Funnel shaped molecules containing benzo/pyrido[1,2,5]thiadiazole functionalities as peripheral acceptors for organic photovoltaic applications

Article abstract of DOI:10.1039/c6ra06616j

A series of four novel funnel shaped triphenylamine core derivatized materials with various combinations of 3,6-di-tert-butyl-9H-carbazole as donors and benzo/pyrido[1,2,5]thiadiazole as peripheral acceptors were designed, synthesized and characterized. O

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ARTICLE
Funnel shaped molecules containing benzo/pyrido[1,2,5]
thiadiazole functionalities as peripheral acceptors for organic
photovoltaic applications
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
c,d
e
*,b,f
DOI: 10.1039/x0xx00000x
Mahalingavelar Paramasivam , Akhil Gupta
,
N. Jagadeesh Babu , K. Bhanuprakash
,
*,c
*,a,f
Sheshanath V. Bhosale and V. Jayathirtha Rao
www.rsc.org/
A series of four novel funnel shaped triphenylamine core derivatized materials with various combinations of 3,6-di-tert-
butyl-9H-carbazole as donors and benzo/pyrido[1,2,5] thiadiazole as peripheral acceptors were designed, synthesized and
characterized. Optical and electrochemical studies revealed that increasing acceptor strength in the structure leads to
bathochromic shift in their absorption profile and stabilizes the lowest unoccupied molecular orbital (LUMO) energy level.
Enhancement in the intensity of an intramolecular charge transfer transition with synergistic downshift of the highest
occupied molecular orbital (HOMO) and LUMO levels was observed by increasing the number of acceptor units.
Thermogravimetric analyses (TGA) indicated that the materials containing pyrido[1,2,5]thiadiazole (PTD) acceptor
functionality have excellent thermal stability compared to the materials comprising benzo[1,2,5] thiadiazole (BTD)
g
functionality. The measurement of glass transition temperature (T ) corroborated TGA results and the values were found
in the range of 148-198 °C. X-ray analyses indicated that more twist observed in a structure leads to absorption at high
energy region and varying the number of electron donor and acceptor groups have impact on the modulation of frontier
energy levels. A power conversion efficiency of 2.21% for the molecule with PTD as an acceptor and bis-carbazole as donor
-2
was achieved for the preliminary photovoltaic devices under simulated AM 1.5 illumination (100 mW cm ).
explored in developing BHJ architecture. Polymers can exert
extended photo-response and good film forming capabilities
with the conventional acceptors such as solvent-dissolvable
Introduction
Current energy and environmental crisis due to the
predicted exhaustion of fossil fuels has drawn a world-wide
attention towards clean, inexhaustible and long term
securable renewable energy sources. Among several
renewable energy technologies, solution-processable bulk-
heterojunction (BHJ) devices and dye-sensitized solar cells
have attracted considerable attention and are the most
fullerene derivatives, [6,6]-phenyl-C -butyric acid methyl ester
6
1
(PC BM) and its C analogue (PC BM). On the other hand
61 71 71
polymeric donor materials pose number of challenges such as
difficulty in purification, end group contamination and broad
molecular weight distribution. Regardless, power conversion
efficiency (PCE) values over 10% have been reported with such
5
1
,
2
polymeric donors. However, in the interim, solution-
studied strategies.
BHJ devices in particular are esteemed
processed small molecular donor-based BHJ devices have also
aroused great interest, mainly due to their advantages of well-
defined chemical structure, convenient purification methods
due to their advantages of being lightweight, low cost and
3, 4
flexibility in large-area applications.
From the materials
perspective, two types of π-conjugated organic materials,
namely polymeric and small molecular entities, have been
6
-9
and monodisperse molecular weight. Such advantages allow
and encourage researchers to exert efforts to design and
develop small molecular donors. Recently, Heliatek, a German
company, announced an unprecedented cell efficiency of 12%
a
Crop Protection Chemicals Division, CSIR- Indian Institute of Chemical Technology,
Hyderabad 500007 India. E-mail: jrao@iict.res.in
b
Inorganic and Physical Chemistry division, CSIR- Indian Institute of Chemical
10
using tandem solar cells featuring small molecules. This
implies that small molecules may surmount their
corresponding polymeric counterparts in the near future.
To improve the photovoltaic performance of BHJ devices,
new synthetic strategies with rational molecular design and
Technology, Hyderabad 500007 India; E-mail: bhanu2505@yahoo.co.in
c
School of Applied Sciences, RMIT University, GPO Box 2476, Melbourne VIC 3001
Australia; Email: sheshanath.bhosale@rmit.edu.au
d
Institute for Frontier Materials, Deakin University, Geelong Victoria 3216 Australia
e
Centre for X-ray Crystallography, CSIR-Indian Institute of Chemical Technology,
6, 11, 12
device optimization have been demonstrated.
Among
Hyderabad, 500607, India.
f
CSIR-Network of Institutes for Solar Energy, New Delhi, India
the several approaches emerged in molecular design, reducing
the optical band gap for robust spectral coverage and tuning
HOMO and LUMO levels of chromophores to achieve better
Electronic Supplementary Information (ESI) available: Details of NMR and Mass
spectral data, absorption and emission profiles of the dyes, X-Ray crystallographic
data of intermediate 6b. See DOI: 10.1039/x0xx00000x
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40
prevent π-π aggregation of the resultant chromophores. We
have demonstrated the effect of pyrido[1,2,5]thiadiazole (PTD)
or benzo[1,2,5]thiadiazole (BTD) units and their influence on
optoelectronic properties. This study allows more comparative
measures in evaluating structure-property relationship.
Results and discussion
Design and synthesis
All of the materials were synthesized according to a multistep
synthetic pathway depicted in scheme 1. Synthesis of 5-
bromobenzo[c][1,2,5]thiadiazole (1a) and 6-bromo[1,2,5]thia-
diazole [3,4-b]pyridine (1b) was carried out by the ring closure
Fig. 1. Molecular structures of the newly designed and investigated
2
reaction of SOCl with 4-bromobenzene-1,2-diamine and 5-
materials
.
bromopyridine-2,3-diamine, respectively, dissolved in pyridine
3
4
at 0 °C. These bromo derivatives, 1a and 1b, were treated
3, with n-butyl lithium at -78°C to eliminate halogen atoms
followed by the addition of 2-isopropoxy-4,4,5,5-tetramethyl-
1
energetic alignment with electron acceptors are paramount.
1
4
Over the last decade, large number of small molecular
1,3,2- dioxa-borolane in order to afford the corresponding
electron donors have been extensively investigated with
various molecular architectures such as donor-acceptor-donor
1
5-17
18-23
A-D-A,
24-27
28-32
(D-A-D),
D-A-A,
star shaped
and X-
3
3
Reagents and conditions: i) SOCl
2
, Pyridine, 0°C; ii) 2-isopropoxy-
shaped. We found that the star shaped molecules offer
potential advantages like better miscibility with electron
acceptors and well defined molecular packing with lesser
extent of π-π aggregation due to their non-planar structure.
However, these materials have received less attention due to
their moderate photovoltaic performance in comparison with
linear/branched molecules. PCE of 4.71% has been reported,
which is the highest efficiency so far for the solution-
4
,4,5,5-tetramethyl-1,3,2-dioxaborolane, n-BuLi, THF, -78°C; iii) t-BuCl,
3
anhyd. AlCl3, CH NO2, 24h, rt; iv) N-Bromosuccinimide, DMF, 24h, rt; v)
2 3 4,
CuI, Cs CO3, 1,10-Phenonthroline, DMF, reflux, 24h; vi) Pd(PPh )
3
0
processable BHJ devices using star shaped molecules. This
implies that further scrutinization based on this module is
needed. It is known that benzo/pyrido[1,2,5]thiadiazole groups
have been used as central acceptor units in the materials for
various organic electronic applications such as organic field
3
4, 35
36
effect transistors (OFET),
2
and BHJ solar cells.
dye-sensitized solar cells (DSSC)
These acceptor units are known to
1, 37, 38
increase electron affinity along with strong intermolecular π–π
interactions which consequences in enhancing photovoltaic
3
9
performance.
Particularly
for
BHJSC
applications,
incorporation of benzo/pyrido[1,2,5]thiadiazole groups as
peripheral units into a star shaped π-conjugated system is less
explored to date. It is expected that incorporation of such
acceptor functionalities can help target chromophore to
acquire merits, such as good miscibility with fullerene
acceptors and better interaction at D-A interface to obtain
optimal charge transport in the desired direction.
Keeping these points in mind, we envisaged to design
molecules with desymmetrization approach on the star shaped
architecture by introducing various acceptors and π-building
blocks. Herein, we report the design and development of a
novel, funnel-shaped molecular framework that contains
triphenylamine as the central core unit, which has been
derivatized with 3,6-di tert-butyl-9H-carbazole (donor part)
and
benzo/pyrido[1,2,5]thiadiazole
(acceptor
part) PhCH / 2M K CO 80°C.
3 2 3,
functionalities (Fig. 1). The selection of tert-butyl group on Scheme 1. Synthetic strategy for the preparation of target
carbazole was made to ensure sufficient solubility and to chromophores
2
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boronate intermediates 2a and 2b. 3,6-di-tert-butyl-9H-
carbazole (3) was prepared according to the reported
4
1
literature. Carbazole substituted dibromo derivative of
triphenylamine (5) was synthesized using the classic Ullmann
4
2
coupling reaction conditions. Syntheses of bromo derivatives
a and 6b were carried out by the palladium catalyzed Suzuki
coupling of boronate derivatives 2a and 2b with tris(4-
6
2
8
bromophenyl)amine. Finally, the target dyes DCTB and DCTP
were synthesized in excellent yields by stoichiometrical
addition of 3 with 6a and 6b, respectively, using CuI/1, 10-
2 3
phenonthroline in the presence of K CO . The dyes CTDB and
CTDP were obtained through the well-known palladium
catalyzed Suzuki coupling of 5 with two equivalents of
4
boronate derivatives 2a and 2b respectively. All these dyes
3
1
were characterized by spectroscopic means using H NMR,
13
C
NMR, IR, EI-MS and MALDI-TOF. The details of the
experimental and characterizaꢀon data is available in the ESI†.
Molecular geometry and crystal structure
To further understand the structure–property relationship
at molecular level, all the molecules were optimized using the
B3LYP functional and 6-311G (d, p) basis set implemented in
the Gaussian 09 program. From the optimized structures (Fig.
2
b
) it was observed that the average value of CNC angle was
Fig. 2 a) ORTEP diagram of CTDP with the atom-numbering scheme
~120°in all the dyes, whereas mean value of dihedral angle
and dihedral angle values. Displacement ellipsoids are drawn at the
between the planes of the central NCCC atoms was found to
be more deviated with the torsion angle value of ~45°. The
carbazole unit attached to the TPA core was twisted in an out-
of-plane fashion with an angle of ~53° which is in accordance
with the X-ray structure of CTDP (Fig. 2a). Comparative
dihedral angles were obtained for the acceptor units attached
to the TPA core and the values are measured to be ~38° for the
DFT optimized geometry and ~34° for the crystal structure
3
0% probability level for non-hydrogen atoms, and H atoms are shown
as small spheres of arbitrary radius;
B3LYP/6-311g(d, p)level of theory.
b) DFT optimized geometries from
to LUMO is a strong indication of better intramolecular charge
transfer transition. The larger transition dipole moment of the
dyes indicates that these molecules are more polarized in
excited state which is in accordance with our emission data
(CTDP). The optimized geometry of four molecules revealed
that significant deviation from co-planarity between donor and
acceptor fragments leads to break in conjugation which
(positive solvatochromism).
4
4
consequences the absorption at low wavelength region. The
bond distances between C-N and C–C bonds in acceptor
functionalities were computed to be 1.306 and 1.365 Å
respectively, as predicted by the B3LYP level of theory. The
variation with respect to acceptor groups is brought out clearly
with shortening of this bond when an electron withdrawing
group is introduced. The C-N bond is almost 0.059 Å shorter
than C–C bond and the values are given in Fig. S1 (ESI†). The
single X-Ray crystal structure of the intermediate 6b is given
Fig. S2 (ESI†). The longer wavelength absorpꢀon is originated
mainly from HOMO-1 and HOMO levels to LUMO level in case
of all the dyes. The molecular orbital amplitude plots of FMOs
such as HOMO-1, HOMO and LUMO, are shown in Fig. 3. The
electron density of HOMO-1 in DCTB and DCTP is
predominantly populated on carbazole unit. On the other hand
in dyes CTDB and CTDP, it is mainly delocalized over the
carbazole and TPA units. In HOMO of all the dyes, electron
density is delocalized over the carbazole and TPA groups,
whereas LUMO is predominantly localized on acceptor units.
The presence of well separated electron density from HOMO
Fig. 3. Computed isodensity (0.02) surfaces of the HOMO-1, HOMO
and LUMO of the target molecules.
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Journal Name
Photophysical Properties
Table 1 Photophysical properties of the dyes
The absorption and emission spectra of these four dyes
recorded in toluene are shown in Fig. 4 and the salient
features of the corresponding spectra are summarized in Table
maxa(nm)
ꢀ1
ꢀ1
filma(nm)
b
Dye
λ
ε(M cm )
λ
E
g, opt (eV)
4
09
16,639
69,138
16,456
79,147
32,459
65,299
30,022
78,770
DCTB
DCTP
CTDB
CTDP
434
450
434
442
2.43
323
4
3
35
21
1
and Table S1 (ESI†) The absorption spectra of these dyes
showed two distinct bands. The first absorption peak at 312–
23 nm is aꢁributed to π → π* transiꢀon of the conjugated
2.30
410
2.49
3
4
3
12
33
16
3
2.27
backbone. The second absorption peak of the dyes DCTB and
CTDB containing BTD unit displayed maximum wavelength
a
-5
Absorption spectra of the dyes (1x10 M) were recorded in toluene at
b
(
λ
max) at 410 nm and the dyes DCTP and CTDP containing PTD
ambient temperature. Estimated from the onset absorption spectra
of the dyes in thin film state.
acceptor unit showed λmax at 435 nm which can be assigned to
4
ICT transition from donor (carbazole) to acceptor part.
5-47
The
bathochromic shift of 25 nm in the λmax of DCTP and CTDP dyes
can be attributed to the presence of electron withdrawing
pyridine in the PTD unit. Moreover, increased number of
acceptor groups in the studied structures has pronounced
effect in the ICT peak intensity i.e., molar extinction
coefficients were increased to two-fold (Fig. 4). It has been
noticed that absorption spectra remain unchanged irrespective
of any variations in donor/acceptor units of the target dyes. As
expected, going from solution to film absorption, the
absorption spectra of all the dyes were red shifted (Fig.S3,
ESI†) which indicate the presence of strong intermolecular
[
ET(30)] such as hexane, toluene, chloroform, THF, acetonitrile,
DMF and methanol (Fig. S3 and S5, ESI†) and the
corresponding data is provided in Tables S2-S4, ESI†. The
absorption spectra of all the dyes are impassive irrespective of
solvent polarity. In emission spectra, significant positive
solvatochromism is observed for these dyes, i.e., emission
maximum becomes broad and red shifted as the polarity of
solvent increases from non-polar to polar aprotic solvent,
implying strong ICT transition between donor and acceptor
counterparts. Non-polar locally excited (LE) state, as seen in
absorption, is transformed into a polar excited state, as
evident from emission, in the presence of apolar aprotic
solvent. On the other hand, the photoluminescence was
completely quenched with polar protic solvents like DMF and
methanol (Tables S3–S4, ESI†). This may be attributed to the
interaction of polar protic solvent dipole with extensive
polarization induced by thiadiazole acceptor unit which lead to
g
packing in the solid state. The optical band gap (E ) values are
calculated using the onsets of thin film absorption spectra.
To examine the electronic and spectroscopic properties,
optimized geometries were used for the computation of
vertical excitation energies using TDDFT with various
functionals such as B3LYP, PBE, M06-2X and 6-311g (d, p) basis
4
8
set. Among them, M06-2X in the framework of PCM using
toluene as solvent showed reasonable agreement with small
underestimation and the trend is consistent with experimental
observation, whereas B3LYP and PBE are highly over-
5
0
non-radiative relaxation.
Fluorescence lifetime is that characteristic time in which a
molecule can undergo conformational changes as well as
interaction with other molecules or diffuse through the local
environment. The performance of organic solar cells is greatly
influenced by the dynamics of excitons which is pertinent to
the fluorescence lifetime of the dyes. The excited-state
lifetimes of all the four dyes were measured in toluene (Fig.
4
9
estimated. The computed energies of vertical excitations
along with oscillator strength and compositions in terms of
molecular orbitals are compiled in Table 2. The comparison of
normalized plots of simulated spectra with oscillation strength
and experimental absorption of dyes are depicted in Fig. S4
ESI†. The oscillaꢀon strength of these dyes gradually increased
by increasing the number of acceptor groups in the molecular
backbone which corroborates our experimental absorption.
To study the effect of solvatochromism, absorption and
emission spectra of all the dyes were recorded in a variety of
solvents with different empirical solvent polarity parameter
5a) and the values were found to be 6.61, 7.29, 5.11 and 7.03
ns for DCTB, DCTP, CTDB and CTDP respectively. The gradual
Table 2 Theoretical data of the dyes
M06ꢀ2X (toluene)
a
b
c
λ
max
ꢁ
g
ꢁ
e
Dye
Composition
(
nm)
λ
max (nm)
f
(Debye) (Debye)
4
3
09
23
Hꢀ>L (67%)
Hꢀ1ꢀ>L (17%)
DCTB
DCTP
CTDB
CTDP
370(3.35 eV)
392(3.17 eV)
375(3.31 eV)
394(3.15eV)
0.59
0.52
0.66
0.69
1.61
3.25
2.52
1.31
1.78
3.68
2.85
1.49
4
3
35
21
Hꢀ>L (76%)
Hꢀ1ꢀ>L (17%)
4
3
10
12
Hꢀ>L (67%)
Hꢀ1ꢀ>L (14%)
4
3
33
16
Hꢀ>L (67%)
Hꢀ1ꢀ>L (19%)
a
b
Absorption values measured in toluene; Ground state dipole
-
5
c
Fig. 4 a) Absorption and
b
) emission spectra of dyes (1 × 10 M) moment of the dyes obtained from B3LYP functional; Transition
dipole moment of the dyes
recorded in toluene at ambient temperature.
4
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Table 3 Electrochemical data of the dyes
aEoxd (V)
1/2
Dye
a
Ered1/2 (V) HOMO
b
(eV) LUMO
b
(eV) Eg, ele (eV)
c
1st
2nd
3rd
DCTB 0.87
DCTP 0.90
CTDB 1.19
CTDP 1.23
1.15
1.14
1.51
1.54
1.28
ꢀ1.47
ꢀ1.18
ꢀ1.20
ꢀ0.91
ꢀ5.27
ꢀ5.30
ꢀ5.59
ꢀ5.64
ꢀ2.93
ꢀ3.20
ꢀ3.19
ꢀ3.49
2.34
2.10
2.40
2.15
1.51
ꢀ
ꢀ
a
2 2
Measured in CH Cl with 0.1 M tetrabutylammonium perchlorate
-
Fig. 5. a) Time-resolved fluorescence decay curves of the dyes in (TBAPC) as supporting electrolyte with a scan rate between 50 mV s
1
b
; Deduced from the formula HOMO = -(4.4 + Eox) and LUMO = -(4.4 +
toluene (λex = 405 nm, λem = 530 nm);
b
) Schematic diagram represents
c
red), Bandgap obtained from electrochemical data
the modulation of energy levels estimated from electrochemical data.
E
increment in emissive lifetime was observed by replacing estimated from the peak potentials of anodic and cathodic
benzene with pyridine in the acceptor unit. The electron- waves based on -4.4 eV as SCE energy level relative to vacuum.
transfer rates (k ) of these molecules were found in the range The first oxidation and reduction peak potentials were used to
ET
8 −1
of 1.4–2.0 × 10 s (Table S1 ESI†)
calculate the HOMO and LUMO energy levels. The CV data of
DCTB-DCTP and CTDB-CTDP transformations was compared to
examine that how the variation in number of units and
acceptor strength involves in modulation of frontier energy
levels. For instance, in DCTB-DCTP and CTDB-CTDP
transformations, the replacement of BTD by PTD unit induced
the anodic peak potential (Epa) with a positive shift of 30–40
mV and 290 mV in cathodic peak potential (Epc).This indicated
that introduction of PTD unit has minute influence on HOMO
level, albeit affects the LUMO level greatly. For DCTB-CTDB
transformation, a shift is observed in anodic peak (Epa) from
Electrochemical Properties
The redox behaviour of target dyes is essential to
investigate the alignment of these frontier energy levels with
respect to the energy levels of electron acceptors, such as
PC BM, for efficient charge transport. Cyclic Voltammetry was
6
1
performed to evaluate oxidation and reduction potentials
using three- electrode configuration. Fig. 6 shows the resulting
cyclic voltammogram (CV) traces. The CVs of dyes DCTB and
DCTP showed three quasi-reversible oxidations which are
originated from two carbazole donor and triphenylamine units.
Whereas dyes CTDB and CTDP showed two quasi-reversible
oxidations and the oxidation peaks are attributed to carbazole
donor and triphenylamine unit. All the dyes showed single
reduction potential. Electrochemical band gaps were
0
.87 to 1.19 V and same pattern is observed in DCTP-CTDP
transformation i.e., anodic peak (Epa) was shifted from 0.9 to
.23 V which indicates the positive shift of 320–330 mV upon
1
increasing the number of acceptor units. The potential of
cathodic peak (Epc) shifted toward more positive values from -
1
.47 to -1.20 V for DCTB-CTDB transformation and -1.18 to -
.91 V for DCTP-CTDP transformation corresponding to the
0
value of ~270 mV. Similar trend was observed in
triphenylamine-thienylvinylene hybrid compounds reported by
2
9, 51-54
Roncali et.al.
The HOMO and LUMO energy levels of
these materials were calculated from the onsets of their
corresponding oxidation and reduction potentials. The results
of CV analyses revealed that these dyes display oxidation and
reduction at more positive potentials with increasing the
number of acceptor units and induce
a simultaneous
downshift of both HOMO and LUMO levels. Observed HOMO
and LUMO energy levels are well matched with the commonly
used acceptor material PCBM. Since the LUMO levels of the
newly synthesized materials are deeper compared to the
threshold value of 0.3 eV required for effective charge
separation, efficient electron transfer is expected from these
materials to PCBM and is beneficial to improve short-circuit
2
1, 55
current (Jsc
)
and open circuit voltage (Voc).
. The
electrochemical band gaps observed for DCTB and DCTP were
comparatively low when compared with CTDB and CTDP,
which indicates that band gap (ΔE) increases with number of
acceptor units and decreases with acceptor strength (Fig. 5b).
It is worth noting that the electrochemical and optical band
gap values are relatively in good agreement. Theoretically
predicted LUMO energy levels are about 0.3 eV low, whereas
Fig.
6
.
Cyclic voltammograms of the dyes recorded in
dichloromethane.
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8
7
6
5
4
3
2
1
CTDP
DCTB
DCTP
0
0
0.2
0.4
0.6
0.8
1
Voltage (V)
Fig. 7. Thermograms (
a
) and DSC traces (
b) of the dyes measured at a
Fig. 8. Characteristic current density versus voltage (J–V) curves for the
heating rate of 10 °C/min under N
2
atmosphere.
best BHJ devices based on CTDP (black), DCTB (red) and DCTP (blue) in
-
2
blends with PC61BM under simulated sunlight (100 mWcm , AM 1.5G).
The device structure is: ITO/PEDOT:PSS (38 nm)/active layer/Ca (20
nm)/Al (100 nm).
the HOMO energy levels match well with the electrochemical
data and the values are summarized in Table 3 and Table S5,
ESI†.
ITO/PEDOT:PSS (38 nm)/active layer/Ca (20 nm)/Al (100 nm),
where the active layer was a solution-processed blend of
either of the donor materials and the solubilised fullerene
PC BM. The active layers were prepared by spin coating the
Thermal Properties
The thermal properties and phase transition nature of all
the dyes were evaluated by thermogravimetric analysis and
differential scanning calorimetry, respectively, under nitrogen
6
1
mixtures of appropriate material with PC BM in
6
1
-1
atmosphere at a heating rate of 10 °C min . All the dyes
exhibited excellent thermal stability. The temperature
corresponding to 5% weight loss for the dyes is in the range of
chlorobenzene under ambient conditions @ 1:1 ratios without
subsequent annealing. The optimum layer thickness was found
to be in the range of approximately 60 nm. We chose a high
boiling solvent for casting an active layer on top of the
PEDOT:PSS surface as our attempts to fabricate devices using
5
6, 57
389–515 °C.
The dyes having PTD unit undergo less
thermal degradation and possess higher decomposition
temperatures (Fig. 7) and the data is provided in Table S6,
ESI†. The high thermal stability of DCTP and CTDP may be
attributed to the well-ordered π-π interaction between the
neighbouring units. In other words, the possibility of
interaction sites in PTD are pyridine and thiadiazole units
which lead to efficient intermolecular interactions. On the
other hand, these intermolecular interactions are expected to
be weak in the case of the dyes containing BTD units. The
observation identified in TGA curves of DCTB and CTDB (Fig.
low boiling solvents, such as CHCl , resulted in very poor
3
photovoltaic performance. This was primarily due to low film
quality. Moreover, the use of high-boiling solvent is preferable
from a processing point of view. Though we fabricated our
devices using all of the materials reported in this study, we
were not able to fabricate CTDB using chlorobenzene,
primarily due to inadequate solubility. CTDB showed enough
solubility in CHCl , however the film interpretation was
3
inconsistent to fabricate any devices. All other materials, CTDP,
DCTB and DCTP, showed miscibility with PC61BM in their blend
solutions, hence used to construct BHJ devices.
7
a
)
may due to solvent/moisture associated with the
chemical, is being ejected. Melting point (T ) values were
found to be in the range of 313–371 °C. The glass-transition
temperature (T ) values were observed in the range of 148–
98 °C which is determined from the second heating scan and
gradually increased in the order of CTDB < CTDP < DCTB<
m
The best photovoltaic devices based on DCTP, where PTD
acceptor was used, gave power conversion efficiency (PCE) of
g
1
2.21% whereas its analogue DCTB afforded 1.06%. The BHJ
devices based on DCTP and DCTB showed similar open circuit
5
8
DCTP. This is attributed to the presence of more tertiary
substitution on carbazole unit which increases the amorphous
voltages (V ) which is an observation consistent with the
oc
measured HOMO values. Devices based on DCTP showed
higher photocurrent than the devices based on DCTB, a finding
that is consistent with the observed bathochromic-shift in the
absorption spectrum of DCTP when compared with DCTB.
5
9
nature of the local system.
Photovoltaic properties
Because of their appropriate optoelectronic features,
adequate solubility and good film-forming capabilities, the
materials reported herein were incorporated as p-type Table 4 Comparative BHJ solar cell performance of the dyes
Blend film thicknessa
V
(mV)
J
sc
(mA/cm )
semiconducting components along with soluble fullerene
derivative [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)
as a n-type semiconductor in BHJ photovoltaic devices. BHJ
architectures typically deliver higher device power conversion
efficiencies by maximizing the surface area of the interface
between the donor and acceptor materials in the active layer.
For all the compounds, the device structure used was
oc
Dye
FF
PCE %
2
(1:1 @ 3000 rpm)
CTDP
63
970
4.55
0.34
1.50
DCTB
DCTP
65
64
870
871
3.80
6.75
0.32
0.38
1.06
2.21
a
BHJ devices with specified weight ratio. Device structure is
ITO/PEDOT:PSS (38 nm)/active layer/Ca (20 nm)/Al (100 nm) with
active layer thickness of about 65 nm.
6
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Overall comparison of the BHJ performance of DCTP and DCTB without further purification. Tetrahydrofuran (THF) and
indicated that the use of PTD accepting unit can indeed toluene were distilled over sodium and benzophenone, and
provide a platform for achieving higher PCE and Jsc, when N,N’-dimethyl-formamide (DMF) was dried over 3 Å molecular
studied for a given donor-acceptor module. Our results clearly sieves under nitrogen atmosphere. All chromatographic
1
advocate the promotion of PTD as an interesting structural separations were carried out on silica gel (60-120 mesh).
1
H
3
concept for the design and development of efficient BHJ and C NMR spectra were measured on a Bruker Advance DPX
materials. By contrast, the maximum PCE obtained for a device spectrometer using CDCl solvent with TMS as internal
3
based on CTDP was 1.50%, when fabricated under similar standard. Mass spectra were obtained using electron
conditions. It is notable to mention that the device based on ionization (EI) mass spectrometry (Thermofinnigan, Sanzox,
CTDP afforded higher photocurrent than the device based on CA), gas chromate-graphy-mass spectrometry (GC-MS) was
DCTB, an apparent observation corroborating the absorption performed on VG70–70H. MALDI-TOF mass spectra were
profile of CTDP when compared with DCTB. The relatively obtained from Axima performance MALDI-TOF/TOF mass
lower efficiency achieved with the use of BTD acceptor when spectrometer (Shimadzu). UV–Vis absorption spectra were
compared with PTD acceptor is likely due to the weaker measured on
a
Perkin-Elmer spectrophotometer and
absorption of the visible spectrum. This suggests that the light fluorescence spectra were recorded using a Spex model
harvesting ability of small molecular donors plays a significant Fluorolog-3 spectrofluorometer. Fluorescence quantum yields
role in achieving their optimum photovoltaic performance. The were estimated by integrating the fluorescence bands and
respective current–voltage (J–V) curves are shown in Fig. 8, using 9,10-diphenyl anthracene (ϕ = 0.82 in toluene) as
and Table
4
represents
a
comparative photovoltaic reference. Fluorescence lifetime measurements were
performed on a picosecond time-correlated single photon
performance data.
counting (TCSPC) setup (Edinburgh Instruments Ltd, Lifespec II
model) employing a picosecond light emitting diode lasers
Conclusions
(Nano LED, λex= 405nm) as the excitation source. The decay
curves were recorded by monitoring the fluorescence emission
maxima of the dyes. The lamp profile was recorded by placing
a scatter (diluted Ludox solution in water) in place of the
sample. The width of the instrument response function (IRF)
was limited by the full width at half maxima (FWHM) of the
excitation source, 625 ps at 405 nm. The quality of the fits was
In summary, we have developed a series of four molecules
with funnel architecture which consists of triphenylamine as
the central unit with various combinations of donor (3,6-di-
tert-butyl-9H-carbazole)
and
acceptor
(benzo/pyrido[1,2,5]thiadiazole) fragments. These materials
were synthesized by classic Ullmann and palladium-catalysed
Suzuki coupling reactions with good yields and were
thoroughly characterized. The optical and electrochemical
properties of the dyes are well tuned based on the strength
and number of donor and acceptor units. The fluorescence
quantum yields and excited state life times, and their electron-
transfer rate constants were calculated to evaluate their
excited state properties. The band gap obtained from cyclic
voltammetry analysis is consistent with optical and theoretical
data. The structure-property relationship of the dyes was
investigated in detail using the results obtained from the
experimental and DFT calculations. When these new materials
were tested in solution-processable BHJ devices, better
performance was observed with materials comprising PTD unit
and these results corroborate the experimental and
theoretical findings. The major highlight of this preliminary
study revealed that nonetheless dyes are absorbing at 435 nm
with low molar absorptivity, it reached the efficiency of 2.21%.
Efforts are ongoing to further improve the PCE of devices and
future studies will look at extending the conjugation in the
studied architecture towards longer wavelength region.
2
judged by χ values and the distribution of residuals. Perkin-
Elmer Spectrum BX spectrophotometer was used to obtain IR
−1
spectra of the dyes at a resolution of 4 cm . Thermo
gravimetric analyses (TGA) were performed with a TGA/SDTA
8
1
51e (Mettler Toledo) thermal analyzer with a heating rate of
-1
0 °C min under nitrogen atmosphere in the temperature
g
range of 33–550 °C. The glass-transition temperatures (T )
−1
were measured at a heating rate of 10 °C min under nitrogen
atmosphere using differential scanning calorimetry (DSC Q200
g
TA). T was determined from the second heating scan. Melting
points were measured with an Electro thermal IA 9100 series
digital melting point instrument and are uncorrected. Cyclic
voltammetry measurements were performed on
a PC-
controlled CH instruments model CHI 620C electrochemical
analyzer using 1 mM dye solution in dichloromethane at a scan
rate of 100 mV/sec using 0.1
M tetrabutylammonium
perchlorate (TBAP) as supporting electrolyte. The glassy
carbon, standard calomel electrode (SCE) and platinum wire
were used as working, reference and auxiliary electrodes
respectively.
X-Ray Crystallographic details
Experimental section
X-ray data for two compounds was collected at room
temperature using a Bruker Smart Apex CCD diffractometer
Materials and General Procedures
with graphite monochromated MoK
α radiation (λ=0.71073Å)
Unless otherwise specified, reactions were performed
under nitrogen atmosphere with standard Schlenk techniques.
All the reagents were of reagent/analytical grade and used
with ω-scan method. Preliminary lattice parameters and
orientation matrices were obtained from four sets of frames.
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Unit cell dimensions were determined using 6154 reflections in PDS-UVT UV/ozone cleaner with the platform set to maximum
2
DCTP and 5956 reflections in intermediate 6b crystal data. height. The intensity of the lamp was > 36 mW/cm at a
Integration and scaling of intensity data were accomplished distance of 10 cm. At ambient conditions, the ozone output of
using SAINT program. Structures were solved by Direct the UV cleaner is greater than 50 ppm. Aqueous solutions of
Methods using SHELXS97 and refinement was carried out by PEDOT/PSS (HC Starck, Baytron P AI 4083) were filtered (0.2
full-matrix least-squares technique using SHELXL97. µm RC filter) and deposited onto glass substrates in air by spin
Anisotropic displacement parameters were included for all coating (Laurell WS-400B-6NPP lite single wafer spin
non-hydrogen atoms. All
H
atoms were positioned processor) at 4000 rpm for 60 s to give a layer having a
geometrically and treated as riding on their parent C atoms thickness of 40 ± 5 nm. The PEDOT/PSS layer was then
with C-H distances of 0.93-0.97 Å, and with Uiso(H) = 1.2Ueq annealed on a hotplate in a glove box at 120 °C for 10 min. For
(C) or 1.5Ueq for methyl atoms. The crystallization solvent OPV devices, the newly synthesized organic p-type materials
toluene was included in the crystal lattice of the compound. and PC61BM (Nano-C) were separately dissolved in individual
The solvent is found to be severely disordered to obtain a vials by magnetic stirring. Blend ratios and solution
reliable structural model. The SQUEEZE utility in PLATON was concentrations were varied to optimize device performance.
used to remove contribution from the disordered solvent. An The solutions were then combined, filtered (0.2 µm RC filter),
electron count of 110 was suggested per unit cell (void volume and deposited by spin coating (SCS G3P spin coater) onto the
3
of 460Å ) which closely relates to two molecules of toluene. ITO-coated glass substrates inside a glove box (with H2O and
The files (.hkl and .ins) generated by PLATON after the O2 levels both < 1 ppm). Film thicknesses were determined on
SQUEEZE treatment were used for the final refinement of identical samples using a Dektak 6M Profilometer. The coated
structures.
substrates were then transferred (without exposure to air) to a
vacuum evaporator inside an adjacent nitrogen-filled glove
box. Samples were placed on a shadow mask in a tray. The
Computational details
area defined by the shadow mask gave device areas of exactly
2
.1 cm . Deposition rates and film thicknesses were monitored
Density Functional Theory (DFT) calculations were
performed using the Gaussian 09 (G09) ab initio quantum
chemical software package. Initially, structural optimizations
were performed without any symmetry constraints in gas
phase. The local minima were further confirmed by vibrational
analysis. For DFT calculations, Beckes three-parameter (B3)
gradient- corrected exchange functional combined with Lee–
0
using a calibrated quartz thickness monitor inside the vacuum
chamber. Aluminium (Al) (3 pellets of 99.999%, KJ Lesker),
from an alumina-coated graphite boat, having thicknesses of
1
00 nm, respectively, were evaporated onto the active layer by
-6
thermal evaporation at pressures less than 2×10 mbar.
Where used, C60 (Nano-C) and 2,9-dimethyl-4,7-diphenyl-
6
0, 61
Yang–Parr (LYP) correlation functional
and 6-311G (d, p)
1,10-phenanthroline (Aldrich) were evaporated from alumina
basis set was used. Methyl groups were introduced instead of
tert-butyl groups on carbazole to reduce computational cost.
The minimized geometries were then used to obtain frontier
molecular orbitals (FMOs) and also subjected to single-
pointtime-dependent DFT (TDDFT) studies. The evaluation of
crucibles. A connection point for the ITO electrode was made
by manually scratching off a small area of the active layers. A
small amount of silver paint (Silver Print II, GC Electronics, part
number: 22-023) was then deposited onto all of the
connection points of both ITO and Al. The completed devices
were then encapsulated with glass and a UV-cured epoxy
25 vertical singlet–singlet transitions was carried out to
simulate the UV-Vis spectra of the dyes. The integral equation
formalism polarizable continuum model (PCM) within the self-
consistent field theory has been used to describe the solvation
(Summers Optical, Lens Bond type J-91) by exposure to 365 nm
UV light inside the glove box for 10 min. The encapsulated
devices were then removed from the glove box and tested in
air within 1 h. Electrical connections were made using alligator
clips. The OPV devices were tested using an Oriel solar
simulator fitted with a 1000W xenon lamp filtered to give an
6
2, 63
of dyes in toluene.
The TDDFT calculations were
performed with various energy functionals such as B3LYP, PBE
6
4
and M06-2X. The simulation of the major portion of
absorption spectrum and interpretation of the nature of
-2
6
5
output of 100 mW cm at simulated AM 1.5. The lamp was
calibrated using a standard filtered silicon (Si) cell from Peccell
transitions was executed using software GaussSum 2.2.5. The
percentage contribution of individual fragments present in a
given dye was calculated against their respective molecular
orbitals.
Limited, which was subsequently cross-calibrated with
a
standard reference cell traceable to the National Renewable
Energy Laboratory. The devices were tested using a Keithley
2
400 source meter controlled by Labview software.
Device fabrication and characterization of photovoltaic devices
Indium tin oxide (ITO)-coated glass (10 Ohms per square)
was cleaned by sonicating in a stirred solution of 5% (v/v)
Deconex 12PA detergent for 30 min. The ITO-coated glass was
then successively sonicated for 10 min each in distilled water,
acetone, and isopropanol. The substrates were then cleaned
by UV/ozone at room temperature for 10 min. The UV/ozone
cleaning of glass substrates was performed using a Novascan
5
-Bromobenzo[c][1,2,5]thiadiazole (1a)
Thionyl chloride (SOCl ) (2.91 mL, 37.4 mmol) was added
2
drop wise to a stirred, cooled solution of 4-bromobenzene-1,2-
diamine (5 g, 26.7mmol) in pyridine (30 mL) at 0 °C and the
reaction mixture was stirred for additional 2h at this
temperature. Then the reaction mixture was heated to reflux
8
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for 3h and poured into water. The crude product was collected silica gel and purified by column chromatography using n-
by filtration and washed with water thoroughly to remove hexane to yield a white solid. The obtained product was
excess of pyridine and SOCl
2 2 2
. The crude solid was purified by further purified by recrystallization from CH Cl /hexane
1
silica gel column chromatography using n-hexane as eluent mixture to give white crystals. (13.2g, 47.3 mmol, 79%)
H
1
affording 1a as a while solid (5.2 g, 91%). H NMR (500 MHz, NMR (500 MHz, CDCl
CDCl ): δ 8.18 (s, 1H), 7.86-7.83 (d, 1H, J = 9.3Hz), 7.65-7.60 (d, 7.43 (d, 2H, J = 8.5 Hz), 7.25-7.22 (d, 2H, J = 8.5 Hz), 1.42 (s,
H, J = 9.3 Hz); C NMR (125 MHz, CDCl
33.1, 124.5, 123.8, 122.1; GC/MS: m/z 214 (M+).
3
): δ 8.07 (s, 2H), 7.66 (s, 1H, (N-H)), 7.45-
3
1
3
1
3
): δ 155.2, 153.3, 18H). GC/MS: m/z 279 (M+).
1
Tris(4-bromophenyl)amine (4)
6
-Bromo-[1,2,5]thiadiazolo[3,4-b]pyridine (1b)
In a two-neck flask, NBS (23.9 g, 134.5 mmol) was added in
portions to a solution of TPA (10.0 g, 40.8 mmol) in DMF (120
mL) at 0 °C. The mixture was warmed to room temperature,
stirred for an additional 12h and then poured into excess
water. This aqueous solution was extracted with chloroform.
The combined extracts were washed with brine and dried over
This compound was synthesized according to the
procedure reported for 1a, using 5-bromopyridine-2,3-diamine
5.0 g, 26.5 mmol), to give the desired product as a light yellow
(
1
solid (5.6 g, 98%). H NMR (500 MHz, CDCl
3
): δ 9.09 (s, 1H),
1
3
8
.56 (s, 1H); C NMR (125 MHz, CDCl
31.2, 121.1; GC/MS: m/z 215 (M+).
3
): δ 159.9, 156.8, 147.8,
2 4
anhydrous Na SO . The the solvent was evaporated under
1
reduced pressure. The residue was purified by silica gel column
chromatography using n-hexane as eluent to yield a white
1
solid. (17.8 g, 37.0 mmol, 91%) H NMR (500 MHz, CDCl
5
-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-
3
): δ
yl)benzo[c][1,2,5]thiadiazole (2a)
7.36-7.33 (dd, 6H, J = 8.9 Hz), 6.93-6.90 (dd, 6H, J = 8.9 Hz).
n-Butyllithium (4.67 mL, 10.24 mmol (2.0 M solution in
hexanes)) was added drop wise to a solution of 1a (2.0 g, 9.34
mmol) in anhydrous THF (25.0 mL) at -78 °C (dry ice-acetone
bath) and the resultant yellow suspension was left to stir at
this temperature for 2h. 3.25 mL (4.19 mmol) of 2-isopropoxy-
GC/MS: m/z 481 (M+).
4
-Bromo-N-(4-bromophenyl)-N-(4-(3,6-di-tert-butyl-9H-
carbzol-9-yl)phenyl)aniline (5)
A mixture of 3,6-di-tert-butyl-9H-carbazole (5.0 g, 17.8
4,4,5,5-tetramethyl-[1,3,2]-dioxaborolane was added rapidly
mmol), tris(4-bromophenyl)amine (10.4 g, 21.4 mmol), copper
to this solution and the resulting reaction solution was stirred
for 1h at -78 °C followed by overnight stirring at room
temperature. Saturated ammonium chloride (40 mL) and
chloroform (50 mL) were added to the reaction mixture and
the organic layer was separated. Organic layer was washed
(
I) iodide (2.1 g, 11.3 mmol), 1,10-phenanthroline (4.2 g, 23.2
mmol) and Cs CO (17.5 g, 53.7 mmol) was dissolved in 80 mL
2
3
of dry DMF and refluxed under nitrogen atmosphere for 12h.
After cooling to room temperature, solvent was removed
under vacuum and the residue was dissolved in chloroform.
The solution was passed through celite pad to remove
inorganic impurities and the filtrate was extracted with
chloroform. The combined organic layer was washed with
4
with brine (30 mL), dried over anhydrous MgSO , concentrated
to a crude and subjected to chromatography on silica eluting
with 5% ethyl acetate in hexanes to provide 2a as a white solid
1
1.9 g, 78%). H NMR (500 MHz, CDCl
(
3
): δ 8.50 (s, 1H), 7.97-
): δ 156.1,
2 4
water, brine and dried over anhydrous Na SO . The solution
1
.91 (m, 3H), 1.39 (s, 12H); C NMR (125 MHz, CDCl
3
7
3
was filtered, concentrated and subjected to column
133.6, 129.4, 120.6, 84.4, 24.9; GC/MS: m/z 262 (M+).
chromatography on silica gel with n-hexane as the eluent, to
1
get white solid. (6.9 g, 10.1 mmol, 47%) H NMR (500 MHz,
6-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-[1,2,5]thiadi-
CDCl
3
): δ 8.14-8.13 (d, 2H, J = 1.5 Hz), 7.49-7.35 (m, 11H), 7.33-
azole[3,4-b]pyridine (2b)
7
1
1
1
.23 (d, 2H, J = 10.6 Hz), 7.21-7.03 (d, 3H, J = 8.7 Hz), 1.46 (s,
13
8H); C NMR (125 MHz, CDCl ): δ 145.9, 145.2, 142.6, 138.9,
3
This compound was synthesized according to the
procedure reported for 2a, using 1b (2.0 g, 9.3 mmol), to give
1
38.2, 132.9, 132.3, 132.1, 127.4, 125.6, 125.3, 124.6, 123.2,
23.0, 115.9, 115.8, 108.9, 34.5, 31.8. MS (EI): m/z 680 (M+).
the desired product as a light yellow solid. (2.1 g, 86%).
NMR (500 MHz, CDCl
H
3
): δ 9.36 (s, 1H), 8.78 (s, 1H), 1.48 (s,
1
3
12H); C NMR (125 MHz, CDCl
4.9; GC/MS: m/z 263 (M+).
3
): δ 159.9, 147.1, 137.5, 83.4,
4
-(Benzo[c][1,2,5]thiadiazol-5-yl)-N,N-bis(4-
2
bromophenyl)aniline (6a)
-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)
5
3
,6-Di-tert-butyl-9H-carbazole (3)
To a stirred solution of carbazole (10.0 g, 59.8 mmol) and
anhydrous AlCl (24.4 g, 179.4 mmol) in nitromethane, tert-
butyl chloride (19.7 ml, 179.4 mmol) was added under N
atmosphere. The solution was stirred at room temperature for
8h. The mixture was poured into water and extracted with
benzo[c][1,2,5]thiadiazole (2 g, 7.6 mmol) and tris(4-
bromophenyl)amine (4.04 g, 8.4 mmol) were dissolved in 30
3
mL of dry toluene. After adding 12 mL of 2M K
2 3
CO solution,
2
the reaction mixture was bubbled with N gas for 15 minutes
2
and tetrakis(triphenylphosphine)palladium (90 mg, 0.001
mmol) was added to the reaction mixture, and then heated to
1
chloroform. The combined organic layer was neutralized with
sodium carbonate solution. Obtained organic solution was
8
0 °C for 24h. After cooling the reaction mixture to room
temperature, it was poured into water, extracted with
chloroform and dried over anhydrous Na SO . The resulting
2 4
washed with brine, dried over anhydrous Na SO , filtered and
2
4
concentrated. The green residue obtained was coated with
solution was filtered and concentrated by rotary evaporation.
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The residue obtained was purified by column chromatography
This compound was synthesized according to the
over silica gel with n-hexane as eluent to give the desired procedure similar to that of CTDB, using 6-(4,4,5,5-
1
product as yellow solid. (3.2 g, 5.95 mmol, 71%). H NMR (500 tetramethyl-1, 3, 2-dioxaborolan-2-yl)[1,2,5]thiadiazolo[3,4-
MHz, CDCl
.86 (d, 1H, J = 7.6 Hz), 7.84-7.83 (d, 2H, J = 8.3 Hz), 7.60-7.58 red solid. The obtained product was recrystallized from
d, 4H, J = 9.1 Hz), 7.40-7.37 (d, 2H, J = 8.3 Hz), 7.17-6.99 (d, toluene/methanol mixture to give red needles. (680 mg, 0.86
3
): δ 8.12 (s, 1H), 8.04-8.01 (d, 1H, J = 9.1 Hz), 7.87- b]pyridine (853 mg, 3.2 mmol), to give the desired product as
7
(
1
3
1
4
1
1
H, J = 9.1 Hz); C NMR (125 MHz, CDCl
3 3
): δ 155.4, 154.0, mmol, 55%) H NMR (500 MHz, CDCl ): δ 9.42-9.41 (d, 2H, J =
47.2, 145.9, 141.5, 133.9, 132.5, 129.8, 128.4, 125.9, 123.9, 2.3 Hz), 8.45 (d, 2H, J = 2.3 Hz), 8.20 (s, 2H), 7.74-7.71 (d, 4H, J
3
21.4, 117.6, 116.2. MS (EI): m/z 537 (M+).
1
= 8.5 Hz), 7.55-7.42 (m, 12H), 1.48 (s, 18H); C NMR (125 MHz,
CDCl ): δ 160.9, 156.1, 148.1, 147.8, 145.0, 142.9, 139.1, 136.5,
3
4
-([1,2,5]Thiadiazolo[3,4-b]pyridin-6-yl)-N,N-bis(4-bromo
134.2, 130.7, 128.8, 127.8, 126.2, 125.0, 124.5, 123.6, 123.3,
116.3, 109.1, 34.7, 31.9. MS (Maldi-TOF): (m/z) calcd: 792.28;
phenyl)aniline (6b)
This compound was synthesized according to the found: 792.59
procedure similar to that of 6a, using 6-(4,4,5,5-tetramethyl-1,
3, 2-dioxaborolan-2-yl)-[1, 2, 5]thiadiazolo[3, 4-b]pyridine (2.0 4-(Benzo[c][1,2,5]thiadiazol-5-yl)-N,N-bis(4-(3,6-di-tert-butyl-
g, 7.6 mmol) to give the desired product as orange solid. The 9H-carbazol-9-yl)phenyl) aniline (DCTB)
obtained product was recrystallized from CH
2
Cl
2
/hexane to get
orange needles. (2.7 g, 5.01 mmol, 61%) H NMR (500 MHz, 4.1mmol),
CDCl
A
mixture of 3,6-di-tert-butyl-9H-carbazole (1.2 g,
1
4-(benzo[c][1,2,5]thiadiazol-5-yl)-N,N-bis(4-
3
): δ 9.35 (s, 1H), 8.38 (s, 1H), 7.62-7.59 (d, 2H, J = 8.7 Hz), bromophenyl) aniline (1g, 1.9 mmol), copper (I) iodide (223
.43-7.40 (d, 4H, J = 8.9 Hz), 7.21-7.19 (d, 2H, J = 8.5 Hz), 7.04- mg, 1.2 mmol), 1,10-phenanthroline (436 mg, 2.41 mmol) and
7
7
1
1
1
.02 (d, 4H, J = 8.7 Hz); C NMR (125 MHz, CDCl
3
3 2 3
): δ 160.9, Cs CO (1.8 g, 5.6 mmol) was dissolved in 20 mL of dried DMF
56.0, 147.9, 147.8, 145.8, 136.4, 132.6, 130.2, 128.6, 126.2, and refluxed under nitrogen atmosphere for 12h. After cooling
24.8, 123.6, 116.6. MS (EI): m/z 538 (M+).
to room temperature, the solvent was removed under vacuum
and the residue was dissolved in chloroform. The chloroform
solution was filtered through celite pad to remove inorganic
impurities and the filtrate was extracted with chloroform. The
combined organic layers are washed with water, brine and
4-(Benzo[c][1,2,5]thiadiazol-5-yl)-N-(4-(benzo[c][1,2,5]thia-
diazol-5-yl)phenyl)-N-(4-(3,6-di-tert-butyl-9H-carbazol-9-
yl)phenyl)aniline (CTDB)
4
-bromo-N-(4-bromophenyl)-N-(4-(3, 6-di-tert-butyl-9H-
carb-azol-9-yl)phenyl)aniline (1 g, 1.5 mmol), 5-(4,4,5, 5- filtered and concentrated by rotary evaporation. The residue
tetramethyl-1, 3, 2-dioxaborolan-2-yl) benzo[c][1,2, was subjected to column chromatography on silica gel with n-
]thiadiazole (847 mg, 3.2 mmol) were dissolved in 15 mL of hexane as the eluent to the desired product get yellow solid.
2 4
dried over anhydrous Na SO . The resulting solution was
5
1
2 3 3
toluene. After adding 8 mL of 2M K CO solution, reaction (1.2 g, 1.28 mmol, 67%). H NMR (500 MHz, CDCl ): δ 8.18-8.16
mixture was bubbled with nitrogen for 15 minutes. (m, 5H), 8.04-8.03 (d, 1H, J = 9.2 Hz), 7.91-7.89 (d, 1H, J = 9.2
Tetrakis(triphenylphosphine)palladium (90 mg, 0.001 mmol) Hz), 7.71-7.69 (d, 2H, J = 8.4 Hz), 7.54-7.39 (m, 20H), 1.48 (s,
1
3
3
was added to the reaction mixture and then heated to 80 °C 36H), C NMR (125 MHz, CDCl ): δ 155.6, 154.2, 147.9, 145.9,
for 24h. After cooling the reaction mixture to room 142.9, 141.8, 139.4, 133.9, 133.5, 129.9, 128.7, 127.8, 125.5,
temperature, it was filtered through celite pad to remove the 124.3, 123.7, 123.4, 121.6, 117.8, 116.4, 109.3, 109.3, 34.8,
inorganic impurities and the filtrate was extracted with 32.1. MS (Maldi-TOF): (m/z) calcd: 933.48; found: 933.75
chloroform. The combined organic layer was washed with
water, brine and dried over anhydrous Na
solution was filtered and concentrated by rotary evaporation tert-butyl-9H-carbazol-9-yl) phenyl)aniline (DCTP)
and the residue obtained was purified by column This compound was synthesized according to the
2 4
SO . The resulting 4-([1,2,5]Thiadiazolo[3,4-b]pyridin-6-yl)-N,N-bis(4-(3,6-di-
chromatography over silica gel with n-hexane as the eluent to procedure similar to that of DCTB, using 4-
give the desired product as yellow solid. (652 mg, 0.82 mmol, ([1,2,5]thiadiazolo[3,4-b]pyridin-6-yl)-N,N-bis(4-
1
Yield 55%). H NMR (500 MHz, CDCl
3
): δ 8.18-8.17 (dd, 4H, J = bromophenyl)aniline (1g, 1.9 mmol), to give the desired
1
9
.6 Hz), 8.16-8.15 (d, 2H, J = 9.2 Hz), 8.07-8.04 (d, 4H, J = 1.7 product as orange solid. (0.78 g, 0.83 mmol, Yield 44%)
H
Hz), 7.93-7.92 (d, 4H, J = 1.7 Hz), 7.91-7.69 (m, 4H), 7.53-7.37 NMR (500 MHz, CDCl
1
3
): δ 9.41-9.39 (d, 1H, J = 2.3 Hz), 8.42-
3
(
3
m, 8H), 1.48 (s, 18H); C NMR (125 MHz, CDCl ): δ 155.5, 8.14 (d, 1H, J = 2.3 Hz), 8.17 (s, 4H), 7.72-7.69 (d, 2H, J=9.1 Hz),
1
3
1
1
1
54.1, 147.5, 145.5, 142.8, 141.6, 139.2, 134.0, 133.5, 129.8, 7.58-7.42 (m, 16H), 7.24-7.14 (m, 2H), 1.48 (s, 36H); C NMR
29.3, 128.5, 127.6, 125.6, 124.5, 123.5, 123.3, 121.5, 117.7, (125 MHz, CDCl ): 160.9, 156.1, 148.6, 147.8, 145.5, 142.9,
16.3, 109.1, 34.7, 31.9. MS (Maldi-TOF): (m/z) calcd: 791.29; 139.2, 136.6, 136.6, 133.8, 130.0, 128.9, 128.7, 128.2, 127.8,
125.8, 125.2, 124.8, 123.9, 123.6, 123.3, 116.3, 109.2, 34.7,
2.0. MS (Maldi-TOF): (m/z) calcd: 934.47; found: 934.80.
3
found: 791.51.
3
4
-([1,2,5]Thiadiazolo[3,4-b]pyridin-6-yl)-N-(4-
[1,2,5]thiadiazole [3,4-b]pyridin-6-yl)phenyl)-N-(4-(3,6-di-
tert-butyl-9H-carbazol-9-yl)phenyl)anilinev (CTDP)
(
Acknowledgements
1
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MP thanks UGC, India for the fellowship. We gratefully 19 J. Zhou, X. Wan, Y. Liu, Y. Zuo, Z. Li, G. He, G. Long, W. Ni, C.
Li, X. Su and Y. Chen, J. Am. Chem. Soc., 2012, 134, 16345-
acknowledge CSIR for TAPSUN, NWP0055 project funding.
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Graphical Abstract
Funnel shaped molecules containing benzo/pyrido[1,2,5]thiadiazole
functionalities as peripheral acceptors for organic photovoltaic
applications
Tuning the energy levels of a series of funnel shaped small molecule electron donors was carried
out by varying different donor and acceptor combinations. A power conversion efficiency of
2.21 % was achieved for the DCTP which contains bis-carbazole as donor and
pyrido[1,2,5]thiadiazole as an acceptor unit.