Bibridged bianchoring metal-free dyes based on phenoxazine and triphenyl amine as donors for dye-sensitized solar cell applications

Article abstract of DOI:10.1039/c3ra42852d

The design and synthesis of two new metal-free bibridged bianchoring ambipolar dyes (D-(π-A)₂) by linking electron-donating triphenyl amine or phenoxazine and electron-accepting cyanoacetic acid through a cyanovinyl thiophene π-bridge for fabrication of metal-free, dye-sensitized solar cells (DSSCs). The structures of the synthesized dyes were confirmed by NMR, mass spectrometry and elemental analysis. Experimental and theoretical (DFT) approaches revealed that the phenoxazine (POX-th-CN) dye exhibits a lower band gap (2.03 eV) compared to the triphenyl amine (TPA-th-CN) dye (2.19 eV). Due to this lower band gap, the electronic absorption of the phenoxazine dye becomes red-shifted, and, in addition, the fabricated dye-sensitized solar cell device based on this dye shows higher current conversion efficiencies (η = 1.78%) compared to the TPA-th-CN dye. The Royal Society of Chemistry 2013.

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Graphical Abstract
Bibridged bianchoring metal free dyes based on Phenoxazine and
Triphenyl amine as donors for dye sensitized solar cell applications
Sekar Ramkumar and Sambandam Anandan*
Nanomaterials & Solar Energy Conversion Lab, Department of Chemistry,
National Institute of Technology, Tiruchirappalliꢀ620 015, India.
*Corresponding author (sanand@nitt.edu)
Designed and synthesized two new metal free bibridged bianchoring ambipolar dyes (Dꢀ(πꢀA)2) by
linking electron donating triphenyl amine/phenoxazine and electronꢀaccepting cyanoacetic acid
through a cyanovinyl thiophene πꢀbridge for fabrication of metal free dyeꢀsensitized solar cells
(DSSC).

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Bibridged bianchoring metal free dyes based on Phenoxazine and
Triphenyl amine as donors for dye sensitized solar cell applications
Sekar Ramkumar and Sambandam Anandan*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Designed and synthesized two new metal free bibridged bianchoring ambipolar dyes (Dꢀ(πꢀA)2) by
linking electron donating triphenyl amine/phenoxazine and electronꢀaccepting cyanoacetic acid through a
cyanovinyl thiophene πꢀbridge for fabrication of metal free dyeꢀsensitized solar cells (DSSC). The
structures of the synthesized dyes were confirmed by NMR, mass and elemental analysis. The
10 experimental and theoretical (DFT) approaches revealed that phenoxazine (POXꢀthꢀCN) dye exhibits
lowered band gap (2.03 eV) compared to triphenyl amine (TPAꢀthꢀCN) dye (2.19 eV). Due to such
lowered band gap electronic absorption of phenoxazine dye get red shifted and in addition the fabricated
dye sensitized solar cell device based on phenoxazine dye shows higher current conversion efficiencies
( = 1.78%) compared to TPAꢀthꢀCN dye.
double πꢀbridge (Dꢀ(πꢀA)2) charge transporting character for the
development of highly efficient DSSCs.15ꢀ17 Such materials can
provide extended πꢀconjugation which increases the ICT
properties, multilinking abilities, enhanced stability and
15 Introduction
Research on dye sensitized solar cells (DSSCs) has been
intensively progressing in recent years due to their low cost,
lightweight and flexible power production applications compared
to conventional pꢀn junction solar cells.1 Alternative generation
20 of new type of sensitizers based on metal and metal free dyes
have been continuously under progress to meet the improvements
particularly in terms of efficiency and stability compared to the
early works on DSSCs.2ꢀ5 In particular, research on metal free
dyes which has the general structure donorꢀπꢀbridgeꢀacceptor (Dꢀ
25 πꢀA) have attracted a great deal of interest nowadays, due to their
high molar extinction coefficient, simple synthesis procedure, and
environmental friendliness.6ꢀ8 Upon illumination, DꢀπꢀA dye
absorbs light, then intramolecular charge transfer (ICT) occurs
from D to A through the πꢀbridge and from the acceptor moiety
30 electrons are injected to the semiconductor nano particles which
are chemisorbed by the acceptors.9 The absorption, ICT
properties and efficiencies of the dyes can be tuned through
chemical and structural modification by altering the donor, πꢀ
bridge and acceptor moieties.
50 efficiency
of
devices
compare
to
monoanchoring
counterparts.15,20,21 Furthermore, in the recent years a series of biꢀ
anchoring metal free dyes based on donor (carbazole and
diphenylamine), πꢀ linker (cyano vinyl thiophene and cyano vinyl
biphenyl) and acceptor (cyanoacetic acid and rhodanineꢀ3ꢀacetic
55 acid) were synthesized to fabricate DSSC which shows
efficiencies in the range 0.56 ꢀ 4.04%.22,23 In continuation,
replacement of carbazole and diphenylamine donors with
triphenyl amine and phenoxazine particularly to alter the HOMO
energy levels and to shift their absorption behavior to the red
60 region which play a crucial role in determining the overall
conversion efficiency.24,25 Hence, biꢀbridged biꢀanchoring metal
free
dyes
namely
(2E,2'E)ꢀ3,3'ꢀ(5,5'ꢀ(1E,1'E)ꢀ2,2'ꢀ(4,4'ꢀ
(phenylazanediyl)bis(4,1ꢀphenylene))bis(1ꢀcyanoetheneꢀ2,1ꢀ
diyl)bis (thiopheneꢀ5,2ꢀdiyl))bis(2ꢀcyanoacrylic acid) (TPAꢀthꢀ
65 CN)
and
(2E,2'E)ꢀ3,3'ꢀ(5,5'ꢀ(1E,1'E)ꢀ2,2'ꢀ(10ꢀhexylꢀ10Hꢀ
phenoxazineꢀ3,7ꢀdiyl)bis(1ꢀcyanoetheneꢀ2,1ꢀdiyl)bis(thiopheneꢀ
5,2ꢀdiyl))bis(2ꢀcyanoacrylic acid) (POXꢀthꢀCN) (Scheme 1)
based on triphenylamine or phenoxazine as donor,
cyanovinylthiophene as π–linker and cyanoacrylic acid as
35
Generally, triphenylamine, phenoxazine, phenothiazine,
carbazole,
diphenylamine,
coumarin,
indoline,
and
tetrahydroquinoline10,11 are the promising electron donor moieties
for DSSC. Recently, considerable attention has been paid toward
the exploration of triphenylamine12,13 and phenoxazine14 based
1
70 acceptor were synthesized and characterized by H, 13CꢀNMR,
mass and elemental analysis. The electronic and structural
properties of the synthesized dyes were investigated by UVꢀvis
and fluorescence spectroscopy. The HOMO and LUMO energy
levels were calculated using cyclic voltammetry and Density
75 Functional Theory (DFT). Finally, the synthesized dyes are used
for the fabrication of DSSCs.
40 chromophores for DSSC due to their potential electronꢀdonating
ability and their holeꢀtransport properties. In addition, fabricated
DSSC with triphenylamine15 and phenoxazine16 donors show
efficiency of about 5.5% and 4.4%. Besides the use of
triphenylamine and phenoxazine as a donor for sensitizer, many
45 research groups synthesized biꢀanchoring organic materials with
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phenyl, thiophene rings are appear at 7.26ꢀ8.47 ppm. 13C NMR
spectrum of the POXꢀthꢀCN in DMSOꢀd6 solvent shows the
chemical shift (δ) of hexyl chain carbons at 14.11−31.59 ppm and
the methylamine carbon (ꢀCH2Nꢀ) in hexyl chain at 44.86 ppm.
55 Further, the aromatic phenyl carbons, thiophene ring carbons and
vinyl carbons adjacent to the phenyl, thiophene rings are appear
at 91.93ꢀ145.07 ppm. The carboxylic acid carbons in
cyanoacrylic acid groups (ꢀCOOH) appear at 160.74 ppm.
5
10
15
1
Similarly, the H NMR specrum of the synthesized TPAꢀthꢀCN
60 dye in DMSOꢀd6 solvent shows the chemical shift (δ) of the
aromatic phenyl protons, thiophene ring protons and vinyl
protons adjacent to the phenyl, thiophene rings at 7.19ꢀ8.16 ppm
and 13C NMR spectrum of the TPAꢀthꢀCN dye in DMSOꢀd6
solvent shows the chemical shift (δ) of aromatic phenyl carbons,
65 thiophene ring carbons and vinyl carbons adjacent to the phenyl,
thiophene rings are appear at 108.09ꢀ146.92 ppm. The carboxylic
acid carbons in cyanoacrylic acid groups (ꢀCOOH) appear at
1
159.53 ppm. Thus from the H NMR and 13C NMR spectral data
described in the experimental section, the assigned peaks are
70 found consistent with the proposed structure.
Optical Characteristics
The absorption and emission spectra of the synthesized dyes
TPAꢀthꢀCN and POXꢀthꢀCN in DMF solution (3 × 10ꢀ4 M) are
shown in Fig. 1 and their data are depicted in Table 1. TPAꢀthꢀ
75 CN and POXꢀthꢀCN shows two absorbance bands in solution (a
high energy peak at 354 nm and 378 nm followed by a low
energy peak at 485 nm and 518 nm) likely arising from the πꢀπ*
transition and intra molecular charge transfer (ICT) transition
from donor amine to cyano acrylic acid acceptor, respectively. To
80 confirm the ICT property, the absorption behaviour of the
preliminary compounds 1c and 2c have been done and compared
Scheme 1. Synthetic pathway of the organic dyes: (i) POCl3, DMF, 95°C,
4h stirring (Yield: 54% for 1a and 59% for 2a). (ii) 2ꢀ(thiophenꢀ2ꢀyl)ꢀ
20 acetonitrile, 6 M NaOH, ethanol, 2h refluxed (Yield: 60% for 1b and 80%
for 2b). (iii) POCl3, DMF, 95°C, 4h stirring (Yield: 32% for 1c and 30%
for 2c). (iv) Cyanoacetic acid, piperidine, acetonitrile, 12ꢀ24 h refluxed
(Yield: 20% for POXꢀthꢀCN and 18% for TPAꢀthꢀCN).
Synthesis and characterization of materials
25 The synthetic pathways of organic dyes POXꢀthꢀCN and TPAꢀthꢀ
CN were depicted in Scheme 1. The VilsmeierꢀHaack
formylation of hexylated phenoxazine (1) and triphenylamine (2)
gave aldehyde compounds 10ꢀhexylꢀ10Hꢀphenoxazineꢀ3,7ꢀ
dicarbaldehyde (1a) and 4,4’ꢀdiformaltriphenylamine (2a)
26
with POXꢀthꢀCN and TPAꢀthꢀCN compounds respectively
(Fig. S1).
30 respectively. The
synthesized aldehydes (1a and 2a) were
85
coupled with active methylene compound thiopheneꢀ2ꢀ
acetonitrile by Knoevenagel condensation to yield (2E,2'E)ꢀ3,3'ꢀ
(10ꢀhexylꢀ10Hꢀphenoxazineꢀ3,7ꢀdiyl)bis(2ꢀ(thiophenꢀ2ꢀ
yl)acrylonitrile)
(1b)
and
(2E,2'E)ꢀ3,3'ꢀ(4,4'ꢀ
35 (phenylazanediyl)bis(4,1ꢀphenylene))bis(2ꢀ(thiophenꢀ2ꢀ
yl)acrylonitrile) (2b) respectively which on then undergo
VilsmeierꢀHaack formylation reaction followed by Knoevenagel
condensation with cyanoacetic acid gave different products
(2E,2'E)ꢀ3,3'ꢀ(5,5'ꢀ(1E,1'E)ꢀ2,2'ꢀ(10ꢀhexylꢀ10Hꢀphenoxazineꢀ3,7ꢀ
40 diyl)bis(1ꢀcyanoetheneꢀ2,1ꢀdiyl)bis(thiopheneꢀ5,2ꢀdiyl))bis(2ꢀ
cyanoacrylic acid) (POXꢀthꢀCN) and (2E,2'E)ꢀ3,3'ꢀ(5,5'ꢀ(1E,1'E)ꢀ
2,2'ꢀ(4,4'ꢀ(phenylazanediyl)bis(4,1ꢀphenylene))bis(1ꢀ
90
95
cyanoetheneꢀ2,1ꢀdiyl)bis(thiopheneꢀ5,2ꢀdiyl))bis(2ꢀcyanoacrylic
acid) (TPAꢀthꢀCN) respectively. The synthesized dyes were
Fig.1 Normalized absorption and fluorescent spectra of TpaꢀthꢀCN and
100 POXꢀthꢀCN dyes recorded in DMF.
45 characterized by H, 13C NMR, Mass and elemental analysis. H
NMR spectrum of the synthesized dye POXꢀthꢀCN in DMSOꢀd6
solvent shows the chemical shift (δ) of hexyl chain protons at
0.92−1.98 ppm and the methylamine protons (ꢀCH2Nꢀ) in hexyl
chain shows triplet at 4.32 ppm. Further, the aromatic phenyl
50 protons, thiophene ring protons and vinyl protons adjacent to the
1
1
The ICT absorption spectrum of the phenoxazine compound is
red shifted about 38 nm and broadened compared to triphenyl
amine dye due to the high electron donating ability of
phenoxazine which induces the electronic transition probability in
2
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conjugated dipolar systems.27,28
40 TPAꢀthꢀCN and POXꢀthꢀCN are 1.10 V and 0.79 V vs
Ag/AgCl(aq) which were determined from the intersection of two
tangents drawn at the rising current and background charging
current of a cyclic voltammogram, may be due to the electron
donating ability of triphenyl amine and phenoxazine. The lower
45 the oxidation potential of phenoxazine based dye indicated that
phenoxazine is a good electron donor compared to triphenyl
amine.32 The HOMO and LUMO energy levels of the synthesized
50
Fig.2 Absorption spectra of TpaꢀthꢀCN and POXꢀthꢀCN dyes adsorbed on
nanocrystalline TiO2 films.
55
5
However the absorption spectrum of the dyes on TiO2 surface
(Fig. 2) shows blue shifted wavelength maximum and broadened
longer wavelength compared to solution spectra due to the Hꢀ
aggregation of dyes on TiO2 surface and dyeꢀdye or dyeꢀTiO2
interactions respectively.29,30 Further, this donors (triphenyl
Fig.3 Cyclic voltammogram measured in DMF (3× 10ꢀ4 M) of POXꢀthꢀ
60 CN and TPAꢀthꢀCN at a Pt working electrode, glassy carbon counter
electrode and Ag/Ag+ reference electrode. (0.1 M TBAP as an electrolyte;
scan rate 0.1V/s).
10 amine (TPA) and phenoxazine (POX)) act as a good light
absorber compared to the donors (carbazole (Car) and diphenyl
amine (Dpa)).22,23 The fluorescence emission spectra of
triphenylamine and phenoxazine derivatives in DMF solution
were recorded at their excitation wavelength shows emission
dyes were calculated based on the relationship mentioned in
supporting information.33 The lower HOMO energy levels of
15 wavelength at 617 nm (λex= 510 nm), 682 nm (λex= 590 nm) 65 POXꢀthꢀCN (ꢀ5.14 eV) compared to TPAꢀthꢀCN (ꢀ5.45 eV)
respectively.
indicates the effective regeneration of dyes from redox electrolyte
which in turn may responsible to improve the device efficiency.34
The LUMO level of dyes was calculated to be ꢀ3.26 eV for TPAꢀ
thꢀCN and ꢀ3.11 eV for POXꢀthꢀCN by subtracting the band gap
70 which was estimated by the intersection between the absorption
and emission spectra, from the HOMO level. 33
Electrochemical Characteristics
The electron injection from the excited dye to conduction band
(CB) of the semiconductor nano particles and the dye
20 regeneration through the electrolytes are mainly depends on the
LUMO energy level relative to reduction potential and HOMO
energy level
relative to oxidation potential of the dyes
respectively. The conduction band energy level of TiO2 (−4.0 eV
vs vacuum) is generally lower than the LUMO level of the dye
25 for electron injection while the redox potential of electrolyte (Iꢀ
/I3ꢀ is −4.6 eV vs vacuum) generally higher than the HOMO level
of the dye for regeneration of the oxidized dye molecule by
iodide ion (Iꢀ) and avoiding the charge recombination between the
oxidized dye molecules and photo‐injected electrons in the
30 nanocrystalline TiO2 film.10 The oxidation peak potentials of Tpaꢀ
thꢀCN and POXꢀthꢀCN dyes are determined by cyclic
voltammogram by comparison with ferrocene (Fc; 4.8 eV)31 in
DMF medium (Fig. 3) and the corresponding data were shown in
Table 1. Cyclic Voltammogram of both dyes (Fig. 3) showed one
35 quasiꢀreversible oxidation peak due to the electron donating
ability of triphenyl amine (1.10 V) and phenoxazine (0.79 V)
groups and one irreversible reduction peak due to the electron
withdrawing ability of cyanoacrylic acid group (ꢀ0.02 V in TPAꢀ
thꢀCN and ꢀ0.01V in POXꢀthꢀCN). The oxidation potential of
75
80
85
90 Fig.4 Schematic experimental energy levels of the dyes with frontier
molecular orbital of HOMO and LUMO at the B3LYP/6ꢀ31G (d).
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Table 1: Photophysical, electrochemical and theoretical data of synthesized dyes.
opt
abs/nm(ε/Mꢀ1cmꢀ1)a
λem
Dye
Eg
Eox/V (vs.
EHOMO
ELUMO
EHOMO
ELUMO
λ
(eV)c
Fc)
(eV)d
(eV)d
(eV)e
(eV)e
/nma,b
POXꢀthꢀCN
TPAꢀthꢀCN
378 (34876)
518 (26219)
682
2.03
0.79
ꢀ5.14
ꢀ3.11
ꢀ5.56
ꢀ3.30
354 (25736)
484 (41702)
617
2.19
1.10
ꢀ5.45
ꢀ3.26
ꢀ5.93
ꢀ3.51
aAbsorption and emission spectra were recorded in DMF solution (3×10ꢀ4 M) at room temperature. bDyes were excited at their absorption maximum value
c
(for POXꢀthꢀCN, λex = 590 nm and for TPAꢀthꢀCN, λex = 510 nm). Optical band gap calculated from intersection between the absorption and emission
d
ox
5
spectra. The values of EHOMO and ELUMO were calculated with the following formula: HOMO (eV) = ꢀe(Eonset V(vs Fc/Fc+) + 4.8 V); LUMO(eV)= E0ꢀ0
(eV) + EHOMO(eV), where E0ꢀ0 is the intersection between the absorption and emission spectra of the sensitizers. eB3LYP/6ꢀ31G (d) calculated values.
Table 2: Photovoltaic performance data of the fabricated devices based on synthesized dyes.
Surface
Dye
JSC (mAcmꢀ2)
VOC (mV)
FF
(%)a
IPCE (%)
concentration Γ
Electron lifetime (τeff) (ms)
(mol/cm2)
POXꢀthꢀCN
TPAꢀthꢀCN
7.4
4.7
631
500
0.32
0.48
1.78
1.32
57.5
48.0
2.92 × 10ꢀ7
2.76 x 10ꢀ7
9.10
2.21
aIllumination: 85 mW cmꢀ2 simulated AM 1.5 G solar light; electrolyte containing: 0.05 M I2/0.5 M KI/0.5 M 4ꢀtertꢀbutyl pyridine in 3ꢀ
10 methoxypropionitrile.
The band gap of the POXꢀthꢀCN is smaller than the TPAꢀthꢀCN;
nano particles.36 Thus the theoretically calculated HOMO, LUMO
this lowers the HOMO–LUMO energy bandgap which inturn 35 and band gap energy levels are in good agreement with the
shifts the absorption region towards longer wavelength of the
solar spectrum.35 Thus, HOMO and LUMO values are in good
15 agreement with the energy level of TiO2 conduction band and
electrolyte redox potential respectively as shown in Figure 4.
experimental values (see Table 1).
Theoritical Characteristics using DFT calculation
40
The optimized electronic structures and frontier molecular orbital
diagrams of the synthesized dyes were determined using density
20 functional theory (DFT) (B3LYP/6ꢀ31G (d) basis set with the
Gaussian 09 program package) and it is shown in Figure 5. In
both TPAꢀthꢀCN and POXꢀthꢀCN, the HOMO is mainly
populated over the amine and cyano vinyl thiophene blocks with
considerable contribution from the former, whereas LUMO is
25 delocalized through the cyano vinyl thiophene and cyanoacrylic
acid fragments with sizable contribution from the latter. The
LUMO of the πꢀbridge is similar to the HOMO of the dyes, but
the lobes do not align. This spatial separation in HOMO and
LUMO orbitals through πꢀbridge is an important characteristic for
30 DꢀπꢀA organic dyes in DSSCs because this structural strategy
will induce the intramolecular charge transfer (ICT) from the
donor to acceptor groups under illumination and increases the
interfacial electron injection from organic dye to semiconductor
45
50 Fig.5 The optimized structure and frontier orbitals of the POXꢀthꢀCN and
TPAꢀthꢀCN dyes calculated with B3LYP/6ꢀ31G (d) level.
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substituent in POXꢀthꢀCN which induces the controlled
aggregation of dyes on TiO2 surface.39
Photovoltaic Characteristics
The currentꢀvoltage characteristics of fabricated DSSCs using
synthesized organic dyes (TPAꢀthꢀCN and POXꢀthꢀCN) as
sensitizers of type (FTO/TiO2/dye/liquid electrolyte/Pt/FTO)
were measured under simulated AM 1.5 irradiation (85 mW/cm2)
and the calculated data such as shortꢀcircuit photocurrent density
(JSC), open circuit photovoltage (VOC), and fill factor (FF) were
determined from Fig. 6 and it is summarized in Table 2. The
fabricated device based on POXꢀthꢀCN dye exhibits a short
60
5
65
10 circuit current density (JSC) of 7.4 mA cmꢀ2, open circuit photo
voltage (VOC) of 631 mV and a fill factor (FF) of 0.32%.
Similarly the device based on TPAꢀthꢀCN dye exhibits a short
circuit current density (JSC) of 4.7 mA cmꢀ2, open circuit photo
voltage (VOC) of 500 mV and a fill factor (FF) of 0.48%. The
15 efficiency ( ) of POXꢀthꢀCN is 1.78% and TPAꢀthꢀCN is 1.32%
70
which was calculated using the following equation:
=
JSC×VOC×FF/light intensity. The reason for the lower efficiency
of these dyes based devices are due to their lower fill factor.37
The efficiency of phenoxazine dye is higher than the triphenyl
20 amine dye may be due to the absorption red shift and as well as
low band gap property of the phenoxazine dye.28 The incident
photonꢀtoꢀcurrent conversion efficiencies (IPCE) as a function of
wavelength for POXꢀthꢀCN and TPAꢀthꢀCN are presented in Fig.
7. Fabricated solar cell based on both the dyes exhibited a broad
25 IPCE spectrum from 350 to 700 nm. POXꢀthꢀCN showed the
IPCE value of 57.5% at 440 nm and 32% at 535 nm whereas
TPAꢀthꢀCN showed the IPCE value of 48.0 % at 475 nm.
Compared to triphenyl amine based device, phenoxazine based
Fig. 7 IPCE spectra of the fabricated devices based on POXꢀthꢀCN and
75 TPAꢀthꢀCN dyes.
Electrochemical Impedance Characteristics
To further understand the internal resistances for the charge
transfer processes such as the charge recombination at the
80
30
35
40
85
90
95
45 Fig. 6 Current densityꢀvoltage characteristics for POXꢀthꢀCN and TPAꢀ
thꢀCN based devices for DSSCs under illumination of simulated solar
light (AM 1.5, 85 mW/cm2).
100
devices exhibited broader wavelength IPCE spectra resemble
with the absorption spectra on dye adsorbed TiO2 film (Fig. 2).
50 Thus the longer wavelength absorption region favors the light
harvesting ability and increases the photocurrent response on the
POXꢀthꢀCN DSSC device. The adsorbed dye amount (Γ) on TiO2
surface was calculated for correlating the efficiency of the
devices by desorbing the dye from the TiO2 surface using 0.1M
55 NaOH in DMF/H2O (1:1) mixture38 indicates that the
phenoxazine based dyes are adsorbed comparatively higher than
the triphenyl amine based dyes due to the presence of hexyl
105
Fig. 8 EIS spectra of DSSCs based on POXꢀthꢀCN and TPAꢀthꢀCN dyes
measured at VOC, 85 mW/cm2. (A) Nyquist plots. (B) Bode phase plots.
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TiO2/dye/electrolyte interface, electron transport in the TiO2
electricallyꢀconnected network of TiO2 particles under tubular
furnace. After the sintering process, when the temperature of the
plate drops to 50° to 70°C, it was immerse into the dye solution
and leave for 24 h. Excess nonꢀadsorbed dye were washed with
−
electrode, electron transfer at the Pt counter electrode and I−/I3
transport at the electrolyte were measured using electrochemical
impedance spectra (EIS)40 at VOC under AM 1.5 with a frequency
5
range of 100ꢀ104 Hz. As shown in Nyquist plots (Fig. 8A) the 60 anhydrous ethanol. And the Platinum catalyst counter electrode
radius of the middle semicircle is in the order of POXꢀthꢀCN <
TPAꢀthꢀCN indicating the electron transport is in the order of
TPAꢀthꢀCN < POXꢀthꢀCN which reflected in the efficiency of
devices.41 The electron lifetime (τeff) in TiO2 conduction band
was prepared by deposition of H2PtCl6.6H2O solution (0.005 mol
dm−3 in isopropanol) onto FTO glass and then sintering at 400oC
for 20 min.46 The DSSC device was fabricated by the following
method: the photo cathode was placed on top of the photo anode
10 were calculated from the Bode phase plots (Fig. 8B) using the 65 and was tightly clipped together. Then, liquid electrolyte
following equation:42 τeff = 1/2πf, where, f is the frequency peak
in Bode phase plot. The electron lifetime was calculated for
POXꢀthꢀCN device (9.10 ms) and TPAꢀthꢀCN device (2.21 ms)
from the corresponding frequency peak maxima in Bode phase
15 plot respectively. Thus, the higher electron life time in TiO2
conduction band for POXꢀthꢀCN device indicates the inhibition
0.05MI2/0.5MKI/0.5M 4ꢀtertꢀbutyl pyridine (TBP) in 3ꢀ
methoxypropionitrile was injected inꢀbetween the two electrodes.
Instruments
1Hꢀ and 13CꢀNMR spectra was realized on a 300/400 MHz
70 BRUKER spectrometer in deuterated chloroform or
dimethylsulfoxide solution at 298K. Chemical shifts (δ values)
were recorded in units of ppm relative to tetramethylsilane (TMS)
as an internal standard. The molecular weight of the dyes were
determined by Micromass QUATTRO 11 ESIꢀMS spectrometer
75 coupled to a Hewlett Packard series 1100 degasser. Elemental
analyses were performed on a Vario EL III elemental analyzer.
Absorption and fluorescence spectra were measured in DMF
solution on a T90+ UVꢀvis spectrometer and Shimadzu RFꢀ5301
PC spectrofluorophotometer respectively. Electrochemical
80 measurements were performed on a Metrohm Autolab PGSTAT
potentiostat/galvanostatꢀ84610. All measurements were carried
out at room temperature with a conventional threeꢀelectrode
configuration consisting of a platinum disc working electrode, a
glassy carbon (GC) auxiliary electrode, and a Ag/AgCl(aq) was
85 used as the reference electrode. The potentials were reported vs
ferrocene as standard using a scan rate of 0.1 Vsꢀ1 and the sample
−
of the electron recombination to the I3 ions and thus enhances
the device efficiency compare to TPAꢀthꢀCN device. 23
FTꢀIR Characteristics
20 To understand the adsorption behavior of synthesized dyes (POXꢀ
thꢀCN and TPAꢀthꢀCN) on the TiO2 nanoparticles for interfacial
electron injection, Fourier Transform Infrared (FTꢀIR)
spectroscopy was performed in ATR mode (Fig. S2 (A) & (B)).
The ATRꢀFTꢀIR spectrum of both dye molecules shows the
25 characteristic peaks of the ꢀC≡N and –C=O of carboxylic acid
group at 2210 cmꢀ1, 1738 cmꢀ1 for POXꢀthꢀCN and 2209 cmꢀ1,
1732 cmꢀ1 for TPAꢀthꢀCN respectively. The large broad peak
above 3000 cmꢀ1 is due to the adsorbed moisture and –OH
stretching of carboxylic acid group. Whereas for the dye adsorbed
30 TiO2 films, the –C=O stretching band around 1730 cmꢀ1 are
disappeared and a new band appeared in 1583 cmꢀ1 indicates the
nature of dye binding on TiO2 surface via the bidendate
carboxylate (ꢀOꢀCꢀOꢀ) due to the deprotonation of –COOH
group.43 In addition, observed peak broadening also supports that
35 the dyeꢀTiO2 interactions have occurred.44
solutions contained 3×10ꢀ4
M
sample and 0.1
M
tetrabutylammonium perchlorate (TBAP) in anhydrous DMF as a
supporting electrolyte under Argon atmosphere. Electrochemical
90 impedance spectroscopy (EIS) measurements were done under 85
mWcmꢀ2 light illumination by using an Autolab PGSTAT
potentiostat/galvanostatꢀ84610. The impedance spectra were
recorded with a frequency ranging between 10 kHz to 1Hz at
their open circuit potential (OCP). The IPCE spectra were
95 recorded using Oriel 300W Xe Arc lamp in combination with an
Oriel Cornerstone 260¼ monochromator. The number of incident
photons was calculated for each wavelength using a calibrated
monocrystalline silicon diode as reference. ATRꢀFTꢀIR spectra
Experimental Section
Materials
All reagents and chemicals were purchased from Alfa Aesar and
SigmaꢀAldrich and used without further purification unless
40 specified otherwise. All solvents were dried by refluxing for at
least 24 h over CaH2 and freshly distilled prior to use. All column
chromatographic separations were carried out on Merck silica gel
(60ꢀ120 mesh). FTO glass plates (sheet resistance 10ꢁ/□) were
were measured with
a Thermo spectrophotometer system
100 equipped with a ZnSe prism.
purchased from BHEL, INDIA The photoanode was prepared by
.
Synthesis
45 the following the procedure as follows45: The glacial acetic acid
(5 ml), 7.5 ml of tetraisopropyl titanate (C12H28O4Ti) and one
drop of Triton Xꢀ100 were mixed with 15 ml of 2ꢀpropanol.
Water (5 ml) was added to the above solution drop wise while
vigorously stirring the solution. The resulting semi colloidal
50 suspension was dispersed on a fluorine doped tin oxide (FTO)
glass plate by glass rod using doctor blade technique. Loose crust
on TiO2 film was removed by wiping smoothly by cotton wool.
The thickness of the TiO2 film was successively controlled, by
repeating the above coating procedure. Then it was sintered at
55 450°C for 1 h for removing the binder, solvent and giving an
Synthesis of 10ꢀhexylꢀ10Hꢀphenoxazine (1)
1ꢀBromohexane (2.2 g, 0.01308 mol), phenoxazine (2.0 g, 0.0109
mol) and potassium hydroxide (0.734 g, 0.01308 mol) were
105 added to 26 mL of dimethylsulfoxide (DMSO) and stirred the
reaction mixture for 4 h at RT. After identifying the completion
of reaction by TLC, the resulting mixture was extracted with
dichloro methane (DCM)/water and then dried the organic
portion with Na2SO4. The solvent was evaporated and the
110 resulting dark brown liquid was purified by column
chromatography on silica by using hexane as solvent to give
6
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DOI: 10.1039/C3RA42852D
colorless liquid (Yield: 63.4%; 1.85 g). 1H NMR (CDCl3, ppm): δ
6.80ꢀ6.76 (m, 2H), 6.63ꢀ6.60 (m, 4H), 6.46 (d, 2H, J=8Hz), 3.46
(t, 2H, J=8Hz), 1.68ꢀ1.56 (m, 2H), 1.43ꢀ1.34 (m, 6H), 0.92 (t, 3H,
J=7.2Hz).
131.94, 131.32, 125.01, 123.42, 117.03, 116.44, 116.35, 49.40,
31.46, 26.67, 26.58, 22.71, 14.07. HRꢀMS Waters (Micromass):
QꢀTof micro (YAꢀ105) (TOF MS ES+) Anal. Calcd. for
60 C34H27N3O3S2: 589.1494. Found: 590.1412 [M+H]+.
5
Synthesis of 10ꢀhexylꢀ10Hꢀphenoxazineꢀ3,7ꢀdicarbaldehyde
(1a)
Synthesis of (2E,2'E)ꢀ3,3'ꢀ(5,5'ꢀ(1E,1'E)ꢀ2,2'ꢀ(10ꢀhexylꢀ10Hꢀ
phenoxazineꢀ3,7ꢀdiyl)bis(1ꢀcyanoetheneꢀ2,1ꢀ
diyl)bis(thiopheneꢀ5,2ꢀdiyl))bis(2ꢀcyanoacrylic acid) (POXꢀthꢀ
CN)
Freshly distilled POCl3 (17.4 ml, 25eq) was added drop wise to
DMF (13.3 ml, 23eq) under N2 atmosphere at 0° C, and then the
solution was stirred for 1 h. Compound 1 (2 g, 7.48 m.mol) was
65 0.4g (0.6312 mmol) of compound 1c, 0.21 g
(2.89 mmol) of
cyanoacetic acid was added to 50 ml of freshly distilled
acetonitrile. 0.181 ml of piperidine was added to this mixture and
refluxed the reaction for 12 h. After cooling to room temperature,
the solvent was removed by distillation. The residue was purified
70 by column chromatography using silica gel and CHCl3:CH3OH
(10:1, v:v) mixed solvent as the eluent to give the dye as red
10 added to the above solution, and the resulting mixture was stirred
for 4 h at 95oC. After cooling to room temperature, the mixture
was poured into a beaker containing iceꢀcube, and basified with
4M NaOH. Filtered the solid and extracted with ethylacetate
(EA)/brine. After evaporating the organic solvent the crude
15 product was purified by column chromatography on silica using a
mixture of EA/Hexane (1:6, v/v), to give a greenish yellow solid
1
colour solid (Yield: 20%; 0.15 g). H NMR (DMSOꢀd6, ppm):
1
8.47ꢀ8.42 (m, 2H), 8.20ꢀ8.17 (m, 2H), 8.14ꢀ8.11 (m, 2H), 7.99ꢀ
7.96 (m, 2H), 7.68 (m, 2H), 7.60ꢀ7.58 (m, 2H), 7.51ꢀ7.47 (m,
75 2H), 7.35ꢀ7.26 (m, 2H), 4.32 (t, 2H,J=8.1 Hz), 1.98ꢀ1.96 (m, 2H),
1.59ꢀ1.53 (m, 2H), 1.44ꢀ1.38 (m, 4H), 0.93 (t, 3H,J=6.9 Hz).13C
NMR (DMSOꢀd6, ppm): δ 160.74, 145.07, 137.56, 131.33,
128.48, 124.28, 121.97, 114.92, 111.70, 109.85, 100.78, 91.93,
44.86, 31.59, 26.59, 25.17, 22.75, 14.11. ATRꢀFTꢀIR: 2210 cmꢀ1,
80 1738 cmꢀ1.Anal. Calcd. for C40H29N5O5S2: C, 66.37; H, 4.04; N,
9.68. Found: C, 66.21; H, 4.14; N, 9.84. m/z (ESI) Anal. Calcd.
for C40H29N5O5S2: 723.161. Found: 723.24 [M]+.
(Yield: 54%; 1.31 g). H NMR (CDCl3, ppm): δ 9.69 (s, 2H),
7.34ꢀ7.32 (m, 2H), 7.25 (s, 2H), 6.59 (d, 2H, J=8Hz), 3.56 (t, 2H,
J=8 Hz), 1.71ꢀ1.63 (m, 2H), 1.45ꢀ1.36 (m, 6H), 0.92 (t, 3H,
20 J=7.2Hz).
Synthesis of (2E,2'E)ꢀ3,3'ꢀ(10ꢀhexylꢀ10Hꢀphenoxazineꢀ3,7ꢀ
diyl)bis(2ꢀ(thiophenꢀ2ꢀyl)acrylonitrile) (1b)
Freshly distilled ethanol (24ml) was taken in a 100 ml single neck
round bottom flask. The compound 1a (1g, 1eq) and thiopheneꢀ2ꢀ
25 acetonitrile (0.84g, 2.2eq) were added to 100 ml single neck
round bottom flask containing 24ml freshly distilled ethanol.
Then, 6M NaOH (0.7 ml) was added to this reaction mixture at
room temperature after that the reaction mixture was refluxed for
2 h. A dark red color solid was formed. It was filtered and
30 recrystallized in chloroform and methanol to give the product
Synthesis of 4,4’ꢀDiformaltriphenylamine (2a)
Freshly distilled POCl3 (28.4 ml, 25eq) was added drop wise to
85 DMF (21.7 ml, 23eq) under an atmosphere of N2 at 0° C, and
then it was stirred for 1 h. Compound 2 (3 g, 12.24 m.mol) was
added to the above solution, and the resulting mixture was stirred
for 4 h at 95oC. After cooling to room temperature, the mixture
was poured into a beaker containing iceꢀcube, and basified with
90 4M NaOH. Filtered the solid and extracted with ethylacetate
(EA)/brine. After evaporating the organic solvent the crude
product was purified by column chromatography on silica using a
mixture of EA/Hexane (1:6, v/v), to give a yellow solid (Yield:
59%; 2.2 g). 1H NMR (CDCl3, ppm): δ 9.95 (s, 2H), 7.85ꢀ7.82
95 (m, 4H), 7.48ꢀ7.44 (m, 2H), 7.34ꢀ7.32 (m, 1H), 7.26ꢀ7.22 (m,
6H).
(Yield: 60%; 1.0 g).
1H NMR (CDCl3, ppm): δ 7.36 (d,
2H, J=8.4 Hz), 7.31 (d, 2H, J=3.2 Hz), 7.25 (d, 2H, J=3.6 Hz),
7.15ꢀ7.09 (m, 2H), 7.05ꢀ7.03 (m, 4H), 6.55ꢀ6.50 (m, 2H),3.52 (t,
2H,J=8 Hz), 1.67 (d, 2H, J=6 Hz),1.43ꢀ1.38 (m, 6H), 0.95ꢀ0.93
35 (m, 3H).13C NMR (CDCl3, ppm): δ 139.11, 138.55, 136.95,
134.20, 132.44, 128.37, 127.97, 127.54, 126.63, 116.73, 105.73,
44.23, 31.55, 27.05, 26.64, 22.81, 14.13.
Synthesis of (2E,2'E)ꢀ3,3'ꢀ(10ꢀhexylꢀ10Hꢀphenoxazineꢀ3,7ꢀ
diyl)bis(2ꢀ(5ꢀformylthiophenꢀ2ꢀyl)acrylonitrile) (1c)
40 DMF (3.3 ml, 23eq) was taken in a 3 neck 50 ml round bottom
flask. To this freshly distilled POCl3 (4.4 ml, 25eq) was added
drop wise under N2 atmosphere at 0° C, and then it was stirred for
1 h. 1g of compound 1b was added to the above solution, and the
resulting mixture was stirred for 4 h at 95°C. It was monitored by
45 TLC. After the completion of the reaction, it was cooled to room
temperature and then the mixture was poured into a beaker
containing iceꢀcube, and basified with 4M NaOH. It was
extracted with dichloromethane/brine. After evaporating the
organic solvent, the crude product was purified by column
Synthesis of (2E,2'E)ꢀ3,3'ꢀ(4,4'ꢀ(phenylazanediyl)bis(4,1ꢀ
phenylene))bis(2ꢀ(thiophenꢀ2ꢀyl)acrylonitrile) (2b)
The compound 2a (1.5g, 1eq) and thiopheneꢀ2ꢀacetonitrile
100 (1.35g, 2.2eq) were added to 100 ml single neck round bottom
flask containing 45 ml freshly distilled ethanol. Then, 6M NaOH
(0.1 ml) was added to this reaction mixture at room temperature
after that the reaction mixture was refluxed for 2 h. It was
monitored by TLC. An orange color solid was formed. It was
105 filtered and recrystallized in dichloromethane and methanol to
give the product (Yield: 80%; 2.26 g). 1H NMR (CDCl3, ppm): δ
7.77 (d, 4H, J=8Hz), 7.38ꢀ7.34 (m, 4H), 7.29ꢀ7.25 (m, 4H), 7.21ꢀ
7.12(m, 7H), 7.07ꢀ7.05 (m, 2H).13C NMR (CDCl3, ppm): δ
148.82, 146.00, 139.76, 138.99, 130.65, 130.03, 128.23, 127.96,
110 126.75, 126.62, 125.85, 125.58, 123.2, 117.43, 103.82.
50 chromatography on silica using
a
mixture of Ethyl
acetate/Hexane (1:4, v/v), to give a dark red color solid (Yield:
1
32%; 0.35 g). H NMR (CDCl3, ppm): 9.81 (s, 2H), 8.37 (d, 4H,
J=9 Hz), 8.26 (s, 2H), 7.51 (d, 2H, J=9 Hz), 7.40 (s, 2H), 7.33ꢀ
7.25 (m, 2H), 7.10ꢀ7.08 (m, 2H), 4.27 (t, 2H,J=8.1 Hz), 1.99ꢀ1.97
55 (m, 2H), 1.59ꢀ1.58 (m, 2H), 1.56ꢀ1.43 (m, 4H), 0.98ꢀ0.94 (m,
3H). 13C NMR (CDCl3, ppm) δ 189.28, 141.69, 135.16, 133.05,
Synthesis of (2E,2'E)ꢀ3,3'ꢀ(4,4'ꢀ(phenylazanediyl)bis(4,1ꢀ
phenylene))bis(2ꢀ(5ꢀformylthiophenꢀ2ꢀyl)acrylonitrile) (2c)
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DOI: 10.1039/C3RA42852D
DMF (3.5 ml, 23eq) was taken in a 3 neck 50 ml round bottom
may be due to their lower fill factor which will be rectified by
adding appropriate electrolytes.
flask. To this freshly distilled POCl3 (4.5 ml, 25eq) was added
drop wise under N2 atmosphere at 0° C, and then it was stirred for
1 h. 1g of compound 2b was added to the above solution, and the
resulting mixture was stirred for 4 h at 95°C. It was monitored by
TLC. After the completion of the reaction, it was cooled to room
temperature and then the mixture was poured into a beaker
containing iceꢀcube, and basified with 4M NaOH. It was
extracted with dichloromethane/brine. After evaporating the
Notes and references
5
60 Nanomaterials
& Solar Energy Conversion Lab, Department of
Chemistry, National Institute of Technology, Tiruchirappalli-620 015,
India. E-mail: sanand@nitt.edu;sanand99@yahoo.com
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
65 DOI: 10.1039/b000000x/
10 organic solvent, the crude product was purified by column
chromatography on silica using a mixture of CHCl3:CH3OH
(20:1, v/v), to give a dark red color solid (Yield: 30%; 0.33 g). 1H
NMR (CDCl3, ppm): δ 10.00 (s, 2H), 7.82 (d, 4H, J=8.7 Hz),
7.55ꢀ7.51 (m, 8H), 7.44ꢀ7.37 (m, 1H), 7.35ꢀ7.26 (m, 2H), 7.20ꢀ
15 7.14 (m, 4H). 13C NMR (CDCl3, ppm): δ 190.66, 153.39, 152.15,
145.64, 140.94, 131.44, 130.29, 127.20, 126.40, 122.90, 108.54,
104.24. HRꢀMS Waters (Micromass): QꢀTof micro (YAꢀ105)
(TOF MS ES+) Anal. Calcd. for C34H21N3O2S2: 567.1075.
Found: 568.1062 [M+H]+.
Acknowledgements
Authors thank DST, New Delhi (SR/S1/PCꢀ49/2009)
for sanctioning major research project and IndiaꢀSpain Joint
Collaborative project (DST/INT/Spain/Pꢀ37/11).
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(2E,2'E)ꢀ3,3'ꢀ(5,5'ꢀ(1E,1'E)ꢀ2,2'ꢀ(4,4'ꢀ
(phenylazanediyl)bis(4,1ꢀphenylene))bis(1ꢀcyanoetheneꢀ2,1ꢀ
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0.5g (0.8808 mmol) of compound 2c, 0.3 g (3.52 mmol) of
25 cyanoacetic acid was added to 50 ml of freshly distilled
acetonitrile. 0.261 ml of piperidine was added to this mixture and
refluxed the reaction for 24 h. After cooling to room temperature,
the solvent was removed by distillation. The residue was purified
by column chromatography using silica gel and CHCl3:CH3OH
30 (10:1, v:v) mixed solvent as the eluent to give the dye as red
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1
colour solid (Yield: 18%; 0.12 g). H NMR (DMSOꢀd6, ppm):
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2009, 48, 2474.
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8.16ꢀ8.13 (m, 2H), 8.01ꢀ7.98 (m, 4H), 7.90ꢀ7.79 (m, 4H), 7.75ꢀ
7.69 (m, 4H), 7.55ꢀ7.50 (m, 2H), 7.40ꢀ7.38 (m, 4H), 7.27ꢀ7.19
(m, 6H). 13C NMR (DMSOꢀd6, ppm): δ 159.53, 146.92, 140.16,
35 134.23, 134.16, 132.41, 132.31, 132.05, 131.50, 131.44, 127.39,
127.35, 126.01, 122.42, 119.10, 118.83, 116.77, 116.19, 109.53,
108.09. ATRꢀFTꢀIR: 2209 cmꢀ1, 1732 cmꢀ1.Anal. Calcd. for
C40H23N5O4S2: C, 68.46; H, 3.30; N, 9.98. Found: C, 68.38; H,
3.36; N, 9.82. m/z (ESI) Anal. Calcd. for C40H23N5O4S2:
40 701.1191. Found: 701.17 [M]+.
15 A. Abbotto, N. Manfredi, C. Marinzi, F.D. Angelis, E. Mosconi, J.H.
Yum, Z. Xianxi, M. K. Nazeeruddin and M. Gratzel, Energy.
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Conclusion
17 S.S. Park, Y.S. Won, Y.C. Choi and J.H. Kim, Energy Fuels, 2009,
23, 3732.
18 Y.S.Won, Y. Yang, J.H. Kim, J.H. Ryu, K.K. Kim and S.S. Park,
In conclusion, designed and synthesized here two new biꢀbridged
biꢀanchoring metal free dyes based on phenoxazine and triphenyl
amine as donor, cyanovinylthiophene as π–linker and
45 cyanoacrylic acid as acceptor for DSSC applications. Among the
two dyes (POXꢀthꢀCN and TPAꢀthꢀCN), phenoxazine based dye
exhibits higher power conversion efficiency due to the higher
electron donating ability of phenoxazine compared to triphenyl
amine because the former has electron rich nitrogen and oxygen
50 atoms while the latter have only nitrogen atom. In addition, such
property is responsible for the decrease in the band gap and as
well as increases the light harvesting property to red region;
brings the HOMO energy level near to redox potential of
electrolytes for effective dye regeneration and the increases
55 electron life time in TiO2 conduction band. The lower the
efficiency of these devices compared to literally available devices
105
110
115
Energy Fuels, 2010, 24, 3676.
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T.W. Huang and C.C. Wu, Org. Lett., 2012, 14, 6338.
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8
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DOI: 10.1039/C3RA42852D
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