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.     

Photochemically and Thermally Driven Full‐Color Reflection in a Self‐Organized Helical Superstructure Enabled by a Halogen‐Bonded Chiral Molecular Switch

Supramolecular approaches toward the fabrication of functional materials and systems have been an enabling endeavor. Recently, halogen bonding has been harnessed as a promising supramolecular tool. Herein we report the synthesis and characterization of a novel halogen‐bonded light‐driven axially chiral molecular switch. The photoactive halogen‐bonded chiral switch is able to induce a self‐organized, tunable helical superstructure, that is, cholesteric liquid crystal (CLC), when doped into an achiral liquid crystal (LC) host. The halogen‐bonded switch as a chiral dopant has a high helical twisting power (HTP) and shows a large change of its HTP upon photoisomerization. This light‐driven dynamic modulation enables reversible selective reflection color tuning across the entire visible spectrum. The chiral switch also displays a temperature‐dependent HTP change that enables thermally driven red, green, and blue (RGB) reflection colors in the self‐organized helical superstructure.     

Halogen‐Bond‐Templated [2+2] Photodimerization in the Solid State: Directed Synthesis and Rare Self‐Inclusion of a Halogenated Product

A ditopic halogen‐bond acceptor organizes a diiodooctafluorostilbene for a [2+2] photodimerization reaction to take place between two stilbene molecules in the solid state. The resultant cyclobutane product is functionalized with halogen atoms and undergoes self‐assembly to form a channel‐type host–guest compound that exhibits a very rare form of self‐inclusion.     

Orientation‐Controlled 2D Anisotropic and Isotropic Photon Transport in Co‐crystal Polymorph Microplates

2D anisotropic transport of photons/electrons is crucial for constructing ultracompact on‐chip circuits. To date, the photons in organic 2D crystals usually exhibit the isotropic propagation, and the anisotropic behaviors have not yet been fully demonstrated. Now, an orientation‐controlled photon–dipole interaction strategy was proposed to rationally realize the anisotropic and isotropic 2D photon transport in two co‐crystal polymorph microplates. The monoclinic microplate adopts a nearly horizontal transition dipole moment (TDM) orientation in 2D plane, exhibiting anisotropic photon–dipole interactions and thus distinct re‐absorption waveguide losses for different 2D directions. By contrast, the triclinic plate with a vertical TDM orientation, shows 2D isotropic photon–dipole interactions and thus the same re‐absorption losses along different directions. Based on this anisotropy, a directional signal outcoupler was designed for the directional transmission of the real signals.     

Halogen Bonding between an Isoindoline Nitroxide and 1,4‐Diiodotetrafluorobenzene: New Tools and Tectons for Self‐Assembling Organic Spin Systems

Radical assembly : Halogen bonding has been observed for the first time between an isoindoline nitroxide and an iodoperfluorocarbon (see figure), which cocrystallize to form a discrete 2:1 supramolecular compound in which N? O^.???I halogen bonding is the dominant intermolecular interaction. This illustrates the potential use of halogen bonding and isoindoline nitroxide tectons for the assembly of organic spin systems.

     

Revealing the Charge‐Transfer Interactions in Self‐Assembled Organic Cocrystals: Two‐Dimensional Photonic Applications

A new crystal of a charge‐transfer (CT) complex was prepared through supramolecular assembly and it has unique two‐dimensional (2D) morphology. The CT nature of the ground and excited states of this new Bpe‐TCNB cocrystal (BTC) were confirmed by electron spin resonance measurements, spectroscopic studies, and theoretical calculations, thus providing a comprehensive understanding of the CT interactions in organic donor–acceptor systems. And the lowest CT₁ excitons are responsible for the efficient photoluminescence (Φ_PL=19 %), which can actively propagate in individual 2D BTCs without anisotropy, thus implying that the optical waveguide property of the crystal is not related to the molecular stacking structure. This unique 2D CT cocrystal exhibits potential for use in functional photonic devices in the next‐generation optoelectronic communications.     

Self‐Assembled Fibers Containing Stable Organic Radical Moieties: Alignment and Magnetic Properties in Liquid Crystals

Macroscopically oriented stable organic radicals have been obtained by using a liquid–crystalline (LC) gel composed of an l ‐isoleucine‐based low molecular weight gelator containing a 2,2,6,6‐tetramethylpiperidine 1‐oxyl moiety. The LC gel has allowed magnetic measurements of the oriented organic radical. The gelator has formed fibrous aggregates in liquid crystals via intermolecular hydrogen bonds. The fibrous aggregates of the radical gelator are formed and oriented on cooling by applying a magnetic field to the mixture of liquid crystals and the gelator. Superconducting quantum interference device (SQUID) measurements have revealed that both oriented and nonoriented fibrous aggregates exhibited antiferromagnetic interactions, in which super‐exchange interaction constant J is estimated as ?0.89 cm^?1.     

Spontaneously Self‐Assembled Naphthalimide Nanosheets: Aggregation‐Induced Emission and Unveiling a‐PET for Sensitive Detection of Organic Volatile Contaminants in Water

A simple design strategy of long alkyl chain substitution was formulated to block the detrimental π–π interaction that potentially transforms the aggregation‐caused quenching (ACQ) chromophores into aggregation‐induced emission (AIE) active smart nanomaterials. The long octadecyl pendant chain substituted naphthalimide (NI) derivatives self‐assembled into fluorescent nanosheets (NS)‐like structures that spontaneously have surfaces coated with NI cores in water. The fluorescent NS were subsequently used to recognize the organic volatile contaminants (OVCs) at ppb levels via an acceptor‐excited photoinduced electron transfer (a‐PET) mechanism, unveiled as the first representative example. A new design strategy is thereby provided to detect toxic xylene derivatives in water using smart nanomaterials.     

Synergistic Spin Transition between Spin Crossover and Spin‐Peierls‐like Singlet Formation in the Halogen‐Bonded Molecular Hybrid System: [Fe(Iqsal)2][Ni(dmit)2]⋅CH3CN⋅H2O

To introduce halogen‐bond interactions between a cation and an anion, a novel Fe^III complex from iodine‐substituted ligands involving a paramagnetic nickel dithiolene anion was prepared and characterized. The compound exhibited the synergy between a spin‐crossover transition and a spin‐Peierls‐like singlet formation. The halogen‐bond interactions between the iodine and the sulfur atoms stabilized the paramagnetic state of π‐spins and played a crucial role in the synergistic magnetic transition between d‐ and π‐spins. In addition, the compound showed the light‐induced excited spin state trapping effect.     

Optical properties of (Z)‐2‐(2‐phenylhydrazinylidene)acenaphthen‐1(2H)‐one: a potential electron donor in organic solar cells

Electron‐donating molecules play an important role in the development of organic solar cells. (Z )‐2‐(2‐Phenylhydrazinylidene)acenaphthen‐1(2H )‐one (PDAK), C₁₈H₁₂N₂O, was synthesized by a Schiff base reaction. The crystal structure shows that the molecules are planar and are linked together forming `face‐to‐face' assemblies held together by intermolecular C—H…O, π–π and C—H…π interactions. PDAK exhibits a broadband UV–Vis absorption (200–648 nm) and a low HOMO–LUMO energy gap (1.91 eV; HOMO is the highest occupied molecular orbital and LUMO is the lowest unoccupied molecular orbital), while fluorescence quenching experiments provide evidence for electron transfer from the excited state of PDAK to C₆₀. This suggests that the title molecule may be a suitable donor for use in organic solar cells.     

Compression of Self‐Assembled Nano‐Objects: 2D/3D Transitions in Films of (Perfluoroalkyl)Alkanes—Persistence of an Organized Array of Surface Micelles

Understanding and controlling the molecular organization of amphiphilic molecules at interfaces is essential for materials and biological sciences. When spread on water, the model amphiphiles constituted by C_nF_2n+1C_mH_2m+1 (FnHm) diblocks spontaneously self‐assemble into surface hemimicelles. Therefore, compression of monolayers of FnHm diblocks is actually a compression of nanometric objects. Langmuir films of F8H16, F8H18, F8H20, and F10H16 can actually be compressed far beyond the “collapse” of their monolayers at ～30 Ų. For molecular areas A between 30 and 10 Ų, a partially reversible, 2D/3D transition occurs between a monolayer of surface micelles and a multilayer that coexist on a large plateau. For A<10 Ų, surface pressure increases again, reaching up to ～48 mN m^?1 before the film eventually collapses. Brewster angle microscopy and AFM indicate a several‐fold increase in film thickness when scanning through the 2D/3D coexistence plateau. Compression beyond the plateau leads to a further increase in film thickness and, eventually, to film disruption. Reversibility was assessed by using compression–expansion cycles. AFM of F8H20 films shows that the initial monolayer of micelles is progressively covered by one (and eventually two) bilayers, which leads to a hitherto unknown organized composite arrangement. Compression of films of the more rigid F10H16 results in crystalline‐like inflorescences. For both diblocks, a hexagonal array of surface micelles is consistently seen, even when the 3D structures eventually disrupt, which means that this monolayer persists throughout the compression experiments. Two examples of pressure‐driven transformations of films of self‐assembled objects are thus provided. These observations further illustrate the powerful self‐assembling capacity of perfluoroalkyl chains.