Organic dyes containing oligo-phenothiazine for dye-sensitized solar cells

Article abstract of DOI:10.1039/c2jm35556f

A series of organic dyes containing oligo-phenothiazine were synthesized and used effectively on the fabrication of dye-sensitized solar cells (DSSCs). In these compounds the phenothiazine moiety functions both as an electron donor and as a π-bridge. Thes

Full text of DOI:10.1039/c2jm35556f

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PAPER
Organic dyes containing oligo-phenothiazine for dye-sensitized solar cells†
Yuan Jay Chang, Po-Ting Chou,b Yan-Zuo Lin,c Motonori Watanabe,d Chih-Jen Yang,d Tsung-Mei Chinc
a
*
bd
*
and Tahsin J. Chow
Received 15th July 2012, Accepted 29th August 2012
DOI: 10.1039/c2jm35556f
A series of organic dyes containing oligo-phenothiazine were synthesized and used effectively on the
fabrication of dye-sensitized solar cells (DSSCs). In these compounds the phenothiazine moiety
functions both as an electron donor and as a p-bridge. These materials exhibit considerably high values
of open-circuit voltage (Voc) ranging from 0.78–0.83 V under an AM1.5 solar condition (100 mW
cmꢀ2). Two kinds of substituents, i.e., hexyl and hexyloxyphenyl groups, were added onto the N(10) of
phenothiazine for comparison. The best device displayed a short-circuit current (Jsc) of 14.3 mA cmꢀ2
,
an open-circuit voltage (Voc) of 0.83 V, a fill factor (FF) of 0.65, corresponding to an overall conversion
efficiency of 7.78%. Their photophysical properties were analyzed with the aid of a time-dependent
density functional theory (TDDFT) model with the B3LYP functional. The electronic nature of the
devices was further elucidated by using electrochemical impedance spectroscopy.
In the design of organic dyes for sensitized solar cells, it is
Introduction
critical to improve not only the value of the short-circuit current
(Jsc) but also the value of the open-current voltage (Voc). For the
former it is necessary to improve the p-conjugation, so that the
organic chromophore can harvest light energy to the largest
extent. For the latter an effort of reducing the rate of charge
recombination is necessary, so that the current leakage can be
minimized. In our previous investigations, we have discovered
that it is possible to use non-conjugated p-chromophores for
DSSCs, i.e., by incorporating a unit of paracyclophene to
interrupt the p-system.7 The Voc value of these materials can
indeed reach a high standard as a result of slow rate of charge
recombination. However, the relatively small value of Jsc became
a drawback, due to the breaking down of the p-conjugation.
In this report we try to re-examine our concept by using
phenothiazine as a building block. Phenothiazine contains elec-
tron-rich nitrogen and sulfur heteroatoms in the central ring, yet
it is not fully conjugated in a classical sense. The geometry of the
molecule is not planar, but slightly bent in the central ring. We
envisioned that the non-planar conformation of phenothiazine
can reduce not only the rate of charge recombination but also the
molecular aggregation. The electron rich nature of a phenothi-
azine makes it a good electron donor during a photo-excited
charge transfer transition. It displays a low and reversible
oxidation potential toward the formation of a stable radical
cation.8–11 By linking more than one unit of phenothiazine, we
can examine the effect of chain length on the performance of
DSSCs.
The increased consumption of fossil and the more serious crisis
of environment pollution have led us to the search for new and
renewable energy sources. Solar energy is widely recognized as
the most promising candidate to help solving this problem. Dye-
sensitized solar cells (DSSCs) are expected to be potential tools
due to their low cost processing and impressive photovoltaic
performance.1 The ruthenium-based dyes, reported by Michael
€
Gratzel, have achieved maximal power conversion efficiencies
over 11%. Yet they have encountered some problems, such as
limited resources, elaborated purification processes, etc.2
Compared with the rare and expensive metal complexes, organic
dyes have the advantages of environmental friendliness, higher
structural flexibility, lower cost, generally high molar extinction
coefficients, easier preparation and purification, etc. Metal-free
organic dyes have been widely investigated recently, many of
which exhibited an energy-to-electricity conversion efficiency
close to that of N719.3–6
aDepartment of Chemistry, Tung Hai University, Taichung 407, Taiwan.
E-mail: jaychang@thu.edu.tw; Fax: +886-4-23590426; Tel: +886-4-
23590428 extn 305
bDepartment of Chemistry, National Taiwan University, No.1, Sec. 4,
Roosevelt Rd., Da’an Dist., Taipei 10617, Taiwan. E-mail: chowtj@gate.
sinica.edu.tw; Fax: +886-2-27884179; Tel: +886-2-27898552
cDepartment of Chemistry, Chinese Culture University, 55 Hwa Kang Rd,
Yang-Ming Shan, Taipei 111, Taiwan
dInstitute of Chemistry, Academia Sinica, No.128, Sec. 2, Academia Rd.,
Three types of compounds composed of phenothiazine units
are prepared, i.e., PT1, PT2, and PT3, as shown in Fig. 1. The
nitrogen atoms in each type of compounds are substituted by two
kinds of linear chains, i.e., derivatives a and b. A cyanoacrylate
Nankang Dist., Taipei 11529, Taiwan
† Electronic supplementary information (ESI) available: Experimental
data (PT1, PT2 and PT3), the 1H and 13C NMR spectra, theoretical
calculation, UV/vis spectra, CV spectra, and EIS spectra, and the
devices incorporating DCA. See DOI: 10.1039/c2jm35556f
21704 | J. Mater. Chem., 2012, 22, 21704–21712
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solutions containing 0.1
M
tetrabutylammonium hexa-
fluorophosphate (TBAPF6) as supporting electrolyte under
ambient conditions after purging for 10 minutes with N2. The
conventional three-electrode configuration was employed, which
consists of a glassy carbon working electrode, a platinum counter
electrode, and a Ag/Ag+ reference electrode calibrated with
ferrocene/ferrocenium (Fc/Fc+) as an internal reference. Mass
spectra were recorded on a VG70-250S mass spectrometer.
The chemicals, i.e., phenothiazine, 4-bromophenol, 1-bromo-
hexane, butyllithium (1.6 M in hexane), tetrakis-(triphenyl-
phosphine) palladium (Pd(PPh3)4), N,N-dimethylformamide,
triisopropylborate, palladium(II) acetate (Pd(OAc)2), 1,10-bis-
(diphenylphosphanyl)ferrocene (dppf), N-bromosuccinimide
(NBS), cyanoacetic acid, ammonium acetate, and acetic acid,
were purchased from ACROS, Alfa, Merck, Lancaster, TCI,
Showa, and Sigma-Aldrich, separately. Chromatographic sepa-
rations were carried out by using silica gel from Merk, Kieselgel
si 60 (40–63 mm).
Fig. 1 Organic dye structures of oligo-phenothiazine.
group is attached to one side of the compound, acting as an
electron acceptor. The synthetic procedures of PT1, PT2, and
PT3 are described in Scheme 1. All structures were confirmed by
their spectroscopic data.
Experimental
Fabrication and characterization of DSSCs
General information
The FTO conducting glass (FTO glass, fluorine doped tin oxide
over-layer, transmission >90% in the visible, sheet resistance 8 U
per square), titania oxide pastes of Ti-nanoxide T/SP and Ti-
nanoxide R/SP were purchased from Solaronix. A thin film of
TiO2 (16–18 mm) was coated on a 0.25 cm2 FTO glass substrate.
It was immersed in a THF solution containing 3 ꢂ 10ꢀ4 M dye
sensitizers for at least 12 h, then rinsed with anhydrous aceto-
nitrile and dried. Another piece of FTO sputtered with 100 nm
thick Pt was used as a counter electrode. The active area was
controlled at a dimension of 0.25 cm2 by adhering 60 mm thick
polyester tape on the Pt electrode. The photocathode was placed
on top of the counter electrode and was tightly clipped together
to form a cell. Electrolyte was then injected into the seam
All reactions and manipulations were carried out under a
nitrogen atmosphere. Solvents were distilled freshly according to
standard procedures. 1H and 13C NMR spectra were recorded on
a Bruker (AV 400/AV 500 MHz) spectrometer in CDCl3 and
DMSO-d6 as solvents. Chemical shifts are reported in d scale
downfield from the peak for tetramethylsilane. Absorption
spectra were recorded on a Jasco-550 spectrophotometer.
Emission spectra were obtained from a Hitachi F-4500 spectro-
fluorimeter. The emission spectra in solutions were measured in a
spectral grade solvent using a 90ꢁ angle detection. The redox
potentials were measured by using cyclic voltammetry on a CHI
620 analyzer. All measurements were carried out in THF
Scheme 1 Synthetic route of organic dyes. Reagents and conditions: (i) n-BuLi, DMF, THF, ꢀ78 ꢁC; (ii) (a) n-BuLi, triisopropylborate, THF, followed
by HCl(aq), (b) Pd(PPh3)4, 3,7-dibromo-10-hexyl-10H-phenothiazine or 3,7-dibromo-10-(4-(hexyloxy)phenyl)-10H-phenothiazine (3) and 7-bromo-10-
hexyl-10H-phenothiazine-3-carbaldehyde or 7-bromo-10-(4-(hexyloxy)phenyl)-10H-phenothiazine-3-carbaldehyde (5), toluene–THF (2/1), K2CO3 (2
M); and (iii) cyanoacetic acid, NH4OAc, AcOH, 90–100 ꢁC.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 21704–21712 | 21705

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3-(3-Bromo-10-hexyl-10H-phenothiazin-7-yl)-10-hexyl-10H-
phenothiazine (3a)
between two electrodes. An acetonitrile solution containing LiI
(0.5 M), I2 (0.05 M) and 4-tert-butylpyridine (TBP) (0.5 M) was
used as the electrolyte 1 (E1). A solution of 3-dimethylimidazo-
lium iodide (1.0 M), LiI (0.05 M), I2 (0.03 M), guanidinium
thiocyanate (0.1 M), and TBP (0.5 M) in MeCN : valeronitrile
(85 : 15, v/v) was used as electrolyte 2 (E2). Devices made of a
commercial dye N719 under the same conditions (3 ꢂ 10ꢀ4 M,
Solaronix S.A., Switzerland) were used as reference. The cell
parameters were obtained under incident light with an intensity
of 100 mW cmꢀ2 measured by a thermopile probe (Oriel 71964),
which was generated by a 300 W (Oriel Class A Solar Simulator
91160A-1000, Newport) passing through an AM 1.5 filter (Oriel
74110). The light intensity was further calibrated by an Oriel
reference solar cell (Oriel 91150) and adjusted to be 1.0 sun. The
monochromatic quantum efficiency was recorded through a
monochromator (Oriel 74100) under short-circuit conditions.
Electrochemical impedance spectra of DSSCs were recorded by
an Impedance/Gain-Phase analyzer (SI 1260, Solartron).
To a three-necked round-bottom flask containing 1a (3.2 g, 8.83
mmol) was added dropwise BuLi (8.3 mL, 13.3 mmol, 1.6 M in
hexane) in dry THF at ꢀ78 ꢁC, after then the solution was
brought up to 0 ꢁC and was stirred by a magnetic bar for 30
minutes. The solution was cooled again to ꢀ78 ꢁC and to it was
added dropwise triisopropyl borate (3.6 mL, 13.3 mmol). The
reaction mixture was warmed up gradually to room temperature
and was stirred overnight. To the reaction mixture was then
added an excess amount of 10% HCl(aq) (30 mL), while the
mixture was stirred for another 1 h. The reaction was quenched
by pouring into distilled water, followed by extraction with ethyl
acetate. The organic layer was dried over anhydrous MgSO4.
Evaporation of the solvent gave a crude product, which was
immediately subjected to the next reaction. It was mixed with 3,7-
dibromo-10-hexyl-10H-phenothiazine (3.89 g, 8.83 mmol),
K2CO3(aq) (2.76 g, 2 mmol) in 10 mL H2O, and Pd(PPh3)4 (310
mg, 0.26 mmol) in dry toluene–THF (2/1). The mixture was
heated to 60 ꢁC for 12 h. After cooling, the products were
extracted with ethyl acetate and the organic layer dried over
anhydrous MgSO4. The crude product was dried in vacuo, and
was purified using a silica gel column chromatograph eluted with
CH2Cl2–hexane (1/1). A yellow solid of 3a was obtained in 55%
yield (3.12 g, 4.85 mmol). dH (400 MHz, CDCl3) 7.20–7.33 (m,
6H), 7.18 (d, 2H, J ¼ 7.2 Hz), 6.95 (t, 1H, J ¼ 7.2 Hz), 6.89 (d,
2H, J ¼ 9.2 Hz), 6.87 (d, 1H, J ¼ 8.8 Hz), 6.70 (d, 1H, J ¼ 8.4
Hz), 3.87 (t, 2H, J ¼ 7.2 Hz), 3.81 (t, 2H, J ¼ 7.2 Hz), 1.77–1.89
(m, 4H), 1.43–1.50 (m, 4H), 1.34–1.37 (m, 8H), 0.91–0.95 (m,
6H). dC (100 MHz, CDCl3) 145.0, 144.3, 143.7, 134.6, 134.0,
129.9, 129.6, 127.4, 127.2, 126.7, 125.4, 125.2, 125.1, 124.5, 124.4,
122.3, 116.4, 115.6, 115.5, 115.3, 114.3, 47.6, 47.5, 31.5, 31.4,
26.9, 26.8, 26.7, 26.6, 14.0. MS (EI, 70 eV): m/z (relative intensity)
642 (M+, 100); HRMS calcd for C36H39N2BrS2: 642.1738, found
642.1741.
N-Hexylphenothiazine-3-carbaldehyde (2a)
To a solution of 3-bromo-10-hexyl-10H-phenothiazine (2.0 g,
5.52 mmol) in THF at ꢀ78 ꢁC was added dropwise n-BuLi
(5.2 mL, 8.28 mmol, 1.6 M in hexane). The mixture was stirred
for 1 h, then to it was added DMF (0.63 mL, 8.28 mmol). The
reaction was stirred with a magnetic bar for 6 h, then was
quenched by adding water, then was extracted with ethyl
acetate. The organic layer was dried over anhydrous MgSO4
and concentrated under reduced pressure to give the crude
product. The product was purified by a silica gel column
chromatograph eluted with dichloromethane–hexane (1/1). A
yellow solid of 2a was obtained in 80% yield (1.37 g, 4.41
mmol). Spectroscopic data of 2a: dH (400 MHz, CDCl3) 9.78 (s,
1H), 7.62 (dd, 1H, J ¼ 1.9, 8.4 Hz), 7.56 (d, 1H, J ¼ 1.8 Hz),
7.13–7.17 (m, 1H), 7.09 (dd, 1H, J ¼ 1.3, 7.6 Hz), 6.95 (t, 1H,
J ¼ 7.5 Hz), 6.86–6.89 (m, 2H), 3.87 (t, 2H, J ¼ 7.2 Hz), 1.76–
1.83 (m, 2H), 1.39–1.46 (m, 2H), 1.29–1.31 (m, 4H), 0.87 (t, 3H,
J ¼ 7.0 Hz); dC (100 MHz, CDCl3) 189.9, 150.7, 143.4, 131.0,
130.0, 128.3, 127.5, 125.0, 123.7, 123.5, 115.9, 114.7, 48.0, 31.3,
26.7, 26.5, 22.5, 13.9; MS (FAB, 70 eV): m/z (relative intensity)
311 (M+, 100); HRMS calcd for C19H21NOS: 311.1344, found
311.1341.
3-(3-Bromo-10-(4-(hexyloxy)phenyl)-10H-phenothiazin-7-yl)-10-
(4-(hexyloxy)phenyl)-10H-phenothiazine (3b)
Compound 3b was synthesized according to the same procedure
as that of 3a. A yellow solid of 3b was obtained in 50% yield. dH
(400 MHz, CDCl3) 7.27 (dt, 2H, J ¼ 8.8, 2.0 Hz), 7.23 (dt, 2H, J
¼ 9.6, 2.0 Hz), 7.06–7.10 (m, 7H), 6.97 (dd, 1H, J ¼ 7.6, 2.0 Hz),
6.86–6.92 (m, 3H), 6.74–6.84 (m, 2H), 6.17 (d, 2H, J ¼ 6.8 Hz),
6.15 (d, 1H, J ¼ 6.8 Hz), 6.01 (d, 1H, J ¼ 8.8 Hz), 4.02 (t, 4H, J ¼
6.4 Hz), 1.80–1.87 (m, 4H), 1.48–1.52 (m, 4H), 1.35–1.39 (m, 8H),
0.91–0.94 (m, 6H). dC (100 MHz, CDCl3) 159.0, 158.9, 144.3,
143.6, 143.0, 134.2, 133.7, 132.9, 132.5, 132.0, 131.8, 129.4, 128.6,
126.8, 126.5, 124.7, 124.5, 124.1, 122.2, 121.4, 120.0, 119.1, 116.7,
116.5, 116.4, 115.9, 115.8, 115.6, 114.1, 68.3, 31.5, 29.2, 25.7,
22.6, 14.0. MS (EI, 70 eV): m/z (relative intensity) 826 (M+, 100);
HRMS calcd for C48H47O2N2BrS2: 826.2262, found 826.2264.
10-(4-(Hexyloxy)phenyl)-10H-phenothiazine-3-carbaldehyde (2b)
Compound 2b was synthesized according to the same procedure
as that of 2a. A light yellow solid of 2b was obtained in 77%
yield. dH (400 MHz, CDCl3) 9.69 (s, 1H), 7.45 (d, 1H, J ¼ 1.6
Hz), 7.28 (dd, 1H, J ¼ 8.2, 2.0 Hz), 7.27 (d, 2H, J ¼ 8.8 Hz),
7.13 (d, 2H, J ¼ 8.8 Hz), 6.96 (dd, 1H, J ¼ 5.6, 3.6 Hz), 6.84
(dd, 2H, J ¼ 5.8, 3.6 Hz), 6.22 (d, 1H, J ¼ 8.8 Hz), 6.17–6.19
(m, 1H), 4.05 (t, 2H, J ¼ 6.4 Hz), 1.84–1.88 (m, 2H), 1.51–1.55
(m, 2H), 1.38–1.42 (m, 4H), 0.93–0.97 (m, 3H). dC (100 MHz,
CDCl3) 189.6, 159.2, 149.5, 142.8, 132.0, 131.6, 130.9, 129.9,
127.3, 127.1, 126.6, 123.5, 120.0, 118.9, 116.7, 116.4, 115.0, 68.4,
31.5, 29.2, 25.7, 22.6, 14.0. MS (FAB, 70 eV): m/z (relative
intensity) 404 ((M + H)+, 100); HRMS calcd for C25H25NO2S:
404.1684, found 404.1694.
10-Hexyl-7-(10-hexyl-10H-phenothiazin-3-yl)-10H-pheno-thiazine-
3-carbaldehyde (4a)
Compound 4a was synthesized according to the same procedure
as that of 2a. A yellow solid of 4a was obtained in 76% yield. dH
(400 MHz, CDCl3) 9.80 (s, 1H), 7.64 (dd, 1H, J ¼ 8.6, 2.0 Hz),
21706 | J. Mater. Chem., 2012, 22, 21704–21712
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7.58 (d, 1H, J ¼ 2.0 Hz), 7.31 (t, 1H, J ¼ 2.0 Hz), 7.30 (d, 2H, J ¼
2.0 Hz), 7.26 (d, 1H, J ¼ 2.0 Hz), 7.16 (d, 2H, J ¼ 8.0 Hz), 6.93 (t,
1H, J ¼ 7.2 Hz), 6.88 (d, 4H, J ¼ 8.4 Hz), 3.88 (t, 2H, J ¼ 6.8 Hz),
3.86 (t, 2H, J ¼ 6.8 Hz), 1.81–1.85 (m, 4H), 1.44–1.48 (m, 4H),
1.30–1.35 (m, 8H), 0.93 (t, 6H, J ¼ 2.0 Hz). dC (100 MHz, CDCl3)
189.9, 150.4, 145.0, 144.4, 142.1, 135.5, 133.6, 131.0, 130.1, 128.3,
127.4, 127.3, 125.4, 125.3, 125.2, 125.2, 125.1, 124.4, 124.3, 124.0,
122.4, 116.0, 115.5, 115.3, 114.6, 48.0, 47.5, 31.5, 31.4, 26.8, 26.6,
26.5, 22.7, 22.6, 14.1, 14.0. MS (FAB, 70 eV): m/z (relative
intensity) 593 ((M + H)+, 100); HRMS calcd for C37H41N2OS2:
593.2661, found 593.2674.
122.2, 120.0, 119.78, 119.7, 119.5, 119.3, 119.2, 116.7, 116.6,
116.5, 116.4, 115.8, 115.6, 114.9, 68.4, 31.6, 29.2, 25.7, 22.6, 14.0.
MS (FAB, 70 eV): m/z (relative intensity) 1149 (M+, 100); HRMS
calcd for C73H71O4N3S3: 1149.4607, found 1149.4608.
(E)-2-Cyano-3-(N-hexylphenothiazin-7-yl)acrylic acid (PT1a)
A mixture of 2a (1.00 g, 3.21 mmol), cyanoacetic acid (355 mg,
4.17 mmol), and ammonium acetate (74 mg, 0.96 mmol) were
placed in a three-necked flask in acetic acid under a nitrogen
ꢁ
atmosphere with heating at 100 C for 12 h. After cooling, the
reaction was quenched by adding water, and extracted with
CH2Cl2. The organic layer was dried over anhydrous MgSO4 and
concentrated under reduced pressure to give the crude product.
The product was purified using a silica gel column chromato-
graph eluted with CH2Cl2–acetic acid (19/1). The deep red solid
was isolated in 70% yield (770 mg, 1.28 mmol), mp: 154–156 ꢁC.
Spectroscopic data of PT1a: dH (400 MHz, DMSO-d6) 8.14 (s,
1H), 7.90 (d, 1H, J ¼ 8.7 Hz), 7.79 (d, 1H, J ¼ 1.7 Hz), 7.22 (t,
1H, J ¼ 8.0 Hz), 7.13–7.16 (m, 2H), 7.06 (d, 1H, J ¼ 8.2 Hz), 6.99
(t, 1H, J ¼ 7.4 Hz), 3.92 (t, 2H, J ¼ 7.2 Hz), 1.63–1.70 (m, 2H),
1.34–1.39 (m, 2H), 1.22–1.24 (m, 4H), 0.81 (t, 3H, J ¼ 6.8 Hz); dC
(100 MHz, DMSO-d6) 163.8, 152.2, 148.8, 142.7, 131.5, 129.0,
128.0, 127.3, 125.6, 123.6, 123.2, 122.1, 116.9, 116.4, 115.6, 100.0,
46.9, 30.7, 26.0, 25.7, 22.0, 13.8; MS (FAB, 70 eV): m/z (relative
intensity) 378 (M+, 100); HRMS calcd for C22H22 N2O2S:
378.1402, found 378.1400.
10-(4-(Hexyloxy)phenyl)-7-(10-(4-(hexyloxy)phenyl)-10H-
phenothiazin-7-yl)-10H-phenothiazine-3-carbaldehyde (4b)
Compound 4b was synthesized according to the same procedure
as that of 2a. An orange solid of 4b was obtained in 82% yield. dH
(400 MHz, CDCl3) 9.68 (s, 1H), 7.44 (d, 1H, J ¼ 2.0 Hz), 7.24–
7.28 (m, 5H), 7.10–7.15 (m, 5H), 7.07 (d, 1H, J ¼ 2.4 Hz), 6.99
(dd, 1H, J ¼ 7.6, 2.0 Hz), 6.93 (dd, 2H, J ¼ 8.4, 2.0 Hz), 6.79–6.84
(m, 2H), 6.19–6.22 (m, 3H), 6.16 (d, 1H, J ¼ 8.4 Hz), 1.85–1.89
(m, 4H), 1.53–1.56 (m, 4H), 1.39–1.44 (m, 8H), 0.96–1.00 (m,
6H). dC (100 MHz, CDCl3) 189.5, 149.2, 144.3, 143.8, 141.6,
135.2, 133.3, 132.8, 132.1, 132.0, 131.5, 130.8, 129.9, 127.4, 126.9,
126.6, 124.6, 124.5, 124.1, 124.0, 122.3, 120.1, 119.7, 119.4, 119.1,
116.7, 116.4, 115.8, 68.3, 31.6, 29.3, 29.2, 25.8, 22.6, 14.1. MS
(FAB, 70 eV): m/z (relative intensity) 777 ((M + H)+, 100);
HRMS calcd for C49H49N2O3S2: 777.3185, found 777.3204.
(E)-2-Cyano-3-(10-(4-(hexyloxy)phenyl)-10H-phenothiazin-7-
yl)acrylic acid (PT1b)
10-Hexyl-7-(10-hexyl-3-(10-hexyl-10H-phenothiazin-7-yl)-10H-
phenothiazin-7-yl)-10H-phenothiazine-3-carbaldehyde (5a)
Compound PT1b was synthesized according to the same proce-
dure as that of PT1a. A red solid of PT1b was obtained in 70%,
mp: 167–169 ꢁC. dH (400 MHz, DMSO-d6) 8.00 (s, 1H), 7.59 (s,
1H), 7.54 (d, 1H, J ¼ 8.8 Hz), 7.26 (d, 2H, J ¼ 8.8 Hz), 7.14 (d,
2H, J ¼ 8.8 Hz), 6.95–6.97 (m, 1H), 6.84–6.86 (m, 2H), 6.17–6.20
(m, 2H), 4.06 (t, 2H, J ¼ 6.4 Hz), 1.85–1.88 (m, 2H), 1.51–1.55
(m, 2H), 1.39–1.42 (m, 2H), 0.95 (t, 3H, J ¼ 6.8 Hz); dC (100
MHz, DMSO-d6) 167.9, 159.4, 154.3, 149.2, 142.3, 131.8, 131.6,
131.3, 129.4, 127.1, 126.6, 125.2, 123.8, 120.1, 118.8, 116.8, 116.6,
115.8, 115.3, 96.9, 68.4, 31.5, 29.1, 25.7, 22.6, 14.0; MS (FAB, 70
eV): m/z (relative intensity) 471 ((M + H)+, 100); HRMS calcd
for C28H27N2O3S: 471.1750, found 471.1742.
Compound 5a was synthesized according to the same procedure
as that of 3a. A yellow solid of 5a was obtained in 65% yield. dH
(400 MHz, CDCl3) 9.81 (s, 1H), 7.66 (dd, 1H, J ¼ 8.4, 2.0 Hz),
7.61 (d, 1H, J ¼ 2.0 Hz), 7.31–7.34 (m, 7H), 7.28 (d, 1H, J ¼ 2.4
Hz), 7.16 (d, 2H, J ¼ 7.6 Hz), 6.87–6.95 (m, 7H), 3.85–3.90 (m,
6H), 1.82–1.87 (m, 6H), 1.45–1.49 (m, 6H), 1.28–1.37 (m, 12H),
0.90–0.92 (m, 9H). dC (100 MHz, CDCl3) 189.9, 150.4, 145.1,
144.2, 143.8, 142.2, 135.5, 134.4, 134.1, 133.7, 131.0, 130.1, 128.4,
127.4, 127.2, 125.4, 125.2, 125.1, 124.8, 124.6, 124.5, 124.4, 124.1,
122.3, 116.0, 115.4, 115.3, 114.6, 48.0, 47.6, 31.4, 31.3, 26.8, 26.6,
26.5, 22.6, 22.6, 14.0. MS (FAB, 70 eV): m/z (relative intensity)
873 (M+, 100); HRMS calcd for C55H59ON3S3: 873.3820, found
873.3807.
(E)-2-Cyano-3-(10-hexyl-3-(10-hexyl-10H-phenothiazin-3-yl)-
10H-phenothiazin-7-yl)acrylic acid (PT2a)
10-(4-(Hexyloxy)phenyl)-7-(10-(4-(hexyloxy)phenyl)-3-(10-(4-
(hexyloxy)phenyl)-10H-phenothiazin-7-yl)-10H-pheno-thiazin-
7-yl)-10H-phenothiazine-3-carbaldehyde (5b)
Compound PT2a was synthesized according to the same proce-
dure as that of PT1a. A red solid of PT2a was obtained in 72%,
mp: 126–128 ꢁC. dH (400 MHz, DMSO-d6) 8.08 (s, 1H), 7.83 (d,
1H, J ¼ 8.0 Hz), 7.36 (d, 2H, J ¼ 8.0 Hz), 7.32 (s, 2H), 7.14 (t, 1H,
J ¼ 7.6 Hz), 7.08 (d, 1H, J ¼ 7.2 Hz), 7.01 (d, 1H, J ¼ 8.8 Hz),
6.86–6.96 (m, 4H), 3.76–3.81 (m, 4H), 1.59–1.61 (m, 4H), 1.28–
1.30 (m, 4H), 1.60–1.62 (m, 8H), 0.75–0.76 (m, 6H); dC (100
MHz, DMSO-d6) 164.5, 151.8, 148.5, 144.8, 144.2, 141.9, 134.6,
133.1, 131.7, 129.1, 127.9, 127.5, 126.1, 125.7, 125.5, 124.7, 124.6,
123.6, 123.2, 123.1, 122.8, 117.7, 116.8, 116.2, 116.0, 115.8, 47.3,
46.9, 31.3, 31.2, 26.6, 26.4, 26.3, 26.1, 22.5; MS (FAB, 70 eV): m/z
(relative intensity) 660 ((M + H)+, 100); HRMS calcd for
C40H41N3O2S2: 660.2732, found 660.2719.
Compound 5b was synthesized according to the same procedure
as that of 3a. A yellow solid of 5b was obtained in 57% yield. dH
(400 MHz, CDCl3) 9.70 (s, 1H), 7.46 (d, 1H, J ¼ 2.0 Hz), 7.25–
7.31 (m, 7H), 7.10–7.15 (m, 9H), 7.08 (d, 1H, J ¼ 2.0 Hz), 7.00
(dd, 1H, J ¼ 7.2, 1.6 Hz), 6.91–6.95 (m, 4H), 6.79–6.85 (m, 2H),
6.15–6.22 (m, 6H), 4.03–4.06 (m, 6H), 1.83–1.88 (m, 6H), 1.52–
1.55 (m, 6H), 1.39–1.42 (m, 12H), 0.93–0.96 (m, 9H). dC (100
MHz, CDCl3) 189.6, 159.3, 159.0, 158.9, 149.2, 144.4, 143.6,
143.1, 141.6, 135.2, 134.0, 133.8, 133.4, 132.9, 132.8, 132.1, 132.0,
131.5, 130.8, 129.9, 127.4, 126.8, 126.5, 124.7, 124.6, 124.5, 124.0,
This journal is ª The Royal Society of Chemistry 2012
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(E)-2-Cyano-3-(10-(4-(hexyloxy)phenyl)-3-(10-(4-(hexyloxy)-
phenyl)-10H-phenothiazin-7-yl)-10H-phenothiazin-7-yl)acrylic
acid (PT2b)
software. The frontier orbital plots of HOMO and LUMO were
drawn by using Gaussian 03.
Results and discussion
Compound PT2b was synthesized according to the same proce-
dure as that of PT1a. A dark red solid of PT2b was obtained in
85%, mp: 172–174 ꢁC. dH (400 MHz, DMSO-d6) 8.02 (s, 1H),
7.71 (s, 1H), 7.54 (d, 1H, J ¼ 9.2 Hz), 7.24–7.31 (m, 6H), 7.17 (d,
4H, J ¼ 8.0 Hz), 7.09 (d, 2H, J ¼ 8.4 Hz), 7.02 (d, 1H, J ¼ 6.8
Hz), 6.88 (t, 1H, J ¼ 7.2 Hz), 6.80 (t, 1H, J ¼ 7.2 Hz), 6.05–6.11
(m, 4H), 4.00–4.03 (m, 4H), 1.72–1.75 (m, 4H), 1.41–1.44 (m,
4H), 1.31–1.33 (m, 8H), 0.87–0.90 (m, 6H); dC (100 MHz,
DMSO-d6) 164.2, 159.2, 158.8, 152.4, 147.5, 144.0, 143.5, 141.2,
134.4, 132.8, 132.5, 132.1, 131.8, 131.6, 131.5, 128.5, 127.5, 126.8,
125.9, 124.9, 123.9, 122.8, 119.7, 119.1, 118.9, 118.8, 117.2, 116.9,
116.1, 115.8, 115.4, 99.7, 68.2, 31.4, 29.1, 25.7, 22.5, 14.3; m/z
(FAB) 844.3264 ((M + H)+, C52H49N3O4S2 requires 844.3242).
Synthesis
The key intermediate in the synthesis of PT1, PT2, and PT3 was
the N-substituted 7-bromophenothiazine derivatives, i.e.,
compounds 1a and 1b that were prepared by N-alkylation. The
phenothiazines with a carbaldehyde functionality (2, 4, and 5)
were prepared from 10H-phenothiazine according to a sequence
of Suzuki–Miyaura coupling12 followed by formylation to give
the products with yields of 57–82%. The presence of an aldehyde
group was verified by absorption at d 9.68–9.81 ppm in 1H NMR.
The final step was a Knoevenagel condensation between the
carbaldehyde and cyanoacetic acid in the presence of ammonium
acetate.13 The synthesis of PT1a was completed by a standard
procedure as described in the ESI.†14 The structures of all new
compounds were characterized by their spectroscopic data.
(E)-2-Cyano-3-(10-hexyl-3-(10-hexyl-3-(10-hexyl-10H-
phenothiazin-7-yl)-10H-phenothiazin-7-yl)-10H-phenothiazin-7-
yl)acrylic acid (PT3a)
Absorption spectra
Compound PT3a was synthesized according to the same proce-
dure as that of PT1a. A red solid of PT3a was obtained in 70%,
The UV-Vis absorption spectra of all dyes in CH2Cl2 solution
and on TiO2 are shown in Fig. 2 and 3, and the parameters are
listed in Table 1. The compounds with multiple units of pheno-
thiazine, i.e., PT2 and PT3, display longer absorption wave-
lengths than the mono-phenothiazine compound, i.e., PT1. All
these dyes exhibit broad absorption in the range of 250–600 nm.
The short wavelength region at 250–330 nm is attributed to
localized p–p* and n–p* transitions, while the long wavelength
region in the range of 459–496 nm is derived from intramolecular
charge-transfer transitions (ICT). The intensity of UV transition
increases with the number of phenothiazine units, i.e., PT3 >
PT2 > PT1. The absorption photocurrent (Jsc) in maxima of ICT
bands exhibits a similar trend in solutions.
ꢁ
mp: 112–114 C. dH (400 MHz, THF-d8) 8.11 (s, 1H), 8.01 (dd,
1H, J ¼ 8.4, 2.0 Hz), 7.80 (d, 1H, J ¼ 2.0 Hz), 7.37–7.44 (m, 8H),
6.96–7.15 (m, 8H), 6.92 (t, 1H, J ¼ 7.2 Hz), 3.61–4.01 (m, 6H),
1.81–1.86 (m, 6H), 1.50–1.52 (m, 6H), 1.32–1.37 (m, 12H), 0.87–
0.93 (m, 9H); dC (100 MHz, THF-d8) 163.3, 151.9, 149.0, 145.2,
144.2, 143.9, 142.0, 135.3, 134.2, 134.1, 133.6, 130.7, 129.7, 127.0,
126.0, 125.3, 125.1, 124.9, 124.8, 124.7, 124.6, 124.0, 123.7, 122.0,
116.1, 115.9, 115.5, 115.3, 115.0, 99.8, 47.4, 47.1, 47.0, 31.5, 31.4,
26.8, 26.6, 26.4, 26.3, 22.5, 13.4; MS (FAB, 70 eV): m/z (relative
intensity) 940 (M+, 100); HRMS calcd for C58H60N4O2S3:
940.3878, found 940.3861.
(E)-2-Cyano-3-(10-(4-(hexyloxy)phenyl)-3-(10-(4-(hexyloxy)-
phenyl)-3-(10-(4-(hexyloxy)phenyl)-10H-phenothiazin-7-yl)-10H-
phenothiazin-7-yl)-10H-phenothiazin-7-yl)acrylic acid (PT3b)
The absorption spectra of all dyes absorbed on the surface of
TiO2 are shown in Fig. 3. The film absorption band displayed a
blue shift of ca. 29–38 nm with respect to those in solutions, and
the spectral difference among all dyes diminished. The blue shift
appears to be a result of deprotonation of the carboxylic acid,
when it is anchored onto the surface of titanium oxide.
Compound PT3b was synthesized according to the same proce-
dure as that of PT1a. A black solid of PT3b was obtained in 77%,
ꢁ
mp: 152–154 C. dH (400 MHz, THF-d8) 8.02 (s, 1H), 7.68 (s,
1H), 7.65 (d, 1H, J ¼ 8.8 Hz), 7.31–7.35 (m, 6H), 7.18–7.23 (m,
10H), 6.96–7.01 (m, 5H), 6.77–6.84 (m, 2H), 6.20–6.25 (m, 6H),
4.08 (t, 6H, J ¼ 6.0 Hz), 1.83–1.88 (m, 6H), 1.56–1.58 (m, 6H),
1.41–1.43 (m, 12H), 0.97 (t, 9H, J ¼ 6.4 Hz); dC (100 MHz, THF-
d8) 159.5, 159.2, 159.1, 151.5, 147.9, 144.4, 143.5, 143.1, 141.6,
133.7, 133.3, 133.0, 131.9, 131.4, 130.4, 128.9, 126.5, 126.1, 124.1,
123.6, 121.9, 120.1, 119.5, 119.3, 116.5, 116.3, 116.2, 115.7, 115.4,
115.1, 67.9, 31.59, 29.6, 29.2, 25.7, 22.5, 13.3; MS (FAB, 70 eV):
m/z (relative intensity) 1216 (M+, 100); HRMS calcd for
C76H72O5N4S3: 1216.4665, found 1216.4678.
Quantum chemistry computations
The structure of dyes was optimized by using the B3LYP/6-31G*
hybrid functional. For the excited states, a time-dependent
density functional theory (TDDFT) with the B3LYP functional
was employed. All analyses were performed using Q-Chem 3.0
Fig. 2 Absorption spectra of dyes in CH2Cl2 solution.
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Table 1 Photochemical and electrochemical parameters of the dyes
Ema
(nm)
lmax
(TiO2)
Eox
(V)
E0–0
(eV)
Ered
(V)
Jsc (mA cmꢀ2
)
Voc (V)
E1/E2
FF
E1/E2
hd (%)
E1/E2
a
lmax (nm)/
b
c
dye
(3 (Mꢀ1 cmꢀ1))
E1/E2e
PT1a
PT2a
PT3a
PT1b
PT2b
PT3b
N719
459(14 000)
477(17 000)
478(16 100)
475(16 200)
495(20 500)
496(19 000)
—
660
676
675
655
662
661
—
428
448
446
437
461
459
—
0.73
0.59
0.58
0.73
0.57
0.50
—
2.32
2.24
2.19
2.15
2.10
2.03
—
ꢀ1.59
ꢀ1.65
ꢀ1.61
ꢀ1.42
ꢀ1.53
ꢀ1.53
—
12.91/11.03
13.20/12.21
12.09/10.25
13.47/12.87
14.48/14.00
11.93/12.98
15.87/18.61
0.68/0.76
0.69/0.77
0.70/0.82
0.70/0.78
0.71/0.82
0.72/0.82
0.74/0.76
0.62/0.65
0.62/0.58
0.59/0.60
0.66/0.65
0.61/0.64
0.58/0.60
0.61/0.63
5.45/5.43
5.63/5.44
5.01/5.05
6.21/6.52
6.30/7.38
5.01/6.44
7.15/8.93
a
Absorption and emission are in CH2Cl2. b Oxidation potential in THF (10ꢀ3 M) containing 0.1 M (n-C4H9)4NPF6 with a scan rate of 50 mV sꢀ1 (vs.
NHE). c Ered calculated by Eox ꢀ E0–0
.
Performance of DSSCs measured in a 0.25 cm2 working area on a FTO (8 U per square) substrate. e Electrolyte
d
1 (E1): LiI (0.5 M), I2 (0.05 M), and TBP (0.5 M) in MeCN. Electrolyte 2 (E2): 3-dimethylimidazolium iodide (1.0 M), LiI (0.05 M), I2 (0.03 M),
guanidinium thiocyanate (0.1 M), and TBP (0.5 M) in MeCN : valeronitrile (85 : 15, v/v).
calculations, which will be discussed in the following section. The
LUMO levels of the dyes were estimated by the values of Eox and
the zero–zero band gaps at the onset of absorption spectra
(Table 1). For a DSSCs to work properly, both the HOMO and
LUMO (lowest unoccupied molecular orbital) levels have to
match the conduction band (ꢀ0.5 V vs. NHE) of the TiO2 elec-
trode and the redox potential (0.4 V vs. NHE) of the iodine/
iodide electrolyte. The energy levels of all dyes agree well with the
requirements for an efficient flow of electrons.
Computational analysis
The electronic nature of the dyes was further explored by theo-
retical models. Full geometrical optimizations were performed
Fig. 3 Absorption spectra of dyes absorbed on TiO2.
by using the B3LYP/6-31G* hybrid functional implanted in Q-
Chem. In the ground state the geometry of phenothiazine is not
totally planar, but rather slightly bent in the center forming the
shape of a butterfly. The dihedral angles are about 32ꢁ to 39ꢁ in
each phenothiazine. Such a slight distortion does not seem to
retard the resonance between the two fused benzene rings across
the central phenothiazine moiety. As depicted in Fig. 5, the
electron density in the HOMO levels of PT2 and PT3 spread over
more than one phenothiazine unit. The strong resonance effect is
beneficial to the migration of electrons upon photo-excitation.
The electron density distribution in the LUMOs of the dyes is
localized mainly on the cyanoacrylate end group as shown in
Fig. 5. For PT1, a significant electronic delocalization appears
Electrochemical properties
The oxidation potentials (Eox), corresponding to the highest
occupied molecular orbital (HOMO) level of the dyes, were
measured by cyclic voltammetry. The number of oxidative waves
corresponds to the number of phenothiazine units in each
compound. The lowest ionization potentials in THF solutions
decreased in the order of PT1 > PT2 > PT3 (Fig. 4). It is clear
that the adjacent phenothiazine moieties linked together in the
same molecule can influence each other effectively. Such a
conclusion is again supported by the result of theoretical
Fig. 5 Frontier molecular orbitals of PT1b, PT2b, and PT3b calculated
Fig. 4 Oxidation potential of the dyes in THF (10ꢀ3 M).
This journal is ª The Royal Society of Chemistry 2012
with TDDFT (B3LYP/6-31G*) in vacuum.
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Fig. 6 Difference of Mulliken charges between ground state (S0) and
excited state (S1), estimated by a time dependent DFT/B3LYP model.
between the cyanoacrylate and the adjacent ring in the nearest
phenothiazine unit. This kind of interaction reduced the elec-
tronic density in the nearest phenothiazine unit and weakened its
electron releasing property. As a result, the HOMO–LUMO
energy gap in PT1 is wider than those in PT2 and PT3, therefore
the ICT absorption band in the former exhibited a blue shift with
respect to the latter. For PT2 and PT3, there exist more than one
phenothiazine units, which are all potential electron donors in
ICT transitions. In PT2 it is clear that the second phenothiazine
moiety bears the most positive charge in the charge-separated
state. For PT3 the answer may not be straightward before a
quantitative analysis on the charge distribution in the excited
state. In Fig. 5, a difference on Mulliken charge distributions
between the ground state (S0) and excited state (S1) is plotted
according to the estimation by the time dependent DFT/B3LYP
model. From the bar chart in Fig. 6, it is clear that the second
phenothiazine moiety carries most of the positive charge in the
ICT state.
Fig. 7 J–V curves and IPCE of the DSSCs devices made with the dyes.
The plots were measured under a light intensity of 1.0 sun.
(Table 1). However in all this studies, the value of Jsc seems to be
the controlling factor on the performances. The trend of relative
quantum efficiency follows the same order as the Jsc value, i.e.,
PT2 > PT1 > PT3. Among all dyes, the devices made with PT2b
performed the best, with Jsc ¼ 14.0 mA cmꢀ2, Voc ¼ 0.82 V, and
FF ¼ 0.64, which summed up to an overall quantum efficiency of
7.38%.
To optimize the performance of DSSCs, it is known that the
alignment of dyes on the surface of TiO2 is a critical factor. The
organic compound needs to be planted vertically on the surface
of metal oxide, yet must not form aggregates of itself. It should
also cover the surface as complete as possible to prevent a direct
contact between TiO2 and the electrolyte. To achieve this goal,
we have made the following designs: (1) to change the electrolyte
system from E1 to E2 to help preventing short-circuit and
increasing the Voc value. The lower concentration of LiI in E2
raises the potential level of conduction band of TiO2, therefore
enlarges the Voc value.15 (2) To add substituents with alkyl chains
to reduce aggregation.16 The substituent size on the nitrogen
atoms in derivatives b (–C6H4OC6H13) is larger than that in
derivatives a (–C6H13). A long chain is able to reduce the dark
current by decreasing the rate of charge recombination. The
effect is supported by the higher resistance in the electrochemical
impedance spectrum (EIS) of derivatives b.17 The Voc values of
derivatives b are considerably higher than those of derivatives a
(Table 1). (3) To adjust the solvent used for anchoring the dyes
onto TiO2. Among the solvents used, i.e., THF, CH2Cl2, EtOH–
CH2Cl2, and MeCN–t-BuOH (1 : 1), the devices made with the
latter gave the best result (Fig. S22, Tables S4, and S5†).
Therefore the mixed solvent system of MeCN and t-BuOH in
1 : 1 ratio is used for the performance parameters listed in Tables
1 and 2.
The bent structure of phenothiazine and its non-conjugation
along the oligo-phenothiazine p-chromophores may slow down
the rate of charge recombination in the excited state. The
conformation of molecules is likely to be re-adjusted upon
photo-excitation to accommodate the charge re-distribution
along the chromophore. Any disturbance on the p-conjugation,
either a distortion on molecular geometry or an interruption of
the conjugation by heteroatoms, may reduce the rate of charge
migration. As long as the charge-separated state can maintain a
longer lifetime, a higher quantum efficiency can be achieved for
the devices.
Photovoltaic performance of DSSCs
The DSSCs devices made with these dyes were fabricated
according to a standard procedure. Two kinds of electrolytes were
used in order to achieve the best result, i.e., system E1 was made of
LiI (0.5 M), I2 (0.05 M), and TBP (4-tert-butylpyridine) (0.5 M) in
MeCN, and system E2 was composed of 3-dimethylimidazolium
iodide (DMII)(1.0 M) and guanidinium thiocyanate (0.1 M), in
addition to LiI (0.05 M), I2 (0.03 M), and TBP (0.5 M) in a mixed
solvent of MeCN and valeronitrile (85 : 15, v/v). The parameters
were measured under AM 1.5 solar light (100 mW cmꢀ2), i.e.,
short-circuit current (Jsc), open-circuit photovoltage (Voc), fill
factor (FF), and solar-to-electrical photocurrent density (h) are
summarized in Table 1. The photocurrent–voltage (J–V) plots of
all devices are shown in Fig. 7. An apparent improvement of Voc
values was observed by using the new type electrolyte E2, e.g. an
increase of ca. 0.10 V was obtained with respect to those using E1
The structure of oligo-phenothiazine is not totally linear,
partly due to the non-planar geometry of the phenothiazine ring,
and also because of the non-symmetrical 3,7-substituted linkages
between the adjacent phenothiazine moieties. The bent shape of
molecules may induce some disturbance on their alignment on
the surface of TiO2. To improve the packing order of the dyes,
adding DCA (deoxycholic acid) to the solution of the dyes may
21710 | J. Mater. Chem., 2012, 22, 21704–21712
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Table 2 Photovoltaic parameters of devices made PT1b, PT2b and
PT3b with and without DCA
Conclusions
In summary, we have successfully demonstrated that high
performance DSSCs can be achieved by using the non-conjugated
spacers constructed by the oligomers of phenothiazine, which act
both as the electron donor and the bridge unit. In the trimer
system, i.e., PT3a and PT3b, the central phenothiazine ring
behaved as the strongest donor, which bears most of the positive
charge in the charge-separated state. The performance of the
dimer system, i.e., PT2a and PT2b, was better than the trimers,
indicating that there exists a limiting factor on the length of the
organic dyes. The promotion of Voc values by the non-conjugate
chromophore was clearly evidenced by the high resistance of the
electrochemical impedance spectrum. The high Voc values reached
a level of >0.83 V, and the best conversion efficiency of 7.78% was
obtained in PT2b. These results are also significantly better than
the previous reports involving phenothiazine derivatives.14,19
Dyea
PT1b
PT2b
PT3b
DCA (mM)
Jsc (mA cmꢀ2
)
Voc (V)
FF
hb (%)
0
10
0
10
0
12.87
13.03
14.00
14.33
12.98
13.33
0.78
0.80
0.82
0.83
0.82
0.83
0.65
0.67
0.64
0.65
0.60
0.62
6.52
6.98
7.38
7.78
6.44
6.87
10
Concentration of dye is 3 ꢂ 10ꢀ4 M in MeCN–t-BuOH (1/1).
a
b
Performance of DSSCs measured in a 0.25 cm2 working area on an
FTO (8 U per square) substrate under AM 1.5 condition with
electrolyte 2.
Acknowledgements
Financial support from the National Science Council of Taiwan
and Academia Sinica are gratefully acknowledged. Special
thanks to Professor C.-P. Hsu at the Institute of Chemistry for
her assistance on quantum chemistry computations.
References
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€
Electrochemical impedance spectroscopy (EIS) analysis was
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