Carborane-Bodipy scaffolds for through space energy transfer

Article abstract of DOI:10.1039/c0cc02656e

Use of a closo-1,12-dicarbadecaborane backbone to link two different organic dyes separated by ca. 37 promotes exclusive through space electronic energy transfer with an efficiency of 81 to 87% with Stoke shifts between 4750 and 2800 cm^-1 depen

Full text of DOI:10.1039/c0cc02656e

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Carborane-Bodipy scaffolds for through space energy transferw
Raymond Ziessel,* Gilles Ulrich,* Jean Hubert Olivier, Thomas Bura and Alexandra Sutter
Received 19th July 2010, Accepted 6th September 2010
DOI: 10.1039/c0cc02656e
Use of a closo-1,12-dicarbadecaborane backbone to link two
different organic dyes separated by ca. 37 A promotes exclusive
through space electronic energy transfer with an efficiency of 81
to 87% with Stoke shifts between 4750 and 2800 cmÀ1 depending
on the pH.
Carboranes have been extensively used as templates for the
preparation of soft matter,1 polymers,2 non-linear materials,3
rigid rods4 and self-assembled molecular structures.5 Their
derivatives such as metallacarboranes are analogues of
metallocenes6 and, like them, find applications, for example,
in medicinal chemistry.7 Carboranes of the closo form, such as
1,12-dicarbadecaborane (C2B10H12), are geometrically rigid
species with an electronic structure making them chemically
and thermodynamically very robust. Furthermore, they are
transparent to visible and UV light down to about 200 nm and
do not exhibit redox activity in standard windows.8 These
properties prompted us to use closo-1,12-dicarbadecaborane
as a platform to link photoactive modules in a rod-like manner
and promote through space Forster resonance energy transfer
¨
(FRET).9
As photoactive modules we chose 4,4-difluoro-4-bora-
3a,4a-diaza-s-indacene (Bodipy) dyes for their valuable
optical properties, such as high absorption coefficients
(B80 000–200 000 MÀ1 cmÀ1), in the visible portion of the
electromagnetic spectrum, narrow emission profiles and out-
standing emission quantum yields reaching 100% for the
best cases.10 Furthermore, we and others have previously
demonstrated that such modules may be conveniently
functionalised using procedures including carbon–carbon
cross-coupling reactions catalyzed by transition metals.11 As
well, substitution at boron can be used both to enhance Stokes
shifts12 and to import functionalities capable of increasing the
solubility and polarity of the dyes.13
Scheme 1 Keys: (i) nBuLi, THF, À78 1C, CuBr, 4-(bromoethynyl)-
iodobenzene, 35%; (ii) dye G, benzene, triethylamine, [Pd(PPh3)4],
60 1C, overnight, 33%; (iii) dye R, benzene, triethylamine, [Pd(PPh3)4],
60 1C, 36 h, 70%.
critical step is kinetically controlled, requiring low temperature
and stepwise addition of the reagents (ESIw). The state of
carborane substitution was followed by 1H NMR clearly showing
the progressive disappearance of the CH–carborane signal at
2.70 ppm and the apparition of the characteristic C–I signal in
the 13C-NMR spectra at 95.2 ppm (in CDCl3). The next step
consisted of linkage of the ethynyl substituted green dye G to the
preformed closo-1,12-(4-ethynyliodophenyl)dicarbadecaborane.
The design of the green Bodipy G had two objectives:
(i) improved solubility and polarity allowing easy purification
by separation of the cross-linked GC from any apolar
carborane 2 and (ii) sensitivity of the absorption and emission
properties to pH, enabling variation of the spectral overlap
between the energy donor and energy acceptor modules
(vide infra). Use of Pd(0) catalysis afforded the mono-grafted
dye GC in 33% and the disubstituted GCG compound in 25%
yield. The presence of the dimethylamino and polyoxo chains
rendered the chromatographic separation facile and the
unreacted starting material 2 was easily recovered and recycled.
The last step used a similar procedure to link the known red
ethynyl Bodipy R16 to GC. The mixed red-carborane-green
(RCG) dye was obtained in 70% yield and displayed a
well-resolved proton NMR spectra in keeping with the
presence of the different modules and excluding the formation
of aggregates.
The present target was to functionalize closo-1,12-dicarba-
decaborane 1 with two 4-ethynylphenyliodo fragments leading
to compound 2 (Scheme 1). The protocol developed was
inspired by related work5,8a,14 and based on that used in the
synthesis of ‘‘nanocars’’ and ‘‘nanotrucks’’, in which
carboranes terminate axles for rolling motion.15 Metallation
of 1 using nBuLi and CuBr at low temperature, followed
by subsequent reaction with 4-(bromoethynyl)-iodobenzene
afforded the di-substituted derivative 2 with two functional
groups at the vertices suitable for further reactions. This
LCOSA, ECPM/ULP, CNRS, 25 rue Becquerel, 67087 Strasbourg
Cedex 02, France. E-mail: ziessel@chimie.u-strasbg.fr,
gulrich@chimie.u-strasbg.fr; Web: http://www-lmspc.u-strasbg.fr/
lcosa; Tel: +33 3 68 85 26 89
w Electronic supplementary information (ESI) available: Complete
experimental part for all novel compounds. See DOI: 10.1039/c0cc02656e
c
7978 Chem. Commun., 2010, 46, 7978–7980
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Table 1 Photophysical properties measured in dioxane, at room temperature
Cmpd
labs/nm
e/MÀ1 cmÀ1
lem/nm
Ffa
ts/ns
kr (10À8 sÀ1 b
)
knr (10À8 sÀ1 b
)
SS (cmÀ1 c
)
R
G
525
693
628
695
628
694
525
628
525
58 400
96 900
100 400
93 300
92 300
81 100
65 200
82 800
62 000
538
726
638
726
638
728
728
638
638
0.80@510 nm
0.31@650 nm
0.52@585 nm
0.25@650 nm
0.39@585 nm
0.30@665 nm
0.22@510 nm
0.42@590 nm
0.37@510 nm
4.7
3.1
3.4
2.8
2.9
3.3
3.3
3.2
3.2
1.7
1.0
1.5
0.9
1.3
0.9
0.7
1.3
1.2
0.4
2.3
1.4
2.7
2.1
2.1
2.4
1.8
2
460
655
250
615
250
670
5300
250
3370
G + H+
GC
GC + H+
RCG
RCG + H+
a
Quantum yield determined in dioxane solution, ca. 5 Â 10À7 M, at a given wavelength using as reference cresyl violet (Ff = 0.50 in ethanol)20 or
tetramethoxydiisoindomethene-difluoroborate.21 All Ff are corrected for changes in refractive index. Calculated using the following equations:
kr = Ff/tF, knr = (1 À Ff)/tF, assuming that the emitting state is produced with unit quantum efficiency. Stokes’ shift.
b
c
Fig. 1 Absorption and emission spectra of GC (left hand side, lexc = 650 nm), protonated form (right hand side, lexc = 585 nm) in dioxane
containing gaseous HCl. Emission of the aggregated species was not observed at the emission experiment concentration.
Fig. 2 Absorption and emission spectra of RCG (left hand side, lexc = 510 nm), and its protonated form (right hand side, lexc = 510 nm),
in dioxane containing vapours of HCl.
The photophysical properties of the model compounds and the
carborane-substituted dyes are gathered in Table 1. The spectra of
the alkyne derivatives G and R are typical of Bodipy dyes with a
strong S0 - S1 (p–p*) transition, respectively, at l = 693 and
525 nm. The linkage of Bodipy dyes G and R to the carborane does
not quench the fluorescence. This is in line with recent results found
with fluorescent nanocars.17 For GC and G, the presence of the
dimethylamino donor groups is responsible for the strong batho-
chromic shift and also for the marked sensitivity to acid (Fig. 1).18
By acidification of GC and G with HCl(g) a strong hypsochromic
shift was observed due to the protonation of the tertiary amine and
annihilation of the nitrogen lone-pair delocalisation, and the band
shifted to 628 nm (Fig. 1).19 At the same time in the high energy
part of the spectra, typical bands assigned to intramolecular charge
transfer disappeared on protonation. Note that at the absorption
experiment concentration (5 Â 10À6 M), a supplementary band was
observed at 674 nm, and is assigned to the formation of aggregates
of the charged species in an apolar solvent like dioxane.
Fig. 3 Spectral overlapping between the emission spectra of R (black)
with absorption spectrum of GC (blue) and GC + H+ (red).
c
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Table 2 Summary of properties relative to ET in RCG
kETb/sÀ1
Jc/cm6 mmolÀ1
c
Ecal (%)
d
Emes (%)
Cmpd.
Forster radius/A
¨
Ffa
tsa/ns
RCG
RCG + H+
47.5
52.1
0.16
0.11
1.5
0.7
6.2 Â 108
82
89
81
87
1.0 Â 10À13
1.7 Â 109
1.7 Â 10À13
a
b
Fluorescence quantum yield and lifetime of the S1 state associated to the R subunit at 541 nm. Calculated with RRCG 36.8 A, estimated by
molecular modelling (SPARTAN), and orientation factor (k2) for Forster transfer of 2/3. Calculated using Photochemcad software and emission
c
¨
d
spectrum of R and absorption spectrum of GC. Energy transfer efficiency calculated from measured quantum yields E = 1 À Ff(RCG)/Ff(R).
The absorption spectra of RCG in its deprotonated and
Notes and references
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for RCG using an excitation wavelength of 19 600 cmÀ1
.
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does not participate in Dexter-type energy transfer but where
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agreement with Forster theory. The chemistry developed using
¨
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at the vertices suitable for post-functionalization. The use of a
novel highly soluble and polar green BODIPY dye emitting in
the near-infra is useful to facilitate the purification process and
opens up the possibility to further develop the chemistry
at the unsubstituted b-pyrrolic positions. Current work is
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to a metallacarborane in order to switch from an innocent to a
non-innocent spacer.
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7980 Chem. Commun., 2010, 46, 7978–7980
This journal is The Royal Society of Chemistry 2010