Carbazole-BODIPY conjugates: Design, synthesis, structure and properties

Article abstract of DOI:10.1039/c4dt00983e

A set of carbazole substituted BODIPYs 2a-2c were designed and synthesized by the Pd-catalysed Sonogashira cross-coupling reaction. The effects of variation in the donor strength of various carbazoles were investigated by photophysical, electrochemical and computational studies. The electronic absorption spectra of BODIPYs 2a and 2c show charge transfer bands, which show red shift in polar solvents. The BODIPYs 2a-2c are highly fluorescent in nonpolar solvents (emission from the localized state) and poorly fluorescent in polar solvents (emission from the charge transfer state). The photophysical and electrochemical studies reveal strong donor-acceptor interaction between carbazole and BODIPY and follows the order 2a > 2c > 2b. The computational calculations show good agreement with the experimental results. The single crystal structures of BODIPYs 2a-2c are reported, which exhibit interesting supramolecular interactions. The packing diagrams of 2a show a zigzag 3D structural arrangement, whereas 2b and 2c show complex 3D structural motifs. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00983e

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1
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DOI: 10.1039/C4DT00983E
CarbazoleꢀBODIPY conjugates: design, synthesis, structure and properties
∗
Rajneesh Misra, Thaksen Jadhav, Bhausaheb Dhokale, Prabhat Gautam, Rekha Sharma,
Ramesh Maragani and Shaikh M. Mobin
Department of Chemistry, Indian Institute of Technology Indore, Indore 452 017, India.
*
Eꢀmail: rajneeshmisra@iiti.ac.in
Toc:
1.0
0.5
0.0
N
N
N
N
F
N
F
N
F
N
F
B
B
N
F
N
F
B
300
400
500
600
700
Wavelength (nm)
1

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DOI: 10.1039/C4DT00983E
Abstract
A set of carbazole substituted BODIPYs 2aꢀ2c were designed and synthesized by the Pdꢀ
catalysed Sonogashira crossꢀcoupling reaction. The effects of variation in the donor strength of
various carbazoles were investigated by photophysical, electrochemical and computational
studies. The electronic absorption spectra of BODIPYs 2a and 2c show charge transfer band,
which show red shift in polar solvents. The BODIPYs 2aꢀ2c are highly fluorescent in nonpolar
solvents (emission from localized state) and poorly fluorescent in polar solvents (emission from
charge transfer state). The photophysical and electrochemical studies reveal strong donorꢀ
acceptor interaction between carbazole and BODIPY and follows the order 2a > 2c > 2b. The
computational calculations show good agreement with the experimental results. The single
crystal structures of BODIPYs 2aꢀ2c are reported, which exhibits interesting supramolecular
interactions. The packing diagram of 2a show zig zag 3D structural arrangement, whereas 2b and
2c show complex 3D structural motifs.
Introduction
There has been continuous growing research interest in 4,4ꢀdifluoroꢀ4ꢀboraꢀ3a,4aꢀdiazaꢀsꢀ
indacene (BODIPY) dyes due to their strong acceptor nature and spectacular photophysical
properties such as strong absorption with high extinction coefficient, high fluorescence quantum
1
yield, and excellent photostability. The BODIPY dyes have been explored in biochemical
2
labeling, laser dyes, molecular electronics, photonics and electroluminescent materials. Our
group has explored the effect of various functional groups at the βꢀpyrrolic and mesoꢀposition of
3
the BODIPY on its photophysical properties. Recently, we have shown the quenching of
fluorescence of the BODIPYs can be used as a measure of relative electron donating strength of
2

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4
different aryl substituents. In continuation of the our previous report, we were further interested
to study the donor strength of different carbazoles.
Carbazole derivatives have been widely studied in optoelectronics due to its high
chemical stability, low redox potential, hole transport property, and strong electron donating
5
ability. Nakamura and coworkers have explored the synthesis, and photophysical properties of
6
thienylcarbazoles, and carbazole dimers.
In order to study the electron releasing behavior of different carbazoles, we have
connected it with mesoꢀethynylꢀBODIPY. The electron releasing strength of the carbazole were
found to be directly proportional to the photophysical properties of carbazoleꢀBODIPY
conjugates. It has been shown that the BODIPYs 2aꢀ2c show strong donorꢀacceptor interaction.
Results and Discussions
The BODIPYs 2aꢀ2c were synthesized by the Pdꢀcatalyzed Sonogashira crossꢀcoupling reaction
7
of 8ꢀchloro BODIPY 1 with respective ethynylcarbazoles aꢀc (Scheme 1). 8ꢀchloro BODIPY 1
6
,8
and the ethynylcarbazoles were synthesized by the reported procedures (Scheme S1).
The Sonogashira crossꢀcoupling reaction of the 8ꢀchloro BODIPY 1, with the respective
ethynylcarbazoles (aꢀc) at 0ꢀ5 °C in dichloromethane with PdCl (PPh ) as catalyst and CuI as
2
3 2
coꢀcatalyst resulted in BODIPYs 2aꢀ2c in 69%, 58% and 70% yields respectively. All the
1
13
BODIPYs were well characterized by H NMR, C NMR, HRꢀMS and single crystal Xꢀray
1
diffraction techniques. In the H NMR spectrum of the BODIPYs, the β pyrrolic protons appear
at the 6.5ꢀ6.6 ppm region, β’ protons appear at 7.4ꢀ7.5 ppm region whereas the pair of two α
protons appears at ~7.8 ppm (see supporting information).
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Ar
B
Cl
Ar
PdCl (PPh ) ,CuI,
2
3 2
N
F
N
F
N
F
N
F
DCM:TEA (10:1),
0 °C, 1 h
B
1
2aꢀ2c
N
N
Ar
=
N
b
c
a
Scheme 1. Synthesis of BODIPYs 2aꢀ2c.
Thermal stability of the organic materials is key requirement for various optoelectronic
applications. The thermal properties of BODIPYs 2aꢀ2c were investigated by the
ꢀ1
thermogravimetric analysis (TGA) at a heating rate of 10 °C min , under nitrogen atmosphere
(
Fig. 1). The thermal decomposition temperature (T ) for 10 % weight loss in the BODIPYs 2aꢀ
d
2c was in the range of 320ꢀ368 °C (Table 1) and follows order 2c > 2b > 2a. The presence of
alkyl groups and steric hindrance lowers the thermal stability of BODIPY 2a. The positional
isomers 2b and 2c show similar thermal stability.
4

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105
2
a
2
2c
b
90
75
60
45
100
200
300
400
500
C)
600
700
800
Temparature (
°
–1
Fig. 1 TGA plots of the BODIPYs 2aꢀ2c at the heating rate of 10 °C min under nitrogen
atmosphere.
Photophysical Properties
The electronic absorption spectra of the BODIPYs 2aꢀ2c in different solvents are shown in Fig. 3
and the corresponding data are summarized in Table 1. The pictures of the BODIPYs 2a–2c in
toluene in day light and UV light are displayed in Fig. 2. The carbazoleꢀBODIPY conjugates 2aꢀ
2
c show strong absorption band between 537–549 nm region with high extinction coefficient.
The absorption spectrum in different solvents do not show significant effect on the band between
37–549 nm region, but show pronounced effect on the shoulder transitions in between 450ꢀ525
5
nm region. The BODIPY 2a show charge transfer (CT) band at around 595 nm which was red
shifted with increasing solvent polarity. This reflects strong donorꢀacceptor interaction between
9
,10
BODIPY and carbazole unit in BODIPY 2a.
The BODIPY 2c shows weak CT in polar
solvents (tetrahydrofuran and dichloromethane) whereas CT band was absent in the case of 2b
irrespective of solvent polarity. The strength of CT band in BODIPYs 2aꢀ2c follows the order as
2a > 2c > 2b.
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−
4
Fig. 2 The BODIPYs 2a–2c in toluene at concentration of 10 M in day light (A) and in UV
light (B).
Table 1. Photophysical and thermal properties of BODIPYs 2aꢀ2c.
4
ꢀ
a
b
c
BODIPY solvent
λabs(nm) ɛ×10
λem
Stokes
ɸF
Td
(°C) band
gap (eV)
Optical
Theoretical
band
gap (eV)
ꢀ1
(
Mol
.
(nm)
583
671
737
ꢀ
Shift(cm )
1229
3378
4916
ꢀ
1
ꢀ1
d
e
cm )
2
a
Cyclohexane 544
13
547
16
541
11
584
5.79
2.85
5.98
3.01
57.1
30.7
1.78
5.85
3.10
0.3537 322 2.11
2.59
5
Toluene
THF
0.0564
5
0.0010
5
DCM
5
5
40
12
2
b
Cyclohexane 543
5.65
2.91
4.34
5.32
3.09
3.62
4.91
3.41
5.40
3.83
3.61
553
562
333
488
0.5512 366 2.21
0.4991
2.80
5
4
13
76
Toluene
547
5
4
14
77
THF
541
10
541
556,
770
ꢀ
499
ꢀ
0.0031
5
DCM
5
4
11
81
2
c
Cyclohexane 539
00
541
09
537
06
537
8.28
7.81
7.89
5.65
8.35
5.37
7.55
4.64
548
558
668
ꢀ
304
563
365
ꢀ
0.3813 368 2.23
0.2252
2.84
5
Toluene
THF
5
0.0030
5
DCM
5
04
a
b
Excitation at λmax, Determined by using Rhodamine B as standard (ɸ = 0.5, in ethanol)
c
d
Decomposition temperature at 10% weight loss, determined by TGA. Determined from the
e
onset wavelength of the UVꢀvis absorption in cyclohexane. Calculated from theoretical study in
vacuum.
6

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(a)
6
5
4
3
2
1
0
(
b)
6
4
2
0
Cyclohexane
Toluene
THF
Cyclohexane
Toluene
THF
DCM
DCM
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
c)
Wavelength (nm)
(
8
7
6
5
4
3
2
1
0
Cyclohexane
Toluene
THF
DCM
300
400
500
600
700
Wavelength (nm)
Fig. 3 Absorption spectra of the BODIPYs 2a (a), 2b (b), and 2c (c) in different solvents.
In order to study the effect of solvent variation on the electronic absorption behavior of
the BODIPYs 2aꢀ2c, TD‐DFT calculations were performed in dichloromethane and cyclohexane
solvents using IEFPCM model. The computed vertical transitions, their oscillator strengths and
the configurations are shown in Table 2. The molecular orbitals involved in the transitions are
displayed in Fig. S6. The contributions from different orbitals were found to be largely
influenced by the polarity of the solvents. The TD‐DFT studies exhibit two major transitions for
BODIPYs 2a and 2c, whereas three major transitions for 2b. TDꢀDFT calculations of the
BODIPY 2a in cyclohexane reveal that the main absorption band at 451 nm and weak absorption
band at 407 nm are mainly from HOMOꢀ4 and HOMOꢀ2 to the LUMO orbital. The BODIPY 2b
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shows main transition at 480 nm from HOMOꢀ3 and HOMOꢀ2 to the LUMO orbital and two
weak bands at 458 and 409 nm from HOMOꢀ3 and HOMOꢀ1 to the LUMO orbital. The
BODIPY 2c shows the main transition at 445 nm and weak transition at 403 nm from HOMOꢀ3
and HOMOꢀ1 to the LUMO orbital.
Table 2. Computed vertical transitions, their oscillator strengths and configurations of BODIPYs
2
aꢀ2c.
DCM
cyclohexane
Compound
a
a
λmax[nm]
f
Configuration
λmax[nm]
f
Configuration
2
a
472.54
0.5174
HOMOꢀ4 ꢀLUMO( 0.19082)
HOMOꢀ2 ꢀLUMO(0.68435)
450.65
0.2777
HOMOꢀ4 ꢀLUMO(0.30622)
HOMOꢀ3 ꢀLUMO(0.24261)
HOMOꢀ2 ꢀLUMO( 0.58965)
4
13.45
0.1205
HOMOꢀ4 ꢀLUMO(0.67993)
HOMOꢀ2 ꢀLUMO(ꢀ0.19424)
407.40
479.85
0.2000
HOMOꢀ4 ꢀLUMO(0.61773)
HOMOꢀ2 ꢀLUMO(0.34555)
2
b
517.22
1.1228
0.4795
HOMOꢀ1ꢀLUMO(0.70434)
0.9528
0.3378
HOMOꢀ2 – LUMO(0.70526)
HOMOꢀ3 ꢀ LUMO( 0.25489)
HOMOꢀ1 ꢀ LUMO( 0.66410)
HOMOꢀ3 ꢀLUMO( 0.65791)
HOMOꢀ1 ꢀLUMO(ꢀ0.25915)
4
80.46
HOMOꢀ3 ꢀ LUMO(ꢀ0.15527) 457.72
HOMOꢀ2 ꢀ LUMO(0.69229)
4
13.07
0.0870
HOMOꢀ3ꢀ LUMO(0.68861)
HOMOꢀ2ꢀ LUMO(0.15815)
409.39
0.1138
2
c
468.27
0.5376
0.1144
HOMOꢀ4 ꢀ LUMO(ꢀ0.17930) 445.38
HOMOꢀ1 ꢀ LUMO(ꢀ0.68721)
0.3512
0.1916
HOMOꢀ3 ꢀ LUMO( ꢀ0.32036)
HOMOꢀ1 ꢀ LUMO(ꢀ0.63474)
HOMOꢀ3 ꢀ LUMO( ꢀ0.62915)
HOMOꢀ1 ꢀ LUMO( 0.32614)
4
08.49
HOMOꢀ4 ꢀ LUMO(ꢀ0.68302) 403.19
HOMOꢀ1 ꢀ LUMO(0.18244)
a
Oscillator strength
The fluorescence spectra of BODIPYs 2aꢀ2c in different solvents are shown in the Fig. 4
and data are summarized in Table 1. The fluorescence spectra of 2aꢀ2c in nonpolar solvent
exhibits characteristic emission of BODIPYs such as narrow bandwidth, small stokes shift and
high quantum yield. The increase in solvent polarity leads to red shift in the fluorescence with
decrease in fluorescence quantum yield and generates a new broad emission band at longer
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wavelength region. The emission band in nonpolar solvent was assigned as locally excited (LE)
emission band while the newly generated longer wavelength emission band in polar solvent was
9
,11
assigned as charge transfer (CT) emission band.
The literature reveals that the contribution of emission from CT state in BODIPYs is
9
directly proportional to the strength of donorꢀacceptor interaction. The emission from charge
transfer state and quenching of fluorescence could be an effective tool to compare the strength of
4
,9
DꢀA interactions in the BODIPYs. The BODIPYs 2aꢀ2c show different emission patterns with
increasing the solvent polarity from cyclohexane to toluene and tetrahydrofuran. The emission is
dominated from CT state with decrease in the quantum yield by increasing the polarity of
solvent. The BODIPYs 2aꢀ2c emit from the LE state in nonꢀpolar solvent (cyclohexane) with
high quantum yields. In toluene the BODIPYs 2b and 2c show emission from LE state, whereas
2a shows emission from CT state. In THF 2b show dual emission LE and CT state whereas 2a
and 2c show emission from CT state. The BODIPY 2a show considerable red shift in the
emission with decrease in quantum yield from cyclohexane to toluene and THF, indicating that
some additional factors are also involved. The two terꢀbutyl groups on the carbazole unit
decreases the quantum yield due to rotational and vibrational relaxations and may have important
1
2,13
role in more donorꢀacceptor character.
The polar dichloromethane solvent stabilizes the
charge transfer state and makes the BODIPYs 2aꢀ2c nonꢀemissive. The emission properties of
BODIPYs 2aꢀ2c in different solvents suggest the trend in strength of DꢀA interaction as 2a > 2c
>
2b. The high sensitivity of the BODIPYs 2aꢀ2c towards solvent polarity make them potential
candidate for application as fluorescent probes for biological species and solvent polarity
1
0
detection.
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(a)
(b)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
Cyclohexane
Toluene
THF
0.8
Cyclohexane
Toluene
THF
0.6
0.4
0.2
0.0
550
600
650
700
750
800
850
550
600
650
700
750
800
850
Wavelength (nm)
Wavelength (nm)
(c)
1.0
0.8
0.6
0.4
0.2
0.0
Cyclohexane
Toluene
THF
5
50
600
650
700
750
800
850
Wavelength (nm)
Fig. 4 Normalized fluorescence spectra at the wavelength of maximum intensity of 2a (a), 2b
b), and 2c (c) in cyclohexane, toluene and THF solvents.
(
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Electrochemical properties
3
2
1
0.0µ
1
0.0µ
0.0µ
0.0µ
0.0
0
.0
ꢀ10.0µ
ꢀ20.0µ
ꢀ30.0µ
ꢀ40.0µ
BODIPY 2a
ꢀ
ꢀ
ꢀ
ꢀ
10.0µ
20.0µ
30.0µ
40.0µ
CV
DPV
BODIPY 2b
CV
DPV
ꢀ2
ꢀ1
0
1
2
ꢀ2
ꢀ1
0
1
2
Potential vs SCE (V)
Potential vs SCE (V)
10.0µ
0.0
ꢀ10.0µ
ꢀ20.0µ
ꢀ30.0µ
BODIPY 2c
CV
DPV
ꢀ2
ꢀ1
0
1
2
Potential vs SCE (V)
Fig. 5 The CV and DPV plots of the BODIPYs 2aꢀ2c.
The electrochemical properties of the BODIPYs 2aꢀ2c were investigated by the cyclic
voltammetric and differential pulse voltammetric (CV and DPV) techniques. The CV and DPV
results are shown in Fig. 5 and the corresponding data are summarized in Table 3. The
electrochemical study reveals that the redox peaks are irreversible in nature. The BODIPY 2a
show two oxidation and two reduction waves corresponding to the formation of dication and
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dianion radicals respectively, whereas BODIPYs 2b and 2c show two oxidation and one
reduction waves corresponding to the formation of dication and anion radicals respectively. The
BODIPY 2a shows the easiest oxidation indicating the pronounced delocalization of the electron
density of the donor carbazole moiety in the acceptor BODIPY. The trend in the first oxidation
potential follows the order as 2b> 2c > 2a. This trend supports the strength of donorꢀacceptor
interaction observed from the photophysical studies.
a
Table 3. Electrochemical properties of the BODIPYs 2aꢀ2c.
2
1
1
2
E
E
E
E
BODIPY
b
b
b
b
red
red
oxid
oxid
2
a
1.54
1.77(broad)
1.58(broad)
1.16
1.24
1.21
ꢀ0.63
ꢀ1.34
ꢀ1.16
ꢀ0.95
2
b
ꢀ
ꢀ
2
c
a
ꢀ1
electrochemical analysis was performed in 0.1 M solution of Bu NPF in DCM at 100 mV s scan rate, versus
4
6
b
SCE. irreversible wave.
Theoretical calculations
In order to understand the geometry and the electronic structure of the BODIPYs 2aꢀ2c density
1
4
functional theory (DFT) calculations were carried out using the Gaussian09W program. The
geometry optimization was carried out without any symmetry constraint in the gas phase. The
density functional theory (DFT) calculation was performed at the B3LYP/6ꢀ31G (d) level of
1
4
15
theory. The TD‐DFT calculations were performed at the B3LYP/6‐31 G(d) level .
The geometry optimization show good correlations with the experimental data obtained
from single crystal structures. The Fig. 6 depicts the frontier molecular orbitals (FMO) of the
BODIPYs 2aꢀ2c. In BODIPY 2a, HOMO is localized on the electron donating carbazole unit
12

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and LUMO on the electron withdrawing BODIPY unit, which reflects strong donorꢀacceptor
interaction. In BODIPYs 2b and 2c HOMO is localized on the BODIPY unit and LUMO on the
BODIPY and carbazole unit, which reveals weak donorꢀacceptor interaction than 2a. The
theoretical band gap values are summarized in Table 1, which shows the trend as 2a < 2b < 2c.
Fig. 6 Frontier molecular orbitals of BODIPYs 2aꢀ2c at the B3LYP/6ꢀ31G(d) level.
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Single crystal X‐ray analysis
The single crystals of the BODIPYs 2a and 2b were obtained by slow diffusion of ethanol in the
dichloromethane solution whereas crystal of 2c was obtained by slow evaporation of
hexane:dichloromethane (6:4) solution at room temperature. The BODIPYs 2a–2c crystallizes
into orthorhombic Pna2 , P2 2 2 , and Pbcn space groups respectively. The crystal structure and
1
1 1 1
data refinement parameters are shown in Table S1 (SI). The front view and side view of the
crystal structures are displayed in Fig. 7. The crystal structures of BODIPYs 2a–2c show planar
structures. The dihedral angle between carbazole and BODIPY units in BODIPYs 2a–2c are
24.77°, 4.04°, and 8.25° respectively.
Fig. 7 Crystal structures of the BODIPYs 2aꢀ2c, (A) front view and (B) side view.
The comparison of the selected bond lengths and bond angles of the single crystal structures and
the DFT optimized structures of BODIPYs 2a–2c show slight variation in bond lengths and bond
angles (Fig. 8 and Fig. S5). In the BODIPYs 2a–2c the two CꢀN bonds of carbazole show
different bond lengths. The CꢀN bond near to BODIPY is shorter than the distant CꢀN bond,
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suggesting higher double bond character of the former, due to the delocalization of lone pair
1
6
electrons of carbazole nitrogen into the electron deficient BODIPY unit.
1
.390
F
F 1.393
1.385
F
F 1.383
1.382
F
F
1.382
B
1.549
1.356
1.407
B
1.551
1
1
.547
B
1.538
1.344
1.350
N2
1.388
.407
.342
1
.341
1
.551
1.405
1
.398
N1 1.549
.391
1.384
1.375
1.396
1.388
N
1
N
2
1
.398
N1
N2
1.388
1.378
1.335
1
1.388
1.393
1.394
1.399
1
1.418
1
.407
1.365
1.390
1.380
1
.370
1.401
1
.407
1.404
.425
1.415
1.398
1.419
1.405
1.400
.423
1
1
1
.200
1.206
1.205
1.431
1.430
1
.429
1
.452
1.392
1.416
1.408
1
.388N
1.403
1
.378
1
.392
1
.389
1.398
1.455
N
1
.376
N 1.461
.394
1.397
1
2
a crystal stucture
2b crystal stucture
2c crystal stucture
F 1.382
1.383
F
F 1.383
1.382
F F
1.382 1.383
F
B
1.569 1.344
B
1.570 1.344
1.344
1.415
1.568
B
1.569
1.343
1
.344
1.569
1
.409
N1
.390
N2
1.409
N1 1.569
1.390
N2
1.409
1.390
1.409
1.344 N1
.390
N2
1.390
1.408
1
.409
1
1.390
1
.389
1
1
.416
1.391
1.415
1.391
1.390
1.414
1
.415
1.391
1.390
1.390
1
.413
1.414
1
.416
1.413
.411
1.415
1.416
1.410
1
1
.411
1.220
1.219
1
.222
1
.417
1.419
1.417
1
.458
1
.421
1
1.409
1
.415
1
.385N
1.397
.386
1
.392
1
.389
1.402
1.456
N
1
.382
N 1.457
.396
1.391
1
2
c DFT optimised structure
2
a DFT optimised structure
2b DFT optimised structure
Fig. 8 Comparison of selected bond lengths of the crystal structures and DFT optimized
structures of the BODIPYs 2aꢀ2c.
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The crystal structures of the BODIPYs 2aꢀ2c show interesting supramolecular interactions. The
distances of the supramolecular interactions in the crystal packing are shown in Table 3. In the
packing diagram of 2a, the interactions C(8)
C(25A) H(25A)‐‐‐F(2) form a trimer like structure which grows along the cꢀaxis to form 2D
sheet (Fig. S4). The two different sheets connect another via C(15) H(15)‐‐‐F(1),
C(22) H(22)‐‐‐F(1) and C(13) H(13)‐‐‐F(2) interactions to form zig zag 3D structural
arrangement (Fig. 9).
‐H(8)‐‐‐F(2), C(13)‐H(13)‐‐‐F(2) and
‐
‐
‐
‐
Fig. 9 Packing diagram of the BODIPY 2a.
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In the packing diagram of BODIPY 2b, the interaction F(1)ꢀꢀꢀH(13)ꢀC(13) and C(2)
‐H(2)‐‐‐
π
(centroid C6, C7, C8, C9, N2) leads to the Zꢀshaped 2D chain. This 2D chain further transforms
into a sheet through interactions C(16)‐H(16)‐‐‐π (centroid C12, C13, C14, C15, C16, C17),
C(8)
C7, C8, C9, N2). This two dimensional chain further extends to complex 3D structural motifs via
two mutual C(1) H(1)‐‐‐F(2) interactions (Fig. 10).
‐H(8)‐‐‐π (centroid C1, C2, C3, C4, N1), π (centroid C1, C2, C3, C4, N1)‐‐‐ π (centroid C6,
‐
Fig. 10 Packing diagram of the BODIPY 2b.
In the packing diagram of BODIPY 2c the interaction C(9)ꢀH(9) ꢀꢀꢀF(1) forms a herringbone
like structure, in which the BODIPY core is connected to each other in head to heat fashion
along cꢀaxis (Fig. 11a). This sheet further grow in above direction via C(21)
‐H(21)‐‐‐F(2),
C(25C) H(25C)‐‐‐ F(2), C(1) H(1)‐‐‐π (centroid C18, C19, C20, C21, C22, C23) interactions and
‐
‐
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in below direction via C(24B)
‐H(24B)‐‐‐F(1), and C(24A)
‐H(24A)‐‐‐π (centroid C6, C7, C8, C9,
N2) interactions to form complex 3D structural arrangement (Fig. 11b).
Fig. 11 Packing diagram of the BODIPY 2c (a) view along cꢀaxis (b) view along bꢀaxis.
18

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Table 3. Selected bond length of intermolecular interactions in the crystal structure of BODIPYs
aꢀ2c.
2
Interaction
Distance (Å)
2
a
C(15)
C(22)
C(13)
‐
‐
‐
H(15)‐‐‐F(1)
H(22)‐‐‐F(1)
H(13)‐‐‐F(2)
2.628
2.626
2.478
2.574
2.552
C(25A)
‐H(25A)‐‐‐F(2)
C(8) H(8)‐‐‐F(2)
‐
2
b
C(16)
‐
H(16)‐‐‐π (centroid C12, C13, C14, C15, C16, C17)
2.806
3.216
2.928
C(8)‐H(8)‐‐‐π (centroid C1, C2, C3, C4, N1)
H(9)‐‐‐π (acetylene)
C(9)
‐
π (centroid C1, C2, C3, C4, N1) ‐‐‐ π (centroid C6, C7, C8, C9, N2) 3.434
C(2)
C(13)
‐
H(2)‐‐‐π (centroid C6, C7, C8, C9, N2)
3.175
2.489
2.602
‐H(13)‐‐‐ F(1)
C(1)
‐
H(1)‐‐‐F(2)
2
c
C(9)
‐
H(9)‐‐‐F(1)
2.426
2.514
2.655
2.532
2.545
3.014
C(24B)
C(24A)
C(21) H(21)‐‐‐F(2)
C(25C) H(25C)‐‐‐ F(2)
C(1) H(1)‐‐‐π (centroid C18, C19, C20, C21, C22, C23)
‐
‐
H(24B)‐‐‐F(1)
H(24A)‐‐‐π (centroid C6, C7, C8, C9, N2)
‐
‐
‐
Conclusion
In summary donorꢀacceptor carbazoleꢀBODIPY conjugates 2aꢀ2c were synthesized by the Pdꢀ
catalysed Sonogashira crossꢀcoupling reaction. The electronic absorption and fluorescence
properties show intramolecular charge transfer from carbazole to BODIPY. The photophysical
properties of BODIPYs 2aꢀ2c in different solvents suggest that the mesoꢀethynylBODIPY can be
used as effective tool to measure small differences in donor strength of different carbazoles. The
donorꢀacceptor interaction in BODIPYs 2aꢀ2c follows the order 2a > 2c > 2b. The presence of
alkyl groups and steric hindrance in BODIPY 2a decreases the thermal stability. Our results
provide important method for screening the donor strength of different donors which will help in
design of strong DꢀA materials.
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Experimental Section
General methods.
Chemicals were used as received unless otherwise indicated. All oxygen or moisture sensitive
1
13
reactions were performed under nitrogen/argon atmosphere. H NMR (400 MHz), and C NMR
(100MHz) spectra were recorded on the Bruker Avance (III) 400 MHz instrument by using
1
CDCl . H NMR chemical shifts are reported in parts per million (ppm) relative to the solvent
3
residual peak (CDCl , 7.26 ppm). Multiplicities are given as: s (singlet), d (doublet), t (triplet), q
3
(quartet), dd (doublet of doublets), dt (doublet of triplets), m (multiplet), and the coupling
1
3
constants, J, are given in Hz. C NMR chemical shifts are reported relative to the solvent
residual peak (CDCl , 77.36 ppm. Thermogravimetric analyses were performed on the Metler
3
Toledo Thermal Analysis system. UV
‐visible absorption spectra were recorded on a Carry‐100
Bio UV visible Spectrophotometer. Cyclic voltamograms (CVs) were recorded on a CHI620D
‐
electrochemical analyzer using Glassy carbon as working electrode, Pt wire as the counter
electrode, and Saturated Calomel Electrode (SCE) as the reference electrode. The scan rate was
1
‐
1
00 mVs . A solution of tetrabutylammonium hexafluorophosphate (TBAPF ) in CH Cl (0.1
6
2
2
M) was employed as the supporting electrolyte. HRMS was recorded on Brucker
micrOTOF Q II mass spectrometer. The density functional theory (DFT) calculation were
carried out at the B3LYP/6 31G(d) level for B, F, C, N, H in the Gaussian 09 program.
‐Daltonics,
‐
‐
Synthesis and Characterization of BODIPYs 2aꢀ2c:
A generalized procedure for the Sonogashira crossꢀcoupling reaction.
8ꢀChloro BODIPY 1 (190 mg, 0.845 mmol) and the corresponding ethynylꢀcarbazole (0.845
mmol) were dissolved in DCM–triethylamine (10 : 1, v/v; 10 ml), and the mixture was cooled to
20

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0
5
°C using an ice bath. The reaction mixture was purged with argon, and Pd(PPh ) Cl (29.6 mg,
3 2 2
mol%), and CuI (16 mg, 10 mol%) were added, followed by stirring at 0ꢀ5 °C for 1 h. Upon
the completion of the reaction, the mixture was evaporated to dryness, and the crude product was
dissolved in CH Cl , chromatographed on silica (5:5; hexanes:dichloromethane), and
2
2
recrystallized from a DCM:Ethanol (8:2) mixture to gave 2aꢀ2c (yield 55–70%) as crystalline
solids.
2a
1
Blue solid. Yield: 69%, Mp: above 250 °C. H NMR (400 MHz, CDCl , 25 °C, ppm): δ 8.27 (d,
3
1H, J=4 Hz), 8.12 (d, 1H, J=4 Hz), 7.86 (s, 2H), 7.75 (d, 1H, J=2 Hz), 7.61 (dd, 1H, J=1.6, 8
Hz), 7.44 (d, 2H, J=4 Hz), 7.39 (d, 1H, J=12 Hz), 6.6 (d, 2H, J=4 Hz), 4.89 (q, 2H, J=8 Hz),
1
3
1
1
1
.50ꢀ1.55 (m, 12H), 1.48 (s, 9H) ppm, 0.00 ppm; C NMR (100 MHz, CDCl , 25 °C, ppm): δ
3
43.1,143.0, 141.9, 138.9, 137.6, 136.4, 130.3, 128.3, 127.9, 124.9, 124.6, 122.2, 121.1, 118.3,
11
18.2, 116.4, 108.4, 106.9, 101.2, 88.1, 38.6, 34.7, 34.6, 32.0, 31.8, 15.0; B NMR (CDCl , 96.3
3
+
MHz, ppm) 0.3 (t, J
‐
= 19.3 Hz); HRMS (ESI
‐
TOF) m/z: calcd. for C H BF N + H =
B F
33 34
2
3
522.2892, found 522.2896.
2b
1
Red solid, Yield: 58%, Mp: above 250 °C. H NMR (400 MHz, CDCl , 25 °C, ppm): δ 8.13ꢀ8.17
3
(m, 2H), 7.85 (s, 2H), 7.71 (s, 1H), 7.46ꢀ7.58 (m, 5H), 7.28ꢀ7.32 (m, 1H), 6.58 (d, 2H, J=4 Hz),
1
3
4
1
1
.44 (q, 2H, J=8 Hz), 1.50 (t, 3H, J=8 Hz) ppm; C NMR (100 MHz, CDCl , 25 °C, ppm): δ
3
43.2, 141.2, 139.3, 136.5, 128.9, 127.4, 125.4, 123.6, 122.3, 121.2, 120.8, 119.7, 118.2, 117.1,
1
1
13.0, 108.9, 108.7, 84.5, 37.8, 13.9, 0.00 ppm; B NMR (CDCl , 96.3 MHz, , ppm) 0.2 (t, J
B F
‐
3
+
=
19.3 Hz); HRMS (ESI
‐
TOF) m/z: calcd. for C H BF N + Na = 432.1458, found 432.1451.
2
5
18
2
3
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2c
1
Red solid Yield: 70%, Mp: 222ꢀ226 °C. H NMR (400 MHz, CDCl , 25 °C, ppm): δ 8.43 (dd,
3
1
1
3
1
1
H, J=2, 0.8 Hz), 8.14ꢀ8.16 (m, 1H), 7.82 (s, 2H), 7.76 (dd, 1H, J=8.8, 1.2 Hz), 7.54ꢀ7.58 (m,
H), 7.45ꢀ7.49 (m, 4H), 7.32ꢀ7.36 (m, 1H), 6.57 (d, 2H, J=4 Hz), 4.42 (q, 2H, J=8 Hz), 1.49 ( t,
1
3
H, J=8 Hz) ppm; C NMR (100 MHz, CDCl , 25 °C, ppm): δ 142.4, 141.3, 140.5, 136.3,
3
30.6, 126.4, 126.9, 126.2, 123.5, 122.3, 120.8, 120.3, 117.9, 110.6, 109.12, 109.06, 84.8, 37.9,
1
1
3.8. B NMR (CDCl , 96.3 MHz, , ppm) 0.3 (t, J
B F
‐
= 19.3 Hz); HRMS (ESI‐TOF) m/z: calcd.
3
+
for C H BF N + Na = 432.1458, found 432.1452.
2
5
18
2
3
Supporting Information
1
13
11
19
General experimental methods, and copies of H, C, B, F NMR, and HRMS spectra of all
new compounds, Crystallographic Information Files (CIFs) for 2aꢀ2c with CCDC numbers
994993, 994994 and 994991 respectively, and DFT calculation data.
Acknowledgment
We thank CSIR, and DST, New Delhi for financial support. TJ and BD thanks, UGC and CSIR
New Delhi for their fellowship. We are thankful to Sophisticated Instrumentation Centre (SIC),
IIT Indore.
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