Nanosecond laser photolysis of Ni(II) tetraphenylporphyrin. Kinetic studies on association and dissociation of aliphatic amines at the axial positions

Article abstract of DOI:10.1021/jp0219000

Nanosecond laser photolysis studies of nickel(II) tetraphenylporphyrin, Ni^IITPP, in toluene solutions containing primary, secondary, and tertiary aliphatic amines were carried out for elucidation of the association and dissociation mechanisms of the axial amines. From the absorption spectroscopic studies, the primary amines were found to react with Ni^IITPP to give the six coordinate species. The secondary and the tertiary amines, however, give no coordinated species. The laser photolysis of Ni^IITPP in toluene containing primary amines revealed that the four- and six-coordinate species undergo mutual exchange upon laser excitation. In secondary amine-toluene solutions, the four coordinated species undergoes light-induced transformation to yield the six-coordinate species. However, no formation of the six coordinate species are observed for tertiary amine-toluene solutions of Ni^IITPP by photolysis. The six and four coordinate species produced by laser photolysis of the amine-toluene solutions decay according to the pseudo-first-order kinetics. The plot of the rate constant, k_obsd, for the decay of these transient species vs the primary amine concentration at 300 K is concave upward: the value k_obsd initially decreases toward a minimum and then increases with an increase in the amine concentration. Because the laser photolysis of Ni^IITPP in primary amine-toluene solutions results in the nonequilibrium state among four and six-coordinate species, k_obsd is the rate constant to reach the equilibrium state. It is assumed that the four and six coordinate species in the nonequilibrium state achieve the equilibrium via the formation of a five coordinate Ni^IITPP as an intermediate. The rate constants, k_obsd, represented as a function of the primary amine concentration are well explained with the use of the steady-state approximation with regard to the five coordinate species. On the basis of the laser photolysis in the temperature range 200-300 K, the reaction mechanisms for the equilibrium reactions between Ni^IITPP and various amines are discussed in detail.

Full text of DOI:10.1021/jp0219000

J. Phys. Chem. A 2003, 107, 4445-4451
4445
Nanosecond Laser Photolysis of Ni(II) Tetraphenylporphyrin. Kinetic Studies on Association
and Dissociation of Aliphatic Amines at the Axial Positions
Hiroyuki Suzuki,†,‡ Haruna Adachi,†,‡ Yoshio Miyazaki,§ and Mikio Hoshino*,†,‡
The Institute of Physical and Chemical Research, Wako, Saitama 351- 0198, Japan, and Interdisciplinary
Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuda-machi,
Midori-ku, Yokohama, Kanagawa 226-8502, Japan, and Department of Chemistry, Faculty of Engineering,
Toyo UniVersity, Kujirai, Kawagoe, Saitama 350-8585, Japan
ReceiVed: August 19, 2002; In Final Form: February 12, 2003
Nanosecond laser photolysis studies of nickel(II) tetraphenylporphyrin, NiIITPP, in toluene solutions containing
primary, secondary, and tertiary aliphatic amines were carried out for elucidation of the association and
dissociation mechanisms of the axial amines. From the absorption spectroscopic studies, the primary amines
were found to react with NiIITPP to give the six coordinate species. The secondary and the tertiary amines,
however, give no coordinated species. The laser photolysis of NiIITPP in toluene containing primary amines
revealed that the four- and six-coordinate species undergo mutual exchange upon laser excitation. In secondary
amine-toluene solutions, the four coordinated species undergoes light-induced transformation to yield the
six-coordinate species. However, no formation of the six coordinate species are observed for tertiary amine-
toluene solutions of NiIITPP by photolysis. The six and four coordinate species produced by laser photolysis
of the amine-toluene solutions decay according to the pseudo-first-order kinetics. The plot of the rate constant,
k
obsd, for the decay of these transient species vs the primary amine concentration at 300 K is concave upward:
the value kobsd initially decreases toward a minimum and then increases with an increase in the amine
concentration. Because the laser photolysis of NiIITPP in primary amine-toluene solutions results in the
nonequilibrium state among four and six-coordinate species, kobsd is the rate constant to reach the equilibrium
state. It is assumed that the four and six coordinate species in the nonequilibrium state achieve the equilibrium
via the formation of a five coordinate NiIITPP as an intermediate. The rate constants, kobsd, represented as a
function of the primary amine concentration are well explained with the use of the steady-state approximation
with regard to the five coordinate species. On the basis of the laser photolysis in the temperature range
200-300 K, the reaction mechanisms for the equilibrium reactions between NiIITPP and various amines are
discussed in detail.
Introduction
and six- coordinate Ni porphyrins are in equilibrium in the
organic solvents containing nitrogen bases.36
Photoinduced dissociation of axial ligands from various
metalloporphyrins has been the subject of extensive studies1-19
partly because of their importance for elucidation of the thermal
association and dissociation mechanisms of the axial ligands
in heme proteins.20-30 In contrast to the photodissociation, only
a few reports have been made for the photoinduced association
of the axial ligand to metalloporphyrins.3,31-33 Among various
metalloporphyrins, Ni(II)tetraphenylporphyrin (NiIITPP) is the
typical molecule which exhibits both association and dissociation
of axial ligands upon laser excitation.3,33
The excited states of nickel(II) porphyrins, NiIIP, responsible
for association and dissociation of the ligand molecule have
been examined by pico- and femtosecond photolysis.3,7,37-40 The
lowest excited state of NiIIP has been identified as the metal
excited state, from which dissociation and association of the
axial ligand takes place. In the present study, we have carried
out laser photolysis studies of NiIITPP in aliphatic amine-
toluene solutions. The dissociation and the association mech-
anisms of aliphatic amines to NiIITPP are investigated on the
basis of the kinetic studies of the transient species.
The central Ni(II) atom in nickel(II) porphyrins has the d8
electron configuration. Four coordinate NiIITPP is known to
Experimental Section
1
2
34,35
2
have the A1g ((dz ) ) in the ground state.
The orange red
toluene solution of NiIITPP exhibits no appreciable change in
color in the presence of 2.0 M pyridine at room temperature.
However, when the solution is cooled to 200 K, the color of
the solution turns from orange red to violet because of the
formation of (Py)2NiIITPP (Py ) pyridine at the axial position),
Reagent grade toluene was used as a solvent without further
purification. Purified NiIITPP was kindly donated by Dr. Yama-
moto of our institute. Aliphatic amines and pyridine derivatives
(2-, 3-, and 4-methylpyridine) were purified by distillation under
reduced pressure before use. The concentrations of NiIITPP used
in the present study are 2.0-3.0 × 10-6 M.
3
21
which has the B1g (dz , dx _y 1) state. The absorption spectro-
2
2
scopic and NMR studies have demonstrated that the four-, five-,
Absorption spectra were recorded on a Hitachi 330 spectro-
photometer. The temperature of the sample solution was
controlled by a cryostat from Oxford Instruments (DN 1001).
The laser photolysis was performed with the use of third
harmonics from the YAG laser (HY500 from JK Lasers Ltd.).
The duration and the energy of the laser pulse are 20 ns and
* To whom correspondence should be addressed. Fax: 048-462-4668.
Phone: 048-467-9426. E-mail: hoshino@postman.riken.go.jp.
† The Institute of Physical and Chemical Research.
‡ Tokyo Institute of Technology.
§ Toyo University.
10.1021/jp0219000 CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/09/2003

4446 J. Phys. Chem. A, Vol. 107, No. 22, 2003
Suzuki et al.
Here D0 and D∞ denote the 433-nm absorbance at [n-PA] ) 0
and at an “infinite” concentration of n-PA, respectively. From
K1 and K2, K is given by K ) K1K2. With the use of the curve
fitting and least-squares method, K is obtained as 0.25 M-2
.
The sum of square residuals, ∑ei2, was 7.29 × 10-5
.
The similar absorption spectral changes of NiIITPP in toluene
solutions were measured by changing the concentration of
aromatic amines, 3- and 4-methylpyridine. The spectral changes
are very close to those observed with primary amines. However,
the addition of 2-methylpyridine gives no effects on the
absorption spectrum of NiIITPP in toluene, indicating that methyl
group at the 2 position prohibited the coordination of 2-meth-
ylpyridine to the axial positions of NiIITPP. The addition of
the secondary and tertiary amines does not affect the absorption
spectrum of NiIITPP in toluene: neither the shift of the
absorption peaks nor the change in the molar absorption
coefficients was detected for NiIITPP even at 8.0 M amines.
Thus, the species in these solutions is concluded to be NiIITPP.
Laser Photolysis of NiIITPP in Amine-Toluene Solutions
at Room Temperature. Laser photolysis studies were carried
out for NiIITPP in primary amine-toluene solutions by changing
the concentration of the amines. With an increase in the amine
concentration, the ratio, [NiIITPP]/[six coordinate NiIITPP],
decreases. When the amine concentration is low, the transient
spectrum observed at 30 ns after the pulse has a positive peak
at ca. 430 nm and a negative one at ca. 415 nm. On the other
hand, at high amine concentrations, the transient spectra
measured at 30 ns exhibit a positive peak at ca. 415 and a
negative one at ca. 430 nm in the Soret band region. Taking
account of the absorption spectral changes of NiIITPP in the
presence of primary amines, we concluded that, upon laser
excitation, four-coordinate species and six-coordinate species
interconvert each other. The transient spectra uniformly decay
according to first-order kinetics over the wavelength region
studied. As an example, the laser photolysis of NiIITPP in
cyclohexylamine-toluene solutions is described below. The
absorption spectrum of NiIITPP changes with an increase in the
concentration of cyclohexylamine, CHA. From the absorption
spectral measurements, the major species at 0.88 and 4.4 M
CHA are NiIITPP and (CHA)2NiIITPP, respectively. Figure 2
shows the transient absorption spectra of NiIITPP in cyclohexyl-
amine-toluene solutions taken at 30 ns after the 355-nm laser
pulse. The spectrum (A) observed for the toluene solution of
NiIITPP containing 0.88 M CHA is in good agreement with the
difference spectrum, (CHA)2NiIITPP minus NiIITPP. When the
toluene solution of NiIITPP contains 4.4 M CHA, the transient
spectrum (B) is in accord with the difference spectrum, NiIITPP
minus (CHA)2NiIITPP. Thus, in appearance, the photochemical
reaction of this system is represented as
Figure 1. Absorption spectral changes observed for NiIITPP in toluene
by changing [n-propylamine]: (1) 0.0, (2) 0.61, (3) 1.2, (4) 2.4, (5)
3.65, (6) 4.9, and (7) 6.1 M. The inset shows the absorbance at 413
nm represented as a function of [n-propylamine]. The solid line is the
calculated curve with the use of K ) 0.25 M-2
.
100 mJ/pulse, respectively. The transient spectra were taken by
the intensified charge coupled device detector.18,19 The decay
of the transient spectrum was measured with a photomultiplier
(Hamamatsu R 758). The monitoring light from a xenon lamp
(Ushio 150 D, 150 W) was led to the entrance slit of the
monochromator. Then, the monochromated light intensity from
the exit of the monochromator was detected by the photomul-
tiplier. The output from the photomultiplier was conducted to
the digital storage oscilloscope (Gould model 630 from Gould
Instrument System, Ltd.). The data were stored on a personal
computer for kinetic analysis.
Results
Absorption Spectral Changes of NiIITPP in Amine-
Toluene Solutions. The absorption spectrum of NiIITPP in tolu-
ene solutions exhibits marked changes by addition of primary
aliphatic amines. The spectral changes are found to be very
similar independent of the nature of the primary amine used.
Figure 1 shows the absorption spectral changes observed for
NiIITPP in toluene containing 0-6.1 M n-propylamine, n-PA.
The absorption peaks of NiIITPP located at 413 and 525 nm
shift toward red with an increase in the concentration of
n-propylamine, [n-PA], and new peaks appear at 433, 563, and
603 nm with isosbestic points. According to earlier spectroscopic
studies,41-46 the new peaks are ascribed to the six coordinate
species, (n-PA)2 NiIITPP. The spectral changes observed for the
n-propylamine-toluene solutions are interpreted in terms of the
equilibrium reactions described below
K1
\
NiIITPP + n-PA {
} (n-PA)NiIITPP
(1)
(2)
K2
(n-PA)NiIITPP + n-PA { } (n-PA)2NiIITPP
\
NiIITPP + 2CHA + hν f (CHA)2NiIITPP
(4)
Because isosbestic points appear in the spectral changes, we
concluded that the equilibrium reaction is composed of two
major species, NiIITPP and (n-PA)2 NiIITPP. Presumably, the
concentration of the five coordinate species, (n-PA)NiIITPP is
too low to be detected by absorption spectroscopy: [(n-PA)-
NiIITPP] , [NiIITPP] + [(n-PA)2NiIITPP]. The absorbance
change at 433 nm is represented as a function of [n-PA] in the
inset of Figure 1. From eqs 1 and 2 and [(n-PA)NiIITPP] ,
[NiIITPP] + [(n-PA)2NiIITPP], the 433-nm absorbance, D, is
formulated as
(CHA)2NiIITPP + hν f (CHA)NiIITPP + 2 CHA (5)
The photochemically produced (CHA)2NiIITPP and NiIITPP
disappear to achieve the equilibrium according to pseudo-first-
order kinetics. It is noted that the transient is hardly detected at
1.7 M CHA, indicating that the photochemical formation of
(CHA)2NiIITPP by eq 4 is canceled by the photochemical
disappearance of NiIITPP (eq 5).
The laser photolysis of NiIITPP in toluene containing 0.5-
8.2 M methylpyridines (3-CH3Py and 4-CH3Py) gives the results
D ) (D0 + D∞K[n-PA]2)/(1 + K[n-PA]2)
(3)

Nanosecond Laser Photolysis of NiIITPP
J. Phys. Chem. A, Vol. 107, No. 22, 2003 4447
Figure 2. Transient absorption spectra observed at 300 K for NiIITPP
in toluene containing (A) 0.88 and (B) 4.4 M cyclohexylamine, at 30
ns after the 355-nm laser pulse. The correct measurements of the
absorbance change in the wavelengths X < 430 nm of the spectrum
(A) could not be made because of the large absorbance of NiIITPP.
Figure 4. Transient absorption spectra observed at (A) 295 and (B)
220 K for NiIITPP in toluene containing 6.16 M 3-methylpyridine.
The photochemical formation of the six coordinate NiIITPP is
also observed for the toluene solutions containing other second-
ary aliphatic amines, di-n-propylamine and di-n-butylamine.
The absorption spectrum of NiIITPP in toluene is not altered
by the addition of tertiary amines similarly to the case of the
secondary amines. Contrary to secondary amine-toluene solu-
tions, no transient was observed for NiIITPP in tertiary amine-
toluene solutions after the laser pulse in the amine concentration
range 1.0-8.0 M. Probably, tertiary amines are unable to
coordinate to the axial positions in both the ground and the
excited state.
Transient Absorption Spectra at Low Temperatures. The
color of the toluene solutions containing 1.0 M primary amines
changes from orange red to violet with decreasing temperature.
This result indicates that the six coordinate NiIITPP is produced
at low temperatures owing to the shift of the equilibrium
reactions (eqs 1 and 2) to the right side. The shift of the
equilibrium at low temperatures is also observed for he toluene
solution of NiIITPP in the presence of pyridine derivatives (3-
and 4-methylpyridine).
Figure 4 shows the transient spectra observed for the toluene
solution containing 6.16 M 3-methylpyridine, 3-MP, at 30 ns
after the 355-nm laser pulse. The transient spectrum measured
at 295 K has a positive peak at 430 nm and a negative one at
410 nm because of the photochemical formation of the six
coordinate species, (3-MP)2NiIITPP. At 220 K, the laser
photolysis gives the transient spectrum identical with the
difference spectrum, NiIITPP minus (3-MP)2NiIITPP, indicating
that (3-MP)2NiIITPP photodissociates the axial ligands, 3-MP.
These results imply that, on going from 295 to 220 K, the major
light absorbing species in the solution changes from NiIITPP to
(3-MP)2NiIITPP.
Figure 3. Transient absorption spectrum observed at 300 K for NiII-
TPP in toluene containing 7.7 M diethylamine at 30 ns after the 355-
nm laser pulse.
similar to those obtained with CHA: the transient species
changes from the six- to the four-coordinate species with an
increase in the methylpyridine concentration.
As mentioned above, the secondary amines afford no effects
on the absorption spectrum of NiIITPP even at high concentra-
tions of the amines. Figure 3 shows the transient spectrum
observed with the laser photolysis of NiIITPP in a diethylamine
(DEA)-toluene solution. Because the spectrum has positive
peaks at 430, 560, and 600 nm and negative ones at 413 and
525 nm, the transient is ascribed to the six coordinate species,
(DEA)2NiIITPP. These results are interpreted by assuming that
NiIITPP associates two DEA molecules at the vacant axial
positions in the excited state to yield (DEA)2NiIITPP
Transient absorption spectra of NiIITPP in secondary amine
(sec-amine)-toluene solutions were measured at low temper-
atures. The spectra taken at 200 K were found to be identical
with those at 295 K. The absorption spectrum of NiIITPP in
sec-amine-toluene at 200 K is identical with that of NiIITPP,
NiIITPP + hν + 2DEA f (DEA)2NiIITPP
(DEA)2NiIITPP f NiIITPP + 2 DEA
(6)
(7)

4448 J. Phys. Chem. A, Vol. 107, No. 22, 2003
Suzuki et al.
TABLE 1: Rate and Equilibrium Constants for Association
and Dissociation of Axial Amines
k1, M-1s-1 k3/k2, M-1
k4, s-1
K,a M-2
n-propylamine(r.t)
n-buthylamine (r.t)
isopropylamine (r.t)
isobuthylamine (r.t)
6.7 × 105
8.7 × 105
5.3 × 105
6.3 × 105
9.5
10.2
10.5
18.6
5.4
2.5 × 107 2.5 × 10-1
1.9 × 107 4.2 × 10-1
2.0 × 108 2.8 × 10-2
6.5 × 107 1.8 × 10-1
6.0 × 107 5.2 × 10-2
4.5 × 107
cycrohexylamine (r.t) 5.8 × 105
diethylamine (r.t)
dipropylamine (r.t)
2.7 × 107
dibuthylamine (280K)
3.0 × 107
3-methylpyridine (r.t) 1.4 × 106
2.6
20.1
9.8 × 107 3.7 × 10-2
2.7 × 108 7.2 × 10-2
4-methylpyridine (r.t) 9.6 × 105
a Experimental errors are wthin (3%.
in Figure 5 is the calculated one by eq 10 with the use of the
kinetic parameters descried above.
Figure 5. Rate constant for the decay of the transient measured at
295 K for NiIITPP in toluene, represented as a function of [n-
propylamine].
The pseudo-first-order rate constants, kobsd, obtained with the
laser photolysis of NiIITPP in secondary amine-toluene solu-
tions were measured in the concentration range 1.0-8.0 M at
295 K. It is found that kobsd is invariable against the concentra-
tion of the amines. This finding suggests that the decay process
of the six coordinate species produced by photolysis of NiII-
TPP in secondary amine-toluene is principally the dissociation
of the axial ligand: k1[amine] and k3[amine] are much smaller
than k2 and k4. Accordingly, eq 10 is approximately changed to
the following equation:
measured at 295 K. Thus, the species absorbing the laser pulse
is solely ascribed to NiIITPP independent of temperature in the
range 200-295 K. Accordingly, the photoreaction in this system
is interpreted in terms of the photochemical formation of the
six-coordinate species, (sec-amine)2NiIITPP, from NiIITPP.
No transient spectra were observed for tertiary amine- and
2-methylpyridine-toluene solutions of NiIITPP in the temper-
ature range 200-300 K. The tertiary amines studied here are
triethylamine and tri-n- propylamine
kobsd ) k4
(11)
Kinetic Studies of the Transient Species at 295 K. Figure
5 shows the pseudo-first-order rate constants, kobsd, for the decay
of the transient spectrum observed at 430 nm with the 355-nm
laser photolysis of n-propylamine (n-PA)-toluene solutions of
NiIITPP, represented as a function of the concentration of n-PA.
The rate constants, kobsd, obtained at 430 nm is almost identical
with those at 410 nm. At 295 K, the rate constant, kobsd, initially
decreases toward a minimum value and then increases with an
increase in [n-PA]. The general features of the plot of kobsd vs
the concentration of other primary amines and pyridine deriva-
tives (3- and 4-methylpyridine) used in the present study are
similar to the case of n-PA.
In Table 1 are listed the kinetic parameters, K, k1, k4, and k3/k2,
obtained with the primary amines, secondary amines, and 3-
and 4-methylpyridine at 295 K.
Rate Constant Measurements in the Temperature Range
220-300 K. As shown in eq 10, kobsd is expressed by the rate
constants, k1-k4. Because the rate constants are the functions
of temperature, the plot of kobsd vs the amine concentration is
expected to display the characteristic curve dominated by the
sample temperature. The plots of kobsd vs the amine concentration
were obtained with every 10 K step in the range 300-200 K
for the primary, secondary, and aromatic amines.
The laser photolysis of NiIITPP in amine-toluene solutions
affords nonequilibrium between four and six coordinate species
of NiIITPP. Thus, the chemical reactions occurring after the
photolysis are the processes to recover the equilibrium
Figure 6 shows the plots of kobsd vs the concentration of
isopropylamine, iso-PA, obtained with the laser photolysis of
NiIITPP in iso-PA-toluene solutions at 295 and 240 K. The
k
obsd values obtained at 295 K decrease with a decrease in [iso-
k1
PA], and those at 220 K increase with an increase in [iso-PA].
To elucidate the plots of kobsd vs [amine] in the temperature
range 200-300 K, the rate constants, k1-k4, are formulated by
the Arrhenius expression: kn ) An exp(-∆En/RT) (n ) 1-4).
Thus, eq 10 is transformed to
NiIITPP + amine {k
\
} (amine)NiIITPP
(8)
(9)
2
k3
(amine)NiIITPP + amine {k } (amine)2NiIITPP
\
4
By assuming the steady-state approximation with regard to
the five coordinate species, (amine)NiIITPP, the pseudo-first-
order rate constant, kobsd, for, achievement of the equilibrium
is formulated as
kobsd
)
A32 exp(-∆E32/RT)A1 exp(-∆E1/RT) [amine]2 + A4
exp{(-∆E4)/RT}
(12)
1 + A32 exp(-∆E32/RT)[Amine]
k1k3[amine]2 + k2k4
kobsd
)
(10)
Here A32 ) A3/A2 and ∆E32 ) ∆E3 - ∆E2. As mentioned above,
the values of k1, k4, and k2/k3 were uniquely determined at T )
295 K. Thus, A1, A4, and A32 are respectively expressed as a
function of ∆E1, ∆E4, and ∆E32, leading to the conclusion that
eq 12 has only three unknown parameters (∆E1, ∆E4, and ∆E32).
Activation energies, ∆E1, ∆E4, and ∆E32, were determined from
the data of kobsd and eq 12 with the use of the nonlinear least-
squares fitting program. Then, pre-exponential factors are
calculated from these activation energies. In the case of
secondary amines, the rate constants, kobsd, are independent of
k2 + k3[amine]
The equilibrium constant K ()k1k3/k2k4) has already been
obtained from the absorption spectral change of NiIITPP in
toluene solution containing amines. With the use of eq 10, K,
and the least-squares fitting program, k1, k4, and k3/(k2k4) are
determined from the plot of kobsd vs [n-PA] in Figure 5: K )
0.25 M-2, k1 ) 6.7 × 105 M-1s-1, k4 ) 2.54 × 107 s-1, and
k3/k2 ) 9.5 M-1. Unfortunately, the absolute values of k2 and
k3 could not be determined in the present study. The solid curve

Nanosecond Laser Photolysis of NiIITPP
J. Phys. Chem. A, Vol. 107, No. 22, 2003 4449
TABLE 2: Kinetic and Thermodynamic Parameters for Association and Dissociation of Axial Amines
∆E1, kcal
A1, M-1s-1
∆E32, kcal
A32 , M-1
∆E4, kcal
A4, s-1
∆H, kcal
∆S, eu
n-propylamine
n-buthylamine
isopropylamine
isobuthylamine
cycrohexylamine
diethylamine
dipropylamine
dibuthylamine
3-methylpyridine
4-methylpyridin
3.1
3.2
2.5
3.7
3.9
1.3 × 108
2.0 × 108
3.4 × 107
3.2 × 108
4.1 × 108
-14.0
-14.0
-0.9
-7.9
-0.4
8.0 × 10-10
4.6 × 10-10
2.3
3.8
3.0
1.6 × 1010
8.7 × 1010
3.2 × 1017
2.1 × 1016
9.3 × 1014
1.5 × 1011
5.6 × 1011
1.3 × 1012
8.6 × 1010
2.3 × 1011
-14.7
-13.8
-11.4
-16.2
-51.5
-55.2
-44.3
-57.0
-27.3
13.0
12.0
9.8
2.7 × 10-5
2.7
-6.3
4.5
5.0
5.7
6.1
5.0
4.5 × 1010
4.6 × 109
-5.7
-4.4
1.7 × 10-4
1.2 × 10-2
4.0
-3.6
-3.4
-18.7
-16.7
4.0
Figure 8. Quantum yields for the formation of (sec-DPA)2NiIITPP
from NiIITPP in toluene, represented as a function of [sec-DPA].
and ∆E4 ) 4.5 × 103 cal/mol. In Table 2 are listed A1, A32, A4,
∆E1, ∆E32, ∆E4, and ∆H for primary, secondary, and aromatic
amines.
In a previous paper,33 we have reported the laser photolysis
studies of NiIITPP in a pyridine-toluene mixture in the
temperature range 300-200 K. The transient observed by
photolysis of (Py)2NiIITPP at 220 K decayed according to
pseudo-first-order kinetics with the rate constant k. On the basis
of the fact that the plot of k vs [Py] gives a straight line with an
intercept, the transient was assigned to the five coordinate
species, (Py)NiIITPP. However, the present result indicates that
the apparent linear plot of k vs [Py] can be obtained with the
case that k3[amine] . k2 (see eq 10). From Table 2, it is
concluded that this case can be attained when the temperature
of the sample solutions containing 1 M primary and aromatic
amine becomes below 240 K. Thus, the earlier result can be
reinterpreted: photolysis of (Py)2NiIITPP gives NiIITPP, which
returns to (Py)2NiIITPP via the five coordinate (Py)NiIITPP.
Figure 6. Rate constants for the decay of the transient measured at
295 and 220 K for NiIITPP in toluene, represented as a function of
[isopropylamine].
Quantum Yield Measurements in Secondary Amine-
Toluene Solutions. The quantum yield measurements for the
formation of the six coordinate NiIITPP in di-n-propylamine
(sec-DPA)-toluene solutions were carried out with the use of
the laser photolysis technique. The method for determination
of the quantum yields has already been described else-
where.22,47,48 The light quanta absorbed by NiIITPP were
measured by monitoring the T-T absorption of ZnIITPP in the
standard benzene solution, which has the absorbance identical
with that of NiIITPP in toluene.
Figure 7. Rate constants for the decay of (DEA)2NiIITPP for NiIITPP
in toluene containing (open circle) 1.93, (closed circle) 2.90, (open
triangle) 3.87, (closed triangle) 5.80, and (open square) 7.74 M
diethylamine, represented as a function of temperature.
the concentration of the amines. From eq 12, the Arrhenius
expression for kobsd is simply written as
Figure 8 shows the plot of the quantum yields for the
formation of the six coordinate species, (sec-DPA)2NiIITPP, at
300 K represented as a function of [sec-DPA]. The yield
monotonically increases with an increase in [sec-DPA]. As will
be discussed later, we consider that the photoinduced association
and dissociation of axial amines take place according to the
sequential mechanism. Thus, the photochemical reaction of
kobsd ) A4 exp(-∆E4/RT)
(13)
Figure 7 exhibits the plots of kobsd represented as a function of
temperature at 1.93, 2.90, 3.87, 5.80, and 7.74 M diethylamine
in toluene solutions. From the plots and the least-squares fitting
program, A4 and ∆E4 are determined as A4 ) 1.5 × 1011 s-1

4450 J. Phys. Chem. A, Vol. 107, No. 22, 2003
Suzuki et al.
NiIITPP in sec-DPA is represented by
systems afford large ∆E4 and A4 values for the dissociation of
an axial amine molecule from the six-coordinate species, (sec-
A)2NiIITPP. According to the transition state theory, k4, is
expressed by
NiIITPP(1A1g) + hν f NiIITPP(3B1g)
(14)
(15)
kd
9
NiIITPP(3B1g)
8 NiIITPP(1A1g)
k4 ) (kT/h)ø exp(∆S4*/R) exp(-∆E4/RT)
(20)
ka
9
NiIITPP(3B1g) + sec-DPA
8 (sec-DPA)NiIITPP(3B1) (16)
where ∆S4* is the activation entropy and other symbols have
their usual meanings. Thus, the large A4 values (A4 . 1013 s-1
)
obtained with the secondary amine systems are explained by a
positive ∆S* value at the transition state responsible for the
dissociation of the axial amine. It is suggested that the transition
state has a loose structure in comparison with that of the reactant,
(sec-A)2NiIITPP. A large activation energies would be necessary
for (sec-A)2NiIITPP to attain such a loose transition state.
In primary amine-toluene solutions, NiIITPP and the six-
coordinate species, (pri-A)2 NiIITPP, exist as the equilibrium
mixture. The laser photolysis of the solutions brings forth the
nonequilibrium state because of the photochemical conversion
of (pri-A)2 NiIITPP to NiIITPP or NiIITPP to (pri-A)2 NiIITPP.
The decay of the transients, therefore, is ascribed to the recovery
of the equilibrium. The plots of the decay rate constants vs [pri-
A] are well explained by assuming the steady-state approxima-
tion with regard to the five-coordinate species, (pri-A)NiIITPP.
These results lead to the conclusion that (pri-A)NiIITPP is highly
unstable. It is likely that (pri-A)NiIITPP is readily transformed
to either NiIITPP by dissociation of the axial pri-A and/or (pri-
A)2NiIITPP by association of pri-A at the axial position. Similar
observation has been made with the laser photolysis of the six
coordinate iron(II) porphyrins. Bis-nitrogeneous base com-
plexes of iron(II) porphyrins, (N)2FeIIP, photodissociate an axial
N base to give the five coordinate species, (N)FeIITPP, which
immediately undergoes “spontaneous dissociation” of another
axial N base, resulting in the formation of the four coordinate
FeIIP.4,13,51 The transient FeIIP returns to (N)2FeIITPP via
(N)FeIITPP.
kb
9
(sec-DPA)NiIITPP (3B1g)
8 NiIITPP (1A1g) + sec-DPA
(17)
kc
(sec-DPA)NiIITPP(3B1) + sec-DPA
98
(sec-DPA)NiIITPP(3B1g) (18)
The five coordinate nickel(II) porphyrins are known to have
the 3B1g ground state.40 From eqs 14-18, the yield, Φ, for the
formation of (sec-DPA)2NiIITPP is formulated as
ka[sec-DPA]
kc[sec-DPA]
Φ ) Φ (3B1g)
(19)
kd + ka[sec-DPA] kb + kc[sec-DPA]
The quantum yield Φ(3B1g) for the formation of the lowest
excited state of NiIITPP is assumed to be unity. With the use of
the least-squares fitting program, we obtain kd/ka ) 25.0 ( 1.0
M and kb/kc < 0.1 M. Because the value, 1/kd, has been
determined as ca. 250 ps by the femtosecond photolysis,38 ka is
obtained as ca. 1.6 × 108 M-1 s-1 49
.
The exact value of kb/kc
could not be determined because of the narrow concentration
range of [sec-DPA].
Discussion
Nickel(II) porphyrins, NiIIP, in noncoordinate solvents exhibit
marked changes in the absorption spectra by the addition of
nitrogeneous bases, N.41-43 The absorption spectroscopic studies
have demonstrated that the coordination of two nitrogeneous
bases at the axial positions of NiIIP results in the change in the
absorption spectra. The four- and six-coordinate species, NiIIP
and NiIIP(N)2, have the spin state S ) 0 and 1 at the ground
states, respectively.50 The NMR measurements of the porphyrin
proton suggest that the intermediate five coordinate species,
NiIIP(N) (either S ) 0 or 1), plays a key role in establishment
of the equilibrium reaction between NiIIP and NiIIP(N)2.36,45
The absorption spectral changes observed for primary amine-
toluene solutions of NiIITPP are in good agreement with those
previously measured for solutions of nickel(II) porphyrins
containing piperidine or pyridine.33,41-43 The equilibrium con-
stant, K, for the formation of bis-amine complexes of NiIITPP
increases in the order n-butylamine > n-propylamine > isobu-
tylamine > cyclohexylamine > isopropylamine. This finding
suggests that the branched-primary amines give the equilibrium
constants smaller than linear-chain amines. It is noteworthy that
the K values obtained with 3- and 4-methylpyridine are close
to those of the branched primary amines.
Photoexcited states of NiIITPP and the six-coordinate com-
plexes, (N)2NiIITPP (N ) nitrogeneous bases), have been well
studied by fast transient absorption and Raman spectro-
scopy.7,37-40,50 The femtosecond photolysis38 confirmed that (1)
3 1
1
II
the lowest excited states, 3B1g ( ( dx _y , dz )), of Ni TPP having
the lifetime of ca. 250 ps are produced within 350 fs after the
pulse and (2) photoexcitation of (N)2NiIITPP initially gives the
2
2
2
ligand excited state which decays to the 1A1g state ( ( dz )) within
20 ps. The lifetime of the 1A1g state of (N)2NiIITPP is too short
to be detected owing to the ultrafast dissociation of the axial
ligands.
1 2
2
It has been recognized that the 3B1g state of NiIITPP can take
two nitrogeneous ligands at the axial positions to give the ground
state of (N)2NiIITPP, and the 1A1g state of (N)2NiIITPP dissoci-
ates two axial nitrogeneous bases to yield NiIITPP (1A1g).38 The
reaction mechanism for the formation of (N)2NiIITPP from
NiIITPP at the lowest excited state is considered to be either
the synchronous or the sequential addition reaction of two
N-base molecules at the axial positions. The synchronous
addition mechanism seems very unlikely because it requires
three body collision among NiIITPP and two amine molecules.
Thus, the sequential addition mechanism is more likely for
interpretation of the formation of (N)2NiIITPP from the excited
state of NiIITPP.
The absorption spectrum of NiIITPP in the toluene solution
is not altered by the addition of secondary and tertiary amines
in the temperature range 300-200 K. Presumably, the forward
rate constant, k3, for the formation of the six-coordinate species
is very small because of crowded structures of the secondary
and tertiary amines that prohibited the thermal coordination of
the amines to the axial position of NiIITPP.
From the earlier and present studies,3,34,35,40 the energetic
diagram of the ground and the lowest excited states of NiII-
TPP, (N)NiIITPP, and (N)2NiIITPP are qualitatively described
as shown in Scheme 1. The ground-state energy of NiIITPP is
6-15 kcal higher than (pri-A)2NiIITPP depending on the nature
We have measured activation energies and frequency factors
for k4 processes. Among the amines studied, secondary amine

Nanosecond Laser Photolysis of NiIITPP
J. Phys. Chem. A, Vol. 107, No. 22, 2003 4451
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of the axial primary amine. In the case of the secondary and
tertiary amines, the six-coordinate species, (sec-A)2NiIITPP and
(tert-A)2NiIITPP, cannot be detected even at 200 K, indicating
that the ground-state energy of NiIITPP is much lower than those
of (sec-A)2NiIITPP and (tert-A)2NiIITPP. Presumably, owing to
the bulky aliphatic groups, the secondary and tertiary amines
suffer the strong strain energy by coordination to NiIITPP, and
thus, the five- or six-coordinate species are hardly produced by
thermal reactions.
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2-methylpyridine hardly reacts with NiIITPP, suggesting that
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2
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According to Scheme 1, the photodissociation of the two axial
primary amine molecules sequentially occurs at the excited
1
state: A1g of (pri-A)2NiIITPP f 1A1 of (pri-A)NiIITPP f 1A1g
of NiIITPP. The sequential addition of two primary amines at
3
the excited state of NiIITPP is represented as: B1g of NiIITPP
3
3
f B1 of (pri-A)NiIITPP f either B1g of (pri-A)2NiIITPP or
1A1g of NiIITPP.
In secondary amine-toluene solutions of NiIITPP, the
relaxation pathways of the excitation energy leading to the
association of the amine molecules at the axial positions are
3B1g of NiIITPP f 3B1g of (sec-A)NiIITPP f either 3B1g of (sec-
1
A)2NiIITPP or A1g of NiIITPP.
It is found that neither the thermal nor photochemical
reactions of NiIITPP in tertiary amine-toluene solutions gives
(tert-A)2NiIITPP. This finding implies that, as shown in Scheme
3
1, the B1 state of (tert-A)NiIITPP is higher in energy than the
3
3B1g state of NiIITPP, and the B1g state of (tert-A)2NiIITPP is
1
higher in energy than the A1g state of NiIITPP.
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(49) We can also obtain kd/ka < 0.1 M and kb/kc ) 25 M from the plot
in Figure 8 and eq 19. The former value gives ka > 4.0 × 1010 M-1 s-1
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which clearly exceeds the limit of the diffusion controlled process. We,
thus, discard the values of kd/ka < 0.1 M and kb/kc ) 25 M.
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