Towards "smart" multiphoton fluorophores: Strongly solvatochromic probes for two-photon sensing of micropolarity

Article abstract of DOI:10.1039/b502585k

New fluorophores, combining broad, very high two-photon absorption in the near-infrared region with a marked dependence of their emission spectra on solvent polarity, have been designed as model probes for two-photon sensing of the chemical environment. T

Full text of DOI:10.1039/b502585k

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Towards ‘‘smart’’ multiphoton fluorophores: strongly solvatochromic
probes for two-photon sensing of micropolarity
C e´ line Le Droumaguet, Olivier Mongin, Martinus H. V. Werts and Mireille Blanchard-Desce*
Received (in Cambridge, UK) 22nd February 2005, Accepted 8th April 2005
First published as an Advance Article on the web 22nd April 2005
DOI: 10.1039/b502585k
New fluorophores, combining broad, very high two-photon
absorption in the near-infrared region with a marked depen-
dence of their emission spectra on solvent polarity, have been
designed as model probes for two-photon sensing of the
chemical environment.
increase the excitonic coupling between the branches (so as to
12
increase the TPA efficiency ) by increasing the transition dipole of
the branches, and to favor the formation of a highly polar emissive
excited-state (so as to enforce strongly polarity-dependent emis-
sion). To do so, we have combined elongated arms, built from
arylene–vinylene moieties, with the powerful electron-withdrawing
Molecular two-photon absorption (TPA) has attracted a lot of
interest over recent years in correlation with the many applications
it offers, both in biological imaging and materials science. These
include two-photon laser scanning microscopy imaging (TPLSM),
2 3 p 2 4 9
groups SO CF (Hammett’s s 5 0.96) or SO C F (Hammett’s
3
D optical data storage, localised photodynamic therapy, micro-
1
fabrication and optical power limitation. This has driven the
designing of compounds displaying enhanced TPA cross-sections
along with specific features, depending on their specific applica-
tion. Whereas a number of one-photon excitable medium-sensitive
probes and highly solvatochromic fluorophores have been deve-
Fig. 1 Octupolar fluorophores for enhanced TPA (excitonic coupling)
and localised emission from a highly solvatochromic dipolar branch.
2
loped because of their numerous potential applications, medium-
sensitive, two-photon excited fluorescence (TPEF) probes are still
largely undeveloped, despite TPEF fluorophores sensitive to ions
3–5
having recently begun to appear. Fluorescent molecules having
very high two-photon absorptivities and showing strongly medium-
sensitive luminescence would provide medium-responsive TPA
fluorophores as micropolarity probes for various media of interest.
With this in mind, we have implemented a molecular
engineering strategy towards fluorophores showing broad and
intense TPEF in the spectral region of interest for applications
(red–NIR region), with a marked dependence of their photo-
luminescence (PL) characteristics on the solvent polarity. A
number of solvatochromic, one-photon fluorescent probes of the
2
,6
donor–acceptor (D–A) type have been reported. These push–
pull molecules, such as Prodan, Nile Red, 7-aminocoumarins and
Fluoroprobe, exhibit intramolecular charge transfer phenomena{
and possess polar emissive states, whose emission is highly
dependent on solvent polarity. However, most dipolar chromo-
phores do not exhibit very high two-photon absorption cross-
4
,7,8
sections
compared to quadrupolar and octupolar derivatives
5,9,10
which do.
Recently, a joint experimental and theoretical study revealed
that combining dipolar branches within a three-branched octupo-
11
lar structure
by means of a common electron-donating
triphenylamine core (D), resulted in excitonic coupling between
the dipolar branches (D–A). This induced an intense TPA band,
with the emission originating from a dipolar emissive excited-state,
localised on one branch{ (Fig. 1). Based on this observation, our
strategy was to modify the octupolar triphenylamine derivatives to
Scheme 1 Synthesis of octupolar fluorophores. i: methyltriphenylpho-
sphonium iodide, NaH, THF, 20 uC, 60 h (yield of 2, 43%); ii: 2 (1 equiv.),
2
R –Ph–X (4 equiv.), Pd(OAc)
2
, P(o-tolyl) , DMF/Et
3
3
N, 100 uC (yield of 3,
, P(o-tolyl)
76%; 4, 55%; 5, 31%); iii: 2 (1 equiv.), 6 (4 equiv.), Pd(OAc)
2
3
,
*mireille.blanchard-desce@univ-rennes1.fr
3
DMF/Et N, 100 uC (yield of 7, 18%). Non 5 n-nonyl.
2
802 | Chem. Commun., 2005, 2802–2804
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Table 1 Photophysical data for fluorophores 3–5 and 7 in toluene
Normalized
solvatochromic
Stokes
shift/10 cm
s
2
at
21
e/M cm
21
3
21a
b
W
c
3
shift/10 cm
21d
e
labs/nm
lem/nm
t/ns
lTPA(max)/nm
lTPA(max)/GM
3
4
5
7
a
430
435
440
435
80800
79000
128000
119000
494
499
517
498
3.0
3.0
3.4
2.9
0.71
0.77
0.84
0.93
1.85
1.84
1.51
1.29
13.1
15.5
20.8
23.2
740
755
800
740
1340
1430
2070
2080
b
Stokes shift 5 (1/labs 2 1/lem). Fluorescence quantum yield in toluene determined relative to Fluorescein in 0.1 M NaOH. Experimental
c
d
fluorescence lifetime measured by time-correlated single photon counting. Value of the slope derived from the linear dependence of the Stokes
2
2
shift on the polarity–polarizability function (Df) of the solvent (Df 5 (e 2 1)/(2e + 1) 2 (n 2 1)/(2n + 1)) where e is the dielectric constant
e
and n the refractive index). 1 GM 5 10
250
4 21
cm s photon .
sp 5 1.09) (Scheme 1), the aim being that they would act as good
conduits for long distance intramolecular charge transfer. These
trigonal chromophores were synthesised by means of a threefold
Heck coupling of trivinyl 2 with its corresponding halo derivative
Moreover, the PL of the octupolar fluorophores is highly solvent
sensitive, demonstrating that the engineering strategy is valid. As
illustrated in Fig. 3 for molecule 5, a marked bathochromic shift of
the emission band is observed with increasing solvent polarity. In
contrast, the absorption is little affected." This suggests that an
intramolecular charge transfer phenomenon takes place after
excitation, prior to emission, and that the emission stems from a
strongly dipolar emissive state. Indeed the solvatochromic
(Scheme 1).
All newly synthesised fluorophores combine a high fluorescence
quantum yield (Table 1) and broad TPA bands in the 700–1000 nm
spectral region (Fig. 2); a region of application interest because of
increased penetration depth in scattering media and the availability
of suitable excitation sources.§ We note that the TPA maxima
occur at less than half the wavelength of the one-photon maximum
wavelength, indicating that there is significant excitonic coupling
13
behaviour was found to follow a Lippert–Mataga relationship;
a linear dependence of the Stokes shift as a function of the
polarity–polarizability function, Df, being observed (Table 1).
Increasing the acceptor strength lead to an increase in the
solvatochromism. Interestingly, further amplification of the PL
sensitivity was achieved by lengthening the conjugated arms of
fluorophores 5 and 7, leading to normalized solvatochromic shifts
11
between the dipolar branches.
As indicated by comparing chromophores 3 and 4, increasing
the peripheral group’s acceptor strength results in a red shift and
enhancement of the TPA bands. Increasing the size of the
compounds leads to a further marked TPA enhancement of
the peak, and to a definite broadening of the TPA spectrum in the
desired spectral range. As a result, fluorophore 7, whose TPEF
21
larger than 20 000 cm . Such PL sensitivity has previously only
6
been reported for smaller D–A fluorophores, and never before for
fluorophores with strong two-photon absorption. This marked
sensitivity of the PL on the micropolarity, in combination with
their high two-photon absorption cross-section in the 700–950 nm
spectral range, makes these derivatives promising environment-
sensitive TPEF probes; in particular, for applications that would
benefit from reduced scattering and background fluorescence (e.g.
action cross-section (s
a very high action cross-section both at 800 nm (1500 GM) and
00 nm (950 GM). To the best of our knowledge, this is the first
example of a dye maintaining such a high TPEF across the whole
00–900 nm spectral range, offering much greater flexibility in
2
W) peaks at 1940 GM (740 nm), maintains
9
1
4
7
originating from endogeneous chromophores ). Two-photon
excitation thus offers both an enhanced selectivity for fluorescence
signals coming from the probes and an intrinsic 3D resolution not
allowed by one-photon excitation.
terms of excitation sources. In contrast, the TPEF cross-section
of coumarin-307, a typical dipolar fluorophore, peaks at 16 GM
8
(800 nm).
Fig. 3 Solvatochromic behaviour of fluorophore 3 (normalized one
photon-induced emission spectra). Quantum yields: 0.40 (hexane), 0.71
Fig. 2 TPA spectra of fluorophores 3–5 and
7
determined by
3 2 2
(toluene), 0.89 (CHCl ), 0.79 (THF), 0.88 (CH Cl ), 0.12 (acetone), 0.04
femtosecond TPEF measurements in toluene.
(CH CN).
3
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" The TPA spectrum was also found to be similar (within experimental
error) in a more polar solvent.
1
(a) W. R. Zipfel, R. M. Williams and W. W. Webb, Nat. Biotechnol.,
003, 21, 1369; (b) D. A. Parthenopoulos and P. M. Rentzepis, Science
Washington D. C.), 1989, 245, 843; (c) J. D. Bhawalkar, N. D. Kumar,
C. F. Zhao and P. N. Prasad, J. Clin. Laser Med. Surg., 1997, 15, 201;
d) S. Kawata, H.-B. Sun, T. Tanaka and K. Takada, Nature, 2001, 412,
97; (e) G. S. He, G. C. Xu, P. N. Prasad, B. A. Reinhardt, J. C. Bhatt,
2
(
(
6
R. McKellar and A. G. Dillard, Opt. Lett., 1995, 20, 435; (f) J. E. Ehrlich,
X. L. Wu, I. Y. S. Lee, Z. Y. Hu, H. R o¨ ckel, S. R. Marder and
J. W. Perry, Opt. Lett., 1997, 22, 1843.
2
(a) B. Valeur, Molecular Fluorescence: Principles and Applications,
Wiley-VCH, Weinheim, 2002; (b) J. R. Lakowicz, Principles of
Fluorescence Spectroscopy, Kluwer Academic/Plenum, New York, 1999.
Fig. 4 Variation of the fluorescence lifetime of fluorophore 3 as a
function of the solvent polarity–polarizability.
3 (a) M. Taki, J. L. Wolford and T. V. O’Halloran, J. Am. Chem. Soc.,
004, 126, 712; (b) H. M. Kim, M.-Y. Jeong, H. C. Ahn, S.-J. Jeon and
2
B. R. Cho, J. Org. Chem., 2004, 69, 5749; (c) S. J. K. Pond, O. Tsutsumi,
M. Rumi, O. Kwon, E. Zojer, J.-L. Br e´ das, S. R. Marder and
J. W. Perry, J. Am. Chem. Soc., 2004, 126, 9291.
S. Charier, O. Ruel, J.-B. Baudin, D. Alcor, J.-F. Allemand, A. Meglio
and L. Jullien, Angew. Chem., Int. Ed., 2004, 43, 4785.
The fluorescence lifetimes also vary as a function of the
environment. As illustrated in Fig. 4 for fluorophore 3, a
linear dependence of the fluorescence lifetime on the polarity–
polarizability function is observed in a range of apolar and
medium polarity solvents. This quasi-linear dependence is lost in
more polar solvents as a result of a drastic increase in the rate of
non-radiative decay. This is also responsible for the decrease of the
fluorescence quantum yield in polar solvents, a feature shared with
most dipolar fluorescent probes.
4
5
6
M. H. V. Werts, S. Gmouh, O. Mongin, T. Pons and M. Blanchard-
Desce, J. Am. Chem. Soc., 2004, 126, 16294.
(a) G. F. Mes, B. de Jong, H. J. van Ramesdonk, J. W. Verhoeven,
J. M. Warman, M. P. de Haas and L. E. W. Horsman-van den Dool,
J. Am. Chem. Soc., 1984, 106, 6524; (b) M. Goes, X. Y. Lauteslager,
J. W. Verhoeven and J. W. Hofstraat, Eur. J. Org. Chem., 1998, 2373.
G. S. He, T.-C. Lin, J. Dai, P. N. Prasad, R. Kannan, A. G.
Dombroskie, R. A. Vaia and L.-S. Tan, J. Chem. Phys., 2004, 120, 5275.
C. Xu and W. W. Webb, J. Opt. Soc. Am. B, 1996, 13, 481.
9 (a) L. Ventelon, M. Blanchard-Desce, L. Moreaux and J. Mertz, Chem.
Commun., 1999, 2055; (b) M. Rumi, J. E. Ehrlich, A. A. Heikal,
J. W. Perry, S. Barlow, Z.-Y. Hu, D. McCord-Maughon, T. C. Parker,
H. R o¨ ckel, S. Thayumanavan, S. R. Marder, D. Beljonne and
J.-L. Br e´ das, J. Am. Chem. Soc., 2000, 122, 9500; (c) O.-K. Kim,
K.-S. Lee, H. Y. Woo, K.-S. Kim, G. S. He, S. H. Guang,
J. Swiatkiewicz and P. N. Prasad, Chem. Mater., 2000, 12, 284; (d)
L. Ventelon, S. Charier, L. Moreaux, J. Mertz and M. Blanchard-Desce,
Angew. Chem., Int. Ed., 2001, 40, 2098; (e) O. Mongin, L. Porr e` s,
L. Moreaux, J. Mertz and M. Blanchard-Desce, Org. Lett., 2002, 4, 719;
(f) A. Abbotto, L. Beverina, R. Bozio, A. Facchetti, C. Ferrante,
G. A. Pagani, D. Pedron and R. Signorini, Org. Lett., 2002, 4, 1495; (g)
S. K. Lee, W. J. Yang, J. J. Choi, C. H. Kim, S.-J. Jeon and B. R. Cho,
Org. Lett., 2005, 7, 323.
0 (a) B. R. Cho, K. H. Son, H. L. Sang, Y.-S. Song, Y.-K. Lee, S.-J. Jeon,
J. H. Choi, H. Lee and M. Cho, J. Am. Chem. Soc., 2001, 123, 10039;
(b) L. Porr e` s, O. Mongin, C. Katan, M. Charlot, T. Pons, J. Mertz and
M. Blanchard-Desce, Org. Lett., 2004, 6, 47; (c) W. J. Yang, D. Y. Kim,
C. H. Kim, M.-Y. Jeong, S. K. Lee, S.-J. Jeon and B. R. Cho, Org.
Lett., 2004, 6, 1389.
1 C. Katan, F. Terenziani, O. Mongin, M. H. V. Werts, L. Porr e` s,
T. Pons, J. Mertz, S. Tretiak and M. Blanchard-Desce, J. Phys. Chem.
A, 2005, 109, 3024.
7
8
In conclusion, we have implemented a successful strategy that
leads to two-photon fluorescent probes which combine intense
two-photon absorption over a broad range of wavelengths (700–
900 nm) and have PL characteristics (emission spectra and to a
lesser extent fluorescence lifetime) that are strongly dependent on
their environment. Such fluorophores can be useful for sensing in
various chemical environments, including scattering media. These
fluorophores may also be derivatised to yield two-photon
fluorescent labels for environment sensing in materials such as
silica and zeolites.
We acknowledge financial support from R e´ gion Bretagne
(«Renouvellement des Comp e´ tences» Program) and Rennes
M e´ tropole. We wish to thank Laurent Porr e` s for his contribution
to TPEF measurements.
1
C e´ line Le Droumaguet, Olivier Mongin, Martinus H. V. Werts and
Mireille Blanchard-Desce*
Synth e` se et Electrosynth e` se Organiques (CNRS, UMR 6510),
Universit e´ de Rennes 1, Campus Scientifique de Beaulieu, B aˆ t. 10A,
F-35042, Rennes Cedex, France.
E-mail: mireille.blanchard-desce@univ-rennes1.fr;
Fax: (+33) 2 99 28 62 77; Tel: (+33) 2 99 28 62 77
1
12 D. Beljonne, W. Wenseleers, E. Zojer, Z. Shuai, H. Vogel, S. J. K. Pond,
J. W. Perry, S. R. Marder and J.-L. Br e´ das, Adv. Funct. Mater., 2002,
Notes and references
1
2, 631.
3 (a) E. Lippert, Z. Naturforsch., A: Astrophys. Phys. Phys. Chem., 1955,
0, 541; (b) N. Mataga, Y. Kaifu and M. Koizumi, Bull. Chem. Soc.
Jpn., 1955, 28, 690.
1
{
In some cases twisted intramolecular charge transfer.
Such excitation localization phenomena have been demonstrated for
1
{
15
several branched structures.
The TPA spectra were determined by investigating their TPEF in toluene,
§
14 W. R. Zipfel, R. M. Williams, R. Christie, A. Y. Nikitin, B. T. Hyman
and W. W. Webb, Proc. Natl. Acad. Sci. U. S. A., 2003, 100, 7075.
15 O. P. Varnavski, J. C. Ostrowski, L. Sukhomlinova, R. J. Twieg,
G. C. Bazan and T. Goodson, III, J. Am. Chem. Soc., 2002, 124,
1736.
8
according to the experimental protocol established by Xu and Webb,
using femtosecond excitation pulses. This experimental protocol avoids the
contribution of excited-state absorption, known to lead to artificially
enhanced TPA cross-sections.
2
804 | Chem. Commun., 2005, 2802–2804
This journal is ß The Royal Society of Chemistry 2005