Article pubs.acs.org/crystal
Asymmetric Restriction of Intramolecular Rotation in Chiral Solvents Young-Jae Jin,† Hyojin Kim,†,‡ Jong Jin Kim,§ Nam Ho Heo,§ Jong Won Shin,*,∥ Masahiro Teraguchi,⊥ Takashi Kaneko,⊥ Toshiki Aoki,*,⊥ and Giseop Kwak*,† †
School of Applied Chemical Engineering, Major in Polymer Science and Engineering and §School of Applied Chemical Engineering, Major in Applied Chemistry, Kyungpook National University, 1370 Sankyuk-dong, Buk-ku, Daegu 702−701, Korea ‡ Daegu Technopark Nano Convergence Practical Application Center, 891-5 Daecheon-dong, Dalseo-ku, Daegu 704−801, Korea ∥ Beamline Department Pohang Accelerator Laboratory, POSTECH, 80 Jigokro-127-beongil, Nam-gu Pohang, Gyeongbuk, 37673, Korea ⊥ Department of Chemistry and Chemical Engineering, Graduate School of Science and Technology, and Center for Transdisciplinary Research, Niigata University, Ikarashi 2-8050, Nishi-ku, Niigata 950-2181, Japan S Supporting Information *
ABSTRACT: Commercially available molecular rotor (MR) compounds were recrystallized using chiral monoterpenes as solvents. The resulting crystals exhibited large circular dichroism signals with opposing signs according to the handedness of the chiral solvent used. X-ray crystallographic analysis showed that the chirality originated from asymmetric restriction of intramolecular rotation in the crystals. The crystals were also highly emissive due to restricted bond rotation, while solutions of the materials were almost nonemissive. The solvent-to-MR chirality transfer approach to crystallization discussed herein should be a convenient, universal way to obtain highly emissive chiral crystals.
emission” (AIE) or “crystallization-induced emission” (CIE); however, they are highly similar phenomena based on the restriction of intramolecular rotation (RIR).19−22 The RIR in MRs causes a significant emission enhancement based on a significant reduction in intermolecular stacking and an efficient suppression of collisional quenching and vibrational relaxation. Tang’s group has also demonstrated a unique combination of AIE and chirality in MRs. When the RIR-active MRs were further combined with certain chiral moieties, excellent CPL activities with high FL efficiencies and large dissymmetries could be attained in the aggregates.23−27 In chiral chemistry, on the other hand, some research groups have discovered a different phenomenon related to crystallization: some compounds have been shown to produce optically active enantiomorphous crystals simply by crystallization in common achiral solvents.28−33 This phenomenon is referred to as chiral crystallization, and it allows a topochemically controlled reaction termed “absolute asymmetric synthesis”.34−36 Chiral crystallization may be one of the easiest methods for developing high performance CD (or CPL)-active crystals, because it can produce target materials using commercially available achiral compounds without any further synthetic efforts. However, both enantiomorphous single
1. INTRODUCTION Optically active materials possessing circular dichroism (CD) and/or circularly polarized luminescence (CPL) have recently attracted considerable interest for potential application to advanced fields such as optoelectronic devices.1−8 These materials require a large dissymmetry factor in the solid phase to give high performance in these technologies. Organic π-conjugated molecules are one of the most promising classes of candidate materials for use in these advanced applications because of the synthetic variety, processability, and wide range of optoelectronic functions. However, organic fluorophores commonly form excimers when in bulk solids (such as crystals and aggregates) owing to the highly dense packing structure based on their intrinsic coplanar geometry, which leads to significant fluorescence (FL) quenching in the solid state. This optical quenching phenomenon may limit their future applications. Tang’s research group has overcome this problem through both synthetic and theoretical strategies related to molecular dynamics to develop many useful advanced materials.9−15 In particular, two series of propeller-like molecular rotor (MR) compounds, tetraphenylethylene (TPE) and silole derivatives, have been extensively studied. These MRs show limited emission in solutions, but when aggregated or crystallized, their FL emission intensity is enhanced by more than 2 orders of magnitude.16−18 Depending on whether the MRs are aggregated or crystallized, this enhancement is referred to as either “aggregation-induced © XXXX American Chemical Society
Received: January 25, 2016 Revised: March 30, 2016
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Figure 1. Chemical structures of (a) MRs and (b) chiral solvents used in this study, and (c) features of the MR crystals obtained from the chiral solvents.
parameters that were mismatched with those of the MRs. Moreover, it was determined that highly promising candidates for the chiral solvents would be commercially available but not expensive, with both (+)- and (−)-enantiomers available. Examining these criteria, monoterpenes, such as limonene and α-pinene, were selected as chiral solvents (Figure 1b) because these oily compounds that come from natural sources are commercially available and inexpensive.37−42 The most critical reason the monoterpenes were used as chiral solvents in this study was the difference in solubility parameters (δ) between the MRs (e.g., δ = 20.4 for TPE)43−45 and the monoterpenes (δ = 16.5 for limonene, 15.6 for α-pinene).46 As expected, the MRs examined did not dissolve in the chiral solvents at room temperature, while they started to dissolve in the solvents on heating to 130 °C, reaching a relatively high concentration of more than 5 wt %. Subsequently, when slowly cooled down to room temperature, highly emissive crystals with good features were obtained (Figure 1c). The TPE appeared as colorless and hemihedral crystals under normal light with large clusters more than 5 mm in diameter, while TPB and PPCPD appeared as colorless needle-like crystals and powdered clumps, respectively. The TPCPD gave nonregular red−violet-colored crystals. Unfortunately, the most aromatic MR used (HPB) hardly dissolved even in hot solvents, and as such was not suitable for recrystallization. Solid-state CD spectra can be measured in either the form of a KBr pellet, or more conveniently as a Nujol mull.47−49 Although the KBr and Nujol methods can discriminate between the enantiomorphs of chiral crystals, artifact CD signals often appear owing to the optical anisotropy of crystals. This means that the CD signals may be inaccurate. In fact, a KBr pellet showed a completely different CD signal as compared to a well polished crystal.50 Also, the KBr and Nujol methods do not guarantee a good CD spectrum owing to accidental experimental errors such as insufficient transparency of the specimen.51 On the other hand, DR-CD allows us to examinine randomly oriented chiral compounds without using either pressure, solvent, or diluting media, all of which can render the sample irrecoverable and may in some cases interact with the sample.52 This means that there are no alternative methods for examining pure solid compounds except DR-CD. Therefore, DR-CD was measured for all crystal samples in this study.
crystals are formed in the crystallization batch. Therefore, obtaining chiral crystals from totally achiral environments is not convenient due to the difficulty of differentiating between both enantiomorphous crystals.37−42 Although one enantiomeric component can be obtained preferentially by using one enantiomorphic crystal as a seed material, it is still hard to predict the possible occurrence of chiral crystallization, and hence, trial and error and many efforts are needed for finding new examples of chiral crystallization.28−33 In the case of MRs, however, if RIR always occurs asymmetrically under certain crystallization conditions, and the handedness can be controlled as desired, the problems mentioned above may be easily solved due to the discriminability and predictability of the induced chirality. The goal of this study was to develop a facile and universal crystallization method for producing highly emissive CD (or CPL)-active chiral crystals. The crystallization was conducted in chiral solvents using commercially available MR compounds. Considering the solubility parameters, monoterpenes were used as chiral solvents. A standard recrystallization technique was employed: hot, saturated solutions of MRs in the chiral solvents were slowly cooled to afford crystals. As desired, both chirality and FL emission were revealed in the condensed phases. The resulting crystals were perfect mirror images of each other, showing right- and left-handedness in diffuse reflectance (DR)CD spectroscopy according to the handedness of the chiral solvents used. X-ray crystallographic analysis showed that the chirality resulted from asymmetric RIR in the chiral environment. The crystals were also emissive due to this RIR. Herein, we describe the preparation, DR-CD, crystal structures, and FL emissions of the MR crystals, and suggest that the solvent-toMR chirality transfer approach to crystallization should be a convenient way to obtain highly emissive chiral crystals.
2. RESULTS AND DISCUSSION Several commercial MRs were tested for chiral crystallization: tetraphenylethylene (TPE), tetraphenylbutadiene (TPB), pentaphenyl cyclopentadiene (PPCPD), tetraphenyl cyclopentadienone (TPCPD), and hexaphenylbenzene (HPB), as shown in Figure 1a. These MRs are all fully aromatic compounds. To achieve chirality induction during the recrystallization, the solvents needed to be chiral with solubility B
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(+)-limonene showed a negative signal at the same wavelength. The crystals obtained from chiral α-pinenes showed almost the same CD spectra as those from chiral limonenes, although the CD intensity was slightly weaker (Figure S2). Other research groups have also reported the CD spectra of TPE crystals obtained from chiral crystallization in achiral solvents.57,58 These CD spectra, measured using KBr and Nujol mull methods, showed split-type signals with several Cotton bands, which were completely different from our results. This is most likely because of the difference in CD measurement method, as mentioned previously. Two other MRs that were successfully recrystallized, TPB and PPCPD, showed similar chiral resolution during crystallization in chiral solvents, while TPCPD showed very limited CD signals (Figure S3, Figure S4). There are several possible situations which would allow transfer of chirality to achiral organic compounds: (i) asymmetric restriction of rotation of bonds, (ii) one-handed helical coiling, and (iii) asymmetric stacking columnar arrangement.59 The examples found in this work likely belong to the first category, because the CD band of the TPE crystal exactly matched the UV absorption band of TPE in dilute solution, meaning that the chirality comes from the individual molecules. As TPE is a typical MR compound, it can be used as a model compound when thinking about the possible conformations of MRs. Figure 3 illustrates the four
Figure 2 shows the DR-CD and DR-UV spectra of the pure TPE crystal cluster obtained by recrystallization from chiral
Figure 2. DR-CD and DR-UV spectra of TPE crystals obtained from chiral limonenes, and CD and UV spectra of TPE in chiral limonene solutions.
limonene in comparison with the CD and UV spectra of TPE in chiral limonene solutions. It should be noted that, in order to obtain a true CD signal from a single crystal plane, the DR-CD was measured very carefully for the most flat plane of the crystal cluster, the area of which needed to be more than 3 × 3 mm2 to fit in the sample holder. Extremely large Cotton effects were seen in the CD spectra despite the fact that the TPE molecule has no stereogenic center. The TPE crystals showed the strongest CD signal at 320 nm as a first Cotton band, with the second Cotton band appearing at 280 nm, indicating a solventto-MR chirality transfer during the crystallization. The absence of significant modifications due to artifacts was confirmed by rotational CD measurements and linear dichroism (LD) responses (Figure S1). The chiral crystals reverted to achiral forms when dissolved in solvent.53 When the crystal was dissolved, the CD signal disappeared, indicating no chirality in the resulting solution. The TPE crystals showed dual UV absorption bands at 320 and 390 nm, while the solution exhibited a single absorption band at 320 nm. This indicates that the 320 nm band was due to the π−π* electronic transition in a single molecule, while the longer-wavelength band at 390 nm is ascribed to electronic transitions based on intermolecular interactions similar to those seen in the case of J-type aggregates.54−56 The DR-CD spectra of the TPE crystals also showed very weak but apparent mirror-image CD bands at around 400 nm, which accord well with the longer-wavelength absorption at 390 nm in the DR-UV spectra. Hence, the 400 nm CD band should be ascribed to an asymmetric intermolecularly stacked structure in the chiral crystal. Notably, the CD band of the TPE crystal matched the absorption band of the solution rather than that of the crystal itself. This indicates that the solvent-induced chirality was due to a conformational variation in the intramolecular structure, not due to an intermolecular hierarchical structure. This is likely due to asymmetric RIR during crystallization, as will be described later in the X-ray crystallographic analysis. The sign of the CD signal, and therefore whether the crystal showed (+)or (−)-handedness, was dependent on the absolute configuration of the chirality of limonene used. The two crystals obtained from (+)- and (−)-limonene showed opposing signs in the CD signals: the crystal obtained from (−)-limonene showed a positive CD signal at 320 nm while the crystal from
Figure 3. Four representative conformations of TPE (sky blue = ethylene plane, pink = plane of one of the phenyl rings).
representative conformations of TPE. Of these conformations, the two conformations having phenyl rings (a) perpendicular to and (c) parallel to the core ethylene plane are achiral due to the existence of a plane of symmetry, while the other two conformations (b and d), having phenyl rings twisted in different directions to each other and to the core plane, are chiral and enantiomeric to each other. At room temperature, any conformation is possible because of the sufficiently low rotational barriers of the phenyl rings.53 However, the fact that the TPE crystal was chiral hints that only conformation (b) or (d) remained after recrystallization from the chiral solvent. Single crystal X-ray diffraction measurements were performed to confirm this idea. Figure 4a show the crystal structures of TPE obtained from (+)- and (−)-limonene, respectively, which are almost same to those reported in the literature.60,61 The crystal structures of TPE belonged to the typical chiral space group P21, No. 4. The ORTEP images of TPE show that a four-winged propeller-like conformation was formed on the basis of the nonequivalent torsion angles between the ethylene core and the side phenyl rings. The schematic illustration of the two enantiomorphic crystals of TPE is shown in Figure 4b. The four CCC6H5 bonds are rotated in opposite directions in the two enantiomorphs, as C
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Figure 4. Crystal structures of TPE obtained from chiral limonenes: (a) ORTEP images with thermal ellipsoids plotted at the 50% probability level. All hydrogen atoms have been omitted for clarity. (b) Schematic illustration.
TPCPD. Moreover, the crystals were highly emissive while the solutions were almost nonemissive (Figure 1c). The absolute FL quantum yields of TPE, TPB, and PPCPD crystals were relatively high (23.4%, 91.2%, and 22.0%, respectively), several hundred or thousand times larger than those of the solutions (Table S2). The probability of nonradiative decay by routes such as collisional quenching and vibrational relaxation should be significantly reduced due to RIR in the solid state. These two features, large CD and intense FL, hint at an effective CPL in the condensed phase. Regrettably, however, the CPL could not be measured in this study owing to the significant anisotropy of the crystal bulk state. For the same reason, CPL of chiral crystals has yet to be measured by any other research groups. To reduce the effect of anisotropy on CPL as far as possible, larger single crystals with a sufficiently flat plane are required for front-face FL emission spectroscopy.
shown by the torsion angles (Table S1). The absolute configurations of TPE from (+)- and (−)-limonene were the (P)- and (M)-conformations, respectively, as determined by synchrotron radiation single crystal X-ray diffraction. The TPE crystals obtained from (+)- and (−)-pinene showed the same absolute configurations as TPE isolated from (+)- and (−)-limonenes (Table S1, Figures S5). Owing to the hierarchical chiral structure, the enantiomorphs of the TPE crystals could be distinguished by optical microscopy (Figure S6), as already reported.57 Differential scanning calorimetry (DSC) showed that the two enantiomorphs of TPE had the same melting point, at 239 °C (Figure S7). Unfortunately, the crystal system and space group of TPB could not be determined by single crystal X-ray diffraction because of the poor crystallinity of TPB. According to the literature, crystal structures of TPB are known to form two polymorphic crystals, the chiral α-phase (monoclinic, P21, No. 4) and the achiral βphase (monoclinic, P21/c, No. 14).62−64 The CD-active PPCPD isolated by recrystallization from chiral solvents showed very weak diffraction, indicating an amorphous state, while PPCPD crystals obtained from tetrahydrofuran are known to belong to the nonchiral space group P21/c (No. 14).65 TPCPD crystallized in the monoclinic C2/c space group (Figure S8), indicating no chirality in the crystal state, which is in accordance with the results of the DR-CD measurements. Similarly, TPCPD crystals obtained from ethyl acetate are known to belong to the nonchiral space group C2/c.66 As previously mentioned, the MR crystals recrystallized from chiral solvents were all CD active with the exception of
3. CONCLUSIONS We have demonstrated that the intramolecular rotation of MRs can be asymmetrically restricted in the recrystallization process as desired if appropriate chiral solvents are used. The MR crystals obtained from the recrystallizations showed large CD and high FL quantum efficiencies owing to asymmetric RIR. Exploring various combinations of MRs and chiral solvents for the recrystallization and developing new fabrication methods for the preparation of chiral crystals as films will further advance our approach, allowing us to provide highly enhanced CD- and CPL-active crystal films. D
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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/acs.cgd.6b00128. Additional data, including experimental section, DRCD, UV−vis, DSC, X-ray crystallography and FL (Figures S1−S8, Table S1−S3) (PDF) Accession Codes
CCDC 1433109−1433114 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through National Research Foundation of Korea (NRF) grants, funded by the Korean government (MEST) (No. 2014R1A2A1A11052446). Single crystal X-ray diffraction with PLS-II 2D-SMC beamline was supported by MSIP and POSTECH.
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