Preparation And Performance Of Electrochromic/ Electrofluorochromic Polyimide Materials

Electrochromic（EC）materials have attracted significant attention due to their environmental friendliness and diverse color variations.However,with the development of science and technology,the EC materials’single-functional color switching is difficult to meet the diversified needs.In this context,the electroflourochromic（EFC）technique,which shows switchable fluorescence intensities,has been found to enrich the optical changes under electrical stimulations.Therefore,a new concept of EC/EFC dual-functional optical materials was proposed by combining EFC with EC materials.The EC/EFC dual-functional materials could exhibit two display modes of switchable color states and fluorescence intensities,wherein their optical properties can switch reversibly with potential changes,which shows great potential for applications in intelligent light windows,optical displays,information encryption,optical sensing,etc.However,the EC/EFC dual-functional materials are still in the early stages of research.The distinction in display principle imparts huge differences in EC and EFC material designs,resulting in challenges for designing high-performance EC/EFC dual-functional materials.For example,difficulties obtaining colorless-to-black color switches,aggregated fluorescence quenching,long response times,and poor cycling stability have severely limited practical applications.Therefore,developing novel high-performance EC/EFC dual-functional polymers is imminent.Triphenylamine（TPA）derivatives have been widely studied in EC and EFC fields due to their colorless neutral state and excellent redox stability.As an EC/EFC control unit,the TPA+unit can change the polymers from colorless to colored and quench the fluorescence from a bright to a dark state.Tetraphenylethene（TPE）,a typical aggregation-induced emission（AIE）unit and an EC chromophore with long wavelength absorptions,is an ideal structure for EC/EFC dual-functional materials.Polyimide is one of the high-performance engineering materials with excellent comprehensive performance,outstanding thermal stability,chemical stability,and tunable molecular structure,which has been widely used in optoelectronics.In this work,a series of optoelectronic polyimides with different TPE-basedπ-bridges connected TPA derivatives have been synthesized,and the electrochemistry,electrospectroscopy,and theoretical calculations were used to study the structure-property relationships of the polyimides systematically,which will be of great significance to the practical application of EC and EFC materials in the future.First,a novel diamine monomer（TPE-2TPANH₂）containing a TPE-basedπ-bridge with two TPA redox centers was designed and synthesized.Two novel polyimides（T6F-TPE-PI and TH-TPE-PI）were obtained by condensation polymerization by TPE-2TPANH₂with 6FDA and HPMDA dianhydride monomers,respectively.All two polyimide films showed excellent transmittance and solubility due to the designed twisted TPE-based structure and well-chosen dianhydrides.Moreover,the polyimides had high thermal stability benefiting from the imide main chain structure.Compared with the fluorinated（-CF3）structure in T6F-TPE-PI,the alicyclic ring in the TH-TPE-PI can efficiently inhibit charge transfer（CT）interactions.Therefore,the TH-TPE-PI exhibited fluorescence,and the TPE structure caused AIE properties.Moreover,the TH-TPE-PI film demonstrated a reversible EC character,allowing the film to change from an almost imperceptible pale yellow to light green and eventually to black.The green state showed a fast response speed(T_colored/T_bleached=1.5/2.5 s),high coloration efficiency(177 cm²C^-1),and highly stable EC cyclability（500 cycles）.However,the black state performed poorly,with almost no cycling ability.As for the EFC performance of TH-TPE-PI,the polyimide film demonstrated a reversible fluorescence switch with fast response speed(T_off/T_on=1.5/2.5 s)and highly stable EFC switching over 300 cycles.As a result,we demonstrated the effectiveness of TPE bridging the electroactive group of TPAs and realized a polyimide material with a dual function of EC/EFC.Next,to further investigate the relationship between conjugation structure change with optical modulation and the desire to obtain a completely colorless polyimide optoelectronic material,a novel diamine monomer（TPE-O-2TPANH₂）containing TPE-etherπ-bridge with two TPAs was designed and further polymerized with HPMDA to a novel polyimide TOH-TPE-PI.Due to the flexible ether linkages between the TPE and TPA that could further inhibit intramolecular CT,the TOH-TPE-PI exhibited a completely colorless nature state.Moreover,the TOH-TPE-PI film demonstrated a reversible EC character,allowing the film to change from colorless to light blue and eventually to dark purple.The blue state showed great stable switching stability（no decay after 200 cycles and only a 15%decrease after 500 cycles）.However,due to the introduction of the rotatable ether linkages,which limited counterion penetration and diffusion rates,the response speed of TOH-TPE-PI was poor(T_colored/T_bleached=10.7/6.8 s).Moreover,the TOH-TPE-PI exhibited an AIE-enhanced EFC character with a"blue-to-dark"fluorescence switching performance,high fluorescence contrast（87）,fast response speed(T_off/T_on=2.5/3.6 s)and stable EFC switching over 300 cycles.Thus,we obtained a completely colorless and blue fluorescence EC/EFC dual-functional polyimide material in this section.Then,to further improve the stability of the"colorless-to-black"EC switching and the fluorescent contrast of EFC,we designed and synthesized a new diamine monomer（TPE-2DPANH₂）containing a TPEπ-bridge with two diphenylamines（DPAs）and further polymerization with HPMDA to a novel polyimide ST-TPE-PI.Due to the shortened conjugation structure improved rigid,which restricts molecular vibration and rotation,the ST-TPE-PI showed a better fluorescence performance.Moreover,the ST-TPE-PI was ring-substituted by two nitrogen redox centers.Hence,as previously reported for TPE systems,TPE analogs can undergo one-electron oxidation and further the disproportionations of the one-electron oxidation product to form dications.As a result,the ST-TPE-PI showed a lower full-oxidation potential,which is beneficial for the"colorless-to-black"EC cyclability,from dozens of improved to 500 cycles.In addition,due to the improvement of fluorescence quantum yield and quenching efficiency,ST-TPE-PI showed a high EFC fluorescence contrast(I_on/I_off=487)with fast switch time(T_off/T_on=1.2/2.1 s)and good stability（300 cycles）.Therefore,ST-TPE-PI is an EC/EFC dual-functional material with excellent overall performance,capable of a"colorless-black"EC switch and high EFC fluorescence contrast with AIE enhancement.Next,on the basis of ST-TPE-PI,the structural changes of TPE and the electrochemical performance of TPE-based polyimides during electrochemical redox were further assessed.We designed and synthesized two new polyimides with two types of TPE-basedπ-bridge connected with DPA electroactive units（pendant-grouped and hyperbranched）.Electrochemistry,electrospectroscopy,and theoretical calculations were used to systematically study the structure-property relationships of the polyimides.We found that the imide backbone structure in PG-TPE-PI did not connect with the DPA redox center could reduce the effect of conformational change on the polymer backbone structure,which enhanced electrochemical cycling stability.As a result,PG-TPE-PI presented a high response speed(T_colored/T_bleached=2.29/2.51 s)and high coloration efficiency(566 cm²C^-1),as well as the highest reported cycling stability for a colorless-to-black polymer material（1000 cycles no decay and 3000cycles with 15%decay in an open-system）,thereby showing excellent application potential.