and chemically independent recognition elements can assemble predictably into a solid.

Molecular Solids from Symmetrical Bis(piperazine-2,5-diones) with Open and Closed Monomer Conformations

The design, synthesis and solid state structures of a new class of xylylene-linked bis(1,4- piperazine-2,5-diones) are reported in an effort to extend the molecular framework of piperazine-2,5-diones. These compounds were derived from piperazine-2,5-dione as the core structure, synthesized via a new efficient route, and their crystal structures were determined. We examined the effects of side chain substitution on conformations of the linked bis-DKPs. Crystallization of 3,3'-[1,4-phenylenebis(methylene)]-bis[6-(hydroxymethyl)-1,4-dimethylpiperazine-2,5-dione] yielded molecular solids with an unusual network of "C"-shaped monomers held together by four intermolecular hydrogen bonds per asymmetric unit. Similarly, intermolecular interactions between the iodomethyl groups in 3,3'-[1,4-phenylenebis(methylene)]-bis[6-(iodomethyl)-1,4-dimethyl-piperazine-2,5-dione] result in the monomers adopting a "C"-shape in the solid state. Assembly of the monomers with side chains converted to methyl groups or tert-butyldimethylsilyl ethers, thereby lacking these stabilizing intermolecular interactions, results in an infinite array of "S"-shaped conformations. These results suggest that the interplay between the attractive intermolecular interactions and repulsive steric interactions of the substituents at the C6 and C6' positions of the diketopiperazine rings is important in determining the solid-state conformations of xylylene-linked bis(piperazine-2,5-diones).

A Theoretical Study of Cohesion, Structural Deformation, Inclusion, and Dynamics in Porous Hydrogen-Bonded Molecular Networks

Molecules with multiple sites of hydrogen bonding attached to suitable cores tend to crystallize as open networks. The resulting crystals can have the following unusual properties: They can include significant amounts of guest molecules; the guests are typically located in channels and can be exchanged without loss of crystallinity; and the geometry of the networks can change in response to new guests. We have found that DFT calculations can provide accurate simulations of the unusual structure and properties of such materials, represented by crystals of prototypic tetrapyridinone 1. These calculations have yielded three key insights that cannot be obtained directly from experiments. (1) The hypothetical porous network obtained by removing guests from crystals of compound 1 is highly flexible, and its deformations are inherently anisotropic, leading to lengthening or shortening of the channels along the c axis and no significant changes along the a and b axes. (2) Quantitative analysis of the total cohesive energy has revealed that hydrogen bonding within the network makes a dominant contribution, along with interactions of guests with the network. (3) Differences in the overall stability of crystals of compound 1 as the guests are varied do not arise primarily from significant changes in the cohesive energy of the network itself; instead, differences in guest-guest interactions play a key role, resulting from the nature of the guests and constraints imposed by the surrounding network. These insights, together with the results of ab initio molecular dynamics, help explain how hydrogen-bonded networks can be robust yet permit molecular movement that underlies the exchange of guests and adaptive porosity. These insights promise to be of general value to scientists studying ordered molecular materials in which strong directional interactions are prominent.