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Mechanochromic and thermochromic fluorescent properties of cyanostilbene derivatives
Dyes and Pigments 98 (2013) 486e492
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Mechanochromic and thermochromic fluorescent properties of cyanostilbene derivatives Yujian Zhang a, b, Guilin Zhuang a, Mi Ouyang a, Bin Hu a, Qingbao Song a, Jingwei Sun a, Cheng Zhang a, *, Cheng Gu c, Yuanxiang Xu c, Yuguang Ma c, * a
State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou, PR China Department of Materials Chemistry, Huzhou University, Xueshi Road 1#, Huzhou, PR China c State Key Lab of Supramolecular Structure and Materials, Jilin University, Changchun 130012, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 December 2012 Received in revised form 18 March 2013 Accepted 20 March 2013 Available online 28 March 2013
Two isomers (TPA-CNa and TPA-CNb) consisting of twisted triphenylamine and cyanostilbene were synthetized by Knoevenagel condensation and CeN coupling reaction. The fluorescent colours of two isomers in crystalline state were sky-blue for TPA-CNa and green for TPA-CNb with quantum yields (Ff) of 44.9% and 7.7%, respectively. The single crystal X-ray diffractometry revealed the fully different molecular conformations and packing modes of TPA-CNa (planar) and TPA-CNb (twisting), which might be the reason for different Ff. The reversible mechanochromic and thermochromic fluorescence switching was observed in crystalline TPA-CNa powders. The sky-blue crystals were changed into the greenemissive solids after grinding, and they recovered their original state upon heating at 60 C over 2 min. The reversible conversion of fluorescence colour was also realized by a pure thermal process. Powder X-ray diffractometry clearly demonstrated that the mechanochromic and thermochromic behaviour could be attributed to the crystal-to-amorphisation phase transition. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Cyanostilbene derivatives Isomers Mechanochromic Thermochromic Crystal structure Phase transition
1. Introduction Stimuli-responsive florescence switching, such as mechano-, thermo- and vapochromism was thought to be very important for developing sensors, memories and rewritable optical media [1e3]. In particular, mechanochromic fluorescent (MCF) materials are of great importance in practical applications [4,5]. The rapid development was achieved in this field in recent years. For instance, the mechanochromic behaviour was observed for liquid-crystalline molecules and crystalline solids containing pyrene core, which were easily distinguished by the naked eye [6e9]. The tetraphenylethylene or cyanostilbene-based crystalline organic solids were also capable of exhibiting mechanochromic fluorescence [10e14]. Moreover, it was proposed that the MCF properties was related to the planarization of molecular conformation in Xu’s group [10,15]. Recently, Park group demonstrated that a reversible fluorescence switching could be attritued to two-directional shear-sliding ability of adjacent molecular sheets, which are formed through
* Corresponding authors. E-mail addresses: [email protected] (C. Zhang), [email protected] (Y. Ma). 0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dyepig.2013.03.017
intermolecular multiple secondary bonds [13]. The altering of dipoleedipole interaction and intermolecular pep interaction was also suggested to account for MCF behaviours [16e19]. At present, whereas the understanding of MCF mechanisms remains unclear at the molecular-level, which requires further debates. For MCF materials, their luminescent colours can be also reversibly switched by other environmental stimulus including organic vapour, heating, acid and base. For example, acid and base treatments effectively cause the fluorescence transition between blue and red colours of CF3-substituted cyanostilbene derivatives [20]. Tang et al. have revealed thermally induced fluorescence switching properties accompanied by the controlling polymorphism [16,21]. However, the reversible colours transformation achieved by a pure thermal treatment has been rarely reported in MCF systems, which would widen the range of application in optical memory systems. Herein, we prepared two electron donoracceptor (DeA) structured isomers containing the twisty arylamine and cyanostilbene derivatives (see Fig. 1). The twisty conformation inducing the weak intermolecular interaction in aggregated state may facilitate the phase transformation [10]. The change in the position of cyano group had great effects on molecular packing and optical properties. The crystal structure of
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2.3. Synthesis and crystal parameters
Fig. 1. Molecular structures of two isomers and their corresponding crystal images under UV irradiation (lex ¼ 365 nm).
TPA-CNa with a “pinwheel” motif was edge-to-face packing, while that of TPA-CNb was parallel stacking. Additionally, the emission colour and efficiency switching of dye TPA-CNa can be reversibly converted by grinding-vapour or only heating process. 2. Experimental 2.1. Chemicals and instruments Tetrahydrofuran (THF) was purchased from Aladdin and distilled from Na/benzophenone under N2 prior to use. Chinese liquor (56 , Peking Erguotou wine) and medicinal alcohol (75%) were purchased from Wal-Mart supermarket in Hangzhou. All other chemicals were purchased from Alfa Aesar or Aladdin and used as received. 1 H and 13C NMR spectra of the desired products were recorded on a Bruker AVANCE III 500-MHz instrument (Bruker, Switzerland) using TMS as the internal standard and the chloroform-d (CDCl3) as the solvent. Mass spectroscopy was recorded with a Thermo LCQ Fleet MS spectrometer. Elemental analyses were performed using the Thermo-Finnigan Flash EA-1112 (CE, Italy) instrument. The UVe vis spectrum was recorded on a Perkin Elmer Lambda 35 spectrophotometer. Fluorescent measurements were recorded on a PerkineElmer LS-55 luminescence spectrophotometer. The Ff of crystal was determined by using a calibrated integrating sphere system. Powder XRD measurements were conducted on X’Pert PRO diffractometer (CuKa) in the range 5 < 2q < 30 (PANalytical, Netherlands). X-ray crystallographic intensity data were collected using a Xcalibur, Eos, Gemini Ultra CCD diffractometer equipped with a graphite monochromated Enhance (Mo) X-ray source A). The time-resolved fluorescence lifetime experi(l ¼ 0.71073 ments were obtained by time-correlated single photo-counting technique with an Flsp920 spectro-fluorometer. Differential scanning calorimetry (DSC) was performed on a TA Instruments DSC2920 at a heating rate of 10 C min-1. Field-emission scanning electron microscopy (SEM) measurements were taken by using a Hitachi S-4800 scanning electron microscopy (Hitachi, Japan). Digital photographs were taken by Canon 550D (Canon, Japan) digital cameras. 2.2. Sample preparation Single crystals were obtained from the mixture of n-hexane and dichloromethane at room temperature. Crystal samples of TPACNa and TPA-CNb were prepared by recrystallization from hot ethanol solutions as sheet-like and needle-like crystallites respectively.
(Z)-2-(4-(diphenylamino)phenyl)-3-(4-methoxyphenyl)-acrylo nitrile (TPA-CNa): 3-(4-bromophenyl)-2-(4-methoxy phenyl) acrylonitrile (CSB-Br) (3.1 g, 10 mml), diphenylamine (25 mmol, 4.23 g), Pd(OAc)2 (45 mg, 0.18 mmol), (t-Bu)3P (0.18 mmol), sodium tert-butoxide (0.86 g, 9.0 mmol) and toluene (45 mL) were mixed in three-necked flask kept under nitrogen. The mixture was heated at reflux for 24 h. after the reaction finished, the mixture was cooled to room temperature and the solvent was removed under vacuum. Subsequently, the residue was extracted with chloroform. The extraction solution was washed with brine. And then, the organic layer was dried over MgSO4 and filtered. The crude product was purified by columm chromatography using dichloromethane/hexanes (1/50) mixture as eluent to obtain the desired compound in 84.5% yield (3.4 g). 1H NMR (500 MHz, CDCl3) d(ppm) 7.87 (d, J ¼ 9.0 Hz, 2H), 7.51 (d, J ¼ 9.0 Hz, 2H), 7.38 (s, 1H), 7.32e7.28 (m, 4H), 7.14(d, J ¼ 7.5 Hz, 4H), 7.10 (d, J ¼ 8.5, 4H), 6.98(d, J ¼ 8.5, 2H), 3.88 (s, 3H); 13C NMR (500 MHz, CDCl3); d 161.2, 148.5, 147.3, 139.7, 131.0, 129.5, 128.2, 126.9, 126.7, 125.2, 125.0, 123.7, 122.8, 118.7, 114.4, 108.5, 55.5; MS(EI): m/e 402.2 (Mþ); Anal. Calcd for C28H22N2O: C, 83.56; H, 5.51; N, 6.96; O, 3.98. Found: C, 83.47; H, 5.62; N, 6.89; O, 4.02. (Z)-3-(4-(diphenylamino) phenyl)-2-(4-methoxyphenyl) acrylonitrile (TPA-CNb) was prepared from the Knoevenagel condensation reaction of 4-(diphenylamino)- benzaldehyde with 2-(4-methoxy phenyl)acetonitrile in anhydrous ethanol in the presence of sodium hydroxide. 1H NMR (500 MHz, CDCl3) d 7.76 (d, J ¼ 9.0 Hz, 2H), 7.59 (d, J ¼ 9.0 Hz, 2H), 7.35 (s, 1H), 7.31e7.34 (m, 4H), 7.17 (d, J ¼ 7.5 Hz, 4H), 7.13 (t, J ¼ 7.5 Hz, 2H), 7.07 (d, J ¼ 9.0 Hz, 2H), 6.96 (d, J ¼ 9.0 Hz, 2H), 3.87 (s, 3H); 13C NMR (500 MHz, CDCl3); d 160.0, 149.6,146.8, 139.8,130.3,129.7,129.5, 127.6,127.0,126.9,125.6,124.2, 118.9, 114.4, 107.6, 55.4; MS(EI) m/e 402.2[Mþ]. Anal. Calcd for C28H22N2O: C, 83.56; H, 5.51; N, 6.96; O, 3.98. Found: C, 83.51; H, 5.55; N, 6.91; O, 4.03. Crystallographic data for TPA-CNa: C28H22N2O, M ¼ 402.48 g/ mol, monoclinic, a ¼ 22.475(3) A, b ¼ 8.7817(10) A, c ¼ 10.7156(13) A3 , A; a ¼ 90.00 , b ¼ 96.981(2) and g ¼ 90.00 , V ¼ 2099.3(4) T ¼ 133(2) K, space group P21/c, DC ¼ 1.273 Mg m3, Z ¼ 4, 14805 reflections collected, 4575 unique reflection (Rint ¼ 0.0392), The final R indices were R1 ¼ 0.0454, wR2 ¼ 0.1253 [I > 2s (I)], CCDC ¼ 872836. Crystallographic data for TPA-CNb: C28H22N2O, M ¼ 402.48 g/ mol, triclinic, a ¼ 6.7788(7) A, b ¼ 17.4776(19) A, c ¼ 20.892(3) A; a ¼ 65.356(2) , b ¼ 89.988(3) , V ¼ 2237.5(5) A3, T ¼ 293(2) K, space group P-1, DC ¼ 1.195 g cm3, Z ¼ 4, 11833 reflections collected, 8189 unique reflection (Rint ¼ 0.0241), The final R indices were R1 ¼ 0.0854, wR2 ¼ 0.2308 [I > 2s(I)], CCDC ¼ 872835. 3. Results and discussion 3.1. Synthesis and characterization The molecular structures of cyanostilbene derivatives TPACNa and (Z)-3-(4-(diphenylamino) phenyl)-2-(4-methoxyphenyl) acrylonitrile (TPA-CNb) were illustrated in Fig. 1. The desired dye TPA-CNb and the key intermediate (Z)-3-(4-bromophenyl)-2-(4methoxyphenyl) acrylonitrile (CSB-Br) were prepared by Knoevenagel condensation reaction of benzyl cyanide derivatives with corresponding aromatic aldehydes. Subsequently, the palladiumcatalysed aromatic CeN coupling reaction of CSB-Br with diphenylamine was carried out at 110 C in toluene, to obtain TPA-CNa with the yields of more than 84.5%. Finally, the resulting compounds were fully characterized by 1H NMR, 13C NMR, MS spectral and elemental analyses. Briefly, a mass of isomerically-
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pure cyanostilbene derivatives can be efficiently and easily obtained.
configurational change could result in the different fluorescence emission behaviours.
3.2. Photophysical properties
3.3. Stimuli-responsive fluorescence switching
As shown in Fig. S1, the absorption spectra of TPA-CNa indicated an obvious peak at 299 nm along with a shoulder peak at around 379 nm, which was assigned to the pep* transition. For TPA-CNb, the shoulder peak became prevailing absorption band at 388 nm. However, their emission spectra were almost the same. Moreover, two isomers were almost nonfluorescent when molecularly dissolved in THF, while highly fluorescent when aggregated. The remarkable fluorescence enhancement can be unambiguously considered as the aggregation-induced emission phenomenon [22e24]. Fig. 1 revealed the fluorescent colours of two dyes recrystallized from ethanol solution were sky-blue for TPA-CNa and green for TPA-CNb. Both TPA-CNa and TPA-CNb in crystalline state emitted an intense light with quantum yields (Ff) of 44.9% and 7.7%, respectively. Upon photoexcitation, the emission peaks (lem) of their crystals were located at 470 nm (TPA-CNa) and 507 nm (TPA-CNb) (see Fig. 2). The fluorescence spectra of blend films of two isomers dispersed in poly (methyl methacrylate) (PMMA, 0.05wt %) were also obtained in inset of Fig. 2. The emission spectrum of TPA-CNa got close to that of crystals. In comparison, the fluorescence peak of TPA-CNb in PMMA at 461 nm was obviously blue-shifted from that in crystalline state. The results indicated that strong intermolecular interactions existed in crystals of TPA-CNb. Additionally, the position change of cyano group resulted in different degrees of distortion between the vinylene and neighbour phenyl ring in crystalline states. Fig. 3a indicated that torsion angles of TPA-CNb are 28.5 and 16.2 between the vinylene and adjacent phenyl ring. However, the corresponding torsion angles for TPA-CNa were reduced to 9.0 and 0.7, respectively (Fig. 3b). Thus, the TPA-CNa had longer effective conjugation than TPA-CNb, with more extended planarity. Based on the resulting optical properties, two interesting features were as follows: (1) dye TPA-CNb possessed significantly lower Ff than TPA-CNa due to less coplanarity and face-to-face stacking interaction (Fig. 3c, d) [25,26]. (2) In terms of the fluorescence spectra of two dyes, the direction of spectral shift was different. As crystals were formed, the peak maximum of TPA-CNb was red-shifted relative to that in THF, while that of TPA-CNa was blue-shifted (see Fig. S1b). The phenomenon indicated even a tiny
Fig. 2. Fluorescence spectra of TPA-CNa (C) and TPA-CNb (-) in crystal, grinding treatment and dispersed in PMMA (Inset).
Aside from the higher Ff, another notable difference was that the dye TPA-CNa exhibited mechanochromic fluorescent behaviour. As shown in Fig. 4, the sky-blue powders changed into relatively weak green-emissive solids (Ff ¼ 21.2%, lem ¼ 502 nm) after grinding by a spatula. Interestingly, this change occurred only at the pressed area. Fig. 4c indicated photographic images of TAPCNa, shaped in ‘ZFL’ on a quartz plate, after pressing with a spatula under natural light (left) and 365 nm UV light (right), respectively. Clearly, the ground dye TPA-CNa exhibited higher lem and lower Ff than that in the crystalline state (Ff ¼ 44.9%, lem ¼ 470 nm). These ground powders could revert to the blue-emitting solids upon heating at about 60 C. The emission spectrum indicated a broad fluorescent band with an obvious peak at 491 nm (Fig. S2). Moreover, the ground sample exhibited the evident vapochromic fluorescence. Fig. S3 showed the change of fluorescence from green to sky-blue was also achieved by the addition of various organic solvents such as ethanol, hexane and acetonitrile. As depicted in Fig. S4, the reversibility of fluorescence conversion was confirmed by the subsequent grinding-vapour exposure processes. The emission peaks did not exhibit notable degradation and shift, even after 5 cycles. Interestingly, when the blue-emitting crystals of TPA-CNa were heated to the melted state and rapidly cooled down at room temperature in air. The solidified powders, termed as crystal-m, showed the light green fluorescence (Fig. 5, Ff ¼ 16.7%, lem ¼ 519 nm). Moreover, if the ground powders were thermally melted and then cooled down rapidly, its fluorescence was also changed into light blue similar to crystal-m. Upon heating at 80 C for 5 h, the fluorescence of the solidified powders returned to original sky-blue. The fluorescence spectra of crystal-h exhibited the emission peak at 495 nm with a shoulder peak at around 473 nm, which resemble that of the ground powder with anneal treatment (Fig. 5, Fig. S2). These observations indicated that TPA-CNa exhibited vapo-, machano- and thermochromic fluorescent behaviours. Uniquely, the harmless ethanol-water mixtures also resulted in fluorescence conversion. As shown in Fig. 6aed, below 30% volume fractions of water (fW), the fluorescence could completely return to the original state. When fW was progressively increased, the ground sample would partially convert to its original phase with cyan fluorescence (Fig. 6f, g). However, the ground TPA-CNa did not show a pronounced colour change under UV light via the addition of mixture with fW >50% (Fig. 6h) due to the inhibition of the crystallization. It was noteworthy that the mixtures such as medicinal alcohol (75%) and Chinese liquor (56 ) were widely used in our daily life, thus the practicality could be further improved. As shown in Fig. 7A, some irregular plates recrystallized from hot ethanol indicated relatively smooth surfaces. The surfaces of plates after grinding became rough and showed disordered granules (Fig. 7B). This rough surface could return to the original state after heating or solvent vapour treatment (Fig. 7C and D). Namely, mechanical stimuli resulted in the change of surface morphology, which was recoverable through annealing or fuming treatment. The various surface morphologies could be attributed to the rearrangement of molecules in the crystal lattice [13]. This proposal was evidenced by time-resolved fluorescence spectroscopy and X-ray diffraction (XRD) analyses. The XRD pattern of crystals TPA-CNa exhibited clear reflection peaks due to highly ordered microcrystalline structures (see Fig. 8A). After grinding, the crystal lattice was significantly disturbed as shown by obviously decreased peak intensities along with several broad peaks. The results indicated
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Fig. 3. The conformations and torsion angles in single-crystal structures of TPA-CNa (a) and TPA-CNb (b); scheme of the CH/O interactions and p∙∙∙p interactions between two adjacent molecules for TPA-CNa(c) and TPA-CNb (d), respectively.
machanochromic behaviour was related to the conversion of a well-ordered phase to a poorly organized phase. Additionally, Fig. S5 exhibited the excited state yielded a single-exponential decay (s ¼ 5.98 ns) in the crystalline state. Upon grinding, the sample decayed bi-exponentially with the lifetime shortened to 3.24 ns. These results indicated the ground powders possessing two distinguished states was a more disordered state, in agreement with the partial amorphisation discussed above. After heating treatment, XRD of the sample obviously indicated the peaks originated from the original crystals along with those from the ground powder, revealing a mixture of two packing modes (see Fig. 8A). Thus, this thermochromic transformation may be an incomplete process, which was further confirmed by the broad fluorescent spectrum of the annealed sample (see Fig. S2). Fig. 8B depicted that the solidified powders (crystal-m) exhibiting very broad diffraction peak was the amorphous states. After heating, the sample crystalm became well-ordered aggregates with sharp and intensive reflection peaks. The results indicated the crystal and amorphous phase were efficiently switched by only thermal stimulation, which induced the change in fluorescence colours. Moreover, the ground sample did not experience an exo/endothermic process at the corresponding temperature (Fig. S6). And its DSC curve was quite similar to that before grinding. Thus, this “abnormal” behaviour was attributed to the absence of (meta)stable state at room pressure [9].
Fig. 4. The photographs of TPA-CNa particles taken at R.T under 365 nm UV light: (A) heating or organic vapour treatments; (B) ground sample; (C) compound TPA-CNa is casted on a quartz substrate and written “ZFL” with a metal spatula under ambient light (left) and UV light (right).
3.4. X-ray crystal structure In addition to photophysical properties, the different molecular packing motifs were also observed in their single crystal. The crystal structure of TPA-CNa containing one discrete molecule was monoclinic with space group P21/c. The twodimensional structure with edge-to-face interactions was an achiral “pinwheel” pattern on the bc plane (see Figs. 3c and 9A), similar to DSB (Distyrylbenzene) [26]. The good coplanarity of cyanostilbene and absence of face-to-face pep interaction resulted in the high Ff in crystal state [26e28]. In the “pinwheel” stacking, Fig. 9A showed the CeH/N interaction, which was longer than the Van der Waals (VDW) radii of 2.75 A [29], existed between two adjacent molecules. The strong Ce H/O interaction with short distance of 2.507 A and angle of 151.7 were also observed. As for dye TPA-CNb, the single crystal consisting of two molecules belonged to Pı triclinic space group. As shown in Fig. 3d and Fig. S7, there was a face-to-face slipped p-stacking motif with a roll angle of 58.7 in the crystal. Another
Fig. 5. Fluorescence spectra and photo of dye TPA-CNa under 365 nm UV light in the different states; Crystal-m: heating powders to melt and solidification at R.T.; Crystalh: the solidified powders annealed above 80 C for 5 h.
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Fig. 6. Photographs showing dye TPA-CNa on an agate mortar under UV irradiation (lex ¼ 365 nm), (A)e(H) TPA-CNa crystals after grinding with a pestle; (a)-(h) the corresponding fluorescence by dropwise addition of the ethanol/water mixtures onto the centre of the ground powders.
notable feature was that the CeH/N distance in TPA-CNb, 2.54(1) A fell well within the sum of VDW radii (see Fig. 9B). In addition, two types of aromatic CeH/p interactions (I and II), making the adjacent molecules of the twisted TPA connect to
each other, were also observed in Fig. 9B. However, such aromatic CeH/p and CeH/N interactions did not existed in crystal of TPA-CNa (see Fig. S8). These various multiple secondary bonding interactions in TPA-CNb enhanced the
Fig. 7. SEM images of TPA-CNa: (A) crystals, (B) ground crystals, (C) annealing sample B at 60 C for 5 min and (D) fuming sample B in ethanol vapor for 30 s.
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Fig. 8. XRD profiles of TPA-CNa in different states.
molecular rigidity and stability in crystal lattice [30,31]. This proposal was further supported by a higher melting point of TPA-CNb (m.p. 139.7 C) than that of TPA-CNa (m.p. 131.2, C see Fig. S6). Moreover, the single crystal with a “pinwheel” motif (edge-to-face), relative to parallel stacking, was quite unstable (see Fig. S7). Briefly, the crystals of TPA-CNa with edge-to-face stacking were readily damaged by force stimuli in the absence
of various secondary bonding interactions. Thus, the dye TPA-CNa presented MCF properties, rather than TPA-CNb. 4. Conclusions In summary, two isomerically-pure isomers exhibiting aggregation-induced enhanced emission behaviour were easily obtained with high yields. The desired dye TPA-CNa presented high Ff (44.9%) and weak intermolecular interactions relative to TPACNb. Moreover, its luminescent efficiency and colour can be reversibly transformed by grinding-vapour or a pure thermal treatment. Based on the PXRD and fluorescence lifetime studies, the mechanochromic and thermochromic fluorescent characteristics were related to the phase transition between the crystalline phase and amorphous phase. In crystalline state, the intermolecular interaction of TPA-CNa was relatively weaker than that of TPACNb, which induced the phase transformation. This material with reversible colour conversion may be employed to prepare temperature- or pressure-sensing in future. Acknowledgements The authors gratefully thank the supporting of National Basic Research Program of China (2010CB635108, 2011CBA00700) and International S&T Cooperation Program, China (2012DFA-51210). Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.dyepig.2013.03.017. References
Fig. 9. Molecular stacking structure with secondary bonding interactions in the crystal of TPA-CNa (A) and TPA-CNb (B).
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Mechanochromic and thermochromic fluorescent properties of cyanostilbene derivatives Yujian Zhang a, b, Guilin Zhuang a, Mi Ouyang a, Bin Hu a, Qingbao Song a, Jingwei Sun a, Cheng Zhang a, *, Cheng Gu c, Yuanxiang Xu c, Yuguang Ma c, * a
State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou, PR China Department of Materials Chemistry, Huzhou University, Xueshi Road 1#, Huzhou, PR China c State Key Lab of Supramolecular Structure and Materials, Jilin University, Changchun 130012, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 December 2012 Received in revised form 18 March 2013 Accepted 20 March 2013 Available online 28 March 2013
Two isomers (TPA-CNa and TPA-CNb) consisting of twisted triphenylamine and cyanostilbene were synthetized by Knoevenagel condensation and CeN coupling reaction. The fluorescent colours of two isomers in crystalline state were sky-blue for TPA-CNa and green for TPA-CNb with quantum yields (Ff) of 44.9% and 7.7%, respectively. The single crystal X-ray diffractometry revealed the fully different molecular conformations and packing modes of TPA-CNa (planar) and TPA-CNb (twisting), which might be the reason for different Ff. The reversible mechanochromic and thermochromic fluorescence switching was observed in crystalline TPA-CNa powders. The sky-blue crystals were changed into the greenemissive solids after grinding, and they recovered their original state upon heating at 60 C over 2 min. The reversible conversion of fluorescence colour was also realized by a pure thermal process. Powder X-ray diffractometry clearly demonstrated that the mechanochromic and thermochromic behaviour could be attributed to the crystal-to-amorphisation phase transition. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Cyanostilbene derivatives Isomers Mechanochromic Thermochromic Crystal structure Phase transition
1. Introduction Stimuli-responsive florescence switching, such as mechano-, thermo- and vapochromism was thought to be very important for developing sensors, memories and rewritable optical media [1e3]. In particular, mechanochromic fluorescent (MCF) materials are of great importance in practical applications [4,5]. The rapid development was achieved in this field in recent years. For instance, the mechanochromic behaviour was observed for liquid-crystalline molecules and crystalline solids containing pyrene core, which were easily distinguished by the naked eye [6e9]. The tetraphenylethylene or cyanostilbene-based crystalline organic solids were also capable of exhibiting mechanochromic fluorescence [10e14]. Moreover, it was proposed that the MCF properties was related to the planarization of molecular conformation in Xu’s group [10,15]. Recently, Park group demonstrated that a reversible fluorescence switching could be attritued to two-directional shear-sliding ability of adjacent molecular sheets, which are formed through
* Corresponding authors. E-mail addresses: [email protected] (C. Zhang), [email protected] (Y. Ma). 0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dyepig.2013.03.017
intermolecular multiple secondary bonds [13]. The altering of dipoleedipole interaction and intermolecular pep interaction was also suggested to account for MCF behaviours [16e19]. At present, whereas the understanding of MCF mechanisms remains unclear at the molecular-level, which requires further debates. For MCF materials, their luminescent colours can be also reversibly switched by other environmental stimulus including organic vapour, heating, acid and base. For example, acid and base treatments effectively cause the fluorescence transition between blue and red colours of CF3-substituted cyanostilbene derivatives [20]. Tang et al. have revealed thermally induced fluorescence switching properties accompanied by the controlling polymorphism [16,21]. However, the reversible colours transformation achieved by a pure thermal treatment has been rarely reported in MCF systems, which would widen the range of application in optical memory systems. Herein, we prepared two electron donoracceptor (DeA) structured isomers containing the twisty arylamine and cyanostilbene derivatives (see Fig. 1). The twisty conformation inducing the weak intermolecular interaction in aggregated state may facilitate the phase transformation [10]. The change in the position of cyano group had great effects on molecular packing and optical properties. The crystal structure of
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2.3. Synthesis and crystal parameters
Fig. 1. Molecular structures of two isomers and their corresponding crystal images under UV irradiation (lex ¼ 365 nm).
TPA-CNa with a “pinwheel” motif was edge-to-face packing, while that of TPA-CNb was parallel stacking. Additionally, the emission colour and efficiency switching of dye TPA-CNa can be reversibly converted by grinding-vapour or only heating process. 2. Experimental 2.1. Chemicals and instruments Tetrahydrofuran (THF) was purchased from Aladdin and distilled from Na/benzophenone under N2 prior to use. Chinese liquor (56 , Peking Erguotou wine) and medicinal alcohol (75%) were purchased from Wal-Mart supermarket in Hangzhou. All other chemicals were purchased from Alfa Aesar or Aladdin and used as received. 1 H and 13C NMR spectra of the desired products were recorded on a Bruker AVANCE III 500-MHz instrument (Bruker, Switzerland) using TMS as the internal standard and the chloroform-d (CDCl3) as the solvent. Mass spectroscopy was recorded with a Thermo LCQ Fleet MS spectrometer. Elemental analyses were performed using the Thermo-Finnigan Flash EA-1112 (CE, Italy) instrument. The UVe vis spectrum was recorded on a Perkin Elmer Lambda 35 spectrophotometer. Fluorescent measurements were recorded on a PerkineElmer LS-55 luminescence spectrophotometer. The Ff of crystal was determined by using a calibrated integrating sphere system. Powder XRD measurements were conducted on X’Pert PRO diffractometer (CuKa) in the range 5 < 2q < 30 (PANalytical, Netherlands). X-ray crystallographic intensity data were collected using a Xcalibur, Eos, Gemini Ultra CCD diffractometer equipped with a graphite monochromated Enhance (Mo) X-ray source A). The time-resolved fluorescence lifetime experi(l ¼ 0.71073 ments were obtained by time-correlated single photo-counting technique with an Flsp920 spectro-fluorometer. Differential scanning calorimetry (DSC) was performed on a TA Instruments DSC2920 at a heating rate of 10 C min-1. Field-emission scanning electron microscopy (SEM) measurements were taken by using a Hitachi S-4800 scanning electron microscopy (Hitachi, Japan). Digital photographs were taken by Canon 550D (Canon, Japan) digital cameras. 2.2. Sample preparation Single crystals were obtained from the mixture of n-hexane and dichloromethane at room temperature. Crystal samples of TPACNa and TPA-CNb were prepared by recrystallization from hot ethanol solutions as sheet-like and needle-like crystallites respectively.
(Z)-2-(4-(diphenylamino)phenyl)-3-(4-methoxyphenyl)-acrylo nitrile (TPA-CNa): 3-(4-bromophenyl)-2-(4-methoxy phenyl) acrylonitrile (CSB-Br) (3.1 g, 10 mml), diphenylamine (25 mmol, 4.23 g), Pd(OAc)2 (45 mg, 0.18 mmol), (t-Bu)3P (0.18 mmol), sodium tert-butoxide (0.86 g, 9.0 mmol) and toluene (45 mL) were mixed in three-necked flask kept under nitrogen. The mixture was heated at reflux for 24 h. after the reaction finished, the mixture was cooled to room temperature and the solvent was removed under vacuum. Subsequently, the residue was extracted with chloroform. The extraction solution was washed with brine. And then, the organic layer was dried over MgSO4 and filtered. The crude product was purified by columm chromatography using dichloromethane/hexanes (1/50) mixture as eluent to obtain the desired compound in 84.5% yield (3.4 g). 1H NMR (500 MHz, CDCl3) d(ppm) 7.87 (d, J ¼ 9.0 Hz, 2H), 7.51 (d, J ¼ 9.0 Hz, 2H), 7.38 (s, 1H), 7.32e7.28 (m, 4H), 7.14(d, J ¼ 7.5 Hz, 4H), 7.10 (d, J ¼ 8.5, 4H), 6.98(d, J ¼ 8.5, 2H), 3.88 (s, 3H); 13C NMR (500 MHz, CDCl3); d 161.2, 148.5, 147.3, 139.7, 131.0, 129.5, 128.2, 126.9, 126.7, 125.2, 125.0, 123.7, 122.8, 118.7, 114.4, 108.5, 55.5; MS(EI): m/e 402.2 (Mþ); Anal. Calcd for C28H22N2O: C, 83.56; H, 5.51; N, 6.96; O, 3.98. Found: C, 83.47; H, 5.62; N, 6.89; O, 4.02. (Z)-3-(4-(diphenylamino) phenyl)-2-(4-methoxyphenyl) acrylonitrile (TPA-CNb) was prepared from the Knoevenagel condensation reaction of 4-(diphenylamino)- benzaldehyde with 2-(4-methoxy phenyl)acetonitrile in anhydrous ethanol in the presence of sodium hydroxide. 1H NMR (500 MHz, CDCl3) d 7.76 (d, J ¼ 9.0 Hz, 2H), 7.59 (d, J ¼ 9.0 Hz, 2H), 7.35 (s, 1H), 7.31e7.34 (m, 4H), 7.17 (d, J ¼ 7.5 Hz, 4H), 7.13 (t, J ¼ 7.5 Hz, 2H), 7.07 (d, J ¼ 9.0 Hz, 2H), 6.96 (d, J ¼ 9.0 Hz, 2H), 3.87 (s, 3H); 13C NMR (500 MHz, CDCl3); d 160.0, 149.6,146.8, 139.8,130.3,129.7,129.5, 127.6,127.0,126.9,125.6,124.2, 118.9, 114.4, 107.6, 55.4; MS(EI) m/e 402.2[Mþ]. Anal. Calcd for C28H22N2O: C, 83.56; H, 5.51; N, 6.96; O, 3.98. Found: C, 83.51; H, 5.55; N, 6.91; O, 4.03. Crystallographic data for TPA-CNa: C28H22N2O, M ¼ 402.48 g/ mol, monoclinic, a ¼ 22.475(3) A, b ¼ 8.7817(10) A, c ¼ 10.7156(13) A3 , A; a ¼ 90.00 , b ¼ 96.981(2) and g ¼ 90.00 , V ¼ 2099.3(4) T ¼ 133(2) K, space group P21/c, DC ¼ 1.273 Mg m3, Z ¼ 4, 14805 reflections collected, 4575 unique reflection (Rint ¼ 0.0392), The final R indices were R1 ¼ 0.0454, wR2 ¼ 0.1253 [I > 2s (I)], CCDC ¼ 872836. Crystallographic data for TPA-CNb: C28H22N2O, M ¼ 402.48 g/ mol, triclinic, a ¼ 6.7788(7) A, b ¼ 17.4776(19) A, c ¼ 20.892(3) A; a ¼ 65.356(2) , b ¼ 89.988(3) , V ¼ 2237.5(5) A3, T ¼ 293(2) K, space group P-1, DC ¼ 1.195 g cm3, Z ¼ 4, 11833 reflections collected, 8189 unique reflection (Rint ¼ 0.0241), The final R indices were R1 ¼ 0.0854, wR2 ¼ 0.2308 [I > 2s(I)], CCDC ¼ 872835. 3. Results and discussion 3.1. Synthesis and characterization The molecular structures of cyanostilbene derivatives TPACNa and (Z)-3-(4-(diphenylamino) phenyl)-2-(4-methoxyphenyl) acrylonitrile (TPA-CNb) were illustrated in Fig. 1. The desired dye TPA-CNb and the key intermediate (Z)-3-(4-bromophenyl)-2-(4methoxyphenyl) acrylonitrile (CSB-Br) were prepared by Knoevenagel condensation reaction of benzyl cyanide derivatives with corresponding aromatic aldehydes. Subsequently, the palladiumcatalysed aromatic CeN coupling reaction of CSB-Br with diphenylamine was carried out at 110 C in toluene, to obtain TPA-CNa with the yields of more than 84.5%. Finally, the resulting compounds were fully characterized by 1H NMR, 13C NMR, MS spectral and elemental analyses. Briefly, a mass of isomerically-
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pure cyanostilbene derivatives can be efficiently and easily obtained.
configurational change could result in the different fluorescence emission behaviours.
3.2. Photophysical properties
3.3. Stimuli-responsive fluorescence switching
As shown in Fig. S1, the absorption spectra of TPA-CNa indicated an obvious peak at 299 nm along with a shoulder peak at around 379 nm, which was assigned to the pep* transition. For TPA-CNb, the shoulder peak became prevailing absorption band at 388 nm. However, their emission spectra were almost the same. Moreover, two isomers were almost nonfluorescent when molecularly dissolved in THF, while highly fluorescent when aggregated. The remarkable fluorescence enhancement can be unambiguously considered as the aggregation-induced emission phenomenon [22e24]. Fig. 1 revealed the fluorescent colours of two dyes recrystallized from ethanol solution were sky-blue for TPA-CNa and green for TPA-CNb. Both TPA-CNa and TPA-CNb in crystalline state emitted an intense light with quantum yields (Ff) of 44.9% and 7.7%, respectively. Upon photoexcitation, the emission peaks (lem) of their crystals were located at 470 nm (TPA-CNa) and 507 nm (TPA-CNb) (see Fig. 2). The fluorescence spectra of blend films of two isomers dispersed in poly (methyl methacrylate) (PMMA, 0.05wt %) were also obtained in inset of Fig. 2. The emission spectrum of TPA-CNa got close to that of crystals. In comparison, the fluorescence peak of TPA-CNb in PMMA at 461 nm was obviously blue-shifted from that in crystalline state. The results indicated that strong intermolecular interactions existed in crystals of TPA-CNb. Additionally, the position change of cyano group resulted in different degrees of distortion between the vinylene and neighbour phenyl ring in crystalline states. Fig. 3a indicated that torsion angles of TPA-CNb are 28.5 and 16.2 between the vinylene and adjacent phenyl ring. However, the corresponding torsion angles for TPA-CNa were reduced to 9.0 and 0.7, respectively (Fig. 3b). Thus, the TPA-CNa had longer effective conjugation than TPA-CNb, with more extended planarity. Based on the resulting optical properties, two interesting features were as follows: (1) dye TPA-CNb possessed significantly lower Ff than TPA-CNa due to less coplanarity and face-to-face stacking interaction (Fig. 3c, d) [25,26]. (2) In terms of the fluorescence spectra of two dyes, the direction of spectral shift was different. As crystals were formed, the peak maximum of TPA-CNb was red-shifted relative to that in THF, while that of TPA-CNa was blue-shifted (see Fig. S1b). The phenomenon indicated even a tiny
Fig. 2. Fluorescence spectra of TPA-CNa (C) and TPA-CNb (-) in crystal, grinding treatment and dispersed in PMMA (Inset).
Aside from the higher Ff, another notable difference was that the dye TPA-CNa exhibited mechanochromic fluorescent behaviour. As shown in Fig. 4, the sky-blue powders changed into relatively weak green-emissive solids (Ff ¼ 21.2%, lem ¼ 502 nm) after grinding by a spatula. Interestingly, this change occurred only at the pressed area. Fig. 4c indicated photographic images of TAPCNa, shaped in ‘ZFL’ on a quartz plate, after pressing with a spatula under natural light (left) and 365 nm UV light (right), respectively. Clearly, the ground dye TPA-CNa exhibited higher lem and lower Ff than that in the crystalline state (Ff ¼ 44.9%, lem ¼ 470 nm). These ground powders could revert to the blue-emitting solids upon heating at about 60 C. The emission spectrum indicated a broad fluorescent band with an obvious peak at 491 nm (Fig. S2). Moreover, the ground sample exhibited the evident vapochromic fluorescence. Fig. S3 showed the change of fluorescence from green to sky-blue was also achieved by the addition of various organic solvents such as ethanol, hexane and acetonitrile. As depicted in Fig. S4, the reversibility of fluorescence conversion was confirmed by the subsequent grinding-vapour exposure processes. The emission peaks did not exhibit notable degradation and shift, even after 5 cycles. Interestingly, when the blue-emitting crystals of TPA-CNa were heated to the melted state and rapidly cooled down at room temperature in air. The solidified powders, termed as crystal-m, showed the light green fluorescence (Fig. 5, Ff ¼ 16.7%, lem ¼ 519 nm). Moreover, if the ground powders were thermally melted and then cooled down rapidly, its fluorescence was also changed into light blue similar to crystal-m. Upon heating at 80 C for 5 h, the fluorescence of the solidified powders returned to original sky-blue. The fluorescence spectra of crystal-h exhibited the emission peak at 495 nm with a shoulder peak at around 473 nm, which resemble that of the ground powder with anneal treatment (Fig. 5, Fig. S2). These observations indicated that TPA-CNa exhibited vapo-, machano- and thermochromic fluorescent behaviours. Uniquely, the harmless ethanol-water mixtures also resulted in fluorescence conversion. As shown in Fig. 6aed, below 30% volume fractions of water (fW), the fluorescence could completely return to the original state. When fW was progressively increased, the ground sample would partially convert to its original phase with cyan fluorescence (Fig. 6f, g). However, the ground TPA-CNa did not show a pronounced colour change under UV light via the addition of mixture with fW >50% (Fig. 6h) due to the inhibition of the crystallization. It was noteworthy that the mixtures such as medicinal alcohol (75%) and Chinese liquor (56 ) were widely used in our daily life, thus the practicality could be further improved. As shown in Fig. 7A, some irregular plates recrystallized from hot ethanol indicated relatively smooth surfaces. The surfaces of plates after grinding became rough and showed disordered granules (Fig. 7B). This rough surface could return to the original state after heating or solvent vapour treatment (Fig. 7C and D). Namely, mechanical stimuli resulted in the change of surface morphology, which was recoverable through annealing or fuming treatment. The various surface morphologies could be attributed to the rearrangement of molecules in the crystal lattice [13]. This proposal was evidenced by time-resolved fluorescence spectroscopy and X-ray diffraction (XRD) analyses. The XRD pattern of crystals TPA-CNa exhibited clear reflection peaks due to highly ordered microcrystalline structures (see Fig. 8A). After grinding, the crystal lattice was significantly disturbed as shown by obviously decreased peak intensities along with several broad peaks. The results indicated
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Fig. 3. The conformations and torsion angles in single-crystal structures of TPA-CNa (a) and TPA-CNb (b); scheme of the CH/O interactions and p∙∙∙p interactions between two adjacent molecules for TPA-CNa(c) and TPA-CNb (d), respectively.
machanochromic behaviour was related to the conversion of a well-ordered phase to a poorly organized phase. Additionally, Fig. S5 exhibited the excited state yielded a single-exponential decay (s ¼ 5.98 ns) in the crystalline state. Upon grinding, the sample decayed bi-exponentially with the lifetime shortened to 3.24 ns. These results indicated the ground powders possessing two distinguished states was a more disordered state, in agreement with the partial amorphisation discussed above. After heating treatment, XRD of the sample obviously indicated the peaks originated from the original crystals along with those from the ground powder, revealing a mixture of two packing modes (see Fig. 8A). Thus, this thermochromic transformation may be an incomplete process, which was further confirmed by the broad fluorescent spectrum of the annealed sample (see Fig. S2). Fig. 8B depicted that the solidified powders (crystal-m) exhibiting very broad diffraction peak was the amorphous states. After heating, the sample crystalm became well-ordered aggregates with sharp and intensive reflection peaks. The results indicated the crystal and amorphous phase were efficiently switched by only thermal stimulation, which induced the change in fluorescence colours. Moreover, the ground sample did not experience an exo/endothermic process at the corresponding temperature (Fig. S6). And its DSC curve was quite similar to that before grinding. Thus, this “abnormal” behaviour was attributed to the absence of (meta)stable state at room pressure [9].
Fig. 4. The photographs of TPA-CNa particles taken at R.T under 365 nm UV light: (A) heating or organic vapour treatments; (B) ground sample; (C) compound TPA-CNa is casted on a quartz substrate and written “ZFL” with a metal spatula under ambient light (left) and UV light (right).
3.4. X-ray crystal structure In addition to photophysical properties, the different molecular packing motifs were also observed in their single crystal. The crystal structure of TPA-CNa containing one discrete molecule was monoclinic with space group P21/c. The twodimensional structure with edge-to-face interactions was an achiral “pinwheel” pattern on the bc plane (see Figs. 3c and 9A), similar to DSB (Distyrylbenzene) [26]. The good coplanarity of cyanostilbene and absence of face-to-face pep interaction resulted in the high Ff in crystal state [26e28]. In the “pinwheel” stacking, Fig. 9A showed the CeH/N interaction, which was longer than the Van der Waals (VDW) radii of 2.75 A [29], existed between two adjacent molecules. The strong Ce H/O interaction with short distance of 2.507 A and angle of 151.7 were also observed. As for dye TPA-CNb, the single crystal consisting of two molecules belonged to Pı triclinic space group. As shown in Fig. 3d and Fig. S7, there was a face-to-face slipped p-stacking motif with a roll angle of 58.7 in the crystal. Another
Fig. 5. Fluorescence spectra and photo of dye TPA-CNa under 365 nm UV light in the different states; Crystal-m: heating powders to melt and solidification at R.T.; Crystalh: the solidified powders annealed above 80 C for 5 h.
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Fig. 6. Photographs showing dye TPA-CNa on an agate mortar under UV irradiation (lex ¼ 365 nm), (A)e(H) TPA-CNa crystals after grinding with a pestle; (a)-(h) the corresponding fluorescence by dropwise addition of the ethanol/water mixtures onto the centre of the ground powders.
notable feature was that the CeH/N distance in TPA-CNb, 2.54(1) A fell well within the sum of VDW radii (see Fig. 9B). In addition, two types of aromatic CeH/p interactions (I and II), making the adjacent molecules of the twisted TPA connect to
each other, were also observed in Fig. 9B. However, such aromatic CeH/p and CeH/N interactions did not existed in crystal of TPA-CNa (see Fig. S8). These various multiple secondary bonding interactions in TPA-CNb enhanced the
Fig. 7. SEM images of TPA-CNa: (A) crystals, (B) ground crystals, (C) annealing sample B at 60 C for 5 min and (D) fuming sample B in ethanol vapor for 30 s.
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491
Fig. 8. XRD profiles of TPA-CNa in different states.
molecular rigidity and stability in crystal lattice [30,31]. This proposal was further supported by a higher melting point of TPA-CNb (m.p. 139.7 C) than that of TPA-CNa (m.p. 131.2, C see Fig. S6). Moreover, the single crystal with a “pinwheel” motif (edge-to-face), relative to parallel stacking, was quite unstable (see Fig. S7). Briefly, the crystals of TPA-CNa with edge-to-face stacking were readily damaged by force stimuli in the absence
of various secondary bonding interactions. Thus, the dye TPA-CNa presented MCF properties, rather than TPA-CNb. 4. Conclusions In summary, two isomerically-pure isomers exhibiting aggregation-induced enhanced emission behaviour were easily obtained with high yields. The desired dye TPA-CNa presented high Ff (44.9%) and weak intermolecular interactions relative to TPACNb. Moreover, its luminescent efficiency and colour can be reversibly transformed by grinding-vapour or a pure thermal treatment. Based on the PXRD and fluorescence lifetime studies, the mechanochromic and thermochromic fluorescent characteristics were related to the phase transition between the crystalline phase and amorphous phase. In crystalline state, the intermolecular interaction of TPA-CNa was relatively weaker than that of TPACNb, which induced the phase transformation. This material with reversible colour conversion may be employed to prepare temperature- or pressure-sensing in future. Acknowledgements The authors gratefully thank the supporting of National Basic Research Program of China (2010CB635108, 2011CBA00700) and International S&T Cooperation Program, China (2012DFA-51210). Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.dyepig.2013.03.017. References
Fig. 9. Molecular stacking structure with secondary bonding interactions in the crystal of TPA-CNa (A) and TPA-CNb (B).
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