Femtosecond time-resolved photo-absorption studies on the excitation dynamics of chromium(III) porphyrin complexes in solution

Article abstract of DOI:10.1016/j.cplett.2007.07.079

The electronic relaxation dynamics of the excited states of the chromium(III) tetraphenylporphyrin complexes, [Cr(TPP)(Cl)(L)] (L = H₂O, pyridine, 1-methylimidazole), in toluene were studied by using femtosecond time-resolved absorption spectroscopy. The excited state (⁴T₁) of 1-methylimidazole and pyridine complexes given by a very rapid intersystem crossing from ⁴S₁ to ⁴T₁ decays to establish the ⁴T₁ ? ⁶T₁ equilibrium in several hundred picoseconds. The ⁴T₁ state of [Cr(TPP)(Cl)(H₂O)] exhibits a biexponential decay composed of the photodissociation process of the axial H₂O ligand and the decay of the photolyzed intermediate to the ground state of [Cr(TPP)(Cl)].

Full text of DOI:10.1016/j.cplett.2007.07.079

Chemical Physics Letters 445 (2007) 167–172
www.elsevier.com/locate/cplett
Femtosecond time-resolved photo-absorption studies on the
excitation dynamics of chromium(III) porphyrin complexes in solution
a,
b
c
b
Masahiko Inamo *, Chie Okabe , Takakazu Nakabayashi , Nobuyuki Nishi ,
d
Mikio Hoshino
a
Department of Chemistry, Aichi University of Education, Kariya 448-8542, Japan
Department of Electronic Structure, Institute for Molecular Science, Okazaki 444-8585, Japan
b
c
Research Institute for Electronic Science, Hokkaido University, Sapporo 060-0812, Japan
d
Institute of Physical and Chemical Research, Wako 351-0189, Japan
Received 9 November 2006; in final form 26 July 2007
Available online 6 August 2007
Abstract
The electronic relaxation dynamics of the excited states of the chromium(III) tetraphenylporphyrin complexes, [Cr(TPP)(Cl)(L)]
(L = H2O, pyridine, 1-methylimidazole), in toluene were studied by using femtosecond time-resolved absorption spectroscopy. The
4
4
excited state (4T1) of 1-methylimidazole and pyridine complexes given by a very rapid intersystem crossing from S1 to T1 decays to
establish the 4T1
ꢀ
6T1 equilibrium in several hundred picoseconds. The 4T1 state of [Cr(TPP)(Cl)(H2O)] exhibits a biexponential decay
composed of the photodissociation process of the axial H2O ligand and the decay of the photolyzed intermediate to the ground state of
[Cr(TPP)(Cl)].
ꢀ 2007 Elsevier B.V. All rights reserved.
1. Introduction
gen, carbon monoxide, or nitrogen monoxide to the heme
proteins [2–4].
The chemistry of metalloporphyrins has been exten-
sively studied for many years in relation to the naturally
occurring tetrapyrrole macrocyclic compounds in various
biological systems, such as electron transfer catalysts of
heme-containing enzymes, oxygen-carrying heme proteins,
and light-harvesting pigments in photosynthetic systems.
In particular, the photochemistry of synthetic metallopor-
phyrins has been a subject of interest in elucidating the var-
ious photophysical and photochemical properties and their
excited state reactivity [1]. Among a variety of photoin-
duced metalloporphyrin reactions, photodissociation and
the subsequent association of the axial ligands have been
investigated to elucidate the binding mechanisms of dioxy-
Recently, we have studied the photochemistry of chro-
mium(III) porphyrin complexes and their axial ligand
substitution reaction using a technique resolved to nano-
seconds [5–11]. During the course of these studies, we
found efficient photoinduced dissociation of the axial
H2O ligand from the tetraphenylporphyrin complex
[Cr(TPP)(Cl)(H2O)] in toluene [5]. The photoreactive
excited state for the dissociation of the axial H2O ligand
in [Cr(TPP)(Cl)(H2O)] was discussed based on the excited
state dynamics and the quantum yield measurement [5,8].
The theoretical and photophysical studies of the chro-
mium(III) porphyrin complexes have shown that the
excited singlet and triplet states of the porphyrin p system
weakly interact with the central chromium(III) (S = 3/2) to
4
2
4
6
give S1, T1, T1, and T1 excited states [12,13]. However,
because the photodissociation of the axial ligand should
take place in the ultrafast time scale, the nature of the
photodissociative excited states and the factors influencing
*
Corresponding author. Fax: +81 566 26 2636.
E-mail address: minamo@auecc.aichi-edu.ac.jp (M. Inamo).
0009-2614/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2007.07.079

168
M. Inamo et al. / Chemical Physics Letters 445 (2007) 167–172
ligand dissociation are still poorly understood. Coordina-
tion and dissociation of the axial H2O ligand play impor-
tant role in the photoreaction of the chromium(III)
porphyrin complexes [5–11], and it is indispensable to
understand the dynamics of the photoexcitation and relax-
ation stages involving photodissociative excited states in
order to elucidate the mechanism of the photodissociation
of the axial H2O ligand. In the present study, we applied a
femtosecond time-resolved absorption spectroscopy to the
photodynamics of [Cr(TPP)(Cl)(L)] (L = H2O, pyridine
(Py), 1-methylimidazole (1-MeIm)) in toluene, and the
decay dynamics of the excited states including the dissoci-
ation of the axial H2O ligand will be discussed based on
the transient absorption spectra.
Industries, special reagent grade) were used as received.
The concentration of a-terthiophene was 1 · 10ꢀ3 M
(1 M = 1 mol dmꢀ3). All experiments were carried out at
293 K.
UV–visible absorption spectra were recorded on a Hit-
achi U-3000 spectrophotometer. The concentration of
water in the toluene solution was determined using a
Karl-Fischer titrator (CA-06, Mitsubishi Chemical Co.,
Ltd.). The concentration of the chromium(III) porphyrin
complex for the time-resolved experiments was ca.
20 lM. The concentration of H2O in the toluene solution
was less than 1 · 10ꢀ2 M.
3. Results
2. Experimental section
3.1. Absorption spectra of the chromium(III) porphyrin
complexes
Aquachloro(5,10,15,20-tetraphenylporphyrinato)chro-
mium(III), [Cr(TPP)(Cl)(H2O)], was prepared according to
the reported procedure [5,14]. Pyridine and 1-MeIm dried
over solid potassium hydroxide were fractionally distilled
before use. Toluene was refluxed on sodium, and then dis-
tilled. The solution of [Cr(TPP)(Cl)(L)] (L = Py and 1-
MeIm) was prepared by adding the ligand L to the toluene
solution of [Cr(TPP)(Cl)(H2O)].
Fig. 1 shows the absorption spectra of [Cr(TPP)(Cl)(L)]
(L = H2O, Py, and 1-MeIm) in toluene containing large
excess of L. It has been demonstrated that a water mole-
cule coordinates to the central chromium atom in
[Cr(TPP)(Cl)(H2O)] [5] and that the axial Cl ligand does
not dissociate in toluene [17]. The axial H2O ligand in
[Cr(TPP)(Cl)(H2O)] is easily replaced with a ligand such
as Py and 1-MeIm, as evidenced by the UV–visible absorp-
tion spectra shown in Fig. 1. The shape of the absorption
spectra is independent of the metalloporphyrin concentra-
tion. The lack of deviation from the Beer–Lambert law in
the concentration range of 3–50 lM for each complex indi-
cates that no dimer or higher aggregate of the metallopor-
phyrins was present in the toluene solution under the
present experimental conditions.
Femtosecond time-resolved absorption spectra were
measured by using a Ti:sapphire regenerative amplifier
(Spectra Physics, Spitfire, wavelength 790 nm, repetition
rate 1 kHz, pulse duration 230 fs) [15]. A portion of the
amplified output was separated and focused in a cell con-
taining water to generate a femtosecond continuum. The
generated continuum was split into two parts: one for a
probe beam and the other for a reference. The remainder
of the amplified output was frequency-doubled to produce
a second harmonic (395 nm). The polarization of the pump
beam was rotated by 54.7ꢁ relative to that of the probe
beam. For measurement of time-resolved absorption spec-
tra, the probe and reference beams were detected by a sin-
gle spectrograph and a CCD camera. The toluene solution
of metalloporphyrins was circulated through a quartz cell
with a 2 mm path length. The measured spectrum of the
probe beam was normalized by the simultaneously mea-
sured spectrum of the reference beam for each exposure.
A chirp correction was performed by the optical Kerr effect
[16]. For the measurement of the time-resolved absorption
intensities at a certain probe wavelength, a mechanical
chopper synchronized with the amplified pulses was placed
in the pump beam line. The probe and reference intensities
were detected by photomultiplier tubes, and the time-
resolved absorption intensities were obtained by a boxcar
integrator with a toggle mode where the pump pulse was
chopped every other shot.
3.0
b
2.5
a
2.0
c
1.5
ε
1.0
0.5
0
350
400
450
500
550
600
650
The response function was determined by sum-fre-
quency mixing in a 0.5 mm BBO crystal and a temporal rise
of a-terthiophene in the S1 state. The function was esti-
mated to be a Gaussian function with a FWHM of 280
fs. a-Terthiophene and solvents (Wako Pure Chemical
Wavelength / nm
Fig. 1. UV–visible absorption spectra of the toluene solution of the
Cr(III)-TPP complex. [Cr(TPP)(Cl)(H2O)] (a), [Cr(TPP)(Cl)(1-MeIm)] (b),
[Cr(TPP)(Cl)(Py)] (c).

M. Inamo et al. / Chemical Physics Letters 445 (2007) 167–172
169
3.2. Photoexcitation dynamics of [Cr(TPP)(Cl) (1-
MeIm)] and [Cr(TPP)(Cl)(Py)]
0.04
500 nm
420 nm
435 nm
The transient absorption spectrum of [Cr(TPP)(Cl)(1-
MeIm)] in toluene observed at 2 ps after photoexcitation
of the femtosecond pump pulse is shown in Fig. 2. The
transient spectrum observed at 34 ns is also displayed for
comparison. The bleaching of the Soret band of
[Cr(TPP)(Cl)(1-MeIm)] and the appearance of a broad
absorption band around 475 nm are commonly observed
for both spectra. The transient spectra observed at 34 ns
0
–0.04
–0.08
–0.12
6
4
6
can be attributed to T1 (4T1 and/or T1). The T1 and T1
states are known to be in thermal equilibrium [8]. The trip-
let excited state decays to the ground state with a time con-
stant of 7.75 · 106 sꢀ1 at 298 K without leaving any long-
lived intermediates. In agreement with the previous study
[8], no photodissociation of the axial 1-MeIm is observed.
The time evolution of the absorbance change (DA) is
shown in Fig. 3. The decay analysis of the transient species
in the time region 0–400 ps was carried out by using a non-
linear least-squares method. A satisfactory fit was obtained
with simultaneous fitting of all the data in Fig. 3 according
to Eq. (1).
450 nm
0
200
400
600
Time / ps
Fig. 3. Transient absorbance of [Cr(TPP)(Cl)(1-MeIm)] in toluene.
A similar photoinduced spectral change was observed
for the toluene solution of [Cr(TPP)(Cl)(Py)]. The time
evolution of the absorbance change (DA) can be expressed
by Eq. (1): the rate constant obtained was k = (2.7
0.5) · 109 sꢀ1. According to a previous Letter [8], the ini-
tially formed singlet excited state 4S1 undergoes either
DAtðkÞ ¼ aðkÞ expðꢀktÞ þ bðkÞ
ð1Þ
Here, DAt(k) represents absorbance change at wavelength k
and time t. The time constant obtained was k = (3.8
0.5) · 109 sꢀ1. Eq. (1) implies that the initially observed ex-
cited state decays with a first order process to yield a long-
4
intersystem crossing to give T1 or the axial Py ligand dis-
6
lived species, T1 (4T1 and/or T1). In the present case, the
sociation to yield the pentacoordinate [Cr(TPP)(Cl)], which
has a Soret band around 435 nm. However, an absorbance
increase of [Cr(TPP)(Cl)] at 435 nm was not detected
experimentally during the decay of the initially observed
excited state, presumably because of the small quantum
yield, 0.18, for the ejection of the axial Py.
transient spectra of the excited states shown in Fig. 2
resemble each other, and the absorbance change associated
with the decay to T1 is relatively small.
3.3. Photoexcitation dynamics of [Cr(TPP)(Cl)(H2O)]
0.04
0
Fig. 4 shows the transient absorption spectrum of
[Cr(TPP)(Cl)(H2O)] observed after the photoexcitation of
the femtosecond pump pulse in toluene together with that
of the ns time resolution [5]. The latter spectrum detected
at 34 ns after the pulse was ascribed to the coordinately
unsaturated complex [Cr(TPP)(Cl)], and this species is
produced by the photodissociation of the axial H2O
ligand. Contrary to the case of [Cr(TPP)(Cl)(1-MeIm)]
and [Cr(TPP)(Cl)(Py)], we did not detect the formation
of the T1 state of [Cr(TPP)(Cl)(H2O)] in the ns time region.
In a previous study, the quantum yield for the photodisso-
ciation of the axial H2O ligand, 0.94, was found to be inde-
pendent of dioxygen concentration [5]. These findings have
been interpreted tentatively in terms of the dissociation of
–0.04
–0.08
–0.12
–0.16
400
450
500
550
600
650
4
the axial H2O in the S1 state.
Wavelength / nm
The time evolution of the absorbance change (DA) for
the toluene solution of [Cr(TPP)(Cl)(H2O)] is shown in
Fig. 5. The decay of the initially generated excited state
in the picosecond time domain can be regarded as consec-
Fig. 2. Transient spectra of [Cr(TPP)(Cl)(1-MeIm)] in toluene. The solid
line was recorded at 2 ps after the laser pulse. The dotted line was the
nanosecond transient spectrum observed at 34 ns after the laser pulse.
C1ꢀMeIm = 3.7 · 10ꢀ3 M.

170
M. Inamo et al. / Chemical Physics Letters 445 (2007) 167–172
and c(k) = A(C, k), where A(A, k), A(B, k), and A(C, k) rep-
0.04
0
resent the absorbance of A, B, and C at wavelength k,
respectively. The rate constants obtained were k1 = (6.8
1.6) · 1010 sꢀ1 and k2 = (1.6 0.2) · 109 sꢀ1. Thus, the
photodynamic processes of [Cr(TPP)(Cl)(H2O)] can be de-
scribed as follows. The initially observed excited state pro-
duced within 2 ps after the pump pulse decays with a rate
constant of k1 to the photolyzed intermediate, which further
decays with a rate constant of k2 to eventually yield the
coordinately unsaturated [Cr(TPP)(Cl)].
The values of a(k) and b(k) were determined to be
ꢀ0.0016 0.0006 and ꢀ0.0015 0.0009 for 420 nm,
0.0006 0.0006 and ꢀ0.051 0.003 for 435 nm, 0.0038
0.0006 and ꢀ0.034 0.002 for 450 nm, and 0.0034
0.0006 and 0.0147 0.0003 for 500 nm. The absorbance
change associated with the first reaction (A ! B), given
by ꢀaðkÞ ꢀ bðkÞkꢀ1 1k2, is obtained as 0.0016, 0.0006,
ꢀ0.0030, and ꢀ0.0037 at 420, 435, 450, and 500 nm, respec-
tively, indicating that the spectral change for the first
process is relatively small. Probably, the absorption spectra
of these species resemble each other. The absorbance
change for the second reaction (B ! C), given by
bðkÞðk2 ꢀ k1Þkꢀ1 1, is calculated as 0.0015, 0.050, 0.033, and
ꢀ0.0144 at 420, 435, 450, and 500 nm, respectively. This
process from the photolyzed intermediate to the ground
state of [Cr(TPP)(Cl)] is therefore accompanied by a rela-
tively large spectral change.
–0.04
–0.08
–0.12
400
450
500
550
600
650
Wavelength / nm
Fig. 4. Transient spectra of [Cr(TPP)(Cl)(H2O)] in toluene. The solid line
was recorded at 2 ps after the laser pulse. The dotted line was the
nanosecond transient spectrum observed at 34 ns after the laser pulse.
420 nm
0.020
0.015
0.010
0.006
0.005
0.004
4. Discussion
500 nm
A number of studies to understand the photodynamics
of porphyrins have been carried out with the use of fs,
ps, and ns time-resolved techniques. In most porphyrins,
excitation of the S2 band leads to rapid electronic relaxa-
tion from S2 to S1 within a femtosecond time period
[17,18]. The S1 state goes to either the triplet state within
a few ns by intersystem crossing or to the ground state
by the radiative process and internal conversion. The life-
times of metalloporphyrins at S1 are known to depend on
the nature of the central metal. For example, the S1 life-
times of zinc and magnesium tetraphenylporphyrins
([Zn(TPP)] and [Mg(TPP)]) are 2 and 9 ns, respectively,
[19]. However, for copper(II) tetraphenylporphyrin
([Cu(TPP)]) the intersystem crossing from the S1 state to
the T1 state occurs very rapidly (<350 fs), followed by the
establishment of the tripdoublet–tripquartet equilibrium
0.005
-0.04
0.003
0.02
435 nm
450 nm
-0.06
-0.08
-0.10
0
-0.02
-0.04
0
200
400
600
Time/ ps
Fig. 5. Transient absorbance of [Cr(TPP)(Cl)(H2O)] in toluene.
4
(2T1 ꢀ T1) with a time constant of 450 ps [20,21]. Pre-
sumably, the interaction between the excited porphyrin
ligand and paramagnetic central copper(II) results in a
utive reactions (A ! B ! C) and the analysis of the decay
curves was carried out with a non-linear least-squares
method with Eq. (2).
2
short lifetime of the S1 state of [Cu(TPP)].
In the present work on the time-resolved studies of chro-
mium(III) porphyrins in toluene with a fs laser pulse at
395 nm, the light pulses were absorbed by the absorption
band located at the wavelength shorter than the Soret band
(S2) of the chromium porphyrins. The vibrational relaxa-
tion in the S2 state and the electronic relaxation from S2
to S1 are completed within several picoseconds in metallo-
AtðkÞ ¼ aðkÞ expðꢀk1tÞ þ bðkÞ expðꢀk2tÞ þ cðkÞ
ð2Þ
where At(k) represents the absorbance of the solution at
wavelength k and time t. The terms a(k), b(k), and c(k)
are expressed as a(k) = A(A, k) + k1(k2 ꢀ k1)ꢀ1A(B,k) +
k2(k1 ꢀ k2)A (C,k), b(k) = k1(k1 ꢀ k2)ꢀ1(A(B,k) ꢀ A(C, k)),

M. Inamo et al. / Chemical Physics Letters 445 (2007) 167–172
171
porphyrins [18]. It has been reported that the paramagnetic
Cr(III)-TPP complex does not show normal (p, p*) fluores-
cence and that extremely weak triplet state fluorescence
4T1 excited state to the photodissociative CT state. It is,
however, difficult to obtain further information on the
properties of the photodissociative excited states due to
the intense porphyrin-centered transition bands, and it
can be provided by picosecond time-resolved Raman
spectroscopy.
4
from T1 is observed in ethanol [13]. These findings were
interpreted by the extremely rapid intersystem crossing
4
4
from S1 to T1 and the decay of the initially populated
transient with a lifetime of 295 ps was attributed to the
Recently, the photoinduced dissociation of the axial
ligand of the chromium(III) tetraphenylporphyrin complex,
[Cr(TPP)(X)] (X = Cl, Br), has been investigated in benzene
and tetrahydrofuran solutions using transient Raman and
absorption spectroscopic techniques [23]. Nanosecond tran-
sient resonance Raman spectroscopic results obtained for
the noncoordinating benzene solution were discussed in
terms of the photodissociation and recombination of the
axial halogen ligand, while a relatively fast photoinduced
reaction in the coordinating tetrahydrofurane solution
was attributed to the excited five-coordinate species,
[Cr(TPP)(THF)]+. The photodissociation of the axial halo-
gen ligand was, however, hardly observed during the photo-
induced processes in toluene in the present study. It is
supposed that the photodissociation of the neutral axial
ligand of H2O is energetically more favorable than the dis-
sociation of the charged halide ion.
6
deactivation of 4T1, resulting in the 4T1 ꢀ T1 equilibrium.
Since this lifetime is comparable to that of the initial tran-
sient observed for [Cr(TPP)(Cl)(1-MeIm)] (260 ps) and
[Cr(TPP)(Cl)(Py)] (370 ps), the transient observed just
after a fs laser pulse in the present study can also be
4
regarded as the T1 excited state. The following spectral
change can be ascribed to the similar energy dissipation
process as reported previously [13].
On the other hand, the photoexcitation of
[Cr(TPP)(Cl)(H2O)] at 395 nm gives the initial transient as
shown in Fig. 4, and the decay of this transient follows a
biphasic path, affording an electronic ground state of the
coordinately unsaturated [Cr(TPP)(Cl)]. The ns time-
resolved study has shown that the quantum yield of the
axial H2O photodissociation to give [Cr(TPP)(Cl)] is 0.94
4
4
[5]. Since the intersystem crossing from S1 to T1 may be
too fast to be observed for the chromium(III) porphyrins,
the initial transient for [Cr(TPP)(Cl)(H2O)] may also be
Acknowledgements
4
4
the T1 state rather than S1. The biphasic path observed
here includes the axial H2O photodissociation and the
energy dissipation process, which leads to [Cr(TPP)(Cl)]
in its electronic ground state. It could be possible that the
This work was supported by a Grant-in-Aid for Scien-
tific Research (No. 18550052) and ‘Nanotechnology Sup-
port Project’ of the Ministry of Education, Science,
Sports, and Culture of Japan.
4
initial decay process is attributed to the cooling of the T1
4
6
state to lead to the T1 ꢀ T1 equilibrium. However, the
lifetime of the initial transient (15 ps) is much shorter than
4
References
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and therefore it is more preferable to ascribe the decay of
the initial transient (k1 path) to the conversion of T1 to
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