Highly Fluorescent Dianionic Polymethines with a 1,3,2-Dioxaborine Core

Article abstract of DOI:10.1021/acs.joc.1c00138

The difluoroboron β-diketonate ring is ever more used for creating bright polymethine-type fluorophores for the visible and NIR range. Here, we report the synthesis and spectral properties of a series of dianionic cyanine dyes of the rare A1-π-A-π-A1 type, with the central dioxaborine ring (A) embedded into the polymethine chain and various electron-acceptor terminal groups A1. Depending on the nature of the end group, the maxima of their intensive (with molar extinctions up to 380 000 M-1 cm-1) and narrow long-wavelength absorption band lie in the range of 530-770 nm. Their absorption and fluorescence bands are nearly mirror-like and characterized by weak solvatochromism; the marked hypsochromic shifts are observed only when going from polar aprotic solvents to methanol. The designed dianionic dyes have fluorescence quantum yields up to 92 % in the visible range, and even for the NIR dyes, the values of 18-37 % are observed in DMF.

Full text of DOI:10.1021/acs.joc.1c00138

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Article
Highly Fluorescent Dianionic Polymethines with a 1,3,2-Dioxaborine
Core
Vladyslav Polishchuk, Andrii Kulinich, Eduard Rusanov, and Mykola Shandura*
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ABSTRACT: The difluoroboron β-diketonate ring is ever more
used for creating bright polymethine-type fluorophores for the
visible and NIR range. Here, we report the synthesis and spectral
properties of a series of dianionic cyanine dyes of the rare A1−π−
A−π−A1 type, with the central dioxaborine ring (A) embedded into
the polymethine chain and various electron-acceptor terminal
groups A1. Depending on the nature of the end group, the maxima
of their intensive (with molar extinctions up to 380 000 M−1 cm−1)
and narrow long-wavelength absorption band lie in the range of
530−770 nm. Their absorption and fluorescence bands are nearly
mirror-like and characterized by weak solvatochromism; the marked hypsochromic shifts are observed only when going from polar
aprotic solvents to methanol. The designed dianionic dyes have fluorescence quantum yields up to 92 % in the visible range, and
even for the NIR dyes, the values of 18−37 % are observed in DMF.
INTRODUCTION
■
The difluoroboron β-diketonate complex (1,3,2-dioxaborine)
is a prominent structural unit of organic dyes, which renders
them many appealing photophysical properties, including large
molar extinctions, strong and tunable fluorescence both in
solutions and in the solid state, and two-photon excited
emission.1 Polymethine dyes comprising the dioxaborine ring
are known as highly luminescent and intensely colored
fluorophores,2 widely used in NLO materials,3 as fluorescent
probes for protein, lipid, or DNA labeling,4−7 as fluorescent
“turn-on” pH sensors, and molecular rotors.8,9
Typically, dioxaborine is the terminal electron-acceptor
group in merocyanines (D−π−A) or anionic polymethines
(A−π−A),2 whereas dyes with the dioxaborine ring incorpo-
Figure 1. Synthetic possibilities of cyano-substitued dioxaborine 1.
rated into the polymethine chain (D−π−A−π−D) have only
recently gained much attention.10 Most of the D−π−A−π−D
dioxaborines are difluoroborinated curcuminoids, used in
bioimaging,11 fluoride-anion detection,12 and as photosensi-
tizers in long-wavelength cationic photopolymerization.13
Synthesis of difluoroborinated curcuminoids usually involves
condensation of the boron difluoride complex of acetylacetone
with benzaldehydes.14 However, this synthetic route is ill-
suited for obtaining dyes with stronger electron-donating
heterocyclic end groups, and hence, polymethines with the
central dioxaborine core remain scarce. As shown in our
previous work, the cyano group in position 5 of dioxaborine
substantially increases activity of both methyl groups in
precursor 1 (Figure 1),15 opening the way to D−π−A−π−D
dyes with various heterocyclic termini. In this paper, we report
a synthesis and characterization of novel dianionic dyes A1−π−
A−π−A1 with the central dioxaborine ring (A) and different
electron-acceptor end groups (A1) (Figure 1). Such dianionic
dyes are little studied; only several structures derived from
squaric and croconic acids having been described so far.16
RESULTS AND DISCUSSION
■
Synthesis of dioxaborine dianionic dyes 3−8 was performed
according to Scheme 1 via condensation of bis-hemicyanine 2
with different CH acids at room temperature in the presence of
Received: January 18, 2021
Published: March 17, 2021
https://doi.org/10.1021/acs.joc.1c00138
© 2021 American Chemical Society
J. Org. Chem. 2021, 86, 5227−5233
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Scheme 1. Synthesis of Dianionic Dyes with the Dioxaborine Core
acetic anhydride and an organic base. Formation of the mono-
substituted anionic dye is the fast step and has been detected
using UV−vis spectral control. Its long-wavelength absorption
band lies halfway between the corresponding bands of bis-
hemicyanine 2 and the final product. It is conceivable that the
second reaction step is the limiting one, since here the
nucleophile attacks the more electron-rich anionic intermedi-
ate. On the basis of the literature data, electron-accepting
abilities of the chosen end groups could be roughly estimated
as malononitrile < 1,3-indandione < 1,3-dimethylbarbituric,
coumarindioxaborine < 1,3-diethylthiobarbituric, tricyanofur-
an. Of these, 1,3-indandione, coumarindioxaborine, and
tricyanofuran have their own extended π-systems, which pave
the path to deeply colored dyes.
dioxaborine meso-C atom and the terminal groups’ electron-
accepting abilities. Thus, for malononitrile-based dye 3, the
chemical shift δC of Cmeso is equal to 76.3 ppm. An increase in
the electron-accepting ability of the end group, when going to
thiobarbituric-based 5, results in a downfield shift of this signal
to δC = 82.9 ppm (Table 1). However, the correlation is
Table 1. Characteristic Signals in 13C and 19F NMR Spectra
of Dyes 3−8
dye
δC(Cmeso) [ppm]
δF(11BF2) [ppm]
3
4
5
6
7
8
76.3
81.2
82.9
80.8
83.15
83.92
−144.63
−143.85
−143.67
−144.26
−144.02
−143.75
According to the 1H NMR data, in dyes 3−7, the spin−spin
coupling constants (3JHH) between the vicinal H atoms of the
polymethine chain range from 13.0 to 14.3 Hz, indicating the
all-trans conformation of their chromophores in solution. For
“dimethine” dianions, the only such 3JHH constant increases in
the series 3 < 6 < 4 < 5, indicating probably a greater
contribution of the limiting structure, in which both negative
violated for dianions 7 and 8, the Cmeso signals of which are
even more downfield than that of dianion 5, while the electron-
accepting abilities of their end groups are similar or weaker
than that of the thiobarbituric moiety.2,18 The explanation here
is typical in 13C NMR spectroscopy long-range structural
effects, since dyes 3−6 has dimethine and 7, 8 trimethine
chains between the dioxaborine core and their end groups. The
19F NMR chemical shifts, though minor, depend more
regularly on the electron-accepting properties of end groups.
The chemical shift δF of the 11BF2 group in the series 3−8
ranges from −144.63 to −143.67 ppm, reaching the maximum
value for thiobarbiturate 5 (Table 1).
Scheme 2. Some Resonance Structures of Dyes 3−8
According to the X-ray data, the all-trans conformation of
dye 3 holds in the solid state (Figure 2). The C and O atoms
of the dioxaborine ring lie in-plane (the RMS deviation of the
charges are localized at the end groups (see Scheme 2). The
1H NMR spectrum of compound 8 is distinguished by the
structureless and broadened signals attributed to H atoms of
the polymethine chain and C(CH3)2 groups of the
tricyanofuran residues (see Supporting Information (SI)).
This is most likely due to relatively fast rotamerization around
the C−C bonds connecting the tricyanofuran end groups and
polymethine chain.17,18 Analysis of the 13C NMR data has
revealed the relation between the chemical shift of the
Figure 2. Arbitrary views of the X-ray structure of dye 3.
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Table 2. UV-vis and Fluorescence Spectral Characteristics of Dyes 3−8
dye
solvent
λa [nm] ε × 10−5 [M−1 cm−1
]
FWHMa [cm−1
]
λf [nm]
Φf [%]
FWHMf [cm−1
]
ΔνS [cm−1
]
ε·Φf × 10−5 [M−1 cm−1
]
3
DMF
methanol
DMF
methanol
DMF
methanol
DMF
methanol
DMF
methanol
DMF
538
530
570
553
597
581
624
606
728
708
767
748
2.38
2.22
2.53
1.94
2.93
2.20
3.26
2.50
3.78
3.02
3.19
2.81
822
1003
793
1143
768
1083
642
957
553
551
586
580
614
610
640
628
757
751
800
796
90
5
3
5
3
4
4
3
0.1
2
1
859
1051
843
1197
815
1121
704
1096
946
504
719
479
842
464
818
401
578
526
809
538
806
2.14
1.44
2.33
1.16
2.55
1.87
1.99
0.025
1.40
0.39
0.57
0.19
65
92
60
87
85
61
1
37
13
18
4
5
6
7
8
814
1172
903
1244
831
1075
1
methanol
1124
6.9 0.4
Figure 3. Normalized UV−vis absorption (a) and fluorescence (b) spectra of dyes 3−8 in DMF.
fitted atoms is equal to 0.0135 Å), while the boron atom is out
of the plane by 0.404 Å. There is also a slight nonplanarity of
the polymethine chromophore: the dihedral angle between the
malononitrile termini planes C6N1N2 and C11N3N4 is equal
to 13.0°. Note that, despite the apparent molecular symmetry
of dianion 3, the nonsymmetric positioning of tetraethylam-
monium counterions results in the crystal structure of triclinic
respective FWHMa values) and less intensive (Table 2). All
these changes indicate the stronger effect of electrophilic
solvation on the electronic structure of the studied dyes, which
agrees with their dianionic nature. Note also that the
solvatochromic shifts in the pair DMF−methanol are larger
for dyes 4−7, comprising carbonyl groups in their acceptor
termini. Indeed, carbonyls are much more prone to H-bond
formation than nitrile groups present in dianions 3 and 8. The
role of this factor is also evidenced by a smaller solvatochromic
shift for coumarindioxaborine-based dye 7, having only one
carbonyl group in each termini, than for 1,3-indandione and
(thio)barbituric derivatives.
The long-wavelength absorption bands of dyes 3−8 are
highly intensive. For all of them, the molar extinctions (ε) are
close to or exceed the value of 200 000 M−1 cm−1 in both DMF
and methanol (Table 2). For dye 7 in DMF, the molar
extinction reaches the value of 378 000 M−1 cm−1, which is
among the highest known for polymethine dyes. By their
shapenarrow, asymmetric, with a less intensive short-
wavelength shoulderthe absorption bands of compounds
3−8 also resemble those of typical symmetric polymethines
(Figure 3). These data, along with their weak solvatochrom-
ism, suggest that all the obtained dianionic dyes are
characterized by a highly symmetric electronic structure, less
dependent on the nature of acceptor end groups.
The fluorescence bands of dyes 3−8 are also very narrow,
with the asymmetry caused by the low long-wavelength
shoulder; i.e., the absorption and emission bands are nearly
mirror-like (Figure 3). The Stokes shifts (ΔνS) are relatively
small, amounting to 400−540 cm−1 in DMF and 580−
840 cm−1 in methanol, while the solvatochromic shifts Δλf in
the pair DMF−methanol are even smaller than the respective
Δλa values, amounting to only 66−300 cm−1. These data imply
that high electronic symmetry of dianions 3−8 is retained in
the first excited state S1.
̅
space group P1. Thereby, dianion 3 becomes slightly
asymmetric in the solid state; for example, the bond lengths
C1−C2 and C2−C3 are equal to 1.422 and 1.415 Å,
respectively. The bond lengths in the polymethine chain lie
within an interval of 1.401 0.021 Å, with the bond length
alternation (BLA), evaluated as a mean absolute deviation
from the average bond length, equal to 0.012 Å. Such a
negligible BLA suggests that the electronic structure of the
studied dianionic dyes is close to the so-called ideal
polymethine structure (Scheme 2),19 implying effective
delocalization of negative charges between the distal end
groups and dioxaborine core.
Depending on the terminal group and solvent, the
absorption maxima of dyes 3−8 range from 530 to 767 nm
(Table 2, Figure 3). At that, their solvatochromism in a series
of aprotic solvents (DMF, acetonitrile, and dichloromethane)
is very small (see SI). For example, for dyes 4 and 8,
bathochromic shifts of the absorption maxima (λa) in going
from acetonitrile to DMF as the solvent amount to only 100−
110 cm−1 and must be explained by an increase in the medium
refractive index.20 The differences in their photophysical
properties in aprotic and protic media are more pronounced,
so the quantitative spectral study was carried out primarily in
DMF and methanol as the solvents (Table 2).
When going from DMF to methanol, the hypsochromic
shifts of dyes’ 3−8 absorption maxima are much larger (Δλa =
8−20 nm, Δνa = 280−540 cm−1) than in the pair DMF−
methanol and cannot, therefore, be explained only by a
decrease in the solvent refractive index. In addition, the
absorption bands in methanol become much broader (cf. the
The fluorescence quantum yields (FQYs) of the obtained
dyes are remarkably high, nearing 90 % for dyes 3, 4, and 5 in
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DMF (Table 2)higher than typical FQYs for symmetric
cationic polymethines absorbing in the same respective
spectral regions. Even deeply colored dyes 7 and 8, whose
spectral bands lie in the NIR region, fluoresce strongly in
DMF. In view of their high molar absorption, the fluorescence
brightness (ε·Φf) of polymethines 3−8 is very high, with a
maximum value of 255 000 M−1 cm−1 for dye 5. In all cases, a
change from DMF to methanol results in smaller FQYs, which
is consistent with the conclusion about stronger specific
solvation of their chromophores in the latter solvent. The most
drastic fluorescence quenching in methanol is observed for 1,3-
indandione derivative 6, whose FQY decreases to a mere 1 %
in contrast to 61 % in DMF. The same phenomenon was
earlier observed for 1,3-indandione-based merocyanines and
anionic oxonols21 and had been explained by a stabilization of
their nonfluorescent ππ*-excited states upon electrophilic
solvation of carbonyl groups of 1,3-indandione by protic
solvents.
The BLAs in the polymethine chains of molecules 3 and 4,
evaluated as a mean absolute deviation from the average bond
lengths, are equal to 0.016 and 0.014 Å, respectively, in the
ground state S0, decreasing to 0.013 and 0.011 Å in the
fluorescent state S1. The maximum C−C bond lengths in the
polymethine chain are observed for internal C−C bonds of
dioxaborine, which is in good agreement with the X-ray data of
dye 3, and the ring blocks isomerization around this elongated
bonds. The calculated NBO charge distributions in chromo-
phores 3 and 4 are very similar (see SI). In the ground state S0
of dianion 3, each malononitrile group C(CN)2 has a total
charge of −0.637, whereas the better electron-accepting
barbituric residue in dye 4 has a slightly larger charge of
−0.664. In both dyes, the vertical S1FC←S0 excitation leads to a
minor charge transfer from the end groups to the dioxaborine
core and a charge alternation in the polymethine chain.
The long-wavelength TD-DFT electronic transitions S1
FC
←
S0 in the studied dianions are allowed polymethine-type ππ*
transitions with the 99 % contributions from the HOMO→
LUMO (Figure 4) excitation. The next two transitions are of
We suppose that the high FQYs of dianionic dyes 3−8
should be explained by a favorable combination of two key
factors: (1) their electronic structure, electronic symmetry
resembles that of typical symmetric ionic polymethines; (2)
the central dioxaborine ring hampers rotations (isomerization)
of their chromophore, thus effectively reducing one of the
principal paths of the nonradiative excited-state decay.
The synthesized dianionic dyes are found to be stable both
in the solid state and in solution. They are better soluble in
DMSO and DMF, less soluble in methanol and dichloro-
methane, and nearly insoluble in low-polarity organic solvents.
Compounds 7 and 8, comprising the most extended
chromophore systems, have the smallest solubility in the
series. Dyes 3−8 do not undergo traceable hydrolysis or
methanolysis in the MeCN−H2O (9:1) mixture or in
methanol in the absence of strong acids or bases. Since their
long-wavelength absorption maxima vary in a wide spectral
range, their relative photostabilities were evaluated via
comparison with several common ionic polymethines absorb-
ing in various spectral regions, such as indodicarbocyanine
(HIDC), indotricarbocyanine (HITC), and bis(1,3-dimethyl
barbituric acid) pentamethine oxonol (diBAC1(5)). The
obtained data (see SI) show that barbituric-based dye 4 is
the most photostable among the obtained dioxaborines,
whereas indandione-based dianion 6 is the least stable.
Compounds 4 and 5 are comparable by photostability with
HIDC, deeply colored dyes 7 and 8, with HITC, and all the
studied dyes are more photostable than diBAC1(5).
The DFT and TD-DFT calculations have been carried out
for dianions 3 and 4, with the B3LYP/6-31G(d,p) level of
theory and the polarizable continuum model (PCM) formal-
ism to account for solvent effects (DMF as the model solvent;
see Computational Details). Taking into account the
asymmetric structure of dye 3 in the crystal, the ground (S0)
and excited (S1) state geometry optimizations have been
performed without any symmetry restrictions. However, in
both cases, for dianions 3 and 4, the final geometries
Figure 4. Selected MOs of dye 4.
the ππ*-type too, with the major contributions from the
HOMO-1→LUMO and HOMO→LUMO+1 excitations
(Figure 4). They both are symmetry-forbidden, having the
oscillator strengths below 0.05. Even in the case of barbituric-
based 4, the nπ* transitions are involved only in higher
electronic excitations.
In summary, we reported the synthesis and characterization
of a series of novel highly fluorescent dianionic polymethine
dyes of the A1−π−A−π−A1 type with the central dioxaborine
ring (A) and different electron-acceptor terminal groups (A1).
Depending on the latter, their absorption and emission ranges
can be varied through the visible and NIR spectrum regions.
Having been less studied hitherto, such dianionic π-systems
have good synthetic flexibility and potential as highly effective
light-energy transforming functional compounds.
EXPERIMENTAL SECTION
■
General Materials and Methods. 1H NMR (400 MHz),
19F NMR (376.5 MHz, CFCl3 as internal standard), and 13C NMR
(75.83 MHz, 100.6 MHz or 125.7 MHz) spectra were recorded with a
Varian VXR-300, Varian VXR-400, or a Bruker 170 Avance 500. The
residual solvent peaks of DMSO-d6 (δH = 2.50 ppm, δC = 39.51 ppm)
or CDCl3 (δH = 7.27 ppm, δC = 77.00 ppm) were used as internal
standards. UV−vis absorption spectra were recorded using a
Shimadzu UV-3100 spectrophotometer. Fluorescence spectra were
recorded with a Solar CM-2203 fluorescence spectrophotometer. To
minimize the inner filter effects, the optical densities of dye solutions
in fluorescence measurements did not exceed 0.1 in the absorption
maximum. The fluorescence quantum yields (Φf) were determined
relative to rhodamine 6G (ΦR = 0.95, ethanol),22 rhodamine 101 (ΦR
= 0.91, acidified EtOH),23 indodicarbocyanine iodide (ΦR = 0.4,
correspond to the CS molecular symmetry, with the Cmeso
−
CN and BF2 groups lying in the principal plane. The optimized
polymethine chromophores 3 and 4 are slightly nonplanar.
Still, in the case of dye 3, the calculated dihedral angle between
the malononitrile termini planes in the ground state is equal to
only 4.8°. Its higher nonplanarity in the solid state can be
explained by the crystal packing effects, which cause also its
symmetry distortion.
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methanol),24 and indotricarbocyanine iodide (ΦR = 0.28, EtOH).25
Dianionic Dye 5. A mixture of compound 2 (0.39 g, 1 mmol), 1,3-
diethylthiobarbituric acid (0.4 g, 2 mmol), acetic anhydride (0.31 g,
3 mmol), and triethylamine (0.61 g, 6 mmol) was stirred in dry
acetonitrile (5 mL) for 2 h at room temperature. The solution was
concentrated under reduced pressure, and the residue was triturated
in methanol. The precipitate was filtered off and purified by
recrystallization from methanol to give 5 as a green solid. Yield:
The values of Φf were corrected taking into account refractive indexes
AR
A
I
IR
η2
of the solvents, Φ = ΦR
×
×
×
, where ΦR is the
ηR2
f
fluorescence quantum yield of a fluorescent standard, I and IR are
the fluorescence intensities of a tested compound and the fluorescent
standard correspondingly (the areas of their fluorescence bands), A
and AR are the optical densities of two solutions at the excitation
wavelength, η and ηR are refractive indexes of two solutions.
Photostability of dianionic dyes 3−8 and polymethines HIDC,
HITC, and diBAC1(5) was checked under controlled conditions. Dye
solutions in MeCN with optical densities of 0.1−0.15 in the
absorption maximum were placed in a 1 cm quartz cuvette in the
fluorescent spectrometer (Solar CM2203, 250 W xenon lamp as an
excitation source). Photodecomposition of the samples was studied in
“kinetics” mode under continuous irradiation with a 300 nm
wavelength and registration of a fluorescence intensity decrease in
time. Slit widths of the excitation monochromator were set to the
value of 6 mm, which corresponds to 28.5 nm of excitation bandwidth
at half-maximum. For control, initial and final fluorescence spectra of
the studied solutions were compared as well.
1
49 % (0.39 g). H NMR (400 MHz, DMSO-d6): δH 8.83 (br.s, 2H),
8.29 (d, J = 14.3 Hz, 2H), 7.47 (d, J = 14.3 Hz, 2H), 4.43 (q, J = 6.8
Hz, 8H), 3.09 (q, J = 7.3 Hz, 12H), 1.17 (t, J = 7.2 Hz, 30H) ppm.
13C{1H} NMR (126 MHz, DMSO-d6): δC 177.3, 176.5, 160.3, 146.5,
117.6, 105.9, 97.9, 82.95, 46.3, 42.6, 42.2, 13.0, 9.1 ppm. 19F NMR
(376 MHz, DMSO-d6): δF −143.60 (10B, 0.2F), −143.67 (11B, 0.8F)
ppm. Anal. Calcd for C36H56BF2N7O6S2: C, 54.33; H, 7.09; N, 12.32.
Found: C, 54.42; H, 7.13; N, 12.12.
Dianionic Dye 6. A mixture of compound 2 (0.39 g, 1 mmol), 1,3-
indandione (0.29 g, 2 mmol), acetic anhydride (0.31 g, 3 mmol), and
N,N-diisopropylethylamine (0.77 g, 6 mmol) was stirred in dry
acetonitrile (8 mL) for 2 h at r.t. The precipitate was filtered off
and washed with acetonitrile to give 6 as a blue solid. Yield: 45 %
1
(0.33 g). H NMR (400 MHz, DMSO-d6): δH 8.14 (br.s., 2H), 7.79
Synthetic Procedures. Bis-hemicyanine 215 and 8-(diethylami-
no)-2,2-difluoro-4-methyl-5-oxo-(5H)-chromeno[4,3-d]-1,3,2-(2H)-
dioxaborine2c were prepared according to published methods.
Malononitrile, 1,3-dimethylbarbituric acid, 1,3-diethylthiobarbituric
acid, and 1,3-indandione were purchased from commercial sources.
Tricyanofuran was prepared with a 73% yield in modified solvent-
free condensation conditions. 2-Dicyanomethylen-3-cyano-4,5,5-
trimethyl-2,5-dihydrofuran: A mixture of 3-hydroxy-3-methylbutan-
2-one (2.04 g, 20 mmol), malononitrile (2.64 g, 40 mmol), and
potassium carbonate (0.04 g, 0.3 mmol) was stirred under heating
on oil bath at 80 °C for 20 min. The completely solidified reaction
mixture was triturated with 15 mL of ethanol and stirred for 20 min at
r.t. The precipitate was removed by suction filtration, washed with
50 % aqueous ethanol and then with ethanol, to afford tricyanofuran
as bright beige solid. Yield: 73 % (2.9 g). mp = 198−199 °C (lit. mp
196−198 °C).26 1H NMR (400 MHz, CDCl3): δH 2.38 (s, 3H), 1.64
(s, 6H) ppm. Anal. Calcd for C11H9N3O: C, 66.32; H, 4.55; N, 21.09.
Found: C, 66.50; H, 4.43; N, 21.17.
Dianionic Dye 3. A mixture of compound 2 (0.39 g, 1 mmol),
malononitrile (0.2 g, 3 mmol), acetic anhydride (0.31 g, 3 mmol), and
triethylamine (0.61 g, 6 mmol) was stirred in dry acetonitrile (5 mL)
for 2 h at r.t. The solution was concentrated under reduced pressure,
and the residue was dissolved in 5 mL of i-PrOH. The precipitated
impurities were removed by suction filtration, and 3 mL of 15 %
aqueous solution of tetraethyl ammonium iodide was added to the
filtrate. After 16 h, the newly formed precipitate was filtered off and
washed with water to give 3 as a brown solid. Yield: 34 % (0.20 g).
1H NMR (400 MHz, DMSO-d6): δH 7.51 (d, J = 13.2 Hz, 2H), 5.57
(d, J = 13.2 Hz, 2H), 3.20 (q, J = 7.3 Hz, 16H), 1.07−1.24 (m, 24H)
ppm. 13C{1H} NMR (101 MHz, DMSO-d6): δC 171.4, 150.1, 120.2,
118.7, 118.2, 96.8, 76.3, 51.9, 51.4, 7.6 ppm. 19F NMR (376 MHz,
DMSO-d6): δF −144.57 (10B, 0.2F), −144.63 (11B, 0.8F) ppm. Anal.
Calcd for C30H44BF2N7O2: C, 61.75; H, 7.60; N, 16.80. Found: C,
61.90; H, 7.51; N, 16.69.
(d, J = 13.9 Hz, 2H), 7.46−7.62 (m, 8H), 7.08 (d, J = 13.9 Hz, 2H),
3.53−3.66 (m, 4H), 3.05−3.17 (m, 4H), 1.15−1.29 (m, 30H) ppm.
13C{1H} NMR (126 MHz, DMSO-d6): δC 190.9, 190.8, 174.5, 141.5,
140.2, 138.2, 133.0, 132.7, 120.6, 120.2, 118.4, 111.1, 101.9, 80.8,
54.1, 42.3, 18.5, 17.2, 12.9 ppm. 19F NMR (376 MHz, DMSO-d6): δF
−144.20 (10B, 0.2F), −144.26 (11B, 0.8F) ppm. Anal. Calcd for
C42H52BF2N3O6: C, 67.83; H, 7.05; N, 5.65. Found: C, 67.95; H,
7.12; N, 5.73.
Dianionic Dye 7. A mixture of compound 2 (0.39 g, 1 mmol), 8-
(diethylamino)-2,2-difluoro-4-methyl-5-oxo-(5H)-chromeno[4,3-d]-
1,3,2-(2H)-dioxaborine (0.65 g, 2 mmol), acetic anhydride (0.31 g,
3 mmol), and N,N-diisopropylethylamine (0.77 g, 6 mmol) was
stirred in dry acetonitrile (8 mL) for 2 h at room temperature. The
solution was concentrated under reduced pressure, and the residue
was suspended in water. The precipitate was filtered off and washed
with hot methanol to give 7 as a green solid. Yield: 33 % (0.36 g).
1H NMR (400 MHz, DMSO-d6): δH 8.32 (t, J = 13.4 Hz, 2H), 8.16
(br.s., 2H), 7.70 (d, J = 9.2 Hz, 2H), 7.07 (d, J = 13.0 Hz, 2H), 6.77
(dd, J = 9.2, 2.3 Hz, 2H), 6.49 (d, J = 2.3 Hz, 2H), 6.03 (d, J =
13.8 Hz, 2H), 3.54−3.66 (m, 4H), 3.42−3.53 (m, 8H), 3.08−3.18
(m, 4H), 1.19−1.28 (m, 30H), 1.15 (t, J = 6.9 Hz, 12H) ppm.
13C{1H} NMR (76 MHz, DMSO-d6): δC 173.7, 168.7, 167.2, 159.8,
156.7, 153.3, 146.6, 126.5, 117.2, 109.9, 107.9, 105.4, 103.4, 96.2,
94.5, 83.2, 54.1, 44.7, 42.3, 18.5, 17.2, 12.8 ppm. 19F NMR (376 MHz,
DMSO-d6): δF −144.11, −144.06, −144.02, −143.96 ppm. Anal.
Calcd for C54H72B3F6N5O10: C, 59.09; H, 6.61; N, 6.38. Found: C,
59.21; H, 6.85; N, 6.10.
Dianionic Dye 8. A mixture of compound 2 (0.39 g, 1 mmol),
tricyanofuran (0.37 g, 2 mmol), acetic anhydride (0.31 g, 3 mmol),
and N,N-diisopropylethylamine (0.77 g, 6 mmol) was stirred in dry
acetonitrile (8 mL) for 16 h at room temperature. The solution was
concentrated under reduced pressure, and the residue was triturated
in hot isopropanol. The mixture was cooled down, and the precipitate
was isolated by suction filtration and purified by preparative thin layer
chromatography on silica gel (dichloromethane/acetone 4:1) to give
8 as a green solid. Yield: 29 % (0.17 g). 1H NMR (400 MHz, DMSO-
d6): δH 8.47 (br.m, 2H), 8.17 (br.s, 2H), 6.05 (d, J = 13.1 Hz, 2H),
5.94 (d, J = 13.4 Hz, 2H), 3.55−3.67 (m, 4H), 3.08−3.19 (m, 4H),
1.50 (br.s, 12H), 1.20−1.29 (m, 30H) ppm. 13C{1H} NMR (76 MHz,
DMSO-d6): δC 177.1, 172.5, 167.2, 144.1, 117.1, 116.5, 115.5, 109.8,
105.2, 94.5, 83.9, 54.1, 42.3, 27.1, 18.5, 17.2, 12.9 ppm. 19F NMR
(376 MHz, DMSO-d6): δF −143.68 (10B, 0.2F), −143.75 (11B, 0.8F)
ppm. Anal. Calcd for C46H58BF2N9O4: C, 65.01; H, 6.88; N, 14.83.
Found: C, 64.92; H, 6.81; N, 14.61.
Dianionic Dye 4. A mixture of compound 2 (0.39 g, 1 mmol), 1,3-
dimethylbarbituric acid (0.31 g, 2 mmol), acetic anhydride (0.31 g,
3 mmol), and triethylamine (0.61 g, 6 mmol) was stirred in dry
acetonitrile (5 mL) for 2 h at room temperature. The solution was
concentrated under reduced pressure, and the residue was triturated
in isopropyl alcohol. The precipitate was filtered off and purified by
recrystallization from methanol to give 4 as a green solid. Yield: 51 %
1
(0.36 g). H NMR (400 MHz, DMSO-d6): δH 8.92 (br.s, 2H), 8.28
(d, J = 14.1 Hz, 2H), 7.32 (d, J = 14.1 Hz, 2H), 3.15 (s, 12H), 3.09
(q, J = 7.3 Hz, 12H), 1.17 (t, J = 7.3 Hz, 18H) ppm. 13C{1H} NMR
(126 MHz, DMSO-d6): δC 175.6, 163.6, 162.6, 152.4, 146.2, 118.3,
103.3, 95.8, 81.2, 46.3, 28.1, 27.4, 9.1 ppm. 19F NMR (376 MHz,
DMSO-d6): δF −143.79 (10B, 0.2F), −143.85 (11B, 0.8F) ppm. Anal.
Calcd for C32H48BF2N7O8: C, 54.32; H, 6.84; N, 13.86. Found: C,
54.51; H, 6.91; N, 13.79.
X-ray Diffraction Measurements. A single crystal of dye 3
suitable for X-ray analysis was obtained by slow evaporation of
acetonitrile solution at room temperature (3 weeks period). All
crystallographic measurements were performed at room temperature
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Article
on a Bruker Smart Apex II diffractometer operating in the ω scan
mode. The intensity data were collected within the θmax ≤ 26.6°
using Mo−Kα radiation (λ = 0.71078 Å). The intensities of 28 364
reflections were collected (6634 unique reflections, Rmerg = 0.0265).
The structure was solved by direct methods and refined by the full-
matrix least-squares technique in the anisotropic approximation for
non-hydrogen atoms using the Bruker SHELXTL program package.27
All CH hydrogen atoms were placed at calculated positions and
refined as “riding” model. Convergence was obtained at R1 = 0.0470
and wR2 = 0.1227 for 5066 observed reflections with I ≥ 2σ(I); R1 =
0.0639 and wR2 = 0.1360, GOF = 1.018 for 6634 independent
reflections, 379 parameters, the largest and minimal peaks in the final
difference map 0.27 and −0.21 e/Å3. Crystal data for 3: triclinic, space
Eduard Rusanov − Institute of Organic Chemistry, National
Academy of Sciences of Ukraine, 02094 Kyiv, Ukraine
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.joc.1c00138
Notes
The authors declare no competing financial interest.
REFERENCES
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ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.joc.1c00138.
Spectra of new compounds, description of molecular
packing of dye 3, calculated electronic transitions and
charge distributions in dianions 3 and 4, and Cartesian
coordinates of their (TD)-DFT optimized structures
(PDF)
Accession Codes
CCDC 2036277 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
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Corresponding Author
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Qian, J. A. A fluorescence turn-on probe for human (bovine) serum
albumin based on the hydrolysis of a dioxaborine group promoted by
proteins. Chem. Commun. 2017, 53 (48), 6432−6435.
Mykola Shandura − Institute of Organic Chemistry, National
Academy of Sciences of Ukraine, 02094 Kyiv, Ukraine;
orcid.org/0000-0003-0896-4376; Email: shandura@
ukr.net
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Ashokkumar, P.; Faklaris, O.; Galli, T.; Danglot, L.; Klymchenko, A.
S. Ultrabright and Fluorogenic Probes for Multicolor Imaging and
Tracking of Lipid Droplets in Cells and Tissues. J. Am. Chem. Soc.
2018, 140 (16), 5401−5411.
Authors
Vladyslav Polishchuk − Institute of Organic Chemistry,
National Academy of Sciences of Ukraine, 02094 Kyiv,
Ukraine; orcid.org/0000-0003-4396-0119
Andrii Kulinich − Institute of Organic Chemistry, National
Academy of Sciences of Ukraine, 02094 Kyiv, Ukraine;
orcid.org/0000-0002-0857-6632
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