Reaction of spironaphthoxazine with acid

Article abstract of DOI:10.1016/j.jphotochem.2012.11.004

The reaction of spironaphthoxazine (SNO) with acid in ethanol was investigated by spectroscopic analyses SNO was protonated and then transformed into a complex in an acidic ethanol solution in the dark. It is presumed that the complex contains a hydrated proton and its structure is similar to that of the metal complex of a merocyanine-type isomer photoderived from SNO based on its UV-vis absorption and fluorescence spectra. The complex dissociated into SNO by adding base. The complex was further changed under acidic conditions after a long time. Highly concentrated acid decomposed SNO to an indoline compound having a carboxyl group and 1-amino-2-naphthol, which was revealed by fluorescence, IR, and GC-MS analyses. These reaction processes were promoted by UV irradiation.

Full text of DOI:10.1016/j.jphotochem.2012.11.004

Accepted Manuscript
Title: Reaction of spironaphthoxazine with acid
Authors: Hiromasa Nishikiori, Suguru Shimamura, Tsuneo
Fujii
PII:
DOI:
Reference:
S1010-6030(12)00543-6
doi:10.1016/j.jphotochem.2012.11.004
JPC 9302
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
22-7-2012
12-10-2012
12-11-2012
Please cite this article as: H.N. *nishiki@shinshu-u.ac.jpS. Shimamura, T. Fujii,
Reaction of spironaphthoxazine with acid, Journal of Photochemistry and Photobiology
A: Chemistry (2010), doi:10.1016/j.jphotochem.2012.11.004
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Reaction of spironaphthoxazine with acid
Hiromasa Nishikiori,* Suguru Shimamura, Tsuneo Fujii
Department of Environmental Science and Technology, Faculty of Engineering,
Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan
Corresponding author: Hiromasa Nishikiori
Tel: +81-26-269-5536
Fax: +81-26-269-5550
E-mail: nishiki@shinshu-u.ac.jp
Department of Environmental Science and Technology, Faculty of Engineering,
Shinshu University, Wakasato, Nagano 380-8553, Japan
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Abstract
The reaction of spironaphthoxazine (SNO) with acid in ethanol was investigated by
spectroscopic analyses SNO was protonated and then transformed into a complex in an
acidic ethanol solution in the dark. It is presumed that the complex contains a hydrated
proton and its structure is similar to that of the metal complex of a merocyanine-type isomer
photoderived from SNO based on its UV-vis absorption and fluorescence spectra. The
complex dissociated into SNO by adding base. The complex was further changed under
acidic conditions after a long time. Highly concentrated acid decomposed SNO to an
indoline compound having a carboxyl group and 1-amino-2-naphthol, which was revealed by
fluorescence, IR, and GC-MS analyses. These reaction processes were promoted by UV
irradiation.
Keywords: Spironaphthoxazine; Acid; Protonation; Complex; Fluorescence
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1. Introduction
Organic dye molecules generally have high absorption efficiencies and some of them
exhibit a specific color compared to inorganic materials. A high number of organic dye
molecules are not required for the coloration of materials, which allow modification of their
photochemical properties and add specific photo-functions without a change in the material
properties. The coloration and photoemission are interesting photochemical phenomena
applicable to the photo-function.
Many promising organic compounds exhibiting a
photochromic behavior have been found and widely investigated in various fields [1].
Photochromism is an important phenomenon that provides a photochemical technique for the
development of electronic devices such as photo-quantum switching and memory [16]. The
photochromic phenomena originate not only from the photo-isomerization of molecules, but
also from the photoinduced metal chelation of organic ligands [713]. The photoreaction and
photochemical properties of the molecules need to be controlled in order to perform a fast and
effective photochromism. Therefore, it is important to study the reaction mechanism of such
organic molecules in solutions under various conditions.
Spirooxazines (SOs) exhibit the well-known photochromic reaction of the photoinduced
interconversion between the original colorless form and the colored merocyanine (MC) form
[3,6]. The MC is easily coordinated with specific metal ions, e.g., Zn2+, in solution and
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forms both fluorescent and non-fluorescent MC-Mn+ complexes; the fluorescent complexes
are stable to heat and visible light while the non-fluorescent complexes are unstable [7,8].
We previously reported the details of the fluorescence and excitation spectra of
spironaphthoxazine (SNO) in various solvents [14]. Two types of fluorescent intermediate
species (Xs and Xh) occur between SNO and MC by cleavage of the spiro CO bond, as
shown by Scheme 1 [1518]. The fluorescence processes of SNO and the intermediate
species are shown in Scheme 2. The intermediate species can be also regard as one of the
MC isomers, i.e., the CCC (cis-cis-cis) or TCC (trans-cis-cis) form for the C=C−N=C bond
[19,20]. MC, we call it in this study, is assigned to TTC (trans-trans-cis) form. In low-polar
solvents, such as benzene, upon the excitation of SNO (E1), the 390-nm fluorescence (F1) is
normally emitted from its excited state (SNO*).
In aprotic polar solvents, such as
acetonitrile, some of the SNO molecules are transformed into a non-planar, strongly solvated
species (Xs). Such aprotic polar molecules strongly interact with the C and O atoms of the
spiro CO moiety and distort the spiro structure. The transition from species Xs to Xs* (E2)
is followed by a 450-nm fluorescent emission (F2). In protic polar solvents, such as ethanol
and water, some of the SNO molecules are normally excited to SNO* (E1) to produce the
hydrogen-bonded species (Xh*) by the CO bond breakage in the excited states, followed by
the 430-nm fluorescent transition to Xh (F3). A protic polar molecule more strongly bonds to
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the O atom of the spiro CO moiety and weakens the CO bond. The Xh species was
immediately transformed into SNO or MC in the ground state because it was unstable at room
temperature. Scheme 2 shows the two extreme fluorescence processes of the intermediate
species. Actually, the two process can occur in polar solvents. The fluorescence of the MC-
Zn2+ complex is observed at around 540 nm by UV irradiation of SNO in aprotic polar
solvents, such as acetone and acetonitrile, in the presence of Zn2+ [7,8,21,22]. The MC-Zn2+
complex was not observed in ethanol because the MC form was stabilized by hydrogen
bonding to the solvent molecules [23]. On the other hand, a small number of protons assisted
the MC-Al3+ complex formation in the ethanol solution containing Al3+ [24].
(Schemes 1 and 2)
In our previous studies, we observed the fluorescence photochromic behavior, that is, the
photoinduced interconversion between Xs and the MC-Zn2+ complex in the hydrophobic
interlayers of montmorillonite clay modified by organic surfactants [15] and in a xerogel
prepared from silicon alkoxide [16–18]. The xerogel system also contained the MC-SiOH
complex, like the MC-Zn2+, because there were many acidic SiOH groups on the surface of
the gel pores in which the organic molecules were encapsulated [16–18,25−28].
The
complex formation is expected to depend on the type of metal ions involving a specific
electron affinity [18,24]. MC can interact not only with the metal ions, but also with the
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protons on the silica gel surface.
In this study, we observed the reactions of SNO in an ethanol solution containing
hydrochloric acid in order to reveal the interaction between SNO and the protons. The dark
and photochemical reaction processes were investigated by measuring the UV-vis absorption
and fluorescence spectra as a function of time.
2. Experimental section
SNO was purchased from Tokyo Chemical Industry and used without further purification.
Ethanol (99.5%, S grade), aluminum chloride hexahydrate (S grade), hydrochloric acid (S
grade), sodium hydroxide (S grade), and 1-amino-2-naphthol hydrochloride (reagent grade)
were purchased from Wako Pure Chemicals and used without further purification.
An SNO ethanol solution and an aluminum chloride ethanol solution were prepared and
adjusted to 9.6  104 and 3.2  104 mol dm3 concentrations, respectively. The SNO
ethanol solutions containing aluminum chloride were prepared by mixing 5.0 cm3 of the
SNO solution and 5.0 cm3 of the aluminum chloride solution. Hydrochloric acid was
adjusted to 3.2  104 and 3.2  102 mol dm3 concentrations. The acidic SNO ethanol
solutions were prepared by mixing 5.0 cm3 of the SNO solution and 5.0 cm3 of the
hydrochloric acid. The concentration of SNO was 4.8  104 mol dm3 in all the solutions.
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A 1-amino-2-naphthol ethanol solution was also prepared in which its concentration was 4.8 
104 mol dm3. The UV-vis absorption spectra were measured as a function of time in the
dark and during UV irradiation at 298 K using a Shimadzu UV-2500 spectrophotometer. The
fluorescence and excitation spectra were also measured in the same way using a Shimadzu
RF-5300 fluorescence spectrophotometer. The UV (35010 nm, 20 mW cm2) irradiation
was conducted using a Shimazdu RF-5000 fluorescence spectrophotometer with a 150-W Xe
short arc lamp (Ushio UXL-155) in order to study the photoreaction.
The UV-vis absorption, fluorescence, and excitation spectra of the acidic SNO solution
allowed to react for 48 h during the UV irradiation were observed before and after adding the
sodium hydroxide solution until the pH value reached 12.
Gas chromatography-mass spectrometry (GC-MS) of the acidic SNO solution allowed to
react for 288 h during the UV irradiation, labeled Solution A, was performed using a gas
chromatograph-mass spectrometer (Shimadzu GCMS-QP5000). The IR spectrum of the
powder sample obtained by evaporating Solution A to dryness was measured using an FTIR
spectrophotometer (Shimazdu IRPrestige-21).
3. Results and discussion
Reaction of SNO with Aluminum Ion
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In the ethanol solution, hydrogen bonding induced the isomerization from SNO to MC in
the ground state via the unstable Xh [14]. The Xh species was immediately transformed into
SNO or MC. However, the equilibrium constant of the reaction from SNO to MC is low at
around room temperature.
The concentration of MC was very low under the present
condition. Fig. 1 shows the change in the UV-vis absorption spectrum of the SNO ethanol
solution containing Al3+ during the UV irradiation. The molar ratio of SNO to Al3+ was 3/1.
The spectral band having peaks at around 300, 320 and 350 nm and a shoulder at around 370
nm originates from the naphthoxazine ring moiety of SNO [29−31].
In addition, the
spectrum observed just after the preparation (at time 0) exhibited a band at 530 nm assigned
to the species protonated at the naphtholate part of the MC (MCH+) [24,32,33]. This
assignment was confirmed by the spectral change in the SNO ethanol solution containing
hydrochloric acid as shown below. The proton assisted in the C−O bond cleavage and ring
opening of the SNO. There is the possible formation of the protonated SNO (SNOH+) and its
fast thermal isomerization to MCH+.
(Fig. 1)
The band intensity at 530 nm increased and then decreased. With this decrease, a band at
450 nm became evident due to the aluminum-chelation of MC. Most of the aluminum ions
are expected to be solvated in the ethanol solution and form hydrogen-bonded complexes
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such as [Al(EtOH)4]3+ [34]. The proton was provided by the reaction of Al3+ with a slight
amount of water contained in the ethanol solvent, and then some aluminum ions formed
[Al(OH)(EtOH)3]2+ [35]. The protonated species was slowly transformed into the aluminum
chelate complex of MC (MC-Al3+) by the ligand exchange of [Al(EtOH)4]3+ for MC. Under
this condition, the reaction rates were faster and the amounts of products were greater than in
the dark although the spectral peak positions and the patterns of increase and decrease in their
intensities were similar to those in the dark. It was previously confirmed that SNO directly
formed the MC-Al3+ complex without forming the protonated species during the UV
irradiation [24]. The protonation and aluminum chelation of SNO easily proceeded under the
present conditions. The chelation was assisted by the addition reaction of protons and
subsequent ring-opening.
The fluorescence and excitation spectra observed as a function of the UV irradiation time
are shown in Fig. 2. Just after the preparation, the peaks of the fluorescence and excitation
spectra observed at 425 nm and 295 and 357 nm, respectively, were assigned to Xh. The
fluorescence lifetime was reported to be 1.1 ns in methanol [36]. The reference stated that
SNO was transformed into MC via an excited state of the intermediate species, which was
non-fluorescent. The solvent-stabilized fluorescent state was formed from the high-energy
excited state of the intermediate species by the vibronic relaxation. The excited state lifetime
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of the solvent-stabilized fluorescent Xh* was determined to be ca. 1 ns, whereas that of the
non-fluorescent Xh* decayed and formed MC within a few ps [36]. The excitation band was
shifted to the longer wavelength side with an increase in the UV irradiation time even though
the related fluorescence band had slightly changed. The set of fluorescence and excitation
bands observed after the UV irradiation are similar to those observed in aprotic polar solvents,
indicating the existence of the certain ground-state species [14]. Therefore, it is suggested
that Xh was transformed into the protonated Xh species (XhH+), i.e., the certain intermediate
state between SNOH+ and MCH+ in the ground state was allowed by the interaction with a
proton. This change was not reflected in the absorption spectra because the amounts of such
intermediate protonated species were much lower than that of SNO. The peaks of the
fluorescence and excitation spectra at 540 nm and 462 nm, respectively, assigned to MC-Al3+,
were observed after the UV irradiation and their intensities increased with an increase in the
irradiation time. No fluorescence was assigned to MCH+ because it is a non-fluorescent
species.
(Fig. 2)
Reaction of SNO with Acid
Changes in the UV-vis absorption spectra were investigated in order to examine the acidic
influence on the SNO behavior in the ethanol solutions. The band assigned to MCH+ was
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observed at 530 nm and its intensity increased with time in the acidic SNO ethanol solution in
which the molar ratio of SNO to H+ was 3/1 as previously reported [24]. Fig. 3 shows the
change in the UV-vis absorption spectrum of the SNO ethanol solution containing
hydrochloric acid in the dark and during the UV irradiation. The molar ratio of SNO to H+
was 3/100. The spectral change was very similar to that of the solution containing Al3+. The
spectrum observed at 0 exhibited a band at 530 nm assigned to MCH+. The band intensity at
530 nm increased and then decreased. The MCH+ band intensity was much higher than that
observed in the solution containing Al3+ due to a higher number of protons. Furthermore, the
maximum MCH+ band intensity observed during the UV irradiation was much greater than
that observed in the dark. The band intensity at 450 nm increased with the 530-nm band
decrease. This band can be assigned to a complex of MC with a proton and water molecule,
MC-H3O+, having a structure similar to the metal chelate complex of MC (MC-Mn+). The
structure of this complex is presumed to be very similar to the MC-SiOH complex observed
in the silica xerogel [16–18].
(Fig. 3)
The time course of the absorbance values for the peaks of MCH+ and MC-H3O+ in Fig. 3 is
plotted in Fig. 4. The reactions of SNO with acid are expressed by the following equations:
k1
SNO + H+ → MCH+
(1)
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k2
MCH+ + H2O → MC-H3O+
(2)
(3)
(4)
k3
SNO + H+ + H2O → MC-H3O+
k4
MC-H3O+ → Products
Reaction (3) in which SNO photochemically forms MC-H3O+ was suggested to be similar to
the MC-Al3+ formation as shown in a previous study [24]. Reaction (4) should be considered
because the other product resulted from MC-H3O+ as stated below. The reaction rate
constants of these four reactions, k1, k2, k3, and k4, were estimated by the first order analysis of
the time course of the absorbance values at the main peaks, 450 and 530 nm, as shown in
Table 1. The concentration of H+ is regarded as being much higher than that of SNO. The
analysis determined the molar extinction coefficient values for the main peaks of each species
obtained in a previous study [24]. The molar extinction coefficient for MC-H3O+ was
assumed to be equal to that for MC-Al3+ in order to compare the reactions of the solution in
the dark and during UV irradiation because the structures of their π conjugated systems are
similar to each other.
(Fig. 4 and Table 1)
The UV irradiation promoted all the reactions by comparison between the reaction rate
constants in the dark and during UV irradiation. The constants observed during the UV
irradiation were 2–6 times higher than those observed in the dark. The changes in the
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concentration of each species and in the number of absorbed photons were complicated as the
reaction proceeded. The rate constants, which were not strict, but apparent values, were
clearly increased by the UV irradiation. With respect to reaction (1), the photoisomerization
from SNO to MC induced the forming of MCH+ because the quantum yield of the
photoisomerization is reported to be a certain value, 0.2−0.4 [37,38]. With respect to reaction
(2), MCH+ absorbs some UV light, which can thermally induce the MC-H3O+ formation to
some extent after the internal conversion because this reaction is suggested to be a ground
state process [24]. The constants for the photochemical process of the MC-H3O+ formation,
k3, during UV irradiation were not significantly higher than those of the other processes. This
is because the steady state UV irradiation also promoted the dissociation of MC-H3O+ to form
MCH+ and consequently decreased the apparent reaction constants of the MC-H3O+
formation.
The changes in the fluorescence and excitation spectra are shown in Fig. 5. These spectra
clearly coincided with those observed in the SNO ethanol solution containing Al3+ during the
UV irradiation for at least a few hours. The fluorescence band at around 425 nm then shifted
to the longer wavelength side and a new excitation band at 333 nm became evident, indicating
the production of new species as explained below.
(Fig. 5)
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The relationships between the absorbance and fluorescence intensities of the MC-Al3+ and
MC-H3O+ complexes were evaluated in order to distinguish their fluorescence properties.
The fluorescence intensity values at 540 nm for the solutions containing SNO and Al3+
(SNO/Al3+ = 3/1) and SNO and H+ (SNO/H+ = 3/100) are plotted versus the related
absorbance values in Fig. 6. The fluorescence quenching due to the self-absorption by the
MCH+ species can be negligible under the present conditions. The slope for the SNO–Al3+
system was ca. five times greater than that for the SNO–H+ system, indicating that the
fluorescence quantum yield of MC-Al3+ is higher than that of MC-H3O+. This is a result
similar to that observed in the sol–gel reaction system of silicon alkoxide, i.e., the
fluorescence quantum yield of MC-Mn+ was higher than that of MC-SiOH and depended on
the metal ion [18]. The fluorescence process of the metal complex depends on the electron
affinity of the metal ion because the metal ion occupies the unshared electron pair on the
imine N which quenches the iminonaphtholate fluorescence [8].
(Fig. 6)
MCH+ rapidly returned to SNO within 1 min when neutralized. The rate constant of this
process was estimated to be >5 × 10−2 s−1. Also, the MC-H3O+ complex can be dissociated
by adding base. MC-H3O+ was gradually dissociated and reproduced SNO when neutralized
with the rate constant 1.45 × 10−4 s−1. The reverse processes of Reactions (2) and (3), which
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were much faster than the regular processes, cannot be distinguished due to the very fast
reverse process of Reaction (1). Fig. S1 shows the UV-vis absorption, fluorescence, and
excitation spectra of the solution allowed to react for 48 h in the dark observed before and
after adding the sodium hydroxide solution. The absorption band due to MCH+ and MC-
H3O+ disappeared within 1 min and the absorbance due to SNO increased by adding base until
the pH value reached 12. With this behavior, the fluorescence and excitation bands assigned
to MC-H3O+ decreased and those assigned to Xh increased. The spectral red-shift compared
to the original Xh spectra is due to the solvent effect by adding an aqueous sodium hydroxide
solution. The dissociation of MC-H3O+ was confirmed to occur by adding base.
Decomposition of SNO
The fluorescence and excitation spectra of the SNO ethanol solution with hydrochloric
acid, which was allowed to react for 24 h, is compared to those of 1-amino-2-naphthol in Fig.
7. In the acidic SNO ethanol solution, the fluorescence peak was observed at 360 nm upon
excitation at 287 nm and the excitation peaks were observed at 266, 276, 287, and 333 nm
when monitoring the 365-nm fluorescence. In addition, the peaks of the fluorescence spectra
were also observed at around 440 nm, which were red-shifted compared to the 425-nm peak
assigned to Xh.
The related excitation band was also observed at 350–400 nm when
monitoring the 445-nm fluorescence. The 540-nm fluorescence peak is due to MC-H3O+
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These bands, except for the 540-nm fluorescence, are very similar to those of 1-amino-2-
naphthol, which exhibits fluorescence and excitation peaks at 357 nm and at 263, 276, 286,
and 333 nm, respectively, assigned to the neutral species of 1-amino-2-naphthol [39,40].
Furthermore, the fluorescence peak at around 450 nm and the related excitation band at 350–
400 nm are assigned to the anion species. These results indicated that SNO was decomposed
and produced 1-amino-2-naphthol. In addition to this, indoline compounds are also expected
to be produced.
(Fig. 7)
The GC-MS and FTIR analyses were conducted in order to confirm the decomposition of
SNO and determination of the products. These results are shown in Figs. S2 and S3,
respectively. Three chromatogram peaks were observed and they were assigned to SNO or
MC, 1-amino-2-naphthol, and an indoline compound based on the related mass spectra. The
FTIR spectra of the powder prepared from Solution A and the 1-amono-2-naphthol powder
are quite similar and some characteristic peaks are observed in both of these spectra. On the
other hand, the peak at 1737 cm–1 is only observed in the spectrum of the powder prepared
from Solution A. This peak was assigned to the C=O stretching vibration of the carboxyl
group. Therefore, it is presumed that the indoline compound contained a carboxyl group.
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The reaction process of SNO with acid is shown in Scheme 3. SNO is protonated in an
acidic solution, forms MCH+, and then is transformed into MC-H3O+ by associating with a
water molecule. In the present systems, a number of water molecules existed around these
species. The water molecules were omitted in the molecular forms in the scheme. However,
an important water molecule to determine the molecular form was shown in the MC-H3O+
form. In a highly acidic solution, the proton catalyzes the isomerization of MC-H3O+
followed by a water molecule addition to the C=N bond, which is cleaved to produce 1-
amino-2-naphtholate and 1,3,3-trimethylindoline carboxylic acid. This process is similar to
the well-known hydrolysis of imines. The negative charge on each atom in MC and MCH+
was calculated by the molecular orbital method.
Their ground state structures were
optimized at the DFT level using the B3LYP functional and the 6-31G basis set. The
Mulliken charge on the oxygen atom of MC was estimated to be −0.494 and it was reasonably
protonated. The charge on the nitrogen atom bonding to the naphthalene ring of MCH+ was
estimated to be −0.442. This nitrogen atom can act as a Schiff base and hydrogen-bond to
water molecule.
The compounds, 1-amino-2-naphtholate and 1,3,3-trimethylindoline
carboxylic acid, were reasonably produced in this study. This reaction having the constant k4
was clearly promoted by 350-nm light irradiation as shown in Table 1. This process can be
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thermally induced after the internal conversion because the absorption band at around 350 nm
is not due to its main π-conjugated skeleton, which exhibits the 450-nm absorption.
(Scheme 3)
Conclusions
The reactions of SNO in an ethanol solution containing hydrochloric acid were observed in
the dark and during UV irradiation by spectroscopic measurements in order to reveal the
interaction between SNO and protons.
SNO was protonated to form MCH+ and then
transformed into a complex in the acidic ethanol solution in the dark. It is presumed that the
complex contains a hydrated proton and its structure is similar to that of the metal complex of
MC, MC-H3O+, based on its UV-vis absorption and fluorescence spectra. The fluorescence
quantum yield of MC-H3O+ was lower than that of MC-Al3+ because the fluorescence process
of such complexes depends on the electron affinity of the metal ion, which occupies the
unshared electron pair on the imine N which quenches the fluorescence. The complex
dissociated and reproduced SNO by adding base. The complex was further changed under
the acidic conditions after a long time. Highly concentrated acid induced the decomposition
of SNO to the indoline compound with a carboxyl group and 1-amino-2-naphthol, which was
revealed by fluorescence, IR, and GC-MS analyses. The first protonation process was
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photochemically promoted, while the others were thermally promoted by UV irradiation.
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Scheme titles and figure captions
Scheme 1 Isomerization and metal chelation of SNO.
Scheme 2 Fluorescence processes of SNO isomers in solvents. The bold arrows indicate
preferential processes.
Scheme 3 Reaction process of SNO with acid.
Figure 1 Change in UV-vis absorption spectrum of the SNO ethanol solution with Al3+
during UV irradiation, in which the molar ratio of SNO to Al3+ was 3/1.
Figure 2 Change in (a) fluorescence and (b) excitation spectra of the SNO ethanol solution
with Al3+ during UV irradiation, in which the molar ratio of SNO to Al3+ was 3/1. The
fluorescence spectra were observed upon excitation at 370 nm. The excitation spectra were
observed at 445 and 540 nm.
23
Page 23 of 38

Figure 3
Change in UV-vis absorption spectrum of the SNO ethanol solution with
hydrochloric acid (a) in the dark, and (b) during the UV irradiation, in which the molar ratio
of SNO to H+ was 3/100.
Figure 4 Time course of the absorbance values for the peaks of MCH+ (530 nm) and MC-
H3O+ (450 nm) observed in the SNO ethanol solution with hydrochloric acid (a) in the dark,
and (b) during the UV irradiation, in which the molar ratio of SNO to H+ was 3/100.
Figure 5 Change in (a) fluorescence, and (b) excitation spectra of the SNO ethanol solution
with hydrochloric acid during the UV irradiation, in which the molar ratio of SNO to H+ was
3/100. The fluorescence spectra were observed upon excitation at 370 nm. The excitation
spectra were observed at 445 and 540 nm.
Figure 6 Fluorescence intensity values at 540 nm for the solutions containing SNO and Al3+
(SNO/Al3+ = 3/1) and SNO and H+ (SNO/H+ = 3/100) plotted versus the related absorbance
values.
24
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Figure 7 Fluorescence and excitation spectra of (a) the acidic SNO ethanol solution allowed
to react for 24 h during UV irradiation compared to those of (b) 1-amino-2-naphthol. The
fluorescence spectra were observed upon excitation at (1) 287, and (2) 370 nm. The
excitation spectra were observed at (3) 365, and (4) 445 nm.
25
Page 25 of 38

Scheme 1
n+
+Mn+
Mn+
Spirooxazineform (SNO) Merocyanine form (MC)
colorless blue
MC-M complex
yellow
hn (UV)
fluorescence
fluorescence
(lmax = 390 nm)
(lmax = 540 nm)
hn (vis)
or D
(in nonpolarsolvents)
fluorescence
(lmax = 420-440 nm)
(in polarsolvents)
Intermediatespecies (Xs and Xh)
Page 26 of 38

Scheme 2
SNO*
Xs*
Xh*
MC*
MC
MC*
MC
F3
E1
F1
F2
E2
Xs
in aproticpolarsolvents
Xh
SNO
in proticpolar solvents
Page 27 of 38

Scheme 3
+H+
+H2O
SNO
MCH+
MC-H3O+
Isomers of MC-H3O+
+H2O
+
Page 28 of 38

Figure 1
2
24 h
18 h
12 h
8 h
0.1 h
0
1
0
0.1 h
8 h
12 h
300
400
500
Wavelength / nm
600
700
Page 29 of 38