Article Cite This: Langmuir 2017, 33, 13515-13521
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Photoluminescence by Intercalation of a Fluorescent β‑Diketone Dye into a Layered Silicate Mutsumi Hirose,† Fuyuki Ito,‡ Tetsuya Shimada,§ Shinsuke Takagi,§ Ryo Sasai,∥ and Tomohiko Okada*,†
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†
Department of Chemistry and Material Engineering, Faculty of Engineering, Shinshu University, 4-17-1, Wakasato, Nagano 380-8553, Japan ‡ Institution of Education, Shinshu University, 6-ro, Nishinagano, Nagano 380-8544, Japan § Department of Applied Chemistry, Graduate Course of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1, Minami-ohsawa, Hachiohji-shi, Tokyo 192-0397, Japan ∥ Department of Physics and Materials Science, Interdisciplinary Graduate School of Science and Engineering, Shimane University, 1060 Nishikawatsu, 690-8504 Matsue, Japan S Supporting Information *
ABSTRACT: A β-diketone dye was packed into the twodimensional nanospace of a synthetic smectite (Sumecton SA), which is a cation-exchangeable layered silicate, to induce strong emission owing to molecular packing of the dye. An emissive dye, 1-(4-methoxyphenyl)-3-(4-pyridyl)-1,3-propandione, was prepared through a Claisen condensation reaction; the dye exhibited aggregation-induced emission, which is enhanced emission owing to clustering of molecules to form aggregates in poor solvents or in the solid state. The dye was nonemissive in solution. However, strong green emission was observed because of the restriction of molecular motion when the protonated dye was accommodated into the interlayer nanospace of the silicate layers through cation-exchange reactions. The restricted motion was confirmed by the smaller nonradiative relaxation rate constant obtained by time-resolved luminescence and quantum yield measurements. A moderate dye packing (0.11 mmol/g) in the interlayer space is important to obtain enhanced emission, whereas the intercalation of a large amount of dye (0.27 mmol/g) resulted in concentration quenching. Therefore, the interlayer space of the layered silicate used here was responsible for the strong emission because of moderate packing of the accommodated β-diketones.
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INTRODUCTION Aggregation-induced emission (AIE) refers to a photophysical phenomenon exhibited by luminogenic molecules that are nonemissive when they are dissolved in good solvents as molecules but become highly emissive when they are clustered as aggregates in poor solvents or in the solid state.1,2 Considering the systematic design of AIE luminogenic dyes, the restriction of the intramolecular motion (e.g., rotation and vibration) by maintaining an appropriate distance between neighboring luminogenic groups is important for the manifestation of AIE characteristics. AIE is antagonistic to quenching caused by aggregation, which is often observed in conventional dye crystals because of strong chromophore interactions (e.g., π−π stacking and C−H···π interactions) that lead to nonradiative relaxation and spectral shifts. Among the AIE luminogenic dyes, 1,1,2,3,4,5-hexaphenylsilole, in which silole core is connected with six phenyl rings in total, is a good example to explain AIE.3 Although the conformation is flexible and nonemissive in solution, in stabilized crystal packing or in aggregates, the molecule exhibits a highly twisted (propeller© 2017 American Chemical Society
like) conformation caused by the steric repulsion between the adjacent phenyl rings, preventing a dense face-to-face packing structure. Clustering of the AIE luminogenic dyes in solid (e.g., lipid membranes4 and mesoporous silica5) and encapsulated matrices (e.g., polymers6,7 and amorphous silica8−12) has been investigated because optically transparent solids in ultraviolet (UV) and visible light are advantageous for evaluating the optical properties of dyes and for applying them to fabrication of optical and bioimaging materials. Organization of the AIE luminogens in a controlled manner (location, orientation, and packing) at solid interfaces is an alternative methodology for pursuing enhanced AIE properties. For using a specific interaction with solid surfaces, negatively/ positively charged inorganic layered solids have been employed to construct supramolecular assembly systems through electroReceived: October 4, 2017 Revised: November 9, 2017 Published: November 13, 2017 13515
DOI: 10.1021/acs.langmuir.7b03460 Langmuir 2017, 33, 13515−13521
Article
Langmuir static interactions.13−17 Cationic and anionic AIE luminogenic dyes (e.g., hexaphenylsilole derivatives) have been developed for immobilizing the dyes onto negatively (layered silicates13 and α-ZrP14,15) and positively charged (layered double hydroxides16,17) inorganic nanosheets. AIE characteristics of the dyes are preserved after intercalation (the molecules are accommodated into the interlayer spaces of the nanosheets). Here, we report the enhancement of the AIE characteristics of a structure-sensitive luminogenic dye (a β-diketone) by means of intercalation into a synthetic smectite, a negatively charged layered silicate. Diaryl β-diketones have been utilized as precursors of difluoroboron-coordinated β-diketonates for the purpose of enhancing emission.18−25 The fluoroboron-free βdiketones (dinaphthoylmethane β-diketones) often exhibit AIE phenomena even in the absence of a pinning substituent to restrict the intramolecular rotational motion; a more densely packed structure results in aggregation-caused quenching in a poor solvent, whereas moderate packing at a lower fraction of the poor solvent (70% in H2O/tetrahydrofuran (THF)) exhibited AIE.25 Our interest is focused on the molecular packing of the β-diketones for organization at an interface of inorganic layered solids to afford AIE. A synthetic smectite (Sumecton SA, a hydrothermally synthesized layered silicate) comprising ultrathin (1.0 nm) crystalline silicate layers separated by hydrated interlayers was used in this study. The cations in the interlayer spaces that compensate for the negatively charged silicate layers can be readily exchanged with various organic cations. The cation-exchange reactions of the layered silicates have been extensively investigated for the organization of various cationic dyes for applications as photochemical and optical functional hybrids.26−30 In the present system, the electrostatic interactions between the dye and the silicate layers, in addition to dye−dye interactions, play an important role in both restricting the intramolecular motion and controlling the distance or packing. We prepared a pyridylβ-diketone (dbmPy; Scheme 1) because the pyridine moiety
Synthesis of 1-(4-Methoxyphenyl)-3-(4-pyridyl)-1,3-propandione (dbmPy) by Claisen Condensation. Methyl isonicotinate (0.68 g, 5 mmol) and 4′-methoxyacetophenone (1.65 g, 11 mmol) in dry THF (25 mL) were added dropwise (1−2 drops/s) into a suspension of sodium amide (0.47 g, 12 mmol) in dry THF at room temperature. The mixture was refluxed for more than 6 h until the mixture turned brown. The Claisen condensation reaction shown in Scheme 2 was quenched by adding 40 mL of 2 mol L−1 aqueous acetic
Scheme 2. Synthesis of dbmPy through a Claisen Condensation Reaction
acid solution. After the addition of 40 mL of diethyl ether to the aqueous phase, the extracted organic phase was washed with brine and dried over anhydrous sodium sulfate. After the insoluble fraction in the organic phase was separated by filtration, the resulting organic supernatant was evaporated under reduced pressure. The crude product was purified by silica gel column chromatography using ethyl acetate as an eluent, followed by distillation under reduced pressure to obtain dbmPy as a powder (0.793 g, 3.12 mmol) in 62% yield. The structure of dbmPy was analyzed by 1H nuclear magnetic resonance (NMR; see the Supporting Information, Figure S2) and mass spectroscopy (MS). An absorption maximum of dbmPy in THF was observed in the UV−Vis spectrum at 355 nm. 1H NMR (500 MHz, CDCl 3, δ): 8.77 (d, J = 6.0 Hz, 2H, Py H), 7.98 (d, J = 9.0 Hz, 2H, Ar H), 7.75 (d, J = 10 Hz, 2H, Py H), 6.97 (d, J = 9.0 Hz, 2H, Ar H), 6.81 (s, 1H, enol CH), 3.88 (s, 3H,−OCH3).31 MS (m/z): [M + H]+ calcd for C15H13NO3, 256.097; found, 256.099. Preparation of Aggregates of dbmPy for AIE Measurements. A 10−3 mol L−1 stock solution of dbmPy was prepared in THF. Aliquots (100 μL) of the stock solution were added to 10 mL volumetric flasks and diluted with THF and deionized water in the appropriate ratios. Each mixture was placed in an ultrasonic bath for 10 min before measurement. To prepare aggregates of protonated dbmPy (dbmPyH+), a small amount of 1 mol L−1 HCl was added to dioxane and deionized water in the appropriate ratios, followed by the ultrasonic agitation. Adsorption of dbmPy onto a Layered Silicate (SA). After dissolution of dbmPy in a mixed solvent of HCl aqueous solution (adjusted to pH 3) and ethanol (3:2 in volume), 0.1 g of the SA was allowed to react under magnetic stirring at room temperature for 12 h. The resulting solid was recovered by filtration and dried at 323 K for 3 h. Hereafter, the dried solids are named as dbmPyH+-S0.1, dbmPyH+S0.5, dbmPyH+-S1, and dbmPyH+-S2, in which the amount of dbmPy added to the solution was 0.1, 0.5, 1, and 2 times the CEC of SA, respectively. The volume of the mixed solvent varied in the range of 11.8 to 237 mL, whereas the concentration of the dbmPy solution remained constant. The amount of adsorbed dbmPy on the SA was determined from the change in the concentration of the supernatant before and after adsorption using a visible absorption spectrophotometer (λabs = 355 nm). Change in the Fluorescence Spectra in Response to Solvent. A supported film was prepared for measuring the photoluminescence spectrum by suction filtration of the 0.5 mass % suspension of the
Scheme 1. Molecular Structure of dbmPy
will be protonated in an acidic aqueous solution and will participate in the cation-exchange reactions with the exchangeable cations in the SA interlayer.
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EXPERIMENTAL SECTION
Reagents and Materials. Sodium amide (NaNH2, Mw = 39.01 g/ mol, Wako Pure Chemical Ind., Ltd.), 4′-methoxyacetophenone (C9H10O2, Mw = 150.18 g/mol, Tokyo Chemical Industry Co., Ltd.), methyl isonicotinate (C7H7NO2, Mw = 137.14 g/mol, Tokyo Chemical Industry Co., Ltd.), and tetrahydrofuran (THF, dehydrated, stabilizer free, Kanto Chemical Co. Inc.) were used without further purification. A layered silicate (Sumecton SA, JCSS-3501; hereafter abbreviated as SA; (Na0.49Ca0.14)0.77+[(Mg5.97Al0.03)(Si7.20Al0.80)O20(OH)4]0.77−, supplied by Kunimine Ind. Co., Ltd.) is a synthetic Na-saponite, which is reference clay sample of the Clay Science Society of Japan. The XRD pattern of SA is shown in the Supporting Information, Figure S1). The cation exchange capacity (CEC) of SA is 71 mequivalent (mEq)/100 g of clay. Ethanol (99.5) and 1 mol L−1 HCl were purchased from Wako Pure Chemical Ind., Ltd. These materials were used as received. 13516
DOI: 10.1021/acs.langmuir.7b03460 Langmuir 2017, 33, 13515−13521
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Figure 1. (a) Photographs of dbmPy in THF/H2O captured under light irradiation (λex = 365 nm), (b) photoluminescence spectra of the dbmPy THF/H2O solutions, and (c) plots of fluorescence intensity versus the water fraction. dbmPyH+-S1 in water (2 mL) on a polytetrafluoroethylene filter disc. Solvents used in the study were water, dimethylformamide (DMF), methanol, ethanol, acetone, and ethyl acetate. Each solvent (20 μL) was dropped onto the supported film, and the photoluminescence spectrum was then recorded immediately. By evaporation in air at room temperature for 1 h, a large portion of solvent was removed for measuring the fluorescence spectrum. Equipment. 1H NMR (500 MHz) spectrum was recorded using a Bruker ASCEND 500 spectrometer in deuterated CDCl3. Mass spectrum was recorded using a Bruker microTOFII/Shimadzu Prominence UFLC XR. UV−vis spectra were recorded on a Shimadzu UV−2450PC spectrophotometer. The visible diffuse reflectance spectra were recorded on a JASCO V-750 spectrophotometer equipped with a PIV-756 integrated sphere and G265 condenser lens units. Steady-state fluorescence spectra were recorded using a Shimadzu RF-5300PC fluorescence spectrophotometer. Solid-state quantum yield measurements were performed using a Hamamatsu photonics C9920-02. XRD patterns were obtained by a Rigaku RINT 2200 V/PC diffractometer (monochromatic Cu Kα radiation), operated at 20 mA and 40 kV. IR spectra were measured on a JASCO FT/IR-4200 spectrophotometer. The time-resolved fluorescence measuring system (Hamamatsu Photonics, C4780) was based on a streak camera and an Nd3+ YAG laser:32 a photon-counting condition (Hamamatsu Photonics, C4334 streak scope, connected with CHROMEX 250IS polychrometer) with EKSPLA PG-432 optical parametric generator (430 nm, 25 ps fwhm, 20 μJ, 1 kHz) pumped by the third harmonic radiation of Nd3+ YAG laser, EKSPLA PL2210JE (355 nm, 25 ps fwhm, 300 mJ, 1 kHz).
Table 1. List of the Adsorbed Amount of dbmPy on SA, Basal Spacing, and Optical Properties adsorbed amount [mEq/100 g] solution (HCl/ethanol) dbmPyH+-S0.1 dbmPyH+-S0.5 dbmPyH+-S1 dbmPyH+-S2
basal spacing [nm]
λabsa [nm] 355
4.2 9.4 11 27
1.21 1.32 1.39 1.39
382 389 392 374, 450
λemb [nm] 450, 530 543 538 535 533
quantum yieldc [%] 0.01 9.8 10.0 10.1 7.2
a
Absorbance maximum. bPhotoluminescence maximum (excited at 380 nm). cMeasured as powder for the intercalation compounds.
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RESULTS AND DISCUSSION Fluorescence Properties of dbmPy in Solution. Blue emission was observed for dbmPy dissolved in THF (Figure 1a), corresponding to the emission band maximum located at approximately 450 nm (excitation wavelength of 365 nm) in the photoluminescence spectrum (Figure 1b). The position of the maximum was in the range of 400−450 nm and shifted gradually to a longer wavelength with increasing H2O/THF fraction. The gradual redshift can be attributed to aggregation and/or increasing solvent polarity due to the addition of H2O, as reported for the AIE properties of related diketonate derivatives (dinaphthoyl β-diketones).25 The solvatochromism of dbmPy was observed in the fluorescence spectra, which
Figure 2. XRD patterns of dbmPyH+-Sn and pristine Sumecton SA (SA).
varied depending on the polarity of the solvent used (the fluorescence spectra are shown in Figure S3). Since the solubility of dbmPy in water was quite low, the aggregates precipitated out of water. An increase in the relative intensity was observed (Figure 1c) when the water fraction of the H2O/ THF solution was increased up to 70%, indicating the AIE 13517
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threshold to appear AIE was likely caused by the difference in solubility of the dye depending on the ligand/substituent.25 A much closer distance between the adjacent dyes by forming the precipitates (like concentration quenching phenomenon) is also a possible reason for the lack of AIE character observed at such a higher water fraction (>80% water). Adsorption of dbmPy onto SA. The adsorbed amount of dbmPy on SA from an aqueous acidic solution and the basal spacing of the product are summarized in Table 1. A gradual increase in the adsorbed amount of dbmPy was displayed when the volume of the dye solution was increased. The adsorbed amount was within the CEC (71 mEq/100 g of SA). The FTIR absorption band attributed to the pyridine ring in dbmPy shifted from 1514 to 1501 cm−1 because the pyridine moiety forms an adduct with Brönsted acidic sites33 (the IR spectra are shown in Figure S4). The spectral difference indicated that protonation of dbmPy was required for it to be adsorbed through the cation-exchange reactions of SA with dbmPyH+Cl−. The XRD patterns of the resulting solids (dbmPyH+-Sn) are shown in Figure 2, together with that of pristine SA. Intercalation of dbmPyH+ into the interlayer space of SA was confirmed by the shifting of the (001) diffraction peak to a lower 2θ region with increasing amount of dbmPyH+. The basal spacing of pristine SA (1.21 nm) increased to the values listed in Table 1 and finally to 1.39 nm for the dbmPyH+-S1 and -S2 samples. The peak became sharp with increasing amount of dbmPyH+ probably due to developing stacked layers by electrostatic and π interactions between the silicate layer and dbmPyH+. The ratio of the dbmPyH+ intercalated (or high local concentration of dbmPyH+) to the pristine SA phases changed depending on the amount of intercalated dbmPyH+, as usually observed in dye−clay intercalation compounds, in which dye aggregation occurs even at a lower concentration (segregation phenomenon).34 AIE Properties of dbmPyH+-Sn Intercalation Compounds. Strong green emission (λem = 540 nm) was observed for dbmPyH+-Sn (Figure 3a), whose color was different from that of the neutral dbmPy in solution (λabs = 353 nm and λem = 450 nm). At a low dye loading (dbmPyH+-S0.1), weak emission appeared at approximately 450 nm in addition to the strong green emission band at approximately 540 nm owing to the monomer. The absorption band maxima in the diffuse reflectance spectra (Figure 3b) were observed at approximately 390 nm and were located in the longer wavelength region compared with that in an acidic solution (355 nm, adjusted using HCl). In the acidic solution, fine soluble aggregates of dbmPyH+Cl− formed out of the solution to exhibit green emission in a lower water/dioxane fraction (Figure S5). The green color is assumed on the basis of an intermolecular chargetransfer state,25 which appears in dbmPyH+ from the methoxysubstituted phenyl ring (electron donor) of one molecule to the electron withdrawing pyridinium group of another molecule. A slight hypsochromic shift depending on the loading amount (Table 1) presumably reflects the structures of the aggregates. In the diffuse reflectance spectra, the bandwidth broadened with increasing amount of dbmPyH+. A broader spectrum was obtained for dbmPyH+-S2 because of an emerging peak in the longer wavelength region (∼450 nm), indicating two different aggregates in the interlayer space. Time-resolved fluorescence spectra were obtained to study the photophysical behavior of the system in detail (Figure S6). The decay curve for dbmPyH+-S1 was successfully fitted to a
Figure 3. (a) Photoluminescence (λex = 380 nm) and (b) diffuse reflectance spectra of the dbmPyH+-Sn.
Figure 4. Schematic illustrations of (top) possible orientation of dbmPyH+ and (bottom) the spatial dye distributions in the interlayer space; (a) a moderate packing forms, whereas a densely packed aggregate is included in (b).
characteristics. Above 80% H2O, the intensity dropped to be nearly nonemissive to the eye. The gradual redshift in the emission wavelength was eventually observed owing to the aggregation effect. At 80% water fraction, aggregates reached a critical size and started to precipitate. The difference in the 13518
DOI: 10.1021/acs.langmuir.7b03460 Langmuir 2017, 33, 13515−13521
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Figure 5. Change in the photoluminescence spectra of the dbmPyH+-S1-supported film (λex = 380 nm) before and after contact with solvents, followed by evaporation. The solvents used are (a) water, (b) DMF, and (c) methanol.
single exponential decay (the excitation lifetime, τf, was determined to be 0.88 ns in λem = 560−615 nm), whereas that for dbmPyH+-S2 was double exponential decay, which can be divided into the longer emissive component (τf = 0.88 ns, 65%) and the shorter lifetime component (τf = 0.39 ns, 35%), which was due to self-quenching. The present series of dbmPyH+-Sn complexes exhibited AIE characteristics, as evidenced by the fluorescence quantum yields (ϕf) in the range of 0.07−0.10 (Table 1, which were larger than the ϕf < 0.01 for dbmPyH+ in solution (ethanol/HCl aq. mixture). The value of ϕf of approximately 0.10 did not decrease even if the amount of intercalated dbmPyH+ was increased up to 0.11 mmol/g (dbmPyH+-S0.1, -S0.5, and -S1), whereas concentration quenching occurred at a high loading of dbmPyH+ in SA (0.27 mmol/g dbmPyH+-S2), which resulted in a decrease of ϕf. Further evidence that a compound becomes highly emissive upon aggregation is indicated by the restriction of intramolecular motion (e.g., rotation and vibration), which can be described by the decrease of the nonradiative deactivation rate constant for fluorescence (knr) given by ϕf (Table 1) and τf (0.88 ns), according to eq 1.35 k nr =
interlayer space of 0.43 nm) for dbmPyH+-S1 and -S2 indicate a monomolecular coverage, as schematically illustrated in Figure 4. Since the basal spacing of dbmPyH+-S2 was close to that of dbmPyH+-S1, the packing density of dbmPyH+ varied depending on the amount of dbmPyH+ that was intercalated (if not adsorb on the external surface of SA). A moderate dye packing (Figure 4a) in dbmPyH+-S1 may be allowed to enhance green emission, whereas dbmPyH+-S2 presumably includes a densely packed dye aggregate (Figure 4b), which is a possible origin of the aggregation-caused quenching. The threshold of the packing density for the presence of AIE characteristics in the noncoordinated diketone (without difluoroboron ligands) incorporated in the layered silicate was found to be in a range of 0.11−0.27 mmol/g. Change in the Fluorescence Spectra in Response to Solvent. As shown in Figure 5, the immersing solvent (water, DMF, and methanol) gave rise to the change in the fluorescence spectra in the dbmPyH+-S1-supported film. Upon contact with water droplets (Figure 5a), a negligible shift of the emission band at 540 nm was observed. There was a significant change in the spectrum (an approximately 100 nm hypsochromic shift) after immersing the film in DMF. This response was partially irreversible, which was revealed after DMF was evaporated. On account of the strong base character of DMF, the deprotonation of dbmPyH+ after it was dissolved in DMF caused a large one-way blueshift resulting in blue emission, as observed in the THF solution of dbmPy (Figure 1). Reversible responses were displayed in methanol immersion (Figure 5c); a slight hypsochromic shift (∼10 nm) was observed upon contact with methanol, followed by a bathochromic shift to the original position after the removal of methanol. The reversible responses were also observed in other organic solvents (e.g., ethanol, acetone, and ethyl acetate), irrespective of the solvent polarity (Figure S7). Because the increase in polarity leads to a bathochromic shift in the solution system (Figure S3), the reversible changes can be explained in terms of the change in the aggregates of dbmPyH+ from moderate to dense (and vice versa) by the intercalation of methanol.
(1 − ϕf ) τf
(1)
The calculated knr values for dbmPyH+-S1 and -S2 samples were 1.0 × 109 and 1.2 × 109 s−1, respectively. These values should be substantially smaller, considering the smaller ϕf value in the solution (
