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Nanoparticles, Helical Fibers, and Nanoribbons of an Achiral Twin-Tapered Bi-1,3,4-oxadiazole Derivative with Strong Fluorescence Songnan Qu, Lianjiu Zhao, Zhixin Yu, Ziyu Xiu, Chengxiao Zhao, Peng Zhang, Beihong Long, and Min Li* Key Laboratory of Automobile Materials (Jilin UniVersity), Ministry of Education, Institute of Materials Science and Engineering, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed September 12, 2008. ReVised Manuscript ReceiVed December 9, 2008 A twin-tapered bi-1,3,4-oxadiazole derivative (BOXD-T8) showed a monomeric feature and intramolecular charge transition at concentrations lower than 10-5 mol/L. BOXD-T8 molecules self-assembled to nanoparticles and further to helical nanofibers with blue fluorescence emission in DMSO, while nanoribbons resulted in an emission-enhanced gel in ethanol. The strong fluorescent emissions of BOXD-T8 in an isolated state in apolar solvents were attributed to the coplanar conformation of the rigid backbone and the strong fluorescent emissions of BOXD-T8 in the aggregation states were attributed to the coplanar conformation of the rigid backbone and J aggregation.
Introduction The self-assembly of linear π-conjugated molecules to form nanoarchitectures with different morphologies has attracted much interest, because it is important in various fields of nanoscience and technology, particular in the emerging field of supramolecular electronics.1 In this context, the control of the supramolecular organization of linear π-conjugated molecules into helices with nanoscopic dimensions has attracted considerable attention as these helices are candidate materials for use in biological and electro-optical applications.2 Generally, chiral assemblies are often achieved through noncovalent interactions between chiral molecules or between chiral building blocks. However, a few reports have described the formation of an artificial helix from achiral molecules.3 Most π-conjugated molecules show high fluorescence in dilute solutions but become weakly luminescent in the solid state due to the formation of less emissive species such as excimers, which certainly restrict the applications of these molecules in high* To whom correspondence should be addressed. Fax: 86 431 85168444. E-mail:
[email protected]. (1) (a) Schenning, A. P. H. J.; Meijer, E. W. Chem. Commun. 2005, 3245. (b) Meijer, E. W.; Schenning, A. P. H. J. Nature 2002, 419, 353. (c) Auweraer, M. V. D.; De Schryver, F. C. Nat. Mater. 2004, 3, 507. (d) Ajayaghosh, A.; Varghese, R.; Praveen, V. K.; Mahesh, S. Angew. Chem., Int. Ed. 2006, 45, 3261. (e) Ajayaghosh, A.; Varghese, R.; George, S. J.; Vijayakumar, C. Angew. Chem., Int. Ed. 2006, 45, 1141. (f) Ajayaghosh, A.; Vijayakumar, C.; Varghese, R.; George, S. J. Angew. Chem., Int. Ed. 2006, 45, 456. (2) (a) Schoonbeek, F. S.; van Esch, J. H.; Wegewijs, B.; Rep, D. B. A.; de Haas, M. P.; Klapwijk, T. M.; Kellogg, R. M.; Feringa, B. L. Angew. Chem., Int. Ed. 1999, 38, 1393. (b) Wu¨rthner, F.; Thalacker, C.; Sautter, A. AdV. Mater. 1999, 11, 754. (c) Prins, R. B.; Brunsveld, L.; Meijer, E. W.; Moore, J. S. Angew. Chem., Int. Ed. 2000, 39, 228. (d) Wu¨thner, F.; Yao, S.; Beginn, U. Angew. Chem., Int. Ed. 2003, 42, 3247. (3) (a) Sone, E. D.; Zubrarev, E. R.; Stupp, S. I. Angew. Chem., Int. Ed. 2002, 41, 1705. (b) Yang, W.; Chai, X.; Chi, L.; Liu, X.; Cao, Y.; Lu, R.; Jiang, Y.; Tang, X.; Fuchs, H.; Li, T. Chem.sEur. J. 1999, 5, 1144. (c) Siemeling, U.; Scheppelmann, I.; Neumann, B.; Stammel, A.; Stammler, H.-G.; Frelek, J. Chem. Commun. 2003, 2236. (d) Ogata, D.; Shikata, T.; Hanabusa, K. J. Phys. Chem. B 2004, 108, 15503. (e) Shikata, T.; Ogata, D.; Hanabusa, K. J. Phys. Chem. B 2004, 108, 508. (f) Ko¨hler, K.; Fo¨ester, G.; Hauster, A.; Dobner, B.; Heiser, U. F.; Ziethe, F.; Richter, W.; Steiniger, F.; Drechsler, M.; Stettin, H.; Blume, A. J. Am. Chem. Soc. 2004, 126, 16804. (g) Pelzl, G.; Diele, S.; Weissflog, W. AdV. Mater. 1999, 11, 707. (h) Jeong, K.-U.; Knapp, B. S.; Ge, J. J.; Jin, S.; Graham, M. J.; Harris, F. W.; Cheng, S. Z. D. Chem. Mater. 2006, 18, 680. (i) Bao, C.; Lu, R.; Jin, M.; Xue, P.; Tan, C.; Xu, T.; Liu, G.; Zhao, Y. Chem.sEur. J. 2006, 12, 3287. (j) Qu, S.; Wang, H.; Yu, Z.; Bai, B.; Li, M. New J. Chem. 2008, 32, 2023. (k) Yuan, J.; Liu, M. J. Am. Chem. Soc. 2003, 125, 5051. (l) Zhang, L.; Lu, Q.; Liu, H. J. Phys. Chem. B 2003, 107, 2565. (m) Huang, X.; Li, C.; Jiang, S.; Wang, X.; Zhang, B.; Liu, M. J. Am. Chem. Soc. 2004, 126, 1322.
Scheme 1. Molecular Structure of BOXD-T8
density optical systems.4 It is thus highly desirable to develop novel π-conjugated molecules with high solid-state luminescence efficiency. 1,3,4-Oxadiazole derivatives (OXDs) have enjoyed widespread use as electron-transporting/hole-blocking (ECHB) materials, emitting layers in electroluminescent diodes and nonlinear optical materials, due to their electron-deficient nature of the heterocycle, high photoluminescence quantum yield, and good thermal and chemical stabilities of the materials.5 1,3,4Oxadiazole derivatives in solid states (such as crystals6 and liquid crystals7) have been well investigated. However, reports on the nanoarchitectures of 1,3,4-oxadiazole derivatives are very limited.8 Recently, we reported the synthesis and mesomorphic properties of two series of bi-1,3,4-oxadiazole derivatives (2,2′bis(4-alkoxyphenyl)bi-1,3,4-oxadiazole, BOXD-n, n ) 1, 3, 4, 5, 6, 7, 10, 16, and 2,2′-bis(3,4,5-trialkoxyphenyl)bi-1,3,4oxadiazole, BOXD-Tn, n ) 3, 4, 5, 6, 7, 8, 10, 14).7f,h They have been confirmed to give rise to a nematic phase, a smectic C phase with a large tilt angle (θ ) 50°), a highly ordered smectic X phase, and columnar mesophases through tailing either the length (4) (a) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537. (b) Introduction to Organic Light-Emitting Materials and DeVices; Huang, C. H.; Li, F. Y.; Huang, W., Eds.; Fudan University Press: Shanghai, 2005. (5) (a) Schultz, B.; Bruma, M.; Brehmer, L. AdV. Mater. 1997, 9, 601. (b) Thelakkat, M.; Schmidt, H.-W. Polym. AdV. Technol. 1998, 9, 429. (c) Hughes, G.; Bryce, M. R. J. Mater. Chem. 2005, 15, 94. (6) Schulz, B.; Orgzall, I.; Freydank, A.; Xu¨, C. AdV. Colloid Interface Sci. 2005, 116, 143–164, and references therein. (7) (a) Kim, B. G.; Kim, S.; Park, S. Y. Tetrahedron Lett. 2001, 42, 2697. (b) Lai, C. K.; Ke, Y. C.; Su, J. C.; Lu, C. S.; Li, W. R. Liq. Cryst. 2002, 29, 915. (c) Wen, C. R.; Wang, Y. J.; Wang, H. C.; Sheu, H. S.; Lee, G. H.; Lai, C. K. Chem. Mater. 2005, 17, 1646–1654. (d) Zhang, Y. D.; Jespersen, K. G.; Kempe, M.; Kornfield, J. A.; Barlow, S.; Kippelen, B.; Marder, S. R. Langmuir 2003, 19, 6534. (e) Tokuhisa, H.; Era, M.; Tsutsui, T. AdV. Mater. 1998, 10, 404. (f) Qu, S.; Li, M. Tetrahedron 2007, 63, 12429. (g) Seo, J.; Kim, S.; Gihm, S. H.; Park, C. R.; Park, S. Y. J. Mater. Chem. 2007, 17, 5052. (h) Qu, S.; Chen, X.; Shao, X.; Li, F.; Zhang, H.; Wang, H.; Zhang, P.; Yu, Z.; Wu, K.; Wang, Y.; Li, M. J. Mater. Chem. 2008, 18, 3954. (8) Ryu, S. Y.; Kim, S.; Seo, J.; Kim, Y.-W.; Kwon, O.-H.; Jang, D.-J.; Park, S. Y. Chem. Commun. 2004, 70.
10.1021/la802995b CCC: $40.75 2009 American Chemical Society Published on Web 01/06/2009
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Qu et al. Table 1. Photophysical Properties of BOXD-T8 in Various Solvents (1 × 10-5 mol/L) at Room Temperature solvent
λabs (nm)
λem (nm)
Stokes shift (cm-1)
ΦF
cyclohexane benzene chloroform DMF DMSOa ethanol
329 335 338 328 326 329
378-395 421 455 487 499 495
5078.7 6097.8 7607.8 9953.9 10634.8 10193.1
96 × 10-2 77 × 10-2 60 × 10-2 23 × 10-2 11 × 10-2 2 × 10-2
a
BOXD-T8 in DMSO was measured at 35 °C.
Figure 1. PL spectra of BOXD-T8 in different solvents at a concentration of 1 × 10-5 mol/L at room temperature (BOXD-T8 in DMSO was measured at 35 °C).
or the number of terminal alkyl chains and exhibit strong blue fluorescent emissions either in cyclohexane or in the bulk. In this work, we report the interesting properties of BOXD-T8 (Scheme 1): (1) an unprecedented hierarchical self-assembly of BOXDT8 into nanoparticles and helical fibers with blue fluorescence emission in DMSO; (2) a highly luminescent gel from ethanol. To the best of our knowledge, this is the first case of evolution of nanoparticles to helical fibers from an achiral linear π-conjugated molecule.
Experimental Section Characterization. UV-vis absorption spectra were recorded on a Shimazu UV-160 spectrometer, and photoluminescence was measured on a Perkin-Elmer LS 55 spectrometer. The absorption measurements and fluorescence measurements were carried out using a 1 × 1 cm cuvette with a thermistor directly attached to the wall of the cuvette holder for controlling the temperature. The size distribution of the micelles was determined by dynamic light scattering (DLS) with a vertically polarized He-Ne laser (DAWN EOS, Wyatt Technology). The scattering angle was fixed at 90°, and the measurement was carried out at a constant temperature of 25 °C. Transmission electron microscopy (TEM) images were taken with a JEOL2010 microscope, operating at an acceleration voltage of 100 kV. Field emission scanning electron microscopy (FE-SEM) images were taken with a JSM-6700F apparatus. For the observation of nanoparticles, a drop of BOXD-T8 solution was cast onto carboncoated copper grids or silicon plates and the solvent was quickly evaporated under vacuum. Samples for FE-SEM measurement were prepared by wiping a small amount of gel onto a silicon plate followed by evaporating the solvent at ambient temperature. X-ray diffraction (XRD) measurement was carried out with a Bruker Avance D8 X-ray diffractometer.
Figure 2. Temperature-dependent (a) UV-vis absorption spectra and (b) emission spectra of BOXD-T8 at a concentration of 3 × 10-5 mol/L in DMSO (solid line). PL spectra of BOXD-T8 precipitates developed from DMSO (dashed line).
Solvent-Polarity-Dependent Fluorescence Emissions. The optical properties of BOXD-T8 in various solvents (1 × 10-5 mol/L) are shown in Figure 1 and Table 1. BOXD-T8 exhibited strong fluorescence (ΦF ≈ 0.96) with vibronic structure in cyclohexane (1 × 10-5 mol/L).9 The emission maximum redshifted from 395 nm (in cyclohexane) to 495 nm (in ethanol) with an increase of the polarity of the solvent, while the fluorescence quantum yields decreased from 0.96 (in cyclohexane) to 0.02 (in ethanol), and no vibronic structures were observed
in polar solvents. Both the shapes and the maximum of the emissions remained unchanged, whereas the intensity of the emissions was nearly proportional to the concentrations within the range of 10-5-10-7 mol/L, indicating its monomeric feature. Thus, observations of the large red-shifted and decreased fluorescence emissions and the unresolved vibronic structure in the photoluminescence (PL) spectra in polar solvents suggested the formation of intramolecular charge transitions.10 Self-Assembly Behaviors. Fluorescent nanoparticles of BOXDT8 were formed in DMSO at concentrations above 1.0 × 10-5 mol/L, whose sizes are dependent on the concentration and holding time (see the Supporting Information). The self-assembly process of BOXD-T8 in DMSO at 3 × 10-5 mol/L was envisioned by the changes of the UV-vis absorption and emission as a function
(9) Ye, K. Q.; Wang, J.; Sun, H.; Liu, Y.; Mu, Z. C.; Li, F.; Jiang, S. M.; Zhang, J. Y.; Zhang, H. X.; Wang, Y.; Che, C. M. J. Phys. Chem. B 2005, 109, 8008.
(10) Metivier, R.; Amengual, R.; Leray, I.; Michelet, V.; Genet, J.-P. Org. Lett. 2004, 739.
Results and Discussion
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Figure 5. FE-SEM image of BOXD-T8 precipitates developed from DMSO at (a) 3 × 10-5 mol/L and (b) 1 × 10-4 mol/L.
Figure 3. (a) TEM image and (b) size distribution histograms of BOXDT8 nanoparticles in DMSO at 3 × 10-5 mol/L with a 30 min holding time.
of temperature (Figure 2). The main absorption peak at 326 nm decreased and slightly red-shifted, while a new shoulder band at 366 nm intensified, showing level-off tails in the visible spectral region with a decrease of temperature from 40 to 19 °C, which suggested the characteristic J-type aggregation and nanoaggregation formation.11 The process was completely reversible upon heating. BOXD-T8 in DMSO (3 × 10-5 mol/L) exhibited a fluorescence band centered at 500 nm at 40 °C (ΦF ≈ 0.08),
Figure 4. FE-SEM image of BOXD-T8 in DMSO at 3 × 10-5 mol/L with (a) 60 min and (b) 90 min holding times and at 1 × 10-4 mol/L with (c) 60 min and (d) 90 min holding times.
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Figure 7. XRD patterns of BOXD-T8 (a) precipitates developed from DMSO and (b) xerogel from ethanol (0.7 wt %).
Figure 6. (a) Fluorescence emission spectra of BOXD-T8 in ethanol (0.7 wt %) in isotropic solution, gel, and xerogel. (b) FE-SEM image of BOXD-T8 xerogel from 0.7 wt % gel in ethanol.
which shifted to 442 nm with enhanced emission at 19 °C (ΦF ≈ 0.31). The BOXD-T8 nanoparticles in DMSO were not stable, and precipitates formed during storage at room temperature for more than 2 h. Time-dependent DLS (see the Supporting Information), TEM, and SEM were carried out to investigate the peculiar aggregating behaviors. Nanoparticles with dimensions of 20-100 nm developed from BOXD-T8 in DMSO (3 × 10-5 mol/L) at 30 min (Figure 3), which grew to particles with dimensions of 0.5-3 µm at 60 min (Figure 4a) and superhelical fibers coupled with particles with dimensions of 0.5-3 µm at 90 min (Figure 4b). Similar morphologies were observed in BOXD-T8 in DMSO at 1 × 10-4 mol/L (Figure 4c,d). BOXDT8 precipitated after being held in DMSO for more than 2 h. The FE-SEM images of precipitates developed from DMSO displayed both right- and left-handed superhelical fibers with diameters of about 0.2-1 µm (Figure 5). It seemed that most of the superhelical fibers were composed of several thinner twisted fibers with diameters of 0.2-0.4 µm, which intertwined into a helix with nonuniform helical pitches. In some special cases, helical reversal was observed in one fiber. Because both leftand right-handed helical ribbons were present in equal quantities, no CD signals were detected in it. The XRD profiles of BOXD-T8 precipitates developed from DMSO consist of several sharp strong diffractions in both the low-angle range
and high-angle range, indicating its crystalline feature (Figure 7a). The reason for evolution of nanoparticles to helical fibers of achiral BOXD-T8 molecules is still unclear. The BOXD-T8 precipitates developing from DMSO showed blue fluorescence emission (λmax ) 398, 410 nm) with an efficiency ΦF of 28% (ΦF in the solid state is obtained in a calibrated integrating sphere),12 as shown in Figure 2b. Furthermore, BOXD-T8 can gel ethanol. BOXD-T8 was completely dissolved in ethanol when heated to its boiling point. Upon cooling to room temperature, immobile gels formed within 1/2 h at contents above 0.5 wt % (1.4 × 10-2 mol/L), showing enhanced blue-shifted fluorescence emission (about 140-fold) at 424 nm in contrast to its isotropic solution (Figure 6a). The FE-SEM image of BOXD-T8 xerogel from ethanol displayed three-dimensional networks of flat ribbons with widths of 0.10.4 µm (Figure 5b). The XRD profile of the BOXD-T8 xerogel from ethanol consists of a sharp strong diffraction (22.6 Å) and up to fourth-order diffractions, suggesting a layer structure,7f,h which is different from its precipitates developed from DMSO, indicating different packing modes (Figure 7). The layer spacing from XRD is much smaller than the calculated full extended molecular length (about 36 Å), thus BOXD-T8 molecules’ large tilt angle in layers. The fluorescent emission band of BOXD-T8 xerogel from ethanol blue-shifted to 408 nm with an efficiency ΦF up to 40%, which is higher than that of BOXD-T8 precipitates developed from DMSO. From computer calculations and the molecular structure of BOXD-1 in the single-crystal state,7h the rigid backbone of the BOXD-T8 molecule is coplanar and fully conjugated, which might account for the high fluorescence efficiency in its monomeric states in apolar solvent and aggregation states. Considering that molecules of BOXD-T8 are donor-acceptoracceptor-donor (DAAD) type and thus would favor electron donor-acceptor interaction to give rise to slipped stacks, a packing mode for BOXD-Tn xerogel in layers was depicted (11) An, B.-K.; Jung, S.-D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410. (12) Kawamura, Y.; Sasabe, H.; Adachi, C. Jpn. J. Appl. Phys., Part 1 2004, 43, 7729.
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Scheme 2. Packing Mode for BOXD-T8 Xerogel in Layers
in Scheme 2. The enhanced fluorescent emissions of BOXD-T8 aggregation either in DMSO or in ethanol might be attributed to the close aggregated fluorophore parts which prevent their interactions with solvent molecules and thus decrease the rate of solvent relaxation, as well as J-type aggregation.
Conclusion In summary, BOXD-T8 showed a monomeric feature and intramolecular charge transition at concentrations lower than 1 × 10-5 mol/L. BOXD-T8 molecules self-assembled to nanoparticles and further to helical nanofibers with blue fluorescence emission in DMSO, while nanoribbons resulted in an emissionenhanced gel of ethanol. The strong fluorescent emissions of BOXD-T8 in an isolated state in apolar solvents were attributed to the coplanar conformation of the rigid backbone, and the strong fluorescent emissions of BOXD-T8 in the aggregation states were attributed to the coplanar conformation of the rigid backbone and J aggregation. To the best of our knowledge, this
is the first observation of evolution of nanoparticles to helical fibers of an achiral linear π-conjugated molecule. Acknowledgment. We thank Prof. Dr. Yuguang Ma of Jilin University for fluorescence quantum yield measurements. We are grateful to the National Science Foundation Committee of China (Project No. 50873044), Program for New Century Excellent Talents in Universities of the China Ministry of Education, Special Foundation for PhD Program in Universities of the China Ministry of Education (Project No. 20050183057), and Project 985-Automotive Engineering of Jilin University for their financial support of this work. Supporting Information Available: Figures showing the UV-vis absorption spectra, size distribution histograms, temperaturedependent UV--vis absorption and emission spectra, PL spectra, E-SEM and FE-SEM images, CD spectra, and voltammograms of BOXD-T8 and calculated conformation of BOXD-T3. This material is available free of charge via the Internet at http://pubs.acs.org. LA802995B
