800
H. Koshima, N. Ojima / Dyes and Pigments 92 (2012) 798e801
Fig. 4. Dependence of irradiation time of UV light on the displacement angle of the
Fig. 6. Dependence of irradiation time on the XRD peak intensity of trans-1 micro-
crystals. (a) Blue: UV light was irradiated for 60 s and then irradiation was stopped.
3
platelike microcrystal (200 ꢁ 25 ꢁ 1.2
mm ) of trans-1.
(b) Green: UV light was irradiated for 60 s and then visible light at 530 nm was irra-
diated. (For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
photoisomerization between the trans and cis isomers in benzene
via alternate irradiation with UV (365 nm) and visible (530 nm)
light. In contrast, the absorption spectrum of trans-1 microcrystals
did not change after UV irradiation at 365 nm for 30 s (Fig. 5b),
suggesting that the photoisomerization occurred near the crystal
surface.
Fig. 6 shows the dependence of irradiation time on the relative
XRD peak intensity of trans-1 microcrystals. The (20-2) peak
intensity decreased to 87% of the original value upon UV irradiation
2
(
40 mW/cm ) for 60 s because of the transecis photoisomerization
and deterioration in crystallinity (Fig. 2b). However, no shift of
peaks or new peaks were observed in the XRD profile, even after
continued irradiation, suggesting that the cis-1 product did not
form a new crystalline phase. After irradiation was stopped, the
XRD peak intensity increased to 94% after 4 min and recovered after
1
5 min due to cisetrans thermal isomerization. In contrast, irradi-
2
ation with visible light at 530 nm (10 mW/cm ) recovered the
initial intensity after 4 min due to cisetrans photoisomerization
resulting from (n,p*) excitation.
As discussed above, stopping the illumination returned the
crystal to its initial linear shape after 4 min (Fig. 3f). In contrast,
visible light irradiation for 1 min recovered the initial straight
crystal (Fig. 3l), revealing that both bending and straightening of
microcrystals were controlled by irradiation with UV and visible
light.
The (10-1) top surface of the trans-1 microcrystal was smooth
before irradiation, as observed with an AFM (Fig. 7a). After UV
irradiation for 10 s, uneven features appeared with a height of 3 nm
and a relative roughness of 0.4% of the crystal thickness (840 nm)
(Fig. 7b). The uneven features decreased slightly upon visible light
irradiation at 530 nm for 10 min, but the initial smooth surface was
not recovered (Fig. 7c).
In the trans-1 crystal, the amino group of trans-1 molecule is
disordered in 50% occupancy and two independent planar mole-
ꢀ
cules exist at a dihedral angle of 70.60 at the (10-1) face, forming
two kinds of herringbone structures (red and blue) along the b axis
(Fig. 7def) [14]. Upon UV irradiation, the planar trans-1 molecules
underwent photoisomerization to cis-1 on the (10-1) crystal
surface, resulting in an increased torsional conformation due to
repulsion of two phenyl planes. Crystalline cis-azobenzene mole-
ꢀ
cules normally exhibit a dihedral angle of 64.26 between the two
Fig. 5. (A) Absorption spectra of trans-1 (0.05 M) in benzene before (yellow) and after
UV irradiation at 365 nm for 30 s (red). Stopping irradiation for 10 min recovered the
initial spectrum in benzene (red yellow). (B) Absorption spectra of the trans-1
microcrystals before (yellow) and after UV irradiation for 30 s (red). (For interpretation
of the references to colour in this figure legend, the reader is referred to the web
version of this article.)
phenyl planes [16]. The transecis photoisomerization elongated the
unit cell length along the b, a, and c axes near the (10-1) crystal
surface, giving rise to the uneven features. In contrast, the unit cell
dimension remained constant at the nonirradiated surface, result-
ing in bending of the microcrystal.
Koshima, Hideko
