Polyhalogenated aminobenzonitriles vs. their co-crystals with 18-crown-6: amino group position as a tool to control crystal packing and solid-state fluorescence. Issue 5 (10th January 2022)

• Record Type: Journal Article
• Authors: Vaganova, Tamara A.; Benassi, Enrico; Gatilov, Yurij V.; Chuikov, Igor P.; Pishchur, Denis P.; Malykhin, Evgenij V.
• Is Part Of: CrystEngComm. Volume 24:Issue 5(2022)
• Page Start: 987
• Page End: 1001
• Publication Date: 2022-01-10
• Subjects: Crystals -- Periodicals; Crystal growth -- Periodicals; Crystallography -- Periodicals; Cristaux -- Périodiques; Cristaux -- Croissance -- Périodiques; Cristallographie -- Périodiques; 548
• DOI: 10.1039/d1ce01469b
• Languages: English
• ISSNs: 1466-8033

Abstract

Abstract : Strengthening ( para -isomers) or weakening ( ortho -isomers) of π-electron aggregation due to the crystal structure rearrangement results in the bathochromic or hypsochromic shift of the fluorescence maximum. Abstract : A series of para - and ortho -aminobenzonitriles differing in the nature and number of halogen substituents were used to synthesize 2:1 co-crystals with 18-crown-6 ether. The supramolecular structure of the obtained co-crystals as well as aminobenzonitrile crystals was studied in detail using single-crystal X-ray diffraction. Incorporation of the crown ether into the crystal matrix of arylamine results in the replacement of the H-bonds between amine molecules (N–H⋯NC and N–H⋯F) by the bond with a crown oxygen atom (N–H⋯Ocr ). The crystal packing rearrangement modifies the π-electron interactions between aminobenzonitrile molecules both in the type of contact (C–F⋯π, C–Cl⋯π, CN⋯π, π⋯π) and mutual arrangement of the stacked molecules (parallel/anti-parallel, displaced/rotated). These transformations cause a change in the solid-state fluorescence characteristics of aminobenzonitriles: co-crystallization is accompanied by a bathochromic shift of the fluorescence maximum in the case of para -isomers and by a hypsochromic shift in the case of ortho -isomers; the magnitude of this effect depends on the number of halogen substituents. Exploration of the nature of the intra- and intermolecular interactions, along with the excited states of the molecules in the gas phase, in aminobenzonitrile crystals and their co-crystals, using state-of-the-art TD-DFT calculations evidences that, depending on the NH2 group position, insertion of the crown ether causes either an increase in the change of the dipole moment upon photo excitation/emission with a subsequent increase in the Stokes shifts ( para -aminobenzonitriles) or a decrease in these characteristics ( ortho -aminobenzonitriles). This is consistent with the strengthening or weakening of π-electron aggregation in pairs of para - or ortho -aminobenzonitrile molecules, respectively, upon co-crystallization. A quantitative model that can clearly distinguish the different behaviours of ortho - and para -substituted molecules and provides an analytical tool of wide-ranging validity was proposed. The central importance of the mutual arrangement of the functions playing the role of the H-bond donor and acceptor was established; this finding may be exploited as a design tool to purposefully modify the molecular packing and tune the solid-state photophysical properties. Using DSC, the co-crystals' structure was found to self-organize in the same way upon crystallization from solution and from the melt and to regenerate in the melting–crystallization cycle.