Cooperative two-photon absorption enhancement by through-space interactions in multichromophoric compounds

Article abstract of DOI:10.1002/anie.200903519

Spaceinvaders:Aseriesofmultichromophores(fromdinnerstodendritic24mers) issynthesized,spectroscopicallycharacterized,andtheoreticallymodeled. Enhancementofthetwo photonabsorptionresponsedependsonthenumberanddistributionofchromophoricsubun its(seepicture,re

Full text of DOI:10.1002/anie.200903519

Angewandte
Chemie
DOI: 10.1002/anie.200903519
Multichromophores
Cooperative Two-Photon Absorption Enhancement by Through-Space
Interactions in Multichromophoric Compounds**
Francesca Terenziani, Venkatakrishnan Parthasarathy, Anna Pla-Quintana, Tarun Maishal,
Anne-Marie Caminade, Jean-Pierre Majoral, and Mireille Blanchard-Desce*
In the last decade, a major effort has been devoted to the
achievement of efficient organic materials for nonlinear
optics (NLO). Optimization has been sought at both the
molecular and the supramolecular level. Of major importance
is the awareness that the NLO properties of organic-based
materials can be different from the sum of the properties of
the isolated molecules.[1–4] In many cases, intermolecular
interactions are found to be detrimental to the achievement of
good performances for either second-order (electro-optical
organic materials)[3] or third-order materials (e.g., two-
photon absorption, TPA).[5] It has been shown, however,
that significant cooperative TPA enhancement can be
obtained in conjugated branched[6–9] or conjugated dendritic
structures[10–12] as a result of through-bond coherent coupling
between chromophoric subunits. Enhancement can also be
achieved in the solid state through strong environment
(charges) effects.[13,14] In contrast, the role of through-space
interactions as a means to enhance TPA responses has seldom
been considered.
Dipole–dipole interactions were found to be responsible for
a reduction of the TPA response per chromophoric unit, while
modeling suggested that changing the relative orientation/
distance of the chromophores would allow cooperative TPA
enhancement to be achieved.[15] This prompted us to design
and investigate multichromophoric molecular structures in
which through-space interactions—instead of through-bond
interactions[16,17]— would be exploited to increase the TPA
response. In contrast to supramolecular chemistry, our aim
was to use covalent bonding to confine chromophores and
control their number and relative distance/positioning by
grafting on suitable platforms.
Following this strategy, herein we report the amplification
of the TPA response of a series of well-defined and soluble (as
opposed to aggregates or nanocrystals) multistilbazole archi-
tectures (Scheme 1), and investigate how the different top-
ologies and number of dipolar chromophores influence the
Recently, we reported a strongly nonadditive behavior in
covalent antiparallel dimers of a dipolar chromophore.[15]
[*] Dr. V. Parthasarathy, Dr. T. Maishal, Dr. M. Blanchard-Desce
CNRS, Chimie et Photonique Molꢀculaire (CPM)
35042 Rennes (France)
and
Universitꢀ de Rennes 1, CPM, Campus de Beaulieu
Bꢁtiment 10A, 35042 Rennes (France)
E-mail: mireille.blanchard-desce@univ-rennes1.fr
Dr. F. Terenziani
Dipartimento di Chimica GIAF & INSTM UdR-Parma, Parma
University, Parco Area delle Scienze 17/a, 43100 Parma (Italy)
Dr. A. Pla-Quintana, Dr. A.-M. Caminade, Dr. J.-P. Majoral
CNRS; LCC (Laboratoire de Chimie de Coordination)
205 route de Narbonne, 31077 Toulouse (France)
and
Universitꢀ de Toulouse; UPS, INPT; LCC, 31077 Toulouse (France)
[**] F.T. and M.B.-D. acknowledge the Italo-French University and Egide
for funding through the Galileo Program. M.B.-D. thanks the Indo-
French Center for Promotion of Advanced Research (IFCPAR)/
Centre Franco-Indien pour la Promotion de la Recherche Avancꢀe
(CEFIPRA) for financial support and a PDF to V.P. M.B.-D.
acknowledges the CNRS for a PDF to T.M. Thanks are due to the
Fundaciꢂn Ramꢂn Areces for a grant to A.P.-Q. M.B.-D., J.-P.M., and
A.-M.C. acknowledge financial support from ANR (Project “Bio-
dendridot”). M.B.-D. also acknowledges financial support (equip-
ment grants) from the CPER “Photonique aux Interfaces” (FEDER
and Rꢀgion Bretagne). We thank J.-M. Vabre for contributions to the
synthesis.
Scheme 1. A series of multichromophoric compounds built from the
confinement of push–pull stilbazole chromophores: reference mono-
mer (M); dimers obtained by grafting two chromophoric subunits to a
central benzenic platform in the para (Dp), meta (Dm), and ortho (Do)
positions; trimer (T); dendrimers of the first (G1) and second
generation (G2) bearing 12 and 24 chromophoric subunits, respec-
tively.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200903519.
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ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Scheme 2. Synthesis of multistilbazoles: a) N-hexylaniline, K2CO3, DMF, 90–1108C, 2–3 d, 28–86%; b) I2, aq. NaHCO3, CH2Cl2, 108C, 1 h, then RT,
0.5 h, 55–97%; c) N-iodosuccinimide, DMF, 08C–RT, 1.5 h, 59%; d) vinylpyridine, Pd(OAc)2, PPh3, K2CO3, nBu4NBr, DMF, 50–708C, 3 h, 60–74%;
e) Cs2CO3, THF/acetone, 458C, 1 d, 58%; f) Cs2CO3, THF/acetone, RT, 60 h, 70%.
magnitude of the cooperative effect. Dimers and a related
trimer were prepared in good yields according to the multi-
step synthetic path described in Scheme 2 (see also the
Supporting Information). Multichromophoric dendritic archi-
tectures confining 12 or 24 chromophores were also prepared
in moderate to good yields (see the Supporting Information)
by reaction of an excess of reagent 1 on the dendritic
platforms P1 and P2 (Scheme 2).
All of the compounds were characterized by linear one-
photon absorption (OPA) and fluorescence spectroscopy, as
well as by the two-photon excited fluorescence technique
(using femtosecond excitation) to extract the corresponding
TPA spectra.
Figure 1 (top panels). A slight hypsochromic shift as well as
a definite hypochromic shift of the OPA band is observed for
all multimeric derivatives as a result of interchromophoric
interactions. The reduction of the molar extinction coefficient
per chromophoric subunit is particularly remarkable for Do,
T, and the dendrimeric derivatives, that is, for compounds
providing the closest proximity between subchromophores or
with a larger number of chromophores. This finding can be
ascribed to steric crowding, possibly leading to partial loss of
planarity, as well as to inhomogeneous broadening effects. All
of these effects would be expected to lead to a reduction of
the TPA response as well. In contrast, we observe that the
TPA maximum cross section, sm2 ax, per chromophoric subunit
increases from monomer to multimers, even though the
maximum molar absorptivity emax decreases. This behavior is
The spectroscopic properties of the compounds are
summarized in Table 1, and the OPA and TPA spectra
normalized per chromophoric subunit are reported in
summarized in Figure 1 (bottom panel), where the ratio s2max
max is reported as a function of the number of chromophoric
subunits.
/
e
All multichromophoric compounds perform better than
the model chromophore.[18] In addition, we observe that the
response of the dimers increases from Dp to Dm to Do, which
corresponds to increasing proximity of the chromophoric
subunits in the multichromophoric structure and hence to
stronger interactions. The meta trimer T is found to lead to a
slightly larger sm2 ax/emax ratio than its corresponding dimer Dm.
Analogously, the second-generation dendrimer G2 leads to a
higher ratio than the first-generation dendrimer G1, thus
indicating that increasing the number of chromophores in
related multichromophoric structures yields stronger effects.
An amplification factor of up to 2.5 per chromophoric subunit
Table 1: Spectroscopic properties of monomer M and N-meric multi-
chromophoric derivatives in DMSO.
À
Á
max
OPA
max
em
[a]
N
l
e lmax =N
l
f
sm2 ax=N
[nm]
[mꢀO1PcAmꢀ1
]
[nm]
[%]
[GM][b]
M
Dp
Dm
Do
T
1
2
2
2
3
386
384
384
384
384
381
381
31000
29400
30750
21400
26200
23300
20140
495
490
490
490
488
488
491
9.4
7.4
7.1
5.3
6.2
6.0
5.9
40
55
60
68
57
51
58
G1
G2
12
24
[a] Fluorescence quantum yield. [b] 1 GM=10ꢀ50 cm4 sphotonꢀ1
.
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Angewandte
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Figure 2. Calculated OPA and TPA per chromophoric subunit for
monomer, dimers of varying interchromophore angle (a, see legend
and inset), and a 12-chromophore multimer of icosahedral symmetry.
The exciton coupling was fixed to Sj¼iJij =0.05 eV.
of the considered dimer is sketched in the inset of Figure 2. A
multichromophoric architecture having icosahedral symme-
try, with 12 equidistant chromophores lying in the directions
connecting the center to the vertexes of the icosahedron
(interchromophore angle of about 638, see the Supporting
Information), was also considered. The results in Figure 2
clearly show that in order to achieve TPA enhancement (per
chromophoric unit) in dimers with respect to the model
monomer, an angle a > 1208 is required. This is consistent
with expected geometries based on electrostatic repulsions.
Under these conditions TPA is enhanced whereas the OPA
intensity decreases. We stress that this trend is independent of
the specific value of the Jij terms, but the behavior is
accentuated if the exciton coupling is increased, for example,
as a consequence of decreased interchromophore distances
(see the Supporting Information). This is consistent with the
observation of higher sm2 ax/emax ratios for Do with respect to
the other dimers, and for dimers and trimers with respect to
dendrimers (spacers between the platform—benzene core or
dendritic architecture—and the subchromophores are longer
in the latter case).
Interestingly, a reduction of the TPA cross section is
predicted for the multimer of icosahedral symmetry (again
independently of the specific value and sign of the Jij), while a
clear cooperative enhancement is experimentally observed
for dendrimer G1 (as well as G2). This result suggests that
chromophoric subunits in the dendrimers are not equidistant,
but rather form pairs of closer-interacting chromophores only
slightly interacting with other chromophores from proximal
pairs. This is consistent with the dendritic architecture, in
which two chromophores are branched to each peripheral
phosphorus unit (Scheme 1). Hence, the branched geometry
provided by the dendritic platform supplying AB2 connection
is of particular interest for TPA enhancement. Moreover, we
observe that the TPA enhancement increases from G1 to G2,
Figure 1. Top panels: experimental OPA and TPA per chromophoric
subunit, measured in dimethyl sulfoxide (DMSO) solution. Bottom
panel: the sm2 ax/emax ratio as a function of the number N of chromo-
phoric subunits.
is obtained for the s2max/emax response, and up to 1.7 for the
bare TPA cross section, s2. To the best of our knowledge, this
is the first time that a cooperative enhancement (up to 70%)
of the TPA response has been observed in soluble multi-
chromophoric structures in which dipolar chromophoric
subunits are connected through saturated bonds and hence
interact only via through-space interactions.
The results presented here can be interpreted by means of
the theoretical model first presented in reference [4], based
on a two-level description of chromophoric subunits. Only
electrostatic interactions are taken into account between
chromophoric subunits, which gives rise to the well-known
excitonic term (exciton hopping), plus mean-field interaction,
plus other contributions describing the exciton–exciton inter-
action and the interaction between states having a different
exciton number.[1] These last two contributions are negligible
in the specific case of weakly polar and polarizable chromo-
phores, so that discussion can be restricted to the standard
excitonic Hamiltonian (see the Supporting Information),
where the exciton-transfer interaction between chromo-
phores i and j is labeled Jij.[4]
Theoretical results for OPA and TPA spectra are reported
in Figure 2 for dimers of increasing interchromophore angle
(a) and for a model 12-chromophore multimer. The geometry
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purely electrostatic through-space interchromophoric inter-
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[18] We stress that an alternative model compound bearing a phenyl
moiety as a pendant group (compound M’) was found to have a
similar TPA response to that of chromophore M (see the
Supporting Information).
Received: June 29, 2009
Revised: August 17, 2009
Published online: October 13, 2009
Keywords: chromophores · dendrimers · nonlinear optics ·
.
through-space interactions · two-photon absorption
[1] V. M. Agranovich, M. D. Galanin, Electronic Excitation Energy
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