Table 1. Absorption Maxima and Coefficients of 1-6 and the Quantum Yields and Conversions in Ethyl Acetate
λmax/nm (ꢀ × 103/M-1 cm-1
)
quantum yield
a
b
Φafb (313 nm)
Φbfa (517 nm)
conversion (calcd)
1a
2
258 (16)
267 (20), 320 (sh), 460 (sh)
297 (20), 377 (10), 600 (0.35), 645 (0.35)
265 (34), 300 (sh), 450 (sh)
289 (34), 376 (11), 450 (sh), 596 (0.41), 648 (0.40)
309 (34), 377 (16), 598 (0.63), 646 (0.63)
351 (12), 523 (10)
360 (16), 537 (11)
380 (23), 543 (15)
381 (20), 551 (14)
390 (23), 558 (15)
400 (20), 565 (15)
0.31
0.14
0.011
0.078
0.041
0.040
0.28
0.43 (0.50)
0.61 (0.77)
0.79 (0.71)
0.85 (0.89)
1.00 (0.99)
1.00 (0.99)
0.054
0.010
0.012
0.0010
0.0010
3a
4
5
6a
a Taken from ref 3b.
value of 1. The cyclization quantum yields of monoimino-
nitroxyl-substituted 2a and diiminonitroxyl-substituted 4a are
47% and 19% of that of nonsubstituted diarylethene 1. This
result indicates that the quantum yields are reduced depend-
ing on the number of radical moieties. The cyclization
quantum yields of monoiminonitroxyl-substituted 2a and
mononitronylnitroxyl-substituted 3a are 47% and 3.5% of
that of 1. This result suggests that nitronyl nitroxyl radical
is more effective to suppress cyclization reaction in com-
parison with imino nitroxyl radical.
The main cause of the suppression of the cyclization
reaction is considered to be the energy transfer from the
diarylethene core to the radical substituents. Therefore the
absorption of these radicals were investigated.9 Absorption
spectra of 3a, 5a, and 6a have absorption band around 360
nm, while the band is not discerned in 1a, 2a, and 4a. Figure
3 shows the absorption spactra of p-bromophenyl nitronyl
nitroxide has a strong absorption band at 367 nm (ꢀ )
16 000) and imino nitroxide has a weak absorption band at
311 nm (ꢀ ) 4000). These absorption bands are assigned to
π-π* transitions. The absorption bands in the visible region
are assigned to n-π* transitions. The absorption character-
istics were reproduced by time-dependent density-functional
theory (TD-DFT) calculations with UB3LYP functional and
6-31G basis set.10 The calculation indicated that nitronyl
nitroxide has a 360 nm absorption band with an f value of
0.0699 and imino nitroxide has a 338 nm absorption band
with an f value of 0.0144. The radical moieties are considered
to act as the energy acceptors. Since the cyclization reaction
is reported to be very fast (<2 ps),11 the cyclization was not
completely quenched. The energy transfer from diarylethene
core to nitronyl nitroxyl radical is more effective than the
energy transfer to imino nitroxyl radical.
On the other hand, the suppression of the cycloreversion
reactions cannot be explained by the energy transfer because
the absorption bands of the closed-ring isomers have lower
energy than the π-π* absorption band of the radicals. An
important factor affecting the cycloreversion quantum yield
is the resonant quinoid structures (Scheme 1). The cyclo-
reversion quantum yields of 5b and 6b are exceptionally
small. This is due to the formation of quinoid structures. In
the study of biradicals and triradicals, nitronyl nitroxide is
known to have stronger intramolecular magnetic interaction
than imino nitroxide because of the formation of quinoidal
structures.12 In the case of 4 which has two imino nitroxyl
moieties, contribution of the quinoid form is considered to
be small. Therefore, imino nitroxide radical did not affect
(7) Ratera, I.; Ruiz-Molina; Vidal-Gancedo, J.; Wurst, K.; Daro, N.;
letard, J.-F.; Rovira, C.; Veciana, J. Angew. Chem., Int. Ed. 2001, 40, 919-
922.
(8) Hanazawa, K.; Sumiya, R.; Horikawa, Y.; Irie, M. J. Chem. Soc.,
Chem. Commun. 1992, 206-207.
(9) (a) Zoppellaro, G.; Enkelmann, V.; Geies, A.; Baumgarten, M. Org.
Lett. 2004, 6, 4929-4932. (b) Zoppellaro, G.; Ivanova, A.; Enkelmann,
V.; Geies, A.; Baumgarten, M. Polyhedron 2003, 22, 2099-2110.
(10) The calculation was done by Gaussian 03, Revision B.04; Gaussian,
Inc.: Wallingford CT, 2004. See the Supporting Information for the full
reference.
Figure 3. Absorption spectra of p-bromophenyl nitronyl nitroxide
7 (blue line) and p-bromophenyl imino nitroxide 8 (orange line) in
ethyl acetate solution.
(11) Tamai, N.; Saika, T.; Shimidzu, T.; Irie, M. J. Phys. Chem. 1996,
100, 4689-4692.
nitroxide 7 and p-bromophenyl imino nitroxide 8. Nitronyl
(12) (a) Lahti, P. M.; Esat B.; Walton, R. J. Am. Chem. Soc. 1998, 120,
5122-5123. (b) Hosokoshi, Y.; Takizawa, K.; Nakano, H.; Goto, T.;
Takahashi, M.; Inoue, K. J. Magn. Magn. Mater., 1998, 177-181, 634-
635. (c) Hosokoshi, Y.; Nakazawa, Y.; Inoue, K.; Takizawa, K.; Nakano,
H.; Takahashi, M.; Goto, T. Phys. ReV. B 1998, 60, 12924-12932. (d)
Tanaka, M.; Matsuda, K.; Itoh, T.; Iwamura, H. J. Am. Chem. Soc. 1998,
120, 7168-7173.
(4) Teki, Y.; Miyamoto, S.; Nakatsuji, M.; Miura, Y. J. Am. Chem. Soc.
2001, 123, 294-305.
(5) Hamachi, K.; Matsuda, K.; Itoh, T.; Iwamura, H. Bull. Chem. Soc.
Jpn. 1998, 71, 2937-2943.
(6) Kaneko, T.; Akutsu, H.; Yamada, J.; Nakatsuji, S. Org. Lett. 2003,
5, 2127-2129.
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