White-light-emitting self-assembled nanofibers and their evidence by microspectroscopy of individual objects

Article abstract of DOI:10.1021/ja106807u

The self-assembly of a blue-emitting light-harvesting organogelator and specifically designed highly fluorescent tetracenes yields nanofibers with tunable emissive properties. In particular, under near-UV excitation, white light emission is achieved in or

Full text of DOI:10.1021/ja106807u

Published on Web 12/23/2010
White-Light-Emitting Self-Assembled NanoFibers and Their
Evidence by Microspectroscopy of Individual Objects
Carlo Giansante, Guillaume Raffy, Christian Scha¨fer, Hakim Rahma, Min-Tzu Kao,
Alexandre G. L. Olive, and Andre´ Del Guerzo*
UniVersite´ Bordeaux 1, CNRS, Institut des Sciences Mole´culaires, NEO Nanostructures
Organiques, 351 crs de la Liberation, 33405 Talence ce´dex, France
Received July 30, 2010; E-mail: a.del-guerzo@ism.u-bordeaux1.fr
Abstract: The self-assembly of a blue-emitting light-harvesting organogelator and specifically designed
highly fluorescent tetracenes yields nanofibers with tunable emissive properties. In particular, under near-
UV excitation, white light emission is achieved in organogels and dry films of nanofibers. Confocal
fluorescence microspectroscopy demonstrates that each individual nanofiber emits white light. A kinetic
study shows that an energy transfer (ET) occurs between the blue-emitting anthracene derivative and the
green- and red-emitting tetracenes, while inter-tetracene ETs also take place. Moreover, microscopy unravels
that the nanofibers emit polarized emission in the blue spectral region, while at wavelengths higher than
500 nm the emission is not significantly polarized.
removable fluid and a network of self-assembled nanofibers.8
These fiber preparation methods exploiting solution processing
Introduction
Self-assembly of chemical species by weak noncovalent
interactions is a widespread strategy exploited by nature in its
forms and functions1 and is attracting considerable interest in
the creation of artificial systems.2 Self-assembly has, for
example, brought significant advantages in the bottom-up
approach to realize highly emissive multichromophoric nano-
structures for light-harvesting and optoelectronic purposes.3-5
Quasi-one-dimensional systems have also been attained by
exploiting the programmed self-assembly of low-molecular-
weight organic chromophores.6,7 In the particular case of
organogels, the heterogeneous soft material is composed of a
are an alternative to vapor deposition of low-molecular-weight
organic fluorophores9 or electrospinning of molecular, macro-
molecular, or hybrid organic/inorganic constituents.10-13 Inter-
estingly, the latter multicomponent systems have been suggested
as white-light-emitting materials, a property only rarely explored
in organogels.14-16 In all these cases9-16 the bulk material
composed of multiple components and of randomly dispersed
nano- or micro-objects was investigated to characterize the
emissive properties. Nevertheless, this resulted only in the
determination of averaged macroscopic properties without full
apprehension of the implications of heterogeneity and random
orientations of objects. Besides, the influence of the anisotropic
shape of the fibers on the emission of white light has not been
studied so far, a feature that could be exploited in macroscopic
materials. Thus, the particular characteristics of an indiVidual
multicomponent nanofiber have not been addressed yet. In this
work we aimed to realize white-light-emitting self-assembled
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316 J. AM. CHEM. SOC. 2011, 133, 316–325
10.1021/ja106807u  2011 American Chemical Society

White-Light-Emitting Nanofibers
A R T I C L E S
Figure 1. (a) Pictures of a 2 mM B gel in DMSO, 10 µM G solution in THF, and 10 µM R solution in THF under UV light (λex ) 365 nm) and the
molecular structures. (b) Corrected fluorescence spectra of B, G, and R (λex ) 365 nm).
nanofibers and ascertain by confocal fluorescence microspec-
troscopy17 the spectral, photophysical, and anisotropic features
of the individual nano-objects.
nanofibers of B are exploited as light-harvesting matrixes
capable of hosting and sensitizing by excitation energy transfer22
green and red emitters that have been specifically designed to
be co-self-assembled with B. The photophysics and anisotropic
properties of the three-component self-assembled nanofibers are
revealed by optical microscopy.
The above-mentioned optical properties are in general
obtained by blending three to four components, for example,
using molecules that display similar chemical structures and
interact favorably in assembled multicomponent systems. This
necessitates the use of a family of molecules with optical
properties that can be tuned with small modifications of the
structure. n-Acene derivatives are very good candidates as they
can be tailored chemically, exhibit high fluorescence quantum
yields, and record charge mobility in single crystals.18 The
HOMO-LUMO gap can be reduced by extending the π-con-
jugation along the long axis of the core by adding a conjugated
aromatic ring or along the short axis with phenyl or alkynyl-
trialkylsilyl substituents. They constitute an alternative to
oligo(phenylenevinylene), oligothiophene, perylene, or other
fluorescent π-conjugated organogelators. In particular, the
anthracene derivative B (2,3-bis(decyloxy)anthracene; Figure
1) displays a robust self-assembling ability in various organic
solvents such as alcohols or DMSO,19 even in the presence of
organic or inorganic guests.20,21 B differs by its particular
packing through weak interactions excluding H-bonds, while
displaying attractive emissive properties. In this work, the
Results and Discussion
Synthesis and Solution Properties of New Tetracenes. As
energy acceptors we have prepared new n-acenes that display
red-shifted emission, high fluorescence quantum yields, and
structural similarities to B. Thus, 2,3-bis(hexadecyloxy)-5,12-
diphenyltetracene (G; Figure 1) and 2,3-bis(hexadecyloxy)-
5,6,11,12-tetraphenyltetracene (R; Figure 1) have been synthe-
sized. G and R were obtained by addition of phenylmagnesium
bromide or phenyllithium to the 2,3-bis(hexadecyloxy)tetracene-
5,12-dione and 2,3-bis(hexadecyloxy)-6,11-diphenyltetracene-
5,12-dione derivatives, respectively, using hydriodic acid to
promote final aromatization of the dihydroxy intermediates
(Scheme 1). The dione precursors with hexadecyl chains were
previouslyobtainedbydemethylationofthemethoxyanalogue22,23
rapidly followed by a substitution reaction. In THF solutions,
G and R show green (λem ) 502 nm) and orange-red (λem
)
555 nm) fluorescence with quantum yields reaching 53% and
69%, respectively (Table 1, Figure 1). The red shift relative to
B is achieved using a tetracene core and the HOMO-LUMO
gap is tuned with two or four phenyl substituents. These also
raise the quantum yields as compared to those of 2,3-dialkox-
ytetracenes by reducing the singlet-triplet intersystem crossing,
as previously described in other substituted n-acenes.24
(17) (a) Eisele, D. M.; Knoester, J.; Kirstein, S.; Rabe, J. P.; Van den Bout,
D. A. Nat. Nanotechnol. 2009, 4, 658. (b) Lin, H.; Camacho, R.; Tian,
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Engelkamp, H.; Schenning, A. P. H. J.; Christianen, P. C. M.; Maan,
J. C.; Meijer, E. W. Angew. Chem., Int. Ed. 2005, 45, 1232. (d) Lang,
E.; Sorokin, A.; Drechsler, M.; Malyukin, Y. V.; Ko¨hler, J. Nano Lett.
2005, 5, 2635. (e) Hofkens, J.; Latterini, L.; Vanoppen, P.; Faes, H.;
Jeuris, K.; De Feyter, S.; Kerimo, J.; Barbara, P. F.; De Schryver,
F. C.; Rowan, A. E.; Nolte, R. J. M. J. Phys. Chem. B 1997, 101,
10588.
(21) (a) Sangeetha, N. M.; Bhat, S.; Raffy, G.; Belin, C.; Loppinet-Serani,
A.; Aymonier, C.; Terech, P.; Maitra, U.; Desvergne, J.-P.; Del Guerzo,
A. Chem. Mater. 2009, 21, 3424. (b) Del Guerzo, A.; Olive, A. G. L.;
Reichwagen, J.; Hopf, H.; Desvergne, J.-P. J. Am. Chem. Soc. 2005,
127, 17984.
(18) (a) Anthony, J. F. Angew. Chem., Int. Ed. 2008, 47, 452. (b) Bendikov,
M.; Wudl, F.; Perepichka, D. F. Chem. ReV. 2004, 104, 4891.
(19) (a) Desvergne, J.-P.; Brotin, T.; Meerschaut, D.; Clavier, G.; Placin,
F.; Pozzo, J.-L.; Bouas-Laurent, H. New J. Chem. 2004, 28, 234. (b)
Placin, F.; Desvergne, J.-P.; Lasse`gues, J.-C. Chem. Mater. 2001, 13,
117.
(22) Reichwagen, J.; Hopf, H.; Del Guerzo, A.; Belin, C.; Desvergne, J.-
P.; Bouas-Laurent, H. Org. Lett. 2005, 7 (6), 971.
(23) Paraskar, A. S.; Reddy, A. R.; Patra, A.; Wijsboom, Y.-H.; Gidron,
O.; Leitus, G.; Bendikov, M. Chem.sEur. J. 2008, 14, 10639.
(24) Komfort, M.; Lo¨hmannsro¨ben, H.-G.; Salhammer, T. J. Photochem.
Photobiol., A 1990, 51, 215.
(20) Desvergne, J.-P.; Del Guerzo, A.; Bouas-Laurent, H.; Belin, C.;
Reichwagen, J.; Hopf, H. Pure Appl. Chem. 2006, 78, 707.
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J. AM. CHEM. SOC. VOL. 133, NO. 2, 2011 317

A R T I C L E S
Giansante et al.
Scheme 1. Final Steps of the Synthesis of G and Ra
a Reagents and conditions: (a) BBr3, DCM; (b) K2CO3, n-C16H33Br, DMF; (c) PhMgBr (for G) or PhLi (for R), THF; (d) HI, Et2O.
Table 1. Absorption and Emission Properties of B, G, and R in Solution and in Organogels (Deoxygenated): Lowest Energy Absorption
Wavelength, λabs, Extinction Coefficient, ε, Emission Wavelength, λem, and Quantum Yield, Φem, Decay Time, τ, and Radiative Constant, kr
solvent
λ
abs, nm
ε × 103, M-1 cm-1
λ
em, nm
τ, ns
kr × 107, ns-1
Φem
B
G
R
B-gel
B-gel
W-gel
W-gel
THF
THF
THF
DMSO
MeOH
DMSO
MeOH
366/376
489
523
7.0/4.8
8.9
9.6
7b
389/410
502
555
c/412
c/412
d
5.0
13.8
13.6
e
e
e
5.0
3.8
5.1
0.25
0.53
0.69
0.24f
0.08f
0.26f
0.11f
385
385
7b
385/488a
385/488a
7b
7b
d
e
a At 515-525 nm an additional shoulder is seen. b For ε at 385 nm, the experimental error increases to ∼15% due to high absorbance and excitation
light diffusion. c Band around 389 nm not entirely measurable due to reabsorption (fluorimetry) and laser light scattering (microscopy). d White
spectrum. e Multiexponential decays; see the text and Figure 6. f Excitation at 365 nm, measured using an integration sphere.
UV light, the blue emission decreases while those of G and R
are observed. The partial excitation energy transfer from B to
the energy acceptors can be modulated by changing the
concentration of the acceptors, and using 0.012 equiv of G and
R, a white-light-emitting “W-gel” is obtained (Figure 2b).
Indeed, the emission mostly consists of the addition of the
spectral features of the B-gel and of slightly narrowed and 2
nm red-shifted spectra of G and R (Figure 3). This slight band
narrowing is reminiscent of emitters confined in a lattice with
a high degree of orientation and a reduced vibrational degree
of freedom, as well as more alike energy levels.26 Only the near-
UV emission is partly reabsorbed by the gel that is translucent
to visible light (see Figure 2c). The corresponding CIE 1931
chromaticity coordinates for a two-degree field of view (x )
Figure 2. (a, b) Pictures of cuvettes with 2.0 mM B with 0.012 equiv of
0.319, y ) 0.332; Figure 3b) nearly match those of the most
common white light standard (daylight simulator D65, x )
0.31292, y ) 0.32933). In addition, the fluorescence quantum
yield25 of the W-gel reaches 26%, indicating the low loss of
excited states in the gel. Interestingly, while a W-gel can be
obtained also using another solvent, some of the spectral
properties change. Indeed, when methanol is gelled by B, G,
and R in adequate proportions, although the spectrum shows
slightly more G and less R emission (Figure 3a), white light is
obtained (0.012 equiv of G and R, CIE x ) 0.306, y ) 0.331).
Besides, the quantum yield is lower (Φem ) 11%), in accordance
with the fact that the B-gel of MeOH emits less than the B-gel
of DMSO (Φem ) 8% vs 24%, Table 1). It has to be noticed
that previous studies by small-angle X-ray scattering had shown
that B-gels of DMSO show a high degree of crystallinity,27 while
no intense scattering for MeOH gels has been reported.
G and R under UV light, λex ) 365 nm: (a) THF solution, (b) DMSO
organogel (W-gel). (c) Pictures of quartz cuvettes containing pristine B
organogel (B-gel) and W-gel under daylight (the logo of our institute can
be seen in the background through the translucent gels).
Spectroscopy of Gels. Fluorescent nanofibers emitting blue
light are obtained by gelating DMSO with B. This “B-gel”
displays a structured emission spectrum and no undesired
excimer-like emission (Figure 1, λex ) 365 nm, λem ) 412 nm,
Φem ) 24%).25 When both G and R are added in small
quantities (2.4 × 10-5 M) to a 2.0 × 10-3 M THF solution of
B, only the latter emits (Figure 2a). In contrast, when a 120 °C
hot DMSO solution of B, G, and R is air-cooled to room
temperature, the three molecules self-assemble to form a
translucent organogel (Figure 2c) with a melting temperature,
Tm, of 37 °C (for B-gel Tm ) 38 °C, Figure SI.2, Supporting
Information). As a result, upon selective excitation of B with
Spectroscopy of Individual Nanofibers. Spatial, spectral, and
temporal resolution is achieved by confocal fluorescence
(25) Quantum yields Φem of gels measured using an integrating sphere
according to the following:(a) Pålsson, L.-O.; Monkman, A. P. AdV.
Mater. 2002, 14, 757. (b) Porre`s, L.; Holland, A.; Pålsson, L.-O.;
Monkman, A. P.; Kemp, C.; Beeby, J. J. Fluoresc. 2006, 16, 267.
Quantum yields Φem in solution using as a reference fluorescein at
pH 11 according to the following: Crosby, G. A.; Demas, J. N. J.
Phys. Chem. 1971, 75, 991.
(26) Bloess, A.; Durand, Y.; Matsushita, M.; Verbeck, R.; Groenen, E. J. J.;
Schmidt, J. J. Phys. Chem. A 2001, 105, 3016.
(27) Terech, P.; Aymonier, C.; Loppinet-Serani, A.; Bhat, S.; Banerjee,
S.; Das, R. K.; Maitra, U.; Del Guerzo, A.; Desvergne, J.-P. J. Phys.
Chem. B 2010, 114, 11409.
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A R T I C L E S
Figure 3. (a) Corrected fluorescence spectra (λex ) 365 nm) of (full line) a W-gel/DMSO and (dashed line) a W-gel/MeOH. (b) CIE 1931 chromaticity
coordinates of W-gel/DMSO: x ) 0.319, y ) 0.332. (c) CIE 1931 chromaticity coordinates of W-gel/MeOH: x ) 0.306, y ) 0.331.
Figure 4. (a-d) λex ) 385 nm, λem > 405 nm: (a, b) fluorescence intensity confocal microscopy image (30 × 30 µm, 80 × 80 nm/pixel) and (c, d) typical
intensity profile of a fiber of (a, c) a B-gel and (b, d) a W-gel of DMSO at 22 °C. (e) Intensity-corrected fluorescence spectrum of a 5 µm × 200 nm portion
of the W-gel (λex ) 385 nm; see ref 29). For comparison, the same spectrum as Figure 3a is drawn in gray (intensities matched, au). (f) Intensity-corrected
fluorescence spectra integrated over a 10 × 10 µm area of a W-gel: (full line) λex 385 ) nm, (dashed line) λex ) 488 nm, λem > 500 nm.
microscopy and is used to characterize the origin of the white
light emission. The deoxygenated solvated W-gel and B-gel
using the less volatile solvent DMSO are formed directly on a
cover glass, while the transfer of a three-component MeOH gel
was less successful. Unless otherwise specified, all the B-gels
and W-gels described below are obtained from degassed DMSO
under continuous nitrogen flow to limit the photobleaching due
to oxygen. At close proximity of the glass-gel interface,
emissive three-dimensional fiber networks surrounded by a
nonemissive bulk are observed. The networks are undistin-
guishable (Figure 4a,c), and the optical diffraction-limited cross-
section of the fibers is typically ∼200 nm, while some larger
fiber bundles of widths within ∼500 nm are less frequently
observed. These quite homogeneous and long fibers distribute
randomly or form domains tens of micrometers in diameter of
partially aligned fibers, but do not form big amorphous
aggregates. These nanofiber networks are very similar to that
of B-gels previously characterized by electron microscopy
displaying fibers 80-200 nm and bundles 0.5-1 µm wide.19b
A model has been proposed in which B molecules are assembled
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J. AM. CHEM. SOC. VOL. 133, NO. 2, 2011 319

A R T I C L E S
Giansante et al.
Figure 5. (a) Fluorescence intensity confocal microscopy image (20 × 20 µm) of a W-gel (DMSO) at 22 °C (λex ) 385 nm, λem > 405 nm). (b) Distribution
of the CIE 1931 chromaticity coordinates of 500 fluorescence spectra, similar to Figure 4a, acquired while scanning the nanofibers in an area of 20 × 20
µm. The z-scale represents the number of photons yielding the given coordinates.
into coaxial triads, stacked head-to-tail into sheets forming the
fibers (see Figure SI.3, Supporting Information).28 X-ray and
neutron scattering studies have confirmed a hexagonal molecular
packing of B in organo- and aerogels,27 in agreement with the
crystal structures of the model analogues (P3 and R3 space
groups28). In W-gels, G and R do thus not significantly alter
the nanostructures induced by B, in line with the expectations
arising from an alkyloxy substitution of all three components
in positions 2 and 3.21 The following studies support this
statement (vide infra).
Fluorescence microspectroscopy reveals that the W-gel is
composed of white-light-emitting self-assembled nanofibers (W-
SANFs). Indeed, the spectra of individual W-SANFs under near-
UV excitation (λex ) 385 nm) are very similar to those of the
bulk (Figure 4e,f). Figure 4e shows a typical spectrum obtained
accumulating light emitted from a 5 µm × 200 nm line, the
equivalent of a segment of an individual isolated W-SANF.29
A statistical analysis on 500 such segments within a W-gel
(Figure 5a) reveals that the occurrence of colors (Figure 5b) is
peaking in the white light coordinate region and is moderately
distributed in the “off-white” light areas. The color coordinates
fluctuate mostly on a line connecting those of the B component
and those of the G/R mixture. This shows that the nanofibers
behave like a two-component system, with slightly varying
contributions of B and of the G/R mixture. This also shows
that G and R are quite homogeneously dispersed in the B light-
harvesting matrix.
sufficiently large for energy transfers on the nanometer scale,
with values of JBG ) 1.5 × 1014 M-1cm-1 nm4 between B and
G and JBR ) 1.2 × 1014 M-1cm-1 nm4 between B and R. Time-
resolved confocal fluorescence lifetime imaging microscopy
(FLIM) for B emission (400 nm < λem < 450 nm) in B- and
W-SANFs reveals complex excited-state decays that can be
represented by triexponential functions. The decay times, τ, are
significantly shorter in W-SANFs than in B-SANFs (Figure 6c),
with a distribution of values essentially around weighted means
of 3.1 and 4.9 ns, respectively. Besides, the cumulated fluo-
rescence of G and R in the W-SANFs rises with time upon
selective excitation of B with a distribution of τ around a
weighted mean value of 3.5 ns (λex ) 385 nm, λem > 500 nm;
Figure 6e′,f). Their emission decay occurs at a longer time scale
with the same decay time as when G and R are directly excited
(τ ) 15 ns, λex ) 488 nm; compare parts d, e′′, and f of Figure
6). The concurrent quenching of B and emission rise for G and
R on comparable time scales (Figure 6b,e′) clearly indicates an
excitation energy transfer taking place from B to G and R upon
selective excitation of B.29 Moreover, at λex ) 488 nm both G
and R can absorb, but the emission reflects exclusively the
spectral features of R (Figure 4f), suggesting a hetero-ET, G
f R (JGR ) 3.8 × 1014 M-1cm-1 nm4). The unambiguous
determination of the relative contributions of B f G, B f R,
and G f R hetero-ETs (see the spectral overlaps, Figure SI.4,
Supporting Information) requires a more detailed study that is
beyond the scope of this paper, but their occurrence is confirmed
by the results presented below. The overall B to acceptors
transfer is efficient in the W-gel with an average estimate of
the energy transfer kinetics obtained from the lifetime distribu-
tions of 〈kET〉 ) 1.2 × 108 s-1. Within the doping range used
(2.4% in total), no large phase separation and inhomogeneous
acceptor dispersion is favored, as confirmed by quite homoge-
neous FLIM images.
j
j
Photophysics within Individual Nanofibers. The key process
that contributes to achievement of white light emission in
W-SANFs is a partial excitation energy transfer (ET) from the
light-harvesting matrix B to the energy acceptors G and R. The
spectral overlaps for Fo¨rster-type resonant energy transfer are
(28) Olive, A. G. L.; Raffy, G.; Alouchi, H.; Le´ger, J.-M.; Del Guerzo,
A.; Desvergne, J.-P. Langmuir 2009, 25, 8606.
Polarization Microscopy of Individual Nanofibers. The par-
ticular interest in using anisotropic nano-objects is confirmed
on individual fibers by confocal fluorescence polarization (P)
imaging under linearly polarized laser excitation. Molecules
selectively absorb linearly polarized light when their absorption
(29) Each spectrum results from 20 pixels taken every 250 nm and is
corrected for instrumental response. Selective excitation of B at λex
) 385 nm in W-SANFs is achieved since B absorbs at least 100 times
more than G or R. At λex ) 488 nm, both G and R are excited, and
two-photon excitation of B has been ruled out experimentally. An
excitation wavelength of 490 nm < λex < 700 nm is not available.
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White-Light-Emitting Nanofibers
A R T I C L E S
Figure 6. (a, b, d, e) Confocal FLIM images (30 × 30 µm) with color codes green, yellow, and red representing average decay times from shorter to longer:
(a) B-gel, (b, d, e) W-gel, (a, b) λex ) 385 nm, 400 nm < λem < 450 nm, (d) λex ) 488 nm, λem > 500 nm, (e′, e′′) λex ) 385 nm, λem > 500 nm, (e′) rise,
(e′′) decay. (c) Histograms of decay time τ distributions obtained by triexponential fitting of decays of (yellow) image a and (green) image b. (f) τ distributions
obtained by (red) exponential fitting of decays of image d and (green) biexponential fitting of transient emission of image e. Decay ) positive, and rise )
negative.
Figure 7. Fluorescence polarization confocal images (30 × 30 µm) under horizontally polarized excitation (east/west, λex ) 385 nm) of a W-gel: (a) 400
nm < λem < 450 nm, (b) λem > 500 nm. Color codes correspond to the polarization P: from negative to positive, blue, green, and red.
dipole, which in B, G, and R is along the short molecular axis,
is oriented parallel to the axis of the excitation beam (photo-
selection). The resulting emission, stemming from the same
dipolar transition, is characterized by the polarization P,
calculated at each image pixel using the following equation:
ization) and P < 0 (change of orientation of the polarization)
indicate homo- or heteromolecular ETs.
In W-SANFs the B component emits light moderately
polarized perpendicular to the long axis of the fibers (400 nm
< λem < 450 nm). In our experimental setup, this corresponds
to a positively polarized emission with P ) 0.25 for north/south
fibers that display an angle of θ ) 90 ( 5° relative to the laser
polarization (Figure 7a, red color code) and a negative polariza-
tion for east/west fibers (P ) -0.10, blue, θ ) 0 ( 5°). The
polarization varies monotonously between these two extremes
when the fibers are oriented with an intermediate angle θ (Figure
8a), as expected for a set of fibers displaying the same
emission.30 The occurrence of polarized emission changing
strongly with the angle θ indicates a high degree of molecular
I| - I⊥
P )
(1)
I| + I⊥
where I| is the intensity of linearly polarized emission measured
parallel to the excitation beam and I⊥ is the intensity of polarized
emission measured on the perpendicular axis. P can vary from
-1 to +1 and with the angle θ of the nanofiber with respect to
the orientation of the laser beam polarization. Indeed, a strong
variation of P vs θ reveals a preferential orientation of the
molecules within the nanofiber, while P ≈ 0 (overall depolar-
(30) A quantitative analysis of P, beyond our scope, requires taking into
account molecular packing, effects of the substrate, and optics.
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J. AM. CHEM. SOC. VOL. 133, NO. 2, 2011 321

A R T I C L E S
Giansante et al.
Figure 8. Polar plots of average P for individually picked SANF segments (2-5 µm long) as a function of the angle θ of the fiber axis with respect to the
laser polarization (east/west): (a) λex ) 385 nm, (gray circles) B-SANFs, λem > 405 nm, (blue circles) W-SANFs, 400 nm < λem < 450 nm, (orange squares)
λem > 500 nm; (b) λex ) 488 nm, λem > 500 nm, (black squares) W-SANFs, (green tilted squares) B + G (1.2%), (full red circles) B + R (1.2%), (red
triangles) B + R (0.05%). Full lines are guides to the eye only and are obtained by fitting P with a simple sinusoidal function.
Figure 9. Schematic representation of some photophysical processes in B-, BR-, BG-, and W-gels: (top) east/west fibers, (bottom) north/south fibers. Wavy
arrows represent absorbed and emitted colors. Double arrows represent the polarization of the excitation and of the emission, as determined in the latter by
the orientation of the molecules. Multiple arrows schematically suggest that several orientations are possible: for example, in B-gel the B molecules emit
blue light preferentially perpendicular to the fiber axis, while in BR-gel R emission is on average more random. Yellow arrows represent energy transfer.
In east/west fibers of BG- and W-gels, G absorbs 488 nm light less efficiently than R (the orientation of the G dipole and polarization of excitation match
less). See the text.
order and a preferential average orientation of the B molecules
within the W-SANFs. The results are very similar to those
obtained for B-SANFs (Figure 8a, -0.08 e P e +0.22), further
confirming that the self-assembly of B is not affected by the
presence of both G and R. Besides, an important contribution
to the generation of negative P values could be the partial
rotation of the emission polarization after excitation by an
efficient exciton hopping processes, i.e., B f B donor-donor
excitation energy transfers shown to occur in organogels31 and
J-aggregates.17b In contrast, the fluorescence arising from the
G and R components in the W-SANFs is almost not polarized
when sensitized by B (λex ) 385 nm, λem > 500 nm, Figure
8a).32
To gain further insight into the polarized emission of the
acceptors within the nanofibers, we have also excited gels at
488 nm, a wavelength at which only G and R can absorb, and
measured P of the resulting emission (λem > 500 nm; see the
spectrum in Figure 4f). The results are not as clear-cut as under
UV excitation, but interesting indications on the tetracenes can
be deduced. Figure 8b shows the polarization plots for a W-gel,
a gel of B with only R (BR-gel), and a gel of B with only G
(BG-gel). Figure 9 is a schematic representation of some of
the photophysical processes occurring in these SANFs and can
be used as a simplified guide. For the BR-gel, two cases are
examined with 0.05% or 1.2% R. When R is diluted into B at
0.05% and excited directly, it emits always with a positive P
almost independently of the orientation θ of the SANF,
(31) (a) Ajayaghosh, A.; Praveen, V. K.; Vijayakumar, C.; George, S. J.
Angew. Chem., Int. Ed. 2007, 46, 6260. (b) Herz, L. M.; Daniel, C.;
Silva, C.; Hoeben, F. J. M.; Schenning, A. P. H. J.; Meijer, E. W.;
Friend, R. H.; Phillips, R. T. Phys. ReV. B 2003, 68, 045203.
(32) Arithmetically estimated, depending on the polarization of the detected
light, the emitted “off-white” light should vary between a yellow tone
(CIE 0.33, 0.35) and a blue tone (CIE 0.30, 0.30).
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White-Light-Emitting Nanofibers
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Figure 10. (a, b) Fluorescence intensity confocal microscopy image (65 × 65 µm, 127 × 127 nm/pixel) of a dry SANF network obtained by drop-casting
of a deoxygenated 0.1 mM B/DCM solution (a) without and (b) with 25 and 75 µM G and R, respectively, at 22 °C (λex ) 385 nm, λem > 405 nm). (c)
Corrected fluorescence spectra collected on (blue line) area a and (black line) area b (intensities (au) are matched). The inset shows the CIE 1931 chromaticity
coordinates corresponding to area b: x ) 0.29, y ) 0.33.
indicating multiple (or random) orientations of R molecules in
the fiber and the absence of strong depolarization processes.
When R is present at a higher concentration of 1.2%, P is also
positive but with lower values (P ≈ 0.13), suggesting a partial
concentration-dependent depolarization of R emission due to a
homo-ET, R f R. This ET most probably results from a
resonant Fo¨rster-type interaction which is disfavored in the B
+ 0.05% R gel due to the higher dilution and distance between
the chromophores. A low P value (P < 0.1) is also observed
when a 1.2% concentration of only G is mixed with B,
suggesting also that a G f G ET is present. In this case,
however, the variation of P with θ indicates that G molecules
are preferentially oriented like the B molecules in the B-gel.
The emission polarization in W-SANFs with 1.2% G and R
results from the combination of several effects. As mentioned
previously, at λex ) 488 nm the emission stems exclusively from
R (Figure 4f). For east/west fibers (θ ≈ 0°), the emission and
P value of the W-SANF are similar to those of a BR-SANF,
suggesting that the emission of R results essentially from the
direct excitation of R and negligibly from a G f R sensitization.
In contrast, for north/south fibers (θ ≈ 90°) the P values are
almost reduced to zero. In this case, we suggest that R is excited
not only by direct absorption of the laser light, but also due to
G f R sensitization. Because multiple energy transfers occur
(G f G, G f R, and R f R) and the R molecules are
randomly oriented, the emission of R is almost totally depo-
larized. The difference between these two cases is due to a
photoselection that affects G (preferentially oriented, selectively
absorbs) and not R (more randomly oriented, always absorbs).
In the W-SANFs, upon excitation of B at λex ) 385 nm, the
emission of G + R is observed and the overall polarization is
close to zero for each angle θ. In this case, G and R are both
excited by B f G and B f R ETs, and in addition
intra-acceptor ETs can occur (G f R, G f G, and R f R).
The combination of the efficient B f B exciton hopping, the
previously mentioned ETs, and the lack of preferential orienta-
tion of R results in an absence of overall polarization of the G
+ R emission.
Dry Films of Nanofibers. To determine whether desolvated
SANFs present the same properties as the solvated SANFs, we
have prepared films on a surface by depositing a solution of B,
G, and R on a glass slide using a volatile solvent that does not
lead to bulk gelification of B. The formation of nanofibers
directly on the surface is observed by microscopy after rapid
evaporation of the dichloromethane solvent (Figure 10), although
only dilute solutions of B (0.10 mM), G (25 µM), and R (75
µM) have initially been used. The concentration at which fibers
are formed during solvent evaporation could not be determined,
but it is expected that fiber formation and blending of the three
components occur at a concentration higher than in the case of
DMSO gels. Microspectroscopy reveals that these W-SANFs
also emit white light, as shown in Figure 10, when using doping
percentages of G (0.25 equiv) and R (0.75 equiv). This result
suggests that the overall energy transfer process and/or that the
blending of the acceptors within the nanofibers are less efficient
in this case.33 Interestingly, we have also observed that on many
parts of the film the fibers spontaneously align on areas
surpassing 100 µm2.
General Discussion and Conclusion
The organogels composed of B, G, and R yield color-tunable
fluorescent fibers, with spectral features varying with the
proportions of the three components, as well as the wavelength
of irradiation. At a composition of 1.2% G and 1.2% R, while
only R fluorescence occurs when exciting at 488 nm, the three
components all emit when irradiating at 385 nm. The light-
harvesting and blue-emitting anthracene matrix sensitizes the
green and red tetracene acceptors by the partial energy transfer
processes B f G and B f R. According to the spectral overlap
values, G should be more sensitized than R, although with the
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Giansante et al.
spectrometer (Bruker, Advance 300), and HRMS measurements
were performed using a QStar mass spectrometer (Applied Bio-
systems) equipped with an ESI source and a time-of-flight detector.
additional G f R ET white light emission is achieved. The
microspectroscopy studies have revealed that all the nanofibers
individually emit white light and that the photophysical proper-
ties are similar in all the fibers. The tetracenes are homoge-
neously dispersed into or onto the fibers, as shown by optical
microscopy, although the polarization studies reveal that while
B and G display a similar preferential orientation, R is more
randomly oriented. Two molecular characteristics distinguish
B, G, and R. First, the alkyl chains of the tetracenes are slightly
longer than those of B, and second, the phenyl substitutions of
the tetracenes are bulky. The phenyls can disfavor the insertion
of the tetracenes into the tightly packed B matrix, in which
anthracene cores interact closely along their unsubstituted edges
(see Figure SI.3, Supporting Information) and in which it has
been shown previously that tetracenes without substitutions on
the edges integrate more efficiently.21 This effect is more
pronounced for the bulky R, which could thus have a tendency
to partially self-aggregate or even interact on the outskirts of
the fibers. Interestingly, the resulting energy transfer is still
efficient with a small doping ratio, and only the polarization of
the emission at λem > 500 nm is influenced. The high degree of
order at the molecular level of the B constituent in the
nanofibers26,27 is not perturbed and is important, as it constitutes
the basis for a robust self-assembly, induces a polarized blue
emission, and favors a B f B exciton hopping that could
contribute to the light-harvesting process. The organization of
the self-assembly is also influenced by the solvent. Indeed, the
high quantum yields of emission measured in B- and W-gels
of DMSO can be related to the partial crystallinity27 of the self-
assembly and the resulting reduction of defect-induced quench-
ing processes. In the case of the dried fibers, it can be
furthermore suggested that the blending of the three components
into SANFs is affected by the conditions in which the self-
assembly occurs. In particular, the temperature is lower, the
critical concentration is higher, and the surface can influence
the fiber growth, also inducing a very interesting self-alignment
process. Only very recently a similar alignment has been
observed for another gel on mica and attributed to epitaxial self-
assembly.34
2. Synthesis of 2,3-Bis(hexadecyloxy)-5,12-diphenyltetracene
(G). 2,3-Bis(hexadecyloxy)tetracene-5,12-dione was synthesized
according to the literature procedures.22 2,3-Bis(hexadecyloxy)tetra-
5,12-quinone (200 mg, 2.71 × 10-4 mol) was suspended in 30 mL
of THF under argon and cooled in an ice bath. A 1.36 mL volume
of phenylmagnesium bromide (1.0 M in THF) was added dropwise.
The reaction was stirred for 1 h at 0 °C, allowed to reach room
temperature, and stirred for another 48 h. Afterward, the reaction
was added to 150 mL of water and extracted with diethyl ether.
The organic layer was dried over MgSO4, and the solvent was
removed in vacuo. The residue was dissolved in 20 mL of diethyl
ether, protected from light, and heated to reflux. A 4 mL volume
of HI (57% in water) was added dropwise under reflux and the
mixture stirred at reflux for another 20 min. The reaction was
allowed to cool, added to a 10% Na2S2O5 solution, and extracted
with diethyl ether. The organic layer was dried over MgSO4, and
the solvent was evaporated. The product was isolated by column
chromatography on silica gel (eluent dichloromethane (DCM)/
petroleum ether, 1:4) Yield: 37%. To obtain the product in a solid
1
state, it can be crystallized from DCM/MeOH. H NMR (CDCl3,
3
300 MHz): δ (ppm) ) 0.87 (t, J ) 6.7 Hz, 6 H, CH3), 1.2-1.4
3
(m, 52 H, CH2), 1.75 (m, 4 H, CH2), 3.85 (t, J ) 6.5 Hz, 4 H,
OCH2), 6.79 (s, 2 H, ArH), 7.25 (m, 2 H, ArH), 7.5-7.7 (m, 10 H,
ArH), 7.75 (m, 2 H, ArH), 8.19 (s, 2 H, ArH). 13C NMR (CDCl3,
300 MHz): δ (ppm) ) 14.12 (CH3), 22.69, 26.00, 28.69, 29.37,
29.41, 29.63, 29.67, 29.73, 31.93 (CH2), 68.35 (OCH2), 104.36
(CArH), 124.62 (CArH), 124.83 (CArH), 126.96 (CAr,q), 127.42
(CArH), 128.29 (CArH), 128.55 (CAr,q), 128.62 (CArH), 130.38 (CAr,q),
131.42 (CArH), 134.05 (CAr,q), 139.85 (CAr,q), 149.49 (CAr,q). Note
that not all signals of the CH2 groups are visible due to overlapping.
HRMS (ESI, positive ions): m/z calcd for [C62H84O2]+ 860.6465,
measd 860.6471.
3. Synthesis of 2,3-Bis(hexadecyloxy)-5,6,11,12-tetraphenyltet-
racene (R). 2,3-Dimethoxy-6,11-diphenyltetracene-5,12-dione was
synthesized according to the literature procedures.23 2,3-Bis(hexa-
decyloxy)-6,11-diphenyltetracene-5,12-dione was synthesized ac-
cording to the following procedure: 2,3-Dimethoxy-6,11-diphe-
nyltetracene-5,12-dione (190 mg, 4.04 × 10-4 mol) was dissolved
in 10 mL of DCM under argon and cooled in an ice bath. BBr3 (1
mL of a 1 M solution in DCM, 9.7 × 10-4 mol) was added dropwise
and the reaction stirred for 30 min at 0 °C, for 1.5 h at room
temperature, and finally for 2 h under reflux and was allowed to
cool. The reaction was added to water, and a few drops of
concentrated hydrochloric acid were added. The product was
extracted with diethyl ether, and the organic phase was washed
with water and brine and dried over MgSO4, and the solvent was
removed in vacuo. The residue was dissolved in 15 mL of DMF
under argon, and potassium carbonate (0.5 g, 3.62 mmol) was added
followed by 1-bromohexadecane (290 mg, 9.48 × 10-4 mol). The
reaction was stirred at 130 °C for 14-15 h. The solvent was
removed in vacuo, and the residue was absorbed on silica gel. The
product was isolated by column chromatography on silica gel
(eluent DCM/petroleum ether, 1:1). Yield: 33%. 1H NMR (CDCl3,
In conclusion, using microspectroscopy, this work has shown
that self-assembly is an efficient method to accurately blend
three components into highly organized and anisotropic nano-
objects. Original perspectives could derive from the use of
individual white-light-emitting nanofibers and can also be
deduced from the observation that self-assembled nanofibers
are adapted for the preparation of desolvated thin films34,35 or
aerogels,21 for macroscopic alignment yielding highly organized
materials,36 or for semiconductivity.34
Experimental Section
Synthesis. 1. General Procedures. 2,3-Bis(decyloxy)anthracene
as well as the precursors for the green and red compounds were
synthesized following literature procedures.22,23 All solvents and
chemicals were commercially available and used without further
purification unless otherwise stated. THF was distilled over sodium
prior to use. NMR spectra were recorded on a 300 MHz NMR
3
300 MHz): δ (ppm) ) 0.87 (t, J ) 6.4 Hz, 6 H, CH3), 1.2-1.5
3
(m, 52 H, CH2), 1.82 (m, 4 H, CH2), 4.03 (t, J ) 6.6 Hz, 4 H,
OCH2), 7.28-7.34 (m, 4 H, ArH), 7.42-7.64 (m, 12 H, ArH). 2,3-
Bis(hexadecyloxy)-6,11-diphenyltetracene-5,12-dione (385 mg, 4.32
× 10-4 mol) was dissolved in 10 mL of THF under argon and
cooled in an ice bath. A 1.2 mL volume of PhLi (1.8 M in dibutyl
(33) The dry fibers are more exposed to residual and detrimental oxygen
than the previously degassed DMSO gels, impeding more detailed
investigations of this system under the current microscopy setup.
(34) (a) Prasanthkumar, S.; Saeki, A.; Seki, S.; Ajayaghosh, A. J. Am. Chem.
Soc. 2010, 132, 8866. (b) Hong, J.-P.; Um, M.-C.; Nam, S.-R.; Hong,
J.-I.; Lee, S. Chem. Commun. 2009, 310.
(36) (a) Shklyarevskiy, I. O.; Jonkheijm, P.; Christianen, P. C. M.;
Schenning, A. P. H. J.; Del Guerzo, A.; Desvergne, J.-P.; Meijer,
E. W.; Maan, J. C. Langmuir 2005, 21, 2108. (b) Hung, A. M.; Stupp,
S. I. Langmuir 2009, 25, 7084. (c) Hirai, Y.; Babu, S. S.; Praveen,
V. K.; Yasuda, T.; Ajayaghosh, A.; Kato, T. AdV. Mater. 2009, 21,
4029.
(35) Giansante, C.; Olive, A. G. L.; Scha¨fer, C.; Raffy, G.; Del Guerzo,
A. Anal. Bioanal. Chem. 2010, 396, 125.
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A R T I C L E S
ether) was added dropwise and the reaction stirred in the melting
ice bath overnight. Afterward, the solution was added to a saturated
NH4Cl solution and extracted with diethyl ether twice. The
combined organic phases were washed with water and dried over
MgSO4, and the solvent was evaporated. The residue was dissolved
in diethyl ether, protected from light, and refluxed. A 6 mL volume
of HI (57% in water) was added slowly under reflux, which was
continued for another 25 min. Afterward, the reaction was allowed
to cool, added to a 10% Na2S2O5 solution, and extracted with diethyl
ether. The organic phase was washed with water and dried over
MgSO4, and the solvent was removed in vacuo. The product was
isolated by column chromatography on silica gel (eluent DCM/
petroleum ether, 1:4). Yield: 34% of a red oil. To obtain the product
in a solid state, it can be crystallized from DCM/MeOH to give a
red-orange powder. 1H NMR (THF-d8, 300 MHz): δ (ppm) ) 0.89
(t, 3J ) 6.8 Hz, 6 H, CH3), 1.2-1.4 (m, 52 H, CH2), 1.64 (m, 4 H,
CH2), 3.65 (t, 3J ) 6.3 Hz, 4 H, OCH2), 6.51 (s, 2 H, ArH),
6.82-6.88 (m, 8 H, ArH), 6.98-7.12 (m, 14 H, ArH), 7.31 (m, 2
H, ArH). 13C NMR (THF-d8, 200 MHz): δ (ppm) ) 14.66 (CH3),
23.76, 27.14, 29.89, 30.51, 30.79, 30.87, 33.07 (CH2), 68.82
(OCH2), 104.88 (CArH), 125.20 (CArH), 126.59 (CArH), 126.64
(CArH), 127.39 (CArH), 128.19 (CArH), 128.38 (CArH), 129.47
(CAr,q), 129.51 (CAr,q), 131.02 (CAr,q), 133.15 (CArH), 133.36 (CArH),
135.11 (CAr,q), 137.27 (CAr,q), 143.37 (CAr,q), 143.80 (CAr,q), 151.08
(CAr,q). Note that not all signals of the CH2 groups are visible due
to overlapping. HRMS (ESI, positive ions): m/z calcd for
[C74H92O2]+ 1012.7091, measd 1012.7101.
Confocal Fluorescence Microscopy. The samples were prepared
using deoxygenated solvents and then placed under a continuous
light N2 gas flow. We used a Picoquant Microtime 200 with two
MPD SPADs and a Ti-Sa laser chain (Coherent) yielding 4-6 ps
pulses at 4.75 MHz. An 80% T/20% R spectrally flat beam splitter
was used in combination with a microscope objective (100×
UPLSAPO, NA 1.4), suitable interferential filters, a 50 µm pinhole,
and, when specified, a polarizing beam splitter and two Glan-
Thompson polarizers. For polarization microscopy, the instrumental
G-factor was measured for each spectral range with a crystal (2,3-
bis(heptyloxy)anthracene, 400-450 nm) or a solution of G, R, or
G + R (500-700 nm). Typical laser powers of ∼30 W/cm2 at
385 nm and ∼130 W/cm2 have been used. Spectroscopy was
performed with an Andor system, collecting after the pinhole. A
distance of 250 nm between each pixel and an acquisition time of
0.6 ms per pixel have been used, and each spectrum is the sum of
the fluorescence of 20 pixels. The spectra were intensity corrected
for instrumental response, measuring suitable reference solutions
that cover the whole spectral range. FLIM results were obtained
using the Symphotime fitting software, which performs a multiex-
ponential fitting (one, two, or three exponentials depending on the
sample) at each pixel. In Figure 6a,b, the color codes represent the
average lifetime resulting from a triexponential fitting. Figure 6d:
the color codes represent the value of the monoexponential decay
time. In Figure 6e′,e′′, the color codes represent the rise time (5,
e′) and the decay time (5, e′′) resulting from biexponential transient
emission (a triexponential fitting by FLIM, with two short rise times
and one long decay time, is difficult because the long decay
component has by far the highest contribution to emission). Parts
c and f of Figure 6 are histograms of photons counted for each
occurrence of a determined decay time: when the fitting is
triexponential using ∑Ai exp(-t/τi), the counts of each pixel are
divided into three weighted contributions, A1τ1/∑Aiτi, A2τ2/∑Aiτi,
and A3τ3/∑Aiτi.
Fluorescence. Bulk fluorescence spectra were obtained on a
Horiba Jobin-Yvon Fluorolog spectrofluorimeter. Fluorescence
quantum yields of G and R were determined in deoxygenated THF
using as a reference fluorescein at pH 11.25 A Labsphere optical
Spectralon integrating sphere (diameter 100 mm), which provides
a reflectance of >99% over the 400-1500 nm range, has been used
to measure quantum yields of deoxygenated B- and W-gels (light
intensity measured, not power).25
Acknowledgment. We thank M. Cantuel and N. McClenaghan.
Financial support was provided by Project ANR-06-JCJC-0030,
Re´gion Aquitaine (Ph.D. fellowship to M.-T.K.; equipment, Grant
20051304008AB), GIS AMA, CNRS, and the French Ministry of
Education and Research.
Supporting Information Available: Synthesis, structures, and
additional spectra. This material is available free of charge via
the Internet at http://pubs.acs.org.
JA106807U
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