Nanocarbon materials show attractive functions in device applications, however the structural deviations and ambiguity disturb understanding between the functions and the structures. We have worked on the synthesis of molecular nanocarbon materials based on a simple strategy of macrocyclization of aromatic units. The molecular structures and supramolecular integrated structures can be fully accessed at the precision of molecular level, and unparalleled functions derived from the unique structures were found.

For example, a macrocyclic hydrocarbon molecule, [6]cyclo-2,7-naphthylene ([6]CNAP), synthesized by single bond linkage of six naphthylene units (Figure 1a) has a cyclic structure equivalent to an atom-defective structure of graphene [1]. In this study, [6]CNAP was applied to a negative electrode active material for a rechargeable lithium battery, where graphite is conventionally and commercially used as the material. All-solid-state lithium battery with LiBH₄ as electrolyte was constructed with three layers simply by uni-directional pressing: the composite electrode with [6]CNAP, acetylene black (AB) and LiBH₄ | LiBH₄ | Li (Figure 1b and c). Depending on purification methods, the recyclability of the rechargeable batteries largely differed. Surprisingly, highly purified specimen by sublimation method showed poor recyclability, and the recrystallized specimen from organic solvents showed stable recyclability up to 65 discharge-charge cycles and around twice battery capacity than a graphite electrode. We found that the differences in battery performance were originated from the molecular packing structures in solid states by powder X-ray structural analyses with Rietveld refinement. The key for the high battery performance is the one-dimensional nanopores constructed from the assembly of the central pore of [6]CNAP and π-stacks of naphthylene units. The quantitative battery performance results and the precisely determined packing structures showed that lithium ion is stored by the intercalation between naphthylene units and also in the one-dimensional nanopores to afford the high battery capacity. We successfully revealed the relationship between unique packing structures and battery performance [2].

[1] Nakanishi, W., Yoshioka, T., Taka, H., Xue, J. Y., Kita, H. & Isobe, H. (2011). Angew. Chem. Int. Ed. 50, 5323.

[2] Sato, S., Unemoto, A., Ikeda, T., Orimo, S. & Isobe, H. (2016). Small 12, 3381.

Recently, the traditional way of perceiving crystalline matter as static and brittle has started to change, and nowadays we are witnessing a growing number of examples where crystals display a plethora of flexible response to a variety of stimuli. They were found to move, jump, split, flex, twist, curl, explode, or to display a salient behaviour under UV radiation or heating, but lately, they were also found to respond to the applied external mechanical force [1]. Organic molecular crystals present a majority of examples of crystal adaptability to external stimuli, whilst metal-organic adaptable crystals are still quite rare. In the first report on the mechanical flexibility of coordination polymers, we have shown that crystals of a family of Cd(II) coordination polymers are capable of displaying not only exceptional mechanical elasticity but also variable flexible responses to applied external pressure [2]. They can actually differently tolerate exerted force and the different tolerability is a result of slight differences in the importance of intermolecular interactions in crystal packing.

We aim to understand the feature more deeply and to shed light on the underlying principles of the phenomenon, we have recently discovered unprecedented difference in plasticity of crystals of closely related class of Cd(II) coordination polymers [3]. In addition to variable plasticity, crystals also display remarkable pliability and ductility, not hitherto observed for metal-containing molecular crystals, which we present herein. To understand the phenomenon and rationalize observations, in addition to micro-focus SCXRD and AFM, we have also performed a series of custom-designed experiments and complemented those with an in-depth theoretical analysis. The results pointed at intermolecular interactions as the crucial structural feature in determining the type and extent of these highly unusual mechanical responses of crystalline metal-based polymeric materials.

[1] Commins, P., Israel Tilahun Desta,†Durga Prasad Karothu,† Panda, M. K. & Naumov, P. (2016) Chem. Commun. 52,13941.

[2] Đaković, M., Borovina, M., Pisačić, M., Aakeröy, C. B., Soldin, Ž., Kukovec., B.-M., Kodrin, I. (2018) Angew. Chem. Int. Ed. 130, 15017.

[3] Pisačić, M., Biljan, I., Kodrin, I., Popov, N., Soldin, Ž., Đaković, M. Chem. Mat. accepted.

This work has been fully supported by the Croatian Science Foundation under Project IP-2019-04-1242.

At present, the functional materials structurally switchable by stimuli such as heat, the addition of cations, changes of pH, pressure, or light are the motive of innumerable studies to be ideal models to investigate the relation structure-function and new properties derived from that change. In this work, we studied a family of spirorhodamines (SRAs) in solid-state photochemical reactions. These are photochromic molecules with a switching mechanism based on the differences in the fundamental electronic state between isomers.[1] It involves changes in the molecule structure and is thermally reversible.[2,3] In this work, assuming as the hypothesis that in solid-phase the permanence time in the optically active isomer is associated with its structural characteristics, a family of compounds modifying the substituent was synthesized.

These equilibria were characterized in solid-state by reflection, absorption and emission fluorescence spectroscopy, single-crystal X-ray diffraction,[4] atomic force microscopy coupled to infrared spectroscopy[4] and computational calculations, evaluating the changes produced after irradiating the corresponding close isomer with ultraviolet light for each compound.

[1] Dürr, H., Bouas-Laurent, H. (2003) Photochromism: Molecules and Systems, Eds.

[2] Di Paolo, M., Bossi, M. L., Baggio, R. and Suarez, S. A. (2016) Acta Crystallogr. Sect. B Struct. Sci. Cryst. Eng. Mater., 72, 684.

[3] Di Paolo, M., Boubeta, F., Alday, J., Michel Torino, M., Aramendía, P., Suarez, S.* and Bossi, M.* (2019) J. Photochem. Photobiol. A,. 384, 112011.

[4] Brazilian Synchrotron Light Laboratory (LNLS) beamlines MX2 and IR1.

In this work, the effects of pressure and temperature on the nonlocalized two-electron multicentric covalent bonds (‘pancake bonding’) in closely bound radical dimers were probed by single-crystal X-ray diffraction on a 4-cyano-N-methylpyridinium salt of 5,6-dichloro-2,3-dicyanosemiquinone (DDQ∙4CN) and I-methylpyridinium salt of tetrabromosemiquinone radical anion (Br₄Q∙NMePyr) as the sample compounds.

The DDQ∙4CN crystal structure can be described as closely bound stacked dimers of radical anions with interplanar separation <3.2 Å, which is known as non-localized two electron covalent bonding. At ambient conditions, the stacks of pancake bonded radical anions are formed by two types of distances: short intra-dimer and long inter-dimer contacts. On cooling, the anisotropic structural compression was accompanied by continuous changes in molecular stacking; the discontinuities in the changes in volume and b and c cell parameters suggest that a phase transformation occurs between 210 and 240 K. At a pressure of 2.55 GPa, both distances between radical dimers shortened to 2.9 Å, and become roughly equal, which corresponds to distances observed in extended-bonded polymers. Increasing pressure further to 6 GPa reduced the interplanar separation of the radicals to 2.75 Å, which may indicate that the covalent component of the interaction significantly increased [1]. The linear strain analysis shows that most deformations of pressure and temperature occur in the direction of pancake bonding.

The Br₄Q∙NMePyr crystal structure is built of infinite stacks of equidistant radical anions with no Peierls distortion [2]. On cooling the structure is compressed monotonically, the distance between radicals changes non-linearly, compress to <3.3 Å, but the space group remains the same. Upon pressure, the structure is compressed monotonically with no phase transformations in all the pressure range (0 – 6.0 GPa), the lowest interplanar distance is <2.9 GPa, which may indicate the increase of the covalent component in pancake bond and a significant decrease of the electron jumping barrier which may influence semiconductivity.

Azulene is a dark-blue, polar, bicyclic aromatic hydrocarbon (Figure 1) that is a non-benzenoid isomer of naphthalene. In addition to its long-standing medicinal and pharmaceutical relevance, the polar nonbenzenoid aromatic framework of azulene constitutes an attractive building block in the design of redox-addressable, optoelectronic, and conductive materials. This presentation will highlight our recent developments in the chemistry of hybrid metal/azulene platforms featuring isocyanide and thiolate junctions X along their molecular axis (Figure 2).

Figure 1. Electronic structure of azulene: resonance forms Figure 2. Two ways of functionalization of azulene at 2- and and origin of a molecular dipole. 6- positions that are important for its fixation on a solid support.

Single crystal X-ray structural analysis of a series of novel 2,6-functionalized azulenes will be presented [1,2]. In particular, heterobimetallic ensembles that incorporate the first examples of a conjugated p-bridge equipped with both isocyanide and thiol junction groups in the same molecular linker will be discussed (e.g., Figure 3B).

Figure 3. Two different functional groups – isonitrile and thiol – used for chemical modification of azulenes.

[1] Applegate, J.C.; Okeowo, M.K.; Erickson, N.R.; Neal, B.M.; Berrie, C.L.; Gerasimchuk, N.N.; Barybin, M.V. (2016) Chem. Sci., 7, 1422–1429. [2] Hart, M.D.; Meyers, J.J.; Wood, Z.A.; Nakakita, T.; Applegate, J.C.; Erickson, N.R.; Gerasimchuk, N.N.; Barybin, M.V. (2021). Molecules, 26, 981. https://doi.org/10.3390/molecules26040981

To investigate the salt formation of acridine with 4-amino salicylic acid (I), 5-chloro salicylic acid (II) and hippuric acid (III), the single crystal X-ray structure analysis have been performed. The present study allows to understandthe effect of molecular conformation adopted by acridine with hydroxyl group during the stabilization of crystal packing of these salt molecules, and to quantify the propensity of the intermolecular interactions to form the supramolecular assembly. The analysis of atom to atom or residue to residue contacts remains a favoured mode of analyzing the molecular packing in crystals. More importantly, they complement each other and are giving the complete picture of how these molecules assemble in molecular crystals. Hirshfeld surfaces, fingerprint plots and enrichment ratios were generated and further analyzedthe intermolecular interactions, and evaluatedtheir quantitative contributions to the crystal packing of the above saidthree salt molecules (I,II & III). The non-covalent interactionisosurfaceshave employed here, which allowsvisualizing where the hydrogen bonding and dispersion interactions contribute within the crystal.