Novel excited quintet state in porphyrin: Bis(quinoline-TEMPO)-yttrium- tetraphenylporphine complex

Article abstract of DOI:10.1021/ic0511827

New mono- and bis[4-(3-hydroxy-2-methyl-4-quinolinoyloxy)-2,2,6,6- tetramethylpiperidin-1-oxyl](meso-tetraphenylporphyrinato)yttrium-(III) complexes have been synthesized, and the properties of the excited states generated by photoexcitation of porphyrin were studied by time-resolved (TR) and pulsed two-dimensional electron paramagnetic resonance (EPR) spectroscopy. A TR-EPR spectrum was observed in the quartet (S = 3/2) or quintet (S = 2) states generated from interactions of one or two radicals with the photoexcited triplet state of the porphyrin. The zero-field splitting D values of these states were analyzed in terms of those of the triplet and the radical-triplet pair. The spin states of the excited states were definitely assigned by measuring the nutation frequencies with pulsed EPR.

Full text of DOI:10.1021/ic0511827

Inorg. Chem. 2005, 44, 9125−9127
Novel Excited Quintet State in Porphyrin:
Bis(quinoline TEMPO) yttrium tetraphenylporphine Complex
−
−
−
Luca Maretti,† Saiful S. M. Islam,† Yasunori Ohba,†,§ Takashi Kajiwara,‡ and Seigo Yamauchi*,†
Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Katahira
2-1-1, Aoba-ku, Sendai 980-8577, Japan, Department of Chemistry, Graduate School of Science,
Tohoku UniVersity, Aramaki, Aoba-ku, Sendai 908-8578, Japan, and CREST, JST, Kawaguchi-shi,
Saitama 332-0012, Japan
Received July 14, 2005
New mono- and bis[4-(3-hydroxy-2-methyl-4-quinolinoyloxy)-2,2,6,6-
tetramethylpiperidin-1-oxyl](meso-tetraphenylporphyrinato)yttrium-
(III) complexes have been synthesized, and the properties of the
excited states generated by photoexcitation of porphyrin were
studied by time-resolved (TR) and pulsed two-dimensional electron
paramagnetic resonance (EPR) spectroscopy. A TR-EPR spectrum
that are covalently connected to a porphyrin experience an
interaction with the photoexcited triplet porphyrin, a new
series of excited states are generated. While the ground state
1
has a doublet character (S ) /2) due to noninteracting
doublets, the resulting excited states are singlet (S ) 0), triplet
(S ) 1), or quintet (S ) 2) states. A porphyrin system that
efficiently generates a quintet state becomes a candidate for
a new concept photoreaction center in which the conversion
of the quintet radical-ion pair to the ground state of the
doublet nature becomes spin forbidden.
was observed in the quartet (S
) ) 2) states
3/2) or quintet (S
generated from interactions of one or two radicals with the
photoexcited triplet state of the porphyrin. The zero-field splitting
D values of these states were analyzed in terms of those of the
triplet and the radical-triplet pair. The spin states of the excited
states were definitely assigned by measuring the nutation frequen-
cies with pulsed EPR.
In general, the photoexcited quintet state has its own
history especially related to pure organic materials suitable
for light-switched molecular magnets. In 1995, Corvaja’s
group opened this new frontier on the study of excited states
having spin multiplicity higher than the triplet state.2 A
quartet state was observed in a fluid solution by excitation
of mononitroxide-linked fullerene. Later, this system was
advanced by introducing a second nitroxide, and the pho-
toexcited quintet state was achieved in a frozen solution3
and recently in a fluid solution.4 Teki’s group also achieved
this goal by connecting two nitroxide moieties to diphenyl-
anthracene.5 Whereas purely organic materials have suc-
ceeded, in organometallic systems, no excited quintet state
has been generated until now. In our laboratories, several
years have been spent on the study of the triplet-radical
interactions in the system of pyridyl nitroxides coordinated
to zinc porphyrin,6 where one radical can coordinate to the
Mimicking the photosynthetic reaction center to produce
long-lived radical-ion pairs has been a subject of great interest
in recent years. Dyads and triads based on porphyrin electron
donors connected to electron acceptors such as fullerene or
quinone molecules have been extensively studied for their
photophysical and photochemical properties.1 These studies
have shown that most of the electron-transfer pathways go
through the photoexcited singlet state of porphyrins, leading
to a radical-ion pair that quickly recombines as a result of
spin-allowed processes.
Introduction of stable free radicals in the photoreaction
center allows one to generate excited states of new high-
spin-multiplicity pathways for electron transfer and to control
recombination processes between states having different spin
quantum numbers. When two uncoupled stable free radicals
(2) Corvaja, C.; Maggini, M.; Prato, M.; Scorrano, G.; Venzin, M. J. Am.
Chem. Soc. 1995, 117, 8857-8858.
(3) (a) Conti, F.; Corvaja, C.; Toffoletti, A.; Mizuochi, N.; Ohba, Y.;
Yamauchi, S.; Maggini, M. J. Phys. Chem. A 2000, 104, 4962-4967.
(b) Mizuochi, N.; Ohba, Y.; Yamauchi, S. J. Phys. Chem. A 1999,
103, 7749-7752.
(4) Franco, L.; Mazzoni, M.; Corvaja, C.; Gubskaya, V. P.; Berzhnaya,
L. S.; Nuretdinov, I. A. Chem. Commun. 2005, 2128-2130.
(5) (a) Teki, Y.; Miyamoto, S.; Nakatsuji, M.; Miura, Y. J. Am. Chem.
Soc. 2001, 123, 294-305. (b) Teki, Y.; Nakatsuji, M.; Miura, Y. Mol.
Phys. 2002, 100, 1385-1394.
(6) (a) Ishii, K.; Fujisawa, J.; Ohba, Y.; Yamauchi, S. J. Am. Chem. Soc.
1996, 118, 13079-13080. (b) Fujisawa, J.; Ishii, K.; Ohba, Y.;
Yamauchi, S.; Fuhs, M.; Mo¨bius, K. J. Phys. Chem. A 1999, 103,
213-216. (c) Fujisawa, J.; Iwasaki, Y.; Ohba, Y.; Yamauchi, S.; Fuhs,
M.; Mo¨bius, K.; Weber, S. Appl. Magn. Reson. 2001, 21, 483-493.
* To whom correspondence should be addressed. E-mail: yamauchi@
tagen.tohoju.ac.jp.
† Institute of Multidisciplinary Research for Advanced Materials, Tohoku
University.
‡ Department of Chemistry, Graduate School of Science, Tohoku
University.
§ CREST, JST.
(1) (a) Luo, C.; Guldi, D. M.; Imahori, H.; Tamaki, K.; Sakata, Y. J. Am.
Chem. Soc. 2000, 122, 6535-6551. (b) Kodis, G.; Liddell, P. A.; de
la Garza, L.; Clausen, P. C.; Lindsey, J. S.; Moore, A. L.; Moore, T.
A.; Gust, D. J. Phys. Chem. A 2002, 106, 2036-2048.
10.1021/ic0511827 CCC: $30.25
Published on Web 11/12/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 25, 2005 9125

COMMUNICATION
central metal ion and excited quartet and doublet states have
been generated.
Here we report the synthesis and characterization of the
excited states of mono[4-(3-hydroxy-2-methyl-4-quinolinoy-
loxy)-2,2,6,6-tetramethylpiperidin-1-oxyl](aqua)(meso-tet-
raphenylporphyrinato)yttrium(III) (QTYTPP) and bis[4-(3-
hydroxy-2-methyl-4-quinolinoyloxy)-2,2,6,6-tetramethyl-
piperidin-1-oxyl](meso-tetraphenylporphyrinato)yttrium-
(III), silver(I) salt (QT2YTPPAg). The radical ligand used
to connect paramagmetic fragments to the heterocyclic
porphyrin through a central metal ion was the 4-(3-hydroxy-
2-methyl-4-quinolinoyloxy)-2,2,6,6-tetramethylpiperidin-1-
oxyl free radical (QT). This kind of modified TEMPO radical
has been extensively used as a prefluorescent probe by
Scaiano’s group in several systems.7 QT is, by the way, also
an efficient ligand for transition metals such as Ni(II) and
Co(II) and for lanthanide metals8 thanks to the enolic part
of its structure, which can be easily activated by adding a
suitable base such as potassium tert-butoxide in 2-propanol.
Yttrium metal was selected for its diamagnetic property, high
coordination number (>6), and ability to connect TPP to an
organic ligand like 2,4-pentanedionato.9 Its large size does
not allow it to enter the porphyrin ring and makes it possible
to coordinate quite large bidentate ligands such as QT. Both
the mono- and bisradical complexes were prepared from bis-
[4-(3-hydroxy-2-methyl-4-quinolinoyloxy)-2,2,6,6-tetrameth-
ylpiperidin-1-oxyl](meso-tetraphenylporphyrinato)yttrium-
(III), potassium(I) salt (QT2YTPPK). QT2YTPPK was
prepared by a ligand-exchange reaction from 2,4-pentanedi-
onato(meso-tetraphenylporphyrinato)yttrium(III) by adding
QT and potassium tert-butoxide in 2-propanol.10 For the
monoradical complex, QT2YTPPK was dissolved in 1:1
water/acetone, and after evaporation of acetone, a precipitate
was formed. After filtration, single crystals were obtained
from 2-propanol/chloroform and characterization was achieved
by means of X-ray crystallography. The crystal structure
shows that Y is in heptacoordination by four nitrogen atoms
(N1-N4) from the TPP ligand, two carbonyl oxygen atoms
(O1 and O2) from QT ligand, and one coordinating water
molecule (O5), with distances of 2.230(7)-2.387(7) Å
(Figure 1).11
Figure 1. ORTEP diagram of QTYTPP with thermal ellipsoids at 50%
probability. QT and TPP moieties are represented with filled and open bonds,
respectively. Protons are omitted for clarity. Selected bond distances (Å):
Y-O1, 2.230(7); Y-O2, 2.351(6); Y-O5, 2.372(6); O4-N6, 1.272(10);
Y-N1, 2.387(7); Y-N2, 2.352(6); Y-N3, 2.361(7); Y-N4, 2.348(8).
Figure 2. Molecular structure of QT2YTPPAg.
(EPR) spectra were then recorded at room temperature for
QTYTPP and QT2YTPPAg, and they showed the three
characteristic lines of the TEMPO radical, evidencing the
presence of doublet character in the ground states. Quantita-
tive analyses of the spectral intensities confirmed the
presence of one QT for QTYTPP and two QT’s for
QT2YTPPAg. In this bisradical complex, the large distance
between the two radicals avoids a strong exchange interaction
and the possibility of having a triplet ground state, which
would dramatically affect the deactivation processes, result-
ing in the fast conversion from the excited state to the triplet
ground state. This has already been realized in a system of
silicon phthalocyanine covalently linked to two nitroxides,
where the short distance between two nitroxides leads to a
triplet ground state and no quintet state could be observed
by time-resolved (TR)-EPR.12
QT2YTPPAg (Figure 2) was synthesized by adding silver
nitrate to a 2-propanol solution of QT2YTPPK. This resulted
in precipitation, yielding pure material of QT2YTPPAg.
Continuous-wave (CW) electron paramagnetic resonance
(7) Aspe´e, A.; Grac´ıa, O.; Maretti, L.; Sastre, R.; Scaiano, J. C.
Macromolecules 2003, 36, 3550-3556.
TR-EPR spectra of the excited states of QTYTPP and
QT2YTPPAg were observed in toluene at 30 K and 0.3 µs
after the laser pulse (Figure 3). The irradiation wavelength
was selected at 585 nm.
(8) Maretti, L. MSc. Dissertation, University of Ottawa, Ottawa, Canada,
2002.
(9) Wong, C.; Horrocks, W. D., Jr. Tetrahedron Lett. 1975, 31, 2637-
2640.
(10) Single crystals were obtained in chloroform: Maretti, L.; Islam, S.
M. S.; Ohba, Y.; Kajiwara, T.; Yamauchi, S., unpublished data.
Crystallographic data for QT2YTPPK‚7CHCl3: monoclinic, C2/c, a
) 17.673(4) Å, b ) 18.486(8) Å, c ) 32.090(13) Å, â ) 99.317-
(14)°, V ) 10346(6) Å3, Z ) 4, Dcalcd ) 1.469 g/cm3, R1 ) 0.072 (I
> 2σ(I)).
(11) Crystallographic data for 1: monoclinic, space group C2/c, a ) 44.979-
(6) Å, b ) 13.4637(18) Å, c ) 23.867(3) Å, â ) 107.803(3)°, V )
13761(3) Å3, T ) 200(2) K, Z ) 8, Dcalcd ) 1.238 g/cm3, 31 651
reflections were observed, of which 9029 were independent; R1 )
0.073 (I > 2σ(I)), wR2 ) 0.198 (all data) for 754 parameters.
Both spectra show electron spin polarization as an absorp-
tion (A) and an emission (E) of the microwave having an
A/E type, that is, due to the spin-selective intersystem
crossing process from the photoexcitation state. For QTYT-
PP, three kinds of signals were observed; one is a pair signal
(12) Ishii, K.; Hirose, Y.; Kobayashi, N. J. Am. Chem. Soc. 1998, 120,
10551-10552.
9126 Inorganic Chemistry, Vol. 44, No. 25, 2005

COMMUNICATION
Table 1. Nutation Frequencies of the Complexes
B/mT
νn/MHz
νn/ν1
assignment
QTYTPP
1 (1)a
326
300
356
311
344
22.5 ( 1
31.2 ( 1
31.2 ( 1
39.1 ( 1
38.1 ( 1
D0
1.39 (x2)
T ((1-0)
1.39 (x2)
1.74 (x3)
1.69 (x3)
T ((1-0)
Qa ((3/2 to (1/2)
Qa ((3/2 to (1/2)
QT2YTPPAG
1 (1)
326
320
335
20.2 ( 1
40.7 ( 1
40.0 ( 1
D0
2.01 (2)
1.98 (2)
Qi ((2 to (1)
Qi ((2 to (1)
a The values in parentheses are calculated from eq 3.
spectroscopy. This technique is based on the pulsed spin-
echo method; the first microwave pulse length is variable,
and the echo signal, which is obtained by a second
microwave pulse, is Fourier transformed with respect to the
first pulse width, giving a nutation frequency. This experi-
ment was carried out at 30 K for both complexes on a pulsed
EPR spectrometer of our own design.15 The nutation
frequency for each signal was measured at the magnetic fields
whose positions are indicated by arrows in Figure 3. The
spin multiplicity and the magnetic transitions are assigned
by using the following equation:
Figure 3. TR-EPR specta of (a) QTYTPP and (b) QT2YTPPAg. Spectra
were observed at 30 K and 0.3 µs after the laser pulse (585 nm). The arrows
indicate the positions where the nutation frequencies were observed by
pulsed EPR.
giving an A/E polarization pattern, which shows A and E at
the lower magnetic field (ca. 296 mT) and higher magnetic
field (ca. 358 mT) sides, respectively. The second signal also
has an A/E pattern (A, 307 mT; E, 349 mT), and the third
one is a sharper signal having A polarization positioned
almost at the center of the spectrum. For QT2YTPPAg, two
kinds of signals were observed. One is a pair signal giving
an A/E pattern (A, 314 mT; E, 340 mT), and the other is a
relatively sharp signal, being similar to that of QTYTPP.
The sharper signals are easily assigned to the ground state
having a doublet nature (D0) by comparing the TR spectrum
at later times (>9 µs) with the CW-EPR spectrum observed
in a separate measurement.
νn ) [S(S + 1) - MS(MS - 1)]1/2ν1
(3)
Here ν1 is the nutation frequency of a S ) 1/2 system under
the same experimental conditions. The obtained nutation
frequencies are summarized in Table 1.
By comparing the calculated and experimental nutation
frequencies, it is clearly concluded that monoradical QTYT-
PP generates mainly a quartet excited state under photoex-
citation of the porphyrin ring. The observed triplet is
considered to be due to a small amount of diamagnetic
impurity, probably produced during the synthetic procedure.
Bisradical QT2YTPPAg instead generates an excited quintet
state, whose magnetic transitions of |MS ) (2 S |MS )
(1 were observed.
In summary, we have obtained the excited quintet state
for the first time for a porphyrin metal complex and hopefully
introduced a new strategy in the design of more efficient
photoreaction centers. Using spin-forbidden transitions in-
stead of increasing the distances between electron donors
and acceptors, longer-lived radical-ion pairs could be ob-
tained.
The assignment of the other signals is made by analyzing
the zero-field-splitting parameter D. For strongly coupled
systems, the following equations are obtained for the excited
quartet (Qa) and quintet (Qi) states, respectively, in a coupled
triplet (T)-radical (R) system:13
DQ ) (1/3)(DT + DRT)
(1)
(2)
a
DQ ) (1/6)(DT + DR1T + DR2T) + (1/12)DR1R2
i
The D value of the excited triplet state is already obtained
as 31.1 mT.14 From the obtained structure, DRT and DRR are
estimated to be much smaller than DT; namely, DQ and DQ
Acknowledgment. This work was supported by Grant-
in-Aid for Scientific Researches No. 165500031 from the
Ministry of Education, Science, Culture and Sports, Japan.
a
i
1
1
are ca. /3 (10.4 mT) and /6 (5.2 mT) of DT. The obtained
values (from Figure 3) are 10.3 and 4.3 mT, providing the
assignments of the signals as those of the quartet and quintet
states, respectively. These assignments of the spin states were
confirmed more definitely by means of pulsed EPR nutation
Supporting Information Available: Synthesis information,
X-ray crystal data of QTYTPP, and one-dimensional nutation
frequency spectra for QTYTPP and QT2YTPPAg. This material
is available free of charge via the Internet at http://pubs.acs.org.
(13) Bencini, A.; Gatteschi, D. Electron Paramagnetic Resonance of
IC0511827
Exchange Coupled Systems; Springler-Verlag: Berlin, 1990; Chapter
3.
(14) Ishii, K.; Ohba, Y.; Iwaizumi, M.; Yamauchi, S. J. Phys. Chem. 1996,
100, 3839-3846.
(15) Hanaishi, R.; Ohba, Y.; Yamauchi, S.; Iwaizumi, M. J. Chem. Phys.
1995, 103, 4819.
Inorganic Chemistry, Vol. 44, No. 25, 2005 9127