Facile access to highly fluorescent nanofibers and microcrystals via reprecipitation of 2-phenyl-benzoxazole derivatives

Article abstract of DOI:10.1021/la2036554

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.

Full text of DOI:10.1021/la2036554

ARTICLE
pubs.acs.org/Langmuir
Facile Access to Highly Fluorescent Nanofibers and Microcrystals via
Reprecipitation of 2-Phenyl-benzoxazole Derivatives
Abdelhamid Ghodbane,† Sꢀebastien D’Altꢀerio,† Nathalie Saffon,§ Nathan D. McClenaghan,||
Luca Scarpantonio,|| Pascale Jolinat,‡ and Suzanne Fery-Forgues*,†
†Laboratoire des Interactions Molꢀeculaires Rꢀeactivitꢀe Chimique et Photochimique, CNRS UMR 5623, ‡Laboratoire LAPLACE,
CNRS UMR 5213, and §Service commun RX, Institut de Chimie de Toulouse, ICT- FR2599, Universitꢀe Paul Sabatier 31062
Toulouse cedex 9, France
Institut des Sciences Molꢀeculaires, CNRS UMR 5255, University of Bordeaux, 351 Cours de la Libꢀeration, 33405 Talence cedex, France
S Supporting Information
b
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 photo-
luminescence 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
The aim of the present work is to investigate the relationship
between the molecular structure, the solid-state optical proper-
ties, and the morphology of the particles spontaneously obtained
using a simple solvent-exchange method. The 2-phenyl-benzox-
azole series was chosen for the following reasons: (i) Compounds
of this family show excellent fluorescence properties when dis-
solved 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 well-
known.5a,7a,8 (iii) Curiously, to our knowledge, solid-state spec-
troscopic 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 lumines-
cent 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 com-
pounds readily form single crystals, which are suitable for standard
X-ray crystallographic analysis. This allows elucidation of the
There has been growing interest in developing organic nano-
and 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 com-
pounds 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, be-
cause this allows easy and cheap preparation of materials. Unfor-
tunately, 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 fluores-
cence properties still relies on an empirical approach.
Received: September 19, 2011
Revised:
November 4, 2011
Published: November 30, 2011
r
2011 American Chemical Society
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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, PheꢀCH2), 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 cmꢀ1; νCdC 1577 cmꢀ1
;
1556 cmꢀ1
.
Figure 1. Structural formulas of the 2-phenyl-benzoxazole derivatives
(1ꢀ6).
ν
ꢀNdCꢀOꢀ
1
2-(40-n-Octylphenyl)-benzoxazole (6). Mp: 57 °C. H 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, PheꢀCH2), 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 =
molecular arrangement, which is extremely valuable for under-
standing the photophysical properties in the solid state. (v) They
are generally poorly soluble in water. Therefore, they lend them-
selves 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 suspen-
sions 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 fluor-
escence 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.
308 (M + H+); 336 (M + C2H5 ). IR: νCdN 1619 cmꢀ1; νCdC
+
1576 cmꢀ1; ν
1556 cmꢀ1
.
ꢀNdCꢀOꢀ
Crystallographic Data. Data for compounds 2 and 3 were col-
lected on a Bruker-AXS APEX II diffractometer using a 30 W air-cooled
microfocus source (ImS) with focusing multilayer optics at a tempera-
ture of 193(2 )K for 2 and on a Bruker-AXS SMART APEX II diffracto-
meter at a temperature of 213(2) K for 3, with graphite-monochromated
Mo Kα radiation (wavelength = 0.71073 Å) by using phi- and omega-
scans. The structures were solved by direct methods (SHELXS 97),12
and all non-hydrogen atoms were refined anisotropically using the least-
squares method on F2.13
’ EXPERIMENTAL SECTION
Materials. N-Methylpyrrolidinone, 2-aminophenol, and acid chlor-
ides (benzoyl, 4-methoxy-benzoyl, p-toluoyl, 4-n-butyl-benzoyl, 4-t-butyl-
benzoyl, and 4-n-octyl-benzoyl chloride) used for synthesis were ob-
tained 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 Deriva-
tives (1ꢀ6). An aliquot of 2-aminophenol (9.16 mmol, 1 g) was dis-
solved in 5 mL of N-methylpyrrolidinone. The orange solution was dea-
erated with nitrogen and cooled to 0 °C. The acid chloride (9.16 mmol),
previously dissolved when necessary in a minimum N-methylpyrrolidi-
none, 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 vary-
ing 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.
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 Åꢀ3
.
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 Åꢀ3
.
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, deposit@ccdc.cam.ac.uk).
Apparatus. Mass spectra were obtained at the “Service Commun de
Spectromꢀetrie de masse de l’Universitꢀe Paul Sabatier de Toulouse” with
For spectroscopic measurements, subsequent purification was per-
formed 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.
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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 temperature-
controlled cell. UVꢀvis absorption spectra were recorded on a Hewlett-
Packard 8452A diode array spectrophotometer. For solutions and
suspensions, corrected steady-state fluorescence spectra were registered
with a Photon Technology International (PTI) Quanta Master 1 spec-
trofluorometer, using cells of 1 cm or 1 mm optical pathlength. The
fluorescence quantum yields (ΦF) were determined using the classical
filters. Alternatively, samples were illuminated at 450ꢀ490 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’Universitꢀe 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 col-
lected in transmission mode, on capillary samples, on a θꢀθ 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.
2
formula: ΦFx = (As ꢁ Fx ꢁ nx ꢁ 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 Jobin-
Yvon Fluorolog-3 fluorometer equipped with a Labsphere optical
Spectralon integrating sphere (diameter of 100 mm), which provides
a reflectance >99% over the 400ꢀ1500 nm range (>95% within
250ꢀ2500 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:
’ RESULTS
Preparation of the 2-Phenyl-benzoxazole Derivatives and
Observation with the Polarization Microscope. Compounds
1ꢀ6 were synthesized according to a standard procedure, by
condensing 2-aminophenol with an acyl chloride, in a 1:1 stoich-
iometry (see Experimental Section). Observation with the polari-
zation microscope confirms that the solids obtained are crystal-
line. Compounds 1ꢀ4 gave needle-like microcrystals, while the
t-butyl (5) and n-octyl (6) derivatives gave platelets. Two exam-
ples 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 mono-
clinic 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 πꢀπ
interactions. The molecule pairs are arranged perpendicularly to
each other (Figure 3b). Each molecule has only two short con-
tacts (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 Eb
3
ΦP
with
α ¼ 1 ꢀ
¼
La α
3
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 fluor-
escence 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 350ꢀ380 nm,
and the emission wavelength was set at above 400 nm, using suitable
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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).
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 in-
volved 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 Meth-
od. To obtain aqueous suspensions of small particles, a repreci-
pitation 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 (1ꢁ 10ꢀ2 and 1ꢁ 10ꢀ3 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 ꢁ 10ꢀ4 M (concentrated
sample) or 2 ꢁ 10ꢀ5 M (dilute sample), the proportion of
ethanol in water being 2%, v/v. The process was monitored for
dilute samples by UVꢀvis absorption spectroscopy, to determine
the time necessary for its completion (see Supporting Informa-
tion 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 reprodu-
cible measurements to be performed.
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 (350ꢀ380 nm) and recording emission above 400 nm.
Alternatively, irradiation was also performed with blue light
(450ꢀ490 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 ꢁ 10ꢀ4 M), which gave numerous objects with a size easily
observable with our fluorescence microscope. Samples 1 and
4ꢀ6 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 tetrahy-
drofuran (THF)) and its concentration (2%, 5%, and 10%) in
the reprecipitation medium, while keeping the same dye con-
centration (2 ꢁ 10ꢀ4 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 1ꢀ3 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 2ꢀ4 μm wide, depending on the
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Figure 4. Fluorescence microscopy images of aqueous suspensions of the 2-phenylbenzoxazole derivatives 2, 3, and 4 (2 ꢁ 10ꢀ4 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 dis-
tinguished with the fluorescence microscope and is thus parti-
cularly adapted for the analysis of the dilute samples (dye con-
centration: 2 ꢁ 10ꢀ5 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 (20ꢀ30 nm) that
stacked together and a few microcrystals, up to 800 nm. The
crystallinity was confirmed by a clear electron diffraction spec-
trum (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 to-
gether. The methyl (2) and methoxy (3) derivatives led to regular,
acicular structures, which surprisingly hardly fixed the contrast-
ing agent. The size was identical to that measured by fluorescence
microscopy for dilute samples. Concentrated (2 ꢁ 10ꢀ4 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 Micro-
scopy. The particles resulting from the reprecipitation process of
compounds 2 and 3 at 2 ꢁ 10ꢀ4 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
30ꢀ40 μm long and 0.7ꢀ1.5 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 cylin-
drical. 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 50ꢀ100 nm. No individual particles were
Figure 5. Transmission electron microscopy images of the samples
made from aqueous suspensions of the 2-phenyl-benzoxazole derivatives
(1ꢀ6) (2 ꢁ 10ꢀ5 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 sub-
sequently 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 Deriva-
tives Dissolved in n-Heptane and in Ethanol. The behavior of
compounds 1ꢀ3 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
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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 ꢁ 10ꢀ4 M) after reprecipitation in water with 2% v/v ethanol.
the progress of the reprecipitation process. Consequently, mea-
surements 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 cmꢀ1) for 6. For all com-
pounds, no changes were detected in the shape and posi-
tion of the emission spectra when varying the excitation wave-
Figure 7. Red line: Experimental XRPD pattern obtained for the solid
formed by reprecipitation of 2-(4-methyl-phenyl)benzoxazole (3) at 2 ꢁ
10ꢀ4 M in water with 2% v/v ethanol, and subsequent filtration. Blue
line: Theoretical XRPD pattern calculated from the corresponding X-ray
analysis.
length, 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 hydro-
phobic 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.
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 vibra-
tional 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 pre-
sence 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 3ꢀ6 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 Deriva-
tives in Aqueous Suspensions. Suspensions of the 2-phenyl-
benzoxazole derivatives at 2 ꢁ 10ꢀ5 M in water with 2% v/v
ethanol were studied after completion of the reprecipitation pro-
cess, 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
3. Photoluminescence Properties of the 2-Phenyl-benzoxa-
zole Derivatives in the Microcrystalline State. The photolumi-
nescence 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.
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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
λabs (nm) λem (nm)
solution in ethanol
λabs (nm) λem (nm)
aqueous suspension (2% ethanol)
solid state
λem (nm)
350s, 360, 378s 0.26
ØF
ØF
λabs (nm)
λem (nm)
ØP
BzxꢀH (1)
292, 298, 312 316, 330, 346 0.69 292, 298, 310 322, 336, 350 0.73
292s, 300, 316
306
328s, 344, 352
338s, 356, 364
328s, 344, 358
328s, 346, 360
328s, 344, 360
350, 370, 382
BzxꢀOCH3 (2) 298, 306, 320 324, 340, 356 0.72 298, 306, 320 332, 348, 360 0.89
382
0.38
0.34
0.38
0.33
0.35
BzxꢀCH3 (3)
BzxꢀnBu (4)
BzxꢀtBu (5)
BzxꢀnOct (6)
294, 302, 314 318, 334, 350 0.74 292, 302, 314 322, 338, 354 0.72
294, 302, 316 320, 334, 352 0.74 292, 302, 314 324, 340, 356 0.80
294, 302, 316 318, 334, 352 0.77 292, 302, 314 324, 340, 354 0.75
294, 302, 316 320, 334, 352 0.77 292, 302, 314 324, 340, 354 0.83
300, 306, 320
300, 308, 322
300, 306, 320
300, 312, 326
369, 383
378, 392s
346, 362, 374
356, 374, 393
a 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%.
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, surpris-
ingly 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-hydro-
xyphenyl)-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 πꢀπ 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
Figure 8. (a) From left to right: Fluorescence emission spectra (λex
=
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 photolumines-
cence quantum yield.
Substitution brought some difference in the photolumines-
cence 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 alkyl-
substituted 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
306 nm) of the methoxy derivative 2 in n-hexane, ethanol, and water/
ethanol 98:2, v/v after reprecipitation, and photoluminescence spec-
trum (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, espe-
cially for dichroism and wave-guiding. However, manipulation
and development of efficient fluorescent systems typically proves
challenging. Indeed, phenomena like exciton and excimer for-
mation, which strongly compete with fluorescence, frequently
occur in crystals. Surface defects and impurities act as energy
traps for excited species. Moreover, strong intermolecular πꢀπ
interactions or continuous intermolecular hydrogen bonding
between neighboring fluorophores have been suggested as the
main factors of fluorescence quenching.3 Consequently, knowl-
edge of the crystal packing mode is the first requirement to
understand the solid-state photophysical properties.
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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 amorphi-
zation 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 pre-
sence 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 macro-
scopic 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 photo-
luminescence 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 extre-
mities of the fibers. This phenomenon can be attributed to wave-
guiding,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 pheno-
menon 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 forma-
tion of this network. Fluorescent reticulated materials are of great
demand for applications as chemical and biochemical sensors.24a,26
Finally, it is noteworthy that compound 4 gave beautiful
needle-like crystals by recrystallization in methanol, while repre-
cipitation 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 un-
complexed 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 photo-
luminescence 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 pro-
gress to prepare nanocrystals of these derivatives in view of bio-
logical applications, as well as other benzoxazole derivatives with
original optical properties.
’ ASSOCIATED CONTENT
S
Supporting Information. Additional figures and tables.
b
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’Universitꢀe 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 Rꢀegion
Aquitaine is gratefully acknowledged.
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