Selective Formation and Optical Property of a 21-Helical Columnar Fluorophore Composed of Achiral 2-Anthracenecarboxylic Acid and Benzylamine Yoshitane Imai,*,† Katuzo Murata,† Natsuyo Asano,‡ Yoko Nakano,§ Kakuhiro Kawaguchi,† Takunori Harada,⊥ Tomohiro Sato,⊥ Michiya Fujiki,§ Reiko Kuroda,‡,⊥ and Yoshio Matsubara*,†
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 9 3376–3379
Department of Applied Chemistry, Faculty of Science and Engineering, Kinki UniVersity, 3-4-1 Kowakae, Higashi-Osaka 577-8502, Japan, and Department of Life Sciences, Graduate School of Arts and Sciences, The UniVersity of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo, 153-8902, Japan, and Graduate School of Materials Science, Nara Institute of School and Technology Takayama, Ikoma, Nara, 630-0192, Japan, and JST ERATO-SORST Kuroda Chiromorphology Team, 4-7-6, Komaba, Meguro-ku, Tokyo, 153-0041, Japan. ReceiVed April 2, 2008; ReVised Manuscript ReceiVed May 20, 2008
ABSTRACT: By using achiral fluorescent 2-anthracenecarboxylic acid with achiral benzylamine, a chiral 21-helical columnar organic fluorophore having circularly polarized luminescence (CPL) in the solid state is created. Although this system exhibits polymorphism, the polymorphism can be controlled by changing the crystallization method. Introduction The origin and amplification of chirality that leads to an overwhelming enantioenrichment of organic molecules is a significant topic of interest in the field of chemistry. One of the proposed theories for the origin of chirality is the generation of chiral crystals from achiral molecules with each crystal exhibiting one of the two possible enantiomers.1 In other words, enantioenriched organic molecules can be formed and augmented by asymmetric reactions using chiral crystals obtained from achiral molecules. The second method involves the formation of enantioenriched molecules from racemic compounds by using interstellar circularly polarized luminescence (CPL).2 Recently, we developed a chiral supramolecular organic fluorophore having CPL properties in the solid state by combining two types of organic moleculessfluorescent 2-anthracenecarboxylic acid (1) and chiral (1R,2R)-1,2-diphenylethylenediamine.3 This complex has a 21-helical columnar hydrogen- and ionic-bonded network formed by the carboxylate oxygen of a carboxylic acid anion and the ammonium hydrogen of a protonated amine. Therefore, if a chiral complex is formed by using an achiral amine molecule instead of chiral 1,2diphenylethylenediamine, the obtained complex may have a 21helical column structure and CPL property without an outside chiral source.4 In this paper, we report the selective formation and solidstate optical properties of a chiral supramolecular organic fluorophore composed of two types of achiral organic molecules. Until now, although chiral supramolecular complexes composed of achiral organic molecules have been reported, the solid-state CPL spectrum of such a chiral supramolecular fluorophore has not been measured and its circular anisotropy factor has not appeared. Therefore, this study is a significant advancement in the research on solid-state chiral supramolecular fluorophores. * To whom correspondence should be addressed. (Y.I.) E-mail: y-imai@ apch.kindai.ac.jp. Fax: +81-6-6727-2024. Tel: +1-6-6730-5880 (Ext. 5241). (Y.M.) E-mail:
[email protected]. † Kinki University. ‡ The University of Tokyo. § NAIST. ⊥ JST.
Two achiral molecules are used in this studys2-anthracenecarboxylic acid (1), a fluorescent carboxylic acid, and benzylamine (2), an achiral amine molecule.
Experimental Section General Methods. All reagents were used directly as obtained commercially. Component molecules 1 and 2 were purchased from Tokyo Kasei Kogyo Co., Ltd. Solvent was purchased from Wako Pure Chemical Industry. Formation of Complex by Crystallization from Solution. 1 (10 mg, 0.045 mmol) and 2 (5 mg, 0.047 mmol) were dissolved in EtOH (2 mL). After a few days, colorless crystals I and II were deposited and collected. The total weight of all the crystals obtained in a batch is 6-8 mg. Formation of Complex by Solid-Vapor Crystallization. 1 (10 mg, 0.045 mmol) was left to stand in a vapor of 2 (100 mg, 1.0 mmol) in a sealed bottle at room temperature for a few days. Formation of Complex by Cogrinding Crystallization. 1 (10 mg, 0.045 mmol) and 2 (5.4 mg, 0.05 mmol) were directly ground together in an agate mortar at room temperature for a few minutes. The sample was then left to stand at room temperature for some duration to remove surplus 2. X-ray Crystallographic Study of Crystal I. X-ray diffraction data for single crystals were collected using BRUKER APEX. The crystal structures were solved by the direct method5 and refined by full-matrix least-squares using SHELX97.6 The diagrams were prepared using PLATON.7 Absorption corrections were performed using SADABS.8 Nonhydrogen atoms were refined with anisotropic displacement parameters, and hydrogen atoms were included in the models in their calculated positions in the riding model approximation. Crystallographic data of I: C15H10O2 · C7H9N, M ) 392.38, monoclinic, space group P21, a ) 9.3866(8), b ) 5.7927(5), c ) 15.8363(13) Å, β ) 106.3640(10)°, V ) 826.20(12) Å3, Z ) 2, Dc ) 1.324 g cm-3, µ(Mo KR) ) 0.085 mm-1, 5181 reflections measured, 3192 unique, final R(F2) ) 0.0422 using 2905 reflections with I < 2.0σ(I), R(all data) ) 0.0472, T ) 100(2) K, CCDC 672456. Crystallographic data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB21EZ, UK; fax: (+44)1223-336-033;
[email protected]). Measurement of XRD Spectra. X-ray powder patterns of crystals were corrected on a Rigaku RINT2500.
10.1021/cg800335t CCC: $40.75 2008 American Chemical Society Published on Web 07/26/2008
Properties of a 21-Helical Columnar Fluorophore
Figure 1. Crystal structures of complex I. (a) 21-Helical columnar hydrogen-bonded and ionic-bonded network parallel to the b-axis. (b) Packing structure observed along the b-axis.
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Figure 2. X-ray powder diffraction patterns of complexes (a) I and (b) II.
1 H NMR Study of Complex. 1H NMR spectra were recorded on a Varian Mercury M300 Spectrometer. Measurement of Solid-State Fluorescence Spectra. A solid-state fluorescence spectra and absolute photoluminescence quantum yield were measured by Absolute PL Quantum Yield Measurement System (C9920-02, HAMAMATSU PHOTONICS K. K.) under air atmosphere at room temperature. The excited wavelength is 322 nm in both complexes. Measurement of Solid-State CD and CPL Spectra. The CD spectrum was measured using a Jasco J-800KCM spectrophotometer. The CPL spectrum was measured using a Jasco CPL-200 spectrophotometer. The excited wavelength is 350 nm. The CPL spectrum is approached by simple moving average (SMA).
Results and Discussion The formation of a chiral supramolecular fluorophore was attempted via crystallization from an ethanol (EtOH) solution. A mixture of 1 and 2 was dissolved in the EtOH solution and left to stand at room temperature. After a few days, this system exhibited a type of polymorphism, and two kinds of colorless crystals composed of 1 and 2 without the EtOH molecule, I and II, were obtained. In order to study the crystal structure of these complexes, the X-ray crystallographic analysis of complex I was attempted. The crystal structure of complex I is shown in Figure 1. From the X-ray analysis, it is observed that this complex is a chiral crystal; that is, the stoichiometry of complex I is 1:2 ) 1:1 and the space group is P21. Expectedly, this crystal has a 21-helical columnar hydrogen- and ionic-bonded network along the b-axis (Figure 1a). This column is mainly formed by the carboxylate oxygen of a carboxylic acid anion (Figure 1, indicated by blue molecules) and the ammonium hydrogen of a protonated amine (Figure 1, indicated by green molecules). The complex is formed by the self-assembling of this 21-column (Figure 1b). Each column interacts via four types of anthraceneanthracene edge-to-face interactions (2.78, 2.80, 2.83, and, 2.85 Å,indicatedbyredarrowsinFigure1b)andonebenzene-anthracene edge-to-face interaction (2.69 Å, indicated by a purple arrow in Figure 1b).
Figure 3. CD spectra of complex I (solid line) and complex I′ (dotted line) in the solid state (KBr pellets).
On the other hand, since complex II was of an inferior quality, it was not possible to obtain its structural information from X-ray crystallographic analysis. Therefore, X-ray powder diffraction patterns of these crystals were measured and compared with that of complex I (Figure 2). Although 1H NMR analysis reveals that the stoichiometry of complex II is 1:2 ) 1:1, the X-ray powder diffraction pattern of complex II is different from that of complex I. Moreover, when a circular dichroism (CD) spectrum of crystal II was measured using a KBr pellet,9 its CD was not observed. These results suggest that although the stoichiometries of complexes I and II are the same, complex II has a different structure with complex I and is a racemic crystal. The most serious problem in solid-state organic fluorophores is fluorescence quenching in the crystalline state. In order to study the solid-state optical properties of chiral complex I, its solid-state fluorescence spectrum was measured. Complex I exhibited fluorescence without quenching in the solid state. The maximum value of the solid-state fluorescence for complex I (λem) is 446 nm and a shift (18 nm) to a lower wavelength is observed relative to that of fluorescent molecule 1. Interestingly,
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Figure 4. CPL and fluorescence spectra of complex I in the solid state (KBr pellet).
the absolute value of the photoluminescence quantum yield in complex I (ΦF ) 0.16) is 4 times greater than that of molecule 1 in the solid state. Although the crystal structure of molecule 1 is not known, molecule 1 may exist as a dimer having a hydrogen bond in the solid state. From the crystal structure of complex I, it can be inferred that one of the reasons for an increase in the photoluminescence quantum yield by complexation is the suppression of the concomitant nonradiative processes by changing the bonding style of the fluorescence molecule 1 from a hydrogen bond to a hydrogen- and ionicbonded network. In the same matter, when the solid-state fluorescence spectrum of complex II was measured, the solidstate fluorescence maximum and the photoluminescence quantum yield in complex II are almost same as those in chiral complex I (λem ) 446 nm and ΦF ) 0.14, respectively). This result suggests that there is no significant difference in the relative position of the anthracene ring between complexes I and II. Fluorescent complex I is a chiral crystal; therefore, its fluorescence may display CPL. In order to study the CPL of complex I, the solid-state CD spectrum of complex I was measured by using KBr pellets (Figure. 3). The solid-state samples were prepared according to the standard procedure for obtaining glassy KBr matrices.9 Although the complex had very little interaction with the KBr matrix in this system and the maximum value of the solid-state fluorescence for complex I with the KBr matrix shifted slightly to 438 nm, the features in the CD spectrum originating from the anthracene unit are observed at 416 nm (solid line in Figure 3). The circular anisotropy (gCD ) ∆OD/OD) factor of the Cotton effect (λCD ) 416 nm) is approximately -1.0 × 10-3. In order to check if the crystal caused any artifacts in the spectrum, the CD spectrum of complex I′ with chirality opposite to that of complex I was measured.10 The spectrum was a mirror image of that of complex I (dotted line in Figure 3).
Consequently, the measurement of a solid-state CPL spectrum of complex I with a negative Cotton effect was attempted using KBr pellets.11 We successfully obtained the CPL spectrum of complex I, as shown in Figure 4. A negative CPL spectrum was obtained for complex I. The circular anisotropy [gem ) 2(IL - IR)/(IL + IR)] factor of complex I is approximately -1.1 × 10-3. The sign of the CPL spectrum is the same as that of the corresponding CD spectrum at the longest wavelength (416 nm); that is, the chirality of this complex in the ground-state is the same as that in the excited state. The origin of the CPL property can be explained by the crystal structure and by the theory of oscillator coupling,12 which suggest that the features of the CPL spectrum originating from the anthracene ring are mainly caused by the interactions of the anthracene rings between adjoining 21-helical columns. In this system, the occurrence of crystal polymorphism (crystals I and II) causes a problem. In order to control polymorphism, two other crystallization methods were attempted, as opposed to the typical method of crystallization using a solution. In the first method, crystallization was attempted using vapor conditions; that is, the sample was obtained by leaving molecule 1 to stand at room temperature in a vapor of liquid molecule 2. After a few days, the X-ray powder diffraction pattern of the sample was measured and compared with that of the chiral and racemic complexes I and II (Figure 5). Interestingly, it was found that the peaks observed in molecule 1 disappeared completely and a new set of peaks corresponding to the racemic complex II had appeared (Figure 5c). The second crystallization method used a mixture of 1 and 2 that was directly ground in an agate mortar. After grinding of the sample for a few minutes, the X-ray powder diffraction pattern of the sample was measured. Even in this case, the peaks of molecule 1 disappeared completely. Interestingly, as opposed to solid-vapor crystallization, the powder X-ray diffraction pattern showed that
Properties of a 21-Helical Columnar Fluorophore
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expected that such chiral supramolecular complex and complexation will be useful in the development of the novel solidstate chiral fluorophores. Acknowledgment. This work was supported by the Kansai Research Foundation for technology promotion. Supporting Information Available: X-ray crystallographic reports (CIF) of complex I. This information is available free of charge via the Internet at http://pubs.acs.org.
References
Figure 5. X-ray powder diffraction patterns of crystals (a) I, (b) II, (c) by solid-vapor crystallization, and (d) by cogrinding crystallization.
the obtained complex is chiral complex I (Figure 5d). These show that the polymorphism of this system can be perfectly controlled by the type of crystallization method. The detailed mechanism involved in solid-vapor crystallization is not known. However, it is inferred that since a hydrogen- and ionic-bonded network in complex is stronger than a hydrogen bond in only molecule 1, the complexation is automatically proceeded under solid-vapor condition. On the other hand, in the case of crystallization by the grinding of molecules or crystallization using a solution, that is, when molecule 1 is dissolved in liquid 2 or a crystallization solvent, it is believed that the formation of the chiral complex is caused by the polar effect of the surrounding molecules. Conclusions A chiral 21-helical columnar fluorophore was successfully created using achiral fluorescent 2-anthracenecarboxylic acid with achiral benzylamine. The solid-state photoluminescence quantum yield of this fluorophore was increased by a supramolecular complexation and solid-state CPL was successfully observed. Moreover, the polymorphism of this system could be controlled by changing the crystallization method. It is
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