Facile synthesis of highly fluorescent Boranil complexes

Article abstract of DOI:10.1021/ol2011665

Complexation of a large variety of Anils (aniline-imines) with boron(III) precursors provides stable Boranils, some of which have been structurally characterized. Analysis of their optical properties reveals that the fluorescence stems from an intraligand

Full text of DOI:10.1021/ol2011665

ORGANIC
LETTERS
2011
Vol. 13, No. 13
3414–3417
Facile Synthesis of Highly Fluorescent
Boranil Complexes
†
†
,†
‡
ꢀ
Denis Frath, Sebastien Azizi, Gilles Ulrich,* Pascal Retailleau,
and Raymond Ziessel*,†
ꢀ
Laboratoire de Chimie Organique et Spectroscopies Avancees (LCOSA),
ꢁ
ꢀ
UMR7515 au CNRS, Ecole de Chimie, Polymeres, Materiaux de Strasbourg (ECPM),
25 rue Becquerel, 67087 Strasbourg, Cedex 02, France, and Laboratoire de Crystallochimie,
^
ICSN - CNRS, Bat 27 - 1 avenue de la Terrasse, 91198 Gif-sur-Yvette, Cedex, France
ziessel@unistra.fr
Received May 2, 2011
ABSTRACT
Complexation of a large variety of Anils (aniline-imines) with boron(III) precursors provides stable Boranils, some of which have been structurally
characterized. Analysis of their optical properties reveals that the fluorescence stems from an intraligand charge transfer (ILCT) state with the best
quantum yields reaching 90%. Chemistry on the Boranils allows grafting of photoactive modules acting as energy antennae for
borondipyrromethene (Bodipy) and subphtalocyanine (SubPc) fluorophores.
To date, considerable effort has been devoted to the
development of innovative fluorescent dyes for applica-
tions as labels in biomedical analysis,1 molecular sensors,2
and light emitting devices.3 Efficient photoluminescence
and ambipolar charge transport properties have been
reportedfor someof these dyes.4 Ofthe manyconventional
fluorophores, those of the Bodipy family are most intri-
guing due to their outstanding optical properties, extra-
ordinary chemical versatility, and variety of applications
spanning from biolabeling5 to solar cells6 and nanoparticle
engineering.7 Yet, despite these attractive features, the
synthesis of such dyes remains tedious, involving linear
stepwisesyntheseswithrelativelylowglobal yields. Thus, it
is significant to note that other N∧N bidentate ligands
(e.g., deprotonated benzimidazoles, indoles, azaindoles,
and pyrazoylanilines) form stable boron complexes with
interesting luminescence properties.8
The search for alternative fluorescent dyes prompted us
to envisage changes in, first, the nature of the chelate unit
and, second, the nature of the B(III) fragment [BF2,
B(Ar)2, B(ArF5)2, B(OAr)2]. Coordinately saturated (and
essentially tetrahedral) B complexes have been studied
using N∧N∧O∧O-tetradentate ligands,9 O∧N∧O tridentate
ligands,10 or N∧O-bidentate ligands (Figure 1). This group
can be divided into those forming five-membered rings with
quinolines11 or six-membered rings with salicylaldehydes,12
oxazolylphenolates,13 acylpyrrole,14 or pyridinephenolates.15
† LCOSA, CNRS, ECPM.
‡ Laboratoire de Crystallochimie, ICSN - CNRS.
(1) (a) Yang, W.; He, H.; Drueckhammer, D. G. Angew. Chem., Int.
Ed. 2001, 40, 1714. (b) Killoran, J.; Allen, L.; Gallagher, J. F.; Gallagher,
W. M.; O’Shea, D. F. Chem. Commun. 2002, 1862.
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Thummel, R.; Tao, Y.; Wang, S. Adv. Funct. Mater. 2005, 15, 143.
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Chem. 2010, 3721.
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U. E. Org. Lett. 2008, 10, 3299. (b) Kumaresan, D.; Thummel, R. P.;
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r
10.1021/ol2011665
Published on Web 06/07/2011
2011 American Chemical Society

1À4 being distinctive. In the BF2-containing products,
coupling of the imine proton to B is observed, leading to
the appearance of a broad quartet due to the 11B coupling
(3JHÀB = 3.7 Hz) (Figure S1). This coupling is absent in the
BPh2 analogues owing to the longer NÀB distance as ob-
served in the X-ray molecular structure (vide infra).
The X-ray molecular structures of 6 and 5 are shown in
Figures 2 and 3, respectively, with selected geometric
values. In both structures, the B has a slightly distorted
tetrahedral geometry with angles ranging from 106.43(15)°
and 115.31(16)° and from 104.6(11)° and 112.2(11)° in 6
Figure 1. Various chelating modes of B(III) complexes.
More exotic N,O-chelates forming six-membered rings
have been obtained from phenalene-1,3-dione and
perylenediimides.16 Also, dinuclear B-complexes have
been prepared with binaphthyl, salen, and para-phenyl
frameworks.17 Yet, the luminescence quantum yields for
such derivatives are typically low, and little information
about their photostability is available.
Here we disclose the facile preparation in two steps of
novel B(III) complexes of various substituted Anils, giving
Boranilcomplexes bearing substituentsengenderingstrong
luminescence in both solution and the solid state. Such
complexesoffer interesting prospects becauseoftheir facile
synthesis on a large scale from common precursors and
their possible use as synthons for connection of an addi-
tional chromophoric subunit or other module of interest.
Some of the complexes without donor/acceptor modules
have previously been prepared.12a,18
∧
and 5, respectively. Surprisingly, the N1 O1 fragment
appears quasi-planar in 6, with an rms deviation of 0.0110
˚
˚
A, while B1 lies only 0.0313(28) A out of the least-squares
mean plane defined by the nine atoms including N1 and O1.
This is unlike any of the 11 structures of diphenylboron
Schiff base derivatives found to date in the CSD,20 in which
all the chelate rings are markedly distorted from planarity.
Such distortions are seen in the difluoroboron derivative 5,
˚
where the B-atom is displaced 0.435(16) A from the hetero-
cycle mean plane. With the exception of the difluoroboron
chelate of N-methyl salicylaldimine,21 all six other struc-
tures of difluoroboron chelates found within the CSD also
show strong distortions of the chelate rings from planarity.
The B1ÀO1 and B1ÀN1 distances are significantly
longer in 6 than the corresponding distances in 5 (1.639(3)
˚
˚
vs 1.503(3) A and 1.552(15) vs 1.420(20) A, respectively),
presumably in reflection of the electronegativity of the
fluorine substituents. As a consequence of the congested
substitution at the B-center by two phenyl groups, the
methoxyphenyl substituent at the N1 atom is more tilted in
6 than in 5, as indicated by the respective dihedral angles
between the least-squares mean plane defined by the
heterobicycle and that of the phenyl ring, 63.46(9)° vs
48.70(60)°. The dihedral angles between the phenyl (C21/
C26 and C15/C20) B-substituents and the heterobicycle in
6 are 58.57(9)° and 76.21(10)°, respectively. The crystal
packing of both molecules is described in detail in the
Supporting Information (Figure S2 and S3).
Preparation of the Anil19 derivatives 1À4 is straightfor-
ward starting from the aldehydes A and anilines B in re-
fluxing ethanol with trace amounts of p-TsOH (Scheme 1).
During the course of the reaction the desired imines pre-
cipitate pure out of the reaction mixture.
Scheme 1. Synthesis of the Anils and their Boranil Complexes
(12) (a) Umland, F.; Hohaus, E.; Brodte, K. Chem. Ber. 1973, 106,
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J. S. J. Org. Chem. 2004, 69, 5354. (c) Bhaumik, J.; Yao, Z.; Borbas, E.;
Taniguchi, M.; Lindsey, J. S. J. Org. Chem. 2006, 71, 8807.
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H.; Wang, Y.; Wang, Y.; Chen, Z.; Ma, D. Inorg. Chem. 2009, 48, 7230.
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J.; Lia,g, B.; Wang, D.; Xue, D.; Li, X. Org. Lett. 2008, 10, 4437.
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Yoon, Y. Chem. Commun. 2010, 46, 6512. (b) Wei, P.; Atwood, D. A.
Inorg. Chem. 1997, 36, 4060.
Subsequent B(III) complexation is feasible with BF3 Et2O
3
under basic conditions (diisopropylethylamine) to quench the
nascent HF or with BPh3 in hot toluene. No base is required in
this latter case, but the phenyl is used as the proton scavenger,
producing benzene. The reactions were readily monitored by
1H NMR spectroscopy, the loss of the signal near δ 13 ppm
due to the intramolecularly H-bonded phenolic proton of
(18) (a) Tripathi, S. M.; Tandon, J. P. Inorg. Nucl. Chem. Lett. 1978,
14, 97. (b) Okada, M.; Hatori, O. PCT WO2011/013474 A1.
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G.; Tao, Y.; D’Iorio, M.; Wang, S. Chem. Mater. 2000, 12, 79. (b) Qin,
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€
Org. Lett., Vol. 13, No. 13, 2011
3415

Scheme 2. Synthesis of Bis-Boranils and Borondipyrromethane
(BODIPY) Based Boranil Derivatives
Figure 2. ORTEP view for 6 showing the atom-labeling scheme.
Scheme 3. Synthesis of Functionalized Boranil Derivatives
(SubPc accounts for Boranil)
Figure 3. Left: ORTEP view for 5 showing the atom-labeling
scheme. Right: perpendicular view to highlight the distortion
at the B-atom.
standard conditions (Scheme 3). The stable and soluble
dyad 18 was isolated in 85% yield. In 15 and 18, the linkage
of two photoactive moieties via an unsaturated spacer
provides interesting spectroscopic features (vide infra).
The photophysical properties of the new dyes are sum-
marized in Table 1. The absorption maxima and molar
absorption coefficients (ε) were found to be dependent on
the para substituents on both the phenol and imine sides and
ranged from 372 to 443 nm and from 13 000 to 66 000 MÀ1
cmÀ1, respectively. Note that the absorption spectra of dyes
5 and 6 present two transitions, reduced ε and low quantum
yield compared to the other dyes (Table 1). This is typical of
aggregation, further confirmed by a weak overlap between
the excitation and absorption spectra (Figures S4,S5). On
increasing the electronic density on the phenol side by adding
a NEt2 substituent in the para position (dyes 7, 8, and 10), all
the spectroscopic characteristics are dramatically improved.
Switching from a fluoro to a phenyl group shifts the
absorption by ca. 16 nm (compare 8/9 and 10/11). By
increasing the pushÀpull character along the main
Developing upon these facile syntheses, we extended the
method to dinuclear B-complexes (Scheme 2). The use of
stoichiometric amounts of 4-diethylamino-2-hydroxy ben-
zaldehyde provides derivatives 12 and 14 in 82% yield.
Conversion to the respective B(III) complexes 13 and 15
proceeds efficiently, with no need for optimization.
In seeking to investigate a broader range of substituents,
we found that a cross-coupling reaction using Boranil 7,
and promoted by low-valent palladium, is efficient using,
for instance, trimethylsilylacetylene. No decomplexation
of B in favor of Pd is observed, in keeping with the good
stability of the B-complex. Compound 16 was found to
be deprotected to the terminal alkyne using a slight excess
of KF in polar solvents. The pivotal building block
17 was allowed to react with a prefunctionalized sub-
phtalocyanine22 bearing a phenyliodo residue under
(22) Camerel, F.; Ulrich, G.; Retailleau, P.; Ziessel, R. Angew.
Chem., Int. Ed. 2008, 47, 8876.
(23) Olmsted, J. J. Phys. Chem. 1979, 83, 2581.
3416
Org. Lett., Vol. 13, No. 13, 2011

Table 1. Selected Optical Data Measured in Solution
λabs
ε
λem
Δ
τ
dye (nm) (MÀ1 cmÀ1
)
(nm) (cmÀ1
)
jfa
(ns) solvent
3
b
5
6
342
372
338
403
403
396
409
427
422
429
429
443
428
407
504
409
414
12 600
13 000
9600
471 8000
471 5650
0.01
0.02
0.02
À
À
À
Toluene
Toluene
539 11 000
539 8600
455 2850
445 2800
492 4100
474 2300
473 2600
507 3600
4200
0.06 1.76
7
69 000
44 000
32 000
66 000
49 000
55 000
48 000
46 000
68 000
51 000
75 000
77 000
60 000
0.09 0.27 Toluene
0.05 0.12 Toluene
0.13 0.77 Toluene
0.60 1.61 Toluene
0.41 1.29 Et2O
Figure 4. Absorption, emission, and excitation spectra of dyes
(a) 10 and (b) 11 in toluene at rt.
8
9
10
10
10
10
11
13
15
0.13 0.78 THF
496
À
<0.01
À
MeCN
506 2800
506 3600
514 5100
0.61 2.73 Toluene
0.90 1.14 Toluene
0.45
0.49 2.81
0.09 0.29 Toluene
À
Toluene
514
464 2900
476
574 6730
574
À
16
18
À
<0.01
0.15
À
À
Toluene
567
76 000
À
0.17 2.02
Figure 5. Absorption, emission, and excitation spectra of dyes
15 (a) and 18 (b) in toluene at rt.
a Using quinine sulfate as reference Φ=0.55inH2SO4 1N, λex = 366 nm
for dyes emitting below 480 nm; Rhodamine 6G as reference, Φ = 0.88 in
ethanol, λex = 488 nm for dyes emitting between 480 and 570 nm; and cresyl
violet as reference, Φ = 0.55, λex = 546 nm in ethanol for dyes emitting
above 570 nm.23 b Below detection limit.
that a much stronger ILCT character and larger dipole
moment relative to the ground state are effective in leading
to unusual complexes displaying optical properties depen-
dent on the electronic modification of the chelate core.
These assumptions are further confirmed with the di-
nuclear dye13which exhibits a 90% quantum yield and the
highest radiative rate of all dyes. In case of the dual dye 15,
the absorption profile is a linear combination of the
absorption of the Boranil subunit and the Bodipy residues,
a situation expected to arise due to the orthogonality of the
phenyl ring. Interestingly, by irradiation in the N∧O-boron
fragment at 380 nm, quantitative energy transfer occurs,
leading to the exclusive fluorescence of the Bodipy unit at
514 nm. This is supported by an excellent match between
the excitation and absorption spectra over the entire
wavelength range studied (Figure 5a).
Finally, similar results were obtained with the mixed dye
18, where the absorption, emission, and excitation spectra
confirm that the N∧O-boron moiety could be used as an
input energy antenna for cascade energy transfer to the
fluorescent subphthalocyanine residue at 574 nm. Very
little residual emission from the Boranil moiety is found at
476 nm probably due to a poorer spectral overlap of the
Boranil subunit with the SubPc absorption (Figure 5b).
Efforts to increase the dipole moment in the excited state
and thus to shift the optical properties in the red part of the
spectrum are currently in progress.
molecular axis spanning over the ligand, a 55 nm bath-
ochromic shift of the absorption is observed for 10 with
respect to dye 5 (Figures 4a,S4). The absorption spectra
consist of intraligand (IL) bands with additional πÀπ*
transitions at 258 nm when using the BPh2 fragment in Et2O.
The simultaneous grafting of NEt2 and NO2 fragments
onto the ligand provides bright luminescence with quan-
tum yields reaching 61% for dyes 10 and 11 (Table 1). The
high quantum yield does not depend on the nature of the
B-substituents (compare 10 and 11 in toluene). The mod-
erate Stokes shift from the longest-wavelength absorption
to the most energetic emission is consistent with the
assumption that the luminescence is fluorescence. This
conclusion is supported by the observation that the emis-
sion is not quenched by oxygen and by the short lifetimes.
However, the shape of the emission band with several
shoulders on the low energy side and the nonmirror sym-
metry with the absorption bands are suggestive of an
intraligand charge transfer emissive state (ILCT).24 By
increasing the solvent polarity, the full width at half-max-
imum of the emission bands increases and the quantum
yields dramatically decrease to insignificant values in polar
acetonitrile (Table 1 and Figure S6). However, the dinuclear
derivative 13 and the bipartite dyes 15 and 18 are less
sensitive to the solvent polarity and less prone to a quantum
yield decrease in polar solvents. This is a clear indication
Supporting Information Available. Synthetic proce-
dures and analytical data. This material is available free
of charge via the Internet at http://pubs.acs.org.
(24) Kollmannsberger, M.; Rurack, K.; Resch-Genegr, U.; Daub, J.
J. Phys. Chem. 1998, 102, 10211.
Org. Lett., Vol. 13, No. 13, 2011
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