Cyclohexadienone core 3,6-di-tert-butylcarbazole decorated triazole bridged dendrimers: synthesis, photophysical and electrochemical properties and application as an additive in dye-sensitized solar cells

Article abstract of DOI:10.1039/C8NJ05986A

Cyclohexadienone core 3,6-di-tert-butylcarbazole decorated triazole bridged dendrimers are synthesised up to the second generation using click chemistry via a convergent approach. The molar extinction coefficient and the intensity of the emission band inc

Full text of DOI:10.1039/C8NJ05986A

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Cyclohexadienone core 3,6-di-tert-butylcarbazole
decorated triazole bridged dendrimers: synthesis,
photophysical and electrochemical properties and
application as an additive in dye-sensitized solar cells†
Cite this:DOI: 10.1039/c8nj05986a
Janakiraman Babu,a Shanmugam Ganesan,b Kaliamurthy Ashok Kumar,c
a
Masiyappan Karuppusamy,de Arumugam Pandurangan c and Perumal Rajakumar
*
Cyclohexadienone core 3,6-di-tert-butylcarbazole decorated triazole bridged dendrimers are synthesised
up to the second generation using click chemistry via a convergent approach. The molar extinction
coefficient and the intensity of the emission band increase with an increase in the dendrimer generation.
The HOMO–LUMO energy level and the energy gap Eg obtained from photophysical and electrochemical
parameters coincide with the results obtained from DFT calculations. The higher generation dendrimer 3
(G2) shows a higher Voc value and an increased efficiency of 9.01% when used as additive in DSSCs
compared to the lower generation dendrimers 2 (G1) and 1 (G0) under 100 mW cmÀ2 illumination.
Further, Nyquist plots obtained from electrochemical impedance measurements also show that the
higher generation dendrimer G2 provides an increased number of charge transport pathways, which
enhances interfacial electron transport and reduces the recombination dynamics.
Received 26th November 2018,
Accepted 3rd February 2019
DOI: 10.1039/c8nj05986a
rsc.li/njc
mechanism to how photosynthesis happens in leaves. Dye
molecules are present at the interface between transparent nano-
Introduction
As mankind has depleted non-renewable sources of energy, crystalline TiO2 and an electrolyte, which absorb solar photons and
energy crises are faced by domestic and industrial consumers. result in the excitation of the molecules. The excited molecules
Many researchers have focused on harvesting solar energy as an inject electrons into the conduction band of semiconducting TiO2.
alternate route. Mankind started to tap solar energy (a renew- The electrons are conducted quickly and that gÀoverns the behavior
able source) through silicon based solar panels. But the cost of external circuits. The conversion of an IÀ/I3 redox mediator is
and manufacturing difficulties associated with silicon based the main focus for the enhancement of DSSC efficiency; this
solar panels have provided the big challenge to the scientific mainly bridges the photoanode and counter electrode that form
community of providing low cost solar cells as a permanent the internal circuit in DSSCs. To obtain high performance DSSCs,
solution to the energy crisis. The introduction of dye-sensitized the counter electrode also plays a key role by regenerating the
1
À
solar cells (DSSCs) by Gratzel and O’Regan was a remarkable IÀ/I3 redox mediator in the cell, which allows the flow of
¨
breakthrough in the history of the development of solar cells electrons into the internal circuit of a DSSC. The electrocatalytic
and they have gained much research interest since 1991 due to reduction and regeneration of the IÀ/I3 mediator via the flow
À
their advantages over other available solar cells. A DSSC is a of electrons from the external circuit into the internal circuit
typical photoelectrochemical cell,2 which works with a similar is done by a platinum (Pt) counter electrode, due to its high
conductivity and electrocatalytic activity. For the past two
decades, the replacement of expensive Pt counter electrodes
happens to be the main focus of research, owing to their
a Department of Organic Chemistry, University of Madras, Chennai-600 025,
Tamil Nadu, India. E-mail: perumalrajakumar@gmail.com
b Department of Chemistry, SRM Institute of Science and Technology,
stability issues. Recently, carbon-based materials, like graphene
and CNTs, are widely used as alternatives to Pt counter electrodes,
and the efficiency of carbon-based electrodes can be enhanced
through the doping of non-metallic ions such as N, P, B, S and Se.3
Organic molecules are explored either in the form of metal-
based4 or metal-free dye5 molecules to increase the efficiency of
DSSCs. Ru(II) based dye molecules are the most commonly used
Kattankulathur-603 203, Tamil Nadu, India.
E-mail: ganesan.sh@ktr.srmuniv.ac.in
c Department of Chemistry, Anna University, Chennai-600 025, Tamil Nadu, India
d Inorganic and Physical Chemistry Laboratory (IPCL), CSIR-Central Leather
Research Institute, Adyar, Chennai-600 020, India
e Academy of Scientific and Innovative Research (AcSIR), CSIR-CLRI Campus,
Chennai 600020, India
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nj05986a dyes throughout the world that result in better DSSC performance.
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Other than dye molecules, electrolytes present in cells also play a corresponding alcohol 7 via reduction with NaBH4. The reaction of
vital role in determining the solar energy harvesting efficiency.6 the alcoholic compound 7 with SOCl2/pyridine gave the respective
They are responsible for the inner electron flow and rapid chloro-compound 8 and, further, the chloro-compound 8 was
regeneration of the dye molecules. The leakage and evaporation converted into the azido compound (G0-N3 dendron) 9 via a
of the electrolyte are the major problems faced when liquid reaction with 1.2 equiv. of NaN3 in acetone under refluxing
electrolytes are used, thus reducing the durability of cells and conditions. In the 1H NMR spectrum, the azido dendron (G0-N3)
resulting inÀvery low performance when harvesting solar energy. 9 displayed a singlet at d 4.26 ppm from the benzylic protons
When IÀ/I3 redox mediators are used in the electrolyte, the attached to the azido group, in addition to signals from the
sublimation of iodine from the cell affects drastically the perfor- other aliphatic and aromatic protons. In the 13C NMR spectrum,
mance of DSSCs. Nitrogen-based heterocycles, such as pyrazole, the azido dendron 9 showed a signal at d 54.5 ppm from the
imidazole, triazole, pyridine, pyrimidine, and pyrazine, are employed benzylic CH2 carbon attached to the azido group, along with the
as additives in redox couple electrolyte solutions to improve the other aliphatic and aromatic carbon signals.
efficiency of DSSCs.7
We then focused on the synthesis of the first and second
Dendrimers8 are a class of macromolecule, with a well-defined generation dendritic wedges. The reaction of 1.1 equiv. of the
tree-like monodisperse structure, that has received much interest zeroth generation azido dendron 9 with 0.5 equiv. of 3,5-bis-
in the fields of material and biological science. They are used (propargyloxy)benzyl chloride 10 under click reaction conditions,
as scaffolds for delivering drugs to targets,9 as biomimetic i.e., CuSO4Á5H2O (5 mol%) and sodium ascorbate (10 mol%) in a
catalysts,10 in organic light emitting diodes11 and in light mixture of THF and water (1 : 1) at room temperature, gave the first
harvesting devices.12 Vogtle et al.,13 Tomalia et al.,14 Frechet generation chloro-dendritic wedge (G1–Cl) 11 in good yield. The
et al.,15 and Newkome et al.16 are pioneers in developing corresponding first generation azido dendron 12 was synthesised
dendrimer chemistry over the last decade. Yamamoto et al. as a colourless solid via the reaction of 1.2 equiv. of NaN3 with
synthesised fifth generation hole transporting dendrimers with 1.0 equiv. of the chloro-dendritic wedge 11 under refluxing
phenylazomethine on the surface and triphenylamine at the core conditions in acetone. The 1H NMR spectrum of 12 (G1-N3)
position and used these as additives to increase the efficiency of a displayed three singlets at d 4.26, 5.21 and 5.61 ppm from benzylic
solar energy harvesting system with N719, a Ru(II)-based dye chloromethane, N-methylene and O-methylene protons, respectively,
molecule.17 Dendrimers with an imine (azomethine) linkage are and the triazole protons appeared as a singlet at d 7.66 ppm in
an excellent electron transporting material and are considered an addition to signals from the other aliphatic and aromatic
interesting class of molecules for new material applications.18 An protons. The 13C NMR spectrum of 12 (G1-N3) showed signals
alternative to the azomethine linkage is 1,2,3-triazole, which is at d 53.7, 54.8 and 62.6 ppm, which correspond to benzylic,
synthesised via Cu(I) catalysed 1,3-dipolar cycloaddition between N-methylene and O-methylene carbon, respectively, along with
an alkyne and an azide (click chemistry).19 Rajakumar et al. have signals from other aliphatic and aromatic carbons. The conversion
applied click chemistry to the synthesis of 1,2,3-triazole-based of the first generation chloro-dendron 11 to the azido dendron
dendrimers with various electron transporting and electron 12 was confirmed via the change in the chemical shift of the
accepting molecules, such as phenothiazine,20 diphenyl amine,21 benzylic carbon from d 46.2 ppm in 11 (G1–Cl) to d 54.8 ppm for
pyrene,22 dimethyl isophthalate,23 benzoheterazole,24 and triphenyl the azido benzylic CH2 carbon in 12 (G1-N3).
amine with chalcone,25 at the periphery and used these as additives
to increase the power conversion efficiency (PCE) of solar cells.
Similarly, the second generation azido dendron 14 was
synthesised from the click reaction of 0.5 equiv. of 3,5-bis(pro-
Carbazole has excellent electron transporting properties and pargyloxy)benzyl chloride 10 with 1.1 equiv. of the first generation
great stability, with unique photophysical and luminescence azido dendron 12, in the presence of CuSO4Á5H2O (5 mol%) and
properties; hence it is used in OLEDs26 and DSSCs.27 Due to its sodium ascorbate (10 mol%) in a mixture of THF and water (1 : 1)
increased performance in electronic devices, we are prompted at room temperature, to give the second generation dendritic
to use carbazole dendrimers as additives in DSSCs. Herein, we wedge 13 (G2-Cl) in good yield, which was then converted to the
report the synthesis of the cyclohexadienone core 3,6-di-tert- second generation azido dendron 14 via a reaction with NaN3 in
butylcarbazole decorated triazole bridged dendrimers 1, 2 and refluxing acetone.
1
3 (Fig. 1) through a convergent approach and investigate their
In the H NMR spectrum, the dendritic azide 14 displayed
photophysical and electrochemical properties along with their benzylic, O-methylene, two different N-methylene protons (namely
application as an additive to improve solar energy harvesting methylene protons near benzene ring and another methylene
efficiency in DSSCs.
protons near carbazole unit) as four different singlets at d 4.16,
5.12, 5.38 and 5.57 ppm, respectively. The outer and inner
triazole ring protons appeared as two different singlets at d
7.57 ppm for two protons and at d 7.66 ppm for four protons, in
addition to signals from other aliphatic and aromatic protons.
Results and discussion
The alkylation of carbazole 4 with t-butyl chloride in the presence In the 13C NMR spectrum, the second generation dendritic azide
of ZnCl2 gave 3,6-di-tert-butylcarbazole 5, which was reacted with G2-N3 14 displayed signals at d 53.7, 53.9, 54.7 and 62.4 ppm for
4-bromobenzaldehyde to give 4-carbazolyl benzaldehyde 6 as a four different types of methylene carbon, viz., benzylic, O-methylene,
yellow coloured solid. The aldehyde 6 was converted to the two different N-methylene carbons (namely methylene carbons near
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Fig. 1 Molecular structures of the cyclohexadienone core carbazole dendrimers 1, 2 and 3.
benzene ring and another methylene carbons near carbazole aliphatic and aromatic proton signals. The 13C NMR spectrum of
unit), along with signals from other aliphatic and aromatic 16 also confirmed the presence of a propargylic unit via the
carbons. The reaction sequence for the synthesis of the zeroth, appearance of signals at d 55.8, 75.9, and 78.2 ppm, in addition to
first and second generation carbazole dendritic wedges is other aliphatic and aromatic carbon signals.
shown in Scheme 1.
The reaction of 1.0 equiv. of the bispropargyloxy compound
The dienone core unit 16 was synthesised as a yellow coloured 16 with 2.2 equiv. of each of the dendritic azides 9, 12 and 14
solid in 86% yield via the aldol condensation of 1.0 equiv. of under click reaction conditions, viz., 5 mol% CuSO4Á5H2O and
cyclohexanone 15 with 2.1 equiv. of 4-(prop-2-ynyloxy)benzalde- 10 mol% sodium ascorbate in THF : H2O (1 : 1) for 12 h,
hyde in ethanol in the presence of NaOH (Scheme 2). The afforded the zeroth, first and second generation dendrimers
1H NMR spectrum of the dienone core compound 16 displayed 1, 2 and 3 in 90%, 83% and 77% yields, respectively (Scheme 2).
1
a triplet at d 2.55 ppm (J = 2.1 Hz) and a doublet at d 4.72 ppm
The H NMR spectrum of the second generation carbazole
(J = 2.1 Hz) from the propargylic protons, in addition to other dendrimer 3 showed signals from the triazolyl protons and b
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Scheme 1 (i) tBuCl, ZnCl2, CH3NO2, N2 atm, 5 h, 5 (86%); (ii) Cu2O, DMAc, 170 1C, N2 atm, 24 h, 6 (58%); (iii) NaBH4, MeOH/DCM, 6 h, 7 (95%); (iv) SOCl2,
pyridine, DCM, overnight, 8 (90%); (v) NaN3, acetone, reflux, 12 h, 9 (98%), 12 (95%), 14 (94%); (vi) CuSO4Á5H2O (5 mol%), sodium ascorbate (10 mol%),
THF : H2O (1 : 1, v/v), rt, 12 h, 11 (85%), 13 (79%).
protons of the enone double bond at d 7.50 and 7.61 ppm, dendrimer 3 showed the molecular ion peak (M+) at m/z =
respectively, along with other aliphatic and aromatic proton 5114.35. The structure of dendrimer 3 was also confirmed
signals. The 13C NMR spectrum of the G2 dendrimer 3 showed from elemental analysis. Similarly, the structures of the zeroth
N-methylene and O-methylene carbon signals at d 53.7, 53.9, generation carbazole dendrimer 1 (G0) and first generation
61.8 and 61.9 ppm, in addition to other aliphatic and aromatic carbazole dendrimer 2 (G1) were also confirmed from spectral
carbon signals. The mass spectrum (MALDI-TOF) of the G2 and analytical data.
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Scheme 2 (i) 4-(Prop-2-ynyloxy)benzaldehyde (2.1 equiv.), NaOH, EtOH,
rt, 12 h, 16 (86%); (ii) CuSO4Á5H2O (5 mol%), sodium ascorbate (10 mol%),
THF : H2O (1 : 1), (v/v), rt, 12 h, 1 (90%), 2 (83%) and 3 (77%).
Photophysical properties
Fig. 3 Emission spectra of the carbazole dendrimers 1, 2 and 3 in DCM
(1 Â 10À5 M).
UV-visible absorption spectra of the dendrimers 1, 2 and 3 were
recorded at a concentration of 1 Â 10À5 M in DCM (Fig. 2), and
the data is summarised in Table 1. The synthesised dendrimers
1, 2 and 3 show almost identical absorption spectra, with two
absorption bands at 297 and 346 nm. The absorption band at
297 nm is due to p–p* transitions of localized electrons of
carbazole moieties present at the peripheries of the dendrimers.
Another absorption band at 346 nm relates to the dienone core
unit. It is clear from the absorption spectra that as the generation
of the dendrimers increases, the number of carbazole units on
the periphery increases; this results in an increase in the molar
extinction coefficient. The increase in absorbance from a lower
generation to a higher generation amplifies the light absorbing
capacity of the dendrimer and is known as the valence effect in
dendrimer chemistry;28 hence dendrimers 1, 2 and 3 can act as
good light harvesting materials.
synthesised dendrimers at 297 nm (Fig. 3), and the data are
summarised in Table 1. Dendrimers 1, 2 and 3 show emission
bands at around 355 and 370 nm. These dual emission bands
correspond to 9-phenyl carbazole, which gives rise to twisted
intramolecular charge transfer (TICT)29 in the polar solvent.
Fig. 3 clearly shows that the emission intensities of the dendrimers
increase as the generation increases, which is consistent with the
fluorophoric dienone and the increased number of carbazole units;
this is described as the valence effect in dendrimer chemistry.28
Electrochemical properties
The electrochemical properties of the synthesised carbazole
dendrimers 1, 2 and 3 were studied via recording cyclic voltam-
metry data in dichloromethane at a concentration of 1 Â
10À3 M, using a 0.1 M solution of n-Bu4NPF6 as the supporting
electrolyte; the results are tabulated in Table 2 and the voltammo-
grams are shown in Fig. 4. The experiments were carried out at
ambient temperature under a nitrogen atmosphere with a conven-
tional three electrode configuration consisting of a gold disc as
the working electrode, platinum wire as the counter electrode and
Ag/AgCl as the reference electrode. The oxidation and reduction
potentials for all the dendrimers appear in the potential range of
À2.0 to +2.0 V. In the cyclic voltammetry data, two oxidation peaks
appear between À0.66 and 1.58 V and two reduction peaks appear
between 1.32 to À0.98 V for all the dendrimers. CV plots of all the
dendrimers show the irreversible redox potential. The oxidation
and reduction peaks for dendrimers 1, 2 and 3 are attributed to the
presence of electron donating carbazole moieties and electron
accepting dienone units. It is clear from Fig. 4 that upon increasing
the number of carbazoles at the periphery, the anodic peak
potential is shifted towards a slightly more negative potential.
The oxidation of carbazole molecules to the respective radical
cations is observed as the first oxidation wave for dendrimers 1, 2
and 3, which appears at the lower potential of À0.66 to À0.59 V.
Electrochemical coupling between carbazole units at the 3 and 6
positions is prevented by the presence of bulky tert-butyl units,
which was confirmed by multiple CV scans. From the cyclic
voltammograms (Fig. 4), it is clear that upon increasing the
number of carbazole units, the DEp2 value increases from the G0
The emission spectra of dendrimers 1, 2 and 3 are recorded
at a concentration of 1 Â 10À5 M in DCM upon exciting the
Fig. 2 UV-vis absorption spectra of the carbazole dendrimers 1, 2 and 3 in
DCM (1 Â 10À5 M).
Table 1 The photophysical properties of dendrimers 1, 2 and 3
Dendrimer
lmax (nm)
e (Â 104 L molÀ1 cmÀ1
)
lem (nm)
Dendrimer 1
Dendrimer 2
Dendrimer 3
297, 346
297, 346
297, 346
4.75, 3.31
7.90, 4.13
12.60, 4.13
355, 370
355, 371
356, 371
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Table 2 The electrochemical parameters of dendrimers 1, 2 and 3
Dendrimer (generation)
Epa1 (V)
Epa2 (V)
Epc1 (V)
Epc2 (V)
DEp1 (V)
DEp2 (V)
Dendrimer 1
Dendrimer 2
Dendrimer 3
À0.59
À0.64
À0.66
1.51
1.47
1.58
1.30
1.27
1.32
À0.89
À0.98
À0.92
1.89
1.91
1.98
2.40
2.45
2.50
6-311+G(d,p) basis set with B3LYP/6-31G(d,p) optimized geometry
to calculate the optical and electrochemical properties for the ten
lowest spin allowed transitions, employing dichloromethane
(DCM, e = 8.93) as the solvent. The polarizable continuum model
(PCM) was adopted to include solvation effects.32 All density
functional theory (DFT) calculations were performed using the
Gaussian 09 (Revision C.01) suite of programs.33
The ground state optimized geometry of dendrimer 1 and
the corresponding electron density distributions of the frontier
molecular orbitals (FMOs) are depicted in Fig. 5 and 6, respectively.
The calculated HOMO–LUMO energy gap (Eg) is found to be
3.12 eV, with a highest occupied molecular orbital (HOMO)
energy value of À5.64 eV and a lowest unoccupied molecular
orbital (LUMO) energy value of À2.52 eV. Effective intra-
molecular charge transfer (ICT) from donor to acceptor during
electron excitation is necessary to achieve high photo-to-electric
conversion efficiency. It is clear from Fig. 6 that the electron
density of the HOMO of dendrimer 1 is primarily located on the
carbazole motif (donor) and partially found on the benzene
linker. The LUMO of dendrimer 1 is primarily found on the
cyclohexanone ring (acceptor) with a minor portion on the benzene
linker. These results clearly indicate that there is a high probability
for charge separation upon excitation in dendrimer 1.
Fig. 4 Cyclic voltammograms of the carbazole dendrimers 1, 2 and 3 in
DCM (1 Â 10À3 M) scanned at 100 mV sÀ1
.
to the G2 dendrimer, i.e., in the order: 3 (G2) 4 2 (G1) 4 1 (G0). The
Ep2 peak shows a gradual increase in value from 2.4 to 2.50 V.
The energy level parameters were obtained from CV and UV
data and are summarized in Table 3. The HOMO energy levels
of the carbazole dendrimers 1, 2 and 3 are in the range of À5.49
to À5.61 eV; these values were obtained from the onset oxidation
ox
potential, i.e., HOMO = À(4.41 + E
). Further, the LUMO energy
onset
level values of the synthesised carbazole dendrimers were calcu-
lated from the HOMO energy level (LUMO = HOMO + Eg) and the
optical band gap (Eg) values were determined from the UV-visible
absorption spectra. The higher generation carbazole dendrimer 3
was found to possess a reduced band gap of 2.82 eV when compared
to the lower generation dendrimer with a band gap of 2.89 eV.
The calculated vertical excitation energy (E), absorption
wavelength (lmax), oscillator strength ( f ) and molar extinction
coefficient (e) from TD-DFT calculations at the B3LYP/6-311+G(d,p)
level of theory in dichloromethane solvent are given in Table 4.
Dendrimer 1 exhibits an absorption maximum (lmax) of 407 nm
with an oscillator strength of 1.3482 and a molar extinction
Computational details
The bulky alkyl chains were replaced with methyl groups to
improve the computational efficiency. The ground state geometry
of the dendrimer was optimized with the Becke three-parameter
and Lee–Yang–Parr correlation hybrid (B3LYP) functional30 in
conjunction with the 6-31G(d,p) basis set.31 Vibrational frequency
analysis was also carried out at the same level of theory to confirm
the minimum energy values on the potential energy surface. Time-
dependent density functional theory (TD-DFT) calculations were
carried out with the B3LYP functional in conjunction with the
Fig. 5 The optimized geometry of dendrimer 1 computed at the B3LYP/
6-31G(d,p) level of theory. The long alkyl chain is replaced with methyl
groups to improve the computational efficiency [color indication: gray –
carbon; white – hydrogen; blue – nitrogen; and red – oxygen].
Table 3 The energy level parameters of dendrimers 1, 2 and 3
Eogpt b (eV)
E
(V)
HOMOd (eV)
LUMOe (eV)
a
ox
onset
c
Dendrimer (generation)
lonset (nm)
Dendrimer 1
Dendrimer 2
Dendrimer 3
428
434
439
2.89
2.86
2.82
1.20
1.15
1.08
À5.61
À5.56
À5.49
À2.72
À2.70
À2.67
a
b
c
Onset absorption maximum from UV-vis absorption spectra. Optical band gap calculated using the formula Eg = 1240/UVonset
.
Onset oxidation
potential. d HOMO = À[4.41 + E
]. e LUMO = [HOMO + Eogpt].
ox
onset
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interface; and (ii) the dye regeneration efficiency. Moreover,
such charge recombination at the TiO2/N3 dye/electrolyte inter-
face leads to losses in the short-circuit current (Jsc) and the open-
circuit voltage (Voc), resulting in a decrease in power conversion
efficiency. It is well known that the addition of organic nitrogenous
compounds to the IÀ/I3 electrolyte system would improve the
À
photovoltage significantly; this is commonly attributed to a shift in
the conduction band edge of titania towards more negative
potentials.34 This can probably be attributed to higher rates of
electron recombination between TiO2 and the electrolyte in dye-
sensitized solar cells.
Fig. 7 shows a typical dye sensitized solar cell with the
synthesized dienone core triazole bridged carbazole dendrimer
serving as an additive to the redox electrolyte. The nanocrystalline
TiO2 photoelectrode was prepared as described in the literature.35
Fig. 8 shows current–voltage ( J–V) curves from the bare cell and
after the addition of carbazole dendrimers 1, 2 and 3 to the
redox electrolyte. The photovoltaic performances of the DSSCs
(short-circuit current density ( Jsc), open-circuit voltage (Voc), fill
factor (ff), and electric energy efficiency (Z)) are summarized in
Table 5. For cells constructed with the synthesized carbazole
dendrimers, an increase in current is observed, along with an
increase in Voc. The redox electrolyte IÀ/I3À, with the newly
synthesized dendrimers, shows good stability and improved
efficiency owing to its quasi-gel type behaviour. It overcomes
the problems of leakage and evaporation shown by standard KI
electrolyte. The synthesized organic dendrimers interacted well
with iodine in the redox couple and decreased the sublimation
of iodine, which enhances the stability and efficiency. The cell
involving a carbazole doped redox couple based DSSC is
remarkably stable and the photocurrent maintains 92% of the
initial value ever after 10 days of storage. On the contrary, cells
without carbazole dendrimers in the redox electrolyte are much
less stable and show a significant decrease in the open circuit
Fig. 6 Frontier molecular orbital (HOMO and LUMO) contour images of
dendrimer 1 with orbital energy values. The isodensity surface value is fixed
at 0.03 a.u.
Table 4 The calculated vertical excitation energy E (eV), absorption wavelength
(lmax), oscillator strength (f), and molar extinction coefficient (e) of dendrimer 1 at
the TD-PCM-B3LYP/6-311+G(d,p) level of theory, in dichloromethane
Dendrimer
E (eV)
lmax (nm)
f
e (MÀ1 cmÀ1
)
1
3.05
3.87
3.21
406.75
320.47
386.82
1.3482
0.2363
0.1358
103873.30
23902.73
67680.16
coefficient of 103873.3 MÀ1 cmÀ1. Similarly, for the dendrimer 1
with different oscillator strength ( f ) values the corresponding
excitation energy, absorption wavelength and molar extinction
co-efficient are calculated and summarized in Table 4. It can be
clearly understood from the above analysis that dendrimer 1
could be a good light harvesting material.
DSSC studies
The DSSC performance is mainly dependent on: (i) the minimization voltage (Voc) and short circuit current ( Jsc) over a short period
of charge recombination losses at the TiO2/N3 dye/electrolyte of time.
Fig. 7 A schematic representation of the use of carbazole dendrimers as an additive for DSSC performance.
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couple and decrease the sublimation of iodine, enhancing the
lifetime of the redox couple and reducing charge recombination
occurring at the TiO2/N3 dye/redox electrolyte interface.
Electrochemical impedance spectroscopy and Nyquist plots
The interfacial charge transport kinetics of the fabriÀcated DSSCs
based on dendrimer addition along with an IÀ/I3 electrolyte
were studied via EIS Nyquist plots, as shown in Fig. 9a. For
the applied open circuit potential (OCP), measurements were
taken using a two electrode solar device configuration over the
frequency range of 100 kHz to 0.1 Hz under ambient conditions.
This resulted in two examples of semicircle behaviour, respectively
in the high and mid-frequency regions of the Nyquist plots, affirm-
ing the presence of two interfacial charge transport resistance
processes in the fabricated devices. The intersections of these
Fig. 8 J–V curves of DSSCs with the dienone core carbazole dendrimers
1, 2 and 3 under simulated solar light at 1 sun (100 mW cmÀ2).
As the generation increases, the number of carbazole and semicircles with the real axis of impedance give the total charge
triazole units increases, which results in an increase in the transport resistance at every interface. The semicircular arc in the
light-to-electric energy conversion. The carbazole dendrimer 3 high frequency region corresponds to the charge transport resis-
shows 9.01% solar power conversion efficiency, with high Jsc tance at the Pt/KI electrolyte interface (Rct1) and the mid-frequency
and Voc, due to the presence of a greater number of carbazole semicircle represents the charge transport resistance (recombina-
units, which in turn increases the chemical capacitance and tion resistance) at the TiO2/N3/KI + dendrimer electrolyte interface
eventually the photocurrent of the cells with dendrimer additives (Rct2). The existence of two semicircles was further confirmed from
(Table 5). The presence of a well conjugated triazole group in the the phase angle shifts in the Bode phase plots (Fig. 9b). Using the
second generation dendrimer 3 controls the back reactions of the equivalent circuit model shown in the inset of Fig. 9a, the obtained
IÀ/I3À redox mediator and thus results in high current conversion semicircles were fitted and the corresponding electrochemical
compared to the other dendrimers. The DSSC without carbazole parameters are summarized in Table 6, where Rs represents the
derivatives shows low efficiency and stability. Further, the COOH series resistance, which seems to be nearly equal for all devices (this
moiety in the N3 dye gets deprotonated while anchored on the is ascribed to the sheet resistance of fluorinated tin oxide (FTO)),
surface of the TiO2 photoanode and the conduction band of TiO2 and Cm represents the total chemical capacitance of the photoanode.
gets shifted to a more positive potential as a result of possible
From Table 6, it is noted that pure KI electrolyte has higher
protonation. The potential difference between the Fermi level of Rct1 and Rct2 values compared to all dendrimer based devices.
TiO2 and the HOMO level of the redox electrolyte with a dendrimer The sublimation of iodine over a short time period in aqueous
additive is responsible for the increased Voc. It is confirmed that the KI electrolyte is responsible for the high charge transport
carbazole dendrimer additives interact well with iodine in the redox resistance in the pure KI based device.36 In the case of the
Table 5 The photoelectrochemical properties of carbazole doped redox couple (IÀ/I3À) based DSSCs under 1 sun illumination (100 mW cmÀ2
)
System
Current ( Jsc) (mA)
Voltage (Voc) (mV)
Fill factor (ff) %
Efficiency (Z) %
TiO2/N3dye/KI/I2/Pt
TiO2/N3dye/KI/I2/1/Pt
TiO2/N3dye/KI/I2/2/Pt
TiO2/N3dye/KI/I2/3/Pt
10.1 (Æ0.2)
12.4 (Æ0.2)
16.4 (Æ0.2)
18.3 (Æ0.3)
780 (Æ3)
825 (Æ4)
870 (Æ4)
895 (Æ5)
51 (Æ3)
53 (Æ1)
54 (Æ2)
55 (Æ5)
4.01 (Æ0.02%)
5.42 (Æ0.01%)
7.70 (Æ0.03%)
9.01 (Æ0.04%)
Fig. 9 (a) EIS Nyquist plots (the inset shows the high frequency region and the equivalent circuit model) and (b) Bode phase plots for the fabricated DSSCs.
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Table 6 The electrochemical parameters of the fabricated DSSCs
conversion of 9.01%, with a short circuit current density ( Jsc) of
18.3 mA cmÀ2, an open circuit voltage (Voc) of 895 mV and a fill
factor (ff) of 55%, when used as an additive with a redox mediator
in a DSSC, compared to the other lower generation dendrimers G0
(1) and G1 (2). Further, detailed analysis of the interfacial electron
transport kinetics, such as the charge transport resistance,
chemical capacitance and electron relaxation lifetimes, was
conducted via electrochemical impedance measurements.
w2
System
KI
Rs (O) Rct1 (O) Rct2 (O) Cm (mF) te (ms) (Â 10À3
)
2.21
61.21
12.23
2.37
334.5
232.1
1.214
1.306
0.42
0.31
0.08
0.04
1.01
0.92
1.43
3.70
Dendrimer 1 + KI 1.52
Dendrimer 2 + KI 1.2
Dendrimer 3 + KI 0.97
36.14 2.038
15.12 2.805
2.13
dendrimer + KI electrolyte based devices, the interfacial charge
transport resistances were reduced owing to the usage of a
quasi-gel type dendrimer electrolyte. This can reduce the sub-
limation of iodine in the electrolyte system and increase the
charge transportation in a device. It is noteworthy that Rct1 gets
reduced for all dendrimer based devices, which reveals that
efficient charge transportation occurred at the Pt/electrolyte
interface. In particular, dendrimers 2 and 3 being added into
the KI electrolyte system resulted in low Rct1 values of 2.37 O
and 2.13 O, respectively, due to the presence of an anchoring
cyclohexadienone unit with a greater number of carbazole dendron
units. This can offer efficient charge transportation at the
Pt/electrolyte interface and reduce the recombination dynamics.
Further, the obtained Rct2 and Cm values of the fabricated DSSCs
determine the charge injection efficiency and recombination
possibility, which can be estimated using the electron relaxation
lifetime (te). This can be calculated from the peak frequency
( fmid) in the mid-frequency region of the EIS Bode phase plot
(Fig. 9b) using the relationship te = 1/2pfmid. The standard KI and
dendrimer 1 + KI based devices revealed high electron relaxation
lifetimes of 0.42 and 0.31 ms, respectively, owing to their high Rct2
values. Even though the chemical capacitances of the dendrimer 2
and 3 + KI based devices were high, their very low Rct2 values can
decrease the electron relaxation lifetime (te) values, which results in
them having more significant charge injection processes than charge
recombination.37 The increased chemical capacitance could possibly
originate from the number of p-conjugation units (carbazole), which
increase the current densities of devices fabricated with dendrimer 2
and 3 + KI based electrolyte systems (as evidenced from the obtained
high Jsc values). In particular, the device fabricated with dendrimer 3
+ KI realized a very low te value of 0.04 ms owing to its reduced Rct2. A
larger number of anchoring units in dendrimer 3 can provide an
increased number of charge transport pathways, which enhances the
charge injection efficiency and reduces the recombination dynamics.
Experimental section
Materials and methods
All reagents and solvents were obtained from the chemical
companies Sigma-Aldrich, LOBA, SRL and Avra. The known products
were identified via a comparison of their melting points and from
spectral data already reported in the literature. All melting points
reported are uncorrected and were determined using Toshniwal
melting point apparatus via the open capillary tube method.
The UV-vis spectra were recorded using an Agilent 8453 spectro-
photometer. The emission spectra were recorded using a
HORIBA JOBIN YVON Fluoromax-4 spectrofluorometer. The
1H and 13C NMR spectra were recorded using Bruker 300 MHz
and 75 MHz spectrometers. The chemical shifts are reported in
ppm (d) with TMS as an internal standard, and the coupling
constants ( J) are expressed in Hz. MALDI-TOF mass spectra
were recorded using a Voyager-DE PRO mass spectrometer
using an a cyano-4-hydroxy cinnamic acid (CHCA) matrix. Elemen-
tal analyses were performed with a PerkinElmer 240B elemental
analyzer. Electrochemical studies were carried out using a CH
Instruments electrochemical analyzer. Current–voltage ( J–V) curves
and electrochemical impedance measurements were performed
using an Autolab PGSTAT 302N potentiostat/galvanostat with an
FRA32M module controlled by NOVA 1.10 software. TLC was
performed on glass plates coated with silica gel-G (ACME) of about
0.25 mm thickness and visualized with iodine. Column chromato-
graphy was carried out with silica gel (ACME, 100–200 mesh).
General procedure for the formation of the 1,2,3-triazole ring
(procedure A)
A mixture of CuSO4Á5H2O (5 mol%) and sodium ascorbate
(10 mol%) was added to a stirred solution of acetylenic compound
(0.5 equiv.) and azide compound (1.1 equiv.) in THF : H2O (1:1)
solvent mixture, and this was further stirred at room temperature
for 12 h. THF was removed from the reaction mixture under
reduced pressure and the residue obtained was diluted with CHCl3.
The organic layer was washed with water (2 Â 50 mL) and brine
solution (1 Â 50 mL) and dried over Na2SO4, and the solvent was
evaporated under vacuum. The crude product obtained was purified
via column chromatography using SiO2. The corresponding triazole
compounds were eluted from the column using a CHCl3:MeOH
solvent mixture.
Conclusions
In summary, we have synthesized a series of cyclohexadienone
core, triazole bridged carbazole decorated dendrimers, up to
the second generation, using a convergent approach. As the
number of carbazole units increases from lower (G0) to higher
(G2) generation dendrimers, the molar extinction coefficients
and intensities of the emission bands also increase, which is
supported by DFT calculations. From the CV studies, all the
dendrimers show irreversible redox signals, with the first oxidation
General procedure for the synthesis of azido dendrons (procedure B)
peak corresponding to the formation of a radical cation of the To a stirred solution of the chloro-dendritic wedge (1.0 equiv.)
carbazole unit. The G2 dendrimer 3 shows the best power energy in acetone, NaN3 (1.2 equiv.) was added and then this was
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refluxed for 12 h. The solvent was evaporated under vacuum for C320H328N50O15: C, 75.15; H, 6.46; N, 13.69%; found: C, 74.86;
and the residue obtained was dissolved in CHCl3 (100 mL). The H, 6.15; N, 13.49%.
organic layer was washed with water (2 Â 50 mL) and brine
3,6-Di-tert-butyl-9H-carbazole 5. Anhydrous ZnCl2 (12.23 g,
solution (1 Â 50 mL) and dried over anhydrous sodium 89.70 mmol) was added to carbazole (5.0 g, 29.90 mmol) in
sulphate, and the solvent was distilled out under vacuum to dissolved nitromethane (100 mL) under a N2 atmosphere and
afford the corresponding dendritic azide as a colourless solid. this was stirred for 10 min followed by the dropwise addition of
Dendrimer 1. Following general procedure A, dendrimer 1 t-butyl chloride (9.88 mL, 89.70 mmol). The reaction mixture
was synthesised from the acetylenic dienone core 16 (0.3 g, was allowed to stir at room temperature for 5 h. The reaction
0.78 mmol) and the azido dendron 9 (G0-N3) (0.71 g, 1.73 mmol). mixture was then slowly added to water (200 mL) and then
The crude product obtained was purified via column chromato- extracted with CHCl3 (3 Â 100 mL). The combined organic layer
graphy using silica gel with CHCl3 : MeOH (9 : 1) as eluent (CHCl3) was washed with water (3 Â 100 mL) and brine solution
to afford dendrimer 1 as a yellow coloured solid. Yield: 90% (100 mL) and dried over anhydrous sodium sulphate. The organic
(0.85 mg); melting point: 158–160 1C; 1H NMR (300 MHz, layer was evaporated, and the crude product was recrystallised
CDCl3): dH 1.46 (s, 36H), 1.72–1.79 (m, 2H), 2.88 (t, 4H, J = 6 Hz), using hexane to afford 3,6-di-tert-butyl-9H-carbazole 5 as a colour-
5.26 (s, 4H), 5.63 (s, 4H), 7.03 (d, 4H, J = 8.4 Hz), 7.33 (d, 4H, less solid. Yield: 85% (7.09 g); melting point: 217–219 1C; 1H NMR
J = 8.4 Hz), 7.46–7.49 (m, 12H), 7.57 (d, 4H, J = 8.1 Hz), 7.68 (300 MHz, CDCl3): dH 1.44 (s, 18 H), 7.24 (d, 2 H, J = 8.4 Hz),
(s, 2H), 7.75 (s, 2H), 8.14 (s, 4H) ppm; 13C NMR (75 MHz, CDCl3): 7.44 (dd, 2H, J = 8.4 Hz, 1.5 Hz), 7.69 (s, 1 H), 8.07 (d, 2H,
dC 22.9, 28.5, 31.9, 34.7, 53.8, 62.1, 109.1, 114.7, 116.3, 122.8, 123.5, J = 1.2 Hz) ppm; 13C NMR (75 MHz, CDCl3): dC 32.0, 34.7, 110.0,
123.7, 127.1, 129.3, 129.5, 132.3, 132.9, 134.6, 136.3, 138.8, 138.9, 116.1, 123.2, 123.5, 138.0, 142.2 ppm.
143.2, 144.4, 158.5, 190.1 ppm; MALDI-TOF m/z: 1203.56 (M)+;
anal. calc. for C80H82N8O3: C, 79.83; H, 6.87; N, 9.31%; found: of 3,6-di-tert-butyl-9H-carbazole 5 (3.0 g, 10.73 mmol), 4-bromo
C, 78.86; H, 6.96; N, 9.49%. benzaldehyde (1.98 g, 10.73 mmol) and Cu2O (3.07 g, 21.47 mmol)
4-(3,6-Di-tert-butyl-9H-carbazol-9-yl)benzaldehyde 6. A mixture
Dendrimer 2. Following general procedure A, dendrimer 2 in DMAc (20 mL) was heated at 170 1C under a N2 atmosphere for
was synthesised from the acetylenic dienone core 16 (0.3 g, 24 h. The reaction mixture was filtered through Celite to remove
0.78 mmol) and the azido dendron 12 (G1-N3) (1.83 g, 1.73 mmol). the inorganic residue, and it was then washed with ethyl acetate
The crude product obtained was purified via column chromato- (2 Â 150 mL). The combined organic layer was washed with
graphy using silica gel with CHCl3 : MeOH (4 : 1) as eluent to afford water (3 Â 50 mL) and brine solution (50 mL) and dried over
dendrimer 2 as a yellow coloured solid. Yield: 83% (1.64 g); melting anhydrous sodium sulphate; it was then evaporated under
1
point: 170–172 1C; H NMR (300 MHz, CDCl3): dH 1.45 (s, 72H), reduced pressure to give the crude product. The crude product
1.69–1.77 (m, 2H), 2.84 (t, 4H, J = 5.1 Hz), 5.14 (s, 8H), 5.19 (s, 4H), thus obtained was purified through column chromatography
5.43 (s, 4H), 5.59 (s, 8H), 6.52 (s, 4H), 6.65 (s, 2H), 6.98 (d, 4H, using SiO2, elution with hexane: CHCl3 (7 : 3) gave 4-(3,6-di-tert-
J = 8.4 Hz), 7.32 (d, 8H, J = 8.7 Hz), 7.38–7.47 (m, 20H), 7.55 (d, 8H, butyl-9H-carbazol-9-yl)benzaldehyde 6 as a yellow coloured
1
J = 7.8 Hz), 7.60 (s, 2H), 7.68 (s, 4H), 7.71 (s, 2H), 8.13 (s, 8H) ppm; solid. Yield: 58% (2.38 g); melting point: 134–136 1C; H NMR
13C NMR (75 MHz, CDCl3): dC 22.9, 28.5, 32.0, 34.8, 53.8, 54.1, 62.0, (300 MHz, CDCl3): dH 1.47(s, 18 H), 7.44–7.50 (m, 4H), 7.78 (d,
62.1, 102.1, 107.5, 109.1, 114.8, 116.3, 123.1, 123.2, 123.6, 123.7, 2H, J = 8.4 Hz), 8.10 (d, 2H, J = 8.1 Hz), 8.14 (s, 2H), 10.09 (s, 1H)
127.1, 129.3, 129.6, 132.3, 132.9, 134.6, 136.4, 136.9, 138.8, 138.9, ppm; 13C NMR (75 MHz, CDCl3): dC 31.9, 34.8, 109.3, 116.5,
143.3, 144.0, 144.3, 158.5, 159.9, 190.1 ppm; MALDI-TOF m/z: 124.0, 124.1, 126.2, 131.4, 134.2, 138.4, 143.9, 144.0, 191.0 ppm.
2507.16 (M)+; anal. calc. for C160H164N22O7: C, 76.65; H, 6.59;
N, 12.29%; found: C, 75.86; H, 6.15; N, 12.09%.
(4-(3,6-Di-tert-butyl-9H-carbazol-9-yl)phenyl)methanol 7. NaBH4
(0.36 g, 9.38 mmol) was added in portions to a solution of the
Dendrimer 3. Following general procedure A, dendrimer 3 was aldehyde 6 (3.0 g, 7.822 mmol) in methanol/DCM (50 mL, 1 : 1) and
synthesised from the acetylenic dienone core 16 (0.2 g, 0.52 mmol) this was stirred at room temperature for 6 h. The reaction mixture
and the azido dendron 14 (G2-N3) (2.72 g, 1.15 mmol). The crude was then poured into water and extracted with CHCl3 (2 Â
product obtained was purified via column chromatography using 100 mL). The combined organic layer was washed with water
silica gel with CHCl3 : MeOH (7 : 3) as eluent to afford dendrimer 3 (3 Â 50 mL) and brine solution (50 mL) and dried over
as a yellow coloured solid. Yield: 77% (2.06 g); melting point: anhydrous sodium sulphate. The organic layer was evaporated
204–206 1C; 1H NMR (300 MHz, CDCl3): dH 1.35 (s, 144H), to give (4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)methanol 7
1.56–1.58 (m, 2H), 2.75 (t, 4H), 5.00 (s, 28H), 5.23 (s, 4H), 5.29 as a colourless solid. Yield: 95% (2.86 g); melting point: 188–
(s, 8H), 5.49 (s, 16H), 6.33 (s, 4H), 6.38 (s, 8H), 6.48 (s, 2H), 6.52 190 1C; 1H NMR (300 MHz, CDCl3): dH 1.47 (s, 18 H), 4.61 (s, 2H)
(s, 4H), 6.85 (d, 4H, J = 8.4 Hz), 7.22 (d, 16H, J = 8.7 Hz), 7.44–7.50 (m, 4H), 7.78 (d, 2H, J = 8.4 Hz), 8.10 (d, 2H,
7.30–7.38 (m, 36H), 7.45 (d, 16H, J = 8.1 Hz), 7.50 (s, 6H), 7.58 (s, J = 8.1 Hz), 8.14 (s, 2H) ppm; 13C NMR (75 MHz, CDCl3): dC
8H), 7.61 (s, 2H), 8.04 (s, 16H) ppm; 13C NMR (75 MHz, CDCl3): 31.9, 34.8, 64.5,109.3,116.5, 124.0, 124.1, 126.2, 131.4, 134.2,
dC 22.7, 29.7, 32.0, 34.7, 53.7, 53.9, 61.8, 61.9, 101.9, 102.0, 138.4, 143.9, 144.0 ppm.
107.4, 107.6, 109.1, 114.1, 114.8, 116.3, 123.3, 123.5, 123.8,
9-(4-(Chloromethyl)phenyl)-3,6-di-tert-butyl-9H-carbazole 8.
127.0, 129.2, 129.6, 132.3, 133.0, 134.6, 136.4, 136.9, 137.1, The alcoholic compound 7 (3.0 g, 7.78 mmol) was dissolved
138.7, 138.9, 139.3, 143.3, 143.8, 143.9, 144.1, 158.5, 159.7, in DCM (100 mL) and stirred at 0 1C, thionyl chloride (0.67 mL,
159.9, 190.1 ppm; MALDI-TOF m/z: 5114.35 (M)+; anal. calc. 9.34 mmol) was added, followed by the addition of pyridine
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(0.75 mL, 9.34 mmol), and then the reaction mixture was stirred 7.66 (s, 4H), 8.13 (s, 8H) ppm; 13C NMR (75 MHz, CDCl3): dC
overnight. It was then poured over crushed ice and extracted 31.9, 34.7, 46.2, 53.7, 54.0, 61.9, 101.8, 102.0, 107.5, 108.1, 109.1,
with DCM (2 Â 100 mL). The combined organic layer was 116.3, 123.2, 123.5, 123.7, 127.0, 129.5, 132.9, 136.9, 138.7,
washed with water (3 Â 100 mL) and brine (100 mL). The organic 138.8, 139.7, 143.2, 143.9, 159.4, 159.8 ppm.
layer was dried over anhydrous sodium sulphate and evaporated
Dendritic azide 14 (G2-N3). Following general procedure B,
under vacuum to afford the desired chloro-compound 8 as a the second generation dendritic azide 14 (G2-N3) was obtained
1
colourless gel. Yield: 90% (2.83 g); H NMR (300 MHz, CDCl3): as a colourless solid by reacting the second generation dendritic
d
H 1.46 (s, 18H), 4.60 (s, 2H), 7.36 (d, 2H, J = 8.7 Hz), 7.46 (d, 2H, chloride 13 (G2-Cl) (1 g, 0.42 mmol) with NaN3 (1.2 equiv.). Yield:
J = 8.7 Hz), 7.54 (d, 2H, J = 8.1 Hz), 7.59 (d, 2H, J = 8.1 Hz), 8.14 (s, 94% (940 mg); melting point: 180–182 1C; 1H NMR (CDCl3,
2H) ppm; 13C NMR (75 MHz, CDCl3): dC 32.0, 32.9, 34.7, 109.2, 300 MHz): dH 1.44 (s, 72H), 4.16 (s, 2H), 5.12 (s, 12H), 5.38
116.3, 123.5, 123.7, 126.8, 130.5, 136.2, 138.3, 138.9, 143.1 ppm.
(s, 4H), 5.57 (s, 8H), 6.49 (s, 6H), 6.57 (s, 1H), 6.62 (s, 2H), 7.30
9-(4-(Azidomethyl)phenyl)-3,6-di-tert-butyl-9H-carbazole 9. (d, 8H, J = 8.4 Hz), 7.42–7.46 (m, 16H), 7.54 (d, 8H, J = 7.8 Hz),
Following general procedure B, the dendritic azide 9 (G0-N3) 7.57 (s, 2H), 7.66 (s, 4H), 8.12 (s, 8H) ppm; 13C NMR (75 MHz,
was obtained as a colourless gel by reacting 9-(4-(chloromethyl)- CDCl3): dC 31.9, 34.7, 53.7, 53.9, 54.7, 62.4, 102.3, 102.7, 107.9,
phenyl)-3,6-di-tert-butyl-9H-carbazole 8 (2.5 g, 6.19 mmol) with 108.1, 109.1, 116.2, 122.9, 123.7, 123.8, 127.2, 129.4, 133.2,
NaN3 (1.2 equiv.). Yield: 98% (2.48 g); 1H NMR (300 MHz, 137.1, 137.9, 139.0, 139.2, 143.4, 144.1, 144.3, 159.8, 160.1 ppm.
CDCl3): dH 1.46 (s, 18H), 4.26 (s, 2H), 7.34 (d, 2H, J = 9 Hz),
Cyclohexadienone core 16. To a stirred solution of cyclo-
7.47 (d, 2H, J= 8.7 Hz), 7.50 (d, 2H, J = 8.4 Hz), 7.57 (d, 2H, hexanone 15 (2.0 mL, 19.31 mmol) in ethanol, NaOH (1.55 g,
J = 8.7 Hz), 8.14 (d, 2H, J = 1.8 Hz) ppm; 13C NMR (75 MHz, 38.71 mmol) was added, and this was stirred for 15 minutes.
CDCl3): dC 32.1, 34.8, 54.5, 109.3, 116.4, 123.6, 123.8, 127.0, 4-(Prop-2-ynyloxy)benzaldehyde (6.82 g, 42.58 mmol) was added
129.6, 134.2, 138.3, 139.2, 143.1 ppm.
dropwise to the reaction mixture and it was stirred for 6 h.
Dendritic chloride 11 (G1-Cl). From a stirred solution of It was then poured over water and extracted using CHCl3 (2 Â
1-(chloromethyl)-3,5-bis(prop-2-ynyloxy)benzene 10 (0.5 g, 2.13 mmol) 100 mL), and the combined organic layer was washed with
and the zeroth generation dendritic azide 9 (G0-N3) (1.92 g, water (2 Â 50 mL) and brine (1 Â 50 mL) and dried over sodium
4.68 mmol), and by following general procedure A, the first sulphate. The solvent was distilled and the crude product was
generation dendritic chloride 11 (G0-Cl) was obtained as a purified using column chromatography using hexane : CHCl3
colourless solid after elution from a column using CHCl3 : (1 : 4) as eluent to give the pure dienone 16, which was eluted
MeOH (9 : 1). Yield: 85% (1.86 g); melting point: 124–126 1C; from the column as a yellow colored solid. Yield: 86% (6.35 g);
1H NMR (300 MHz, CDCl3): dH 1.45 (s, 36H), 4.48 (s, 2H), 5.19 melting point: 124–126 1C; 1H NMR (300 MHz, CDCl3): dH 1.75–
(s, 4H), 5.59 (s, 4H), 6.63 (s, 1H), 6.65 (s, 2H), 7.32 (d, 4H, 1.84 (m, 2H), 2.55 (t, 2H, J = 2.1 Hz), 2.91 (t, 4H, J = 5.4 Hz), 4.72
J = 8.7 Hz), 7.43–7.46 (m, 8H), 7.55 (d, 4H, J = 8.4 Hz), 7.66 (d, 4H, J = 1.8 Hz), 7.01 (d, 4H, J = 8.7 Hz), 7.45 (d, 4H, J =
(s, 2H), 8.13 (d, 4H, J = 1.2 Hz) ppm; 13C NMR (75 MHz, CDCl3): 8.4 Hz), 7.54 (d, 2H, J = 8.1 Hz), 7.59 (d, 2H, J = 8.1 Hz), 7.75
dC 32.0, 34.8, 46.2, 53.8, 62.2, 101.9, 108.1, 109.1, 116.3, 123.0, (s, 2H) ppm; 13C NMR (75 MHz, CDCl3): dC 23.0, 28.5, 55.8, 75.9,
123.6, 126.8, 127.1, 129.5, 132.9, 138.8, 138.9, 139.9, 143.2, 78.2, 114.8, 129.6, 132.1, 134.7, 136.4, 157.8, 190.2 ppm.
144.4, 159.6 ppm.
Fabrication of dye-sensitized solar cells
Dendritic azide 12 (G1-N3). Following general procedure B,
the first generation G1 dendritic azide 16 (G1-N3) was obtained
as a colourless solid by reacting the first generation dendritic
chloride 11 (G1-Cl) (1 g, 0.94 mmol) with NaN3 (1.2 equiv.). Yield:
Prepared TiO2 electrodes on a fluorinated tin oxide (FTO)
substrate were immersed in a 5 Â 10À5 M solution of photo-
sensitized dye [cis-dithiocynato-N,N-bis(2,2-bipyridyl-4,4-dicarboxylic
acid)ruthenium(II)] (N3 dye) in ethanol for 24 h. The dye coated
photoelectrode was dried and used for the measurement of the
solar energy-to-electric energy conversion efficiency. Initially
30 mg of synthesized dendrimer was made into a paste with
DMF solvent, and this was added to 0.3 g of potassium iodide
(KI) and 0.03 g of iodine (I2) in 25 mL of an acetonitrile + tert-
butyl alcohol mixture in a 1 : 1 volume ratio and stirred for 2 h
at 50 1C to obtain a homogeneous quasi-gel type dendrimer
electrolyte. Finally, the prepared quasi-electrolyte was spread
over a dye-sensitized TiO2 photoanode (active area of 1 Â 1 cm2)
and this was sandwiched with a platinum counter electrode for
the fabrication of a DSSC.
1
98% (976 mg); melting point: 142–144 1C; H NMR (300 MHz,
CDCl3): dH 1.45 (s, 36H), 4.26 (s, 2H), 5.21 (s, 4H), 5.61 (s, 4H),
6.58 (d, 2H, J = 1.5 Hz), 6.64 (t, 1H), 7.33 (d, 4H, J = 8.7 Hz),
7.43–7.48 (m, 8H), 7.57 (d, 4H, J = 8.4 Hz), 7.67 (s, 2H), 8.13 (d,
4H, J = 1.2 Hz) ppm; 13C NMR (75 MHz, CDCl3): dC 31.9, 34.7,
53.7, 54.8, 62.6, 102.5, 108.1, 109.1, 116.2, 122.7, 123.6, 123.8,
127.2, 129.3, 133.2, 138.0, 139.1, 139.3, 143.4, 144.5, 160.0 ppm.
Dendritic chloride 13 (G2-Cl). From 1-(chloromethyl)-3,5-
bis(prop-2-ynyloxy)benzene 10 (0.5 g, 2.13 mmol) and the first
generation dendritic azide 12 (4.91 g, 4.68 mmol), and by
following general procedure A, the second generation dendritic
chloride 13 (G2-Cl) was obtained as colourless solid after elution
from a column using CHCl3 : MeOH (4 : 1). Yield: 79% (2.39 g);
1
melting point: 198–200 1C; H NMR (CDCl3, 300 MHz): dH 1.44
(s, 72H), 4.38 (s, 2H), 5.09 (s, 12H), 5.37 (s, 4H), 5.56 (s, 8H),
6.49 (s, 4H), 6.55 (s, 3H), 6.61 (s, 2H), 7.29 (d, 8H, J = 8.7 Hz),
Conflicts of interest
7.41–7.44 (m, 16H), 7.53 (d, 8H, J = 8.1 Hz), 7.59 (s, 2H), The authors declare that they have no conflicts of interest.
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12 (a) X. Zhang, Y. Zeng, T. Yu, J. Chen, G. Yang and Y. Li,
J. Phys. Chem. Lett., 2014, 13, 2340–2350; (b) T. H. Kwon,
M. K. Kim, J. Kwon, D. Y. Shin, S. J. Park, C. L. Lee, J. J. Kim and
J. I. Hong, Chem. Mater., 2007, 19, 3673–3680; (c) Y. Zeng, Y. Ying
Li, J. Chen, G. Yang and Y. Li, Chem. – Asian J., 2010, 5, 992–1005.
Acknowledgements
The authors thank DST-FIST, New Delhi for the NMR spectral
facility and the Department of Organic Chemistry, University of
Madras, and JB thanks UGC (F17-42/08 (SA-I); Dt: 24-09-2013)
for financial assistance in the form of JRF. SG thanks DST-Fast
Track (SB/FT/CS-063/2013), New Delhi for dye-sensitized solar cell
studies. The authors KA and AP thank DST-SERB for financial
support through N-PDF (file no: PDF/2016/003878; Dt: 23-06-2017).
Also, the authors sincerely thank Dr. Venkatesan Subramanian,
CSIR-CLRI, Chennai for the DFT calculations.
¨
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