Surface Engineering of the Mechanical Properties of Molecular Crystals via an Atomistic Model for Computing the Facet Stress Response of Solids.

Invited for the cover of this issue are Mubarak Almehairbi, Vikram C. Joshi, Changquan Calvin Sun and Sharmarke Mohamed. The image depicts the digital exploration of the mechanical properties of crystals on specific facets that may be of interest for materials applications by "dialing-in" their stress response. Read the full text of the article at 10.1002/chem.202400779.

Surface Engineering of the Mechanical Properties of Molecular Crystals via an Atomistic Model for Computing the Facet Stress Response of Solids.

Dynamic molecular crystals are an emerging class of crystalline materials that can respond to mechanical stress by dissipating internal strain in a number of ways. Given the serendipitous nature of the discovery of such crystals, progress in the field requires advances in computational methods for the accurate and high-throughput computation of the nanomechanical properties of crystals on specific facets which are exposed to mechanical stress. Here, we develop and apply a new atomistic model for computing the surface elastic moduli of crystals on any set of facets of interest using dispersion-corrected density functional theory (DFT-D) methods. The model was benchmarked against a total of 24 reported nanoindentation measurements from a diverse set of molecular crystals and was found to be generally reliable. Using only the experimental crystal structure of the dietary supplement, L-aspartic acid, the model was subsequently applied under blind test conditions, to correctly predict the growth morphology, facet and nanomechanical properties of L-aspartic acid to within the accuracy of the measured elastic stiffness of the crystal, 24.53±0.56 GPa. This work paves the way for the computational design and experimental realization of other functional molecular crystals with tailor-made mechanical properties.

Transformation between 2D and 3D Covalent Organic Frameworks via Reversible [2 + 2] Cycloaddition.

We report the first transformation between crystalline vinylene-linked two-dimensional (2D) polymers and crystalline cyclobutane-linked three-dimensional (3D) polymers. Specifically, absorption-edge irradiation of the 2D poly(arylenevinylene) covalent organic frameworks (COFs) results in topological [2 + 2] cycloaddition cross-linking of the π-stacked layers in 3D COFs. The reaction is reversible, and heating to 200 °C leads to a cycloreversion while retaining the COF crystallinity. The resulting difference in connectivity is manifested in the change of mechanical and electronic properties, including exfoliation, blue-shifted UV-vis absorption, altered luminescence, modified band structure, and different acid-doping behavior. The Li-impregnated 2D and 3D COFs show a significant room-temperature ion conductivity of 1.8 × 10-4 S/cm and 3.5 × 10-5 S/cm, respectively. Even higher room-temperature proton conductivity of 1.7 × 10-2 S/cm and 2.2 × 10-3 S/cm was found for H2SO4-treated 2D and 3D COFs, respectively.

A Pure-Red Doublet Emission with 90 % Quantum Yield: Stable, Colorless, Iodinated Triphenylmethane Solid.

Red luminescence is found in off-white tris(iodoperchlorophenyl)methane (3I-PTMH ) crystals which is characterized by a high photoluminescence quantum yield (PLQY 91 %) and color purity (CIE coordinates 0.66, 0.34). The emission originates from the doublet excited state of the neutral radical 3I-PTMR , which is spontaneously formed and becomes embedded in the 3I-PTMH matrix. The radical defect can also be deliberately introduced into 3I-PTMH crystals which maintain a high PLQY with up to 4 % radical concentration. The immobilized iodinated radical demonstrates excellent photostability (estimated half-life >1 year under continuous irradiation) and intriguing luminescent lifetime (69 ns). TD-DFT calculations demonstrate that electron-donating iodine atoms accelerate the radiative transition while the rigid halogen-bonded matrix suppresses the nonradiative decay.

2D Poly(arylene vinylene) Covalent Organic Frameworks via Aldol Condensation of Trimethyltriazine.

Designing structural order in electronically active organic solids remains a great challenge in the field of materials chemistry. Now, 2D poly(arylene vinylene)s prepared as highly crystalline covalent organic frameworks (COFs) by base-catalyzed aldol condensation of trimethyltriazine with aromatic dialdehydes are reported. The synthesized polymers are highly emissive (quantum yield of up to 50 %), as commonly observed in their 1D analogues poly(phenylene vinylene)s. The inherent well-defined porosity (surface area ca. 1000 m2 g-1 , pore diameter ca. 11 Šfor the terephthaldehyde derived COF-1) and 2D structure of these COFs also present a new set of properties and are likely responsible for the emission color, which is sensitive to the environment. COF-1 is highly hydrophilic and reveals a dramatic macroscopic structural reorganization that has not been previously observed in framework materials.

Meso Alkynylated Tetraphenylethylene (TPE) and 2,3,3-Triphenylacrylonitrile (TPAN) Substituted BODIPYs.

The tetraphenylethylene (TPE) substituted BODIPY 2a, and 2,3,3-triphenylacrylonitrile (TPAN) substituted BODIPYs 2b and 2c were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. Their photophysical and electrochemical properties were investigated. The BODIPY 2a exhibits strong D-A interaction with poor fluorescence quantum yield. The BODIPYs 2b and 2c show red-shifted absorption and emission with higher fluorescence quantum yield compared to BODIPY 2a. The photonic properties of BODIPYs 2a-2c were compared with 4-ethynylbenzonitrile substituted BODIPY 3 and phenylacetylene substituted BODIPY 4. The results reveal that the electron donating group at the meso position of BODIPY blue shifts the absorption and emission with decreased fluorescence quantum yield, whereas the electron withdrawing group at the meso position of BODIPY red shifts the absorption and emission with enhanced quantum yields. The single crystal structures of BODIPYs 2a and 2b reflect the planar orientation of meso substituent and the BODIPY core, which leads to close π-π stacking. The extensive π-π stacking and strong donor-acceptor (D-A) interaction makes these BODIPYs AIE inactive. The experimental observations were supported by DFT calculation.

Bulk heterojunction organic solar cells based on carbazole-BODIPY conjugate small molecules as donors with high open circuit voltage.

In this study, we have used three D-A type carbazole substituted BODIPY (carbazole connected to the meso position of BODIPY) small molecules as donors along with PC71BM as an electron acceptor for the fabrication of solution processed bulk heterojunction organic solar cells. The devices based on the as cast active layer showed power conversion efficiency in the range of 2.20-2.70%, with high open circuit voltage (Voc) in the range of 0.94-1.08 V. The high Voc is related to the deeper highest occupied molecular orbital energy level of these small molecules. The power conversion efficiency (PCE) of devices based on thermally annealed and solvent vapor annealed (TSVA) :PC71BM and :PC71BM processed active layers improved up to 5.05% and 4.80%, respectively, attributed to the improved light harvesting ability of active layers, better phase separation for exciton dissociation and balanced charge transport, induced by the TA and TSVA treatment.

Dynamic Covalent Chemistry of Enamine-ones: Exploring Tunable Reactivity in Vitrimeric Polymers and Covalent Organic Frameworks.

Dynamic covalent chemistry (DCC) has revolutionized the field of polymer science by offering new opportunities for the synthesis, processability, and recyclability of polymers as well as in the development of new materials with interesting properties such as vitrimers and covalent organic frameworks (COFs). Many DCC linkages have been explored for this purpose, but recently, enamine-ones have proven to be promising dynamic linkages because of their facile reversible transamination reactions under thermodynamic control. Their high stability, stimuli-responsive properties, and tunable kinetics make them promising dynamic cross-linkers in network polymers. Given the rapid developments in the field in recent years, this review provides a critical and up-to-date overview of recent developments in enamine-one chemistry, including factors that control their dynamics. The focus of the review will be on the utility of enamine-ones in designing a variety of processable and self-healable polymers with important applications in vitrimers and recyclable closed-loop polymers. The use of enamine-one linkages in crystalline polymers, known as COFs and their applications are also summarized. Finally, we provide an outlook for future developments in this field.

Donor-acceptor ferrocenyl-substituted benzothiadiazoles: synthesis, structure, and properties.

This article reports the design, and synthesis of D-π1-A-π2-D unsymmetrical, and D-π1-A-π2-A-π1-D symmetrical type of ferrocenyl-substituted benzothiadiazoles by the Pd-catalyzed Sonogashira, and Stille coupling reactions. The photophysical and electrochemical behavior of the ferrocenyl-substituted benzothiadiazoles show strong donor-acceptor interaction. The increase in the number of acceptor benzothiadiazole unit, results in the lowering of the energy gap, which leads to the bathochromic shift of the absorption spectrum. The single crystal X-ray structures of 3a, 5a, and 5g were obtained which show interesting supramolecular interactions.

Tetracyanobutadiene functionalized ferrocenyl BODIPY dyes.

Tetracyanobutadiene (TCBD) derivatives of ferrocenyl BODIPYs 2a-2c were designed and synthesized by [2 + 2] cycloaddition-retroelectrocyclization reaction of tetracyanoethylene (TCNE) with meso alkynylated ferrocenyl BODIPYs. The TCBD substituted ferrocenyl BODIPYs were designed in such a way that the distance between the ferrocenyl unit and the TCBD remains constant, whereas the distance between the BODIPY and the TCBD unit varies. The TCBD and BODIPY units were connected directly through a single bond (in 2a), through a phenylacetylene linkage (in 2b) and through a vinyl linkage (in 2c). The photonic and electrochemical properties of ferrocenyl BODIPYs were strongly perturbed by the incorporation of TCBD. The TCBD derivatives 2a-2c show red shifted absorption compared to their precursors 1a-1c. The single crystal structures of TCBD functionalized ferrocenyl BODIPYs 2a and 2c reveal extensive intermolecular hydrogen bonding but lack π-π stacking interactions.

Meso enyne substituted BODIPYs: synthesis, structure and properties.

We report the synthesis of meso enyne substituted BODIPYs by the reaction of 8-chloro BODIPY with terminal alkynes under Sonogashira coupling conditions, and by Pd-Cu catalyzed hydroalkynylation reaction of terminal alkynes, across the -C[triple bond, length as m-dash]C- bond of meso alkynylated BODIPYs. The scope of reaction was explored by reacting different meso alkynylated BODIPYs with various terminal alkynes, which results in meso enyne substituted BODIPYs with different substituents. The meso enyne substituted BODIPYs show blue shifted absorption and red shifted emission with large Stokes shift compared to meso alkynylated BODIPYs. The single crystal structures of BODIPYs , , and are reported. Their packing diagram exhibits extensive intermolecular C-Hπ, C-HF hydrogen bonding and ππ stacking interactions, leading to 1D supramolecular frameworks extending into the complex 3D structural frameworks.

Colorimetric and fluorimetric detection of fluoride and cyanide ions using tri and tetra coordinated boron containing chromophores.

Two organoboron based fluorophores pyrazabole and BODIPY have been designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction and successfully employed for fluoride and cyanide ion sensing. Pyrazabole acts as a fluorimetric sensor, whereas BODIPY acts as a fluorimetric as well as colorimetric sensor for fluoride and cyanide ions with ratiometric response. The photophysical properties of pyrazabole and BODIPY exhibit good electronic communication between triarylborane and pyrazabole/BODIPY. The single crystal X-ray structure of the pyrazabole shows a chair conformation for the pyrazabole core. The packing in pyrazabole and BODIPY shows interesting supramolecular structures. The computational studies show good agreement with the experimental results.

Ferrocenyl pyrazaboles: design, synthesis, structure, and properties.

This article reports the design and synthesis of donor-acceptor-donor type ferrocenyl-substituted pyrazaboles by the Pd-catalyzed Suzuki and Sonogashira coupling reactions. The photophysical and electrochemical behaviors of the ferrocenyl-substituted pyrazaboles were found to be dependent on the nature and length of the spacer groups. The single crystal X-ray structures of ferrocenyl-substituted pyrazaboles 11a, 11b, and 11d were obtained, which reveals that the pyrazabole core exists in chair, planar, and boat conformations. The crystal structures show marvelous supramolecular interactions. The pyrazaboles 11a-11g show good thermal stability, which increases with the length of spacer. The computational study reveals donor-acceptor interactions in ferrocenyl pyrazaboles. The computational results show good agreement with the experimental results.

Meso enamine substituted BODIPYs.

Different enamines were introduced at the meso position of the BODIPY by catalyst free oxidation of tert-amines and in situ cross coupling with 8-chloro BODIPY. The reaction conditions were optimized to achieve better yields. The reaction works well with aliphatic tert-amines bearing an N-(CH-CH-) backbone. The N-alkyl substituents perturb the optical properties of enamine substituted BODIPYs.

Reversible mechanochromism and enhanced AIE in tetraphenylethene substituted phenanthroimidazoles.

Tetraphenylethene (TPE) substituted phenanthroimidazoles 3a and 3b were designed and synthesized by the Suzuki cross-coupling reaction. They show reversible mechanochromic behavior with contrast colors between sky-blue and yellow green. The powder XRD studies show that destruction of a crystalline state into an amorphous state is responsible for mechanochromism. Hydrogen bonding interaction of a cyano-group in 3b results in enhanced AIE and improved thermal stability.

The quenching of fluorescence as an indicator of donor-strength in meso arylethynyl BODIPYs.

A series of meso arylethynyl BODIPYs (2a-2h) were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. The effects of the donor on the photophysical properties of the BODIPYs were explored. The DFT optimized structures and crystal structures show the planar orientation of the donor group with respect to the acceptor BODIPY, which favors a high degree of conjugation and induces strong donor-acceptor interactions. The quenching of fluorescence was correlated with the electron donating strength of the donor. The anthracene, pyrene and triphenylamine were found to have a stronger electron donating ability than the p-methoxyphenyl, phenanthrene, 1-naphthalene, biphenyl, and 2-naphthalene moieties. This was further supported by computational calculations and electrochemical analysis. The single crystal structures of BODIPYs 2d and 2e are reported, which show marvellous supramolecular structures.

Carbazole-BODIPY conjugates: design, synthesis, structure and properties.

A set of carbazole substituted BODIPYs 2a-2c were designed and synthesized by the Pd-catalysed Sonogashira cross-coupling reaction. The effects of variation in the donor strength of various carbazoles were investigated by photophysical, electrochemical and computational studies. The electronic absorption spectra of BODIPYs 2a and 2c show charge transfer bands, which show red shift in polar solvents. The BODIPYs 2a-2c are highly fluorescent in nonpolar solvents (emission from the localized state) and poorly fluorescent in polar solvents (emission from the charge transfer state). The photophysical and electrochemical studies reveal strong donor-acceptor interaction between carbazole and BODIPY and follows the order 2a > 2c > 2b. The computational calculations show good agreement with the experimental results. The single crystal structures of BODIPYs 2a-2c are reported, which exhibit interesting supramolecular interactions. The packing diagrams of 2a show a zigzag 3D structural arrangement, whereas 2b and 2c show complex 3D structural motifs.