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.

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).

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.

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.

Two structural-simple 3-aryl-2-cyano acrylamide derivatives (CDPA-E and CDPA-P) were synthesized and characterized by NMR, MS, IR, and single crystal. It was found that both of them exhibited obvious piezofluorochromic properties. The as-prepared yellow-green compounds were both converted to orange-red after grinding with a spatula. All the ground samples can be recovered to its original color by heating at 80 °C over 5 min or fuming with solvents like DCM, ethanol, ethyl acetate etc. The changes in fluorescence color can be attributed to the phase transition between crystalline (order) state and amorphous (disorder) state according to the Powder XRD spectrum. At the molecular level, the extension of the conjugation length and the formation of excimers are confirmed to account for the color transition of CDPA-P and CDPA-E, respectively. In addition, CDPA-P exhibited AIE activity which was caused by the restricted rotation of the molecules in the crystal state.

Two new electrochromic materials consisting of carbazole and phenyl-methanone units, 4-(9H-carbazol-9-yl)-phenyl-methanone (CPM) and 4-(3,6-di(thiophen-2-yl)-9H-carbazol-9-yl)-phenyl-methanone (TCPM), were synthesized and characterized. Both the compounds show well-defined oxidation and reduction processes. Spectroelectrochemical analysis reveals that their polymers display reasonable optical contrast (25%, 41%) and fast switching time (3 s, 2 s). Cyclic voltammogram study confirms that the polymer with the existence of thiophene units is more stable in electrochemical environment.Graphical abstractHighlights► Two novel compounds, CPM and TCPM, were synthesized successfully, and their polymers were obtained by electrochemical method. ► The PCPM film showed reasonable optical contrast and two colors under different potentials. ► The PTCPM film not only displayed good optical contrast and different colors, but also possessed better stability.

•ETCB and NDTC were synthesized. And their polymers were obtained by electropolymerization.•Both the polymer films showed well-defined oxidation and reduction process.•The PETCB and PNDTC films showed reasonable optical contrast and two colors under different potentials.•The copolymer film based on ETCB and EDOT could display five colors and higher optical contrast.Two novel donor–acceptor type monomers, ethyl 4-(3,6-di(thiophen-2-yl)-9H-carbazole-9-yl)-benzoate (ETCB) and 9-(4-nitrophenyl)-3,6-di(thiophen-2-yl)-9H-carbazole (NDTC), were synthesized and characterized. Both the monomers show good electrochemical activity. UV–vis absorption studies reveal that the spectra of them are obviously different due to the introduction of the acceptor groups with different polarity, and the compound with –NO2 group has lower band gap. Fluorescent spectral studies indicate that the solution of ETCB in DCM exhibits sky-blue emission, while the NDTC hardly displays the fluorescence because of stronger intramolecular charge transfer. Their polymers can be synthesized by electropolymerization, and both the films show well-defined oxidation and reduction process. Spectroelectrochemical analysis reveals that PETCB film displays the color change from yellow–green (neutral) to blue–purple (oxidized), while the color change of PNDTC film is from yellow (neutral) to gray (oxidized). Both the polymer films exhibit reasonable optical contrast and switching time. Moreover, the copolymer based on ETCB and 3,4-ethylenedioxythiophene (EDOT) is also investigated. The copolymer could show five colors change under different applied potentials and higher optical contrast (50% of 1100 nm) and coloration efficiency (356.88 cm2 C−1).Graphical abstract

Cyanostilbene derivatives with the aggregation-induced emission enhancement (AIEE) activity are prepared by Knoevenagel and Suzuki reactions. Among them, the dye (Z)-2,3-bis(4′-(diphenylamino)-[1,1′-biphenyl]-4-yl)acrylonitrile (CNS-4) nanoparticle suspension shows the polarity-dependent characteristics of the fluorescence properties. By the fluorescence spectroscopy and transmission electron microscopy (TEM) analysis, the restriction of transfer from the local excited (LE) state to the intramolecular charge-transfer (ICT) state and crystal formation results in a blue-shift in emission and enhances the intensity in the aggregate state. Additionally, the luminophors CNS-3 and CNS-4 possess the AIEE effect as well as mechanochromic fluorescent properties. This mechanofluorochromic behavior originates from the change between the crystalline and the amorphous state.

Copolymer of perylene (PE) with 3,4-ethylenedioxythiophene (EDOT) was electrochemically synthesized and characterized. The resultant copolymer film not only has orange-light emitter property, but also possesses distinct electrochromic properties. It shows four colors at different doped states (from red–brown to light-green). Double potential step chronoamperometry experiment reveals that the copolymer film has high optical contrast (25% at 516 nm, 42% at 1100 nm) and fast switching time (2 s). Cyclic voltammograms indicate that the polymer has reasonable stability because of the existence of EDOT units.Graphical abstractHighlights► Obtained a novel copolymer based on PE and EDOT via electropolymerization. ► The copolymer showed orange-emitter property. ► Possessed distinct color changes between four colors. ► High optical contrast and fast switching time.

Two blue-emitting oligomers, namely FDPA1 and FDPA2 containing 9,10-diphenylanthracene core end-capped with triphenylamine-substituted fluorene has been synthesized and characterized. The spiro-configuration end-capping groups imparts two compounds with pronounced morphological stability (Tg > 185 °C, Td > 420 °C) and excellent hole injection ability (EHOMO > −5.27 eV) with the advantageous optical characteristics of corresponding core. Scanning electron microscope (SEM) and X-ray diffraction (XRD) reveal that the two oligomers form excellent amorphous films and possess good morphological stability after annealing.Highlights► Novel nonplanar oligomers containing DPA core were facilely prepared. ► Possessing good thermal and morphological stabilities. ► Maintaining the high fluorescence quantum yield and wide energy gap of the DPA core. ► Excellent hole-injection and -transporting ability of the resulting materials.

Two conducting polymer films based on N-phenylnaphthalen-2-amine and N-(4-(1H-pyrrol-1-yl)phenyl)-N-phenylnaphthalen-2-amine were successfully prepared via constant potential electrolysis. These two polymer films both exhibited five various colors under different potentials. Cyclic voltammograms showed that the both polymer films have well-defined redox process and reasonable stability. Besides, these two polymer films showed comparatively fast switching time.Graphical abstractHighlights► BNA, NAP and their polymers can be easily obtained. ► Film PBNA and PNAP both exhibit five colors under different potentials. ► Film PBNA and PNAP have reasonable good stability. ► Film PBNA and PNAP show comparatively fast switching time.

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.

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.