Boron-containing two-photon-absorbing chromophores: Electronic interaction through the cyclodiborazane core

Article abstract of DOI:10.1002/anie.200602266

(Chemical Equation Presented) Through-bond coupling across a cyclodiborazane core results in large absorption cross sections in a new type of boron-containing two-photon-absorbing chromophore (see picture; B pink, N blue, C gray, H other than BH (white) o

Full text of DOI:10.1002/anie.200602266

Communications
chromophores.[7] Experimentally, symmetrical one-dimen-
sional chromophores typically consist of two donor or
acceptor groups that interact through a conjugated system.
For example, the donor–acceptor–donor and acceptor–
donor–acceptor chromophores described by Marder et al.
can be regarded as p-conjugated molecules that undergo large
changes in quadrupole moment on excitation.[8] Another
approach was proposed by Andraud, Baldeck et al., who
pointed out that TPA cross sections of oligomers are
enhanced by biexcitonic coupling between two weakly con-
jugated monomers.[9] This theoretical approach led to the
molecular engineering of several new efficient TPA chromo-
phores and, in our hands, to water-soluble TPEF chromo-
phores for bioimaging.[10]
In most cases the electronic interaction proceeds via
conjugated systems. Recently, some computational studies
have shown that the traditional p-conjugated transmitter in
hyperpolarizable donor–acceptor systems could be replaced
by a system of s bonds to give p–s–p through-bond
coupling.[11] Furthermore, experimental data on 1,3-bis(di-
phenylmethylene)cyclobutane (Scheme 1a) have shown that
Two-Photon Absorption
DOI: 10.1002/anie.200602266
Boron-Containing Two-Photon-Absorbing
Chromophores: Electronic Interaction through
the Cyclodiborazane Core**
Ali Hayek, Jean-François Nicoud,* FrØdØric Bolze,
Cyril Bourgogne, and Patrice L. Baldeck
Scheme 1. 1,3-Bis(diphenylmethylene)cyclobutane (a) and cyclodibora-
zanes (b).
Conception of novel chromophores exhibiting two-photon
absorption (TPA) is of growing importance nowadays due to
the great variety of applications of this nonlinear optical
(NLO) property,[1] which include two-photon excited fluores-
cence (TPEF) microscopy of biological systems,[2] three-
dimensional microfabrication,[3] optical limiting,[4] data stor-
age, and photodynamic therapy.[5] Since the prediction of this
phenomenon by Göppert-Mayer in 1931,[6] many theoretical
studies have been made on mono- and multidimensional
in such a structure the two p systems interact electronically
through the 1,3-dimethylenecyclobutane core.[12] Cyclodibor-
azanes (Scheme 1b), widely studied by Chujo et al., mainly
for the preparation of conjugated polymers, are heteroatomic
analogues of 1,3-dimethylenecyclobutane derivatives.[13]
Many of the cyclodiborazane-based polymers are fluorescent
and electroluminescent. They were also expected to show
interesting third-order NLOproperties. [13b] These experimen-
tal results suggested that the cyclodiborazane core maintains
some kind of conjugation (1,3-p-type interaction) between
the p systems across its square structure.
We report here the synthesis and characterization of a new
TPA chromophore containing a cyclodiborazane central core.
Our aim was to check the influence of this unusual structure
on the efficiency of TPA properties due to electronic coupling
through such a pseudoconjugated system. Our target chro-
mophore 1 is shown in Figure 1.
Bis(4-iodophenyl)cyclodiborazane core 4 was obtained in
60% yield by classical hydroboration dimerization of 4-
iodobenzonitrile (2) with mesitylborane (3; Scheme 2).[13a]
Product 4 was obtained as a mixture of two stereoisomers in
a 1:1 ratio (1H NMR spectrum in the Supporting Informa-
tion). Considering the different substituents in the cyclo-
diborazane structure, 4 can exist as four stereoisomers
(Scheme 3).[14] The mesityl groups can be cis or trans with
respect to each other, and the Ar groups can be in syn or anti
configurations. Thus, cis-syn (A), cis-anti (B), trans-syn (C),
[*] A. Hayek, Prof. Dr. J.-F. Nicoud, Dr. F. Bolze, Dr. C. Bourgogne
Institut de Physique et Chimie des MatØriaux de Strasbourg
Groupe des MatØriaux Organiques (UMR 7504)
23 rue du Loess, BP 43, 67034 Strasbourg cedex 2 (France)
Fax: (+33)388-107-246
E-mail: nicoud@ipcms.u-strasbg.fr
Dr. P. L. Baldeck
Laboratoire de SpectromØtrie Physique
UniversitØ Joseph Fourier, CNRS (UMR 5588)
BP 87, 38402 Saint Martin d’H›res (France)
Fax: (+33)476-635-495
E-mail: baldeck@ujf-grenoble.fr
[**] Boron-Containing Two-Photon-Absorbing Chromophores, Part 1.
This work was supported by the CNRS and University Louis Pasteur
of Strasbourg. A.H. is grateful to the French ministry of research for
a fellowship. We thank Prof. Dr. R. Welter and Dr. A. De Cian for the
solution of the X-ray crystal structure.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
6466
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Chemie
(TMS) group was removed by
tetrabutylammonium fluoride
(TBAF) in THF to yield eth-
ynyl derivative 9. Sonogashira
coupling of 4 with 4-dipheny-
lamino-4’-ethynylstilbene (9)
led to a mixture of two stereo-
isomers, from which stereo-
pure chromophore 1 was iso-
lated in 55% yield after
Figure 1. Structure of chromophore 1.
Scheme 2. Synthesis of bis(4-iodophenyl)cyclodiborazane core 4.
a) THF, 08C, 15 min; 258C, 12 h; 60%.
Scheme 4. Synthesis of chromophore 1 and model compound 10.
ꢁ
a) NaH, THF, 3 h, 55!668C, 85%; b) Me3SiC CH, [PdCl2(PPh3)2]
6 mol%, CuI 10 mol%, THF/iPr2NH, RT, 48 h, 90%; c) TBAF, THF,
15 min, RT, 80%; d) 4-ethynylstyrene, [PdCl2(PPh3)2] 6 mol%, CuI
10 mol%, NEt3, RT, 70%; e) PdCl2 10 mol%, Cu(OAc)2·H2O 1.5 mol%,
PPh3 40 mol%, THF, Et3N, 808C, 12 h, 70%.
column chromatography. It was fully characterized by 1H,
11B, and 13C NMR and IR spectroscopy and HRMS. In the
1H NMR spectrum, the imino resonance is located at d =
8.1 ppm, as already reported for other cyclodiborazane
structures.[16] The protons linked to boron give a very broad
signal around 5.3 ppm. The 11B NMR signal appears at d =
5.6 ppm. Complete NMR characteristics are given in the
Scheme 3. Possible stereoisomers of tetrasubstituted cyclodibora-
zanes.
and trans-anti (D) stereoisomers are possible. The trans-syn
and trans-anti isomers (C and D) have been proposed as the
most stable, but with no experimental evidence. We calcu-
lated the heats of formation for each stereoisomer by the
AM1 method and found 269.0, 247.5, 238.7, and
232.0 kJmolꢀ1 for A, B, C, and D respectively.[15] These
computational results clearly show that the cis arrangement of
the mesityl groups in A and B is disfavored. Since we obtained
only two stereoisomers for 4, we can deduce that it consists of
a mixture of C and D.
The synthesis of chromophore 1 is shown in Scheme 4.
(E)-4-Iodo-4’-diphenylaminostilbene (7) was prepared by
Wadsworth–Emmons condensation between commercially
available 4-(diphenylamino)benzaldehyde (5) and diethyl 4-
iodobenzylphosphonate (6) in the presence of sodium hy-
dride. Trimethylsilylacetylene was subjected to Sonogashira
coupling with 7 to give 8 in 85% yield. The trimethylsilyl
=
Supporting Information. In the IR spectrum, the C N band at
1641 cmꢀ1 and B H band at 2390 cmꢀ1 are in agreement with
ꢀ
previous reports.[14a] The stereochemistry of 1 was definitively
proved by X-ray crystallography. X-ray-quality crystals were
grown by slow diffusion of pentane into a concentrated
chloroform solution. It was identified as the trans-anti D-type
isomer. This confirms the stereochemistry of the most stable
isomer predicted by our computational studies and the earlier
predictions by Hawthorne.[14a] An ORTEP plot of 1 is
presented in Figure 2. Color figures are available in the
Supporting Information.[17]
This is, to our knowledge, the first structure showing the
actual stereochemistry of a tetrasubstituted cyclodiborazane.
The central square ring is totally planar (torsion angle B1-N1-
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Figure 2. ORTEP plot of chromophore 1 showing the D-type stereo-
chemistry.
B2-N2 0.08). The phenyl rings attached to the cyclodiborazane
moiety slightly deviate from planarity (torsion angle B1-N1-
ꢀ
C1-C2 7.28). The N N distance of 2.17 Š is short enough to
permit electronic coupling between the two conjugated
ꢀ
systems. This N N distance is similar to those reported for
ꢀ
symmetric cyclodiborazanes and is close to the Csp2 Csp2
distance of 2.12 Š reported for 1,3-dialkylidene cyclobu-
tanes.[18]
Given that the starting diiodo compound 4 is most
probably a mixture of C and D structures, we can assume
that the other stereoisomer of 1 has C stereochemistry. It has
been isolated and its study is in progress.
Figure 3. AM1 calculated molecular orbitals of chromophore 1:
a) LUMO+1 (E=ꢀ0.9984 eV); b) LUMO (E=ꢀ1.0054 eV); c) HOMO
(E=ꢀ7.8874 eV); d) HOMOꢀ1 (E=ꢀ7.8885 eV).
The optical properties of 1 were investigated in CH2Cl2 by
UV/Vis absorption and fluorescence emission spectroscopy
(all spectra in the Supporting Information). For comparison
we also prepared the styrene “monomer” 10, that is, a
conjugated system linked to the nitrogen atoms of the
cyclodiborazane core by a double bond. Compound 10 was
obtained from 7 by Sonogashira coupling with 4-ethynylsty-
rene (Scheme 4). The absorption maxima are located at
403 nm (e = 1.0 ” 105 mꢀ1 cmꢀ1) and 392 nm (e = 4.3 ”
104 mꢀ1 cmꢀ1) for 1 and 10, respectively. Thus, chromophore
1 exhibits a slight bathochromic shift of 11 nm in comparison
with the model “monomer” 10. Both chromophores are
luminescent: lmax = 557 nm for 1 and lmax = 491 nm for 10. A
marked bathochromic shift of the emission band is observed
for 1 in comparison with 10. These results show a large
difference (D = 1718 cmꢀ1) in the Stokes shifts of 1 and 10.
This indicates that the two conjugated systems linked to the
cyclodiborazane core are strongly coupled in the excited state,
but weakly coupled in the ground state. Such behavior has
already been reported for the coupling of phenylacetylene
chromophores in meta conjugation around a phenyl ring.[19]
This led us to consider the photophysical properties of
chromophore 1 in the light of the exciton model. The use of
the exciton model to understand the photophysics of cova-
lently bonded conjugated molecules is not new.[20]
nylamino moieties, while the LUMOs show clearly that an
electronic delocalization actually occurs along the whole
conjugation pathway of the molecule. In the LUMO + 1
(Figure 3a) orbital overlap is present across the cyclodibor-
azane ring and thus allows conjugation between the two
donor groups in the excited state.
Two-photon absorption properties are directly related to
the third-order nonlinearity g. Thus, we calculated the
hyperpolarizability g of chromophore 1 using the finite-field
method, which has been already used to calculate the
hyperpolarizabilities of boron-containing NLOchromo-
phores.[21] We found g0 ꢂ 1730 ” 10ꢀ36 esu. This high g value
is in agreement with the TPA cross section s2 of 1 that was
experimentally determined in dichloromethane by the way of
its two-photon induced fluorescence emission.[22] Indeed, 1
exhibits a strong two-photon absorption band in the near-IR
region, centered at 830 nm with s2 = 1350 GM (1 GM =
10ꢀ50 cm4 sphotonꢀ1). The TPA spectrum is shown in Figure 4.
According to the theory, purely excitonic coupling should
lead to large shifts in both absorption and emission spectra.
Since we observed a large shift only for the emission, it
appears that a through-bond electronic interaction likely
occurs essentially in the excited state.[19] This interaction was
evidenced by molecular modeling. Selected AM1 calculated
molecular orbitals of 1 are depicted in Figure 3. The
HOMOꢀ1 and HOMO are quasidegenerate in energy, and
this is also the case for the LUMOand LUMO + 1. These two
pairs of orbitals differ essentially in symmetry properties. We
can observe from the HOMOs that in the fundamental state
the electronic density is located around the terminal diphe-
Figure 4. Two-photon absorption cross section of 1 in CH2Cl2.
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Chemie
[5] J. Liu, Y. W. Zhao, J. Q. Zhao, A. D. Xia, L. J. Jiang, S. Wu, L.
A TPA chromophore having the same carbon skeleton as
1, but without the cyclodiborazane central core and with
NOct2 donor groups in place of NPh2, has s2 = 950 GM at
800 nm.[23] Given that NR2 instead of NAr2 does not affect
significantly the TPA cross section,[8b] we conclude that the
electronic effect induced by the cyclodiborazane core signifi-
cantly increases the TPA cross section in comparison with two
conjugated systems linked only by a s bond.
In conclusion, we have prepared a new type of efficient
centrosymmetric one-dimensional TPA chromophore based
on the interaction between two donor groups through the
incompletely conjugated cyclodiborazane core (p–s–p
through-bond coupling). This strategy afforded a fluorophore
with a TPA maximum in the range of laser wavelengths used
in two-photon microscopy. This opens the way to the use of
new boron-containing TPA chromophores in biological
sciences. The trans-anti stereochemistry around the central
core was proved by X-ray crystallography. The study of the
other stereoisomer of chromophore 1 that is in progress will
provide information on geometrical effects on the electronic
coupling and its influence on the TPA properties.
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Experimental Section
Syntheses: Full synthetic experimental details and analytical data can
be found in the Supporting Information.
X-ray crystal structure of chromophore 1: Details on data
collection, structure solution, and additional ORTEP plots are
given in the Supporting Information.
Computational studies: Geometry optimization of the chromo-
phore was performed with the MOPAC program within the InsightII
and Cerius2 interfaces from Accelrys (http://www.accelrys.com) with
the AM1 Hamiltonian. Static hyperpolarizabilities were calculated by
the finite-field method incorporated in the MOPAC program.
The TPA cross-section spectra were obtained by up-conversion
fluorescence measurements (details in the Supporting Informa-
tion).[22]
Received: June 6, 2006
Published online: September 4, 2006
Keywords: boron · donor–acceptor systems · heterocycles ·
.
luminescence · nonlinear optics
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 6469