Blue-emitting butterfly-shaped 1,3,5,9-tetraarylpyrenes: Synthesis, crystal structures, and photophysical properties

Article abstract of DOI:10.1021/ol4002653

The first example of aryl-functionalized, butterfly-shaped, highly fluorescent and stable blue-emitting monomers, namely, 7-tert-butyl-1,3,5,9- tetrakis(p-R-phenyl)pyrenes, were synthesized by the Suzuki-Miyaura cross-coupling reaction from a novel bromide precursor of 1,3,5,9-tetrabromo-7- tert-butylpyrene. The crystal structures and optical and electronic properties have been investigated.

Full text of DOI:10.1021/ol4002653

ORGANIC
LETTERS
2013
Vol. 15, No. 6
1318–1321
Blue-Emitting Butterfly-Shaped
1,3,5,9-Tetraarylpyrenes: Synthesis, Crystal
Structures, and Photophysical Properties
Xing Feng,† Jian-Yong Hu,*,†,‡ Fumitaka Iwanaga,† Nobuyuki Seto,† Carl Redshaw,§
Mark R. J. Elsegood, and Takehiko Yamato*,†
Department of Applied Chemistry, Faculty of Science and Engineering, Saga University,
Honjo-machi 1, Saga-shi, Saga 840-8502, Japan, Department of Organic Device
Engineering, Yamagata University, Yonezawa, Yamagata 992-8510, Japan,
Department of Chemistry, The University of Hull, Hull HU6 7RX, U.K., and
Chemistry Department, Loughborough University, Leicestershire LE11 3TU, U.K.
yamatot@cc.saga-u.ac.jp; hujianyong@yz.yamagata-u.ac.jp
Received January 29, 2013
ABSTRACT
The first example of aryl-functionalized, butterfly-shaped, highly fluorescent and stable blue-emitting monomers, namely, 7-tert-butyl-1,3,5,9-
tetrakis(p-R-phenyl)pyrenes, were synthesized by the SuzukiꢀMiyaura cross-coupling reaction from a novel bromide precursor of 1,3,5,9-
tetrabromo-7-tert-butylpyrene. The crystal structures and optical and electronic properties have been investigated.
The development of efficient optoelectronic materials
based on polycyclic aromatic hydrocarbons (PAHs) has
been extensively investigated in the past two decades.1
Indeed, many members of PAHs have been employed
in organic light-emitting diodes (OLEDs)2,3 and other
optoelectronic devices,4 as well as fluorescence probes.5
Pyrene and its derivative are important members of PAHs
that have exhibited several advantages: (1) solution pro-
cessable, (2) good thermal stability, (3) enhanced charge
carrier mobility, and (4) intense luminescence efficiency.
However, the use of pyrenes as efficient emitters in OLEDs
has been somewhat limited,2 primarily because the planar
structure of pyrene has a strong tendency to form
π-aggregates/excimers, thereby quenching the fluorescence
in concentrated solution or in the solid-state. To suppress
the passive aggregation, several research groups have
been focused on exploring the availability of methods for
the functionalization of the pyrene core. In general, the
1-, 3-, 6-, and 8-positions of pyrene preferentially undergo
electrophilic aromatic substitution (SEAr) reactions.
Thus, various pyrene derivatives2 (Supporting Information)
can be easily accessed depending on the experimental
conditions.6
† Saga University.
‡ Yamagata University.
§ The University of Hull.
Loughborough University.
(1) (a) Havey, R. G. Polycyclic Aromatic Hydrocarbons; Wiley-VCH:
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Y.-Q.; Guo, Y.-L.; Yu, G.; Hu, W.-P. Chem. Rev. 2011, 111, 3358–3406.
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r
10.1021/ol4002653
Published on Web 03/05/2013
2013 American Chemical Society

Scheme 1. Synthesis of Compounds 4 and 5
On the other hand, the 4-, 5-, 9-, and 10-positions (i.e.,
K-region) of the pyrene are facile to bromination in the
presence of iron powder if the sterically bulky tert-butyl
groups are located at the 2- and 7-positions.7 Interestingly,
on further prolonging the reaction time, the ipso-bromination
productof 4,5,7,9,10-pentabromopyrene canbeobtained.8
Besides the electrophilic substitution of pyrene, Hu et al.
reported an efficient, one-step synthetic approach to cata-
lyze the oxidation of the K-region of pyrene using ruthe-
the pyrene ring has been pretected, four bromines atoms
can be introduced at the 1-, 3-, 5-, and 9-positions by
electrophilic bromination of 2-tert-butylpyrene (1). Herein,
we succeeded in developing a new bromide precursor,
1,3,5,9-tetrabromo-7-tert-butylpyrene (2), in excellent
yield (Scheme 1). To the best of our knowledge, this is
the first example of a method to halogenate the pyrene
ring both in the activated sites (1- and 3-positions) and in
the K-region (5- and 9-positions).
The precursor 2 has two advantages: (1) the active sites
at the 1- and 3-positions could give C-functionalized
pyrene by the cross-coupling reaction to suppress the
aggregation,11a11a and (2) the K-region(5-and 9-positions)
affords a strategy to extend conjugated systems to larger
PAHs by cyclization.13 Accordingly, in this study, by using
2 as an intermediate, we syntheized novel butterfly-shaped,
highly fluorescent, and stable blue-emitting monomers,
namely, 1,3,5,9-tetraaryl-7-tert-butylpyrenes (4), which
were characterized by X-ray diffraction, absorption and
fluoresence spectra, electrochemical data, and density
functional theory (DFT).
As described in Scheme 1, the mono-tert-butylated
product, 2-tert-butylpyrene (1),14 treated with Br2 (6 equiv)
in CH2Cl2 at room temperature in the presence of iron
powder yielded the expected tetrabromopyrene 2 in a high
yield of 84%. Then, Suzuki cross-coupling reaction of 2
with the corresponding arylboronic acids afforded the
1,3,5,9-tetraaryl-7-tert-butylpyrenes 4 in isolated yields of
65ꢀ72%. As a comparison, we synthesized the Schiff base
5 from the aromatic aldehyde 4c (Scheme 1) and 6 [1,3,6,8-
tetrakis(4-methoxyphenyl)pyrene] according to the reported
literature procedure.15
9
10
€
nium chloride. Mullen et al. also developed an asym-
metric functionalization method to direct bromination to
the K-regionof the pyrene without the protectivetert-butyl
groups. Recently, it was found that the active sites of the
1,3-positions of pyrene could be brominated from starting
compound 1 because the tert-butyl group protects the ring
from electrophilic attack at the 6,8-positions.11 In addi-
tion, we reported the selective formation of the 5-mono-
and 5,9-disubstitution products from 7-tert-butyl-1,3-
dimethylpyrene by formylation and acetylation depending
on the Lewis acid catalysts used.12 Thus, based on the
above-mentioned research, we attempted to exploit a new
intermediate in order to develop a series of pyrene related
materials for further applications. Our initial attempt was
to synthesize 1,3,4,5,9,10-hexabromo-7-tert-butylpyrene
(3) using iron powder to catalyze its formation from
2-tert-butylpyrene (1) in different solvents; however,
efforts using CH2Cl2, nitrobenzene, and benzene all failed.
Probably, the bromo atom substituted at the 1- and
3-positions of the pyrene would sterically hinder the
4- and 10-positions, thereby enabling regioselective sub-
stitution at the 5- and 9-positions. Since the 7-position of
(7) Yamato, T.; Fujimoto, M.; Miyazawa, A.; Matsuo, K. J. Chem.
Soc., Perkin Trans. 1 1997, 1201–1207.
(8) Hu, J.-Y.; Ni, X.-L.; Feng, X.; Era, M.; Elsegood, M. R. J.; Teatd,
S. J.; Yamato, T. Org. Biomol. Chem. 2012, 10, 2255–2262.
(9) Hu, J.; Zhang, D.; Harris, F. W. J. Org. Chem. 2005, 70, 707–708.
The molecular structures of compounds 4ꢀ6 were char-
1
acterized by H/13C NMR spectra, single-crystal X-ray
(13) (a) Paudel, A.; Hu, J.-Y.; Yamato, T. J. Chem. Res. 2008, 457–
460. (b) Hu, J.-Y.; Paudel, A.; Seto, N.; Feng, X.; Era, M.; Matsumoto,
T.; Tanaka, J.; Elsegood, M. R. J.; Redshaw, C.; Yamato, T. Org.
Biomol. Chem. 201310.1039/C3OB27320B.
€
(10) Zophel, L.; Beckmann, D.; Enkelmann, V.; Chercka, D.; Rieger,
€
R.; Mullen, K. Chem. Commun. 2011, 47, 6960–6962.
(11) (a) Figueira-Duarte, T. M.; Simon, S. C.; Wagner, M.; Druzhi-
€
nin, S. I.; Zachariasse, K. A.; Mullen, K. Angew. Chem., Int. Ed. 2008,
(14) Miura, N.; Yamano, E.; Tanaka, A.; Yamauchi, J. J. Org. Chem.
1994, 59, 3294–3300.
(15) Halleux, V. de.; Callbert, J.-P.; Brocorens, P.; Cornil, J.;
47, 10175–10178. (b) Feng, X.; Hu, J.-Y.; Yi, L.; Seto, N.; Tao, Z.;
Redshaw, C.; Elsegood, M. R. J.; Yamato, T. Chem.;Asian J. 2012, 7,
2854–2863.
ꢀ
Declercq, J.-P.; Bredas, J.-L.; Geerts, Y. Adv. Funct. Mater. 2004, 14,
649–659.
(12) Hu, J.-Y.; Paudel, A.; Yamato, T. J. Chem. Res. 2008, 308–311.
Org. Lett., Vol. 15, No. 6, 2013
1319

Table 1. Photophysical and Electrochemical Properties of Compounds 4, 5 and 6
λ
max abs (nm)
λ
max PL (nm)
LUMO
(eV)
HOMO
(eV)
HOMOꢀLUMO
ΔE (eV)
compd
solnsa/filmsb
solnsa/filmsb
Φfc solns/thin films
Tmd(°C)
Tde (°C)
4a
4b
4c
5
373
379
386
355
391
380
412
421
469
467
434
410
443
471
nd
0.92/0.75
0.90/0.72
0.56/0.48
nd/nd
ꢀ1.58
ꢀ1.36
ꢀ2.45
ꢀ2.12
ꢀ1.47
ꢀ5.01
ꢀ4.76
ꢀ5.66
ꢀ5.25
ꢀ4.71
3.43
3.40
3.21
3.13
3.24
335
330
302
266
271
350
410
329
430
nd
369
400
nd
6
405
488
0.94/nd
a Maximum absorption wavelength measured in dichloromethane at room temperature. b Measured in thin neat films. c Measured in dichloro-
methane and in neat thin films, respectively. d Melting temperature (Tm) obtained from differential scanning calorimetry (DSC) measurement.
e Decomposition temperature (Td) obtained from thermogravimetric analysis (TGA). nd: no determination.
diffraction, FT-IR spectroscopy, mass spectroscopy, as
well as elemental analysis. All compounds 4 were soluble
in common organic solvents such as toluene, CH2Cl2,
CHCl3, THF, acetonitrile, and N,N-dimethylformamide
(DMF) and exhibited excellent thermal stability under
air/N2. The key thermal data for the pyrenes 4 are sum-
marized in Table 1.
solid-state structure. However, the compound 6 exhibited
a sandwich-like 3D structure self-assembled in the solid-
state via CꢀH π bond interactions overlapping the
3 3 3
porous two-dimensional networks. A chloroform mole-
cule was encapsuled in the molecular channel by a hydro-
˚
gen bonding C45ꢀH45 O4 (2.506 A) interaction
3 3 3
(see the Supporting Information).
Figure 1. X-ray structure of compounds 4a and 4b.
Single crystals of 4a (CCDC 917256), 4b (CCDC
917257), and 6 (CCDC 915429) were grown from a
mixture of CH2Cl2 and MeOH and investigated by
X-ray crystallography to establish the structure. All crystal
structures were found to belong to the monoclinic crystal
system with space group P21/c for 4a and 6 and P21/n for
4b. As shown in Figure 1, these terminal moieties adopt a
reasonably twisted conformation with a substantial dihe-
dral angle relative to the pyrene ring (46.2ꢀ68.0°), and
torsion angles of 49.4ꢀ70.5° between the pyrene and
phenyl rings, while for the pyrene and methoxylphenyl
rings, the torsion angles were 48.1ꢀ56.2°. In compounds 4,
the four terminal moieties twist with reasonable dihedral
angles relative to the pyrene cores by the proximal hydro-
gen atoms. This conformation effectively suppresses πꢀπ
stacking and releases the steric interactions in the solid
Figure 2. (a) Normalized UVꢀvis absorption and (b) emission
spectra of compounds 4 recorded in dichloromethane at ca.
∼10ꢀ5ꢀ10ꢀ6 M at 25 °C. (c) Emission spectra of 4 in thin film (d)
photographs of blue emission from the films of 4 (left f right).
The UV absorption and fluorescence spectra of 4ꢀ6
were investigated in dichloromethane and in a thin film
(Figure 2). For 4aꢀc and 6, two prominent absorp-
tion bands were observed in between 298ꢀ308 nm and
373ꢀ391 nm, and the absorption spectrum of 4c was
broader and less well-resolved in the range of 330ꢀ420
nm. The compounds 4 have a slight red-shift order of 4c
(CHO) > 4b (OMe) > 4a (H), which are in agreement
with our previous reports.8,11b For the Schiff base 5, the
broader absorption spectrum was observed at 355 nm,
arising from the more delocalized chromophore (red shift)
with a possibility for charge transfer (broadened spectrum).
In the thin film absorptions (Table 1 and Supporting
state. OnlyCꢀH π interactionswere observed, and they
3 3 3
impact on molecular geometries as well as prevent excimer
formation in the solid state.16 The packing of molecules
of 4a and 4b revealed the 2D self-assembled planar
€
(16) (a) Wurthner, F.; Thalacker, C.; Diele, S.; Tschierske, C.
Chem.;Eur. J. 2001, 7, 2245–2253. (b) Jenekhe, S. A.; Osaheni, J. A.
Science 1994, 265, 765–768.
1320
Org. Lett., Vol. 15, No. 6, 2013

Information), the compounds 4 have slightly broader
spectra with red-shifts (5ꢀ14 nm) in comparison to that
observed in solution, which can be explained by re-
absorption in the solid-state. For 6, however, an unusual
zigzag-type absorption spectrum was observed with a
maximum at 391 nm, indicating its trend to form a single
crystal in the solid state causes a high noise level.15
Upon excitation, the emission maxima at 412 nm for 4a
and 421 nm for 4breveala smaller red shift thancompound
6 (λmax = 434 nm). For 4c and 5, however, the presence of
the CHO moiety and the 4-tert-butylphenyl iminophenyl
group caused a significant red-shift and a broadened
emission maximum (λmax) at 469 and 467 nm, respectively.
Nevertheless, no excimer emissions were observed in these
newly developedbutterfly-shapedtetraarylpyrene systems.
The thin-solid film fluorescence spectrum of 4 presented a
prominent maximum emission in the blue region at 410 nm
for 4a, 443 nm for 4b, and 471 nm for 4c, respectively.
Interestingly, the emission spectrum of 4a shows a slight
hypsochromic shift with respect to the spectrum in solu-
tion. This difference might be due to the different dielectric
constants.17 Compound 4b displayed a slight red-shift
(∼22 nm) compared to the spectra recorded in solution
(Table 1), thought to be due to the aggregation. For 4c,
there is a slight red-shift (increased by 2 nm) in solution
versus thin film. However, the fluorescence emission spec-
trum of 6 showed broad emission bands at 488 nm in the
solid state. A 54 nm red-shift was observed with respect to
the solution without the tert-butyl group. The substituents
at the 5,9-positions of pyrene gave the structure more
symmetry and played a key role for influencing S1 r S0
excitation.18 Furthermore, parts b and c of Figure 2
revealed that these butterfly-shaped compounds 4 emit
very bright, sharp, and solid-state blue fluorescence. All
compounds 4 have very high quantum yields (Φfc) in
range of 0.56ꢀ0.92 in solution and 0.72ꢀ0.78 in the solid-
state, resepctively (Table 1). There is thus potential for
these to be used as efficient blue emitters in OLEDs.
DFT calculations for 4ꢀ6 were performed using the
Gaussian 03 program. The calculated density surfaces of
the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular obital (LUMO) of 4a, 4c,
and 5 are shown in Figure 3 and the Supporting Informa-
tion. The HOMO is located over the entire pyrene frame-
work in each molecule, whereas the aryl substituent groups
or extended chains have limited contributions to this
system. However, it is remarkable that the LUMO of 4c
spreads over the entire pyrene framework, including the
4-formylphenyl unit. It is noteworthy that the terminal
phenylimino groups do not contribute to the construction
of the LUMO in 5, despite of the introduction the extended
π-conjugated by CdN bond.
Figure 3. Spatial distributions of the compounds 4a, 4c, and 5
frontier orbitals. The lower plots represent the HOMOs, and the
upper plots represent the LUMOs.
oxidation waves located at 1.48 V for 4a, 1.35 V and 1.73 V
for 4b, and 1.75 V for 4c (vs Fc/Fcþ). All compounds
displayed oxidation waves originating from the conjuga-
tion system of pyrene. The HOMO energy levels were
calculated to be ꢀ5.49 eV for 4a, ꢀ5.36 eV for 4b, and
ꢀ5.73 eV for 4c, respectively, from the onset of the first
oxidation wave. The energy gap (Eg) was estimated from
UVꢀvis absorption (3.11 eV for 4a, 3.01 eV for 4b, and
2.88 eV for 4c). The experimental data was found to
closely correspond to the theoretically calculated data
(see the Supporting Information). The slight difference
was caused by the theoretical calculations performed in
the gas phase. The high reversibility of their redox
processes demonstrated that the butterfly-shaped pyr-
enes were stable and suggested that the current mole-
cular design may be suitable for OLED-like optoelec-
tronic devices.
In conclusion, this work provides a promising strategy
to halogenate both at the active sites and in the K-region.
We successfully developed a novel bromide precursor of
1,3,5,9-tetrabromo-7-tert-butylpyrene. Withthistetra bro-
mopyrene as a key starting material, butterfly-shaped,
highly efficient blue-emitting pyrene derivatives were
synthesized. Both single-crystal X-ray analysis and photo-
physical properties for these pyrenes have been fully
investigated. Further investigation of their applications
in OLEDs is in progress in our laboratory.
Acknowledgment. This work was performed under the
Cooperative Research Program of “Network Joint Re-
search Center for Materials and Devices (Institute for
Materials Chemistry and Engineering, Kyushu University)”.
We thank the EPSRC (overseas travel grant to C.R.) and
The Royal Society for financial support.
The electrochemical properties of 4 were investigated
by cyclic voltammetry (CV). All compounds showed
Supporting Information Available. Detailed descrip-
tions of experimental procedures, X-ray crystallographic
data, NMR data, and DFT. This materials is available
free of charge via the Internet at http://pubs.acs.org.
(17) Tyagi, P.; Venkateswararao, A.; Thomas, K. R. J. J. Org. Chem.
2011, 76, 4571–4581.
(18) Crawford, A. G.; Dwyer, A. D.; Liu, Z.-Q.; Steffen, A.; Beeby,
A.; Palsson, L.-O.; Tozer, D. L.; Marder, T. B. J. Am. Chem. Soc. 2011,
133, 13349–13362.
The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 6, 2013
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