Ring-fluorinated fluoresceins as an organic photosensitizer for dye-sensitized solar cells using nanocrystalline zinc oxide

Article abstract of DOI:10.1016/j.jfluchem.2005.12.009

Four kinds of ring-fluorinated fluoresceins and sulfofluorescein from tetrafluororesorcinol and/or tetrafluorophthalic anhydride have been synthesized and the photochemical properties of the zinc oxide nanocrystalline electrode sensitized by the ring-fluorinated fluoresceins were investigated.

Full text of DOI:10.1016/j.jfluchem.2005.12.009

Journal of Fluorine Chemistry 127 (2006) 257–262
www.elsevier.com/locate/fluor
Ring-fluorinated fluoresceins as an organic photosensitizer for
dye-sensitized solar cells using nanocrystalline zinc oxide
a
a
b
Kazumasa Funabiki a, , Naoyuki Sugiyama , Hidenori Iida , Ji-Ye Jin ,
*
Tsukasa Yoshida c, Yasutake Kato c, Hideki Minoura c, Masaki Matsui a
a Department of Materials Science and Technology, Faculty of Engineering, Gifu University, 1-1, Yanagido, Gifu 501-1193, Japan
b Department of Chemistry, Faculty of Science, Shinshu University, 3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan
c Environmental and Renewable Energy Systems (ERES) Division, Graduate School of Engineering,
Gifu University, 1-1, Yanagido, Gifu 501-1193, Japan
Received 28 November 2005; received in revised form 6 December 2005; accepted 12 December 2005
Available online 30 January 2006
Abstract
Four kinds of ring-fluorinated fluoresceins and sulfofluorescein from tetrafluororesorcinol and/or tetrafluorophthalic anhydride have been
synthesized and the photochemical properties of the zinc oxide nanocrystalline electrode sensitized by the ring-fluorinated fluoresceins were
investigated.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Dye-sensitized solar cells; Fluorinated fluoresceines; Zinc oxide
1. Introduction
sensitized solar cells, has been developed [9]. During the course
of our search for the suitable dyes for this porous ZnO [10], we
would like to report here on the synthesis of four kinds of ring-
fluorinated fluoresceins including fluorinated sulfofluoresceine,
the application to the dye-sensitized solar cell with the porous
ZnO, and the estimation of relationship between the efficiency
and the position of the fluorine atoms.
Much attention has been addressed to the investigation on
new metal-free organic dyes such as merocyanines [1],
coumarins [2], styryl [3], and indoline dyes [4], for the dye-
sensitized solar cells with nanocrystalline semiconductors due
to the advantages, such as the wide versatility for the synthesis
of organic dyes, the ease of the tuning of molecular structure,
¨
and its low cost, since Gratzel and O’Regan have been
published highly efficient ruthenium complex-sensitized
titanium oxide solar cells [5].
2. Results and discussion
Various halogenated 9-fluoresceins have also been
employed as one of the most effective metal-free organic
sensitizer for the dye-sensitized solar cells [6]. However,
although ring-fluorinated fluoresceines are widely used as
fluorescence probes for nitric oxide and metal anion [7], only
20,70-difluorinated fluoresceine derivative has been examined as
a sensitizer for the dye-sensitized solar cell [8].
2.1. Synthesis of ring fluorinated fluoresceins
Four kinds of ring-fluorinated fluoresceins were prepared
according to literature [11]. First, 2,4-difluororesorcinol (2)
was prepared in four steps from commercially available
2,3,4,5-tetrafluoronitrobenzene (1), as follows (Scheme 1): (1)
nucleophilic substitution of fluorine atoms, which are activated
by the nitro group, with methoxide ion; (2) reduction of the
nitro group with Pd/C in EtOAc under hydrogen atmosphere
(one atom); (3) hydrodediazonation of the difluorinated aniline
using the combination of HNO3 and H3PO2; (4) BBr3-
promoted deprotection of methyl ether producing difluorinated
resorcinol.
Recently, a novel method for the preparation of porous zinc
oxide (ZnO), which could be used as a semiconductor for dye-
* Corresponding author. Tel.: +81 58 293 2599; fax: +81 58 293 2599.
E-mail address: kfunabik@apchem.gifu-u.ac.jp (K. Funabiki).
0022-1139/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2005.12.009

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K. Funabiki et al. / Journal of Fluorine Chemistry 127 (2006) 257–262
ophthalic anhydride (3b) under the same conditions to produce
the corresponding 4,5,6,7-tetrafluorofluorescein 4b (4b0) in
74% yield. Unfortunately, 2,4-difluororesorcinol (2a) did not
participate well in the reaction with tetrafluorophthalic
anhydride (3b) to give only 19% yield of the corresponding
fluorescein 4c (4c0). The reaction of difluororesorcinol with
sulfophthalic anhydride under the same conditions produced
20,40,50,70-tetrafluorosulfofluorescein (4d) in 31% yield.
Scheme 1. Reagents and conditions: (a) 2.2 equiv, MeONa, MeOH, 4 8C
(95%); (b) 10 mol% Pd/C, one atom H2, EtOAc, rt (95%); (c) (1) 1.05 equiv,
NaNO2, H2O–conc HCl (v/v = 2:1) and (2) 20 equiv, H3PO2, rt (71%); (d)
3 equiv, BBr3, CH2CI2, rt (90%).
2.2. Absorption, emission, and electroproperties of
fluorinated fluoresceins
As shown in Scheme 2, the obtained 2,4-difluororesorcinol
(2a) reacted smoothly with phthalic anhydride (3a) in the
methanesulfonic acid, which acts as both a solvent and a
Blønsted acid, at 80–85 8C to give the corresponding 20,40,50,70-
tetrafluorofluorescein 4a (4a0) in 74% yield. Resorcinol (2b)
also reacted well with commercially available tetrafluor-
The UV–vis absorption, fluorescence spectra, and electro-
properties of fluorinated fluoresceins (4) as well as fluorine-free
fluorescein as a reference in phosphite buffer solution (pH 9.18)
are listed in Table 1.
The absorption maxima (lmax) of 4 and fluorescein were
observed between 490 and 525 nm. The emission maxima (lem
)
Scheme 2. Reagents and conditions: (a) MeSO3H, 80–85 8C.

K. Funabiki et al. / Journal of Fluorine Chemistry 127 (2006) 257–262
259
Table 1
Properties of fluorescein and fluorinated fluoresceines 4
Entry
Dye
l
max(e) (nm)a
lem (nm)a
Ered (V)b
Eox (V)b
1
2
3
4
5
Fl
490 (69300)
508 (75600)
508 (71200)
525 (75100)
515 (63200)
517
535
531
550
537
ꢀ1.77
ꢀ1.63
ꢀ1.68
ꢀ1.39
ꢀ1.35
0.70
0.76
0.76
0.92
1.02
4a
4b
4c
4d
a
Measured in buffer (pH 9.18) solution at the concentration of
1 ꢁ 10ꢀ5 mol dmꢀ3
.
b
vs. SCE in MeCN.
of 4 and fluorescein were around 515 and 550 nm. Both
absorption and emission maxima shifts to the longer wavelength
in the following order (4c > 4a = 4b > fluorescein) by increas-
ing the number of a fluorine atom. Their molar absorption
coefficient (e) were in the range of 69,200–75,600 nm.
Fig. 1. Schematic energy diagram of semiconductors, fluorinated fluorescein 4,
and electrolyte (I3ꢀ/Iꢀ).
As shown in Table 1, the reduction (Ered) and oxidation
potential (Eox) of the dyes 4 are also described. Electrochemical
measurements of the dyes 4 in DMSO gave the only oxidation
potential. Therefore, the Ered of 4 is calculated using by the
equations using l that means crossing point of the absorption
point and emission spectra [12], as described below.
2.3. Preparations of ZnO solar cell
Detailed procedures for the preparation of nanocrystalline
ZnO film electrodes have been reported in the previous report
[9]. After drying thus prepared film at 150 8C for 1 h, the film
Eox ðV versus Ag in acetonitrileÞ
¼ Eox ðV versus Ag in DMSOÞ þ 0:22 V ;
Eox ðV versus SCE in acetonitrileÞ
¼ Eox ðV versus Ag in acetonitrileÞ ꢀ 0:16 V;
DE ¼ hc=l;
was immersed in the solvent (5 ml) of the dyes
4
(500 mmol dmꢀ3) under the described conditions in Tables 2
and 3.
2.4. Photovoltaic performance of DSSCs with the
fluorinated fluoresceins 4
Ered ðV versus SCE in acetonitrileÞ
¼ Eox ðV versus SCE in acetonitrileÞ þ DE
As shown in Table 2, an effect of the solvents on an
adsorption of the dye 4b on the ZnO was observed.
Fig. 1 illustrates the energy diagram in a system of ZnO solar
cell with fluoresceins 4 in tetrabutylammonium iodide and
iodine solution.
Among the solvent examined, such as toluene, acetonitrile,
ethanol (EtOH), and dimethylformamide (DMF) under the
same conditions, the value of the solar-light-to-electricity
conversion efficiency (h) were not so different and the order is
the following EtOH > DMF > acetonitrile > toluene (entries
1–4). Amount of the adsorbed dyes 4b on the surface of ZnO
increased, accompanied by the decrease of the polarity of the
The Eox and Ered correspond to HOMO and LUMO energy
levels, respectively. The Ered of the dyes are negative enough to
inject electrons into the conduction band of ZnO. On the other
hand, the energy gap between Eox of the dyes and the electrolyte
triiodide/iodide (I3ꢀ/Iꢀ) was small.
Table 2
Photochemical properties of fluorinated fluoresceines 4b on ZnO
IPCEb (%)
Jsc
(mA cmꢀ2
ffe
hf (%)
Dye loading
(nmol cmꢀ2
)
c
d
Voc (V)
Entry
Solvent
Solvent
polaritya
Immersion
conditions
)
1
2
3
4
5
PhMe
MeCN
EtOH
DMF
0.0135
0.2744
0.2886
0.3055
0.2886
80 8C, 1 h
80 8C, 1 h
80 8C, 1 h
80 8C, 1 h
rt, 24 h
18
21
23
21
25
1.42
1.58
1.70
1.48
1.84
0.41
0.42
0.47
0.51
0.51
0.49
0.55
0.57
0.58
0.59
0.28
0.36
0.45
0.43
0.55
66
49
32
18
28
EtOH
a
Lippert values (Ref. [13]).
b
c
d
e
f
IPCE: incident photon-to-current conversion efficiency.
Jsc: shortcircuit photocurrent density.
Voc: open circuit photovoltage.
ff: fill factor.
h: solar-light-to-electricity conversion efficiency.

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K. Funabiki et al. / Journal of Fluorine Chemistry 127 (2006) 257–262
Table 3
Photochemical properties of fluoresceins and fluorinated fluoresceines 4 on zinc oxide
Dyea
IPCEb (%)
Jsc
(mA cmꢀ2
ffe
hf (%)
Dye loading
(nmol cmꢀ2
Jsc per mol
(107 mA molꢀ1
)
c
d
Voc (V)
Entry
)
)
1
2
3
4
5
Fl
38
18
25
3
2.40
1.77
1.84
0.24
1.17
0.51
0.44
0.51
0.44
0.47
0.58
0.62
0.59
0.53
0.57
0.71
0.48
0.55
0.06
0.31
36
32
28
6
6.7
5.5
6.6
4.0
5.6
4a
4b
4c
4d
20
21
a
Dyes were adsorbed on zinc oxide in EtOH at rt for 24 h.
IPCE: incident photon-to-current conversion efficiency.
Jsc: shortcircuit photocurrent density.
Voc: open circuit photovoltage.
b
c
d
e
f
ff: fill factor.
h: solar-light-to-electricity conversion efficiency.
solvents. Immersion at lower temperature for a prolonged time
slightly increased the all photochemical properties (entry 5).
Action–photocurrent spectra, UV–vis absorption spectra of
thin film of fluorinated fluorescein 4b/ZnO under the best
immersion conditions (EtOH, rt, 24 h), and that in EtOH are
shown in Fig. 2.
dyes’ ff and h were also calculated by measuring the I–V curve
of other dyes 4 and fluoresciene. The performance character-
istics of ZnO cells with fluorinated fluoresceines 4 as well as
fluoresceine as a reference are summarized in Table 3.
From the adsorption amounts in Table 3, the amount of dye
on ZnO film was in the following order: Fl (36) > 4a (32) > 4b
(28) > 4d (21) > 4c (6). These results indicate that the acidity
of the anchoring group, which is originated from the number of
the fluorine atoms as well as the difference of the functional
group such as a carboxy or sulfo group, has an important effect
on the amount of dye on ZnO. That is: (1) the introduction of
fluorine atoms in the molecules causes the increase of the
acidity of the anchoring group, because of the strongest
electronegativity of the fluorine atom. In fact, Sun et al.
reported that pKa (3.3) of 4,5,6,7,20,40,50,70-octafluorofluor-
escein (4c) was much smaller than those of 4,5,6,7-
tetrafluorofluorescein (4b) (6.1) [11] and (2) pH (1.6) of sulfo
group is smaller than that of carboxy one (12.6) [14].
Consequently, the desorption of the dye from the surface of
ZnO seems to be faster than the absorption in the absorption–
desorption equilibrium.
Absorption maximum (lmax) of 4b/ZnO is shifted to longer
wavelength and the absorption peak is slightly broadened,
compared with its corresponding absorption peak in EtOH. The
observation can be attributed to the formation of the dimer and/
or aggregate of the dye molecule on the surface of ZnO. The
action–photocurrent spectrum resembles the absorption band of
4b on ZnO. This result indicates that the photocurrent is
generated by the injection of electrons from the adsorbed
fluorinated fluorescein 4b into the conduction band of ZnO.
Photocurrent–photovoltage (I–V) curve of 4b is appeared in
Fig. 3.
The shortcircuit photocurrent density (Jsc) and open circuit
photovoltage (Voc) of 4b were observed at 1.84 mA cmꢀ2 and
0.51 V, respectively. The fill factor ( ff) was calculated to be
0.62 and the solar-light-to-electricity conversion efficiency (h)
was calculated to be 0.59, respectively, using the equations
described below.
As described in Table 3, the contribution of one mole of the
dye molecules to the short-circuit photocurrent could also be
calculated and estimated. The order is Fl (6.7) = 4b (6.6) > 4d
(5.6) = 4a (5.5) > 4c (4.0).
For the fluorinated fluoresceins 4a, 4b and 4c, the
introduction of fluorine atoms into 4,5,6,7-position of the
ff ¼ Pmax=ðVocJscÞ; h ¼ ðVocJsc ffÞ=Pin
In these equations, Pmax and Pin stand for the largest output
power of solar cells and the input power, respectively. Other
Fig. 2. Action and UV–vis absorption spectra of 4b.
Fig. 3. I–V curve of 4b.

K. Funabiki et al. / Journal of Fluorine Chemistry 127 (2006) 257–262
261
fluorescein hardly have an influence on the short-circuit
photocurrent. On the other hand, fluorine atoms at 20,40,50,70-
position slightly caused the reduction of not only the Jsc but Voc.
Highly fluorinated fluoresceins 4c also decreased the values of
the short-circuit photocurrent. In other words, as shown in
Table 1, fluorination of 20,40,50,70-position of fluorescein (4a:
fluorophthalic anhydride was purchased from Acros organics.
All chemicals were of reagent grade and, if necessary, were
purified in the usual manner prior to use. 3,5-Difluororesorci-
nol, 4,5,6,7-tetrafluorofluorescein, and 4,5,6,7, 20,40,50,70-octa-
fluorofluorescein were prepared according to literature [11].
E
red = ꢀ1.63) resulted in a lowering of the Ered (LUMO) than
4.3. Procedure for the synthesis of 20,40,50,70-
tetrafluorofluorescein (4a)
that of 4,5,6,7-position (4b: Ered = ꢀ1.68), and further
fluorination enormously lowers the Ered of the dye 4c (4c:
E
red = ꢀ1.39). That is, the photochemical properties, such as
The mixture of phthalic anhydride (1 mmol, 0.150 g),
methanesulfonic acid (10 ml), 2,4-difluoroesorcinol (2 mmol,
0.292 g) was heated at 80–85 8C for 40 h under argon. The
resultant mixture was quenched with water (70 ml). The
precipitate was corrected by filtration and dried under vacuum.
The precipitate was purified by column chromatography on
silica gel using hexane–ethyl acetate (v/v = 1/3) to give
20,40,50,70-tetrafluorofluorescein in 74% yield (0.304 g).
Yield 74%; mp > 300 8C; Rf 0.50 (hexane/ethyl acetate = 1/
Voc and Jsc, may have a crucial relation to the LUMO of the
dyes. Fluorination of fluorescein may cause a lowering of the
Ered (LUMO) as well as a reduction of the photochemical
properties.
3. Conclusion
In summary, four kinds of ring-fluorinated fluoresceins were
prepared and employed as a sensitizer for ZnO solar cell.
Among the dyes, 4,5,6,7-tetrafluorofluorescein gave highest
solar-light-to-electricity conversion efficiency (h), which is less
efficient than that of fluorine-free fluorescein. As an anchoring
group, the carboxy group is better than sulfo group.
1
3); H NMR (DMSO-d6) d 6.50 (2H, dd, J = 11.22, 1.95 Hz),
7.38 (1H, d, J = 7.69 Hz), 7.75 (1H, t, J = 7.69 Hz), 7.81 (1H, t,
J = 7.69 Hz), 8.01 (1H, d, J = 7.69 Hz), 11.2 (2H, s); 13C NMR
(DMSO-d6) d 168.15 (s), 151.36 (s), 148.52 (dd, J = 240.67,
4.96 Hz), 140.58 (dd, J = 244.80, 6.62 Hz), 136.63 (d,
J = 9.10 Hz), 136.25 (dd, J = 17.37, 12.41 Hz), 135.85 (s),
130.60 (s), 125.49 (s), 125.20 (s), 123.98 (s), 108.87 (d,
J = 7.45 Hz), 108.24 (d, J = 21.50 Hz), 80.28 (s); 19F NMR
(DMSO-d6) d ꢀ57.26 (2F, t, J = 11.22 Hz), ꢀ74.84 (2F, dd,
J = 11.22, 1.95 Hz); HRMS (EI): m/z calcd. for C20H8O5F4
(M), 404.0308; found, 404.0305.
4. Experimental
4.1. Instruments
Melting points were obtained on a Yanagimoto MP-S2
micro-melting point apparatus and are uncorrected. Infrared
spectra (IR) were recorded on a Perkin-Elmer System 2000 FT-
4.4. Procedure for the synthesis of 20,40,50,70-
tetrafluorosulfofluorescein (4d)
1
IR spectrometer using KBr. H NMR spectra were measured
with a JEOL a-400 (400 MHz) FT-NMR spectrometers in
CDCl3 or DMSO-d6 solutions with tetramethylsilane as the
internal standard. 13C NMR spectra were obtained on a JEOL
a-400 (100 MHz) FT-NMR spectrometers in CDCl3 or DMSO-
d6 solutions. High resolution mass spectra (HRMS) were taken
on a JEOL JMS-700 mass spectrometer operating at an
ionization potential of 70 eV. UV–vis absorption and fluores-
cence spectra were taken on Hitachi U-3500 and F-4500
spectrophotometers, respectively. Electrochemical measure-
ments of dyes were measured with a model 800 electrochemical
workstation (CH Instruments, Austin, TX, USA). One-step
cathode electrodeposition was taken on Hokuto-Denko HSV-
100 potentiostat system. The photoelectrochemical measure-
ments (two-electrode system) of solar cells were performed on
a Bunko-Keiki CEP-2000 system (AM 1.5).
The mixture of phthalic anhydride (1 mmol, 0.150 g),
methanesulfonic acid (10 ml), 2,4-difluoroesorcinol (2 mmol,
0.292 g) was heated at 80–85 8C for 2 days under argon. The
resultant mixture was quenched with water (70 ml), extracted
with ethyl acetate (50 ml ꢁ 3 ml), dried over Na2SO4, and
concentrated under vacuum. The precipitate was purified by
column chromatography on silica gel using methanol–
dichloromethane (v/v = 1/5) to give 20,40,50,70-tetrafluorosulfo-
fluorescein (4d) in 31% yield (0.136 g).
Yield 37%; mp > 300 8C; Rf 0.03 (MeOH–CH2Cl2 = 1/5);
1H NMR (DMSO-d6) d 6.25 (2H, d, J = 12.39 Hz), 7.13 (2H, d,
J = 7.59 Hz), 7.48 (1H, t, J = 7.59 Hz), 7.56 (1H, t, J =
7.39 Hz), 7.96 (1H, d, J = 7.59 Hz); 13C NMR (DMSO-d6) d
109.99 (d, J = 21.49 Hz), 113.66 (d, J = 9.04 Hz), 127.60 (s),
128.27 (s), 129.27 (s), 129.08 (s), 129.39 (s), 129.83 (s), 139.95
(s), 140.18 (dd, J = 244.07, 7.83 Hz); 19F NMR (DMSO-d6) d
ꢀ55.44 (2F, dd, J = 19.46, 12.20 Hz), ꢀ86.28 (2F, d,
J = 19.46 Hz); HRMS (FAB): m/z calcd. for C19H9 O6F4S
(M + H), 441.0056; found, 441.0057.
4.2. Materials
The isolation of pure products was carried out by column
chromatography using silica gel (Wakogel C-200, 100–200
mesh, Wako Pure Chemical Ind., Ltd.). Analytical TLC was
done on Merck precoated (0.25 mm) silica gel 60 F254 plates.
Ten percent Pd on carbon was purchased from Merck Co. Ltd.
2,3,4,5-Tetrafluoronitrobenzene and tetrafluorosulfobenzoic
anhydride were purchased from Tokyo Kasei Kogyo. Tetra-
4.5. Preparation of dye-sensitized solar cells with ZnO
Preparation of ZnO thin film as well as the dye-sensitized
solar cells was prepared as described in the previous paper

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K. Funabiki et al. / Journal of Fluorine Chemistry 127 (2006) 257–262
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Acknowledgements
(and references cited therein).
This work was partially supported by the Industrial
Technology Research Grant Program from New Energy and
Industrial Technology Development Organization (NEDO) of
Japan (01B64002c). We thank Professors H. Yamanaka and T.
Ishihara as well as Dr. T. Konno of the Kyoto Institute of
Technology for the HRMS measurements of 4a and 4d.
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