Photochromism of a spiroperimidine compound in polymer matrices

Article abstract of DOI:10.1039/b822914g

The photochromic behavior of a spiroperimidine compound, 2,3-dihydro-2-spiro-7′-(8′-imino-7′,8′- dihydronaphthalen-1′-amine)perimidine (PNI), doped in various polymers has been studied. Upon irradiation with 405 nm light, PNI exhibits photochromism, and a

Full text of DOI:10.1039/b822914g

View Article Online / Journal Homepage / Table of Contents for this issue
LETTER
www.rsc.org/njc | New Journal of Chemistry
Photochromism of a spiroperimidine compound in polymer matrices
a
a
ab
Yasuo Norikane,* Riju Davisw and Nobuyuki Tamaoki*
Received (in Montpellier, France) 19th December 2008, Accepted 5th February 2009
First published as an Advance Article on the web 17th February 2009
DOI: 10.1039/b822914g
The photochromic behavior of a spiroperimidine compound, 2,3-
bond at the spiro carbon, followed by the formation of an
0
0
0
0
0
dihydro-2-spiro-7 -(8 -imino-7 ,8 -dihydronaphthalen-1 -amine)-
perimidine (PNI), doped in various polymers has been studied.
Upon irradiation with 405 nm light, PNI exhibits photo-
chromism, and a color change (light yellow to brown) is observed.
The initial (closed) and colored (open) forms exhibit absorption
maxima (kmax) around 400 and 500 nm, respectively. The open
form possesses a wide absorption band, extending to B750 nm.
Little effect of the polymer matrix on the kmax of either the
closed or open forms was observed, and these values were also
similar to those in solution. The lifetime of the open form
open form with extended p-conjugation (Scheme 1). Their
colored open forms absorb across almost the entire visible
light spectrum. These compounds exhibit so-called ‘T-type’
photochromism, where the reverse reaction proceeds ther-
mally and photochemically. PNI and PNO-p possess primary
amine groups, which enables their chemical modification and
1
1
also coupling with other functional groups.
The possibly significant advantages of PNI and PNO-p over
other organic photochromic compounds are the low cost of
their synthesis and color neutrality, and thus they are
good candidates for photochromic materials. In particular,
potential applications of these compounds are in the fields of
photochromic lenses and light-controlling materials, where
light irradiation (such as by sunlight) causes a deep coloration
(preventing light transmission), which returns to the original
color (enabling light transmission) in the dark. The color
neutrality of photochromic glasses depends on the broadness
of their visible light absorption spectrum, and is usually
achieved by mixing compounds absorbing in different regions
g
depends mainly on the glass transition temperature (T ) of the
polymer, and ranges from, for example, 11 min to 5.4 d. This
indicates that the lifetime of the open form can be readily tuned
by simply choosing a suitable polymer matrix. Photochromism
in different polymer substrates are demonstrated herein.
Photochromism is a reversible change in an absorption
spectrum due to the transformation of a chemical species that
is induced, at least in one direction, by irradiation with light.
Numerous organic photochromic compounds and their
derivatives have been reported, and they are potential candi-
dates for diverse applications such as photochemical
1
2,13
of the visible spectrum.
can be achieved by a single compound, due to the broad
In the cases of PNI and PNO-p, it
9
,10
absorption of their open forms.
In our previous papers, we have reported the photochromic
1
–8
memories, molecular switches and light control materials.
9,10
One of the challenges of organic photochromic compounds in
relation to these applications is their synthesis, as a multiple-
step preparation from commercially available starting
compounds is generally required. Complexity of synthesis
raises the cost of compound production, which is a very
important factor in commercial applications.
properties of PNI and PNO-p in solutions. It was revealed that
the rate of the thermal back reaction (from open to closed form) of
these molecules strongly depends on the nature of the solvent; the
rate is greatly enhanced in protic solvents. The lifetime, defined as
t = 1/k (where k is the rate constant of the thermal back reaction),
Recently, we have reported photochromic compounds (PNI
and PNO-p) that can be synthesized via a one step reaction
9
from inexpensive commercially available precursors. They
can be easily synthesized by stirring a toluene or benzene
solution of 1,8-diaminonaphthalene in the presence of MnO
2
under ambient temperature for 3–4 h, although a subsequent
separation on silica gel is required to obtain pure PNI (15%)
9
,10
and PNO-p (18%).
Upon irradiation with UV or blue light,
the initial yellow color of solutions of PNI and PNO-p turn
brown and deep blue, respectively. The photochromism of
these molecules is attributable to photocleavage of the C–N
a
Nanotechnology Research Institute, National Institute of Advanced
Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba,
Ibaraki, 305-8565, Japan. E-mail: y-norikane@aist.go.jp
Research Institute for Electronic Science, Hokkaido University, N20,
b
W10, Kita-ku, Sapporo, 001-0020, Japan.
E-mail: tamaoki@es.hokudai.ac.jp
w Current address: Adhesive Technologies, Henkel AG & Co. KGaA,
Dusseldorf, 40589, Germany
¨
Scheme 1
This journal is ꢀc The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009 New J. Chem., 2009, 33, 1327–1331 | 1327

View Article Online
of the open form of PNI is 600 ms in methanol, whereas it is 20 d in
Upon irradiation of the polymer films using 405 nm light,
the photochromic reaction was observed and the absorption
band around 500 nm appeared, with a concomitant decrease in
the absorption at 405 nm (Fig. 1). This absorption band at
500 nm, which is attributed to the open form, is significantly
broad and extends to 750 nm. Together with this band,
another band at around 590 nm appeared as a shoulder in
most polymers. However, it was not observed in hydrophilic
polymers such as PVA, HEMA and PVP. This shoulder was
also prominent in toluene, but absent in alcoholic solvents,
10
acetonitrile. The –OH groups of alcohol solvents are understood
to play an important role in catalyzing the thermal back
1
0
reaction. Therefore, the lifetime of the open form can be tuned
by simply choosing the right solvent. The observations in our
solution phase studies suggested that these lifetimes could also be
controlled in polymer substrates by the rational design of
an environment for the photochromic compound. Optimization
of the photochromic reactions in polymer substrates, including
the thermal back reaction, is essential and brings significant
advancement in the application of PNI and PNO-p to photo-
chromic lenses and light-controlling materials. The purpose of this
study is to investigate the effect of a polymer matrix on photo-
chromic reactions, especially the thermal back reaction, and to
demonstrate the performance of photochromic films that suggest
similar applications. The lifetime of the open form was dependent
9
,10
DMSO and acetonitrile.
Thus, we understand that this
absorption band is dependent on the polarity of the medium.
This band might be attributed to the existence of another
chemical species that is in equilibrium with the open form.
A possible candidate is the open isomer, a geometrical isomer
of open form where the CQN bond of the imine moiety is
1
0
mainly on the glass transition temperature (T ) of the polymer,
g
inverted by 1801. In this case, the thermal back reaction
process consists of at least two events: the isomerization
around the CQN bond and ring-closure. One might expect
that spectral changes should deviate the isosbestic point.
However, we did not observe a deviation of the isosbestic
point during the thermal isomerization. The reason for this is a
relatively faster equilibrium between syn and anti isomers (fast
thermal isomerization around the CQN bond) than the ring-
closure reaction. A shift of the transient absorption has been
observed by transient absorption spectroscopy, and the shift is
and it was observed to range from 11 min to 5.4 d.
The absorption spectra of PNI in polymers (polycarbonate,
HEMA and EVA) and in toluene are shown in Fig. 1, and the
absorption maxima (lmax) of its closed and open forms are
listed in Table 1. The non-irradiated solutions exhibited a lmax
around 405 nm, which corresponds to that of the closed form.
The lmax values and shape of the absorption spectra of
the closed form observed in polymers resemble those in
9
,10
solutions, as reported previously.
As can be seen in Fig. 1
1
0
and Table 1, the absorption spectra of the closed form is little
affected by the polymer substrate. Similarly, in solutions, the
absorption spectra were not affected significantly by the polar
or protic nature of the solvents.
possibly attributed to isomerization around the CQN bond;
this event occurs on a microsecond timescale. Thus, a fast
equilibrium should be established within a second. Although
the transient absorption was measured in solution, we presume
that a similar reaction takes place in polymer matrices.
The existence of a similar species, in equilibrium with the
open form, has been proposed in the case of photochromic
1
1
spiroperimidines, which are derivatives of PNI.
Although the photochromic reaction was observed in most
polymers, no photochromic reaction was observed in PEG
(
MWB3000) film, even with a transparent light yellow film on
a glass substrate. This is probably due to crystallization of the
PEG, thereby preventing molecular motion due to insufficient
local free volume around the PNI molecules.
PNI exhibited a ‘T-type’ photochromism in polymers as well
as in solution. When an irradiated polymer film (brown color)
was kept in the dark, the original color (light yellow) returned.
The lifetime, t (t = 1/k, where k is the rate constant for the
decay of the open form), varied with the polymer, as listed in
Table 1. In polymers such as PMMA, polystyrene and poly-
carbonate, the lifetimes were as long as about 100 h. On
the other hand, in PDMS, EVA and 1,2-polybutadiene, shorter
lifetimes were observed, especially in the case of 1,2-
polybutadiene, where the lifetime was as short as 11 min.
Two rate constants for the thermal back reaction were
observed for films of polycarbonate, HEMA and SBBC,
whereas the other films of polymers displayed single rate
constants. In general, the rate of the decoloration reaction of
photochromic compounds is affected by several factors, such
as residual strain in the photogenerated molecules, static
distribution of local free volume and dynamic aspects of the
Fig. 1 The UV-vis absorption spectra of PNI in (a) polycarbonate,
1
4–16
(b) HEMA, (c) EVA and (d) toluene upon irradiation with 405 nm
molecular environment.
In addition, the volume required
light. The concentration of PNI in the polymer film is 3 wt%.
(sweeping volume) for isomerization often determines the
1
328 | New J. Chem., 2009, 33, 1327–1331
This journal is ꢀc The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009

View Article Online
Table 1 Absorption maxima of the closed and open forms, reaction rates for the thermal back reaction, and the lifetime of the open form in
polymers and solutions
ꢁ1
Polymer
T
g
/1C
Coating solvent
lmax closed/nm
lmax open/nm
k/s
Lifetime (t)
ꢁ
ꢁ
ꢁ
ꢁ
6
6
6
6
PMMA
82
Toluene
407
406
409
409
491
2.4 ꢂ 10
3.3 ꢂ 10
2.9 ꢂ 10
130 h
100 h
93 h
81 h
3.5 h
26 h
25 h
2.6 h
57 min
27 h
18 min
78 min
33 min
24 min
11 min
Polyvinyl chloride
Polystyrene
Polycarbonate
1,4-Dioxane
Toluene
Chloroform
491, B590 (sh)
484, B590 (sh)
490, B590 (sh)
100
145–150
3.4 ꢂ 10 (86%)
ꢁ
5
7
.7 ꢂ 10 (14%)
ꢁ
5
PVA
HEMA
99
55
H
2
O/ethanol
406
407
508
500
1.1 ꢂ 10
ꢁ
5
Methanol
1.1 ꢂ 10 (61%)
ꢁ
4
1
.1 ꢂ 10 (39%)
ꢁ
4
SIBC
SBBC
Toluene
Toluene
411
411
493, B590 (sh)
493, B590 (sh)
3.0 ꢂ 10
ꢁ
5
1.1 ꢂ 10 (54%)
ꢁ
4
9
.1 ꢂ 10 (46%)
ꢁ4
PVP
PDMS
EVA
1,2-Polybutadiene
PEG
175
H
2
O/ethanol
408
400
410
409
409
410
403
403
516
2.2 ꢂ 10
ꢁ
4
Toluene
Toluene
Toluene
1,4-Dioxane
480, B590 (sh)
501, B590 (sh)
494, B590 (sh)
5 ꢂ 10
ꢁ
ꢁ
4
3
Bꢁ35
ꢁ95
6.7 ꢂ 10
1.5 ꢂ 10
a,b
ꢁ5
ꢁ7
3
Toluene
Acetonitrile
MeOH
490, 590 (sh)
489
500
6.3 ꢂ 10
5.2 ꢂ 10
1.7 ꢂ 10
4.4 h
22 d
600 ms
b
b
a
a
b
Ref. 9. Ref. 10.
1
4
reaction rate. Due to these factors, the reaction rate deviates
from single exponential kinetics. Although further studies
remain to be completed to examine the matrix effect on PNI,
it should be noted that this compound exhibits anomalously
simple single- or double-exponential behaviors.
The rate-determining step seems to be different in polymers
and solutions for the thermal back reaction of PNI. In
polymers, the rate depends on the rigidity of the medium.
The ring-closure motion of the molecule, such as twisting and
rotation of the bonds connecting the two aromatic rings, is
In solution, the lifetime strongly depends on the nature of
g
the rate-determining step. Polymers having a low T value
1
0
the solvent. For example, in toluene, the lifetime is 4.4 h.
On the other hand, in alcoholic solvents, its lifetime decreases
significantly (600 ms in methanol) as the size of the alkyl group
of the solvent decreases. In polar aprotic solvents such as
DMSO and acetonitrile, lifetimes are 10 and 22 d, respectively.
Thus, in solution, the proton of the –OH group plays an
important role in catalyzing the thermal back reaction. In line
with previous observations, similar effects were expected
in polymers, where polymers possessing –OH groups may
facilitate the thermal back reaction. However, as can be seen
in Table 1, the lifetime did not follow this trend. In PVA and
HEMA, the lifetimes are both about 1 d, although these
polymers have –OH groups. The decay profile of the open
form in HEMA consists of another component with a
relatively shorter lifetime of 2.6 h; however, the effect of the
provide a larger free volume for the ring-closure reaction of
the PNI molecules. The –OH groups of the polymer may
facilitate the proton shift from the amine to the imine group
for the ring-closure. However, the ring-closure molecular
motion is significantly slow and thus may be considered the
rate-determining step. In solutions, on the other hand, the
rate-determining step is the proton shift from the amine
nitrogen on the naphthalene group to the imine nitrogen.
The –OH groups of alcoholic solvents assist the proton shift,
and molecular motions involved in the ring-closure process
occur easily and hence becomes relatively faster in solution.
Upon irradiation using 405 nm light through a photomask,
polymer films of PNI dispersed in polycarbonate gave patterns
with a good color contrast (Fig. 2). The irradiated area turned
brown, while the non-irradiated regions remained light yellow.
In addition, by using another photomask, we were able
to write patterns onto a polystyrene film that depict high
resolution imaging (Fig. 3). These images remained unchanged
1
0
–
OH group seems to be small compared to that in solutions.
On the other hand, it is noteworthy that shorter lifetimes were
observed in PDMS, EVA and 1,2-polybutadiene. These
polymers have elastic properties, and their T values are below
g
room temperature. The lifetime is enhanced by about
7
00 times only by changing the polymer from 1,2-
polybutadiene (11 min) to PMMA (130 h). Rigid polymers
with higher T values seem to prevent the motion of the
g
dispersed molecules, causing a slow thermal back reaction.
Such effects of polymer matrices have been reported for the
thermal back reaction of photochromic spirooxazines and
8
,17,18
spiropyranes.
Similar to these reports, the mobility of
the photochromic molecules in the switching process is
important, and it affects the rate of the thermal back reaction.
However, the effect of T is very small when spirooxazine is
g
Fig. 2 A photograph of a polymer film (22 mm ꢂ 22 mm) having PNI
(3 wt%) dispersed in polycarbonate after 405 nm light irradiation
through a photomask. The irradiated area turns to brown.
1
7
dispersed in polystyrene and 1,2-polybutadiene, which is in
contrast to the case of PNI in this study.
This journal is ꢀc The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009 New J. Chem., 2009, 33, 1327–1331 | 1329

View Article Online
Fig. 3 (a) A photograph of a polymer film (22 mm ꢂ 22 mm) having PNI (3 wt%) dispersed in polystyrene after 405 nm light irradiation through
a photomask. (b) and (c) Enlarged photographs of the indicated regions of the film.
for a week at room temperature in the dark. Similar results
were also obtained using a PMMA film.
A demonstration of the ‘fast’ thermal back reaction in
1
1
,2-polybutadiene is shown in Fig. 4. A film of PNI in
,2-polybutadiene (1% w/w) was irradiated using 405 nm light
with a photomask for 3 min. Immediately after the irradiation,
the photomask was removed and the film exhibited a pattern
with good color contrast. Leaving the film on the bench, it
gradually lost its color contrast of the pattern and returned to
the initial light yellow color within 1 h.
These polymers exhibit a photochromic reaction, even under
sunlight (Fig. 5). Rapid photocoloration was observed within
1
min because the photochemical forward (ring-opening)
reaction is more efficient than the photochemical back (ring-
Fig. 5 Photographs of polymer films having PNI (1 wt%) dispersed
in (a, b) 1,2-polybutadiene (10 mm ꢂ 17 mm) and (c, d) polycarbonate
9
closure) reaction. Thus, these polymers containing PNI can
(
14 mm ꢂ 14 mm). (a, c) Before exposure to light and (b, d) after 2 min
function as light-controlling materials triggered by sunlight.
In summary, the photochromic properties of PNI in
polymers have been studied, and it has been revealed that the
lifetime of its open form is sensitive to the polymer matrix,
although its absorption spectrum is little affected. We have
exposure to sunlight.
controlling materials, the optimization and tuning of the
photochromic reactions are fundamental requirements. The
tuning of the thermal back reaction rate can be achieved by
simply selecting a suitable polymer substrate. PNI was stable
upon photoirradiation for several hours in most of the polymer
matrices studied. The fatigue resistance in polymer matrices is
currently being studied and will be reported elsewhere.
We thank Ryota Itoh, Tomomi Namekawa, Masamichi
Takemoto (Nihon University) and Tadashi Miyagi (Ibaraki
University) for their help in the synthesis and photochemical
experiments as part of Internship programs.
g
observed that the lifetime depends mainly on the T of the
polymer, which is in contrast to the solvent effects reported
previously, where –OH groups of alcoholic solvents play a
significant role in catalyzing the thermal back reaction. Thus,
in polymers, photochromic reactions depend on the available
free volume in the medium, which affects the mobility of the
photochromic molecule. As for applications leading to light-
Experimental
9
,10
PNI was synthesized as previously reported
and recrystal-
lized from ethyl acetate. Polystyrene, polyvinyl alcohol (PVA),
poly(2-hydroxyethyl methacrylate) (HEMA), styrene/isoprene
ABA block copolymer (SIBC), styrene/butadiene ABA block
copolymer (SBBC), ethylene/vinyl acetate copolymer (EVA)
and 1,2-polybutadiene were purchased from Scientific Polymer
Products, Inc. Poly(methyl methacrylate) (PMMA) was
purchased from Lancaster. Polyvinyl chloride was purchased
from the Wako Chemicals Co. Poly(bisphenol A carbonate)
(
pyrrolidone (PVP) was purchased from TCI. Polydimethyl-
Polycarbonate) was purchased from Aldrich. Polyvinyl
s
siloxane (PDMS) was purchased from Dow Corning (Sylgard
84 silicone elastomer kit). Polyethylene glycol (PEG) was
Fig. 4 Photographs of a polymer film (22 mm ꢂ 22 mm) having PNI
1
(
3 wt%) dispersed in 1,2-polybutadiene. (a) Before irradiation and (b) after
3
min of 405 nm irradiation. The film was then kept in the dark for
purchased from the Kishida Chemical Co. Spectroscopic grade
solvents were used for the preparation of polymer films.
(c) 5 min, (d) 10 min, (e) 30 min and (f) 50 min after the irradiation.
1
330 | New J. Chem., 2009, 33, 1327–1331
This journal is ꢀc The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009

View Article Online
The general procedure for the preparation of cast films was as
follows. Solutions containing PNI and polymer were mixed at
appropriate ratio to a final concentration of 1–3 wt% of PNI in
the polymer substrate. Solvents used for sample preparation are
listed in Table 1. The solution was drop-cast on a cover glass
3 (a) K. Aminoto and T. Kawato, J. Photochem. Photobiol., C, 2005,
2
2
07–226; (b) E. Hadjoudis and I. M. Mavridis, Chem. Soc. Rev.,
004, 33, 579–588.
4
S. Delbaere and G. Vermeersch, J. Photochem. Photobiol., C, 2008,
, 61–80.
5 Y. Yokoyama, Chem. Rev., 2000, 100, 1717–1739.
9
6
K. G. Yager and C. J. Barrett, J. Photochem. Photobiol., A, 2006,
82, 250–261.
V. I. Minkin, Chem. Rev., 2004, 104, 2751–2776.
(
22 mm ꢂ 22 mm), and the solvent was allowed to evaporate at
1
room temperature under a continuous flow of dry nitrogen.
After coarse removal of the solvent, the film was annealed in a
vacuum oven at 100 1C for 1 h. After cooling to room
temperature, the film was used for photochromic experiments.
UV-vis absorption spectroscopy was carried out using a
JASCO V-570 spectrophotometer. Irradiation was carried out
using a high pressure mercury lamp coupled with appropriate
7
8 G. Such, R. A. Evans, L. H. Yee and T. P. Davis, J. Macromol.
Sci., Part C, 2003, 43, 547–579.
9
R. Davis and N. Tamaoki, Org. Lett., 2005, 7, 1461–1464.
1
0 Y. Norikane, R. Davis, Y. Nishimura, T. Arai and N. Tamaoki,
submitted.
11 R. Davis and N. Tamaoki, Chem.–Eur. J., 2007, 13,
26–631.
6
ꢁ2
12 J. C. Crano, T. Flood, D. Knowles, A. Kumar and B. V. Gemert,
Pure Appl. Chem., 1996, 7, 1395–1398.
13 C. Salemi-Delvaux, E. Pottier, R. J. Guglielmetti, R. Dubest and
J. Aubard, Dyes Pigm., 1999, 40, 157–162.
filters (405 nm, B1 mW cm ) or by LED (lmax = 410 nm,
1
ꢁ2
8 nm fwhm, B7 mW cm ). Rate constants for the thermal
back reaction from open to closed forms were determined by
monitoring the temporal change in absorption at 500 nm of
the open form. These measurements were carried out at 295 K.
1
1
1
1
4 J. G. Victor and J. M. Torkelson, Macromolecules, 1987,
0, 2241–2250.
5 I. Mita, K. Horie and K. Hirao, Macromolecules, 1989, 22,
58–563.
6 Y. Munakata, T. Tsutsui and S. Saito, Polym. J., 1990, 22,
43–848.
2
5
8
References
7 K. Kojima, N. Hayashi and M. Toriumi, J. Photopolym. Sci.
Technol., 1995, 8, 47–54.
1
K. Matsuda and M. Irie, J. Photochem. Photobiol., C, 2004, 5,
69–182.
F. M. Raymo and M. Tomasulo, Chem. Soc. Rev., 2005, 34,
27–336.
1
18 R. A. Evans, T. L. Hanley, M. A. Skidmore, T. P. Davis,
G. K. Such, L. H. Yee, G. E. Ball and D. A. Lewis, Nat. Mater.,
2005, 4, 249–253.
2
3
This journal is ꢀc The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009 New J. Chem., 2009, 33, 1327–1331 | 1331