Materials Today Chemistry
Novel polyamides with pendant p-phenylenediamine and α-/β-substituted naphthalene: synthesis, characteristics, and effects of substitution sites on electro-switchable optical behaviors
Introduction
Electrofluorochromic (EFC) materials, which show reversible fluorescence modulation by electrochemical redox reactions, have received significant research interest spanning the field of optical displays, sensors, and information encryption [[1], [2], [3], [4]]. Several examples of organometallic complexes [5,6], quantum dots [7], organic small molecules [[8], [9], [10], [11], [12], [13]], and polymers [[14], [15], [16], [17], [18], [19]] have successfully demonstrated EFC behaviors. The requisite optoelectronic characteristics, including working voltage, switching time, fluorescent colors, and brightness, may vary for different applications, which opens up the necessity to finely tune the corresponding performances. For example, Zhang et al. [1] fabricated an RGB color-tunable turn-on EFC device by designing several pH-sensitive photoluminescent molecules, exhibiting great potential for application in encrypted information storage and display. Liou et al. [20] synthesized cyanostilbene-based triphenylamine-containing derivatives and demonstrated that the incorporation of bromide and dimethoxy groups would result in enhanced red-shift emission and higher solid-state fluorescence. Sun et al. [21] achieved same color dual-switching of electrochromic (EC)/EFC polymer through the introduction of ether linkage between tetraphenylethylene and triphenylamine units, greatly promoting the development of dual-mode displays with coloration and emission. Despite lots of efforts have been made to develop EFC devices, some technical challenges, including dissatisfactory cyclability, slow response speed, low fluorescence contrast, and poor processability, have hindered their further real-life applications. Further optimizations of EFC materials are still needed to improve the comprehensive performances. Benefiting from the facile structural modifications of organic EFC materials, the EFC performances can be improved by the structural design of electroactive fluorophores [11,[22], [23], [24], [25]]. Therefore, carefully studying and understanding of structure-property relationships is of great significance to precisely regulate the corresponding performances of EFC devices.
Naphthalene with unique photoelectric properties has been widely applied in solar cells, fluorescence sensors, and memory devices [[26], [27], [28]]. Owing to its high fluorescence quantum yield, excellent photo-/chemo-stability, and easy functionalization, naphthalene will be a promising candidate as the fluorophore in designing EFC materials [29,30]. More interestingly, the positions of ten carbon atoms on naphthalene are not equivalent, divided into α-position (1, 4, 5, 8) and β-position (2, 3, 6, 7) [27]. In-depth exploration of the impact of the two substitution sites on performances will help guide structure design for the development of new EFC materials. In addition, to improve the electrochemical stability of EFC materials, p-phenylenediamine has been recently demonstrated as an ideal electroactive unit because of its strong electronic coupling of the mono-cation radical [31]. Considering the EC nature of the p-phenylenediamine unit, the combination of fluorescent naphthalene and electroactive p-phenylenediamine may not only provide a model to study the effects of substitution sites of naphthalene but also achieve excellent and adjusted EC/EFC performances, greatly promoting the development and applications of EC/EFC devices.
Herein, we reported two novel p-phenylenediamine–based semi-aromatic polyamides (α-HPA and β-HPA) bearing a same pendant naphthalene with a methoxy group but different substitution sites (α- and β-substitution). Polymerized materials are designed to achieve excellent thermal stability, excellent film-forming ability, and competitive processability. The alicyclic structure was chosen to reduce the strong charge transfer effect caused by fully aromatic polyamides, increasing the fluorescence intensity and further improving the fluorescence contrast. Twisted electroactive p-phenylenediamine protected with methoxy units will enable the resulting polyamides with good solubility, fast response speed, and excellent electrochemical stability and cyclability. The distinct photophysical behaviors and the structure–property relationships of the α-HPA and β-HPA will be carefully investigated.
Section snippets
Synthesis and characterization
The synthetic routes of two novel diamines/polyamides and model compounds were shown in Scheme 1 and scheme S1, and the synthetic methods were described in detail in the supplementary information. Two novel diamines (α-2NH2 and β-2NH2) were synthesized individually via a five-step route. First, 4-nitro-4-methoxydiphenylamine was synthesized from an amination reaction of p-phenylenediamine and 4-fluoronitrobenzene as per the previously reported procedure [14]. Second, α-NO2/β-NO2 were obtained
Conclusion
Herein, we have developed two high-performance bifunctional EC/EFC polyamides (α-HPA and β-HPA) containing pendant p-phenylenediamine protected by methoxy units and α-/β-substituted naphthalene. In addition to good solubility and thermal stability, the two polyamides exhibited excellent overall performance including long-term electrochemical/EC/EFC stability (>500 cycles) and high color/fluorescence contrast. These two substitution sites influence the coplanarity and charge-transfer strength of
Author contributions
Kaixin Su: Conceptualization, Investigation, Methodology, Writing - original draft, Data curation. Ningwei Sun: Conceptualization, Investigation, Methodology, Writing - review & editing, Data curation. Xuzhou Tian: Methodology, Formal analysis. Xiaoqian Li: Formal analysis. Danming Chao: Validation, Project administration. Daming Wang: Conceptualization, Writing - review & editing, Supervision. Hongwei Zhou: Data curation, Validation. Chunhai Chen: Data curation, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article.
Acknowledgments
The authors graciously acknowledge the National Natural Science Foundation of China for funding (grant No. 21774046) and Jilin Provincial Science and Technology Department, China (grant Nos. 20200801057GH and 20190201075JC).
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