owing to the small free volume in the environment, as shown
in Fig. 4(a). On the other hand, the E–Z reaction with the
pulsed laser excitation of rather high excitation intensity
could be interpreted as due to the simultaneous production
of excited states of many azobenzene moieties, leading to
cooperative photo-isomerization, as shown in Fig. 4(b).
The upper abscissa of Fig. 3 indicates the ratio NP/NM.
Here, NP and NM are the number of photons in the exposed
volume and the number of azobenzene moieties, respectively.
As clearly shown in Fig. 3, the threshold of the isomerization
reaction corresponds to NP/NM = ca. 2. This large value
of the photon number may support the cooperative photo-
isomerization in the present gel state.
This work was supported by Grants-in-Aid for Scientific
Research on Priority Area ‘‘New Frontiers in Photochromism
(No. 471)’’ and (B) (18350101) from the Ministry of
Education, Culture, Sports, Science, and Technology
(MEXT), Japan.
Notes and references
1 (a) P. Terech and R. G. Weiss, Chem. Rev., 1997, 97, 3133–3159;
(b) L. Estroff and A. D. Hamilton, Chem. Rev., 2004, 104,
1201–1217.
2 (a) K. Murata, M. Aoki, T. Susuki, T. Harada, H. Kawabata,
T. Komori, F. Ohseto, K. Ueda and S. Shinkai, J. Am. Chem. Soc.,
1994, 116, 6664–6667; (b) J. J. D. de Jong, L. N. Lucas,
R. M. Kellogg, J. H. van Esch and B. F. Feringa, Science, 2004,
304, 278–281; (c) M. Akazawa, K. Uchida, J. J. D. de Jong,
J. M. Areephong, M. Stuart, G. Caroli, W. R. Browne and
B. L. Feringa, Org. Biomol. Chem., 2008, 6, 1544–1547;
(d) T. Hirose, K. Matsuda and M. Irie, J. Org. Chem., 2006, 71,
7499–7508; (e) S. Yagai, T. Karatsu and A. Kitamura, Chem.–Eur.
J., 2005, 11, 4054–4063.
3 (a) H. Bouas-Laurent, in Photochromism Molecules and Systems,
ed. H. Duerr, Elsevier, Amsterdam, 1990; (b) K. G. Yager and
C. Barrett, J. Photochem. Photobiol., A, 2006, 182, 250–261.
4 (a) K. Murata, M. Aoki, T. Nishi, A. Ikeda and S. Shinkai,
J. Chem. Soc., Chem. Commun., 1991, 1715–1718; (b) T. Ikeda
and A. Kanazawa, Liquid Crystal Photonics Opto-photochemical
Effects in Photoresponsive Liquid Crystals, in Molecular Switches,
ed. B. L. Feringa, Wiley-VCH, Weinheim, Germany, 2001, ch. 12;
(c) K. Ichimura, S.-K. Oh and M. Nakagawa, Science, 2000, 288,
1624–1626; (d) Y. Yu, M. Nakano and T. Ikeda, Nature, 2003, 425,
145; (e) Y. Yu and T. Ikeda, Angew. Chem., Int. Ed., 2006, 45,
5416–5418; (f) M. Kondo, Y. Yu and T. Ikeda, Angew. Chem., Int.
Ed., 2006, 45, 1378–1382; (g) M. Yamada, M. Kondo, J. Mamiya,
Y. Yu, M. Kinoshita, C. Barrett and T. Ikeda, Angew. Chem., Int.
Ed., 2008, 47, 4986–4988.
5 (a) S. Yagai, T. Karatsu and A. Kitamura, Langmuir, 2005, 21,
11048–11052; (b) X. Song, J. Perlstein and D. G. Whitten, J. Am.
Chem. Soc., 1995, 117, 7816–7817; (c) T. Kawasaki, M. Tokuhiro,
N. Kimizuka and T. Kunitake, J. Am. Chem. Soc., 2001, 123,
6792–6800; (d) M. Shimomura and T. Kunitake, J. Am. Chem.
Soc., 1987, 109, 5175–5183.
6 H. Rau and E. Lueddecke, J. Am. Chem. Soc., 1982, 104,
1616–1620.
7 (a) T. Fujino and T. Tahara, J. Phys. Chem. A, 2000, 104, 4203–4210;
(b) T. Fujino, S. Y. Arzhantsev and T. Tahara, J. Phys. Chem. A,
2001, 105, 8123–8129; (c) T. Fujino, S. Y. Arzhantsev and T. Tahara,
Bull. Chem. Soc. Jpn., 2002, 75, 1031–1040.
As was mentioned in the introductory part, the isomerization
reaction of azobenzene in solution phase takes place in
short region of o1 ps and this time is much shorter than the
pulse duration of the present ns laser pulse (fwhm, 6 ns). If the
lifetime of the excited state of the azobenzene unit is o a few
ps in the present gel, the Np/Nm value to produce the
dense population of the excited state by a ns pulsed laser
should be much larger than the present value of 2, owing
to the short lifetime of the excited state. The present result
thus strongly suggests that some intermediate state between
the excited state of the E-form and the Z-form in the
ground state may have a lifetime at least several hundreds
of ps, allowing the dense population of the intermediate
species.
Finally, it is worth noting the thermal recovery of 1Z to 1E
after the laser induced isomerization in the gel state. No
significant recovery to 1E in the gel state after the 355 nm
laser exposure was detected in 65 h at room temperature, while
at an elevated temperature of 50 1C, slow thermal recovery
was observed (ESIw). This result confirms again that the
H-aggregates are not decomposed by the laser induced
morphology change and the Z-isomer in the gel state is
packed very closely together, leading to the very slow
recovery. In addition, this result is also consistent with the
above result that only pulse laser irradiation with high photon
density allowed E–Z isomerization of the azobenzene
molecules to occur, even in a stacked situation, by sponta-
neous excitation.
8 (a) M. Suzuki, T. Asahi and H. Masuhara, Phys. Chem. Chem.
Phys., 2002, 4, 185–192; (b) T. Asahi, M. Suzuki and H. Masuhara,
J. Phys. Chem. A, 2002, 106, 2335–2340; (c) M. Suzuki, T. Asahi,
K. Takahashi and H. Masuhara, Chem. Phys. Lett., 2003, 368,
384–392.
9 D. L. Ross and J. Blanc, Photochromism by Cis–Trans-Isomerization,
in Photochromism, ed. G. H. Brown, Wiley-Interscience, New York,
1971, ch. 5.
10 S. Kume, K. Kuroiwa and N. Kimizuka, Chem. Commun., 2006,
2442–2444.
11 The peak intensity of the nanosecond laser pulse can be estimated
as 8.2 ꢀ 1023 photon sꢁ1 mmꢁ2, by dividing the number of the
photons by the pulse duration of 6 ns.
In conclusion, a photoreactivity threshold was observed
in H-aggregates of 1E in the gel state with intermolecular
hydrogen-bonding networks that usually prohibits the individual
E–Z isomerization of the azobenzene moiety. It was found
upon high enough photon density, a cooperative effect of the
E–Z isomerization of adjacent azobenzene moieties occurs,
leading to photo-functionality with the threshold of the light
intensity.
ꢂc
This journal is The Royal Society of Chemistry 2009
4422 | Chem. Commun., 2009, 4420–4422