Molecular crystals: mechanics and photonics

Abstract

Focusing on the mechanics and photonics of molecular crystals, Rajadurai Chandrasekar, Panče Naumov, Xue-Dong Wang and Kristin Hutchins introduce a themed collection showcasing a series of articles that delve into their structural properties and broad range of functional applications.

Rajadurai Chandrasekar

Rajadurai Chandrasekar is a Professor in the School of Chemistry at the University of Hyderabad (UoH), Hyderabad, India. He received his PhD from the Max Planck Institute for Polymer Research, Mainz. His group at UoH pioneered two new research fields, namely, (i) mechanophotonics – a domain that integrates mechanical micromanipulation of single-crystal photonic modules towards circuits, and (ii) the crystal photonics foundry – a novel approach involving focused ion-beam milling of organic single crystals to produce various organic single-crystal photonic modules. He has been an elected Fellow of the Optical Society of India, the Royal Society of Chemistry, the Indian Academy of Sciences, and the National Academy of Sciences. He is an International Advisory Board Member of journals such as Journal of Materials Chemistry C, Materials Advances, Advanced Optical Materials, ACS Applied Engineering Materials, and Small.

Panče Naumov

Panče Naumov is a full professor of chemistry at New York University Abu Dhabi (NYUAD) and a Global Network Professor at NYU’s Faculty of Arts and Sciences. He also holds a cross-appointment at the Molecular Design Institute and serves as the Director of the NYUAD Center for Smart Engineering Materials. Leading the Smart Materials Lab at NYUAD, Naumov’s team is recognized as a leader in chemistry and materials science in the UAE. He earned his PhD in chemistry and materials science from the Tokyo Institute of Technology and has previously held research fellow and faculty positions in Japan. Additionally, he is involved as an associate editor with multiple prestigious journals such as JACS, and as an advisory board member in several journals, and is the founder of the UAE Chapter of the American Chemical Society, as well as the President of the Emirates Crystallographic Society.

Xue-Dong Wang

Xue-Dong Wang is a full professor in the Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University. He received his PhD in Physical Chemistry at the Institute of Chemistry, Chinese Academy of Sciences (ICCAS). His group at Soochow University is committed to the precise creation of low-dimensional crystalline materials for molecular photonics including lasers, optical waveguides, and optical detectors. In the field of organic low-dimensional materials and photonics, they have carried out basic scientific research from precise synthesis and functional realization to theoretical mechanism elucidation. He is also a young editorial board member of the journal of Moore and More, and has obtained several awards such as the Chinese Chemical Society nanochemistry emerging award.

Kristin M. Hutchins

Kristin Hutchins is an associate professor in the Department of Chemistry at the University of Missouri (MU) and a core faculty member in the MU Materials Science & Engineering Institute. She earned her PhD in chemistry from the University of Iowa and was a postdoctoral associate at the University of Illinois Urbana-Champaign. Kristin previously held faculty positions at Texas Tech University before moving to her current position. Hutchins’ group uses supramolecular strategies to design stimuli-responsive organic solids, pharmaceutical materials, and polymers that capture valuable resources. She is an editorial advisory board member for the journals CrystEngComm and Structural Dynamics, and she serves on the advisory board for the Women in Supramolecular Chemistry (WISC) Network.

The structural dynamics of molecular crystals endow them with exceptional mechanical and optical properties—both independently and synergistically—positioning them as standout materials for applications such as photodetection, scintillation detection, phototransistors, semiconductor devices, near-infrared (NIR) phosphors, thermoelectric devices, optical displays, self-healing devices, nonlinear optical devices, waveguides, cavities and polariton lasers.

Precise tuning of functional groups, dihedral angles, conformational rigidity or flexibility, local disorder, polarization, charge transfer pathways, chirality, doping, and polymorphism imparts molecular crystals with highly tailored optomechanical properties. These modifications give rise to unusual mechanical responses, including flexibility activated by light, heat, or mechanical force, thermosalient and photosalient (explosive) effects, and molecular and macroscopic motions.

On the photonic front, molecular crystals can generate electromagnetic energy across a wide spectral range through diverse photophysical and photochemical processes operating on different timescales.

They also exhibit stable exciton formation, efficient charge transport, ferroelectric polarization, magnetoresistance, effective light-guiding and lasing capabilities.

For real-world device applications, while bottom-up approaches leverage structurally induced dynamics, it is equally imperative to technically develop the controlled manipulation of tiny crystals through top-down methods.

This themed collection presents a selection of representative articles highlighting the key attributes of molecular crystals discussed above. In the context of structural effects on crystal mechanics, Thakuria and Reddy et al. (https://doi.org/10.1039/D5TC00313J) report on twist-elasticity resulting from the cocrystallization of odd–even aliphatic dicarboxylic acids with the fluorophore (2Z)-3-(biphenyl-4-yl)-2-(pyridin-3-yl)prop-2-enenitrile. This cocrystallization induces changes in crystal packing that lead to elastic deformation. Matsuo and Hayashi (https://doi.org/10.1039/D5TC00817D) describe three polymorphic forms of the luminescent compound 1,4-bis(benzofuran-2-yl)-2,3,5,6-tetrafluorophenylene, yielding distinctly different mechanical behaviors: flexible fibers, rigid blocks, and flexible plates, depending on the packing motif. Skoko et al. (https://doi.org/10.1039/D4TC04615C) explored a series of molecular alloys composed of 1,2,4,5-tetrabromobenzene and 1,2,4,5-tetrachlorobenzene. They demonstrate that the temperature of the thermosalient phase transition increases linearly with the tetrabromobenzene content, ranging from 30.13 °C to 45.33 °C. Hutchins et al. (https://doi.org/10.1039/D5TC00576K) demonstrated the use of a mixed cocrystal strategy to modulate the thermal and thermomechanical properties—specifically thermal expansion—within hydrogen-bonded solids.

In the realm of photonic properties of molecular crystals, by tuning the balance between molecular twist and conformational rigidity in imidazo[1,2-a]pyridines, Saha et al. (https://doi.org/10.1039/D5TC01128K) developed dual-state emissive luminogens suitable for applications in OLEDs and cellular imaging. Sun and co-workers (https://doi.org/10.1039/D4TC04812A) reported pronounced magnetoresistance and promising thermoelectric performance in a quasi-skutterudite Ca₃Pt₄Sn₁₃ single crystal. Zhang et al. (https://doi.org/10.1039/D5TC00112A) present a modular strategy for imparting multicolor luminescence to organic single crystals while maintaining their crystallinity. By coating semiconductor quantum dots onto flexible organic crystals using a layer-by-layer electrostatic self-assembly technique, they fabricated flexible optical waveguides with tunable light output. Koide et al. (https://doi.org/10.1039/D5TC00891C) reported flexible organic–inorganic hybrid crystals of tin(IV) chloride and naphthalenediimide exhibiting elasticity, mechanochromism, and photothermal conversion. Chandrasekar et al. (https://doi.org/10.1039/D5TC02090E) demonstrated the use of two optically distinct, flexible copper coordination crystal waveguides for the mechanophotonic construction of a three-port Y-splitter, enabling the generation, splitting, and routing of different fluorescence signals into selected output ports. Hu et al. (https://doi.org/10.1039/D5TC00456J) introduced an interfacial heteroepitaxy strategy to fabricate molecularly thin, single-crystalline p–n heterojunctions with precise orientation control. This approach enables the development of polarization-sensitive photodetectors with enhanced performance.

This themed collection on the mechanics and photonics of molecular crystals offers a concise overview of recent advances, highlighting progress in structural understanding, underlying mechanisms, and emerging applications.

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