Borondipyrromethene dyes with pentane-2,4-dione anchors

Article abstract of DOI:10.1021/ol902386u

[Chemical equation presented] New, acetylacetone-linked borondipyrromethene (BODIPY) dyes were readily obtained from BODIPY cores by various protocols involving direct grafting with acetylacetone or cross-coupling from a preorganized phenylacacH derivative bearing either an iodo or an ethynyl function. Facile anchoring on TiO₂ powder is obtained and scrutinized by FT-IR spectroscopy.

Full text of DOI:10.1021/ol902386u

ORGANIC
LETTERS
2010
Vol. 12, No. 3
408-411
Borondipyrromethene Dyes with
Pentane-2,4-dione Anchors
Jean-Hubert Olivier,† Alexandre Haefele,† Pascal Retailleau,‡ and
Raymond Ziessel*,†
Laboratoire de Chimie Organique et Spectroscopies AVance´es (LCOSA), CNRS, Ecole
de Chimie, Polyme`res, Mate´riaux de Strasbourg (ECPM), 25 rue Becquerel, 67087
Strasbourg, Cedex 02, France, and Laboratoire de Crystallochimie, ICSN - CNRS,
Baˆt 27 - 1 aVenue de la Terrasse, 91198 Gif-sur-YVette, Cedex, France
ziessel@unistra.fr
Received October 16, 2009
ABSTRACT
New, acetylacetone-linked borondipyrromethene (BODIPY) dyes were readily obtained from BODIPY cores by various protocols involving
direct grafting with acetylacetone or cross-coupling from a preorganized phenylacacH derivative bearing either an iodo or an ethynyl function.
Facile anchoring on TiO2 powder is obtained and scrutinized by FT-IR spectroscopy.
Acetylacetonate anion (acac-) and its derivatives and analogues
are versatile ligands for a wide variety of metal ions.1 They
possess several advantages over phosphine or oligopyridine
ligands due to their ease of synthesis, their anionic nature, and
the ready formation of their complexes. The various function-
alizations provide a means to control the electronic properties
of their complexes as well as their physical characteristics (e.g.,
solubility, volatility). Particular examples are provided by their
use in dendrimers to anchor a phosphorescent label2 and as
intermediates in the synthesis of liquid crystalline materials.3
Nonetheless, very few hybrid acac ligands combining electro-
active,4 photoactive, or additional chelating fragments have been
described up to now.5 In part, this reflects the dominance of
the interest in acacH () acetylacetone ) 2,4-pentanedione) and
related species as extremely useful precursors for the synthesis
of numerous heterocycles, amino alcohols, and amino acids.6
It is known, however, that copper salts promote the arylation
of activated methylene groups such as those of malononitrile,
ethylcyanoacetate, and acetylacetone,7 and the recent discovery
that a Cu(I)/L-proline mixture8 efficiently catalyzes the arylation
of acetylacetone prompted us to attempt the substitution of
borondipyrromethene dyes (BODIPYs) with acacH fragments.
The objective of the present work was therefore to develop
procedures as part of a battery of methods for introducing acacH
residues onto these fluorescent dyes to link them to transition
metals or inorganic semiconductor particles or surfaces.
To this end, we first explored the synthesis of 2 by a
conventional route (Scheme 1). Arylation of acetylacetone with
† Laboratoire de Chimie Organique et Spectroscopies Avance´es
(LCOSA).
(4) Massue, J.; Bellec, N.; Chopin, S.; Levillain, E.; Roisnel, T.; Clrac,
R.; Lorcy, D. Inorg. Chem. 2005, 44, 8740–8748.
‡ Laboratoire de Crystallochimie.
(5) McNamara, W. R.; Snoeberger, R. C., III; Li, G.; Schleicher, J. M.;
Cady, C. W.; Poyatos, M.; Schmuttenmaer, C. A.; Crabtree, R. H.; Brudvig,
G. W.; Batista, V. S. J. Am. Chem. Soc. 2008, 130, 14329–14338.
(6) Attanasi, O. A.; Filippone, P.; Fiorucci, C.; Foresti, E.; Mantellini,
F. J. Org. Chem. 1998, 63, 9880–9887.
(1) Siedle, A. R. Diketone and Related Ligands, in ComprehensiVe
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10.1021/ol902386u  2010 American Chemical Society
Published on Web 12/28/2009

Scheme 1. Synthesis of the acacH BODIPY 2
the phenyliodo-substituted BODIPY 1 under anhydrous condi-
tions provided the target ligand 2 in moderate yield. In the
presence of water, 2 was not isolated, but the ketone 3 was
obtained in low yield. Compound 2 appears stable in ordinary
daylight and is highly fluorescent in solution (discussed below).
This compound was also structurally characterized by single-
crystal X-ray diffraction. It crystallizes in the space group C2/
c, the molecules lying in stacked columns down a such that
the BODIPY core planes are parallel (though remote, with an
interplanar spacing >4 Å). Down a, each consecutive molecule
within a column is rotated by 120° relative to the preceding,
placing the acetylacetone unit of every third molecule on a site
of 2-fold symmetry. Thus, there are two inequivalent molecules
present in the latttice, though the differences between them
dimensionally are slight.
Figure 1. ORTEP view of 2 with displacement ellipsoids shown
at the 30% probability level.
was successful also with the NIR emitter 7. The isolated yields
are acceptable in all cases (Scheme 2). It is worth noting that
conventional Pd catalysts are not deactivated by the acac
residues despite the basic conditions.
Scheme 2. Synthesis of the acacH BODIPYs
Within both acetylacetone units, the dimensions and planarity
are consistent with their adoption of a delocalized, enol form
(Figure 1), in keeping with that of related aromatic deriva-
tives.9-11 Thus, bond lengths within the OCCCO units are
(molecule 1) C(17)-O(3) 1.306(8), C(17)-C(16) 1.399(9),
C(16)-C(19) 1.393(8), C(19)-O(2) 1.317(8) (molecule 2,
2-fold symmetric), C(19′)-O(2′) 1.306(6), C(19′)-C(18′)
1.392(7) Å. The acetylacetone segments are nearly coplanar
(dihedral angle, molecule 1, 11.9°, molecule 2, 2.5°) with
their linked Bopdipy cores, though the phenyl linker is nearly
orthogonal (dihedral angle molecule 1, 78.1°, molecule 2,
70.8°) to the BODIPY unit. The O· · · O separations are 2.409(7)
Å (molecule 1) and 2.450(6) Å (molecule 2 in Figure 1), consistent
with (symmetrical) bridging by the enolic proton.
Unfortunately, the cross-coupling of acacH modules to more
electron-rich BODIPYs having an extended dipyrromethene
core provided by two styryl fragments in the 3,5-substitution
positions failed under various experimental conditions. Thus,
a new strategy based on the cross-coupling of the acacH-linked
phenyliodo derivative 4 was devised. Substitution of one iodo
group of 1,4-di-iodobenzene by an acacH unit proved facile,
yielding 4 in 65% yield under the experimental conditions of
Scheme 1. Cross-coupling of 4 with an ethynyl-substituted
BODIPY was found not to be limited to simple BODIPY
species (yellow and green emitters 5 and 6, respectively) but
We have previously noted that some particular ethynyl-
substituted BODIPY derivatives could not be fully purified
owing to their high reactivity and tendency to polymerize
rapidly.12 This limits the cross-coupling strategy of Scheme 2
to stable ethynyl-grafted BODIPYs.
Our next objective was the synthesis of an acacH building
block bearing a reactive terminal alkyne. This scenario would
help to prepare many various dyes by shortcutting the synthesis
of the BODIPY alkyne derivatives. TMS-acetylene reacts under
standard conditions to afford the protected compound 8.
Unfortunately, this compound appears to be relatively unstable,
and its deprotection was unsuccessful probably because of the
presence of fluoride or water (Scheme 3). In situ deprotection
of the trimethylsilyl group enabled the cross-coupling to
iodophenyl BODIPYs, but the reaction product was the keto
derivative 9. Attempts to deprotonate the methylene group and
to acylate it with an ester failed in all cases providing an
intractable mixture of compounds (Scheme 3).
(9) Allen, F. H. Acta Crystallogr., Sect. B: Struct. Sci. 2002, 58, 380–
388
.
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.
Org. Lett., Vol. 12, No. 3, 2010
409

Scheme 3
.
One-Pot Synthesis Attempts to Prepare acacH
BODIPYs
Scheme 5. Molecular Formulas of BODIPYs Derivatives 14-16
2b). The fluorescence spectra showed relatively good mirror
symmetry with respect to the lowest-energy absorption
transitions, confirming that the same optical transitions are
involved in both absorption and emission processes. These
transitions were weakly polarized, as is typical for S1
emitters. Furthermore, the fluorescence decay profiles fitted
a single exponential, with fluorescence lifetimes in the 3-15
ns range. The fact that the radiative rate constant (about 1
× 108 s-1) is much greater than the nonradiative rate constant
(about 5 × 107 s-1) shows that the molecular excited state
preferentially deactivates radiatively. The small Stokes shift
observed may be explained by the substantial invariance of
the molecular geometric structure in the ground (S0) and the
first excited (S1) states, while the perfect match between the
excitation and absorption spectra points to the efficient
radiative deactivation of the excited electronic state.
Thus, an alternative strategy investigated was the substitution
of the iodo group in 4 with acetylene (used to degass the reaction
mixture), followed by a one-pot cross-coupling reaction with
iodophenyl-BODIPY dyes after removing all residual acetylene
from the reaction mixture. Dyes 6 and 10 with an acetylenic
linker were successfully isolated in good yields by this one-pot
protocol (Scheme 4).
Scheme 4. Synthesis of the acacH BODIPYs 6 and 10
Finally, after some experimentation, we succeeded in linking
in a covalent fashion dyes 2 (orange), 10 (blue), and 7 (green)
to TiO2 powder (Degussa, P-25). By mixing a dichloromethane
solution of the dye with TiO2 dispersed in a small volume of
isopropanol at 60 °C, the powder progressively took on the color
of the solution. After 2 h, the color of the solution had
diminished in intensity by about 50%. After vigorous washing
of the colored powder under sonication, the dried samples were
orange, blue, and green, demonstrating efficient linking of the
BODIPY dyes (Figure 3). Treatment of TiO2 powders with dyes
carrying an iodo function in place of the acacH fragment does
not provide colored powders, excluding possible adsorption of
the dyes on the particle surface (Figure 3, left-hand side). FT-
IR spectra of the powders were also in keeping with complex-
ation of 1,3-diketonate units to Ti centers, with a carbonyl region
absorption (νacac ) 1599 cm-1) very close to that of soluble
Ti-acac complexes (Figure 4) but absent in genuine TiO2.17 It
is well established that grafting dyes to inorganic surfaces by
pincer type of anchors is more efficient than single tethers like
carboxylates or phosphonates. The binding energies of acac
In extension of this one-pot reaction, we performed more
sophisticated syntheses in which two sites were functionalized
(Scheme 5). While the ethynyl derivatives of dyes 11-13 are
too unstable to be used in coupling reactions to 4, the procedure
defined in Scheme 4 enabled isolation of compounds 14-16
in appreciable yields.
The absorption spectra of 14-16 in dilute dichloromethane
solutions showed two main absorption peaks. That at 300 nm
is common to the whole series, while the lower-energy
absorption varies with the BODIPY core (Figure 2a). The
absorption bands located at higher energies are assigned to
π-π* transitions localized on the acacH fragment, and its
intensity is proportional to the number of units. The absorption
bands localized at 515, 643, and 709 nm, respectively, for 14,
15, and 16 are safely assigned, in light of previous data, to
S0fS1 transitions.13–15 Interestingly, the weak absorption in
the 360-400 nm range, probably due to S0fS2 transitions,
showed, in contrast, only a weak dependence on an increase
in the core delocalization of the BODIPY unit. Excitation
in the low-energy absorption bands of 14, 15, and 16 led to
strong emissions at 533, 672, and 749 nm, respectively, with
quantum yields in the range 50-70% (Table 1 and Figure
(13) Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed 2008,
47, 1184–1201. Loudet, A.; Burgess, K. Chem. ReV. 2007, 107, 4891–4932.
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Y.-W.; Spieles, M.; Li, Y.-Z.; Rurack, K.; You, X.-Z. Chem. Asian J. 2006,
1-2, 176. Erlen-Ela, S.; Yilmaz, M. D.; Icli, B.; Dede, Y.; Icli, S.; Akkaya,
E. U. Org. Lett. 2008, 10, 3299–3302.
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D.; Van Averbeke, M.; Boens, N. J. Phys. Chem. A 2007, 111, 8588–8597.
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2844.
410
Org. Lett., Vol. 12, No. 3, 2010

Figure 3. TiO2 powders labeled with dyes 2, 10, and 7 and their
dichloromethane solution. Solution A is an equimolar solution of the
phenyliodo BODIPYs 11, 12, and 13. Solutions B, C, and D are
micromolar solutions of the acacH dyes 2, 10, and 7. The bottom glass
slides contain the dry powders of TiO2 which have been treated from
left to right with solutions A, B, C, and D, respectively, highlighting
the covalent linking of the dye to the semiconductor.
Figure 2. Data measured in dichloromethane at rt for dyes 14 (orange),
15 (blue), and 16 (green). (a) Absorption at c ≈ 10-5 M. (b) Emission
at c ≈ 10-6 M, excitation wavelength 480 nm for 14, 595 nm for 15,
and 630 nm for 16.
Table 1. Optical Data Measured in Dichloromethane Solution
b
λabs
ε
λF
τF
(ns)
krb
knr
a
(nm) M1.cm-1 (nm)
ΦF
108 s-1 108 s-1
2
3
5
6
7
525
525
524
502
705
526
645
515
643
709
70,000
71,300
75,000
76,000
103,000
68,000
132,000
73,000
75,000
88,000
540
539
541
514
764
541
662
533
672
749
0.90
0.91
0.60
0.53
0.25
0.63
0.55
0.70 15.0
0.66 13.0
0.50
8.0
9.0
8.0
3.0
3.0
7.0
9.0
1.12
1.01
0.75
1.77
0.83
0.90
0.61
0.47
0.51
0.56
0.13
0.10
0.50
1.57
2.5
0.53
0.50
0.20
0.26
0.56
9
10
14
15
16
Figure 4. FT-IR of the TiO2 powders grafted with the acacH-dyes
dispersed in KBr pellets: genuine TiO2 (black); TiO2 + dye 2 (orange);
TiO2 + dye 7 (green); TiO2 + dye 10 (blue).
9.0
a Determined at rt, using Rhodamine 6G as reference (ΦF ) 0.78 in water,
λexc ) 488 nm).16 All ΦF are corrected for changes in refractive index.
b Calculated using the following equations: kr ) ΦF/τF, knr ) (1 - ΦF)/τF,
assuming that the emitting state is produced with unit quantum efficiency.
cent and stable under various conditions. Anchoring these novel
dyes to TiO2 nanoparticles is facile, leading to highly colored
powders which may find interesting applications in solar cells
or related energy conversion devices. The fluorescence of the
dyes linked to the semiconductor is completely quenched which
is a good indication of electron injection in the conduction band
of the semiconductor. We are currently using these dyes in
electrochemical solar cells.
fragments to oxygen vacancy sites of TiO2 is by far more
suitable than carboxylates.5 Furthermore, we believe that acac
derivatives will be less prone to hydrolysis versus surface
Ti(IV)-carboxylate esters and should provide more robust solar
cells.
In conclusion, we have developed a new series of 1,3-
diketone-substituted BODIPYs based either on direct substitu-
tion with Cu(I)-L proline catalysts or on Pd(0)-promoted cross-
coupling reactions either with ethynyl-substituted BODIPYs or
with ethynyl-substituted acacH compounds. The chemical scope
of these pentadione compounds could be enlarged by reaction
with functionalized aldehyde to provide novel energy transfer
cassettes and third-order nonlinearities.18 The introduction of
acacH fragments does not, as expected, significantly perturb
the optical properties of the dyes which remain highly fluores-
Acknowledgment. We also acknowledge the CNRS pro-
viding research facilities and the ANR COPROP-NANO-N°
NT05-2_42256 for financial support and Professor Jack Har-
rowfield (ISIS in Strasbourg) for commenting on the manuscript
before publication.
Supporting Information Available: Synthetic procedures
and analytical data reported herein. CIF file deposited at
Cambridge Crystallographic Data Centre under the number
740694. This material is available free of charge via the Internet
at http://pubs.acs.org.
(18) Hales, J. M.; Zheng, S.; Barlow, S.; Marder, S. R.; Perry, J. W.
J. Am. Chem. Soc. 2006, 128, 11362–11363.
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