Anomalously selective quenching of S2 fluorescence from upper excited state of zinc 5-(l′-pyrenyl)-10,15,20-triphenylporphyrin derivatives through intramolecular charge transfer state

Article abstract of DOI:10.1021/jp903743w

Zinc 5-(1′-pyrenyl)-10,15,20-tris(p-methoxyphenyl)porphyrin (ZnP-Py) and zinc 5-(1′-pyrenyl)-10,15,20tris(pentafluorophenyl)porphyrin (ZnFP-Py), in which a pyrenyl group is directly connected at the mesopositions of their corresponding porphyrins, were sy

Full text of DOI:10.1021/jp903743w

8852
J. Phys. Chem. A 2009, 113, 8852–8856
Anomalously Selective Quenching of S2 Fluorescence from Upper Excited State of Zinc
5-(1′-Pyrenyl)-10,15,20-triphenylporphyrin Derivatives through Intramolecular Charge
Transfer State
Kazutaka Hirakawa,*,† Keiko Saito,‡ and Hiroshi Segawa*,§
Department of Basic Engineering (Chemistry), Faculty of Engineering, Shizuoka UniVersity, Johoku 3-5-1,
Naka-ku, Hamamatsu, Shizuoka 432-8561, Japan, Department of Chemistry, Faculty of Science, Kitasato
UniVersity, Kitasato 1-15-1, Sagamihara, Kanagawa 228-8555, Japan, and Research Center for AdVanced
Science and Technology, The UniVersity of Tokyo, Komaba 4-6-1, Meguro-ku, Tokyo 153-8904, Japan
ReceiVed: April 22, 2009; ReVised Manuscript ReceiVed: June 17, 2009
Zinc 5-(1′-pyrenyl)-10,15,20-tris(p-methoxyphenyl)porphyrin (ZnP-Py) and zinc 5-(1′-pyrenyl)-10,15,20-
tris(pentafluorophenyl)porphyrin (ZnFP-Py), in which a pyrenyl group is directly connected at the meso-
positions of their corresponding porphyrins, were synthesized and investigated on the photophysical properties.
While the S1 fluorescence of the porphyrin moieties was not quenched by the pyrenyl group at all, the S2
fluorescence was remarkably quenched. The anomalously selective quenching of the S2 fluorescence was
dependent on the solvent polarity, and the quenching was enhanced in the polar solvent. The S2 fluorescence
quenching is considered to take place through an intramolecular charge transfer state. Interestingly, the charge
transfer state relaxes to the S1 state through the rapid charge recombination with high quantum yield. The
charge transfer rate of ZnFP-Py, which was calculated from the S2 fluorescence quenching, was larger than
that of ZnP-Py, possibly due to the differences in the free-energy changes of the electron transfer and the
electron densities at their meso-positions. This study has suggested that ultrafast electron transfer from the
upper excited state and charge recombination to S1 can occur in a directly connected electron donor-acceptor
system.
Introduction
cence is a useful probe to investigate the ultrafast ET from the
upper excited state.
An enormous number of molecular systems composed of
porphyrins have been investigated on the intramolecular electron
transfer (ET) from the lowest singlet excited state (S1), which
is a notation of electronic states populated by excitation in the
Q-band (500-650 nm).1-3 On the other hand, the porphyrin
derivatives have a characteristic intense absorption, namely the
Soret-band (400-450 nm) owing to a transition to upper excited
states with a large energy gap to S1. The upper excited states
have been denoted as the second excited singlet state (S2), and
some of the metalloporphyrins and diacid porphyrins emit an
S2 fluorescence whose quantum yields4-8 and lifetimes6c,9-14
were in the region of 10-3-10-5 and 0.8-4.5 ps, respectively.
Although the lifetimes of S2 are remarkably shorter than those
of S1, intermolecular9,15 and intramolecular12-14 ET from S2
within the S2 lifetimes have been reported. When the ET from
S2 occurs against internal conversion (IC) from S2 to S1, the
ET must be an ultrafast process, which is an attractive phe-
nomenon in both basic and applied photochemistry. For
example, the photosensitized DNA damage by using highly
oxidative porphyrin S2 has been investigated.15 The S2 fluores-
The first investigation on intermolecular ET from S2 by
Chosrowjan et al.9 was from the S2 state of zinc tetraphenylpor-
phyrin to solvent dichloromethane, which included undefined
and irreversible solvent decomposition. Concerning the intramo-
lecular ET from S2 reported by LeGourrie´rec et al.,12 a formation
of a 50% charge separated state with a 50% S1 state was
observed. In this molecule, intramolecular energy transfer from
S2 should be excluded because it was reported for a molecule
composed of similar units that can be both the energy acceptor
and donor.11 Mataga et al. reported the intramolecular ET in
the zinc porphyrin-electron acceptor dyad systems.13 Using these
dyad systems, the bell-shaped energy gap law in the ET from
porphyrin S2 has been demonstrated. Recently, Fujitsuka et al.
reported that the S2 fluorescence quenching of tetraphenylpor-
phyrin Sb(V) and P(V) complexes through ET from their axial
ligands.14 Although, the ET from porphyrin S1 state can occur
other than the ET from S2 in the reported many molecular
systems, the ultrafast ET from the S2 is the possible photo-
chemical process.
In the present study, we synthesized zinc meso-(1′-pyrenyl)-
triphenylporphyrin derivatives (Chart 1) as simple models to
investigate ET from S2. In the molecules, (i) direct linkage
between porphyrin and pyrene enhances intramolecular ET
relative to intermolecular ET to solvent, (ii) the pyrene unit
cannot become an energy acceptor of porphyrin S2, and (iii)
the energy level of the charge transfer (CT) state is below S2
and above S1; therefore, ET cannot occur from S1. These
molecules may be used as a model of photooxidative electron
transfer to the porphyrin S2. If a rapid ET from S2 occurs in
* To whom correspondence should be addressed. (K.H.) Address:
Department of Basic Engineering (Chemistry), Faculty of Engineering,
Shizuoka University, Johoku 3-5-1, Naka-ku, Hamamatsu, Shizuoka 432-
8561, Japan. Tel.: +81-53-478-1287. Fax: +81-53-478-1287. E-mail: tkhirak
@ipc.shizuoka.ac.jp. (H.S.) Address: Research Center for Advanced Science
and Technology, The University of Tokyo, Komaba 4-6-1, Meguro-ku,
Tokyo 153-8904, Japan. Tel.: +81-3-5452-5295. Fax: +81-3-5452-5299.
E-mail: csegawa@mail.ecc.u-tokyo.ac.jp.
† Shizuoka University.
‡ Kitasato University.
§ The University of Tokyo.
10.1021/jp903743w CCC: $40.75  2009 American Chemical Society
Published on Web 07/09/2009

Anomalously Selective Quenching of S2 Fluorescence
J. Phys. Chem. A, Vol. 113, No. 31, 2009 8853
CHART 1
p-methoxybenzaldehyde (WAKO Pure Chem. Ind.) were dis-
solved in 200 mL of propionic acid (WAKO Pure Chem. Ind.,
Osaka, Japan), and 2.1 mL of pyrrole (WAKO Pure Chem. Ind.)
was added to the reflux for 1 h. The solvent propionic acid was
removed by vacuum evaporation after the reaction. The crude
product was washed by methanol several times and purified by
column chromatography on silica gel with chloroform-hexane
(9/1, v/v) as an eluent several times, the result being a pure
product with a 3.3% (0.21 g) yield. ZnP-Py was prepared using
1
0.027 g of H2P-Py. H NMR (CDCl3, TMS) δ 4.06 (s, 6H,
methoxy-H of 10,20-phenyl), 4.11 (s, 3H, methoxy-H of 15-
phenyl), 7.25 (d, 4H, JH-H ) 10.5 Hz, 10,20-m-phenyl-H), 7.30
(d, 2H, JH-H ) 7.5 Hz, 15-m-phenyl-H), 7.44 (d, 1H, JH-H
)
9.5 Hz, pyrenyl-H), 7.67 (d, 1H, JH-H ) 9.0 Hz, pyrenyl-H),
8.03-8.17 (m, 8H, 10,15,20-o-phenyl-H (6H) and pyrenyl-H
(2H)), 8.32 (t, 2H, pyrenyl-H), 8.40 (d, 1H, JH-H ) 9.0 Hz,
pyrenyl-H), 8.51 (d, 1H, JH-H ) 7.5 Hz, pyrenyl-H), 8.57 (d,
2H, JH-H ) 4.5 Hz, 3,7-ꢀH), 8.80 (d, 1H, JH-H ) 7.5 Hz,
pyrenyl-H), 8.87 (d, 2H, JH-H ) 4.5 Hz, 2,8-ꢀH), 8.98-9.01
(m, 4H, 12,13,17,18-ꢀH). FAB-MS: m/z 889.9 (M+). UV-vis
absorption peaks (λmax/nm) in dichloromethane: 310, 322, 338,
428, 556, 598.
Synthesis of ZnFP-Py. Free-base 5-(1′-pyrenyl)-10,15,20-
tris(pentafluorophenyl)porphyrin (H2FP-Py) was synthesized
by the following procedure: 3.9 g of 1-pyrenecarboxyaldehyde
and 10.0 g of pentafluorobenzaldehyde (Aldrich Chem. Com.
Inc.) were dissolved in 500 mL of propionic acid, and 5.0 mL
of pyrrole was added to the reflux for 1 h. The solvent propionic
acid was removed by vacuum evaporation after the reaction.
The crude product was washed by methanol/water (9/1, v/v)
several times and purified by column chromatography on silica
gel with chloroform-hexane (9/1, v/v) as an eluent several
times, and we obtained a mixture of H2FP-Py and tetrakis(pen-
tafluorophenyl)porphyrin (H2FP). Zinc(II) ion was incorporated
into H2FP-Py and H2FP, and pure ZnFP-Py was obtained by
high-performance liquid chromatography (LC-10, Shimadzu,
Kyoto, Japan, using a silica gel column) with chloroform-hexane
(6/94, v/v) as an eluent. The purification of ZnFP-Py was
difficult, and only a small amount of the sample could be
these molecules, the S2 fluorescence quenching becomes an
effective probe of the ET process under various conditions. Thus,
the S2 fluorescence quantum yields and its solvent dependence
were investigated.
Experimental Section
Measurements. The absorption and fluorescence spectra were
taken on a V-570 UV-vis-NIR spectrophotometer (JASCO,
Tokyo, Japan) and a FP-777 spectrophotometer (JASCO), res-
pectively. The fluorescence quantum yields of the porphyrin
moieties were determined relative to zinc tetraphenylporphyrin
in benzene (S1 fluorescence: Φf ) 3.0 × 10-2, S2 fluorescence:
Φf ) 3.5 × 10-4 7c
)
and corrected by the refractive index of
solvents.16 Cyclic voltammograms were measured with a three-
electrode system using a platinum working, a platinum counter,
and a saturated calomel reference electrode (SCE), which were
assembled with a HABF501 potentiogalvanostat (Hokuto Denko,
Tokyo, Japan). All the electrolysis solutions were purged with
nitrogen.
1
obtained. H NMR (CDCl3, TMS) δ 7.33 (d, 1H, JH-H ) 9.5
Hz, pyrenyl-H), 7.73 (d, 1H, JH-H ) 9.5 Hz, pyrenyl-H),
8.07-8.14 (m, 2H, pyrenyl-H), 8.35-8.38 (m, 2H, pyrenyl-
H), 8.43 (d, 1H, JH-H ) 9.0 Hz, pyrenyl-H), 8.56 (d, 1H, JH-H
) 8.0 Hz, pyrenyl-H), 8.71 (d, 2H, JH-H ) 4.5 Hz, 3,7-ꢀH),
8.78 (d, 2H, JH-H ) 4.5 Hz, 2,8-ꢀH), 8.79 (d, 1H, JH-H ) 8.0
Hz, pyrenyl-H), 8.99 (s, 4H, 12,13,17,18-ꢀH). FAB-MS: m/z
1070.0 (M+). UV-vis absorption peaks (λmax/nm) in dichlo-
romethane: 322, 341, 422, 552, 582.
Materials. The spectroscopic grade solvents of acetonitrile,
dichloromethane, toluene (Dojin Chem. Ind., Kumamoto, Japan),
and hexane (Kanto Chem. Com. Inc., Tokyo, Japan) were used
for the measurement as received. Zinc 5-(1′-pyrenyl)-10,15,20-
tris(p-methoxyphenyl)porphyrin (ZnP-Py) and zinc 5-(1′-
pyrenyl)-10,15,20-tris(pentafluorophenyl)-porphyrin (ZnFP-Py)
were synthesized by the following procedures. At first, free-
base meso-(1′-pyrenyl)triphenylporphyrin derivatives were syn-
thesized using a modified version of the procedure of Adler
and co-workers.17 Zinc ion was incorporated into the porphyrins
according to the literature.18 The products were identified by
1H NMR, FAB-MS, and UV-vis spectroscopy (described in
the following sections). Zinc 5,10,15,20-tetrakis(p-methoxyphe-
nyl)porphyrin (ZnP)17,18 and zinc 5,10,15,20-tetrakis(pentafluo-
rophenyl)porphyrin (ZnFP)19 were synthesized according to the
literature.
Calculations. The equilibrium geometries of ZnP-Py and
ZnFP-Py were calculated by the ab initio molecular orbital
(MO) calculation at the Hartree-Fock 6-31G* level. The
electron densities of the porphyrin and the pyrene moieties were
also estimated from this method. These calculations were
performed on Spartan 08 (Wavefunction Inc. CA).
Results
Absorption and Fluorescence Spectra of ZnP-Py and
ZnFP-Py. The absorption feature of ZnP-Py (Figure 1a)
coincides with the superposition of ZnP and pyrene, indicating
that the π-π interaction between the porphyrin and the pyrene
moieties is weak.20 The weak π-π interaction is due to steric
rotational hindrance of the pyrene moiety around the meso-
position of the porphyrin, which keeps the two π-electronic
systems nearly orthogonal to each other. The absorption feature
Synthesis of ZnP-Py. Free-base 5-(1′-pyrenyl)-10,15,20-
tris(p-methoxyphenyl)porphyrin (H2P-Py) was synthesized by
the following procedure: 1.73 g of 1-pyrenecarboxaldehyde
(Aldrich Chem. Com. Inc., Milwaukee, WI) and 2.7 mL of

8854 J. Phys. Chem. A, Vol. 113, No. 31, 2009
Hirakawa et al.
TABLE 2: Estimated Energies of S1, S2, and CTa
compound solvent ES1 (eV) ES2 (eV) CT1 (eV) CT2 (eV)
ZnP-Py
toluene
CH2Cl2
CH3CN
2.06
2.04
2.04
2.13
2.10
2.08
2.07
2.83
2.83
2.86
2.97
2.84
2.89
2.88
3.06
2.72
2.63
2.80
2.68
2.34
2.25
3.39
3.05
2.96
4.04
3.92
3.58
3.49
ZnFP-Py hexane
toluene
CH2Cl2
CH3CN
a ES1 and ES2 indicate the S1 and S2 energies of the porphyrin
moieties, respectively. CT1 is the energy level of the porphyrin
anion-pyrene cation pair. CT2 is the energy level of the porphy-
rin cation-pyrene anion pair.
both the S2 and S1 fluorescence were observed (Figure 1b).
Although the S1 fluorescence quantum yields were almost the
same as those of the corresponding reference porphyrins, the
S2 fluorescence was clearly quenched by the pyrene moiety
(Figure 1b and Table 1).
Energies of Porphyrin S2 and S1 and of the CT State. The
S2 and S1 states energies of porphyrins relative to the ground
states were estimated from their fluorescence peaks and are listed
in Table 2. The reduction potentials (vs SCE) of ZnP (-1.28
V) and ZnFP (-0.90 V), pyrene (-2.21 V),21 and the oxidation
potentials (vs SCE) of ZnP (0.84 V), ZnFP (1.37 V), and pyrene
(1.44 V) in benzonitrile were obtained from cyclic voltammo-
grams. The CT state energies relative to the ground states were
estimated from these redox potentials using dielectric continuum
theory (Table 2).22,23 The energy levels of the CT1 state (the
porphyrin anion-pyrene cation pair) were lower than the
porphyrin S2 energy, suggesting that ET from the pyrene
to porphyrin S2 is possible from an energetic viewpoint.
The CT1 state energy of ZnP-Py in toluene is apparently higher
than that of S2, possibly due to an underestimation of the
solvation energy of a less-polar solvent, toluene. The energy
levels of the CT2 states (the porphyrin cation-pyrene anion
pair) were higher than those of the CT1, supporting that the
ET from the pyrene to porphyrin S2.
Calculated Electron Densities of ZnP-Py and ZnFP-Py.
Ab initio MO calculations indicated that the HOMO and the
next HOMO of ZnP-Py are a2u and a1u symmetries, respec-
tively, whereas those of ZnFP-Py are reversed, similarly to
the case of other tris(pentafluorophenyl)porphyrin deriva-
tives.24-26 The a2u orbital has electron density at the meso
positions and the central nitrogens, whereas the a1u orbital has
nodes at these positions. The electron density at 5-C of the next
HOMO of ZnFP-Py is significantly higher (6.4 × 10-2) than
that of ZnP-Py (6.2 × 10-6). Since the electron densities at
1′-C of the pyrene moieties of ZnP-Py and ZnFP-Py are
almost the same (8.9 × 10-2 and 8.5 × 10-2, respectively), the
electronic communication between the next HOMO of porphy-
rin and the HOMO of pyrene in ZnFP-Py becomes larger than
that of ZnP-Py.
Figure 1. (a) Absorption spectra of ZnP-Py and ZnP in dichlo-
romethane. (b) Fluorescence spectra of ZnP-Py and ZnP at 400 nm
excitation in dichloromethane. The absorbance of these porphyrins was
adjusted to 0.020 at the excitation wavelength.
TABLE 1: Fluorescence Quantum Yields
a
b
c
compound
ZnP
solvent
ΦS2-S2
ΦS2-S1
ΦS1-S1
toluene
CH2Cl2
CH3CN
toluene
CH2Cl2
CH3CN
hexane
toluene
CH2Cl2
CH3CN
hexane
toluene
CH2Cl2
CH3CN
2.90 × 10-4
4.10 × 10-4
8.80 × 10-4
1.40 × 10-4
0.98 × 10-4
0.86 × 10-4
1.00 × 10-4
1.30 × 10-4
0.41 × 10-4
1.40 × 10-4
0.44 × 10-4
0.50 × 10-4
0.05 × 10-4
0.20 × 10-4
0.027
0.033
0.017
0.030
0.031
0.017
0.020
0.017
0.012
0.011
0.019
0.019
0.013
0.013
0.027
0.035
0.018
0.031
0.037
0.020
0.021
0.019
0.014
0.011
0.021
0.019
0.016
0.013
ZnP-Py
ZnFP
ZnFP-Py
a S2 fluorescence quantum yields at S2 excitation (Ex ) 400 nm).
b S1 fluorescence quantum yields at S2 excitation (Ex ) 400 nm).
c S1 fluorescence quantum yields at S1 excitation (Ex ) 550 nm).
of ZnFP-Py is similar to that of ZnP-Py. An ab initio MO
calculation also indicated that the planes of the pyrene moieties
are perpendicular to those of the porphyrins. This calculation
showed that the highest occupied MO (HOMO) and lowest
unoccupied MO (LUMO) of these porphyrins and pyrenes were
localized on their corresponding moieties, supporting the results
of absorption measurements.
The S1 fluorescence spectra of ZnP-Py and ZnFP-Py were
similar to those of the corresponding reference porphyrins (ZnP
and ZnFP), indicating that the contribution of the CT state in
S1 is negligibly small. The fluorescence quantum yields of S1
of the porphyrin moieties were almost the same as those of the
corresponding reference porphyrins in any solvents when the
porphyrins were photoexcited at the Q-bands (Table 1).
Consequently, it is confirmed that the S1 of the porphyrin
derivatives is not quenched by the pyrene moiety. When
ZnP-Py and ZnFP-Py were photoexcited at the Soret band,
Discussion
The present study has demonstrated that the S2 fluorescence
of the porphyrin is quenched by the pyrene moiety, whereas
the S1 fluorescence is not. Since the energy levels of the excited
state of the pyrene moiety and the CT state of these pyrenylpor-
phyrins are higher than that of S1 of the porphyrin, quenching
through energy transfer and ET does not occur from the S1. In
the S2, ET from the pyrene moiety to the porphyrin S2 is
energetically possible because the energy level of the CT state
of a porphyrin anion and a pyrene cation pair is located between

Anomalously Selective Quenching of S2 Fluorescence
J. Phys. Chem. A, Vol. 113, No. 31, 2009 8855
TABLE 3: Quantum Yields of Deactivation Processes and
Estimated ET Rate Constants from the S2 State
compound
solvent
ΦIC
ΦET
ΦCRfS1
kET/1012 s-1
ZnP-Py
toluene
CH2Cl2
CH3CN
hexane
toluene
CH2Cl2
CH3CN
0.50
0.24
0.10
0.42
0.39
0.13
0.15
0.50
0.76
0.90
0.58
0.61
0.87
0.85
0.92
0.81
0.82
0.84
0.96
0.80
0.96
0.88
2.00
2.60
3.30
3.10
ZnFP-Py
40.00
10.00
Consequently, the quantum yields of the ET (ΦET), the IC (ΦIC),
and the CR to the porphyrin S1 (ΦCRfS1) can be estimated by
the following equations:
Figure 2. Dependence of the fluorescence quantum yield ratio
(Φ/Φ0) on the solvent polarity parameter (f ) (2(ε - 1))/(2ε + 1)
-(n2 - 1)/(2n2 + 1), where ε is the dielectric constant, n is the
refractive index): S1 fluorescence of ZnP-Py (O) and ZnFP-Py
(0) and S2 fluorescence of ZnP-Py (b) and ZnFP-Py (9).
ΦS2-S2
ΦET ) 1 -
(1)
(2)
Φ0
S2-S2
ΦIC ) 1 - ΦS2-S2 - ΦET
ΦS2-S1
- ΦIC
ΦS1-S1
ΦCRfS1
)
(3)
ΦET
where Φ0
is the S2 fluorescence quantum yield of the
S2-S2
reference porphyrin by the S2 excitation. The quantum yields
of these processes are listed in Table 3. The ΦET become large
in polar solvent. For further investigation, the relative ET rate
constant (kET) was roughly estimated from the following
equation:
Figure 3. Schematic energy diagram of the deactivation processes. P
and P•--Py•+ indicate the porphyrin moiety and the porphyrin
anion-pyrene cation pair in ZnP-Py or ZnFP-Py, respectively.
1
1
kET
)
-
kf
(4)
Φ0
Φ
(
)
S2-S2
S2-S2
the S2 and the S1 energy levels of the porphyrins. The transient
absorption spectrum measurement showed the absorption peak
around 460 nm (Supporting Information). This absorption could
be assigned to the radical cation of pyrene,27 and did not coincide
with the reported absorption peak of the radical anion of
pyrene,28 strongly suggesting the ET from the pyrene to
porphyrin S2. If the S2 quenching is due to the ET, dependence
of the fluorescence quenching on the solvent polarity is expected.
The fluorescence quantum yields ratio (Φ/Φ0), which results
from the division of the fluorescence quantum yields of ZnP-Py
and ZnFP-Py by those of the corresponding reference porphy-
rins, was plotted against the solvent polarity parameter (Figure
2). The Φ/Φ0 of S2 clearly decreased with increasing the solvent
polarity, whereas the Φ/Φ0 of S1 was not affected. If the S2
quenching process is due to enhancement of direct IC to S1,
the solvent dependence of S2 fluorescence cannot be explained.
For the purpose of careful investigation, the following deactiva-
tion processes from the S2 of ZnP-Py and ZnFP-Py were
assumed: (1) direct IC from S2 to S1 and (2) ET between pyrene
and the porphyrin S2, which is followed by charge recombination
(CR) to the porphyrin S1 or S0, as shown in Figure 3. Since the
S1 fluorescence quantum yields of reference porphyrins (ZnP
and ZnFP) are independent of the excitation wavelengths within
experimental error (Table 1) similar to that of previous
reports,6b,c,7b the main pathway of the deactivation processes of
S2 of these reference molecules are IC and S2 emission.
where kf is the rate constant of S2 fluorescence.29 The kET values
are mostly increasing with increasing the solvent polarity (Table
3), almost coinciding with the dependency of the Φ/Φ0 on the
solvent polarity. The kET value of ZnFP-Py in dichloromethane
is larger than that in relatively high polar acetonitrile. This
complicated result might be explained by the affect of charge
transfer interaction between the porphyrin S2 and solvent
dichloromethane molecule.9 The values of kET for ZnFP-Py
are much larger than those of ZnP-Py. The ET should occur
from the HOMO of the pyrene to the next HOMO of the
porphyrin. Electron density at 5-C of the next HOMO of
ZnFP-Py is larger than that of ZnP-Py, possibly resulting in
an enhancement of electronic communication24-26 between the
porphyrin and the pyrene. Furthermore, the free energy changes
(-∆G) of ET of ZnFP-Py, which were estimated from the
values in Table 2 (0.16 eV in toluene, 0.55 eV in dichlo-
romethane, and 0.63 eV in acetonitrile), were larger than those
of ZnP-Py (-0.23 eV in toluene, 0.11 eV in dichloromethane,
and 0.23 eV in acetonitrile). These findings support the
observation that the ET from the porphyrin S2 of ZnFP-Py is
faster than that of ZnP-Py.
In addition, the high values of ΦCRfS1 have indicated that
the CT state predominantly decays into S1. The -∆Gs of the
CR to S0, which were estimated from the values in Table 2
(2.63-3.06 eV for ZnP-Py and 2.25-2.80 eV for ZnFP-Py),
are much larger than the -∆Gs of the CR to S1 (0.59-1.00 eV

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(17) Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour,
J.; Korsakoff, L. J. Org. Chem. 1967, 32, 476.
(18) Buchler, J. W. Static Coordination Chemistry of Metalloporphyrins.
Porphyrins and Metalloporphyrins; Smith, K. M. , Ed.; Elsevier Scientific
Publishing Company: New York, 1975; Chapter 5.
(19) Spellane, P. J.; Gouterman, M.; Antipas, A.; Kim, S.; Liu, Y. C.
Inorg. Chem. 1980, 19, 386.
(20) Hirakawa, K.; Segawa, H. J. Photochem. Photobiol. A Chem. 1999,
123, 67.
(21) The reduction potential of pyrene was estimated from the oxidation
potential (1.44 V vs SCE) and the 0-0 band of absorption maximum (340
nm).
(22) Weller, A. Z. Phys. Chem. Neue Folge 1982, 133, 93.
(23) The CT state energy in solvent x (ECT)x was calculated from the
following equation:
for ZnP-Py and 0.18-0.67 eV for ZnFP-Py), indicating that
CR to S0 is a highly exothermic process. Therefore, CR to S1
becomes the predominant process.
In summary, the present study has shown that the S2 fluo-
rescence of porphyrin is quenched through rapid ET from the
pyrenyl group directly connected at the meso position, whereas
the S1 fluorescence cannot be quenched. The CT state relaxes
to the S1 state through rapid CR with a high quantum yield.
The ET from the electron donor to the porphyrin S2 can occur,
suggesting that the photooxidation of various molecules by using
porphyrin S2 state, which has higher oxidative ability than the
S1, is possible.
Acknowledgment. This work was supported by grants from
the Ministry of Education, Science and Culture of the Japanese
Government and from the Japan Science and Technology
Corporation.
Supporting Information Available: The transient absorp-
tion spectra of ZnP-Py. This material is available free of charge
via the Internet at http://pubs.acs.org.
e2
1
1
1
1
e2
εxa
References and Notes
+
(ECT)x ) (E1/2 - E1/2-) +
+
-
-
(
)(
)
2 rpor rpy εx εm
(1) (a) Wasielewski, M. R. Distance Dependencies of Electron Transfer
Reactions. Photoinduced Electron Transfer; Fox, M. A., Chanon, M. , Eds.;
Elsevier: Amsterdam, The Netherlands, 1988; Chapter 1.4. (b) Wasielewski,
M. R. Chem. ReV. 1992, 92, 435.
(2) Kurreck, H.; Huber, M. Angew. Chem., Int. Ed. Engl. 1995, 34,
849.
where E1/2+, E1/2-, e, εx, and εm are half-wave one-electron oxidation and
reduction potentials in solvent m, the electronic charge, and the static
dielectric constants of solvent x and m, respectively. The rpor (6.2 Å) and
the rpy (4.0 Å) are the effective ionic radius of porphyrin and pyrene,
respectively. These values were obtained by the ab initio MO calculation
at the Hartree-Fock 6-31G* level. Distance “a” is the center-to-center
distance between the porphyrin and the pyrene moieties (7.7 Å). The
following values were used as static dielectric constants: 36.0 (acetonitrile),
25.2 (benzonitrile), 9.10 (dichloromethane), 2.38 (toluene), and 1.88
(hexane).
(3) Piotrowiak, P. Chem. Soc. ReV. 1999, 28, 143.
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A. M. P. J. Phys. Chem. 1976, 80, 2389.
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K. N.; Kachura, T. F. Chem. Phys. Lett. 1980, 73, 80. (b) Stelmakh, G. F.;
Tsvirko, M. P. Opt. Spectrosc. 1983, 55, 518. (c) Aaviksoo, J.; Freiberg,
A.; Savikhin, S.; Stelmakh, G. F.; Tsvirko, M. P. Chem. Phys. Lett. 1984,
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1984, 81, 2962. (b) Kurabayashi, Y.; Kikuchi, K.; Kokubun, H.; Kaizu,
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(29) The kf was roughly estimated to be 2.5 × 108 s-1 from the
fluorescence quantum yield (Φ0S2-S2) and the lifetime (τ0) of S2 state of
zinc tetraphenylporphyrin derivatives in acetonitrile by the following
equation:
(8) Fujitsuka, M.; Cho, D. W.; Solladie´, N.; Troiani, V.; Qiu, H.;
Majima, T. J. Photochem. Photobiol. A: Chem. 2007, 188, 346.
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Φ0
S2-S2
kf )
τ0
(12) LeGourrie´rec, D.; Andersson, M.; Davidsson, J.; Mukhtar, E.; Sun,
L.; Hammarstro¨m, L. J. Phys. Chem. A 1999, 103, 557.
(13) (a) Mataga, N.; Shibata, Y.; Chosrowjan, H.; Yoshida, N.; Osuka,
A. J. Phys. Chem. B 2000, 104, 4001. (b) Mataga, N.; Chosrowjan, H.;
Shibata, Y.; Yoshida, N.; Osuka, A.; Kikuzawa, T.; Okada, T. J. Am. Chem.
Soc. 2001, 123, 12422. (c) Mataga, N.; Chosrowjan, H.; Taniguchi, S.;
For the calculation, the values for Φ0
of ZnP and τ0 of zinc
S2-S2
tetraphenylporphyrin (τ0 ) 3.5 ps)9 were used. The kf was assumed to be
the constant in all solvents for the calculation of the kET
.
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