Aggregation-Induced Emission Enhancement from Disilane-Bridged

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Aggregation-Induced Emission Enhancement from Disilane-Bridged Donor−Acceptor−Donor Luminogens Based on the Triarylamine Functionality Tsukasa Usuki,† Masaki Shimada,† Yoshinori Yamanoi,*,† Tatsuhiko Ohto,‡ Hirokazu Tada,‡ Hidetaka Kasai,§ Eiji Nishibori,§ 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 § Division of Physics, Faculty of Pure and Applied Sciences, Tsukuba Research Center for Interdisciplinary Materials Science (TIMS) & Center for Integrated Research in Fundamental Science and Engineering (CiRfSE), University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan ‡

S Supporting Information *

ABSTRACT: Six novel donor−acceptor−donor organic dyes containing a Si−Si moiety based on triarylamine functionalities as donor units were prepared by Pd-catalyzed arylation of hydrosilanes. Their photophysical, electrochemical, and structural properties were studied in detail. Most of the compounds showed attractive photoluminescence (PL) and electrochemical properties both in solution and in the solid state because of intramolecular charge transfer (ICT), suggesting these compounds could be useful for electroluminescence (EL) applications. The aggregation-induced emission enhancement (AIEE) characteristics of 1 and 3 were examined in mixed water/ THF solutions. The fluorescence intensity in THF/water was stronger in the solution with the highest ratio of water because of the suppression of molecular vibration and rotation in the aggregated state. Single-crystal X-ray diffraction of 4 showed that the reduction of intermolecular π−π interaction led to intense emission in the solid state and restricted intramolecular rotation of the donor and acceptor moieties, thereby indicating that the intense emission in the solid state is due to AIEE. An electroluminescence device employing 1 as an emitter exhibited an external quantum efficiency of up to 0.65% with green light emission. The emission comes solely from 1 because the EL spectrum is identical to that of the PL of 1. The observed luminescence was sufficiently bright for application in practical devices. Theoretical calculations and electrochemical measurements were carried out to aid in understanding the optical and electrochemical properties of these molecules. KEYWORDS: disilane, donor−acceptor, aggregation-induced emission enhancement (AIEE), intramolecular charge transfer (ICT), organic light-emitting diodes (OLEDs) the aggregated or solid states.9−16 This phenomenon paves the way to the construction of highly efficient solid-state luminescent materials, and to improve the efficiency of OLEDs, the external quantum efficiency (EQE). To achieve high fluorescence efficiency in the solid state, bulky groups are commonly introduced to decrease π−π interactions and thus suppress ACQ.17 Symmetrical diaryldisilanes (Ar−Si−Si−Ar) can be regarded as a class of compounds showing intense photoluminescence in the solid state.18,19 Si−Si σ bond shows the similar chemical and physical properties to CC π bond due to almost the same ionization potential. Accordingly, these compounds secure extension of

1. INTRODUCTION There has been considerable interest recently in luminescence materials because of their varied applications in organic lightemitting diodes (OLEDs).1−4 One of popular designs for luminescent materials is based on the D−π−A system, in which the electron-donating (D) and accepting (A) moieties are linked by π-conjugated spacers.5,6 Their potential utility is attributed to their ambipolar character, and the efficient transport of charge carriers from the donor to the acceptor is performed by intramolecular charge transfer (ICT). However, these π-conjugated compounds exhibit low fluorescence quantum efficiency in the solid state or at high concentration state, which is mainly caused by aggregation-caused quenching (ACQ) due to π−π stacking of the planar backbone architecture.7,8 Unlike conventional fluorescent molecules, luminophores displaying AIEE have recently received attention for use in OLEDs because they emit with high quantum yield in © XXXX American Chemical Society

Special Issue: AIE Materials Received: September 29, 2017 Accepted: December 22, 2017

A

DOI: 10.1021/acsami.7b14802 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Chemical structures of disilane-bridged donor−acceptor−donor compounds in (a) previous work and (b) this work.

Figure 2. Chemical structures and yields of compounds 1−6.

compounds containing triarylamines as electron-donating terminal groups and benzothiazole or thienopyrazine as electron-withdrawing core groups (Figure 1b). The triarylamine motif is one of the most extensively studied donor units in D− A dyes due to stronger electron-push ability to tune its low energy bandgap and electrochemical stability in the oxidized form compared with previous study (OMe and NMe2 as the donor substitute).21−27 The effects of donor substituents on the optical properties and crystal packing of these dyes were also investigated. In addition, quantum chemical calculation was employed to understand the correlation between optical behavior and packing arrangement in the solid state. Finally, the OLED composed of Si−Si bridged compound 1 as an emission layer was successfully fabricated and demonstrated to emit green electroluminescence.

conjugation system and the suppression of intermolecular interaction by introducing substituents on aromatic rings. We recently reported the synthesis and photophysical properties of D−Si−Si−A−Si−Si−D and A−Si−Si−D−Si−Si−A bridges.20 These compounds displayed bright solid state emission and AIEE arising from the suppression of nonradiative relaxations and intermolecular π−π interactions in the aggregated state. However, compounds in previous work (Figure 1a) did not exhibit electroluminescence properties in spite of their high fluorescence quantum yields and stability in the solid film. Given these bridges’ high structural variability, their electronic and optical properties can be tuned by attaching conjugating substituents to the silicon atoms. It is important to understand the way in which various substituents and heterocycles affect the optical properties of each compound. Consequently, we were motivated to compare in detail the solid state properties of disilane-bridged D−A−D B


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ACS Applied Materials & Interfaces Table 1. Photophysical Properties of Compounds 1−6 in CH2Cl2a −1

−1 b

in the solid state

compd

λabs (nm)

ε (× 10 M cm )

λem (nm)

ΦFc

τ (ns)d

λex (nm)e

λem (nm)

ΦFc

τ (ns)d

1 2 3 4 5 6

386 367 396 369 385 367

6.20 6.62 5.10 6.74 4.26 5.59

524 615 620 670 488 480

0.016 0.018 0.007 0.003 0.072 0.004

1.3 7.6

444 426 450 450 436 404

495 510 526 547 485 504

0.247 0.040 0.068 0.044 0.174 0.073

2.7 7.4 12.5 11.0 4.2 3.3

3

a

b

f f

1.7 f

c

Measured in anhydrous degassed dichloromethane. Molar extinction coefficient. Absolute quantum yields determined using an integrating sphere system. dFluorescence lifetime detected at the maximum fluorescence wavelengths. The excitation wavelength was 365 nm (1−6 in dichloromethane and 2, 5, 6 in the solid state) or 464 nm (1, 3, 4 in the solid state). eThe excitation wavelength in the solid state was determined from excitation spectra. fFluorescence lifetime was below the detection limit.

Figure 3. Aggregation effect of 1 and 3. Definition of f w is as follows: f w = Vwater/(VTHF + Vwater). Fluorescence photographs of (a) 1 and (e) 3 in water/THF with different f w under UV illumination (λex = 365 nm). Emission spectra of (b) 1 and (f) 3 in THF/water mixtures with different water fractions ( f w). Plots of the fluorescence intensity (area) of (c) 1 and (g) 3 vs f w. Plots of maximum emission wavelength of (d) 1 and (h) 3 vs f w.

2. RESULTS AND DISCUSSION

relationship between donor−acceptor substituents and changes in the optical properties of organic materials, we began to rationally design and investigate the effects of triarylamine as the donor and heterocyclic groups as acceptors on the photophysical characteristics of disilane-bridged dye systems. Figure 2 shows the structures of 1−6 prepared using a Pd-

2.1. Synthesis and Structural Characterization. Narrow bandgaps in organic molecules are typically achieved by constructing conjugated scaffolds possessing strong electrondonating and accepting groups. To better understand the C


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ACS Applied Materials & Interfaces catalyzed method.28,29 These compounds were all isolated as pale yellow solids, and were structurally characterized by 1H NMR, 13C NMR, and HRMS. 2.2. Photophysical Properties. The photophysical properties of 1−6 were determined by UV−vis absorption and fluorescence spectroscopy, collected both in solution and in the solid state at room temperature. The results are summarized in Table 1, Tables S1 and S2, and Figures S1−S12. All compounds displayed broad and weak absorption bands at 350−450 nm and sharp absorption peaks at 250−350 nm. There were no notable differences in the absorption bands of the various compounds. The intense absorption bands below 350 nm correspond to the π−π* transition of the donor moiety, whereas the weak bands that extend to the visible region can be assigned to ICT. Changes in solvent polarity did not affect the absorption bands. All investigated D−Si−Si−A−Si−Si−D molecules gave emission in the visible light region when irradiated with UV light. The emission peaks of 1−4 were red-shifted and broader as solvent polarity increased (toluene < CH2Cl2 < acetone). Compounds 3 and 4 displayed dual emission: the emission at ca. 400 nm is from the excited triarylamine moiety (local excitation; LE) and the broad emission above 500 nm is due to ICT. The emissions of carbazole-based molecules 5 and 6 behave differently from those of 1−4. In particular, the optical properties of compound 5 slightly depended on the solvent. These results can be explained by twisted ICT (TICT).30,31 In the excited state (the charge separation state due to ICT) of compounds 1−4, the triarylamine moieties are twisted to have a more stable conformation in polar solvents, which leads to redshifted emission because of the lower energy gap of the twisted state. On the other hand, the carbazole moieties of compounds 5 and 6 cannot twist because of the connection between the two aromatic rings. The installation of an electron-donating moiety on the arylamine resulted in red-shifted absorption and emission properties due to enhancement of the push−pull system and the twisted excitation state compared with compounds described in the previous work (absorption at 360 nm, emission at 490 nm).20 In the solid state, all compounds displayed blueish-green to greenish-yellow emission at around 500−550 nm arising from the ICT effect. Notably, all compounds showed higher quantum yields in the solid state than in solution. To better understand this observation, the aggregation effect was investigated by the fluorescence spectrum in the mixed solvent of water and THF. In the more polar solvent, emission intensity of these compounds should decrease to insignificant values because of positive solvato kinetic effect and other possible interactions such as photoinduced electron transfer. However, aggregation of these compounds in THF/H2O mixed solvent resulted in increased emission intensities due to aggregation-induced emission. Figure 3a−d show the optical behavior of 1 dissolved in THF/water solutions with different water fractions (f w). Increasing the proportion of water had little effect on the spectroscopic properties until f w = 60%. The emission intensity increased upon the addition of more than 70% water to THF. The fluorescence intensity reached a maximum value at a water content of 90% and a slight blue shift was observed, from 512 to 497 nm. In contrast, the emission wavelength of 3 in mixed solvent changed dramatically. The emission in pure THF ( f w = 0%) was red due to TICT. The photoluminescence intensity

decreased up to a water concentration of about f w < 60%. Compound 3 showed a strong nonradiative relaxation effect from the TICT state in THF. The addition of a small amount of water increased the polarity of the microenvironment, the fluorescence intensity of compound 3 dropped dramatically, and subsequently the emission color changed to green and intensified due to aggregate formation. The emission band significantly blue-shifted, from 627 to 540 nm, as the solvent polarity increased (Figure 3h). These hypsochromic shifts in the fluorescence could be a result of aggregation-induced blueshifted emission.32,33 The observed phenomenon in the aggregated state originate from noncoplanaity in the excited state and freezing of the TICT transition state. The present results obtained using solvents of various polarities agree well with those obtained for the compounds in the solid state. The emission intensities increased 6-fold for 1 and doubled for 3 upon progressing from a pure THF (100%) solution to a 10%:90% THF:water mixture (Figure 3c, g). It is noted that concentration quenchings of 1 and 3 were not observed by the measurement of fluorescence spectra of 1 and 3 in dichloromethane at various concentrations (Figures S13 and 14). 2.3. Crystal Analysis. The packing style and intermolecular interactions of crystalline compounds are related to their solid state physical properties. To investigate the decisive factor determining AIEE, we aimed to study the single-crystal structures of the above compounds, because X-ray crystal structures provide useful information on spatial arrangements in the structure. The X-ray-quality crystals of 4 were obtained by recrystallization from acetonitrile. The structure was determined by X-ray diffraction (Figure 4). The X-ray crystal structure of 4 contains no notable intermolecular π−π interactions. The disilane-bridged groups resulted in increased steric hindrance between pairs of adjacent molecules, preventing the fluorescence quenching typically observed in the solid or aggregated state. Consequently, compound 4 in the solid state resulted in AIEE, thereby overcoming fluorescence quenching caused by aggregation. 2.4. Electrochemical Properties. The redox properties of 1 were investigated by cyclic voltammetry (CV) in CH2Cl2. The redox potential and CV chart are shown in Table S6 and Figure S16, respectively. The CV profile of 1 shows one reversible electron oxidation wave at 0.51 V vs the ferrocene/ ferrocenium (Fc/Fc+) couple. The oxidation process is assigned as the terminal triphenylamine segment to give a monocation radical.34,35 This oxidation value is lower than that observed for our previous disilane-bridged D−A−D compound (Figure 1a), as expected given the effect of the stronger electron donor group. An irreversible reduction wave was observed at −2.10 V vs Fc/Fc+ in the cathodic potential region. This reduction process is assigned to the reduction of the central acceptor moiety.36 The values of HOMO and LUMO for 1 were estimated to be −5.3 and −2.5 eV, respectively, and were calculated from an energy level of 4.8 eV relative to vacuum vs Fc/Fc+.37 The HOMO−LUMO energy gap was determined to be 2.8 eV, indicating that 1 is suitable for application in OLEDs. 2.5. Thermal Stability. The thermal behavior of 1−6 was studied by thermal gravimetric analysis (TGA). The weight loss of the compound due to thermal instability was determined under a nitrogen atmosphere by heating from 30 to 500 °C at a rate of 10 °C per min. The weight of 2 and 3 were gradually decreased blow 180 and 120 °C, respectively (Figures S18 and S19). On the other hand, weight change of 1, 4, and 5 was not D


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of compound 1, there was no signal change between before and after heating (Figure S23), whereas new signals appeared in the methyl region after heating in NMR spectra of compound 2 (Figures S24 and 25). Those mean that 1 is chemically stable but 2 decomposes at 200 °C under inert condition. These results correspond to TGA and indicate that compound 1 has good thermal stability, which is important to device fabrication. 2.6. Theoretical Calculations. In an attempt to gain more insight into the electronic structures of compounds 1−6, we explored them through DFT and TD-DFT calculations performed at the B3LYP/6-31G(d) level of theory.38 The frontier molecular orbitals (HOMO and LUMO) and their energies are summarized in Figure S28. The HOMO and LUMO were spatially well separated. The HOMOs were delocalized over the triarylamine units, whereas the LUMOs were located at the acceptor moiety. The TD-DFT calculations indicate that the low energy side of the absorption spectra (S0 → S1 transition) is associated with HOMO−LUMO excitation (Tables S7−S12), in good agreement with the longestwavelength absorption peaks ascribed to ICT transitions. The addition of an electron-donating substituent (methoxy functionality) on the triarylamine groups decreased the HOMO−LUMO energy gaps, resulting in red-shifted absorbance and emission maxima. These tendencies agree well with the observed UV−vis spectra. In comparison with previous work, incorporation of a triarylamine functionality raises the HOMO energy level, while the bridged disilane unit is beneficial for stabilizing the LUMO energy level. Combining these two structural functionalities results in a relatively low-band gap emitter. This reduction in the HOMO−LUMO band gap agrees well with the bathochromic shifts observed in the UV−vis spectra. 2.7. OLED Device Fabrication. The prepared disilane compounds exhibited promising optical properties, such as high luminescence quantum yields, which encouraged us to use them as doped and undoped emitters for the preparation of OLEDs. Compound 1 was selected as an emitter to investigate its application in electroluminescent devices because of the most efficient solid state emission in 1−6 and high thermal stability. Two different OLED devices, ITO/α-NPD/1/BCP/ LiF/Al (undoped) and ITO/α-NPD/DPVBi:1(3%)/BCP/ LiF/Al (doped), were fabricated by thermal deposition in an ultrahigh vacuum multichamber system. An energy level diagram of the devices and the thicknesses of each layer are

Figure 4. (a) Single-crystal X-ray structure of 4. Hydrogen atoms have been omitted for clarity, and all thermal ellipsoids are displayed at 50% probability. (b) Packing structure of 4.

observed below 370 °C (Figures S17, S20, 21). Moreover, 6 showed no weight change to be 400 °C (Figure S22). In addition, to examine thermal stability, 1H NMR of 1 and 2 before and after heating was measured. These compounds were heated at 200 °C for 1 h under vacuum conditions. In the case

Figure 5. (a) EL spectra of undoped (blue line) and 1-doped (orange line) devices. (b) Photographs of EL devices at a driving voltage of 18 V. Undoped EL device: ITO/α-NPD/1/BCP/LiF/Al. Doped EL device: ITO/α-NPD/DPVBi:1 (3%)/BCP/LiF/Al. E


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water, with blue-shifted emission providing bright greenish fluorescence. Donor−acceptor interaction in these compounds is supported by the results of DFT calculations. The single crystal X-ray structure of 4 revealed the suppression of π−π stacking in the solid state. Cyclic voltammetry of 1 showed a reversible oxidation process at 0.51 V vs Fc/Fc+, corresponding to the formation of triphenylamine radical cation. The triphenylamine substituent is highly oxidizable and leads to a cathodic shift in the oxidation potential. Undoped OLEDs were fabricated using compound 1 as emitter, and the devices based on 1 exhibited green electroluminescence with a maximum external quantum efficiency of 0.65%. OLED performance was enhanced in the absence of the host material (DPVBi) as the emitting layer.

shown in Figures S27 and S29, respectively. The results obtained using the doped and undoped devices suggested that these materials can efficiently transport both electrons and holes. In these fabricated OLED devices, N,N′-di-1-naphthylN,N′-diphenylbenzidine (α-NPD) was used as hole-transporting layer, bathocuproine (BCP) was as an electrontransporting layer, and 4,4′-bis(2,2-diphenylvinyl)-1,1′-biphenyl (DPVBi) in doped OLED was utilized as a host material in an emissive layer. The EL emission spectra of undoped and DPVBi-doped devices are shown in Figure 5a. The electroluminescence spectra of the fabricated devices exhibited maxima close to that observed in the solid-state photoluminescence spectrum of 1. These devices exhibited green emission, as shown in Figure 5b, indicating that the green emission is the intrinsic emission of 1. Compound 1 has good film-forming ability, and an OLED without host material (DPVBi) can be prepared. The performances of these devices are summarized in Figure S30. Their current density−voltage curves in Figure S30a indicate that the current increases nonlinearly as the applied voltage is increased, thereby confirming the semiconducting properties of the investigated compound. The optical power of the OLED is shown in Figure S30b: the turn-on voltages of these devices were in the range 9−10 V. For the undoped device, the external quantum efficiency rises sharply at low current densities to a maximum of 0.65% at 1 × 10−3 A/cm2, then falls off rapidly at higher current densities (Figure S30c). Commercially available DPVBi is the most commonly used host material for blue to green fluorescent dopants in the fabrication of OLEDs.39 The fluorescence and absorption spectra of 1 partially overlap, and green fluorescence can be obtained using an appropriate ratio of 1 mixed with DPVBi. The maximum external quantum efficiency of this doped device exhibited 0.47%. Moreover, we are aware that the doped device is rather unstable even in an inert atmosphere. Compound 1 showed a higher quantum yield when dispersed in poly(methyl methacrylate) (PMMA) (Table S4, ΦF: 0.35) than when in the solid state (ΦF: 0.25), although the emission wavelength was almost the same (Figure S15, λem: 491 nm). However, the external quantum efficiency of the device fabricated using 1 doped in DPVBi (η: 0.47%) was lower than that of the undoped device (η: 0.65%). We assume that energy is lost during the energy transfer between the dopant and host material. The introduction of triarylamine groups as electron donors resulted in significant separation of the HOMO and LUMO. Complete separation can be helpful in improving the carrier current and electroluminescence of 1.40−43

4. EXPERIMENTAL SECTION 4.1. Materials and Instruments. All chemicals and reagents were obtained from commercial sources and used without additional purification, except for THF and toluene used for synthesis, which were purified using an organic purifier. Optical spectra of the samples were measured using spectroscopically pure solvents. ITO-coated glass with a sheet resistance of ca. 15 Ω/square was purchased from Kuramoto Co., Ltd. Prior to use, the glass was cleaned by sonication in a detergent solution and then rinsed in ultrapure water, acetone, and 2propanol. All melting points were taken on a Yanaco melting point apparatus and were uncorrected. 1H NMR (500 MHz) and 13C NMR (125 MHz) spectra were recorded on Bruker US500 spectrometer using CDCl3 as the solvent and TMS as an internal standard. Chemical shifts of NMR spectra were calibrated as TMS = 0.00 ppm for 1H NMR and CDCl3 solvent peak = 77.0 ppm for 13C NMR, respectively. FAB mass spectra were measured with a JEOL MStation JMS-700 spectrometer. ESI-TOF mass spectra were recorded on a Micromass LCT spectrometer. UV−vis absorption spectra were measured with a JASCO V-570 spectrometer. Fluorescence spectra and absolute quantum yields were measured with a JASCO FP-8600 spectrometer and a Hamamatsu C9920−02 spectrometer. Fluorescence lifetimes were measured with a Hamamatsu Quantaurus-Tau C11367 spectrometer. The thermal stabilities of the samples under a nitrogen atmosphere were determined by measuring their weight loss while heating at a rate of 10 °C/min from 25 to 500 °C. The most stable conformers and energy transitions were calculated by DFT and TDDFT computations performed using the Gaussian 09 Revision E.01 program and the B3LYP/6-31G(d) basis set.38 4.2. General Procedure for the Synthesis of D−Si−Si−A−Si− Si−D Molecules. To a solution of 5,7-diiodo-2,3-dimethylthieno[3,4b]pyradine (417.4 mg, 1.0 mmol), Pd(P(t-Bu)3)2 (24.6 mg, 0.05 mmol) in toluene (3.00 mL), and N,N-diisopropylethylamine (0.78 mL, 4.0 mmol) was added toluene solution of N,N-diphenyl-4(1,1,2,2-tetramethyldisilanyl)aniline (2.2 mmol, 4.50 mL) for 4 days at 0 °C under an argon atmosphere. The mixture was quenched with water and extracted with CH2Cl2 three times. The combined extract was dried over Na2SO4. The solvent was evaporated under reduced pressure and roughly purified by column chromatography on SiO2 (eluent: hexane/EtOAc = 1/1). The analytically pure compound was obtained by GPC column (eluent: CHCl3) to afford 4,4′-((2,3dimethylthieno[3,4-b]pyrazine-5,7-diyl)bis(1,1,2,2-tetramethyldisilane2,1-diyl))bis(N,N-diphenylaniline) (1) as pale yellow solid in 39%. 4,4′-((2,3-Dimethylthieno[3,4-b]pyrazine-5,7-diyl)bis(1,1,2,2-tetramethyldisilane-2,1-diyl))bis(N,N-diphenylaniline) (1). Yield 39%. Pale yellow solid. Mp: 53.4−55.0 °C. 1H NMR (500 MHz, CDCl3): δ 7.24−7.21 (m, 12H), 7.06 (dd, 8H, J = 8.7, 1.1 Hz), 7.00 (t, 4H, J = 7.3 Hz), 6.93 (d, 4H, J = 8.5 Hz), 2.54 (s, 6H), 0.49 (s, 12H), 0.32 (s, 12H). 13C NMR (125 MHz, CDCl3): δ 150.6 (Cq), 149.8 (Cq), 147.9 (Cq), 147.6 (Cq), 135.9 (Cq), 134.8 (CH), 132.0 (Cq), 129.2 (CH), 124.5 (CH), 122.8 (CH), 122.4 (CH), 23.4 (CH3), −2.6 (CH3), −3.3 (CH3). FAB−LRMS m/z 882 (M+). FAB−HRMS Calcd for C52H58N4SSi4: 882.3459. Found: 882.3479 (M+).

3. CONCLUSION In summary, new D−Si−Si−A−Si−Si−D luminogenic compounds, comprising triarylamines as electron-donating terminal groups and benzothiazole or thienopyrazine as electronwithdrawing core groups, have been designed and synthesized by Pd-catalyzed arylation of hydrosilanes. The structural and optical properties of these compounds were investigated, and their photophysical properties were controlled by selecting appropriate donor and acceptor structures. These compounds were all categorized as green-light emitters in the solid state. Their maximum emission wavelengths were observed at 485− 547 nm in the solid state and at 480−670 nm in solution. The compounds exhibited high fluorescence efficiencies, with quantum yields (ΦF) of up to 0.25. Compounds 1 and 3 displayed remarkable AIEE in mixed solvents of THF and F


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and supporting electrolyte concentrations were 5 × 10−4 mol/L and 0.1 mol/L, respectively. The typical scan rate of 0.1 V/s was used. All electrochemical experiments were carried out at room temperature under nitrogen atmosphere. 4.5. OLED Device Fabrication. We fabricated multilayer OLEDs on ITO-coated glass substrates. The substrates were cut and cleaned with detergent, ultrapure water, acetone, and 2-propanol. The substrates were then treated with UV−ozone for 30 min before being loaded into the vacuum chamber. The organic layers and the Li/ Al cathode were then thermally evaporated onto the substrates. The effective area of the emitting diode was 6.0 mm2. An optical fiber guided the EL light to a spectrograph (Oriel, model FICS 77441) connected to a CCD camera (Andor, model DU420OE) to measure the spectra. The current−voltage (I−V) characteristics of OLEDs were measured using a Keithley 2400 SourceMeter. The power of EL light was measured with Thorlabs S120 Optical Power Meter simultaneously with the electric current. The external quantum efficiencies of the EL devices were estimated by considering the area of detection (solid angle of 32.34°). All measurements were carried out under ambient conditions.

4,4′-(Benzo[c][1,2,5]thiadiazole-4,7-diylbis(1,1,2,2-tetramethyldisilane-2,1-diyl))bis(N,N-diphenylaniline) (2). Yield 33%. Pale yellow solid. Mp: 126.2−128.4 °C. 1H NMR (500 MHz, CDCl3): δ 7.52 (s, 2H), 7.23 (d, 8H, J = 7.9 Hz), 7.15 (d, 4H, J = 8.2 Hz), 7.05 (d, 8H, J = 7.6 Hz), 7.00 (t, 4H, J = 7.3 Hz), 6.90 (d, 4H, J = 8.5 Hz), 0.48 (s, 12H), 0.28 (s, 12H). 13C NMR (125 MHz, CDCl3): δ 158.0 (Cq), 147.9 (Cq), 147.6 (CH) 135.1 (Cq), 134.8 (Cq), 134.6 (CH), 131.8 (Cq), 129.2 (CH), 124.6 (CH), 122.9 (CH), 122.4 (CH), −3.3 (CH3), −3.5 (CH3). FAB−LRMS m/z 854 (M+). FAB−HRMS Calcd for C50H54N4SSi4: 854.3146. Found: 854.3136 (M+). 4,4′-((2,3-Dimethylthieno[3,4-b]pyrazine-5,7-diyl)bis(1,1,2,2-tetramethyldisilane-2,1-diyl))bis(N,N-bis(4-methoxyphenyl)aniline) (3). Yield 15%. Pale yellow solid. Mp: 53.3−55.1 °C. 1H NMR (500 MHz, CDCl3): δ 7.17 (d, 4H, J = 8.5 Hz), 7.02 (d, 8H, J = 9.1 Hz), 6.82− 6.78 (m, 12H), 3.78 (s, 12H), 2.54 (s, 6H), 0.48 (s, 12H), 0.30 (s, 12H). 13C NMR (125 MHz, CDCl3): δ 155.8 (Cq), 150.5 (Cq), 149.8 (Cq), 148.8 (Cq), 140.8 (Cq), 135.9 (Cq), 134.6 (CH), 129.2 (Cq), 126.8 (CH), 119.2 (CH), 114.6 (CH), 55.4 (CH3), 23.4 (CH3), −2.6 (CH3), −3.2 (CH3). ESI−LRMS m/z 1002 (M+). ESI−HRMS Calcd for C56H66N4O4SSi4: 1002.3882. Found: 1002.3879 (M+). 4,4′-(Benzo[c][1,2,5]thiadiazole-4,7-diylbis(1,1,2,2-tetramethyldisilane-2,1-diyl))bis(N,N-bis(4-methoxyphenyl)aniline) (4). Yield 44%. Pale yellow solid. Mp: 156.5−157.5 °C. 1H NMR (500 MHz, CDCl3): δ 7.52 (s, 2H), 7.09 (d, 4H, J = 8.5 Hz), 7.02 (d, 8H, J = 9.2 Hz), 6.81 (d, 8H, J = 9.2 Hz), 6.76 (d, 4H, J = 8.5 Hz), 3.78 (s, 12H), 0.47 (s, 12H), 0.26 (s, 12H). 13C NMR (125 MHz, CDCl3): δ 158.0 (Cq), 155.9 (Cq), 148.9 (Cq), 140.8 (CH), 135.1 (Cq), 134.8 (Cq), 134.5 (CH), 129.0 (Cq), 126.8 (CH), 119.2 (CH), 114.6 (CH), 55.6 (CH3), −3.2 (CH3), −3.4 (CH3). ESI−LRMS m/z 974 (M+). ESI−HRMS Calcd for C54H62N4O4SSi4: 974.3569. Found: 974.3550 (M+). 5,7-Bis(2-(4-(9H-carbazol-9-yl)phenyl)-1,1,2,2-tetramethyldisilaneyl)-2,3-dimethylthieno[3,4-b]pyrazine (5). Yield 30%. Pale yellow solid. Mp: 87.4−90.2 °C. 1H NMR (500 MHz, CDCl3): δ 8.13 (d, 4H, J = 7.9 Hz), 7.59 (d, 4H, J = 8.2 Hz), 7.42 (d, 4H, J = 8.2 Hz), 7.38− 7.36 (m, 8H), 7.28−7.26 (m, 4H), 2.55 (s, 6H), 0.57 (s, 12H), 0.44 (s, 12H). 13C NMR (125 MHz, CDCl3): δ 150.9 (Cq), 149.9 (Cq), 140.7 (Cq), 138.8 (Cq), 137.7 (Cq), 135.7 (Cq), 135.4 (CH), 125.9 (CH), 125.8 (CH), 123.4 (Cq), 120.2 (CH), 119.8 (CH), 109.8 (CH), 23.5 (CH3), −2.6 (CH3), −3.4 (CH3). FAB−LRMS m/z 878 (M+). FAB− HRMS Calcd for C52H54N4SSi4: 878.3146. Found: 878.3136 (M+). 4,7-Bis(2-(4-(9H-carbazol-9-yl)phenyl)-1,1,2,2tetramethyldisilaneyl)benzo[c][1,2,5]thiadiazole (6). Yield 29%. Pale yellow solid. Mp: 83.1−85.1 °C. 1H NMR (500 MHz, CDCl3): δ 8.11 (d, 4H, J = 7.6 Hz), 7.59 (s, 2H), 7.49 (d, 4H, J = 7.9 Hz), 7.40−7.37 (m, 12H), 7.28−7.25 (m, 4H), 0.54 (s, 12H), 0.39 (s, 12H). 13C NMR (125 MHz, CDCl3): δ 158.0 (Cq), 140.7 (CH), 138.7 (Cq), 137.7 (Cq), 135.2 (Cq), 135.1 (CH), 134.6 (Cq), 125.9 (CH), 125.8 (CH), 123.4 (Cq), 120.3 (CH), 119.9 (CH), 109.8 (CH), −3.4 (CH3), −3.5 (CH3). FAB−LRMS m/z 850 (M+). FAB−HRMS Calcd for C50H50N4SSi4: 850.2833. Found: 850.2819 (M+). 4.3. Crystallography. Single crystal of 4 for X-ray diffraction was prepared by the recrystallization from acetonitrile. A suitable crystal was mounted on a glass fiber using grease. Synchrotron radiation (SR) X-ray diffraction data were collected on a CCD detector at SPring-8 beamline BL26B2 (Hyogo, Japan) at 100 K. The total number of observed reflections and the number of unique reflections were 15324 (2θ < 20.135°; d > 0.60 Å) and 9020, respectively. The structure was solved by direct method using SIR-2008 program,44 and was refined by full-matrix least-squares techniques against F2 by implementing SHELXL-2016/6.45 Crystallographic data of 4 have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 1575433. 4.4. Electrochemical Measurements. Cyclic voltammograms were recorded with BAS ALS-650DT using Ag/Ag+ reference, glassycarbon working, and Pt wire counter electrodes. Potentials are reported with reference to an internal standard of ferrocenium/ ferrocene. A three-electrode one-compartment cell was used containing disilane compounds and supporting electrolyte (nBu4NPF6) in CH2Cl2. Deaeration of the solution was performed by bubbling an inert gas for 10 min before measurement. The compound

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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/acsami.7b14802. Optical properties of 1−6 in toluene and acetone, relative fluorescence quantum yields of 1−6 in dichloromethane, UV−vis and FL spectra of 1−6, fluorescence spectrum of 1 and 3 in dichloromethane at various concentration, fluorescence spectrum of 1 in PMMA, crystallographic data of 4, cyclic voltammogram of 1, thermal stability of 1−6, energy diagram of OLED, theoretical calculations of 1−6, OLED characteristics, copies of 1H & 13C NMR of 1−6 (PDF) Crystallographic information file for C56H62N4O4SSi4 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yoshinori Yamanoi: 0000-0002-6155-2357 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported in part by CREST from JST (H.N.), Nippon Sheet Glass Foundation for Materials Science and Engineering (Y.Y.), Mitutoyo Association for Science and Technology (MAST) (Y.Y.), and Grants-in-Aid for Scientific Research (C) (15K05604; Y.Y.), Scientific Research on Innovative Areas “Molecular Architectonics: Orchestration of Single Molecules for Novel Functions” (area 2509, Nos. 25110012, 26110505, 26110506, 16H00957, and 16H00958; H.T., H.N., and Y.Y.), and “Soft Crystals: Science and Photofunctions of Easy-Responsive Systems with Flexibility and Higher-Ordering” (area 2903, 17H06369; Y.Y.) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. The synchrotron single-crystal X-ray analysis was carried out on the BL26B2 beamline of SPring-8 with the approval of RIKEN (proposal 20176763). G


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Aggregation-Induced Emission Enhancement from Disilane-Bridged

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