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Spontaneous Formation of Fluorescent Nanofibers and Reticulated Solid from Berberine Palmitate: A New Example of Aggregation-Induced Emission Enhancement in Organic Ion Pairs Joe Chahine,† Nathalie Saffon,‡ Martine Cantuel,§ and Suzanne Fery-Forgues*,† †
Laboratoire des Interactions Moleculaires Reactivite Chimique et Photochimique, UMR CNRS 5623, Universite Paul Sabatier, 31062 Toulouse cedex 9, France ‡ Service commun RX, Structure Federative Toulousaine en Chimie Moleculaire, SFTCM FR2599, Universite Paul Sabatier, 31062 Toulouse cedex 9, France § Institut des Sciences Moleculaires, UMR CNRS 5255, Universite Bordeaux 1, 351 Cours de la Liberation, 33405 Talence cedex, France
bS Supporting Information ABSTRACT: The salt formed between the large aromatic berberine cation and the long-chain palmitate anion was synthesized and used to prepare aqueous suspensions of particles owing to a solvent-exchange method. Under these conditions, elongated particles were readily obtained. They were studied by transmission microscopy with polarized light, as well as by fluorescence and electron microscopy. They were shown to be probably crystallized nanofibers, which were stable in suspension. Unexpectedly, upon filtration and drying, these fibers evolved to give a reticulated solid. The fluorescence properties of the compound were analyzed in solution, in aqueous suspension and in the powder crystalline state. Interestingly, berberine palmitate is virtually not fluorescent in aqueous solution because of the quenching effect of water, but transition to the solid state was accompanied by a strong increase in fluorescence intensity. This phenomenon was explained by the original molecular arrangement in the solid state. Actually, in the crystal, the anions form a distinct layer, which limits parallel-stacking of the fluorescent cations. Moreover, the berberine cations are protected from the access of water molecules, and so no quenching effect can take place. This example confirms that the newly introduced concept of ion-pair aggregation-induced fluorescence enhancement can be extended to a variety of structures. It also shows the interest of ion pairs for preparing fluorescent nanofibers and reticulated solids using a solvent-exchange method that is particularly easy to implement.
’ INTRODUCTION Because of the recent development of microtechnologies, the design and preparation of nano- and microsized luminescent materials made of crystallized organic dyes is becoming of acute interest. In particular, nanofibers are of high demand for miniaturized optoelectronic devices where they could be used as active components to generate or transmit light.1,2 However, the preparation of these nanomaterials is still quite delicate. On the one hand, they must be homogeneous in size, deprived of defects, and prepared in sufficient amount with relatively low costs. Unfortunately, the number of preparation methods is limited because of the thermal fragility of organic compounds. On the other hand, the nanofibers must exhibit good optical properties, while the solid state generally precludes organic dyes from emitting efficiently. Different strategies can be implemented to answer each of these requirements. For instance, crystallized fluorescent nanofibers of high quality can be obtained by adapting the preparation procedure to the chemical structure of the dye, and conversely. For example, very small molecules are often reluctant to readily r 2011 American Chemical Society
form fibers, and elaborated preparation methods such as vapor deposition methods with epitaxial crystal growth must be used.3 When softer preparation methods are preferable, their success depends on the association properties of the dyes. There are of course some rare examples of small molecules that arrange as nanofibers via simple solvent-exchange methods,4 but this behavior is hardly predictable. However, the self-association properties are markedly improved when the molecule bears both a large aromatic system and a bulky aliphatic moiety, which respectively induce π-π stacking and additional van der Waals interactions. Such molecules spontaneously form nanofibers in solvents or solvent mixtures, although this sometimes occurs to the detriment of crystallinity.5 It must be noted that until now in the literature, for this type of molecules, the aromatic and aliphatic moieties are linked by covalent bonds.
Received: May 6, 2010 Published: February 22, 2011 2844
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two questions: will the propensity to form nanofibers be retained with an ion pair system? Can the recently reported phenomenon of AIEE be extended to other types of ion pairs? Evidence would thus be given that the versatility of the system is very interesting for the design of new fluorescent organic nanofibers.
’ EXPERIMENTAL SECTION Figure 1. Chemical structure of the berberine palmitate salt (1).
Solving the problem of light emission in the solid state also is a complex task. Actually, it is well-known that most of organic dyes are highly fluorescent in solvents in which they are well soluble, but lose their fluorescence efficiency when aggregated.6 A high degree of crystallinity, which favors some optical properties such as waveguiding and dichroism, often makes things worse as long as fluorescence is considered. This effect is due to the numerous intermolecular interactions that take place in the solid state and quench the dye emission. It is particularly strong when the dye molecules lie parallel to each other. However, particular molecular arrangements are known to favor fluorescence emission.7 Very recently, we showed that such an original arrangement was taking place for a hydrophobic organic salt formed between a fluorescent phenolate and various spectroscopically silent ammonium derivatives.8 In these crystals, anions and cations arrange into layers, preventing the parallel-stacking of the fluorophores. A particularity of this system is that the crystals were fluorescent while the dissolved phenolate was almost nonemissive in water, due to quenching by water molecules. Consequently, we proposed this system as a new example of aggregation-induced emission enhancement (AIEE),9 a recently discovered phenomenon that had not been reported until then to take place in organic ion pairs. In the present work, it was decided to combine what is known about the formation of nanofibers with our concept of AIEE in organic ion pairs. The idea to associate a large aromatic ring system with an aliphatic moiety was thus retained, and it was imagined to separate them via transformation into ion pair. To do so, berberine was chosen as the fluorescent cation (Figure 1). This natural alkaloid, present in many species of the Papaveraceae, Berberidaceae, and Fumariaceae plant families,10 has played a prominent role in herbal healing in traditional and Eastern medicines for ages,11 owing to its wide biological and pharmaceutical activities.12,13 Berberine is a bright yellow compound, whose intense color is easily explained by the chemical structure: This isoquinolinium cation is almost planar, except the half-chair conformation of the nitrogen-containing nonaromatic cycle,14-16 and has an extended conjugated electron system. It exhibits interesting photophysical properties. It is almost nonfluorescent in water, but it fluoresces appreciably when dissolved in some organic solvents,17-20 placed in the contact of lipid phases,21 intercalated in DNA,13,22 and incorporated in cyclodextrins,23 calixarenes,24 and micelles.18,25 In the literature, the berberine cation has been associated with various organic and inorganic anions,19,26,27 and it has been observed that many of these water-soluble salts easily gave nice single crystals, the structure of which has been reported.14-16 In the present paper, our idea was to associate the berberine cation with a carboxylate anion provided with a long alkyl chain, specifically palmitate, in order to obtain a hydrophobic salt (Figure 1) and with the hope that this salt would crystallize in water. The following work describes the preparation and characterization of berberine palmitate, as well as its spectroscopic behavior in solution and in the solid state. The aim is to answer
Materials. Absolute ethanol (Prolabo and VWR) and high-pressure demineralized water (resistivity 16 MΩ cm) prepared with a Milli-Q apparatus (Millipore) were used as solvents. Berberine chloride (9,10-dimethoxy2,3-methylenedioxy-5,6-dihydrodibenzo[a,g] quinolizinium chloride) was purchased from Sigma. Sodium palmitate (Fluka) was used as received. Preparation of Berberine Palmitate. Berberine chloride (100 mg, 2.68 10-4 mol) was dissolved in 50 mL of water by gentle heating at 40 °C. Similarly, 74.8 mg (2.68 10-4 mol) of sodium palmitate was dissolved in 20 mL of water at 60 °C. Upon mixing of the two hot solutions, a precipitate formed. After cooling down under stirring, it was separated by filtration on a B€uchner apparatus, extensively rinsed with water, and then dried under vacuum at 50 °C, yielding 137.8 mg of 1 as a bright yellow powder. Berberine Palmitate (1). Yield: 82%. 1H NMR (CD3OD): δ ppm =0.91 (t, J = 6.5 Hz, 3H, CH3), 1.30 (m, 24H, CH2), 1.60 (m, J = 7.5 Hz, 2H, CH2-CH2-COO-), 2.16 (t, J = 7.5 Hz, 2H, CH2-COO-), 3.28 (t, J = 6.3 Hz, 2H, H10), 4.13 (s, 3H, C20-OCH3), 4.23 (s, 3H, C19-OCH3), 4.94 (t, J = 6.3 Hz, 2H, H13), 6.13 (s, 2H, H2), 6.98 (s, 1H, H6), 7.68 (s, 1H, H7), 8.02 (d, J = 9.1 Hz, 1H, H18), 8.14 (d, J = 9.1 Hz, 1H, H21), 8.73 (s, 1H, H14), 9.80 (s, 1H, H15). Anal. Calcd for C36H49NO6 3 2H2O: C, 68.87; H, 8.51; N, 2.23. Found: C, 69.20; H, 9.03; N, 2.46. ES-MS: [M]þ = 336.4; [M]- = 255.6. Crystallographic Data. Data were collected at low temperature T = 193(2) K on a Bruker-AXS APEX II diffractometer with MoKR radiation (λ = 0.71073 Å). The structure was solved by direct methods (SHELXS 97)28 and all non-hydrogen atoms were refined anisotropically using the least-squares method on F2.29
Crystal data for 1: C36H49NO6, 2H2O, M = 627.79, triclinic, P1, a = 6.430(2), b = 10.419(4), c = 26.210(10) Å, R = 98.166(14)°, β = 90.558(15)°, γ = 91.150(13)°, V = 1737.6(11) Å3, Z = 2. A total of 13893 reflections (4102 independent, Rint = 0.2827) were collected. 425 parameters, 218 restraints, R1 [I > 2σ(I)] = 0.1377, wR2 [all data] = 0.4222, largest diff. peak and hole: 0.683 and -0.600 eÅ-3. CCDC XXXX contains the supplementary crystallographic data for the structure. These data can be obtained free of charge via www.ccdc. cam.ac.uk/conts/retrieving.html (or for the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax þ441223 336033; e-mail deposit@ccdc. cam.ac.uk). Apparatus. Mass spectra were obtained at the “Service Commun de Spectrometrie de masse de l’Universite Paul Sabatier de Toulouse” with a Waters LCT spectrometer using the electrospray ionization technique. The 1H NMR spectra were recorded on a Bruker AC300 spectrometer operating at 300.13 MHz. The microanalyses were obtained 2845
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Langmuir with an EA1112 elemental analyzer from CE Instruments in the “Service inter-universitaire de micro-analyses de l’ENCIACET”. The melting point was measured on a Stuart Automatic SMP40 apparatus. Spectroscopic measurements were conducted at 22 °C in a temperaturecontrolled cell. UV-vis absorption spectra were recorded on a HewlettPackard 8452A diode array spectrophotometer. Corrected steady state fluorescence spectra were recorded with a Photon Technology International (PTI) Quanta Master 1 spectrofluorometer. Cells of 1 cm and 1 mm optical pathway were used for solutions and suspensions, respectively. The fluorescence quantum yields (Φ) were determined using the classical formula:30 Φx = (As Fx nx2 Φs)/(Ax Fs ns2) where A is the absorbance at the excitation wavelength, F the area under the fluorescence curve and n the refraction index. Subscripts s and x refer to the standard and to the sample of unknown quantum yield, respectively. Coumarin 6 in ethanol (Φ = 0.78) was taken as the standard.30 The fluorescence quantum yields were measured by exciting the samples near their absorption maximum. Fluorescence decay was measured with the stroboscopic technique utilizing a Strobe Master fluorescence lifetime spectrophotometer from PTI. The excitation source was a flash lamp filled with a mixture of nitrogen and helium (30/70). Data were collected over 200 channels with a time-base of 0.1 ns per channel. Excitation was performed at 337 nm, and emission was measured at the emission maximum. Analysis of fluorescence decay was performed using the multiexponential method software from PTI. Photoluminescence quantum yields in the solid state were measured using a Jobin-Yvon Fluorolog-3 fluorometer equipped with an F-3018 integrating sphere and a UV-vis detector R2658. Samples were mounted as pure solids between two glass coverslips and inserted in the integrating sphere using the solid sample holder. The quantum yield was determined using the method developed by de Mello et al.31 Excitation was performed at two different wavelengths (400 and 420 nm). Three measurements were performed, corresponding to different experimental conditions: (a) the integrating sphere is empty; (b) the sample is placed inside the sphere, but the xenon lamp beam is directed onto the sphere wall; (c) the lamp beam is directed onto the sample. The absolute quantum yield value was calculated using the expression: Pc -ð1-AÞPb Φ¼ ALa where A = 1 - Lc/Lb, L is the area under the lamp profile, P is the area under the curve of the emitted light and subscripts refer to the three types of measurements. In each case, the area under the lamp profile is proportional to the amount of unabsorbed light. To record the signals Li, a neutral density filter (0.5%) was used to attenuate the very high intensity of the lamp profile and its effect was taken into account in the calculation of Φ. Before integrating each signal, correction functions accounting for the whole spectrometer assembly and the integrating sphere were applied. In addition, blank spectra were recorded using glass coverslips and subtracted from Pb and Pc. The size and shape of the micro/nanoparticles were observed with a Zeiss Axioskop fluorescence microscope equipped with an Andor Luca camera. The excitation wavelength was 430-450 nm, and the emission wavelength was set at around 500-530 nm, using suitable filters. Transmission electron microscopy was performed at the Service Commun de Microscopie Electronique de l’Universite Paul Sabatier, using a JEOL JEM 1011 microscope equipped with a SIS Megaview III camera. To prepare the samples, the carbon grid was soaked into the aqueous suspension containing the berberine derivative, after reprecipitation was complete. The sample was revealed with a drop of an ammonium molybdate aqueous solution (2%, pH 5) as a contrasting agent and allowed to dry for 48 h under vacuum at 60 °C. Scanning electron microscopy (SEM) was carried out on a JEOL 6320F microscope, at the Centre Interdisciplinaire de Nanoscience de Marseille (CiNAM). The X-ray powder diffraction pattern was performed in the Service Diffraction X du Laboratoire de Chimie de Coordination
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de Toulouse. It was collected in transmission mode, on capillaries samples, on a θ-θ XPert Pro Panalytical diffractometer, with λ (Cu KR1, KR2) 1.54059, 1.54439 Å. The extraction of peak positions for indexing was performed with the fitting program, available in the PC software package Highscoreþ supplied by Panalytical. Observations of the particles with polarized light were carried out using a Leitz Orthoplan microscope equipped with an Olympus 32 objective, N.A. = 0.60, and a webcam.
’ RESULTS Preparation and Characterization of the Berberine Salt. An aliquot of the sodium salt of palmitic acid previously dissolved in a small volume of water was added to a concentrated berberine chloride aqueous solution in stoichiometric proportions (see Experimental Section). The organic salt obtained was poorly soluble in water and a bright yellow precipitate readily appeared. This precipitate was collected by filtration, rinsed and dried at 50 °C under vacuum. The characterization was made by mass spectrometry (electrospray ionization technique, positive and negative modes) and 1H NMR spectroscopy. The berberine NMR signals were attributed according to bibliographic data,32 and the integration confirmed that the organic anion and the berberine cation were in stoichiometric proportion. Elemental analysis showed that compound 1 was hydrated, which was first explained by the presence of residual water due to the synthesis procedure. Crystal Packing Mode. Single crystals of 1 were grown from aqueous solutions by very slow ripening at 4 °C and X-ray diffraction analysis was performed. The crystal that was selected for this study diffracted poorly and the analysis was not of high quality. However, it can be seen clearly that compound 1 crystallizes in a triclinic system (space group P1). It is striking to see that cations and anions are arranged into layers. All the fatty chains are parallel and organized head-to-tail (Figure 2a). In the berberine layers, all the heterocycles lay flat in the same plane. The cations that are situated just below the fatty chain layer are oriented in the same direction and are shifted one with respect to the next one (Figure 2b). The intermolecular distance is 3.4 Å, which allows strong π-π interaction. Considering now the second cation layer, the heterocycles are arranged top-to-tail with respect to those of the upper layer (Figure 2c). The latter arrangement is highly favorable to fluorescence. It can be noted that the compound is a hydrate, with two water molecules per asymmetric unit. Interestingly, these water molecules only establish H-bonds with the carboxylate function of the palmitate anion. It is interesting to compare this packing mode with that reported for berberine salts formed with other anions. When associated with the azide, thiocyanate, chloride, bromide, iodide, hydrogen sulfate, and sulfate anions, the berberine cations pack in centrosymmetric pairs, which in turn form columns, all aligned along the same axis. The spaces between the columns are occupied by anions and most often by several water molecules.15 A similar arrangement was found for the salt formed by the berberine cation and a formate anion, crystallizing in the presence of one equivalent succinic acid.14 In this case, the formate anions and succinic acid molecules fill up the space between the columns, and interact with each other and with the berberine cations through weak hydrogen bonds. Therefore, it seems that the centrosymmetric arrangement is quite usual for the berberine cations, and it is not surprising that it has been retained during the formation of the palmitate salt. However, the arrangement in layers encountered here is unprecedented for a berberine salt. 2846
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Figure 3. Evolution of the UV-vis absorption spectrum of berberine palmitate 1 (6.17 10-5 M) during the reprecipitation process at 22 °C in water containing 1.2% v/v ethanol. One measurement was taken every 90 s.
Figure 2. Crystal cell (a) and molecular packing (b and c) of the berberine palmitate hydrate 1 3 2H2O.
Dye Reprecipitation Monitored by UV-Vis Absorption Spectroscopy. The berberine palmitate prepared as described
above was obtained as a powder. The observation with the fluorescence microscope showed well-separated needle-like microcrystals, the size of which reached 400 μm 30 μm (see Supporting Information, Figure S1). To obtain smaller particles, the reprecipitation method was used.33 This simple and soft method, based on a solvent exchange, has been widely developed during the past few years and has proved to be very useful in the group to prepare nano- and microcrystals of various organic dyes.34,35 It generally consists in dissolving the organic compound in a hydrophilic solvent, and then pouring a small volume
of this concentrated solution into a large volume of water. The organic compound then precipitates, thus leading to the formation of particles in aqueous suspension. In the present case, a stock solution of berberine palmitate (5 10-3 M) in ethanol was prepared. Then, 25 μL of this solution was injected into a cell containing 2 mL of water, and the mixture was left under stirring at room temperature. The dye concentration in the mixture was 6.17 10-5 M, the proportion of ethanol in water being 1.2% v/v. The mixture rapidly got cloudy. A fluffy precipitate even deposited on the cell walls when stirring was too weak. The process was monitored by UV-vis absorption spectroscopy (Figure 3). The initial spectrum was as expected, showing the four characteristic bands of the berberine cation (see Supporting Information for a detailed UV-vis absorption study). During the reprecipitation process, absorbance was markedly decreased, which indicates the formation of particles. Meanwhile, the shape of the absorption spectrum hardly changed with time, as already observed during the formation of nanocrystals from other organic dyes.4c,35 No further evolution of the spectrum was detected after 10 min, which suggests that the reprecipitation process was achieved. Observations by Microscopy. 1. Fluorescence Microscopy. In order to visualize the particle formation, a drop of the suspension at 6.17 10-5 M was taken at various times after the beginning of the reprecipitation process, placed between two slides of glass, and observed with a fluorescence microscope. Elongated structures that emit green fluorescence were observed. For the sample taken 5 min after mixing the solutions, most of these particles were very thin and measured about 3 μm, although a few longer particles (about 8 μm) were also observed (Figure 4a). In the sample taken at 20 min, the average particle size was 8 ( 2 μm, with a weaker population measuring about 3 μm. The samples taken at longer times (Figure 4b) showed almost no variation in the size and shape of the particles (at 3 h 40 min, two populations were still encountered, with average size about 10 and 3 μm, the width of the longest particles being below 0.6 μm). This confirms that the reprecipitation process is quite fast. It must also be noted that the particles tend to form agglomerates. This apart, the aqueous suspensions were steady and observation after 1 week revealed no further evolution. 2. Fluorescence Microscopy with Polarized Light. With the aim to obtain more information about the molecular arrangement in the particles, observations were made with the microscope on single particles placed between two crossed polarizers, using white light illumination. Each particle appeared dark when 2847
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Figure 4. Fluorescence microscopy images of aqueous suspensions of berberine palmitate (1) (6.17 10-5 M) after reprecipitation in water with 1.2% v/v ethanol. Observations were made 5 (a) and 40 min (b) after the beginning of the reprecipitation process.
Figure 5. Transmission microscopy images of the samples made from aqueous suspensions of berberine palmitate (1) after reprecipitation in water with 1.2% v/v ethanol. [1] = 6.17 10-5 M (a) and 2.39 10-4 M (b-d).
one of the polarizers was oriented in the direction of its long or short axis. In contrast, the particle was bright when its long axis was around 45° with respect to the orientation of the crossed polarizers. These observations indicate that the particles observed exhibit two neutral extinction lines, oriented parallel and perpendicular to the long axis. The homogeneous and complete extinction of the particles suggests their single crystal character, or at least a high degree of order. 3. Transmission Electron Microscopy. Observations were also made by transmission electron microscopy. In this case, a drop of suspension taken 40 min after the beginning of the reprecipitation process was deposited on a grid, dried and stained by a contrasting agent. Two samples with different dye concentrations were prepared. The first sample (6.17 10-5 M) displayed elongated particles (Figure 5a), with a length comprised between 3 and 10 μm and a width ranging between 0.16 and 0.8 μm. All these particles seemed to be very thin. They were very close to those observed in the same sample by fluorescence microscopy. A sample of higher concentration (2.39 10-4 M) was also prepared. In this case, the particles were wider (Figure 5b). It can
be noted that for both samples no electron diffraction spectrum could be obtained. Around the large particles, very small ones were also distinguished (Figure 5c), but it is difficult to say whether these particles are present in the suspension or formed during sample drying. It is interesting to note that an alteration of the shape of the particles was sometimes observed (Figure 5d), the structures obtained evoking reticulated fibers. 4. Scanning Electron Microscopy. Preparing the samples for SEM required the particles to be filtered and dried. Here again, two samples were examined to see the effect of concentration. For instance, dilute (6.17 10-5 M) and concentrated (2.39 10-4 M) berberine palmitate suspensions were filtered 1 h after the beginning of the reprecipitation process. Unexpectedly, in all cases, the observation of the dry samples by SEM revealed the formation of a reticulated microporous solid. No individual particles were observed any longer. In the sample issued from dilute suspensions, elongated structures that measured about 10 μm long and 0.4-0.8 μm wide were easily distinguished (Figure 6a). Most likely, they correspond to the particles observed in the suspensions. But, it seems that these particles 2848
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Figure 6. Scanning electron microscopy images of the solid formed by reprecipitation of berberine palmitate (1) in water with 1.2% v/v ethanol, and subsequent filtration 1 h after the beginning of the process. [1] = 6.17 10-5 M (a and b) and 2.39 10-4 M (c and d).
have coalesced (Figure 6b). When they were laid parallel to each others, they resulted in notched plates of various widths. When oriented in different directions, they gave a network. The concentrated sample lead to shorter and wider particles, and therefore to a more compact solid (Figure 6, parts c and d). The effect of aging time of the suspensions was also investigated. To do so, the suspensions were kept in the dark at 4 °C during 3 days after the beginning of the reprecipitation process. The aging time only had a small effect, resulting in solids of slightly different porosity (see Supporting Information, Figure S3). This is in line with the previously noted stability of the suspensions. 5. X-ray Powder Diffraction Spectrum. At this stage of the work, checking the crystallinity of the samples observed by SEM seemed interesting. An XRPD spectrum was performed on the sample obtained by reprecipitation of 1 at 6.17 10-5 M and filtration after 1 h. The spectrum was typically that of a crystallized compound although the deviation of the baseline revealed the presence of some amorphous compound (Figure 7). This spectrum was compared with that calculated from the X-ray analyses performed on the monocrystal. The fit was not good (see Supporting Information, Figure S4). This probably comes from the bad resolution of the X-ray analysis, which did not allow a good XRPD pattern to be generated. Another possibility is that the reprecipitated compound crystallizes in a different system. Study of the Emission Properties. 1. Berberine Palmitate Dissolved in Aqueous Medium and in Alcohols. Before studying the behavior of 1 in the solid state, its spectroscopic behavior was investigated at low concentration in aqueous medium and in various alcohols, in order to clarify the fluorescence quenching effect that has been commonly reported for the berberine cation in polar media. For the sake of comparison, fluorescence measurements were first carried out in a medium identical to the one used for reprecipitation
Figure 7. XRPD pattern obtained for the solid formed by reprecipitation of berberine palmitate (6.17 10-5 M) in water with 1.2% v/v ethanol, and subsequent filtration 1 h after the beginning of the process.
experiments. The dilute solution of 1 in water containing 1.2% ethanol was virtually not fluorescent, whatever the excitation wavelength. This behavior was not surprising since a closely related salt, berberine chloride, is known to be almost nonfluorescent in water, its quantum yield being 4.7 10-4 in D2O.17 The fluorescence of berberine palmitate was also investigated in six different alcohols ranging from methanol to hexanol, upon excitation at 430 nm. All the emission spectra obtained were similar in position and shape: they displayed only one unresolved band at 540 nm, whatever the alcohol used. However, a drastic decrease of the fluorescence intensity was observed when passing from hexanol to methanol, the fluorescence quantum yield decreasing from 5.3 10-2 to 7.5 10-3, respectively. This behavior is in line with the one already reported for berberine chloride by Iwunze, who assigned this fluorescence quenching to a polarity effect that would destabilize the excited state,18 which is less polar than the ground state. To investigate more thoroughly the origin of fluorescence extinction in aqueous medium, small 2849
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Figure 8. Variation of the fluorescence intensity of berberine palmitate (1) (8.8 10-6 M) in 2 mL 2-butanol upon addition of different volumes of water (from top to bottom: 0, 2, 4, 6, 10, 12, 16, 20, 25, 30, 35, 40, 50, 60, 80, 100, 120, and 140 μL). Corrected from dilution effect, λex = 430 nm. Inset: Stern-Volmer plot showing the variation of the fluorescence intensity ratio I0/I versus water concentration.
aliquots of water were added to a solution of 1 in 2-butanol and emission was recorded. A drastic decrease in emission was observed when increasing the proportion of water in 2-butanol from 0 to 3.63 M (Figure 8). Meanwhile, the fluorescence lifetime was reduced from 3.48 ( 0.33 ns to 1.55 ( 0.20 ns. This phenomenon may be due to a polarity change of the medium, but it could also result from the direct interaction of water molecules with the berberine cation in its excited state (dynamic quenching). In the last case, the kinetics of this process can be described by the classical Stern-Volmer relationship: I0/I = 1 þ KSV[Q] = 1 þ kqτ0[Q], where I0 and I are respectively the fluorescence intensity in the absence and presence of quencher Q, KSV is the Stern-Volmer constant, kq the quenching constant and τ0 the fluorescence lifetime in the absence of quencher. To verify this hypothesis, the fluorescence intensity variation was plotted versus the water concentration in the medium. A straight line was indeed obtained, the slope of which was KSV = 0.5532 M-1 (Figure 8, inset). From the known lifetime of 1 in 2-butanol, the value of the quenching constant was thus calculated to be kq = 1.589 108 M-1 s-1. It can be noted that this value is lower than the diffusion constant that is around 109-1010 M-1 s-1 in most liquids.36 This means that the intermolecular quenching process is not limited by diffusion. Moreover, the linearity of the variation reveals that static quenching is not involved in this process. In summary, the fluorescence extinction observed in water may result from two effects, the destabilization of the excited state due to an increase of the medium polarity and a direct quenching effect, involving in particular the nonradiative deactivations that occur via vibrations on the frequency of the hydroxyl group.37 2. Fluorescence Study of Berberine Palmitate Aqueous Suspensions. Suspensions of berberine palmitate were studied after completion of the reprecipitation process, using cells of 1 mm optical pathway, so that absorbance was approximately 0.035 at 420 nm. While the dissolved salt was almost not fluorescent in water, the suspension displayed an intense emission spectrum with a single unresolved band, peaking at 520 nm (Figure 9, blue lines). The shape and position of this band were not affected by changing the excitation wavelength. The excitation spectrum, which gives the image of the emitting species, displayed bands centered at 334 and 454 nm and was unaltered by changing the emission wavelength. This indicates that only one fluorescent species is detected in the suspension. A slight
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Figure 9. Blue lines: Excitation and emission fluorescence spectra of an aqueous suspension of berberine palmitate (1) at 6.17 10-5 M after reprecipitation in water with 1.2% v/v ethanol. λex = 420 nm, λem = 520 nm. Red lines: Excitation and emission photoluminescence spectra of the microcrystalline powder of 1. λex = 420 nm, λem = 530 nm. All spectra are corrected and normalized. Inset: Evolution of the fluorescence intensity at 520 nm versus time, after injecting 25 μL of 1 at 5 10-3 M into 2 mL water and stirring at 20 °C (λex = 420 nm).
shift can be noted in the position of excitation bands with respect to the corresponding absorption bands, displayed in Figure 3. An explanation is that the suspension contains a proportion of dissolved salt that contributes to the absorption spectrum but is practically not fluorescent. The lifetime was measured and found to be 6.4 ( 0.3 ns. This value can be compared with the fluorescence lifetime of berberine chloride dissolved in water, which was measured to be 0.6 ( 0.3 ns. The long lifetime found in the suspension indicates a strong stabilization of the excited state. Since the dissolved salts are almost not fluorescent, it is very likely that fluorescence arises from the particles formed in the suspension. This was confirmed very easily, by monitoring the fluorescence intensity of the sample from the beginning of the reprecipitation process (inset of Figure 9). To perform efficient stirring, a 1 cm-cell was used. It can be seen that fluorescence intensity increased very quickly after injecting the ethanol solution of salt into water. Then the curve leveled off, and finally decreased, due to the formation of fluffy aggregates on the cell walls. 3. Photoluminescence Properties of Berberine Palmitate in the Microcrystalline State. The photoluminescence properties of the berberine salt in the solid state were investigated using a spectrofluorometer equipped with an integration sphere. The measurement was performed on the microcrystalline powder directly issued from synthesis. The corresponding spectra are given in Figure 9 (red lines). Microcrystals of 1 exhibited photoluminescence, with an unresolved single band centered at 516 nm. Although it was narrower, this emission spectrum was quite close to that recorded on the suspension, confirming that the particles are the emitting species in the latter medium. The quantum yield was found to be 1.8 10-2. It can be noted that both bands visible in the excitation spectrum at 368 and around 460 nm were red-shifted with respect to those of the absorption spectrum of the dissolved compound.
’ DISCUSSION In the literature, the association of the berberine cation with organic anions has only been studied in solution, for analytical purpose. Actually, the ion pairs26 and three-partner complexes27 thus formed are strongly colored. Their extraction in a biphasic 2850
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Langmuir medium, and subsequent titration by optical methods allowed the analysis of quaternary ammonium salts of biological relevance and optically inactive surfactants. However, to our knowledge, the solid-state properties of these salts have not been investigated until now, as it is also the case for the salts formed with inorganic anions. The present work shows the interest of these species. From a photophysical viewpoint, the berberine cation in solution loses its fluorescence with increasing the polarity of the solvent. It is almost not fluorescent in water and it was shown here that this may be due to dynamic fluorescence quenching by water molecules. In contrast, in the solid state, the berberine palmitate salt is photoluminescent. Its quantum yield is not very high, but it is comparable to that of the cation dissolved in an organic solvent such as butanol. The comparison of the fluorescence lifetime values measured in solution and in suspension reveals a strong stabilization of the chromophore in the solid state. The fluorescence revival in the solid state can be explained by two reasons. First, the chromophore is now isolated from water molecules. Although the salt is a hydrate, the berberine cation is not involved in H-bonds with water molecules. Second, the original packing mode favors the emission of photoluminescence. The bulky organic anions form a thick sheet between the cation layers. In the latter, the neighboring berberine heterocycles are either shifted one with respect to the other, or in a head-to-tail configuration. Side-by-side π-π stacking between fluorophores, which is the most current cause of fluorescence deactivation in solids, is thus reduced. One of the initial aims of this work was to see whether the aggregation-induced emission enhancement, previously observed for the salt formed by a fluorescent phenolate and a quaternary ammonium, could be found for other types of organic ion pairs. Actually, the crystal packing mode of berberine palmitate is quite close to that previously reported by us8 and by another team38 for salts made of an aromatic anion and an aliphatic cation: all these species form well separated layers. The associated photophysical effect (a photoluminescence revival in the solid state) is also similar. Therefore, the present work brings definite evidence that these are generic processes that should be widely encountered for ion pairs, one ion being an aromatic planar structure and the other one being a bulky aliphatic molecule. The first question being now answered, another very interesting point is that berberine palmitate forms nanofibers, which probably are very thin and elongated monocrystals. TEM measurements brought no proof about the crystallinity of these fibers, but it often happens that very thin crystallized organic particles generate no electron diffraction pattern. In contrast, the observation with crossed polarizers indicated a high degree of order and suggested that the fibers are crystallized. The XRPD pattern obtained with the filtered reticulated material showed clear diffraction lines, characteristic of a crystallized compound. It is not sure that the microcrystals issued from synthesis, the nanofibers produced by reprecipitation, and the reticulated materials crystallize in the same system. But, all of them are intensely fluorescent in the same color range. This suggests that the molecular arrangement is very close in the three cases. Various types of fluorescent nanofibers exist, based on polymers, peptides, and DNA or made of small organic molecules that are more or less prone to self-organization according to their structure,2 as already said above. To our knowledge, the present case is the first example of fluorescent nanofibers obtained from an organic salt, in which the fluorophore is associated with an oppositely charged aliphatic moiety. Moreover, it must be noted
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that these nanofibers were spontaneously formed using the simple reprecipitation method, for which only a few examples of fiber formation have been reported until now and only for covalent compounds.4 The reprecipitation method is easy to implement and generally allows good amounts of nanoparticles to be obtained for various applications. Most often, particles are filtered and dried for subsequent use. Curiously, in the present case, the nanofibers were only stable in suspension and coalesced upon filtration and drying. This phenomenon is encountered for low melting point compounds, such as paraffin. Actually, the melting point of 1 was tentatively measured, and it was found that the compound darkened and probably decomposed around 114.4 °C. It would be interesting to understand why coalescence occurs in our case. No coalescence was observed during the synthesis of berberine palmitate, which leads to distinct microcrystals (see Figure S1, Supporting Information). It can thus be thought that experimental conditions such as concentration, temperature, and the crystallization rate play an important role in the occurrence of this phenomenon. It would also be interesting to know if coalescence will be observed when varying the nature of the aliphatic counterion and other experimental conditions. Controlling this phenomenon will have two important issues. The berberine palmitate nanofibers are regular in shape and exhibit good fluorescence properties. Efforts will be made to prepare dry individual nanofibers that retain these properties. On the other hand, developing the use of the reprecipitation method as an original way to obtain reticulated fluorescent materials could be a distinct advantage. As a matter of fact, these materials are of very high interest in a wide range of applications where a high ratio of surface area to volume is needed—for example in the field of chemical and biochemical sensors.4a,5a Their easy preparation could lead to new developments.
’ CONCLUSION Aggregation-induced emission enhancement is quite a rare phenomenon, which is actively searched for applications in the field of fluorescent materials. It has been reported a few times in nanofibers, which present then a particular interest for incorporation in microsystems. But, this is the first time that the phenomenon is shown to occur in nanofibers made of ion pairs. The fluorescent ion-pair concept presented here is obviously of high interest because of its versatility and ease of implementation. Various ion pairs can be tested, knowing that minor structural modifications will affect the crystal packing mode, the luminescence efficiency, and the morphology of the nano-object formed. Work is underway in our team with different ion pairs to get a deeper understanding in the structure/property relationship for this type of system. ’ ASSOCIATED CONTENT
bS
Supporting Information. Fluorescence microscopy images, a UV-vis absorption study, scanning electron microscopy, an XRPD pattern, and a cif file containing crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail: sff@chimie.ups-tlse.fr. Fax: þ335-6155-8155. 2851
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’ ACKNOWLEDGMENT We would like to thank Dr. Jean-Pierre Galaup (Laboratoire Aime Cotton, Orsay) for performing the experiments with crossed polarizers, Mr. Laurent Weingarten (Service Commun de Microscopie Electronique de l’Universite Paul Sabatier) for his kind help in TEM measurements, Dr. Laure Vendier (Service Diffraction X du Laboratoire de Chimie de Coordination de Toulouse) for the XRPD pattern, Mr. Abdelhamid Ghodbane for the measurement of the melting point, and Dr. Nathan McClenaghan (Universite Bordeaux 1) for the lending of the Fluorolog-3 fluorometer. We are also indebted to Mr. Serge Nitsche (Service de Microscopie Electronique du CINaM) for the beautiful SEM images and Dr. Gerard Pepe for fruitful discussions and access to SEM facilities. M.C. also thanks CNRS for a postdoctoral fellowship. ’ REFERENCES (1) (a) Schiek, M.; Balzer, F.; Al-Shamery, K.; Brewer, J. R.; L€utzen, A.; Rubahn, H.-G. Small 2008, 4, 176-181 and references cited therein. (b) Brewer, J.; Schiek, M.; Luetzen, A.; Al Shamery, K.; Rubahn, H.-G. Nano Lett. 2006, 6, 2656–2659. (2) Fery-Forgues, S.; Fournier-No€el, C. Organic fluorescent nanofibers and submicrometer rods. In Nanofibers; Kumar, A., Ed.; In-Techweb: 2010. Available on the Internet: http://www.sciyo.com/books/show/ title/nanofibers. (3) (a) Yanagi, H.; Ohara, T.; Morikawa, T. Adv. Mater. 2001, 13, 1452–1455. (b) Balzer, F.; Rubahn, H.-G. Adv. Funct. Mater. 2005, 1, 17–24. (c) Hernandez-Sosa, G.; Simbrunner, C.; Sitter, H. Appl. Phys. Lett. 2009, 95, 013306. (d) Lee, J. K.; Koh, W. K.; Chae, W. S.; Kim, Y. R. Chem. Commun 2002, 138–139. (4) (a) An, B.-K.; Gihm, S. H.; Chung, J. W.; Park, C. R.; Kwon, S.-K.; Park, S. Y. J. Am. Chem. Soc. 2009, 131, 3950–3957. (b) Fu, H.; Xiao, D.; Yao, J.; Yang, G. Angew. Chem., Int. Ed. 2003, 42, 2883–2886. (c) Mille, M.; Lamere, J.-F.; Rodrigues, F.; Fery-Forgues, S. Langmuir 2008, 24, 2671–2679. (d) Zhang, X. J.; Zhang, X. H.; Shi, W. S.; Meng, X. M.; Lee, C.; Lee, S. T. J. Phys. Chem. B 2005, 109, 18777–18780. (e) Yu, H.; Qi, L. Langmuir 2009, 25, 6781–6786. (f) Li, S.; He, L.; Xiong, F.; Li, Y.; Yang, G. J. Phys. Chem. B 2004, 108, 10887–10892. (g) Zhao, Y. S.; Peng, A.; Fu, H.; Ma, Y.; Yao, J. Adv. Mater. 2008, 20, 1661–1665. (h) Onodera, T.; Oshikiri, T.; Katagi, H.; Kasai, H.; Okada, S.; Oikawa, H.; Terauchi, M.; Tanaka, M.; Nakanishi, H. J. Cryst. Growth 2001, 229, 586–590. (5) For example: (a) Che, Y.; Zang, L. Chem. Commun. 2009, 34, 5106–5108. (b) Del Guerzo, A.; Olive, A. G. L.; Reichwagen, J.; Hopf, H.; Desvergne, J. -P. J. Am. Chem. Soc. 2005, 127, 17984–17985. (c) Everett, T. A.; Twite, A. A.; Xie, A.; Battina, S. K.; Hua, D. H.; Higgins, D. A. Chem. Mater. 2006, 18, 5937–5943. (d) W€urthner, F.; Yao, S.; Beginn, U. Angew. Chem., Int. Ed. 2003, 42, 3247–3250. For an exhaustive bibliography, see ref 2. (6) (a) Birks, J. B. Photophysics of Aromatic Molecules: Wiley: London, 1970. (b) Silinsh, E. A. Organic Molecular Crystals: SpringerVerlag: Berlin, 1980. (7) Cornil, J.; Beljonne, D.; Dos Santos, D. A.; Calbert, J. P.; Shuai, Z.; Bredas, J. L. C. R. Acad. Sci. Paris 2000, Ser. IV, 403–408. (8) Lamere, J.-F.; Saffon, N.; Dos Santos, I.; Fery-Forgues, S. Langmuir 2010, 26, 10210–10217. (9) For example: (a) An, B. K.; Kwon, S. K.; Jung, S. D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410–14415. (b) Lim, S. J.; An, B. K.; Jung, S. D.; Chung, M. A.; Park, S. Y. Angew. Chem., Int. Ed. 2004, 43, 6346– 6350. (c) Oelkrug, D.; Trompert, A.; Gierschner, J.; Egelhaaf, H.-J.; Hanack, M.; Hohloch, M.; Steinhuber, E. J. Phys. Chem. B 1998, 102, 1902–1907. (d) Xu, J.; Liu, X.; Lv, J.; Zhu, M.; Huang, C.; Zhou, W.; Yin, X.; Liu, H.; Li, Y.; Ye, J. Langmuir 2008, 24, 4231–4237. (e) Bhongale, C. J.; Chang, C. W.; Lee, C. S.; Diau, E. W. G.; Hsu, C. S. J. Phys. Chem. B 2005, 109, 13472–13482. (f) Dong, S.; Li, Z.; Qin, J. J. Phys. Chem. B
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