Molecular Motion In Solid State Research Articles

Bifurcated Polymorphic Transition and Thermochromic Fluorescence of a Molecular Crystal Involving Three-Dimensional Supramolecular Gear Rotation.

The ability of molecules to move and rearrange in the solid state accounts for the polymorphic transition and stimuli-responsive properties of molecular crystals. However, how the crystal structure determines the molecular motion ability remains poorly understood. Here, we report that a three-dimensional (3D) supramolecular gear network in the green-emissive polymorph 1G of a dialkylamino-substituted anthracene-pentiptycene π-system (1) enables an unusual bifurcated polymorphic transition into a yellow-emissive polymorph (1Y) and a new green-emissive polymorph (1G*) via 3D correlated supramolecular rotation. The 90° forward correlated rotation causes the molecular conformation between the octyl and the anthracene units to change from syn to anti, the ladder-like supramolecular columns to constrict, and the gear network to disengage. This cooperative molecular motion is marked by the gradual formation of an intermediate state (1I) across the entire crystal from 170 to 230 °C, which then undergoes bifurcated (forward or backward rotation) and irreversible transitions to form polymorphs 1Y and 1G* at 230-235 °C. Notably, 1G* is similar to 1G but lacks gear engagement, preventing its transformation into 1Y. Nevertheless, 1G can be restored by grinding 1Y or 1G* or fuming with dichloromethane (DCM) vapor. This work illustrates the correlation between the crystal structure and solid-state molecular motion behavior and demonstrates how a 3D molecular gear system efficiently transmits thermal energy to drive the polymorphic transition and induce fluorochromism through significant conformational and packing changes.

Photo-thermo-induced room-temperature phosphorescence through solid-state molecular motion

The development of smart-responsive materials, in particular those with non-invasive, rapid responsive phosphorescence, is highly desirable but has rarely been described. Herein, we designed and prepared a series of molecular rotors containing a triazine core and three bromobiphenyl units: o-Br-TRZ, m-Br-TRZ, and p-Br-TRZ. The bromine and triazine moieties serve as room temperature phosphorescence-active units, and the bromobiphenyl units serve as rotors to drive intramolecular rotation. When irradiated with strong ultraviolet photoirradiation, intramolecular rotations of o-Br-TRZ, m-Br-TRZ, and p-Br-TRZ increase, successively resulting in a photothermal effect via molecular motions. Impressively, the photothermal temperature attained by p-Br-TRZ is as high as 102 °C, and synchronously triggers its phosphorescence due to the ordered molecular arrangement after molecular motion. The thermal effect is expected to be important for triggering efficient phosphorescence, and the photon input for providing a precise and non-invasive stimulus. Such sequential photo-thermo-phosphorescence conversion is anticipated to unlock a new stimulus-responsive phosphorescence material without chemicals invasion.

Aggregate Materials beyond AIEgens

ConspectusAggregate materials focus on the materials that are constituted by more than a single entity. It could be dimers, trimers, or multimers formed by the group of same molecules or mixtures formed by different molecules. Although materials have different forms such as liquid, hydrogel, colloidal suspension, and powder, aggregates undisputedly are among the most commonly existing forms, which involve many aspects of human daily life such as the clothes we wear, the drugs we take, the food we eat, and the tools and devices we use. Sciences related to aggregate materials are also rich. From molecules to aggregates, different aspects of science such as crystallography, interfacial science, supramolecular science, and engineering science may be involved. Therefore, research on aggregate materials and related mechanisms that dominate the properties of aggregate materials is significant.However, it is noteworthy that reductionism is the dominant research philosophy for the design of traditional materials, and it claims that a molecule is the smallest particle of a substance that retains all of the properties of the substance. Consequently, scientists have expended a lot of effort to develop new molecules and to study the relationships between molecular structure and properties without paying attention to aggregate tuning. Actually, more and more research supports the fact that aggregate control such as the tuning of the packing mode, self-assembly, and morphology could work as a more versatile and effective strategy to endow materials with rich properties and functions. A prototype of the aggregate research is the aggregation-induced emission (AIE) phenomenon and materials. AIE research mainly focuses on the luminescence change accompanied by the aggregation process. After more than 20 years of flourishing development, AIE research has made great advances in new material development, mechanism studies, and high-technology applications. More importantly, AIE research opens the window to aggregate science and demonstrates that new behaviors and functions that are absent in single molecular species can be generated in molecular aggregates.In this Account, we highlight our efforts toward aggregate materials beyond AIE luminogens (AIEgens), with a special focus on new properties in addition to luminesce endowed by aggregation. We first summarize the chirality that is induced by the aggregation process of prochiral molecules. Then we present the aggregation resulting from the transformation between hydrophilicity and hydrophobicity. After that, the aggregation-induced generation of reactive oxygen species (ROS) is described and the regulation of the aggregation state for tuning the ROS generation efficiency is discussed. We also discuss how to achieve the switch between radiative decay and nonradiative decay through the regulation of the aggregation extent to afford luminescence and photothermy (photothermal effect), respectively. Additionally, different strategies to accelerate the solid-state molecular motion are presented and could be used to produce efficient photothermal materials. In particular, the concept of intramolecular motion-induced photothermy has been proposed, which worked as a reliable strategy to generate photothermal materials. At the end of the Account, a brief summary of this area and outlooks in this field are presented.

Derivatives of Bis(trifluoromethyl)biphenyl and Various Donor Noieties Exhibiting Dual State Emission

A series of bis(trifluoromethyl)biphenyl derivatives bearing carbazole, dimethylacridan, and phenothiazine donor moieties were designed and synthesized via Ullman coupling reaction. The compounds were found to be capable of forming molecular glasses with glass transition temperatures ranging from 152 °C to 244 °C. Significant dual-state emission was revealed for the synthesized compounds. Radiative decay of singlet and triplet states emerged from restricted molecular motion in solid state. The phenothiazinyl-substituted bis(trifluoromethyl)biphenyl derivative induces particular interest, as it exhibits room-temperature phosphorescence. This makes it promising for oxygen sensing applications. The theoretical work revealed that carbazolyl disubstituted bis(trifluoromethyl)biphenyl has the largest deviation of the planes of donor and acceptor fragments from 90°, which leads to larger overlap of electron density in the ground and excited states and larger oscillator strength. The calculated values of dipole moment and charge transfer during excitation were found to be the lowest for phenothiazinyl disubstituted bis(trifluoromethyl)biphenyl. This result explains the experimental fact that the solvent polarity has the least effect on the fluorescent spectrum of this compound.

Molecular Motions in AIEgen Crystals: Turning on Photoluminescence by Force-Induced Filament Sliding.

Life process is amazing, and it proceeds against the eternal law of entropy increase through molecular motion and takes energy from the environment to build high-order complexity from chaos to achieve evolution with more sophisticated architectures. Inspired from the elegance of life process and also to effectively exploit the undeveloped solid-state molecular motion, two unique chiral Au(I) complexes were elaborately developed in this study, in which their powders could realize a dramatic transformation from nonemissive isolated crystallites to emissive well-defined microcrystals under the stimulation of mechanical force. Such an unusual crystallization was presumed to be caused by molecular motions driven by the formation of strong aurophilic interactions as well as multiple C-H···F and π-π interactions. Such a prominent macroscopic off/on luminescent switching could also be achieved through extremely subtle molecular motions in the crystal state and presented a filament sliding that occurred in a layer-by-layer molecular stacking fashion with no involvement of any crystal phase transition. Additionally, it had been demonstrated that the manipulation of the solid-state molecular motions could result in the generation of circularly polarized luminescence.

Time-dependent solid-state molecular motion and colour tuning of host-guest systems by organic solvents

Host-guest complex solid state molecular motion is a critical but underexplored phenomenon. In principle, it can be used to control molecular machines that function in the solid state. Here we describe a solid state system that operates on the basis of complexation between an all-hydrocarbon macrocycle, D4d-CDMB-8, and perylene. Molecular motion in this solid state machine is induced by exposure to organic solvents or grinding and gives rise to different co-crystalline, mixed crystalline, or amorphous forms. Distinct time-dependent emissive responses are seen for different organic solvents as their respective vapours or when the solid forms are subject to grinding. This temporal feature allows the present D4d-CDMB-8⊃perylene-based system to be used as a time-dependent, colour-based 4th dimension response element in pattern-based information codes. This work highlights how dynamic control over solid-state host-guest molecular motion may be used to induce a tuneable temporal response and provide materials with information storage capability.

Visualization and Manipulation of Molecular Motion in the Solid State through Photoinduced Clusteroluminescence.

The development of molecular machines has long been a dream of scientists and is expected to revolutionize many aspects of technology and medicine. As the prerequisite of a practicable molecular machine, studies on the solid-state molecular motion (SSMM) are not only of scientific importance but also practically useful. Herein, two nonconjugated molecules, 1,2-diphenylethane (s-DPE) and 1,2-bis(2,4,5-trimethylphenyl)ethane (s-DPE-TM), are synthesized, and their SSMM is investigated. Experimental and calculation results reveal that s-DPE and s-DPE-TM are capable of performing light-driven SSMM to form excited-state through-space complexes (ESTSC). The radiative decay of ESTSC generates an unexpected visible emission termed clusteroluminescence, which serves as a tool to visualize the process of SSMM. Meanwhile, the original packing structure can be recovered from ESTSC after the removal of light irradiation. This work provides a new strategy to manipulate and "see" the SSMM and gains new insights into clusteroluminescence.

Molecular Motion in the Solid State

All matter is in motion, and such behavior has been well studied in the gas and liquid states. However, the study of solid-state molecular motion is still in its infancy, mainly, because of the abs...

Relationship between Solid-State Molecular Motion and Morphology for Ultrahigh Molecular Weight Polyethylene Crystallized under Different Conditions

Morphological effects on molecular mobility have been studied for solid ultrahigh molecular weight polyethylenes (UHMW-PE) crystallized from the melt and from solution or during polymerization. On the basis of transmission electron microscopic (TEM) observation, a crystalline domain structure was well identified for nascent UHMW-PE powders, which is quite different from regular lamellar stacking for solution-crystallized samples and the usual spherulites for melt-crystallized samples. Nuclear magnetic resonance (NMR) results showed that the amorphous chains between these crystalline domains in nascent powders were constrained, as well as those sandwiched between stacked crystalline lamellae for the solution-crystallized sample. Also, the existence of three regimes was recognized in the relaxation behavior of the crystalline phase, as revealed by 1H pulse NMR measurements. In process 1 (heating from room temperature), activation of molecular motion at the boundary between crystal/amorphous regions takes pl...

Relaxation Phenomena and Molecular Motion in Solid State of High Polymers

Nuclear magnetic resonance absorption (n-m-r) is of value in the study of high polymers because it can give information concerning the motion of the nuclei, and therefore of atoms or molecules, or parts of molecules. In favorable cases one can, within certain limits, identify the moving entity, the way in which it moves, and measure its rate of motion. On the other hand, the motion of molecules or parts of molecules in solid state is also revealed in their dielectric and mechanical properties. It is, therefore, of interest to compare the results of nuclear magnetic resonance absorption measurements with those of dielectric and mechanical measurements wherever possible.