Development of Novel Solid-State Light-Emitting Materials Based on

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Article Cite This: ACS Omega 2018, 3, 9105−9113

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Development of Novel Solid-State Light-Emitting Materials Based on Pentafluorinated Tolane Fluorophores Shigeyuki Yamada,*,† Akira Mitsuda,† Kazuya Miyano,† Tsuyoshi Tanaka,† Masato Morita,† Tomohiro Agou,‡ Toshio Kubota,‡ and Tsutomu Konno† †

Faculty of Molecular Chemistry and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Department of Quantum Beam Science, Graduate School and Engineering, Ibaraki University, 4-12-1 Nakanarusawa, Hitachi, Ibaraki 316-8511, Japan

‡

ACS Omega 2018.3:9105-9113. Downloaded from pubs.acs.org by 95.181.217.57 on 08/31/18. For personal use only.

S Supporting Information *

ABSTRACT: We herein describe the synthesis of novel pentafluorinated tolane fluorophores, which possess an extended π-conjugated structure with a large molecular dipole moment along the longitudinal axis. We also report a detailed evaluation of both the photophysical and thermal behaviors of these fluorophores. All molecules displayed photoluminescence (PL) characteristics in both the crystalline state and in dilute solutions. The large longitudinal dipole moment induced solvatochromic PL behavior, which switched sensitively with changes in the solvent polarity. In addition, incorporation of the fluorinated tolane-based solid-state light-emitting moiety into the polymer side chain was found to be responsible for the PL characteristics observed in the solid state. It was also noteworthy that the polymerization protocol led to a significant enhancement in the thermal stability, with the thermal decomposition temperature increased by 90 °C. Accordingly, novel solid-state light-emitting materials with high thermal stabilities were successfully developed as promising candidates for use in light-emitting and optoelectronic applications.

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unique ability of the fluorine atom to dramatically alter the chemical reactivity of compounds in addition to both their physical and chemical properties.10 On the basis of our accumulated knowledge into synthetic methodologies toward fluorinated organic molecules, we recently focused on the development of functional materials, that is, liquid-crystalline and photoluminescence (PL) materials, containing fluorinated partial structures. As a result, we discovered novel fluorinecontaining π-conjugated molecules with an intriguing PL characteristic. More specifically, we reported PL-emitting polyfluorinated bistolanes whose color could be tuned by varying the electron density distributions and aggregation structures in condensed phases.11 However, despite their unique PL properties, these bistolane derivatives required long reaction sequences for their preparation. For the purpose of this study, we therefore selected polyfluorinated tolane (diphenylacetylene) derivatives as alternative candidates because of their ready availability and their potent PL characteristics originating from their extended π-conjugated structures and their fluorine-induced electron density distribution. Indeed, few studies have been published into fluorine-containing tolane derivatives with solid-state PL

INTRODUCTION Light-emitting molecules that luminesce in the solid state are important materials because of their broad application in lighting and optoelectronic devices.1 In general, owing to their intense fluorescent characteristic in dilute solutions, fluorescent dye molecules consisting of a simple π-conjugated structure, for example, pyrene, coumarin, or boron dipyrromethene, have been applied as fluorescent brightening agents, diagnostic fluorescent markers, and fluorescent indicators. In contrast, the fluorescence of such compounds is easily quenched at high concentrations or in the solid state because of aggregationcaused quenching.2,3 However, the rapid development of luminescent molecules that function in the solid state has been reported since the discovery of two new luminescent phenomena, namely crystallization-induced emission enhancement4,5 and aggregation-induced emission enhancement,6,7 both of which dramatically enhance emission intensities because of the inhibition of nonradiative decay through the restricted intramolecular rotation caused by molecular aggregation. Thus, owing to the growing demand for solidstate light-emitting materials for use in various optical applications, the discovery of novel solid-state fluorophores remains both a challenge and a priority. Over the past few decades, we have focused on the development of efficient synthetic procedures toward versatile fluorine-containing molecules,8,9 as we are interested in the © 2018 American Chemical Society

Received: June 29, 2018 Accepted: July 31, 2018 Published: August 14, 2018 9105

DOI: 10.1021/acsomega.8b01490 ACS Omega 2018, 3, 9105−9113

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with (trimethylsilyl)acetylene, the trimethylsilyl-protecting group was removed to give (4-alkoxyphenyl)acetylenes 2d− 2f, and a Sonogashira cross-coupling reaction with iodopentafluorobenzene produced the desired tolanes 1d−1f. Purification of all compounds 1a−1f was achieved via a double purification technique involving column chromatography followed by recrystallization from CH2Cl2/MeOH (v/v = 1:1). The molecular structures of 1a−1f were fully characterized by 1H, 13C, and 19F nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, and highresolution mass spectrometry (HRMS). Importantly, the NMR results confirmed that the prepared compounds were of sufficient purity to evaluate their photophysical properties. Among the obtained crystalline products, 1b and 1f, which bear ethoxy and hexyloxy substituents, respectively, furnished single crystals appropriate for X-ray crystallographic analysis. The crystal structures, packing structures, and observed shortcontact distances for these two compounds are shown in Figure 2. As indicated in Figure 2, the two aromatic rings in the diphenylacetylene (tolane) scaffold formed a coplanar structure in the crystalline lattice, which facilitated the construction of antiparallel dimeric structures through electrostatic π−π interactions between the alkoxy-substituted electron-rich aromatic ring and the electron-deficient pentafluorobenzene moiety (Figure 2A,C). The closest interatomic distance between two aromatic rings involved in π−π stacking was 349 pm for 1b and 341 pm for 1f. These values are comparable for the sum of the van der Waals (vdW) radii for two carbon atoms (i.e., CvdW = 170 pm).14 According to the packing structures shown in Figure 2B,D, both 1b and 1f formed ordered structures through intermolecular interactions such as hydrogen bonding, vdW interactions, and π−π interactions. Taking the vdW radii for hydrogen (HvdW = 120 pm) and fluorine (FvdW = 147 pm) into consideration, it was apparent that two H···F hydrogen bonds were present in the crystal structures of 1b and 1f, thereby promoting the formation of stable packed structures. In the case of the hexyloxy-substituted 1f, a relatively short Halkyl···Halkyl contact (286 pm) was also observed between two neighboring hexyloxy-chains, which is likely because of the formation of ordered crystal structures. Photophysical Properties in Solution. With the novel pentafluorinated tolanes in hand, dilute solutions of the various compounds were prepared in CH2Cl2, and their UV−vis absorption and PL spectra were recorded to investigate their photophysical properties in the solution state. Thus, Figure 3A shows the obtained absorption and PL spectra; the photophysical data are summarized in Table 1. As shown in Figure 3A, all pentafluorinated tolane derivatives exhibited absorption bands between 240 and 350 nm. According to a previous report into the photophysics of unsubstituted tolanes,15 the absorption band at 250 nm can be attributed to the π−π* transition from the ground S0 state to the excited singlet S2 state because of the forbidden direct excitation from S0 to the lowest S1 state. In addition, the absorption band observed at ∼300 nm is likely to be a peculiar intramolecular charge transfer (ICT) band originating from the CC triple bond stretching and aromatic ring-breathing vibrations of unsymmetrically disubstituted tolane derivatives with both electron-donating and -withdrawing substituents.12a,16

characteristics, although several analogs have been reported so far.12 Thus, to discover promising fluorinated organic materials with intensive solid-state PL characteristics, we report the systematic preparation of novel-fluorinated tolane derivatives and subsequent evaluation of their photophysical and thermal characteristics (Figure 1). In addition, we wish to disclose the development of novel light-emitting materials with high thermal stabilities, which are based on the polyfluorinated tolane scaffold.

Figure 1. Chemical structures for solid-state light-emitting candidates used in this study.

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RESULTS AND DISCUSSION Synthesis and Crystal Structures. Pentafluorinated tolanes 1a−1f, which bear a range of flexible alkoxy chains, were synthesized according to the synthetic procedure outlined in Scheme 1. Scheme 1. Synthetic Procedure for the Preparation of Pentafluorinated Tolanes 1a−1f

As indicated in the scheme, pentafluorinated tolanes 1a−1c bearing short alkoxy substituents, that is, CH3O (n = 1), C2H5O (n = 2), and n-C3H7O (n = 3), were synthesized in isolated yields of 61−72% via a facile single-step protocol, namely a Pd(0)-catalyzed Sonogashira cross-coupling reaction of commercially available (4-alkoxyphenyl)acetylenes 2a−2c with iodopentafluorobenzene. In contrast, compounds 1d−1f, which bear longer alkoxy chains (i.e., n = 4−6), were effectively prepared from readily available 1-alkoxy-4-bromobenzenes 3d−3f over a three-step manipulation according to the reported procedure.13 More specifically, following a Pd(0)catalyzed Sonogashira cross-coupling reaction of 3d, 3e, or 3f 9106


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Figure 2. (A,C) Crystal structures of the ethoxy-substituted 1b and the hexyloxy-substituted 1f. (B,D) Packing structures of 1b and 1f, and the short contacts observed in the crystal structures.

Figure 3. (A) Absorption and PL spectra of pentafluorinated tolanes 1a−1f in dilute CH2Cl2 solutions (concentration: 10−5 mol L−1 for the absorption spectra and 10−6 mol L−1 order for the PL spectra). (B) PL spectra for the hexyloxy-substituted 1f dissolved in different solvents. Values in parentheses are ET(30) value that are an indicator for solvent polarity. (C) Commission Internationale de l’Eclairage (CIE) color diagram calculated from the PL spectra. (D) CIE color diagram for solvatochromic PL spectra.

To confirm the vertical electronic transition, we performed quantum chemical calculations using the Gaussian 09W software package.17 The ground-state geometries were optimized using the density functional theory (DFT) approach with the CAM-B3LYP functional18 and the 6-31+G(d) basis set with the implicit solvation model, the conductor-like polarizable continuum model (CPCM),19 for CH2Cl2. Excited structures were also calculated by time-dependent (TD)-DFT calculations using the same functional and basis set. The calculated data, including the excitation energies, oscillator strengths (f), and electronic transitions, are listed in Table 1.

For all compounds (1a−1f), the TD-DFT calculations indicated the existence of a single possible vertical electronic excitation with a large oscillator strength (f), which was assigned as a direct transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied MO (LUMO). Upon careful examination of the frontier molecular orbital contributions for both HOMO and LUMO (Figures S48−S59 in the Supporting Information), we found that the orbital lobe at the HOMO was mainly distributed over the alkoxy-substituted electron-rich aromatic ring, whereas the orbital lobe at the LUMO was localized over the electrondeficient pentafluorophenyl ring, thereby indicating that the 9107


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Table 1. Theoretical Evaluation of the Vertical Excitation in Pentafluorinated Tolanes 1a−1f and Experimentally Obtained Photophysical Data Measured in Dilute Solutions excitation energy [eV/nm] (f)a

electronic transitiona (contribution, %)

ex μgr ∥ /μ∥ [D]a

1a

4.17/297 (1.26)

HOMO → LUMO (92%)

4.84/6.34

1b

4.16/298 (1.29)

HOMO → LUMO (92%)

5.16/6.72

1c

4.16/298 (1.30)

HOMO → LUMO (92%)

5.31/6.88

1d

4.16/298 (1.32)

HOMO → LUMO (92%)

5.16/6.74

1e

4.16/298 (1.32)

HOMO → LUMO (92%)

5.17/6.75

1f

4.16/298 (1.33)

HOMO → LUMO (92%)

5.08/6.65

λabs [nm]b (ε × 10−3) 252 (0.93), (23.6) 254 (4.64), (39.1) 253 (0.12), (25.4) 252 (1.04), (24.5) 253 (1.44), (41.3) 254 (1.24), (23.1)

λPL [nm]c

ΦPLd

Stokes shift [cm−1]

298 (21.6), 312

396

0.21

6798

303 (34.2), 314

386, 404

0.09

7095

301 (24.4), 313

408

0.17

7439

302 (21.1), 314

407

0.06

7277

304 (37.4), 314

401

0.07

6909

301 (20.7), 314

404

0.13

7095

a Calculated by TD-DFT calculations using the CAM-B3LYP/6-31+G(d) level of theory and the CPCM model (CH2Cl2) for the optimized geometry using the same functional and basis set. bMeasured in CH2Cl2 (10−5 mol L−1). cMeasured in CH2Cl2 (10−6 mol L−1). dThe absolute quantum yield was determined using a calibrated integrating sphere system. f = oscillator strength. μgr ∥ = longitudinal molecular dipole moment in −1 cm−1]. the ground state. μex ∥ = longitudinal molecular dipole moment in the excited state. ε = molar extinction coefficient [L mol

major HOMO → LUMO transition likely originates from the ICT transition. In addition, molecular dipole moments along the longitudinal axis (μgr ∥ ) in the ground state were 4.84−5.31 D, which suggests that both the electron-rich and -deficient aromatic rings in 1a−1f caused intramolecular charge separation in CH2Cl2. These theoretical evaluations were therefore consistent with the experimental results obtained by UV−vis absorption experiments. Subsequently, the steady-state PL spectra (see Figure 3A) for dilute CH2Cl2 solutions of 1a−1f (i.e., 10−6 mol L−1) were examined by irradiation at the absorption maximum wavelength (i.e., λex = 311−314 nm). In all cases, the maximum PL wavelengths were between 396 and 408 nm, and all compounds exhibited a large Stokes shift20 (6700−7400 cm−1), despite a small Stokes shift (∼1000 cm−1) being previously reported in the case of nonsubstituted tolanes in solution.12a,15 Considering the longitudinal molecular dipole moment present in the excited S1 state (μex ∥ = 6.34−6.88 D) as calculated by TD-DFT calculations, it is reasonable to assume that the PL band moves to longer wavelengths through a radiationless stabilization of excited molecules via solvent reorganization. The large μex ∥ as well as free rotation of aromatic rings also seem to affect the luminescence efficiency because of enhancement of S1 → T1 intersystem crossing, resulting in low ΦPL.21 To further confirm that all PL in solution originated from ICT transitions, the PL behavior was examined using alternative solvents with different ET(30) values,22 which is one of the promising indicators for solvent polarity, that is, hexane (Hex, 31.0), ethyl acetate (AcOEt, 38.1), acetone (acetone, 40.7), N,N-dimethylformamide (DMF, 43.2), dimethylsulfoxide (DMSO, 45.1), and acetonitrile (MeCN, 45.6) (see Figure 3B). Upon comparison of the various PL spectra obtained for 1f, the lower ET(30) value for hexane resulted in the emission signals shifting to lower wavelengths, whereas the solvent with higher ET(30) value, for example, DMF, DMSO, and MeCN, led to a shift to longer wavelengths. The theoretical calculations and experimental results thereby indicated that the PL of 1 in the solution state can be assigned as an ICT transition. Furthermore, as shown in the CIE color diagram (Figure 3D), deep blue PL was observed for compounds 1a−1f in CH2Cl2, although this color changed

depending on the solvent polarity, as MeCN resulted in lightblue PL. Photophysical Properties in the Crystal Form. Our attention was then directed toward the solid-state PL behavior of compounds 1a−1f. All PL measurements were performed for crystalline samples prepared by recrystallization and held between two quartz plates. The obtained PL spectra and the corresponding photophysical data are shown in Figure 4A and summarized in Table 2.

Figure 4. (A) PL spectra of 1a−1f in the crystal form. (B) CIE color diagram of the solid-state PL of 1a−1f.

As indicated, pentafluorinated tolane 1a, which bears a methoxy substituent at the longitudinal terminal, gave a broad PL band with an emission maximum wavelength (λPL) of 477 nm. In sharp contrast, 1b−1f possessed sharp PL bands at shorter wavelengths (λPL = 398−403 nm). As shown in Figure 4B, comparison of the PL colors showed that crystalline 1a 9108


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process, resulting in a lower ΦPL value, although the twisted bistolane scaffold led to high ΦPL values through effective suppression of the nonradiative process. Development of Light-Emitting Polymers Bearing the Pentafluorinated Tolane Scaffold. We then decided to focus on the evolution of solid-state luminescent pentafluorinated tolane derivatives to give light-emitting materials with high thermal stabilities, as organic light-emitting diodes can be prepared through repeated thermal deposition cycles and annealing processes at temperatures >150 °C. Indeed, we found that the thermal decomposition temperatures (Tdec) of 1c and 1f, that is, the temperature where 5% weight loss had taken place, were 158 and 181 °C, respectively (Figure S47). Thus, as neither of these compounds would be suitable for thermal treatment processes, we examined the development of novel solid-state light-emitting molecules with high thermal stabilities. In this context, we conducted the novel preparation of light-emitting polymer materials containing the structural motifs of 1c and 1f in the side chain, as polymer materials are known to possess independent characteristics originating from both the main chain and the side chain. In this case, polymers bearing the pentafluorinated tolane scaffold in the side chain would likely be solid-state light-emitting materials that exhibit high thermal stabilities. Indeed, novel pentafluorinated tolane derivatives bearing a polymerizable functional group (i.e., the methacryloyl group) in the flexible moiety were prepared, and subsequent radical polymerization in the presence of a catalytic amount of a radical initiator were demonstrated. The synthetic procedure for preparation of the desired polymers 5 is outlined in Scheme 2. Thus, upon the treatment of 4-[3-(methoxymethoxy)propoxy]phenylacetylene (6c), which was readily accessible from 4-bromophenol, with iodopentafluorobenzene (1.5 equiv) in the presence of 5 mol % Cl2Pd(PPh3)2 and PPh3 and 10 mol % CuI in Et3N at 60 °C over 20 h, the Sonogashira cross-coupling reaction took place efficiently to give the corresponding methoxymethyl-terminated pentafluorinated tolane 7c in 68% yield. Subsequent treatment with concentrated hydrochloric acid smoothly cleaved the C−O bond to give the corresponding OH-terminated product 8c in 60% yield. The condensation of 8c with methacryloyl chloride in the presence of excess Et3N in tetrahydrofuran (THF) at 25 °C over 24 h successfully provided the corresponding propylene-linked monomer 9c in 88% isolated yield. Under

Table 2. Photophysical Data for Crystals of 1a−1f compounds 1a 1b 1c 1d 1e 1f

λPL [nm] (λex [nm]) 477 398 403 400 400 402

(354) (312) (312) (316) (312) (316)

ΦPL 0.14 0.07 0.21 0.19 0.09 0.16

emitted blue-white PL with CIE coordinates of (0.23, 0.29), whereas crystalline 1b−1f exhibited deep-blue PL similar to that observed in CH2Cl2. After measuring PL lifetime (τ) with monitoring at the emission maximum wavelength, the τ for 1a−1f was found to be on the order of nanoseconds, which indicates that the luminescence at the λPL is fluorescence (Table S2, Figure S45). Considering the crystal-packing structures of 1b and 1f (Figure 2), intermolecular π−π interactions between the electron-rich and -deficient aromatic rings to form ordered layered structures promoted the nonradiative deactivation of the excited molecules in the crystal, thereby resulting in the PL band being shifted to significantly longer wavelengths in comparison with the excitation wavelengths. As shown in Figure S44, excitation spectra for 1b−1f showed sharp signal with a relatively large long-wavelength shift compared to the corresponding absorption band, which creates discrete excitation structure to emit luminescence in sharp spectral band. In contrast, the 1a showed broad excitation band with close wavelength to the absorption band, which can be expected to exist multiple structures with a different crystal system involving a weak intermolecular interaction in the ground state. Because of the multiple crystalline structures in the ground state, it can be considered that various excimer sites catch the exciton to emit luminescence with a broad spectrum shape. Although this difference has not yet been fully accounted for, it is anticipated that 1a may possess polymorphic aggregates through the numerous intermolecular interactions present because of the small methoxy substituent. In addition, the PL efficiency (ΦPL) in the crystalline state was 0.07−0.21, which was somewhat lower than that of the pentafluorinated bistolane (≤0.66) reported previously by our group.11a As previously mentioned, compounds 1a−1f formed ordered packing structures in the crystal state, which promoted the nonradiative deactivation

Scheme 2. Synthetic Procedure for the Preparation of Light-Emitting Polymers 5 Bearing the Pentafluorinated Tolane Scaffold

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Table 3. Polymer Characterization and Their Photophysical and Thermal Properties

5c 5f

Mna

Mwa

Mw/Mna

Tg [°C]b

d −3 c in CH2Cl2 solution λmax abs [nm] (ε × 10 ) /λPL [nm] (ΦPL)e

in solid state λex [nm]/λPL [nm] (ΦPL)e

Tdec [°C]f

3500 10 400

6500 21 900

1.89 2.10

31.0 10.3

313 (22.1)/398 (0.07) 313 (19.1)/407 (0.18)

320/416 (0.03) 312/414 (0.09)

245 272

Determined by GPC using CHCl3 as an eluent. bDetermined by DSC (N2 atmosphere, scan rate: 10 °C min−1) on the second heating cycle. Measured in CH2Cl2 (10−5 mol L−1 concentration). dMeasured in CH2Cl2 (10−6 mol L−1). eMeasured using a calibrated integrating sphere system. fMeasured by thermogravimetric analysis. Heating rate: 10 °C min−1. a c

identical conditions, methacrylate monomer 9f bearing a hexylene linkage was obtained from 6f in a 41% yield over three steps. With monomers 9c and 9f in hand, the N,N′azobis(isobutyronitrile) (AIBN)-initiated radical polymerization was examined; vigorous stirring of 9c or 9f in the presence of 5 mol % AIBN in degassed THF at 60 °C for 24 h, followed by reprecipitation by the addition of MeOH yielded 5c (or 5f) as a yellow solid in 64% (or 86%) yield. Polymer characterization, that is, determination of the average molecular weight, the polydispersity, and the glass transition temperature (Tg), was carried out by gel-permeation chromatography (GPC) and differential scanning calorimetry (DSC), as summarized in Table 3. GPC measurements using CHCl3 as the eluent revealed that the number average molecular weight (Mn) and the weight average molecular weight (Mw) of polymer 5c were 3500 and 6500, respectively, which gave a polydispersity (Mw/Mn) of 1.89. Similarly, the Mn, Mw, and Mw/Mn values for polymer 5f were 10 400, 21 900, and 2.10, respectively. DSC measurements also revealed that the two polymers had glass transition temperatures (Tg) of 31.0 °C (5c) and 10.3 °C (5f) on the second heating cycle, which correlates with an increased flexibility in the side chain, lowering the value of Tg. With polymer samples 5c and 5f obtained from reprecipitation, we moved on to evaluate their photophysical properties. The polymers 5 also displayed PL characteristics not only in dilute CH2Cl2 solution but also in the solid state. In addition, the photophysical properties of polymers 5c and 5f in solution were similar to those of the corresponding lowmolecular-weight molecules 1c and 1f (Figure S43), and these polymers bearing the luminescent partial structures of 1c and 1f in their side chains exhibited light-emitting properties even in the solid state (Figure 5A). Comparison of the solid-state PL spectra for low-molecular-weight 1 and polymeric compound 5 indicated slight variations in the maximum PL wavelengths and the spectral shapes. More specifically, incorporation of the light-emitting unit into the polymer side chains shifted the PL maximum wavelengths to longer wavelength regions (i.e., up to a 27 nm shift) and caused broadening of the spectral shape, leading to a dramatic PL color change from deep blue for 1c and 1f to blue-white for 5c and 5f (see Figure 5B). Such dramatic changes in the solidstate PL behavior following polymerization could be attributed to the formation of amorphous-like molecular aggregation with various different intermolecular interactions. In this case, the disordered amorphous aggregation is likely to cause a significant decrease in the PL efficiency (ΦPL). Subsequently, the thermal stabilities of 5c and 5f were evaluated by measuring their thermal decomposition temperatures (Tdec) using thermogravimetric analysis (TGA). On the basis of the TGA measurements, it was found that polymerization contributed to a dramatic enhancement in the thermal stability. More specifically, the Tdec values for polymers 5c and

Figure 5. (A) Solid-state PL spectra, (B) CIE color diagrams, and (C) TGA thermograms of polymers 5c and 5f, and comparison with the corresponding low-molecular-weight molecules 1c and 1f.

5f were 245 and 272 °C, respectively, which are >90 °C higher than the corresponding values for 1c and 1f (Figure 5C). Taking into consideration a previous report into thermal decomposition mechanism for alkynes,23 it appears that 1c and 1f also thermally decompose via the same mechanism, that is, the nth order of mechanism. Thus, one explanation for the dramatic increase in Tdec for the polymers containing 1c or 1f moieties is that the main polymeric chains encompass the alkyne moiety, and so a higher energy is required to release the alkyne moiety from the polymeric aggregations. This effect therefore rendered the development of novel solid-state lightemitting molecules with higher thermal decomposition temperatures successful.

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CONCLUSIONS We herein reported the successful synthesis of novel pentafluorinated tolane derivatives via a facile manipulation from readily available (4-alkoxyphenyl)acetylenes and disclosed their intriguing photophysical behavior. More specifically, we found that the prepared pentafluorinated tolane derivatives displayed PL characteristics both in dilute solutions and in the solid state (i.e., crystalline aggregated structures). These results indicate that the present fluorine-containing πconjugated molecules can be considered promising fluorophores for use in a range of applications. It was of interest that the luminescent behavior, for example, emission maximum, spectral shape, was dramatically changed by incorporating the 9110


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atmospheric filtration and the filtrate was poured into a saturated aqueous NH4Cl solution. The crude product was then extracted with AcOEt (×3), and the combined organic layer was washed with brine, dried over anhydrous Na2SO4, filtered, and the solvent was removed using a rotary evaporator. The resulting residue was subjected to column chromatography using a mixed solvent (hexane/CH2Cl2 = 5:1) as the eluent to produce 1f, which was further purified by recrystallization from CH2Cl2/MeOH (1:1) to give the desired product as white solids in 61% yield (0.36 g, 0.97 mmol). mp 60 °C (determined by DSC); 1H NMR (CDCl3): δ 0.91 (t, J = 6.8 Hz, 3H), 1.29−1.39 (m, 4H), 1.41−1.52 (m, 2H), 1.80 (quin, J = 6.4 Hz, 2H), 3.98 (t, J = 6.4 Hz, 2H), 6.89 (d, J = 8.8 Hz, 2H), 7.50 (d, J = 8.8 Hz, 2H); 13C NMR (CDCl3): δ 14.0, 22.6, 25.7, 29.1, 31.6, 68.2, 71.8 (q, J = 3.6 Hz), 100.7 (tq, J = 19.7, 3.6 Hz), 102.1 (q, J = 2.1 Hz), 113.3, 114.7, 133.5, 137.6 (dtq, J = 251.6, 16.9, 2.9 Hz), 139.6−142.6 (dm, J = 256.0 Hz, 1C for C6F5), 147.0 (ddq, J = 256.7, 11.8, 4.4 Hz), 160.3; 19F NMR (CDCl3): δ −137.02 (dd, J = 21.8, 6.8 Hz, 2F), −154.10 (t, J = 20.3 Hz, 1F), −162.61 (ddd, J = 21.8, 20.3, 6.8 Hz, 2F); IR (KBr): ν 3080, 2952, 2936, 2230, 1604, 1566, 1528, 1496 cm−1; HRMS (FAB) m/z: [M+] calcd for C20H17F5O, 368.1200; found, 368.1205. X-ray Crystallography. Single crystals of the pentafluorinated diphenylacetylenes (i.e., 1b and 1f) were obtained by slow evaporation from a mixed solvent system (CH2Cl2/ MeOH = 1:1) and were mounted on a glass fiber. All measurements were carried out using a diffractometer with filtered Mo Kα radiation (λ = 0.71075 Å) and equipped with a rotating anode generator using a VariMax with PILATUS/DW (Rigaku, Japan). All calculations were performed using the CrysAlisPro software package (Rigaku)24 and the refinement was carried out using SHELXL-97.25 The structures were solved by direct methods and expanded using Fourier techniques. The crystal data are deposited in the Cambridge Crystallographic Data Center (CCDC) database (CCDC 1847249 for 1b and 1847250 for 1f). Photophysical Properties. UV−vis absorption spectra were recorded using a JASCO V-500 absorption spectrometer (JASCO, Japan). Steady-state PL spectra were obtained using a Hitachi F-7000 (Hitachi, Japan) or a JASCO FP-8500 fluorescence spectrometer (JASCO, Japan). PL quantum yields were estimated using a calibrated integrating sphere system. Thermal Properties. Phase-transition properties were observed by polarizing optical microscopy using an Olympus BX51 microscope (Olympus, Japan) equipped with a temperature-controlled stage (Instec HCS302 microscope, hot and cold stages, and an mK1000 temperature controller, Instec Inc. USA). The thermodynamic behavior of each compound was determined using DSC [SII X-DSC7000 (Seiko Instruments Inc., Japan) or SHIMADZU DSC-60 Plus (Shimadzu, Japan)] at heating and cooling rates of 5.0 °C min−1 for low-molecularweight molecules 1a−1f and 10 °C min−1 for polymers 5c and 5f. To evaluate the thermal stabilities of these compounds, TGA was carried out using a Discovery TGA (TA instruments, Japan) at a heating rate of 10 °C min−1. Computation. All DFT computations were performed using the Gaussian 09 (rev. C.01) program package.17 Geometric optimizations were executed using the CAMB3LYP hybrid functional and the 6-31+G(d) basis set with the implicit solvation model, CPCM,18 for CH2Cl2. Excitation energies, oscillator strengths, and molecular dipole moments for the optimized structures were calculated using the TD-SCF

fluorescent moiety into the polymer side chain. In addition, the incorporation of solid-state light-emitting molecules into polymer side chains led to a dramatic enhancement in the thermal stability by >90 °C, whereas maintaining the solidstate light-emitting properties, although the luminescent efficiency was slightly reduced because of disordered amorphous molecular aggregation. Our obtained results therefore clearly demonstrate that the incorporation of functional molecules into polymer side chains is promising for the development of thermally stable functional materials. Thus, with the ultimate aim of developing highly efficient and thermally stable solid-state light-emitting materials, we are currently investigating the development of light-emitting polymer materials bearing highly efficient fluorophores in the side chain, and our results will be disclosed in due course.

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METHODS General. 1H and 13C NMR spectra were obtained using an AVANCE III 400 NMR spectrometer (1H: 400 MHz and 13C: 100 MHz) in chloroform-d (CDCl3) (Bruker, Germany), and the chemical shifts are reported in parts per million (ppm) based on the residual proton signal of the NMR solvent. 19F NMR (376 MHz) spectra were obtained using AVANCE III 400 NMR spectrometer in CDCl3 with CFCl3 (δF = 0 ppm) as an internal standard (Bruker, Germany). IR spectra were recorded using the KBr method with FT/IR-4100 typeA spectrometer (JASCO, Japan); all spectra are reported in wave numbers (cm−1). High-resolution mass spectra were recorded on a JMS-700MS spectrometer (JEOL, Japan) using the fastatom bombardment (FAB) method. Materials. Starting materials for the preparation of 1 were obtained from the following commercial source; 4-ethynylanisole (2a), p-ethynylpropoxybenzene (2c), p-bromoalkoxybenzene (3d−3f), and pentafluoroiodobenzene were purchased from FUJIFILM Wako Pure Chemical Corporation. 4Ethoxyphenylacetylene (2b) was available from Tokyo Chemical Industry Co., Ltd. A palladium catalyst, that is, Cl2Pd(PPh3)2, was purchased from Aldrich Fine Chemicals. PPh3, CuI, and AIBN were also purchased from FUJIFILM Wako Pure Chemical Corporation. THF used for the radical polymerization was commercially available from FUJIFILM Wako Pure Chemical Corporation, which was freshly distilled from sodium benzophenone ketyl before use. Column chromatography was carried out on silica gel (Wakogel 60N, 38−100 μm) and thin-layer chromatographic (TLC) analysis was performed on silica gel TLC plates (Merck, Silica gel 60F254). All reactions were carried out using dried glassware and magnetic stirrer bars. Preparation of the pentafluorinated tolanes was carried out according to the synthetic procedure outlined in Scheme 1. Typical synthetic procedures of 1f from the corresponding precursor 2f and the characterization data are shown below as a selected example. Details of the synthetic methods employed for 1a−1e, 5c, and 5f are described in the Supporting Information. Typical Procedure for the Preparation of 2,3,4,5,6Pentafluoro-1-[2-(4-hexyloxyphenyl)ethyn-1-yl]benzene (1f). In a 50 mL two-necked round-bottomed flask was placed iodopentafluorobenzene (0.71 g, 2.4 mmol), Cl2Pd(PPh3)2 (0.056 g, 0.080 mmol), PPh3 (0.021 g, 0.080 mmol), CuI (0.017 g, 0.16 mmol), and Et3N (10 mL). To this solution was added 2f (0.33 g, 1.6 mmol) and Et3N (6.0 mL), and the resultant mixture was heated at 60 °C for 17 h. After this process, the resultant precipitate was separated by 9111


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method at the same level of theory. To obtain the structure at the lowest excited (S1) state, optimization calculations were executed using the same functional and basis set as those for the structure obtained from the TD-SCF calculations.

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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/acsomega.8b01490. Details of synthetic procedures, characterization, and NMR spectra (1H, 13C, and 19F) for the pentafluorinated tolanes 1a−1f and polymers 5c and 5f; crystallographic details for 1b and 1f, DSC thermograms, UV−vis spectra, PL spectra in dilute solutions, excitation and PL spectra in the solid state for 1a−1f, 5c, and 5f; TGA thermograms for 1c, 1f, 5c, and 5f; and Cartesian coordinates for the calculated 1a−1f in the ground and excited states (PDF) Crystal data of the ethoxy-substituted 1b (CIF) Crystal data of the hexyloxy-substituted 1f (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.Y.). ORCID

Shigeyuki Yamada: 0000-0002-6379-0447 Tomohiro Agou: 0000-0002-9133-190X Tsutomu Konno: 0000-0002-5146-9840 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Iketani Science and Technology Foundation (No. 0291019-A), the Foundation for the Promotion of Ion Engineering, and JSPS KAKENHI, a Grant-in-Aid for Scientific Research (C), Grant Number JP18K05262. We express our gratitude to Profs. Osamu Tsutsumi (Ritsumeikan University), Kensuke Naka, Hiroaki Imoto, Masaki Shimizu, and Mitsuhiko Morisue (Kyoto Institute of Technology) for the photophysical and powder X-ray diffraction measurements and their meaningful discussions. We also thank to Prof. Kaoru Adachi (Kyoto Institute of Technology) for determination of the average molecular weights using GPC.

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REFERENCES

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