Electrochemical behavior and electrogenerated chemiluminescence of star-shaped D-A compounds with a 1,3,5-triazine core and substituted fluorene arms

Article abstract of DOI:10.1021/ja104160f

We report the synthesis, electrochemistry, and electrogenerated chemiluminescence of a series of star-shaped donor-acceptor (D-A) molecules. The star-shaped molecules consist of an electron-deficient 1,3,5-triazine core with three fluorene arms substituted with diarylamino (TAM1-TAM3) or carbazolyl (TAM4) electron donors. Cyclic voltammetry of TAM1-TAM3 shows that the reduction consists of one wave of single electron transfer to the core, while the oxidation exhibits a single peak of three sequential electron-transfer processes, with the formation of a trication. The carbazole-containing molecule TAM4 after oxidation undergoes a subsequent rapid chemical reaction to produce a dimer (via the overall coupling of two radical cations with the loss of two protons). The dimer electrooxidizes more easily than the monomer of TAM4. With continuous cycling on the oxidation side, a conductive polymer film is formed on the surface of the working electrode. Because of the presence of the acceptor (triazine) center and strong donors in the arms (diarylamine or carbazole), TAM1-TAM3 exhibit large solvatochromic effects with emissions ranging from deep blue (428 nm) to orange-red (575 nm) depending on the solvent polarity. These star-shaped molecules show high PL quantum yields of 0.70-0.81. The electrogenerated chemiluminescence (ECL) of TAM1-TAM3 in nonaqueous solutions showed strong ECL that could be seen with the naked eye in a well-lit room. Because the enthalpy of annihilation is higher than the energy required for formation of the singlet excited state, the ECL emission is believed to be generated via S-route annihilation. However, TAM4 shows weak annihilation ECL because of the production of polymer film on the electrode surface during oxidation cycles. However, by limiting the potential region only to the reduction side and using benzoyl peroxide (BPO) as a coreactant, strong ECL of TAM4 can be obtained.

Full text of DOI:10.1021/ja104160f

Published on Web 07/15/2010
Electrochemical Behavior and Electrogenerated
Chemiluminescence of Star-Shaped D-A Compounds with a
1,3,5-Triazine Core and Substituted Fluorene Arms
Khalid M. Omer,† Sung-Yu Ku,†,‡ Yu-Chen Chen,‡ Ken-Tsung Wong,*,‡ and
Allen J. Bard*,†
Center for Electrochemistry, Department of Chemistry and Biochemistry, UniVersity of Texas at
Austin, Austin, Texas 78712, and Department of Chemistry, National Taiwan UniVersity,
106 Taipei, Taiwan
Received May 14, 2010; E-mail: ajbard@mail.utexas.edu; kenwong@ntu.edu.tw
Abstract: We report the synthesis, electrochemistry, and electrogenerated chemiluminescence of a series
of star-shaped donor-acceptor (D-A) molecules. The star-shaped molecules consist of an electron-deficient
1,3,5-triazine core with three fluorene arms substituted with diarylamino (TAM1-TAM3) or carbazolyl (TAM4)
electron donors. Cyclic voltammetry of TAM1-TAM3 shows that the reduction consists of one wave of
single electron transfer to the core, while the oxidation exhibits a single peak of three sequential electron-
transfer processes, with the formation of a trication. The carbazole-containing molecule TAM4 after oxidation
undergoes a subsequent rapid chemical reaction to produce a dimer (via the overall coupling of two radical
cations with the loss of two protons). The dimer electrooxidizes more easily than the monomer of TAM4.
With continuous cycling on the oxidation side, a conductive polymer film is formed on the surface of the
working electrode. Because of the presence of the acceptor (triazine) center and strong donors in the
arms (diarylamine or carbazole), TAM1-TAM3 exhibit large solvatochromic effects with emissions ranging
from deep blue (428 nm) to orange-red (575 nm) depending on the solvent polarity. These star-shaped
molecules show high PL quantum yields of 0.70-0.81. The electrogenerated chemiluminescence (ECL)
of TAM1-TAM3 in nonaqueous solutions showed strong ECL that could be seen with the naked eye in a
well-lit room. Because the enthalpy of annihilation is higher than the energy required for formation of the
singlet excited state, the ECL emission is believed to be generated via S-route annihilation. However,
TAM4 shows weak annihilation ECL because of the production of polymer film on the electrode surface
during oxidation cycles. However, by limiting the potential region only to the reduction side and using benzoyl
peroxide (BPO) as a coreactant, strong ECL of TAM4 can be obtained.
Introduction
In the present work, we report the synthesis, electrochemical,
and photochemical characterization and electrogenerated chemi-
luminescence of star-shaped molecules based on a 1,3,5-triazine
core with three arms built with 9,9-diethylfluorene end-capped with
diarylamine or carbazole, as shown in Figure 1. Star-shaped donor-
π-acceptor (D-A) systems are considered promising candidates for
different applications, such as organic light-emitting devices,1
photovoltaic applications,2 and organic thin film transistors.3
Recently, we reported blue electrogenerated chemilumines-
cence (ECL) of star-shaped molecules based on truxene-
† University of Texas at Austin.
‡ National Taiwan University.
(1) (a) Lai, W.-Y.; He, Q.-Y.; Zhu, R.; Chen, Q.-Q.; Huang, W. AdV.
Funct. Mater. 2008, 18, 265. (b) Xu, T.; Lu, R.; Liu, X.; Chen, P.;
Qiu, X.; Zhao, Y. J. Org. Chem. 2008, 73, 1809. (c) Kreger, K.; Ba¨te,
M.; Neuber, C.; Schmidt, H.-W.; Strohriegl, P. AdV. Funct. Mater.
2007, 17, 3456. (d) Zhou, X-.H.; Yan, J.-C.; Pei, J. Org. Lett. 2003,
5, 3543. (e) Wu, I.-Y.; Lin, J. T.; Tao, Y.-T.; Balasubramaniam, E.;
Su, Y. Z.; Ko, C.-W. Chem. Mater. 2001, 13, 2626.
Figure 1. Star-shaped 1,3,5-triazine derivatives.
oligofluorene.4 The truxene-oligofluorene compounds show C3
symmetry; however, in the hydrocarbon structure, there were
no strong D and A groups to stabilize the hole or the electron,
i.e., to stabilize the radical cation and anion. Therefore, we
designed and synthesized a C3 symmetry system by introducing
strong donor groups (diarylamine for TAM1-TAM3 or car-
(2) (a) Cremer, J.; Bauerle, P. J. Mater. Chem. 2006, 16, 874. (b) Roquet,
S.; Cravino, A.; Leriche, P.; Ale’veque, O.; Fre‘re, P.; Roncali, J.
J. Am. Chem. Soc. 2006, 128, 3459. (c) Thelakkat, M. Macromol.
Mater. Eng. 2002, 287, 442.
9
10944 J. AM. CHEM. SOC. 2010, 132, 10944–10952
10.1021/ja104160f  2010 American Chemical Society

Star-Shaped D-A Compounds
A R T I C L E S
Scheme 1. Electrooxidation of Triphenylamine and Following
Dimerization of Triphenylamine
Mechanism I:
A•--D-A•- + A-D•+-A f A•--D-A + (A-D-A)*
(5)
A•--D-A + A - D•+-A f A-D-A + (A-D-A)*
(6)
Mechanism II:
A•--D - A•- + A-D-A f 2A•--D-A
A•--D-A + A-D•+-A f A-D-A + (A-D-A)*
(7)
(8)
However, in the presence of strong acceptor and donor, an
intramolecular charge-transfer emission is possible as radical
anion and radical cation collide.
bazole for TAM4) and the strong electron acceptor (1,3,5-
triazine) bridged by the 2,7-fluorenes. Diarylamine or carbazole
with lone pair electrons on the nitrogen makes these derivatives
easy to oxidize. Shirota et al.5 designed various systems of star-
shaped triarylamine-based molecules, which have different
effects on lowering the potential for oxidation based on their
conjugation lengths. Unsubstituted triphenylamine (TPA) un-
dergoes dimerization to form tetraphenylbenzidine as shown by
ESR and cyclic voltammetry (Scheme 1).6 The dimer is oxidized
more easily than the monomer of TPA.
1,3,5-Triazine molecules show good optical and electro-
chemical properties.7 Due to their electron deficiency (i.e.,
triazine is a typical acceptor unit), radical anions are stabilized,
and reduction occurs reversibly. Such radical anion stability is
a fundamental requirement of generating efficient ECL.
Electrogenerated chemiluminescence (ECL) arises from the
electron transfer between a cation radical and an anion radical.8
The simplest ECL mechanism is the direct annihilation reaction
between the cation radical and the anion radical as follows:
A•--D-A•- + A-D•+-A f (A•--D•+-A)* + A-D-A
(9)
(A•--D•+-A)* f A-D-A + hν
(10)
Another ECL pathway, often used when lack of stability of one
radical ion or the limitation of the potential window to prevent
its generation, involves adding a coreactant, which can produce
either a strong reducing agent or strong oxidizing agent
depending on the nature of the coreactant. For example, benzoyl
peroxide (BPO) is one of the common coreactants used if the
radical cation is not stable. Upon reduction, BPO forms a strong
oxidizing agent (E° ) +1.5 V),10 which can directly react with
an anion to produce an excited state.
A - e f A•+ (radical cation formation)
A + e f A•- (radical anion formation)
(1)
(2)
A•- + A•+
f
1A* + A
(singlet state formation by direct annihilation) (3)
1A* f A + hν (ECL emission)
(4)
However, asymmetric annihilation, for example between a
dianion and monocation may occur to produce the excited state
by the following two proposed mechanisms:9
BPO• + A•- f A* + BPO-
A* f A + hν
(12)
(13)
(3) (a) Kim, K. H.; Chi, Z.; Cho, M. J.; Jin, J.-I.; Cho, M. Y.; Kim, S. J.;
Joo, J.-S.; Choi, D. H. Chem. Mater. 2007, 19, 4925. (b) Ponomarenko,
S. A.; Tatarinova, E. A.; Muzafarov, A. M.; Kirchmeyer, S.; Brassat,
L.; Mourran, A.; Moeller, M.; Setayesh, S.; De Leeuw, D. Chem.
Mater. 2006, 18, 4101. (c) Horowitz, G. AdV. Mater. 1998, 10, 365.
(4) Omer, K. M.; Kanibolotsky, A. L.; Skabara, P. J.; Perepichka, I. F.;
Bard, A. J. J. Phys. Chem. B 2007, 111, 6612.
D-A molecules are sensitive to solvent polarity, due to the
intramolecular charge transfer that occurs upon excitation. As
a result, they are potential solvent polarity probes.11
In this paper, we describe the synthesis of TAM1-TAM4.
Electrochemistry and spectroscopy are thoroughly studied.
TAM1-TAM3 show bright ECL, which could be seen with
the naked eye under ambient light.
(5) (a) Shirota, Y.; Kageyama, H. Chem. ReV. 2007, 107, 953. (b) Shirota,
Y. J. Mater. Chem. 2005, 15, 75. (c) Shirota, Y. J. Mater. Chem.
2000, 10, 1.
(6) (a) Nelson, R. F.; Adams, R. N. J. Am. Chem. Soc. 1968, 90, 3925.
(b) Seo, E. T.; Nelson, R. F.; Fritsch, J. M.; Marcoux, L. S.; Leedy,
D. W.; Adams, E. N. J. Am. Chem. Soc. 1966, 88, 3498.
(7) (a) Fang, Q.; Yamamoto, T. Macromol. Chem. Phys. 2004, 205, 795.
(b) Cui, Y. Z.; Fang, Q.; Lei, H.; Xue, G.; Yu, W. T. Chem. Phys.
Lett. 2003, 377, 507. (c) Pang, J.; Tao, Y.; Freiberg, S.; Yang, X.-P.;
D’Iorio, M.; Wang, S. J. Mater. Chem. 2002, 12, 206. (d) Cherioux,
F.; Audebert, P.; Hapiot, P. Chem. Mater. 1998, 10, 1984. (e) Fink,
R.; Frenz, C.; Thelakkat, M.; Schmidt, H.-W. Macromolecules 1997,
30, 8177.
Experimental Section
Synthesis. TAM1-TAM4 consist of a 1,3,5-triazine core and
three arms of 9,9-diethylfluorene end-capped with diarylamine or
carbazole. The syntheses of TAM1-TAM4 start from 2-bromo-
9,9-diethylfluorene to give 2-cyano-9,9-diethylfluorene, followed
by an acid-promoted triazene formation and C-N bond cross-
coupling reaction. This synthetic route was shown in Scheme 2.
Treatment of 2-bromo-9,9-diethylfluorene12 with 2 equiv of CuCN
9
J. AM. CHEM. SOC. VOL. 132, NO. 31, 2010 10945

A R T I C L E S
Omer et al.
Scheme 2. Synthetic Pathway of TAM Compounds
in DMF at reflux temperature gave 2-cyano-9,9-diethylfluorene
(compound 1) in an isolated yield of 92%. Compound 1 was
regioselectively brominated at C-7 with 2 equiv of bromine in the
presence of Lewis acid (FeCl3) to give compound 2 in 99%. Then,
CF3SO3H-promoted cyclization of compound 2 in dichloromethane
generated 1,3,5-triazine T-Br in an isolated yield of 81%. Finally,
Pd-catalyzed C-N bond formation of T-Br with a different
diarylamine using NaOtBu as base at 110 °C afforded
TAM1-TAM3. The same procedure was not feasible for the C-N
bond coupling with carbazole substitution. K2CO3 was employed
as base, instead of NaOtBu. The mixture was stirred at 160 °C in
o-xylene in the presence of Pd(OAc)2 and PtBu3 generated the
TAM4 in a moderate yield.
Compound 2. Compound 1 (9.8 g, 40 mmol) was dissolved in
chloroform (20 mL), and the mixture was lined with aluminum
foil to keep away from light. Then bromine (4 mL, 80 mmol) was
added slowly into the flask, and the reaction mixture was stirred at
the reflux temperature for 12 h. The mixture was extracted with
CHCl3/water. The organic solution was collected, dried with
MgSO4, and concentrated with a rotary evaporator. The product
was purified by vacuum system to get compound 2 as a brown
solid: (13.0 g, 99%). mp 95-96 °C; IR (KBr) ν 2979, 2230, 1607,
1448, 824, 519 cm-1; 1H NMR (CDCl3, 400 MHz) δ 7.74 (d, J )
8.0 Hz, 1H), 7.66-7.59 (m, 3H), 7.53-7.50 (m, 2H), 2.05-1.99
(m, 4H) 0.29 (t, J ) 7.4 Hz, 6H); 13C NMR (CDCl3, 100 MHz) δ
152.5, 150.0, 144.7, 138.3, 131.3, 130.4, 126.4, 126.3, 123.0, 121.9,
120.1, 119.4, 110.3, 56.8, 32.5, 8.4; MS (m/z, FAB+) 326 (52),
307 (18), 217 (24), 154 (100), 136 (77); HRMS ((M + H)+, FAB+)
Calcd C18H1779BrN 326.0544, found 326.0545; calcd C18H1781BrN
328.0524, found 328.0533. Anal. Calcd C, 66.27; H, 4.94; Br, 24.49;
N, 4.29; found C, 66.49; H, 4.95; N, 4.03.
with CH2Cl2/water. The organic solution was collected and dried
with MgSO4, and the organic solvents were removed with a rotary
evaporator. The product was purified by recrystallization with
methanol to get T-Br as a white solid: (10.4 g, 81%). mp 214 °C
1
(DSC); IR (KBr) ν 2979, 1613, 1593, 1249, 811, 751 cm-1; H
NMR (CDCl3, 400 MHz) δ 8.86 (dd, J ) 8.0, 1.6 Hz, 3H), 8,74
(d, J ) 0.8 Hz, 3H), 7.91 (d, J ) 8.0 Hz, 3H), 7.70 (d, J ) 7.6 Hz,
3H), 7.56 -7.53 (m, 6H), 2.29-2.24 (m, 6H), 2.16-2.10 (m, 6H),
0.435 (t, J ) 7.4 Hz, 18H); 13C NMR (CDCl3, 100 MHz) δ 171.5,
153.1, 149.8, 144.8, 139.6, 135.4, 130.2, 128.5, 126.4, 123.2, 122.2,
121.7, 119.8, 56.7, 32.8, 8.6; MS (m/z, FAB+) 980 (43), 978 (43),
326 (24), 307 (60), 289 (52); HRMS ((M + H)+, FAB+) Calcd
C54H49N379Br81Br2 980.1436, found 980.1433; calcd C54H49N3-
79Br281Br 978.1456, found 978.1460.
TAM1. Pd(OAc)2 (23 mg, 0.1 mmol), diphenylamine (700 mg,
4 mmol), NatBuO (460 mg, 6 mmol), and T-Br (980 mg, 1 mmol)
were added to a 100 mL two-neck flask with a septum. The flask
was evacuated and backfilled with argon. Toluene (20 mL) and
PtBu3 (8 mL, 0.4 mmol, 0.05 M in toluene) were added. The
reaction mixture was heated at 110 °C for 48 h. The reaction mixture
was allowed to cool to room temperature, quenched with water
(40 mL), and extracted twice with CH2Cl2.The combined organic
solution was washed with brine and dried over MgSO4. The product
was isolated as yellow crystals with recrystallization from methanol
to afford TAM1 (890 mg, 72%). IR (KBr) ν 2959, 1600, 1487,
1368, 811, 745 cm-1; 1H NMR (CDCl3, 400 MHz) δ 8.83 (d, J )
8.0 Hz, 3H), 8.71 (s, 3H), 7.83 (d, J ) 8.0 Hz, 3H), 7.69 (d, J )
8.4 Hz, 3H), 7.29 (d, J ) 8.0 Hz, 9H), 7.15 (d, J ) 8.0 Hz, 15H),
7.10-7.03 (m, 12H), 2.19-2.16 (m, 6H), 2.02-1.99 (m, 6H), 0.46
(t, J ) 7.2 Hz, 18H); 13C NMR (CDCl3, 100 MHz) δ 171.4, 152.4,
150.0, 148.0, 147.7, 145.6, 135.5, 134.3, 129.1, 128.4, 124.0, 123.2,
123.0, 122.7, 121.1, 119.0, 118.8, 56.3, 32.7, 8.8; MS (m/z, FAB+)
1243 (50), 1242 (32), 415 (48), 385 (35), 371 (100); HRMS ((M
+ H)+, FAB+) Calcd C90H79N6 1243.6366, found 1243.6379.
TBr. Compound 2 (12.8 g, 40 mmol) and trifluoromethane-
sulfonic acid (3.6 mL, 40 mmol) were dissolved in dichloromethane
(40 mL). The reaction mixture was stirred at 0 °C for 1 h and then
stirred at room temperature for 24 h. The mixture was extracted
9
10946 J. AM. CHEM. SOC. VOL. 132, NO. 31, 2010

Star-Shaped D-A Compounds
A R T I C L E S
TAM2. A procedure similar to that for TAM1 was applied. A
mixture of Pd(OAc)2 (23 mg, 0.1 mmol), biphenyl-4-ylphenylamine
(1.0 g, 4 mmol), NatBuO (460 mg, 6 mmol), T-Br (980 mg, 1 mmol),
toluene (20 mL), and PtBu3 (8 mL, 0.4 mmol, 0.05 M in toluene)
gave TAM2 (1.2 g, 82%) as yellow crystals with recrystallization from
The Netherlands) coupled with a photomultiplier tube (PMT, Ham-
mamatsu R4220p, Japan) held at -750 V with a high-voltage power
supply (Kepco, Flushing, NY). The photocurrent produced at the PMT
was transformed into 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. The
ECL spectra were taken using a liquid nitrogen-cooled charge-coupled
device (CCD) camera (Princeton Instrument, SPEC-32), which was
cooled to -100 °C and calibrated with a mercury lamp. Absorption
spectra were recorded with a Milton Roy Spectronic 3000 array
spectrophotometer. Fluorescence spectra were acquired on a Quanta
Master spectrofluorimeter (Photon Technology International, Birming-
ham, NJ). Differential scanning calorimetry (DSC) was recorded with
JADE DSC (PerkinElmer).
DigiSim 3.03 (Bioanalytical Systems, Inc., West Lafayette, IN)
was used to simulate the cyclic voltammograms. The double-layer
capacitance (∼40 nF) and uncompensated resistance (1200 Ω) were
determined from the current response to a potential step made in a
nonfaradic region. The diffusion coefficients, D, were determined
from the Randles-Sevic equation and a Cottrell plot of an oxidizing
potential step. The electrode surface area was determined from a
Cottrell plot of a potential step experiment in 1 mM ferrocene in
MeCN (D ) 1.2 × 10-5 cm2/s).13
1
methanol. IR (KBr) ν 2979, 1607, 1514, 1282, 824, 765 cm-1; H
NMR (CDCl3, 400 MHz) δ 8.86 (dd, J ) 9.2, 1.2 Hz, 3H), 8.74 (d,
J ) 0.8 Hz, 3H), 7.86 (d, J ) 8.0 Hz, 3H), 7.72 (d, J ) 8.4 Hz 3H),
7.63-7.61 (m, 6H), 7.53 (d, J ) 8.2 Hz, 6H), 7.45 (t, J ) 7.6 Hz,
6H), 7.35-7.28 (m, 9H), 7.25-7.19 (m, 15H), 7.15 (dd, J ) 10.4
Hz, 2.4 Hz, 3H), 7.08 (t, J ) 7.4 Hz, 3H), 2.24-2.19 (m, 6H),
2.06-2.01 (m, 6H), 0.49 (t, J ) 7 Hz, 18H); 13C NMR (CDCl3, 100
MHz) δ 171.4, 152.5, 150.0, 147.8, 147.5, 147.0, 145.6, 140.4, 135.7,
135.1, 134.3, 129.2, 128.6, 128.4, 127.7, 126.7, 124.3, 123.8, 123.4,
122.9, 121.2, 119.0, 56.3, 32.8, 8.9; MS (m/z, FAB+) 1472 (100), 1471
(68), 492 (18), 447 (32), 307 (55); HRMS (M+, FAB+) Calcd
C108H90N6 1470.7227, found 1470.7202.
TAM3. A procedure similar to that for TAM1 was applied. A
mixture of Pd(OAc)2 (23 mg, 0.1 mmol), naphthalen-2-ylphenyl-
amine (900 mg, 4 mmol), NatBuO (460 mg, 6 mmol), T-Br (980 mg,
1 mmol), toluene (20 mL), and PtBu3 (8 mL, 0.4 mmol, 0.05 M in
toluene) gave TAM3 (700 mg, 53%) as yellow crystals using column
chromatography with hexane/toluene (3/1) and then recrystallization
from methanol. IR (KBr) ν 2966, 1600, 1520, 1375, 818, 791 cm-1
.
Results and Discussion
1H NMR (CDCl3, 400 MHz) δ 8.81 (dd, J ) 9.2, 1.2 Hz, 3H), 8.68
(s, 3H), 7.96 (d, J ) 8.4 Hz, 3H), 7.91 (d, J ) 8.4 Hz, 3H), 7.81-7.78
(m, 6H), 7.61 (d, J ) 8.4 Hz, 6H), 7.48 (m, 6H), 7.38-7.32 (m, 6H),
7.25-7.23 (m, 3H), 7.13-7.11 (m, 9H), 7.01-6.97 (m, 6H), 2.16-2.12
(m, 6H), 1.95-1.89 (m, 6H), 0.41 (t, J ) 7.2 Hz, 18H); 13C NMR
(CDCl3, 100 MHz) δ 171.4, 152.3, 149.8, 148.8, 148.4, 145.8, 143.5,
135.1, 134.5, 134.0, 130.8, 129.0, 128.3, 126.7, 126.2, 126.1, 125.9,
124.2, 122.9, 122.1, 121.8, 121.1, 119.5, 118.8, 116.4, 56.2, 32.7, 8.8;
MS (m/z, FAB+) 1394 (15), 1393 (12), 460 (16), 307 (100), 289 (45);
HRMS ((M + H)+, FAB+) Calcd C102H85N6 1393.6836, found
1393.6853.
Electrochemistry. Cyclic voltammograms for TAM1-TAM4
are shown in Figure 2, and Table 1 summarizes the electro-
chemical results. All TAM compounds show one reversible
reduction peak at (1 to 4) -1.79, -1.78, -1.78, and -1.74 V
vs SCE, respectively. The reduction peaks were reversible even
at a slow scan rate (50 mV/s), indicating a long-lived radical
anion. The radical anion is reasonably assigned the 1,3,5-triazine
(A) core and is relatively insensitive to the substituent groups.
The reduction peak potential separation between the forward
scan and reverse scan was ∼65 mV at 100 mV/s, slightly greater
than ∼58 mV expected for a one-electron process.14 The
additional 6 mV is attributed to iR drop in the highly resistive
solvent, so the reduction is believed to be a single electron
transfer. Moreover, the same peak splitting was observed for
the reference material, ferrocene, which is known to undergo
one-electron oxidation. Digital simulation of reduction CV at
different scan rates was performed to confirm the reduction
mechanism (Supporting Information, Figure S3). The best fit
between the experimental and simulated CVs was observed from
50 mV/s to 1 V/s, assuming the reduction mechanism is a simple
electron transfer with no coupled homogeneous chemical
reactions. The best fit was obtained, after correction for Ru, a
heterogeneous electron transfer rate constant, ko ∼ 0.02 cm/s.
This relatively slow electron transfer for a simple outer sphere
reaction might be caused by a large conformational change in
the molecule upon reduction.
TAM4. A mixture of Pd(OAc)2 (23 mg, 0.1 mmol), carbazole (900
mg, 4 mmol), K2CO3 (830 mg, 6 mmol), and T-Br (980 mg, 1 mmol)
were added to o-xylene (20 mL) and PtBu3 (8 mL, 0.4 mmol, 0.05 M
in toluene). The reaction mixture was heated at 160 °C for 5 days.
The reaction mixture was allowed to cool to room temperature,
quenched with water (40 mL), and extracted twice with CH2Cl2.The
combined organic solution was washed with brine and dried over
MgSO4, and the product was isolated as yellow crystals from column
chromatography with hexane/toluene (3/1) and then recrystallized from
methanol to afford TAM4 (630 mg, 51%). IR (KBr) ν 2986, 1620,
1527, 1235, 831, 751 cm-1; 1H NMR (CDCl3, 400 MHz) δ8.98 (dd,
J ) 9.6, 1.6 Hz, 3H), 8.87 (d, J ) 1.2 Hz, 3H), 8.20 (d, J ) 7.6 Hz,
6H), 8.07 (t, J ) 8.4 Hz, 6H), 7.65-7.63 (m, 6H), 7.49-7.47 (m,
12H), 7.36-7.32 (m, 6H), 2.37-2.33 (m, 6H), 2.24-2.20 (m, 6H),
0.58 (t, J ) 7.2 Hz, 18H); 13C NMR (CDCl3, 100 MHz) δ 171.5,
152.9, 150.5, 145.1, 140.8, 139.8, 137.3, 135.3, 128.6, 125.9, 123.3,
121.8, 121.6, 120.3, 119.9, 109.7; MS (m/z, FAB+) 1238 (20), 1237
(12), 460 (15), 391 (28), 307 (100); HRMS ((M + H)+, FAB+) Calcd
C90H73N6 1237.5897, found 1237.5897.
Characterization. Electrochemical measurements were per-
formed in a conventional electrochemical cell consisting of a
working electrode with a ∼1 mm inlaid platinum disk, a Ag wire
as a quasireference electrode (referenced verse internal standard,
ferrocene/ferrocenium then converting to SCE), and a Pt wire coil
as counter electrode. For ECL, the working electrode was a 2 mm
platinum disk J-type (bent to face the detector). The working
electrode was polished with 1 µm alumina (Buehler, Ltd., IL), and
then 0.3 µm and 0.05 µm, followed by sonication in deionized water
and acetone for 5 min each. This series of D-A molecules was
tested in 1:1 benzene:MeCN, with 0.1 M Bu4NPF6 as a supporting
electrolyte. Cyclic voltammograms (CVs) were recorded on a model
660 electrochemical workstation (CH Instruments, Austin, TX).
(8) For review on ECL: (a) Bard, A. J. Electrogenerated Chemilumines-
cence; Marcel Dekker: New York, 2004. (b) Miao, W. Chem. ReV.
2008, 108, 2506. (c) Knight, A. W.; Greenway, G. M. Analyst 1994,
119, 879. (d) Richter, M. M. Chem. ReV. 2004, 104, 3003.
(9) (a) Lai, R. Y.; Fabrizio, E. F.; Jenekhe, S. A.; Bard, A. J. J. Am. Chem.
Soc. 2001, 123, 9112. (b) Rashidnadimi, S.; Hung, T.-H.; Wong, K.-
T.; Bard, A. J. J. Am. Chem. Soc. 2008, 130, 634.
(10) Chandross, E.; Sonntag, F. J. Am. Chem. Soc. 1966, 88, 1089.
(11) Reichardt, C. SolVents and SolVent Effect in Organic Chemistry; Wiley-
VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2003.
(12) Saroja, G.; Pingzhu, Z.; Ernsting, N. P.; Liebscher, J. J. Org. Chem. 2004,
69, 987.
(13) Kadish, K.; Ding, J.; Malinski, T. Anal. Chem. 1984, 56, 1741.
(14) The exact difference in the peak potentials for a nernstian reaction
with no resistive effects depends slightly on the potential for scan
reversal past the wave of interest. Bard, A. J.; Faulkner, L. R.
Electrochemical Methods; Wiley: New York, 2001, p 242.
Electrochemical current and ECL transients were simultaneously
recorded using an Autolab electrochemical workstation (Eco Chemie,
9
J. AM. CHEM. SOC. VOL. 132, NO. 31, 2010 10947

A R T I C L E S
Omer et al.
Figure 2. Cyclic voltammograms of TAM1-TAM4 in 1:1 Bz:MeCN, 0.1 M Bu4NPF6, scan rates 100 mV/s. Working electrode: Pt inlaid disk (diameter
∼ 0.5 mm), counter electrode: Pt coil, reference electrode: Ag QRE.
Table 1. Electrochemical Resultsa
was stable at scan rates as low as 50 mV/s; even after 100 cycles
there was no evidence of decomposition or dimer formation.
oxidation
potential
E1/2 (V)
reduction
potentials
E1/2 (V)
HOMOb
(eV)
LUMOb
(eV)
Eg
(eV)
TAM4 is oxidized at 1.18 V, which is considerably more
positive than the potential for oxidation of the diarylamino-
capped species. However, the radical cation is unstable, so that
the reverse wave is much smaller than for the other TAM
compounds, and a new reduction wave of a product appears on
the reverse scan. This could be a dimer that forms through the
coupling of the two carbazole units at the C3 or C6 position
(and, as described in the next section, some polymer).16 The
dimer can be electrochemically oxidized more easily, at 0.88
V (Figure 3). Scheme 3 shows a possible, but still speculative,
mechanism for the formation of a dimeric compound from the
two bulky, star-shaped carbazole molecules. However, the first
oxidation of carbazole is a mixture of ECE and EC mechanisms,
in which the following chemical reaction is very fast.17,6b That
means the oxidation of carbazole after the first cycle is an ECE
mechanism (Scheme 3).17 Continuous cycling leads to an
increasing growth of the smaller oxidation peak, which indicates
a polymer film is formed on the surface of the electrode,
consistent with previous studies of a related species.18
D (cm2/s)
oxd
red1
TAM1
TAM2
TAM3
TAM4
PTAM4
0.81
0.82
0.85
1.18
0.93
1.15
-1.79,
-1.78,
-1.78,
-1.74,
-
4 × 10-6 -5.27 -2.67 2.60
5 × 10-6 -5.27 -2.67 2.60
5 × 10-6 -5.23 -2.67 2.80
3 × 10-6 -5.63 -2.71 2.92
a All potentials are versus SCE. HOMO ) -Eox
-4.8 eV.
b
vs Fc/Fc+
LUMO ) Eg - HOMO. Eg ) Eox + Ered
.
TAM1-TAM3 show chemically reversible oxidation peaks at
0.81, 0.82, 0.82 V vs SCE, respectively (Figure 1). The magnitude
of anodic peak current (ipa ∼ 12 µA) was three times that of the
cathodic peak current (ipc∼ 4 µA), suggesting that the oxidation
process is an overall three-electron transfer process and attributed
to the oxidation of each donor group essentially simultaneously.
The CV simulation results (Supporting Information, Figure S4)
suggest three sequential electron transfers with the ∆Eo between
each wave of 20-40 mV, with Do ) 6 × 10-6 cm2/s and with a
fast heterogeneous rate constant (simulation ko ) 1 × 104 cm/s),
e.g., nernstian reactions. That the three donor groups oxidize at
very nearly the same potential suggests lack of electronic com-
munication among the D groups (diarylamino), implying the
triazene core and the fluorene linkers serve as good electron
delocalization blocking groups. However, the fact that the peak
splitting is larger than ∼58 mV suggests that there is some
interaction, perhaps electrostatic.15 The electrogenerated trication
PTAM4 (Polymer of TAM4). TAM4 can electropolymerize
to give a coating on the surface of the electrode.16 After 10
cycle scans (scanning first to the oxidation potential of the
(16) (a) Marrec, P.; Dano, C.; Simonet, N. G.; Simonet, J. Synth. Met. 1997,
89, 171. (b) Abe, S. Y.; Bernede, J. C.; Delvalle, M. A.; Tregouet,
Y.; Ragot, F.; Diaz, F. R.; Lefrant, S. Synth. Met. 2002, 126, 1. (c)
Inzelt, G. J. Solid State Electrochem. 2003, 7, 503.
(17) (a) Ambrose, J. F.; Nelson, R. F. J. Electrochem. Soc. 1968, 115, 1159.
(b) Ambrose, J. F.; Carpenter, L. L.; Nelson, R. F. J. Electrochem.
Soc. 1975, 122, 876.
(15) Flanagan, J. B.; Margel, S.; Bard, A. J.; Anson, F. C. J. Am. Chem.
Soc. 1978, 100, 4248.
(18) Natera, J.; Otero, L.; Sereno, L.; Fungo, F.; Wang, N.-S.; Tsai, Y.-
M.; Hwu, T.-Y.; Wong, K.-T. Macromolecules 2007, 40, 4456–4463.
9
10948 J. AM. CHEM. SOC. VOL. 132, NO. 31, 2010

Star-Shaped D-A Compounds
A R T I C L E S
Figure 3. Cyclic voltammograms of 1 mM TAM4 in dry acetonitrile, 0.1 M Bu4NPF6. Working electrode: Pt inlaid disk (diameter ∼ 2 mm), counter
electrode: Pt coil, reference electrode: Ag QRE. (a) Comparison between first and second cycle. (b) After 10 cycles.
Scheme 3. Electrooxidation of TAM4 and the Following Dimerization Reaction
monomer of TAM4), an orange thin film could be seen to build
up on the surface of the working electrode during this cycling.
Both the original wave seen on the first scan, and the second
that builds up at less positive potentials, increased with the
number of scans. This suggests the formation of an electroactive
and conducting polymer upon oxidation of TAM4 (Figure 3a
and 3b). The working electrode coated with PTAM4 was
washed with the same solvent, dried in air, and then placed in
a fresh electrolyte solution without monomer. The new polymer-
modified electrode produced two distinct and reversible oxida-
tion peaks, the first at 0.93 V and the second at 1.15 V vs SCE
(Figure 4a). The scan rate dependence of the polymer film
(Figure 4b) shows a peak current directly proportional to the
scan rate at low scan rates, which deviates from this trend at
scan rates above about 0.3 V/s, suggesting kinetic limitations
in the oxidation, perhaps associated with bringing counteranions
into the film. The PTAM4 shows electrochromic behavior and
is pale orange in the neutral state, changing to dark green when
it is oxidized. The PTAM4 electrochromic behavior was stable
for at least 2000 cycles, suggesting the possibility of applications.
A question of interest is why the radical cation of the
carbazole-substituted species, TAM-4 undergoes rapid dimer-
ization, while those of the diarylamine-substituted ones,
TAM1-TAM3, do not. Related parent compounds, e.g.,
triphenylamines and carbazoles, have been studied by Adams,
Nelson, and co-workers,6,17 While the simple parent triphenyl-
amine dimerizes rapidly, the rate of dimerization depends
strongly on the nature of para-substitution; for example,
monosubstituted p-tolyldiphenylamine is much more stable than
the parent, and radical cation of trip-tolylamine is very stable.17
Thus, the effective para-substitution by the fluorene and triazine
as well as the considerably increased steric hindrance together
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J. AM. CHEM. SOC. VOL. 132, NO. 31, 2010 10949

A R T I C L E S
Omer et al.
Figure 4. PTAM4 on Pt electrode in MeCN, 0.1 M Bu4NPF6. (a) Anodic oxidation of PTAM4, scan rate: 100 mV/s. (b) Scan rate dependence of the
polymer film oxidation from 10 mV/s to 500 mV/s.
act to stabilize the radical cations, even for the more highly
oxidized states. The carbazolium radical cations, even substituted
ones, are less stable.17,18
indication of the polar character in the excited states. Therefore,
the TAM compounds have strong electronic interactions
between the donor and acceptor subunits in the excited state
but weak ones in the ground state. This solvatochromism
indicates that the stabilization of the excited state is larger than
that of ground state by the surrounding solvent molecules.20
Electrogenerated Chemiluminescence (ECL). The TAM com-
pounds (except for TAM4) are good candidates for ECL genera-
tion, because they show high PL quantum yields and exhibit stable
radical ion species, as shown in the cyclic voltammograms. Figure
6 displays the ECL spectra of these molecules. In the simple ECL
mechanism, light is produced during annihilation between the cation
radical and the anion radical. However, for these D-A molecules,
the annihilation can also occur between the anion radical and the
radical bi- and trication species.
The substituents affect the energy of the highest occupied
molecular orbital (HOMO) as derived from the half wave peak
potentials, taking the energy of the HOMO ) -Eoxvs Fc/Fc+ -4.8
eV.19 From the results in Table 1, for the star-shaped D-A
molecules (TAM1-TAM3) with diarylamino end-capping
groups, the frontier energy levels are similar but very different
from that of carbazole-containing compound (TAM4). This
result is strongly related to the relative electron-donating abilities
of the diarylamino groups and carbazole (that can be considered
to be a radical cation localized inside the carbazole ring because
of its coplanar structure.) The lowest unoccupied molecular
orbital (LUMO) is the about the same for TAM1-TAM4 since
reduction mainly involves the triazine group in all of these.
Photophysical Properties. Absorption and fluorescence spectra
of the TAM compounds were taken in the same solvent as used
for electrochemistry and ECL experiments (1:1 MeCN:benzene).
The spectra are shown in Figure 5a and 5b, respectively. Table
2 summarizes the photophysical properties of TAM1-TAM4.
The absorption spectra of TAM1-TAM3 are very similar and
gave λmax centered at ca. 420 nm, while TAM4 shows a λmax at
370 nm. Similarly (Figure 5b) TAM1-TAM3 gave emission
maxima at about the same λmax centered at 565 nm, showing
that the different substituent groups have only a small effect
on electron-donating ability toward the triazene core. In contrast,
TAM4 gave an emission maximum centered at 505 nm, blue-
shifted from the others by 60 nm due to the weaker electron-
donating ability of carbazole. These results agree with the
electrochemical results, showing that diarylamino end-capping
groups extend the π-conjugation along the fluorene bridge to
the central triazene better than that of the carbazole. The PL
quantum yields of TAM1-TAM4 were estimated using rubrene
as a standard. For TAM1-TAM3, these were ∼80% in 1:1
Bz:MeCN, while that of TAM4 was about 70% in the same
solvent.
TAM1-TAM3 give relatively strong ECL, which could be
seen by the naked eye in a well-lit room. Figure 6 shows the
ECL spectra, which are similar to their fluorescence spectra,
showing that the emission is mainly from the excited monomer.
The very small red-shift of the ECL spectra relative to the PL
spectra normally encountered in ECL studies is attributed to an
inner filter effect owing to the high sample concentration used
for ECL and also to a small instrumental offset. The ECL
spectrum of TAM2 taken with a dilute solution (0.2 µM), the
same as that used in the PL, was obtained with the same CCD
detector used for ECL experiments, showed that emission shifted
by only 3 nm. The ECL probably occurs via the S-route, since
ox
red
the enthalpy of annihilation (∆Hann ) E1/2 - E1/2 - 0.1) is
larger than the energy required to generate the excited singlet
state Es. In contrast, TAM4 shows only a very weak and broad
ECL peak (Figure 7). The weak ECL emission of TAM4 is
ascribed to instability of the radical cation and the formation
of the polymer film on the electrode surface. The broad emission
band (bandwidth ∼ 330 nm) might indicate ECL emission by
the dimer formed during the electrolysis. However, when
benzoyl peroxide (BPO) was used as a coreactant with only
reduction, a strong ECL peak was observed (Figure 6d) with
the coreactant-induced ECL spectrum of TAM4 narrower and
similar to the PL spectrum. This suggests that the annihilation
ECL of TAM4 (i.e., pulsing from Epa + 80 mV to Epc - 80
mV) is mainly due to the emission of the monomer, dimer, and
perhaps the polymer. We attempted to obtain the ECL spectrum
of an electrochemically produced polymer film (PTAM4) in a
monomer-free solution with and without coreactant. Although
the emission was too weak to obtain an ECL spectrum with a
These D-A molecules show large solvatochromic effects in
the emission, while the absorption spectra are independent of
solvent polarity. A small Stokes shift is observed in less polar
solvents such as hexane (λmax,PL ) 428 nm), while a large Stokes
shift is observed in polar solvents such as DMF (λmax,PL ) 575
nm), as shown in Figure 5c-e. Additionally, the intensities of
PL emission decrease with increasing solvent polarity, a strong
(19) (a) Li, Y.; Cao, Y.; Gao, J.; Wang, D.; Yu, G.; Heeger, A. J. Synth.
Met. 1999, 99, 243. (b) Gritzner, G.; Kuta, J. Pure Appl. Chem. 1984,
56, 462.
(20) Brunel, J.; Mongin, O.; Jutand, A.; Ledoux, I.; Zyss, J.; Blanchard-
Desce, M. Chem. Mater. 2003, 15, 4139.
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10950 J. AM. CHEM. SOC. VOL. 132, NO. 31, 2010

Star-Shaped D-A Compounds
A R T I C L E S
Figure 5. (a) Absorption spectra of TAMs in 1:1 MeCN:Bz. (b) Fluorescence spectra. (c) Absorption spectra of TAM2 in different solvents as indicated
in the figure. (d) Fluorescence spectra of TAM2 in different solvents as indicated in the figure. Absorption and emission spectra are normalized. (e) Picture
of TAM2 in different solvents with PL maxima in each solvent shown.
Table 2. Spectroscopic and ECL Results of TAM1-TAM4 in 1:1
MeCN:Bz
reaction involves the trication reacting with the monoanion. This
involves several different routes, such as
λmax,ECL
(nm)
Es
b
c
λAbs
λPL
(eV)a
∆H°ann
ΦPL
ΦECL
D3+• + D•- f D/ + D2+•
(14)
max
max
TAM1
TAM2
TAM3
TAM4
418
562
565
562
505
574
575
575
518
2.20
2.19
2.20
2.45
2.50
2.50
2.50
2.72
0.81
0.85
0.81
0.70
0.5
0.5
0.5
along with various comproportionation reactions, e.g.,
418
418
370
D3+• + D f D+• + D2+•
(15)
-
a Es ) 1239.85/λmax
.
b Compared to rubrene. c Relative to the ECL
PL
followed by radical annihilation. In work to be published
elsewhere, we have shown through digital simulation that such
behavior is expected when there are significant differences in
the concentrations of oxidizing and reducing equivalents. In this
case, the availability of a higher concentration of one of the
reactants diffusing away from the annihilation zone and back during
the following pulse will lead to a larger emission and a more
extended transient decay, in this case occurring during the cathodic
pulse when the trication formed upon oxidation reacts with the
anion.
of DPA taking ΦECL (DPA) as 1.
CCD, with a more sensitive photomultiplier tube (PMT) as a
detector, ECL light from the polymer could be recorded; we
did not attempt to obtain a spectrum with band-pass filters.
Figure 8 shows the profile of TAM2 ECL intensity versus
time for individual potential steps. The alternation of potential
was pulsing from Epa + 100 mV to Epc - 100 mV, which is
sufficient to oxidize and reduce TAM2. Unlike the usual profiles
from annihilation of stable radical monoions, where equal
emission is found on anodic and cathodic steps, the ECL
emission here shows much larger emission in the reduction steps
(Figure 8). This behavior was independent of the direction of
the first potential step. The difference here is that the annihilation
Conclusion
A series of highly fluorescent, star-shaped donor-acceptor
molecules with an electron-accepting triazine core bridged by
fluorene linked to electron-donating diarylamino and carbazolyl
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A R T I C L E S
Omer et al.
Figure 6. ECL and PL spectra for the 1 mM star-shaped molecules in 1:1 MeCN:Bz, 0.1 M Bu4NPF6 as a supporting electrolyte. TAM1-TAM3 are direct
annihilation ECL (Epc - 80 mV and Epa + 80 mV). TAM4 is coreactant ECL using 10 mM BPO. WE is J-type ∼ 2 mm platinum electrode, RE is Ag wire
as a quasireference electrode, CE is Pt coil.
Figure 8. i-t curve for TAM2, pulse time: 0.1 s, step pattern: Epa + 100
mV to Epc -100 mV vs SCE. ECL photocurrent (red line); electrochemical
current (black line). Anodic pulse is positive; cathodic pulse is negative.
TAM1-TAM3 show relatively strong ECL, while TAM4 only
shows weak ECL via the direct annihilation reaction. However
with BPO as a coreactant, bypassing the electrochemical
oxidation step, TAM4 produced strong ECL.
Figure 7. ECL spectrum of TAM4, by direct annihilation. Applied potential
pulsed between the reduction potential (Epc - 80 mV and Epa + 80 mV) of
the dimer formed on the oxidation side.
groups were synthesized and characterized. The donor-acceptor
configuration produces a strong photoinduced charge separation
in the excited state, and strong solvatochromism in the PL
spectra. The electrochemical oxidation of the triarylamino
terminus of TAM1-TAM3 gave rise to stable radical trications,
whereas a stable radical anion was formed upon the reduction
of the triazene core. Dimerization of the carbazole-substituted
TAM4 was observed during the electrochemical oxidation,
producing a polymer film on the electrode surface. The polymer
film (PTAM4) exhibits electrochromic behavior: pale orange
in the neutral state and dark green in oxidized state.
Acknowledgment. We thank Roche, Inc. and the Robert A. Welch
Foundation (F-0021) for support of this research. We also thank the
National Science Council of Taiwan for financial support.
Supporting Information Available: Text giving additional
synthesis information for TAM1-TAM4; figure showing the
numbering of the carbons in TAM1-TAM4. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA104160F
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10952 J. AM. CHEM. SOC. VOL. 132, NO. 31, 2010