A kinetic study on the spin polarization switching of benzil in the presence of triethylamine

Article abstract of DOI:10.1021/jp0033768

Dual spin polarization of CIDEP signal of radicals produced in benzil/triethylamine in the benzene system was investigated by the time-resolved ESR and transient absorption technique. CIDEP signals showed absorption or emission depending on the concentration of triethylamine. Emissive CIDEP signal was observed below 10^-3 M of triethylamine, whereas absorptive signal was observed above 10^-1 M. The dual CIDEP signal was explained by the triplet mechanism (TM) and the radical-triplet pair mechanism (RTPM); the absorptive polarized TM signal was generated from the reaction of free triplet benzil with triethylamine, whereas the emissive polarized CIDEP signal was generated through the interaction between triplet benzil and benzil ketyl radical produced from the benzil-triethylamine complex.

Full text of DOI:10.1021/jp0033768

J. Phys. Chem. A 2001, 105, 3741-3744
3741
A Kinetic Study on the Spin Polarization Switching of Benzil in the Presence of
Triethylamine
Tetsuo Okutsu,* Masaki Ooyama, Katsuhiko Tani, and Hiroshi Hiratsuka
Department of Chemistry, Gunma UniVersity, Kiryu Gunma 376-85l5, Japan
Akio Kawai
Department of Chemistry, Tokyo Institute of Technology, 2-12-1 Ohokayama, Meguro-ku,
Tokyo 152-8551, Japan
Kinichi Obi
Department of Chemical and Biological Sciences, Japan Women’s UniVersity, Mejirodai, Bunkyo-ku,
Tokyo, 112-8681, Japan
ReceiVed: September 18, 2000; In Final Form: January 10, 2001
Dual spin polarization of CIDEP signal of radicals produced in benzil/triethylamine in the benzene system
was investigated by the time-resolved ESR and transient absorption technique. CIDEP signals showed absorption
or emission depending on the concentration of triethylamine. Emissive CIDEP signal was observed below
10-3 M of triethylamine, whereas absorptive signal was observed above 10-l M. The dual CIDEP signal was
explained by the triplet mechanism (TM) and the radical-triplet pair mechanism (RTPM); the absorptive
polarized TM signal was generated from the reaction of free triplet benzil with triethylamine, whereas the
emissive polarized CIDEP signal was generated through the interaction between triplet benzil and benzil
ketyl radical produced from the benzil-triethylamine complex.
Introduction
In our previous study,3 the dual spin polarization of benzil
in the presence of amines or phenols was interpreted to result
Benzil is a typical R-dicarbonyl molecule. Many studies on
the photochemical reactions have been reported on benzil. The
photochemical reactions of benzil have been studied by flash
photolysis1,2 and time-resolved ESR.3-5 Benzil has attracted our
attention because the CIDEP signals of produced radicals show
dual spin polarization in the presence of various amines and
phenols. For example, spin polarization of benzil ketyl and
counter radicals produced in the benzil/aniline system is inverted
twice depending on the concentration of quenchers, net emission
at l × l0-3 M of aniline, net absorption at l × l0-1 M, and
again inverted to emission at 10 M.3
from two different precursors. Absorptive polarized radicals
were formed in the reaction of free triplet benzil whose ∆ms )
2 transition was known to be absorptive in the low-temperature
experiment.3 Emissive polarized radicals were produced through
the photoreaction of benzil-quencher complex formed in the
ground state. Assuming that the excited triplet complex gave
an emissive CIDEP signal by the triplet mechanism (TM), it
was successfully demonstrated that the polarization inversion
occurred twice by changing the quencher concentration.
In a benzene solution of benzil in the presence of triethyl-
amine (TEA), spin polarization inversion was also observed.14
The inversion, however, could not be explained by the above
mechanism. The inversion occurred only once; emission was
observed at a very low concentration of TEA (<l0-3 M) and
absorption at a high concentration (>l0-l M). Mukai et al. also
demonstrated dual spin polarization in benzil/TEA in 2-propanol
solution.15 They concluded that the different electronic states
of benzil yielded different intermediate species with opposite
spin polarization. They assigned that the absorptive polarized
radical was the benzil anion radical produced through a one
photon process, while the emissive polarized radical was the
benzil ketyl radical formed from the higher triplet state Tn
through a two photon process. The spin sublevel population of
Tn produced from a higher singlet state through intersystem
crossing (ISC) was assumed to be different from that of Tl
generated from Sl. This Tn state was thought to react with the
solvent 2-propanol and produced the emissive polarized ketyl
radical.
It is well-known that benzil has two rotational isomers.6-8
These isomers have been thought to be responsible for the dual
spin polarization of CIDEP. Several studies have been made
on the conformations of benzil. Benzil in the ground state has
a nonplanar (skew) conformation in which the central carbonyl-
carbonyl bond is twisted at an angle of 111˚.9 Therefore, the
two benzoyl moieties are not in conjugation with each other.
In the first excited singlet state, benzil has a trans-planar
conformation. Morants and Wright reported that the skew benzil
relaxed to the trans-planar conformation after the excitation.10
Miyasaka and Mataga reported that conformational relaxation
in the Sl state was completed within a few picoseconds,11 though
the lifetime of the S1 state was fairly long (ca. 2 ns). The
conformation of the triplet state (T1) was reported to be identical
to that of the Sl state; i.e., the conformations in the Sl and Tl
states are thought to be unique.12,13 Therefore, the dual CIDEP
signal must have nothing to do with the existence of rotational
isomers.
Recently, the quartet-precursor radical-triplet pair mechanism
(RTPM) was revealed to play an important role in the spin
* Corresponding author. E-mail: okutsu@chem.gunma-u.ac.jp.
10.1021/jp0033768 CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/27/2001

3742 J. Phys. Chem. A, Vol. 105, No. 15, 2001
Okutsu et al.
Figure 2. Transient absorption spectra obtained by laser photolysis in
benzil (2 × 10-2 M)/TEA (5 × 10-4 M) in benzene at (a) 200, (b)
400, (c) 800, (d) 1600, and (e) 3200 ns after the laser pulse.
Figure 1. Concentration dependence of the CIDEP spectra on TEA
obtained by laser photolysis in benzil (2 × 10-3 M)/TEA in benzene.
Concentrations of TEA are (a) 1 × 10-3 M and (b) 10-1 M.
polarization.16-18 Bla¨ttler and Paul reported that the benzil ketyl
radical in 2-propanol solution was produced by a single-photon
process from the lowest triplet and the two-photon process was
via a higher triplet state. And they demonstrated that emissive
polarization of benzil ketyl was due to RTPM.19
In this study, we have reinvestigated the switching mechanism
of spin polarization in the photoreactions of benzil in the
presence of TEA and revealed that, in addition to TM, RTPM
also plays an important role in the spin polarization inversion;
the net emissive CIDEP signal is due to RTPM, and the net
absorptive signal is due to TM.
Figure 3. Laser power dependence of the production yield of benzil
ketyl radical for (9) benzil anion radical, (O) benzil ketyl radical
produced from the benzil/TEA system, and (2) benzil ketyl radical
produced from benzil in 2-propanol.
Experimental Section
Time-resolved ESR measurements were carried out using a
Varian E-112 spectrometer at X-band without field modulation.
Signals were accumulated in a boxcar integrator (Stanford SR-
250). The gate of the boxcar was opened for 1.1-2.1µs after
the laser pulse. An excimer laser (Lambda Physik LPX100) was
used as an excitation light source. The details of the experimental
apparatus for time-resolved ESR measurements were described
CIDEP signal is inverted to absorption. According to TM, the
polarization of produced radicals reflects that of the triplet
precursor. Though the triplet state of benzil has absorptive
character as known in the low-temperature experiments, the
produced radicals in our system show both emissive and
absorptive polarization depending on experimental conditions.
Transient absorption experiments were carried out to examine
the generation mechanism of these radicals. Figure 2 shows the
transient absorption spectra of the benzil in the presence of TEA
in benzene. The spectrum obtained at 200 ns has peaks at 370
and 480 nm and a broad absorption around 600 nm. The peak
at 480 nm is attributed to the T-T absorption of benzil, because
the transient absorption spectrum of triplet benzil has a typical
peak at 480 nm.2 The peak at 370 nm is assigned to the benzil
ketyl radical because the transient absorption spectrum of benzil
ketyl radical shows a peak at 370 nm.14 The longer wavelength
transient absorption around 600 nm is ascribed to the benzil
anion radical. We measured laser power dependence of the
transient absorption signals of the ketyl and anion radicals in
benzil/TEA in benzene and benzil in 2-propanol.
Figure 3 shows the logarithmic plots of the benzil ketyl and
anion radicals vs laser power. Laser power dependencies gave
slopes of 0.9 for the ketyl radical, 1.1 for the anion radical in
the benzil/TEA system, and 1.1 for the ketyl radical in the benzil/
2-propanol system. It is concluded that all radicals are produced
through a one photon process with excitation at 355 nm. Since
the laser power dependence on the CIDEP signal of the emissive
polarized ketyl radical was 2, another photon should take part
in the generation of the emissive spin polarization.
previously.20-22 For transient absorption measurements, a Nd3+
:
YAG laser (Quanta-Ray GCR-130, 355 nm) was used as an
excitation light source. The samples were flowed through a
quartz cell with a flow rate of ca. 30 mL min-1. A Unisoku
TSP-601H laser system was used as signal detection.
GR grade benzil (Tokyo Kasei) was used in CIDEP and
transient absorption experiments. GR grade triethylamine (Tokyo
Kasei) was used without further purification. GR grade benzene
(Kanto Kagaku) and GR grade 2-propanol (Kanto Kagaku) were
used as solvents without further purification.
Results and Discussion
Figure 1 shows the CIDEP spectra obtained in laser photolysis
of benzil in the presence of TEA. Benzene was used as a solvent.
The spectrum in Figure la obtained at concentrations of 2.0 ×
10-3 M benzil and l0-3 M TEA shows net emissive signals
without hyperfine structure. This signal was assigned to the
benzil ketyl radical. There is little difference in the g-values
between the benzil ketyl radical and triethylamine radical, and
thus, the net emissive CIDEP signal is produced by TM or
RTPM. Figure lb shows a CIDEP spectrum obtained in the same
system at a high concentration of TEA (0.1 M). The phase of

Spin Polarization Switching of Benzil
J. Phys. Chem. A, Vol. 105, No. 15, 2001 3743
Figure 5. Kinetic simulation of the CIDEP intensities at t ) 1.5 µs as
a function of initial concentration of TEA. Emissive polarization due
to RTPM is indicated by the solid line and absorptive polarization due
to TM is indicated by the broken line.
d[3Babs*]
Figure 4. Reaction scheme to product benzil ketyl radical.
) -(ksp + ket[TEA])[3Babs*]
(9)
(10)
(11)
dt
A detailed kinetic analysis was reported in the previous
paper.22 The whole reaction scheme and kinetic data in the
benzil/TEA system are summarized in Figure 4. There is
chemical equilibrium in the ground-state molecules,
d[(B-‚‚‚TEA+)abs
]
) ket[3Babs*][TEA]
dt
•
d[BHabs
]
) RkptΦr[(B-‚‚‚TEA+)abs
]
B + TEA {K
\
} B‚‚‚TEA
(1)
dt
where R is a factor for spin polarization generation. The value
of R is not known and assumed as unity in the following
calculation. [3Babs*] at t ) 0 is considered to be equal to the
initial concentration of triplet benzil, which was calculated from
the molecular extinction coefficient of the ground state, the
intersystem crossing quantum yield24 (Φisc ) 0.92), the equi-
librium constant, and the excitation laser power. [3Babs*] )
1.5 × l0-5 M at [benzil] ) 9 × l0-3 M and 25 mJ pulse-1. Φr
is a quantum yield of the ketyl radical formation which is 0.31.22
The concentration of BHabs• was obtained by solving the above
rate equations numerically up to t ) 1.5 µs. The results are
plotted as a broken line in Figure 5 against initial concentration
of TEA.
The CIDEP signals shown in Figure 1b was measured by
opening the gate of the boxcar integrator from 1.1 to 2.1 µs. In
this time range, only 13% of initially produced anion radicals
survive in CIP as an average and 87% CIP dissociates to the
ketyl radicals. The CIDEP spectrum in Figure 1b is, therefore,
mainly due to the ketyl radical with some contribution of the
anion radical and reflects the absorptive spin polarization of
triplet benzil through TM.
where B is benzil and K is the equilibrium constant. Equilibrium
constant K was obtained as ca. 15 M-1 22
Both chemical species,
.
free benzil and its complex with TEA, are responsive to
photoabsorption,
B + hν f 1B*
(B‚‚‚TEA) + hν f 1(B‚‚‚TEA)*
(2)
(3)
and the singlet excited states of both species undergo intersystem
crossing to the triplet states,
1B* ISC8 3Babs
1(B‚‚‚TEA)* ISC8 3(B‚‚‚TEA)*
*
(4)
(5)
where the subscript “abs” represents absorption of spin polariza-
tion.
Free triplet benzil yields the contact ion pair (CIP) of benzil
anion (B-‚‚‚TEA+), which decays with the rate constant of
1.5 × 106 s-1 forming the ketyl radicals with absorptive
polarization through TM.
Next, we will discuss the ketyl formation from the complex
in the ground state.
ksp
9
3Babs
*
8 3Bth*
(6)
(7)
(8)
3(B‚‚‚TEA)* f BH• + TEA•
(12)
ket
3Babs* + TEA
9
8 (B-‚‚‚TEA+)abs
The concentration of excited-state complex was calculated from
com
the molecular extinction coefficient ꢀ353
) 490 M-1 cm-1
kpt
9
of the complex22 and excitation laser power by using the
following equation:
(B-‚‚‚TEA+)abs
8 BHabs‚ + TEA‚
where 3Bth* is thermally distributed triplet benzil. ksp is the spin
relaxation rate of the triplet benzil which was measured as 1 ×
1010 s-1 by optical induced magnetization.23 ket and kpt are the
CIP formation and proton-transfer rate constants, and their
values are 6.0 × l08 M-1 s-1 and 1.5 × l06 s-1, respectively.
The decay of the triplet benzil is not needed to be included in
the reaction scheme of the spin polarized ketyl formation due
to the slow decay rate, 2.3 × 104 s-1, compared with spin
relaxation rate. These kinetic parameters are listed in our
previous study.22 The rate equations for the above scheme are
expressed as
I0(1 - 10-ꢀcl
)
[3(B‚‚‚TEA)*] )
(13)
NAV
Here I0 is the excitation laser power and was estimated as
4.4 × l016 photons when there is a 25 mJ pulse-1 at 355 nm,
NA is Avogadro number, and V is the volume of the cell used.
The quantum yield of ketyl formation from the complex was
confirmed to be unity.22
Now, we will focus our interest on the mechanism of spin
polarization inversion. In our previous paper, we proposed that

3744 J. Phys. Chem. A, Vol. 105, No. 15, 2001
Okutsu et al.
the spin polarization inversion is due to the complex forma-
tion: free benzil gives absorptive CIDEP through TM, whereas
the complex of benzil with TEA yields the emissive one through
TM, too, which is opposite to the spin polarization of the triplet
produces benzil ketyl in our system, Mz(t ) 0)/Peq ) 0 M was
chosen as the initial condition.
The equations were solved numerically up to t ) 1.5 µs. [BH•]
was calculated from eq 13. The results are shown in Figure 5
as a solid line against the initial concentration of TEA. Since
the concentrations of the complex and hence produced ketyl
are low at low concentration of TEA, RTPM does not work
efficiently and emissive CIDEP signals are weak. The chemical
equilibrium (1) moves to the right with the increase in TEA
concentration, and the ketyl concentration becomes high enough
to promote RTPM, where triplet benzil still survives. When TEA
concentration is higher than 10-2 M, triplet benzil reacts with
TEA to yield CIP and the efficiency of RTPM goes down. These
are the reasons the emissive CIDEP intensity curve shows a
peak around 10-2 M of TEA.
3
state measured at 77 K. We should, however, take back this
idea because that spin polarization by TM from the photoex-
citation of the complex cannot explain the second-order laser
power dependence of the CIDEP signal intensity in this system.
Another mechanism to give emissive polarization is RTPM16-18
which works between a spin relaxed triplet molecule and a free
radical. According to RTPM, radicals always show emissive
polarization regardless of the triplet spin polarization. The singlet
excited state of the complex produced in reaction (3) yields the
ketyl radical following ISC. These processes are thought to be
too fast compared with Larmor precession rate to generate spin
polarization and, hence, the produced radicals would not show
CIDEP. The ketyl radicals produced in reaction (12) interact
with spin relaxed triplet benzil formed in process (6), and
emissive polarization is generated on the free radicals through
RTPM.
Absorptive polarized CIDEP is also plotted in the figure as
a broken line in arbitrary unit. From the curves in Figure 5, it
is easily understood that the polarization changes from emission
to absorption with increase in TEA concentration.
These demonstrations reproduce the experimental results
about the spin polarization inversion depending on the TEA
concentrations.
RTPM
3B* + BH•
8 1B + BHem
(14)
•
Summary
Intensity of the spin polarization due to RTPM is already
calculated by Bla¨ttler and Paul, and we carried out the spin
polarization estimation after their method.19 These are ex-
pressed as
Mechanism of spin polarization generation in photoreaction
of the benzil/triethylamine system was investigated by using
the time-resolved ESR and transient absorption techniques. The
benzil ketyl radicals produced show spin polarization inversion
depending on TEA concentration, emission at low and absorp-
tion at high concentrations. We have analyzed this double-faced
polarization of CIDEP signal with kinetic data and revealed that
photoexcitaion of the ground-state complex yields the ketyl
radicals with emissive polarization through RTPM, and the
triplet state of free benzil forms those with absorptive polariza-
tion through TM. This study demonstrates that RTPM readily
occurs in an ordinary photochemical system under laser excita-
tion condition.
dM
y ) -T2-1My - ω1Mz
(15)
dt
dM
dt
z ) ω1My - T1-1(Mz - Peq[BH•]) + PkRTPM[3Bth*][BH•]
(16)
The triplet state of benzil decays,
k0
9
3B*
8 1B
(17)
(18)
(19)
References and Notes
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kT-T
23B*
8 21B
ket
3B* + TEA
9
8 B- + TEA+
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where k0 is the single-exponential triplet decay rate k0
)
2 × 104 s-1, kT-T is the rate constant of the T-T annihilation
2kT-T ) 1.3 × 108 M-1 s-1, kRTPM is the rate of the quenching
of the triplet benzil by benzil ketyl radical kRTPM ) 1010 s-1
which is the value of the diffusion-controlled rate, and ket is
the rate constant of electron transfer ket ) 6.0 × 108 s-1 22
. Spin
polarization intensity by RTPM is the simulated ESR time
profile by numeric integration by solving the Bloch equations
and kinetic treatments. The magnetization induced by RTPM
in the z-component is coupled with microwave field amplitude
ω and created the magnetization in the y-component. The
parameters are unknown for T1 and T2 in our system. These
values are quoted from the literature T1 ) 6 µs and T2 ) 0.25
µs-1 19
which are the values in 2-propanol at -50 °C. These
,
must be different from the value at our conditions in benzene
solution at 25 °C; however, we can get some qualitative
information of the CIDEP intensity which corresponds to the
concentration of TEA. ω ) 1.9 × 105 rad s-1 was obtained in
our system. P/Peq ) -60 is used which is reported to be the
adequate ratio of RTPM polarization. Peq is the spin polarization
of the Boltzmann distribution. Because of no two-photon process
(23) Takagi, Y. Chem. Phys. Lett. 1985, 119, 5
(24) Carmichel, I.; Hug, G. L,; Murov, S. L. Handbook of Photochem-
istry; Marcel Dekker Inc.: New York, 1992.