Switchable metallacycles and metallacages.

Two-dimensional metallacycles and three-dimensional metallacages constructed by coordination-driven self-assembly have attracted much attention because they exhibit unique structures and properties and are highly efficient to synthesize. Introduction of switching into supramolecular chemistry systems is a popular strategy, as switching can endow systems with reversible features that are triggered by different stimuli. Through this strategy, novel switchable metallacycles and metallacages were generated, which can be reversibly switched into different stable states with distinct characteristics by external stimuli. Switchable metallacycles and metallacages exhibit versatile structures and reversible properties and are inherently dynamic and respond to artificial signals; thus, these structures have many promising applications in a wide range of fields, such as drug delivery, data processing, pollutant removal, switchable catalysis, smart functional materials, etc. This review focuses on the design of switchable metallacycles and metallacages, their switching behaviours and mechanisms triggered by external stimuli, and the corresponding structural changes and resultant properties and functions.

A Redox-active Phenothiazine-based Pd2L4-type Coordination Cage and Its Isolable Crystalline Polyradical Cations.

Polyradical cages are of great interest because they show very fascinating physical and chemical properties, but many challenges remain, especially for their synthesis and characterization. Herein, we present the synthesis of a polyradical cation cage 14•+ through post-synthetic oxidation of a redox-active phenothiazine-based Pd2L4-type coordination cage 1. It's worth noting that 1 exhibits excellent reversible electrochemical and chemical redox activity due to the introduction of a bulky 3,5-di-tert-butyl-4-methoxyphenyl substituent. The generation of 14•+ through reversible electrochemical oxidation is investigated by in situ UV-vis-NIR and EPR spectroelectrochemistry. Meanwhile, chemical oxidation of 1 can also produce 14•+ which can be reversibly reduced back to the original cage 1, and the process is monitored by EPR and NMR spectroscopies. Eventually, we succeed in the isolation and single crystal X-ray diffraction analysis of 14•+, whose electronic structure and conformation are distinct to original 1. The magnetic susceptibility measurements indicate the predominantly antiferromagnetic interactions between the four phenothiazine radical cations in 14•+. We believe that our study including the facile synthesis methodology and in situ spectroelectrochemistry will shed some light on the synthesis and characterization of novel polyradical systems, opening more perspectives for developing functional supramolecular cages.

Light-Responsive Supramolecular Liquid-Crystalline Metallacycle for Orthogonal Multimode Photopatterning.

The development of multimode photopatterning systems based on supramolecular coordination complexes (SCCs) is considerably attractive in supramolecular chemistry and materials science, because SCCs can serve as promising platforms for the incorporation of multiple functional building blocks. Herein, we report a light-responsive liquid-crystalline metallacycle that is constructed by coordination-driven self-assembly. By exploiting its fascinating liquid crystal features, bright emission properties, and facile photocyclization capability, a unique system with spatially-controlled fluorescence-resonance energy transfer (FRET) is built through the introduction of a photochromic spiropyran derivative, which led to the realization of the first example of a liquid-crystalline metallacycle for orthogonal photopatterning in three-modes, namely holography, fluorescence, and photochromism.

Symmetry-Broken Intermolecular Charge Separation of Cationic Radicals.

A quadrupolar compound Pyr-BA with two pyrrole-type nitrogen atoms doped externally was prepared in this work. In high contrast with other π ionic radicals, its cationic radical Pyr-BA⋅+ undergoes unusual symmetry-broken charge separation (SB-CS), generating the mixed valence complex of Pyr-BA+1-q ⋅⋅⋅Pyr-BA+1+q , where q is the degree of charge transfer. Variable-temperature (VT) single-crystal analysis, absorption and EPR experiments all confirmed that aggregation and lower temperature would help to facilitate this SB-CS process. Gibbs energy calculations and gauge-including magnetically induced current simulation both validate that, for Pyr-BA⋅+ , SB-CS behavior is more favorable than the conventional dimerization mode. To the best of our knowledge, this is the first study that shows solid single-crystal evidence for spontaneous SB-CS between identical ionic radicals. Such a unique phenomenon is of great significance both in terms of fundamental aspects and uncharted material science.

Functionalization of Pentacene: A Facile and Versatile Approach to Contorted Polycyclic Aromatic Hydrocarbons.

In this work, a facile and versatile strategy for the synthesis of contorted polycyclic aromatic hydrocarbons (PAHs) starting from the functionalized pentacene was established. A series of novel PAHs 1-4 and their derivatives were synthesized through a simple two-step synthesis procedure involving an intramolecular reductive Friedel-Crafts cyclization of four newly synthesized pentacene aldehydes 5-8 as a key step. All the molecules were confirmed by single-crystal X-ray diffraction and their photophysical and electrochemical properties were studied in detail. Interestingly, the most striking feature of 1-4 is their highly contorted carbon structures and the accompanying helical chirality. In particular, the optical resolution of 2 was successfully achieved by chiral-phase HPLC, and the enantiomers were characterized by circular dichroism and circularly polarized luminescence spectroscopy. Despite the highly nonplanar conformations, these contorted PAHs exhibited emissive properties with moderate-to-good fluorescence quantum yields, implying the potential utility of this series PAHs as high-quality organic laser dyes. By using a self-assembly method with the help of epoxy resin, a bottle microlaser based on 3 a was successfully illustrated with a lasing wavelength of 567.8 nm at a threshold of 0.3 mJ/cm2 . We believe that this work will shed light on the chemical versatility of pentacene and its derivatives in the construction of novel functionalized PAHs.

TEMPO Radical-Functionalized Supramolecular Coordination Complexes with Controllable Spin-Spin Interactions.

The topic of noncovalent spin-spin interactions is of increasing general interest in supramolecular radical chemistry. In this report, a series of exo- and endo-TEMPO radical-functionalized metallacycles 1-4 and metallacages 5 and 6 were constructed via coordination-driven self-assembly, wherein the number, location, and distance of the spins were precisely controlled. Their intriguing spin-spin interactions were systematically investigated by electron paramagnetic resonance (EPR) and were well interpreted at the molecular level assisted by X-ray crystallography analysis. The results revealed their distinct spin-spin interactions in the solution state, wherein the spin-spin interaction of metallacycle 3 was much stronger than that of the other five assemblies mainly due to its shorter intramolecular spin-spin distance. In the solid state, 1-6 exhibited obvious spin-spin (dipole-dipole) interactions because of the close arrangement of TEMPO units as indicated in their single crystals. Specifically, a large zero-field splitting (ZFS; D = 17.5 mT) was observed in the crystalline form of metallacycle 4, which arose from the strong intermolecular spin-spin coupling. Interestingly, when the counterion of PF6- in 4 was changed to BF4-, the BF4- counterion analog 4a also exhibited a large ZFS, but the ZFS originated from the intramolecular spin-spin interaction due to a small variation in its crystal conformation. Moreover, the reversible on-off switching of the ZFS in 4 and 4a via the crystal-to-amorphous transformation induced by mechanical grinding and solvent vapor stimuli was also successfully realized. The unique and controllable inter- and intramolecular spin-spin interactions in this work reveal new insights for the understanding and manipulation of spin-spin interactions and may open up a new way to develop organic spin materials in the future.

Orthogonal Self-Assembly of a Two-Step Fluorescence-Resonance Energy Transfer System with Improved Photosensitization Efficiency and Photooxidation Activity.

During the past few decades, fabrication of multistep fluorescence-resonance energy transfer (FRET) systems has become one of the most attractive topics within supramolecular chemistry, chemical biology, and materials science. However, it is challenging to efficiently prepare multistep FRET systems with precise control of the distances between locations and the numbers of fluorophores. Herein we present the successful fabrication of a two-step FRET system bearing specific numbers of anthracene, coumarin, and BODIPY moieties at precise distances and locations through an efficient and controllable orthogonal self-assembly approach based on metal-ligand coordination and host-guest interactions. Notably, the photosensitization efficiency and photooxidation activity of the two-step FRET system gradually increased with the number of energy transfer steps. For example, the two-step FRET system exhibited 1.5-fold higher 1O2 generation efficiency and 1.2-fold higher photooxidation activity than that of its corresponding one-step FRET system. This research not only provides the first successful example of the efficient preparation of multistep FRET systems through orthogonal self-assembly involving coordination and host-guest interactions but also pushes multistep FRET systems toward the application of photosensitized oxidation of a sulfur mustard simulant.

Extending eyebox with tunable viewpoints for see-through near-eye display.

The Maxwellian display presents always-focused images to the viewer, alleviating the vergence-accommodation conflict (VAC) in near-eye displays (NEDs). However, the limited eyebox of the typical Maxwellian display prevents it from wider applications. We propose a Maxwellian see-through NED based on a multiplexed holographic optical element (HOE) and polarization gratings (PGs) to extend the eyebox by viewpoint multiplication. The multiplexed HOE functions as multiple convex lenses to form multiple viewpoints, which are copied to different locations by PGs. To mitigate the imaging problem that multiple viewpoints or no viewpoints enter the eye pupil, the viewpoints can be tuned by mechanically moving a PG. We implement our method in a proof-of-concept system. The optical experiments confirm that the proposed display system provides always in-focus images within a 12 mm eyebox in the horizontal direction with a 32.7° diagonal field of view (FOV) and a 16.5 mm eye relief (ERF), and its viewpoints are tunable to match the actual eye pupil size. Compared with other techniques to extend the eyebox of Maxwellian displays, the proposed method shows competitive performances of a large eyebox, adaptability to the eye pupil size, and focus cues within a large depth range.

Design of a compact waveguide eyeglass with high efficiency by joining freeform surfaces and volume holographic gratings.

In this paper, a compact waveguide eyeglass integrating freeform surfaces and volume holographic gratings (VHGs) is proposed for full-color display with high energy utilization. The in-coupler with four freeform surfaces collimates the light emitting from the micro image source (MIS) and couples them into the waveguide. The six-layer VHGs as an outcoupler are designed to modulate the light propagating toward the user's eye. The chromatic aberrations and aberrations are well optimized and compensated by the in-coupler. The diffraction angular bandwidth of the gratings matches the angular range of the light propagating in the waveguide. The simulation results show that our proposed eyeglass achieves a diagonal field of view (FOV) of 39.5°, the average diffraction efficiency of the outcoupler achieves 95.22%, and the diffraction uniformity is about 0.95. Because of the integrated designs and compact stable structures, the optimized display system is expected to be flexibly used in various applications.

Effect of surface roughness of optical waveguide on imaging quality and a formula of RSE tolerance and incident angle.

The optical waveguide is a lightweight and portable scheme for augmented reality near-eye display devices. However, the surface roughness of the waveguide affects its imaging performance, which has not been studied. In this work, we investigate the light scattering caused by the root-mean-square roughness of the waveguide surface and present two methods to numerically analyze the modulation transfer function (MTF) of the display system. Here, we consider the effects of different surface roughness, incident angle, and incident wavelength on the scattering distribution when other conditions are constant. For a simplified optical waveguide display system, the MTF degradation and the variation of the tolerance is calculated. And when the MTF (@ 40 cycles/mm) is required to be 0.3 and the incident angles of the total reflection surface are 45°, 55°, 65° and 75°, the random surface error (RSE) tolerances are 0.207λ0, 0.255λ0, 0.347λ0 and 0.566λ0 (λ0=0.5461µm), respectively. We find a formula descripting the relationship between RSE tolerance and incident angle. If the RSE tolerance exceeds the value of the formula at an angle, the imaging quality of the system will drop significantly. The formula can predict tolerances and incident angles and provide basic tool for imaging quality analysis and manufacturing for optical waveguide AR/VR display systems.

Full-color see-through near-eye holographic display with 80° field of view and an expanded eye-box.

A full-color see-through near-eye holographic display is proposed with 80° field of view (FOV) and an expanded eye-box. The system is based on a holographic optical element (HOE) to achieve a large FOV while the image light is focused at the entrance to human pupil and the image of entire field enters human eye. As we know, one of the major limitations of the large FOV holographic display system is the small eye-box that needs to be expanded. We design a double layer diffraction structure for HOE to realize eye-box expansion. The HOE consists of two non-uniform volume holographic gratings and a transparent substrate. Two fabricated holographic gratings are attached to front and back surfaces of the substrate to multiplex image light and achieve eye-box expansion. Simultaneously, the HOE is also manufactured for RGB colors to realize full-color display. The experiment results show that our proposed display system develops 80° round FOV and an enlarged eye-box of 7.5 mm (H) ×5 mm (V) at the same time. The dynamic display ability is also tested in the experiments. The proposed system provides a new solution for the practical application of augmented reality display.

Radical-Induced Hierarchical Self-Assembly Involving Supramolecular Coordination Complexes in Both Solution and Solid States.

To explore a new supramolecular interaction as the main driving force to induce hierarchical self-assembly (HSA) is of great importance in supramolecular chemistry. Herein, we present a radical-induced HSA process through the construction of well-defined rhomboidal metallacycles containing triphenylamine (TPA) moieties. The light-induced radical generation of the TPA-based metallacycle has been demonstrated, which was found to subsequently drive hierarchical self-assembly of metallacycles in both solution and solid states. The morphologies of nanovesicle structures and nanospheres resulting from hierarchical self-assembly have been well-illustrated by using TEM and high-angle annular dark-field STEM (HAADF-STEM) micrographs. The mechanism of HSA is supposed to be associated with the TPA radical interaction and metallacycle stacking interaction, which has been supported by the coarse-grained molecular dynamics simulations. This study provides important information to understand the fundamental TPA radical interaction, which thus injects new energy into the hierarchical self-assembly of supramolecular coordination complexes (SCCs). More interestingly, the stability of TPA radical cations was significantly increased in these metallacycles during the hierarchical self-assembly process, thereby opening a new way to develop stable organic radical cations in the future.

Design and modeling of a transmission and reflection switchable micro-focusing Fresnel device based on phase-change materials.

In this paper, a switchable micro-focusing Fresnel device based on phase-change materials (PCMs) is proposed, which can selectively display the functions of transmission and reflection without the use of mechanical adjustment on micro scale. The switchable function is realized by combining Fresnel structure with PCM. A four-level switchable Fresnel device consisting of a typical PCM Ge3Sb2Te6 (GST-326) is designed to focus light into a focal length of 30 µm at wavelength of 3.1 µm. The optical performance of the switchable device has been analyzed by using finite-difference time-domain (FDTD) method, showing bright convergence point near pre-designed focal length with focusing efficiencies larger than 18%, depth of focus (DOF) less than 4.65 µm and the full width at half-maximum (FWHM) not larger than 1.30 µm. Furthermore, by precisely manipulating the variation of PCM thickness, we also obtain a device that possesses the characteristics of a transmission-reflection focusing beam splitter. The devices show good potential for the combination of traditional binary optical devices and PCM to produce new functions, and provides a promising innovative approach for miniature focal length switching device.

On-axis near-eye display system based on directional scattering holographic waveguide and curved goggle.

The contradiction between the field of view (FOV), luminance uniformity (LU) and weight has always restricted the development of augmented reality display systems. An on-axis near-eye display (NED) system based on directional scattering holographic waveguide (DSHW) and curved goggle is proposed in order to realize a large FOV with high LU, light weight, and conformal design capability. The DSHW which consists of a linear volume holographic grating and holographic diffuser is used to deliver the virtual image and construct a transparent directional emission display screen with high LU. The curved goggle is used to project the image on the display screen into human eye and form a large FOV, with a suitable exit pupil diameter (EPD) and eye relief distance (ERF) and while keeping the external scene visible. Our proposed NED achieved an FOV of 44° horizontal (H) × 12° vertical (V) easily, which is almost consistent with the theoretical design. The EPD is 6 mm, ERF is 18.6 mm, and LU is about 88.09% at full viewing angle. The system is lightweight and flexible, which can be further applied in the next-generation, integrated protection-display helmet system through conformal optical design.

A full-color compact 3D see-through near-eye display system based on complex amplitude modulation.

For complex amplitude modulation (CAM)-based three-dimensional (3D) near-eye systems, it is a challenge to realize colorful 3D display by using spatial light modulator (SLM) and grating. Here, a full-color compact 3D see-through near-eye display (NED) system by CAM is proposed. Computer generated holograms (CGHs) for different wavelengths are calculated separately. Each CGH contains two position-shifted sub-holograms and the separated distance is carefully calibrated to eliminate chromatic aberration. Colorful 3D images are synthesized through time-multiplexing. Color managements are performed and chromatic aberration of the system is analyzed to provide better colorful effect. The system structure is integrated to be compact and a prototype is implemented. Pre-compensation is added on CGHs to offset the system's assembling errors. Optical experiment results show that the proposed system can provide good 3D full-color see-through performance without vergence-accommodation conflict (VAC). Dynamic colorful display ability is also tested, which shows good potential for interactive NED in the future.

Dithienopicenocarbazole-Based Acceptors for Efficient Organic Solar Cells with Optoelectronic Response Over 1000 nm and an Extremely Low Energy Loss.

Two cheliform non-fullerene acceptors, DTPC-IC and DTPC-DFIC, based on a highly electron-rich core, dithienopicenocarbazole (DTPC), are synthesized, showing ultra-narrow bandgaps (as low as 1.21 eV). The two-dimensional nitrogen-containing conjugated DTPC possesses strong electron-donating capability, which induces intense intramolecular charge transfer and intermolecular π-π stacking in derived acceptors. The solar cell based on DTPC-DFIC and a spectrally complementary polymer donor, PTB7-Th, showed a high power conversion efficiency of 10.21% and an extremely low energy loss of 0.45 eV, which is the lowest among reported efficient OSCs.

Non-classical S-Heteroacenes with o-Quinoidal Conjugation and Open-Shell Diradical Character.

A series of non-classical S-heteroacenes were synthesized and exhibited intriguing physical properties and chemical reactivities that are very different from classical acenes. X-ray crystallographic analyses revealed that all acenothiophene derivatives Ph-AT-1-Ph-AT-3 had an o-quinoidal π-conjugation with large bond-length alternation, whereas the acenodithiophene derivative Ph-ADT-3 easily dimerized or reacted with oxygen to form a peroxy-bridged dimer. The long acenothiophene Ph-AT-4 was also highly reactive. The origin of these unique properties was investigated carefully by both experiments and theoretical calculations. The high reactivity of the long non-classical S-heteroacenes can be explained by their intrinsic open-shell diradical character as well as the o-quinoidal conjugation.

Toward Tetraradicaloid: The Effect of Fusion Mode on Radical Character and Chemical Reactivity.

Open-shell singlet diradicaloids display unique electronic, nonlinear optical, and magnetic activity and could become novel molecular materials for organic electronics, photonics, and spintronics. However, design and synthesis of diradicaloids with a significant polyradical character is a challenging task for chemists. In this Article, we report our efforts toward a tetraradicaloid system. A series of potential tetraradicaloids by fusion of two p-quinodimethane (p-QDM) units with naphthalene or benzene rings in different modes were synthesized. Their model compounds containing one p-QDM moiety were also prepared and compared. Their ground-state structures, physical properties, and chemical reactivity were systematically investigated by various experimental methods such as steady-state and transient absorption, two-photon absorption, X-ray crystallographic analysis, electron spin resonance, superconducting quantum interference device, and electrochemistry, assisted by density functional theory calculations. It was found that their diradical and tetraradical characters show a clear dependence on the fusion mode. Upon the introduction of more five-membered rings, the diradical characters greatly decrease. This difference can be explained by the pro-aromaticity/antiaromaticity of the molecules as well as the intramolecular charge transfer. Our comprehensive studies provide a guideline for the design and synthesis of stable open-shell singlet polycyclic hydrocarbons with significant polyradical characters.

Different Strategies for the Stabilization of Acenes and Acene Analogues.

Acenes, a type of polycyclic aromatic hydrocarbon containing linearly fused benzene rings, have received much attention from organic chemists, physical chemists, and materials scientists, due to their intriguing properties and potential applications in organic electronics. Without doubt, acene chemistry has been one of the hottest topics among the π-conjugated systems. However, poor stability of acenes is the prominent issue that limits their applications. In this personal account, we summarize different strategies developed in our group to construct and stabilize acenes and acene analogues. In addition, the unique properties and applications of some molecules will be discussed.

Pro-aromatic and anti-aromatic π-conjugated molecules: an irresistible wish to be diradicals.

Aromaticity is an important concept to understand the stability and physical properties of π-conjugated molecules. Recent studies on pro-aromatic and anti-aromatic molecules revealed their irresistible tendency to become diradicals in the ground state. Diradical character thus becomes another very important concept and it is fundamentally correlated to the physical (optical, electronic and magnetic) properties and chemical reactivity of most of the organic optoelectronic materials. Molecules with distinctive diradical character show unique properties which are very different from those of traditional closed-shell π-conjugated systems, and thus they have many potential applications in organic electronics, spintronics, non-linear optics and energy storage. This critical review first introduces the fundamental electronic structure of Kekulé diradicals within the concepts of anti-aromaticity and pro-aromaticity in the context of Hückel aromaticity and diradical character. Then recent research studies on various stable/persistent diradicaloids based on pro-aromatic and anti-aromatic compounds are summarized and discussed with regard to their synthetic chemistry, physical properties, structure-property relationships and potential material applications. A summary and personal perspective is given at the end.