During a SCSC solid-state Diels–Alder reaction, voids are created, followed by conformational change and crystal annealing that lead to the formation of new weak interactions.
Electron donor-to-acceptor interactions
between 9-methylanthracene
and bis(N-cyclobutylimino)-1,4-dithiin lead to the
formation of chiral charge-transfer (CT) crystals. The structure consists
of charge-transfer stacks where these two molecules arrange in a 1:1
alternating arrangement. These undergo a topochemical thermal single-crystal-to-single-crystal
(SCSC) [2 + 4] Diels–Alder reaction in the solid state. CT
crystals were reacted at 40 °C, their structures were determined
by X-ray diffraction at various degrees of conversion, and they were
examined using Hirshfeld surfaces and lattice energy calculations
to find evidence of reaction cooperativity and feedback mechanisms.
The results show that steric effects between product molecules and
reactant molecules during the SCSC reaction influence the formation
of products along the b axis, resulting in a more
ordered structure than initially suggested by the crystal structure
analysis. A maximum reaction conversion of around 96% was obtained,
which indicates that the reaction is also nonrandom within the charge-transfer
stacks. Lattice and intramolecular energy calculations show that the
energy of an inherently metastable crystal obtained via the SCSC reaction
is slightly higher compared to that of the recrystallized product
crystal. Finally, structural analysis using CrystalExplorer shows
that the shape, size, and surface curvature of the Hirshfeld surface
are not much changed by the reaction, indicating that the reaction
cavity remains relatively constant and that the reaction is under
topochemical control.
A single-crystal-to-single-crystal solid-state reaction involving the 2 : 1 charge-transfer complex of 9-bromoanthracene and bis(N-cyclobutylimino)-1,4-dithiin leads to a synthetic co-crystal composed of the Diels-Alder cycloadduct and unreacted 9-bromoanthracene molecules. Analysis of close contacts in the product crystal and DFT energy calculations indicate an ordered arrangement of product and unreacted molecules due to cooperative effects during the reaction.
Cocrystallization
of two or more molecular compounds can dramatically
change the physicochemical properties of a functional molecule without
the need for chemical modification. For example, coformers can enhance
the mechanical stability, processability, and solubility of pharmaceutical
compounds to enable better medicines. Here, we demonstrate that amino
acid cocrystals can enhance functional electromechanical properties
in simple, sustainable materials as exemplified by glycine and sulfamic
acid. These coformers crystallize independently in centrosymmetric
space groups when they are grown as single-component crystals but
form a noncentrosymmetric, electromechanically active ionic cocrystal
when they are crystallized together. The piezoelectricity of the cocrystal
is characterized using techniques tailored to overcome the challenges
associated with measuring the electromechanical properties of soft
(organic) crystals. The piezoelectric tensor of the cocrystal is mapped
using density functional theory (DFT) computer models, and the predicted
single-crystal longitudinal response of 2 pC/N is verified using second-harmonic
generation (SHG) and piezoresponse force microscopy (PFM). The experimental
measurements are facilitated by polycrystalline film growth that allows
for macroscopic and nanoscale quantification of the longitudinal out-of-plane
response, which is in the range exploited in piezoelectric technologies
made from quartz, aluminum nitride, and zinc oxide. The large-area
polycrystalline film retains a damped response of ≥0.2 pC/N,
indicating the potential for application of such inexpensive and eco-friendly
amino acid–based cocrystal coatings in, for example, autonomous
ambient-powered devices in edge computing.
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