Difluoroborates of phenyl-substituted aza-dipyrromethenes: Preparation, spectral properties, and stability in solution

Article abstract of DOI:10.1134/S1070363215120130

Boron(III) complexes with phenyl-substituted meso-aza-dipyrromethenes (one of them 4,4'-brominated) have been prepared. The bromination has resulted in weakening of the compound fluorescence by more than ten-fold. The studied complexes are stable for a lo

Full text of DOI:10.1134/S1070363215120130

ISSN 1070-3632, Russian Journal of General Chemistry, 2015, Vol. 85, No. 12, pp. 2739–2742. © Pleiades Publishing, Ltd., 2015.
Original Russian Text © N.A. Dudina, M.B. Berezin, A.S. Semeikin, E.V. Antina, 2015, published in Zhurnal Obshchei Khimii, 2015, Vol. 85, No. 12,
pp. 2007–2010.
Difluoroborates of Phenyl-Substituted Aza-Dipyrromethenes:
Preparation, Spectral Properties, and Stability in Solution
N. A. Dudinaa, M. B. Berezina, A. S. Semeikinb, and E. V. Antinaa
a Krestov Institute of Solutions Chemistry, Russian Academy of Sciences, ul. Akademicheskaya 1, Ivanovo, 153045 Russia
e-mail: nad@isc-ras.ru
b Ivanovo State University of Chemistry and Technology, Ivanovo, Russia
Received April 20, 2015
Abstract—Boron(III) complexes with phenyl-substituted meso-aza-dipyrromethenes (one of them 4,4'-
brominated) have been prepared. The bromination has resulted in weakening of the compound fluorescence by
more than ten-fold. The studied complexes are stable for a long time in the solutions in non-polar solvents both
in the dark and exposed to light. The bromine-containing complex rapidly decomposes in the polar media.
Keywords: ms-aza-dipyrromethene, BODIPY, synthesis, absorption spectrum, fluorescence, stability
DOI: 10.1134/S1070363215120130
The interest towards 4,4-difluoro-4-bora-3a,4a-diaza-
s-indacenes (difluoroborates of dipyrromethenes also
known as BODIPY dyes) has emerged in the recent
years; these compounds have been recognized as
universal phosphors exhibiting excellent thermal [1]
and photochemical stability, a high quantum yield of
fluorescence (most often independent of the solvent
nature), and a high molar absorptivity [2]. BODIPY
compounds have been applied as fluorescence labels,
switches, chemosensors, and laser dyes [3–5]. Owing
to the evident advantages of BODIPY over the
conventional phosphors, new strategies of BODIPY
structural modification and the respective properties
optimization (including introduction of various
substituents at pyrrole rings and the spacer of the
dipyrromethene scaffold [6–8]) have been widely
studied.
In this work we studied the effect of the substitution
with heavy atoms (bromine) on spectral properties and
stability in the solution of boron(III) complexes with
phenyl-substituted aza-dipyrromethenes (compounds 1
and 2).
Complexes 1 and 2 were prepared from chalcone 3:
the interaction of the latter with nitromethene in the
presence of diethylamine afforded 1-nitro-2,4-di-
phenylbutan-4-one 4, and its high-temperature intra-
molecular condensation in the presence of ammonium
acetate gave 3,3',5,5'-tetraphenyl-ms-aza-2,2'-dipyrro-
methene 5. Compound 5 was transformed into the
difluoroborate complex 1 via treatment with boron
trifluoride etherate in the presence of triethylamine; the
subsequent bromination with N-bromosuccinimide
yielded the corresponding dibrominated derivative 2
(Scheme 1).
The dyes exhibiting strong absorption and fluore-
scence at the long-wave (600–800 nm) spectral range
can be used as sensibilizers for photodynamic therapy
and metabolism monitoring in biological systems [9].
The 4,4'-halogenated aza-derivatives of BODIPY are
of specific interest in this regard. Besides the light
absorption and emission efficiency, the stability in
different media in the dark and at light is an important
phosphors property in view of their practical
application.
Electron absorption spectra of complexes 1 and 2 in
different organic solvents contained a strong band
[λmax = 643–653 (1) and 641–646 (2) nm], a weak
broadened band [λmax = 475–478 (1) and 504–511 (2)
nm], and a near-UV band [λmax = 307–308 (1) and
282–287 (2) nm] typical of the aryl-substituted
compounds (Figs. 1 and 2). The spectra shape of
complexes 1 and 2 was practically identical in all
studied solvents: the non-polar (cyclohexane and
dichloromethene) as well as polar proton-donor
2739

2740
DUDINA et al.
Scheme 1.
Ph
Ph
CH3NO2
Ph
Ph
Ph
CH2NO2
Et3NH
MeOH
O
O
4
3
Br
Ph
Ph
Ph
Ph
Ph
Ph
Ph
N
N
NH
N
N
N
BF3 Et2O
AcONH4
190oC
NBS
BF2
BF2
N
N
N
Et3N, CH2Cl2
CH2Cl2
Ph
Ph
Ph
Ph
Br
2
5
1
(propanol-1, ethanol, and methanol) and electron-
donor (DMF) ones. The solvatochromism was reflected
in a slight blue (up to 4 nm, compound 1) or red (up to
5 nm, compound 2) shift of the maximum of the
stronger absorption band for the solutions in cyclo-
hexane as compared to the corresponding solutions in
alcohols.
within less than 24 h (Fig. 2b, curve 4). For example,
the absorption band with maximum at 641 nm became
weaker upon storage of ethanol solution of compound
2, and the absorbance in the near-UV region (λmax
=
286 nm) grew stronger; those changes evidenced
practically complete decomposition of the chromo-
phore into the colorless unsaturated or aromatic
compounds. As seen from the data plotted in Fig. 4,
the major changes of the electron absorption spectrum
of ethanol solution of complex 2 were observed during
the first 3 h of the exposure to light.
Fluorescence spectra of solutions of complexes 1
and 2 in cyclohexane (c ≈ 1 × 10–6 mol/L) contained a
single band with a maximum at 674 nm (Fig. 3), the
fluorescence being by more than an order of magnitude
stronger in the case of complex 1.
Most likely, the decomposition of complex 2 in the
polar reactive solvents (alcohols and DMF) occurred
via the nucleophilic substitution of bromine atoms
followed by the reduction of the meso-aza bridge imine
bond with bromide anions and oxidative decom-
position of the aza-dipyrrin scaffold and the sub-
stituents with the formed bromine. Elucidation of the
detailed mechanism of the process requires additional
experiments falling outside the scope of this study.
Solutions of complex 1 in cyclohexane and alcohols
were stable at light for over several weeks. Solutions
of complex 2 in cyclohexane and other non-polar
solvents were stable as well (Fig. 2a), whereas its
solutions in alcohols and DMF became colorless
A
To summarize, the bromination of compound 1 at
the 4,4′ positions of the aza-dipyrrin scaffold sig-
nificantly decreased the stability of that BODIPY dye
in the polar electron- and proton-donor solvents and
weakened its fluorescence, having no effect on the
excellent stability of the studied compounds in inert
non-polar organic media.
EXPERIMENTAL
Chalcon 3 and boron trifluoride etherate (both from
Sigma-Aldrich) were used as received. Organic
solvents (dichloromethane, ethanol, and DMF) were
λ, nm
Fig. 1. Electron absorption spectra of complex 1 solutions
in (1) cyclohexane and (2) propanol.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 85 No. 12 2015

DIFLUOROBORATES OF PHENYL-SUBSTITUTED AZA-DIPYRROMETHENES
2741
(a)
(b)
A
A
1–4
λ, nm
λ, nm
Fig. 2. Electron absorption spectra of complex 2 solutions in (a) cyclohexane and (b) propanol: (1) freshly prepared solution and the
solution stored at light during (2) 3 h, (3) 1 day, and (4) 3 week.
purified by conventional methods [10]. Propanol-1
(UVIR-HPLC-HPLC preparative, PAI) and cyclo-
hexane (Panreac) were used as received. Absorption
and luminescence (λex = 615 nm) spectra were
registered using a СМ2203 (SOLAR) spectrofluori-
3,3',5,5'-Tetraphenyl-ms-aza-2,2'-dipyrromethene
difluoroborate (1). 1.32 g (13.1 mmol) of triethyl-
amine and 1.86 g (13.1 mmol) of boron trifluoride
etherate were added to a solution of 0.588 g
(1.31 mmol) of 3,3',5,5'-tetraphenyl-ms-aza-2,2'-dipyrro-
methene in 40 mL of dichloromethane (room tem-
perature, at stirring). The solution was stirred during
3 h, washed with water, and dried over sodium sulfate.
After the solvent removal the residue was purified by
chromatography on silica gel eluting with dichloro-
methane. The solvent was evaporated, and the product
was precipitated with methanol upon cooling. Yield
1
meter. H NMR spectra of the solutions in CDCl3
(298 K, 500 MHz) were recorded using an AVANCE
III-500 (Bruker) spectrometer installed at the Upper-
Volga Regional Center for Physicochemical Studies.
1-Nitro-2,4-diphenylbutan-4-one (4) was prepared
via the procedure adopted from [9]. A solution of
10.0 g (48.0 mmol) of compound 3, 10.4 mL
(0.193 mol) of nitromethane, and 15.0 mL (0.145 mol)
of diethylamine in 100 mL of methanol was refluxed
during 24 h. After the solvent removal, the residue was
recrystallized from methanol. Yield 8.14 g (63%), mp
97–99°С.
0.426 g (65.4%). Electron absorption spectrum, λmax
,
nm (log ε): 653(5.02), 478(4.05), 308(4.49) (benzene);
643, 475, 307 (cyclohexane); 649(4.91), 478, 308
(chloroform); 647, 478, 307 (propanol-1); 645, 477,
fl
307 (ethanol). Fluorescence spectrum: λ
674 nm
max
(cyclohexane). 1Н NMR spectrum, δ, ppm: 7.07 s (2H,
3,3',5,5'-Tetraphenyl-ms-aza-2,2'-dipyrromethene
(5) was prepared via the procedure adopted from [11].
A mixture of 3 g (3.71 mmol) of compound 4 and 25 g
of ammonium acetate was heated at 190°C during 2 h.
The reaction mass was then cooled to ambient and
diluted with water; the precipitate was filtered off,
washed with water several times, and dried in air. The
so obtained crude product was dissolved in dichloro-
methane and twice purified by chromatography on
silica gel (L 40/100) collecting the medium part of the
major elution zone. The solvent was evaporated, and
the product was precipitated with methanol on cooling.
I
1
Yield 2.35 g (70.2%). H NMR spectrum, δ, ppm: 7.24 s
(2H, 4,4'-H), 7.35–7.69 m (12H, m,p-H-Ph), 7.94–8.17 m
(8H, o-H-Ph), 14.43 br.s (1H, NH). Found %: H 5.26; C
85.59; N 9.24. C32H23N3. Calculated %: H 5.16; C
85.50; N 9.35.
λ, nm
Fig. 3. Fluorescence spectra of solutions of complexes (1) 1
and (2) 2 in cyclohexane (equal concentration of the
complexes).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 85 No. 12 2015

2742
DUDINA et al.
(a)
(b)
0.5
0.4
0.3
τ, min
0.2
0.1
0.0
250
350
450
550
650
750
λ, nm
Fig. 4. Evolution of electron absorption spectrum of ethanol solution of complex 2 in the course of 210 min keeping at (a) light and
(b) the corresponding change of the absorbance (A641/A641,0).
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Peng, X., J. Org. Chem., 2009, vol. 74, p. 7675. DOI:
10.1021/jo901149y.
4,4'-H), 7.44–7.58 m (12H, m,p-H-Ph), 8.04–8.15 m
(8H, o-H-Ph). Found, %: H 4.22; C 76.85; N 8.32.
C32H22BF2N3. Calculated, %: H 4.46; C 77.28: N 8.45.
4,4'-Dibromo-3,3',5,5'-tetraphenyl-ms-aza-2,2'-
dipyrromethene difluoroborate (2). 0.143
g
(0.804 mmol) of N-bromosuccinimide was added to a
solution of 0.1 g (0.201 mmol) of compound 4 in
20 mL of anhydrous dichloromethane. The solution
was refluxed during 1 h, cooled to ambient, and
subjected to chromatography on silica gel (L 40/100)
eluting with dichloromethane. The solvent was evapo-
rated, and the product was precipitated with methanol
upon cooling. Yield 0.088 g (67%). Electron absorp-
tion spectrum, λmax, nm: 646, 507, 287 (cyclohexane);
649 (4.91), 509, 287 (chloroform); 643, 510, 286
(propanol-1); 641, 511, 286 (ethanol). Fluorescence
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C2RA20219K.
fl
max
1
spectrum: λ
674 nm (cyclohexane). H NMR spec-
trum, δ, ppm: 7.46–7.51 m (12H, m,p-H-Ph), 7.74–
7.91 m (8H, o-H-Ph). Found %: H 3.10; C 58.59; N
6.34. BF2C32H20Br2N3. Calculated %: H 3.08; C 58.67;
N 6.41.
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