A modular approach to two-photon absorbing organic nanodots: Brilliant dendrimers as an alternative to semiconductor quantum dots?

Article abstract of DOI:10.1039/b517270e

Nanoscopic fluorescent dendrimers having up to 96 two-photon chromophores and showing very large two-photon absorption cross-sections (up to 56 000 GM) were designed as a complementary "organic" alternative to quantum dots. The Royal Society of Chemistry

Full text of DOI:10.1039/b517270e

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A modular approach to two-photon absorbing organic nanodots:
brilliant dendrimers as an alternative to semiconductor quantum dots?{
Olivier Mongin,a Thatavarathy Rama Krishna,b Martinus H. V. Werts,a Anne-Marie Caminade,b
Jean-Pierre Majoral*b and Mireille Blanchard-Desce*a
Received (in Cambridge, UK) 6th December 2005, Accepted 20th December 2005
First published as an Advance Article on the web 19th January 2006
DOI: 10.1039/b517270e
photophysical properties, (ii) modulation of solubility in various
environments (via addition of external solubilising groups) and (iii)
further covalent functionalisation for additional functionality.
Recent work has shown that conjugated dendrimers based upon
the assembly of TP chromophores in dendrimeric architectures9
exhibit very large TPA cross-sections (up to 11 000 GM).
Pioneering work by Prasad, Fre´chet and co-workers shows that
covalent assembling of TP chromophores within a more flexible
dendrimeric assembly leads to high TPA cross-sections (up to
2600 GM for 8 chromophores{).10 Such dendritic systems can be
derivatised by using an acceptor core leading to interesting systems
based on Fo¨rster resonance energy transfer for two-photon
induced photoluminescence11 or even singlet oxygen formation.12
Aiming at the design of an ‘‘all-organic’’ object of similar size to
inorganic QDs with similar TPEF cross-sections, we have designed
dendritic systems bearing up to 96 strongly TPA-active fluoro-
phores (Fig. 1). These fluorophores were quasi-1D quadrupolar
systems built from the grafting of donor moieties via conjugated
spacers onto a fluorenyl core. Such systems are known to exhibit
large TPA cross-sections in the spectral range of interest for
bioimaging.13 The triple bond was chosen as a connector to allow
better photostability as compared to the double bond thanks to the
absence of photoisomerisation.13c
Nanoscopic fluorescent dendrimers having up to 96 two-
photon chromophores and showing very large two-photon
absorption cross-sections (up to 56 000 GM) were designed as a
complementary ‘‘organic’’ alternative to quantum dots.
For more than a decade, two-photon absorption (TPA) has
attracted increasing attention in relation with various applications,
such as 3-D microfabrication,1 optical data storage,2 photo-
dynamic therapy,3 and optical power limiting.4 Two-photon-
excited fluorescence (TPEF) has been found to be of particular
interest for the biological community: two-photon laser scanning
fluorescence microscopy offers the advantages of imaging deeper
in living tissues (down to 500 mm), with reduced photodamage and
background fluorescence and with 3-D spatial resolution.5 In
addition to biological imaging, TPEF can also be of interest for
3-D imaging in materials. These applications call for the design of
new luminophores whose TPA cross-sections are optimised in the
spectral range of interest: 700–1200 nm corresponding to the
combination of reduced scattering and minimal absorption in
(bio)organic materials. Recently semiconductor nanocrystals
(quantum dots: QDs) have been shown to provide a particularly
effective approach to fluorescent labels with extremely large TPEF
cross-sections (up to 47 000 GM).6 Indeed these brilliant
nanoobjects (typically of 3 nm radius without the encapsulating
polymers) have been heralded as useful materials for biological
imaging and have gained overwhelming popularity.7,8 However,
these inorganic systems (such as those made from zinc sulfides and
cadmium selenides6) suffer from several drawbacks including
toxicity and blinking.
The ‘‘nanodots’’ were built by grafting an exponentially
increasing number of TP-active chromophores 1 on the periphery
of phosphorus dendrimers of generation 1 to 4 decorated with
terminal P(S)Cl2 groups.14 Chromophore 1 was prepared in a
three-step sequence starting from commercially available aniline
2a, involving iodination, Mitsunobu reaction with hydroquinone
and Sonogashira coupling with alkyne 4 (Scheme 1). Nucleophilic
substitutions involving P(S)Cl2 end groups15 and 1 take place
easily leading to dendrimers G1, G2, G3 and G4 bearing
respectively 12, 24, 48 and 96 chromophores (Scheme 2).
In this paper we demonstrate that an ‘‘all organic’’ alternative
approach can lead to nanoobjects with similar geometry and size
as semiconductor QDs showing competitive TPEF performance.
Organic ‘‘nanodots’’ are built from the grafting of a discrete and
large number of optimised two-photon (TP) chromophores on the
periphery of dendrimers. In addition this ‘‘organic nanodots’’
strategy offers a modular route for (i) molecular control of
aSynthe`se et Electrosynthe`se Organiques (CNRS, UMR 6510),
Universite´ de Rennes 1, Campus Scientifique de Beaulieu, Baˆt. 10A,
F-35042, Rennes Cedex, France.
E-mail: mireille.blanchard-desce@univ-rennes1.fr;
Fax: (+33) 2 23 23 62 77; Tel: (+33) 2 23 23 62 77
bLaboratoire de Chimie de Coordination, CNRS, 205 route de
Narbonne, F-31077, Toulouse Cedex 4, France.
E-mail: majoral@lcc-toulouse.fr; Fax: (+33) 5 61 55 30 03
{ Electronic supplementary information (ESI) available: Preparation of
chromophore 1 and dendrimers and experimental details of spectroscopic
measurements. See DOI: 10.1039/b517270e
Fig. 1 Schematic representation of dendrimeric two-photon absorbing
fluorescent organic ‘‘nanodots’’.
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Chem. Commun., 2006, 915–917 | 915

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Scheme 1 i, I2, NaHCO3, CH2Cl2 (86%); ii, hydroquinone (3 equiv.),
DEAD, PPh3, THF, rt, 16 h (51%); iii, 3, Pd(PPh3)2Cl2, CuI, toluene/
Et3N, 40 uC, 16 h (57%).
Fig. 2 Normalized absorption and emission spectra in toluene of
dendrimers G1–G4 and model chromophore 1.
Dendrimers G1–G4 show strong one-photon absorption in the
near UV with a nearly linear increase of the extinction coefficient
with the number of chromophores (Table 1) as expected from
quantitative functionalisation of the dendrimer periphery. As
desired, all dendrimers maintain reasonably high quantum yields,
even in the highest generation G4, which validates our strategy. A
clear loss of fine structure and broadening of the emission band (in
particular in the red) is observed (Fig. 2). This change in the shape
of the emission spectra with increasing generation suggests that
interchromophoric interactions occur in the excited state. In
contrast, the absorption spectra remain virtually unchanged
pointing to the lack of significant interactions in the ground state.
A modest decrease in fluorescence quantum yield is also observed
with increasing the generation, most probably in relation to
interchromophoric interactions in the excited state. The dendri-
mers G1–G4 display much lower steady-state fluorescence
anisotropies than the parent chromophore 1. Since rotational
depolarization in these bigger molecules is expected to be much
slower, the depolarization must be due to rapid energy transfer
between chromophores. Preliminary time-resolved measurements
show that this process is much faster than the limited time-
resolution of our flashlamp-based TCSPC apparatus (instrumental
response 2 ns FWHM). This also explains the fact that we do not
observe any fast components in the fluorescence lifetime measure-
ments, only monoexponential decays. Further investigations of the
photophysical dynamics are planned.
The TPA of the dendrimers was studied by investigating the
TPEF of dendrimers G1–G4 in toluene.§ TPEF measurements
allow for direct measurement of the TPEF action cross-section
s2W, the relevant figure of merit for imaging applications. In
addition this method has been recognized as more reliable than
nonlinear transmission measurements.16 The quadratic dependence
of the TPEF signal on the excitation intensity was checked for each
data point, indicating that no photodegradation or saturation
Scheme 2 Synthesis of the 4th generation dendrimer bearing 96 fluorophore end groups.
Table 1 Photophysical data for model fluorophore 1 and dendrimers G1–G4 in toluene
Number of fluorophores
labs/nm
e/M21 cm21
lem/nm
Wa
t/nsb
rc
lTPA(max)/nm
s2 at lTPA(max)/GMd
1
1
12
24
48
96
386
385
386
386
386
84 900
420, 444
423, 446
426, 445
441
0.83
0.75
0.71
0.62
0.48
0.67
0.71
0.69
0.71
0.66
0.178
0.032
0.019
0.013
0.012
b
702
701
701
701
705
765
8880
17 700
29 800
55 900
G1
G2
G3
G4
a
1 004 000
2 035 000
3 785 000
7 101 000
445
Fluorescence quantum yield in toluene determined relative to fluorescein in 0.1 N NaOH. Experimental fluorescence lifetime measured by
time-correlated single photon counting. Steady-state fluorescence anisotropy. 1 GM = 10250 cm4 s photon21
.
c
d
916 | Chem. Commun., 2006, 915–917
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(France) for the post-doctoral grant to TRK. We wish to thank
Ce´dric Rouxel for his contribution to synthesis, and Adrien Gilot
and Manuel Parent for their help in setting up the TPEF
experiment.
Notes and references
{ TPA data derived from femtosecond measurements.
§ TPEF measurements were performed under excitation with 150 fs pulses
from a Ti:sapphire laser, using the protocol of Xu and Webb,21 taking into
account refractive index effects.22
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occurs. The TPA spectrum of monomeric chromophore 1 was also
determined for comparison. As clearly seen from Fig. 3, the TPA
cross-section increases linearly with the growing number of
chromophores in each generation, leading to very large TPA
cross sections, comparable to inorganic QDs. The additive
behaviour indicates that the molecular responses of each individual
chromophore located on the periphery remain unaffected (as could
be expected from the absence of significant interactions in the
ground state). In contrast molecular aggregation of individual
chromophores (in a solvent promoting aggregation) has been
shown to lead to TPA loss.17 The confinement strategy based on
the covalent grafting on a dendritic architecture thus allows
avoidance of any deleterious effects on TPA as a result of the
aggregation of chromophores in a spontaneous manner. As a
result, the maximum value (55 900 GM at 705 nm) obtained for
G4 is comparable to the s2 value measured by Webb and co-
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QDs is viable. In addition, the phosphorous dendrimers have been
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bioimaging applications.
In conclusion, the present results demonstrate that organic
‘‘nanodots’’ represent a promising approach towards fluorescent
markers competitive and complementary to semiconductor QDs
for TPEF imaging-based applications. In addition the modular
route allows access to a variety of organic nanodots with different
fluorophores, allowing tuning of the photoluminescence proper-
ties. We have validated the proposed strategy with quadrupolar
type chromophores that are blue-emitters. Such a route could also
be applied to the grafting of quadrupoles that are green-emitters or
yellow-emitters and show broader (and even higher) TPA.13b
Further functionalisation may also be achieved. In particular
adding supplementary layers and decorating the periphery with
ammonium groups would yield hydrosoluble dendrimers.20
We gratefully acknowledge financial support from Rennes
Me´tropole (‘‘Allocation d’Installation Scientifique’’ awarded to
MBD and MHVW). JPM thanks the Ministe`re de la Recherche
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