Highly emissive hand-shaped π-conjugated alkynylpyrenes: Synthesis, structures, and photophysical properties

Article abstract of DOI:10.1039/c2ob06865f

Three alkynyl-functionalised, hand-shaped, highly fluorescent and stable emitters, namely, 2-tert-butyl-4,5,7,9,10-pentakis(p-R-phenylethynyl)pyrenes have been successfully synthesized via a Pd/Cu-catalysed Sonogashira cross-coupling reaction. The chemical structures of the alkynylpyrenes were fully characterized by their ¹H/¹³C NMR spectra, mass spectroscopy and elemental analysis. Synchrotron single-crystal X-ray analysis revealed that there is a 1-D, slipped, face-to-face motif with off-set, head-to-tail stacked columns, which are clearly influenced by the single, bulky, tert-butyl group in the pyrene ring at the 2-position. Detailed studies on the photophysical properties in both solutions and thin films strongly indicate that they might be promising candidates for optoelectronic applications, such as organic light-emitting devices (OLEDs) or as models for investigating the fluorescent structure-property relationship of the alkynyl-functionalised pyrene derivatives. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2ob06865f

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PAPER
Highly emissive hand-shaped π-conjugated alkynylpyrenes: Synthesis,
structures, and photophysical properties†
Jian-Yong Hu,*a,b Xin-Long Ni,a Xing Feng,a Masanao Era,a Mark R. J. Elsegood,c Simon J. Teatd and
Takehiko Yamato*a
Received 4th November 2011, Accepted 21st December 2011
DOI: 10.1039/c2ob06865f
Three alkynyl-functionalised, hand-shaped, highly fluorescent and stable emitters, namely, 2-tert-butyl-
4,5,7,9,10-pentakis(p-R-phenylethynyl)pyrenes have been successfully synthesized via a Pd/Cu-catalysed
Sonogashira cross-coupling reaction. The chemical structures of the alkynylpyrenes were fully
characterized by their 1H/13C NMR spectra, mass spectroscopy and elemental analysis. Synchrotron
single-crystal X-ray analysis revealed that there is a 1-D, slipped, face-to-face motif with off-set, head-to-
tail stacked columns, which are clearly influenced by the single, bulky, tert-butyl group in the pyrene ring
at the 2-position. Detailed studies on the photophysical properties in both solutions and thin films strongly
indicate that they might be promising candidates for optoelectronic applications, such as organic light-
emitting devices (OLEDs) or as models for investigating the fluorescent structure–property relationship of
the alkynyl-functionalised pyrene derivatives.
alkynyl-functionalised banana-shaped,4 cruciform-shaped,5 as
Introduction
well as star-shaped6 π-conjugated fluorophores are of particular
Recently, carbon-rich organic compounds with a high degree of
π-conjugation have attracted great attention due to their interest-
ing optoelectronic properties as ideal materials in modern
electronic and optoelectronic devices.1–3 Generally, the optoelec-
tronic effects of these organic π-conjugated compounds are
highly dependent on their molecular structure design. Major
issues for the organic π-conjugated compounds used in elec-
tronic and optoelectronic devices are their ability to form mor-
phologically stable and homogeneous films, and excellent
efficiencies with respect to high quantum yields, charge-carrier
transport, etc. To fulfil these and other requirements, several
families of organic π-conjugation compounds with interesting
molecular architectures have been exploited. Among them,
interest because they exhibit unique optical and electronic prop-
erties deriving from their individual multiply-conjugated
pathway structures. The representative examples of the banana-
shaped fluorophores are the oligo(arylene ethynylene)s of Yama-
guchi et al.,4 the cruciform-shaped 1,2,4,5-tetra-substituted(phe-
nylethynyl) benzenes of Haley et al.,5a and the star-shaped
hexaethynylbenzene derivatives of Vollhardt et al.6a
Pyrene is one of the most important large π-conjugated aro-
matics, which has high fluorescence quantum yield in solution
and shows efficient excimer emission.7 Pyrene and its derivatives
have been widely used as photophysical probes,8 as liquid crys-
talline materials,9 as well as in photonic devices.10 Despite its
wide uses, the fact that the absorption and emission properties of
the pyrene core are confined to the UV region, remains a major
drawback. Recently, several alkynyl-functionalised pyrenes have
been fully studied where the introduction of alkynyl groups
induced effective extension of the π-conjugation into the visible
region and a large increase in fluorescence intensity.11 For
example, Ziessel and co-workers reported two acetylene-linked
dendrimers by combining ethynylphenylaminoacyls with pyrene
cores and studied their liquid crystal and gel properties in opto-
electronic devices.11e Hence, there exist more intensive interests
in developing new alkynylpyrenes as model systems or promis-
ing candidates for practical uses in optoelectronic devices.
aDepartment of Applied Chemistry, Faculty of Science and Engineering,
Saga University, Honjo machi 1, Saga-shi, Saga, Japan.
E-mail: yamatot@cc.saga-u.ac.jp; Fax: +81-952-288548
bDepartment of Organic Device Engineering, Yamagata University,
Yonezawa, Yamagata992-8510, Japan. E-mail: hujianyong@
yz.yamagata-u.ac.jp; Fax: +81-238-263412
cChemistry Department, Loughborough University, Loughborough,
Leicestershire, UK, LE11 3TU
dAdvanced Light Source, Berkeley Lab., 1 Cyclotron Road, Berkeley,
CA94720, USA
†Electronic supplementary information (ESI) available: 1H/13C NMR
spectra of 4a–c and 6, TGA curves of 4a–c and 6, cyclic voltammogram
(CV) of 4a–c and 6, the effect of concentration on the fluorescence emis-
sion spectra of 4c, and photoluminesence (PL) spectra of 4a–c in 1 wt
%-doped into PMMA films. CCDC reference number 811807. For ESI
and crystallographic data in CIF format see DOI: 10.1039/c2ob06865f
As a part of our research program on the construction of
pyrene-based extended π-conjugation compounds,12 we recently
reported a series of alkynyl-functionalised, cruciform-shaped
light-emitting monomers, which revealed highly pure-blue
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emissions, high fluorescence quantum yields, and a low degree
solid-state π-aggregation. These properties are completely attrib-
uted to the bulky tert-butyl groups substituted in the pyrene ring
at the 2- and 7-positions.12b Along this line, in this paper, we
present the synthesis, structures and photophysical properties of
three alkynyl-functionalised π-conjugated emitters of 2-tert-
butyl-4,5,7,9,10-pentakis(p-R-phenylethynyl)pyrenes. We inves-
tigated the single-crystal X-ray structures and also the photophy-
sical properties of the pyrenes in solution and thin films.
Although these newly synthesized compounds differ from the
former cruciform-shaped emitters, in which the two bulky tert-
butyl groups played the key roles in suppressing the π-aggrega-
tions of pyrene units both in solution and in solid-states,12b we
surmised that the single tert-butyl group at the 2-position would
also play an important role in the molecular arrangement and
orientation in solution and the solid-state, which might lead to
interesting optoelectronic properties.
Scheme 2 Synthesis of 2-tert-butyl-4,5,7,9,10-pentakis(phenylethy-
nyl)pyrene derivatives 4a–c. Reagents and conditions: (a) [PdCl2
(PPh3)2], CuI, PPh3, Et3N/DMF (1 : 1), 24–48 h, 100 °C.
Results and discussion
Synthesis and characterization
Following our previously reported approach, we readily obtained
the intermediate 4,5,9,10-tetrabromo-2,7-di-tert-butylpyrene 1
by the Lewis acid-catalysed bromination of 2,7-di-tert-butylpyr-
ene.13 Interestingly, when the reaction time was prolonged to
8 h, the Lewis acid-catalysed ipso-bromination of the tetrabro-
mide 1 took place, affording the pentabromide 2 in high yield of
85% (Scheme 1), and it served as the starting material for the
synthesis of the 4,5,7,9,10-pentakis(p-substituted phenylethy-
nyl)pyrene derivatives.
We carried out the modified Sonogashira cross-coupling reac-
tion of the pentabromide 2 with various kinds of phenylacety-
lenes 3, and succeeded in producing the corresponding 2-tert-
butyl-4,5,7,9,10-pentakis(p-substituted phenylethynyl)pyrenes 4
in good yields (recrystallization yields) (Scheme 2). As a com-
parison, we prepared a non-conjugated pyrene 6, [2-tert-butyl-
4,5,7,9,10-pentakis(4-methoxyphenyl)pyrene] by the standard
Suzuki cross-coupling reaction14 (Scheme 3).
We fully determined the molecular structures and purities of
these new pyrenes 4 and 6 by H NMR spectra, FT-IR spec-
Scheme 3 Synthesis of 2-tert-butyl-4,5,7,9,10-pentakis(4-methoxy-
phenyl)pyrene (6). Reagents and conditions: (a) [Pd(PPh3)4], Na2CO3,
toluene/EtOH, 24 h, 70 °C, 36%.
1
troscopy, mass spectroscopy, as well as elemental analysis. All
results were consistent with the proposed hand-shaped structures
(see ESI†). All compounds are soluble in common organic sol-
vents, such as CH2Cl2, CHCl3, tetrahydrofuran (THF), and
toluene. Thermal properties of 4 and 6 were investigated by
thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) measurements. The decomposition tempera-
tures (Td) of the pyrenes 4 and 6 were in the range of
433–507 °C corresponding to a 5% weight loss. No glass tran-
sition temperatures (Tg) of these pyrenes 4 and 6 were detected
in the DSC measurements, and the melting points of 4a–c were
in the range of 315–383 °C except for 6, which shows no
melting point. The key thermal data of the pyrenes 4 and 6 are
listed in Table 2.
X-Ray molecular structure and crystal packing of 4c
The performance of the organic compounds in optoelectronic
devices strongly relies on the intermolecular order in the active
layer. We further confirmed the molecular structure of 4c by
Scheme 1 Synthesis of 4,5,7,9,10-pentabromo-2-tert-butylpyrene (2).
Reagents and conditions: (a) Br2, Fe-powder, CH2Cl2, rt 4 h, 90% (1).
(b) Br2, Fe-powder, CH2Cl2, rt 8 h, 85% (2).
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Table 1 Summary of the crystal data of 4c
Parameter
4c
Empirical formula
C65H48O5·0.07CH2Cl2
914.80
100(2)
Formula weight (g mol−1
T/K
)
Wavelength (Å)
Crystal system
Space group
Crystal color and size (mm)
a/Å
b/Å
c/Å
0.7749
Triclinic
ˉ
P1
Yellow, 0.25 × 0.03 × 0.02
10.0392(8)
15.7261(12)
16.0211(11)
81.677(5)
82.511(5)
79.608(6)
2447.5(3)
2
α (°)
β (°)
γ (°)
Volume (Å3)
Z
Density, calcd. (g m−3
)
1.241
0.101
962
Absorption coefficient (mm−1
F(000)
)
θ range for data collection
Reflections collected
Independent reflections
Observed data (F2 > 2σ(F2))
Rint
Restraints/parameters
Goodness-of-fit on F2
R1 (F2 > 2σ(F2))
2.48 to 25.59
16532
6986
4579
0.0373
0/645
1.035
0.0723
wR2 (all data)
0.2097
synchrotron single-crystal X-ray analysis.15 The yellow, needle-
shaped crystals were obtained by slow evaporation of a of
mixture concentrated CHCl3/CH2Cl2 solutions. The crystallo-
graphic data for this compound are summarized in Table 1.
Structure diagrams of 4c are shown in Fig. 1. There is one com-
plete molecule and a small amount of CH2Cl2 (see structure
refinement details†) in the asymmetric unit. The ten ethynyl
carbons are essentially coplanar with the central pyrene ring.
Two C6H4 rings are close to being co-planar with the central
pyrene ring with C(41) > C(46) and C(59) > C(64) twisted by
0.8 and 6.1° respectively, while three are substantially twisted
with C(23) > C(28), C(32) > C(37) and C(50) > C(55) twisted
by 31.7, 47.3 and 30.4° respectively. The crystal packing dia-
grams of 4c are shown in Fig. 2. The molecules adopt a 1-D
slipped face-to-face motif with off-set head-to-tail stacked
columns. Each 4c molecule displays 24-point π–π stacking with
molecules above and below using both the pyrenyl and ethynyl
carbons,12b with intermolecular distances of ca. 3.42–3.58 Å.16
As we expected, the individual stacking arrangement in the
current crystal of 4c is clearly influenced by the single, bulky,
tert-butyl group in the pyrene ring at the 2-position. Thus, these
newly developed alkynylpyrenes with unique intermolecular
interactions suggests that they might be advantageous to high
charge-carrier transport in optoelectronic device applications.17
Fig. 1 X-ray structure diagrams of compound 4c: (above) top view;
(below) side view of 1-D slipped four molecules.
in thin neat films are shown in Fig. 3. For the non-conjugated
pyrene 6, the absorption spectra reveal a vibronic feature that is
characteristic of the unsubstituted parent 2,7-di-tert-butylpyrene
with a short wavelength absorption maximum at ca. 290 nm,
and a long wavelength absorption maximum at ca. 355 nm, indi-
cating the five introduced 4-methoxyphenyl groups are only
slightly conjugated to the central pyrene core. This is also
observed in the 1,3,6,8-tetraarylpyrenes.18 Interestingly, for the
hand-shaped alkynylpyrenes 4, although the profiles of the
optical absorption spectra are similar to those of our previously
reported cruciform-shaped alkynylpyrenes,12b red-shifts by ca.
25 nm in each spectrum were observed, which are attributed to
the additional phenylacetylenic substituents attached at the 7-
position. On the other hand, the longest wavelength bands of the
alkynylpyrenes 4 are largely red-shifted by ca. 100 nm, located
at 433 nm for 4a, 438 nm for 4b, and 443 nm for 4c, with
respect to those of the parent pyrene19 (λabs = 336 nm in
CH2Cl2) and 6, respectively. This can be attributed to the
increased conjugation length arising from the introduction of the
five phenylacetylenic units. In particular, 4c displayed the largest
bathochromic shift due to the strongest electron-donating ability
of the methoxy group.12b In the case of thin neat films, for 6,
compared to the corresponding spectrum in solution, a small
bathochromic shift (∼2 nm) was observed in the UV/Vis absorp-
tion spectrum, which indicates that this compound exhibited a
very similar conformation in both states.20 However, for the
alkynylpyrenes 4a–c, the UV/Vis absorption spectra completely
Photophysical and electrochemical properties
We measured the UV/Vis absorption spectra of these new alky-
nylpyrenes 4 both in dilute CH2Cl2 solutions and in thin neat
films at room temperature, and the results are presented in
Table 2, together with those of the non-conjugated pyrene 6. The
normalized UV/Vis absorption spectra of 4 and 6 in solution and
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lost the vibronic features compared to the corresponding spectra
in solution. These hypsochromic shifts going from solution to
the thin neat films are probably due to both the presence of
strong intermolecular π–π aggregations and different molecular
conformations.
We investigated the electrochemical characteristics of the
pyrenes 4 and 6 by using cyclic voltammetry (CV). The pyrenes
were scanned both positively and negatively, separately, in 0.10
M tetrabutylammonium perchlorate (Bu4NClO4) in anhydrous
dichloromethane with a scan rate of 50 mV s−1 at room tempera-
ture. We find that the oxidation and reduction of the pyrenes
were quasi-reversible or reversible processes (see the ESI†). The
onset potentials for oxidation were 0.86, 0.65, 0.62, and 1.42 V
(vs SCE) for 4a, 4b, 4c, and 6, respectively. The onset potentials
for reduction were −1.68, −1.47, −1.48, and −1.98 V (vs SCE),
respectively. From the onset potentials of the oxidation and
reduction processes, the band gaps (Eg) of the pyrenes were
Fig. 2 Packing diagrams of compound 4c: (above) view parallel to a
highlighting π–π stacking; (below) view parallel to b showing herring-
bone packing motif.
Fig. 3 Normalised UV/Vis absorption spectra of 4 and 6 recorded in
CH2Cl2 (a) and in thin neat films (b).
Table 2 Photophysical and electrochemical properties of hand-shaped molecules 4 and 6
Hand-
shaped
molecules
λmax abs (nm)
λmax PL (nm)
d
g
h
Φfc solns/thin neat
films/doped films
Eg
HOMOe
(eV)
LUMOf
(eV)
Tm
Td
solnsa
filmsb
solnsa
filmsb
(eV)
(°C)
(°C)
4a
4b
4c
6
433
438
443
355
428
425
431
357
454
456
463
411
558
546
553
414
0.46/0.12/0.38
0.72/0.32/0.56
0.86/0.16/0.45
0.24/0.17/0.15
2.54
2.12
2.10
3.40
−5.26
−5.05
−5.02
−5.82
−2.72
−2.93
−2.92
−2.42
326
383
315
nd
507
457
453
433
a Measured in dichloromethane at room temperature. b Measured in thin neat film. c Measured in dichloromethane, in neat thin film and 1 wt% doped
polymethylmethacrylate (PMMA) films, respectively. d Determined from the LUMO and HOMO energy levels. e Calculated from the oxidation
potentials. f Calculated from the reduction potentials. g Melting temperature (Tm) obtained from differential scanning calorimetry (DSC)
measurement. h Decomposition temperature (Td) obtained from thermogravimetric analysis (TGA).
2258 | Org. Biomol. Chem., 2012, 10, 2255–2262
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2.54, 2.12, 2.10, and 3.40 eV for 4a, 4b, 4c, and 6. Thus,
emission difference between 4 and 6 should be ascribed to the
ethynylene linkages that elongate the conjugation length of the
molecules and make the molecules more planar with expanded
π-delocalization. In addition, the emission spectra of the alkynyl-
pyrenes 4 systematically varied in agreement with the electronic
absorption spectra, implying that the energy gap between ground
and excited states decreases in the order of 4a > 4b > 4c
(Table 2). The emission spectra are independent of the excitation
wavelength. We further examined the effects of concentration on
the fluorescence emission of 4c in CHCl3. Upon increasing the
concentrations from 1.0 × 10−8 M to 1.0 × 10−4 M, we observed
the intensities of the emission bands gradually increase, and the
emissions correspond to only the monomer emission at 463 nm
with shoulders at 444 and 488 nm (see ESI†). This result indi-
cates that the presence of the single, sterically bulky, tBu group
at the 2-position can prevent two molecules of 4c from getting
close enough to result in excimer emission even at high
concentrations.12b
In the thin neat films, the emission peak of the pyrene 6
shows a slight red-shift by ca. 3 nm, located at 414 nm, which
points to only a marginal difference compared with that in sol-
ution. However, the main emission peaks of the alkynylpyrenes
4a, 4b, and 4c were located at 558, 546, and 553 nm, respect-
ively, with the disappearance of the fine structures of the spectra.
The emissions are largely red-shifted by ca. 100 nm compared to
those in solution. These large red-shifts of 4 in the thin neat
films are probably due to the facile formation of strong intermo-
lecular π–π aggregations between pyrene units23 and the differ-
ence in dielectric constants of the environment,24 which caused
the low-energy emissions. The results are also consistent with
the synchrotron single-crystal X-ray structure determination of
the representative compound 4c.
ox
according to the equations,21,22 IP = − ([Eonset
]
+ 4.4) eV and
red
ox
red
EA = − ([Eonset
]
+ 4.4) eV, where [Eonset
]
and [Eonset
]
are
the onset potentials for the oxidation and reduction of the
pyrenes versus the reference electrode. The lowest unoccupied
molecular orbital (LUMO) and highest occupied molecular
orbital (HOMO) of the pyrenes were LUMO: −2.72, −2.93,
−2.92, −2.42; HOMO: −5.26, −5.05, −5.02, and −5.82 eV for
4a, 4b, 4c, and 6, respectively. It was observed that the Eg values
of 4a–4c varied slightly due to the difference of the p-R-substitu-
ents, and indicates that different substituents in these pyrenes can
inevitably result in different energy states of the whole molecule.
Moreover, the calculated HOMO values of 4a–c vary slightly in
the range −5.26 to −5.02 eV, indicating that the materials are
suitable for application in OLED-like optoelectronic devices.18c
All data of electrochemical properties for the pyrenes 4 and 6 are
summarized in Table 2.
The normalized PL emission spectra of the pyrenes 4 and 6,
upon excitation at the corresponding wavelength, both in
CH2Cl2 solutions and in the thin neat films are shown in Fig. 4.
For the non-conjugated pyrenes 6, a deep-blue emission was
observed with a long wavelength emission maximum at 411 nm
and a shoulder band at ca. 430 nm in solution. However, for the
alkynylpyrenes 4, all emission spectra are largely bathochromi-
cally red-shifted into the sky-blue visible region; 4a, 4b, and 4c
showed a main emission at 454, 456, and 463 nm with a
shoulder peak at 481, 484, and 488 nm, respectively. The
The fluorescence quantum yields of the pyrenes 4 and 6
recorded both in dilute CH2Cl2 solution and in the thin neat
films at room temperature are also summarized in Table 2. For 6,
low quantum yields were obtained in solution and the thin neat
films, which indicates that excitons were not confined to the
whole backbone of 6 due to its non-conjugated molecular struc-
ture. Some energy loss might happen during the exciton
migration,25 thereby resulting in low quantum yields. Interest-
ingly, for the hand-shaped alkynylpyrenes 4, high fluorescence
quantum yields were found in the range of 0.46–0.86 in solution,
indicating the excitons were completely confined to the whole
backbone of 4, arising from the significantly expanded π-deloca-
lization between the phenylacetylenic groups and the central
pyrene core, thereby giving higher quantum yields. However, in
the case of the thin neat films, the lower fluorescence quantum
yields might be due to the facile formation of intermolecular π–π
aggregations between pyrene units. Therefore, we further exam-
ined the photoluminescence (PL) spectra and the fluorescence
quantum yields of 4 and 6 in thin films doped into polymethyl-
methacrylate (PMMA) at the 1 wt% level.26 We observed a blue
emission with a maximum peak at 448, 454, and 463 nm for 4a,
4b, and 4c, respectively, which is similar to the corresponding
emission in dilute solution (see ESI†). On the other hand, for the
alkynylpyrenes 4, higher quantum yields in the range 0.38–0.56
were observed in the 1 wt%-doped PMMA films, compared to
the corresponding thin neat films, indicating the formation of
strong intermolecular π–π aggregations of pyrene units can be
Fig. 4 Normalised fluorescence emission spectra of 4 and 6 recorded
in CH2Cl2 (a) and in thin neat films (b).
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Lewis acid-catalysed ipso-bromination of 2,7-di-tert-butyl-
4,5,9,10-tetrabromopyrene (1). To a mixture of 2,7-di-tert-butyl-
pyrene (500 mg, 1.59 mmol), dry CH2Cl2 (250 mL) and 250 mg
iron powder, a solution of Br2 (1.0 g, 8.70 mmol) and CH2Cl2
(25 mL) was added drop wise over 1 h at 0 °C with stirring.
After this addition, the mixture was warmed to room temperature
and stirred for 8 h. Then, the mixture was poured into a
large amount of ice water. The organic layer was washed succes-
sively with 10% aq. sodium thiosulfate (50 mL) and water
(100 mL × 3), brine, dried (Na2SO4), and concentrated. The
residue was washed with hot hexane (100 mL) and filtered to
remove the byproducts. Then, the precipitate was washed with
CH2Cl2 (50 mL) to give the desired compound 2 as a light-
yellow solid (886 mg, 85%). M.p. 261 °C. 1H NMR
(300 MHz, CDCl3): δ = 1.64 (s, 9 H, tBu), 8.87 (s, 2 H, Py-
avoided in the doped films. This result suggests the new alkynyl-
pyrenes 4 might be promising dopants in the fabrication of host–
guest-based organic light-emitting devices.27 The quantum
yields obtained for the pyrenes 4 are respectably high, which
make them good candidates further investigations in optoelectro-
nics with improved properties. All data are presented in Table 2.
Conclusions
In conclusion, we have developed three new alkynyl-functiona-
lised, hand-shaped, π-conjugated emitters via the Sonogashira
cross-coupling reaction in good yields and fully characterised the
special chemical structures. The results obtained through inspect-
ing the photophysical properties indicate that the extension of
π-conjugation for these pyrene chromophores through phenylace-
tylenic substituents serves to shift the wavelength of absorption
and fluorescence emission into the blue visible region with high
fluorescence quantum yields. Synchrotron single-crystal X-ray
analysis revealed that the single bulky tBu group in the pyrene
ring at the 2-position played a significant role in the molecular
organization of the π stacking of the pyrene units. Thus, the
herein presented molecules are exciting new materials that
combine excellent optical features and improved thermal stab-
ilities, which make them potential candidates in optoelectronic
applications such as OLED-like devices, or models to investigate
the fluorescent structure–property relationship of alkynylpyrenes.
Further explorations into this area are ongoing.
H
6,8), 8.90 (s, 2 H s, 2 H, Py-H1,3) ppm. MS (EI): m/z 651.75
[M]+. C20H13Br5 (652.84): calcd. C 36.80, H 2.01; found C
37.06, H 2.12.
General procedure for the Sonogashira cross-coupling reac-
tion towards the synthesis of 4. 4,5,7,9,10-Pentabromopyrene 2
(0.46 mmol), [PdCl2(PPh3)2] (0.023 mmol), CuI (0.023 mmol),
PPh3 (0.046 mmol) and the phenylacetylenes 3 (4.60 mmol)
were added to a degassed solution of DMF (20 mL) and triethyl-
amine (20 mL) under argon. The resulting mixture was stirred at
100 °C for the time mentioned in the individual cases. The reac-
tion mixture was then cooled to room temperature and quenched
with Et2O and extracted. Solvent was removed to give the crude
reaction mixture, which was further worked up as indicated in
the individual cases.
Experimental
Synthesis of 2-tert-butyl-4,5,7,9,10-pentakis-phenylethynylpyr-
ene (4a). To a stirred mixture of 4,5,7,9,10-pentabromopyrene
(300 mg, 0.46 mmol), Et3N (20 mL) and DMF (20 mL), was
added PdCl2(PPh3)2 (17.3 mg, 0.023 mmol), CuI (4.6 mg,
0.023 mmol) and PPh3 (11.80 mg, 0.046 mmol) stirring for
30 min at 0 °C under argon. Then, phenylacetylene (467 mg,
4.6 mmol) was added, the mixture was heated to 100 °C with
stirring for 24 h. After cooling, the mixture was diluted into
CH2Cl2 (200 mL) and washed successively with saturated
NH4Cl solution, H2O and brine. The organics were dried
(MgSO4) and evaporated. The crude product was washed with
ethyl acetate (150 mL) and separated by filtration. The filtrate
was completely dissolved in CH2Cl2 and purified by column
chromatography, eluting with hexane : CH2Cl2 (5 : 1) and recrys-
tallization from toluene in hexane gave the desired product as a
red-orange solid (168 mg, 48%); m.p. 326 °C. IR (KBr): 2955,
2198 (–CuC–), 1655, 1510, 1268, 1018, 880, 835, 772,
General
All the palladium-mediated cross-coupling reactions were per-
formed under an argon gas atmosphere in oven-dried pressure
tubes. Unless otherwise noted, all materials were obtained from
commercial suppliers and used without further purification. The
1H and 13C NMR spectra were recorded on JEOL 300
(300 MHz and 75 MHz) spectrometers. The IR spectra were
obtained as KBr pellets on a Nippon Denshi JIR-AQ2OM spec-
trometer. Mass spectra were obtained on a Nippon Denshi
JMS-HX110A Ultrahigh Performance Mass Spectrometer at
75 eV using a direct-inlet system. Elemental analyses were per-
formed with a Yanaco MT-5 analyser. Thermogravimetric analy-
sis (TGA) was undertaken using a SEIKO EXSTAR 6000 TG/
DTA 6200 unit under nitrogen atmosphere at a heating rate of
10 °C min−1. Differential scanning calorimetery (DSC) was per-
formed using a Perkin–Elmer Diamond DSC Pyris instrument
1
656 cm−1. H NMR (300 MHz, CDCl3): δ = 1.68 (s, 9 H, tBu),
under nitrogen atmosphere at a heating rate of 10 °C min−1
.
7.36–7.44 (m, 15 H, Ph), 7.63–7.82 (m, 10 H, Ph), 8.78 (s, 2 H,
Py-H1,3), 8.85 (s, 2 H, Py-H6,8). 13C NMR of this could not be
determined due to its low solubility. MS (EI): m/z 758.36 [M]+.
C60H38 (758.94): calcd. C 94.95, H 5.05; found C 94.88, H
5.12.
UV-Vis spectra were measured using a Shimadzu UV-3150 UV-
vis-NIR spectrophotometer. Photoluminescence spectra were
obtained using a FluroMax-2 (Jobin-Yvon-Spex) luminescence
spectrometer. Electrochemical properties of HOMO and LUMO
energy levels were determined by Electrochemical Analyzer.
Synthesis of 2-tert-butyl-4,5,7,9,10-pentakis(4-tert-butylphenyl
ethynyl)pyrene (4b). To a stirred mixture of 4,5,7,9,10-pentabro-
mopyrene (300 mg, 0.46 mmol), Et3N (20 mL) and DMF
(20 mL), was added PdCl2(PPh3)2 (17.3 mg, 0.023 mmol), CuI
(4.6 mg, 0.023 mmol) and PPh3 (11.80 mg, 0.046 mmol) stirring
Materials
The preparations of 2,7-di-tert-butylpyrene and 4,5,9,10-tetrabro-
mopyrene (1) were previously described.13
2260 | Org. Biomol. Chem., 2012, 10, 2255–2262
This journal is © The Royal Society of Chemistry 2012

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1
for 30 min. at 0 °C under argon. Then, 4-tert-butylphenylacety-
lene (726 mg, 4.60 mmol) was added and the mixture was
heated to 100 °C with stirring for 48 h. After cooling, the
mixture was diluted with Et2O (150 mL) and washed succes-
sively with saturated NH4Cl solution (100 mL), H2O and brine.
The organics were dried (MgSO4) and evaporated. The crude
products were purified by column chromatography eluting with
hexane and recrystallization from hexane gave the desired
product as a yellow solid (200 mg, 42%); m.p. 383 °C. IR
(KBr): ν = 2956, 2199 (–CuC–), 1652, 1504, 1267, 1016, 879,
833, 761, 559 cm−1. 1H NMR (300 MHz, CDCl3): δ = 1.32 (s, 9
H, tBu), 1.39 (s, 36 H, tBu), 1.68 (s, 9 H, tBu), 7.40–7.51 (m,
10 H, Ph), 7.63–7.78 (m, 10 H, Ph), 8.86 (s, 2 H, Py-H1,3), 8.90
(s, 2 H, Py-H6,8) ppm. 13C NMR (300 MHz, CDCl3): δ = 31.3,
31.9, 34.8, 34.9, 35.6, 87.1, 87.2, 87.6, 89.1, 100.5, 100.7,
120.5, 120.6, 120.7, 120.8, 125.3, 125.4, 125.45, 125.47,
125,50, 125.52, 125.54, 129.7, 130.0, 130.1, 130.6, 131.7,
131.75, 131.83, 151.5, 151.7, 151.8, 151.9 ppm. MS (EI): m/z
1038.67 [M]+. C80H78 (1039.48): calcd. C 92.44, H 7.56; found
C 92.26, H 7.46.
with hexane to give 6 as a white solid (105 mg, 36%); H NMR
(300 MHz, CDCl3): δ = 1.26 (s, 9 H, tBu), 3.81 (s, 3 H, OMe),
3.83 (s, 6 H, OMe), 3.84 (s, 6 H, OMe), 6.86 (d, J = 8.79 Hz, 4
H, Ph), 6.87 (d, J = 8.79 Hz, 4 H, Ph), 6.92 (d, J = 8.79 Hz, 2
H, Ph), 7.20 (d, J = 8.61 Hz, 4 H, Ph), 7.21 (d, J = 8.58 Hz, 4
H, Ph), 7.45 (d, J = 8.79 Hz, 2 H, Ph), 7.93 (s, 2 H, Py-H1,3),
8.02 (s, 2 H, Py-H6,8) ppm.13C NMR (300 MHz, CDCl3): δ =
31.7, 35.4, 55.2, 55.4, 113.2, 113.3, 113.5, 114.3, 122.0, 122.3,
122.8, 123.0, 128.9, 129.0, 131.0, 131.4, 132.0, 132.2, 132.3,
134.6, 137.7, 137.8, 138.3, 148.5, 158.0, 159.1 ppm. MS (EI):
m/z 789.82 [M]+. C55H48O5 (789.97): calcd. C 83.73, H 6.13;
found C 83.62, H 6.15.
Crystal data and refinement details for 4c
Diffraction data were collected at the Advanced Light Source
(ALS) Station 11.3.1 using a Bruker SMART APEX II CCD dif-
fractometer using narrow frames to θmax = 26.46°.28 Data were
corrected for absorption on the basis of symmetry equivalence
and repeated data (min. and max. transmission factors: 0.975,
0.998) and Lp effects. The structure was solved by direct
methods and refined on F2 using all data.29 H atoms were con-
strained in a riding model. Over a centre of symmetry, two
residual peaks, with a separation consistent with that of the two
chlorine atoms in dichloromethane, were successfully modelled
as a small amount of CH2Cl2 of crystallisation. Further details
can be found in Table 1 and ref. 15
Synthesis of 2-tert-butyl-4,5,7,9,10-pentakis(4-methoxyphenyl-
ethynyl)pyrene (4c). To a stirred mixture of 4,5,7,9,10-penta-
bromopyrene (300 mg, 0.46 mmol), Et3N (20 mL) and DMF
(20 mL), added PdCl2(PPh3)2 (17.3 mg, 0.023 mmol), CuI
(4.6 mg, 0.023 mmol) and PPh3 (11.80 mg, 0.046 mmol) stirring
for 30 min. at 0 °C under argon. Then, 4-ethynylanisole
(611 mg, 4.6 mmol) was added, the mixture was heated to
100 °C and stirred for 48 h. After cooling, the mixture was
diluted into CH2Cl2 (200 mL) and washed successively with
saturated NH4Cl solution (100 mL), H2O and brine. The organic
layer was dried (MgSO4) and evaporated. The crude products
were washed with ethyl acetate (45 mL) and purified by column
chromatography eluting with hexane : CH2Cl2 6 : 1 and recrystal-
lization from toluene in hexane gave the desired compound as a
yellow solid (192 mg, 46%); m.p. 315 °C. IR (KBr): ν = 2958,
2195 (–CvC–), 1762, 1610, 1512, 1255, 1176, 1031, 831,
Acknowledgements
This work was performed under the Cooperative Research
Program of the Network Joint Research Center for Materials and
Devices (Institute for Materials Chemistry and Engineering,
Kyushu University). The Advanced Light Source is supported
by the Director, Office of Science, Office of Basic Energy
Science, of the US Department of Energy under Contract No.
DE-AC02-05CH11231. We thank Dr Yong-Jin Pu (Department
of Organic Device Engineering, Yamagata University) for fruit-
ful discussions.
1
537 cm−1. H NMR (300 MHz, CDCl3) δ = 1.68 (s, 9 H, tBu),
3.88 (s, 3 H, OMe), 3.89 (s, 12 H, OMe), 6.94–6.99 (m, 10 H,
Ph), 7.63–7.76 (m, 10 H, Ph), 8.86 (s, 2 H, Py-H1,3), 8.89 (s, 2
H, Py-H6,8) ppm. 13C NMR (300 MHz, CDCl3): δ = 31.9, 35.7,
55.4, 86.6, 86.8, 89.3, 90.5, 99.7, 100.2, 114.2, 114.3, 114.4,
115.9, 116.1, 120.8, 121.9, 123.0, 123.1, 123.3, 124.9, 128.1,
129.8, 130.0, 133.3, 133.4, 133.5, 150.3, 159.9, 160.0, 160.1,
160.2 ppm. MS (EI): m/z 908.34 [M]+. C65H48O5 (909.07):
calcd. C 85.88, H 5.32; found C 85.82, H 5.26.
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