Cai Sun

Conductance Switch of a Bromoplumbate Bistable Semiconductor by Electron-Transfer Thermochromism

Electron-transfer (redox) thermochromism was successfully used for switching the conductance of semiconductors, by introducing a thermally active organic component into an inorganic semiconducting framework. A moisture-resistant semiconductor {(MV)2 [Pb7 Br18 ]}n (MV2+ =methyl viologen cation) has been prepared through an in situ synthetic method for MV2+ . It features a rare 3D haloplumbate open framework and unprecedented electron-transfer thermochromic behavior in haloplumbates. The electrical conductivity of this compound dropped significantly after coloration and restored after decoloration, which was satisfactorily explained by valence band XPS and theoretical data. This work not only offers a new approach to modify electrical properties of semiconductors without altering components or structures, but may lead to the development of over-temperature color indicators, circuit overload protectors or photovoltaic materials.

An electron-transfer photochromic metal–organic framework (MOF) compound with a long-lived charge-separated state and high-contrast photoswitchable luminescence

Long-lived charge-separated states and photoswitchable photoluminescence are important for many photochemical and photophysical applications. Recent studies have demonstrated the effectiveness of MOFs to achieve long-lived charge-separated states; however, the related studies are still rather rare owing to the limited numbers of photoresponsive MOFs. In this work, a new electron-transfer photochromic MOF compound, encapsulating photoactive viologen cations, was found to exhibit a charge-separated state with a lifetime value of more than 2 months, exceeding the reported values of the analogues. In addition, the photochromic process can afford a luminescence contrast up to 10 times between two stable states, which is higher than those of most known pyridine derivative-based photochromic compounds.

Design Strategy for Improving Optical and Electrical Properties and Stability of Lead-Halide Semiconductors

Broad absorption, long-lived photogenerated carriers, high conductance, and high stability are all required for a light absorber toward its real application on solar cells. Inorganic-organic hybrid lead-halide materials have shown tremendous potential for applications in solar cells. This work offers a new design strategy to improve the absorption range, conductance, photoconductance, and stability of these materials. We synthesized a new photochromic lead-chloride semiconductor by incorporating a photoactive viologen zwitterion into a lead-chloride system in the coordinating mode. This semiconductor has a novel inorganic-organic hybrid structure, where 1-D semiconducting inorganic lead-chloride nanoribbons covalently bond to 1-D semiconducting organic π-aggregates. It shows high stability against light, heat, and moisture. After photoinduced electron transfer (PIET), it yields a long-lived charge-separated state with a broad absorption band covering the 200-900 nm region while increasing its conductance and photoconductance. This work is the first to modify the photoconductance of semiconductors by PIET. The observed increasing times of conductivity reached 3 orders of magnitude, which represents a record for photoswitchable semiconductors. The increasing photocurrent comes mainly from the semiconducting organic π-aggregates, which indicates a chance to improve the photocurrent by modifying the organic component. These findings contribute to the exploration of light absorbers for solar cells.

Grinding size-dependent mechanoresponsive luminescent Cd(II) coordination polymer

In this work, we report a new mechanoresponsive luminescent Cd(ii) coordination polymer that exhibits a sensitivity to mechanical stimulation. The luminescence colour changed gradually from weak yellowish-green to bright cyan in response to different grinding sizes, thereby showing interesting mechanochromic properties, namely a grinding-enhanced luminescence and good crystalline maintenance ability.

Viologen-templated bromoplumbate: a new in situ synthetic method and energy gap engineering

Changing N-substituents to modify energy gaps is important for the diverse applications (photochromism, photocatalysis, etc.) of viologen-based compounds. In situ N-substitution using alcohols under solvothermal and acidic conditions is an effective and common synthetic method for viologen-templated metal halides. In this work, we found that replacing alcohols with alkenes could reduce the reaction temperature dramatically to room temperature. A new cis/trans-viologen-templated bromoplumbate with a novel 1-D chain structure built by edge-sharing planar [Pb3Br9]3− oligomers was successfully obtained. This bromoplumbate hybrid has a narrower energy gap compared with bulk PbBr2, which has been well explained by linear optical response spectra calculations and PDOS analysis.

Nitrogen-Rich Tetranuclear Metal Complex as a New Structural Motif for Energetic Materials

For designing energetic materials (EMs), the most challenging issue is to achieve a balance between energetic performance and reliable stability. In this work, we employed an efficient and convenient method to synthesize a new class of EMs: nitrogen-rich tetranuclear metal complexes [M(Hdtim)(H2O)2]4 (M = Zn 1, Mn 2; H3dtim = 1H-imidazol-4,5-tetrazole) with the N content of >46%. The structural analyses illustrate that isomorphous compounds 1 and 2 feature isolated hollow ellipsoid tetranuclear units, which are linked by both π–π interactions and hydrogen-bonding interactions to give a 3D supramolecular architecture. Compounds 1 and 2 exhibit prominent energetic characteristics: excellent detonation performances and reliable thermal, impact, and friction stabilities. Being nitrogen-rich tetrazolate compounds, the enthalpies of combustion of 1 (−11.570 kJ g–1) and 2 (−12.186 kJ g–1) are higher than those of classical EMs, RDX and HMX, and they possess high positive heats of formation. Sensitivity tests demonstrate that 1 and 2 are insensitive to external mechanical action. Excellent energetic performances and low sensitivities promote 1 and 2 to serve as a new class of promising EMs with a desirable level of safety.

An enhanced extended hook method to realize tetranuclear metal clusters embedded in energetic metal–organic framework channels

Rare tetranuclear Zn clusters were successfully embedded into 3D energetic metal–organic framework channels by an enhanced extended hook method. The obtained metal–organic framework exhibits not only high enthalpy of combustion and low sensitivity but also strong green fluorescence.

Improving coloration time and moisture stability of photochromic viologen–carboxylate zwitterions

Viologen (N,N′-disubstituted bipyridinium) compounds are well known for their electron-transfer photochromic properties and many potential applications. However, viologen-based organic molecules, especially those with aliphatic hydrocarbon substituents, are inclined to be hygroscopic, and most of the known viologen compounds need dozens of seconds or even several minutes to exhibit photoinduced color change. The viologen–carboxylate zwitterion ligand, N,N′-dipropionate-4,4′-bipyridinium (L), is highly hygroscopic and its shortest coloration time is 5 s upon irradiation of a Xe lamp (ca. 110 mW cm−2). After coordination with ZnCl2, the obtained new compound [Zn2LCl4]n (ZnLCl) is highly stable in a wet environment (e.g. 60% relative humidity), and its shortest coloration time is 1 s upon irradiation of the same Xe lamp. X-ray diffraction analysis coupling with multiple experimental spectra (UV-vis, ESR, XPS, and IR) and density of states (DOS) calculations revealed that the photoinduced coloration of L and ZnLCl is contributed mainly by O → pyridinium and Cl → pyridinium electron transfer, respectively. The lower electronegativity of Cl over O and the shorter donor–acceptor separation may explain why ZnLCl has a shorter coloration time. Moreover, enhancement of molecular rigidity by the coordination structure may account for the improvement of moisture stability.