Δ1-pyrroline based boranyls: Synthesis, crystal structures and luminescent properties

Article abstract of DOI:10.1016/j.dyepig.2014.05.026

Two new Δ¹-pyrroline ligands and their corresponding boron complexes have been synthesized and characterized. Their luminescent properties were investigated, the boron complexes emitting brightly around 410 nm. The presence of the exocyclic dou

Full text of DOI:10.1016/j.dyepig.2014.05.026

Dyes and Pigments 111 (2014) 16e20
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
D
1-pyrroline based boranyls: Synthesis, crystal structures and
luminescent properties
a
b
a
,
, b, **
Francisco Cardona , Joao Rocha , Artur M.S. Silva *, Samuel Guieu a
~
a QOPNA, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
b CICECO, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new
D
1-pyrroline ligands and their corresponding boron complexes have been synthesized and
Received 28 March 2014
Received in revised form
17 May 2014
Accepted 19 May 2014
Available online 29 May 2014
characterized. Their luminescent properties were investigated, the boron complexes emitting brightly
around 410 nm. The presence of the exocyclic double bond has no influence on their electronic prop-
erties, meaning that this part of the complex could be used to fine tune their properties without dis-
turbing their fluorescence.
© 2014 Elsevier Ltd. All rights reserved.
Keywords:
Pyrroline ligand
Heterocycle
Boron complex
Fluorescence
Crystal structure
Dyes
1. Introduction
We disclose here the straightforward synthesis of versatile
boranils 1 and 2 (Fig. 2) based on a
D
1-pyrroline core. These com-
The development of innovative fluorescent dyes is of great sig-
nificance, given their potential applications as labels in biomedical
analysis [1], molecular sensors [2,3], and light emitting devices
[4,5]. Boron dipyrromethene (BODIPY) [6e8] and azadipyrrome-
thene (aza-BODIPY) [6,9e12] dyes (Fig. 1) have arisen as very
promising due to their outstanding chemical and photophysical
properties [8]. Their chemical versatility allowed a variety of ap-
plications covering from biolabeling [13] to solar cells [14] and
nanoparticle engineering [15]. Nevertheless, the synthesis of such
dyes remains tedious, involving linear stepwise syntheses with
relatively low global yields [11]. Recently, the search for alternative
fluorescent dyes led to a new family of fluorescent boron com-
plexes, based on N- and O-donor ligands, so-called boranils [16e22]
(Fig. 1). This kind of organoboron complexes is easily synthesized,
potentially on a large scale. They can be modified using common
reaction procedures, which allow a fine-tuning of their photo-
physical properties in order to adapt them to a given application
[11,23,24].
plexes were obtained from common precursors, and could easily be
modified to be used as scaffolds for connecting an additional
chromophoric moiety or reactive centre.
2. Material and methods
2.1. General
Melting points were measured in a Reichert Thermovar appa-
ratus fitted with a microscope and are uncorrected. NMR spectra
were recorded with Bruker DRX 300 spectrometers (300 for 1H,
75 MHz for 13C and 282 MHz for 19F), in CDCl3 as solvent, if not
stated otherwise. Chemical shifts (d) are reported in ppm and
coupling constants (J) in Hz; internal standard was residual peak of
the solvent. Unequivocal 13C assignments were made with the aid
of 2D gHSQC and gHMBC (delays for one-bond and long-range J C/H
couplings were optimised for 145 and 7 Hz, respectively) experi-
ments. Positive-ion ESI mass spectra were acquired using a Q-TOF 2
instrument [Nitrogen was used as nebuliser gas and argon as
collision gas. The needle voltage was set at 3000 V, with the ion
source at 80 ꢀC and desolvation temperature at 150 ꢀC. Cone voltage
was 35 V]. High resolution mass spectra analysis (HRMS-ESIþ) were
performed on a microTOF (focus) mass spectrometer. Ions were
* Corresponding author.
** Corresponding author. CICECO, Department of Chemistry, University of Aveiro,
3810-193 Aveiro, Portugal.
E-mail addresses: francisco.cardona@ua.pt (F. Cardona), rocha@ua.pt (J. Rocha),
artur.silva@ua.pt (A.M.S. Silva), sguieu@ua.pt (S. Guieu).
http://dx.doi.org/10.1016/j.dyepig.2014.05.026
0143-7208/© 2014 Elsevier Ltd. All rights reserved.

F. Cardona et al. / Dyes and Pigments 111 (2014) 16e20
17
and Fe (45 equiv.) successively at room temperature, and the
resulting mixture was heated at 65 ꢀC for 10 h under nitrogen at-
mosphere. After cooling down to room temperature, the reaction
mixture was filtrated through celite, rinsed with AcOEt. The whole
mixture was washed with a saturated aqueous solution of NaHCO3,
brine, dried over Na2SO4 and concentrated under reduced pressure.
The residue was purified by silica flash column chromatography
[petroleum ether/EtOAc (8:2)].
Fig. 1. BODIPY and boranil chelating complexes.
2.2.3.2. Procedure B. Compound 4, 8 or 9 (1 equiv.), zinc powder
(10 equiv.), ammonium acetate (10 equiv.) and dry methanol (so
that final concentration of Michael adduct is 0.1 mol Lꢁ1) were
added to a flask under nitrogen. The slurry was stirred at room
temperature for 12 h. Then the solid was removed by filtration and
the solvent evaporated under reduced pressure affording the crude
product, which was purified by silica flash column chromatography
[petroleum ether/EtOAc (9:1)].
generated using an ApolloII (ESI) source. Ionization was achieved by
electrospray, using a voltage of 4500 V applied to the needle, and a
counter voltage between 100 and 150 V applied to the capillary.
Elemental analyses were obtained with Carlo Erba 1108 and LECO
932 CHNS analysers. Preparative thin-layer chromatography was
performed with Merck silica gel (60 DGF254). Column chromatog-
raphy was performed with Merck silica gel 60 (70ꢁ230 mesh). All
other chemicals and solvents used were obtained from commercial
sources and were either used as received or dried by standard
procedures.
2.2.4. General procedure for the synthesis of compounds 1 and 2
The appropriate D
1-pyrroline 10 or 11 (1 equiv) was dissolved in
dry 1,2-dichloroethane (so that the concentration is 0.04 mol.Lꢁ1).
Dry triethylamine (10 equiv) was added, and the resulting solution
was stirred for 10 min at 80 ꢀC. Boron trifluoride-diethyl etherate
(18 equiv) was added dropwise and the final solution was stirred for
30 min at 80 ꢀC under nitrogen atmosphere and then cooled to
room temperature. CH2Cl2 (4 mL) was added and the crude mixture
was washed with water (3 ꢂ 2 mL). The organic layer was sepa-
rated, dried over Na2SO4 and evaporated to dryness. The residue
was purified by silica flash column chromatography [petroleum
ether: EtOAc (7:3)] to afford the pure compounds.
Compounds 3 [25], 4 [11], 5 [26], 6 [27], 7 [28], 8 [29], 9 [30],
have already been described, and their spectroscopic data were
consistent with the literature.
2.2. General procedures
2.2.1. General procedure for the synthesis of compounds 3, 6 and 7
An aqueous solution of sodium hydroxide (30%, 25 mL) was
slowly added to a methanol solution (30 mL) of the appropriate
acetophenone (5.0 mmol). After the solution had been cooled to
room temperature, benzaldehyde or cinnamaldehyde (6.0 mmol)
was added. The mixture was stirred at room temperature for 20 h
and was then poured into water (100 mL), ice (100 g), and conc.
hydrochloric acid (pH adjusted to ca. 2). The obtained solid was
removed by filtration, dissolved in dichloromethane (50 mL), and
washed with an aqueous solution of sodium hydrogen carbonate
(5%, 30 mL). The organic layer was collected and dried with anhy-
drous sodium sulfate, and the solution was concentrated to dry-
ness. The residue was recrystallized from ethanol.
2.3. Synthesis
2.3.1. 2-(2-Hydroxyphenyl)-4-phenyl-
Employing the general procedure
hydroxyphenyl)-4-nitro-3-phenylbutan-1-one
D
1-pyrroline 10
and using 1-(2-
(59 mg,
B
8
0.21 mmol), ammonium acetate (158 mg, 2.06 mmol), and zinc
powder (134 mg, 2.06 mmol) in dry methanol (1.2 mL) gave pure 2-
(2-hydroxyphenyl)-4-phenyl-
D
1-pyrroline 10 (35 mg, 0.15 mmol).
1
Yield: 60%; pale brown solid; mp 91e93 ꢀC; H NMR (300 MHz,
CDCl3)
d
7.44-7.16 (m, 7H, H-40, H-60, H-200, H-600, H-300, H-500, H-400),
7.03 (d, J ¼ 8.2 Hz, 1H, H-30), 6.86 (dd, J ¼ 7.5, 7.5 Hz, 1H, H-50), 4.53
(dd, J ¼ 16.1, 8.2 Hz, 1H, H-5), 4.13 (dd, J ¼ 16.1, 5.6 Hz, 1H, H-5),
2.2.2. General procedure for the synthesis of compounds 4, 8 and 9
To a solution of the appropriate substrate (1 equiv.) in nitro-
methane (so that final concentration is 0.3 mol.Lꢁ1) at room tem-
perature, 1,8-diazabicyclo[5.4.0]undec-7-ene (0.2 equiv.) was
added dropwise. The resulting solution was stirred for 2 h at room
temperature, and concentrated under vacuum. The residue was
purified by flash chromatography [(petroleum ether/EtOAc 9:1)] to
afford pure compounds 4, 8 and 9.
3.72e3.45 (m, 2H, H-4, H-3), 3.15 (dd, J ¼ 16.6, 5.6 Hz, 1H, H-3). 13
C
NMR (75 MHz, CDCl3) d 176.1, 160.9, 144.3, 132.5, 129.2, 128.8, 126.8,
126.7, 118.3, 117.2, 116.8, 67.7, 43.7, 41.2. ESI(þ)ꢁMS: m/z: 238.1
[MþH]þ. ESI(þ)ꢁHRMS calcd. for C16H16NO: 238.1226, found:
238.1225.
2.3.2. (E)-2-(2-Hydroxyphenyl)-4-styryl-
Employing the general procedure
hydroxyphenyl)-3-(nitromethyl)-5-phenylpent-4-en-1-one
D
1-pyrroline 11
and using (E)-1-(2-
B
2.2.3. General procedure for the synthesis of compounds 5, 10 and
11
9
(36 mg, 0.11 mmol), ammonium acetate (86 mg, 1.12 mmol), and
zinc powder (73 mg, 1.12 mmol) in dry methanol (1.2 mL) gave pure
2.2.3.1. Procedure A. To a stirred solution of nitro derivative 4, 8 or
9 (1 equiv.) in THF/MeOH (2/1) were added acetic acid (16 equiv.)
(E)-2-(2-hydroxyphenyl)-4-styryl-
D
1-pyrroline
11
(20
mg,
0.076 mmol). Yield: 68%; pale green oil; 1H NMR (300 MHz, CDCl3)
d
13.71 (s,1H, OH), 7.58e7.14 (m, 7H, H-40, H-60, H-200, H-600, H-300, H-
500, H-400), 7.02 (d, J ¼ 8.2 Hz,1H, H-30), 6.87 (dd, J ¼ 7.5, 7.5 Hz,1H, H-
50), 6.49 (d, J ¼ 15.8 Hz, 1H, H-
b
), 6.22 (dd, J ¼ 15.8, 8.2 Hz, 1H, H-
a),
4.34 (dd, J ¼ 16.6, 7.6 Hz, 1H, H-5), 3.93 (dd, J ¼ 16.6, 5.4 Hz, 1H, H-
5), 3.45e3.15 (m, 2H, H-4, H-3), 3.01e2.89 (m, 1H, H-3). 13C NMR
(75 MHz, CDCl3)
d 176.4, 160.9, 136.9, 132.5, 131.4, 130.6, 129.3,
128.6, 127.5, 126.1, 118.3, 117.2, 116.9, 65.5, 41.6, 39.8. ESI(þ)ꢁMS: m/
z: 264.1 [MþH]þ. Anal. Calcd for C18H17NO: C 82.10, H 6.51, N 5.32;
found: C 82.25, H 6.82, N 5.38%.
Fig. 2.
D
1-Pyrroline based boranils 1 and 2.

18
F. Cardona et al. / Dyes and Pigments 111 (2014) 16e20
2.3.3. 5,5-difluoro-2-phenyl-1,2,3,5-tetrahydrobenzo[e]pyrrolo[1,2-
c][1,3,2]oxazaborinin-4-ium-5-uide 1
Employing the general procedure and using 2-(2-
hydroxyphenyl)-4-phenyl-D
1-pyrroline 10 (70 mg, 0.295 mmol),
triethylamine (299 mg, 0.41 mL, 2.95 mmol), and boron trifluoride-
diethyl etherate (754 mg, 0.65 mL, 5.31 mmol) in dry 1,2-
dichloroethane (1.5 mL) gave pure 5,5-difluoro-2-phenyl-1,2,3,5-
tetrahydrobenzo[e]pyrrolo[1,2-c][1,3,2]oxazaborinin-4-ium-5-uide
1 (79 mg, 0.277 mmol). Yield: 94%; white solid; mp 123e125 ꢀC; 1H
NMR (300 MHz, CDCl3)
d
7.57 (ddd, 1H, J ¼ J ¼ 8.5, 7.5, 1.2 Hz, H-40),
Fig. 3. Molecular structure of compound 4 (only one enantiomer is shown for clarity)
(a) and unit cell content (b). Thermal ellipsoids are shown at the 50% probability level,
hydrogen atoms are shown with an arbitrary radius (0.30Å). C, grey; O, red; N, blue; H,
white. (For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
7.50-7.19 (m, 6H, H-60, H-200, H-600, H-300, H-500, H-400), 7.12 (d,
J ¼ 8.5 Hz, 1H, H-30), 6.95 (dd, J ¼ 7.5, 7.5 Hz, 1H, H-50), 4.58 (dd,
J ¼ 15.0, 8.2 Hz, 1H, H-5), 4.27 (dd, J ¼ 15.0, 5.7 Hz, 1H, H-5),
3.96e3.78 (m, 2H, H-4, H-3), 3.41 (dd, J ¼ 18.1, 5.7 Hz, 1H, H-3). 13
C
NMR (75 MHz, CDCl3)
d
175.8, 159.3, 140.8, 137.9, 129.2 (2 ꢂ C),
128.9, 127.7, 126.7 (2 ꢂ C), 119.8, 119.7, 112.9, 60.3, 42.0, 39.9. 19F
Michael addition of nitromethane to chalcone 3 was attempted
using two different kinds of base sources, a secondary amine
(Et2NH) and an amidine (diazabicyclo[5.4.0]undec-7-ene). This
second one gave the better yields. The structure of the racemic
product 4 was confirmed by single crystal X-ray diffraction (Fig. 3).
The reduction of the nitro group could lead to side reactions, due
to the presence of a ketone and to the in situ formation of an imine.
To accomplish this reduction two experimental conditions already
presented in the literature were tested [31,32]: i) zinc, ammonium
acetate, palladium on carbon (10%w/w), MeOH, 12 h, rt; and ii) iron,
acetic acid, THF:MeOH/(2:1), 12 h, 65 ꢀC. The best results were
obtained using iron as the reducing agent and acetic acid as the
NMR (282 MHz, CDCl3)
d
ꢁ138.03 (dq, J ¼ 29.5, 14.4 Hz, 1F), -138.68
(dq, J ¼ 29.5,14.4 Hz,1F). ESI(þ)-MS: m/z: 266.1 [MꢁF]þ. Anal. Calcd
for C18H17NO: C 67.41, H 4.95, N 4.91; found: C 67.26, H 5.03, N
5.02%.
2.3.4. (E)-5,5-difluoro-2-styryl-1,2,3,5-tetrahydrobenzo[e]pyrrolo
[1,2-c][1,3,2]oxazaborinin -4-ium-5-uide 2
Employing the general procedure and using (E)-2-(2-
hydroxyphenyl)-4-styryl-D
1-pyrroline 11 (42 mg, 0.159 mmol),
triethylamine (161 mg, 0.22 mL, 1.595 mmol), and boron
trifluoride-diethyl etherate (408 mg, 0.35 mL, 2.87 mmol) in dry
1,2-dichloroethane (1.0 mL) gave pure (E)-5,5-difluoro-2-styryl-
1,2,3,5-tetrahydrobenzo[e]pyrrolo[1,2-c][1,3,2]oxazaborinin-4-
ium-5-uide 2 (45 mg, 0.143 mmol). Yield: 90%; pale brown solid;
proton source. D
1-Pyrroline derivative 5 was obtained as a racemic
mixture in 77% yield.
The boron complexes 1 and 2 were obtained following this
optimized strategy (Scheme 2). Michael addition of nitromethane
to 20-hydroxychalcone 6 [30] or 20-hydroxycinamylideneaceto
phenone 7 [26] using DBU as base gave the corresponding Michael
adducts 8 and 9 in nearly quantitative yield [33].
mp 129e131 ꢀC; 1H NMR (300 MHz, CDCl3)
d
7.59 (ddd, J ¼ 8.4, 7.4,
1.2 Hz, 1H, H-40), 7.43 (dd, J ¼ 7.8, 1.2 Hz, 1H, H-60), 7.42-7.22 (m, 5H,
H-200, H-600, H-300, H-500, H-400), 7.14 (d, J ¼ 8.4 Hz, 1H, H-30), 6.97 (dd,
J ¼ 7.4, 7.8 Hz, 1H, H-50), 6.58 (d, J ¼ 15.7 Hz, 1H, H-
b), 6.21 (dd,
For the synthesis of
D
1-pyrroline derivatives 10 and 11, the
J ¼ 15.7, 8.1 Hz,1H, H- ), 4.44 (dd, J ¼ 14.8, 8.2 Hz,1H, H-5), 4.08 (dd,
a
reduction of the nitro group followed by nucleophilic intra-
molecular ring cyclization/dehydration sequence was attempted
using the iron/acetic acid strategy. Unfortunately this strategy was
not the best for these derivatives. The reduction with zinc and
ammonium acetate in methanol gave better yields and an easier
purification. The use of palladium was avoided due to the potential
reduction of the double bond present in compound 11. The corre-
J ¼ 14.8, 6.5 Hz, 1H, H-5), 3.77-3.46 (m, 2H, H-4, H-3), 3.24 (dd,
J ¼ 17.8, 6.0 Hz, 1H, H-3). 13C NMR (75 MHz, CDCl3)
d 176.0, 159.3,
137.9, 135.9, 132.8, 128.9, 128.7 (2 ꢂ C), 128.1, 127.7, 126.3 (2 ꢂ C),
119.9, 119.7, 113.0, 58.7, 40.3, 38.5. 19F NMR (282 MHz, CDCl3)
d
ꢁ161.25 (dq, J ¼ 29.3, 14.2 Hz), -161.82 (dq, J ¼ 28.3, 14.2 Hz).
ESI(þ)ꢁMS: m/z: 292.2 [MꢁF]þ. ESI(þ)ꢁHRMS calcd. for
C
18H16BFNO: 292.1307; found: 292.1303.
sponding
D
1-pyrrolines 10 and 11 were obtained in 60% and 68%
yields, respectively.
3. Results and discussion
Boron complexation was achieved using BF3eOEt2 and anhy-
drous triethylamine in 1,2-dichloroethane at 40 ꢀC, to give com-
plexes 1 and 2 in 94% and 90% yield, respectively. The structure of
the complexes was confirmed by NMR, MS, and elemental analysis.
3.1. Synthesis and characterization
The synthesis of boranils 1 and 2 involved the preparation of the
D
1-pyrroline core. As this synthesis proved to be temperamental it
was first optimized on a model compound (Scheme 1).
Scheme 1. Optimization of the conditions on a model compound.
Scheme 2. Synthesis of the boranil derivatives 1 and 2.

F. Cardona et al. / Dyes and Pigments 111 (2014) 16e20
19
Schiff-base ligands 10 and 11 present an absorption band cen-
tred at ca. 310 nm. The absorption spectra of complexes 1 and 2
exhibit a major absorption band at ca. 340 nm. The coordination of
10 and 11 with boron seems to have a bathochromic effect on the
absorption properties, without interfering much with their in-
tensities. Even if some Schiff-base containing compounds have
been reported to be luminescent, ligands 10 and 11 are non-
emissive in solution. On the contrary, boron complexes 1 and 2
are photoluminescent with a maximum emission at 410 nm, with
low quantum yields of 2.6%. The introduction of a fluoroborate
centre probably restricts the non-emissive relaxation pathways,
such as internal charge transfer and torsional vibrations, thus
promoting the fluorescence. The excitation spectra of 1 and 2 match
their absorption spectra, and are nearly mirror images of their
emission spectra, which indicate that the emission happens
through fluorescence from S1 to S0 energy levels. The conjugation
between the boron complex and the phenyl ring or the styrene is
Fig. 4. Molecular structure of compound 1 (only one enantiomer is shown for clarity)
(a) and unit cell content (b). Thermal ellipsoids are shown at the 50% probability level,
hydrogen atoms are shown with an arbitrary radius (0.30Å). C, grey; O, red; N, blue; H,
white; B, pink; F, yellow. (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
Table 1
Selected Optical data measured in dichloromethane.
disrupted by the
extra double bond has almost no effect on the absorption and
D
1-pyrroline core, thus the introduction of the
a
Dye
labs (nm)
ε(Mꢁ1 cmꢁ1
)
lem (nm)b
4
f (%)c
10
11
1
313
314
343
341
5300
7300
4000
5700
n.a.
n.a.
410
409
<0.1
<0.1
2.6
emission properties of the fluorophores.
4. Conclusion
2
2.6
a
ε (Mꢁ1 cmꢁ1) is determined by linear regression of 4 measurements in the range
In conclusion, the synthesis of D
1-pyrrolines boranil complexes
10-4 M to 10-6 M in dichloromethane.
b
has been successfully achieved, leading to new fluorophores in a
straightforward manner and with high overall yields. Efforts to
extend their emission to the red part of the electromagnetic spec-
trum are currently underway. In order to tune their interaction with
biological systems, an enantioselective version of these new fluo-
rophores is also being developed in our laboratory.
Excitation at 340 nm.
Quantum yields are determined with excitation at 340 nm, by linear regression
c
of 4 measurements in the range of absorption 0 to 0.2, by comparison with fluo-
rescein (quantum yield 0.90 at excitation 470 nm in a solution of NaOH 0.01 M in
water).
Complex 1 gave single crystals by slow evaporation of a satu-
rated solution in dichloromethane (Fig. 4). Analysis of the crystal
structure confirmed the structure of the compound, with the
planarity of the 6-membered ring containing the boron atom. The
Acknowledgement
Thanks are due to the University of Aveiro and the Portuguese
D
1-pyrroline core disrupts the conjugation between the emissive
~
^
Fundaçao para a Ciencia e a Tecnologia (FCT) for funding the
Organic Chemistry Research Unit (project PEst-C/QUI/UI0062/
2013), the CICECO Associate Laboratory (PEst-C/CTM/LA0011/
2013) and the Portuguese National NMR Network (RNRMN). SG
also thanks the FCT for a postdoctoral grant (SFRH/BPD/70702/
2010).
centre and the phenyl ring, which should prevent any bath-
ochromic effect.
3.2. Absorption and emission properties
Photophysical data were recorded in dichloromethane for all the
Appendix A. Supplementary data
new fluorophores (Table 1 and Fig. 5).
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2014.05.026. CCDC-990043 and CCDC-
990044 contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge at www.ccdc.cam.a
c.uk/conts/retrieving.html (or from the Cambridge Crystallographic
Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK. Fax: C44 1223
336 033. E-mail: deposit@ccdc.cam.ac.uk).
References
[1] Killoran J, Allen L, Gallagher JF, Gallagher WM, O'shea DF. Synthesis of BF2
chelates of tetraarylazadipyrromethenes and evidence for their photodynamic
therapeutic behaviour. Chem Commun; 2002:1862e3.
[2] Beer G, Niederalt C, Grimme S, Daub J. Redox switches with chiroptical signal
expression based on binaphthyl Boron dipyrromethene conjugates. Angew
Chem Int Ed 2000;3:3252e5.
[3] Yamaguchi S, Akiyama S, Tamao K. Colorimetric fluoride ion sensing by boron-
containing
p-electron systems. J Am Chem Soc 2001;123:11372e5.
[4] Wang S. Luminescence and electroluminescence of Al(III), B(III), Be(II) and
Zn(II) complexes with nitrogen donors. Coord Chem Rev 2001;215:79e98.
[5] Liu Y, Guo J, Zhang H, Wang Y. Highly efficient white organic electrolumi-
nescence from a double-layer device based on a boron hydroxyphenylpyr-
idine complex. Angew Chem Int Ed 2002;41:182e4.
Fig. 5. Absorption (0.1 mM) spectra of dyes 10, 11, 1 and 2, normalized excitation
(0.001 mM, emission at 410 nm) and normalized emission (0.001 mM, excitation at
340 nm) spectra of dyes 1 and 2 in dichloromethane.
[6] Loudet A, Burgess K. BODIPY dyes and their derivatives: syntheses and spec-
troscopic properties. Chem Rev 2007;107:4891e932.

20
F. Cardona et al. / Dyes and Pigments 111 (2014) 16e20
[7] Boens N, Leen V, Dehaen W. Fluorescent indicators based on BODIPY. Chem
[21] Frath D, Azizi S, Ulrich G, Ziessel R. Synthesis of luminescent 2-(20-Hydrox-
yphenyl)benzoxazole (HBO) borate complexes. Org Lett 2012;14:4774e7.
[22] Frath D, Azizi S, Ulrich G, Retailleau P, Ziessel R. Facile synthesis of highly
fluorescent boranil complexes. Org Lett 2011;13:3414e7.
Soc Rev 2012;41:1130e72.
[8] Ulrich G, Ziessel R, Harriman A. The chemistry of fluorescent bodipy dyes:
versatility unsurpassed. Angew Chem Int Ed 2008;47:1184e201.
[9] Rogers MAT. Tetra-arylazadipyrromethines: a new class of synthetic colouring
matter. Nature 1943;151. 504e504.
[23] Jameson LP, Dzyuba SV. Aza-BODIPY: improved synthesis and interaction
with soluble Ab1e42 oligomers. Bioorg Med Chem Lett 2013;23:1732e5.
[10] Rogers MAT. 2,4-Diarylpyrroles. Part I. Synthesis of 2,4-diarylpyrroles and
2,20,4,40-tetra-arylazadipyrromethines. J Chem Soc (Resumed); 1943:590e6.
[11] Gorman A, Killoran J, O'Shea C, Kenna T, Gallagher WM, O'Shea DF. In vitro
demonstration of the heavy-atom effect for photodynamic therapy. J Am
Chem Soc 2004;126:10619e31.
[24] Tasior M, O'Shea DF. BF2-Chelated tetraarylazadipyrromethenes as NIR fluo-
rochromes. Bioconjug Chem 2010;21:1130e3.
[25] Liu D-N, Tian S-K. Stereoselective synthesis of polysubstituted alkenes
through a phosphine-mediated three-component system of aldehydes, a-Halo
carbonyl compounds, and terminal alkenes. Chem A Eur J 2009;15:4538e42.
[12] Grossi M, Palma A, McDonnell SO, Hall MJ, Rai DK, Muldoon J, et al. Unrav-
elling the reaction mechanism of the reductive ring contraction of 1,2-
pyridazines. J Org Chem 2012;77:9304e12.
[13] Wu L, Loudet A, Barhoumi R, Burghardt RC, Burgess K. Fluorescent cassettes
for monitoring three-component interactions in vitro and in living cells. J Am
Chem Soc 2009;131:9156e7.
[26] Liang Y, Dong D, Lu Y, Wang Y, Pan W, Chai Y, et al. One-pot synthesis of
substituted
D
1-pyrrolines through the Michael addition of nitroalkanes to
chalcones and subsequent reductive cyclization in aqueous media. Synthesis;
2006:3301e4.
[27] Krohn K, Ahmed I, John M. Enantioselective synthesis of flavan-3-ols using a
mitsunobu cyclization. Synthesis; 2009:779e86.
[14] Rousseau T, Cravino A, Ripaud E, Leriche P, Rihn S, De Nicola A, et al. A tailored
hybrid BODIPYeoligothiophene donor for molecular bulk heterojunction solar
cells with improved performances. Chem Commun 2010;46:5082e4.
[15] Tokoro Y, Nagai A, Chujo Y. Nanoparticles via H-aggregation of amphiphilic
BODIPY dyes. Tetrahedron Lett 2010;51:3451e4.
[28] Silva AMS, Pinto DCGA, Tavares HR, Cavaleiro JAS, Jimeno ML, Elguero J. Novel
(E)- and (Z)-2-Styrylchromones from (E,E)-20-Hydroxycinnamylideneaceto
phenones e xanthones from daylight photooxidative cyclization of (E)-2-
Styrylchromones. Eur J Org Chem; 1998:2031e8.
[29] Loudet A, Bandichhor R, Burgess K, Palma A, McDonnell SO, Hall MJ, et al. B,O-
chelated azadipyrromethenes as near-IR probes. Org Lett 2008;10:4771e4.
[30] Oliva CG, Silva AMS, Resende DISP, Paz FAA, Cavaleiro JAS. Highly enantio-
selective 1,4-Michael additions of nucleophiles to unsaturated aryl ketones
with organocatalysis by bifunctional cinchona alkaloids. Eur J Org Chem;
2010:3449e58.
[16] Rao Y-L, Wang S. Four-coordinate organoboron compounds with a
p-conju-
gated chelate ligand for optoelectronic applications. Inorg Chem 2011;50:
12263e74.
[17] Massue J, Frath D, Ulrich G, Retailleau P, Ziessel R. Synthesis of luminescent 2-
(20-Hydroxyphenyl)benzoxazole (HBO) borate complexes. Org Lett 2011;14:
230e3.
[18] Zhou Y, Kim JW, Kim MJ, Son W-J, Han SJ, Kim HN, et al. Novel bi-nuclear
boron complex with pyrene ligand: red-light emitting as well as electron
transporting material in organic light-emitting diodes. Org Lett 2010;12:
1272e5.
[31] Kawai H, Yuan Z, Kitayama T, Tokunaga E, Shibata N. Efficient access to tri-
fluoromethyl diarylpyrrolines and their N-Oxides through enantioselective
conjugate addition of nitromethane to b,b-disubstituted enones. Angew Chem
Int Ed 2013;52:5575e9.
[32] Guo S, Xie Y, Hu X, Huang H. Highly diastereo- and enantioselective tandem
reaction toward functionalized pyrrolidines with multiple stereocenters. Org
Lett 2011;13:5596e9.
[19] Zhang Z, Bi H, Zhang Y, Yao D, Gao H, Fan Y, et al. Luminescent boron-
contained ladder-type
p-conjugated compounds. Inorg Chem 2009;48:
7230e6.
[33] Oliva CG, Silva AMS, Paz FAA, Cavaleiro JAS. Highly enantioselective and
regioselective conjugate addition of nitromethane to 1,5-Diarylpenta-2,4-
dien-1-ones using bifunctional cinchona organocatalysts. Synlett; 2010:
1123e7.
[20] Tokoro Y, Nagai A, Chujo Y. Synthesis of
p
-conjugated polymers containing
organoboron Benzo[h]quinolate in the main chain. Macromolecules 2010;43:
6229e33.