Organic Styryl Dye Nanoparticles: Synthesis and Unique

Dec 16, 2008 - The observed fluorescence would come from an intramolecular charge-transfer (ICT) excited state stabilized by the matrix of TPB or TFPB...
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Langmuir 2009, 25, 1131-1137

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Organic Styryl Dye Nanoparticles: Synthesis and Unique Spectroscopic Properties Hiroshi Yao,* Minami Yamashita, and Keisaku Kimura Graduate School of Material Science, UniVersity of Hyogo, 3-2-1 Koto, Kamigori-cho, Ako-gun, Hyogo, 678-1297 Japan ReceiVed September 3, 2008. ReVised Manuscript ReceiVed October 31, 2008 We report the synthesis and unique spectroscopic properties of organic styryl dye nanoparticles. Aqueous-phase ion association between a cationic styryl dye 2-(4-(dimethylamino)styryl)-1-ethylpyridinium (DASPE), possessing both electron donor and acceptor groups in its molecule, and tetraphenylborate (TPB) or tetrakis(4-fluorophenyl)borate (TFPB) anion, in the presence of poly(vinylpyrrolidone), produces the ion-based dye (DASPE) nanoparticles of ∼30-100 nm in diameter. Absorption spectra of the DASPE nanoparticles show a large bathochromic shift in comparison with that of the dye monomer in water. Quantum chemical calculations demonstrate that ion-pair formation brings about a large internal rotation around a single bond in DASPE, and this internal twisting as well as local polarity of the counteranion have a strong influence on the red shift of the optical spectra. Furthermore, nanoparticle formation results in enhanced fluorescence of DASPE: more than a 20-fold enhancement in the fluorescence quantum yield as compared to that of the dye monomer in water, giving a new methodology for the synthesis of fluorescent organic nanoparticles. The observed fluorescence would come from an intramolecular charge-transfer (ICT) excited state stabilized by the matrix of TPB or TFPB, and the enhancement is due to both the high rotational resistance for the single bond in DASPE and the matrix polarity effect that can suppress the nonradiative processes.

Introduction The successful development of molecular devices for applications in photochemical energy conversion/storage and optoelectronic systems requires fundamental research of photofunctional organic materials.1 Recent research entails photophysical/photochemical investigations of noncovalent organic assemblies or aggregates on nanometer scales because their photofunctionality significantly differs from that of the monomers,2 presenting processing and stability challenges in the production of organic nanomaterials. In particular, fluorescent organic nanomaterials or nanoparticles have inspired growing research interest as a result of their large diversity in molecular structure and optical properties that are of potential use in, for example, sold-state light-emitting diodes, organic lasers, and sensors.3,4 For constructing fluorescent organic nanoparticles, the design of highly emissive “bulk” materials that can fluoresce even in the solid state is an important requirement. Whereas a number of molecules are known to be highly fluorescent in dilute solution, most of them tend to show a decreased fluorescence in the solid state because of certain intermolecular interactions that result in significant quenching of the emission.5 A problem that has to be overcome to achieve an intense solid-state emission is how to suppress the self-quenching or nonradiative processes. * To whom correspondence should be addressed. Tel.: +81-791-58-0160. Fax: +81-791-58-0161. E-mail: [email protected]. (1) (a) Aviram, A.; Ratner, M. A. Chem. Phys. Lett. 1974, 29, 277. (b) Keyes, R. W. Phys. Today 1992, 45, 42. (2) (a) Belanger, S. S.; Hupp, J. T. Angew. Chem., Int. Ed. 1999, 38, 2222. (b) Stang, P. J.; Olenyuk, B. Acc. Chem. Res. 1997, 30, 502. (c) Rotomskis, R.; Augulis, R.; Snitka, V.; Valiokas, R.; Liedberg, B. J. Phys. Chem. B 2004, 108, 2833. (d) Marks, T. J. Science 1985, 227, 881. (3) (a) Chen, C.-T.; Chiang, C.-L.; Lin, Y.-C.; Chan, L.-H.; Huang, C.-H.; Tsai, Z.-W.; Chen, C.-T. Org. Lett. 2003, 5, 1261. (b) Kim, Y.; Bouffard, J.; Kooi, S. E.; Swager, T. M. J. Am. Chem. Soc. 2005, 127, 13726. (c) Langhals, H.; Krotz, O.; Polborn, K.; Mayer, P. Angew. Chem., Int. Ed. 2005, 44, 2427. (4) (a) Horn, D.; Rieger, J. Angew. Chem., Int. Ed. 2001, 40, 4330. (b) An, B.-K.; Kwon, S.-K.; Park, S. Y. Angew. Chem., Int. Ed. 2007, 46, 1978. (5) Friend, R. H.; Gymer, R. W.; Holms, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bre´das, J. L.; Lo¨gdlund, M.; Salaneck, W. R. Nature 1999, 397, 121.

So far, there are only a few convenient methods for controlling fluorescence properties in the solid state:6 For example, oxidized anthracene derivatives exhibit solid-state fluorescence enhancement depending on inclusion of toluene molecules.6a The aggregate-induced enhanced emission from natively emissive molecules, such as siloles, biphenylethylene, tetraphenylbutadiene, and fluorenones, is also found.7 Ionic styryl dyes such as 4-(dimethylamino)styryl-1-alkylpyridinium have attracted widespread interest because they can be used as excellent molecular fluorescence probes.8,9 Fluorescence properties of these dyes are characterized by molecular internal twisting motion; that is, internal rotation of at least one single bond of the molecule is the main route for nonradiative deactivation. Photoexcitation of the dyes induces the internal twisting motion of single bonds that lead to the twisted intramolecular charge-transfer (TICT) states without emission, resulting in a very small fluorescence quantum yield in fluid media.9 The fluorescence quantum yield thus strongly depends on the solvent nature such as viscosity and polarity,10 so that the increase in viscosity around the probe molecule toward immobilization can suppress this nonradiative process by frictional resistance and can increase its fluorescence. This emission (6) (a) Fei, Z.; Kocher, N.; Mohrschladt, C. J.; Ihmels, H.; Stalke, D. Angew. Chem., Int. Ed. 2003, 42, 783. (b) Yoshida, K.; Ooyama, Y.; Miyazaki, H.; Watanabe, S. J. Chem. Soc., Perkin Trans. 2 2001, 708. (7) (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) An, B.-K.; Kwon, S.-K.; Jung, S.-D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 144410. (c) Chen, J.; Xu, B.; Ouyang, X.; Tang, B. Z.; Cao, Y. J. Phys. Chem. A 2004, 108, 7522. (d) Liu, Y.; Tao, X.; Wang, F.; Shi, J.; Sun, J.; Yu, W.; Ren, Y.; Zou, D.; Jiang, M. J. Phys. Chem. C 2007, 111, 6544. (8) (a) Mujumdar, S. R.; Mujumbar, R. B.; Grant, C. M.; Waggoner, A. S. Bioconjugate Chem. 1996, 7, 356. (b) Mishra, A.; Behera, R. K.; Behera, P. K.; Mishra, B. K.; Behera, G. B. Chem. ReV. 2000, 100, 1973. (9) (a) Cao, X.; Tolbert, R. W.; MaHale, J. L.; Edwards, W. D. J. Phys. Chem. A 1998, 102, 2739. (b) Strehmel, B.; Seifert, H.; Rettig, W. J. Phys. Chem. B 1997, 101, 2232. (c) Sczepan, M.; Rettig, W.; Tolmachev, A. I.; Kurdyukov, V. V. Phys. Chem. Chem. Phys. 2001, 3, 3555. (d) Ramadass, R.; Ju¨rgen-Hahn, J. J. Phys. Chem. B 2007, 111, 7681. (e) Hachisako, H.; Murakami, R. Chem. Commun. (Cambridge) 2006, 1073. (10) Gorner, H.; Gruen, H. J. Photochem. 1985, 28, 329.

10.1021/la802879e CCC: $40.75  2009 American Chemical Society Published on Web 12/16/2008

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Figure 1. (a) Absorption spectrum of DASPE-I in water. (b) Absorption spectra of DASPE-X (X ) I, TPB, and TFPB) in chloroform. The chemical structure of DASPE is also shown.

behavior allows us to consider that the ionic styryl dye is one of the potential candidates for fluorescent organic materials. Meanwhile, we have developed a simple and versatile method for preparing organic nanoparticles in aqueous solution using “ion association” technique.11 This method is based on the formation of water-insoluble ion pairs in aqueous solution by association of a functional organic cation with a hydrophobic anion such as tetraphenylborate to fabricate organic nanoarchitectures. If a cationic styryl dye molecule is immobilized to form organic nanoparticles through the ion association, highly fluorescent ion-based organic nanoparticles can be synthesized. In this paper, we report the synthesis and optical properties of styryl dye nanoparticles that are prepared by the ion association technique.2-(4-(Dimethylamino)styryl)-1-ethylpyridinium(DASPE) is selected as a cationic styryl dye and is associated with a different type of hydrophobic borate anions in the presence of a neutral polymer stabilizer poly(vinylpyrrolidone).11d Well-dispersed DASPE nanoparticles that ranged in about 30-100 nm in diameter are successfully synthesized, and the optical properties of these nanoparticles are investigated.

Experimental Section Materials. Iodide of 2-(4-(dimethylamino)styryl)-1-ethylpyridinium (abbreviated as DASPE; chemical structure is shown in Figure 1) was purchased from Sigma-Aldrich Chemical Co. and used as received. Poly(vinylpyrrolidone) (abbreviated as PVP; average MW ) 10000, Aldrich) was used as a neutral stabilizer to prevent particle agglomeration. Sodium tetraphenylborate (NaTPB, Aldrich) and sodium tetrakis(4-fluorophenyl)borate dihydrate (NaTFPB · 2H2O, Aldrich) were of the highest commercial grade available and used (11) (a) Yao, H.; Ou, Z.; Kimura, K. Chem. Lett. 2005, 34, 1108. (b) Ou, Z.; Yao, H.; Kimura, K. Chem. Lett. 2006, 35, 782. (c) Ou, Z.; Yao, H.; Kimura, K. Bull. Chem. Soc. Jpn. 2007, 80, 295. (d) Ou, Z.; Yao, H.; Kimura, K. J. Photochem. Photobiol., A 2007, 189, 7.

Yao et al. as received without further purification. These tetrasubstituted borate salts contain hydrophobic bulky anions that are often used for ionpair extraction. Pure water was obtained by an Advantec GS-200 automatic water-distillation supplier. Methods. (a) Preparation of DASPE Nanoparticles. Organic nanoparticles of the styryl dye DASPE were prepared by means of the ion association technique we have developed.11 A typical preparation procedure is depicted as follows: At room temperature, rapid addition of 6.0-4.0 mL of aqueous DASPE-I ()DASPE iodide) solution (0.025 mM) into the ultrasonicated aqueous solution of 3.0-4.0 mL of NaTPB or NaTFPB of 0.05 mM in the presence of PVP (0.2 mg/mL) produced an almost clear suspension of nanoparticles. Namely, ion association between the DASPE cations and TPB (or TFPB) anions leads to water-insoluble nanoparticle formation. Ultrasonication was further continued for 10 min. Herein we call the particles “DASPE nanoparticles” because their spectroscopic properties are dominated by the DASPE chromophore. To be concise, the DASPE nanoparticle sample prepared using TPB or TFPB is referred to as DASPE-a or DASPE-b, respectively. The final molar ratio, or, net charge ratio (determined as F) of the loaded borate anions to DASPE, that is, [TPB-] (or [TFPB-])/[DASPE+], was 1 or 2. The nanoparticle sample was filtered by a 200 nm pore size membrane filter (Sartorious, mini-star RC-15). Since mixing of aqueous NaTPB (or NaTFPB) and DASPE-I solutions at the same molar fraction in the absence of PVP yielded the orange opaque solid dispersion composed of the anion-exchanged dye species DASPE-TPB (or DASPE-TFPB),12 we isolated these precipitates that were purified by three repeated cycles of centrifugation-washing with water followed by drying under vacuum. The DASPE-TPB and DASPE-TFPB solids were insoluble in water but soluble in chloroform, so their spectroscopic properties were evaluated in chloroform. (b) Computational Methods. Ground-state geometry calculations were carried out with the Gaussian 03 program13 at the density functional theory (DFT) level using B3LYP functional and a LanL2DZ basis set for an iodine atom and a 6-31G(d,p) basis set for all other atoms.14 In the iodide atom, an effective core potential was used. The initial geometry for the B3LYP-level structure optimizations was assisted by semiempirical AM1 level calculations. For the calculations of ion-pair complexes, we have chosen initial guess geometries of DASPE+ with one of the counteranions (I-, TPB-, or TFPB-) positioned at the vicinity of the positively charged pyridinium position. Low-lying excitation energies and corresponding oscillator strengths were computed by employing a time-dependent DFT (TD-DFT) approach with a hybrid functional B3LYP.15 (c) Morphology and Size Determination. Morphology and size of the DASPE nanoparticles synthesized were examined with a Hitachi S-4800 scanning transmission electron microscope (STEM). A specimen for STEM observations was prepared by dropping the suspension on an amorphous carbon-coated copper mesh. The (12) We verified that the products (precipitates) were composed of DASPE and TPB (or TFPB) moieties with a 1:1 stoichiometry by the measurements of FT-IR and 1H NMR spectra. See the Supporting Information for more detail. (13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03; Gaussian: Wallingford, CT, 2004. (14) Pichierri, F. J. Mol. Struct. (THEOCHEM) 2002, 581, 117. (15) (a) Casida, M. E. In Recent AdVances in Density Functional Methods; Chong, D. P., Ed.; World Scientific: Singapore, 1995; Vol. 1. (b) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett. 1996, 256, 454. (c) van Gisbergen, S. J. A. Molecular Response Property Calculations Using Time Dependent Density Functional Theory in Chemistry; Vrije Universiteit: Amsterdam, 1998; p 190.

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measurements of the hydrodynamic diameter of nanoparticles on the basis of dynamic light scattering (DLS) in aqueous solution were conducted with an Otsuka ELS-800 light scattering spectrophotometer with a 10 mW He-Ne laser. (d) Spectroscopic Measurements. UV-visible absorption spectra were recorded on a Hitachi U-4100 spectrophotometer. Fluorescence spectra and fluorescence anisotropy spectra were obtained with a Hitachi F-4500 spectrofluorometer. The fluorescence anisotropy (r) is calculated using the formula,

r)

IVV - GIVH , IVV + 2GIVH

G)

IHV IHH

where G is the grating factor that has been included to correct for wavelength response to polarization of the emission optics and detector. IVV and IVH are the fluorescence intensities measured parallel and perpendicular to the vertically polarized excitation, respectively. Similarly, IHH and IHV are the fluorescence intensities measured parallel and perpendicular to the parallelly polarized excitation, respectively.16 Fluorescence quantum yields (φf) were determined by comparing the emission spectra of Ru(bpy)32+ (bpy ) 2,2′-bipyridine) in aerated water (φf ) 0.028) obtained with excitation at 450 nm.17 In the measurements, we set the absorbance of 450 nm at around 0.1 and calculated φf values with corrections for the absorbances of all solutions or dispersions.

Results and Discussion Optical Properties of DASPE-X (X ) I, TPB, and TFPB) Dissolved in Solution. Absorption spectrum of DASPE-I in bulk water (Figure 1a) and those of DASPE-X (X ) I, TPB, and TFPB) in chloroform were measured (Figure 1b). The absorption maximum (λmax) of DASPE-I varied from 434 nm (water, dielectric constant ε ) 78.4) to 478 nm (chloroform, ε ) 4.8), reasonably consistent with the behavior of negatiVe solvatochromism; that is, the absorption is shifted to blue in more polar solvent.9 More interestingly, λmax was dependent on the counteranion in chloroform solution; the peaks appeared at 470 nm for TPB-, 478 nm for I-, and 491 nm for TFPB-. This finding indicates that DASPE-X (X ) I, TPB, or TFPB) is present in the form of an ion-pair complex in low-polarity solvents such as chloroform. The absorption peak position of DASPE as a function of solvent polarity has been so far interpreted in terms of the highly polar nature of this molecule since it has electron-donor (dimethylamino group) and electron-acceptor (ethylpyridinium group) substituents at the opposite ends of the rod-shaped conjugated π-electron system.18 In addition, the effect of local electric field induced by the molecular charge (that is, internal Stark or electrochromic modulation effect) is known to cause a spectral shift of a chromophore in low-polarity solvents.19 In the case of DASPE, stabilization of the initial Franck-Condon state with a significant charge separation by such electrochromic modulation is able to make λmax shift to the red.19 Therefore, the observed counteranion dependence of λmax can be attributed to (16) (a) Ramadass, R.; Bereiter-Hahn, J. Biophys. J. 2008, 95, 4068. (b) Ramadass, R.; Bereiter-Hahn, J. J. Phys. Chem. B 2007, 111, 7681. (17) (a) Nakamura, K. Bull. Chem. Soc. Jpn. 1982, 55, 2697. (b) Plevoets, M.; Vo¨gtle, F.; De Cola, L.; Balzani, V. New J. Chem 1999, 23, 63. (18) (a) Clarke, R. J.; Zouni, A.; Holzwarth, J. F. Biophys. J. 1995, 68, 1406. (b) Klymchenko, A. S.; Duportail, G.; Me´ly, Y.; Demchenko, A. P. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 11219. (c) According to the Onsager’s reaction field model, negative solvatochromism is expected for transitions from a state with a high dipole moment to a state with a smaller one. An enhanced stabilization of the ground state by polar solvents leads to an increased transition energy. (d) Suppan, P.; Ghoneim, N. SolVatochromism; The Royal Society of Chemistry: London, 1997. (e) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry, 3rd ed.; Wiley-VCH: Weinheim, Germany, 2003. (19) (a) Klymchenko, A. S.; Demchenko, A. P. J. Am. Chem. Soc. 2002, 124, 12372. (b) Liptay, W. Angew. Chem., Int. Ed. Engl. 1969, 8, 177.

Figure 2. (a) Fluorescence spectra of DASPE-X (X ) I, TPB, and TFPB) in water or in chloroform (excitation wavelength ) 450 nm). (b) Photographs of the solid-state (i) DASPE-TPB, (ii) DASPE-TFPB, and (iii) DASPE-I. The upper and lower images were obtained under normallight and UV (365 nm) irradiation, respectively, showing an intense emission from the solid-state DASPE-TPB and DASPE-TFPB with excitation at 365 nm. Table 1. Fluorescence Quantum Yields (Of) of Various Samples Containing DASPE Chromophore under an Aerated Condition samples

phase

fluorescence, nm

φf

DASPE-I DASPE-I DASPE-TPB DASPE-TFPB DASPE-a (F ) 1) DASPE-a (F ) 2) DASPE-b (F ) 1) DASPE-b (F ) 2)

aqueous chloroform chloroform chloroform nanoparticles nanoparticles nanoparticles nanoparticles

583 556 556 579 603 603 598 583

(3-5) × 10-3 0.036 0.039 0.083 0.085 0.092 0.080 0.12

(i) an effect of “local polarity” of the counteranion present in the vicinity of DASPE, (ii) an internal Stark effect modulated by the proximal counteranion, and/or (iii) a geometrical change with regard to a single-bond twisting of DASPE itself. Note that the local polarity is a molecular-level electrostatic property that involves the polarizability of the counteranion. We will discuss this issue again in a later section. Figure 2a displays the fluorescence spectrum of DASPE-I in water and those of DASPE-X (X ) I, TPB, and TFPB) in chloroform (excitation wavelength ) 450 nm). The fluorescence peak positions, showing a large Stokes shift and counteranion dependence, are summarized in Table 1 along with their fluorescence quantum yields (φf). It is known that the fluorescence yield φf of ionic styryl dyes decreases as the solvent polarity increases due to the decrease in a polarity-dependent potential barrier against the rotation of a certain single bond responsible for the fast radiationless processes,9c resulting in a very low φf value in aqueous solution.9,10 On the basis of Table 1, φf was dependent on the type of the counteranion and had the largest

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Figure 3. (a) Photograph (two left-side vials) showing the Tyndall scattering of a laser light of 632.8 nm from the aqueous DASPE-a nanoparticles. No scattering could be observed from the aqueous solution of DASPE-I (right-side vial). (b) STEM images of the prepared DASPE-a (i and ii) and DASPE-b (iii and iv) nanoparticle samples. (c) Size distributions of the corresponding DASPE-a (i and ii) and DASPE-b (iii and iv) nanoparticle samples characterized by DLS.

value for DASPE-TFPB in chloroform. This indicates that TFPBbestows the highest barrier for the radiationless route and thus possesses the smallest polarity of the counteranions examined.9c,20 The effect of I- and TPB- on the fluorescence properties was so similar that they could have a comparable polarity with each other.20 It should be worth noting here that solids of DASPE-TPB and DASPE-TFPB were still brightly emissive, whereas that of DASPE-I was almost nonemissive. Photographs of these solids obtained under normal illumination and UV-light irradiation at 365 nm are shown in Figure 2b. This interesting result indicates that, in the ion-pair complex between DASPE+ and TPB- or TFPB-, concentration quenching is particularly suppressed, and more importantly, such ion-pair complexes can be emissive organic solids that would lead to the formation of emissive organic nanoparticles. The solids of DASPE-TPB and DASPE-TFPB were revealed to be amorphous or very low crystalline from the X-ray diffraction (XRD) measurements. See the Supporting Information for more detail. Formation of DASPE Nanoparticles. A series of DASPE nanoparticles were successfully prepared in aqueous solution in the presence of PVP by the ion association method.11 Figure 3a (20) Internal Stark effect and the related screening of the positive charge on DASPE by the counteranions would also influence the fluorescence properties of the chromophore. However, since the φf value of DASPE-I was almost identical with that of DASPE-TPB in chloroform, the influence of screening and the consequent electrochromic modulation on φf would be very small.

shows a typical photograph of the aqueous DASPE-a nanoparticle dispersions exhibiting the Tyndall effect, together with the aqueous DASPE-I solution without the Tyndall scattering. No Tyndall scattering could be observed for the DASPE-TPB and DASPE-TFPB chloroform solutions. Figure 3b shows typical STEM images of the organic nanoparticles of DASPE-a (i and ii) and DASPE-b (iii and iv) prepared at F ) 1 and 2, respectively. In both samples, the particle size increased with an increase in the net charge ratio F. The DLS measurements gave us their size distribution as shown in Figure 3c, showing that the average diameter (and the standard deviation) of DASPE-a nanoparticles is 31.4 (9.7%) or 72.6 nm (8.9%) at F ) 1 or 2, respectively, and that of DASPE-b nanoparticles is 46.5 (8.8%) or 98.0 nm (13%) at F ) 1 or 2, respectively. The size distribution is relatively narrow with standard deviations of 9-13% of the average diameter in all nanoparticle samples. The diameters estimated by STEM accord well with those determined by DLS. It is expected that particle formation processes begin with rapid nucleation and generation of small clusters followed by the slow coalescence of these initial clusters into larger particles.11 The DASPE cation and a hydrophobic borate anion first contact with each other to form an ion-pair complex due to the strong electrostatic attraction. The contact ion pairs will aggregate themselves by their van der Waals interaction to produce embryos or nuclei, followed by the growth of nuclei into clusters and subsequently larger particles.4 It is generally accepted that PVP

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Figure 4. (a) Absorption and (b) fluorescence spectra of DASPE nanoparticles. The upper and lower spectra stand for the DASPE-a and DASPE-b nanoparticle samples, respectively. (c) Fluorescence anisotropy of DASPE-a (F ) 1) and DASPE-b (F ) 1) nanoparticles as a function of wavelength.

is a common colloid stabilizer for many metal and semiconductor nanoparticles because of its steric effects,21 so this polymer would stabilize the dispersion of organic DASPE nanoparticles. In the DASPE nanoparticles, their average size weakly depended on the type of borate counteranions and strongly on F. The counteranion dependence implies the difference in the flocculation force or van der Waals attractive force between the DASPE-TPB and DASPE-TFPB ion-pair species. The reason for the F dependence of the average particle diameter is not clear at present, but an effect in the surface electric double layer screening by the excess electrolytes would play a role for the growth of the DASPE nanoparticles. Optical Properties of DASPE Nanoparticles in Aqueous Solution. A series of UV-vis absorption spectra of DASPE-a and DASPE-b nanoparticles in aqueous solution were shown in Figure 4a. Their spectral shapes were quite similar to that of the solution-phase DASPE monomer, suggesting a monomeric distribution in the nanoparticles. The absorption peaks (λmax) were ranged in 463-470 nm, exhibiting a large bathochromic shift compared to that of DASPE in water (434 nm) and a hypsochromic shift compared to that of DASPE-X (X ) TPB or TFPB) in chloroform (470 or 491 nm, respectively). The shift in λmax proves the electrostatic interaction between the DASPE cation and the borate anion (TPB or TFPB), and the behavior (21) (a) Si, R.; Zhang, Y.; You, L.; Yan, C. J. Phys. Chem. B 2006, 110, 5994. (b) Vinodgopal, K.; He, Y.; Ashokkumar, M.; Grieser, F. J. Phys. Chem. B 2006, 110, 3849. (c) Paredes, J. I.; Suarez-Garcia, F.; Villar-Rodil, S.; Martinez-Alonso, A.; Tascon, J. M. D. J. Phys. Chem. B 2003, 107, 8905. (d) Gabaston, L. I.; Jackson, R. A.; Armes, S. P. Macromolecules 1998, 31, 2883.

is again reminiscent of the solvatochromic effect for DASPE. However, unlike the counteranion dependence of λmax observed for the chloroform solution samples, the positions of λmax for the nanoparticle samples were almost independent of the type of the counteranion (TPB- or TFPB-). This implies the possibility that the overall electrostatic/dipole-dipole interactions between the dye and its peripheral counteranion ensemble have a considerable influence on the absorption properties of DASPE in the nanoparticles.22 Figure 4b shows typical fluorescence spectra of the DASPE nanoparticles in water excited at 450 nm. Their fluorescence properties (peak position and fluorescence quantum yield φf) are also summarized in Table 1. In the nanoparticle samples having the same counteranion, it appears that φf increases with an increase in the particle size. Note that addition of PVP neither quenched the fluorescence of solution-phase DASPE nor altered its spectral shape. In the present nanoparticles, the most interesting feature is the notable enhancement of φf (more than 20-fold enhancement for DASPE-b nanoparticles) in comparison with that of the monomeric DASPE in water. In addition, fluorescence spectra of all DASPE nanoparticle samples exhibited a red shift compared to those of the monomeric species in both water and chloroform. The large Stokes shift (∼20-30 nm) proves the absence of selfaggregation into J aggregates, and the more red-shifted emission should then come from an intramolecular CT (ICT) state in the DASPE chromophore. As stated before, the solvent polarity has an influence on the internal motion barrier around a single bond (22) Kim, J.; Lee, M. J. Phys. Chem. A 1999, 103, 3378.

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in DASPE on the excited-state potential, resulting in the influence on the radiationless channel through the ICT state toward the full TICT state.9 Moreover, the singly bridged derivatives of ionic styryl dye exhibit small changes of the fluorescence quantum yields as compared to the unbridged compounds, whereas the doubly bridged derivatives (that is, inhibition of the internal rotation) show strongly increased fluorescence with φf of about 0.2-0.3 at room temperature in fluid media.9c Using these arguments, the relatively strong emission of DASPE in the nanoparticles suggests the high potential barrier along the internal single-bond twisting of DASPE, which would be caused by both the high rotational friction for the single bond in DASPE and the relatively low polarity of phenylborate matrices. The validity of the above-mentioned scheme was confirmed by the measurements of fluorescence anisotropy for the DASPE nanoparticles prepared at F ) 1 (Figure 4c). In Figure 4c, a relatively high anisotropy was found for the nanoparticles, suggesting that DASPE chromophore is very hard to change its molecular configuration during emission. In addition, we observed that anisotropy values slightly and gradually (nonuniformly) decreased (depolarized) with increasing wavelength from ∼0.3 to ∼0.1. This indicates the change in the transition dipole moment from an initially excited Franck-Condon level, that is, formation of the stable ICT states. The fluorescence observed at a longer wavelength is expected to come from a state with stronger ICT nature, and this may be ascribed to a slight torsional motion about the single bond in DASPE.16 In any case, these unique optical properties demonstrate the versatility of the ionic styryl dye molecule for attaining organic nanoparticles with an intense solid-state emission. Quantum Chemical Investigations. The DASPE molecule is nonplanar, and the rotation angle (dihedral angle, ψ; see Figure 5) of two aromatic rings (ethylpyridinium and styryl moieties) has been estimated to be ∼40° on the basis of semiempirical MO calculations.23 To examine whether the molecular structure of DASPE is changeable by the presence of counteranions, we optimized the ground-state geometries of ion-pair complexes of DASPE-X (X ) I, TPB, and TFPB) as well as a liberated DASPE monomer using the B3LYP-DFT level calculations with the Gaussian 03 program.13 The optimized free DASPE monomer was reasonably nonplanar with the dihedral angle ψ ∼ 29° on the basis of this calculation (Figure 5a), smaller than that obtained by the previous semiempirical calculations. In the ion-pair complexes, on the other hand, the stable geometries of the DASPE part strikingly changed to have the ψ value of ∼45, ∼43, or ∼41° for DASPE-X, where X denotes I, TPB, or TFPB, respectively. The optimized structure of DASPE-I is representatively shown in Figure 5a. The angle ψ greatly increased with an attachment of a monovalent counteranion and had a similar value with each other. The HOMO and LUMO of DASPE suggest that electrons of HOMO are localized in the molecular part which bears the dimethylamino group, while those of the LUMO in the other part of the twisted molecule;9b that is, HOMO-LUMO transition involves an ICT between different fragments (see the Supporting Information for more detail). Therefore, we next calculated the excitation energy for variously twisted conformations of DASPE, with a twist angle of ψ ) 90° for the limiting full TICT conformation, at the TD-DFT B3LYP/6-31G(d,p) level. Figure 5b shows the ψ dependence of the first excitation energy and the transition oscillator strength of DASPE. Note that the influence of solvents was not taken into account and thus it represents only (23) Hachisako, H.; Murata, Y.; Ihara, H. J. Chem. Soc., Perkin Trans. 2 1999, 2569.

Yao et al.

Figure 5. (a) Optimized geometries of the liberated DASPE+ molecule and DASPE+-I- ion-pair complex. The dihedral angle ψ between two aromatic rings (1-ethylpyridinium and styryl groups) is also shown. (b) Angle ψ dependence of the first excitation energy and the transition oscillator strength of DASPE. (c) Scheme for the photophysical behavior of DASPE in the present dye nanoparticles.

a qualitative trend; however, the obtained ψ dependence is useful for comparing the excitation energies of DASPE in these ionpair complexes. From the figure, we could know that twisting of the 1-ethylpyridinium group monotonically led to the first excited state that is lower in energy than the nontwisted conformation. In the presence of an ion-associable counteranion, the angle ψ increased in comparison with that in the liberated DASPE cation, so that the counteranion undoubtedly influences the bathochromic shift of λmax of DASPE through the internal single-bond twisting. Photophysical Scheme of DASPE in the Dye Nanoparticles. In the case of DASPE-X (X ) I, TPB, and TFPB) ion-pair complexes in chloroform, the wavelength of the absorption maximum λmax followed the order of TFPB > I > TPB (shorter wavelength), whereas the dihedral angles ψ obtained by the quantum chemical calculations were similar but precisely had the order of I > TPB > TFPB (smaller angle), implying that the effect of counteranions on λmax does not limit to the dihedral angle rotation. In particular, the deviation on “TFPB” (or the position of “TFPB” in the orders) is suggestive; that is, it is possible that (i) local polarity of “TFPB” or (ii) electrochromic modulation by “TFPB” additionally contributes to the absorption property of DASPE. The former means that TFPB should have the lowest local polarity of the counteranions examined. The latter, as described in the preceding section, is based on the fact that substitution of I- for the large counteranion such as TFPB,

Organic Styryl Dye Nanoparticles

which screens the pyridinium cation with less efficiency, increases the internal Stark effect or electrochromic modulation of the positive charge on DASPE, resulting in more red-shifted absorption.19 Considering here that the size of TFPB is close to that of TPB, the degree of screening of the positive charge on DASPE, which is associated with the degree of electrochromic modulation, should be similar between TPB and TFPB. However, the absorption spectra did not satisfy this criterion so that the local polarity of TFPB would contribute more largely to the absorption property of DASPE. The lower local polarity can make the rotation barrier around ψ higher, and as a consequence, it makes the fluorescence quantum yield φf higher, which is supported by the data of φf shown in Table 1. In the case of DASPE nanoparticles, on the other hand, the counteranion TPB or TFPB had little influence on λmax, despite the expectation that the DASPE-b nanoparticles should have larger λmax values than the DASPE-a nanoparticles (Figure 4). Hence the overall electrostatic/dipole-dipole interactions between the chromophores and its peripheral counteranion ensemble considerably contribute to the spectroscopic properties. Note that the local polarity of counteranions obviously affects the fluorescence intensity of nanoparticles, similarly to the case of the chloroform samples; for example, DASPE-b nanoparticles (at F ) 2, in particular) had a larger φf value than the DASPE-a sample. In addition, the TFPB environment with low polarity can shift the fluorescence peak position of DASPE to a higher energy in comparison with the TPB environment. With closer inspection of the absorption/fluorescence spectral band widths, we can obtain further insight on a photophysical scheme for DASPE in the nanoparticles. In Figure 4, the fluorescence band widths of DASPE-b nanoparticles are broader than those of DASPE-a nanoparticles, whereas their absorption band widths show a slightly opposite trend. This indicates that the broad fluorescence comes from the inhomogeneity of the stabilized fluorescence states (or ICT states) of DASPE rather than from the distribution of ψ.24 Since the full TICT states (ψ ) 90°) are known to be nonfluorescing,22 a photophysical behavior of DASPE in the nanoparticles can be thus depicted on the basis of Figure 5c, and inhibition of the internal rotation of the single bond in DASPE as well as the local polarity of the counteranions (24) (a) Rettig, W. Angew. Chem., Int. Ed. Engl. 1986, 25, 971. (b) Bhattacharyya, K.; Chowdhury, M. Chem. ReV. 1993, 93, 507.

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bring about the relatively strong fluorescence in the organic dye nanoparticles.

Conclusion Ion association between a cationic styryl dye (2-(4-(dimethylamino)styryl)-1-ethylpyridinium; DASPE) and tetraphenylborate (TPB) or tetrakis(4-fluorophenyl)borate (TFPB) anion in water, in the presence of poly(vinylpyrrolidone) (PVP), produced the organic nanoparticles of DASPE (30-100 nm in diameter). Absorption peaks of DASPE nanoparticles exhibited a large bathochromic shift in comparison with that of the dye monomer in water, and an internal rotation around the single bond in DASPE as well as the local polarity of the counteranion had a strong influence on the red shift of absorption. Moreover, formation of the DASPE nanoparticles resulted in an enhanced fluorescence as compared to that of the dye in water; >20-fold enhancement in the fluorescence quantum yield, giving a new methodology for the synthesis of fluorescent organic nanoparticles. The enhancement was dependent on the counteranion used and can be attributed to both the high frictional resistance for the internal single-bond twisting of DASPE and the matrix polarity effect around DASPE, which can suppress the deactivation processes from the excited fluorescence states of the chromophore. We expect that this “ion association” technique will play a vital role for the syntheses of various useful organic nanoparticles in the future. Acknowledgment. The present work was financially supported by Grants-in-Aid for Scientific Research (B, 19310076 (H.Y.); S, 16101003 (K.K.)) from the Japan Society for the Promotion of Science (JSPS). We thank Naoki Nishida and Noriyuki Kitaoka for the XRD and IR spectroscopic measurements. We highly appreciate Dr. Morifumi Fujita (University of Hyogo) for helpful advice on measurements of 1H NMR spectra. Supporting Information Available: FT-IR spectra of DASPE-X (X ) I, TPB, and TFPB), NaTPB, and NaTFPB; 1H NMR spectra of DASPE-TPB and DASPE-TFPB in CDCl3; XRD patterns of the DASPE-X powders obtained with Cu KR radiation (0.154 nm); and HOMO and LUMO of DASPE+ obtained on the basis of B3LYP/631G(d,p) calculations (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. LA802879E