Unsymmetrical p-carborane backbone as a linker for donor-acceptor dyads

Article abstract of DOI:10.1002/chem.201103307

Fluorescent nanorods: Donor-acceptor dyads based on novel unsymmetrically disubstituted closo-1,12-dicarbadecaboranes have been prepared in a completely controlled manner by using a three-step procedure. Dyads with different donor-acceptor spacing were th

Full text of DOI:10.1002/chem.201103307

DOI: 10.1002/chem.201103307
Unsymmetrical p-Carborane Backbone as a Linker for Donor–Acceptor
Dyads
Delphine Hablot,[a] Alexandra Sutter,[a] Pascal Retailleau,[b] and Raymond Ziessel*[a]
Dedicated to Dr. Hubert le Bozec on the occasion of his 60th birthday
The chemistry of polyhedral boron hydrides has been
widely investigated[1] and closo-boranes in particular are
known to show high kinetic stability favouring various appli-
cations. Such applications range from those of BNCT
(boron neutron-capture therapy) in medicine[2] to those in
the synthesis of superacids.[3] Partial substitution of boron by
carbon gives rise to the related carboranes, which, in func-
tionalised forms, are important building blocks for the engi-
neering of soft matter,[4] polymers,[5] nonlinear materials,[6]
rigid rods[7] and self-assembled molecular structures.[8] Car-
boranes of the closo form, such as 1,12-dicarbadecaborane
(C2B10H12 in Scheme 1) are geometrically rigid species with
an electronic structure making them optically transparent
and redox inactive.[1,9] However, most methods currently
available for the synthesis of derivatives of this carborane
provide only symmetically disubstituted species, typically
those bearing either two alkyne functions,[10] two pyridines,[8]
two phosphanes[8] or two 4-iodophenyl[11,12] residues. In the
last case, metallation of the para-carborane and transforma-
tion to its cuprate at low temperature, and subsequent reac-
tion with 4-(bromoethynyl)iodobenzene provides a homo
Scheme 1. Synthesis of the unsymmetrically functionalized p-carborane.
disubstituted derivative with the functional groups at the
vertices of the carborane cluster.[10,12] Although conversion
of such a species to an unsymmetrical derivative is possible,
this requires statistical cross-coupling reactions that limit the
isolated yields, produce disubstituted derivatives that are
useless and consume the precious starting material.[12] Note
that monosubstituted carboranes are available but hetero-
disubstituted carboranes are elusive and have not been ob-
tained in a controlled process mostly because of the reactivi-
ty of the first functional group towards nucleophiles.[13]
Thus, the present challenge was seen as that of functionaliz-
ing closo-1,12-dicarbadecaborane 1 with two orthogonal
functions in a procedure of minimum length.
To this end, we first explored the synthesis of 3 by using
closo-1,12-(4-ethynyliodophenyl)dicarbadecaborane 1 as the
starting material. Compound 3 was prepared by a three-step
procedure as outlined in Scheme 1. The choice of triethylsi-
lylacetylene (TES-acetylene) was based on its inertness to-
wards nBuLi or cuprates.[14] Indeed the pivotal compound 3
was obtained in 82% yield by formation of the cuprate at
the unsubstituted carborane C atom followed by reaction
with 4-(bromoethynyl)iodobenzene. Subsequent transforma-
tion to the terminal alkyne required cross-coupling with tri-
methylsilylacetylene (TMS-acetylene) and selective removal
of the TMS protecting group, leading to 5 in excellent yield
(Scheme 1). Alternative routes using acetylene gas[15] or
propargyl alcohol do not give the pivotal derivative 5 in ac-
ceptable yields.
[a] D. Hablot, A. Sutter, Dr. R. Ziessel
Laboratoire de Chimie Molꢁculaire et Spectroscopies Avancꢁes
(LCOSA)
Ecole Europꢁenne de Chimie, Polymꢂres et Matꢁriaux
CNRS, 25 rue Becquerel, 67087 Strasbourg Cedex 02 (France)
E-mail: ziessel@unistra.fr
[b] Dr. P. Retailleau
Laboratoire de Cristallochimie
ICSN—CNRS, Bꢃt 27-1 avenue de la Terrasse
91198 Gif-sur-Yvette, Cedex (France)
Crystal-structure determinations were performed for 1
and 3 (Figures 1 and 2). Both molecules have an essentially
cylindrical shape and carborane 3 is approximately twice as
long (at ca. 24 ꢀ) as carborane 1. The carborane units in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103307.
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Chem. Eur. J. 2012, 18, 1890 – 1895

COMMUNICATION
both are almost identical; superposition of one on the other
results in an overall r.m.s. deviation of 0.12 ꢀ. Extension of
1 through two additional ethynyl linkers slightly increases
the overall deviation from linearity by about 108 (supple-
mentary angle of I1-C11-Si1). The two phenyl rings of 3 are
nearly coplanar (dihedral angle 18.48), with their mean
plane orthogonal to the bc plane, which contains the long
molecular axis.
Both compounds crystallize in the monoclinic space group
P21/c. The lattice of 1 is layered and consists of carborane
bilayers parallel to the bc plane (Figure 1b). The nuclei are
not situated on the glide mirrors and the iodophenylethynyl
groups are arranged between the bilayers in a herring-bone
fashion viewed down the a axis, the phenyl rings being in
general positions, with a dihedral angle of 71.38 between
1
1
¯
those at x, = -y, z- = (Figure 1c). Along the [011] and [011]
2
2
directions, the inversion-related rings stack parallel with an
interplanar distance of 3.407(2) ꢀ but with an offset of
À
2.171 ꢀ and are further sandwiched by C H···aryl (centroid)
interactions (d=3.616 ꢀ and ]=172.68; Figure 1d).
For compound 3, the molecules are also layered in the bc
plane in a herring-bone fashion, the triethylsilane group
lying over the iodophenyl of a neighbouring molecule with a
708 angle of attack (Figure 2b). In this crystal structure, the
carborane nuclei are isolated from each other by surround-
ing triethylsilane groups and phenyl groups.
As photoactive modules to be attached, we chose a bor-
ondipyrromethene (Bodipy; A and/or B) for its labs/lem at
645/659 nm,[16]
a subphthalocyanine (SubPc; C) for its
labs/lem at 565/574 nm,[17] or a diketopyrrolopyrrole (DPP;
D) for its labs/lem at 483/547 nm.[18] These choices were based
on the potential to provoke energy transfer by a resonance
mechanism[19] due to overlap of the emission and absorption
properties of each module.
Our immediate target became the linking of the first fluo-
rophore B to the carborane 3 under conventional Pd cataly-
sis. The TES-protected dye 6a was isolated in excellent
yields and deprotection was straightforward in the presence
of KF under polar conditions (Scheme 2). Cross-coupling of
6b to a second fluorophore such as C or D afforded the
mixed dyes 7 and 8 in excellent yields.
As expected from earlier work on photoactive molecular-
scale wires, a key objective for such artificial systems is to
determine the efficiency of unidirectional information trans-
fer in the donor–acceptor system as a function of the length
of the spacing unit.[20] Here, with a carborane spacer, the sit-
uation is highly interesting due to the inability to transfer in-
formation (in the form of an electron, a hole or a charge)
through the cluster. Thus, an important aspect of the present
work was our utilization of the chemistry developed to
extend the length of the carborane spacer by formation of a
Figure 1. a) ORTEP view of carborane 1. Displacement ellipsoids are
drawn at the 50% probability level. b) Projection of the layered structure
¯
of the carborane 1 in the (011) plane. c) Lattice structure down the a
axis. d) Close-up of intermolecular interactions (dotted lines), viewed
¯
down the [011] direction.
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R. Ziessel et al.
Subsequently, deprotection of the TES group in 9a
proved to be difficult and intense experimentation was re-
quired to reach a maximum yield of 50%. The use of basic
conditions (KOH or K2CO3) decomposes the SubPc chro-
mophore even at room temperature. The optimal conditions
eventually developed required a large excess of KF in meth-
anol, THF, trace amounts of water and a short reaction time
under mild heating (Table 1).
Table 1. Experimental conditions for the deprotection of the TES group
of dye 9a.[a]
Solvent
Base
(equiv)
t
T
Yield
[%][e]
(ratio v/v)[b]
G
[h][c]
[8C][d]
CH2Cl2/MeOH (1:1)
NaOH (2)
K2CO3 (1)
KF (40)
KF (100)
KF (400)
KF (400)
KF (100)
KF (200)
KF (50)
0.5
15
21
15
6
0.75
2
2
RT
RT
RT
RT
RT
50
50
50
RT
decomp
decomp
15
30
50
50
42
36
<10
THF/MeOH/H2O (3:1:e)
THF/MeOH/H2O (5:2:e)
THF/MeOH/H2O (10:3:e)
THF/MeOH/H2O (6:5:e)
THF/MeOH/H2O (6:5:e)
THF/MeOH/H2O (1:1:e)
THF/MeOH/H2O (5:4:e)
CH2Cl2/MeOH (1:1)
15
[a] Concentration of 9a about 4ꢅ10À3 molLÀ1. [b] e=one drop of water.
[c] Reaction time. [d] RT=room termperature. [e] decomp=decomposi-
tion.
Note that switching from the SubPc to a Bodipy end-
group generates a situation in which deprotection of the
TES group is impossible due to the instability of the Bodipy
in the presence of large quantities of KF. With compound
9b in hand, it was easy to cross-link a second carborane resi-
due using 3 and conventional Pd-catalyzed coupling to
obtain compound 10a. After deprotection with KF and
cross-linking 10b with A, the mixed dyad 11 was obtained in
an acceptable overall yield (Scheme 3).
Figure 2. a) ORTEP view of carborane 3. Displacement ellipsoids are
drawn at the 50% probability level. b) Projection of the lattice structure
down the a axis.
The photophysical properties of the model compounds
and the carborane-substituted dyes are summarized in
Table 2. The spectra of the building blocks D, 6a, 9a and
10a are typical of DPP, Bodipy and SubPc dyes with a
Table 2. Photophysical properties measured in THF at room tempera-
ture.
labs
e
lem
Ff[a]
ts
kenf or
[nm]
A
]
[nm]
[ns]
ken(t) [sÀ1
]
D
6a
7
483
645
645
566
645
498
565
565
645
566
19,900
127600
153900
112800
142700
37331
105000
86400
127000
96000
547
659
659
573
659
566
574
574
659
575
0.90
0.47
0.47
0.03
0.47
0.01
0.11
0.11
0.36
0.07
6.40
4.78
4.80
0.46
4.82
<0.1
1.80
1.78
4.81
1.18
–
–
ken(f) 1.7ꢅ109
ken(t) 1.6ꢅ109
8
ken(f) 1.2ꢅ1010
–
–
–
9a
10a
11
ken(f) 3.2ꢅ108
ken(t) 2.9ꢅ108
double carborane unit before attachment of the second fluo-
rophore. This, we chose to do first by connecting the C to
carborane 5, leading to derivative 9a in 86% isolated yield
(Scheme 3).
[a] Quantum yield determined in THF solution, about 5ꢅ10À7 m at a
given wavelength using as reference tetramethoxydiisoindomethenedi-
fluoroborate (ff =0.51 in methanol).[22] All FF are corrected for changes
in refractive index. [b] Calculated using the following equations : kenf
[(F0/F)À1]/t0, ken(t) =1/tÀ1/t0).
=
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Donot–Acceptor Systems
COMMUNICATION
the DPP moiety (Table 1). The
excitation spectrum matches
the absorption spectrum fairly
well over the entire absorption
window (Figure 3b).
On increasing the number of
carborane units in the SubPc
precursors 9a and 10a the
transitions corresponding to
the SubPc group in the 450 to
570 nm region remain identi-
cal, while the absorption due
to the carboraneethynylphenyl
group between 250 and 355 nm
increases accordingly (Fig-
ure 4a). On linking both scaf-
foldings to a Bodipy fragment,
the absorption spectra are
almost linear combinations of
the absorptions of the respec-
tive modules (Figure 4b), but
the emission spectra showed
dual emissions at 575 and
659 nm. In case of dyad 7,
linked with one carborane
unit, the residual emission at
575 nm is very weak, highlight-
ing an almost quantitative
energy-transfer process de-
duced from the red emitter at
Scheme 2. Synthesis of Bodipy–carborane-bridged SubPc or DPP dyads.
strong p–p* transitions in the
visible region. The linkage of
carborane subunits to the
Bodipy or SubPc dyes does
not significantly perturb the
fluorescence properties. This is
in line with results found with
fluorescent “nanocars”,[21] and
related dyads.[12] For the dyad
8, the absorption spectrum
(Figure 3b) is almost a linear
combination of the absorption
of 6a (Figure 3a) and of DPP.
Irradiation in the DPP absorp-
tion band at 480 nm resulted
in very efficient electronic
energy transfer (EET>99%)
to the red emitter. The rate
constant for this EET is about
1.2ꢅ1010 sÀ1, taking into ac-
count the quantum yields and
the excited state lifetime of
Scheme 3. Synthesis of the Bodipy/SubPc dyad spaced by two carborane subunits.
Chem. Eur. J. 2012, 18, 1890 – 1895
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1893

R. Ziessel et al.
Figure 3. Absorption (black line), emission (dashed line) and excitation
(dotted line) spectra of a) dye 6a and b) dyad 8 in THF at room temper-
ature.
Figure 4. a) Absorption (black line), emission (dashed line) and excita-
tion (dotted line) spectra of dyes 9a and 10a. b) Absorption and excita-
tion spectra of dyad 7 (dotted line) and 11 (dashed line): inset fluores-
cence of 7 (dotted line) and 11 (dashed line) in THF at room tempera-
ture.
659 nm. When two carborane moieties are linked in dyad
11, a non-quantitative energy-transfer process, reaching
76% efficiency was calculated. In both cases, the excitation
spectra overlap with the absorption spectra, proving that the
SubPc units are involved in the energy transfer. The rates
have been calculated by taking into account the quantum
yields and lifetimes of the model compounds 9a and 10a
(ken(f) =1.7ꢅ109 sÀ1 and ken(f) =3.2ꢅ108 sÀ1 in dyes 7 and 11,
respectively). The rates calculated from the residual life-
times are in excellent agreement with those previously cal-
culated from the quantum yields (ken(t) =1.6ꢅ109 sÀ1 and
different photoactive modules displaying specific spectro-
scopic features suitable for efficient singlet energy transfer.
This chemistry paves the way for the engineering of larger
rigid systems.
Experimental Section
1
Selected data for 7: H NMR (400 MHz, CD2Cl2): d=8.80–8.90 (4 line m,
k
en(t) =2.9ꢅ108 sÀ1 , respectively, in dyes 8 and 11; Table 2).
6H), 7.89–7.99 (4 line m, 6H), 7.67 (d, J=8.3 Hz, 2H), 7.52–7.63 (4 line
m, 6H), 7.47 (d, J=8.6 Hz, 2H), 7.37 (d, J=8.3 Hz, 2H), 7.19–7.34
(5 line m, 8H), 7.10 (d, J=8.6 Hz, 2H), 6.96 (d, J=8.6 Hz, 4H), 6.71 (d,
J=8.6 Hz, 2H), 6.66 (s, 2H), 4.11–4.21 (3 line m, 4H), 3.79–3.88 (3 line
m, 4H), 3.63–3.72 (4 line m, 4H), 3.60–1.55 (large m, 10H, BH), 3.50–
3.59 (4 line m, 4H), 3.35 (s, 6H), 1.50 ppm (s, 6H); 13C NMR (100 MHz,
CD2Cl2): d=160.45, 153.32, 151.09, 142.57, 137.96, 136.59, 136.12, 133.53,
132.86, 132.46, 132.32, 132.06, 131.91, 131.57, 131.41, 130.37, 130.14,
129.55, 124.34, 124.21, 123.28, 122.74, 122.49, 121.72, 121.47, 118.15,
117.47, 115.50, 91.56, 90.71, 90.34, 87.47, 80.67, 72.53, 71.26, 70.14, 68.26,
59.25, 15.20 ppm; ESI-MS m/z (%): 1620.6 (100), 1622.6 (70); elemental
analysis calcd (%) for C97H80B12F2N8O6: C 71.85, H 4.97, N 6.91; found:
C 71.56, H 4.75, N 6.71.
Note that the fluorescence is insensitive to the presence
of oxygen and the short lifetimes (ns) are consistent with a
singlet-state emission. The measured Stokes shifts are small
(<330 cmÀ1) for the model dyes 6a, 9a and 10a, but are sig-
nificantly increased in the dyads, reaching 4900 cmÀ1 in the
best cases.
In summary, we have employed an inert closo-carborane
spacer that has been functionalized, for the first time, at the
vertices with two different functional groups of different re-
activity. This strategy allows the stepwise post-linking of two
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Donot–Acceptor Systems
COMMUNICATION
Selected date for 8: 1H NMR (400 MHz, CD2Cl2): d=7.78 (d, J=8.1 Hz,
2H), 7.73 (d, J=8.6 Hz, 2H), 7.68 (d, J=7.6 Hz, 2H), 7.43–7.64 (10 line
m, 14H), 7.38 (d, J=8.1 Hz, 2H), 7.22–7.35 (m, 12H), 7.18 (d, J=7.6 Hz,
4H), 6.97 (d, J=8.6 Hz, 4H), 6.66 (s, 2H), 4.98 (s, 4H), 4.12–4.22 (3 line
m, 4H), 3.78–3.90 (3 line m, 4H), 3.63–3.72 (4 line m, 4H), 3.52–3.59
(4 line m, 4H), 3.49 (s, 2H), 3.62–1.57 (large m, 10H, BH), 3.35 (s, 6H),
2.34 (s, 6H), 1.51 ppm (s, 6H); 13C NMR (100 MHz, CD2Cl2): d=163.05,
160.45, 153.32, 148.75, 148.40, 142.57, 138.11, 137.96, 136.59, 136.13,
133.52, 132.86, 132.48, 132.40, 132.17, 132.08, 130.13, 129.63, 129.55,
129.34, 128.36, 127.97, 127.15, 127.13, 126.97, 126.36, 124.37, 124.21,
121.88, 121.72, 118.15, 117.46, 115.50, 110.91, 110.75, 92.05, 91.58, 90.34,
87.62, 80.67, 72.53, 71.26, 70.14, 68.26, 66.85, 59.26, 49.04, 46.12, 44.48,
15.21 ppm; ESI-MS m/z (%): 1675.7 (85), 1674.7 (60), 1673.7 (100); ele-
mental analysis calcd (%) for C102H94B11F2N5O8: C 73.15, H 5.66, N 4.18;
found: C 72.84, H 5. 43, N 3.76.
Selected data for 11: 1H NMR (400 MHz, CD2Cl2): d=8.79–8.90 (4 line
m, 6H), 7.87–7.99 (4 line m, 6H), 7.68 (d, J=8.3 Hz, 2H), 7.53–7.64
(3 line m, 6H), 7.49 (d, J=8.3 Hz, 2H), 7.18–7.46 (m, 18H), 7.11 (d, J=
8.3 Hz, 2H), 6.97 (d, J=8.3 Hz, 4H), 6.72 (d, J=8.3 Hz, 2H), 6.66 (s,
2H), 4.12–4.21 (3 line m, 4H), 3.80–3.88 (3 line m, 4H), 3.64–3.72 (4 line
m, 4H), 3.62–1.52 (large m, 20H, BH), 3.51–3.59 (4 line m, 4H), 3.36 (s,
6H), 1.51 ppm (s, 6H); 13C NMR (100 MHz, CD2Cl2): d=160.47, 153.35,
151.09, 142.56, 137.96, 136.61, 136.14, 133.55, 132.87, 132.48, 132.43,
132.32, 132.09, 132.06, 131.92, 131.85, 131.58, 131.41, 130.36, 130.16,
129.56, 124.40, 124.26, 123.30, 122.76, 122.49, 121.76, 121.49, 118.18,
117.51, 115.52, 91.63, 91.46, 90.73, 90.38, 87.65, 87.50, 80.69, 72.54, 71.28,
70.16, 68.34, 68.28, 66.86, 59.27, 34.72, 26.17, 22.92, 15.22, 14.39 ppm;
ESI-MS m/z (%): 1987.0 (100); elemental analysis calcd (%) for
C117H98B22F2N8O6: C 70.69, H 4.97, N 5.64; found: C 70.49, H 4.76, N
5.48.
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We acknowledge the CNRS for provision of research facilities and finan-
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commenting on the manuscript before publication.
Keywords: alkynes · carboranes · donor–acceptor systems ·
energy transfer · fluorescence · phthalocyanines
[1] a) N. N. Greenwood, Boron, Pergamon, Oxford, 1975; b) E. L.
Muetterties, Boron Hydride Chemistry, Academic Press, New York,
1975; c) J. Plesek, Chem. Rev. 1992, 92, 269–278; d) R. B. King,
Chem. Rev. 2001, 101, 1119–1152.
Received: October 20, 2011
Published online: January 16, 2012
Chem. Eur. J. 2012, 18, 1890 – 1895
ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1895