Fig. 2 The E-to-Z photoisomerisation of azobenzene in a hexane
solution and an aqueous dispersion of bead-milled Az crystals upon
irradiation with 313 nm light. (a) The first order plots for E-to-Z
photoisomerisation in a hexane solution (J) and in an aqueous
dispersion (K). (b) The Z-fractions as a function of time during
313 nm light irradiation for the solution (J) and the dispersion (K).
Fig. 3 The reversible changes in particle size (observed (+) and
averaged (K) values) of Az crystals dispersed in water upon alter-
nating exposure to 313 nm (odd numbers) and 435 nm light
(even numbers).
dispersions with respect to solution photochemistry, in a
manner similar to photoisomerisation in polymer solids. The
fractions of the Z-isomer in photoirradiated dispersions were
estimated by using extracted hexane solutions. Fig. 2(b) shows
the fractions of the Z-isomer in both solution and disper-
sion as a function of exposure time. While about 90% of the
Z-isomer was formed at the photostationary state in hexane
solution, the fraction of the Z-isomer in aqueous dispersion at
the photostationary state levelled off at about 30%. Accord-
ingly, it is very likely that a photoisomerisable fraction
arises from the surface regions of the Az particles dispersed
in water.
observation of a crystal surface of Az revealed9 that the
thickness of a terrace corresponding to bilayered Az molecules
at the topmost crystal surface of the (001) face is reduced
under UV irradiation, while the edge of the terrace moves to
expand the area to compensate the volume change, approxi-
mately in line with the specific density data. Consequently, the
particle size reduction by UV irradiation requires another
interpretation, and there are two possibilities. In analogy to
some drugs exhibiting mechanochemical amorphisation,15 the
milling may generate an amorphous state, at least at the
surface region of the E-Az crystals, resulting in photo-
isomerisability and volume changes. The second possibility
involves dissolution of the Az at the surface regions of the
crystals into the PVA chains surrounding crystals, forming
thin composite layers that behave similarly to polymeric
materials,16 with Az chromophores demonstrating shrinkage
due to E-to-Z photoisomerisation. Studies on the relationship
between the crystallinity and photoisomerisability of dispersed
Az nanoparticles are in progress.
As pointed out above, the baselines of the spectra are
modified during photoirradiation, suggesting that the photo-
isomerisation causes light scattering. In order to eliminate the
influence of light scattering, the absorption spectra shown in
Fig. 1 were transformed into their first derivatives.w While the
crossings of the derivative spectra in the hexane solution
exposed to 313 nm light lay on a ‘‘zero’’-line, the crossings
of the spectra in the aqueous dispersion were shifted from a
zero-line, confirming that the lack of isosbestic points was due
to light scattering.12 In other words, the results suggested that
the particle sizes of the Az were modified by photoirradiation.
In this context, particle size measurements by means of
dynamic light scattering (Sysmex, Nano-Z) were performed
for aqueous dispersions after alternating exposure to 313 nm
and 435 nm light for enough time to reach photostationary
states. As shown in Fig. 3, the particle size of a dispersed
solution (252 Æ 2 nm) at the initial state was markedly reduced
upon irradiation with 313 nm light, whereas the increment of
particle size was observed after subsequent 435 nm light
irradiation, which regenerated ca. 83% of E-Az. The changes
in particle size distribution took place reversibly thereafter,
and particle sizes upon exposure to 313 and 435 nm light were
223 Æ 1 and 231 Æ 1 nm, respectively. We have no reasonable
interpretation for the fact that particles sizes after 435 nm light
exposure are smaller than those before photoirradiation. The
ca. 3.5% volume change may be ascribable to the transforma-
tion between E-Az, with a rod-like shape, and Z-Az, with a
bent shape. However, it is hard to ascribe the volume change
to crystal structural alteration because the calculated values of
the specific density of E-Az13 and Z-Az14 are 1.230 and 1.123,
respectively, so the crystal volume of the Z-isomer should be
slightly larger than that of the E-isomer. The previous AFM
The reversible photoisomerisability of E-Az crystals has
been displayed by downsizing of the crystals in water. The
fraction of the Z-isomer is levelled off upon exposure to
313 nm light at about 30% in the photostationary state. The
photoisomerisation leads to reversible changes of the size of
the Az crystals, leading to about a 3.5% reduction in particle
diameter upon 313 nm light irradiation, possibly due to the
formation of thin composite layers of Az and PVA as a
dispersion stabilizer on particles.
Notes and references
1 G. S. Hartley, Nature, 1937, 14, 281.
2 (a) H. Rau, in Photochromism: Molecules and Systems, ed. H. Durr
and H. Bouas-Laurent, Elsevier, Amsterdam, The Netherlands,
1990, pp. 165–192; (b) C.-W. Chang, Y.-C. Lu, T.-T. Wang and
E. W.-G. Diau, J. Am. Chem. Soc., 2004, 126, 10109 and references
therein.
3 K. G. Yager and C. J. Barrett, J. Photochem. Photobiol., A, 2006,
182, 250 and references therein.
4 (a) A. V. Lyubimov, N. L. Zaichenko and V. S. Marevtsev,
J. Photochem. Photobiol., A, 1999, 120, 55; (b) K. Ichimura,
in Organic Photochromic and Thermochromic Compounds,
ed. J. Crano and R. J. Guglielmetti, Springer, New York, USA,
vol. 2, 1999, pp. 9–63; (c) A. Natansohn and P. Rochon, Chem.
Rev., 2002, 102, 4139.
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 1496–1498 | 1497
Ichimura, Kunihiro
