Novel Means of Controlling the Solid-State Circular Dichroism

Article pubs.acs.org/crystal

Novel Means of Controlling the Solid-State Circular Dichroism Property in a Supramolecular Organic Fluorophore Comprising 4-[2-(Methylphenyl)ethynyl]benzoic Acid by Varying the Position of the Methyl Substituent Noriaki Nishiguchi,† Takafumi Kinuta,† Tomohiro Sato,† Yoko Nakano,‡ Takunori Harada,§ Nobuo Tajima,⊥ Michiya Fujiki,‡ Reiko Kuroda,§ Yoshio Matsubara,† and Yoshitane Imai*,† †

Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan ‡ Graduate School of Materials Science Nara Institute of School and Technology Takayama, Ikoma, Nara 630-0192, Japan § Department of Life Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan ⊥ First-Principles Simulation Group, Computational Materials Science Center, NIMS, Sengen, Tsukuba, Ibaraki 305-0047, Japan S Supporting Information *

ABSTRACT: The solid-state circular dichroism (CD) of a 4-[2-(methylphenyl)ethynyl]benzoic acid/amine supramolecular organic fluorophore can be controlled by changing the position of the methyl substituent of the methylphenylethynyl unit on the achiral 4-[2-(methylphenyl)ethynyl]benzoic acid component molecule, instead of changing the chirality of the chiral amine component molecule.

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INTRODUCTION Generally, a chiral compound with the opposite chirality is required to control the sign of the optical properties of a given chiral compound. However, a chiral compound with opposite chirality may not be readily available. Therefore, it is desirable to develop a method of tuning the optical properties of a chiral compound without using an opposite chiral source when developing novel chiral systems. Solid-state fluorophores have recently attracted considerable attention, particularly two-component supramolecular organic fluorophores because their physical and chemical properties are easily controlled by changing the component molecules.1 However, most of the supramolecular organic fluorophores that have been reported are not chiral. Consequently, there have been few investigations involving solid-state circular dichroism (CD), which is one of the most basic and important solid-state chiral properties.2 We recently developed a solid-state π-conjugated chiral supramolecular organic fluorophore composed of chiral 1phenylethylamine (1) as the chiral component molecule and 4[2-(4-methylphenyl)ethynyl]benzoic acid (2) as the π-conjugated fluorescent component molecule.3 Because solid-state molecules are rigidly constrained, neighboring molecules exert a strong influence over them. Therefore, it is expected that control of the © 2012 American Chemical Society

solid-state CD of these supramolecular organic complexes may be accomplished by changing the packing arrangement, rather than the chirality, of the component molecules. In this study, we report the unconventional control of the solid-state CD of a 1/4-[2-(methylphenyl)ethynyl]benzoic acid supramolecular organic complex. The effect is achieved by changing the position of the methyl substituent on the methylphenylethynyl group of the achiral 4-[2-(methylphenyl)ethynyl]benzoic acid component, instead of changing the chirality of the amine component 1. We used (R)-(+)-1-phenylethylamine [(R)-1] as the chiral amine component molecule. The following two positional isomers of 2 were used as the achiral component molecules: 4-[2-(3-methylphenyl)ethynyl]benzoic acid (3) and 4-[2-(2-methylphenyl)ethynyl]benzoic acid (4).

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EXPERIMENTAL SECTION

General Methods. Component molecule (R)-1 was purchased from Tokyo Kasei Kogyo Co. The crystallization solvent was purchased from Wako Pure Chemical Industry. The solvent was used directly as commercially obtained. Received: November 14, 2011 Revised: February 15, 2012 Published: February 27, 2012 1859

dx.doi.org/10.1021/cg201496j | Cryst. Growth Des. 2012, 12, 1859−1864

Crystal Growth & Design

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geometries were fully optimized using this theory and basis set. These calculations were carried out with the Gaussian 0311 program.

Chart 1

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RESULTS AND DISCUSSION The (R)-1/3 [or (R)-1/4] chiral supramolecular organic complex was prepared by crystallization from an ethanol (EtOH) solution. Each complex was dissolved in EtOH and left to stand at room temperature. After one week, a large quantity of crystals of complex II [comprising (R)-1/3], or crystals of complex III [comprising (R)-1/4], was obtained. To study the control of the solid-state CD of the 1/4-[2(methylphenyl)ethynyl]benzoic acid supramolecular complex by changing the position of the methyl group on 4-[2-(methylphenyl)ethynyl]benzoic acid, we obtained the solid-state CD spectra of complexes II and III using KBr pellets. The obtained spectra were then compared with the spectrum of complex I. The solid-state CD and absorption spectra of complexes I,3 II, and III (indicated by black lines) are shown in Figure 1. The shapes of the CD spectra are similar. Characteristic peaks caused by the ethynylphenylene unit are observed between 299 and 332 nm. The circular anisotropy factors (|gCD = ΔOD/OD|) of the last Cotton effect (λCD = 320 nm for I, 319 nm for II, and 332 nm for III) for I, II, and III are approximately 1.0 × 10−3, 2.2 × 10−3, and 4.6 × 10−4, respectively. To check whether complexes I−III introduced any artifacts into the spectra, complexes I′−III′ were prepared, in which (R)-1 was replaced with (S)-(−)-1-phenylethylamine [(S)-1]. The solidstate CD spectra of complexes I′,3 II′, and III′ were then measured (indicated by gray lines in Figure 1). These CD spectra were found to be mirror images of the CD spectra of complexes I−III. These results indicate that chromophoric molecules in crystals have a chiral arrangement in complexes II and III, as in the case of complex I. Interestingly, the signs of the CD spectra of chiral complexes I−III (or I′−III′) are not the same, even though they are composed of the same chiral amine component molecule. That is, the sign of the CD spectrum of complex I at the longest wavelength is positive (+) (indicated by the black line in Figure 1a), whereas that of complex II is negative (−) (indicated by the black line in Figure 1b). Moreover, the longest wavelength of the Cotton effect of complex III (λCD = 332 nm) is clearly longer than that of complex II (λCD = 319 nm), and the sign of the CD spectrum of complex III at the longest wavelength is negative (−) (indicated by the black line in Figure 1c). To control the sign of the chiral optical properties, a chiral compound with opposite chirality is usually used. However, this shows that the solid-state CD of a 4-[2-(methylphenyl)ethynyl]benzoic acid/amine supramolecular organic fluorophore can be controlled by changing the position of the methyl group on the methylphenylethynyl unit of the achiral component molecule. To study the origin of the solid-state CD of these complexes, X-ray crystallographic analyses of complexes II and III were carried out, and the crystal structures of complexes I,3 II, and III were compared. The crystal structures of I are shown in Figure 2. The stoichiometry of I is (R)-1:2 = 1:1, and its space group is P21. This complex has a characteristic 21-helical columnar network structure along the b axis (Figure 2a,b). This column is mainly composed of carboxylate oxygen from the carboxylic acid anions and ammonium hydrogen from the protonated amine. Moreover, intracolumnar benzene−benzene edge-to-face interactions (2.96 Å, indicated by solid arrows A in Figure 2a) between the hydrogen atom of the benzene ring in 2 and the

Synthesis of Compounds 3 and 4. Component molecules 3 and 4 were prepared via a typical Sonogashira electronic cross-coupling reaction.4 1H NMR spectra were recorded using a Varian Mercury M300 spectrometer in acetone-d6 using tetramethylsilane as the internal standard. Component molecule 3: 1H NMR (300 MHz, CD3COCD3): δ = 8.07 (d, J = 8.1 Hz, 2H), 7.68 (d, J = 8.4 Hz, 2H), 7.42−7.25 (m, 4H) 2.38 (s, 3H). Component molecule 4: 1H NMR (300 MHz, CD3COCD3): δ = 8.08 (d, J = 8.4 Hz, 2H), 7.71 (d, J = 7.8 Hz, 2H), 7.55 (d, J = 7.2 Hz, 2H),7.35−7.24 (m, 2H), 2.53 (s, 3H). Formation of Complex by Crystallization from EtOH. (R)-1 (11 mg, 0.10 mmol) and 3 (19 mg, 0.08 mmol) [or 4 (19 mg, 0.08 mmol)] were dissolved in EtOH (3 mL) and left to stand at room temperature. After one week, a large quantity of crystals of II (16 mg) comprising (R)-1 and 3 (or crystals of III (15 mg) comprising (R)-1 and 4) was obtained. The weights given represent the total crop of the crystals obtained in a single batch. Measurement of Solid-State CD and Absorption Spectra. The solid-state CD and absorption spectra were measured using a Jasco J-800KCM spectrophotometer. The solid-state samples were prepared according to the standard procedure for obtaining glassy KBr matrices.5 X-ray Crystallographic Studies of Crystals II and III. X-ray diffraction data for single crystal of II were collected using BRUKER APEX. X-ray diffraction data for single crystals of III were collected using RIGAKU SATURN 70R. The crystal structures were solved by the direct method6 and refined by full-matrix least-squares using SHELXL97. 6 The diagrams were prepared using PLATON.7 Absorption corrections of II were performed using SADABS.8 Absorption corrections of III were performed using multiscan. 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 for II: C24H23N1O2, M = 357.43, space group P212121, a = 5.9953(5), b = 7.2024(6), c = 44.908(4) Å, V = 1939.2(3) Å3, Dc = 1.224 g/cm3, z = 4, μ(Mo Kα) = 0.077 mm−1, 17000 reflections measured, 4533 unique, final R(F2) = 0.0418 using 4312 reflections with I > 2.0σ(I), R(all data) = 0.0441, T = 115(2) K. CCDC 800734. Crystallographic data for III: C24H23N1O2, M = 357.43, space group P21, a = 7.2507(17), b = 6.0916(15), c = 22.013(5) Å, β = 92.607(3)°, V = 971.2(4) Å3, Dc = 1.222 g/cm3, z = 2, μ(Mo Kα) = 0.077 mm−1, 8724 reflections measured, 5161 unique, final R(F2) = 0.0391 using 4872 reflections with I > 2.0σ(I), R(all data) = 0.0413, T = 100(2) K. CCDC 800735. 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 Solid-State Fluorescence Spectra. A solidstate fluorescence spectra and absolute photoluminescence quantum yields 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 326 and 352 nm for complexes II and III, respectively. Calculation Method of HOMO−LUMO Gaps of Component Molecules 3 and 4. The HOMO−LUMO gaps of component molecules 3 and 4 were calculated by the hybrid density functional theory (B3LYP functional9) with the cc-pVDZ10 basis set. For calculating the HOMO−LUMO gaps of moleucles in the isolated state, the molecular 1860



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Figure 2. Crystal structures of I. (a) 21-helical columnar network structure observed along the b axis.13 Solid arrows A indicate the intracolumnar benzene−benzene edge-to-face interactions. (b) View along the a axis. (c) Packing structure along the b axis. Solid arrows B−D indicate the intercolumnar benzene−benzene edge-to-face interactions. (d) View along the a axis.

indicated by solid arrows B−D, respectively, in Figure 2c) (Figure 2c,d).14 The crystal structures of II are shown in Figure 3. The stoichiometry of II is (R)-1:3 = 1:1, and its space group is P212121. This complex also has a 21-helical columnar network structure along the a axis, as in complex I (Figure 3a,b). This complex also features intracolumnar benzene−benzene edgeto-face interactions (2.95 Å, indicated by solid arrows A in Figure 3a) between the hydrogen atom of the benzene ring in 3 and the benzene ring of (R)-1.14 The self-assembly of these 21helical columns through four types of intercolumnar benzene− benzene edge-to-face interactions (2.96, 2.84, 2.71, and 2.99 Å; indicated by solid arrows B−E, respectively, in Figure 3a) result in the formation of complex II (Figure 3a,b).14 Interestingly, X-ray crystallographic analyses revealed that although the structures of the 21-helical columns in complexes I and II are similar (Figures 2c and 3a), the packing arrangements of the shared 21-helical columns are considerably different (indicated by solid rectangles in Figures 2d and 3b). The crystal structures of III are shown in Figure 4. The stoichiometry of III is (R)-1:4 = 1:1, and its space group is P21, which is the same as that of complex I. In complex III, intracolumnar interactions were not observed.14 III is formed by the self-assembly of 21-helical columns with intercolumnar

Figure 1. (a−c) CD and absorption spectra of complexes I−III (black lines) and complexes I′−III′ (gray lines) in the solid state (KBr pellets).12

benzene ring of (R)-1 are also observed.14 I is formed by the selfassembly of these 21-helical columns by three intercolumnar benzene−benzene edge-to-face interactions (2.93, 2.73, and 2.75 Å; 1861



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opposite sign, that is, negative and positive from the longer wavelength side. These CD intensities are absent in the spectra of the solutions of 2−4 and are apparently induced by the molecular arrangement in the crystals. Molecular packing induces CD intensities of solids via (i) distortion or fixation of conformations of chromophore molecules; and (ii) intermolecular coupling of electronic transitions of chromophore molecules which constitutes clockwise or anticlockwise screw senses. We suggest that the opposite CD signs of complexes I and II result from the influence of (ii) rather than that of (i). Molecules 2 and 3 are far from planar in the respective crystals, with the carboxyl group twisted relative to the molecular plane of C6H4−C≡C−C6H4Me, producing CD intensities via (i). The carboxyl groups of 2 and 3 twist in the same direction however (the OCCC torsion angles between the carboxyl group and the adjacent 6-member ring are 37.8° in 2, and 39.0° in 3) which should induce CD intensities of the same sign in complexes I and II. The opposite signs observed for complexes I and II should therefore be attributed to the influence of (ii) rather than that of (i). The shapes of the CD spectra of I and III are similar, probably because the packing structures in complexes I and III are similar. Complex III has just additional weak negative CD intensity at 332 nm compared with complex I. The OCCC torsion angles between the carboxyl group and the adjacent 6member ring are of the same sign (37.8° in 2 and 33.3° in 4). A significant bending of 4 in crystal is observed for complex III unlike 2 and 3 in I and II, respectively (Figure 5). While

Figure 3. Crystal structures of II. (a) Packing structure of 21-helical columnar network structure observed along the a axis. (b) View along the b axis. Solid arrows A indicate the intracolumnar benzene− benzene edge-to-face interactions. Solid arrows B−E indicate the intercolumnar benzene−benzene edge-to-face interactions.

benzene−benzene edge-to-face (2.70 Å, indicated by solid arrow A in Figure 4a) and CH-π interactions (2.72 Å, indicated by solid arrow B in Figure 4a), respectively (Figure 4a,b).14 The

Figure 5. Crystal structures of (a) 2, (b) 3, and (c) 4 in complexes I−III.

comparing the parameters of complex I with complex III, the distance between the 4-(2-methylphenylethynyl)-benzene units along the 21-helical column increases from 5.98 Å for complex I to 6.09 Å for complex III (AA, Figures 2d and 4b). As the position of the methyl substituent on the methylphenylethynyl group changes from p-position to o-position (from 2 to 4), the distance between the 21-helical columns along the a-axis (BB, Figures 2d and 4b) increases from 7.18 Å in complex I to 7.25 Å in complex III. On the other hand, the distance between the 21-helical columns along the c-axis (CC, Figures 2d and 4b) decreases from 22.81 Å in complex I to 22.01 Å in complex III. These structural differences would result in the minor difference of the CD spectra of I and III. The solid-state fluorescence spectra of complexes II and III were measured. Because 4-[2-(methylphenyl)ethynyl]benzoic acid is a fluorescent molecule, it was anticipated that complexes II and III would display solid-state fluorescence similar to that of complex I. Although the foremost problem with solid-state organic fluorophores is the loss of fluorescence in the crystalline state, both complexes exhibited fluorescence in the solid state.

Figure 4. Crystal structures of III. (a) Packing structure of 21-helical columnar network structure observed along the b axis. Solid arrows A and B indicate the intercolumnar benzene−benzene edge-to-face and intercolumnar CH-π interactions, respectively. (b) View along the a axis.

packing structures of the shared 21-helical columns in complexes I and III are similar (indicated by solid rectangles in Figures 2d and 4b); on the other hand, those in complexes II and III are clearly different (indicated by solid rectangles in Figures 3b and 4b). The observed CD spectra and electronic absorptions of complexes I−III are shown in Figure 1. These complexes have a set of positive and negative CD intensities in the range 260− 330 nm, which originate from the electronic absorption of component molecules 2−4 in this wavelength region. The CD intensities of complexes I and III are positive and negative from the longer wavelength side, whereas that of complex II has an 1862



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The solid-state fluorescence maxima (λem) of complexes II and III were observed at 373 and 379 nm, respectively, and the absolute values of the photoluminescence quantum yield (ΦF) in II and III were 0.49 and 0.68 in the solid state, respectively. In comparison, the solid-state λem and ΦF of complex I are 379 nm and 0.58, respectively,3 and no dramatic change of solid-state fluorescence was observed. It is well-known that a small highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) gap induces a red shift in the emission wavelength.15 The HOMO−LUMO gaps of the gas-phase molecules of 2,3 3, and 4 in the isolated state were calculated to be 3.98, 4.03, and 4.00 eV, respectively; HOMO− LUMO gaps of these three component molecules have a similar value.

CONCLUSIONS A chiral supramolecular organic fluorophore was successfully prepared using achiral fluorescent 4-[2-(3-methylphenyl)ethynyl]benzoic acid [or 4-[2-(2-methylphenyl)ethynyl]benzoic acid] and chiral (R)-(+)-1-phenylethylamine. Conventionally, the sign of the chiral properties is controlled using a chiral compound with an opposite chirality. By changing the position of the methyl group on the methylphenylethynyl group of the achiral molecule from the 4-position to the 3-position, the signs of the solid-state CD spectra of these supramolecular fluorophores were reversed, despite using the same chiral amine molecule. Moreover, by changing the position of the methyl group from the 4-position to the 2position, the additional positive CD intensity at 332 nm in complex III appeared. Conventionally, the solid-state CD of the fluorophore is controlled by changing the chirality of the chiral amine component molecule. These results indicate that the solid-state chiral properties of a 4-[2-(methylphenyl)ethynyl]benzoic acid/amine supramolecular organic fluorophore can be controlled by varying the position of the methyl group on the methylphenylethynyl group of the achiral 4-[2-(methylphenyl)ethynyl]benzoic acid component molecule in the solid state. Supramolecular organic fluorophores offering this functionality are expected to be useful in the development of novel solidstate chiral supramolecular fluorophores, particularly because chiral compounds with opposite chiralities may not be readily available. ASSOCIATED CONTENT

S Supporting Information *

Crystallographic reports (CIF) of complexes II and III; 1H NMR spectra of compounds 3 and 4. This information is available free of charge via the Internet at http://pubs.acs.org/.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-6-6730-5880 (Ext. 5241). Fax: +81-6-6727-2024. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by a Grant-in-Aid for Scientific Research (Nos. 22750133 and 23111720) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and a research grant from Support Center for Advanced Telecommunications Technology Research, Foundation (SCAT). 1863



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Novel Means of Controlling the Solid-State Circular Dichroism

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