Xiangfeng Shao

Ring Reconstruction on a Trichalcogenasumanene Buckybowl: A Facile Approach to Donor–Acceptor‐Type [5‐6‐7] Fused Planar Polyheterocycles

The transformation of trichalcogenasumanene buckybowls into donor-acceptor-type [5-6-7] fused polyheterocycles is disclosed. The strategy involves a highly efficient ring-opening of the flanking benzene upon oxidation at room temperature, and facile ring closure by functional-group transformation. Crystallographic studies indicate that the resulting [5-6-7] fused polyheterocycles possess a planar conformation owing to the release of ring strain by expansion of one of the six-membered flanking rings to the seven-membered one. Additionally, the [5-6-7] fused polyheterocycles bear electron-withdrawing groups, which reduce the HOMO-LUMO energy gap, and display broad absorption bands extending to λ=590 nm. Consequently, these compounds show strong red emission with fluorescence quantum yields of up to 38 %.

Honeycomb supramolecular frameworks of organic–inorganic hybrid cluster composed of cation radical and Keggin-type polyoxometalate

Keggin-type phosphomolybdic acid (H3PMo12O40) promoted the formation of a cation radical of tetra(thienylthio)-tetrathiafulvalene (TT-TTF), consequently resulting in a cation radical complex [(TT-TTF)+•]3[(PMo12O40)3−](CH2Cl2), which shows self-assembly in solution and forms microtubes. The molecular level structure of the microtube is evaluated by X-ray diffraction analysis, which reveals that the microtube is produced via various non-covalent interactions in the hierarchical assembly process: the electrostatic interactions between (TT-TTF)+• and (PMo12O40)3− afford an organic–inorganic hybrid triangular nanocluster [(TT-TTF)+•]3[(PMo12O40)3−] as the building block; the strong π–π interactions between (TT-TTF)+• result in the cluster-based two-dimensional honeycomb nanosheet; the van der Waals forces between the nanosheets lead to the propagation along the third direction to form a three-dimensional supramolecular framework possessing nano-sized channels.

4,5,9,10-Pyrene Diimides: A Family of Aromatic Diimides Exhibiting High Electron Mobility and Two-Photon Excited Emission

The design and synthesis of high-performance n-type organic semiconductors are important for the development of future organic optoelectronics. Facile synthetic routes to reach the K-region of pyrene and produce 4,5,9,10-pyrene diimide (PyDI) derivatives are reported. The PyDI derivatives exhibited efficient electron transport properties, with the highest electron mobility of up to 3.08 cm2  V-1  s-1 . The tert-butyl-substituted compounds (t-PyDI) also showed good one- and two-photon excited fluorescence properties. The PyDI derivatives are a new family of aromatic diimides that may exhibit both high electron mobility and good light-emitting properties, thus making them excellent candidates for future optoelectronics.

Inclusion complexes of fullerenes with flexible tetrathiafulvalene derivatives bearing four aryls through sulfur bridges

TTF derivatives decorated with four aryls through the sulfur bridges are employed to form the donor–acceptor type inclusion complexes with fullerenes. The key factor for the formation of inclusion complexes is the introduction of structural flexibility in TTF molecules along with the molecular size matching with fullerenes. A crystallographic study indicates that the structures of the resulting complexes are stabilized by a multidimensional intermolecular interaction network consisting of TTF cores, peripheral aryls, and fullerenes, which in turn gives rise to the electronic communication between the donor and the acceptor as proved by the solid state absorption spectra. Moreover, the fullerene molecules form the two-dimensional sheet structure in the complexes.

Surgery on Trichalcogenasumanenes: from π-Bowl to Planar, Invertible Curvature, and Chiral Polycycles

The buckybowl trichalcogenasumanenes show cleavage of flanking benzene ring upon oxidation, which leads their dissection by fusing various amidine moieties onto peripheral region. By gradually increasing the ring size of amidine from five- to six- and seven-membered, the molecule engineering results in the [7-5-6]-, [7-6-6]-, and [7-7-6]-fused polycycles. Three systems are distinct in the molecular geometries, packing motifs, and optoelectronic properties. The [7-5-6]-fused case adopts the flat backbone, displays strong emission with the fluorescence quantum yield up to 52.3 %, and undergoes a two-photon absorption process. The [7-6-6]-fused one is of a curvature with molecular geometry inversion, forms a tight stack of curved π-system, shows broad absorption extended to 700 nm, and exhibits the p-type semiconducting behavior with hole mobility of 4.4×10-3  cm2  V-1  s-1 . The [7-7-6]-fused one possesses the highly twisted skeleton to show stable chirality, and exhibits red emission in both solution and solid state. The surgery on trichalcogenasumanene is a promising approach to create polycycles with diverse functionalities.

Driving π-plane to π-bowl through lateral coordination at room temperature

A series of hetero polycycles (5a-6b) are created through the hybridization of the bowl-shaped trichalcogenasumanenes and planar triazacoronene. Among them, 6a and 6b bear the 2,2′-bipyridine (bpy) ligand in the lateral direction. The crystallographic analyses show that 6a and 6b possess planar cores, whereas they are driven to bowl-shaped (bowl depth, 0.38–0.50 A) via chelation of Zn2+ by the bpy unit at room temperature (RT). This is caused by the formation of a five-membered ring upon coordination with Zn2+. The coordination of Zn2+ also brings distinct variation of the optical properties, i.e., the crystals of 6a and 6a[ZnCl2] respectively show green and red fluorescence. Moreover, 5a-6b exhibit a reversible color change and a fluorescence OFF/ON response upon acidification and neutralization. Owing to the multiple protonation process, 6a and 6b display a gradient color change from yellow to magenta, violet, and blue as the amount of acid increases. These new hetero polycycles (5a-6b) have potential applications as chemosensors and/or switches.

Distinguishing Diketopyrrolopyrrole Isomers in Single-Molecule Junctions via Reversible Stimuli-Responsive Quantum Interference

Distinguishing structural isomers at the single-molecule level remains a challenge. We report the single-molecule recognition of two diketopyrrolopyrrole containing isomers (SDPP and SPPO) employing the mechanically controllable break junction technique. The single-molecule conductances of the two isomers are indistinguishable under normal conditions. However, reversible protonation and deprotonation of the SPPO in molecular junction result in more than 1 order of magnitude conductance change, which dramatically enhances the conductance difference between the two isomers. Theoretical study reveals that the dramatic conductance switching is due to reversible quantum interference effect. It is suggested that combination of stimuli-response and quantum interference can be an efficient strategy to enhance isomer recognition and conductance switching in single-molecule junctions.

Broadband optical limiting of a novel twisted tetrathiafulvalene incorporated donor–acceptor material and its Ormosil gel glasses

To accomplish broadband optical limiting with high visible-light region (ca. 400–700 nm) transmittance, as well as to satisfy the requirement of practical applications, a novel molecule, TTF-Pt(bzimb), has been rationally designed and synthesized by connecting an excellent electron donating tetrathiafulvalene and a Pt(II)-incorporated electron withdrawing Pt(bzimb) with a C–C triple bond. Chlorine substituted Cl-Pt(bzimb) was presented for comparison. As expected, the theoretical calculation results clearly demonstrate that TTF-Pt(bzimb) exhibits a twisted conformation with a dihedral angle of 50° at the C–C triple bond. The intramolecular charge transfer existing between the donor and the acceptor is well evidenced by a series of experimental results such as UV-vis spectra, fluorescence spectra, and CV. Benefiting from a unique conformation, TTF-Pt(bzimb) and its doped Ormosil gel glasses in a methyltriethoxysilane matrix exhibit broadband (532 and 1064 nm) optical limiting behaviour with high visible-light transmittance. Furthermore, TTF-Pt(bzimb) shows more remarkable optical limiting behaviour than the state-of-the-art optical limiting material, C60, under 532 nm. Thus, our results disclose a rational molecular design strategy for the accomplishment of remarkable optical limiting. TTF-Pt(bzimb) and its gel glasses can be considered as promising candidates for the applications of optical limiting and for use in nonlinear optical devices.

Remarkable Nonlinear Optical Response of Pyrazine-fused Trichalcogenasumanenes and Their Application for Optical Power Limiting

The C3v-symmetric hetera buckybowl trichalcogenasumanenes have attracted recent interest due to their unique features in chemical and optoelectronic aspects. To gain a nonlinear optical (NLO) response, the electron-withdrawing unit pyrazine was introduced onto tricalcogenasumanenes to give the pyrazine-fused trithiasumanene (1) and triselenasumanene (2) as reported herein. Compounds 1 and 2 exhibit typical NLO properties which are verified using open-aperture Z-scan technology under a 532 nm nanosecond laser pulse. It is found that the NLO performance of these two molecules is much better than the state-of-the-art NLO material C60. The time-resolved transient absorption clearly verified that the NLO properties of 1 and 2 originate from the reverse saturable absorption (RSA) of the triplet excited-state. Owing to their remarkable NLO performance, 1 and 2 are further employed as optical power limiting (OPL) materials. Compounds 1 and 2 exhibit OPL behavior with a limiting threshold of 1.78 J cm−2 and 2.43 J cm−2, respectively; this performance is better than that of C60 (12.85 J cm−2). This work indicates that the sumanene derivatives are promising candidates for NLO and OPL materials.