Strong inhibition of singlet oxygen sensitization in pyridylferrocene-fluorinated zinc porphyrin supramolecular complexes

Article abstract of DOI:10.1021/jp034920q

A supramolecular pyridylferrocene (PyFc)-fluorinated zinc porphyrin (ZnTFPP) dyad system was developed, and its photodynamics was investigated in dichloromethane and acetonitrile. The nitrogen of PyFc coordinates to the metal center of zinc porphyrin to form a supramolecular donor-acceptor complex. Formation of the pyridylferrocene-zinc porphyrin supramolecule was probed by ¹H NMR and UV-vis spectroscopy. The one-electron reduction potential of ZnTFPP in dichloromethane is shifted to a negative direction by 0.08 V due to formation of the supramolecular complex with PyFc. The binding constant of ZnTFPP^.- with PyFc in the supramolecular complex determined from the potential shift is much smaller than the binding constant of the neutral form, ZnTFPP. The fluorescence of ZnTFPP is strongly quenched by electron transfer from PyFc to the singlet excited state (¹ZnTFPP*) in the supramolecular complex of ZnTFPP with PyFc. The lifetime of ¹ZnTFPP* is also shortened significantly in the supramolecular complex. No transient absorption spectrum due to the charge-separated state (ZnTFPP^.-PyFc⁺) or the triplet excited state (³ZnTFPP*-PyFc) was detected even at the picosecond time scale due to the ultrafast back electron transfer from ZnTFPP^.- to PyFc⁺ to yield the ground state supramolecular complex rather than the triplet excited state. This leads to strong inhibition of singlet oxygen sensitization of the zinc porphyrin by formation of the supramolecular complex.

Full text of DOI:10.1021/jp034920q

J. Phys. Chem. A 2003, 107, 5515-5522
5515
Strong Inhibition of Singlet Oxygen Sensitization in Pyridylferrocene-Fluorinated Zinc
Porphyrin Supramolecular Complexes
Yukiyasu Kashiwagi,† Hiroshi Imahori,*,‡ Yasuyuki Araki,§ Osamu Ito,*,§ Koji Yamada,|
Yoshiteru Sakata,| and Shunichi Fukuzumi*,†
Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity, CREST,
Japan Science and Technology Corporation (JST), Suita, Osaka 565-0871, Japan, Department of Molecular
Engineering, Graduate School of Engineering, Kyoto UniVersity, PRESTO, JST, Sakyo-ku,
Kyoto 606-8501, Japan, Fukui Institute for Fundamental Chemistry, Kyoto UniVersity, 34-4,
Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan, Institute for Multidisciplinary Research for
AdVanced Materials, Tohoku UniVersity, CREST, JST, Katahira, Aoba-ku, Sendai 980-8577, Japan, and The
Institute of Scientific and Industrial Research, Osaka UniVersity, Ibaraki, Osaka 567-0047, Japan
ReceiVed: April 8, 2003
A supramolecular pyridylferrocene (PyFc)-fluorinated zinc porphyrin (ZnTFPP) dyad system was developed,
and its photodynamics was investigated in dichloromethane and acetonitrile. The nitrogen of PyFc coordinates
to the metal center of zinc porphyrin to form a supramolecular donor-acceptor complex. Formation of the
1
pyridylferrocene-zinc porphyrin supramolecule was probed by H NMR and UV-vis spectroscopy. The
one-electron reduction potential of ZnTFPP in dichloromethane is shifted to a negative direction by 0.08 V
due to formation of the supramolecular complex with PyFc. The binding constant of ZnTFPP•- with PyFc in
the supramolecular complex determined from the potential shift is much smaller than the binding constant of
the neutral form, ZnTFPP. The fluorescence of ZnTFPP is strongly quenched by electron transfer from PyFc
to the singlet excited state (1ZnTFPP*) in the supramolecular complex of ZnTFPP with PyFc. The lifetime
of 1ZnTFPP* is also shortened significantly in the supramolecular complex. No transient absorption spectrum
due to the charge-separated state (ZnTFPP•--PyFc+) or the triplet excited state (3ZnTFPP*-PyFc) was detected
even at the picosecond time scale due to the ultrafast back electron transfer from ZnTFPP•- to PyFc+ to yield
the ground state supramolecular complex rather than the triplet excited state. This leads to strong inhibition
of singlet oxygen sensitization of the zinc porphyrin by formation of the supramolecular complex.
Introduction
rins, mimicking the biological electron transfer systems.5-9 An
increasing number of noncovalently linked supramolecular
Porphyrin-based photosensitizers have been the subject of
enormous interest due to their potential use in photodynamic
therapy (PDT).1 The photosensitizing agent is photoexcited by
visible light to the singlet excited state which is efficiently
converted by intersystem crossing to the triplet excited state,
followed by energy transfer to molecular oxygen to produce
the singlet oxygen which is a putative cytotoxic agent.1,2 On
the other hand, porphyrins with highly delocalized π systems
are suitable for efficient electron transfer, because the uptake
or release of electrons results in minimal structural change upon
electron transfer.3 Rich redox properties render porphyrins and
metalloporphyrins as essential components in important biologi-
cal electron transport systems including photosynthesis and
respiration.4 Thus, continuous efforts have been devoted to study
electron transfer reactions of covalently linked electron donor-
acceptor ensembles containing porphyrins and metalloporphy-
assemblies containing porphyrins and metalloporphyrins have
also been developed extensively.10-16 The main objective of
studies on covalently or noncovalently linked donor-acceptor
ensembles has so far been focused on creating long-lived charge-
separated (CS) states to be utilized for light energy conversion
purposes.5-16 Such CS states, which are highly reactive, can
also act as putative cytotoxic agents. When the triplet excited
states of porphyrins are lower in energy as compared to the CS
states of donor-acceptor dyads containing porphyrins, the back
electron transfer from the charge-separated states normally
results in formation of either the ground state or the triplet
excited state of porphyrins, which leads to generation of singlet
oxygen.17,18 In general, the larger the reorganization energy (λ)
of electron transfer, the slower the forward photoinduced
electron transfer of the singlet excited state of porphyrins in
the Marcus normal region19 leading to the higher triplet yield
of porphyrins because of the competing intersystem crossing
to the triplet state.7 Conversely, the smaller the λ value of
electron transfer, the faster the forward photoinduced electron
transfer but the slower the back electron transfer to the ground
state in the Marcus inverted region. When the triplet excited
state is lower in energy than the radical ion pair, the back
electron transfer to the triplet excited state in the Marcus normal
region becomes faster than the back electron transfer to the
ground state in the Marcus inverted region. In each case, the
* To whom correspondence should be addressed. E-mail: imahori@scl.
kyoto-u.ac.jp; fukuzumi@ap.chem.eng.osaka-u.ac.jp; ito@tagen.tohoku.ac.jp.
† Department of Material and Life Science, Graduate School of Engineer-
ing, Osaka University, CREST, Japan Science and Technology Corporation
(JST).
‡ Department of Molecular Engineering, Graduate School of Engineering,
Kyoto University, PRESTO, JST, and Fukui Institute for Fundamental
Chemistry, Kyoto University.
§ Institute for Multidisciplinary Research for Advanced Materials, Tohoku
University, CREST, JST.
| The Institute of Scientific and Industrial Research, Osaka University.
10.1021/jp034920q CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/28/2003

5516 J. Phys. Chem. A, Vol. 107, No. 29, 2003
Kashiwagi et al.
CHART 1
were measured on a Perkin-Elmer FT-IR 2000 spectrophoto-
meter as KBr disks. Elemental analyses were performed on a
Perkin-Elmer model 240C elemental analyzer.
Materials. All solvents and chemicals were of reagent grade
quality, obtained commercially and used without further puri-
fication unless otherwise noted (vide infra). Tetrabutylammo-
nium perchlorate (TBAP) used as a supporting electrolyte in
dichloromethane and acetonitrile for the electrochemical mea-
surements was obtained from Tokyo Kasei Organic Chemicals.
Tetrahexylammonium perchlorate (THAP) used as a supporting
electrolyte in benzene was prepared by the addition of NaClO4
to the tetrahexylammonium bromide in acetone, followed by
recrystallization from acetone. Dichloromethane (CH2Cl2) was
purchased from Wako Pure Chemical Ind., Ltd., and purified
by successive distillation over calcium hydride. Thin-layer
chromatography (TLC) and flash column chromatography were
performed with an Art. 5554 DC-Alufolien Kieselgel 60 F254
(Merck) and a Fujisilicia BW300, respectively.
formation of the triplet excited state of porphyrins leads to
generation of singlet oxygen in the presence of oxygen via
efficient energy transfer from the triplet excited state to
oxygen.17 The efficient generation of singlet oxygen is of
primary importance to develop photosensitizers for their use in
PDT. It is also important to find a way to inhibit generation of
singlet oxygen whenever it is desired after PDT, since the
exposure to light needs to be prohibited for a long time.
Carotenoid pigments are known to quench the chlorophyll
singlet excited state both in vivo and in vitro,20 preventing the
chlorophyll- and porphyrin-sensitized formation of singlet
oxygen by intercepting the triplet states via photoinduced
electron transfer.21-23 In this context, carotenoid-porphyrin
linked systems have been extensively studied to limit damage
in tumor imaging applications related to PDT.24,25 However,
there has so far been no report on a supramolecular porphyrin
system which inhibits the generation of singlet oxygen in
sensitization of porphyrins.
Such a supramolecular porphyrin system to inhibit generation
of singlet oxygen would be attained provided that one can design
a system in which the λ value is small enough to achieve
efficient forward photoinduced electron transfer of the singlet
excited state of porphyrins but the λ value becomes larger for
the back electron transfer from the radical ion pair to facilitate
the back electron transfer to the ground state rather than to the
triplet state. In principle, such a drastic change in the λ value
between the forward electron transfer and back electron transfer
is possible, since the coordination binding constant, which is
related to the binding distance, is known to be changed
drastically depending on the oxidation state of metalloporphy-
rins26 and the λ value increases with increasing the donor and
acceptor binding distance.17 However, the effects of the change
in the binding constants upon electron transfer in supramolecular
complexes have yet to be examined.
Zn(II) Complex of 5,10,15,20-Tetrakis(pentafluorophen-
yl)porphyrin (1). A solution of pyrrole (0.70 mL, 10 mmol)
and pentafluorobenzaldehyde (1.28 mL, 10 mmol) in CH2Cl2
(400 mL) was degassed by bubbling with nitrogen for 30 min.
The reaction vessel was shielded from ambient light. Then,
boron trifluoride diethyl etherate (0.30 mL, 3.0 mmol) was added
in one portion. The solution was refluxed for 4 h under a
nitrogen atmosphere. To the reaction mixture, p-chloranil (1.85
g, 6.4 mmol) was added and refluxed for 12 h. After cooling,
the reaction mixture was washed with a saturated NaHCO3
aqueous solution and water. After standing over anhydrous Na2-
SO4, the organic extract was concentrated to dryness. The
organic layer was evaporated. Flash column chromatography
on silica gel with CHCl3-hexane (2:3) as the eluent gave a
pale red solid. A saturated methanol solution of Zn(OAc)2 (5.0
mL) was added to a CHCl3 solution of the pale red solid (50
mL) and refluxed for 10 h. After cooling, the reaction mixture
was washed with water twice and dried over anhydrous Na2-
SO4, and the solvent was evaporated. Column chromatography
on silica gel with CHCl3 as an eluent and subsequent repre-
cipitation from CHCl3-hexane gave 1 as a pink solid (181 mg,
1
0.175 mmol, 7% yield). H NMR (270 MHz, CDCl3) δ 9.01
(8H, s); FABMS m/z 1036 (M+).
4-Pyridylmethyl Ferrocenylcarboxylate (2a). Oxalyl chlo-
ride (1.0 mL, 12 mmol) was added to a stirred solution of
ferrocenylcarboxylic acid (500 mg, 2.17 mmol) in dry CH2Cl2
(20 mL) and pyridine (0.01 mL). The resulting solution was
stirred for 14 h at room temperature and refluxed for 5 h. Excess
oxalyl chloride and solvents were removed under reduced
pressure, and the residue was redissolved in dry CH2Cl2 (20
mL). A CH2Cl2 solution of 4-pyridinecarbinol (2.20 g, 20.2
mmol) was added to the resulting solution, and the solution was
stirred for 12 h. The reaction mixture was washed with a
saturated NaHCO3 aqueous solution and water. The organic
layer was dried over anhydrous Na2SO4, and then the solvent
was evaporated. The residue was purified by silica gel column
chromatography (CHCl3/NEt3 ) 19/1 (v/v) as eluent) to afford
2a as a orange solid (314 mg, 0.977 mol, 45% yield). mp 83.5-
We report herein a novel supramolecular system consisting
of pyridylferrocenes and a fluorinated zinc porphyrin (Chart 1),
in which the generation of singlet oxygen in sensitization of
the zinc porphyrin is strongly inhibited by formation of the
supramolecular complex. The fluorinated zinc porphyrin is used
to facilitate the photoinduced electron transfer from the ferrocene
moiety to the zinc porphyrin moiety by the electron-withdrawing
effect of fluoro-substituents. The change in the binding constant
of the supramolecular complex depending on the oxidation state
of zinc porphyrins is investigated by spectroscopic and elec-
trochemical measurements. The inhibition mechanism is re-
vealed from a detailed study on the electrochemical and
photophysical properties of the supramolecular complexes.
1
84.5 °C (dec); H NMR (270 MHz, CDCl3) δ 8.64 (d, J ) 6
Experimental Section
Hz, 2H), 7.35 (d, J ) 6 Hz, 2H), 5.28 (s, 2H), 4.87 (t, J ) 2
Hz, 2H), 4.45 (t, J ) 2 Hz, 2H), 4.18 (s, 5H); FT-IR (KBr)
1704 cm-1 (νCdO); UV-vis (CH2Cl2) λmax (log ꢀ) ) 447.2
(1.41), 311.6 (2.07), 255.1 (2.87); EI MS m/z 321 (M+). Anal.
Calcd for C17H15FeNO2: C, 63.58; H, 4.71; N, 4.36. Found:
C, 63.32; H, 4.47; N, 4.15.
General. Melting points were recorded on a Yanagimoto
micromelting point apparatus and not corrected. 1H NMR spectra
were measured on a JEOL EX-270 (270 MHz) or a JEOL JMN-
AL300 (300 MHz). Fast atom bombardment mass spectra
(FABMS) were obtained on a JEOL JMS-DX300. IR spectra

Inhibition of Singlet Oxygen Sensitization
J. Phys. Chem. A, Vol. 107, No. 29, 2003 5517
Benzyl ferrocenylcarboxylate (2b), ferrocenylamido-4-pyri-
dine (3a), and ferrocenylamido-4-benzene (3b) were also
synthesized by acid chloride-mediated condensation and char-
acterized by 1H NMR, UV-vis, FABMS spectra, and elemental
analysis (see Supporting Information S1 and S2).
SCHEME 1
Spectral Measurements. Time-resolved fluorescence spectra
were measured by a single-photon counting method using a
second harmonic generation (SHG, 410 nm) of a Ti:sapphire
laser (Spectra-Physics, Tsunami 3950-L2S, 1.5 ps full width at
half-maximum (fwhm)) and a streakscope (Hamamatsu Photo-
nics, C4334-01) equipped with a polychromator (Acton Re-
search, SpectraPro 150) as an excitation source and a detector,
respectively.18
The picosecond laser flash photolysis was carried out using
a second harmonic generation (SHG, 388 nm) of a mode-locked
Ti:sapphire laser (Clark-MXR CPA-2001) as an exciting source.
The excitation laser was depolarized. A white continuum pulse
generated by focusing the fundamental of a Nd:sapphire laser
on a H2O cell was used as monitoring light. A pump-probe
system with a dual MOS detector (Hamamatsu Photonics,
C6140) equipped with a polychromator (Acton Research,
SpectraPro 150) was applied for the measurements of the
transient absorption spectra.18
Nanosecond transient absorption measurements were carried
out using an OPO laser (Continuum, Surelite OPO, fwhm 4 ns,
555 nm) pumped by a Nd:YAG laser (Continuum, SLII-10, 4-6
ns fwhm) as an excitation source. Photoinduced events in
nanosecond time regions were monitored by using a continuous
Xe-lamp (150 W) and an Si-APD photodiode (Hamamatsu
2949) as a probe light and a detector, respectively. The output
from the photodiodes was recorded with a digitizing oscilloscope
(HP, 54810A, 500 MHz). All the samples in a quartz cell (1 ×
1 cm) were deaerated by bubbling argon through the solution
for 15 min.18
Steady-state absorption spectra were measured on a Hewlett-
Packard 8452A diode array spectrophotometer at 298 K. Steady-
state fluorescence spectra were measured on a Perkin-Elmer
LS50B fluorescence spectrophotometer. Quenching experiments
of the fluorescence of ZnTFPP-PyFc complexes were per-
formed on a SHIMADZU spectrofluorophotometer (RF-
5000PC). The excitation wavelength of ZnTFPP and ZnTFPP-
PyFc complexes was 555 nm in deaerated CH2Cl2 and CH3CN.
The solutions were deaerated by argon purging for 7 min prior
to the measurements.
Figure 1. UV-visible spectra of ZnTFPP (1.0 × 10-6 M) in the
presence of various concentrations of 2a (1.0 × 10-6 to 2.0 × 10-5
M) in CH2Cl2. Inset: Plot of [ZnTFPP]0/(A0 - A) vs [2a]-1 at 413 nm.
ecules were well-characterized by 1H NMR, UV-vis, FABMS
spectra, and elemental analysis (see Experimental Section).
Formation of a ZnTFPP-PyFc Complex. The UV-vis
spectra of ZnTFPP in CH2Cl2 at 298 K are changed upon
addition of 2a as shown in Figure 1, where the Soret and
Q-bands are red-shifted with isosbestic points. The absorbance
change exhibits a saturation behavior with increasing 2a
concentration. This indicates that 2a forms a 1:1 complex with
ZnTFPP (eq 1). Such nitrogenous bases readily bind to zinc
ZnTFPP + PyFc {K
\
} (ZnTFPP‚‚‚PyFc)
(1)
porphyrins with a 1:1 stoichiometry.30,31 According to eq 1, the
absorbance change is given by eq 2,32 which predicts a linear
correlation between [ZnTFPP]0/(A0 - A) and [PyFc]-1, where
Electrochemical Measurements. Cyclic voltammetry mea-
surements were performed at 298 K on a BAS 100W electro-
chemical analyzer in deaerated CH2Cl2 containing 0.1 M TBAP
as a supporting electrolyte. A conventional three-electrode cell
was used with a platinum button working electrode and a
platinum wire as the counter electrode. The measured potentials
were recorded with respect to the Ag/AgNO3 reference elec-
trode. All potentials (vs Ag/Ag+) were converted to values
versus saturated calomel electrode (SCE) by adding 0.29 V.27
The E1/2 value of ferrocene used as a standard is 0.37 V versus
SCE in CH2Cl2 under the present experimental conditions. All
electrochemical measurements were carried out under an
atmospheric pressure of argon.
[ZnTFPP]0/(A0 - A) ) 1/(ꢀc - ꢀp) + (K[PyFc](ꢀc - ꢀp))-1
(2)
A0 and A are the absorbance of ZnTFPP at 413 nm in the
absence and presence of PyFc, and ꢀp and ꢀc are the molar
absorption coefficients of ZnTFPP at 413 nm in the absence
and presence of PyFc, respectively. From a linear plot of
[ZnTFPP]0/(A0 - A) versus [PyFc]-1 is determined the forma-
tion constant as 1.2 × 105 M-1 in CH2Cl2. The formation
constants (K) were also determined for 2a and 3a in benzene,
acetonitrile (CH3CN) and CH2Cl2 as listed in Table 1.
The binding constant of the ZnTFPP-3a complex in CH2-
Cl2 is larger than that of the ZnTFPP-2a complex. This can
be ascribed to the higher basicity of the pyridine moiety resulting
from the electron-donating group at the 4-position. The binding
constant of 2a in CH3CN is 1.9 × 103 M-1, which is
significantly smaller than the value in CH2Cl2 because of the
competing coordination of CH3CN. The smaller K value in
benzene than in CH2Cl2 may be attributed to the π interaction
with benzene.
Results and Discussion
Synthesis. Tetrakis(pentafluorophenyl)porphyrinatozinc (1,
ZnTFPP) was prepared according to Lindsey’s method.28
Pyridylferrocenes (2a and 3a, PyFc) and their reference
compounds 2b and 3b were also synthesized by acid chloride-
mediated condensation as shown in Scheme 1.29 These mol-

5518 J. Phys. Chem. A, Vol. 107, No. 29, 2003
Kashiwagi et al.
TABLE 1: Binding Constants (K) of the ZnTFPP (1)
Complexes with PyFc (2a and 3a), One-Electron Oxidation
Potentials (E0ox) of PyFc, and Reduction Potentials (E0red) of the
1-Pyridine Complex at 298 K
K, M-1 a
E0ox vs SCE, V Er0ed vs SCE, V
solvent
2a
3a
2a
3a
1-pyridine
CH2Cl2 1.2 × 105 2.2 × 105 0.74b
0.73b
0.61b
0.67c
-1.03b
-1.04b
-0.97c
CH3CN 1.9 × 103 8.3 × 103 0.63b
C6H6
1.9 × 104 3.7 × 104 0.68c
a The experimental error is (10%. b In the presence of 0.1 M TBAP
as supporting electrolyte. c In the presence of 0.1 M THAP as supporting
electrolyte.
Figure 3. Dependence of the one-electron reduction potential of
ZnTFPP (1.0 × 10-4 M) on the concentration of 2a (0-4.0 × 10-3
M) in CH2Cl2 containing 0.1 M TBAP at 298 K. Scan rate ) 100 mV/
s.
Figure 2. 1H NMR spectra of 2a in the presence of various
concentrations of ZnTFPP in CD2Cl2. Peaks a and b are the protons in
the pyridine ring, peak c is in the spacer CH2, peaks d-f are in the
ferrocene ring, and peak a′ is in the â-position of ZnTFPP.
1
The H NMR signals of free 2a exhibit upfield shifts upon
complexation with ZnTFPP in CD2Cl2 as shown in Figure 2.
The signals of the free 2a and the complexed 2a always
coalesced into a single signal. This indicates that the complex-
ation occurs faster than the NMR time scale. The large upfield
shift of the pyridyl aromatic protons of 2a (3.28 ppm) induced
by the complexation is ascribed to the large porphyrin aromatic
ring-current.33 This clearly indicates that the pyridyl group of
2a coordinates axially to the central zinc ion of ZnTFPP.
Change in the Binding Constant Depending on the
Oxidation State. The change in the one-electron reduction
potential of ZnTFPP by the supramolecular complex formation
with 2a was examined by cyclic voltammetry in CH2Cl2
containing 0.1 M TBAP. The one-electron reduction potential
of ZnTFPP is shifted to a negative direction in the presence of
2a. The potential shift increases with increasing 2a concentration
to reach the one-electron reduction potential of the ZnTFPP-
2a complex as shown in Figure 3. The potential shift (∆E1/2) is
given by the Nernst equation (eq 3),
Figure 4. The phosphorescence spectra of singlet oxygen sensitized
by irradiation of ZnTFPP (1.1 × 10-5 M) at 551 nm in the presence of
various concentrations of 2a in oxygen-saturated C6D6 at 298 K.
that the binding in the ZnTFPP-2a complex becomes much
weaker upon the one-electron reduction.
2a exhibits the reversible wave for the one-electron oxidation
to the ferricenium cation (PyFc+). The one-electron oxidation
potentials of 2a and 3a are shifted cathodically by about 0.2 V
as compared with those of the nonsubstituted ferrocene because
of the electron-withdrawing carbonyl function. The one-electron
oxidation potentials are also listed in Table 1.
Inhibition of Singlet Oxygen Sensitization by Supramo-
lecular Complex Formation. Photoirradiation of ZnTFPP with
light of λ ) 551 nm in O2-saturated benzene-d6 results in
generation of singlet oxygen which is detected by the 1O2
phosphorescence at 1270 nm35 as shown in Figure 4. Upon
1
addition of 2a, the O2 phosphorescence intensity decreases
significantly (Figure 4). The phosphorescence quantum yields
(Φ) were determined using tetraphenylporphyrin (TPP) as a
standard.36 The Φ values are summarized in Table 2. The Φ
value (75%) is drastically reduced to 2.3% in the presence of
2a (7.8 × 10-4 M), where most ZnTFPP molecules form the
complex with 2a judging from the K value in benzene (Table
1).37 In contrast, the Φ value is only slightly decreased to 52%
when 2a is replaced by 2b which has no coordination site to
ZnTFPP. The Φ value of the ZnTFPP-pyridine complex is
60%. Thus, singlet oxygen sensitization of ZnTFPP is strongly
∆E1/2 ) (2.3RT/F) log(K/K-)
(3)
where K- is the binding constant of ZnTFPP•- with 2a. The
K- value is determined from the ∆E1/2 and the K value as 5.5
× 103 M-1, which is much smaller than the K value (1.2 × 105
M-1). Similarly, the K- value of 3a in CH2Cl2 was determined
as 6.3 × 103 M-1 which is also much smaller than the
corresponding K value (2.2 × 105 M-1).34 The much smaller
K- values as compared with the corresponding K values indicate

Inhibition of Singlet Oxygen Sensitization
J. Phys. Chem. A, Vol. 107, No. 29, 2003 5519
TABLE 2: Relative Quantum Yields (Φ) of Singlet Oxygen
Generated by Irradiation of ZnTFPP (1) in
Oxygen-Saturated C6D6 at 298 K ([1] ) 1.1 × 10-5 M, [2a]
) [2b] ) 7.8 × 10-4 M, [Pyridine] ) 2.1 × 10-2 M)
compound
Φ,a %
compound
1-pyridine
1-pyridine + 2b
Φ,a %
TPP (standard)
1
1-2a
62b
75
2.3
60
52
a Excitation at λ ) 551 nm, using TPP (Φ ) 0.62 in C6D6) as
standard. b Taken from ref 36.
Figure 6. Fluorescence decay profiles of supramolecular complexes
[ZnTFPP-2a complex (closed circle), ZnTFPP-pyridine complex
(open square)] in deaerated benzene by excitation (555 nm) at emission
wavelength ) 600 nm. [ZnTFPP] ) 1.0 × 10-5 M, [2a] ) 1.0 × 10-3
M, [pyridine] ) 2.1 × 10-2 M.
TABLE 3: Fluorescence Lifetimes (τ) and Quenching Rate
Constants (kq) of the ZnTFPP (1) and 1-PyFc Complexes at
298 K ([1] ) 1.0 × 10-5 M, [2] ) [3] ) 1.0 × 10-3 M,
[Pyridine] ) 2.1 × 10-2 M)
C6H6
kq, s-1
CH2Cl2
kq, s-1
CH3CN
kq, s-1
Figure 5. Fluorescence spectra of ZnTFPP (5.0 × 10-6 M) in the
presence of various concentrations of 2a (0-1.0 × 10-3 M) in deaerated
benzene. The arrows indicate the direction of change.
τ, ns
τ, ns
τ, ns
1
1.4
1.6
1.4
1.6
1-pyridine
1-2a
1-3a
6.3 × 108 1.4
7.1 × 108 1.5
6.7 × 108
0.96 4.2 × 108 0.52 1.2 × 109 0.38 2.0 × 109
inhibited by complexation with 2a. To clarify the origin of such
strong inhibition of singlet oxygen sensitization, the photo-
physical properties of the ZnTFPP-PyFc complex are examined
in comparison with those of ZnTFPP (vide infra).
1.5
4.2 × 107 1.3
5.5 × 107
1-pyridine + 2b 1.6
1-pyridine + 3b 1.6
0
0
1.4
1.4
0
0
1.4
4.8 × 107
ZnTFPP, the fluorescence decay consists of two lifetimes (1.6
and 0.96 ns). The shorter lifetime component increases with
increasing 2a concentration as shown in Figure 6. Under the
conditions where all ZnTFPP molecules form the complex with
2a, the fluorescence decay exhibits a single-exponential decay
with the lifetime of 0.96 ns which corresponds to the fluores-
cence lifetime of the ZnTFPP-2a complex. In contrast to the
case of the ZnTFPP-2a complex, the fluorescence lifetime of
the ZnTFPP-3a complex (1.5 ns) is virtually the same as that
of the ZnTFPP-pyridine complex (1.6 ns). This is consistent
with the little quenching of the steady-state fluorescence in the
ZnTFPP-3a complex due to the larger distance between
ZnTFPP and 3a as compared with the case of the ZnTFPP-2a
complex (vide supra).
The fluorescence lifetimes were also determined in CH3CN
and in CH2Cl2. The fluorescence lifetimes are summarized in
Table 3. The fluorescence lifetime in each solvent is shortened
only in the supramolecular complex (ZnTFPP-2a) as compared
to the lifetime of ZnTFPP. The quenching rate constants (kq)
are determined from the fluorescence lifetimes (τ) in the
presence of PyFc and the lifetime (τ0) in the presence of pyridine
using eq 4. The kq values are also listed in Table 3.
Photophysical Properties of the ZnTFPP-PyFc Complex.
Photoexcitation of the Q-band of ZnTFPP at 555 nm in benzene
results in fluorescence at λmax ) 585 and 638 nm as shown in
Figure 5. Addition of 2a to a benzene solution of ZnTFPP results
in a significant change in the fluorescence spectrum of ZnTFPP.
The fluorescence intensity of ZnTFPP decreases with increasing
2a concentration and the fluorescence maxima are red-shifted
to λmax ) 599 and 651 nm, when ZnTFPP is converted
completely to the ZnTFPP-2a complex (Figure 5). When 2a
is replaced by 3a, the fluorescence quenching of ZnTFPP by
3a is much less efficient as compared with the case of 2a,
although the binding constant of the ZnTFPP-3a complex (3.7
× 104 M-1) is larger than the binding constant of the ZnTFPP-
2a complex (1.9 × 104 M-1). The amide spacer of 3a is rigid,
whereas the ester spacer of 2a is flexible. Such a flexibility of
the spacer of 2a enables formation of the ZnTFPP-2a complex
in which the distance between the ferrocene and the porphyrin
moiety is closer than the distance in the ZnTFPP-3a complex
with the rigid spacer, resulting in the more efficient quenching
in the ZnTFPP-2a complex. In the noncoordinative reference
system, that is, the ZnTFPP-pyridine complex with 2b or 3b,
however, the fluorescence of the ZnTFPP-pyridine complex
is hardly quenched by the intermolecular reaction with 2b or
3b.
kq ) τ-1 - τ0
(4)
-1
The fluorescence lifetimes of the ZnTFPP-PyFc complexes
and the reference systems were determined using a single-photon
counting method. The fluorescence of ZnTFPP in benzene
exhibits a single-exponential decay with a lifetime of 1.4 ns.
The fluorescence lifetime remains approximately the same when
ZnTFPP forms a complex with pyridine (2.1 × 10-2 M) in
benzene (1.6 ns). When 2a is added to a benzene solution of
To clarify the fluorescence quenching process of ZnTFPP
by PyFc, the picosecond transient absorption spectra of the
ZnTFPP-2a complex are measured in CH3CN as shown in
Figure S3a (Supporting Information). The picosecond transient
absorption spectrum of ZnTFPP exhibits the S1-Sn absorption
spectrum at λmax ) 450 nm. When ZnTFPP forms the complex

5520 J. Phys. Chem. A, Vol. 107, No. 29, 2003
Kashiwagi et al.
Figure 8. Plots of the initial absorbance vs [2a] for the triplet-triplet
absorption due to 3ZnTFPP* at 470 nm in (a) the ZnTFPP-2a complex
and (b) the ZnTFPP-pyridine complex in the presence of 2b after laser
excitation at 555 nm in deaerated benzene.
SCHEME 2
Figure 7. Decay dynamics of the triplet-triplet absorption due to
3ZnTFPP* at 470 nm (a) in the ZnTFPP-2a complex and (b) in the
ZnTFPP-pyridine complex in the presence of 2b after laser excitation
at 555 nm in deaerated benzene. [ZnTFPP] ) 1.0 × 10-5 M, [2a] )
0-1.0 × 10-3 M, [pyridine] ) 2.1 × 10-2 M, [2b] ) 0-1.1 × 10-3
M.
with 2a (1.0 × 10-2 M), the S1-Sn absorbance disappears with
the lifetime of 0.30 ns which agrees well with the fluorescence
lifetime (vide supra). No T1-Tn absorption appears, ac-
companied by disappearance of the S1-Sn absorbance. This is
in sharp contrast with the case of the ZnTFPP-pyridine complex
which exhibits the conversion from the S1 state to the triplet
excited state (Figure S3b, Supporting Information).38 Similar
results were also obtained in benzene.
The photodynamics of the triplet excited state (3ZnTFPP*)
was also investigated by nanosecond laser flash photolysis in
benzene and CH3CN. The T1-Tn absorbance of ZnTFPP decays
obeying first-order kinetics. The addition of 2a to a benzene
solution of ZnTFPP results in a decrease in the initial T1-Tn
absorbance and the initial absorbance drop increases with
increasing 2a concentration as shown in Figure 7a. In addition,
the triplet decay rate increases with increasing 2a concentration
(Figure 7a).
3
dependence of the decay rate constant on [2a] or [2b] confirms
the bimolecular quenching of ZnTFPP* by 2a or 2b. No new
transient absorption band is observed, accompanied by decay
of the triplet excited state. Similar results are obtained in CH3-
CN (see Supporting Information S4 and S5).
Fluorescence Quenching Mechanism. The energy diagram
of the ZnTFPP-2a complex is summarized in Scheme 2, where
the energies were determined from the one-electron redox
potentials (Table 1) and the singlet excited state energies
obtained from the absorption and fluorescence maxima. The
triplet excited energies were determined from the phosphores-
cence spectrum of ZnTFPP observed in deaerated frozen
2-methyltetrahydrofuran at 77 K.
In contrast, no initial drop in absorbance due to ZnTFPP*
3
is observed in the case of the ZnTFPP-pyridine complex in
the presence of 2b which has no coordination site to ZnTFPP
(Figure 7b). Such a difference between 2a and 2b is shown as
plots of the initial drop in absorbance (∆A) versus 2a or 2b
concentration in Figure 8. This indicates that the complex
formation of ZnTFPP with 2a results in strong inhibition for
the triplet generation and thus no singlet oxygen is formed
(Figure 4).
The increase in the decay rate with increasing 2a or 2b is
ascribed to the intermolecular quenching of the triplet excited
state of ZnTFPP by 2a or 2b. The decay rate constant increases
linearly with increasing 2a or 2b concentration. Such a linear

Inhibition of Singlet Oxygen Sensitization
J. Phys. Chem. A, Vol. 107, No. 29, 2003 5521
Since the singlet excited state of ferrocene (2.46 eV)39 is much
higher in energy than the singlet excited state of ZnTFPP (2.02
eV), no singlet energy transfer from ZnTFPP* to PyFc is
between the zinc porphyrin π radical anion and the nitrogenous
base of the electron donor moiety, leading to the facile back
electron transfer to the ground state. Such a finding was
unprecedented since the present study is the first example of a
supramolecular system of porphyrins in which porphyrins act
as an electron acceptor.
1
expected to occur. In such a case, an intramolecular electron
1
transfer from 2a to ZnTFPP* in the ZnTFPP-2a complex,
which is energetically feasible, may be the only pathway for
the fluorescence quenching (Scheme 2a). Such an electron
transfer quenching of the singlet excited state porphyrin was
reported for a carotenoid-porphyrin linked compound.23 The
formation of the charge-separated state formed by an electron
transfer from the carotenoid to the singlet excited state porphyrin
was confirmed by the characteristic transient absorption of the
carotenoid radical cation.23 In the case of ZnTFPP-2a, however,
no transient absorption due to ZnTFPP•- is observed, ac-
companied by the disappearance of the absorption due to
1ZnTFPP* (Figure S3a, Supporting Information). No transient
absorption due to 3ZnTFPP* is observed at 3ZnTFPP-2a, either
(Figure S3a, Supporting Information). This indicates that the
lifetime of the CS state is too short to be detected. The CS state
decays at a much faster rate than the formation rate of the CS
state, thus leading to regeneration of the ground state pair,
ZnTFPP-2a, rather than to generation of the triplet excited state,
3ZnTFPP*-2a, which is lower in energy than the CS state
(Scheme 2a).40
The bimolecular quenching of the triplet excited state may
also be caused by electron transfer from free 2a (or 2b) to
3ZnTFPP* as shown in Scheme 2b. Although the CS state is
3
higher in energy than ZnTFPP*, the triplet excited state is
quenched by 2a or 2b via the rate-determining electron transfer
because of the much faster back electron transfer to the ground
state than the triplet decay (Scheme 2b). In the case of
bimolecular electron transfer reactions, a larger solvent reor-
ganization energy at a large encounter distance is favorable to
keep the rate constant large for a larger driving force, resulting
in the lack of observation of the Marcus inverted region in the
photoinduced bimolecular CS reactions.41,45,46
In conclusion, the supramolecular complex formation between
ZnTFPP and 2a provides a unique way to inhibit the singlet
oxygen sensitization due to efficient intracomplex photoinduced
1
electron transfer from 2a to ZnTFPP* and the facile back
electron transfer from ZnTFPP•- to 2a+ to the ground state
rather than to the triplet excited state.
The reason the back electron transfer to the ground state is
much faster than to the triplet excited state is rationalized by a
large reorganization energy involved in the back electron
transfer, which may be comparable to the driving force of back
electron transfer to the ground state (1.67 eV). In such a case,
the back electron transfer to the ground state may be close to
the Marcus top region19 and thereby the back electron transfer
rate to the ground state is much faster than the rate of electron
transfer to the triplet excited state, which has a much smaller
driving force (0.16 eV). The large reorganization energy required
for the back electron transfer results from the significant change
in the binding constant associated with the electron transfer
reduction of ZnTFPP (vide supra). Upon photoinduced electron
transfer in the ZnTFPP-2a complex, the binding between
ZnTFPP•- and 2a+ becomes smaller to facilitate the dissociation
of the supramolecular complex. Such a dissociation process
results in an increase in the distance between ZnTFPP•- and
2a+, leading to the larger solvent organization associated with
the back electron transfer. The larger reorganization energy of
electron transfer at the larger distance between the donor and
acceptor molecules has now been well-established.7,41
Acknowledgment. This work was partially supported by a
Grant-in-Aid for Scientific Research Priority Area (13440216)
and the Development of Innovative Technology (No. 12310)
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan. H.I. thanks the Nagase Foundation for
financial support.
Supporting Information Available: Synthetic procedure
and characterization of 2b, 3a, and 3b (S1 and S2) and the
photodynamics of 1ZnTFPP* in CH3CN (S3) and 3ZnTFPP* in
CH3CN (S4 and S5). This material is available free of charge
via the Internet at http://pubs.acs.org.
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