FULL PAPER DOI: 10.1002/asia.201402739
Mechanochromic Luminescence Characteristics of Pyridine-Terminated Chromophores in the Solid State and in a Poly(vinyl alcohol) Matrix Mizuho Kondo,* Seiya Miura, Kentaro Okumoto, Mayuko Hashimoto, and Nobuhiro Kawatsuki*[a] Abstract: Mechanoresponsive luminophores containing different substituted pyridine rings at the molecular terminus are synthesized and their photoluminescence properties are investigated. The solid chromophore with a 4-substituted pyridine ring exhibits a reversible photoluminescent color change, while the 2-substituted chromophore shows
only a small change in luminescence, and the 3-substituted chromophore displays an irreversible photoluminescent color change with mechanical grinding. Keywords: mechanochromic luminescence · sensors · solid-state photoemission · solvatochromism
Introduction
unit, which exhibits a wide change in fluorescence emission from green to red by exploiting several aggregation states of the solid.[9] They also reported multi-stimuli-responsive fluorescence in the anthracene derivatives with fluorescence changes under pressure, heat, acid, and base stimuli.[10] Responsivity to acid stimulus was achieved by changing the electronic state at the pyridyl molecular terminal by disrupting the electronic push–pull properties. Acid-responsive behavior has also been reported in other luminophores with benzoxazole[11] or juroidine[12] moieties. Recently, Sun and co-workers reported a novel MCL behavior for an anthracene derivative without an acid-responsive unit; however, the dyes exhibited an acid-induced color change due to a phase transition from a crystalline powder to an amorphous solid.[13] Changes in the PL properties by acidic coordination enables fine-tuning of the color with a change in the acid species. In addition, properties such as film preparation, self-assembly, and liquid-crystallinity have been successfully achieved by the introduction of an acidic polymer film with a suitable acidic functional unit as the host material.[14] We have previously explored the color-tuning of pyridyl-terminated PL compounds via acidic noncovalent bonding, for example, hydrogen bonding and protonation,[15] and successfully achieved light-driven simultaneous patterning of molecular alignment and the PL properties.[16] We have also investigated the effect of the molecular structure of acid-responsive termini on the PL properties. The electronic push–pull structure and aggregation play an important role for solid-state PL properties; therefore, MCL compounds are strongly affected by the structure of the molecular terminus. Zhou et al. prepared an electronic push–pull type Schiff base terminated with different substituted pyridyl rings and reported that only the 4-substituted compound exhibited MCL behavior, which suggests that the rotational position of the nitrogen atom strongly affects the
Stimuli-responsive luminescent materials that demonstrate a switch in emission due to light, temperature, acid, mechanical force, and electricity have attracted much attention in various fields. In particular, the tuning and switching of fluorescence in organic solid-state materials based on environmental factors have been intensively studied from a fundamental research perspective and with respect to potential sensor applications. Mechanochromic luminescence (MCL) refers to the phenomenon whereby an isothermal change in solid-state photoluminescence (PL) occurs where the material has two states, and at least one pathway is induced by mechanical stimuli like grinding, shearing, pressing, and stretching.[1] Plausible mechanisms for MCL indicate changes in the photophysical state of the chromophore, such as changes in the chemical structure,[2] aggregation,[3] intramolecular conformation,[4] and formation of intermolecular excimers.[5] MCL is expected to be applied to mechanical sensors, the detection of microenvironmental changes,[6] and optical memory.[7] Various types of MCL compounds have been reported[8] and some mechanochromic luminophores have had pyridine introduced at the molecular terminus to function as an electronical tuning unit. Dong et al. have reported MCL behavior in an anthracene derivative containing a pyridyl [a] Dr. M. Kondo, S. Miura, K. Okumoto, M. Hashimoto, Dr. N. Kawatsuki Department of Materials Science and Chemistry University of Hyogo 2167 Shosha, Himeji, 671-2280 (Japan) Fax: (+ 81) 79-267-4014 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402739.
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A change of the sample color in response to mechanical grinding is also achieved for a dye-dispersed poly(vinyl alcohol) film. Furthermore, a simultaneous acid and mechanoresponsive photoemission color change is achieved in the dye-dispersed film.
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MCL properties.[7] Herein, we report the synthesis of three novel pyridine-terminated compounds that exhibit different MCL behavior, and investigate the changes in their photophysical properties under mechanical stimulus. In addition, we report simultaneous acid and mechanoresponsive photoemission color change patterning using a dye-dispersed polymer film.
Results and Discussion 1. Mechanochromic photoluminescence Asymmetric chromophores prepared with different pyridine carboxaldehyde substitutions were used to change the rotational position of the nitrogen atom on the pyridine ring. Figure 1 shows the chemical structures of the compounds
Figure 1. Chemical structures of the compounds used in this study.
and the detailed procedures for each reaction are described in the Experimental section. The number in the nomenclature of the compounds indicates the substitute position of the pyridine ring. Figure 2 (a) summarizes the changes in the photophysical properties of T4 in response to grinding. T4 was comprised of whitish crystals and exhibits blue emission in the initial state, while it became yellow upon grinding with a spatula and had a green PL emission [Figure 2 (a) inset]. T4 then reverted to its initial color by heating above the melting point. The mechanoresponsive behavior could be repeated several times with additional grinding. The color change was also confirmed using diffusion and PL spectroscopy. The PL spectral peak of T4 was at 483 nm in the initial state and shifted to 519 nm with grinding, while the shoulder of the diffusion spectrum peak at long wavelength was red-shifted after grinding. The T3 powder changed its PL emission color from blue (456 nm) to green (505 nm), and the ground powder did not revert to the initial color after annealing for 10 min. The MCL behavior of T2 was similar to that of T4, although the change was small
Figure 2. Mechano-induced change in the diffusion (black) and PL (colored) spectra of (a) T4, (b) T3, and (c) T2 powder. Triangles, circles, and squares represent the spectra measured for the initial, ground, and annealed powder samples, respectively. The insets show the color of the initial and ground luminophore powders under ambient (top) and UV (bottom) light.
[Figure 2 (c)]. Mechano-induced changes of the PL decay profile were also examined and revealed an extension of the PL lifetime after grinding for all the compounds (Figure 3). The decay time for T2 and T4 reverted to the initial state upon annealing, while that for T3 did not. The extension of the PL decay time indicates that a change in the intra- or intermolecular photophysical interaction of the solid-state structure was induced.[3–5, 17] In addition, the irreversible extension observed for T3 suggests that the dye maintained a similar structure after grinding and annealing. Optical micrographs of the ground and annealed T3 powder indicated that no obvious crystalline structure was formed after annealing (see Figure S1, Supporting Information), which suggests that T3 was converted to an amorphous structure by annealing. We also attempted to prepare a dispersion of the T4 dye in a polymer film as an MCL ink. Poly(vinyl alcohol) (PVA) was used as an incompatible polymer matrix for T4 powder, which resulted in blue emission from the opaque dye-dispersed film, as shown in Fig-
Abstract in Japanese:
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Figure 3. Mechano-induced changes in fluorescent decay time profiles for the initial, ground, and annealed T4 (a), T3 (b), and T2 (c) powders upon exposure to 370 nm light.
Figure 5. XRD patterns for the T4/PVA (a), T3/PVA (b), and T2/PVA films (c). The intensity was normalized with respect to the halo at 2q/q 208 due to the PVA matrix.
mechano-induced color change of the PVA film dispersion was due to a change in the crystal structure of the solid. X-ray diffraction (XRD) measurements were conducted to elucidate the different photophysical properties of the dyes, and the results are presented in Figure 5. By fixing the luminophore powder in a polymer matrix, the change in the diffraction intensity under static conditions with mechanical stimulus could be observed. The XRD profiles for the films were normalized at 2q/q 208 based on scattering from the undoped PVA film. The T4 powder was crystalline, as indicated by the many sharp and intense reflection peaks, and the T4-dispersion film showed similar peaks. The ground film showed rather weak signals and a broad halo associated with PVA was observed, which indicates disordered molecular packing of the solids [Figure 5 (a)]. A similar decrease in the diffraction peak intensity after grinding was also observed for the other dyes [Figs. 5 (b) and 5 (c)]. When the ground T2- and T4-dispersion films were annealed at the melting point of the luminophore, a slight increase was observed in the two strongest XRD peaks, although the profile did not revert to the initial profile. In addition, no obvious peaks were observed for the T3-dispersion film after annealing, which suggests that T3 tends to form an amorphous structure in the polymer matrix. The annealed film shown in Figure 4 was slightly greener than the initial film and the written character with further grinding was less obvious (see Figure S2, Supporting Information). It can be presumed that
Figure 4. Photographs of PVA films with dispersed T4 (a), T3 (b), and T2 (c); as-prepared (left) after writing (center), and after annealing (right).
ure 4 (a). After grinding the character “T” in the film with a pen to induce MCL, a green emission in the shape of the character was observed. In addition, the written character could be easily erased by heating the film above the melting point of the T4 solid. However, a PVA film with dispersed T3 exhibited a green photoemission after annealing that could not be shifted to a blue emission, while the T2-dispersion film showed only a small color change, similar to that observed for the solid-state material [Figure 4 (b,c)]. When poly(methyl methacrylate) was used instead as the polymer matrix, the T4-dispersion polymer film became homogenously transparent and colored with a green emission. However, this film did not exhibit MCL behavior; therefore, the
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files for the luminophores before and after grinding. When the ground sample was heated, an exothermic process occurred at relatively low temperatures compared with that for the initial solid, and the solid melted at a temperature identical to the melting point of the heated sample. In addition, the ground dyes showed similar properties to the initial solid in the second scan (see Figure S3, Supporting Information), which was the same as the first scan of the initial solid for T4, while T3 showed an endothermic peak and T2 showed no obvious peak below the melting point. These observations suggest that the metastable structure produced by the mechanical stimulus converted to the initial phase through an exothermic process.[19, 20] Furthermore, the second heating of the T3 solid showed an exothermic peak at 95 8C which could be restored to the original crystalline state at the second scan. When the ground or melted T3 sample was annealed at 95 8C for a long time (> 2 h) the solid was partially recrystallized and the emission color was partly changed to blue, which indicates that the peak was associated with an amorphous–solid phase transition, and the mechanoresponsive behavior of T3 could be repeated by further annealing. Figure 7 summarizes the changes in the absorbance and PL spectra of the dyes dissolved in various solvents. The maximum absorbance (lAbs) and PL (lPL) wavelengths, and the maximum absorption coefficients (e) and quantum yields (Ff) of the solutions under UV irradiation are summarized in Table 1. The lPL for the dyes changed upon dissolution in a polar solvent, and T4 exhibited a larger change in lPL than the other compounds. In addition, both the absorption and PL emission maxima were shifted to longer wavelengths when the dyes were dissolved in acid, and T4 showed the largest change. In addition, T4 exhibited almost no photoemission when dissolved in formic acid. It has been reported that protonation with strong acids causes a large absorption red-shift in p-conjugated compounds, and we have previously reported changes in lAbs and lPL for pyridine-terminated oligothiophenes with protonation.[15a] Simultaneous acid- and mechanoresponsive characteristics are demonstrated in Figure 8 for the T4-dispersed PVA film. When the film was placed under an acidic atmosphere for 10 s, the film became slightly yellow colored and the photoemission intensity was reduced [Figure 9 (b)]. As shown in Figure 9 (c), the character written on the protonated film appeared orange. In addition, the color of the film
part of the well-ground T4 powder introduced into the polymer matrix formed hydrogen bonds with PVA upon annealing.[18] 2. Study of the MCL mechanism - effects of the nitrogen position The XRD analysis indicated that the position of the nitrogen atom at the molecular terminus affects the solid-state aggregation and the MCL behavior. The nitrogen atom also plays an important role in the photophysical, thermal, and electronic properties of the compounds, which are strongly correlated with the mechanism for MCL behavior. Figure 6 shows first scan differential scanning calorimetry (DSC) pro-
Figure 6. First scan DSC thermograms for T4 (a), T3 (b), and T2 (c) powders before and after grinding.
Table 1. Maximum absorption, photoemission wavelength, maximum absorption coefficient, and quantum efficiency of the chromophore solutions.[a]
Tetrahydrofuran Chloroform Acetone Dimethyl sulfoxide Methanol Acetic acid Formic acid
T4 lAbs [nm]
e [m1 cm1]
lPL [nm]
Ff[b]
T3 lAbs [nm]
e [m1 cm1]
lPL [nm]
Ff[b]
T2 lAbs [nm]
e [m1 cm1]
lPL [nm]
Ff[b]
366 375 362 369 367 381 425
26 800 18 600 27 300 24 600 24 700 18 800 22 000
492 492 507 534 536 569 566
6.95 9.32 6.59 6.79 4.18 0.238 0.028
363 368 359 367 363 370 390
22 400 24 700 27 300 27 400 23 600 22 000 16 100
471 472 493 507 506 548 566
7.63 7.85 6.20 7.00 7.84 1.13 0.184
366 369 362 370 364 372 415
27 200 21 900 26 600 24 400 22 600 15 600 13 500
471 478 491 478 511 567 609
6.46 8.55 6.51 7.68 7.52 0.421 0.181
[a] Concentration of the solution: 1 104 mol/L. [b] Excitation at 365 nm.
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lar behavior, the film used in the current study only red-shifted homogeneously. In addition, the acid/dye complex solid prepared from the solution of luminogen and acid did not exhibit MCL properties, which suggests that the initial crystal and molecular structure plays an important role for the MCL behavior. A plausible mechanism can be elucidated from the difference in the acid-absorption ability between the initial and ground powder; the initial luminophore solid absorbed the acid only at the surface, which caused a loss of MCL and photoemission with a redshift of the absorbance and a reduction of contrast. Acid on the surface of the initial solid was easily evaporated, while the ground powder remained as the protonated form, whereby the contrast of the ground powder PL image became clear. When a T4-dispersed film was kept in an acidic atmosphere for a long time (10 min), the film exhibited a weak orange emission and Figure 7. Changes in the absorption (black) and PL (colored) spectra of T4 (a), T3 (b), and T2 (c) dissolved in the written character was diffivarious solvents; (1) tetrahydrofuran (closed circles), (2) chloroform (open triangles), (3) acetone (crosses), cult to distinguish, which indi(4) dimethyl sulfoxide (closed squares), (5) methanol (open squares), (6) acetic acid (closed triangles), and cates that the enhancement of (7) formic acid (open circles). Photographs show the luminophore solutions irradiated with UV light. color contrast is due to the difference in acid absorption between the initial solid and the ground powder. and the written character were blue-shifted when the film Density functional theory (DFT) calculations for the comwas left in air for over 1 week. Similar acid- and mechanorpounds were performed to explain the effect of the chemical esponsive behavior was induced in the T3-dispersed film, structure on the PL spectra of the solutions. The calculations while the T2-dispersed film showed only as slight change were performed using DFT with the restricted CAM-B3LYP upon exposure to fuming acid, which reflects the result of the solvent effect (see Figure S4, Supporting Information). Xue and co-workers have previously reported multicolor MCL characteristics for a benzoxazole unit. They demonstrated simultaneous acid- and mechanoresponsive behavior and succeeded in improving the contrast of the initial and ground sample.[9] Although the film with the written character pattern was exposed to fuming acetic acid to induce simi-
Figure 9. HOMO (left) and LUMO (right) electron densities for (a) T4, (b) T3 and (c) T2. The red and green lobes represent the positive and negative signs of the coefficients of the molecular orbitals. The size of each lobe is proportional to the MO coefficient.
Figure 8. Simultaneous acid- and mechanoresponsive changes in PL images of the T4-dispersed film. (a) Initial film, (b) after acid fuming, (c) after writing the character “H”, and (d) after 9 days.
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functional and the 6-31G(d) basis set, as implemented in ited reversible mechano-induced changes in PL, of which Gaussian 09.[21] Figure 9 shows that all the luminophores the 4-pyridyl terminated chromophore had the largest. In have similar highest occupied molecular orbitals (HOMOs) addition, MCL patterning was successfully achieved using and lowest unoccupied molecular orbitals (LUMOs). The dye dispersed in PVA film. Further investigations on the inelectron clouds in the HOMO levels are mainly located on fluence of the polymer matrix type and content on the phothe electron-donating thiophene and bridged p-units, while tophysical properties and applications of acid-responsive those of the LUMO levels are predominantly placed on the chromophores are currently in progress. pyridine moiety, revealing the intramolecular charge transfer (ICT) characteristics in the molecules, which is consistent with the spectroscopic results described by Gong et al.[22] Experimental Section The dipole moment of T4 is 5.78 D, which is over twice that of T2 (2.18 D), and T3 is between them (3.27 D). DFT calculations for the chromophores suggest that T4 would have the largest dipole moment and a strong push–pull interaction in the p-conjugated structure along the molecular long axis, assuming that the photo- Scheme 1. Synthetic route of the chromophores. emission characteristics of T4 are easily affected by the solSynthesis vent polarity. One of the origins of luminescence switching 4-Bromophenylacetonitrile (1.8 g, 9.2 mmol) and 4-pyridine carboxyaldein the solid state is the change in conformation. Cyanostyrhyde (1.0 g, 9.3 mmol) were dissolved in 2 mL of methanol. Potassium [7] [3, 22] yl or phenylthylene moieties tend to form twisted contert-butoxide (0.2 g, 0.89 mmol) dissolved in 2 mL of methanol was formations and become planarized through facile mechaniadded dropwise to the solution with stirring, and the reaction mixture was mixed at room temperature for 20 h. The crude product was filtered cal stimuli, and can then be restored by thermal annealing. and recrystallized from methanol to give Br-4 as yellowish needle-like Zhang and co-workers reported that dyes with twisted stackcrystals (1.4 g, mp 138–140 8C, 52 % yield). 1H NMR (CDCl3): d = 7.45 (s, ing architectures result in much shorter effective conjugation 1 H), 7.56–7.55 (d, J = 4.2 Hz, 2 H), 7.61–7.59 (d, J = 4.2 Hz, 2 H), 7.69– lengths and bluer emissions, while planarized conformations 7.68 ppm (d, J = 2.9 Hz, 2 H), 8.75–8.74 (d, J = 2.8 Hz, 2 H). IR (KBr): n˜ = 3058, 2220, 1593, 1540, 1489, 1419, 1409, 1374, 1234, 1077 cm1. generate increased conjugation, thus producing much redder [22] Compound Br-3 was synthesized by a procedure similar to that for Br-4, emissions. Although we could not obtain single-crystal difbut with 3-pyridine carboxyaldehyde, and was obtained as yellowish fraction data for T4, T3 or T2, the twisted structure is preneedle-like crystals (1.1 g, 3.9 mmol, mp 130–132 8C, 87 % yield). dicted in the simulated structure and a similar MCL mecha1 H NMR (CDCl3): d = 7.43–7.40 (m, 3 H), 7.50 (s, 1 H), 7.55–7.53 (d, J = nism can be induced in compounds with similar chemical 4.3 Hz, 2 H), 7.60–7.58 (d, J = 4.2 Hz, 2 H), 8.47–8.45 (d, J = 4.1 Hz, 2 H), structures. However, DFT calculations for the luminophores 8.65–8.64 (d, J = 2.4 Hz, 2 H), 8.83 ppm (s, 1 H). IR (KBr): n˜ = 3055, 2218, 1588, 1563, 1492, 1415, 1409, 1371, 1234, 1081 cm1. used in this study indicate a similar twisted conformation to Compound Br-2 was synthesized by a procedure similar to that for Br-4, that of the cyanostyryl unit (see Figure S5, Supporting Inforbut with 2-pyridine carboxyaldehyde, and was obtained as yellowish mation); therefore, another effect should exist for MCL beneedle-like crystals (1.1 g, 3.9 mmol, mp 140–141 8C, 82 % yield). havior in this study. Ooyama and Harima evaluated various 1 H NMR (CDCl3): d = 7.32–7.30 (m, 3 H), 7.56 (s, 1 H,), 7.58–7.57 (d, J = mechanoreponsive electron push–pull compounds and dem3.3 Hz, 2 H), 7.61–7.59 (d, J = 4.5 Hz, 2 H), 7.80–7.77 (m, 3 H), 7.91–7.90 onstrated that MCL is attributed to a reversible switching (d, J = 4.0 Hz, 2 H), 8.75–8.74 ppm (d, J = 2.4 Hz, 2 H). IR (KBr): n˜ = 3047, 2216, 1577, 1558, 1487, 1473, 1402, 1348, 1409, 1242, 1003 cm1. between crystalline and amorphous states with changes of 5-Hexyl-2-thiopheneboronic acid pinacol ester (0.82 g, 2.8 mmol) and Brmolecular arrangement and intermolecular interactions.[19] 4 (0.70 g, 2.5 mmol) were placed under nitrogen with stirring, to which They also reported that the dipole of the compounds has N2-purged tetrahydrofuran (THF; 15 mL) and K2CO3 aqueous solution a strong influence on the MCL characteristics, in agreement (1 m, 5 mL) were added. Tetrakis(triphenylphosphine)palladium(0) with our results. It can be speculated that H-aggregation (91 mg, 0.079 mmol) dissolved in N2-purged THF (5 mL) was then added, occurs in the crystal structure, which induces a blue-shift of and the reaction mixture was heated to 65 8C for 17 h. After the mixture was cooled to room temperature, 60 mL of chloroform and 50 mL of the absorbance and PL, thereby shortening the lifetime, water were added. The solution mixture was extracted with chloroform which is strongly affected by the molecular packing distance and washed with water. The organic layer was dried with sodium sulfate. and dipole moment. After the solvent was removed under reduced pressure, T4 was obtained by recrystallization from hexane (0.33 g, mp 128 8C, 36 % yield). 1H NMR (CDCl3): d = 0.90–0.87 (t, 4.7 Hz, 3 H), 1.40–1.29 (m, 6 H), 1.72–1.66 (m, 5 H), 2.84–2.81 (t, 2 H), 6.77–6.76 (d, J = 1.8 Hz, 2 H), 7.21–7.20 (d, J = 1.8 Hz, 2 H), 7.45 (s, 1 H), 7.64–7.62 (d, J = 4.3 Hz, 2 H), 7.68–7.66 (d, J = 4.3 Hz, 2 H), 7.70–7.69 (d, J = 3.1 Hz, 2 H), 8.74–8.73 ppm (d, J = 2.9 Hz, 2 H). 13C NMR (126 MHz, CDCl3): d = 14.16, 22.63, 28.82, 30.35, 31.61, 115.90, 116.89, 122.68, 123.95, 125.47, 125. 80, 126.72, 131.48, 136.61, 137.66, 139.99, 140.69, 147.34, 150.68 ppm. IR (KBr): n˜ = 2953, 2922,
Conclusions Three pyridine-terminated MCL compounds were prepared by changing the rotational position of the nitrogen atom in the pyridine ring at the molecular terminus. The dyes exhib-
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2852, 2227, 1591, 1536, 1513, 1467, 1256 cm1. Elemental anal. calcd for C24H24N2S: C, 77.38; H, 6.49; N, 7.52; found: C, 77.55; H, 6.66; N, 7.50. MS (GC-MS, intensity) m/z 372 (M +).
Acknowledgements This work was supported in part by a Grant-in-Aid for Young Scientists B (No. 25810079) from the Japan Society for the Promotion of Science nd a research grant from the University of Hyogo.
Compound T3 was synthesized by a procedure similar to that for T4, but using Br-3, and was obtained as yellowish needle-like crystals (0.37 g, 0.99 mmol, mp 95–98 8C, 44 % yield). 1H NMR (CDCl3): d = 0.90–0.87 (t, 4.6 Hz, 3 H), 1.39–1.31 (m, 6 H), 1.72–1.66 (m, 5 H), 2.83–2.80 (t, J = 5.1 Hz, 2 H), 6.76–6.75 (d, J = 1.7 Hz, 2 H), 7.20–7.19 (d, J = 1.8 Hz, 2 H), 7.31–7.29 (m, 4.1 Hz, 3 H), 7.61 (s, 1 H), 7.64–7.63 (d, J = 2.2 Hz, 2 H), 7.73–7.71 (d, J = 4.2 Hz, 2 H), 7.80–7.77 (m, J = 4.8 Hz, 3 H), 7.96–7.94 (d, J = 4.0 Hz, 2 H), 8.75–8.74 ppm (d, J = 2.2 Hz, 2 H). 13C NMR (126 MHz, CDCl3): d = 14.19, 22.65, 28.85, 30.32, 31.58, 31.61, 113.50, 117.35, 123.71, 123.75, 125.41, 125. 67, 126.39, 129.74, 131.80, 134.53, 136.03, 136.85, 140.07, 147.02, 150.87, 151.37 ppm. IR (KBr): n˜ = 2953, 2922, 2852, 2226, 1606, 1581, 1513, 1468, 1422, 1379, 1223, 1024 cm1. Elemental anal. calcd for C24H24N2S: C, 77.38; H, 6.49; N, 7.52; found: C, 77.67; H, 6.21; N, 7.35. MS (GC-MS, intensity) m/z 372 (M+).
[1] a) Y. Sagara, T. Kato, Nat. Chem. 2009, 1, 605 – 610; b) S. Yamane, Y. Sagara, T. Mutai, K. Araki, T. Kato, J. Mater. Chem. C 2013, 1, 2648 – 2656. [2] a) D. A. Davis, A. Hamilton, J. Yang, L. D. Cremar, D. Gough, S. L. Potisek, M. T. Ong, P. V. Braun, T. J. Martnez, S. R. White, J. S. Moore, N. R. Sottos, Nature 2009, 459, 68 – 72; b) X. Fang, H. Zhang, Y. Chen, Y. Lin, Y. Xu, W. Weng, Macromolecules 2013, 46, 6566 – 6574; c) M. Shiotsuki, T. Endo, J. Polym. Sci. Part A 2014, 52, 1392 – 1400. [3] a) C. Dou, D. Chen, J. Iqbal, Y. Yuan, H. Zhang, Y. Wang, Langmuir 2011, 27, 6323 – 6329; b) R. Rao M, C. Liao, S. Sun, J. Mater. Chem. C 2013, 1, 6386 – 6394; c) Y. Zhang, J. Sun, G. Zhuang, M. Ouyang, Z. Yu, F. Cao, G. Pan, P. Tang, C. Zhang, Y. Ma, J. Mater. Chem. C 2014, 2, 195 – 200; d) N. Mizoshita, T. Tani, S. Inagaki, Adv. Mater. 2012, 24, 3350 – 3355;e) S. Yagai, S. Okamura, Y. Nakano, M. Yamauchi, K. Kishikawa, T. Karatsu, A. Kitamura, A. Ueno, D. Kuzuhara, H. Yamada, T. Seki, H. Ito, Nat. Commun. 2014, 5, 4013. [4] a) Q. Qi, J. Zhang, B. Xu, B. Li, S. X. Zhang, W. Tian, J. Phys. Chem. C 2013, 117, 24997 – 25003; b) Q. Qi, Y. Liu, X. Fang, Y. Zhang, P. Chen, Y. Wang, B. Yang, B. Xu, W. Tian, S. X. Zhang, RSC Adv. 2013, 3, 7996 – 8002. [5] Y. Sagara, T. Kato, Angew. Chem. Int. Ed. 2011, 50, 9128 – 9132; Angew. Chem. 2011, 123, 9294 – 9298. [6] Y. Sagara, T. Komatsu, T. Ueno, K. Hanaoka, T. Kato, T. Nagano, Adv. Funct. Mater. 2013, 23, 5277 – 5284. [7] M. Yang, D. Xu, W. Xi, L. Wang, J. Zheng, J. Huang, J. Zhang, H. Zhou, J. Wu, Y. Tian, J. Org. Chem. 2013, 78, 10344 – 10359. [8] a) Y. Sagara, T. Mutai, I. Yoshikawa, K. Araki, J. Am. Chem. Soc. 2007, 129, 1520 – 1521; b) K. Nagura, S. Saito, H. Yusa, H. Yamawaki, H. Fujihisa, H. Sato, Y. Shimoikeda, S. Yamaguchi, J. Am. Chem. Soc. 2013, 135, 10322 – 10325; c) W. Z. Yuan, Y. Tan, Y. Gong, P. Lu, J. W. Y. Lam, X. Y. Shen, C. Feng, H. H. Sung, Y. Lu, I. D. Williams, J. Z. Sun, Y. Zhang, B. Z. Tang, Adv. Mater. 2013, 25, 2837 – 2843; d) S. Wang, S. Xiao, X. Chen, R. Zhang, Q. Cao, K. Zou, Dyes Pigm. 2013, 99, 543 – 547; e) Y. Zhang, J. Sun, G. Bian, Y. Chen, M. Ouyang, B. Hua, C. Zhang, Photochem. Photobiol. Sci. 2012, 11, 1414 – 1421; f) X. Y. Shen, W. Z. Yuan, Y. Liu, Q. Zhao, P. Lu, Y. Ma, I. D. Williams, A. Qin, J. Z. Sun, B. Z. Tang, J. Phys. Chem. C 2012, 116, 10541 – 10547; g) R. Wei, P. Song, A. Tong, J. Phys. Chem. C 2013, 117, 3467 – 3474. [9] Y. Dong, B. Xu, J. Zhang, X. Tan, L. Wang, J. Chen, H. L., Shanpeng, W. Bao, L. Ling, Y. B. Zou, W. Tian, Angew. Chem. Int. Ed. 2012, 51, 10782 – 10785; Angew. Chem. 2012, 124, 10940 – 10943. [10] Y. Dong, J. Zhang, X. Tan, L. Wang, J. Chen, B. Li, L. Ye, B. Xu, B. Zou, W. Tian, J. Mater. Chem. C 2013, 1, 7554 – 7559. [11] P. Xue, P. Chen, J. Jia, Q. Xu, J. Sun, B. Yao, Z. Zhang, R. Lu, Chem. Commun. 2014, 50, 2569 – 2571. [12] C. Dou, L. Han, S. Zhao, H. Zhang, Y. Wang, J. Phys. Chem. Lett. 2011, 2, 666 – 670. [13] R. Rao M, C. Liao, S. Sun, J. Mater. Chem. C 2013, 1, 6386 – 6394. [14] a) T. Kato, N. Mizoshita, K. Kishimoto, Angew. Chem. Int. Ed. 2006, 45, 38 – 68; Angew. Chem. 2006, 118, 44 – 74; b) T. Kato, N. Hirota, A. Fujishima, J. M. J. Frchet, J. Polym. Sci. Part A 1996, 34, 57 – 62. [15] a) M. Kondo, J. Miyake, K. Okumoto, H. Shoji, N. Kawatsuki, Chem. Lett. 2013, 42, 891 – 893; b) N. Kawatsuki, Y. Minami, J. Lee, Chem. Lett. 2009, 38, 298 – 299. [16] N. Kawatsuki, R. Ando, R. Ishida, M. Kondo, Y. Minami, Macromol. Chem. Phys. 2010, 211, 1741 – 1749. [17] A. Pucci, G. Ruggeri, J. Mater. Chem. 2011, 21, 8282 – 8291. [18] S. Deng, T. Chen, W. Chien, J. Hong, J. Mater. Chem. C 2014, 2, 651 – 659. [19] Y. Ooyama, Y. Harima, J. Mater. Chem. 2011, 21, 8372 – 8380.
Compound T2 was synthesized by a procedure similar to that of T4, but using Br-2, and was obtained as yellowish needle-like crystals (0.44 g, 1.2 mmol, mp 118–119 8C, 52 % yield). 1H NMR (CDCl3): d = 8.75–8.74 (d, J = 2.2 Hz, 2 H), 7.96–7.94 (d, J = 4.0 Hz, 2 H), 7.80–7.77 (m, J = 4.8 Hz, 3 H), 7.73–7.71 (d, J = 4.2 Hz, 2 H), 7.64–7.63 (d, J = 2.2 Hz, 2 H), 7.61 (s, 1 H), 7.31–7.29 (m, 4.1 Hz, 3 H), 7.20–7.19 (d, J = 1.8 Hz, 2 H), 6.76–6.75 (d, J = 1.7 Hz, 2 H), 2.83–2.80 (t, J = 5.1 Hz, 2 H), 1.72–1.66 (m, 5 H), 1.39–1.31 (m, 6 H), 0.90–0.87 ppm (t, 4.6 Hz, 3 H). 13C NMR (126 MHz, CDCl3): d = 14.20, 22.66, 28.86, 30.34, 31.60, 31.63, 114.19, 117.39, 123.73, 124.18, 124.23, 125. 40, 125.66, 126.72, 132.11, 136.08, 136.75, 139.85, 140.21, 146.94, 149.97, 152.17 ppm. IR (KBr): n˜ = 2953, 2921, 2849, 2215, 1578, 1555, 1510, 1467, 1428, 1367, 1215, 1053 cm1. Elemental anal. calcd for C24H24N2S: C, 77.38; H, 6.49; N, 7.52; found: C, 77.38; H, 6.70; N, 7.27. MS (GC-MS, intensity) m/z 372 (M +). Experimental Setup The synthesized compounds were characterized using Fourier transform nuclear magnetic resonance (FT-NMR; Bruker, DRX-500) and Fourier transform infrared (FT-IR; Jasco, FT-IR 410) spectroscopies. 1H and 13 C NMR spectra were measured in CDCl3 at room temperature with tetramethylsilane (TMS; d = 0.00) as an internal standard. Electrospray ionization (ESI) mass spectra and total C, H and N content of the final compounds were measured with a gas chromatograph mass spectrometer (GC/MS; Shimadzu, GCMS-QP5050 A) and an elemental analyzer (Yanaco, MT-5), respectively. The thermal properties of the compounds were evaluated using differential scanning calorimetry (DSC; Seiko, SSC5200) at a heating/cooling rate of 0.5 8C min1. UV-vis absorption and diffusion spectra were measured with a UV spectrometer (Hitachi, U3010) and fluorescence spectra were obtained with a fluorescence spectrometer (Hitachi, F-4500). The fluorescence quantum yields of the dye solutions were estimated by comparison with a quantum yield standard, where the quantum yield (Ff) of a material is calculated using Eq. (1):
Ff ¼ Fs
I As n 2 Is A n2s
ð1Þ
where I is the standard emission intensity, A is the absorbance, and n is the refractive index. The subscript s refers to the standard solution of quinine sulfate. The diffusion spectra of the solids were measured with the solid pasted on a tape. The PL lifetimes of the composite films were measured with a nanosecond spectrofluorometer (Horiba, FluoroCube) with 370 nm irradiation. The dye-dispersed films were spin-coated on glass substrates from an aqueous solution containing 4 wt % PVA (Kuraray Co., DP: 500, saponification > 97 %) and the unground dye powder (approximately dye/polymer = 0.14 (w/w)). The films were then dried for 1 day at room temperature. X-ray diffractometry (XRD; Rigaku, SmartLab) was performed with a standard small angle diffractometry setup.
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[20] a) T. Tsukuda, M. Kawase, A. Dairiki, K. Matsumoto, T. Tsubomura, Chem. Commun. 2010, 46, 1905 – 1907; b) J. Mizuguchi, N. Tanifuji, K. Kobayashi, J. Phys. Chem. B 2003, 107, 12635 – 12638. [21] Gaussian 09, Revision C.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant,
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S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian, Inc., Wallingford CT, 2010. [22] Y. Gong, Y. Tan, J. Liu, P. Lu, C. Feng, W. Z. Yuan, Y. Lu, J. Z. Sun, G. He, Y. Zhang, Chem. Commun. 2013, 49, 4009 – 4011. Received: June 27, 2014 Published online: September 5, 2014
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Mechanochromic Luminescence Characteristics of Pyridine-Terminated Chromophores in the Solid State and in a Poly(vinyl alcohol) Matrix Mizuho Kondo,* Seiya Miura, Kentaro Okumoto, Mayuko Hashimoto, and Nobuhiro Kawatsuki*[a] Abstract: Mechanoresponsive luminophores containing different substituted pyridine rings at the molecular terminus are synthesized and their photoluminescence properties are investigated. The solid chromophore with a 4-substituted pyridine ring exhibits a reversible photoluminescent color change, while the 2-substituted chromophore shows
only a small change in luminescence, and the 3-substituted chromophore displays an irreversible photoluminescent color change with mechanical grinding. Keywords: mechanochromic luminescence · sensors · solid-state photoemission · solvatochromism
Introduction
unit, which exhibits a wide change in fluorescence emission from green to red by exploiting several aggregation states of the solid.[9] They also reported multi-stimuli-responsive fluorescence in the anthracene derivatives with fluorescence changes under pressure, heat, acid, and base stimuli.[10] Responsivity to acid stimulus was achieved by changing the electronic state at the pyridyl molecular terminal by disrupting the electronic push–pull properties. Acid-responsive behavior has also been reported in other luminophores with benzoxazole[11] or juroidine[12] moieties. Recently, Sun and co-workers reported a novel MCL behavior for an anthracene derivative without an acid-responsive unit; however, the dyes exhibited an acid-induced color change due to a phase transition from a crystalline powder to an amorphous solid.[13] Changes in the PL properties by acidic coordination enables fine-tuning of the color with a change in the acid species. In addition, properties such as film preparation, self-assembly, and liquid-crystallinity have been successfully achieved by the introduction of an acidic polymer film with a suitable acidic functional unit as the host material.[14] We have previously explored the color-tuning of pyridyl-terminated PL compounds via acidic noncovalent bonding, for example, hydrogen bonding and protonation,[15] and successfully achieved light-driven simultaneous patterning of molecular alignment and the PL properties.[16] We have also investigated the effect of the molecular structure of acid-responsive termini on the PL properties. The electronic push–pull structure and aggregation play an important role for solid-state PL properties; therefore, MCL compounds are strongly affected by the structure of the molecular terminus. Zhou et al. prepared an electronic push–pull type Schiff base terminated with different substituted pyridyl rings and reported that only the 4-substituted compound exhibited MCL behavior, which suggests that the rotational position of the nitrogen atom strongly affects the
Stimuli-responsive luminescent materials that demonstrate a switch in emission due to light, temperature, acid, mechanical force, and electricity have attracted much attention in various fields. In particular, the tuning and switching of fluorescence in organic solid-state materials based on environmental factors have been intensively studied from a fundamental research perspective and with respect to potential sensor applications. Mechanochromic luminescence (MCL) refers to the phenomenon whereby an isothermal change in solid-state photoluminescence (PL) occurs where the material has two states, and at least one pathway is induced by mechanical stimuli like grinding, shearing, pressing, and stretching.[1] Plausible mechanisms for MCL indicate changes in the photophysical state of the chromophore, such as changes in the chemical structure,[2] aggregation,[3] intramolecular conformation,[4] and formation of intermolecular excimers.[5] MCL is expected to be applied to mechanical sensors, the detection of microenvironmental changes,[6] and optical memory.[7] Various types of MCL compounds have been reported[8] and some mechanochromic luminophores have had pyridine introduced at the molecular terminus to function as an electronical tuning unit. Dong et al. have reported MCL behavior in an anthracene derivative containing a pyridyl [a] Dr. M. Kondo, S. Miura, K. Okumoto, M. Hashimoto, Dr. N. Kawatsuki Department of Materials Science and Chemistry University of Hyogo 2167 Shosha, Himeji, 671-2280 (Japan) Fax: (+ 81) 79-267-4014 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402739.
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A change of the sample color in response to mechanical grinding is also achieved for a dye-dispersed poly(vinyl alcohol) film. Furthermore, a simultaneous acid and mechanoresponsive photoemission color change is achieved in the dye-dispersed film.
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MCL properties.[7] Herein, we report the synthesis of three novel pyridine-terminated compounds that exhibit different MCL behavior, and investigate the changes in their photophysical properties under mechanical stimulus. In addition, we report simultaneous acid and mechanoresponsive photoemission color change patterning using a dye-dispersed polymer film.
Results and Discussion 1. Mechanochromic photoluminescence Asymmetric chromophores prepared with different pyridine carboxaldehyde substitutions were used to change the rotational position of the nitrogen atom on the pyridine ring. Figure 1 shows the chemical structures of the compounds
Figure 1. Chemical structures of the compounds used in this study.
and the detailed procedures for each reaction are described in the Experimental section. The number in the nomenclature of the compounds indicates the substitute position of the pyridine ring. Figure 2 (a) summarizes the changes in the photophysical properties of T4 in response to grinding. T4 was comprised of whitish crystals and exhibits blue emission in the initial state, while it became yellow upon grinding with a spatula and had a green PL emission [Figure 2 (a) inset]. T4 then reverted to its initial color by heating above the melting point. The mechanoresponsive behavior could be repeated several times with additional grinding. The color change was also confirmed using diffusion and PL spectroscopy. The PL spectral peak of T4 was at 483 nm in the initial state and shifted to 519 nm with grinding, while the shoulder of the diffusion spectrum peak at long wavelength was red-shifted after grinding. The T3 powder changed its PL emission color from blue (456 nm) to green (505 nm), and the ground powder did not revert to the initial color after annealing for 10 min. The MCL behavior of T2 was similar to that of T4, although the change was small
Figure 2. Mechano-induced change in the diffusion (black) and PL (colored) spectra of (a) T4, (b) T3, and (c) T2 powder. Triangles, circles, and squares represent the spectra measured for the initial, ground, and annealed powder samples, respectively. The insets show the color of the initial and ground luminophore powders under ambient (top) and UV (bottom) light.
[Figure 2 (c)]. Mechano-induced changes of the PL decay profile were also examined and revealed an extension of the PL lifetime after grinding for all the compounds (Figure 3). The decay time for T2 and T4 reverted to the initial state upon annealing, while that for T3 did not. The extension of the PL decay time indicates that a change in the intra- or intermolecular photophysical interaction of the solid-state structure was induced.[3–5, 17] In addition, the irreversible extension observed for T3 suggests that the dye maintained a similar structure after grinding and annealing. Optical micrographs of the ground and annealed T3 powder indicated that no obvious crystalline structure was formed after annealing (see Figure S1, Supporting Information), which suggests that T3 was converted to an amorphous structure by annealing. We also attempted to prepare a dispersion of the T4 dye in a polymer film as an MCL ink. Poly(vinyl alcohol) (PVA) was used as an incompatible polymer matrix for T4 powder, which resulted in blue emission from the opaque dye-dispersed film, as shown in Fig-
Abstract in Japanese:
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Figure 3. Mechano-induced changes in fluorescent decay time profiles for the initial, ground, and annealed T4 (a), T3 (b), and T2 (c) powders upon exposure to 370 nm light.
Figure 5. XRD patterns for the T4/PVA (a), T3/PVA (b), and T2/PVA films (c). The intensity was normalized with respect to the halo at 2q/q 208 due to the PVA matrix.
mechano-induced color change of the PVA film dispersion was due to a change in the crystal structure of the solid. X-ray diffraction (XRD) measurements were conducted to elucidate the different photophysical properties of the dyes, and the results are presented in Figure 5. By fixing the luminophore powder in a polymer matrix, the change in the diffraction intensity under static conditions with mechanical stimulus could be observed. The XRD profiles for the films were normalized at 2q/q 208 based on scattering from the undoped PVA film. The T4 powder was crystalline, as indicated by the many sharp and intense reflection peaks, and the T4-dispersion film showed similar peaks. The ground film showed rather weak signals and a broad halo associated with PVA was observed, which indicates disordered molecular packing of the solids [Figure 5 (a)]. A similar decrease in the diffraction peak intensity after grinding was also observed for the other dyes [Figs. 5 (b) and 5 (c)]. When the ground T2- and T4-dispersion films were annealed at the melting point of the luminophore, a slight increase was observed in the two strongest XRD peaks, although the profile did not revert to the initial profile. In addition, no obvious peaks were observed for the T3-dispersion film after annealing, which suggests that T3 tends to form an amorphous structure in the polymer matrix. The annealed film shown in Figure 4 was slightly greener than the initial film and the written character with further grinding was less obvious (see Figure S2, Supporting Information). It can be presumed that
Figure 4. Photographs of PVA films with dispersed T4 (a), T3 (b), and T2 (c); as-prepared (left) after writing (center), and after annealing (right).
ure 4 (a). After grinding the character “T” in the film with a pen to induce MCL, a green emission in the shape of the character was observed. In addition, the written character could be easily erased by heating the film above the melting point of the T4 solid. However, a PVA film with dispersed T3 exhibited a green photoemission after annealing that could not be shifted to a blue emission, while the T2-dispersion film showed only a small color change, similar to that observed for the solid-state material [Figure 4 (b,c)]. When poly(methyl methacrylate) was used instead as the polymer matrix, the T4-dispersion polymer film became homogenously transparent and colored with a green emission. However, this film did not exhibit MCL behavior; therefore, the
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files for the luminophores before and after grinding. When the ground sample was heated, an exothermic process occurred at relatively low temperatures compared with that for the initial solid, and the solid melted at a temperature identical to the melting point of the heated sample. In addition, the ground dyes showed similar properties to the initial solid in the second scan (see Figure S3, Supporting Information), which was the same as the first scan of the initial solid for T4, while T3 showed an endothermic peak and T2 showed no obvious peak below the melting point. These observations suggest that the metastable structure produced by the mechanical stimulus converted to the initial phase through an exothermic process.[19, 20] Furthermore, the second heating of the T3 solid showed an exothermic peak at 95 8C which could be restored to the original crystalline state at the second scan. When the ground or melted T3 sample was annealed at 95 8C for a long time (> 2 h) the solid was partially recrystallized and the emission color was partly changed to blue, which indicates that the peak was associated with an amorphous–solid phase transition, and the mechanoresponsive behavior of T3 could be repeated by further annealing. Figure 7 summarizes the changes in the absorbance and PL spectra of the dyes dissolved in various solvents. The maximum absorbance (lAbs) and PL (lPL) wavelengths, and the maximum absorption coefficients (e) and quantum yields (Ff) of the solutions under UV irradiation are summarized in Table 1. The lPL for the dyes changed upon dissolution in a polar solvent, and T4 exhibited a larger change in lPL than the other compounds. In addition, both the absorption and PL emission maxima were shifted to longer wavelengths when the dyes were dissolved in acid, and T4 showed the largest change. In addition, T4 exhibited almost no photoemission when dissolved in formic acid. It has been reported that protonation with strong acids causes a large absorption red-shift in p-conjugated compounds, and we have previously reported changes in lAbs and lPL for pyridine-terminated oligothiophenes with protonation.[15a] Simultaneous acid- and mechanoresponsive characteristics are demonstrated in Figure 8 for the T4-dispersed PVA film. When the film was placed under an acidic atmosphere for 10 s, the film became slightly yellow colored and the photoemission intensity was reduced [Figure 9 (b)]. As shown in Figure 9 (c), the character written on the protonated film appeared orange. In addition, the color of the film
part of the well-ground T4 powder introduced into the polymer matrix formed hydrogen bonds with PVA upon annealing.[18] 2. Study of the MCL mechanism - effects of the nitrogen position The XRD analysis indicated that the position of the nitrogen atom at the molecular terminus affects the solid-state aggregation and the MCL behavior. The nitrogen atom also plays an important role in the photophysical, thermal, and electronic properties of the compounds, which are strongly correlated with the mechanism for MCL behavior. Figure 6 shows first scan differential scanning calorimetry (DSC) pro-
Figure 6. First scan DSC thermograms for T4 (a), T3 (b), and T2 (c) powders before and after grinding.
Table 1. Maximum absorption, photoemission wavelength, maximum absorption coefficient, and quantum efficiency of the chromophore solutions.[a]
Tetrahydrofuran Chloroform Acetone Dimethyl sulfoxide Methanol Acetic acid Formic acid
T4 lAbs [nm]
e [m1 cm1]
lPL [nm]
Ff[b]
T3 lAbs [nm]
e [m1 cm1]
lPL [nm]
Ff[b]
T2 lAbs [nm]
e [m1 cm1]
lPL [nm]
Ff[b]
366 375 362 369 367 381 425
26 800 18 600 27 300 24 600 24 700 18 800 22 000
492 492 507 534 536 569 566
6.95 9.32 6.59 6.79 4.18 0.238 0.028
363 368 359 367 363 370 390
22 400 24 700 27 300 27 400 23 600 22 000 16 100
471 472 493 507 506 548 566
7.63 7.85 6.20 7.00 7.84 1.13 0.184
366 369 362 370 364 372 415
27 200 21 900 26 600 24 400 22 600 15 600 13 500
471 478 491 478 511 567 609
6.46 8.55 6.51 7.68 7.52 0.421 0.181
[a] Concentration of the solution: 1 104 mol/L. [b] Excitation at 365 nm.
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lar behavior, the film used in the current study only red-shifted homogeneously. In addition, the acid/dye complex solid prepared from the solution of luminogen and acid did not exhibit MCL properties, which suggests that the initial crystal and molecular structure plays an important role for the MCL behavior. A plausible mechanism can be elucidated from the difference in the acid-absorption ability between the initial and ground powder; the initial luminophore solid absorbed the acid only at the surface, which caused a loss of MCL and photoemission with a redshift of the absorbance and a reduction of contrast. Acid on the surface of the initial solid was easily evaporated, while the ground powder remained as the protonated form, whereby the contrast of the ground powder PL image became clear. When a T4-dispersed film was kept in an acidic atmosphere for a long time (10 min), the film exhibited a weak orange emission and Figure 7. Changes in the absorption (black) and PL (colored) spectra of T4 (a), T3 (b), and T2 (c) dissolved in the written character was diffivarious solvents; (1) tetrahydrofuran (closed circles), (2) chloroform (open triangles), (3) acetone (crosses), cult to distinguish, which indi(4) dimethyl sulfoxide (closed squares), (5) methanol (open squares), (6) acetic acid (closed triangles), and cates that the enhancement of (7) formic acid (open circles). Photographs show the luminophore solutions irradiated with UV light. color contrast is due to the difference in acid absorption between the initial solid and the ground powder. and the written character were blue-shifted when the film Density functional theory (DFT) calculations for the comwas left in air for over 1 week. Similar acid- and mechanorpounds were performed to explain the effect of the chemical esponsive behavior was induced in the T3-dispersed film, structure on the PL spectra of the solutions. The calculations while the T2-dispersed film showed only as slight change were performed using DFT with the restricted CAM-B3LYP upon exposure to fuming acid, which reflects the result of the solvent effect (see Figure S4, Supporting Information). Xue and co-workers have previously reported multicolor MCL characteristics for a benzoxazole unit. They demonstrated simultaneous acid- and mechanoresponsive behavior and succeeded in improving the contrast of the initial and ground sample.[9] Although the film with the written character pattern was exposed to fuming acetic acid to induce simi-
Figure 9. HOMO (left) and LUMO (right) electron densities for (a) T4, (b) T3 and (c) T2. The red and green lobes represent the positive and negative signs of the coefficients of the molecular orbitals. The size of each lobe is proportional to the MO coefficient.
Figure 8. Simultaneous acid- and mechanoresponsive changes in PL images of the T4-dispersed film. (a) Initial film, (b) after acid fuming, (c) after writing the character “H”, and (d) after 9 days.
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functional and the 6-31G(d) basis set, as implemented in ited reversible mechano-induced changes in PL, of which Gaussian 09.[21] Figure 9 shows that all the luminophores the 4-pyridyl terminated chromophore had the largest. In have similar highest occupied molecular orbitals (HOMOs) addition, MCL patterning was successfully achieved using and lowest unoccupied molecular orbitals (LUMOs). The dye dispersed in PVA film. Further investigations on the inelectron clouds in the HOMO levels are mainly located on fluence of the polymer matrix type and content on the phothe electron-donating thiophene and bridged p-units, while tophysical properties and applications of acid-responsive those of the LUMO levels are predominantly placed on the chromophores are currently in progress. pyridine moiety, revealing the intramolecular charge transfer (ICT) characteristics in the molecules, which is consistent with the spectroscopic results described by Gong et al.[22] Experimental Section The dipole moment of T4 is 5.78 D, which is over twice that of T2 (2.18 D), and T3 is between them (3.27 D). DFT calculations for the chromophores suggest that T4 would have the largest dipole moment and a strong push–pull interaction in the p-conjugated structure along the molecular long axis, assuming that the photo- Scheme 1. Synthetic route of the chromophores. emission characteristics of T4 are easily affected by the solSynthesis vent polarity. One of the origins of luminescence switching 4-Bromophenylacetonitrile (1.8 g, 9.2 mmol) and 4-pyridine carboxyaldein the solid state is the change in conformation. Cyanostyrhyde (1.0 g, 9.3 mmol) were dissolved in 2 mL of methanol. Potassium [7] [3, 22] yl or phenylthylene moieties tend to form twisted contert-butoxide (0.2 g, 0.89 mmol) dissolved in 2 mL of methanol was formations and become planarized through facile mechaniadded dropwise to the solution with stirring, and the reaction mixture was mixed at room temperature for 20 h. The crude product was filtered cal stimuli, and can then be restored by thermal annealing. and recrystallized from methanol to give Br-4 as yellowish needle-like Zhang and co-workers reported that dyes with twisted stackcrystals (1.4 g, mp 138–140 8C, 52 % yield). 1H NMR (CDCl3): d = 7.45 (s, ing architectures result in much shorter effective conjugation 1 H), 7.56–7.55 (d, J = 4.2 Hz, 2 H), 7.61–7.59 (d, J = 4.2 Hz, 2 H), 7.69– lengths and bluer emissions, while planarized conformations 7.68 ppm (d, J = 2.9 Hz, 2 H), 8.75–8.74 (d, J = 2.8 Hz, 2 H). IR (KBr): n˜ = 3058, 2220, 1593, 1540, 1489, 1419, 1409, 1374, 1234, 1077 cm1. generate increased conjugation, thus producing much redder [22] Compound Br-3 was synthesized by a procedure similar to that for Br-4, emissions. Although we could not obtain single-crystal difbut with 3-pyridine carboxyaldehyde, and was obtained as yellowish fraction data for T4, T3 or T2, the twisted structure is preneedle-like crystals (1.1 g, 3.9 mmol, mp 130–132 8C, 87 % yield). dicted in the simulated structure and a similar MCL mecha1 H NMR (CDCl3): d = 7.43–7.40 (m, 3 H), 7.50 (s, 1 H), 7.55–7.53 (d, J = nism can be induced in compounds with similar chemical 4.3 Hz, 2 H), 7.60–7.58 (d, J = 4.2 Hz, 2 H), 8.47–8.45 (d, J = 4.1 Hz, 2 H), structures. However, DFT calculations for the luminophores 8.65–8.64 (d, J = 2.4 Hz, 2 H), 8.83 ppm (s, 1 H). IR (KBr): n˜ = 3055, 2218, 1588, 1563, 1492, 1415, 1409, 1371, 1234, 1081 cm1. used in this study indicate a similar twisted conformation to Compound Br-2 was synthesized by a procedure similar to that for Br-4, that of the cyanostyryl unit (see Figure S5, Supporting Inforbut with 2-pyridine carboxyaldehyde, and was obtained as yellowish mation); therefore, another effect should exist for MCL beneedle-like crystals (1.1 g, 3.9 mmol, mp 140–141 8C, 82 % yield). havior in this study. Ooyama and Harima evaluated various 1 H NMR (CDCl3): d = 7.32–7.30 (m, 3 H), 7.56 (s, 1 H,), 7.58–7.57 (d, J = mechanoreponsive electron push–pull compounds and dem3.3 Hz, 2 H), 7.61–7.59 (d, J = 4.5 Hz, 2 H), 7.80–7.77 (m, 3 H), 7.91–7.90 onstrated that MCL is attributed to a reversible switching (d, J = 4.0 Hz, 2 H), 8.75–8.74 ppm (d, J = 2.4 Hz, 2 H). IR (KBr): n˜ = 3047, 2216, 1577, 1558, 1487, 1473, 1402, 1348, 1409, 1242, 1003 cm1. between crystalline and amorphous states with changes of 5-Hexyl-2-thiopheneboronic acid pinacol ester (0.82 g, 2.8 mmol) and Brmolecular arrangement and intermolecular interactions.[19] 4 (0.70 g, 2.5 mmol) were placed under nitrogen with stirring, to which They also reported that the dipole of the compounds has N2-purged tetrahydrofuran (THF; 15 mL) and K2CO3 aqueous solution a strong influence on the MCL characteristics, in agreement (1 m, 5 mL) were added. Tetrakis(triphenylphosphine)palladium(0) with our results. It can be speculated that H-aggregation (91 mg, 0.079 mmol) dissolved in N2-purged THF (5 mL) was then added, occurs in the crystal structure, which induces a blue-shift of and the reaction mixture was heated to 65 8C for 17 h. After the mixture was cooled to room temperature, 60 mL of chloroform and 50 mL of the absorbance and PL, thereby shortening the lifetime, water were added. The solution mixture was extracted with chloroform which is strongly affected by the molecular packing distance and washed with water. The organic layer was dried with sodium sulfate. and dipole moment. After the solvent was removed under reduced pressure, T4 was obtained by recrystallization from hexane (0.33 g, mp 128 8C, 36 % yield). 1H NMR (CDCl3): d = 0.90–0.87 (t, 4.7 Hz, 3 H), 1.40–1.29 (m, 6 H), 1.72–1.66 (m, 5 H), 2.84–2.81 (t, 2 H), 6.77–6.76 (d, J = 1.8 Hz, 2 H), 7.21–7.20 (d, J = 1.8 Hz, 2 H), 7.45 (s, 1 H), 7.64–7.62 (d, J = 4.3 Hz, 2 H), 7.68–7.66 (d, J = 4.3 Hz, 2 H), 7.70–7.69 (d, J = 3.1 Hz, 2 H), 8.74–8.73 ppm (d, J = 2.9 Hz, 2 H). 13C NMR (126 MHz, CDCl3): d = 14.16, 22.63, 28.82, 30.35, 31.61, 115.90, 116.89, 122.68, 123.95, 125.47, 125. 80, 126.72, 131.48, 136.61, 137.66, 139.99, 140.69, 147.34, 150.68 ppm. IR (KBr): n˜ = 2953, 2922,
Conclusions Three pyridine-terminated MCL compounds were prepared by changing the rotational position of the nitrogen atom in the pyridine ring at the molecular terminus. The dyes exhib-
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2852, 2227, 1591, 1536, 1513, 1467, 1256 cm1. Elemental anal. calcd for C24H24N2S: C, 77.38; H, 6.49; N, 7.52; found: C, 77.55; H, 6.66; N, 7.50. MS (GC-MS, intensity) m/z 372 (M +).
Acknowledgements This work was supported in part by a Grant-in-Aid for Young Scientists B (No. 25810079) from the Japan Society for the Promotion of Science nd a research grant from the University of Hyogo.
Compound T3 was synthesized by a procedure similar to that for T4, but using Br-3, and was obtained as yellowish needle-like crystals (0.37 g, 0.99 mmol, mp 95–98 8C, 44 % yield). 1H NMR (CDCl3): d = 0.90–0.87 (t, 4.6 Hz, 3 H), 1.39–1.31 (m, 6 H), 1.72–1.66 (m, 5 H), 2.83–2.80 (t, J = 5.1 Hz, 2 H), 6.76–6.75 (d, J = 1.7 Hz, 2 H), 7.20–7.19 (d, J = 1.8 Hz, 2 H), 7.31–7.29 (m, 4.1 Hz, 3 H), 7.61 (s, 1 H), 7.64–7.63 (d, J = 2.2 Hz, 2 H), 7.73–7.71 (d, J = 4.2 Hz, 2 H), 7.80–7.77 (m, J = 4.8 Hz, 3 H), 7.96–7.94 (d, J = 4.0 Hz, 2 H), 8.75–8.74 ppm (d, J = 2.2 Hz, 2 H). 13C NMR (126 MHz, CDCl3): d = 14.19, 22.65, 28.85, 30.32, 31.58, 31.61, 113.50, 117.35, 123.71, 123.75, 125.41, 125. 67, 126.39, 129.74, 131.80, 134.53, 136.03, 136.85, 140.07, 147.02, 150.87, 151.37 ppm. IR (KBr): n˜ = 2953, 2922, 2852, 2226, 1606, 1581, 1513, 1468, 1422, 1379, 1223, 1024 cm1. Elemental anal. calcd for C24H24N2S: C, 77.38; H, 6.49; N, 7.52; found: C, 77.67; H, 6.21; N, 7.35. MS (GC-MS, intensity) m/z 372 (M+).
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Compound T2 was synthesized by a procedure similar to that of T4, but using Br-2, and was obtained as yellowish needle-like crystals (0.44 g, 1.2 mmol, mp 118–119 8C, 52 % yield). 1H NMR (CDCl3): d = 8.75–8.74 (d, J = 2.2 Hz, 2 H), 7.96–7.94 (d, J = 4.0 Hz, 2 H), 7.80–7.77 (m, J = 4.8 Hz, 3 H), 7.73–7.71 (d, J = 4.2 Hz, 2 H), 7.64–7.63 (d, J = 2.2 Hz, 2 H), 7.61 (s, 1 H), 7.31–7.29 (m, 4.1 Hz, 3 H), 7.20–7.19 (d, J = 1.8 Hz, 2 H), 6.76–6.75 (d, J = 1.7 Hz, 2 H), 2.83–2.80 (t, J = 5.1 Hz, 2 H), 1.72–1.66 (m, 5 H), 1.39–1.31 (m, 6 H), 0.90–0.87 ppm (t, 4.6 Hz, 3 H). 13C NMR (126 MHz, CDCl3): d = 14.20, 22.66, 28.86, 30.34, 31.60, 31.63, 114.19, 117.39, 123.73, 124.18, 124.23, 125. 40, 125.66, 126.72, 132.11, 136.08, 136.75, 139.85, 140.21, 146.94, 149.97, 152.17 ppm. IR (KBr): n˜ = 2953, 2921, 2849, 2215, 1578, 1555, 1510, 1467, 1428, 1367, 1215, 1053 cm1. Elemental anal. calcd for C24H24N2S: C, 77.38; H, 6.49; N, 7.52; found: C, 77.38; H, 6.70; N, 7.27. MS (GC-MS, intensity) m/z 372 (M +). Experimental Setup The synthesized compounds were characterized using Fourier transform nuclear magnetic resonance (FT-NMR; Bruker, DRX-500) and Fourier transform infrared (FT-IR; Jasco, FT-IR 410) spectroscopies. 1H and 13 C NMR spectra were measured in CDCl3 at room temperature with tetramethylsilane (TMS; d = 0.00) as an internal standard. Electrospray ionization (ESI) mass spectra and total C, H and N content of the final compounds were measured with a gas chromatograph mass spectrometer (GC/MS; Shimadzu, GCMS-QP5050 A) and an elemental analyzer (Yanaco, MT-5), respectively. The thermal properties of the compounds were evaluated using differential scanning calorimetry (DSC; Seiko, SSC5200) at a heating/cooling rate of 0.5 8C min1. UV-vis absorption and diffusion spectra were measured with a UV spectrometer (Hitachi, U3010) and fluorescence spectra were obtained with a fluorescence spectrometer (Hitachi, F-4500). The fluorescence quantum yields of the dye solutions were estimated by comparison with a quantum yield standard, where the quantum yield (Ff) of a material is calculated using Eq. (1):
Ff ¼ Fs
I As n 2 Is A n2s
ð1Þ
where I is the standard emission intensity, A is the absorbance, and n is the refractive index. The subscript s refers to the standard solution of quinine sulfate. The diffusion spectra of the solids were measured with the solid pasted on a tape. The PL lifetimes of the composite films were measured with a nanosecond spectrofluorometer (Horiba, FluoroCube) with 370 nm irradiation. The dye-dispersed films were spin-coated on glass substrates from an aqueous solution containing 4 wt % PVA (Kuraray Co., DP: 500, saponification > 97 %) and the unground dye powder (approximately dye/polymer = 0.14 (w/w)). The films were then dried for 1 day at room temperature. X-ray diffractometry (XRD; Rigaku, SmartLab) was performed with a standard small angle diffractometry setup.
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