An aggregation-induced blue shift of emission and the self-assembly of nanoparticles from a novel amphiphilic oligofluorene

Article abstract of DOI:10.1039/b815431g

An aggregation-induced blue shift of emission was observed for the first time from a novel amphiphilic trifluorene, TFOH, and tentatively attributed to a similar TICT mechanism; the optical properties and self-assembly behaviors of TFOH, along with their relationships, are presented. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b815431g

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
An aggregation-induced blue shift of emission and the self-assembly
of nanoparticles from a novel amphiphilic oligofluorenew
Linna Zhu, Chuluo Yang* and Jingui Qin
Received (in Cambridge, UK) 4th September 2008, Accepted 3rd October 2008
First published as an Advance Article on the web 24th October 2008
DOI: 10.1039/b815431g
An aggregation-induced blue shift of emission was observed for
the first time from a novel amphiphilic trifluorene, TFOH, and
tentatively attributed to a similar TICT mechanism; the optical
properties and self-assembly behaviors of TFOH, along with
their relationships, are presented.
quite similar, with absorption peaks at 349 and 352 nm,
respectively. In addition, the absorption spectra of TFOH
and TFP exhibit little change with solvent polarity. However,
their fluorescence spectra in THF solution and in the film state
show a remarkable difference. TFP exhibits an emission max-
imum of 398 nm in THF solution and 421 nm in the film state,
which resembles those of oleophilic oligofluorenes and poly-
fluorenes. Unexpectedly, TFOH exhibits an emission max-
imum of 494 nm in THF, which is red-shifted by about 70 nm
compared to that of TFP. In addition, the emission maximum
shows almost no shift with THF solution concentration vary-
ing from 1.0 Â 10À7 to 5.0 Â 10À4 mol LÀ1, suggesting no
aggregation in THF solution. Moreover, the film emission of
TFOH is blue-shifted by about 60 nm relative to its THF
solution, and thus the possibility of long wavelength emission
originating from aggregation can be excluded. On the con-
trary, it seems that aggregation induces a blue shift of the
emission in this situation.
Polyfluorenes and oligofluorenes have emerged as a very
important class of optoelectronic materials in virtue of their
high luminescent efficiency, good charge-carrier mobility and
convenient color tunability.1 Water-soluble polyfluorenes with
cationic side chains have been reported for the detection of
biomacromolecules.2 By modifying them with hydrophilic
groups, oligofluorenes/polyfluorenes tend to assemble into
nanoparticles of various morphologies, with the formed struc-
tures showing characteristic optical properties.3–5 For exam-
ple, rod–coil graft copolymers containing oligofluorene/
polyfluorene backbones and poly(ethylene oxide) side chains
can form fluorescent micellar structures in aqueous solution;4
amphiphilic trifluorenes with n-hexyl and hydroxyethoxy-
methyl side chains can form self-condensed nanoparticles in
THF/H2O.5 In both cases, aggregation arising from self-
assembly results in excimer formation, and consequently
causes a red shift of their fluorescence emission.
Considering the structural differences between TFP and
TFOH, an analogue of a twisted intramolecular charge-
transfer state (TICT) is proposed as a possible explanation
for these observations.6 For TFOH, the electron-donating
bis-(2-hydroxyethyl) amino group is able to internally rotate
with respect to the benzene ring. In the low polarity medium of
toluene, the emission spectrum of TFOH is structured and
behaves like a locally excited (LE) state (Fig. 1); in the polar
solvent THF, the emission spectrum is remarkably red-shifted
and becomes structureless, which indicates a large dipole
In this Communication, we report a novel amphiphilic
trifluorene, TFOH, having n-hexyl and para-N,N-bis-
(2-hydroxyethyl)aminophenyl side chains, as well as its
hydrophobic analogue, TFP, with n-hexyl and phenyl pendant
groups, for comparison. Interestingly, the ‘‘pure’’ fluorene
derivative, TFOH, shows a clear green emission in single
solvent THF; moreover, its emission spectrum shows an
obvious blue shift in the THF/H2O system or in the film state.
This represents the first observation of an aggregation-induced
blue shift of emission for oligofluorenes. The optical properties
and self-assembly behaviours of TFOH, along with their
relationships, will be discussed herein.
TFOH and TFP were synthesized by a Suzuki coupling
reaction, as shown in Scheme 1, and structurally characterized
by 1H NMR, 13C NMR, FT-IR spectroscopy, elemental
analysis and mass spectroscopy (see ESIw). We initially in-
vestigated the optical properties of TFOH and TFP in THF
solution and in the film state. As shown in Fig. 1, the UV-vis
absorption spectra of TFOH and TFP in THF solution are
Department of Chemistry, Hubei Key Lab on Organic and Polymeric
Optoelectronic Materials, Wuhan University, Wuhan 430072,
P. R. China. E-mail: clyang@whu.edu.cn
w Electronic supplementary information (ESI) available: Synthetic
procedures, detailed UV-vis and fluorescence emission spectra, and
SEM images. See DOI: 10.1039/b815431g
Scheme 1 Synthetic routes for the trimeric fluorenes TFOH and TFP.
Chem. Commun., 2008, 6303–6305 | 6303
ꢀc
This journal is The Royal Society of Chemistry 2008

View Article Online
addition, the emission dependence on the viscosity also pro-
vides evidence for the TICT analogue mechanism. Similarly,
when glycerol was added to the solution of TFOH in metha-
nol, a significant blue shift of the emission was observed due to
the suppression of rotation of the amino groups.
Finally, we investigated the self-assembly behavior of am-
phiphilic TFOH in the THF/H2O system by FE-SEM and
TEM images, and the corresponding optical properties. As
shown in Fig. 3, with increasing water fraction, the maximum
emission wavelength first shows a slight red shift, then a
prominent blue shift. Most likely, when the THF/H2O ratio
is higher than 6 : 4, the increasing water fraction enhances the
polarity of the system, consequently bringing about the red
shift in emission. By the time water dominates in the system,
micellization may occur, with the formed nanoparticles
restricting the rotation of the bis-(2-hydroxyethyl)amino
group with respect to the benzene ring, as in the rigid glass
or film state, consequently resulting in a blue shift of the
emission. As for TFP, the emission spectrum showed a slightly
red shift when the THF/H2O ratio was changed from 5 : 5 to
4 : 6, while no blue shift occurred, even when the THF/H2O
ratio reached 1 : 99 (see ESIw).
Fig. 4(a) shows an SEM image of a molecular aggregate of
TFOH after transferring from a THF/H2O (1 : 9) solution
onto a quartz substrate; nanoparticles of radius of 80–250 nm
were clearly observed. The TEM image (Fig. 4(b)) further
shows the solid inner structure of the nanoparticles, and the
high-resolution TEM image (inset of Fig. 4(b)) clearly shows
molecular self-assembly patterns. It is noteworthy that the
diameters of the nanoparticles measured by DLS (dynamic
light scattering) are correlated with the THF/H2O ratio.3c
With the increasing proportion of water, the mean diameter
of the nanoparticles decreased (THF/H2O 4 : 6, 342 nm; 2 : 8,
255 nm; 1 : 9, 170 nm; 1 : 99, 122 nm), which suggests a more
compact aggregation. It is also important to note, from the
DLS measurements, that the nanoparticles cannot form when
the THF/H2O ratio is higher than 4 : 6. This ratio is close to
the ratio (6 : 4) that initially caused the blue shift of the
emission.
Fig. 1 UV-vis absorption and fluorescence spectra of (a) TFP
(5.0 mM) and (b) TFOH (5.0 mM) in THF, toluene and in the film
state. The excitation wavelengths for TFP and TFOH are 360 and
350 nm, respectively.
moment of the emitting state that could be assigned to a
phenomenon similar to TICT emission.7 In contrast, TFP does
not exhibit dual fluorescence and red-shifts as the solvent
polarity increases from toluene to THF, obviously owing to
the absence of amino substitution on the pendant benzene
ring. In addition, the quantum yield of TFOH is remarkably
increased by 86.1% in toluene from 10.2% in THF, while the
quantum yield of TFP shows a relative small variation, from
77.9% in THF to 93.9% in toluene.
To further prove our assumption, we measured the fluores-
cence spectra of TFOH in THF in the temperature range of
77–300 K (Fig. 2). A clear critical temperature was observed at
157 K, which is close to the freezing point of THF (165 K).
Above 157 K, the maximum emission was red-shifted with
decreasing temperature, possibly due to increasing solution
polarity when the temperature dropped.8 Below 157 K, the
maximum emission shifted to higher energies with decreasing
temperature, from about 530 nm at 157 K to 420 nm at 77 K.
This may be attributed to the restricted rotation of
bis-(2-hydroxyethyl) amino groups with respect to the benzene
ring in rigid glass below the freezing point of THF.9 In
We note that the aggregation-induced blue shift (AIBS)
phenomenon is somewhat homothetic to that of aggregation-
induced emission (AIE)10 because of its rotation restrainability
due to low temperature, high viscosity or high aggregation
state. However, we have to point out that both mechanisms
Fig. 3 The maximum emission wavelength of TFOH in the THF/H2O
system (5.0 mM) at different solvent ratios. The excitation wavelength
was 350 nm.
Fig.
2 The maximum emission wavelength of TFOH in THF
(5.0 mM) at different temperatures. The excitation wavelength was 350 nm.
ꢀc
This journal is The Royal Society of Chemistry 2008
6304 | Chem. Commun., 2008, 6303–6305

View Article Online
20874077) and the Ministry of Education of China for New
Century Excellent Talents in University (NCET-04-0682).
Notes and references
1 (a) A. Kraft, A. C. Grimsdale and A. B. Holmes, Angew. Chem., Int.
Ed., 1998, 87, 402; (b) C. D. Muller, A. Falcou, N. Reckefuss,
¨
M. Rojahn, V. Wiederhirn, P. Rudati, H. Frohne, O. Nuyken,
H. Becker and K. Meerholz, Nature, 2003, 421, 829; (c) Z. Bao,
A. J. Lovinger and J. Brown, J. Am. Chem. Soc., 1998, 120, 207;
(d) U. Scherf and E. J. W. List, Adv. Mater. (Weinheim, Ger.), 2002,
14, 477; (e) D. Neher, Macromol. Rapid Commun., 2001, 22, 1366.
2 (a) B. S. Gaylord, A. J. Heeger and G. C. Bazan, J. Am. Chem.
Soc., 2003, 125, 896; (b) E. S. Baker, J. W. Hong, B. S. Gaylord,
G. C. Bazan and M. T. Bowers, J. Am. Chem. Soc., 2006, 128,
8484; (c) S. J. Dwight, B. S. Gaylord, J. W. Hong and G. C. Bazan,
J. Am. Chem. Soc., 2004, 126, 16850; (d) F. He, Y. Tang, M. Yu,
S. Wang, Y. Li and D. Zhu, Adv. Funct. Mater., 2006, 16, 91.
3 (a) W. C. Wu, Y. Tian, C.-Y. Chen, C. S. Lee, Y. L. Sheng,
W.-C. Chen and A. K.-Y. Jen, Langmuir, 2007, 23, 2805;
(b) S. Wang, B. Liu, S. Gaylord and G. C. Bazan, Adv. Funct.
Mater., 2003, 13, 463; (c) F. Wang and G. C. Bazan, J. Am. Chem.
Soc., 2006, 128, 15786; (d) H. D. Burrows, V. M. M. Lobo, J. Pina,
M. L. Ramos, J. S. Melo, A. J. M. Valente, M. J. Tapia,
S. Pradhan and U. Scherf, Macromolecules, 2004, 37, 7425;
(e) G. Tu, H. Li, M. Forster, R. Heiderhoff, L. J. Balk, R. Sigel
and U. Scherf, Small, 2007, 3, 1001.
4 J. H. Yao, K. Y. Mya, L. Shen, B. P. He, L. Li, Z. H. Li,
Z. K. Chen, X. Li and K. P. Loh, Macromolecules, 2008, 41, 1438.
5 Y. Koizumi, S. Seki, A. Tsukuda, S. Sakamoto and S. Tagawa,
J. Am. Chem. Soc., 2006, 128, 9036.
6 For a review, see: Z. R. Grabowski, K. Rotkiewicz and W. Rettig,
Chem. Rev., 2003, 103, 3899.
7 (a) B. Valeur, Molecular Fluorescence: Principles and Applications,
VCH, Weinheim, Germany, 2001, pp. 63; (b) A. Siemiarczuk,
Z. R. Grabowski, A. Krowczynski, M. Asher and M. Ottolenghi,
´ ´
Fig. 4 The (a) SEM image and (b) TEM image of TFOH prepared
from THF/H2O, 1 : 9. The inset of (b) shows a high resolution TEM
image of an edge of the nanoparticles.
are largely different. In the AIE system, the molecules are out
of coplanarity in the single-molecular state (dilute solution),
which is weakly emissive. In the aggregated state, intramole-
cular rotation is restricted, resulting in fluorescence emission
enhancement. AIE is usually correlated to the intramolecular
rotation of phenyl rings, and no charge-transfer is involved. In
contrast, in our case, the rotation of the phenyl rings has no
influence on fluorescence, as the comparative analogue, TFP,
does not show a similar phenomenon to TFOH. Instead, the
blue shift of the emission originates from the restriction of
rotation of the amino groups, and an analogue of twisted
intramolecular charge-transfer is involved.
In conclusion, we have presented an unusual aggregation-
induced blue shift of emission from a new amphiphilic tri-
fluorene. This phenomenon is tentatively attributed to a
similar TICT mechanism. We note that there is only one
report about the green emission of oligofluorene attributed
to its aggregation behavior in a THF/H2O system,5 while this
work is the first report that trimeric fluorenes show green
emission in a single polar solvent, providing a new way to tune
the emission spectra of oligofluorenes. The unique AIBS
property may find an application in the detection of micro-
environments, especially in biological processes. We are cur-
rently investigating the scope and generality of this
phenomenon and its applications.
Chem. Phys. Lett., 1977, 51, 315; (c) E. Lippert, W. Luder and
¨
H. Boos, in Advances in Molecular Spectroscopy, ed. A. Mangini,
Pergamon Press, Oxford, 1962, pp. 443.
8 C. Reichardt, Solvents and Solvent Effects in Organic Chemistry,
VCH, Weinheim, Germany, 3rd edn, 2003, pp. 428.
9 A. Siemiarczuk, Z. R. Grabowski and A. Krowczynski, Chem.
´ ´
Phys. Lett., 1977, 51, 315.
10 (a) J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu,
H. S. Kwok, X. Zhan, Y. Liu, D. Zhu and B. Z. Tang, Chem.
Commun., 2001, 1740; (b) Y. Hong, M. Haubler, J. W. Y. Lam,
¨
Z. Li, K. K. Sin, Y. Dong, H. Tong, J. Liu, A. Qin, R. Renneberg
and B. Z. Tang, Chem.–Eur. J., 2008, 14, 6428.
We are grateful for financial support from the National
Natural Science Foundation of China (nos. 50773057 and
ꢀc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 6303–6305 | 6305