Aggregation-Induced Fluorescence Emission Properties of

Mar 6, 2015 - A series of dicyanomethylene-1,4-dihydropyridine (DCMP) derivatives with aggregation-induced emission (AIE) were designed and synthesize...
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Aggregation-Induced Fluorescence Emission Properties of Dicyanomethylene-1,4-dihydropyridine Derivatives Hui Li,† Yang Guo,‡ Guoxing Li,† Hongping Xiao,† Yunxiang Lei,† Xiaobo Huang,*,† Jiuxi Chen,† Huayue Wu,*,† Jinchang Ding,† and Yixiang Cheng*,‡ †

College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, P. R. China School of Chemistry and Chemical Engineering, Nanjing University Nanjing, 210093, P. R. China

‡

S Supporting Information *

ABSTRACT: A series of dicyanomethylene-1,4-dihydropyridine (DCMP) derivatives with aggregation-induced emission (AIE) were designed and synthesized. These target compounds emit low fluorescence in THF solutions because of the free rotation of the phenyl and dihydropyridine rings about the axes of the olefinic double bonds and the resultant nonradiative decay process, but they exhibit strong fluorescence in the aggregate state because of the restriction of intramolecular rotation (RIR), as confirmed by solution thickening and cooling experiments. Compared with the almost-planar dicyanomethylene-4H-pyran (DCM) derivative DCM-1 with aggregation-caused quenching (ACQ), DCMP5 with AIE takes a highly twisted conformation by replacing the oxygen atom of the DCM skeleton with an N-ethyl group, as revealed by crystallographic data and theoretical calculations. Depending on the different electron-donating substituents, the solids of these compounds emit yellow or orange fluorescence. Moreover, the cyano group endows DCMP-5 with a strong self-assembly ability under proper conditions, and the obtained regular microparticles emit bright orange fluorescence. These materials will broaden the new family of AIE-exhibiting fluorophores.

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INTRODUCTION Most organic fluorophores contain planar π-conjugated aromatic rings, which can increase the chances for molecular aggregation to form excimers and exciplexes. Such molecular aggregation will cause the fluorescence quenching of fluorophores in the solid state or in concentrated solutions due to the nonradiative deactivation of the excited state, which is known as aggregation-caused quenching (ACQ). Normally, ACQ greatly limits the applications of the aromatic hydrocarbon fluorophores and their derivatives for light-emitting materials and devices in the aggregated state.1,2 In 2001, Tang and co-workers3,4 first reported a class of interesting tetraphenylsilole derivatives that were nonemissive in dilute solutions but exhibited intense emission in concentrated solutions or in the solid state. This is a unique phenomenon that is called aggregation-induced emission (AIE) and is exactly opposite to the ACQ effect. Attracted by the fascinating application prospects, many efforts in the past decade have been dedicated to the development of novel organic fluorophores with AIE properties. So far, most of them are aromatic hydrocarbons, and they often show blue fluorescence, such as tetraphenylethenes,5−7 8,8a-dihydrocyclopenta[a]indenes,8 1,4-distyrylbenzenes,9 9,10-distyryl-anthracenes,10−13 butadienes,14 fulvenes,15 and arylbenzenes.16 To further enrich the known types of AIE-active molecules and widen the range of their emission colors, one of the most important methods is to introduce heteroatoms into the AIE skeleton to construct a © 2015 American Chemical Society

donor−acceptor (D−A) structure that then leads to significantly red-shifted emission through intramolecular charge transfer (ICT).17 To date, only a few heteroatom-containing fluorophores with AIE properties have been reported, such as phosphole derivatives,18−20 tetraphenylthiophene derivatives,21 dithiole nitrofluorene derivatives,22 boron dipyrromethene derivatives,23 and indolo[3,2-b]carbazole derivatives.24 Therefore, the development of novel heteroatom-containing AIEactive fluorophores is very attractive topic. Dicyanomethylene-4H-pyran (DCM) derivatives, as typical D−π−A conjugated groups, have been reported to be used as excellent fluorophores in red fluorescence materials because of their long emission wavelengths and high fluorescence yields arising from ultrafast ICT.25 Recently, a series of DCM derivatives were reported to show AIE activities in the aggregate state as a result the restriction of intramolecular rotation (RIR), which fixes their molecular conformations and endows them with AIE activities. Some of these species are shown in Scheme 1.26−31 In particular, DCM-1, which contains dimethyl amino groups, shows ACQ behavior upon aggregation; in this case, the strong push−pull interactions associated with the amino−cyano pairs lead to fluorescence emission quenching owing to nonradiative relaxation of its ICT states.27 Received: November 5, 2014 Revised: March 2, 2015 Published: March 6, 2015 6737

DOI: 10.1021/jp511060k J. Phys. Chem. C 2015, 119, 6737−6748

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The Journal of Physical Chemistry C Scheme 1. Chemical Structures of DCM and DCMP Derivatives

C, H, and N elemental analyses were performed on an Elementar Vario MICRO analyzer. UV−vis absorption spectra were recorded on a Perkin-Elmer Lambda 25 spectrometer, and fluorescence spectra were recorded on a RF-5301PC fluorometer. Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer Pyris-1 instrument under a N2 atmosphere. Scanning electron microscopy (SEM) images were recorded on a JEOL JSM-6700F scanning electron microscope. Fluorescence microscopic images were obtained on a Leica DMI3000B inverted optical microscope. Timeresolved emission decay behaviors were measured on a FluoroMax-4 (Horiba Jobin Yvon) fluorometer with excitation at 452 nm. Electrochemical measurements were carried out in anhydrous CH2Cl2 with 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) as the supporting electrolyte at a scan rate of 0.02 V/s at room temperature under the protection of nitrogen. A gold disk was used as the working electrode, platinum wire was used as the counter electrode, and Ag/AgCl (3 M KCl solution) was used as the reference electrode. Synthesis of 2-(2,6-Dimethyl-4H-pyran-4-ylidene)malononitrile (2). 2,6-Dimethyl-4-pyrone (1) (25 mmol, 3.1 g) and malononitrile (250 mmol, 16.5 g) in 15 mL of acetic anhydride were heated under reflux for 2 h. The unreacted acetic anhydride was aspirated off, and the residue was washed with 50 mL of boiling water and collected to give the crude brown product. The crude product was further purified by column chromatography on silica gel (petroleum ether/ethyl acetate, 10:1, v/v) to afford pure compound 2 as a yellow solid (2.86 g, 67%). MS (ESI, m/z): 170.95 (M+ − 1). Synthesis of 2-(2,6-Dimethyl-1-ethylpyridin-4(1H)ylidene)malononitrile (3). A mixture of compound 2 (1.72 g, 10 mmol) and 70% ethylamine aqueous solution (20 mL) in 20 mL of acetonitrile was heated at 80 °C for 12 h. After the mixture had cooled to room temperature, the solvent was removed using a rotary evaporator, and the residue was purified by silica gel column chromatography (petroleum ether/ethyl acetate, 15:1, v/v) to give compound 3 (1.54 g, 77%) as a yellow solid. 1H NMR (CDCl3, 400 MHz): δ 6.66 (s, 2H), 4.05 (q, J = 7.2 Hz, 2H), 2.48 (s, 6H), 1.40 (t, J = 7.2 Hz, 3H). 13C NMR (CDCl3, 75 MHz): δ 156.0, 147.2, 118.6, 113.9, 43.8, 20.3, 14.7. MS (ESI, m/z): 200.10 (M+ + 1).

In this work, we designed and synthesized a series of dicyanomethylene-1,4-dihydropyridine (DCMP) derivatives by replacing the oxygen atom of the DCM skeleton with a nitrogen atom. The introduction of a nitrogen atom can not only broaden the structural types of DCM derivatives, but also significantly modify the photophysical properties of these derivatives by electronic effects, as well as steric effects caused by the short alkyl chain bonded to the nitrogen atom.32,33 Additionally, the introduction of appropriate electron donors into the phenyl ring can effectively tune the range of emission colors of the resulting DCMP dyes. We investigated in detail the absorption and fluorescence features of these compounds and their structure−property relationships. The results indicate that these DCMP derivatives show outstanding AIE properties. Depending on the electron-donating substituents, the solids of these compounds emit yellow or orange fluorescent light. It should be pointed that DCMP-5, which containing dimethyl amino groups, also shows outstanding AIE activity, which is quite different from the behavior of DCM-1, despite their similar structures. As revealed by crystallographic data and theoretical calculations, the introduction of an ethyl group at the nitrogen atom increases the steric hindrance, which prevents the molecules from packing compactly and avoids the formation of excimers or exciplexes. The different optical behaviors of these DCMP derivatives are caused by the RIR process in the aggregate state, as confirmed by solution thickening and cooling experiments. Furthermore, DCMP-5 can self-organize into regular microstructures with different sizes and shapes as a result of dipole−dipole interactions.

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EXPERIMENTAL SECTION Materials and Measurements. The general chemicals employed in this work were purchased from Merck Ltd. or Aldrich Chemical Co. and used as received. All solvents were of analytical reagent grade. 1H and 13C NMR spectra were recorded in solutions of CDCl3 or deuterated dimethyl sulfoxide (DMSO-d6) on a Bruker DRX 300 or DRX 400 NMR spectrometer with tetramethylsilane (TMS) as the internal standard. Electrospray ionization-mass spectrometry (ESI-MS) spectra were recorded on a Finnigan DECAX-30000 LCQ Deca mass spectrometer. Fourier transform infrared (FTIR) spectra were obtained on a Nexus 870 FT-IR spectrometer. 6738

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The Journal of Physical Chemistry C Scheme 2. Synthetic Routes to DCMP Derivatives

Synthesis of DCMP Derivatives. A mixture of compound 3 (172 mg, 1.0 mmol), aryl aldehyde (2.2 mmol), piperidine (0.1 mL), and DMSO (4 mL) was refluxed under argon for 24 h. The reaction mixture was cooled to room temperature, water was added, and the mixture was extracted with ethyl acetate. The combined organic phase was washed with H2O and dried over anhydrous Na2SO4. The solvent was removed by vacuum distillation, and the residues were purified by silica gel column chromatography using petroleum ether/ethyl acetate (5:1, v/v) as the eluent to give the target DCMP derivatives. (E)-2-(1-Ethyl-2,6-distyrylpyridin-4(1H)-ylidene)malononitrile (DCMP-1). Yellow solids (259 mg), 69% yield. 1 H NMR (DMSO-d6, 400 MHz): δ 7.81−7.79 (m, 4H), 7.44− 7.39 (m, 10H), 6.92 (s, 2H), 4.39 (q, J = 6.8 Hz, 2H), 1.32 (t, J = 7.2 Hz, 3H). 13C NMR (DMSO-d6, 75 MHz): δ 154.6, 149.0, 139.0, 135.2, 129.6, 128.8, 127.9, 119.9, 118.7, 110.7, 45.1, 14.5. MS (ESI, m/z): 376.15 (M+ + H). FT-IR (KBr, cm−1): 2189, 1605, 1537, 1470, 1381, 1181, 960, 758, 689. Anal. Calcd for C26H21N3: C, 83.17; H, 5.64; N, 11.19. Found: C, 83.22; H, 5.51; N, 11.02. (E)-2-(2,6-Bis(4-methylstyryl)-1-ethylpyridin-4(1H)ylidene)malononitrile (DCMP-2). Yellow solids (363 mg), 90% yield. 1H NMR (DMSO-d6, 400 MHz): δ 7.69 (d, J = 8.4 Hz, 4H), 7.37 (s, 4H), 7.26 (d, J = 8.0 Hz, 4H), 6.89 (s, 2H), 4.36 (q, J = 7.2 Hz, 2H), 2.35 (s, 6H), 1.31 (t, J = 7.2 Hz, 3H). 13C NMR (DMSO-d6, 75 MHz): δ 154.5, 149.1, 139.4, 138.9, 132.5, 129.4, 127.9, 118.8, 112.7, 110.4, 45.0, 21.0, 14.5. MS (ESI, m/z): 404.10 (M+ + H). FT-IR (KBr, cm−1): 2191, 1601, 1541, 1477, 1381, 1176, 970, 841. Anal. Calcd for C28H25N3: C, 83.34; H, 6.24; N, 10.41. Found: C, 83.47; H, 6.15; N, 10.29. (E)-2-(2,6-Bis(4-methoxystyryl)-1-ethylpyridin-4(1H)ylidene)malononitrile (DCMP-3). Yellow solids (409 mg), 94% yield. 1H NMR (DMSO-d6, 400 MHz): δ 7.76 (d, J = 8.8 Hz, 4H), 7.31 (d, J = 18.0 Hz, 2H), 7.23 (d, J = 18.0 Hz, 2H), 7.01 (d, J = 8.8 Hz, 4H), 6.88 (s, 2H), 4.36 (q, J = 7.2 Hz, 2H), 3.81 (s, 6H), 1.31 (t, J = 7.2 Hz, 3H). 13C NMR (DMSO-d6, 75 MHz): δ 160.5, 154.4, 149.3, 138.6, 129.6, 127.9, 119.0, 117.3, 114.3, 110.3, 55.3, 44.9, 14.5. MS (ESI, m/z): 436.10 (M+ + H). FT-IR (KBr, cm−1): 2195, 1600, 1538, 1475, 1379, 1178, 958, 813. Anal. Calcd for C28H25N3O2: C, 77.22; H, 5.79; N, 9.65. Found: C, 77.39; H, 5.65; N, 9.57. (E)-2-(1-Ethyl-2,6-bis(4-hydroxystyryl)pyridin-4(1H)ylidene)malononitrile (DCMP-4). Yellow solids (163 mg), 40% yield. 1H NMR (DMSO-d6, 400 MHz): δ 9.93 (s, 2H), 7.64 (d, J = 8.7 Hz, 4H), 7.30 (d, J = 15.6 Hz, 2H), 7.18 (d, J = 15.6 Hz, 2H), 6.85−6.81 (m, 6H), 4.34 (q, J = 7.2 Hz, 2H), 1.32 (t, J = 7.2 Hz, 3H). 13C NMR (DMSO-d6, 75 MHz): δ 159.1, 154.4, 149.4, 139.0, 129.8, 126.4, 119.1, 116.2, 115.7, 109.9, 44.9, 14.5. MS (ESI, m/z): 406.10 (M+-H). FT-IR (KBr, cm−1): 3243,

2193, 1613, 1542, 1479, 1376, 1171, 966, 844. Anal. Calcd for C26H21N3O2: C, 76.64; H, 5.19; N, 10.31. Found: C, 76.52; H, 5.29; N, 10.16. (E)-2-(2,6-Bis(4-(dimethylamino)styryl)-1-ethylpyridin4(1H)-ylidene)malononitrile (DCMP-5). Orange-red solids (411 mg), 89% yield. 1H NMR (DMSO-d6, 400 MHz): δ 7.62 (d, J = 9.2 Hz, 4H), 7.27 (d, J = 15.6 Hz, 2H), 7.10 (d, J = 15.6 Hz, 2H), 6.83 (s, 2H), 6.74 (d, J = 9.2 Hz, 4H), 4.34 (q, J = 6.8 Hz, 2H), 2.99 (s, 12H), 1.33 (t, J = 7.2 Hz, 3H). 13C NMR (DMSO-d6, 75 MHz): δ 154.0, 151.2, 149.6, 139.3, 129.4, 122.9, 119.3, 113.9, 111.8, 109.1, 44.6, 14.4. MS (ESI, m/ z): 462.20 (M+ + H). FT-IR (KBr, cm−1): 2184, 1600, 1530, 1472, 1372, 1183, 969, 809. Anal. Calcd for C30H31N5: C, 78.06; H, 6.77; N, 15.17. Found: C, 78.34; H, 6.62; N, 15.29. (E)-2-(1-Ethyl-2,6-bis(2-(thiophen-2-yl)vinyl)pyridin-4(1H)ylidene)malononitrile (DCMP-6). Brown solids (411 mg), 80% yield. 1H NMR (DMSO-d6, 300 MHz): δ 7.70 (d, J = 4.8 Hz, 2H), 7.63 (d, J = 15.3 Hz, 2H), 7.55 (d, J = 3.3 Hz, 2H), 7.16 (dd, J = 4.8 Hz, J = 3.6 Hz, 2H), 7.08 (d, J = 15.6 Hz, 2H), 6.88 (s, 2H), 4.30 (q, J = 7.2 Hz, 2H), 1.31 (t, J = 7.2 Hz, 3H). 13C NMR (DMSO-d6, 75 MHz): δ 154.4, 148.5, 140.0, 132.1, 130.3, 128.8, 128.4, 118.8, 118.1, 110.3, 45.1, 14.4. MS (ESI, m/ z): 388.05 (M+ + H). FT-IR (KBr, cm−1): 2189, 1600, 1539, 1471, 1334, 1180, 848, 710. Anal. Calcd for C22H17N3S2: C, 68.19; H, 4.42; N, 10.84. Found: C, 68.27; H, 4.31; N, 10.69.

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RESULTS AND DISCUSSION Synthesis and Features of DCMP Derivatives. Scheme 2 illustrates the synthetic routes to DCMP derivatives. 2-(2,6Dimethyl-4H-pyran-4-ylidene)malononitrile 2 was synthesized

Figure 1. TGA curves of DCMP derivatives. 6739

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Figure 2. (a) Fluorescence spectra of DCMP-1 (1 × 10−5 mol/L) in THF/water mixtures with different water volume fractions. Inset: Visible fluorescence of DCMP-1 (1 × 10−5 mol/L) in 0% and 90% water under a 365-nm UV lamp. (b) Plot of fluorescence intensity of DCMP-1 in THF/ water against f w (concentration = 1 × 10−5 mol/L). (c) UV−vis spectra of DCMP-1 (1 × 10−5 mol/L) in THF/water mixtures with different water volume fractions.

Table 1. Electrochemical Data, HOMO and LUMO Energies, and Energy Gaps of DCMP Derivatives compound

HOMOa (eV)

LUMOa (eV)

Ega (eV)

λonset (nm)

Egb (eV)

HOMOc (eV)

LUMOc (eV)

Egc (eV)

Egd (eV)

ΦPLe

DCMP-1 DCMP-2 DCMP-3 DCMP-4 DCMP-5 DCMP-6

−5.60 −5.60 −5.56 −5.55 −4.95 −5.56

−3.04 −3.00 −2.95 −3.15 −2.59 −3.50

2.56 2.60 2.61 2.40 2.36 2.06

483 476 472 473 486 492

2.57 2.61 2.63 2.62 2.55 2.52

−5.62 −5.53 −5.44 −5.50 −5.17 −5.60

−2.66 −2.54 −2.37 −2.44 −2.10 −2.70

2.96 2.99 3.07 3.06 3.07 2.90

2.55 2.58 2.67 2.65 2.69 2.49

0.17 0.39 0.20 0.13 0.19 0.05

HOMO energy level calculated from the onset of the oxidation wave; LUMO energy level calculated from the onset of the first reduction wave; b onset energy levels calculated using the following equations: HOMO = −(4.8 + Eonset ox ), LUMO = −(4.8 + Ered ), Eg = LUMO − HOMO. Eg calculated from the absorption of UV−vis spectra in CH2Cl2 solution; band gap energy (eV) = 1240/[λonset (nm)]. cDFT quantum mechanical calculations (B3LYP/6-311+G**). dTD-DFT calculations (B3LYP/6-311+G**). eQuantum yields in THF/water mixtures with f w = 90% determined with anthracene as the fluorescence reference in ethanol (Φ = 0.27). a

soluble in common organic solvents such as CH2Cl2, CHCl3, toluene, DMSO, and tetrahydrofuran (THF), but insoluble in methanol, ethanol, and water. Thermal stability is one of the key requirements for practical applications of organic fluorophores. The thermal properties of DCMP derivatives were investigated by thermogravimetric analysis (TGA), which was carried out under a N2 atmosphere at a heating rate of 10 °C/min. As shown in Figure 1, the TGA curves reveal that the degradation temperatures (Td) of 5% weight loss of the DCMP derivatives are in the range of 173−397 °C. There is a total loss of about 56−80% for these compounds when heated to 700 °C.

by the reaction of malononitrile with 2,6-dimethyl-4-pyrone (1) in acetic anhydride according to the literature.34 Treatment of compound 2 with ethylamine afforded 2-(1-ethyl-2,6-dimethylpyridin-4(1H)-ylidene)malononitrile (3) in CH3CN at 80 °C.35 Then, DCMP derivatives were synthesized by Knoevenagel condensation between compound 3 and various commercially available arylaldehydes in 40−94% yields. According to the 1H NMR spectra of DCMP-3, DCMP-4, and DCMP-5, the chemical shifts of the H atoms in the ethylene units were in the range of 7.10−7.31 ppm, and the 3J values were all greater than 15 Hz, which indicates that these compounds should exist in the E isomeric form. These compounds were found to be 6740

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Figure 3. (a) Fluorescence spectra of compound DCMP-5 (1 × 10−5 mol/L) in THF/water mixtures with different water volume fractions. (b) Maximum emission intensity of DCMP-5 as a function of water volume fraction. (c) UV−vis spectra of compound DCMP-5 (1 × 10−5 mol/L) in THF/water mixtures with f w ranging from 0% to 99%.

Figure 4. (a) Fluorescence photographs recorded under 365-nm UV irradiation for DCMP-5 (1 × 10−5 mol/L) in THF/water mixtures with f w ranging from 0% to 99%. (b) Fluorescence photo of the solids for DCMP-5 under a 365-nm UV lamp.

The results indicate that DCMP derivatives can provide desirable thermal properties for various fluorescent materials. Aggregation-Induced Emission Properties of DCMP Derivatives. DCMP-1 in dilute THF solution (1 × 10−5 mol/ L) emitted very weak light at 475 nm, and almost no fluorescence could be observed under a 365-nm UV lamp (Figure 2a). As shown in Figure 2a, the fluorescence spectra exhibited no obvious changes as the content of water was increased from 0% to 70%, but a dramatic fluorescence enhancement behavior was observed when the water volume fraction ( f w) was 80%. At f w = 90%, the fluorescence of the solution of DCMP-1 increased to a maximum value, and the intensity was 20 times higher than that in pure THF. In addition, the maximum emission wavelength showed a red shift of about 56 nm to 531 nm, which can be related to aggregate formation. The further addition of water to 99 vol %, however,

resulted in an obvious lowering of the emission intensity, which can be attributed to a decreased solubility. The formation of aggregates can also be confirmed by the changes of UV−vis spectra of DCMP-1 in THF/water mixtures with varying water volume fractions (Figure 2c). DCMP-1 exhibited two absorption peaks at 321 and 406 nm in pure THF solution (Table S1, Supporting Information). With an increase in f w from 0% to 70%, these two absorption peaks basically remained at the same positions, and a new absorption peak at about 275 nm appeared at f w = 50%. When f w was further increased to higher values (≥80%), the absorption peaks gradually decreased. At the same time, leveled-off tails appeared in the visible region, which can be ascribed to light-scattering effects and indicate the formation of aggregates.24,36 In the solvent mixtures with water contents of ≤70%, no leveled-off spectral tails in the long-wavelength region were recorded by the UV− 6741

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Figure 5. (a) UV−vis and (b) fluorescence spectra of DCMP-5 in various solvents.

Figure 6. (a) Effect of temperature on the fluorescence spectrum of DCMP-5 (1 × 10−5 mol/L) in THF solution. (b) Fluorescence spectra of DCMP-5 (1 × 10−5 mol/L) in methanol−glycerol mixtures (containing 0.5 vol % THF) with different glycerol volume fractions.

fold, respectively, compared to those in THF (Figures S2 and S3, Supporting Information), and their fluorescence quantum yields (ΦF) were determined to be 0.39 and 0.20, respectively, using anthracene as a fluorescence reference (ΦF = 0.27 in ethanol)37 (Table 1). These results indicate these two compounds showed stronger AIE behaviors than DCMP-1, which should be due to their more twisted conformations caused by substituent groups on the phenyl ring. Hydroxylsubstituted DCMP-4 also exhibited AIE behavior but gave an emission enhancement of only about 5-fold at f w = 99% (Figure S4, Supporting Information). Moreover, the solids of DCMP-4 exhibited weaker yellow fluorescence than those of DCMP-2 and DCMP-3 (Figure S1, Supporting Information). These results further confirm that greater steric hindrance is advantageous to the enhancement of the AIE effect of DCMP derivatives. For DCMP-5, which contained the dimethyl amino group as shown in Figure 3, when the content of water was increased from 0 to 70 vol %, no obvious emission enhancement could be observed because of the free rotation of the phenyl and dihydropyridine rings about the axes of the olefinic double bonds and the resultant nonradiative decay process. In contrast, at f w = 80%, the fluorescence intensity increased dramatically, and a maximum emission intensity as high as 133-fold was then reached at f w = 90%, which should be attributed to the RIR process in the aggregate state, which fixed the molecular

vis spectrometer, confirming that the molecules were dissolved as isolated species in the solvent mixtures. The UV−vis absorption of DCMP-1 in THF/water mixtures showed outstanding changes at f w ≥ 80% that were consistent with the emission spectra. Therefore, the dramatic fluorescence enhancement should originate from the aggregation of the molecules. Furthermore, the solids of DCMP-1 emitted bright orange fluorescence under illumination with a 365-nm UV lamp (Figure S1, Supporting Information). Herein, DCMP-1 showed faint fluorescence in dilute solutions but exhibited strong emission in concentrated solutions and in the solid state, which is consistent with the behavior described by the AIE concept. According to the previous report of AIE-active DCM2,27 the AIE of DCMP-1 can be attributed to its twisted conformations, which are helpful in preventing the formation of excimers or exciplexes and boost the emission in the aggregate state. In this work , we further introduced various substituent groups such as methyl, methoxy, N,N-dimethyl, and hydroxyl groups into the phenyl cycle of the molecules to investigate the influence of electronic and steric hindrance effects on the AIE properties of DCMP-based derivatives. We found that the THF/water solutions of DCMP-2 and DCMP-3 both exhibited outstanding fluorescence enhancement for f w > 70% and emitted yellow fluorescence. For f w = 90%, DCMP-2 and DCMP-3 exhibited emission enhancements of >875- and 12006742

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Figure 7. (a) Single-crystal structure of DCMP-5. Hydrogen atoms are omitted for clarity. (b) Fluorescence image of a crystal of DCMP-5. (c) Molecular packing showing various CH···N hydrogen bonds (red dashed lines).

toluene to DMSO. According to a previous report,27 the emission spectra of DCM-1 showed a red shift of about 100 nm from dioxane to dimethylformamide (DMF), which indicates that DCM-1 displays a stronger solvent dependence than DCMP-5. It can be concluded that the introduction of the Nethyl group in DCMP-5 could decrease the ICT process. In addition, DCM-1 emitted strong red fluorescence in dilute solution, which can be attributed to the strong push−pull interactions of the amino−cyano pair, which could lock the conformation of the dye molecule and show ACQ behavior upon π−π stacking aggregation of the planar molecule.28 In contrast, DCMP-5 displayed obvious AIE behavior because of the highly twisted conformations arising from the steric hindrance of the ethyl unit linked to the nitrogen atom. Replacement of the phenyl ring by a thienyl ring on the DCMP skeleton gives DCMP-6, whose solution at f w = 90% exhibited orange fluorescence (Figure S5, Supporting Information); however, the fluorescence of DCMP-6 in the solid state was obviously weaker than those of the other DCMP derivatives containing phenyl rings (Figure S1, Supporting Information). The weak AIE activity of DCMP-6 can be ascribed to its

conformation, blocked the nonradiative path, and boosted the fluorescence. At the same time, the aggregation of the fluorescent molecules forced them to adopt more planar conformation. The corresponding emission peak for the THF/ water mixture of DCMP-5 showed a 47-nm red shift from 528 to 575 nm as f w was increased from 0% to 99%. Meanwhile, the color of the solution changed from olive to orange and then red under 365-nm UV illumination (Figure 4). Unlike DCMP-1− DCMP-4, the solids of DCMP-5 emitted bright orange fluorescence because of the stronger electron-donating ability of the dimethyl amino group (Figure S1, Supporting Information). These results indicate that DCMP-5 displays outstanding AIE behavior, which is exactly contrary to DCM-1, which exhibits ACQ properties. Considering that the strong electron-donating ability of the dimethyl amino group is helpful for the ICT effect, we investigated the UV−vis and fluorescence spectra of DCMP-5 in various solvents (Figure 5). As shown in Figure 5, the absorption spectra of DCMP-5 exhibited no obvious changes in different polar solvents, and the fluorescence spectra showed only a certain degree of red shift from 533 to 546 nm with increasing solvent polarity from 6743

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Figure 8. Molecular orbital diagrams for the (top) LUMOs and (bottom) HOMOs of DCMP derivatives obtained from DFT calculations.

coplanarity, which was confirmed by the theoretical calculations. This is further evidence that a distorted structure is advantageous to AIE performance. The time-resolved emission decay behaviors of DCMP-1 and DCMP-5 were studied in THF/water mixtures and in the solid state (Table S2, Supporting Information). It was found that the different emission behaviors of the DCMP derivatives in THF/ water mixtures with varying f w values showed different decay dynamics. In pure THF solution and THF/water mixtures with f w < 80%, DCMP-1 showed very weak fluorescence, and the fluorescence lifetime was lower than the limit of the apparatus (about 0.1 ns) and could not been detected, which indicated the decay was extremely fast because of the efficient annihilation process associated with active intramolecular rotation. However, at f w ≥ 80%, the active intramolecular rotations were restricted by the formation of aggregates, and the excited states started to decay by two relaxation pathways, namely, a fast channel and a slow channel. In the solvent mixture with f w = 80%, the emission abruptly increased, with 33% (A1) and 67% (A2) of the excited molecules decaying by the fast and slow channels with lifetimes of 0.39 ns (τ1) and 2.25 ns (τ2), respectively. When f w was increased to 90%, the decay by the fast channel was decreased, but the excited molecules decaying by the slow channel increased, accompanied by increases of the τ1 and τ2 values. At f w = 99%, the decay was close to that of the DCMP-1 solid because of the decreased solubility of the molecules in the THF/water mixture. DCMP-5 exhibited fluorescence decay behaviors similar to those of DCMP-1. These results indicate that the higher emission intensities in the THF/water mixtures with higher f w values should be ascribed to the longer lifetimes of the excited states of the aggregates of the DCMP derivatives.38 Mechanisms of AIE. After studying the optical properties of these DCMP derivatives in THF/water mixtures, we further investigated the mechanisms of AIE. According to a previous

Figure 9. Cyclic voltammograms of DCMP derivatives.

Figure 10. Comparison of experimental energy gaps (from CV and UV−vis spectroscopy) with theoretical values.

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Figure 11. (a,b) SEM images of DCMP-5 in THF/water mixture ( f w = 60%) at different concentrations: (a) 1 × 10−5 and (b) 5 × 10−5 mol/L. (c) Fluorescence microscopy image of DCMP-5 (5 × 10−5 mol/L) at f w = 60%. (d) Fluorescence image of a crystal of DCMP-5.

report,39 if an RIR process is indeed at work, the molecules exhibiting AIE should become more luminescent in solution with increasing solution viscosity or decreasing solution temperature, because both the thickening and cooling processes are known to hamper intramolecular rotations. We first checked the effect of temperature on the emission behavior of DCMP-5 in THF solution. As shown in Figure 6a, the emission spectrum of DCMP-5 in dilute THF solution (1 × 10−5 mol/L) was progressively intensified with almost no shift in the emission peak when the temperature was gradually decreased from 30 to −78 °C, and the intensity at −78 °C was 9.5 times higher than that at 30 °C. Because THF has a low viscosity at low temperature, the enhancement of the emission intensity was predominantly caused by the temperature effect in this case.39 The results thus indicate that cooling could effectively limit the thermally activated intramolecular rotation of DCMP-5 and thereby cause its fluorescence enhancement. We then checked the effect of viscosity on the DCMP-5 emission. Because glycerol is a highly viscous liquid that is fully miscible with many polar solvents, we investigated the emission behaviors of DCMP-5 in methanol/glycerol mixtures (1 × 10−5 mol/L, containing 0.5 vol % THF) with different fractions of glycerol. As shown in Figure 6b, the emission of DCMP-5 was gradually enhanced with increasing amounts of glycerol. The emission intensity of DCMP-5 in the methanol/glycerol mixture with 90% glycerol was about 10 times higher than that in methanol solution. Additionally, the emission wavelengths was about 532 nm and showed little change with increasing glycerol fraction, which indicates that the emission enhancement is predominantly due to the viscosity effect and not the aggregation of the molecules. These results indicate that increasing the viscosity of the solvent mixture can effectively

enhance the DCMP-5 emission, suggesting that the AIE of DCMP-5 was caused by the RIR process. Considering that the structures of the DCMP derivatives contain donor and acceptor units, it is necessary to discuss the possible influence of the twisted-intramolecular-charge-transfer (TICT) process on their AIE behaviors. Similarly to AIE, TICT is associated with intramolecular rotation or twisting; however, TICT results in a red-shifted emission color and decreased emission intensity with increasing solvent polarity, which is quite different from AIE.40 According to a previous report,23 the emission of a fluorophore exhibiting TICT is sensitive to temperature variations, and its emission in THF solution should be intensified and blue-shifted with increasing temperature because the polarity of THF decreased with increasing temperature. As shown in Figure 6a, the emission spectrum of DCMP-5 in THF solution gradually intensified with almost no shift in the emission peak when the temperature was decreased, indicating that the TICT process is probably not be involved in the AIE of DCMP-5. Furthermore, if a TICT process were involved in the emission of compounds exhibiting AIE, these compounds in THF/hexane mixtures should show obvious blue shifts and emission enhancements upon the addition of “small” amounts of hexane as a result of the TICT process, and then the emission should be further enhanced by the AIE process upon the addition of “large” amounts of hexane.23,41 In this case, the polarity of the THF/hexane mixtures gradually decreased with increasing content of hexane. Additionally, the AIE of these compounds in THF/water mixtures should show a particular trend with increasing water volumes, that is, the addition of small amounts of water should weaken and red shift their emission because of the TICT process, whereas the addition of 6745

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properties of DCMP-5 can be attributed to its highly twisted conformations, which favor active intramolecular rotations in dilute solutions, promote nonradiative relaxation processes, and weaken the fluorescence; however, when in the aggregate state, these conformations help to avoid the formation of π−π stacking interactions, prevent fluorescence quenching, and thus boost the emission.44 Electronic Structure. We performed density functional theory (DFT) calculations to gain a better understanding of the geometric and electronic s structure and optical properties of the molecules presented in this study. The DFT calculations were carried out with Gaussian 03, revision C.02,45 using Becke’s three-parameter set with the Lee−Yang−Parr modification (B3LYP) with the 6-311+G** basis set. Figure 8 displays the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) diagrams of DCMP derivatives, and the calculated HOMO and LUMO energies and energy gaps (Eg) are listed in Table 1. According to Figure 8, both the HOMO and LUMO of each of these compounds are basically distributed over the whole molecule including the DCMP core and substituted phenyl ring, which indicates that these DCMP derivatives have no obvious ICT tendency, which is consistent with the observed fluorescence behaviors of DCMP-5 in different polar solvents shown in Figure 5b. As reported in Table 1, we determined that the HOMO energy levels of these DCMP derivatives range from −5.62 to −5.17 eV, the LUMO energy levels range from −2.70 to −2.10 eV, and the band gaps are in the range of 2.90−3.07 eV. Compared with DCMP-1−DCMP-4, DCMP-5 has a higher HOMO energy (−5.17 eV), because of the strongly electrondonating dimethyl amino group, and a higher LUMO energy (−2.10 eV), because the highly distorted structure is unfavorable for decreasing the LUMO energy, which corresponds well with the results estimated using cyclic voltammetry (CV) analyses (Table 1, Figure 9). Moreover, the DFT theoretical energy gaps (Eg) for these DCMP derivatives were found to be between 2.90 and 3.07 eV, with DCMP-6 having the lowest Eg value (2.90 eV) because its relatively good coplanarity decreases the LUMO energy (−2.70 eV) of the compound (Figure S9, Supporting Information). The energy gaps of these compounds were also estimated from UV−vis spectra, and the results are included in Table 1. It can be seen that the energy gaps obtained from the theoretical calculations are consistent in tendency with the experimentally measured results (from CV and UV−vis spectroscopy) (Figure 10), except for that of DCMP-5. Furthermore, we performed time-dependent density functional theory (TD-DFT) calculations of the DCMP derivatives at the B3LYP/6-311+G** level and found that the calculated energy gaps showed greater consistency with those estimated from the absorption edge (Table 1). Therefore, the electronic structure and the optical properties of these DCMP derivatives can be effectively tuned by rational molecular design. Self-Assembly. As discussed above, the calculation results and the experimentally observed AIE phenomena indicate that DCMP derivatives should adopt twisted conformations that prevent intermolecular π−π stacking. Without large planar cores in these molecules, dipole−dipole interactions can also be utilized to achieve ordered self-assemblies, as is commonly observed in cyano-substituted conjugated molecules.46 Thus, scanning electron microscopy (SEM) was used to examine the morphology of the precipitate that formed in THF/water mixtures with high f w values. As displayed in Figure 11a, for f w

large amounts of water should intensify and blue shift their emission as a result of the RIR process.23,41,42 First, we investigated the fluorescence behaviors of DCMP-5 in THF/ hexane mixtures and found that the fluorescence of DCMP-5 in THF/hexane mixtures gradually increased with increasing fraction of hexane, but the emission peak exhibited no obvious changes (Figure S6, Supporting Information). DCMP-1 and DCMP-2 also showed similar results under these conditions (Figure S7, Supporting Information). Second, we found that the emissions of DCMP-1 (Figure 2a), DCMP-2 (Figure S2, Supporting Information), and DCMP-5 (Figure 3a) in THF/ water mixtures exhibited only an obvious fluorescence enhancement with increasing content of water from 0% to 90%, which should mainly originate from the RIR process. As described above, we therefore conclude that the TICT process does not play an important role in the AIE behaviors of these DCMP derivatives. To better understand the relationship between the AIE properties and the molecular packing, we obtained single crystals of DCMP-5 from a CHCl3/CH3OH solution.43 As shown in Figure 7b, the single crystal of DCMP-5 emitted orange fluorescence. Crystal X-ray diffraction intensity data were collected at 100 K on a Bruker−Nonius Smart Apex CCD diffractometer with graphite-monochromated Mo Kα radiation. The intensity data were processed using the SAINT and SADABS routines, and the structure solution and refinement were carried out by the SHELXTL suite of X-ray programs (version 6.10). The crystal data and collected parameters are summarized in Table S3 (Supporting Information). The X-ray crystal structure of DCMP-5 with atom numbering is depicted in triclinic form with the space group P1̅ (Figure 7a). As is evident from Figure 7a, DCMP-5 exists in the E isomeric form, and in its crystal structure, there are two independent molecules (structure A and structure B) that have different conformations with different dihedral angles. In structure A, the dihedral angles between the central DCMP cycle (C3, C4, C5, C6, C7, and N1) and substituted phenyl rings C23−C28 and C13−C18 were 25.07° and 89.16°, respectively. In structure B, the dihedral angles between the DCMP cycle (C33, C34, C35, C36, C37, and N6) and phenyl rings C43−C48 and C53−C58 were 17.16° and 86.81°, respectively. We also measured the torsion angles in DCMP5 using structure A as an example. We found that the torsion angles between the two ethylene units and the DCMP unit were 30.7° and 68.9° and those between the two ethylene units and the two phenyl rings were 6.2° and 18.2° (Table S4, Supporting Information). Additionally, the N-ethyl group had a torsion angle of 87.4° from the DCMP unit. In sharp contrast, DCM-1 adopts an almost-planar conformation with very small torsion angles (Figure S8, Supporting Information). It is obvious that DCMP-5 adopted a highly twisted conformation, which should be ascribed to the introduction of N-ethyl group. As shown in Figure 7c, the molecule of DCMP-5 was restricted by multiple C−H···N hydrogen bonds from adjacent structures A and B in the same layer, and the distances of these H···N bonds were in the range of 2.52−2.69 Å (Table S5, Supporting Information). The large torsion angles between the phenyl ring and the central DCMP cycle resulted in there being no obvious face-to-face π−π stacking interactions between structure A and lower structure B. Instead, the intermolecular C−H···N interactions help to rigidify the twisted conformation and lock the intramolecular rotations of the phenyl and dihydropyridine rings against the double bonds. The AIE 6746

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The Journal of Physical Chemistry C = 60% and a concentration of 1 × 10−5 mol/L, numerous microspheres were observed, and the diameter was hundreds of nanometres. This is because DCMP-5 has sufficient solubility in THF/water solution, and the molecular aggregation process is slow, so ordered microspheres can be generated. When the concentration was increased to 5 × 10−5 mol/L, the precipitate appeared to be withered tree-shaped microparticles, and the sizes increased and the regularity of the morphology decreased concomitantly because of the high concentration (Figure 11b). However, no ordered microstructures were observed at f w = 90% because the molecules quickly aggregated and formed smaller aggregates. Fluorescence microscope images were also taken to investigate the morphology. After being allowed to stand for 24 h, a precipitate of DCMP-5 formed in the THF/ water mixture (5 × 10−5 mol/L) with f w = 60%, and the lamellar microparticles emitted orange fluorescence (Figure 11c). However, when f w was increased to 90%, no ordered particles could be observed (Figure 11d), which was consistent with the SEM observations. The differences in morphology indicate that the size and shape of the microstructures are correlated with the THF/water ratio and the concentration of the solution.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21204066, 51173078, 21474048) and the Commonwealth Project of Science and Technology Department of Zhejiang Province (No. 2012C23030).

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CONCLUSIONS In summary, we designed and synthesized a series of DCMP derivatives and investigated their unique fluorescence properties. These target compounds exhibited very weak fluorescence in dilute solutions because of the free rotation of the phenyl and dihydropyridine rings about the axes of the olefinic double bonds and the resultant nonradiative decay process, but they emitted intense emission in the aggregate state because of the restriction of intramolecular rotation (RIR), as confirmed by solution thickening and cooling experiments. Compared with the almost-planar DCM-1, which exhibits ACQ activity, DCMP-5 exhibits AIE properties and takes a highly twisted conformation upon replacement of the oxygen atom of the DCM skeleton with an N-ethyl group, as verified by crystallographic data and theoretical calculations. Additionally, various substitutents with different electronic effects and steric structures could largely influence the AIE and luminescence properties; that is, bulky groups could increase the AIE characteristics, and strongly electron-donating groups could promote the red shift of the emission wavelength. Furthermore, the self-assembly behavior of DCMP-5 could be tuned by the THF/water ratio and the concentration of the solution. At a low f w and concentration, ordered microspheres were generated because of sufficient solubility and low aggregation speed, whereas no ordered particles were observed because of the rapid aggregation process. These results indicate that these DCMP derivatives can be developed as excellent AIE fluorophores.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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*E-mail: [email protected] (X.H.). *E-mail: [email protected] (H.W.). *E-mail: [email protected] (Y.C.). 6747

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DOI: 10.1021/jp511060k J. Phys. Chem. C 2015, 119, 6737−6748