5-(2,6-Bis((E)-4-(dimethylamino)styryl)-1-ethylpyridin-4(1H)-ylidene)-2,2-dimethyl-1,3-dioxane-4,6-dione: aggregation-induced emission, polymorphism, mechanochromism, and thermochromism

Yibin Zhou a, Lebin Qian a, Miaochang Liu a, Xiaobo Huang *a, Yuxiang Wang b, Yixiang Cheng b, Wenxia Gao a, Ge Wu c and Huayue Wu *a
aCollege of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, 325035, P. R. China. E-mail: xiaobhuang@wzu.edu.cn; huayuewu@wzu.edu.cn
bSchool of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China
cSchool of Pharmacy, Wenzhou Medical University, Wenzhou 325035, P. R. China

Received 20th June 2017 , Accepted 4th August 2017

First published on 7th August 2017


Herein, we report the synthesis of a new 1,4-dihydropyridine derivative containing 2,2-dimethyl-1,3-dioxane-4,6-dione and N,N-dimethylaniline. This compound exhibits aggregation-induced emission properties due to its highly twisted conformation and it has three crystalline polymorphs, which exhibit yellow (DHPM-y), orange (DHPM-o), and red (DHPM-r) fluorescence. The different emissions of the polymorphs mainly depend on their molecular conformations due to the weak intermolecular interactions, as revealed by single crystal structural analysis. More importantly, these three crystalline polymorphs show different mechanochromic phenomena under varying pressure. The fluorescent color changes in DHPM-o and DHPM-r upon gentle grinding are attributed to a crystal-to-crystal transformation which alters their molecular conformations, whereas those of DHPM-y and DHPM-o upon strong grinding are ascribed to the transformation from a crystalline state to an amorphous state. Additionally, DHPM-y and DHPM-o also display thermochromic properties. These results will provide useful information for obtaining intriguing multifunctional fluorescent materials by rational modification of classic luminogens.


Introduction

Stimuli-responsive organic fluorescent compounds display a solid-state fluorescence color change in response to mechanical stimulation and thus, exhibit mechanochromic (MC) properties.1 The fluorescence color change can often be restored by another stimulus or more stimuli, such as heating, organic solvent vapor, and light. Due to this characteristic, MC compounds have potential application in the fields of fluorescent sensors and optical recording, and attract great attention from researchers.2 The primary condition for the development of MC materials is the development of organic molecules with strong fluorescence in the solid state. However, most of the traditional organic fluorescent molecules are rigid planar structures with large π-conjugated skeletons, which make them easily form strong intermolecular π–π stacking interactions. Thus, they emit energies by a non-radiative relaxation process in the excited state, which results in a reduction or absence of fluorescence in the aggregated state.3 The notorious aggregation-induced quenching effect greatly limits the application of traditional organic fluorescent molecules in the field of MC materials. A major breakthrough came from the discovery of fluorescent compounds with aggregation-induced emission (AIE) properties,4 and before that, reports on MC materials were scarce.1 AIE molecules often have highly twisted molecular conformations, which not only enable them to emit strong fluorescence in the aggregated state by weakening intermolecular close stacking and intense π–π interactions, but also easily lead to the formation of MC properties by changing the molecular packing modes upon pressure. As a result, AIE molecules have become a crucial source of MC materials. In fact, another noteworthy advantage of AIE-active molecular structures is that the variable molecular conformation, intermolecular interactions, and packing modes between molecules make it possible to generate crystal polymorphs that exhibit distinctly different solid-state fluorescence, which might lead to multicolor mechanochromism.5 Nevertheless, AIE-active organic molecules with MC and polymorphic properties are still scarce because it is hard to predict whether a chemical structure can form crystalline polymorphs with MC properties.6

Recently, 1,4-dihydropyridine (DHP) has been reported to be a good building block for the construction of AIE organic molecules with multiple functionality.7 However, to the best of our knowledge, AIE-active DHP derivatives with MC and polymorphic properties have not been reported. In this work, a new DHP compound containing 2,2-Dimethyl-1,3-dioxane-4,6-dione (Meldrum's acid) and N,N-dimethylaniline, namely 5-(2,6-bis((E)-4-(dimethylamino)styryl)-1-ethylpyridin-4(1H)-ylidene)-2,2-dimethyl-1,3-dioxane-4,6-dione (DHPM, Scheme 1) is synthesized, in which the oxygen atoms of Meldrum's acid, DHP unit, and the two phenyl rings are advantageous for the formation of weak interactions, such as C–H⋯O bonds, C–H⋯π bonds, and π–π stacking. Furthermore, the introduction of a short ethyl chain onto the DHP ring is not only conducive to obtain a distorted molecular conformation,7d but also to enhance the possibility of obtaining multiple crystal polymorphs by altering the molecular stacking modes and intermolecular interactions.8 The photophysical properties of DHPM are investigated and the results indicate that the target molecule exhibits intriguing multifunctional photophysical properties including AIE phenomena, polymorphism-dependent emission, MC properties, and thermochromic properties.


image file: c7tc02746j-s1.tif
Scheme 1 Synthetic route to DHPM.

Experimental

Measurements and materials

NMR spectra were collected on a Bruker DRX 500 NMR spectrometer. Elemental analysis was carried out on an Elementar Vario MICRO analyzer. MS spectra were measured on a Finngan MAT95XP mass spectrometer. Fluorescence spectra and absorption spectra were measured on a HITACHI F-7000 fluorometer and a Perkin-Elmer Lambda 25 spectrometer, respectively. Solid-state emission quantum yields and fluorescence lifetimes were collected on a FluoroMax-4 (Horiba Jobin Yvon) fluorometer equipped with an integrated sphere. X-ray diffraction (XRD) measurements were conducted on a Bruker X-ray diffractometer (D8 Advance). All single crystal data were collected on a Bruker-Nonius Smart Apex CCD diffractometer with graphite-monochromated MoKα radiation. 2,6-Dimethyl-4H-pyran-4-one (1), 2,2-dimethyl-1,3-dioxane-4,6-dione, 4-dimethylaminobenz-aldehyde, and ethylamine were purchased from Sigma-Aldrich and used as received.

Synthesis of 5-(2,6-dimethyl-4H-pyran-4-ylidene)-2,2-dimethyl-1,3-dioxane-4,6-dione (2)

Compound 1 (3.0 g, 24.2 mmol) and 2,2-dimethyl-1,3-dioxane-4,6-dione (5 g, 34.7 mmol) were dissolved in acetic anhydride (15 mL). The solution was heated at 110 °C for 3.5 h. After cooling, the solution was extracted with dichloromethane three times. Subsequently the combined organic layers were washed with water three times and then dried with anhydrous sodium sulfate. After filtration, the solvent was evaporated under reduced pressure, and the residue was purified by silica gel chromatography using petroleum ether/ethyl acetate (1[thin space (1/6-em)]:[thin space (1/6-em)]4, v/v) as the eluent to afford pure compound 2 as a pink solid (1.28 g), 21.2% yield. 1H NMR (CDCl3, 500 MHz): δ 8.30 (s, 2H), 2.40 (s, 6H), 1.68 (s, 6H). 13C NMR (CDCl3, 125 MHz): δ 164.9, 164.0, 156.8, 110.4, 102.1, 90.2, 26.8, 20.5.

Synthesis of 5-(2,6-bis((E)-4-(dimethylamino)styryl)-4H-pyran-4-ylidene)-2,2-dimethyl-1,3-dioxane-4,6-dione (3)

Compound 2 (248 mg, 1.0 mmol) and 4-dimethylaminobenzaldehyde (894 mg, 6.0 mmol) were dissolved in a solution of piperidine (0.6 mL) and acetonitrile (6 mL). The solution was heated at 80 °C under nitrogen for 24 h. After cooling, the reaction mixture was poured into methanol (50 mL), and a large amount of solid precipitated out from the mixture. After filtration, the precipitate was washed with methanol three times, and then dried to afford pure compound 3 as a purplish red solid (204.7 mg), 40.3% yield. 1H NMR (CDCl3, 500 MHz): δ 8.38 (s, 2H), 7.49 (dd, 3J = 12.0 Hz, 4J = 3.5 Hz, 6H), 6.73–6.67 (m, 6H), 3.06 (s, 12H), 1.72 (s, 6H). 13C NMR (CDCl3, 125 MHz): δ 164.4, 161.3, 156.0, 151.7, 137.8, 129.5, 123.0, 114.8, 112.1, 110.4, 101.8, 89.0, 40.1, 26.8. MS (EI, m/z): 512.22 (M+, 56%), 410.19 (75%), 395.16 (100%), 384.22 (41%), 369.19 (28%), 207.02 (42%), 146.02 (54%), 58.03 (75%). Anal. calcd for C31H32N2O5: C, 72.64; H, 6.29; N, 5.47. Found: C, 72.19; H, 6.24; N, 5.51.

Synthesis of 5-(2,6-bis((E)-4-(dimethylamino)styryl)-1-ethylpyridin-4(1H)-ylidene)-2,2-dimethyl-1,3-dioxane-4,6-dione (DHPM)

Compound 3 (300 mg, 0.6 mmol) and ethylamine (3 mL) were dissolved in acetonitrile (10 mL). The solution was heated at 80 °C under nitrogen for 24 h. The reaction mixture was cooled to room temperature and then refrigerated at −10 °C for 2 h, and a large amount of solid precipitated in the mixture. After filtration, the precipitate was washed with methanol three times, and then dried to afford pure orange DHPM solid (182.9 mg) in 57.8% yield. 1H NMR (CDCl3, 500 MHz): δ 8.98 (s, 2H), 7.43 (d, J = 8.0 Hz, 4H), 7.32 (d, J = 15.5 Hz, 2H), 6.80 (d, J = 15.5 Hz, 2H), 6.72 (d, J = 7.5 Hz, 4H), 4.27 (q, J = 6.5 Hz, 2H), 3.03 (s, 12H), 1.72 (s, 6H), 1.51 (t, J = 7.0 Hz, 3H). 13C NMR (CDCl3, 125 MHz): δ 165.5, 151.3, 148.6, 140.3, 130.4, 129.0, 123.2, 117.5, 113.4, 112.0, 100.9, 81.8, 45.0, 40.2, 26.5, 14.8. MS (EI, m/z): 539.27 (M+, 1%), 410.25 (2%), 382.22 (26%), 366.18 (4%), 207.02 (11%), 146.03 (8%), 58.04 (44%). Anal. calcd for C33H37N3O4: C, 73.44; H, 6.91; N, 7.79. Found: C, 72.95; H, 6.97; N, 7.72.

Results and discussion

Synthesis of DHPM

The synthetic route to DHPM is shown in Scheme 1. 5-(2,6-Dimethyl-4H-pyran-4-ylidene)-2,2-dimethyl-1,3-dioxane-4,6-dione (2) was obtained by reacting 2,6-dimethyl-4-pyrone (1) with 2,2-dimethyl-1,3-dioxane-4,6-dione in acetic anhydride according to a similar method to the previous literature procedure.7d The Knoevenagel reaction of compound 2 with 4-dimethylaminobenzaldehyde using piperidine as a base in acetonitrile afforded 5-(2,6-bis((E)-4-(dimethylamino)styryl)-4H-pyran-4-ylidene)-2,2-dimethyl-1,3-dioxane-4,6-dione (3). Then, the reaction of intermediate 3 with ethylamine gave the target compound DHPM in 57.8% yield. The structure of DHPM was confirmed by 1H NMR and 13C NMR spectroscopy, mass spectrometry, elemental analysis, and single-crystal X-ray diffraction analysis.

AIE properties of DHPM

DHPM has one obvious absorption band centered at 423 nm and it exhibits very weak emission with a fluorescence quantum yield (ΦF) of 1.3% at 567 nm in pure dimethyl sulfoxide (DMSO) solution (1 × 10−5 M) (Fig. S1, ESI). Furthermore, a solvent–nonsolvent fluorescence test was conducted using DMSO and water as the solvent and nonsolvent, respectively. The fluorescence intensities of the DMSO/water mixtures (1 × 10−5 M) remained virtually unchanged with an increase in the water volume fraction (fw) from 10 to 60% (Fig. 1a). However, a dramatic enhancement in emission was observed when fw continued to increase to 70%. Subsequently, an orange emission (ΦF = 14.9%) peak at 598 nm was observed when fw reached 80% and the fluorescence intensity attained a maximum value, which was about 11 times higher than that in pure DMSO. The UV-vis absorption spectra of DHPM in DMSO/water mixtures were also investigated (Fig. 1b). When fw increased to 70 and 80%, the absorption spectra of the mixtures showed obvious level-off tails in the long-wavelength region, indicating the formation of a large number of aggregates.9 It should be noted that the fluorescence spectrum of DHPM in the DMSO/water mixture at fw = 70% showed an obvious broad peak character, which might be ascribed to the formation of aggregates with different sizes.6d,10 At a low water fraction (fw = 0–60%), the DMSO/water mixtures containing isolated DHPM molecules displayed weak fluorescence due to the free rotation of N,N-dimethylaniline and the DHP unit around the C–C bond and the resultant non-radiative decay. In contrast, at a high water fraction (fw = 70 and 80%), the isolated molecules began to aggregate and the free rotation was suppressed effectively, which resulted in a dramatic enhancement in fluorescence.4,7d–g These results indicate that DHPM has an obvious AIE property owing to the restriction of intramolecular rotation (RIR). The RIR mechanism was verified by a solution thickening experiment. With an increase in the volume fraction of glycerol with high viscosity, the fluorescence intensities of the methanol/glycerol mixtures gradually intensified, indicating the thickening process was advantageous to the fluorescence enhancement by hampering the intramolecular free rotations (Fig. S2, ESI).11
image file: c7tc02746j-f1.tif
Fig. 1 (a) Fluorescence spectra of DHPM (1 × 10−5 M) in DMSO/water mixtures with different fractions of water. Insets: Photographs of DHPM at fw = 0% and 80% under 365 nm UV illumination. (b) UV-vis absorption spectra of DHPM (1 × 10−5 M) in DMSO/water mixtures with different fractions of water.

Polymorphic properties of DHPM

Three crystalline polymorphs of DHPM exhibiting different fluorescence colors, namely DHPM-y, DHPM-o, and DHPM-r, were obtained under different crystallization conditions (Fig. 2 and Table 1). When the n-hexane/acetone (3[thin space (1/6-em)]:[thin space (1/6-em)]1, v[thin space (1/6-em)]:[thin space (1/6-em)]v) mixture of the as-synthesized solids was heated at 60 °C and then cooled, needle-like single crystals of DHPM-y were obtained. DHPM-y emits yellow fluorescence at 577 nm with a high ΦF value of 45.5%. The other two flaky single crystals DHPM-o and DHPM-r were cultured from the slow evaporation of a toluene/chloroform (3[thin space (1/6-em)]:[thin space (1/6-em)]1, v[thin space (1/6-em)]:[thin space (1/6-em)]v) mixture and ethyl acetate/chloroform (3[thin space (1/6-em)]:[thin space (1/6-em)]1, v[thin space (1/6-em)]:[thin space (1/6-em)]v) mixture, respectively. DHPM-o and DHPM-r display orange (λem = 598 nm) and red (λem = 633 nm) fluorescence with quantum yields of 21.0% and 12.2%, respectively. The three crystal polymorphs could be interconverted via recrystallization with the corresponding mixed solvent. These results indicate that DHPM has polymorphic properties, and it exhibits multicolor emissions. For these polymorphs, it could be found that the polymorph with the shortest emission wavelength has the highest fluorescence efficiency. To explain this phenomenon, the time-resolved fluorescence decay parameters of the polymorphs were investigated (Table 1). Compared with DHPM-o and DHPM-r, DHPM-y has the largest radiative rate constant (kf = 9.4 × 107 s−1) and the smallest non-radiative rate constant (knr = 1.1 × 108 s−1), which could explain its highest fluorescence efficiency.12
image file: c7tc02746j-f2.tif
Fig. 2 The normalized fluorescence spectra of DHPM-y, DHPM-o, and DHPM-r. Insets: Photographs of the single crystals of these three polymorphs under 365 nm UV illumination.
Table 1 The fluorescence properties and decay parameters of DHPM in different crystal states
Sample λ em (nm) Φ F (%) Fluorescence decayb
A 1/A2 (%) τ 1 (ns) τ 2 (ns) τb (ns) k f (s−1) k nr (s−1)
a Φ F measured using a calibrated integrating sphere. b Determined from I = A1[thin space (1/6-em)]exp(−t/τ1) + A2[thin space (1/6-em)]exp(−t/τ2), where τ1 and τ2 are the lifetimes of the shorter-lived and longer-lived species, and A1 and A2 are their respective fractions, respectively. The weighted mean lifetime 〈τ〉 was obtained from the equation: 〈τ〉 = (A1τ1 + A2τ2)/(A1 + A2). The radiative rate constant kf and non-radiative rate constant knr were obtained from the equations: kf = ΦF/〈τ〉 and knr = (1 − ΦF)/〈τ〉, respectively. c Ground sample coming from DHPM-o.
DHPM-y 577 45.5 7/93 1.15 5.12 4.84 9.4 × 107 1.1 × 108
DHPM-o 598 21.0 20/80 1.10 4.59 3.89 5.4 × 107 2.0 × 108
DHPM-r 633 12.2 40/60 0.69 3.52 2.39 5.1 × 107 3.7 × 108
Gently ground samplec 570 44.3 8/92 0.97 5.15 4.82 9.2 × 107 1.2 × 108
Strongly ground samplec 619 16.6 48/52 1.09 3.51 2.34 7.1 × 107 3.6 × 108


Table 2 Summary of the torsion angles in the single crystals of DHPM-y, DHPM-o, and DHPM-r
Sample θ 1 θ 2 θ 3 θ 4 θ 5
a The torsion angle between the central DHP ring and the ethylene unit. b The torsion angle between the N,N-dimethylaniline unit and the ethylene unit. c The torsion angle between the N-ethyl group and the DHP ring. The torsion angles θ1θ5 are marked in Fig. 3.
DHPM-y 33.19° 32.58° 8.64° 7.09° 84.56°
DHPM-o Structure A 35.67° 31.36° 1.71° 8.40° 84.71°
Structure B 31.59° 37.67° 0.72° 7.25° 82.08°
DHPM-r 35.95° 32.85° 9.36° 7.72° 82.83°


Crystal structures of DHPM

Normally, the optical properties of organic molecules in the aggregated state are closely related to their intramolecular conformations, intermolecular interactions, and packing arrangements of molecules.13 To gain deeper insight into the underlying reasons for the different solid-state emission behaviors of DHPM-y, DHPM-o, and DHPM-r, their single crystals were investigated.14 The crystal structures of these three crystalline polymorphs are shown in Fig. 3 and the respective crystallographic data are depicted in Table S1 (ESI). The unit cell of DHPM-o is triclinic with the P[1 with combining macron] space group. Also, the single crystals of DHPM-y and DHPM-r both belong to monoclinic with the space group P121/c1.
image file: c7tc02746j-f3.tif
Fig. 3 Single-crystal structures of the polymorphic crystals of DHPM: (a) DHPM-y; (b) DHPM-o and (c) DHPM-r. Hydrogen atoms are omitted for clarity.

In DHPM-y, the torsion angles θ1 and θ2 between the central DHP ring and the two ethylene units are 33.19° and 32.58°, and θ3 and θ4 between the N,N-dimethylaniline units and the two ethylene units are 8.64° and 7.09°, respectively (Fig. 3 and Table 2). In DHPM-r, the torsion angles θ1θ4 are 35.95°, 32.85°, 9.36°, and 7.72°, respectively. Obviously, the torsion angles θ1θ4 in DHPM-r are bigger than the corresponding angles in DHPM-y. However, the torsion angle θ5 between the N-ethyl group and the DHP ring is 84.56° in DHPM-y, which is bigger than that in DHPM-r (82.83°). Thus, it is difficult to determine which of the two polycrystalline molecules is more twisted if only the torsion angles are taken into account. We further investigated the intermolecular interactions and packing arrangements of these two polymorphs. As shown in Fig. 4, only very weak π–π stacking interactions are found in the crystals of DHPM-y and DHPM-r owing to their twisted conformations, and their distances are 3.985 and 3.867 Å, respectively. However, there are three types of weak C–H⋯π bonds with the distances of 2.598–3.329 Å in the DHPM-y crystal and 2.504–3.304 Å in the DHPM-r crystal. These weak intermolecular interactions fix the molecular conformations of the polymorphs in the solid state and thus inhibit the intramolecular rotations and block the non-radioactive relaxation, resulting in solid-state fluorescence.6i,l Although the molecules of DHPM-y and DHPM-r both adopt a head-to-tail packing mode in pairs and pack in a stair shape along the b axis, and they display almost identical molecular packing arrangements (Fig. 5), there is a subtle difference in the distances of their C–H⋯π bonds and π–π stacking interactions. The interaction distances of DHPM-y are obviously larger than the corresponding distances of DHPM-r (Fig. 4), which indicates the former has weaker intermolecular interactions and a looser packing mode than the latter. Thus, the molecules in the crystal lattice of DHPM-y have a bigger volume and a smaller density (2968.3 Å3 and 1.208 Mg m−3) compared to those of DHPM-r (2884.6 Å3 and 1.243 Mg m−3) (Table S1, ESI). Herein, since the DHPM-y crystal has weaker intermolecular interactions and a looser packing mode compared with that of DHPM-r, DHPM-y should possess a more twisted conformation, which also indicates that θ5 plays a pivotal role in controlling the degree of molecular distortion as well as contributing to the effective π-electron conjugation of the molecule than the other torsion angles θ1θ4. Since there were no specific strong intermolecular interactions in these crystalline polymorphs, their molecular conformations play a more important role in influencing their photophysical properties than their packing arrangements.6f,g,l,12,15 This is consistent with the fact that DHPM-y with a bigger θ5 value shows a shorter emission wavelength and higher ΦF value in the solid state relative to DHPM-r with a smaller θ5 value. For the DHPM-o crystal, there are two independent conformational molecules marked as structures A and B (Fig. 3). The crystal structure was stabilized by seven types of C–H⋯O bonds (2.430–2.719 Å), two types of C–H⋯π bonds (3.309 and 3.315 Å), and two types of weak π–π stacking interactions (3.724 and 3.760 Å) (Fig. 6), adopting a stair-like stacking mode along the c axis (Fig. 7). Interestingly, according to the θ5 values (Table 2), the conformation of structure A (84.71°) is similar to that of the yellow-emitting DHPM-y (84.56°), whereas structure B (82.08°) is similar to the red-emitting DHPM-r (82.83°), which might account for the orange emission of DHPM-o.


image file: c7tc02746j-f4.tif
Fig. 4 Intermolecular interactions in the crystals of DHPM-y (a) and DHPM-r (b).

image file: c7tc02746j-f5.tif
Fig. 5 Molecular stacks in the crystals of DHPM-y (left) and DHPM-r (right) viewed along the a-axis (a), b-axis (b), and c-axis (c). Hydrogen atoms are omitted for clarity.

image file: c7tc02746j-f6.tif
Fig. 6 Intermolecular interactions in the crystal of DHPM-o.

image file: c7tc02746j-f7.tif
Fig. 7 Molecular stacks in the crystal of DHPM-o viewed along the a-axis (a), b-axis (b), and c-axis (c). Hydrogen atoms are omitted for clarity.

MC and thermochromic properties of DHPM

The MC properties of the three crystalline polymorphs upon grinding in a mortar with a pestle were explored. Interestingly, they displayed different MC phenomena upon gentle grinding and strong grinding (Fig. 8). For the DHPM-o sample, when it was ground with gentle pressure, the resultant gently ground sample exhibited bright yellow fluorescence (ΦF = 44.3%) with an obvious blue-shifted emission (from 598 to 570 nm), which is similar to that of DHPM-y (Fig. 9a and Table 1). Keeping strong pressure on the gently ground sample, a red-emitting strongly ground sample (λem = 620 nm, ΦF = 16.6%) was obtained and the fluorescence spectrum showed a red shift of 22 nm relative to that of the DHPM-o sample. These results indicate that different pressure could lead to a blue shift and red shift in the fluorescence spectra of DHPM-o, which result in different fluorescence colors. For DHPM-y (Fig. 9b and Table 1), although gentle grinding caused a blue shift (12 nm) in its fluorescence spectrum from 577 to 565 nm, no obvious fluorescent color change was observed, whereas, a red-emitting ground sample in an amorphous state was obtained upon strong grinding, which was the same as that for the strongly ground sample of DHPM-o. In the case of DHPM-r (Fig. 9c and Table 1), the emission peaks of the gently ground sample and strongly ground sample were 566 and 618 nm, respectively, where only gentle grinding resulted in a fluorescent color change from red to yellow. According to the results of the XRD measurements (Fig. 10), the diffraction peaks of the gently ground samples of these three crystalline polymorphs were obviously weakened, meanwhile, several peaks disappeared (DHPM-y and DHPM-r) or several new peaks appeared (DHPM-o) relative to that of the original polymorphs. However, their XRD curves still showed obvious crystalline structure characteristics. Accordingly, the blue-shifted emissions of the original polymorphs upon gentle grinding could be ascribed to the fact that gentle grinding destroys the crystalline structure of the polymorphs to some extent and subsequently increases the degree of distortion of their molecular conformations. Since the emissions of these polymorphic crystals mainly depend on their molecular conformations, the gently ground samples exhibited similar emission wavelengths and they all emitted yellow fluorescence even though their XRD curves were somewhat different. Thus, the fluorescent color changes of DHPM-o and DHPM-r upon gentle grinding should be attributed to the phase transition from one crystalline state to another.6k,8,12 Different from gentle grinding, strong grinding completely destroyed the crystalline structures of the polymorphs as confirmed by their XRD curves, and led to the transformation from a crystalline state to an amorphous state, which activated π–π interactions and resulted in a red shift in the fluorescence spectra and fluorescent color changes of DHPM-y and DHPM-o. Interestingly, the red-emitting strongly ground samples in the amorphous state exhibited a higher fluorescence quantum yield compared to the crystalline DHPM-r, which should be ascribed to the larger kf value (7.1 × 107 s−1) and smaller knr value (3.6 × 108 s−1) of the former relative to that of the latter (kf = 5.1 × 107 s−1 and knr = 3.7 × 108 s−1) (Table 1). In general, an amorphous solid would crystallize upon fuming by solvent vapor. In our case, when the strongly ground samples of the polymorphs were fumed with acetone vapor, their emissions and XRD curves were consistent with that of DHPM-y (Fig. 9d and Fig. 10d), which indicates reversible MC properties. Furthermore, by heating the crystals of DHPM-y and DHPM-o at 175 °C for 6 h and 1.5 h, respectively, and cooling to room temperature, both of the resultant annealed samples were crystalline crystals and emitted red fluorescence at about 630 nm, which are consistent with the DHPM-y crystal (Fig. 8h, i and 11). These results reveal that DHPM-y and DHPM-o display thermochromic properties and their fluorescent color change is ascribed to the crystal-to-crystal transformation.
image file: c7tc02746j-f8.tif
Fig. 8 Fluorescence images of the DHPM solid samples in different states recorded under UV light at 365 nm. (a) DHPM-o; (b) left-half of DHPM-o was gently ground; (c) DHPM-y; (d) left-half of DHPM-y was gently ground; (e) DHPM-r; (f) left-half of DHPM-r was gently ground; (g) strongly ground sample; (h) heating DHPM-y at 175 °C for 6 h; and (h) heating DHPM-o at 175 °C for 1.5 h.

image file: c7tc02746j-f9.tif
Fig. 9 Fluorescence spectra of the DHPM solid samples upon exposure to various stimuli: (a) DHPM-o; (b) DHPM-y; (c) DHPM-y; and (d) strongly ground samples of the three crystalline polymorphs.

image file: c7tc02746j-f10.tif
Fig. 10 XRD curves of the DHPM solid samples upon exposure to various stimuli: (a) DHPM-o; (b) DHPM-y; (c) DHPM-y; and (d) strongly ground samples of the three crystalline polymorphs.

image file: c7tc02746j-f11.tif
Fig. 11 Comparison of fluorescence spectra (a) and XRD curves (b) of DHPM-r solid sample and the annealed samples of DHPM-y and DHPM-o.

Conclusions

An AIE-active 1,4-dihydropyridine derivative with a twisted conformation was found to have three crystalline polymorphs, which exhibited yellow (DHPM-y), orange (DHPM-o), and red (DHPM-r) fluorescence. The different emissions of the polymorphs were found to mainly depend on their molecular conformations due to weak intermolecular interactions, as revealed by the single crystal structural analysis. Interestingly, these three crystalline polymorphs show different MC behaviors under different pressure. The fluorescent color changes of DHPM-o and DHPM-r upon gentle grinding are attributed to a crystal-to-crystal transformation which alters their molecular conformations, whereas, that of DHPM-y and DHPM-o upon strong grinding are ascribed to the transformation from a crystalline state to an amorphous state. Additionally, DHPM-y and DHPM-o also display thermochromic properties. These results will provide useful information for obtaining intriguing multifunctional fluorescent materials by rational modification of classic luminogens.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grants 21572165, 21472140, 21372177, and 51673093), the Zhejiang Provincial Natural Science Foundation (Grant LY16B040005), and the Graduate Innovation Foundation of Wenzhou University (No. 3162016036).

Notes and references

  1. Z. G. Chi, X. Q. Zhang, B. J. Xu, X. Zhou, C. P. Ma, Y. Zhang, S. W. Liu and J. R. Xu, Chem. Soc. Rev., 2012, 41, 3878–3896 RSC.
  2. (a) Y. Sagara, S. Yamane, M. Mitani, C. Weder and T. Kato, Adv. Mater., 2016, 28, 1073–1095 CrossRef CAS PubMed; (b) X. Q. Zhang, Z. G. Chi, Y. Zhang, S. W. Liu and J. R. Xu, J. Mater. Chem. C, 2013, 1, 3376–3390 RSC; (c) S. Xue, X. Qiu, Q. Sun and W. Yang, J. Mater. Chem. C, 2016, 4, 1568–1578 RSC; (d) S. Varughese, J. Mater. Chem. C, 2014, 2, 3499–3516 RSC; (e) H. Sun, S. Liu, W. Lin, K. Y. Zhang, W. Lv, X. Huang, F. Huo, H. Yang, G. Jenkins, Q. Zhao and W. Huang, Nat. Commun., 2014, 5, 3601–3609 Search PubMed; (f) Z. Ma, Z. Wang, M. Teng, Z. Xu and X. Jia, ChemPhysChem, 2015, 16, 1811–1828 CrossRef CAS PubMed; (g) S. Mukherjee and P. Thilagar, J. Mater. Chem. C, 2016, 4, 2647–2662 RSC; (h) W. Z. Yuan, Y. Tan, Y. Gong, P. Lu, J. W. Y. Lam, X. Y. Shen, C. Feng, H. H. Y. Sung, Y. Lu, I. D. Williams, J. Z. Sun, Y. Zhang and B. Z. Tang, Adv. Mater., 2013, 25, 2837–2843 CrossRef CAS PubMed; (i) Y. Gong, Y. Zhang, W. Z. Yuan, J. Z. Sun and Y. Zhang, J. Phys. Chem. C, 2014, 118, 10998–11005 CrossRef CAS.
  3. S. A. Jenekhe and J. A. Osaheni, Science, 1994, 265, 765–768 CAS.
  4. J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu, H. S. Kwok, X. Zhan, Y. Liu, D. Zhu and B. Z. Tang, Chem. Commun., 2001, 1740–1741 RSC.
  5. X. Luo, W. Zhao, J. Shi, C. Li, Z. Liu, Z. Bo, Y. Q. Dong and B. Z. Tang, J. Phys. Chem. C, 2012, 116, 21967–21972 CAS.
  6. (a) Z. He, L. Zhang, J. Mei, T. Zhang, J. W. Y. Lam, Z. Shuai, Y. Q. Dong and B. Z. Tang, Chem. Mater., 2015, 27, 6601–6607 CrossRef CAS; (b) X. Cheng, H. Zhang, K. Ye, H. Zhang and Y. Wang, J. Mater. Chem. C, 2013, 1, 7507–7512 RSC; (c) S. J. Yoon and S. Y. Park, J. Mater. Chem., 2011, 21, 8338–8346 RSC; (d) X. Du, F. Xu, M. S. Yuan, P. Xue, L. Zhao, D. E. Wang, W. Wang, Q. Tu, S. W. Chen and J. Wang, J. Mater. Chem. C, 2016, 4, 8724–8730 RSC; (e) J. N. Zhang, H. Kang, N. Li, S. M. Zhou, H. M. Sun, S. W. Yin, N. Zhao and B. Z. Tang, Chem. Sci., 2017, 8, 577–582 RSC; (f) Q. Qi, J. Zhang, B. Xu, B. Li, S. X. A. Zhang and W. Tian, J. Phys. Chem. C, 2013, 117, 24997–25003 CrossRef CAS; (g) X. Mei, G. Wen, J. Wang, H. Yao, Y. Zhao, Z. Lin and Q. Ling, J. Mater. Chem. C, 2015, 3, 7267–7271 RSC; (h) C. Wang, B. Xu, M. Li, Z. Chi, Y. Xie, Q. Li and Z. Li, Mater. Horiz., 2016, 3, 220–225 RSC; (i) Y. Wang, G. Zhang, W. Zhang, X. Wang, Y. Wu, T. Liang, X. Hao, H. Fu, Y. Zhao and D. Zhang, Small, 2016, 12, 6554–6561 CrossRef CAS PubMed; (j) C. Li, X. Luo, W. Zhao, C. Li, Z. Liu, Z. Bo, Y. Dong, Y. Q. Dong and B. Z. Tang, New J. Chem., 2013, 37, 1696–1699 RSC; (k) S. J. Yoon, J. W. Chung, J. Gierschner, K. S. Kim, M. G. Choi, D. Kim and S. Y. Park, J. Am. Chem. Soc., 2010, 132, 13675–13683 CrossRef CAS PubMed; (l) X. Gu, J. Yao, G. Zhang, Y. Yan, C. Zhang, Q. Peng, Q. Liao, Y. Wu, Z. Xu, Y. Zhao, H. Fu and D. Zhang, Adv. Funct. Mater., 2012, 22, 4862–4872 CrossRef CAS; (m) B. Xu, J. He, Y. Mu, Q. Zhu, S. Wu, Y. Wang, Y. Zhang, C. Jin, C. Lo, Z. Chi, A. Lien, S. Liu and J. Xu, Chem. Sci., 2015, 6, 3236–3241 RSC; (n) Y. Qi, Y. Wang, Y. Yu, Z. Liu, Y. Zhang, G. Du and Y. Qi, RSC Adv., 2016, 6, 33755–33762 RSC; (o) C. Botta, S. Benedini, L. Carlucci, A. Forni, D. Marinotto, A. Nitti, D. Pasini, S. Righetto and E. Cariati, J. Mater. Chem. C, 2016, 4, 2979–2989 RSC; (p) Y. Xu, K. Wang, Y. Zhang, Z. Xie, B. Zou and Y. Ma, J. Mater. Chem. C, 2016, 4, 1257–1262 RSC; (q) S. Ito, A. Hirose, M. Yamaguchi, K. Tanaka and Y. Chujo, J. Mater. Chem. C, 2016, 4, 5564–5571 RSC; (r) P. Galer, R. C. Korošec, M. Vidmar and B. Šket, J. Am. Chem. Soc., 2014, 136, 7383–7394 CrossRef CAS PubMed; (s) P. S. Hariharan, D. Moon and S. P. Anthony, J. Mater. Chem. C, 2015, 3, 8381–8388 RSC; (t) Y. X. Li, J. X. Qiu, J. L. Miao, Z. W. Zhang, X. F. Yang and G. X. Sun, J. Phys. Chem. C, 2015, 119, 18602–18610 CrossRef CAS; (u) J. Mei, J. Wang, A. Qin, H. Zhao, W. Yuan, Z. Zhao, H. H. Y. Sung, C. Deng, S. Zhang, I. D. Williams, J. Z. Sun and B. Z. Tang, J. Mater. Chem., 2012, 22, 4290–4298 RSC.
  7. (a) Z. Guo, A. Shao and W. H. Zhu, J. Mater. Chem. C, 2016, 4, 2640–2646 RSC; (b) C. X. Shi, Z. Q. Guo, Y. L. Yan, S. Q. Zhu, Y. S. Xie, Y. S. Zhao, W. H. Zhu and H. Tian, ACS Appl. Mater. Interfaces, 2013, 5, 192–198 CrossRef CAS PubMed; (c) A. Shao, Z. Guo, S. Zhu, S. Zhu, P. Shi, H. Tian and W. Zhu, Chem. Sci., 2014, 5, 1383–1389 RSC; (d) H. Li, Y. Guo, G. Li, H. Xiao, Y. Lei, X. Huang, J. Chen, H. Wu, J. Ding and Y. Cheng, J. Phys. Chem. C, 2015, 119, 6737–6748 CrossRef CAS; (e) Y. Lei, Y. Liu, Y. Guo, J. Chen, X. Huang, W. Gao, L. Qian, H. Wu, M. Liu and Y. Cheng, J. Phys. Chem. C, 2015, 119, 23138–23148 CrossRef CAS; (f) Y. Lei, D. Yang, H. Hua, C. Dai, L. Wang, M. Liu, X. Huang, Y. Guo, Y. Cheng and H. Wu, Dyes Pigm., 2016, 133, 261–272 CrossRef CAS; (g) Y. Liu, Y. Lei, M. Liu, F. Li, H. Xiao, J. Chen, X. Huang, W. Gao, H. Wu and Y. Cheng, J. Mater. Chem. C, 2016, 4, 5970–5980 RSC.
  8. Y. Lei, Y. Zhou, L. Qian, Y. Wang, M. Liu, X. Huang, G. Wu, H. Wu, J. Ding and Y. Cheng, J. Mater. Chem. C, 2017, 5, 5183–5192 RSC.
  9. H. Auweter, H. Haberkorn, W. Heckmann, D. Horn, E. Lüddecke, J. Rieger and H. Weiss, Angew. Chem., Int. Ed., 1999, 38, 2188–2191 CrossRef CAS PubMed.
  10. T. Jadhav, J. M. Choi, B. Dhokale, S. M. Mobin, J. Y. Lee and R. Misra, J. Phys. Chem. C, 2016, 120, 18487–18495 CAS.
  11. J. Chen, C. C. W. Law, J. W. Y. Lam, Y. Dong, S. M. F. Lo, I. D. Williams, D. Zhu and B. Z. Tang, Chem. Mater., 2003, 15, 1535–1546 CrossRef CAS.
  12. Y. Zhang, Q. Song, K. Wang, W. Mao, F. Cao, J. Sun, L. Zhan, Y. Lv, Y. Ma, B. Zou and C. Zhang, J. Mater. Chem. C, 2015, 3, 3049–3054 RSC.
  13. (a) D. Yan and D. G. Evans, Mater. Horiz., 2014, 1, 46–57 RSC; (b) S. Varghese and S. Das, J. Phys. Chem. Lett., 2011, 2, 863–873 CrossRef CAS PubMed.
  14. CCDC 1553600–1553602 (DHPM-y, DHPM-o, and DHPM-r).
  15. K. Wang, H. Zhang, S. Chen, G. Yang, J. Zhang, W. Tian, Z. Su and Y. Wang, Adv. Mater., 2014, 26, 6168–6173 CrossRef CAS PubMed.

Footnote

Electronic supplementary information (ESI) available: Photophysical properties, crystal data, 1H NMR, and 13C NMR. CCDC 1553600–1553602. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7tc02746j

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