Reverse photochromism of spiropyran in silica

Article abstract of DOI:10.1016/j.jphotochem.2010.05.012

The high thermal stability of the photomerocyanine-form (PMC-form) of spiropyran (SP) dispersed in perhydropolysilazane (PHPS), which converted to silica at ambient temperature, was investigated. PHPS was converted to silica by the de-ammonium reaction wi

Full text of DOI:10.1016/j.jphotochem.2010.05.012

Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 136–140
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Reverse photochromism of spiropyran in silica
K. Kinashi, S. Nakamura, Y. Ono, K. Ishida, Y. Ueda∗
Graduate School of Engineering, Department of Chemical Science and Engineering, Kobe University, 1-1, Rokko, Nada, Kobe 675-8501, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The high thermal stability of the photomerocyanine-form (PMC-form) of spiropyran (SP) dispersed in
perhydropolysilazane (PHPS), which converted to silica at ambient temperature, was investigated. PHPS
was converted to silica by the de-ammonium reaction with water vapor within approximately 48 h.
The structure of the converted silica was not only SiO2, but also partially uncondensed Si–OH and O–H.
The PMC-form with high thermal stability was attributed to the protonated form, which was produced
by intermolecular hydrogen bonding between oxide anion generated by cleavage of C–O bonds and the
partially uncondensed Si–OH and O–H of silica. The protonated PMC-form was directly generated from the
SP-form without UV light irradiation, because the singlet ground state of the PMC-form (PMC0) is lower
than that of the SP-form (SP0) in silica. In this mechanism, the singlet ground state of the protonated PMC-
form (H· · ·PMC0), which is significantly stabilized by silica, had polarity similar to water. The stabilization
of the PMC-form was attributed to hydrogen bonding between silica and direct generation from the
SP-form by reverse photochromism.
Received 25 January 2010
Received in revised form 7 May 2010
Accepted 25 May 2010
Available online 31 May 2010
Keywords:
Reverse photochromism
Spiropyran
Perhydropolysilazane
Silica
Thermal stability
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
for the dispersion and stabilization of the PMC-form. Stabilization
of the PMC-form has mainly focused on metal complex [21], and SP
Light-induced reversible transformation between two isomers
with different absorption spectra is referred to as photochromism
[1–4]. Two isomers differ from one another not only in their absorp-
tion properties, but also in refractive indices, dielectric constants,
oxidation–reduction potentials, and geometric structures. There-
fore, these properties can be reversibly switched upon irradiation
with an appropriate wavelength of light. Spiropyrans (SPs) are
well-known photochromic compounds that have attracted much
interest with respect to both the fundamental elucidation of pho-
tochemical reactions and their potential application to optical
memory and photo-optical switching devices [5–10]. The appli-
cation of SPs to such devices is restricted by the short lifetime
of the open photomerocyanine-form (PMC-form), which ther-
mally reverts to the closed colorless spiropyran-form (SP-form)
(Scheme 1). Many studies have been performed to investigate
the lifetime and/or dynamics of the thermal reversion process
by measuring the static absorption/fluorescence [11–14] and the
time-resolved absorption/fluorescence [15–20] during the last cou-
ple of decades. Obviously these studies contributed to solve the
complicated mechanism of PMC-form. For practical applications,
the thermally unstable PMC-form must be dispersed in a solid
matrix, because PMC-form in their crystalline state is usually inac-
tive. In the last two decades, various methods have been reported
has been dispersed in rigid matrices such as polymers [22], sol–gels
[23], organogels [24], inorganic particles [25], clay interlayers [26]
and mesoporous silica [27], whereby the mobility of the PMC-form
is restricted. PMC-form can be either covalently linked to a poly-
mer backbone or dispersed in a polymer matrix, and can also be
entrapped within two- and three-dimensional networks or interca-
late within inorganic nanoparticles. Polysilazane derived silica have
been intensively studied in recent years [28]. Perhydropolysilazane
(PHPS) is considered to be a highly efficient ceramic precursor.
Coatings prepared on commercially available PHPS substrates are
curable by reaction with water vapor at ambient temperature. PHPS
can be converted to silica without lattice defects and with a den-
sity of 1.3 g/cm3 [29]. Moreover, at temperatures of 80–120 ◦C the
curing rate can be increased by the treatment of PHPS films with
pyridine and NH3 [30]. Using methyl-substituted piperidine, alky-
lamines or acids as catalysts, PHPS curing at room temperature is
claimed to provide dense coatings that are impermeable to oxy-
gen, which thereby prevent the oxidation of metal surfaces [31].
Quite recently, Yamano and Kozuka reported a mechanism for the
interaction of the PMC-form with a silica matrix, in which the
absorption process was enhanced by reverse photochromic reac-
tion [32]. We previously reported that the high thermal stability
of the PMC-form in silica has the possibility of being protonated by
interaction between oxide anions generated by cleavage of the C–O
bond, ethanol or the propanoic acid linked N-position, and oxy-
gen of Si–O–Si in silica [33]. However, the detailed mechanisms
of hydrogen bonding and thermal stabilization are yet to be clari-
∗
Corresponding author. Tel.: +81 78 803 6182.
E-mail address: yueda@kobe-u.ac.jp (Y. Ueda).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.05.012

K. Kinashi et al. / Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 136–140
137
Scheme 1. Reversible photochromic reaction of the SP- and PMC-forms.
fied. In this report, we have proposed a mechanism regarding the
generation of the protonated PMC-form from investigations using
UV–vis, infrared (IR) and X-ray photoelectron (XPS) spectroscopies,
in addition to an energy diagram for the mechanism.
Fig. 1. Absorption spectra of methanol solutions of the SP-form (conc. 1.0 × 10−5 M,
solid line) and photostationary PMC-form (broken line) under irradiation with
360 nm light.
2. Experimental
1ꢀ-(2-Hydroxyethyl)-3ꢀ,3ꢀ-dimethyl-6-nitrospiro[1(2H)-
which was generated under UV light irradiation. The absorption
peaks of the PMC-form in methanol and PVA were at 529 and
535 nm, respectively. Thermal decay curves of the absorption inten-
sities in methanol and PVA corresponded well to the following
single exponential decay function.
benzopyran-2,2ꢀ-indoline] (Sp) was synthesized by a coupling
reaction
with
Fischer’s
base
1-(2ꢀ-hydroxyethyl)-2,3,3-
trimethylindolenium iodide and 5-nitrosalicylaldehyde according
to the previous report [33]. A commercially available m-xylene
solution of PHPS (5%, NL110, Clariant Japan Co.) was used as a
precursor to prepare the silica film. The PHPS m-xylene solution
containing 10−4 M Sp was spun at 4000 rpm on a fused silica
and Si(1 0 0) substrate, the latter of which was for FT-IR mea-
surements. In the case of using poly(vinyl alcohol) (PVA; Nacalai
Tesque Co.) as a matrix, a PVA dimethylsulfoxide solution with
same concentration of SP was prepared by spin-coating. The
as-spun film was stored in the dark at room temperature while all
measurements also performed in this paper were carried out in
the dark to inhibit thermal reversion. The excitation wavelength
(ꢀ = 360 nm) from a Xe lamp (MAX-301, Asahi Spectra) was used to
generate the PMC-form. UV–vis absorption spectra were recorded
with a Shimadzu UV-2200 spectrophotometer. IR spectra were
recorded using a Jeol FT/IR-660Plus under vacuum conditions. XPS
spectra were recorded with Shimadzu ESCA3400, using an Al K␣
monochromator X-ray source operated at 10 kV and 20.0 mA under
pressures of 10−5 Pa, and with an analyzer pass energy of 23.3 eV
during spectral acquisition. The takeoff angle of an electron from
the sample was fixed at 45◦ with respect to the specimen surface.
The C 1s core-level spectra were decomposed into three Gaussian
functions using the IGOR Pro Ver. 5.0 program. For peak synthesis,
the full width at half-maximum (FWHM) at 1.7 eV was used for
each Gaussian peak.
ꢀ
ꢁ
At − A
∞
ln
= −kt
(1)
A0 − A
∞
where A is the residual absorbance after long term exposure,
∞
A0 and At are the absorbances immediately after UV light irra-
diation and at any subsequent time t, respectively, and k is the
rate constant for the PMC-form to SP-form. According to the con-
ventional approach for evaluation of a first-order reaction, the
reaction rate constants, k, and half-lifetimes (ꢁ1/2) of the PMC-
form in methanol and PVA were estimated to be 9.6 × 10−5 s−1
(7.2 × 103 s) and 3.2 × 10−6 s−1 (2.2 × 105 s), respectively. ꢁ1/2 of the
PMC-form in PVA was approximately 30-times longer than that in
methanol. In the case of the PHPS matrix, the absorption intensity of
the PMC-form decreased rapidly within approximately 3 h, because
the PMC-form was surrounded by m-xylene in the film containing
non-polar m-xylene at initial state. However the intensity remained
constant for 2–3 days after decreasing by approximately half. It
is a noteworthy feature that the constant intensity subsequently
exhibited a gradual increase from a certain time. The absorption
peak shifted to 510 nm after 2–3 days, which suggests that PHPS
was completely converted to silica within the 2–3 day period. k
and ꢁ1/2 of the PMC-form in PHPS after 2–3 days were estimated
to be approximately 8.7 × 10−9 s−1 and 8.0 × 107 s, respectively.
PHPS would be completely converted into silica within 2–3 days,
3. Results and discussion
3.1. Thermal stability in polar matrices
Fig. 1 shows UV–vis absorption spectra of Sp methanol solution
(1.0 × 10−5 M) before and after 360 nm light irradiation at ambi-
ent temperature. Before light irradiation, main absorption peak
of SP-form appeared around at 330 nm. This absorption band was
assigned to ␲–␲* transitions of chromene and indoline rings. Upon
light irradiation, the solution changed from colorless to red and
a broad absorption band appeared with a maximum at 529 nm.
This absorption band was assigned to the PMC-form, which has
a zwitterionic structure. After stopping the UV light irradiation,
the absorption intensity of the PMC-form gradually decreased over
approximately 10 h at ambient temperature.
Fig. 2 shows the time dependence of the absorption intensity
of the PMC-form at ꢀmax in methanol (as a polar solution), PVA (as
a polar polymer) and in PHPS. The monitored absorption intensity
was defined by the centered absorption peak in the visible region,
Fig. 2. Time dependence of the normalized absorption intensity at ꢀmax for the PMC-
form in various matrices in the dark at 25 ◦C: (ꢁ) silica, (ꢀ) PVA and (ꢂ) methanol.
The inset shows the time dependence of the later time.

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K. Kinashi et al. / Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 136–140
Fig. 3. Time evolution of the absorption behavior of the PMC-form in silica in the
dark at ambient temperature.
Fig. 4. ꢀmax of the PMC-form vs. ET: ꢁ represents experimental; filled symbols
represent ET values that were estimated from the linear fit of the solvents.
at 510 nm was formed directly from the SP-form irradiated with
360 nm light after 312 h.
the conversion mechanism of which is described later in Section
3.2. ꢁ1/2 for the PMC-form in silica is approximately 105 times
longer than that in methanol. It is well known that the PMC-form
is stabilized in polar solvents and that stability is also significantly
enhanced in rigid matrices such as polymer networks [34]. There-
fore, a rigid matrix with polar moieties, such as hydroxyl and
carboxylic groups, is the best candidate for stabilization of the
PMC-form. In a polar medium, the PMC-form is predominantly
comprised of charge localized zwitterionic structures that are sta-
bilized in the polar matrix. Nadolski et al. has suggested that the
singlet ground state is more polar than the singlet excited state
[35]. Therefore, PMC-form is stabilized against external heating
in a polar matrix, which results in a short-wavelength shift of
ꢀmax (hypsochromic shift) [33,34]. In addition, molecular vibra-
tion of the PMC-form in PVA would be strongly restricted due
to steric hindrance, so that the probability of cycloreversion is
restricted. However, in the case of the silica matrix, another sta-
bilization factor in addition to the matrix polarity and steric
hindrance is suggested, due to the significant hypsochromic shift
observed.
Reichardt introduced the transition energy expressed in
kcal/mol in order to explain the solvent effect on ꢀmax for
the pyridium-N-phenoxide betaine dye in various solvents. This
parameter is referred to as the ET value, which is related to ꢀmax
by the following equation [39].
2.859 × 104
ET (kcal/mol) =
ꢀmax (nm)
Fig. 4 shows a linear plot of ꢀmax for the PMC-form versus ET param-
eters in various solvents. ET for PVA is estimated to be 52.6 kcal/mol
from ꢀmax on the straight-line approximation. It is suggested that
PVA behaves as a protic polar matrix with similar properties to alco-
hols (e.g., ethanol or methanol). Each plot of PHPS under conversion
is shown as a closed triangle. From ꢀmax of the PMC-form, the ET
values for PHPS under conversion at 0, 3, 48 and 312 h are estimated
to be 39.1, 44.3, 58.5 and 61.3 kcal/mol, respectively. These results
indicate that the polarity of PHPS is changed from an aprotic non-
polar matrix to a protic polar matrix as the conversion progresses.
The unconverted PHPS changed to a tetrasilanol intermediate
with similar polarity to that of DMSO within approximately 3 h;
however, the tetrasilanol intermediate was very short lived after
hydrolysis with water vapor. The intermediate was gradually con-
verted into silica with a similar polarity to that of methanol/water
(2/3) within ca. 48 h. PHPS was fully converted into silica polarity
similar to that of water within the period of 48–312 h. There-
fore, PHPS would be changed to a protic polar matrix with O–H
groups having an equivalent polarity to water via the short-lived
tetrasilanol intermediate. Fig. 5 shows the IR absorption spectra for
PMC-form/PHPS after spin-coating that was measured in the dark
over time at ambient temperature. The structure of PHPS in the
conversion process and the molecular structure of PHPS with time
dependence are also illustrated. The spectrum of the as-prepared
film shows several absorptions at 3368, 2937, 2857, 2804, 2165,
1172, 1069, 932 and 829 cm−1. These bands are ascribed to N–H
stretching (3368 cm−1), Si–H stretching (2165 cm−1), Si–NH–Si
bending (1172 cm−1), Si–N stretching (932 cm−1) and Si–H wag-
3.2. Mechanism of thermal stability
The absorption spectra of the PMC-form in PHPS following UV
irradiation with 360 nm light after dark conditions are shown in
Fig. 3. The absorption spectrum before irradiation did not appear
in this region. The state just after spin-coating with UV irradiation
is denoted as 0 h, where the PMC-form has ꢀmax at 574 nm. ꢀmax
decreases with external heating (ambient temperature ca. 298 K) to
559 nm with a slight hypsochromic shift after ca. 3 h. After approx-
imately 48 h, ꢀmax has shifted significantly to 518 nm. It should be
noted that the absorption intensity increased with a slight hyp-
sochromic shift to 510 nm after ca. 312 h. These results indicate
that the PHPS structure is converted drastically and its structural
changing carried out an interaction to PMC-form within the period
of 3–48 h. Shimizu et al. and Raymo et al. previously reported
the effect of acids on the properties of SPs and observed similar
absorption spectrum for SPs in acidic media [36–38]. The unique
spectrum was assigned to hydrogen bonding with the PMC-form.
Therefore, the band observed at 510 nm can be attributed to the
PMC-form with a short conjugated structure, which has interacted
with that completely converted to silica. Bleaching of the film was
achieved by irradiation with visible light at >420 nm; the visible
absorption band had completely disappeared. The spectral changes
indicate that the PMC-form returns to the SP-form. However, the
film was then kept in the dark and the absorption band at 510 nm
slowly reappeared. It was noted that the absorption band centered
ging (829 cm−1) of PHPS, and C–H stretching (2937–2804 cm−1
)
of the methylene unit of 4-4ꢀ-trimethylenbis(1-methylpiperidine)
used as catalyst. The absorptions originated from SP appear weakly
in the region from 1700 to 1200 cm−1, due to the low concentration.
After 3 h, the absorption intensities of Si–H (2165 and 829 cm−1
)
and Si–NH–Si decrease and an absorption band ascribed to Si–O–Si
stretching vibration appears at 1069 cm−1. The absorption intensi-
ties of N–H, Si–H, Si–NH–Si and Si–N bands then become weaker
and that of Si–O–Si band becomes stronger as time progresses. After

K. Kinashi et al. / Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 136–140
139
Fig. 5. IR spectra of a film of PHPS with the PMC-form and the molecular structure
of PHPS for various times.
312 h, a strong absorption peak of Si–O–Si appears, which indi-
cates that PHPS has been converted to silica by hydrolysis between
Si–H and Si–N groups and water vapor, with subsequent cross-
linking from condensation reactions. Careful observation reveals
that broad and weak bands appear around 3330 and 949 cm−1 after
312 h, which are ascribed to O–H and Si–OH stretching vibrations,
respectively. It is noteworthy that the FWHM of the absorption peak
at 3330 cm−1 in the spectrum of the film after 24 h is broader than
that of the N–H band, which suggests that this peak is the over-
lapping of O–H and N–H bands. This would indicate that the O–H
group was generated after 24 h. A large amount of the Si–H groups
in PHPS have high reactivity with hydroxyl groups. The conversion
process from PHPS to silica as determined from the IR analysis is in
good agreement with the time-resolved absorption spectra (Fig. 3).
The IR features indicate that silica was subsequently formed by
condensation and cross-linking reactions of PHPS and the result-
ing SiO2 contains residual O–H and Si–OH groups despite complete
conversion.
The structural information of the PMC-form was also investi-
gated using XPS. Fig. 6 shows XPS spectra of the PMC-form in silica
(0 h) and after complete conversion to silica (312 h), in which a rel-
atively broad C 1s signal revealed the presence of several types of
C atoms with small chemical shifts. The signal centered at 285.0 eV
originated from (1) –C–C– and –C–H binding energies. This signal
was used for calibration of the spectra and was decomposed using
the Gaussian method. The best fit of the C 1s spectrum of the PMC-
form at 0 h (Fig. 6a) was obtained using two signals corresponding
to (1) –C–C and –C–H at 285.0 eV and (2) –C–O at 286.2 eV (1.2 eV
shift). However, in the case of the C 1s spectrum at 312 h (Fig. 6b),
a third broad signal appears at (3) 288.5 eV (3.5 eV shift) in addi-
Fig. 6. Decomposition of the C 1s profile for the PMC-form in silica with conversion
at (a) 0 and (b) 312 h. The two or three Gaussian components, 1, 2 and 3, correspond
to –C–C–, –C–H (285.0 eV), –C–O– (286.2 eV), and –O–C O (288.5 eV), respectively.
of the PMC-form has hydrogen bonding between the oxide anion on
C–O and the residual O–H and Si–OH in silica; the protonated PMC-
form is abbreviated as H· · ·PMC-form. The mechanism for thermal
stability and the respective molecular structures, including pho-
tographs showing the film states are summarized in Fig. 7. The
state of the PMC-form just after photochromic reaction is shown
at reaction coordinate 1 (RC-1), which corresponds to 0 h. The
absorption spectrum of the RC-1 PMC-form shows ꢀmax centered
at 574 nm and the molecular structure has a standard zwitterionic
structure. Within approximately 3 h (RC-2), the obtained PMC-form
is reverted rapidly to the initial SP-form, because PHPS is mostly
in the solution state; however, ꢀmax reaches 559 nm with slight
hypsochromic shift and the absorption intensity rapidly decreases.
In RC-2, PHPS is converted into the short-lived tetrasilanol inter-
mediate after hydrolysis with water vapor. After 48 h (RC-3), the
ꢀmax of the PMC-form at ca. 559 nm undergoes a significant hyp-
sochromic shift to 518 nm and the absorption intensity remains
constant. The large spectral shift (RC-2 → RC-3) suggests that the
single ground state of the PMC-form (PMC0) is more polar than the
singlet excited state (PMC1) [44]. The energy gap between PMC0
and PMC1 can be widened by differential stabilization with ambi-
ent polarity; therefore, the electronic states of these molecular
structures have different dipole moments [45]. In RC-3, PHPS is
condensed to promote subsequent cross-linking. A small amount of
the H· · ·PMC-form is formed from the residual PMC-form by hydro-
gen bonding with partially uncondensed Si–OH and O–H. After
approximately 312 h (RC-4), the H· · ·PMC-form is formed directly
from the SP-form, due to protonation with the partially uncon-
densed Si–OH and O–H in silica. The molecular structure of the
H· · ·PMC-form is a protonated zwitterionic structure. In RC-4, the
slow coloration indicates a slow open-ring reaction that leads to
the formation of the H· · ·PMC-form by protonation. The direct gen-
tion to (1) and (2). The broad signal agrees well with the –O–C
O
binding energy, which has been previously reported by several
researchers [40–42]. However, the structure of the PMC-form does
not contain the –O–C O group. The third XPS signal is similar to
the –O–C O bond derived from the hydroxyl group of PVA films
[43], which are reported as 284.6 and 285.3 eV (0.7 eV shift) and
288.2 eV (3.4 eV shift). As a result, a third chemical bond is assigned
to the newly generated hydroxyl group (–C–O–H) with residual
O–H and Si–OH in silica. Spectral analysis infers that the structure

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K. Kinashi et al. / Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 136–140
Fig. 7. (a) Schematic representation of the reaction coordinates (RCs) related to the potential energies of the PMC-form and the state of interaction with the PMC-form
according to PHPS reaction in the system. The color bar indicates the color tone of the actual film. (b) Molecular structure of the PMC- and H· · ·PMC-forms in a film, and
optical photomicrographs of the respective RC-1 and RC-4 states.
eration of the H· · ·PMC-form corresponds to what is referred to as
reverse photochromism in RC-4. The increasing absorption inten-
sity (RC-3 → RC-4) is produced directly from the SP-form by reverse
photochromism, because the energy gap between H· · ·PMC0 and
H· · ·PMC1 is wider and stabilized beyond SP0.
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After the PMC-form was left in PHPS for approximately 48 h,
the ꢀmax of the PMC-form underwent a significant shift from 574
to 518 nm. The IR peaks of Si–H and N–H decreased while the
Si–O–Si bands increased, in which indicated the almost complete
conversion of PHPS to silica, but with partially uncondensed Si–OH
and O–H remaining in the silica. Therefore, it was suggested that
after 48 h, the converted silica had a solvent polarity similar to a
methanol/water mixture. We have characterized the structure of
the PMC-form in silica at 48–312 h and determined a new chemi-
cal bond assigned to hydrogen bonding between the oxide anion on
C–O and Si–OH in silica. After 48–312 h, the H· · ·PMC-form was also
directly generated from the SP-form by protonation with silica. The
thermal stability behavior of the PMC-form is suggested as being
due to reverse photochromism. From a technological viewpoint, the
slow visualization of color change cause by reverse photochromism
can be utilized, for example, in time-variable color sensors and
hygrometers.
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This research was financially supported by a Sasakawa Scientific
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