Mi Ouyang

A donor–acceptor cruciform π-system: high contrast mechanochromic properties and multicolour electrochromic behavior

A donor–acceptor (D–A) cruciform conjugated luminophore DMCS-TPA was designed and synthesized. The DMCS-TPA solid shows both aggregation induced emission (AIE) effect and high contrast mechanochromic (MC) behavior with a remarkable spectral shift of 87 nm. The obvious fluorescence switching from yellowish green to orange can be realized by pressing at only 10 MPa or simply grinding. The photophysical properties, theory calculation and XPS results demonstrate that the extension of the conjugation length and subsequent enhancement of intramolecular charge transfer (ICT) transition are responsible for the improved MC performance. In addition, DMCS-TPA is readily deposited on the ITO electrode surface by the electrochemical method to form an electrochromic (EC) film with multiple colours showing (light green at 0 V, red at 1 V, grey at 1.1 V and blue at 1.45 V) and a high optical contrast of 65% at 769 nm. The results suggest that incorporation of electroactive moieties into luminophores to constitute D–A cruciform conjugated structures is a promising design strategy for preparing dual functional materials combining MC and EC properties.

Heating and mechanical force-induced luminescence on–off switching of arylamine derivatives with highly distorted structures

A triphenylamine-based organic luminophor (TPA-CO) with a highly distorted structure has been designed and effortlessly obtained by an Ullmann reaction. The luminophor exhibits a stimuli-induced emission enhancement effect and intramolecular charge transfer properties. The fluorescence efficiency of its crystals is dramatically increased from 0.4% to 12.3% upon grinding. The emission enhancement is also realized by a heating process. The “bright” state can recover its original state and turn “dark”. The luminescence “on–off” behaviour is repeatedly transformed by a grinding–vapour process or by a heating process. The XRD patterns of the “bright” and “dark” states show that the change of emission intensity is related to the reversible transition between the crystalline state and the metastable amorphous state. At the molecular level, the emission enhancement upon external stimuli may be attributed to conformational planarization and weak intermolecular interactions.

Unique torsional cruciform π-architectures composed of donor and acceptor axes exhibiting mechanochromic and electrochromic properties

A series of unprecedented bifunctional materials response to both mechanical and electrical stimuli have been developed with torsional cruciform π-architectures composed of donor and acceptor axes. These cross-conjugated geometries possess spatially separated HOMO and LUMO located on the donor and acceptor axis, respectively. A unique charge transfer (CT) process from one axis to the other in the excited state is evidenced by theoretical calculations and spectral analysis. This unusual electronic nature along with the conformational flexibility of compounds is found to be significant for their effective mechanochromic (MC) and electrochromic (EC) performances. Through changing substituents on one bar, systematic and comparative studies have been carried out to explore the structural impacts on the MC and EC properties. Based on the structure–property relationships, remarkable MC materials with emission shifts above 70 nm and excellent EC materials with high optical contrast (70%) and fast response time (0.59 s for fading, 1.44 s for colouring) are obtained. Besides, an effective method for selectively modulating the LUMO energy level as well as bandgap is also attained.

Polymeric electrochromic materials with donor–acceptor structures

Conjugated polymers with various electron-donor (D) and -acceptor (A) structures have been an important focus in the field of electrochromic (EC) research. Recent years have witnessed significant advances in the context of the design and synthesis of D–A type conjugated polymers. Most studies have investigated tunable band gap and color changes by introducing appropriate D and A units. However, D–A polymers with specific D units containing A units in the backbone or side chain possess varied ionization potentials, electron affinities and conjugation effects, leading to diverse electrochemical, optical-physical and EC properties. In addition, some innovative D–A structural polymers, such as cruciform and dendritic structures, present superior EC properties as well as multifunctional performance. In this review, our main focus will be placed on summarizing the characteristics of polymeric EC materials with various donor–acceptor structures. The overarching aim is to strengthen the understanding of the relationship between the D–A structure and the EC properties, especially color characteristics, and to provide some suggestions for the design of novel multifunctional D–A polymers for the future.

Heating and mechanical force-induced “turn on” fluorescence of cyanostilbene derivative with H-type stacking

A cyanostilbene-based derivative with aggregation induced-emission was obtained, which exhibited multi-luminescent intensity under external stimuli. The as-prepared crystals of the desired dye showed a rather faint luminescence. Upon heating, its fluorescence could be turned on with the quantum yields of 13.3%. The quantum yields of the melted and solidified powders were further increased to 28.6%. Powder X-ray diffractometry, fluorescence spectroscopy, fluorescence lifetime experiments and single crystal XRD analyses were applied to investigate the changes of molecular packing modes. Such a transformation of fluorescence “dark”–“bright”, easily distinguished by naked eye, was related to the phase transition. The planar cyanostilbene with H-type packing and multiple mutable interactions may be the essential factor for the multi-luminescent intensity properties.

Electric field controlled formation and dissociation of multiwalled carbon nanotube conductive pathways in a polymer melt

Electric field controlled formation and dissociation of multiwalled carbon nanotube (MWCNT) conductive pathways in a polycarbonate (PC) melt are investigated by the dynamic percolation measurement. The results show that field-induced MWCNT alignment causes the decrease in the activation energy of conductive pathway formation. The directional or disordered alignment of MWCNTs in the PC melt results in the transition from a conductor to an insulator as the electric field changes from 500 to 1 V/cm. This electric-controllable directional or disordered alignment technology is promising for the fabrication of low-dimensional conductive materials and applications of voltage-switch devices.

Clear piezochromic behaviors of AIE-active organic powders under hydrostatic pressure

A novel diphenylacrylonitrile derivative (Z)-3-(4′-(diphenylamino)-[1,1′-biphenyl]-4-yl)-2-(4-methoxyphenyl)-acrylonitrile (β-CN-TPA) containing a twisted triphenylamine and diphenylacetonitrile was synthesized via Knoevenagel condensation and Suzuki coupling reactions. These molecules exhibited aggregation enhanced emission (AIE) effects. Interestingly, their mechano-fluorochromic properties were invisible upon grinding with a pestle. However, when hydrostatic pressure in a diamond anvil cell (DAC) was applied on the crystals of β-CN-TPA, the distinct piezochromic behaviors of the compound were observed. The fluorescence color changed from light green (530 nm) to red (665 nm) with a significant red-shift of 135 nm. The powder X-ray diffraction and high-pressure Raman studies indicated that the as-synthesized and ground samples had the same crystalline structures, while the compressed samples had an evident change in inter-molecular interactions. Comparative tests and theoretical analysis further confirmed that the distinct fluorescence behaviors of the desired dye during the different stress conditions were associated with the various inter-molecular interactions that existed with adjacent molecules.

A core–shell composite of porous ZnO nanosheets and a multichromic conducting polymer: enhanced electrochromic performances

A core–shell composite of porous ZnO nanosheets and a multichromic conducting polymer poly(4,4′,4′′-tris[4-(2-bithienyl)pheny]amine) (PTBTPA) was prepared by electrodeposition combined with the electropolymerization method. The composite film exhibits noticeable electrochromism with reversible color changes from orange, olive green to dark gray. An optical contrast of 68.7% and a switching time of 0.96 s are obtained for the composite film, better than that of the pure PTBTPA film, 51.8% and 1.95 s. The cyclic stability studies reveal that the composite film exhibits much more enhanced durability and retains 70% of the electroactivity even after 1000 cycles. However, the pure PTBTPA film loses almost most of its electroactivity after 1000 cycles. The core–shell composite structure is believed to be responsible for the observed enhanced electrochromic performance. On one hand, porous ZnO nanosheets with loose inner space can facilitate the penetration of counterions into the polymer film and shorten the diffusion distance, resulting in the higher optical contrast and faster switching speed; on the other hand, the larger contact area can enhance the adhesion between the polymer and the ITO electrode, contributing to better electrochemical stability.

A novel ferrocene-containing aniline copolymer: its synthesis and electrochemical performance

A novel ferrocene-containing aniline, 6-(2-amino-phenol-9H-yl)-hexyl ferrocenecarboxylate (AnFc) was synthesized via the hydrogenation of 6-(2-nitro-phenol-9H-yl)-hexyl ferrocenecarboxylate (NPFc). Then, the homopolymer of AnFc (PAnFc), copolymers of aniline and AnFc (P(An-co-AnFc)), and polyaniline (PAn) were prepared using a chemical oxidative polymerization process. The structure, morphology, and electrochemical properties of the prepared polymers were characterized by Fourier transform infrared spectroscopy (FTIR), ultraviolet visible spectroscopy (UV-vis), scanning electron microscopy (SEM), cyclic voltammograms (CV) and galvanostatic charge–discharge tests. The results demonstrated that the AnFc functionalized monomer and the corresponding polymer derivatives have been synthesized successfully, and the introduction of the novel functionalized ferrocene-based aniline obviously affected the spectral characteristic, morphology and electrochemical characteristics of the electro-active polymers obtained, as well as its charge migration along the polymer backbone. In addition, the charge–discharge tests showed that P(An-co-AnFc) improved the discharge plateau in the potential range of about 3.0–4.0 V and with an acceptable initial discharge specific capacity of 104.9 mA h g−1 for P(An-co-AnFc) (5 : 1 ([An]/[AnFc])) (compared with 108.2 mA h g−1 for PAn). Furthermore, P(An-co-AnFc) exhibited an even more improved cycling stability than that of PAn, and after 30 cycles the discharge capacity of P(An-co-AnFc) (3 : 1 ([An]/[AnFc])) still maintained 76.3% of the capacity obtained during the initial cycle.

Enhanced electrochromic switching speed and electrochemical stability of conducting polymer film on an ionic liquid functionalized ITO electrode

The 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) functionalized ITO substrate was successfully prepared via a solution immersion method and then incorporated with poly(4,4′,4′′-tris[4-(2-bithienyl)phenyl]amine) (PTBTPA) to form the PTBTPA–[BMIM]BF4 film by electrochemical polymerization, which presents reversible multicolor changes from orange, olive green to dark gray. Interestingly, compared with the bleaching time (tb) and the coloring time (tc) of the pure PTBTPA film (1.76 s and 4.51 s) at 1100 nm, the PTBTPA–[BMIM]BF4 film exhibits shorter tb and tc (0.87 s and 2.90 s) at the same wavelength. Obviously, the switching speed of the PTBTPA–[BMIM]BF4 film has been improved significantly, and it is further supported by the electrochemical impedance spectra which demonstrate that the PTBTPA–[BMIM]BF4 film possesses much lower charge transfer resistance. The reduction of charge transfer resistance could be attributed to (1) the private channel provided by the ionic liquid [BMIM]BF4 as a linker between the polymer and the electrode; (2) the ability of the simultaneous doping and dedoping of ClO4− in the electrolyte and BF4− ions of the ionic liquid. Moreover, the cyclic stability studies reveal that the PTBTPA–[BMIM]BF4 film exhibits better durability and retains 70.4% of its original electroactivity after 500 cycles in ionic liquid solution. The results demonstrate that the electrochemical and the electrochromic performances could be significantly enhanced through incorporating PTBTPA with the ionic liquid ([BMIM]BF4).