Jingwei Sun

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.

Polymorphic crystals and their luminescence switching of triphenylacrylonitrile derivatives upon solvent vapour, mechanical, and thermal stimuli

For piezo-, vapo-, and thermochromic materials, it remains a challenge to figure out the underlying reason for fluorescence color changes upon external stimulation and determine why only some fluorophores reveal emission switching. A novel triphenylacrylonitrile derivative (TPAN-MeO) with remarkably twisted conformations has been carefully prepared via the Suzuki coupling reaction. The fluorescence of TPAN-MeO in the aggregate state depends on the polymorphic forms: three crystalline forms BCrys, SCrys and YCrys exhibit bright blue, sky-blue and yellow emission, respectively; meanwhile the amorphous powders are also strongly fluorescent with green emission. The crystals BCrys and SCrys exhibit mechano- and piezochromism in that grinding and high pressure could alter the emission colour, respectively. In addition, the amorphous film exhibits vapo- and thermochromic behaviour in that organic vapour and heating could change the green colour into sky-blue. Interestingly, the solvent vapour and heating stimuli can trigger a crystal-to-crystal transformation between SCrys form and YCrys form.

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.

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.

Effect of stacking mode on the mechanofluorochromic properties of 3-aryl-2-cyano acrylamide derivatives

Three structurally simple 3-aryl-2-cyano acrylamide derivatives, 2-cyano-3-(2-methoxyphenyl)-2-propenamide (1), 2-cyano-3-(3-methoxyphenyl)-2-propenamide (2) and 2-cyano-3-(4-methoxyphenyl)-2-propenamide (3) were synthesized. They exhibited different optical properties due to their distinct face-to-face stacking mode. The as-prepared crystals of 1 exhibited green luminescence and the emission peak did not change after grinding treatment. However, the emission peaks of 2 (Φf = 12%) and 3 (Φf = 16%) exhibited an obvious red-shift upon grinding, and their corresponding quantum yields decreased to 8% and 10%, respectively. Differential scanning calorimetry and powder X-ray diffractometry data indicated that the optical properties of 2 and 3 could be attributed to the transformation from the crystalline phase to the amorphous phase. X-ray crystal structures, infrared spectroscopy and data from fluorescence lifetime experiments further validated the relationship between fluorescence switching, stacking mode and molecular interactions.

Ratiometric pressure sensors based on cyano-substituted oligo(p-phenylene vinylene) derivatives in the hybridized local and charge-transfer excited state

Ratiometric sensors for visual monitoring of pressure environments are highly valuable in various fields, such as security inks and optoelectronic devices. However, ratiometric pressure sensors remain inadequate due to the lack of piezo-chromic luminescent materials with high-contrast color change and high emission efficiency. A donor–acceptor cyano-substituted oligo(p-phenylene vinylene) derivative mF-TPA exhibits a high luminescence efficiency (Φf = 82.6%), which is related to the hybridized local and charge-transfer (HLCT) states. Moreover, its crystalline particles can sense exact hydrostatic pressure as high as 10 GPa, accompanied by visible color changes (λPL = 146 nm, from light-green to deep-red). Interestingly, the photo-luminescence peak wavelengths have a linear relationship with the external pressure, enabling its use as a ratiometric pressure sensor. In situ Raman spectroscopy confirms that the intermolecular interaction is obviously enhanced due to the closer packing at high pressure, inducing a red-shift of the emission peak and a decrease of the fluorescence intensity.

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.

Highly Twisted Isomers of Triphenylacrylonitrile Derivatives with High Emission Efficiency and Mechanochromic Behavior

Two novel triphenylacrylonitrile derivatives (TPAN-C2 and TPAN-C3) with highly twisted conformations have been successfully prepared by Suzuki coupling reaction. Both of them show mechanochromic (MC) fluorescent behaviour and exhibit remarkable fluorescence redshifts from sky-blue to light green upon fully grinding them. Interestingly, the ground powders recover their original states upon heating at 100 °C for 1 min or fuming with solvents vapour. Such intriguing MC properties of TPAN-C2 and TPAN-C3 are attributed to their phase transitions between crystalline and amorphous states, which are confirmed by powder X-ray diffraction (XRD) data. Moreover, the behaviour of both TPAN-C2 and TPAN-C3 indicates that the aggregation-induced emission (AIE) gives rise to rather high fluorescence quantum efficiencies (Φf ) of 0.78 and 0.97, respectively. The influences of stacking mode and molecular conformation on MC properties and the high Φf of TPAN-C2 and TPAN-C3 are discussed.

Fabrication of Nano/Microstructure of Cyano Substituted Oligo(para-phenylenevinylene) with Aggregation-Induced Emission and Morphology Dependent Luminescence

A new cyanostilbene based molecule, (2Z,2′Z)-3,3′-(4,4′′-dimethyl-[1,1’:4′,1′′-terp-henyl]-2′,5′-diyl)bis(2-(3-methoxyphenyl)acrylonitrile) (DMCS-PMP), has been designed and synthesized. Photoluminescence spectrum (PL) and scanning electron microscopy (SEM) analyses indicated that DMCS-PMP possessed extraordinary aggregation-induced emission behavior as well as morphology dependent luminescence property. Moreover, multiple intriguing nano/microstructures, including hollow sphere, spindle-like net and belt, have been fabricated via a simple reprecipitation method by varying the solvent, concentration and aging time. The hollow spheres and porous spindle-like networks are promising candidates for construction of functional optoelectronic devices, catalysts and sensors.