Novel fluorophores: Efficient synthesis and photophysical properties

Article abstract of DOI:10.1021/ol036399o

The synthesis of new donor-acceptor acetylene-bridged compounds was realized via organophosphorus reactions and Sonogashira couplings. These systems exhibit high fluorescence quantum yields, and excitation induces very efficient charge redistribution in t

Full text of DOI:10.1021/ol036399o

ORGANIC
LETTERS
2
004
Vol. 6, No. 5
39-742
Novel Fluorophores: Efficient Synthesis
and Photophysical Properties
7
†
‡
,†
,‡
R e´ mi M e´ tivier, R e´ mi Amengual, Isabelle Leray,* V e´ ronique Michelet,* and
‡
Jean-Pierre Gen eˆ t
CNRS UMR 8531 Laboratoire de Photophysique et Photochimie Supramol e´ culaires et
Macromol e´ culaires, D e´ partement de Chimie, ENS-Cachan, 61 AVenue du Pr e´ sident
Wilson, F-94235 Cachan Cedex, France, and Laboratoire de Synth e` se Organique,
E.N.S.C.P., UMR 7573, 11 rue P. et M. Curie, F-75231 Paris Cedex 05, France
icmleray@ppsm.ens-cachan.fr; michelet@ext.jussieu.fr
Received December 10, 2003
ABSTRACT
The synthesis of new donor−acceptor acetylene-bridged compounds was realized via organophosphorus reactions and Sonogashira couplings.
These systems exhibit high fluorescence quantum yields, and excitation induces very efficient charge redistribution in the molecules.
3
The design of new organic fluorescent molecules has
cence emission. A series of push-pull chromophores (D-
π-A) containing a polyphenylethynyl unit as a conjugated
1
attracted increasing attention for their potential optical and
2
4
sensing applications. The structural diversities of fluoro-
bridge were already studied (Scheme 1). In these systems,
phores offer widespread opportunities in many fields, but
the photophysical properties always need to be improved and
adjusted. In this context, poly(phenylene ethynylenes) and
other arylethynyl fluorophores are very attractive because
of their electron-transport abilities and their intense fluores-
Scheme 1
†
CNRS UMR 8531 Laboratoire de Photophysique et Photochimie
Supramol e´ culaires et Macromol e´ culaires.
‡
Laboratoire de Synth e` se Organique.
(
1) (a) Denk, W.; Strickler, J. H.; Webb, W. W. Science 1990, 248, 73.
b) Abbotto, A.; Beverina L.; Bozio, R.; Bradamente, S.; Ferrante, C.;
Pagani, G. A.; Signorini, R. AdV. Mater. 2000, 12, 1963.
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A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997,
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(
(
9
(
photoexcitation induces substantial charge separation as
evidenced by the large solvatochromic shift observed on
emission spectra. The nonlinear properties of some of those
4
(
1
0.1021/ol036399o CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/07/2004

Scheme 2
5
related compounds were already investigated. In our con-
tinuing search for novel fluorescent molecular sensors for
organophosphorus reaction either with phosphorus trichloride
or chlorodiphenylphosphine followed by an oxidation reac-
tion starting from the bromophenylacetylene 3.
6
,7
alkaline cations, we focused on the design of new photo-
induced charge-transfer (PCT) fluorophores. We turned our
attention to phosphine oxide derivatives that seem very
The synthesis of the bromo compound 3 was recently
described in the literature starting from 1,4-diiodobenzene
8
11
promising for cation complexation. Taking into account the
via two Pd-catalyzed cross-couplings. We have found that
high fluorescence quantum yield of polyphenylacetylene and
the PCT character of triphenylphosphine oxide substituted
by a donor group, we envisaged preparing new polyphenyl
the second Pd-catalyzed Sonogashira coupling could be
improved in the presence of a water-soluble catalyst (Scheme
2). The new water-soluble carboxylated ligand m-TPPTC
9
10,12
acetylene phosphine oxides bearing a methoxy donor group
2
combined with 1 mol % Pd(OAc) catalyst indeed afforded
(
Scheme 1). In this report, we wish to disclose our
the clean and efficient reaction of 4-bromophenylacetylene
4 with iodide compound 5 in 88% yield. The phosphorylation
was then investigated using chlorodiphenylphosphine and
phosphorus trichloride, respectively. The lithium-bromide
exchange was conducted in the presence of n-BuLi at -78
°C in THF, and the reaction mixture was reacted with
preliminary results in the first synthesis and the photophysical
properties of these new fluorophores.
A general procedure for preparing substituted aryl-
phosphine uses organometallic derivatives and phosphorus
chloride. On the basis of our recent synthesis of water-soluble
phosphine, we envisaged a common strategy for trisubsti-
tuted phosphine oxide derivatives bearing one or three
triphenylacetylene moieties (Scheme 1). Phosphine oxides
1
0
ClPPh
directly oxidized in 29% overall yield. In the same manner,
the reaction of the lithiated anion with PCl afforded the
2
, leading to the corresponding phosphine, which was
3
1
and 2 were expected to be delivered by means of an
phosphine 6, despite a low yield of 15%. Attempts to increase
the yields were unfruitful, probably due to the high polarity
and therefore the low solubility of bromide 3. Anticipating
that phosphine oxide 2 may exhibit higher fluorescent
properties, we turned our attention to an alternate preparation
based on the synthesis of the key intermediate 8 (Scheme
2). The phosphorylation using the Grignard derived from
the commercially available 4-bromo-trimethylsilylphenyl-
acetylene 7, followed by phosphorus oxidation and desilyl-
ation reaction, led to the desired adduct 8 in 45% overall
yield. The Sonogashira coupling of the phosphine oxide 8
with the iodide 5 gave the corresponding triarylphosphine
oxide 2 in 67% isolated yield. UV absorption and steady-
state and time-resolved fluorescence emission properties of
these new fluorophores were then investigated at room
temperature in order to obtain a full characterization of the
(
5) (a) Kondo, K.; Fujitani, T.; Ohnishi, N. J. Mater. Chem. 1997, 7,
29. (b) Stiegman, A. E.; Graham, E.; Perry, K. J.; Khundkar, L. R.; Cheng,
L.-T.; Perry, J. W. J. Am. Chem. Soc. 1991, 113, 7658.
6) (a) Leray, I.; O’Reilly, F.; Habib-Jiwan, J.-L.; Soumillion, J.-P;
4
(
Valeur, B. Chem. Commun. 1999, 795. (b) Leray, I.; Lef e` vre, J. P.; Delouis,
J.-F.; Delaire, J.; Valeur, B. Chem. Eur. J. 2001, 7, 4590. (c) M e´ tivier, R.;
Leray, I.; Valeur, B. Chem. Commun. 2003, 996.
(7) (a) Winkler, J. D.; Bowen, C. M.; Michelet, V. J. Am. Chem. Soc.
1
998, 120, 3237. (b) Collins, G. E.; Choi, L. S.; Ewing, K. J.; Michelet,
V.; Bowen, C. M.; Winkler, J. D. Chem. Commun. 1999, 4, 321. (c) Evans,
L., III; Collins, G. E.; Shaffer, R. E.; Michelet, V.; Winkler, J. D. Anal.
Chem. 1999, 71, 5322. (d) Collins, G. E.; Morris, R. E.; Wei, J.-F.; Smith,
M.; Hammond, M. H.; Michelet, V.; Winkler, J. D.; Serino, P. M.; Guo,
Y. Energy Fuels 2002, 16, 1054.
(
8) (a) Charbonni e` re, L. J.; Ziessel, R.; Montalti, M.; Prodi, L.;
Zaccheroni, N.; Boehme, C.; Wipff, G. J. Am. Chem. Soc. 2002, 124, 7779.
b) Malone, J. F.; Marrs, D. J.; McKervey, M. A.; O’Hagan, P.; Thompson,
(
N.; Walker, A.; Arnaud-Neu, F.; Mauprivez, O.; Schwing-Weill, M.-J.;
Dozol, J.-F.; Rouquette, H.; Simon, N. J. Chem. Soc., Chem. Commun.
1
995, 2151.
9) Changenet, P.; Plaza, P.; Martin, M. M.; Meyer, Y. H.; Rettig, W.
Chem. Phys. 1997, 221, 311.
10) Amengual, R.; Genin, E.; Michelet, V.; Savignac, M.; Genet, J. P.
AdV. Synth. Catal. 2002, 344, 393.
(
(11) Mongin, O.; Gossauer, A. Tetrahedron 1997, 53, 6835.
(12) Amengual, R.; Michelet, V.; Gen eˆ t, J. P. Tetrahedron Lett. 2002,
43, 5905.
(
740
Org. Lett., Vol. 6, No. 5, 2004

3
Figure 1. Absorption spectra of 1 (blue) and 2 (red) in CHCl .
For comparison, right and left ordinate scales for molar extinction
coefficients have a ratio equal to 3.
photophysical behaviors and to evaluate the photoinduced
charge transfer (PCT) that occurs in these molecules.
As shown in Figure 1 and Table 1, the new fluorophores
exhibit an intense absorption band in the UV region. After
normalization to the same number of chromophores, the
molar absorption coefficients at the wavelength of maximal
absorption are similar for 1 and 2. The absorption of 2 is
only slightly red-shifted with regard to the corresponding
band of 1, which reveals a weak interaction between the
Figure 2. Corrected fluorescence spectra of 1 (Panel A) and 2
Panel B) in various solvents (cyclohexane, black; dioxane, gray;
CHCl , purple; CH Cl , blue; DMSO, red; CH CN, orange; EtOH,
(
3
2
2
3
green). Spectra are normalized to the same height at the maximum.
13
chromophores in the ground state for 2.
the formation of an intramolecular charge transition. The
Stokes shifts, obtained for 1 and 2 in different solvents, can
be used to evaluate the difference in dipole moments between
Table 1. Photophysical Data for 1 and 2
solvent
λabs (nm), ꢀa
λem (nm)
ΦFb
τ (ns)
the ground and the excited states (µ
Lippert-Mataga equation:
e
g
- µ ) by using the
1
4
1
1
1
1
1
1
1
2
2
2
2
2
2
2
cyclohex
dioxane
CHCl3
CH2Cl2
DMSO
EtOH
CH3CN
cyclohex
dioxane
CHCl3
CH2Cl2
DMSO
EtOH
333
333
335, 5.9
334, 6.1
336
333
331
335
336
338, 17.7
338, 18.0
338
336
334
364
374
388
402
434
412
422
366
379
392
406
443
418
428
0.77
0.97
0.76
0.78
0.75
0.81
0.94
0.78
0.89
0.77
0.73
0.71
0.86
0.79
0.63
0.73
0.78
0.87
1.17
1.03c
1.13
0.58
0.69
0.74
0.85
1.24
1.02c
1.14
2
2
ν - ν )
(µ - µ ) ∆f + constant
(1)
a
f
e
g
3
hca
where ν
a
and ν
f
are the frequencies of the absorption and
fluorescence maximum, respectively, h is Planck’s constant,
c is the velocity of light, a is the radius of the cavity in which
the solutes reside, and ∆f is the orientation polarizability
defined as:
CH3CN
2
ꢀ
- 1
n - 1
a
Units ) 10 M-1 cm-1. b Fluorescence quantum yield with a 10%
4
∆f )
-
(2)
2
2
ꢀ + 1
c
2n + 1
random error. Additional decay time of 40 ps (fractional intensity fi )
-2%) was found.
1
where ꢀ is the static dielectric constant of the solvent, and n
is the optical refractive index of the solvent.
Taking the Onsager cavity of the fluorophore 1 to be 9 Å,
which is half the length of the long axis of the molecule,
the enhancement of the dipole moment was found to be about
F
As expected, 1 and 2 are strongly fluorescent (Φ varies
from 0.7 to 0.9) in the 350-600 nm region (Figures 2A and
-
1
2
B). A large Stokes shift (up to 7000 cm for 2 in DMSO)
and the unresolved vibronic structure of the fluorescence
spectrum observed for 1 and 2 in a polar solvent suggest
3
0.5 D. Since the dipole moment in the ground state was
evaluated to be 4.9 D using a quantum semiempirical PM3
model, the dipole moment in the excited state may be larger
than 35 D. Such a value points out a significant separation
of the charges upon excitation (∼40% of an electron being
(
13) Aujard, I.; Baltaze, J.-P.; Baudin, J.-B.; Cogn e´ , E.; Ferrage, F.;
Jullien, L.; Perez, E.; Pr e´ vost, V.; Qian, L. M.; Ruel, O. J. Am. Chem. Soc.
001, 123, 8177.
2
Org. Lett., Vol. 6, No. 5, 2004
741

transferred) which is in accordance with the dimethylamino-
cyano-diphenylacetylene behaviors.15,16 The observed Stokes
shifts for 2, and all of the photophysical parameters
mentioned in Table 1, are comparable to those for 1, as in
the case of dimethylamino-substituted triphenylphosphine
10
oxide derivatives. These observations suggest that interac-
tions within the trimer molecule are relatively weak and show
that excitation energy is more likely to be localized on only
one chromophore in the case of 2. Such a property is fully
compatible with the work of Goodson et al.17 The Onsager
radius of the fluorophore 2 is identical to the one calculated
for 1 (a ) 9 Å), and consequently the dipole moment of the
excited fluorophore is also around 35 D. The other two
phenylacetylene fluorophores contained in the trimer
molecule 2 seem to have no significant influence on the
acceptor character of the phosphine oxide function.
Fluorescence decays were measured by the time-correlated
single-photon counting method with a picosecond laser
excitation. For 1 and 2, in all solvents except ethanol, a
monoexponential behavior was found. An example of the
fluorescence decay obtained for 2 in dichloromethane is
given in Figure 3. Furthermore, the excitation spectra were
independent of the selected emission wavelength and very
similar to the absorption spectrum. Those results suggest the
presence of only one emitting species, in contrast to the
3
Figure 3. Fluorescence decay data of compound 2 in CH CN by
excitation at λexc ) 330 nm and collected emission λ > 355 nm.
em
Experimental data (points) and best fit (solid line) were obtained
by considering a single exponential. The obtained weighted residual
is plotted in the bottom panel.
which slightly depends on the polarity of the solvent, the
fluorescence lifetime increases substantially by increasing
the solvent polarity.
18
observation of Garcia-Garibay, and therefore reinforce the
observations made by Beeby et al. on 1,4-bis-(phenyl-
In summary, we have prepared two novel polyphenyl
acetylene phosphine oxides bearing a methoxy group via
organophosphorus reactions and efficient Pd-catalyzed
Sonogashira couplings. These systems exhibit high fluores-
cence quantum yields and excitation inducing very efficient
charge redistribution in the molecules. These novel fluoro-
phores open new opportunities for the conception of new
fluorescent molecular sensors. Further studies are currently
underway for the synthesis of analogues and probes and the
evaluation of photophysical properties and will be reported
in due course.
1
9
ethynyl)benzene. We did not observe any evidence of
different rotamers for 1 and 2.
For 1 and 2 in ethanol, the observed minor decay
component (f
i
) 1-2%) with a decay time of 40 ( 25 ps
(shortest time scale available on our instrumentation) was
interpreted as a dynamic Stokes shift with a decay time
corresponding to the characteristic time of solvent relax-
2
0
ation. In contrast with the fluorescence quantum yield,
(
14) (a) Mataga, N.; Kaifu, Y.; Koizumi, M. Bull. Chem. Soc. Jpn. 1955,
8, 690. (b) Lippert, E. Z. Naturforsch. A 1955, 10, 541.
15) Hirata, Y.; Okada, T.; Nomoto, T. Chem. Phys. Lett. 1997, 278,
33.
2
1
9
(
Acknowledgment. R. Amengual and R. M e´ tivier are
grateful to the Minist e` re de l’Education et de la Recherche
for a grant (1999-2002, 2000-2003). We are grateful to
Prof. J. Delaire and Prof. B. Valeur for fruitful discussions.
(16) Khundkar, L. R.; Stiegman, A. E.; Perry J. W. J. Phys. Chem. 1990,
4, 1224.
(
17) (a) Varnavski, O. P.; Ostrowski, J. C.; Sukhomlinova, L.; Twieg,
R. J.; Bazan, G. C.; Goodson, T., III. J. Am. Chem. Soc. 2002, 124, 1736.
b) Wang, Y.; Ranasinghe, M. I.; Goodson, T., III. J. Am. Chem. Soc. 2003,
25, 9562.
18) Levitus, M.; Schmieder, K.; Ricks, H.; Shimizu, K. D.; Bunz, U.
H. F.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2001, 123, 4259.
19) Beeby, A.; Findlay, K.; Low, P. J.; Marder, T. B. J. Am. Chem.
Soc. 2002, 124, 8280.
20) Horng, M. L.; Gardecki, J. A.; Papazyan, A.; Maroncelli, M. J. Phys.
Chem. 1995, 99, 17311.
(
1
(
Supporting Information Available: General experimen-
tal procedures for 1 and 2. This material is available free of
charge via the Internet at http://pubs.acs.org.
(
(
OL036399O
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Org. Lett., Vol. 6, No. 5, 2004