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Crystal–crystal phase transformation by external stimuli has attracted significant attention for application in switchable materials, which can change their structures and properties. Herein, it is revealed that N-salicylidene-p-aminobenzoic acid crystals undergo a two-step crystal–crystal phase transformation through a gas–solid reaction with aqua–ammonia vapour. The photochromic behaviour of the crystals switched from nonphotochromic to photochromic and back to nonphotochromic via a phase transformation. The two-step phase transformation and photochromic behaviour change were characterized and correlated by X-ray crystal structure analysis, UV–Vis spectroscopy, differential scanning calorimetry and scanning electron microscopy. This article is the first report to capture the stepwise structural change in the gas–solid (acid–base) reaction of ammonia with benzoic acid derivatives.

The enantioselective synthesis of polycyclic aromatic hydrocarbon (PAH)-based planar chiral cyclophanes was achieved for the first time by the rhodium-catalyzed intramolecular regio- and enantioselective [2+2+2] cycloaddition of tethered diyne-benzofulvenes followed by stepwise oxidative transformations. The thus synthesized planar chiral bent cyclophanes, that possess bent p-terphenyl- and 9-fluorenone-cores, were converted to 9-fluorenol-based ones with excellent ee values of >99 % by diastereoselective 1,2-reduction. These 9-fluorenol-based cyclophanes exhibited high fluorescence quantum yields, which were significantly higher than that of an acyclic reference molecule (78-82 % vs. 48 %). The bending effect on the chiroptical property was also examined, which revealed that the anisotropy factors (gabs values) for electronic circular dichroism (ECD) of these 9-fluorenol-based planar chiral bent cyclophanes increase as the tether length becomes shorter.

Single and double cyclophenylene-ethynylenes (CPEs) with axial and helical chirality have been synthesized by the Sonogashira cross-coupling of di- and tetraethynyl biphenyls with a U-shaped prearomatic diiodoparaphenylene followed by reductive aromatization. X-ray crystallographic analyses and DFT calculations revealed that the CPEs possess highly twisted bent structures. Bend angles on the edge of the paraphenylene units were close to the value of [5]cycloparaphenylene (CPP)-the smallest CPP to date. The double and single CPEs possessed stable chirality despite flexible biphenyl structures because of the high strain in the diethynyl-paraphenylene moiety. In both the single and double CPEs, orbital interactions along the biphenyl axis were observed by DFT calculations in LUMO and LUMO+2 of the single CPE and LUMO+1 of the double CPE, which likely cause lowering of these orbital energies. Concerning chiroptical properties: boosting of the gabs value was observed in the biphenyl-based double CPE, as well as the binaphthyl-based single CPE, compared to the biphenyl-based single CPE.

Mechanochromically luminescent cryst. org. materials that can reversibly change the color of their solid-state emissions upon exposure to mech. stimuli have been studied intensely in recent years owing to their wide range of potential applications. Although the mechanism of the mech. induced change of emission color is in most cases attributed to transitions between the cryst. and amorphous phases, the understanding of the relationship between the individual morphol. of a crystal (crystal habit) and its specific response to mech. stimuli still remains elusive. Herein, we demonstrate that differently substituted triphenylimidazolylbenzothiadiazole derivs. exhibit distinct mechanochromic luminescence (MCL) if they crystallize as plate-like crystals, but those that crystallize as needles do not. Specifically, nonsubstituted and fluoro-substituted derivs. form plate-like MCL crystals, whereas needle-like non-MCL microcrystals are obtained for the other derivs. with Me, methoxy, ethoxy, or trifluoromethyl substituents. We have confirmed that the slight modification of the steric bulk of a peripheral substituent dramatically changes both the morphol. and the MCL properties of these derivs. The mol. structures of the constituent compds. of both types of crystals have been detd. by single-crystal and powder X-ray diffraction analyses. A comparison of these structures revealed that a twisted mol. conformation is the origin of the mech.-stimuli-responsive emission color change in the plate-like crystals. In other words, mol. conformations fixed in the assembled structures det. the MCL properties of morphol. different crystals. The knowledge acquired in the present study can be expected to accelerate the development of cryst. MCL materials with controlled nano- or microstructures, with possible applications in microscale mechanosensors. Plate-like or needle-like crystals have been obtained for triphenylimidazolylbenzothiadiazole derivs. depending on the substituents on the Ph group attached to the benzothiadiazole ring. Nonsubstituted or fluoro-substituted derivs. form plate-like crystals and exhibit mechanochromic luminescence. In contrast, the emission color of the other derivs. that form needle-like microcrystals remains virtually unchanged after grinding. [on SciFinder(R)]

Planar chiral zigzag-type [8]- and [12]cyclophenylene (CP) belts have been synthesized in good yields with high ee values of 98% and 83%, respectively, by the rhodium-catalyzed enantioselective intramolecular sequential cyclotrimerizations of the corresponding cyclic polyynes. The observed high enantioselectivity arises from the regioselective formation of a rhodacycle intermediate from an unsymmetric triyne unit. The X-ray crystal structural analysis of the racemic planar chiral zigzag-type [8]CP belt revealed that the uneven molecules mesh with each other to form a one-dimensional columnar packing structure, in which one column contains single enantiomers, giving two types of chiral columns [(S)- and (R)-form columns] arranged alternately. The ring strain of the zigzag-type [8]CP belt was smaller than that of the armchair-type [8]CPP belt despite its smaller ring size, due to the presence of the strain-relieving m-terphenyl moieties. The effect of the number of the benzene rings of the zigzag-type CP belts on absorption and emission peaks was small due to interruption of π-conjugation at the m-phenylene moieties. However, the bending effect on the absolute fluorescence quantum yield as well as absorption and emission peaks was significant. Concerning chiroptical properties, the modest anisotropy dissymmetry factors of ECD and CPL were observed in the [8]CP belt.

Lopinavir (LPV) and ritonavir (RTV) are anti-viral drugs used in combination and commonly prepd. through hot-melt extrusion techniques. Mech. processes are greatly involved, including blending and milling. Therefore, the crystal behavior of LPV and RTV under the mech. process is an interesting study. Here, the LPV, RTV, and their mixts. were processed by two different treatments: grinding and compression processes. The solid-state properties of the drugs were evaluated by powder x-ray diffraction (PXRD) differential scanning calorimetry (DSC), Fourier transform IR spectroscopy (FTIR), Raman spectroscopy. Single-crystal XRD anal. was also carried out to confirm the packing structure in the crystal lattice. It was obsd. that LPV was very sensitive to the grinding process, where it tends to form an amorphous solid in both the pure and mixt. forms. On the other hand, RTV has a very stable cryst. structure and was able to retain its crystallinity even under grinding. Both LPV and RTV were obsd. to be quite stable under compression, where both retained their cryst. form with only slight changes in their d-spacing values. This study highlighted the mol. origin of LPV and RTV crystal behavior after grinding and compression processes. [on SciFinder(R)]

Syn- and anti-isomers of an adamantylideneadamantane 1,2-dioxetane having a phthalimide side chain were prepd. and investigated their cryst.-state chemiluminescence (CL) properties. The isomers showed contrastive CL properties depending on their crystal-structural characteristics, indicating that CL provides an attractive target for real-time monitoring of a chem. reaction in the crystal. [on SciFinder(R)]

Ciprofloxacin hydrochloride (CIP) is a common fluoroquinolone antibiotic agent, and in the marketed product the compound exists as the hydrate phase (1.43 hydrate). Upon heating, the stable hydrate phase dehydrates and converts to a new anhydrous form 1 (AH1). Further heating results in a phase transformation that gives another new anhydrous form 2 (AH2). The crystal structures of CIP 1.43 hydrate and AH2 were solved from single crystal X-ray structure analysis. However, the single crystal of CIP AH1 could not be obtained, thus the crystal structure of CIP AH1 was determined directly from powder X-ray diffraction data using synchrotron technique. The crystal structure of AH1 is partly related to the hydrate as they both show similar one-dimensional (1D) chain and two-dimensional sheet structure. In the AH2, it is only the 1D chain that is retained after phase transformation. The mechanism of dehydration and the phase transformation is determined by correlating structural differences of AH1 and AH2. Each crystalline phase of CIP shows different physical stability, dissolution rate, and colour which can be derived from structural difference among the phases.