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Multi-functional Octamethyltetrasila[2.2]cyclophanes: Conformational Variations, Circularly Polarized Luminescence, and Organic Electroluminescence Masaki Shimada, Yoshinori Yamanoi, Tatsuhiko Ohto, Song-Toan Pham, Ryo Yamada, Hirokazu Tada, Kenichiro Omoto, Shohei Tashiro, Mitsuhiko Shionoya, Mineyuki Hattori, Keiko Jimura, Shigenobu Hayashi, Hikaru Koike, Munetaka Iwamura, Koichi Nozaki, and Hiroshi Nishihara J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b05671 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on July 21, 2017
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Journal of the American Chemical Society
Multi-functional Octamethyltetrasila[2.2]cyclophanes: Conformational Variations, Circularly Polarized Luminescence, and Organic Electroluminescence Masaki Shimada,† Yoshinori Yamanoi,†,* Tatsuhiko Ohto,‡ Song-Toan Pham,‡ Ryo Yamada,‡ Hirokazu Tada,‡ Kenichiro Omoto,† Shohei Tashiro,† Mitsuhiko Shionoya,† Mineyuki Hattori,§ Keiko Jimura,§ Shigenobu Hayashi,§ Hikaru Koike,ǁ Munetaka Iwamura,ǁ Koichi Nozaki,ǁ and Hiroshi Nishihara†,* †
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan ‡
Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
§
National Institute of Advanced Industrial Science and Technology (AIST), AIST Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
ǁGraduate
School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
ABSTRACT: Both symmetrical and unsymmetrical cyclophanes containing disilane units, tetrasila[2.2]cyclophanes 1–9, were synthesized. The syn and anti conformation and the kinetics of inversion between two anti-isomers were investigated by X-ray diffraction and variable-temperature NMR analysis, respectively. The flipping motion of two aromatic rings was affected by the bulkiness of the aromatic moiety (1 vs 6), the phase (solid vs solution), and the inclusion by host molecules (1 vs 1⊂[Ag2L]2+). The photophysical, electrochemical, and structural properties of the compounds were thoroughly investigated. Unsymmetrical tetrasila[2.2]cyclophanes 5–8 displayed blue–green emission arising from intramolecular charge transfer. Compound 6 emitted a brilliant green light in the solid-state under 365 nm irradiation and showed a higher fluorescence quantum yield in the solid state (Φ = 0.49) than in solution (Φ = 0.05). We also obtained planar chiral tetrasila[2.2]cyclophane 9, which showed interesting chiroptical properties, such as a circularly polarized luminescence (CPL) with a dissymmetry factor of |glum| = ca. 2 × 10–3 at 500 nm. Moreover, an organic green light-emitting diode (OLED) that showed a maximum external quantum efficiency (ηext) of ca. 0.4% was fabricated by doping 4,4'-bis(2,2'diphenylvinyl)-1,1'-biphenyl with 6.
Introduction Organic molecules comprising electron donor, π-spacer, and acceptor (D–π–A) units have attracted much attention because of their nonlinear optical properties, and potential applications in electro-optics and molecular photonics.1 The versatile chemical modification of these molecules allows for the improvement of their chemical characteristics and processability. Because the strong intermolecular interactions in the aggregated or solid states often decrease the emission performance, donor– acceptor-based molecules that suppress π–π stacking and display effective solid-state fluorescence are required for organic light-emitting diodes (OLEDs). Interest in circularly polarized luminescence (CPL), which is derived from the difference in emission between right- and left-circularly polarized light by chiral luminophores, has grown in recent years. CPL has a potential for developing advance photonic devices such as 3D displays and enantioselective chemo/biosensors.2 Intense CPL is
mainly achieved from chiral lanthanide complexes and helical molecules.3 However, these compounds are expensive, difficult to synthesize, and poorly soluble in orgnic solvents. Accordingly, simple organic molecules exhibiting CPL are required to overcome these problems. To explore the range of applications of OLEDs and chiral luminophores, we focused on disilane-modified cyclophane derivatives. Introducing a disilane moiety into arenes improves their optical and electronic properties compared with the parent arene compounds owing to σ–π conjugation between the Si–Si σ orbital and the arene π orbital.4 In addition, unsymmetrical cyclophanes are a well-known planar chiral source used for luminescent materials, medicines, and ligands in asymmetrical reactions.5 Therefore, tetrasila[2.2]cyclophanes, which combine Si−Si σ systems with a cyclophane skeleton, are expected to provide materials with interesting emissive properties. Syntheses of symmetrical cyclophanes bridged with
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Scheme 1. Preparation scheme and chemical structures of octamethyltetrasila[2.2]cyclophanes.
Typical reaction conditions: bis(1,1,2,2teramethyldisilylanyl)arenes (1.0 mmol), diiodoarenes (1.0 mmol), Pd(P(t-Bu)3)2 (2.5 mol%), N,Ndiisopropylethylamine (2.0 mmol), m-xylene (15 mL), 0 °C, 2 d. See Supporting Information for detail.
Si−Si bonds using organometallic reagents have been reported.6–10 Despite their importance, efficient modular methods to access disilane-bridged cyclophanes remain challenging, particularly for aromatic systems except for simple, symmetrical aryls. The physical properties of these compounds have not been thoroughly investigated. Unsymmetrical disilane-bridged cyclophanes have not been reported although the donor–acceptor system produced strong emissions arising from intramolecular charge transfer (ICT). Moreover, planar chiral silacyclophanes have not been investigated despite their chiroptical properties. We recently developed an efficient synthetic route to organosilane compounds bearing Si−Si bonds using transition metal-catalyzed processes, and the compounds demonstrated the functional properties including strong solid-state emission and solvatochromism.11 Here, we report Pd-catalyzed approach to tetrasila[2.2]cyclophanes, which are symmetrical and unsymmetrical [2.2]cyclophanes bridged by two Si2Me4 units, to allow facile installation of functional group into [2.2]cyclophanes. This method is particularly useful for the synthesis of diverse donor–acceptor disilane-bridged aromatic heterocycles. Their photophysical properties, crystal structures, molecular dynamics, and theoretically calculated structures are described. We also discuss the chiroptical properties such as CPL in planar chiral compound 9 and we demonstrate an electroluminescent device containing a 6-doped organic light–emitting materials in relation to their practical applications.
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Synthesis and characterization. Octamethyltetrasila[2.2]cyclophanes were prepared by adapting the Pdcatalyzed cyclization of bis(1,1,2,2-teramethyldisilyl)arenes with diiodoarenes (Scheme 1).12 The reaction of bisdisilylthiophene or -ethylenedioxythiophene (EDOT) with 1,4diiodobenzene and 2,5-diiodothiophene provided the desired products in moderate yields (1‒4). These encouraging results prompted us to investigate the synthesis of disilane-bridged heterophanes incorporating donor– acceptor functionalities.13 We designed donor–acceptor chromophores bearing electron-withdrawing groups, such as benzothiadiazole and thienopyrazine, and electrondonating groups, such as thiophene and EDOT (5‒8). The compounds were purified by column chromatography and/or recrystallization and isolated as stable solid in up to 21% yield. The procedures for the synthesis are described in the Supporting Information (SI). All compounds were characterized by NMR, MS and HRMS, and the results indicated that their purity was adequate for the spectroscopic studies. X-ray structural analysis. The solid-state structures of these compounds were determined by single-crystal X-ray diffraction, which is useful for understanding material properties. Suitable single crystals of compounds 1, 5, 6 and 8 were obtained by recrystallization (see SI Chapter 2), allowing their structural characterization by X-ray diffraction.14,15 The main crystallographic data, selected bond distances and angles are listed in Tables S1–S8. The molecular structures for 5 and 6 and those for 1 and 8 were shown in Figure 1 and Figure S1, respectively. The Si–Si bond lengths (2.36−2.38 Å) were similar to typical Si–Si bond length (2.34 Å)16 and the angles of thiophene–Si–Si were ca. 105° owing to the steric hindrance of the two thiophene moieties. Notably, a single isomer was observed as the sole structure for the X-ray diffraction. The results indicate that the rotation about the two Si–Si bonds connecting the two thiophene rings was restricted in the crystal phase.
Figure 1. (a, c) ORTEP drawings (50% probability ellipsoids) and (b, d) crystal packing structures of (a, b) 5 and (c, d) 6. Hydrogen atoms are omitted for clarity.
Results and discussion
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Molecular dynamics and conformational analysis. We investigated the molecular geometry of 1 as a model compound in the conformational mobility study. Tetrasila[2.2]cyclophane 1 has syn and anti conformations (Figures 2(a) and S4). We performed theoretical calculations to estimate the relative stability of both conformations. The theoretical studies suggested that a Gibbs free energy of the anti conformation was lower than that of the syn conformation by 3.5 kcal/mol, and the syn/anti ratio was 0.3/99.7 at 298 K (Table S13). The explanation for this is that the syn conformation structure has electronic repulsion between two S moieties. Therefore, at room temperature, only the anti form exists in solution. Sakurai reported the kinetic parameters of the change in conformation of 1 between the anti-A and the anti-B forms via the syn form in solution (Figure 2(a)).15 We also investigated the inversion of 1 by variable temperature (VT) NMR of 1 in CDCl3 solution (Figure S5). At low temperature, the 1H NMR showed two signals with equal intensity assigned to the Si−CH3 moiety (a and b in Figure 2(a)). As the temperature increased, these signals coalesced at around 310 K and became a broad singlet at 323 K. From the Eyring equation, the inversion barrier for flipping 1 between anti-A and anti-B was determined to be 15.0 kcal/mol (see SI Chapter 5).17 The VT-13C NMR of 1 in CDCl3 also showed the Si−CH3 peak(s) as two signals at 300 K that coalesced above 310 K, similar to the 1H NMR (Figure S6). Moreover, we performed solid-state VT-13C NMR of crystalline 1 (Figure S7). The Si−CH3 peaks in the crystalline state showed a set of signals that did not coalesce until 323 K, demonstrating that the thiophene rings did not flip in the crystalline state. Si−CH3 peaks of compounds 3, 5, and 7 also were broad and coalesced at higher temperature, which indicates that the inversion occurred in solution similar to 1. (Figures S9, S10, and S12) According to the Si−CH3 peaks of 2 and 4, the flipping is in the fast limit at 300 K, and broad Si−CH3 peaks of 2 at 220 K were observed (Figure S8). These results suggest that the faster inversion of 2 and 4 occurred in solution at room temperature than that of 1. On the other hand, 6 and 8 displayed sharp Si−CH3 peaks and Si−CH3 peaks of 6 did not depend on the temperature (Figure S11), which insists that the flipping did not occur owing to the greater steric hindrance of the aryl substituents. Cyclic hosts that entrap organic molecules strongly in solution are important for selective extraction of chemical species or intermediates. We developed several macrocyclic host molecules,18 and [Ag2L]2+ included small organic molecules as guests.18c Here, to demonstrate the interaction between silacyclophane 1 and [Ag2L]2+ and to control the inversion motions of 1, 1⊂[Ag2L]2+ complex was fabricated (Figure 2(b) and Scheme S1). The NMR titration of 1 in [Ag2L]2+ produced peaks at 3.8, −0.5, and −1.1 ppm, which was the characteristic of the inclusion of 1, and indicated that 1 bound to [Ag2L]2+ in a 1:1 ratio (Figure S13). ESI-MS also showed that 1⊂[Ag2L]2+ complex was fabricated (Figure S22). A guest competition experiment with ferrocene demonstrated that the association constant, Ka, between 1 and [Ag2L]2+ was much higher than 1010 M−1,
Figure 2. (a) Conformation variation of 1. (b) Synthesis scheme and molecular structure of 1⊂[Ag2L](SbF6)2. (c) ORTEP drawing (30% probability ellipsoids) of one disordering pattern of 1⊂[Ag2L](SbF6)2. Side alkyl-chains of L, counter anions, methyl groups of 1, solvent molecules, and hydrogen atoms are omitted for clarity. The Ag−C bonds in (b) were drawn based on the crystal structure in (c).
suggesting there was a strong intermolecular interaction between 1 and [Ag2L]2+ (Scheme S2 and Figure S23). 1D and 2D NMR analyses of 1⊂[Ag2L]2+ (Figures S14 and S16– S21) showed that 1⊂[Ag2L]2+ had a C2h-symmetrical structure; thus, the guest molecule included in the host cavity was only in an anti conformation. VT-1H NMR of 1⊂ [Ag2L]2+ indicated that the peaks from the included 1 did not coalesce at higher temperature (323 K) owing to the restriction of the thiophene ring rotation in the cavity (Figure S15). Encapsulation is an effective and simple approach to conformational rigidification of tetrasilacyclophanes. We proposed that the guest spinning motion might be restricted by the interaction between thiophene and Ag+. We also obtained a single crystal of 1⊂[Ag2L](SbF6)2 suitable for X-ray crystallography by slow vapor diffusion of ether into CH2Cl2 solution of an equimolar mixture of [Ag2LX2](SbF6)2 and 1.19 Details of the molecular structure determination are given in Figures 2(c) and S3, and Tables S11 and S12. The structure of 1 in macrocyclic host was in an anti conformation similar to single-crystal 1. Moreover, the intermolecular interaction between host and guest could be investigated. Both Ag+ ions were in η2-typed Agπ coordination geometries and were bridged by a side-on
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Table 1. Data for the absorption and fluorescence spectra observed for compounds 1–8 at room temperature.
a
in cyclohexane
in the solid state
compound
λabs (nm)
ε (10 M cm )
λem (nm)
Φ
1
271
2.41
398
