English

• Circularly polarized luminescence (CPL) can provide significant information for advanced visual perception since it not only reveals excited-state properties of chiral systems, but also has a wide range of application prospects in chiral devices, enantioselective sensing systems, and biomedicine, such as 3D display, OLED materials, information storage and processing, CPL lasers and biological probes [1, 2]. Therefore, this field has received growing attention and become one of the hot spots in recent years. A core issue in developing CPL materials is achieving a large dissymmetric factor g_lum, which is calculated by g_lum = 2(I_L – I_R)/ (I_L – I_R), where I_L and I_R are the intensity of left- and right-handed circularly polarized emission intensity, respectively. The maximum value of g_lum is +2 or −2, which represents complete left- or right-handed CPL, respectively. Though having tunable emission and high emission efficiency, organic CPL systems usually present small g_lum of around 10⁻⁵~10⁻², which hampers their applications. Various approaches have been developed to amplify asymmetric factors in organic systems, such as modification of highly emissive chromophore molecules with chiral groups, CPL induction by doping chiral molecules, assembly in supramolecular structures, photophysical process regulation including cascade energy transfer, charge transfer and photochemical upconversion [3-11].

  To build an efficient CPL system, transfer and control of chirality from the molecular level to nanoscale and even macroscopic materials is the first step and the strategy of chiral co-assembly was proven to be effective [1]. In addition, the magnetic dipole moments and electric dipole moments of the excited organic molecules are sensitive to photophysical processes, through which CPL signals can be amplified [7]. Photon upconversion based on triplet-triplet annihilation upconversion (TTA-UC) involves photosensitizers and triplet energy acceptors and series energy transfer processes such as sensitization, triplet energy transfer and triplet-triplet annihilation [12-15]. Combination of TTA-UC technology and CPL design can construct upconversion CPL systems with amplified g_lum [4].

  With the rising demand for sustainable clean energy technologies, as well as solidified material for practical applications, developing CPL materials with large g_lum through non-sophisticated methods is still an exciting endeavor in CPL research. Herein, we report solid organic CPL systems with large g_lum by combining cellulose matrix and photon upconversion materials. Solid biomaterials such as cellulose are considered as promising chiral optical matrices [16]. Photon upconversion based on TTA-UC proceeds through a series of energy transfer processes where g_lum could be amplified. It is envisioned that integrating the chirality transfer from cellulose matrices and amplification during upconversion would construct materials with intriguing CPL properties. The photosensitizer and triplet energy acceptor of TTA-UC are physically doped into the chiral cellulose chain and solidified in the chiral nematic structure through noncovalent interactions during spin-coating. The chiral matrices transfer chirality to the chromophores which generate amplified CPL via TTA-UC. The g_lum of the solid TTA-UC CPL systems from the left-handed and the right-handed matrix are up to +0.1 and −0.15, respectively (Scheme 1).

  Scheme 1

  

  Scheme 1.  Illustration of the TTA-UC CPL system based on the cellulose matrix and the spin-coating preparation.

  DownLoad: Full-Size Img PowerPoint

  Ethyl cellulose (EC) or (acetyl) ethyl cellulose (AEC) with chiral skeleton and rigid helical structure can form a chiral nematic structure by self-assembling. The TTA-UC photosensitizer is octaethyl porphyrin platinum(Ⅱ) (PtOEP) and the triplet energy acceptor is 9, 10-diphenylanthracene (DPA) which can upconvert green excitation light into blue photons. The chloroform solution of the cellulose, PtOEP and DPA was prepared by spin coating onto glass slides, giving the TTA-UC CPL film. EC usually generates liquid crystal with a left-handed helical structure both in solvent and solid state [17-19]. The EC films without or with PtOEP and DPA doping were firstly investigated with polarized optical microscopy (POM) and scanning electron microscopy (SEM). For all EC films, clear birefringence phenomena displaying planar liquid crystal texture – Grandjean texture were observed under the POM as shown in Figs. 1a-d, indicating the helical axis of a chiral liquid crystal is parallel to the optical axis and the positive birefringence of EC liquid crystal [19]. Moreover, no chromophore crystal was observed in the transparent chiral EC films suggesting the good dispersity of chromophores in EC matrix. Brittle fractured by liquid nitrogen, the cross-sectional SEM images of the EC films present a rough cascaded fracture surface in Figs. 1e-h. The rich pore-like structure in microscale and certain long-range oriented texture in mesoscopic scale is mainly due to the twisted helical morphology and the rigidity of molecular chains of EC [20]. The possible interaction between the chromophores and the cellulose matrix was analyzed by attenuated total refraction Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray diffraction (XRD), respectively. The FTIR spectra of all samples show no evident difference (Fig. S2 in Supporting information) to the pure EC film. The XRD patterns of all samples (Fig. S3 in Supporting information) are broad and exhibit two wide amorphous diffraction peaks of the cellulose Ⅱ at around 10.5° and 20.5° [21, 22]. The results indicate that EC is mainly in an amorphous state. The chromophores interact with the matrix mainly by weak van der Waals forces, to form homogeneous films during solidification by spin-coating [23].

  Figure 1

  

  Figure 1.  The POM images viewed in polarization mode in the dark field (a-d) and cross-sectional SEM images (e-h) of pure EC, PtOEP@EC, DPA@EC, and PtOEP/DPA@EC films prepared by spin-coating. [DPA/EC] = 0.2 wt%, [PtOEP/EC] = 0.01 wt%.

  DownLoad: Full-Size Img PowerPoint

  The upconversion emission spectra of those PtOEP/DPA@EC films were tested upon excitation by a 532 nm laser at 200 mW/cm². A series of transparent PtOEP/DPA@EC films are prepared by fixing the molar ratio of DPA: PtOEP at 500:1, and increasing the doping concentration of DPA/EC from 0.25 wt% to 12.5 wt%. When the doping concentration of DPA ≥ 8.75 wt%, DPA began to segregate in EC film which hinders the chiral assembly process and weakens the TTA-UC intensity (Fig. S4 Supporting information). The chiral properties of PtOEP/DPA@EC film with optimal doping content of [DPA/EC] = 8.75 wt% were further investigated.

  The CD spectra of DPA@EC film, PtOEP@EC film, and PtOEP/DPA@EC film were collected, respectively. In Figs. 2a-c, the CD signal at the maximum absorption of the triplet energy acceptor DPA at 340, 356, 375, 395 nm, and the photosensitizer PtOEP at 381 and 533 nm exhibited positive Cotton effect, respectively, which is consistent with the pure EC film. When a beam of circularly polarized light enters the EC film, the left-handed EC films prefer to reflect the left-handed circularly polarized light, so that the electric vibration vector of the light wave presents a positive left-handed signal [24]. CD spectra reveal the chiral absorption signal of the chromophore induced by the chiral matrix. In general, the interference of macroscopic anisotropies usually presents in solid samples, i.e., linear dichroism (LD) and linear birefringence (LB) properties providing certain contributions to the measured CD spectrum in thin films. Therefore, the crude CD signal of such a solid film is the sum of various contributions [25]. To exclude the interference of linear polarization absorption, CD and its synchronous linear dichroism (LD) were collected when the film is rotated by various degrees of 0°, 90°, 180°, 270° and 360°, respectively (Fig. S5 in Supporting information). The circular dichroic absorption signal of the film is far stronger than its linear polarization signal. And the original CD is similar to the flipped (rotated by 180°) sample, indicative of negligible LD and LB contribution.

  Figure 2

  

  Figure 2.  (a-c) CD spectra of DPA@EC film, PtOEP@EC film, PtOEP/DPA@EC film, respectively. (d-f) CPL spectra of DPA@EC film (λ_ex = 320 nm), PtOEP@EC film (λ_ex = 380 nm), PtOEP/DPA@EC film (λ_ex = 532 nm), respectively.

  DownLoad: Full-Size Img PowerPoint

  The CPL spectra of DPA@EC, PtOEP@EC, and PtOEP/DPA@EC films were measured by a CPL-200 spectrometer, respectively. Obvious positive CPL signals of all EC films are observed as shown in Figs. 2d-f, indicating the CPL activity of chromophores endowed by the EC matrix. The g_lum values of DPA@EC, PtOEP@EC, and PtOEP/DPA@EC films are calculated to be +0.057, +0.04 and +0.10, respectively [26-28]. The TTA-UC film presents a significant enhancement in g_lum value due to the involvement of series energy transfer process during photon upconversion.

  It has practical significance to achieve different chiral properties of chiral systems by controlling the chiral matrix. Since the commercial EC exhibits single left-handed chirality in chloroform. To construct CPL systems with different chirality, we prepared the EC derivative matrix AEC of different chirality by modifying its side chain group through a one-step acetylation reaction (Fig. S6 in Supporting information). FTIR (Fig. S7 in Supporting information) confirmed the -OH group of EC has been partially acetylated to produce AEC. With the increase of acetyl substitution, the helical distance of the cellulose chains increases, which leads to the decreasing twisting power of the chiral nematic mesophase. As a consequence, the originally left-handed helical structure turns into a right-handed arrangement [21]. Whether the CD signal of AEC is negative or not is related to the degree of its acetylation. The AEC sample prepared by 5 h acetylation of EC exhibits extensive negative signal compared to the EC film (Fig. 3a).

  Figure 3

  

  Figure 3.  (a) CD spectra of the pure EC film and AEC film. (b) CD spectrum of PtOEP/DPA@AEC film. (c) Upconversion CPL spectra of PtOEP/DPA@AEC film excited by a 532 nm laser. (d) Comparison of absolute glum values of DPA@EC, PtOEP/DPA@EC, DPA@AEC, PtOEP/DPA@AEC films.

  DownLoad: Full-Size Img PowerPoint

  Chloroform solution of AEC was spin-coated on glass giving a thin film of the right-handed helical structure. Grandjean planar liquid crystal texture can be observed in pure AEC, DPA@AEC, PtOEP@AEC and PtOEP/DPA@AEC films under POM (Fig. S8 in Supporting information). Within the allowable range of errors, the CD signals of series of AEC films are negative (Fig. 3b, Figs. S9a and b in Supporting information), showing a significant negative Cotton effect. The reflective bands of negative chirality of AEC films indicate the characteristics of the right-handed chiral structure, which is opposite to all CD signals of films prepared with EC [29]. The doped AEC films present a negative CPL which is opposite to that of doped EC films. The g_lum of CPL from DPA@AEC, PtOEP@AEC, and PtOEP/DPA@AEC films are measured to be −0.04, −0.017, and −0.15, respectively (Fig. 3c and Fig. S9 in Supporting information). It is worth noticing that both in the EC or the AEC films, the g_lum of the upconversion system is significantly larger than the films doped with DPA or PtOEP only (Fig. 3d). the amplification of g_lum in TTA-UC systems can be explained in the amplified magnetic-dipole transition moment through the TTA process and depressed electric-dipole moments by inter- or intramolecular radiationless energy transfer, both of which resulted in the increase of CPL g_lum in the TTA-UC system [30].

  In summary, organic CPL systems with large g_lum based on cellulose matrix and photon upconversion chromophores were constructed by simply spin-coating. The achiral sensitizer and acceptor of TTA-UC are endowed with chirality from the cellulose matrix and CPL from the chromophore is significantly amplified owing to the energy transfer processes of TTA-UC. By applying the left-handed ethyl cellulose or the right-handed (acetyl) ethyl cellulose as the chiral matrix, CPL g_lum up to +0.1 and −0.15 are achieved, respectively. This work proposes a straightforward approach for constructing solid organic upconversion CPL materials with large g_lum, which expands the research scope and application potentials of organic chiroptical materials.

  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 are grateful for the financial support from the National Natural Science Foundation of China (Nos. 21673264, 21573266, 21672226 and 22090012) and the Youth Innovation Promotion Association of the Chinese Academy of Sciences (Nos. 2017032 and 2020035).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.06.072.

  
  
• 1. [1]

     Y. Sang, J. Han, T. Zhao, P. Duan, M. Liu, Adv. Mater. 32 (2020) e1900110. doi: 10.1002/adma.201900110

  2. [2]

     Z.L. Gong, X.F. Zhu, Z.H. et al., Sci. China Chem. 64 (2021) 2060–2104. doi: 10.1007/s11426-021-1146-6

  3. [3]

     W.L. Zhao, M. Li, H.Y. Lu, C.F. Chen, Chem. Commun. 55 (2019) 13793–13803. doi: 10.1039/c9cc06861a

  4. [4]

     D. Yang, J. Han, M. Liu, P. Duan, Adv. Mater. 31 (2019) e1805683. doi: 10.1002/adma.201805683

  5. [5]

     X.J. Qin, J.L. Han, D. Yang, et al., Chin. Chem. Lett. 30 (2019) 1923–1926. doi: 10.1016/j.cclet.2019.04.035

  6. [6]

     C.Y. Wang, T. Jiang, X. Ma, Chin. Chem. Lett. 31 (2020) 2921–2924. doi: 10.1016/j.cclet.2020.03.021

  7. [7]

     T. Zhao, J. Han, P. Duan, M. Liu, Acc. Chem. Res. 53 (2020) 1279–1292. doi: 10.1021/acs.accounts.0c00112

  8. [8]

     L. Frederic, A. Desmarchelier, L. Favereau, G. Pieters, Adv. Funct. Mater. 31 (2021) e2010281. doi: 10.1002/adfm.202010281

  9. [9]

     X.Y. Zhang, Z.R. Xu, Y. Zhang, Y.W. Quan, Y.X. Cheng, ACS Appl. Mater. Inter. 13 (2021) 55420–55427. doi: 10.1021/acsami.1c18392

  10. [10]

      W. Zhang, B.N. Ni, H.K. et al., Mater. Chem. Front. 5 (2021) 5471–5477. doi: 10.1039/d1qm00482d

  11. [11]

      L. Yuan, Q.J. Ding, Z.L. Tu, et al., Chin. Chem. Lett., 33 (2022) 1459–1462. doi: 10.1016/j.cclet.2021.08.104

  12. [12]

      C.Y. Fan, L.L. Wei, T. Niu, et al., J. Am. Chem. Soc. 141 (2019) 15070–15077. doi: 10.1021/jacs.9b05824

  13. [13]

      L. Li, Y. Zeng, J.P. Chen, et al., J. Phys. Chem. Lett. 10 (2019) 6239–6245. doi: 10.1021/acs.jpclett.9b02393

  14. [14]

      W.X. Yin, T.J. Yu, J.P. Chen, et al., ACS Appl. Mater. Inter. 13 (2021) 57481–57488. doi: 10.1021/acsami.1c19181

  15. [15]

      X. Xiao, W. Tian, M. Imran, H.M. Cao, J.Z. Zhao, Chem. Soc. Rev. 50 (2021) 9686–9714. doi: 10.1039/d1cs00162k

  16. [16]

      C. Duan, Z. Cheng, B. Wang, et al., Small (2021) e2007306.

  17. [17]

      J. Bheda, J.F. Fellers, J.L. White, Colloid Polym. Sci. 258 (1980) 1335–1342. doi: 10.1007/BF01668781

  18. [18]

      L. Yan, Q. Zhu, T. Ikeda, J. Appl. Polym. Sci. 82 (2001) 2770–2774. doi: 10.1002/app.2130

  19. [19]

      V.K. Thakur, Nanocellulose Polymer Nanocomposites: Fundamentals and Applications, Wiley Scrivener, 2014.

  20. [20]

      A. Thomas, M. Antonietti, Adv. Funct. Mater. 13 (2003) 763–766. doi: 10.1002/adfm.200304383

  21. [21]

      O.J. Rojas, Cellulose Chemistry and Properties: Fibers, Nanocelluloses and Advanced Materials, Springer, 2016.

  22. [22]

      W. Li, M. Xu, C. Ma, et al., ACS Appl. Mater. Inter. 11 (2019) 23512–23519. doi: 10.1021/acsami.9b05941

  23. [23]

      B. Wu, H. Wu, Y. Zhou, et al., Angew. Chem. Int. Ed. 60 (2021) 3672–3678. doi: 10.1002/anie.202012224

  24. [24]

      Y. Nishio, Y. Fujiki, J. Macromol. Sci. B 30 (2006) 357–384.

  25. [25]

      G. Albano, G. Pescitelli, L. Di Bari, Chem. Rev. 120 (2020) 10145–10243. doi: 10.1021/acs.chemrev.0c00195

  26. [26]

      J. Wang, G. Zhuang, M. Chen, et al., Angew. Chem. Int. Ed. 59 (2020) 1619–1626. doi: 10.1002/anie.201909401

  27. [27]

      D. Han, X. Yang, J. Han, J. Zhou, T. Jiao, P. Duan, Nat. Commun. 11 (2020) 5659. doi: 10.1038/s41467-020-19479-1

  28. [28]

      S. Huo, P. Duan, T. Jiao, Q. Peng, M. Liu, Angew. Chem. Int. Ed. 56 (2017) 12174–12178. doi: 10.1002/anie.201706308

  29. [29]

      J.X. Guo, D.G. Gray, J. Polym. Sci. Pol. Phys. 32 (1994) 2529–2537. doi: 10.1002/polb.1994.090321510

  30. [30]

      J. Han, P. Duan, X. Li, M. Liu, J. Am. Chem. Soc. 139 (2017) 9783–9786. doi: 10.1021/jacs.7b04611

• 

  Scheme 1  Illustration of the TTA-UC CPL system based on the cellulose matrix and the spin-coating preparation.

  下载: 全尺寸图片 幻灯片
  

  Figure 1  The POM images viewed in polarization mode in the dark field (a-d) and cross-sectional SEM images (e-h) of pure EC, PtOEP@EC, DPA@EC, and PtOEP/DPA@EC films prepared by spin-coating. [DPA/EC] = 0.2 wt%, [PtOEP/EC] = 0.01 wt%.

  下载: 全尺寸图片 幻灯片
  

  Figure 2  (a-c) CD spectra of DPA@EC film, PtOEP@EC film, PtOEP/DPA@EC film, respectively. (d-f) CPL spectra of DPA@EC film (λ_ex = 320 nm), PtOEP@EC film (λ_ex = 380 nm), PtOEP/DPA@EC film (λ_ex = 532 nm), respectively.

  下载: 全尺寸图片 幻灯片
  

  Figure 3  (a) CD spectra of the pure EC film and AEC film. (b) CD spectrum of PtOEP/DPA@AEC film. (c) Upconversion CPL spectra of PtOEP/DPA@AEC film excited by a 532 nm laser. (d) Comparison of absolute glum values of DPA@EC, PtOEP/DPA@EC, DPA@AEC, PtOEP/DPA@AEC films.

  下载: 全尺寸图片 幻灯片
•