Ferrocenyl aza-dipyrromethene and aza-BODIPY: Synthesis and properties

Article abstract of DOI:10.1016/j.jorganchem.2016.10.019

A series of ferrocenyl substituted tetraphenylazadipyrromethane and Aza-BODIPY were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. The effect of the pyrrolic substituents on photophysical and electrochemical properties w

Full text of DOI:10.1016/j.jorganchem.2016.10.019

Accepted Manuscript
Ferrocenyl aza-dipyrromethene and aza-BODIPY: Synthesis and properties
Rekha Sharma, Ramesh Maragani, Rajneesh Misra
PII:
S0022-328X(16)30467-3
10.1016/j.jorganchem.2016.10.019
JOM 19663
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 8 September 2016
Revised Date: 13 October 2016
Accepted Date: 14 October 2016
Please cite this article as: R. Sharma, R. Maragani, R. Misra, Ferrocenyl aza-dipyrromethene and
aza-BODIPY: Synthesis and properties, Journal of Organometallic Chemistry (2016), doi: 10.1016/
j.jorganchem.2016.10.019.
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ACCEPTED MANUSCRIPT
Ferrocenyl Aza-dipyrromethene and Aza-BODIPY: Synthesis and Properties
Rekha Sharma, Ramesh Maragani and Rajneesh Misra*
Department of Chemistry, Indian Institute of Technology Indore, 453552
TOC
N
N
NH
Fe
Fe
Abstract
A series of ferrocenyl substituted tetraphenylazadipyrromethane and Aza-BODIPY were
designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. The effect
of the pyrrolic substituents on photophysical and electrochemical properties was explored. The
results show substantial electronic communication between the ferrocene unit, and Aza-
BODIPY core. The aza-dipyromethane and aza-BODIPY 3–6 are non-emissive in nature and
their absorption maxima exhibit red shifted absorption compared to unsubstituted azabodipy.
The computational calculation on Aza-BODIPY 5 and 6 was performed, which reveals that
electron density transfers from ferrocene (donor) to azabodipy (acceptor) core and the
photophysical properties are in good agreement with the computational data.
Introduction:
Aza-BODIPY dyes have received substantial attention of the scientific community due to their
remarkable spectral properties such as high molar extinction coefficient and large fluorescence
quantum yields.1-2 These dyes have been extensively used in development of sensors, as near-
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infrared absorbers and as a sensitizers in photodynamic therapy.3-4 Aza-BODIPYs are structural
analogues of BODIPYs with a nitrogen at 8- position instead of the carbon. These dyes exhibit
red shifted absorption and emission bands compared to BODIPYs.5 Several strategies have been
explored to tune the photonic properties of Aza-BODIPY core including (a) coordination by
BF2,6-8 (b) extending the conjugation through phenyl rings,9-10 (c) restricting the conformation of
Aza-BODIPY core,11 and (d) replacing the phenyl rings with thiophene rings. 12-14
Our group has explored BODIPYs, functionalized at the meso-position, as well as at the pyrrolic
position.15 The electron withdrawing Aza-BODIPY core is an important building block in a
variety of donor–acceptor molecular systems, which enhances the charge transfer and lowers the
HOMO-LUMO gap. This encouraged us to explore the Aza-BODIPY core and effect of the
pyrrolic substituents on photophysical and electrochemical properties of these molecular
systems.
Ferrocene is undoubtedly a strong electron donor.17 The ferrocenyl derivatives are thermally and
photochemically stable.18 There are few reports where the ferrocenyl derivatives are connected
to the Aza-BODIPY core via spacer at different positions (Chart 1).3b,19-21 To the best of our
knowledge, there are no reports available on Aza-BODIPY dyes with ferrocene groups
connected through pyrrolic position. Hence, in the context of our research on the ferrocenyl
functionalized systems, we have attached the ferrocenyl group to one of the pyrrolic ring and
both the pyrrolic rings of the tetraphenylazadipyrromethane and Aza-BODIPY respectively. The
photophysical and electrochemical properties of all new compounds were studied and DFT
calculations were performed to optimise the geometry and study the frontier molecular orbitals.
F
F
N
B
N
B
N
N
N
N
F
F
F
F
O
C
O
C
O
O
Fe
Fe
Fe
Fe
Chart 1. Structures of reported Ferrocenyl Aza-BODIPY dyes
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Results and discussion:
The synthesis of the ferrocenyl substituted tetraphenylazadipyrromethane and its BF2 chelates 3
–6 are shown in Scheme 2. The ferrocenyl substituted tetraphenylazadipyrromethane and Aza-
BODIPY were synthesized in moderate yield by the Pd-catalyzed Sonogashira cross-coupling
reaction of iodinated Aza-BODIPY(2) with the ethynyl ferrocene.
The iodinated Aza-BODIPY 2 was synthesized by four step reaction sequence starting from
Aldol condensation reaction between benzaldehyde and acetophenone which resulted chalcone.22
The addition of nitromethane to the chalcone in the presence of diethylamine resulted the nitro
compound in 75–80% yield.23 The nitro compound and ammonium acetate were boiled at reflux
in ethanol under an argon atmosphere to obtain Aza-dipyrromethane.23 The Aza-dipyrromethane
precipitated during the reaction was filtered, recrystallized and used in next step without further
purification. Aza-dipyrromethene was subsequently converted into the Aza-BODIPY derivatives
by heating with trifluoroboron etherate and triethylamine in toluene. The iodination of Aza-
BODIPY derivative was achieved using N-iodosuccinimide (NIS) in a mixture of CHCl3 and
acetic acid (3:1) in 80% yield (Scheme 1).23
Scheme 1. Synthesis of iodinated Aza-BODIPY.
The Sonogashira coupling reaction of iodo Aza-BODIPY 2 with ethynylferrocene resulted in
monosubstituted 3 and di-substituted ferrocenyl azapyromethanes 4. It was noted that, contrarily
to C-Bodipy analogues, the Pd-catalyzed cross coupling reaction resulted in the loss of a BF2
fragment.14,24 This was also confirmed by 19F NMR. The monosubstituted and di-substituted
ferrocenyl azapyromethanes were difficult to purify. So, first washing with cold methanol and
diethyl ether removed most impurities,25 including the monosubstituted product, followed by
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column chromatography and recrystallization afforded pure ferrocenyl azapyromethanes 3 and 4.
1
13
The ferrocenyl aza-pyromethanes 3 and 4 were also well characterized by H NMR, C NMR
and high-resolution mass spectrometry (HRMS) techniques. The ferrocenyl aza-pyromethanes 3
and 4 were again treated with trifluoroboron etherate in presence of diisopropylamine and
dichloromethane to afford the ferrocenyl substituted Aza-BODIPY 5 and 6 (Scheme 2).
The BF2 chelates were purified by column chromatography using 1:1 DCM/hexane which
resulted purple-blue ferrocenyl substituted Aza-BODIPY 5 and 6. The ferrocenyl substituted
13
11
19
aza-BODIPY 5 and 6 were well characterized by NMR (1H, C, B and F) analysis and by
high-resolution mass spectrometry (HRMS) (see Figures S5-S19, Supporting Information).
Scheme 2. Synthetic route for Ferrocenyl Aza-dipyrromethene and Aza-BODIPY 3–6.
1
The H NMR peaks for Aza-BODIPY 5 and 6 were shifted upfield compared with ferrocenyl
aza-pyromethanes 3 and 4. The 11B NMR spectra of compounds 5 and 6 showed a triplet at δ =
0.70 and 0.80 ppm, respectively, which corresponds to the tetracoordinate boron center. The 19F
NMR signals of 5 and 6 were observed as a quartet due to the tetracoordinate boron atom at –
131.50 and –131.05 ppm, respectively.10b
Photophysical properties
The electronic absorption spectra of the ferrocenyl substituted tetraphenylazadipyrromethane
and Aza-BODIPY were recorded in chloroform at room temperature (Figure 1), and data are
listed in Table 1. The absorption spectra of the ferrocenyl substituted tetraphenylazadipyrro
methane and Aza-BODIPY exhibit two intense peaks, one in the UV region and another in the
visible region with high extinction coefficient.
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Figure 1: Electronic absorption spectra of the azapyromethane and azabodipy 3–
6 at 1.0 × 10−5 M,
recorded in chloroform.
The ferrocenyl tetraphenylazadipyrromethane 3 and 4 exhibit absorption maxima at 602 nm. The
BF2 coordination of ferrocenyl tetraphenylazadipyrromethane 3 and 4 results in a bathochromic
shift of around 50 nm which is attributed to the extended conjugation. The ferrocenyl Aza-
BODIPY 6 exhibit a shoulder band at 800 nm which is an intramolecular charge transfer from
the ferrocenes to the Aza-BODIPY core.26
The ferrocenyl tetraphenylazadipyrromethane and Aza-BODIPY 3–6 are non emissive in nature
due to the fast non-radiative deactivation of the excited state with intramolecular charge
transfer.27-28
Electrochemistry
The electrochemical behavior of the ferrocenyl substituted tetraphenylazadipyrromethane and
Aza-BODIPY (3–6) were investigated by the cyclic voltammetric analysis in anhydrous
dichloromethane solution at room temperature using tetrabutylammoniumhexafluorophosphate
(TBAPF6) as a supporting electrolyte. The electrochemical data are listed in Table 1, and the
representative cyclic voltammogram of Aza-BODIPY 6 is shown in Figure 2. The cyclic
voltammograms of the Aza-BODIPY 3–5 can be found in the ESI section. All potentials are
corrected to be referenced against FcH/FcH+, as required by IUPAC.29
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Figure 2: Cyclic voltammogram of 1.0×10-4 M solutions of Aza-BODIPY 6 in CH2Cl2 containing 0.1M
Bu4NPF6 as supporting electrolyte, recorded at a scan speed of 100 mVs-1.
The cylic voltammogram of the ferrocenyl substituted tetraphenylazadipyrromethane and Aza-
BODIPY (3–6) show two reversible reduction peaks, one reversible oxidation peak
(ferrocene/ferrocenium) and one irreversible oxidation peak. The ferrocene based reversible
oxidation wave for tetraphenylazadipyrromethane and Aza-BODIPY (3–6) was observed at E1/2
= 0.12–0.14 V. The ferrocenyl moieties in the tetraphenylazadipyrromethane and Aza-BODIPY
(3–6) show higher oxidation potential compared to the free ferrocene. This confirms substantial
electronic communication between ferrocene and Aza-BODIPY core. The second oxidation
becomes easier in ferrocenyl tetraphenylazadipyrromethane and Aza-BODIPY (3–6) than
unsubstituted azadipyrromethane and Aza-BODIPY.
The reduction of ferrocenyl tetraphenylazadipyrromethane and Aza-BODIPY becomes harder
than unsubstituted azadipyrromethane and Aza-BODIPY. The mono ferrocenyl
tetraphenylazadipyrromethane 3 shows two quasi reversible reduction peaks at E1/2 = -1.18 –
-1.51 V and diferrocenyl tetraphenylazadipyrromethane 4 shows at -1.56 and -2.18 V. The
ferrocenyl Aza-BODIPYs 5 and 6 show multiple quasi reversible reduction peaks in the range of
-0.8 – -2.07 V.
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Table 1. Photophysical and electrochemical data of ferrocenyl tetraphenylazadipyrromethane and
Aza-BODIPY 3–6.
Photophysical
dataa
Electro-chemical datab
Compound
λmax, [nm]
Eox(V)
Ered(V)
(ε[Lmol–cm–1])
Ferrocene
-
0.00
-
Azapyromethane
596 (68000)
0.98
1.86
-1.09
-1.41
3
602 (65000)
0.12
1.02
-1.18
-1.51
4
602 (66000)
650 (79000)
0.13
1.03
-1.56
-2.18
Azabodipy
1.10
1.44
-0.70
-1.53
-2.03
5
6
651 (67000)
653 (70000)
0.13
1.09
-0.80
-1.47
-2.02
0.14
1.06
-1.51
-2.07
a Measured in chloroform at 1.0×10-5 M concentration. bRecorded by cyclic voltammetry using 1.0×10-4 M solutions of 3
solution of Bu4NPF6 in DCM at 100 mVs-1 scan rate, vs FcH/FCH+ electrode.
–
6, containing 0.1 M
Computational calculations
The structural and electronic properties of the ferrocenyl substituted Aza-BODIPYs (5 and 6)
were explored by means of Computational calculations. The Aza-BODIPYs 5 and 6 were
optimized by using Gaussian 09W program at the B3LYP/6-31G** level for C, H, N and
LANL2DZ for Fe.30-32
The optimized structure of ferrocenyl substituted Aza-BODIPY 6 show eclipsed conformation of
cyclopentadienyl rings of the ferrocenyl groups (Figure 3). The dihedral angle in unsubstituted
Aza-BODIPY, between phenyl ring (C28, 29, 32, 34, 30, 27) and Aza-BODIPY core was found
36.67°, while in ferrocenyl substituted Aza-BODIPY 6 the dihedral angle between phenyl ring
and Aza-BODIPY core is 23.10° (Figure S3).
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Figure 3. The DFT optimized structure of ferrocenyl substituted Aza-BODIPY 6 using B3LYP level.
The 6-31G** basis set for C, N, H and LANL2DZ for Fe
In order to understand the photophysical properties of azabodipy the time dependent density
functional calculation (TD-DFT) was carried out using B3LYP method for Aza-BODIPY and
ferrocenyl substituted Aza-BODIPY 5 and 6 in chloroform solvent using polarized continuum
model (CPCM) of Gaussian 09 software. The experimental and computational (TD-DFT:
B3LYP) UV-vis absorption spectra are shown in Figure S4. The strong absorption band
calculated at B3LYP level is 588 nm for Aza-BODIPY and 616 nm for ferrocenyl substituted
Aza-BODIPY 5 and 6. The experimental values for Aza-BODIPY and ferrocenyl Aza-BODIPY
5 and 6 are 650 and 651 nm respectively. The experimental values are in good agreement with
theoretical values.
The predicted vertical transitions and oscillator strengths in ferrocenyl substituted Aza-BODIPY
5 and 6 are listed in Table 2. In ferrocenyl substituted Aza-BODIPY 5 the transition is
dominated by HOMO→LUMO+1 transition 68% and oscillator strength is 0.4908. In Aza-
BODIPY 6, the HOMO→LUMO+1 transition contribute to the lowest excited state by 44%,
which belongs to π-π* transition. The Aza-BODIPY 6 shows one more contribution (32%)
between HOMO-1→LUMO, which corresponding to the ICT transition at 779 nm as shown in
Figure 4.
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Table 2. Computed vertical transition energies and their Oscillator strengths (f) and major
contributions for the Aza-BODIPY 5 and 6
TD-DFT/ B3LYP (DCM)
λmax
fa
Major contribution (%)
HOMO→LUMO (100%)
Aza Bodipy
588 nm
0.8351
616 nm
624 nm
0.4908
0.2701
HOMO → LUMO+1(68%)
HOMO-2→LUMO (56%)
5
616 nm
779 nm
0.4178
0.1431
HOMO →LUMO+1 (44%)
HOMO−1→LUMO (32%)
6
af is Oscillator strengths
Figure 4. The major transitions in Aza-BODIPY 6
The Figure 5 shows the electron density distribution of the HOMO and LUMO of the ferrocenyl
substituted Aza-BODIPY 5 and 6 obtained using TD-DFT/B3LYP method. The electron
densities of HOMOs in the Aza-BODIPY 5 and 6 are delocalized over the ferrocenyl and Aza-
BODIPY core, whereas the LUMOs are located only on the Aza-BODIPY core. The HOMO-
LUMO gap is lower in ferrocenyl substituted Aza-BODIPY 6 as compared to the Aza-BODIPY
and monosubstituted Aza-BODIPY 5. These results reveal that by increasing conjugation the
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electronic communication increases in Aza-BODIPY 6 and band gap decreases. The comparsion
of optical band gap with theoretical band gap are listed in the Table S1.
Figure 5. The energy level diagram of the frontier molecular orbitals of the ferrocenyl
substituted Aza-BODIPY 5 and 6 calculated using B3LYP level of TD-DFT theory.
Conclusions
In summary, we have synthesized ferrocenyl substituted tetraphenylazadipyrromethane and Aza-
BODIPY by the Pd-catalyzed Sonogashira cross-coupling reaction. The UV-visible absorption,
electrochemical and computational studies of these molecules show good electronic interaction.
The absorption maxima of Aza-BODIPY 5 and 6 show considerable red shift compared to the
ferrocenyl substituted tetraphenylazadipyrromethane. The DFT and TD-DFT results reveal that
electron density transfers from ferrocene (donor) to azabodipy (acceptor) core. The
computational calculations of Aza-BODIPY 5 and 6 are in good agreement with the
experimental data. The Aza-BODIPYs reported here are potential candidates for biological
imaging and organic photovoltaic applications and their detailed study is currently on going in
our laboratory.
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Experimental section:
General: Chemicals were used as received unless otherwise indicated. All oxygen or moisture
sensitive reactions were performed under nitrogen/argon atmosphere using standard schlenk
method. Triethylamine (TEA) was received from commercial source, and distilled on KOH prior
1
to use. H NMR (400 MHz), and 13C NMR (100MHz) spectra were recorded on the Bruker
Avance (III) 400 MHz, using CDCl3 as solvent. Tetramethylsilane (TMS) was used as reference
1
13
for recording H (of residual proton;
δ = 7.26 ppm), and C (δ = 77.0 ppm) spectra in CDCl3.
UV-visible absorption spectra of all compounds in chlorofom were recorded on a Carry-100 Bio
UV-visible Spectrophotometer. Cyclic voltamograms (CVs) were recorded on a CHI620D
electrochemical analyzer using glassy carbon as a working electrode, Pt wire as the counter
electrode, and Ag/Ag+ as the reference electrode. HRMS was recorded on Brucker-Daltonics,
micrO TOF-Q II mass spectrometer.
Synthesis and Characterization
The ethynyl ferrocene was purchased from Sigma-Aldrich, and iodo Aza-BODIPY was
synthesized according to known methods.
Synthesis of ferrocenyl aza-pyromethanes and aza-BODIPY:
Iodo azabodipy (100 mg, 0.13 mmol) was stirred together with Pd(dba)2 (40 mg, 0.05 mmol) and
AsPh3 (115 mg) in degassed DMF (15 mL) and triethyl amine (15ml) for 30 min at room
temperature before the ethynyl ferrocene (2.5 equivalent) was added. The mixture was heated at
80 ºC for 48 h. The solvent was removed; the remaining residue was suspended in water (50 mL)
and extracted with EtOAc (3 x 20 mL). The combined organic layers were dried (MgSO4) and
concentrated in vacuum. The resulting crude product was purified by column chromatography
on silica gel eluting with CH2Cl2/hexane (2:3).
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Aza-BODIPY BF2.
Ferrocenyl substituted aza-pyromethanes (33 mg, 0.044 mmol) was dissolved in dry
dichloromethane (DCM) (15 mL) and 100 mL of diisopropylethylamine was added.
Trifluoroboron etherate (200 mL) was added to the mixture via a syringe. The reaction mixture
was stirred overnight at room temperature. The solution was washed with distilled water (50
mL) and dried over anhydrous Mg(SO4)2. The solvent was rotary evaporated to obtain a dark
blue solid. The product was further purified by column chromatography on silica gel with 1: 1
DCM/hexanes to afford a purple-blue solid.
Ferrocenyl aza-pyromethanes 3:
purple solid (Yield 30%).1H NMR (400 MHz, CDCl3): δ (ppm) 8.44 (d, 2H, J = 7.66 Hz), 8.21
(d, 2H, J = 7.66 Hz), 8.08 (d, 2H, J = 7.66 Hz), 7.84 (d, 2H, J = 7.66 Hz), 7.61-7.30 (m, 13H),
7.13 (s, 1H), 4.52 (s, 2H), 4.29 (s, 2H), 4,25-4.20 (m, 5H).
13C NMR (100 MHz, CDCl3): δ = 161.49, 153.61, 148.22, 145.12, 144.96, 139.67, 133.66,
133.30, 132.97, 131.22, 130.53, 130.43, 129.62, 129.24, 128.86, 128.59, 128.31, 128.25, 127.91,
127.71, 125.92, 115.28, 111.84, 98.97, 81.85, 71.28, 69.95, 69.12, 65.52. HRMS (ESI) m/z,
calcd for M+H (C44H31FeN3): 658.1941; found: 658.1784.
Ferrocenyl aza-pyromethanes 4:
Blue solid (Yield 35%). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.26 (d, 4H, J = 7.79 Hz), 8.14
(d, 4H, J = 7.79 Hz), 7.52-7.28 (m, 12H), 4,45 (s, 4H), 4.21 (s, 4H), 4.15 (s, 10H).
13C NMR (100 MHz, CDCl3): δ = 154.16, 148.19, 141.17, 138.26, 131.80, 131.26, 129.31,
129.22, 127.72, 127.16, 127.00, 126.69, 114.89, 113.04, 111.27, 96.70, 80.41, 70.24, 68.90,
68.03, 64.54, 52.30. HRMS (ESI) m/z, calcd for M+ H (C56H38Fe2N3): 865.1840; found:
865.1882.
Ferrocenyl aza-BODIPY 5:
Purple solid (Yield 30% ). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.25 (d, 2H, J = 7.50 Hz),
8.10-7.98 (m, 6H), 7.56-7.40 (m, 12H), 4.38 (s, 2H), 4.21 (s, 2H), 4.08 (m, 5H).
13C NMR (100 MHz, CDCl3): δ = 160.68, 160.56, 146.39, 144.67, 144.00, 142.12, 132.05,
132.01, 131.34, 131.19, 130.66, 130.44, 129.73, 129.62, 129.34, 128.66, 128.11, 127.82, 119.24,
114.96, 97.04, 79.39, 71.29, 69.94, 69.04, 64.91, 29.69 . 11B: δ =0.70 (t), 19F: δ =
-131.48.
HRMS (ESI) m/z, calcd for M+ H (C44H30BF2FeN3): 705. 1853; found: 705.1886.
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Ferrocenyl aza-BODIPY 6:
Blue solid (Yield 25%). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.26 (d, 4H, J = 7.79 Hz), 7.99
(d, 4H, J = 7.79 Hz), 7.54-7.41 (m, 12H), 4.37 (s, 4H), 4.20 (s, 4H), 4.07 (s, 10H).
11B: δ =0.70 (t), 19F: δ = -131.48. HRMS (ESI) m/z, calcd for M+ H (C56H37BF2Fe2N3):
913.1832; found: 913.1946.
Acknowledgements
The work was supported by DST and CSIR, Govt. of India, New Delhi. We gratefully
acknowledge Sophisticated Instrumentation Centre (SIC), IIT Indore.
Notes and references
Department of Chemistry, Indian Institute of Technology Indore, 452017, India. Fax: +91 731
2361 482; Tel: +91 731 2438 710 * E-mail rajneeshmisra@iiti.ac.in
†Electronic Supplementary Information (ESI) available: General experimental methods, and
copies of 1H NMR, and HRMS spectra of all new compounds.
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