Solid state photochromism and thermochromism of two related N-salicylidene anilines

Article abstract of DOI:10.1016/j.chemphys.2014.10.007

The crystalline structure and optical properties of N-salicylidene-p-cyanoaniline and N-salicylidene-p-carboxyaniline were investigated in solid state (microcrystalline powder), with the purpose to connect the effects of substitution and crystal packing with their optical properties. Diffuse reflectance and fluorescence spectroscopy were used to study the absorption and emission properties upon photoirradiation and cooling down to the liquid nitrogen temperature. The Stokes-shifted fluorescence, with a quantum yield of about 10^-2-10^-1, is given by the tautomeric cis-keto species formed as a result of the excited state intramolecular proton transfer from the initial enol structure. Despite their similar geometrical parameters, only SA-COOH is photochromic, which is in contradiction with literature. On the other side, the thermochromism is especially strong at SA-CN. The study shows the behavior of SA-CN similar to that of a classical anil, while SA-COOH presents interesting features that contradict the presumed behavior based only on its supramolecular structure.

Full text of DOI:10.1016/j.chemphys.2014.10.007

Chemical Physics 444 (2014) 43–51
Contents lists available at ScienceDirect
Chemical Physics
journal homepage: www.elsevier.com/locate/chemphys
Solid state photochromism and thermochromism of two related
N-salicylidene anilines
a
a
b
Mihaela Avadanei a, , Vasile Cozan , Sergiu Shova , José António Paixão
⇑
a ‘‘P.Poni’’ Institute of Macromolecular Chemistry, Iasi, Romania
b Faculty of Sciences and Technology, Department of Physics, University of Coimbra, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
The crystalline structure and optical properties of N-salicylidene-p-cyanoaniline and N-salicylidene-p-
carboxyaniline were investigated in solid state (microcrystalline powder), with the purpose to connect
the effects of substitution and crystal packing with their optical properties. Diffuse reflectance and fluo-
rescence spectroscopy were used to study the absorption and emission properties upon photoirradiation
and cooling down to the liquid nitrogen temperature. The Stokes-shifted fluorescence, with a quantum
yield of about 10ꢀ2–10ꢀ1, is given by the tautomeric cis-keto species formed as a result of the excited
state intramolecular proton transfer from the initial enol structure. Despite their similar geometrical
parameters, only SA-COOH is photochromic, which is in contradiction with literature. On the other side,
the thermochromism is especially strong at SA-CN. The study shows the behavior of SA-CN similar to that
of a classical anil, while SA-COOH presents interesting features that contradict the presumed behavior
based only on its supramolecular structure.
Received 19 June 2014
In final form 1 October 2014
Available online 13 October 2014
Keywords:
N-salicylidene aniline
Photochromism
Thermochromism
Fluorescence
Reflectance spectroscopy
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
followed by the cis–trans isomerization. Otherwise, the excited
cis-keto species relaxes in a non-radiative way and no photocolor-
The ability of a molecular system to organize itself when the
trigger is the light or temperature is given by a complex assembly
of structural characteristics, supramolecular interactions and elec-
tronic effects on the reactive moiety. Aromatic anils, from which
one can highlight N-salicylidene anilines and N-salicylidene
aminopyridines, constitute a perfect example of a very versatile
system with switching properties in solid state between thermo-
and photochromism [1]. Due to the intramolecular hydrogen bond,
these two properties are based essentially on the same phenome-
non, named the tautomerism between the basic enol–imine and
the keto-amine form obtained by a proton transfer from the
hydroxylic group towards the imine nitrogen atom. In thermochro-
mic crystals, the tautomeric equilibrium between the white cis-
enol and the yellow cis-keto forms changes reversibly according
to the temperature variation [2]. Under UV irradiation of the enol
and cis-keto forms, the photosensitivity of crystalline anils mani-
fests also by a change in color, but as a result of formation of the
trans-keto isomer as ultimate and metastable photoproduct. There-
fore, the photochromism consists of a succession of structural
changes of the molecule, because the tautomerization must be
ation is observed. The photochromic anils are enjoying of particu-
lar interest, because the mechanism of the intramolecular proton
transfer takes place in excited state at the ultrafast time scale (from
nanosecond/picoseconds in solid state [3] to femtoseconds in solu-
tion [4]), which recommends these materials in the creation of
optical switches based on their nonlinear optical properties [5],
data storage devices or molecular machines [6].
In crystalline state, the interplay between the two isomers is
conducted by a proper manipulation of the compound structural
features, by means of the so-called ‘‘crystal engineering’’ [7]. Thus,
stabilization of a certain type of isomer can be obtained either by
controlling the supramolecular architecture [8], or preparing
hybrid materials by inclusion in matrices [9], co-crystallization
[10] or by coordination [11]. The liaisons are the supramolecular
interactions established between compound’s molecules or
between host and guest, with direct influence on the molecular
conformation and energy levels.
Thermochromism is a general property of the salicylidene ani-
lines (SA) derivatives, but the photochromic behavior is less
encountered in solid state. Thermochromic and photochromic
crystals can be then selectively designed by controlling the crystal-
line packing of molecules, as being stated that these two properties
are mutually exclusive due to the steric requirements [1]. An
⇑
Corresponding author.
http://dx.doi.org/10.1016/j.chemphys.2014.10.007
0301-0104/Ó 2014 Elsevier B.V. All rights reserved.

44
M. Avadanei et al. / Chemical Physics 444 (2014) 43–51
almost planar conformation (the dihedral angle <25°) assisted by
or CH– supramolecular interactions is usually consistent
respectively. Because the substitution groups at the aniline ring
possess the electron – withdrawing (SA-CN) or proton donating/
acceptor character (SA-COOH), the intermolecular interactions that
control the supramolecular structure are different with this
respect. From this reason, crystallography was used in conjunction
with diffuse reflectance and emission spectroscopy in crystalline
state in order to explore the thermochromic and photochromic
properties of the two SAs. Although the conformation and molecu-
lar geometrical parameters are comparable, the influence of the
intermolecular interactions and electronic effects is observed in
their dissimilar properties mainly under photoirradiation. Our
results regarding the effects of UV radiation on SA-COOH, being
apparently in contradiction with those of Johmoto et. al. [10], come
to add some new information concerning its photosensitivity. In
the same time, no investigation on the optical properties on crys-
talline SA-CN has been reported, so far as we know.
p–p
p
with a closed packed assembly and is encountered at the most of
thermochromic crystals. Photochromism is associated with a
twisted geometry of the molecule (the dihedral angle between
the two aromatic rings >25°), therefore the molecules are less
crowded and the larger space allows the isomerization towards
the trans-keto photoproduct. This ‘‘general rule’’ has not been
always obeyed, and interesting situations with crystals showing
thermochromism and photochromism despite their closed packed
structure [8d], [12], or crystals with ‘‘open – structure’’ but with no
photosensitivity [12] were reported.
With regard to the anomalous behavior of some planar N-sali-
cylidene anilines (SA), our study was focused on optical properties
of two simple members of the SA family, which differ in substitu-
tion pattern at the aniline ring. N-salicylidene-p-cyanoaniline (SA-
CN) and N-salicylidene-p-carboxyaniline (SA-COOH) are two of the
simplest SAs, but the structural differences as concerning mainly
the intermolecular interactions are shown to leave their mark upon
photoluminescence and photochromism. Therefore, the relation-
ship between the optical properties and the crystalline structure
must be thoroughly comprehended, as most of the application
ideas are focused on the use of photochromic and/or thermochro-
mic SAs in solid state. There are several reports concerning espe-
cially the emission characteristics of SA-CN and SA-COOH. Thus,
details about the excited states and the tautomerism of SA-CN in
solution were obtained by time-resolved fluorometry by Vargas
[13]. The emission properties of SA-COOH in crystalline state have
been investigated by Chen et al., which outlined the benefits of its
1D microrod morphology as a potential optical waveguides mate-
rial [14]. Johmoto et al. reported the absence of photochromism
for the SA-COOH crystals, but the co-crystals of SA-COOH with
some bases showed photochromic reactivity [10].
2. Experimental part
2.1. Synthesis
The two compounds were synthesized in the same manner, by
reacting stoichiometric amounts of salicylaldehyde or o-hydrox-
yaniline with the appropriate p-substituted benzaldehyde. The
synthesis was performed in ethanol at room temperature for 5 h.
The solvent was removed at rotavapor and the remained solid
product was washed with ethanol three times, affording pure
compounds as orange (SA-CN) or yellow (SA-COOH) crystalline
powders. Ethanol (Chimopar SA) and the starting materials
(Sigma–Aldrich) were used as received.
Characterization. Mp. (°C): SA-CN 123.5; SA-COOH > 240. 1H
NMR (400 MHz, CDCl3, 298 K, d ppm): SA-CN: 8.59 (s, 1H, CH@N),
7.71 (d, 2H, ortho to –CN, J 8.0 Hz), 7.44–7.40 (d, 1H, ortho to –CH@,
J 8.0 Hz, t, 1H, metha to –CH@, J 8.0 Hz), 7.32 (d, 2H, ortho to –N@, J
8.0 Hz), 7.03 (d, 1H, ortho to –OH, J 8.0 Hz), 6.96 (t, 1H, para to
–CH@, J 8.0 Hz). SA-COOH: 8.64 (s, 1H, CH@N), 8.16 (d, 2H, ortho
to –COOH, J 8.0 Hz), 7.43–7.40 (d, 1H, ortho to –CH@, t, 1H,
metha to –CH@), 7.33 (d, 2H, ortho to –N@, J 8.0 Hz), 7.04 (d, 1H,
In this paper we present the results concerning the photochro-
mism and thermochromism of SA-CN and SA-COOH obtained here
by steady-state techniques and presented as a comparative analy-
sis. The general mechanism of photo- and thermochromism, and
the molecular structures are presented in Scheme 1 and in Fig. 1,
Scheme 1. The structural formulae of the two N-salicylidene derivatives studied and the activation pathways leading to thermochromism and photochromism, with the
values of the irradiation wavelengths used in this study.

M. Avadanei et al. / Chemical Physics 444 (2014) 43–51
45
Fig. 1. ORTEP drawings at 50% probability level of the asymmetric part of the unit cell of SA-CN and SA-COOH, with the atom numbering scheme.
ortho to –OH), 6.97 (t, 1H, para to –CH@). FTIR (KBr, cmꢀ1): SA-CN:
3. Results
2227 (s, C„N), 1617 (s, C@N), 1595 (s), 1564 (s), 1489 (m), 1454
(s), 1392 (m), 1360 (m, C-Ph), 1274 (s), 1190 (m, N-Ph),1172 (s),
1151 (m), 1030 (m), 971 (m), 908 (m), 857 (s), 761 (s). SA-COOH:
3444 (br, w, OH), 1680 (s, C@O), 1621 (s, C@N), 1601 (s), 1570
(s), 1497 (s), 1458 (m), 1431 (s), 1363 (m), 1321(s), 1288
(s, C–O), 1231 (w), 1177 (s), 1160 (m), 1115 (m), 944 (m), 860
(m), 775 (s), 753 (s), 690 (m).
3.1. Diffuse reflectance spectroscopy
The diffuse reflectance spectra were recorded on the powder of
pure compounds, without dilution in any reference material, as
being reported that the matrix effects might influence the initial
equilibrium between the two tautomers [8c]. When compared to
the absorption spectra of hydrocarbon and alcohol solutions, the
diffuse reflectance spectra of microcrystalline powders of both
compounds are extended up to 700 nm and all the electronic tran-
sitions are found to be displaced towards lower energies.
2.2. X-ray structure data collection and refinement
Crystallographic measurements were carried out with an
Oxford-Diffraction XCALIBUR E CCD diffractometer equipped with
graphite-monochromated Mo Ka radiation. Crystallinity and the
crystals dimensions were checked under a polarized light micro-
scope. The needle-like crystals suitable for single crystal X-ray dif-
fraction studies were grown by slow evaporation from acetonitrile
solution (for SA-COOH) and acetonitrile/chloroform solution (for
SA-CN), at room temperature. The crystals were placed 40 mm
from the CCD detector. The unit-cell determination and data inte-
gration were carried out using the CrysAlis package of Oxford Dif-
fraction. All structures were solved by direct methods using
SHELXS-9733 and refined by full-matrix least squares on Fo 2 with
SHELXL-9733 with anisotropic displacement parameters for non
hydrogen atoms. All H atoms attached to carbon were introduced
in idealized positions using the riding model with their isotropic
displacement parameters fixed at 120% of their riding atom. Posi-
tional parameters of the H attached to O atoms were obtained from
difference Fourier syntheses and verified by the geometric param-
eters of the corresponding hydrogen bonds.
As illustrated in Fig. 2, the common feature is the region below
400 nm, where the signals from the aromatic transitions, originat-
ing from aniline and salicylidene rings, are located at the same
wavelength for both compounds, i.e. 266 nm and 342 nm. Beyond
400 nm, the spectra show the differences in the electronic transi-
tions given by the substitution and by the molecules confinement
in the crystalline lattice. The broadness of the absorption bands
and the peaks locations indicate the existence of intermolecular
interactions and the formation of molecular aggregates in the
microcrystalline powder. In addition, for both compounds one can-
not exclude the possible co-existence of the enol and a trace of cis-
keto species. The last ones, in equilibrium with the zwitterionic
resonance structure, are reported as being especially stabilized in
crystals [15]. The generally assigned enol region is situated in the
UV domain, so the S1 S0
( , p) transition connected to the
p⁄
closed form absorption is observed through the intense peak at
428 nm for SA-CN and the shoulder around 450 nm for SA-COOH.
These values correspond to a spectral difference of more than
5000 cmꢀ1 with respect to hydrocarbon solutions peaks. The possi-
ble contribution from the cis-keto species is seen in the SA-CN
spectrum through the shoulder at 525 nm, fitting with the intense
orange color of this compound. In turn, SA-COOH, which is bright
yellow at room temperature, has the band maximum around
500 nm, also with a contribution from a trace of quinoid forms. It
is worth adding that no evidence for the existence of quinoid struc-
tures in high amount was found from the SC-XRD data and from
FTIR spectroscopy (Fig. S1, ESI), therefore the enol tautomer in
ground state is the most stable form for the both compounds.
When subjected to photoirradiation, SA-CN and SA-COOH have
been found to behave completely different, as observed by the
color variations and the spectral changes associated with the
chemical transformations. Irradiation of SA-CN up to 30 min, with
365 nm for 20 min and afterwards with 436 nm for 10 min more,
led to no change in color and to just a weak increase of absorbance
of the whole spectrum (Fig. 2(a)). The absence of photochromism
at the normal time scale is then concluded for SA-CN.
2.3. Spectral measurements
The diffuse UV–vis reflectance spectroscopy was carried out
with a Specord M-200 spectrometer equipped with an integrating
sphere accessory. The spectra were recorded on the pure com-
pounds, using polytetrafluoroethylene (PTFE) as a reference. Kub-
elka–Munk treatment has been applied. The solid-state emission
spectra were obtained with a Perkin Elmer LS55 fluorescence spec-
trometer and the measurements were carried out at an angle of 29°
between the sample surface and the incident light beam. The fluo-
rescence quantum yield of the powders was measured with the
integrating sphere, by using an FLS908 fluorescence spectrometer
(Edinburgh Instruments). The excitation source was a Xenon arc
lamp at the 350 nm excitation wavelength. For both optical
absorption and fluorescence measurements, the microcrystals
were grinded to a fine powder.
In contrast, exposure of SA-COOH up to 10 min at 365 nm
induced a large increase in absorbance in the whole region of the
spectrum, with a special appearance of a band around 575 nm
and the rising of two shoulders, around 390 and 475 nm
(Fig. 2(b)). The emerging of the 575 nm and 475 nm bands
Photoirradiation. The photochromic properties were investi-
gated under 365 and 436 nm filtered radiation of a 350 W medium
pressure mercury lamp mounted in a LOS-2 irradiation unit (Rus-
sian Federation). The radiation intensity measured at 365 nm
was 7 W/m2.

46
M. Avadanei et al. / Chemical Physics 444 (2014) 43–51
Fig. 2. Diffuse reflectance spectra during irradiation (365 nm followed by 436 nm light) at room temperature at several representative exposure times, of powders of: (a) SA-
CN, and (b) SA-COOH. The arrows show the bands trend under the influence of UV radiation. The vertical dotted lines mark the peaks position. (c) The scale – expanded
spectra showing the difference between the trans-keto form signals under the two irradiation wavelengths. (d) Photographs of SA-CN and SA-COOH powders before and after
irradiation with 365 nm light.
occurred at the expenses of that at 500 nm, and, in accordance, the
powder color turned from yellow to intense orange. The conver-
sion of the closed, enolic form into the cis-keto form is indicated
by the evolving of the 475 nm and 390 nm shoulders. The spectral
changes induced by irradiation continued to show up till the expo-
sure reached 20 min, and when the irradiation time exceeded this
period the diffuse reflectance spectrum showed a photostationary
state. Then, the exposure wavelength has been switched to
436 nm, in order to address the absorption of the cis-keto form.
The spectra in Fig. 2(b) show the enhancement of the absorbance
in the region assigned to the final photoproduct (564 nm) and also
the formation of a weak band around 410 nm. The development of
the band at 410 nm is surprising, but it is intimately correlated
with that of the 564 nm envelope, as their simultaneous growth
and disappearance have been observed when several cycles of irra-
diation/relaxation cycles have been applied. Due to the remarkable
spectral changes induced in SA-COOH by irradiation, the relaxation
back to the initial species was performed by monitoring the time
evolution of the 564 nm band’s envelope. The fading of the trans-
keto photoproducts was seen to be almost completed in a little
more than 4 h with restoring the initial yellow color.
The kinetic analysis for the fading of the 564 nm band in SA-
COOH has been performed by using the Malkin equation for the
first order processes, ln½ðA0 ꢀ A Þ=ðAt ꢀ A Þꢁ ¼ k ꢂ t. Here, A0, At
1
1
and A are the absorbance values just when the irradiation was
1
stopped, taken as zero time, at a time t, and in the stationary state,
when the recovery of the initial state has been considered com-
pleted, respectively; k is the rate constant of the process. As it
could be seen in Fig. 3, the non-linearity of the first – order plot
in the two situations analyzed leads to the conclusion that the
relaxation process consists of two-stage decay. The values of the
rate constants are not very different each other: k1 = 2.31 ꢃ 10ꢀ4 sꢀ1
and k2 = 3.76 ꢃ 10ꢀ4 sꢀ1 for combined 365 nm + 436 nm irradia-
tion, and 3.28 ꢃ 10ꢀ4 sꢀ1 and 7.04 ꢃ 10ꢀ4 sꢀ1, under 365 nm light
only, respectively. The difference between using the two irradia-
tion wavelengths is observed in the slower component, which
decays faster under 365 nm exposure than under the combined
365 and 436 nm light. One could advance the hypothesis of two
colored forms photoinduced by the two successive irradiation
lights. This finding is in conjunction with the diffuse reflectance
spectra (Fig. 2(c)). Here, it is observed that the additional band at
410 nm has developed only under irradiation with 436 nm and

M. Avadanei et al. / Chemical Physics 444 (2014) 43–51
47
SA-COOH, the intensity of main emission peak followed an oscilla-
tory trend, by decreasing in the first 10 min and afterwards
increasing and decreasing at every 5 min of irradiation. Regardless
the intensity variations as a function of irradiation time, the fluo-
rescence spectrum of SA-CN (Fig. 4(a)) showed no visible change
in shape, and the band maximum and the shoulder maintained
their positions.
In contrast, the photophysics and photochemistry of SA-COOH
seem to be more elaborate. As it can be observed in Fig. 4(b), under
irradiation with 365 nm the decreasing of the whole emission band
intensity is accompanied by the enhancement of the shoulder
around 584 nm, which goes in line with the spectral changes seen
in the diffuse reflectance spectra. When the sample was further
subjected to the 436 nm radiation, the spectral changes consist of
the reduction of the central maximum at 550 nm, the clear and
continuous growing of the 584 nm emission and the appearance
of a weak band in the red tail, around 630 nm.
The excitation spectra were recorded at the maximum of the
emission wavelengths and are presented in Fig. 5. In the case of
SA-CN (Fig. 5(a)), according to the weak modifications undergone
by under 365 nm light, the excitation profiles of the 552 nm emis-
sion showed no difference before and after irradiation. Although
the general profile did not entirely follow the shape of the reflec-
tance spectrum, the positions of the peaks in the two spectra
match each other. Therefore, the weak contributions at 480 and
520 nm are assigned to a trace of cis-keto structures, and those
with much higher intensity at 417 and 370 nm come from the enol
structures. It can be then safely stated that the large emission band
with kmax = 552 nm is arising from the radiative de-excitation of
the excited cis-keto tautomer. The structured appearance of the
SA-CN fluorescence band might be thought as originating from
two different vibronic levels or could arise from radiative relaxa-
tion of two different cis-keto species. According to the lack of
photocoloration and no evidence for a trans-keto⁄ form emission,
it seems that the deactivation of the cis-keto⁄ state in SA-CN is
not taking place by isomerisation, but through back proton transfer
into the primary enol form.
Fig. 3. Kinetic analysis by using the Malkin equation for the relaxation of the
photoproduced trans-keto species in SA-COOH.
the centroid of the trans-keto band blueshifted from ꢄ575 nm to
the final 564 nm. This suggests that the direct excitation of the
cis-keto form would provoke the corresponding isomerization to
create a second kind of trans-keto structures, but differing from
the first ones by a small variation in conformation.
The two-stage bleaching of the photoproducts from our study is
not a singular case and has been also observed for several others
N-salicylidene aniline derivatives. Kawato et al. reported a similar
fading process in the case of thermal relaxation of N-salicylidene
aniline itself and of its deuterated derivative [16], and also for
N-(3-tert-butylsalicylidene)-3-nitro-aniline [16] and 4,40-methyl-
enebis(N-salicylidene-2,6-diisopropylaniline) [17], or 4,40-methyl-
enebis{(N-3,5-di-tert-butylsalicylidene)-2,6-dimethylaniline} [18].
Since we monitored the relaxation process at 564 nm, one could
ascribe the two stages of back-isomerisation to two different kinds
of trans-keto species.
As a result of the changes induced by irradiation on the relative
populations of enol and keto species, the excitation spectra of SA-
COOH recorded at kem = 534 nm took different shapes at different
irradiation times. As one can be seen in Fig. 5(b), the contribution
around 500 nm in the excitation spectrum roughly matches the
corresponding peak in the reflectance spectrum. The excitation
spectrum before irradiation consists of three bands, where those
attributable to the cis-keto form (ꢄ470 nm) and the closed enol
form (420 nm) are slightly blueshifted as compared to the bands
obtained in reflectance. Under irradiation with 365 nm light one
can be noted the broad band in the visible region probed, which
presents several unresolved contributions. The weaker band cen-
tered on 500 nm can be assigned to the cis open form, by compar-
ing it to the reflectance spectrum. The wide band below 420 nm
covers the region assigned to the enol form absorption.
3.2. Emission properties under UV irradiation
In a close relationship with the crystalline structure, the lumi-
nescence properties of SA-CN and SA-COOH were found to be very
similar prior to irradiation, but the spectral emission changes are
different upon exposure to UV light. This behavior is in agreement
with the photochemistry monitored by DRS (Section 3.1). The fluo-
rescence spectroscopy in solid state has been carried out at the
excitation wavelengths corresponding to the absorption of the enol
and cis-keto tautomers, which are 350 and 500 nm, respectively.
Fig. 4 presents the emission spectra of SA-CN and SA-COOH
with 350 nm and 500 nm excitation wavelength, at several repre-
sentative irradiation times. The both compounds share the same
profile of the fluorescence spectrum, which is slightly red-shifted
for SA-CN because of the presence of the C„N group at the para
position of the aniline ring, which enlarged the conjugation and
reduced the energy gap between HOMO and LUMO [19]. The
emission profile consists of a single, broad and asymmetric band
ranging from 500 to 700 nm, and whose intensity is higher for
SA-COOH. The emission is centered at 565 nm for SA-CN and at
550 nm for SA-COOH. The quantum yields of fluorescence at the
excitation wavelength of 350 nm were 0.026 for SA-CN and 0.115
for SA-COOH. The origin of the emission band is associated with
the radiative relaxation of the cis-keto⁄ tautomer, thus demonstrat-
ing the efficient proton transfer in the excited state.
3.3. Fluorescence studies in solid state at 77 K
The almost planar conformation of SA-CN and SA-COOH mole-
cules could be regarded as a prerequisite for thermochromism, as
a general rule in the case of SA derivatives. Therefore, the tauto-
meric equilibrium between the closed and open structures was
investigated on cooling from room temperature down to liquid
nitrogen temperature. The intense orange color of the microcrys-
talline powder of SA-CN at ambient temperature changes to light
yellow at 77 K (Fig. 6(c)). The thermal relaxation is quite fast, the
restoring to the original color being completed in a few minutes.
SA-COOH behaves in a similar fashion on cooling and on thermal
recovery.
The photoluminescence observed under photoirradiation has
been monitored at 5 min time intervals. For both SA-CN and

48
M. Avadanei et al. / Chemical Physics 444 (2014) 43–51
Fig. 4. Fluorescence emission spectra, excited with 350 nm (left) and 500 nm (right) under UV irradiation of microcrystalline powders of: SA-CN (a, b) and SA-COOH (c, d).
The samples were irradiated with 365 nm and afterwards with 436 nm light.
Fig. 5. (a) The excitation spectra of SA-CN at room temperature, recorded at the emission wavelength of 552 nm before (blue trace) and after irradiation (red trace), as
compared with the corresponding diffuse reflectance spectrum (black trace); (b) The profiles of excitation spectra in solid state of SA-COOH, at several irradiation times,
recorded at the emission wavelength of 534 nm at room temperature. The traces at 5 and 15 min were obtained under 365 nm irradiation, while that at 30 min is recorded
with 20 min under 365 nm and then under 436 nm light for 10 min longer (see details in text). In comparison with the excitation spectra, the diffuse reflectance spectra
before irradiation and after 15 min under 365 nm are presented with dotted lines. (For interpretation of the references to color in this figure legend, the reader is referred to
the web version of this article.)

M. Avadanei et al. / Chemical Physics 444 (2014) 43–51
49
Fig. 6. The solid state emission spectra of (a) SA-CN and (b) SA-COOH, as follows: at 298 K (black trace), 77 K (blue trace) and back recovery (red trace), together with the
corresponding excitation spectra (with dotted lines), observed at 554 nm (a) and 546 nm (b), respectively. (c) Photographs of powders of SA-CN and SA-COOH at two different
temperatures. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
The thermo-induced structural changes were expected to be
seen also in emission properties. The excitation wavelength was
350 nm, which corresponds to the maximum of the enol
absorption.
accompanied by an enhancement of the Stokes shift when the
temperature increased.
In the emission spectrum of SA-COOH, the two peaks are less
resolved. The peak on the low energy wing is best seen by Gaussian
spectral decomposition and is almost in the same position with the
maximum of the band at 293 K, i.e. at 546 nm. The calculated spec-
tral distance between the two peaks is 980 cmꢀ1. In the excitation
spectra at the two different temperatures, the band around 470 nm
that could be assigned to a small population of cis-keto species in
ground state decreased on cooling. Accordingly, the enol excitation
band at 400 nm increased when the temperature was lowered, so
that the tautomeric equilibrium shifts towards enolic form at 77 K.
In the low temperature spectrum of SA-CN (Fig. 6(a)), the band
profile is similar to those reported for other SAs [20]. The fluores-
cence intensity increased of about 3 times and the vibronic struc-
ture of the emission band is now resolved, together with a blue
shift of the high energy maximum and with the edge sharpening
in the same region. This contribution is due to the radiative de-
excitation of the cis-keto⁄ species. The energy difference between
the two resolved peaks (537 and 587 nm) is about
1440 100 cmꢀ1, which corresponds to an aromatic stretching
vibration. The whole profile change of emission is accompanied
by the enhancement of a shoulder around 628 nm, which is the
thermochromic band and is indicated by a dotted ellipse in Fig 6(a).
At room temperature, the SA-CN excitation spectrum of the
emission maximum at 554 nm shows a broad region with two
main peaks, which is extended from UV to ꢄ450 nm and it was
already assigned to the enolic form absorption. At 77 K, the two
enol peaks show a combined red shift and intensity changes, which
seems to be connected with the splitting of the main emission
band. The cis-keto trace in ground state decreased on cooling and
the increased intensity of the enol excitation band at low temper-
ature is in accordance with the color whitening, therefore the
shifted tautomeric equilibrium towards the enol species at 77 K
is clearly evidenced. High temperatures are favorable to the trans-
fer of the hydrogen atom along the O1–H1ꢂ ꢂ ꢂN1 bond, giving rise to
a small amount of the cis-keto tautomer. All these changes are
4. Discussion
The two N-salicylidene aniline derivatives analyzed show dif-
ferent optical properties, which may seem surprising if only their
similar structural characteristics are taken into account. Appar-
ently, SA-CN is behaving like a classical anil, showing only thermo-
chromism and no observable photochromism in powder form.
Microcrystalline SA-COOH presents both thermo- and photochro-
mism, although it was claimed the absence of its photosensitivity
[10]. Moreover, its fluorescence at room temperature is stronger
as compared to that of SA-CN, while in cold conditions, the
situation is reversed.
The internal H-bond is a prerequisite for the proton transfer and
the further tautomerization from the initial enol-imine into the
keto-amine form upon photoexcitation. The emission peaks

50
M. Avadanei et al. / Chemical Physics 444 (2014) 43–51
observed above 500 nm for both SA-CN and SA-COOH are consis-
tent with the radiative relaxation of an excited cis-keto species cre-
ated through the excited state intramolecular proton transfer,
therefore the ESIPT process is extremely effective in both com-
pounds. In a smaller amount, some cis-keto⁄ structures produced
from the direct excitation of the cis-keto species in ground state
also show a radiative deactivation.
The color change for both compounds when the temperature
was lowered is connected with the splitting of the emission band.
The two resolved maxima of SA-CN fluorescence band may origi-
nate from two different conformations as regarding the molecule
planarity [20]: the more planar, but less relaxed conformation flu-
oresces at the higher energy side, i.e. emits a pure emerald green
color at 537 nm, and becomes visible when the temperature is low-
ered. The other cis-keto form is characterized by a more stable and
relaxed conformation and this structure could be observed by the
corresponding emission maximum at 582 nm, which maintained
its position from the room temperature. The transfer of the hydro-
gen atom is thermally activated for both SA-CN and SA-COOH so
we can conclude their similar thermochromic behavior. The ther-
mochromic emission band that emerges around 628 nm only for
SA-CN at 77 K might be given by a cis-keto⁄ species with the lowest
energy, therefore it must have the most stable conformation. The
origin of the 628 nm band is yet to be clarified.
the presence of the terminal cyano group, would allow SA-CN
acquiring of a higher structural rigidity by suppressing the rotation
around the C2–C6 bond, also showed by the lower value of the
dihedral angle between the salicylidene and aniline rings, as com-
pared to that in SA-COOH. This fact implies that the increased
rigidity would results in a higher fluorescence yield, because the
non-radiative decay of the cis-keto⁄ state would have been
reduced. The positive thermobehavior of SA-CN points to the exis-
tence of a small population of cis-keto structures in ground state at
room temperature, and the thermo-induced discoloration results
in converting them into the initial enol–imine structure at nitrogen
liquid temperature. However, the number of molecules in the cis-
ketone configuration is very small, as supplementary confirmed
by the SC-XRD data.
In contrast, the SA-COOH molecules are stacked by means of
one CH–
p
contact assisted by two strong O–Hꢂ ꢂ ꢂH bonds between
two in line/adjacent molecules. From this point of view, one could
categorize the SA-COOH structure to a more ‘‘open’’ and relaxed
kind, even when the interplanes distance is smaller than that of
SA-CN and is corresponding to a ‘‘close-packed’’ configuration.
Also, its density (1.411 g/cm3) is greater than that of SA-CN
(1.252 g/cm3), and that would normally correspond to a crowded
molecular arrangement. Because of the COOH terminal group, the
p-conjugation is limited only at the salicylidene ring level, and
The lack of sensitivity of SA-CN to the irradiation with 365 nm
could lead to the conclusion that the photo- and thermochromism
are mutually exclusive properties in its case and the cis–trans
isomerisation is prohibited. We cannot disregard the possibility
that the photochromic cycle in the case of SA-CN is actually occur-
ring at an ultrafast time scale. In contrast, the photochromism of
SA-COOH apparently follows the general rule of the photoreactiv-
ity of N-salicylidene anilines, despite of the SA-COOH planar geom-
etry and the tight supramolecular arrangement. In addition, the
strong fluorescence and the photochromism of SA-COOH could
be viewed as antagonistic properties, as the isomerization into
the trans form is an alternative route in the cis-keto⁄ relaxation
and it means the quenching of fluorescence.
Therefore, the reason behind the different photochromic prop-
erties must be looked for in the supramolecular architecture and
environment of the molecules. According to single crystal X-ray
diffraction data, the geometrical parameters are similar for the
two compounds. The dihedral angle between the two aromatic
rings is of close values (6.2(4)° for SA-CN and 7.8(3)° for SA-COOH),
while the crystalline packing shares as well comparable values of
the interplanes distances between two stacked molecules in the
cell unit (3.42 Å in SA-CN and 3.48 Å in SA-COOH). The stronger
intramolecular hydrogen bond of SA-COOH (1.86 Å) as compared
to that of SA-CN (1.88 Å) and the smaller values of the bonds
lengths that compose the pseudo-cycle show a strained configura-
tion. The intramolecular pseudo-cycle in the SA-CN molecule is a
little more relaxed. So, the close packing leaving no virtual room
for the cis–trans isomerization combined with the molecules pla-
nar conformation was initially seen as a strong indication of the
probable absence of photochromism in solid state for both SA-CN
and SA-COOH.
the molecule is less rigid than SA-CN’s, but it seems improbable
that the close contact between the molecular layers would provide
the necessary space for the free rotation of the C7–N12 plane or
around the C7–C9 bond. The cis–trans isomerisation is going along
with a quite efficient emission, showed by the 2 order of magni-
tude difference between the values of quantum yield of SA-COOH
and SA-CN. Preliminary studies by means of time resolved fluores-
cence spectroscopy measured using the TCSPC technique revealed
the radiative rate constants of close values for SA-COOH and
SA-CN (0.0474 nsꢀ1 and 0.0336 nsꢀ1, respectively), while the non-
radiative rate constant of SA-COOH is almost 2.5-fold lower than
that of SA-CN (0.521 nsꢀ1 as compared with 1.277 nsꢀ1) [22]. This
finding supports the less fluorescent character of SA-CN. Therefore,
SA-COOH is behaving like a very efficient thermochromic and
photochromic SA derivative. One can tentatively explain the
photochromism by the creation of the red photoproduct through
the ‘‘pedal motion’’ process, as it has been stated for the most SA
derivatives [21], because of the little available space in the crystal.
The studies aiming to go deep into the ultrafast processes and
photophysics of SA-CN and SA-COOH through time resolved spec-
tral methods are currently undertaken in our laboratory.
The photosensitivity of SA-COOH, in contradiction with the
report of Johmoto et. al. [10], improves its capability of tuning
the emitting properties by ‘‘crystal engineering’’. However, the
changes from the planar to a non-planar conformation do not guar-
antee a photochromic crystal. The threshold value for the dihedral
angle necessary for a SA derivative to show photochromism has
been stated to be around 30° [23]. This is because the optical prop-
erties, in fact the balance between the radiative and non-radiative
decay routes, are depending of many factors and the large dihedral
angle between the aromatic rings is not the key factor in the SAs
photosensitivity. The cis–trans isomerization may occur even in
crowded surroundings, if the molecular environment allows the
twisting motions of the C–Ph and N–Ph bonds.
Therefore, one step closer into revealing the opposed photo-
behavior of SA-CN and SA-COOH may stem in analyzing the nature
of intermolecular interactions in crystalline structure. SA-CN pack-
ing is driven by two strong
p–p stacking interactions, while two
individual molecules are coupled by dipolar interactions. This
closed – packed arrangement of SA-CN molecules in the crystal
hinders the C2–N1 torsional motion or the rotations around the
C2–C6 and/or N1–C9 bonds in the cis-ketone form that would ren-
der the final photochromic product. Therefore, at the first glance,
5. Conclusion
Two salicylidene anilines derivatives, N-salicylidene-p-cya-
noaniline (SA-CN) and N-salicylidene-p-carboxyaniline (SA-COOH),
have been investigated in solid state with the purpose of getting a
closer picture of their photophysical properties. The planar
the
p–p
interactions abolished the photochromism. Moreover,
conjugation across the entire molecule, given by
the extended
p

M. Avadanei et al. / Chemical Physics 444 (2014) 43–51
51
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The compounds showed a moderate fluorescence in solid state at
room temperature, which gets stronger once the temperature
was lowered (77 K), accompanied by a markedly change of color,
especially in the case of SA-CN. The anomalous Stokes shift of the
emission band with respect to the absorption of the enol form
was explained by the radiative deactivation of the excited cis-keto
species. These structures originate from a fast intramolecular pro-
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to a keto-amine form.
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in a cis–keto state. The photochromic behavior of SA-COOH was
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