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Flexible stimuli-responsive metal–organic frameworks have become promising candidates for numerous applications in gas-related technologies; however, the methods of fine tuning their responses are still limited and sought after. In this work, we demonstrate control over the adsorption properties of a flexible platform by incorporating halogen substituents (X = F, Cl, Br, I) into an eightfold interpenetrated isoreticular series [Zn­(oba)­(X-pip)] n (JUK-8X; X-pip = 4-pyridyl-functionalized benzene-1,3-dicarbo-5-halogenohydrazide; oba2– = 4,4′-oxydibenzoic carboxylate). The introduced halogen atoms allow for precise tuning of CO2 gate-opening pressures from p/p 0 = 0.08 for the parental JUK-8 to 0.78 for the chlorine-functionalized JUK-8Cl. The presence of fluorine or chlorine substituent in the X-pip linker practically does not influence the maximum molar CO2 uptake as compared to JUK-8, whereas larger bromine or iodine atoms increase this uptake by 59 and 48%, respectively. Utilizing in situ powder X-ray diffraction (PXRD) during CO2 adsorption for a model JUK-8F, we propose a detailed mechanism of phase transitions including positions of the adsorbed gas molecules for the two loaded phases. Density functional theory calculations supported by in situ PXRD measurements at a saturation pressure shed light on the unusual CO2 adsorption properties of JUK-8Br and JUK-8I. Overall, our report demonstrates the use of halogen interactions for the control of a gas-responsive system and provides insightful guidance for the further development of flexible, adaptable materials.

The influence of π–π overlap in centrosymmetric dimers on the aggregation type, single-crystal absorption and fluorescence anisotropy of the new heterocyclic system 5,6,10b-tri­aza­acephenanthrylene is presented.

This paper presents the relationship between the fluorescent properties of (E/Z)-(N-alkylpyridyl)­enamino-pyrrolo­[2,3-b]­quinoxalin-2-one derivatives in the crystalline state and the molecular packing governed by the conformation of the molecules, hydrogen bonding, and π–π interactions. In particular, the type of 2-pyridyl alkyl chain (2-Py­(CH2) n with n = 0,1,2) is responsible for the molecular packing and influences the fluorescence properties of crystalline pyrrolo­[2,3-b]­quinoxalines. The molecules studied exhibit permanent dipole moments that, in the crystalline phases, promote either the formation of dimers with an antiparallel orientation or stacks with a parallel dipole moment orientation. In dimers and stacks, the main observed interaction is the π–π type. The conformations of both (E)- and (Z)-enamines are stabilized by intramolecular hydrogen bonds from the NH group of the enamine to either the nitrogen atom N4 of the quinoxaline (E)-diastereoisomer or the oxygen atom of the amide carbonyl group in the (Z)-diastereoisomer. When an additional intramolecular hydrogen bond from the enamine NH group to the pyridyl nitrogen atom forms, it affects the conformation of the molecules, causing the reorientation of the molecular permanent dipole moment. Density functional theory (DFT) calculations performed for two neighboring molecules in dimers or stacks indicate two charge transfer mechanisms: intra- and intermolecular mechanisms. For centrosymmetric dimers, charge transfer occurs within each component molecule (intramolecular charge transfer). For stacks with molecules arranged by translation and noncentrosymmetric dimers, charge transfer is of an intermolecular nature. Higher absolute fluorescence quantum yields (Φf = 12.06–13.77%) are exhibited by the (E-diastereoisomers, which contain methylene or ethylene chains and form either translational stacks of the molecules or noncentrosymmetric dimers. The lowest absolute fluorescence quantum yields (Φf = 3.80–4.00%) are typical for the centrosymmetric dimers present in crystalline (Z)-(N-pyridyl)­enaminopyrrolo­[2,3-b]­quinoxaline and (E)-(N-ethylpyridyl)­enamino-pyrrolo­[2,3-b]­quinoxaline. The fluorescence liftimes τ were determined for single crystals of the studied phases (from 13.3 to 15.6 ns) and revealed one single process of radiative energy transfer.

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