Article pubs.acs.org/Organometallics
Trimethylsilyl Group Assisted Stimuli Response: Self-Assembly of 1,3,6,8-Tetrakis((trimethysilyl)ethynyl)pyrene Feng Xu,† Takanori Nishida,† Kenta Shinohara,† Lifen Peng,† Makoto Takezaki,† Takahiro Kamada,‡ Haruo Akashi,‡ Hiromu Nakamura,§ Kouki Sugiyama,§ Kazuchika Ohta,§ Akihiro Orita,*,† and Junzo Otera† †
Department of Applied Chemistry and Biotechnology and ‡Research Institute of Natural Sciences, Okayama University of Science, 1-1 Ridai-cho, Kita-ku, Okayama 700 0005, Japan § Smart Material Science and Technology, Interdisciplinary Graduate School of Science and Technology, Shinshu University, 1-15-1 Tokida, Ueda 386 8567, Japan S Supporting Information *
ABSTRACT: Substitution of 1,3,6,8-tetraethynylpyrene with sterically bulky trimethylsilyl groups enabled four modes of molecular packing, two polymorphs (triclinic system (Y-form) and “loose” crystal Colho (OK-form)), a ”rigid” liquid crystalline phase Colho (OC-form), and an amorphous phase (OA-form), which emitted fluorescence at different wavelengths under UV light. The four phases could be interconverted by physical stimuli such as heating, grinding by mortar and pestle, and exposing to the vapor of organic solvents.
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INTRODUCTION Because trialkyl(aryl)silyl groups exhibit attractive steric and electronic effects, they play a pivotal role in organic synthesis and organic materials.1 For instance, the bulky trialkylsilyl groups are used as protecting groups of terminal ethynes and hydroxyl groups, and the steric bulkiness of the silyl group can be tuned by changing the substituents on the silicon atom to realize selective protection and deprotection.1c The electronwithdrawing effect of the silyl groups stabilizes α-carbanions and facilitates Peterson olefination.1d This electronic effect is enhanced by substituting phenyl group(s) on the silicon atom, and regioselective hydroalumination and carbolithiation were achieved in 3-hydroxy-1-silylpropynes.1e Although a number of applications of silicon chemistry have been reported so far, a new silicon reagent is still being developed.1f,g Siliconcontaining organic materials have also attracted great attention. Substitution of fluorophores with trialkylsilyl groups enhanced the emission intensity,1h and in silole, a participation of the antibonding orbital of the C−Si bond in the butadiene 4π system stabilized the LUMO of silole to achieve a small HOMO−LUMO band gap.1i,j In order to expand the utility of silicon chemistry in organic materials, we planned to take advantage of the steric bulkiness of trialkylsilyl group in the control of morphologies of a silicon-substituted fluorophore which possesses stimuli-responsive properties in the solid state.2 So far several stimuli-responsive pyrene derivatives have been reported. These pyrenes underwent phase transitions when external stimuli emerged (for instance, cubic to columnar phase2f and columnar assembly to amorphous2g), and their fluorescent properties were tuned in accordance with the resulting molecular assemblies. We therefore designed 1,3,6,8tetrakis((trimethylsilyl)ethynyl)pyrene (1) as a fluorescent © XXXX American Chemical Society
material, which enabled a response to various physical stimuli in the solid state (Scheme 1). Compound 1 was composed of Scheme 1. Structure and Plausible (Meta)stable States of 1
an expanded π system (tetraethynylpyrene) and four bulky substituents (trimethylsilyl groups). In the solid state of 1, the expanded pyrene π system would enable intense fluorescence, and its fluorescent color could be tuned by a change in the molecular assembly and/or the formation of a pyrene excimer.3 As has been well documented in crystals, organic molecules tend to pack tightly to fill spaces; therefore, we envisaged that 1 would align linearly to achieve efficient π−π stacking between disciform pyrene-containing π systems (forms A−C in Scheme 1).4,5 Although forms A−C might be discriminative stable/ metastable states of 1, if free rotation of the bulky trimethylsilyl groups enabled smooth slippage of the pyrene cores, interconversion of 1 among A−C would be triggered easily Received: October 10, 2016
A
DOI: 10.1021/acs.organomet.6b00781 Organometallics XXXX, XXX, XXX−XXX
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Organometallics by physical stimuli, leading to a change in the fluorescent color and intensity.6,7 Herein, we report that 1 shows four (meta)stable states, the Y-, OC-, OA-, and OK-forms, which exhibit different fluorescent colors. The four states of 1 can be reversibly interconverted by physical stimuli such as heating, shearing, and exposure to organic solvents.
Surprisingly, in the solid state, four assembly modes which were different in morphology were observed, as summarized in Table 1. When 1 was subjected to recrystallization and reprecipitation, two different types of solventless forms were obtained in accordance with the solvents used: yellow prisms (Y-form) by recrystallization from acetone and orange narrow needles (OC-form) by reprecipitaion from hexane.9 The Y- and OC-forms exhibited different UV−vis absorption profiles (diffuse reflectance method), showing absorption termini at 550 and 620 nm, respectively (Table 1 and Figure 2). When the Y- and OC-forms were ground with a mortar and pestle, an orange powder bearing an absorption terminus at 605 nm (OAform) was obtained. When the the OA-form was heated over 90 °C, the OA-form was transformed to another orange powdery state, the OKform, the absorption terminus of which was observed at 620 nm.10 On irradiation with UV light, all of the solid Y-, OC-, OA-, and OK-forms emitted strong fluorescence, and the Emax values were observed at 524 nm (ΦF = 0.66), 600 nm (0.49), 586 nm (0.57), and 594 nm (0.17), respectively (Figure 2). The OC-, OA-, and OK-forms exhibited rather long fluorescence lifetimes (>48 ns) diagnostic of the formation of an excimer in photoexcitation.11 Because the Y-form emitted fluorescence at a wavelength longer than that in solution and exhibited a shorter lifetime (2.2 ns), we concluded that an intermolecular interaction such as exciton coupling would occur in the emission of the Y-form. In order to shed light on the morphologies of 1 in the four solid states, all of the forms were subjected to X-ray diffraction analyses. Recrystallization of 1 from acetone afforded single crystals of the Y-form suitable for X-ray structure analysis. The crystallographic data and ORTEP drawing of 1 are shown in Table 1 and Figure 3, respectively.12 The Y-form was depicted as form B in Scheme 1 and exhibited a triclinic crystal system (P1̅ (No. 2)) with the center of symmetry residing on the midpoint of the biphenyl unit. In the crystal of the Y-form, only half-planes of the pyrene cores were overlapped to the proximate molecule, and one of the three methyl groups on each silyl group was inserted between silyl groups of the proximate molecule. Because of the insufficient overlapping of the proximate molecules, Y-form could not form an excimer in the photoexcited state, leading to the monomeric emission on irradiation by UV light. When a powder X-ray diffraction (XRD) analysis was undertaken, the OC-form demonstrated clear reflection peaks (peaks 1−10) at room temperature (Figure 4a). This result indicated a Colho arrangement of 1, depicted as form A in Scheme 1. The sharp peaks 1−9 were observed at 16.0, 9.2, 8.0, 6.1, 5.4, 4.6, 4.5, 4.1, and 3.7 Å with a reciprocal d spacing ratio of 1:√3:2:√7:3:√12:√13:4:√19 that corresponded to (100), (110), (200), (210), (300), (220), (310), (400), and (320) reflections, respectively. The reflections indicate a twodimensional hexagonal lattice (a = 18.5 Å) and showed good agreement with the simulated pattern. Peak 10 in the high-angle region was assigned to a stacking distance (h = 3.37 Å), and this distance was appropriate as an intracolumnar distance of closely packed aromatic rings.13 Although we assigned the OC-form as a discotic columnar mesophase, the OC-form showed no spontaneous fluidity as for conventional discotic liquid crystals and could be isolated as a crystal-like solid by reprecipitation at room temperature. Even at higher temperature, the OC-form showed no fluidity14 and decomposed at 296 °C. In Figure 4a, the OC-form exhibited sharp diffractions, as did the crystalline
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RESULTS AND DISCUSSION Synthesis and Structure Elucidations. First, 1 was synthesized from commercially available pyrene in two steps: the tetrabromination of pyrene and successive Sonogashira coupling of the resulting tetrabromide with (trimethylsilyl)acetylene (Scheme 2).8 Compound 1 was easily purified by column chromatography on silica gel using hexane as the eluent and, after evaporation, was isolated as an orange powder in 89% yield. Scheme 2. Synthesis of 1
Compound 1 showed no clear melting point or decomposition up to 270 °C, and thermogravimetric analysis (TGA) demonstrated that 1 underwent thermal decomposition above 296.0 °C (Figure S29 in the Supporting Information). In Figure 1 are shown normalized UV−vis absorption (CH2Cl2, 1.0 ×
Figure 1. UV−vis absorption and photoluminescence spectra of 1 in CH2Cl2 (1.0 × 10−6 and 1.0 × 10−2 M).
10−6 M) and photoluminescence spectra (CH2Cl2, 1.0 × 10−6 and 1.0 × 10−2 M). In CH2Cl2, 1 indicated the largest absorption band at 439 nm (ε = 58 × 104 L mol−1 cm−1). When 1 was irradiated with UV light in CH2Cl2 (1.0 × 10−6 M), a strong emission was observed (Emax 441 nm, ΦF = 0.90). The high fluorescence quantum yield indicated that efficient expansions of the π system and the silyl groups play pivotal roles in enhancing the emission in 1.8 In contrast to this, when UV light was irradiated to a 10−2 M CH2Cl2 solution of 1, broad excimer emission was recorded at 557 nm. This result demonstrated that 1 could undergo an intermolecular interaction despite the steric hindrance of trimethylsilyl groups. B
DOI: 10.1021/acs.organomet.6b00781 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Table 1. Selected Physical Properties of Y-, OC-, OA-, and OK-Forms description form in Scheme 1 preparation phase, space group (No.) or phasea a (Å) b (Å) c or h (Å) α (deg) β (deg) γ (deg) d (Å)b UV (nm)c PL (nm)d/ΦFe τ (ns)f kr/knr (106 s−1)g
Y-form
OC-form
OA-form
OK-form
yellow prism B recryst from acetone crystal, P1̅ (2) 6.316(2) 10.023(3) 15.378(5) (c) 70.431(9) 81.290(12) 80.513(13) ca. 3.34 550 524/0.66 2.2 300/155
orange needle A reprecipitated from hexane “rigid” mesophase, Colho 18.5
orange powder C shearing of Y-, OC-, or OK-form amorphous
orange powder A’ heating of OA-form “loose” crystal, Colho 18.8
3.37 (h)
3.47 (h)
3.37 620 600/0.49 48.6 10.1/10.5
605 586/0.57 70.8 8.1/6.1
3.47 620 594/0.17 66.8 2.6/12.4
a
Phase nomenclature: Colho = hexagonal ordered columnar mesophase. bdistance between pyrenes. cAbsorption terminus. dEmax. eAbsolute fluorescence quantum yield. fEmission lifetime. gSee the Supporting Information.
Figure 2. UV−vis absorption and photoluminescence spectra of 1 in Y-, OC-, OA-, and OK-forms.
Figure 3. Space-filling model of Y-form 1.
Figure 4. XRD patterns recorded at 20 °C for (a) OC-form, (b) OAform, (c) OK-form, and (d) Y-form.
Y-form (Figure 4d), and this result also diagnosed the crystallike high 2D and 1D regularities of 1 in the OC-form. The “rigid” discotic columnar mesophase similar to that of the crystal was reported in octaphenoxy-substituted phthalocyaninato copper(II) derivatives.15 The XRD pattern of the OA-form (the ground samples of Y- or OC-forms) showed a significant decrease in peak intensities (Figure 4b). The XRD pattern indicated that a phase conversion to amorphous form occurred on grinding (forms A and B to form C in Scheme 1). In contrast, when the OA-form was heated on a glass plate at 260 °C for 150 s, an amorphous to columnar phase transition (OA-
form to OK-form) occurred, and clear reflection peaks (peaks 1−11) were recorded at room temperature (Figure 4c). The diffractions (peaks 1−4 and 7−10) observed at 16.34, 9.35, 8.11, 6.16, 4.65, 4.52, 3.74, and 3.47 Å could be ascribed to the (100), (110), (200), (210), (220), (310), (320), and (001) reflections, respectively, of the hexagonal columnar phase (Colho, a = 18.8 Å, h = 3.47 Å) which is depicted as form A′ in Scheme 1. However, this phase (OK-form) exhibited several diffractions which were inconsistent with Colho at 5.51, 5.11, and 3.42 Å C
DOI: 10.1021/acs.organomet.6b00781 Organometallics XXXX, XXX, XXX−XXX
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Organometallics (peaks 5, 6 and 11). These reflections may be attributed to three-dimensionality. When peak 11 was ascribed to the (101) reflection, the spacing could be calculated from the values of a = 18.8 and h = 3.47 Å to be 3.39 Å, which is almost the same as the observed value of 3.42 Å. Therefore, the OK-form of 1 was assigned to a crystal form bearing a Colho molecular arrangement having some three-dimensional regularity. Although both OK- and OC-forms indicated Colho arrangement, the OK-form showed a longer molecular distance h diagnostic of the weaker π−π interaction between adjacent molecules 1: h = 3.37 for the OC-form and 3.47 Å for the OK-form. In XRD patterns (Figure 4a,c), the OK-form indicated peaks somewhat broader than those of the OC-form,16 and this phenomenon also demonstrated the loose arrangement of 1 in the OK-form. From these results, we concluded that although the OK-form is “crystalline” in form, it would have higher free energy at room temperature than the “rigid liquid crystalline” OC-form because of the looseness of the molecular arrangement in OK-form.17 Thermochromism. The thermal properties of the four forms of 1 were investigated in terms of differential scanning calorimetry (DSC) by heating each form to 270 °C, and all of the thermochromic phase-transition sequences are summarized in Figure 5.
Figure 7. Photoluminescence spectra of OC-forms which were obtained by reprecipitation from hexane and by thermal phase transition from Y-form. The inset shows the color change of Y-form upon heating to 250 °C.
resulting orange form was consistent with that of the OC-form which was obtained by reprecipitation from hexane (Figure 7). This result demonstrated that the Y-form would undergo a thermochromic phase conversion to OC-form upon heating. The Y−OC transition enthalpy ΔH was estimated as 38.0 J/g from the DSC curve, suggesting that the Y-form was thermodynamically more stable than the OC-form at room temperature. When a virgin sample of the OA-form was heated, an ambiguous exothermic profile was recorded on the drifting baseline at 73.2 °C in the first run, and the phase transition from OA- to OK-form proceeded sluggishly (Figure 6b). Although in this OA−OK conversion, which was promoted by heating at 250 °C for 24 h, no change was observed in color and appearance, the emission maximum was slightly shifted from 586 to 594 nm with a large decrease in fluorescence quantum yield (ΦF = 0.57 to ΦF = 0.17) (Figure 8).
Figure 5. Thermochromic phase transition sequences for the Y-, OC-, OA-, and OK-forms.
In the DSC profile of the Y-form, a clear endothermic peak was observed at 166.4 °C only in the first heating (Figure 6a). Both in the following cooling process and in the second cycle no peak was observed (Figure S9 in the Supporting Information). When it was heated, the Y-form changed rapidly from yellow to orange (Figure 7, inset). The fluorescence of the
Figure 8. Photoluminescence spectra of OK-form which was obtained by thermal phase transition from OA-form.
Vapochromism. Because the Y- and OC-forms were prepared selectively by recrystallization from acetone and reprecipitation from hexane, respectively, vapochromic phase conversion was investigated by exposing each form to a vapor of acetone or hexane. When the OC-form on the glass plate was exposed to acetone vapor at 30 °C, the color gradually changed from orange to yellow (Figures 9a,b). Upon irradiation of the resulting yellow form with UV light, green emission was observed at 525 nm (Figures 9b,c). This emission profile was identical to that of the Y-form which was obtained by recrystallization from acetone. DSC analysis of the yellow form indicated a profile similar to
Figure 6. DSC thermograms of (a) Y-form and (b) OA-form. D
DOI: 10.1021/acs.organomet.6b00781 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Figure 9. (a) The OC-form before exposure to acetone vapor under room light (left) and under UV irradiation (right). (b) The Y-form undergoing phase conversion from the OC-form by exposure to acetone vapor under room light (left) and under UV irradiation (right). (c) Photoluminescence spectrum of the Y-form undergoing phase conversion from the OC-form.
Figure 11. Photoluminescence spectra of OC-forms which were obtained by reprecipitation from hexane and by vapochromic phase transition from OA- and OK-forms.
that of the Y-form and exhibited an exothermic peak at 167 °C (Figure S18 in the Supporting Information). These results showed that the OC-form underwent acetone-promoted vapochromic phase conversion leading to the Y-form. Because the crystal shapes remained intact during the phase conversion, the phase conversion could be regarded as crystal to crystal conversion. Vapochromic phase conversions occurred from the other Oforms, as shown in Figure 10. When OA- and OK-forms were
and OK-forms. When the Y-form was sheared with a mortar and pestle at room temperature, the color changed rapidly from yellow to orange (Figure 12, inset). When the OC- and OK-
Figure 12. Photoluminescence spectra of OA-forms which were obtained by shearing Y-, OC- and OK-forms. (inset: color change of Yform by shearing). Figure 10. Vapochromic phase transition sequences for the Y-, OC-, OA-, and OK-forms.
forms were subjected to grinding on the mortar, the orange color remained intact, and a remarkable color change was not observed. Photoluminescence analyses of the resulting three orange forms which were thus obtained by shearing Y-, OC-, and OK-forms exhibited similar profiles (Emax = 586 nm), which were assignable to the amorphous form OA-form (Figure 12). The mechanochromic phase conversions are summarized and shown in Figure 13. Fluorescence Lifetime. Fluorescence decays of 1 in CH2Cl2 and Y-, OC-, OA-, and OK-forms were measured using an apparatus including a femtosecond laser system (Figure 14).
exposed to acetone vapor overnight, they underwent a color change from orange to yellow to furnish the Y-form, as observed in the phase conversion of the OC-form. The yellow forms thus obtained from OA- and OK-forms also showed emission and DSC profiles which were identical with those of the Y-form which was derived by recrystallization from acetone (Figure S16 in the Supporting Information). On the other hand, when the OA- and OK-forms were exposed to hexane vapor at 30 °C overnight, there was no noticeable change in color; however, analyses of the photoluminescence revealed that the OA- and OK-forms were completely converted to the OC-form (Figure 11). In sharp contrast, the hexane-promoted phase conversion from Y- to OC-form was not observed: when the Y-form was exposed to hexane vapor at 30 °C overnight, the Y-form remained yellow, and no change was observed in the fluorescence spectrum. The low reactivity of the Y-form in hexane-promoted phase conversion could be explained in terms of thermodynamic stability of the Y-form. Because the Y-form is more stable than the OC-form at room temperature as DSC analysis indicated, hexane vapor could not rearrange the Y-form array of 1 to the hexagonal molecular array of the OC-form, which was thermodynamically unfavorable. Mechanochromism. Mechanochromic phase conversion was investigated by providing a mechanical stimulus to Y-, OC-,
Figure 13. Mechanochromic phase transition sequences for the Y-, OC-, OA- and OK-forms. E
DOI: 10.1021/acs.organomet.6b00781 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
524 nm (ΦF = 0.66) for the Y-form and 600 nm (0.49) for the OC-form. Grinding of the two forms with a mortar and pestle furnished the orange amorphous solid OA-form. The OA-form emitted fluorescence at 586 nm (ΦF = 0.57) under UV light. When it was heated, the OA-form underwent rapid phase conversion to the OK-form, which exhibited emission at 594 nm (ΦF = 0.17) while the Y-form led to the OC-form. The single-crystal X-ray diffraction analysis of the Y-form revealed that, in the Y-form, only half-planes of 1 were overlapped with the proximate molecule, leading to monomeric photoluminescence. The shorter fluorescence lifetime (τ = 2.2 ns) also indicated the monomeric emission of the Y-form. Although the XRD analyses demonstrated the hexagonal columnar arrangement of the OC- and OK-forms, the former was characterized as a “rigid” liquid crystalline form, and the latter as a “loose” crystalline form. Because rather long fluorescence lifetimes (τ > 48 ns) were recorded for the OC-, OA-, and OK-forms, their emissions were assigned as excimeric photoluminescence. In the solid state of 1 were also observed vapochromic phase transitions. Exposing the OC-, OA-, and OK-forms to acetone vapor provided the Y-form, and exposing the OA- and OKforms to hexane yielded the OC-form. Assuming grinding would enable a phase transition to a higher energy level of phase, the correlations between the free energy and temperature can be depicted in the order of OA-, OK-, OC-, and Yforms (Figure 15).18 Further investigation of expanded π systems with bulky trialkyl(aryl)silyl substituents and their applications as organic materials are underway.
Figure 14. Fluorescence decay profile of 1 in CH2Cl2 (1.0 × 10−6 M) and Y-, OC-, OA-, and OK-forms.
As shown in Figure 14, 1 in CH2Cl2 and Y-form showed rather short fluorescence lifetimes (48 ns). The short lifetimes and the emission maxima of 1 in CH2Cl2 and Y-form recorded at shorter wavelengths (441 and 524 nm, respectively) were ascribable to monomeric emission. Although the X-ray singlecrystal analysis disclosed that the adjacent molecules 1 in the Yform were located at 3.34 Å, only halves of the adjacent molecules 1 were overlapped with each other, and therefore, the formation of a photoexcited excimer was prohibited, leading to the predominant monomeric photoluminescence of the Yform. In contrast to this, the rather longer lifetimes which were observed in a series of orange forms, OC-, OA-, and OK-forms, diagnosed that they would emit excimeric photoluminescence. These three orange forms showed their emission maxima at wavelengths longer (580−600 nm) than that of the Y-form, and this result also might support the excimer formation of OC-, OA-, and OK-forms in photoexcitation. The reaction rate constants for radiative (kr) and nonradiative processes (knr) were calculated by the following equations and are summarized in Table 1. k r = ΦF /τ k nr = (1 − ΦF)/τ
The Y-form showed large reaction constants of radiative and nonradiative processes (>1.0 × 108 s−1), while the OC-, OA-, and OK-forms had rather smaller values (
