Molecular engineering of simple phenothiazine-based dyes to modulate dye aggregation, charge recombination, and dye regeneration in highly efficient dye-sensitized solar cells

Article abstract of DOI:10.1002/chem.201304897

A series of simple phenothiazine-based dyes, namely, TP, EP, TTP, ETP, and EEP have been developed, in which the thiophene (T), ethylenedioxythiophene (E), their dimers, and mixtures are present to modulate dye aggregation, charge recombination, and dye regeneration for highly efficient dye-sensitized solar cell (DSSC) applications. Devices sensitized by the dyes TP and TTP display high power conversion efficiencies (PCEs) of 8.07 (J_sc=15.2mA cm ^-2, V_oc=0.783V, fill factor (FF)=0.679) and 7.87 % (J _sc=16.1mA cm^-2, V_oc=0.717V, FF=0.681), respectively; these were measured under simulated AM 1.5 sunlight in conjunction with the I^-/I₃^- redox couple. By replacing the T group with the E unit, EP-based DSSCs had a slightly lower PCE of 7.98 % with a higher short-circuit photocurrent (J_sc) of 16.7mA cm^-2. The dye ETP, with a mixture of E and T, had an even lower PCE of 5.62 %. Specifically, the cell based on the dye EEP, with a dimer of E, had inferior J_sc and V_oc values and corresponded to the lowest PCE of 2.24 %. The results indicate that the photovoltaic performance can be finely modulated through structural engineering of the dyes. The selection of T analogues as donors can not only modulate light absorption and energy levels, but also have an impact on dye aggregation and interfacial charge recombination of electrons at the interface of titania, electrolytes, and/or oxidized dye molecules; this was demonstrated through DFT calculations, electrochemical impedance analysis, and transient photovoltage studies. Shape sensitivity: A solar cell sensitized with a phenothiazine-based dye containing a bithiophene unit (TTP) as the electron donor has a high power conversion efficiency (PCE) of 7.87 %, whereas a related dye containing an ethylenedioxythiophene and thiophene unit (ETP) and that containing an ethylenedioxythiophene dimer (EEP) have much lower PCE values. The photovoltaic performance is sensitive to structural modifications (see picture).

Full text of DOI:10.1002/chem.201304897

DOI: 10.1002/chem.201304897
Full Paper
&
Phenothiazine-Based Dyes
Molecular Engineering of Simple Phenothiazine-Based Dyes To
Modulate Dye Aggregation, Charge Recombination, and Dye
Regeneration in Highly Efficient Dye-Sensitized Solar Cells
Yong Hua,[a] Shuai Chang,[b] Jian He,[b] Caishun Zhang,[c] Jianzhang Zhao,[c] Tao Chen,*[b] Wai-
Yeung Wong,*[a, c] Wai-Kwok Wong,*[a] and Xunjin Zhu*[a]
Abstract: A series of simple phenothiazine-based dyes,
namely, TP, EP, TTP, ETP, and EEP have been developed, in
which the thiophene (T), ethylenedioxythiophene (E), their
dimers, and mixtures are present to modulate dye aggrega-
tion, charge recombination, and dye regeneration for highly
efficient dye-sensitized solar cell (DSSC) applications. Devices
sensitized by the dyes TP and TTP display high power con-
version efficiencies (PCEs) of 8.07 (Jsc =15.2 mAcmꢀ2, Voc =
0.783 V, fill factor (FF)=0.679) and 7.87% (Jsc =16.1 mAcmꢀ2,
Voc =0.717 V, FF=0.681), respectively; these were measured
under simulated AM 1.5 sunlight in conjunction with the
dye ETP, with a mixture of E and T, had an even lower PCE
of 5.62%. Specifically, the cell based on the dye EEP, with
a dimer of E, had inferior Jsc and Voc values and correspond-
ed to the lowest PCE of 2.24%. The results indicate that the
photovoltaic performance can be finely modulated through
structural engineering of the dyes. The selection of T ana-
logues as donors can not only modulate light absorption
and energy levels, but also have an impact on dye aggrega-
tion and interfacial charge recombination of electrons at the
interface of titania, electrolytes, and/or oxidized dye mole-
cules; this was demonstrated through DFT calculations, elec-
trochemical impedance analysis, and transient photovoltage
studies.
ꢀ
Iꢀ/I3 redox couple. By replacing the T group with the E unit,
EP-based DSSCs had a slightly lower PCE of 7.98% with
a higher short-circuit photocurrent (Jsc) of 16.7 mAcmꢀ2. The
Introduction
O’Regan and Grꢀtzel reported an efficient dye-sensitized solar
cell (DSSC) system with a polypyridyl ruthenium(II) complex ad-
sorbed on a nanocrystalline semiconductor TiO2 electrode in
1991,[1] DSSCs have received considerable attention as promis-
ing alternatives to conventional silicon-based photovoltaic de-
vices due to their low cost, easy manufacturing processes, and
high power conversion efficiencies (PCEs).[2] The advancement
of the PCE (h) is critically dependent on the development of
new photosensitizers and electrolytes;[3] this led to the best re-
corded PCE of 12.3% based on a porphyrin dye in a cobalt(II/
III)-based redox electrolyte.[4] In recent years, many metal-free
organic dyes as an alternative to ruthenium complex sensitiz-
ers have been obtained with impressive efficiencies.[5] These
sensitizers exhibit many advantages, such as lower material
costs, higher molar extinction coefficients, and tunable spectral
responses. However, metal-free organic-dye-based DSSCs are
known to generate lower PCEs than those of RuII-based poly-
pyridyl complexes; this is probably a result of the formation of
dye aggregates on the semiconductor surface, leading to the
self-quenching of excited states and the reduction in the yield
of electron injection from the dye into the conduction band
(CB) of TiO2.[6] Another disadvantage of organic dyes in DSSCs
is faster interfacial charge recombination between the injected
Currently, available fossil fuel resources are limited and deplet-
ed rapidly during recent decades. Hence, there is an increasing
global awareness concerning the crisis to find alternative re-
newable energy sources, such as solar energy, to meet the re-
quirements and developments of modern society. Since
[a] Dr. Y. Hua, Prof. W.-Y. Wong, Prof. W.-K. Wong, Dr. X. Zhu
Department of Chemistry and Institute of Advanced Materials
Institute of Molecular Functional Materials and HKBU
Institute of Research and Continuing Education (Shenzhen)
Hong Kong Baptist University, Waterloo Road
Kowloon Tong, Hong Kong (S.A.R. China)
Fax: (+86)852-3411-7348
E-mail: rwywong@hkbu.edu.hk
wkwong@hkbu.edu.hk
xjzhu@hkbu.edu.hk
[b] S. Chang, J. He, Dr. T. Chen
Department of Physics
The Chinese University of Hong Kong
Shatin, New Territories, Hong Kong (S.A.R. China)
E-mail: taochen@phy.cuhk.edu.hk
[c] C. Zhang, Prof. J. Zhao, Prof. W.-Y. Wong
State Key Laboratory of Fine Chemicals
School of Chemical Engineering
ꢀ
electron and I3 ions in the electrolyte or dye cations formed
in DSSCs, resulting in a reduced Voc value.[2a,6b,7] With regard to
Dilian University of Technology, 116024 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304897.
these disadvantages, the molecular engineering of organic sen-
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sitizers should focus on two fundamental aspects: 1) the
ꢀ
HOMO and LUMO of the dye are required to suit the Iꢀ/I3
redox potential and the CB edge level of the TiO2 semiconduc-
tor, which have a significant influence on electron injection,
open-circuit voltage decay, and dye regeneration, and 2) the
fine structural tuning of the organic sensitizers to regulate the
dye aggregation and charge recombination processes in
DSSCs. Moreover, the rational option of electron donor and p-
conjugated moiety to construct organic sensitizers is of pro-
found importance in control of the photovoltaic cell per-
formance. In this respect, thiophene (T) and its derivatives
have been widely introduced into the p-bridged conjugated
skeletons of organic sensitizers in DSSC applications.[8] Various
thiophene derivatives can give flexible molecular structural op-
timization of sensitizers correlated to the photovoltaic per-
formance. It is well known that phenothiazine is a good elec-
tron-donating group because it contains electron-rich nitrogen
and sulfur heteroatoms in the central ring, and also its nonpla-
nar butterfly conformation can sufficiently inhibit molecular ag-
gregation. Moreover, the N(10)-substituted alkyl chain on the
nitrogen atom of the phenothiazine core can impede current
leakage by retarding charge recombination.[5i] These advantag-
es make phenothiazine derivatives potential candidates for
highly efficient DSSCs.
Herein, we present five new organic dyes (Figure 1), namely,
TP, EP, TTP, ETP, and EEP, in which substitutions of thiophene
(T), ethylenedioxythiophene (E), their dimers (TT and EE), and
a mixture (ET) on the C(7) atom of the phenothiazine core
serve as electron donors, electron-rich phenothiazine features
a hexyl chain at the N(10) atom as a p-conjugated segment,
and cyanoacrylic acid at the C(3) position of the phenothiazine
ring is an electron acceptor. We have systematically studied
the effects of subtle structural changes upon the photophysi-
cal, photochemical, and photovoltaic properties of the DSSC
devices utilizing theoretical calculations, electrochemical impe-
dance analysis, and transient photovoltage decay measure-
ments. To the best of our knowl-
Figure 1. Structures of the phenothiazine-based dyes TP, EP, TTP, ETP, and
EEP.
tions with appropriate stannyl reagents and 1. Compound 6
(or 7) was prepared from 4 (or 5) through bromination with
NBS, followed by Stille coupling reactions to afford the alde-
hyde precursors 8–10. Finally, the Knoevenagel condensation
of 2 and 3 (or 8–10) with cyanoacetic acid in acetic acid under
reflux afforded the target dyes TP, EP, TTP, ETP, and EEP. The
chemical structures of these intermediates and target dyes
edge, there have been no such
systematic investigations on the
introduction of T and its deriva-
tives into metal-free organic
dyes as the electron donors for
DSSCs.
Results and Discussion
Synthesis and characterization
The synthetic routes for the new
dyes based on the phenothia-
zine core are illustrated in
Scheme 1. From the intermedi-
ate 7-bromo-N-hexyl-10H-pheno-
thiazine-3-carbaldehyde
(1),[5c]
compounds 2–5 were synthe-
sized by conventional palladium-
Scheme 1. Synthetic routes for phenothiazine-based dyes TP, EP, TTP, ETP, and EEP. a) [Pd(PPh3)4], nBu3SnR, tolu-
catalyzed Stille coupling reac- ene, reflux; b) N-bromosuccinimide (NBS), THF, 08C; c) CH3COOH, CH3COONH4, 1208C.
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TP and EP dyes are l=457 and 466 nm, respectively. The
slight redshift for EP relative to TP can be ascribed to the
greater electron-donating ability of the E unit. Similarly, the ICT
band of dye ETP (l=481 nm), containing a mixture of E and T,
as the electron donor displays a large redshift of 11 nm com-
pared with the analogous bithiophene-based dye TTP (l=
470 nm). For the same reason, the appended dimer of E in dye
EEP (lmax =489 nm) leads to a larger redshift of 19 nm in com-
parison with TTP (lmax =470 nm). In addition, the larger batho-
chromic shift is also because appending a dimer of E or a mix-
ture of E and T induces a large degree of molecular planarity
through the intramolecular O···S interactions between the
oxygen atom of E and the sulfur atom of T or E in the p-conju-
gated system.[9] When the dyes are adsorbed on TiO2 nanopar-
ticle films (Figure 2b), slight hypochromic shifts were observed
for these dyes with respect to their solutions; this may be
mainly attributed to deprotonation of the carboxylic acid. Spe-
cifically, the absorption bands of ETP (14 nm) and EEP (17 nm)
adsorbed TiO2 films display larger blueshifts compared with TP
(5 nm), EP (6 nm), and TTP (4 nm); this indicates that the
dimer of E or the mixture of E and T are more liable to form p-
stacked aggregates on TiO2 films.[10] Notably, all of these dyes
exhibit apparent redshifted absorption thresholds of above
650 nm on TiO2 films compared with those in solution; this
matches well with the solar spectrum in the visible region.
Figure 2. The UV/Vis absorption spectra of the phenothiazine-based dyes in
CH2Cl2 (a) and on TiO2 films (b).
Electrochemical properties
Table 1. Photophysical and electrochemical properties of the phenothia-
zine-based dyes.
To evaluate the thermodynamic possibility of electron injection
and sensitizer regeneration, cyclic voltammograms were re-
corded in dichloromethane to determine the oxidation poten-
tials (Eox; Figure 3). The oxidation potentials (Eox) correspond to
the HOMOs. As listed in Table 1, the HOMO levels of dyes TP,
EP, TTP, ETP, and EEP were 0.64, 0.57, 0.62, 0.51, and 0.42 V, re-
[b]
[c]
[d]
Dye
lmax[a] [nm]
(e [ꢂ10ꢀ4 mꢀ1 cmꢀ1])
lmax
Eox
E0-0
Eox*[e]
[V]
[nm]
[V]
[eV]
TP
EP
TTP
ETP
EEP
457 (2.57)
466 (2.60)
470 (2.24)
481 (2.39)
489 (2.52)
452
460
466
467
472
0.64
0.57
0.62
0.51
0.42
2.27
2.19
2.23
2.12
2.04
ꢀ1.63
ꢀ1.62
ꢀ1.61
ꢀ1.61
ꢀ1.62
spectively; these values are more positive than the Iꢀ/I3 redox
ꢀ
potential value (0.4 V vs. NHE), indicating that the oxidized
dyes could be efficiently regenerated by the electrolyte. The
resulting E0-0 values are 2.27, 2.19, 2.23, 2.12, and 2.04 eV,
which were estimated from the onset of the UV/Vis absorption
spectra of these dyes. The LUMOs were calculated from the
values of Eox and the zero–zero band gaps (E0-0). As a result,
the LUMO levels of dyes TP, EP, TTP, ETP and EEP are ꢀ1.63,
ꢀ1.62, ꢀ1.61, ꢀ1.61, and ꢀ1.62 V, respectively, which are more
negative than the CB edge of TiO2 (ꢀ0.5 V vs. NHE), providing
the thermodynamic feasibility of electron injection from the
excited dye molecules into the CB of TiO2. According to
a report in the literature, the stronger electron-donating ability
of the donor group results in a higher HOMO energy level.[5a]
As expected, the HOMO level of TTP is higher than that of TP,
because the TT dimer is more inclined to give an electron than
the T monomer. For the same reason, replacing T with E for EP
and ETP further pushes the HOMO levels to higher levels due
to the stronger electron-donating ability of the E group. Nota-
bly, dye EEP appended with two E units exhibits the highest
HOMO level of 0.42 V. Although significant upgrading of the
HOMO level for EEP can narrow the HOMO–LUMO band gap
and produce a redshifted absorption spectrum in solution, the
[a] Absorption maximum measured in CH2Cl2 (1ꢂ10ꢀ5 m). [b] Absorption
maximum of dye adsorbed on TiO2. [c] Oxidation potential measured in
CH2Cl2 containing 0.1m (n-C4H9)4NPF6 with a scan rate of 100 mVsꢀ1 (vs.
a normal hydrogen electrode (NHE)). [d] Zero–zero band gap determined
from the onset of absorption spectrum. [e] Eox*=EoxꢀE0-0
.
1
were fully characterized by H and13C NMR spectroscopy and
MALDI-TOF mass spectrometry.
Spectroscopic studies
The UV/Vis absorption spectra of the new dyes were recorded
in CH2Cl2 (Figure 2a), and the detailed spectroscopic data are
summarized in Table 1. Two major broad absorption bands
were observed in the range of l=300–600 nm with peak
molar extinction coefficients (e) of 2.24–2.60ꢂ104 mꢀ1 cmꢀ1. The
high-energy bands located at l=300–380 nm are attributed to
localized aromatic p–p* electronic transitions of the chromo-
phores, whereas those with low energy at l=400–600 nm are
ascribed to intramolecular charge transfer (ICT) from the donor
to the acceptor. The maximum absorption wavelength of the
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In the optimized ground-state geometries shown in Table 2,
the dihedral angles between the donor units of T and E and
the phenothiazine core are computed to be 22.1 and 20.18 for
TP and EP, respectively. The ground-state geometry of TTP
possesses a 10.58 twist between the adjacent two T units,
whereas much smaller dihedral angles of 2.3 and 1.18 were ob-
served for ETP and EEP, respectively. These results indicate that
the mixture of E and T or the dimer of E tend to produce
a higher degree of molecular coplanarity through intramolecu-
lar O···S interactions between the oxygen atom of E and the
sulfur atom of T or E. Specifically, the intramolecular O···S inter-
actions are maximized when two E groups are adjacent, result-
ing in a high degree of coplanarity comprised of the dimer of
E and the phenyl ring of phenothiazine core. The results are
consistent with the UV/Vis absorption spectra observed in so-
lution. When compared with TP, the dyes of TTP (lmax
=
470 nm), ETP (lmax =481 nm), and EEP (lmax =489 nm) show
gradually increasing redshifted CT bands of 13, 24, and 32 nm,
respectively, due to continuously strengthened p conjugation
in the chromophores.
Photovoltaic performance
The DSSCs were fabricated with these phenothiazine-based
dyes on nanocrystalline titania films in conjunction with the
ꢀ
Iꢀ/I3 electrolyte, and the important parameters, namely, short-
Figure 3. Cylic voltammograms (a) and an energy diagram (b) of dyes TP, EP,
TTP, ETP, and EEP.
circuit current density (Jsc), open-circuit photovoltage (Voc), fill
factor (FF), and total PCE (h), were measured under standard
AM 1.5 solar light (100 mWcmꢀ2) and are summarized in
Table 3, and the photocurrent–voltage (J–V) plots are shown in
resulting smaller difference between the HOMO level of the
ꢀ
dye and the redox potential of the Iꢀ/I3 couple is unfavorable
for fast dye regeneration.[5g] As a result, slower dye regenera-
tion would lead to potential back reactions at the TiO2/dye/
electrolyte interface, as demonstrated by the smallest charge
recombination resistance in DSSCs based on EEP (see below).
Theoretical studies
To understand the electronic distribution in the frontier molec-
ular orbitals and electronic processes upon photoexcitation,
DFT calculations were performed by using Gaussian 03 pro-
gram package at the B3LYP/6-31 G(d)* hybrid functional for
full geometry optimization (Table 2). The electron density distri-
butions of the HOMOs for the five phenothiazine-based dyes
are delocalized throughout the entire molecular systems from
the donor to the cyanoacrylic acid acceptor, whereas the elec-
tron density distributions of the LUMO are mainly located on
the cyanoacrylic acid segment and partly on the phenothiazine
core. The partial orbital overlap between the donor and ac-
ceptor indicates a fast charge transition. When these dyes are
applied in DSSCs, the photoinduced electron can be effectively
injected into the CB of the TiO2 semiconductor from the donor
unit via the terminal cyanoacrylic acid acceptor. At the same
time, fast charge transfer could avoid geminate charge recom-
bination between the oxidized dye molecules and the photoin-
jected electrons at the TiO2/dye/electrolyte interface.[5c]
Figure 4. Current–voltage curves (a) and incident photocurrent efficiency
(IPCE; b) curves of DSSCs based on the dyes TP, EP, TTP, ETP, and EEP.
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Table 2. Calculated frontier orbitals and optimized geometries.
Dye
HOMO
LUMO
Optimized structure
TP
EP
TTP
ETP
EEP
shows
a much lower PCE of 5.62%, a lower Jsc of
Table 3. Photovoltaic parameters in DSSCs based on the new dyes in full
12.01 mAcmꢀ2 and Voc of 0.672 V. Surprisingly, the cell sensi-
tized with the dye EEP, comprising the dimer of E, shows an in-
ferior Jsc of 5.93 mAcmꢀ2 and Voc of 0.559 V, corresponding to
the lowest PCE of 2.24%, regardless of its much higher molar
extinction coefficient and broader absorption spectral response
in solution. Clearly, the subtle structural differences in these
phenothiazine-based dyes led to different performances in
DSSCs based on these dyes. The results demonstrate that the
replacement of T with electron-rich E in these phenothiazine-
based dyes results in deleterious effect on the value of Jsc. The
dramatically reduced Jsc values for cells based on ETP and EEP
can also be predicted by the low IPCE spectra (Figure 4b) with
65 and 40% at 475 nm, respectively, in contrast to the IPCE of
72% at 470 nm for TTP. The main reason for the low Jsc value
should be that dyes ETP and EEP, with a high level of coplanar-
ity arising from O···S interactions, are liable to form p-stacked
aggregates on the TiO2 surface, which can lead to lower effi-
ciency of electron injection. It has been well documented in
the literature that the aggregation of organic dyes on TiO2
films quenches the excited states of the dyes, resulting in low
efficiency of electron injection.[5a,6b,11] As mentioned previously,
the upshift of the HOMO energy levels for ETP and EEP led to
the weak driving force for dye regeneration by Iꢀ, which also
contributed to the low photocurrent in DSSCs. In addition, the
sunlight (AM 1.5 G, 100 mWcmꢀ2).[a]
Dye
Jsc
Voc
[V]
FF
h
Rrec
[Wcmꢀ1
]
[mAcmꢀ2
]
[%]
TP
EP
TTP
ETP
EEP
15.18
16.65
16.13
12.01
5.93
0.783
0.746
0.717
0.672
0.559
0.679
0.642
0.681
0.696
0.676
8.07
7.98
7.87
5.62
2.24
262.3
252.8
121.8
92.34
45.80
[a] The effective areas of all of the DSSCs are 0.24 cm2. Dyes were main-
tained at 0.5 mm in acetonitrile/tert-butanol (1/1, v/v). Electrolyte: LiI
(0.05m), I2 (0.1m), and 1,2-dimethyl-3-propylimidazolium iodide (DMPII;
0.6m) in acetonitrile/tert-butanol (1/1, v/v).
Figure 4a. The PCE values strongly depend on the arrangement
of the substituents of the thiophene analogues in these organ-
ic dyes. The cell based on the dye TP exhibits the highest h of
8.07% with Jsc of 15.18 mAcmꢀ2 and Voc of 0.783 V. The cell
based on EP shows a similar performance of a higher Jsc of
16.65 mAcmꢀ2, a lower Voc of 0.746 V and h of 7.98%. Under
the same conditions, the cell based on dye TTP gives an effi-
ciency of 7.87% with a higher Jsc of 16.13 mAcmꢀ2 relative to
TP due to extension of the p-conjugated system. On the other
hand, the dye ETP, with a mixture of E and T, as the donor
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amount of dyes TTP and EEP adsorbed on the TiO2 films was
6.39ꢂ10ꢀ8 and 11.9ꢂ10ꢀ8 molcmꢀ2, respectively; this indicates
that the amount of dye adsorbed is not the reason for the
lower Jsc value of the device based on the dye EEP.
Transient photovoltage measurements
To further confirm the electron recombination kinetics be-
tween the photoinjected electrons at TiO2 and the oxidized
electrolyte in DSSCs, open-circuit voltage decay (OCVD) was re-
corded to illustrate the recombination lifetime of Voc from the
illuminated quasi-equilibrium state for 20 s under open-circuit
conditions to dark equilibrium decaying for 80 s (Figure 6). The
Electrochemical impedance spectroscopy
To understand the significant decrease in the value of Voc from
TP- to EEP-based DSSCs, electrochemical impedance spectros-
copy (EIS) was performed to analyze interfacial charge recom-
bination and carrier transportation processes under dark condi-
tions. The interfacial charge recombination reaction of elec-
trons with I3ꢀ ions can be described by a charge recombination
resistance (Rrec). As shown in Figure 5, a major intermediate
Figure 6. OCVD profiles enlarged from 20 to 90 s for DSSCs based on the
new dyes TP, EP, TTP, ETP, and EEP.
Voc decay can be related to the electron lifetime (tn) through
Equation (1):
Figure 5. EIS Nyquist plots for DSSCs based on the new dyes TP, EP, TTP,
ETP, and EEP.
KBT dVoc
e
Þꢀ1
ð1Þ
tn ¼ ꢀ
ð
dt
semicircle for each dye is observed in the EIS Nyquist plot,
which is related to the charge recombination resistance on the
TiO2/dye/electrolyte interface (Rrec). The calculated Rrec values
are listed in Table 3. A smaller Rrec value means a larger charge
recombination rate.[12] The Rrec values in the order of TP
(262.3 W)>EP (252.8 W)>TTP (121.8 W)>ETP (92.34 W)>EEP
(45.80 W). The larger Rrec values for TP and EP imply a smaller
in which KB is the Boltzmann constant, T is the temperature,
and e is the electron charge.[5c,13] Therefore, the electron life-
times can be derived from the slope of Voc decay curves. As
shown in Figure 6, the slopes become steeper and steeper for
the cells in the sequence of TP, EP, TTP, ETP and EEP; this indi-
cates that the electron lifetimes are gradually reduced in the
dark measurements, showing the order of the charge recombi-
nation in the electrolyte, which are consistent with those ob-
tained from the EIS spectra. As illustrated, the Voc values of the
DSSCs based on the dyes of ETP and EEP show faster decay
rates than that based on TTP, which indicates a higher electron
recombination rate and shorter electron lifetime for the first
two cells. It is clear that the electron lifetime in the order of
TP>EP>TTP>ETP>EEP implies that expanding the p conju-
gation by appending the E unit is disadvantageous to sup-
pressing charge recombination. Specifically, the two adjacent E
units can facilitate charge recombination between the injected
electrons and the oxidized electrolyte species, which may be
attributed to stable complex formation by means of Columbic
interactions between the hydrophilic oxygen atoms of E and
ꢀ
charge recombination possibility with I3 in the electrolyte,
which was reflected in the better performance of DSSCs. The
results are consistent with the order of the Voc values in DSSCs
based on these dyes. Notably, both cells based on the dyes
ETP and EEP, with at least one E unit, showed an inferior value
of Voc relative to that of the dye TTP containing bithiophene.
For the ground-state optimized geometries, TTP possesses an
18.98 twist angle between the phenyl ring of the phenothia-
zine core and the adjacent T unit, whereas the dihedral angle
of the two T units is 10.58. In contrast, the corresponding dihe-
dral angles were determined to be 11.6 and 2.38 in ETP, and
4.3 and 1.18 in EEP. In other words, the appending of the
dimer of E or the mixture of E and T led to enhanced molecu-
ꢀ
lar coplanarity, which might facilitate the approach of I3 ions
ꢀ
to the TiO2 surface, resulting in faster charge recombination
and lower values of Voc. More importantly, dye aggregation
arising from the coplanar structure should be the main reason
for the low value of Voc.
the oxidized electrolyte species (I3 or I2),[8d,14] which can rapid-
ly bring the oxidized electrolyte species close to the TiO2 sur-
face and promote photoexcited electron recombination, result-
ing in faster OCVD and lower Voc value. The rate of the oxi-
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column chromatography on silica gel by using a 1:4 mixture of
hexane and CH2Cl2 as the eluent to afford corresponding products
2–5, respectively.
dized electrolyte species approaching the TiO2 surface can be
speeded up with an increasing number of E units; thus en-
hancing the charge recombination rate and penalizing the
overall conversion efficiency.
10-Hexyl-7-(5-hexylthiophen-2-yl)-10H-phenothiazine-3-
carbaldehyde (2): Red solid, 89% yield; 1H NMR (400 MHz,
CDCl3): d=9.77 (s, 1H), 7.63–7.61 (m, 1H), 7.56 (d, J=2.0 Hz,
1H), 7.38–7.32 (m, 1H), 7.30 (d, J=2.0 Hz, 1H), 7.23–7.20 (m,
1H), 7.19–7.14 (m, 1H), 7.07–7.05 (m, 1H), 6.88–6.83 (m, 1H),
3.84 (t, J=7.2 Hz, 2H), 2.79 (t, J=6.4 Hz, 2H), 1.87–1.75 (m,
2H), 1.70–1.64 (m, 2H), 1.42–1.28 (m, 12H), 0.92–0.86 ppm (m,
6H); 13C NMR (100 MHz, CDCl3): d=190.0, 150.5, 145.6, 141.2,
139.8, 132.2, 130.8, 130.4, 125.1, 125.0, 124.6, 124.0, 123.3,
123.1, 122.3, 116.0, 114.3, 48.2, 31.63, 31.62, 31.4, 31.1, 30.3,
28.8, 26.54, 26.51, 22.7, 14.2, 14.0 ppm; HRMS (MALDI-TOF):
m/z calcd for C29H35NOS2: 477.2215 [M+]; found: 477.2234.
10-Hexyl-7-(7-hexyl-2,3-dihydrothieno[3,4-b][1,4]dioxin-5-
yl)-10H-phenothiazine-3-carbaldehyde (3): Red solid, 86%
Conclusion
We designed and synthesized a series of new phenothiazine-
based organic dyes featuring different numbers of T and/or E
units as the donor, and cyanoacrylic acid as the acceptor, with
the aim of investigating the structure–property relationship of
these dyes in DSSC applications. The stepwise incorporation of
the electron-rich E group resulted in an increase in the
ground-state redox potential and a redshift of the ICT absorp-
tion peak, accompanied by dye aggregation, faster interfacial
charge recombination, and slower dye regeneration. Devices
sensitized by dyes TP and TTP displayed high PCEs of 8.07 and
7.87%, respectively, measured under simulated AM 1.5 sunlight
1
yield; H NMR (400 MHz, CDCl3): d=9.78 (s, 1H), 7.64–7.61 (m,
ꢀ
in conjunction with a Iꢀ/I3 redox couple; these high PCEs
1H), 7.57 (d, J=2.0 Hz, 1H), 7.48–7.44 (m, 2H), 6.89–6.82 (m,
2H), 6.26 (s, 1H), 4.31 (s, 2H), 4.24 (s, 2H), 3.87 (t, J=7.2 Hz,
2H), 2.63 (t, J=7.2 Hz, 2H), 1.83–1.79 (m, 2H), 1.45–1.42 (m,
2H), 1.31–1.26 (m, 12H), 0.92–0.86 ppm (m, 6H); 13C NMR
(100 MHz, CDCl3): d=190.0, 154.5, 148.7, 139.8, 137.7, 137.2,
132.2, 130.3, 129.6, 125.0, 124.6, 123.4, 122.9, 116.7, 115.7,
114.5, 111.4, 64.9, 64.5, 48.2, 31.6, 31.2, 30.1, 29.2, 28.9, 26.5,
25.8, 24.5, 22.6, 14.1, 14.0 ppm; HRMS (MALDI-TOF): m/z calcd
for C31H37NO3S2: 535.2235 [M+]; found: 535.2239.
were attributed to fast dye regeneration dynamics and slow
charge recombination kinetics. Upon replacing T with E units,
the EP-based DSSC had a good PCE (7.98%) with a much
higher short-circuit current density of 16.65 mAcmꢀ2 due to
the broader spectral response. The dye ETP, with a mixture of
E and T, had a lower PCE value of 5.62% compared with the
dye TTP. In particular, the cell sensitized with the dye EEP, with
a dimer of E units, displayed inferior values of Jsc and Voc, lead-
ing to a significantly reduced PCE of 2.24% compared with
that of TTP. Our primary results shed light on the principle and
strategy of the design and synthesis of phenothiazine-based
organic dyes with a balance between dye aggregation and in-
terfacial charge recombination and enhancing the absorption
spectral response window.
10-Hexyl-7-(thiophen-2-yl)-10H-phenothiazine-3-carbalde-
1
hyde (4): Red solid, 79% yield; H NMR (400 MHz, CDCl3): d=
9.77 (s, 1H), 7.63–7.61 (m, 1H), 7.55 (d, J=2.0 Hz, 1H), 7.37–
7.34 (m, 1H), 7.30 (d, J=2.0 Hz, 1H), 7.24–7.20 (m, 1H), 7.20–
7.19 (m, 1H), 7.05–7.03 (m, 1H), 6.87–6.81 (m, 2H), 3.84 (t, J=
7.2 Hz, 2H), 1.43–1.31 (m, 2H), 1.30–1.27 (m, 4H), 0.87 ppm (d,
J=2.0 Hz, 3H); 13C NMR (100 MHz, CDCl3): d=190.0, 150.3,
142.9, 142.5, 131.1, 130.3, 130.1, 128.4, 128.1, 126.7, 125.1,
124.7, 124.5, 124.3, 124.2, 116.1, 114.7, 48.1, 31.4, 26.7, 26.5,
22.6, 14.0; HRMS (MALDI-TOF): m/z calcd for C23H23NOS2:
393.1236 [M+]; found: 393.1249.
Experimental Section
General
7-(2,3-Dihydrothieno[3,4-b][1,4]dioxin-5-yl)-10-hexyl-10H-
phenothiazine-3-carbaldehyde (5): Red solid, 70% yield;
1H NMR (400 MHz, CDCl3): d=9.78 (s, 1H), 7.64–7.61 (m, 1H),
7.57 (d, J=6.4 Hz, 1H), 7.48–7.44 (m, 2H), 6.89–6.82 (m, 2H),
6.26 (s, 1H), 4.30 (t, J=7.2 Hz, 2H), 4.25 (t, J=7.2 Hz, 2H), 3.87
(t, J=4.0 Hz, 2H), 1.83–1.79 (m, 2H), 1.45–1.42 (m, 2H), 1.31–
1.26 (m, 4H), 0.88 ppm (t, J=4.0 Hz, 3H); 13C NMR (100 MHz,
CDCl3): d=190.0, 150.2, 141.8, 140.2, 137.2, 131.1, 130.3, 128.1,
124.9, 124.7, 124.6, 116.4, 115.9, 114.7, 114.2, 110.3, 64.6, 64.1,
48.1, 31.6, 26.7, 22.8, 14.2, 14.0 ppm; HRMS (MALDI-TOF): m/z
calcd for C25H25NO3S2: 451.1344 [M+]; found: 451.1346.
All solvents and reagents were commercially available and used as
received without further purification. Phenothiazine and 2-cyano-
acetic acid were purchased commercially from Meyers Chemicals,
Inc. TiO2 paste and iodide-based liquid electrolyte (HL-HPE) were
purchased from Dyesol company. Tributyl(thiophen-2-yl)stan-
nane,[15]
tributyl-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)stan-
nane,[15] tributyl(5-hexylthiophen-2-yl)stannane,[16] and tributyl(7-
hexyl-2,3- dihydrothieno[3,4-b][1,4]dioxin-5-yl)stannane[16] were syn-
thesized according to methods reported in the literature. The fabri-
cation and characterization of cells were performed as previously
described (also see the Supporting Information).[5]
7-(5-Bromothiophen-2-yl)-10-hexyl-10H-phenothiazine-3-
carbaldehyde (6): Compounds 6 and 7 were prepared accord-
ing to reported procedures.[5a] Dark yellow solid, 77% yield;
1H NMR (400 MHz, CDCl3): d=9.76 (s, 1H), 7.62–7.59 (m, 1H),
7.53 (d, J=2.0 Hz, 1H), 7.24–7.21 (m, 1H), 7.17 (d, J=2.0 Hz,
1H), 6.96 (d, J=2.0 Hz, 1H), 6.90 (d, J=2.0 Hz, 1H), 6.84 (d, J=
2.0 Hz, 1H), 6.78 (d, J=2.0 Hz, 1H), 3.83 (t, J=7.2 Hz, 2H),
1.78–1.74 (m, 2H), 1.43–1.38 (m, 2H), 1.30–1.25 (m, 4H),
General synthetic procedure for 2–5
Under a nitrogen atmosphere, a mixture of 1 (0.30 mmol), tribu-
tyl(5-hexylthiophen-2-yl)stannane,
tributyl(7-hexyl-2,3-dihydro-
thieno[3,4-b][1,4]dioxin-5-yl)stannane, tributyl(thiophen-2-yl)stan-
nane, or tributyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)stannane
(0.40 mmol) and [Pd(PPh3)4] (10 mg, 0.0016 mmol) in toluene
(20 mL) was heated at reflux for about 12 h. After cooling, the sol-
vent was removed under vacuum and the residue was purified by
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0.86 ppm (t, J=7.2 Hz, 3H); 13C NMR (100 MHz, CDCl3): d=
189.9, 150.1, 144.4, 142.8, 131.2, 130.9, 130.3, 129.2, 128.3,
124.8, 124.4, 124.23, 124.15, 122.7, 116.1, 114.8, 111.0, 48.1,
31.6, 26.6, 22.6, 14.2, 14.0 ppm; HRMS (MALDI-TOF): m/z calcd
for C23H22BrNOS2: 471.0316 [M+]; found: 471.0323.
TOF): m/z calcd for C37H41NO5S3: 675.2153 [M+]; found:
675.2146.
General synthetic procedure for dyes TP, EP, TTP, ETP, and
EEP
7-(7-Bromo-2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-10-
hexyl-10H-phenothiazine-3-carbaldehyde (7): Dark yellow
solid, 80% yield; 1H NMR (400 MHz, CDCl3): d=9.78 (s, 1H),
7.64–7.61 (m, 1H), 7.56 (d, J=2.0 Hz, 1H), 7.40 (d, J=2.0 Hz,
1H), 7.36–7.34 (m, 1H), 6.81 (m, 2H), 4.30 (d, J=4.0 Hz, 4H),
3.86 (t, J=7.2 Hz, 2H), 1.82–1.78 (m, 2H), 1.45–1.41 (m, 2H),
1.32–1.26 (m, 4H), 0.88 ppm (t, J=7.2 Hz, 3H); 13C NMR
(100 MHz, CDCl3): d=190.0, 150.3, 141.8, 140.5, 137.2, 131.1,
130.3, 128.1, 125.0, 124.7, 124.4, 124.1, 116.4, 115.9, 114.7,
114.3, 110.9, 64.9, 64.7, 48.1, 31.6, 26.7, 22.7, 14.2, 14.0 ppm;
HRMS (MALDI-TOF): m/z calcd for C25H24BrNO3S2: 529.0426
[M+]; found: 529.0433.
Under a nitrogen atmosphere, a mixture of the precursor carbalde-
hyde (0.20 mmol), cyanoacetic acid (100 mg, 1.30 mmol), and am-
monium acetate (200 mg) in acetic acid (20 mL) was heated at
reflux overnight. After cooling, the reaction mixture was poured
into water (50 mL) and extracted with CH2Cl2. The combined or-
ganic layers were evaporated to dryness and purified by column
chromatography on silica gel with CH2Cl2/MeOH (20:1, v/v) as the
eluent to afford the target compound.
(E)-2-Cyano-3-[10-hexyl-7-(5-hexylthiophen-2-yl)-10H-phenothia-
zin-3-yl]acrylic acid (TP): Dark red solid, 61% yield; m.p. 173–
1758C; 1H NMR (400 MHz, CDCl3): d=8.04 (s, 1H), 7.89–7.86 (m,
1H), 7.61 (d, J=2.0 Hz, 1H), 7.29–7.27 (m, 1H), 7.21 (d, J=2.0 Hz,
1H), 7.00 (d, J=2.0 Hz, 1H), 6.79 (t, J=2.0 Hz, 2H), 6.70 (d, J=
2.0 Hz, 1H), 3.61 (s, 2H), 2.76 (t, J=7.6 Hz, 2H), 1.70–1.63 (m, 4H),
1.35–1.24 (m, 12H), 0.91–0.84 ppm (m, 6H); 13C NMR (100 MHz,
CDCl3): d=168.6, 154.6, 149.6, 145.7, 141.1, 139.9, 132.1, 130.8,
130.4, 125.2, 125.1, 124.6, 124.0, 123.8, 123.4, 123.2, 122.4, 115.9,
114.7, 97.2, 48.3, 31.63, 31.62, 31.4, 31.2, 30.3, 28.8, 26.6, 26.5, 22.6,
14.1, 14.0 ppm; HRMS (MALDI-TOF): m/z calcd for C32H36N2O2S2:
545.2290 [M+]; found: 545.2312.
10-Hexyl-7-(5’-hexyl-[2,2’-bithiophen]-5-yl)-10H-phenothia-
zine-3-carbaldehyde (8): Red solid, 74% yield; 1H NMR
(400 MHz, CDCl3): d=9.70 (s, 1H), 7.56–7.53 (m, 1H), 7.49 (d,
J=4.0 Hz, 1H), 7.27–7.24 (m, 1H), 7.21 (d, J=4.0 Hz, 1H), 7.00
(d, J=4.0 Hz, 1H), 6.94 (d, J=4.0 Hz, 1H), 6.90 (d, J=4.0 Hz,
1H), 6.78 (d, J=4.0 Hz, 1H), 6.73 (d, J=4.0 Hz, 1H), 6.60–6.59
(m, 1H), 3.78 (t, J=7.2 Hz, 2H), 2.71 (t, J=7.2 Hz, 2H), 1.76–
1.69 (m, 2H), 1.63–1.54 (m, 2H), 1.38–1.29 (m, 4H), 1.27–1.16
(m, 8H), 0.87–0.79 ppm (m, 6H); 13C NMR (100 MHz, CDCl3): d=
190.0, 150.3, 150.2, 145.6, 142.4, 140.9, 137.1, 134.7, 131.1,
130.2, 129.9, 128.4, 125.1, 124.8, 124.5, 124.2, 123.8, 123.3,
123.2, 116.1, 114.7, 48.1, 31.6, 31.6, 31.4, 30.2, 28.8, 26.7, 26.5,
22.7, 22.6, 14.1, 14.0 ppm; HRMS (MALDI-TOF): m/z calcd for
C33H37NOS3: 559.2033 [M+]; found: 559.2039.
(E)-2-Cyano-3-[10-hexyl-7-(7-hexyl-2,3-dihydrothieno[3,4-
b][1,4]dioxin-5-yl)-10H-phenothiazin-3-yl]acrylic acid (EP): Dark
red solid, 64% yield; m.p. 181–1838C; 1H NMR (400 MHz, CDCl3):
d=8.01 (s, 1H), 7.82 (d, J=4.8 Hz, 1H), 7.59 (s, 1H), 7.35 (d, J=
4.8 Hz, 2H), 6.77–6.74 (m, 2H), 4.27 (d, J=2.0 Hz, 2H), 4.21 (d, J=
2.0 Hz, 2H), 3.79 (t, J=7.2 Hz, 2H), 2.62 (t, J=7.2 Hz, 2H), 1.79–1.76
(m, 2H), 1.64–1.56 (m, 2H), 1.42–1.31 (m, 12H), 0.90–0.87 ppm (m,
6H); 13C NMR (100 MHz, CDCl3): d=168.6, 154.5, 149.7, 139.9,
138.0, 137.9, 132.2, 130.3, 129.6, 125.0, 124.6, 124.2, 123.8, 122.9,
116.7, 116.0, 115.7, 114.5, 111.4, 97.0, 64. 9, 64.4, 48.2, 31.6, 31.4,
30.4, 29.2, 28.9, 26.5, 25.8, 24.5, 22.6, 14.1, 14.0 ppm; HRMS
(MALDI-TOF): m/z calcd for C34H38N2O4S2: 603.2346 [M+]; found:
603.2324.
10-Hexyl-7-[5-(7-hexyl-2,3-dihydrothieno[3,4-b][1,4]dioxin-
5-yl)thiophen-2-yl]-10H-phenothiazine-3-carbaldehyde (9):
Red solid, 72% yield; 1H NMR (400 MHz, CDCl3): d=9.76 (s,
1H), 7.61–7.58 (m, 1H), 7.54 (d, J=4.0 Hz, 1H), 7.34–7.31 (m,
1H), 7.28 (d, J=4.0 Hz, 1H), 7.08 (d, J=4.0 Hz, 1H), 7.04 (d, J=
4.0 Hz, 1H), 6.85–6.77 (m, 2H), 4.31 (t, J=7.2 Hz, 2H), 4.21 (t,
J=7.2 Hz, 2H), 3.82 (t, J=6.4 Hz, 2H), 2.65–2.61 (d, J=4.0 Hz,
2H), 1.68–1.60 (m, 2H), 1.40–1.45 (m, 2H), 1.33–1.27 (m, 12H),
0.93–0.85 ppm (m, 6H); 13C NMR (100 MHz, CDCl3): d=190.0,
150.2, 142.1, 139.9, 137.6, 137.5, 134.5, 131.0, 130.22, 130.17,
128.3, 124.5, 124.3, 124.1, 124.0, 122.8, 122.7, 116.5, 116.0,
114.7, 108.0, 65.2, 64.5, 48.1, 31.6, 31.4, 30.4, 28.9, 26.7, 26.5,
25.8, 22.64, 22.62, 17.4, 14.2, 14.0 ppm; HRMS (MALDI-TOF):
m/z calcd for C35H39NO3S3: 617.2146 [M+]; found: 617.2148.
10-Hexyl-7-[7’-hexyl-2,2’,3,3’-tetrahydro-(5,5’-bithieno[3,4-
b][1,4]dioxin)-7-yl]-10H-phenothiazine-3-carbaldehyde (10):
Red solid, 73% yield; 1H NMR (400 MHz, CDCl3): d=9.77 (s,
1H), 7.63–7.61 (m, 1H), 7.57 (d, J=6.4 Hz, 1H), 7.50–7.44 (m,
2H), 7.18–7.15 (m, 1H), 6.88–6.80 (m, 2H), 4.36–4.30 (m, 6H),
4.29–4.22 (m, 2H), 3.86 (t, J=7.0 Hz, 2H), 2.64 (t, J=6.4 Hz,
2H), 1.82–1.77 (m, 2H), 1.65–1.59 (m, 2H), 1.34–1.22 (m, 12H),
0.90–0.86 ppm (m, 6H); 13C NMR (100 MHz, CDCl3): d=190.0,
150.3, 142.1, 139.1, 137.4, 137.5, 134.5, 130.3, 130.22, 130.16,
128.4, 124.8, 124.7, 124.6, 124.1, 122.8, 122.6, 116.2, 116.0,
114.7, 107.6, 65.5, 65.2, 64.5, 64.2, 48.1, 31.6, 31.4, 30.4, 28.9,
26.6, 26.1, 25.5, 22.8, 22.6, 17.3, 14.1, 14.0 ppm; HRMS (MALDI-
(E)-2-Cyano-3-[10-hexyl-7-(5’-hexyl-[2,2’-bithiophen]-5-yl)-10H-
phenothiazin-3-yl]acrylic acid (TTP): Dark red solid, 68% yield;
m.p. 197–1998C;1H NMR (400 MHz, CDCl3): d=7.97 (s, 1H), 7.73 (d,
J=7.2 Hz, 1H), 7.43 (d, J=7.2 Hz, 1H), 7.22–7.09 (m, 2H), 7.01–6.97
(m, 1H), 6.94–6.92 (m, 2H), 6.30 (d, J=7.2 Hz, 2H), 6.56 (d, J=
8.0 Hz, 1H), 3.61 (s, 2H), 2.76 (t, J=7.2 Hz, 2H), 1.70–1.63 (m, 4H),
1.39–1.24 (m, 12H), 0.91–0.84 ppm (m, 6H); 13C NMR (100 MHz,
CDCl3): d=168.2, 153.5, 148.2, 145.3, 141.1, 140.7, 136.8, 134.7,
131.9, 129.5, 128.1, 125.4, 124.8, 124.3, 123.8, 123.5, 123.2, 123.1,
122.7, 122.5, 116.9, 115.4, 114.4, 98.9, 48.2, 31.6, 31.6, 31.4, 30.2,
28.9, 26.5, 26.3, 22.7, 22.6, 14.2, 14.1 ppm; HRMS (MALDI-TOF): m/z
calcd for C36H38N2O2S3: 627.2168 [M+]; found: 627.2194.
(E)-2-Cyano-3-{10-hexyl-7-[5-(7-hexyl-2,3-dihydrothieno[3,4-
b][1,4]dioxin-5-yl)thiophen-2-yl]-10H-phenothiazin-3-yl}acrylic
acid (ETP): Dark red solid, 61% yield; m.p. 204–2068C; 1H NMR
(400 MHz, CDCl3): d=8.01 (s, 1H), 7.84 (d, J=7.6 Hz, 1H), 7.59 (s,
1H), 7.29–7.32 (m, 1H), 7.23 (d, J=2.0 Hz, 1H), 7.08 (d, J=2.0 Hz,
1H), 7.04 (d, J=2.0 Hz, 1H), 6.76–6.79 (m, 2H), 4.32 (t, J=7.6 Hz,
2H), 4.23 (t, J=7.6 Hz, 2H), 3.81 (t, J=7.6 Hz, 2H), 2.62 (t, J=
7.6 Hz, 2H), 1.80–1.77 (m, 2H), 1.62–1.58 (m, 2H), 1.43–1.30 (m,
12H), 0.90–0.87 ppm (m, 6H); 13C NMR (100 MHz, CDCl3): d=168.0,
154.3, 149.4, 141.2, 139.7, 137.6, 137.5, 134.6, 132.0, 130.41, 130.35,
125.3, 124.5, 123.9, 123.7, 123.4, 122.8, 122.8, 116.6, 116.1, 115.9,
114.7, 108.0, 97.5, 65.2, 64.5, 48.2, 31.6, 31.5, 31.4, 30.4, 28.8, 26.5,
Chem. Eur. J. 2014, 20, 6300 – 6308
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[4] A. Yella, H. W. Lee, H. N. Tsao, C. Y. Yi, A. K. Chandiran, M. K. Nazeeruddin,
E. W. G. Diau, C. Y. Yeh, S. M. Zakeeruddin, M. Grꢀtzel, Science 2011, 334,
629–634.
25.8, 22.6, 14.1, 14.0 ppm; HRMS (MALDI-TOF): m/z calcd for
C38H40N2O4S3: 684.2145 [M+]; found: 684.2117.
(E)-2-Cyano-3-{10-hexyl-7-[7’-hexyl-2,2’,3,3’-tetrahydro-(5,5’-
bithieno[3,4-b][1,4]dioxin)-7-yl]-10H-phenothiazin-3-yl}acrylic
acid (EEP): Dark red solid, 58% yield; m.p. 218–2208C; 1H NMR
(400 MHz, CDCl3): d=7.98 (s, 1H), 7.81 (d, J=8.0 Hz, 1H), 7.59 (s,
1H), 7.46–7.42 (m, 2H), 6.78–6.74 (m, 2H), 4.34–4.23 (m, 8H), 3.80
(t, J=6.4 Hz, 2H), 2.62 (t, J=7.6 Hz, 2H), 1.76 (d, J=7.4 Hz, 2H),
1.62–1.58 (m, 2H), 1.42–1.29 (m, 12H), 0.91–0.86 ppm (m, 6H);
13C NMR (100 MHz, CDCl3): d=168.3, 154.3, 140.0, 137.8, 137.1,
137.0, 136.8, 136.6, 132.0, 130.3, 129.3, 125.1, 124.7, 124.1, 123.7,
122.9, 117.2, 116.1, 115.7, 114.5, 112.5, 108.6, 105.6, 97.4, 65.2, 65.1,
64.8, 64.6, 48.2, 31.6, 31.4, 30.5, 29.7, 29.2, 28.9, 26.5, 25.9, 22.6,
14.2, 14.0 ppm; HRMS (MALDI-TOF): m/z calcd for C40H42N2O6S3:
743.2277 [M+]; found: 743.2240.
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Acknowledgements
We thank the National Natural Science Foundation of China
(NSFC; grant 91222201), the Hong Kong Research Grants Coun-
cil (grants HKBU202210, HKBU202811, and HKBU203011), and
Hong Kong Baptist University (grants FRG2/12–13/050) for fi-
nancial support. W.-K.W. and W.-Y.W. are also grateful for
a grant from the Areas of Excellence Scheme, University Grants
Committee, Hong Kong (project no. [AoE/P-03/08]). X.Z. and
W.-Y.W. thank The Science, Technology and Innovation Commit-
tee of Shenzhen Municipality (JCYJ20120829154440583,
JCYJ20120615155451326) for financial support. W.-Y.W. also
thanks Dalian University of Technology (Haitian Scholarship for
W.-Y.W.). T.C. acknowledges financial support from the CUHK
Group Research Scheme, CUHK Focused Scheme B Grant
“Center for Solar Energy Research” and the University Direct
Grant (4053012).
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Keywords: aggregation
pigments · phenothiazine · solar cells
·
charge recombination
·
dyes/
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Received: December 14, 2013
Published online on April 8, 2014
Chem. Eur. J. 2014, 20, 6300 – 6308
www.chemeurj.org
6308
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