Redox and photochemical behaviour of a porphyrin monolayer on an indium-tin oxide electrode

Article abstract of DOI:10.1016/j.electacta.2005.05.030

In order to investigate photoluminescence behaviour of an ordered molecular porphyrin monolayer and its quenching properties by oxygen gas, a porphyrin with long alkyl chains, 5,10,15,20-tetrakis[4-(11-carboxylundecane-1-oxy)phenyl] porphyrin (4), was synthesized and adsorbed onto an indium-tin oxide (ITO) substrate by a chemical dipping method. Cyclic voltammetry was used to analyze the ITO electrode coated with 4. The peak current of the first oxidation was proportional to the sweep rate, and the surface coverage was estimated to be 2.3-2.5 × 10^-10 mol cm^-2. The UV-vis spectrum of the monolayer showed a broadened Soret band, which shifted to longer wavelength. These features suggest that the porphyrin moieties of 4 are packed to form a J-type structure. The oxygen quenching ratio of the porphyrin 4 monolayer on the ITO electrode, I₀/I₁₀₀, was estimated to be 1.25, where I₀ and I₁₀₀ are, respectively, luminescence intensity values in 100% argon and 100% oxygen. On repeated step cycling between 100% argon and 100% oxygen atmospheres, the response times of luminescence quenching were 10 s (argon to oxygen) and 23 s (oxygen to argon). These findings suggest that a monolayer of sensing dye is applicable for oxygen sensing system without deterioration of size-accuracy of models.

Full text of DOI:10.1016/j.electacta.2005.05.030

Electrochimica Acta 51 (2005) 677–683
Redox and photochemical behaviour of a porphyrin monolayer
on an indium-tin oxide electrode
Naoko Arakia,∗, Makoto Obataa, Akio Ichimurab, Yutaka Amaoc,
Kazunori Mitsuod, Keisuke Asaie, Shigenobu Yanoa,∗
a
Division of Natural Science and Ecological Awareness, Graduate School of Humanities and Sciences, Nara Women’s University,
Kita-uoya-nishi-machi, Nara-shi 630-8506, Japan
Department of Chemistry, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
b
c
Department of Applied Chemistry, Oita University, Dannoharu 700, Oita 870-1192, Japan
Aerodynamics Research Group, Institute of Space Technology and Aeronautics, Japan Aerospace Exploration Agency,
d
7-44-1, Jindaiji Higashi Chofu City, Tokyo 182-8522, Japan
Department of Aeronautics and Space Engineering, Tohoku University, 01 Aoba, Aoba-ku, Sendai 980-8579, Japan
e
Received 13 January 2005; received in revised form 19 April 2005; accepted 15 May 2005
Available online 5 July 2005
Abstract
In order to investigate photoluminescence behaviour of an ordered molecular porphyrin monolayer and its quenching properties by oxy-
gen gas, a porphyrin with long alkyl chains, 5,10,15,20-tetrakis[4-(11-carboxylundecane-1-oxy)phenyl]porphyrin (4), was synthesized and
adsorbed onto an indium-tin oxide (ITO) substrate by a chemical dipping method. Cyclic voltammetry was used to analyze the ITO electrode
coated with 4. The peak current of the first oxidation was proportional to the sweep rate, and the surface coverage was estimated to be
2.3–2.5 × 10−10 mol cm−2. The UV–vis spectrum of the monolayer showed a broadened Soret band, which shifted to longer wavelength.
These features suggest that the porphyrin moieties of 4 are packed to form a J-type structure. The oxygen quenching ratio of the porphyrin 4
monolayer on the ITO electrode, I0/I100, was estimated to be 1.25, where I0 and I100 are, respectively, luminescence intensity values in 100%
argon and 100% oxygen. On repeated step cycling between 100% argon and 100% oxygen atmospheres, the response times of luminescence
quenching were 10 s (argon to oxygen) and 23 s (oxygen to argon). These findings suggest that a monolayer of sensing dye is applicable for
oxygen sensing system without deterioration of size-accuracy of models.
© 2005 Elsevier Ltd. All rights reserved.
Keywords: Porphyrin monolayer; Oxygen quenching; Photochemistry; Indium-tin oxide; Cyclic voltammetry
1. Introduction
rials [1,2]. Porphyrins on gold surfaces using S–Au link-
ages have been widely studied for application to molecu-
lar devices. One reason for this interest is the porphyrin’s
large molar absorption coefficient in the visible light region.
Another reason is efficient electron transfer from the excited
state of porphyrin to grand state of various acceptors and
donors such as ferrocene and fullerene derivatives [3–7].
Furthermore, the oxygen-induced luminescence quenching
properties have been examined for thin films consisting of
porphyrins, such as polymer films, Langmuir-Blodget (LB)
films and chemisorption films [8–12]. However, the photolu-
minescence behaviour and its quenching properties by oxy-
gen gas remain to be fully understood for ordered molecular
Self-assembled monolayers (SAMs) have emerged as an
alternative and useful strategy for assembling molecular com-
ponents on surface. SAMs have been used to develop novel
surface-patterning methodologies, to fabricate new types of
chemically sensitive devices, to study interfacial electron
transfer processes and to prepare a variety of unusual and
potentially useful electronic, photonic and redox-active mate-
∗
Corresponding authors. Tel.: +81 742 20 3392; fax: +81 742 20 3392.
E-mail addresses: xan.araki@cc.nara-wu.ac.jp (N. Araki),
yano@cc.nara-wu.ac.jp (S. Yano).
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2005.05.030

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N. Araki et al. / Electrochimica Acta 51 (2005) 677–683
porphyrin SAMs. In this study, we focused on the photolu-
minescence behaviour of a porphyrin SAMs. While the most
established SAM system is alkanethiols on gold surfaces
[13,14], a strong quenching of the porphyrin excited sin-
glet state by the gold surface enhances non-radiative losses.
Indium-tin oxide (ITO)isa promising candidate asa substrate
for photoluminescence study because the ITO electrode has
high optical transmission and suppresses the quenching of
the porphyrin excited states [13]. Therefore, we investigated
photoluminescence behaviour of porphyrin SAM on an ITO
electrode and its quenching properties by oxygen gas.
Spontaneous adsorption of long-chain n-alkanoic acid
(CnH2n+1COOH) on AgO, Al2O3 and gold surfaces, has
been studied in the past few years [15–18]. However, we
are not aware of any studies on stable porphyrin SAMs
formed from n-alkanoic acids on an ITO surface. We report,
herein the synthesis of a functionalized porphyrin deriva-
tive, namely 5,10,15,20-tetrakis[4-(11-carboxylundecane-
1-oxy)phenyl]porphyrin, and the preparation of a self-
assembled monolayer of the porphyrin on an ITO electrode.
The monolayer on the electrode was characterized by cyclic
voltammetry and UV–vis absorption spectroscopy. The pho-
toluminescence behaviour of the porphyrin monolayer and
its quenching property by oxygen gas are discussed on the
basis of luminescence measurements in the presence of argon
and oxygen gases.
2.2. Synthesis
2.2.1. General
Porphyrin derivative 4 was prepared according to
Scheme 1. 1H NMR spectra were recorded on a Varian
Gemini 2000 (300 MHz) instrument. Fast atom bombard-
ment mass spectra (FAB-MS) were measured on a JMS-700T
spectrometer (Osaka City University) using 3-nitrobenzyl
alcohol as a matrix. Elemental analysis was carried out
using a Perkin-Elmer 240C Elemental Analyzer (Osaka City
University).
2.2.2. Methyl 12-(4-formylphenoxy)dodecanate (1)
A solution of p-hydroxybenzaldehyde (6.07 g, 49.7 mmol)
in 5% NaOH (aq) (70 mL) was vigorously stirred at room
temperature with CH2Cl2 (50 mL) and tetrabutylammonium
bromide (3.07 g, 9.52 mmol). To the mixture was added a
solution of 12-bromododecanoic acid (2.34 g, 8.38 mmol)
in CH2Cl2 (20 mL) and the whole was stirred at room
temperature for 60 h. The organic layer was washed with
5% NaOH (aq) and water, and then dried over Na2SO4,
and the solvent was removed under reduced pressure. The
residue was reprecipitated from hot MeOH to afford crude
12-(4-formylphenoxy)dodecanoic acid as a white powder
(1.80 g). The crude product (5.82 g) was taken up in a mix-
ture of MeOH (500 mL) and benzene (100 mL). Hydro-
gen chloride gas was introduced into the mixture for 2 h,
and then the mixture was refluxed at 90–95 ◦C for 3 days
with azeotropic distillation of water. After cooling, the mix-
ture was poured into water and extracted with CHCl3. The
organic layer was washed with saturated NaHCO3 (aq) and
water, dried over Na2SO4, and evaporated. The residue
was purified by silica gel column chromatography with
ethyl acetate/hexane (5/12, v/v) as an eluent (Rf = 0.58), fol-
lowed by precipitation from hot hexane to afford methyl
12-(4-formylphenoxy)dodecanate(1)asawhitesolid(1.39 g,
2. Experimental
2.1. Materials
All solvents and chemicals were of reagent grade quality
obtained from Wako Pure Chemical Industries Ltd., Osaka,
Japan, and were used without further purification unless
otherwise noted. The solvents for cyclic voltammetry were
passed through a short alumina column just before use.
Tetrabutylammonium hexafluorophosphate (Bu4NPF6) was
obtained from Tokyo Kasei Kogyo Co. Ltd., Tokyo, Japan,
and recrystallized from EtOH just before use.
1
4.37 mmol, 19.1%). H NMR (300 MHz, CDCl3): δ 9.88
3
(s, 1H, −CHO), 7.81 (d, J = 8.85 Hz, 2H, ArH), 6.98 (d,
3J = 8.54 Hz, 2H, ArH), 4.03 (t, 3J = 6.68 Hz, 2H, −OCH2−),
Scheme 1. Reagents and conditions: (a) 12-bromododecanoic acid, Bu4NBr, 5 wt% NaOH (aq), CH2Cl2, r.t., (b) MeOH, HCl (gas), benzene, reflux, (c) pyrrole,
Zn(OAc)2·2H2O, BF3·OEt2, CHCl3, p-chloranil, (d) 6 M HCl and (e) KOH, MeOH, THF.

N. Araki et al. / Electrochimica Acta 51 (2005) 677–683
679
3.66 (s, 3H, −COOCH3), 2.28 (t, 3J = 7.63 Hz, 2H,
−CH2COOCH3), 1.85–1.28 (m, 18H, −OCH2(CH2)9−).
subsequent filtration resulted in 5,10,15,20-tetrakis[4-(11-
carboxylundecane-1-oxy)phenyl]porphyrin (4) as a purple
solid (0.69 g, 0.44 mmol, 97.3%). 1H NMR (300 MHz, THF-
3
d8): δ 8.83 (s, 8H, pyrrole-βH), 8.05 (d, J = 8.54 Hz, 8H,
2.2.3. 5,10,15,20-Tetrakis[4-(11-methoxycarbonylun-
decane-1-oxy)phenyl]porphyrinato zinc(II) (2)
ArH), 7.27 (d, 3J = 8.85 Hz, 8H, ArH), 4.20 (t, 3J = 6.41 Hz,
3
8H, −OCH2−), 2.20 (t, J = 7.32 Hz, 8H, −CH2COOH),
Pyrrole (0.3 mL, 4.36 mmol), 1 (1.01 g, 3.03 mmol) and
Zn(OAc)2·2H2O (2.7 g, 12.3 mmol) were added to CHCl3
(300 mL) that had been purged with argon for 30 min.
BF3·OEt2 (0.2 mL, 1.60 mmol) was added to the mixture
to initiate the reaction. After introduction of additional
argon gas for 10 min, the mixture was stirred at 30 ◦C for
3.5 h, and then p-chloranil (0.56 g, 2.27 mmol) was added.
The mixture was refluxed for 1 h, washed with saturated
NaHCO3 (aq) and water, and dried over Na2SO4. Silica
gel (5 g) was added to the dark solution, and all solvents
were removed by evaporation. The silica gel containing
the absorbed products was placed on top of a silica gel
column and elution was carried out with a mixture of
CH2Cl2/acetone (12/1, v/v). The red band (Rf = 0.79) was
collected. Evaporation of the solvent and precipitation
from CH2Cl2/MeOH afforded 5,10,15,20-tetrakis[4-(11-
methoxycarbonylundecane-1-oxy)phenyl]porphyrinato
zinc(II) (2) as a purple solid (0.41 g, 0.26 mmol, 34.7%). 1H
NMR (300 MHz, CDCl3): δ 8.97 (s, 8H, pyrrole-βH), 8.09 (d,
3J = 8.54 Hz, 8H, ArH), 7.25 (d, 3J = 8.85 Hz, 8H, ArH), 4.24
(t, 3J = 6.41 Hz, 8H, −OCH2−), 3.65 (s, 12H, −COOCH3),
1.97–1.36 (m, 72H, −OCH2(CH2)9−), −2.67 (s, 2H,
−2NH). FAB-MS for C92H118O12N4: m/z calcd., 1471.9
[M]+: found, 1471.8. Anal. Calcd. for C92H118N4: C,
75.07; H, 8.08; N, 3.80. Found: C, 74.60; H, 8.09;
N, 3.79.
2.3. Preparation of a porphyrin 4 monolayer
ITO electrodes (on grass, 10 ꢀ cm−2) were pur-
chased from Sanyoshinku Co. Ltd., cut into slices (ca.
0.5 cm × 3.0 cm), rinsed with acetone, and dried just before
being soaked in a porphyrin solution. The porphyrin solution
was prepared as follows: porphyrin derivative 4 and EtOH
(porphyrin concentration was 1 mM) were mixed, and
then EtOH solution containing 1% KOH was added to
the solution until 4 was dissolved completely. An ITO
electrode was immersed in the solution for 2 days at 25 ◦C
to form a monolayer. The ITO electrode was washed with
EtOH and dried under air. The monolayer of 4 on the ITO
electrode surface was characterized by cyclic voltammetry
and UV–vis spectroscopy.
3
2.28 (t, J = 7.33 Hz, 8H, −CH2COOCH3), 2.01–1.34 (m,
72H, −OCH2(CH2)9−). FAB-MS for Zn1C96H124O12N4:
m/z calcd., 1591.4 [M]+: found, 1590.9.
2.4. Spectroscopic measurements
2.4.1. Electrochemical measurements
2.2.4. 5,10,15,20-Tetrakis[4-(11-methoxycarbonylun-
decane-1-oxy)phenyl]porphyrin (3)
Electrochemical measurements of
4
in solution
(c = 2.6 × 10−4 M) and of the monolayer deposited on
the ITO electrode were carried out by cyclic voltammetry
using a CV50W voltammetric analyzer (Version 2.0)
(BAS Inc., Tokyo, Japan). Voltammetric studies were
performed under a nitrogen atmosphere using a conven-
tional three-electrode cell. The working electrode was a
Pt disk (diameter 1.8 mm) for solution of 4. A Pt wire
and an Ag/AgPF6 were the counter and reference elec-
trodes, respectively. All potentials are referenced to Ag/
AgPF6.
A solution of 2 (0.36 g, 0.23 mmol) in CH2Cl2 (50 mL)
was washed several times with 6 M HCl (aq), satu-
rated NaHCO3 (aq) and dried over Na2SO4. Evapora-
tion of the solvent and precipitation from CH2Cl2/MeOH
gave 5,10,15,20-tetrakis[4-(11-methoxycarbonylundecane-
1-oxy)phenyl]porphyrin (3) as a purple solid (0.32 g,
0.21 mmol, 92.6%). 1H NMR (300 MHz, CDCl3): δ 8.86 (s,
3
8H, pyrrole-βH), 8.09 (d, J = 8.54 Hz, 8H, ArH), 7.25 (d,
3J = 8.85 Hz, 8H, ArH), 4.24 (t, 3J = 6.41 Hz, 8H, −OCH2−),
3.67 (s, 12H, −COOCH3), 2.30 (t, 3J = 7.63 Hz, 8H,
−CH2COOCH3), 2.00–1.34 (m, 72H, −OCH2(CH2)9−),
−2.66 (s, 2H, −2NH). FAB-MS for C96H126O12N4: m/z
calcd., 1529.0 [M + H]+: found, 1529.0.
2.4.2. Absorption, excitation, and emission spectroscopy
The UV–vis absorption spectra of 4 dissolved in THF
(c = 2.4 × 10−6 M) and of the monolayer on an ITO electrode
were recorded using a V-570 UV–vis/NIR spectrophotometer
(JASCO International Co. Ltd., Tokyo, Japan). The steady-
state emission and excitation spectra of 4 dissolved in THF
(c = 2.1 × 10−6 M) were measured on a RF-5300PC spec-
trofluorophotometer (Shimadzu Corporation, Kyoto, Japan).
A 150 W xenon lamp with a cut filter of 450 nm, Toshiba UV
cut filter V-Y 45 (Toshiba, Tokyo, Japan), was used as a vis-
ible emission light source. The excitation and emission band
pass widths were 3 nm.
2.2.5. 5,10,15,20-Tetrakis[4-
(11-carboxylundecane-1-oxy)phenyl]porphyrin (4)
Porphyrin 3 (0.72 g, 0.45 mmol) was dissolved in THF
(150 mL) and stirred at room temperature. To this stirred
mixture was added a solution prepared by mixing MeOH
(150 mL) and KOH (5.26 g, 93.7 mmol). The solution was
stirred at room temperature for 24 h, and then acidi-
fied with 1 M HCl (aq) to adjust the pH to 4. Then
the reaction mixture was diluted with water (1 L) and

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N. Araki et al. / Electrochimica Acta 51 (2005) 677–683
2.5. Oxygen quenching behaviour
2.5.1. Oxygen quenching behaviour for porphyrin 4 on
an ITO electrode
The oxygen quenching behaviour of 4 on an ITO electrode
were studied by using a Shimadzu RF-5300PC spectrofluo-
rophotometer with a 150 W xenon lamp source equipped with
a cut filter of 390 nm, Toshiba UV cut filter V-Y 39, as a visi-
ble excitation light source. An ITO plate modified with 4 (ca.
13 mm × 30 mm) was placed diagonally relative to the quartz
cell and was exposed to gas mixtures containing various con-
centrations of oxygen (in the range of 0–100%) produced by
controlling the flow rates of oxygen and argon gases with a
gas flow meter, Kofloc M1N1-GASCOM (Kojima Instrument
Inc., Tokyo, Japan). The oxygen concentration was calcu-
lated by dividing the oxygen flow rate by the total flow rate
of the mixed gases. The total pressure was maintained at
101,333 Pa (760 Torr) and all experiments were carried out
at room temperature. The excitation and emission band pass
widths were 5 nm. The oxygen quenching behaviour of a por-
phyrin 4 monolayer on the ITO electrode were characterized
by the Stern–Volmer quenching constant, KSV, obtained from
Eq. (1):
Fig. 1. Cyclic voltammograms of 4 in THF containing 0.1 M Bu4NPF6 as an
electrolyte at scan rates of (a) 50 mV s−1, (b) 100 mV s−1, (c) 500 mV s−1
(d) 1000 mV s−1 and (e) 2000 mV s−1
,
.
ducible cyclic voltammograms were obtained after multiple
scan cycles (as shown in Fig. 2). A significant symmetrical
oxidation peak, corresponding to the one-electron oxidation
oftheporphyrinmoiety, isobservedat792 mV. Onthereverse
scan, three re-reduction peaks appear at 3, 239 and 714 mV.
The sum of these reduction peak areas is almost the same
as the area of the main oxidation peak. This suggests the
presence of three different adsorption states of the oxidation
species, which have the possible redox centre of porphyrin
moiety, in the monolayer on the ITO electrode. The peak
current of the main oxidation peak increases linearly with
increase of the potential scan rate, as expected from Eq. (2),
which was derived for adsorbed redox species:
I0
= 1 + KSV[O2]
(1)
I
where I0 is the luminescence intensity in the absence of oxy-
gen, I is the intensity in the presence of oxygen and [O2] is
the oxygen concentration in percentage [19,20].
n2ΓF2v
ip =
(2)
2.5.2. Dynamic response of oxygen quenching for
porphyrin 4 on an ITO electrode
4RT
where ip, n, Γ , F and v are the peak current, the number
of electrons, the surface coverage, the Faraday constant and
the potential scan rate, respectively (Fig. 3) [21]. The charge
required for the complete oxidation of 4 adsorbed on an ITO
electrode, measured from the peak area, gives the surface
coverage (Γ ) of 2.3 × 10−10 mol cm−2 when n is equal to 1.
Furthermore, the Γ values of three samples fell in the narrow
range from 2.3 × 10−10 to 2.5 × 10−10 mol cm−2. The simple
deposition method using the porphyrin with four carboxyl
Dynamic response of oxygen quenching of the porphyrin
4 on an ITO electrode were detected conventionally with a
spectrofluorophotometer, while the gas flow was switched
between 100% oxygen and 100% argon using a gas flow
meter, Kofloc M1N1-GASCOM [9]. All experiments were
carried out at 101,333 Pa (760 Torr) and at room temperature.
3. Results and discussion
3.1. Electrochemical properties of the ITO electrode
modified with porphyrin 4
The redox properties of 4 deposited on an ITO electrode
were characterized by cyclic voltammetry. Figs. 1 and 2 show
the cyclic voltammograms of 4 in THF and 4 on an ITO
electrode at different potential scan rates, respectively. As
shown in Fig. 1, the cyclic voltammetric curve displayed
characteristic features of a diffusion-controlled redox couple
with the half-wave potential of 753 mV. The ratio of the re-
reduction peak current to the oxidation peak current, ipc/ipa,
is less than unity and increases with the scan rate, indicat-
ing the instability of the oxidation product in THF solution.
For 4 deposited as a monolayer on an ITO electrode, repro-
Fig. 2. Cyclic voltammograms of an ITO electrode modified with 4 in
CH3CN containing 0.1 M Bu4NPF6 as an electrolyte at scan rates of
(a) 100 mV s−1, (b) 200 mV s−1, (c) 300 mV s−1, (d) 400 mV s−1 and (e)
500 mV s−1
.

N. Araki et al. / Electrochimica Acta 51 (2005) 677–683
681
Fig. 5. Excitation (solid line) and emission spectra (broken line) of 4 in
THF. λem = 658 and λex = 422 nm were used for excitation and emission,
respectively.
Fig. 3. Dependence of the peak current (ip) on the potential scan rate (v) for
a monolayer of 4 on an ITO electrode. Peak current was calculated from the
cyclic voltammograms.
concentration:
10−3A
groups leads to effective and reproducible modification of
the monolayer on the ITO electrode.
Γ =
(3)
ε
where Γ , A and ε are the surface coverage, absorbance
of the monolayer and molar absorption coefficient, respec-
tively [25,26]. Using Eq. (3), the surface coverage (Γ )
of 4 deposited on the ITO electrode is calculated to be
2.19 × 10−10 mol cm−2. This value is consistent with that
derived from cyclic voltammetry. Therefore, the redox num-
ber of electrons for 4 on an ITO electrode was estimated
to be 1. The luminescence of porphyrin 4 dissolved in THF
(c = 2.1 × 10−6 M) was observed at 658 and 722 nm, on irra-
diation at 422 nm (Soret band), as shown in Fig. 5.
3.2. Optical properties of the ITO electrode modified
with porphyrin 4
Fig. 4 shows the UV–vis absorption spectra of 4 dis-
solved in THF and 4 on an ITO electrode in air, in the
transmission mode. The Soret band of 4 on the ITO elec-
trode (Soret band at 435.5 nm and four Q-bands at 523.0,
561.0, 599.0 and 654.5 nm) is broadened and red shifted
by 15 nm relative to that in THF solution (Soret band at
420.0 nm and four Q-bands at 520.0, 553.5, 594.5 and
651.5 nm). Also, similar broadening and red shift were
reported for porphyrin SAMs on gold electrodes, LB mono-
layers of porphyrins on glass or semiconductors and por-
phyrin aggregates in solutions [1,14,22–24]. A stacked
face-to-face porphyrin π-aggregation (sandwich-type, H-
aggregate) leads to a blue shift, while side-by-side porphyrin
π-aggregation (J-aggregate) leads to a red shift [24]. Thus,
the observed spectral change is due to partially J-aggregate
like stacked structure of the porphyrins in the monolayer
microenvironment.
3.3. Structure of the porphyrin 4 monolayer
Fig. 6 shows possible models of 4 immobilized on an
ITO electrode. In model A, the macrocyclic plane of 4
is almost perpendicular to the ITO surface, and the pro-
2
˚
jection area of a molecule 4 is calculated at ca. 58 A .
In contrast, if the plane of 4 is parallel to the electrode
2
˚
surface, it would cover the area of 174 A per molecule.
These occupied areas of molecules lead to a maximum
surface coverage of 4 of 2.9 × 10−10 mol cm−2 for model
A and of 9.5 × 10−11 mol cm−2 for model B. The sur-
face coverage of the perpendicular model of 2.9 × 10−11
Also, the surface coverage of the monolayer was evalu-
ated from the absorption spectrum on the basis of Eq. (3),
which is Lambert–Beer’s law modified for two-dimensional
Fig. 6. Possible structures of a monolayer of 4 immobilized on an ITO
electrode in which one or two carboxylic groups bind to the surface (model
A, loose binding model) or four carboxylic groups bind to the surface (model
B, tight binding model) (P, porphyrin core).
Fig. 4. UV–vis absorption spectra of 4 in THF (broken line) and on an ITO
electrode (solid line).

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N. Araki et al. / Electrochimica Acta 51 (2005) 677–683
Curve fitting to Eq. (5) gave the parameters KSV1
=
0.170%−1 and f1 = 0.224 (broken line, Fig. 7). Eq. (5) repro-
duces the non-linearity of the plot well. However, there is
still significant deviation from observation in the higher oxy-
gen concentration region, where the observed luminescence
intensity from the monolayer of 4 is lower than that predicted
by Eq. (5). In order to explain this phenomenon, we take into
account adsorbed oxygen molecules on the monolayer. The
adsorption equilibrium is defined by Eq. (6):
[MO2]
[M]0 − [M]
Keq
=
=
(6)
[M][O2]
[M][O2]
where Keq, [MO2], [M] and [M]0 are the equilibrium con-
stant, the amount of dye statically quenched by oxygen
molecules, the amount of free dye and the total amount of
dye, respectively. In this scheme, the dye in the MO2 region
is no longer available for oxygen quenching. Hence, Eq. (6)
is modified as follows.
Fig. 7. Stern–Volmer plot for 4 immobilized on an ITO electrode. (Broken
line; KSV1 = 0.170%−1, KSV2 = 0%−1 and f1 = 0.224. Solid line; KSV1 and
Keq = 0.073 and 0.078%−1, KSV2 = 0%−1 and f1 = 0.210).
I0
I
1
=
(7)
(8)
φf1/(1 + KSV1[O2]) + (1 − f1)
mol cm−2 is almost the same as the surface coverage
of (2.3–2.5) × 10−10 mol cm−2 estimated from the cyclic
voltammogram. Hence, the major binding structure of 4
should be that shown in model A.
[M]
1
φ =
=
[M]0
1 + Keq[O2]
The result of curve fitting to Eq. (7) is shown in Fig. 8 as
a solid line. The parameter f1 is equal to 0.210 and values of
0.073 and 0.078%−1 are obtained for KSV1 and Keq. In Eq.
(7), the two parameters KSV1 and Keq are identical mathemat-
ically, soitisimpossibletoassignthemindividually. Theterm
φ, improves the fitting to the plots (solid line, Fig. 7), suggest-
ing an influence of adsorbed oxygen molecules. Even though
the Stern–Volmer plot of the monolayer shows the signif-
icant non-linearity, the modified two-site quenching model
represents the oxygen sensing properties well. Hence, the
monolayer of porphyrin derivatives 4 is an ultra thin film
capable of oxygen sensing.
3.4. Oxygen quenching behaviour of the porphyrin 4
monolayer on an ITO electrode
Fig. 7 shows the intensity changes of the luminescence
from a monolayer of 4 as a function of oxygen concen-
tration. The oxygen quenching ratio of the monolayer was
I0/I100 = 1.25, where I0 and I100 are the luminescence inten-
sityintheabsenceandinthepresenceofoxygen, respectively.
While the resulting plot shows considerable linearity at lower
oxygen concentration, non-linearity is observed at higher
oxygen concentration. Such behaviour can be attributed to
heterogeneity of the quenching sites of the sensing compo-
nent; each type of site has its own Stern–Volmer constant. In
general, the observed plots fit very well to a two-site quench-
ing model described by the following modified Stern–Volmer
Eq. (4):
3.5. Dynamic oxygen quenching behaviour for the
porphyrin 4 monolayer on an ITO electrode
Fig. 8 shows a typical dynamic response of the oxygen
quenching when 100% oxygen and 100% argon gases were
I0
I
1
=
(4)
f1/(1 + KSV1[O2]) + (1 − f1)/(1 + KSV2[O2])
where f1 is the fractional intensity contribution of the compo-
nentoftheStern–VolmerconstantKSV1 intheabsenceofoxy-
gen [12,27,28]. In our system, curve fitting to Eq. (4) gave the
parameters KSV1 = 0.121%−1, KSV2 = −0.000513%−1 and
f1 = 0.263. The Stern–Volmer constant must be positive and
KSV2 is negligibly small. On the assumption that KSV2 is
negligibly small and can be regarded as zero, Eq. (4) can be
modified to yield Eq. (5).
I0
I
1
Fig. 8. Dynamic response times for 4 immobilized on an ITO electrode.
(Response times were 10 and 23 s, respectively.)
=
(5)
f1/(1 + KSV1[O2]) + (1 − f1)

N. Araki et al. / Electrochimica Acta 51 (2005) 677–683
683
switched for 600 s. The response times for luminescence
quenching are defined as the times taken to reach the 90% of
the ultimate response. The response times of the porphyrin 4
monolayer onto ITO electrode are 10 s on going from argon to
oxygen and 23 s on going from oxygen to argon. As shown in
Fig. 8, alargenoisesignal(indicatedbyanarrow)isobserved;
this is caused by the non-ideal behaviour of the gas-switching
mechanism used to purge the sample container.
15033246, 16350032, 17029040 and 17036041 from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy (MEXT) of the Japanese Government, and grants from
Osaka Gas and San-EiGen Foundation for chemical research,
and also in part by Special Coordination Funds for ‘Molecu-
lar Sensors for Aero-Thermodynamic Research (MOSAIC)’
from the Ministry of Education, Culture, Sports, and Science
and the Technology Agency.
4. Conclusion
References
A porphyrin with long alkyl chains 4 was synthesized and
immobilized on an ITO electrode using a dipping method.
Cyclic voltammetry indicated that the immobilization was
successful and showed fairly good reproducibility. Further-
more, the UV–vis absorption spectrum of the monolayer of
4 on the ITO electrode indicated the presence of J-aggregate
like partially stacked structures. The major binding structure
of 4 on the electrode was a perpendicular one in which one or
two carboxylic groups bind to the surface. A monolayer of 4
on an ITO electrode was tested for oxygen quenching ability.
For the luminescence quenching ability by oxygen gas of the
porphyrin 4 monolayer, the Stern–Volmer quenching coeffi-
cient (KSV) and the equilibrium constant (Keq), were 0.073
and 0.078%−1, these two parameters are impossible to distin-
guish. Theresponsetimesfortheporphyrin4monolayerwere
10 s on going from argon to oxygen and 23 s on going from
oxygen to argon. This is the first time that a monolayer has
been studied for a luminescence quenching by oxygen gas.
A precise reduced scale model coated with luminescence
dye is used in wind tunnel experiment based on oxygen-
induced luminescence quenching mechanism, for aircraft and
rockets [29]. In this technology, the coating deteriorates the
size-accuracy of the model, even though the thickness is the
order of a few micrometer. In this point of view, the mono-
layer of sensing dye, which has thickness of the order of a
few nanometer, is the ultimate sensing system for keeping
the size-accuracy of the model. The coatings of well-packed
dyes are thought to be useless for oxygen sensing because
of luminescence quenching by the self-aggregation of chro-
mophores. However, the results presented indicate that a por-
phyrinmonolayeremitsaluminescenceandcanbeapplicable
for oxygen sensing. In order to improve sensing properties,
the sensing monolayer using metalloporphyrins and other
luminescence dye is under investigation.
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Acknowledgements
This work was supported by Grants-in-Aid for Scien-
tific Research on Priority Areas (417, 434) and 13557211,