Synthesis, photophysical, electrochemical, and DSSC application of novel donor-acceptor triazole bridged dendrimers with a triphenylamine core and benzoheterazole as a surface unit

Article abstract of DOI:10.1039/C6NJ02126C

Triazole bridged novel donor-acceptor dendrimers with triphenylamine as the core and benzoheterazole as the surface unit have been synthesized by click chemistry via a convergent approach. All the dendrimers exhibited excellent optical and electrochemical

Full text of DOI:10.1039/C6NJ02126C

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Synthesis, photophysical, electrochemical and DSSC application of novel
donorꢀacceptor triazole bridged dendrimers with triphenylamine core and
benzoheterazole as surface unit
Kannan Rajavelu,a Perumal Rajakumar,*a Mandal Sudip b and Ramanujam Kothandaramanb
aDepartment of Organic Chemistry, University of Madras, Guindy Campus, Chennai 600 025,
Tamilnadu, India and bDepartment of Chemistry, Indian Institute of Technology Madras,
Chennai-600 036, India.
*Corresponding Author: Tel No: +91 44 2220 2810; EꢀMail: Perumalrajakumar@gmail.com
Abstract:
Triazole bridged novel donorꢀacceptor dendrimers with triphenylamine as core and
benzoheterazole as surface unit has been synthesized by click chemistry via convergent
approach. All the dendrimers exhibited excellent optical and electrochemical properties,
supported by DFT calculations. Lowerꢀgeneration dendrimers exhibit better current generating
capacity and also shows better power conversion efficiency than higherꢀgeneration dendrimers
when used as additives in dyeꢀsensitized solar cells.
Introduction
The development of new organic materials for the application in molecular organic
electronics,1 nanotechnology,2 and optoelectronic devices3 such as organic solar cells and
organic light emitting diodes have shown a remarkable scientific activity due to their relevance
in the generation and rational use of energy. Great challenge to the research community to
prepare low cost renewable energy sources could also find a niche as power supplies in the
emerging area of flexible optoelectronics. With more recent development, dendrimers are used as
hole transporting material in electronics. Dendrimers are mono dispersed macromolecules
1

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possessing wellꢀdefined structure that can be precisely tailored with discrete functionalities to
create multifunctional materials. These molecules, with highly branched globur threeꢀ
dimensional architecture, present multiple chains emanating from a single core. Challenges in the
field of dendrimers lie in the synthesis using modern techniques and most frequently in the
purification of the synthesized nano molecules. Dendrimers have been extensively used in
pioneering work by Vögtle, Tomalia, Fréchet and Newkome4. Dendrimers have received much
attention in the areas of material science and biological science such as in versatile biomimetic
catalysis, drug delivery systems,5 sensors, solar cell additives6 and efficient light harvesting
antenna, multivalent diagnostics for magnetic resonance imaging (MRI),7 molecular
encapsulation and in bioꢀconjugate chemistry.8 Recent work in this area focus on the synthesis of
new donorꢀacceptor dendrimers with triazole bridging unit. If such system has an efficient
electron transfer from donor to acceptor upon photoꢀexcitation of the sensitizer, the negative
charge accumulated on the acceptor can be injected into the conduction band (CB) of a metal
oxide semiconductor.9 Such supramolecular architecture with combined donor and acceptor
blocks can reduce band gap energy also. Triphenylamine is an excellent electronꢀdonating group
with aggregationꢀresistant and nonplanar molecular shape which makes it as an important donor
in many systems.10 Organic glasses can be derived from highly electron donating triphenylamine
derivatives.11 Many factor plays an important role in increasing the energy harvesting efficiency
of triphenylamine dendrimers by intramolecular charge transfer (ICT), 12 molecular coplanarity,
vibronic coupling, good stability to oxidation and charge carrier mobility. Triphenylamine
possesses a propeller shaped, electron rich structure13 which can maintain uninterrupted
conjugation with the help of lone pair of electrons at the central nitrogen atom, to enhance the
electronic communication along with macromolecular chains and make them to behave as strong
2

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electron donor, and radical cations that help to enhance electrical or photochemical stabilities for
charge transfer from the donor to acceptor. 14 Two electron acceptors can be introduced into the
organic donor viz: triphenylamine framework. The two electron acceptor groups can affect not
only the absorption spectra or molar extinction coefficients and also the HOMO and LUMO
levels of the sensitizers. Research efforts to increase the power conversion efficiency in DSSC
using nitrogen containing heterocycles such as pyrazole, imidazole, triazole, pyridine,
pyrimidine, and pyrazine as additives show an increase in the Voc value and improve the
efficiency.15 In 1991 Michael Gratzel and Brian O’Regan reported DSSC capable of 10% energy
conversion efficiency.16 Unlike ruthenium sensitizers N719 or N3 is commonly employed in
DSSCs and most of the triphenylamine dye possess one single group for TiO2 grafting. From the
literature survey, there is no report on the synthesis of dendrimers through click chemistry with
benzoheterazole unit as attractive acceptor at periphery with electron donating triphenylamine
unit as the central core in order to study their application in material chemistry and biology.
Thus, to fully explore the potential of the benzoheterazole17 unit, synthesis of such novel class of
molecule is highly desirable. Benzoheterazole has interesting biological applications such as
anticonvulsant, anticancer, antiꢀHIV, antidiabetic, antialzheimer, antihypertensive and
antioxidant properities.18 Nowadays, click chemistry has emerged as an efficient tool for the
rapid synthesis of triazole bridged dendrimers via Cu (I)ꢀcatalyzed Huisgen 1,3ꢀdipolar
cycloaddition introduced by Sharpless and coꢀworkers which has been widely used in the design
and synthesis of optoelectronic materials and affords a good way to achieve target molecules
under mild reaction conditions with high efficiency.19 Generally, click chemistry provides
excellent regioselectivity and high yield without any side reactions. Recently, we have used
dendrimers as additives to improve the efficiency of DSSCs. In our laboratory dendrimers with
3

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20
dimethyl isophthalate and pyrene21 at the periphery has been used as additive in DSSCs. The
present investigation mainly focus on the synthesis and photophysical, electrochemical
properties and DSSCs application of dendrimers 1ꢀ8 having benzoheterazole surface group
(Fig.1).
4

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Fig. 1 Molecular structure of benzoheterazole dendrimers 1ꢀ8
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Results and discussion
In order to explore the utility of benzoheterazole as a surface unit in dendrimers, the
dendritic wedge 13 and 14 were synthesized as outlined in Scheme 1. Our synthesis starts from
the reaction of 4ꢀmethyl cinnamic acid with 2ꢀaminophenol /2ꢀamino thiophenol in POCl3 at 100
oC for 5 h to give 2ꢀ[4ꢀ(methyl)styryl]benzoxazole 9 and 2ꢀ[4ꢀ(methyl)styryl]benzothiazole 10 in
1
85% and 90% yields, respectively. In the H NMR spectrum 2ꢀ(4ꢀ(methyl)styryl)benzothiazole
10, the olefinic hydrogens are cis to each other due to twisted intramolecular charge transfer
from donor 4ꢀmethylbenzene and acceptor benzothiazole and also hydrogen bonding interaction
in the presence of polar solvents changes the geometry from trans to cis form in benzothiazole.22
The bromides 11 and 12 were obtained in 65% and 70% yields by the reaction of 2ꢀ[4ꢀ
(methyl)styryl]benzoheterazole 9 and 10 with NBS. Azidation of 11 and 12 with 1.2 equiv. of
o
NaN3 in a mixture of acetone/H2O (4:1) at 60 C for 6 h gave the azides 13 and 14 in 91% and
89% yields, respectively.
We then focused our attention on the synthesize of the first generation dendritic
chlorides, the building unit viz. 3,5ꢀbis(propargyloxy)benzyl chloride 16, was obtained as
follows: the bisꢀpropargyl derivative 15 was obtained by the reaction of methyl 3,5ꢀ
dihydroxybenzoate with propargyl bromide in dry DMF in the presence of K2CO3 followed by
reduction with LiAlH4 to give the corresponding alcohol which was then reacted with SOCl2/ Py
in DCM to give 3,5ꢀbis(propargyloxy)benzyl chloride 16. Reaction of the chloride 16 with 2.1
equiv. of 2ꢀ[4ꢀ(azidomethyl)styryl]benzoheterazole 13 and 14 under the click reaction conditions
viz., CuSO4.5H2O and sodium ascorbate in a 1 : 1 mixture of tꢀBuOH and water at room
temperature afforded first generation dendritic chlorides (G1ꢀCl) 17 and 18 in 90% and 87%
1
yields, respectively. In the H NMR spectrum dendritic chloride 17 displayed three sets of
6

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singlets at δ 4.47, 5.16 and 5.55 for benzylic chloromethane, Nꢀmethylene and Oꢀmethylene
protons respectively, in addition to the signals for the aromatic protons. In the 13C NMR
spectrum 17 showed signals at δ 46.0, 53.8 and 62.1 for benzylic chloromethane, Nꢀmethylene
and Oꢀmethylene carbons respectively, in addition to the signals for the aromatic carbons. The
dendritic chloride 17 and 18 was further converted into the first generation dendritic azides (G1ꢀ
N3) 19 and 20 in 87% and 86% yields, respectively by the reaction of the chloride 17 and 18
with 1.2 equiv. of NaN3 in DMF at room temperature for 24 h.
o
Scheme 1: (i) 1.2 equiv. NꢀBromosuccinimide, benzoyl peroxide, benzene, 60 C, 8 h. (ii) 1.5
o
equiv. sodium azide, acetone/water (4:1), 60 C, 4 h. (iii) LAH, THF, rt then followed by
SOCl2/Py. (iv) CuSO4.5H2O (5 mol %), NaAsc (10 mol %), THF/H2O (1:1), rt, 12 h. (v) 1.2
equiv. sodium azide, DMF, rt, 24 h.
The core unit diyne 25 and triyne 26 can be synthesized by multiꢀstep procedure as
reported in the literature23.via simple bromination of triphenylamine with 2.1/3.1 equivalents of
Nꢀbromosuccinimide in DMF followed by Sonogashira coupling of 3.0/4.5 equivalents of
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trimethylsilylacetylene in a mixture of THF/Et3N to give the bis silylated acetylene 23 and tris
silylated acetylene 24 in 70% and 82% yields, respectively. Removal of TMS protecting group
using K2CO3 in methanol gave the diyne 25 and the triyne 26 in 90% and 80% yields,
respectively (Scheme 2).
R
N
R
N
R
N
i
ii
Br
Br
Me3Si
SiMe3
21, R = H
22, R = Br
23, R = H
24, R =
25, R = H
26, R =
SiMe3
Scheme 2: (i) 3.0 equiv. / 4.5 equiv. trimethylsilylacetylene, 6% [PdCl2(PPh3)2], 3% PPh3, 3%
CuI, Et3N/THF, 90 oC. (ii) Anhydrous potassium carbonate, methanol, rt, 24 h.
Treatment of 1.0 equiv. of diyne 25 with 2.1 equiv. of 2ꢀ(4ꢀ
(azidomethyl)styryl)benzoheterazole 13 and 14 in the presence of CuSO4.5H2O (5 mol%) and
sodium ascorbate (10 mol%) in a mixture of THF and H2O (1:1) at room temperature for 12 h
1
afforded dendrimers 1 and 2 in 65% and 70% yields, respectively (Scheme 3). In the H NMR
spectrum, dendrimer 1 showed a four proton singlet at δ 5.60 for Nꢀ methylene proton and two
proton singlet at δ 7.63 for triazole proton in addition to the signals for aromatic protons. In the
13C NMR spectrum, dendrimer 1 displayed Nꢀmethylene carbon at δ 53.8 in addition to the
signals for the aromatic carbons. In the ESI mass spectrum dendrimer, 1 showed a molecular ion
peak at m/z 846 [M + H]+. Similarly, the structure of the zeroth generation dendrimer 2 was also
confirmed from spectral and analytical data.
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Scheme 3: (i) CuSO4.5H2O (5 mol%), NaAsc (10 mol%), THF/H2O (1:1), rt, 12 h.
In order to synthesize the first generation dendrimers 3 and 4, 1.0 equiv. of the diyne 25
was reacted with 2.1 equiv. of each dendritic azides 19 and 20 in 5 mol% CuSO4.5H2O with 10
mol% sodium ascorbate in a mixture of DMF and H2O (4:1) for 12 h at room temperature to
afford the dendrimers 3 and 4 in 58% and 53% yields, respectively (Scheme 3). In the 1H NMR
spectrum, dendrimer 3 displayed three singlets at δ 5.12, 5.53 and 5.62 for two types of benzylic
Nꢀmethylene protons and Oꢀmethylene proton and two signals at δ 8.30, 8.53 for the triazole
protons of two types of triazole ring systems viz: outer, at the dendritic wedge and inner, near the
13
core unit respectively. In the C NMR spectrum, dendrimer 3 displayed three signals at δ 52.5,
61.5 and 79.1 for two type’s benzylic Nꢀmethylene carbons and Oꢀmethylene carbon in addition
to the signal for the aromatic carbons. The ESI mass spectrum of 3 showed the molecular ion
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peak at m/z 1881 [M + H]+. Similarly the structure of the first generation dendrimer 4 was also
confirmed from spectral and analytical data.
The triyne 26 was then utilized for the synthesis of dendrimers 5, 6, 7 and 8. Reaction of
3.1 equiv. of the dendritic wedge 13 with 1 equiv. of the triyne 26 under click chemistry
conditions gave the dendrimer 5 in 60% yield (Scheme 4). In the 1H NMR spectrum, dendrimer
5 showed a six proton singlet at δ 5.62 for Nꢀ methylene protons and a three proton singlet at δ
7.80 for triazole protons in addition to the signals for the aromatic protons. In the 13C NMR
spectrum, dendrimer 5 displayed Nꢀmethylene carbon at δ 52.6 in addition to the signals for the
aromatic carbons. The mass spectrum of 5 showed a molecular ion peak at m/z 1146 [M + H]+.
Similar method was employed for the synthesis of dendrimer 6, 7 and 8 in 67%, 55% and 50%
yields from 1 equiv. of the triyne 26 with 3.3 equiv. of the dendritic wedge 14, 19 and 20,
1
respectively. In the H NMR spectrum, dendrimer 8 displayed three singlets at δ 5.13, 5.54 and
5.61 two types of benzylic Nꢀmethylene protons and Oꢀmethylene protons, a singlet
corresponding to six protons at δ 7.59 for the triazolyl protons at the dendritic wedge adjacent to
the benzothiazole unit and a singlet corresponding to three protons at δ 8.53 for the triazolyl
protons near to the core unit in addition to the signals for aromatic protons. In the 13C NMR
spectrum, dendrimer 8 displayed three signals at δ 52.5, 61.1 and 79.0 for two types of Nꢀ
methylene and one type of Oꢀmethylene carbons in addition to the signals for aromatic carbons.
The ESI mass spectrum of 8 showed the molecular ion peak at m/z 2796 [M + H]+. The
elemental composition also supported the molecular formula of the dendrimer 8. Similarly,
spectroscopic and analytical methods were employed to confirm the structure of the dendrimers
6 and 7.
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Scheme 4: (i) CuSO4.5H2O (5 mol%), NaAsc (10 mol%), DMF/H2O (4:1), rt, 24 h.
Investigation on cisꢀtrans isomerization of benzothiazole in polar solvents
The presence of double bond incorporated between the electron donor and electron
acceptor can lead to trans to cis isomerization due to twisted intramolecular charge transfer
(TICT) state formation. The transꢀ2ꢀ[4ꢀ(methyl)styryl]benzothiazole 10 is a pushꢀpull aromatic
olefin, which has benzothiazole ring (acceptor) on one side of the olefin bond and 4ꢀmethyl
benzene ring (donor) on the other side. The donor and acceptor groups are perpendicular to each
other in molecular frame. Fayed et al24 first reported dimethylaminostyrylbenzothiazole
(DMASBT) based intramolecular charge transfer in donor and acceptor system evidenced by the
increase in the dipole moment. Saha et al.25 also reinvestigated the structure of transꢀ2ꢀ[4ꢀ
(Dimethylamino)styryl]benzothiazole both experimentally and theoretically proposed that
DMASBT emits from TICT state in polar solvents. To determine the high dipole moment in
excited state with twisting of 4ꢀmethyl benzene ring and thereby getting stabilized in polar
solvents (Chloroformꢀd, Dimethyl sulfoxideꢀd6) the trans geometry changes to cis form due to
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the interaction of the lone pair of electrons on the nitrogen atom of benzothiazole unit resulting
in hydrogen bonding with polar solvents (Fig. 2). The dipole moment of their excited state is
greater than that of the ground state. Theoretical calculations clearly suggest that HOMO is more
localized on the donor 4ꢀmethyl benzene ring and the LUMO is localized on the rest of the
benzothiazole and there is a minimum overlap between these molecular orbitals. The DFT
calculations predict a rotational barrier of 0.34 eV for the conversion of one geometrical isomer
to the other as reported earlier by krishnamoorthy et al22. The optimized geometrical parameters
for both the cis and trans isomers of 10 are compiled in Table 1. The trans form of benzothiazole
unit is most stable at room temperature. Further, energy minimization study was carried out and
the energy difference between the cis and trans isomer is ꢀ6.06 kcal/mol. The energy value of
trans is higher than cis value proving that the trans isomer is more stable compared to the cis
isomer. This result clearly reveals that the olefinic double bond is only trans, however gets
converted to cis form for benzothiazole system alone in the presence of polar solvents.
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Fig. 2 Shows geometrical changes trans to cis form of benzothiazole group in 1H NMR spectrum
(300 MHz, DMSOꢀd6).
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Table 1 Frontier molecular energy levels, energy gaps and dipole moments of benzoheterazole
molecule.
Molecule
HOMO (eV)
LUMO (eV)
Eg (eV)
Dipole moment
µ (Debye)
2.58
9 (cis form)
ꢀ6.00
ꢀ5.91
ꢀ6.08
ꢀ5.90
ꢀ2.05
ꢀ2.11
ꢀ2.01
ꢀ2.18
3.95
3.80
4.07
3.73
9 (trans form)
10 (cis form)
10 (trans form)
2.42
2.33
1.99
Photophysical studies
The UVꢀvis absorption and emission spectra of dendrimers 1ꢀ8 were recorded in DMF at
room temperature and the λmax values are summarized in Table 2. Dendrimers 1ꢀ8 exhibited
strong absorption bands in the range of 324ꢀ346 nm. The absorption maxima observed for all the
dendrimers could be due to πꢀπ* transition. The absorption band could be due to the presence of
benzoheterazole and donor triphenylamine unit in the molecular system. In fact this observation
suggests that both the chromophores involving benzoheterazole and triphenylamine could be
responsible for the broad absorption bands for the dendrimers 1ꢀ8. Molar absorption coefficient
increases for the bis substituted dendritic wedges with benzoxazole and benzothiazole group as
the dendrimer generation increases. The benzoxazole and benzothiazole dendrimers 3 and 4
shows an increase in molar absorption coefficient than that of the zeroth generation benzoxazole
and benzothiazole dendrimers 1 and 2 due to valence effect26. However in the case of tris
substituted dendritic wedges the molar absorption coefficient decreases for both the benzoxazole
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and benzothiazole dendrimer as the dendrimer generation increases from the zeroth to first
generation. The molar absorption coefficient of tris substituted benzoxazole dendrimer 7 and tris
substituted benzothiazole dendrimer 8 shows decreases in molar absorption coefficient which
could be due to excess molecular crowding in tris substituted dendritic wedges. In the
fluorescence spectra, the emission values were obtained by exciting all the dendrimers 1ꢀ8 at the
corresponding absorption maximum. The dendrimers 1ꢀ8 exhibited strong florescence emission
in the region of 388ꢀ414 nm and the λem values for the fluorescence spectra are shown in Table
2. The emission spectra showed that the benzoheterazole dendrimers show gradual decrease in
emission intensity as the dendrimer generation increases. The decreasing fluorescence intensity
is due to fluorescence quenching with increase in number of benzoheterazole unit for selfꢀ
quenching and the enhancement of nonꢀradioactive transitions.27
Table 2 Photo physical properties of dendrimers 1ꢀ8 in DMF (1 X 10ꢀ3 mol Lꢀ1)
Dendrimer
λ
max (nm)
ε (mol/L)
λem (nm)
1
2
3
4
5
6
7
8
324
344
328
346
328
345
327
339
571
566
695
675
645
730
625
620
407
405
388
401
406
406
414
403
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Electrochemical studies
The electrochemical property of the benzoheterazole dendrimers 1ꢀ8 was studied by
recording cyclic voltammetry in DMF at room temperature at a scan rate of 100 mV/s with nꢀ
Bu4NPF6 as supporting electrolyte, Glassy carbon electrode as working electrode, Ag/AgCl as
reference electrode and Pt electrode as counter electrode. All the synthesized benzoheterazole
dendrimers exhibited electrochemical response in cyclic voltammetry and the electrochemical
parameters are shown in Table 3. The redox behavior of dendrimers 1, 3, 5, 7 and 2, 4, 6, 8 are
shown in Fig. 3a and 3b. Oxidation and reduction potential for all the dendrimer appeared in the
potential range of ꢀ2.0 to +2.0 V. In the cyclic voltammetry two oxidation peaks appeared
Fig. 3 Cyclic voltammogram of dendrimers a) 1, 3, 5, 7 b) 2, 4, 6, 8 in DMF (1 X 10ꢀ3 mol Lꢀ1),
scanned at 100 mV sꢀ1
between ꢀ1.51 to 1.34 V and two reduction peaks appeared between ꢀ0.73 to ꢀ1.55 V for all the
dendrimers. Oxidation and reduction peaks for all dendrimers 1ꢀ8 were attributed to the presence
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of strong electron donating triphenylamine and electron withdrawing benzoheterazole groups.
All the dendrimer exhibited the first oxidation potential, which is attributed to the oxidation of
the triphenylamine core which is quasi reversible28 and benzoheterazole can undergoes quasi
reversible single electron transfer process in cathodic scan direction. From the cyclic
voltammetry in Fig. 3a it is clear that on increasing the number of benzoxazole at periphery, the
anodic peak potential which is shifted towards slightly higher positive potential in benzoxazole
dendrimer 1, 3, 5 and 7 is due to the presence of triazole and heterocyclic ring. Dendrimers 1, 3,
5 and 7 showed a gradual increase is Epa from 1.20, 1.34, 1.23 and 1.34 V and Epc increases
slightly from ꢀ0.73, ꢀ0.76, ꢀ0.75 and ꢀ0.81 V. For the benzothiazole dendrimers 2, 4, 6 and 8
there is no potential shift upon increasing the generation growth and anodic peak potential
abruptly decreases which could be intramolecular interactions within benzothiazole moiety. The
oxidation and reduction potential of the dendrimers from cyclic voltammetry provides strong
evidence for the utility of the synthesized molecules in DSSC application.
Table 3 Electrochemical parameters of dendrimers 1ꢀ8 in DMF (1 X 10ꢀ3 mol Lꢀ1)
dendrimer
Epa1 (V)
-0.57
-0.57
-0.55
-0.53
-0.53
-0.55
-0.52
-0.51
Epa2 (V)
1.20
1.23
1.34
1.22
1.23
1.14
1.34
1.26
Epc1 (V)
-0.73
-0.75
-0.76
-0.76
-0.75
-0.78
-0.81
-0.79
Epc2 (V)
-1.55
-1.48
-1.48
-1.44
-1.54
-1.53
-1.54
-1.47
∆Ep1(V)
0.16
0.18
0.21
0.23
0.22
0.23
0.29
0.28
∆Ep2 (V)
2.75
1
2
3
4
5
6
7
8
2.71
2.82
2.66
2.77
2.67
2.88
2.73
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Theoretical calculations
In order to characterize the optical and electronic properties, it is essential to examine the
HOMOs, LUMOs, energy gaps and oscillator strength. To gain insight into the molecular
structures and electron distribution, the geometries of the triphenylamine containing
benzoheterazole unit were fully optimized and the density functional theory calculations was
carried out employing the B3LYP/6ꢀ31G level with Gaussian 09 programme package.29 Effect of
the solvents was simulated by employing DFT calculations using the polarizable continuum
model (PCM). As shown in Fig. 4, for the zeroth generation dendrimers 1, 2, 5 and 6 the excited
state LUMO was only localized on a heterocyclic ring and also self charge transfer by the
benzoheterazole group is observed. From the HOMO level of the zeroth generation dendrimers
1, 2, 5 and 6, electron density is homogeneously distributed on the electron donor triphenylamine
and triazole as linker system. However in the case of dendrimer 1 and 2, the electron density is
distributed to the withdrawing group of phenyl ring due to the conjugation between the phenyl
rings in the core unit. It may imply that the planar benzoheterazole blocked by triphenylamine
can make electron distribution of HOMO to the electron distribution of the LUMO. The well
overlapped HOMO and LUMO orbitals on the linker triazole unit suggest the inductive or
withdrawing electron tendency from triphenylamine donor unit to the benzoheterazole unit.
Thus, the HOMO and LUMO excitation induced by light irradiation could move the excited
electron distribution or charge transfer across the triazole bridge from donor unit to the acceptor
unit. 30 The oscillator strength can also prove the charge transfer from donor to the periphery and
the oscillator strength, excitation energy values support this observation. The frontier molecular
orbital of the zeroth generation dendrimers reveals that HOMOꢀLUMO excitation moves the
electron density distribution from the donor triphenylamine to the acceptor benzoheterazole at
the periphery through triazole bridging unit (Table 4).
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Dendrimer
Optimized structure
HOMO
LUMO
1
2
5
6
Fig. 4 Frontier molecular orbitals (HOMO and LUMO) of benzoheterazole dendrimer 1, 2, 5 and
6 along with the orbital energies obtained at B3LYP/6ꢀ311+G(d,p)//6ꢀ31G(d,p).
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Table 4 Orbital energy levels of benzoheterazole dendrimer 1, 2, 5 and 6
Dendrimer
EHOMO (eV)
ELUMO (eV)
Eg (eV)
ꢀ5.02
ꢀ5.02
ꢀ5.00
ꢀ4.99
ꢀ2.09
ꢀ2.13
ꢀ2.08
ꢀ2.13
2.93
2.89
2.92
2.86
1
2
5
6
Dye Adsorption on TiO2 surface
The role of adsorption in device performance is based on the molecular volumes of the
dendrimers. In donorꢀacceptor benzoheterazole dendrimers of higher generation the low
efficiency is due to the total amount of dye absorbed on TiO2 surface31 which have a major
influence on the solar cell performance and the summarized data are listed in Table 5. The higher
generation dendrimers 3, 4, 7 and 8 can be bind with TiO2 surface and hence molecular volume
of dendrimer increases and also amount of dye absorbed on TiO2 surface will play important role
in solar cell performance in DSSCs. When the generation of the benzoheterazole dendritic
acceptor increases the molecular volume of the dyes increase about 2 to 3ꢀfold axis. It is clear
that molecular volume or bulkiness of the acceptor moiety plays an important role in the dye
uptake.32 Upon increasing from lower to higher generation the molecular volume of acceptor unit
increases and the amount of dye uptake decreases in benzoxazole dendrimers with the following
order 1 (3.5 × 10ꢀ6 mol cm−2) > 5 (3.4 × 10ꢀ6 mol cm−2) > 3 (3.1 × 10ꢀ6 mol cm−2) > 6 (3.0 ×10ꢀ6
mol cm−2) and also in the case of benzothiazole dendrimers the dye uptake gradually decrease in
the following order 2 (3.3 × 10ꢀ6 mol cm−2) > 6 (3.0 × 10ꢀ6 mol cm−2) > 4 (3.0 × 10ꢀ6 mol cm−2) >
8 (3.0 ×10ꢀ6 mol cm−2). As a result in the higher generation dendrimers 3, 4, 7 and 8 the amount
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of dye adsorption on TiO2 surface is directly proportional to the high molecular volume of
acceptor unit and weak intermolecular πꢀπ interaction.
DSSC studies
Dye sensitized solar cells appear to be highly promising alternatives to more expensive
solar cell technologies. The organic nitrogenous compounds have influence on the efficiency of
DSSCs which can be explained by the shift of the conduction band potential and the
improvement of solar cell performance by the addition of dendrimer additive to the redox
Fig. 5 Currentꢀvoltage curves of DSSCs with dendrimer additives (a) 1, 3, 5, 7 (b) 2, 4, 6, 8 in
both cases experiments were conducted under simulated solar light at 70 mW cmꢀ2.
electrolyte.33 Hence, we have planned and synthesized a new type of donorꢀacceptor triazole
dendrimers 1ꢀ8 and employed them as additives in DSSCs. Fig. 5a and 5b shows the currentꢀ
voltage (JꢀV) curves for the undoped electrolyte system and the doped electrolyte system with
the benzoheterazole dendrimers 1ꢀ8. The photovoltaic performance of shortꢀcircuit current
density (Jsc), the openꢀcircuit voltage (Voc), and the fill factor and the electric energy efficiency
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(η) are summarized in Table 5. The present investigations mainly focus on the newly
synthesized donorꢀacceptor triazole bridged dendrimers and the highest power conversion
efficiency of 8.02 was observed when the dendrimer 1 was used as an additive in the DSSC. In
donorꢀacceptor dendrimer system on increasing the generation the power conversion efficiency
(η) decreases due to aggregation of the dendrimers on the TiO2 surface which enhance
considerably the degree of charge recombination between the injected electrons and the oxidized
dendrimers.34 The variation of Voc with increase in generation is mostly associated with charge
recombination between the injected electrons in the positive shift of the conduction band and
electron acceptor in the electrolyte interface.35 The open circuit current density increases with
increasing generation and the power conversion efficiency gradually decreases with benzoxazole
dendrimers 1, 3, 5 and 7 when used as additive which could be due to back electron transfer to
the semiconductorꢀelectrolyte interface.36 In the case of benzothiazole dendrimers 2, 4, 6 and 8
the Voc decreases with increasing generation and the power conversion efficiency increases due
to cation regeneration and reduce the back current, delocalizing πꢀelectrons on the conjugated
donor moiety and the bridged triazole can generate more excited electrons under the same light
excitation.37 It is very clear that the photovoltaic performances of the DSSCs can be affected by
increasing the generation growth. The number of acceptor unit increases while increasing the
generation and the secondary transition progressively gains intensity and shifts to lower energy
due to charge transfer from the central donor unit to the two distinct couples of branches which
are simultaneously involved in the excitation. An increased dark current by planar organic
molecules has been observed by Snaith et al.38 The formation of πꢀstacked aggregation and selfꢀ
quenching are the major factors for low conversion efficiency of DSSC.39 The aggregation of the
dendrimers may lead to intermolecular quenching or molecules residing in the system that are
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not attached to the semiconductor surface.40 Therefore, when the dendrimers are anchored on the
surface of TiO2, the LUMO centered on the anchoring moiety should enhance the orbital overlap
with the titanium 3d orbital and subsequently favor the electron injection to the conduction band
of TiO2.
Table 5 Photovoltaic performances of DSSCs with doped as well as undoped benzoheterazole
ꢀ
dendrimers 1ꢀ8 in the electrolyte system (Iꢀ/I3 ) under illumination of 70 mW cmꢀ2 at AM 1.5
with N719 dye.
DSSC configurations
Fill
Factor
(FF)
0.39
0.45
0.67
0.59
0.45
0.59
0.70
0.32
0.67
VOC
JSC
(mA cmꢀ2
Efficiency
(ɳ) (%)
Dye loading
(moles cmꢀ2)
(mV)
TiO2/N719/LiI/I2/Pt
TiO2/N719/LiI/I2/1/Pt
TiO2/N719/LiI/I2/2/Pt
TiO2/N719/LiI/I2/3/Pt
TiO2/N719/LiI/I2/4/Pt
TiO2/N719/LiI/I2/5/Pt
TiO2/N719/LiI/I2/6/Pt
TiO2/N719/LiI/I2/7/Pt
TiO2/N719/LiI/I2/8/Pt
792
764
791
801
774
745
731
811
757
8.18
16.20
5.94
3.60
8.02
4.51
3.97
6.80
7.63
5.80
3.22
5.70
3.2 x 10ꢀ6
3.5 x 10ꢀ6
3.3 x 10ꢀ6
3.1 x 10ꢀ6
3.0 x 10ꢀ6
3.4 x 10ꢀ6
3.0 x 10ꢀ6
2.9 x 10ꢀ6
3.0 x 10ꢀ6
5.85
14.19
12.21
7.87
8.95
7.87
Electrochemical impedance spectroscopy
Electrochemical impedance spectroscopy (EIS) is performed to elucidate the interfacial
charge recombination processes in DSSCs based on the eight dendrimer additives under dark
condition. The Nyquist plot for dendrimer 1ꢀ8 are shown in Fig. 6a and 6b and the
corresponding data are listed in Table 6. The Nyquist plots have two semicircles and the larger
semicircle at lower frequencies indicates charge recombination resistance at the
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TiO2/dendrimer/electrolyte interface, while the smaller semicircle at higher frequencies
corresponds to the charge recombination processes at the Pt/electrolyte interface. The large
semicircles, which indicate that charge transfer behavior between TiO2 and dendrimer
electrolytes was significantly due to the increasing surface moiety for higher generation
dendrimer.41 As shown in Fig. 6a and 6b that the radius of the larger semicircle increases for
lower generation dendrimers in the order of 1 > 5 > 6 > 2 indicating that the charge
recombination resistance gradually increases in zeroth generation dendrimer compared with that
of the higher generation dendrimer. The RCT values of benzoxazole dendrimer shows charge
transfer resistance for dendrimers in the following order 1 (71.2 ꢁ) >5 (56.5 ꢁ) > 3 (38.6 ꢁ) > 7
(16.2 ꢁ) and also the benzothiazole dendrimer exhibited charge transfer resistance in the
following order 2 (13.5 ꢁ) >6 (25.3 ꢁ) > 4 (46.4 ꢁ) > 8 (21.7 ꢁ). The low RCT value indicates
the high charge loss in the TiO2/electrolyte interface and also faster charge recombination and a
larger dark current.42 The large value of RCT indicates more effective suppression of the
ꢀ
recombination of the injected electron with Iꢀ/I3 in the electrolyte.43 Zeroth generation
dendrimers 1, 2, 5, and 6 shows a significantly larger charge recombination resistance (RCT)
value than the higher generation dendrimers, implying small charge recombination at the
TiO2/electrolyte interface, with a consequent high Voc for the lower generation dendrimers with
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Fig. 6 Electrochemical impedance spectra (Nyquist plot) of the DSSCs a) 1, 3, 5, 7 b) 2, 4, 6, 8
fabricated using with dendrimers measured under dark conditions.
the following order of decreasing Voc is 1 (764 mV) > 5 (745 mV) > 6 (731 mV). The
recombination rate caused by backward electron transfer was estimated using the Bodeꢀphase
spectra for the DSSCs, as shown in Fig. 7a and 7b and the electron lifetime calculated are shown
in Table 6. The EIS bode plot exhibits two Bode frequency peaks for electron transfer at the
TiO2/dendrimer/electrolyte interface and redox charge transfer at the counter electrode. The peak
frequency at the lower frequency region is related to the charge recombination rate and the
reciprocal is regarded as electron lifetime. The peak at lower frequency corresponds to the large
semicircle in the Nyquist plot of dendrimer which is shifted to higher frequency when the
molecular volume of higher generation dendrimer increases. On increasing the dendritic
Fig. 7 Electrochemical impedance spectra (Bode plot) of the DSSCs a) 1, 3, 5, 7 b) 2, 4, 6, 8
fabricated using with dendrimers measured under dark conditions.
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generation the electron lifetime gradually decrease44 which shows that dendrimer 2 (4.8 ms) > 6
(2.2 ms) > 4 (1.6 ms) > 8 (1.4 ms). For the benzothiazole dendrimers 2, 4, 6 and 8, the electron
−
recombination to the I3 ions enhance the electron lifetime to confirm the better DSSCs
−
performance. The high molecular volume of dendrimers 3, 4, 7 and 8 could not prevent the I3
ions reaching the TiO2 surface, thus leading to the enlarged charge recombination or dark current
and shortened electron life time.45 It is believed that with maximum amount of dye adsorbed on
TiO2 film and more number of benzoheterazole units in higher generation appear to raise the
chances of the electrolyte being close to the TiO2 surface.
Table 6 Parameters obtained by fitting the electrochemical impedance spectra of the DSSCs with
dendrimers 1ꢀ8.
dendrimers
RS (ꢁ)
RCT1 (ꢁ) RCT2 (ꢁ)
C1 (ꢂF)
C2 (ꢂF)
τn (ms)
18.6
21.9
22.7
19.4
20.3
19.2
20.0
18.0
23.3
6.94
71.2
13.5
38.6
46.4
56.5
25.3
16.2
21.7
8.14
18.2
10.2
18.6
32.8
35.1
36.6
13.2
31.8
6.85
5.61
15.4
4.33
7.16
9.04
5.74
2.91
15.6
516
76.1
473
4.2
1.3
4.8
1.4
1.4
1.5
2.2
4.3
1.6
TiO2/N719/LiI/I2/Pt
TiO2/N719/LiI/I2/1/Pt
TiO2/N719/LiI/I2/2/Pt
TiO2/N719/LiI/I2/3/Pt
TiO2/N719/LiI/I2/4/Pt
TiO2/N719/LiI/I2/5/Pt
TiO2/N719/LiI/I2/6/Pt
TiO2/N719/LiI/I2/7/Pt
TiO2/N719/LiI/I2/8Pt
75.6
43.8
45.3
61.7
330
51.9
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Conclusions
In conclusion, triphenylamine cored dendrimers with benzoheterazole surface unit and
triazole bridging unit were synthesized in high yields by convergent synthetic strategy via click
chemistry approach. All the dendrimers exhibited excellent optical and electrochemical
properties. The cyclic voltammetric studies reveal that the triphenylamine core unit showed
quasireversible behavior. Zeroth generation dendrimers 1 and 5 exhibited better power
conversion efficiency of 8.0 % and 7.6 %, respectively, when used as additives in DSSC than
the control (N719 dye/LiI/I2) as well as other benzoheterazole dendrimers are used as additives.
DSSCs studies clearly revealed that as the number of benzoheterazole and triazole units
increases from the zeroth to the first generation dendrimers, the DSSCs performance decreases
due to aggregation and back electron transfer to the conduction band of TiO2. The density
functional theory is also supported by optical, electrochemical as well as DSSCs studies. Further
studies to increases the efficiency in DSSCs with benzoheterazole dendrimers as additives is
underway.
Experimental Section
General Information
All the reagents and solvents were obtained from Sigma Aldrich, Alfa Aesar, Merck and
Avra chemical companies. The known products were identified by the comparison of their
melting points and spectral data is already reported in the literature. Melting points were
uncorrected and were determined using Toshniwal melting point apparatus by the open capillary
tube method. The UVꢀVis spectra were recorded on a Hitachi Uꢀ3210 spectrophotometer. The
emission spectra were recorded on a HORIBA JOBIN YVON Fluoromaxꢀ4 spectrophotometer.
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1
The H and 13C NMR spectra were recorded on a Bruker 300 MHz NMR spectrometer. The
chemical shifts are reported in ppm (δ) with TMS as an internal standard and the coupling
constants (J) are expressed in Hz. Elemental analyses were performed on a PerkinꢀElmer 240B
elemental analyzer. Electrochemical studies were carried out on a CH Instrument
electrochemical analyzer. Electrospray ionization mass spectrometry (ESIꢀMS) spectra were
measured with a LCQ AD mass spectrometer. The Gaussian 09 package was used for Density
functional theory (DFT) calculations and their using the polarizable continuum model (PCM).
Currentꢀvoltage (IꢀV) measurement was performed using Metrohm Autolab 302N
potentiostat/galvanostat with FRA32M module controlled by NOVA 1.10 software.
Fabrication and Characterization of DSSCs
The nanocrystalline TiO2 photoelectrode was prepared as described earlier.46 The
prepared TiO2 electrodes were immersed in a 1mM solution of the sensitized dye Diꢀ
tetrabutylammonium cisꢀbis(isothiocyanato)bis(2,2′ꢀbipyridylꢀ4,4′ꢀdicarboxylato)ruthenium(II)
(N719 dye) in absolute ethanol for one day. The dye coated photoelectrode was washed, dried
and immediately used for the measurement of solar cell performance. A sandwich type
photoelectrochemical cell was used, which was composed of a dyeꢀcoated TiO2 photoanode.
Platinum coated fluorinated tin oxide (FTO) acted as a counter electrode. The electrolyte solution
was injected into the space between photoanode and counter electrode. The electrolyte solution
was composed of lithium iodide (LiI, 0.1 M); iodine (0.01 M), and dendrimer (0.1 mM) additives
in 10 mL DMF solvent. The photovoltaic properties were measured under simulated solar light at
70 mWcm.ꢀ2 Currentꢀvoltage (JꢀV) measurement was performed using Metrohm Autolab 302N
potentiostat/galvanostat with FRA32M module controlled by NOVA 1.10 software. The apparent
cell area of the TiO2 photoelectrode was 1 cm2 (1 cm X 1 cm).
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General procedure for zeroth generation dendrimers
A
mixture of diyne 25 and triyne 26 (1.0 equiv.) and 2ꢀ(4ꢀ
(azidomethyl)phenyl)benzoheterazole 13 and 14 (2.2 equiv./ 3.3 equiv.) was dissolved in a
mixture of THF and H2O (1:1; 20 mL) and sodium ascorbate (10 mol%) was added followed by
the addition of CuSO4.5H2O (5 mol%). The reaction mixture was stirred overnight at room
temperature. The residue obtained after evaporation of the solvent was washed thoroughly with
water and dissolved in CHCl3 (3 x 100 mL). The organic layer was washed with water (200 mL)
and brine (50 mL), dried over Na2SO4, and then solvent was evaporated to give the crude triazole
product, which was purified by column chromatography (SiO2) with CHCl3 or CHCl3ꢀMeOH as
eluent to give the corresponding to the triazole dendrimers.
Dendrimer 1
Yield: 65%; mp: 250ꢀ254 oC; 1H NMR (300 MHz, CDCl3): δH 5.60 (s, 4H); 7.05 (s, 2H);
7.10 (d, J = 2.4 Hz, 4H); 7.13 (s, 3H); 7.26 (d, J = 15.6 Hz, 2H); 7.33 (d, J = 7.5 Hz, 8H); 7.53 (t,
J = 5.1 Hz, 2H); 7.60 (d, J = 8.1 Hz, 4H); 7.63 (s, 2H); 7.67 (d, J = 8.7 Hz, 2H); 7.71 (t, J = 4.2
13
Hz, 3H); 7.77 (d, J = 16.5 Hz, 2H). C NMR (75 MHz, CDCl3): δc 53.8, 109.0, 110.3, 114.9,
118.9, 123.4, 124.0, 124.6, 124.7, 124.8, 125.4, 126.7, 128.1, 128.5, 129.4, 135.7, 136.2, 138.2,
142.1, 147.1, 147.5, 148.1, 150.4, 162.4. ESIꢀMS m/z 846 [M + H]+. Anal.Calcl.for C54H39N9O2:
C, 76.67; H, 4.65; N, 14.90. Found: C, 75.68; H, 4.73; N, 14.79%.
Dendrimer 2
o
1
Yield: 70%; mp: 258ꢀ260 C; H NMR (300 MHz, CDCl3): δH 5.59 (s, 4H); 7.04 (t, J =
7.2 Hz, 1H); 7.11 (d, J = 8.7 Hz, 6H); 7.26 (d, J = 15.6 Hz, 4H); 7.33 (d, J = 7.8 Hz, 4H); 7.36 (s,
1H); 7.43 (t, J = 7.5 Hz, 3H); 7.52 (d, J = 11.1 Hz, 3H); 7.59 (d, J = 8.1 Hz, 4H); 7.63 (s, 2H);
29