Photochromic radical compounds based on a naphthopyran system

Article abstract of DOI:10.1021/ol034574v

(Matrix presented) Naphthopyran derivatives with aminoxyl substituents (4a,b) gave the corresponding open-formed isomers (5a,b) by irradiation, which could be changed back to the starting closed-formed naphthopyrans by the treatment with SiO₂ as a catalyst. The tuning of intermolecular magnetic interactions between the isomer couples was found to be possible in these reversible systems.

Full text of DOI:10.1021/ol034574v

ORGANIC
LETTERS
2003
Vol. 5, No. 12
2127-2129
Photochromic Radical Compounds
Based on a Naphthopyran System
Takashi Kaneko, Hiroki Akutsu, Jun-ichi Yamada, and Shin’ichi Nakatsuji*
Department of Material Science, Graduate School of Science,
Himeji Institute of Technology, 3-2-1 Kouto, Kamigori, Hyogo 678-1297, Japan
nakatuji@sci.himeji-tech.ac.jp
Received April 1, 2003
ABSTRACT
Naphthopyran derivatives with aminoxyl substituents (4a,b) gave the corresponding open-formed isomers (5a,b) by irradiation, which could be
changed back to the starting closed-formed naphthopyrans by the treatment with SiO as a catalyst. The tuning of intermolecular magnetic
2
interactions between the isomer couples was found to be possible in these reversible systems.
Aminoxyl radicals have been widely used as spin labeling
reagents1 as impressively exemplified in recent reports on
their usage for the elucidation of the helical structure of
several oligo(m-phenyleme ethynylene)s2 or as the radical
probe for fluorescence imaging.3 Simultaneously, their usage
in the field of materials chemistry as the building blocks for
molecular based-magnetic materials has been well recognized
in recent years.1
stable radicals, especially aminoxyl radicals, as spin sources.6
As for the spin systems with photofunctionality, several
organic/organometallic systems have recently been developed
by some research groups.7 So far, we have also proposed
our own spin systems with photofunctionality by using such
photochromic systems as norbornadiene/quadricyclane, spiro-
pyran/merocyanine, anthracene/dimer, or styrylpyrylium/
dimer systems.8
In the course of our studies on the development of novel
organomagnetic materials,5 we have been interested in
preparing multifunctional spin systems with conductivity,
liquid crystalline properties, or photofunctionality by using
In this communication, we wish to report our further
approach to the access of new photochromic spin systems,
naphthopyran derivatives9 with aminoxyl radical substituents.
(6) (a) Nakatsuji, S. AdV. Mater. 2001, 13, 1719-1724. (b) Nakatsuji,
S. In Recent Research DeVelopments in Organic & Bioorganic Chemistry;
Pandalai, S. G., Ed.; Transworld Research Network: Trivandrum, India,
2002; pp 1-26. (c) Nakatsuji, S. J. Synth. Org. Chem. Jpn. 2003, in press.
(7) (a) Hamachi, K.; Matsuda, K.; Itoh, T.; Iwamura, H. Bull. Chem.
Soc. Jpn. 1998, 71, 2937-2943. (b) Matsuda, K.; Irie, M. J. Am. Chem.
Soc. 2000, 122, 7195-7201. (c) Matsuda, K.; Irie, M. Chem. Eur. J. 2001,
7, 3466-3473. (d) Matsuda, K.; Matsuo, M.; Irie, M. J. Org. Chem. 2001,
66, 8799-8803. (e) Ratera, I.; Ruiz-Molina, D.; Vidal-Gancedo, J.; Wurst,
K.; Daro, N.; Le`tard, J.-F.; Rovira, C.; Veciana, J. Angew. Chem., Int. Ed.
2001, 40, 919-922. (f) Be´nard, S.; Rivie`re, E.; Yu, P.; Nakatani, K.;
Delouis, J. F. Chem. Mater. 2001, 13, 159-162.
(8) (a) Nakatsuji, S.; Ogawa, Y.; Takeuchi, S.; Akutsu, H.; Yamada, J.;
Naito, A.; Sudo, K.; Yasuoka, N. J. Chem. Soc., Perkin Trans. 2 2000,
1969-1975. (b) Nakatsuji, S.; Ojima, T.; Akutsu, H.; Yamada, J. J. Org.
Chem. 2002, 67, 916-921. (c) Nakatsuji, S.; Ogawa, Y.; Akutsu, H.;
Yamada, J. Bull. Chem. Soc. Jpn. 2003, in press.
(1) Cf. (a) Nitroxide Spin Labels, Reactions in Biology and Chemistry;
Kocherginsky, N., Swartz, H. M., Eds.; CRC Press: Boca Raton, FL, 1995.
(b) Biological Magnetic Resonance; Berliner, L., Ed.; Plenum Press: New
York, 1998; Vol. 14.
(2) Matsuda, K.; Stone, M. T.; Moore, J. S. J. Am. Chem. Soc. 2002,
124, 11836-11837.
(3) Coenjarts, C.; Garcia, O.; Llauger, L.; Palfreyman, J.; Vinette, A.
L.; Scaiano, J. C. J. Am. Chem. Soc. 2003, 125, 620-621.
(4) Cf. (a) Magnetic Properties of Organic Materials; Lahti, P. M., Ed.;
Marcel Dekker, Inc.: New York, 1999. (b) Molecular Magnetism; Itoh,
K., Kinoshita, M., Eds.; Kodansha/Gordon and Breach Science Publishers:
Tokyo, 2000. (c) Veciana, J., Ed. Structure and Bonding; Springer-Verlag:
Berlin, 2001; Vol. 100. (d) Magnetism: Molecules to Materials; Miller, J.
S., Drillon, M., Eds.; Wiley-VCH: Weinheim, Germany, 2001-2002; Vols
I-IV.
(5) Natatsuji, S.; Anzai, H. J. Mater. Chem. 1997, 7, 2161-2174.
10.1021/ol034574v CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/10/2003

It was well anticipated that the dimethylamino groups on
the p-position of two phenyl rings would stabilize the
resonance hybrids with a betaine structure in the open-formed
species (IIa and IIb).
using DCC and DMAP to give the desired esters 4a or 4b.
A single crystal of the latter radical was obtained by the
recrystallization from dichloromethane-ethanol solution, and
its structure was confirmed by a preliminary X-ray analysis.
Irradiation of 4a with light of 365 nm wavelength in
carbon tetrachloride gradually gave a reddish precipitate,
which was found to be the desired compound 5a, being
elucidated by the spectroscopic measurements together with
an elemental analysis (Scheme 2). Similarly, the open-formed
Scheme 2
The ethynyl alcohol derivative obtained by the ethynyl-
ation of Michler’s ketone10 was heated in toluene with
equimolecular amounts of 2,6-dihydroxylnaphthalene in the
presence of a catalytic amount of p-TsOH to furnish, after
Meyer-Schuster/Claisen-type rearrangement and the suc-
cessive cyclization process, the naphthopyran derivative 3
with a hydroxyl group (Scheme 1).11 The naphthopyran 3
compound 5b was obtained by the irradiation of 4b in the
same solvent. The successful precipitation and isolation of
5a and 5b in a nonpolar solvent is thought to be due to the
presence of polar structures in the open-formed species, but
their unstable nature has prevented further recrystallization
in polar solvents such as methanol.
Scheme 1
After several unsuccessful attempts, it was found that the
open-formed species (5a,b) could be changed back to the
original naphthopyrans (4a,b) by the treatment with SiO2 as
a solid-phase catalysis12 in acetonitrile at ambient tempera-
ture. The recovery 4a and 4b could be easily monitored by
the electronic spectral change to the original spectra, and
the yields of these forward and backward reactions are shown
in Scheme 2. Consequently, although the efficiency is not
so high, reversible systems could be constructed in principle
in this case.
To see the magnetic properties of the photochromic
couples 4a/5a and 4b/5b, their magnetic susceptibilities were
measured by a SQUID susceptometer within the temperature
range of 2-300 K, and the data are summarized in Table 1.
Table 1. Magnetic Data of Photochromic Couples 4a/5a and
4b/5b
was then condensed with 3-carboxy-PROXYL (2,2,5,5-
tetramethyl-1-pyrrolidinyloxy) or 4-carboxy-TEMPO (2,2,6,6-
tetramethyl-1-piperidinylinyloxy) in dichloromethane by
compd
magnetic interactiona
Cb (emu K mol-1
)
θc (K)
4a
5a
4b
5b
antiferromagnetic
antiferromagnetic
ferromagnetic
0.31 (83)
0.10 (27)
0.34 (91)
0.08 (21)
-0.65
-0.35
0.05
(9) Concerning naphthopyran photochromic systems, see for example:
van Gemert, B. In Organic Photochromic and Thermotropic Compounds;
Crano, J. C., Guglielmetti, R. J., Eds.; Plenum Press: New York, 1999; pp
111-140.
(10) (a) Nakatsuji, S.; Nakashima, K.; Iyoda, M.; Akiyama, S. Bull.
Chem. Soc. Jpn. 1988, 61, 2253-2255. (b) Nakatsuji, S.; Yahiro, T.;
Nakashima, K.; Akiyama, S., Nakazumi, H. Bull. Chem. Soc. Jpn. 1991,
64, 1641-1647.
antiferromagnetic
-0.20
a Fitting for Curie-Weiss law. b Curie constant. Values in parentheses
denote the estimated spin concentration. c Weiss temperature.
2128
Org. Lett., Vol. 5, No. 12, 2003

Although antiferromagnetic interactions have been observed
for both 4a and 5a, an apparent difference is discerned for
their Curie constants as well as Weiss temperatures. Although
the reason for the rather low C value of 5a is not clear yet
because of the lack of its crystal structure, it is thought to
be due, at least partially, to the singlet formation between
the spin centers in the solid-state of this radical. A similar
but larger difference has been found between the Curie
constants of the couple 4b/5b. Moreover, the signs of the
Weiss constants are different in this case. Thus, the tuning
of their Curie constants together with their Weiss tempera-
tures is apparently possible in these two reversible couples,
in principle, by choosing the outer stimuli of light and a
catalyst.
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research (Grant 13440213) from the
Ministry of Education, Science, Sports and Culture, Japan,
which is gratefully acknowledged.
Supporting Information Available: Figure of the mo-
lecular structure obtained by X-ray analysis for 4b, in-
cluding a table summarizing the crystal data, and a figure
of its magnetic data, tables of FAB-HRMS, and elemental
analysis data for the radicals. This material is available free
of charge via the Internet at http://pubs.acs.org.
(11) Tanaka, K.; Aoki, H.; Hosomi, H.; Ohba, S. Org. Lett. 2000, 2,
2133-2134. In our case, the reaction in toluene was slightly more efficient
than that in the solid state.
(12) It was found to be sufficient for the conversion to pass the solution
through a column of SiO2.
OL034574V
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