Synthesis and spectral properties of non-symmetrical red and near IR emitter dibenzoBODIPYs

Article abstract of DOI:10.1016/j.tetlet.2018.01.047

New symmetrical and non-symmetrical benzoBODIPYs have been synthesized from diketones. For the two series the 3 and 5 positions have been substituted by different aromatic rings and onto benzo sub-units different groups have been introduced. The methodology of diketones self-condensation provides symmetrical dyes. By cross-condensation reaction, these positions can be differentiated and specific functions connected to the desired positions. These molecules have been fully characterized and their optical properties analyzed by both experimental and theoretical means. They are red to NIR emitters with a range of emission from 679 to 780 nm in CH₂Cl₂. They show maxima of absorption between 651 nm and 732 nm, strong ε of around 100,000 M^?1 cm^?1 and quite good quantum yields from 16% to 75%. The thienyl moiety on α-positions of the nitrogens generates the highest red shifts. Meanwhile dimethylamino groups in the same positions bring, besides chemical properties, proton sensitive dyes. The bromine atom onto the dibenzo sub-units exhibits good reactivity through Sonogashira coupling reactions. This approach provides multifunctional red to NIR dyes with endless possibilities of combination of chemical properties.

Full text of DOI:10.1016/j.tetlet.2018.01.047

Accepted Manuscript
Synthesis and Spectral Properties of Non-symmetrical Red and Near IR Emitter
DiBenzoBODIPYs
Mustapha Khelladi, Nicolas Leclerc, Denis Jacquemin, Antoinette De Nicola,
Gilles Ulrich
PII:
S0040-4039(18)30083-2
https://doi.org/10.1016/j.tetlet.2018.01.047
TETL 49634
DOI:
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
30 November 2017
16 January 2018
18 January 2018
Please cite this article as: Khelladi, M., Leclerc, N., Jacquemin, D., De Nicola, A., Ulrich, G., Synthesis and Spectral
Properties of Non-symmetrical Red and Near IR Emitter DiBenzoBODIPYs, Tetrahedron Letters (2018), doi:
https://doi.org/10.1016/j.tetlet.2018.01.047
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Tetrahedron Letters
Synthesis and Spectral Properties of Non-symmetrical Red and Near
IR Emitter DiBenzoBODIPYs
a
a
b
a,*
a,*
Mustapha Khelladi , Nicolas Leclerc, Denis Jacquemin , Antoinette De Nicola and Gilles Ulrich
a
b
ICPEES, UMR 7515 CNRS-Unistra, ECPM, 25 Rue Becquerel, 67087 Strasbourg Cedex 02, France; Laboratoire CEISAM, UMR CNRS 6230, Université de
Nantes, 2, rue de la Houssinière, 44322 Nantes Cedex 03, France.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
New symmetrical and non-symmetrical benzoBODIPYs have been synthesized from
diketones. For the two series the 3 and 5 positions have been substituted by different
aromatic rings and onto benzo sub-units different groups have been introduced. The
methodology of diketones self-condensation provides symmetrical dyes. By cross-
condensation reaction, these positions can be differentiated and specific functions
connected to the desired positions. These molecules have been fully characterized and
their optical properties analyzed by both experimental and theoretical means. They are
Available online
red to NIR emitters with a range of emission from 679 to 780 nm in CH
2 2
Cl . They
show maxima of absorption between 651 nm and 732 nm, strong  of around 100000
-1
-1
M cm and quite good quantum yields from 16 % to 75 %. The thienyl moiety on -
positions of the nitrogens generates the highest red shifts. Meanwhile dimethylamino
groups in the same positions bring, besides chemical properties, proton sensitive dyes.
The bromine atom onto the dibenzo sub-units exhibits good reactivity through
Sonogashira coupling reactions. This approach provides multifunctional red to NIR
dyes with endless possibilities of combination of chemical properties.
Keywords:
Synthesis
Fluorophore
BenzoBODIPY
Red emitters
TD-DFT
2
009 Elsevier Ltd. All rights reserved.
Introduction
new applications. Despite this, only few procedures have been
developed. In 1998 in the literature the first example was the
Red and near infrared dyes are widely studied due to their
potential applications in chemical and biological probes, light
emitting diodes,
diarylbenzoBODIPYs are attractive red and near-infrared organic
fluorophores because of their photophysical properties such as
high molar extinction coefficients, intense fluorescence emission
6a
synthesis of monobenzoBODIPYs by reaction of pyrrole on
1
6b,c
isoindole followed by more recent approachs
. However
2
3,4
photovoltaic cells.
Among them
current methods of preparation of dibenzoBODIPYs allow
obtaining symmetrical dyes. Preparation from condensation of
7a
diketones led to symmetrical dyes by self-condensation and non
7b
symmetrical ones are resulted from cross-condensation. But
5
and chemical and photo-stability. Their red shift is achieved via
from this "diketone method" the most common version is the
desymmetrization of symmetrical benzoBODIPYs, for instance
the fusion of the benzene moieties onto ß,ß'-pyrrolic positions
and the extension of the conjugation at 3 and 5 positions of the
BODIPY's skeleton. Modifications of these two fragments allow
modulation of their optical properties and introduction of specific
functionalities. Access to non symmetrical benzoBODIPYs could
expand the range of structural changes and hence pave the way to
8
9
dihalo dye or diacetyl derivatives. More recent approach can
give non symmetrical dyes from condensation of precursors of
10
isoindole the tetrahydroisoindole.

2
Tetrahedron Letters
In this work the synthesis of the benzoBODIPYs (Scheme 1)
3
Aa
OMe
H
H
Br
Br
H
87
73
95
90
73
90
98
89
95
61
38
19
7
was inspired by the literature procedure. It allows preparing dyes
in 4/5 steps in gram-scale quantity from inexpensive reagents.
Another advantage of the presented method is the absence of
potentially instable pyrrole intermediate. The key molecules here
3Ab
3Bb
H
1 2
are the diketones 4 bearing the R , R and Ar groups which
3Ca
3Cb
4Aa
4Ab
4Bb
OMe
H
become the substituents of the benzoBODIPYs skeleton.
Therefore it is possible to modulate the spectroscopic and
chemical properties of the dyes 6 by combining different
substituents. The desired functions association can be achieved
when forming hydrazones 3 and during the reaction of
condensation of diketones 4. For this last step two strategies have
been considered: i) the original synthesis giving symmetrical
benzoBODIPYs, by self-condensation of diketones 4, and ii) an
approach supplying non-symmetrical benzoBODIPYs by cross-
condensation of two different diketones 4. For this purpose new
Br
H
OMe
H
Br
Br
H
H
4Ca
4Cb
OMe
H
1
diketones have been synthesized. R is either H or Br. The
withdrawing bromine atom modifies the electronic density on the
BODIPY core and allows the introduction of further
functionalities by pallado-catalysed coupling reactions.
Br
H
6
Aa
Ab
OMe
H
Conversely methoxy group as R
2
enables to increase the
6
Br
electronic density onto the dyes. It is noteworthy that the aryl
groups (Ar) on -positions of the nitrogens, in this case thienyl,
anisyl and dimethylaminophenyl, do result in a red shift in
absorption and emissionwhereas the dimethylaminophenyl
fragment provides specific properties like a pH responsive
systems.
6
6
Bb
Ca
H
Br
H
46
23
OMe
Our goal in this work was the preparation of non-symmetrical
dyes. The first approach was the functionalization of the
dibromo-BODIPYs 6Ab and 6Bb (Scheme 2). Two types of non-
symmetrical molecules were obtained: monobromide one 8B,
provided by the introduction of a single substituent and 9,
substituted twice by two different groups. To optimize the
conditions of coupling reactions homo-bis-coupling products 7A,
The precursors 2C and 2B are commercially available and
1
1
11
12
2
A, 3Aa and 3Bb are identical of those described in the
11
literature. The hydrazones 3 were prepared by refluxing, with a
catalytic amount of acid, the 2-hydroxyacetophenone derivatives
and carbohydrazides 2 in n-propanol. Derivatives 4 were
1
3
obtained through a transposition reaction with lead tetraacetate
on to the hydrazone 3. During this reaction hydroxy group is
replaced by the aryl moiety very effectively (4, 89-98% yields).
For the synthesis of the compound 4Ca the control of the
reaction time was very important. With more than 3 hours of
reaction, demethylation of amino group was observed, attributed
to oxidation of the tertiary amine. With these diketones a series
of new symmetrical benzoBODIPYs was synthesized. The
dipyrromethenes 5 arose from the self-condensation of indoles
intermediates in a mixture of EtOH/AcOH as a solvent at 90
7B1 and 7B2 were prepared initially. The Stille and Sonogashira
cross-coupling reactions allowed us to get these compounds and
by this additional conjugation a red shift in absorption and
emission for the functionalized productsis expected. An excess of
5
-hexyl-2-tributylstannylthiophene
Pd(PPh as catalyst in toluene at reflux were the conditions to
get a quite soluble benzoBODIPY 7A with 79 % of yield from
Ab. For the Sonogashira reaction the alkynes concerned were
(5
equivalents)
with
3 4
)
6
the 3-dimethylamino-1-propyne and the (triethylsilyl)acetylene.
The former contributed to higher polarity and potentially to water
solubilization of the molecules after the quaternarization of
amino group. The deprotection of the second ethyne would
provide a new grafting function on these multifunctional dyes.
1
4
°
C. The nature of the alcohol as co-solvent has been changed
from the original synthesis. EtOH instead of MeOH gave better
results in our cases. After a rapid extraction with CH Cl and a
2
2
basic treatment the residue was filtered over silica gel and used
without further purification. The complexation of boron was
carried out in anhydrous CH Cl with BF .OEt . The yields of
2 2 3 2
The best results were obtained with Pd(dppf)Cl
2
as a catalyst
with CuI and Et N in toluene. Because of the poor solubility of
3
these two steps were quite good (6, 19 % - 46 %) and consistent
the dibromo-BODIPYs their total conversion was achieved when
the temperature reached 100 °C. 7B1 and 7B2 were isolated with
excellent yields of 83 % and 80 % respectively from 6Bb. Then
the dibromo-BODIPYs were reacted with 1,1 equivalent of
propyne under Sonogashira coupling conditions described above.
From 6Bb the mono-coupling product 8B was obtained with 30%
of yield. Starting material was recovered and bis-coupling
compound (7B1) was isolated too. The introduction of a single
dimethylaminopropyne group was observed onto 6Ab but the
compound 8A was not stable in solution. For that reason it was
not characterized. The attempts to synthesize mono-
triethylsilylacetyl derivatives from the dibromo-BODIPYs led to
a mixture of mono and bis-coupling products which were
impossible to separate. Although, when 6Bb was treated first
with 1 equivalent of dimethylaminopropyne overnight under the
same conditions of the Sonogashira reaction and then with the
silylated alkyne compound 9 could be readily isolated. This two-
step sequence was somewhat more efficient than the single
coupling reaction (preparation of 8B) with two-steps yield of 25
with those reported in the literature.7
Scheme 1. Synthesis of symmetrical BenzoBODIPYs 6
Compds
Yields (%)
2
A
OMe
H
70
%
for 9. In fact the composition of the crude product showed
three compounds (including 9) with clearly different polarities

and therefore easily separable: the bis-amino compounds 7B1
and the bis-triethylsilyl 7B2 isolated with respective yields of 15
and 10 %.
highly polar side products. The following combinations of
diketones allowed distinguishing not only the 3 and 5 positions of
the BODIPYs but in the meantime the dibenzo sub-units. Hence
the structures of 11, 12 and 13 (Scheme 3) featured larger
number of functionnalities. With similar yields the cross-
condensations approach permitted to simultaneously tune the
optical properties and the presence of specific functions onto the
NIR emitter BODIPYs.
Scheme 2. Synthesis of non symmetrical BenzoBODIPYs
from dibromoBODIPYs.
Scheme 3. Synthesis of non symmetrical BenzoBODIPYs by
diketones cross-condensations.
Yields
Compds
(%)
Compds
Yields (%)
10
OMe
OMe
OMe
H
H
H
OMe
H
H
Br
Br
H
15
15
14
10
7
A
_
_
_
_
_
79
1
1
1
1
2
3
7
7
B1
B2
83
80
H
H
8
A
B
Not isolated
8
30
25
Br
OMe
9
a) For compound 7A: 5-hexyl-2-tributylstannylthiophene (5
equivalents), Pd(PPh , toluene, reflux; for compounds 7B1 and
B2: Alkyne, Pd(dppf)Cl , CuI, toluene, Et N, 100 °C.
All these new BODIPYs have been fully characterized by
NMR, IR and Mass spectroscopy. Their optical properties have
been investigated and are shown in Table 1 (Table S1). Their
3 4
)
7
2
3
This post-functionalization approach allowed with moderate
maxima of absorption are in the range of 651-732 nm in CH
2 2
Cl
yields to change the nature of the substituents essentially onto the
phenyl rings of the dibenzo sub-units but not of the aromatic
rings on -positions of the nitrogens. To this end the synthetic
methodology is still the same but instead of proceeding by self-
condensations of diketones 4, cross-condensations of different
diketones (Scheme 3) offer in straightforward ways non
symmetrical BenzoBODIPYs. Compound 10 (Scheme 3) resulted
from the reaction between diketones 4Ca with 4Aa that only
differed by the nature of one aromatic ring. Thereby this
BODIPY showed same dibenzo sub-units but different aryl
moieties in 3,5-positions. The yield obtained is 15 %, but self-
condensation occurred too and the symmetrical bis-
which are assigned to the S transition. Their maxima of
emission are in the range of 679-780 nm and they exhibit a quite
large quantum yields for molecules emitting in this spectral
0
S
1
region 16 to 75 % in CH
2 2
Cl . For symmetrical BODIPY (like
6Ca) as well as non symmetrical ones (like 10-13) these maxima
almost don't move regardless of the solvents used. For
compounds with 4-(dimethylamino)phenyl substituent in on -
position(s) of the nitrogen the fluorescence quantum yields (Ф)
decrease slightly when the dielectric constant of the solvent
increases. Nevertheless, in acetone after protonation, Ф are
restored concomitantly with an emission blue shift (Figure 1).
thienylBODIPY forms in 10
%
yield. The bis-
dimethylaminophenyl derivative could not be isolated from other
Absorption
Emission
Toluene
CH Cl
Acetone
Φ = 27 %
Φ = 19 %
Φ = 14 %
Φ = 22 %
2
2
+
Acetone + H
Figure 1. Absorption (left) and emission (right) spectra of 12 in different solvents, protonation with excess HClg.
Table 1. Optical properties of BODIPYs

4
Tetrahedron
c
a
a
d
d
e
λ
abs
ε
λ
em
τ
F
k
7
r
k
7
nr
Δ
s
b
Compds
Ф
F
Solvant
1
1
1
1
1





(
nm) (M .cm ) (nm)
(ns) (10 s ) (10 s ) (cm )
6
6
6
6
6
6
7
7
7
Ab
Bb
Ca
Ca
Ca
Ca
A
B1
B2
700
87800
86000
95100
69600
86100
75700
102500
117300
118600
125300
124100
91900
80400
67800
69200
88100
85500
84000
78900
88000
79600
83900
86100
85600
74700
73500
71500
741
679
762
769
773
711
765
698
703
687
699
763
774
766
738
725
728
725
692
754
759
753
721
758
765
761
712
41
75
28
21
18
30
24
49
52
54
50
28
22
17
24
36
34
27
40
27
19
14
22
29
23
18
22
5.99
5.52
5.30
4.50
4.70
4.40
5.18
6.76
7.37
5.89
7.26
4.29
3.41
3.52
4.23
6.67
6.38
4.61
4.73
4.40
4.14
3.37
5.99
5.13
5.12
3.96
4.37
6.80
13.6
5.30
4.60
3.80
6.70
4.60
7.30
7.10
9.10
6.89
6.53
6.45
4.83
5.67
5.40
5.33
5.86
8.96
6.14
4.59
4.15
3.67
5.65
4.49
4.55
5.03
9.80
4.53
13.5
17.3
17.4
15.7
14.7
7.60
6.50
7.81
6.89
16.8
22.9
23.6
18.0
9.60
10.3
15.8
12.7
16.6
19.6
25.5
13.0
13.8
15.0
20.7
17.8
790
633
843
943
1088
491
654
532
524
549
486
725
912
989
735
699
820
869
609
821
949
1025
707
773
933
983
574
CH
CH
2
Cl
Cl
2
2
651
716
717
713
687
729
673
678
662
676
723
2
Toluène
CH Cl
Acetone
2
2
+
Acetone +H
CH
CH
CH
CH
CH
2
Cl
Cl
Cl
Cl
Cl
2
2
2
2
2
2
2
8
B
9
2
2
1
0
1
2
3
Toluene
CH Cl
Acetone
7
7
7
23
19
00
2
2
+
Acetone +H
Toluene
1
1
1
690
6
6
6
87
82
64
2 2
CH Cl
Acetone
Acetone +H
Toluene
+
710
7
6
6
08
99
86
2 2
CH Cl
Acetone
Acetone +H
Toluene
+
716
7
7
6
14
08
84
2 2
CH Cl
Acetone
Acetone +H
+
a
b
F
Error ±2 nm. Tetramethoxy-BODIPY, Φ = 0.49 in dichloromethane (λexc = 650 nm) was used as reference. All ΦF were corrected with the
d
F F F F
refractive index of the medium, error ±10%. c Error ±0.5 ns. Calculated using the following equations: kr = Φ /τ and knr = (1−Φ )/τ ,
assuming that the emitting state is produced with unit quantum efficiency. Stokes shift: Δs = (1/λabs) − (1/λem).
e
To complement the experimental characterizations, we have
performed theoretical calculations using an approach combining
Time-Dependent Density Functional Theory (TD-DFT) and a
second-order wavefunction method, namely CC2. This
1
5
procedure, described in previous works and summarized in the
SI was applied to all 6Xx and 1x dyes in their neutral and
protonated forms. The theoretical calculations nicely reproduce
the experimental 0-0 energies as well as the variations of the
energies following substitutions (see Table S1). More
importantly, they reveal that, for a given dye, the ground and
excited-state dipole moments are similar with a maximal
difference of 1.4 D (6Ca), explaining the above-mentioned very
small solvatochromic effects. The density difference plots (EDD)
of four representative dyes are displayed in Figure 2 whereas
Figure S28 provides complete data. The EDD are typical of those
16
found in BODIPY derivatives, with the core of the BODIPY
concentrating the largest density variations. In 6Ca, one notices
that the methoxy and amino groups appear with rather small blue
lobes in Figure 2, confirming that they act as donor groups.
However, the changes of density on these groups are rather
limited, explaining why all dyes absorb and emit in similar
spectral regions: the auxochromes added at the periphery do not
play a major role. The small blueshifts induced by protonation of
Figure 2. EDD for four dyes. The red (blue) zones indicate
increase (decrease) of density upon electronic transition.
Cyclic voltammetry of 3 prototypical compounds (6Ab, 6Ca &
the amino groups is also readily explained by the EDD plots: 10) has been recorded in dichloromethane (Table S2 lists the
protonation of this group renders it insignificant in the excited- potentials relative to the SCE reference electrode). For 6Ab a first
state. Eventually, by comparing the EDD of 6Ca and 10, we quasi-reversible
oxidation
.
is
assigned
to
the
note that they present very similar topologies, i.e., the nature of
+
5c
(
BODIPY/BODIPY ) couple at +0.58, for 6Ca & 10 bearing a
the excited-state is not strongly impacted by the (a)symmetry of
the system for the present derivatives. This is confirmed by the
computed charge-transfer parameters that do not reveal any
significant CT character even in non symmetric compounds
dimethylamino function a first oxidation is observed respectively
at +0.20 & +0.26 V attributed to the amine radical cation
formation, and a second probably localized on the BODIPY core
at +0.50 & +0.66 V (Fig S29-S31). Both waves have a shape
characteristic of a Nernstian one-electron process. The 3 dyes
present reduction to the radical anion centered on BODIPY in the
range -0.97 to -1.26 V.
(
Table S1 in the SI).

We have succeeded in the synthesis of dissymmetric
dibenzoBODIPY dyes by two synthetic pathways: a posteriori
dissymetrization via Pd catalyzed cross-coupling reaction or by
aromatic diketones cross-condensation. By introduction of a
single aromatic tertiary amine we have obtained a pH responsive
fluorophore in the NIR region, which could pave the way to the
development of NIR ratiometric fluorescent probes. Moreover,
such dissymmetric BODIPYs are fundamental precursors for the
construction of chiral Boron based dyes with a stereogenic Boron
center. Works are ongoing to reach these targets.
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Acknowledgments
We acknowledge the CNRS providing research facilities and
financial support and the Ministère de l'Enseignement Supérieur
et de la Recherche for a MENRT fellowship for M. Khelladi. We
also thank Drs C. Pham-Huu, P. Adam and P. Shaeffer for their
kindness and the huge help during these last months. The
theoretical calculations used resources of the CCIPL in Nantes as
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Graphical Abstract
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Synthesis and Spectral Properties of Non-
symmetrical Red and Near IR Emitter
DiBenzoBODIPYs
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Mustapha Khelladi, Nicolas Leclerc, Denis Jacquemin, Antoinette De Nicola and Gilles Ulrich

6
Tetrahedron
Highlights
Synthesis of non-symmetrical BenzoBODIPY
Cross-condensation of diketones
Red NIR emitters
pH responsive dyes