Carbazole Substituted BODIPYs: Synthesis, Computational, Electrochemical and DSSC Studies

Article abstract of DOI:10.1007/s10895-017-2152-9

Carbazole and p-anisyl substituted BODIPY dyes with a cyanoacetic acid anchoring group have been prepared and their spectral, electrochemical properties and photosensitizing potential in DSSC have been evaluated. X-ray structure of N-phenylcarbazole subst

Full text of DOI:10.1007/s10895-017-2152-9

J Fluoresc
DOI 10.1007/s10895-017-2152-9
ORIGINAL ARTICLE
Carbazole Substituted BODIPYs: Synthesis, Computational,
Electrochemical and DSSC Studies
Praseetha E. Kesavan1 · Raghu Nath Behera2 · Shigeki Mori3 · Iti Gupta1
Received: 5 May 2017 / Accepted: 31 July 2017
© Springer Science+Business Media, LLC 2017
Abstract Carbazole and p-anisyl substituted BODIPY
dyes with a cyanoacetic acid anchoring group have been
prepared and their spectral, electrochemical properties and
photosensitizing potential in DSSC have been evaluated.
X-ray structure of N-phenylcarbazole substituted BODIPY
revealed lower torsion angle between BODIPY plane and
carbazole plane, suggesting increased communication
between the two units. DFT studies indicated effective elec-
tronic interactions between the BODIPY unit and carbazole
substituents. The N-butylcarbazole and N-phenylcarbazole
substituted BODIPYs showed anodic shifts in their reduc-
tion potentials, indicating facile reduction process. The
predicted HOMO–LUMO gaps are in agreement with the
electrochemical result and the lower band gap was observed
for the carbazole substituted BODIPYs.
Introduction
The rapid increase in energy demand with the decrease in
conventional fossil fuel reserves led the scientific world to
think of an alternate. Conversion of solar energy by photo-
electrochemical cells was found to be a good alternate for
near future [1]. This can also help the world to reduce the
emission of greenhouse gases from electricity generation by
fossil fuels [2]. However, the cost of photovoltaic electricity
production [3] is still too high to be competitive with nuclear
or fossil energy. Since the discovery of dye-sensitized solar
cell (DSSC) by Grätzel and Brain O’Regan in 1991 [4],
DSSCs have gained significant commercial and academic
interests. The photovoltaic cells generate electricity in a
process where the sunlight is absorbed by the sensitizer
adsorbed on the semiconducting nanocrystalline TiO2 sur-
face [5]. Exposure to sunlight excites sensitizer’s electrons
from ground to an excited state, these are further injected
into the TiO2 conduction band and travel through semi-
conductor network to the transparent electrode to produce
electricity [6]. Thus, the choice of dye is very important in
the DSSC; it should be stable and must bound strongly to
the semiconductor to inject electrons to the TiO2 conduc-
tion band [7–9]. BODIPY dyes are well-known fluorophore
with high absorption coefficient (>80,000 cm−1M−1), strong
emission and higher quantum yield, high photostability and
good solubility [10, 11]. Thus, they have been applied in
fluorescent switches [12], biochemical labelling agents [13],
electroluminescent diodes [14], photodynamic therapy [15].
The first study of BODIPY as a sensitizer for DSSC was
done by Nagano and co-workers in 2005 [16]. Followed by
this work, a number of reports have come up with the high-
est sunlight to energy conversion efficiency (η) up to 6.06%
reported by Kubo and co-workers [17–19]. Recent develop-
ments in BODIPY chemistry allows synthetic modifications
Keywords BODIPYs · Carbazole-BODIPYs ·
X-ray structure · DFT studies · DSSC studies · Cyclic
voltammetry
Electronic supplementary material The online version
of this article (doi:10.1007/s10895-017-2152-9) contains
supplementary material, which is available to authorized users.
* Iti Gupta
iti@iitgn.ac.in
1
Indian Institute of Technology Gandhinagar, Village Palaj,
Simkheda, Gandhinagar, Gujarat 382355, India
2
Department of Chemistry, BITS Pilani-K K Birla Goa
Campus, Zuarinagar 403726, Goa, India
3
Integrated Centre for Sciences, Ehime University,
Matsuyama 790-8577, Japan
Vol.:(0123456789)
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on the core structure, through which absorption and emis-
sion properties of these dyes can be modulated [20–22].
Ooyama et al. have synthesied donor–acceptor BODIPYs
(D-A type) having diphenylaminocarbazole group attached
to the meso-phenyl position of boron dipyrrin core for DSSC
application [23, 24]. The substitution of electron rich bulky
donor groups like N-butylcarbazole at BODIPY core can
result in significantly red-shifted emission with large Stokes
shifts [25, 26]. Also, previous reports suggest that the bulky
electron donor groups increase the stability of oxidised dye
molecules [27]. Although BODIPY based sensitizers have
shown power efficiency lower than zinc-porphyrins and
ruthenium based compounds; the metal free organic photo-
sensitizers are more practical to use in DSSCs. Due to their
desirable spectroscopic properties: strong absorption cross
section in visible region, large quantum efficiency and good
solubility in organic solvents; BODIPYs are potential can-
didates as photosensitizing agents. The BODIPY dyes have
better photostability than that of many proposed sensitiz-
ers. The previous research in this area suggest that, the C-8
position in the BODIPY is optimal position for the charge
injection upon photoexcitation [28]. Thus, substitution at
C-8 position on the BODIPY core is a popular strategy to
synthesize verity of derivatives for photovoltaics applica-
tions. Also, the electron-donating/electron-withdrawing
groups on the BODIPYs core can affect their performance
in DDSC application. The various donor–acceptor type or
pyrrole-substituted BODIPY derivatives have displayed
power efficiency in between 0.13 and 6.06 in DSSCs [28].
Herein, we report the synthesis and photosensitizing
application of D-A type BODIPYs having bulky aromatic
donor groups like: p-anisyl, N-butylcarbazole and N-phenyl-
carbazole at their C-8 position. The cyanoacetic acid on the
C-2 position of the BODIPY core is acting as an anchoring
group (Fig. 7). How the optical, electrochemical and pho-
tosensitizing application of the BODIPYs are altered by the
presence of bulky electron donor groups at their C-8 position
is described in this work.
performed using Horiba-Jobin Yvon Fluolorog-3 Spectrom-
eter at IIT Gandhinagar. Cyclic voltammetric (CV) studies
were carried out with an electrochemical system utilizing
the three electrode configuration consisting of a glassy car-
bon (working electrode), platinum wire (auxiliary electrode)
and saturated calomel (reference electrode) electrodes. The
experiments were done in dry dichloromethane using 0.1 M
tetrabutylammonium perchlorate as supporting electrolyte
at IIT Bombay. All potentials were calibrated vs. saturated
calomel electrode by the addition of ferrocene as an internal
standard, taking E½ (Fc/Fc+)=0.51 V vs. SCE. The LUMO
energy levels were calculated using the equation: ELUonMseOt
(eV) = ‒ (Eredonset ‒ EFc/Fc+ onset) − 4.8 eV, where Ered
onset
and EFc/Fc+
are the onset reduction potentials for the
sensitizer and the ferrocene against the SCE reference elec-
trode, while −4.8 eV is the energy value of ferrocene with
respect to the vacuum level which is defined as zero [29].
The HOMO energy levels were calculated with the LUMO
levels obtained by CV measurements and the optical band
gap (E0−0) obtained by UV–vis measurements: EHOMO =E
LUMO + E0−0
.
Photovoltaic Device Fabrication
The procedure for preparation of TiO2 electrodes and fab-
rication of the sealed cells for photovoltaic measurements
were adapted from reported procedures [30]. A S11 screen-
printed double layer of TiO2 particles was used as the photo-
electrode. A 12-μm thick film of 13-nm-sized TiO2 particles
was first printed on the FTO conducting glass, which was
kept in a clean box for 5 min, and then dried at 125 °C up to
6 min, and further coated by a 5-μm thick second layer of
400-nm light-scattering anatase particles. Finally, the elec-
trodes coated with the TiO2 pastes were gradually sintered in
a muffle furnace at different temperature (275, 325, 375, and
450 °C with 5 min interval and at 500 °C for 15 min). These
films were immersed into aqueous TiCl4 solution (40 mM)
at 70 °C for 30 min, washed with water and ethanol, and
then heated again at 450 °C for 0.5 h. The films were put
into the dye solution (mixture of toluene:ethanol= 1:4) at
room temperature and kept for 12 h. The sensitized elec-
trode was rinsed with ethanol and dried. For the assembly
of DSSCs, the dye-covered TiO2 electrode and Pt-counter
electrode were assembled into a sandwich type cell. The
electrolyte solution consists of 0.1 M LiI, 0.05 M I2, 0.6 M
1-methyl-3-propyl-imidazolium iodide (PMII), and 0.5 M
4-tert-butylpyridine (TBP) in a mixture of acetonitrile and
valeronitrile (volume ratio of 85:15).
Experimental
General Methods and Reagents
Unless otherwise mentioned, all the reagents and solvents
were purchased from Aldrich, Acros Organics or Merck and
used without further purification. Pyrrole was distilled under
vacuum prior to use. Silica gel (60–120 mesh size) used
for column chromatography were procured from Merck.
The solution NMR spectra of compounds were recorded
with Bruker Avance III 500 MHz NMR spectrometer at IIT
Gandhinagar. Absorption spectra were recorded with Shi-
madzu UV-1700 and Fluorescence emission studies were
Computational Calculation Details
The geometry of all the compounds was optimized with
Density Functional Theory (DFT) using the hybrid (B3LYP)
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J Fluoresc
exchange functional [30–32] with 6-31G(d) basis set. Also,
the frequency of all the molecules was calculated at the
same level of theory to check (no imaginary frequencies)
the stationary points as minima on the potential energy sur-
face. The vertical excitation energies were calculated using
the time-dependent density functional theory (TD-DFT)
approach at the same functional and basis sets. Solvent
effects were studied using the Polarized Continuum Model
[33]. The calculations were performed using Gaussian09
suite of program [34].
Compound 4 Column Condition: Silica gel (60–120
mesh) column. Desired product was eluted with 55% DCM/
hexane mixture. Yield: 600 mg (52%). 1H NMR (500 MHz,
CDCl3, δ ppm): 8.14 (d, J=8 Hz, 2H, Ar-H), 8.04 (s, 2H,
3
pyrrole N-H), 7.48 (t, J = 8.5 Hz, 2H, Ar-H), 7.40 (m,
6H, Ar-H), 7.28 (m, 2H, Ar-H), 6.76 (d, J = 1.5 Hz, 2H,
pyrrole α-H), 6.21 (q, J=3 Hz, 2H, pyrrole β-H), 6.00 (s,
2H, pyrrole β-H), 5.60 (s, 2H, C-H). HRMS (ESI-Q-TOF):
C27H20N3+ [M-1]+:calcd m/z 386.1657 found m/z 386.1673.
General Synthetic Procedure for BODIPYs (5–7)
General Synthetic Procedure for Dipyrromethanes
(1–4)
Dipyrromethane (1–4) (0.25 mmol) was dissolved in 30 mL
DCM and oxidized with DDQ (0.30 mmol, 1.2 equiv) for
half an hour. To this mixture TEA (1.5 mL, 40 equiv) and
BF3.OEt2 (1.6 mL, 50 equiv) were added without any time
delay under N2 atm. and allowed to stir at room temperature
for 1 h. After 1 h, the solvents were removed under reduced
pressure and subjected to silica gel column chromatography
in order to obtain the desired pure compound.
Aldehyde (3 mmol) and pyrrole (30 mmol, 10 equiv) were
stirred for 5 min at room temperature under N2 atmosphere.
After 5 min, added 0.1 equiv of TFA and allowed the reac-
tion mixture to stir for half an hour. Completion of the reac-
tion was confirmed by checking TLC. Removed excess pyr-
role by vacuum distillation and purified the compound by
silica gel column chromatography (DCM/hexane).
PS1 Column Condition: Silica gel (60–120 mesh) column.
Desired product was eluted with 90% DCM/hexane mixture.
Yield: 10 mg (13%). 1H NMR (500 MHz, CDCl3, δ ppm):
8.28 (d, J=8 Hz, 2H, Ar-H), 7.98 (s, 2H, pyrrole α-H), 7.69
(d, J = 8 Hz, 2H, Ar-H), 6.89 (d, J = 3.5 Hz, 2H, pyrrole
β-H), 6.57 (d, J=3.5 Hz, 2H, pyrrole β-H). HRMS (ESI-Q-
TOF): C16H11BN2O2F+ [M-F]+:calcd m/z 293.0898 found
m/z 293.0884.
Compound 1 Column Condition: Silica gel (60–120
mesh) column. Desired product was eluted with 80% DCM/
hexane mixture. Yield: 407 mg (51%). 1H NMR (500 MHz,
CDCl3, δ ppm): 8.05 (d, J= 8.5 Hz, 2H, Ar-H), 7.96 (bs,
2H, Pyrrole N-H), 7.32 (d, J = 8 Hz, 2H, Ar-H), 6.72 (d,
J = 1.5 Hz, 2H, α-H), 6.17 (d, J = 3 Hz, 2H, β-H), 5.90
(d, J=2 Hz, 2H, β-H), 5.55 (s, 1H, C-H). HRMS (ESI-Q-
TOF): C16H13N2O2+ [M-1]+:calcd m/z 265.0977, found m/z
265.0985.
Compound 5 Column Condition: Silica gel (60–120
mesh) column. Desired product was eluted with 30%
1
Compound 2 Column Condition: Silica gel (60–120
mesh) column. Desired product was eluted with 50% DCM/
hexane mixture. Yield: 440 mg (58%). 1H NMR (500 MHz,
CDCl3, δ ppm): 7.92 (s, 2H, Pyrrole N-H), 7.13 (d,
J=8.5 Hz 2H, Ar-H), 6.85 (d, J=8.5 Hz 2H, Ar-H), 6.68 (d,
J=1.5 Hz, 2H, α-H), 6.15 (d, J=2.5 Hz, 2H, β-H), 5.91 (s,
2H, β-H), 5.43 (s, 1H, C-H), 3.79 (s, 3H, -O-CH3). HRMS
(ESI-Q-TOF): C16H15N2O+ [M-1]+:calcd m/z 251.1184
found m/z 251.1209.
DCM/hexane mixture. Yield: 50 mg (67%). H NMR
(500 MHz, CDCl3, δ ppm): 7.92 (s, 2H, pyrrole α-H), 7.54
(q, J = 2 Hz, 2H, Ar-H), 7.05 (d, J = 8.5 Hz, 2H, Ar-H),
6.97 (d, J=3.5 Hz, 2H, pyrrole β-H), 6.54 (d, J=3 Hz, 2H,
pyrrole β-H), 3.92 (s, 1H, O-CH3). 13C NMR (125.7 MHz,
CDCl3 δ ppm): 162.12, 147.45, 143.43, 134.88, 132.42,
131.34, 126.36, 118.27, 114.08, 55.53. HRMS (ESI-Q-
TOF): C16H13BN2OF+ [M-F]+:calcd m/z 279.1105 found
m/z 279.1113.
Compound 3 Column Condition: Silica gel (60–120
mesh) column. Desired product was eluted with 50% DCM/
hexane mixture. Yield: 610 mg (55%). 1H NMR (500 MHz,
CDCl3, δ ppm): 7.97 (m, 4H, Ar-H), 7.42 (m, 2H, Ar-H),
Compound 6 Column Condition: Silica gel (60–120
mesh) column. Desired product was eluted with 35% DCM/
hexane mixture. Yield: 52 mg (50%). 1H NMR (500 MHz,
CDCl3, δ ppm): 8.35 (s, 1H, Ar-H), 8.13 (d, J=8 Hz, 1H,
ArH), 7.94 (s, 2H, pyrrole α-H), 7.72 (d, J = 8 Hz, 1H,
3
7.29 (m, 2H, pyrrole N-H), 7.18 (t, J = 7 Hz, 1H, Ar-H),
3
6.68 (s, 2H, pyrrole α-H), 6.17 (s, 2H, pyrrole β-H), 5.98 (s,
2H, pyrrole β-H), 5.66 (s, 1H,C-H), 4.28 (t, 3J=6.5 Hz, 2H,
N-CH2-C-), 1.84 (q, J = 7.5 Hz, 2H, C-CH2-C-), 1.40 (m,
2H, -C-CH2-CH3), 0.94 (t, 3J=7 Hz, 3H, -C-CH3). HRMS
(ESI-Q-TOF): C25H24N3+ [M-1]+:calcd m/z 366.1970 found
m/z 366.1992.
Ar-H), 7.56–7.48 (m, 3H, Ar-H), 7.32–7.29 (t, J = 7 Hz,
1H, Ar-H), 7.04 (s, 2H, pyrrole β-H), 6.58 (s, 2H, pyrrole
β-H), 4.39 (t, 3J=7 Hz, 2H, N-CH2-C-), 1.94 (t, 3J=5 Hz,
2H, -C-CH2-C-), 1.47 (m, 2H, -C-CH2-CH3), 1.00 (t,
3J=7 Hz, 3H, -C-CH3). 13C NMR (125.7 MHz, CDCl3, δ
ppm): 142.85, 142.09, 141.1, 135.2, 131.81, 128.71, 126.72,
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3
124.71, 123.55, 123, 122.53, 120.72, 119.93, 118.16,
109.29, 108.67, 43.22, 31.15, 20.6, 13.87. HRMS (ESI-Q-
TOF): C25H22BN3F+ [M-F]+:calcd m/z 394.1891 found m/z
394.1909.
β-H), 7.34 (t, J = 7 Hz, 1H, Ar-H) 7.25 (d, J = 8 Hz, 1H,
pyrrole β-H), 6.75 (d, J = 3 Hz, 1H, pyrrole β-H), 4.41 (t,
J = 7 Hz, 2H, N-CH2-C-), 1.95 (m, 2H, -C-CH2-C-), 1.48
3
(m,2H, -C-CH2-CH3), 1.02 (t, J = 7 Hz, 3H, -C-CH3).
13C NMR (125.7 MHz, CDCl3, δ ppm): 184.57, 151.73,
147.99, 146.96, 142.53, 141.17, 137.1, 136.83, 135.34, 129,
128.91, 127.06, 124.32, 124.07, 123.28, 122.43, 121.23,
120.81, 120.29, 118.1, 109.49, 109.03, 52.33, 43.30, 31.15,
Compound 7 Column Condition: Silica gel (60–120 mesh)
column. Desired product was eluted with 30% DCM/hexane
mixture. Yield: 60 mg (55%). 1H NMR (500 MHz, CDCl3, δ
ppm): 8.17 (d, 2H, Ar-H), 8.01 (s, 2H, pyrrole α-H) 7.82 (dd,
J=8.5 Hz, 4H, ArH), 7.57 (d, J=8 Hz, 2H, Ar-H), 7.46 (q,
J=7.5 Hz 2H, Ar-H), 7.35 (t, 3J=7.5 Hz, 2H, Ar-H), 7.09
(d, J=3.5 Hz, 2H, pyrrole β-H), 6.62 (d, J=2.5 Hz 2H, pyr-
role β-H). HRMS (ESI-Q-TOF): C27H18BN3F+ [M-F]+:calcd
m/z 414.1578 found m/z 414.1597.
+
29.70, 20.6, 13.87. HRMS (ESI-Q-TOF): C27H23BN3OF2
[M+H]+:calcd m/z 442.1092 found m/z 442.1890.
Compound 10 Column Condition: Silica gel (60–120
mesh) column. Desired product was eluted with 80% DCM/
hexane mixture. Yield: 73 mg (23%). 1H NMR (500 MHz,
CDCl3, δ ppm): 9.93 (s, 1H, CHO), 8.34 (s, 1H, pyrrole
β-H), 8.24 (s, 1H, pyrrole α-H), 8.18 (d, J = 7.5 Hz, 2H,
Ar-H), 7.86 (s, 4H, Ar-H), 7.58 (d, J=8.5 Hz, 2H, Ar-H),
7.48 (d, J=8 Hz, 3H, Ar-H and pyrrole α-H), 7.35 (m, 3H,
pyrrole Ar-H and β-H), 6.80 (d, J = 3.5 pyrrole β-H). 13C
NMR (125.7 MHz, CDCl3, δ ppm):184.78, 149.71, 143.25,
141.38, 140.19, 134.78, 132.24, 131.24, 128.49, 126.83,
126.34, 124.02, 120.90, 120.60, 109.72. HRMS (ESI-Q-
TOF): C28H19BN3OF2+ [M+H]+:calcd m/z 462.1589 found
m/z 462.1582.
General Synthetic Procedure for 2‑formyl BODIPY
(8–10)
DMF (11.62 mL) and POCl3 (12 mL) were mixed and stirred
in an ice bath for 5 min under nitrogen atmosphere. The
mixture was warmed to room temperature and stirred for
another half an hour. BODIPY (0.69 mmol) in 60 mL of
1,2-dichloroethane was added to this mixture and the result-
ing mixture was stirred for 2 h at 50 °C. After the reaction
mixture cooled to room temperature, poured slowly to satu-
rate sodium bicarbonate solution at 0 °C. After the mixture
was warmed to room temperature, it was further stirred
for 30 min and then washed with water. The organic layer
was collected, dried over anhydrous sodium sulphate and
evaporated at a reduced pressure. The crude compound was
purified by silica gel column chromatography using DCM/
hexane as a solvent mixture.
General Synthetic Procedure for Sensitizers (PS2‑4)
Compounds 8–10 (0.1 mmol), cyanoacetic acid (0.13 mmol)
ammonium acetate (0.03 mmol) were refluxed in 2 mL ace-
tic acid under nitrogen atmosphere for 12 h. After cooling,
the reaction was quenched by adding water and extracted
with dichloromethane. The organic layer was dried over
Na2SO4 and concentrated under reduced pressure to give
the crude product. Pure compound was separated by silica
gel column chromatography and eluted with DCM/hexane
to MeOH/DCM mixture.
Compound 8 Column Condition: Silica gel (60–120
mesh) column. Desired product was eluted as orange solid
with 70% DCM/hexane mixture. Yield: 142 mg (63%).1H
NMR (500 MHz, CDCl3, δ ppm): 9.88 (s, 1H, CHO), 8.27
(s, 1H, pyrrole β -H), 8.14 (s, 1H, pyrrole α-H), 7.58 (d,
J = 8.5 Hz, 2H, Ar-H), 7.35 (s, 1H, pyrrole α-H), 7.18 (d,
J = 4.5 Hz, 1H, pyrrole β -H), 7.08 (d, J = 8.5 Hz, 2H,
PS2 Column Condition: Silica gel (60–120 mesh) column.
Desired product was eluted with 15% MeOH/DCM mix-
ture. Yield: 25 mg (63%). 1H NMR (500 MHz, CD3OD, δ
ppm): 8.33 (s, 1H,), 8.15 (s, 1H,), 8.05 (s, 1H,), 7.66–7.64 (t,
J=7 Hz, 3H), 7.25 (d, J=4 Hz,1H), 7.18–7.16 (d, J=8 Hz,
2H), 6.77 (d, J = 4 Hz, 1H), 3.93 (s, 3H). HRMS (ESI-Q-
TOF): C20H14BN3O3F+ [M-F]+:calcd m/z 374.1112 found
m/z 374.1110.
Ar-H), 6.72 (d, 1H, pyrrole β-H), 3.94 (s, 3H, O-CH3). 13
C
NMR (125.7 MHz, CDCl3, δ ppm): 184.85, 162.92, 149.61,
148.37, 142.49, 134.63, 132.70, 128.37, 125.56, 121.08,
114.52, 55.66. HRMS (ESI-Q-TOF): C17H14BN2O2F+
[M+H]+:calcd m/z 327.1116 found m/z 327.1099.
Compound 9 Column Condition: Silica gel (60–120
mesh) column. Desired product was eluted with 80% DCM/
hexane mixture. Yield: 165 mg (54%). 1H NMR (500 MHz,
CDCl3, δ ppm): 9.89 (s, 1H, CHO), 8.36 (s, 1H, Ar-H), 8.29
(s, 1H, pyrrole α-H), 8.18 (d, J=7.5 Hz, 2H, pyrrole α-H
and Ar-H), 7.74 (d, J=8 Hz, 1H, Ar-H), 7.57 (t, 3J=8.5 Hz,
2H, Ar-H), 7.51 (d, J=8 Hz, 1H, Ar-H), 7.41 (s, 1H, pyrrole
PS3 Column Condition: Silica gel (60–120 mesh) column.
Desired product was eluted with 20% MeOH/DCM mixture.
Yield: 42 mg (82%). 1H NMR (500 MHz, CD3OD, δ ppm):
8.48 (s, 1H), 8.37 (s, 1H), 8.216-8.200 (d, J = 8 Hz, 1H),
8.15 (s, 1H), 8.071 (s, 1H), 7.79 (d, J=8.5 Hz, 1H), 7.74 (s,
1H), 7.69 (d, J=8.5 Hz, 1H), 7.58 (d, J=8 Hz, 1H), 7.54
(d, J = 7.5 Hz, 1H), 7.27 (m, 2H), 6.79 (d, J = 4 Hz, 1H),
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4.41 (t, J = 7 Hz, 2H), 1.88 (m, 2H), 1.44 (m, 2H), 0.97
(t, 3J=7.5 Hz, 3H). HRMS (ESI-Q-TOF): C29H23BN4O2F+
[M-F]+:calcd m/z 489.1898 found m/z 489.1862.
Photosensitizer PS1 was obtained in 13% yield, after neutral
alumina column chromatography using 80% DCM/hexane
mixture.
As shown in Schemes 1 and 2, the stepwise synthetic
strategies were employed for the preparation of other sen-
sitizers PS2‑4; in the first step, N-butylcarbazolylaldehyde
and N-phenylcarbazolylaldehyde were synthesized as per the
literature reports [37, 38]. In the second step, anisaldehyde
and other bulky aromatic aldehydes were used to synthesize
dipyrromethanes 2–4 as per the standard procedure [39].
Pure dipyrromethanes 2–4 were obtained in 52–58% yields
after silica gel column chromatographic purification using
50–55% DCM/hexane mixture. In the third step, 2–4 were
oxidized to dipyrromethene using DDQ and finally treated
with BF3.OEt2 to get the BODIPYs 5–7. Silica gel column
chromatography in 35% DCM/hexane solvent mixture pro-
duced BODIPYs 5–7 in 50–67% yields. The BODIPYs 5–7
were exposed to Vilsmeier-Haack formylation [40] to obtain
2-formyl BODIPYs 8–10. The crude formyl BODIPY deriv-
atives 8–10 were purified by silica gel column chromatog-
raphy in 23–63% yields. The cyanoacetic acid functionality
can act as a good anchoring group for DSSC applications.
Thus, compounds 8–10 were further converted to photosen-
sitizers PS2‑4 by treating them with cyanoacetic acid in the
presence of base [41]. Silica gel column chromatographic
purification in 15–20% methanol/DCM mixture afforded
PS2‑4 in 44–82% yields using.
PS4 Column Condition: Silica gel (60–120 mesh) column.
Desired product was eluted with 20% MeOH/DCM mix-
1
ture. Yield: 46 mg (44%). H NMR (500 MHz, CD3OD,
δ ppm):8.44 (s, 1H), 8.26 (s, 1H), 8.17 (d, J =8 Hz, 2H,),
8.00 (m, 3H), 7.90 (d, J = 7 Hz, 2H), 7.79 (s, 1H), 7.61
(d, J = 8 Hz, 2H), 7.45 (m, 3H), 7.31 (t, J = 7.5 Hz, 2H),
+
6.85 (d, J=4 Hz). HRMS (ESI-Q-TOF): C31H19BN4O2F2
[M+H]+:calcd m/z 529.1647 found m/z 529.1609.
Results and Discussion
Synthesis
All the photosensitizers (PS1‑4) were synthesized in the
stepwise manner. PS1 was prepared by following earlier
reports. 5-Carboxyphenyl dipyrromethane was obtained by
the condensation of pyrrole and 4-carboxyphenylbenzal-
dehyde in presence of trifluoroacetic acid [35]. This was
purified by silica gel column chromatography in 51% yield
using 90% DCM (dichloromethane)/hexane mixture. In the
second step, dipyrromethane was oxidized to dipyrrome-
thene using DDQ (2,3-dichloro-5,6-dicyanao-1,4-benzo-
quinone) and finally reacted with BF3.OEt2 to get PS1 [36].
All compounds 1–10 and PS1‑4 were characterized
1
by HRMS, H, 13C NMR spectroscopy (supplementary
Scheme 1 Synthesis of
BODIPY precursors
1 3

J Fluoresc
Scheme 2 Synthesis of target
molecules PS2‑4
information). The corresponding molecular ion peaks of tar-
get photosensitizers PS1‑4 in HRMS spectra, confirmed the
formation of these compounds. The 1H-NMR spectra (ESI)
of compounds 2–10 were recorded in CDCl3 and CD3OD
was used for PS1‑4 NMR analysis, due to the solubility
issue.
olefinic proton on the carbon linking the BODIPY core and
cyanoacetic acid group appeared as a singlet at 8.15 ppm.
The four aromatic protons of anisyl group appeared as two
sets of doublets at 7.16 and 7.64 ppm respectively. The two
α-pyrrole protons showed up as separate signals at 7.66 and
8.33 ppm respectively. The three β-pyrrole protons gave sep-
arate signals at 6.78, 7.25 and 8.05 ppm respectively. The
other sensitizers PS3 and PS4 showed characteristic proton
signal between 7.44 and 8.15 ppm for the alkene proton of
the cyanoacetic acid group present on BODIPY core.
The β-formyl-BODIPYs (8–10) showed characteristic
singlet around 9.87–9.93 ppm corresponding to the alde-
hyde proton. The representative proton NMR spectrum
of molecule PS‑2 is shown in Fig. 1. The characteristic
Fig. 1 1H-NMR spectrum
of sensitizer PS2 recorded in
CD3OD
1 3

J Fluoresc
Table 1 Crystal parameters of BODIPY 7
X‑ray Diffraction Studies
Crystal parameters
The X-ray structure of 7 (CCDC 1,015,966) was determined;
ORTEP views are given in Fig. 2. The crystal structure
refinement parameters are provided in Table 1. The single
crystals of BODIPY 7 were obtained by slow evaporation
of DCM /hexane solution over a period of 2 weeks. It gave
orange colored, orthorhombic crystals having Pbca (#61)
space group. The values of N-B-N and F-B-F angles were
105.30° (10) and 109.04° (11) respectively. The value of the
dihedral angle between the boron dipyrrin unit and phenyl
ring (C4-C5-C10-C11) was 55.46° (16). The observed dihe-
dral angle was comparatively lower than the meso-phenyl
BODIPY (reported value was 60.80°) [42]. The torsion
angle between the carbazole unit and phenyl ring (C16-N3-
C13-C12) was 48.04° (16). These torsion angles clearly
indicate that the carbazole moiety and boron dipyrrins unit
were aligned in the same plane and the phenyl ring was on
a different plane.
Empirical formula
Formula weight
Crystal color, Habit
Crystal dimensions
Crystal system
C27H18BF2N3
433.27
Orange, block
0.200×0.120×0.100 mm
Orthorhombic
a=10.5211(17) Å
b=15.372(2) Å
c=25.047(4) Å
V=4050.9(11) Å3
Primitive, Pbca (#61)
8
0.0473
0.1159
1.133
Lattice parameters
Lattice type, Space group
Z value
R1
wR2
GOF
spectra of compounds PS2‑4 are presented in Fig. 3. All
the compounds PS1‑4 exhibited a strong absorption band at
500–550 nm and a weak band around at 380–430 nm.
The broader and weaker band centered at 350 nm in PS1
and around 380–430 nm in PS2‑4 was assigned to the higher
energy S0−S2 transition. The major absorption band between
500 and 550 nm is attributed to S0−S1 transition. Except
for PS1, the wavelengths of the major absorption band of
PS2‑4 were 22–26 nm red-shifted as compared to the meso-
tolyl BODIPY (496 nm in methanol) [43]. Similarly, the
wavelengths of the major absorption band of PS3‑4 were
also 20–30 nm red-shifted as compared to the correspond-
ing meso-carbazole BODIPY (497 nm). All these sensitizers
exhibited a high molar extinction coefficient in their absorp-
tion maxima around 500 to 530 nm. The higher extinction
Absorption and Emission Studies
The absorption, emission, electrochemical studies and theo-
retical studies were performed to understand the potential of
these dyes in DSSC application. The UV/vis absorption and
fluorescence spectra of compounds PS1‑4 were examined in
methanol and the photophysical properties are summarized
in Table 2. The solvatochromic study in different solvents
was not possible due to poor solubility of target molecules in
other solvents. Photophysical studies show that alterations of
BODIPY core by substituting at beta position significantly
affect the absorption and emission properties of BODIPYs.
The comparison of normalized absorption and emission
Fig. 2 ORTEP diagrams of
BODIPY 7 a top view, b side
view. Thermal ellipsoids are
shown at 50% probability levels
1 3

J Fluoresc
ϕf
Table 2 Absorption and
emission data of PS1‑4 in
methanol
Compounds
λabs (nm)
log ε
λem (nm)
Δvst
(nm)
(cm−1
)
PS1
PS2
PS3
PS4
498
524
518
529
4.73
4.35
4.43
4.25
518
565
567
569
20
41
49
40
775
1385
1668
1329
0.025a
0.015b
0.015b
0.008b
aThe fluorescence quantum yield was calculated by using fluorescein in EtOH (ϕf =0.79) as the standard
bThe fluorescence quantum yield was calculated by using Rhodamine B in EtOH (ϕf =0.7) as the standard
Fig. 3 Comparison of a absorption spectra; b emission spectra of PS2‑4 in methanol, λex =500 nm
coefficient is a desirable property for dye molecules to make
them suitable for DSSC application.
Electrochemical and Photovoltaic Studies
As clear from the Fig. 3, a typical mirror-image relation-
ship was observed for the lowest energy absorption band and
emission spectra for all the sensitizers PS1‑4. On comparing
the emission spectra of sensitizers PS1‑4 we can see that
except PS1, all other sensitizers are emitting in the region
around 565 nm. Except for PS1, emission maxima for PS2‑4
were 52–56 nm red-shifted as compared to that of meso-tolyl
BODIPY (513 nm in methanol).
The cyclic voltammetry measurements of sensitizers PS1‑4
were carried out at the scan rate of 50 mV/s using tetrabuty-
lammonium perchlorate (TBAP) as supporting electrolyte.
The redox potential data, HOMO–LUMO energy levels and
band gap are presented in Table 3. The comparison of reduc-
tion waves is shown in Fig. 4. All the sensitizers showed two
reduction waves except PS3, the first reduction potentials
for all sensitizers were the reversible type. PS1 and PS2
showed first reduction potentials at − 0.53 and − 0.50 eV
respectively. PS3 and PS4 showed first reduction potentials
at −0.46 and −0.41 eV respectively. The sensitizers PS1,
PS2 and PS4 showed irreversible type second reduction
potentials in the range of −1.24 to −1.31 eV.
From Table 2, it is clear that the beta substitution is
affecting the fluorescence quantum yield of BODIPY. PS1
exhibited the highest quantum yield (ϕf) around 0.025 and
it is comparable with the previously reported data. How-
ever, in the case of PS2‑4, there is a clear quenching of
fluorescence quantum yield compared to its corresponding
parent BODIPY. PS4 showed the lowest fluorescence quan-
tum yield (0.008). The lower quantum yields for compounds
PS2‑4 indicates dominant non-radiative decay channels in
these molecules.
The reported meso-tolyl BODIPY showed two reduction
peaks at −0.788 and −1.81 eV. As compared to the reduc-
tion potentials of meso-tolyl BODIPY the reduction poten-
tial of all the sensitizers exhibited anodic shift; i.e. they are
much easier to reduce compared to the meso-tolyl BODIPY
1 3

J Fluoresc
Table 3 Reduction potentials,
HOMO/LUMO energy levels
and the band gap of PS1‑4 from
electrochemical, optical and
DFT data
Compound Ered (V vs.
SCE)
LUMO (eV) Optical HOMO (eV) ΔEHOMO−LUMO
ΔEHOMO−LUMO (in
Methanol) (eV)
E0−0
(eV)
(in gas phase)
(eV)
I
II
PS1
PS2
PS3
PS4
−0.53 −1.24 −3.76
−0.50 −1.27 −3.79
2.44
2.27
2.28
2.26
−6.20
−6.06
−6.11
−6.14
3.01
2.96
2.90
2.44
3.04
3.07
2.63
2.39
−0.46
–
−3.83
−0.41 −1.31 −3.88
Fig. 4 Comparison of reduction waves of PS1-4 in DCM using 0.1 M TBAP as supporting electrolyte, recorded at 50 mV/s scan speed (V vs.
SCE)
[43]. The electrochemical study of sensitizers suggests that
compared to the p-carboxyphenyl and p-anisyl groups (in
PS1 and PS2 respectively), the N-butylcarbazole and N-phe-
nylcarbazole groups (in PS3 and PS4 respectively) caused
much lower reduction potentials and they were very easy
to reduce. From the absorption studies, optical band gap
has been calculated for sensitizers PS1‑4 [29]. Band gap
and HOMO–LUMO energy levels are important for a dye to
consider as a potential sensitizer in DSSC [44].
DFT calculation, it is found that the HOMO–LUMO
gap decreases progressively from PS1 (3.01 eV) to PS4
(2.44 eV) and almost similar to that of the optical band gap
(Table 3 and supplementary information S20). The energy
level diagram of sensitizers PS1‑4 (supplementary infor-
mation S20) illustrates the HOMO/LUMO energy levels of
each sensitizer with respect to the conduction band (CB) of
TiO2. Upon photoexcitation of the sensitizer, the electron
is excited from HOMO to LUMO of the sensitizer and then
further injected from LUMO to the CB of TiO2. Thus, the
sensitizer’s LUMO energy levels should be higher in energy
Generally, the theoretical calculation method is one
of mode to find out the HOMO–LUMO gap. From the
1 3

J Fluoresc
than that of the CB of TiO2. The energy of the conduction
band of the TiO2 semiconductor was estimated to be 4.2 eV
[44]. As clear from the figure (supplementary informa-
tion S20), LUMO energy levels of PS1‑4 were higher than
the energy level of the CB of TiO2. This will help for easy
injection of electron from the LUMO level of the sensitizer
to the CB of the semiconductor. Oxidized state of the sen-
sitizers (HOMO) should have a lower energy level than the
redox couple energy level (I−/I− 3) which was calculated to
be 4.9 eV [44]. It is observed that all our sensitizers have
HOMO energy level lower than that of the redox couple.
These results show the applicability of our molecules PS1‑4
as dyes in DSSC.
conversion yield (PCE) was calculated; these values are
tabulated in Table 4.
Figure 5 shows the photocurrent-voltage curves of the
DSSCs measured under simulated AM1.5 G irradiation
(100 mW cm− 2). The solar-to-electric power conversion
efficiencies (PCE or η%) for these sensitizers lie in the range
of 0.05–0.09%. The PS3 showed relatively higher photo-
current density (Jsc) and open-circuit photovoltage (Voc)
among all the dyes. PS3 and PS4 have N-phenylcarbazole
and N-butylcarbazole moieties at 8-position respectively, but
former showed better photovoltaic performance than latter;
which could be linked to the higher torsion angle in PS3
compared to PS4. The higher torsion angle between carba-
zole and borondipyrrin planes in PS3 may help to reduce
the dye aggregation which in turn increases the dye’s overall
performance [30]. The Voc and PCE values of PS2‑4 are
comparable the previously reported triphenylamine substi-
tuted BODIPYs [31]; which showed Voc values between
0.35 and 0.56 and PCE values between 0.015–0.12%. This
study leaves room for improvement of cell performance by
carefully refining molecular designs with better electron
donating groups and increasing conjugation in BODIPY
dyes.
The photovoltaic performances of dyes PS1‑4 were evalu-
ated by using them in DSSCs with a redox mediator based
on iodine electrolyte. The electrolyte consist of 0.1 M LiI,
0.05 M I2, 0.6 M 1-methyl-3-propyl-imidazolium iodide
(PMII) and 0.5 M 4-tert-butylpyridine (TBP) in a mixture
of acetonitrile and valeronitrile (volume ratio of 85:15)
[45]. The short-circuit photocurrent (Jsc) density, the open-
circuit photovoltage (Voc), the photocurrent-voltage curve
(J-V curve), fill factor (ff) were measured first. Then, with
the help of these parameters solar energy-to-electricity
Computational Studies
Table 4 The photovoltaic parameters of PS1‑4 sensitized TiO2 based
solar cells
To gain better insight into the electronic structures of sen-
sitizers PS1‑4, we optimized all the structures with Den-
sity Functional Theory (DFT) using the hybrid (B3LYP)
exchange functional [32, 46, 47] with 6-31G(d) basis set. The
electronic spectra of all the four compounds were generated
in the gas phase as well as in methanol with time-dependent
density functional theory (TD-DFT) at B3LYP/6-31G(d)
Compound
Jsc (mA·cm2)
Voc (V)
ff (%)
η(%)
PS1
PS2
PS3
PS4
0.28
0.26
0.41
0.26
0.37
0.33
0.36
0.34
50.7
54.3
58.5
55.4
0.05
0.05
0.09
0.05
Fig. 5 Current–Voltage graph
of PS1‑4 sensitized solar cells
1 3

J Fluoresc
Table 5 Electronic transitions
of PS1‑4; geometry optimized
at B3LYP/6-31G(d) level of
theory
Compound Experiment (in
methanol)
Calculated (in methanol)
Calculated (in gas phase)
λabs (nm) Energy (eV)
λabs (nm) log ε λabs (nm) Energy (eV)
f
f
PS1
PS2
PS3
PS4
498
524
518
529
4.73 424.54
4.35 457.74
4.43 441.78
4.25 449.69
2.920
2.709
2.806
2.757
0.4005 412.40
0.4425 446.50
0.6120 438.33
0.6374 449.74
3.006
2.776
2.829
2.757
0.2792
0.4911
0.2892
0.4454
Absorption maxima: (λabs), oscillator strength: (f)
level. The solvent effect of methanol was taken into account
by the polarizable continuum model (PCM).
extending the π-conjugation of the system. While compar-
ing the optimized geometry parameters of PS4 with the crys-
tal parameters of its precursor BODIPY 7, the N-B-N bond
angle decreased from 105.30 to 104.47° and the bond angle
for F-B-F got increased from 109.04 to 112.57°. Also, the
dihedral angle between the phenyl ring and the BODIPY unit
(C4-C5-C10-C11) got deviated from 55.46 to 53.26° and the
dihedral angle between the carbazole unit and the phenyl
ring (C16-N3-C13-C12) got deviated from 48.04 to 23.98°.
The gas phase optimized molecular structures are shown
in Fig. 7 and the optimized coordinates are provided in sup-
plementary information (Table 1 and S20). In the optimized
structures, the phenyl ring of PS1, PS2 and the carbazole
ring of PS3 slightly deviated from the plane of BODIPY.
Whereas, in PS4 the carbazole ring and the BODIPY core
are almost perpendicular and the central phenyl ring is
Fig. 6 Molecular orbital pat-
terns and energy level diagram
of PS3 (left) and PS4 (right) in
the gas phase
1 3

J Fluoresc
Fig. 7 BODIPY based photosensitizers and their optimized structures reported in this work
1 3

J Fluoresc
computational facility of the chemistry department, BITS Pilani-K K
Birla Goa Campus.
The electronic spectra simulated using TD-DFT-
B3LYP/6-31G(d) level of theory showed more than one
absorption peaks and the peaks with maximum absorption
wavelength (λabs) were selected for comparison. The absorp-
tion spectra offer excitation energies and frontier molecular
orbitals (FMOs) involved in the absorption process. Calcu-
lated electronic transition energies along with the oscillator
strengths are presented in Table 5.
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(EMR/2015/000779) Govt. of India is greatly acknowledged. PEK
thanks IIT Gandhinagar for fellowship and infrastructural support.
IG is appreciative to Prof. H. Furuta, Kyushu University for X-ray
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