Liquid crystalline properties and photochromism of 4-halogeno-N-(4- alkoxysalicylidene) anilines

Article abstract of DOI:10.1080/10587250210540

A new homologous series of 4-halogeno-N-(4-alkoxysalicylidene) anilines was synthesized. The phase transitions of these homologues were determined using a differential scanning calorimeter and an ordinary polarizing microscope. It is shown that the homolo

Full text of DOI:10.1080/10587250210540

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Liquid Crystalline Properties
and Photochromism
of 4-Halogeno-N-(4-
alkoxysalicylidene) anilines
S. Sakagami a , T. Koga a & A. Takase a
a National Institute of Advanced Industrial Science
and Technology , Kyushu, Tosu, Saga, Japan
Published online: 18 Oct 2010.
To cite this article: S. Sakagami , T. Koga & A. Takase (2002) Liquid Crystalline
Properties and Photochromism of 4-Halogeno-N-(4-alkoxysalicylidene) anilines,
Molecular Crystals and Liquid Crystals, 373:1, 71-81, DOI: 10.1080/10587250210540
To link to this article: http://dx.doi.org/10.1080/10587250210540
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Mol. Cryst. and Liq. Cryst., 2002, Vol. 373, pp. 71-81
# 2002 Taylor and Francis, Inc.
Liquid Crystalline Properties and
Photochromism of 4-Halogeno-N-
(4-alkoxysalicylidene) anilines
S.SAKAGAMI, T.KOGA and A.TAKASE
National Institute of Advanced Industrial Science and Technology, Kyushu,
Saga, Tosu, Japan
A new homologous series of 4-halogeno-N-(4-alkoxysalicylidene) anilines was synthesized. The
phase transitions of these homologues were determined using a differential scanning calorimeter
and an ordinary polarizing microscope. It is shown that the homologues exhibit liquid crys-
talline phases as the terminal alkoxyl chain is increased. A nematic phase is found for only a
fluoro-derivative. All the other homologues show a smectic A phase. Furthermore, it is in-
dicated that some members of these homologous series exhibit a photochromic property in the
solid state.
Keywords Liquid crystal, smectic A, nematic, photochromism, salicylideneaniline-derivatives
INTRODUCTION
It is well recognized that N-salicylideneaniline (SA) and its derivatives have
photochromic and=or thermochromic properties in the crystal and glass
state [1–6]. In recent years the study of solid photochromic materials has
grown because of their possible application in the optical processing of in-
formation. On the other hand, a number of investigations have been un-
dertaken about the relationship between molecular structure and liquid
crystallinity [7–11]. Therefore, certain guidelines exist regarding the feature
of the molecular structure that favors liquid crystalline formation. The vast
majority of compounds exhibiting a liquid crystalline phase may be re-
garded as a rigid central group (mesogen) and one or two flexible terminal
alkyl or alkoxyl chains. Furthermore, SA is one of the good mesogens in
liquid crystalline materials because intrahydrogen bonding stabilizes the anil
linkage [12].
Received 6 February 2001; accepted 13 June 2001.
Address correspondence to S. Sakagami, National Institute of Advanced Industrial Science
and Technology, Kyushu, Saga, Tosu, 841-0052, Japan. Email: s.sakagami@aist.go.jp
71

72
S. SAKAGAMI et al.
SCHEME 1
The main purpose of this study is to find new mesomorphic compounds
having a skeleton of SA. It is well known that the halogen atom is one of the
terminal substituents favorable for liquid crystalline formation [7, 10]. Thus,
a new homologous series of 4-halogeno-N-(4-alkoxysalicylidene) anilines
was synthesized and examined for phase transition behavior and a photo-
chromic property. The members of this homologous series having the fol-
lowing generalized structural formula have been referred to as abbreviations
SAFL (n), SACL (n), SABR (n), and SAIO (n), respectively. The n in par-
entheses denotes the carbon number of the terminal alkoxyl chain, and the
value of n ranges from 1 to 8 (see Scheme 1).
EXPERIMENTAL
4-Alkoxy-2-hydroxybenzaldehydes and 4-halogenoanilines were commer-
cially obtained. The substituted benzaldehydes were coupled with the cor-
responding 4-halogenoanilines by refluxing for about two h in an ethanol
solution. The crude product of SAFL (n) and SACL (n) was purified by
successive recrystallization from ethanol. SABR (n) and SAIO (n) were re-
crystallized from a mixture of ethanol and benzene by use of activated
charcoal. The structure of the products was confirmed, mainly by means of
infrared spectroscopy.
The liquid crystalline texture and phase transition temperatures were
determined using a Nikon polarizing microscope equipped with a Mettler
FP-52 microfurnace for sample-temperature control. Checks on the transi-
tion temperatures were made using a Shimazu differential scanning calori-
meter (DSC). All measurements were carried out under a flow of nitrogen
gas to prevent decomposition of the sample.

LIQUID CRYSTALLINE PROPERTIES AND PHOTOCHROMISM
73
Ultraviolet photoirradiation was performed by using a Moritex high-
pressure mercury lamp (200 W) filtered through a band pass glass filter
(transmission maximum, around 365 nm). The irradiation time was 5 min.
The absorption spectra in the spectral range of 350–700 nm were obtained
with JASCO Ubest spectrometer at room temperature. For UV irradiation
and absorption measurements, the polycrystalline film specimen prepared
from the melt was held between a pair of quartz plates separated by about
10 mm. The film was allowed to stand overnight in a dry box before mea-
surements.
RESULTS AND DISCUSSION
The phase transition temperatures and photochromism of all the homo-
logues studied here are summarized in Tables 1 through 4. First of all, it
should be described that all phase transition temperatures stated above on
the basis of the texture change are in good accord with those measured from
DSC thermograms. As shown in Table 1, the first three homologues of
SAFL (n) do not show a liquid crystalline phase. SAFL (4) exhibits both
nematic and smectic phases that take place monotropically. Furthermore, it
TABLE 1 The photochromism and the phase transition temperatures of SAFL (n) (ꢀC)
n
Smectic A
Nematic
Isotropic
Photochromism
1
2
3
4
5
6
7
8
101
90
82
77
70
92
74
76
ꢀ
ꢀ
(62)
(68)
(63)
(73)
64
The parentheses show the monotropic transitions.
TABLE 2 The photochromism and the phase transition temperatures of SACL (n) (ꢀC)
n
Smectic A
Isotropic
Photochromism
1
2
3
4
5
6
7
8
124
132
114
119
122
124
123
123
ꢀ
ꢀ
ꢀ
ꢀ
(111)
95
84
77
74
ꢀ
ꢀ
67

74
S. SAKAGAMI et al.
TABLE 3 The photochromism and the phase transition temperatures of SABR (n) (ꢀC)
n
Smectic A
Isotropic
Photochromism
1
2
3
4
5
6
7
8
154
143
123
128
129
128
131
132
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
(122)
107
91
80
83
ꢀ
ꢀ
73
TABLE 4 The photochromism and the phase transition temperatures of SAIO (n) (ꢀC)
n
Smectic A
Isotropic
Photochromism
1
2
3
4
5
6
7
8
188
150
146
132
131
132
131
133
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
127
118
114
110
109
should be stressed that, among all the homologues studied here, the nematic
phase is observed only for SAFL (4). This characteristic behavior, that the
liquid crystalline phase is observed for homologues with a relatively long
terminal alkoxyl chain, is not surprising, because the appearance of the
mesomorphic phase is not unusual; it has frequently been observed in other
homologous series of liquid crystalline materials [10]. Under polarizing
microscopic observation, this smectic phase exhibits a fan-shaped texture or
focal conic texture when formed on cooling the nematic phase. In addition,
the focal conic or fan textures are easily changed to a pseudoisotropic tex-
ture (homeotropic alignment) by cover-slip displacement. These character-
istic textures show that this smectic phase can be identified as smectic A [13].
As the end alkoxyl chain is increased, SAFL (8) exhibits the enantiotropic
smectic A phase. This finding, that an enantiotropic mesomorphic phase
occurs as the terminal alkoxyl chain is increased, is not unusual, because it
has been frequently observed for the homologous series of the other liquid-
crystalline materials [10]. However, it is unusual that SAFL (6) exhibits no
mesomorphic phase, even though unusual phase transitions can rarely be
observed for derivatives of SA [14]. This finding is likely to be associated
with the different crystal structure of SAFL (6) as compared to that of the
other homologues.

LIQUID CRYSTALLINE PROPERTIES AND PHOTOCHROMISM
75
Table 2 shows the phase transition temperatures of SACL (n). The first
two homologues exhibit no mesomorphic phase. The members with n ꢁ 3
exhibit a smectic A phase, although SACL (3) shows a monotropic smectic
A phase. The finding that a monotropic smectic phase is observed for SACL
(3), which has a relatively short terminal alkoxyl chain, is not unique, as
described previously. Furthermore, it is worthwhile to note that the phase
transition temperatures of SACL (n) are higher than those of SAFL (n)
because of strong intermolecular attractive force, which is due to the large
and heavy substituent of the chlorine atom compared with that of fluorine.
The phase transition temperatures of SABR (n) are shown in Table 3.
The phase transitions of this homologous series are quite similar to those of
SACL (n). The first two homologues show no liquid crystalline phase, and
all the higher members exhibit an enantiotropic smectic A phase except for
SABR (3), which indicates a monotropic transition.Furthermore, as is
expected from comparison between chlorine and bromine atoms, the phase
transition temperatures of SABR (n) are higher than those of SACL (n).
Table 4 shows the phase transition temperatures of SAIO (n). In the case
of this homologous series the first three homologues exhibit no meso-
morphic phase. The members with n ꢁ 4 show an enantiotropic smectic A
phase. The finding that there exists no monotropic transition in this series is
different from the other three homologous series described previously. In
addition, the fact that all the phase transitions are enantiotropic might likely
be associated with the very high phase transition temperatures, which are
due to the terminal large iodine atom. However, a more detai1ed discussion
is impossible at present.
A photochromic property of each homologue is also given in Tables 1
through 4. The open circles in the Tables indicate that photochromism can
be observed for the members with the naked eye, regardless of its photo-
chromic intensity. It should be emphasized that the photochromic property
cannot be observed in the liquid crystalline phases, i.e., the photochromism
can be found only in the solid phase. Among SAFL (n), only SAFL (2) and
SAFL (3) show the photochromic property. The other homologues do not
indicate a photochromic property. Figure 1 shows the absorption spectra in
the visible region of SAFL (3) measured before and after irradiation with
ultraviolet light at room temperature. Furthermore, it is worthwhile to note
that the photochromic-color sample produced by means of irradiation of
ultraviolet light reverts to original color after several minutes if irradiation is
discontinued. All homologues studied here are yellow or pale yellow when
freshly prepared. At room temperature the color of these homologues
exhibiting photochromism deepened to reddish orange by irradiation with

76
S. SAKAGAMI et al.
FIGURE 1 Absorption spectra of SAFL (3). Curve (a) before irradiation, curve (b) after
irradiation.
ultraviolet light. The fact that photochromism is not found for any homo-
logues described above is likely to be associated with the different crystal
structure from that of the other photochromic homologues. In the case of
SACL (n), all members except for SACL (1) and SACL (6) exhibit a pho-
tochromic property, as shown in Table 2. Figure 2 shows the absorption
spectra in the visible region of SACL (4) measured at room temperature. In
addition, among SABR (n) all the homologues except for SABR (6) indicate
a photochromic property. Figure 3 shows the absorption spectra in the
visible region of SABR (7) measured before and after irradiation with
ultraviolet light. As described so far, it is curious and not easily understood
that some homologues show photochromic behavior while others in the
same homologous series do not. This finding is likely to be associated with

LIQUID CRYSTALLINE PROPERTIES AND PHOTOCHROMISM
77
FIGURE 2 Absorption spectra of SACL (4). Curve (a) before irradiation, curve (b) after
irradiation.
the complicated differences between them, such as conformational and
configurational changes and the different crystalline structure. The photo-
chromic property of SAIO (n) is shown in Table 4. It should be noted that in
this homologous series all of the homologues indicate a photochromic
property. In addition, it should be stated that the photochromic property
observed for the five members from SABR (3) to SABR (7) is relatively
weak compared with that of the other three homologues. Figure 4 shows the
absorption spectra of SAIO (8) measured at room temperature. With regard
to this Figure 4 it should be stressed that a clear photochromic change can
be observed with the naked eye, although only a slight change is observed in
the absorption spectra.

78
S. SAKAGAMI et al.
FIGURE 3 Absorption spectra of SABR (5). Curve (a) before irradiation, curve (b) after
irradiation.
In addition, it is interesting to note that Table 5 shows the photochromic
property as well as the phase transition temperatures of N-(4-alkoxy-2-
hydroxybenzylidene) anilines (AHBA (n)), which have no substituent of a
halogen atom. It is easily understood that no mesomorphic phase is found in
this homologous series because no terminal substituent exists. Among these
homologues, four members exhibit the photochromic property (shown in
Table 5). These findings indicate that the photochromic property is deli-
cately dependent upon the terminal substituent. Figure 5 indicates the
absorption spectra in the visible region of AHBA (1).
It has been indicated that photochromic salicylideneaniline exists in
enol form in the crystalline state, and that the photochromic change occurs
through a hydrogen transfer to the imine nitrogen atom followed by

LIQUID CRYSTALLINE PROPERTIES AND PHOTOCHROMISM
79
FIGURE 4 Absorption spectra of SAIO (8). Curve (a) before irradiation, curve (b) after
irradiation.
TABLE 5 The photochromism and the phase transition temperatures of AHBA (n) (ꢀC)
n
Isotropic
Photochromism
1
2
3
4
5
6
7
8
62
75
61
69
62
63
52
54
ꢀ
ꢀ
ꢀ
ꢀ
geometrical molecular rearrangement [1, 15]. Potashnik and Ottolenghi
have suggested that the molecular rearrangement is the cis-trans iso-
merization due to rotation about the bond, including the benzene ring [4].

80
S. SAKAGAMI et al.
FIGURE 5 Absorption spectra of AHBA (1). Curve (a) before irradiation, curve (b) after
irradiation.
From the comparisons among Tables 1 to 5, it is evident that a larger
terminal halogen atom is much more favorable for the appearance of
photochromism than small hydrogen or fluorine atoms. In other words,
the proton transfer and molecular rearrangement are easy for the homo-
logues with large and heavy terminal substituents. Recently, it has been
shown by means of X-ray diffraction analysis that an interconversion
between the enol and transketo form is responsible for the color change
[16]. The finding that no photochromism is observed for any members
studied here suggests that in the case of these homologues it is difficult for
the transketo molecular isomerization to occur. Finally, it must be pointed
out that there is no definite correlation between the length of the terminal
alkoxyl chain and the photochromism.

LIQUID CRYSTALLINE PROPERTIES AND PHOTOCHROMISM
81
CONCLUSION
The experimental results on the new liquid crystalline homologous series of
4-halogeno-N-(4-alkoxysalicylidene) anilines reveal the following salient
features:
1. A Relatively low members of every halogeno-derivative exhibit no
mesomorphic phase.
2. The smectic A phase appears as the terminal alkoxyl chain is increased.
3. The nematic phase is observed for only one homologue of the fluoro-
derivatives.
4. All the members of the iodo-derivatives exhibit a photochromic property,
while no homologues of the other three, halogeno-derivatives show
photochromism.
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