Laser photolysis studies of the photochemical reactions of chromium(III) octaethylporphyrin and tetramesitylporphyrin complexes in toluene solution

Article abstract of DOI:10.1021/ic0342696

A nanosecond laser photolysis study was carried out for the Cr(III) porphyrin complexes of 2,3,7,8,12,13,17,18-octaethylporphyrin, [Cr(OEP)(Cl)(L)], and of 5,10,15,20-tetramesitylporphyrin, [Cr(TMP)(Cl)(L)], in toluene containing water and an excess amount of L (L = axial ligand). The laser photolysis generates the triplet excited state of the parent complex as well as a five-coordinate complex, [Cr(porphyrin)(Cl)], produced by the photodissociation of the axial ligand L. The yields for the formation of the triplet state and the photodissociation of L are found to markedly depend on the nature of both L and porphyrin ligand. The five-coordinate [Cr(porphyrin)(Cl)] readily reacts with both H₂O and L in the bulk solution to give [Cr(porphyrin)(Cl)(H₂O) and [Cr(porphyrin)(Cl)(L)], respectively. The axial H₂O ligand in [Cr(porphyrin)(Cl)(H ₂O)] is then substituted by the ligand L to regenerate the original complex [Cr(porphyrin)(Cl)(L)]. In principle, the substitution reaction takes place by the dissociative mechanism: the first step is the dissociation of H₂O from [Cr(porphyrin)(Cl)(H₂O)], followed by the reaction of the five-coordinate [Cr(porphyrin)(Cl)] with the ligand L to regenerate [Cr(porphyrin)(Cl)(L)]. The rate constants for this ligand substitution reaction are found to exhibit bell-shaped ligand concentration dependence. The detailed kinetic analysis revealed that both ligands L and H₂O in toluene make a hydrogen bond with the axial H₂O ligand in [Cr(porphyrin)(Cl)(H₂O)] to yield dead-end complexes for the substitution reaction. The reaction mechanisms are discussed on the basis of the substituent effects of the porphyrin peripheral groups and the kinetic parameters determined from the temperature dependence of the rate constants.

Full text of DOI:10.1021/ic0342696

Inorg. Chem. 2003, 42, 6095−6105
Laser Photolysis Studies of the Photochemical Reactions of
Chromium(III) Octaethylporphyrin and Tetramesitylporphyrin Complexes
in Toluene Solution
Masahiko Inamo,* Kosuke Eba, Kayoko Nakano, and Norio Itoh
Department of Chemistry, Aichi UniVersity of Education, Kariya, Aichi 448-8542, Japan
Mikio Hoshino
The Institute of Physical and Chemical Research, Wako, Saitama 351-0189, Japan
Received March 12, 2003
A nanosecond laser photolysis study was carried out for the Cr(III) porphyrin complexes of 2,3,7,8,12,13,17,18-
octaethylporphyrin, [Cr(OEP)(Cl)(L)], and of 5,10,15,20-tetramesitylporphyrin, [Cr(TMP)(Cl)(L)], in toluene containing
water and an excess amount of L (L ) axial ligand). The laser photolysis generates the triplet excited state of the
parent complex as well as a five-coordinate complex, [Cr(porphyrin)(Cl)], produced by the photodissociation of the
axial ligand L. The yields for the formation of the triplet state and the photodissociation of L are found to markedly
depend on the nature of both L and porphyrin ligand. The five-coordinate [Cr(porphyrin)(Cl)] readily reacts with
both H O and L in the bulk solution to give [Cr(porphyrin)(Cl)(H O)] and [Cr(porphyrin)(Cl)(L)], respectively. The
2
2
axial H O ligand in [Cr(porphyrin)(Cl)(H O)] is then substituted by the ligand L to regenerate the original complex
2
2
[Cr(porphyrin)(Cl)(L)]. In principle, the substitution reaction takes place by the dissociative mechanism: the first
step is the dissociation of H O from [Cr(porphyrin)(Cl)(H O)], followed by the reaction of the five-coordinate
2
2
[Cr(porphyrin)(Cl)] with the ligand L to regenerate [Cr(porphyrin)(Cl)(L)]. The rate constants for this ligand substitution
reaction are found to exhibit bell-shaped ligand concentration dependence. The detailed kinetic analysis revealed
that both ligands L and H O in toluene make a hydrogen bond with the axial H O ligand in [Cr(porphyrin)(Cl)(H O)]
2
2
2
to yield dead-end complexes for the substitution reaction. The reaction mechanisms are discussed on the basis of
the substituent effects of the porphyrin peripheral groups and the kinetic parameters determined from the temperature
dependence of the rate constants.
Introduction
intersystem crossing, and intramolecular energy dissipation
processes. Thus, studies on the excited states of metallopor-
phyrins are indispensable for full understanding of photo-
induced axial ligand dissociation and association reactions.
We have been particularly interested in the dynamics of
the photoinduced axial ligand dissociation and association
reaction of the chromium(III) porphyrins. Theoretical and
photophysical studies revealed that the excited singlet and
triplet states of the porphyrin π system weakly interact with
The roles of natural metalloporphyrins are very important
in various biological systems such as electron transfer
catalysts of heme-containing enzymes, active sites in heme
proteins which reversibly bind dioxygen and carbon mon-
oxide, and the light-harvesting pigments in photosynthetic
processes. The photochemistry of synthetic metalloporphyrins
has been extensively investigated to elucidate the biological
functions of natural metalloporphyrins.1 Photochemical
processes such as photoinduced ligand dissociation are
strongly linked with various photophysical processes of the
excited-state molecules as luminescence, internal conversion,
4
2
4
the central chromium atom (S ) 3/2) to give S1, T1, T1,
and 6T1 excited states and that the lowest and second lowest
6
4
excited states, T1 and T1, are in thermal equilibrium.2,3
Meanwhile, laser photolysis of chromium(III) porphyrins in
* To whom correspondence should be addressed. E-mail: minamo@
auecc.aichi-edu.ac.jp.
(1) Sima, J. Struct. Bonding 1995, 84, 135.
(2) Gouterman, M.; Hanson, L. K.; Khalil, G.-E.; Leenstra, W. R.; Buchler,
J. W. J. Chem. Phys. 1975, 62, 2343.
(3) Harriman, A. J. Chem. Soc., Faraday Trans. 1 1982, 78, 2727.
10.1021/ic0342696 CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/27/2003
Inorganic Chemistry, Vol. 42, No. 19, 2003 6095

Inamo et al.
tylporphyrinato)chromium(III), [Cr(TMP)(Cl)(H2O)] (2), were pre-
pared and purified according to the synthetic procedure for the
chromium(III) 5,10,15,20-tetraphenylporphyrin complex [Cr(TPP)-
(Cl)(H2O)].6,26 Pyridine (Py, Wako Pure Chemicals) and 1-meth-
ylimdazole (1-MeIm, Wako Pure Chemicals) were dried over solid
potassium hydroxide and then distilled. 3-Cyanopyridine (3-CNPy,
Wako Pure Chemicals) was purified by vacuum sublimation.
Toluene (Wako Pure Chemicals) was dried over sodium metal and
then distilled. Anal. Calcd (found) for 1 (C36H46ClCrN4O): C, 67.17
(67.75); H, 7.26 (7.32); N, 8.78 (8.43); Cl, 5.55 (5.67). Calcd
(found) for 2 (C56H54ClCrN4O‚0.27CHCl3): C, 73.56 (73.53); H,
5.95 (6.29); N, 6.10 (6.02).
solution causes photoinduced axial ligand ejection as well
as other various photoinduced phenomena.4-11 Since the
photodissociated ligand undergoes a recombination reaction
to yield the parent metalloporphyrins, laser photolysis is one
of the key methods to elucidate the dynamics of the axial
ligand binding. An earlier study has shown that the axial
ligand of chromium(III) porphyrins is fairly labile compared
with those of other chromium(III) complexes having non-
porphyrin ligands.6 The high reactivity of the chromium-
(III) porphyrins coupled with the facile ligand substitution
reaction at the axial coordination site is of fundamental
interest for their chemistry in solution.12-24
The electronic structure and the reactivity of the metal-
loporphyrins in the excited and ground states are strongly
affected by the nature of the axial ligand as well as the
molecular structure of the porphyrin ligand.25 However, little
information is available on the relationship between the
porphyrin structure and the photochemical and photophysical
properties of the chromium(III) porphyrins. As a continuing
effort to develop our understanding of chromium(III) por-
phyrin chemistry, we have investigated the photochemistry
of complexes of 2,3,7,8,12,13,17,18-octaethylporphyrin,
[Cr(OEP)(Cl)(L)], and of 5,10,15,20-tetramesitylporphyrin,
[Cr(TMP)(Cl)(L)], in toluene (L ) axial ligand) using a
nanosecond laser photolysis technique. In this paper, we
describe the mechanism of the photoinduced reactions of
these chromium(III) porphyrins on the basis of the kinetics
of the reaction and the quantum yield measurements to gain
further insight into the properties of the excited states
responsible for the photoinduced dissociation of the axial
ligand.
Absorption spectra were recorded on a Hitachi U-3000 spectro-
photometer. A cryostat (Oxford DN1704) was used to measure the
absorption spectra at low temperatures. Laser photolysis studies
were carried out with a Nd:YAG laser (Surelite, Hoya-Continuum)
equipped with the second (532 nm) harmonic generator. The
duration of the 532 nm pulse was 6 ns. The detection system of
the transient spectra was described previously.6 The concentration
of the chromium(III) porphyrin complex in toluene was less than
1.0 × 10-5 mol kg-1, and that of the the axial ligand, more than
1.0 × 10-4 mol kg-1. The concentration of water in the toluene
solution was determined by a Karl Fischer titrator (CA-06,
Mitsubishi Chemicals). The O2 concentration in toluene solutions
was determined by measuring the O2 pressure with a mercury
manometer. The Bunsen coefficient of O2 in toluene is 0.22 at 25.0
°C.27 Polystyrene films dissolved chromium(III) porphyrins were
prepared according to the method previously reported.7
The experimental pseudo-first-order rate constant kobsd was
obtained from the nonlinear least-squares analysis of the transient-
decay curve observed after the laser pulse. The decay curves were
averaged several times on the digital oscilloscope. The estimated
standard deviation of kobsd was less than (3%.
Results
Experimental Section
Chloro(2,3,7,8,12,13,17,18-octaethylporphyrinato)chromium-
(III), [Cr(OEP)(Cl)(H2O)] (1), and chloro(5,10,15,20-tetramesi-
Photoreaction of [Cr(OEP)(Cl)(H2O)] and [Cr(TMP)-
(Cl)(H2O)]. We previously reported that the chromium(III)
tetraphenylporphyrin complex exists as [Cr(TPP)(Cl)(H2O)]
(TPP ) 5,10,15,20-tetraphenylporphyrin) in a toluene solu-
tion containing 1 × 10-3 mol kg-1 water.6 Laser irradiation
causes the photodissociation of H2O from [Cr(TPP)(Cl)-
(H2O)], followed by the recombination reaction with H2O
to regenerate the initial complex. In the present study, similar
experiments were performed for the OEP and TMP com-
plexes. UV-visible absorption spectra are shown in Figure
1 for [Cr(TMP)(Cl)(H2O)] and in Figure S1 for [Cr(OEP)-
(Cl)(H2O)], together with the transient spectra observed after
the laser pulse. The spectrum of [Cr(TMP)(Cl)(H2O)] revers-
ibly changes with temperature, and a new Soret band appears
in the 430 nm region with an increase in the temperature. In
the inset of Figure 1, the transient spectrum observed after
the laser pulse is shown together with the difference spectrum
obtained by subtracting the spectrum measured at -40 °C
from that at 40 °C. These two spectra are almost identical,
and therefore the blue-shifted band observed at the higher
temperatures can be attributed to the five-coordinate species,
(4) Yamaji, M.; Hama, Y.; Hoshino, M. Chem. Phys. Lett. 1990, 165,
309.
(5) Yamaji, M. Inorg. Chem. 1991, 30, 2949.
(6) Inamo, M.; Hoshino, M.; Nakajima, K.; Aizawa, S.; Funahashi, S.
Bull. Chem. Soc. Jpn. 1995, 68, 2293.
(7) Hoshino, M.; Tezuka, N.; Inamo, M. J. Phys. Chem. 1996, 100, 627.
(8) Hoshino, M.; Nagamori, T.; Seki, H.; Chihara, T.; Tase, T.; Wakatsuki,
Y.; Inamo, M. J. Phys. Chem. A 1998, 102, 1297.
(9) Inamo, M.; Hoshino, M. Photochem. Photobiol. 1999, 70, 596.
(10) Inamo, M.; Nakaba, H.; Nakajima, K.; Hoshino, M. Inorg. Chem. 2000,
39, 4417.
(11) Jeoung, S. C.; Kim, D.; Cho, D. W.; Yoon, M. J. Phys. Chem. A 2000,
104, 4816.
(12) Fleischer, E. B.; Krishnamurthy, M. J. Am. Chem. Soc. 1971, 93, 3784.
(13) Fleischer, E. B.; Krishnamurthy, M. J. Coord. Chem. 1972, 2, 89.
(14) Krishnamurthy, M. Inorg. Chim. Acta 1978, 26, 137.
(15) Ashley, K. R.; Leipoldt, J. G.; Joshi, V. K. Inorg. Chem. 1980, 19,
1608.
(16) Leipoldt, J. G.; Basson, S. S.; Rabie, D. R. J. Inorg. Nucl. Chem.
1981, 43, 3239.
(17) O’Brien P.; Sweigart, D. A. Inorg. Chem. 1982, 21, 2094.
(18) Leipoldt, J. G.; van Eldik, R.; Kelm, H. Inorg. Chem. 1983, 22, 4146.
(19) Leipoldt, J. G.; Meyer, H. Polyhedron 1987, 6, 1361.
(20) Ashley, K. R.; Kuo, J. Inorg. Chem. 1988, 27, 3556.
(21) Ashley, K. R.; Trent, I. Inorg. Chim. Acta 1989, 163, 159.
(22) Inamo, M.; Sumi, T.; Nakagawa, N.; Funahashi, S.; Tanaka, M. Inorg.
Chem. 1989, 28, 2688.
(23) Inamo, M.; Sugiura, S.; Fukuyama, H.; Funahashi, S. Bull. Chem. Soc.
Jpn. 1994, 67, 1848.
(24) Inamo, M.; Nakajima, K. Bull. Chem. Soc. Jpn. 1998, 71, 883.
(25) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic
Press: New York, 1978; Vol. III, Chapter 1.
(26) Summerville, D. A.; Jones, R. D.; Hoffman, B. M.; Basolo, F. J. Am.
Chem. Soc. 1977, 99, 8195.
(27) Battino, R., Ed. IUPAC Solubility Data Series; Pergamon Press:
Oxford, U.K., 1981; Vol. 7.
6096 Inorganic Chemistry, Vol. 42, No. 19, 2003

Chromium(III) Octaethylporphyrin Complexes
Figure 1. UV-visible absorption spectra of the toluene solution of the
Cr(III)-TMP complex. T/°C ) -40 (a), -20 (b), 0 (c), 20 (d), and 40 (e).
[H2O] ) 4.24 × 10-3 mol kg-1. In the inset, the transient spectrum of the
toluene solution of the Cr(III)-TMP complex observed just after the laser
irradiation (b) and the differences between the spectrum of the Cr(III)-
TMP complex measured at 40 °C and that measured at -40 °C (solid line)
are shown.
[Cr(TMP)(Cl)]. On the other hand, the spectral changes are
not found for [Cr(OEP)(Cl)(H2O)] in the temperature range
from -40 to 40 °C. These findings indicate that the H2O
molecule in [Cr(OEP)(Cl)(H2O)] binds to the central Cr(III)
atom more tightly than that in [Cr(TMP)(Cl)(H2O)].
The transient spectra shown in Figure 1 and Figure S1
decay according to first-order kinetics without the formation
of permanent photoproducts. Figure 2 shows the plots of the
pseudo-first-order rate constants, kobsd, for the decay of the
transient spectra represented as a function of the water
concentration. The pseudo-first-order rate constant increases
with an increase in the water concentration. These observa-
tions indicate that the five-coordinate species, [Cr(TMP)-
(Cl)] and [Cr(OEP)(Cl)], produced by the laser photolysis
undergo the recombination reaction with H2O, returning
to the six-coordinate species, [Cr(TMP)(Cl)(H2O)] and
[Cr(OEP)(Cl)(H2O)]. The decay of the transient spectrum
can be attributed to the following:
Figure 2. Dependence of the pseudo-first-order rate constant kobsd of the
decay of the transient spectrum for [Cr(porphyrin)(Cl)(H2O)] on the
concentration of H2O in toluene for the TMP (A) and OEP (B) complexes.
T/°C ) 15.0 (a), 25.0 (b), and 35.0 (c). Solid and dotted lines represent the
calculated curves for eq 12 and the second term of the right-hand side of
eq 12, respectively.
axial H2O in [Cr(TMP)(Cl)(H2O)] dissociates more easily
than that in [Cr(OEP)(Cl)(H2O)]. This is supported by the
facts that, as shown in Figure 1, the TMP complex in toluene
exists as the mixture of [Cr(TMP)(Cl)(H2O)] and [Cr(TMP)-
(Cl)] when [H2O] ) 4.24 × 10-3 mol kg-1 and that the OEP
complex exists solely as [Cr(OEP)(Cl)(H2O)].
kH2
[Cr(porphyrin)(Cl)] + H2O y Oz [Cr(porphyrin)(Cl)(H2O)]
k-H
O
2
(1)
We observed merely the photodissociation of the axial H2O
from [Cr(porphyrin)(Cl)(H2O)]. No any other transient
species such as the excited state of the five-coordinate
complex, [Cr(porphyrin)(Cl)], or the triplet excited state of
[Cr(porphyrin)(Cl)(H2O)] could be detected. These findings
indicate that the lifetimes of these excited states are too short
to be observed by the present nanosecond laser photolysis
technique.
Photoreaction of [Cr(OEP)(Cl)(L)] (L ) Py, 1-MeIm).
The chromium(III) porphyrins exist as [Cr(porphyrin)(Cl)-
(L)] in the toluene solution containing an excess amount of
a ligand L such as Py and 1-MeIm. Figure 3 shows the
transient spectra for [Cr(OEP)(Cl)(Py)] and [Cr(OEP)(Cl)-
(1-MeIm)] in toluene and in polystyrene film observed after
a 532-nm laser irradiation. In the polystyrene film, the
transient species of the 1-MeIm complex has a strong
Thus, kobsd is given by
kobsd ) kH O[H2O] + k-H O
(2)
2
2
where kH O and k-H O represent the second-order rate constant
2
2
for the water association reaction and the first-order rate
constant for the water dissociation reaction, respectively. The
plots of kobsd vs [H2O] in Figure 2 give straight lines in
appearance. However, as will be mentioned later, the reaction
1 is more or less complicated due to the existence of
[Cr(porphyrin)(Cl)(HOH‚‚‚OH2)], in which a water molecule
is bound to the coordinate H2O ligand by hydrogen bonding.
The detailed kinetic analysis will be described later.
The intercept of the line in Figure 2 for the TMP complex
is larger than that of the OEP complex, indicating that the
Inorganic Chemistry, Vol. 42, No. 19, 2003 6097

Inamo et al.
The latter species corresponds to the difference between the
transient spectrum in the polystyrene film (spectrum c in
Figure 3A) and that in the toluene solution (spectrum a in
Figure 3A). The second transient spectrum of [Cr(OEP)(Cl)-
(1-MeIm)] in the toluene solution measured at 690 ns after
the laser pulse (spectrum b in Figure 3A) is in good
agreement with the difference spectrum between [Cr(OEP)-
(Cl)(H2O)] and [Cr(OEP)(Cl)(1-MeIm)] in toluene as shown
in Figure S2. Thus, the second transient species is ascribed
to [Cr(OEP)(Cl)(H2O)], which is produced by the reaction
between the transient [Cr(OEP)(Cl)] and H2O. The interme-
diate, [Cr(OEP)(Cl)(H2O)], eventually returns to [Cr(OEP)-
(Cl)(1-MeIm)] by the axial ligand substitution reaction with
1-MeIm in the solution. The photophysical and photochemi-
cal processes of [Cr(OEP)(Cl)(1-MeIm)] are similar to that
of [Cr(TPP)(Cl)(Py)],10 and the reaction mechanism can be
expressed by Scheme 1.
According to Scheme 1, the fast process includes four
processes, i.e., the dissociation of the axial ligand from the
triplet excited state (kdiss), intersystem crossing of the triplet
excited state to the ground singlet state (kT-S), and the
reactions of the five-coordinate complex [Cr(OEP)(Cl)] with
H2O (kH O) and 1-MeIm (kL).
2
The transient spectrum observed for [Cr(OEP)(Cl)(Py)]
decays via a biphasic process as shown in Figure 3. The
typical absorption band of the triplet excited state around
480 nm was not observed in the transient spectrum taken at
29 ns after the laser pulse (spectrum a in Figure 3B), while
the absorption band around 430 nm, which can be attributed
to the five-coordinate [Cr(OEP)(Cl)], is much more intense
than that observed for [Cr(OEP)(Cl)(1-MeIm)]. These find-
ings indicate that the quantum yield for the photodissociation
of the axial Py ligand in the singlet excited state is so high
that the intersystem crossing leading to the generation of the
triplet excited state becomes negligible or that the lifetime
of the triplet excited state is extremely short due to the
efficient photodissociation of the axial Py ligand. From these
results, we conclude that the first transient detected at 29 ns
after the pulse is solely ascribed to the five-coordinate
[Cr(OEP)(Cl)]. Since the second transient spectrum taken
at 725 ns after the pulse is quite similar to the difference
spectrum between [Cr(OEP)(Cl)(H2O)] and [Cr(OEP)(Cl)-
(Py)], the second transient is attributed to [Cr(OEP)(Cl)-
(H2O)]. The decay of the first transient spectrum to the
second one obeys first-order kinetics with respect to the
complex, and it corresponds to the reaction of [Cr(OEP)-
(Cl)] with Py and H2O in the bulk solution to generate
[Cr(OEP)(Cl)(Py)] and [Cr(OEP)(Cl)(H2O)], respectively.
Reaction paths (kdiss and kT-S) for the triplet excited state
are missing from the fast process of the Py complex due to
the quite low quantum yield of the triplet excited state.
Photoreaction of [Cr(TMP)(Cl)(L)] (L ) Py, 1-MeIm).
The transient spectra observed for [Cr(TMP)(Cl)(Py)] in
toluene and in a polystyrene film after a 532-nm laser pulse
are shown in Figure 4. In the polystyrene film, the transient
species ascribed to the triplet excited states (6T1 and 4T1) of
[Cr(TMP)(Cl)(Py)] decays according to first-order kinetics
with a rate constant of 8.0 × 106 s-1 at 25.0 °C without the
Figure 3. Transient spectra observed for the toluene solution of [Cr(OEP)-
(Cl)(1-MeIm)] in the presence of 5.82 × 10-3 mol kg-1 1-MeIm and
5.87 × 10-3 mol kg-1 H2O (A) and for that of [Cr(OEP)(Cl)(Py)] in the
presence of 4.03 × 10-3 mol kg-1 Py and 5.29 × 10-3 mol kg-1 H2O (B)
at 25.0 °C. The spectra were taken at 29 ns (a) and 690 ns (b) for (A) and
at 29 ns (a) and 725 ns (b) for (B) after a 532 nm laser irradiation. The
transient spectra observed for the polystyrene film containing [Cr(OEP)-
(Cl)(1-MeIm)] taken at 59 ns and that of [Cr(OEP)(Cl)(Py)] taken at 229
ns are also shown as spectrum c for each case.
negative absorption around 440 nm. Since the photodisso-
ciation of the axial ligand of the complex does not occur in
the polystyrene film, this species can be ascribed to the 6T1
excited state of [Cr(OEP)(Cl)(1-MeIm)], which is in thermal
4
equilibrium with the T1 state.2,7 The triplet excited state,
6T1, decays according to first-order kinetics with a rate
constant of 3.7 × 106 s-1 at 25.0 °C. On the other hand, the
decay of the transient spectrum in the toluene solution was
found to be biphasic. The faster step of the decay, which
we call the “fast process”, is completed within 10-6 s after
the laser pulse, and a much slower process (“slow process”)
follows it. As shown in Figure 3A, the transient spectrum in
the toluene solution measured at 29 ns after the laser pulse
is similar to that in the polystyrene film with a slight
difference around the 430 nm region. These findings indicate
that, as in the case of [Cr(TPP)(Cl)(Py)],9 the transient
spectrum measured at 29 ns can be ascribed to both the trip-
let excited state of [Cr(OEP)(Cl)(1-MeIm)] and the five-
coordinate species [Cr(OEP)(Cl)] given by the photodisso-
ciation of 1-MeIm from [Cr(OEP)(Cl)(1-MeIm)] in toluene.
6098 Inorganic Chemistry, Vol. 42, No. 19, 2003

Chromium(III) Octaethylporphyrin Complexes
Scheme 1
[Cr(TMP)(Cl)(Py)]. Scheme 1 shows the mechanism of these
photophysical and photochemical processes.
The laser photolysis of [Cr(TMP)(Cl)(1-MeIm)] in the
toluene solution gives solely the triplet excited state, which
decays according to first-order kinetics with a rate constant
of 1.01 × 107 s-1 at 25.0 °C. Evidence for the photodisso-
ciation of the axial 1-MeIm ligand was hardly obtained by
the present nanosecond laser photolysis. Presumably, the
strong bond between Cr and 1-MeIm prohibits the photo-
dissociation of the axial 1-MeIm.
Kinetics of the Fast Process. Figure 5 shows the time
profiles of the absorbance changes for the fast process of
[Cr(OEP)(Cl)(1-MeIm)] in the toluene solution. The absor-
bance at 420 nm initially increases and then decreases with
time, while those at 442 and 480 nm monotonically change
with time, indicating that consecutive reactions are included
in the fast process. On the basis of the mechanism shown in
Scheme 1, the first and second of these consecutive reactions
are attributed to the decay of the triplet excited state and the
reaction of the five-coordinate intermediate [Cr(OEP)(Cl)]
with 1-MeIm or H2O, respectively. The spectral changes in
the range between 380 and 520 nm were analyzed using the
SPECFIT/32 program with a kinetic model of consecutive
reactions, A f B f C, and the rate constants of each process
were determined.28,29 The values of the pseudo-first-order
rate constants of each reaction, i.e., kobsd1 for the reaction
A f B and kobsd2 for the reaction B f C, were determined
as kobsd1 ) 3.9 × 107 s-1 and kobsd2 ) 9.2 × 106 s-1 (T )
25.0 °C). kobsd1 can be expressed as kobsd1 ) kT-S + kdiss on
the basis of the mechanism of Scheme 1. The rate constant,
3.7 × 106 s-1, for the decay of the triplet excited state of
[Cr(OEP)(Cl)(1-MeIm)] in the polystyrene film is smaller
than kobsd1 in toluene because the reaction in the solid matrix
Figure 4. Transient spectra observed for the toluene solution of [Cr(TMP)-
(Cl)(Py)] in the presence of 7.55 × 10-3 mol kg-1 Py and 5.38 × 10-3
mol kg-1 H2O at 25.0 °C. The spectra were taken at 29 ns (a) and 483 ns
(b) after a 532 nm laser irradiation. The transient spectrum observed for
the polystyrene film containing [Cr(TMP)(Cl)(Py)] taken at 39 ns is also
shown (c).
dissociation of the axial ligand. On the other hand, as in the
case of the OEP complex, the decay of the transient spectrum
in the toluene solution containing excess water and pyridine
was found to be biphasic. The fast step of the decay is
completed within 10-6 s after the laser pulse, followed by a
much slower reaction. The first transient spectrum measured
at 29 ns after the laser pulse indicates that both the triplet
excited state of [Cr(TMP)(Cl)(Py)] and the five-coordinate
species [Cr(TMP)(Cl)] are produced by the laser photolysis
of [Cr(TMP)(Cl)(Py)]. The second transient spectrum in the
toluene solution measured at 483 ns after the laser pulse
is in good agreement with the difference spectrum be-
tween [Cr(TMP)(Cl)(H2O)] and [Cr(TMP)(Cl)(Py)]. Thus,
the second transient species is ascribed to [Cr(TMP)(Cl)-
(H2O)]. The axial ligand substitution reaction of [Cr(TMP)-
(Cl)(H2O)] with pyridine regenerates the parent complex,
(28) Gampp, H.; Maeder, M.; Meyer, C. J.; Zuberbu¨hler, A. D. Talanta
1985, 32, 95.
(29) SPECFIT/32; Spectrum Software Associates: Marlborough, MA.
Inorganic Chemistry, Vol. 42, No. 19, 2003 6099

Inamo et al.
Figure 6. Plot of the pseudo-first-order rate constant kobsd of the decay of
[Cr(TMP)(Cl)(H2O)] as a function of the concentration of pyridine for the
laser photolysis of [Cr(TMP)(Cl)(Py)] in toluene. CH O/mol kg-1 ) 3.1 ×
and 1.91 × 10-2 (f). T ) 25.0 °C.
Figure 5. Change in absorbance of the toluene solution of [Cr(OEP)(Cl)-
(1-MeIm)] caused by a 532 nm laser irradiation. Conditions are the same
as those of Figure 3.
2
10-3 (a), 4.7 × 10-3 (b), 7.1 × 10-3 (c), 9.1 × 10-3 (d), 1.11 × 10-2 (e),
substitution reaction of [Cr(TPP)(Cl)(H2O)] by a ligand L
such as pyridine.10 This reaction was found to occur
according to the dissociative mechanism with a dead-end
complex formation as a preequilibrium of the substitution
reaction:
does not include the dissociation of the axial 1-MeIm ligand.
Meanwhile, kobsd2 is given by kH O[H2O] + k1-MeIm[1-MeIm].
2
Second-order rate constants, kH O and k1-MeIm, can be deter-
2
mined from the kinetic analysis of the slow process for the
photoreaction of [Cr(OEP)(Cl)(1-MeIm)]. As will be men-
tioned in the next section, we obtained kH O ) 1.52 × 109
2
KL
mol-1 kg s-1 and k1-MeIm ) 5.56 × 108 mol-1 kg s-1 at T )
25.0 °C. The kobsd2 value, 1.2 × 107 s-1, calculated by using
these values, [H2O] ) 5.87 × 10-3 mol kg-1 and [1-MeIm]
) 5.82 × 10-3 mol kg-1, agrees well with the value
determined by the present SPECFIT/32 analysis.
[Cr(TPP)(Cl)(H2O)] + Py y\z [Cr(TPP)(Cl)(HOH‚‚‚Py)]
(3)
k-H
[Cr(TPP)(Cl)(H2O)] y 2Oz [Cr(TPP)(Cl)] + H2O (4)
kH2
O
This analysis also provides the spectra of each species as
the difference spectrum (transient species minus [Cr(OEP)-
(Cl)(1-MeIm)]). The transient spectra obtained are shown
in Figure S5. It is found that the triplet excited state of
[Cr(OEP)(Cl)(1-MeIm)] in toluene (spectrum a in Figure S5)
has almost the same spectrum as that in the polystyrene film
(spectrum d in Figure S5). The spectrum of [Cr(OEP)(Cl)]
(spectrum b in Figure S5) has a positive peak around 420
nm and bleaching of the Soret band of the parent complex,
indicating that the Soret band is blue shifted due to the
dissociation of the axial ligand.
For the fast process of the photoreaction of [Cr(TMP)-
(Cl)(Py)] in toluene, SPECFIT/32 analysis could not be
successful probably because of the complicated decay profile
owing to the overlaps of the fast process and the following
ligand substitution reaction, which will be mentioned later.
Kinetics of the Slow Process. As mentioned above, the
slow process can be ascribed to the axial water substitution
reaction of the intermediate, [Cr(porphyrin)(Cl)(H2O)], by
the nitrogenous base L in the solution. The pseudo-first-order
rate constant, kobsd, of the axial substitution reaction was
measured in the presence of an excess amount of L. Figure
6 shows the plots of the rate constant kobsd vs [Py] at several
concentrations of H2O, obtained for the slow process of the
photoreaction of [Cr(TMP)(Cl)(Py)] in toluene at 25.0 °C.
Results obtained at 15.0 and 35.0 °C are shown in Figure
S6. We previously reported the kinetics of the axial water
kL
[Cr(TPP)(Cl)] + Py
98 [Cr(TPP)(Cl)(Py)]
(5)
The dead-end complex, [Cr(TPP)(Cl)(HOH‚‚‚Py)], represents
the complex in which the Py molecule binds to the hydrogen
atom of the coordinating H2O ligand in [Cr(TPP)(Cl)(H2O)]
by hydrogen bonding. By application of the steady-state
approximation to the five-coordinate intermediate, [Cr(TPP)-
(Cl)], the pseudo-first-order rate constant, kobsd, is formulated
as
kobsd ) k-H OkL[Py](kH O[H2O] + kL[Py])-1(1 + KL[Py])-1
2
2
(6)
The axial water substitution reactions of the TMP and OEP
complexes by L are expected to occur via a similar
dissociative mechanism. However, the kinetic results shown
in Figure 6 and Figure S6 could not be satisfactorily
explained by eq 6,30 indicating that the steady-state ap-
proximation to the five-coordinate intermediate is inappropri-
ate in the present case. This is because the assumptions used
in the steady-state approximation, i.e., k-H O , kH O[H2O]
2
2
and k-H O , kL[L], are not satisfied due to relatively large
k-H O value for the TMP complex. Instead, the consecutive
2
2
reactions were considered:
(30) Maximum deviation between the obtained rate constant and the
calculated value based on eq 6 exceeds 30%.
6100 Inorganic Chemistry, Vol. 42, No. 19, 2003

Chromium(III) Octaethylporphyrin Complexes
Table 1. Kinetic and Thermodynamic Parametersa for the Water
Substitution Reaction of [Cr(porphyrin)(Cl)(H2O)] by the Nitrogen Base
L at T ) 25.0 °Cb
k12
k23
98 A3
A1 y
\
k21
z A2
(7)
porphyrin
Here A1, A2, and A3 correspond to [Cr(TMP)(Cl)(H2O)],
[Cr(TMP)(Cl)], and [Cr(TMP)(Cl)(L)], respectively.31,32 In
eq 7, the backward reaction from A3 to A2 was neglected
because the rate constant is extremely smaller than k12, k21,
and k23.33 When the concentration of Py is high and thus the
chromium species is completely transformed to [Cr(TMP)-
(Cl)(L)], the absorbance of the reacting solution, A, can be
expressed as
TMP
OEP
k-H O/s-1
6.96 × 106
43.9 ( 2.0
33.3 ( 6.5
4.86 × 105
60.5 ( 1.3
66.8 ( 4.5
1.52 × 109
6.6 ( 1.2
2
q
∆H-H O
2
q
∆S-H O
2
k
H O/mol-1 kg s-1 1.39 × 109
2
q
∆HH O
3.1 ( 1.3
2
q
∆SH O
-59.3 ( 4.2
-47.0 ( 3.9
2
kL/mol-1 kg s-1
4.62 × 108 (L ) Py)
3.88 × 108 (L ) 3-CNPy)
6.6 ( 1.5 (L ) Py)
8.0 ( 1.7 (L ) 3-CNPy)
-56.8 ( 5.1 (L ) Py)
7.42 × 108 (L ) Py)
5.56 × 108 (L ) 1-MeIm)
9.3 ( 1.3 (L ) Py)
8.6 ( 1.3 (L ) 1-MeIm)
-43.8 ( 4.4 (L ) Py)
q
∆HL
A ) P exp(-λ2t) + Q exp(-λ3t) + R
(8)
q
∆SL
-53.5 ( 5.6 (L ) 3-CNPy) -48.7 ( 4.2 (L ) 1-MeIm)
λ2 ) 0.5{k12 + k21 + k23 + [(k12 + k21 + k23)2 - 4k12k23]1/2}
KL/mol-1 kg
∆HL°
8.30 × 102 (L ) Py)
1.65 × 102 (L ) Py)
7.85 × 10 (L ) 3-CNPy)
-38.3 ( 2.0 (L ) Py)
5.15 × 102 (L ) 1-MeIm)
-29.9 ( 1.3 (L ) Py)
(9)
-29.6 ( 2.2 (L ) 3-CNPy) -28.7 ( 1.2 (L ) 1-MeIm)
-72.5 ( 6.5 (L ) Py) -57.8 ( 4.4 (L ) Py)
-62.9 ( 7.3 (L ) 3-CNPy) -44.2 ( 3.8 (L ) 1-MeIm)
λ3 ) 0.5{k12 + k21 + k23 - [(k12 + k21 + k23)2 - 4k12k23]1/2}
∆SL°
(10)
K
H O/mol-1 kg
4.92 × 10
-32.8 ( 3.4
-78 ( 11
c
c
c
2
∆HH O
°
°
2
where terms P, Q, and R are the functions of the rate
constants (k12, k21, k23), molar absorption coefficients of each
species, and the initial concentration of A1 (see Supporting
Information). Typical concentration-time curves for the
present consecutive reactions are shown in Figure S7 using
the rate constants, which will be given later in Table 1. The
first step of the consecutive reactions ends within 500 ns.
Furthermore, the reaction curve of this first step is compli-
cated because of the overlaps of the fast process. Accord-
ingly, we determined the rate constant, λ3, from the kinetic
analysis of the slower decay, which strictly followed first-
order kinetics, by omitting the initial part of the reaction
curve of the slow process. The pseudo-first-order rate
constant, kobsd, shown in Figure 6 corresponds to the rate
constant, λ3.
In the present case, preequilibrium of the dead-end
complex formation between [Cr(TMP)(Cl)(H2O)] and an
external ligand such as Py should be considered as in the
case of the TPP complex.10 Furthermore, if another dead-
end complex formation between [Cr(TMP)(Cl)(H2O)] and
an H2O molecule (eq 11) is considered, the obtained pseudo-
first-order rate constant is more satisfactorily explained.
∆SH O
2
a ∆H/kJ mol-1; ∆S/J mol-1 K-1
.
b Values of the rate and equilibrium
constants were calculated by using the determined ∆Hq, ∆Sq, ∆H°, and
∆S°. c Term KH O is not necessarily included in the rate law to explain the
2
kinetic results.
Chart 1
The kH O and k-H O terms included in eq 1 are identical
2
2
with those in the present substitution reaction. The second
term of the right-hand side of eq 2 should be modified
because of the formation of the dead-end complex, [Cr(TMP)-
(Cl)(HOH‚‚‚OH2)], and the pseudo-first-order rate constant
kobsd in eq 2 can be rewritten as
kobsd ) kH O[H2O] + k-H O(1 + KH O[H2O])-1 (12)
2
2
2
The kinetic results of the slow process of the photoreaction
of [Cr(TMP)(Cl)(Py)] as well as [Cr(TMP)(Cl)(3-CNPy)]
(Figure S8) were simultaneously analyzed together with those
of the photoreaction of [Cr(TMP)(Cl)(H2O)] using a weighted-
least-squares fitting calculation based on eqs 10 and 12. The
kinetic and thermodynamic parameters obtained are listed
in Table 1. Solid lines in Figures 2, 6, S6, and S8 were
calculated using the kinetic parameters obtained. A similar
calculation was applied to the slow process of [Cr(OEP)-
(Cl)(Py)] and [Cr(OEP)(Cl)(1-MeIm)] and the photoreaction
of [Cr(OEP)(Cl)(H2O)], and the kinetic parameters obtained
are shown in Table 1. The ligand concentration dependence
of the obtained rate constant kobsd for the OEP complexes is
shown in Figures 2, S9, and S10.
KH
[Cr(TMP)(Cl)(H2O)] + H2O y 2Oz
[Cr(TMP)(Cl)(HOH‚‚‚OH2)] (11)
The proposed structure of the dead-end complexes is
shown in Chart 1. Taking the formation of these two dead-
end complexes into account, the rate constants, k12, k21, and
k23, in the pseudo-first-order rate constant λ3 are formulated
as k12 ) k-H O(1 + KL[L] + KH O[H2O])-1, k21 ) kH O[H2O],
2
2
2
and k23 ) kL[L].
(31) Matsen, F. A.; Franklin, J. L. J. Am. Chem. Soc. 1950, 72, 3337.
(32) Moore, J. W.; Pearson, R. G. Kinetics and Mechanism; Wiley-
Interscience Publications: New York, 1981.
(33) The rate constant of the dissociation of the axial Py ligand from
[Cr(TMP)(Cl)(Py)] was determined as 2.6 s-1 (T ) 25.0 °C) in the
kinetic study on the axial substitution reaction of [Cr(TMP)(Cl)(Py)]
by 1-MeIm in the toluene solution. These results will be published
elsewhere.
The presence of the dead-end complex, [Cr(TMP)(Cl)-
(HOH‚‚‚OH2)], changes the formulation of kobsd from eq 2
to eq 12. The contribution of the second term of the right-
hand side of eq 12 to the rate constant kobsd is shown by the
Inorganic Chemistry, Vol. 42, No. 19, 2003 6101

Inamo et al.
dotted lines in Figure 2. The dead-end complex formation
clearly affects the kinetics for the recombination reaction
between [Cr(TMP)(Cl)] and H2O. On the other hand, the
formation of this type of dead-end complex is negligible for
the OEP complex under the present experimental conditions
probably due to a much smaller KH O value (see Table 1).
2
Quantum Yield Measurements. The quantum yield, Φdiss
,
for the photodissociation of the axial ligand L (L ) H2O,
Py, and 1-MeIm) from the OEP complex [Cr(OEP)(Cl)(L)]
in toluene was determined by laser flash photolysis. The
yield, Φdiss, of [Cr(OEP)(Cl)(1-MeIm)] was evaluated from
the yield, ΦH O, for the formation of the intermediate species,
2
[Cr(OEP)(Cl)(H2O)]. Equation 13 represents the relationship
between Φdiss and ΦH O
:
2
Figure 7. Effect of dioxygen on the quantum yield for the photodisso-
ciation of L from [Cr(OEP)(Cl)(L)] in toluene at 25.0 °C.
Φdiss ) ΦH O(kH O[H2O] + kL[L])kH O-1[H2O]-1 (13)
2
2
2
Φdiss for the photodissociation of L from [Cr(OEP)(Cl)(L)]
(L ) H2O and Py) is expressed as
The initial transients observed at 29 ns after the laser pulse
are the triplet states of [Cr(OEP)(Cl)(1-MeIm)] and [Cr(OEP)-
(Cl)]. After the completion of the fast process, the residual
intermediate is solely ascribed to [Cr(OEP)(Cl)(H2O)]. Thus,
the absorbance change ∆A obtained immediately after the
disappearance of the initial transients is given by
Φdiss ) ∆Adiss(∆ꢀ2-1)Iabs-1NA
(17)
where ∆Adiss and ∆ꢀ2 are the absorbance change just after
laser excitation and the difference in the molar absorption
coefficient between [Cr(OEP)(Cl)] and [Cr(OEP)(Cl)(L)]
(L ) H2O and Py) at a monitoring wavelength, respectively.
The molar absorption coefficients of the five-coordinate
intermediate [Cr(OEP)(Cl)] at 470 nm was determined by
the method previously described.6 Figure S11 shows the
absorption spectrum of [Cr(OEP)(Cl)] as well as those of
[Cr(OEP)(Cl)(L)] (L ) H2O and Py). By using the benzene
-1
∆A ) ΦH O(∆ꢀ1)IabsNA
(14)
2
where Iabs, ∆ꢀ1, and NA represent the number of photons
absorbed by [Cr(OEP)(Cl)(1-MeIm)], the difference in the
molar absorption coefficients between [Cr(OEP)(Cl)(H2O)]
and [Cr(OEP)(Cl)(1-MeIm)], and Avogadro’s number, re-
spectively. The value of ∆ꢀ1 was determined as -1.14 ×
105 M-1 cm-1 at 444 nm from the absorption spectra of
[Cr(OEP)(Cl)(L)] (L ) H2O and 1-MeIm) in toluene. A
benzene solution of zinc(II) tetraphenylporphyrin, [Zn(TPP)],
which has the same absorbance at the laser excitation
wavelength (532 nm) as that of the toluene solution of
[Cr(OEP)(Cl)(1-MeIm)], was used as a reference for the
quantum yield measurements. When the benzene solution
of [Zn(TPP)] is subjected to the laser pulse, the triplet excited
state of [Zn(TPP)], which has the absorption band around
the 470 nm region, is produced. The absorbance change,
∆AT(470nm), at 470 nm after the laser pulse is expressed as
solution of [Zn(TPP)] as a reference, the quantum yield, Φdiss
,
is readily obtained:
Φdiss ) ∆Adiss(∆AT(470nm)-1)ꢀT(∆ꢀ2-1)ΦT
(18)
Figure 7 shows the dependence of Φdiss on the concentra-
tion of dioxygen in the toluene solution. Dioxygen has no
effect on the quantum yield of the photodissociation of H2O
and Py, while Φdiss gradually decreases with an increase in
the dioxygen concentration in the case of the 1-MeIm
complex. The effect of dioxygen is also observed for the
decay of the triplet excited state of [Cr(OEP)(Cl)(1-MeIm)]
in toluene. The decay process was monitored at 480 nm
where only the triplet excited state has an absorption band.
Figure 8 shows the rate constant, kobsd, for the decay of the
triplet excited state as a function of the dioxygen concentra-
tion. The plot of kobsd vs [O2] gives a straight line with an
intercept:
-1
∆AT(470nm) ) ΦTꢀTIabsNA
(15)
where ΦT (0.83) and ꢀT (7.3 × 104 M-1 cm-1) are the triplet
yield of [Zn(TPP)] and the molar absorption coefficient of
the triplet excited state of [Zn(TPP)] at 470 nm, respec-
tively.34 From eqs 14 and 15, eq 16 is derived:
kobsd ) kT + kq[O2]
(19)
ΦH O ) ∆A(∆AT(470 nm)-1)ꢀT(∆ꢀ1-1)ΦT
(16)
2
Here kT and kq are the rate constants for the decay of the
triplet excited state in the absence of dioxygen and the
bimolecular quenching rate constant of the excited state by
dioxygen, respectively: kT ) (2.4 ( 0.1) × 107 s-1 and
kq ) (1.3 ( 0.2) × 109 mol-1 kg s-1 (25.0 °C).
The laser photolysis of [Cr(OEP)(Cl)(H2O)] and [Cr(OEP)-
(Cl)(Py)] gives solely the initial transient species, [Cr(OEP)-
(Cl)], immediately after the pulse. The formation of the triplet
excited states could not be detected. Thus, the quantum yield
As shown in Figure 7, Φdiss observed for [Cr(OEP)(Cl)-
(1-MeIm)] asymptotically decreases with an increase in [O2]
toward a constant value. This result suggests that the
(34) Hurley, J. K.; Sinai, N.; Linschitz, H. Photochem. Photobiol. 1983,
38, 9.
6102 Inorganic Chemistry, Vol. 42, No. 19, 2003

Chromium(III) Octaethylporphyrin Complexes
Figure 8. Plot of the first-order rate constant kobsd for the decay of the
6T1 state of [Cr(OEP)(Cl)(1-MeIm)] represented as a function of [O2] in
toluene at 25.0 °C.
photodissociation of the ligand may occur in both the singlet
and triplet excited states, the former with a very short lifetime
(,1 ns) and the latter with a lifetime long enough to be
Figure 9. Energy level diagram for the Cr(III) porphyrin complexes in
toluene.
quenched by dioxygen. By assuming these mechanisms, Φdiss
can be expressed as
,
2
2
(π, dz ) charge transfer and (d, dz ) excited states may be
responsible for the dissociation of σ-bonded axial ligands
Φdiss ) Φdiss(4S1) + Φdiss(6T1)
2
because increasing electron density in the dz orbital will
weaken the bond between the metal and axial ligand. In the
case of the chromium(III) porphyrins, theoretical investiga-
tions revealed that vacant dz and half-filled dxy, dyz, and dxz
) Φdiss(4S1) + Φ(6T1)kdiss(kT + kq[O2])-1
(20)
2
where Φdiss(4S1), Φdiss(6T1), and Φ(6T1) represent the quantum
orbitals of a chromium atom are located between the HOMO
4
6
yields for the ligand dissociation at the S1 and T1 states
and the formation of the triplet excited state, respectively.
The solid line in Figure 7 is calculated using a least-squares
fitting analysis with values of kT and kq obtained from Figure
8. The determined values are Φdiss(4S1) ) (0.30 ( 0.02) and
a2u(π) and LUMO eg(π*) orbitals of the porphyrin ring in
2
2
energy, whereas an empty dx -y orbital lies well above the
eg(π*) orbital.2 Recent transient absorption and resonance
Raman studies suggest that the normally emissive excited
tripmultiplet (π, π*) of the chromium(III) porphyrins has a
radiationless transition pathway via the low-lying (π, dπ) CT
state.11 In this study, the quenching experiment using
-1
Φ(6T1)kdisskq ) (8.7 ( 0.4) × 10-3 mol kg-1. Since Φdiss
determined in the absence of oxygen is 0.77, the quantum
yield of the ligand dissociation reaction in the triplet excited
state, Φdiss(6T1), can be estimated to be 0.47 from eq 20.
4
dioxygen revealed that both S1 and 4,6T1 excited states are
responsible for the photodissociation of the axial ligand.
Presumably, the dissociation of the axial ligand occurs from
Discussion
2
the (π, dπ) or (π, dz ) CT states, which locate lower in energy
4
than the S1 and 4,6T1 excited states. Figure 9 shows the
Electronic State of the Complex and Reactivity of the
Photodissociation of the Axial Ligand. One of the most
outstanding characteristics of the photochemistry of the
Cr(III) porphyrin complexes is the efficiency of the photo-
dissociation of the axial ligand. The reactive excited states
of metalloporphyrins with the first-row transition metals are
expected to be (1) intraligand S1 state, (π, π*), (2) intraligand
T1 state, (π, π*), (3) ligand field excited states, (d, d), (4)
ligand to metal charge-transfer states, (π, d), and (5) metal
to ligand charge transfer states, (d, π*).1 In particular, as
mentioned for the porphyrin complexes of transition metals
such as Fe(II), Co(II), Co(III), and Ni(II),35-44 the low-lying
electronic energy dissipation diagram of the Cr(III) porphyrin
complexes. By assuming that the dissociation of the axial
ligand at the CT states occurs with an efficiency of unity,
kdiss in eq 20 is interpreted as the rate constant from the triplet
excited state to the CT state.
Quantum yield for the photodissociation of the axial ligand
is influenced by the nature of the porphyrin and axial ligands
(38) Tetreau, C.; Lavalette, D.; Momenteau, M. J. Am. Chem. Soc. 1983,
105, 1506.
(39) Tait, C. D.; Holten, D.; Gouterman, M. Chem. Phys. Lett. 1983, 100,
268.
(40) Tait, C. D.; Holten, D.; Gouterman, M. J. Am. Chem. Soc. 1984, 106,
6653.
(41) Hoshino, M.; Kogure, M.; Asano, K.; Hinohara, T. J. Phys. Chem.
1989, 93, 6655.
(42) Kim, D.; Holten, D. Chem. Phys. Lett. 1983, 98, 584.
(43) Kim, D.; Kirmaier, C.; Holten, D. Chem. Phys. 1983, 75, 305.
(44) Rodriguez, J.; Holten, D. J. Chem. Phys. 1990, 92, 5944.
(35) Lavalette, D.; Tetreau, C.; Momenteau, M. J. Am. Chem. Soc. 1979,
101, 5395.
(36) Dixon, D. W.; Kirmaier, C.; Holten, D. J. Am. Chem. Soc. 1985, 107,
808.
(37) Maillard, P.; Schaeffer, C.; Te´treau, C.; Lavalette, D.; Lhoste, J.-M.;
Momenteau, M. J. Chem. Soc., Perkin Trans. 2 1989, 1437.
Inorganic Chemistry, Vol. 42, No. 19, 2003 6103

Inamo et al.
coordinated to the central metal. In a previous study, we
found that [Cr(TPP)(Cl)(1-MeIm)] does not dissociate the
axial 1-MeIm by the laser flash photolysis.9 However, the
OEP complex, [Cr(OEP)(Cl)(1-MeIm)], releases 1-MeIm
with the quantum yields of 0.30 (Φdiss(4S1)) and 0.47
(Φdiss(6T1)). Such a difference in the quantum yield for the
photodissociation of the axial ligand was also seen for the
Py complexes, [Cr(TPP)(Cl)(Py)] and [Cr(OEP)(Cl)(Py)].45
These findings indicate that the conversion of the porphyrin-
is concluded that the difference in the porphyrin structure
affects the reactivity of the axial ligand dissociation in the
excited state more markedly than in the ground state.
Another aspect that should be pointed out is the difference
in the photodissociation efficiency between the Py and
1-MeIm complexes. The quantum yield of the photodisso-
ciation of Py is larger than that of 1-MeIm for both cases of
TPP and OEP. These differences are considered to originate
from the nature of the bond between the metal and axial
ligand. The molecular structure of the complex clearly
demonstrates that the bond length between the chromium
and the axial Py nitrogen atoms in [Cr(TPP)(Cl)(Py)]
(rCr-N(Py) ) 2.140(5) Å) is longer than the corresponding bond
length of 2.103(4) Å in [Cr(TPP)(Cl)(1-MeIm)].6,24 This
result can reasonably be explained by the steric effect: the
six-membered pyridine ring at the axial site of the metal-
loporphyrins is expected to suffer greater steric hindrance
than the five-membered imidazole ring due to greater
repulsion between the pyridine R-hydrogen atoms and the
porphyrin core. Judging from the bond lengths, we consider
that the bond dissociation energy of the axial ligand of
[Cr(porphyrin)(Cl)(Py)] should be smaller than that of
[Cr(porphyrin)(Cl)(1-MeIm)], and thus, the quantum yield
for the photodissociation of the former is much larger than
that of the latter.
4
centered S1 excited state to the states having dissociative
character competes with the rate of the intersystem crossing
4
4
from S1 to T1, and the former process should be more
efficient for the OEP complex as compared with the TPP
complex. There should be substantial differences in the
porphyrin molecular orbital between OEP and TPP that affect
the electron density on the metal and the kinetics of the decay
of the excited states of the complexes.
The rate constant of the axial ligand dissociation at the
triplet excited states can be estimated from the quantum yield
measurements. The quantum yield, Φdiss(6T1), of [Cr(OEP)-
(Cl)(1-MeIm)] is markedly larger than that of [Cr(TPP)(Cl)-
(1-MeIm)]: Φdiss(6T1) ) 0.47 for [Cr(OEP)(Cl)(1-MeIm)]
and Φdiss(6T1) < 0.01 for [Cr(TPP)(Cl)(1-MeIm)].9 From eq
20, the quantum yield, Φdiss(6T1), in the absence of dioxygen
is written as
Steric Effect on the Axial Substitution Reaction. As
shown in Table 1, the five-coordinate intermediate, [Cr(por-
phyrin)(Cl)], produced by the photodissociation of the axial
ligand has a high reactivity toward the ligand association
reaction. The small activation enthalpy and negative activa-
tion entropy for the axial ligand binding of Cr(III) porphyrins
indicate that the energy required for orientation of the
reacting molecules in a favorable configuration seems small,
like other metalloporphyrins with very large rate constants
(in the range of 108-109 mol-1 kg s-1) for the axial ligand
rebinding for Fe(II)35-37 and Co(III)39-41 porphyrins. Due to
the high reactivity of the five-coordinate intermediate, there
exists little difference in the kinetic parameters of the ligand
rebinding reaction between OEP and TMP complexes.
It was found that the peripheral substituents on the
porphyrin ligand affect the dynamics of the axial water
dissociation reaction from [Cr(porphyrin)(Cl)(H2O)]. The
-1
Φ
diss(6T1) ) [Φ(6T1)]kdisskT
(21)
The term kT can be expressed as kT ) kdiss + k, where k is
the rate constant for the decay from the triplet excited state
to the ground state in the dioxygen-free toluene solution.
For [Cr(TPP)(Cl)(1-MeIm)], we obtain kdiss < 9 × 104 s-1
(25.0 °C) using the values of Φ(6T1) ) 0.88, Φdiss(6T1) <
0.01, and kT ) 7.75 × 106 s-1 at 25.0 °C.9 In the case of the
OEP complex, the kdiss value can be estimated from the decay
6
rate constant of the T1 excited state in the dioxygen-free
toluene solution (kdiss + k ) 2.4 × 107 s-1 at 25.0 °C) and
that in the polystyrene film (k ) 3.7 × 106 s-1 at 25.0 °C)
on the assumption that the rate constant k is almost the same
for both conditions, giving the kdiss value of 2.0 × 107 s-1 at
25.0 °C.46 The rate constant, kdiss, for the dissociation of the
axial 1-MeIm via the triplet excited state of the OEP complex
is at least 200 times larger than that of the TPP complex. In
contrast, we could not find appreciable difference in the rate
constant for the axial 1-MeIm dissociation reaction at the
ground state between [Cr(TPP)(Cl)(1-MeIm)] and [Cr(OEP)-
(Cl)(1-MeIm)]. The former gives the rate constant 2.6 × 10-2
s-1 (25.0 °C),23 and the latter, 7.4 × 10-2 s-1 (25.0 °C).47 It
k-H O value for the TMP complex is greater than that of the
2
OEP complex by a factor of 20. The substituent effect can
also be found in the activation enthalpy; i.e., ∆Hq-H O is 43.9
2
kJ mol-1 for the TMP complex and 60.5 kJ mol-1 for the
OEP complex. A similar effect was found for the water-
exchange reaction of water-soluble iron(III) porphyrins in
aqueous solution studied by a high-pressure 17O NMR
technique.48 The R-methyl groups of the meso-aryl groups
of the porphyrin in [Fe(TMPS)(H2O)2]3- (TMPS ) 5,10,-
15,20-tetrakis(sulfonatomesityl)porphyrin) enhance the rate
of the axial water exchange reaction by a factor of 10 in
comparison with [Fe(TPPS)(H2O)2]3- (TPPS ) 5,10,15,20-
tetrakis(4-sulfonatophenyl)porphyrin). A larger activation
(45) Quantum yields of the Py dissociation, Φdiss(4S1) and Φdiss(6T1), were
determined to be 0.18 and 0.47 for [Cr(TPP)(Cl)(Py)],9 while Py
dissociates almost quantitatively at the excited state derived from 4S1
for [Cr(OEP)(Cl)(Py)] (Φdiss(4S1) ) 0.89).
(46) In this study, we could not determine the triplet yield, Φ(6T1), of
[Cr(OEP)(Cl)(1-MeIm)] from the transient spectrum observed im-
mediately after the pulse, and therefore, the kdiss value cannot be
calculated directly. We, however, can estimate the triplet yield from
the assumption of Φdiss(4S1) + Φ(6T1) ∼ 1.0. From Φdiss(4S1) ) 0.3,
Φ(6T1) is obtained as ca. 0.7. With the use of eq 21, Φ(6T1) ) 0.7,
Φ
diss(6T1) ) 0.47, and kT ) 2.4 × 107 s-1, kdiss is calculated as 1.6 ×
(47) Unpublished data.
(48) Schneppensieper, T.; Zahl, A.; van Eldik, R. Angew. Chem., Int. Ed.
2001, 40, 1678.
107 s-1 at 25 °C, which is in good agreement with the value of
kdiss ) 2.0 × 107 s-1 mentioned in the text.
6104 Inorganic Chemistry, Vol. 42, No. 19, 2003

Chromium(III) Octaethylporphyrin Complexes
volume as well as a smaller activation enthalpy due to the
bulky R-methyl groups has been discussed in terms of the
effect of steric decompression on a dissociative activation
processes.
and 4T1 states and the binding of the axial ligand L or water
molecules to the five-coordinate [Cr(porphyrin)(Cl)]. The
slow decay process is due to the axial H2O ligand substitution
reaction of the transient species, [Cr(porphyrin)(Cl)(H2O)],
by the ligand L to regenerate the initial complex. The
mechanism of the axial H2O substitution reaction was
confirmed to be the dissociative mechanism. The laser
photolysis of [Cr(porphyrin)(Cl)(H2O)] in toluene causes the
dissociation of the axial H2O to yield [Cr(porphyrin)(Cl)],
followed by the recombination of the H2O molecule, and
any other excited states were not observed. The reactivity
of the excited states of the Cr(III) porphyrin markedly
depends on the nature of the porphyrin ligand. Actually, the
triplet excited state of [Cr(OEP)(Cl)(1-MeIm)] is found to
dissociate the axial ligand L more efficiently than that of
[Cr(TPP)(Cl)(1-MeIm)]. This finding is explained by as-
suming that the OEP complex undergoes the conversion from
the triplet states to the dissociative excited states much faster
than the TPP complex. The dissociative excited states
responsible for the ligand dissociation are suggested to be
the CT excited states.
On the other hand, the acceleration effect due to the bulky
R-methyl groups on the peripheral phenyl groups was not
observed for the dissociation reaction of the axial ligand L
from [Cr(porphyrin)(Cl)(L)] (porphyrin ) TPP, OEP, TMP;
L ) Py, 1-MeIm); i.e., the dissociation rate constants of Py
and 1-MeIm at 25.0 °C were determined to be 8.2 and
2.6 × 10-2 s-1 for the TPP complex,23 19 and 7.4 × 10-2
s-1 for the OEP complex,45 and 2.6 and 2.4 × 10-2 s-1 for
the TMP complex,47 respectively. These findings indicate
that the acceleration effect on the H2O dissociation reaction
of the TMP complex may be caused by the difference in the
environment of the axial H2O ligand; i.e., the more hydro-
phobic environment of the axial coordinating site of the TMP
complex due to the R-methyl groups makes the H2O ligand
less stable, and this effect causes the faster rate of the water
dissociation reaction.
Conclusions
Supporting Information Available: Figures reporting the UV-
visible absorption spectra of [Cr(OEP)(Cl)(L)] (L ) H2O, 1-MeIm,
Py) and [Cr(TMP)(Cl)(L)] (L ) H2O, Py), transient spectra of
[Cr(OEP)(Cl)(L)] (L ) H2O, 1-MeIm), ligand concentration
dependence of kobsd for the reaction of [Cr(TMP)(Cl)(H2O)] with
Py in toluene, concentration-time curves for the consecutive
reactions, ligand concentration dependence of kobsd for the reaction
of [Cr(TMP)(Cl)(H2O)] with 3-CNPy in toluene, ligand concentra-
tion dependence of kobsd for the reaction of [Cr(OEP)(Cl)(H2O)]
with L (L ) Py, 1-MeIm) in toluene, and the UV-visible absorption
spectrum of the five-coordinate [Cr(OEP)(Cl)] and rate equations
for the consecutive reactions. This material is available free of
charge via the Internet at http://pubs.acs.org.
Photochemical reactions of [Cr(OEP)(Cl)(L)] and
[Cr(TMP)(Cl)(L)] (L ) H2O, Py, 1-MeIm) were investigated
in the toluene solution using a nanosecond laser flash
photolysis technique. The laser photolysis of [Cr(porphyrin)-
(Cl)(L)] in toluene generates the triplet excited state of the
six-coordinate complex as well as a coordinately unsaturated
complex, [Cr(porphyrin)(Cl)]. On the basis of the effect of
dioxygen on the quantum yield, the photodissociation of the
axial ligand was confirmed to occur at the dissociative states
4
6
4
via both the excited S1 and T1 (and/or T1) states. For the
Py and 1-MeIm complexes, the transient spectra observed
immediately after the laser pulse exhibit the biphasic decay.
6
The fast process is attributed to the deactivation of the T1
IC0342696
Inorganic Chemistry, Vol. 42, No. 19, 2003 6105