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Elastic Bending Flexibility of Fluorescent Organic Single Crystal: New Aspects of Commonly Used Building Block “4,7-Dibromo-2,1,3-benzothiadiazole” Shotaro Hayashi, Toshio Koizumi, and Natsumi Kamiya Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00992 • Publication Date (Web): 19 Oct 2017 Downloaded from http://pubs.acs.org on October 20, 2017
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Elastic Bending Flexibility of Fluorescent Organic Single Crystal: New Aspects of Commonly Used Building Block “4,7-Dibromo-2,1,3benzothiadiazole” Shotaro Hayashi,* Toshio Koizumi and Natsumi Kamiya Department of Applied Chemistry, National Defense Academy, 1-10-20, Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan
Supporting Information Placeholder
Commercially available 4,7-dibromo2,1,3-benzothiadiazole can be used as a building block for the organic synthesis of high performance optoelectronics devices. Herein, we report that a single crystal of 4,7-dibromo-2,1,3-benzothiadiazole displays elastic bending flexibility and crystalline-state fluorescence. A large needle-shaped single crystal (over 1 cm long) exhibited a fibril lamella type structure based on slipstacked (J-aggregated) molecular wires. The crystal was capable of bending under applied stress and quickly reverts to its original shape upon relaxation. The crystal shows a reversible fluorescence change via mechanical bending–relaxation that can be performed many times. ABSTRACT:
Organic materials possessing mechanical flexibility and low band gaps (ca. < 3 eV) have received considerable interest due to their potential use in materials science.1–3 4,7Dibromo-2,1,3-benzothiadiazole, DBBT (Figure 1A), is a commercially available building block used for organic semiconducting materials that results in high performance flexible optoelectronic devices.4–11 The benzothiadiazole unit works as an electron-acceptor in π-conjugated oligomers4–6 and polymers.7–11 Thus, various donor-acceptor molecules based on this unit have been synthesized by palladium-catalyzed cross-coupling reactions of DBBT and their optoelectronic properties have been studied for the fabrication of various organic devices such as solar cells4,5,7–9 and field-effect transistors.6 Thus, DBBT is crucially important; to the best of our knowledge, however, there are no reports on the physical properties of DBBT. Herein, we report the discovery of new aspects of DBBT. Organic single crystals with elastic (reversible) bending flexibility are a rare phenomenon; a few types of crystals have discovered and studied recently.12–16 Surprisingly, a single crystal of DBBT exhibits elastic bending flexibility. The centimeter-scale straight-shaped crystal of DBBT bent under applied stress in the b direction and recovered its shape upon stress reduction (Figure 1B-C). The bending motion of the crystals is similar to bending rubber (Figure
1D). More interestingly, the crystal shows elastic bending flexibility and fluorescent properties in the crystalline state. Organic single crystals assembled using π-conjugated molecules are highly important in materials science because of characters such as their ability to form a densely packed 3D morphology and anisotropy.17–21 However, organic single crystals are often brittle (no flexibility) due to their densely packed structure. On the other hand, polymers in the solid state (films and fibers) are typical flexible materials;22–24 however, in polymer science, the polymer materials are amorphous and increasing the crystalline state in polymer matrix decreases flexibility.25 The trade-off between flexibility and crystallinity of materials is often a fatal problem in solid-state materials science. The study on the fabrication of flexible crystals with optical and/or electrochemical properties is important to overcome the problem. A
B
S N
120 µm
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b Br
Br H
140 µm
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H
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C Bending b Relaxation
c a
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Relaxation
Figure 1. (A) 4,7-Dibromo-2,1,3-benzothiadiazole, DBBT. (B) Illustration of the crystal. (C) Elastic bending motion of the crystal. (D) Bending−relaxation cycles of rubber.
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Organic crystals with elastic bending flexibility, namely elastic crystals, are an ideal material for electronic devices.12–15 However, elastic crystals with functionalities such as light absorption and fluorescence are extremely rare. We reported an elastic organic single crystal based on a πconjugated oligomer in recent years.12 The crystal exhibited fluorescence (λfl = 500 nm, Φ = 25%) due to the πconjugated structure. Elastic (flexible) single crystals (densely packed and anisotropic materials) with solid-state fluorescence (functionality) can be used in a variety of flexible optical devices in the near future. From these viewpoints, we have searched for the single crystals using a “trial and error” approach for new designs. During our synthetic studies of π-conjugated molecules for functional elastic crystals, we noticed that a commercially available molecule gave a centimeter-scale crystal that displayed solidstate fluorescence and elastic bending flexibility. Herein, we report the study of a single crystal of DBBT. Centimeter-scale straight-shaped (or needle-shaped) single crystals of DBBT were obtained from an ethanol solution containing DBBT (1.0 M). Slow crystallization of DBBT for 1 day gave centimeter-scale needle-type crystals (thickness > 100 µm, width > 120 µm, length 10–25 mm, Figure S1). The 3D size indicates that the crystal growing ability of DBBT is high and the material is suitable for a mechanical bending test using visual inspection. A crystal of DBBT (thickness 142 µm, width 168 µm, length 21 mm) with two faces, (010) and (001) determined using an optical microscope, showed a macroscopic characteristic surface morphology (Figure S2A and S2B). These faces show lamellar morphology in the a axis direction because of the refraction of light. The crystal structure of DBBT is displayed in Figure 2. Polymorphism was not observed in the crystals of the compound. The structure of DBBT in the crystal planes are shown in Figure 2A and 2B. Packing of the molecules show a slip-stacked assembly (J-aggregation) in the x and z axis directions (Figure 2C). The center-to-center separation of the DBBT planes equaled 1.172, 1.144, and 1.144 Å. The crystal structure of DBBT contains Br−Br (3.547 Å) intramolecular halogen interaction and hydrogen bonding H−N (2.649 Å) that are significantly shorter than the sums of their van der Waals radii (dBr-Br = rBr + rBr = 3.70 Å, dH-N = rH + rN = 2.75 Å) (Figure S3). Besides, the distance between N and S (3.631 Å) is close their sum of the radii (3.35 Å). These interactions would be important to produce elastic bending flexibility. The crystal showed a fibril lamella (or columnar-type) structure when observed from the (100) face (Figure 2D). DBBT showed four types of molecular orientation (A, B, C, D) and overlap in the a axis of the crystal (Figure S4). Red and blue lines display the lamella patterns. Both the (010) and (001) faces show no spaces, and DBBT molecular wires grew by π-π stacking on the a axis. Criss-cross packing can be viewed on the (010) face. To further analyze the crystal structure, X-ray diffraction (XRD) analysis of the crystal was performed. Patterns displayed for the (100) face (red and blue lines in Figure S4) were not observed when the crystals were
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aligned parallel to the substrate (Figure S2C). This indicates that the lamella space patterns do not exist in the (010) and (001) faces. When the crystals were randomly set, many diffraction peaks are observed (Figure S2D). According to the Bragg equation, the length was calculated to be 10.5 and 5.25 Å, which corresponds to one lamella layer (red line) and lengths of 10.2 and 5.10 Å were also observed as another lamella layer (blue line). A
top view
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y x
side view x y
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y
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z
1.144 A
y
o
z
1.144 A
D
a c (100)
b
(010)
a a
b
c b
(001)
c
Figure 2. (A) Top view of DBBT in the crystal structure. (B) Side view of DBBT in the crystal structure. (C) Stacking of the molecule. (D) Crystal morphology of the crystal. DBBT in the crystal shows an enhanced slip-stacked π–π overlap and lamellar structure comprising slip-stacked molecular wires along the a axis direction.
An individual crystal (thickness 142 µm, width 168 µm, length 21 mm) was subjected to a mechanical test to assess the response. Figure 3 and Video 1 display the mechanical bending performance of the crystal. The crystal was fixed with an adhesive tape on a sample glass tube (Figure 3A). Stress was applied by pushing the crystal end with a forceps. The straight-shaped crystal (Figure 3B) bent under applied stress in the b direction (Figure 3C-D) and relaxed upon stress reduction (Figure 3E) to recover its original straight-shape. Interestingly, reversible bending–relaxation of the crystal can be cycled many times (Figure 3E-G, G-I, and I-K). This mechanical motion indicates that the crystal of DBBT is an elastic (bending) organic single crystal. The crystal bending angle exceeded ca. 30°. The elastic strain (εn) of the crystal was estimated using the curvature of the bent crystal (Figure S6) and εn = d/2r, 26 where d corresponds to the length of the (001) plane (d = 142 µm) and r is the radius (r = 18 mm). Thus, εn of the crystal (Figure 3J) was 0.39%. Of note, the crystal can be bent in the b direction; however, the crystal is brittle when bending in the c
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direction. Because organic crystals are anisotropic, elastic bending flexibility of the crystal was shown in one direction. The mechanism of the mechanical bending is not clear but is probably due to the slip-stacked (J-aggregated)structured molecular wires. These may act as a flexible wire, which undergo expansion (⇦ ⇨) and contraction (⇨ ⇦) on the basis of the changing center-to-center separation length (Figure 2C, 3L). When the changes occur, DBBT molecules in the crystal move to their original thermodynamic positions, which are the most stable center-to-center separation length based on suitable intermolecular and interactions (π-π stacking, halogen interaction and hydrogen bond) as driving force. To evaluate this phenomenon, we measured the single crystal XRD (SCXRD) of both original and bent crystals (thickness 150 µm, width 177 µm, length 24 mm). The original crystal was set on the tip of a glass capillary using adhesive. Then, a bent crystal was fabricated and fixed using an amorphous polyethylene (PE) film (Figure S7 and S8). The εn of the crystal was calculated to be ca. 0.4%. Diffraction spots from the bent crystal are in different positions compared with the original crystal (Figure S9), implying that expansion and contraction cause movement of the molecules. If the molecular packing (Jaggregated structure) in the crystal changes with applied stress, a change in the optical properties should also be observed. Thus, spectroscopic measurements of the crystals are important for interpreting the reason behind the mechanical bending flexibility. A
top view
side view
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B
b
C
18 mm crystal 21 mm
D
E
F
G
H
I
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K
L
was applied by pushing at the end of the crystal with a forceps. (L) Plausible mechanism of elastic bending. Illustration of centerto-center separation length changes causing a change in the length of the molecular wires. Zig-zag and criss-cross packings between the molecular wires prevent slipping of the planes.
Because DBBT is only use as a building block for various π-conjugated oligomers and polymers, 4–11 there is no report on its optical properties in the solution or solid states. The absorption spectra of DBBT in dichloromethane (DCM) and the solid state (both powders and crystals) are shown in Figure 4. A powder sample was prepared by grinding a single crystal for 5 min. The absorption spectrum of DBBT in DCM shows peaks at 301 and 347 nm (Figure 4). In comparison, the spectrum of the powder shows a broad absorption band with a peak at 305 nm. The band in the crystal has a lower energy absorption maximum (λmax = 360 nm) than that of the powder. The optical band gap of the crystal, calculated from the onset of absorption profile, was 2.35 eV. The fluorescence spectrum of DBBT in DCM showed a band with a peak at 434 nm (Figure 4); blue colored fluorescence was observed (Figure 4). To estimate the fluorescence efficiency, we measured quantum yields (Φ) of the solution, crystal, and powder of DBBT. The yields were measured based on an absolute method using an integrating sphere equipped with a multichannel spectrometer. The quantum yield of DBBT in DCM was ca. 5% (Figure 4). The fluorescence spectrum of the crystal showed a remarkably red-shifted band with a peak at 513 nm and higher efficiency than in DCM (Φ = ca. 9%). The crystal packing of DBBT (Figure 2) is typical J-aggregation. This structure type normally causes a red-shift when moving from solution to solid states and an increase in the quantum yield. 26 The spectrum of DBBT in ground powder (λfl = 490 nm) showed a blue-shifted band compared with that in crystal. The quantum yield slightly decreased to ca. 7%. The XRD pattern of the crystal exhibited clear diffraction peaks due to ordered lamella layer of the structure, but the powder was significantly disordered molecular packing (Figure S10). These results indicate that the ground powder was polycrystalline state; the grinding of the crystal causes disorder in molecular packing and/or changes the center-tocenter separation of the molecular packing.28 The fluorescence properties of DBBT are helpful to investigate the bending flexibility and are of interest for mechanofluorochromic behavior.29 Thus, we next performed detection of fluorescence in the original (straight), bent, and relaxed crystals.
b c
a Bending b Relaxation
Figure 3. (A) Illustration of the crystal fixed on a glass tube. (B) Image of the crystal fixed on a glass tube. (C–K) Mechanical bending and relaxation of the crystal. Stress for the bending test
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700
crystal
8.7%
500 550 600 wavelength (nm)
620 600 580 560 540 520 500 480
Int 0.50
0 greenish-blue
Figure 4. (A) UV–vis absorption spectra of DBBT in DCM, powder, and crystal states. (B) Fluorescence spectra of DBBT in DCM, powder, and crystal states. (C) Fluorescence images of the samples. Quantum yield results are inserted in the photographs.
Of note, the crystal showed a slightly different fluorescence spectrum when it was mechanically bent nears its elastic bending limit (30°). The spectra of original, bent, and relaxed crystals were obtained using a fiber probe system with a LED light (Figure 5A). The crystal (εn = ca. 0.4%) was set on the tip of a glass capillary using a bonding material as in the SCXRD measurement. A nonfluorescent PE film was used for fixing the crystal. The spectrum of the bent crystal showed a slightly blue-shifted fluorescence band (λfl = 504 nm) compared with the original (Figure 5B). The spectrum of the bent crystal is similar to that of the powder. When the bent crystal relaxed upon force removal (removing of PE film), the spectrum exhibited a red-shifted band that was identical to the original. The original and relaxed crystals show greenish–blue fluorescence, but the bent crystal shows sky-blue fluorescence (Figure 5C). The change in fluorescence is probably due to changing center-to-center separation length in the slipstacked (J-aggregated) molecular packing. The packing of DBBT shifts from stable to metastable via mechanical bending, resulting in a blue-shifted fluorescence band. Relaxation of the bent crystal allows for recovery of the stable packing mode. This mechanofluorochromic phenomenon is induced by the mechanical bending–relaxing motion and can be observed for many cycles. The fluorescence band of the crystal was reversibly shifted 5 times and we plotted the cycles vs. the three wavelength points of normalized intensity, 1.0 (λmax), 0.5, and 0.75, respectively (Figure 5D).
sky-blue
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Int 0.75
2 3 cycles
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4
Figure 5. (A) Illustration and photograph of the setup for fluorescence measurements of the crystals. (B) Fluorescence spectra of original, bent, and relaxed crystals. (C) Photograph of original (relaxed) and bent crystals. (D) Spectral change induced by the bending–relaxation cycles.
In conclusion, we have studied a crystal of commercially available 4,7-dibromo-2,1,3-benzothiadiazole, DBBT. Unlike common organic crystals, a centimeter-scale needleshaped single crystal of the molecule bends under applied stress and quickly reverts to its original shape upon relaxation. Namely, the material is an elastic organic single crystal. Moreover, the crystal shows greenish–blue colored fluorescence (λ = 513 nm, Φ = ca. 9%). The unique mechanical and fluorescent properties of the crystal include mechanofluorochromism based on mechanical bending– relaxation cycles. This work turns the next page of materials chemistry for “flexible crystals with optical functionality.” Further studies of these molecules and their derivatives are now in progress.
ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Experimental details, X-ray analysis data and Crystallographic data.
AUTHOR INFORMATION Corresponding Author
*
[email protected] ORCID
Shotaro Hayashi: 0000-0001-8703-6740; Toshio Koizumi: 0000-0001-8017-2395; Natsumi Kamiya: 0000-0001-76623595.
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ACKNOWLEDGMENT This work was supported by a Grant-in-aid for Young Scientists (B) 15K17872 and Grant-in Aid for Scientific Research on Innovative Areas “π-figuration” 17H05171. This work was performed under the Cooperative Research Program of "Network Joint Research Center for Materials and Devices".
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Elastic Bending Flexibility of Fluorescent Organic Single Crystal: New Aspects of Commonly Used Building Block “4,7Dibromo-2,1,3-benzothiadiazole” Shotaro Hayashi,* Toshio Koizumi and Natsumi Kamiya
A large single crystal (over 1 cm long) of 4,7-dibromo-2,1,3-benzothiadiazole was found to be an elastic crystalline material. The straight needle-shaped crystal was capable of bending under applied stress and then quickly reverted to its original shape upon relaxation. The reversible motion can be performed for many times. Surprisingly, the crystal showed fluorescence and mechanical bending–relaxation resulted in reversible fluorescence change.
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