Monofunctionalized 1,3,5,7-tetraarylazaBODIPYs and their application in the synthesis of AzaBODIPY based conjugates

Article abstract of DOI:10.1021/acs.joc.9b01311

A series of monofunctionalized 1,3,5,7-tetraarylazaBODIPYs containing functional groups such as p-hydroxymethyl phenyl, p-hydroxyphenyl, p-cyanophenyl, p-nitrophenyl, and p-formylphenyl groups at the 1-position of the azaBODIPY core were synthesized by mi

Full text of DOI:10.1021/acs.joc.9b01311

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Article
Mono-functionalized 1,3,5,7-TetraarylAzaBODIPYs and Their
Application in the Synthesis of AzaBODIPY Based Conjugates
Angira Koch, and Mangalampalli Ravikanth
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b01311 • Publication Date (Web): 12 Aug 2019
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Mono-functionalized 1,3,5,7-TetraarylAzaBODIPYs and Their Application in
the Synthesis of AzaBODIPY Based Conjugates
Angira Koch and Mangalampalli Ravikanth*
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076,
India. E-mail: ravikanth@chem.iitb.ac.in
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ABSTRACT:
A series of mono-functionalized 1,3,5,7-tetraarylazaBODIPYs containing functional
groups such as p-hydroxymethyl phenyl, p-hydroxyphenyl, p-cyanophenyl, p-nitrophenyl and p-
formylphenyl groups at 1-position of azaBODIPY core were synthesized by mixed condensation
of two different nitrochalcones in n-butanol in the presence of CH3COONH4 at reflux followed
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by complexation with BF3 OEt2. The mixed condensation of nitrochalcones resulted in the
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formation of three different dipyrromethenes, which was treated with BF3 OEt2 to afford the
desired mono-functionalized tetraarylazaBODIPYs in 30-35 % yields. To demonstrate the
application of mono-functionalized tetraarylazaBODIPYs, we reacted mono-formyl
functionalized tetraarylazaBODIPY with excess pyrrole to afford mono-dipyrromethanyl
substituted tetraarylazaBODIPY, which was used as a key precursor to prepare novel covalently
linked azaBODIPY based conjugates. The mono-dipyrromethanyl azaBODIPY was in situ
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oxidized with DDQ and either reacted with BF3 OEt2 to afford azaBODIPY-BODIPY conjugate
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or reacted with metal salt such as Pd(acac)2 to afford azaBODIPY-Pd(II)dipyrrin conjugate.
Alternately, the dipyrromethanyl-substituted azaBODIPY was condensed with dipyrromethane
dicarbinol or 16-oxatripyrrane under mild acid catalyzed conditions followed by oxidation and
chromatographic purification to afford azaBODIPY-porphyrin or azaBODIPY-oxasmaragdyrin
conjugates, respectively. The photophysical studies on conjugates revealed that azaBODIPY is a
good energy acceptor and invoked the possibility of energy transfer from donor to acceptor in
covalently linked conjugates.
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INTRODUCTION
4,4-Difluoro-4-bora-3a, 4a-diaza-s-indacenes (BODIPYs, 1)1 are one of the most
attractive fluorescent dyes used as energy/electron donor/acceptor systems for various
applications because of their favorable spectral and electrochemical properties viz., high photo
stability, high extinction coefficients, decent quantum yields and redox properties.2 BODIPY
dyes have been used in several research fields including drug discovery, biomedical imaging and
optical sensing. BF2-chelated tetraarylazadipyrromethene derivatives (azaBODIPYs, 2)3 are the
structural analogues of BODIPYs derived by replacing the meso “C” of BODIPYs with “N” and
exhibits much better photophysical properties than BODIPYs. For example, BODIPYs show
absorption and emission bands in the visible region with decent quantum yields whereas aza-
BODIPYs absorb and emit in the NIR region with much better extinction coefficients and
quantum yields. AzaBODIPYs have also been used in various applications such as
photosensitizers in photodynamic therapy,4 in sensing anions, cations5 and as photosynthetic
model systems capable of harvesting energy.6 However, compared to BODIPYs, which have
been investigated extensively over the years, the research on azaBODIPYs is relatively less
because of the involvement of more number of synthetic steps and extensive purifications in
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obtaining the intermediate precursors needed for the synthesis of azaBODIPYs. In fact, the
azaBODIPYs were generally synthesized as symmetrical 1,3,5,7-tetraaryl azaBODIPYs and
interestingly; there are very few reports available on unsymmetrical functionalized
azaBODIPYs. A perusal of literature revealed that only few synthetic reports are available on
difunctionalized azaBODIPYs where the functional groups were either introduced at the aryl
groups present at the 3,5-positions of azaBODIPYs7,8,9 3 or at the direct 2, 6-positions of aza-
BODIPY core 4 and 5 (Chart 1). The reports on functionalized azaBODIPYs where functional
groups present at the 1,7-positions are very few and the mono-functionalized azaBODIPYs
having one functionalized aryl group at 1 or 7 positions of azaBODIPY core is very scarce.
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Chart 1: Structures of BODIPY (1) and aza-BODIPYs (2-5).
O’Shea and co-workers10 reported the synthesis of unsymmetrical tetraarylazadipyrrins
by cross condensation of nitroso derivatives of 2,4-diaryl pyrrole containing two different aryl
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groups with different 2,4-diarylpyrroles in acetic anhydride/acetic acid mixture at 100 C. The
research group of O'Shea successfully prepared one example of mono-functionalized aza-
dipyrrin containing bromo-aryl group (Scheme 1) which was subsequently used by Parisotto et
al.11 to carry Heck coupling reactions and prepared a series NIR absorbing azaBODIPYs.
Borisov and co-workers12 adopted mixed condensation approach to prepare a series
unsymmetrical azaBODIPYs by condensing two different nitro chalcones.
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Scheme 1: Synthetic route for synthesis of unsymmetrical azaBODIPYs: (a) acetic
anhydride/acetic acid, 100 0C, 1h; (b) aryl bromide, t-Bu acrylate, TBAB, DMF, 100 0C, stirring,
8 h; (c) CH2Cl2, TEA, BF3.OEt2.
under base catalyzed conditions followed by BF2 complexation. Except couple of these scattered
reports, to the best of our knowledge, there is no report on tetraarylazaBODIPYs that are mono-
functionalized on one of the four aryl rings. The mono-functionalized tetraarylazaBODIPYs, if
easily accessible, can be used as building blocks to synthesize fluorescent azaBODIPY based
systems and test their potential applications in various research fields. Herein, we report the
synthesis of unsymmetrical mono-functionalized tetraarylazaBODIPYs 6-10 having the
functionalized aryl group at the 1-position of azaBODIPY core by adopting mixed condensation
approach. We have successfully introduced five different functional groups such as -CH2OH, -
OH, -CN, -NO2 and -CHO groups at the aryl group present at 1-position of azaBODIPY core.
The mono-functionalized tetraraylazaBODIPYs were used further to synthesize various novel
azaBODIPY based systems such as azaBODIPY-BODIPY dyad 12, azaBODIPY-metal dipyrrin
dyad 13, azaBODIPY-porphyrin dyad 14 and azaBODIPY-smaragdyrin dyad 15 (Chart 2). The
spectral, electrochemical and photophysical properties of mono-functionalized azaBODIPYs and
their dyads were also described.
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Chart 2: Structures of the monofunctional azaBODIPYs (6 - 10) and azaBODIPY-BODIPY
dyads (12), azaBODIPY-Pd dipyrrin (13), azaBODIPY-porphyrin dyad (14) and azaBODIPY-
oxasmaragdyrin dyad (15).
RESULTS AND DISCUSSION:
To synthesize the desired mono-functionalized azaBODIPYs, we need an access for
various functionalized nitrochalcones, which were synthesized over a sequence of two steps as
shown in Scheme 2. In the first step, aldol condensation of functionalized benzaldehyde and 4-
methyl acetophenone under basic conditions in ethanol at room temperature followed by
recrystallization afforded chalcones 16a-e as off-white solids in 75-80 % yields. The chalcones
were subjected to nitration under Michael addition conditions to afford nitrochalcones 17a-e in
70-80 % yields. The mono-functionalized azaBODIPYs were prepared by mixed condensation
method as shown in Scheme 3.
The condensation of desired functionalized nitrochalcones (17b-17e) with 3-(tolyl)-4-
nitro-1-(p-tolyl)butan-1-one (17a) in n-butanol, in the presence of ammonium acetate at reflux
temperature for 10 h resulted in the formation of a mixture of three dipyrromethenes as judged
by TLC analysis.
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Scheme 2: Synthesis of chalcones 16(a-e) and nitrochalcones 17(a-e).
The solvent was removed on rotatory evaporator under high vacuum and the resulted
pasty solid was washed with water and filtered to obtain crude dark brown solid mixture of three
azadipyrrins. Without further purification and separation, the crude solid in CH2Cl2 was treated
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with BF3 OEt2 in the presence of triethylamine at room temperature in open air for 30 min
followed by addition of water and left the reaction for overnight under stirring. The reaction
mixture was subjected to standard work-up to afford the crude solid mixture of three
azaBODIPYs: tetratolyl azaBODIPY, mono-functionalized azaBODIPY and difunctionalized
azaBODIPY. TLC analysis clearly indicates the presence of three well-separated azaBODIPYs.
The crude solid was subjected to silica gel column chromatographic purification and the desired
pure mono-functionalized azaBODIPYs were separated from the other two azaBODIPYs and
isolated as brown crystalline solids in 9-15 % yields. Thus, by following the mixed condensation
approach, we have prepared four mono-functionalized azaBODIPYs containing functional
groups such as -CH2OH, -OH, -CN and -NO2 groups at the aryl group present at 1-position of
azaBODIPY core. The mono-formyl functionalized azaBODIPY was prepared by oxidizing
compound 6 in CH2Cl2 with MnO2 followed by column chromatographic purification. The
mono-functionalized azaBODIPYs were found to be freely soluble in common organic solvents
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and their identities were confirmed by molecular ion peak in HR-MS, 1D and 2D NMR
techniques.
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Scheme 3: Synthesis of monofunctionalized azaBODIPYs 6-10.
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The partial H NMR spectrum along with H-1H COSY and NOESY spectra of
compound 10 is shown in figure 1. The unsymmetrical nature of the functionalized azaBODIPYs
was clearly noted in appearance of two sets of singlets for nine tolyl -CH3 protons. The pyrrole
type a and type b protons appeared as two singlets at 7.04 and 7.07 ppm. The 1H NMR spectra of
all mono-functionalized azaBODIPYs were clean and the meso-aryl protons showed cross-peak
correlations in 2D NMR spectra.
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Figure 1. (a) 1H, (b)1H-1H COSY and (c)1H-1H NOESY NMR spectra of compound 10 recorded
in CDCl3.
The mono-functionalized azaBODIPYs are very useful synthons to prepare a range of
fluorescent compounds which we have demonstrated here by taking mono-formyl functionalized
azaBODIPY 10 as shown in Scheme 4.
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Scheme 4: Synthesis of azaBODIPY conjugates 11-15
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Figure 2. 1H NMR spectra of compounds : (a) 11, (b) 12 and (c) 15 recorded in CDCl3
The compound 10 was treated with excess pyrrole in CH2Cl2 in the presence of catalytic
amount of BF3.OEt2 at room temperature for 30 mins followed by simple column
chromatographic purification which afforded the mono-dipyrromethanyl azaBODIPY 11 in 80%
yield. The molecular ion peak at 684.3117 and appearance of NMR resonances at 5.60 ppm
(Figure 2c) corresponding to meso -CH proton and two sets of resonances in 5.90-6.30 ppm
region corresponding to four pyrrole protons of dipyrromethanyl moiety along with other
resonances corresponding to azaBODIPY moiety confirmed the identity of mono-
dipyrromethanyl azaBODIPY 11. The compound 11 was oxidized with DDQ in CH2Cl2 at room
temperature in open air for 20 min and the resulted azaBODIPY appended with dipyrrin moiety,
without isolating was treated with BF3.OEt2 in the presence of Et3N for additional 30 min at room
temperature. The crude compound was subjected to silica gel column chromatographic
purification to afford pure azaBODIPY appended with BODIPY unit 12 in 74% yield. The
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formation of 12 was confirmed by HR-MS, H (Figure 2b), B and F NMR techniques. The
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azaBODIPY appended with Pd(II) complex of dipyrrin 13 was prepared in two steps. In the first
step, the compound 11 was oxidized with DDQ in CH2Cl2 followed by flash column
chromatographic purification to isolate azaBODIPY appended with dipyrrin moiety. In the
second step, the isolated azaBODIPY appended with dipyrrin moiety was treated with excess
Pd(acac)2 in toluene/Et3N (5:1) at reflux temperature for 2 h. The crude compound, after standard
work-up, was passed through celite bed followed by evaporation of solvent to afford
azaBODIPY-Pd(II) dipyrrin conjugate 13 as blue solid in 52 % yield. The identity of compound
13 was confirmed by HR-MS and NMR spectroscopic techniques. The azaBODIPY-porphyrin
conjugate 14 was prepared by condensing one equivalent of compound 11 with one equivalent of
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dipyrromethane dicarbinol 18 in CH2Cl2 in the presence of catalytic amount of BF3 OEt2 for 2 h
under an inert atmosphere followed by oxidation with DDQ at room temperature in open air for
additional 30 min. The progress of the reaction was followed by TLC analysis and absorption
spectroscopy. The crude compound was purified by silica gel column chromatography and
afforded pure azaBODIPY-porphyrin conjugate 14 as bright purple coloued solid in 14% yield.
The formation of 14 was also confirmed by HRMS, NMR and absorption spectroscopic
techniques. We have also succeeded in synthesizing azaBODIPY-oxasmaragdyrin conjugate 15
by condensing 11 with 16-oxatripyrrane under mild acid catalyzed conditions followed by
oxidation with DDQ to afford the conjugate 15 in 28% yield. The identity of compound 15 was
also confirmed by molecular ion peak at 1082.4537 in HR-MS and a clean NMR spectrum
presented in Figure 2.
PHOTOPHYSICAL AND ELECTROCHEMICAL STUDIES:
The azaBODIPYs strongly absorb and emit in the region of 650-670 nm. The mono-
functionalized tetraarylazaBODIPYs 6-10 showed strong absorption and emission in the region
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of 660-710 nm and the relevant data are tabulated in Table 1. The absorption band position and
extinction coefficients were slightly varied depending upon the kind of functionalized aryl
groups present at the 1-position of azaBODIPY core. For example, the mono-formyl
functionalized tetraarylazaBODIPY 10 showed absorption band at 675 nm whereas it was
observed at 678 nm in mono-nitro functionalized tetraarylazaBODIPY 9. The mono-
dipyrromethanyl azaBODIPY 11 showed slight hypsochromically shifted absorption band at 667
nm. The azaBODIPY-BODIPY conjugate 12 showed a moderately intense band with a shoulder
at 506 nm corresponding to BODIPY moiety and one intense band at 677 nm corresponding to
azaBODIPY moiety (Figure 3a). The compound 13 containing azaBODIPY and Pd (II) dipyrrin
units showed one broad band 497 nm corresponding to Pd(II) dipyrrin moiety and a strong band
at 670 nm due to azaBODIPY moiety (Figure 3b). The azaBODIPY-porphyrin conjugate 14
showed one strong Soret band at 421 nm corresponding to porphyrin unit, two weak Q-bands at
517 and 554 nm due to porphyrin unit and one broad intense band at 670 nm due to both
azaBODIPY and porphyrin moieties (Figure 3c). The azaBODIPY-oxasmaragdyrin conjugate
showed less number of bands due to strong interaction between azaBODIPY and
oxasmaragdyrin moieties. The strong Soret band at 448 nm and weak Q-band at 556 nm are due
to smaragdyrin moiety whereas moderately intense broad band at 666 nm is due to both
azaBODIPY and smaragdyrin moieties (Figure 3d). Due to strong interaction between the two
chromophoric moieties in compound 15, the absorption band in the visible-NIR region
experienced hypsochromic shift, broadened with reduction in extinction coefficient compared to
independent azaBODIPY and oxasmaragdyrin.
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The steady state and time resolved fluorescence properties of compounds 6-15 were
investigated and the data is included in table 1. The mono-functionalized tetraarylazaBODIPYs
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6-10 and mono-dipyrromethanyl azaBODIPY 11 showed one strong fluorescence band in the
region of 690-710 nm with quantum yields in the range of 0.34-0.53. The singlet state lifetimes
of compounds 6-11 measured by time-resolved fluorescence single photon counting method
were fitted to the single exponential decay and measured singlet state lifetimes are in the range of
1.60-3.0 ns. The functionalized tetraarylazaBODIPY having hydroxyphenyl group at 1-position
showed low quantum yield and singlet state lifetime owing to the presence of hydroxyl group.
The dyad azaBODIPY-Pd (II) dipyrrin 13 showed very weak fluorescence with emission peak
maxima at 670 nm (Figure 3b) with a quantum yield of 0.06 and singlet state lifetime of 1.74 ns
(Figure 3b inset) due to presence of heavy Pd (II) ion, which quenches the fluorescence of
azaBODIPY moiety.
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The dyads azaBODIPY-BODIPY 12, azaBODIPY-porphyrin 14 and azaBODIPY-
oxasmaragdyrin 15 act as donor-acceptor systems and showed energy transfer from one unit to
another. In dyad 12, the BODIPY unit absorbs at higher energy and acts as donor whereas the
azaBODIPY unit absorbs at lower energy and acts as energy acceptor. The dyad 12 on excitation
at 500 nm, where BODIPY unit absorbs very strongly, the expected emission from BODIPY unit
at 515 nm was completely quenched and a strong emission at 708 nm (Figure 3a) corresponding
to azaBODIPY unit was observed with a quantum yield of 0.46. The time-resolved studies on
compound 12, upon excitation at 635 nm and emission collected at 708 nm showed single
exponential decay with a lifetime of 2.88 ns (Figure 3a inset) due to azaBODIPY unit. These
results support intramolecular energy transfer from BODIPY unit to azaBODIPY unit in dyad
12.
In azaBODIPY-porphyrin conjugate 14, the porphyrin unit absorbs at higher energy and
acts as donor whereas azaBODIPY unit acts as energy acceptor. The conjugate 14 upon
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excitation at 415 nm where porphyrin unit absorbs strongly, the emission from porphyrin unit at
650 was almost quenched and a strong emission at 699 nm due to azaBODIPY unit was observed
(Figure 3c). The quantum yield estimated was 0.47 due to azaBODIPY unit. The singlet state
lifetime collected with the emission wavelength of azaBODIPY unit at 699 nm was fitted to
single exponential decay (Figure 3c inset) and lifetime was found to be 2.19 ns. These
observations also support an efficient energy transfer from porphyrin unit to azaBODIPY unit in
conjugate 14. In azaBODIPY-oxasmaragdyrin conjugate 15, both azaBODIPY and
oxasmaragdyrin moieties absorb and emit in the overlapping region. The conjugate 15 upon
excitation at 440 nm showed weak emission band at 698 nm (Figure 3d) with a quantum yield of
0.06. The weak emission of conjugate was due to strong interaction between azaBODIPY and
oxasmaragdyrin moieties which led to enhancement of non-radiative decay channels in dyad 15.
The singlet state decay of conjugate 15 collected with emission wavelength of 698 nm was fitted
to single exponential (Figure 3d inset). This is in agreement with the quantum yield data of
compound 15. Thus, the preliminary photophysical studies on compounds 12, 14 and 15
indicated the possibility of energy transfer at singlet state from energy donor to energy acceptor.
Since dyad 12 showed very efficient energy transfer from BODIPY unit to azaBODIPY unit in
CHCl3, we have investigated absorption and fluorescence properties of dyad 12 in five different
solvents such as toluene, CH2Cl2, CHCl3, THF and CH3CN as shown in Figure 4 and the
relevant data is tabulated in Table 2. As clear from figure 4(a) and data in table 2, the dyad 12
showed similar absorption features in all solvents with slight changes in peak maxima and
extinction coefficients. The steady state fluorescence spectra (Figure 4b) of dyad 12 in five
different solvents were recorded at excitation wavelength of 490 nm where BODIPY unit
exclusively absorbs strongly. The azaBODIPY-BODIPY dyad 12 on excitation at 490 nm in five
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different solvents where BODIPY unit absorbs strongly, the emission was observed in the range
of 702-712 nm, which corresponds to azaBODIPY unit indicating a possibility of energy transfer
from BODIPY unit to azaBODIPY unit in dyad 12. However, the preliminary data presented in
table 2 indicates that the energy transfer from BODIPY unit to azaBODIPY unit in dyad 12 was
less efficient in CH3CN but more efficient in CHCl3. Detailed photophysical investigation will be
required to understand the energy transfer dynamics observed in these three dyads.
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Table 1: Photophysical and Electrochemical Data of compounds 6-15.
Compounds
λabs(nm)
[logε(mol-1dm3cm-1)]
λem
(nm)
Electrochemical data
E1/2red (V)
Φ
τ
(ns)
E1/2ox(V)
6
7
666 [4.85]
668 [4.94]
674 [4.80]
678 [4.70]
675 [4.82]
668 [4.85]
697
700
705
710
707
687
0.48
0.34
0.47
0.53
0.52
0.46
2.09
1.64
2.75
2.95
2.58
2.26
+1.35
+1.29
+1.42
+1.43
+1.40
+1.35
-0.31, -1.07
-0.36, -1.31
-0.23, -0.91
-0.20, -0.70
-0.23, -0.88
-0.36, -1.26
8
9
10
11
12
13
14
506 [4.50], 676 [4.80]
497 [4.43], 670 [4.72]
421 [4.93], 670 [4.46]
708
700
699
0.46
0.06
0.47
2.88
1.74
2.19
+0.16, + 1.40
-0.12, + 1.39
-0.69, +1.39
-0.23, -0.57, -1.24
-0.30, -1.15
-0.17, -1.30, -1.52
-1.77
15
448 [4.80], 666 [4.40]
698
0.06
2.11
+0.73, +1.37
-0.25, -0.41, -1.09,
-1.6
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Figure 3: Overlay of absorption (blue line) and emission (green line); decay curve (inset) of
compounds (a) Compound 12, (b) Compound 13, (c) Compound 14, (d) compound 15 recorded
in CHCl3 (5×10-6 M); (e) the cyclic (blue line) and differential pulse voltammograms (red line) of
compound 10 recorded in CH2Cl2 at a scan rate of 50 mV/s using tetrabutylammonium
perchlorate as supporting electrolyte (f).
1.0
(a)
(b)
CH2Cl2
CH Cl
2 2
800
600
400
200
0
CH CN
3
Toluene
THF
0.8
0.6
0.4
0.2
0.0
Toluene
THF
CHCl3
CH CN
3
CHCl
3
400
500
600
700
800
500
600
700
800
Waveleng th(nm)
Waveleng th(nm)
Figure 4: Comparison of (a) absorption spectra and (b) emission spectra of dyad 12 in five
different solvents.
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Table 2: Photophysical data of compound 12 in different solvents.
5
6
7
8
9
Solvents
λabs(nm)
λem(nm)
Φ
[logε(mol-1dm3cm-1)]
507 [4.70 ], 678 [4.95]
506 [4.70], 676[4.95]
505 [4.73], 673[4.95]
504 [4.67 ], 676 [4.91]
500 [4.57], 667[4.77]
Toluene
CHCl3
CH2Cl2
THF
712
708
710
709
702
0.28
0.46
0.25
0.21
0.10
10
11
12
13
14
15
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CH3CN
The electrochemical properties of compounds 6-15 were determined by cyclic
voltammetry at a scan rate of 50 mV s-1 using tetrabutylammonium perchlorate (TBAP) as
supporting electrolyte and the relevant data is included in Table 1. In general, azaBODIPYs are
electron deficient and show one irreversible oxidation and two quasi-reversible reductions. Due
to electron deficient nature of compounds 6-10, the compounds are difficult to oxidize but easier
to reduce. For example, the compound 6 showed irreversible oxidation at 1.35 V and two quasi-
reversible reductions at -0.31 and -1.07 V, whereas the compound 9 having electron withdrawing
nitrophenyl group shows irreversible oxidation at 1.43 V and quasi-reversible reductions at -0.20
and -0.70 V. Thus, due to presence of nitrophenyl group, the compound 9 is harder to oxidize but
easier to reduce as compared to compound 6. The compound azaBODIPY-BODIPY dyad 12
showed three quasi-reversible reductions at -0.23, -0.57 and -1.24 V. In this, the reductions at -
0.23 and -1.24 V were due to azaBODIPY moiety whereas the reduction at -0.57 V was due to
BODIPY moiety. Similarly, in azaBODIPY porphyrin dyad 14, we observed two oxidations at +
0.69 and +1.39 V and four reductions at -0.17, -1.30, -1.52 and -1.77 V. In this case also, the
oxidations were mainly due to porphyrin unit; the reductions at -0.17 and -1.30 were due to
azaBODIPY moiety and the reductions at -1.52 and -1.77 V were due to porphyrin unit. The
azaBODIPY-smaragdyrin dyad also showed oxidations and reductions corresponding to both the
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constituted units. Thus, the dyads 12-15 showed redox behaviour of both the constituted moieties
with slight shifts in their redox potentials indicating that the constituted units retain their
individual characteristic features in dyads.
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CONCLUSIONS:
The mono-functionalized tetraarylazaBODIPYs are highly desirable building blocks for
the synthesis of different novel azaBODIPY based systems for various studies and applications.
The mixed condensation approach of condensing two different nitrochalcones is the most
straightforward method to afford mono-functionalized tetraarylazaBODIPYs. The condensation
of two different nitrochalcones resulted in mixture of three azadipyrrins which on complexation
with BF2 unit afford desired mono-functionalized tetraarylazaBODIPYs. We have successfully
synthesized five different mono-functionalized tetraarylazaBODIPYs in decent yields by
adopting the mixed condensation approach. The use of mono-functionalized
tetraarylazaBODIPYs was demonstrated by taking mono-dipyrromethanyl azaBODIPY as a key
precursor and synthesized very novel four different azaBODIPY based systems such as
azaBODIPY-BODIPY dyad, azaBODIPY-Pd(II) dipyrrin, azaBODIPY-porphyrin and
azaBODIPY-oxasmaragdyrin conjugates in good yieds. The preliminary photophysical studies
on conjugates revealed that azaBODIPY can act as good energy acceptor and there is a
possibility of efficient intramolecular energy transfer within the dyads. Thus, the mono-
functionalized azaBODIPYs open up possibilities to prepare a range fluorescent azaBODIPY
based systems with multi-level applications.
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MATERIALS AND METHODS:
General Experimental:
All the starting materials and solvents were procured from commercial sources and used
without further purification. The NMR spectra were recorded in CDCl3 at room temperature
using a Bruker 400 MHz and 500 MHz instruments with tetramethylsilane (TMS) as an internal
standard. The HR-Mass spectra were recorded with a Bruker maxis Impact and Q-Tof micro
mass spectrometer using electron spray ionization method, TOF analyzer. Cyclic voltammetric
and Differential Pulse Voltammetric (DPV) studies were carried out with an electrochemical
system utilizing a three-electrode configuration consisting of a glassy carbon(working electrode),
platinum wire (auxiliary) electrode and a saturated calomel (reference) electrode using TBAP as
the supporting electrolyte. The experiments were done in dry dichloromethane using 0.1 M
tetrabutylammonium perchlorate as supporting electrolyte. Absorption and steady state
fluorescence spectra were obtained by Varian instruments. For UV-Vis and fluorescence
titrations, the stock solution for all compounds (5×10-6 M) was prepared by using spectroscopic
grade CHCl3 solvent. The fluorescence quantum yields (Φ) were estimated from the emission
and absorption spectra by comparative method at the excitation wavelength of 648 nm using 3,5-
dianisyl-1, 7-di(p-phenyl) azaBODIPY (Φ = 0.36 in chloroform)12 as standard. The time resolved
fluorescence decay measurements were carried out at magic angle using a picosecond diode laser
based time correlated single photon counting (TCSPC) fluorescence spectrometer from IBH,
UK. All the decays were fitted to single exponential. The good fit criteria were low chi-square
(1.0) and random distributions of residuals.
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General method for the synthesis of compounds 16(a-e):
Compound 16a (3-(tolyl)-1-(p-tolyl)prop-2-en-1-one) was synthesized according to the
reported literature procedure.13 4-methylacetophenone (1.00 g, 5.20 mmol) and 4-
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methylbenzaldehyde (1.00 g, 5.33 mmol) were dissolved in 5 mL EtOH in a round bottom flask.
5 mL of an aqueous potassium hydroxide solution (875.5 mg, 15.60 mmol) was added dropwise
which resulted in the formation of off white coloured solid. The reaction mixture was allowed to
stir overnight. The product was filtered, washed with water and dried. The off white coloured
solid was used for further synthesis without purification. Similarly, the compounds 16b-16e7, 14,15
were synthesized by the same procedure by reacting the corresponding aldehyde with 4-methyl
acetophenone and the resulting product was used for next step without further purification.
General method for the synthesis of compounds 17(a-e):
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Compounds 17 (a-e) were synthesized according to the literature procedure16 by the
reaction of respective chalcones with nitromethane in presence of diethylamine.
To the methanolic solution of compound 17a (2 g, 0.50 mmol) taken in a round bottomed
flask, diethylamine (2 g, 0.50 mmol) was added followed by the addition of nitromethane ( 2 g,
0.50 mmol) under an inert atmosphere. The reaction mixture was refluxed overnight. After the
completion of reaction (checked by TLC), the reaction mixture was neutralized with dilute
hydrochloric acid and extracted with dichloromethane and water. The organic layers were
collected and dried over anhydrous sodium sulphate. The solvent was evaporated under vacuum
to afford the compound (3-(tolyl)-4-nitro-1-(p-tolyl)butan-1-one) 17a as a pasty material which
was used for further reaction without purification.
Compounds7, 17 17b-17e were synthesized by following the above procedure and used
without further purification.
Compound 17d (3-(4-(cyano)phenyl)-4-nitro-1-(p-tolyl)butan-1-one): viscous pale yellow
liquid. Yield: 1 g (80%). 1H NMR (500 MHz, CDCl3, δ in ppm): 2.40 (s, 3H), 3.42-3.43 (m, 2H),
4.28 (p, 1H), 4.67-4.71 (dd, 1H), 4.82-4.86 (dd, 1H), 7.24 (d, 3J = 8.0 Hz, 2H), 7.41 (d, 3J = 8.0
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Hz, 2H), 7.61(d, J = 8.0 Hz, 2H), 7.78(d, J = 8.0 Hz, 2H); C NMR (100 MHz, CDCl3, δ in
ppm): 21.6, 38.8, 41.1, 128.1, 129.2, 129.4, 132.1, 138.2, 144.7, 196.0; HRMS Calcd for
C18H16N2O3Na 331.1053 (M+Na)+, found 331.1056 (M+Na)+.
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General method for the synthesis of compounds 6 - 9:
The compounds 6 - 9 were prepared by the mixed condensation of compound 17a with
the compounds 17b, 17c, 17d and 17e.
Synthesis of compound 6 (4,4-Difluoro-1-(4-hydroxymethyl)benzene-3,5,7-tri-p-tolyl-4-bora-
3a,4a,8-triaza-s-indancene):
In order to synthesize compound 6, compounds 17a and 17b in butanol were taken in a
round-bottomed flask equipped with a magnetic stirrer and reflux condenser. To this mixture,
ammonium acetate was added and the reaction mixture was heated under reflux temperature for
12 h, which resulted in the formation of a mixture of dipyrromethenes. The reaction mixture was
cooled and solvent was evaporated under vacuum. The blackish brown solid was precipitated by
the addition of water to the reaction mixture. The solid was air-dried. To the solution of the
above solid (1 g) in dichloromethane, triethylamine was added followed by the addition of
BF3.OEt2. The colour of the solution changed from blue to greenish blue. The completion of the
reaction was monitored using thin layer chromatography. The TLC showed three clear spots
indicating the formation of three mixed condensation products. The reaction mixture was
extracted with dichloromethane and water. The organic layers were collected and dried over
anhydrous sodium sulphate. Solvent was evaporated under vacuum to obtain the crude product as
brown coloured solid. Purification of the crude compound by column chromatography gave
compound 6 as second moving band. The solvent was evaporated and brown crystalline solid
1
0
was obtained. Yield: 187 mg (30%); mp: 228-235 C; H NMR (500 MHz, CDCl3, δ in ppm):
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2.41 (s, 6H), 2.44 (s, 3H), 4.78 (s, 2H), 7.00 (s, 1H), 7.01 (s, 1H), 7.26 - 7.29 (m, 6H), 7.4 (d, 3J
5
6
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= 8.0 Hz, 3H), 7.94 -7.98 (m, 8H), 8.06 (d, J = 8.1 Hz); F NMR (470.59 MHz, CDCl3, δ in
7
8
9
3
11
3
ppm) -131.5 (q, J (B, F) = 80.0 Hz, 2H); B NMR (160.46 MHz, CDCl3, δ in ppm) 1.21 (t, J
(B, F) = 32 Hz); 13C{1H} NMR (125 MHz, CDCl3, δ in ppm): 21.4, 21.5, 21.6, 60.8, 118.5,
127.0, 128.8, 128.9, 129.2, 129.4, 129.5, 129.6, 131.9, 139.9, 141.3, 141.5, 142.0; HRMS calcd
for C36H30BF2KN3O 608.2088 (M+K)+, found 608.2083 (M+K)+.
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Compounds 7-9 were prepared using the above procedure by the mixed condensation
reaction of compound 17a with the corresponding compounds 17b, 17c, 17d and 17e,
respectively, followed by BF2 complexation to obtain the desired products 7-10.
Compound
7
(4,4-Difluoro-1-(4-hydroxy)benzene-3,5,7-tri-p-tolyl-4-bora-3a,4a,8-triaza-s-
1
0
indancene): Colour: Dark brown crystalline solid; Yield: 60 mg (36%); 242-250 C H NMR
(500 MHz, CDCl3, δ in ppm): 2.41 (s, 6H), 2.42 (s, 2H), 6.91 (s, 1H), 6.97 (s, 1H), 7.26 -7.28 (m,
3
3
19
6H), 7.95 (d, J = 8.1 Hz, 6H), 8.01 (d, J = 8.3 Hz, 2H); F NMR (470.59 MHz, CDCl3, δ in
ppm) -131.2 (q, 3J (B, F) = 32.9 Hz); 11B NMR (160.46 MHz, CDCl3, δ in ppm) 1.23 (t, 3J (B, F)
= 32.0 Hz); 13C{1H} NMR (125 MHz, CDCl3, δ in ppm): 21.4, 21.6, 115.6, 117.5, 118.1, 125.5,
129.0, 129.2, 129.3, 129.5, 129.8, 131.1, 139.5, 141.2, 141.3, 156.9; HRMS calcd for
C35H29BF2N3O 556.2372 (M+H)+, found 556.2377 (M+H)+.
Compound
8
(4,4-difluoro1-1-(1-yl)benzonitrile-3,5,7-tri-p-tolyl-4-bora-3a,4a,8-triaza-s-
1
0
indancene): Colour: Dark blue amorphous solid; Yield: 65 mg (35 %); 240-245 C H NMR
(400 MHz, CDCl3, δ in ppm): 2.41(s, 3H), 2.42 (s, 3H), 2.46 (s, 2H), 7.02 (s, 1H), 7.05 (s, 1H),
7.27 - 7.31 (m, 7H), 7.72 (d, 3J = 8.4Hz, 2H), 7.92(d, 3J = 8.0 Hz, 4H), 8.00 (d, 3J = 8.4 Hz, 2H),
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8.14 (d, 3J = 8.4 Hz, 2 H); 19F NMR (470.59 MHz, CDCl3, δ in ppm) -131.6 (q, 3J (B, F) = 32.9
Hz); 11B NMR (160.46 MHz, CDCl3, δ in ppm) 1.21 (t, 3J (B, F) = 32.0 Hz); 13C{1H} NMR (100
MHz, CDCl3, δ in ppm): 21.5, 21.6, 21.7, 22.3, 29.7, 111.8, 128.3, 128.7, 129.2, 129.3, 129.4,
129.5, 129.9, 132.1, 137.0, 139.3, 140.6, 141.3, 142.5, 144.4, 145.5,146.6, 157.1, 162.0; HRMS
calcd for C36H28BF2N4 565.2376 (M+H)+, found 565.2375 (M+H)+.
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Compound
9
(4,4-Difluoro-1-(4-nitrophenyl)-3,5,7-tri-p-tolyl-4-bora-3a,4a,8-triaza-s-
1
0
indancene): Colour: Dark brown amorphous solid; Yield: 55mg (32 %); 245-250 C H NMR
(400 MHz, CDCl3, δ in ppm): 2.42 (s, 2H), 2.43 (s, 2H), 2.47 (s, 2H), 7.0 (s, 2H), 7.28-7.32 (m,
7H), 7.93 (d, 3J = 7.2 Hz, 4H), 8.01 (d, 3J = 8.4 Hz, 2H), 8.22 (d, 3J = 8.8 Hz, 2H), 8.30 (d, 3J =
8.8 Hz, 2H); 19F NMR (470.59 MHz, CDCl3, δ in ppm) -131.7 (q, 3J (B, F) = 28.2 Hz); 11B NMR
(160.46 MHz, CDCl3, δ in ppm) 1.18 (t, 3J (B, F) = 35.3 Hz); 13C{1H} NMR (100 MHz, CDCl3,
δ in ppm): 21.6, 21.7, 25.3, 77.2, 123.7, 128.2, 128.7, 129.1, 129.3, 129.4, 129.5, 129.6, 129.9,
139.0, 140.8, 141.3, 142.7, 147.4; HRMS calcd for C35H27BF2KN4O2 623.1833 (M+K)+, found
623.1827 (M+K)+.
Compound 10 (4,4-difluoro1-1-(1-yl)benzaldehyde-3,5,7-tri-p-tolyl-4-bora-3a,4a,8-triaza-s-
1
indancen): Colour: Dark brown crystalline solid; Yield 89 mg (89%); mp: 235-250 0C H NMR
(400 MHz, CDCl3, δ in ppm): 2.42 (s, 3H), 2.43 (s, 3H), 2.45 (s, 3H), 7.04 (s, 1H), 7.07 (s, 1H),
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3
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7.29 (t, J = 5 Hz, 6H), 7.96 (m, 9H), 8.22 (d, J = 8.6, 2H), 10.08 (s, 1H); F NMR (470.59
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MHz, CDCl3, δ in ppm): -131.6 (q, J (B, F) = 32.9 Hz); B NMR (160.46 MHz, CDCl3, δ in
ppm): 1.21 (t, 3J (B, F) = 30.0 Hz); 13C{1H} NMR(100 MHz, CDCl3, δ in ppm) 21.6, 21.7, 25.3,
123.7, 128.2, 128.7, 129.1, 129.3, 129.4, 129.5, 129.6, 129.9, 139.0, 140.8, 141.3, 142.7, 147.4;
HRMS calcd for C36H28BF2KN3O 606.1931 (M+K)+, found 606.1938 (M+K)+.
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Synthesis of mono-dipyrromethanyl azaBODIPY 11:
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To a solution of compound 10 (100 mg, 0.17 mmol) in dichloromethane (100 ml), pyrrole
(1.7 mmol) was added, followed by the addition of catalytic amount of BF3.OEt2 (0.017 mmol).
The reaction mixture was stirred at room temperature for 15 min. The completion of the reaction
was checked by TLC, which indicates the disappearance of the spot corresponding to the reactant
and formation of a new polar spot corresponding to the desired compound. The reaction was
quenched with 0.1 M NaOH and extracted with dichloromethane and dried over anhydrous
sodium sulphate. Evaporation of the solvent under vacuum afforded the crude product, which is
purified by silica gel column chromatography (ethyl acetate/petroleum ether: 30/70%) to obtain
the tritolyl monodipyrromethanyl azaBODIPY as deep blue coloured amorphous solid. Yield: 82
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mg (73 %). mp: decomposes at ~235 C; H NMR (400 MHz, CDCl3, δ in ppm): 2.41 (s, 9H),
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5.10 (s, 1H), 6.0 (s, 2H), 6.19 (s, 2H), 6.75 (s, 2H), 6.97 (d, J = 8.4 Hz, 2H), 7.23 - 7.32 (m, 8
H), 7.93 - 8.02 (m, 10 H); 19F NMR (470.59 MHz, CDCl3, δ in ppm) -131.5 (q, 3J (B, F) = 28.0
Hz); 11B NMR (160.46 MHz, CDCl3, δ in ppm) 1.23 (t, 3J (B, F) = 32.0 Hz); 13C{1H} NMR (100
MHz, CDCl3, δ in ppm): 18.2, 29.7, 42.1, 108.1, 128.8, 129.4; HRMS calcd for C44H37BF2N5
684.3112 (M+H)+, found 684.3117 (M+H)+.
Synthesis of azaBODIPY-BODIPY dyad 12 :
To a solution of compound 11 (50 mg, 0.07 mmol) in dichloromethane (100 mL), DDQ
(24 mg, 0.10 mmol) was added with continuous stirring at room temperature. This mixture was
stirred for around 15 min., followed by the successive addition of triethylamine (0.4 mL, 2.8
.
mmol) and BF3 OEt2 (0.4 mL, 2.8 mmol). The reaction mixture was allowed to stir for further 30
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min. The solvent was evaporated under vacuum and the crude product was purified by silica gel
column chromatography using ethyl acetate/petroleum ether (40/60 %) to obtain compound 12 as
greenish blue amorphous solid. Yield: 38 mg (71%). mp: >300 0C; 1H NMR (400 MHz, CDCl3,
δ in ppm): 2.40 (s, 3H), 2.42 (s, 6H), 6.5 (s, 2H), 7.05 (s, 3 H), 7.08 (s, 1H), 7.28 - 7.32 (m, 6H),
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7.67 (d, J = 8.4 Hz, 2H), 7.96 - 8.01 (m, 8H), 8.22 (d, J = 8.3, 2H); F NMR (376.46 MHz,
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CDCl3, δ in ppm): -131.5 (q, J (B, F) = 32.9), -144.9 (q, J (B, F) = 28.2); B NMR (160.46
MHz, CDCl3, δ in ppm) 1.26 (t, 3J (B, F) = 32.0 Hz), 0.59 (t, 3J (B, F) = 28.8 Hz); 13C{1H} NMR
(100 MHz, CDCl3, δ in ppm): 21.5, 21.6, 21.7, 118.6, 119.1, 128.5, 128.8, 129.1, 129.3, 129.5,
129.8, 130.8, 131.5, 134.1, 134.8, 135.3, 140.4, 140.8, 141.4, 142.2, 144.1, 144.8, 145.0, 146.3,
146.9; HRMS calcd for C44H33B2F4KN5 768.2504 (M+ K)+, found 768.2510 (M+ K)+.
Synthesis of azaBODIPY-Pd(II) dipyrrin dyad 13:
To a solution of compound 11 (30 mg, 1.50 mmol) in toluene taken in a round bottomed
flask equipped with a magnetic stirrer and reflux condenser, triethylamine was added, followed
by the addition of Pd(acac)2. The reaction mixture was heated at reflux temperature under N2
atmosphere. The progress of the reaction was monitored by thin layer chromatography. After 3 h,
there was complete consumption of the reactant and the spot corresponding to the starting
material completely disappeared and a new spot was formed. The solvent was evaporated under
vacuum and the reaction mixture was extracted with dichloromethane and water. The organic
layer was collected and passed through celite. Solvent was removed in a rotatory evaporator to
0
1
obtain compound 13 as blue coloured amorphous solid. Yield: 18 mg (52 %), mp: >300 C; H
NMR (500 MHz, CDCl3, δ in ppm): 2.08 (s, 6H), 2.38 (s, 3H), 2.42 (s, 3H), 6.43 (s, 2H), 6.74 (s,
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2H), 7.01 (s, 1H), 7.07 (s, 1H), 7.23-7.30 (m, 8H), 7.50 (d, J = 8.0, 2H), 7.95-7.97 (d, J = 8.1,
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2H), 7.98 (d, J = 8.1, 2H); F NMR (470.54 MHz, CDCl3, δ in ppm): -131.6 (q, J (B, F) =
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28.2); B NMR (160.46 MHz, CDCl3, δ in ppm) 1.26 (t, J (B, F) = 30.5 Hz); C{1H} NMR
(100 MHz, CDCl3, δ in ppm): 21.5, 21.6, 22.3, 22.6, 26.4, 30.9, 31.5, 34.1, 116.9, 128.1, 129.2,
129.4, 130.8, 131.4, 133.3, 146.4, 187.2; HRMS C49H40BF2N5NaO2Pd (M+Na)+ calcd for
908.2195, found 908.2193 (M+Na)+.
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Synthesis of azaBODIPY-porphyrin dyad 14:
Diacyl dipyrromethane was reduced to the corresponding dicarbinol 18 with NaBH4. The
dicarbinol so obtained was immediately condensed with compound 11 (2.5 mmol) in
dichloromethane using catalytic amount of TFA at room temperature, followed by oxidation with
DDQ. After 2h, the solvent was evaporated and the crude compound was purified using silica gel
column chromatography using ethyl acetate/petroleum ether (25/75) as eluent to obtain
compound 14 as bright purple crystalline solid. Yield: 9 mg (14 %). mp: >300 0C 1H NMR (400
MHz, CDCl3, δ in ppm): -2.73 (s, 2H), 2.21 (s, 3H), 2.45 (s, 3H;), 2.46 (s, 3H), 2.71 (s, 6H), 4.10
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(s, 3H), 7.06 (bs, 2H), 7.37-7.26 (m, 12H), 7.56 (d, J = 7.7 Hz, 4H), 8.03 (d, J = 8.2 Hz, 2H),
8.07-8.15 (m, 8H), 8.34 (d, 3J = 8.0, 2H), 8.49 (d, 3J = 8.0, 2H), 8.87 (bs, 2H), 8.91 (d, 3J = 4.9
Hz, 2H); 8.97 (d, 3J = 6.49 Hz, 2H); 19F NMR (470.54 MHz, CDCl3, δ in ppm): -131.5 (q, 3J (B,
F) = 32.9), -144.9 (q, 3J (B, F) = 28.2); 11B NMR (160.46 MHz, CDCl3, δ in ppm) 1.26 (t, 3J (B,
F) = 32.0 Hz), 0.59 (t, 3J (B, F) = 28.8 Hz); HRMS calcd for C76H59BF2N7O 1134.4849 (M+ H)+,
found 1134.4843 (M+ H)+.
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Synthesis of azaBODIPY-oxasmaragdyrin dyad 15:
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Solution of compound 11 (50 mg, 0.073 mmol) in DCM was taken in a round-bottomed
flask under an inert atmosphere. To this, oxatripyrrane 19 (29 mg, 0.073) was added followed by
the addition of catalytic amount of trifluoroacetic acid. The reaction mixture was stirred at room
temperature for 2 h. After two hours, DDQ was added and the reaction mixture was further
stirred for another half an hour. The solvent was evaporated in a rotatory evaporator. The crude
product was purified by silica gel column chromatography using ethyl acetate/petroleum ether
(25/75) mixture as eluent to obtain compound 15 as greenish blue solid. Yield: 37 mg (28%).
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mp: >300 C; H NMR (500 MHz, CDCl3, δ in ppm): 2.33 (s, 3H), 2.48 (s, 6H), 2.49 (s, 6H),
7.07 (s, 2H), 7.31 - 7.38 (m, 6H), 7.65 (d, 3J = 7.4 Hz, 5H), 8.01 (d, 3J = 8.00 Hz, 2H), 8.05 (d, 3J
= 8.0 Hz, 2H), 8.13 (t, 3J = 16 Hz, 6H), 8.82 (s, 2H), 9.13 (s, 2 H), 9.47 (s, 2H), 9.54 (s, 2H); 19F
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NMR (376.46 MHz, CDCl3, δ in ppm): -131.4 (q, J (B, F) = 32.9); B NMR (160.46 MHz,
CDCl3, δ in ppm) 1.33 (t, 3J (B, F) = 32.0 Hz); HRMS calcd for C72H55BF2N7O 1082.4535 (M+
H)+, found 1082.4537 (M+ H)+.
ASSOCIATED CONTENT:
*Supporting Information:
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The supporting information contains the characterization data (HRMS, H, B, F, C NMR
spectra) and cyclic voltammograms of all new compounds.
ACKNOWLEDGEMENTS:
M. R. thanks the Science and Engineering Research Board (SERB), Government of India
for financial support (Grant No. EMR/2015/002196). A. K. would like to acknowledge the
SERB for NPDF fellowship (File Number: PDF/2017/000540).
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References:
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