Synthesis and photophysical properties of novel butterfly-shaped blue emitters based on pyrene

Article abstract of DOI:10.1039/c3ob41350k

Using 1,3,5,9-tetrabromo-7-tert-butylpyrene as the bromide precursor, a series of novel butterfly-shaped 1,3,5,9-tetraaryl substituted pyrene derivatives were synthesized by the Suzuki-Miyaura cross-coupling reaction. Their thermal, photophysical, electrochemical and related properties were systematically investigated. All compounds were found to exhibit high thermal stabilities with decomposition temperatures (T_d) of up to 300 °C. All compounds show highly blue fluorescence emissions in the spectral region of 412-469 nm in solution (Φ_f: 0.45-0.92) and 410-470 nm in the solid- state (Φ_f: 0.48-0.75). It is noteworthy that these butterfly-shaped pyrenes 4 possess low-lying HOMO levels ranging from -4.76 to -5.93 eV, which make them promising candidates in OLED applications.

Full text of DOI:10.1039/c3ob41350k

Organic &
Biomolecular Chemistry
View Article Online
View Journal | View Issue
PAPER
Synthesis and photophysical properties of novel
butterfly-shaped blue emitters based on pyrene†
Cite this: Org. Biomol. Chem., 2013, 11,
8366
Xing Feng,a Jian-Yong Hu,*a,b Hirotsugu Tomiyasu,a Nobuyuki Seto,a Carl Redshaw,c
Mark R. J. Elsegoodd and Takehiko Yamato*a
Using 1,3,5,9-tetrabromo-7-tert-butylpyrene as the bromide precursor, a series of novel butterfly-shaped
1,3,5,9-tetraaryl substituted pyrene derivatives were synthesized by the Suzuki–Miyaura cross-coupling
reaction. Their thermal, photophysical, electrochemical and related properties were systematically investi-
gated. All compounds were found to exhibit high thermal stabilities with decomposition temperatures
(Td) of up to 300 °C. All compounds show highly blue fluorescence emissions in the spectral region of
412–469 nm in solution (Φf: 0.45–0.92) and 410–470 nm in the solid- state (Φf: 0.48–0.75). It is note-
worthy that these butterfly-shaped pyrenes 4 possess low-lying HOMO levels ranging from −4.76 to
−5.93 eV, which make them promising candidates in OLED applications.
Received 2nd July 2013,
Accepted 10th October 2013
DOI: 10.1039/c3ob41350k
www.rsc.org/obc
utilization as fluorescence probes. Recently, a number of syn-
thetic methods for the synthesis of novel PAHs based on
Introduction
The design and synthesis of efficient organic semiconductors pyrene have been reported,5 and their potential uses as chemo-
based on polycyclic aromatic hydrocarbons (PAHs) has sensors were exploited.6
attracted considerable interest, owing to their potential appli-
However, the flat structure of pyrene has restricted its use
cations in optoelectronic devices,1 such as organic light-emit- for developing EL materials for OLED applications, primarily
ting diodes (OLEDs), organic photovoltaics (OPVs), organic because of the tendency towards π-stacking and excimer for-
field-effect transistors (OFETs), etc. In the past two decades, mation, which would quench the emission intensity with low
significant efforts have been devoted to exploring novel electro- fluorescence quantum efficiency. One strategy to suppress the
luminescent (EL) materials with specific optoelectronic pro- passive π–π stacking interactions in condensed media is to
perties.1,2 To realize full-color OLED displays the development introduce appropriate substituents into the pyrene core. The
of efficient and pure red, green and blue emitters is a signifi- methods tend to attach substituents at the 1-,3-,6- and 8-posi-
cant milestone. However, only the red and green materials tions of the pyrene so as to affect both the overall photo-
have been implemented in devices capable of sufficient physical properties and the geometrical structures. For
efficiencies and lifetimes for commercial value,3 whilst the example, 1,3,6,8-tetraphenylpyrene (TPPy) exhibited a pure
availability of blue emitters with satisfactory multifunctional blue fluorescence with a high quantum yield (Φf = 0.90 in
properties for high-performance OLEDs still remains
challenge.
a
cyclohexane).7 Bulky aryl groups have been located at the
1,3,6,8-positions for the preparation of tetraaryl-functionalized
Pyrenes4 as classical members of the PAHs have attracted pyrenes which showed high efficiencies with deep-blue emis-
great interest from the scientific community, not only sion, low turn-on voltages and high brightness in OLED
because of their planar structure, but also because of their devices.8 Sotoyam and co-workers have investigated different
excellent fluorescence properties, which has led to their wide phenyl moieties of 1,3,6,8-tetrasubstituted pyrene and how
they affect the fluorescent properties by molecular orbital
(MOs) methods.9 The tert-butyl groups located at the 2- or the
aDepartment of Applied Chemistry, Faculty of Science and Engineering, Saga
2- and 7-positions of the pyrene core play an important role in
University, Honjo-machi 1, Saga 840-8502, Japan. E-mail: yamatot@cc.saga-u.ac.jp
inhibiting the undesirable π-stacking interactions in the solid-
state. Thus, the cruciform-, hand-, and Y-shaped pyrene deriva-
tives were reported and can be potentially used as blue emit-
ters in OLEDs.10
Previously, a novel bromide precursor 1,3,5,9-tetrabromo-
7-tert-butylpyrene was prepared from 2-tert-butylpyrene and
Br2 in CH2Cl2 in the presence of iron powder (6 equiv.) at
bEmergent Molecular Function Research Group, RIKEN Center for Emergent Matter
Science (CEMS), Wako, Saitama 351-0198, Japan. E-mail: jian-yong.hu@riken.jp
cDepartment of Chemistry, The University of Hull, Cottingham Road, Hull, Yorkshire
HU6 7RX, UK
dChemistry Department, Loughborough University, Loughborough, LE11 3TU, UK
†Electronic supplementary information (ESI) available: 1H/13C NMR spectra of
4a–f, 5 and 6, the effect of different solvents on the fluorescence emission
spectra of 4 and 5, etc. See DOI: 10.1039/c3ob41350k
8366 | Org. Biomol. Chem., 2013, 11, 8366–8374
This journal is © The Royal Society of Chemistry 2013

View Article Online
Organic & Biomolecular Chemistry
Paper
room temperature.11 Based on this key intermediate, herein,
we access a series of butterfly-shaped compounds of 1,3,5,9-
tetraarylpyrenes with deep-blue fluorescence properties.
Several 4-substituted phenyl groups were successfully intro-
duced into the pyrene core at the 1,3,5,9-positions. Not only
does this category of materials have great potential in organic
materials, such as blue emitters for OLED applications, but
will also be useful to understand the structure–property
relationships of the current butterfly-shaped 1,3,5,9-tetraaryl-
pyrenes relative to the other arylpyrenes.
Results and discussion
Synthesis
The synthetic routes to the butterfly-shaped pyrene derivatives
4a–f are illustrated in Scheme 1. By using the 1,3,5,9-tetra-
bromo-7-tert-butylpyrene (2)11 as the key intermediate, the
compounds of type 4 were synthesized by the Suzuki–Miyaura
cross-coupling reaction with the corresponding arylboronic
acids (3) in 48–72% yield. To investigate the effect of a term-
inal moiety on the structure–property relationships in these
butterfly-shaped pyrene systems, the reaction of 4f with 17.0
equiv. of hydroxylamine hydrochloride in ethanol solution
afforded the corresponding oxime 5 in 76% yield according to
the literature approach.12 Furthermore, in order to investigate
the effect of the tert-butyl group of the pyrene on photophysical
properties, 6 was synthesized from 4a, in which the bulky steric
tert-butyl group has been removed by Nafion-H catalysis.13 The
chemical structures of all synthesized compounds were
confirmed by their 1H/13C NMR spectra, FT-IR spectroscopy,
mass spectrometry, as well as elemental analysis.
Fig. 1 TGA curves of compounds 4 and 5.
scanning calorimetry (DSC) under a nitrogen atmosphere at a
heating rate of 10 °C min−1. The TGA data revealed that the
butterfly-shaped compounds 4 showed high thermal stabilities
in the range of 250–450 °C, with a 5% weight loss (Fig. 1), and
the following stability order CN > OMe > t-Bu > H > CHO > CF3
> 5. From the TGA data, the amount of carbonized residue
(char yield) of 4c, 4e, 4f and 5 under a nitrogen atmosphere
was higher (20% for 4c, 4f, 37% for 4e and 64% for 5) than 4a,
4b and 4d with almost complete loss, which means that 4c, 4e,
4f and 5 possess more aromatic components than 4a, 4b and
4d,14 due to the substituent group at the para-position of the
phenyl would affect the thermal properties. No glass transition
temperature (Tg) was observed for any of the compounds.
Thermal properties
Photophysical properties
Thermal stability is an important physical parameter to evalu-
ate EL materials for OLED applications. We investigated the
thermal stability of these butterfly-shaped pyrene derivatives
4 and 5 by thermogravimetric analysis (TGA) and differential
We measured the UV-vis absorption and fluorescence (FL)
spectra of compounds 4–6 in both dichloromethane solution
(∼10−5 M) and thin films. The key data are summarized in
Table 1.
Fig. 2 illustrates the normalized absorption and FL spectra
of 4, 5 and 6 in dilute dichloromethane solutions. The absorp-
tion spectra of these butterfly-shaped pyrene derivatives 4 and
6 exhibited well-resolved profiles in the range of 280–480 nm.
The short wavelength absorption peak located at 277–279 nm
and the long wavelength maximum (λmax
)
occur at
370–381 nm, assigned to the π–π* and the n–π* transitions
(Fig. 2a), respectively. Noticeably, 4f and 5 showed slight red
shifts of ∼10 nm compared to the other compounds, due to
the effective increase in the conjugation length. It is note-
worthy that the compound 6 exhibited a blue shift (8 nm) after
the removal of the tert-butyl group from the 2-position of the
pyrene core compared to 4c, arising from the improved ener-
gies of both S1 ← S0 excitations and S2 ← S0 excitations.15
Upon excitation at the absorption maximum in CH2Cl2, all
compounds exhibited deep-blue to sky-blue FL with sharp
emission maximum peaks at 412 nm for 4a, 416 nm for 4b,
421 nm for 4c, 418 nm for 4d, 437 nm for 4e, 440 nm for 5 and
Scheme 1 Synthetic route to the compounds 4,
5 and 6. Reagents and
conditions: (a) iron powder, Br2, CH2Cl2, room temp., 4 h, 84%. (b) Pd(PPh3)4,
toluene, K2CO3, 90 °C, 24 h. (c) NH2OH·HCl, ethanol, NaOH, reflux, 12 h. (d)
Nafion-H, o-xylene, 160 °C, 24 h.
This journal is © The Royal Society of Chemistry 2013
Org. Biomol. Chem., 2013, 11, 8366–8374 | 8367

View Article Online
Paper
Organic & Biomolecular Chemistry
Table 1 The photophysical and electrochemical properties of compounds 4, 5 and 6
λmax abs (nm)
λmax PL (nm)
Φf c solns/thin
films
Stokes shift (cm−1
)
Compound
solnsa/filmsb
solnsa/filmsb
solnsa/filmsb
τ (ns)
Tm d/Td e (°C)
4a
4b
4c
4d
4e
4f
5
373/380
378/379
379/369
370/358
378/387
386/400
381/nd
371/nd
412/410
416/435
421/443
418/466
437/460
469/471
440/nd
411/nd
0.92/0.75
0.45/nd
0.90/0.72
0.91/nd
0.89/nd
0.56/0.48
0.63/nd
0.26 /nd
39/30
38/56
42/74
48/108
59/73
83/71
59/nd
40/nd
4.57
5.18
5.82
12.2
7.51
4.04
4.53
5.10
335/350
332/407
330/410
258/328
378/450
302/329
214/250
244/nd
6
a Maximum absorption wavelength measured in dichloromethane at room temperature. b Measured as thin neat films. c Measured in
dichloromethane and as neat thin films, respectively. d Melting temperature (Tm) obtained from differential scanning calorimetry (DSC)
measurements. e Decomposition temperature (Td) obtained from thermogravimetric analysis (TGA). nd: not determined.
changing the terminal group at the para-position of the phenyl
moiety can affect the electron transition absorption, which
also agrees with our previous report.10a Furthermore, a substi-
tuent effect involving S1 ← S0 was also observed in the emis-
sion spectra, and without the tert-butyl group in compound 6,
the emission maximum displayed a lower red shift of ∼10 nm
compared to the corresponding compound 4c.
We also investigated the photophysical properties of 4 in
the solid-state. The UV-vis absorption and photoluminescence
(PL) spectra data are also listed in Table 1. As shown in Fig. 3,
the absorption spectra of these butterfly-shaped pyrenes 4,
possessing similar frameworks (pyrene-based core), exhibited
a similar absorption band with a slight red shift (∼14 nm)
to those in dilute solutions. Compared with the FL,
Fig. 2 (a) Normalized UV-vis absorption and (b) fluorescence emission spectra
of compounds 4 and 5 recorded in dichloromethane solutions at ∼10−5–10−6
M
at 25 °C.
411 nm for 6 (Fig. 2b). In the case of 4f, which showed a
maximum peak at 469 nm, there was a larger Stokes shift of
83 nm relative to the other butterfly-shaped compounds,
suggesting that its electronic structure in the ground state is
significantly different from that in the excited state. It is noted
that 4e and 5 showed almost the same emission peaks,
suggesting that the –CuN and –CvN–OH groups make
similar electronic contributions. This result implies that
Fig. 3 (a) Normalized UV-vis absorption and (b) emission spectra of 4a–f as
thin neat films.
8368 | Org. Biomol. Chem., 2013, 11, 8366–8374
This journal is © The Royal Society of Chemistry 2013

View Article Online
Organic & Biomolecular Chemistry
Paper
the PL spectra of 4a–f and 5 in thin films exhibited intense blue
emissions with a maximum peak in the range of 410–471 nm,
without extra excimer emissions, while compounds 4b–e dis-
played a red-shift (∼48 nm), ascribed to the tert-butyl group
located at the 7-position and the tetraaryl moieties at the
1,3,5,9-positions which can prevent π-aggregations via the
intra/intermolecular interactions in the solid-state. The single-
crystal packing revealed that no π–π stacking was observed.11
However, the aggregations cannot be sufficiently suppressed16
due to the small blocking group of the arylphenyl located at
the 1,3,5,9-positions. Thus, larger red-shifts occurred in 4c and
4d. Fig. 2b and 3b revealed that these butterfly-shaped com-
pounds 4 and 5 emit very bright and sharp blue fluorescence
in both the solution and the solid-state. The newly developed
butterfly-shaped pyrenes 4a–e and 5 have very high quantum
yields (Φf) in the range of 0.89–0.92 in solution and 0.72–0.78
in the solid-state. The presence of –CHO in 4f led to exhibiting
lower quantum yields of 0.56 in solution and 0.48 in the solid-
state due to contributions from other non-emissive deactivation
pathways. In contrast 6 exhibited a lower quantum yield of 0.26
with the TPPy (Φf = 0.90),7 which is probably due to the strong
intermolecular interactions possible without the bulky steric
tert-butyl group. Fluorescence lifetimes of 4–6 were determined
in dichloromethane solution at 10−5 M with τ = 4.04–12.2 ns.
Concentration-dependent UV-vis absorption and FL spectra
studies in CH2Cl2 confirmed that these compounds did not
change with concentration in solution, even on increasing the
concentrations from 10−6 M to 10−3 M (See ESI†). On the other
hand, we examined the solvatochromism of 4 and 5 in various
solvents, such as cyclohexane, tetrahydrofuran (THF), CH2Cl2,
CH3CN, and N,N-dimethylformamide (DMF) (Fig. 4a). In the
compounds 4a–e and 5, the emission maxima were slightly
shifted by ∼5 nm depending on the solvent polarity. However,
although the absorption spectrum of compound 4f does not
show any change, the solvent dependence in the emission
spectra of 4f is remarkable with large bathochromic shifts of
∼36 nm on going from non-polar solvents (cyclohexane) to
polar solvents (DMF). The linear relationship of stoke shift
(Δνst) against the solvent parameter Δf (ε,n)18 was determined
by the Lippert–Mataga plot (Fig. 4b).19 This solvatochromism
can be attributed to the decrease in the energy of the singlet-
excited states as a function of an increase in the polarity of the
solvents.20
Fig. 4 (a) Emission spectra of 4f in cyclohexane, THF, CH2Cl2, CH3CN and DMF
at 25 °C. (b) Lippert–Mataga plots for compound 4c (CHO): (a) cyclohexane, (b)
CH2Cl2, (c) THF, (d) DMF, and (e) CH3CN.
where ε is the static dielectric constant and n is the refractive
index of the solvent.
Electrochemical properties
We investigated the electrochemical properties of the com-
pounds 4 and 5 by cyclic voltammetry (CV) recorded in CH2Cl2
solution using ferrocene (Fc/Fc+) as the internal standard with
a scan rate of 100 mV s−1 at room temperature. Fig. 5 and S4†
1
2Δμ2
Δv ¼
Δ f þ const
ð1Þ
4πε0hca3 hcR3
where Δv = vabs − vem stands for Stokes shift, vabs is the wave-
number of the maximum absorption, vem is the wavenumber of
the maximum emission, Δμ = μe − μg is the difference in the
dipole moment of solute molecule between excited (μe) and
ground (μg) states, h is Planck’s constant, R is the radius of the
solvent cavity in which the fluorophore resides (Onsager cavity
radius), and Δf is the orientation polarizability given by (eqn (2))
ε ꢀ 1
n2 ꢀ 1
Δ f ¼
ꢀ
ð2Þ
2ε þ 1 2n2 þ 1
Fig. 5 Cyclic voltammograms recorded for compounds 4.
Org. Biomol. Chem., 2013, 11, 8366–8374 | 8369
This journal is © The Royal Society of Chemistry 2013

View Article Online
Paper
Organic & Biomolecular Chemistry
display the cyclic voltammograms (CVs) of 4 and 5. All com- group in 6, the energy gap resembles that of 4a, but with a
pounds exhibited quasi-reversible oxidations in CH2Cl2 except lower LUMO energy level.
5. Interestingly, compounds 4b and 4c, with electron-donating
groups, showed two reversible oxidation waves, while com-
pounds 4d–f, with electron-withdrawing groups, have only one
Conclusion
reversible oxidation each. These results differ from the Y-shaped
compounds that we recently reported.10c Compound 5 showed
In summary, in this study, a series of novel butterfly-shaped
unclear irreversible oxidations peak which might be due to its
limited solubility in solvents employed. Obviously, the substi-
tuent positions play a significant role in affecting the elec-
tronic structure and reversible oxidation characteristics. They
are also clearly affected by the donor/acceptor groups of the
arylphenyl moieties at the 5,9-positions of the pyrene core in
the current butterfly-shaped pyrenes.15 Accordingly, the
HOMO and LUMO energy levels were estimated from the CVs
and the UV-vis absorption and the data are summarized in
Table 2. The HOMO energy levels increased from −5.36 to
−5.79 eV in the order: 4e > 4d > 4f > 4a > 4b = 5 > 4c, and their
LUMO energy levels were in the range −2.36 to −2.96 eV.
Based on the HOMO and LUMO energy levels, it can be pro-
posed that these newly developed blue emitters 4 and 5 could
be used as blue emitters in OLEDs.
pyrene derivatives were successfully synthesized by the classical
Suzuki–Miyaura cross-coupling reaction. The introduction of
several arylphenyl substituents located at the 1,3,5,9-positions
of the pyrene influenced the FL emissions in solution and in
the solid-state. The structure–property relationships of the
donor–acceptor interactions play a crucial role in determining
the thermal, UV-vis absorption, electrochemical behavior, etc.
These pyrenes possess high solubility, high stability and blue
emissions with quantum efficiency of up to 70%, and long
fluorescence lifetimes in solution. These excellent qualities
make them very promising for applications in OLED devices.
Experimental
General
To further understand the electronic properties of 4, 5 and
6, DFT calculations (B3LYP/6-31G* basis set) were carried out
using Gaussian 03 for the geometry optimization.21 The opti-
mized structures and orbital distributions of the HOMO and
LUMO of these butterfly-shaped pyrenes are shown in both
Fig. 6 and the ESI.† The dihedral angles between the arylphe-
nyl and pyrene are in the range of 55–58°, which suggested
that the different substituents equally contribute to the steric
congestion in these systems. The HOMO levels of 4, 5 and 6
mainly involve contributions of π-orbitals from the pyrene
core, while the substituent aryl groups have limited contri-
butions to these molecules. Similarly, the LUMO levels are also
spread across the entire pyrene frameworks; however, with the
π-conjugation enlarging, the LUMOs spread to entire mole-
cules in the order from –CF3, –CN, –CHO to –CvN–OH.
The HOMO–LUMO energy gaps were in the range of 3.21 eV to
3.43 eV in 4 and 5. Therefore, these theoretical calculations
provided clear evidence that the red shift of the absorption
and emission bands observed in compounds 4 and 5 are con-
sistent with the experimental results. Without the tert-butyl
All melting points (Yanagimoto MP-S1) are uncorrected. 1H
NMR spectra (300 MHz) were recorded on a Nippon Denshi
JEOL FT-300 NMR spectrometer with SiMe4 as an internal
reference: J-values are given in Hz. IR spectra were measured
for samples as KBr pellets in a Nippon Denshi JIR-AQ2OM
spectrophotometer. UV-Vis spectra were measured using a
Shimadzu UV-3150UV-vis-NIR spectrophotometer. Fluo-
rescence spectroscopic studies of compounds in solution were
performed in various organic solvents in a semimicro fluo-
rescence cell (Hellma®, 104F-QS, 10 × 4 mm, 1400 μL) with a
Varian Cary Eclipse spectrophotometer. Fluorescence
quantum yields were measured in solution and in the solid-
state (Hamamatsu Photonics K. K., Quantaurus-QY A10094)
using the integrated sphere absolute PL quantum yield
measurement method. The luminescence lifetimes of the
butterfly-shaped compounds were obtained with Hamamatsu
Photonics OB920 by using nano LED excitation sources at 373 nm
(ligand) or the maximum λem of compounds (Saga University,
Japan). The optical characteristics of compound 4 films were
Table 2 Optical and electrochemical properties of compounds 4, 5 and 6
b
b
e
λedge a/nm
E1/2
Eonsetox
Egap(opt) c/eV
EHOMO(CV) d/eV
EHOMO(cal) e/eV
Egap(calcd)
4a
4b
4c
4d
4e
4f
5
399
405
411
403
418
431
419
397
1.43
1.35
1.27
1.66
1.72
1.66
1.01
—
1.35
1.27
1.20
1.58
1.63
1.58
1.02
—
3.11
3.06
3.01
3.08
2.97
2.88
2.96
3.12
−5.56
−5.51
−5.44
−5.82
−5.87
−5.82
−5.47
—
−5.01
−4.90
−4.76
−5.61
−5.93
−5.66
−5.25
−5.06
3.43
3.40
3.40
3.40
3.32
3.21
3.24
3.43
6
a Measured in CH2Cl2. b CV measured in 0.1 M n-Bu4NPF6–CH2Cl2 with a scan rate of 100 mV s−1
.
c Calculated from λedge
.
d Values calculated
using the ferrocene HOMO level: calculated from the empirical formulae HOMO = −(4.8 + Eonsetox − Eonsetox(Fc)).17 e DFT/B3LYP/6-31G* using
Gaussian 03.
8370 | Org. Biomol. Chem., 2013, 11, 8366–8374
This journal is © The Royal Society of Chemistry 2013

View Article Online
Organic & Biomolecular Chemistry
Paper
Fig. 6 Computed molecular orbital plots (B3LYP/6–31G*) of 4 and 5.
estimated using UV spectra and PL spectra were obtained slowly added to the stirred solution. The reaction mixture was
using a FluroMax-2 (Jobin-Yvon-Spex) luminescence spectro- continuously stirred for 3 h at room temperature and the reac-
meter (Yamagata University, Japan). The thin films were pre- tion process was tracked by GC, and then poured into a large
pared by the solution process. Dissolve 10 mg of sample in 1 mL excess of ice/water. The reaction mixture was extracted with
toluene solution, the solution is placed on the substrate, dichloromethane (2 × 50 mL). The combined organic extracts
which is then rotated at high speed in order to spread the were washed with water and brine, dried with anhydrous
fluid by centrifugal force. Thermogravimetric analysis (TGA) MgSO4 and evaporated. The residue was crystallized from
was carried out using a SEIKO EXSTAR 6000 TG/DTA 6200 unit hexane to afford pure 2-tert-butylpyrene 1 (4.56 g, 71.4%) as a
under a nitrogen atmosphere at a heating rate of 10 °C min−1
.
gray powder. Recrystallization from hexane gave 1 as colorless
Differential scanning calorimetry (DSC) was performed using a prisms. M.p. 111.5–113.2 °C (lit.22 M.p. 110–112 °C). The
Perkin-Elmer Diamond DSC Pyris instrument under a nitrogen 1H NMR spectrum completely agreed with the reported values.
atmosphere at a heating rate of 10 °C min−1. Mass spec- 1H NMR (300 MHz, CDCl3): δ = 1.59 (s, 9H, tBu), 8.18 (d, J =
trometry of target compounds was carried out with a Nippon 9.2 Hz, 2H, pyrene-H), 8.30 (s, 2H, pyrene-H), 8.37 (d, J =
Denshi JMS-HX110A Ultrahigh Performance Mass Spectro- 9.2 Hz, 2H, pyrene-H) and 8.47 (s, 1H, pyrene-H) ppm.
meter at 75 eV using a direct-inlet system. Elemental analyses
Lewis acid-catalyzed bromination of 2-tert-butylpyrene
were performed using a Yanaco MT-5 analyser. Cyclic voltam- (1). A mixture of 2-tert-butylpyrene (0.512 g, 2.0 mmol) and
metry measurements were conducted using an ALS/H CH iron powder (0.56 g, 10 mmol) was added while stirring for
Instruments Electrochemical Analyzer 660B with a standard 15 min in CH2Cl2 (10 mL) at room temperature. Then, a solu-
three-electrode configuration. Typically, a three-electrode cell tion of Br2 (0.63 mL, 12.1 mmol) in CH2Cl2 (15 mL) was slowly
equipped with a Pt disk working electrode, a Pt wire counter added dropwise with vigorous stirring. After this addition, the
electrode and an Ag/AgNO3 (0.01 M in anhydrous acetonitrile) reaction mixture was continuously stirred for 4 h at room
reference electrode was employed. The measurements were temperature. The mixture was quenched with Na2S2O3 (10%)
made in anhydrous dichloromethane (for oxidation) with and extracted with dichloromethane (2 × 50 mL). The com-
0.1 M n-tetrabutylammonium hexafluorophosphate as the bined organic extracts were washed with water and brine and
supporting electrolyte under an argon atmosphere at a scan then evaporated. The crude product exhibits a gray color. The
rate of 100 mV s−1. The potential of the Ag/AgNO3 reference crude product was insoluble in general common organic sol-
electrode was internally calibrated by using the ferrocene/ferro- vents, such as benzene, hexane, methanol, etc. and only
cenium redox couple (Fc/Fc+).
slightly dissolved in CH2Cl2 or CHCl3. Thus, the residue was
dissolved in hot CHCl3 and filtered, and then crystallized from
CHCl3 to give pure 7-tert-butyl-1,3,5,9-tetrabromopyrene
Materials
Unless otherwise stated, all other reagents used were pur- 2 (978 mg, 84%) as colourless prisms. M.p. 303.4–305.0 °C; IR:
chased from commercial sources and used without further νmax (KBr)/cm−1: 2962, 2365, 1579, 1523, 1461, 1425, 1392,
purification. The preparation of 2-tert-butylpyrene (1) was 1363, 1267, 1195, 1132, 1027, 1012, 941, 877, 809, 655 and 474;
described previously.22
1H NMR (300 MHz, CDCl3): δ = 1.65 (s, 9H, tBu), 8.47 (s, 1H,
Synthesis of 2-tert-butylpyrene (1). A mixture of pyrene pyrene-H2), 8.71 (s, 2H, pyrene-H) and 8.79 (s, 2H, pyrene-H)
(5.0 g. 24.2 mmol) and 2-chloro-2-methylpropane (2.62 g, ppm. Due to the poor solubility, it was not further identified
3.23 mL) was added in 40 mL of CH2Cl2 at 0 °C and stirred for by 13C NMR spectroscopy. FABMS: m/z: 573.62 [M+]. C20H14Br4
15 min. Powdered anhydrous AlCl3 (3.62 g, 27.2 mmol) was (573.78): calcd C 41.85, H 2.46; found: C 42.05, H 2.53.
This journal is © The Royal Society of Chemistry 2013
Org. Biomol. Chem., 2013, 11, 8366–8374 | 8371

View Article Online
Paper
Synthesis of 1,3,5,9-tetraaryl-7-tert-butylpyrenes (4). The
Organic & Biomolecular Chemistry
7-tert-Butyl-tetrakis-1,3,5,9-(4-trifluoromethylphenyl)pyrene (4d).
series of butterfly-shaped compounds 4a–f were synthesized Colourless prisms (CH2Cl2–methanol, 2 : 1) (183 mg, 63%).
from 1,3,5,9-tetrabromo-7-tert-butylpyrene 2 with the corres- M.p. 258 °C. IR: νmax (KBr)/cm−1: 2961, 1614, 1403, 1324, 1256,
1
ponding arylboronic acid 3 by a Suzuki–Miyaura cross-coup- 1167, 1123, 1067, 1020, 852, 623 and 436. H NMR (400 MHz,
ling reaction in high yield.
CDCl3): δ = 1.38 (s, 9H, tBu), 7.73–7.84 (m, 16 H, Ar–H), 7.94 (s,
Synthesis of 7-tert-butyl-1,3,5,9-tetraphenylpyrene (4a). 1 H, pyrene-H), 8.05 (s, 2 H, pyrene-H) and 8.25 (s, 2 H, pyrene-
7-tert-Butyl-1,3,5,9-tetrabromopyrene 2 (200 mg, 0.35 mmol), H) ppm; 13C NMR (100 MHz, CDCl3): δ = 149.7, 144.4, 144.1,
phenylboronic acid 3a (245 mg, 2.11 mmol), Pd(PPh3)4 (70 mg, 139.3, 136.2, 130.9, 130.8, 130.3, 130.0, 129.3, 127.4, 125.6,
0.061 mmol), and K2CO3 (500 mg, 3.6 mmol) were mixed in a 125.54, 125.51, 125.4, 125.3, 125.2, 125.16, 124.7, 123.9, 122.8,
flask containing argon saturated toluene (10 mL) and ethanol 121.8, 35.5 and 31.6 ppm. FABMS: m/z: 834.29 [M+]. C48H30F12
(4 mL). The reaction mixture was stirred at 90 °C for 24 h. After (834.73): calcd C 69.07, H 3.62; found: C 68.97, H 3.83.
it was cooled to room temperature, the reaction mixture was
7-tert-Butyl-tetrakis-1,3,5,9-(4-cyanophenyl)pyrene
(4e).
extracted with dichloromethane (3 × 50 mL), and the organic Colourless prisms (CH2Cl2–methanol, 2 : 1) (144 mg, 62%).
layer was washed with H2O and brine, and then dried with M.p. 378 °C. IR: νmax (KBr)/cm−1: 2954, 2230, 1683, 1604, 1504,
anhydrous MgSO4 and evaporated. The crude product was 1480, 1256, 1178, 1109, 995, 898, 837 and 562. 1H NMR
purified by column chromatography using dichloromethane as (400 MHz, CDCl3): δ = 1.37 (s, 9H, tBu), 7.73 (d, J = 8.2 Hz, 4H,
an eluent to afford a deep yellow solid. Recrystallization from a Ar–H), 7.78 (d, J = 8.2 Hz, 4H, Ar–H), 7.87 (d, J = 8.2 Hz, 4H,
mixture of CH2Cl2–methanol (2 : 1) gave 4a (128 mg, 65%) as Ar–H), 7.88 (d, J = 8.1 Hz, 4H, Ar–H), 7.92 (s, 1H, pyrene-H),
colourless needles. M.p. 335 °C. IR: νmax (KBr)/cm−1: 3422.5, 7.98 (s, 2H, pyrene-H) and 8.21 (s, 2H, pyrene-H) ppm. 13C
2961.0, 2366.6, 1593.2, 1496.6, 1363.8, 1252.8, 1177.6, 1070.3, NMR (100 MHz, CDCl3): δ = 147.5, 142.7, 142.4, 136.8, 133.4,
894.7, 762.3, 701.3, 626.1 and 483.1. 1H NMR (400 MHz, 129.9, 128.5, 128.1, 127.1, 126.4, 124.8, 122.4, 122.2, 121.2,
CDCl3): δ = 1.36 (s, 9H, tBu), 7.40–7.54 (m, 12H, Ar–H), 119.4, 116.0, 115.96, 109.4, 109.2, 33.0 and 29.0 ppm. FABMS:
7.63–7.70 (m, 8H, Ar–H), 7.98 (s, 1H, pyrene-H), 8.16 (s, 2H, m/z: 662.31 [M+]. C48H30N4 (662.78): calcd C 86.98, H 4.56;
pyrene-H) and 8.29 (s, 2H, pyrene-H) ppm. 13C NMR (100 MHz, found: C 86.68, H 4.88.
CDCl3):
δ
=
148.6, 141.4, 141.1, 139.8, 137.2, 130.6,
7-tert-Butyl-1,3,5,9-tetrakis(4-formylphenyl)pyrene
(4f).
130.5, 130.1, 129.7, 128.4, 128.3, 127.4, 127.3, 127.2, 125.5, Yellow prisms (CH2Cl2–methanol, 2 : 1) (168 mg, 72%).
124.5, 124.1, 121.4, 35.4 and 31.6 ppm. FABMS: m/z: M.p. 302 °C. IR: νmax (KBr)/cm−1: 3444, 2957, 1703, 1607, 1564,
562.28 (M+). C44H34 (562.74): calcd C 93.62, H 5.38; found: 1385, 1313, 1209, 1167, 819 and 490. 1H NMR (400 MHz,
C 93.25, H 5.25.
CDCl3): δ = 1.37 (s, 9H, tBu), 7.81 (d, J = 8.1 Hz, 4H, Ar–H), 7.87
7-tert-Butyl-1,3,5,9-tetrakis(4-tert-butylphenyl)pyrene (4b). (d, J = 8.1 Hz, 4H, Ar–H), 8.00 (s, 1H, pyrene-H), 8.07–8.09 (m,
Colourless prisms (CH2Cl2–methanol, 2 : 1) (132 mg, 48%). 8H, Ar–H, 2H, pyrene-H), 8.28 (s, 2H, pyrene-H), 10.14 (s, 2H,
M.p. 332 °C. IR: νmax (KBr)/cm−1: 2964, 1596, 1510, 1460, 1396, CHO) and 10.16 (s, 2H, CHO) ppm. 13C NMR (100 MHz,
1363, 1270, 1109, 1023, 901, 833, 572 and 482. 1H NMR CDCl3):δ = 191.82, 191.77, 149.8, 147.1, 146.8, 139.6, 136.5,
(400 MHz, CDCl3): δ = 1.39 (s, 9H, pyrene-tBu), 1.42 (s, 18H, 135.7, 135.5, 131.2, 130.7, 130.0, 129.9, 129.0, 128.9, 127.5,
phenyl-tBu), δH: 1.44 (s, 18H, pyrene-tBu), 7.54 (d, J = 8.4 Hz, 125.3, 124.8, 123.9, 122.0, 35.5 and 31.6 ppm. FABMS: m/z:
4H, Ar–H), 7.57 (d, J = 8.8 Hz, 4H, Ar–H), 7.62 (d, J = 7.6 Hz, 674.28 [M+]. C48H34O4 (674.78): calcd C 85.44, H 5.08; found:
4H, Ar–H), 7.63 (d, J = 7.6 Hz, 4H, Ar–H), 7.98 (s, 1H, pyrene- C 85.34, H 5.35.
H), 8.26 (s, 2H, pyrene-H) and 8.33 (s, 2H, pyrene-H) ppm. 13C
Synthesis of 7-tert-butyl-1,3,5,9-tetrakis(4-hydroxyimino-
NMR (100 MHz, CDCl3): δ = 150.2, 149.9, 148.3, 139.3, 138.5, phenyl)pyrene (5). A mixture of 7-tert-butyl-1,3,5,9-tetrakis-(4-
138.2, 137.0, 130.6, 130.3, 130.0, 129.9, 128.9, 127.2, 125.7, formylphenyl)pyrene 4f (30 mg, 0.045 mmol), hydroxylamine
125.3, 125.2, 124.4, 124.2, 121.2, 35.4, 34.7, 34.6, 31.7 and hydrochloride (60 mg, 0.86 mmol), sodium hydroxide (30 mg,
31.4 ppm. FABMS: m/z: 786.58 [M+]. C60H66 (786.52): C 91.55, 0.75 mmol) and 10 mL ethanol were refluxed for 12 h, and
H 8.45; found: C 91.25, H 8.55.
then cooled to room temperature. The solvent was removed
7-tert-Butyl-1,3,5,9-tetrakis(4-methoxyphenyl)pyrene
(4c). in vacuo to leave the residue, which was washed with water and
Yellow prisms (CH2Cl2–methanol, 2 : 1) (154 mg, 65%). recrystallized from acetone to give 5 (23 mg, 70%) as a yellow
M.p. 330 °C. IR: νmax (KBr)/cm−1: 3437, 2957, 1610, 1507, 1457, solid. M.p. 214 °C. IR: νmax (KBr)/cm−1: 33 465, 2974, 1756,
1285, 1246, 1174, 1106, 1038, 830, 583 and 540. 1H NMR 1630, 1371, 1249, 1218, 1072, 1009, 974, 943, 836, 730, 607,
1
(400 MHz, CDCl3): δ = 1.38 (s, 9H, tBu), 3.90 (s, 6H, OMe), 3.92 568, 524 and 466. H NMR (400 MHz, CDCl3): δ = 1.29 (s, 9H,
(s, 6H, OMe), 7.06 (d, J = 8.8 Hz, 4H, Ar–H), 7.07 (d, J = 8.8 Hz, tBu), 2.22 (s, 4H, OH), 7.34 (d, J = 8.0 Hz, 4H, Ar–H), 7.45 (d, J =
4H, Ar–H), 7.56 (d, J = 8.6 Hz, 4H, Ar–H), 7.60 (d, J = 8.8 Hz, 8.0 Hz, 4H, Ar–H), 7.53 (d, J = 8.0 Hz, 4H, Ar–H), 7.62, (s, 2H,
4H, Ar–H), 7.92 (s, 1H, pyrene-H), 8.12 (s, 2H, pyrene-H) and pyrene-H), 7.66 (d, J = 8.0 Hz, 4H, Ar–H), 7.67 (s, 2H, –CHvN–
8.30 (s, 2H, pyrene-H) ppm; 13C NMR (100 MHz, CDCl3) δ = H), 8.05 (s, 2H, –CHvN–H), 8.14 (d, J = 9.2 Hz, 2H, pyrene-H)
159.0, 158.9, 148.5, 139.1, 136.7, 133.9, 133.7, 131.7, 131.2, and 8.24 (s, 2H, pyrene-H) ppm; 13C NMR (100 MHz, CDCl3):
130.8, 129.7, 127.3, 125.6, 121.2, 113.9, 113.8, 55.4 and δ = 115.02, 150.0, 149.8, 143.3, 142.9, 140.7, 138.1, 133.9,
31.7 ppm; FABMS: m/z: 682.19 [M+]. C48H42O4 (682.31): calcd C 133.6, 131.9, 131.8, 131.5, 131.3, 130.0, 129.8, 128.3, 128.0,
84.43, H 6.20; found: C 84.01, H 5.93.
127.9, 126.4, 125.7, 125.4, 122.7, 36.4 and 32.1 ppm. FABMS:
8372 | Org. Biomol. Chem., 2013, 11, 8366–8374
This journal is © The Royal Society of Chemistry 2013

View Article Online
Organic & Biomolecular Chemistry
Paper
m/z: 734.52 [M+]. calcd C48H38N4O4 (734.84): C 78.45, H 5.21;
found: C 78.20, H 5.25.
5 (a) Y.-L. Fogel, M. Kastler, Z.-H. Wang, D. Andrienko,
G. J. Bodwell and K. Müllen, J. Am. Chem. Soc., 2007, 129,
11743–11749; (b) K. Mochida, K. Kawasumi, Y. Segawa and
K. Itami, J. Am. Chem. Soc., 2011, 133, 10716–10719;
(c) B.-X. Gao, M. Wang, Y.-X. Cheng, L.-X. Wang, X.-B. Jing
and F.-S. Wang, J. Am. Chem. Soc., 2008, 130, 8297–8306.
6 (a) X.-L. Ni, S. Wang, X. Zeng, Z. Tao and T. Yamato, Org.
Lett., 2011, 13, 552–555; (b) X.-L. Ni, X. Zeng, C. Redshaw
and T. Yamato, J. Org. Chem., 2011, 76, 5696–5702;
(c) X.-L. Ni, X. Zeng, C. Redshaw and T. Yamato, Tetra-
hedron, 2011, 67, 3248–3253.
7 I. B. Berlman, J. Phys. Chem., 1970, 74, 3085–3093.
8 P. Sonar, M. S. Soh, Y. H. Cheng, J. T. Henssler and
A. Sellinger, Org. Lett., 2010, 15, 3292–3295.
9 W. Sotoyama, H. Sato, M. Kinoshita, T. Takahashi,
A. Matsuura, J. Kodama, N. Sawatari and H. Inoue, SID
Symposium Digest of Technical Papers, 2003, 34, 1294–
1297.
Synthesis of 1,3,5,9-tetraphenylpyrene (6). A mixture of
7-tert-butyl-1,3,5,9-tetraphenylpyrene 4a (50 mg, 0.089 mmol),
Nafion-H (50 mg), and 2 mL o-xylene was refluxed for 24 h,
and then cooled to room temperature. The solid was removed
in vacuo and the mother solution collected. The crude product
was purified by column chromatography using dichloro-
methane–hexane (1 : 1) as an eluent to afford a yellow solid.
Recrystallization from a mixture of CH2Cl2–ethanol (2 : 1) gave
6 (23 mg, 51%) as light yellow prisms. M.p. 244 °C. IR:
νmax (KBr)/cm−1: 3058, 3032, 1600, 1486, 1442, 1071, 1034, 893,
1
790, 759, 740, 700, 620 and 475. H NMR (400 MHz, CDCl3): δ
= 7.42–7.45 (m, 4H, Ar–H), 7.49–7.53 (m, 8H, Ar–H), 7.59–7.61
(m, 4H, Ar–H), 7.66–7.68 (m, 4H, Ar–H), 7.85–7.89, (m, 1H,
pyrene-H), 8.00 (s, 1H, pyrene-H), 8.17 (s, 2H, pyrene-H) and
8.14 (d, J = 8 Hz, 2H, pyrene-H) ppm; 13C NMR (100 MHz,
CDCl3): δ = 141.2, 141.0, 139.6, 137.4, 130.7, 130.6, 130.1,
130.0, 128.4, 128.3, 127.4, 127.39, 127.2, 125.7, 125.5 and 10 (a) J.-Y. Hu, M. Era, M. R. J. Elsegood and T. Yamato,
124.1 ppm. FABMS: m/z: 506.12 (M+). C40H26 (506.20): calcd C
94.83, H 5.17; found: C 94.77, H 5.29.
Eur. J. Org. Chem., 2010, 72–79; (b) J.-Y. Hu, X.-L. Ni,
X. Feng, M. Era, M. R. J. Elsegood, S. J. Teat and T. Yamato,
Org. Biomol. Chem., 2012, 10, 2255–2262; (c) X. Feng,
J.-Y. Hu, L. Yi, N. Seto, Z. Tao, C. Redshaw,
M. R. J. Elsegood and T. Yamato, Chem.–Asian J., 2012, 7,
2854–2863.
Acknowledgements
This work was performed under the Cooperative Research 11 X. Feng, J.-Y. Hu, F. Iwanaga, N. Seto, C. Redshaw,
Program of “Network Joint Research Center for Materials and
M. R. J. Elsegood and T. Yamato, Org. Lett., 2013, 15, 1318–
Devices (Institute for Materials Chemistry and Engineering,
1321.
Kyushu University)”. We would like to thank the OTEC at Saga 12 M. Tasashi and T. Yamato, J. Org. Chem., 1983, 48, 1461–
University and the International Collaborative Project Fund of 1468.
Guizhou province at Guizhou University for financial support. 13 (a) T. Yamato, C. Hideshima, A. Miyazawa, M. Tashiro,
We also would like to thank the EPSRC and the Royal Society
for financial support (travel grants to C.R.).
G. K. S. Prakash and G. A. Olah, Catal. Lett., 1990, 6, 345–
348; (b) G. A. Olah, G. K. S. Prakash, P. S. Iyer, M. Tashiro
and T. Yamato, J. Org. Chem., 1987, 52, 1881–1884;
(c) T. Yamato, C. Hideshima, M. Tashiro, G. K. S. Prakash
and G. A. Olah, J. Org. Chem., 1991, 56, 6248–6250;
(d) T. Yamato, J. Synth. Org. Chem. Jpn., 1995, 53, 487–499;
(e) T. Yamato and J. Hu, J. Chem. Res., 2006, 762–765.
Notes and references
1 (a) P. Tyagi, A. Venkateswararao and K. R. J. Thomas, J. Org.
Chem., 2011, 76, 4571–4581; (b) L. Schmidt-Mende, 14 C.-J. Chen, H.-J. Yen, W.-C. Chen and G.-S. Liou, J. Polym.
A. Fechtenkötter, K. Müllen, E. Moons, R. H. Friend and
J. D. MacKenzie, Science, 2001, 293, 1119–1122.
2 (a) X.-L. Feng, W. Pisula and K. Müllen, Pure Appl. Chem.,
2009, 81, 2203–2224; (b) H. Klauk, D. J. Gundlach,
Sci., Part A: Polym. Chem., 2011, 49, 3709–3718.
15 A. G. Crawford, A. D. Dwyer, Z.-Q. Liu, A. Steffen, A. Beeby,
L.-O. Pålsson, D. L. Tozer and T. B. Marder, J. Am. Chem.
Soc., 2011, 133, 13349–13362.
J. A. Nichols and T. N. Jackson, IEEE Trans. Electron Devices, 16 J. N. Moorthy, P. Natarajan, P. Venkatakrishnan,
1999, 46, 1258–1263; (c) S.-L. Suraru, U. Zschieschang, D.-F. Huang and T. J. Chow, Org. Lett., 2007, 9, 5215–5218.
H. Klauk and F. Würthner, Chem. Commun., 2011, 47, 17 C. M. Cardona, W. Li, A. E. Kaifer, D. Stockdale and
11504–11506. G. C. Bazan, Adv. Mater., 2011, 23, 2367–2371.
3 (a) M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, 18 E. M. S. Castanheira, M. S. D. Carvalho, D. J. G. Soares,
S. Sibley, M. E. Thompson and S. R. Forrest, Nature, 1998,
395, 151–154; (b) C.-T. Chen, Chem. Mater., 2004, 16, 4389–
4400.
4 (a) T. M. Figueira-Duarte and K. Müllen, Chem. Rev., 2011,
111, 7260–7314; (b) J.-Y. Hu and T. Yamato, Org. Light Emit-
ting Diode – Mater. Process Devices, 2011, 21–60; (c) J.-Y. Hu,
P. J. G. Coutinho, R. C. Calhelha and M.-J. R. P. Queiroz,
J. Fluoresc., 2011, 21, 911–922.
19 (a) F. Han, L. Chi, W. Wu, X. Liang, M. Fu and J. Zhao,
J. Photochem. Photobio., A, 2008, 196, 10–23; (b) G.-J. Zhao,
J.-Y. Liu, L.-C. Zhou and K.-L. Han, J. Phys. Chem. B, 2007,
111, 8940–8945.
Y.-J. Pu, G. Nakata, S. Kawata, H. Sasabe and J. Kido, Chem. 20 J. C. Sciano, Handbook of Organic Photochemistry, CRC
Commun., 2012, 48, 8434–8436.
Press, Boca Raton, FL, 1989.
This journal is © The Royal Society of Chemistry 2013
Org. Biomol. Chem., 2013, 11, 8366–8374 | 8373

View Article Online
Paper
Organic & Biomolecular Chemistry
21 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery Jr.,
T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam,
S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci,
M. Cossi, G. Scalmani, N. Rega, G. A. Petersson,
H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
V. G. Zakrzewski, S. Dapprich, A. D. Daniels,
M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck,
K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui,
A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov,
G. Liu, A. Liashenko, P. Piskorz, I. Komaromi,
R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham,
C. Y. Peng, A. Nanayakkara, M. Challacombe,
P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong,
C. Gonzalez and J. A. Pople, GAUSSIAN 03, (Revision
C.02), Gaussian, Inc., Wallingford CT, 2004.
R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, 22 Y. Miura, E. Yamano, A. Tanaka and J. Yamauchi, J. Org.
K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg,
Chem., 1994, 59, 3294–3300.
8374 | Org. Biomol. Chem., 2013, 11, 8366–8374
This journal is © The Royal Society of Chemistry 2013