Magnetic field effects on the decay rates of triplet biradical photogenerated from intramolecular electron-transfer in a zinc- tetraphenylporphyrin-fullerene linked compound

Article abstract of DOI:10.1016/j.cplett.2003.12.112

Transient absorption spectra of a zinc-tetraphenylporphyrin(ZnP)- fullerene(C₆₀) linked compound with flexible eight methylene groups indicated that intramolecular electron-transfer occurred in benzonitrile, while not in toluene. In benzonitrile, the decay rate constant of biradical decreases quickly in lower magnetic fields (<0.1 T), and then increases gradually and finally become almost constant in higher magnetic fields (0.1≤H≤1.2 T). The magnetic field effects (MFEs) verified that the triplet biradical was generated from the intramolecular electron-transfer of the triplet-excited states of ZnP and C₆₀. The reverse phenomenon of the MFEs around 0.1 T is most likely ascribed to the properties of C₆₀ moiety.

Full text of DOI:10.1016/j.cplett.2003.12.112

Chemical Physics Letters 385 (2004) 417–422
www.elsevier.com/locate/cplett
Magnetic field effects on the decay rates of triplet biradical
photogenerated from intramolecular electron-transfer in a
zinc-tetraphenylporphyrin-fullerene linked compound
a,
a
a
a
Hiroaki Yonemura *, Hideki Nobukuni , Shinya Moribe , Sunao Yamada ,
b
Yoshihisa Fujiwara , Yoshifumi Tanimoto
c
a
Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
b
Department of Mathematical and Life Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
c
Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan
Received 16 September 2003
Published online: 29 January 2004
Abstract
Transient absorption spectra of a zinc-tetraphenylporphyrin(ZnP)-fullerene(C60) linked compound with flexible eight methylene
groups indicated that intramolecular electron-transfer occurred in benzonitrile, while not in toluene. In benzonitrile, the decay rate
constant of biradical decreases quickly in lower magnetic fields (<0.1 T), and then increases gradually and finally become almost
constant in higher magnetic fields (0:1 6 H 6 1:2 T). The magnetic field effects (MFEs) verified that the triplet biradical was gen-
erated from the intramolecular electron-transfer of the triplet-excited states of ZnP and C60. The reverse phenomenon of the MFEs
around 0.1 T is most likely ascribed to the properties of C60 moiety.
Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction
powerful for verifying the mechanism of photochemical
reactions. Previously, we have obtained photogenerated
triplet biradicals of donor-acceptor linked compounds,
and found that the lifetimes of the biradicals are re-
markably extended in the presence of magnetic fields up
to 1 T with electromagnet [11–15].
High MFEs on the dynamics of radical pairs or bi-
radicals generated by photochemical reactions have
been extensively investigated to establish the measure-
ment of MFEs under high magnetic fields above 1 T
[8,10,17–25]. The reverse phenomena in the MFEs have
been reported in some radical pairs or biradicals. The
reverse phenomena of the MFE on the lifetime, in the
linked biradicals, have been observed only in high
magnetic fields above 1 T [19–24].
Recently, photochemical and photoelectrochemical
properties of fullerene (C60) have been widely studied
[1]. Photoinduced electron-transfer reactions of donor-
C60 linked molecules have been also reported [1–7]. In a
series of donor-C60 linked systems, some of the com-
pounds show novel properties, which accelerate photo-
induced charge separation and decelerate charge
recombination [3–5]. Those properties have been ex-
plained by the remarkably small reorganization energy
in their electron-transfer reactions. The porphyrin–C60
linked compounds, where the porphyrin moieties act as
both donors and sensitizers, have been extensively
studied [3–5].
Magnetic field effects (MFEs) on the reaction kinetics
or yields of photochemical reactions in the condensed
phase have been studied [8–16]. The MFEs have been
As to the MFEs in donor-C60 linked systems, C60 has
interesting properties. First, C60 has no magnetic nuclei
based on hydrogen, and thus there are little hyperfine
interactions due to the C60 radical contributing to the
MFEs. Second, C60 is a highly symmetric molecule, and
therefore anisotropic Zeeman interaction, due to the C60
* Corresponding author. Fax: +81-92-642-3611.
E-mail address: yonetcm@mbox.nc.kyushu-u.ac.jp (H. Yonemura).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2003.12.112

418
H. Yonemura et al. / Chemical Physics Letters 385 (2004) 417–422
radical, is very small. Third, the g-value (g ¼ 1:9982)
tial pulse voltammetry (DPV), and MFEs were
measured according to the previous papers [13–15].
Fluorescence lifetimes were measured by a single-
photon-counting system using a dye laser (423 nm) with
a N2 laser. Toluene and benzonitrile were used as
received.
ꢀꢀ
[26] of C60 radical (C60 ) is smaller than that of the
donor radical such as phenothiazine cation radical
(g ¼ 2:0052) [27]. Therefore, the difference of g-values
between the donor and C60 is expected to be large. On
the basis of the three points, the MFEs in various donor-
C60 linked compounds are expected to provide useful
information for studying the spin chemistry of C60.
From these considerations, we recently examined pho-
toinduced electron-transfer reactions and MFEs on the
photogenerated biradicals in phenothiazine–C60 linked
compounds and found the MFEs on the intramolecluar
electron-transfer reactions in benzonitrile, ascribing to
the formation of the triplet biradicals [13–15]. However,
MFEs on the decay of biradicals in porphyrin–C60
linked compounds have not yet been reported.
In the present Letter, we have examined photoin-
duced electron-transfer reactions and MFEs on the
photogenerated biradicals in a zinc-tetraphenylpor-
phyrin (ZnP)–C60 linked compound (ZnP(8)C60).
Contribution of a triplet biradical, generated from
intramolecular electron-transfer reaction, to those
reactions was verified by the MFEs in toluene and
benzonitrile.
3. Results and discussion
3.1. Solvent effect on photoinduced electron-transfer
Absorption spectrum of ZnP(8)C60 in benzonitrile is
almost the superposition of the spectra of the individual
compounds (ZnPref and C60ref). A similar result was
also obtained in toluene. These results indicate that
there are no appreciable intramolecular electronic in-
teractions between the ZnP and the C60 moieties at the
ground states in both solvents.
N
N
N
N
O
(CH2)8
O
H3CO
Zn
N
CH3
N
CH3
ZnP(8)C60
C60ref
2. Experimental
Fluorescence lifetimes (s) of ZnP(8)C60 and ZnPref
were measured by a time-correlated single-photon-
counting apparatus and monitoring at 610 nm (benzo-
nitrile) and 600 nm (toluene). The fluorescence decay
curve was well fitted with a single exponential approxi-
mation. The s values of ZnP(8)C60 were 1.1 ns in ben-
zonitrile and 1.0 ns in toluene, respectively, and were
considerably shorter than those of ZnPref (1.8 ns in
benzonitrile and 1.8 ns in toluene). Thus, the rate con-
Synthetic procedure of ZnP(8)C60 is shown in Scheme
1. ZnPref was prepared according to the previous paper
[28]. The structure of ZnP(8)C60 was confirmed by H-
NMR and MALDI-TOF MS spectra and elemental
analysis.
1H-NMR, steady-state absorption, and transient ab-
sorption spectra, cyclic voltammetry (CV) and differen-
1
N
N
N
N
O
(CH2)8
Br
+
CHO
HO
Zn
ZnPref
N
N
N
N
CHO
O
(CH2)8
O
Zn
K2CO3
DMF
1
MeNHCH2CO2H + C60
+
ZnP(8)C 60
1
toluene
Scheme 1.

H. Yonemura et al. / Chemical Physics Letters 385 (2004) 417–422
419
Scheme 2.
stants for the quenching of the singlet-excited state of
ZnP (1ZnPꢁ) in benzonitrile and toluene were estimated
to be 3.5 ꢂ 108 and 4.4 ꢂ 108 sꢀ1, respectively. These rate
constants are comparable with that (kisc1 ¼ 4:0 ꢂ 108 sꢀ1
in Scheme 2) of the intersystem crossing for ZnP [29].
Transient absorption spectra of ZnP(8)C60 were
measured following laser excitation at 532 nm, where
absorption ratios of the ZnP and the C60 moieties were
72:28 in benzonitrile or 81:19 in toluene. The results are
shown in Fig. 1. In toluene, two broad absorption bands
were observed (Fig. 1a). One (kmax, 700 nm) was mainly
assignable to the T1–Tn absorption of the C60 moiety [1],
and the other (k ¼ 450–550 nm) to that of the ZnP
moiety. The triplet-excited state of ZnP (3ZnPꢁ) was
probably generated via intersystem crossing from
1ZnPꢁ. The triplet-excited state of C60 (3C60ꢁ) was also
likely generated via the intersystem crossing from sin-
glet-excited state of C60 (1C60ꢁ), that was formed by the
direct excitation of C60 and by the energy-transfer from
1ZnPꢁ. No appreciable absorption signals due to charge
separated ZnP cation radical (ZnPꢀþ) and C60 were
observed.
ꢀꢀ
In contrast, the T1–Tn absorption of the C60 and the
ZnP moieties disappeared in benzonitrile (Fig. 1b), while
the bands due to the ZnPꢀþ (kmax, 650 nm) [3–7] and the
ꢀꢀ
C60 (>600 nm) [1] were observed. The spectrum is in
good agreement with that of the charge-separated state
ꢀꢀ
(ZnPꢀþ–C60 ), as reported in the ZnP–C60 dyads [3–
7,29]. The results clearly indicate that the quenching of
3
ꢁ
3ZnPꢁ by C60 and/or of C60 by ZnP occurs and sub-
3
ꢀꢀ
sequently triplet biradical (ZnPꢀþ–C60 ) forms in ben-
zonitrile as shown in Scheme 2. The singlet biradical
might be also generated by the intramolecular electron-
1
1
ꢁ
transfer from ZnPꢁ to C60 and/or from ZnP to C60
,
though it was hardly detectable in the present system.
Accordingly, intramolecular electron-transfer occurs for
the four excited states of ZnP–C60 system as: 1ZnPꢁ–C60,
ꢁ
3ZnPꢁ–C60, ZnP–1C60ꢁ, and ZnP–3C60 in benzonitrile,
0.08
0.06
0.04
0.02
0
(a)
resulting formation of the biradical. The decays of those
ꢀꢀ
radicals (ZnPꢀþ and C60 ) were found to follow the first-
order reaction kinetics. The decay profiles of transient
absorption at 460 nm at 288 K are shown in Fig. 2.
The decay curves could be expressed by the following
450
550
650
λ /nm
750
0.08
0.1
0.08
0.06
0.04
0.02
0
(b)
0.06
0.1 T
1.2 T
0T
0.04
0.02
0
0.5
Time / µs
0
1
1.5
450
550
650
λ /nm
750
Fig. 1. Transient absorption spectra of ZnP(8)C60 in: (a) toluene (at 1.0
ls) and (b) benzonitrile (at 0.25 ls) after laser excitation at 288 K.
Fig. 2. Decay profiles of transient absorption at 460 nm in benzonitrile
at 0, 0.1, and 1.2 T at 288 K.

420
H. Yonemura et al. / Chemical Physics Letters 385 (2004) 417–422
equation: AðtÞ ¼ A0 expðꢀkdtÞ þ C, where AðtÞ is tran-
sient absorption at tðsÞ, A0 and C are time-independent
constants, and kd (sꢀ1) is the decay rate constant for the
biradical. These parameters were calculated by the non-
liner least-squares method. In the absence of magnetic
field (0 T) the lifetime of the biradical was estimated to
be 231 ns for the observed kd-value (4.3 ꢂ 106 sꢀ1). It
was longer than that (118 ns) for phenothiazine–C60
linked compounds [13–15], while shorter than that (780
ns) for other ZnP–C60 dyads [3–5] in benzonitrile.
Energy diagram and electron-transfer processes for
ZnP(8)C60 in benzonitrile and toluene are summarized
in Scheme 2. Thermodynamic data for the present in-
tramolecular electron-transfer reactions in ZnP(8)C60
were evaluated as reported previously [14,15]. In the
case of benzonitrile, the Gibbs free energy change
(DGCR) for the intramolecular charge recombination
Fig. 2. With the increase of the magnetic field, the kd-
value decreased steeply at lower magnetic fields (<0.1
T), and then recovered gradually (0:1 6 H < 0:6 T) and
became almost constant in higher magnetic fields (>0.6
T) as shown in Fig. 3. As a result, this reverse phe-
nomenon of the MFEs around low magnetic field (ꢃ0.1
T) strongly indicates that the back electron-transfer
occurs via the triplet state of biradical. The kd-values
were evaluated to be 4.3 ꢂ 106 sꢀ1 at 0 T and 3.1 ꢂ 106
sꢀ1 at 0.08 T, respectively. Thus the decay rate of the
biradical was reduced by 1.4 times on going from 0 to
0.08 T. The degree of MFEs on the kd-value was smaller
than those (1.8–2.2 times) in the case of phenothiazine–
C60 linked compounds [13–15]. This reverse phenome-
non is interesting and unprecedented, because that in the
other linked compounds were observed around high
magnetic fields (1–2 T) [19–24].
ꢀꢀ
(CR) process from C60 to ZnPꢀþ was calculated by
The MFEs for a biradical can be explained by spin
rephrasing (Dg), hyperfine coupling (hfc), and spin-
lattice relaxation (SLR) mechanisms [8–10]. The rate
constant kisc due to the Dg mechanism at 1.2 T is eval-
uated to be of the order of 108 sꢀ1 from the g values of
using the redox potentials (E1=2ðZnPꢀþ=ZnPÞ ¼ 0:26 V
ꢀꢀ
and E1=2ðC60=C60 Þ ¼ ꢀ1:02 V vs. Fcþ/Fc) of ZnP and
C60 moieties in ZnP(8)C60. The driving force (ꢀDGCS
)
for the intramolecular charge separation (CS) processes
1
ꢀꢀ
was determined by using of the state energy for ZnPꢁꢁ
ZnPꢀþ (g ¼ 2:0025) [30] and C60 (g ¼ 1:9982) [26]
3
1
ꢁ
3
(2.04 eV), ZnPꢁ (1.53 eV), C60 (1.75 eV), and C60
(1.50 eV) [3]. In benzonitrile, the Coulombic terms can
be negligible. Center to center distance (Rc) between ZnP
radicals, which is much larger than the kd-value. Thus,
the Dg mechanism is ineffective where the magnetic fields
are <1.2 T.
ꢀ
and C60 is estimated to be 24.0 A for ZnP(8)C60 by as-
suming the conformation based on AM1 analysis. The
The hfc mechanism can be discussed by a semitheo-
retical value (B1=2) of the half width of the MFE, where
B1=2 is given by B1=2 ¼ 2ðB21 þ B22Þ=ðB1 þ B2Þ. B1 and B2
refer to the hyperfine (hf) interactions between the nu-
clear spins and unpaired electron spins of each radical
[8]. The B1=2 value is estimated to be 0.9 mT using hfc
constants of the ZnPꢀþ [30], because the C60 moiety
cannot contribute to the hf interaction. The B1=2 value of
0.9 mT is smaller than the experimental value of 2.5 mT
(Fig. 3). In addition, the rate constant (kisc3 in Scheme 2)
for intersystem crossing (ISC) due to the isotropic hf
effective ionic radii of the donor and acceptor radicals
ꢀþ
ꢀꢀ
are rþ ¼ 5:0 A for ZnP [29] and rꢀ ¼ 4:4 A for C60
ꢀ
ꢀ
[29], respectively. Free energy changes, ꢀDGCS1
,
ꢀDGCS2, ꢀDGCS3, and ꢀDGCS4 are the free energy
changes for electron-transfer from ZnPꢁ/3ZnPꢁ to C60
1
and from ZnP to C60ꢁ/3C60 (kCS1, kCS2, kCS3, and kCS4
in Scheme 2), respectively. In toluene, the free energy
changes were calculated from Rehm–Weller and Born
equations by using the redox potentials in benzontrile.
1
ꢁ
Free energy changes, ꢀDGCS1
,
ꢀDGCS2
,
ꢀDGCS3
,
ꢀDGCS4, and ꢀDGCR were estimated to be 0.76, 0.25,
0.47, 0.22, and 1.28 eV in benzonitrile, respectively,
while to be )0.14, )0.67, )0.45, )0.70, and 2.20 eV in
toluene, respectively.
6
5
4
5
Thus, the intramolecular electron-transfer from
1ZnPꢁ or 3ZnPꢁ to C60 and from ZnP to 1C60ꢁ or 3C60ꢁ is
thermodynamically favorable in benzonitrile but is un-
favorable in toluene. The thermodynamic consider-
ations are in good agreement with the solvent effect on
the photochemical reactions in the fluorescence and the
transient absorption spectra as described above.
3
0
0.1
0.2
4
3
0
0.2 0.4 0.6 0.8
H/T
1
1.2
3.2. Magnetic field effects on the decay of biradical
Fig. 3. MFEs on the decay rate constants (kd) for the biradical as
evaluated from the transient absorption at 460 nm upon laser excita-
tion of ZnP(8)C60 in benzonitrile at 288 K. The magnetic field de-
pendence of kd is calculated using Eq. (3) and the parameters
{Hloc ¼ 1:35 mT, ðg : gÞav ¼ 6 ꢂ 10ꢀ4, sc ¼ 20 ps, kSLRðddÞ ¼ 0, and
MFEs on the photoinduced electron-transfer in
ZnP(8)C60 were examined in benzonitrile or toluene. In
benzonitrile, the decay at 460 nm was retarded in the
presence of lower magnetic field (0.1 T) as shown in
ksoc ¼ 2:7ꢂ106
s
ꢀ1}.

H. Yonemura et al. / Chemical Physics Letters 385 (2004) 417–422
421
interaction is estimated to be of the order of 108 sꢀ1
from the B1=2 value, which is much larger than the kd-
value. These results indicate that the MFE cannot be
explained only by the isotropic hf mechanism.
In ZnP(8)C60, the exchange interaction in the birad-
ical is negligibly small in comparison with the hf inter-
where c and Hloc are the magnetogyric ratios of the
electron and the locally fluctuating magnetic fields due
to anisotropic hf interactions, sc the correlation time,
and H the external magnetic field. The Hloc value for
ZnPꢀþ is estimated to be 1.35 mT from the electron spin
density and isotropic hfc constants [32]. In addition, the
rate constant of SLR (kSLR) induced by g anisotropy is
expressed as follow [10,21,24]:
ꢀ
action, since the distance (Rc ¼ 24:0 A) between ZnP
and C60 moities is large. At zero magnetic field, the kd-
value for the triplet biradical is controlled by the iso-
tropic hf-induced ISC, the back electron-transfer from
the singlet biradical, and the SO-induced ISC processes
(kisc3, kbet, and ksoc in Scheme 2). The back electron-
transfer and the SO-induced ISC processes are inde-
pendent of the magnetic field. In lower magnetic fields
(H K 2 mT), the isotropic hf-induced ISC process be-
tween Tþ1 and S (singlet) or Tꢀ1 and S states decreases
kSLRðdgÞ ¼ f4p2ðg : gÞ b2H2=ð5h2Þgsc=ð1 þ c2H2s2cÞ;
av
ð2Þ
where ðg : gÞ and sc are mean values averaged in two
av
radicals for the inner product of the anisotropic g tensor
and the correlation time for the anisotropic Zeeman
interaction, respectively, and b the Bohr magnetron.
The observed decay rate constant (kd) for the birad-
ical is expressed as Eq. (3) [10,21,24]:
due to the Zeeman splitting of the triplet sublevels (Tþ1
,
T0, and Tꢀ1). In the magnetic field region of
ꢃ 2 mT 6 H < 0:1 T, the MFE on the kd-value is most
kd ¼ ð1=2ÞkSLRðdhf Þ þ kSLRðddÞ þ kSLRðdgÞ þ ksoc
;
ð3Þ
likely explained by the SLR in the three sublevels (Tþ1
,
T0, and Tꢀ1) of T state and the S state of the biradical
due to the anisotropic hf and interradical electron di-
pole–dipole interactions. In the magnetic field region of
0:1 6 H < 0:6 T, the MFE can be interpreted by the
SLR due to the anisotropic Zeeman interaction. In
higher magnetic fields (H P 0:6 T), the kd-value may be
governed by the magnetic-field-independent SO-induced
ISC and the saturated SLR processes. As a consequence,
the kd-value became almost constant above ꢃ0.6 T.
The MFEs in ZnP(8)C60 are in contrast with those in
ZnP–viologen linked compounds. In the ZnP–viologen
systems, similar reverse phenomena of the MFEs have
been reported in higher magnetic field (>1 T) [22], but
not in lower magnetic field (<1 T) [16]. Similar phe-
nomena were observed in phenothiazine–C60 linked
compounds in lower magnetic fields (<0.2 T) [14,15],
while in higher magnetic field (>1 T) for phenothiazine–
viologen linked compound [21]. From the comparison
with donor-C60 and donor-viologen linked compounds,
the C60 moiety in ZnP(8)C60 is most likely responsible
for the present unusual phenomenon on the MFE.
The reverse phenomena of the MFEs around 1–2 T
have been reported and the predominant contribution of
the anisotropic Zeeman interaction to the SLR has been
verified in a variety of biradicals and radical pairs
[8,10,17–25]. Therefore, the major contribution to the
SLR mechanism may be changed from anisotropic hf to
anisotropic Zeeman interactions at the low magnetic
fields. On the basis of the above considerations, the
MFEs in ZnP(8)C60 are mainly governed by the
SLR mechanism due to anisotropic hf and Zeeman
interactions.
where kSLRðddÞ is the rate constant of SLR (kSLR) in-
duced by the dipole–dipole interaction. Using Eqs. (1)
and (2), we used the value of 1.35 mT for Hloc and the
literature value of ðg : gÞ ¼ 7:44 ꢂ 10ꢀ7 using (g : g)
av
ꢀꢀ
values of ZnPꢀþ (1.35 ꢂ 10ꢀ7) [33] and C60 (1.35 ꢂ 10ꢀ6
)
[34] in the region of sc ¼ 1–100 ps to estimate kSLRðdgÞ
and kSLRðdhf Þ. They were estimated to be 0.11–3.3 ꢂ 102
and 0.054–1.6 ꢂ 106 sꢀ1 at 0.1 T, respectively, but
kSLRðdgÞ was always smaller than kSLRðdhf Þ. Thus, the
experimental result (Fig. 3) cannot be simulated from
those parameters. However, if we assume ðg : gÞ
¼
av
6 ꢂ 10ꢀ4 for the biradical, sc ¼ 20 ps, and ksoc ¼ 2:7 ꢂ
106
s
ꢀ1, respectively, kSLRðdgÞ becomes larger than
kSLRðdhf Þ at 0.2 T. Thus, the present experimental result
could be simulated by using Eq. (3), as shown by a solid
line (Fig. 3). In this case, the kSLRðddÞ-value becomes
negligibly small as compared with the kSLRðdhf Þ-value,
since the distance (Rc) between ZnP and C60 moities in
ꢀ
ZnP(8)C60 is 24.0 A. Accordingly, the large anisotropy
of g-tensor {ðg : gÞ ¼ 6 ꢂ 10ꢀ4} may be ascribed by
av
taking the rod-like extended conformation and/or the
ꢀꢀ
interaction between ZnPꢀþ and C60 in the biradical
ꢀꢀ
(ZnPꢀþ–C60 ).
MFEs on the decay of the radical pair between a C60
cluster anion and a pyrene cation have been observed in
a micellar system [25]. In that case, decay rates of the
radicals increased with increasing the magnetic field, and
MFEs above 2 T were ascribed to the SLR mechanism
due to anisotropic Zeeman interaction. MFEs at 77 K
on the decay of biradical also have been reported in a
carotenoid–porphyrin–C60 triad system, but no MFEs
were observed at 298 K [31]. Those MFEs are different
from the present case.
The rate constant of SLR (kSLR) due to anisotropic hf
interactions is expressed as follow [10,21,24]:
On the contrary, no MFEs at the whole absorption
spectral region were observed in toluene. The result is
consistent with the fact that the triplet biradical was not
kSLRðdhf Þ ¼ c2Hl2ocsc=ð1 þ c2H2sc2Þ;
ð1Þ

422
H. Yonemura et al. / Chemical Physics Letters 385 (2004) 417–422
3
generated due to the endothermic reaction from ZnPꢁ
or ZnP to C60 or C60 in toluene.
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3
ꢁ
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4. Conclusion
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11771.
Laser excitation of ZnP(8)C60 afforded the T1–Tn
absorption of both ZnP and C60 moieties in toluene,
while the T1–Tn absorption disappeared and the ab-
sorption due to the photogenerated biradical (ZnPꢀþ–
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C60 ), was observed in benzonitrile. In benzonitrile, the
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decay rate of the biradical varied with the magnetic field.
The present MFEs strongly indicates that the photoin-
duced intramolecular electron-transfer from 3ZnPꢁ to
3
ꢁ
C60 or from ZnP to C60 takes place, generating triplet
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69.
3
ꢀꢀ
biradical (ZnPꢀþ–C60 ) sensitive to the magnetic field.
The reverse phenomenon of MFEs, that is, the decay
rate constant of the biradical decreases and increases
with increase of the magnetic field, was clearly observed
around low magnetic field (ꢃ0.1 T). This novel MFE
was interpreted by that the contribution of anisotropic
Zeeman interaction to the SLR was more pronounced
by using C60 as an acceptor in the donor–acceptor
linked compound. The present study provides useful
information for designing the donor-C60 systems for the
development of novel molecular spin systems. Further
investigations on the ZnP–C60 linked systems with dif-
ferent chain lengths and quantitative analysis of the
MFEs are now in progress.
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Acknowledgements
The authors are grateful to Mr. H. Horiuchi for the
preparation of quartz cells for fluorescence and transient
absorption spectral experiments. The authors also thank
The Center of Advanced Instrumental Analysis, Kyushu
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M. Takagi and S. Takenaka for MS measurements. The
present study was financially supported by the Grant-
in-Aids for Scientific Research: Priority Area of ÔFun-
damental Science and Technology of Photofunctional
InterfacesÕ (Area 417, No. 14050076) and ÔInnovative
Utilization of Strong Magnetic FieldsÕ (Area 767, No.
15085203), the Grant-in-Aids for Young Scientists (A)
(No. 14703023), and for 21st Century COE Program
ÔFunction Innovation of Molecular InformaticsÕ from
MEXT of the Japan.
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