Mechanically controlled multifaceted dynamic transformations in twisted organic crystal waveguides.

This study introduces mechanically induced phenomena such as standing, leaning, stacking, and interlocking behaviors in naturally twisted optical waveguiding microcrystals on a substrate. The microscale twisted crystal self-assembled from 2,4-dibromo-6-(((2-bromo-5-fluorophenyl)imino)methyl)phenol is flexible and emits orange fluorescence. Mechanistic analysis reveals the strain generated by the intergrowing orientationally mismatched nanocrystallites is responsible for the twisted crystal growth. The crystal's mechanical flexibility in the perpendicular direction to (001) and (010) planes can be attributed to intermolecular Br···Br, F···Br, and π···π stacking interactions. Through a systematic process involving step-by-step bending and subsequent optical waveguiding experiments at each bent position, a linear relationship between optical loss and mechanical strain is established. Additionally, the vertical standing and leaning of these crystals at different angles on a flat surface and the vertical stacking of multiple crystals reveal the three-dimensional aspects of organic crystal waveguides, introducing light trajectories in a 3D space. Furthermore, the integration of two axially interlocked twisted crystals enables the coupling of polarization rotation along their long axis. These crystal dynamics expand the horizons of crystal behavior and have the potential to revolutionize various applications, rendering these crystals invaluable in the realm of crystal-related science and technology.

Highly efficient color-tunable organic co-crystals unveiling polymorphism, isomerism, delayed fluorescence for optical waveguides and cell-imaging.

Photofunctional co-crystal engineering strategies based on donor-acceptor π-conjugated system facilitates expedient molecular packing, consistent morphology, and switchable optical properties, conferring synergic 'structure-property relationship' for optoelectronic and biological functions. In this work, a series of organic co-crystals were formulated using a twisted aromatic hydrocarbon (TAH) donor and three diverse planar acceptors, resulting in color-tunable solid and aggregated state emission via variable packing and through-space charge-transfer interactions. While, adjusting the strength of acceptors, a structural transformation into hybrid stacking modes ultimately results in color-specific polymorphs, a configurational cis-isomer with very high photoluminescence quantum yield. The cis-isomeric co-crystal exhibits triplet-harvesting thermally activated delayed fluorescence (TADF) characteristics, presenting a key discovery in hydrocarbon-based multicomponent systems. Further, 1D-microrod-shaped co-crystal acts as an efficient photon-transducing optical waveguides, and their excellent dispersibility in water endows efficient cellular internalization with bright cell imaging performances. These salient approaches may open more avenues for the design and applications of TAH based co-crystals.

Crystal photonics foundry: geometrical shaping of molecular single crystals into next generation optical cavities.

The advancements in organic photonics have reached new heights in the recent past with the demonstration of diverse organic crystal optical components and circuits. However, the development of industrially viable manufacturing of organic optical components is the need of the hour for finding an alternative to silicon-based photonics. Here, we demonstrate focused ion beam (FIB) milling as a tool to shape organic single crystals into optical cavities of diverse geometries and dimensions. The generality of FIB milling was tested on perylene and coumarin-153 microcrystals. The microcrystals obtained by self-assembly of perylene and sublimation of coumarin-153 were carved into desired disc-, ring- and rectangular shapes. These shaped crystals act as cavities exhibiting sharp resonance modes in the fluorescence spectrum, confirming optical interference. The FDTD numerical calculations support the light electric field distribution in these optical cavities. This unprecedented single crystal processing technique enables industrial-scale production of optical components and circuits and acts as a foundry for crystal photonics.

Mechanical properties and peculiarities of molecular crystals.

In the last century, molecular crystals functioned predominantly as a means for determining the molecular structures via X-ray diffraction, albeit as the century came to a close the response of molecular crystals to electric, magnetic, and light fields revealed that the physical properties of molecular crystals were as rich as the diversity of molecules themselves. In this century, the mechanical properties of molecular crystals have continued to enhance our understanding of the colligative responses of weakly bound molecules to internal frustration and applied forces. Here, the authors review the main themes of research that have developed in recent decades, prefaced by an overview of the particular considerations that distinguish molecular crystals from traditional materials such as metals and ceramics. Many molecular crystals will deform themselves as they grow under some conditions. Whether they respond to intrinsic stress or external forces or interactions among the fields of growing crystals remains an open question. Photoreactivity in single crystals has been a leading theme in organic solid-state chemistry; however, the focus of research has been traditionally on reaction stereo- and regio-specificity. However, as light-induced chemistry builds stress in crystals anisotropically, all types of motions can be actuated. The correlation between photochemistry and the responses of single crystals-jumping, twisting, fracturing, delaminating, rocking, and rolling-has become a well-defined field of research in its own right: photomechanics. The advancement of our understanding requires theoretical and high-performance computations. Computational crystallography not only supports interpretations of mechanical responses, but predicts the responses itself. This requires the engagement of classical force-field based molecular dynamics simulations, density functional theory-based approaches, and the use of machine learning to divine patterns to which algorithms can be better suited than people. The integration of mechanics with the transport of electrons and photons is considered for practical applications in flexible organic electronics and photonics. Dynamic crystals that respond rapidly and reversibly to heat and light can function as switches and actuators. Progress in identifying efficient shape-shifting crystals is also discussed. Finally, the importance of mechanical properties to milling and tableting of pharmaceuticals in an industry still dominated by active ingredients composed of small molecule crystals is reviewed. A dearth of data on the strength, hardness, Young's modulus, and fracture toughness of molecular crystals underscores the need for refinement of measurement techniques and conceptual tools. The need for benchmark data is emphasized throughout.

Dimension Engineering of Stimuli-Responsive 1D Molecular Crystals into Unusual 2D and 3D Zigzag Waveguides.

We demonstrate an innovative technique to achieve organic 2D and 3D waveguides with peculiar shapes from an acicular, stimuli-responsive molecular crystal, (2Z,2'Z)-3,3'-(anthracene-9,10-diyl)bis(2-(3,5-bis(trifluoromethyl)phenylacrylonitrile), Ant-CF3 . The greenish-yellow fluorescent (FL) Ant-CF3 molecular crystals exhibit laser power-dependent permanent mechanical bending in 2D and 3D. Investigation of a single-crystal using spatially-resolved Raman/FL/electron microscopy, and theoretical calculations revealed photothermal (Z,E)/(E,E) isomerization-assisted transition from crystalline to amorphous phase at the laser-exposed regions. This phenomenon facilitates the dimension engineering of a 1D crystal waveguide into 2D waveguide on a substrate or a 3D waveguide in free space. The bends can be used as interconnection points to couple different optical elements. The presented technique has broader implications in organic photonics and other crystal-related photonic technologies.

Amphibian-like Flexible Organic Crystal Optical Fibers for Underwater/Air Micro-Precision Lighting and Sensing.

Visible light guiding optical fibers with underwater operational capability are highly desired for subaquatic communication and sensing technologies. Herein, we present mechanically flexible, blue-violet fluorescent (4,4'-bis(2,6-di(1H-pyrazol-1-yl)pyridin-4-yl)biphenyl) (BPP) crystal waveguides with high-aspect ratio. These milli-meter-long BPP crystals guide light actively and passively in ambient and underwater conditions demonstrating their amphibian-like character. Due to the crystal's high flexibility, the optical fiber's output light direction in submerged and ambient states can be altered mechanically for high-precision lighting and sensing applications. The development of such multi-environment-compatible and mechanically flexible organic optical fibers acting as sensing materials possess enormous potential for short-range underwater photonic technologies.

A Broadband, Multiplexed-Visible-Light-Transport in Composite Flexible-Organic-Crystal Waveguide.

We report the construction of an organic crystal multiplexer using three chemically and optically different acicular, flexible organic crystals for a broadband, visible light signal transportation. The mechanical integration of a highly flexible crystal waveguide of (Z)-2-(3,5-bis(trifluoromethyl)phenyl)-3-(7-methoxybenzo[c][1,2,5]thiadiazol-4-yl)acrylonitrile (BTD2CF3 ) displaying bright yellow (λ1 ) fluorescence with blue-emitting (λ2 ) BPP and cyan emitting (λ3 ) DBA crystals using AFM-tip provides a composite organic crystal multiplexer. The constructed hybrid single crystal multiplexer effectively transduces three optical signals (λ1 +λ2 +λ3 ) covering the 420-750 nm region as a composite output signal. The presented proof-of-principle experiment demonstrates the real potential of organic flexible crystal waveguides for visible light communication technologies.

Hot-exciton harvesting via through-space single-molecule based white-light emission and optical waveguides.

Through-space donor-alkyl bridge-acceptor (D-σ-A) luminogens are developed as new organic single-molecule white light emitters (OSMWLEs) involving multiple higher lying singlet (S n ) and triplet (T m ) states (hot-excitons). Experimental and theoretical results confirm the origin of white light emission due to the co-existence of prompt fluorescence from locally excited states, thermally activated delayed fluorescence (TADF), and fast/slow dual phosphorescence color mixing simultaneously. Notably, the fast phosphorescence was observed due to trace amounts of isomeric impurities from commercial carbazole, while H-/J-aggregation resulted in slow phosphorescence. Crystal structure-packing-property analysis revealed that the alkyl chain length induced supramolecular self-assembly greatly influenced the solid-state optical properties. Remarkably, the 1D-microrod crystals of OSMWLEs demonstrated the first examples of triplet harvesting waveguides by self-guiding the generated phosphorescence through light propagation along their longitudinal axis. This work thus highlights an uncommon design strategy to achieve multi-functional OSMWLEs with in-depth mechanistic insights and optical waveguiding applications making them a potentially new class of white emissive materials.

Adaptable Optical Microwaveguides From Mechanically Flexible Crystalline Materials.

Flexible organic crystals (elastic and plastic) are important materials for optical waveguides, tunable optoelectronic devices, and photonic integrated circuits. Here, we present highly elastic organic crystals of a Schiff base, 1-((E)-(2,5-dichlorophenylimino)methyl)naphthalen-2-ol (1), and an azine molecule, 2,4-dibromo-6-((E)-((E)-(2,6-dichlorobenzylidene)hydrazono)methyl)phenol (2). These microcrystals are highly flexible under external mechanical force, both in the macroscopic and the microscopic regimes. The mechanical flexibility of these crystals arises as a result of weak and dispersive C-H⋅⋅⋅Cl, Cl⋅⋅⋅Cl, Br⋅⋅⋅Br, and π⋅⋅⋅π stacking interactions. Singly and doubly-bent geometries were achieved from their straight shape by a micromechanical approach using the AFM cantilever tip. Crystals of molecules 1 and 2 display a bright-green and red fluorescence (FL), respectively, and selective reabsorption of a part of their FL band. Crystals 1 and 2 exhibit optical-path-dependent low loss emissions at the termini of crystal in their straight and even in extremely bent geometries. Interestingly, the excitation position-dependent optical modes appear in both linear and bent waveguides of crystals 1 and 2, confirming their light-trapping ability.

Micromechanically-Powered Rolling Locomotion of a Twisted-Crystal Optical-Waveguide Cavity as a Mobile Light Polarization Rotor.

We demonstrate mechanically-powered rolling locomotion of a twisted-microcrystal optical-waveguide cavity on the substrate, rotating the output signal's linear-polarization. Self-assembly of (E)-2-bromo-6-(((4-methoxyphenyl)imino)methyl)-4-nitrophenol produces naturally twisted microcrystals. The strain between several intergrowing, orientationally mismatched nanocrystalline fibres dictates the pitch lengths of the twisted crystals. The crystals are flexible, perpendicular to twisted (001) and (010) planes due to π⋅⋅⋅π stacking, C-H⋅⋅⋅Br, N-H⋅⋅⋅O and C-H⋅⋅⋅O interactions. The twisted crystals in their straight and bent geometries guide fluorescence along their body axes and display optical modes. Depending upon the degree of mechanical rolling locomotion, the crystal-waveguide cavity correspondingly rotates the output signal polarization. The presented twisted-crystal cavity with a combination of mechanical locomotion and photonic attributes unfolds a new dimension in mechanophotonics.

Mechanophotonics - a guide to integrating microcrystals toward monolithic and hybrid all-organic photonic circuits.

Molecular crystals are emerging as a non-silicon alternative for the construction of all-organic photonic integrated circuits (OPICs). The advent of flexible molecular crystals and the development of atomic force microscopy tip-based mechanical micromanipulation (mechanophotonics) techniques facilitate the construction of many proof-of-principle OPICs. This article validates the reason for using organic crystals as alternate non-silicon materials for OPIC fabrication. It also guides the readers by introducing several crystal-based photonic modules and OPIC prototypes, their passive and active light transduction potentials, and the possibility of implementing well-known photo-physical concepts viz. optical energy transfer and reabsorbance mechanisms. There is also an urgent need to develop a suitable technique for creating geometrically and dimensionally well-defined organic crystals displaying photonic attributes. Finally, the goal should be to build a library of selected optical crystals to facilitate the construction of OPICs with a pick-and-place approach.

Polarised Optical Emission from Organic Anisotropic Microoptical Waveguides Grown by Ambient Pressure Vapour-deposition.

Ambient pressure chemical vapour deposition of 5,5'-bis((2-(trifluoromethyl)phenyl)ethynyl)-2,2'-bithiophene provides ultrapure needle-shaped crystals. The crystal's supramolecular structure consists of an array of hydrogen bonds and π-π interactions leading to anisotropic arrangements. The cyan emitting crystals exhibit an optical waveguiding tendency with guided polarised optical emissions due to anisotropic molecular arrangements.

Mechanophotonics-Mechanical Micromanipulation of Single-Crystals toward Organic Photonic Integrated Circuits.

The advent of molecular crystals as "smart" nanophotonic components namely, organic waveguides, resonators, lasers, and modulators are drawing wider attention of solid-state materials scientists and microspectroscopists. Crystals are usually rigid, and undeniably developing next-level crystalline organic photonic circuits of complex geometries demands using mechanically flexible crystals. The mechanical shaping of flexible crystals necessitates applying challenging micromanipulation methods. The rise of atomic force microscopy as a mechanical micromanipulation tool has increased the scope of mechanophotonics and subsequently, crystal-based microscale organic photonic integrated circuits (OPICs). The unusual higher adhesive energy of the flexible crystals to the surface than that of crystal shape regaining energy enables carving intricate crystal geometries using micromanipulation. This perspective reviews the progress made in a key research area developed by my research group, namely mechanophotonics-a discipline that uses mechanical micromanipulation of single-crystal optical components, to advance nanophotonics. The precise fabrication of photonic components and OPICs from both rigid and flexible microcrystal via AFM mechanical operations namely, moving, lifting, cutting, slicing, bending, and transferring of crystals are presented. The ability of OPICs to guide, split, couple, and modulate visible electromagnetic radiation using passive, active, and energy transfer mechanism are discussed as well with recent literature examples.

Mechanical Processing of Naturally Bent Organic Crystalline Microoptical Waveguides and Junctions.

Precise mechanical processing of optical microcrystals involves complex microscale operations viz. moving, bending, lifting, and cutting of crystals. Some of these mechanical operations can be implemented by applying mechanical force at specific points of the crystal to fabricate advanced crystalline optical junctions. Mechanically compliant flexible optical crystals are ideal candidates for the designing of such microoptical junctions. A vapor-phase growth of naturally bent optical waveguiding crystals of 1,4-bis(2-cyanophenylethynyl)benzene (1) on a surface forming different optical junctions is presented. In the solid-state, molecule 1 interacts with its neighbors via CH⋅⋅⋅N hydrogen bonding and π-π stacking. The microcrystals deposited at a glass surface exhibit moderate flexibility due to substantial surface adherence energy. The obtained network crystals also display mechanical compliance when cut precisely with sharp atomic force microscope cantilever tip, making them ideal candidates for building innovative T- and Δ-shaped optical junctions with multiple outputs. The presented micromechanical processing technique can also be effectively used as a tool to fabricate single-crystal integrated photonic devices and circuits on suitable substrates.

Laser intensity-dependent nonlinear-optical effects in organic whispering gallery mode cavity microstructures.

Nonlinear microresonators are very desired for a wide variety of applications. Up-conversion processes responsible for the transformation of IR laser radiation into visible are intensity-dependent and thus rather sensitive to all involved effects, which can mask each other. In this work we study the phenomena that are the most important for possible lasing in 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4 H-pyran dye spherical microresonators: the two-photon absorption and photobleaching. Based on the suggested model of the threshold-like dependence of the two-photon luminescence (TPL) on pump power, we demonstrate the role of intensity-dependent photobleaching in the appearance of the TPL and find a good agreement with the experiment. This finding is important for the analysis of lasing in nonlinear dye-based resonators.

Mechanophotonics: Flexible Single-Crystal Organic Waveguides and Circuits.

We present the one-dimensional optical-waveguiding crystal dithieno[3,2-a:2',3'-c]phenazine with a high aspect ratio, high mechanical flexibility, and selective self-absorbance of the blue part of its fluorescence (FL). While macrocrystals exhibit elasticity, microcrystals deposited at a glass surface behave more like plastic crystals due to significant surface adherence, making them suitable for constructing photonic circuits via micromechanical operation with an atomic-force-microscopy cantilever tip. The flexible crystalline waveguides display optical-path-dependent FL signals at the output termini in both straight and bent configurations, making them appropriate for wavelength-division multiplexing technologies. A reconfigurable 2×2-directional coupler fabricated via micromanipulation by combining two arc-shaped crystals splits the optical signal via evanescent coupling and delivers the signals at two output terminals with different splitting ratios. The presented mechanical micromanipulation technique could also be effectively extended to other flexible crystals.

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.

Direct micro-scale monitoring of molecular aggregation, its growth and diffusion via aggregation-induced emission.

Time-dependent monitoring of aggregation-induced fluorescence of a model compound namely, (Z)-3-(3',5'-bis(trifluoromethyl)-[1,1'-biphenyl]-4-yl)-2-(4-bromophenyl)acrylonitrile unearth hitherto unknown molecular level events such as onset of molecular aggregation, their growth, size, and diffusion dynamics. The presented generalized approach can also be extended to in situ monitoring and controlling of various biological aggregation processes down to a single-cell level and all aspects of materials chemistry, as well.

Mechanical Actuation and Patterning of Rewritable Crystalline Monomer-Polymer Heterostructures via Topochemical Polymerization in a Dual-Responsive Photochromic Organic Material.

The dark-orange monomer single crystals of 1,1'-dioxo-1H-2,2'-biindene-3,3'-diyldidodecanoate (BIT-dodeca2) convert to a transparent single-crystalline polymer (PBIT-dodeca2) material via a single-crystal-to-single-crystal (SCSC) polymerization reaction under sunlight, which then undergoes reverse thermal transformation into BIT-dodeca2 single crystals, leading to reversible photo-/thermochromism, coupled with mechanical actuation. We exploit the properties of this unique material to demonstrate the formation of monomer-polymer heterostructures in selected regions of single crystals with micrometer-scale precision using a laser. This is the first example of heterostructure patterning involving monomer-polymer domains in single crystals. We reveal that the speed of photomechanical bending induced by the polymerization reaction in this example is comparable to those of the well-known diarylethene derivatives, in which electrocyclic ring-closing-ring-opening reactions operate. Furthermore, we characterize the distinct mechanical properties of the monomer and polymer using a quantitative nanoindentation technique as well as demonstrate photopatterning on a monomer-coated paper for potential use in security devices. These crystals with several advantages, such as photomechanical bending (weight lifting) even when the crystal size is large, responsiveness to both UV and visible light, distinct solubilities (the polymer is insoluble, whereas the monomer is soluble in most organic solvents) and colors, provide unique opportunities for their use at different length scales of the sample (µm to mm) for various purposes.

Mechanophotonics: precise selection, assembly and disassembly of polymer optical microcavities via mechanical manipulation for spectral engineering.

The advancement of nanoscience and technology relies on the development and utility of innovative techniques. Precise manipulation of photonic microcavities is one of the fundamental challenges in nanophotonics. This challenge impedes the construction of optoelectronic and photonic microcircuits. As a proof-of-principle, we demonstrate here that an atomic force microscopy cantilever and confocal microscopy can be used together to mechanically micromanipulate polymer-based whispering gallery mode microcavities or microresonators into well-ordered geometries. The micromanipulation technique efficiently assembles or disassembles resonators and also produces well-ordered dimer, trimer, tetramer, and pentamer assemblies of resonators in linear and bent geometries. Interestingly, an intricate L-shaped coupled-resonator optical waveguide (CROW) comprising a pentamer assembly effectively transduces light through a 90° bend angle. The presented new research direction, which combines mechanical manipulation and nanophotonics, is also expected to open up a plethora of opportunities in nano and microstructure-based research areas including nanoelectronics and nanobiology.