Synthesis and photophysical properties of a highly fluorescent ditopic ligand based on 1,6-bis(ethynyl)pyrene as central aromatic core

Article abstract of DOI:10.1002/ejoc.200500059

A simple synthetic route for the efficient preparation and purification of two regioisomers of a pyrene derivative containing two ethynyl groups - 1,6- and 1,8-bis(ethynyl)pyrene - is described. The former compound was used as a building block for the ste

Full text of DOI:10.1002/ejoc.200500059

FULL PAPER
Synthesis and Photophysical Properties of a Highly Fluorescent Ditopic Ligand
Based on 1,6-Bis(ethynyl)pyrene as Central Aromatic Core
Stéphanie Leroy-Lhez*[a] and Frédéric Fages*[b]
Keywords: Pyrene / 2,2Ј-Bipyridine / Fluorescence / Synthesis / Conjugated oligomer
A simple synthetic route for the efficient preparation and pu-
rification of two regioisomers of a pyrene derivative contain-
ing two ethynyl groups—1,6- and 1,8-bis(ethynyl)pyrene—is
described. The former compound was used as a building
block for the stepwise synthesis of a highly conjugated, di-
topic bis(2,2Ј-bipyridine) ligand (2) containing the pyrene
nucleus at the central core. The fluorescence properties of
ligand 2 are reported.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
ments. As such, triad 2 was anticipated to act not only as a
nanoscale fluorophore,[17] but also as a rigid photonic wire
for the bridging of two luminophoric transition metal cen-
ters.[18] Polyaromatic systems have been used as energy relay
subunits within conjugated backbones.[19] Moreover, conju-
gated oligomers and polymers incorporating ligand units
have received great attention as photoconducting, photonic,
or sensory materials.[20]
Introduction
Ever since it was first isolated from coal tar in 1837,[1]
pyrene has been the subject of tremendous investigation.
Indeed, this polycyclic aromatic hydrocarbon exhibits a set
of many interesting electrochemical[2] and photophysical[3]
attributes, which have resulted in its utilization in a variety
of scientific areas. Some recent advanced applications of py-
rene include fluorescence labeling of oligonucleotides for
DNA assay,[4] electrochemically generated luminescence,[5]
carbon nanotube functionalization,[6] fluorescence chemo-
sensing,[7] design of luminescent liquid crystals,[8] supramo-
lecular self-assembly,[9] etc.
Pyrene-containing receptors for transition metal ions
were recently reported as a versatile class of photoactive
supramolecular systems.[10–12] In that connection, we had
reported previously on dyad and triad molecules in which
pyren-1-yl or pyrene-1,6-diyl units, respectively, are con-
nected to the 2,2Ј-bipyridine (bpy) moiety by a single C–C
bond.[13,14,17] Recently, rod-like bpy-pyrene dyads, such as
compound 1, containing a phenylene ethynylene unit as ri-
gid bridge were shown to display outstanding photophysical
features owing to extended conjugation.[15,16] In particular,
the singlet and triplet excited state properties of 1 were
found to be dramatically sensitive to the binding of metal
ions, such as zinc(ii) and ruthenium(ii). This prompted us
to synthesize the related ditopic ligand 2, in which the cen-
tral pyrene ring is symmetrically substituted in the 1,6-posi-
tions with two bpy-terminated phenylene ethynylene seg-
Critical to the synthesis of ligand 2 is the efficient prepa-
ration of the acetylenic precursor 1,6-bis(ethynyl)pyrene
(5A; Scheme 2, below), which requires regiochemical con-
trol of the aromatic substitution on the pyrene ring. Direct
electrophilic substitution occurs exclusively at the electron-
rich 1-, 2-, 3-, and 4-positions, affording isomeric mixtures
in variable ratios depending on the experimental conditions.
Friedel–Crafts monoacetylation of pyrene is known to af-
ford the 1-substituted monoacetyl derivative specifically un-
less an excess of acetyl chloride is used. In the latter case
the 1,3-, 1,6-, and 1,8-diacetyl isomers are formed and can
be separated by a sequence of tedious purification steps in-
volving both chromatography and crystallization.[21] Simi-
larly, dibromination of pyrene with Br2 gives a mixture of
[a] CIMMA UMR 6200 CNRS, Université d’Angers,
2 Bd Lavoisier, 49045 Angers Cedex, France
Fax: +33-2-41-73-54-05
E-mail: stephanie.lhez@univ-angers.fr
[b] GCOM2 UMR 6114 CNRS, Université de La Méditerranée,
Faculté des Sciences de Luminy
Case 901,
13288 Marseille Cedex 9, France
E-mail: fages@luminy.univ-mrs.fr
2684
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejoc.200500059
Eur. J. Org. Chem. 2005, 2684–2688

Highly Fluorescent Ditopic Ligand Based on 1,6-Bis(ethynyl)pyrene
FULL PAPER
1,6- and 1,8-dibromopyrenes, which can be resolved by attempts to resolve the two regioisomers by crystallization
fractional crystallization.[22] In our hands, this method of- were unsuccessful. Eventually the mixture was directly sub-
fered the less soluble 1,6-bromopyrene in pure form but low jected to an ethynylation reaction, under Sonogashira con-
yield, but not the pure 1,8-isomer.[14] In addition, 2,7-sub- ditions, with 2-methylbut-3-yn-2-ol, which resulted in the
stituted pyrenes also represent attractive scaffolds on which formation of the corresponding diols 4A and 4B
to base the design of conjugated oligomers with a well de- (Scheme 1). It was anticipated that the divergent or conver-
fined straight shape.[23] These, however, are much less solu- gent orientations of the hydrogen-bonding hydroxy func-
ble, unless alkyl chains are introduced at the 4- and 9-posi- tions in 4A or 4B, respectively, could endow such com-
tions, and substitution at the 2,7-positions of pyrene re- pounds with markedly different solubility properties in or-
quires tedious indirect routes.,[24,25] Moreover, the molecu- ganic solvents. Indeed, the somewhat more centrosymmetric
lar orbital coefficients of the 2- and 7-centers are nearly compound 4A was found to be much less soluble than its
zero, which induces electronic decoupling of the pyrene ring 1,8-isomer, precipitating almost quantitatively upon tritura-
with the π-delocalized substituent. Although this peculiar tion of the crude product with dichloromethane. After fil-
property may impart new electronic features to the conju- tration, compound 4A, obtained in 21% overall yield from
gated rods, it has scarcely been investigated so far.[26]
pyrene, could be used in the following step without further
Here we report an efficient and expeditious method al- purification. Column chromatography of the soluble frac-
lowing both the pure 1,6- and 1,8-isomers of bis(ethynyl)- tion allowed the 1,8-isomer to be obtained in pure form.
pyrene, 5A and 5B, respectively, to be obtained in good
After conventional treatment under basic conditions,
yield as precursors of phenylene ethynylene-based conju- compounds 4A and 4B yielded the corresponding bis(ethy-
gated systems. The synthesis of the ditopic ligand 2 from nyl)pyrene derivatives 5A and 5B, respectively. 1,6-Bis(ethy-
5A is described. Photophysical measurements show that 2 nyl)pyrene (5A) afforded oligomer 2 after a sequence of Pd-
strongly absorbs and emits in the visible region.
mediated cross-coupling reactions of ethynyl derivatives
with the corresponding haloarenes (Scheme 2), similarly to
the synthetic route previously described for compound 1.[15]
Thanks to the presence of branched alkyl chains, com-
pound 2 is soluble in common organic solvents.
Results and Discussion
Synthesis
The synthetic route to triad 2 involved iodination of py-
rene by a standard procedure,[27,28] which afforded a mix-
ture of the 1,6- and 1,8-diiodo derivatives in a 7:3 ratio ac-
Spectroscopic Properties
Triad compound 2 does indeed behave as a strongly vis-
cording to 1H NMR spectroscopy (Scheme 1). Several ible absorbing and emitting chromophore in fluid solution
Scheme 1. a) I2/KIO3 (1 and 0.4 equiv.), CH3COOH/H2O/H2SO4 (100:10:1 ratio), 40 °C, 4 h, 32%. b) HCϵC(CH3)2OH (2.3 equiv.),
Et2NH, [PdII(PPh3)2Cl2] (2.2 mol%), CuI (2.8 mol%), 50 °C, 20 h.
Scheme 2. a) NaOH (9 equiv.), toluene, reflux, 3 h, 58%. b) Compound 6 (2 equiv.), Et2NH, [PdII(PPh3)2Cl2] (10 mol%), CuI (10 mol%),
50 °C, 20 h, 61%. c) KOH (13 equiv.), toluene, reflux, 1 h, 53%. d) 4-Bromo-2,2Ј-bipyridine (2.2 equiv.), toluene, iPr2NH, [Pd(PPh3)4]
(13 mol%), 80 °C, 24 h, 61%.
Eur. J. Org. Chem. 2005, 2684–2688
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. Leroy-Lhez, F. Fages
FULL PAPER
at room temperature. Indeed, its absorption maximum for matic pyrene nucleus.[15b] The Stokes’ shift values in both
the lowest-energy transition band is red-shifted by about toluene and THF are in the 700–850 cm–1 range and so are
60 nm relative to dyad 1 (Table 1, Figure 1) and the molar significantly lower than those obtained for ligand 1
absorption coefficient is nearly twice as high as that of 1 at (1760 cm–1 in both solvents). These results, together with
the maximum wavelength.[15] The oscillator strength, calcu- those obtained from electronic absorption spectroscopy,
lated with the assumption of a single transition for the low- point to 2 having a more rigid structure than 1 in both
est-energy absorption,[14,15] is 2.42 in THF as solvent, which the ground and the excited states. Compound 2 is strongly
is remarkably high in comparison with the value obtained emissive in the visible region, as evidenced by the high value
for 1 (0.80).[15b] The electronic absorption features of 2 were of the fluorescence quantum yield (Table 1). The excited
found to be independent of the solvent polarity. This obser- state of 2 was observed to decay monoexponentially and to
vation, indicative of the absence of any significant charge display a very short fluorescence lifetime (1 ns in THF).
transfer in the ground state, is in agreement with previous The radiative rate constants—kf and its reduced value
observations on bpy-pyrene ligands.[14,15] Moreover the kf/n3νf3, which takes the wavelength dependence on the
longest-wavelength absorption band is observed to be more emission probability in account—were calculated in THF
structured than those of monosubstituted pyrene analogue (Table 1). Both values were found to be higher for 2 than
derivatives.
for 1. All together, the photophysical data obtained for 2
are consistent with the occurrence of a highly allowed ππ*
singlet excited state in which the excited state of the pyrene
chromophore exhibits a strong 1La character. Moreover,
such stabilization of the long-axis polarized transition
seems to be more effective in the case of disubstitution at
the 1- and 6-positions of the pyrene nucleus.
Table 1. Photophysical properties of ligands 1 and 2 at room tem-
perature in toluene and THF; “–” = not determined.
Ligand 2
Ligand 1[15b]
Toluene
THF
Toluene
THF
ε [m–1·cm–1]
λmax (abs.)
λmax (em.)
Δν [cm–1]
78300
94600
47600
411
443
1760
0.52
–
52600
411
443
1760
0.68
1.7
462, 441 461, 440
478
703
0.50
–
480
837
0.60
1.0
Conclusions
Φfluo
τ [ns]
We have described a very efficient method to introduce
a pyrene chromophore in the central core of a phenylene
ethynylene oligomer. The approach relies on the function-
alization of a mixture of 1,6- and 1,8-dihalogenated pyrene
derivatives with 2-methylbut-3-yn-2-ol under Sonogashira
conditions. The resulting mixture of dialcohol isomers is
simply resolved by crystallization, to yield the 1,6- and 1,8-
bis(ethynyl)pyrene difunctional building blocks. As an ex-
ample, the use of the 1,6-isomer allowed to us construct the
conjugated ditopic ligand 2, which exhibits an intense vis-
ible fluorescence emission. Use of the same precursor for
the preparation of conjugated polymers with unique photo-
physical properties could also be envisaged. Moreover, the
1,8-isomer, also obtained in this study, could represent a
precursor of choice for the generation of conjugated macro-
cycles.
kf [108 s–1]
kf/νf3n3 [10–5 s–1·cm3]
–
–
6.0
–
4.0
2.4
–
1.3
Experimental Section
General Remarks: All solvents used for spectroscopic measure-
ments were of spectroscopic grade and were used as commercially
available. Electronic absorption spectra were recorded with a Hita-
chi U3300 spectrophotometer. The integrated absorption intensity,
͐εdνa, which is proportional to the oscillator strength (f) was calcu-
lated from the absorption spectra for the lowest-energy transition.
Fluorescence spectra were recorded in nondeoxygenated solvents
at 20 °C with either a Hitachi F4500 or a Spex Fluorolog spectro-
fluorimeter. We did not find any difference in emission properties
with or without degassing the solutions. Fluorescence quantum
yields were determined with quinine sulfate as a standard (Φf =
0.55 at 25 °C in 1 n H2SO4). The relative error in the quantum
yields is 5%. The laser setup used in these experiments consists
Figure 1. Absorption (–) and corrected fluorescence emission (···)
spectra of 2 in THF at room temperature (c = ca. 3×10–6 mol·L–1).
The corrected fluorescence emission spectrum of 2 (Fig-
ure 1 and Table 1) was found to be independent both of the
polarity of the solvent and of the excitation wavelength.
The fluorescence excitation spectrum matched the absorp-
tion profile over the entire wavelength range. Consistently
with the case of monobpy-pyrene systems, the fluorescence
emission spectrum of 2 displays a clear vibrational struc-
ture. The vibronic spacing is found to be ca. 1300 cm–1,
which is in agreement with the stretching modes of the aro- of a hybrid mode-locked dye laser associated with an actively
2686 © 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjoc.org
Eur. J. Org. Chem. 2005, 2684–2688

Highly Fluorescent Ditopic Ligand Based on 1,6-Bis(ethynyl)pyrene
FULL PAPER
mode-locked cw-pumped Nd:YAG laser (Coherent “Antares 76-
S”), a dye amplifier and a Nd:YAG regenerative amplifier. Pulses
of 1 ps (1 mJ, 10 Hz repetition rate) tuned to 600 nm were gener-
ated. The samples were excited with 1 ps pulses at 300 nm gener-
ated by frequency doubling of the fundamental 600 nm laser pulse
in a KDP crystal. Fluorescence was collected by use of achromatic
lenses and detected through a polarizer set at magic angle (54.7°)
with a Hamamatsu C5680 streak camera and a Chromex 250IS
spectrograph.
from hot toluene to afford 5A as brown needles (200 mg, 58%
yield). 1H NMR (250 MHz, CDCl3): δ = 3.64 (s, 2 H, CϵC–H),
8.13–8.22 (m, 6 H, Hpyr), 8.62 (d, 2 H, J = 9.1 Hz, H2 and H7) ppm.
13C NMR (50.3 MHz, CDCl3): δ = 83.0 (CϵC), 125.1, 126.3, 128.3,
130.6 ppm. MS (EI), m/z: 250 [M]+.
Diethynyl Compound (7): A solution of 5A (0.401 g, 1.6 mmol), 6
(1.701 g, 3.13 mmol), and freshly distilled diethylamine (50 mL)
was freeze–pump–thaw-degassed and transferred to a mixture of
Pd[PPh3]2Cl2 (115.5 mg, 0.16 mmol) and CuI (30.8 mg, 0.16 mmol)
under argon atmosphere. The reaction mixture was heated at 50 °C
for 20 h under inert atmosphere. After the mixture had cooled
down to room temperature, the solvent was removed under vac-
uum. The crude product obtained was dissolved in dichlorometh-
ane and filtered. The filtrate was concentrated and purified by
chromatography on a silica gel column, with elution with dichloro-
methane, to yield the coupling product as an orange solid (1.03 g,
All chemicals were purchased from Aldrich Chemical Co. and were
used as received. Solvents were distilled prior to use. 1H and 13C
NMR spectra were recorded with a Bruker AC 200 or a Bruker
AC 250 spectrometer. Mass spectra were obtained with a VG Au-
tospec-Q Micromass spectrometer either in the EI or the LSIM+
(NBA matrix) mode.
Diiodopyrene (3A, 3B): Pyrene (12 g, 59.3 mmol) was dissolved in
acetic acid (400 mL) at 90 °C. The reaction mixture was cooled
to 40 °C, and then water (40 mL), iodine (15.07 g, 59.36 mmol),
potassium iodate (5.14 g, 24 mmol), and concentrated H2SO4
(4 mL) were added. The mixture was stirred at 40 °C for 4 h. The
brown solid was filtered, washed with dichloromethane and water,
and dried under vacuum to afford 3A and 3B as a mixture after
recrystallization from hot toluene (8.8 g, 32% yield). M.p. 120–
140 °C. 1H NMR (200 MHz, CDCl3): δ = 7.92 (d, J = 8.1 Hz, 2
H, H3 and H8), 8.1 (d, J = 9.1 Hz, 2 H, H4 and H9), 8.34 (d, J =
9.1 Hz, 2 H, H5 and H10), 8.54 (d, J = 8.1 Hz, 2 H, H2 and H7) ppm
for 3A. 1H NMR (200 MHz, CDCl3): δ = 7.91 (d, J = 8.1 Hz, 2
H, H3 and H6), 8.07 (s, 2 H, H4 and H5), 8.39 (s, 2 H, H9 and H10),
8,54 (d, J = 8.2 Hz, 2 H, H2 and H7) ppm for 3B.
1
61% yield). M.p. 150 °C. H NMR (200 MHz, CDCl3): δ = 0.85–
1.03 (m, 24 H, CH3), 1.2–1.85 (m, 44 H, –CH2–), 1.66 [s, 12 H,
C(CH3)2], 3.93–4.0 (m, 8 H, –O–CH2), 7.0 (s, 2 H, Hphen), 7.14 (s,
2 H, Hphen), 8.10–8.21 (m, 6 H, Hpyr), 8.80 (d, J = 9 Hz, 1 Hz, 2
H, H2 and H7) ppm. MS (LSIMS+), m/z: 1079 [M]+. Analysis:
calcd. (found) for C74H94O6 (1078): C 82.33 (82.65), H 8.78
(8.72)%.
A mixture of this protected compound (380 mg, 0.35 mmol) and
KOH (260 mg, 4.63 mmol) in toluene (20 mL) was heated at reflux
for 1 h and was then filtered. The filtrate was washed with water
until pH 7. The organic layer was dried over Na2SO4. After re-
moval of the solvent, the crude product was purified by column
chromatography (silica gel), with elution with petroleum ether/
dichloromethane (1 to 25%) to yield 7 as a yellow oil (0.180 g, 53%
yield). 1H NMR (250 MHz, CDCl3): δ = 0.80–1.05 (m, 24 H, CH3),
1.2–2.0 (m, 44 H, CH2), 3.37 (s, 2 H, CϵCH), 3.95–4.05 (m, 8 H,
OCH2), 7.06 (s, 2 H, Hphen), 7.17 (s, 2 H, Hphen), 8.12–8.21 (m, 6
H, Hpyr), 8.81 (d, J = 9.1 Hz, 2 H, H2 and H7) ppm. MS (LSIMS+),
m/z: 963 [M + 1]+.
Disubstituted Pyrene (4A, 4B): A Schlenk flask was charged with
a solution of 2-methyl-but-3-yn-2-ol (1 mL, 10.3 mmol) in freshly
distilled diethylamine (60 mL). The solution was freeze–pump–
thaw degassed and transferred to a mixture of 3A and 3B (2 g,
4.4 mmol), Pd[PPh3]2Cl2 (68 mg), and CuI (0.12 mmol) under ar-
gon atmosphere. The reaction mixture was heated at 50 °C under
argon atmosphere for 20 h. The solvent was removed under vac-
uum and the insoluble product was filtered, washed with dichloro-
methane, and dried. Compound 4A was obtained as a brownish
solid (1.07 g, 66%). The filtrate was evaporated and the crude pro-
duct was then subjected to column chromatography (silica gel) with
elution with dichloromethane/methanol (0 to 1%). Compound 4B
was obtained as a yellow solid (538 mg, 33% yield).
Bis-bipyridine Pyrene Ligand (2):
A solution of 7 (150 mg,
0.15 mmol) and 4-bromo-2,2Ј-bipyridine (78 mg, 0.33 mmol) in
freshly distilled toluene (10 mL) and diisopropylamine (4 mL) was
freeze–pump–thaw degassed and transferred by cannula to the cat-
alyst Pd[PPh3]4 (23 mg, 0.02 mmol) under argon. The reaction mix-
ture was heated at 80 °C under inert atmosphere for 24 h. After the
mixture had cooled to room temperature, the solvent was removed
under vacuum. The crude product was purified by column
chromatography (silica gel) with petroleum ether/dichloromethane
(50 to 100%) and then dichloromethane/methanol (0 to 2%) as
eluent. Compound 2 was obtained as an orange solid (120 mg, 61%
yield). 1H NMR (250 MHz, CDCl3): δ = 0.79–2.04 (m, 60 H, alkyl
chains), 4.02–4.09 (8 H, OCH2), 7.14 (s, 2 H, Hphen), 7.22 (s, 2 H,
Hphen), 7.33–7.46 (m, 4 H, Hbpy), 7.84–7.91 (m, 2 H, Hbpy), 8.14–
8.27 (m, 6 H, Hpyr), 8.38–8.43 (m, 2 H, Hbpy), 8.57 (s, 3 H, Hbpy),
8.66–8.78 (m, 4 H, Hbpy), 8.84 (d, J = 9.1 Hz, 2 H, H2 and
H7) ppm. MS (LSIMS+), m/z: 1271 [M + 1]+. Analysis calcd.
(found) for C88H94N4O4 (1270): C 83.11 (82.01); H 7.45 (7.77); N
4.41 (3.56).
1
Compound 4A: M.p. 237–238 °C. H NMR (200 MHz, CDCl3): δ
= 1.81 (s, 12 H, CH3), 8.09–8.13 (m, 6 H, Hpyr), 8.52 (d, J = 9.1 Hz,
2 H, H2 and H7) ppm. 13C NMR (50.3 MHz, CDCl3): δ = 32.1
(CH3), 82.4 (CϵC), 82.9 (CϵC), 117.2, 125.0, 126.2, 128.2, 130.4,
131.3, 132.4, 132.7 ppm. MS (EI): m/z: 366 [M]+. Analysis: calcd.
(found) for C26H22O2 (198): C 85.24 (83.91), H, 6.01 (6.06).
1
Compound 4B: M.p. 95–100 °C. H NMR (250 MHz, CDCl3): δ =
1.80 (s, 12 H, CH3), 8.04 (s, 2 H, H3 and H6), 8.11 (s, 4 H, H4, H5,
H9 and H10), 8.60 (s, 2 H, H2 and H7) ppm. 13C NMR (50.3 MHz,
CDCl3): δ = 31.8 (CH3), 66.1 (–C–CϵC), 81.1 (CϵC), 100 (CϵC),
117.9, 125.0, 126.2, 127.9, 131.3, 132.8 ppm. MS (EI), m/z: 366
[M]+; Analysis: calcd. (found) for C26H22O2 (198): C 85.24 (84.10),
H 6.01 (6.02)%.
1,6-Diethynyl-pyrene (5A): A solution of 4A (500 mg, 1.34 mmol)
and NaOH (480 mg, 12 mmol) in freshly distilled toluene (20 mL)
was heated at reflux for 3 h. This solution was filtered while hot.
After cooling down to room temperature, the organic layer was
washed with water until pH = 7 and dried over sodium sulfate, and
the solvents were evaporated. The crude product was recrystallized
Acknowledgments
We are grateful to Dr. Jean Oberlé (Université Bordeaux 1, Talence,
France) for his kind assistance in time-resolved fluorescence spec-
troscopy measurements.
Eur. J. Org. Chem. 2005, 2684–2688
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. Leroy-Lhez, F. Fages
FULL PAPER
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