Xu-Hui Jin

Highly Coplanar Very Long Oligo(alkylfuran)s: A Conjugated System with Specific Head-To-Head Defect

Well-defined monodisperse conjugated oligomers, which have planar backbones and are free from the disturbance of substituents, attract broad interest. Herein, we report a series of symmetrical, isomerically pure oligofurans, namely, the 16-mer 16F-6C6 together with the related nF-2C6 (n = 4, 6, 8). Through computational studies and detailed spectroscopic and X-ray characterization, for the first time, we show that the planarity of the furan backbone is almost unaffected by the head-to-head defect which is known to cause considerable twists in its oligo- or polythiophene analogues. We present that the properties of these rigid oligo(alkylfuran)s are strongly influenced by the conjugation length. As the longest monodisperse α-oligofuran synthesized to date, 16F-6C6 was observed to be stable and highly fluorescent. Experimental and computational studies of the redox states of these oligo(alkylfuran)s reveal that 16F-6C6 has singlet biradical (polaron-pair) character in the doubly oxidized ground state: the open-shell singlet (⟨S2⟩ = 0.989) is 3.8 kcal/mol more stable than the closed-shell dication.

Supramolecular isomer-dependent photochromism and emission color tuning of bipyridinium salts

A study of the supramolecular isomer-dependent photoresponse and emission behaviors of a carboxybenzyl-substituted bipyridinium salt in the solid state is presented. Two supramolecular isomers (A1 and A2) are obtained by self-assembly with the same starting materials at different concentrations. The flexible conformation of the bipyridinium molecule results in different crystal packing geometries, which further influence and alter their photochromic behaviors owing to the crucial differences in the distance and orientation between the donor and acceptor moieties. Remarkably, the introduction of carboxyl groups into the bipyridinium molecule leads to intramolecular charge transfer (ICT) emissions in the visible region. A strong fluorescence emission can be detected in A1, and reversible on/off and multicolor (yellow, olivine and green) fluorescence switching, via photo-irradiation in the solid state, has been observed. Considering the easy chemical modification and the intriguing multicolor photoswitching properties, the present system shows great potential for use in solid photonic devices.

Reversible luminescence switching between single and dual emissions of bipyridinium-type organic crystals

An excitation-wavelength-dependent luminescence behavior of a bipyridinium derivative, together with its luminescence switching between single- and dual-emissions upon reversible solid state structural transformation, has been presented.

Thermally triggered reversible transformation between parallel staggered stacking and plywood-like stacking of 1D coordination polymer chains.

An unusual example showing reversible interconversion of chain-like isomers under controlled experimental settings is reported, which illustrates the key role of assembly conditions for the target packing architecture with related properties. The reaction of Mn(II) ions with an organic ligand 2-hydroxypyrimidine-4,6-dicarboxylic acid (H(3)hpdc) at room temperature gives a coordination polymer {[Mn(3)(hpdc)(2)(H(2)O)(6)] x 2 H(2)O}(n) containing parallel staggered stacking, whereas the reaction under hydrothermal conditions at 150 degrees C affords a compound {[Mn(3)(hpdc)(2)(H(2)O)(6)] x H(2)O}(n) possessing plywood-like stacking. Interestingly, two compounds contain similar one-dimensional chain components with different orientations that can be controlled by thermodynamic factors. Thermally triggered reversible interconversion of the two compounds was verified by X-ray powder, IR, and element analysis. The spin-canted antiferromagnetic behaviors are observed in as-synthesized samples, and the influence of chain orientations on magnetic properties has been detected.

Photo-facilitated aggregation and correlated color temperature adjustment of single component organic solid state white-light emitting materials

A color tuning strategy has been developed based on the N-alkylation of the terminal 4-pyridyl groups of 4′-(4-pyridyl)-2,2′:6′,2′′-terpyridine molecule to adjust the desired properties of luminescence materials. The fluorescence emission of the parent chromophore can be successfully extended into the long-wavelength region in solution, and an unprecedented aggregation facilitated by light irradiation occurs in n-butanol to result in almost white-light emission due to the generation of dual-color fluorescence. By controlling solid-state molecular structures, four homologous single-crystal materials with intra- and intermolecular charge transfer synergy-induced dual emissions, which render not only ideal white light chromaticity, but also tunable correlated color temperature ranging from warm-white to cool-white light, have been obtained.

Long-range exciton transport in conjugated polymer nanofibers prepared by seeded growth

A longer exciton pathway Organic semiconductors typically exhibit exciton diffusion lengths on the order of tens of nanometers. Jin et al. prepared nanofibers from block polymers consisting of emissive polyfluorene cores surrounded by coronas of polyethylene glycol and polythiophene (see the Perspective by Holmes). Excitons generated in the polyfluorene cannot enter the polyethylene glycol layer and so diffuse more than 200 nm. This distance can be tuned by varying the length of the polyethylene glycol—a feature that could potentially be exploited in the development of organic devices such as photovoltaics. Science, this issue p. 897; see also p. 854 Block-copolymer core-shell fibers show extremely efficient exciton transport, with almost complete incoming light absorption. Easily processed materials with the ability to transport excitons over length scales of more than 100 nanometers are highly desirable for a range of light-harvesting and optoelectronic devices. We describe the preparation of organic semiconducting nanofibers comprising a crystalline poly(di-n-hexylfluorene) core and a solvated, segmented corona consisting of polyethylene glycol in the center and polythiophene at the ends. These nanofibers exhibit exciton transfer from the core to the lower-energy polythiophene coronas in the end blocks, which occurs in the direction of the interchain π-π stacking with very long diffusion lengths (>200 nanometers) and a large diffusion coefficient (0.5 square centimeters per second). This is made possible by the uniform exciton energetic landscape created by the well-ordered, crystalline nanofiber core.