Donor-acceptor star-shaped conjugated macroelectrolytes: Synthesis, light-harvesting properties, and self-assembly-induced f?rster resonance energy transfer

Article abstract of DOI:10.1021/acs.jpcb.5b02851

A novel series of donor-acceptor star-shaped conjugated macroelectrolytes (CMEs), denoted as 4FTs, including anionic carboxylic acid sodium groups (4FNaT), neutral diethanolamine groups (4FNOHT), and cationic ammonium groups (4FNBrT), were designed, synth

Full text of DOI:10.1021/acs.jpcb.5b02851

Article
pubs.acs.org/JPCB
Donor−Acceptor Star-Shaped Conjugated Macroelectrolytes:
Synthesis, Light-Harvesting Properties, and Self-Assembly-Induced
̈
Forster Resonance Energy Transfer
Li Zhao,† Cheng-Fang Liu,† Wei-Dong Xu,† Yi Jiang,† Wen-Yong Lai,*,†,‡ and Wei Huang*,†,‡
†Key Laboratory for Organic Electronics and Information Displays (KLOEID) & Institute of Advanced Materials (IAM), Jiangsu
National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9
Wenyuan Road, Nanjing 210023, China
‡Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation
Center for Advanced Materials (SICAM), Nanjing Tech University, 30 South Puzhu Road, Nanjing 211816, China
S
* Supporting Information
ABSTRACT: A novel series of donor−acceptor star-shaped conjugated macroelectrolytes
(CMEs), denoted as 4FTs, including anionic carboxylic acid sodium groups (4FNaT),
neutral diethanolamine groups (4FNOHT), and cationic ammonium groups (4FNBrT),
were designed, synthesized, and explored as an excellent platform to investigate the impact
of various polar pendent groups on self-assembly behaviors. The resulting CMEs with
donor−acceptor star-shaped architectures exhibited distinct light-harvesting properties. The
interactions between 4FTs and TrNBr, a star-shaped monodisperse CME grafted with
cationic quaternary ammonium side chains, were investigated in H2O and CH3OH using
steady-state, time-resolved fluorescence, dynamic light scattering (DLS), and transmission
electron microscopy (TEM). Highly favored energy transfer has been proven by the
excellent spectral overlap between TrNBr fluorescence and 4FTs absorptions which can be
tuned by adjusting the pendent polar groups and solvents. It is suggested that self-
assembled structures are formed between TrNBr and 4FNaT, while there is no obvious
change for TrNBr/4FNOHT and TrNBr/4FNBrT in both H2O and CH3OH at low concentrations (<10−6 M). This result is
confirmed by the change of the TrNBr and 4FTs fluorescence properties and the time-resolved fluorescence data. The overall
results manifest that at low concentrations the self-assembly between TrNBr and 4FTs is dominated by the electrostatic
interactions. This study suggests that the functionalization of pendent polar groups of star-shaped CMEs has proven to be
effective to modulate the self-assembly behaviors in dilute solutions and thus provide a strategy to further manage the
optoelectronic properties.
conjugated materials.15−18 FRET promotes the possibility of
INTRODUCTION
■
obtaining self-assembled structures comprising energy donors
Organic optoelectronics is the focus of increasing research
activity owing to its potential application in flexible, low-cost,
and acceptors for applications in light-harvesting systems.9,11
and large-area consumptive productions.1−3 In recent years,
These studies not only provide fundamental understanding in
the mechanism of exciton migration but also offer practical
great attention has been paid to conjugated polyelectrolytes
(CPEs), which are composed of π-conjugated polymer
utilization in light-emitting diodes, organic lasers, and chemical
and biological sensors.19−22 As is well-known, fluorene-based
backbones and charged pendent groups to achieve excellent
solubility in polar solvents.4−6 Owing to their combination of
polymers are mostly studied in FRET because of their high
fluorescence quantum yields and high energy blue emissions
optoelectronic properties of traditional conjugated polymers
that allow efficient energy transfer to the acceptors.23,24
with the electrostatic behaviors of electrolytes, they have
emerged as attractive materials for various applications.7,8 In
CPEs have also been employed as probes to detect the
mechanisms of FRET25−28 and self-assembly.29−31 Although
previous several years, CPE-based fluorescent probes have been
CPEs have been investigated extensively and have brought
great development in the application, the detailed process
regarding the energy transfer remains obscure.4,32 In contrast to
CPEs, monodisperse conjugated macroelectrolytes (CMEs),
utilized as biosensors for the detection of protein and DNA/
RNA via electron or energy transfer mechanisms.9,10 Very
recently, their applications as charge-collecting layers or
interfacial layers in organic semiconductor devices, e.g., organic
light-emitting devices and organic photovoltaics, have aroused
intense interest.11−14
Received: March 25, 2015
Revised: May 12, 2015
Published: May 18, 2015
Forster resonance energy transfer (FRET) is an attractive
̈
strategy to modulate the optical and electronic behaviors of
© 2015 American Chemical Society
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Scheme 1. Chemical Structures of 4FTs (4FNaT, 4FNOHT, 4FNBrT) and TrNBr
which are monodisperse macromolecules with multidimen-
sional topology bearing pendent polar groups, provide many
advantages due to their well-defined structures and lack of
batch-to-batch variations.33 CMEs have received wide attention
because of their various potential applications such as organic
light-emitting devices3 and organic photovoltaics.34−37 Addi-
tionally, different from linear CPEs, these branched molecular
structures are believed to be able to reduce the π−π interaction
and aggregate formation in polar solvents, showing much
stronger photoluminescence intensity, which renders them
rather attractive in chemical sensing or bioimaging applica-
tions.33,35 Great attention has been paid to the fundamental
properties and potential applications of CME-based self-
assembled systems,37,38 which are utilized as valuable platforms
to explore charge transport and energy transfer. Hence, taking
advantage of these properties, one can be able to modulate the
self-assembly behaviors of CMEs by adjusting the pendent
polar groups to monitor the donor−acceptor interactions
CME systems with broad response covering the whole visible
and near-infrared spectra.12,41
A detailed study of FRET in three distinct systems between
the cationic TrNBr as an energy donor and 4FTs (anionic
4FNaT, neutral 4FNOHT, and cationic 4FNBrT) as acceptors
(Scheme 1) was carried out, providing an ideal platform for
clarifying the distance and interaction dependence of energy
transfer. The excellent spectral overlap between the cationic
TrNBr fluorescence and 4FTs absorption indicates that the
energy transfer process is highly favored. In the presence of
4FNaT acceptor, the emission quantum yield of TrNBr sharply
decreases, suggesting the existence of FRET process. However,
there is no obvious change in TrNBr/4FNOHT and TrNBr/
4FNBrT. On the basis of these observations, the strong
interactions between the cationic and anionic groups enable
self-assembly that render the distance smaller than R0, further
boosting FRET. Together with dynamic light scattering (DLS)
and transmission electron microscopy (TEM), the self-
assembled structures are analyzed in detail. In this way, studies
of energy transfer in this system are likely to be relevant to
optimizing energy migration between CMEs with different
ionic polar groups. Therefore, modification of the chemical
structures by varying the pendent polar groups allows the
control of the self-assembly behaviors, which render the
handling of FRET a key factor in these systems. From a
positive perspective, it has offered crucial information for the
rational design of light-harvesting CME materials with fine-
tuning optical properties for optoelectronic applications.
falling within the Forster distance (R ),26 thus facilitating FRET
̈
0
in CME systems to fine manage the optoelectronic properties.
In this contribution, we report the synthesis of a novel series
of star-shaped CMEs (4FNaT, 4FNOHT, and 4FNBrT) and
the investigation of the impact of pendent polar groups on
various physical and optical properties. To probe the self-
assembled behaviors, a star-shaped CME, TrNBr, which shows
super high photoluminescence quantum yield (PLQY) in polar
solvents, was selected as an energy donor. All the chemical
structures of CMEs studied here are depicted in Scheme 1.
TrNBr consists of a truxene core and oligofluorene arms,
tethered with cationic ammonium groups. Oligofluorene
segments were selected as the arms because of their high
energy blue emission and high fluorescence quantum yields that
allow efficient energy transfer to the acceptors,25,39 and good
charge-carrier transport properties in deep-blue devices.5,40
4FTs comprise a 4,7-(4-(diphenylamino)phenyl)-2,1,3-benzo-
thiadiazole (TPABT) core and fluorene-based arms, tethered
with anionic carboxylic acid sodium groups (4FNaT), neutral
diethanolamine groups (4FNOHT), and cationic ammonium
groups (4FNBrT), respectively. TPABT was chosen as the core
due to its incorporation of two electron-withdrawing groups
and its orange emission, which were prone to intra/
intermolecular energy transfer leading to light-harvesting
EXPERIMENTAL DETAILS
■
Purification. All chemical reactions were monitored by a
thin-layer chromatography silica gel plate (TLC) with 254 and
365 nm ultraviolet lamp. The silica gel (100−200 mesh and
300−400 mesh) of column chromatography used in the
experiments was purchased from Qingdao Marine chemical
Co., Ltd. NMR spectra were recorded on a Bruker Ultra shield
Plus 400 MHz NMR (1H. 400 MHz; 13C, 100 MHz).
Instruments. UV−vis absorption spectra were recorded on
a Shimadzu UV-3600 UV−vis−NIR spectrophotometer with a
minimum resolution of 0.5 nm. The photoluminescence (PL)
spectra were collected with a Shimadzu Luminescence
Spectrometer LS 50. The fluorescence quantum yields were
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Scheme 2. Synthetic Routes toward 4FTs (4FNaT, 4FNOHT, and 4FNBrT)
determined by a full-featured steady state/transient fluores-
cence spectrometer FLS-920 from Edinburgh Instruments. PL
decays were measured with an Edinburgh FLS-920 spectrom-
eter. The excitation source was a nanosecond flash lamp
operating in an atmosphere of H2 gas (0.40−0.50 bar, 1.2 ns
full width at half-maximum (fwhm), 40 kHz repetition rate),
whose output was filtered through a monochromator prior to
the sample excitation. All fluorescent lifetimes were determined
from the data using the Edinburgh Instruments software
package. The hydrodynamic sizes were determined by dynamic
light scattering (DLS) using a 90 Plus particle size analyzer
(Brookhaven Instruments). The transmission electron micros-
copy (TEM) images were performed using a HT7700
transmission electron microscope at an accelerating voltage of
100 kV. The prepared samples were dropped onto the copper
grid before measurements.
48 h to afford 4FNaT in 95% yield. The precursor 4FNT was
treated with diethanolamine and H2O2 in a THF/dimethylfor-
mamide (DMF) mixture in a sealed vessel under microwave
(MW) irradiation at 75 °C for 40 min to afford 4FNOHT in
93% yield. The precursor 4FNT was also dissolved in THF and
then treated with trimethylamine (TMA) solution in ethanol
(33%) in a sealed vessel at room temperature for 24 h to afford
4FNBrT in 90% yield. The intermediates and the star-shaped
macromolecules were purified by chromatography, and the
1
well-defined structures were adequately verified by H and 13C
NMR spectroscopy and MALDI-TOF mass spectrometry.
4FNOHT is soluble in common polar solvents such as
methanol, ethanol, and dimethyl sulfoxide (DMSO), but not
H2O; nevertheless, 4FNaT and 4FNBrT are soluble in
common polar solvents inclusive of H2O.
Absorption and Photoluminescence Properties. The
UV−vis absorption and photoluminescence (PL) spectra of
4FNaT, 4FNBrT, and TrNBr were measured in H2O and
CH3OH, while for 4FNOHT the spectral data were only
recorded in CH3OH because of its limited solubility in H2O.
Figure 1 shows the UV−vis absorption spectra of 4FTs and the
PL spectra of TrNBr recorded in H2O and CH3OH solutions.
The absorption spectra of 4FTs comprise two distinct π−π*
transitions: a strong peak around 370 nm and a weaker peak at
around 530 nm both in H2O and in CH3OH. The peaks at
about 370 nm arise from the fluorene moieties, and the other
peaks at around 530 nm are ascribed to the intramolecular
charge transfer (ICT) from the donor−π-acceptor type TPABT
core that are composed of triphenylamine (TPA) and benzene
thiazole−thiophene (BT). 4FTs exhibit almost the same optical
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The synthetic proce-
dures toward star-shaped 4FTs (4FNaT, 4FNOHT, 4FNBrT)
are depicted in Scheme 2. Star-shaped TrNBr was synthesized
according to the procedures described in our previous
contribution.35 The key steps of the synthesis of star-shaped
4FTs include Suzuki coupling reaction of monomer 3 or
monomer 4 (Scheme 2) with tris(4-bromophenyl)amine
(monomer 5) to afford monomer 6 or 7 and subsequent Stille
coupling reaction using Pd(II) as the catalyst to afford
precursor conjugated macromolecules (4FCT, 4FNT). The
precursor 4FCT was treated with sodium hydroxide (NaOH)
in tetrahydrofuran (THF) in a sealed container and refluxed for
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CH3OH, in sharp contrast to those of the common linear CPEs
whose PL emission is weak and largely quenched in aqueous
solutions. 4FTs exhibit broad and intense absorption behaviors
but weak PL emissive properties (Supporting Information,
Figure S1b) with much lower ΦF values in comparison to that
of TrNBr because of the ICT characteristics of the TPABT core
structure. The fluorescence quantum yield (ΦF) is comparable
to or slightly higher than the ΦF values reported for the
corresponding truxene-cored three-armed dendrimers or
TPABT-cored four-armed dendrimers.42−44 This result sug-
gests that 4FTs are efficient light-harvesting conjugated
systems.
Figure 1. Normalized UV−vis absorption spectra of 4FTs (4FNaT,
4FNBrT) in H2O and 4FTs (4FNaT, 4FNOHT, 4FNBrT) in
Self-Assembled TrNBr and 4FTs Systems. The inter-
actions between TrNBr and the 4FTs in CH3OH and H2O
were studied with an increment of their concentrations at a
constant concentration ratio ([TrNBr]:[4FTs] = 1:1). The
maximum emission intensity varied by gradually increasing
their concentrations at a constant concentration ratio. The
results are summarized in Figures 2 (H2O) and 3 (CH3OH). As
shown in Figure 2a, the PL intensity of TrNBr/4FNaT
increased progressively in very dilute solutions (1 × 10−7−1.5 ×
10−6 M) with an increment of the concentration and then
decreased (1.6 × 10−6−1.7 × 10−5 M) gradually. It became
completely quenched when the solution concentration was up
to 1.7 × 10−5 M. In the whole concentration range, the solution
of TrNBr/4FNaT is much less emissive than that of TrNBr
alone (Figure 2a and Figure S8a−c (Supporting Information)),
indicating that adding even a very small amount of light-
harvesting anionic 4FNaT into the highly emissive cationic
TrNBr dilute solution leads to obvious energy transfer, causing
significant luminescence quenching. In contrast, in the presence
of light-harvesting cationic 4FNBrT, the solution of TrNBr/
4FNBrT in H2O maintains almost the same emissive properties
of TrNBr without any obvious quenching in dilute solutions (1
× 10−7−7 × 10−7 M) (Figure 2b and Figure S8a,d,e
(Supporting Information)). As shown in Figure 2b, a
decreasing trend similar to that of the TrNBr/4FNaT system
is also observed for TrNBr/4FNBrT with increasing concen-
trations (3 × 10−6−10−4 M). This phenomenon can be
explained by the fact that the repellent interactions between the
two molecules TrNBr and 4FNBrT with the same charged
ionic groups in dilute solutions will result in increasing the
intermolecular distances, which falls out of the range for
efficient FRET. However, by means of increasing the
concentrations up to 3 × 10−6 M or higher, obvious emission
quenching occurs, indicating efficient FRET happens in higher
CH3OH, and PL spectra of TrNBr (λex = 361 nm in H2O, λex
360 nm in CH3OH).
=
behaviors. Only very slight changes are observed in the
absorption wavelength maxima of 4FTs by changing the
pendent polar groups and varying the solvents used. As shown
in Figure 1, the absorption spectra of 4FTs matched well with
the PL spectra of TrNBr, indicating possible efficient energy
transfer would happen in TrNBr/4FTs systems. The
fluorescence quantum yields (ΦF) in H2O and CH3OH were
measured by using an integrating sphere. The corresponding
properties of TrNBr and 4FTs are summarized in Table 1. It is
Table 1. Molar Extinction Coefficients (ε), Absorption (λabs)
a
and Fluorescence Wavelength Maxima (λem), and
b
Fluorescence Quantum Yields (ΦF) for CMEs (TrNBr,
4FTs) in H2O and CH3OH
compd (soln)
ε (L mol−1 cm−1
)
λabs (nm) λem (nm)
ΦF
TrNBr (H2O)
6.8 × 105
2.6 × 105
7.5 × 105
9.2 × 105
9.5 × 105
1.0 × 106
1.1 × 106
361
402, 425
397, 418
688
0.86
0.90
0.03
0.02
0.01
0.02
0.01
TrNBr (CH3OH)
4FNaT (H2O)
360
371, 534
374, 537
371, 532
375, 538
372, 536
4FNBrT (H2O)
4FNaT (CH3OH)
4FNOHT (CH3OH)
4FNBrT (CH3OH)
697
698
687
684
a
All the spectra of TrNBr (5 × 10−7 M) and 4FTs (1 × 10−6 M) were
b
measured in H2O or CH3OH solutions at room temperature. The
fluorescence quantum yields (ΦF) were measured with the absolute
methods by using an integrating sphere, and the excitation wavelength
of 4FTs was the corresponding maximum absorption peak.
interesting to note that the star-shaped TrNBr exhibits a very
high ΦF of 0.86 in H2O and an even higher ΦF of 0.90 in
Figure 2. Dependence of the emission intensity (corresponding intensity was recorded from the emission spectra with peaks at λem = 402 nm and
λem = 425 nm, respectively) for solutions (a) [TrNBr] and [TrNBr]:[4FNaT] = 1:1 and (b) [TrNBr] and [TrNBr]:[4FNBrT] = 1:1 in H2O with
simultaneous increasing concentrations. The emission data were collected at λex = 361 nm.
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Figure 3. Dependence of the emission intensity (corresponding intensity was recorded from the emission spectra with peaks at λem = 397 nm and
λem = 418 nm, respectively) for the solutions (a) [TrNBr] and [TrNBr]:[4FNaT] = 1:1, (b) [TrNBr] and [TrNBr]:[4FNOHT] = 1:1, and (c)
[TrNBr] and [TrNBr]:[4FNBrT] = 1:1 in CH3OH with simultaneous increasing concentrations. The emission data were collected at λex = 360 nm.
Figure 4. Emission spectra of TrNBr (5 × 10−7 M) with increasing concentrations of (a) 4FNaT and (b) 4FNBrT in H2O and (c) 4FNaT, (d)
4FNOHT, and (e) 4FNBrT in CH3OH. The emission spectra were collected at λex = 361 nm in H2O and λex = 360 nm in CH3OH.
concentrations. Based on these results, it is suggested that the
driving force for the self-assembly between 4FNaT and TrNBr
in very dilute solutions is mainly the electrostatic interactions,
which is effective to diminish the intermolecular distance of
TrNBr and 4FNaT, further inducing lower luminescence
efficiency. By increasing their concentrations (>1.5 × 10−6
M), not only electrostatic interactions but also intermolecular
interactions (i.e., π−π stacking) contribute to the FRET
process that causes notable luminescence quenching. Moreover,
the same investigation of TrNBr and 4FTs in CH3OH (Figure
3 and Figure S9 (Supporting Information)) follows the same
trends as the results in H2O. It is worthwhile to mention that
the critical point of the concentration to achieve complete
quenching in CH3OH is found to be around 1.1 × 10−6, 3 ×
10−5, and 1 × 10−4 M for TrNBr/4FNaT, TrNBr/4FNOHT,
and TrNBr/4FNBrT, respectively, following the trend of
[TrNBr/4FNaT] < [TrNBr/4FNOHT] < [TrNBr/4FNBrT].
In this way, the molecular distances at a low concentration are
determined by the electrostatic interactions. However, at a high
concentration, in addition to intermolecular interactions, there
exists aggregation of molecules due to the increase of
concentrations. Therefore, we can draw the conclusion that
the electrostatic interactions are the main force in the process
in dilute solutions with a concentration lower than 10−6 M. To
get further understanding, the dependence of the optical
spectral properties on increasing concentrations was inves-
tigated at length.
Electronic energy transfer from donors (TrNBr) to acceptors
(4FTs) was then studied in detail at a constant TrNBr
concentration (5 × 10−7 M) by varying concentrations of 4FTs
acceptors in a range of (0−9) × 10−7 M (Figure 4 and Figure
S10 (Supporting Information). The absorption spectra of
TrNBr (5 × 10−7 M) vary by increasing the concentrations of
4FTs in H2O (Supporting Information, Figure S10a,b) and
CH3OH (Supporting Information, Figure S10c−e), resulting in
an increase of the absorption intensity at the maximum
wavelength (λambasx) and an appearance of a new absorption band
at about 540 nm. Similarly, as shown in the Supporting
Information (Figure S5a,c), gradually increasing the concen-
trations of TrNBr at a constant 4FNaT (Supporting
Information, Figure S11a) or 4FNBrT (Supporting Informa-
tion, Figure S11c) concentration causes an increase of the
absorption at the wavelength maximum (λambasx) and a slight
hypsochromic shift of λambasx. Comparing the absorption spectrum
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of the TrNBr (5 × 10−7 M)/4FNaT (10−6 M) self-assembled
system and the sum of the absorption spectra of isolated TrNBr
(5 × 10−7 M) and 4FNaT (10−6 M) systems, it is possible to
infer that the new absorption band at about 543 nm may arise
from 4FNaT due to the ICT characteristic core structure
(Supporting Information, Figure S12a, in H2O and Supporting
Information, Figure S12c, in CH3OH). The absorption bands
of the mixtures are slightly broader and red-shifted relative to
the sum of the two components, suggesting the formation of
Either in H2O or in CH3OH, the solutions of TrNBr/4FNaT
system are not able to emit blue light, suggesting the existence
of FRET in this system due to the self-assembly induced by the
interaction of oppositely charged ionic pendent groups.
Table 2 shows the ΦF values of the two systems (TrNBr/
4FNaT, TrNBr/4FNBrT) in H2O and the three systems
Table 2. Fluorescence Quantum Yields of the Systems (λex =
361 nm) TrNBr/4FTs (4FNaT, 4FNBrT) (5 × 10−7 M) in
H2O and the Systems (λex = 360 nm) TrNBr/4FTs (4FNaT,
TrNBr/4FNaT self-assembled structures. The absorption
a
abs
max
4FNOHT, 4FNBrT) (5 ×10−7 M) in CH3OH
intensity of the TrNBr/4FNaT mixtures at λ
is much
smaller than the sum of these two isolated components,
indicating strong intermolecular interactions and efficient
FRET processes between TrNBr and 4FNaT induced by the
self-assembly. In contrast, when we compare the absorption
spectrum of the mixture of TrNBr/4FNBrT and TrNBr/
4FNOHT with the sum of the absorption spectra of isolated
4FNBrT, 4FNOHT, and TrNBr solution, the absorption bands
in terms of intensity and positions are almost identical
(Supporting Information, Figure S12b,d,e). The results suggest
that efficient FRET process happens in the TrNBr/4FNaT
system even in very dilute solution, but not in TrNBr/4FNBrT
and TrNBr/4FNOHT systems. This highlights the influence of
the pendent polar ionic groups on FRET through the self-
assembled system, as revealed by the difference in the spectra
features.
The effect of TrNBr on the spectral properties of 4FTs is
more evident in the PL spectra and vice versa (Figure 4a,b in
H2O and Figure 4c−e in CH3OH). Upon addition of 4FNaT, a
decrease in intensity of TrNBr fluorescence is observed,
together with the appearance of new emission bands in the
range 640−770 nm assigned to 4FNaT in both H2O (Figure
4a) and CH3OH (Figure 4c). The PL spectra of TrNBr are
obtained at the maximum absorption wavelength about at 360
nm, which is also within the absorption region of 4FNaT.
Hence, the appearance of the PL band might be attributed to
the direct excitation of the 4FNaT rather than any excitation
energy transfer (FRET) from TrNBr to 4FNaT. In fact, both
processes were occurring, as shown in Figure S11b (Supporting
Information). The gradual addition of TrNBr results in an
increase of the PL intensity in the range 640−770 nm, assigned
to the 4FNaT, which implies the energy transfer from TrNBr to
4FNaT. Nevertheless, the PL intensity of TrNBr/4FNBrT at
691 nm assigned to 4FNBrT gets weak, and the PL intensity at
λem = 402 nm assigned to TrNBr first increases and then
decreases with a gradual increment of TrNBr concentration
(Supporting Information, Figure S11d,e). These results reflect
the aggregation of 4FNBrT and TrNBr. Furthermore, the
pictures of different solutions under UV light give us an
intuitive impression about the actual FRET process (Figure 5).
system
ΦF(370−608 nm)
ΦF(608−750 nm)
TrNBr/4FNaT (H2O)
0.46
0.85
0.42
0.85
0.84
0.05
0.02
0.03
0.02
0.00
TrNBr/4FNBrT (H2O)
TrNBr/4FNaT (CH3OH)
TrNBr/4FNOHT (CH3OH)
TrNBr/4FNBrT (CH3OH)
a
The fluorescence quantum yields (ΦF) were measured with the
absolute methods by using an integrating sphere, and the excitation
wavelength was the corresponding maximum absorption peak.
(TrNBr/4FNaT, TrNBr/4FNOHT, TrNBr/4FNBrT) in
CH3OH at a total constant concentration (5 × 10−7 M). In
the presence of the 4FNaT acceptors, the ΦF value of the
TrNBr sharply decreases, while the ΦF values of the TrNBr/
4FNaT systems are still much higher than those of 4FNaT
alone (Table 1) in both H2O and CH3OH solutions. This
phenomenon can be attributed to the self-assembled structures
facilitated by the strong ionic attractive interactions of
oppositely charged ionic groups that boost FRET. On the
contrary, in the presence of the 4FNBrT, the ΦF values have no
obvious changes both in H2O and CH3OH, manifesting no
efficient FRET in this process. Moreover, we also investigated
the variation of ΦF values of TrNBr/4FNOHT in CH3OH.
The results show trends similar to that of TrNBr/4FNBrT. We
believe that this may originate from the fact that the different
charged ions induce different intermolecular interactions,
although there may be some contribution from direct excitation
of 4FNaT, 4FNOHT, and 4FNBrT at the wavelength about
360 nm. Based on these analyses, we can infer that oppositely
charged ionic groups have strong electrostatic interactions to
boost the self-assembly that induces obvious FRET even in very
dilute solutions. Consequently, modulating the ionic pendent
groups has provided an effective strategy to adjust the self-
assembly behaviors in the systems.
To confirm the self-assembly in these systems and provide
further insight on the luminescence quenching according to
analysis of the fluorescence behaviors, the Stern−Volmer plots
of TrNBr/4FTs in H2O (Figure 6a) and CH3OH (Figure 6b),
respectively, were established as shown in Figure 6. As shown in
the plots, the I0/I ratio is given as a function of the TrNBr
concentration, where I0 and I are the maximum PL intensities
in the absence and presence of 4FTs.
Φ0
Φ
I0
I
=
= 1 + kqτ0[A] = 1 + KSV[A]
where kq is the quenching constant, KSV is the Stern−Volmer
constant, and τ0 is the TrNBr lifetime in the absence of 4FTs.
The plots of I0/I for TrNBr/4FTs are shown in Figure 6a in
H2O and Figure 6b in CH3OH. Self-assembled systems of
TrNBr/4FNaT (Figure 6, squares) reveal that they do not obey
the linear Stern−Volmer relation expected for collisional
Figure 5. Schematic representation of TrNBr (5 × 10−7 M), 4FTs
(4FNaT, 4FNOHT, 4FNBrT) (10−6 M), and the mixture of TrNBr (5
× 10−7 M)/4FTs (10−6 M) in H2O and CH3OH under UV light.
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Time-Resolved Fluorescence Measurements on the
Self-Assembled Systems of TrNBr/4FNaT. The time-
resolved fluorescence of TrNBr/4FNaT was plotted as shown
in Figure 7. It was measured for the samples in H2O or CH3OH
Figure 6. Stern−Volmer plots of TrNBr in the presence of 4FTs. (a)
4FNaT (squares) and 4FNBrT (circles) in H2O. (b) 4FNaT
(squares), 4FNBrT (circles), and 4FNOHT (triangles) in CH3OH.
The Stern−Volmer plots of 4FNBrT (circles) and 4FNOHT
(triangles) in CH3OH are shown in the inset.
quenching in all of the concentration range. Instead, the plot of
I0/I of TrNBr/4FNaT is linear for low quencher concentration
((0−5) × 10−7 M) and then has a rapidly upward curvature,
due to combined dynamic and static quenching, where the
fluorophore can be quenched not only by collision but also by
complex formation with the quencher. Besides, the plot of I0/I
of the TrNBr/4FNBrT system (Figure 6, circles) follows the
linear Stern−Volmer relation expected for collisional quench-
ing. In addition, we also obtain the plot of I0/I of TrNBr/
4FNOHT in CH3OH, which is similar to that of the TrNBr/
4FNBrT system obeying the linear Stern−Volmer relation. KSV
is determined from the slope of the linear region in this case,
and kq is calculated from these values, using τ0 as the lifetime
for TrNBr (1.06 ns in H2O, 0.85 ns in CH3OH) in the absence
of quencher. Comparing the TrNBr/4FNaT system in the two
solvents, it can be seen from the initial slope of the Stern−
Volmer plots that the quenching efficiency is higher in H2O
than in CH3OH (Figure 6 and Table 3). The results suggest
Figure 7. Fluorescence emission decays of (a) TrNBr (5 × 10−7 M),
(b) 4FNaT (10−6 M), and TrNBr/4FNaT (5 × 10−7 M), collected at
402 and 688 nm in H2O (c) and collected at 397 and 698 nm in
CH3OH (d).
following 1 MHz, 70 ps pulse excitation. The emission decays
of TrNBr and 4FNaT were recorded by using two discrete
exponentials both in H2O and in CH3OH with excellent fits
(Figure 7a,b). The short lifetime of τ1(TrNBr) (0.88 ns in H2O,
0.69 ns in CH3OH) is similar to those reported for truxene-
cored molecules.39 The long lifetime of τ1(TrNBr) (2.03 ns in
H2O, 2.20 ns in CH3OH) is linked with the decay of the
relaxed structures. On the contrary, the long lifetime consisting
of τ2(4FNaT) (4.37 ns in H2O, 3.11 ns in CH3OH) dominates
the decay, which is generally believed to include contributions
from possible intramolecular energy migration from the
fluorene-based arms to the TPABT core. As shown in Figure
7a,b, it is noted that the different decay behaviors of TrNBr and
4FNaT are found to depend on the solvent polarity.
Figure 7c,d shows the fluorescence decays of TrNBr/4FNaT
self-assembly system (5 × 10−7 M), collected at the emission of
TrNBr and 4FNaT. Comparing Figure 7c,d with Figure 7a,b,
the decay times collected at 688 nm in H2O (1.25, 4.00 ns) and
698 nm in CH3OH (0.83, 3.45 ns) are associated with the
emission decays from 4FNaT and the decay times collected at
402 nm in H2O (0.86 ns) and 397 nm in CH3OH (0.87 ns)
may be correlated with the emission decays from TrNBr.
However, the long decay time in TrNBr/4FNaT collected at
402 nm in H2O (3.36 ns) and at 397 nm in CH3OH (2.52 ns)
does not occur in either TrNBr or 4FNaT, manifesting FRET
from the donors to the acceptors in the self-assembled system
to produce new chromophores. Although all these processes are
possible in the present self-assembly systems, we believe that
the increase of 4FNaT emission accompanied by the reduction
of TrNBr implies a significant contribution from intermolecular
energy transfer from TrNBr to adjacent 4FNaT molecules.
Dynamic Light Scattering (DLS) and Transmission
Electron Microscopy (TEM) Measurements on TrNBr/
Table 3. Quenching Constant (kq), Stern−Volmer Constant
a
b
(K ), Spectral Overlap (J), and Forster Distance (R ) of
̈
SV
0
TrNBr/4FNaT in H2O and CH3OH
J
system
(M−1 cm−1
)
R0 (Å) KSV (M−1
)
kq (M−1 s−1
)
TrNBr/4FNaT
(H2O)
8.5 × 1023
38.08
5.80 × 106
6.59 × 1015
TrNBr/4FNaT
(CH3OH)
1.1 × 1023
27.23
3.14 × 106
4.55 × 1015
a
b
J(λ) = ∫ ∞0 F(λ) ε(λ)λ4 dλ. R0 = 0.211(k2n−4ΦDJ(λ))1/6
.
that CMEs intend to ionize easier in H2O than in CH3OH and
thus corroborate the idea again that TrNBr and 4FNaT
experience self-assembly through electrostatic interactions (ion
pairing). From the inset figure of Figure 6b, we can see that the
quenching constant of TrNBr/4FNOHT is slightly higher than
that of TrNBr/4FNBrT, suggesting collisional quenching has
something to do with the intermolecular force.
In order to further confirm the FRET process, the TrNBr/
4FNaT complex spectral overlap (J), the donor fluorescence
yield, and the lifetime were used to calculate the Forster
̈
distance (R0, distance at resonance energy transfer is 50%) of
the TrNBr/4FNaT system as 38.08 Å in H2O and 27.23 Å in
CH3OH, respectively. Results of these calculations are listed in
Table 3. According to the calculation, the donor−acceptor
distances of TrNBr/4FNaT system fall within the Forster
̈
distance, ensuring the existence of FRET.
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4FTs. In order to gain further insights into the self-assembly
systems, dynamic light scattering (DLS) and transmission
electron microscopy (TEM) were used to investigate the
samples (Figure S13 (Supporting Information), Figure 8, and
larger than those of TrNBr or 4FNaT with the same
concentrations, which might be attributed to the self-assembled
aggregates of TrNBr and 4FNaT in both H2O and CH3OH.
On the contrary, the mean diameters of TrNBr/4FNOHT and
TrNBr/4FNBrT keep almost the same particle sizes as their
discrete solutions (TrNBr, 4FNOHT, and 4FNBrT) in both
H2O and CH3OH. All these phenomena have confirmed the
fact that the self-assembly behaviors do exist in the TrNBr/
4FNaT system due to the strong electrostatic interaction of
oppositely charged ionic groups. Moreover, TEM images
provide further clear evidence supporting this hypothesis in
the mixed systems (Figures 8 and 9). Particles are easily
identified and counted to follow similar trends. For example,
TEM analysis conducted on TrNBr/4FNaT finds larger
particles compared with that of TrNBr or 4FNaT alone in
both H2O and CH3OH. However, in both H2O and CH3OH,
the particle sizes in TrNBr/4FNOHT and TrNBr/4FNBrT are
nearly the same as those of TrNBr, 4FNOHT, and 4FNBrT
alone. Bearing these results in mind, we deduce that the self-
assembled structures occur in the TrNBr/4FNaT system,
further consolidating our previous statements.
CONCLUSIONS
■
In summary, we have designed and synthesized a series of
donor−acceptor star-shaped CMEs with various polar pendent
groups, including anionic 4FNaT, neutral 4FNOHT, and
cationic 4FNBrT, which exhibit distinct light-harvesting
properties. This set of materials has been explored as acceptors
to investigate the energy transfer process, where a star-shaped
cationic CME, TrNBr, has been adopted as an energy donor.
Among these, the cationic TrNBr as an energy donor and
anionic 4FNaT as acceptors are observed to self-assemble in
H2O and CH3OH even at very dilute concentrations (<10−6
M), where energy transfer is efficient in this self-assembled
system. The functionalization for the pendent groups of star-
shaped CMEs has proved to be effective to modulate the self-
assembly behaviors in dilute solutions and thus provide a
Figure 8. DLS analysis of 4FTs (4FNaT, 4FNBrT) and mixed systems
of TrNBr/4FNaT and TrNBr/4FNBrT in H2O. TEM images are also
shown in the insets of the corresponding DLS figures.
Figure 9). It is widely accepted that the particle diameter is one
of specific structural indicators for the self-assembly process. As
a powerful technology, DLS allows the fundamental size scales
of the self-assembled particles to be observed. The size
distributions of 4FTs (4FNaT, 4FNBrT, 4FNOHT), TrNBr,
and their mixed systems in both H2O (Figure 8, Figure S13a
(Supporting Information)) and CH3OH (Figure 9, Figure S13b
(Supporting Information)) are facilely recorded. By compar-
ison, the mean diameters of TrNBr/4FNaT are almost 2 times
Figure 9. DLS analysis of 4FTs (4FNaT, 4FNBrT, 4FNOHT) and mixed systems of TrNBr/4FNaT, TrNBr/4FNOHT, and TrNBr/4FNBrT in
CH3OH. TEM images are also shown in the insets of the corresponding DLS figures.
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strategy to further manage the optoelectronic properties. This
hypothesis is supported by the distinct decrease of the
fluorescence band of the TrNBr donor and the increase in
fluorescence of the 4FNaT acceptors. Furthermore, DLS and
TEM measurements also match well with the relevant analysis
about the self-assembly in TrNBr/4FNaT. However, there is no
obvious energy transfer in TrNBr/4FNOHT and TrNBr/
4FNBrT, as revealed by the constant emission quantum yields
of TrNBr in those investigated systems. As a result, by means of
adjusting the pendent polar functional groups, it is possible to
induce self-assembly to further promote the energy transfer
even in very dilute solutions, which provides useful information
and design guidelines for the exploration of a suitable CME
materials system for specific light-harvesting and/or chemical/
biomedical sensing applications. Further effort on this direction
is in progress.
Institutions (PAPD), the Six Talent Plan (2012XCL035), and
the Qing Lan Project of Jiangsu Province.
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ASSOCIATED CONTENT
* Supporting Information
■
S
NMR and MALDI-TOF mass spectra of 4FNT, 4FCT, 4FNaT,
4FNOHT, 4FNBrT, and TrNBr; absorption spectra of TrNBr
in H2O and CH3OH; emission spectra of 4FTs
(4FNaT,4FNBrT) in H2O and 4FTs (4FNaT, 4FNOHT,
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H2O and CH3OH along with increasing concentrations;
emission spectra of the systems ([TrNBr], [TrNBr]:[4FNaT]
= 1:1, [TrNBr]:[4FNOHT] = 1:1, and [TrNBr]:[4FNBrT] =
1:1) with their simultaneous increasing concentrations in H2O
and CH3OH; absorptioin spectra of TrNBr (5 × 10−7 M) with
increasing concentrations of 4FTs in H2O and CH3OH;
absorption and emission spectra of 4FNaT and 4FNBrT (10−5
M) in H2O with increasing concentrations of TrNBr;
absorption spectra of TrNBr, 4FTs, and TrNBr/4FTs mixed
system; DLS analysis of TrNBr in H2O and CH3OH with the
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: iamwylai@njupt.edu.cn (W.-Y.L.).
*E-mail: wei-huang@njtech.edu.cn (W.H.).
■
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript. L.Z. and C.-F.L. contributed equally.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We acknowledge financial support from the National Key Basic
Research Program of China (973 Program, 2014CB648300),
the National Natural Science Foundation of China (21422402,
20904024, 51173081, 61136003), the Natural Science
Foundation of Jiangsu Province (BM2012010, BK20140060,
BK20130037, BK20140865), the Program for New Century
Excellent Talents in University (NCET-13-0872), the Speci-
alized Research Fund for the Doctoral Program of Higher
Education (20133223110008), the Ministry of Education of
China (IRT1148), the Synergetic Innovation Center for
Organic Electronics and Information Displays, the Priority
Academic Program Development of Jiangsu Higher Education
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