Armando Navarro-Huerta

In this work, we report a detailed investigation on the factors affecting the in‐plane motion of tetrafluoro‐1,4‐benzoquinone (TFBQ) within a series of charge‐transfer (CT) cocrystals formed with various fused aromatic donors. Six crystalline materials were obtained, including four cocrystals, a polymorph, and a solid solution. These were characterized by single‐crystal X‐ray diffraction (SC XRD), solid‐state NMR, FTIR, UV‐Vis spectroscopy, and DFT calculations. All cocrystals exhibited short π‐stacking interactions and varied noncovalent interactions (NCI) modulating the in‐plane motion of TFBQ. ¹⁹ F T ₁ relaxation measurements allowed the quantification of the activation enthalpies ( ΔH ^‡ ) and entropies ( ΔS ^‡ ), with theoretical calculations backing these results. Cocrystal 4 is the solid with the most restricted motions due to the presence of strong hydrogen bonds. In contrast, cocrystal 3a depicts the highest in‐plane motional frequencies (up to 2.2 MHz at 300 K), owing to the weak contacts around TFBQ. Surprisingly, a solid solution ( 5 ), which resembled a mixture of cocrystals 2 and 3a , displayed no significant in‐plane motions, despite having similar packing and relaxation behavior to the binary cocrystals. This work illustrates how subtle variations in donor structure affect the in‐plane motions and photophysical properties of CT cocrystals, providing valuable insight into the rational design of functional materials.

The photothermal conversion properties of two charge-transfer organic cocrystals are linked to their dynamic behavior.

Here, we present the first example of a binary optical waveguiding (OWG) cocrystal with large anisotropy featuring a fluorinated acceptor molecule (CPP-TFPN, 1) with on-plane rotational dynamics, confirmed by solid-state NMR (¹⁹F T₁) and theoretical calculations. Spatially resolved microphotoluminescence and variable-temperature photoluminescence experiments allowed us to examine the OWG performance and photophysical properties of both single crystals and bulk microcrystalline samples. A comparison with an analogous cocrystal containing a regioisomeric acceptor (CPP-TFTN, 2) revealed that the photoluminescence characteristics of 1 are associated with the rotational motions of the acceptor, offering insights into how the molecular motion changes this property.

The study reports on the rotational dynamics of acetone‐d₆ (Form II) and DMSO‐d₆ (Form III) within the crystalline structures of two solvates of 5α‐bromo‐6β,19‐epoxy‐androstan‐3β,17β‐diol diacetate (Compound 1) by means of solid‐state Nuclear Magnetic Resonance through the quadrupolar ²H spin‐echo technique. The spectral data allowed the determination of the activation barriers (E_a) for rotation of the solvent molecules, with 6.24 kcal mol⁻¹ for deuterated acetone in the Form II‐d₆ and 8.19 kcal mol⁻¹ for the case of deuterated dimethylsulfoxide in Form III‐d₆. The use of calculations and the Transition State theory through the linear Eyring equation suggested that although the acetone molecules experience a low activation energy (E_a = 6.24 kcal mol⁻¹), a highly ordered transition state during the molecular motion reduces its rotational rate. Conversely, the DMSO molecules, with a higher activation barrier (E_a = 8.19 kcal mol⁻¹) attributed to a denser packing coefficient, have faster motional rates. Based on complementary X‐ray and NMR spectroscopy techniques, this work provides detailed insights into the mechanistic phenomena involved in the mobility of small molecules inside crystalline arrangements.

Herein, we report the characterization of two hydrogen-bonded cocrystals, termed AC-TFTA and AC-PFBA, with mechanofluorochromism upon grinding.

The dimeric steroid SMR-3, featuring a 1,4-phenyldiboronic ester flanked by two pregnan-triol frameworks, was synthesized to explore the intramolecular dynamics of its central component. The structural data from single-crystal X-ray diffraction studies and the Hirshfeld analyses indicate small steric effects around the aromatic ring that should favor the intended motion. However, solid-state NMR data obtained through VT ¹³C{¹H} CPMAS and ²H spin-echo experiments, using the deuterated analogue SMR-3D₄, revealed that this component is rigid even at temperatures where other reported steroidal molecular rotors experience fast rotation (85 °C). A combination of classical molecular dynamics, molecular mechanics, and correlated ab initio calculations allowed us to distinguish the steric and electronic factors that restrict the potential motion in this compound. The experimental and computational data reveal that electronic components dominate the behavior and are responsible for the high rotational barrier in the SMR-3 crystal.

We report here six mechanofluorochromic crystals: three tetraphenylethylene difluoroboronates (–BF₂, 3a–c) and three diphenylboronate derivatives (–B(Ph)₂, 4a–c) that change emission upon grinding. The corresponding photophysical studies allowed us to identify that most of them show emission in the 500–750 nm range, with the exchange of the –BF₂ moiety for the –B(Ph)₂ moiety causing an improvement in their quantum yields (Φ_f). Halogen substitution in their structures caused a decrease of the Φ_f going from –Cl to –Br to –I. Combining single crystal and powder X-ray analyses, we determined that they are highly crystalline but prone to amorphization via mechanical stress, with a noticeable bathochromic shift of their emission upon grinding. Density functional theory computations indicate that conformational changes upon grinding could cause the observed changes. They undergo reversible changes upon exposure to vapors of dichloromethane, setting the path to develop bistable switches in the future.