Modulation of Properties by Ion Changing Based on Luminescent Ionic Salts Consisting of Spirobi(boron ketoiminate).

We report development of luminescent ionic salts consisting of the boron ketoiminate structure, which is one of the robust skeletons for expressing aggregation-induced emission (AIE) properties. From the formation of the boron-centered spiro structure with the ketoiminate ligands, we obtained stable ionic salts with variable anions. Since the ionic salts show Tms below 100 °C, it was shown that these salts can be classified as an ionic liquid. By using PF6 anion, the single crystal-which is applicable for X-ray crystallography-was obtained. According to the optical measurements, it was proposed that electronic interaction should occur through the boron center. Moreover, intense emission was observed both in solution and solid. Finally, we demonstrated that the emission color of the PF6 salt was altered from crystal to amorphous by adding mechanical forces. Based on boron complexation and intrinsic solid-state luminescent characters, we achieved obtainment of emissive ionic materials with environmental responsivity.

1. Introduction

Most organic luminescent dyes show poor emission properties in solids due to aggregation-caused quenching (ACQ) induced mainly by non-specific intermolecular interactions. Therefore, for designing film-type sensors and devices based on organic materials, it is essential to load some mechanisms for suppressing ACQ. One of the promising strategies is to apply the class of molecules possessing AIE properties. AIE-active molecules can show intense emission only when they are aggregates. In the solution state, excited states are readily decayed by intramolecular interaction, while emission can be recovered in solid by suppressing molecular motions and intermolecular interactions. As a result, AIE behaviors can be realized. Furthermore, on the basis of potential environmental sensitivity of AIE-active molecules, various types of stimuli-responsive luminochromic materials and sensors have been developed by employing AIE-presenting skeletons [1,2,3]. For instance, by utilizing aggregation behaviors for signal amplification, a trace amount of water can be detected [4,5]. From this viewpoint, heteroatom-containing molecules with AIE properties are attractive candidates because potential environmental sensitivity of heteroatoms is available for expressing stimuli responsiveness [6,7,8,9]. We have also discovered AIE-active boron complexes and developed stimuli-responsive materials by using these molecules as an element-block [10,11,12,13], which is a building block containing heteroatoms for constructing functional materials [14,15,16,17]. For example, simply by connecting AIE-active boron complexes, AIE-active and solid-state luminescent polymers can be fabricated from heteroatom-containing molecules including boron complexes [18,19,20,21,22]. In particular, since stimuli responsiveness, such as luminochromism as well as intensity changes, was often observed, the series of luminescent sensors can be obtained [23,24,25,26,27,28,29]. By replacing boron with a different element, such as other group-13 elements, stimuli responsiveness and/or unique luminescent properties were observed [30,31,32,33]. From the changes in emission intensity and color, the target molecules or alteration of environmental factors can be monitored.

In conventional boron complexes, difluoride is very common because of its high stability and low synthetic difficulty. Meanwhile, some research groups tried replacing the two fluoride atoms with another unit [34,35,36,37,38,39,40,41,42]. The resulting spiro complexes have various unique features originating from cationic character and steric structures. For instance, boron complexes with two diketone ligands were synthesized [34,35]. These compounds showed interesting stimuli responsiveness, such as solvatochromism and thermochromism. Another paper reported that the central boron atom in the bis(1,9-oxido-phenalenyl)boron complex has an intriguing property [40,41,42]. In these research papers, boron complexes have the spiro structure, where each ligand perpendicularly bridges through the central boron atom. These non-planar spiro structure play key roles in organic electronic devices, such as OLEDs [43], organic phototransistors (OPTs) [44], and organic solid-state lasers (OSSLs) [45]. Spiro structures show chemical and thermal stability and good solubility because of their steric architectures. Further, their bulky structures play a role in lowering melting temperatures (Tms) and crystallinity. Therefore, spirobifluorene derivatives were used for OLEDs as amorphous emissive materials [45]. Therefore, we designed the new boron complexes consisting of the spiro structure to obtain solid-state luminescent properties and expected stimuli responsiveness in condensed states.

Herein, we report ionic salts consisting of the spiro structure with luminescent properties in solution and solid states. On the basis of the spiro structure with luminescent boron complexes, four types of ionic salts with variable counter anions were prepared. From the thermal analyses, it was observed that all salts have melting temperatures below 100 °C, indicating that the products can be classified as an ionic liquid according to the conventional definition. The ionic salts exhibit intense emission in solid as well as solution. In particular, the PF6 salt can form single crystal and show different luminescent colors between crystal and amorphous states. Finally, mechanochromic luminescence was observed from the PF6 salt. We can demonstrate the design of luminescent ionic materials based on boron coordination properties.

2. Results and Discussion

Synthesis of the spirobi(boron ketoiminate) (BBK) structure and salt exchanges were performed according to Scheme 1 according to the previous study [20]. Boron trichloride was added to mixture solution of the ketoimine ligand 1 in the presence of triethylamine in CH2Cl2 and stirred at r.t. for 24 h. The salt BBK-Cl was obtained as a yellow solid after freeze-drying. We also prepared BKI as a model compound M [20]. From the characterization with 1H, 13C and 11B NMR spectroscopy and a mass measurement, we obtained the expected data. Further, by using BBK-Cl as a platform, we prepared the series of salt compounds with variable anion species. The DMSO solution of BBK-Cl was added to AgOTf in MeCN and stirred at r.t. for 3 h. After removal of the precipitated AgCl using filtration, the yellow solid was obtained through extraction followed by freeze-drying. With this protocol, three kinds of salts were obtained with trifluorosulfonate (OTf−), hexafluorophosphate (PF6−) and bis(trifluoromethanesulfonyl)imide (TFSI−) anions (Scheme 1). All products were characterized with NMR and mass spectrometry, and we concluded that the products have enough purity for further optical and thermal analyses.

Scheme 1

Syntheses of ionic salts.

Fortunately, it should be emphasized that BBK-PF6 formed a good single green crystal for analysis with X-ray crystallography. It was clearly shown that the compound has the expected structure and axial chirality (Figure 1). Accordingly, each Ra (light blue) and Sa (pink) formed dimer pairs and the layered structure, proposing that both π-conjugated planes should have an interaction. The distance between the closest planes was 3.46 Å, which is reasonable for the formation of π−π interaction. The structural data suggest that the symmetric property of PF6 anion and hydrogen bonds between fluoride and hydrogen could support crystallization.

Figure 1

(a) ORTEP drawings of BBK-PF6 (black—C; blue—N; red—O; white—H; light green—F; orange—P). (b) Axial chirality pair of the BBK unit; eliminated PF6−. (c) Packing structure of chiral pair of the BBK unit; eliminated PF6− (blue—Ra, pink—Sa).

Thermal properties, such as decomposition temperatures (Tds) and melting temperatures (Tms), were measured with thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively (Table 1 and Figures S1 and S2). The Td value of BBK-TFSI was higher than that of others. It is suggested that thermal motions should be suppressed in BBK-TFSI because the molecular weight of TFSI− is the largest of the four anions. In addition, we estimated their weight losses from the TGA profiles and observed that each decrease was equal to molecular weight of anion species, meaning that initial degradation occurs at the anion moieties. In the DSC results, it was clearly shown that all compounds have Tm below 100 °C, indicating that all ionic salts can be classified as an ionic liquid [46,47,48]. Two ligands with the spiro structures could play a critical role in lowering Tms by disturbing intermolecular interaction in the crystalline state. Moreover, symmetric structures of cations might contribute to lowering Tms, as observed in the nano-cluster-containing ionic liquids [49,50].

Table 1

Optical properties of the ionic salts with variable anion.

	BBK-Cl	BBK-PF6	BBK-OTf	BBK-TFSI	M f
λabs,sol [nm] a	391	391	393	394	380
ε [M−1cm−1] a	39,200	19,000	33,300	34,800	22,300
λem,solution [nm] a	453	453	455	453	460
Φsolution a,b	0.26	0.24	0.26	0.26	0.35
τ [ns] c	1.3	1.4	1.4	1.4	0.5
	(χ2 = 1.12)	(χ2 = 1.15)	(χ2 = 1.03)	(χ2 = 1.15)	(χ2 = 1.01)
kf [× 108 s−1]	1.8	1.7	1.8	1.8	3.7
knr [× 108 s−1]	5.7	5.4	5.2	5.3	18.1
Td (°C) d	214	218	193	233
Tm (°C) e	75	65	68	55

Measured in CH2Cl2 (1.0 × 10−5 mol/L). Calculated as an absolute value determined using an integrated sphere method. Fluorescence lifetime: excited at 375 nm, detected at λem,sol. Determined from the onset in TGA profiles. Determined from DSC. Reprinted with permission from ref. [20]. Copyright 2017 John Wiley and Sons.

To investigate electronic structures in the ground state, UV−vis absorption spectra in CH2Cl2 solution were measured. The peak wavelength of BBK-Cl was found at 392 nm (Figure 2a and Table 1). Compared with ketoiminate difluoride M as a model compound, the peak appeared in the longer-wavelength region. This result suggests that electronic interaction should be caused through the central boron atom. We also examined absorption properties through anion exchanges (Figure 2a and Table 1). Accordingly, significant changes were hardly observed, indicating anion species hardly affect the electronic structures of the ionic salts.

Figure 2

(a) UV−vis absorption and (b) emission spectra of the ionic salts in CH2Cl2 solution (1.0 × 10−5 mol/L). Emission spectra were obtained with the excitation light at λabs,max.

We measured emission spectra with three kinds of the ionic salts in dichloromethane (Figure 2b and Table 1). BBK-Cl showed the emission band in the blue region in the solution state, and slight differences were observed through anion exchange, indicating that anion hardly played a role in optical properties. Similar to the absorption properties, anion species hardly influence electronic structures of the ligand moieties in solution (Figure 2b and Table 1). Meanwhile, emission bands of the ionic salts were observed in the shorter wavelength region compared to that of M. According to the previous reports, the optical properties can be rationally explained [19,20]. The ligand potential shows a larger degree of structural relaxation in the excited state. Through the formation of the spiro structures, structural relaxation could be disturbed, followed by emission bands in the shorter-wavelength region. The smaller rate constant of non-radiation decay of the ionic salts comparing to that of M strongly supports the suppression of molecular motions in the excited state by the spiro formation.

Since the crystalline sample was possible to obtain from BBK-PF6, we monitored changes in luminescent properties in solid (Figure 3 and Figure S4, Table 1 and Table S2). It should be emphasized that BBK-PF6 can exhibit emission in crystal (ΦPL = 0.10, Table 1 and Table S1). The crystal sample of BBK-PF6 exhibited green emission with the peak at 507 nm (Figure 3a). When the sample was scratched, the emission band shifted to the blue region by 30 nm and emission color was changed to light blue (Figure 3b,c). These data represent that BBK-PF6 has a mechanochromic fluorescent property. This specific property should originate from changes in intermolecular interaction through π−π stacking during the morphology change from crystal to amorphous (Figure S3, Supplementary Materials) [51,52]. In the crystalline state, BBK-PF6 formed ordered structures and had strong π−π intermolecular interaction between each chirality pair (Figure 1c). As a result, the emission band was observed in the longer-wavelength region. On the other hands, when that ordered packing was crumbled by mechanical stress, intermolecular interaction should decrease. Consequently, the emission band appeared in the shorter wavelength region. Their quantum yields before and after grinding were 0.10 and 0.15, respectively. These data support that loss of π−π intermolecular interaction should be responsible for emission in the condensed state. It should be mentioned that the mechanochromic luminescent property was obtained only from BBK-PF6 which can form single crystals. Relatively higher crystallinity of the PF6 salt should be favorable for exhibiting luminochromism through molecular environmental changes.

Figure 3

(a) Fluorescent spectra of BBK-PF6 in the solid states (orange—crystalline pristine sample; blue—ground sample). Photographs of BBK-PF6 under UV irradiation (b) before and (c) after grinding.

3. Experimental Section

General: 1H (400 MHz), 11B (128 MHz), and 13C (100 MHz) NMR spectra were recorded on a JEOL JNM-EX400 or a JEOL JNM-AL400 spectrometers (JEOL ltd., Tokyo, Japan). In 1H and 13C NMR spectra, tetramethylsilane (TMS) was used as an internal standard in CDCl3, and 11B NMR spectra were referenced externally to BF3∙OEt2 (sealed capillary). UV−vis absorption spectra were recorded on a SHIMADZU UV-3600 spectrophotometer (Shimadzu Corporation, Kyoto, Japan). Photoluminescence (PL) spectra were measured with a HORIBA JOBIN YVON Fluorolog spectrofluorometer (HORIBA, Ltd., Kyoto, Japan), and photoluminescence quantum yields were calculated by the integrating sphere method. Fluorescence lifetime analyses were carried out on a Horiba FluoreCube spectrofluorometer system (HORIBA, Ltd., Kyoto, Japan); excitation at 375 nm using a UV diode laser (NanoLED-375L). Elemental analysis was performed at the Microanalytical Center of Kyoto University. DSC thermograms were carried out on a SII DSC 6220 instrument (Seiko Instruments Inc., Chiba, Japan). The sample on the aluminum pan was heated at the rate of 10 °C/min under nitrogen flowing (50 mL/min). Thermogravimetric analysis (TGA) was recorded on a Seiko Instruments Inc. (Chiba, Japan) EXSTAR TG/DTA6000. X-ray crystallographic analyses were carried out by Rigaku R-AXIS RAPID-F graphite-monochromated Mo Kα radiation diffractometer with an imaging plate. A symmetry-related absorption correction was carried out using the program ABSCOR [53]. The analysis was carried out with direct methods (SHELX-97 [54] or SIR92 [55]) using Yadokari-XG [56]. The program ORTEP35 was used to generate the X-ray structural diagram [57,58]. Powder X-ray diffraction (PXRD) patterns were taken by using CuKα radiation with Rigaku Miniflex (Rigaku Corporation, Tokyo, Japan).

Synthesis of M: BF3⋅Et2O (6.2 mL, 7.11 g, 50.1 mmol) was added to the solution of 1 [20] (1.37 g, 5.0 mmol) in the mixed solvent of CH2Cl2 (30 mL) and NEt3 (6 mL). The reaction mixture was refluxed under Ar atmosphere for 24 h and then cooled at r.t. The organic layer was washed with water (100 mL × 2) and brine (100 mL), dried over anhydrous magnesium sulfate, and concentrated by a rotary evaporator. The resulting solid was purified by silica gel column chromatography eluted with hexane/AcOEt (2/1). The product M was obtained after it was recrystallized from ethanol as an orange crystal (0.81 g, 50%). 1H NMR (CDCl3): δ 8.47 (1H, d, J = 6.2 Hz, Ar-H), 7.97 (2H, dd, J = 6.8, 1.7 Hz), 7.70 (2H, dd, J = 5.9, 2.1 Hz), 7.56 (1H, s), 7.55 (2H, d, J = 2.6 Hz), 7.49 (2H, d, J = 5.3 Hz), 7.45 (3H, m), 6.44 (1H, s) ppm. 13C NMR (CDCl3): δ 163.1, 153.7, 151.9, 140.2, 136.1, 134.3, 130.9, 130.8, 129.5, 128.5, 127.3, 126.6, 119.5, 118.5, 93.4 ppm. 11B NMR (CDCl3): δ 1.47 ppm. HRMS (ESI): Calculated for [M + H]+, 322.1209; found, m/z 322.1209.

Synthesis of BBK-Cl: BCl3 in CH2Cl2 solution (1.6 mL, 187 mg, 1.60 mmol) was added to the solution of 1 (918 mg, 3.36 mmol) in the mixed solvent of CH2Cl2 (24 mL) and NEt3 (6.4 mL). The reaction mixture was stirred at r.t. under Ar atmosphere for 12 h. After removing solvents using a rotary evaporator, the yellow residue was dissolved into DMSO and precipitated into a large amount of Et2O. The precipitation was recrystallized from Et2O and MeOH. The product BBK-Cl was obtained as a yellow solid (326 mg, 36%). 1H NMR (DMSO-d6): δ 8.44 (1H, d, J = 6.6 Hz, Ar-H), 8.34 (1H, d, J = 2.0 Hz, Ar-H), 8.01 (2H, dd, J = 5.1, 1.7 Hz, Ar-H), 7.94–7.90 (3H, m, Ar-H), 7.70–7.50 (6H, m, Ar-H), 7.33 (1H, s, Ar-H) ppm. 13C NMR (DMSO-d6): δ 153.7, 151.0, 141.8, 134.6, 132.8, 131.5, 131.4, 129.6, 129.0, 127.6, 127.1, 126.0, 120.8, 120.3, 95.0 ppm. 11B NMR (DMSO-d6): δ 4.12 ppm. HRMS (ESI): Calculated for [M + H]+, 555.2238; found, m/z 555.2235.

Synthesis of BBK-PF: BBK-Cl (200 mg, 0.34 mmol) was dissolved into MeOH (5 mL) and added to the solution of silver hexafluorophosphate (105 mg, 0.37 mmol) in acetonitrile (5 mL). After stirring at r. t. for 2 h, the solution was filtered to remove white precipitate. The obtained precipitate was extracted with CH2Cl2 and washed with water twice. The resulting yellow solution was concentrated using a rotary evaporator and redissolved into benzene. After freeze-drying, BBK-PF6 was obtained as a yellow solid (80%, 190 mg). 1H NMR (CDCl3): δ 8.08 (1H, d, J = 6.6 Hz, Ar-H), 7.92 (2H, dd, J = 7.8, 1.2 Hz, Ar-H), 7.80–7.76 (3H, m, Ar-H), 7.94–7.90 (1H, dd, J = 6.6, 2.0 Hz, Ar-H), 7.60–7.44 (6H, m, Ar-H), 6.81 (1H, s, Ar-H) ppm. 13C NMR (DMSO-d6): δ 153.8, 151.0, 141.8, 134.6, 132.8, 131.5, 131.4, 129.6, 129.0, 127.6, 127.1, 126.0, 120.8, 120.3, 95.1 ppm. 11B NMR (DMSO-d6): δ 4.12 ppm. HRMS (ESI): Calculated for [M + H]+, 555.2238; found, m/z 555.2235. Calculated for [PF6]−, 144.9647; found, m/z 144.9644. CCDC #: 2071316.

Synthesis of BBK-OTf: BBK-OTf was prepared from BBK-Cl (200 mg, 0.34 mmol) and silver trifluoromethanesulfonate (96 mg, 0.37 mmol) as yellow solid according to the same method with BBK-PF6. 1H NMR (DMSO-d6): δ 8.44 (1H, d, J = 6.6 Hz, Ar-H), 8.32 (1H, s, Ar-H), 8.01 (2H, dd, J = 5.1, 1.7 Hz, Ar-H), 7.94–7.90 (3H, m, Ar-H), 7.70–7.50 (6H, m, Ar-H), 7.31 (1H, s, Ar-H) ppm. 13C NMR (DMSO-d6): δ 153.7, 151.0, 144.0, 141.8, 134.6, 132.8, 131.5, 131.4, 129.6, 129.0, 127.7, 127.2, 126.0, 120.8, 120.3, 95.1 ppm. 11B NMR (DMSO-d6): δ 4.12 ppm. HRMS (ESI): Calculated for [M + H]+, 555.2238; found, m/z 555.2235. HRMS (ESI): Calculated for [M + H]+, 555.2238; found, m/z 555.2236. Calculated for [OTf]−, 148.9526; found, m/z 148.9522.

Synthesis of BBK-TFSI: BBK-TFSI was prepared from BBK-Cl (200 mg, 0.34 mmol) and silver bis(trifluoromethanesulfonyl)imide (144 mg, 0.37 mmol) as yellow solid according to the same method with BBK-PF6. 1H NMR (DMSO-d6): δ 8.44 (1H, d, J = 6.6 Hz, Ar-H), 8.32 (1H, d, J = 2.0 Hz, Ar-H), 8.01 (2H, dd, J = 7.3, 3.6 Hz, Ar-H), 7.94–7.90 (3H, m, Ar-H), 7.70–7.50 (6H, m, Ar-H), 7.31 (1H, s, Ar-H) ppm. 13C NMR (DMSO-d6): δ 158.6, 153.8, 141.8, 140.2, 134.6, 132.8, 131.5, 131.4, 129.6, 129.0, 127.6, 127.1, 126.0, 120.8, 120.3, 95.1 ppm. 11B NMR (DMSO-d6): δ 4.12 ppm. HRMS (ESI): Calculated for [M + H]+, 555.2238; found, m/z 555.2235. HRMS (ESI): Calculated for [M + H]+, 555.2238; found, m/z 555.2236. Calculated for [TFSI]−, 279.9178; found, m/z 279.9175.

4. Conclusions

By employing the luminescent boron complex structure, we obtained luminescent ionic salts with various kinds of anions. Since the Tm values were found below 100 °C, these salts can be classified as an ionic liquid. All molecules show intense emission not only in solution but also in solid. By replacing the counter anion, the crystallinity can be altered. In particular, it was found that the PF6 salt can form a single crystal and an X-ray analysis was applicable. Furthermore, we accomplished detection of luminochromism from crystal to amorphous state using mechanical stimuli. It is proposed that our stimuli-responsive luminescent materials might be potentially applicable to introduce environment-monitoring ability in the conventional usages of ionic liquids, such as electrolytes in lithium batteries and thermal-resistant reaction solvents.