English

• 1. Introduction

  Diketopyrrolopyrroles (DPPs) were firstly used as pigments in industrial applications since 1974 [1]. This is due to their favorable characteristics, such as bright red color, good optical and thermal stabilities, and excellent photophysical properties. In recent years, DPP and its derivatives have been extensively used in organic semiconducting devices [2,3], such as light-emitting diodes [4], field-effect transistors [5-8], photovoltaics [9-15], thermoelectric materials [16-18], electrochemical transistors [19] and single-molecular devices [19].

  Understanding the developments in DPP's syntheses could help people in designing high performance optical or electronic materials. There are mainly two approaches for DPP modification with functional groups: (1) extending the DPP skeletons through the carbon (C) positions by attaching various aromatic groups [20-22]; (2) chemical modification of DPP at nitrogen (N) positions, including N-alkylation and N-arylation (Scheme 1). Many studies revealed that functionalizing the nitrogen atom of DPP is an efficient way to improve the solubilities and optical/electrical properties [23-25]. Among them, N-alkylation of DPP is easily accessible [20-22], which endows DPP derivatives with solution processable abilities, better electronic properties, and new functionalities [23-30]. This is due to the flexible characteristics of the alkyl chains, the lamellar stacking of which can promote the ordered packing of DPP-based molecules, thereby regulating their electronic properties [23-25]. However, the solid-state luminescence is limited because the relatively small steric hindrance of the alkyl chain makes DPP-based molecules prone to intermolecular π-π stacking, thereby forming exciplexes or excimers. Unlike the alkyl chain, the large steric hindrance of the aryl group of N-aryl DPP inhibits the π-π stacking of DPP-based molecules, resulting in better luminescence. Besides, the N-aryl DPP-based π-expanded derivatives own greater conjugation degree than normal DPPs, which may promote the intermolecular packing, resulting in better electrical performances. However, the synthesis of N-aryl DPPs is complex compared to N-alkylation. Most reactions require that the aryl halides should have electron withdrawing groups, which results in limited reaction substrates and affects their further investigations.

  Scheme 1

  

  Scheme 1.  Chemical structure of DPP, N-aryl DPPs and correlated π-expanded derivatives.

  DownLoad: Full-Size Img PowerPoint

  Herein, the developments in the syntheses of N-aryl DPP derivatives and correlated π-expanded DPP derivatives are summarized (Scheme 1). We first discuss the synthesis of N-aryl DPPs, including condensation of lactone with arylamine, and nucleophilic aromatic substitution of N—H containing DPP. Second, the synthetic developments of π-expanded derivatives on the basis of N-aryl DPPs are introduced. The optical/electronic properties of them are also given. All these studies demonstrate that N-aryl DPPs show potentials in functional materials, which deserve to be further investigated. Finally, the existing problems and future perspectives of N-aryl DPPs and the π-expanded derivatives are discussed.

  2. Synthesis of N-aryl DPPs

  2.1 Bis-N-aryl DPPs

  The synthesis of N-aryl DPPs was started in the 1980s. In 1976, Sprake and coworkers reported N-aryl DPP precursor 4 (2,5-diphenyl-3,6-di(pyridin-2-yl)hexahydropyrrolo[3,4-c]pyrrole-1,4‑dione) by reacting 2-(N-phenylformimidoyl)pyridine (1) with diethyl succinate (2) under base in 36% yield (Scheme 2) [31]. Half-fused pyrrolidone 3 was also obtained, which was transformed to compound 4 when refluxed in toluene in 93% yield. In 1984, Iqbal and coworkers from Ciba-Geigy found that compound 4 and other similar structures with four aromatic groups can be transformed to N-aryl DPPs in the presence of oxidant 1,2-dichloro-4,5-dicyanobenzoquinone (DDQ) (Scheme 2) [32,33]. This method was convenient for those tetraaryl DPPs that were difficult to synthesize. Unfortunately, no further development was observed in this method since the yield at the early stage was not satisfactory.

  Scheme 2

  

  Scheme 2.  Synthetic route for aryl DPPs developed by Sprake et al. [31] and Iqbal et al. [32,33].

  DownLoad: Full-Size Img PowerPoint

  In 2002, Chamberlain and Thornley from Sun Chemical Corporation developed a synthetic method for the N-arylation of DPPs [34]. As shown in Scheme 3, β-ketoamide 5 was transformed to brominated derivative 6 in the presence of bromine and sodium acetate. Diaryl succinamide 7 was obtained by the alkylation of 5 with 6 under the base. Finally, N-aryl DPPs were obtained by the condensation of 7 with phosphorus oxychloride or zinc acetate in 46%−76% yields.

  Scheme 3

  

  Scheme 3.  Synthetic route to aryl DPPs developed by Sun Chemical Corporation [34].

  DownLoad: Full-Size Img PowerPoint

  The most commonly used synthetic method for N-aryl DPPs was developed by Rubin, Langhals and coworkers in 1980 [35,36]. As shown in Scheme 4a, the key precursor was diketofurofurans (DFFs) [35]. They used two synthetic methods to get DFFs from bisdiazotetraketone. One was explosive decomposition by directly heating bisdiazotetraketones (8) at 150–160 ℃ to synthesize four DFFs with different aromatic groups in 25%−50% yields. The other used less harsh conditions to get the same DFFs under reflux in toluene with higher yields (59%−89%). DFFs were also obtained by directly heating diacylsuccinates (10), which were transformed by the corresponding β-ketoesters (9) [37]. However, Rubin and coworkers failed to synthesize N-aryl DPP from DFFs with aniline when heated under steam-bath and only monolactones were obtained.

  Scheme 4

  

  Scheme 4.  A common synthetic route for N-arylated DPPs (DPP1–5), developed by Rubin et al. [35] and Langhals et al. [36]. (a) The synthetic route for the key precursor DFF. (b) The synthetic method of N-arylated DPPs by the condensation of DFFs and anilines.

  DownLoad: Full-Size Img PowerPoint

  In 1996, Langhals and coworkers improved the synthetic method by the condensation of DFFs and anilines in the presence of dicyclohexylcarbodiimide (DCC) and trace amount of trifluoroacetic acid (TFA) (Scheme 4b) [36]. N-aryl DPPs with the same aromatic groups on N positions referred as to symmetric N-aryl DPPs (DPP1–4), were obtained with 35.5%−56% yields. Although N-aryl DPP with different aromatic groups on two N positions referred as to non-symmetric N-aryl DPP (DPP5), were also obtained in only 2.47% yield. It should be noted that this is a relatively convenient method for N-aryl DPPs. Since then, various symmetric N-aryl DPPs and their correlated polymers were synthesized by this synthetic method, and their optical/electronic properties were investigated (Scheme 5) [38-44].

  Scheme 5

  

  Scheme 5.  Chemical structures of N-aryl DPPs monomers (DPP6–19) and corresponding polymers(PDPP1–18), developed byTieke et al. (a-d) [38-42] and Bronstein et al. (e, f) [43,44].

  DownLoad: Full-Size Img PowerPoint

  As shown in Scheme 5a, Tieke and coworkers reported N-aryl DPP monomers DPP6 and DPP7 with bromine groups on flanked benzyl groups and N-atom attached benzyl groups, respectively [38]. Alternating DPP-based polymer PDPP1 and random polymer PDPP2 were prepared from DPP6, while PDPP3 was obtained from DPP7. As shown in Fig. 1, all polymers exhibited strong fluorescence in solutions with high quantum efficiencies (ϕ_Fs) up to 81% (PDPP1). Because the conjugated backbone of PDPP3 was interrupted by the lactam N-atoms, the absorption and emission spectra of PDPP3 showed blue shifts compared with those of PDPP1 and PDPP2. Therefore, PDPP1 and PDPP2 showed red and yellow colors under visible and UV light, whereas PDPP3 exhibited yellow and green colors under visible and UV light. This study demonstrated the potential of N-aryl DPPs in optical materials. Skabara, Tieke, and coworkers synthesized 3,4-ethylenedioxy-thiophene (EDOT) containing N-aryl DPP monomers DPP8 and DPP9, and then prepared two polymers PDPP4 and PDPP5 by electrochemical polymerization method (Scheme 5b) [39]. The electrochemical investigation of synthesized polymers revealed that PDPP4 showed reversible electrochromic properties and a low oxidation potential, whereas the conjugation interrupted PDPP5 exhibited irreversible redox behavior and a high oxidation potential. They further synthesized microporous polymers PDPP6–12 from DPP10–13 monomers with four reactive sites on N-aryl DPPs via Yamamoto or Sonogashira cross-coupling polymerizations (Scheme 5c) [40,41]. Obtained polymers PDPP9–12 were red powders with strong fluorescence, and exhibited high gas storage capabilities with Brunauer-Emmett-Teller surface areas up to ~ 500 m²/g [41]. Bradley, Tieke, and Skabara synthesized star-shaped and linear compounds DPP14 and DPP15, respectively, with N-aryl DPP as a core and quarter-fluorene as an arm (Scheme 5d) [42]. Both compounds exhibited different optical behaviors. For example, a thin film of DPP14 exhibited as higher ϕ_F (DPP excitation) of 20% with similar fluorescence maxima at 634 nm compared with those of DPP15 (ϕ_F = 11%, 633 nm). Besides, thin film of linear DPP15 exhibited amplified spontaneous emission behavior, which was useful for lasing applications.

  Figure 1

  

  Figure 1.  Photo images of polymers PDPP1, PDPP2 and PDPP3 and monomers DPP6 and DPP7 in solutions under visible light (up) and UV light (down). Reproduced with permission [38]. Copyright 2008, Wiley-VCH.

  DownLoad: Full-Size Img PowerPoint

  Conjugated polymers with intense emission in solid-state, especially in the red region, are important for next-generation light-emitting devices. In 2018, Bronstein and coworkers reported encapsulated DPP-based polymers PDPP13–15 with DPP18 or DPP19 as monomers (Scheme 5e) [43]. The polymers were encapsulated by alkyl chains between the two N-attached aromatic groups to minimize the fluorescence quenching effect of π-π stacking interactions in the solid-state. Therefore, they showed very high ϕ_Fs up to 94.8%±1.0% in solutions and 27.8%±1.3% in thin films. Notably, thin films of PDPP14 and PDPP15 displayed bright red emission colors with emission maxima at 641 nm. They further modulated the distances between the encapsulation units (PDPP16–18) to control the encapsulation density (Scheme 5f) [44]. The density increased in the following order: PDPP18 > PDPP16 > PDPP17. Since the densely encapsulated polymers were more shielded, their quenching effect was eliminated. Therefore, PDPP18 exhibited the highest thin-film ϕ_F of 41% ± 4%, culminating in the most efficient ϕ_F in solid state red conjugated polymer at that time.

  2.2 Mono-N-aryl DPPs

  Except for symmetric N-aryl DPPs, the syntheses of non-symmetric N-aryl DPPs and mono-N-aryl DPPs were also developed. In 2005, Smith, Slawin and coworkers designed and synthesized mono-N-aryl and non-symmetric N-aryl DPPs (Schemes 6a and b) [45]. The key compounds were N-aryl-benzimidoyl chlorides 12 with aryl groups at the N position. The reaction of 11 and 12 in strong base condition led to the formation of DPP20 with one unsubstituted N atom and 13 which further transformed to DPP20 in the presence of H₃PO₄ and P₂O₅. Treatment of DPP20 with methyl p-toluenesulfonate or benzyl bromide in an alkaline environment gave the corresponding N-alkyl and N-aryl DPP21–28 (mono-N-aryl DPPs) in 56%−95% yields. Non-symmetric N-aryl DPP30, DPP31 were obtained from N-arylpyrrolinones and imidoyl chlorides. As shown in Scheme 6b, reactions of compounds 14 with benzoyl chloride or N-phenylbenzimidoyl chloride in the presence of LiHMDS formed half-DPPs 15 and 17, which were transformed to 16 by microwave irradiation (MW) in 52% yield, whereas the yield of DPP29 under H₃PO₄ and P₂O₅ was only 7%. Similar to those in Scheme 4b, DPP29–31 can be obtained by reacting 16 with corresponding anilines in 25%−50% yields (Scheme 6b). The aryl groups modulated the optical properties and the ϕ_Fs varied from 5% to 48% for mono-N-aryl/alkyl DPP21–28, while 12%−32% were obtained for tetraaryl DPP29–31.

  Scheme 6

  

  Scheme 6.  Synthetic routes for mono-N-aryl DPPs and non-symmetric N-aryl DPPs. (a, b) Synthetic routes for mono-N-aryl DPPs (DPP21–28) and non-symmetric N-aryl DPPs (DPP30 and DPP31), developed by Smith et al. [45]. (c) Synthetic routes for mono-N-aryl DPP (DPP32) and chemical structures of two N-alkyl DPPs (DPP33 and DPP34), developed by Vullev et al. [46].

  DownLoad: Full-Size Img PowerPoint

  In 2016, Vullev, Gryko and coworkers reported mono-N-aryl DPP32 from 4-(trifluoromethyl)benzoic acid as a precursor, different from 12 reported by Smith et al. as shown in Scheme 6c [46]. The reaction of 18 with 4-(trifluoromethyl)benzoic acid in the alkaline conditions led to the formation of 19 in 23% yield, which was then transformed to 20 under MW in 74% yield. Finally, mono-N-aryl/alkyl DPP32 was obtained by N-alkylation and then N-arylation with a yield of 17%. They further investigated the charge recombination (CR) and charge separation (CS) behaviors of DPP32 with the two other N-alkyl DPPs (DPP33 and DPP34) in solutions of different viscosities. They found that the k_CS/k_CR was the largest for most and the least viscous solvents, regardless of the polarity or proticity. This property is important since impeding CR, while conserving the CS efficiency, which is vital for organic electronics.

  The methods discussed above involved multiple steps, long reaction times, and harsh reaction conditions (Schemes 4-6). Therefore, other efficient methods, especially for direct N-substitution of aryl groups, have also been developed [45,47-49]. In 2005, Smith, Slawin and coworkers synthesized N-aryl DPP36 by using N–H containing DPP35 and electron-deficient 1-fluoro-2,4-dinitrobenzene with K₂CO₃ in 86% yield (Scheme 7a) [45]. After that, Gryko, Jacquemin and coworkers synthesized DPP38–41 from highly activated aryl fluorides [47]. Tetrafluoro- and pentafluoroarenes containing additional electron-deficient cyano or pyridine groups reacted with DPPs to obtain the corresponding products in 14%−36% yields under basic conditions (Scheme 7b). It should be noted that these reactions work only with aryl halides containing electron-withdrawing substituents.

  Scheme 7

  

  Scheme 7.  Synthetic routes for N-aryl DPPs developed by Smith et al. (a) [45], Gryko et al. (b-d) [47-49] and Zhang et al. (e) [50].

  DownLoad: Full-Size Img PowerPoint

  In 2020, Gryko, Jacquemin and coworkers prepared the N-aryl DPP43–48 using N–H containing DPP42 and aromatictriflates (23) with allylpalladium(Ⅱ) chloride dimer, JackiePhos, Cs₂CO₃ and 3 Å molecular sieves (Scheme 7c) [48]. Noting that they used aromatictriflates instead of electron-withdrawing aryl halides. The aromatic substrates can be extended to non-electron-withdrawing aryl groups, such as anthracene and naphthalene. Coumarin groups can also work. In the same year, Gryko and coworkers reported that DPP49 reacted easily with 3‑bromo-2-fluoro-5-nitropyridine (24) in the presence of K₂CO₃ in DMF at 70 ℃ to give the corresponding monoarylated N-arylated DPPs (Scheme 7d) [49]. Monoarylated DPP50 was obtained from the reaction with 1.2 equiv. of the aryl fluoride 24, and bis-N,N-arylated DPP51 was prepared by increasing the amount of aryl fluoride to 4 equivalents. However, this synthetic method was not suitable for benzyl triflates and its derivatives. In 2021, some of us reported a simple and easy method for N-arylated DPPs (Scheme 7e) [50]. DPP52 reacted with diaryliodonium salts 25, generating the corresponding 16 products (DPP53–68) in 28%−78% yields, in the presence of CuI and K₂CO₃. The DPP attached aryl groups can be furan, thiophene, and benzene, showing broad substrate scope. Their optical properties revealed that DPP55, DPP59, DPP63 and DPP67 showed fluorescence with ϕ_Fs up to 96% and 40% in solutions and solid states, respectively. The solid-state color tuned from yellow to dark red under UV irradiation, illustrating potentials in optical application (Fig. 2).

  Figure 2

  

  Figure 2.  UV–vis absorption and fluorescence spectra of DPP52, DPP56, DPP60 and DPP64, corresponding to 3c, 3g, 3k and 3o in solutions (a) and in solid states (b). Reproduced with permission [50]. Copyright 2021, Wiley-VCH.

  DownLoad: Full-Size Img PowerPoint

  To sum up, we have summarized the development of synthesis of N-aryl DPPs, and their photophyical properties. We can see that N-aryl DPPs are often used in the study of light-emitting materials, but less in electrical research, mainly due to the large steric hindrance inhibits the intermolecular π-π stacking, resulting in poor electrical conductivity. It should be noted that further reaction of N-aryl DPPs can extend their π-conjugation lengths, and makes the molecule planar, which may be beneficial for intermolecular π-π stacking and thus electrical property.

  3. Synthesis of π-expanded derivatives based on N-aryl DPPs

  Compared to N-aryl DPPs, π-expanded compounds derived from them are less reported [45,49-52]. In 2014, Würthner and coworkers reported the synthesis of π-expanded DPP70–73 with modifications of both the π-extension through N positions and the carbonyl groups (Scheme 8a) [51]. The reaction precursors DPP69 were prepared by the synthetic method reported by Smith, Slawin and coworkers as introduced above (Scheme 7a) [45]. Subsequent reduction of DPP69 with SnCl₂·2H₂O and then intramolecular condensation reaction under TiCl₄ and DABCO in mesitylene gave the azahexacene analogs DPP70–73 (14%−20% yields). They showed low-lying frontier molecular orbitals, and broad absorption in the visible region. Yue and coworkers also designed DPP75–77, with extended conjugations from one side (Scheme 8b) [52]. DPP75–77 were obtained by Würthner's reaction conditions based on the mono-alkyl/aryl DPP74 (25%−36% yields). The DPP77-based polymer showed intense absorption at visible range.

  Scheme 8

  

  Scheme 8.  Synthetic routes for π-expanded DPPs based on N-aryl DPPs developed byWürthner et al. (a) [51], Yue et al. (b) [52], Gryko et al. (c) [49] and Zhang et al. (d) [50]. (e) Synthetic routes for the symmetric pyridine-flanked DPP complexes developed by Yamagata et al. [53].

  DownLoad: Full-Size Img PowerPoint

  In 2020, Gryko, Guldi, Jacquemin, and coworkers reported a new type of π-expanded DPP78 and DPP79 via intramolecular C–H arylation (Scheme 8c) [49]. DPP78 and DPP79 were obtained based on DPP50 and DPP51 using Pd(PPh₃)₄ with KOAc in toluene in 80% and 86% yields, respectively. DPP78 and DPP79 exhibited red-emissive fluorescence reaching 750 nm. In 2021, some of us used C–H activation to synthesize π-expanded DPP81–89 by using N-unsubstituted DPP (DPP80) and diaryliodonium salts with Pd(OAc)₂ and KOAc in 24%−61% yields (Scheme 8d) [50]. Single crystal OFETs of DPP82 based on benzene flanked DPP showed good p-type mobility up to 0.71 cm² V⁻¹ s⁻¹, demonstrating their potentials as organic semiconductors. However, the reaction of thiophene and furan-flanked DPP could not work due to the reactive C–H bonds at 5-positions, limiting further invesitigations. For heterocyclic ring containing π-expanded DPP, in 2014, Yamagata et al. reported that the coordination of a DPP ligand with boron produced the corresponding DPP complexes (DPP93–95) (Scheme 8e) [53]. The symmetric pyridine-flanked DPP complexes (DPP93–95) were obtained from the reaction of the corresponding DPP90–92 with BPh₃ in toluene at the reflux temperature in 71%, 21% and 60% yields, respectively. The crystal structures of DPP93 showed that the DPP core and 2-pyridyl groups were coplanar, which extended the π-conjugation. To sum up, the synthesis of π-expanded derivative based on N-aryl DPP is later than that of N-aryl DPPs, and it is limited by the DPP substrate. Moreover, the research of it on semiconducting devices has just started. Therefore, the synthesis of π-expanded N-aryl DPPs and their functions still need to be further developed.

  4. Summary and outlooks

  DPPs have attracted significant attentions in organic optical and electronic materials. Chemical modification of DPPs at N positions (N-alkylation and N-arylation) is an important way to design new DPP derivatives. An in-depth understanding of the DPP derivatives and the structure-property relationships is essential for future developments. However, N-arylation of DPPs is challenging compared to N-alkylation. This review summarizes the recent synthetic developments of N-aryl DPP derivatives and correlated π-expanded DPPs. The optical or electronic properties can be successfully modulated by attaching different aryl groups to the N position of DPPs. The research of N-aryl DPPs mainly focuses on the optical properties, while π-expanded DPPs show potential in electric properties, but the studies have just started. This is attributed to the aryl groups on the N position of DPPs, which affect the intramolecular interactions, intermolecular packings molecular, and thus physical properties. There are still problems for further development of N-aryl DPPs. These include: (1) A universal and easily handled N-arylation method for DPP, which is independent of the DPP substrates and reactants; (2) New synthetic methods for π-expanded DPPs, which are less reported so far. Therefore, more molecular examples of N-aryl DPPs are needed, as well as the π-expanded derivatives. We hope that this review can inspire relevant researchers to jointly promote the progress of the organic optoelectronics.

  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 paper.

  Acknowledgments

  We thank the financial support from National Natural Science Foundation of China (NSFC, Nos. 22175081 and 21833005), Beijing National Laboratory for Molecular Sciences (No. BNLM202010), State Key Laboratory of Physical Chemistry of Solid Surfaces (No. 202108), and Guangdong Provincial Key Laboratory of Catalysis (No. 20210701).

  
  
• 1. [1]

     A. Padwa, J. Smolanoff, A. Tremper, Tetrahedron Lett. 15 (1974) 29–32. doi: 10.1016/S0040-4039(01)82128-6

  2. [2]

     Q. Liu, S.E. Bottle, P. Sonar, Adv. Mater. 32 (2020) 1903882. doi: 10.1002/adma.201903882

  3. [3]

     Q. Ma, X. Sun, W. Wang, et al., Chin. Chem. Lett. 33 (2022) 1681–1692. doi: 10.1016/j.cclet.2021.10.054

  4. [4]

     Q. Liu, S. Chavhan, H. Zhang, et al., Adv. Electron. Mater. 7 (2021) 2000804. doi: 10.1002/aelm.202000804

  5. [5]

     Z. Yi, S. Wang, Y. Liu, Adv. Mater. 27 (2015) 3589–3606. doi: 10.1002/adma.201500401

  6. [6]

     C.B. Nielsen, M. Turbiez, I. McCulloch, Adv. Mater. 25 (2013) 1859–1880. doi: 10.1002/adma.201201795

  7. [7]

     Y. Li, P. Sonar, L. Murphy, W. Hong, Energy Environ. Sci. 6 (2013) 1684–1710. doi: 10.1039/c3ee00015j

  8. [8]

     Z.T. Huang, C.C. Fan, G.B. Xue, et al., Chin. Chem. Lett. 27 (2016) 523–526. doi: 10.1016/j.cclet.2016.01.054

  9. [9]

     A. Tang, C. Zhan, J. Yao, E. Zhou, Adv. Mater. 29 (2017) 1600013. doi: 10.1002/adma.201600013

  10. [10]

      C. Zhao, Y. Guo, Y. Zhang, et al., J. Mater. Chem. A 7 (2019) 10174–10199. doi: 10.1039/C9TA01976F

  11. [11]

      W. Zhao, J. Ding, Y. Zou, C.A. Di, D. Zhu, Chem. Soc. Rev. 49 (2020) 7210–7228. doi: 10.1039/D0CS00204F

  12. [12]

      F. Ullah, S. Qian, W. Yang, et al., Chin. Chem. Lett. 28 (2017) 2223–2226. doi: 10.1016/j.cclet.2017.08.009

  13. [13]

      X. Yan, M. Xiong, J.T. Li, et al., J. Am. Chem. Soc. 141 (2019) 20215–20221. doi: 10.1021/jacs.9b10107

  14. [14]

      H. Guo, C.Y. Yang, X. Zhang, et al., Nature 599 (2021) 67–73. doi: 10.1038/s41586-021-03942-0

  15. [15]

      X. Wu, Q. Liu, A. Surendran, et al., Adv. Electron. Mater. 7 (2021) 2000701. doi: 10.1002/aelm.202000701

  16. [16]

      N. Wang, L. Xie, H. Ling, et al., J. Mater. Chem. C 9 (2021) 4060–4266.

  17. [17]

      J.J. Samuel, A. Garudapalli, A.A. Mohapatra, et al., Adv. Funct. Mater. 31 (2021) 2102903. doi: 10.1002/adfm.202102903

  18. [18]

      Z. Tan, W. Jiang, C. Tang, et al., CCS Chem. 3 (2021) 929–937.

  19. [19]

      Y.P. Zang, S. Ray, E.D. Fung, et al., J. Am. Chem. Soc. 140 (2018) 13167–13170. doi: 10.1021/jacs.8b06964

  20. [20]

      A. Iqbal, M. Jost, R. Kirchmayr, et al., Bull. Soc. Chim. Belg. 97 (1988) 615–643.

  21. [21]

      M. Grzybowski, D.T. Gryko, Adv. Opt. Mater. 3 (2015) 280–320. doi: 10.1002/adom.201400559

  22. [22]

      N. Luo, G. Zhang, Z. Liu, Org. Chem. Front. 8 (2021) 4560–4581. doi: 10.1039/D1QO00613D

  23. [23]

      Y. Yang, Z. Liu, G. Zhang, X. Zhang, D. Zhang, Adv. Mater. 31 (2019) 1903104. doi: 10.1002/adma.201903104

  24. [24]

      N. Luo, P. Ren, Y. Feng, et al., J. Phys. Chem. Lett. 13 (2022) 1131–1146. doi: 10.1021/acs.jpclett.1c03909

  25. [25]

      T. Lei, J.Y. Wang, J. Pei, Chem. Mater. 26 (2014) 594–603. doi: 10.1021/cm4018776

  26. [26]

      B.C. Schroeder, T. Kurosawa, T. Fu, et al., Adv. Funct. Mater. 27 (2017) 1701973. doi: 10.1002/adfm.201701973

  27. [27]

      H.W. Luo, Z.T. Liu, Chin. Chem. Lett. 27 (2016) 1283–1292. doi: 10.1016/j.cclet.2016.07.003

  28. [28]

      A. Giovannitti, R.B. Rashid, Q. Thiburce, et al., Adv. Mater. 32 (2020) 1908047. doi: 10.1002/adma.201908047

  29. [29]

      J. Yao, C. Yu, Z. Liu, et al., J. Am. Chem. Soc. 138 (2016) 173–185. doi: 10.1021/jacs.5b09737

  30. [30]

      Z.T. Huang, C.C. Fan, G.B. Xue, et al., CCS Chem. 1 (2019) 632–641.

  31. [31]

      J.M. Sprake, K.D. Watson, J. Chem. Soc. Perkin Trans. 1 (1976) 5–8.

  32. [32]

      M. Jost, A. Iqbal, A.C. Rochat, (Ciba-Geigy AG), Eur. Pat. Appl. 133156, 1984.

  33. [33]

      M. Jost, A. Iqbal, A.C. Rochat, (Ciba-Geiyg AG), US Patent 4585878, 1986.

  34. [34]

      T.R. Chamberlain, C. Thornley, US Patent 6388093, 2002.

  35. [35]

      M.B. Rubin, M. Bargurie, S. Kosti, M. Kaftory, J. Chem. Soc., Perkin Trans. 1 (1980) 2670–2677.

  36. [36]

      H. Langhals, T. Grundei, T. Potrawa, K. Polborn, Liebigs Ann. 1996 (1996) 679–682.

  37. [37]

      R. Pfleger, F. Reinhardt, Chem. Ber. 90 (1957) 2404–2411. doi: 10.1002/cber.19570901042

  38. [38]

      K. Zhang, B. Tieke, Macromolecules 41 (2008) 7287–7295. doi: 10.1021/ma801376r

  39. [39]

      K. Zhang, B. Tieke, J.C. Forgie, P.J. Skabara, Macromol. Rapid Commun. 30 (2009) 1834–1840. doi: 10.1002/marc.200900442

  40. [40]

      K. Zhang, B. Tieke, J.C. Forgie, et al., Polymer 51 (2010) 6107–6114. doi: 10.1016/j.polymer.2010.10.054

  41. [41]

      K. Zhang, B. Tieke, F. Vilela, P.J. Skabara, Macromol. Rapid Commun. 32 (2011) 825–830. doi: 10.1002/marc.201100045

  42. [42]

      A.L. Kanibolotsky, F. Vilela, J.C. Forgie, et al., Adv. Mater. 23 (2011) 2093–2097. doi: 10.1002/adma.201100308

  43. [43]

      A. Leventis, J. Royakkers, A.G. Rapidis, et al., J. Am. Chem. Soc. 140 (2018) 1622–1626. doi: 10.1021/jacs.7b13447

  44. [44]

      J. Royakkers, A. Minotto, D.G. Congrave, et al., Chem. Mater. 32 (2020) 10140. doi: 10.1021/acs.chemmater.0c03604

  45. [45]

      R.L. Riggs, C.J.H. Morton, A.M.Z. Slawin, et al., Tetrahedron 61 (2005) 11230–11243. doi: 10.1016/j.tet.2005.09.005

  46. [46]

      A. Purc, E.M. Espinoza, R. Nazir, et al., J. Am. Chem. Soc. 138 (2016) 12826–12832. doi: 10.1021/jacs.6b04974

  47. [47]

      K. Gutkowski, C. Azarias, M. Banasiewicz, et al., Eur. J. Org. Chem. 47 (2018) 6643–6648.

  48. [48]

      K. Gutkowski, K. Skonieczny, M. Bugaj, D. Jacquemin, D.T. Gryko, Chem. Asian J. 15 (2020) 1369–1375. doi: 10.1002/asia.202000129

  49. [49]

      K. Skonieczny, I. Papadopoulos, D. Thiel, et al., Angew. Chem. In. Ed. 59 (2020) 16104–16113. doi: 10.1002/anie.202005244

  50. [50]

      W. Jiang, Z. Liu, D. Zhu, et al., Angew. Chem. Int. Ed. 60 (2021) 10700–10708. doi: 10.1002/anie.202102131

  51. [51]

      W. Yue, S.L. Suraru, D. Bialas, M. Müller, F. Würthner, Angew. Chem. In. Ed. 53 (2014) 6159–6162. doi: 10.1002/anie.201403227

  52. [52]

      Y. Wang, Y. Xu, M.K. Ravva, et al., Org. Chem. Front. 6 (2019) 2974–2980. doi: 10.1039/C9QO00645A

  53. [53]

      T. Yamagata, J. Kuwabara, T. Kanbara, Tetrahedron 70 (2014) 1451–1457. doi: 10.1016/j.tet.2013.12.090

• 

  Scheme 1  Chemical structure of DPP, N-aryl DPPs and correlated π-expanded derivatives.

  下载: 全尺寸图片 幻灯片
  

  Scheme 2  Synthetic route for aryl DPPs developed by Sprake et al. [31] and Iqbal et al. [32,33].

  下载: 全尺寸图片 幻灯片
  

  Scheme 3  Synthetic route to aryl DPPs developed by Sun Chemical Corporation [34].

  下载: 全尺寸图片 幻灯片
  

  Scheme 4  A common synthetic route for N-arylated DPPs (DPP1–5), developed by Rubin et al. [35] and Langhals et al. [36]. (a) The synthetic route for the key precursor DFF. (b) The synthetic method of N-arylated DPPs by the condensation of DFFs and anilines.

  下载: 全尺寸图片 幻灯片
  

  Scheme 5  Chemical structures of N-aryl DPPs monomers (DPP6–19) and corresponding polymers(PDPP1–18), developed byTieke et al. (a-d) [38-42] and Bronstein et al. (e, f) [43,44].

  下载: 全尺寸图片 幻灯片
  

  Figure 1  Photo images of polymers PDPP1, PDPP2 and PDPP3 and monomers DPP6 and DPP7 in solutions under visible light (up) and UV light (down). Reproduced with permission [38]. Copyright 2008, Wiley-VCH.

  下载: 全尺寸图片 幻灯片
  

  Scheme 6  Synthetic routes for mono-N-aryl DPPs and non-symmetric N-aryl DPPs. (a, b) Synthetic routes for mono-N-aryl DPPs (DPP21–28) and non-symmetric N-aryl DPPs (DPP30 and DPP31), developed by Smith et al. [45]. (c) Synthetic routes for mono-N-aryl DPP (DPP32) and chemical structures of two N-alkyl DPPs (DPP33 and DPP34), developed by Vullev et al. [46].

  下载: 全尺寸图片 幻灯片
  

  Scheme 7  Synthetic routes for N-aryl DPPs developed by Smith et al. (a) [45], Gryko et al. (b-d) [47-49] and Zhang et al. (e) [50].

  下载: 全尺寸图片 幻灯片
  

  Figure 2  UV–vis absorption and fluorescence spectra of DPP52, DPP56, DPP60 and DPP64, corresponding to 3c, 3g, 3k and 3o in solutions (a) and in solid states (b). Reproduced with permission [50]. Copyright 2021, Wiley-VCH.

  下载: 全尺寸图片 幻灯片
  

  Scheme 8  Synthetic routes for π-expanded DPPs based on N-aryl DPPs developed byWürthner et al. (a) [51], Yue et al. (b) [52], Gryko et al. (c) [49] and Zhang et al. (d) [50]. (e) Synthetic routes for the symmetric pyridine-flanked DPP complexes developed by Yamagata et al. [53].

  下载: 全尺寸图片 幻灯片
•