Letter Cite This: ACS Macro Lett. 2018, 7, 364−369
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Circularly Polarized Luminescent Triptycene-Based Polymers Tomoyuki Ikai,*,† Takumu Yoshida,† Seiya Awata,† Yuya Wada,† Katsuhiro Maeda,† Motohiro Mizuno,† and Timothy M. Swager*,‡ †
Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan Department of Chemistry, Massachusetts Institute of Technology (MIT), 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States
‡
S Supporting Information *
ABSTRACT: A series of chiral fluorescent polymers containing optically active triptycene units in the main chain were synthesized via Sonogashira−Hagihara coupling copolymerizations of (R,R)- or (S,S)-2,6-diethynyltriptycene with a range of diiodoaryls. Their optical and chiroptical properties were investigated under various solution conditions. We observe that these polymers emitted circularly polarized light owing to the chiral triptycene framework with a handed twisted rigid structure, and the fluorescence colors could be tuned in the whole visible range from blue to red by replacing the achiral comonomer units. They possessed luminescence dissymmetry factors of approximately 1.0 × 10−3, regardless of the incorporated comonomer units.
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control of the temperature, solvent, and concentration to optimally produce the CPL active aggregates. From a practical viewpoint, it is important to develop new molecular designs to create materials displaying intrinsically high CPL that is independent of extrinsic influences. In this study, we report triptycene-based functional materials with environment-independent chiroptical properties, and the synthesis of a series of optically active polymers (poly-2−poly5) via copolymerization of enantiopure 2,6-diethynyltriptycene ((R,R)- or (S,S)-1) with a range of diiodoaryls (Figure 1A). We demonstrate that the chiral 2,6-diethynyltriptycene framework with a handed twisted rigid structure shows great potential for the development of the CPL materials with desirable fluorescence colors ranging from blue to red. (R,R)- and (S,S)-1 were synthesized from rac-2,6-diiodotriptycene via a resolution step using chiral high-performance liquid chromatography as detailed in Scheme S1. Baseline resolution of rac-1 was achieved using Chiralpak IG as shown in Figure 2C, and both fractions were collected. We confirmed that the two isolated components exhibited the mirror image circular dichroism (CD) spectra (Figure 2D), both with optical purities greater than 99% ee (Figure 2A,B). The absolute configurations of the first- and second-eluted components in Figure 2C were assigned to be 9R,10R and 9S,10S, respectively. This assignment was based on the CD spectral analysis of (S,S)1 separately prepared from a reported triptycene derivative with a 9S,10S configuration, whose absolute configuration had been
ircularly polarized luminescence (CPL) can enable a variety of technologies, including 3D displays, optical storage devices, security tags, and biological probes.1−5 To date, the range of materials that exhibit CPL include optically active lanthanide complexes1,4 and organic (macro)molecules.3,6−10 Nevertheless, CPL materials are still at a relatively early stage and further development is required to understand the relationships between molecular structures and the magnitude of CPL in order to design materials with performance necessary to enable demanding applications. Triptycene is an aromatic hydrocarbon that consists of three homoconjugated benzene rings fixed in a [2.2.2] ring system that constitutes a paddle-wheel shape.11 Taking advantage of its unique structural features, including a bulky and robust framework and extremely limited conformational freedom, a larger number of triptycene-based functional materials,12−16 such as sensors,17−21 gas transport/storage,22−26 functional ligands,27−33 liquid crystals,34−38 and molecular machines,39−44 have been developed. These properties have further inspired the development of optically active triptycenes for chiral recognition and asymmetric synthesis.32,45 To create novel functional materials containing a triptycene framework, we have recently synthesized optically active triptycene derivatives containing pyrene-based π-conjugated units.46 We demonstrated that the resulting molecules emitted left- or right-handed circularly polarized light upon UV irradiation as a result of supramolecular chirality induced in hydrogen-bonded aggregates. However, for this small molecule emitter, CPL was only observed under the specific conditions wherein the materials are in an aggregated state. This feature complicates the formation of optical elements and limits application possibilities because of the requirement for precise © XXXX American Chemical Society
Received: February 7, 2018 Accepted: March 2, 2018
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DOI: 10.1021/acsmacrolett.8b00106 ACS Macro Lett. 2018, 7, 364−369
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ACS Macro Letters
Figure 1. (A) Synthesis of optically active poly-2−poly-5. (B) Structure of (S,S)-mono-2.
(DIPA) mixture at 60 °C (Figure 1A). The polymerization results are summarized in Table S1. The optically active materials, poly-2−poly-5, containing the chiral triptycene units in the main chain were obtained in moderate yields, with molecular masses (Mn) greater than 0.9 × 104 g mol−1 (estimated using size-exclusion chromatography (SEC)). These polymers contain a novel main-chain chirality derived from the triptycenes and display good solubility in common organic solvents, such as THF, chloroform, and chlorobenzene. For comparative studies, a monomeric model compound ((S,S)-mono-2, Figure 1B) related to (S,S)-poly-2 was also prepared, which contained the corresponding p-phenylenebased pendants at the 2,6-positions of the triptycene unit (Scheme S1). CD and absorption spectra of optically active poly-2−poly-5 in THF are shown in Figure 3. These polymers exhibited characteristic Cotton effects in the distinct absorption regions, reflecting the spectroscopic differences imparted by the incorporated comonomer units. We confirmed that the CD spectral patterns of (R,R)-poly-2−(R,R)-poly-5 were mirror images to those of (S,S)-poly-2−(S,S)-poly-5, as was anticipated from their enantiomeric relationships. As expected,
Figure 2. Elution profiles of (R,R)-1 (A), (S,S)-1 (B), and rac-1 (C) on Chiralpak IG (column, 25 cm × 0.46 cm (i.d.). The chromatograms depict UV traces recorded at 254 nm; eluent, hexane/ethyl acetate (9/ 1, v/v); flow rate, 0.5 mL min−1; temperature, ca. 20 °C). (D) CD and absorption spectra of the first- (red line) and second-eluted (blue line) components in THF at 25 °C. [1] = 1.0 × 10−4 M.
determined by the single crystal X-ray structure analysis (Figure S1).46 (R,R)- and (S,S)-1 were copolymerized with a series of diiodoaryl compounds (2−5) using Sonogashira−Hagihara coupling in a tetrahydrofuran (THF)/diisopropylamine
Figure 3. CD and absorption spectra of (S,S)-poly-2−(S,S)-poly-5 (solid line) and (R,R)-poly-2−(R,R)-poly-5 (dashed line) in THF at 25 °C. [Triptycene unit] = 1.0 × 10−5 M. 365
DOI: 10.1021/acsmacrolett.8b00106 ACS Macro Lett. 2018, 7, 364−369
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ACS Macro Letters
Figure 4. (A) Absorption (bottom), CD (middle), and gabs (top) spectra of (S,S)-poly-2 (red line) and (S,S)-mono-2 (blue line) in THF at 25 °C. [Triptycene unit] = 1.0 × 10−5 M. (B) Schematic illustration showing the chiral influence of the optically active triptycene units within (S,S)-poly-2. (C, D) SEC traces of as-synthesized (S,S)-poly-2 (Mn: 1.0 × 104 g mol−1, Mw/Mn: 1.7) (C) and its fractionated components with different molecular masses (D) (eluent, chloroform; polystyrene standards). (E) Molecular-mass dependence of the CD and absorption spectra of (S,S)-poly-2 in THF at 25 °C.
rac-poly-2, which was prepared from rac-1 and 2, had a similar absorption profile to those of the corresponding optically active polymers ((R,R)- and (S,S)-poly-2), but did not show any CD signals (Figure S2). On the other hand, (S,S)-mono-2 exhibited Cotton effects in the absorption region of the p-phenylenebased pendants (Figure 4A). The CD absorption of (S,S)mono-2 was most likely derived from the asymmetric arrangement of the π-conjugated pendants at the 2,6-positions of the triptycene unit. Considering the fact that the CD patterns observed for (S,S)-poly-2 and (S,S)-mono-2 were similar to one other, the chirality of (S,S)-poly-2 is expected to be exclusively caused by the optically active triptycene-based repeating units. In other words, a specific secondary structure (i.e., helical conformation) is not expected to be present in the (S,S)-poly-2 backbone. We also found that the CD intensity of (S,S)-poly-2 per molar absorption coefficient, defined as gabs (ΔAbs/Abs), was decidedly higher than that of (S,S)-mono-2
(Figure 4A). This is probably because individual chromophore units in the polymer chain were subject to chiral influences from the two adjacent triptycene units as illustrated in Figure 4B. This implies that the |gabs| value of the polymer would increase with increasing molecular mass until the effect of the chain end units can be ignored. If the molecular mass is sufficiently high, the |gabs| value of (S,S)-poly-2 is expected to be twice that of (S,S)-mono-2. To investigate the influence of the molecular mass on the chiroptical properties of the polymer, five samples of (S,S)-poly-2 with different molecular masses were prepared by SEC fractionation, and their CD spectra were measured (Figure 4C−E). The maximum |gabs| value of (S,S)poly-2 indeed changed depending on the molecular mass and approached an almost constant value when the molecular mass was greater than 1.0 × 104 g mol−1 (Figure S3). As anticipated, the saturation gabs value (−1.7 × 10−3) was approximately double that of the monomeric compound (S,S)-mono-2 (−0.9 366
DOI: 10.1021/acsmacrolett.8b00106 ACS Macro Lett. 2018, 7, 364−369
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ACS Macro Letters × 10−3). This result also supported the absence of a secondary helical structure in (S,S)-poly-2. To gain further insight into the chiral structure contained within the (S,S)-poly-2 backbone, a molecular model of (S,S)mono-2 was constructed using the Density Functional Theory method at the B3LYP level and the 6-31G (d,p) basis set.47 Two π-conjugated pendants on the triptycene unit were found to arrange in the manner as shown in Figure S4A. An asymmetric structure similar to the computer-generated model is considered to propagate along the (S,S)-poly-2 main chain, affording the nonlinear polymer backbone that is bent with a precise twist angle at every triptycene units (Figure S4B). Based on the fact that (S,S)-poly-2 does not form a regular higherorder structure, the chiral influence of the triptycene units is expected to be propagated only to the adjacent p-phenylene units, and importantly the torsion angles between the repeating units are not restricted. If the torsion angles were restricted by the introduction of intramolecular interactions, such as a hydrogen-bonding, metal coordination, and π−π interaction, the resulting polymers are anticipated to have one-handed helical structure and show unique characteristics. This work is now in progress in our laboratory. The optical and chiroptical properties of the polymers were found to be almost independent of the conditions under which they were measured (solvent, temperature, and concentration, Figures S5−S7). This implied that the relative arrangement of a pair of transition dipole moments around triptycene (Figure S4B) were unchanged regardless of the exterior environments, and are even immune from changing torsion angles between the repeating units, which are expected to vary temperature and solvent. Thus, the shape-persistent framework of triptycene appears to have a dominant role to provide the environmentindependent chiroptical properties for these polymers. Photographs of (S,S)-poly-2−(S,S)-poly-5 in solution under irradiation at 365 nm are shown in Figure 5A. These polymers exhibited blue, green, or red emission, which determined by the π-conjugated unit contained in the main chain. The fluorescence quantum yields of (S,S)-poly-2−(S,S)-poly-5 in THF were determined to be 61, 17, 24, and 40%, respectively. Given the chiroptical and photoluminescence (PL) behavior of the polymers, we investigated their CPL performance. The PL, CPL, and luminescence dissymmetry factor (glum) spectra of (R,R)- and (S,S)-poly-2−poly-5 in solution are shown in Figure 5B. Here, glum = 2(IL − IR)/(IL + IR), where IL and IR are the PL intensities of the left- and right-handed circularly polarized light, respectively. (R,R)- and (S,S)-Poly-2−poly-5 emitted leftand right-handed circularly polarized light, respectively, in a corresponding fluorescence region arising from the triptycene chirality. Again, the performance of the CPL from (S,S)-poly-2 (glum = −1.6 × 10−3) was superior to that of (S,S)-mono-2 (glum = −0.7 × 10−3), reflecting the effects of molecular mass as discussed above (Figure S8). This latter feature highlights the advantages of polymeric materials over low molar mass CPL molecules. As the glum value of (S,S)-poly-2 was similar to the corresponding gabs value, the chiral nature in the ground state is likely retained in the excited state. In addition, these triptycenebased polymers possessed almost constant |glum| values of approximately 1.0 × 10−3, regardless of the incorporated comonomer units. This indicates that the color of the fluorescence can be changed simply by replacing the achiral comonomer units without compromising the CPL properties. In summary, we have demonstrated new optically active polymers containing a chiral triptycene framework in the main
Figure 5. (A) Photograph of the THF solutions of (S,S)-poly-2−(S,S)poly-5 under irradiation at 365 nm. (B) PL (bottom), CPL (middle), and glum (top) spectra of (S,S)-poly-2−(S,S)-poly-5 (solid line) and (R,R)-poly-2−(R,R)-poly-5 (dashed line) in THF at room temperature. λex = 320 (poly-2 and poly-3), 302 (poly-4), or 420 nm (poly5). [Triptycene unit] = 1.0 × 10−5 M.
chain. These polymers exhibited CPL with glum values greater than 1.0 × 10−3 and the fluorescence color can be changed from blue to red by replacing the achiral comonomer units without any noticeable deterioration of the CPL performance. Chiral triptycenes possess highly desirable features, including a robust chiral structure with limited conformational freedom. Thus, we believe that triptycene-based optically active polymers capable of being applied to chiral sensing, asymmetric catalysis, and resolution can be rationally synthesized through an appropriate design of repeating units. Detailed investigations into the other novel features of these polymers, in addition to CPL, are currently in progress and will be reported in due course.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00106. Experimental procedures, characterizations of monomers and polymers, and additional spectroscopic and computational data (PDF). 367
DOI: 10.1021/acsmacrolett.8b00106 ACS Macro Lett. 2018, 7, 364−369
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Tomoyuki Ikai: 0000-0002-5211-2421 Katsuhiro Maeda: 0000-0002-5341-8799 Timothy M. Swager: 0000-0002-3577-0510 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Japan Society for the Promotion of Science (JSPS) Program for Advancing Strategic International Networks to Accelerate the Circulation of Talented Researchers (R2702) and the Grants-in-Aid for Scientific Research (C), Grant No. 17K05875. Work at MIT was supported in part by the National Science Foundation, DMR-1410718.
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