π-topology and spin alignment utilizing the excited molecular field: Observation of the excited high-spin quartet (S = 3/2) and quintet (S = 2) states on purely organic π-conjugated spin systems

Article abstract of DOI:10.1021/ja001920k

As a model system for the photoinduced/photoswitched spin alignment in a purely organic π-conjugated spin system, 9-[4-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]-anthracene (1a), 9-[3-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthracene

Full text of DOI:10.1021/ja001920k

294
J. Am. Chem. Soc. 2001, 123, 294-305
π-Topology and Spin Alignment Utilizing the Excited Molecular
Field: Observation of the Excited High-Spin Quartet (S ) 3/2) and
Quintet (S ) 2) States on Purely Organic π-Conjugated Spin Systems
Yoshio Teki,*,†,‡ Sadaharu Miyamoto,‡ Masaaki Nakatsuji,§ and Yozo Miura§
Contribution from Structure and Transformation, PRESTO JST, the Department of Material Science,
Graduate School of Science, and the Department of Applied Chemistry, Faculty of Engineering, Osaka
City UniVersity, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
ReceiVed May 31, 2000. ReVised Manuscript ReceiVed October 6, 2000
Abstract: As a model system for the photoinduced/photoswitched spin alignment in a purely organic
π-conjugated spin system, 9-[4-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthracene (1a), 9-[3-
(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthracene (1b), 9,10-bis[4-(4,4,5,5-tetramethyl-1-yloxy-
imidazolin-2-yl)phenyl]anthracene (2a), and 9,10-bis[3-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]-
anthracene (2b) were designed and synthesized. In these spin systems, 9-phenylanthracene and 9,10-
diphenylanthracene were chosen as photo spin couplers and iminonitroxide was chosen as a dangling stable
radical. Time-resolved electron spin resonance (TRESR) spectra of the first excited states with resolved fine-
structure splittings were observed for 1a and 2a in an EPA or a 2-MTHF rigid glass matrix. Using the spectral
simulation based on the eigenfield method, the observed TRESR spectra for 1a and 2a were unambiguously
assigned as an excited quartet (S ) 3/2) spin state (Q) and an excited quintet (S ) 2) spin state (Qu), respectively.
The g value and fine-structure splitting for the quartet state of 1a were determined to be g(Q) ) 2.0043, D(Q)
) 0.0235 cm-1, and E(Q) ) 0.0 cm-1. The relative populations (polarization) of each MS sublevel in Q were
determined to be P+1/2′ ) P-1/2′ ) 0.5 and P+3/2′ ) P-3/2′ ) 0.0 with an increasing order of energy in zero
magnetic field. The spin Hamiltonian parameters for Qu are g ) 2.0043, D ) 0.0130 cm-1, and E ) 0.0
cm-1, and the relative populations in Qu were determined to be P0′ ) 0.30, P-1′ ) P+1′ ) 0.35 and P-2′
)
P+2′ ) 0.0. These are the first observations of a photoexcited quartet and a quintet high-spin state in π-conjugated
triplet-radical pair systems. In contrast high-spin excited states were not observed for 1b and 2b, the
π-topological isomers of 1a and 2a, showing the role of π-topology in the spin alignment of the excited states.
Since a weak antiferromagnetic exchange interaction was observed in the ground state of 2a, the clear detection
of the excited quintet high-spin state shows that the effective exchange coupling between the two dangling
radicals through the diphenylanthracene spin coupler has been changed from antiferromagnetic to ferromagnetic
upon photoexcitation. Thus, a photoinduced spin alignment utilizing the excited triplet molecular field was
realized for the first time in the purely organic π-conjugated spin system. Furthermore, the mechanism for the
generation of dynamic electron spin polarization was investigated for the observed quartet and quintet states,
and a plausible mechanism of the enhanced selective intersystem crossing was proposed. Ab initio molecular
orbital calculations based on density functional theory were carried out to determine the electronic structures
of the excited high-spin states and to understand the mechanism of the spin alignment utilizing the excited
molecular field. The role of the spin delocalization and the spin polarization mechanisms were revealed on the
photoexcited state.
1. Introduction
alignment through π-conjugation between dangling stable
iminonitroxide radicals (S ) 1/2) and the excited triplet (S )
1) state of a phenyl- or diphenylanthracene derivative.
Recently, radical (R)-excited triplet (T) pairs in solution have
been extensively investigated by the time-resolved electron spin
Intramolecular spin alignment and exchange interactions
through π-conjugation in purely organic spin systems are quite
important in the field of molecule-based magnetism.1 However,
most studies are limited to ground-state systems. The studies
of the spin alignment between stable radicals and a metastable
excited triplet state will give very important information on the
novel spin alignment, and could lead to a new strategy for
photoinduced/photoswitching magnetic spin systems. In a previ-
ous paper,2 we reported the first observation of a purely organic
excited quartet (S ) 3/2) state and a quintet (S ) 2) state in
π-conjugated spin systems which were generated by a spin
(1) (a) Miller, J. S., Dougherty, D. A., Eds. Mol. Cryst. Liq. Cryst. 305/
306, Proceedings of the Symposium on Ferromagnetic and High-Spin
Molecular Materials, 197th National Meeting of American Chemical Society,
Dallas, TX, Fall 1989. (b) Iwamura, H., Miller J. S., Eds. Mol. Cryst. Liq.
Cryst. 232/233, Proceedings of the Symposium on the Chemistry and
Physics of Molecular-based Magnets, Tokyo, Oct 25-30, 1993. (c) Miller,
J. S., Epstein, A. J., Eds. Mol. Cryst. Liq. Cryst. 271/274, Proceedings of
the IVth International Conference on Molecule-based Magnets, Salt Lake
City, UT, Oct 16-21, 1995. (d) Itoh, K., Miller, J. S., Takui, T., Eds. Mol.
Cryst. Liq. Cryst. 305/306, Proceedings of the Vth International Conference
on Molecule-based Magnets, Osaka, July 15-20, 1996. (e) Kahn, O., Ed.
Mol. Cryst. Liq. Cryst. 334/ 335, Proceedings of the VIth International
Conference on Molecule-based Magnets, Seignosse, Sept 12-17, 1998.
† Structure and Transformation.
‡ Department of Material Science.
§ Department of Applied Chemistry.
10.1021/ja001920k CCC: $20.00 © 2001 American Chemical Society
Published on Web 12/16/2000

π-Topology and Spin Alignment
J. Am. Chem. Soc., Vol. 123, No. 2, 2001 295
resonance (TRESR) technique,3 and the contribution of the high-
spin quartet state to the chemically induced dynamic electron
polarization (CIDEP) mechanism has been pointed out.4-6
However, only a few examples involving the direct observation
of an excited high-spin states (S g 3/2) generated from a
radical-excited triplet pair have been reported. The first direct
TRESR detection of an excited quartet state of a radical-excited
triplet pair was reported for a fullerene-mononitroxide radical
system in solution by Corvaja et al.7a and for a tetraphenylpor-
philinatozinc(II) (ZnTPP) coordinated by p-pyridyl nitronyl-
nitroxide (nitpy) in the solid phase by Yamauchi et al.8 For
homologous systems,7b,c,8b,9 several excited quartet states have
been reported since their pioneering works. For the fullerene-
dinitroxide system an excited quintet state was also very recently
reported.10 In these systems, a stable radical couples weakly
with the triplet excited chromophore through σ-bonds or
coordination. On the other hand in our phenylanthracene-
iminonitoroxide systems, the stable radicals couple with the
phenyl- or diphenylanthracene spin coupler through π-conjuga-
tion, making a strong exchange interaction possible. Further-
more, as shown in this paper, the topology of the π-electron
network is important in determining the sign of the exchange
interaction and in the molecular design of the photoexcited states
on the π-conjugated spin systems.
We have prepared 9-[4-(4,4,5,5-tetramethyl-1-yloxyimida-
zolin-2-yl)phenyl]anthracene (1a), 9-[3-(4,4,5,5-tetramethyl-1-
yloxyimidazolin-2-yl)phenyl]anthracene (1b), 9,10-bis[4-(4,4,5,5-
tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthracene (2a), and
9,10-bis[3-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]-
anthracene (2b) in which 9-phenylanthracene and 9,10-diphen-
ylanthracene were chosen as the spin couplers and the imino-
nitroxide was chosen as the dangling stable radical. Their
molecular structures are shown in Figure 1. In 1a and 2a, the
stable radicals couple strongly with the excited triplet state of
the phenyl- or diphenylanthracene moiety upon photoexcitation,
and their triplet moieties are expected to be a suitable photo-
coupler, causing a ferromagnetic exchange interaction. This
leads to the excited quartet (S ) 3/2) state for 1a and the excited
quintet (S ) 2) state for 2a. On the other hand, for 1b and 2b,
topological isomers of 1a and 2a, respectively, such a high-
spin state cannot be expected on the basis of the π-topological
rule established for the organic high-spin molecules.11-15 In this
paper we describe the photoexcited quartet state of 1a and the
Figure 1. Molecular structure of the phenylanthracene-iminonitroxide
and diphenylanthracene-bis(iminonitroxide) compounds studied in this
work.
photoexcited quintet state of 2a, detected by TRESR, together
with the results of the TRESR experiments for 1b and 2b.
Furthermore, the relationship between the spin alignment in the
excited states and topology on the π-electron networks is
discussed, and ab initio molecular orbital calculations for the
excited spin couplers were performed to clarify the mechanism
of the intramolecular spin alignment.
2. Experimental Section
2.1. TRESR Measurement. The TRESR method3 utilizing CIDEP
has been widely used in the study of chemical reactions and short-
lived excited states. A conventional X-band ESR spectrometer (JEOL
TE300) was used without field modulation in measurements of the
TRESR spectra. TRESR signals were amplified by a wide-band
preamplifier, transferred to a high-speed digital oscilloscope (LeCroy
9350C) and accumulated, typically, 400 times for each point. Excitation
of 1 and 2 was carried out at 355 nm light using a YAG laser
(Continuum Surelite II-10). The typical laser power used in the
experiments was ca. 5-10 mJ. The temperature was controlled using
an Oxford ESR 910 cold He gas flow system. All TRESR experiments
were carried out using an EPA or 2-MTHF rigid glass matrix. The
glass solvents of the highest commercially available purity were purified
by the usual procedure. Samples were degassed by repeated freeze-
pump-thaw cycles using a high vacuum line system. TRESR measure-
ments were carried out at 20-40 K.
2.2. Optical Spectrum Measurements and General Method. UV-
vis absorption spectra were measured using a HITACHI U-3000
absorption spectrometer at room temperature. Fluorescence emission
spectra were obtained at liquid N2 temperature with a HITACHI
F-4500T spectrometer using a quartz Dewar with rectangular suprazil
quartz windows. Melting points were recorded on a Yanagimoto micro
melting point apparatus and are uncorrected. IR spectra were obtained
on a JASCO FT/IR-230 spectrophotometer. 1H NMR spectra were
measured with a JEOL R-400 (400 MHz) or Varian λ-300 (300 MHz)
spectrometer, and the chemical shifts are expressed in parts per million
(δ) using Me4Si as an internal standard.
(2) Teki, Y.; Miyamoto, S.; Iimura, K.; Nakatsuji, M.; Miura, Y. J. Am.
Chem. Soc. 2000, 122, 984.
(3) Kim, S. S.; Weissman, S. I. J. Magn. Reson. 1976, 24, 167.
(4) Imamura, T.; Onitsuka, O.; Obi, K. J. Phys. Chem. 1986, 90, 6471.
(5) Bla¨tter, C.; Paul, J. F. Chem. Phys. Lett. 1990, 166, 375
(6) Kobori, Y.; Mitsui, M.; Kawai, A.; Obi, K. Chem. Phys. Lett. 1996,
252, 355 and references therein.
(7) (a) Corvaja, C.; Maggini, M.; Prato, M.; Scorrano, G.; Venzin, M. J.
Am. Chem. Soc. 1995, 117, 8857. (b) Corvaja, C.; Maggini, M.; Ruzzi, M.;
Scorrano, G.; Toffoletti, A. Appl. Magn. Reson. 1997, 12, 477. (c) Mizouchi,
N.; Ohba, Y.; Yamauchi, S. J. Chem. Phys. 1997, 101, 5966. (d) Ceroni,
P.; Conti, F.; Corvaja, C.; Maggini, M.; Paolucci, F.; Roffia, S.; Scorrano,
G.; Toffoletti, A. J. Phys. Chem. A 2000, 104, 156.
(8) (a) Ishii, K.; Fujiwara, J.; Ohba, Y.; Yamauchi, S. J. Am. Chem. Soc.
1996, 118, 13079. (b) Ishii, K.; Fujisawa, J.; Adachi, A.; Yamauchi, S.;
Kobayashi, N. J. Am. Chem. Soc. 1998, 120, 3152.
(9) (a) Ishii, K.; Hirose, Y.; Kobayashi, N. J. Am. Chem. Soc. 1998,
120, 10551. (b) Ishii, K.; Hirose, Y.; Kobayashi, N. J. Phys. Chem. 1999,
103, 1986.
(10) (a) Mizouchi, N.; Ohba, Y.; Yamauchi, S. J. Phys. Chem. 1999,
103, 7749. (b) Conti, F.; Corvaja, C.; Toffoletti, A.; Mizouchi, N.; Ohba,
Y.; Yamauchi, S.; Maggini, M. J. Phys. Chem. 2000, 104, 4962. During
the review period of our previous paper2 about the excited quartet and quintet
states, the first excited quintet state of a fullerene nitroxide derivative10a
was reported.
(12) (a) Itoh, K. Chem. Phys. Lett. 1967, 1, 235. (b) Wasserman, E.;
Murray, R. W.; Yager W. A.; Trozzolo, A. M.; Smilinsky, G. J. Am. Chem.
Soc. 1967, 89, 5076. (c) Itoh, K. Pure Appl. Chem. 1978, 50, 1251. (d)
Teki, Y.; Takui, T.; Itoh, K.; Iwamura, H.; Kobayashi, K. J. Am. Chem.
Soc. 1986, 108, 2147. (e) Teki, Y.; Itoh, K. In Magnetic Properties of
Organic Materials; Lahti, P. M., Ed.; Marcel-Dekker: New York, 1999;
pp 237-265 and references therein.
(13) Berson, J. A. In Magnetic Properties of Organic Materials; Lahti,
P. M., Ed.; Marcel-Dekker: New York, 1999; pp 7-26 and references
therein.
(14) (a) Ovchinnikov, A. A. Theor. Chim. Acta 1978, 47, 297. (b) Klein,
D. J.; Nelin, C. J.; Alexander, S.; Matsen, A. J. Chem. Phys. 1982, 77,
3101.
(11) (a) Longuett-Higgins, H. C. J. Chem. Phys. 1950, 18, 265. (b)
Mataga, N. Theor. Chim. Acta 1968, 10, 372.
(15) Borden W. T.; Davidson, E. R. J. Am. Chem. Soc. 1977, 99, 4587.

296 J. Am. Chem. Soc., Vol. 123, No. 2, 2001
Teki et al.
m, ArH), 7.61-7.64 (4H, m, ArH), 7.68 (4H, d, J ) 7.8 Hz, ArH),
8.15 (4H, d, J ) 7.8 Hz, ArH), 10.21 (2H, s, CHO). Anal. Calcd for
C28H18O2: C, 87.02; H, 4.69. Found: C, 86.79; H, 4.60.
Scheme 1
9,10-Bis(3-formylphenyl)anthracene (4b). A mixture of 1.35 g (4.0
mmol) of 9,10-dibromoanthracene, 1.56 g (10.4 mmol) of 3-formyl-
phenylboronic acid, 0.29 g (0.25 mmol) of (PPh3)4Pd, and 4.4 g of
K2CO3 in benzene (60 mL)-EtOH (12 mL)-H2O (24 mL) was refluxed
for 24 h under nitrogen.17 After 50 mL of benzene was added, the
organic layer was extracted, washed with brine, and dried (MgSO4).
Evaporation, column chromatography (silica gel, CH2Cl2), and crystal-
lization (EtOH) gave 4b in 86.8% yield (1.34 g, 3.47 mmol) as slightly
1
yellow prisms: mp > 300 °C; IR (KBr) 1710 cm-1 (CHO); H NMR
(CDCl3) δ 7.37-7.39 (4H, m ArH), 7.60-7.63 (4H, m, ArH), 7.78-
7.83 (4H, m, ArH), 8.02 (2H, d, J ) 1.5 Hz, ArH), 8.12 (2H, dd, J )
1.5, 8.8 Hz, ArH), 10.16 (2H, s, CHO). Anal. Calcd for C28H18O2: C,
87.03; H, 4.69. Found: C, 86.82; H, 4.75.
Scheme 2
9-[4-(4,4,5,5-Tetramethyl-1,3-dihydroxyimidazolidin-2-yl)phenyl]-
anthracene (5a). A mixture of 0.71 g (2.5 mmol) of 4a, 0.74 g (5.0
mmol) of 2,3-bis(hydroxyamino)-2,3-dimethylbutane, and 0.070 g (0.28
mmol) of 2,3-bis(hydroxyamino)-2,3-dimethylbutane dihydrogen sulfate
in MeOH (20 mL)-THF (20 mL) was stirred for 72 h. After
evaporation under reduced pressure, 5 mL of MeOH was added, and
then 20 mL of H2O was added to the resulting homogeneous solution.
The deposited powder was collected by filtration, washed with H2O
(20 mL) and MeOH (10 mL), and dried in vacuo to give 5a as a slightly
yellow powder in 64% yield (0.66 g, 1.6 mmol).
9-[3-(4,4,5,5-Tetramethyl-1,3-dihydroxyimidazolidin-2-yl)phenyl]-
anthracene (5b). A mixture of 0.71 g (2.5 mmol) of 4b, 0.74 g (5.0
mmol) of 2,3-bis(hydroxyamino)-2,3-dimethylbutane, and 0.070 g (0.28
mmol) of 2,3-bis(hydroxyamino)-2,3-dimethylbutane dihydrogen sulfate
in MeOH (20 mL)-THF (20 mL) was stirred for 72 h. After
evaporation under reduced pressure, 5 mL of MeOH was added, and
then 20 mL of H2O was added to the resulting homogeneous solution.
The deposited powder was collected by filtration, washed with H2O
(20 mL) and MeOH (10 mL), and dried in vacuo to give 5b as a slightly
yellow powder in 51% yield (0.54 g, 1.3 mmol).
9,10-Bis[4-(4,4,5,5-tetramethyl-1,3-dihydroxyimidazolidin-2-yl)-
phenyl]anthracene (6a). A mixture of 0.48 g (1.25 mmol) of 5a, 0.74
g (5.0 mmol) of 2,3-bis(hydroxyamino)-2,3-dimethylbutane, and 0.070
g (0.28 mmol) of 2,3-bis(hydroxyamino)-2,3-dimethylbutane dihydro-
gen sulfate in MeOH (20 mL)-CHCl3 (20 mL) was refluxed for 72 h.
After evaporation under reduced pressure, 5 mL of MeOH was added,
and then 20 mL of H2O was added to the resulting homogeneous
solution. The deposited powder was collected by filtration, washed with
H2O (20 mL) and MeOH (10 mL), and dried in vacuo to give 6a as a
slightly yellow powder in 44% yield (1.04 g, 0.55 mmol).
2.3. Materials. 2,3-Bis(hydroxyamino)-2,3-dimethylbutane was
prepared according to the literature method.16 Other reagents were used
as purchased. Column chromatography was performed using Fuji Silysia
BW-127ZH silica gel (Fuji-Davison Chemical Co., Ltd.). The stable
radicals 1 and 2 were synthesized according to the procedures shown
in Schemes 1 and 2, respectively.
9-(4-Formylphenyl)anthracene (3a). A mixture of 5.14 g (20 mmol)
of 9-bromoanthracene, 3.90 g (26 mmol) of 4-formylphenylboronic acid,
0.69 g (0.60 mmol) of (PPh3)4Pd, and 11 g of K2CO3 in benzene (120
mL)-EtOH (20 mL)-H2O (40 mL) was refluxed for 24 h under
nitrogen.17 After 50 mL of benzene was added, the organic layer was
extracted, washed with brine, and dried with MgSO4. Evaporation,
column chromatography (silica gel, benzene), and crystallization (EtOH)
gave 3a in 73% yield (4.12 g, 1.46 mmol) as slightly yellow prisms:
1
mp 144-146 °C; IR (KBr) 1710 cm-1 (CHO); H NMR (CDCl3) δ
7.35-7.64 (8H, m, ArH), 8.10-8.12 (4H, m, ArH), 8.54 (1H, s, ArH),
10.19 (1H, s, CHO). Anal. Calcd for C21H14O: C, 89.33; H, 5.00.
Found: C, 89.07; H, 4.98.
9-(3-Formylphenyl)anthracene (3b). A mixture of 1.03 g (4.0
mmol) of 9-bromoanthracene, 0.78 g (5.2 mmol) of 3-formylphenyl-
boronic acid, 0.14 g (0.12 mmol) of (PPh3)4Pd, and 2.2 g of K2CO3 in
benzene (30 mL)-EtOH (6 mL)-H2O (12 mL) was refluxed for 24 h
under nitrogen.17 After 30 mL of benzene was added, the organic layer
was extracted, washed with brine, and dried (MgSO4). Evaporation,
column chromatography (silica gel, CH2Cl2), and crystallization (EtOH)
gave 3b in 72.6% yield (0.82 g, 2.90 mmol) as yellow prisms: mp
9,10-Bis[3-(4,4,5,5-tetramethyl-1,3-dihydroxyimidazolidin-2-yl)-
phenyl]anthracene (6b). A mixture of 0.48 g (1.25 mmol) of 5b, 0.74
g (5.0 mmol) of 2,3-bis(hydroxyamino)-2,3-dimethylbutane, and 0.070
g (0.28 mmol) of 2,3-bis(hydroxyamino)-2,3-dimethylbutane dihydro-
gen sulfate in MeOH (20 mL)-CHCl3 (20 mL) was refluxed for 72 h.
After evaporation under reduced pressure, 5 mL of MeOH was added,
and then 20 mL of H2O was added to the resulting homogeneous
solution. The deposited powder was collected by filtration, washed with
H2O (20 mL) and MeOH (10 mL), and dried in vacuo to give 6b as a
slightly yellow powder in 49% yield (1.04 g, 1.61 mmol).
9-[4-(4,4,5,5-Tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthra-
cene (1a). To a suspension of 3.17 g (7.68 mmol) of 5a in 80 mL of
benzene was added a solution of 4.71 g (22 mmol) of NaIO4 in 80 mL
of H2O, and the mixture was stirred for 2 h. The organic layer (dark
blue) was separated, washed with brine, dried (MgSO4), and evaporated.
After 60 mL of CH2Cl2 and 9.94 g (0.14 mmol) of NaNO2 in CH2Cl2
(20 mL)-H2O (20 mL) were added to the residue, 1 N HCl was added
dropwise to the mixture cooled to 0 °C until the organic layer turned
orange.18 The CH2Cl2 layer was then separated, washed with brine,
dried (MgSO4), evaporated, and column chromatographed (silica gel,
1
120-122 °C; IR (KBr) 1700 cm-1 (CHO); H NMR (CDCl3) δ 7.37
(2H, t, J ) 8.0 Hz, ArH), 7.48 (2H, t, J ) 8.0 Hz, ArH), 7.56 (2H, d,
J ) 8.8 Hz, ArH), 7.72 (1H, d, J ) 7.8 Hz, ArH), 7.77 (1H, t, J ) 7.8
Hz, ArH), 7.96 (1H, s, ArH), 8.07 (2H, d, J ) 8.8 Hz, ArH), 8.08 (1H,
d, J ) 7.8 Hz, ArH), 8.55 (1H, s, ArH), 10.12 (1H, s, CHO). Anal.
Calcd for C21H14O: C, 89.33; H, 5.00. Found: C, 88.86; H, 4.99.
9,10-Bis(4-formylphenyl)anthracene (4a). A mixture of 5.04 g (15
mmol) of 9,10-dibromoanthracene, 6.0 g (40 mmol) of 4-formylphen-
ylboronic acid, 1.31 g (1.20 mmol) of (PPh3)4Pd, and 16.69 g of K2-
CO3 in benzene (200 mL)-EtOH (40 mL)-H2O (80 mL) was refluxed
for 48 h under nitrogen and cooled to room temperature.17 The powder
deposited was collected by filtration and washed with water (50 mL)
and EtOH (50 mL). Recrystallization from EtOH-benzene gave 4a in
78% yield (4.52 g, 11.70 mmol) as slightly yellow prisms: mp > 300
°C; IR (KBr) 1710 cm-1 (CHO); 1H NMR (CDCl3) δ 7.36-7.40 (4H,
(18) (a) Ullman E. F.; Call L.; Osiecki, J. H. J. Org. Chem. 1970, 35,
3623. (b) Ullman E. F.; Osiecki, J. H.; Boocock, D. G. B.; Darcy, R. J.
Am. Chem. Soc. 1972, 94, 7079. (c) Gatteschi, D.; Rey, P. In Magnetic
Properties of Organic Materials; Lahti, P. M., Ed.; Marcel-Dekker: New
York, 1999; pp 601-627.
(16) Lamchen, M.; Mittag, T. W. J. Chem. Soc. C 1966, 2, 300.
(17) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457.

π-Topology and Spin Alignment
J. Am. Chem. Soc., Vol. 123, No. 2, 2001 297
CH2Cl2), and recrystallization from EtOH gave 1a in 26.5% yield (0.8
g, 2.03 mmol) as red prisms: mp 190-193 °C; IR (KBr) 3050, 2980,
1610, 1440, 1440, 1360, 1290, 1160, 890, 850, 740 cm-1. Anal. Calcd
for C27H25N2O: C, 82.41; H, 6.40; N, 7.12. Found: C, 82.17; H, 6.46,
N; 7.01.
are allowed group-theoretically, are usually small in magnitude for
purely organic molecules consisting of light atoms.
The resonance field BM TMS+1(θ,φ) for each transition was directly
S
calculated by solving the following eigenfield equation:19
A‚Z ) BC‚Z
where A and C are given by the following superoperators:
A ) ωE X E - F X E + E X F
and
(2)
(3)
(4)
9-[3-(4,4,5,5-Tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthra-
cene (1b). To a suspension of 0.63 g (1.53 mmol) of 5b in 20 mL of
benzene was added a solution of 0.83 g (3.90 mmol) of NaIO4 in 20
mL of H2O, and the mixture was stirred for 2 h. The organic layer
(dark blue) was separated, washed with brine, dried (MgSO4), and
evaporated. After 20 mL of CH2Cl2 and 2.50 g (36.23 mmol) of NaNO2
in H2O (20 mL) were added to the residue, 1 N HCl was added dropwise
to the mixture cooled to 0 °C until the organic layer turned orange.18
The CH2Cl2 layer was then separated, washed with brine, dried
(MgSO4), evaporated, and column chromatographed (silica gel, CH2-
Cl2), and recrystallization from EtOH gave 1b in 65.0% yield (0.39 g,
1.0 mmol) as red prisms: mp >300 °C; IR (KBr) 2980, 2920, 1610,
1440, 1020, 940, 830, 770, 660, 420 cm-1. Anal. Calcd for
C27H25N2O: C, 82.41; H, 6.40; N, 7.12. Found: C, 82.21; H, 6.29, N;
7.09.
9,10-Bis[4-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]an-
thracene (2a). To a suspension of 1.04 g (1.61 mmol) of 6a in 30 mL
of benzene was added a solution of 1.00 g (4.70 mmol) of NaIO4 in 60
mL of H2O, and the mixture was stirred for 2 h. The organic layer
(dark blue) was separated, washed with brine, dried (MgSO4), and
evaporated. After 20 mL of CH2Cl2 and 1.10 g (1.59 mmol) of NaNO2
in H2O (20 mL) were added to the residue, 1 N HCl was added dropwise
to the mixture cooled to 0 °C until the organic layer turned orange.18
The CH2Cl2 layer was then separated, washed with brine, dried
(MgSO4), evaporated, and column chromatographed (silica gel, CH2-
Cl2), and recrystallization from EtOH gave 2a in 5.6% yield (60 mg,
0.102 mmol) as red prisms: mp >300 °C; IR (KBr) 2980, 2920, 1540,
1480, 1380, 1300, 1140, 780, 700, 680, 420 cm-1. Anal. Calcd for
C40H40N4O2·H2O: C, 76.65; H, 6.75; N, 8.94. Found: C, 76.65; H,
6.63, N; 8.73.
C ) G X E - E X G*
Here, E is a unit matrix and ω is the given microwave frequency. The
operators G and F are the field-dependent and -independent parts of
the spin Hamiltonian given in eq 1 (H′spin ) F + BG), respectively.
By solving the above eigenfield equation (eq 2), the resonance field B
is obtained directly as eigenvalues. The transition probabilities I(θ,φ,æ)
were evaluated by numerically diagonalizing the spin Hamiltonian
energy matrix of eq 1, in which the calculated resonance eigenfield
was given,20 where θ, φ, and æ are the Euler angles. Since the resonance
field is independent of the third Euler angle, æ, the above procedure
practically saves the computing time for the simulation. The line-shape
function of the TRESR spectrum in the glass matrix is given by
g(B) ) N
dæ dφ dθ sin θ P
(θ,φ) I(θ,φ,æ)
MSTMS+1
∫ ∫ ∫
∑
f[B - BM TM (θ,φ)] (5)
S
S+1
In this equation, the ESP value between |S,MS and |S,MS+1 is expressed
by
PM TM (θ,φ) ) PM - PM
S+1
(6)
S
S+1
S
9,10-Bis[3-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]an-
thracene (2b). To a suspension of 1.04 g (1.61 mmol) of 6b in 30 mL
of benzene was added a solution of 1.00 g (4.70 mmol) of NaIO4 in 60
mL of H2O, and the mixture was stirred for 2 h. The organic layer
(dark blue) was separated, washed with brine, dried (MgSO4), and
evaporated. After 20 mL of CH2Cl2 and 1.10 g (1.59 mmol) of NaNO2
in H2O (20 mL) were added to the residue, 1 N HCl was added dropwise
to the mixture cooled to 0 °C until the organic layer turned orange.18
The CH2Cl2 layer was separated, washed with brine, dried (MgSO4),
evaporated, and column chromatographed (silica gel, CH2Cl2), and
recrystallization (EtOH-benzene) gave 2b in 5.6% yield (60.0 mg,
0.102 mmol) as red prisms: mp >300 °C; IR (KBr) 2980, 2920, 1610,
1440, 1370, 1020, 940, 830, 770, 660, 420 cm-1. Anal. Calcd for
C40H40N4O2‚C6H6: C, 80.43; H, 6.75; N, 8.16. Found: C, 80.05; H,
6.72, N; 8.30.
2.4. Analysis of TRESR Spectra of High-Spin Excited States.
The spectral simulation was carried out by the eigenfield/exact-
diagonalization hybrid method,19,20 taking electron spin polarization
(ESP) into account. The effective exchange interaction between the
triplet state of the phenyl- or diphenylanthracene moiety and the
dangling radicals is undoubtedly much larger than the Zeeman and fine-
structure interactions in the present π-conjugated systems. We can
therefore use an ordinary spin Hamiltonian of a pure spin state
(negligible quantum mixing of the different spin states) for the analysis,
which is given by
In the simulation, the ESP value on each spin sublevel in zero magnetic
field was given as a parameter. A Gaussian function was employed
for the line-shape function with a constant line width. The simulation
was carried out using a program written by Y.T. on a personal computer.
3. Results and Discussion
3.1. Optical Spectra. UV-vis spectra of 1a, 1b, 2a, and 2b
are shown in Figure 2, along with that of anthracene. The
spectral patterns with vibronic splitting in the region of 300-
400 nm are very similar to that of anthracene. A slight red shift
observed for 1 and 2 was interpreted as the result of π-conjuga-
tion between the anthracene moiety and side phenyl rings, and
the magnitude of the red shift observed for 1 and 2 was in
agreement with that of phenylanthracene or diphenylanthracene.
The very broad and weak absorption peaks in the range of 400-
500 nm were attributed to a well-known localized nπ*-transition
of the dangling iminonitroxide radicals. The nπ* absorption
peaks overlap slightly with the absorption due to the anthracene
moiety. A strong fluorescence was observed for 1a, 1b, 2a, and
2b (Figure 3). From the fluorescence spectra, the location of
the excited doublet state arising from the π f π* excitation of
the phenyl or diphenylanthracene moiety was determined to be
ca. 3.0 eV above the ground state.
3.2. Conventional ESR Spectra. Conventional X-band ESR
spectra of 1 and 2 in the ground state are shown in Figure 4.
As shown in Figure 4, the ESR spectra of 1a and 1b show well-
resolved hyperfine splitting arising from the nitrogen atom in
the NO group. The temperature dependence of the signal
H′spin ) âeB·g·S + S·D·S ) âeB·g·S + D[SZ2 - S(S + 1)/3] +
2
E(SX2 - SY ) (1)
(20) Teki, Y.; Fujita, I.; Takui, T.; Kinoshita, T.; Itoh, K. J. Am. Chem.
Soc. 1994, 116, 11499. In the spectral simulation of the TRESR spectra,
the calculation of ESP has been added as a subroutine to the computer
program. More detailed procedures of the spectral simulation for high-spin
organic molecules are similar to those described in our previous paper for
the perturbation approach: Teki, Y.; Takui, T.; Yagi, H.; Itoh, K.; Iwamura,
H. J. Chem. Phys. 1985, 83, 539.
where g and D are the electron g tensor and fine-structure tensor,
respectively. In this effective spin Hamiltonian, the higher order fine-
structure terms such as BS3, S4, etc. are ignored since these terms, which
(19) Belford, G. G.; Belford, R. L.; Burkhalter, J. F. J. Magn. Reson.
1973, 11, 251.

298 J. Am. Chem. Soc., Vol. 123, No. 2, 2001
Teki et al.
Figure 2. Optical absorption spectra of 1a, 1b, 2a, and 2b at room
temperature. EPA was used for 1a and 1b, and 2-MTHF was used for
2a and 2b as the solvent. The molar absorption coefficients for 1a and
2a are included in the spectra.
Figure 4. Conventional X-band cw-ESR spectra of 1a, 1b, 2a, and
2b observed at 30 K in a rigid glass matrix. (a) Observed cw-ESR
spectra. The microwave frequencies are 9081.32 MHz for 1a, 9084.01
MHz for 1b, 9055.15 MHz for 2a, 9062.66 MHz for 2b, respectively.
(b) Temperature dependence of the signal intensity of 2a.
K for 2a. For 2b, the Jintra/kB value could not be determined
accurately, since the magnitude of the exchange interaction was
very small.
3.3. TRESR Spectra and Excited Spin States of 1a. A
typical X-band TRESR spectrum of 1a observed at 30 K is
shown in Figure 5 together with its simulation spectrum. Figure
5a shows the TRESR spectrum of 1a at 1.0 µs after the laser
excitation. The spin Hamiltonian parameters for 1a determined
by the simulation are listed in Table 1, along with those of 2a.
In the present system the triplet state of the phenylanthracene
moiety couples with the doublet state of the dangling radical
by the spin-exchange interaction. In such an exchange-coupled
system the wave function of the whole molecule |Ψ(S,M) is
approximately given by the direct product of the wave functions
of the two isolated moieties, |T(SA,mA) and |R(SB,mB) , as
|Ψ(S,M ) ) C(SASBS;m m M)|T(SA,m ) |R(SB,m )
∑
S
A
B
A
B
(7)
Figure 3. Fluorescence emission spectra of 1a, 1b, 2a, and 2b observed
at 77 K. (a) 1a (solid line) and 1b (broken line) in EPA. (b) 2a (solid
line) and 2b (broken line) in 2-MTHF.
for SA ) 1 and SB ) 1/2, where C(SASBS;mAmBM) is the
Clebsch-Gordan coefficient. One quartet (Q) spin state and one
doublet (D) spin state were constructed from the radical-triplet
pair. The g and D tensors for Q are given by the following:21
intensity follows simple Curie law. On the other hand in the
ESR spectra of 2a and 2b, the hyperfine splitting was smeared
out and the signal intensities became weak with decreasing
temperature. This indicates that a weak intramolecular antifer-
romagnetic interaction exists between the two dangling imino-
nitroxide radicals through the ground-state diphenylanthracene
moiety as a closed shell spin coupler. To determine the
magnitude of the antiferromagnetic intramolecular exchange
interaction (Jintra), the temperature dependence of the ESR signal
intensity (see Figure 4b) was examined in the range of 3-90
K, and the value of Jintra/kB was determined to be -5.8 ( 0.2
g(Q) ) (2/3)g(T) + (1/3)g(R)
(8)
and
D(Q) ) (1/3)(D(T) + D(RT))
(9)
where D(Q), D(T), and D(RT) are the fine-structure tensors for
the excited quartet, the excited triplet, and magnetic dipolar-
dipolar interaction between the radical and the excited triplet,
(21) Bencini, A.; Gatteschi, D. EPR of Exchange Coupled Systems;
Springer-Verlag: Berlin, 1990.

π-Topology and Spin Alignment
J. Am. Chem. Soc., Vol. 123, No. 2, 2001 299
Table 1. g Values and Zero-Field Splitting Parameters and ISC Ratio of the Quartet (Q) and the Quintet (Qu) Excited States
g
D/cm-1
E/cm-1
ISC ratio
Q of 1a
Qu of 2a
2.0043
2.0043
0.0235
0.0130
0.0
0.0
P+3/2′:P+1/2′:P-1/ 2′:P-3/2′ ) 0.0:0.5:0.5:0.0
P+2′:P+1′:P0′:P-1′:P-2′ ) 0.0:0.35:0.30:0. 35:0.0
of the excited triplet state of free anthracene23 and D(RT) )
(3/2)DZZ(RT) ) 0.0057 cm-1 calculated by the point dipole
approximation. Here DZZ(RT) is the matrix element of D(RT)
in the direction along the Z principal axis of D(Q), which is
approximated to be parallel to the direction for D(T) to give
the largest fine-structure splitting because |D(T)| . |D(RT)|.
In this estimation, we used the D value of anthracene instead
of the value for phenylanthracene, since the D value of the T1
state of phenylanthracene was unknown. It was impossible to
detect the T1 state of phenylanthracene due to the low efficiency
of intersystem crossing from S0 to the T1 state. The experimen-
tally determined g and D values for the excited quartet states
of 1a (g(Q) ) 2.0043 and D(Q) ) 0.0235 cm-1) shown in Table
1 are in excellent agreement with the calculations.24 This means
that the excited quartet state was generated by the spin-exchange
coupling between the ππ* excited triplet state of the diphenyl-
anthracene moiety and the doublet ground state of the dangling
stable radical. The observed TRESR shows an A/E pattern as
shown in Figure 5a. The relative populations (polarization) of
each MS sublevel in Q have been determined to be P+1/2′
)
P-1/2′ ) 0.5 and P+3/2′ ) P-3/2′ ) 0.0 with an increasing order
of energy in zero magnetic field from the spectral simulation
shown in Figure 5b. Here, we assumed that D > 0, because Q
is expected to have the same sign of D as that of the triplet
state of anthracene according to eq 9. The relative populations
show that a selective intersystem crossing (ISC) to the MS )
(1/2 sublevels occurs from the ππ* excited doublet state (Dn).
Hereafter, Dn stands for the photoexcited doublet state arising
from the radical-excited singlet pair in which the dangling
iminonitroxide radical couples with the photoexcited ππ* singlet
state of the phenylmethylene moiety generated by a vertical π
f π* photoexcitation. The observed ESP is discussed in section
3.7.
The signal with emissive polarization (E), which can be
attributable to the excited doublet state, D, arising from the
radical-excited triplet pair, was very weak (see the inset in
Figure 5a). The g value for D was determined to be g(D) )
2.002 ( 0.001, which is in agreement with the value of 2.0020
estimated using eq 10. As pointed out in the case of the excited
doublet and quartet states of ZnTPP-nitpy,8b the emissive
polarization indicates that the energy level of the excited quartet
state is lower than that of the excited doublet spin state.
Therefore, the sign of the exchange interaction between the
excited triplet state (S ) 1) of the anthracene moiety and the
doublet spin state (S ) 1/2) of the dangling iminonitroxide
radical would be positive (ferromagnetic). The positive sign also
supported by the theoretical calculations will be discussed in
section 3.9. The weak signal due to the excited doublet state
can be explained in terms of a large energy separation between
Q and D due to a larger spin-exchange interaction because the
ESP of the excited doublet state is generated from the perturbed
mixing of the quartet wave functions (|Q,(1/2′ ) with the
doublet wave functions (|D,(1/2′ ). The larger spin-exchange
interaction arises from the π-conjugation. Thus, in the excited
quartet state, an intramolecular ferromagnetic spin alignment
between the excited triplet chromophore and the dangling radical
spin occurs through spin polarization in the dangling phenyl
Figure 5. Typical TRESR spectra of 1a and anthracene at 30 K in an
EPA rigid glass matrix. The microwave frequencies are 9040.6 MHz
for 1a and 9030.6 MHz for anthracene. (a) Observed spectrum of 1a
at 1.0 µs after laser excitation. The inset shows an expanded spectrum
in the g ≈ 2 region. In the inset Q and D denote the signals due to the
excited quartet and doublet states, respectively. (b) Simulated spectrum
of the excited quartet state using the spin Hamiltonian parameters
described in text. The line width ∆Bmsl was determined to be 0.7 mT
by the simulation. Here, X, Y, and Z are the signals corresponding to
the canonical orientation of the fine-structure tensor. “A” denotes the
off-axis extra line, and θex indicates the angle corresponding to the
extra line.38 (c) Calculated angular dependence of the resonance field.
(d) Observed spectrum of anthracene at 1.0 µs after laser excitation.
respectively. For D arising from the radical-excited triplet pair,
the g tensor is given by21
g(D) ) (4/3)g(T) - (1/3)g(R)
(10)
From eq 8, the averaged g value for the excited quartet state of
1a was estimated to be g(Q) ) 2.0040 using the g values of the
iminonitroxide radical (g(R) ) 2.006)22 and the excited ππ*
triplet state of free anthracene (g(T) ) 2.003). The fine-structure
parameter D of the excited quartet state was also estimated to
be D(Q) ) 0.0256 cm-1 from eq 9 using D(T) ) 0.0710 cm-1
(23) The g, D, and E values for the T1 state of anthracene in an EPA
rigid glass matrix were determined to be g ) 2.003, D ) 0.0710 cm-1, and
E ) 0.0070 cm-1. Also the relative populations of the sublevels were PX
) 0.45, PY ) 0.55, and PZ ) 0.0.
(24) Since the π spin density on phenyl or diphenylanthracene is slightly
delocalized to the phenyl ring, an estimated value of D is slightly smaller.
(22) The g value and isotropic hyperfine coupling constants of the two
nitrogen atoms were determined to be g ) 2.0060, aN(1) ) 0.92 mT, and
aN(2) ) 0.42 mT from simulation of the room-temperature ESR spectrum
of 1a in toluene solution, although in our previous paper2 g ) 2.007, which
was an averaged value for the various nitroxide radicals, was used.

300 J. Am. Chem. Soc., Vol. 123, No. 2, 2001
Teki et al.
Figure 8. Schematic diagram of the spin alignment utilizing the excited
triplet molecular field in 2a. In the ground state, the spin polarization
mechanism is dominant, leading to the antiferromagnetic coupling
between the two dangling radical spins, because the diphenylanthracene
spin coupler is a closed shell. On the other hand, in the photoexcited
states, the spin delocalization mechanism overcomes the spin polariza-
tion effect in the anthracene moiety, because the spin coupler becomes
an open-shell triplet excited state. This leads to the ferromagnetic
coupling between the two dangling radical spins in the photoexcited
state.
Figure 6. Schematic diagram of the spin alignment in the doublet
ground state and the excited quartet state of 1a. In the ground state,
the radical spin couples with the closed-shell singlet state of the
phenylanthracene moiety. On the other hand, in the photoexcited states,
the radical spin couples ferromagnetically with the triplet excited state
of the phenylanthracene moiety by the spin polarization through
π-conjugation.
this exchange-coupled system, the wave function for the whole
molecule |Ψ(S,M) is given approximately as a direct product
of the wave functions for the three isolated moieties, |T(SA,mA) ,
|R1(SB,mB) , and |R2(SB,mB) using Racah coefficients. Accord-
ing to the projection theorem of the angular momentum,25 the
D tensor for Qu is obtained using the D tensor for the excited
triplet state of the isolated anthracene moiety and the dipolar
interaction between the radical and the excited triplet by
D(Qu) ) (1/6)(D(T) + D(R1T) + D(R2T))
(11)
Here, the contribution arising from the direct dipolar interaction
between the dangling radical spins is ignored because the values
are 2 orders of magnitude smaller than the intrinsic fine-structure
interaction of the excited triplet anthracene moiety. Under the
similar approximation used in the case of the quartet state, the
fine-structure parameter D of the quintet state was estimated to
be D(Qu) ) 0.0137 cm-1 from eq 11, using D(T) ) 0.0710
cm-1 for the excited triplet state of free anthracene and D(R1T)
) D(R2T) ) 0.0057 cm-1 calculated in the direction of the Z
principal axis of D(Qu) by the point dipole approximation. The
experimentally determined D value (0.0130 cm-1) for the quintet
state of 2a is in excellent agreement with the calculation,
indicating that the observed excited quintet state (S ) 2) arises
from a ππ* excited triplet state of the diphenylanthracene moiety
and two dangling radical spins.
The clear detection of the excited quintet (S ) 2) state
indicates that the intramolecular ferromagnetic (parallel) spin
alignment between the two doublet spins (S ) 1/2) in the
dangling phenyl iminonitroxide radicals is realized by the
photoexcited triplet (S ) 1) molecular field of the diphenyl-
anthracene moiety. Since there is no reason for the different
sign of the exchange coupling for 2a, it was concluded that the
sign of the exchange interaction is also ferromagnetic in 2a.
Thus, the intramolecular ferromagnetic exchange interaction
among the excited triplet chromopore and two dangling radicals
arises in 2a.
Figure 7. Typical TRESR spectra of 2a at 30 K in a 2-MTHF rigid
glass matrix. The microwave frequency is 9135.7 MHz. (a) Observed
spectrum at 30 K and 1.0 µs after laser excitation. (b) Simulation of
the excited quintet spectrum using the spin Hamiltonian parameters
described in text. The line width ∆Bmsl was also determined to be 0.7
mT by the spectral simulation. (c) Calculated angular dependence of
the resonance field.
group shown in Figure 6. The details of the spin alignment are
discussed in section 3.8.
3.4 TRESR Spectra and Excited Spin States of 2a. A
typical TRESR spectrum of 2a observed at 30 K is shown in
Figure 7 together with its simulation spectrum. Figure 7a shows
the TRESR spectrum observed at 1.0 µs after the laser excitation.
The spin Hamiltonian parameters determined by the spectral
simulation are listed in Table 1. The observed TRESR shows
an A/E pattern (Figure 7a), and the determined relative
populations (polarization) of each MS sublevel in the quintet
state (Qu) are also shown in Table 1. The relative population
shows that the selective ISC to the MS ) 0, (1 sublevels occurs
from the ππ* excited singlet state (Sn) which is generated by
the π f π* photoexcitation of the diphenylanthracene moiety.
The triplet state of the diphenylanthracene moiety couples
with the doublet state of the two dangling stable radicals. In
Figure 8 shows the schematic diagram of the spin alignment
utilizing the excited molecular field in 2a. There are two
dominant mechanisms for the intramolecular spin alignment.
One is a spin polarization mechanism, and the other is a spin
(25) (a) Rose, M. E. Elementary Theory of Angular Momentum; John
Wiley & Sons: New York, 1957. (b) Judd, B. R. Operator Techniques in
Atomic Spectroscopy; McGraw-Hill: New York-San Francisco-Toronto-
London, 1963.

π-Topology and Spin Alignment
J. Am. Chem. Soc., Vol. 123, No. 2, 2001 301
canonical transitions in the observed TRESR spectrum of 1a in
Figure 5a show a similar time-decay behavior, indicating that
all signals come from a unique quartet spin state. The dynamic
electron spin polarization decayed within 10 µs, and no further
signals could be detected after 10 µs. Although the time profile
of a very weak doublet, shown by arrow D in the inset of Figure
5a, could not be clearly detected, it seemed to decay more
rapidly than the quartet signals. The ESP of Qu also decayed
within 10 µs, and all canonical transitions in the observed
TRESR spectrum of 2a (Figure 7a) showed a similar time-
decay behavior (Figure 9b). No other signals arising from
different excited spin states were detected during the decay time
of ESP, suggesting that the excited triplet species arising from
the radical-triplet-radical system of 2a decayed rapidly within
0.2 µs or the population was less than the sensitivity of our
TRESR spectrometer. Furthermore, an inversion of the ESP
signal was observed for 2a in the long time region (t > 3.5 µs).
This finding shows that selective ISC toward the ground state
occurs from the MS ) (1 and 0 sublevels of the quintet state,
in a similar manner to the selective ISC from the Sn state
mentioned above. In addition, the absorptive (A) doublet-like
signal of the ground state (G) (Figure 9c) appears with weak
signal intensity in the longer time region. This signal may be
assigned to the lowest triplet state (T0) polarized by an ISC
from Qu to T0, which is closely located above the singlet ground
state (S0) (section 3.2).
3.6. Spin-Orbit Wave Functions of the Ground and
Excited Spin States. To discuss the mechanism of the dynamic
ESP, the spin-orbit wave functions were investigated. For the
convenience of this discussion, the direct products of the wave
functions for the isolated phenylanthracene/diphenylanthracene
moiety and the iminonitroxide radicals were approximately used
for the wave function for the whole molecule. The wave
functions are assumed to be orthogonal to each other although
they are actually slightly mixed through π-conjugation. The
mixing is considered in the later discussion. Only the highest
occupied MO (π-HOMO) and the lowest unoccupied MO (π*-
LUMO) of phenyl- and diphenylanthracene moieties, the singly
occupied MOs (SOMOs) of the radicals, and the lone-pair
nonbonding orbital at the oxygen sites were treated. R and n
denote the SOMO of the radical and the nonbonding lone-pair
orbital of the oxygen site, respectively. Hereafter, we give only
the explicit expressions for the spin-orbit wave functions of
the ground and excited states that play important roles in the
generation of ESP in our systems, even though there are various
excited states constructed from π, π*, R, and n.
Figure 9. Time profiles observed at each canonical resonance field of
the fine-structure transitions of (a) the excited quartet state of 1a and
(b) the excited quintet state of 2a. Assignment of each transition is
shown in Figures 5 and 7. The time profiles were observed at 30 K.
(c) TRESR spectrum of 2a observed at 4 µs after the laser excitation.
The signal indicated by “G” may come from the ground state.
delocalization mechanism.26 The ferromagnetic spin alignment
in 2a in the excited state shows that the spin delocalization
mechanism is much more dominant than in the ground state.
Since 2a is a symmetrical diradical, the spin polarization
mechanism leads to an antiferromagnetic spin configuration
between the dangling radicals, according to the topological rule
for the π-electron network.11-15 Actually, a weak antiferromag-
netic exchange interaction was observed for this system in the
ground state as described in section 3.2 because the diphenyl-
anthracene moiety has a closed-shell spin configuration in the
ground state as shown in Figure 8. However, in the open-shell
T1 state of the anthracene moiety, the spin delocalization
mechanism would overcome the spin polarization effect as
shown in section 3.9. This leads to a large positive spin density
at the symmetrical carbon positions, 9 and 10, of the anthracene
moiety. As a result, all unpaired spins in the photoexcited state
of 2a are parallel to each other, giving an excited quintet high-
spin state (S ) 2) as shown in Figure 8. As already pointed out
in our previous paper,2 these results show that the sign of the
effective exchange coupling between two dangling stable
radicals through the spin coupler becomes ferromagnetic through
the photoexcitation. The spin distribution, which is shown in
section 3.9, confirms this finding. Thus, the photoinduced spin
alignment utilizing the excited triplet molecular field of the
purely organic π-conjugated system has been achieved for the
first time in this spin system (Figure 8). A similar phenomenon
has been very recently reported in a covalently linked (σ-
conjugated) binitroxide-fullerene derivative system.10b
Spin-Orbit Wave Functions of 1a. According to the
antisymmetrical requirement for the total wave function with
respect to permutation of any pair of electrons and the
requirement of the good quantum numbers of S and SZ (eq 7),
the wave functions are written using Slater determinants. The
spin-orbit wave functions for the electronic ground state are
|D0,+1/2 ) |ππRnn|
|D0,-1/2 ) |ππRnn|
(12a)
(12b)
The wave functions of the excited doublet states constructed
from the first ππ* excited singlet state of the phenylanthracene
moiety and the ground-state doublet of the dangling radical are
|Dn,+1/2 ) (1/x2)(|ππ*Rnn| - |ππ*Rnn|) (13a)
|Dn,-1/2 ) (1/x2)(|ππ*Rnn| - |ππ*Rnn|) (13a)
3.5. Time Profiles of the Observed High-Spin Excited
States. The time profiles for the TRESR signals of 1a and 2a
at each canonical resonance position of the fine-structure tensor
are shown in parts a and b, respectively, of Figure 9. All
The ππ* excited doublet state is 3.02 eV above the ground state.
This was determined by the fluorescence spectrum. The Q state
constructed from the ππ* excited triplet state and the dangling
radical was detected by TRESR. The spin-orbit wave functions
(26) Yamashita, S.; Okamura, M.; Nagao, H.; Yamauchi, K. Chem. Phys.
Lett. 1995, 233, 88.

302 J. Am. Chem. Soc., Vol. 123, No. 2, 2001
Teki et al.
are
|Quππ*,+2 ) |ππ*R1R2nn|
(17a)
|Quππ*,+1 ) (1/2)(|ππ*R1R2nn| + |ππ*R1R2nn| +
|ππ*R1R2nn| + |ππ*R1R2nn|) (17b)
|Qππ*,+3/2 ) |ππ*Rnn|
(14a)
|Qππ*,+1/2 ) (1/x3)(|ππ*Rnn| + |ππ*Rnn| +
|Quππ*,0 ) (1/x6)(|ππ*R1R2nn| + |π π*R1R2nn| +
|ππ*R1R2nn| + |ππ*R1R2nn| +
|ππ*Rnn|) (14a)
|Qππ*,-1/2 ) (1/x3)(|ππ*Rnn| + |ππ*Rnn| +
|ππ*R1R2nn| + |ππ*R1R2nn|) (17c)
|ππ*Rnn|) (14c)
|Quππ*,-1 ) (1/2)(|ππ*R1R2nn| + |ππ*R1R2nn| +
|ππ*R1R2nn| + |ππ*R1R2nn|) (17d)
|Qππ*,-3/2 ) |ππ*Rnn|
(14d)
UHF ab initio calculation of 1a (not shown) using the
Becke3LYP density functional theory (DFT) indicated that the
ππ* excited quartet state is the lowest quartet state. A charge-
transfer (CT)-type nπ* excited quartet state constructed from
the dangling radical SOMO and a CT-type nπ* excited triplet
state arises from the one-electron transfer from the lone pair of
the oxygen atom to the π*-orbital of the phenylanthracene. The
CT-type nπ*-state is different from the nπ*-state localized on
the radical site.27 The spin-orbit wave functions are
|Quππ*,-2 ) |ππ*R1R2nn|
(17e)
The explicit expressions of the spin-orbit wave functions of
other spin states were obtained in a manner similar to that of
1a (not included).
3.7. Selective Enhanced Intersystem Crossing and Electron
Spin Polarization for High-Spin Excited States. ESP Gen-
eration Mechanism of 1a. The relative populations (polariza-
tions) of each MS sublevel in Q have been determined to be
P+1/2′ ) P-1/2′ ) 0.5 and P+3/2′ ) P-3/2′ ) 0.0 with an increasing
order of energy in zero magnetic field from the spectral
simulation shown in Figure 5b.28 As shown in Figure 10, this
ESP pattern can be interpreted by assuming an MS conserved
selective ISC (Dn f Qππ*) between the MS ) (1/2 sublevels
of Dn and Qππ* in zero magnetic field. The origin of the ISC
should be attributed to the spin-orbit coupling (SOC) of the
dangling N-O group for the following reasons: (i) The
possibility of a dominant ISC process by a dipolar mixing
between Dππ* and Qππ* was easily ruled out because the
magnitude of the intramolecular exchange interaction (J) is much
larger than that of the dipolar interaction (D) in the π-conjugated
spin systems.29 (ii) It is known that diphenylanthracene has a
very low efficiency for ISC from S1 to T1.30 (iii) There are no
atoms, other than the oxygen in the radical, with localized
unpaired electrons that have large SOC. Thus, for understanding
of the selective ISC in our systems, it is necessary to consider
an enhanced ISC mechanism, which is caused by the presence
of a dangling radical with a 1/2 spin.
The most plausible mechanism for the dynamic ESP is an
intramolecular electron transfer (ET) through π-conjugation and
SOC on the local N-O group (Figure 11).31 Direct SOC
between the Dn and Qππ* states vanishes because the SOC
Hamiltonian, HSO, is a one-electron operator, and SOC between
the Dn and Qππ* states is also negligible as a result of the
symmetry between |Dn,(1/2 and |Qππ*,(3/2 .32 On the other
hand, the SOC interaction between the MS ) (1/2 sublevels
of Qnπ* (|Qnπ*,(1/2 ) and the charge-transfer state (|DCT*,(1/
2 ) in eqs 16a and 16b has a large matrix element which is
given as follows:
|Qnπ*,+3/2 ) |πππ*Rn|
(15a)
|Qnπ*,+1/2 ) (1/x3)(|πππ*Rn| + |πππ*Rn| +
|πππ*Rn|) (15b)
|Qnπ*,-1/2 ) (1/x3)(|πππ*Rn| + |π ππ*Rn| +
|πππ*Rn|) (15c)
|Qnπ*,-3/2 ) |πππ*Rn|
(15d)
The Qnπ* state can mix with the Qππ* state through π-conjuga-
tion, causing the ESP in the observed Q state. In addition to
these states, it is necessary to consider the following photoin-
duced CT excited doublet state:
|DCT*,+1/2 ) |πππ*nn|
|DCT*,-1/2 ) |πππ*nn|
(16a)
(16b)
In this CT state, one electron is transferred from the SOMO of
the radical to the phenylanthracene moiety. Unfortunately, the
CT state could not be clearly observed in the optical spectra.
However, this is not in conflict with our consideration described
later because the CT absorption will overlap with the observed
transitions and has much smaller intensity.
Spin-Orbit Wave Functions of 2a. The spin-orbit wave
functions of the ground state and various excited states were
constructed using the π- and π*-orbitals of the diphenylan-
thracene moiety and the SOMOs of the two dangling radicals
(R1 and R2). The spin-orbit wave functions of the ππ* excited
quintet state (S ) 2), which was detected by TRESR experi-
ments, are
DCT*,(1/2|HSO|Qnπ*,(1/2 ) (1/x3)ê n|lZ|R
≈ (1/x3)êγ n|lZ|pπ(O) (18)
All other matrix elements are insignificant. Here, the Z axis
was chosen to be along the N-O bond axis, and γ is a
coefficient of the pπ orbital for the oxygen atom in the SOMO.33
The magnitude of γ is roughly estimated to be ca. 0.57 from
the ab initio MO calculation (STO-3G basis set).34 Equation
18 shows that the SOC leads to a selective ISC from DCT* to
|Qnπ*,(1/2 . ESP transfer from Qnπ* to Qππ* occurs easily since
(27) The quartet TRESR spectrum was also observed by the photoex-
citation to the localized nπ*-state using 445 nm light from OPO (Continuum
Surelite OPO) pumped by a YAG laser. This finding indicates that the
localized nπ*-state is higher in energy than the nπ* (CT-type) and ππ*
quartet states because the energy transfer occurs from higher energy states
toward lower energy states. Since the ππ* doublet excited state exists higher
in energy than the localized nπ*-state, as shown in the optical spectra of 1
and 2 in Figure 2, the nπ* (CT-type) and ππ* quartet states are lower in
energy than the ππ* doublet excited state.
(28) For P+1/2′, P-1/2′, P+3/2′, and P-3/2′, we employed the same notation
used in the work of Ishii et al.8b

π-Topology and Spin Alignment
J. Am. Chem. Soc., Vol. 123, No. 2, 2001 303
conserved in Qππ*, since | Qnπ*,MS|H|Qππ*,MS | * 0 only
between the spin sublevels with the same MS values. In addition,
the nπ* excited state is expected to be located slightly above
the ππ* excited quartet state. The small nonvanishing matrix
element between Qnπ* and Qππ* causes mixing of both states.
Therefore, if there is any nonzero matrix element between the
Dn and DCT* states, selective ISC occurs from Dn to the MS )
(1/2 sublevels of Q (|Qππ*,(1/2 ), according to the pathway
in Figure 11a. The most reasonable candidate is an ET
interaction (HET) from the SOMO of the radical to the π-HOMO
of the phenylanthracene moiety. The matrix element is roughly
estimated by
HET D fCT ) | DCT*,(1/2|H|Dn,(1/2 | ≈ (1/x2)| π|H|R |
n
(19)
Figure 10. Schematic illustration of the energy diagram, selective
intersystem crossing, and energy relaxation process. The MS ) (1/2
sublevels of the quartet state are selectively populated by ISC. In this
diagram, R is the doublet ground state of the isolated dangling radical.
Since the π-HOMO interacts with the radical site through
π-conjugation, this matrix element has a large value, leading
to the mixing between the Dn and DCT* states. According to
first-order perturbation theory, the perturbed doublet state (Dn′)
is described by35
0
0
S0 , S1 , and T10 are the ground and the excited singlet and triplet states
of the phenylanthracene moiety, respectively.
0
|Dn′(1/2 ) |Dn ,(1/2 +
DCT*,(1/2|H|Dn,(1/2
|CT,(1/2 (20)
ECT - ED
n
This state has a nonzero matrix of the SOC for |Qnπ*,(1/2 ,
which leads to selective ISC from Dn to Qnπ* (Figure 11b).
According to the MS conservation rule, the ESP of Qnπ* is
transferred to the MS ) (1/2 sublevels of Qππ* (|Qππ*,(1/2 )
through an energy-transfer process. This process may be the
most plausible mechanism for the enhanced selective ISC
between Dn and the observed ππ* quartet states (Qππ*).
However, we cannot completely rule out the possibility of
another ESP mechanism because no direct evidence for the DCT*
state was obtained from optical spectra.
ESP Generation Mechanism of 2a. In a manner similar to
that of 1a, the ESP for Qu in 2a can be interpreted as a selective
enhanced ISC (Sn f Qu) (Figure 12) generated through SOC
on the N-O groups. However, the quantitative calculation for
the ESP for Qu is much more difficult than for Qππ*. In an
alternative way, the dominant ESP of Qu was estimated by
assuming the quintet state as an exchange-coupled spin system
Figure 11. Most plausible mechanism for the dynamic electron spin
polarization of the quartet excited state (Q). (a) Selective intersystem
crossing from the perturbed excited doublet state (Dn′) to the MS
)
(31) A novel spin polarization mechanism by an intramolecular photo-
induced energy transfer and the selective enhanced intersystem crossing
were recently reported in a covalently linked copper(II) porphyrin-free-
base porphyrin dimer: (a) Asano-Someda, M.; van der Est, A.; Kru¨ger,
U.; Stehlik, D.; Kaizu, Y.; Levanon, H. J. Phys. Chem. A 1999, 103, 6704.
(b) Toyama, N.; Asano-Someda, M.; Ichino, T.; Kru¨ger, Kaizu, Y. J. Phys.
Chem. A 2000, 104, 4857. However, the mechanism works in the case of
quantum mixing in the triplet-doublet pair and cannot be straightforwardly
applicable to our system since the spin states in our systems are pure spin
states, indicating negligible quantum mixing.
(1/2 sublevels of the nπ* quartet state (Qnπ*) followed by the internal
conversion to the observed ππ* quartet excited state (Qππ*). (b)
Schematic illustration of the energy diagram and the ESP transfer
process. Only the MS ) (1/2 sublevels of Qnπ* have a nonzero SOC
matrix element (HSO) with the component of the CT state in the
perturbed doublet state (Dn′) (see the text). The ESP generated in Qnπ*
is transferred to Qππ* through the IC process induced by the electronic
coupling (He-e ) electron transfer or exchange interaction). A typical
spin configuration of MS ) +1/2 sublevel is also shown as an example.
(32) The negative sign between two Slater determinants in eq 13 and
positive sign for the same Slater determinants in eq 14.
(33) It is well-known that N-O radicals have the largest g value for
this direction along the N-O bond axis and the magnitude is typically gZZ
) 2.010. (Capiomont, A.; Chion, B.; Lajzerowicz-Bonneteau, J.; Lemaire,
H. J. Chem. Phys. 1974, 60, 2350. In this paper, the X direction has been
chosen along the N-O bond axis.) For the other two directions, gYY is 2.007
and gXX is 2.003 (pπ-direction) typically. The large deviation of gZZ from
the free spin g value means that the SOC matrix element (ê n|lZ|pπ(O) )
between the lone pair orbital (n) and the pπ-orbital on the oxygen site does
not vanish and the other SOC elements are negligible.
this process is an internal conversion between states with the
same spin multiplicity. In this process, the ESP of Qnπ* is
(29) The value of Jintra/kB was determined to be -5.8 ( 0.2 K for the
ground state of 2a as described in section 3.2. The magnetic interaction
arises from the indirect interaction through the closed-shell anthracene
moiety. The magnitude of the direct exchange interaction through π-con-
jugation between the radical and the open-shell triplet state of the anthracene
moiety is expected to be much larger, though the magnitude of the
intramolecular exchange interaction on the photoexcited states of 1a was
not determined.
(34) The STO-3G minimum basis set was used since the use of the larger
basis sets gave more than one set of artificial p-orbitals, complicating the
estimation.
(30) Actually, the direct detection via TRESR of the T1 state of
phenylanthracene was not successful under the same conditions used to
detect the quartet state.
(35) Since the quantitative estimation of the matrix element in eq 20 is
actually very difficult, it is not presented in this paper, even though it can
help to validate the proposed mechanism.

304 J. Am. Chem. Soc., Vol. 123, No. 2, 2001
Teki et al.
Figure 13. Typical TRESR spectrum of 1b, which is the π-topological
isomer of 1a. The microwave frequency is 9087.56 MHz. The spectrum
was observed in a 2-MTHF rigid glass matrix at 30 K and 1.0 µs after
laser excitation by accumulation 400 times.
excited state, TRESR experiments were performed for 1b and
2b, which are the π-topological isomers of 1a and 2a,
respectively. In contrast to the result of 1a, no TRESR signal
attributable to the high-spin excited state was observed for 1b
and only a very weak absorptive signal due to the doublet state
was observed (Figure 13). The g value for 1b was g(D) ) 2.001
(arrow in Figure 13) and is similar to the value (g ) 2.0020)
estimated from eq 10, indicating that the excited doublet state
arises from the radical-excited triplet pair. Although the
observed signal shows all characteristics expected for the excited
D of 1b, we cannot completely rule out the overlapping or the
possibility of a signal originating from a defect in the quartz
insert, which has been observed in the g ≈ 2 region, because
the signal in Figure 13 was obtained as a result of the long
accumulation time. In any event, no high-spin TRESR signal
for 1b agrees with the π-topological rule, indicating that the
lowest excited state of 1b is a low-spin doublet (S ) 1/2) spin
state and the sign of the intramolecular exchange is antiferro-
magnetic. A similar change in the TRESR spectrum was also
observed for 2b, the π-topological isomer of 2a. No TRESR
signal for the excited spin state was observed in 2b, indicating
that the lowest excited state is a low-spin singlet (S ) 0) spin
state, which is ESR silent. Therefore, the intramolecular
exchange is antiferromagnetic, similar to that of 1b. These
findings show that the sign of the intramolecular exchange
interaction and the spin multiplicity of the lowest excited spin
state change drastically depending on the topological nature of
the π-electron network. Thus, the π-topological rule of the spin
polarization established for the ground states of the aromatic
hydrocarbons11-15 also plays an important role in the spin
alignment on the excited states. These findings also indicate
the ferromagnetic intramolecular exchange for 1a and 2a and
the antiferromagnetic one for 1b and 2b.
Figure 12. Schematic illustration of the energy diagram, selective
intersystem crossing, and energy relaxation process. In this diagram,
R1 and R2 are the doublet ground states of the two dangling radicals.
S0 , S1 , and T10 are the ground and the excited singlet and triplet states
of the diphenylanthracene moiety, respectively. The MS ) (1 and 0
sublevels of the quintet state are selectively populated by ISC. In the
ground state, the singlet state and the triplet state are nearly degenerated
and the energy splitting is only 2Jintra/kB ) 12 K (see section 3.2 in the
text). The spin state selective decay also occurs from the excited quartet
state (Q) to the ground triplet state, which is nearly degenerate to the
singlet ground state (see section 3.5 in the text).
0
0
between the excited quartet state (Qππ*) and the dangling doublet
radical (R). The spin-orbit wave functions for Qu in eqs 17a-e
can be rewritten as follows:
|Quππ*,+2 ) Aˆ{|Qππ*,+3/2 |R }
(21a)
|Quππ*,+1 ) Aˆ{(1/2)|Qππ*,+3/2 |R +
(x3/2)|Qππ*,+1/2 |R } (21b)
|Quππ*,0 ) Aˆ{(1/x2)|Qππ*,+1/2 |R +
(1/x2)|Qππ*,-1/2 |R } (21c)
|Quππ*,-1 ) Aˆ{(1/2)|Qππ*,-3/2 |R +
(x3/2)|Qππ*,-1/2 |R } (21d)
|Quππ*,-2 ) Aˆ{|Qππ*,-3/2 |R }
(21e)
Here, the operator “Aˆ{...}” means an antisymmetric operation
with respect to permutation of any pair of electrons. As
mentioned, only the MS ) (1/2 sublevels of Qππ* (|Qππ*,(1/
2 ) are populated in the selective enhanced ISC mechanism.
Therefore, the population of Qππ* can be transferred only to
the MS ) (1 and 0 sublevels of Qu, leading to the selective
ISC observed in the quintet TRESR spectrum. The calculated
ESP ratios are P0′ ) 0.40, P-1′ ) P+1′ ) 0.30 and P-2′ ) P+2′
3.9. Electronic Structures and Mechanism of Spin Align-
ment on Excited States. To confirm the physical picture of
the spin alignment, ab initio molecular orbital calculations based
on DFT were performed. The electronic structures of the first
excited triplet states (T1) of the phenylanthracene and diphe-
nylanthracene molecules were calculated using Gaussian 98.36
The minimum energy structures were determined by a semiem-
pirical molecular orbital calculation (MNDO/AM1 method). In
the ab initio calculations, the STO 6-31G basis set and UHF
Becke 3LYP hybrid method were employed. Figure 14 shows
the one-electron molecular orbital energies of the NHOMO,
SOMO, and LUMO and the calculated total spin densities on
the carbon sites. In the anthracene moiety of phenyl- or
diphenylanthracene molecules, the spin density distributions are
) 0.0 and are close in magnitude to the observed ones (P0′
)
0.30, P-1′ ) P+1′ ) 0.35 and P-2′ ) P+2′ ) 0.0), although the
calculated P0′ is larger. For more satisfactory agreement, the
other minor ISC pathways must be taken into account.
3.8. Excited Spin States of the Topological Isomers, 1b
and 2b. It is well-known that π-topology plays a very important
role in spin alignment in the ground state of the alternant
hydrocarbons.11-15 In the ground state, the spin polarization
effect governs the spin alignment and the π-topological rule is
well established. The spin multiplicity of the ground state
depends drastically on the topology of the π-electron network.12
To test the role of the π-topology on the spin alignment in the

π-Topology and Spin Alignment
J. Am. Chem. Soc., Vol. 123, No. 2, 2001 305
dominant for intramolecular spin alignment in the excited state
than in the ground state. The unpaired spin in the iminonitroxide
group is coupled to the phenyl- or diphenylanthracene moiety
through π-conjugation according to the π-topological rule for
the spin polarization.11-15 The observation of Qu in 2a and
theoretical calculations show that spin delocalization overcomes
spin polarization in the excited triplet state of the diphenylan-
thracene spin coupler, giving the same sign of the spin densities
at the 9 and 10 positions of the anthracene moiety. As a result,
all the unpaired spins are aligned parallel in the photoexcited
state of 2a, giving Qu detected by the TRESR experiments.
Since a weak antiferromagnetic exchange interaction was
observed in the ground state of this system, the effective
exchange coupling between the two dangling radicals through
the diphenylanthracene spin coupler changed from antiferro-
magnetic to ferromagnetic by photoexcitation (Figure 8). This
phenomenon originates from the results that the photoexcited
triplet state of the anthracene moiety couples ferromagnetically
to the radical spins and the coupling is larger than the direct
interactions between the two dangling radical spins. Thus,
photoinduced spin alignment and sign inversion of the effective
exchange coupling utilizing the excited triplet molecular field
were achieved for the first time on a purely organic π-conjugated
system. In contrast to 1a and 2a, no such high-spin excited state
was detected for their topological isomers, 1b and 2b, under
the same experimental conditions. This result clearly shows that
the drastic change in the spin states occurs in the excited state,
depending on the topological nature of the π-electron network.
Although the importance of the π-topology is well established
for the spin alignment in the ground state, it has not been tested
for the excited state. This work is the first systematic study on
the role of the π-topology in the photoexcited state, and TRESR
experiments for the π-conjugated spin systems have clearly
demonstrated the importance of the π-topology in the spin
alignment of the excited states.
Figure 14. Spin distributions and the location of the UHF one-electron
molecular orbitals of the excited triplet states of (a) phenylanthracene
and (b) diphenylanthracene. The occupied R- and â-orbitals are shown
by the up and down arrows, respectively.
nearly symmetrical (Figure 14). Only the 11, 12, 13, and 14
positions have negative spin densities, and the other carbon sites
all have positive spin densities. This spin density distribution
shows that spin delocalization overcomes the spin polarization
effect on the anthracene moiety. In contrast, the alternating sign
of the spin density is realized on the dangling phenyl groups of
phenyl- and diphenylanthracenes (Figure 14). This sign alterna-
tion of the spin densities shows that the spin polarization
mechanism overcomes the spin delocalization within the
dangling phenyl groups even in the excited triplet state. The
large positive spin densities at the 9 and 10 positions caused
by spin delocalization in the anthracene moiety couple with the
dangling localized radical spins (S ) 1/2) through spin polariza-
tion of the dangling phenyl groups. This mechanism leads to a
ferromagnetic exchange between excited triplet state and radical
spins in 1a and 2a and to an antiferromagnetic exchange in 1b
and 2b. The direct overlap between the triplet excited state of
the anthracene moiety and the dangling radicals is not dominant
in determining the sign of the intramolecular exchange because
the closest distance between them is ca. 3.67 Å in 2b. The long
distance leads to a weak interaction compared with that through
π-conjugation.37 The ab initio molecular orbital calculations are
consistent with the experimental results and the schematic
diagrams of the spin alignment shown in Figures 6 and 8.
Therefore, it was concluded that a photoinduced intramolecular
ferromagnetic spin alignment occurs for 1a and 2a.
Acknowledgment. We thank Professor Seigo Yamauchi
(Tohoku University) and Professor Toshio Matsushita (Osaka
City University) for their helpful discussions. We also thank
Mr. Koji Iimura for his earlier assistance in the set up of our
TRESR apparatus and Dr. B. K. Breedlove for a careful reading
of our final manuscript and for kind suggestions about wording.
This work was financially supported by PRESTO from the Japan
Science and Technology Corp.
JA001920K
4. Conclusions
Direct detection of the excited quartet (S ) 3/2) and quintet
(S ) 2) states shows that the intramolecular ferromagnetic spin
alignment between the doublet spins (S ) 1/2) in the dangling
iminonitroxide radicals and the ππ* excited triplet state of the
phenyl- or diphenylanthracene moiety occurs by photoexcitation.
The ferromagnetic spin alignment in the excited state of 2a
indicates that a spin delocalization mechanism is much more
(37) The iminonitroxide radical couples with the anthracene moiety
through a π-conjugated spin polarization on the dangling phenyl group.
The magnitude of J was estimated by the following equation: Jeff ≈ Jcoupler
-
F(i) F(j)/4SASB. F(i) and F(j) are the spin densities at the connecting positions
of the triplet anthracene derivative and the iminonitroxide radical. Jcoupler
is the effective exchange of the phenyl coupler and is estimated to be larger
than 2000 cm-1. (See: Ishida, T.; Iwamura, H. J. Am. Chem. Soc. 1991,
113, 4238. In this paper, the magnetic exchange was determined to be 500
K for m-phenylenebis(nitroxide).) In 2b, the spin densities were estimated
to be ca. 0.68 at the 9 and 10 positions of the triplet anthracene (see Figure
14b) and ca. 0.09 at the linking position of the iminonitroxide radical (not
shown in this paper). The spin densities were obtained by the ab initio MO
calculations. Jeff(π-π) through π-conjugation was estimated to be ca. 60
cm-1, when the moieties were coplanar. Actually, Jeff(π-π) will be 10-
30 cm-1 for 2b as a result of the torsion angle. The maximum exchange
through the direct overlap was estimated using (1/4SASB)(4t2/U) F(C) F(O)
cos2 θ and t ) -âC-N exp(-d/1.45). F(C) and F(O) are the spin densities
at the overlapping carbon position (F(C) ) 0.25; see Figure 14b) of the
triplet anthracene derivative and at the oxygen atom (F(O) ) 0.50) of the
iminonitroxide radical. Using U ≈ 10 eV and âC-N ≈ 1.5 eV, Jeff due to
the direct overlap was estimated to be smaller than 3 cm-1 (∼2.5 cm-1 for
cos2 θ ) 1) for 2b.
(36) The DFT calculation was carried out using Gaussian 98, Revision
A.7: Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-
Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe,
M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.;
Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A., Gaussian,
Inc., Pittsburgh, PA, 1998.
(38) Teki, Y. Takui, T., Itoh, K. J. Chem. Phys. 1988, 88, 6134.