Electronic and Functional Scope of Boronic Acid Derived Salicylidenehydrazone (BASHY) Complexes as Fluorescent Dyes

Article abstract of DOI:10.1021/acs.joc.7b00601

A series of boronic acid derived salicylidenehydrazone (BASHY) complexes was prepared and photophysically characterized. The dye platform can be modified by (a) electronic tuning along the cyanine-type axis via modification of the donor-acceptor pair and (b) functional tuning via the boronic acid residue. On the one hand, approach (a) allows the control of photophysical parameters such as Stokes shift, emission color, and two-photon-absorption (2PA) cross section. The resulting dyes show emission light-up behavior in nonpolar media and are characterized by high fluorescence quantum yields (ca. 0.5-0.7) and brightness (ca. 35000-40000 M^-1 cm^-1). Moreover, the 2PA cross sections reach values in the order of 200-300 GM. On the other hand, the variation of the dye structure through the boronic acid derived moiety (approach (b)) enables the functionalization of the BASHY platform for a broad spectrum of potential applications, ranging from biorelevant contexts to optoelectronic materials. Importantly, this functionalization is generally electronically orthogonal with respect to the dye's photophysical properties, which are only determined by the electronic structure of the cyanine-type backbone (approach (a)). Rare exceptions to this generalization are the presence of redox-active residues (such a triphenylamine or pyrene). Finally, the advantageous photophysics is complemented by a significant photostability.

Full text of DOI:10.1021/acs.joc.7b00601

Article
pubs.acs.org/joc
Electronic and Functional Scope of Boronic Acid Derived
Salicylidenehydrazone (BASHY) Complexes as Fluorescent Dyes
María M. Alcaide,† Fabio M. F. Santos,‡ Vania F. Pais,† Joana Ines Carvalho,‡ Daniel Collado,§,∥
̂
̂
Ezequiel Perez-Inestrosa,§,∥ Jesus F. Arteaga,† Francisco Bosca,⊥ Pedro M. P. Gois,*,‡
́
́
́
and Uwe Pischel*,†
†CIQSO − Center for Research in Sustainable Chemistry and Department of Chemistry, University of Huelva, Campus de El Carmen
s/n, E-21071 Huelva, Spain
‡Research Institute for Medicines (iMed.ULisboa), Faculty of Pharmacy, Universidade de Lisboa, 1600-276 Lisbon, Portugal
§Department of Organic Chemistry, University of Mal
́
aga, IBIMA, Campus Teatinos s/n, E-29071 Mal
́
aga, Spain
∥Andalusian Center for Nanomedicine and Biotechnology − BIONAND, Parque Tecnolog
́
ico de Andalucía, E-29590 Malaga, Spain
́
⊥Institute of Chemical Technology (CSIC-UPV), Polytechnical University of Valencia, Av. de los Naranjos s/n, E-46022 Valencia,
Spain
S
* Supporting Information
ABSTRACT: A series of boronic acid derived salicylidenehydrazone (BASHY)
complexes was prepared and photophysically characterized. The dye platform
can be modified by (a) electronic tuning along the cyanine-type axis via
modification of the donor−acceptor pair and (b) functional tuning via the
boronic acid residue. On the one hand, approach (a) allows the control of
photophysical parameters such as Stokes shift, emission color, and two-photon-
absorption (2PA) cross section. The resulting dyes show emission light-up
behavior in nonpolar media and are characterized by high fluorescence quantum
yields (ca. 0.5−0.7) and brightness (ca. 35000−40000 M−1 cm−1). Moreover,
the 2PA cross sections reach values in the order of 200−300 GM. On the other
hand, the variation of the dye structure through the boronic acid derived moiety (approach (b)) enables the functionalization of
the BASHY platform for a broad spectrum of potential applications, ranging from biorelevant contexts to optoelectronic
materials. Importantly, this functionalization is generally electronically orthogonal with respect to the dye’s photophysical
properties, which are only determined by the electronic structure of the cyanine-type backbone (approach (a)). Rare exceptions
to this generalization are the presence of redox-active residues (such a triphenylamine or pyrene). Finally, the advantageous
photophysics is complemented by a significant photostability.
variable component, without having to worry about unwanted
modifications of the photophysical properties.
INTRODUCTION
■
The development of fluorescent dyes that correspond to the
demands of optoelectronic or biomedical applications is an
evergreen research topic at the intersection of synthetic
chemistry and photosciences. There have been considerable
efforts to improve archetypal fluorophore platforms such as
fluoresceins,1−3 rhodamines,1−4 and Bodipy dyes2,5−13 with
respect to their photophysical performance, e.g., excitation with
visible light, high emission quantum yields, and fluorescence at
wavelengths larger than 500 nm. The applied strategies imply
often the cumbersome redesign of the synthetic route to
achieve modifications of the skeleton itself or its substitution
pattern. Unfortunately, frequently the modification of func-
tional aspects such as the introduction of a recognition motif or
bio-orthogonal handles may also influence the electronic
properties of the dye in an unwanted manner. It is therefore
desirable to get hands on a platform where the same ligand
system could give rise to fluorescent dyes with a large molecular
diversity by means of a straightforward assembly reaction with a
Boron(III) architectures, especially as tetracoordinated
complexes, have enjoyed special attention for the design of
fluorescent dyes,14−28 not just limited to perhaps the most
widely known case of the Bodipy dye family.2,5−13 Building on
the rich coordination chemistry of boronic acids,28−34 we have
recently developed a new fluorophore platform that can be
accessed by a diversity-oriented synthetic strategy.35 The dyes
rely on the condensation of boronic acids (BAs) with a
salicylidenehydrazone (SHY) ligand system, leading to BASHY
dyes with intriguing photophysical properties and applications
in bioimaging.35,36 In this work we explore the photophysical
scope of the dyes through the variation of both the push−pull
charge-transfer character at the ligand backbone and the
organic residue that is introduced with the boronic acid
component. The orthogonal electronic and functional tuning
Received: March 13, 2017
© XXXX American Chemical Society
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(see Scheme 1) and the ease of preparation, employing
standard reactions with only minimal requirements for
hydrazones 15a−d and 4-diethylaminosalicylaldehyde, follow-
ing a described procedure (see Experimental Section).35 The
dyes were characterized for their purity and identity by 1H, 11B,
and 13C NMR spectroscopy as well as by elemental analysis or
high-resolution mass spectrometry. The tetracoordinate nature
of the dye was confirmed by 11B NMR, yielding signals in the
range of 3−6 ppm that are in accordance with sp3 boron; see
Experimental Section and Supporting Information for charac-
terization data.
Scheme 1. Representation of the Structural Diversity of
BASHY Dyes
One of the aims of this study was to explore the functional
diversity of the BASHY platform by means of variations of the
boronic acid derived moiety (R2 in Chart 1 and Scheme 2). The
triphenylamine moiety, installed in dye 1, is a known strong
electron donor. The opposite electronic situation is encoun-
tered for dye 7, containing a strongly electron-accepting
pentafluorophenyl residue, also known for its importance as
fluorous tag. Dye 5 features a phenylsulfonamide, a key
structural motif of carbonic anhydrase inhibitors. Nitrogen-
containing heterocyclic moieties such as the 4-isoquinolinyl
(dye 4) or the 4-pyridyl (dye 6) were chosen for their possible
implication in metal−ligand assemblies. Fluorene is a
prominent chromophore in organic light-emitting diodes,
motivating the preparation of the dyes 3 and 8. The integration
of these diverse motifs generally does not alter the photo-
physical properties of the electronically orthogonal boron
salicylidenehydrazone backbone. Finally, the electronic demand
of the distant phenyl substituent was varied through R1 (R1 =
H, CF3, CN, NO2; see Chart 1). This provides a way to control
the fluorescence properties by means of the degree of
intramolecular charge transfer (ICT) along the cyanine-type
backbone in 9−12.40
Photophysical Properties of the Dyes 1−8. The photo-
physical data of the dyes in air-equilibrated toluene and
acetonitrile solutions are summarized in the Supporting
Information, and example spectra in toluene are shown in the
Figures 1 and 2. The dyes 3−8 feature very similar UV−vis
absorption spectra with a maximum at about 465−475 nm and
high molar absorption coefficients (generally 50,000−60,000
M−1 cm−1; except for the dimeric BASHY 8, for which values as
high as 121,000 M−1 cm−1 were observed) both in toluene and
additional purification, make BASHY dyes an interesting target.
The structures of the investigated dyes (1−12) are shown in
Chart 1.
The obtained dyes have very advantageous fluorescence
properties including high emission quantum yield and bright-
ness that are matched with pronounced solvatochromic effects
and an good photochemical stability. The combination of these
attributes is lacking for many dyes, for example, most often
solvatochromic dyes are characterized by relatively low
brightness and moderate photochemical stability.37 In addition,
we show herein for the first time that BASHY dyes are very
efficient two-photon-absorbing chromophores with cross
sections that are larger than those of traditional dyes used in
multiphoton-excitation confocal fluorescence microscopy.38,39
RESULTS AND DISCUSSION
■
Synthesis and Scope. The synthesis of fluorescent BASHY
dyes permits the flexible integration of a large variety of
moieties in the last step of the reaction, consisting in the
condensation of a boronic acid component with a non-
fluorescent salicylidenehydrazone ligand (see Scheme 2).35
This reaction is facile, straightforward, and often column
chromatography is not required to obtain pure products in high
yields (generally >96%). Attending the requirements on a
push−pull charge-transfer architecture, we selected a ligand
system (16a−d) with a strong electron-donating N,N-
diethylamino group. The ligands are accessible from the
a
Chart 1. Structures of the Dyes 1−12
a
Note that the formal charges in the BASHY structures have been omitted for the sake of clarity.
B
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Scheme 2. Synthesis of the Dyes 1−12
acetonitrile, corroborates the at least partial involvement of ICT
in the excited state, as has been suggested recently by quantum-
chemical calculations of BASHY dyes.40 Also the emission
quantum yields (Φfluo) and lifetimes (τfluo) show parallel trends
in dependence on the solvent. For toluene high quantum yields
of about 0.5−0.7 were observed, while in acetonitrile, a drop to
values of about 0.05−0.12 was noted. The fluorescence of
BASHY dyes is not quenched by oxygen, as shown in quantum
yield measurements with nitrogen-purged solutions of the
representative dyes 2, 5, and 9 (quantum yields remain the
same within an error margin of 10%). The fluorescence
lifetimes are situated at ca. 2.4−3.0 ns in toluene and are
significantly shorter in acetonitrile (ca. 0.3−0.5 ns). The
proportionality between Φfluo and τfluo points to a rather
solvent-independent radiative rate constant (kr = Φfluo/τfluo),
being generally in the order of 108 s−1 for the herein
investigated series (see Supporting Information). However,
the nonradiative deactivation [knr = (1 − Φfluo)/τfluo] is by 1
order of magnitude faster in acetonitrile as compared to toluene
(ca. 109 s−1 versus 108 s−1; see Supporting Information). This
may be reasoned with the energy-gap law, predicting a faster
S1−S0 deactivation for energetically lower lying excited singlet
states. An alternative explanation could be a more efficient
intersystem crossing to a close lying excited triplet state (see
laser-flash photolysis study of dye 2 in Supporting Informa-
tion). However, a recent theoretical study on BASHY
complexes, including 9 and 12, revealed that this is not a
general photophysical pattern of these dyes.40 These joint
observations demonstrate that in general the electronic
properties of the BA-derived aromatic moiety (R2; see Scheme
1) have only minor influence on the photophysical properties
of the BASHY dye. Gratifyingly, this allows the orthogonal
assembly of BASHY dyes, where the photophysical properties
are practically only determined by the electronic properties of
the boron salicylidenehydrazone backbone (see below for the
dyes 9−12). The invariability of the electronic structure of the
BASHY backbone was confirmed by electrochemical measure-
ments. The reduction of the BASHY core proceeded at peak
potentials that are located for most of the dyes at ca. − 1.1 V
(in dichloromethane); see Supporting Information.
Figure 1. UV−vis absorption (black) and fluorescence spectra (red) of
the dyes 6 (a), 7 (b), and 9 (c) in air-equilibrated toluene. The
fluorescence spectra were obtained by excitation at the maximum of
the long-wavelength absorption band.
Figure 2. Normalized UV−vis absorption (solid lines) and
fluorescence spectra (dashed lines) of dye 9 (black), 10 (blue), 11
(green), and 12 (red) in air-equilibrated toluene.
acetonitrile. The fluorescence emission maximum is located at
505−510 nm in toluene and at 540−545 nm in acetonitrile.
The bathochromic shift of the emission, observed for the polar
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The brightness (ε × Φfluo) of the dyes is especially
pronounced in nonpolar media, reaching values of up to
37,000 M−1 cm−1 (for dye 6) or even 76,000 M−1 cm−1 for the
dimeric BASHY dye 8 in toluene. These values are comparable
to or even larger than those of bright-emitting traditional dyes,
such as fluoresceins, rhodamines, cyanines, boron dipyrrome-
thene (Bodipy) dyes, or 7-nitrobenzooxadiazole (NBD) ICT
dyes (see Supporting Information for a comparison of the
photophysical properties of BASHY dyes with NBD or
Bodipy).2
However, there are exceptions which are reflected in the
observations that were made for the dyes 1 and 2. These dyes
show very low fluorescence quantum yields in toluene (≤0.02)
and are essentially nonfluorescent in acetonitrile. It is proposed
that photoinduced electron transfer interferes, either in the
excited singlet or triplet state. The involvement of an excited
triplet state, populated via intersystem crossing, was shown for
the example of dye 2 by performing nanosecond laser-flash
photolysis experiments (see Supporting Information).
It is often observed that the photostability of solvatochromic
dyes is considerably reduced in nonpolar solvents, implying
triplet state formation and thereby photooxidations.37 For this
reason we tested this practically relevant aspect for some
selected BASHY dyes in long-term irradiations of air-
equilibrated toluene solutions with a 150 W Xe-lamp, exciting
at >455 nm. As judged from the changes in the UV−vis
absorption spectrum, the dyes 1, 3, 9, 11, and 12 showed very
minor decomposition (6% for 1, 8% for 3, and 3% for 9, 11,
12) after 1 h of irradiation under comparable conditions. This
very reduced photoreactivity is in accordance with the high
fluorescence quantum yield in toluene. As an exception, dye 2
showed a pronounced tendency to photodecompose in air-
equilibrated solution (61% after 1 h of irradiation); see
Supporting Information.
One- and Two-Photon Absorption Properties of the Dyes
9−12. As discussed above, there is no bond-mediated
electronic interaction between the BA-derived residue and the
boron salicylidenehydrazone skeleton (except for dyes 1 and
2). However, along the conformationally locked π-conjugated
cyanine-type axis such communication is possible, especially
when the extremes are substituted with an electron donor/
acceptor (D/A) pair. This is manifested for the dyes 9−12,
featuring the NEt2 donor moiety and a distant phenyl ring with
varying electron-accepting character. In line with this is the
observation that incremented electron-acceptor strength of the
phenyl para-substituent [Hammett constants σpara: 0 (H), 0.54
(CF3), 0.66 (CN), 0.78 (NO2)]41 leads to increased bath-
ochromic shifts of the absorption spectrum which is even more
pronounced for the fluorescence spectrum (see Figure 2).
Thus, on moving from dye 9 to dye 12 (containing the
strongest electron-withdrawing substituent in the series), a
bathochromic shift of the emission band by 27 nm was noted in
toluene and even by 92 nm in acetonitrile. Again, this solvent
dependence is in line with the proposed ICT character of the
emissive state.40 The quantum yields remain high for toluene
throughout this dye series (Φfluo ca. 0.55−0.65), but show a
pronounced drop in polar solvents such as acetonitrile (Φfluo ca.
0.05−0.08 for 9−11 and <0.01 for 12). This is in accordance
with the observations made for the dyes 3−8 (see above). With
the intention to showcase the solvatochromic character37 of
BASHY dyes, the fluorescence of 10−12 was investigated in a
series of solvents (toluene, chloroform, tetrahydrofuran, ethyl
acetate, acetone, acetonitrile, and dimethylformamide. All dyes
showed gradual bathochromic shifts of the emission spectrum
and a decrease of the emission quantum yield upon increasing
the solvent polarity (see Table 1 and Figure 3 as well as
Table 1. Optical Properties of 10, 11, and 12 in Different
Solvents
λabs,max (nm)
λfluo,max (nm)
Φfluo
Dye 10
toluene
478
484
475
478
477
476
481
516
525
534
537
552
559
566
0.65
0.68
0.27
0.30
0.09
0.05
0.08
chloroform
ethyl acetate
tetrahydrofuran
acetone
acetonitrile
dimethylformamide
Dye 11
toluene
484
492
481
483
483
482
482
525
537
546
549
567
577
567
0.57
0.56
0.25
0.28
0.09
0.05
0.07
chloroform
ethyl acetate
tetrahydrofuran
acetone
acetonitrile
dimethylformamide
Dye 12
toluene
489
496
486
489
488
488
494
535
555
577
579
622
632
636
0.55
0.47
chloroform
ethyl acetate
tetrahydrofuran
acetone
0.16
0.23
<0.01
<0.01
<0.01
acetonitrile
dimethylformamide
Supporting Information). The corresponding Lippert−Mataga
plots42−44 confirm the increasing push−pull character in the
order 10 < 11 < 12 (see Supporting Information).
Recent time-dependent density functional theory calculations
on BASHY dyes, including 9 and 12, have shown that the sp3-
hybridized boron distorts the structure somewhat out of
planarity with the dihedral angle that characterizes the relative
Figure 3. Fluorescence spectra of (a) 10, (b) 11, and (c) 12 in toluene
(red), chloroform (green), tetrahydrofuran (blue), acetone (black),
and acetonitrile (magenta). The spectra for each compound were
recorded with optically matched solutions at identical excitation
wavelength. Note that the spectra of 12 in acetone and acetonitrile
coincide practically with the x-axis.
D
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orientation of the diethylamino group and the keto group being
ca. 30°.40 Based on the combined results of the theoretical
spectroscopy study, a mixed cyanine/charge-transfer character
of the first singlet excited state of BASHY dyes was proposed.
Naturally, the charge-transfer character depends on the
electron-donor and -acceptor substitution, being in accordance
with the herein presented experimental observations for the
dyes 9−12.
Based on the D−π−A architecture we expected the
observation of significant two-photon-absorption (2PA)
phenomena.22,45−49 Hence, the 2PA spectra and cross sections
(σ2) for 9−12 were measured; see Figure 4 and Table 2. Strong
tion of BASHY dyes without compromising their photophysical
properties. The electronic fine-tuning along the cyanine-type
backbone of the dyes opens possibilities for constructing D−π−
A wires with tailored emission properties as well as significant
solvatochromic and two-photon-absorption behavior.
EXPERIMENTAL SECTION
■
Materials and General Methods. The reactions were performed
in flame-dried single-neck round-bottom flasks fitted with rubber septa
and maintaining a positive pressure of argon, except for the synthesis
of the dyes which were carried out in sealed tubes. Air- and moisture-
sensitive liquids were transferred with the aid of a syringe or stainless
steel cannules. All solvents were used as received, being of
spectroscopic quality when employed for photophysical measure-
ments.
Thin-layer chromatography was performed using silica gel 60 F254
aluminum plates, visualized by UV light or revealed with an ethanolic
solution of phosphomolybdic acid or bromocresol and heat. Column
chromatography was performed using standard silica gel 60.
The NMR spectra (δ values in ppm) were recorded on standard
instrumentation, operated at 400 or 300 MHz (1H), 100 or 75 MHz
(13C), and 128 MHz (11B), using CDCl3 or (CD3)2SO as deuterated
solvent. All coupling constants are expressed in Hz. High-resolution
mass spectra were recorded on a triple quadrupole/linear ion trap
mass spectrometer, using electrospray ionization (EI). The elemental
composition was determined with a standard CHN analyzer. Infrared
spectra of samples dispersed in KBr disks were recorded on a Fourier
transform spectrometer with ceramic infrared light source and a
deuterated L-alanine doped triglycine sulfate detector.
Electrochemical measurements were done in dichloromethane
containing 0.1 M Bu4NPF6 and at a scan rate of 100 mVs−1. The
potentials are reported versus aqueous Ag/AgCl electrode and are not
corrected for the junction potential. The Eo′ value for the
ferrocenium/ferrocene couple used in this study was 0.46 V for
dichloromethane solutions; glassy carbon electrode.
Figure 4. Two-photon absorption spectra of the dyes 9 (black), 10
(blue), 11 (green), and 12 (red) in air-equilibrated toluene.
Table 2. Photophysical Data Related to Two-Photon
The employed boronic acid derivatives, hydrazine hydrate, 13b
[ethyl oxo-(4-trifluoromethylphenyl)acetate], 13c (ethyl 4-cyanoben-
zoylformate), 14a (phenylglyoxylic acid), 14d (4-nitrophenylglyoxylic
acid), 4-nitrosalicylaldehyde, and amberlyst 15 are commercially
available in high purity and were used as received. The ligands 16a
and 16d and the BASHY dyes 9 and 12 are known compounds and
were available from an earlier project.35
Preparation of the Ligand Precursors (16b and 16c).
Phenylglyoxylic Acids 14. The compounds 14b and 14c were
prepared according to literature-known procedures.51,52 In detail, the
corresponding phenylglyoxylic ester (5.0 mmol) 13b or 13c was
dissolved in 4.0 mL of an aqueous solution of NaOH (2.5 M) and
stirred at room temperature for 2 h. Then the resulting mixture was
acidified with 1 M HCl until pH < 7 and extracted with ethyl acetate
(2 × 50 mL). The combined organic layers were dried over anhydrous
magnesium sulfate, and then the volatiles were evaporated. The
phenylglyoxylic acids were obtained as yellowish solids in excellent
yields of 94% (14b) and 90% (14c), corresponding to 1.03 and 0.79 g,
respectively. The crude was used immediately in the next reaction
without further purification or characterization.
Hydrazonophenylacetates 15. Hydrazine hydrate (12.0 mmol)
and the corresponding phenylglyoxylic acid 14b or 14c (4.0 mmol)
were each dissolved in a separate portion of ethanol (2.0 mL). Then
the two solutions were mixed in a round-bottom flask, and the reaction
was continued for 18 h at 4 °C. After this time, the reaction mixture
was filtered, and the obtained solid was washed twice with 700 μL of
cold ethanol. The hydrazonophenylacetates 15b and 15c were
obtained in yields of 40% (422 mg) and 50% (442 mg), respectively,
and used immediately in the next step without further purification or
characterization.
Salicylidenehydrazones 16. Hydrazonophenylacetate 15b or 15c
(2.0 mmol) was dissolved in 3.0 mL water, and then 22.0 mL
methanol was added. This solution was slowly passed through a
column packed with amberlyst 15 (3.0 mL of dry resin wetted with
Absorption of 9−12 in Toluene
two-photon process
2PE
dye
λfluo (nm)
σ2 (GM)
9
508
518
528
537
194 (890 nm), 117 (950 nm)
214 (890 nm), 128 (950 nm)
176 (890 nm), 164 (970 nm)
283 (910 nm), 295 (980 nm)
10
11
12
2PA bands with maxima between ca. 900 and 1000 nm were
observed. The 2PA cross sections reached values of almost 300
GM for dye 12 and ca. 200 GM for the other three dyes. These
values are larger than observed for comparable push−pull dyes
such as NBD dyes,50 absorbing and emitting in the same
spectral region as BASHY dyes (see Supporting Information).
The two-photon excitation (2PE) led to the population of the
lowest excited singlet state, resulting in the observation of the
same fluorescence as observed in conventional one-photon
spectroscopy (see Table 2 as well as Supporting Information).
CONCLUSIONS
■
A series of BASHY dyes with varying electronic structure and
functional aspects was prepared and photophysically charac-
terized. Except for dyes that experience photoinduced electron-
transfer phenomena, the electronic nature of BA-derived
moieties has no influence on the UV−vis absorption and
fluorescence properties. This is a direct consequence of the
electronic isolation of these moieties from the boron
salicylidenehydrazone backbone itself. This constitutes an
advantage for the flexible molecular and functional diversifica-
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methanol) into a round-bottom flask containing a solution of 4-
diethylaminosalicylaldehyde in methanol (2.0 mmol in 3.0 mL). The
resulting mixture was stirred for 2 h at room temperature and then left
at 4 °C for further 18 h. Afterward the reaction mixture was filtered,
and the obtained solid was washed with cold dichloromethane (3 × 3.0
mL) and cold methanol (2 × 1.0 mL). The ligands 16b and 16c were
obtained as orange solids in 57% and 62% yield, respectively.
7.37 (m, 5H), 7.31−7.20 (m, 3H), 6.38 (d, J = 8.9 Hz, 1H), 6.23 (s,
1H), 3.79 (s, 2H), 3.51−3.40 (m, 4H), 1.24 (t, J = 6.9 Hz, 6H) ppm;
13C NMR (100 MHz, CDCl3) δ 161.7, 157.4, 156.0, 154.3, 153.7,
143.5, 142.7, 142.1, 141.5, 134.7, 132.8, 131.0, 129.6, 129.4, 128.3,
127.5, 126.6, 126.4, 125.0, 119.8, 119.2, 107.8, 106.5, 98.9, 45.5, 37.0,
12.9 ppm; 11B NMR (128 MHz, CDCl3) δ 4.1 ppm; FTIR (KBr,
cm−1) 2974, 2927, 2870, 2360, 1771, 1760, 1749, 1733, 1703, 1623,
1591, 1526, 1505, 1472, 1455, 1440, 1383, 1352, 1260, 1226, 1210,
1189, 1166, 1146, 1077, 1025, 976, 947, 918, 828, 800, 768, 712, 691,
419; HRMS (ESI) calcd for C32H29BN3O3 [M + H]+ 514.2296. Found
514.2290.
1
Ligand 16b. Orange solid, yield 57% (464 mg): H NMR (300
MHz, (CD3)2SO) δ 8.77 (s, 1H), 7.96 (d, J = 8.7 Hz, 2H), 7.89 (d, J =
8.7 Hz, 2H), 7.38 (d, J = 9.0 Hz, 1H), 6.37 (dd, J = 9.0 and 2.1 Hz,
1H), 6.11 (d, J = 2.1 Hz, 1H), 3.41 (q, J = 6.9 Hz, 4H), 1.12 (t, J = 6.9
Hz, 6H) ppm; 13C NMR (75 MHz, (CD3)2SO) δ 166.3, 165.3, 161.6,
157.3, 152.3, 135.5, 134.5, 127.5, 126.2, 126.14, 126.09, 126.0, 106.0,
104.8, 96.8, 44.1, 12.5 ppm; Elemental analysis calcd (%) for
C20H20F3N3O3 C 58.97, H 4.95, N 10.31. Found C 58.63, H 4.88,
N 10.19.
1
Dye 4. Orange solid, yield 75% (36 mg): H NMR (400 MHz,
CDCl3) δ 9.06 (s, 1H) 8.74 (d, J = 8.2 Hz, 1H), 8.47 (s, 1H), 8.08 (s,
1H), 7.97 (d, J = 8.8 Hz, 2H), 7.88 (d, J = 8.0 Hz, 1H), 7.75 (dt, J =
7.0 and 1.3 Hz, 1H), 7.56 (t, J = 7.0 Hz, 1H), 7.44−7.34 (m, 3H), 7.25
(d, J = 8.8 Hz, 1H), 6.34 (dd, J = 9.2 and 2.3 Hz, 1H), 6.07 (d, J = 2.2
Hz, 1H), 3.46−3.33 (m, 4H), 1.18 (t, J = 7.1 Hz, 6H) ppm; 13C NMR
(100 MHz, CDCl3) δ 161.4, 157.5, 155.5, 154.1, 153.4, 152.9, 144.8,
138.7, 134.9, 132.5, 131.1, 130.0, 129.7, 128.7, 128.3, 128.1, 126.6,
108.2, 106.8, 98.7, 45.4, 12.8; 11B NMR (128 MHz, CDCl3) δ 5.5
ppm; FTIR (KBr, cm−1) 2973, 2924, 2870, 2360, 1771, 1760, 1749,
1733, 1622, 1592, 1526, 1504, 1475, 1455, 1441, 1383, 1350, 1261,
1223, 1189, 1166, 1146, 1077, 1054, 1025, 977, 826, 799, 782, 754,
714, 691, 420; HRMS (ESI) calcd for C28H26BN4O3 [M + H]+
477.2092. Found 477.2089.
1
Ligand 16c. Orange solid, yield 62% (452 mg): H NMR (300
MHz, (CD3)2SO) δ 8.77 (s, 1H), 7.98 (d, J = 8.7 Hz, 2H), 7.90 (d, J =
8.7 Hz, 2H), 7.38 (d, J = 9.0 Hz, 1H), 6.38 (dd, J = 9.0 and 2.1 Hz,
1H), 6.11 (d, J = 2.1 Hz, 1H), 3.41 (q, J = 6.9 Hz, 4H), 1.12 (t, J = 6.9
Hz, 6H) ppm; 13C NMR (75 MHz, (CD3)2SO): δ 166.1, 165.4, 161.7,
157.1, 152.4, 135.9, 134.5, 133.1, 127.4, 118.5, 113.0, 106.1, 104.8,
96.8, 44.1, 12.6 ppm; Elemental analysis calcd (%) for C20H20N4O3 C
65.92, H 5.53, N 15.38. Found C 65.95, H 5.53, N 15.04.
General Procedure for the Preparation of the BASHY Dyes.
The synthesis of the dyes was performed by reaction of the
corresponding ligand 16 and the appropriate boronic acid derivative.35
For this purpose stoichiometric amounts of ligand 16 and the boronic
acid (0.1 mmol in all cases, except for the synthesis of 8, where 0.05
mmol of the boronic acid derivative were used) were dissolved in 1.0
mL acetonitrile and stirred at 80 °C for 2 h in a sealed tube. Then the
volatiles were evaporated. In the case of the dyes 2, 3, 5−8, 10, and 11,
this procedure yielded high purity products in near quantitative yield
without the need for further purification. Eventual trace impurities,
that were observed in very few occasions, can be removed by washing
the solid with a small amount of cold methanol. Exceptions from this
very direct and straightforward procedure are the dyes 1 and 4 which
required additional purification by preparative thin-layer chromatog-
raphy for 1 using dichloromethane as eluent or flash column
chromatography using n-hexane/CH2Cl2 (1:9) for 4.
1
Dye 5. Orange solid, yield 98% (49 mg): H NMR (400 MHz,
CDCl3) δ 8.34 (s, 1H), 7.97 (d, J = 8.5 Hz, 2H), 7.73 (d, J = 8.4 Hz,
2H), 7.51 (d, J = 8.3 Hz, 2H), 7.47−7.37 (m, 3H), 7.24 (d, J = 9 Hz,
1H), 6.40 (dd, J = 9.1 and 2.4 Hz, 1H), 6.20 (s, 1H), 4.68 (s, 2H),
3.52−3.42 (m, 4H), 1.25 (t, J = 7.1 Hz, 6H) ppm; 13C NMR (100
MHz, CDCl3) δ 161.4, 157.7, 155.5, 153.9, 153.7, 141.0, 134.8, 132.5,
131.6, 131.2, 129.6, 128.4, 125.6, 108.3, 106.5, 98.8, 45.6, 12.9 ppm;
11B NMR (128 MHz, (CD3)2SO) δ − 0.2 ppm; FTIR (KBr, cm−1)
2975, 2929, 2870, 2360, 1771, 1760, 1749, 1733, 1705, 1621, 1589,
1530, 1503, 1473, 1456, 1443, 1385, 1348, 1261, 1223, 1187, 1165,
1144, 1075, 1025, 975, 943, 828, 802, 759, 724, 713, 690, 420; HRMS
(ESI) calcd for C25H25BN4O5NaS [M + Na]+ 527.1531. Found
527.1523.
1
Dye 6. Orange solid, yield 99% (42 mg): mp 246−248 °C; H
NMR (300 MHz, CDCl3) δ 8.42 (d, J = 5.7 Hz, 2H), 8.32 (s, 1H),
7.98 (d, J = 6.9 Hz, 2H), 7.46−7.38 (m, 3H), 7.29−7.21 (m, 3H), 6.40
(dd, J = 9.3 and 2.0 Hz, 1H), 6.21 (d, J = 2.0 Hz, 1H), 3.54−3.40 (m,
4H), 1.26 (t, J = 6.9 Hz, 6H) ppm; 13C NMR (75 MHz, CDCl3) δ
161.3, 157.7, 155.4, 153.8, 153.7, 148.4, 134.9, 132.4, 131.2, 129.5,
128.4, 126.1, 108.3, 106.4, 98.7, 45.6, 12.8 ppm; 11B NMR (128 MHz,
CDCl3) δ 3.5 ppm; FTIR (KBr, cm−1) 2973, 2933, 2870, 2360, 1771,
1760, 1749, 1733, 1712, 1626, 1592, 1527, 1506, 1472, 1456, 1441,
1386, 1353, 1263, 1220, 1187, 1170, 1145, 1079, 826, 798, 754, 714,
691, 420; Elemental analysis calcd (%) for C24H23BN4O3 C 67.62, H
5.44, N 13.14. Found C 67.17, H 5.38, N 12.85.
1
Dye 1. Orange solid, yield 71% (42 mg): H NMR (400 MHz,
CDCl3) δ 8.17 (s, 1H), 7.94 (d, J = 7.9, 2H), 7.40−7.31 (m, 3H),
7.24−6.81 (m, 15H), 6.28 (d, J = 7.6 Hz, 1H), 6.16 (d, J = 1.6 Hz,
1H), 3.45−3.30 (m, 4H), 1.16 (t, J = 7.0 Hz, 6H) ppm; 13C NMR
(100 MHz, CDCl3) δ 161.7, 157.4, 156.0, 154.1, 153.6, 148.0, 147.5,
134.7, 132.8, 131.8, 131.0, 129.6, 129.3, 129.1, 128.3, 124.3, 123.3,
122.5, 107.7, 106.4, 98.7, 45.5, 12.9 ppm; 11B NMR (128 MHz,
CDCl3) δ 4.3 ppm; FTIR (KBr, cm−1) 2973, 2925, 2870, 2360, 1771,
1760, 1749, 1733, 1703, 1621, 1590, 1524, 1505, 1474, 1455, 1440,
1384, 1351, 1260, 1226, 1189, 1164, 1143, 1074, 1026, 971, 942, 828,
800, 789, 753, 715, 693, 420; HRMS (ESI) calcd for C37H34BN4O3 [M
+ H]+ 593.2718. Found 593.2707.
1
Dye 7. Orange solid, yield 96% (49 mg): mp 235−237 °C; H
NMR (400 MHz, CDCl3) δ 8.24 (s, 1H), 8.02 (d, J = 7.2 Hz, 2H),
7.49−7.41 (m, 3H), 7.28 (s, 1H), 6.44 (d, J = 9.0 Hz, 1H), 6.21 (s,
1H), 3.54−3.43 (m, 4H), 1.27 (t, J = 7.0 Hz, 6H) ppm; 13C NMR
(100 MHz, CDCl3) δ 160.9, 157.7, 155.1, 154.7, 153.4, 134.7, 132.2,
131.3, 129.6, 128.5, 108.4, 106.4, 98.5, 45.6, 12.9 ppm; 11B NMR (128
MHz, CDCl3) δ 2.2 ppm; FTIR (KBr, cm−1) 2979, 2932, 2875, 2360,
1771, 1760, 1749, 1733, 1714, 1625, 1594, 1527, 1508, 1458, 1384,
1353, 1289, 1262, 1220, 1189, 1168, 1147, 1025, 969, 927, 829, 801,
759, 745, 715, 692, 420; HRMS (ESI) calcd for C25H19BF5N3O3Na
[M + Na]+ 538.1332. Found 538.1331.
1
Dye 2. Orange solid, yield 99% (54 mg): mp 255−258 °C; H
NMR (300 MHz, CDCl3) δ 9.08 (d, J = 9.3 Hz, 1H), 8.54 (s, 1H),
8.16 (t, J = 7.8 Hz, 2H), 8.08 (d, J = 6.8 Hz, 1H), 7.97−7.91 (m, 6H),
7.65 (d, J = 7.6 Hz, 1H), 7.42−7.31 (m, 3H), 7.24 (d, J = 9.2 Hz, 1H),
6.29 (dd, J = 9.2 and 2.4 Hz, 1H), 6.09 (d, J = 2.1 Hz, 1H), 3.42−3.25
(m, 4H), 1.13 (t, J = 7.1 Hz, 6H) ppm; 13C NMR (75 MHz, CDCl3) δ
161.6, 157.3, 155.7, 154.0, 153.7, 134.6, 134.3, 132.7, 131.4, 131.2,
130.9, 129.6, 129.3, 129.1, 128.3, 127.5, 127.1, 126.5, 125.6, 125.3,
125.2, 124.7, 124.3, 123.8, 108.0, 106.8, 98.8, 45.3, 12.7 ppm; 11B
NMR (128 MHz, CDCl3) δ 6.1 ppm; FTIR (KBr, cm−1) 2975, 2927,
2870, 2360, 1771, 1760, 1749, 1733, 1703, 1621, 1590, 1524, 1506,
1473, 1455, 1440, 1396, 1351, 1259, 1224, 1188, 1166, 1145, 1075,
1025, 974, 940, 829, 801, 793, 755, 714, 691, 420; Elemental analysis
calcd (%) for C35H28BN3O3 C 76.51, H 5.14, N 7.65. Found C 76.21,
H 5.07, N 7.60.
1
Dye 8. Orange solid, yield 99% (107 mg): H NMR (300 MHz,
CDCl3) δ 8.24 (s, 2H), 7.94 (d, J = 8.2 Hz, 4H), 7.41−7.18 (m, 14H),
6.35 (dd, J = 9.1 and 2.2 Hz, 2H), 6.24 (d, J = 2.0 Hz, 2H), 3.47−3.39
(m, 8H), 1.80−1.64 (m, 4H), 1.22 (t, J = 7.0 Hz, 12H), 1.17−0.23 (m,
30H) ppm; 13C NMR (75 MHz, CDCl3) δ 161.8, 157.2, 156.2, 154.5,
153.5, 150.0, 141.1, 134.6, 132.7, 130.9, 129.4, 129.1, 128.3, 125.4,
118.9, 107.7, 106.4, 98.8, 54.7, 45.5, 40.3, 31.9, 30.3, 29.5, 29.4, 24.0,
22.8, 14.3, 12.8 ppm; 11B NMR (128 MHz, CDCl3) δ 4.2 ppm; FTIR
(KBr, cm−1) 2925, 2851, 2870, 1705, 1622, 1592, 1526, 1502, 1472,
1
Dye 3. Orange solid, yield 96% (49 mg): mp 235−238 °C; H
NMR (400 MHz, CDCl3) δ 8.34 (s, 1H), 7.99 (d, J = 7.0 Hz, 2H),
7.69 (d, J = 7.6 Hz, 1H), 7.63 (d, J = 7.7 Hz, 1H), 7.59 (s, 1H), 7.48−
F
DOI: 10.1021/acs.joc.7b00601
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
1441, 1383, 1350, 1259, 1188, 1164, 1144, 1075, 1024, 971, 944, 923,
826, 801, 787, 753, 713, 691, 429; Elemental analysis calcd (%) for
C67H78B2N6O6 C 74.17, H 7.25, N 7.75. Found C 73.72, H 7.42, N
7.50.
ORCID
Joana Ines Carvalho: 0000-0002-9848-0203
̂
Jesus F. Arteaga: 0000-0001-8153-6621
́
Dye 10. Red solid, yield 99% (49 mg): mp 228−230 °C; 1H NMR
(300 MHz, CDCl3) δ 8.30 (s, 1H), 8.12 (d, J = 8.1 Hz, 2H), 7.64 (d, J
= 8.1 Hz, 2H), 7.39−7.36 (m, 2H), 7.25−7.19 (m, 4H), 6.39 (dd, J =
9.0 and 2.1 Hz, 1H), 6.20 (d, J = 2.1 Hz, 1H), 3.53−3.39 (m, 4H),
1.24 (t, J = 7.2 Hz, 6H) ppm; 13C NMR (75 MHz, CDCl3) δ 161.9,
157.8, 155.7, 153.9, 152.1, 136.2, 134.9, 132.4, 132.0, 130.8, 129.9,
128.1, 127.8, 125.8, 125.2 (q, CF3), 108.3, 106.8, 98.8, 45.6, 12.9 ppm;
11B NMR (128 MHz, CDCl3) δ 4.2 ppm; FTIR (KBr, cm−1) 2981,
2931, 2870, 2360, 2330, 1771, 1760, 1749, 1733, 1699, 1623, 1590,
1527, 1504 1475, 1455, 1440, 1407, 1382, 1354, 1322, 1262, 1227,
1188, 1169, 1149, 1129, 1070, 1018, 977, 944, 853, 843, 822, 795, 714,
699, 420; Elemental analysis calcd (%) for C26H23BF3N3O3 C 63.31, H
4.70, N 8.52. Found C 63.72, H 4.87, N 8.47.
Uwe Pischel: 0000-0001-8893-9829
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The financial support by the Spanish MINECO (grants
CTQ2014-54729-C2-1-P, CTQ2014-54729-C2-2-P,
CTQ2013-41339-P, and CTQ2015-71896-REDT), the Junta
́
de Andalucia (grant P12-FQM-2140), and Portuguese FCT
(grants SFRH/BD/94779/2013, PTDC/QEQ-QOR/1434/
2014, iMed.ULisboa grant UID/DTP/04138/2013) is grate-
fully acknowledged.
Dye 11. Red solid, yield 99% (45 mg): mp 287−289 °C; 1H NMR
(300 MHz, CDCl3) δ 8.29 (s, 1H), 8.13 (d, J = 9.0 Hz, 2H), 7.66 (d, J
= 9.0 Hz, 2H), 7.37−7.34 (m, 2H), 7.24−7.19 (m, 4H), 6.40 (dd, J =
9.0 and 2.1 Hz, 1H), 6.20 (d, J = 2.1 Hz, 1H), 3.52−3.43 (m, 4H),
1.25 (t, J = 7.2 Hz, 6H) ppm; 13C NMR (75 MHz, CDCl3) δ 162.0,
157.9, 155.5, 153.8, 151.1, 137.0, 135.1, 131.9, 130.8, 130.0, 128.1,
127.8, 118.6, 113.8, 108.5, 107.0, 98.7, 45.7, 12.9 ppm; 11B NMR (128
MHz, CDCl3) δ 4.2 ppm; FTIR (KBr, cm−1) 2983, 2931, 2870, 2360,
2227, 1771, 1760, 1749, 1733, 1703, 1624, 1588, 1525, 1506, 1472,
1434, 1379, 1263, 1224, 1188, 1166, 1148, 1114, 1096, 1077, 1020,
974, 944, 823, 420; Elemental analysis calcd (%) for C26H23BN4O3 C
69.35, H 5.22, N 12.21. Found C 69.43, H 5.22, N 12.21.
Photophysical Measurements. The photophysical measure-
ments were performed with air-equilibrated solutions at room
temperature, using quartz cuvettes (1 cm optical path length), except
otherwise indicated. The UV−vis absorption spectra were obtained
with a standard spectrophotometer. The fluorescence spectra were
recorded with a fluorimeter equipped with a xenon flash-lamp as light
source and corrected for the spectral sensitivity of the photomultiplier
detector. The authenticity of the emission was confirmed by recording
excitation spectra, monitoring the emission maximum. In all cases the
excitation and absorption spectra matched each other. The
fluorescence quantum yields were measured with 4-amino-N-propyl-
1,8-naphthalimide (Φfluo= 0.48 in acetonitrile)35 as reference and
corrected for refractive index differences of the used solvents. The
lifetime measurements were done with the time-correlated single-
photon-counting technique. For this purpose a picosecond pulsed
diode laser (λ = 442 nm, pulse width fwhm 78.3 ps) was used as
excitation source. The detector was a multichannel photomultiplier,
and the time response of the system was <50 ps.
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ASSOCIATED CONTENT
* Supporting Information
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ACS Publications website at DOI: 10.1021/acs.joc.7b00601.
1
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́
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AUTHOR INFORMATION
Corresponding Authors
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