Synthesis of pyrene-based ester dendrimers for applications in dye-sensitized solar cells

Article abstract of DOI:10.1246/bcsj.20110280

Synthesis of dendrimers with pyrene at the surface and benzenetricarboxylic acid and phloroglucinol at the core and with ester functionality as branching points has been described. When such dendrimers are used as an additive in DSSC, power conversion eff

Full text of DOI:10.1246/bcsj.20110280

902
Bull. Chem. Soc. Jpn. Vol. 85, No. 8, 902-911 (2012)
© 2012 The Chemical Society of Japan
Synthesis of Pyrene-Based Ester Dendrimers for Applications
in Dye-Sensitized Solar Cells
Perumal Rajakumar,*1 Kathiresan Visalakshi,1 Shanmugam Ganesan,2
Pichai Maruthamuthu,2 and Samuel Austin Suthanthiraraj2
1Department of Organic Chemistry, University of Madras, Guindy Campus, Chennai-600 025, India
2Department of Energy, University of Madras, Guindy Campus, Chennai-600 025, India
Received September 30, 2011; E-mail: perumalrajakumar@gmail.com
Synthesis of dendrimers with pyrene at the surface and benzenetricarboxylic acid and phloroglucinol at the core and
with ester functionality as branching points has been described. When such dendrimers are used as an additive in DSSC,
power conversion efficiency of 6.78% is observed.
Dye-sensitized solar cells (DSSCs) have attracted world
the synthesis of pyrene-based dendrimers 1-6 (Chart 1) having
ester functionalization as branching point, benzenetricarboxylic
acid and phloroglucinol as the core unit and a study of their
photophysical properties as well as solar cell performance
when used as an additive in the redox couple of DSSCs is also
described.
wide attention for its low production cost, easy handling, and
relatively high electrical power conversion efficiency. The first
efficient DSSC was reported by Grätzel in 1991.1 DSSCs are
composed of nanocrystalline mesoporous photoanodes2 coated
with a suitable dye molecule,3 electrolyte containing a redox
couple,4 and counter electrode.5 The electrolyte is the charge-
transfer media between photoanode and counter electrode.
The most efficient and widely used redox couple used in
Results and Discussion
Syntheses. The pyrene-based ester dendrimers 1, 2, and
3 were synthesized by convergent approach (Scheme 1).
Tris(pyren-1-ylmethyl) benzene-1,3,5-tricarboxylate (1) was
obtained in 88% yield by the condensation of one equiv of
pyren-1-ylmethanol (7) with 0.3 equiv of benzene-1,3,5-tri-
carboxylic acid (8) in dry dichloromethane (DCM) in the
presence of diisopropylcarbodiimide (DIPC) and 4-(N,N-di-
methylamino)pyridinium toluene-4-sulfonate (DPTS) respec-
tively. DPTS23 in turn was prepared by stirring N,N-dimethyl-
4-aminopyridine (DMAP) and p-toluenesulfonic acid mono-
hydrate in THF. The 1H NMR spectrum of 1 displayed a singlet
at ¤ 5.99 for O-methylene protons integrating for six protons,
in addition to the thirty aromatic protons ranging from ¤ 7.93 to
8.79. In the 13C NMR spectrum, 1 showed the O-methylene and
carbonyl carbons at ¤ 63.6 and 163.9 respectively, in addition
to the signals for eighteen aromatic carbons. In IR, the ester
carbonyl group appeared at 1720.6 cm¹1. Further FAB-MS and
elemental analysis also confirmed the structure of 1.
¹
¹
DSSCs is I /I3 . Solar cell performance has been remarkably
enhanced by the addition of various additives to the electrolytic
solution.6,7
Dendrimers, being regularly branched and three-dimensional
macromolecules, have attracted scientific attention exception-
ally, because of their size, shape, dimension, flexibility, and
functionality, which can be controlled at molecular level.8-12
Pyrene derivatives due to their richness in photophysical
properties, find application in microenvironment sensors,13
liquid crystals,14 and organic light-emitting diodes.15 The
pyrene units, attached to the dendrimer skeleton through imine
bonds play a pivotal role in enhancing the aggregation pro-
pensity of the PAMAM dendrimers.16 Crooks et al.17 have
synthesized four generations of PPI dendrimers fully substituted
with pyrene and the fluorescence spectroscopy reveals that with
the increase in dendrimer generation, the excimer to monomer
ratio for pyrene emission also increases. The pyrene-containing
eight-arm star-shaped dendrimer-like copolymer18 exhibited
unique thermal properties and crystalline morphologies, distinct
from that of the linear polymer. Dendrimeric compounds bear-
ing pyrene units were synthesized by Cicchi et al.19 to afford
light-harvesting antennae based on the formation of intra-
molecular excimers. Dendrimers with ester linkage20,21 provides
easy access, facile branching, versatility, solubility, applicabil-
ity, and processability from inexpensive raw materials.
Addition of additives possessing N-containing heterocycles
and ester groups could enhance the efficiency of DSSCs due
to valence effect.22 Fascinated by the applications of ester
groups in improving solar cell performance, we report herein
Pyren-1-ylmethyl 4-formylbenzoate (10) was obtained in
92% yield by the coupling of one equiv of 4-formylbenzoic
acid (9) with one equiv of 7 in dry DCM using DIPC and DPTS.
The reduction of 10 with sodium borohydride at 0 °C in a 1:1
mixture of THF and ethanol gave pyren-1-ylmethyl 4-hydroxy-
methylbenzoate (11) in 89% yield. One equiv of 11 underwent
DIPC coupling with 0.3 equiv of 8 to give tris[4-(pyren-1-
ylmethoxycarbonyl)phenylmethyl] benzene-1,3,5-tricarboxyl-
1
ate (2) in 85% yield. The H NMR spectrum of 2 displayed
two singlets at ¤ 5.32 and 6.01 for O-methylene protons
integrating for six protons each, a doublet at ¤ 7.66-7.69 (J =
8.1 Hz) integrating for six protons, in addition to the aromatic
Published on the web August 8, 2012; doi:10.1246/bcsj.20110280

P. Rajakumar et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 8 (2012)
903
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
1
2
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
4
3
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
5
6
Chart 1.
protons. In the 13C NMR of 2 showed the O-methylene carbons
at ¤ 65.5 and 66.6 and the carbonyl carbons at ¤ 166.1 and 170.1
along with the signals for nineteen aromatic carbons which
confirmed the structure of 2. The structure of 2 was further
confirmed from FAB-MS and elemental analysis also.
The bis(pyren-1-ylmethyl) 5-(tert-butyldimethylsiloxy)iso-
phthalate (14) was obtained in 87% yield by the treatment of
0.5 equiv of 5-(tert-butyldimethylsiloxy)isophthalic acid (13)24
with one equiv of 7 using DIPC and DPTS. The reaction of 14
with 1 M HCl in acetone yielded the bis(pyren-1-ylmethyl) 5-

904
Bull. Chem. Soc. Jpn. Vol. 85, No. 8 (2012)
Synthesis of Pyrene-Based Ester Dendrimers for DSSCs
O
O
i
O
O
O
O
1
O
O
O
O
O
O
OH
O
i
O
ii
O
O
E
O
O
2
7
E = CHO 10
iii
E = CH2OH 11
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
iv
i
O
OR
O
R = TBDMS 14
R = H 15
O
O
v
O
O
O
O
3
Scheme 1. Synthesis of pyrene-based ester dendrimers 1-3. Reagents and conditions: (i) 0.3 equiv of benzene-1,3,5-tricarboxylic
acid (8), DIPC, DPTS, dry CH2Cl2, 24 h, 1 (88%), 2 (85%), 3 (84%); (ii) 1 equiv of 4-formylbenzoic acid (9), DIPC, DPTS, dry
CH2Cl2, 24 h, 10 (95%); (iii) THF/EtOH (1:1), NaBH4, 0 °C, 3 h, 11 (89%); (iv) 0.5 equiv of 5-(tert-butyldimethylsiloxy)iso-
phthalic acid (13), DIPC, DPTS, dry CH2Cl2, 24 h, 14 (87%); (v) acetone/1 M HCl, 12 h, 15 (85%).
hydroxyisophthalate (15) in 85% yield. DIPC coupling of one
equiv of 15 with 0.3 equiv of benzene-1,3,5-tricarboxylic acid
(8) in dry DCM in the presence of DPTS gave nonaester 3
Pyrene-1-carboxylic acid (17) was obtained by the oxidation
of pyrene-1-carbaldehyde using potassium permanganate,
pyridine and water at 100 °C for 12 h. The pyrene-based ester
dendrimers 4, 5, and 6 were synthesized by divergent approach
(Scheme 2).
The triester 4 having three pyrene units was obtained in 91%
yield by the DIPC coupling of one equiv of benzene-1,3,5-triol
(16) with three equiv of 17 (Scheme 2). The 1H NMR spectrum
of triester 4 displayed a singlet at ¤ 7.53 for the central
phloroglucinol core integrating for three protons and two
doublets at ¤ 8.96-8.99 (J = 8.1 Hz) and 9.45-9.48 (J = 9.3
Hz) integrating for three protons each, in addition to the
aromatic protons. In the 13C NMR spectrum, triester 4 showed
the carbonyl carbon at ¤ 162.4 along with the signals for
1
having six pyrene units. The H NMR spectrum of nonaester
3 displayed a singlet at ¤ 5.30 for O-methylene protons
integrating for twelve protons, a doublet at ¤ 8.29-8.32 (J =
9.3 Hz) integrating for six pyrenyl protons, in addition to
the aromatic protons. In the 13C NMR spectrum, nonaester 3
showed the O-methylene carbon at ¤ 67.1 and the carbonyl
carbons at ¤ 160.9 and 166.8 along with the signals for twenty
aromatic carbons. Two sharp bands appeared for the nonaester
¹1
3 at 1733.3 and 1719.0 cm in IR indicating the presence of
ester carbonyl group. FAB-MS and elemental analysis are also
in accordance with the proposed structure 3.

P. Rajakumar et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 8 (2012)
905
O
O
O
O
i
O
O
4
O
O
E
O
O
E
O
O
O
O
O
O
O
O
HO
OH
ii
i
O
O
O
O
OH
16
E
O
E = CHO 18
O
iii
E = CH2OH 19
5
O
O
OR
O
O
O
OR
O
O
RO
O
O
O
O
O
O
O
O
iv
OR
i
O
O
O
O
O
O
O
RO
OR
O
O
R = TBDMS 21
R = H 22
v
6
Scheme 2. Synthesis of pyrene-based ester dendrimers 4-6. Reagents and conditions: (i) 3/3/6 equiv of pyrene-1-carboxylic acid
(17), DIPC, DPTS, dry CH2Cl2, 24 h, 4 (91%), 5 (71%), 6 (78%); (ii) 3 equiv of 4-formylbenzoic acid (9), DIPC, DPTS, dry
CH2Cl2, 24 h, 18 (81%); (iii) THF/EtOH (1:1), NaBH4, 0 °C, 3 h, 19 (68%); (iv) 3 equiv of 3,5-di(tert-butyldimethylsiloxy)benzoic
acid (20), DIPC, DPTS, dry CH2Cl2, 24 h, 21 (79%); (iv) acetone, 1 M HCl, 12 h, 22 (74%).
eighteen aromatic carbons. In IR, the ester carbonyl group
appeared at 1673.5 cm¹1. The structure of the triester 4 was also
confirmed from FAB-MS and elemental analysis.
ing for three protons each, in addition to the aromatic protons.
In the 13C NMR spectrum, hexaester 5 showed the O-methylene
carbon at ¤ 66.1 and the carbonyl carbons at ¤ 157.5 and 170.2
along with the signals for sixteen aromatic carbons. Two sharp
The trialdehyde 18 was obtained by the coupling of three
equiv of 4-formylbenzoic acid (9) with one equiv of 16 in dry
DCM using DIPC and DPTS. The reduction of trialdehyde 18
with sodium borohydride at 0 °C in a 1:1 mixture of THF and
ethanol gave the triol 19 in 68% yield. One equiv of the triol
19 underwent DIPC coupling with three equiv of 17 to give
¹1
bands appeared for the hexaester 5 at 1618.3 and 1659.8 cm
in IR indicating the presence of ester carbonyl group. FAB-MS
and elemental analysis are also in accordance with the proposed
structure.
The hexasilyloxy triester 2125 was obtained in 79% yield
by the treatment of three equiv of 3,5-di(tert-butyldimethyl-
siloxy)benzoic acid (20)26 with one equiv of 16 as mentioned
earlier. The reaction of hexasilyloxy triester 21 with 1 M HCl in
acetone yielded the triester 2225 in 84% yield. DIPC coupling
of six equiv of 17 with one equiv of triester 22 in dry DCM in
1
hexaester 5 having three pyrene units. The H NMR spectrum
of hexaester 5 displayed two singlets at ¤ 4.76 and 7.36 for
the O-methylene and the phloroglucinol protons respectively,
integrating for six and three protons each, two doublets at
¤ 8.69-8.72 (J = 8.1 Hz) and 9.35-9.38 (J = 9.3 Hz) integrat-

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Bull. Chem. Soc. Jpn. Vol. 85, No. 8 (2012)
Synthesis of Pyrene-Based Ester Dendrimers for DSSCs
2
3
1
2
1
3
Wavelength/nm
Wavelength/nm
Figure 2. Fluorescence spectra of 1 in DMSO; 2 and 3
in CHCl3.
Figure 1. UV-visible absorption spectra of 1 in DMSO;
2 and 3 in CHCl3.
Table 1. Photophysical Data for the Dendrimers 1, 4, 5, and
6 in DMSO and 2 and 3 in CHCl3
and carbonyl group is absent in dendrimers 1, 2, and 3 due to
the presence of intervening methylene groups.
Emission
max/nm
Dendrimers UV -max/nm
-
Phloroglucinol as the Core Unit: Ester dendrimers 4, 5,
and 6 show two absorption bands and two shoulders which are
attributed to the ³-³* transitions of the pyrene moiety and the
values are listed in Table 1. The dendrimers 4, 5, and 6 show
monomeric pyrene emission around 420-422 nm (Table 1)
purely because the pyrenyl units do not have flexibility due to
the absence of an intervening methylene group and hence they
are not in close proximation as that of the dendrimers 1, 2, and
3. Further more, the absorption of 4, 5, and 6 show red shift of
about 45 nm when compared with the dendrimers 1, 2, and 3
due to the presence of carbonyl group conjugated to the pyrene
pendant.
1
2
3
4
5
6
245.3, 266.8, 278.0, 329.1, 345.7
487
245.8, 267.2, 278.0, 315.2, 329.1, 345.4 490
241.1, 245.3, 267.7, 278.3, 329.5, 346.3 492
245.9, 284.0, 357.5, 390.1
246.0, 283.2, 354.6, 389.0
259.0, 285.1, 357.2, 389.0
402, 420
410, 421
411, 422
the presence of DPTS gave the nonaester 6 with six pyrene
units showing complete conjugation. The H NMR spectrum
1
of nonaester 6 displayed a singlet at ¤ 7.53 for the aromatic
protons of the central core integrating for three protons, three
doublets at ¤ 7.04-7.05 (J = 2.4 Hz), 8.96-8.99 (J = 8.1 Hz),
and 9.34-9.37 (J = 9.3 Hz) integrating for six protons each
and a triplet at ¤ 6.47-6.48 (J = 1.8 Hz) integrating for three
protons, in addition to the aromatic protons. Two sharp bands
DSSC Studies of Ester-Linked Dendrimers Having
Pyrene Surface.
The current-voltage characteristics was
measured under AM 1.5 illumination (100 mW cm¹2) using a
BAS100A Electrochemical Analyzer. A 150-W tungsten
halogen lamp (OSRAM, Germany) served as the light source.
The values are derived from the equation © = (Voc © Jsc © ff)/
Ps, where Voc is open circuit potential (V), Jsc is short circuit
current density (mA cm¹2), ff is the fill factor, and Ps is the
power of the incident (solar) light.
¹1
appeared for the nonaester 6 at 1685.2 and 1615.0 cm in IR
indicating the presence of ester carbonyl group. The mass
spectrum (FAB-MS) of nonaester 6 showed a molecular ion
peak at m/z 1902. The structure of the nonaester 6 was further
confirmed by elemental analysis.
In general organic nitrogenous compounds6a,28 are intro-
duced as additives in the redox couple of DSSC, however
introduction of dendrimers containing oxygen atoms and
pyrene units resulted in good performance. Both Voc and Jsc
increased in the doped electrolyte systems when compared to
the undoped redox counterparts. The increase in Voc is due to
the effective formation of charge-transfer complex of the n
and ³ electrons present in the dendrimer with iodine in the
redox couple. Iodide is regenerated in turn by the reduction of
triiodide at the counter electrode. The circuit is completed via
electron migration as given in the following equation.
ꢀ
Photophysical Properties of Ester-Linked Dendrimers
Having Pyrene Surface. Benzenetricarboxylic Acid as the
Core Unit: The electronic absorption spectra of the pyrene-
based ester dendrimers 1 (in DMSO), 2, and 3 (in CHCl3)
shows five sharp characteristic transitions of the pyrene unit
with maxima between 241 and 346 nm (Figure 1 and Table 1).
The excimer emission (Figure 2) of the pyrene-based ester
dendrimers 1 (in DMSO), 2, and 3 (in CHCl3) was observed at
487, 490, and 492 nm respectively. Pyrene-containing com-
pounds can form excited-state dimers (excimers) by stacking
interaction between excited-state and ground-state monomers.
Excimer formation requires close proximity between the pyrene
moeities.27 The pyrene-based ester dendrimers 1, 2, and 3 show
excimer emission owing to the flexibility caused by the O-
methylene carbons which could result in close proximity of the
pyrenyl units. Further the conjugation between the pyrene unit
Iꢀ þ I2 ꢀ I3
ð1Þ
ð2Þ
ð3Þ
3Iꢀ þ 2dyeþ ! I3ꢀ þ 2dye ðTiO2 electrodeÞ
I3ꢀ þ 2eꢀ ꢀ 3Iꢀ ðcounter electrodeÞ
To investigate the effects of pyrene-based ester dendrimers 1-6
as an additive in the redox couple, UV-visible absorption of

P. Rajakumar et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 8 (2012)
907
1.8
1.6
1.4
1.2
1
Table 2. Performance of the Cells Fabricated with and
without the Additives in the Redox Couple
¹2
Electrolyte systems
Jsc/mA cm
Voc/mV
ff
©/%
¹
¹
1 + I /I3 redox couple
4.10
4.50
6.10
600
630
710
0.51 3.14
0.52 3.69
0.51 5.52
5
3
4
6
2
1
¹
¹
2 + I /I3 redox couple
¹
¹
3 + I /I3 redox couple
¹
¹
I /I3 redox couple
Without additive
0.8
0.6
0.4
0.2
1.60
430
0.45 0.95
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0.000
1
2
3
0
270
320
370
Wavelength/nm
420
Figure 3. UV-visible absorption spectra of KI and I2 with
additives 1-6.
the prepared electrolytes was measured. The peaks around 290
and 360 nm indicate the presence of I3
¹ 29 The intensity and the
width of the absorption peaks at 296 and 364 nm in the spectra
(Figure 3) increased with the addition of dendrimers 1-6,
.
¹
implying an increase in I3 concentration. With an increase
0
100
200
300
400
500
600
700
in iodide ion concentration due to the addition of additives in
the redox couple, the regeneration of oxidized dye may be
accelerated and the accumulated electrons in TiO2 via charge
injection from the excited dye molecule increase, resulting in
high Voc. The pyrene dendrimers work as a hole transporter to
shuttle a hole from the oxidized dye to the counter electrode by
forming complex with iodine. The pyrene-based ester dendri-
mers 1-6 act as an additive in the redox couple and efficiently
convert light into electrical current. A high photocurrent is
obtained with the increase in the number of pyrene units, since
the pyrene is a potent photoreductant and the pyrene radical
cation is a strong oxidant that is expected to oxidize iodide
rapidly.30
Voltage/mV
Figure 4. I-V curve for 1, 2, and 3 as an additive in the
redox couple.
Table 3. Performance of the Cells Fabricated with and
without the Additives in the Redox Couple
¹2
Electrolyte systems
Jsc/mA cm
Voc/mV
ff
©/%
¹
¹
4 + I /I3 redox couple
5.10
5.30
6.60
655
685
790
0.48 4.01
0.48 4.30
0.52 6.78
¹
¹
5 + I /I3 redox couple
¹
¹
6 + I /I3 redox couple
¹
¹
I /I3 redox couple
Without additive
1.60
430
0.45 0.95
Charge-transfer complex formation of the additives with
iodine prevents the facile sublimation of iodine thus enhancing
the stability of the electrolyte. In addition to this, the additive
interacting with iodine in the redox couple may shift the
semiconductor band edge toward negative potential by adsorp-
tion on to the TiO2 surface in DSSC and enhance the effi-
ters. These include the open circuit photovoltage (Voc), the short
circuit photocurrent (Jsc), and the overall efficiency (©). The
first two parameters, Voc and Jsc, are readily obtained from the
current-voltage characteristic (I-V) of the DSSC. Table 3
shows the photovoltaic performance of DSSCs based on the
incorporation of pyrene-based ester dendrimers 4, 5, and 6 in
the redox couple.
¹
ciency.31 The concentrations of I , I2, and the additives used
(3.2 © 10¹3 M of KI, 4.1 © 10¹4 M of I2, and the pyrene-based
ester dendrimers of 6.4 © 10¹6 M) resulted in better perform-
ance in DSSC.
Benzenetricarboxylic Acid as the Core Unit: Table 2
shows the photovoltaic performance of DSSCs based on the
incorporation of pyrene-based ester dendrimers 1, 2, and 3 in
the redox couple.
The comparison of DSSCs using pyrene-based ester dendri-
¹
¹
mers 4, 5, and 6 as additives along with I /I3 redox couple
and without additives (Figure 5) reveals that dendrimer 6
shows higher efficiency than that of the dendrimers 1-5
because of the increase in number of pyrene pendants with
carbonyl substitution. However, all the pyrene-based ester
dendrimers 1-6 gives high efficiency when compared to the
undoped redox couple. The cell fabricated without the additive
in the redox couple shows the efficiency of 0.95%. The
From the results it is clear that with the increase in the
number of pyrene unit, © value, open circuit voltage (Voc), and
short circuit current density (Jsc) of the DSSCs increases.
Figure 4 shows the current versus voltage curve of a DSSC
using pyrene-based ester dendrimers 1, 2, and 3 as an additive
¹
¹
dendrimer 3 in I /I3 redox couple shows the second highest
efficiency among the six dendrimers which has been screened.
This is due to the presence of six pyrene pendants which are
not directly linked with carbonyl function and hence results
¹
¹
along with the I /I3 redox couple.
Phloroglucinol as the Core Unit:
The photovoltaic
performance of the DSSC is evaluated based on three parame-

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Bull. Chem. Soc. Jpn. Vol. 85, No. 8 (2012)
Synthesis of Pyrene-Based Ester Dendrimers for DSSCs
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0.000
(ACME, 100-200 mesh). All solvents used were purified as
per standard procedure.32
5
6
4
General Procedure for Esterification Reactions (Proce-
dure A). To a solution of alcohol (2.2/1.8/2.6/6.5/0.2 mmol)
in dry dichloromethane (150 mL) was added the carboxylic
acid (0.7/1.8/1.2/1.6/1.2 mmol), followed by 4-(N,N-di-
methylamino)pyridinum toluene-4-sulfonate (DPTS) (2.2/1.8/
2.5/1.5/0.5 mmol). The mixture was stirred at room temper-
ature under nitrogen for 15 min. Diisopropylcarbodiimide
(DIPC) (2.2/2.1/2.6/1.6/1.3 mmol) was then added and stir-
ring continued at room temperature until the completion of
the reaction. The reaction mixture was diluted with CH2Cl2
(2 © 50 mL) and washed with water (50 mL). The organic layer
was separated, dried over MgSO4, and solvent was removed by
rotary evaporation. The residue, thus obtained after evaporation
of the solvent was purified by column chromatography using
suitable eluent as mentioned for each compound to give the
corresponding ester.
0
100
200
300
400
500
600
700
800
Voltage/mV
Figure 5. I-V curve for 4, 5, and 6 as an additive in the
redox couple.
General Procedure for Sodium Borohydride Reduction
(Procedure B). To a solution of aldehyde (0.7 mmol) in a
mixture of THF and ethanol (1:1, 20 mL) was added in portions
NaBH4 (1.4 mmol) at 0 °C. The reaction mixture was then
stirred at room temperature for 3 h, after which concd HCl
(3-5 drops) was added. The inorganic residue obtained was
filtered off. Evaporation of the solvent from the reaction
mixture in vacuo gave a residue, which was purified by column
chromatography using suitable eluent as mentioned for each
compound to give the corresponding alcohol.
in breakdown of conjugation between pyrene and carbonyl
group. The dendrimer 5 used as an additive in the redox couple
shows the third highest efficiency among all the compounds
that are screened due to the presence of three pyrene units. The
dendrimer 4 in I /I3 redox couple shows the fourth highest
efficiency among all the tested compounds due to the presence
of three pyrene pendants with carbonyl substitution. Finally the
dendrimers 1 and 2 having three pyrene units show moderate
efficiency since there is a breakdown in conjugation between
pyrene pendants and carbonyl group.
¹
¹
General Procedure for TBDMS Deprotection of an
Alcohol (Procedure C). The silyl-protected alcohol (1.2 g)
was heated at 50 °C with acetone (100 mL) and 1 M HCl
(30 mL) for 12 h. The solvent was removed under reduced
pressure. The residue was diluted with CHCl3 (100 mL),
washed with water (3 © 50 mL) and brine (50 mL). The organic
layer was separated, dried over MgSO4, and the solvent was
removed by rotary evaporation. The residue, thus obtained after
evaporation of the solvent was purified by column chromatog-
raphy using suitable eluent to give the corresponding alcohol.
Tris(pyren-1-ylmethyl) Benzene-1,3,5-tricarboxylate (1).
Following the general procedure A, tris(pyren-1-ylmethyl)
benzene-1,3,5-tricarboxylate (1) was obtained as a colorless
solid from benzene-1,3,5-tricarboxylic acid (8) (0.15 g, 0.7
mmol) and pyren-1-ylmethanol (7) (0.5 g, 2.2 mmol). The ester
was chromotographed over SiO2 using hexane/ethyl acetate
(4:1) as an eluent. Yield: 88%; mp: 240-242 °C. IR (KBr,
Conclusion
In conclusion, we have synthesized pyrene-based dendrimers
with ester branching points which shows the power conversion
efficiency was sensitive to the effect of the number of pyrene
pendants and its conjugation with carbonyl group. Efficient
power conversion will be possible through the structural
modifications on conjugated pyrene units. Further our results
suggests that ³-conjugated systems might show better light
harvesting ability and hence better performance in the solar
cells than other types of molecular systems.
Experimental
General Considerations.
All melting points were
uncorrected. The 1H and 13C NMR spectra were recorded
on Bruker 300 spectrometers. The chemical shifts are reported
in ppm (¤) with TMS as internal standard and coupling con-
stant (J) are expressed in Hz. MALDI-TOF mass spectra were
recorded on a Voyager-DE PRO mass spectrometer using ¡-
cyano-4-hydroxycinnamic acid (¡-CHCA) as matrix and EI-
MS spectra on a JEOL DX-303 mass spectrometer. The FAB-
MS spectra were recorded on a JEOL SX 102/DA-6000 mass
spectrometer using p-nitrobenzyl alcohol (NBA) as matrix.
Elemental analyses were performed on a Perkin-Elmer 240B
elemental analyzer. The UV-vis spectra were recorded on a
Shimadzu 260 spectrophotometer. The emission spectra were
recorded on a Perkin-Elmer LS-5B spectrophotometer. Glass
plates coated with silica gel-G (ACME) of about 0.25 mm
thickness were used for TLC and visualized with iodine.
Column chromatography was carried out with silica gel
1
cm¹1): 1720.6 (C=O); H NMR (300 MHz, CDCl3): ¤ 5.99 (s,
6H), 7.93-8.25 (m, 27H), 8.79 (s, 3H). 13C NMR (75 MHz,
DMSO-d6): ¤ 63.6, 121.6, 121.9, 122.1, 122.3, 122.7, 122.9,
130.3, 130.5, 130.9, 132.1, 132.3, 132.7, 132.8, 133.4, 133.5,
133.6, 133.9, 134.0, 163.9. Mass spectrum (FAB-MS): m/z 852
(M+). Elemental Anal. calculated for C60H36O6: C, 84.49; H,
4.25%. Found: C, 84.59; H, 4.17%.
Pyren-1-ylmethyl 4-Formylbenzoate (10). Following the
general procedure A, pyren-1-ylmethyl 4-formylbenzoate (10)
was obtained as a colorless solid from pyren-1-ylmethanol
(7) (0.48 g, 2.1 mmol) and 4-formylbenzoic acid (9) (0.26 g,
1.7 mmol). The ester 10 was chromatographed using hexane/
CHCl3 (3:2) as an eluent. Yield: 95%; mp: 136-138 °C.
1
IR (KBr, cm¹1): 1721.2, 1704.6 (C=O). H NMR (300 MHz,

P. Rajakumar et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 8 (2012)
909
CDCl3): ¤ 5.93 (s, 2H), 7.66-7.69 (d, 2H, J = 8.1 Hz), 7.85-
8.07 (m, 10H), 8.16-8.19 (d, 1H, J = 9.0 Hz), 9.85 (s, 1H).
13C NMR (75 MHz, CDCl3): ¤ 64.8, 121.6, 123.4, 123.5, 123.9,
124.5, 124.6, 125.1, 126.2, 126.8, 126.9, 127.2, 127.3, 128.3,
128.6, 129.1, 129.5, 130.1, 130.8, 133.9, 138.0, 164.4, 190.4.
Mass spectrum (EI, 70 eV): m/z 364 (M+). Elemental Anal.
calculated for C25H16O3: C, 82.40; H, 4.43%. Found: C, 82.34;
H, 4.50%.
alcohol 15 was chromotographed using hexane/CHCl3 (3:1)
as an eluent. Yield: 85%; mp: 136-138 °C. IR (KBr, cm¹1):
1735.4 (C=O). H NMR (300 MHz, CDCl3): ¤ 5.34 (s, 4H),
5.82 (s, 1H), 7.96-8.05 (m, 11H), 8.13-8.18 (m, 8H), 8.33-
8.36 (d, 2H, J = 9.3 Hz). 13C NMR (75 MHz, CDCl3): ¤ 64.7,
122.9, 123.6, 124.5, 124.6, 125.2, 125.5, 126.0, 127.3, 127.5,
127.6, 127.7, 127.8, 128.2, 129.5, 129.6, 130.6, 130.9, 131.3,
131.4, 131.8, 171.0. Mass spectrum (EI, 70 eV): m/z 610
(M+). Elemental Anal. calculated for C42H26O5: C, 82.61; H,
4.29%. Found: C, 82.66; H, 4.21%.
1
Pyren-1-ylmethyl 4-Hydroxymethylbenzoate (11). Fol-
lowing the general procedure B, pyren-1-ylmethyl 4-hydroxy-
methylbenzoate (11) was obtained as a colorless solid from
pyren-1-ylmethyl 4-formylbenzoate (10) (0.26 g, 0.7 mmol)
and NaBH4 (0.06 g, 1.5 mmol). The alcohol 11 was purified
by column chromatography using hexane/CHCl3 (2:3) as an
eluent. Yield: 89%; mp: 114-116 °C. IR (KBr, cm¹1): 1715.3
Nonaester 3. Following the general procedure A, nonaester
3 was obtained as a colorless solid from bis(pyren-1-ylmethyl)
5-hydroxyisophthalate (15) (0.53 g, 0.73 mmol) and benzene-
1,3,5-tricarboxylic acid (8) (0.05 g, 0.24 mmol). The nonaester
3 was recrystallized from CHCl3. Yield: 84%; mp: 168 °C;
1
1
(C=O). H NMR (300 MHz, CDCl3): ¤ 2.08 (bs, 1H), 4.77 (s,
IR (KBr, cm¹1): 1733.3, 1719.0 (C=O). H NMR (300 MHz,
2H), 6.06 (s, 2H), 7.25-7.34 (d, 2H, J = 8.1 Hz), 7.98-8.20
(m, 10H), 8.33-8.36 (d, 1H, J = 9.3 Hz). 13C NMR (75 MHz,
CDCl3): ¤ 64.6, 65.3, 122.9, 124.6, 124.9, 125.5, 125.6, 126.1,
126.5, 127.4, 127.8, 127.9, 128.3, 129.2, 129.7, 130.0, 130.7,
131.2, 131.8, 146.2, 166.4. Mass spectrum (EI, 70 eV): m/z
366 (M+). Elemental Anal. calculated for C25H18O3: C, 81.95;
H, 4.95%. Found: C, 82.01; H, 4.88%.
CDCl3): ¤ 5.30 (s, 12H), 7.91-8.01 (m, 38H), 8.08-8.13 (m,
22H), 8.29-8.32 (d, 6H, J = 9.3 Hz). 13C NMR (75 MHz,
CDCl3): ¤ 67.1, 113.1, 122.6, 123.5, 124.2, 124.9, 126.3,
126.4, 126.5, 126.7, 126.8, 127.3, 127.8, 127.9, 128.6, 128.7,
129.9, 130.2, 130.3, 130.4, 138.3, 160.9, 166.8. Mass spectrum
(FAB-MS): m/z 1986 (M+). Elemental Anal. calculated for
C135H78O18: C, 81.56; H, 3.95%. Found: C, 81.48; H, 4.03%.
Tris[4-(pyren-1-ylmethoxycarbonyl)phenylmethyl] Ben-
zene-1,3,5-tricarboxylate (2). Following the general proce-
dure A, tris[4-(pyren-1-ylmethoxycarbonyl)phenylmethyl] ben-
zene-1,3,5-tricarboxylate (2) was obtained as a colorless solid
from benzene-1,3,5-tricarboxylic acid (8) (0.05 g, 0.21 mmol)
and pyren-1-ylmethyl 4-hydroxymethylbenzoate (11) (0.23 g,
0.63 mmol). The ester 2 was chromotographed using hexane/
ethyl acetate (4:1) as an eluent. Yield: 85%; mp: 188-190 °C.
Pyrene-1-carboxylic Acid (17).
A 100 mL RB flask
charged with pyrene-1-carbaldehyde (1 g, 4.37 mmol), water
(16 mL), and pyridine (16 mL) was heated at 100 °C. A solution
of potassium permanganate (1 g, 6.45 mmol) in water (10 mL)
was added dropwise and the reaction mixture was heated at
100 °C for 12 h. The solvent was removed with a rotary evapo-
rator and then added water (100 mL) to the residue and was
heated on a water bath for 15 min and filtered. The filtrate was
acidified with concentrated HCl and the pyrene-1-carboxylic
acid (17) precipitated out was filtered, washed with water and
dried. Yield: 85%; mp: 270 °C (lit.33 268-269 °C).
Triester 4. Following the general procedure A, triester
4 was obtained as a pale yellow solid from benzene-1,3,5-triol
(16) (0.063 g, 0.65 mmol) and pyrene-1-carboxylic acid (17)
(0.4 g, 1.6 mmol). The ester 4 was chromotographed using
hexane/CHCl3 (3:2) as an eluent. Yield: 91%; mp: 180-182 °C.
IR (KBr, cm¹1): 1673.5 (C=O). 1H NMR (300 MHz, CDCl3): ¤
7.53 (s, 3H), 8.08-8.34 (m, 21H), 8.96-8.99 (d, 3H, J =
8.1 Hz), 9.45-9.48 (d, 3H, J = 9.3 Hz). 13C NMR (75 MHz,
CDCl3): ¤ 119.0, 122.6, 123.7, 124.2, 126.4, 126.6, 126.7,
126.9, 127.1, 129.0, 129.3, 129.5, 130.3, 130.5, 131.1, 131.9,
133.1, 156.9, 162.4. Mass spectrum (FAB-MS): m/z 810 (M+).
Elemental Anal. calculated for C57H30O6: C, 84.43; H, 3.73%.
Found: C, 84.55; H, 3.64%.
1
IR (KBr, cm¹1): 1677.0, 1718.0 (C=O). H NMR (300 MHz,
CDCl3): ¤ 5.32 (s, 6H), 6.01 (s, 6H), 7.36-7.39 (d, 6H,
J = 8.1 Hz), 7.94-8.31 (m, 36H). 13C NMR (75 MHz, CDCl3):
¤ 65.5, 66.6, 122.9, 124.6, 125.5, 125.6, 126.1, 127.4, 127.8,
128.0, 128.3, 128.8, 130.2, 130.7, 131.0, 131.2, 131.8, 140.4,
153.4, 156.4, 164.4, 166.1, 170.1. Mass spectrum (FAB-MS):
m/z 1255 (M+). Elemental Anal. calculated for C84H54O12: C,
80.37; H, 4.34%. Found: C, 80.29; H, 4.44%.
Bis(pyren-1-ylmethyl)
5-(tert-Butyldimethylsiloxy)iso-
phthalate (14). Following the general procedure A,
bis(pyren-1-ylmethyl) 5-(tert-butyldimethylsiloxy)isophthalate
(14) was obtained as a colorless liquid from 5 (tert-butyl-
dimethylsiloxy)isophthalic acid (13) (0.5 g, 1.7 mmol) and
pyren-1-ylmethanol (7) (0.78 g, 3.4 mmol). The ester 14 was
chromotographed using hexane/DCM (1:1) as an eluent. Yield:
87%; IR (KBr, cm¹1): 1727.2 (C=O). 1H NMR (300 MHz,
CDCl3): ¤ 0.19 (s, 6H), 0.96 (s, 9H), 5.67 (s, 4H), 7.83-8.09
(m, 19H), 8.14-8.17 (d, 2H, J = 9.3 Hz). 13C NMR (75 MHz,
CDCl3): ¤ ¹4.0, 16.3, 20.9, 64.2, 121.9, 122.2, 122.4, 122.6,
123.6, 123.9, 124.0, 124.2, 125.7, 126.1, 126.7, 126.8, 127.0,
127.6, 130.1, 130.4, 130.7, 130.9, 132.3, 168.0. Mass spectrum
(FAB-MS): m/z 724 (M+). Elemental Anal. calculated for
C48H40O5Si: C, 79.53; H, 5.56%. Found: C, 79.44; H, 5.62%.
Bis(pyren-1-ylmethyl) 5-Hydroxyisophthalate (15). Fol-
lowing the general procedure C, bis(pyren-1-ylmethyl) 5-
hydroxyisophthalate (15) was obtained as a colorless solid from
bis(pyren-1-ylmethyl) 5-(tert-butyldimethylsiloxy)isophthalate
(14) (2.0 g), acetone (100 mL), and 1 M HCl (13 mL). The
Trialdehyde 18.
Following the general procedure A,
trialdehyde 18 was obtained as a colorless solid from benzene-
1,3,5-triol (16) (0.5 g, 4.0 mmol) and 4-formylbenzoic acid (9)
(1.86 g, 12.4 mmol). The trialdehyde 18 was chromotographed
using hexane/CHCl3 (1:4) as an eluent. Yield: 81%; mp: 182-
184 °C. IR (KBr, cm¹1): 1618.4, 1676.6 (C=O). 1H NMR
(300 MHz, CDCl3): ¤ 7.25 (s, 3H), 8.03-8.06 (d, 6H, J =
7.8 Hz), 8.35-8.38 (d, 6H, J = 7.8 Hz), 10.16 (s, 3H). 13C NMR
(75 MHz, CDCl3): ¤ 113.3, 130.8, 133.7, 139.8, 151.3, 157.0,
163.4, 191.4. Mass spectrum (EI, 70 eV): m/z 522 (M+).
Elemental Anal. calculated for C30H18O9: C, 68.97; H, 3.47%.
Found: C, 68.90; H, 3.52%.

910
Bull. Chem. Soc. Jpn. Vol. 85, No. 8 (2012)
Synthesis of Pyrene-Based Ester Dendrimers for DSSCs
Triol 19. Following the general procedure B, triol 19 was
obtained as a colorless solid from trialdehyde 18 (0.52 g,
1.0 mmol) and NaBH4 (0.23 g, 6.0 mmol). The triol 19 was
purified by column chromatography using hexane/CHCl3 (2:3)
as an eluent. Yield: 68%; mp: 184-186 °C. IR (KBr, cm¹1):
4.1 © 10¹4 M of I2, and the pyrene based ester dendrimers
of 6.4 © 10¹6 M as additives in DMF (10 mL). The solar to
electric energy conversion efficiency was measured under
simulated solar light at 40 mW cm¹2. The photocurrent-photo-
voltage was measured using a BAS 100A Electrochemical
analyzer. The apparent cell area of TiO2 photoelectrode was
1 cm2 (1 cm © 1 cm).
1
1615.3 (C=O). H NMR (300 MHz, CDCl3): ¤ 3.62 (bs, 3H),
4.78 (s, 6H), 7.27 (s, 3H), 7.43-7.46 (d, 6H, J = 8.7 Hz), 8.02-
8.05 (d, 6H, J = 8.1 Hz). 13C NMR (75 MHz, CDCl3): ¤ 64.9,
126.4, 126.6, 128.5, 129.7, 130.1, 140.3, 157.5. Mass spectrum
(EI, 70 eV): m/z 528 (M+). Elemental Anal. calculated for
C30H24O9: C, 68.18; H, 4.58%. Found: C, 68.09; H, 4.66%.
Hexaester 5. Following the general procedure A, hexaester
5 was obtained as a pale yellow solid from triol 19 (0.10 g,
0.2 mmol) and pyrene-1-carboxylic acid (17) (0.15 g, 0.6
mmol). The hexaester 5 was chromatographed using hexane/
CHCl3 (3:2) as an eluent. Yield: 71%; mp: 180-182 °C; IR
(KBr, cm¹1): 1618.3, 1659.8 (C=O). 1H NMR (300 MHz,
CDCl3): ¤ 4.76 (s, 6H), 7.36 (s, 3H), 8.03-8.11 (m, 11H), 8.17-
8.20 (m, 11H), 8.24-8.28 (m, 11H), 8.69-8.72 (d, 3H, J =
8.1 Hz), 9.35-9.38 (d, 3H, J = 9.3 Hz). 13C NMR (75 MHz,
CDCl3): ¤ 66.1, 124.1, 124.2, 124.4, 124.8, 125.2, 126.0,
126.1, 126.2, 127.2, 128.8, 129.0, 129.4, 130.3, 130.9, 131.0,
134.1, 157.5, 170.2. Mass spectrum (MALDI-TOF): m/z 1212
(M+). Elemental Anal. calculated for C81H48O12: C, 80.19; H,
3.99%. Found: C, 80.25; H, 3.88%.
Hexasilyloxy Triester 21. Following the general proce-
dure A, hexasilyloxy triester 21 was obtained as a colorless
liquid from 3,5-di(tert-butyldimethylsiloxy)benzoic acid (20)26
(8.14 g, 21.3 mmol) and benzene-1,3,5-triol (16) (0.89 g, 7.1
mmol). The ester 2125 was chromatographed using hexane/
DCM (1:1) as an eluent. Yield: 79%.
Triester 22. Following the general procedure C, triester 22
was obtained as a pale yellow solid from hexasilyloxy triester
21 (7.5 g), acetone (150 mL), and 1 M HCl (275 mL). The
triester 22 was chromotographed using hexane/EtOAc (3:2) as
an eluent. Yield: 84%; mp: 226-228 °C (lit.25 225 °C).
The authors thank DST-FIST, New Delhi for NMR spectral
facility given to the Department. KTV thank CSIR for the
award of SRF.
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1615.0 (C=O). H NMR (300 MHz, DMSO-d6): ¤ 6.47-6.48
(t, 3H, J = 1.8 Hz), 7.04-7.05 (d, 6H, J = 2.4 Hz), 7.53 (s, 3H),
7.80-8.86 (m, 42H), 8.96-8.99 (d, 6H, J = 8.1 Hz), 9.34-9.37
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3.49%. Found: C, 81.45; H, 3.37%.
General Procedure for Dye-Sensitized Solar Cell Studies.
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