Electrochemistry and electrogenerated chemiluminescence of a spirobifluorene-based donor (Triphenylamine)-acceptor (2,1,3-Benzothiadiazole) molecule and its organic nanoparticles

Article abstract of DOI:10.1021/ja2000825

A new D-A-π-A-D molecule (Spiro-BTA) containing two 2,1,3- benzothiadiazole (BTA) as the acceptor (A) and triphenylamine as the donor (D) bridged by a spirobifluorene moiety has been synthesized. The novel D-A molecule shows intense red emission (612 nm)

Full text of DOI:10.1021/ja2000825

ARTICLE
pubs.acs.org/JACS
Electrochemistry and Electrogenerated Chemiluminescence of a
Spirobifluorene-Based Donor (Triphenylamine)ꢀAcceptor
(2,1,3-Benzothiadiazole) Molecule and Its Organic Nanoparticles
Khalid M. Omer,† Sung-Yu Ku,†,‡ Jian-Zhang Cheng,‡ Shu-Hua Chou,‡ Ken-Tsung Wong,*,‡ and
Allen J. Bard*,†
†Center for Electrochemistry, Department of Chemistry and Biochemistry, and Center for Nano-and Molecular Science and
Technology, University of Texas at Austin, Austin, Texas 78712, United States
‡Department of Chemistry, National Taiwan University, 106 Taipei, Taiwan
S Supporting Information
b
ABSTRACT: A new DꢀAꢀπꢀAꢀD molecule (Spiro-BTA)
containing two 2,1,3-benzothiadiazole (BTA) as the acceptor
(A) and triphenylamine as the donor (D) bridged by a spirobi-
fluorene moiety has been synthesized. The novel DꢀA mole-
cule shows intense red emission (612 nm) with a high PL
quantum yield (ΦPL = 0.51) in a solid film. A cyclic voltammo-
gram of Spiro-BTA in 1:2 MeCN:benzene/0.1 M Bu4NPF6
shows two reversible oxidation waves and one reversible
reduction wave. The first oxidation wave and reduction wave
were assigned as two successive electron transfer peaks sepa-
rated by ∼50 mV related to the oxidation of the two noninteracting donors and the reduction of the two noninteracting acceptors,
respectively. Electrogenerated chemiluminescence (ECL) of Spiro-BTA upon cyclic oxidation and reduction in MeCN:benzene 1:2
shows a very bright and stable red emission that could be seen in a well-lit room. Using a reprecipitation method, well-dispersed
organic nanoparticles (NPs) of the Spiro-BTA were prepared in aqueous solution. The nanoparticles were analyzed by dynamic light
scattering (DLS) and scanning electron microscopy (SEM), yielding a NP size (without surfactant) of 130 ( 20 nm, while with
surfactant, 100 ( 20 nm. Bathochromic shifts of absorption spectra (∼16 ( 2 nm), as compared to that of the dissolved Spiro-BTA
in THF, were observed for both NPs in water and as a thin film. While blue shifts (14 ( 2 nm) were observed for the
photoluminescence (PL). The PL intensity of the Spiro-BTA nanoparticles was slightly enhanced (ΦPL of nanoparticles in water =
48%) over that of the dissolved Spiro-BTA in THF. The ECL of the organic Spiro-BTA nanoparticles in aqueous solution could be
observed upon oxidation with tri-n-propylamine as a coreactant.
’ INTRODUCTION
emission was observed by using surfactant to stabilize the Spiro-
BTA NPs and produce smaller particle sizes. More remarkably,
those particles can produce ECL emission after reacting with
tripropylamine as a coreactant. Such observation may lead to
important applications of using ECL of organic NPs in aqueous
media as the emissive tag for the analysis of biologically inter-
esting molecules.
The electrochemical and emission properties of organic NPs
have been the subject of relatively few studies compared to
inorganic NPs. For inorganic materials the physical and chemical
properties observed in the corresponding bulk forms are very
different when the particle size is reduced to the nanometer
regime. The size effect of the properties of organic NPs is not
expected to produce large changes in characteristics compared to
the bulk crystal, because electrons are largely confined within
We report the synthesis, spectroscopic and electrochemical
characterization, as well as the electrogenerated chemilumines-
cence (ECL) of a new highly fluorescent molecule (Spiro-BTA)
(structure shown in Scheme 1). Spiro-BTA is a C2-symmetric
donorꢀacceptor (DA) compound, consisting of spirobifluorene
as the π-bridge core connected to two 2,1,3-benzothiadiazole
(BTA) units as the acceptors (A) and end-capped with triphe-
nylamines as the donors (D), with the overall structure
DꢀAꢀπꢀAꢀD (see Scheme 1). There have been a number
of studies of the electrochemical, spectroscopic, and ECL
behavior of DA molecules.1 While this paper considers analogous
studies of this species in nonaqueous solutions, in addition, well-
dispersed organic nanoparticles (NPs) with a narrow size distribu-
tion have been prepared in aqueous solution using a reprecipita-
tion method.3 The Spiro-BTA NPs in aqueous solution show
enhanced fluorescence emission over the fluorescence in organic
solution. Particularly interesting, a strong enhancement of the
Received: January 4, 2011
Published: March 17, 2011
r
2011 American Chemical Society
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Scheme 1. Synthetic Route to Spiro-BTA
single molecules and do not delocalize or transfer over nan-
ometer and micrometer domains.2 Inorganic NPs have been
suggested as biological fluorescent labels, but they are difficult to
functionalize on a specific site and tend to be toxic and unstable
under intense radiation. In contrast, organic molecules have the
advantage of being easily functionalized with versatile synthetic
strategies and, in addition, the possibility of tailoring the electro-
nic and optical properties of organic molecules to fit a desired
application. In principle, organic NPs might have practical
applications because their fluorescence spans a wide wavelength
region. The tailor-made molecular design strategy has triggered
recent research interests in organic NPs generated from small
organic molecules.3 Because of the very small background light in
the ECL technique, the ECL of organic NP labels in aqueous
solution has potential applications in the future, for example, in
bioanalytical methods. We recently reported ECL for organic
NPs from the well-known ECL emitters 9,10-diphenylanthra-
cene (DPA) and rubrene in aqueous media.4 In addition, the
ECL from nanobelts spontaneously formed from ionic metallic
complexes with organic ligands dispersed in water has also been
observed.5 In these earlier works, control of the particle size of
organic NPs was the main challenge, since the larger the particles,
the smaller the diffusion coefficient and the weaker the ECL.
However, the synthesis and control of the size and shape of
organic nanoparticles are not as straightforward as the inorganic
counterparts. A small diffusion coefficient is one of the crucial
hurdles in electrochemical measurements of organic NPs and is
still an issue that requires further study.
over MgSO4 and concentrated in vacuum. Spiro-BTA (600 mg, 82%
yield) was purified by chromatography with silica gel (CHCl3:hexane:
toluene 1:2:1): Tg 274 °C (DSC); mp >500 °C (DSC); H NMR
1
(CDCl3, 400 MHz) δ 8.21 (d, J = 8.0, 2H), 8.09 (d, J = 8.0 Hz, 2H), 7.87
(d, J = 7.6 Hz, 2H), 7.80 (d, J = 8.4 Hz, 4H), 7.60 (d, J = 8.0 Hz, 2H), 7.55
(d, J = 7.2 Hz, 2H), 7.38 (t, J = 7.6 Hz, 4H), 7.29 (d, J = 8.0 Hz, 6H),
7.19ꢀ7.13 (m, 16H), 7.05 (t, J = 6.8 Hz, 4H), 6.92 (d, J = 7.2 Hz, 2H);
13C NMR (CDCl3, 100 Hz) δ 154.0, 153.9, 149.8, 148.5, 148.0, 147.4,
141.8, 141.4, 137.3, 132.6, 132.3, 130.8, 129.8, 129.5, 129.3, 128.1, 127.9,
127.8, 127.0, 124.8, 124.4, 124.3, 123.3, 122.8, 120.2, 120.0; MS (m/z,
FABþ) 1071.3 (50), 907.1 (100); HRMS calcd for C73H46N6S2
1071.3304, found 1071.3308.
Synthesis of Nanoparticles. The reprecipitation method was
used for nanoparticle synthesis.3 Spiro-BTA NPs were synthesized by
injecting 100 μL of Spiro-BTA in THF (5 ꢁ 10ꢀ4 M) into 10 mL of
deionized water (or to a 100 mM Triton X-100 aqueous solution when
using surfactant) under sonication at room temperature. The resulting
NP solution, after filtering through a 0.22 μm pore-sized filter (Millex
GP, PES membrane) had a pale yellow color. The particles are stable for
1 week based on DLS measurements.
Instrumentation. UVꢀvis spectra were recorded on a DU-800
spectrophotometer (Beckman Coulter, Inc., Brea, CA). Fluorescence
spectra were recorded on a Fluorolog-3 spectrofluorimeter (ISAJobin
Yvon Horiba, Edison, NJ), using a 1 cm path length quartz cuvette, and
the samples were excited at their absorption maxima, with a slit width of
50 nm. Scanning electron microscopy (SEM) was performed with a
LEO 1450VP microscope at an accelerating voltage of 20 kV and linked
with an Oxford Instruments X-ray analysis system. ζ-Potential analysis
was carried out with a Zeta Plus (Brookhaven Instruments Corp.,
Holtsville, NY). Particle size distribution and measurement utilized a
90Plus/BI-MAS (Brookhaven Instruments Corp., Holtsville, NY)
instrument.
’ EXPERIMENTAL SECTION
Synthesis of Spiro-BTA. Pd(PPh3)4 (81 mg, 0.07 mmol), 21c (507
mg, 0.68 mmol), and N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolan-2-yl)aniline (3)6 (780 mg, 2.10 mmol) were added to a 50-mL
two-neck flask with a septum. The flask was evacuated and backfilled
with argon. Toluene (10 mL), PtBu3 (1.2 mL, 0.07 mmol, 0.05 M in
toluene) and K2CO3 (2.7 mL, 1.0 M, 2.72 mmol) were added in order by
a syringe at room temperature. The reaction mixture was heated at
110 °C for 72 h. The reaction mixture was allowed to cool to room
temperature, water (30 mL) was added, and the mixture was extracted
with CH2Cl2 (40 mL ꢁ 3). The combined organic solution was dried
Electrochemical methods such as multipotential step and cyclic
voltammetry (CV) were performed with a conventional three-electrode
cell linked to a CH Instruments electrochemical workstation model 660
(Austin, TX). The electrochemical/ECL cell contained an L-type inlaid
platinum disk as a working electrode, which was polished with alumina of
different sizes between each experiment. The other electrodes were a
platinum coiled wire counter electrode and a Ag wire quasireference
electrode (for solution electrochemistry and ECL) or, in aqueous
solutions, a Ag/AgCl reference electrode (nanoparticle experiments).
Current and ECL transients were simultaneously recorded using an
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probably composed of two overlapping sequential one-electron
transfer reactions, and the second wave also involves a one-
electron transfer. From the chemical structure and the groups in
the Spiro-BTA, the first oxidation waves can be assigned to the
oxidation of the two terminal triphenylamine groups. The two
closely spaced waves clearly indicate that there is negligible
electronic communication between the two donor groups. The
reversibility of these waves, even at a scan rate of 50 mV/s,
indicates a high stability of the dication (Dþ•∼A∼F∼A∼Dþ•).
With ΔEo = 50 mV between the two reversible 1e oxidations of the
triphenylamines and the heterogeneous electron transfer rate
constant showing Nernstian behavior, the digital simulation of
the oxidation CVs over a broad range of scan rates (50ꢀ1000 mV/
s) (Figure 2) shows a good fit to the experimental CV, consistent
with the assumption of a weak electronic coupling of two terminal
donor groups.
Figure 1. Cyclic voltammogram of ∼1 mM Spiro-BTA in Bz:MeCN
(2:1): electrolyte, 0.1 M Bu4NPF6; scan rate, 100 mV/s; working
electrode, ∼1 mm Pt disk; counter electrode, platinum coil; reference
electrode, Ag wire (calibrated against Fc/Fcþ). Initial scan direction is
cathodic.
The second one-electron transfer oxidation wave is reversible,
even at a scan rate of 50 mV/s. This oxidation potential is
consistent with that of the central fluorene moiety,7 which now
has extended π-conjugation via 2,7-substitution. Spiro-BTA
shows a reduction peak at ꢀ1.45 V vs SCE (ipc = 6 μA), which
exhibits the same peak height as the first 2e oxidation wave.
Therefore, we assigned this to a Nernstian two-electron transfer
peak, originating from the sequential reductions of the two
acceptor 2,1,3-benzothiadiazole groups. As with the oxidation,
digital simulation of the reduction CVs with ΔEo = 60 mV
between the two reversible 1e waves was carried out. The
heterogeneous electron transfer rate constant employed in
simulated CVs that fit the experimental data best was ko ∼
0.01 cm/s over scan rates of 50ꢀ1000 mV/s (Figure 3). Thus,
the reduction of both benzothiadiazole groups at relatively the
same potential is also indicative of the lack of electronic com-
munication between the two benzothiadiazoles via the fluorene
linkage, but it is slower than the oxidations, perhaps because
these A-groups are sterically slightly less accessible than the
D-groups. However, the slightly larger ΔEo (Eo2,red ꢀ Eo1,red) =
60 mV might also suggest a stronger electrostatic repulsion
between the two acceptor groups because of the smaller separa-
tion distance as compared to that of the donor groups.8 Scan rate
(v) studies show that the anodic and cathodic currents of the first
oxidation and reduction peaks of Spiro-BTA were proportional
to v1/2. The peak current ratio is approximately unity down to a
scan rate of 50 mV/s, indicating the absence of a following
chemical reaction. This suggests that the oxidation to a stable
radical dication and the reduction to a stable dianion are
essentially Nernstian.
Figure 4 depicts the theoretical calculation of optimized
ground-state geometry of Spiro-BTA calculated by B3LYP/
STO-3G. The dihedral angle between spirobifluorene and
2,1,3-benzothidiazole is 17°, and a twisted conformation be-
tween triphenylamine and benzothiadizole with a dihedral angle
of 15° is observed. The optimized geometry indicates Spiro-BTA
is not coplanar, which may explain the small electronic coupling
indicated by the redox processes. This agrees with the experi-
mental and simulation results that conclude the electronic
communication between the two donor groups and between
the two acceptors via the fluorene bridge is weak.
Spectroscopic Characterization of Spiro-BTA Solutions
and NPs. Figure 5 shows the SEM image of fresh Spiro-BTA
nanoparticles dispersed in aqueous solution with an average
diameter of ∼130 nm. Dynamic light scattering (DLS) of the
Spiro-BTA NPs solution confirms the observation obtained by
Autolab electrochemical workstation (Eco Chemie) coupled with a
photomultiplier tube (PMT, Hammamatsu R4220p) held at ꢀ750 V
with a high-voltage power supply (Kepco, Flushing, NY). The photo-
current produced at the PMT was converted to a voltage signal by an
electrometer/high-resistance system (Keithley, Cleveland, OH) and fed
into the external input channel of an analog-to-digital converter (ADC)
of the Autolab. Emission spectra were obtained with a charge coupled
device (CCD) camera (Princeton Instruments, SPEC-10) that was
cooled to ꢀ100 °C. The CCD camera and monochromator wavelengths
were calibrated with a mercury lamp prior to each measurement.
To generate ECL via ion annihilation, the working electrode potential
was pulsed between the first oxidation and reduction peak potentials
with a pulse width of 0.1 s. To generate ECL with a coreactant, the
working electrode potential was pulsed between zero and the first
oxidation potential of both the coreactant and Spiro-BTA NPs. Digital
simulations of cyclic voltammograms were performed with Digisim
Software package 3.0 (Bioanalytical Systems, Inc., West Lafayette, IN),
to investigate the mechanisms of the electrochemical processes.
Chemicals. Anhydrous acetonitrile (MeCN, 99.8% in a sure-sealed
bottle) and anhydrous benzene (Bz, 99.9% in a sure-sealed bottle) were
obtained from Aldrich. Tetra-n-butylammonium hexafluorophosphate
(Bu4NPF6) was obtained from Fluka and transferred directly into a
helium atmosphere glovebox. Tripropylamine (98%) was obtained from
Aldrich.
’ RESULTS AND DISCUSSION
Synthesis of Spiro-BTA. The titled compound Spiro-BTA,
consisting of a spirobifluorene core, two benzothiadiazole moi-
eties, and two triphenlyamines, was synthesized from 4,7-
dibromobenzo[1,2,5]thiadiazole and 9,90-spirobifluorene 2,7-
diboronic ester (1) through a Suzuki coupling reaction. Scheme 1
depicts the synthetic route of Spiro-BTA from compound 1.
Selective Suzuki coupling of the diboronate 1 with an excess
amount (6 equiv) of dibromobenzothiadiazole afforded the
dibromo 21c in 30% yield. The dibromo compound 2 was
subjected to coupling with N,N-diphenyl-4-(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)aniline (3)6 in the presence of
a catalytic amount of Pd(PPh3)4 and cocatalyst PtBu3 to give
Spiro-BTA in 82% yield.
Electrochemistry. Figure 1 shows the cyclic voltammogram
(CV) of a 1 mM Spiro-BTA solution in benzene:MeCN
(v/v, 2:1)/0.1 MBu4NPF6. The resultsare summarizedinTable1.
The oxidation scan shows two waves, at Eo = 0.88 V (ipa
=
1,ox
6 μA) and Eo2,ox = 1.30 V (ipa = 3 μA) vs SCE. The peak current of
the first wave is twice that of the second, so the first peak is
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Table 1. Electrochemical (CV) and Spectroscopic Results for Spiro-BTA in Solution (1:2 MeCN:Bz)
Eo1,ox (V vs SCE)
Eo2,ox (V vs SCE)
Eo1,red (V vs SCE)
λmaxAbs (nm)
λmaxFL (nm)
λmaxECL (nm)
ΦPL
ΦPL
film
0.88
1.30
ꢀ1.45
450
622
635
0.47
0.51
Figure 2. Simulation of ∼1 mM Spiro-BTA oxidation at different scan rates. Simulation parameters involve two one-electron oxidations with DO
=
6.5 ꢁ 10ꢀ6 cm2/s, Eo1,ox = 0.86 V vs SCE, Eo2,ox = 0.91 V vs SCE, ko = 1 ꢁ 104 cm/s, R = 0.5, uncompensated resistance Ru = 2 KΩ, and double layer
capacitance Cd = 50 nF at (a) 0.05 V/s, (b) 0.1 V/s, (c) 0.25 V/s, (d) 0.5 V/s, (e) 0.75 V/s, (f) 1 V/s. Simulation (dotted line), experimental (solid line).
SEM (Supporting Information, Figure S2). The particle size was
stable even after 1 week under ambient conditions (in the dark
and under sunlight) without adding any surfactant to provide
extra stabilization. However, when prepared in the presence of
Triton X-100, the Spiro-BTA NPs size decreased to ∼100 nm,
and the distribution of particle sizes was more uniform with no
aggregation detected by DLS (Supporting Information, Figure S3).
The electrochemistry in an aqueous solution of the Spiro-BTA
NPs did not show a distinctive redox peak because of the low
concentration of the NPs and their small diffusion coefficient.
Absorption and emission spectra of the Spiro-BTA dissolved
in a solution of benzene:MeCN 2:1 are shown in Figure 6.
The absorption spectrum shows two maxima, one at shorter
wavelength (305 nm), which is attributed to the πꢀπ* transition
of the fluorene,9 while the other at longer wavelength (445 nm) is
ascribed to the πꢀπ* transition of Spiro-BTA. Spiro-BTA shows
a PL emission maximum centered at 612 nm with a quantum
yield of 0.45 in THF solution and 0.51 for a solid film measured
by a calibrated integrated sphere. The observed large Stokes shift
implies a possible conformational change upon photoexcitation.
Since Spiro-BTA is composed of donor and acceptor subunits,
emission from a photoexcited intramolecular charge transfer
(ICT) can also account for the evident Stokes shift. The
solvent-polarity-independent UVꢀvis absorption spectra of
Spiro-BTA as shown in Figure 6c indicate that there is no strong
charge transfer in the ground state. However, the PL emission
spectra of Spiro-BTA show significant red-shifts from low-polarity
to high-polarity solvents along with decreased PL intensity, e.g.,
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Figure 3. Simulation of ∼1 mM Spiro-BTA reduction at different scan rates. Simulation parameters for two one-electron reductions with DO = 6.5 ꢁ
10ꢀ6 cm2/s, Eo1,red = ꢀ1.42 V vs SCE, Eo2,red = ꢀ1.48 V vs SCE ko = 0.01 cm/s, R = 0.5, Ru = 2 KΩ, and Cd = 50 nF at scan rates of (a) 0.05 V/s,
(b) 0.1 V/s, (c) 0.25 V/s, (d) 0.5 V/s, (e) 0.75 V/s, (f) 1 V/s. Simulation (dotted line), experimental (solid line).
Figure 4. Optimized ground-state geometry calculated by B3LYP/STO-3G: (a) light blue indicates the dihedral angle between the benzothiadiazole
and spirobifluorene planes and (b) light blue indicates the dihedral angle between triphenylamine and benzothiadiazole planes.
green emission (537 nm) in cyclohexane to orange-red emission
(624 nm) in CH2Cl2 (Figure 6b,d). The strong solvent-polarity-
dependent PL indicates that that Spiro-BTA has a highly charged
excited state due to photoinduced charge transfer.
We prepared three types of samples to study the photophysical
properties of Spiro-BTA NPs. Sample A consisted of fresh Spiro-
BTA NPs, sample B was NPs with Triton X-100 in water, and
sample C was a solution of 100 μL of 1 mM Spiro-BTA in 10 mL
of THF. The absorption and emission spectra of Spiro-BTA NPs
with and without surfactant (Triton X-100) as compared to that
in solution are shown in Figure 7. The absorption maximum of
Spiro-BTA in THF solution (sample C) is located at 450 nm,
while the absorption maxima are red-shifted by 18 nm (at 468 (
2 nm) for Spiro-BTA NPs (sample A). The UVꢀvis spectra of
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Spiro-BTA NPs strongly suggest that Spiro-BTA is not molecu-
larly dissolved but rather well-dispersed as colloidal particles.
This UVꢀvis spectral shift indicates that the particles are self-
assembled as aggregates during the recrystallization process.10 In
contrast, the emission of Spiro-BTA NPs colloid solution is blue-
shifted by 10 nm as compared to that of Spiro-BTA in THF.
More interestingly, the PL intensity of Spiro-BTA NPs is
increased over the one in solution, especially in the presence of
surfactant (Figure 7b). The blue-shifted emission is due to the
restricted conformational relaxation in the NPs, which also leads
to the enhancement of emission intensity as reported for
aggregation-induced emission enhancement (AIEE).11 Similar
behaviors, such as red-shifted absorption and blue-shifted emis-
sion, were observed in a thin solid film of Spiro-BTA (Figure S4,
Supporting Information), which was prepared by spin-coating of
a 1 ꢁ 10ꢀ3 M solution (THF) at 1000 rpm.
Electrogenerated Chemiluminescence (ECL) in Solution.
Annihilation ECL was obtained by alternating the potentials of
ox
the working electrode from the first oxidation potential (Ep
=
red
þ50 mV) to the first reduction potential (Ep = ꢀ50 mV).
Spiro-BTA dissolved in organic solution (Bz:MeCN) produced
strong ECL emission that was easily seen with the naked eye in a
well-lit room. The ECL spectrum of Spiro-BTA, shown in
Figure 8, is characterized by an ECL maximum centered at
635 nm, red-shifted by 13 nm from the fluorescence maximum.
Such a red-shift in ECL emission is often encountered in ECL
spectra and is attributed to an inner filter effect because of the
different concentrations used for PL (μM) and ECL (mM). An
additional small factor in the red-shifted ECL peak is that
different instruments were used for acquiring the PL and ECL.
The enthalpy of radical ion annihilation (2.23 eV) is calculated
as12 ΔHann = Eo ꢀ Eo ꢀ 0.1, where Eo is the anodic formal
a
c
a
potential, Eo is the cathodic formal potential, and ΔHann is
c
greater than the energy required to produce the excited state
Es = 1239 (λmax nm) (∼2 eV). This result indicates that the ECL
emission is directly formed via an annihilation reaction to give the
singlet excited state, which is also called an energy-sufficient or
S-route. Transient ECL (i-t-ECL) depicted in Figure 9 shows
that equal ECL intensity is seen upon each anodic to cathodic
pulse cycle, indicating that the same amount of cation/dication
and anion/dianion are formed upon each pulse. In addition, all
radicals and ions are stable within the time scale of the ECL
experiment.
ECL of Organic Nanoparticles. Although the electrochem-
istry of the NPs could not be observed, they did produce ECL
upon electrochemical excitation. Because of the limited potential
window at negative potentials in aqueous solution, we were
limited to seek ECL by oxidation of the NPs. For this reason, we
Figure 5. SEM image of organic NPs of Spiro-BTA prepared in water.
Scale marker is 300 nm.
Figure 6. (a) Absorbance (solid line) and fluorescence spectra (dotted line) of Spiro-BTA in a solution of benzene:MeCN 2:1. (b) Emission color of
Spiro-BTA in different solvents. (c) Absorption and (d) photoluminescence spectra of 9 ꢁ 10ꢀ5 M Spiro-BTA in different solvents.
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Figure 7. (a) Absorption and (b) emission spectra: (A, black) fresh Spiro-BTA NPs, (B, red) NPs with Triton X-100 in water, and (C, green) 100 μL of
1 mM Spiro-BTA in 10 mL of THF.
Figure 9. i-t-ECL of ∼1 mM Spiro-BTA, pulsing was from Epa = þ70
mV to Epc = ꢀ70 mV (black line is current, red line is ECL intensity).
Figure 8. ECL and fluorescence spectra of Spiro-BTA in Bz:MeCN 2:1
solution: ECL integration time, 1 min; slit width, 0.5 mm; pulsing range,
E
pa = þ70 mV to Epc = ꢀ70 mV.
used a coreactant that produces a strong reducing agent, tri-n-
propylamine (TPrAH), a well-known coreactant that oxidizes
electrochemically at 0.9 V vs SCE, producing, upon loss of a
proton, a strong reducing agent (ꢀ1.7 V versus SCE).13 The
ECL from well-dispersed Spiro-BTA NPs in the absence of
surfactants in aqueous medium is shown in Figure 10 when the
electrogenerated oxidized NPs (NPþ•) on the surface of the
particles reacted with the strong reducing agent (TPrA•)
NP ꢀ e f NPþ•
TPrAH ꢀ e f TPrAHþ•
Figure 10. i-t-ECL Spiro-BTA NPs in water with 0.1 M TPrA as a
coreactant, pulsing from 0.0 V to þ1.2 V vs Ag/AgCl.
TPrAHþ þ NP f NPþ• þ TPrAH
•
TPrAHþ f TPrA• þ Hþꢀ
in the aqueous solution. NPs could adsorb on the surface of the
electrode, so electrode fouling is one of the reasons for the weak
ECL light.
NPþ• þ TPrA• f TPrAþ• þ NP
ꢂ
ꢂ
NP f NP þ hν
’ CONCLUSIONS
There is no evidence of electrode fouling, and the increasing
ECL signal on cycling perhaps suggests that adsorption with
formation of an emitting film is a possibility. However, the ECL
signal was not strong, and although it could be recorded with a
photomultiplier tube, it was not sufficiently stable to obtain an
ECL spectrum. The relatively weak ECL of Spiro-BTA NPs can
be attributed to the small diffusion coefficient of the particles to
the electrode surface as well as the instability of the cationic states
A new red-emitter (Spiro-BTA) based on a DꢀAꢀπꢀAꢀD
structure, where D is triphenylamine, A is 2,1,3-benzothiadiazole,
and π is spirofluorene, has been synthesized and characterized.
The dianion and dication of Spiro-BTA is highly stable upon
electrochemical reduction and oxidation, respectively. The ab-
sence of D/A charge transfer in the ground state was verified by
the solvent-polarity-independent UVꢀvis absorption spectra of
Spiro-BTA, whereas the strongly red-shifted PL in polar solvents
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Journal of the American Chemical Society
ARTICLE
confirms the charge separation in the excited state. Well-dis-
persed Spiro-BTA nanoparticles were prepared in aqueous
solution using a reprecipitation method. Spiro-BTA has a high
quantum yield in solution and even higher as nanoparticles, and
as the particles size becomes smaller in the presence of surfactant,
the quantum yield becomes higher. ECL of Spiro-BTA in organic
solution is intense and can be seen with the naked eye in ambient
light. The ECL of Spiro-BTA NPs can be observed with the aid of
TPrA as a coreactant by the oxidation of both coreactant and
nanoparticles simultaneously. As with inorganic NPs, organic
nanoparticle ECL as emissive tags for bioanalytical applications
in aqueous solution would be of interest, if the intensities could
be improved.
(9) Kalsi, P. S. Spectroscopy of Organic Compounds, 6th ed.; New Age
International Ltd.: New Delhi, 2004; pp 13.
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Nanotechnol. 2009, 14, 89. (b) Kitahama, Y.; Takazawa, K. Bull. Chem.
Soc. Jpn. 2008, 81, 1282.
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Soc. 2000, 122, 8565. (b) Zeng, Q.; Li, Z.; Dong, Y.; Di, C.; Qin, A.;
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’ ASSOCIATED CONTENT
S
Supporting Information. Detailed synthetic procedure
b
and spectroscopic characterization of Spiro-BTA, DLS measure-
ments for Spiro-BTA NPs, absorption and photoluminescence
spectra of Spiro-BTA thin film, and a Lippert plot. This material
is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Authors
ajbard@mail.utexas.edu; kenwong@ntu.edu.tw
’ ACKNOWLEDGMENT
This work was financially supported by the National Science
Foundation (CHE-0808927), the Robert A. Welch Foundation
(F-0021), Roche Diagnostics, Inc., and the National Science
Council, Taiwan.
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