Novel BODIPY preparations from sterically hindered pyrroles. Synthesis and photophysical behavior in solution, polystyrene nanoparticles, and solid phase

Article abstract of DOI:10.1134/S1070363208110467

Trimesityl-BODIPY (TMB), a new derivative of 4,4-difluoro-4-bora-3a,4a- diaza-s-indacene belonging to fluores-cent nanostructures series, was synthesized from the corresponding pyrrole by the Trofimov reaction. This reaction was also employed to obtain 2-[2.2]paracyclophanylpyrrole from 5-acetyl[2.2]-paracyclophane. The spectral properties of TMB have been investigated in dichloromethane, nanolatex (polystyrene) films prepared by rapid solvent evaporation, and microcrystals. Comparative analysis of TMB properties with those of mesityl-BODIPY (MB) was performed. TMB was prepared to minimize π-π interactions in order to preserve luminescence in the aggregate state. Both fluorophores were shown to form fluorescing aggregates in the amorphous state (film). Fluorescence spectra (extinction and lifetime) were also studied. In crystal, MB shows a weaker fluorescence, while TMB behaves as a single fluorescing aggregate with a lifetime of 9.5 ns.

Full text of DOI:10.1134/S1070363208110467

ISSN 1070-3632, Russian Journal of General Chemistry, 2008, Vol. 78, No. 11, pp. 2247–2256. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © R. Meallet-Renault, G. Clavier, C. Dumas-Verdes, S. Badre, E. Yu. Shmidt, A.I. Mikhaleva, C. Laprent, R. Pansu, P. Audebert,
B.A. Trofimov, 2008, published in Rossiiskii Khimicheskii Zhurnal, 2008, Vol. 52, No. 1, pp. 91–99.
Novel BODIPY Preparations from Sterically Hindered Pyrroles.
Synthesis and Photophysical Behavior in Solution,
Polystyrene Nanoparticles, and Solid Phase
R. Meallet-Renaulta, G. Claviera, C. Dumas-Verdesa, S. Badrea, E. Yu. Shmidtb,
A. I. Mikhalevab, C. Laprenta, R. Pansua, P. Audeberta, and B. A. Trofimovb
a PPSM, ENS Cachan, CNRS, UniverSud, 61 av. President Wilson, F-94230 CACHAN, France
b Favorskii Irkutsk Institute of Chemistry, Siberian Branch, Russian Academy of Sciences,
ul. Favorskogo 1, Irkutsk, 664033 Russia
e-mail: boris_trofimov@irioch.irk.ru
Received February 15, 2008
Abstract―Trimesityl-BODIPY (TMB), a new derivative of 4,4-difluoro-4-bora-3а,4а-diaza-s-indacene
belonging to fluores-cent nanostructures series, was synthesized from the corresponding pyrrole by the
Trofimov reaction. This reaction was also employed to obtain 2-[2.2]paracyclophanylpyrrole from 5-acetyl[2.2]-
paracyclophane. The spectral properties of TMB have been investigated in dichloromethane, nanolatex
(polystyrene) films prepared by rapid solvent evaporation, and microcrystals. Comparative analysis of TMB
properties with those of mesityl-BODIPY (MB) was performed. TMB was prepared to minimize π–π
interactions in order to preserve luminescence in the aggregate state. Both fluorophores were shown to form
fluorescing aggregates in the amorphous state (film). Fluorescence spectra (extinction and lifetime) were also
studied. In crystal, МВ shows a weaker fluorescence, while TMB behaves as a single fluorescing aggregate
with a lifetime of 9.5 ns.
DOI: 10.1134/S1070363208110467
fluorescent polymers [8], zeolites [9] or activated latex
spheres, as well as inorganic nanocrystals (quantum
points) [10]. Organic crystals or pigments [11] are, in
principle, quite attractive candidates, since they allow
one to create a high local fluorophore concentration
and ultrabright objects [12], but, unfortunately, very
little fluorophores are able to form radiating crystals.
Actually, aggregation suppresses fluorescence [13] due
to interactions between molecular electron clouds,
which increases the probability of radiationless
transitions [14].
INTRODUCTION
Fluorescence spectroscopy is used for structural
and dynamic studies on both organic and inorganic
systems. The success of this technique is due to a great
sensitivity of fluorophores to their immediate
environment [1]. Fluorescence can be used for high-
precision physicochemical and biochemical
measurements. The technical possibilities of
fluorescence spectroscopy allow spectra of a single
molecule to be measured [2]. The fluorescence
technique has found application in biochemical [3],
medical chemical [4], and environmental analysis [5].
Over some years we have synthesized various
fluorescing crystals, aimed at obtaining novel radiating
derivatives of 4,4-difluoro-4-bora-3а,4а-diaza-s-
indacene which is better known under the registered
trade mark BODIPY®.
The popularity of fluorescence spectroscopy
favored extension of the range of organic and
inorganic fluorophores. They can be used in a variety
of ways, mostly as labels [6]. However, certain
limitations can arise because of fluorescence
quenching and decreased quantum yields. Alternative
approaches were developed, such as use of dendrimers
[7] which still remain fairly expensive materials,
These chromophores and, especially, their
polysubstituted derivatives possess optimal spectral
characteristics (high fluorescence quantum yield, high
radiative stability) [15], and, at the same time, are
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MEALLET-RENAULT et al.
properties of the products in solution and in the solid
phase is also presented.
Synthesis of BODIPY
Synthesis of BODIPY was performed in three
stages: condensation of aldehyde with two pyrrole
molecules, oxidation, and complex formation with
boron trifluoride (Scheme 1). Intermediate products
were not isolated.
readily synthesized from pyrrole derivatives [16]. Our
approach is based on the use of BODIPY nanocrystals
or dispersions in latex nanospheres and aimed at
producing ultrasensitive fluorescent sensors.
Burgess and co-workers [20] suggested a new
approach, by replacing aldehyde with acid chloride
(Scheme 2).
It was shown that fluorescence quenching in the
solid phase can be limited or avoided if fluorophores
are locates at a certain distance from each other [17].
With this in mind we synthesized sterically hindered
pyrroles and then used them as starting materials for
preparing BODIPY derivatives capable of forming
fluorescing crystals. These compounds can also be
introduced in polymer nanoparticles [18] without the
risk of fluorescence quenching on aggregation.
Moreover, substitution in the pyrrole ring shifts the
absorption spectrum of the resulting derivatives to
lower energies [19].
The molecular geometries were calculated
quantum-chemically for structures with the lowest gas-
phase energy.
This procedure does not involve isolation of the
intermediate indacene and oxidation with DDQ
(dichlorodicyanobenzoquinone), which offers great
advantages by excluding by-product formation. The
procedure involves prolonged heating (24 h with
stirring in a solvent), but the reaction yield is generally
higher.
The first BODIPY derivative (МВ) was prepared
by condensation of cryptopyrrole with mesitylaldehyde
(Scheme 3) in 70% yield [21].
Here we report on the synthesis of pyrroles with
sterically hindered substituents by the Trofimov
reaction and their subsequent transformation into
BODIPY. Detailed information on the photophysical
Solid-phase photophysical properties of the
resulting derivative proved to be quite reasonable (vide
Scheme 1. Synthesis of BODIPY by condensation of pyrrole with aldehyde.
Scheme 2. Synthesis of BODIPY by Burgess by condensation of pyrrole with acid chloride.
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NOVEL BODIPY PREPARATIONS FROM STERICALLY HINDERED PYRROLES
2249
Scheme 3. Synthesis of МВ.
involving condensation of oximes with acetylenes in a
superbasic medium (Scheme 4) [22].
supra). In the meantime, pyrroles with bulkier
substituents were prepared by the Trofimov reaction,
Scheme 4. Trofimov reaction.
aromatization
found that ethyl mesityl ketone (I) is quite poorly
reactive toward hydroxylamine. Oximation by reflux-
ing the ketone with hydroxylamine hydrochloride
under varied conditions, including microwave
activation, proved ineffective. “Lethargic oximation” (over
the course of 8 months) gave as little as 30% of ethyl
mesityl ketone oxime (II) (Scheme 5).
This reaction allows an almost infinite range of
pyrroles to be synthesized from acyl oximes and
acetylene.
In the present we took two aromatic compounds:
mesitylene and [2.2]paracyclophane. Difficulties arose
in the synthesis of ethyl mesityl ketone oxime. It was
Scheme 5. “Lethargic” synthesis of ethyl mesityl ketone oxime.
A different synthetic approach to oxime II was also
used: Mesitylmagnesium bromide (IV) was reacted
with propionitrile to form the corresponding imine
whose oximation gave the target oxime (total yield
~40%). The whole procedure can be accomplished
within two days.
Scheme 6. Alternative synthesis of ethyl mesityl ketone oxime.
HCl
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MEALLET-RENAULT et al.
Fig. 1. Gas-phase structures of МВ and ТМВ, obtained by
В3LYP/6-31G٭(d) minimization.
Fig. 2. (a) ОН and (b) и BV molecular orbitals of ТМВ.
In 5-acetyl[2.2]paracyclophane, steric hindrances
are expressed to a lesser extent. In this case, oxime can
be easily prepared in high yield (97%) by direct
oximation of ketone with hydroxylamine.
Comparison of the two structures shows that the
three bulky substituents in the ТМВ molecule should
hinder intermolecular interactions. It can be expected
that ТМВ should surpass MB in solid-phase photo-
physical properties. The S0 → S1 transition in BODIPY
occurs between the HO and BV orbitals (see Fig. 2)
[26].
Pyrroles were synthesized by reacting oximes with
acetylene in the presence of the KOH/DMSO
superbase. With ethyl mrsityl ketone oxime, the
corresponding pyrrole was obtained in 23% yield at an
atmospheric pressure of acetylene. Under the same
conditions, the yield of pyrrole from 5-acetyl[2.2]-
paracyclophane oxime was as little as 10–15%. This
result can be explained by deoximation of the target
oxime under the reaction conditions. An alternative
approach was suggested, that involved initial forma-
tion of anhydrous cesium oximate and subsequent
reaction in the presence of acetylene at p 14 atm to
obtain pyrrole in a total yield of 41% [23].
In ТМВ, these two orbitals are localized on the
central indacene and have nothing in common with the
aromatic rings at the molecular periphery. The same
result is observed with МВ. Substituents have no effect
on the positions of the absorption and emission bands
of МВ and ТМВ derivatives.
Spectrometric Studies
Fluorescence in Solution
The absorption and fluorescence spectra of МВ and
ТМВ are shown in Fig. 3.
Similarly, BODIPY ТМВ was synthesized by
condensation of 2-mesityl-3-methylpyrrole in 45%
yield.
The absorption maxima are at 526 and 543 nm and
correspond to the S0 → S1 transition There is also one
more absorption band near 350 nm, assignable to the
S0 → S2 transition. the emission maxima (S1 → S0) are
at 535 and 556 nm for МВ and ТМВ, respectively. The
red shift of the maximum is due to the inductive effect
of mesityl groups. Such spectral pattern is
characteristic of all molecules of the BODIPY family.
The molecular extinction coefficients of МВ and ТМВ
and the fluorescence quantum yields are fairly high
(Table 1).
Molecular Modeling
The two obtained BODIPY derivatives were
modeled (Fig. 1) by the DFT method [24].
The central indacene in both molecules is planar,
and the mesityl rings embraced between the 2- and 6-
Me groups are inclined to the indacene plane by 90°.
Thus, the aromatic systems serve as good bulky
substituents that do not violate the fine structure of the
central fragment in BODIPY. These results are
consistent with data Kee et al. [25], who showed that
the steric hihindrances associated with the substituent
R4 in the BODIPY molecule much affect the
fluorescence properties of BODIPY.
The excitation spectra coincide with the absorption
spectra. The fluorescence lifetime in dichloromethane
is ~6 ns ( 0.1). Both fluorophores are stable under
light irradiation (no changes were observed under light
irradiation at 200 mW cm–2).
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NOVEL BODIPY PREPARATIONS FROM STERICALLY HINDERED PYRROLES
2251
Fig. 3. (a) Absorption and (b) emission spectra in dichloromethane: (dashed line) МВ and (solid line) ТМВ.
R = H or CH=CH2
Fig. 4. Scheme of synthesis of polystyrene nanoparticles with cyclam groups.
Earlier a BODIPY containing a 4'-iodophenyl
substituent in the 4 position of the pyrrole ring was
synthesized. The properties of this BODIPY in
chloroform are quite similar to those of МВ. Burghart
and co-workers [27] synthesized an analog of ТМВ
containing a 4'-iodophenyl substituent in the 4 position
and a phenyl substituent in the 1 position. However,
this latter BODIPY has a much lower fluorescence
quantum yield (0.2 in CHCl3), since phenyl rotation
gives rise to radiationless transitions. Blocking such
rotation much enhances the quantum yield [25, 27].
Rotation of mesityl groups about the C–aryl bond in
ТМВ is hindered. This fact explains the observed high
fluorescence quantum yield.
Latex 15 nm in diameter is stable in aqueous sus-
pension in the presence of a surfactant (dodecyltrime-
thylammonium bromide, 15–0.05 wt %).
Balls have different colors depending on the nature
of the metal ion in the solution (Fig. 5). They are
colorless in straight water and get bluish violet, if Cu
(II) ions are added. Balls were impregnated with МВ
molecules by swelling in dichloromethane. Organic
Table 1. Characteristics of the absorption and fluorescence
spectra in dichloromethanea
Fluore-
scence
Extinction
coefficient,
ε,
Absorption
maximum,
λads, nm
Life- Quantum
time, τ, yield,
BODIPY
derivative
Activated Polystyrene Nanoparticles
maximum,
ns
Φf
λem, nm l mol–1 cm–1
We embedded МВ molecules in polymer balls
(Fig. 4).
MB
526
543
535
556
58000
80000
6.10
6.30
0.72
0.90
These particles contain both surface and central
cyclam groups (L) that can associate with Zn(II), Cu(II),
and Ni(II) ions (Table 2).
TMB
a χ2 (MB) 1.058; χ2 (TMB) 0.969.
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2252
MEALLET-RENAULT et al.
10–9 M
1.0
Сu2+
concentration
0.5
0.0
5×10–6
M
Fig. 5. Absorption spectra of cyclams: (1) free cyclam
ligands; (2) cyclam–nickel complex; (3) cyclam–zinc
complex; and (4) cyclam–copper complex.
Fig. 6. Fluorescence spectrum of nanolatex containing МВ
molecules in the presence of Сu2+.
solvent discloses nanoparticles, the hydrophobic
fluorophore escapes the aqueous medium and
penetrates into balls that contract when the solvent is
evaporated. Therewith, the spectral properties of МВ
change only slightly (λads 528.5 nm, λem 540 nm,
fluorescence quantum yield 0.77). It was shown that
one ball can accommodate 70–80 МВ molecules
without changing its size and color. When more MB
molecules are inserted (up to 110 molecules per ball),
the system gets less stable (it loses color within several
months). Red shifts of the absorption and emission
spectra and decreased fluorescence lifetimes and
quantum yields were also noted, imply-ing formation
of nonradiating aggregates.
fluorophore can be present in two different states. The
fluorescence of 80% chromophore molecules is
quenched by the cyclam–copper complex, and the rest
20% preserve their initial properties. This finding
allows us to state that 20% fluorophore molecules
reside in the very center of the particles. On the other
hand, fluorescence quenching is quite efficient (9%
quenching at the ion concentration 5 nmol l–1), which
can be explained by the antenna effect (energy jump
on Forester’s transfer) between BODIPY molecules
enclosed in the particle. Evidence for this conclusion
comes from fluorescence anisotropy measurements.
By embedding BODIPY family particles in nano-
particles, we could enhance the sensitivity of a
colorimetric sensor toward Сu2+ ions.
The samples containing 75 fluorophore molecules
per ball are selective toward Cu(II). Actually, it is only
these ions that are able to quench fluorescence (Fig. 6);
thus, the detection limit of copper ions is about 1 nmol l–1
(0.03 copper ions per ball).
We also embedded ТМВ molecules in nano-
particles whose surface was modifies with cyclam
groups [28]. These nanoparticles have almost the same
spectral properties as free ТМВ molecules (λads
545 nm, λem 559 nm, τF 7 ns), but no more than 40 mole-
cules can be embedded in one ball, and, therewith,
reduced solubility of the dye outside the matrix is
observed. The molecular size of ТМВ (ca. 15×10×
5 Å) is larger compared to МВ (ca. 10×10×5 Å), and
the polymer has a different structure if the cyclam
Nickel and zinc ions have no effect on fluorescence.
Such selectivity can be explained in terms of energy
transfer from fluorophore to colored complex, since
the spectrum of the cyclam–copper complex much
overlaps with the emission spectrum of МВ.
Analysis of spectral data shows that inside particles
Table 2. Association constants of cyclam with metal ions
Parameter
Association constant
Ball color in solution
Cu(II)
1027
Ni(II)
1020
Zn(II)
1020
Co(II)
1013
–
Cd(II)
1011
Bluish violet
Bluish green
Colorless
Colorless
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NOVEL BODIPY PREPARATIONS FROM STERICALLY HINDERED PYRROLES
2253
Fig. 7. (a) Absorption and (b) emission spectra of ТМВ: (1) in film; (2) in dichloromethane; and (3) in monocrystal.
ligand is grafter exclusively to the surface and is not
contained inside balls. By this reason, one nanolatex
particle binds more МВ than ТМВ molecules.
Experiments on TMB fluorescence quenching with
metal ions are in progress.
In the lower part of the figure we present a scheme
of dipole orientation in the ground and higher excited
fluorophore states. The absorption spectrum is shifted
red, relaxation is faster, and lifetime is shorter than for
the monomer in solution. The first absorption range is
contributed by monomeric ТМВ molecules, and the
second is definitely associated with J-type aggregates.
As mentioned above, aggregation of balls with high
MB contents took place, which followed from changes
in certain fluorescence parameters. The properties of
МВ and ТМВ in amorphous films and crystal powders
were also studied.
Our aim was to decrease the “monomer contribu-
tion” (λem ~ 569 nm) into image formation over the
fluorescence lifetime of the aggregates (λem.~ 619 nm)
(Fig. 9). To this end, we made use of filters to “isolate”
accessible wavelength ranges.
Amorphous Films
Amorphous МВ and ТМВ films can be obtained by
rapid solvent evaporation. Inspection of the fluore-
scence spectra reveals micrometer-sized drops at a
distance of a few nanometers from each other.
Table 3. Spectral data for amorphous films and crystal
powders
Figure 7 shows the absorption and emission spectra
of a ТМВ film.
Object
λads , nm
λem, nm
562
635
739 (peak)
<τ> , ns
1. 36
Compared with the dichloromethane solution, the
absorption spectrum is broader, and its maximum is
shifted red (Δλ 10 nm). The fluorescence spectra contain
two maxima: 569 4 and 619 2.9 nm (Table 3).
MB film
542
608
739
MB powder
1. 28
Multiexponential fluorescence decay is observed,
and the apparent lifetime is shorter than for monomeric
molecules in solution (from 0.5 to 1.5 ns).
569
619
TMB film
553
–
0. 5–1. 5
1. 77
TMB powder
TMB microcrystal
598
608
According to Kasha’s aggregation model [28], the
molecular dipoles in J-type aggregates are oppositely
directed (Fig. 8): An allowed transition from the
ground state to a lower energy level takes place.
9.5a
–
[1.087]b
Monoexponential decay. b χ2.
a
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MEALLET-RENAULT et al.
H-type aggregates
Intermediates
J-type aggregates
The “aggregate contribution” (619-nm filter) into
the intensity of the image is nonuniform over the
sample (Fig. 9c), implying that the aggregate is
inhomogeneous; the lifetime is longer than for the
monomer.
Excited state
Ground state
Excited state
Together the above evidence suggests that the
principal form here is the monomer. Intensity visualiza-
tion shows that it is distributed homogeneously, and its
emission is quenched due to aggregate formation.
(...
...)
...)
|
|
|
|
(--------------)
(--------------)
(--------------)
(--------------)
(...
|
|
|
|
(...
Ground state
...)
|
|
|
|
The absorption spectrum of МВ films is broader
compared to the monomer and shifted red (Table 3).
The fluorescence spectra contain a broad band with at
562 3.5 nm (in some cases, additional broadening at
635 nm is observed) and a band at 739.5 1 nm. The
mean lifetime is 1.36 0.07 ns. Thus, we can suggest
that J-type aggregates are also formed in amorphous
films.
Fig. 8. Diagram of exciton states and schematic representa-
tion of dipole orientation in aggregates in different states
[29].
The “monomer contribution” (560-nm filter) can be
observed in Fig. 9b: a homogeneous image over the
entire sample, both in terms of intensity and in terms
of lifetime. It should be noted that the lifetime for the
monomer is much smaller than in the case of the total
fluorescence yield (entire wavelength range, Fig. 9a).
Probably, the monomer monotonically decreases
emission: fluorescence quenching occurs.
Microcrystals
We obtained fluorescence spectra for MB and TMB
crystal powders. Compared with the above-described
amorphous films, the fluorescence spectra of ТМВ
crystal powders are much broader and contain a single
Fig. 9. Images of amorphous films of ТМВ: (а) full spectrum; (b) monomer contribution; (c) aggregate contribution; and (d)
fluorescence decay for image (○) a; (□) b; and (Δ) c.
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NOVEL BODIPY PREPARATIONS FROM STERICALLY HINDERED PYRROLES
CONCLUSION
2255
Novel sterically hindered BODIPY derivatives
were prepared by the Trofimov reaction. This synthetic
approach contributed to enhanced diversity of
derivatives of the BODIPY family. One of such
derivatives (МВ), water-soluble and sensitive to Cu+2
ions in concentrations of about 1 nM, was used for the
creation of a fluorescent nanosensor. Research into
solid-phase optical properties of МВ and ТМВ showed
that such bulky substituents, as mesityl groups, in the
pyrrole rings are big with consequences, by contrast
alkyl substituents. Aromatic substituents favor
formation of the crystal structure of ТМВ.
The behavior of ТМВ in the solid phase differs
from that in solution, which differentiates this
compound from МВ. At the same time, the radiating
ability is much decreased by defects in the solid phase.
To approach this problem, we are presently developing
new BODIPY derivatives having bulkier substituents.
We tend to find a simple formula and develop an
effective procedure for preparing fluorophore nano-
ensembles. The aim of our work is to obtain ultra-
sensitive nanosensors for determining in vivo such
biologically important cations as intercellular calcium
or such specific molecules as ATP.
Fig. 10. Fluorescence of ТМВ crystal.
maximum at 598 12 nm. The decay is multiexpo-
nential, and the mean lifetime is 1.77 0.8 ns. With
МВ, two bands are observed: at 608 16 and 739.5
1 nm. Again, the decay is multiexponential, lifetime
1.28 0.23 ns.
TMB microcrystals could also be obtained by
evaporation of
a
concentrated dichloromethane
solution. Figure 10 shows a fluorescing TMB crystal
50 μm in size.
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