Effects of hydrogen bonding interaction and solvent polarity on the competition between excitation energy and photo-induced electron transfer processes in hydroxy(1-pyrenebutoxy)phosphorus(V)porphyrin

Article abstract of DOI:10.1016/j.jphotochem.2010.05.002

The solvent effects on an excitation energy transfer and a photo-induced electron transfer processes were examined using synthesized hydroxy(1-pyrenebutoxy)phosphorus(V)porphyrin. In the photoexcited state of the pyrene moiety, the intramolecular energy transfer to the porphyrin competed with the electron transfer from the pyrene to the porphyrin. The quantum yield of energy transfer in non-alcoholic solvents decreased with an increase of the solvent polarity due to the enhancement of electron transfer. However, the energy transfer was predominant process in alcoholic solvents with high polarity. The energy transfer yield increased with an increase in the ratio of methanol in the mixture of acetonitrile and methanol, of which solvent polarity is almost the same as that of acetonitrile. The redox potential measurements and ab initio molecular orbital calculation at Hartree-Fock 6-31G* level have shown that the electron affinity of the porphyrin moiety decreased through the hydrogen bonding with alcoholic solvents, resulting in that the electron transfer is suppressed and the excitation energy transfer becomes the predominant process. In conclusion, the hydrogen bonding interaction with alcoholic solvent contributes to the competition between energy and electron transfer by the changing of the energy level of charge transfer state rather than the effect of solvent polarity.

Full text of DOI:10.1016/j.jphotochem.2010.05.002

Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 73–79
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Effects of hydrogen bonding interaction and solvent polarity on the competition
between excitation energy and photo-induced electron transfer processes in
hydroxy(1-pyrenebutoxy)phosphorus(V)porphyrin
Kazutaka Hirakawaa,∗, Hiroshi Segawab,∗∗
a Department of Basic Engineering (Chemistry), Faculty of Engineering, Shizuoka University, Johoku 3-5-1, Naka-ku, Hamamatsu, Shizuoka 432-8561, Japan
b Research Center for Advanced Science and Technology, The University of Tokyo, Komaba 4-6-1, Meguro-ku, Tokyo 153-8904, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The solvent effects on an excitation energy transfer and a photo-induced electron transfer processes
were examined using synthesized hydroxy(1-pyrenebutoxy)phosphorus(V)porphyrin. In the photoex-
cited state of the pyrene moiety, the intramolecular energy transfer to the porphyrin competed with the
electron transfer from the pyrene to the porphyrin. The quantum yield of energy transfer in non-alcoholic
solvents decreased with an increase of the solvent polarity due to the enhancement of electron trans-
fer. However, the energy transfer was predominant process in alcoholic solvents with high polarity. The
energy transfer yield increased with an increase in the ratio of methanol in the mixture of acetonitrile
and methanol, of which solvent polarity is almost the same as that of acetonitrile. The redox potential
measurements and ab initio molecular orbital calculation at Hartree-Fock 6-31G* level have shown that
the electron affinity of the porphyrin moiety decreased through the hydrogen bonding with alcoholic
solvents, resulting in that the electron transfer is suppressed and the excitation energy transfer becomes
the predominant process. In conclusion, the hydrogen bonding interaction with alcoholic solvent con-
tributes to the competition between energy and electron transfer by the changing of the energy level of
charge transfer state rather than the effect of solvent polarity.
Received 29 January 2010
Received in revised form 20 April 2010
Accepted 1 May 2010
Available online 9 May 2010
Keywords:
Phosphorus(V)porphyrin
Pyrene
Energy transfer
Electron transfer
Hydrogen bonding
Solvent effect
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
electronic transition processes are selected by the control of the
ELT rate depending on the charge transfer (CT) state energy level
Excitation energy transfer (ENT) and photo-induced elec-
tron transfer (ELT) are essential processes for photochemical
processes [1–3], photo-information operations [4–7], biological
system [8–10], and medicinal application such as photodynamic
therapy [11,12]. Intramolecular ENT and ELT systems between
two chromophores involving the porphyrin chromophore have
received much attention in elucidation of the natural photo-
synthetic system [13–18]. The effects of the surroundings of
chromophores play an important role in ENT and ELT processes. Sol-
vent properties such as polarity [19,20], hydrogen bonding [21], pH
[4–7,22,23], and ion strength [4–7,22] are closely correlated with
the governing factor of ENT and ELT rates. The competition between
ENT and ELT in the photoexcited pyrene to the P(V)porphyrin
in bispyrenylP(V)porphyrins was previously reported [19]. These
due to solvent polarity. In general, alcohols are high polar solvents,
and exhibit a hydrogen bonding interaction with the donor and/or
the acceptor molecules. It has been reported that hydrogen bond-
ing interaction significantly affects the electronic state of porphyrin
derivatives [21]. Therefore, evaluation of contributions of solvent
polarity and hydrogen bonding interaction to the governing factor
of ENT and ELT would provide us the significance of an effect of
surroundings on the electronic transition processes.
In this study, hydroxy(1-pyrenebutoxy)P(V)porphyrin (Py-POH,
Fig. 1) was synthesized to investigate the solvent effects on the
selectivity of ENT and ELT from the pyrene to the porphyrin. The
main reasons for using pyrene and hydroxyP(V)porphyrin are that
both ENT and ELT from the pyrene to the porphyrin are ener-
getically possible in the photoexcited state of the pyrene [19],
and the axial hydroxyl group can act as hydrogen bonding site
with solvent molecules. Solvent effects on the photochemical and
electrochemical properties of Py-POH were examined by spectro-
scopic methods and redox potential measurement. The interaction
between Py-POH and alcohol molecule was investigated using ab
initio molecular orbital (MO) calculation.
∗
Corresponding author. Tel.: +81 53 478 1287; fax: +81 53 478 1287.
Corresponding author. Tel.: +81 3 5452 5295; fax: +81 3 5452 5299.
E-mail addresses: tkhirak@ipc.shizuoka.ac.jp (K. Hirakawa),
∗∗
csegawa@mail.ecc.u-tokyo.ac.jp (H. Segawa).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.05.002

74
K. Hirakawa, H. Segawa / Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 73–79
Table 1
Absorption properties of Py-POH in various solvents.
Solvent
Absorption (ꢁmax/nm)
Pyrene moiety
Soret band
Q band
AN
Pyr
DCM
THF
TOL
MT
ET
312, 327, 343
317, 330, 347
314, 328, 345
314, 328, 344
316, 330, 346
312, 326, 342
313, 326, 343
312, 326, 343
314, 327, 344
441
448
444
442
443
436
435
434
438
556, 612
568, 645
567, 614
566, 612
566, 612
561, 605
559, 605
558, 600
562, 608
PR
BT
AN: acetonitrile, Pyr: pyridine, DCM: dichloromethane, THF: tetrahydrofuran, TOL:
toluene, MT: methanol, ET: ethanol, PR: 2-propanol, and BT: iso-butylalcohol.
Fig. 1. Structure of Py-POH.
phenyl-OCCCH2C), 1.85–1.95 (m, 10H, meso-phenyl-OCCH2CC (8H)
and P-OCCCCH2-pyrenyl (2H)), 4.16 (t, 8H, JH–H = 6.2 Hz meso-
phenyl-OCH2CCC), 6.91 (d, 1H, JH–H = 7.5 Hz, 2ꢀ-pyrenyl-H), 7.16 (d,
1H, JH–H = 9.5 Hz, 10ꢀ-pyrenyl-H), 7.18 (d, 8H, JH–H = 8.5 Hz, meso-
m-phenyl-H), 7.74 (d, 1H, JH–H = 7.5 Hz, 3ꢀ-pyrenyl-H), 7.78 (d, 1H,
JH–H = 9.5 Hz, 9ꢀ-pyrenyl-H), 7.89 (d, 9H, JH–H = 8.5 Hz, 4ꢀ-pyrenyl-H
(1H) and meso-o-phenyl-H (8H)), 7.96 (d, 1H, JH–H = 8.5 Hz, 5ꢀ-
pyrenyl-H), 7.98 (t, 1H, JH–H = 7.8 Hz, 7ꢀ-pyrenyl-H), 8.10–8.14 (m,
2H, 6ꢀ,8ꢀ-pyrenyl-H), 8.85 (br. s, 8H, ␤H). FAB-MS: m/z 1221.4 (M+).
UV–vis absorption spectrum in dichloromethane: ꢁmax/nm 314,
328, 345, 444, 567, 614.
2. Experimental details
2.1. Measurements
1H NMR spectra were taken on a JNM-A 500 (500 MHz) FT-NMR
spectrometer (JEOL, Tokyo, Japan). Chemical shifts of 1H were mea-
sured in ı (ppm) units relative to the tetramethylsilane internal
standard. Absorption and fluorescence spectra were taken on a V-
570 UV/VIS/NIR spectrophotometer (JASCO, Tokyo, Japan) and a
FP-777 spectrophotometer (JASCO), respectively. The fluorescence
quantum yields (˚) of the pyrene and porphyrin moieties were
determined relative to pyrene (˚ = 0.65 in acetonitrile) [24] and
diphenoxyP(V)porphyrin (˚ = 0.037 in acetonitrile) [25–27]. These
values were corrected by refractive index of solvents [28]. The flu-
orescence lifetime (ꢀf) was measured by the previously reported
method [25–27]. All the samples for the emission measurement
were purged with nitrogen. Cyclic voltammograms were mea-
sured 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. To compare the redox potentials in different
solvents, these values were corrected by the redox potential of one-
electron oxidation of ferrocene (fc/fc+ vs. SCE: 0.43 V in acetonitrile
and 0.46 V in methanol).
2.3. Ab initio MO calculation of Py-POH
The equilibrium geometry of intermolecular complex of Py-POH
with alcoholic molecules and the energy of the lowest unoccu-
pied MO (LUMO) were estimated from ab initio MO calculation at
Hartree-Fock 6-31G* level. The calculations were performed on a
Spartan 08ꢀ Windows (Wavefunction Inc, CA, USA).
3. Results
3.1. Absorption property of Py-POH in various solvents
The absorption spectrum of Py-POH is almost a superposition of
the pyrene moiety and Soret and Q bands of the P(V)porphyrin moi-
ety. The absorption peaks of the Soret and the Q bands in alcoholic
solvents were blue-shifted compared to those in non-alcoholic sol-
vents (Table 1).
2.2. Materials
The
spectroscopic
grade
solvents
of
acetonitrile,
dichloromethane, tetrahydrofuran, toluene, methanol, iso-
butylalcohol (Dojin Chem. Ind., Kumamoto, Japan), ethanol,
2-propanol, and pyridine (Kanto Chem. Com. Inc., Tokyo, Japan)
were used for the measurement as received. 1-Pyrenebutanol was
from Aldrich Chemical Co. (Milwaukee, WI, USA). Silver nitrate
was from Wako Pure Chemical Ind. (Tokyo, Japan).
Py-POH was synthesized by the following procedure accord-
ing to the previously reported [29]. DichloroP(V)tetrakis(n-p-
butoxyphenyl)porphyrin chloride [30] (0.100 g), 1-pyrenebutanol
(0.200 g), and silver nitrate (0.500 g) were dissolved in 11 mL of ace-
tonitrile and stirred at room temperature for 3 h. The reaction was
supervised by absorption spectra and thin-layer chromatograph.
At the end of the reaction, the solvent was removed by vacuum
evaporation. The product was purified by column chromatogra-
phy on silica gel with a chloroform-methanol (10/1, v/v) as an
eluent, the result being a pure product in an 11% (0.013 g) yield.
1H NMR (CDCl3, TMS): ı −2.41 (dt, 2H, JP–H = 12 Hz, JH–H = 6.0 Hz,
P-OCH2CCC-pyrenyl), −1.96 (d, 1H, JP–H = 26 Hz, P–OH), −1.29 (tt,
2H, P-OCCH2CC-pyrenyl), −0.39 (tt, 2H, P-OCCCH2C-pyrenyl), 1.05
(t, 12H, JH–H = 7.2 Hz, meso-phenyl-OCCCCH3), 1.59 (m, 8H, meso-
3.2. Fluorescence property of Py-POH in various solvents
The fluorescence spectrum of Py-POH was shown in Fig. 2 (A).
The shapes of fluorescence spectra of the pyrene and the por-
phyrin moieties were quite similar to those of 1-pyrenebutanol and
alkoxyP(V)porphyrin derivatives. Fluorescence from the pyrene
moiety (quantum yield in dichloromethane: ˚py→py = 0.0099)
was remarkably decreased compared to the 1-pyrenebutanol
(˚py0 = 0.50) by the ENT and the ELT (described later). The fluores-
cence lifetime of the pyrene moiety was also decreased compared
to the reference pyrene (e.g. 60.6 ns in acetonitrile) (Table 2). The
fluorescence from the porphyrin moiety was observed clearly in the
wavelength region of 570–750 nm by excitation of the pyrene moi-
ety. The absorption feature of the pyrene moiety was observed in
the fluorescence excitation spectrum by monitoring this porphyrin
fluorescence, indicating the ENT from the pyrene to the porphyrin
(Fig. 2(B)).
Fluorescence peak of the pyrene was not significantly affected
by the kinds of solvents, whereas those of the porphyrin was blue-
shifted in alcoholic solvents compared to those in non-alcoholic

K. Hirakawa, H. Segawa / Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 73–79
75
Fig. 3. Dependence of ˚ENT and ˚ELT on solvent polarity parameter (f).
Fig. 2. Fluorescence and fluorescence excitation spectra of Py-POH in methanol.
The fluorescence spectrum was measured by pyrene excitation (Ex = 337 nm) (A).
The fluorescence excitation spectrum was measured by monitoring of the porphyrin
fluorescence (Em = 630 nm) (B).
and the P(V)porphyrin radical through ELT by photo-excitation of
pyrenebutoxyP(V)porphyrin [19]. Thus, the quantum yield of the
ENT (˚ENT) can be calculated by following equation:
˚
py→por
solvents, similar to the absorption spectra (Table 2). Fluorescence
quantum yields of the porphyrin moiety in alcoholic solvents also
become larger than those in non-alcoholic solvents. The fluores-
cence lifetime of the porphyrin moiety in non-alcoholic solvents
was slightly shorter than those in alcoholic solvents. These results
have shown that an interaction with alcoholic solvent molecules
affects the electronic states of the porphyrin moiety of Py-POH.
˚
ENT
=
(1)
˚
por→por
where ˚py→por and ˚por→por are fluorescence quantum yields from
the porphyrin by photo-excitation of the pyrene (Ex = 337 nm) and
the porphyrin (Ex = 560 nm), respectively. Since the fluorescence
quenching is caused by both of the ENT and the ELT, the quantum
yields of the ELT (˚ELT) can be calculated by following equation
assuming that the radiative and nonradiative rates of the pyrene
moiety in Py-POH are not affected by the presence of the porphyrin
moiety:
3.3. Solvent effect on the competition between ENT and ELT from
the photoexcited pyrene moiety to the porphyrin moiety
˚
py→py
py0
˚
ELT
= 1 −
− ˚ENT
(2)
In the photoexcited pyrene, both the ENT and the ELT are ener-
getically possible. The ENT was demonstrated as described in the
above section. Previous study using transient absorption spectrum
measurement has shown the formation of the pyrene cation radical
˚
The calculated values of ˚ENT decreased with the solvent polar-
ity parameter (f) [31], and ˚ELT increased with f in non-alcoholic
Table 2
Fluorescence properties of Py-POH in various solvents.
Solvent
Pyrene moiety
Porphyrin moiety
a
b
c
ꢁmax/nm
˚
py→py
ꢀf/ns
ꢁmax/nm
˚
por→por
˚
py→por
␶f/ns
AN
Pyr
DCM
THF
TOL
MT
ET
378, 397
378, 398
378, 398
378, 397
378, 398
378, 396
377, 396
377, 396
377, 396
0.020
0.0067
0.0099
0.012
0.033
0.037
0.011
0.0075
0.012
1.4
1.0
0.9
1.4
3.1
1.6
1.2
1.1
1.2
630
630
632
628
627
619
617
614
618
0.014
0.016
0.018
0.018
0.028
0.037
0.039
0.031
0.036
0.0065
0.0095
0.012
0.0095
0.025
0.031
0.029
0.028
0.026
0.8
0.9
1.0
1.0
1.6
2.1
2.2
1.7
2.0
PR
BT
Abbreviations of solvents are the same as Table 1.
a
Fluorescence quantum yield of the pyrene by the pyrene excitation.
Fluorescence quantum yield of the porphyrin by the porphyrin excitation.
Fluorescence quantum yield of the porphyrin by the pyrene excitation.
b
c

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K. Hirakawa, H. Segawa / Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 73–79
Scheme 1.
and
˚
ELT
kELT
=
,
(4)
ꢀf
respectively. The estimated values of kELT increased with f in non-
alcoholic solvents, similarly to those of ˚ELT (Fig. 4). On the other
hand, clear relationship between the kENT and f was not observed.
3.4. Effect of methanol on the photophysical property of Py-POH
in acetonitrile/methanol
The absorption and the fluorescence spectra were measured in
the mixture of acetonitrile and methanol. Since the solvent polarity
of acetonitrile is almost the same as that of methanol, the polarity
of this mixture does not significantly depend on the ratio of these
solvents. The absorption and fluorescence peaks shifted to a shorter
wavelength region as the ratio of methanol was increased. At the
same time, the fluorescence quantum yields were also increas-
ing. The ENT quantum yields estimated from Eq. (1) increased
depending on the ratio of methanol as the same tendency of the
fluorescence peaks changing (Fig. 5). On the other hand, the ELT
yields decreased with an increase of the ratio of methanol. These
findings have shown that the ELT is inhibited and the ENT becomes
favorable in methanol.
Fig. 4. Dependence of kENT and kELT on solvent polarity parameter (f).
solvents (Fig. 3). This solvent effect can be reasonably explained
by that ELT is favorable in high polar solvent due to lowering of
CT state energy. The values of ˚ENT and ˚ELT in alcoholic solvents
deviated from the relation between those in non-alcoholic solvents
and f, showing that ENT preferentially occurs in alcoholic solvent
compared to non-alcoholic solvent.
3.5. Redox potential of Py-POH in acetonitrile and methanol
Redox potentials of Py-POH were measured by the cyclic
voltammogram in acetonitrile or methanol. Reduction potentials of
the porphyrin moiety were −1.00 V (in acetonitrile) and −1.17 V (in
methanol) vs. fc/fc+. Oxidation potentials of the pyrene moiety were
0.82 V (in acetonitrile), 0.85 (in methanol) vs. fc/fc+. The energy lev-
els of the CT states (ECT) of Py-POH in the solvents were estimated
from the difference in the reduction potential of porphyrin and the
oxidation potential of pyrene with ion-pair formation free enthalpy
[32,33]. The value of ECT in acetonitrile was calculated to be 1.82 eV
and slightly lower than that in methanol (2.02 eV), suggesting that
ELT is favorable in acetonitrile rather than in methanol.
The rate constants of ENT (kENT) and ELT (kELT) were estimated
using the following equations:
˚
ENT
kENT
=
(3)
ꢀf
3.6. Calculation study of effect of hydrogen bonding between the
axial hydroxyl group of Py-POH and alcoholic solvent molecule
The interaction between Py-POH and alcoholic solvent molecule
was examined by ab initio MO calculation at Hartree-Fock 6-31G*
Table 3
Center-to-center distances between the oxygen and the hydrogen atoms of axial
hydroxyl group of Py-POH (dOH) and LUMO energy estimated from ab initio MO
calculation.
Solvent
dOH (Å)
LUMO (eV)
ꢂE (eV)
No Solvent
MT
ET
PR
BT
0.965
0.989
0.991
0.991
0.993
−2.75
−2.53
−2.56
−2.58
−2.56
+0.22
+0.19
+0.17
+0.19
Fig. 5. Dependence of fluorescence peaks, ˚ENT, and ˚ELT on the ratio of methanol
in the acetonitrile/methanol solvent. The fluorescence peaks are represented as the
photon energy.
Abbreviations of solvents are the same as Table 1. ꢂE: Energy gap between LUMO
of Py-POH with alcohol molecule and that without solvent molecule.

K. Hirakawa, H. Segawa / Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 73–79
77
Table 4
Parameters for ENT and ELT from the photoexcited pyrene moiety to the porphyrin
ring.
Solvent
J (10−11 cm6 mol−1
)
−ꢂG (eV)
ꢁ (eV)
AN
Pyr
DCM
THF
TOL
MT
ET
5.89
5.69
5.73
5.94
5.53
5.86
5.52
5.84
5.43
1.51
1.47
1.45
1.43
1.21
1.31
1.29
1.28
1.28
0.952
0.644
0.695
0.674
0.047
0.963
0.897
0.849
0.825
PR
BT
Abbreviations of solvents are the same as Table 1.
phyrin moiety of Py-POH decreases due to the hydrogen bonding
interaction with alcoholic solvent molecule. Indeed, the reduction
potential of the porphyrin moiety increases in methanol than that
in acetonitrile, indicating that the electron affinity of the porphyrin
decreases in methanol than that in acetonitrile. These results can
be reasonably explained by that the axial hydroxyl group of Py-POH
acts as proton donor and the positive charge of the P(V)porphyrin is
partially transferred to alcoholic molecule, resulting in an increase
of reduction potential as shown in Fig. 7. This is supported by that
negative charge of the axial ligand of P(V)porphyrin causes blue-
shift of the absorption spectrum [36].
Fig. 6. Schematic diagram of deactivation processes of the photoexcited pyrene
moiety of Py-POH. IC and CR indicate the internal conversion and the charge recom-
bination, respectively.
level. The calculation of the equilibrium geometry of intermolecular
complex of Py-POH with alcoholic solvent has shown that hydro-
gen bonding interaction between Py-POH and solvent molecule
(Scheme 1). The bond length between H and O atoms of the
hydroxyl group of Py-POH becomes larger by the interaction with
alcoholic solvent molecule (Table 3). The energy of LUMO of Py-POH
increases through the interaction with alcoholic solvent (Table 3).
These results suggest that the axial hydroxyl group of Py-POH acts
as proton donor, resulting in an increase of reduction potential.
According to the Förster mechanism [34], the ENT rate constant
depends on the spectral overlap (J) between the fluorescence of
donor and the absorption of the acceptor as follows:
ꢀ
J =
F(ꢃ)ε(ꢃ)ꢃ−4dꢃ.
(5)
Although the absorption spectra of the porphyrin moiety were
slightly blue-shifted through the interaction with alcoholic sol-
vents, the calculated values of J were almost the same in the all
solvents used in this study (Table 4). Therefore, the switching of
ENT and ELT should be due to the change of ELT rate constant
rather than the increase of ENT rate constant. The interaction
between the porphyrin and alcoholic solvent decreases the ELT rate,
resulting in an enhancement of the ENT. Indeed, the ENT rate con-
stants were rarely affected by the kind of solvents. The ELT rate
constants in non-alcoholic solvents became larger than those in
alcoholic solvents. The P(V)porphyrin is a strong electron acceptor
by the central cationic phosphorus atom [37–40]. The strength as
an electron acceptor depends on the electron affinity of an axial
ligand connected at the central phosphorus atom. The CT state
energies of Py-POH in various solvents calculated from the experi-
mental value in acetonitrile using the dielectric continuum theory
[32,41] decreases with dielectric constant of solvents (Fig. 8). The
CT state energies in alcoholic solvents estimated from the experi-
mental value in methanol by the above method are clearly higher
than those in non-alcoholic solvents and comparable with that in
toluene, least polar solvent used in this study. These findings sug-
gest that the hydrogen bonding interaction affects the increase of
4. Discussion
In the photoexcited state of the pyrene of Py-POH, ENT to the
porphyrin competes with ELT to the porphyrin as shown in Fig. 6.
The ˚ELT increased with an increase in the solvent polarity in non-
alcoholic solvents due to the lowering of CT state energy. ENT
preferentially occurs in alcoholic solvents independent of solvent
polarity. The calculated critical distance of ENT from the photoex-
cited pyrene moiety to the P(V) porphyrin through dipole-dipole
interaction (Förster mechanism) is 42 Å [34,35]. Because the center-
to-center distance between the pyrene moiety and the porphyrin
ring, which was estimated from the ab initio MO calculation, is 8.5 Å,
the high yield of ENT reasonably occurs in this molecular system in
the absent of ELT process. The ˚ENT increased with an increase of
the ratio of methanol in the mixture of acetonitrile/methanol. The
absorption and fluorescence peaks of the porphyrin ring in alcoholic
solvents were blue-shifted compared to those in non-alcoholic
solvents, suggesting that the electronic state of the porphyrin moi-
ety is affected by the interaction with alcoholic solvents. Ab initio
MO calculation has shown that the electron affinity of the por-
Fig. 7. Proposed scheme of the hydrogen bonding interaction between the axial hydroxyl group of Py-POH and alcoholic solvent molecule.

78
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Fig. 8. Dependence of the calculated CT state energy on dielectric constant of sol-
vent.
CT energy level rather than the effect of solvent polarity. The driving
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of the pyrene moiety, which was estimated from the fluorescence
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theory (Table 4) [42,43], suggesting that the ELT of this system is in
the inverted region. However, the ELT of this system was enhanced
depending on the −ꢂG. The flexibility of the bridge butoxy chain
between the pyrene and porphyrin might change the ꢁ, resulting
in the enhancement of ELT depending on the −ꢂG.
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In summary, this study has shown that hydrogen bonding
between the axial hydroxyl group of Py-POH and surroundings can
control the competition process between the intramolecular ENT
and ELT. Hydrogen bonding interaction showed larger contribution
to the selection of these processes rather than solvent polarity.
Acknowledgement
This work was supported by a Grant-in-Aid from the Ministry of
Education, Culture, Sports, Science and Technology (MEXT) of the
Japanese Government.
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1/2
oxidation and reduction potentials, respectively, Z1 is the charge of the electron
donor, Z2 is the charge of the electron acceptor, and ε0 is vacuum permeability.
The distance between the pyrene and porphyrin moieties (r = 8.5 Å) was esti-
mated from the ab initio MO calculation. Because the positive charge on the
P(V) porphyrin ring is neutralized by the ELT, the Z2 becomes zero and ECT can
be calculated by the simple equation: ECT = E1+/2 − E1−/2
.

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)x was calculated from the following equa-
(128ꢄ5NAn4) F(ꢃ)ε(ꢃ)ꢃ−4dꢃ where Ä is geometrical factor, NA is the Avogadro
tion: (ECT
)
= (ECT
)
+ (e2/4ꢄε0)((1/εx) − (1/εm))(1/2rpy) where (ECT
)
m
is the
x
m
constant, F(ꢃ) is the normalized fluorescence spectrum of the pyrene moiety,
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calculation. Because the positive charge on the P(V) porphyrin ring is neutral-
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ꢁ
was calculated from the following equation:
ꢁ = (e2/4ꢄε0)((1/n2) − (1/ε))(1/2rpy). According to the above mentioned
reason, the factors of the porphyrin ionic radius and the distance between the
pyrene and porphyrin moieties were neglected.