Two squares in a barrel: An axially disubstituted conformationally rigid aliphatic binding motif for cucurbit[6]uril

Novel binding motifs suitable for the construction of multitopic guest-based molecular devices (e.g., switches, sensors, data storage, and catalysts) are needed in supramolecular chemistry. No rigid, aliphatic binding motif that allows for axial disubstitution has been described for cucurbit[6]uril (CB6) so far. We prepared three model guests combining spiro[3.3]heptane and bicyclo[1.1.1]pentane centerpieces with imidazolium and ammonium termini. We described their binding properties toward CB6/7 and α-/β-CD using NMR, titration calorimetry, mass spectrometry, and single-crystal X-ray diffraction. We found that a bisimidazolio spiro[3.3]heptane guest forms inclusion complexes with CB6, CB7, and β-CD with respective association constants of 4.0 × 104, 1.2 × 1012, and 1.4 × 102. Due to less hindering terminal groups, the diammonio analogue forms more stable complexes with CB6 (K = 1.4 × 106) and CB7 (K = 3.8 × 1012). The bisimidazolio bicyclo[1.1.1]pentane guest forms a highly stable complex only with CB7 with a K value of 1.1 × 1011. The high selectivity of the new binding motifs implies promising potential in the construction of multitopic supramolecular components.Internal Funding Agency of Tomas Bata University, (IGA/FT/2023/001); Ministerstvo Školství, Mládeže a Tělovýchovy, MŠMT, (LM2023042, LUAUS23144); Ústav organické chemie a biochemie Akademie věd České republiky, ÚOCHB AV ČR, (RVO: 61388963)Internal Funding Agency of Tomas Bata University in Zlin [IGA/FT/2023/001]; MEYS CR [LM2023042]; Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences [RVO: 61388963]; Ministry of Education, Youth and Sports [LUAUS23144

Structure and photophysics of indigoids for singlet fission: Cibalackrot

We report an investigation of structure and photophysics of thin layers of cibalackrot, a sturdy dye derived from indigo by double annulation at the central double bond. Evaporated layers contain up to three phases, two crystalline and one amorphous. Relative amounts of all three have been determined by a combination of X-ray diffraction and FT-IR reflectance spectroscopy. Initially, excited singlet state rapidly produces a high yield of a transient intermediate whose spectral properties are compatible with charge-transfer nature. This intermediate more slowly converts to a significant yield of triplet, which, however, does not exceed 100% and may well be produced by intersystem crossing rather than singlet fission. The yields were determined by transient absorption spectroscopy and corrected for effects of partial sample alignment by a simple generally applicable procedure. Formation of excimers was also observed. In order to obtain guidance for improving molecular packing by a minor structural modification, calculations by a simplified frontier orbital method were used to find all local maxima of singlet fission rate as a function of geometry of a molecular pair. The method was tested at 48 maxima by comparison with the ab initio Frenkel-Davydov exciton model. Published under license by AIP Publishing

Triptycene‐Based Molecular Rods for Langmuir‐Blodgett Monolayers

Herein we introduce fully modular synthesis leading to three representative examples of rigid molecular rods that are intended to form sturdy monolayers on various surfaces. These molecules contain two triptycene units that are designed to interlock into a compact "double-decker" structure. Two of the three final products provided suitable crystals for X-ray diffraction (analyzed on synchrotron), allowing deeper insight into packing in the 3-D crystal lattice. The acidity of all three compounds were determined by capillary electrophoresis, and the pKa values ranged between 2.06-2.53. All three rigid rods easily formed Langmuir-Blodgett monolayers (LBMs) on the water-air interfaces, with the area per molecule equal to 55-59 Å2 /molecule, suggesting tight intermolecular packing. The thickness of all three films reached ∼19 Å after transfer to a gold (111) surface, meaning that individual molecules are tilted maximally 38° from the axis perpendicular to the surface. The structure of one of these films on a gold (111) surface was visualized by AFM. These geometrically unique molecules represent promising platforms with a wide scope of applicability in the supramolecular architecture

Structure of a monolayer of molecular rotors on aqueous subphase from grazing-incidence X-ray diffraction.

In situ grazing-incidence X-ray scattering shows that a monolayer of artificial rod-shaped dipolar molecular rotors produced on the surface of an aqueous subphase in a Langmuir trough has a structure conducive to a 2D ferroelectric phase. The axes of the rotors stand an average of 0.83 nm apart in a triangular grid, perpendicular to the surface within experimental error. They carry 2,3-dichlorophenylene rotators near rod centers, between two decks of interlocked triptycenes installed axially on the rotor axle. The analysis is based first on simultaneous fitting of observed Bragg rods and second on fitting the reflectivity curve with only three adjustable parameters and the calculated rotor electron density, which also revealed the presence of about seven molecules of water near each rotator. Dependent on preparation conditions, a minor and variable amount of a different crystal phase may also be present in the monolayer

Phenyl-Substituted Cibalackrot Derivatives: Synthesis, Structure, and Solution Photophysics

Three symmetrically and three unsymmetrically substituted cibalackrot (7,14-diphenyldiindolo[3,2,1-de:3\u27,2\u27,1\u27-ij][1,5]naphthyridine-6,13-dione, 1) dyes carrying two derivatized phenyl rings have been synthesized as candidates for molecular electronics and especially for singlet fission, a process of interest for solar energy conversion. Solution measurements provided singlet and triplet excitation energies and fluorescence yields and lifetimes; conformational properties were analyzed computationally. The molecular properties are close to ideal for singlet fission. However, crystal structures, obtained by single-crystal X-ray diffraction, are rather similar to those of the polymorphs of solid 1, in which formation of a charge-separated state followed by intersystem crossing, complemented with excimer formation, outcompetes singlet fission. Results of calculations by the approximate SIMPLE method suggest which ones among the solid derivatives are the best candidates for singlet fission, but it appears difficult to change the crystal packing in a desirable direction. We also describe the preparation of three specifically deuteriated versions of 1, expected to help sort out the mechanism of fast intersystem crossing in its charge-separated state

Benchmark dynamics of dipolar molecular rotors in fluorinated metal-organic frameworks

Fluorinated metal-organic frameworks (MOFs), comprising a wheel-shaped ligand with geminal rotating fluorine atoms, produced benchmark mobility of correlated dipolar rotors at 2 K, with practically null activation energy (Ea=17 cal/mol). 1H T1 NMR revealed multiple relaxation phenomena due to the exchange among correlated dipole-rotor configurations. Synchrotron radiation X-ray diffraction at 4 K, Density Functional Theory, Molecular Dynamics and phonon calculations showed the fluid landscape and pointed out a cascade mechanism converting dipole configurations into each other. Gas accessibility, shown by hyperpolarized-Xe NMR, allowed for chemical stimuli intervention: CO2 triggered dipole reorientation, reducing their collective dynamics and stimulating a dipole configuration change in the crystal. Dynamic materials under limited thermal noise and high responsiveness enable the fabrication of molecular machines with low energy dissipation and controllable dynamics

The stability of covalent dative bond significantly increases with increasing solvent polarity

Non covalent complexes are often considerably destabilized in the solvent. Here the authors combine vibrational Raman and NMR spectroscopy with a coupled-cluster computational investigation to show that the solvent polarity enhance the complex stability of a Me3NBH3 complex. It is generally expected that a solvent has only marginal effect on the stability of a covalent bond. In this work, we present a combined computational and experimental study showing a surprising stabilization of the covalent/dative bond in Me3NBH3 complex with increasing solvent polarity. The results show that for a given complex, its stability correlates with the strength of the bond. Notably, the trends in calculated changes of binding (free) energies, observed with increasing solvent polarity, match the differences in the solvation energies (Delta E-solv) of the complex and isolated fragments. Furthermore, the studies performed on the set of the dative complexes, with different atoms involved in the bond, show a linear correlation between the changes of binding free energies and Delta E-solv. The observed data indicate that the ionic part of the combined ionic-covalent character of the bond is responsible for the stabilizing effects of solvents.Web of Science131art. no. 210

Structure of a monolayer of molecular rotors on aqueous subphase from grazing-incidence X-ray diffraction

In situ grazing-incidence X-ray scattering shows that a monolayer of artificial rod-shaped dipolar molecular rotors produced on the surface of an aqueous subphase in a Langmuir trough has a structure conducive to a 2D ferroelectric phase. The axes of the rotors stand an average of 0.83 nm apart in a triangular grid, perpendicular to the surface within experimental error. They carry 2,3-dichlorophenylene rotators near rod centers, between two decks of interlocked triptycenes installed axially on the rotor axle. The analysis is based first on simultaneous fitting of observed Bragg rods and second on fitting the reflectivity curve with only three adjustable parameters and the calculated rotor electron density, which also revealed the presence of about seven molecules of water near each rotator. Dependent on preparation conditions, a minor and variable amount of a different crystal phase may also be present in the monolayer