Organic nanodots for multiphotonics: Synthesis and photophysical studies

Article abstract of DOI:10.1039/b702452p

Organic nanodots based on the gathering of an exponentially increasing number of two-photon fluorophores on a dendritic platform of controlled size and symmetry represent a promising non-toxic alternative to quantum dots for (bio)imaging purposes. This mo

Full text of DOI:10.1039/b702452p

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Organic nanodots for multiphotonics: synthesis and photophysical
studiesw
Olivier Mongin,a Anna Pla-Quintana,b Francesca Terenziani,ac Delphine Drouin,a
a
b
b
´
Celine Le Droumaguet, Anne-Marie Caminade, Jean-Pierre Majoral* and
Mireille Blanchard-Desce*a
Received (in Montpellier, France) 15th February 2007, Accepted 31st May 2007
First published as an Advance Article on the web 8th June 2007
DOI: 10.1039/b702452p
Organic nanodots based on the gathering of an exponentially increasing number of two-photon
fluorophores on a dendritic platform of controlled size and symmetry represent a promising non-
toxic alternative to quantum dots for (bio)imaging purposes. This modular route offers a number
of advantages in terms of versatility but also raise a number of questions to be addressed. In
particular, possible interactions between fluorophores, due to confinement effects, have to be
taken into account. With this aim in mind we have investigated and compared the photophysical
and two-photon absorption (TPA) properties of two series of organic nanodots of different
geometries: spherical-like organic nanodots derived from a dendritic scaffold built from a
cyclotriphosphazene core and dumbbell-like organic nanodots derived from a dendritic scaffold
built from an elongated rod-like chromophore. The study provides evidence that the different
topology and nature of the dendritic architecture lead to significant changes in photoluminescence
characteristics as well as to subtle variations of the TPA efficiency. As a result, the dumbbell-like
nanodots although less promising in terms of two-photon induced fluorescence (due to partial
quenching of fluorescence efficiency) also demonstrate that improvement of the TPA efficiency can
be achieved by playing on the nature and topology of the dendritic scaffold of the nanodots.
by two-photon excitation in the visible red–NIR region, owing
Introduction
in particular to the reduction of scattering losses.
Molecular two-photon absorption (TPA) has attracted a lot of
interest over the last decade owing to its applications in
various fields including spectroscopy,1,2 three-dimensional
optical data storage,3–5 microfabrication,6–8 laser up-conver-
sion,9,10 high-resolution three-dimensional imaging of biolo-
gical systems,11–13 and photodynamic therapy.14 Among these,
two-photon-excited fluorescence (TPEF) has gained wide-
spread popularity in the biology community and given rise
to the technique of two-photon laser scanning fluorescence
microscopy.11–13 Using a two-photon excitation process (i.e. a
nonlinear process involving the simultaneous absorption of
two photons) instead of a conventional one-photon excitation
offers a number of advantages. These include the ability for a
highly confined excitation (and intrinsic three-dimensional
resolution) and increased penetration depth by replacing
typical one-photon excitation in the UV–visible blue region
Most of these applications strongly benefit from the use of
chomophores displaying very high TPA cross-sections in
order to significantly decrease the excitation intensity or
provide improved excitation efficiency and selectivity. In-
deed, in recent years a considerable effort has been devoted
to the design and investigation of chromophores with large
TPA cross-sections, exploring in particular dipolar10,15–20
and quadrupolar,17,21–38 structures. Lately, attention has
turned towards multipolar39–43 and conjugated branched
structures44–51 including dendrimers.52–55
Depending on the targeted applications, two-photon chro-
mophores also have to satisfy additional requirements. For
instance, the use of such chromophores for biological multi-
photonic imaging calls for the design of fluorophores combin-
ing a high fluorescence quantum yield (F) and TPA cross-
section (s2) in the spectral range of interest (i.e. 700–1200
nm)56 several orders of magnitude larger than endogenous
chromophores (whose TPA cross-sections range between 10ꢀ5
to about 1 GM).13 This would open the route for selective
excitation of two-photon chromophores leading to signifi-
cantly improved signal to noise ratio (reduction of back-
ground fluorescence) and reduction of photodamage.57 Low
(photo)toxicity and high photostability are important addi-
tional criteria.
a
`
Synthese et Electrosynthese Organiques (CNRS, UMR 6510),
Institut de Chimie, 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
b Laboratoire 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
Dipartimento di Chimica GIAF, Universita di Parma, Parco Area
c
`
Recently semiconductor nanocrystals (quantum dots:
QDs) have been shown to provide a particularly effective
approach to fluorescent nano-objects with extremely large
delle Scienze 17/A, 43100 Parma, Italy
w This paper was published as part of the special issue on Dendrimers
and Dendritic Polymers: Design, Properties and Applications.
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TPA cross-sections (s2F up to 47 000 GM)58 overcoming the
TPA responses of molecular two-photon fluorophores. QDs
have gained tremendous popularity for imaging applications
due to their intense, tunable (e.g. depending on the size and
composition) and robust photoluminescence properties.59
However, these inorganic systems suffer from several draw-
backs such as biological toxicity60 (due in particular to the
presence of heavy metals such as cadmium) and blinking. In
addition their surface functionalization for different pur-
poses such as targeting, recognition, conjugation to biomo-
lecules or specific labeling is possible but not undemanding.
These inorganic nano-objects also raise a number of ques-
tions with respect to environmental issues. The implementa-
tion of alternative strategies towards ‘‘soft’’ and
biocompatible substitutes is thus of high interest. Recently,
we have developed a promising route towards all-organic
nanodots showing competitive two-photon brightness (com-
parable to the highest reported for QDs).61 This route is
based on the confinement of a large number of two-photon
fluorophores showing significant TPA cross-sections within
single spherical nano-objects. Such nanodots can be ob-
tained by grafting two-photon (TP) fluorophores on the
surface of a dendrimeric platform (Fig. 1). This modular
strategy offers several potential advantages: photolumines-
cence (PL) characteristics can, in principle, be tuned by
playing on the nature of the fluorophore, while the dendri-
mer scaffold can be chosen so as to minimize toxicity effects
and control clearance ability.62 However, implementing such
modular approach also requires taking into account inter-
actions between the various molecular building blocks
including possible interactions between decorating fluoro-
phores. The confinement of fluorophores within the nanodot
imposes close proximity between chromophores which fa-
vors interchromophoric interactions: (i) in the ground state
with potential effects on the absorption characteristics and
(ii) in the excited state with impending marked effects on PL.
Such interactions can significantly affect the optical re-
sponses and photophysical properties of the nanodots which
will then differ from the mere addition of the contributions
of isolated single fluorophores. For instance formation of
dimers or aggregates between adjacent fluorophores could
lead to fluorescence quenching.63 Also interactions in the
excited state can lead to changes of PL characteristics due to
various phenomena (energy transfer, excimer formation or
exciton annihilation which is an important decay process in
chromophore decorated dendrimers at high excitation in-
tensity64).
Also it has been shown that interactions between chromo-
phores within dimers65 or aggregates36,66 can lead to signifi-
cant change in TPA responses. Such effects are expected to be
strongly dependent on both the nature of the chromophores
and their proximity and relative orientations. In this respect
the geometry and symmetry of the dendritic scaffold are
expected to play an important role.
In this prospect, we have investigated and compared the
photophysical and TPA properties of two series of organic
nanodots of different geometries: spherical-like organic nano-
dots (SOND, Fig. 1) and dumbbell-like organic nanodots
(DOND, Fig. 2) built from the gathering of an exponentially
increasing number of two-photon active fluorophores F on the
periphery of dendrimeric platforms of different symmetry. The
SOND series is the prototypical organic dots series that was
recently prepared as a proof of concept test series.61 In this
work, we examine how the two-photon absorption and PL
properties are affected by the nature and symmetry of the
dendrimeric scaffold. The SOND series G1–G4 is derived from
a dendritic scaffold built from a C3 symmetry cyclotriphos-
phazene core whereas the DOND series G10–G02 is derived
from a dendritic scaffold built from an elongated rod-like
fluorophore. The different symmetry of the cores induces a
difference in both the number and the symmetry of the
arrangement of the two-photon fluorophores on the surface
of the organic nanodot. In the case of SOND series, the two-
photon fluorophores on the surface experience similar envir-
onment in relation with the symmetry of the core of the
dendrimeric scaffold which controls the overall geometry
and topology (Fig. 1). In contrast, the decorating fluorophores
of the DOND series clearly experience inhomogeneous envir-
onments due to different nature and topology of the core (Fig.
2). This can play a significant role in terms of interchromo-
phoric interactions. If this is the case, the PL characteristics of
compounds of the SOND and DOND series can significantly
differ for a similar number of decorating fluorophores but a
different topology. This is expected to affect the TPEF effi-
ciency of the nanodot. Another important issue with respect to
the TPEF efficiency is the variation of the overall TPA cross-
Fig. 1 Schematic representation of homochromophoric spherical-type organic nanodots (SOND).
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Fig. 2 Schematic representation of heterochromophoric dumbbell-like organic nanodots (DOND).
Scheme 1 Synthesis of spherical dendrimer G1.
section as a function of the topology. Additive contributions of
the two-photon fluorophores to the overall TPA efficiency of
the nanodot was observed in the case of the SOND series.61
Since the environment and the packing of the decorating
fluorophores may differ in the two series, modification of their
TPA efficiency cannot be excluded. Finally whereas in the case
of the SOND series, energy transfer can occur among similar
fluorophores on the periphery (excitation energy migration),
energy transfer can in principle (depending on the nature and
spectroscopic characteristics of the core chromophore) occur
both along the surface (homotransfer) and between the core and
the surface of the nanodot (heterotransfer) in the DOND case.
chromophore F to the P(S)Cl2 end groups of the first genera-
tion phosphorus dendrimer67–69 built from a cyclotriphospha-
zene core70 (Scheme 1). The same reaction carried out with the
second, third and fourth generations of the same series of
dendrimers afforded the spherical dendrimers G2, G3 and G4,
respectively (Fig. 3), as previously reported.61
Two series of dumbbell-like dendrimeric compounds were
synthesized, both issued from the same biphenolic chromo-
phore 271 (Scheme 2) as the starting compound. The first series
(C0, C1) was synthesized to serve as model for the photo-
physical properties of the core fluorophores of the other series
(i.e. DOND series). The synthesis of the generation 0 of the
monochromophoric series C0 necessitates first the isolation of
the pentasubstituted cyclotriphosphazene 1, which is then
reacted with chromophore 2. The substitution of the sixth Cl
of the cyclotriphosphazene with such bulky substituent is slow
and needs 7 days to go to completion (Scheme 2). 31P NMR is
particularly suitable to monitor this reaction; indeed the
Results and discussion
Synthesis
The synthesis of the spherical-like first generation dendrimer
G1 was carried out by grafting 12 equivalents of the TPA
Fig. 3 Structures of spherical-type dendrimers G2–G4.
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Scheme 2 Synthesis of monochromophoric model C0.
spectrum of compound 1 displays an ABB0 system, whereas
the sixth substitution induces an AA0A00 system for C0,
affording a narrow multiplet.
pound possesses two types of chromophoric units: one at the
core and 40 as end groups (Scheme 5, Fig. 2).
The generation 0 of the multichromophoric series of dumb-
bell-like dendrimers (i.e. DOND series) is synthesized using a
different route. Indeed, in this case N3P3Cl6 (large excess) is
reacted with the biphenol 2 to afford compound 3.71 In the
second step, 10 equivalents of the two-photon fluorophore F61
are grafted to afford after a prolonged heating (37 days) the
generation 0 of the dumbbell-like multichromophoric dendri-
mer G00 (Scheme 3). In this case also, 31P NMR is an
invaluable tool, since the completion of the reactions is shown
by the appearance of a narrow multiplet, observed after a
series of intermediate compounds displaying ABB0 type
spectra.
Photophysical properties
One-photon absorption. All dendrimers show a strong ab-
sorption band in the near UV–visible-blue region (Table 1).
The normalized absorption spectra of the multichromophoric
dendrimers of the SOND series overlap almost perfectly with
the absorption spectrum of the isolated decorating fluoro-
phore F with only a slight broadening at the red tail (Fig. 4).
The molar extinction coefficients increase almost linearly with
the number of decorating fluorophores leading to giant ab-
sorption coefficients for the G4 SOND dendrimer (Table 1).72
These trends suggest that no noticeable interactions occur in
the ground state.
First generation compounds of both monochromophoric
and multichromophoric dumbbell-like dendrimers are synthe-
sized by grafting phenols to the P(S)Cl2 end groups of
dendrimer 4.71 The monochromophoric model compound
C1 is obtained by reaction with HO–C6H4–OMe, whereas
the multichromophoric compound G01 is obtained by reaction
with fluorophore F (Scheme 4, Fig. 2). In both cases, the
narrow multiplet (due to the unsymmetrical substitution on
the N3P3 groups) observed at 62.4 ppm for the P(S)Cl2 end
groups by 31P NMR is replaced by another narrow multiplet
at 64.5 ppm. An intermediate signal observed at 69.4 ppm and
corresponding to the monosubstitution (P(S)ClOAr) totally
disappears when the reaction is over. The same method is
applied for the synthesis of the second generation of the
dumbbell-like multichromophoric dendrimer G02. This com-
The absorption spectra of dendrimers of the dumbbell-like
series (DOND dendrimers) are broader and slightly blue-
shifted with respect to that of the SOND dendrimers (Table
1, Fig. 4). The absorption spectra of the core chromophores
(C0 for G00 and C1 for G01 and G02) mostly overlap, have
comparable extinction coefficient and are only slightly blue-
shifted with respect to that of peripheral fluorophores F (Table
1, Fig. 5). However, comparison of the absorption spectra of
dendrimers of the DOND series with that corresponding to
that expected for an additive contributions of core and per-
ipheral chromophores F (Fig. 6) shows that the assembly of
two-photon fluorophores F in the DOND series leads to
broader spectra and lower effective absorption coefficient
per fluorophore. This indicates a larger inhomogeneous
Scheme 3 Synthesis of dumbbell-like multichromophoric compound G00 (only half of the molecule is represented for G00).
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Scheme 4 Synthesis of monochromophoric dendrimer C1 and dumbbell-like dendrimer G01 (only half of the molecule is represented for G01).
broadening suggesting that the decorating fluorophores of the
DOND series experience a larger range of environments (and
disorder) in relation with the different geometry and lower
symmetry of their arrangement within the nanodots.
onment of the emitting fluorophore as compared to an isolated
fluorophore surrounded only by solvent molecules. The loss of
fine vibronic structure, red-shift and spectral broadening of the
emission band with increasing generation number suggests
that the polarity of the environment of the emitting fluoro-
phores is increasing. It should be stressed that the emission of
quadrupolar chromophore F is very sensitive to polarity73
allowing its use as a probe of its local environment. Fig. 7
shows emission spectra calculated by the aid of the model
reported in ref. 73, for different environmental reorganization
energies (eor). Parameters used are the same as reported in ref.
73, since the chromophore is the same. In the adopted model,
the evolution of position and shape (together with inhomoge-
neous broadening effects) of the emission band of some
quadrupolar chromophores (belonging to class I as defined
in ref. 73), is ascribed to symmetry breaking in the relaxed
excited state. Polar environments stabilize broken-symmetry
states, corresponding to dipolar states. The dipolar nature
of the broken-symmetry relaxed excited state is thus respon-
sible for the solvatochromic behavior of fluorescence bands. In
Fluorescence. The emission spectra of dendrimers of the
SOND series clearly show a change of the emission band shape
and position with increasing generation number (Fig. 4). A
change of the vibronic relative intensities is observed along
with a red-shift of the emission maxima (Table 1). Whereas the
0–0 transition is clearly the more intense vibronic band in the
emission spectrum of the parent isolated fluorophore F, the
0–1 transition becomes more intense in the dendrimers and
this effect becomes stronger on going from G1 to G2 com-
pounds (Fig. 4). The change in vibronic structure reveals an
increase in reorganization energy in the dendrimers as com-
pared to the isolated fluorophore, indicating that the emitting
fluorophore has stronger interactions with its environment
when gathered on the surface of the dendrimer. Indeed the
proximity of adjacent chromophores modifies the local envir-
Scheme 5 Synthesis of dumbbell-like dendrimer G02 (only half of the molecule is represented for G02).
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Table 1 Photophysical data for fluorophore F and dendrimers G1–G4 (SOND), C0–C1 (model dumbbell-like chromophores) and G00–G02
(DOND) in toluene
No. of fluorophores
MW/g molꢀ1
labs/nm
e/Mꢀ1 cmꢀ1
lem/nm
Fa
t/nsb
rc
s2 (max.)d/GM
F
1 F
12 F
24 F
48 F
96 F
1 C0
1 C1
1 C0 + 10 F
1 C1 + 20 F
1 C1 + 40 F
841
11 485
24 102
49 335
99 804
2014
5377
9184
19 698
40 726
386
385
386
386
386
377
378
382
382
384
84 900
1 004 000
2 035 000
3 785 000
7 101 000
73 100
71 500
805 000
1 537 000
3 000 000
420
423
426
441
445
410
412
443
444
445
0.83
0.75
0.71
0.62
0.48
0.81
0.68
0.44e
0.11e
0.26e
0.67
0.71
0.69
0.71
0.66
0.82
0.85
0.88f
0.71f
0.65f
0.18
0.03
0.02
0.01
0.01
0.22
765
8880
17 700
29 800
55 900
124
146
7100
14 300
32 800
G1
G2
G3
G4
C0
C1
G00
G01
G02
0.28
g
—
—
—
g
g
a
b
Fluorescence quantum yield determined relative to fluorescein in 0.1 M NaOH. Experimental fluorescence lifetime measured by time-correlated
single photon counting. Steady-state fluorescence anisotropy (lexc = 382 nm). 1 GM = 10ꢀ50 cm4 s photonꢀ1
l
. Determined for lexc =
c
d
e
f
g
abs(max.). Determined for lem = lem(max.). Anisotropy is wavelength-dependent (see Fig. 9).
Fig. 7, the variation of eor nicely describes the evolution of the
emission spectrum in different dendrimer generations and
symmetries (this correspondence is summarized in the legend).
Increasing dendrimer generation corresponds to increased eor
values: chromophores act as dielectrics, more polar than the
dissolving solvent (toluene). The effect of solvent polarity and
that of dendrimer generation/shape are very similar, on the
band position and shape. For example, we can estimate that
the polarity of the nanodots varies from lower than that of
CHCl3 for low-generation nanodots, up to almost that of
acetone for high-generation nanodots.
A distinct feature of the emission characteristics of dendri-
mers of the SOND series is their low anisotropy values. Parent
fluorophore F exhibits anisotropy of 0.18 in toluene, whereas
Fig. 4 Normalized absorption and emission spectra (lexc = labs(max.)) in toluene of SOND G1–G4, and DOND G00–G02. The absorption and
emission spectra of fluorophore F in toluene are also included for evaluation of the effect of the gathering of fluorophores F on the surface of
dendritic nanodots.
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Fig. 5 Normalized absorption and emission spectra (lexc = labs
(max.)) in toluene of model chromophore F and model dendrimers
C0–C1.
G1–G4 display much lower anisotropies, with values ranging
between 0.03 and 0.01 (Table 1). Since rotational diffusion of
the SOND dendrimers is expected to be much slower than that
of F due to their size (the radius of dendrimer G4 is estimated
to about 4 nm), the depolarization can be ascribed to fast
energy transfer between adjacent decorating fluorophores74–79
(excitation energy migration or homotransfer).80 We observe
that the fluorescence lifetimes remain roughly unchanged
(Table 1) which is consistent with the hypothesis of homo-
chromophoric energy transfer. Indeed homotransfer is possi-
ble via a Forster mechanism81 given the non-negligible overlap
between absorption and emission spectra of decorating fluoro-
phores. We also observe a continuous decrease of the fluores-
cence quantum yield with increasing generation number (Ta-
ble 1). This could be due to the occurrence of traps leading to
fluorescence quenching or formation of excimers (consistent
with an increasing red-tail) with lower fluorescence quantum
yields.82 The number of traps is expected to statistically
increase with increased generation number which could ex-
plain the variation of fluorescence quantum yield.
Fig. 7 Emission spectra calculated according to model presented in
ref. 73 using parameters relevant to the model chromophore (F) for
different solvent reorganization energies (see legend, eV units).
citation of only one type of chromophore unattainable. In
addition, we observe that the emission spectra of model
compounds in toluene show a significant overlap, the emission
of fluorophore F being slightly red-shifted (Fig. 5). Emission
spectra of DOND dendrimers G00–G02 show significant differ-
ences as compared to their SOND analogues bearing similar
number of decorating fluorophores (i.e. G1–G3). The emission
band of DOND dendrimers G00–G02 are clearly red-shifted
and broadened (in particular in the red side) with respect to
their SOND analogues G1–G3 (Fig. 8). A possible explanation
is that the emitting fluorophores in the DOND dendrimers
experience a different environment than in the corresponding
SOND dendrimers. This could be attributed to the proximity
not only of adjacent identical decorating fluorophores but also
of the core chromophore. The decorating fluorophores located
close to the core (shown in hashed in Fig. 2) clearly experience
different environment. Simulations based on the model re-
ported in ref. 73 also indicate that DONDs are more polar
objects than corresponding SONDs (see Fig. 7), suggesting a
closer packing of chromophores in DONDs. This finds a
possible confirmation in the lower fluorescence quantum yield
of DONDs with respect to corresponding (i.e. containing
approximately the same number of chromophores) SONDs.
In addition, interchromophoric interactions in the excited
state (leading to formation of excimer, or hetero-excimer if
the core chromophore is involved) can be facilitated in the
case of the DOND series due to the larger inhomogeneous
broadening and difference (disorder) in packing of the
decorating fluorophores on the surface of the DOND den-
drimers. Such phenomenon could explain the even more
pronounced red tail in the emission band of dendrimers of
the DOND series (Fig. 8). Interestingly, the more pro-
nounced red tail is observed for G01 which also gives rise
to the largest inhomogeneous broadening of the absorption
spectra (Fig. 4).
The analysis of the variation of the emission of dendrimers
of the DOND series is more intricate because their emission
stems from both core and peripheral fluorophores (Fig. 4). The
low-energy absorption bands of model core (C0, C1) and
peripheral (F) compounds show a major overlap (Fig. 5) and
similar intensities (Table 1) making selective one-photon ex-
As opposed to the case of homochromophoric dendrimers
of the SOND series whose anisotropy values remain constant
in the whole emission range (400–600 nm), a continuous
Fig. 6 Comparison of experimental and theoretical (evaluated from
additive contributions of isolated constituting chromophores F and
C1) absorption spectra of dendrimer G01.
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Fig. 8 Comparison of the emission spectra of corresponding spherical-type and dumbbell-like nanodots in toluene.
decrease of the anisotropy value with increasing emission
wavelength (from B0.1 at 400 nm to B0.02 at 600 nm) is
observed in the case of heterochromophoric dendrimers of the
DOND series (Fig. 9). This can be attributed to the contribu-
tion of two types of fluorophores (namely the core chromo-
phore and the peripheral fluorophores) with different
individual anisotropies, the combination of which results in a
wavelength-dependent global anisotropy. Whatever the wave-
length, the anisotropy values are lower than that of their
constituting chromophoric units (see r values for F and C0
or C1 in Table 1).83 These low values for such large nano-
objects indicate that fast energy transfer also takes place in the
DONDs. For wavelengths higher than 550 nm, where emission
stems only from the peripheral chromophores (see Fig. 4 and
5), the anisotropy values are similar to that of homochromo-
phoric SONDs indicative of fast energy transfer involving the
peripheral chromophores. The larger values obtained at lower
wavelength can be related to the increasing contribution of the
core chromophore, whose emission is blue-shifted with respect
to that of peripheral fluorophores (Fig. 6).84
adjacent chromophores and formation of (red-shifted) exci-
mers or hetero-excimer with lower fluorescence quantum yield.
This is consistent with the rise of a red tail in the emission band
which is even broader than in the case of the SOND dendri-
mers (Fig. 8). Strikingly dendrimer G01 which is the less
fluorescent of the DOND series shows the more pronounced
red tail. Also, the larger disorder (as testified by the inhomo-
geneous broadening) within decorating fluorophores—due to
the different geometry of the dendrimers of the DOND series
as compared to the SOND series-can be responsible for
different overlap and interactions between the chromophores
on the periphery.
Two-photon absorption
The TPA of the dendrimers was studied by investigating the
TPEF of SOND and DOND dendrimers in toluene. TPEF
measurements allow for direct measurement of the TPEF
action cross-section s2F, the relevant figure of merit for
imaging applications.
The TPA spectra of dendrimers C0 and C1 were determined
as reference for the TPA response of core chromophores in the
DOND series. These model chromophores clearly show much
lower TPA cross-section than decorating fluorophore F over
the whole spectral range (Fig. 10) and their maximum TPA
cross-sections are more than five times smaller (Table 1). As a
Another important feature is the much lower fluorescence
quantum yield of the dendrimers of the DOND series as
compared to those of the SOND series (Table 1).85 This could
be related to interactions between chromophores giving rise to
competing processes such as fast energy transfer between
Fig.
9
Emission anisotropy of monochromophoric dendrimers
Fig. 10 Two-photon absorption spectra of model fluorophore F and
monochromophoric dendrimers C0–C1 determined by femtosecond
TPEF measurements in toluene.
C0–C1 and dumbbell-like nanodots G00–G02 in toluene (lexc
382 nm).
=
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Fig. 11 Two-photon absorption spectra of SOND (G1–G4) and
DOND (G00–G02) dendrimers determined by femtosecond TPEF
measurements in toluene.
Fig. 12 Comparison of the TPA efficiency (normalized by the
number of TP fluorophores per nanodot) of SOND and DOND.
Inset: zoom of the 720–860 nm region (in the same units).
result, the TPA response of dendrimers of the DOND series is
expected to be dominated by the contribution of decorating
fluorophores given their larger molecular TPA response and
large excess (with a F/C ratio of, respectively 10/1, 20/1 and
40/1 for G00, G01 and G02). In addition the TPA response of
reference compounds C0 and C1 over 750 nm is clearly
negligible with respect to that of fluorophore F. Hence
although selective one-photon excitation of decorating fluoro-
phores of the DOND series was not permitted (similar absorp-
tion coefficient and almost complete overlap of the low energy-
absorption bands), selective two-photon excitation of the
decorating fluorophores of heterochromophoric DOND den-
drimers becomes possible. This is a major advantage of TP
excitation.
fluorophores due to the different topology of the dendritic
architecture or to direct interactions between the core and
decorating fluorophores located close to it. Hence, the present
study demonstrates that the different topology of the dendri-
mers of the SOND and DOND series modifies both the PL
and TPA properties of the organic nanodots, providing evi-
dence that arrangement and/or environment of the two-
photon fluorophores in the organic nanodots can significantly
affect their response.
Conclusion
The TPA spectra of dendrimers of the SOND and DOND
series are shown in Fig. 11. The TPA cross-section in each
series of dendrimers increases almost linearly with the number
of decorating fluorophores leading to giant TPA cross-sec-
tions. This is apparent from the quasi overlap of the TPA
spectra normalized by the number of decorating chromo-
phores in the 700–740 nm region (Fig. 12).86 We observe
however a noticeable difference in the TPA spectra of den-
drimers of the SOND87 and DOND series at wavelengths
higher than 740 nm. In that spectral range, dendrimers of the
DOND series show definitely higher TPA cross-sections than
dendrimers of the SOND series. For example at 760 nm,
dendrimers of the DOND series show a TPA cross-section
normalized by the number of chromophores which is 60%
larger than dendrimers of the SOND series. This indicates that
the decorating fluorophores (or at least some of them) of the
DOND dendrimers show a definitely larger TPA cross-section
in the 750–850 nm region. This difference between DOND and
SOND nanostructures can be directly related to their different
topology and thus to the presence of the core chromophore.
Hence although the core chomophores (C0 or C1) show much
smaller TPA activity than decorating fluorophores (Fig. 10),
their presence affects the TPA response of peripheral fluoro-
phores. This can possibly be attributed to a change of
arrangement (relative orientation, packing) of decorating
The present study demonstrates that the topology and nature
of the dendritic platform considerably influences the PL and
TPA properties of the organic nanodots. For comparable
numbers of decorating two-photon fluorophores, the PL
efficiency of the DOND dendrimers is much poorer due most
probably to interactions between the core chromophore and
proximal excited fluorophores (combined with fast energy
transfer which allows excitation energy migration towards
those traps). In contrast, the TPA efficiency is increased for
the DOND dendrimers as compared to the SOND dendrimers
in a large portion of the investigated spectral range (740–900
nm). Such phenomenon may have different causes. It might be
related to the change in topology caused by the different
geometry of the core, thus resulting in a different packing
and self-orientation of the decorating two-photon fluoro-
phores on the surface of the nanodots. Such effect can indeed
lead to changes of the TPA response of decorating fluoro-
phores as compared to isolated ones.66 Alternatively, the
change in TPA response could be due to a direct interaction
of the core with its proximal decorating fluorophores. Repla-
cing the chromophoric core by a non-conjugated core having
the same shape would allow to discriminate these causes. In
any case, this raises an important question since the global
TPA efficiency is found to be different for the two series. This
opens an interesting route towards improved TPA efficiency of
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Fig. 13 Numbering used for NMR assignments (only half of the molecule is represented).
the nanodots by controlling the relative orientation of the
decorating fluorophores and or their environment.
(31P NMR monitoring), then the salts were centrifuged, and
the solvent was removed. The pentasubstituted core was
separated from the tetra- and hexasubstituted analogues by
column chromatography on silica gel using mixtures of hex-
anes and ethyl acetate of increasing polarity (9.1 to 8.2). The
pure product (0.50 g, 57% yield) was obtained as a colorless
oil.
1H NMR (CDCl3, 300.1 MHz), d (ppm) = 3.78 (s, 15H,
OCH3), 6.72 (d, J = 9.0 Hz, 2H, C30A–H), 6.73 (d, J = 9.0 Hz,
4H, C30B–H), 6.79 (d, J = 9.0 Hz, 4H, C30B–H), 6.87 (dd, J =
9.0 Hz/J = 1.8 Hz, 2H, C02A–H), 6.92 (d, J = 9.0 Hz, 4H,
C20B–H), 7.06 (d, J = 9.0 Hz, 4H, C20B–H). 31P{1H} NMR
(CDCl3, 121.5 MHz), d (ppm) = 8.07 (d, J = 81.5 Hz, PB0 ),
8.08 (d, J = 78.0 Hz, PB0 ), 23.10 (dd, J = 78.0 Hz/J = 81.5 Hz,
PA0 ). 13C NMR (CDCl3, 75.5 MHz), d (ppm) = 157.04 (d, J =
2.26 Hz, C40A), 156.87 (C04B), 156.74 (C40B), 144.01 (pseudo t,
J = 3.81 Hz, C10B), 143.85 (pseudo t, J = 3.81 Hz, C10B), 143.75
(pseudo d, J = 9.81 Hz, C10A), 122.15 (pseudo t, J = 2.26 Hz,
C20B), 122.09 (m, C20A), 121.84 (pseudo t, J = 2.26 Hz, C20B),
114.39 (C30), 55.53 (OCH3), 55.51 (OCH3).
We have also demonstrated in this study that all-organic
nanodots can exhibit two-photon absorption and two-photon
brightness comparable to the highest values reported for
quantum dots58 and represent thus a promising complemen-
tary approach towards fluorescent labels for TPEF imaging
applications. The highly modular design of nanodots allows
adjunction of supplementary layers, decoration of the peri-
phery with hydrosolubilizing groups, and insertion of func-
tional groups that could be utilized for bioconjugation
purposes. In that way, we have very recently reported71
biocompatible water-soluble monochromophoric nanodots
with shielding dendrimeric layers and peripheral ammonium
groups, that maintain their quantum efficiency in water and in
physiological environment, and the efficiency of which as
contrast agents has been demonstrated for in vivo TPEF
imaging on living animal.
Experimental
Model compound C0. The pentasubstituted cyclotriphospha-
zene 1 (0.11 g, 0.14 mmol) and dry THF (2.5 mL) were
introduced into a 50-mL Schlenk. The biphenolic chromo-
phore 271 (0.036 g, 0.07 mmol) and caesium carbonate (0.05 g,
0.14 mmol) were added to this solution, and the resulting
mixture was stirred at room temperature for two days, then
five more days at 50 1C (31P NMR monitoring). The salts were
centrifuged off and the solvent was removed by vacuum
evaporation. The pure product (0.10 g, 55% yield) was
obtained after a column chromatography on silica gel using
mixtures of hexane–ethyl acetate of increasing polarity as
eluent (9 : 1 - 8 : 2 - 7 : 3 - 5 : 5).
1H NMR (CDCl3, 300.1 MHz), d (ppm) = 0.63–0.75 (m,
10H, Hq, Ho), 1.14 (m, 4H, Hp), 2.07 (br m, 4H, Hn), 3.76 (s,
12H, CH3O), 3.80 (s, 18H, CH3O), 6.69–6.75 (m, 20H, C30–H),
6.84–6.93 (m, 20H, C20–H), 6.98 (d, J = 8.6 Hz, 4H, Hb), 7.16
(m, 4H, He, Hf), 7.37 (d, J = 8.6 Hz, 4H, Hc), 7.50 (s, 2H, Hk),
7.53 (d, J = 8.6 Hz, 2H, Hh), 7.71 (d, J = 7.8 Hz, 2H, Hi).
31P{1H} NMR (CDCl3, 121.5 MHz), d (ppm) = 9.86 (m, P0).
13C{1H} NMR (CDCl3, 62.9 MHz), d (ppm) = 13.89 (Cq),
23.14 (Cp), 26.03 (Co), 40.44 (Cn), 54.97 (Cm), 55.53 (OCH3),
114.34 (C30), 120.01 (Ci), 120.67 (Ck), 121.34 (m, Cb), 121.92
(m, C20), 125.66 (Ch), 127.02 (Ce), 127.33 (Cc), 129.16 (Cf),
134.22 (Cg), 136.24 (Cd), 140.71 (Cj), 144.30 (m, C10), 150.07
(m, Ca), 151.60 (Cl), 156.56 (C40).
General
All manipulations were carried out with standard high vacuum
and dry-argon techniques. The solvents were freshly dried and
distilled (THF and ether over sodium/benzophenone, pentane
and CH2Cl2 over phosphorus pentoxide). Reactions were
monitored by thin layer chromatography on Merck silica gel
60 F254 precoated aluminum sheets. Column chromatogra-
phy: SDS silica gel Si 60 A CC (70–200 mm, 230–400 mesh) or
Florisils (60–100 mesh). Classical H, 13C, 31P NMR spectra
1
were recorded with Bruker AC 200, AC 250, or Avance 300,
500 spectrometers. References for NMR chemical shifts are
1
85% H3PO4 for 31P NMR, SiMe4 for H and 13C NMR. The
attribution of 13C NMR signals has been done using Jmod, two
dimensional HMBC, and HMQC, Broad Band or CW 31P
decoupling experiments when necessary. The multiplicity of
13C NMR signals is indicated only when the signal is not a
singlet. The numbering used for the assignment of NMR
signals is shown in Fig. 13.
Syntheses
Compound 1. Sodium hydride (0.18 g, 7.04 mmol) and dry
THF (20 mL) were introduced into a 50 mL Schlenk. 4-
Hydroxyanisole (0.74 g, 5.91 mmol) was added to this suspen-
sion. Once the solution was clear, it was cooled to 0 1C in a
water–ice bath and hexachlorocyclotriphosphazene (0.40 g,
1.14 mmol) was added at once. The reaction was stirred while
allowing the temperature to rise slowly to room temperature.
The reaction was considered to be finished after 40 h
Model compound C1. The first generation dendrimer 471
with the chromophore immobilized in the core (0.038 g, 10.68
mmol), 4-hydroxyanisole (0.030 g, 240.04 mmol), caesium
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0
(CL ), 140.72 (Cj), 144.56 (m, CB ), 147.37 (CH ), 147.94 (CQ ),
0
0
0
carbonate (0.150 g, 457.00 mmol), and an acetone–THF (1 : 1)
mixture (8 mL) were introduced in a 50-mL Schlenk. The
resulting heterogeneous solution was stirred at 50 1C for 20 h.
The salts were filtered and the solvent removed when the
reaction was finished (31P NMR monitoring). The crude
product was dissolved in minimum amount of chloroform
and added dropwise to 300 mL of pentane under vigorous
stirring, and the precipitated product recovered by filtration
(for three times) to afford the pure chromophoric core model
C1 (0.051 g, 89% yield).
1H NMR (CDCl3, 300.1 MHz), d (ppm) = 0.53–0.69 (m,
10H, Hq, Ho), 1.09 (m, 4H, Hp), 2.02 (br m, 4H, Hn), 3.25 (d,
3JHꢀP = 10.2 Hz, 30H, CH3N), 3.70 (s, 20H, CH3O), 3.72 (s,
10H, CH3O), 3.73 (s, 30H, CH3O), 6.78 (d, J = 9.0 Hz, 40H,
C13–H), 6.99–7.16 (m, 68H Hc, He, Hf, C02–H, C21–H), 7.36–7.44
(m, 8H, Hh, Hk, Hb), 7.56–7-66 (m, 32H, Hi, C30–H, CHQN).
31P{1H} NMR (CDCl3, 121.5 MHz), d (ppm) = 8.47 (m, P0),
64.30 (s, P1), 64.40 (s, P1). 13C{1H} NMR (CDCl3, 62.9 MHz),
00
0
0
150.12 (m, Ca), 150.93 (CK , CO ), 151.71 (Cl), 155.65 (CC ).
Dendrimer G01. The first generation dendrimer 471 (0.010 g,
2.83 mmol), the terminal chromophore F61 (0.049 g, 58.2
mmol), caesium carbonate (0.038, 113.94 mmol) and tetrahy-
drofuran (7 mL) were introduced in a 50 mL Schlenk, and
stirred at room temperature overnight then for 6 days at 45 1C.
The salts were filtered off and the solvent removed when the
reaction was finished (31P NMR monitoring). The product was
purified by column chromatography (Florisils, CHCl3 to
THF–CH3OH as eluent mixtures) and further diethyl ether
washings to afford the orange bichromophoric product
(0.0191 g, 34% yield).
1H NMR (CDCl3, 500.3 MHz), d (ppm) = 0.57–0.63 (m,
84H, HS, Ho), 0.63–0.70 (m, 126H, HU, Hq), 0.86–0.96 (m,
0
120H, HZ ), 1.04–1.12 (m, 84H, HT, Hp), 1.12–1.23 (m, 60H,
HG), 1.30–1.39 (m, 240H, HX, HY, HZ), 1.52–1.67 (m, 80H,
HW), 1.91–2.06 (m, 84H, HR, Hn), 3.18–3.30 (m, 30H,
P1–N–CH3), 3.27–3.36 (m, 80H, HV), 3.37–3.49 (m, 40H,
HF), 3.60–3.74 (m, 40H, HE), 3.92–4.06 (m, 40H, HD),
6.53–6.69 (m, 80H, HH, HQ), 6.69–6.83 (m, 40H, HC),
7.02–7.16 (m, 68H, HB, C20–H, Hb, He, Hf), 7.37–7.44 (m,
84H, HI, HP, Hc), 7.44–7.51 (m, 84H, HJ, HL, HN, HO, Hh,
Hk), 7.48–7.53 (m, 30H, C03–H, CHQN), 7.58–7.69 (m, 42H,
HK, HM, Hi). 31P{1H} NMR (CDCl3, 202.5 MHz), d (ppm) =
8.58 (m, P0), 64.58 (m, P1). 13C{1H} NMR (CDCl3, 125.8
d (ppm) = 13.91 (Cq), 23.08 (Cp), 26.00 (Co), 33.11 (d, JCꢀP
=
12.1 Hz, CH3N), 40.69 (Cn), 55.00 (Cm), 55.52 (OCH3), 114.49
(C31), 120.06 (Ci), 120.67 (Ck), 121.12 (m, Cb), 121.41 (m, C02),
122.28 (d, JCꢀP1 = 4.4 Hz, C21), 125.54 (Ch), 126.73 (Ce),
127.50 (Cc), 129.41 (C03), 129.41 (Cf), 132.24 (m, C40), 134.72
(Cg), 136.02 (Cd), 138.28 (d, JCꢀP1 = 13.8 Hz, C0HQN),
140.72 (Cj), 144.14 (d, JCꢀP1 = 6.92 Hz, C11), 149.57 (m, Ca),
151.24 (m, C10), 151.56 (Cl), 156.98 (C41).
0
MHz), d (ppm) = 12.22 (CG), 13.86 (CU, Cq), 14.05 (CZ ),
22.68 (CZ), 23.09 (CT, Cp), 25.88 (CS, Co), 26.80 (CX), 27.17
Dendrimer G00. The zeroth generation dendrimer 371 (0.005
g, 4.40 mmol), the terminal chromophore F61 (0.039 g, 46.34
mmol), caesium carbonate (0.029, 89.03 mmol), and a 1 : 1
tetrahydrofuran–acetone mixture (6 mL) were introduced in a
50 mL Schlenk, and stirred at 45 1C for 20 days, then for 17
days at 60 1C. The salts were filtered off and the solvent was
removed when the reaction was finished (31P NMR monitor-
ing). The product was purified by washing the orange solid
with a 1 : 1 acetone–methanol mixture to afford the orange
bichromophoric product (0.0182 g, 45% yield).
(CW), 31.70 (CY), 33.05 (m, P1–N–CH3), 40.30 (CR, Cn), 45.60
0
(CF), 49.50 (CE), 50.99 (CV), 55.05 (CR , Cm), 65.69 (CD), 88.54
00 0
00
00
00 0
0
0
(CI , CN ), 91.03 (CI , CN ), 108.81 (CP ), 109.91 (CI ), 111.41
(CH, CQ), 115.04 (CC), 119.72 (CK, CM), 121.36 (m, C20, Cb, Ci,
0
00
Ck), 122.37 (m, CB), 122.75 (CK , CO ), 125.53 (CL, CO, Ch),
127.17 (m, Cc, Ce), 128.38 (C30), 129.01 (Cf), 130.37 (CJ, CN),
132.17 (m, C40), 132.88 (CP, CI), 133.81 (Cg), 136.14 (Cd),
0
0
0
140.17 (m, CL , CN , C0HQN, Cj), 144.27 (m, CB ), 147.33
1
(CH ), 147.85 (CQ ), 150.93 (m, CK , CO , C0, Ca, Cl), 156.03
0
0
00
0
1H NMR (CDCl3, 500.3 MHz), d (ppm) = 0.60–0.64 (m,
44H, HS, Ho), 0.65–0.70 (m, 66H, HU, Hq), 0.94 (br s, 60H,
0
(CC ).
Dendrimer G02. The second generation dendrimer 571 (0.078
g, 0.91 mmol), the terminal chromophore F61 (0.031 g, 36.48
mmol), caesium carbonate (0.026 g, 78.09 mmol) and a 1 : 1
acetone–THF mixture (6 mL) were introduced in a 50-mL
Schlenk, and stirred at 50 1C for 21 h. The salts were filtered
and the solvent removed when the reaction was finished (31P
NMR monitoring). The orange solid obtained was repeatedly
washed with diethyl ether until no F chromophore was ob-
served by TLC. Then the solid was taken in dichloromethane,
filtered, and the solvent was removed to afford the orange
bichromophoric product (0.034 g, 91% yield).
0
HZ ), 1.10 (m, 44H, HT, Hp), 1.22 (m, 30H, HG), 1.36 (m,
120H, HX, HY, HZ), 1.62 (m, 40H, HW), 1.99 (m, 44H, HR,
Hn), 3.31 (br s, 40H, HV), 3.42–3.57 (m, 20H, HF), 3.63–3.79
(m, 20H, HE), 3.97–4.14 (m, 20H, HD), 6.61 (d, J = 7.6 Hz,
20H, HQ), 6.65 (m, 20H, HH), 6.70 (d, J = 7.4 Hz, 20H, HC),
6.91 (m, 20H, HB), 7.00 (m, 4H, Hb), 7.17 (m, 4H, He, Hf), 7.44
(m, 44H, HI, HP, Hc), 7.46–7.52 (m, 44H, HJ, HL, HN, HO, Hh,
Hk), 7.63 (m, 22H, HK, HM, Hi). 31P{1H} NMR (CDCl3, 121.5
MHz), d (ppm) = 9.68 (m, P0). 13C{1H} NMR (CDCl3, 125.8
0
MHz), d (ppm) = 12.28 (CG), 13.88 (CU, Cq), 14.08 (CZ ),
22.71 (CZ), 23.12 (CT, Cp), 25.89 (CS, Co), 26.84 (CX), 27.23
(CW), 31.74 (CY), 40.25 (Cn), 40.34 (CR), 45.63 (CF), 49.59
1H NMR (CDCl3, 500.3 MHz), d (ppm) = 0.50–0.74 (m,
0
(CE), 50.98 (CV), 55.06 (CR , Cm), 65.82 (CD), 88.22 (CN ),
00
0
410H, HS, HU, Ho, Hq), 0.85–0.96 (m, 240H, HZ ), 0.99–1.19
00 0
88.61 (CI ), 90.78 (CI ), 91.22 (CN ), 108.76 (CP ), 110.07
00
00 0
0
(m, 284H, HG, HT, Hp), 1.24–1.29 (m, 480H, HX, HY, HZ),
1.52–1.67 (m, 160H, HW), 1.85–2.09 (m, 164H, HR, Hn),
3.12–3.49 (m, 330H, HF, HV, P1-N-CH3, P2-N-CH3),
3.53–3.72 (m, 80H, HE), 3.87–4.07 (m, 80H, HD), 6.52–6.67
(m, 160H, HH,, HQ), 6.68–6.81 (m, 80H, HC), 7.00–7.25 (m,
148H, HB, C20–H, C12–H, Hb, He, Hf), 7.34–7.44 (m, 164H, HI,
0
(CI ), 111.23 (CQ), 111.46 (CH), 114.97 (CC), 119.73 (CK, CM),
120.09 (Ci), 120.71 (Ck), 121.24 (m, Cb), 121.97 (m, CB), 122.51
00
0
(CO ), 122.80 (CK ), 125.54 (CL, CO, Ch), 126.89 (Ce), 127.38
(Cc), 129.17 (Cf), 130.37 (CJ, CN), 132.89 (CP), 133.03 (CI),
0
134.31 (Cg), 136.12 (Cd), 136.12 (Cd), 140.00 (CN ), 140.22
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HP, Hc), 7.44–7.54 (m, 194H, HJ, HL, HN, HO, C0HQN,
C1HQN, Hh, Hk), 7.54–7.72 (m, 142H, HK, HM, C30–H, C13–H,
Hi). 31P{1H} NMR (CDCl3, 121.5 MHz), d (ppm) = 8.43 (m,
P0), 62.34 (br s, P1), 64.62 (br s, P2).13C{1H} NMR (CDCl3,
125.8 MHz), d (ppm) = 12.24 (CG), 13.87 (CU, Cq), 14.07
largely overestimated TPA cross-sections. Fluorescein in 0.01
M NaOH, whose TPEF cross-sections are well-known,90
served as the reference, taking into account the necessary
corrections for the refractive index of the solvents.91 The
quadratic dependence of the fluorescence intensity on the
excitation intensity was verified for each data point, indicating
that the measurements were carried out in intensity regimes in
which saturation or photodegradation do not occur. More
details about the experimental setup have been previously
published.91
0
(CZ ), 22.70 (CZ), 23.09 (CT, Cp), 25.88 (CS, Co), 26.82 (CX),
27.22 (CW), 31.72 (CY), 33.02 (m, P1–N–CH3, P2–N–CH3),
0
40.30 (CR, Cn), 45.59 (CF), 49.51 (CE), 50.96 (CV), 55.04 (CR
,
00
Cm), 65.72 (CD), 88.22 (CN ), 88.54 (CI ), 90.87 (CI ), 91.23
00 0
00
00 0
0
0
(CN ), 108.74 (CP ), 109.87 (CI ), 111.22 (CQ), 111.40 (CH),
115.03 (CC), 119.72 (CK, CM), 121.86 (m, C20, C12, Cb, Ci, Ck),
0
00
122.42 (m, CB), 122.78 (CK , CO ), 125.51 (CL, CO, Ch), 127.11
(Cc, Ce), 128.38 (C30, C13), 129.05 (Cf), 130.36 (CJ, CN), 131.59
(C41), 132.34 (C04), 132.88 (CP), 132.98 (CI), 133.67 (Cg), 136.18
Acknowledgements
F. T. acknowledges Universite de Rennes 1 for an invited
assistant professor position. We acknowledge financial sup-
port from CNRS, Region Bretagne (‘‘Renouvellement des
Competences’’ Program) and Rennes Metropole. Thanks are
due to the Fundacion Ramon Areces for a grant to A.
Pla-Quintana.
0
0
(Cd), 139.98 (CN ), 140.17 (m, CL , CHQN, Cj), 144.25 (m,
1
CB ), 147.33 (CH ), 147.93 (CQ ), 150.92 (m, CK , CO , C0, Ca,
1
Cl), 151.25 (m, C1), 156.01 (CC ).
0
0
0
00
0
0
Photophysical studies
Optical absorption and emission spectroscopy. All photophy-
sical measurements have been performed with freshly-pre-
pared solutions in air-equilibrated toluene at room
temperature (298 K). UV/Vis absorption spectra were re-
corded on a Jasco V-570 spectrophotometer. Steady-state
and time-resolved fluorescence measurements were performed
on dilute solutions (ca. 10ꢀ6 M chromophore concentration,
optical density o0.1) contained in standard 1 cm quartz
cuvettes using an Edinburgh Instruments (FLS920) spectro-
meter in photon-counting mode. Emission spectra were ob-
tained, for each compound, under excitation at the wavelength
of the absorption maximum. Fluorescence quantum yields
were measured according to literature procedures using fluor-
escein in 0.1 M NaOH as a standard (quantum yield F =
0.90).88,89 The lifetime values were obtained from the recon-
volution fit analysis (Edinburgh F900 analysis software) of
decay profiles obtained using the FLS920 instrument under
excitation with a nitrogen-filled nanosecond flash-lamp. The
quality of the fits was evidenced by the reduced w2 value
(w2 o1.1).
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83 Model chromophore F exhibits an anisotropy of 0.18, and refer-
ence monochromophoric compounds C0 and C1 display values of
0.22 and 0.28, respectively. Since these two dendrimer-embedded
chromophores differ only in generation number, the increase in
anisotropy value can be related to the slowing-down of the
rotational diffusion for increasing molecule size.
84 DOND G00 shows the smaller anisotropy value at low wavelength
(0.07), which might indicate that energy transfer from core chro-
mophore to peripheral ones also takes place in that case (in relation
with the smaller distance between core and peripheral chromo-
phores). Wavelength-dependent time-resolved anisotropy measure-
ments are required to derive final conclusion.
number of chromophores) of excitation of each type of chromo-
phores. The values given in Table 1 correspond to excitation at the
absorption maximum.
86 Except for dendrimer G02 that shows slightly higher TPA cross-
sections.
87 Even if their PL characteristics show that emitting fluorophores of
SOND dendrimers experience a more polar environment, the
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to that of an isolated molecule (Fig. 12), which was also the case
for one-photon absorption (Fig. 4).
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