Tunable Multicolor Circularly Polarized Luminescence via Co-assembly of One Chiral Electron Acceptor with Various Donors

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLES8 Aug 2022Tunable Multicolor Circularly Polarized Luminescence via Co-assembly of One Chiral Electron Acceptor with Various Donors Fang Wang†, Chengshuo Shen†, Fuwei Gan, Guoli Zhang and Huibin Qiu Fang Wang† School of Chemistry and Chemical Engineering, Zhangjiang Institute for Advanced Study, Frontiers Science Center for Transformative Molecules, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240 Key Laboratory of Specially Functional Polymeric Materials and Related Technology (Ministry of Education), School of Material Science and Engineering, East China University of Science and Technology, Shanghai 200237 †F. Wang and C. Shen contributed equally to this work.Google Scholar More articles by this author , Chengshuo Shen† School of Chemistry and Chemical Engineering, Zhangjiang Institute for Advanced Study, Frontiers Science Center for Transformative Molecules, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240 †F. Wang and C. Shen contributed equally to this work.Google Scholar More articles by this author , Fuwei Gan School of Chemistry and Chemical Engineering, Zhangjiang Institute for Advanced Study, Frontiers Science Center for Transformative Molecules, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240 Google Scholar More articles by this author , Guoli Zhang School of Chemistry and Chemical Engineering, Zhangjiang Institute for Advanced Study, Frontiers Science Center for Transformative Molecules, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240 Google Scholar More articles by this author and Huibin Qiu *Corresponding author: E-mail Address: [email protected] School of Chemistry and Chemical Engineering, Zhangjiang Institute for Advanced Study, Frontiers Science Center for Transformative Molecules, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202202024 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Chiroptical materials with multicolor and sign-invertible circularly polarized luminescence (CPL) are important for advanced optical devices and display technologies. Here, a general strategy is developed to generate CPL with highly tunable emission bands and handedness through charge-transfer (CT) complexation and co-assembly of one chiral non-emissive tetranitrofluorene-based acceptor with various achiral purple to blue emissive donors. The resulting assemblies exhibit intense CPL with a rich array of colors (519–668 nm) and prominent dissymmetry factors (|glum|) in the range of 10−3–10−2. Notably, the CPL sign can be readily inverted by slightly changing the molecular structure of achiral donors. Single-crystal analysis reveals that the donor and acceptor molecules are alternately and asymmetrically packed in a lamellar fashion through CT interactions, leading to efficient transfer of chirality. Furthermore, the refined packing is mediated by the intensity and manner of CT interactions, rendering an inversion of chirality. The chiral co-assembly not only occurs for planar achiral donor molecules, but is also accessible to nonplanar conjugated molecules such as [4]helicene derivatives. Thus, the CPL feature of the resulting products can be easily and broadly manipulated, aiming at advanced chiroptical systems. Download figure Download PowerPoint Introduction Materials with circularly polarized luminescence (CPL) have attracted great attention due to their numerous potential applications in optical sensors, three-dimensional (3D) displays, optical data storage, photoelectric devices, asymmetric synthesis, and so on.1–7 One general route to prepare CPL-active materials is to combine chiral moieties with organic or inorganic luminophores, including small molecules,8–11 polymers,12–14 and metal complexes,15,16 through covalent bonds. This method requires a tedious synthetic process and the CPL characteristic is sometimes unpredictable. Self-assembly based on noncovalent interactions such as hydrogen bonding, π–π interactions, and electrostatic interactions offers an innovative access to CPL materials.17–22 Following this approach, not only chiral but also achiral luminophores can be self-assembled into chiral nanoassemblies to become CPL active.23,24 Nevertheless, the CPL color is currently critically limited in a single system due to the strong dependence of emission wavelength on the chemical structure of fluorescent molecules, and the CPL handedness is challenging to invert if using a constant chiral source.25–39 Charge-transfer (CT) interaction-based co-assembly of electron-rich donor and electron-deficient acceptor is emerging as a prospective approach in developing CPL materials.40–42 It offers distinctive advantages over the conventional self-assembly systems because (1) the luminescence dissymmetry factor (glum) values can be enhanced by the large magnetic dipole transition moment in the CT state,42 and (2) the assembly manner of the donor and acceptor molecules allows the manipulation of the CPL emission color and handedness by altering the CT interactions.43,44 Recently, Nishimura et al.41 developed a variety of oxygen-bridged diphenylnaphthylamine derivatives in which the CPL wavelength was dependent on the intramolecular CT state. Notably, Han et al.42 reported that the intermolecular CT complex consisting of a chiral emissive donor and an achiral electron acceptor shows bright CPL with a large glum value. Nevertheless, only minor success has been achieved in CPL active CT assemblies due to the structural mismatch between diverse molecules, which renders fluorescence quenching or inhibits the transfer of chirality.45–47 Moreover, the previously reported CPL active CT assemblies were solely formed by achiral acceptors with chiral luminescent donors.42,43 Herein, we developed a highly flexible and efficient strategy to produce finely tunable CPL in CT assemblies composed of one chiral non-emissive acceptor and various achiral donors. A chiral non-emissive acceptor methyl 2-(((2,4,5,7-tetranitro-9H-fluoren-9-ylidene)amino)oxy)propanoate ( A) was employed to form CT assemblies with a rich array of achiral planar ( P1– 9) and nonplanar ( NP1– 9) donors with short wavelength photoluminescence (PL). These CT assemblies exhibited multicolor CPL with relatively high dissymmetry factor (glum) values. Remarkably, the CPL emission color and handedness can be simultaneously manipulated by slightly changing the molecular structure of achiral donors (Figure 1a). This was predominately favored by the highly preferred and remarkably tunable chiral alternative packing of the donor and acceptor molecules (Figure 1b). Figure 1 | (a) Molecular structures of non-emissive chiral acceptor (S-A) and planar achiral donors (P), and photographs of corresponding CT aggregates formed in DMF/H2O (1/19, v/v) (S-A∶P = 2.6 mM∶2.6 mM, wavelength of excitation light λex = 365 nm) with different CPL color and handedness. l-CPL and r-CPL denote left- and right-handed CPL, respectively. (b) Illustration of a possible formation process of CT aggregates. Download figure Download PowerPoint Experimental Methods Preparation of aggregates Co-assembly of donor and acceptor molecules was triggered by a nanoprecipitation method. To a minimal aliquot of the concentrated stock solution of the donor and acceptor molecules in N,N-dimethylformamide (DMF) (good solvent) was added a large amount of deionized water (poor solvent) to create a supersaturating situation and initiate the co-assembly. In a typical process, the donor or acceptor molecules were first dissolved in 50 μL of DMF by vigorous shaking. Subsequently, 950 μL of water was added and the mixture was allowed to age at room temperature for at least 8 h before testing. Taking the S- A/ P1 aggregates as an example, 1.2 mg of S- A (2.6 × 10−3 mmol) and 0.3 mg of P1 (2.6 × 10−3 mmol) were dissolved in 50 μL of DMF by vigorous shaking in a septum-capped 5 mL glass vial. By adding 950 μL of water into the mixture, followed by aging at room temperature for at least 8 h, yellow-color aggregates were eventually obtained. Characterization NMR spectra were recorded on Bruker Advance III HD 600, HD 500, and HD 400 spectrometers (Bruker, Karlsruhe, Germany). UV–vis spectra were recorded on a Shimadzu UV-2600 spectrometer (Shimadzu, Kyoto, Japan) at 20 °C in a 10 mm quartz cell. Fluorescence spectra were measured using a Perkin Elmer LS 55 spectrometer (Perkin Elmer, Waltham, America). The absolute fluorescence quantum yield was measured using a PTI-QM/TM/IM steady-state and time-resolved fluorescence spectrofluorometer (PTI, America) with a calibrated integrating sphere and optical attenuator. Circular dichroism (CD) and CPL spectra of the suspension and solution samples were measured in quartz cuvettes (light path length 1 mm) on JASCO J-1500 and JASCO CPL-300 spectrophotometers (JASCO, Tokyo, Japan), respectively. Scanning electron microscopy (SEM) was performed on a JEOL JSM-7800F microscope (JEOL, Beijing, China) with an accelerating voltage of 10 kV. One drop of the as-prepared colloidal dispersion was deposited on a polished silicon wafer, followed by drying and coating with a thin layer of Au to enhance the contrast. Fluorescence microscopy images were obtained using a Leica TCS SP8 STED 3X confocal laser scanning microscope (Leica, Wetzlar, Germany). Crystallographic data were collected on a Bruker SMART Apex II CCD-based X-ray diffractometer (Bruker, Karlsruhe, Germany) with Mo-Kα radiation (λ = 0.71073 Å). Powder X-ray diffraction data were measured on a Bruker D8 Advance X-ray diffractometer (Bruker, Karlsruhe, Germany) operated in the 2θ range from 3.0° to 30.0° at room temperature. Results and Discussion Co-assembly of chiral electron acceptor with planar achiral donors Chiral acceptor molecules ( S- and R- A) were synthesized by integration of a chiral side group with an electron-deficient 2,4,5,7-tetranitrofluorene moiety ( Supporting Information Figures S1–S4 and Tables S1 and S2).48 In consideration of the chemical and structural compatibility, a series of relatively electron-rich planar molecules ( P) including naphthalene ( P1), phenanthrene ( P2), benzo[f]quinoline ( P3), phenanthridine ( P4), acridine ( P5), benzo[a]phenanthrene ( P6), naphtho[2,1-f]quinoline ( P7), triphenylene ( P8), and 1,2-bis(1-naphthyl)ethane ( P9) with purple to blue PL were screened to form CT complexes with the acceptor (Figure 1a). To prepare the co-assembled aggregates, S- or R- A and P (molar ratio 1∶1) were first dissolved in DMF, in which no emission was observed owing to the electron transfer ( Supporting Information Figure S13),49 and then an excessive amount of water was added (DMF/H2O = 1/19, v/v). Subsequently, colloidal suspensions with green to red emissions appeared, which were apparently different from the chiral acceptor (non-emission) or the achiral donors (emission wavelength range 320–426 nm) (Figure 1a and Supporting Information Figures S14 and S15). UV–vis absorption spectra revealed largely red-shifted bands in the region of 400–585 nm (Figure 2a and Supporting Information Figure S16), which can be attributed to the CT interactions from the P donors to S- A. For instance, the CT aggregates of S- A and P4 (abbreviated as S- A/ P4) showed a CT band in the region of 413–475 nm, while a CT band located in longer wavelength region (448–541 nm) was observed for the S- A/ P5 aggregates. The various CT aggregates were further confirmed by fluorescence spectra (Figure 2b and Supporting Information Figure S17), which revealed structureless and red-shifted emission bands. The extremum PL emission color of the CT aggregates varied from green (519 nm for P4) to yellow (552 nm for P3) to orange (588 nm for P2) to orange-red (598 nm for P5 and 612 nm for P1) as the donors were changed. As the number of aromatic rings of the donors increased to four ( P6, P7, P8, and P9), red color emission was observed for the CT aggregates with slightly different maximum emission peaks located at 634, 626, 627, and 624 nm, respectively. The quantum yields of the CT aggregates ranged from 0.35% to 1.33% ( Supporting Information Table S3). Apparently, the PL performance was critically dependent on the characteristics of CT interactions, which were finely modulated by the molecular structure of donor components including the number and spatial arrangement of the aromatic rings and the presence and position of the nitrogen atom in the backbone. Figure 2 | (a) UV–vis and (b) fluorescence spectra of P2–6 in DMF (λex = 280 nm for P2 and λex = 320 nm for P3–6, 0.04 mM) and corresponding CT aggregates formed with S-A in DMF/H2O (1/19, v/v) (λex = 360 nm, S-A∶P = 2.6 mM∶2.6 mM). (c) CD and (d) CPL spectra of CT aggregates formed by S- or R-A with P2–6 in DMF/H2O (1/19, v/v) (λex = 360 nm, S- or R-A∶P = 2.6 mM∶2.6 mM, S- or R-A:P5 = 0.26 mM∶0.26 mM for CD spectra). Download figure Download PowerPoint We further studied the chiroptical properties of the CT aggregates by CD. Mirror-image CD signals were observed for S- and R- A in the region of 250–410 nm ( Supporting Information Figure S18). No CD signal was observed for the individual P1– 9 donors due to their achiral nature ( Supporting Information Figure S19). When mixing S- A with P1– 9 in DMF, the CD spectra only reflected the characteristics of S- A ( Supporting Information Figure S20). However, in DMF/H2O (1/19, v/v, total concentration of 5.2 mM), strong mirror-image CD signals were detected, in which an obvious CD signal was observed corresponding to the wide CT absorption band beyond 410 nm for all the CT aggregates (Figure 2c and Supporting Information Figure S21). It appeared that the chirality localized at the carbon center can only be effectively transferred when S- or R- A form tightly packed aggregates with the donor molecules. To eliminate the possible influence of linear dichroism (LD) and light scattering, we evaluated the CD spectra of the CT aggregates at a lower concentration of 0.52 mM (DMF/H2O = 1/19, v/v). The lower-concentration CT aggregates showed almost identical Cotton effect bands, whereas the intensity of the baseline returned to a normal (low) level (Figure 2c and Supporting Information Figures S21 and S22). Notably, the CD bands for the CT aggregates became well resolved, probably due to elimination of the unfavorable scattering effects. The efficient transfer of chirality also rendered the fluorescent CT aggregates CPL active. As shown by the CPL spectra (Figure 2d and Supporting Information Figure S23), the S- A/ P2 aggregates showed negative CPL emissions (right-handed) with maximum at 592 nm, which switched to yellow (550 nm for S- A/ P3) and green (525 nm for S- A/ P4) CPLs with the same handedness when the phenanthrene skeleton was doped by nitrogen at position 1 and 9, respectively. By changing the spatial arrangement or the number of aromatic rings of the donors, CPL emissions were further moved to the red region (607 nm for S- A/ P5, 645 nm for S- A/ P6, and 636 nm for S- A/ P7) with positive signals (left-handed). Meanwhile, the S- A/ P1 and S- A/ P9 aggregates displayed positive CPL signals at 619 and 629 nm, respectively, while the S- A/ P8 aggregates showed a negative CPL signal at 617 nm, again reflecting an inversion of CPL handedness. These behaviors were further confirmed by the corresponding CT aggregates derived from the other enantiomer R- A. Notably, the S- or R- A/ P5 aggregates showed relatively high |glum| values of 2.1 × 10−2 while the other CT aggregates afforded glum at a 10−3 order of magnitude. We also found that the CT aggregates retained the CPL signs when they were recorded at lower concentration ( Supporting Information Figure S24). Overall, the CT aggregates not only exhibited CPL signals in the emission bands with extremum wavelength ranging from 525 to 645 nm, but also the intensity and handedness of CPL could be efficiently tuned by changing the molecular structures of the achiral donors. Exploring the mechanism of CPL emission Because the CT complexes exhibited intense ground- and excited-state chirality after aggregation, their structural features were also investigated. SEM images revealed that irregular nanostructures were formed for the individual P1– 7 and P9 aggregates, while hollow tubes with a diameter of ca. 340 nm were obtained for the P8 aggregates ( Supporting Information Figure S25). Meanwhile, the S- A and R- A aggregates were presented as long nanoribbons ( Supporting Information Figure S26). After co-assembly, nanoribbons were also observed for the CT aggregates of S- or R- A with P1, P3, P6, P7, or P8, in which S- or R- A/ P7 revealed the largest width of ca. 220 nm; the width of other CT aggregates essentially fell between 45 and 86 nm (Figure 3a and Supporting Information Figure S27). Plates with different widths ranging from 86 to 880 nm formed in the CT aggregates of S- or R- A with P2, P4, P5, or P9 (Figure 3a and Supporting Information Figure S28). Furthermore, these CT aggregates were also observed under fluorescent microscope and again showed relatively regular morphologies with shining green to red fluorescence (Figure 3b and Supporting Information Figure S29). Such well-defined aggregation shapes strongly indicate that the donors and acceptors may pack together in a highly ordered fashion. Figure 3 | (a) SEM and (b) fluorescence microscopy images of CT aggregates formed by S-A and P2–6 (S-A∶P = 2.6 mM∶2.6 mM) in DMF/H2O (1/19, v/v). Download figure Download PowerPoint To elucidate the origin of CPL, we attempted to grow single crystals of the CT complexes. Fortunately, single crystals of the S- A/ P2, S- A/ P4, S- A/ P5, and S- A/ P6 complexes were obtained from the dichloromethane/n-pentane mixtures by gaseous phase diffusion ( Supporting Information Tables S4–S7).a The single crystals and their corresponding powder-like aggregates showed analogous X-ray diffraction (XRD) patterns, strongly demonstrating the same types of molecular stacking (Figure 4c,f). Crystal structure analysis showed that all the repeating units consisted of S- A and donor molecules in a 1∶1 ratio (Figure 4a,b,d,e and Supporting Information Figures S30–S33). The donor and acceptor molecules solely formed regular, alternatively packed layers with relatively short intermolecular distances of 3.37–3.53 Å, reflecting a typical CT complexation feature. Notably, the alternative packings of the S- A/ P2, S- A/ P3, and S- A/ P4 aggregates were nearly identical and the handedness of their CPL emission stayed the same (Figures 2d and 4a–4f). In contrast, the S- A/ P5 and S- A/ P6 aggregates revealed distinct molecular stacking and the handedness of CPL was inverted. Figure 4 | (a, b, d, and e) Molecular packing structures of S-A/P2, S-A/P4, S-A/P5, and S-A/P6 crystals formed in dichloromethane/n-pentane. (c and f) Simulated powder XRD patterns of S-A/P2, S-A/P4, S-A/P5, and S-A/P6 crystals and measured powder XRD patterns of S-A/P2, S-A/P4, S-A/P5, and S-A/P6 aggregates (S-A∶P = 2.6 mM∶2.6 mM) obtained in DMF/H2O (1/19, v/v). (g) Calculated energy level diagrams and molecular orbital diagrams of different species. DFT calculations were performed at the PBE0-D3(BJ)/def2-SVP level of theory. Download figure Download PowerPoint We then studied the molecular orbitals of the CT complexes using simplified dimer models by density functional theory (DFT) calculations. Energy diagrams are shown in Figure 4g, Supporting Information Figures S34 and S44 and Table S8, where the highest occupied molecular orbitals (HOMOs) of the CT dimers lie closer to the HOMOs of the individual donors, whereas the lowest unoccupied molecular orbitals (LUMOs) are closer to the LUMOs of the individual acceptors, which indicate a strong CT transition. Moreover, the HOMO level of the CT complexes increases from −6.97 to −6.12 eV as the electron-donating ability becomes stronger in an order of P4, P2, P5, and P6. Consequently, the HOMO–LUMO gap can be tuned from 3.18 to 2.44 eV, and the emission colors of the CT aggregates can be switched from green to red. The distinct CT interactions between the chiral acceptor and various donors substantially determined the CPL properties of the aggregates. Co-assembly of chiral electron acceptor with nonplanar donors The general applicability of this strategy was further verified by nonplanar helically twisted donors with dynamical chirality, that is, [4]helicene ( NP1) and its derivatives including 2-aza[4]helicene ( NP2), 4-aza[4]helicene ( NP3), 4-ethynyl[4]helicene ( NP4), benzo[4]helicene ( NP5), 4-bromo[4]helicene ( NP6), 2-bromo[4]helicene ( NP7), 2-(4-pyridyl)[4]helicene ( NP8), and 2-((trimethylsilyl)ethynyl)[4]helicene ( NP9) ( NP, Figure 5a and Supporting Information Figures S5–S12). NP1– 9 revealed blue emission (λmax = 385–431 nm) in solution and aggregation states, whereas the CT complexes S- or R- A/ NP1– 9 showed almost no emission in DMF ( Supporting Information Figures S45 and S46). However, the CT aggregates ( S- and R- A/ NP4 formed gels) formed in DMF/H2O (1/19, v/v) were red emissive with quantum yields of 0.40–1.89% ( Supporting Information Table S9). UV–vis and fluorescence spectra support the formation of different CT interactions in these aggregates (Figure 5b,c and Supporting Information Figures S47 and S48). Furthermore, the morphology of the CT aggregates changed from rods ( NP1), to twisted ribbons ( NP2), to fusiform belts ( NP3), to intertwine fibers ( NP4, NP5, and NP8), and to flat ribbons ( NP6, NP7, and NP9) (Figure 5d,e and Supporting Information Figures S49–S52). Compared to the aggregates of sole NP molecules and the CT complexes in DMF ( Supporting Information Figures S53 and S54), the CT aggregates again showed significantly enhanced CD signals corresponding to the CT bands (Figure 5f and Supporting Information Figures S55 and S56). Figure 5 | (a) Chemical structures of nonplanar donor NP1–4 with blue emission. (b) UV–vis spectra, (f) CD spectra, (c) fluorescence spectra and photographs, and (g) CPL spectra and corresponding (h) glum values of CT aggregates formed by chiral S- or R-A acceptors with achiral NP1–4 donors (λex = 360 nm). (d) SEM images of S-A/NP1–4 aggregates. (e) Fluorescence microscopy images of S-A/NP1–3 aggregates. All the CT aggregates (S- or R-A∶NP = 2.6 mM∶2.6 mM) were prepared in DMF/H2O (1/19, v/v). Download figure Download PowerPoint As expected, the CT aggregates revealed red-color CPLs with tunable chirality upon slight change of the molecular structures of nonplanar donors. Left-handed CPLs were obtained by the CT aggregates of S- A with NP1, NP3, NP5, or NP9 at extremum wavelengths of 663, 631, 645, and 642 nm, respectively. While right-handed CPLs were obtained from the CT aggregates of S- A with NP2, NP4, NP6, NP7, or NP8 at extremum wavelengths of 641, 668, 663, 648, and 634 nm, respectively (Figure 5g,h and Supporting Information Figures S57 and S58). Mirror-image CPL signals were observed for the S- and R- A/ NP1– 9 aggregates and their corresponding CPL glum was 10−3–10−2 order of magnitude. Moreover, the possible experimental errors in the CD and CPL measurements (such as LD or light scattering) were eliminated by reducing the total concentration of the CT aggregates to 0.52 mM. Notably, the introduction of nonplanar donors in CT systems not only enriched the co-assembled morphology (e.g., quadrangular rods for S- or R- A/ NP1), but also allowed the generation of longer wavelength CPL (maximum = 668 nm vs 645 nm for the S - or R -A/P CT aggregates) with tunable intensity and handedness. Conclusion We have developed a facile and general strategy for rational preparation of color- and handedness-tunable CPL materials by combining one chiral acceptor with various achiral fluorescent donors through CT interaction-driven complexation and co-assembly. The obtained CT aggregates showed multicolor CPL with large |glum| in the range of 10−3–10−2. The tunable CT interactions between the chiral acceptor and achiral donors offer a highly flexible method to precisely control the color and handedness of CPL. Furthermore, regarding the abundant presence of variable commercially available donor molecules, numerous CPL materials can be expected by taking advantage of this demonstrated strategy. Footnote a CCDC 2121040–2121045 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif Supporting Information Supporting Information is available and includes experimental and computational details, crystallographic data, and supplementary tables and figures of all phases. Conflict of Interest There is no conflict of interest to report. Funding Information This work was financially supported by the National Key R&D Program of China (grant no. 2020YFA0908100), the National Natural Science Foundation of China (grant nos. 92056110 and 22075180), the Innovation Program of Shanghai Municipal Education Commission (grant no. 202101070002E00084), the Science and Technology Commission of Shanghai Municipality (grant nos. 20JC1415000 and 21XD1421900), and the China Postdoctoral Science Foundation (grant no. 2019M661480).