Green electrogenerated chemiluminescence of highly fluorescent benzothiadiazole and fluorene derivatives

Article abstract of DOI:10.1021/ja904135y

A group of highly fluorescent 2,1,3-benzothiadiazole derivatives (BH0-BH3), including two fluorene derivatives (AB2 and C01) were synthesized and characterized. The electrochemical, spectroscopic, and electrogenerated chemiluminescence (ECL) properties of

Full text of DOI:10.1021/ja904135y

Published on Web 07/06/2009
Green Electrogenerated Chemiluminescence of Highly
Fluorescent Benzothiadiazole and Fluorene Derivatives
Khalid M. Omer,† Sung-Yu Ku,‡ Ken-Tsung Wong,*,‡ and Allen J. Bard*,†
Center for Electrochemistry and Department of Chemistry and Biochemistry, The UniVersity of
Texas at Austin, Austin, Texas 78712, and Department of Chemistry, National Taiwan
UniVersity, 106 Taipei, Taiwan
Received May 21, 2009; E-mail: ajbard@mail.utexas.edu; kenwong@ntu.edu.tw
Abstract: A group of highly fluorescent 2,1,3-benzothiadiazole derivatives (BH0-BH3), including two
fluorene derivatives (AB2 and C01) were synthesized and characterized. The electrochemical, spectroscopic,
and electrogenerated chemiluminescence (ECL) properties of the compounds were determined. Ben-
zothiadiazole derivatives BH1, BH2, and BH3 show reversible oxidation and reduction waves and produce
strong green ECL in nonaqueous solutions. This ECL could be seen by the naked eye, even in a well lit
room. The fluorene derivatives, C01 and AB2, also produce bright, easily observable ECL. Since the ECL
spectra are at essentially the same wavelengths as the photoluminescence (PL) spectra, and the energies
of the electron transfer reactions are greater than the singlet state energies, we propose direct formation
of the excited singlet state during ion annihilation. BH0, which shows a quasi-reversible oxidation wave,
only produced weak ECL via direct annihilation but gave strong ECL with benzoyl peroxide (BPO) as a
coreactant. The ECL quantum efficiencies of the series, compared to that of 9,10-diphenylanthrace, was
estimated to range from 0.05 to 7%. This series shows rare green photoluminescence (λPL ) 490-556
nm) with a high PL quantum efficiency in solution (Φ ) 5 to 90%).
Fluorene-based materials, such as terfluorenes,3 oligofluo-
Introduction
rene,4 and polyfluorenes,5 have emerged as promising candidates
for OLEDs due to their high photoluminescence (PL) and
electroluminescence (EL) efficiencies, good thermal stability,
and color tunability across the full visible range. Tuning of the
emission can be achieved by adding more units of conjugation
or modulation of the donor and acceptor strength in bipolar
compounds.6 Generally, fluorene derivatives emit blue light due
to their large energy gap. By introducing a unit with a narrower
energy gap, like the electron-deficient 2,1,3-benzothiadiazole
group into the fluorene backbone, the emission color can be
In this work, we report the synthesis, electrochemical, and
photophysical characterization, as well as the electrogenerated
chemiluminescence (ECL) of a series of novel, low-molecular
weight, highly fluorescent 2,1,3-benzothiadiazole derivatives
(Scheme 1). ECL is a unique type of luminescence in which
the electron transfer between electrogenerated ion radicals
produces an electronically excited product in the vicinity of the
electrode, with the emission of light.1
As shown here and in previous publications, ECL and
electrochemistry are versatile and sensitive tools that can be
used to obtain valuable information about radical ion formation
and stability and about often subtle interactions within mol-
ecules. Such information is of use in the design of organic light
emitting devices (OLEDs), where charge creation, migration,
and recombination are determined by analogous charged states
in the active organic layer.2 Another use of ECL involves
development of new ECL emitters for labels at different
wavelengths for bioanalytical applications, i.e., finding ECL
emitters of wavelengths different from the currently used
species.
(3) (a) Wong, K.-T.; Chien, Y.-Y.; Chen, R.-T.; Wang, C.-F.; Lin, Y.-T.;
Chiang, H.-H.; Hsieh, P.-Y.; Wu, C.-C.; Chou, C. H.; Su, Y. O.; Lee,
G.-H.; Peng, S.-M. J. Am. Chem. Soc. 2002, 124, 11576. (b) Wu, F.-
I.; Dodda, R.; Reddy, D. S.; Shu, C.-F. J. Mater. Chem. 2002, 12,
2893. (c) Tsolakis, P. K.; Kallitsis, J. K. Chem.sEur. J. 2003, 9, 936.
(d) Wu, C.-C.; Lin, Y.-T.; Wong, K.-T.; Chen, R.-T.; Chien, Y.-Y.
AdV. Mater. 2004, 16, 61. (e) Chochos, C. L.; Kallitsis, J. K.;
Gregoriou, V. G. J. Phys. Chem. B 2005, 109, 8755. (f) Chochos,
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V. G. J. Phys. Chem. B 2006, 110, 4657. (g) Chen, A. C.-A.; Wallace,
J. U.; Klubek, K. P.; Madaras, M. B.; Tang, C. W.; Chen, S. H. Chem.
Mater. 2007, 19, 4043.
† The University of Texas at Austin.
‡ National Taiwan University.
(4) (a) Lee, S. H.; Tsutsui, T. Thin Solid Films 2000, 363, 76. (b) Geng,
Y.; Katsis, D.; Culligan, S. W.; Ou, J. J.; Chen, S. H.; Rothberg, L.
J. Chem. Mater. 2002, 14, 463. (c) Li, Y.; Ding, J.; Day, M.; Tao, Y.;
Lu, J.; D’iorio, M. Chem. Mater. 2003, 15, 4936. (d) Culligan, S. W.;
Geng, Y.; Chen, S. H.; Klubek, K.; Vaeth, K. M.; Tang, C. W. AdV.
Mater. 2003, 15, 1176. (g) Kanibolotsky, A. L.; Berridge, R.; Skabara,
P. J.; Perepichka, I. F.; Bradley, D. D. C.; Koeberg, M. J. Am. Chem.
Soc. 2004, 126, 13695. (h) Liu, Q.-D.; Lu, J.; Ding, J.; Day, M.; Tao,
Ye.; Barrios, P.; Stupak, J.; Chan, K.; Li, J.; Chi, Y. AdV. Funct. Mater.
2007, 17, 1028. (i) Kong, Q.; Zhu, D.; Quan, Y.; Chen, Q.; Ding, J.;
Lu, J.; Tao, Y. Chem. Mater. 2007, 19, 3309.
(1) For review on ECL, see: (a) Electrogenerated Chemiluminescence;
Bard, A. J.; Ed.; Marcel Dekker, Inc.: New York, 2004. (b) Miao, W.
Chem. ReV. 2008, 108, 2506. (c) Richter, M. M. Chem. ReV. 2004,
104, 3003. (d) Knight, A. W.; Greenway, G. M. Analyst 1994, 119,
879. (e) Bard, A. J.; Debad, J. D.; Leland, J. K.; Sigal, G. B.; Wilbur,
J. L.; Wohlstadter, J. N. In Encyclopedia of Analytical Chemistry:
Applications, Theory and Instrumentation; Meyers, R. A., Ed.; John
Wiley & Sons: New York, 2000; Vol. 11, p 9842.
(2) Yasuda, T.; Imase, T.; Yamamaoto, T. Macromolecules 2005, 38,
7378.
9
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J. AM. CHEM. SOC. 2009, 131, 10733–10741 10733

A R T I C L E S
Omer et al.
Scheme 1
Scheme 2. Synthetic Route for BH0
We previously reported the electrochemical behavior and ECL
of ter-9,9-diarylflourenes,8 spirobifluorene-bridged bipolar sys-
tems with stable radical ions,9 star-shaped truxene core-
oligofluorene,10 and N-phenylcarbazole bridged dispirobifluo-
rene.11
ECL involves electron transfer between electrochemically
generated species, often radical ions, which results in an excited
species that emits light.1 The simplest ECL process is the radical
ion annihilation reaction, which can be represented as follows
tuned to the green region.7 Therefore, the ability to tune the
luminescence of these systems also makes them of interest for
developing new, multicolor, light-emitting materials which are
needed for full color displays and also to achieve multiple
wavelength ECL labels.
D + e- f D•- (formation of radical anion)
(1)
(2)
D - e- f D•+ (formation of radical cation)
1
D•- + D•+ f D* + D (excited state formation) (3)
(5) (a) Chang, S.-C.; Li, Y.; Yang, Y. J. Phys. Chem. B 2000, 104, 11650.
(b) Lupton, J. M.; Craig, M. R.; Meijer, E. W. Appl. Phys. Lett. 2002,
80, 4489. (c) Wong, K.-T.; Chien, Y.-Y.; Chen, R.-T.; Wang, C.-F.;
Lin, Y.-T.; Chiang, H.-H.; Hsieh, P.-Y.; Wu, C.-C.; Chou, C. H.; Su,
Y. O.; Lee, G.-H.; Peng, S.-M. J. Am. Chem. Soc. 2002, 124, 11576.
(d) Ego, C.; Grimsdale, A. C.; Uckert, F.; Yu, G.; Srdanov, G.; Mullen,
K. AdV. Mater. 2002, 14, 809. (e) Culligan, S. W.; Geng, Y.; Chen,
S. H.; Klubek, K.; Vaeth, K. M.; Tang, C. W. AdV. Mater. 2003, 15,
1176. (f) Wu, W.; Inbasekaran, M.; Hudack, M.; Welsh, D.; Yu, W.;
Cheng, Y.; Wang, C.; Kram, S.; Tacey, M.; Bernius, M.; Fletcher,
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Mater. 2004, 16, 708. (h) Wong, K.-T.; Liao, Y.-L.; Lin, Y.-T.; Su,
H.-C.; Wu, C.-C. Org. Lett. 2005, 7, 5131. (i) Chen, A. C.-A.; Wallace,
J. U.; Wei, S. K.-H.; Zeng, L.; Chen, S. H. Chem. Mater. 2006, 18,
204. (j) Fan, S.; Sun, M.; Wang, J.; Yang, W.; Cao, Y. Appl. Phys.
Lett. 2007, 91, 213502.
1D* f D + hν (ECL emission)
(4)
The energy available for excited state generation is ap-
proximately equal to the enthalpy of annihilation: -∆Ho )
-∆Go - T∆So. If -∆Ho is greater than the energy of an excited
state, Es (singlet) or ET (triplet), a molecule can achieve that
state upon radical ion annihilation. If the singlet state is directly
populated, then the ECL mechanism is referred to as S-route
ECL. If the triplet state is populated, triplet-triplet annihilation
can occur to create a singlet state in what is known as T-route
ECL.12,1a
(6) (a) Joshi, H. S.; Jamshidi, R.; Tor, Y. Angew. Chem., Int. Ed. 1999,
38, 2721. (b) Yoshida, Y.; Tanigaki, N.; Yase, K.; Hotta, S. AdV.
Mater. 2000, 12, 1587. (c) Tsuzuki, T.; Shirasawa, N.; Suzuki, T.;
Tokito, S. AdV. Mater. 2003, 15, 1455. (d) Montes, V. A.; Li, G.;
Pohl, R.; Shinar, J.; Anzenbacher, P. AdV. Mater. 2004, 16, 2001. (e)
Gana, J.-A.; Song, Q. L.; Hou, X- Y.; Chen, K.; Tian, H. J. Photochem.
Photobiol. A 2004, 162, 399.
(8) Choi, J. P.; Wong, K. T.; Chen, Y. M.; Yu, J. K.; Chou, P. T.; Bard,
A. J. J. Phys. Chem. B 2003, 107, 14407.
(9) Fungo, F.; Wong, K.-T.; Ku, S.-Y.; Hung, Y.-Y.; Bard, A. J. J. Phys.
Chem. B 2005, 109, 3984.
(10) Omer, K. M.; Kanibolotsky, A. L.; Skabara, P. J.; Perepichka, I. F.;
Bard, A. J. J. Phys. Chem. B 2007, 111, 6612.
(7) (a) Millard, I. S. Synth. Met. 2000, 111-112, 119. (b) Muller, C. D.;
Falcou, A.; Reckefuss, N.; Rojahn, M.; Wiederhirn, V.; Rudati, P.;
Frohne, H.; Nuyken, O.; Becker, H.; Meerholz, K. Nature 2003, 421,
829. (c) Justin Thomas, K. R.; Lin, J. T.; Velusamy, M.; Tao, Y.-T.;
Chuen, C.-H. AdV. Funct. Mater. 2004, 14, 83. (d) Lin, J.; Dong, J.;
Zhou, Q.; Geng, Y.; Ma, D.; Wang, L.; Jing, X.; Wang, F. J. Mater.
Chem. 2007, 17, 2832.
(11) Rashidnadimi, S.; Hung, T.-H.; Wong, K.-T.; Bard, A. J. J. Am. Chem.
Soc. 2008, 130, 634.
(12) (a) Sartin, M. M.; Zhang, H.; Zhang, J.; Zhang, P.; Tian, W.; Wang,
Y.; Bard, A. J. J.Phys. Chem. C 2007, 111, 16350. (b) Lai, R. Y.;
Fleming, J. J.; Merner, B. L.; Vermeij, R. J.; Bodwell, G. J.; Bard,
A. J. J. Phys. Chem A 2004, 108, 376.
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Benzothiadiazole and Fluorene Derivatives
A R T I C L E S
Scheme 3. Synthetic Routes for BH1, BH2, and BH3
Scheme 4. Synthetic Route of AB2
If either the cation radical or anion radical is unstable (e.g.,
if the radical cation or anion is unstable or cannot be generated
prior to the oxidation or reduction of solvent or supporting
electrolyte), then ECL can often be generated by use of a
coreactant, which can form either a strong reducing agent when
oxidized (as oxalate or tripropylamine) or a strong oxidizing
agent when reduced (as persulfate or benzoyl peroxide). For
example, the oxidation wave of BH0 is not chemically reversible
at scan rates of 100 mV/s (i.e., the cation radical is not stable),
so benzoyl peroxide (BPO) was used as a coreactant for ECL
studies of this species. BPO forms a strong oxidizing agent (Eoxd
) +1.5 V vs SCE)13,1a and leads to the singlet excited state of
BH0 (D*) as follows
BPO + e- f BPO•-
(5)
BPO•- f BPO- + BPO•
(6)
(7)
BPO• + D•- f BPO- + D*
Here we describe the synthesis, electrochemistry, spectros-
copy, and ECL of a series of benzothiadiazole and fluorene
derivatives. The electrochemical reduction, oxidation, and radical
ion annihilation ECL of the benzothiadiazole derivatives
(BH0-BH3) and fluorene derivatives (AB2 and C01) are
described below. Photophysical measurements were carried out,
and a comparison of their ECL spectra and PL is discussed.
Synthesis of Benzothiadiazole Derivatives. Details of the
synthetic procedures and product characterizations are given in
the Supporting Information. Scheme 2 shows the synthesis of
BH0 by Suzuki coupling of dibromobenzothiadiazole with
4-tert-butylphenylboronic ester in the presence of a catalytic
amount of Pd(PPh3)4 and PtBu3, leading to BH0 in an isolated
yield of 80% after purification through recrystallization.
Figure 1. Cyclic voltammograms for the compounds, supporting electrolyte
0.1 M Bu4NPF6:1 mM (BH0, BH1, BH2, BH3) in MeCN, 1 mM (C01,
AB2) in 1:1 MeCN:Bz. Scan rate: 100 mV/s, WE:Pt disk electrode (∼1
mm diameter), CE:Pt coiled electrode.
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A R T I C L E S
Omer et al.
Table 1. Electrochemical Data
c
d
oxidation waves, V vs SCE
reduction waves, V vs SCE
D ( × 10-6
cm2/s
)
Egb
EHOMO
E(LUMO)
Cpda
E1/2
E1/2
E1/2
E1/2
E1/2
(eV)
(eV)
(eV)
oxd1
oxd2
red1
red2
red3
BH0
BH1
BH2
BH3
C01
1.64(Ep)
1.37
1.42
1.33
1.46
1.83(Ep)
1.49
1.54
1.45
1.61
-1.40
-1.40
-1.38
-1.42
-1.39
-1.44
-2.18
-2.18
-2.18
-2.21
-1.93
-2.05
-
-
-
-
-
9.0
9.0
9.0
9.0
7.5
7.0
3.04
2.77
2.80
2.75
2.85
2.87
-6.02
-5.75
-5.79
-5.71
-5.84
-5.81
-2.93
-2.93
-2.92
-2.92
-2.89
-2.88
AB2
1.43
1.56
-2.20
a Concentrations, 1 mM. b From the CV. c The HOMO values are calculated based on the value of -4.8 eV for ferrocene with respect to the
vacuum.17,9 d From the first reduction wave.
Autolab electrochemical workstation (Eco Chemie, The Nether-
lands) coupled with a photomultiplier tube (PMT, Hammamatsu
R4220p, Japan) held at -750 V with a high-voltage power supply
(Kepco, Flushing, NY). The photocurrent produced at the PMT
was converted to a voltage signal with 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. The ECL spectra were taken using a calibrated charge
coupled device (CCD) camera (Princeton Instruments, SPEC-32).
Absorption spectra were recorded with a Milton Roy Spectronic
3000 array spectrophotometer. Fluorescence spectra were acquired
on a QuantaMaster spectrofluorimeter (Photon Technology Inter-
national, Birmingham, NJ). DigiSim 3.03 (Bioanalytical Systems,
Inc., West Lafayette, IN) was used to simulate the cyclic
voltammograms.
Electrochemistry. Oxidation and reduction potentials provide
information concerning the relative energetic positions of the
frontier orbitals. Cyclic voltammetry was used to probe the
electrochemistry of the compounds, assess the stability of the radical
ions in the solution, and find the energy of the radical ion
annihilation in ECL. CVs of these compounds are shown in Figure
1, and the electrochemical results are summarized in Table 1.
Benzothiadiazole Derivatives (BH0, BH1, BH2, and BH3).
Oxidation. These four compounds differ only in the substituents
on each phenyl and generally show similar behavior. The electro-
chemical results are summarized in Table 1. Upon scanning to
positive potentials, all show two consecutive, closely spaced, one-
electron transfer peaks. These peaks correspond to the formation
of the radical cation and dication since the consecutive peaks have
similar peak currents, ∼5 µA. The first oxidation waves for BH1,
BH2, and BH3 were chemically reversible at scan rates as low as
50 mV/s, showing that the radical cations were stable in the time
scale of the experiment. The peak separation for reversible oxidation
peaks was ∼70 mV, slightly larger than the expected value for
nernstian behavior for a one-electron transfer reversible peak, about
59 mV. However, the internal reference, ferrocene, which is known
to produce a nernstian one-electron wave, gave the same peak
potential separation under the same conditions, which is usual for
aprotic media where the ohmic drop can be high (∼1 kΩ) without
positive feedback. Scan rate studies showed that the anodic and
cathodic peak currents (ipa, ipc) of the first oxidation wave were
proportional to the square root of the scan rate (ν1/2). Additionally,
the peak current ratio (ipa/ipc) was approximately unity down to a
scan rate of 50 mV/s, indicating the absence of a following chemical
reaction. This suggests that the first oxidation to the stable cation
radical is near nernstian for BH1-3.
Scheme 3 depicts the synthetic routes for BH1, BH2, and
BH3.14 A Suzuki coupling reaction of 4,7-dibromo-2,1,3-
benzothiadiazole with 4-methoxyphenylboronic acid in the
presence of a catalytic amount of Pd(PPh3)4 and PtBu3 gave
BH3 in an isolated yield of 90% after purification through
recrystallization. Subsequent selective monodeprotection or
dideprotection of BH3 afforded B1 and B2 by means of excess
amounts of HBr (33% in HOAc). Then, treatment of B1 and
B2 with excess amounts of propagyl bromide, K2CO3, and a
catalytic amount of KI (10 mol %) in acetonitrile at 60 °C for
5 h led to the desired compound BH1 and BH2 in a good yield
of 80% and 99%, respectively.
Synthesis of AB2. AB2, which consisted of a spirobifluorene
core, benzothiadiazole moieties, and tert-butylbenzene, was
synthesized from 4,7-dibromo-2,1,3-benzothiadiazole and 9,9′-
spirobifluorene 2,7-diboronic ester (1) through a Suzuki coupling
reaction. Scheme 4 depicts the synthetic route of AB2 from
compound 1. Selective Suzuki coupling of the diboronate 1 with
a large amount (6 equiv) of dibromo-benzothiadiazole afforded
the dibromo 2 in 30% yields. Then Suzuki coupling of the
dibromo 2 with 4-tert-butylphenylboronic ester afforded AB2
in 45% yield.
C01 was previously used as an efficient green emitter in an
organic light-emitting diode.15
Experimental Section
Materials. Anhydrous acetonitrile (MeCN, 99.93%, in a sure-
sealed bottle) and anhydrous benzene (Bz, 99.8% in a sure-sealed
bottle)wereobtainedfromAldrich.Tetra-n-butylammoniumhexafluo-
rophosphate (n-Bu4NPF6) was used as received from Fluka and
benzoyl peroxide (BPO) (reagent grade, 97%) from Aldrich. All
solutions were prepared in a glovebox (Vacuum Atmospheres Corp.)
and placed in airtight vessels for electrochemical and ECL
measurements outside of the glovebox.
Characterization. The electrochemical cell consisted of a
working electrode with a ∼1 mm diameter inlaid platinum disk, a
Ag wire quasireference electrode (referenced after each series of
experiments against the ferrocene/ferrocenium couple to obtain the
potentials vs SCE), and a Pt wire counter electrode. For ECL,
the working electrode was a 2 mm inlaid J-type (bent to face the
detector) platinum disk. The working electrode was polished with
1 µm alumina (Buehler, Ltd., IL), then 0.3 µm, and 0.05 µm,
followed by sonication in DI water and ethanol for 5 min each.
Cyclic voltammograms (CVs) were recorded on a model 660
electrochemical workstation (CH Instruments, Austin, TX). Faradaic
current and ECL transients were simultaneously recorded using an
The second oxidation wave is only slightly chemically reversible.
Taking BH3 as an example, we could simulate the observed
behavior by an EC mechanism, with a homogeneous forward rate
constant, kf ) 10 s-1, Keq ) 0.6, and a heterogeneous rate constant,
kop, of about 5 × 10-3 cm2/s. Figure 2 and Figure 3 show the
simulation of both oxidation waves and the reduction side of BH3
at several scan rates.
The oxidized forms of BH0 are less stable since they show two
sequential chemically quasireversible peaks at 1.64 and 1.83 V.
As shown in Table 1, the trend in the potentials for oxidation of
(13) (a) Chandross, E.; Sonntag, F. J. Am. Chem. Soc. 1966, 88, 1089. (b)
Santa Cruz, T. D.; Akins, D. L.; Brike, R. L. J. Am. Chem. Soc. 1976,
98, 1677.
(14) Ku, S.-Y.; Wong, K.-T.; Bard, A. J. J. Am. Chem. Soc. 2008, 130,
2392.
(15) Ku, S.-Y.; Chi, L.-C.; Hung, W.-Y.; Yang, S.-W.; Tsai, T.-C.; Wong,
K.-T.; Chen, Y.-H.; Wu, C.-I. J. Mater. Chem. 2009, 19, 773.
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Benzothiadiazole and Fluorene Derivatives
A R T I C L E S
Figure 2. Simulation of oxidation of 1 mM BH3. The simulation is corrected for resistance and capacitance: experimental (solid line), simulation (dotted
line). The first wave is completely reversible with one-electron transfer (ko ) 1 × 104 cm/s), the second wave one-electron transfer of EC mechanism. Ru
) 1.2 kΩ, Cd ) 10 nF, R ) 0.5, D ) 9 × 10-6 cm2/s.
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J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009 10737

A R T I C L E S
Omer et al.
Figure 3. Simulation of reduction of 1 mM BH3. The simulation is corrected for resistance and capacitance: experimental (solid line), simulation (dotted
line). The reduction is a one-electron transfer process (k ) 1 × 104 cm/s). Ru ) 1.2 kΩ, Cd ) 10 nF, R ) 0.5, D ) 9 × 10-6 cm2/s.
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10738 J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009

Benzothiadiazole and Fluorene Derivatives
A R T I C L E S
Table 2. Spectroscopy Results
λ
max(abs) (nm),
[log ε]
Es (eV)a 0-0
transition
Stoke’s shifts
∆λst (nm)
b
λ
max(em) (nm)
ΦPL %
BH0 388,[4.33]
BH1 404,[4.43]
BH2 400,[4.40]
BH3 406,[4.44]
512
546
542
556
544
535
2.78
2.67
2.71
2.60
2.60
2.63
5.0
5.5
5.2
7.0
90
75
124
142
142
150
132
118
C01
412,[4.61]
AB2 417,[4.65]
a All spectra recorded in the described solvent at room temperature.
Calculated from the wavelength where excitation and emission spectra
cross. b Quantum yield calculated relative to the fluorescein in solution.
Figure 4. Absorption (dotted lines) and emission (solid lines) spectra. (a)
BH0-BH3 in MeCN. (b) C01 and AB2 in 1:1 Bz:MeCN. Emission spectra
were excited at the absorption maxima.
Table 3. ECL Data
c
d
compda λmaxECL/nm λemFL/nm Es (ev)b 0-0 transition ∆Hann ΦPL
%
ΦECL %
the first wave is BH3 < BH1 < BH2 < BH0. BH3 with two
methoxy groups, which are strong electron donors, is easiest to
oxidize followed by BH1 with one methoxy group. The absence
of stabilizing or bulky substituents in BH0 makes the oxidation
more difficult with lesser stability for the radical cation. In fact,
because of instability of the cation radical, the wave is shifted to
BH0
BH1
BH2
BH3
518
560
556
555
512
546
542
556
2.78
2.67
2.71
2.60
3.00
2.80
2.83
2.75
2.0
0.05
4.5
4.2
5.0
1.40
1.35
2.35
C01
AB2
548
536
544
535
2.60
2.63
2.87 75
2.91 65
2.7
7.50
less positive potentials with respect to the thermodynamic E1/2
,
which is thus somewhat more positive than the value shown (Table
1).
a All concentrations ∼1 mM. b Es ) hc (ν˜abs + ν˜PL)/2. c -∆H )
-(Epa - Epc) - 0.1. d Quantum efficiency calculated relative to DPA,
assuming DPA is 8%.18
BH0 oxidation is distinguishable from that of the other BHs.
The oxidation waves are less chemically reversible at V ) 50 mV/
s. Equal anodic peak currents indicate the same number of electrons
participate in each oxidation step. However, the first oxidation,
which was irreversible at a scan rate V ) 50 mV/s, became
reversible at V > 500 mV/s, indicating that a homogeneous chemical
reaction follows the heterogeneous electron transfer. Simulation of
the oxidation wave of BH0 is shown (Figure S3, Supporting
Information). The best fit was an EC mechanism, i.e., a chemical
reaction following an electrochemical electron transfer with a
heterogeneous rate constant ko ) 0.025 cm/s and a homogeneous
forward rate constant (kf ) 3.6 s-1, keq ) 0.5).
Reduction. In scanning negatively, all BHs show two reversible
reduction waves, as tabulated in Table 1 and shown in Figure 1. A
CV of 2,1,3-benzothiadiazole, the central group, taken under the
same conditions (Figure S2, Supporting Information), showed a
single reversible reduction peak at -1.47 V vs SCE. 2,1,3-
Benzothiadiazole is a well-known heteroatomic compound with a
high electron-accepting part because it has two electron-withdrawing
imine groups (CdN).16 Thus, this peak for the BH0 to BH3 can
be attributed to the benzothiadiazole group. The fact that all four
compounds show essentially the same potential for reduction
demonstrates that there is little effect from the electron-donating
substituents.
In addition to the benzothiadiazole peak, all BHs show another
one-electron reduction peak, which probably corresponds to a
second electrochemical reduction of the same benzothiadiazole
group for all BHs at around -2.18 V vs SCE. The absence of this
peak in unsubstituted benzothiadiazole and the presence in the BH
series might be due to slightly better electron delocalization in the
phenyl groups.
Fluorene Derivatives (C01 and AB2). AB2 involves the coupling
of two benzothiadiazole groups through a spirobifluorene, while
CO1 involves two spirobifluorenes coupled through a benzothia-
diazole group.
C01 for the first oxidation peaks (1.43 and 1.56 V vs SCE). The
small difference is attributed to the greater delocalization in the
positive AB2 cation than the C01 cation. This delocalization is
slightly greater for the AB2 dication, with a shift of 50 mV
compared to the dication of C01. These differences imply that the
HOMO energy is raised slightly from C01 to AB2.
Reduction. C01 reduction produces two reversible peaks at
-1.39 and -1.93 V. Both reductions are expected to occur at the
benzothiadiazole group, which is a good acceptor, as discussed
previously. Unsubstituted benzothiadiazole does not exhibit a second
reduction peak. Here, the second reduction peak of C01 is less
negative than BH0 by 250 mV. This potential shift indicates greater
delocalization onto the two fluorene moieties. In AB2, the potential
difference is 130 mV because AB2 possesses only a single fluorene
moiety. The first AB2 reduction peak occurs at -1.44 V and
represents a two-electron transfer (ipc ) 10 µA, compared with ipc
) 5 µA for the first C01 reduction and the other peaks), indicating
minimal conjugation (electronic communication) through the fluo-
rene moiety so that both benzothiadiazole moieties are reduced at
the same potential. However, the second and third reductions of
AB2 show a small splitting of about 150 mV, indicating that in
contrast to the dianion radical the benzothiadiazole radical trianion
is delocalized across the spirobifluorene moiety. The two processes
do not occur at the same potential because the electrostatic repulsion
from the second reduction (-2.05 V) shifts the third to more
negative potentials (-2.20 V).
Spectroscopy. Absorption and fluorescence spectra for all the
compounds are shown in Figure 4. The absorption and emission
spectra were obtained in the same solvent mixture used in the
electrochemical measurements for the sake of consistency. The
optical properties, absorption and emission maxima, extinction
coefficient, fluorescence quantum yield, and the optical energy gap
are given in Table 2. Structureless π-π* absorption bands were
observed for fluorene derivatives and BH0-BH3 compounds,
suggesting free rotation of the rings in the aromatic system.
Compared with BH0, BH1-BH3 are red-shifted by 12 nm (BH2),
16 nm (BH1), and 18 nm (BH3), due to electron donor properties
of the terminal groups (OMe > propogyl-O > tert-butyl). The
emission spectra of BH0-3 show the same trend. Compared to
BH0, BH1-3, are red-shifted by 30 nm (BH2), 34 nm (BH1), and
44 nm (BH3). BH0 fluoresces with a λmax ) 512 nm and a Stokes
shift of 124 nm, while larger Stokes shifts are observed in BH1, 2,
and 3, as summarized in Table 2.
Oxidation. A scan toward positive potentials for a C01 solution
shows two sequential reversible waves at 1.46 and 1.61 V vs SCE,
respectively (Figure 1). Both have equal peak currents ∼5 µA,
consistent with one-electron transfers for the electrochemical
oxidation processes. The first peak is chemically reversible down
to 50 mV/s, demonstrating a fairly stable cation radical.
The oxidation of AB2 shows similar behavior as C01, with two
consecutive, reversible peaks 30 mV less positive than those of
(16) Yasuda, T.; Imase, T.; Yamamaoto, T. Macromolecules 2005, 38, 7378.
9
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A R T I C L E S
Omer et al.
red
Figure 5. (a) ECL spectra of BHs in MeCN, BH1, BH2, and BH3 were taken by annihilation by stepping between (Ep -50 mV) to the first oxidation
oxd
potential (Ep +50 mV). Integration time, 1 min; slit width, 0.5 mm. BH0 were recorded using BPO (1 0 mM) as a coreactant. (b) ECL of fluorene
red
oxd
derivatives by annihilation by stepping between (Ep -50 mV) to the first oxidation potential (Ep +50 mV).
Figure 6. Stepping ECL (annihilation) of BH3 (1 mM), in MeCN. Pulse width, 0.05 s; stepping direction is from anodic to cathodic. (a) From the first
oxidation peak (1.35 V) (n ∼ 1) to the first reduction peak (-1.50 V) (n ) 1). (b) From both oxidation peaks (1.50 V) (n ) 2) to the first reduction peak
(-1.50 V) (n ) 1), pulse width ) 0.05 s. (As plotted here, the positive electrochemical current is the anodic pulse, and the negative electrochemical current
is the cathodic pulse.)
The absorption λmax for AB2 and C01 are slightly red-shifted
because of their greater conjugation. There is a slightly smaller
Stokes shifts for AB2 and C01 compared to the BHs, which could
be due to the smaller solvent polarity used for these compounds,
Bz:MeCN, compared to pure MeCN in the case of BHs.
Electrogenerated Chemiluminescence (ECL). All BHs and the
fluorene derivatives produced strong ECL emission for radical ion
annihilation, which can be seen with the naked eye in a dimly lit
room; typical results are given in Table 3 and Figure 5. In all cases,
the estimated annihilation energy is sufficient to populate the excited
singlet state directly, classifying the ECL of these systems as energy
sufficient or S-route systems. All ECL spectra were close to the
PL spectra; the 5-15 nm difference between the PL and ECL is
probably caused by instrumental differences and the self-absorption
(inner filter effect) from the high concentrations of the solutions
used for ECL vs those for PL studies.
BH0 produced only weak ECL by direct annihilation (sweeping
or stepping potentials between the first oxidation and reduction
peaks). This results from instability of the radical cation, as seen
in the oxidative cyclic voltammogram. Because of this instability
of the BH0 radical cation, the ECL intensity decreased with time,
and a spectrum could not be obtained with the liquid nitrogen cooled
CCD camera as a detector during extended pulsing. However, in
this case, a coreactant approach could be used. Since the radical
anion of BH0 is stable, BPO was used as a coreactant for reductive
ECL. BPO, upon reducing electrochemically, produces a strong
oxidizing agent (Eoxd ) 1.5 V vs SCE),13,1a which can react with
the radical anion of BH0 populating the excited state. Under these
conditions, strong ECL was observed (Figure 5a).
To estimate the stability of the ECL and the radical ion species
involved, potential pulsing ECL transients were collected. For
example BH3 shows two consecutive oxidation waves and one
reduction peak (one-electron transfers). When the potential was
stepped from the first oxidation wave (Epa,1 +50 mV) to the
reduction wave (Epc,1 -50 mV), approximately equal ECL peaks
were observed on the cathodic and anodic pulses, consistent with
the same quantities of radical anions and cations formed per pulse
and stability of the radical ions during the time of the experiment
(Figure 6a). When the potential was stepped to the second oxidation
wave (Epa,2 +50 mV) producing the dication A2+, the anodic current
was smaller than the cathodic current and the ECL maximum
intensities were about the same as for pulsing to the first oxidation
wave (Figure 6b). However, the decay rate of the anodic ECL
transients is higher (Figure 6b). Moreover, the ECL intensity of
the anodic pulse decreased with time, while in the cathodic, ECL
intensity was constant. There have not been many studies of ECL
generation between radical anions and dications, and the situation
is complicated because of the occurrence of a comproportionation
reaction between the dication and parent and also by the fact that
all of the electrogenerated species are potential quenchers. Although
we have tried to simulate the observed behavior by choosing
different rate constants for the possible reactions, we were not
successful in matching the observed experimental results. Interpret-
ing these kinds of asymmetric ECL generation will probably be
easier in a steady state (two-electrode) mode, e.g., by scanning
electrochemical microscopy (SECM).
The relative quantum efficiencies were determined for all the
ECL emitters, as calculated by the number of photons emitted per
injected electron. These were compared to 9,10-diphenylanthracene
(DPA) as a standard ECL emitter, even though the emission (λmax
) 420 nm) is quite blue-shifted compared to the compounds of
interest here. These results are given in Table 4, taking the quantum
yield of DPA in acetonitrile as 8%.13 The ECL emission intensity
of AB2 is about the same as that of DPA but in the green region
of the spectrum.
Conclusions
We have synthesized a series of highly fluorescent and
strong, green ECL emitters. These compounds were charac-
terized by electrochemistry, spectroscopy, and thermal analy-
sis. All the compounds, with the exception of BH0, give
reversible oxidation and reduction waves in their CV and
form stable radical ions. Due to their high PL quantum yield
9
10740 J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009

Benzothiadiazole and Fluorene Derivatives
A R T I C L E S
Koley for assistance with the digital simulation of the ECL
experiments, and the financial support from the National Science
Council of Taiwan.
and stable radicals upon electrooxidizng and reducing, these
gave strong and stable green ECL emission. The ECL
quantum yield of AB2 is close to that of the well-known
ECL emitter, DPA. Such fluorene derivatives are promising
for future applications as nanomaterials and biological
luminescent labels.
Supporting Information Available: Details of characterization
of the compounds and additional experimental results. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Acknowledgment. We thank the National Science Foundation
(CHE 0451494 and 0808927), Roche BioVeris, and the Robert A.
Welch Foundation (F-0021) for support of this research, Dipankar
JA904135Y
(17) Gritzner, G.; Kuta, J. Pure Appl. Chem. 1984, 56, 462.
(18) Beideman, F. E.; Hercules, M. J. Am. Chem. Soc. 1979, 83, 2203.
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