Carbazole-Based Cyano-Stilbene Highly Fluorescent Microcrystals

Jan 10, 2011 - Seung-Heon Lee , Min-Jeong Koo , Mojca Jazbinsek , and O-Pil Kwon ... Seung-Jun Lee , Bong Joo Kang , Myeong-Hoon Shin , Seung-Chul ...
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Carbazole-Based Cyano-Stilbene Highly Fluorescent Microcrystals Karasinghe A. N. Upamali,† Leandro A. Estrada,† Puran K. De,† Xichen Cai,† Jeanette A. Krause,‡ and Douglas C. Neckers*,† †

Center for Photochemical Sciences, Department of Chemistry, 132 Overman Hall, Bowling Green State University, Bowling Green, Ohio 43403, United States, and ‡Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221-0172, United States Received May 13, 2010. Revised Manuscript Received December 7, 2010 Aggregation-induced enhanced emission (AIEE) is reported for 1-cyano-trans-1,2-bis-(4-carbazolyl)phenylethylene (CN-CPE). The weak luminescence of dilute CN-CPE solutions is enhanced upon aggregate formation into 2-3 μm sized crystals. In contrast to general observations, crystal formation of CN-CPE causes a blue-shift in emission and enhances the intensity. X-ray cryatallographic analysis revealed that key factors causing high luminescence efficiency in the crystal are a lack of strong cofacial π-π alignment and the existence of the strong supramolecular interactions due to the intermolecular H-bonding. These factors seem to be responsible for the AIEE phenomenon as molecules of CN-CPE are held in a rigid twisted conformation, thereby increasing the fluorescence intensity in the solid or aggregated states. Accordingly, conformational twisting in the crystal packing process may be responsible for the unusual emission blue-shift in the aggregate.

1. Introduction Organic dyes that can assemble into nano- and micrometer-sized crystals have been found useful in varied applications as organic electronics.1-5 While nano- and micrometer-sized crystals often exhibit different physical properties relative to macrocrystalline solids,6 strong fluorescence from such assemblies is uncommon.7 Consequently, organic luminophoric materials face major challenges due to low emission efficiency in the aggregate and solid states.8 Recent work by Tang and co-workers has shown that some dyes exhibit enhanced fluorescence in the aggregate or solid states, in contrast to the common fluorescence quenching. This unusual *To whom correspondence should be addressed. E-mail: neckers@ photo.bgsu.edu. (1) Biological Sensing: (a) Wang, L.; Wang, L.; Xia, T.; Dong, L.; Bian, G.; Chen, H. Anal. Sci. 2004, 20, 1013. (b) Zhou, Y.; Bian, G.; Wang, L.; Dong, L.; Wang, L.; Kan, J. Spectrochim. Acta, Part A 2005, 61, 1841. (2) Photocatalysis: Kim, H. Y.; Bj€orklund, T. G.; Lim, S. H.; Bardeen, C. J. Langmuir 2003, 19, 3941. (3) OLED Materials: (a) Mal’tsev, E. I.; Lypenko, D. A.; Shapiro, B. I.; Brusentseva, M. A.; Berendyaev, V. I.; Kotov, B. V.; Vannikov, A. V. Appl. Phys. Lett. 1998, 73, 3641. (b) Jagannathan, R.; Irvin, G.; Blanton, T.; Jagannathan, S. Adv. Funct. Mater. 2006, 16, 747. (c) Quian, G.; Dai, B.; Luo, M.; Yu, D.; Zhan, J.; Zhang, Z.; Ma, D.; Wang, Z. Y. Chem. Mater. 2008, 20, 6208. (4) Optical Waveguides: (a) Kaneko, Y.; Onodera, T.; Kasai, H.; Okada, S.; Oikawa, H.; Nakanishi, H.; Fukuda, T.; Matsuda, H. J. Mater. Chem. 2005, 15, 253. (b) Yanagi, H.; Ohara, T.; Morikawa, T. Adv. Mater. 2001, 13, 1452. (5) Memory Systems: Lim, S. J.; An, B. K.; Jung, S. D.; Chung, M. A.; Park, S. Y. Angew. Chem., Int. Ed. 2004, 43, 6346. (6) (a) Bucar, D.-K.; MacGillivray, L. R. J. Am. Chem. Soc. 2007, 129, 32. (b) Hamilton, B. D.; Weissbuch, I.; Lahav, M.; Hillimyer, M. A.; Ward, M. D. J. Am. Chem. Soc. 2009, 131, 2588. (7) (a) Wilson, J. N.; Smith, M. D.; Enkelmann, V.; Bunz, U. H. F. Chem. Commun. 2004, 1700. (b) Han, M. R.; Hirayama, Y.; Hara, M. Chem. Mater. 2006, 18, 2784. (c) Xiao, D.; Yang, L.; Xi, W.; Shuai, H.; Fu, Z.; Fang, Y.; Yao, J. J. Am. Chem. Soc. 2003, 125, 6740. (d) Patra, A.; Anthony, S. P.; Radhakrishnan, T. P. Adv. Funct. Mater. 2007, 17, 2077. (e) Abyan, M.; Bertorelle, F.; Fery-Forgues, S. Langmuir 2005, 21, 6030. (8) (a) Yang, J. S.; Yan, J. L. Chem. Commun. 2008, 1501. (b) Lee, Y. T.; Chiang, C. L.; Chen, C. T. Chem. Commun. 2008, 217. (c) Lim, S. H.; Friend, R. H.; Rees, I. D.; Ma, J.; Li, Y.; Robinson, K.; Holmes, A. B.; Hennebicq, E.; Beljonne, D.; Cacialli, F. Adv. Funct. Mater. 2005, 15, 981. (9) (a) Luo, J.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Chen, H.; Qui, C.; Kwok, H. S.; Zhan, X.; Liu, Y.; Zhu, D.; Tang, B. Z. Chem. Commun. 2001, 1740. (b) Dong, Y.; Lam, J. W. Y.; Peng, H.; Cheuk, K. K. L.; Kwok, H. S.; Tang, B. Z. Macromolecules 2004, 37, 6408. (c) Lam, J. W. Y.; Tang, B. Z. Acc. Chem. Res. 2005, 38, 745. (d) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Chem. Commun. 2009, 4332.

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phenomenon was identified as aggregation-induced enhanced emission (AIEE) or aggregation-induced emission (AIE).9 The development of novel compounds showing strong fluorescence as aggregates allows investigation of fundamental mechanisms involved in fluorescence enhancement in the aggregate state. The theories suggested for AIEE are assigned to the restriction of intramolecular rotation, conformational planarization, J-aggregate formation, prevention of exciton diffusion, or a combination of these.8,9 Recently, Park and co-workers reported that highly fluorescent cyano-stilbene derivatives exhibit remarkable luminescence properties.10 Fluorescence enhancement as a result of nanoparticle formation and aggregation into nanowires attributed to J-type stacking combined with molecular planarization in the aggregate form has been reported. In dyes with donor-acceptor units, AIEE is likely to arise from a combination of both restricted intramolecular rotation and intramolecular charge transfer (ICT) or twisted intramolecular charge transfer (TICT) states.11 Even though the molecular structure and the packing arrangement in the crystals are each essential for understanding enhanced emission in the solid state, a correlation between these properties and the AIEE phenomenon has seldom been investigated.12 Herein, we provide a new carbazole-based cyano-stilbene derivative, 1-cyano-trans-1,2-bis-(4-carbazolyl)phenylethylene (CN-CPE) (Figure 1), whose light emission is enhanced by (10) (a) An, B.-K.; Kwon, S. K.; Jung, S. D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410. (b) An, B.-K.; Lee, D. S.; Lee, J. S.; Park, Y. S.; Song, H. S.; Park, S. Y. J. Am. Chem. Soc. 2004, 126, 10232. (c) An, B.-K.; Gihm, S. H.; Chung, J. W.; Park, C. R.; Kwon, S. K.; Park, S. Y. J. Am. Chem. Soc. 2009, 131, 3950. (d) Lim, S. J.; An, B.-K.; Jung, S. D.; Chung, M. A.; Park, S. Y. Angew. Chem., Int. Ed. 2004, 43, 6346. (g) An, B.-K.; Kwon, S. K.; Park, S. Y. Angew. Chem., Int. Ed. 2007, 46, 1978. (e) Chung, J. W.; An, B.-K.; Park, S. Y. Chem. Mater. 2008, 20, 6750. (f) Chung, J. W.; You, Y.; Huh, H. S.; An, B.-K.; Yoon, S. J.; Kim, S. H.; Lee, S. W.; Park, S. Y. J. Am. Chem. Soc. 2009, 131, 8163. (11) (a) Hu, R.; Lager, E.; Aguilar-Aguilar, A.; Liu, J.; Lam, J. W. Y.; Sung, H. H. Y.; Williams, I. D.; Zhong, Y.; Wong, K. S.; Pea-Cabrera, E.; Tang, B. Z. J. Phys. Chem. C 2009, 113, 15845. (b) Gao, B. R.; Wang, H.-Y.; Hao, Y.-W.; Fu, L.-M.; Fang, H.-H.; Jiang, Y.; Wang, L.; Chen, Q.-D.; Xia, H.; Pan, L.-Y.; Ma, Y.-G.; Sun, H.-B. J. Phys. Chem B 2010, 114, 128. (12) (a) Qian, L.; Tong, B.; Shen, J.; Zhi, J.; Dong, Y.; Yang, F.; Dong, Y.; Lam, J. W. Y.; Liu, Y.; Tang, B. Z. J. Phys. Chem. B 2009, 113, 9098. (b) Lamere, J.-F.; Saffon, N.; Dos Santos, I.; Fery-Forgues, S. Langmuir 2010, 26, 10210.

Published on Web 01/10/2011

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Figure 1. (A) Chemical structure of 1-cyano-trans-1,2-bis-(4-carbazolyl)phenylethylene (CN-CPE). (B) Single crystal structure of CN-CPE.

aggregation into microcrystals. Restriction of the ICT or TICT state in the aggregate state may also contribute to the AIEE property of CN-CPE. The emission enhancement with the formation of microcrystals can be further explained by detailed analysis of its crystal structure.

2. Experimental Section 2.1. General. All chemicals and solvents were obtained from commercial suppliers and used without further purification unless otherwise noted. Reactions that required anhydrous conditions were conducted under an inert atmosphere of argon in flame-dried glassware. NMR was acquired in spectrometers with working frequencies of 300 MHz for 1H and 75.5 MHz for 13C experiments with tetramethylsilane (TMS) as the internal standard. UV-visible absorption spectra were recorded using a Shimadzu UV-vis spectrometer. Fluorescence spectra were recorded using a Horiba Jobin-Yvon spectrometer. Thin layer chromatography was carried out on MERCK F250 silica gel 60 M analytical plates with UV detection (λ = 254 and 365 nm). Silica gel (60 A˚, 40-63 μm) was used as the stationary phase for column chromatography. Melting points (uncorrected) were measured using a Thomas-Hoover capillary melting point apparatus. High resolution mass spectra were obtained from the University of Illinois SCS mass spectrometry laboratory. Scanning electron microscopy (SEM) images were recorded on a FEI-FP2031/11 microscope at 15 eV using the INCA penta FETX3 detector. SEM samples were prepared by placing a few drops of the microcrystalline suspension onto a glass coverslip placed on an aluminum stub. The samples were allowed to dry at room temperature before viewing under an electron microscope. To enhance the contrast and quality of the SEM images, the samples prepared were sputter-coated with gold/palladium (the concentration of CN-CPE in final solution was 10 μM). Fluorescence images of the microcrystals were obtained from an Olympus FV 1000 fluorescence microscope using a Macrofire camera. The excitation source was a mercury lamp. The samples were prepared by putting a drop of microcrystalline suspension on a glass slide and covering it with a coverslip. 2.2. Fluorescence Lifetime Measurement. Fluorescence decay curves were recorded in a time-correlated single photon counting spectrofluorimeter. A 370 nm nano-LED was used as the excitation source. Decays were monitored at the corresponding emission maxima of the solution. In-built software allowed the fitting of the decay spectra (χ2 = 1.0-1.5) which yielded the fluorescence lifetime. Decay in the fluorescence intensity (I) with time (t) was fitted by a double-exponential function (eq 1) and the weighted mean lifetime Æτæ was calculated according to eq 2. IðtÞ ¼ A1 expð - t=τ1 Þ þ A2 expð - t=τ2 Þ

ð1Þ

Æτæ ¼ ðA1 τ1 þ A2 τ2 Þ=ðA1 þ A2 Þ

ð2Þ

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2.3. Preparation of Microcrystals. Nanopure distilled water was rapidly injected into a CN-CPE solution in acetone or acetonitrile while the solution was being swirled. Formation of microcrystals, clearly detectable under 365 nm light, proceeds as the volumetric water fraction increases. The concentration of CNCPE in the final solutions was 10 μM. 2.4. Absolute Quantum Yield Measurement. The fluorescence quantum yields of the solution-containing microcrystals were measured using an integrating sphere and a 370 nm laser diode as in previous reports.13 2.5. Preparation of Thin Film. A concentrated dichloromethane (DCM) solution of CN-CPE was casted as thin film on quartz plates and allowed to dry at room temperature. 2.6. Crystallographic Data for CN-CPE. For X-ray examination and data collection, a pale yellow needle, approximate dimensions 0.170  0.020  0.005 mm3, was mounted in a loop with paratone-N and transferred immediately to the goniostat bathed in a cold stream. Intensity data were collected at 150 K on a Bruker APEX2 CCD detector at Beamline 11.3.1 at the Advanced Light Source (Lawrence Berkeley National Laboratory) using synchrotron radiation tuned to λ = 0.77490 A˚. For data collection, frames were measured for a duration of 6 s at 0.3 intervals of ω with a maximum 2θ value of ∼60. The data frames were collected using the program APEX2 and processed using the program SAINT routine within APEX2. The data were corrected for absorption and beam corrections based on the multiscan technique as implemented in SADABS. The structure was solved by a combination of direct methods SHELXTL v6.14 and the difference Fourier technique and refined by full-matrix least-squares on F2. Non-hydrogen atoms were refined with anisotropic displacement parameters. H-atom positions were calculated and treated with a riding model in subsequent refinements. The isotropic displacement parameters for the H-atoms were defined as 1.2 Ueq of the adjacent atom. The refinement converged with crystallographic agreement factors of R1 = 4.82%, wR2 = 10.82% for 3155 reflections with I > 2σ(I) (R = 10.61%, wR2 = 13.25% for all data) and 380 variable parameters. Crystal data for CN-CPE: C39H25N3. M = 535.62, monoclinic, C2/c, a = 36.027(8) A˚, b = 8.1258(17) A˚, c = 18.646(4) A˚, R = γ = 90, β = 90.241(6), V = 5459(2) A˚3, Z = 8, T = 150(2) K. 2.7. Synthesis. 1-Cyano-trans-1,2-bis-(40 -bromophenyl)ethylene (CN-BPE). 4-Bromobenzaldehyde (2.0 g, 10 mmol),

4-bromophenylacetonitrile (2.5 g, 10 mmol), and methanol were added in a dry round-bottom flask. Sodium methoxide (1.5 mL, 25% wt) was added dropwise with vigorous stirring. After the addition, the mixture turned cloudy and the suspension was stirred for another 2 h. Upon cooling in an ice/water bath, the newly formed white precipitate was separated by filtration. (13) (a) Pa˚lsson, L.; Monkman, A. P. Adv. Mater. 2002, 14, 757. (b) PerezBolívar, C.; Llovera, L.; Lopez, S. E.; Anzenbacher, P., Jr. J. Lumin. 2010, 130, 145.

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The crude product was recrystallized from ethanol (yield 80% of CN-BPE). Mp: 120-122 C. 1H NMR (300 MHz, CDCl3, δ in ppm): 7.85 (d, 2H), 7.63 (m, 6H), 7.45 (s, 1H). 13C NMR (300 MHz, CDCl3, δ in ppm): 111.4, 117.3, 123.8, 125.2, 127.5, 130.7, 132.4, 133.2, 141.1. HRMS(EI) m/z [M]þ calcd for C15H9NBr2, 360.9102; found, 360.9101.

1-Cyano-trans-1,2-bis-(4-carbazolyl)phenylethylene (CNCPE). A dry round-bottom flask was charged with CN-BPE (500 mg, 1.37 mmol), carbazole (503 mg, 3.01 mmol), Pd(OAc)2 (9.0 mg, 0.04 mmol), P(t-Bu)3 (1.94 mg, 0.01 mmol), Cs2CO3 (1.89 g, 5.34 mmol), and dry toluene (60 mL). The mixture was then refluxed under nitrogen until the reaction was completed (30 h). On completion, the reaction mixture was cooled to room temperature and filtered to get a crude solid. The crude product was recrystallized from the hexane-DCM mixture (1:1) to obtain the yellow colored product in pure form (yield 55%), of CN-CPE. Mp: >230 C. 1H NMR (300 MHz, CDCl3, δ in ppm): 8.23 (m, 6H), 8.00 (d, 2H), 7.75 (m, 5H), 7.55 (m, 8H), 7.35 (t, 4H). 13C NMR (300 MHz, CDCl3, δ in ppm) 109.6, 109.7, 109.8, 120.5, 120.6, 123.7, 123.8, 126.2, 126.7, 127.1, 127.5, 127.6, 130.6, 131.0, 131.4, 140.4. HRMS(EI) m/z [M]þ calcd for C39H25N3, 535.2049; found, 535.2052. 2.8. DFT Calculations. Computations at the density functional theory (DFT) level were carried out using Becke’s threeparameter functional14 hybridized with the Lee-Yang-Parr correlation functional15 (B3LYP). The 6-31G* split-valence basis set was selected for all DFT computations.16 TDDFT level of theory was used to calculate the vertical transitions of the first five states of CN-CPE in the gas phase.17 The Gaussian03 program package was used to perform all the DFT calculations.18

3. Results 3.1. Synthesis. CN-BPE was synthesized by means of Knoevenagel condensation of commercially available 4-bromobenzaldehyde with 4-bromophenylacetonitrile (Scheme 1). The product (14) Becke, A., D. J. Chem. Phys. 1993, 98, 5648. (15) (a) Lee, C.; Yang, W.; Parr, R., G. Phys. Rev. B 1988, 37, 785. (b) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989, 157, 200. (16) Davidson, E. R.; Feller, D. Chem. Rev. 1986, 86, 681. (17) (a) Stratmann, R., E.; Scuseria, G., E.; Frisch, M., J. J. Chem. Phys. 1998, 109, 8218. (b) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett. 1996, 256, 454. (c) Casida, M., E.; Jamosrski, C.; Casida, K., C.; Salahub, D., R. J. Chem. Phys. 1998, 108, 4439. (18) Frisch, M. J. et al. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. See full reference in the Supporting Information.

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Table 1. Photophysical Properties of CN-CPE solvents

λabs max (nm)

λem max (nm)a

ΦFb

carbon tetrachloride 374 445, 465 0.060 toluene 369 453, 470 0.050 chloroform 367 480 0.042 tetrahydrofuran 365 483 0.055 dichloromethane 365 497 0.020 acetone 358 518 0.036 dimethylformamide 359 529 0.028 acetonitrile 356 537 0.021 a λexc = 360 nm. b The ΦF values in solution were measured following a general method using 9,10-diphenylanthracene (ΦF = 0.84 in dichloromethane) as the standard.20

was further functionalized with carbazole via palladium-catalyzed amination as previously described.19 The products were characterized spectroscopically, and detailed crystallographic information was obtained (vide infra). 3.2. Optical Properties and Solvent Effects. CN-CPE is stable and soluble in common organic solvents such as carbon tetrachloride (CCl4), dichloromethane (DCM), acetone, and acetonitrile. The photophysical properties of CN-CPE in solvents of varying polarity are summarized in Table 1. Upon changing solvent from nonpolar CCl4 to increasingly more polar solvents (chloroform, acetone, and acetonitrile), a negative solvatochromism in the π-π* absorption of the molecular structure is observed (Figure 2A). Contrarily, the emission spectra of CN-CPE exhibits positive solvatochromism with increasing solvent polarity (Figure 2B) while the quantum yield of emission decreases (Table 1). The large bathochromic shift combined with the decrease in quantum yield might be due to stabilization of the excited state (ES) upon increasing the solvent polarity. Such solvatochromic behavior could be well explained by the ICT or TICT mechanism, since CN-CPE is composed of donor and acceptor units. The solvent dependency of the CN-CPE emission, described as the plotting of the Stokes shift against orientational polarizability (viz. Lippert-Mataga plot), is depicted in Figure S3 in the (19) Palayangoda, S. S.; Cai, X.; Adhikari, R. M.; Neckers, D. C. Org. Lett. 2008, 10, 281. (20) Adhikari, R. M.; Shah, B. K.; Neckers, D. C. J. Org. Chem. 2007, 72, 4727.

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Figure 2. (A) Normalized absorption spectra of CN-CPE in solvents of varying polarity. (B) Normalized emission spectra of CN-CPE in solvents of varying polarity (λexc = 360 nm).

Figure 3. (A) PL spectra of CN-CPE in acetone-water mixtures; concentration = 10 μM, λexc = 360 nm. (B) Variation in the integrated PL intensity of CN-CPE with increasing water fractions in acetone.

Supporting Information.21The Kirkwood-Onsager parameter, Δf, is calculated via eq 3.22 The nonlinearity of this LippertMataga plot indicates specific solvent effects (chlorinated solvents are known for this) and the existence of multiple ESs (i.e., nonrelaxed and relaxed ICT).23 Δf ¼ ðε - 1Þ=ð2ε þ 1Þ - ðn2 - 1Þ=ð2n2 þ 1Þ

ð3Þ

3.3. Aggregation-Induced Emission Enhancement. To investigate AIEE in CN-CPE, photoluminescence (PL) spectra were obtained in acetone and in acetone-water mixtures (Figure 3A). Upon photoexcitation at 360 nm, a solution (10 μM) of CN-CPE in acetone emits light at 518 nm. With the addition of water (up to 40% v/v), the emission profile is red-shifted and the intensity is reduced due to the increased solvent polarity. However, the emission intensity starts to increase after 50% v/v water, achieving an intensity maximum at a water content of 60% v/v under identical concentration and measurement conditions (Figure 3A). This effect is depicted as well in Figure 3B as the plot of the ratio I/I0 versus the % v/v of water content. Quantification of the ratio I/I0 reveals that when the water fraction is increased to 60% v/v, the (21) Mataga, N.; Kaifu, Y.; Koizumi, M. Bull. Chem. Soc. Jpn. 1956, 29, 465. (22) Lackowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.; Springer: New York, 2006. (23) Adhikari, R. M.; Neckers, D. C. J. Phys. Chem. A 2009, 113, 417.

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fluorescence intensity is ca. 5-fold higher than that in pure acetone solution (I0). The emission intensity decreases with higher percentages of water volume (>60% v/v) in the acetone-water mixture. To investigate the AIEE property of CN-CPE in different solvent PL spectra were also obtained in acetonitrile-water mixture (Figure 4). Interestingly, the fluorescence enhancement was further increased in acetonitrile-water (10-fold) than in acetonewater mixtures (5-fold). Upon photoexcitation at 360 nm, a solution (10 μM) of CN-CPE in acetonitrile emits light at 537 nm. With the addition of water (up to 60% v/v), the emission profile redshifts while reducing its intensity. However, the emission intensity abruptly increases after addition of >60% v/v water, achieving an intensity maximum at a water content of ca. 70% v/v under identical concentration and measurement conditions. The AIEE effect can be quantified by the extent of emission enhancement (RAIEE), as defined by the following equation: RAIEE ¼ ΦFðaggrÞ =ΦFðsolnÞ

ð4Þ

where ΦF(aggr) and ΦF(soln) are the quantum yields in the aggregate and solution states, respectively.24 Absolute quantum yields of the solution were measured in order to obtain the enhancement factor. Such measurement of CN-CPE, initially yielding 3.1% in (24) Zhao, Z.; Chen, S.; Shen, X.; Mahtab, F.; Yu, Y.; Lu, P.; Lam, J. W. Y.; Kwok, H. S.; Tang, B. Z. Chem. Commun. 2010, 686.

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Figure 4. (A) PL spectra of CN-CPE in acetonitrile-water mixtures; concentration = 10 μM, λexc = 360 nm. (B) Variation in the integrated PL intensity of CN-CPE with increasing water fractions in acetonitrile.

Figure 5. (A) Picture of CN-CPE taken in an acetone-water (40:60 v/v) mixture (left) and in pure acetone (right) under 365 nm UV light

(concentration of CN-CPE = 10 μM). (B) SEM image of CN-CPE in acetone-water (40:60 v/v) mixture. (C) Fluorescence microscopy image of microcrystals of CN-CPE in acetone-water (40:60 v/v) mixture.

acetone, increases to 20% in the aggregate state (60% v/v water). This implies that RAIEE = 7 for this system. However, the absolute quantum yield of CN-CPE in acetonitrile is increased from 2.1% in solution to 33% as aggregate (70% v/v water), giving RAIEE = 16. This comparison further confirms the polaritydependent characteristics of the AIEE property of CN-CPE. Emission enhancement can be attributed to the formation of aggregates in the acetone-water mixtures containing >40% v/v water, where water acts as a nonsolvent for CN-CPE and induces aggregation. Molecules aggregate into 2-3 μm sized, orthorhombicshaped crystals, as confirmed from their SEM image (Figure 5B). Aside from fluorescence enhancement, an apparent blue-shift in the emission maxima is observed for both acetone-water and acetonitrile-water mixtures. The emission maximum of CN-CPE shifts 23 nm hypsochromically from 100% acetone to 40:60 acetone-water mixtures (from 518 to 495 nm for 0 to 60% v/v water content, respectively). In the case of acetonitrile-water mixtures, the emission maximum blue-shift is more evident (41 nm) than the acetone-water mixture (from 537 to 496 nm for 0 to 70% v/v water content, respectively). Is spite of the difference in solvent polarity, the microcrystal suspension shows the same emission maximum in both cases (495-496 nm). However, the decreasing emission intensity with water content beyond 60% v/v could be attributed to the CN-CPE molecules agglomerating to form less crystalline aggregates in solvent mixtures with high water content (Supporting Information Figure S4).12a The absorption is red-shifted with the aggregation (Supporting Information Figure S2). 3.4. Crystal Structure of CN-CPE. To gain more insight about the AIEE of the CN-CPE, X-ray quality crystals were Langmuir 2011, 27(5), 1573–1580

grown in a 1:1 dichloromethane-hexane solution and the single crystal structure was obtained. Solid-state photophysical properties such as fluorescence emission of a luminophore are closely related to molecular stacking.25 Therefore, both the fluorescent enhancement and the PL blue-shift can be interpreted by detailed studies of the crystal structure. The crystallographically determined structure of CN-CPE is shown in Figure 1B. The interesting feature of the molecule is the intramolecular twist conformation in the solid state. While the carbazole groups are nearly planar (deviations from planarity of 0.025 and 0.039 A˚), the backbone of the molecule is twisted with respect to the stilbene (dihedral angles of 54.29(6) and 54.44(6) for carbazole N1-C12 and phenyl C13-C18 and carbazole N2-C39 and phenyl C22-C27, respectively). In addition, the phenyl rings form a dihedral angle of 36.00(8). A notable feature of the crystal packing (Figure 6) is the relatively strong C-H 3 3 3 N interaction between the nitrile group of one CN-CPE and a neighboring carbazole (C11 3 3 3 N2 = 3.457(3) A˚, H 3 3 3 N2 = 2.56 A˚, C11-H11 3 3 3 N2 = 158) forming discrete dimeric pairs (Figure 6B). Fluorescence quenching of most chromophores in the solid state is mainly due to strong intramolecular π-π stacking interactions.25 The large deviation from coplanarity within the CN-CPE molecule does not allow the typical cofacial π-π stacking. As a result of this intermolecular H-bonding, molecules (25) (a) Yu, G.; Yin, S.; Liu, Y.; Chen, J.; Xu, X.; Sun, X.; Ma, D.; Zhan, X.; Peng, Q.; Shuai, Z.; Tang, B. Z.; Zhu, D.; Fang, W.; Luo, Y. J. Am. Chem. Soc. 2005, 127, 6335. (b) Tang, B. Z.; Geng, Y.; Lam, J. W. Y.; Li, B.; Jing, X.; Wang, X.; Wang, F.; Pakhomov, A. B.; Zhang, X. Chem. Mater. 1999, 11, 1581. (c) Jenekhe, S. A.; Osaheni, J. A. Science 1994, 265, 765. (d) Li, Y.; Li, F.; Zhang, H.; Xie, Z.; Xie, W.; Xu, H.; Li, B.; Shen, F.; Ye, L.; Hanif, M.; Ma, D.; Ma, Y.-G. Chem. Commun. 2007, 231.

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Figure 6. Molecular packing diagram showing hydrogen-nitrogen interactions (A) and crystal packing arrangement of CN-CPE in the ac plane (B).

of CN-CPE are held in a rigid intramolecular twisted conformation thereby increasing the fluorescence intensity in the solid or aggregated states. The noticeably enhanced fluorescence quantum yield in the solid state (thin film) of the CN-CPE (0.82) was determined using the integrated sphere method.20 Accordingly, conformational twisting in the crystal packing process (reduced conjugation) may be responsible for the unusual blue-shift.26 3.5. Fluorescence Decay Dynamics. Time resolved fluorescence spectra of CN-CPE were measured in acetone-water mixtures and acetonitrile-water mixtures to understand the dynamics of the ES decay process. The emission enhancement of CN-CPE in acetone-water and acetonitrile-water mixtures is accompanied by a corresponding change in lifetime (Table 2). The decay curves of CN-CPE in acetone-water mixtures and acetonitrile-water mixtures are best fitted by a double-exponential function. The ES decays via fast and slow channels. With an increase in the water content in the solvent mixture, the decay via the fast channel is slowed down and the decaying via the slow channel is populated (Figure 7). 3.6. DFT Calculations. The computationally optimized structure of CN-CPE (Supporting Information Figure S6) presented similar features to those obtained from the crystal structure with the sole difference that the carbazole groups are far from being periplanar (71.3 between Cz-planes). Carbazole groups are twisted from the stilbene backbone (∼50-54 dihedral angles between Cz and Ph for both cases) and both phenyls are twisted from each other (51 dihedral angle between phenyls). The structural features presented in the DFT calculations suggest that the crystalline packing involves planarization of the carbazole units. This planarization helps with accommodating the molecules in a tight pocket wherein H-bonding between the aforementioned discrete dimeric pairs is favored. While this should bring consequences in the optical properties of the bulk, it negligibly affects the angles between donor and acceptor units. Consequently, it is expected that the orbital distribution be similar among these two structures in the crystal. The spatial distribution of the frontier orbitals (HOMO and LUMO) of CN-CPE is depicted in Figure 8. As expected, the HOMO is localized in the carbazole donors whereas LUMO is localized in the central cyanostilbene core. TDDFT calculations revealed that the first four vertical (26) (a) Li, Z.; Dong, Y.; Mi, B.; Tang, Y.; Haeussler, M.; Tong, H.; Dong, Y.; Lam, J. W. Y.; Ren, Y.; Sung, H. H. Y.; Wong, K. S.; Gao, P.; Williams, I. D.; Kwok, H. S.; Tang, B. Z. J. Phys. Chem. B 2005, 109, 10061. (b) Chen, J.; Law, C. C. W.; Lam, J. W, Y.; Dong, Y.; Lo, S. M. F.; Williams, I. D.; Zhu, D.; Tang, B. Z. Chem. Mater. 2003, 15, 1535.

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Table 2. CN-CPE Fluorescence Lifetimes of Aggregates in Different Solvent Mixtures solvents

A1 (%)

A2 (%)

τ1 (ns)

τ2 (ns)

Æτæ

acetone-water (10:0) acetone-water (9:1) acetone-water (4:6) acetonitrile-water (10:0) acetonitrile-water (8:2) acetonitrile-water (3:7)

80.48 67.60 56.51 93.68 97.12 51.79

19.52 32.40 43.49 6.32 2.88 48.21

0.24 0.62 1.70 0.32 0.32 1.61

5.84 2.47 3.69 2.98 2.96 5.02

1.33 1.22 2.57 0.49 0.40 3.25

transitions (HOMO f LUMO, HOMO-1 f LUMO, HOMO-2 f LUMO, and HOMO-3 f LUMO) involve HOMOs localized in the carbazole units (Supporting Information Figure S7). This is consistent with the charge transfer character of the optical transitions as evidenced by the previously described experiments.

4. Discussion The facile synthetic accessibility plus the notable optical properties make CN-CPE an interesting probe for AIEE. Among the important optical features in solution, we found a negative solvatochromism in the UV-vis absorption spectra and a positive solvatochromism in the PL spectra. The first feature is indicative of a stabilized ground state (GS) energy upon increasing solvent polarity as all Franck-Condon lines are solvent insensitive given the lack of time for reorientation of solvent dipoles to occur.27 This implies a remarkably polar character of the GS. As time progresses after photoexcitation (0.1 ps to 1 ns), CN-CPE experiences internal reorganization whereupon alignment of the solvent dipoles ensues. This implies lowering of the zero vibrational energy level of the S1 state upon increasing solvent polarity and thus a smaller emission band gap, consistent with a charge-transfer character of the ES. Indeed, analysis of the Lippert-Mataga plot reveals a difference between dipole moments of Δμ = 36 D, assuming an Onsager radius of 9.5 A˚ based on our DFT calculations (notwithstanding specific effects from the used chlorinated solvents). Such level of theory predicts a GS dipole moment of μGS = 7.2 D, thus implying μES = 43 D. These values are characteristic of ICT transitions from other triarylamine-based donor-acceptor derivatives.28 The orbital mapping computed at the B3LYP/6-31G* level is also consistent with this picture (vide supra). Assuming that the DFT-optimized geometry corresponds to the minimum energy structure in solution, the conformational (27) Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Principles of Molecular Photochemistry: An Introduction; University Science Books: Sausalito, CA, 2009. (28) Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Chem. Rev. 2003, 103, 3899.

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Figure 7. Fluorescence decay curves of CN-CPE in acetone-water mixtures (A) and in acetonitrile-water mixtures (B).

Figure 8. Calculated spatial distributions of frontier MOs of CN-CPE at the B3LYP/6-31G* level (isodensity value = 0.03).

twisting observed in the X-ray structure clearly correlates with the blue-shifted emission spectra from the aggregates compared to those from the solution phase. The lack of structural planarity of CN-CPE is not compatible with the formation of H-aggregates with face-to-face intermolecular association. Furthermore, the bulky nitrile group prevents parallel π-π intermolecular stacking.10 The structural planarization upon crystallization is suggested to be responsible for the emission enhancement with red-shifting absorbance for the case of 1-cyano-trans-1,2-bis-(40 methylbiphenyl)-ethylene, CN-MBE, nanoparticles.10 Contrary to this case, the carbazole and stilbene units from CN-CPE still remain in twisted conformation even after crystallization. Therefore, in the case of CN-CPE, structural rigidification instead of planarization seems to be responsible for the enhanced emission. The size and shape of the aggregates was similar from acetoneto acetonitrile-based solutions (compare Figure 5B and Supporting Information Figure S5). This observation highlights that the higher absolute quantum yield value found in acetonitrile-based solutions might be due to a more favored process of microcrystal formation in such a system than for the acetone-based solutions. The solvent polarity dependent characteristic of CN-CPE emission has been linked to a possible involvement of another operating mechanism in the ES deactivation dynamics other than the restriction of the nonradiative torsional/vibrational relaxation channel.11b If the AIEE of CN-CPE is only because of restriction of nonradiative relaxation channel, it should be independent of the solvent polarity. Our steady-state fluorescence results in diverse solvents combined with the emission enhancement dependent on polarity suggest that ICT is inherent of the S1 potential energy surface (PES) of CN-CPE regardless of the state, as solution or aggregate. The results of our DFT calculations are also in line with this as the orbital mapping of CN-CPE was performed in vacuum (gas phase). The narrower spectral features of the aggregate state in comparison with those from solution Langmuir 2011, 27(5), 1573–1580

state are linked to the allowed vibronic transitions between excited and ground states, as stated by the Franck-Condon principle, and give an idea about the steepness of both GS and ES potential energy surfaces (PESs).26 Indeed, a narrower fluorescence spectrum indicates a lower number of allowed vibronic transitions in line with a conformational restricted assembly. The difference we find here is that the conical intersection (CI) between ES and GS PESs might have a higher energy than that of the minimum well of the S1 PES and, consequently, such geometry is reachable in solution but not in the conformational restricted aggregate state.29 This should favor luminescence over a nonradiative decay mechanism. Further investigation is underway to understand these alternative mechanisms involved in the microcrystal formation and the AIEE phenomenon for this compound, as they are of primary importance for the establishing of proper design parameters that help improving the solid state emission quantum yield and other bulk properties for the next generation of compounds.

5. Conclusions In this work, we designed and synthesized a new type of AIEE active compound which can assemble into microcrystals and enhance the emission with a blue-shift. Restriction of the nonradiative decay channel of the ICT or TICT states of CN-CPE was found as a primordial contributor toward the AIEE phenomenon. Such emission enhancement in the aggregate state is due to the lack of strong π-π molecular stacking in the crystal lattice, impeded by intermolecular H-bonding interactions between the discrete dimers of CN-CPE, which might be hampering exciton diffusion through the molecular assembly. Conformational twist-

(29) Sanchez-Galvez, A.; Hunt, P.; Robb, M. A.; Olivucci, M.; Vreven, T.; Schlegel, H. B. J. Am. Chem. Soc. 2000, 122, 2911.

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ing in the crystal packing seems to be responsible for the unusual blue-shift in the emission maximum. In solution, this novel AIEE active compound shows emission enhancement with UV-irradiation. Moreover, CN-CPE microcrystals function as a highly selective chemosensor for the detection of Cr(VI) in aqueous solution through fluorescence quenching. These phenomena will be discussed separately in due course. Acknowledgment. This work was supported by the Endowment Fund for Photochemical Sciences. We thank the donors of this fund. K.U thanks Dr. R. M. Wilson, Dr. T. H. Kinstle, and Mr. G. K. Kole for helpful discussions. L.A.E thanks the McMaster Endowment for a Fellowship and Prof. Massimo Olivucci for granting access to computational tools. The following

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are acknowledged for technical assistance: Dr. Fengyu Li (FM), Ms. Selin Ergun, and Dr. Cesar Perez. DFT calculations were possible due to the generous allotted time by the Ohio Supercomputer Center. Crystallographic data were collected at Beamline 11.3.1 at the Advanced Light Source (ALS). The ALS (Lawrence Berkeley National Laboratory) is supported by the U.S. Department of Energy, Office of Energy Sciences Materials Sciences Division, under Contract DE-AC02-05CH11231. Supporting Information Available: Absorption spectra of CN-CPE in acetone-water and acetonitrile-water mixtures. NMR spectra and crystallographic data (in CIF format) of CN-CPE. SEM images. DFT calculation details. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2011, 27(5), 1573–1580