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Facile Access to Highly Fluorescent Nanofibers and Microcrystals via Reprecipitation of 2-Phenyl-benzoxazole Derivatives Abdelhamid Ghodbane,† Sebastien D’Alterio,† Nathalie Saffon,§ Nathan D. McClenaghan,|| Luca Scarpantonio,|| Pascale Jolinat,‡ and Suzanne Fery-Forgues*,† Laboratoire des Interactions Moleculaires Reactivite Chimique et Photochimique, CNRS UMR 5623, ‡Laboratoire LAPLACE, CNRS UMR 5213, and §Service commun RX, Institut de Chimie de Toulouse, ICT- FR2599, Universite Paul Sabatier 31062 Toulouse cedex 9, France Institut des Sciences Moleculaires, CNRS UMR 5255, University of Bordeaux, 351 Cours de la Liberation, 33405 Talence cedex, France
)
†
bS Supporting Information ABSTRACT: 2-Phenyl-benzoxazole and five derivatives bearing an alkyl or alkoxy substituent on the phenyl ring were used to prepare aqueous suspensions of particles via a solvent-exchange method. In these conditions, the methyl and methoxy derivatives spontaneously gave nanofibers, while the other compounds led to microcrystals. This shows that minor chemical changes are enough to direct the formation of a given type of particle. From a spectroscopic viewpoint, all compounds strongly emit blue light in the solid state, with spectra much broader than those registered in n-heptane and ethanol solutions. The photoluminescence quantum yields reached 38% and were slightly affected in aqueous suspension by the polarity of the environment. The molecular arrangement, deduced from X-ray analysis for the methyl and methoxy derivatives, was used to explain the fluorescence properties in the solid state. This work shows that 2-phenyl-benzoxazole derivatives are interesting candidates for applications as fluorescent nanomaterials, including in aqueous and biological media.
’ INTRODUCTION There has been growing interest in developing organic nanoand microstructured materials that display good fluorescence properties in the solid state, due to their potential use in new technological devices,1 as well as in the field of chemical and biochemical sensors.2 One distinct advantage of organic compounds is that synthesis allows the physical properties of the molecules to be finely modulated. However, molecular-level design of truly fluorescent materials remains difficult. While the link between the optical properties and the chemical structure of a fluorophore is generally well-known, as long as the fluorophore is dissolved in a solvent or dispersed in a matrix, there is a large difference between the optical properties of a dissolved molecule and those of the same molecule in the condensed state. This is explained by the fact that the optical properties in the solid state depend not only on the dye structure, but also vary strongly with the intermolecular arrangement. For example, the best dyes may become virtually nonfluorescent if they display the frequently encountered face-to-face stacking arrangement in the solid state.3 Moreover, ideally, molecules should also spontaneously lead to the desired particles, such as nanofibers and platelets, because this allows easy and cheap preparation of materials. Unfortunately, very little information is available to cope with this paradigm. Engineering molecules to get materials that exhibit both the required morphology and the good solid-state fluorescence properties still relies on an empirical approach. r 2011 American Chemical Society
The aim of the present work is to investigate the relationship between the molecular structure, the solid-state optical properties, and the morphology of the particles spontaneously obtained using a simple solvent-exchange method. The 2-phenyl-benzoxazole series was chosen for the following reasons: (i) Compounds of this family show excellent fluorescence properties when dissolved in organic solvents or dispersed in a matrix. They serve as near-UV dyes in many applications, such as scintillation counters,4 laser dyes,5 optical brighteners for textiles,6 and biological probes.7 (ii) The photophysical behavior, studied in solution, is wellknown.5a,7a,8 (iii) Curiously, to our knowledge, solid-state spectroscopic properties in the 2-phenyl-benzoxazole series have hardly been investigated yet. The only reports deal with the 20 hydroxyl derivative, used as a chelating agent that gives luminescent metal complexes usable in organic light-emitting diodes (OLEDs),9 and the properties of the free ligand are not discussed. However, many 2-phenyl-benzoxazole derivatives are fluorescent as powders when illuminated with a hand-hold UV lamp, and the solid-state characteristics reported for closely related structures, specifically benzobisoxazoles, are promising.10 (iv) These compounds readily form single crystals, which are suitable for standard X-ray crystallographic analysis. This allows elucidation of the Received: September 19, 2011 Revised: November 4, 2011 Published: November 30, 2011 855
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Compound 3 crystallized in the medium upon cooling, and 4 crystallized after addition of methanol. 2-(40 -n-Butylphenyl)-benzoxazole (4). Mp: 50.4 °C. 1H NMR (CDCl3): δ ppm = 8.19 (d, 2H, J = 9 Hz, Phe), 7.78 (m, 1H, H7), 7.60 (m, 1H, H4), 7.38 (m, 2H, H5 and H6), 7.36 (d, 2H, J = 9 Hz, Phe), 2.72 (t, 2H, J = 7.5 Hz, PheCH2), 1.66 (m, 2H, CH2), 1.41 (m, 2H, CH2), 0.97 (t, 3H, J = 7.5 Hz, CH3). Anal. Calcd for C17H17NO: C, 81.24; H, 6.82; N, 5.57. Found: C, 80.90; H, 6.95; N, 5.75. MS (DCI): m/z 252 (M + H+), 280 (M + C2H5+). IR: νCdN 1620 cm1; νCdC 1577 cm1; νNdCO 1556 cm1. 2-(40 -n-Octylphenyl)-benzoxazole (6). Mp: 57 °C. 1H NMR (CDCl3): δ ppm = 8.19 (d, 2H, J = 9 Hz, Phe), 7.78 (m, 1H, H7), 7.60 (m, 1H, H4), 7.38 (m, 2H, H5 and H6), 7.36 (d, 2H, J = 9 Hz, Phe), 2.71 (t, 2H, J = 7.5 Hz, PheCH2), 1.68 (m, 2H, CH2), 1.30 (m, 10H, CH2), 0.90 (t, 3H, J = 7.5 Hz, CH3). Anal. Calcd for C21H25NO: C, 82.04; H, 8.20; N, 4.56. Found: C, 81.59; H, 8.23; N, 4.31. MS (DCI): m/z = 308 (M + H+); 336 (M + C2H5+). IR: νCdN 1619 cm1; νCdC 1576 cm1; νNdCO 1556 cm1. Crystallographic Data. Data for compounds 2 and 3 were collected on a Bruker-AXS APEX II diffractometer using a 30 W air-cooled microfocus source (ImS) with focusing multilayer optics at a temperature of 193(2 )K for 2 and on a Bruker-AXS SMART APEX II diffractometer at a temperature of 213(2) K for 3, with graphite-monochromated Mo Kα radiation (wavelength = 0.71073 Å) by using phi- and omegascans. The structures were solved by direct methods (SHELXS 97),12 and all non-hydrogen atoms were refined anisotropically using the leastsquares method on F2.13
Figure 1. Structural formulas of the 2-phenyl-benzoxazole derivatives (16).
molecular arrangement, which is extremely valuable for understanding the photophysical properties in the solid state. (v) They are generally poorly soluble in water. Therefore, they lend themselves well to the preparation of aqueous suspensions of micro-/ nanoparticles via the solvent-exchange method currently used in our group. (vi) Finally, some recrystallized 2-phenyl-benzoxazole derivatives give acicular crystals, and it is interesting to see if this tendency will be retained in microparticles. Straightforward access to nanofibers could be of high interest for subsequent applications.11 Consequently, 2-phenyl-benzoxazole and five of its derivatives were synthesized and compared in solution, as aqueous suspensions and in the microcrystalline solid state. The compounds differ by the nature of the aliphatic group in the para position of the phenyl ring. Only subtle variations are expected in the fluorescence properties of these dyes dissolved in a solvent. However, this work is aimed at seeing whether the variations brought to the chemical structure will affect the size and shape of the particles formed and influence the solid-state fluorescence characteristics.
’ EXPERIMENTAL SECTION Crystal Data for 2. C14H11NO2, M = 225.24, monoclinic, P21/c, a = 26.401(2) Å, b = 3.9063(3) Å, c = 21.6976(15) Å, α = γ = 90°, β = 105.161(4)°, V = 2159.8(3) Å3, Z = 8. Reflections collected: 17 709 (4365 independent, Rint = 0.0956). 347 parameters, 36 restraints, R1 [I > 2σ(I)] = 0.0567, wR2 [all data] = 0.1352, largest diff. peak and hole: 0.188 and 0.264 e Å3.
Materials. N-Methylpyrrolidinone, 2-aminophenol, and acid chlorides (benzoyl, 4-methoxy-benzoyl, p-toluoyl, 4-n-butyl-benzoyl, 4-t-butylbenzoyl, and 4-n-octyl-benzoyl chloride) used for synthesis were obtained from Aldrich. For reprecipitation and spectroscopic work, absolute ethanol (Prolabo-VWR), tetrahydrofuran, dimethylformamide and n-heptane (SDS), and high-pressure demineralized water (resistivity 18.3 MΩ cm) prepared with a Milli-Q apparatus (Millipore) were used as solvents. General Synthesis of the 2-Phenyl-benzoxazole Derivatives (16). An aliquot of 2-aminophenol (9.16 mmol, 1 g) was dissolved in 5 mL of N-methylpyrrolidinone. The orange solution was deaerated with nitrogen and cooled to 0 °C. The acid chloride (9.16 mmol), previously dissolved when necessary in a minimum N-methylpyrrolidinone, was added dropwise. After being stirred at 0 °C for 1 h, pyridine (11.4 mmol, 0.9 g) was added, and the mixture was refluxed for 2 h. The mixture was then cooled, and 10 mL of a water/methanol (80:20) mixture was added. The formed suspension was cooled to 4 °C, and the precipitate was filtered on a B€uchner funnel, washed with 10 mL of chilled water/methanol (80:20, v/v) mixture, and then with 10 mL of ice-cold water. After recrystallization in methanol, the solid was filtered and dried under vacuum to give the desired compounds with a yield varying between 20% (compound 6) and 70% (compound 1). Compounds 1, 2, 3, and 5 (previously described in the literature) were as expected (Figure 1). Their melting points were 103.7, 98.0, 115.6, and 107.5 °C, respectively. For spectroscopic measurements, subsequent purification was performed to obtain perfectly white compounds. Compounds 1 and 2 were sublimated under a vacuum. Compounds 3 and 4 were refluxed for 30 min in tetrahydrofuran in the presence of activated charcoal, the hot solution was filtered, and one-half of the solvent was evaporated.
Crystal Data for 3. C14H11NO, M = 209.24, monoclinic, P21/c, a = 17.7321(19) Å, b = 4.9836(5) Å, c = 12.4973(13) Å, α = γ = 90°, β = 100.166(8)°, V = 1087.0(2) Å3, Z = 4. Reflections collected: 9844 (1826 independent, Rint = 0.1847). 166 parameters, 29 restraints, R1 [I > 2σ(I)] = 0.0561, wR2 [all data] = 0.1610, largest diff. peak and hole: 0.157 and 0.191 e Å3. Crystallographic data for the structures 2 and 3 are available. 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, UK; fax, +441223 336033; e-mail,
[email protected]). Apparatus. Mass spectra were obtained at the “Service Commun de Spectrometrie de masse de l’Universite Paul Sabatier de Toulouse” with 856
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Figure 2. Compounds 4 and 5 recrystallized in methanol, observed with a polarization microscope. a Waters GCT Premier spectrometer using the chemical ionization technique with CH4. The 1H NMR spectra were recorded on a Bruker AC300 spectrometer operating at 300.13 MHz. The microanalyses were obtained with an EA1112 elemental analyzer from CE Instruments in the “Service inter-universitaire de micro-analyses de l’ENCIACET”. Melting points were measured on a Stuart Automatic SMP40 apparatus. Spectroscopic measurements were conducted at 22 °C in a temperaturecontrolled cell. UVvis absorption spectra were recorded on a HewlettPackard 8452A diode array spectrophotometer. For solutions and suspensions, corrected steady-state fluorescence spectra were registered with a Photon Technology International (PTI) Quanta Master 1 spectrofluorometer, using cells of 1 cm or 1 mm optical pathlength. The fluorescence quantum yields (ΦF) were determined using the classical formula: ΦFx = (As Fx nx2 ΦFs)/(Ax Fs ns2), where A is the absorbance at the excitation wavelength, F is the area under the fluorescence curve, and n is the refraction index.14 Subscripts “s” and “x” refer to the standard and to the sample of unknown quantum yield, respectively. 2-Phenyl-benzoxazole in cyclohexane (ΦF = 0.78) was taken as the standard.8a The fluorescence quantum yields were measured by exciting the samples near their absorption maximum. Solid-state photoluminescence quantum yields were recorded on a Horiba JobinYvon Fluorolog-3 fluorometer equipped with a Labsphere optical Spectralon integrating sphere (diameter of 100 mm), which provides a reflectance >99% over the 4001500 nm range (>95% within 2502500 nm) with PMT detection by a Hamamatsu R2658. Solid samples were mounted on a Teflon support with glass coverslips, and luminescence spectra were recorded and corrected for the luminescence fluctuations of the sphere and for the neutral density filter.15 The absolute quantum yield value (ΦP) could be determined by a method based on the one developed by de Mello et al.16 This value is given by: ΦP ¼
filters. Alternatively, samples were illuminated at 450490 nm, and the emission wavelength was collected above 500 nm. Transmission and scanning electron microscopy was performed at the “Service Commun de Microscopie Electronique de l’Universite Paul Sabatier”. For TEM, a JEOL JEM 1400 electron microscope equipped with a SIS Megaview III camera was used. To prepare the samples, the carbon grids were soaked in an aqueous suspension containing the benzoxazole derivatives, after reprecipitation was complete. The samples were revealed with a drop of ammonium molybdate aqueous solution (2%, pH 5) as a contrasting agent and allowed to dry for 48 h under vacuum at 60 °C. SEM was carried out with a JEOL JSM 6700F apparatus. The powder sample was deposited on a self-adhesive tape and not metalized. The X-ray powder diffraction patterns were performed in the “Service Diffraction X du Laboratoire de Chimie de Coordination de Toulouse”. They were collected in transmission mode, on capillary samples, on a θθ XPert Pro Panalytical diffractometer, with λ (Cu Kα1, Kα2) 1.54059, 1.54439 Å. The extraction of peak positions for indexing was performed with the fitting program, available in the PC software package Highscore+ supplied by Panalytical.
’ RESULTS Preparation of the 2-Phenyl-benzoxazole Derivatives and Observation with the Polarization Microscope. Compounds
16 were synthesized according to a standard procedure, by condensing 2-aminophenol with an acyl chloride, in a 1:1 stoichiometry (see Experimental Section). Observation with the polarization microscope confirms that the solids obtained are crystalline. Compounds 14 gave needle-like microcrystals, while the t-butyl (5) and n-octyl (6) derivatives gave platelets. Two examples are provided in Figure 2; the other images can be seen in the Supporting Information (Figure S1). Crystal Packing Mode. Single crystals of 2 and 3 were slowly grown in methanol at room temperature, and X-ray diffraction analysis was performed. Both compounds crystallize in the monoclinic system (space group P21/c). For the methyl derivative (3), each crystal unit contains four molecules, which are displayed by pairs. As seen in Figure 3a, the molecules whose planes are parallel display a head-to-tail arrangement. The distance between the planes is greater than 7.7 Å, therefore not allowing ππ interactions. The molecule pairs are arranged perpendicularly to each other (Figure 3b). Each molecule has only two short contacts (2.5 Å), which involve the nitrogen atom of the heterocycle and one hydrogen atom of the aromatic cycle. These contacts are established with molecules that are oriented perpendicularly. The observation of packing along the b axis reveals that the molecules whose planes are parallel are displayed like bricks in a wall, with little overlapping and a distance between planes of around 3.5 Å.
Ec ð1 αÞ 3 Eb La 3 α
with α ¼ 1
Lc Lb
Thus, the excitation source was scanned to evaluate the reflected light for the empty sphere (La), the samples facing the source light (Lc), and the sample out of the irradiation beam (Lb). Neutral density (ND) filters were used to protect the detector from saturation resulting from direct irradiation while the excitation light is recorded.15 The following fluorescence spectra were recorded: the sample facing the source light (Ec), and that out from the direct irradiation (Eb). The ND filters were chosen to have the emission intensity of the samples at λem on the same order of magnitude as La. The recrystallized compounds were observed with an Olympus BX50 polarization microscope. 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 350380 nm, and the emission wavelength was set at above 400 nm, using suitable 857
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Figure 3. Crystal unit cell of 2-(40 -tolyl)-benzoxazole (3) (a and b). Crystal unit cell (c) and short contacts (d) for 2-(40 -methoxy-phenyl)-benzoxazole (2).
1. Observations by Fluorescence Microscopy. A drop of the reprecipitation mixtures was taken at least 1 h after the beginning of the reprecipitation process, placed between two glass slides, and observed with a fluorescence microscope by exciting with UV light (350380 nm) and recording emission above 400 nm. Alternatively, irradiation was also performed with blue light (450490 nm) (for example, see samples 2 and 3 in Figures S4 and S6). The concentrated and dilute samples provided the same type of particles, although of different size and abundance. The following description is that of the concentrated samples (2 104 M), which gave numerous objects with a size easily observable with our fluorescence microscope. Samples 1 and 46 gave microcrystals. These microcrystals measured up to 50 μm for compounds 1 and 4 (Figures 4 and S5), only 10 μm for the t-butyl derivative 5 (many of them being twinned, Figure S5), and even less for the octyl derivative (6), with a tendency to agglomerate. In contrast, the methoxy and methyl derivatives spontaneously organized in nano-/microfibers (Figures 4 and S4), which measured several tens of micrometers. For 2, fibers were straight and discrete. For 3, they seemed to be flexible, probably made of smaller juxtaposed entities. The influence of experimental conditions on the formation of nanofibers was investigated by varying the nature of the organic solvent (ethanol, dimethylformamide (DMF), and tetrahydrofuran (THF)) and its concentration (2%, 5%, and 10%) in the reprecipitation medium, while keeping the same dye concentration (2 104 M). Detailed data are gathered in Tables S1 and S2. From a general point of view, the length and width of the particles increased with the proportion of solvent in the medium, except for DMF. For compound 2, rod-like fibers were observed using the three solvents at low concentration. They were between 30 and 60 μm long, and 13 μm wide. Their extremities were far brighter than other regions of the fibers. It may be noted that with 10% THF, the fibers evolved toward flat microcrystals (Figure S6). For compound 3, with the three solvents, the fibers appeared as very thin ribbons. They measured between 50 and 100 μm long, and 24 μm wide, depending on the
The molecular arrangement of 2 is quite different. There are eight molecules per crystal unit, displayed on tilted planes that form angles of about 106° (Figure 3c). Molecules have up to seven short contacts (2.8 Å), but mainly with molecules situated in a different plane (Figure 3d). Between two unit cells, the molecules situated on parallel planes are only separated by 3.5 Å, but they are not superimposed. It is noteworthy that for 2, the heterocyclic oxygen atom, as well as the methoxy group, is involved in short contacts with adjacent molecules. It must also be noted that for both molecules 2 and 3, two different positions for the O and N atoms with an inversion of these two atoms were observed (see the Supporting Information). Preparation of Particles Using the Reprecipitation Method. To obtain aqueous suspensions of small particles, a reprecipitation method was implemented.17 This simple and mild method, based on a solvent exchange, has been widely developed during the past few years and has proved to be very useful in preparing nano- and microcrystals of various organic dyes.18 It consists of 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, two stock solutions of 2-phenyl-benzoxazole derivatives (1 102 and 1 103 M) in ethanol were prepared. Next, 40 μL of these solutions was very rapidly injected into a cell containing 1.96 mL of water, and the mixture was left under stirring at room temperature. The dye concentration in the mixture was 2 104 M (concentrated sample) or 2 105 M (dilute sample), the proportion of ethanol in water being 2%, v/v. The process was monitored for dilute samples by UVvis absorption spectroscopy, to determine the time necessary for its completion (see Supporting Information Figures S2 and S3). Actually, according to our experience, after the initial reprecipitation process, the suspensions remain stable during many hours. Crystal growth actually continues by Ostwald ripening, but it is quite slow and allows quite reproducible measurements to be performed. 858
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Figure 4. Fluorescence microscopy images of aqueous suspensions of the 2-phenylbenzoxazole derivatives 2, 3, and 4 (2 104 M) after reprecipitation in water with 2% v/v ethanol. Excitation with UV light.
reprecipitation conditions. At low solvent concentration, the fibers were very similar and of regular shape, independent of the solvent nature. The quality of the fibers remained satisfactory even with 10% ethanol. In contrast, some small differences in their morphology were noticed with increasing proportion of the other solvents, and the fibers tended to be strands of fibrils, with a branched appearance. It is noteworthy that the fibers were brightly emissive on their edges and dark in the center (Figure S6). 2. Observations by Transmission Electron Microscopy. TEM allows the observation of particles much smaller than those distinguished with the fluorescence microscope and is thus particularly adapted for the analysis of the dilute samples (dye concentration: 2 105 M). A drop of suspension taken 1 h or more after the beginning of the reprecipitation process was deposited on a grid, dried, and stained by a contrasting agent. Particles were found in all of the samples (Figure 5). The 2-phenyl-benzoxazole (1) sample contained both small nanocrystals (2030 nm) that stacked together and a few microcrystals, up to 800 nm. The crystallinity was confirmed by a clear electron diffraction spectrum (Figure S7). Samples of the butyl (4 and 5) and octyl (6) derivatives also revealed the presence of microcrystals, which often seemed to be composed of smaller crystals stacked together. The methyl (2) and methoxy (3) derivatives led to regular, acicular structures, which surprisingly hardly fixed the contrasting agent. The size was identical to that measured by fluorescence microscopy for dilute samples. Concentrated (2 104 M) samples of compounds 2 and 3 reprecipitated with 2% DMF, as well as with 2% and 10% THF were also observed. The size and shape of the particles were in good agreement with former observations. 3. Observations of the Fibers by Scanning Electron Microscopy. The particles resulting from the reprecipitation process of compounds 2 and 3 at 2 104 M in water containing 2% ethanol were filtered and dried under vacuum at 45 °C, before observation by SEM (Figures 6 and S8). For 2, the method showed the presence of elongated particles that measured about 3040 μm long and 0.71.5 μm wide. This is in total agreement with the size of the fibers observed by fluorescence microscopy for the concentrated sample. SEM allows additional details to be seen. For instance, it appeared that the fibers are not cylindrical. They are flat structures, the thickness of which seems to be below 200 nm. Their extremities are irregular, and it seems that most of the fibers can be split lengthwise, as if they were made by small fibrils stuck together. For 3, SEM revealed the formation of a reticulated microporous solid, whose cylindrical fibers have a diameter of about 50100 nm. No individual particles were
Figure 5. Transmission electron microscopy images of the samples made from aqueous suspensions of the 2-phenyl-benzoxazole derivatives (16) (2 105 M) after reprecipitation in water with 2% v/v ethanol.
observed any longer. It seems that the particles observed in the suspensions have formed thread-like structures, which subsequently coalesced. X-ray Powder Diffraction (XRPD) Spectra. The crystallinity of the nanofibers observed by SEM was checked by XRPD. Both patterns were typical of crystalline compounds. These spectra were compared to those calculated from the X-ray analyses performed on the monocrystals. The obtained fit was very good for 3 (Figure 7), and less satisfying for 2. In both cases, the deviation of the baseline revealed the presence of some amorphous matter. Fluorescence Properties. 1. 2-Phenyl-benzoxazole Derivatives Dissolved in n-Heptane and in Ethanol. The behavior of compounds 13 dissolved in cyclohexane, hexane, and ethanol has been reported in the literature.5a,8a,8b To complete these data and extend the study to the other compounds in the series, spectroscopic measurements were made in n-heptane and ethanol, and the results are gathered in Table 1. For all six 859
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Figure 6. Scanning electron microscopy images of the samples made from aqueous suspensions of the 2-phenyl-benzoxazole derivatives 2 and 3 (2 104 M) after reprecipitation in water with 2% v/v ethanol.
the progress of the reprecipitation process. Consequently, measurements were performed more than 1 h after the beginning of the reprecipitation process, which allows the spectral variations to be reduced. Moreover, measurements were repeated many times on different samples, and so the spectrum given in the figure is representative of most of the samples. The excitation and fluorescence spectra showed that the vibrational resolution was lost for the unsubstituted and alkyl derivatives, and barely visible for the methoxy derivative (Figure 8a). The spectra were shifted to the red with respect to those taken in organic solvents, the difference reaching 30 nm (2400 cm1) for 6. For all compounds, no changes were detected in the shape and position of the emission spectra when varying the excitation wavelength, and, conversely, the excitation spectra were unaffected by a variation of the emission wavelength. This was surprising, because suspensions obviously contain a proportion of dissolved dye as well as particles. The apparent fluorescence quantum yields of these suspensions were tentatively measured for hydrophobic compounds 4 and 5, assuming that most dyes are present as microparticles. They were found to be 0.59 and 0.60, respectively, which is slightly lower than those of the dyes dissolved in organic solvents. 3. Photoluminescence Properties of the 2-Phenyl-benzoxazole Derivatives in the Microcrystalline State. The photoluminescence properties of the 2-phenyl-benzoxazole derivatives in the solid state were investigated using a spectrofluorometer equipped with an integration sphere. All compounds exhibited strong photoluminescence upon excitation at 310 nm (Table 1). The shape of the emission and excitation spectra varied from one compound to another. The vibrational resolution was totally lost for the unsubstituted (Figure 8b) and methoxy derivatives (Figure 8a); it was more or less clearly distinguished for the other compounds, both in the emission (Figure 8b) and in the excitation spectra (see the example of 3 and 4 in Figure S9). The maximum of the emission spectra varied from 360 to 393 nm. Interestingly, the superposition of the normalized spectra (Figure 8b) shows that the spectrum of 1 is quite narrow and centered on the UV region, while the spectra of the substituted compounds were much broader and extended significantly beyond the blue region of the visible. This trend is particularly strong with the n-butyl and n-octyl derivatives 4 and 6. The photoluminescence spectra were therefore significantly red-shifted with respect to the fluorescence spectra of the corresponding dissolved compounds. The photoluminescence quantum yield varied from 0.26 for 1 to 0.38 for 2 and 4.
Figure 7. Red line: Experimental XRPD pattern obtained for the solid formed by reprecipitation of 2-(4-methyl-phenyl)benzoxazole (3) at 2 104 M in water with 2% v/v ethanol, and subsequent filtration. Blue line: Theoretical XRPD pattern calculated from the corresponding X-ray analysis.
compounds, the absorption spectra were the same in n-heptane and in ethanol, and the excitation spectra were identical to the absorption spectra. The emission spectra, which displayed vibrational fine structure, were slightly red-shifted in ethanol. The fluorescence quantum yields were high in both solvents. This behavior is characteristic of compounds that are unlikely to be involved in hydrogen bonding, and whose polarity is low in both ground and excited states. In other words, they are largely insensitive to the polarity and proticity of the medium. The spectroscopic properties hardly varied with substitution; the substituted compounds absorbed and emitted at only slightly longer wavelength than unsubstituted compound 1. The presence of a bulky alkyl group in the 40 -position was not expected to change markedly the optical properties in solution, with respect to those of the methyl derivative. In fact, the absorption and emission properties of 36 were very similar. Regarding the methoxy derivative, its spectra were slightly shifted to the red with respect to those of the other compounds, due to the donor character of the substituent. 2. Fluorescence Study of the 2-Phenyl-benzoxazole Derivatives in Aqueous Suspensions. Suspensions of the 2-phenylbenzoxazole derivatives at 2 105 M in water with 2% v/v ethanol were studied after completion of the reprecipitation process, using cells of 1 cm optical path length for compounds 4 and 5, and cells of 1 mm for the other compounds, so that absorbance was below 0.05. The samples were used without further dilution. It must be underlined that the shape of the spectra depends on 860
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Table 1. Spectroscopic Characteristics of 2-Phenyl-benzoxazole Derivatives Dissolved in Organic Solvents, in Aqueous Suspensions, and in the Solid Statea solution in n-heptane
solution in ethanol
aqueous suspension (2% ethanol)
solid state
λabs (nm)
λem (nm)
ØF
λabs (nm)
λem (nm)
ØF
λabs (nm)
λem (nm)
λem (nm)
ØP 0.26
BzxH (1)
292, 298, 312
316, 330, 346
0.69
292, 298, 310
322, 336, 350
0.73
292s, 300, 316
328s, 344, 352
350s, 360, 378s
BzxOCH3 (2)
298, 306, 320
324, 340, 356
0.72
298, 306, 320
332, 348, 360
0.89
306
338s, 356, 364
382
0.38
BzxCH3 (3) BzxnBu (4)
294, 302, 314 294, 302, 316
318, 334, 350 320, 334, 352
0.74 0.74
292, 302, 314 292, 302, 314
322, 338, 354 324, 340, 356
0.72 0.80
300, 306, 320 300, 308, 322
328s, 344, 358 328s, 346, 360
369, 383 378, 392s
0.34 0.38
BzxtBu (5)
294, 302, 316
318, 334, 352
0.77
292, 302, 314
324, 340, 354
0.75
300, 306, 320
328s, 344, 360
346, 362, 374
0.33
BzxnOct (6)
294, 302, 316
320, 334, 352
0.77
292, 302, 314
324, 340, 354
0.83
300, 312, 326
350, 370, 382
356, 374, 393
0.35
Maximum absorption wavelength (λabs), maximum emission wavelength (λem), and fluorescence and photoluminescence quantum yields (ØF and ØP, respectively). The most intense band is underlined. For solutions and suspensions, excitation near the absorption maximum. For solid samples, λex = 310 nm. The error on the fluorescence and photoluminescence quantum yields is estimated to be 10%. a
Benzoxazoles are widely used as chelating agents, and many crystal structures have been reported for the complexes formed with platinium, zinc, and beryllium cations. However, surprisingly little data concerning the crystal structure of metal-free 2-phenyl-benzoxazole derivatives can be found in the literature. 2-Phenylbenzoxazole has been reported to crystallize in the orthorhombic system,19 but very little information is available for this structure. A closely related compound, 2-(20 -hydroxyphenyl)-1,3-benzoxazole, also crystallizes in the orthorhombic system.20 In the latter case, the adjacent molecules are displayed in a head-to-head style, and strong ππ stacking interactions take place. It is well-known that this arrangement, similar to that encountered in H-aggregates, impedes fluorescence emission. Assuming some similarity in the packing mode, quite a low photoluminescence quantum yield was expected for 1 in the solid state. This is far from being the case, even if this compound is slightly less emissive than the others. In the present work, the crystal packing mode of 2 and 3 was obtained. In both cases, the molecules lie parallel and are arranged like bricks in a wall, which implies that the transition dipole moments of the fluorophores are stabilized. Short contacts are established with adjacent molecules that are situated in a very different plane (the angle between the planes is close to 90° for 2 and 106° for 3). This is allowed by quite a rare molecular arrangement, which is highly favorable to light emission because the transition dipole moments of the molecules are crossed and thus have little interaction between them.21 This explains why the photoluminescence quantum yield was quite high. Besides, molecules are almost symmetrical and can display two different positions in the crystal unit, with an inversion of the O and N atoms. This could lead to crystal imperfections, and hence to slightly different energy levels for the molecules that compose the crystal. This could also disfavor the formation of excimers and the migration of excitons, and therefore increase the photoluminescence quantum yield. Substitution brought some difference in the photoluminescence efficiency. The unsubstituted compound 1 is less emissive than the substituted compounds, and its emission spectrum is narrow and located in the UV. This can be attributed to both the intrinsic properties of 1 that emits less than the others in solution and to differences in the crystal packing mode. Among the alkylsubstituted compounds, the n-butyl and n-octyl derivatives have the broadest spectra and best emission efficiency. An explanation may be found in the loose crystal packing mode suggested by the low melting points. The presence of the bulky t-butyl group leads
Figure 8. (a) From left to right: Fluorescence emission spectra (λex = 306 nm) of the methoxy derivative 2 in n-hexane, ethanol, and water/ ethanol 98:2, v/v after reprecipitation, and photoluminescence spectrum (λex = 310 nm). (b) Normalized photoluminescence emission spectra of 2-phenyl-benzoxazole derivatives 1 (green line), 3 (red line), 4 (orange line), 5 (blue line), and 6 (purple line). λex = 310 nm.
’ DISCUSSION Crystalline materials show interesting optical properties, especially for dichroism and wave-guiding. However, manipulation and development of efficient fluorescent systems typically proves challenging. Indeed, phenomena like exciton and excimer formation, which strongly compete with fluorescence, frequently occur in crystals. Surface defects and impurities act as energy traps for excited species. Moreover, strong intermolecular ππ interactions or continuous intermolecular hydrogen bonding between neighboring fluorophores have been suggested as the main factors of fluorescence quenching.3 Consequently, knowledge of the crystal packing mode is the first requirement to understand the solid-state photophysical properties. 861
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Langmuir to no improvement with respect to the other compounds. This point is noteworthy, because among other strategies to improve the photoluminescence quantum yields,21,22a the introduction of nonplanar or bulky substituents is aimed at inhibiting the close packing of molecules that cause fluorescence quenching.22 These substituents often induce the amorphization of the compound.23 In the present case, all of the studied compounds are crystalline, and it is possible that the presence of moderately bulky alkyl groups does not significantly modify the molecular arrangement, which is already very favorable for the methyl derivative. It could be interesting to introduce much bulkier groups and to see if amorphization leads to additional improvement of the solid-state properties. Another important point is that, among the six compounds investigated, only 2 and 3 spontaneously formed nanofibers via the reprecipitation method. The formation of nanofibers with small molecules via the reprecipitation method is quite rare18e,h,24 and still poorly understood. In our case, it is noteworthy that the presence of a small aliphatic group on a rod-like molecule favors the formation of nanofibers, as compared to the unsubstituted molecule 1. This trend is lost when increasing the size of the alkyl chain, because the butyl and octyl compounds only lead to the formation of microcrystals. The nature (methyl or methoxy) of the substituent closely determines the morphology and stability of the fibers. Crystalline materials were obtained in both cases, with a packing mode close to that of the corresponding macroscopic crystal. The experimental conditions of reprecipitation also affect the morphology of the fibers. When small proportions of organic solvent are used, the nature of the solvent plays a minor role. Increasing the solvent proportion leads to clear changes in the fiber morphology, especially when using DMF and THF. This can be explained by solubility differences. With a significant amount of organic solvent (10%) in the reprecipitation medium, the solubility of the benzoxazole derivative is increased, and thus supersaturation is decreased. Consequently, crystals grow more slowly and become bigger than in a medium that contains almost only some water. Both types of fibers are interesting. The methoxy derivative (2) leads to stable, quite regular, elongated structures. Their optical properties are remarkable: the photoluminescence quantum yield is the highest among all of the compounds studied here, and the emission spectrum is wide and red-shifted. The emitted light is mainly transmitted to the extremities of the fibers. This phenomenon can be attributed to waveguiding,25 which is particularly efficient due to good crystallinity. SEM observations suggested that fibers obtained at high dye concentration could be strands of fibrils. The bright points observed here and there on the fibers thus probably result from light emerging from the extremity of these fibrils, or at least from surface defects. In suspension, the methyl derivative (3) forms nanofibers that almost resemble nanoribbons. These fibers also show a wave-guiding phenomenon that explains their bright and dark areas, light being now transmitted to every edge of the fiber. The most interesting point with compound 3 is that the flat fibers evolve toward cylindrical fibers arranged in a tight network upon drying. This behavior is strongly reminiscent of that encountered in a previous work with berberine palmitate.18f While this phenomenon had seemed unique, it now appears to be more frequent than we had originally thought. Obviously, recrystallization of compound 3 in organic solvents does not lead to reticulated material (Figure S1), and it would be interesting to investigate the exact role played by the reprecipitation method in the formation of this network. Fluorescent reticulated materials are of great demand for applications as chemical and biochemical sensors.24a,26
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Finally, it is noteworthy that compound 4 gave beautiful needle-like crystals by recrystallization in methanol, while reprecipitation only led to shapeless microcrystals. Consequently, the habit of the crystals obtained by recrystallization in organic solvents does not allow prediction of the shape of the particles obtained by reprecipitation, probably because the latter system is very far from equilibrium.
’ CONCLUSION 2-Phenyl-benzoxazole derivatives have been known for a long time for their excellent optical properties in organic solution, and in the solid state after chelation with a metal cation. The uncomplexed molecules had not been investigated until now in the solid state for applications as fluorescent nanomaterials. This study shows that they could be particularly interesting for this field. (i) The substituted compounds exhibit an original packing mode that allows efficient light emission in the solid state. (ii) Their emission spectrum in the solid state is broad, allowing detection from blue to green. (iii) The quantum yield of photoluminescence of the micro- and nanocrystals is high, whatever the polarity of the surrounding medium. This allows deployment in very different media and especially in water, on the contrary to nanoparticles made of more polar dyes, the luminescence of which is quenched in aqueous media.18e (iv) The compounds that bear a methyl and a methoxy substituent spontaneously arrange as nanofibers, which display interesting wave-guiding properties. One of these compounds spontaneously leads upon drying to a fluorescent reticulated material. Work is now in progress to prepare nanocrystals of these derivatives in view of biological applications, as well as other benzoxazole derivatives with original optical properties. ’ ASSOCIATED CONTENT
bS Supporting Information. Additional figures and tables. This material is available free of charge via the Internet at http:// pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author
*Fax: +335-6155-8155. E-mail: sff@chimie.ups-tlse.fr.
’ ACKNOWLEDGMENT We would like to thank Mr. Laurent Weingarten and Mr. Lucien Datas (Service Commun de Microscopie Electronique de l’Universite Paul Sabatier) for their kind help in TEM and SEM measurements, and Dr. Laure Vendier (Service Diffraction X du Laboratoire de Chimie de Coordination de Toulouse) for the XRPD patterns. Financial support from the CNRS and Region Aquitaine is gratefully acknowledged. ’ REFERENCES (1) (a) Lim, S.-J.; An, B.-K.; Jung, S.-D.; Chung, M.-A.; Park, S.-Y. Angew. Chem., Int. Ed. 2004, 43, 6346–6350. (b) Jagannathan, R.; Irvin, G.; Blanton, T.; Jagannathan, S. Adv. Funct. Mater. 2006, 16, 747–753. (c) Quian, G.; Dai, B.; Luo, M.; Yu, D.; Zhan, J.; Zhang, Z.; Ma, D.; Wang, Z. Y. Chem. Mater. 2008, 20, 6208–6216. (d) Botzung-Appert, E.; Zaccaro, J.; Gourgon, C.; Usson, Y.; Baldeck, P. L.; Ibanez, A. J. Cryst. Growth 2005, 283, 444–449. (e) Kaneko, Y.; Onodera, T.; Kasai, H.; Okada, S.; Oikawa, H.; Nakanishi, H.; Fukuda, T.; Matsuda, H. J. Mater. 862
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