Fluorescence quenching and enhancement of vitrifiable oligofluorenes end-capped with tetraphenylethene

Article abstract of DOI:10.1039/c2jm30261f

We report the synthesis of novel amorphous fluorene-based fluorophores that have been end-capped with tetraphenylethene (TPE). Although in the solid state the fluorophores show bright cyan fluorescence with high (68%) photoluminescence quantum efficiency (PLQE), strong fluorescence quenching is observed in solution with low PLQE values ranging from 0.3% to 2.1%. When the fluorophores were added to a 90% water-THF solvent mixture nanoparticles were formed, which was confirmed by Dynamic Light Scattering (DLS) and Atomic Force Microscopy (AFM). Subsequent fluorescence measurements reveal that all the fluorophores exhibit aggregation induced emission (AIE) with high PLQE (41%). We have carried out a comparative study of 5 fluorene-based TPE materials (F(1-5)TPE), in which the fluorene core is extended, and investigated their thermal, optical, electrochemical and electroluminescence properties. From fluorescence quantum yield data we have discovered that the AIE effect decreases as the fluorene core increases both in the nanoparticle and in the solid state. Thermal analysis reveals that all fluorophores are amorphous with high thermal stabilities. Potential application in solid state vapour sensing has been demonstrated using dichloromethane that shows "on" and "off" fluorescence behaviour. Finally, Organic Light Emitting Diodes (OLEDs) have been fabricated with device configuration ITO/PEDOT/F(1-5)TPE/TPBi/LiF/Al. The best OLED device that incorporates F1-TPE as the emitter exhibits a turn-on voltage of 5.8 V, L_max = 1300 cd m^-2, η_P,max = 1 lm W ^-1 and η_C,max = 2.6 cd A^-1.

Full text of DOI:10.1039/c2jm30261f

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2012, 22, 7515
www.rsc.org/materials
PAPER
Fluorescence quenching and enhancement of vitrifiable oligofluorenes
end-capped with tetraphenylethene†
a
a
a
a
Matthew P. Aldred,‡ Chong Li,‡ Guo-Feng Zhang, Wen-Liang Gong, Alexander D. Q. Li, b Yanfeng Dai,c
*
c
Dongge Ma and Ming-Qiang Zhu
a
*
*
Received 14th January 2012, Accepted 14th February 2012
DOI: 10.1039/c2jm30261f
We report the synthesis of novel amorphous fluorene-based fluorophores that have been end-capped
with tetraphenylethene (TPE). Although in the solid state the fluorophores show bright cyan
fluorescence with high (68%) photoluminescence quantum efficiency (PLQE), strong fluorescence
quenching is observed in solution with low PLQE values ranging from 0.3% to 2.1%. When the
fluorophores were added to a 90% water–THF solvent mixture nanoparticles were formed, which was
confirmed by Dynamic Light Scattering (DLS) and Atomic Force Microscopy (AFM). Subsequent
fluorescence measurements reveal that all the fluorophores exhibit aggregation induced emission (AIE)
with high PLQE (41%). We have carried out a comparative study of 5 fluorene-based TPE materials
(F(1–5)TPE), in which the fluorene core is extended, and investigated their thermal, optical,
electrochemical and electroluminescence properties. From fluorescence quantum yield data we have
discovered that the AIE effect decreases as the fluorene core increases both in the nanoparticle and in
the solid state. Thermal analysis reveals that all fluorophores are amorphous with high thermal
stabilities. Potential application in solid state vapour sensing has been demonstrated using
dichloromethane that shows ‘‘on’’ and ‘‘off’’ fluorescence behaviour. Finally, Organic Light Emitting
Diodes (OLEDs) have been fabricated with device configuration ITO/PEDOT/F(1–5)TPE/TPBi/LiF/
Al. The best OLED device that incorporates F1-TPE as the emitter exhibits a turn-on voltage of 5.8 V,
Lmax ¼ 1300 cd mꢀ2, hP,max ¼ 1 lm Wꢀ1 and hC,max ¼ 2.6 cd Aꢀ1
.
stability. PF homopolymers also exhibit thermotropic liquid
crystallinity2 and can be aligned on rubbed polyimide resulting in
polarised light emission with a polarisation of 15.3 In contrast to
PF, oligofluorenes exhibit monodispersity and improved chem-
ical purity. Due to their linear structures and high aspect ratios
oligofluorenes and fluorene-based low molecular weight mate-
rials have been reported to exhibit smectic and glassy nematic
liquid crystalline states.4 Also, aligned thin films have been
reported to exhibit linearly5 and circularly polarised light emis-
sion6 and high field-effect mobilities.7 Additionally, oligo-
fluorenes show attractive properties for cholesteric liquid crystal
lasers.8 Vitrification above room temperature is a very important
prerequisite for certain optoelectronic applications, and both
oligofluorene and polyfluorene can form high quality thin glassy
films without crystal grain boundaries.
Aggregation induced emission (AIE) was discovered in 2001,9
and within the last few years a great deal of effort has been
undertaken to study this phenomenon. AIE-active materials are
chromophores that exhibit quenching in the solution, contrary to
its solid (or ‘‘aggregated’’) state, which shows luminescence. This
is opposite to aggregation caused quenching (ACQ), which
demonstrates larger quantum efficiencies in solution compared
to its solid state. Restriction of intramolecular rotation (RIR) is
Introduction
Fluorene has attracted interest for many years in academia and
industry for applications in electronics and optoelectronics, such
as OLEDs, Organic Field-Effect Transistors (OFETs) and solar
cells.1 Its near planar biphenyl unit ensures a high degree of
conjugation and its synthetic versatility at the benzylic 9-position
and aromatic 2,7-positions allows ease of design and synthesis.
Polyfluorenes (PFs) and their co-polymers have become a major
class of solution processable organic semiconductors and display
high fluorescence quantum yields in solution and solid state,
good solubility, high charge-carrier mobilities and high thermal
aWuhan National Laboratory for Optoelectronics, Huazhong University of
Science and Technology, Wuhan, Hubei, China 430074. E-mail: mqzhu@
hust.edu.cn; Fax: + 86 27 87793419; Tel: + 86 27 87793419
bDepartment of Chemistry, Washington State University, Pullman, WA,
99164, United States. E-mail: dequan@wsu.edu
cChangchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, Jilin, China 130022. E-mail: mdg1014@ciac.jl.cn
† Electronic Supplementary Information (ESI) available: full
instrumentation details, experimental procedures/characterisation, and
the 1H NMR and mass spectra of all final materials. See DOI:
10.1039/c2jm30261f
‡ Authors have equal contributions.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 7515–7528 | 7515

View Article Online
a common structural feature of these materials and has been used
to explain the AIE phenomenon.10 In AIE-active materials these
low frequency modes have a large influence on the internal
conversion and, accordingly, in the solid state they contribute
less to the internal conversion resulting in the prevalence of
radiative decay.11 In the non-aggregated state the light emission
is turned ‘‘off’’, opposite to its aggregated state that turns ‘‘on’’
the light emission. This ‘‘on–off’’ fluorescence behaviour is
important for sensing applications. Due to these novel and
attractive properties, AIE-active materials have found wide-
spread applications, ranging from chemosenors,12 fluorescent
assays,13 solvent vapour/explosive sensors,14 OLEDs15 and two-
photon absorption and excitation.16 The most common class of
materials reported in the literature that exhibit AIE properties
and have been researched for such applications incorporate
silole17 and TPE18 groups. However, the majority of these
previously reported materials tend to be non-linear in structure
with a low aspect ratio.
below room temperature, compared to shorter alkyl chains.4a
Indeed, due to the decrease of the van der Waals forces and hence
the free volume shorter chains will result in an increase of the Tg,
however, at the expense of solubility. Therefore, octyl chains
were viewed as a reasonable compromise. F(1–5)TPE were syn-
thesised according to Schemes 1 and 2. A convergent approach
was adopted from two different pathways, involving a boronic
acid or ester tetraphenylethene derivative and a brominated
fluorene core. Ultimately the Pd(0)-catalysed Suzuki cross
coupling was used to perform the end capping. Dialkylation of 1
at the fluorene benzylic 9-position was carried out using the
KOH/KI/DMSO method and 1-bromooctane as the alkylating
agent. The crude product was purified by recrystallisation using
ethanol to give 2 in excellent yield (84.1%). Commercially
available 2-bromofluorene was alkylated using the same condi-
tions used to prepare compound 2. Reaction of 3 in dry THF
with n-BuLi in standard subambient conditions (ꢀ78 ꢁC),
followed by quenching using distilled tri(isopropyl)borate and
subsequent acidification afforded the boronic acid 4. Bis-fluorene
(5) and ter-fluorene (6) were subsequently synthesised by Suzuki
cross couplings of compounds 3 and 4 or 2 and 4, respectively, in
toluene and water (2 : 1 ratio). Controlled bromination at the
active 2 and 7 positions yielded the dibrominated compounds 7
and 8. Diphenyllithium was easily prepared from diphenyl-
methane (9) and n-BuLi in THF at 0 ꢁC, which was subsequently
added to 4-bromobenzophenone and dehydrated using the
acid catalyst p-toluenesulfonic acid (PTSA) to give the desired
Br-TPE (10). Similar conditions used for the synthesis of
compound 4 were followed to make the boronic acid 11. Finally,
F(1–3)TPE were prepared by Suzuki cross couplings of the TPE-
boronic acid 11 and the brominated fluorene cores (2, 7 and 8). A
selective Suzuki cross coupling was designed to synthesise the
important intermediate 15. Due to the weaker nature of the C–I
bond compared to the C–Br bond the coupling reaction at the
former should dominate, reducing the amount of di-coupling
product. Iodination of 2-bromofluorene (12) using iodine,
potassium iodate in acetic acid yielded compound 13 in excellent
yield (81.1%). Selective Suzuki cross coupling of compounds 11
and 14 yielded the mono-TPE (15) in good yield (66.1%). This
was functionalised with a boronic ester group by reacting with
bispinacolatodiboron, KOAc, PdCl2(dppf) in dry dioxane to
yield the boronic ester (16) in excellent yield (82.7%). Similar
Pd(0)-catalysed Suzuki cross coupling protocols were used to
synthesise F4-TPE and F5-TPE.
It was our interest to synthesise a material that possesses the
attractive properties of oligofluorene, such as high thermal
stability, good solution processability and glassy behaviour,
which is able to quench its emission in solution but retain the
high PLQE and AIE properties of TPE in the solid state. Here,
we report the synthesis of a series of amorphous fluorene oligo-
mers (F(1–5)TPE; Fig. 1) that exhibit fluorescence quenching in
solution, in contrast to its nanoparticle and solid states that
exhibit bright cyan luminescence. A detailed comparative study
has been carried out with analogous materials that differ only by
the number of fluorene rings in its core. With regards to F5-TPE,
the PLQE in solution is reduced from 96% (oligo-5-fluorene,
without TPE groups) to 2.1% by simply end-capping with TPE.
Solid state amorphous patterns have been fabricated that exhibit
‘‘on–off’’ fluorescence switching when exposed to solvent vapour.
Finally, OLEDs have been fabricated with device configuration
ITO/PEDOT/F(1–5)TPE/TPBi/LiF/Al.
Results and discussion
Synthesis
The structure of fluorene can be easily functionalised by nucle-
ophilic substitution at the 9-position, due to the acidity of
the benzylic protons, and electrophilic substitution at the 2- and
7-positions due to the reactivity of electrophiles with the elec-
tron-rich aromatic ring. Long alkyl chains enhance the solubility
of extended fluorene oligomers and can effectively increase the
viscosity, which is necessary to inhibit crystallisation and
encourage glassy behaviour above room temperature. If the alkyl
chains are too long at the fluorene 9-position the glass transition
temperature (Tg) will have a tendency to be lower, and possibly
Thermal properties
Thermal properties of F(1–5)TPE were evaluated by differential
scanning calorimetry (DSC) and differential thermal analysis
(DTA). As expected the DSC scans (see Fig. 2) of all 5 fluo-
rophores show a first order transition temperature above room
temperature, which represents a glass transition temperature
(Tg). No crystallisation exotherms are observed on cooling and
F(1–5)TPE were isolated as amorphous materials due to the
absence of an endothermic melting peak on the first heating
cycle. This is probably due to the high viscosity of the amorphous
state. All materials exhibit elevated Tg values compared to the
corresponding fluorene-only materials that are not end-capped
(see Table 1).8 Interestingly, for this series of TPE end-capped
Fig. 1 Structures of F(1–5)TPE.
7516 | J. Mater. Chem., 2012, 22, 7515–7528
This journal is ª The Royal Society of Chemistry 2012

View Article Online
Scheme 1 (a) C8H17Br, KOH, KI, DMSO, RT, (b) (i) n-BuLi (2.5 M), THF (dry), ꢀ78 ꢁC, (ii) triisopropylborate, (iii) HCl (20% v/v), (c) Na2CO3 (aq),
Pd(PPh3)4, DME, reflux, (d) Br2, DCM (dry), (e) (i) n-BuLi (2.5 M), THF (dry), 0 ꢁC, (ii) 4-bromobenzophenone, (iii) PTSA, toluene (Dean–Stark), (f)
Pd(PPh3)4, K2CO3, toluene:H2O (2 : 1), PTC, 90 ꢁC.
oligo-fluorenes the Tg values decrease as the fluorene core
is extended, therefore, F1-TPE exhibits the largest Tg value
(77.0 ꢁC) and F5-TPE exhibits the lowest (55.4 ꢁC). This trend is
opposite to the analogous oligo-n-fluorenes, which show an
increase in the Tg values as the conjugation length is increased. In
this case, if we imagine the molecules sliding past each other in
the melt, the molecules without TPE groups can freely slide past
each other and exhibit a decrease in the free volume as the
conjugation length is increased, and therefore an increase in the
Tg. However, after end-capping the rigid and bulky TPE end
groups of F(1–5)TPE may obstruct this sliding process
decreasing the free volume compared to its fluorene-only
analogue, thus exhibiting a comparatively higher Tg value. From
this situation it can be envisaged that the free volume could
increase when the fluorene core is elongated because when the
fluorene core is large the TPE end groups have less effect on this
sliding process and, as a result, the Tg value decreases. This
aforementioned obstruction has a big effect on polymeric
materials and generally elevates the Tg value.20 In any case, this
shows that TPE end-capping imposes a strong influence on the
thermal properties of the fluorene core and the obstruction of the
sliding process might explain why the Tg values of F(2–5)TPE are
higher than the oligo-n-fluorenes. All the fluorene-TPE materials
form stable glassy films after spin coating from toluene on quartz
substrates and no crystallisation is observed for more than
3 months at room temperature. In contrast, oligo-5-fluorene
(Tg ¼ 17 ꢁC) spontaneously crystallises by aging at room
temperature. High Tg values above room temperature are
desirable for uniform and defect-free thin films because they are
more likely to stay in their glassy state without crystallisation,
which can be detrimental to optoelectronic devices such as
OLEDs. The thermal degradation properties of these end-capped
TPE fluorenes are also superior compared to the analogous
oligo-n-fluorenes (see Table 1). All five materials exhibit negli-
gible differences in Td values. We have also confirmed the
amorphous nature of these materials by XRD analysis (see
Fig. 3), in which the diffraction curve shows the characteristic
amorphous halo without sharp crystalline diffractions.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 7515–7528 | 7517

View Article Online
Scheme 2 (a) Iodine, potassium iodate, acetic acid, H2O, 90 ꢁC, (b) C8H17Br, KOH, KI, DMSO, RT, (c) K2CO3, Pd(PPh3)4, toluene:H2O (2 : 1), PTC,
90 ꢁC, (d) bispinacolatodiboron, KOAc, PdCl2(dppf), dioxane (dry), 70 ꢁC to 90 ꢁC.
Table 1 Thermal propertiesa
Material
Tg (ꢁC)
Td (ꢁC)
F1-TPE
F2-TPE
F3-TPE
F4-TPE
F5-TPE
77.0
425
68.0 (ꢀ20)8
65.4 (ꢀ1)8
61.4 (11)8
55.4 (17)19
423 (374)8
425 (417)8
435 (421)8
429
a
Abbreviations: Tg ¼ Glass transition temperature, Td ¼ thermal
decomposition temperature. For comparison the parentheses denote
the values for the analogous fluorene-only materials, oligo-n-fluorene (n
¼ 2, 3, 4 and 5).
hexane and THF by 20 nm. Additionally, the molar absorptivity
also increases as the degree of conjugation is increased. The
increase in the conjugation reduces the optical band gap and,
therefore, both bathochromic and hyperchromic shifts in
absorption are observed, consistent with the particle-in-a-box
model. The absorption spectrum is broad and featureless with no
vibrational fine structure, which is common for fluorene-type
chromophores. Decreasing the solvent polarity from THF to
hexane blue-shifts the labs for all the Fn-TPE materials because
a less polar solvent can increase the energy of the p* orbital.
However, this is not the case for the emission event (see later).
The labs in the solid state (see Table 2) follows a similar trend
compared to solution, however, the bandwidth (see Fig. 5) is
Fig. 2 DSC thermograms showing first-order transitions on the second
heating cycle (10 ꢁC minꢀ1).
Optical properties
Steady state UV-VIS absorption and PL emission spectra
The UV-VIS absorption spectra, shown in Fig. 4, shows peaks
related to the p–p* transition of the chromophore and demon-
strates the influence of conjugation on the maximum absorption
wavelength (labs) and molar absorptivity. When the number of
fluorene units in the core is enlarged, from F1-TPE to F5-TPE,
the p-electron system is increased and the labs red shifts in both
7518 | J. Mater. Chem., 2012, 22, 7515–7528
This journal is ª The Royal Society of Chemistry 2012

View Article Online
oligofluorene-TPE materials exhibit bathochromic shifts in
absorption compared to the oligo-n-fluorenes that do not
incorporate TPE groups, which reflects an increase in the
conjugation length due to the TPE end-capping.
Although the magnitude of the Stokes shifts for oligo-n-fluo-
renes changes very little from the solution to the solid state, the
F(1–5)TPE materials exhibit comparatively large changes. The
most striking feature of Fig. 4 and Fig. 5 is the large Stokes shifts
in the solid state compared to the small Stokes shifts in solution
(THF), in which F1-TPE exhibits a Stokes shift of 137 nm (solid
state). Provided there is a normal absorption edge, a large Stokes
shift means a small overlap between the absorption and emission
spectra, which is desirable for optoelectronic applications such as
OLEDs, where re-absorption can reduce the device efficiency. In
solution oligo-n-fluorenes display small Stokes shifts ranging
from 35 nm for oligo-2-fluorene to 41 nm for oligo-5-fluorene,
which are similar to the values obtained for our Fn-TPE mate-
rials in solution (THF). Conjugated materials favour planar
conformations in the excited state compared to the ground
state,22 therefore, these large Stokes shifts in the solid state may
be due to conformational changes of the TPE end groups, in
which the geometry of the molecule in the ground state is con-
verted to a more planar excited state.23 Also, the large red-shifts
in the PL of the nanoparticle and solid state compared to the
solution (THF) suggests that the F(1–5)TPE molecules in solu-
tion are more strongly deviated from planarity with a lower
degree of conjugation. F5-TPE exhibits a PLmax of 489 nm in the
solid state (thin film) and a PLmax of 413 nm in the solution state
(THF) with a calculated red-shift of 76 nm. In comparison, the
analogous oligo-5-fluorene shows only a 7 nm red-shift of
the PLmax in the solid state (418 nm, thin film) compared to the
solution state (411 nm, THF). In the case of TPE in solution,
bond rotational motions are unrestricted, and it is this aspect
that results in poor fluorescence quantum efficiencies. Molecular
planarisation is more likely to happen in the solution state where
molecules are more able to undergo changes in their confirma-
tion. Also, twisted conformations in the ground state usually
encourage a planar excited state conformation, i.e., some planar
and rigid materials such as ladder type polyparaphenylene are
unable to undergo conformational changes in the excited state.24
Therefore, it appears that the rotational and torsional motions of
the TPE end-caps are so excessive in THF solution that planar-
isation in the excited state becomes so difficult. In the solid state
Fig. 3 XRD pattern of the as-synthesised F1-TPE after 6 months of the
initial isolation is featureless. The lack of diffraction peaks indicates that
F1-TPE remains as an amorphous material for a long period of time
without crystallisation.
Fig. 4 UV-VIS absorption spectra (with molar absorptivity, 3 in Mꢀ1
cmꢀ1) and emission spectra in THF, concentration ¼ 1 ꢂ 10ꢀ6 M, small
peak at 405 nm in the emission spectra for F(1–3)TPE is the Raman peak.
slightly more broadened, perhaps due to the formation of ground
state aggregates by interchain p–p* interactions that increases
the effective conjugation within the molecule.21 These
Table 2 Optical propertiesa
Slabs (nm)
Slem (nm)
Flabs
(nm)
Flem
(nm)
Nlem
(nm)
Fluorophore
Hexane
THF
Hexane
THF
F1-TPE
F2-TPE
F3-TPE
F4-TPE
F5-TPE
353
360
367
370
373
357
365
369
374
377
355
364
370
372
374
525
530
535
530
525
n.d
n.d
n.d
411
413
492, 492b
493
488
489
489
494
498
502
489
431 (459, 490)
a
Abbreviations and definitions: Slabs ¼ maximum absorption wavelength in solution (hexane and THF), Flabs ¼ maximum absorption wavelength in
film (prepared by spin-casting 0.01 g/0.3 ml from dry toluene solution on quartz substrate), Slem ¼ maximum emission wavelength in solution (hexane
F
N
and THF), lem ¼ maximum emission wavelength in film (prepared by spin-casting 0.01 g/0.3 ml from toluene solution on quartz substrate), lem
¼
maximum emission wavelength in nanoparticle (THF–H2O mixture fw ¼ 90%), all measurements at an excitation of 361 nm, n.d ¼ not detected
b
(fluorescence quenching is observed and the signal is too weak to be accurately determined). Maximum emission wavelength in powder form.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 7515–7528 | 7519

View Article Online
Fig. 5 The UV-VIS and PL spectra of F(1–5)TPE measured from thin films prepared by spin casting from toluene solution on quartz substrates.
the TPE end-caps are more constrained and rotational motions
are more hindered (compared to THF solution), however, its
geometry is in fact still non-planar and thus perhaps allows
a greater ease for planarisation. The ability of TPE to adopt
a large number of interconvertable conformations has been
reported previously.25 Although the emission peaks of F(1–3)
TPE in THF solution are not clear due to considerable
quenching, with respect to F(4–5)TPE the emission peaks are
dominated by the defined vibronic bands of the fluorene core.
The PL emission of the fluorophores in hexane seems to be
a special case. The absorption maxima of the fluorophores in
hexane are blue shifted compared to the absorption maxima in
THF. Oppositely, the PL emission profiles for the fluorophores
F(1–5)TPE in hexane exhibit huge bathochromic shifts
compared to the fluorophores in THF solution. Consequently,
the fluorophores exhibit very large Stokes shifts (F1-TPE ¼
172 nm), which is unusually greater than the Stokes shifts for the
solid state. Again, these huge Stokes shifts could be due to a more
planar geometry of the molecule in the excited state compared to
its ground state. In hexane solution the intramolecular rotations
of the TPE end-caps in some of the molecules are more hindered,
i.e., they exhibit conformations that are less twisted, compared to
the TPE end-caps in THF solution and, therefore, planarisation
occurs and red-shifts the PL emission. However, most of the
molecules contain more twisted conformations of the TPE
groups, which exhibit unhindered intramolecular rotations and
experience difficulty in planarising successfully. Consequently
the FS values in hexane are still very low (see Table 3). Similar
behaviour has been previously reported for tetraphenylethene in
hexane, which exhibits a weak fluorescence emission at approx-
imately 530 nm (PLmax),26 compared to 445 nm in the solid state.
Excimer formation in these materials is highly unlikely due to the
large octyl chains situated at the fluorene 9-position, which
weakens the ground state electronic interactions between mole-
cules, which are usually responsible for aggregate formation.
Additionally, in our case the fluorescence efficiency is enhanced
in the solid state, opposite to excimer formation, which in most
cases drastically quenches the fluorescence. Large Stokes shifts
and large differences in the solution and solid state PL emission
have been reported for other TPE-based materials.27
Although F5-TPE is an oddity (see later), on close inspection
of the PL emission peaks for the nanoparticles in 90% water–
THF solvent mixture of F(1–4)TPE, it can be observed that they
are similar to the solid state. With respect to F(1–3)TPE the
PLmax of the nanoparticles are red-shifted compared to the solid
state, which maybe due to the greater aggregation of the mole-
cules in the confined space of the nanoparticles. Red-shifts in the
PL of nanoparticles compared to the solid state, for silole and
germole materials, have also been previously reported.28
The reported F(1–5)TPE materials contain two different
chromophores. Therefore, in principle the PL spectra could
exhibit two emission profiles. It is well known that the fluorene
core exhibits three emissive transitions with defined vibronic
structures, representing the (0,0), (0,1) and (0,2) transition peaks,
with the (0,0) transition being the highest in energy. The emission
profile changes only slightly from solution to the solid state. In
contrast tetraphenylethene (TPE) exhibits only one broad peak
(PLmax ¼ 445 nm) with ill-defined vibronic structure. In the solid
state F5-TPE exhibits an emission profile with characteristics
from both the central fluorene core and the TPE peripheral
groups. A comparison of these emission characteristics are
shown in Fig. 6. In solution the PL for all F(1–5)TPE materials is
barely detectable, however, in the solid state the PL emission
profiles for all 5 materials are dominated by TPE. From the solid-
state PL spectra of F(1–3)TPE, shown in Fig. 5, it is clear that the
emission profiles are not reminiscent of the fluorene core due to
the lack of defined vibronic peaks. The emission profiles are
broad and featureless with a bathochromic shift, compared to
fluorene-only and TPE-only cores, due to the increase in conju-
gation length. Surprisingly, even when the core of the molecule
contains four and five fluorene rings, the broad and featureless
emission characteristics of TPE are still present. However, a well-
defined vibronic transition around 421 nm is observed, which is
characteristic of the (0,0) vibronic peak of the fluorene core. The
same peak, albeit weak, is observed for F3-TPE and grows in
intensity from F3-TPE to F5-TPE as the fluorene core increases.
7520 | J. Mater. Chem., 2012, 22, 7515–7528
This journal is ª The Royal Society of Chemistry 2012

View Article Online
Table 3 Fluorescence quantum yields, nanoparticle dimensions and AIE effecta
FS
Fluorophore
Hexane
THF
FF
FN
D (nm)
aF
aN
F1-TPE
F2-TPE
F3-TPE
F4-TPE
F5-TPE
0.015
0.018
0.019
0.018
0.024
0.003
0.008
0.011
0.016
0.021
0.59
0.56
0.64
0.62
0.68
0.38
0.25
0.31
0.39
0.41
186 (0.124)
195 (0.165)
247 (0.143)
216 (0.169)
226 (0.129)
196.7
70
58.2
38.8
32.4
125.3
31.4
28.2
24.4
19.7
a
Abbreviations and definitions: FS ¼ relative fluorescence quantum yield in solution estimated using 9,10-diphenylanthracene (F ¼ 0.90 in
cyclohexane29) as standard, FF ¼ relative fluorescence quantum yield in PMMA films estimated using 9,10-diphenylanthracene (FF ¼ 0.83 in
PMMA30), FN ¼ relative fluorescence quantum yield for nanoparticles (THF–H2O mixture fw ¼ 90%) estimated using 9,10-diphenylanthracene
(FF ¼ 0.90 in cyclohexane), D ¼ overall average nanoparticle diameter for THF–H2O mixture fw ¼ 90% at T ¼ 25 ꢁC, (parentheses) ¼
polydispersity index (PDI), aF ¼ extent of emission enhancement (AIE effect) in the film (FF/FS), aN ¼ extent of emission enhancement (AIE
effect) in the nanoparticle (FN/FS).
Most conventional fluorophores exhibit aggregation caused
quenching (ACQ) and thus upon aggregation exhibit a reduction
in fluorescence intensity compared to its solution state where the
molecules are more isolated. For example, a dilute THF solution
of oligo-5-fluorene exhibits a reduction in PL intensity after
aggregation in 90% water (see Fig. 7). Despite the diminished
fluorescence intensity, the long alkyl chains at the fluorene
9-position are very effective in reducing aggregation in the solid
state, which minimises the quenching and thwarts significant
reduction in the fluorescence intensity. The PL shown in Fig. 8
demonstrates that F(1–5)TPE exhibit the opposite effect and all
the end-capped fluorenes show typical AIE behaviour (see Table
3). At water fractions (fw) between 50–90% all the aggregates
were homogeneous with no precipitate. At the critical concen-
tration of water the PL intensity starts to increase and continues
Fig. 6 Comparison of the solid state PL spectra of oligo-5-fluorene, TPE
to increase with higher water contents. The relationship of I/I0
and F5-TPE.
with water fraction (vol%), see Fig. 8f, shows that the PL
intensity is enhanced as the water fraction increases without any
reduction in intensity at higher water fractions. Investigation of
For F4-TPE and F5-TPE an additional peak at 450 nm and 452
the ascending PL spectra at low fw and from the photos shown in
nm, respectively, is observed, which could be the (0,2) transition
Fig. 9, it can be observed that the critical water concentration for
of the fluorene ring. The greater the fluorene character of the
fluorescence ‘‘turn-on’’ is proportional to the molecular weight of
molecule the more defined the emission profile and an increase in
the material, with F1-TPE showing the highest critical water
the (0,0) transition is observed.
concentration of approximately 70% fw. This is reasonable
Aggregation-induced emission (AIE) properties
The AIE behaviour was investigated by monitoring the change in
PL intensity with the addition of increasing amounts of water in
a THF–water solvent mixture. Here, the THF acts as the solvent
and water acts as the antisolvent. Typical AIE behaviour is
observed by a transition from isolated molecules to nano-
aggregates at a critical concentration of water. This results in
a corresponding increase in the PL intensity. Further increasing
the water fraction results in a further increase in PL intensity due
to further aggregation and the resulting nanoparticles emit light
at a higher PL intensity. In most cases the higher the water
content the higher the PL intensity. However, in some cases, after
an initial rise in PL intensity, a drop is observed at approximately
70% fw and above.24a This has been attributed to molecular
Fig. 7 The PL spectra of oligo-5-fluorene (R ¼ C8H17) in THF solution
agglomeration and the formation of random, amorphous
aggregates.31
and in a 90% water–THF solution (excitation wavelength ¼ 361 nm,
concentration ¼ 1 ꢂ 10ꢀ6 M).
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 7515–7528 | 7521

View Article Online
Fig. 8 Emission spectra of (a) F1-TPE, (b) F2-TPE, (c) F3-TPE, (d) F4-TPE and (e) F5-TPE in THF–water mixtures with different water contents at
concentration ¼ 1 ꢂ 10ꢀ6 M, excitation wavelength ¼ 361 nm, (f) Changes in PL peak intensities (I/I0) of F(1–5)TPE, in different THF–water mixtures
(I0 ¼ PL intensity in pure THF solution and intensity recorded at 500 nm).
because the molecular weight of F1-TPE is the lowest with the
highest solubility in the solvent mixture thus being able to
withstand high water concentrations without nucleation. From
the photoluminescence quantum efficiency (PLQE) values of the
materials in solution and nanoparticle state, we can easily
calculate the AIE effect and state the following trends (nano-
particle, aN): F1-TPE (125.3) > F2-TPE (31.4) > F3-TPE (28.2)
> F4-TPE (24.4) > F5-TPE (19.7). As the fluorene core is
7522 | J. Mater. Chem., 2012, 22, 7515–7528
This journal is ª The Royal Society of Chemistry 2012

View Article Online
Fig. 11 The fluorescence imaging confirmed AFM results, which indi-
cated F1-TPE forms nanoparticles in a 9 : 1 water:THF mixture.
For F(1–4)TPE at 90% fw the PL emission spectra of the
nanoparticles are similar to the solid film. However, with respect
to F5-TPE the emission spectrum is noticeably different with
well-defined vibronic features reminiscent of oligo-fluorenes (see
Fig. 8). On closer analysis of Fig. 8e, the PL spectrum at 80% fw is
similar to its related solid state spectrum and to F4-TPE
(Fig. 8d). However, the dramatic change in the emission profile
occurs at 85% and 90% fw. Although not clear from the photos in
Fig. 9, in 90% fw F5-TPE exhibits bright white fluorescence.
Similar to polyfluorene nanoparticles prepared in a similar
manner32 (reprecipitation using solvent and antisolvent system)
the F(1–4)TPE nanoparticles reported here are in the amorphous
state, like that of the solid state. The PL emission peaks of these
nanoparticles are indeed similar to the solid state, although with
a trivial slight bathochromic shift. The phase and morphological
behaviour that results in changes of the fluorene core confor-
mation has become an essential factor in the photophysics
of polyfluorene and to a lesser extent oligofluorene.19,32 Also,
F5-TPE exhibits the lowest Tg of all the fluorophores. All these
aspects may tentatively point to crystallisation as the cause of the
unusual PL emission that is shown in Fig. 8e. Interestingly, when
the same experiment is performed at a higher F5-TPE concen-
tration (1 ꢂ 10ꢀ5 M) in 90% fw the TPE PL emission is not
suppressed and the PL looks similar to that shown for F4-TPE in
90% fw. Therefore polarity factors due to the increase of the
water content are unlikely. Morphology dependent optical
properties of AIE-active materials have been reported by other
groups.33
Fig. 9 Photos of 1 mM F(1–5)TPE in THF–water mixtures with different
water fraction increasing from left to right (fw ¼ 0% to 90%), while
excited over 365 nm illumination. A ¼ brighter fluorescence with exten-
sion of the central fluorene core, B ¼ higher MW Fn-TPE require less
water for fluorescence ‘‘turn-on’’.
elongated the TPE end groups have a weaker influence on the
AIE behaviour of the compound. Accordingly, in pure THF
solutions F5-TPE is the most efficient emitter (see Fig. 9). Ideally,
an efficient AIE material exhibits a large PLQE in the nano-
particle state (or solid state) and a low PLQE in solution.
Surprisingly, F5-TPE still exhibits a significant AIE effect even
with such a large fluorene core. The AIE effect in the solid state
(aF) follows a similar trend. Nanoparticle dimensions were
confirmed by DLS and the size, morphology and fluorescent
images of the F1-TPE nanoparticles were also confirmed by
AFM and fluorescent microscopy (see Fig. 10 and 11). The
nanoparticles of all fluorophores are approximately 200 nm in
size.
The AIE behaviour can be more quantitatively demonstrated
by comparing the PLQE values (see Table 3) of the materials in
solution (FS), nanoparticles (FN) and solid state (FF). In THF
solution the PLQE values increase with elongation of the fluo-
rene core with values calculated at 0.003, 0.008, 0.011, 0.016 and
0.021, for F(1–5)TPE, respectively. In the nanoparticles the
PLQE values are enhanced to 0.38 (F1-TPE), 0.25 (F2-TPE),
0.31 (F3-TPE), 0.39 (F4-TPE) and 0.41 (F5-TPE), showing
a large increase in the quantum yield. This is a clear demon-
stration of AIE activity and is opposite to the ACQ effect, in
which the addition of water and subsequent aggregation results
in lower quantum yields compared to their solution. Some
caution must be taken regarding the relative PLQE values of the
nanoparticles as they have been shown to be much lower than
their absolute values due to the overestimation of the UV
Fig. 10 AFM shows that the nanoparticles formed from F1-TPE in
a mixture of THF–water with 90% water fraction have similar sizes to
that obtained from DLS.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 7515–7528 | 7523

View Article Online
absorbance caused by light scattering or the Mie effect.34 The
PLQE in the ‘‘solid state’’ are the highest with F5-TPE reaching
68%. The effectiveness of TPE end-capping at inducing fluores-
cence quenching can be demonstrated by comparing the PLQE
values of the oligo-n-fluorenes with and without TPE end-
capping. The PLQEs of the oligo-n-fluorenes without TPE
end-capping, are 0.638 (n ¼ 2), 0.758 (n ¼ 3), 0.778 (n ¼ 4)
and 0.96 (n ¼ 5), compared to 0.008 (n ¼ 2), 0.011 (n ¼ 3), 0.016
(n ¼ 4) and 0.021 (n ¼ 5) for F-n-TPE.
compared to its pristine thin film, however, with a slight blue-
shift. Similar experiments using other AIE-active materials have
been carried out by other research groups that involve frozen
dilute solutions in dioxane in which the PL intensity is observed
to increase when the temperature is decreased to ꢀ20 ꢁC.28 Also,
solvents with high viscosity such as paraffin oil have been used to
confirm that the AIE phenomenon is due to intramolecular
effects.38
In an attempt to further understand the nature and importance
of the internal rotations of the TPE moiety and its influence on
the optical properties of our end-capped oligomers, we designed
and synthesised F1-PP. Reducing the internal rotations of tet-
raphenylethane by locking two phenyl rings together, as shown
in Scheme 3, should prevent fluorescence quenching and enhance
photoluminescence properties in solution. Compound 10 can be
oxidised to the corresponding phenyl phenanthrene derivative
using DDQ as the oxidant.39 From GC-MS analysis no starting
material was observed and, surprisingly, 10% of the chloro
derivative was detected, presumably picking up the chlorine
atom from the DDQ molecule. However, this impurity is not
detrimental to the Suzuki coupling and was not removed. Using
quinine sulfate as the standard (F ¼ 0.54 in 0.1 M H2SO440) the
PLQE of F1-PP was calculated at 53%, which is much higher
than the PLQE of F1-TPE. In addition F1-PP does not exhibit
AIE properties. Of course high PLQE in solution of rigid
molecules is not a new insight, nonetheless in this respect the
comparison with F1-TPE is relevant because it demonstrates the
importance of internal rotations of the TPE groups and its effect
on fluorescence quenching. Therefore, we can safely confirm that
the internal rotations of the TPE terminal groups play an
important role in determining the materials fluorescence
efficiencies. In comparison with F1-TPE, F1-PP exhibits blue-
shifts in its absorption and emission maxima (see Fig. 12), which
The AIE phenomenon of TPE-based materials can be best
explained by intramolecular effects, and not intermolecular
effects, such as p–p stacking or J-aggregation. In our case
intramolecular effects are more likely because of the amorphous
nature of the materials and, secondly, because of the long alkyl
chains at the fluorene 9-position. The alkyl chains effectively
increase the intermolecular distance, which reduces the van der
Waals forces and thus prevents strong intermolecular interac-
tions and close packing of the TPE chromophores. Also, TPE
itself is non-planar, in which the dihedral angles of its phenyl
rings are around 60ꢁ, thus preventing close p–p stacking.
Another important AIE-active material is 1,1,2,3,4,5-hex-
aphenylsilole, which exhibits an intermolecular distance of
ꢀ
7.61 A and is clearly non-planar, which prevents close packing of
the molecules.35 All these factors seem to be in contradiction to
a recent publication that suggests the AIE phenomenon to be due
to intermolecular effects.36 The PL emission is enhanced in the
solid state because of the restricted rotational and vibrational
motions of the phenyl rings around the TPE double bond, thus
blocking non-radiative pathways. In solution fast non-radiative
relaxation is dominant because the vibrational and rotational
motions of the phenyl rings are significant. Accordingly, the
PLQEs are enhanced in the solid state compared to the solution.
The importance of intramolecular effects in explaining the
enhanced fluorescence emission can be found in examining the
optical properties of dilute dispersions (Abs ¼ <0.1) of F(1–5)
TPE materials in a rigid polymethylmethacrylate (PMMA)
matrix. These blend films were prepared by spin casting
approximately 0.4 to 1% by weight ratios of F(1–5)TPE materials
in PMMA (0.1 g mlꢀ1 DCM). It is worth mentioning that in
much larger weight ratios (10 to 16%) using oligofluorenes Clark
and co-workers regarded the oligo-n-fluorene molecules in
PMMA blends as isolated and non-interacting.37 We were able to
repeat the PLQE of oligo-5-fluorene (1 wt%) in PMMA (96%,
using diphenylanthracene as the standard), with good agreement
to the literature value (95%, absolute value). The PLQE of oligo-
5-fluorene in THF was calculated at 96% and identical to the
PMMA matrix, which also suggests the molecules to be isolated
in PMMA at this concentration. Therefore, with similar weight
ratios of F(1–5)TPE in PMMA, compared to oligo-5-fluorene,
we can also regard the molecules as isolated. Isolated molecules
in dilute solutions of THF are unhindered and free to rotate.
However, in a rigid PMMA matrix internal rotations are
restricted due to the dramatic reduction in the free volume and
subsequently the PLQE of F1-TPE reaches 0.59. This is
dramatically higher compared to the solution state (0.003) and is
reminiscent of a molecular rotor, therefore, suggesting intra-
molecular processes being responsible for the enhanced lumi-
nescence. Furthermore, for the case of F1-TPE in a PMMA
matrix, the PL spectrum shows an identical emission profile
Scheme 3 (a) DDQ, DCM (dry), CH3SO3H, 0 ꢁC to RT, (b) n-BuLi
(2.5 M), THF (dry), ꢀ78 ꢁC, (ii) 2-isopropoxy-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane, (c) K3PO4, Pd(PPh3)4, DMF (dry), 90 ꢁC.
7524 | J. Mater. Chem., 2012, 22, 7515–7528
This journal is ª The Royal Society of Chemistry 2012

View Article Online
When dried patterns made of F1-TPE were exposed to organic
solvent vapour, the PL intensity of such patterns would become
dimmer. Such intriguing properties of Fn-TPEs have the poten-
tial to be used as solid-state chemical sensors for volatile organic
vapours (Fig. 14).
Electrochemical properties
The electrochemical behaviours of these compounds were
determined by cyclic voltammetry using a standard three-elec-
trode electrochemical cell in an electrolyte solution of 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) dissolved
in distilled dichloromethane. The working electrode was glassy
carbon, the counter electrode was a platinum wire, and the
reference electrode was Ag/Ag+ (0.1 M). Thus, the HOMO levels
were calculated to be ꢀ5.86, ꢀ5.87, ꢀ5.84eV, ꢀ5.82 and ꢀ5.83
eV by the method ꢀ(Eox + 4.70) reported by Li et al.41 (Table 4).
These HOMO levels are very similar to those of oligofluorene.42
The HOMO levels of these compounds are even close to that of
the most widely used NPB (5.4 eV), suggesting the hole-injection
barriers from NPB to the compounds are minimum. Their
LUMOs were obtained from optical band gap energies (esti-
mated from the onset wavelength of their UV absorptions) and
HOMO values, which gave LUMO values in the range of ꢀ2.78
to ꢀ2.84 eV, which is close to the electron affinity of tri(phenyl-2-
benzimidazolyl)benzene (TPBi) (2.7 eV).
Fig. 12 UV-VIS absorption of F1-PP in THF, PL spectra of F1-PP in
THF solution (excitation ¼ 345 nm) and in powder form (excitation ¼
361 nm).
interestingly implies less overlap of the p-electrons and a reduc-
tion in the conjugation length.
Real-time fluorescence imaging and solid-state vapour sensing
The fact that F(1–5)TPE exhibits AIE properties with bright
cyan emission enabled real-time monitoring of the morphology
evolution during solvent evaporation. Fig. 13 reveals that
nucleation sites formed first in the earlier stage, followed by
ripple-like growth. The individual ripple-like growth further
enlarged and became elongated as time lapsed, and eventually
emerged into the morphology of ‘‘sardine bait balls’’ rounded up
by dolphins. This structure, which resembles massive sardines
swarming in a tight ball, suddenly disappeared and was replaced
with large spherical-like structures. Some cores of the spheres still
contained solvent and thus appeared dark in the fluorescence
imaging while others were free of solvent, thus emitting a bright
cyan colour. Finally, all the spheres were dried into discs and at
this moment, some much smaller spheres appeared. A portion of
the smaller spheres again was not completely free of solvent and
thus they had dark cores. This is probably the very first example
to reveal morphology changes as molecules assemble into
nanostructures and microstructures using fluorescence imaging.
Electroluminescence properties
Electroluminescence of F(1–5)TPE was investigated due to their
high solid-state photoluminescence, amorphous behaviour and
high thermal stabilities. We fabricated multilayer OLED devices
with configuration ITO/PEDOT (40 nm)/Fn-TPE (ꢃ50 nm)/
TPBi (40nm)/LiF (1 nm)/Al (100 nm) by spin coating deposition
processes, in which Fn-TPE materials serves as the emitting
layer, PEDOT functions as the hole-transporting layer and TPBi
functions both as the hole-blocking and electron transporting
layer. As shown in Fig. 15, after being fabricated into LED
devices, the EL spectra of F3-TPE have emission characteristics
Fig. 14 Patterns formed from F1-TPE were deposited on glass
substrates (left: control; right: sample). The control was never exposed to
organic vapour, but the sample was exposed to dichloromethane with
exposure time listed on the left. After 20 s exposure, the photo-
luminescence of F1-TPE was completely turned off, demonstrating its
ability to detect organic vapours.
Fig. 13 Real-time fluorescence imaging of F1-TPE during solvent
evaporation, solvent ¼ DCM, concentration ¼ 10ꢀ12 M.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 7515–7528 | 7525

View Article Online
Table 4 EL propertiesa
lEL
(nm)
Lmax
(cd mꢀ2
hP,max
(lm Wꢀ1
hC,max
(cd Aꢀ1
Fn-TPE
Von (V)
)
)
)
HOMO (eV)
LUMO (eV)
Eg (eV)
F1-TPE
F2-TPE
F3-TPE
F4-TPE
F5-TPE
468
468
488
476
480
5.8
5.5
6.4
6.2
4.8
1300
1100
1050
1050
120
1.0
0.55
0.5
0.6
0.06
2.6
1.5
1.6
1.8
0.2
ꢀ5.858
ꢀ5.868
ꢀ5.842
ꢀ5.816
ꢀ5.826
ꢀ2.779
ꢀ2.836
ꢀ2.829
ꢀ2.813
ꢀ2.830
3.079
3.032
3.013
3.003
2.996
a
Abbreviations: lEL ¼ EL max, Von ¼ turn on voltage, Lmax ¼ maximum brightness, hP,max ¼ maximum power efficiency, hC,max ¼ maximum current
efficiency, HOMO ¼ ionisation potential, LUMO ¼ electron affinity, Eg ¼ optical band gap.
similar to the PL spectra. The devices based on F(1–5)TPE
exhibits a maximum luminance (Lmax) of 1300, 1100, 1050, 1050
and 120 cd mꢀ2, a maximum current efficiency (hc,max) of 2.6, 1.5,
From the EL data, it can be concluded that F1-TPE is the best
OLED emitter and the turn-on voltages and efficiencies can be
tuned by the length of the fluorene core.
1.6, 1.8 and 0.2 cd Aꢀ1 and a maximum power efficiency (hP,max
)
of 1.0, 0.55, 0.5, 0.6 and 0.06 lm Wꢀ1, respectively. The detailed
EL performances are listed in Table 4. The materials LUMO
levels seem to be well matched with the electron affinity of TPBi.
However, the large barriers for hole injection at the anode due to
the high ionisation potentials of the fluorophores result in rather
high turn-on voltages (higher than 4.8 V). The barriers for charge
injection seem to be the lowest for F5-TPE due to the compar-
atively lower turn on voltage for its OLED device. However, in
contrast the current efficiency is the lowest with hc,max ¼ 0.2 cd
Aꢀ1. The Lmax appears to follow a trend of decreasing brightness
as the fluorene core is extended from one fluorene unit to five
fluorene units, with F1-TPE exhibiting the highest brightness.
Conclusions
We have synthesised a series of amorphous fluorene-based
materials end-capped with TPE groups that exhibit superior
thermal properties compared to their analogous fluorene-only
materials that are not end-capped with TPE. Interestingly, TPE
end-capping activates ‘‘aggregation induced emission’’ and the
materials quench in solution, contrary to the nanoparticles and
solid state that exhibit strong fluorescence. Comparative studies
of their AIE effect reveal that as the fluorene core is elongated the
TPE end groups have a weaker influence on the AIE behavior of
the compound. Therefore, F1-TPE is the most efficient AIE
Fig. 15 (A) EL spectra of F3-TPE, (B) change in luminance and current density with the applied bias, (C) luminance versus power efficiency, (D)
luminescence versus current efficiency.
7526 | J. Mater. Chem., 2012, 22, 7515–7528
This journal is ª The Royal Society of Chemistry 2012

View Article Online
fluorophore compared to F(2–5)TPE. From THF–water solvent
mixture experiments we observe a gradual increase of the PL
intensity with increasing water concentrations, reaching
a maximum at 90% v/v water content with high PLQE. At this
water concentration the particles are of nano-size dimensions
with an overall average diameter of approximately 200 nm.
Additionally, the highest solid state PLQE was observed for F5-
TPE, which reached a value of 68%. We have shown that F1-TPE
can be used as a solid state solvent vapour sensor with very
visible ‘‘on–off’’ fluorescence behaviour. Encouraged by their
high PLQEs and amorphous behaviour OLEDs were fabricated
and from the EL data F1-TPE exhibits the best OLED proper-
2H), 7.61–7.66 (m, 8H), 7.73 (d, 2H, J ¼ 8.4 Hz), 7.77 (d, 2H, J ¼
7.6 Hz), 7.80 (d, 2H, J ¼ 7.6 Hz).
MS m/z (APCI): 1828.6 (M+).
Elemental analysis, expected (%): C 91.29, H 8.71, obtained
(%): C 91.11, H 8.75.
F4-TPE
1H NMR (400 MHz, CDCl3): d (ppm) 0.73–0.87 (m, 40H), 1.08–
1.25 (m, 80H), 2.04–2.13 (m, 16H), 7.05–7.18 (m, 34H), 7.46 (d,
4H, J ¼ 8.4 Hz), 7.55–7.70 (m, 16H), 7.75–7.82 (m, 8H).
MS m/z (MALDI-TOF): 2217.1 (M+).
Elemental analysis, expected (%): C 91.00, H 9.00, obtained
(%): C 90.91, H 8.96.
ties, with a turn on voltage of 5.8 V, Lmax ¼ 1300 cd mꢀ2, hP,max
¼
1 lm Wꢀ1 and hC,max ¼ 2.6 cd Aꢀ1. These TPE end-capped oli-
gofluorenes represent an interesting class of linear, amorphous
AIE-active materials that exhibit intriguing optical properties.
F5-TPE
1H NMR (400 MHz, CDCl3): d (ppm) 0.76–0.85 (m, 50H), 1.09–
1.26 (m, 100H), 2.05–2.12 (m, 20H), 7.06–7.17 (m, 34H), 7.46 (d,
4H, J ¼ 8.0 Hz), 7.56–7.71 (m, 20 H), 7.75–7.85 (m, 10H).
MS m/z (MALDI-TOF):2605.3 (M+).
Experimental
General procedure for the Pd-catalysed Suzuki cross-coupling
Elemental analysis, expected (%): C 90.79, H 9.21, obtained
(%): C 90.59, H 9.09.
Dibromo-n-fluorene (2, 7 or 8, limiting reactant), boronic acid/
ester (11 or 16, 2.1–2.4 eq.), tetrabutylammonium hydrogen
sulfate (10 mol%) and K2CO3 (5 eq.) were added to a tolue-
ne:H2O (2 : 1 v/v ratio) mixture and the system was evacuated,
with the aid of a vacuum pump, and filled with nitrogen 3 times.
Pd(PPh3)4 (5 mol%) was added and again the reaction vessel was
evacuated and purged with nitrogen. The reaction was heated at
90 ꢁC for 24 h. Water (20 ml) and toluene (10 ml) were added and
the toluene layer separated and subsequently removed using
a rotary evaporator. The crude product was purified by gravity
column chromatography (silica gel) eluting with DCM–hexane
solvent mixture to yield a yellow-green solid.
F1-PP
1H NMR (400 MHz, CDCl3): d 0.61–0.74 (m, 4H), 0.80 (t, 6H),
1.08–1.22 (m, 20H), 2.06–2.10 (m, 4H), 7.20–7.38 (m, 14H), 7.46–
7.75 (m, 20H), 7.80 (d, 2H, J ¼ 8.0 Hz), 8.85 (d, 4H, J ¼ 8.3 Hz).
MS m/z (APCI): 1047.9 (M+).
Elemental analysis, expected (%): C 92.88, H 7.12, obtained
(%): C 92.55, H 6.98.
HPLC ¼ 96.4% (2 : 8, DCM:hexane, CN column).
Acknowledgements
F1-TPE
This work was financially supported by the NSFC (20874025,
21174045), NSFC Research Fellowship for International Young
Scientists (21150110141), the Key Fellowship of China Post-
doctoral Science Foundation, the Open Program for Beijing
National Laboratory for Molecular Sciences (BNLMS) and the
Fundamental Research Funds for the Central Universities
(HUST2010MS101). We also thank the Analytical and Testing
Center of Huazhong University of Science and Technology.
1H NMR (400 MHz, CDCl3): d (ppm) 0.65 (quint, 4H), 0.79 (t,
6H), 1.03–1.18 (m, 20H), 1.96–2.00 (m, 4H), 7.06–7.14 (m, 34H),
7.43 (d, 4H, J ¼ 8.4 Hz), 7.52 (m, 4H), 7.70 (d, 2H, J ¼ 8.0 Hz).
MS m/z (APCI): 1051.9 (M+).
Elemental analysis, expected (%): C 92.52, H 7.48, obtained
(%): C 92.27, H 7.33.
F2-TPE
Notes and references
1H NMR (400 MHz, CDCl3): d (ppm) 0.77–0.81 (m, 20H), 1.07–
1.19 (m, 40H), 2.01–2.05 (m, 8H), 7.04–7.16 (m, 34H), 7.45 (d,
4H, J ¼ 8.4 Hz), 7.54 (overlapping d, 2H), 7.56 (d, 2H, J ¼ 1.2
Hz), 7.61 (overlapping d, 2H), 7.64 (dd, 2H, J ¼ 1.2 Hz, 7.6 Hz),
7.73 (m, 4H).
1 (a) M. Fukuda, K. Sawada and K. Yoshini, Jpn. J. Appl. Phys., 1989,
28, L1433; (b) D. Neher, Macromol. Rapid Commun., 2001, 22, 1366;
(c) M. O’Neill and S. M. Kelly, Adv. Mater., 2011, 23, 566; (d)
M. T. Bernius, M. Inbasekaran, J. O’Brien and W. S. Wu, Adv.
Mater., 2000, 12, 1737.
2 M. Grell, D. D. C. Bradley, M. Inbasekaran and E. P. Woo, Adv.
Mater., 1997, 9, 798.
MS m/z (APCI): 1440.9 (M+).
Elemental analysis expected (%): C 91.74, H 8.26, obtained
(%): C 91.67, H 8.24.
3 M. Grell, W. Knoll, D. Lupo, A. Meisel, T. Miteva, D. Neher,
H. G. Nothofer, U. Scherf and A. Yasuda, Adv. Mater., 1999, 11, 671.
4 (a) M. P. Aldred, A. J. Eastwood, S. M. Kelly, P. Vlachos,
A. E. A. Contoret, S. R. Farrar, B. Mansoor, M. O’Neill and
W. C. Tsoi, Chem. Mater., 2004, 16, 4928; (b) K. L. Woon,
M. P. Aldred, P. Vlachos, G. H. Mehl, T. Stirner, S. M. Kelly and
M. O’Neill, Chem. Mater., 2006, 18, 2311; (c) Y. H. Geng,
S. W. Culligan, A. Trajkovska, J. U. Wallace and S. H. Chen,
Chem. Mater., 2003, 15, 542; (d) R. Guntner, T. Farrell, U. Scherf,
T. Miteva, A. Yasuda and G. Nelles, J. Mater. Chem., 2004, 14, 2622.
F3-TPE
1H NMR (400 MHz, CDCl3): d (ppm) 0.77–0.80 (m, 30H), 1.07–
1.19 (m, 60H), 2.03–2.11 (m, 12H), 7.05–7.14 (m, 34H), 7.44 (d,
4H, J ¼ 8.4 Hz), 7.54 (overlapping d, 2H), 7.56 (overlapping d,
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 7515–7528 | 7527

View Article Online
5 M. P. Aldred, A. E. A. Contoret, S. R. Farrar, S. M. Kelly,
D. Mathieson, M. O’Neill, W. C. Tsoi and P. Vlachos, Adv. Mater.,
2005, 17, 1368.
6 K. L. Woon, M. O’Neill, G. J. Richards, M. P. Aldred, S. M. Kelly
and A. M. Fox, Adv. Mater., 2003, 15, 1555.
18 (a) W. X. Xue, G. X. Zhang, D. Q. Zhang and D. B. Zhu, Org. Lett.,
2010, 12, 2274; (b) M. Wang, X. G. Gu, G. X. Zhang, D. Q. Zhang
and D. B. Zhu, Anal. Chem., 2009, 81, 4444; (c) X. Q. Zhang,
Z. G. Chi, H. Y. Li, B. J. Xu, X. F. Li, S. W. Liu, Y. Zhang and
J. R. Xu, J. Mater. Chem., 2011, 21, 1788.
7 T. Yasuda, K. Fujita, T. Sutsui, Y. H. Geng, S. W. Culligan and
S. H. Chen, Chem. Mater., 2005, 17, 264.
19 W. C. Tsoi, A. R. Buckley and D. G. Lidzey, Chem. Phys. Lett., 2009,
468, 32.
8 L. S. Chinelatto Jr, J. D. Barrio, M. Pinol, L. Oriol, M. A. Matranga,
M. P. De Santo and R. Barberi, J. Photochem. Photobiol., A, 2010,
210, 130.
9 J. D. Luo, Z. L. Xie, J. W. Y. Lam, L. Cheng, H. Y. Chen, C. F. Qiu,
H. S. Kwok, X. W. Zhan, Y. Q. Liu, D. Zhu and B. Z. Tang, Chem.
Commun., 2001, 1740.
10 (a) A. J. Qin, J. W. Y. Lam, F. Mahtab, C. K. W. Jim, L. Tang,
J. Z. Sun, H. H. Y. Sung, I. D. Willams and B. Z. Tang, Appl.
Phys. Lett., 2009, 94, 253308; (b) M.-Q. Zhu, L. Zhu, J. Han,
W. Wu, J. K. Hurst and A. D. Q. Li, J. Am. Chem. Soc., 2006, 128,
4303.
20 J. M. G. Cowie and V. Arrighi, Polymers: Chemistry and Physics of
Modern Materials, 3rd edn (CRC Press, 2007).
21 J. Teetsov and M. A. Fox, J. Mater. Chem., 1999, 9, 2117.
22 M. I. Sluch, A. Godt, U. H. F. Bunz and M. A. Berg, J. Am. Chem.
Soc., 2001, 123, 6447.
23 P. Changenet, P. Plaza, M. M. Martin and Y. H. Meyer, Chem. Phys.,
1997, 221, 331.
24 S. I. Hintschich, F. B. Dias and A. P. Monkman, Phys. Rev. B:
Condens. Matter Mater. Phys., 2006, 74, 045210.
25 H. Tong, Y. N. Hong, Y. Q. Dong, M. Haussler, Z. Li, J. W. Y. Lam,
Y. P. Dong, H. H. Y. Sung, I. D. Williams and B. Z. Tang, J. Phys.
Chem. B, 2007, 111, 11817.
26 C. E. Bunker, N. B. Hamilton and P. Y-Sun, Anal. Chem., 1993, 65,
3460.
27 (a) W. Z. Wang, T. T. Lin, M. Wang, T. X. Liu, L. L. Ren, D. Chen
and S. Huang, J. Phys. Chem. B, 2010, 114, 5983; (b) H. Tong,
Y. N. Hong, Y. Q. Dong, M. Haussler, Z. Li, J. W. Y. Lam,
Y. P. Dong, H. H. Y. Sung, I. D. Williams and B. Z. Tang, J. Phys.
Chem. B, 2007, 111, 11817; (c) Z. J. Zhao, S. M. Chen, C. M. Deng,
J. W. Y. Lam, C. Y. K. Chan, P. Lu, Z. M. Wang, B. B. Hu,
X. P. Chen, P. Lu, H. S. Kwok, Y. G. Ma, H. Y. Qiu and
B. Z. Tang, J. Mater. Chem., 2011, 21, 10949.
28 H. J. Tracy, J. L. Mullin, W. T. Klooster, J. A. Martin, J. Haug,
S. Wallace, I. Rudloe and K. Watts, Inorg. Chem., 2005, 44, 2003.
29 S. Hamai and F. Hirayama, J. Phys. Chem., 1983, 87, 83.
30 D. Katsis, Y. H. Geng, J. J. Ou, S. W. Culligan, A. Trajkovska,
S. H. Chen and L. J. Rothberg, Chem. Mater., 2002, 14, 1332.
31 Y. Q. Dong, J. W. Y. Lam, A. J. Qin, Z. Li, J. Z. Sun, H. H. Y. Sung,
I. D. Williams and B. Z. Tang, Chem. Commun., 2007, 40.
32 C. F. Wu and J. McNeill, Langmuir, 2008, 24, 5855.
33 X. L. Luo, J. N. Li, C. H. Li, L. P. Heng, Y. Q. Dong, Z. P. Liu,
Z. S. Bo and B. Z. Tang, Adv. Mater., 2011, 23, 3261.
34 A. J. Qin, L. Tang, J. W. Y. Lam, C. K. W. Jim, Y. Yu, H. Zhao,
J. Z. Sun and B. Z. Tang, Adv. Funct. Mater., 2009, 19, 1891.
35 Y. Ren, J. W. Y. Lam, Y. Q. Dong, B. Z. Tang and K. S. Wong, J.
Phys. Chem. B, 2005, 109, 1135.
11 Q. Peng, Y. P. Yi, Z. G. Shuai and J. S. Shao, J. Am. Chem. Soc.,
2007, 129, 9333.
12 (a) Y. Liu, Y. H. Tang, N. N. Barashkov, I. S. Irgibaeva,
J. W. Y. Lam, R. R. Hu, D. Birimzhanova, Y. Yu and B. Z. Tang,
J. Am. Chem. Soc., 2010, 132, 13951; (b) Z. J. Ning, Z. Chen,
Q. Zhang, Y. L. Yan, S. X. Qian, Y. Cao and H. Tian, Adv. Funct.
Mater., 2007, 17, 3799; (c) L. H. Peng, M. Wang, G. X. Zhang,
D. Q. Zhang and D. B. Zhu, Org. Lett., 2009, 11, 1943; (d) Y. Liu,
C. M. Deng, L. Tang, A. J. Qin, R. R. Hu, J. Z. Sun and
B. Z. Tang, J. Am. Chem. Soc., 2011, 133, 660; (e) C. N. Park and
J. I. Hong, Tetrahedron Lett., 2010, 51, 1960; (f) Y. Q. Dong,
J. W. Y. Lam, A. J. Qin, Z. Li, J. Z. Liu, J. Z. Sun, Y. P. Dong and
B. Z. Tang, Chem. Phys. Lett., 2007, 446, 124.
13 (a) M. Wang, X. G. Gu, G. X. Zhang, D. Q. Zhang and D. B. Zhu,
Anal. Chem., 2009, 81, 4444; (b) M. Zhao, M. Wang, H. J. Liu,
D. S. Liu, G. X. Zhang, D. Q. Zhang and D. B. Zhu, Langmuir,
2009, 25, 676; (c) W. X. Xue, G. X. Zhang, D. Q. Zhang and
D. B. Zhu, Org. Lett., 2010, 12, 2274.
14 (a) J. W. Chen, B. Xu, X. Y. O. Yang, B. Z. Tang and Y. Cao, J. Phys.
Chem. A, 2004, 108, 7522; (b) B. K. An, S. K. Kwon and S. Y. Park,
Bull. Korean Chem. Soc., 2005, 26, 1555; (c) S. J. Toal, D. Magde and
W. C. Trogler, Chem. Commun., 2005, 5465; (d) P. Lu, J. W. Y. Lam,
J. Z. Liu, C. K. W. Jim, W. Z. Yuan, N. Xie, Y. C. Zhong, Q. Hu,
K. S. Wong, K. K. L. Cheuk and B. Z. Tang, Macromol. Rapid
Commun., 2010, 31, 834.
15 (a) W. Z. Yuan, P. Lu, S. M. Chen, J. W. Y. Lam, Z. M. Wang,
Y. Liu, H. S. Kwok, Y. G. Ma and B. Z. Tang, Adv. Mater., 2010,
22, 2159; (b) S. J. Liu, F. Li, Q. Diao and Y. G. Ma, Org. Electron.,
2010, 11, 613.
36 N. B. Shustova, B. D. McCarthy and M. Dinca, J. Am. Chem. Soc.,
2011, 133, 20126.
37 J. Clark, L. Bazzana, D. D. C. Bradley, J. C. Gonzalez, G. Lanzani,
D. G. Lidzey, J. Morgado, A. Nocivelli, W. C. Tsoi, T. Virgili and
R. D. Xia, J. Nanophotonics, 2008, 2, 023504.
38 M. H. Lee, D. Kim, J. W. Y. Lam and B. Z. Tang, J. Kor. Phys. Soc.,
2004, 45, 329.
39 T. S. Navale, K. Thakur and R. Rathore, Org. Lett., 2011, 13,
1634.
40 W. H. Melhuish, J. Phys. Chem., 1961, 65, 229.
16 (a) H. H. Fang, Q. D. Chen, J. Yang, H. Xia, B. R. Gao, J. Feng,
Y. G. Ma and H. B. Sun, J. Phys. Chem. C, 2010, 114, 11958; (b)
M. Q. Zhu, G.-F. Zhang, C. Li, M. P. Aldred, E. Chang,
R. A. Drezek and A. D. Q. Li, J. Am. Chem. Soc., 2011, 133, 365.
17 (a) M. Wang, D. Q. Zhang, G. X. Zhang and D. B. Zhu, Chem. Phys.
Lett., 2009, 475, 64; (b) J. W. Chen, C. C. W. Law, J. W. Y. Lam,
Y. P. Dong, S. M. F. Lo, I. D. Williams, D. B. Zhu and
B. Z. Tang, Chem. Mater., 2003, 15, 1535; (c) L. H. Peng,
M. Wang, G. X. Zhang, D. Q. Zhang and D. B. Zhu, Org. Lett.,
2009, 11, 1943.
41 Q. Sun, H. Wang, C. Yang and Y. Li, J. Mater. Chem., 2003, 13, 800.
42 G. X. Jiang, J. Wu, B. Yao, Y. H. Geng, Y. X. Cheng, Z. Y. Xie,
L. X. Wang, X. B. Jing and F. S. Wang, Macromolecules, 2006, 39,
7950, and references therein.
7528 | J. Mater. Chem., 2012, 22, 7515–7528
This journal is ª The Royal Society of Chemistry 2012