Enhanced electrogenerated chemiluminescence of phenylethynylpyrene derivatives: Use of weakly electron-donating group as a substituent

Article abstract of DOI:10.1021/jo3010974

A weakly donating group (n-propyl) has been used as a substituent at the para-position of the phenyl group for a series of phenylethynylpyrene derivatives where the number of phenylethynyl peripheral arms appended to the pyrene core is varied. This system

Full text of DOI:10.1021/jo3010974

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Enhanced Electrogenerated Chemiluminescence of
Phenylethynylpyrene Derivatives: Use of Weakly Electron-Donating
Group as a Substituent
Yeon Ok Lee,†,⊥ Tuhin Pradhan,†,‡,⊥ Sangwook Yoo,§,⊥ Tae Hyun Kim,∥ Joohoon Kim,*,§
and Jong Seung Kim*,†
‡
†Department of Chemistry and Research Institute for Natural Sciences, Korea University, Seoul 136-701, Korea
§Department of Chemistry, Research Institute for Basic Sciences, Kyung Hee University, Seoul 130-701, Korea
∥Department of Chemistry, Soonchunghyang University, Asan 336-745, Korea
S
* Supporting Information
ABSTRACT: A weakly donating group (n-propyl) has been used
as a substituent at the para-position of the phenyl group for a
series of phenylethynylpyrene derivatives where the number of
phenylethynyl peripheral arms appended to the pyrene core is
varied. This system markedly improved the concurrent stability of
both cation and anion radicals and consequently greatly improved
electrogenerated chemiluminescence (ECL). Density functional
theory (DFT)-based theoretical calculations supported the
associated photophysical and electrochemical properties of the
series compounds.
challenging task especially for pyrene derivatives because
increasing number of electron-donating groups in a compound
not only enhances the cation radical stability but also reduces
the anion radical stability, as we have observed in our previous
work for tetrakis(ethynyl)pyrene series.9 Similarly, the electron-
withdrawing group enhances anion radical stability but also
reduces cation radical stability at the same time.9 Therefore, we
require a judicial choice for the substituent group in order to
achieve a balanced stability between cation and anion radicals
simultaneously.
In this study, we prepared a series of phenylethynylpyrene
derivatives using the n-propyl group as a substituent at the
para-position of the phenyl group (1−5, Scheme 1). In
comparison with our previous series,8a we replaced the
dimethylamine (−NMe2) group by the n-propyl group, which
is a weakly electron-donating group, as to avoid the reciprocal
relationship mentioned above in radical ions stability with a
strongly electron-donating group. We have also varied the
number of peripheral arms to verify and improve the stability of
radical ions. We then performed the photophysical studies for
the series to test the quantum efficiency and π-conjugated
network in the molecules, the electrochemical studies to
compare the radical stability, and finally the ECL studies. We
also compared the ECL properties of the present series with the
INTRODUCTION
■
Electrogenerated chemiluminescence (ECL), as a powerful
analytical technique, is widely used for many purposes, such as
immunoassays, DNA analyses, and molecular diagnosis of
clinically important compounds (e.g., steroidal hormones,
thyrotopin, digoxin, etc.), in combination with flow injection
analysis (FIA), high-performance liquid chromatography
(HPLC), capillary electrophoresis, and micro total analysis
(μTAS).1−4 ECL has also promising applications in OLEDs
(organic light emitting diodes),5 which are currently used in
long-lived and highly efficient color displays. ECL is the process
in which radical ions generated at electrodes undergo electron-
transfer reactions to form excited states that emit light.1 The
generated radical ions are required to be stable enough to form
emissive excited states and thus to exhibit a strong ECL because
decomposition of the radical ions causes one of the redox
processes to be chemically irreversible.6,7 There have been
efforts to enhance ECL efficiency by improving the radical
stability.6c,8 Bard et al. have improved the stability of radical
cations generated from fluorene compounds for ECL enhance-
ment by modifying the fluorene ring to 9,9′-spirobifluorene
derivatives.6c In our previous study, we have also improved the
radical cation stability of pyrene derivatives by increasing the
number of peripheral donors appended to pyrene core.8a In this
regard, we expect even greater ECL efficiency to be achieved if
both the cation and anion radicals are concurrently made
enough stable at the electrode surfaces. This is a very
Received: June 11, 2012
© XXXX American Chemical Society
A
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the product mixture was separated by recrystallization followed
by column chromatography.
Scheme 1. Chemical Structures of 1−6
Absorption and emission spectra of 1−5 in CH2Cl2 exhibit
bathochromic shifts as the number of peripheral arms in this
series increase, implying that effective extension of the
conjugation occurs with the addition of phenylethynyl moieties
(Figure 1a). The absorption spectra of 1−5 show a
resemblance to the spectrum of pyrene in terms of band
shape because there are no specific functional groups, which
can cause the different excitation mode. Likewise, the emission
spectra (Figure 1b) show a systematic bathochromic shift with
the conjugation length increase, that is, 1 < 2 = 3 < 4 < 5,
suggesting that the energy gap between the ground and excited
states decreases in this order. The emission spectra of 1−5 were
characterized by two defined vibrational structures observed in
a typical fluorescence spectrum of pyrene. A relatively small
Stokes shift of ∼10−20 nm was observed between the
absorption and emission maxima of all compounds (Table 1).
It reflects a small difference between the dipole moments in the
ground state and Franck−Condon (FC) excited state (or
locally excited (LE) state), suggesting a negligible charge
transfer (CT) character in the FC excited state. Furthermore,
all compounds are strongly fluorescent with high fluorescence
quantum yield, which is a fundamental requirement of
generating efficient ECL (Table 1.).
From the analysis of NMR spectra for 1, we observed that
the 2-H pyrene peak (ortho-position to the substituent) exhibits
a downfield shift from 8.0 to 8.8 ppm, which suggests
delocalization of electron density from the pyrene core to the
peripheral phenylethynyl moiety affecting the nearby proton
most (Figure 1c). This proton even shows a greater downfield
shift when the 1- and 3-positions of the pyrene core are
substituted by phenylethynyl groups (in 4 and 5, for example).
Additionally, the 2-H pyrene peak intensity greatly increased in
5 as 2-H and 7-H are equivalent protons in 5 (Figure 1c). The
delocalization of the electron density can be understood from
the frontier orbital profiles of the compounds 1−5 (Figure 3).
Both HOMO and LUMO are located in pyrene core as well as
in peripheral phenylethynyl groups. The delocalization of the
electron density increases with increasing the peripheral arms
(1→5). It is indicated that a highly conjugated network is
developed in 5. Furthermore, the single-crystal X-ray structure
(CCDC no. 881405) of 5 shows a nearly planar structure,
which proved the presence of highly π-conjugated network in
compound 5 (Figure S1, Supporting Information).
strongest ECL-active compound (6) in our previous series
reported.8a We finally presented the DFT (density functional
theory)-based theoretical calculations to achieve better under-
standing of the experimentally observed properties of the
present series.
RESULTS AND DISCUSSION
■
Pyrene derivatives (1−5) were synthesized by the Sonogashira
coupling reactions10 of bromopyrenes (7−11) with 4-
ethynylpropylbenzene as shown in Scheme 2. 1-Bromopyrene
a
Scheme 2. Preparation of Compounds 1−5
The electrochemical behavior of 1−5 and 6 was investigated
by cyclic voltammetry (CV) with Bu4NPF6 in CH2Cl2 (0.1 M)
on a Pt electrode, and the results show two distinctive trends as
summarized in Table 1 and Figure 2. First, the electrochemical
HOMO−LUMO band gap (ΔEgapelec) gradually decreases as
the number of substitution increases in the compounds 1−5, in
good agreement with energy gap trends obtained from the
lowest UV/vis absorption values and DFT calculations
a
opt
(ΔEgap and ΔEgapcalc, respectively, Table 1). Those results
Reagents and conditions: (a) PdCl2(PPh3)2, CuI, PPh3, Et3N,
toluene, 80 °C, 2 h.
suggest that more effective conjugation in 1−5 occurs by
increasing the number of peripheral arms. Second, the
reversibility of the reduction and oxidation curves improves
with increasing the number of the substituent group (1→5),
which indicates that the extension of conjugation via a C−C
triple bond alters the reversibility favorably. This is attributable
to the highly conjugated network of 5, giving an extraordinary
stability of both cation and anion radicals in oxidation and
reduction process, respectively. For 5, we also observed that the
(7), a mixture of 1,6- and 1,8-dibromopyrenes (8 and 9), 1,3,6-
tribromopyrene (10), and 1,3,6,8-tetrabromopyrene (11), were
prepared by the bromination of pyrene with 1−4 equiv of
bromine.11 Mono-, tris-, and tetrakis((4-propylphenyl)-
ethynyl)pyrene were prepared by the ethynylation reaction.12
To obtain 2 and 3, a mixture of 7 and 8 was reacted with A, and
B
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Figure 1. (a) UV/vis spectra of 1−5 in CH2Cl2. (b) Normalized fluorescence spectra of 1−5 in CH2Cl2. (c) 1H NMR (CDCl3, 300 MHz) spectra of
pyrene and 1−5.
Table 1. Photophysical, Electrochemical, and Theoretical
DATA of 1−5
compd
1
2
3
4
5
a
abs
λmax (nm)
387
398
550
713
152
0.78
417
434
588
938
154
0.99
419
434
582
825
152
0.82
445
464
607
920
147
0.89
472
489
612
737
123
0.87
em
λmax (nm)
ECL
λmax
(nm)
Stokes shift (cm−1
)
ECL
em
λmax
− λmax (nm)
b
Φ
Epc (V)
−2.045 −1.961 −1.916 −1.868 −1.733
Epa (V)
0.878
2.923
3.12
0.856
2.821
2.84
0.884
2.80
2.82
0.842
2.71
2.64
2.70
0.806
2.539
2.52
c
elec
ΔEgap (eV)
d
opt
ΔEgap (eV)
calc
ΔEgap (eV) (DFT)
3.27
2.93
2.92
b
2.50
Figure 2. (a) Cyclic voltammograms of 1−6 (0.5 mM) for a Pt
electrode with Bu4NPF6 in CH2Cl2 (0.1 M). Scan rate = 100 mV s−1.
(b) Comparison between first and 30th cycle of 6. (c) Comparison
between first and 30th cycle of 5.
a
Recorded at the longest wavelengths. Using 9,10-diphenyl
anthracene (for 1−5) in CH2Cl2. Φf = (9,10-diphenylanthracene)0.95
c
in EtOH.13 Electrochemical band gap calculated as the difference
d
between the two peak potentials. HOMO − LUMO gap calculated
from the oneset of the UV−vis absorption.
To get deeper insight into the radical stability, we have
performed DFT-based theoretical calculations using the
Gaussian 09 program package.14 Figure S2 (Supporting
Information) gives the spin density isosurfaces of singly
occupied molecular orbitals (SOMO) of cations and anions
for 1−5. In the case of cation/anion radicals, SOMOs
correspond to their HOMOs (Figures S2 and 3, Supporting
Information). As shown in Figure S2 (Supporting Informa-
tion), the net spin density in the SOMOs of 1-5 resides on
pyrene core and peripheral phenylethynyl moiety. This overall
electron spin density delocalization in the SOMO surfaces
supports the enhanced cation (anion) radical stability at the
molecular level, because electron-deficient (rich) cation (anion)
radical produced after oxidation (reduction) needs immediate
electron density delocalization to get thermodynamic stability
at the molecular level, which can be achieved from overall well
conjugated system. It can also be supported from calculated
electrostatic potential (ESP) density distribution. The electro-
static potential density surfaces of the neutral molecule
peak separations (ΔEpp) for the reversible reduction and
oxidation waves are ∼90 and ∼100 mV, respectively, which are
comparable to the peak separation of electrochemically
reversible ferrocene, known to show Nernstian behavior
under the same conditions. In comparison to 5, compound 6
has better peak reversibility for radical cation but worse peak
reversibility for the radical anion, which indicates that
dimethylamine (−NMe2) groups help to enhance the cation
radical stability but reduce anion radical stability at the same
time in 6 (Figures 2a).9 On the other hand, 5 has good peak
reversibility for both cation and anion radicals, which suggests
that the replacement of a strongly electron-donating group
(e.g., −NMe2) with a weakly electron-donating group (e.g., n-
propyl) in the peripheral arms helps to get better anion radical
stability in 5 (Figure 2a). That is, 5 is better than 6 from the
viewpoint of generating both stable cation and anion radicals
simultaneously.
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positive charge in 6 is more concentrated (deep blue) at the
end of the peripheral donors (−NMe2 groups) of cation
radicals than that in 5, which supports the better positive charge
distribution due to the presence of electron donating groups
(−-NMe2) at the end of the peripheral donors of 6 and hence
explains the better oxidation peak reversibility of 6 compared to
5 (Figure 2). The calculated non- adiabatic reduction (cation→
neutral) potential (NRP)8a,9 for the cation radicals of 1−5 are
respectively, −6.20, −5.90, −5.91, −5.72, and −5.57,
implicating that cations with more number of peripheral
groups are more stable than the cations with less number of
peripheral groups (1→5), which are consistent with the
analysis based on photophysical and electrochemical measure-
ments. NRP of 6 (−4.90 eV) is larger than NRP of 5 (−5.57
eV), which also predicts higher cation radical stability of 6 than
5. On the other hand, the calculated vertical detachment energy
(VDE)8a,9 (anion→neutral) of anion radicals 1-5 are
respectively, 0.75, 1.02, 1.11, 1.34, and 1.52 eV. This implies
that anion radical stability increases on going from 1 to 5, which
is also consistent with our experimental observations. It is
notable that VDE of 5 is larger than 6 (1.12 eV), which predicts
better anion radical stability of 5 than 6. These results have
verified that tetrakis could give a good platform to ensure stable
electrochemical properties. In addition, these results clearly
indicate that 5 is better than 6 with respect to simultaneous
radical ion stability.
The ECL spectra were recorded in dichloromethane at 0.5
mM concentration with 0.1 M Bu4NPF6 as supporting
electrolyte. Compound 5 shows very strong ECL emission,
whereas 1−4 show very weak ECL emission (Figure 5). As
Figure 3. HOMO (left) and LUMO (right) orbital surfaces of 1−5.
Green and red correspond to the different phases of the molecular
wave functions for the HOMOs and LUMOs, and the isovalue is 0.02
au.
(center), cation (left), anion (right) of 5 and 6 have been
shown in Figure 4 (For 1−4, ESP density surfaces have been
Figure 5. ECL spectra of 0.5 mM 1−6 in 0.1 M TBAPF6 in CH2Cl2
with pulsing (10 Hz) between peak potentials for reduction and
oxidation of compounds, respectively. Inset: ECL spectra of 1−3.
shown in the spectrum of 5, there are defined emission peaks
around 490 and 610 nm. The weak ECL peaks of 5 around at
490 nm are similar to its fluorescence spectra that are mainly
from the excited monomer. Pyrene derivatives are well-known
to exhibit excimer emission due to its longer lifetime of the
excited states.15 Being the pyrene derivatives, 1−5 have a large
possibility to form excimers when they are annihilated at the
electrode surfaces during electrochemical events.1b,16 A very
strong and broad emission band (bandwidth ∼200 nm) appears
in ECL of 5 at 610 nm (∼120 nm red-shifted from monomer
emission). We attributed this ECL emission to the emission
from the normal excimer formed by annihilation of radical ions
of 5 generated electrochemically. Interestingly, the ECL
intensities of 1−5 enhance remarkably as the number of
Figure 4. Top: Calculated electrostatic potential (ESP) density of 5
(middle) after adding one electron to 5 (left) and removing one
electron from 5 (right). Bottom: Calculated electrostatic potential
(ESP) density of 6 (middle) after adding one electron to 6 (left) and
removing one electron from 6 (right). The more blue and more red,
respectively, indicate more positive and more negative electrostatic
potentials. Isovalue of the isosurfaces is 0.0004 au.
shown in Figure S3, Supporting Information). In Figure 4, we
observed that positive charge (blue) of cation radicals as well as
negative charge (red) of anion radicals reside mostly on pyrene
core and phenylethynyl arms, indicating that phenylethynyl
arms along with pyrene core are responsible for both cation and
anion radical stability of 1−5. It is important to note that
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substitution increases. Although 4 and 5 have similar quantum
yields, 5 shows ∼16 times enhanced ECL intensity compared to
4. This is due to the enhanced radical stability of 5 by highly
conjugated network, which is supported by photophysical and
electrochemical data. The compound 5 shows even stronger
ECL intensity (∼5 times) than 6 under the same experimental
conditions (Figure 5). This may be mainly due to the following
reasons. First, concurrent stability of both radical ions (cation
and anion) of 5 provides better ECL intensity since the chances
of fruitful annihilations for formation of emissive excited states
between cation and anion radicals are increased. Second, the
formal potential difference (ΔE0) of 5 is larger than that of 6
(2.51 and 2.13 V, respectively), which provides comparatively
greater driving force in the annihilation reaction for sufficient
formation of emissive singlet excited states at a given time
frame1d,17 and consequently increases ECL efficiency. This
becomes more significant when both the ionic radicals are
stable. Third, upon continuous and repeated cycling (30 times),
the reduction wave of 6 became irreversible at a fixed scan rate
(Figure 2b) and a red thin film was formed on the electrode
surface during the cycling. However, the reduction curves of 5
remained reversible even after repeated cycling (30 times),
indicating a long-lived stable radical anion (Figure 2c).
EXPERIMENTAL SECTION
■
S y n t h e s i s o f P h e n y l e t h y n y l p y r e n e s . 1 - ( 4 -
Propylphenylethynyl)pyrene (1). Bromopyrene (100 mg, 0.357
mmol), PdCl2(PPh3)2 (24 mg, 0.036 mmol), CuI (6.7 mg, 0.036
mmol), PPh3 (9.3 mg, 0.036 mmol), and 1-ethynyl-4-propylbenzene
(103 mg, 0.714 mmol) were added to a degassed solution of
triethylamine (10 mL) and toluene (50 mL) under Ar. After the
mixture was stirred at 80 °C for 5 h, the product was poured into
CH2Cl2 (200 mL) and water (200 mL). The organic layer was
separated and dried over anhydrous MgSO4, and then the solvent was
removed in vacuo. Column chromatography using silica gel with
hexane only gave 41 mg (34%) of brown oil. Mp: 82−88 °C. IR (KBr
pellet, cm−1): 2200 (CC), 1594, 1511, 1H NMR (300 MHz,
CDCl3): δ 8.68 (d, J = 9.1 Hz, 1H), 8.23−8.00 (m, 7H), 7.66 (d, J =
7.7 Hz, 2H), 7.26 (d, J = 7.9 Hz, 2H), 2.67 (t, J = 7.3 Hz, 2H), 1.54
(m, 2H), 0.98 (t, J = 7.3 Hz, 3H). 13C NMR (100 MHz, CDCl3): δ
143.6, 132.1, 131.8, 131.5, 131.3, 129.7, 128.9, 128.47, 128.27, 127.5,
126.4, 125.8, 125.8, 125.6, 124.8, 125.6, 95.6, 88.2, 38.3, 24.7, 14.0.
HRMS (FAB-DFMS) m/z: [M]+ calcd for C27H20 344.1565, found
344.1566.
1,6-Bis(4-propylphenylethynyl)pyrene (2) and 1,8-Bis(4-
propylphenylethynyl)pyrene (3). A mixture of 7 and 8 (500 mg,
1.40 mmol), PdCl2(PPh3)2 (98 mg, 0.140 mmol), CuI (6.7 mg, 0.140
mmol), PPh3 (9.3 mg, 0.140 mmol), and 1-ethynyl-4- propylbenzene
(503 mg, 3.49 mmol) was added to a gassed solution of triethylamine
(50 mL) and toluene (200 mL) under Ar. After being stirred at 80 °C
for 3 h, the reaction mixture was poured into CH2Cl2 (200 mL) and
water (200 mL). The organic layer was separated and dried over
anhydrous MgSO4, and the solvent was removed in vacuo. A 200 mL
solution of toluene was added, and the yellow solid was precipitated
from solution. Filtration of the solid gave 352 mg (52%) of 2 as a
yellow solid. The filtrate was evaporated in vacuo, and the crude
product was then subjected to column chromatograph (silica gel) with
hexane only to give 204 mg (30%) of 3 as a yellow solid.
CONCLUSIONS
■
In conclusion, a series of phenylethynylpyrene derivatives has
been synthesized by varying phenylethynyl peripheral arms
appended to the pyrene core, where a weakly electron-donating
group (n-propyl) has been used as a substituent at the para-
position of the phenyl group (1−5). The photophysical studies
showed that 1−5 have large quantum yields and π-conjugated
network increases with the number of peripheral arms
appended to the pyrene core. The crystal structure of 5
indicated that 5 has nearly planar structure, which strengthened
the notion of a well-conjugated π-network in 5. As evidenced
from the electrochemical studies, the stability of the radical ions
produced electrochemically at the electrode surface enhances
with the number of peripheral arms appended to the pyrene
core, and both cation and anion radicals of 5 are very stable. In
comparison to 6 reported previously,8a 5 is much better than 6
in respect of concurrent cation and anion radical stability. The
replacement of strongly electron-donating group (−NMe2) by
a weakly electron-donating group (n-propyl) reduces the
instability of radical anions and simultaneously balances the
cation radical stability of 5. As a result, 5 exhibited
extraordinary ECL enhancement, which is ∼16 times compared
to 4 and ∼5 times compared to 6. DFT-based theoretical
calculations, such as frontier molecular orbital (MO) energy
and surfaces, spin density distribution (SDD) of cation/anion
radicals, electrostatic potential (ESP) density distribution,
nonadiabatic reduction potentials (NRP) for cation radicals,
and vertical detachment energy (VDE) for anion radicals,
supported the experimental observations. Our results will be
useful for designing new efficient ECL materials by improving
the concurrent stability of both cation and anion radicals. As 5
simultaneously provides monomer and excimer emissions, it
can be used as single-dopant emitter in “small molecule”
OLEDs to produce white color light by mixing a range of blue-
green (monomer peak at ∼490 nm) and yellow-orange
(excimer peak at ∼610 nm) wavelength lights.18 This approach
may simplify the architecture of OLED devices.
Compound 2. Mp: 98−104 °C. IR (KBr pellet, cm−1): 2200 (C
C), 1593, 1513. 1H NMR (300 MHz, CD2Cl2): δ 8.72 (d, J = 9.1 Hz,
2H), 8.24−8.20 (m, 6H), 7.66 (d, J = 8.2 Hz, 4H), 7.29 (d, J = 8.1 Hz,
4H), 2.68 (t, J = 7.6 Hz, 4H), 1.71 (m, 4H), 0.99 (t, J = 7.3 Hz, 6H).
13C NMR (100 MHz, CDCl3): δ 143.3, 131.8, 131.5, 130.9, 129.7,
128.5, 127.9, 126.1, 124.9, 124.1, 120.5, 118.6, 95.7, 87.8, 37.9, 24.3,
13.7. HRMS (FAB-DFMS) m/z: [M]+ calcd for C38H30 486.2348,
found 486.2348.
Compound 3. Mp: 98−104 °C. IR (KBr pellet, cm−1): 2200 (C
1
C), 1597, 1511. H NMR (300 MHz, CD2Cl2): δ 8.80 (s, 2H), 8.22
(d, J = 9.8 Hz, 2H), 8.12 (s, 2H), 7.68 (d, J = 8.4 Hz, 4H), 7.29 (d, J =
7.9 Hz, 4H), 2.68 (t, J = 7.62 Hz, 4H), 1.71 (m, 4H), 0.99 (t, J = 7.3
Hz, 6H). 13C NMR (100 MHz, CDCl3): δ 143.7, 131.9, 131.9, 131.4,
130.0, 128.6, 128.1, 126.6, 125.2, 124.4, 120.9, 118.1, 96.1, 88.2, 38.3,
24.6, 14.1. HRMS (FAB-DFMS) m/z: [M]+ calcd for C38H30
486.2348, found 486.2347.
1,3,6-Tris(4-propylphenylethynyl)pyrene (4). 1,3,6-Tribromopyr-
ene (100 mg, 0.229 mmol), PdCl2 (PPh3)2 (16 mg, 0.022 mmol), CuI
(5.5 mg, 0.022 mmol), PPh3 (6.0 mg, 0.022 mmol), and 1-ethynyl-4-
propylbenzene (165 mg, 1.15 mmol) were added to a degassed
solution of triethylamine (10 mL) and toluene (50 mL) under Ar. The
resulting mixture was stirred at 80 °C for 3 h. The solvent was
removed under vacuum to give 3. The crude product was then
subjected to column chromatography (silica gel, hexane only) to yield
3 (52 mg, 36%) as a yellow powder. Mp: 150−190 °C. IR (KBr pellet,
1
cm−1): 2202 (CC), 1596, 1513. H NMR (300 MHz, CD2Cl2): δ
8.75 (s, 2H), 8.69 (d, J = 9.1 Hz, 1H), 8.23−8.14 (m, 3H), 7.68 (m,
6H), 7.25 (d, J = 8.0 Hz, 6H), 2.68 (t, J = 7.57 Hz, 6H), 1.71−1.64 (m,
6H), 1.70 (m, 4H), 0.98 (t, J = 7.8 Hz, 9H). 13C NMR (100 MHz,
CDCl3): δ 143.5, 133.2, 131.8, 131.7, 131.5, 131.1, 130.0, 128.7, 128.5,
126.1, 125.5, 124.2, 123.9, 120.5, 124.45, 120.43, 119.2, 118.51, 96.17,
95.95, 95.82, 87.9, 87.2, 38.0, 24.4, 13.8. HRMS (FAB-DFMS) m/z:
[M]+ calcd for C49H40 628.3130, found 628.3130.
1,3,6,8-Tetrakis(4-propylphenylethynyl)pyrene (5). 1,3,6,8-Tetra-
bromopyrene (200 mg, 0.389 mmol), PdCl2(PPh3)2 (27 mg, 0.039
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mmol), CuI (7.0 mg, 0.039 mmol), PPh3 (10.0 mg, 0.039 mmol), and
1-ethynyl-4-propylbenzene (280 mg, 1.95 mmol) were added to a
degassed solution of triethylamine (20 mL) and toluene (80 mL)
under Ar. The organic layer was separated and dried over anhydrous
MgSO4, and the solvent was removed in vacuo. A 200 mL solution of
dichloromethane was added, and an orange solid was precipitated from
solution. Filtration of the solid gave 157 mg (68%) of 5 as an orange
solid. Mp: 256−276 °C. IR (KBr pellet, cm−1): 2199 (CC), 1594,
1511. 1H NMR (300 MHz, CD2Cl2): δ 8.78 (s, 4H), 8.43 (s, 2H), 7.67
(d, J = 7.7 Hz, 8H), 7.30 (d, J = 8.0 Hz, 8H), 2.68 (t, J = 7.55 Hz, 8H),
1.77−1.65 (m, 8H), 0.98 (t, J = 7.35 Hz, 12H). 13C NMR (100 MHz,
CDCl3): δ 143.9, 133.8, 131.91, 131.87, 128.9, 127.0, 124.3, 120.6,
119.3, 96.5, 87.4, 38.3, 24.6, 14.1. HRMS (FAB-DFMS) m/z: [M]+
calcd for C60H50 770.3913, found 770.3915.
Spectroscopic Measurements. Absorption spectra were recorded
on a diode array spectrophotometer with 20 μM solution of the 1−6
in CH2Cl2, and photoluminescence spectra were obtained with
fluorescence spectrometer with a 1 cm standard quartz cell using 3
μM solution conditions in various solvents, respectively. The
fluorescence quantum yields were determined by using Rhodamine
6G as the reference by the literature method.19
Electrochemistry and Electrogenerated Chemiluminescence
Measurements. All electrochemical measurements were carried out
using an electrochemical analyzer. A Pt disk (2 mm diameter) working
electrode was polished with 0.3 μm alumina powder on a polishing pad
and then rinsed with deionized water. Residual alumina particles were
thoroughly removed by sonicating the working electrode successively
in deionized water and absolute ethanol. Then, the working electrode
was blown dry with a N2 stream. A Pt wire and an Ag wire were used
as a counter and a quasi-reference electrode, respectively. All potentials
were calibrated by the addition of ferrocene as an internal standard
using E°(Fc+/Fc) = 70 mV vs Ag/Ag+. ECL spectra were obtained
using a monochromator and a charge-coupled device camera. A two-
step potential of 10 Hz was alternatively applied with the cathodic and
anodic peak potentials of the each compound during 60 s to generate
ECL via radical ion annihilation. Dichloromethane solutions for ECL
measurement contained 0.5 mM alkynylpyrenes at respectively and 0.1
M TBAPF6 as an electrolyte.
Computational Details. The geometries of 1−5 and the geometries
of their corresponding cation/anion radical species were optimized at
B3LYP/6-31G(d) level of theory. No imaginary frequencies were
available after vibration analysis of the optimized structures, which
indicates that each of the optimized structures is at the real minimum
on the potential energy surfaces (PES). The frontier molecular orbital
(MO) energy and orbital surfaces, spin density distribution (SDD)
surfaces, electrostatic potential (ESP) density surfaces, nonadiabatic
reduction potential (NRP), and vertical electron detachment energy
(VDE) were also calculated at the same level of theory using optimized
geometries. The nonadiabatic reduction potential (NRP) for cation
radicals and vertical electron detachment energy (VDE) for anion
radicals were calculated by using the following relationships8,9
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jongskim@korea.ac.kr; jkim94@khu.ac.kr.
Author Contributions
⊥These authors contributed equally to this research.
ACKNOWLEDGMENTS
■
This work was supported by the CRI program (no.
20120000243) (J.S.K.), the Agency for Defence Development
through Chemical and Biological Defence Research Center
(J.K.), and the Basic Research Program (no. 2011-0027269
(J.K.) and no. 2010-0005574 (T.H.K.)) of the National
Research Foundation of Korea. T.P. acknowledges Priority
Research Centers Program through the NRF funded by the
Ministry of Education, Science and Technology
(NRF20120005860).
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