Yuyu Dai

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.

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.

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).

A fast electrochromic polymer based on TEMPO substituted polytriphenylamine.

A novel strategy to obtain rapid electrochromic switching response by introducing 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) moiety into polytriphenylamine backbone has been developed. The electrochromic properties of the integrated polymer film are investigated and a possible mechanism is proposed with TEMPO as a counterion-reservoir group to rapidly balance the charges during electrochromic switching, which leads to significantly improved electrochromism performance.

Photo-irradiated E/Z Isomerization Reaction of Star-shaped Isomers Containing Two Cyanostilbene Arms with Charge Transfer Excited States

The E/Z isomerization reaction of the multi-cyanostilbene molecule is still not clear. Herein, we have designed and synthesized three star-shaped molecular isomers with a triphenylamine core linked to two cyanostilbene groups with E/Z isomerization, Z,Z-TPDCF, Z,E-TPDCF and E,E-TPDCF, possessing three different isomeric molecular configurations, to investigate the specific E/Z isomerization reaction of the cyanostilbene groups in the two molecular arms. The in situ UV, 1H NMR and HPLC spectra under UV-irradiation clearly showed that the E/Z isomerization reactions of both E,E-TPDCF and Z,Z-TPDCF firstly turned them into Z,E-TPDCF, and the Z,E-TPDCF was almost simultaneously turned into more E,E-TPDCF and less Z,Z-TPDCF due to the calculated lowest unoccupied molecular orbitals of Z,E-TPDCF on the cyanostilbene arm with the Z-configuration. In general, Z,E-TPDCF exhibited a relatively better configurational stability than Z,Z-TPDCF or E,E-TPDCF under the photo-irradiation conditions. Further research demonstrated that all three isomers exhibited excellent aggregation-induced emission (AIE) properties.