Highly Elastic Organic Crystals for Flexible Optical Waveguides

The study of elastic organic single crystals (EOSCs) has emerged as a cutting‐edge research of crystal engineering. Although a few EOSCs have been reported recently, those suitable for optical/optoelectronic applications have not been realized. Here, we report an elastic crystal of a Schiff base, (E)‐1‐(4‐(dimethylamino)phenyl)iminomethyl‐2‐hydroxyl‐naphthalene. The crystal is highly bendable under external stress and able to regain immediately its original straight shape when the stress is released. It displays bright orange–red emission with a high fluorescence quantum yield of 0.43. Intriguingly, it can serve as a low‐loss optical waveguide even at the highly bent state. Our result highlights the feature and utility of “elasticity” of organic crystals.     

2D Organic Photonics: An Asymmetric Optical Waveguide in Self‐Assembled Halogen‐Bonded Cocrystals

Anisotropic organic molecular construction and packing are crucial for the optoelectronic properties of organic crystals. Two‐dimensional (2D) organic crystals with regular morphology and good photon confinement are potentially suitable for a chip‐scale planar photonics system. Herein, through the bottom‐up process, 2D halogen‐bonded DPEpe‐F₄DIB cocrystals were fabricated that exhibit an asymmetric optical waveguide with the optical‐loss coefficients of R_Backward=0.0346 dB μm^?1 and R_Forward=0.0894 dB μm^?1 along the [010] crystal direction, which can be attributed to the unidirectional total internal reflection caused by the anisotropic molecular packing mode. Based on this crystal direction‐oriented asymmetric photon transport, these as‐prepared 2D cocrystals have been demonstrated as a microscale optical logic gate with multiple input/out channels, which will offer potential applications as the 2D optical component for the integrated organic photonics.     

Organic Crystals with Near‐Infrared Amplified Spontaneous Emissions Based on 2′‐Hydroxychalcone Derivatives: Subtle Structure Modification but Great Property Change

A series of highly efficient deep red to near‐infrared (NIR) emissive organic crystals 1 – 3 based on the structurally simple 2′‐hydroxychalcone derivatives were synthesized through a simple one‐step condensation reaction. Crystal 1 displays the highest quantum yield (Φ_f) of 0.32 among the reported organic single crystals with an emission maximum (λ_em) over 710 nm. Comparison between the bright emissive crystals 1 – 3 and the nearly nonluminous compounds 4 – 7 clearly gives evidence that a subtle structure modification can arouse great property changes, which is instructive in designing new high‐efficiency organic luminescent materials. Notably, crystals 1 – 3 exhibit amplified spontaneous emissions (ASE) with extremely low thresholds. Thus, organic deep red to NIR emissive crystals with very high Φ_f have been obtained and are found to display the first example of NIR fluorescent crystal ASE.     

Efficient Two‐Photon Excited Amplified Spontaneous Emission from Organic Single Crystals

E, E‐1, 4‐bis[4′‐(N,N‐dibutylamino)styryl]‐2,5‐dimethoxy‐benzene (DBASDMB) organic crystals with high crystalline quality, large size and excellent optical properties are prepared. The linear and nonlinear properties in the crystal are comparatively studied. The relaxation dynamics pumped by two‐photon are very similar with that pumped by one‐photon. The crystal exhibits very strong two‐photon excited fluorescence and amplified spontaneous emission. Efficient two‐photon absorption, reasonably high fluorescent quantum efficiency, and high crystal quality together with stimulated emission make organic crystals ideal for the application in frequency upconversion and other optoelectronic fields.     

Micromanipulation of Mechanically Compliant Organic Single‐Crystal Optical Microwaveguides

Flexible organic single crystals are evolving as new materials for optical waveguides that can be used for transfer of information in organic optoelectronic microcircuits. Integration in microelectronics of such crystalline waveguides requires downsizing and precise spatial control over their shape and size at the microscale, however that currently is not possible due to difficulties with manipulation of these small, brittle objects that are prone to cracking and disintegration. Here we demonstrate that atomic force microscopy (AFM) can be used to reshape, resize and relocate single‐crystal microwaveguides in order to attain spatial control over their light output. Using an AFM cantilever tip, mechanically compliant acicular microcrystals of three N‐benzylideneanilines were bent to an arbitrary angle, sliced out from a bundle into individual crystals, cut into shorter crystals of arbitrary length, and moved across and above a solid surface. When excited by using laser light, such bent microcrystals act as active optical microwaveguides that transduce their fluorescence, with the total intensity of transduced light being dependent on the optical path length. This micromanipulation of the crystal waveguides using AFM is non‐invasive, and after bending their emissive spectral output remains unaltered. The approach reported here effectively overcomes the difficulties that are commonly encountered with reshaping and positioning of small delicate objects (the “thick fingers” problem), and can be applied to mechanically reconfigure organic optical waveguides in order to attain spatial control over their output in two and three dimensions in optical microcircuits.     

Elastic Organic Crystals Based on Barbituric Derivative: Multi-faceted Bending and Flexible Optical Waveguide

Elastic organic single crystals with light-emitting and multi-faceted bending properties are extremely rare. They have potential application in optical materials and have attracted the extensive attention of researchers. In this paper, we reported a structurally simple barbituric derivative DBDT , which was easily crystallized and gained long needle-like crystals (centimeter-scale) in DCM/CH₃OH (v/v=2/8). Upon applying or removing the mechanical force, both the (100) and (040) faces of the needle-like crystal showed reversible bending behaviour, showing the nature of multi-faceted bending. The average hardness (H) and elastic modulus (E) were 0.28±0.01 GPa and 4.56±0.03 GPa for the (040) plane, respectively. Through the analysis of the single crystal data, it could be seen that the van der waals (C−H⋅⋅⋅π and C−H⋅⋅⋅C), H-bond (C−H⋅⋅⋅O) and π⋅⋅⋅π interactions between molecules were responsible for the generation of the crystal elasticity. Interestingly, elastic crystals exhibited optical waveguide characteristics in straight or bent state. The optical loss coefficients measured at 627 nm were 0.7 dBmm⁻¹ (straight state) and 0.9 dBmm⁻¹ (bent state), while the optical loss coefficient (α) were 1.5 dBmm⁻¹ (straight state) and 1.8 dBmm⁻¹ (bent state) at 567 nm. Notably, the elastic organic molecular crystal based on barbituric derivative could be used as the candidate for flexible optical devices.     

Anisotropic Poisson Effect and Deformation‐Induced Fluorescence Change of Elastic 9,10‐Dibromoanthracene Single Crystals

Elastic organic crystals have attracted considerable attention as next‐generation flexible smart materials. However, the detailed information on both molecular packing change and macroscopic mechanical crystal deformations upon applied stress is still insufficient. Herein, we report that fluorescent single crystals of 9,10‐dibromoanthracene are elastically bendable and stretchable, which allows a detailed investigation of the deformation behavior. We clearly observed a Poisson effect for the crystal, where the short axes (b and c‐axes) of the crystal are contracted upon elongation along the long axis (a‐axis). Moreover, we found that the Poisson's ratios along the b‐axis and c‐axis are largely different. Theoretical molecular simulation suggests that the tilting motion of the anthracene may be responsible for the large deformation along the c‐axis. Spatially resolved photoluminescence (PL) measurement of the bent elastic crystals reveals that the PL spectra at the outer (elongated), central (neutral), and inner (contracted) sides are different from each other.     

Atomic Force Microscopy Investigation of Poly[(R)‐3‐hydroxybutyrate] Lamellar Single Crystals: Relationship between Molecular Weight and Enzymatic Degradation Behavior

Enzymatic degradation behavior of a lamella of single crystals of poly(R)‐3‐hydroxybutyrate (P(3HB)) with an extracellular polyhydroxybutyrate (PHB) depolymerase purified from Alcaligenes faecalis T1 has been investigated by atomic force microscopy (AFM) in order to obtain further information for the chain packing state of P(3HB) in a lamellar single crystal. Two kinds of P(3HB) single crystals with different molecular weights, denoted respectively as H‐ and L‐P(3HB) for high and low molecular weights, respectively, were prepared. The enzymatic treatment was conducted for P(3HB) single crystals adsorbed on a surface of highly ordered pyrolytic graphite. The enzymatic degradation of both P(3HB) single crystals generates several crevices crosswise across the crystal at an early stage. Subsequently, the enzymatic degradation yields numbers of cracks lengthwise along the crystal. In addition to these common features, the interval between cracks crosswise across a lamella in H‐P(3HB) single crystal is longer than that in L‐P(3HB) single crystal, and each crack has V‐shaped and rectangular shaped morphology for H‐ and L‐P(3HB) single crystals, respectively. Based on these results, it is concluded that a lamella of P(3HB) single crystal has straight degradation pathways, that may correspond to a switchboard region, along the long axis of the crystal, independent of molecular weight of P(3HB) samples, and that a H‐P(3HB) single crystal has broader degradation pathways with longer intervals crosswise across the crystal than a L‐P(3HB) single crystal.     

Synthesis, growth and characterization of a new nonlinear optical material: 4-phenylpyridinium hydrogen squarate (4PHS)

A novel organic non-linear optical organic single crystal of 4-phenylpyridinium hydrogen squarate (4PHS) has been synthesized and successfully grown from aqueous solutions by slow evaporation solution growth method. In the present investigation the title compound has been synthesized by taking equimolar quantity of 4-phenylpyridine and squaric acid and mixed thoroughly using double distilled water as the solvent. The prepared concentrated solution was placed in an undisturbed condition, and then the solution was periodically inspected. The good quality single crystals have been harvested in a time span of 3 weeks. Then the grown crystal was characterized as single crystal XRD, differential thermal analysis, thermogravimetric analysis, FTIR, UV-vis-NIR, SHG, (1)H NMR and (13)C NMR analyses, respectively. The observed results from the characterization analyses show its suitability for NLO applications when compared with some of the existing organic crystals. The relative second harmonic generation of this grown crystal was found to be five times higher than that of KDP crystal. The UV cut-off wavelength and decomposition temperature of this grown crystal were also comparatively better. (1)H NMR and (13)C NMR spectroscopic studies were employed to elucidate the structure of the grown specimen.     

Synthesis,crystal growth,structural, spectral,optical, thermal and dielectric studies of a new nonlinear optical material: 4-Hydroxy-l-proline-l-tartaric acid (1:1)

Single crystals of a new organic material, 4-hydroxy-l-proline-l-tartaric acid (HPTA) adduct, were grown by slow evaporation of an aqueous solution containing equimolar concentrations of 4-hydroxy-l-proline and l-tartaric acid at room temperature. Formation of new crystal has been confirmed by single crystal X-ray diffraction and NMR spectroscopic techniques. The single crystal XRD analysis shows that a molecule of 4-hydroxy-l-proline and a molecule of free l-tartaric acid both of which are interlinked to each other by two types of H-bonding interactions, namely O(6)–H(6)⋯O(2) and N(1)–H(1B)⋯O(10). The title compound (HPTA) crystallizes in monoclinic crystal system with non-centrosymmetric space group P2₁. The suitability of the crystal for optical applications was studied by UV–Vis–NIR spectroscopy. Fourier transform infrared (FT-IR) spectral analysis was used to confirm the presence of various functional groups in the grown crystals. The thermal stability of the compound was investigated by thermogravimetric and differential thermal analyses. The dielectric constant and dielectric loss of the crystal were studied as a function of frequency and temperature. The fluorescence spectral analysis of the title crystal shows an indigo emission. The second order nonlinear optical (NLO) property of the crystal was confirmed by the modified Kurtz–Perry powder second harmonic generation (SHG) test.     

Solution‐Grown Organic Single‐Crystalline Donor–Acceptor Heterojunctions for Photovoltaics

Organic single crystals are ideal candidates for high‐performance photovoltaics due to their high charge mobility and long exciton diffusion length; however, they have not been largely considered for photovoltaics due to the practical difficulty in making a heterojunction between donor and acceptor single crystals. Here, we demonstrate that extended single‐crystalline heterojunctions with a consistent donor‐top and acceptor‐bottom structure throughout the substrate can be simply obtained from a mixed solution of C₆₀ (acceptor) and 3,6‐bis(5‐(4‐n‐butylphenyl)thiophene‐2‐yl)‐2,5‐bis(2‐ethylhexyl)pyrrolo[3,4‐c]pyrrole‐1,4‐dione (donor). 46 photovoltaic devices were studied with the power conversion efficiency of (0.255±0.095) % under 1 sun, which is significantly higher than the previously reported value for a vapor‐grown organic single‐crystalline donor–acceptor heterojunction (0.007 %). As such, this work opens a practical avenue for the study of organic photovoltaics based on single crystals.     

Molecular‐Level Understanding of Structural Changes of Organic Crystals Induced by Macroscopic Mechanical Stimulation

Structural changes to molecular crystals upon mechanical stimulation have attracted attention for sensing, recording, and microactuation. Comprehensive structure information is required to understand relationships between the mechanical force applied, the crystal structure, and the bulk property changes in order to develop general design concepts for mechanoresponsive compounds. Unfortunately, mechanical stimulation of organic crystals typically deteriorates their integrity, preventing detailed structure analyses by single‐crystal X‐ray diffraction (XRD) methods. However, in the past three years, several interesting studies have been reported in which molecular crystals retain their integrity even after a mechanically induced crystalline structure change. These materials have allowed us to investigate how macroscopic mechanical forces affect the microscopic structures of molecular crystals by single‐crystal XRD analyses. This Minireview summarizes current knowledge of mechanically induced structure changes in molecular crystals, which will facilitate research in this field.     

Using Crystal Optics to Demonstrate Single‐Layer Localization of a Solid‐State Chain Reaction

Upon warming to 225 K, single crystals of 11‐bromoundecanoyl peroxide (BrUP), in which radicals have been created by photolysis at lower temperature, undergo partial decomposition by a radical chain reaction ca. 40 cycles long. FTIR allowed monitoring two chain products: CO₂ and an α‐lactone that decomposes further at 260 K. When initiation is confined to alternate molecular layers by polarized photoselection, the chain reaction reduces the crystal symmetry from tetragonal to monoclinic. Desymmetrization is easily observed by optical microscopy, although it is difficult to detect by X‐ray diffraction. Accurate monitoring of birefringence using a Sénarmont 1/4‐wave plate, and comparison with FTIR kinetics, proves that the chain reaction occurs within single molecular layers 2 nm thick.     

Growth and characterization of 2-amino-4-picolinium toluene sulfonate single crystal

2-Amino-4-picolinium toluene sulfonate (2A4PTS), a new organic material, was synthesized and grown as single crystals in room temperature by slow evaporation solution growth technique using water as solvent. The crystal structure of 2A4PTS has been determined using single crystal X-ray diffraction studies. 2A4PTS belongs to monoclinic crystal system. The molecular arrangements in the crystal were studied. The structural perfection of the grown crystals has been analysed by high-resolution X-ray diffraction (HRXRD) rocking curve measurements. Fourier transform infrared (FTIR) spectral studies have been performed to identify the functional groups. The optical transmittance window and the lower cutoff wavelength of the 2A4PTS have been identified by UV–Vis–NIR studies. The nonlinear optical properties have been investigated by Z-scan method. The nonlinear refractive index and linear absorption coefficient of the 2A4PTS are found to be in the order of 10⁻⁸ cm²/W and 10⁻⁴ cm/W, respectively. The laser induced surface damage threshold for the grown crystal was measured using Nd:YAG laser. Thermal analysis carried out on the compound reveals that 2A4PTS is stable up to 133 °C. The microhardness test was carried out and the load dependent hardness was measured.     

Metal–Organic Organopolymeric Hybrid Framework by Reversible [2+2] Cycloaddition Reaction

Organic polymers are usually amorphous or possess very low crystallinity. The metal complexes of organic polymeric ligands are also difficult to crystallize by traditional methods because of their poor solubilities and their 3D structures can not be determined by single‐crystal X‐ray crystallography owing to a lack of single crystals. Herein, we report the crystal structure of a 1D Zn^II coordination polymer fused with an organic polymer ligand made in situ by a [2+2] cycloaddition reaction of a six‐fold interpenetrated metal–organic framework. It is also shown that this organic polymer ligand can be depolymerized in a single‐crystal‐to‐single‐crystal (SCSC) fashion by heating. This strategy could potentially be extended to make a range of monocrystalline metal organopolymeric complexes and metal–organic organopolymeric hybrid materials. Such monocrystalline metal complexes of organic polymers have hitherto been inaccessible for materials researchers.     

SHG Materials Based on the AlPO4-5 Single Crystals

Large AlPO4-5 molecular sieve single crystals with high optical quality were synthesized hydrothermally by using TPA as template. As-synthesized crystals were calcined under O2 atmosphere to remove the organic templates in the channels. Disperse-Red-1 (DR1) and p-nitroaniline (pNA) molecules have been successfully incorporated into the one-dimensional channels of AlPO4-5 single crystals respectively by means of vapor phase diffusion. XRD patterns reveal that the loading of organic molecules has not destroyed the structures of AlPO4-5 crystals. Polarizing microscope and SHG results indicate that the DR1 and pNA molecules are well aligned in a preferred direction along the crystal channels. The different polarization-dependence SH intensity shows that different SHG processes occur in the DR1- and pNA-loaded AlPO4-5 crystals.     

Polarized Absorption,Spontaneous and Stimulated Blue Light Emission of J‐type Tetraphenylbutadiene Monocrystals

Blue amplified spontaneous emission at room temperature is demonstrated from the exposed face of the strongly emitting organic semiconductor 1,1,4,4‐tetraphenyl‐1,3‐butadiene in single crystal form. The symmetry of the crystal and calculation of lattice sums indicate the J‐type organization of the molecular transition moments. The minimum in the lowest exciton dispersion branch, from which emission takes place, is found at the edge of the Brillouin zone leading to a dominant vibronic emission since the zero‐phonon line is forbidden. The observed gain narrowed line is attributed to the vibronic replica which becomes amplified with increased pumping. The reported emission is along the normal to the exposed crystal face, important for the development of vertical cavity geometry lasers based on organic single crystals. The threshold excitation fluence of 400 μJ cm^?2 is comparable to other organic crystalline systems, even if the amplification path is much reduced as a consequence of the vertical geometry. Considering these relevant aspects, the optical characterization of this material is provided. The polarized absorption spectra are reported and the properties of the lowest‐energy excitonic state investigated. Calculation of the electronic transitions for the isolated molecule, lattice sums for the transition at lowest energy, and the symmetry of the crystal allow attributing the largest face of the samples and the observed optical bands in the spectra. Polarized time‐resolved spectra are also reported allowing to identify the intrinsic excitonic emission.     

Crystalline Behavior and Structure of a Liquid Crystal Compound, 5‐{[4′‐(((Pentyl)oxy)‐4‐biphenylyl)carbonyl]oxy}‐1‐pentyne

Single crystals of two liquid crystal compounds, 5‐{[4′‐(((pentyl)oxy)‐4‐biphenylyl)carbonyl]oxy}‐1‐pentyne (A3EO5) and 5‐{[(4′‐nonyloxy‐4‐biphenylyl)carbonyl]oxy}‐1‐pentyne (A3EO9), have been prepared by solution growth technique. The morphologies and structures of A3EO5 and A3EO9 crystals were investigated by wide angle X‐ray diffraction (WXRD), atom force microscope (AFM) and transmission electron microscope (TEM). In contrast to the same series of compounds which have a longer alkyl tail, 5‐{[(4′‐heptoxy‐4‐biphenylyl)carbonyl]oxy}‐1‐pentyne (A3EO7), 5‐{[(4′‐heptoxy‐4‐biphenylyl)oxy]carbonyl}‐1‐pentyne (A3E′O7) and A3EO9, A3EO5 shows strikingly different crystalline behavior. The former three compounds have only one crystal form, whereas A3EO5 exhibits polymorphism. Specifically, A3EO5 crystals grown from toluene solution show two crystal forms. The first one is crystal I which adopts a monoclinic P112/m space group with unit cell parameters of a?5.79 Å, b?8.34 Å, c?43.92 Å, γ?96°, and the other one is crystal II which adopts a monoclinic P112 space group with unit cell parameters of a?5.55 Å, b?7.38 Å, c?31.75 Å, γ?94°. When using dioxane as the solvent to grow A3EO5 crystal, we can selectively obtain crystal I. A3EO5 melt‐grown crystals also have two crystal forms which derive from crystal I and crystal II, respectively. The different crystalline behavior of the compounds should correlate with their different electron dipole moment resulting from the different length of alkyl tail.     

A Multidirectional Superelastic Organic Crystal by Versatile Ferroelastical Manipulation

Mechanical twinning changes atomic, molecular, and crystal orientations along with directions of the anisotropic properties of the crystalline materials while maintaining single crystallinity in each domain. However, such deformability has been less studied in brittle organic crystals despite their remarkable anisotropic functions. Herein we demonstrate a direction‐dependent mechanical twinning that shows superelasticity in one direction and ferroelasticity in two other directions in a single crystal of 1,3‐bis(4‐methoxyphenyl)urea. The crystal can undergo stepwise twinning and ferroelastically forms various shapes with multiple domains oriented in different directions, thereby affording a crystal that shows superelasticity in multiple directions. This adaptability and shape recoverability in a ferroelastic and superelastic single crystal under ambient conditions are of great importance in future applications of organic crystals as mechanical materials, such as in soft robotics.     

The Emergence of Organic Single‐Crystal Electronics

Organic semiconducting single crystals are perfect for both fundamental and application‐oriented research due to the advantages of free grain boundaries, few defects, and minimal traps and impurities, as well as their low‐temperature processability, high flexibility, and low cost. Carrier mobilities of greater than 10 cm² V^?1 s^?1 in some organic single crystals indicate a promising application in electronic devices. The progress made, including the molecular structures and fabrication technologies of organic single crystals, is introduced and organic single‐crystal electronic devices, including field‐effect transistors, phototransistors, p‐n heterojunctions, and circuits, are summarized. Organic two‐dimensional single crystals, cocrystals, and large single crystals, together with some potential applications, are introduced. A state‐of‐the‐art overview of organic single‐crystal electronics, with their challenges and prospects, is also provided.