Synthesis of luminescent 2-(2′-hydroxyphenyl)benzoxazole (HBO) borate complexes

Article abstract of DOI:10.1021/ol203014e

Complexation of boron trifluoride by a series of electron donor/acceptor substituted 2-(2′-hydroxy phenyl)benzoxazole (HBO) derivatives yields luminescent B(III) complexes with an emission wavelength ranging from 385 to 425 nm in dichloromethane or toluene. Appropriate chemical functionalization of these new dyes allows connection to different photoactive subunits (Boranil, BODIPY), endowing an efficient cascade energy transfer.

Full text of DOI:10.1021/ol203014e

ORGANIC
LETTERS
2012
Vol. 14, No. 1
230–233
Synthesis of Luminescent
2-(20-Hydroxyphenyl)benzoxazole
(HBO) Borate Complexes
Julien Massue,† Denis Frath,† Gilles Ulrich,*,† Pascal Retailleau,‡ and
Raymond Ziessel*,†
ꢀ
Laboratoire de Chimie Organique et Spectroscopies Avancees (LCOSA), UMR 7515
ꢁ
ꢀ
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
gulrich@unistra.fr; ziessel@unistra.fr
Received November 8, 2011
ABSTRACT
Complexation of boron trifluoride by a series of electron donor/acceptor substituted 2-(20-hydroxy phenyl)benzoxazole (HBO) derivatives yields
luminescent B(III) complexes with an emission wavelength ranging from 385 to 425 nm in dichloromethane or toluene. Appropriate chemical
functionalization of these new dyes allows connection to different photoactive subunits (Boranil, BODIPY), endowing an efficient cascade energy transfer.
Fluorophores incorporating a four-coordinate boron
bound to a π-conjugated chelate have been extensively
studied for various applications ranging from biological
sensing and imaging to the search for new electrolumi-
nescent devices.1 Among them, boron dipyrromethene
(BODIPY) dyes have emerged as very promising due to
their outstanding chemical and photophysical properties.2
In addition to these extensively studied derivatives, new
chelating groups for the B(III) fragment have recently
emerged in the literature; prominent examples include
N∧O bidentate π-conjugated fragments coordinated to
various boron-containing entities such as BF2, B(Ar)2,
B(ArF5)2, and B(OAr)2.3
2-(20-Hydroxyphenyl)benzoxazole (HBO) derivatives,
known for over 40 years,4 are an interesting class of
fluorophores due to their intrinsic Excited-State Intramo-
lecular Proton Transfer (ESIPT) property which leads to a
major structural reorganization of the molecules upon
photoexcitation and large Stokes shifts. This interesting
feature has found applications for pH sensing, chemical
detection of divalent metal cations,5 and anion sensing, in-
cluding that of pyrophosphate6 and fluoride.7 HBO-based
† LCOSA, ECPM, CNRS.
‡ Laboratoire de Crystallochimie, ICSN - CNRS.
(1) Rao, Y.-L.; Wang, S. Inorg. Chem. 2011, 50, 12263.
(2) (a) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891. (b) Boens,
N.; Leen, V.; Dehaen W. Chem. Soc. Rev. 2012, DOI: 10.1039/
c1cs15132k. (c) Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem.,
Int. Ed. 2008, 47, 1184. (d) Ziessel, R.; Ulrich, G.; Harriman, A. New J.
Chem. 2007, 31, 496.
(3) (a) Wu, Q.; Esteghamatian, M.; Hu, N.-X.; Popovic, Z.; Enright,
G.; Tao, Y.; D’Iorio, M.; Wang, S. Chem. Mater. 2000, 12, 79. (b) Cui,
Y.; Wang, S. J. Org. Chem. 2006, 71, 6485. (c) Zhang, Z.; Bi, H.; Zhang,
Y.; Yao, D.; Gao, H.; Fan, Y.; Zhang, H.; Wang, Y.; Wang, Y.; Chen,
Z.; Ma, D. Inorg. Chem. 2009, 48, 7230. (d) Kim, N. G.; Shin, C. H.; Lee,
M. H.; Do, Y. J. Organomet. Chem. 2009, 694, 1922. (e) Liu, S.-F.;
Seward, C.; Aziz, H.; Hu, N.-X.; Popovic, Z.; Wang, S. Organometallics
2000, 19, 5709. (f) Tokoro, Y.; Nagai, A.; Chujo, Y. Macromolecules
2010, 43, 6229. (g) Son, H.-J.; Han, W.-S.; Wee, K.-R.; Chun, J.-Y.;
Choi, K.-B.; Han, S. J.; Kwon, S.-N.; Ko, J.; Lee, C.; Kang, S. O. Eur. J.
Inorg. Chem. 2009, 1503. (h) Murale, D. P.; Lee, K. M.; Kim, K.;
Churchill, D. G. Chem. Commun. 2011, 47, 12512.
(4) Williams, D. L.; Heller, A. J. Phys. Chem. 1970, 74, 4473.
(5) (a) Taki, M.; Wolford, J. L.; O’Halloran, T. V. J. Am. Chem. Soc.
2004, 126, 712. (b) Ohshima, A.; Momotake, A.; Arai, T. Tetrahedron
Lett. 2004, 9377. (c) Tian, Y.; Chen, C.-Y.; Yang, C.-C.; Cody Young,
A.; Jang, S.-H.; Chen, W.-C.; Jen, A. K.-Y. Chem. Mater. 2008, 20, 1977.
(d) Santra, M.; Roy, B.; Ahn, K. H. Org. Lett. 2011, 13, 3422.
(6) Chen, W.-H.; Xing, Y.; Pang, Y. Org. Lett. 2011, 13, 1362.
(7) Chu, Q.; Medvetz, D. A.; Pang, Y. Chem. Mater. 2007, 19, 6421.
r
10.1021/ol203014e
Published on Web 12/16/2011
2011 American Chemical Society

dyes are also excellent N∧O chelates for various metallic
centers such as Re(I),8 Ru(II),9 Ir(III),10 Be(II),11 Sc(III),12
and Ln(III),13 chosen with the objective of obtaining new
optically tunable luminescent materials. Since, to the best
of our knowledge, there is only one literature report of the
coordination of a BF2 fragment to 2-(20-hydroxyphenyl)-
benzoxazole,14 the influence of molecular engineering on
the photoluminescent properties of such tetrahedral boron
complexes still needs to be explored.
Here, wereportonthe two-step preparation of a series of
neutral tetrahedral B(III) complexes bearing various elec-
tron-donating/-withdrawing substituents that enable mod-
ulation of the photophysical properties. We show also that
these new dyes can be connected to other photoactive sub-
units such as BODIPY or Boranil cores to afford sophis-
ticated molecular cassettes. Preparation of the substituted
2-(20-hydroxyphenyl)benzoxazole (HBO) 1ꢀ9 derivatives
and their corresponding B(III) complexes 10ꢀ18 is sum-
marized in Scheme 1. Depending on the nature of the
substituents present on the core of the starting substituted
2-aminophenol I and 2-hydroxybenzaldehyde II, two dif-
ferent routes were chosen. Route A involved heating I and
II together in ethanol at reflux to give the cyclic carbino-
lamines, which precipitated from the reaction mixture.
After collection, these compounds were oxidized with a
slight excess of 2,3-dichloro-5,6-dicyano-1,4-benzoqui-
none (DDQ). The milder one-pot Route B, used when
oxidation-sensitive substituents were present, such as
diethylamino groups, required potassium cyanide to pro-
mote benzoxazole cyclization in the presence of phenyl-
boronic acid.15
HBO dyes 1ꢀ9 display a distinctive downfield 1H
NMR signal (11 to 13 ppm) for the H-bonded phenolic
proton (see Supporting Information (SI) for full char-
acterization).
Boron complexation was achieved using BF3 Et2O in
3
the presence of N,N-diisopropylethylamine (DIEA) in
anhydrous 1,2-dichloroethane to yield B(III) complexes
10ꢀ18 as off-white powders after purification by filtration
through a basic Al2O3 column. The complexation reaction
can be readily monitored by the loss of the downfield
proton signal as well as the appearance of a triplet in the
11B NMR spectrum (see SI) due to coupling with two
equivalent fluorine nuclei (JBꢀF ∼10 Hz). Note that the
coupling constant is significantly lower than those re-
ported for similar fluorescent B(III) dyes (in the range of
JBꢀF = 33 Hz for BODIPY16 and 16 Hz for Boranil17
dyes).
Building upon this straightforward manner to obtain a
series of luminescent HBO-based complexes, we decided to
investigate their connection to other photoactive subunits.
Scheme 2. Synthesis of Dye 22
Scheme 1. Synthesis of Substituted 2-(20-Hydroxyphenyl)-
benzoxazole (HBO) 1ꢀ9 Derivatives and Their Corresponding
B(III) Complexes 10ꢀ18
Carboxylic acid functionalized derivative 19 was readily
obtainedfrom HBO 6 bya simple saponification stepusing
a 3 M NaOH solution. Peptidic coupling of BODIPY 2018
bearing an aminophenyl group on position 8 with 19
proceeded under standard conditions (DMAP, EDCI).
Complexation of the resulting dye 21 with BF3 Et2O
3
afforded, in 88% yield, the highly soluble dye 22 incorpor-
ating the two photoactive units of interest (Scheme 2).
Boranil dyes are also very promising fluorescent N∧O
chelates for a BF2 fragment.17 Functionalization of the
HBO ring with a boranil core requires 3 steps (Scheme 3).
(10) (a) You, Y.; Seo, J.; Kim, S. H.; Kim, K. S.; Ahn, T. K.; Kim, D.;
Park, S. Y. Inorg. Chem. 2008, 47, 1476. (b) Liu, Y.; Li, M.; Zhao, Q.;
Wu, H.; Huang, K.; Li, F. Inorg. Chem. 2011, 50, 5969.
(11) Tong, Y.-P.; Zheng, S.-L.; Chen, X.-M. Inorg. Chem. 2005, 44,
4270.
(12) Katkova, M. A.; Balashova, T. V.; Llichev, V. A.; Konev, A. N.;
Isachenkov, N. A.; Fukin, G. K.; Ketkov, S. Y.; Bochkarev, M. N.
Inorg. Chem. 2010, 49, 5094.
(13) Pang, Y.; Chu, Q. PCT WO2010/075003 A1.
(14) Kwak, M. J.; Kim, Y. Bull. Korean Chem. Soc. 2009, 30 (12),
2865.
~
(15) Lopez-Ruiz, H.; Briseno-Ortega, H.; Rojas-Lima, S.; Santillan,
ꢁ
R.; Farfan, N. Tetrahedron Lett. 2011, 52, 4308.
(16) Ulrich, G.; Ziessel, R. J. Org. Chem. 2004, 69, 2070.
(17) Frath, D.; Azizi, S.; Ulrich, G.; Retailleau, P.; Ziessel, R. Org.
Lett. 2011, 13, 3414.
(18) Ziessel, R.; Bonardi, L.; Retailleau, P.; Ulrich, G. J. Org. Chem.
2006, 71, 3093.
(8) Czerwieniec, R.; Kapturkiewicz, A.; Anulewicz-Ostrowska, R.;
Nowacki, J. J. Chem. Soc., Dalton Trans. 2002, 3434.
(9) Keyes, T. E.; Leane, D.; Forster, R. J.; Coates, C. G.; McGarvey,
C. G.; Nieuwenhuyzen, M. N.; Figgemeier, E.; Vos, J. G. Inorg. Chem.
2002, 41, 5721.
Org. Lett., Vol. 14, No. 1, 2012
231

Selectively reduction of the nitro group of HBO 9 under
standard conditions afforded compound 23, which was
further condensed with 2-hydroxy-4-diethylamino ben-
zaldehyde in refluxing ethanol with a trace amount of
p-TsOH to afford the desired imine 24. Both N∧O chelat-
ing sites react withexcess BF3 Et2O under basic conditions
to yield soluble, stable dyad 25 in 93% yield.
3
Scheme 3. Synthesis of the Dyad 25
Figure 2. ORTEP view for 13 showing the atom-labeling
scheme. Thermal ellipsoids are plotted at the 50% level.
B1ꢀN1, B1ꢀO1, and N1ꢀC13 bond lengths are 1.560(3),
˚
1.441(3), and 1.405(2) A, and the NꢀBꢀO, NꢀBꢀF, and
OꢀBꢀF angles are 107.57(17)°, 108.84(17)°/109.5(2)°, and
109.9(2)°/110.41(19)°.
The X-ray molecular structures of12 and 13 are depicted
in Figures 1 and 2, respectively, with selected geometric
values.
The absorption spectra of complexes 10ꢀ16 exhibit a
major absorption band between 336 and 375 nm with
extinction coefficients in the range 13000ꢀ30000 Mꢀ1
cmꢀ1, whereas complexes 17ꢀ18 bearing a diethylamino
group on the phenolic side have a red-shifted absorption
maximum (385ꢀ401 nm) along with higher extinction
coefficients (60000 to 80000 Mꢀ1 cmꢀ1). Irradiation in
3
the lower energy absorption band gives rise to an intense
emission band in the 385ꢀ439 nm range. These dyes
exhibit quantum yields in the range 17ꢀ73% with good
chemical and photochemical stability in apolar media.
In strong contrast, for 16 and 18 the fluorescence is
heavily quenched, likely by a photoinduced electron trans-
fer (PET) process with the neighboring nitro group.20 Note
that dissolving these complexes in polar solvents such as
alcohols or DMSO leads to decomplexation of boron, as
shown by the recovery of the typical ESIPT emission of the
HBO ligand.21
Figure 1. ORTEP view for 12 showing the atom-labeling
scheme. Thermal ellipsoids are plotted at the 50% level.
B1ꢀN1, B1ꢀO1, N1ꢀC1, and N1ꢀC13 bond lengths are
The broad shape of the emission band for complexes
10ꢀ15, the nonmirror symmetry with the absorption
˚
1.576(3), 1.444(3), 1.317(2) and 1.408(2) A, and the NꢀBꢀO,
bands, and a significant Stokes shift (3300 to 4900 cmꢀ1
)
NꢀBꢀF, and OꢀBꢀF angles are 108.01(17)°, 108.17(18)°/
109.23(18)°, and 110.25(19)°/111.13(19)°.
are suggestive of an intraligand charge transfer (ICT)
emissive state (Figure 3a).22 Furthermore, the nanosecond
lifetime regime is in keeping with such a polarized excited
state. In the case of dye 17, bearing a diethylamino moiety,
a structured, narrow, blue-shifted emission band along
with a reduced Stokes shift (600 to 900 cmꢀ1) is observed,
consistent with a weakly polarized excited state with pro-
nounced singlet state character (Figure 3b). In this case,
pronounced internal cyanine character from the diethyla-
mino group totheimine moietycoordinatedtothe boron is
suggested.23 Interestingly, in the case of the naphthyl dye
In both, all the atoms are coplanar (with an overall rmsd
˚
of 0.0429 and 0.0202 A respectively) except for the fluor-
ines, which contribute to the fairly regular tetrahedral
geometry of the boron center (with angles ranging from
107.9(2)° and 110.3 (2)° and from 108.9(2)° and 110.5(2)°
in 12 and 13, respectively). Unlike the 9 structures of
difluorobenzooxazaborinin derivatives found to date in
the CSD,19 planarity appears to be imposed in the present
instance by the fact that the N1ꢀC1 bond is part of a five-
membered ring. The crystal packing of both molecules is
described in detail in the SI.
(20) Ziessel, R.; Bonardi, L.; Ulrich, G. Dalton Trans. 2006, 2913 and
references cited therein.
(21) Iijima, T.; Momotake, A.; Shinohara, Y.; Sato, T.; Nishimura,
Y.; Arai, T. J. Phys. Chem. A 2010, 114, 1603.
(22) Kollmannsberger, M.; Rurack, K.; Resch-Genegr, U.; Daub, J.
Photophysical data in toluene and dichloromethane
for all the new fluorophores are gathered in Table 1.
J. Phys. Chem. A 1998, 102, 10211.
(23) Mishra, A.; Behera, R. K.; Behera, P. K.; Mishra, B. K.; Behera,
G. E. Chem. Rev. 2000, 100, 1973.
(19) The CSD 3D Graphics Search System v5.32 (November 2010
including the 2011 updates): Allen, F. H. Acta Crystallogr., Sect. B 2002,
58, 380.
232
Org. Lett., Vol. 14, No. 1, 2012

Table 1. Selected Optical Data Measured in Solution
a
dye λabs (nm) ε (Mꢀ1.cmꢀ1) λem (nm) Δ (cmꢀ1) Φ f τ (ns) solvent
10
11
12
13
14
15
349
344
362
359
342
339
375
373
342
341
365
364
371
385
387
399
347
526
366
526
414
15600
16100
13000
13700
29700
33200
20700
19700
29200
29700
11300
13100
12700
79300
67200
59600
17100
50400
18200
53700
106200
401
395
417
422
385
380
422
425
387
384
425
432
439
395
3700 0.26 1.26 Toluene
3800 0.22 0.93 CH2Cl2
3600 0.36 2.03 Toluene
4300 0.31 2.19 CH2Cl2
3300 0.48 1.40 Toluene
3200 0.39 1.39 CH2Cl2
3000 0.17 2.19 Toluene
3300 0.28 1.63 CH2Cl2
3400 0.36 1.45 Toluene
3300 0.37 1.36 CH2Cl2
3900 0.73 2.85 Toluene
4300 0.58 3.36 CH2Cl2
4200 0.02 0.11 Toluene
16
17
660
900
-
0.43 1.47 Toluene
0.42 2.92 CH2Cl2
401
b
18
21
-
-
Toluene
Figure 3. Absorption, emission, and excitation of dyes (a) 11, (b)
17, (c) 13, and (d) 22 in CH2Cl2 at rt.
538
538
537
537
473
1000 0.63 6.14 CH2Cl2
4200 0.64
22
25
8700 0.93 6.76 CH2Cl2
390
0.95
3000 0.62 1.45 CH2Cl2
the energy transfer process. This favorable situation is a
consequence of the spectral overlap between the emission
of the HBO and the absorption of the second excited state
of the BODIPY fragment.25
a Quantum yields determined in solution, using quinine sulfate as
reference φ = 0.55 in H2SO4 1 N, λex = 366 nm for dyes emitting below
480 nm and Rhodamine 6G φ = 0.88 in ethanol, λex = 488 nm for dyes
emitting between 480 and 570 nm. b non fluorescent.
Another interesting situation is generated in the case of
the mixed dyad 25 which exhibits a high extinction coeffi-
cient (>100000 Mꢀ1 cmꢀ1) and strong emission at 473
nm with a quantum yield of 62% (Table 1).
3
13, the molecular core is more rigid and vibronic structure
is observed in the absorption and emission spectra, a
situation in favor of a singlet excited state decay. The short
lifetime and weak Stokes shift are in keeping with this
(Figure 3c).
In the case of the dyad 22, the absorption spectrum is a
linear combination of those of the BODIPY core and the
HBO borate subunit, a situation favored by the orthogon-
ality of the two fragments.24 By excitation of mixed dye 22
at 366 nm, insignificant residual emission of the HBO
fragment was observed at 420 nm (Figure 3d) but strong
emission of the BODIPY subunit at 537 nm (φem = 0.93 in
CH2Cl2) was observed, indicating a 98% efficiency in
energy transfer from the HBO borate to the BODIPY
moiety. The perfect match between absorption and excita-
tion over the entire wavelength range confirms the effi-
ciency and the participation of the HBO borate residue in
In summary, we have synthesized a series of novel
fluorescent boron complexes by chelation of HBO plat-
forms featuring interesting spectroscopic properties in
nonpolar solvents. Current work is focused on investigat-
ing their electrochemical properties in solution and optical
properties in the solid state with the goal to embed them
into optoelectronic devices.
Acknowledgment. The Centre National de la Recherche
Scientifique (CNRS) is acknowledged for financial sup-
port and for providing research facilities.
Supporting Information Available. Synthetic proce-
dures, NMR spectra, crystallographic and spectroscopic
data. This material is available free of charge via the
Internet at http://pubs.acs.org.
ꢀ
(24) Ziessel, R.; Goze, C.; Ulrich, G.; Cesario, M.; Retailleau, P.;
(25) Harriman, T.; Izzet, G.; Ziessel, R. J. Am. Chem. Soc. 2006, 128,
Harriman, A.; Rostron, J. P. Chem.;Eur. J. 2005, 11, 7366.
19868.
Org. Lett., Vol. 14, No. 1, 2012
233