Generation of a triplet diradical from a donor-acceptor cross conjugate upon acid-induced electron transfer

Article abstract of DOI:10.1039/b804999h

Enhancement of the electron acceptor ability of a para-quinodiimine unit by double protonation leads to the proton-induced intramolecular electron transfer from the donor unit to the cross-conjugated acceptor, giving rise to ground state triplet diradical reversibly. The Royal Society of Chemistry.

Full text of DOI:10.1039/b804999h

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Generation of a triplet diradical from a donor–acceptor cross conjugate
upon acid-induced electron transferw
Mats O. Sandberg, Osami Nagao, Zhikun Wu, Michio M. Matsushita and
Tadashi Sugawara*
Received (in Cambridge, UK) 25th March 2008, Accepted 6th May 2008
First published as an Advance Article on the web 18th June 2008
DOI: 10.1039/b804999h
Enhancement of the electron acceptor ability of a para-
quinodiimine unit by double protonation leads to the proton-
induced intramolecular electron transfer from the donor unit to
the cross-conjugated acceptor, giving rise to ground state triplet
diradical reversibly.
role of donor or acceptor is specified to the appropriate
molecular unit within a cross-p-conjugated molecular frame-
work. Such conversion of spin states of the cross-p-conjugated
molecule can be performed in a PVC film reversibly, accom-
panied by the color change. Accordingly, the current system
can be utilized to develop a protonation-induced magnetic
memory when the protonation-induced electron transfer
occurs within one molecule.
A cyclic voltammogram of cross-p-conjugate QA6 in an
acetonitrile solution in the presence of tetra-n-butylammo-
nium perchlorate as an electrolyte showed a redox wave at
0.95 V vs. Ag/AgCl. Since the oxidation potential is close to
that of trianisylamine (TA) (E1/2 = 0.84 V vs. Ag/AgCl), it is
Cooperative transportation of a proton and an electron is
often observed in biological systems, as observed in redox
reactions carried out by flavoenzymes.1 Such a phenomenon
also plays a crucial role in the physical properties of molecular
systems. Emeraldine, which consists of an equimolar mixture
of poly-p-phenylenediamine and poly-p-quinodiimine, shows
high electrical conductivity when it is protonated.2 Di-proto-
nation on the p-quinodiimine unit induces an electron transfer
from the p-phenylenediamine unit, providing two equivalent
cation-radicals of p-phenylenediamine as an electronic carrier
along the polymer chain.
Such a protonation-induced electron transfer can be utilized
for generating a high spin molecule. As an example of the
protonation-induced spin manipulation, Ishiguro et al.
demonstrated that the spin multiplicity of a cross-p-conjugate
of a phenoxyl radical and a nitronyl nitroxide could be
changed reversibly between the diradical and closed shell state
when the pH of the solution was changed.3 Morita et al.
reported the similar phenomenon in a phenalenoxyl radical
carrying a nitronyl nitroxide.4 On the other hand, Wienk and
Janssen observed the generation of ground state triplet dica-
tion-diradical species through an intermolecular electron
transfer by adding acid to an equimolar mixture of fully
reduced and fully oxidized forms of an oligoaniline in which
two para-substituted rings are linked by a m-phenylene ring
which act as a ferromagnetic coupler, although this is an
intermolecular process.5
Here we designed a manipulating spin system in terms of
protonation-induced electron transfer as an unimolar event,
affording a ground state triplet species. As shown in Fig. 1, the
Fig. 1 Schematic drawing of a protonation-induced electron transfer
and generation of high-spin species in QA. (a) The neutral dimer
consists of a donor, an acceptor precursor (pro-acceptor) and a
ferromagnetic coupler. (b) The p-quinodiimine moiety acquires the
acceptor ability, after being doubly-protonated at the two nitrogen
atoms, and the intramolecular electron transfer takes place from the
dianisylphenylamine moiety. (c) A ground state triplet diradical is
generated from the protonation-induced electron transfer and from
the ferromagnetic interaction via the m-phenylene coupler between
two unpaired electrons
Department of Basic Science, Graduate School of Arts & Sciences,
The University of Tokyo, Komaba, Meguro-Ku, Tokyo 153-8902,
Japan. E-mail: suga@pentacle.c.u-tokyo.ac.jp; Fax: +81-3-5454-
6997; Tel: +81-3-5454-6742
w Electronic supplementary information (ESI) available: Preparation
methods for compounds QA, the details of the UV-vis spectral change
upon titration of QA with TFA, ESR spectrum of QAꢀH22+ in a PVC
matrix, calculated spin distribution of QAꢀH22+, IR spectrum of QA
and QAꢀH2ꢀ(ClO4)2 with the theoretical simulations. See DOI:
10.1039/b804999h
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quinodiimine unit. The UV spectrum of the acidic solution
was returned to the original one by the addition of the excess
amount of triethylamine (TEA). This reversible color change
was observed repeatedly.
Formation of the radical species upon protonation was ob-
served by ESR measurement. An ESR spectrum of the frozen
solution of diprotonated QA in dichloromethane containing TFA
([QA] : [TFA] = 1 : 5) showed a broad peak centered at
g = 2.0036 flanked by two shoulders separated by 4.1 mT
(Fig. 3). These fine structures can be assigned to a triplet
diradical species with the zero field splitting parameters of
D/hc = 0.0019 cmꢂ1 E/hc = 0.0008 cmꢂ1 on the basis of the
second order perturbational simulation. Furthermore, the half-
field resonance line due to the forbidden transition (DMs = 2)
was observed. These results are strong support for the generation
of the triplet diradical species upon the di-protonation of QA.
Since the intensity of the triplet signal increased monoto-
nously with decreasing temperature, the triplet is the ground
state for the diradical. According to the MO calculation,
coefficients of SOMOs distribute on the phenylenediamine or
dianysilphenylamine moiety, respectively, and the distribution of
both orbitals overlap on the m-phenylene ring. This electronic
feature strongly suggests the presence of the ferromagnetic
interaction between unpaired electrons.9 The small D value of
QAꢀH22+ was also reproduced on the basis of the spin distribu-
tion calculated by the above molecular orbital calculation.10
Fig. 2 UV-Vis spectral change of QA in dichloromethane solution
titrated by trifluoroacetic acid (TFA). (a) UV-Vis spectra of QA and
doubly protonated QA (QAꢀH22+) in acetonitrile. (b) Acetonitrile
2+
solutions of QA (left: yellow colored solution) and QAꢀH2
(right:
deep blue). (c) Protonation induced electron transfer was also per-
formed in a PVC film reversibly.
2+
Formation of the diradical electronic structure of QAꢀH2
was also supported by magnetic susceptibility data. Although
the chloride salt of QAꢀH22+ was isolated as green precipitate
by bubbling HCl gas into the trichloroethane solution of QA,
the magnetic susceptibility of the salt was only 0.10 emu
attributed to the oxidation process of the triarylamine unit of
QA. On the other hand, the reduction wave could not be
observed down to ꢂ1 V. In order to estimate the redox
potential of the diprotonated form of the p-quinodiimine unit,
the oxidation waves of N,N0-diphenyl-p-phenylenediamine
(PA) were measured because it was a reference compound
for the di-protonated and doubly reduced form of p-benzo-
quinodiimine (QI). The oxidation waves of PA were observed
at +0.71 and +1.11 V in acetonitrile. The data clearly showed
that the acceptor ability of the p-quinodiimine moiety was
drastically increased by di-protonation referring to the second
oxidation potential of PA (+1.11 V). Since this oxidation
potential is higher than the oxidation potential (+0.84 V) of
TA, the electron transfer from the triarylamine unit to the
K molꢂ1 because of the charge transfer interaction between
2+
a highly electron deficient QAꢀH2
moiety and a chloride
anion with a relatively low redox potential. To avoid the
charge donation from the counter anion, the perchlorate
2+
(ClO4ꢂ) salt of QAꢀH2
(QAꢀH2ꢀ(ClO4)2) was prepared
as a deep blue powder by addition of an etheral solution
saturated by perchloric acid. The chemical composition of the
salt was confirmed by the elemental analysis. The QAꢀH2ꢀ
(ClO4)2 salt showed paramagnetic property with the small
2+
diprotonated quinodiimine unit of QAꢀH2 must take place,
affording the dication diradical species.7
The change in the electronic structure of QA upon titration
with trifluoroacetic acid (TFA) to the dichloromethane solu-
tion of QA was monitored by UV-vis absorption spectroscopy
as shown in Fig. 2. A yellow colored solution of cross-
conjugate QA in dichloromethane shows two absorption
peaks at 305 and 442 nm, the shorter peak being characteristic
to the p-benzoquinodiimine unit. When the two equivalent
TFA was added to the solution, the color of the solution
turned to deep blue. Accompanied by the color change, a
strong absorption band at 757 nm with a shoulder at 640 nm
grew. Further addition of TFA did not cause any changes in
the absorption spectrum. Since the band at 757 nm, which
appears by the addition of TFA, is similar to the cation radical
of trianisylamine,8 it is certain that the single electron transfer
occurs from triarylamine to the doubly protonated
Fig. 3 (a) ESR spectrum of the doubly protonated QA (QAꢀH22+) in
a frozen solution of dichloromethane. Marked peaks are assigned as
the z lines of the fine structure of the triplet state of the doubly
protonated species. The inset shows the temperature dependence of the
intensity of the triplet signal.
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antiferromagnetic intermolecular interaction (y = ꢂ7 K).
Although the Curie constant of the salt (0.52 emu K molꢂ1
2+
2 (a) W.-S. Huang, B. D. Humphrey and A. G. MacDiarmid, J.
Chem. Soc., Faraday Trans. 1, 1986, 82, 2385; (b) F. Wudl, R. O.
Angus, F. L. Lu, P. M. Allemand, D. J. Vachon, M. Nowak, Z. X.
Liu and A. J. Heeger, J. Am. Chem. Soc., 1987, 109, 3677; (c) A. G.
MacDiarmid and A. J. Epstein, Faraday Discuss. Chem. Soc., 1989,
88, 317.
3 Although the protonation-induced electron transfer process is not
involved, there are excellent examples in which manipulation of
spin states is conducted in terms of protonation or deprotonation.
(a) K. Ishiguro, M. Ozaki, N. Sekine and Y. Sawaki, J. Am. Chem.
Soc., 1997, 119, 3625; (b) V. V. Martin and J. F. W. Keana, J.
Chem. Soc., Chem. Commun., 1995, 723.
)
was smaller than the predicted value for the QAꢀH2
diradical (1.0 emu K molꢂ1), the value is enough larger than
that for the 1/2 spin complex (0.375 emu K molꢂ1), suggesting
that the diprotonated dication has the diradical electronic
structure. The small Curie constant for the diradical structure
might be due to the chemical bond formation between neigh-
2+
boring QAꢀH2 molecules because the Curie constant of the
salt approached to 0.375 emu K molꢂ1 after annealing at
400 K.
4 S. Nishida, Y. Morita, K. Fukui, K. Sato, D. Shiomi, T. Takui and
K. Nakasuji, Angew. Chem., Int. Ed., 2005, 44, 7277.
5 M. M. Wienk and R. A. J. Janssen, J. Am. Chem. Soc., 1996, 118,
10626.
We also formed a polyvinylchloride film containing con-
jugate QA (1 ꢃ 10ꢂ4 mol gꢂ1). The reversible color change of
the film upon addition of TFA and TEA was the same as that
of the solution (Fig. 2c). The green colored film sample, made
by treating with TFA, also exhibited the triplet ESR signal of
the same D value with that in the frozen solution. These results
suggest that the acid-induced electron transfer occurred as a
unimolecular process, whereas Janssen’s system requires the
intermolecular collaboration.
6 Synthesis of the cross-conjugate QA was described in the ESIw: (a)
R. J. Bushby, D. R. McGill, K. M. Ng and N. Taylor, J.
Chem. Soc., Perkin Trans. 2, 1997, 1405; (b) N. Miyaura, T.
Yanagi and A. Suzuki, Synth. Commun., 1981, 513; (c) M. M.
Wienk and R. A. J. Janssen, J. Am. Chem. Soc., 1996, 118
10626; (d) M. Sandberg and T. Hjertberg, Synth. Met., 1989, 29,
E257.
7 The difference between redox potentials of PA+/PA2+ and TA/
TA+ redox couples is 0.27 V. Judging from the difference of
0.27 eV in the redox potentials, the population of the PA+
+
In conclusion, cross-conjugate moleule QA turned out to
produce a triplet dication diradical when strong acid was added.
Such a type of acceptor, the acceptor ability of which is switch-
able by protonation, can be utilized widely to create manipulat-
ing functional molecular systems, provided that the cooperative
proton–electron transfer takes place triggered by protonation.
The authors express thank to Prof. Y. Kawada of Ibaraki
University for his helpful discussion about synthetic method of
the cross-p-conjugate QA. M.O.S. was funded by the CREST
(JST) project ‘‘Hyper-Structured Molecules for Quantum
Devices’’. O.N. is financially supported by 21Century COE.
This work was supported by Grant-in-Aid for Scientific
Research (13304056) from Ministry of Education, Science,
Technology, Sports and Culture, Japan.
TA+ is 3.0 ꢃ 105 times larger than that of PA2+ + TA at
300 K. Thus the electron transfer will take place almost completely
when the PA2+ is generated by protonation of the QI in the
presence of TA.
8 The cation radical of trianisylamine was generated in a fused
cell for galvanometric and spectroscopic measurements. The
absorption spectrum of the species generated in the cell at
the oxidation potential of +0.84 V showed an absorption
band at 700 nm, which was assigned to the cation radical of
trianisylamine.
9 (a) H. Sakurai, A. Izuoka and T. Sugawara, J. Am. Chem. Soc.,
2000, 122, 9723–9734; (b) R. J. Bushby, D. R. McGill, K. M. Ng
and N. Taylor, J. Chem. Soc., Perkin Trans. 2, 1997, 1405; (c) T. D.
Selby and S. C. Blackstock, J. Am. Chem. Soc., 1999, 121, 7152; (d)
A. Ito, H. Ino, K. Tanaka, K. Kanemoto and T. Kato, J. Org.
Chem., 2002, 67, 491.
10 The small D value of QAꢀH2
(D/hc = 0.0019 cmꢂ1) was
2+
well reproduced on the basis of a simple point-spin model
(D/hc using the conformation and the
=
0.0019 cmꢂ1
)
spin distribution calculated by the density functional theory.
The detail is described in the ESI.w (a) D. Feller, W. T.
Borden and E. R. Davidson, J. Chem. Phys., 1981, 74,
2256; (b) R. S. Hutton and H. D. Roth, J. Am. Chem. Soc.,
1982, 104, 1421.
Notes and references
1 F. Muller, Free flavins: syntheses, chemical and physical proper-
¨
ties, in Chemistry and Biochemistry of Flavoenzymes, ed. F. Muller,
¨
CRC Press, Boca Raton, vol. I, 1991.
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3740 | Chem. Commun., 2008, 3738–3740