Photoinduced diffusive mass transfer in o-Cl-HABI amorphous thin films

Article abstract of DOI:10.1039/b919180a

The first demonstration of photoinduced surface relief grating formation using amorphous thin films composed of a radical dissociative photochromic compound, 2,2′-di(ortho-chlorophenyl)-4,4′,5,5′- tetraphenylbiimidazole, show that mass transfer occurred from the UV-light-irradiated area to the shaded area by patterned light irradiation (365 nm).

Full text of DOI:10.1039/b919180a

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Photoinduced diffusive mass transfer in o-Cl-HABI amorphous
thin filmsw
Azusa Kikuchi,*a Yukari Harada,a Mikio Yagi,a Takashi Ubukata,a
Yasushi Yokoyamaa and Jiro Abeb
Received (in Cambridge, UK) 16th September 2009, Accepted 21st December 2009
First published as an Advance Article on the web 22nd January 2010
DOI: 10.1039/b919180a
The first demonstration of photoinduced surface relief grating
formation using amorphous thin films composed of a radical
dissociative photochromic compound, 2,20-di(ortho-chlorophenyl)-
4,40,5,50-tetraphenylbiimidazole, show that mass transfer
occurred from the UV-light-irradiated area to the shaded area
by patterned light irradiation (365 nm).
In this study, we describe the first SRG formation on radical
dissociative photochromic compounds in order to study the
relationship between SRG formation and diffusive mass transfer
caused by a chemical potential gradient arising from a difference
in the amount of substance. The amount of substance is identical
to the number of moles. Chemical potential plays a role in the
discussion of material spontaneous diffusion, because it carries
the information about how Gibbs energy (G) changes as the
amount of substance changes, and hence how G changes as the
composition of a system changes.7 2,20-Di(ortho-chlorophenyl)-
4,40,5,50-tetraphenylbiimidazole (o-Cl-HABI) can be useful for
examining the relationship between SRG formation produced by
diffusive mass transfer and the chemical potential gradient,
because the amount of substance changes from 1 mol of
o-Cl-HABI to 2 mol of o-Cl-TPIRs by radical dissociative
photochromic reaction. Thus, the changes in chemical
potential gradient would be considered to affect photoinduced
mass transfer during SRG formation. A detailed study of the
relationship between SRG formation and diffusion would help
increase our understanding of the response to light.
In the past decade, surface relief gratings (SRGs) have
attracted attention from the viewpoint of mass transfer. SRGs
formed on azobenzene-functionalized polymer films have
attracted the interest of many researchers since the first reports
by Natansohn et al. and Tripathy et al. in 1995.1 SRGs formed
on films composed of various amorphous polymers, liquid
crystalline polymers and sol–gel matrices having azobenzene
moieties, along with the origin of SRG formation, have also
been described.2 Recently, SRGs have also been formed on
crystals and amorphous films composed of molecules containing
azobenzene units.3 SRG formation on photochromic materials
other than azobenzene-based compounds has also been a topic
of interest.4
Research on SRG formation on various photochromic
materials is expected to provide a useful suggestion to design
more suitable molecular structures for SRG formation.
It has been reported a number of times that SRGs on
azobenzene-functionalized polymer films are formed by efficient
photoisomerization cycles and photoinduced reorientation of
The photochromism of hexaarylbiimidazoles (HABIs) was
discovered by Hayashi and Maeda in the 1960’s. Several
studies of the photochemical and photophysical properties of
HABI and its derivatives have been reported.8 We investigated
SRG formation on amorphous thin films consisting of
o-Cl-HABI. The photochromic behaviour of o-Cl-HABI can
be attributed to photoinduced homolytic reversible cleavage of
the C–N bond between the imidazole rings in an inert
environment without other reactions (Scheme 1). Though
photochromic properties of various HABI derivatives have
been studied extensively, there are essentially no reports for
the direct SRG formation on amorphous thin films of them to
open up a new field of unique photoresponsive materials.
o-Cl-HABI (Wako Pure Chemical Industries, Ltd.) was
recrystallized from a 1 : 1 mixture of dichloromethane and
acetonitrile to yield pale-yellow block crystals. Dichloro-
methane and acetonitrile for recrystallization and benzene
(Wako Super Special Grade) for film preparation were used
without further purification.
azobenzene moieties.2,3 Reversible trans
2 cis photo-
isomerization of azobenzene groups leads to mass transport
processes upon irradiation with polarized light at an appropriate
wavelength. The influence of polymer structures and properties,
such as molecular weight, glass transition temperature Tg and
azobenzene content, on the efficiency of SRG formation have
also been investigated.5,6 Although many experimental and
theoretical studies have been reported in relation to SRG
formation, the mechanism of its formation on photochromic
materials is not yet fully understood.
a Department of Advanced Materials Chemistry, Graduate School of
Engineering, Yokohama National University, 79-5, Tokiwadai,
Hodogaya, Yokohama, 240-8501, Kanagawa, Japan.
E-mail: akikuchi@ynu.ac.jp; Fax: +81 45 339 3948;
Tel: +81 45 339 3944
b Department of Chemistry, School of Science and Engineering,
Aoyama Gakuin University, 5-10-1, Fuchinobe, Sagamihara,
229-8558, Kanagawa, Japan. Fax: +81 42 759 6225;
Tel: +81 42 759 6225
w Electronic supplementary information (ESI) available: Experimental
details of o-Cl-HABI amorphous thin film, the time profile of the
absorbance of o-Cl-TPIR in the amorphous thin film at 383 K. See
DOI: 10.1039/b919180a
Scheme 1 Reversible photodissociation reaction of o-Cl-HABI into
two o-Cl-TPIRs.
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Transparent amorphous thin films (ca. 120 nm thickness)
were prepared by spin coating on a clean glass substrate from
o-Cl-HABI benzene solution (16.3 mmol dmꢁ3). The thickness
of the films was calculated from curve fitting of reflection
fringe obtained from a UV-Vis-NIR spectrometer (JASCO,
V-670) equipped with an absolute reflectance measurement
unit (ARSN-733). The first differential scanning calorimetry
(DSC) scan of o-Cl-HABI microcrystals displayed the melting
point (462 K), whereas no peak due to amorphism was
observed in cooling process. Therefore, the second DSC
heating scan displayed no peak for the melting point of
o-Cl-HABI microcrystals. Tg of o-Cl-HABI films was detected
to be around 366 K by DSC (Fig. S1, ESIw).
Fig. 1(a) shows absorption spectra for a o-Cl-HABI
amorphous thin film before and after UV light irradiation at
365 nm. Before irradiation, the main absorption band is at
270 nm. After irradiation, new bands appear at 350 and 550 nm,
ascribed to the radical form of o-Cl-HABI, o-Cl-TPIR,
produced by photoinduced homolytic dissociation. Then,
under dark conditions, o-Cl-TPIRs thermally recombine at
room temperature to reproduce the dimer form, o-Cl-HABI.
o-Cl-TPIRs recombine within ca. 6–10 s in an amorphous film
at 383 K (Fig. S3, ESIw). This photochromic behaviour is in
good agreement with that observed in o-Cl-HABI acetonitrile
solution. Fig. 1(b) shows UV-Vis absorption spectral change
for o-Cl-HABI films during UV light irradiation. In order to
reduce film decomposition, SRG formation was achieved
within 30 min of irradiation under an Ar atmosphere. After
film preparation, the glass substrate was maintained at 383 K
Fig. 2 AFM images (left) and cross-sectional height profiles for the
solid line (right) of surface relief structures produced by patterned
UV light irradiation (365 nm, 22 mW cmꢁ2, 20 min, 383 K, under Ar
atmosphere) on amorphous thin films of o-Cl-HABI (ca. 120 nm
thickness) through different types of photomask: (a) through a
stripe-type photomask with the period of 2 mm and (b) through a
grid-type photomask with transparent (2 mm) and opaque (1 mm) lines.
using a hotplate. This temperature is optimal for SRG formation
of o-Cl-HABI. UV light irradiation was performed using
a UV-LED lamp (Keyence UV-400 series, UV-50H type,
365 nm). Light was collimated to obtain an intensity of
22 mW cmꢁ2 over the entire irradiation area. Patterned light
irradiation was performed through a custom-made photomask
(Toppan Printing Co.) composed of stripes with the 2 mm widths
of transparent and opaque lines, and orthogonal grids with
transparent (2 mm) and opaque (1 mm) lines. After the films were
irradiated for 20 min, the surface relief structures were observed
through an AFM using a SHIMADZU SPM-9500 equipped
with cantilever (OLYMPUS OMCL-TR800PSA-1).
Fig. 2 shows SRG structures formed by patterned UV light
irradiation on the film through the photomask. Regular
surface modulation was achieved with a spatial period of
2 mm, in accordance with the photomask pattern, and observed
peak-to-trough modulation depth of ca. 20–30 nm (Fig. 2(a)).
To investigate the direction of mass transfer, we irradiated
the film through a photomask with just one slit line (4 mm
wide; Fig. 3). The cross-sectional topography shows that the
convex is higher and the concave is lower than the initial
surface level, which clearly demonstrates that lateral mass
transfer occurred on the film surface. On the surface of the
irradiated areas, o-Cl-HABIs are readily cleaved photo-
chemically into o-Cl-TPIRs; in contrast, on the surface of
the shaded areas, almost all molecules are present in their
dimer form, o-Cl-HABI. Mass transfer induced as a consequence
of the radical dissociative photochromic reaction on the film
surface can be explained as follows. Photochromic reaction
of o-Cl-HABI increases the amount of o-Cl-TPIRs in the
irradiated area, which causes the chemical potential of the
irradiated area to differ from that of the shaded area and in
turn drives the mass transfer. Molecular diffusion can be
induced by chemical potential gradients. That is, the material
moved from the irradiated areas to the shaded areas. We
therefore propose that the chemical potential gradient may be
a driving force for mass transfer.
Fig. 1 (a) UV-Vis absorption spectral change in the o-Cl-HABI film (ca.
500 nm thickness) at 300 K: (dotted line) before irradiation and (solid
line) immediately after UV light irradiation (365 nm, 11 mW cmꢁ2, 5 s
irradiation). (b) UV-vis absorption spectral change in the o-Cl-HABI film
(ca. 200 nm thickness) after a variety of time periods of UV light
irradiation (365 nm, 22 mW cmꢁ2, 383 K) under Ar and air atmospheres.
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to elucidate the mechanism of SRG formation for HABI
derivatives.
The authors wish to express their thanks to the Instrumental
Analysis Center, Yokohama National University, for the use
of the NMR spectrometer. They also thank Mr Yusuke
Moriya for the help in the preliminary AFM measurement.
This work was partly supported by a Grant-in-Aid Science
Research in Priority Areas ‘‘New Frontiers in Photochromism
(No. 471)’’ from the Ministry of Education, Culture, Sports,
Science, and Technology (MEXT), Japan.
Fig. 3 AFM images of UV light irradiation (365 nm, 22 mW cmꢁ2
,
20 min, 383 K, under Ar atmosphere) on amorphous thin films of
o-Cl-HABI (ca. 120 nm thickness) through a photomask (slit width
4 mm). The images on the right are cross-sectional height profiles for
the solid lines shown in the AFM images.
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Fig. 4 (a) Modulation depth in the o-Cl-HABI film (ca. 120 nm
thickness) as a function of irradiation time with different 365-nm
UV light intensities at 383 K. (b) Time profiles of ESR intensities
(monitored at 336.7 mT) of o-Cl-TPIRs in o-Cl-HABI amorphous
films (10 mg) irradiated with different UV light intensities at 300 K.
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This journal is The Royal Society of Chemistry 2010
2264 | Chem. Commun., 2010, 46, 2262–2264