3-/3,5-Pyrrole-substituted BODIPY derivatives and their photophysical and electrochemical studies

Article abstract of DOI:10.1007/s12039-016-1134-9

Nucleophilic substitution on 3-bromo/3,5-dibromo-4,4′-difluoro-8-(aryl)-4-bora-3a,4a-diaza-s-indacene (BODIPY), substituted with anisyl or thienyl at meso positions, with neat pyrrole afforded the mono and di-pyrrole substituted BODIPYs 1–4 in good yields

Full text of DOI:10.1007/s12039-016-1134-9

c
J. Chem. Sci. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-016-1134-9
3-/3,5-Pyrrole-substituted BODIPY derivatives and their photophysical
and electrochemical studies
KARTHIKA J KADASSERYa,#, AKANKSHA NIMESHa,b, SANOJ RAJa,$ and
NEERAJ AGARWALa,∗
aUM-DAE, Centre for Excellence in Basic Sciences, Health center building, Kalina campus, Santacruz (E),
Mumbai, 400 098 India
bDepartment of Chemistry, Indian Institute of Technology Kanpur, Kanpur, 208016 India
#Current address: Department of Chemistry, University at Buffalo, Buffalo, NY 14260, USA
$Current address: Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60607, USA
e-mail: na@cbs.ac.in
MS received 7 February 2016; revised 1 July 2016; accepted 3 July 2016
Abstract. Nucleophilic substitution on 3-bromo/3,5-dibromo-4,4ꢁ-difluoro-8-(aryl)-4-bora-3a,4a-diaza-s-
indacene (BODIPY), substituted with anisyl or thienyl at meso positions, with neat pyrrole afforded the
mono and di-pyrrole substituted BODIPYs 1–4 in good yields. Large bathochromic shifts, upto ∼180 nm in
absorption maxima (581–682 nm), and fluorescence maxima (606–695 nm) were observed for these BODIPYs.
Absorption and fluorescence properties were studied in different solvents to compare the effect of mono and di
substitution on BODIPY. The Lippert-Mataga equations were used which predict strong polarization of mono
substituted BODIPYs. Electrochemical studies were carried out to find the oxidation potential and HOMO
energy levels were calculated. Theoretical studies of 1–4 provide the insight on the electron density distribu-
tion in 1–4. Theoretical and experimental photo-physical studies in different solvents were correlated to find
the substituent effects on BODIPY.
Keywords. BODIPY; charge transfer; lippert-mataga equation; solvatochromism.
1. Introduction
properties could be altered by changing the substituents
on its core. Various parameters such as distribution
4,4ꢁ-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) of electron density, effective conjugation length, sol-
is a popular dye family having high molar absorption ubility in different types of solvents, conjugates with
coefficient, sharp absorption and fluorescence spectra other chromophores were used to tune the properties of
and has been studied by many research groups.1 3 Tun- BODIPYs.13 20 Coupling reactions have been employed
able photophysical and electrochemical properties of on BODIPY to introduce aryl substituents in the pres-
BODIPY dyes make them attractive for chemosensors, ence of transition metal catalysts and recently without
photovoltaic applications, fluorescence probes and bio- using metal catalyst.21 33
logical applications.4 11 BODIPYs are widely used as
labeling dyes or as fluorescence sensors in biological
systems owing to their excellent photophysical proper-
ties and stability in physiological conditions. The devel-
opment of near-infrared (NIR) derivatives of BODIPY
have recently gained much attention for their appli-
cations in fluorescence imaging and in photodynamic
therapy. For example, pyrrole-containing BODIPYs
such as BODIPY 576/589 and BODIPY 650/665 have
been commercialized and their analogues have been
widely used as red fluorescent dyes. Another recent
application of BODIPY is their use in the up-conversion
of energy by triplet-triplet annihilation due to their low
lying triplet energy levels.12 For various applications
of BODIPYs, photophysical as well as electrochemical
The recent developments in direct functionaliza-
tion rather than the inter-conversion of organic groups
have opened up a rapidly expanding field of BODIPY
chemistry.34,35 Introduction of nucleophiles via nucleo-
philic aromatic substitution (SNAr) have been reported
recently on halogen substituted BODIPY.34,35 The same
group showed the oxidative nucleophilic substitution at
3-position of non-halogenated BODIPY.36 Lijuan Jiao
et al., reported the direct oxidative nucleophilic sub-
stitution of the 3-hydrogen of BODIPY by pyrrole
under reflux condition in oxygen atmosphere.37 Only
mono substitution of pyrrole could be achieved by
this method which was further shown to be useful to
synthesize the carboxaldehyde derivative of BODIPY
by Vilsmeier-Haack reaction. Recently, Hao et al.

Karthika J Kadassery et al.
reported regioselective substitution on BODIPYs with (s, 1H, NH), 7.53 (d, 2H, J = 8.6 Hz), 7.71 (s, 1H), 7.22
pyrrole under mild conditions without using any (s, 1H), 7.01–7.06 (m, 4H), 6.95 (d, 1H, J = 4.6 Hz),
catalyst.38
6.73 (d, 1H, J = 3.6 Hz), 6.50 (m, 1H), 6.42 (m, 1H),
In this article, we report synthesis of both mono and 3.93 (s, 3H, OCH3) ppm. MALDI-TOF mass calcd. for
dipyrrole substituted BODIPY derivatives. Halogenated C20H16BF2N3O: 363.1; found: 363.5 (M)+.
3,5-dibromo BODIPYs were used with neat pyrrole
for the synthesis of mono and di-pyrrolyl BODIPY
derivatives 1–4 at 80 and 140◦C, respectively. We
2.2b 3-pyrrolyl-4,4-difluoro-8-(2-thienyl)-4-bora-3a,
4a-diaza-s-indacene, 2: In a reaction flask, 7b (0.055 g,
observed the stepwise dipyrrolyl substitution which is
facilitated by halogenated BODIPY and higher temper-
ature. Density functional theory calculations along with
0.20 mmol) and pyrrole (0.5 mL, 7.4 mmol) were
stirred at 80◦C for 4 h. Excess pyrrole was evapo-
rated under reduced pressure and crude compound
photo-physical and electrochemical properties of these
obtained was purified using silica gel chromatography,
BODIPY derivatives were studied to calculate the vari-
ous photophysical and electrochemical parameters.
1:1 hexane and dichloromethane, to obtain the pure
1
2 as purple solid. [Yield: 57%]. H NMR (500 MHz,
CDCl3): δ = 10.59 (s, 1H, NH), 7.70 (s, 1H), 7.63 (d,
1H, J = 5.1 Hz), 7.48 (s, 1H), 7.32 (d, 1H, J = 4.5 Hz),
2. Experimental
7.25 (s, 2H), 7.08 (s, 1H), 7.01 (s, 1H), 6.98 (d, 1H,
J = 4.5Hz), 6.51 (s, 1H), 6.43 (s, 1H) ppm. 13C NMR
2.1 Chemicals and instrumentation
(500 MHz, CDCl3): δ = 151.5, 137.5, 136.5, 135.1,
All general chemicals and solvents were procured from
S D Fine Chemicals, India and Sigma-Aldrich. Col-
umn chromatography was performed using silica gel
133.0, 131.3, 129.3, 127.6, 126.6, 125.0, 123.6, 121.2,
118.7, 115.9, 111.7 ppm. MALDI-TOF mass calcd. for
C17H12BF2N3S: 339.17; found: 338.47 (M)+, 318.65
(M-F)+.
of 100–200 mesh size. The H and 13C-NMR (δ in
1
parts per million) spectra were recorded using a Bruker
500 MHz spectrometer. Tetramethylsilane (TMS) was
used as an internal reference for recording H-NMR
1
2.2c 3-bis-(pyrrolyl)-4,4-difluoro-8-(4-methoxyphenyl)-
4-bora-3a,4a-diaza-s-indacene, 3: In a reaction flask,
7b (0.06 g, 0.20 mmol) and pyrrole (1.2 mL, 18 mmol)
were stirred at 140◦C for 22 h. Excess pyrrole was
evaporated at reduced pressure and crude compound
obtained was purified using silica gel chromatogra-
phy, using 1:2 hexane and dichloromethane, to obtain
compound 3 as greenish purple solid. [Yield: 22%]. 1H
NMR (500 MHz, CDCl3): δ = 10.28 (br s, 1H, NH),
7.71 (d, 2H, J = 8.6 Hz), 7.15 (s, 2H), 7.06 (d, 2H, J =
8.6 Hz), 6.91 (s, 2H), 6.81–6.83 (m, 4H), 6.40 (s, 2H),
3.63 (s, 3H, OCH3) ppm. MALDI-TOF mass calcd. for
C24H19BF2N4O: 428.2; found: 428.6 (M)+.
spectra (residual proton; δ = 7.26 ppm) in CDCl3.
The MS spectra were recorded on Bruker MALDI-TOF.
UV–Vis spectra were acquired on Shimadzu 1800. Flu-
orescence measurements were carried out using Horiba
Fluoromax 4. Cyclic voltammetry measurements were
carried out using the electrochemical analyzer (620D,
CH Instruments Co.) at room temperature by utiliz-
ing the three-electrode configuration consisting of Pt
disc (working electrode), platinum wire (auxiliary elec-
trode), and standard calomel (reference electrode) elec-
trodes. The experiments were done in dry chloroform
with 0.1 M tetrabutylammoniumhexafluoro phosphate
as the supporting electrolyte.
2.2d 3-bis-(pyrrolyl)-4,4-difluoro-8-(2-thienyl)-4-bora-
3a,4a-diaza-s-indacene, 4: A mixture of 8b (54 mg,
0.20 mmol) and pyrrole (1.2 mL, 18 mmol) were stirred
at 140◦C for 23 h. Excess pyrrole was evaporated at
2.2 Synthesis and characterization of BODIPY
derivatives (1-4)
2.2a 3-pyrrolyl-4,4-difluoro-8-(4-methoxyphenyl)-4- reduced pressure and crude compound obtained was
bora-3a,4a-diaza-s-indacene, 1: In a reaction flask, purified using silica gel chromatography, using 1:2
7a (0.060 g, 0.20 mmol) and pyrrole (0.5 mL, 7.4 mmol) hexane and dichloromethane, to get the greenish-
1
were stirred at 80◦C for 3 h. Excess pyrrole was purple solid of compound 4. [Yield: 17%]. H NMR
evaporated at reduced pressure and crude compound (500 MHz, CDCl3): δ 10.29 (s br, 2H, NH), 7.55–7.60
obtained was purified using silica gel chromatogra- (m, 2H), 7.43 (d, 2H, J = 2.2 Hz), 7.17 (s, 2H), 7.12
phy, using 1:1 hexane and dichloromethane, to get the (d, 1H, J = 4.2 Hz), 6.93 (s, 2H), 6.86 (d, 2H, J = 4.2
pure compound 1 as purple solid. [Yield: 62%]. M.p.: Hz), 6.41 (s, 2H) ppm. MALDI-TOF mass calcd. for
1
>200◦C. H NMR (500 MHz, CDCl3): δ = 10.57 C21H15BF2N4S: 404.2; found: 404.6 (M)+.

Heteroaryl BODIPY derivatives
increasing the reaction temperature in a step of 10◦C,
3. Results and Discussion
no extra spot other than monosubstituted product was
observed till 130◦C. At 140◦C, a new greenish blue col-
ored compound start forming which later were charac-
terized as disubstituted products, 3 and 4. The progress
of all these reactions were monitored by thin layer
chromatography, which clearly indicated the formation
of new products and further supported by red shift
in absorption bands corresponding to compounds 1–4.
Mono-substituted reaction completed within 2 h while
disubstitued reaction progressed slowly and after about
22 h showed almost complete consumption of bromo-
BODIPY reactant. A very small amount of mono sub-
stituted products were also observed in the reactions
at higher temperature (140◦C). The crude product was
purified by chromatography on silica gel column to
afford dark blue-black solids of BODIPYs 1–4. Mono
and disubstituted products moved quite close to each
other in column chromatography. Absorption spec-
troscopy was used to monitor the fractions containing
mono and di substituted BODIPY. The yields of com-
pounds 1 and 2 were good (∼60%) while it was poor
(20%) for 3 and 4.
3.1 Synthesis and characterization
Compounds 1–4 were synthesized as outlined in
Scheme 1. Meso-substituted dipyrromethanes 5 (meso
anisyl) and 6 (meso thienyl) were synthesized.39 Purifi-
cation by column chromatography afforded 5 and 6 as
white solids. These dipyrromethanes 5 and 6 were used
to synthesize the bromo BODIPYs 7a–b and 8a–b in
a sequence of reactions.40 Bromination of 5 and 6 with
one or two equivalents of N-bromosuccinimide at low
temperature (−78◦C) in THF afforded the mono and
dibromo-dipyrromethanes, respectively, which were
further reacted with DDQ (2,3-Dichloro-5,6-dicyano-
1,4-benzoquinone) at room temperature to get the mono
and dibromo-dipyrromethenes. The obtained crude
products were quickly passed through the silica gel col-
umn to remove the excess DDQ and purified product
was reacted with BF3-etherate to get the mono/dibromo
BODIPY dye (7a–8a and 7b–8b).
Compounds 7a–b, 8a–b were subsequently used for
nucleophilic substitution with pyrrole to synthesize
BODIPY derivatives 1–4 (Scheme 1). Compounds 7a
and 8a were reacted with pyrrole at 80◦C to get mono
pyrrole substituted products 1 and 2.37,38 To synthesize
Compounds 1–4 are fairly soluble (∼10 mg/mL) in
common organic solvents such as toluene, chloroform,
the dipyrrole substituted BODIPY, 7b–8b were reacted dichloromethane, acetonitrile and dimethylformamide.
with pyrrole at 80◦C with excess of pyrrole. Further, on The identities of the compounds were confirmed by
i) NBS, THF, -78
ii) DDQ, THF, RT
C
Ar
Ar
InCl3
iii) TEA, BF3-etherate,
+
Ar-CHO
N
N
N
H
Toluene, 90
C
NHHN
B
X1
X2
F F
Ar = Anisyl, 5
Ar = Thienyl, 6
Ar = Anisyl, X1 = H, X2 = Br, 7a
Ar = Anisyl, X1 = X2 = Br, 7b
Ar = Thienyl, X1 = H, X2 = Br, 8a
Ar = Thienyl, X1 = X2 = Br, 8b
Ar
Ar
80 C
+
N
N
N
N
B
N
H
B
Br
F F
F F
NH
7a or 8a
Ar = Anisyl, 1
Ar = Thienyl, 2
Ar
Ar
140 C
+
N
N
N
N
N
H
B
B
Br
Br
F F
F F
NH
HN
7b or 8b
Ar = Anisyl, 3
Ar = Thienyl, 4
Scheme 1. Synthetic scheme for the BODIPY derivatives 1–4.

Karthika J Kadassery et al.
Figure 1. 1H NMR spectrum of compound 2 in CDCl3.
spectroscopic techniques. The 1H NMR spectra of com-
pounds 1–4 showed the characteristic NMR signals
corresponding to BODIPY core with additional hetero
pyrrolyl proton signals of substituted groups and broad
on the extent of de-localization of π electrons of pyrrole
groups on BODIPY core. The broad band observed in
the high energy region 410–470 nm corresponds to the
S0-S2 (π−π∗) transition of the boradiazaindacene. Sub-
stitution of one pyrrole produced bathochromic shift
of around 85-95 nm where as substitution at both 3-
and 5- position by pyrrole produced large red shift of
∼180 nm in absorption maximum, in comparison to un-
substituted BODIPY.43 Various photophysical parame-
ters for 1–4, determined by absorption and emission
spectroscopies, are given in Table 1.
Emission spectra of compounds 1–4 recorded in
toluene are shown in Figure 2. Compounds 1–4 showed
emission peak maxima between ∼600–700 nm with a
broad tail further in lower energy region (in NIR).
A large bathochromic shift of ∼180 nm was observed
1
NH protons. A representative H NMR spectrum for
compound 2 recorded in CDCl3 is shown in Figure 1.
Analysis of 1H NMR revealed the unsymmetrical nature
of compound 2 as all BODIPY protons appeared as dif-
ferent positions and showed separate signals. Signal for
NH proton was observed at 10.56 ppm. Observation of
molecular ion peak for 1–4 confirms the formation of
these BODIPY derivatives.
3.2 Photophysical studies
Photophysical properties of BODIPY has been stud-
ied extensively in various non-polar and polar sol- in comparison to unsubstituted BODIPY.43 Absorption
vents using absorption and fluorescence techniques.41,42 and emission bands of the new BODIPY 1 and 2 are
To investigate the photophysical properties of 1–4, we well-separated from each other and showed large Stoke
recorded their steady-state absorption and fluorescence shift compared to unsubstituted BODIPY. The BOD-
excitation and emission spectra in several solvents. IPYs 3 and 4 which are symmetrical in nature showed
Also we recorded the fluorescence decay in toluene very small Stokes shift. The small Stokes shift in BOD-
using TCSPC technique and determined the position of IPY 3 and 4 suggests small change in geometry of
the absorption and emission maxima, the fluorescence ground and excited states. Fluorescence quantum yields
quantum yields (φ), the rate constants of radiative (kr) of compounds 1–4 were calculated with respect to Rho-
and non-radiative (knr) deactivation, Stokes shifts, flu- damine B (φ = 0.68 in ethanol)44 (Table 1). Fluores-
orescence lifetimes (τ), excited state dipole moments cence quantum yields were found to be as high as 0.8.
and molar absorption coefficients (ε) of these com- Single exponential fluorescence decays were observed
pounds (1–4). Absorption spectra of 1–4 in toluene for 1–4. Fluorescence life time, radiative decay (kr) and
are shown in Figure 2. Compounds 1–4 showed strong non-radiative decay (knr) constants were calculated and
absorption maxima in the range of 584–682 nm and are summarized in Table 1.
is assigned for S0-S1 (π − π∗). This band is assigned
Solvatochromism was studied for compounds 1–4
to diisoindolomethene core of compounds 1–4. The using absorption and emission spectroscopy in several
absorption maximum of this band is directly dependent polar and non-polar solvents such as hexane, toluene,

Heteroaryl BODIPY derivatives
solvents. The Lippert-Mataga equations45 (1 and 2)
were used to study the solvent polarity behavior on
absorption and emission properties of 1–4. Also they
were used to calculate the ratio of dipole moments in
ground and excited state as described below.
ꢀ
ꢁ
2(μe − μg)2 ε − 1
n2 − 1
(va − vf ) =
(va + vf ) =
−
+ K
(1)
hca3
2ε + 1 2n2 + 1
ꢀ
ꢁ
2(μ2e − μ2g)
ε − 1
n2 − 1
+
+ Kꢁ
hca3
2ε + 1 2n2 + 1
(2)
In the above equations, va and νf are the absorption
and emission frequencies (in cm−1), μe and μg are the
dipole moments of molecule in excited state and ground
state, ε and n are the dielectric constant and refrac-
tive index of the solvent, respectively, h is the Planck’s
constant, c is the speed of light and ‘a’ is the cavity
radius of the molecule. K and Kꢁ are constants. Figure 3
shows the plot of (νa − νf ) versus the polarity param-
eter F1 ≡ {(ε − 1)/(2ε + 1) − (n2 − 1)/(2n2 + 1)},
and plot of (νa + νf ) versus the polarity parameter
F2 ≡ {(ε − 1)/(2ε + 1) + (n2 − 1)/(2n2 + 1)} for 1–4.
A better linear fit was observed if one excludes the
data point corresponding to methanol from the Lippert–
Mataga plot as assessed by the correlation coefficient.
Ratio of dipole moments (μe/μg), calculated using the
slopes were found to be 3.14, 4.79, 4.41 and 4.23 for
1–4. It will be worth to note the linear relationship cor-
relation coefficient r = 0.71 for 1, r = 0.64 for 2,
r = 0.88 for 3, and r = 0.83 for 4 in Figure 3(a). The
use of Lippert-Mataga equations is limited to transitions
where the excited state is emissive state and the excited
state dipole moment is independent of solvent polar-
ity. The Lippert-Mataga expression of the Stokes shift
depends heavily on the solute’s dipole moment upon
excitation (ꢀμge = μe − μg) and the size of the cavity
radius (a). The assumptions made in the derivation of
the Lippert-Mataga equation is that only dipole-dipole
interactions (point dipoles) are taken into account and
the solute polarizability is neglected. The uncertainty
over the size of the cavity radius and strong polariza-
tion in polar solvents for 1–4 explains why the deter-
mined ꢀμge is unreliable for 1–4. Photophysical studies
in different solvents suggest the possible charge trans-
fer character in these compounds, more efficiently in
mono-pyrrole substituted BODIPYs 1 and 2.
(a)
(b)
12
0.001
0
1
2
3
4
(c)
Time (ns)
Figure 2. (a) Absorption, (b) normalized emission spectra
of compounds 1–4, and (c) fluorescence decay profile of 3 in
toluene.
DCM, chloroform, ethyl acetate, methanol and DMF
(See Supplementary Information). It is evident from
these studies that a large Stokes shift was observed for
compounds 1 and 2 as compared 3 and 4. This trend
provides information about considerable change in the
dipole moment in excited state of 1 and 2. Large Stokes
3.3 Cyclic voltammetry
shift is observed when polarity of solvent is increased Compounds 1–4 were investigated in CHCl3 at a
which suggests the more interaction of excited state scan rate of 100 mV/s using tetrabutylammoniumhex-
in polar solvents and high sensitivity towards polar afluorophosphate (TBAHFP) as supporting electrolyte.

Karthika J Kadassery et al.
Table 1. Photophysical properties of compounds 1–4 in toluene.
λabs
nm
λem
nm
ꢁ
τ
kr
knr
Stoke shift
nm
μe
Debye
aμg
Debye
Comp
ns
109s−1
109s−1
μe/μg
Std
1
2
3
4
501
516
606
633
674
695
–
0.84
0.1
0.24
0.14
–
–
–
15
25
37
11
13
1.17
3.14
4.79
4.41
4.23
–
–
412, 544, 581
421, 557, 596
461, 585, 663
472, 603, 682
2.87
0.32
2.68
3.49
0.29
0.31
0.09
0.04
0.05
2.81
0.28
0.24
16.80
18.94
20.01
10.19
5.34
3.95
4.53
2.40
Std-4,4ꢁ-difluoro-8-anisyl-4-bora-3a,4a-diaza-s-indacene, aDetermined using DFT calculations.
Figure 3. (Right) (νa + νf ) versus the polarity parameter F2 = {(ε − 1)/(2ε + 1) + (n2 − 1)/(2n2 + 1)} for compounds
1-4. (Left) (νa − νf ) versus the polarity parameter F1 = {(ε − 1)/(2ε + 1) − (n2 − 1)/(2n2 + 1)} for compounds 1–4.
Compounds 1–4 showed oxidation waves and they
could not be reduced in these experimental condi-
tions. A comparison of oxidation wave of compounds
mono and di pyrrole substituted thienyl BODIPY 2,
4 is shown in Figure 4 and oxidation potentials for
compounds 1–4 are presented in Table 2. Monopyr-
role substituted compounds 1 and 2 showed irreversible
oxidation peaks at ∼0.97 V while disubstituted com-
pounds 3 and 4 showed it at ∼0.62 V. This suggests
that substitution of BODIPY by two pyrroles make
them electron rich and hence make them easy to oxi-
dize. Irreversible oxidation peak suggest the instabil-
ity of oxidized BODIPYs. Low oxidation potentials
make compounds 1–4 suitable for electron donating
agents in electronic/photovoltaic devices and biological
applications.46 48
Figure 4. Cyclic voltammograms of compounds 2 and 4 in
CHCl3 at a scan rate of 100 mV/s using tetrabutylammoni-
umhexafluorophosphate (TBAHFP) as supporting electrolyte.
3.4 Density functional theory (DFT) calculations
The geometry optimization of the structures was car- based on density functional theory predict that the S0-
ried using the Becke’s three-parameter exchange func- S1 transition of compounds 1–4 is responsible for these
tional and Lee-Yang-Parr correlation (B3LYP) method absorption and emission bands involving the HOMO
using GAMESS software.49,50 Theoretical calculations and LUMO. EHOMO, ELUMO and band gap calculated

Heteroaryl BODIPY derivatives
Table 2. Energy levels of 1–4 calculated using DFT and electrochemical methods.
Compound aEHOMO(eV) aELUMO (eV) Band gap (eV) bEox (V vs SCE) cEHOMO (eV)
1
2
3
4
−5.12
−5.21
−4.72
−4.80
−2.62
−2.83
−2.54
−2.73
2.50
2.38
2.18
2.07
0.97
0.99
0.60
0.62
−5.69
−5.71
−5.32
−5.34
aFrom DFT method; From cyclic voltammetry w.r.t. SCE; Eox for Fc/Fc+ w.r.t SCE = 0.41V;
b
cEHOMO calculated from oxidation potentials (vs ferrocene) i.e., EHOMO = - (ox w.r.t. Fc/Fc+
5.13 eV).51 54
+
form DFT calculations are summarized in Table 2. localized on the BODIPY core and further extended to
Values of EHOMO determined by DFT calculation dif- the pyrrole groups at the 3- and 5-positions. We believe
fers substantially from that of calculated using CV data. this high delocalization of the π-electrons is responsi-
DFT calculations were performed in the gas phase con- ble for the large optical red shifts observed for these
ditions and effect of solvent is not considered in DFT, as dyes. Figure 5 also supports the extension of electron
is the case in CV. The solvent polarization effect could density in whole molecules with unequal distribution of
not be included in theoretical calculation. This could be electron density.
one of the reasons of difference in the calculated and
In compounds 1–4, pyrrole substituents at 3- or/and
experimental EHOMO values. Trend obtained in calcu- 5- positions showed strong red shift and quenching of
lated EHOMO for 1–4 is similar to experimental values. fluorescence quantum yield with solvent polarity. Large
The electron density maps of these orbitals are shown in bathochromic shifts of BODIPYs by the substitution of
Figure 5. It is observed that the electronic π-system is pyrrole groups at 3- and 5- positions of the BODIPY.
(i)
(ii)
1
2
3
4
Figure 5. Optimized structures of compounds 1–4. (i) HOMO and (ii) LUMO generated using density functional
calculations.

Karthika J Kadassery et al.
8. Carlson J C T, Meimetis L G, Hilderbrand S A and
Interaction between electron donating group (pyrrole)
and π electron clouds of BODIPY shifts electronic den-
sity from the donor substituent to the BODIPY core
leading to the charge transfer state, which quenches the
fluorescence emission efficiently, mainly in polar sol-
vents. Fluorescence emission spectra of compounds 1–4 11. Kumaresan D, Agarwal N, Gupta I and Ravikanth M
retained cyanine-like delocalization of BODIPY unit
and show broadening of spectra in polar solvents.
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4. Conclusions
14. Vincent M, Beabout E, Bennett R and Hewavitharanage
P 2013 Tetrahedron Lett. 54 2050
To summarize, we have synthesized pyrrole substituted
BODIPY derivatives using a simple one step nucleo-
philic substitution. Photophysical studies showed large
bathochromic shifts in absorption and emission maxima
as compared to the unsubstituted BODIPY. Solvato-
chromism was studied for 1–4 using the Lippert-Mataga
equations which suggest strong polarization of mono-
substituted BODIPYs as compared to di-substituted
BODIPYs. Large bathochromic shift, their solvent de-
pendence and DFT calculations suggest that pyrrole
substituted BODIPY 1 and 2 are showing the properties
of charge transfer states.
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Yang Y and Yang Z 2013 Dyes Pigm. 96 328
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Supplementary information (SI)
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Supplementary data of absorption and emission spectra,
and CV data of BODIPY derivatives 1–4 in different
solvents are available at www.ias.ac.in/chemsci.
Acknowledgements
KKJ, AN and Sanoj thank DST Inspire scholarships.
Partial funding was provided by Department of Science
and Technology, India (SR/FT/CS-87/2010). We also
thank Tata Institute of Fundamental Research, Mum-
bai for providing the NMR, MALDI-TOF and TCSPC
facilities.
29. Yakubovskyi V P, Shandura M P and Kovtun Y P 2009
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