ARTICLE pubs.acs.org/JPCB
Piezofluorochromic Properties and Mechanism of an Aggregation-Induced Emission Enhancement Compound Containing N-Hexyl-phenothiazine and Anthracene Moieties Xiqi Zhang, Zhenguo Chi,* Jianyong Zhang, Haiyin Li, Bingjia Xu, Xiaofang Li, Siwei Liu, Yi Zhang, and Jiarui Xu* PFCM Lab, DSAPM Lab, and KLGHEI of Environment and Energy Chemistry, FCM Institute, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China
bS Supporting Information ABSTRACT: A fluorescent compound, 9,10-bis(2-(10-hexyl10H-phenothiazin-3-yl)vinyl) anthracene, has been synthesized and studied. The results show that the compound possesses piezofluorochromic properties as well as aggregation-induced emission enhancement effect. The spectroscopic properties and morphological structures are reversibly exhibited upon pressing (or grinding) or annealing (or fuming). The piezofluorochromic nature is generated through phase transformation under the stimulus of external pressure. The reason for the phase transformation caused by external pressure is ascribed to the twisted conformation of the molecule which leads to poor solid molecular packing and weak interactions in the interfaces of lamellar layers confirmed by its single-crystal X-ray diffraction analysis.
’ INTRODUCTION Piezofluorochromic or piezochromic fluorescent material is a “smart” material whose fluorescent properties can be changed in response to external pressure stimuli. Although modification or alteration of molecular chemical structures is the most common approach to control their fluorescence properties, however, for dynamic controlling of solid state fluorescence with high efficiency, reversibility, and reproducibility, successful examples are quite limited because most chemical reactions in the solid state frequently encounter insufficient conversion, irreversible reactions, or loss of their fluorescence ability.1,2 To overcome this problem, a very attractive approach is to control the fluorescence properties dynamically by altering the mode of solid state molecular packing without changing the chemical structure of the constituting molecules. However, piezofluorochromic materials depending on the change of physical molecular packing modes remain extremely rare.37 Recently, many wholly aromatic aggregation-induced emission enhancement (AIEE) compounds containing phenylvinylanthracene have been synthesized in our laboratory, and it is very interesting to find that some of them have piezofluorochromic nature.810 These compounds have been called piezofluorochromic aggregation-induced emission or piezofluorochromic aggregation-induced emission enhancement (PAIE) materials by us as they possess both piezochromic fluorescence and aggregation-induced emission enhancement properties. AIEE materials are an important class of antiaggregation-caused-emission-quenching materials first reported by Tang11 and have attracted considerable research r 2011 American Chemical Society
attention due to their potential application in various fields such as organic light-emitting devices and chemosensors.1218 Although Park’s group19in 2010 reported an AIEE compound [DBDCS, 2,20 -(1,4-phenylene)bis(3-(4-butoxyphenyl) acrylonitrile] possessing piezofluorochromic property, they did not recognize the existence of a structural relationship between the AIEE compound and the piezofluorochromic nature. In this article, we find that 9,10-bis(2-(10-hexyl-10H-phenothiazin-3-yl)vinyl)anthracene, whose structure has already appeared in a patent,20 has PAIE properties and the piezofluorochromic nature of AnPh is generated through phase transformation under the stimulus of external pressure.
’ EXPERIMENTAL SECTION Materials and Measurements. 9,10-Bis(chloromethyl)anthracene, phenothiazine, and 1-bromohexane purchased from Alfa Aesar were used as received. All other reagents and solvents were purchased as analytical grade from Guangzhou Dongzheng Co. (China) and used without further purification. Tetrahydrofuran (THF) was distilled from sodium/benzophenone. Ultrapure water was used in the experiments. Intermediates 1,21 2,22 and 322 were synthesized according to the literature methods. Received: March 5, 2011 Revised: April 21, 2011 Published: May 18, 2011 7606
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H NMR and 13C NMR spectra were measured on a MercuryPlus 300 spectrometer [CDCl3 as solvent and tetramethylsilane (TMS) as the internal standard]. Mass spectra (MS) were measured on a Thermo DSQ MS spectrometer. Elemental analyses (EA) were performed with an Elementar Vario EL elemental analyzer. Fluorescence spectra were measured on a Shimadzu RF-5301pc spectrometer with a slit width of 1.5 nm for both excitation and emission. Differential scanning calorimetry (DSC) curves were obtained with a NETZSCH thermal analyzer (DSC 204 F1) at heating and cooling rates of 10 °C/min under N2 atmosphere. Wide-angle X-ray diffraction (WAXD) measurements were performed by using a Rigaku X-ray diffractometer (D/max-2200) with an X-ray source of Cu KR (λ = 0.15406 nm) at 40 kV and 30 mA at a scan rate of 4° (2θ) per 1 min. Pressed samples or ground samples were prepared by pressing in an IR pellet at 1500 psi for 5 min or by grinding using a mortar and pestle. Annealing experiments were done on a hot stage with an automatic temperature control system. Single crystals of the compound were grown from ethyl acetate solvent. X-ray crystallographic intensity data were collected at 110 K using a Bruker Smart 1000 CCD diffractometer equipped with a graphitemonochromated Enhance (Mo) X-ray source (λ = 0.71073 Å). The structures were solved by direct methods following difference Fourier syntheses and refined by the full-matrix leastsquares method against F02 using the SHELXTL software.23 The water/THF mixtures with different water fractions were prepared by slowly adding ultrapure water into the THF solution of samples under ultrasound at room temperature. For example, a 70% water fraction mixture was prepared in a volumetric flask by adding 7 mL of ultrapure water into 3 mL of THF solution of the sample. The concentrations of all samples were adjusted to 10 μM after adding ultrapure water. Synthesis of 9,10-Bis(2-(10-hexyl-10H-phenothiazin-3yl)vinyl) Anthracene (AnPh). Tetraethyl anthracene-9,10diylbis(methylene)diphosphonate (1) (0.10 g, 0.21 mmol) and 10-hexyl-10H-phenothiazine-3-carbaldehyde (3) (0.157 g, 0.50 mmol) were dissolved in THF (20 mL), and then t-BuOK (0.2 g) was added under argon. The solution was stirred at room temperature overnight. After removing the solvent under reduced pressure, the residue was recrystallized with THF/EtOH to give AnPh (0.16 g, 97% yield). 1H NMR (300 MHz, CDCl3) δ: 0.91 (t, 6 H, J = 6.6 Hz), 1.301.52 (m, 12 H), 1.87 (qui, 4 H, J = 7.2 Hz), 3.90 (t, 4 H, J = 6.9 Hz), 6.77 (s, 1 H), 6.83 (s, 1 H), 6.866.97 (m, 6 H), 7.127.21 (m, 4 H), 7.387.52 (m, 8 H), 7.75 (s, 1 H), 7.80 (s, 1 H), 8.35 (q, 4 H, J = 3.3 Hz). 13C NMR (75 MHz, CDCl3) δ (ppm): 145.15, 136.35, 132.83, 132.05, 129.79, 127.68, 127.47, 126.66, 126.13, 125.53, 125.36, 125.28, 124.53, 123.60, 122.68, 115.60, 47.94, 31.85, 27.28, 27.04, 22.99, 14.39. MS (EI), m/z: 792, calcd for C54H52N2S2, 792. Anal. Calcd for C54H52N2S2: C, 81.77; H, 6.61; N, 3.53; S, 8.09. Found: C, 81.69; H, 6.58; N, 3.49; S, 8.15.
’ RESULTS AND DISCUSSION The target compound 9,10-bis(2-(10-hexyl-10H-phenothiazin-3yl)vinyl) anthracene (AnPh) was synthesized according to the routes depicted in Scheme 1. The synthesis of AnPh started from 9,10bis(chloromethyl)anthracene and phenothiazine, which were purchased from a commercial source. The intermediates tetraethyl anthracene-9,10-diylbis(methylene)diphosphonate (1),21 10-hexyl10H-phenothiazine (2),22 and 10-hexyl-10H-phenothiazine-3-carbaldehyde (3)22 were synthesized according to literature methods.
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Scheme 1. Synthetic Routes for AnPha
a
The lowest energy conformer was optimized using Gaussian 03 for the compound (bottom).
WittigHorner reaction of 1 and 3 afforded the target product AnPh in high yield (97%). The chemical structures of these products were confirmed with proton and carbon-13 nuclear magnetic resonance spectra (1H NMR and 13C NMR), mass spectrometry, and elementary analysis. To confirm whether a compound is AIEE active or not, the fluorescent behaviors are usually studied with a poor solvent added to a solution of the compound. As the compound is insoluble in water, increasing the water fraction in the mixed solvents can change its existing form from a solution or welldispersed state in pure tetrahydrofuran (THF) to aggregated particles in mixtures with high water content. The photoluminescence (PL) spectra of 10 μM of AnPh in the water/THF mixtures with different water contents are shown in Figure 1, which shows that the PL intensity is very weak and observed almost no change as the water fraction of the water/THF mixture increases from 0% to 50%. However, a significant enhancement of luminescence is observed in the water/THF mixture with water fraction >50%. The 5.0 PL intensity (au) in pure THF is elevated to 125.0 in the water/THF mixture with a water fraction of 95%, about 25 times higher than that in the pure THF solvent, which indicates that the compound has strong AIEE activity. However, the PL spectra with 5565% water fractions are unusual either in intensity or in wavelength. The PL spectra exhibit a significant blue shift with two peaks at 530 and 556 nm, respectively. However, the other PL spectra show only one peak around 570 nm. The emission images of AnPh in pure THF and 60% and 95% water fraction mixtures under 365 nm ultraviolet (UV) illumination are also shown in Figure 1 (inset). In pure THF, the emission of the compound is very weak and virtually invisible. However, in 60% and 95% water fractions of water/THF mixtures the compound exhibits green and strong yellow emissions, respectively. The images and PL spectra of the compound 7607
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Figure 1. PL spectra of dilute solutions of the compound in water/THF mixtures with different water fractions (concentration, 10 μM; excitation wavelength, 400 nm). (Inset) Changes in PL peak intensity (up) and emission images of the compound in pure THF, 60% and 95% water fraction mixtures under 365 nm UV illumination (10 μM) (down).
Figure 2. AnPh taken at room temperature under (A) natural light and (B) UV light. Samples obtained (a) as-synthesized or fumed sample, (b) pressed or ground sample, (c) annealed sample, and (d) pressed or ground sample.
in the mixture with 60% and 95% water fractions are different because of the difference in molecular packing mode in the aggregates. Molecules of AnPh form crystal-like aggregates and amorphous aggregates in mixtures with 60% and 95% water fractions, respectively. This phenomenon is evident in many AIEE systems, but the reason for this is still not clear yet. It is generally believed that in the mixture with a “low” water fraction the solute molecules steadily assemble in an ordered fashion to form more emissive, bluer crystalline aggregates, and in the mixture with the “high” water content the solute molecules quickly agglomerate in a random way to form less emissive, redder amorphous particles.24 The common structural feature for the reported AIEE compounds, such as triphenylethylene, tetraphenylethylene, silole and cyano distyrylbenzene, and distyrylanthracene derivatives,2534 is that the multiple phenyl peripheries are linked to an olefinic core via rotatable CC single bonds to form an AIEE moiety. The steric effect between the phenyl rings forces the AIEE moieties or the molecules to take a twisted conformation. The optimized geometry of AnPh by Gaussian 0335 at the B3LYP/6-31G level also shows a twist configuration. Between two phenyl rings as defined in Scheme 1 (bottom), dihedral angles for AB (BC) and AC are 66° and 54°, respectively. The twisted configuration makes it difficult for the AnPh molecules to assume a dense
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Figure 3. Images taken at room temperature under natural light and 365 nm UV light: (a) a drop of dichloromethane was dropped onto the pressed or ground sample ; (b) AnPh is casted on a filter paper and written “A&P” with a metal spatula.
packing structure. Thus, the AnPh crystals formed have low lattice energy and some structural defects (cavities) and are easily destroyed by external pressure. It is suggested that this “destroy” may be triggered by slip deformation under external pressure. Destruction of crystalline structure leads to the release of binding energy (lattice energy) and thus planarization of the molecular conformation resulting in a red shift in the PL spectrum. Therefore, we propose that if an AIEE compound exists in two different pressure-dependent stable or metastable states it will exhibit piezofluorochromic activity. As expected, this AIEE compound does exhibit significant piezofluorochromic property. Under 365 nm UV light, the assynthesized sample (also the annealed sample or fumed sample with a good solvent) shows strong green emission and the pressed (or ground) sample shows orangered emission (Figure 2, right). In addition, the colors of the samples are also different: the former is yellow and the latter is red (Figure 2, left). This indicates that the pressing (grinding) treatment changes not only its emission spectrum but also its absorption spectrum. The two colors, either emission or absorption, are completely reversible under treatment with annealing (or fuming) and pressing (or grinding). Therefore, the reversible color change (switchable) features of the compound make it a promising candidate as optical recording, anticounterfeiting, and temperature- or pressure-sensing systems. If a drop of dichloromethane was dropped onto the surface of pressed or ground sample, the soaked area immediately changed its emission from orangered to green as well as sample color from red to yellow (Figure 3a). Interestingly, a strong greenemitting yellow solid was converted to an orangered solid with strong red luminescence even when pressed simply with a metal spatula. The change occurred only at the written area (Figure 3b). Under natural light, it can also be seen red “A&P” on yellow “paper”. The results suggest that the material has potential for application as an optical recording material. Figure 4 shows the PL spectra of AnPh after grinding and annealing treatments. The wavelength changes in solid-state emission could be repeated many times without fatigue, suggesting excellent reversibility in the switching processes. The cycle could also be achieved when ground and fumed with dichloromethane vapor. 7608
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Figure 4. Normalized PL spectra of AnPh: (a) grinding, (b) annealing a at 100 °C for 1 min, (c) grinding b sample, and (d) annealing c at 100 °C for 1 min.
Figure 5. WXRD of AnPh: (a) as-synthesized sample, (b) ground sample, (c) annealing b at 100 °C for 1 min, and (d) fuming b in CH2Cl2 vapor for 1 min.
To determine the mechanism of the piezofluorochromic effect, we carried out wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) measurements for the samples. According to the WAXD measurements (Figure 5), the compound exhibited different structures of molecular aggregation before and after pressing or grinding treatment. The diffraction patterns of the as-synthesized, annealed, and fumed samples display sharp and intense reflections at 2θ = 8.3°, 12.7°, 21.3°, and 36.4°, which are indicative of some crystalline order. The diffraction pattern of the sample after pressing or grinding shows a weak, broad, and diffuse peak except for a peak at 36.4°, indicating that after pressing or grinding most of the ordered structures have been destroyed. However, the ordered structures destroyed by pressing or grinding treatment can be restored by annealing or fuming with a good solvent such as dichloromethane for a short time (i.e., 1 min). In the AIEE section we mentioned that the molecules of AnPh form crystal-like aggregates and amorphous aggregates in mixtures with 60% and 95% water fractions, respectively. We tried to consider the difference in the driving forces of microphase separation between “low” and “high” water fraction systems to
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Figure 6. DSC of AnPh: (a) ground sample, (b) annealing a at 100 °C for 1 min, and (c) fuming a in CH2Cl2 vapor for 1 min.
Figure 7. 1H NMR of AnPh in CDCl3: (a) as-synthesized sample, (b) ground sample, and (c) annealing b at 100 °C for 1 min.
explain these findings. In the “high” water fraction system, the driving force must be greater because of the lower solubility of solute molecules in the mixtures. The driving force should play the same role as the external macroscopic pressure. As the compound is piezoresponsive and the external macroscopic pressure causes a change from the crystalline state to the amorphous state, we assume that the driving force of the microphase separation causes formation of amorphous aggregates in mixtures with a high water fraction. The heating DSC results of the compound (Figure 6) show that after being pressed or ground the samples reproducibly observed a meltingcrystallization transition before melting, indicating that the aggregation structure of the pressed sample is in a metastable state which would transfer to a more stable state referring to the crystals of the main melting peak around 202 °C. After being treated by annealing at 100 °C or solvent fuming by dichloromethane for 1 min, the meltingcrystallization transition disappears and the shapes of the two DSC curves are almost the same as that obtained from the as-synthesized sample. These results indicate that pressing or grinding treatment causes the change of morphology of the compound, and the change was recoverable through annealing or fuming. 7609
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Table 1. Crystallographic Data of AnPh empirical formula
C54 H52 N2 S2
fw
793.10
temp., K
110(2)
wavelength, Å
0.71073
cryst syst space group
triclinic P-1
a, Å
5.7587(10)
b, Å
17.681(3)
c, Å
21.772(4)
R, deg
111.793(3)
β, deg
91.700(3)
γ, deg
92.295(3)
V, Å3 Z
2054.4(6) 2
d(calcd), Mg/m3
1.282
abs coeff, mm1
0.171
F(000)
844
θ range, deg
1.2427.07
index ranges
7 eh e 7
index ranges
18 ek e 22
index ranges no. of reflns collected
27 el e 27 17985
no. of independent reflns
8776 [R(int) = 0.0472]
completeness to θ = 27.07°
97.0%
no. of data/restraints/params
8776/0/523
goodness-of-fit on F2
0.925
final R indices [I > 2σ(I)]
R1 = 0.0517, wR2 = 0.1190
R indices (all data)
R1 = 0.1077, wR2 = 0.1463
largest diff. peak and hole
0.458 and 0.282 e 3 A3
To verify whether the chemical structure of AnPh was changed after pressing (grinding) and annealing, 1H NMR experiments in CDCl3 were conducted in Figure 7, which show that the 1H NMR spectra are the same as the as-synthesized sample, indicating that the chemical structure did not change during treatment. This further implies that the different spectroscopic properties of AnPh were caused by physical processes (i.e., by changing the mode of molecular packing). A single crystal of AnPh was successfully grown from the ethyl acetate solvent, and its crystalline structure was determined by single-crystal X-ray diffraction analysis. The X-ray crystallographic intensity data were collected at 110 K using a Bruker Smart 1000 CCD diffractometer equipped with a graphitemonochromated Enhance (Mo) X-ray source (λ = 0.71073 Å). The structures were solved by direct methods following difference Fourier syntheses and refined by the full-matrix leastsquares method against F02 using the SHELXTL software. Selected crystallographic data are given in Table 1. The crystalline structure shows the AnPh molecules are packed in a head-to-head fashion (Figure 8a). Due to the highly twisted conformation and steric hindrance of the bulky phenothiazinyl groups in the molecule, the backbone of the molecule largely deviates from a plane and typical cofacial ππ stacking becomes impossible. Instead, the molecules are first packed in clusters via synergetic weak CH 3 3 3 S, S 3 3 3 π, and CH 3 3 3 π interactions, and the clusters are followed forming lamellar layer structures via CH 3 3 3 π interactions. From Figure 9 it can be
Figure 8. Molecules packing in a crystal cell (a), and the schematic intermolecular interactions in a cluster (green dashed lines, hydrogen atoms omitted for clarity) (b).
Figure 9. Molecules packing in single-crystal structure.
seen that the interaction between each cluster is not the same. The layers are subsequently connected via the phenyl rings in the phenothiazinyl groups with weaker sparse ππ interactions. Due to the twisted conformation and weaker ππ interactions, the interfaces between the layers are relatively loose with some defects and the interfaces are the feeblest parts in the crystalline structures. Thus, the interfaces are readily destroyed through slip deformation by external pressure. The dihedral angle of the AB pair in the single-crystal structure is 71°, which is 7° higher that calculated (64°). The difference between the single-crystal structure and the isolated molecule may suggest that great twist stress exists in the molecule of the crystalline state. Such twist stress is believed to be released when triggered by external pressure, which causes the loss of the long-ranged order of the crystal structure and planarization of molecular conformation. Planarization of the molecular conformation results in an increase of the molecular conjugation and a red shift of the PL spectrum. Although the structural data of the single crystal were obtained at 110 K and the piezofluorochromic experiments were done at room temperature, according to the X-ray diffraction patterns of the single crystal at 110 K and the powder sample at 7610
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’ CONCLUSIONS A piezofluorochromic compound was synthesized. The spectroscopic properties and morphological structures were reversibly and repeatedly exhibited upon pressing (or grinding) or annealing (or fuming). The piezofluorochromic nature is believed to be generated through phase transformation. It is proposed that the twisted conformation of an aggregationinduced emission enhancement compound is the key factor in determining its piezofluorochromic activity, and the common structureproperty relationship will guide researchers in identifying and synthesizing more piezofluorochromic materials. ’ ASSOCIATED CONTENT
bS
Supporting Information. CIF file of the compound and Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected];
[email protected].
’ ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (grant nos. 50773096 and 51073177), the Start-up Fund for Recruiting Professionals from “985 Project” of SYSU, the Science and Technology Planning Project of Guangdong Province, China (grant nos. 2007A010500001-2, 2008B090500196), Construction Project for University-Industry cooperation platform for Flat Panel Display from The Commission of Economy and Informatization of Guangdong Province (grant no. 20081203), the Fundamental Research Funds for the Central Universities, the Opening Fund of Laboratory Sun Yat-sen University (KF201026), and the Fund for Innovative Chemical Experiment and Research of School of Chemistry and Chemical Engineering. ’ REFERENCES (1) Irie, M.; Fukaminato, T.; Sasaki, T.; Tamai, N.; Kawai, T. Nature 2002, 420, 759–760. (2) Papaefstathiou, G. S.; Zhong, Z.; Geng, L.; MacGillivray, L. R. J. Am. Chem. Soc. 2004, 126, 9158–9159. (3) Sagara, Y.; Kato, T. Nat. Chem. 2009, 1, 605–610. (4) Kunzelman, J.; Kinami, M.; Crenshaw, B. R.; Protasiewicz, J. D.; Weder, C. Adv. Mater. 2008, 20, 119–122. (5) Sagara, Y.; Mutai, T.; Yoshikawa, I.; Araki, K. J. Am. Chem. Soc. 2007, 129, 1520–1521. (6) Mutai, T.; Satou, H.; Araki, K. Nat. Mater. 2005, 4, 685–687. (7) Mizukami, S.; Hojou, H.; Sugaya, K.; Koyama, E.; Tokuhisa, H.; Sasaki, T.; Kanesato, M. Chem. Mater. 2005, 17, 50–56. (8) Li, H. Y.; Zhang, X. Q.; Chi, Z. G.; Xu, B. J.; Zhou, W.; Liu, S. W.; Zhang, Y.; Xu, J. R. Org. Lett. 2011, 13, 556–559. (9) Zhang, X. Q.; Chi, Z. G.; Li, H. Y.; Xu, B. J.; Li, X. F.; Zhou, W.; Liu, S. W.; Zhang, Y.; Xu, J. R. Chem. Asian J. 2011, 6, 808–811.
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