Synthesis and photophysical properties of pyrene-based light-emitting monomers: Highly pure-blue-fluorescent, cruciform-shaped architectures

Article abstract of DOI:10.1002/ejoc.200900806

A new series of pyrene-based, pure-blue, fluorescent, stable monomers, namely 2,7-di-tert-butyl-4,5,9,10-tetrakis(p-R-phenylethynyl)pyrenes, have been successfully synthesised by Pd/Cu-catalysed Sonogashira coupling in excellent yield. The cruciform-shaped, π-conjugated structures were fully characterised by ¹H/¹³C NMR and IR spectroscopy, mass spectrometry and elemental analysis. As revealed from single-crystal X-ray analysis, there is a herringbone pattern between stacked columns, but the π-π stacking distance of adjacent pyrene units was not especially short at about 5.82 A due to the introduction of the two bulky tBu groups in the pyrene rings at the 2-and 7-positions. The photophysical properties of these monomers were carefully examined in different organic solvents, and these data strongly indicate their promising application as blue-emitting materials in organic light-emitting diodes (OLEDs).

Full text of DOI:10.1002/ejoc.200900806

FULL PAPER
DOI: 10.1002/ejoc.200900806
Synthesis and Photophysical Properties of Pyrene-Based Light-Emitting
Monomers: Highly Pure-Blue-Fluorescent, Cruciform-Shaped Architectures
Jian-yong Hu,[a] Masanao Era,[a] Mark R. J. Elsegood,[b] and Takehiko Yamato*[a]
Keywords: Fluorescence / Conjugation / Photochemistry / Organic light-emitting diodes
A new series of pyrene-based, pure-blue, fluorescent, stable
monomers, namely 2,7-di-tert-butyl-4,5,9,10-tetrakis(p-R-
phenylethynyl)pyrenes, have been successfully synthesised
by Pd/Cu-catalysed Sonogashira coupling in excellent yield.
The cruciform-shaped, π-conjugated structures were fully
characterised by 1H/13C NMR and IR spectroscopy, mass
spectrometry and elemental analysis. As revealed from sin-
gle-crystal X-ray analysis, there is a herringbone pattern be-
tween stacked columns, but the π-π stacking distance of ad-
jacent pyrene units was not especially short at about 5.82 Å
due to the introduction of the two bulky tBu groups in the
pyrene rings at the 2- and 7-positions. The photophysical
properties of these monomers were carefully examined in
different organic solvents, and these data strongly indicate
their promising application as blue-emitting materials in or-
ganic light-emitting diodes (OLEDs).
Pyrenes belong to the class of polycyclic aromatic hydro-
carbons (PAHs) and have been widely used in many appli-
cations as fluorescent probes[9] or fluorescent sensors[10] by
virtue of their excellent fluorescence properties. However,
the use of pyrenes as emitters in OLEDs is limited because
pyrenes easily form π aggregates/excimers in concentrated
solution and the solid state, and the formation of π aggre-
gates/excimers leads to long-wavelength excimer emission
with low quantum efficiency. Recently, some pyrene deriva-
tives have been developed as hole-transporting materials[11]
or host blue-emitting materials[12] in OLEDs. Thus, there is
substantial interest in designing and developing new py-
rene-based blue-light-emitting molecules with high stability
and efficiency for OLED applications.
Accordingly, our previous report on the synthesis of
4,5,9,10-tetrabromo-2,7-di-tert-butylpyrene[13] prompted us
to explore 4,5,9,10-tetrakis(phenylethynyl)pyrenes as emis-
sive materials. We surmised that the sterically bulky tBu
groups in pyrene rings at the 2- and 7-positions would help
inhibit undesirable face-to-face π stacking in solution and
the solid state.[14] In addition to the ready synthetic accessi-
bility by the Sonogashira coupling, the phenylacetylenic
groups were a priori anticipated to facilitate the construc-
tion of the cruciform-shaped structure and further extend
the conjugation length of the pyrene chromophore, re-
sulting in a shift of the wavelength of absorption and fluo-
rescence emission into the visible region of the electromag-
netic spectrum.[15] Herein, we report the synthesis and pho-
tophysical properties of a new series of pyrene-based, cruci-
form-shaped, π-conjugated, blue-light-emitting monomers
with a low degree of π stacking in the solid state and pure-
blue emission by various spectroscopic techniques.
Introduction
In recent years, carbon-rich organic compounds with a
high degree of π conjugation have received much attention
because of their unique properties as ideal materials for ad-
vanced electronic and photonic applications, such as or-
ganic light-emitting diodes (OLEDs), liquid-crystal dis-
plays, thin-film transistors, solar cells and optical storage
devices.[1–3] Among them, functionalised, cruciform-shaped,
conjugated fluorophores are well-known because they exhi-
bit interesting optical and electronic properties due to their
unique, multiply-conjugated-pathway structures. Examples
of cruciform-shaped phores are the 1,2,4,5-tetrasubstituted
(phenylethynyl)benzenes of Haley et al.,[4] the X-shaped
1,2,4,5-tetravinyl-benzenes of Marks et al.,[5] the 1,4-bis(ar-
ylethynyl)-2,5-distyrylbenzenes of Bunz et al.[6] and other
cross-shaped fluorophores developed by Nuckolls et al.[7]
and Scherf et al.[8] In particular, the tetradonor-substituted
(phenylethynyl)benzenes of Haley and coworkers showed
excellent photophysical properties with the highest quan-
tum yield. Therefore, their seminal studies on the structure-
property relationships for these materials provided valuable
information for the molecular design of high-performance
material.
[a] Department of Applied Chemistry, Faculty of Science and
Engineering, Saga University,
Honjo machi 1, Saga-shi, Saga 840-8502, Japan
Fax: +081-952-288548
E-mail: yamatot@cc.saga-u.ac.jp
[b] Chemistry Department, Loughborough University,
Loughborough, Leicestershire, LE11 3TU, UK
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.200900806.
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Pyrene-Based Light-Emitting Monomers
Results and Discussion
Synthesis
Following our previous approach, we readily obtained
4,5,9,10-tetrabromo-2,7-di-tert-butylpyrene (2) by the
Lewis-acid-catalysed bromination of 2,7-di-tert-butylpyr-
ene,[13] (Scheme 1), and it served as the starting material for
the synthesis of 4,5,9,10-tetrakis(p-R-phenylethynyl)pyrene
derivatives.
Scheme 3. Synthesis of 1,3,6,8-tetrakis[(4-methoxyphenyl)ethynyl]-
pyrene (5). Reagents and conditions: (a) [PdCl2(PPh3)2], diiso-
propylamine, PPh3, CuI, 24 h, 60–70 °C, 92 %.
Scheme 1. Synthesis of 4,5,9,10-tetrabromo-2,7-di-tert-butylpyrene
(2). Reagents and conditions: (a) Br2, Fe powder, CH2Cl2, room
temp. for 4 h, 90%.
though we introduced the same numbers of p-functionalised
phenylacetylenic groups into 4a–c and 5, their molecular
structures were quite different, a result of introducing sub-
stituents at different positions on the pyrene scaffold. Com-
pounds 4 and 5 also differ in the length and shape of their
conjugation pathways. The 1H NMR spectra of 4 and 5
were very simple due to their C2h- and D2h-symmetric struc-
tures, respectively. For example, 2,7-di-tert-butyl-4,5,9,10-
tetrakis(4-methoxyphenylethynyl)pyrene (4c) displayed a
singlet at δ = 8.89 ppm for the pyrene ring protons at the
1-, 3-, 6- and 8-positions, a pair of doublets in a 1:1 ratio
at δ = 6.99 and δ = 7.72 ppm for the aromatic protons, a
singlet at δ = 3.90 ppm for the protons of the four MeO
groups, and the protons of the two tBu groups appeared as
a singlet at δ = 1.68 ppm. Furthermore, the FT-IR spectra
of 4 and 5 showed characteristic peaks at 2991–2961 (for
C–H stretching of aliphatic segments), 2195–2198 (for
–CϵC–), ca. 1600, 1500 and 1460 (aromatic) cm–1. In ad-
dition, 4c and 5 displayed strong absorption peaks at ca.
1170 and 1030 cm–1 (for C–O–C stretching). Simulta-
neously, we also established the structures of 4 and 5 on the
basis of the molecular ions at m/z 714, 939, 835 and 722 in
their mass spectra, respectively. All results were consistent
with the proposed cruciform-shaped structures (see the
Supporting Information). The 4,5,9,10-tetrakis(phenylethy-
nyl)pyrenes 4 were stable solids that could be stored in air
at room temp. for a prolonged period of time. Compounds
4 had good solubility in all common organic solvents and
high melting points up to 260 °C.
The modified Sonogashira coupling reaction of the tetra-
bromide 2 with various phenylacetylenes 3 produced the
corresponding 2,7-di-tert-butyl-4,5,9,10-tetrakis(p-R-phen-
ylethynyl)pyrenes 4 in excellent yields (recrystallisation
yields) as light-green fluorescent solids (Scheme 2). As a
comparison, we synthesised 5, [1,3,6,8-tetrakis(4-methoxy-
phenylethynyl)pyrene] according to the literature procedure
(Scheme 3).[15]
Scheme 2. Synthesis of 4,5,9,10-tetrakis(phenylethynyl)pyrene de-
rivatives 4a–c. Reagents and conditions: (a) [PdCl2(PPh3)2], CuI,
PPh3, Et3N/DMF (1:1), 24–48 h, 100 °C.
Molecular Structure and Crystal Packing of 4c
We fully determined the structures of these new pyrenes
We further confirmed the molecular structure of 4c by
4 and 5 by 1H NMR, 13C NMR, FT-IR spectroscopy, mass single-crystal X-ray analysis.[16] We grew the crystals by
spectroscopy as well as elemental analysis. Interestingly, al- slow evaporation of a CHCl3/CH2Cl2 solution. We obtained
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73

J. Hu, M. Era, M. R. J. Elsegood, T. Yamato
FULL PAPER
this monomer as yellow needles, which provided excellent-
quality data. The crystallographic data for this monomer
are presented in Table 1.
Table 1. Summary of the crystal data of 4c.
Parameter
4c
Empirical formula
Formula weight [gmol–1]
Temperature [K]
Wavelength [Å]
Crystal system
Space group
Crystal colour and size [mm]
a [Å]
b [Å]
C60H50O4
835.00
150(2)
0.71073
monoclinic
P21/c
yellow, 2.00ϫ0.05ϫ0.02
12.865(3)
5.9903(13)
28.642(6)
90
c [Å]
α [°]
β [°]
90.033(4)
90
2207.3(8)
2
1.256
0.077
γ [°]
Volume [Å3]
Z
Density, calcd. [g m–3]
Absorption coefficient [mm–1]
F(000)
884
θ range for data collection [°]
Reflections collected
Independent reflections
Observed data [F2 Ͼ 2σ(F2)]
Rint
Restraints/parameters
Goodness-of-fit on F2
R1 [F2 Ͼ 2σ(F2)]
wR2 (all data)
1.42 to 26.46
18599
4544
2297
0.1134
0/295
0.978
0.0566
0.1463
Figure 1. X-ray crystal-structure diagrams of 4c (i) top view; (ii)
side view.
Structure diagrams of 4c are shown in Figure 1. The
molecule lies on a two-fold axis; thus, half is crystallograph-
ically unique. The four ethynyl carbons are essentially co-
planar with the central pyrene ring, while the four C6H4
rings are not coplanar with the central pyrene ring but have
twist angles of 18.3° for ring C(15)–C(20) and 23.6° for ring
C(24)–C(29), relative to the central, pyrene core. In ad-
dition, the relative twist angle between these two C6H4
groups was 39.5°. The methoxyl groups were twisted by
14.6° and 2.5° relative to their respective rings of attach-
ment.
The crystal packing of pyrene molecules has been studied
previously. A card-packed structure was observed, and the
molecules exhibited an interplanar separation of ca. 3.5 Å
with strong π-π stacking through the involvement of 14 car-
bons in each pyrene molecule.[17] As shown in Figure 2, the
present cruciform-shaped molecules of 4c are packed in a
herringbone arrangement in the crystal. In addition, each
4c molecule displays a 24-point, π-π stacking interaction
with molecules above and below (Figure 2, structure i)
using both the pyrenyl carbons and phenylethynyl carbons
for both interactions. Interestingly, the closest intermo-
lecular C–C distance between the central pyrenyl planes of
two adjacent 4c molecules is not especially short at about
5.82 Å in this crystal lattice. These results strongly indicate
that the two bulky tBu groups attached to the pyrene rings
at the 2- and 7-positions play an important role in sup-
pressing the undesirable, face-to-face, π-π stacking in the
solid state.
Figure 2. Packing diagrams of 4c: (i) view parallel to b, highlighting
the π-π stacking; (ii) view parallel to c, showing the herringbone
packing motif.
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Pyrene-Based Light-Emitting Monomers
Hence, because efficient π stacking in emitting molecules
can lead to extensive excimer formation in the solid state or
in thin films, with a consequent shift in the emission spec-
trum and a reduction in the quantum yield of fluores-
cence,[18] the low degree of π stacking in the crystal lattice in
these current pyrene-based monomers 4 suggests that they
should be robust blue-light-emitting materials.
Photophysical Properties
We measured the UV/Vis absorption and fluorescence
spectroscopic data of these new pyrenes with cruciform-
shaped conjugation in dilute CH2Cl2 solution at room
temp. and the results are presented in Table 2, together with
those of pyrenes 1 and 5. The normalised UV/Vis absorp-
tion spectrum is shown in Figure 3. For pyrene 1, the ab-
sorption spectra was almost identical to that of the parent
Figure 3. Normalised UV/Vis absorption spectra of 4 recorded in
CH2Cl2 at about 10–5  and 25 °C, compared with those of pyrenes
pyrene,[19] with three well-resolved, sharp, absorption bands 1 and 5.
observed in the 300–350 nm region. The slight bathoch-
romic shift (ca. 3 nm) we ascribe to the increased π-electron
introduced at the 4-, 5-, 9- and 10-positions. Interestingly,
density on the pyrene ring, arising from the electron-donat-
ing nature of the two tBu groups at the 2- and 7-posi-
tions.[20] In contrast, the optical absorption spectrum of 5
was very broad and less well resolved. The longest wave-
length band was substantially bathochromically shifted by
ca. 140 nm in comparison to pyrene 1 due to the extended
conjugation length of the pyrene chromophore with the
phenylacetylenic units at the 1-, 3-, 6- and 8-positions. In
addition, it reveals a vibronic feature characteristic of the
unsubstituted parent pyrene with a short-wavelength ab-
sorption maximum at ca. 350 nm and the conjugation seg-
ments with a long wavelength absorption maximum at ca.
480 nm.
although the vibronic features of 4a–c were more similar to
those of 5 than to those of 1 (Figure 3), the spectra of 4
were less red-shifted than that of 5, despite the presence of
the two electron-donating tBu groups in 5.[20] A reasonable
explanation for these different shifts between 4 and 5 is
their quite different conjugation pathways. For 4, the four
phenylacetylenic units are connected with the central pyr-
ene moieties at the nearby 4-, 5-, 9- and 10-positions to
afford a short, cruciform, π-conjugated, molecular struc-
ture. Hence, short, cruciform, π conjugation occurs; for 5,
however, these four phenylacetylenic units are connected
with pyrene moieties at the more distant 1-, 3-, 6- and 8-
positions, resulting in a longer cruciform, π-conjugated
structure. Hence, the conjugation length of 5 is larger than
that of 4, which leads to a larger red-shift to ≈ 500 nm.
Additionally, among 4a–c, the [(p-methoxyphenyl)ethynyl]-
pyrene 4c displayed the largest bathochromic shift of the
absorption bands, which could be attributed to the methoxy
group having the strongest electron-donating ability of the
three different substituents.
Table 2. Optical absorption and emission spectroscopic data for
4a–c in CH2Cl2 (ca. 10–5–10–6 ) and room temp., compared with
that of pyrenes 1 and 5.[a]
Entry
Absorption[b]
λabs [nm]
Fluorescence[c]
ε [–1 cm–1] λmax [nm] (λex)[d] [cm–1] Φf[e]
Stokes shift
1
2
3
4
5
1
339
410
413
415
477
26300
36300
38900
45700
88900
378 (252)
441 (324)
448 (328)
453 (333)
496 (260)
3043
1714
1892
2021
803
0.12
0.66
0.72
0.98
0.96
4a
4b
4c
5
Upon excitation, a dilute solution (ca. 10–6 ) of 4 and 5
in CH2Cl2 at room temp. showed pure-blue and green emis-
sion, respectively (Figure 4). For example, compared with
the lowest-energy emission band of pyrene 5 at 496 nm, the
lowest-energy emission band of 4a, 4b and 4c occurred at
441, 448 and 453 nm, respectively. The fluorescence emis-
sion bands of 4a, 4b and 4c were almost identical. Only one
emission band was observed in the visible pure-blue region,
which indicated all these emissions occur from the lowest
[a] All measurements were performed under degassed conditions.
[b] λabc is the absorption band appearing at the longest wavelength
at ≈ 10–5  in CH2Cl2. [c] λex is the fluorescence band appearing at
the shortest wavelength at ≈ 10–6  in CH2Cl2. [d] Wavelength of
excitation. [e] Fluorescence quantum yields; the values (Ϯ0.01–
0.03) are relative to that of diphenylanthracene (0.90 in cyclohex-
ane).
On the other hand, compared with that of pyrene 1, all excited state with the largest oscillator strength. All spectra
the UV/Vis absorption spectra of 4a–c were broad and less are bathochromically red-shifted into the pure-blue visible
well-resolved, and the longest–wavelength, π-π*, absorption region. Additionally, the fluorescence spectra of these cruci-
maximum of 4a, 4b and 4c occurred at 410, 413 and form-shaped compounds systematically varied in agreement
415 nm, respectively (Table 2); these bands were batho- with the electronic absorption spectra, implying that the en-
chromically red-shifted by 71–76 nm from the correspond- ergy gap between the ground and excited states decreases in
ing band of 1 as a result of the extended conjugation length the order of 4a Ͼ 4b Ͼ 4c (Table 2). The fluorescence spec-
of the pyrene chromophore with the phenylacetylenic units tra were independent of the excitation wavelength. The
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J. Hu, M. Era, M. R. J. Elsegood, T. Yamato
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fluorescence Stokes shifts increased in the order of 4a Ͻ 4b trations. Therefore, we examined the effect of concentration
Ͻ 4c. The fluorescence quantum yields of 1, 4a–c and 5 on the fluorescence emission of 4c in CH2Cl2. By increasing
recorded in dilute CH2Cl2 solution at room temp. are also the concentration from 1.0ϫ10–8  (Figure 5, line 1) to
listed in Table 2. We found the Φf values of 1, 4a–c and 1.0ϫ10–4  (Figure 5, line 9), we observed the intensity of
5 to be in the range of 0.12–0.98 relative to that of 9,10- this emission band to gradually increase, and we observed
diphenylanthracence (0.90 in cyclohexane).[21]
the emission corresponding to only the monomer at 453 nm
(Figure 5). This evidence also indicated that the attachment
of sterically bulky tBu groups at the 2- and 7-positions can
prevent two molecules of 4 from getting close enough to
result in excimer emission at high concentrations.
Figure 4. Normalised fluorescence emission spectra of 4a–c and 5
recorded in CH2Cl2 at ca. 10–6  and 25 °C.
Figure 5. Effect of concentration on the fluorescence emission spec-
tra of 4c, recorded in CH2Cl2 at room temp. (1) 1.0ϫ10–8 , (2)
2.5ϫ10–8 , (3) 1.0ϫ10–7 , (4) 2.5ϫ10–7 , (5) 1.0ϫ10–6 , (6)
2.5ϫ10–6 , (7) 5.0ϫ10–6  (8) 2.5ϫ10–5  and (9) 1.0ϫ10–4 M.
λex = 333 nm.
We emphasise that several reported 1,3,6,8-tetrakis(phen-
ylethynyl)pyrenes[15] show very low values of quantum effi-
ciency in solution due to the possibility that free rotation
of the phenyl substituents in conjugation with the central
pyrene chromophore would allow competing nonradiative
In order to obtain more insight into the photophysical
pathways for the decay of the excited singlet state. Interest- properties of these new cruciform-shaped, conjugated pyr-
ingly, in the current cruciform-shaped 4 and 5, all com- enes 4, we examined the normalised absorption spectra and
pounds had dramatically high quantum yields. Even though emission spectra of 4a and 4c in various solvents, and the
4 and 5 share this potential for nonradiative decay, they optical data are summarised in Table 3. It is well-known
differ in important ways from previously reported struc- that the solvatochromic effect is not only dependent on mo-
tures. Compounds 4a–c have a short, cruciform-shaped lecular structure but also on the nature of the chromophore
structure, similar to that of the tetra-donor-substituted and the solvent.[23] Each monomer showed a certain solva-
(phenylethynyl)benzenes of Haley and coworkers.[4c] Com- tochromism in both the absorption and the emission spec-
pound 5 contains strongly electron-donating methoxyl trum. For example, for 4c, a change of solvent from nonpo-
groups.[15] These structural differences relative to the low- lar cyclohexane to polar DMF caused only a very slight,
quantum-yield structures provide good explanations for the positive, bathochromic shift in the π-π* absorption band
high quantum yields observed.
from 413 to 417 nm (Figure 6). On the other hand, in the
Pyrene itself did not exhibit excimer emission at a con- case of the emission spectrum of 4c, we observed a substan-
centration of 10–5  and below, but both 1-(phenylethynyl)- tial positive bathochromism with a peak around 425 nm
pyrenes[22] and a 1,3,6,8-tetrakis(phenylethynyl)pyrene[15] and a shoulder around 450 nm in cyclohexane (Figure 7),
showed excimer emission at 10–5  and even higher concen- while we observed a broad and red-shifted emission with
Table 3. Optical absorption and emission spectroscopic data for 4a and 4c in various solvents (ca. 10–5–10–6 ) at room temp.[a]
4a
4c
λabs max
λem max
Stokes shift
[cm–1]
λabs max
λem max
Stokes shift
[cm–1]
Solvent
[nm][b]
[nm][c]
Φf[d]
[nm][b]
[nm][c]
Φf[d]
Cyclohexane
THF
CH2Cl2
DMF
408
410
411
412
419
424
441
449
643
746
1714
2000
0.58
0.62
0.66
0.60
413
415
416
417
425
435
453
464
684
0.73
0.84
0.98
0.82
1050
2021
2429
[a] All measurements were performed under degassed conditions. [b] λabs is the absorption band appearing at the longest wavelength at
about 10–5  in the different solvents. [c] λex is the fluorescence band appearing at the shortest wavelength at about 10–6  in the different
solvents. [d] Fluorescence quantum yields; the values (Ϯ0.01–0.03) are relative to that of diphenylanthracene (0.90 in cyclohexane).
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Pyrene-Based Light-Emitting Monomers
only one peak at λmax = 464 nm in the solvent of high po- monomers with cruciform-shaped, π-conjugated structures
larity, DMF. We also observed similar results for 4a in both in excellent yield and fully characterised the special cruci-
the absorption and emission spectra (see the Supporting In- form-shaped structures. The results obtained through in-
formation). Although the dipole moments of 4a–c in either specting the absorption and emission spectra of these pure
the ground or excited states were indeed zero because they monomers, indicate that the extension of π conjugation in
possess C2h symmetry, the results obtained above indicated these pyrene chromophores through phenylacetylenic sub-
that the Qe (the quadrupole moments of 4 in the excited stituents serves to shift the wavelength of absorption and
state) should be larger than the Qg (the quadrupole mo- fluorescence emission into the pure-blue visible region. Sin-
ments of 4 in the ground state) because we observed posi- gle-crystal X-ray analysis indicated the two bulky tBu
tive solvatochromic effects in both the absorption and the groups on the pyrene molecule at the 2- and 7-positions
emission spectra; this implies that there is a change in the play a very important role in inhibiting the π-stacking inter-
quadrupole moment upon excitation.[24] On the other hand, actions between neighbouring pyrene units. These mole-
the fact that solvatochromic effects are more important for cules emit very bright, pure-blue fluorescence and have
emission than for absorption suggests that these new pyr- good solubility in common organic solvents and high sta-
enes 4 are more solvated in the excited state than in the bility. Hence, they are promising as blue organic light-emit-
ground state.[25]
ting materials for the fabrication of OLED devices.
Experimental Section
Genenal Remarks: All melting points are uncorrected. The 1H
NMR spectra were recorded at 300 MHz on a Nippon Denshi
JEOL FT-300 NMR spectrometer in deuteriochloroform with
TMS as an internal reference. The IR spectra were obtained as
KBr pellets with a Nippon Denshi JIR-AQ2OM spectrometer. UV/
Vis spectra were obtained with a Perkin–Elmer Lambda 19 UV/
Vis/NIR spectrometer in various organic solvents. Fluorescence
spectroscopic studies were performed in various organic solvents
in a semimicro fluorescence cell (Hellma®, 104F-QS, 10ϫ4 mm,
1400 µL) with a Varian Cary Eclipse spectrophotometer. Fluores-
cence quantum yields were measured using absolute methods. Mass
spectra were obtained with a Nippon Denshi JMS-HX110A Ultra-
high Performance Mass Spectrometer at 75 eV using a direct-inlet
system. Elemental analyses were performed with a Yanaco MT-5
analyser.
Figure 6. Normalised absorption spectra of 4c recorded in (a) cy-
clohexane, (b) THF, (c) CH2Cl2 and (d) DMF at ca. 10–5  and
25 °C.
Materials: The preparation of 2,7-di-tert-butylpyrene (1) was de-
scribed previously.[13]
Acid-Catalysed Bromination of 2,7-Di-tert-butylpyrene: To a mix-
ture of 2,7-di-tert-butylpyrene (500 mg, 1.60 mmol), CH2Cl2
(250 mL) and Fe powder (250 mg), a solution of Br2 (1,400 mg,
8.7 mmol) in CH2Cl2 (25 mL) was added dropwise over 1 h at 0 °C
with stirring. After this addition, the mixture was warmed to room
temp. and stirred for 4 h. The mixture then was poured into a large
amount of ice-water and extracted with CH2Cl2 (250 mL). The or-
ganic extracts were washed with aqueous Na2S2O3 (10%), water
and brine, dried (Na2SO4), and the solvents were evaporated. The
residue was dissolved in CH2Cl2 and purified by column
chromatography, eluting with hexane, to give a pale-yellow solid
(980 mg, 90%); m.p. 286–288 °C (ref.[13] 286–288 °C). 1H NMR
(300 MHz, CDCl3): δ = 1.62 (s, 18 H, tBu), 8.87 (s, 4 H, pyrene-
H) ppm.
General Procedure for the Sonogashira Coupling Reaction Towards
the Synthesis of 4a–c: 4,5,9,10-Tetrabromo-2,7-di-tert-butylpyrene
2 (0.32 mmol), [PdCl2(PPh3)2] (0.016 mmol), CuI (0.016 mmol),
PPh3 (0.032 mmol) and a phenylacetylene (2.56 mmol) were added
to a degassed solution of DMF (10 mL) and Et3N (10 mL) under
argon. The resulting mixture was stirred at 100 °C for the time
mentioned in the individual cases. The reaction mixture was then
cooled to room temp. and quenched with Et2O and extracted. The
solvent was removed to give the crude reaction mixture, which was
Figure 7. Normalised fluorescence spectra of 4c recorded in (a) cy-
clohexane, (b) THF, (c) CH2Cl2 and (d) DMF at ca. 10–6  and
25 °C.
Conclusions
By using a modified Sonogashira coupling reaction, we
have prepared a new series of well-defined, pyrene-based further worked up as indicated in the individual cases.
Eur. J. Org. Chem. 2010, 72–79
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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77

J. Hu, M. Era, M. R. J. Elsegood, T. Yamato
FULL PAPER
2,7-Di-tert-butyl-4,5,9,10-tetrakis(phenylethynyl)pyrene (4a): To a CDCl3): δ = 160.0, 149.9, 143.1, 133.2, 129.6, 123.8, 123.0, 116.4,
stirred solution of 4,5,9,10-tetrabromo-2,7-di-tert-butylpyrene (2,
200 mg, 0.32 mmol), Et3N (10 mL) and DMF (10 mL), was added
[PdCl2(PPh3)2] (12 mg, 0.016 mmol), CuI (3.2 mg, 0.016 mmol) and
PPh3 (8.2 mg, 0.032 mmol), and the mixture was stirred for 30 min
at 0 °C under argon. Phenylacetylene (260 mg, 2.56 mmol) was
then added, and the mixture was heated to 100 °C with stirring for
24 h. After it was cooled, the mixture was diluted into Et2O
(150 mL) and washed successively with saturated aqueous NH4Cl,
H2O and brine. The organics were dried (MgSO4), and the solvents
were evaporated. In order to obtain pure product, the crude prod-
uct was purified twice by column chromatography, eluting with
hexane/CH2Cl2 (9:1), and recrystallised from hexane to afford the
desired compound as pale-green prisms (149 mg, 65%); m.p. Ͼ
114.3, 99.8, 86.8, 55.4, 35.6, 32.0 ppm. MS (EI): m/z 835.04 [M]+.
C60H50O4 (834.40): calcd. C 86.30, H 6.04, O 7.66; found C 86.15,
H 6.10, O 7.56.
1,3,6,8-Tetrakis[(4-methoxyphenyl)ethynyl]pyrene (5):[15] 1,3,6,8-
Tetrabromopyrene (200 mg, 0.39 mmol), [PdCl2(PPh3)2] (14 mg,
0.0195 mmol), CuI (4 mg, 0.0195 mmol), PPh3 (10 mg,
0.039 mmol) and 4-ethynylanisole (412 mg, 3.12 mmol) were added
to a degassed solution of diisopropylamine (10 mL) and THF
(10 mL) under argon. The resulting mixture was heated at 70 °C
with stirring for 24 h. After the mixture was cooled, the red-orange
precipitate was filtered and washed with CHCl3 (50 mL) and ben-
zene (50 mL) and then recrystallisation from CHCl3 to afford the
pure desired compound as a red-orange powder (259 mg, 92%);
300 °C. IR (KBr): ν = 2961, 2197 (–CϵC–), 1597, 1493, 1442, 1362,
˜
1
m.p. 236–238 °C. H NMR (300 MHz, CDCl3): δ = 3.89 (s, 12 H,
877, 755, 690, 561, 530 cm–1. 1H NMR (300 MHz, CDCl3): δ =
1.69 (s, 18 H, tBu), 7.45–7.54 (m, 12 H, Ar-H), 7.80 (d, J = 8.7 Hz,
8 H, Ar-H), 8.93 (s, 4 H, pyrene-H) ppm. 13C NMR (75 MHz,
CDCl3): δ = 149.8, 131.8, 129.5, 128.7, 128.6, 124.1, 123.7, 123.0,
121.3, 99.6, 87.8, 35.7, 32.0 ppm. FAB-HRMS: calcd. for C56H42
[M]+ 714.92; found 714.38. C56H42 (714.38): calcd. C 93.62, H 5.38;
found C 93.59, H 5.90.
OMe), 6.97 (d, J = 9.0 Hz, 8 H, Ar-H), 7.67 (d, J = 8.7 Hz, 8 H,
Ar-H), 8.40 (s, 2 H, pyrene-H2,7), 8.74 (s, 4 H, pyrene-H4,5,9,10
)
ppm. 13C NMR of this compound could not be determined due to
its low solubility. MS (EI): m/z 722.82 [M]+. C52H34O4 (722.35):
calcd. C 86.41, H 4.74, O 8.85; found C 86.35, H 4.75, O 8.90.
Crystal Data and Refinement Details for 4c: Diffraction data were
collected using a Bruker SMART APEX II CCD diffractometer
using narrow frames to θmax = 26.46°.[26] Data were corrected for
absorption on the basis of symmetry-equivalent and repeated data
(minimum and maximum transmission factors: 0.861, 0.999) and
Lp effects. The structure was solved by direct methods and refined
on F2 using all the data.[27] H atoms were constrained in a riding
model. Further details can be found in Table 1 and ref.[16]
2,7-Di-tert-butyl-4,5,9,10-tetrakis[(4-tert-butylphenyl)ethynyl]pyrene
(4b): To a stirred mixture of 4,5,9,10-tetrabromo-2,7-di-tert-butyl-
pyrene 2 (200 mg, 0.32 mmol), Et3N (10 mL) and DMF (10 mL),
was added [Pd(PPh3)2Cl2] (12 mg, 0.016 mmol), CuI (3.2 mg,
0.016 mmol) and PPh3 (8.2 mg, 0.032 mmol), and the mixture was
stirred for 30 min at 0 °C under argon. (4-tert-Butylphenyl)acetyl-
ene (404 mg, 2.56 mmol) was then added to the mixture, which was
heated to 100 °C with stirring for 48 h. After it was cooled, the
mixture was diluted into Et2O (200 mL) and washed successively
with saturated aqueous NH4Cl, H2O and brine. The organics were
dried (MgSO4), and the solvents were evaporated. The crude prod-
uct was purified by column chromatography, eluting with hexane/
ethyl acetate (9:1), and recrystallised from hexane to give the de-
sired product as pale-green prisms (204 mg, 68%); m.p. Ͼ 300 °C.
Supporting Information (see also the footnote on the first page of
1
this article): H NMR spectra of compounds 4a–4c and 5, UV/Vis
spectra of 4a, fluorescence spectra of 4a.
Acknowledgments
We thank the CANON Company and the Royal Society of Chemis-
try for financial support.
IR (KBr): ν = 2962, 2198 (–CϵC–), 1601, 1516, 1505, 1464, 1394,
˜
1363, 1327, 1268, 1247, 1096, 1016, 880 cm–1. 1H NMR (300 MHz,
CDCl3): δ = 1.40 (s, 36 H, tBu), 1.68 (s, 18 H, tBu), 7.49 (d, J =
8.2 Hz, 8 H, Ar-H), 7.74 (d, J = 8.5 Hz, 8 H, Ar-H), 8.92 (s, 4 H, [1] a) A. de Meijere (Ed.), Topics in Current Chemistry, Carbon-
pyrene-H) ppm. 13C (75 MHz, CDCl3): δ = 152.0, 149.7, 131.6,
129.6, 125.6, 124.0, 122.8, 121.2, 120.7, 99.8, 87.3, 35.6, 34.9, 32.0,
31.3 ppm. MS (EI): m/z 939.36 [M]+. C72H74 (938.60): calcd. C
92.06, H 7.94; found C 92.02, H 8.01.
Rich Compounds, I, Springer, Berlin, Germany, 1998, vol. 196;
b) A. de Meijere (Ed.), Topics in Current Chemistry, Carbon
Rich Compounds, II, Springer, Berlin, Germany, 1999, vol. 201;
c) F. Diederich, P. J. Stang, R. R. Tykwinski (Eds.), Acetylene
Chemistry: Chemistry, Biology, and Materials Science, Wiley-
VCH, Weinheim, Germany, 2005; d) M. M. Haley, R. R. Tyk-
winski (Eds.), Carbon-Rich Compounds: From Molecules to
Materials, Wiley-VCH, Weinheim, Germany, 2006.
2,7-Di-tert-butyl-4,5,9,10-tetrakis[(4-methoxyphenyl)ethynyl]pyrene
(4c): To a stirred mixture of 4,5,9,10-tetrabromo-2,7-di-tert-butyl-
pyrene 2 (200 mg, 0.32 mmol), Et3N (10 mL) and DMF (10 mL),
was added [PdCl2(PPh3)2] (12 mg, 0.016 mmol), CuI (3.2 mg,
0.016 mmol) and PPh3 (8.2 mg, 0.032 mmol), and the mixture was
stirred for 30 min at 0 °C under argon. 4-Ethynylanisole (340 mg,
2.56 mmol) was then added, and the mixture was heated to 100 °C
with stirring for 48 h. After it was cooled, the mixture was diluted
into Et2O (150 mL) and washed successively with saturated aque-
ous NH4Cl, H2O and brine. The organics were dried (MgSO4), and
the solvents were evaporated. The crude product was purified by
column chromatography, eluting with a mixture of hexane/CH2Cl2
(5:1), and recrystallised from ethyl acetate to afford the pure desired
compound as a pale-green solid (200 mg, 75%); m.p. 262–264 °C.
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Received: July 18, 2009
Published Online: November 11, 2009
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