Spectroscopic and photophysical properties of salicylaldehyde azine (SAA) as a photochromic Schiff base suitable for heterogeneous studies

Article abstract of DOI:10.1016/j.cplett.2008.09.030

The photochromic cycle of the salicylaldehyde azine (SAA) has been investigated by means of the stationary and time-resolved UV-vis spectroscopy in a number of differently interacting solvents and micellar systems. The primary enol form of this Schiff base is much more energetically stabilized than in the other aromatic Schiff bases, which is reflected in remarkable resistance of SAA to the hydrolysis process. The fluorescence decay measured in highly viscous solvents as well as in micellar systems has proved the existence of different conformers of the cis-keto tautomer. Three different routes of the photochrome (trans-keto tautomer) decay have been also observed.

Full text of DOI:10.1016/j.cplett.2008.09.030

Chemical Physics Letters 464 (2008) 181–186
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Chemical Physics Letters
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Spectroscopic and photophysical properties of salicylaldehyde azine (SAA) as a
photochromic Schiff base suitable for heterogeneous studies
a,c,*
b
b,c
Marcin Ziółek
, Katarzyna Filipczak , Andrzej Maciejewski
a
Quantum Electronics Laboratory, Faculty of Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan, Poland
Photochemistry Laboratory, Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland
Center for Ultrafast Laser Spectroscopy, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan, Poland
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 May 2008
In final form 15 September 2008
Available online 19 September 2008
The photochromic cycle of the salicylaldehyde azine (SAA) has been investigated by means of the station-
ary and time-resolved UV–vis spectroscopy in a number of differently interacting solvents and micellar
systems. The primary enol form of this Schiff base is much more energetically stabilized than in the other
aromatic Schiff bases, which is reflected in remarkable resistance of SAA to the hydrolysis process. The
fluorescence decay measured in highly viscous solvents as well as in micellar systems has proved the
existence of different conformers of the cis-keto tautomer. Three different routes of the photochrome
(trans-keto tautomer) decay have been also observed.
Ó 2008 Elsevier B.V. All rights reserved.
1
. Introduction
iting a characteristic, strongly Stokes shifted, fluorescence. It has
been concluded that despite the two possible proton transfer cen-
tres, the excitation of symmetric Schiff bases in solution is local-
ized on one salicylidene subunit and the single proton transfer
occurs [3,4,17]. Also, the semi-empirical calculations for SAA pre-
dict the excited state single proton transfer rather than the double
one [15]. Therefore, for simplicity, the excited SAA monoketo-tau-
The best known photochromic Schiff base, salicylideneaniline
SA), as well as the related compounds for many decades attracted
(
much interest [1–9] and nowadays are widely studied because of
possible applications, e.g. in molecular memories and switches
[
10,11]. The salicylaldehyde azine (SAA), see Fig. 1, is the smallest
possible symmetric aromatic Schiff base (with two hydrogen-
bonding centres) belonging to the ‘SA family’.
tomer will be called ‘keto’ throughout the article. The lifetime of
*
the S
1
(p,p
) state of SAA cis-keto tautomer equals 19 ps in acetoni-
According to our knowledge, there are only a few reports on
SAA properties in solid state [12–15]. The SAA molecules are nearly
planar with strong intramolecular hydrogen bonds and are packed
in the plane-to-plane stacks within the crystal, which accounts for
its thermochromic properties [12,13]. Even fewer reports have
been published on SAA properties in solution [16–18] in which
state it reveals the photochromic behaviour (like all other Schiff
bases). In acetonitrile solution, the lifetime of the ground state of
the photochrome (trans-keto tautomer) was found to be nearly
two orders of magnitude shorter than that for the other aromatic
Schiff bases [17].
trile [18]. Finally, after the structural changes in this tautomer
(involving the cleavage of the intramolecular hydrogen bond)
about 20 ± 10% of excited molecules are transferred to the long-
lived ground state of the photochromic form (probably trans-keto
tautomer, see Fig. 1) [18].
In this Letter, we present and discuss the solvatochromic mea-
surements of the spectroscopic and photophysical properties of all
the SAA tautomers in 15 differently interacting homogeneous
solvents and three different micro-heterogeneous micellar sys-
tems, performed by means of the stationary and time-resolved
UV–vis spectroscopy. In the three parts of Section 3, the properties
of the enol, cis-keto and photochrome (trans-keto) tautomer are
investigated.
Recently, we have reported the studies of SAA in femto- and
picosecond time domains in the same solvent and found that the
results are very similar to those obtained for SA [18]. After the exci-
*
tation of the enol form (Fig. 1) to the first singlet state S
1
(p
,p
) an
2
. Experimental
ultrafast excited state intramolecular proton transfer reaction
takes place with the characteristic time below 50 fs [18]. As a re-
SAA was synthesized by condensation of hydrazine with salicyl-
*
sult, a cis-keto tautomer (Fig. 1) in S
1
(p
,p
) state is created exhib-
aldehyde and was recrystallized from ACN. The solvents used were
the same as in our previous paper [19], their abbreviations are
given in Table 1 and their solvatochromic parameters (Kamlet-Taft
*
Corresponding author. Address: Quantum Electronics Laboratory, Faculty of
[
20] and Catalán [21] scale) are presented in Table S1 (in Supple-
Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan, Poland.
E-mail address: marziol@amu.edu.pl (M. Ziółek).
mentary material). The concentration of SAA was between
0
009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2008.09.030

1
82
M. Ziółek et al. / Chemical Physics Letters 464 (2008) 181–186
H
N
The equipment for stationary and time-resolved measurements
was the same as described in our previous papers [18,19,24]. The
stationary UV–vis absorption spectra were measured with a UV–
VIS-550 (Jasco) spectrophotometer and fluorescence emission
spectra were recorded with a modified SPF-500 (Aminco–Bow-
man) spectrofluorimeter. The time-resolved emission measure-
ments (time correlated single photon counting) were carried out
at the magic angle and the pump wavelength was 380 nm (Ti: Sap-
phire laser). The pump pulse wavelength in the nanosecond tran-
sient absorption set-up was 355 nm (Q-switched Nd:YAG laser),
the pump pulse energy was about 1 mJ and the kinetics were re-
corded in the spectral range 300–600 nm for every 25 nm. All mea-
surements were performed at room temperature.
O
H
C
C
H
N
H
O
H
N
O
H
C
C
H
N
H
O
O
3. Results and discussion
H
N
O
H
C
3.1. Enol tautomer
N
The stationary absorption spectrum, in particular the long
wavelength maximum at around 360 nm, originating from the
C
H
H
*
transition S
1
S
0
(p
,p
) of the primary enol form is very similar
Fig. 1. Formula of SAA and its transient tautomeric forms. From up to down: enol,
cis-keto and trans-keto (photochrome) tautomers.
for all solvents except the most protic ones (TFE and HFIP), see
Fig. 2 and Table 1. Although the correlation with the solvatochro-
mic scales is not very good (Table S2) the fit to Kamlet-Taft or
Catalán scales reflects the main dependence of the maximum on
different kinds of solvent interactions. Increasing the polarity of
the solvent results in a slight blue shift, the proton accepting prop-
erties (in terms of hydrogen-bonding ability) have no influence on
the maximum, while the proton donating abilities of the strong
alcohols cause a relatively significant blue shift, see Fig. 2a. An in-
crease in the solvent polarity as well as proticity results also in a
greater absorbance at the minimum near 320 nm (Fig. 2a). The
slight red shift of the maximum with increasing refractive index
(and thus the polarizability function) for non-polar hydrocarbons
(Table 1 and Fig. S1) is probably a manifestation of the role of dis-
persion interactions. It should be noted that the SAA cation mea-
sured in TFA has a much different absorption spectrum (Fig. S2)
with the maximum at around 400 nm and, therefore, the blue shift
of the absorption maximum in TFE and HFIP is certainly not due to
the protonation of SAA. Moreover, the SAA anion measured in
ꢁ5
ꢁ4
1
ꢀ10 and 1 ꢀ10 M. In this range no changes in the absorp-
tion and emission spectra as well as in the fluorescence quantum
yields were observed (checked in HEX and HEXD). Cetyltrimethyl-
ammonium bromide (CTAB, Fluka), Triton-X 100 (TX, Aldrich) and
sodium dodecyl sulphate (SDS, Aldrich) were used as micelle form-
ing surfactants. CTAB forms cationic, SDS anionic and TX non-ionic
micelles in aqueous solutions. The concentration of the surfactants
in water (purified and deionised) was 0.15 M. The maximum con-
centration of SAA in the micellar solution (reached after several
ꢁ4
hours of stirring) was about 1 ꢀ10 M. Taking into account that
ꢁ7
SAA is almost insoluble in water (<2 ꢀ10 M) it can be safely as-
sumed that all of the SAA molecules are inside the micelles and not
more than one SAA molecule per micelle is present in the above
conditions [22,23] (see also the calculations in the Supplemen-
tary material).
Table 1
F
Photophysical properties of SAA in different solvents: maximum of stationary absorption (kmax), maximum of stationary emission (mmax), fluorescence quantum yield (u ),
fluorescence lifetime (
s
) and radiative rate constant (k
R
)
max (cm 1) (exc. 360 nm)
ꢁ
u
(ꢀ10ꢁ3) (exc. 360 nm)
s
(ps)a (exc. 380 nm)
k
= u
/
s
s
Solvent
Absorption kmax (nm)
Emission
m
F
R
F
(±100 cm 1)
ꢁ
(±20%)
(±2 ps)
(ꢀ10
7
ꢁ1
)
(±1 nm)
n-Hexane (HEX)
Nonane (NON)
Hexadecane (HEXD)
Squalane (Sq)
361
362
363
363
359
355
356
355
357
350
345
17 300
17 200
17 200
17 150
17 100
17 000
17 250
17 450
17 100
17 450
17 500
2.3
3.7
4.5
3.9
1.6
0.7
0.6
0.7
2.6
1.6
2.7
76
3.0
130
140b
50
3.5
1
-Chloropropane (ClP)
3.2
3.7
Acetonitrile (ACN)
Ethanol (EtOH)
Methanol (MeOH)
Ethylene glycol (EG)
Trifluoroethanol (TFE)
Hexafluoroisopropanol
19
17
4.1
b
76
47
67
3.4
4.0
(
HFIP)
Diethyl ether (DEE)
Triethylamine (TEA)
TX micelle
SDS micelle
CTAB micelle
358
359
360
357
360
404
17 000
17 350
17 300
17 250
17 200
18 500
1.1
2.1
4.5
2.7
6.2
5.6
200b
b
110
240b
NaOH in water
Trifluoroacetic acid (TFA) 398
a
2
The fitting quality
v
< 1.2.
Average from multicomponent fit: h
s
i= P
A
i
s
i
/P
A (A is the amplitude of ith component of time constant s ), for the detailed data, see Table 2.
i i i
b

M. Ziółek et al. / Chemical Physics Letters 464 (2008) 181–186
183
for SAA the hydrolysis process does not occur (see Figs. S3 and
S4). This confirms the stability of the SAA enol form and makes this
molecule a very good candidate for studying the photochromic
Schiff bases in micro-heterogeneous micellar systems (formed in
aqueous solution) as well as in mesoporous and microporous
materials in which the traces of water are often hard to remove.
Having in mind this remarkable feature of SAA we performed its
studies in water solutions of micelle forming surfactants. The sta-
tionary absorption spectra of SAA in TX, CTAB and SDS, presented
in Fig. 2b, are similar to that in homogeneous solutions (note that
in TX the surfactant absorbs up to 320 nm) and the shape of the
absorption spectrum indicates that the SAA molecules are located
in the polar part of the micelles. In the spectrum of CTAB additional
long-wavelength band occurs. The profile of this band is close to
the absorption spectra of SAA cation and anion and, thus, its prob-
able explanation is the ion formation due to the interaction of the
SAA molecules with CTAB or even acid or basic impurities.
3.2. cis-keto tautomer
Due to the ultrafast proton transfer reaction [18] the short-
wavelength emission of the primary enol form is very weak and
only the long-wavelength fluorescence with a maximum around
5
60–580 nm originating from the cis-keto tautomer was investi-
gated in this study. The spectral position of the cis-keto stationary
emission is nearly the same for all solvents, while the fluorescence
quantum yield varies within one order of magnitude, see Table 1.
The maximum of the emission spectrum is red shifted in polar
non-protic solvents, and in these solvents the smallest fluores-
cence quantum yield is observed. Changing the environment to
both the non-polar and more protic ones results in an increase in
the quantum yield and a small blue shift of the fluorescence profile
(Figs. S5 and S6). In hydrocarbons the fluorescence quantum yield
increases with increasing polarizability function and/or viscosity of
the solvent. Like we previously reported for SAA in ACN [18], the
fluorescence excitation spectra measured in all other solvents at
Fig. 2. Stationary absorption spectra of SAA in homogeneous solvent (a) and
micellar solutions (b). The spectrum in HEXD is also added to (b) to show the
differences between the spectrum in micelles and hydrocarbons.
the cis-keto emission match the long-wavelength part of the
*
S
1
S
0
(p,p
) enol tautomer absorption spectrum (see Fig. S7).
The fluorescence decay in all homogeneous solvents studied ex-
cept Sq and EG is mono-exponential and the lifetime varying from
7 to 130 ps is consistent with the fluorescence quantum yield
(within the limit of its determination). The average radiative rate
1
ꢁ3
NaOH (5 ꢀ10 M) solution in water has the absorption spectrum
similar to that of the cation. Thus, despite the strong basicity of
TEA, no anion is formed in this solvent (Fig. S2).
7
ꢁ1
constant for all solvents equals (3.5 ± 0.5) ꢀ10 s . However, in
much more viscous non-polar (Sq) and polar (EG) solvents the suf-
ficient quality of the fit can be obtained only assuming that the
fluorescence decay is two-exponential (Table 2). It should be noted
that two-exponential decay was also observed previously for SA in
polar and viscous cyclohexanol [2,8].
In the absorption spectra of the other photochromic Schiff bases
an additional long-wavelength band was observed in protic sol-
vents (its intensity increased with the solvent proticity) ascribed
*
to the S
1
S
0
(p
,p
) transition of the cis-keto tautomer stabilized
by intermolecular hydrogen bond with the solvent [19,24]. For
SAA, however, no indication of such a band is found even in HFIP,
which means that the energy of the enol tautomer is much lower
than that of the cis-keto tautomer and any enol-keto equilibrium
in the ground state cannot be formed. The probable explanation
of this fact is the presence of a stronger intramolecular hydrogen
bond in the enol form of SAA, following from the enhanced basicity
of the nitrogen atom, due to the absence of the conjugation with
the aromatic ring bound to it [25]. It is our first evidence that the
SAA enol form in solution is much more energetically stabilized
than the other photochromic Schiff bases.
As presented in Table 2, we have also found two- or even three-
exponential fluorescence decays for SAA in micellar systems. The
shortest component of about 60 ps is similar in TX, SDS and CTAB,
while the duration of the longer one significantly changes for dif-
ferent surfactants. The contribution of the short component
slightly decreases with increasing wavelength. As evidenced in
the stationary absorption spectrum (see Fig. 2) the shortest life-
time observed in SDS can be rationalized by the presence of SAA
molecules in a more polar micro-environment in this micellar sys-
tem than in TX and CTAB. The multi-exponential fluorescence de-
cay observed in micelles and high-viscous solvents can be
It is a known problem for Schiff bases that even small impurities
of water can cause the process of their hydrolysis in protic environ-
ment [19,26,27]. For example, the water attack on the SA molecule
causes its decomposition into salicylaldehyde and aniline [27]. We
have measured the temporal absorption changes for the photo-
chromic Schiff bases (studied by us previously [18,19,24]) in the
rationalized by the existence of at least two conformers of cis-keto
*
tautomer which have similar S1 S0 (
p,p
) emission spectra. Some
details of the justification of this hypothesis are presented in the
Supplementary material.
It is most likely that the origin of the third and the longest fluo-
rescence decay component of duration of about 1.5 ns observed in
CTAB is different. The contribution of its amplitude decreases with
2
H O:MeOH mixtures (volume ratio 1:4) and have found that only

1
84
M. Ziółek et al. / Chemical Physics Letters 464 (2008) 181–186
Table 2
recovery time constants differ by three orders of magnitude. No ef-
i i
Parameters (amplitudes a and lifetimes s ) of multi-exponential fit of SAA fluores-
fect of oxygen was observed (checked in ACN) which means that
the transient absorption from the triplet state does not contribute
to the signals measured. The trans-keto tautomer lifetimes are gen-
erally about 10 times smaller than that of the other photochromic
Schiff bases in the same solvents [4,6,7,19]. Significant residual
negative signals of the depopulation band were frequently ob-
served for the Schiff bases and explained by the anti–syn isomeri-
sation within the enol form competing with the photochromic
cycle [5,7,19] while in SAA the final transient absorbance of the
ground state recovery represents no more than 10% of the initial
transient absorbance in all solvents. Thus, the anti–syn isomerisa-
tion (around the N@C bond) of the enol tautomer plays a minor
role in the deactivation scheme. These two facts (faster decay
and small residual signals) confirm that the ground state of initial
enol form (anti-enol) is more stable relative to the other tautomers,
than in the other aromatic Schiff bases studied so far.
cence in high viscous solvents and micellar systems
Solvent/
micelle
Emission
wavelength
s
1
(ps)
a
1
s
2
(ps)
a
2
s
3
(ps)
a
3
(±10 ps)
(±20 ps)
(±100 ps)
(nm)
Sq
550
90
90
90
0.29 160
0.29 160
0.28 160
0.71
0.71
0.72
–
–
–
–
–
–
575
00
6
EG
550
60
60
60
0.66 110
0.65 110
0.65 110
0.34
0.35
0.35
–
–
–
–
–
–
575
00
6
TX
550
00
60
60
0.33 230
0.29 230
0.58
0.62
500
500
0.09
0.09
6
SDS
525
60
60
60
60
0.64 160
0.57 160
0.54 160
0.51 160
0.36
0.43
0.46
0.49
–
–
–
–
–
–
–
–
550
575
600
CTAB
525
60
60
60
60
0.54 320
0.49 320
0.46 320
0.45 320
0.41 1500
0.48 1500
0.51 1500
0.53 1500
0.05
0.04
0.03
0.02
The decay pattern of the transient absorption of SAA was inves-
tigated by fitting a model function to the experimental data at par-
ticular wavelengths and in a global multi-exponential analysis
550
575
600
ꢁ5
approach. For SAA in the concentration of 5 ꢀ10 M the half-lives
The amplitudes are normalized to 1.
are 8 s in HEX and 95 s in ACN, which reflects greater stabiliza-
l
l
tion of the trans-keto tautomer in polar solvents [6,7]. In both sol-
vents at a constant pump intensity, the lifetime increases in less
concentrated solutions. This can be rationalized by the deactiva-
tion process that involves the complex of two SAA molecules in
the trans-keto tautomer and, similarly as for related systems
increasing wavelength. Thus, this decay component should be
probably assigned to the emission of SAA ion which is present in
CTAB (see previous section) and its fluorescence band is signifi-
cantly blue shifted with respect to SAA cis-keto tautomer, see Table
[
6,28,29] the second-order double proton transfer reenolization
1, Figs. S6 and S8.
can be proposed, see Fig. S14.
However, both pure first-order as well as pure second-order
models (exponential and reciprocal functions, respectively, see
Eqs. (S4) and (S5) in theSupplementary material) fail to adequately
depict the kinetics measured in HEX and ACN. Therefore, a mixed
first and second order case was assumed and then the kinetics
can be expressed as follows [30]:
3
.3. trans-Keto tautomer
The transient absorption studies of the SAA photochrome
(trans-keto tautomer) in the nano- and microsecond time scale
were performed in HEX, ACN, MeOH, TFE, HFIP and the solutions
of three micelle forming surfactants. The shape of the transient
absorption spectra is similar in all solvents (a positive band with
a maximum at around 475 nm assigned to the absorption from
1
ꢁm
D
AðtÞ¼A ek1 t
þA
0
;
ð1Þ
ꢁm
the S
00 nm due to the ground state depopulation of the initial enol
tautomer, see Fig. 3) while the transient decay and ground state
0
state of the trans-keto tautomer and a negative band below
4
0
where A and A denote the amplitudes of the decay and the constant
offset signal, k
1
denotes the first-order rate constant (rate-deter-
mining back trans–cis isomerisation), m = 2c /(k + 2c ) is the
shape parameter (m = 1 for pure second-order reaction and m = 0
for pure first-order reaction), c is the concentration of the trans-
keto tautomer and k is the second-order rate constant. Eq. (1) very
P
k
2
1
P 2
k
P
2
well describes the kinetics observed for ACN (Fig. 4). The triplet
state of benzophenone [31] was used as the reference for the calcu-
lation of the extinction coefficients of the SAA transients. When the
stationary absorption band is normalized to the initial negative
band (Fig. 3) the absorption coefficient of the depopulation band
ꢁ
1
ꢁ1
at 350 nm is equal to about 2300 M cm , while the stationary
ꢁ1
ꢁ1
absorption coefficient for SAA is 23000 M cm near this wave-
length [18]. This means that about 10% of the molecules are trans-
P
ferred to the trans-keto tautomer, thus c = 0.1c, where c is the SAA
concentration. Having this in mind, the fitted rate constants (Fig. 4)
3
ꢁ1
9
ꢁ1 ꢁ1
are k
1
= 1.5 ꢀ10 s and k
2
= 0.7 ꢀ10 M
s
in ACN, while the
ꢁ
5
parameter m is: m = 0.82 for c=5 ꢀ10 M, m = 0.70 for c = 2.5 ꢀ
ꢁ5
ꢁ5
1
0
M and m = 0.50 for c = 1.1 ꢀ10 M.
For SAA in HEX the trans-keto tautomer decay kinetics is also
1 2
well described by Eq. (1), however the fitted values of k and k
are not constant for different concentrations. Since the decay is fas-
ter by approximately one order of magnitude than in ACN, a possi-
ble explanation is that the k
2
value is close to the diffusion rate
= 8RT/3
= 2 ꢀ10
) and therefore, the reaction cannot be described by
the simple second-order kinetics [32].
Fig. 3. Transient absorption of SAA in ACN (c = 5 ꢀ10ꢁ5 M) for selected time delays.
constant in HEX at room temperature (k
d
g
ꢁ
1 ꢁ1
1
0 M
s
The stationary absorption spectra normalized to the measured initial depopulation
bands are also shown.

M. Ziółek et al. / Chemical Physics Letters 464 (2008) 181–186
185
ler than that of CTAB and TX ones [22,23] and the free volume for
the deactivation mechanism is more restricted, therefore the SAA
trans-keto tautomer lifetime might be longer in SDS. In alcohols
and micelles an extra transient absorption feature with a maxi-
mum between 400 and 425 nm can be also recognized as an addi-
tional component of minor amplitude and lifetime in the range of
single ls (Figs. S11 and S13). Its possible origin is the open trans-
enol structure (with respect to C–C (phenyl) bond) created after
reenolization, which then converts to the initial closed (with intra-
molecular hydrogen bond) enol tautomer.
To summarize, it has been found that three depopulation routes
are responsible for the deactivation of the ground state of the SAA
trans-keto tautomer in solution: ‘normal’ first-order process via
the ground state of cis-keto tautomer (back trans–cis isomerisation
followed by back intramolecular proton transfer), second-order
double proton transfer in the hydrogen-bonded complex of two
trans-keto tautomers and pseudo first-order solvent-assisted reeno-
lization via the intermolecular proton transfer. The last mechanism
operates very efficiently in alcohols and micellar systems. The
proposed deactivation scheme for SAA is also presented in Fig. S14.
Fig. 4. Kinetic curves of the transient absorption signals of SAA in ACN for different
concentration with the fitted function according to Eq. (1). The fitted rate constants
3
ꢁ1
9
ꢁ1 ꢁ1
are:
k
1
= 1.5 ꢀ10
s
and
k
2
= 0.7 ꢀ10
M
s . For clarity, only every 150
experimental point is shown.
4
. Conclusions
On the contrary, in alcohols the trans-keto tautomer lifetime is
significantly decreased as observed for some other Schiff bases
The photochromic cycle of SAA as well as spectroscopic and
[
7,19], there is no influence on the SAA concentration and the de-
cay is mono-exponential (time constants: 11 s in TFE, 9 s in
MeOH and 0.15 s in HFIP). This effect might be explained by the
photophysical properties of the SAA tautomers participating in this
cycle has been investigated by means of the stationary and time-
resolved UV–vis spectroscopy. The similarities and differences be-
tween the SAA dynamics in homogeneous solvents and micellar
systems are summarized in Table 3. SAA can be proposed as a con-
venient probe for studying the properties of photochromic Schiff
bases in heterogeneous environments due to a remarkable resis-
tance to hydrolysis, a considerable stabilization of the primary enol
form and a relatively simple deactivation scheme.
l
l
l
solvent assisted reenolization involving the complex with one or
more solvent molecules, like proposed for salicylidene-1-naphtyl-
amine [6]. More detailed studies of 2-(2 -hydroxyphenyl)-3-H-in-
dole [33] indicated the participation of two alcohol molecules in
the catalysis of the deactivation rate of trans-keto tautomer. By
analogy with those studies, the back proton transfer in the SAA
molecule probably takes place through the intermolecular ex-
change of the hydrogen atom from the alcohols’ hydroxyl groups,
see Fig. S14. Very short trans-keto tautomer lifetimes were also ob-
0
Acknowledgements
served for SAA in the micelles: 0.45
ls in SDS, 0.25 ls in TX and
This work was performed under financial support of the Minis-
try of Science and Higher Education (MNiSW) Poland project N204
0
.20 s in CTAB (Fig. S10). This should be probably explained by
l
the water-assisted process (water has proton donating ability com-
parable to that of strong alcohols – see Table S1) and confirms that
the SAA is located in the polar part of the micelles, where the water
molecules are present. The molecular size of SDS micelles is smal-
1
49 32/3777. Dynamic measurements were made at the Center for
Ultrafast Laser Spectroscopy at the A. Mickiewicz University in
Poznan, Poland. The authors gratefully acknowledge Dr. Jerzy
Karolczak assistance with time-resolved fluorescence measure-
ments and Dr. Gotard Burdzinski assistance with nanosecond tran-
sient absorption measurements. We also thank Dr. Krzysztof
Dobek and Dr. Dariusz Komar for permission to use their fitting
program for time-resolved emission data.
Table 3
Summary of the main SAA tautomers’ properties in solution and in micellar systems
Tautomers SAA in solution
SAA in micelles
Enol
Stationary absorption band: red
shift with increasing
Stationary absorption band
similar to that in polar solvents.
Appendix A. Supplementary material
polarizability and blue shift with
increasing polarity and proticity.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.cplett.2008.09.030.
cis-Keto
The shortest lifetime in polar
solvents, the lifetime increases
with increasing polarizability,
viscosity and proticity. Single-
exponential fluorescence decay
Lifetime comparable or longer
than that in highly viscous
solvents. Multi-exponential
fluorescence decay with
wavelength-dependent
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