Enhanced Two-Photon Absorption with Novel Octupolar Propeller-Shaped Fluorophores Derived from Triphenylamine

Article abstract of DOI:10.1021/ol036041s

(Equation presented) Novel octupolar fluorophores derived from the symmetrical functionalization of a triphenylamine core with strong acceptor peripheral groups via phenylene-ethynylene linkers have been synthesized and shown to exhibit high fluorescence quantum yields, very large TPA cross-sections in the red-NIR region, and suitable photostability.

Full text of DOI:10.1021/ol036041s

ORGANIC
LETTERS
2004
Vol. 6, No. 1
47-50
Enhanced Two-Photon Absorption with
Novel Octupolar Propeller-Shaped
Fluorophores Derived from
Triphenylamine
Laurent Porre`s,† Olivier Mongin,† Claudine Katan,† Marina Charlot,†
,†
Thomas Pons,‡ Jerome Mertz,‡ and Mireille Blanchard-Desce*
Synthe`se et ElectroSynthe`se Organiques (CNRS, UMR 6510),
Institut de Chimie, UniVersite´ de Rennes 1, Campus de Beaulieu, Baˆt. 10A,
F-35042 Rennes Cedex, France, and Neurophysiologie et NouVelles Microscopies
(INSERM EPI 00-02, CNRS FRE 2500), Ecole Supe´rieure de Physique et Chimie
Industrielles, 10 rue Vauquelin, F-75231 Paris Cedex 05, France
mireille.blanchard-desce@uniV-rennes1.fr
Received October 17, 2003
ABSTRACT
Novel octupolar fluorophores derived from the symmetrical functionalization of a triphenylamine core with strong acceptor peripheral groups
via phenylene-ethynylene linkers have been synthesized and shown to exhibit high fluorescence quantum yields, very large TPA cross-
sections in the red−NIR region, and suitable photostability.
Molecular two-photon absorption (TPA) has attracted grow-
ing interest over recent years due to the many applications
it offers both in material science and in biological imaging.
These include microfabrication,1 three-dimensional optical
data storage,2 optical power limitation,3 localized photo-
dynamic therapy,4 and two-photon laser scanning fluores-
cence imaging.5 These applications call for the design of
specifically engineered compounds displaying enhanced TPA
cross-sections. In the case of two-photon excited fluorescence
(2) (a) Parthenopoulos, D. A.; Rentzepis, P. M. Science 1989, 245, 843-
845. (b) Strickler, J. H.; Webb, W. W. Opt. Lett. 1991, 16, 1780-1782. (c)
Dvornikov, A. S.; Rentzepis, P. M. Opt. Commun. 1995, 119, 341-346.
(d) Belfield, K. D.; Schafer, K. J. Chem. Mater. 2002, 14, 3656-3662. (e)
Belfield, K. D.; Liu, Y.; Negres, R. A.; Fan, M.; Pan, G.; Hagan, D. J.;
Hernandez, F. E. Chem. Mater. 2002, 14, 3663-3667.
(3) (a) He, G. S.; Xu, G. C.; Prasad, P. N.; Reinhardt, B. A.; Bhatt, J.
C.; Dillard, A. G. Opt. Lett. 1995, 20, 435-437. (b) Ehrlich, J. E.; Wu, X.
L.; Lee, I.-Y. S.; Hu, Z.-Y.; Ro¨ckel, H.; Marder, S. R.; Perry, J. W. Opt.
Lett. 1997, 22, 1843-1845.
† Universite´ de Rennes 1.
‡ Ecole Supe´rieure de Physique et Chimie Industrielles.
(1) (a) Maruo, S.; Nakamura, O.; Kawata, S. Opt. Lett. 1997, 22, 132-
134. (b) Cumpston, B. H.; Ananthavel, S. P.; Barlow, S.; Dyer, D. L.;
Ehrlich, J. E.; Erskine, L. L.; Heikal, A. A.; Kuebler, S. M.; Lee, I.-Y. S.;
McCord-Maughon, D.; Qin, J.; Ro¨ckel, H.; Rumi, M.; Wu, X. L.; Marder,
S. R.; Perry, J. W. Nature 1999, 398, 51-54. (c) Kawata, S.; Sun, H.-B.;
Tanaka, T.; Takada, K. Nature 2001, 412, 697-698. (d) Zhou, W.; Kuebler,
S. M.; Braun, K. L.; Yu, T.; Cammack, J. K.; Ober, C. K.; Perry, J. W.;
Marder, S. R. Science 2002, 296, 1106-1109.
(4) Bhawalkar, J. D.; Kumar, N. D.; Zhao, C. F.; Prasad, P. N. J. Clin.
Laser Med. Surg. 1997, 15, 201-204.
(5) (a) Denk, W.; Strickler, J. H.; Webb, W. W. Science 1990, 248, 73-
76. (b) Xu, C.; Zipfel, W.; Shear, J. B.; Williams, R. M.; Webb, W. W.
Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 10763-10768. (c) Larson, D. R.;
Zipfel, W. R.; Williams, R. M.; Clark, S. W.; Bruchez, M. P.; Wise, F. W.;
Webb, W. W. Science 2003, 300, 1434-1437.
10.1021/ol036041s CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/10/2003

(TPEF) microscopy, high-performance fluorophores must
exhibit both high fluorescence quantum yields (φ) and large
TPA cross-sections (σ2) in the red-NIR range (700-1200
nm), corresponding to the biological optimal window for
reduced photodamage.5 Photostability is also an important
feature to consider for TPEF-based applications.
their TPEF cross-section (σ2φ) in the NIR region, as well as
ensuring suitable photostability.
Our trigonal derivatives are derived from the symmetrical
functionalization of an electron-donating triphenylamine core
with three conjugated branches (Scheme 1) bearing electron-
The optimization of molecular TPA has largely focused
on one-dimensional dipolar6,7 or quadrupolar3,7,8 structures,
and so far little attention has been devoted to the design of
optimized octupolar structures,9 although it has been realized
that increased dimensionality and branched structures10,11
could lead to highly effective multiphoton absorption. In this
perspective, we have investigated a series of novel three-
branched octupolar fluorophores with the aim of optimizing
Scheme 1a
(6) (a) He, G. S.; Yuan, L.; Cheng, N.; Bhawalkar, J. D.; Prasad, P. N.;
Brott, L. L.; Clarson, S. J.; Reinhardt, B. A. J. Opt. Soc. Am. B 1997, 14,
1079-1087. (b) Belfield, K. D.; Hagan, D. J.; Van Stryland, E. W.; Schafer,
K. J.; Negres, R. A. Org. Lett. 1999, 1, 1575-1578. (c) Belfield, K. D.;
Schafer, K. J.; Mourad, W.; Reinhardt, B. A. J. Org. Chem. 2000, 65, 4475-
4481. (d) Abbotto, A.; Beverina, L.; Bozio, R.; Bradamante, S.; Ferrante,
C.; Pagani, G. A.; Signorini, R. AdV. Mater. 2000, 12, 1963-1967. (e)
Antonov, L.; Kamada, K.; Ohta, K.; Kamounah, F. S. Phys. Chem. Chem.
Phys. 2003, 5, 1193-1197.
(7) (a) Reinhardt, B. A.; Brott, L. L.; Clarson, S. J.; Dillard, A. G.; Bhatt,
J. C.; Kannan, R.; Yuan, L.; He, G. S.; Prasad, P. N. Chem. Mater. 1998,
10, 1863-1874. (b) Kim, O.-K.; Lee, K.-S.; Woo, H. Y.; Kim, K.-S.; He,
G. S.; Guang, S. H.; Swiatkiewicz, J.; Prasad, P. N. Chem. Mater. 2000,
12, 284-286. (c) Strehmel, B.; Sarker, A. M.; Detert, H. ChemPhysChem
2003, 4, 249-259.
(8) (a) Albota, M.; Beljonne, D.; Bre´das, J.-L.; Ehrlich, J. E.; Fu, J.-Y.;
Heikal, A. A.; Hess, S. E.; Kogej, T.; Levin, M. D.; Marder, S. R.; McCord-
Maughon, D.; Perry, J. W.; Ro¨ckel, H.; Rumi, M.; Subramaniam, G.; Webb,
W. W.; Wu, X.-L.; Xu, C. Science 1998, 281, 1653-1656. (b) Ventelon,
L.; Blanchard-Desce, M.; Moreaux, L.; Mertz, J. Chem. Commun. 1999,
2055-2056. (c) Ventelon, L.; Charier, S.; Moreaux, L.; Mertz, J.; Blanchard-
Desce, M. Angew. Chem., Int. Ed. 2001, 40, 2098-2101. (d) Frederiksen,
P. K.; Jørgensen, M.; Ogilby, P. R. J. Am. Chem. Soc. 2001, 123, 1215-
1221. (e) Ventelon, L.; Moreaux, L.; Mertz, J.; Blanchard-Desce, M. Synth.
Met. 2002, 127, 17-21. (f) Mongin, O.; Porre`s, L.; Moreaux, L.; Mertz, J.;
Blanchard-Desce, M. Org. Lett. 2002, 4, 719-722. (g) Pond, S. J. K.; Rumi,
M.; Levin, M. D.; Parker, T. C.; Beljonne, D.; Day, M. W.; Bre´das, J.-L.;
Marder, S. R.; Perry, J. W. J. Phys. Chem. A 2002, 106, 11470-11480. (h)
Abbotto, A.; Beverina, L.; Bozio, R.; Facchetti, A.; Ferrante, C.; Pagani,
G. A.; Pedron, D.; Signorini, R. Org. Lett. 2002, 4, 1495-1498. (i) Yang,
W. J.; Kim, D. Y.; Jeong, M.-Y.; Kim, H. M.; Jeon, S.-J.; Cho, B. R. Chem.
Commun. 2003, 2618-2619. (j) Iwase, Y.; Kamada, K.; Ohta, K.; Kondo,
K. J. Mater. Chem. 2003, 13, 1575-1581.
a Reagents and conditions: (a) P(OEt)3, reflux, 10 h (80-84%);
(b) H2O2, AcOH, reflux, 4 h (77-82%); (c) 4-iodobenzaldehyde,
NaH, THF, rt, 15 h (51-61%); (d) KI (2 equiv), KIO3 (1 equiv),
AcOH, 85 °C, 5 h (88%); (e) HCtCSiMe3, Pd(PPh3)2Cl2, CuI,
toluene/Et3N, 40 °C, 2.5 h (96%); (f) TBAF, R-π-X (1-3, 6, or 7,
3.5 equiv), Pd2dba3, PPh3, CuI, toluene/Et3N, 35 °C (72-89%).
(9) (a) Joshi, M. P.; Swiakiewicz, J.; Xu, F.; Prasad, P. N.; Reinhardt,
B. A.; Kannan, R. Opt. Lett. 1998, 23, 1742-1744. (b) Chung, S.-J.; Kim,
K.-S.; Lin, T.-C.; He, G. S.; Swiatkiewicz, J.; Prasad, P. N. J. Phys. Chem.
B 1999, 103, 10741-10745. (c) He, G. S.; Swiatkiewicz, J.; Jiang, Y.;
Prasad, P. N.; Reinhardt, B. A.; Tan, L.-S.; Kannan, R. J. Phys. Chem. A
2000, 104, 4805-4810. (d) Cho, B. R.; Son, K. H.; Sang, H. L.; Song,
Y.-S.; Lee, Y.-K.; Jeon, S.-J.; Choi, J. H.; Lee, H.; Cho, M. J. Am. Chem.
Soc. 2001, 123, 10039-10045. (e) Beljonne, D.; Wenseleers, W.; Zojer,
E.; Shuai, Z.; Vogel, H.; Pond, S. J. K.; Perry, J. W.; Marder, S. R.; Bre´das,
J.-L. AdV. Funct. Mater. 2002, 12, 631-641.
(10) (a) Chung, S.-J.; Lin, T.-C.; Kim, K.-S.; He, G. S.; Swiatkiewicz,
J.; Prasad, P. N.; Baker, G. A.; Bright, F. V. Chem. Mater. 2001, 13, 4071-
4076. (b) Drobizhev, M.; Karotki, A.; Kruk, M.; Mamardashvili, N. Z.;
Rebane, A. Chem. Phys. Lett. 2002, 361, 504-512. (c) Abbotto, A.;
Beverina, L.; Bozio, R.; Facchetti, A.; Ferrante, C.; Pagani, G. A.; Pedron,
D.; Signorini, R. Chem. Commun. 2003, 2144-2145. (d) Yoo, J.; Yang, S.
K.; Jeong, M.-Y.; Ahn, H. C.; Jeon, S.-J.; Cho, B. R. Org. Lett. 2003, 5,
645-648. (e) Zhang, B. J.; Jeon, S.-J. Chem. Phys. Lett. 2003, 377, 210-
216. (f) Mongin, O.; Porre`s, L.; Katan, C.; Pons, T.; Mertz, J.; Blanchard-
Desce, M. Tetrahedron Lett. 2003, 44, 8121-8125.
withdrawing (A) end-groups (12-15). Although octupolar
derivatives based on triphenylamine cores have been shown
to lead to high quadratic polarizabilities,12 only a few
examples of such derivatives have been shown to display
large TPA cross-sections,9a-c calling for further engineering
of the basic structures.
To examine the role of octupolar symmetry, we also have
investigated a structurally related analogue bearing only
electron-releasing D peripheral groups (11). Following the
same line of reasoning, we have selected strong acceptor
peripheral groups using, in particular, the triflyl moiety
(compounds 13, 15), a powerful acceptor,13 which has not
been considered yet in molecular engineering of TPA. Rigid
(11) (a) McDonagh, A. M.; Humphrey, M. G.; Samoc, M.; Luther-Davies,
B. Organometallics 1999, 18, 5195-5197. (b) Adronov, A.; Fre´chet, J. M.
J.; He, G. S.; Kim, K.-S.; Chung, S.-J.; Swiatkiewicz, J.; Prasad, P. N.
Chem. Mater. 2000, 12, 2838-2841. (c) Drobizhev, M.; Karotki, A.;
Rebane, A.; Spangler, C. W. Opt. Lett. 2001, 26, 1081-1083. (d) Drobizhev,
M.; Karotki, A.; Dzenis, Y.; Rebane, A.; Suo, Z.; Spangler, C. W. J. Phys.
Chem. B 2003, 107, 7540-7543. (e) Mongin, O.; Brunel, J.; Porre`s, L.;
Blanchard-Desce, M. Tetrahedron Lett. 2003, 44, 2813-2816.
(12) (a) Stadler, S.; Feiner, F.; Bra¨uchle, C.; Brandl, S.; Gompper, R.
Chem. Phys. Lett. 1995, 245, 292-296. (b) Stadler, S.; Bra¨uchle, C.; Brandl,
S.; Gompper, R. Chem. Mater. 1996, 8, 414-417. (c) Lambert, C.; Gaschler,
W.; Schma¨lzlin, E.; Meerholz, K.; Bra¨uchle, C. J. Chem. Soc., Perkin Trans.
2 1999, 577-587. (d) Lambert, C.; Gaschler, W.; Schma¨lzlin, E.; Meerholz,
K.; Bra¨uchle, C. J. Chem. Soc., Perkin Trans. 2 1999, 577-587.
(13) Hammett constants: σp ) 0.96 for SO2CF3 and σp ) 0.72 for SO2Me
(Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165-195).
48
Org. Lett., Vol. 6, No. 1, 2004

Table 1. Photophysical and Two-Photon Absorption Characteristics of Fluorophores 11-15
OPA
abs
OPA
em
λ
(nm)a
λ
(nm)c
σ2(GM)b,g
OPA
abs
b,d
compd
toluene
CHCl3
log ꢀb
toluene
CHCl3
Stokes shift (cm-1
)
φb,e
τ (ns)b,f
at 2 λ
at 740 nm
11
12
13
14
15
385
388
405
397
408
384
392
409
399
409
5.06
4.91
4.78
5.13
4.97
411
424
450
446
473
450
449
502
500
542
1600
2200
2500
2800
3400
0.77
0.77
0.78
0.79
0.72
0.88
1.27
1.49
1.07
1.29
15
150
420
660
740
30
160
495
1065
1080
a One-photon absorption maximum. b In toluene. c One-photon emission maximum. d Stokes shift ) (1/λabs - 1/λem). e Fluorescence quantum yield relative
to fluorescein in 0.1 N NaOH as a standard (λex ) 470 nm). f Experimental fluorescence lifetime. g TPA cross-sections; 1 GM ) 10-50 cm4 s photon-1
TPEF measurements were performed using a mode-locked Ti:sapphire laser delivering 80 fs pulses at 80 MHz, calibrating with fluorescein.
;
(11-13) or semirigid (14, 15) conjugated rods built from
phenylene-ethynylene and phenylene-vinylene units were
used to allow Intramolecular Charge Transfer (ICT) to take
place between the core and the peripheral groups while
maintaining fluorescence. To enhance the TPA cross-section,
we have investigated molecules of increasing size from 2.9
to 4.3 nm in diameter. B3LYP ab initio calculations indicate
that the molecules adopt a propeller-shaped structure (Figure
1), the core nitrogen being trigonal and the conjugated
branches twisted by about 40°.
The photophysical properties of the new series of trigonal
fluorophores are shown in Table 1 and Figure 2.
Figure 2. Normalized absorption and emission spectra of fluoro-
phores 11-15 in toluene.
Figure 1. B3LYP-optimized geometry of fluorophore 13.
We observe that all fluorophores display a broad absorp-
tion in the near UV-visible violet region and exhibit high
fluorescence quantum yield (about 0.8) independently of their
end-groups (D or A) or conjugated branch nature. In contrast,
their fluorescence lifetimes are significantly affected by the
nature of the peripheral groups. We observe that the octupolar
derivatives (12-15) display longer fluorescence lifetimes
than the analogue bearing only donor peripheral groups (11).
Interestingly, the powerful triflyl acceptor leads to a notice-
able increase of the fluorescence lifetime, while an increase
in the length of the branches by the addition of a phenylene-
vinylene unit results in a decrease of the fluorescence
lifetime.
Increasing the polarity of the solvent leads to a slight
bathochromic shift of the absorption band of octupolar
fluorophores (12-15) and to a more pronounced red shift
in their emission band (Table 1). Such positive solvatochro-
mic behavior can be related to an ICT phenomenon taking
place between the fluorophore center and periphery.18,19
The TPA spectra of fluorophores 11-15 in the 740-950
nm range were determined by investigating their TPEF in
The synthesis of these fluorophores is based on the
threefold Pd(0)-catalyzed cross-coupling reaction of the
nitrogen-cored building block 10 (Scheme 1) with aromatic
iodide (1,14 6, 7) or bromide (215, 316) derivatives. The
synthesis of the triphenylamine reagent 10 was achieved by
triiodination17 of triphenylamine, followed by a triple
Sonogashira coupling with ethynyltrimethylsilane. The ex-
tended building blocks 6 and 7 were obtained from the halo
derivatives 4 and 5, respectively, in a three-step sequence
(Michaelis-Arbuzov reaction, oxidation, and Horner-
Wadsworth-Emmons condensation with 4-iodobenzalde-
hyde). All new fluorophores have been fully characterized
by NMR, HRMS, and elemental analysis data.
(14) Ka¨pplinger, C.; Beckert, R. Synthesis 2002, 1843-1850.
(15) Nemoto, N.; Abe, J.; Miyata, F.; Shirai, Y.; Nagase, Y. J. Mater.
Chem. 1998, 8, 1193-1197.
(16) Nodiff, E. A.; Lipschutz, S.; Craig, P. N.; Gordon, M. J. Org. Chem.
1960, 25, 60-65.
(17) Shirota, Y.; Kobata, T.; Noma, N. Chem. Lett. 1989, 1145-1148.
Org. Lett., Vol. 6, No. 1, 2004
49

solution according to the protocol described by Webb and
co-workers20 in the femtosecond regime. We note that the
which bear similar acceptor end-groups: increasing the
molecular weight of the molecule by 30% (and increasing
the size by about 50%) leads to a 4-fold (and 6-fold,
TPA spectra of the octupolar fluorophores 12-15 (Figure
OPA
abs
respectively) enhancement in the TPA cross-section at
3) exhibit a relative maximum close to 2λ , indicating
OPA
abs
that the lowest energy excited-state is both one-photon and
two-photon allowed.21
2λ
(respectively 740 nm). This also leads to a red-shift
and broadening of both absorption and emission bands
(Figure 2), as well as to an increase in the Stokes shifts,
consistent with an increase of the core-to-periphery ICT. A
similar trend is noted when comparing fluorophores 13 and
15 (Table 1) (but to a lesser extent), while the main effect
lies in the major broadening of the TPA spectrum toward
the NIR region. As a result, the elongated octupolar fluoro-
phore 15 bearing strong acceptor peripheral groups maintains
a sizable TPA cross-section at 900 nm (Figure 3). Finally,
we stress that the use of phenylene-ethynylene linkers in the
conjugated branches (in place of the more classical phen-
ylene-vinylene linkers) leads to improved photostability, as
indicated by the absence of photodegradation either upon
pulsed-laser or prolonged lamp irradiation for derivatives 12
and 13.
In summary, we have designed and investigated novel
multipolar fluorophores derived from a triphenylamine core.
By enhancing the multidimensional ICT between the core
and the peripheral groups, we produced fluorophores that
combine very large TPA cross-sections in the red-NIR
region, high fluorescence quantum yields, and suitable
photostability. This strategy led to octupolar fluorophores
that exhibit some of the highest TPA cross-sections measured
to date in the NIR region and in the femtosecond regime.24
Furthermore, our study provides evidence that the TPA
cross-sections of such derivatives can be significantly
enhanced in the NIR region with elongated derivatives
bearing strong electron-withdrawing peripheral groups, lead-
ing to new directions for further engineering of their TPA
properties. In particular by modifying and lengthening the
conjugated branches, even larger TPA cross-sections should
be attainable.25
Figure 3. TPA spectra of fluorophores 11-15 in toluene.
We note that the character of the end-groups significantly
affects the TPA response. The tris-acceptor octupolar fluoro-
phores display much larger TPA cross-sections than their
tris-donor counterpart (11). For instance, molecule 13 shows
a TPA cross-section more than one-order of magnitude larger
than molecule 11 of similar size. Moreover, we observe from
the comparison of fluorophores 12 and 13 that increasing
the acceptor character of the end-group does lead to an
important increase in the TPA cross-section (Figure 3), in
correlation with a concomitant increase in the Stokes shift
(Table 1) and a red shift of both the absorption and emission
spectra (Figure 2).
By increasing the distance between the core and peripheral
electroactive groups, we aimed at extending the multi-
dimensional charge-transfer phenomenon and increasing the
nonlinear response.23 This strategy turned out to be effective,
as observed from the comparison of fluorophores 12 and 14,
Acknowledgment. We acknowledge financial support
from CNRS (ATIP and NOI programs) and Rennes Me´tro-
pole. L.P. and M.C. received fellowships from MENRT and
DGA, respectively. We thank K. Rousseau and E. Ronceray
for assistance in the synthesis. Calculations were supported
by the “Centre Informatique National de l’Enseignement
Supe´rieur” (CINES-France).
(18) This is corroborated by HOMO-LUMO calculations, which indicate
that a significant electronic redistribution takes place from the core to the
branches upon excitation.
(19) On the basis of the low sensitivity of the one-photon absorption
spectra to solvent polarity, we expect the TPA spectra in the NIR region to
depend only weakly on solvent polarity, except for possible spectral
broadening due to inhomogeneous broadening.
(20) Xu, C.; Webb, W. W. J. Opt. Soc. Am. B 1996, 13, 481-491.
(21) This excited state is doubly degenerated, and our experimental results
are in agreement with models presented in the literature for trigonal octupolar
molecules.9e,22
(22) Lee, W.-H.; Lee, H.; Kim, J.-A.; Choi, J.-H.; Cho, M.; Jeon, S.-J.;
Cho, B. R. J. Am. Chem. Soc. 2001, 123, 10658-10667.
Supporting Information Available: Computational de-
tails, photophysical methods (absorption, fluorescence, and
TPEF), and descriptions of all new fluorophores. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL036041S
(24) PRL-701: σ2 ) 600 GM at 796 nm (Z-scan).9b Crystal violet: σ2
) 1980 GM at 752 nm, (TPEF, φ ) 0.015).9e Compound 9 from ref 10f:
σ2 ) 1265 GM at 770 nm, (TPEF, φ ) 0.80).
(23) Such a strategy has proven to be valid in the case of nanoscale
octupoles derived from triphenylbenzene leading to enhanced first-order
hyperpolarizability: Brunel, J.; Mongin, O.; Jutand, A.; Ledoux, I.; Zyss,
J.; Blanchard-Desce, M. Chem. Mater. 2003, 15, 4139-4148.
(25) In particular by replacing the phenyl moieties with a fluorene unit10f
and/or triple bonds with double bonds (but at the expense of reduced
photostability).
50
Org. Lett., Vol. 6, No. 1, 2004