Synthesis, photo-sensitive and electrochemical properties of rod-like aromatic aldehyde with azo bridge

Article abstract of DOI:10.1515/chem-2015-0122

Eight new rod-like liquid crystal molecules composed by a long rigid core of three six-member rings (cyclohexane ring or benzene ring), azo, ester and terminal aldehyde groups have been prepared. These rod-like liquid crystalline molecules were designed to construct new structures to further study photo-isomerization in their mesophases. All the compounds have been characterized based on their basic spectral data, differential scanning calorimeter (DSC) and hot stage polarizing optical microscope (HS-POM). The result showed that all the molecules, even those with the shortest terminal methyl group, have liquid crystalline properties. Their mesophases are nematic within the temperature ranges from 85 to 145°C. They exhibit photo-sensitivities not only in methanol solutions but also in a mesophase when exposed to UV light. The highest occupied orbital (HOMO) and the lowest unoccupied orbital (LUMO) and the differences between the frontier molecular orbitals (E_g) of these compounds were determined by cyclic voltammetry. The effect of even-odd carbon number of the terminal straight alkyl chain on the UV spectral data and the E_g were observed. The difference between the E_g of these compounds are in excellent agreement with the difference of their maximum absorption wavelength in UV spectra.

Full text of DOI:10.1515/chem-2015-0122

Open Chem., 2015; 13: 1071–1080
Research Article
Open Access
MSiynynantZhheengs*i, sYo,ngPshhenogtWoei-, Wseei Gnesngitive and Electrochemical
Properties of Rod-like Aromatic Aldehyde with Azo
Bridge
DOI: 10.1515/chem-2015-0122
1 Introduction
received December 10, 2014; accepted June 9, 2015.
Abstract: Eight new rod-like liquid crystal molecules Modifying molecules to make them respond to external
composed by a long rigid core of three six-member stimuli, for instance magnetism, heat and light, is an
rings (cyclohexane ring or benzene ring), azo, ester and important way to obtain new materials. To study the
terminal aldehyde groups have been prepared. These interrelationship between structures of molecules and
rod-like liquid crystalline molecules were designed their properties is an essential task of material chemistry
to construct new structures to further study photo- [1]. Some aromatic azo compounds are ofen widely used
isomerization in their mesophases. All the compounds as main raw materials in industry, for example as organic
have been characterized based on their basic spectral dyes [2,3] food additives [4], radical reaction initiators [5]
data, differential scanning calorimeter (DSC) and hot and therapeutic agents [6]. In recent years a great interest
stage polarizing optical microscope (HS-POM). The result has been placed on molecules which have two or more
showed that all the molecules, even those with the shortest forms, the interconversion of which can be induced by
terminal methyl group, have liquid crystalline properties. external stimulus [7-9]. Azo compounds are well known for
Their mesophases are nematic within the temperature their trans-cis isomerization. Azo functionalized materials
ranges from 85 to 145°C. They exhibit photo-sensitivities are of special interest due to their photosensitive nature,
not only in methanol solutions but also in a mesophase which can be exploited for optical and optoelectronic
when exposed to UV light. The highest occupied orbital devices [10]. It is well known that azobenzene light-
(HOMO) and the lowest unoccupied orbital (LUMO) and induced isomerization [11-14] leads to formation of new
the differences between the frontier molecular orbitals functional azo compounds [15,16]. The growing number
(Eg) of these compounds were determined by cyclic of azo compounds have been used in photo switching,
voltammetry. The effect of even-odd carbon number of information storage, holography, etc. [17,18]. Some
the terminal straight alkyl chain on the UV spectral data research has even been focused on the effect of UV-light
and the Eg were observed. The difference between the Eg on solid state of substances [19].
of these compounds are in excellent agreement with the
Generally speaking, methyl group as a terminal chain
difference of their maximum absorption wavelength in UV is too short to form a mesophase in rod-like molecules
spectra.
therefore little attempt has been done so far to include
methyl as a terminal chain in the rod-like liquid crystalline
molecules. Additionally, some publications have reported
the effect of different polar end groups on the mesomorphic
property [20]. Our previous publications have described
the design and synthesis of several series of liquid crystals
with terminal alkyls and a polar end group of CHO [21],
confirming that molecules with an end group of CHO were
easier to form a mesophase. These molecules have enough
thermostability and exhibit a wider temperature range of
mesophase than molecules containing other common
functional groups, for example alkyl, alkoxy or cyano
group. Moreover, an aldehyde can be converted into an
aromatic acid [22], a Schiff base [23] or other functional
Keywords: liquid crystal, photo-sensitive, frontier
molecular orbital, azo group, aromatic aldehyde
*Corresponding author: Minyan Zheng: Corresponding author:
School of Chemistry and Chemical Engineering, Xianyang Normal
University, Shaanxi, China, E-mail: zmy71@126.com
Yongsheng Wei, Wei Geng: School of Chemistry and Chemical
Engineering, Xianyang Normal University, Xianyang, Shaanxi,
712000, China
© 2015 Minyan Zheng, Yongsheng Wei, Wei Geng, licensee De Gruyter Open.
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License.
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1072ꢀ
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Minyan Zheng, Yongsheng Wei, Wei Geng
with UV light, are summarized in Table 1; the maximum
times of photoisomerization were detected at the
photostationary states of every compound. The maximum
photoisomerization time of each target molecule has been
achieved within 70 min for illumination intensities of
105 μW cm-2 (Table 1). The spectrum variations are
the evidence of the trans-cis isomerization of the azo
chromophores. The data depicted in Table 1 show that
the compounds with the same terminal rings have the
same number and the similar position of the peaks in the
UV spectra before and afer irradiation. Fig. 1 shows the
absorption spectral change of compound 3a (5 × 10-5 mol L-1
in methanol, quartz cell, length 1.0 cm) afer irradiation with
365 nm UV-light. Every peak in the UV spectra can be related
to the molecular structure. The small shoulder at ~430 nm
is assigned to a n-π* transition and the maximum of the
absorbance in the range of 290−380 nm corresponds to a
strong π-π* electronic transition of the azo-moiety, while the
H
,
O
a
x
,
N
2S
H2N
H
H
C O
H
N
O
3C
O2
DMF Et
a
N N
O2
H l
C
,
O
,
-
0 5°C
H
N N
H
C O
Et3N
H
C
l RT
2C 2
,
l
SOC 2
A
1
R
H
COO
A
2
R
l
COC
o uene
t l
DMF
O
O
A
R
N N
H
C O
3
a: R = methyl, A = 1,4-disubsituted cyclohexyl; b: R = ethyl,
A =1,4-disubsituted cyclohexyl; c: R = n-propyl, A =1,4-disubsituted
cyclohexyl; d: R = methyl, A = phenyl; e: R = ethyl, A = phenyl;
f: R = n-propyl, A = phenyl; g: R = n-butyl, A = phenyl; h: R = n-pentyl,
A = phenyl.
Scheme 1: Synthetic route.
groups [24]. Liquid crystal of formylazobenzene can even peak at 230 nm is caused by the aromatic ring system.
be used as a stationary phase in separation of special The two isosbestic points, located at nearby 288
mixtures [25]. Based on these ideas, we have synthesized and 390 nm in Fig. 1, confirm the existence of only
eight rod-like liquid crystalline molecules bonded with two compounds − the cis and trans isomers [26]. Afer
shorter terminal alkyls (5 ≥ n ≥ 1), aldehyde, and azo bridge irradiation with UV light the first peaks of 3a, 3b, 3c
bonding. The effect of the terminal cyclohexyl or phenyl (compounds with a terminal cyclohexane ring) are
ring on the liquid crystalline property and photosensitivity unchanged, while both peaks of 3d, 3e, 3f, 3g and 3h
of the compounds was also observed. Furthermore, we (compounds with a terminal benzene ring) change their
wanted to know whether rod-like molecules with the positions. According to the data presented in Table 1
shortest methyl chain would exhibit a mesophase and the terminal alkyl chains promote two effects on the
if there was a relationship between their absorption λmax of the last five compounds. The first is a shif of the
wavelengths in the UV spectra, their frontier molecular absorption bands, more pronounced in the azo band
orbitals and their molecular structures.
than in the aromatic ring. Under exposure to UV light,
the second peak of the first three compounds listed in
Table 1 exhibits a small hypsochromic shif in the range
of 4−8 nm, while the first peak of the benzene absorption
band stays unaltered. Afer radiation with UV light, the
first peak of the last five compounds exhibits a small
bathochromic shif between 3 and 4 nm, while the change
of the second peak is related to the structure of the alkyl
2 Results and discussion
2.1 Photo responsive properties in solution
The synthesis of all target compounds is outlined in chain. In compounds 3d, 3f and 3h, which have an odd
Scheme 1 and their structural data are shown above. number of carbon atoms in the alkyl chain, the second
The spectral values are in accordance with the assigned peak exhibits a slight hypsochromic shif in the range
structure.
of 7−8 nm, while in 3e and 3g, compounds with an even
It is known that the azo group can isomerize from the carbon number alkyl chain, a small bathochromic shif
more stable trans isomer to the cis one under exposure between 3 and 4 nm is observed. This means that the
to UV light. The solutions used in the experiments were number of carbon atoms of the terminal alkyl chain
kept in the dark for two days so that the compounds affects the position of a π-π* electronic transition of the
were exclusively in the trans form. The UV spectra azo-moiety in trans-cis isomerization. The second effect,
were recorded over the same time interval until the before or afer irradiation, is weaker for compounds 3d,
photostationary states of trans-cis isomerization. The 3f, 3h (with an alkyl chain of odd number) than for 3e, 3g
changes of optical absorbances of all these compounds in (with an alkyl chain of even number). The above tendency
a methanol solution, recorded before and afer irradiation was not observed for the first three compounds with
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Photo & Electrochemical Properties of Azoaldehyde
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Table 1: The λmax (absorption maximum, nm) of trans and cis isomers of target molecules
Before irradiation λmax [nm]
(εmax) [L mol-1 cm-1]
Afer irradiation λmax [nm]
Time of isomerization [min]
(εmax) [L mol-1 cm-1]
I
Compd.
I
II
II
in solution
in mesophase
228
(8794)
228
(37908)
230
(5138)
239
(18397)
240
(13926)
240
(8929)
240
(21056)
334
(16106)
330
(17444)
332
(10714)
332
(20556)
338
(14232)
330
(8023)
337
(25971)
228
(9035)
228
(39693)
230
(5341)
242
(19467)
242
(12546)
243
(9421)
243
(23235)
330
(8562)
325
(9688)
325
(5557)
325
(10554)
342
(9740)
322
(5372)
340
(16991)
3a
3b
3c
3d
3e
3f
70
70
70
60
80
80
80
80
90
60
50
70
60
60
70
70
3g
3h
241
(18745)
333
(24712)
244
(20782)
326
(11504)
0 min
10 min
20 min
30 min
40 min
50 min
60 min
70 min
0.8
B
70 min
A
0.8
0.6
0.4
0.2
0.0
60 min
50 min
40 min
30 min
20 min
10 min
0 min
0.6
0.4
0.2
0.0
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
400
450
500
250
300
350
400
450
500
550
250
300
350
400
450
500
550
Wavelength / nm
Wavelength / nm
Figure 1: UV-Visible absorption spectrum of 3a in methanol during trans-cis isomerization. The inset illustrates the small increase of absorption
in 430 nm. (A, under irradiation of UV light, B, under irradiation of sunlight)
terminal cyclohexane rings, indicating that the terminal 2.2 Electrochemical analysis
alkyls have an effect only on the electronic transition of
compounds with terminal benzene ring. This electronic The molecular energy level of the HOMO and LUMO can be
effect can be transmitted through a conjugated system exactly calculated by the cyclic voltammetric method. The
between benzene ring and other moieties so that affects energy levels and energy gaps of the eight compunds were
the position of the π-π* transitions of azo-moiety. This determined by the three electrodes system (SCE, platiunm
even-odd carbon number effect of the alkyl chain in wire and glassy carbon serving as the reference, counter
micromolecules on the UV spectra has not been reported and working electrodes respectively) with THF as the
until now. The original absorbance of 3a is A1 = 0.79527 at solvent(concentrationis4×10-3)andtetrabutylammonium
334 nm (Fig. 1A). Subsequent irradiation with sunlight for perchlorate (t-Bu4NClO4) as a supporting elecrolyte. The
70 min causes the absorbance at 334 nm to increase back HOMO, LUMO and Eg were calculated using the following
to A2 = 0.78397 (Fig. 1B). It is seen that A2/A1 is 99%, i.e., the equations. EHOMO = -e[EonsetOX + 4.4(eV)], ELUMO = -e[Eonsetred
azo compounds isomers do recover in 99%.
+ 4.4(eV)] and Eg = -e∆Φ, ∆Φ = Φp - Φn [27], where e is
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Table 2: Redox potentials, EHOMO, ELUMO and energy gaps of target compounds
Compd.
3a
3b
3c
3d
3e
3f
3g
3h
I
0.59
0.58
0.56
0.53
0.52
0.58
0.44
0.58
Eox
II
0.45
0.40
-0.26
0.48
0.48
-0.25
0.48
0.48
-0.23
0.42
0.42
-0.23
0.41
0.41
-0.22
0.46
0.46
-0.18
-
0.43
0.43
-0.23
Φp
0.44
-0.19
Ered (Φn)
HOMO(eV)
LUMO(eV)
Eg
-4.85
-4.14
0.71
-4.88
-4.15
0.73
-4.88
-4.17
0.69
-4.82
-4.17
0.65
-4.81
-4.18
0.63
-4.86
-4.22
0.64
-4.84
-4.21
0.63
-4.83
-4.17
0.66
carbon number. The effect of carbon number on Eg is in
an excellent agreement with that of the second maximum
absorption wavelength in their UV Spectra (Table 1). The
data from Tables 1 and 2 show that, if the Eg of any target
compound is lower than of other compounds (Table 2), its
second maximum absorption wavelength in UV spectra
is longer than others, which means that the energy of the
second peak in UV spectra is lower than others. The results
indicated that because the second peak of each compound
has lower energy than the first one (Table 2), the electronic
transition between the frontier molecular orbital (Eg) is
associated with the second maximum absorption peak in
the UV spectra. The Eg and absorption wavelengths can
be modified by the terminal alkyl group, which requires
further study in order to change the photo-sensitive
property of molecules within a certain range.
3h
Blank
0.00004
0.00002
0.00000
-0.00002
-0.00004
-0.00006
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Voltage(V vs SCE)
Figure 2: the cyclic voltammogram of 3b in dilute THF solution with
tetrabutylammonium perchlorate as a supporting electrolyte. The
dotted line shows CV of the blank run under identical condition
elementary charge, Φp is the onset oxidation potential
and Φn is the onset reduction potential. The typical 2.3 Liquid crystalline properties and photo
voltammogram of 3b is shown in Fig. 2. The HOMO, LUMO
responsive properties in mesophase
and the corresponding band gap Eg of every compound are
summarized in Table 2.
The transition temperatures, associated with enthalpy
Three trends in Table 2 can be found as follows. The values obtained from DSC studies and the ranges of
first trend is that the Eg of the first three compounds are decomposition from TG for the target compounds are
all greater than that of the last five compounds; the last summarized in Table 3. DSC thermograms were measured
five compounds, with the terminal benzene ring, have at the rate of 10°C min-1 for the heating process. The
lower energy gaps than the first three compounds with melting temperatures of these compounds are observed in
the terminal cyclohexane ring. The results agree with the the range of 84.6−136.6°C and clearing temperatures are
common conception that the electronic effect of benzene 190.7−270°C. The ranges of their decomposition begin from
(as a congurate system) can narrow the difference between 205 to 287.2°C and end from 367.5 to 408.6°C, suggesting
the energy levels. On the other hand Eg (3b) is greater than the thermostability of these compounds. Among them,
Eg (3a) or Eg (3c), coincided with the fact that the energy of 3a with the terminal methylcyclohexane ring is the least
the second peak of 3b is greater than those of 3a and 3c stable one, while 3d with the terminal ethylbenzene ring
(Table 1). The third trend, that the Eg of 3e being greater than is the most stable. The DSC analysis reveals that melting
the Eg of 3d or 3f and the Eg of 3g being greater than the Eg temperatures of compounds 3d−3h decrease with an
of 3f or 3h, indicates that among the last five compounds, increase of the length of the terminal alkyl chain. The
the compound with a straight alkyl chain of an even transition temperature observed from POM is in a good
carbon number has lower Eg than the compounds with odd agreement with a result obtained from the DSC analysis.
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Photo & Electrochemical Properties of Azoaldehyde
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Table 3: The results of DSC measurement
Compound
3a
3b
3c
3d
3e
3f
3g
3i
Ratios of Length to width 4.28
4.58
4.81
4.19
4.49
4.66
5.03
5.23
M.p.(°C)
106.5
67.45
191.5
1.546
84.6
98.8
136.6
70.96
230.6
1.231
94.0
132.2
52.96
271.0
1.360
138.8
122.5
55.25
267.9
1.335
145.4
106.0
35.74
256.5
1.368
140.5
104.9
51.19
245.9
1.149
141.0
ΔH of M.p. (J g-1)
35.56
190.7
1.332
106.1
49.90
225.6
1.538
126.8
C.p.(°C)
ΔH of C.p. (J g-1)
Range of mesophase (°C) 85.0
D.p.(°C)
221.3-377.5 205-388.0
229.4-381.4 238.3-367.5 287.2-395.0 280.1-404.3 259.2-381.9 255.7-408.6
*m.p. - melting point, c.p. - clearing point, d.p - decomposing point
The mesogenic behaviors were investigated by POM. When the sample of 3a, as the first sample, was heated
The POM textures of the typical compounds 3a and 3b to 152°C at the thermoplate, a texture of schlieren type
are shown in Fig. 3, respectively. These compounds can be observed (Fig. 4A). Keeping the temperature
all exhibit the appearance that should belong to the constant, the texture was exposed to UV light of 365 nm
nematic phase of schlieren type. There is no other type of with illumination intensities of 105 μW cm-2 within the
mesophase observed during heating or cooling process. time interval of 10 min. The changes of the texture were
The temperature ranges of their mesophases are from 85 then observed by POM (Fig. 4B-G). From these pictures we
to 145°C. The temperature ranges of the mesophase of can find that the areas of texture are gradually decreasing
both compounds with the terminal n-propyl are 126.8 and as the exposure time increases until they completely
145.4°C, the widest values in their own series, respectively. disappear, i.e. the azo compound did change its shape in a
In other words, the terminal n-propyl creates the widest mesophase. The exposure time of other compounds were
mesophase in both series. Although there was little report also recorded (Table 1), showing that the change of the
focused on the rod-like liquid crystal molecules designed molecular shapes is more difficult in a mesophase than in
with a terminal methyl chain, both 3a and 3d with the solutions. It is because the molecules in a mesophase still
methyl exhibit a mesophase with temperature ranges of 85 need to overcome larger intermolecular forces than in a
and 94°C, respectively. The ratios of the molecular length solution. From the results shown in Table 1, we also found
to the width of these two compounds are 4.28 and 4.19 that the molecules with the terminal benzene ring were so
(Table 3), respectively, slightly more than 4 (the lowest difficult to change their configurations that they needed
value for a molecule to exhibit a mesophase), proving more time of exposure to UV light.
that the methyl, as a shortest alkyl chain, can form a
mesophase in the rod-like molecules of an appropriate
structures. By comparing compounds with the same alkyl
3 Experimental
chain, we found out that 3a, 3b and 3c with terminal
cyclohexane rings have lower melting points, clearing
points and narrower ranges of mesophase than 3d, 3e and 3.1 Materials and Characterization
3f, respectively. Among them, 3d has the highest c.p and
widest range of the mesophase. On one hand, these azo All initial intermediates used in the synthesis were
aromatic aldehydes have wider ranges of a mesophase prepared in our laboratory with purity higher than 99%,
than other liquid crystals with an aldehyde group [28]. On and characterized by IR, GC-MS and 1H NMR. Other reagents
the other hand, they have wider temperature range of a were from commercial sources and used without further
mesophase than other polymers with an azo group [26].
purification. The purity of the compounds was determined
Theoretically, the cis isomer with a bent shape should using LC-10A (Shimadzu) instrument with methanol as
have no mesophase observed. If so, when UV light changes eluent and flowing rate was 1 mL min-1. Elemental analyses
the structure of the compound from the trans to the cis- were conducted using PE-2400 analyzer (Perkin Elmer).
isomer, the textures should completely disappear afer UV spectra were recorded on Agilent 8453 Spectrometer
enough exposure time.
(Agilent Technologies). IR (KBr) spectra were recorded on
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1076ꢀ
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Minyan Zheng, Yongsheng Wei, Wei Geng
(A): The schlieren texture of 3a taken at 130°C, ×300;
(B): The schlieren texture of 3b taken at 142°C, ×300.
Figure 3: The textures of typical compounds observed under POM during heating process
a Vertex 70 spectrophotometer (Bruker). Mass data were 3.2.3 Synthesis of compound 3
recorded on a GCMS-QP2010 (Shimadzu) with IE of 70 ev.
1H NMR were obtained on a BRUKER Avance 400 (400 MHz, 3a was obtained by dissolving 2a in CH2Cl2
solvent CDCl3). Cyclic voltammetric measurements were (20 mL) and adding the mixture into a solution of
performed on a BAS Epsilon (BAS, USA) which was run by 4ˊ-hydroxybenzeneazo-4-benzaldehyde (1.13 g, 5 mmol)
General Purpose Electrochemical System (GPES) sofware. and triethylamine (0.9 mL, 5 mmol) in CH2Cl2 (40 mL) at
The conventional three electrode system was measured ambient temperature for 12 h under vigorous stirring. The
against SCE, platinum wire and glassy carbon serving as solvent was then removed under vacuum condition. The
the reference, counter and working electrodes respectively. obtained precipitate was washed with 5% (mass fraction)
NaOH and water. Recrystallization using THF afforded an
orange powder (3a, 1.50 g) with 86% yield (two steps). The
3.2 Synthesis
3b, 3c, 3d, 3e, 3f, 3g and 3h were also prepared the same
way, and their yields were 84%, 80%, 82%, 83%, 80%,
3.2.1 Synthesis of 4ˊ-Hydroxybenzeneazo-4-benzaldehyde 81% and 85%, respectively.
4-(4-(4-methylhexylacyloxy)phenyldiazenyl)
4ˊ-Hydroxybenzeneazo-4-benzaldehyde was prepared benzaldehyde (3a), relative molecular mass: 350.41, the
using the method reported elsewhere [28]; It was obtained mass fraction detected by LC is 98.9%, yield: 85%, orange
as orange crystals with the yield of 81%.
powder, m.p 107−108°C; Anal. Calcd for C21H22N2O3: C 71.98,
1H NMR (400 MHz, CDCl3, ppm): 9.874 (s, H), 6.003 (s, H 6.33, N 7.99, found: C 72.03, H 6.12, N 7.80; IR (KBr)ν: 2941,
H), 8.124 (d, 2H, J = 9.0 Hz), 7.763 -8.012 (m, 4H), 6.937 (d, 2863 (s, C-H), 2848, 2744 (s, C-H of aldehyde), 2679, 2491,
2H, J = 9.0 Hz).
2353, 1756 (vs, C=O), 1689 (vs, C=O), 1653, 1636 (m, N=N),
FT-IR (KBr, cm-1): 3421, 1671, 1599, 1588, 1559, 1542, 1597, 1541, 1494, 1455 (m, ArH), 1411, 1376, 1316, 1292,1248,
1466, 1398, 1355, 1205, 1138, 1102, 936, 851, 840, 660.
1203, 1172, 1153, 1122, 1083, 1006 (vs, C-O-C), 977, 925, 866 (s,
trans -R-N=N-R), 844, 818 (w, 1,4-Ar), 791, 762, 727, 661 (w,
(CH2)n) cm-1; MS (70 ev) m/z (%): 350 (M+, 5.03), 349 (0.07),
335 (0.03), 322 (0.02), 290 (0.02), 270 (0.08), 251 (0.38),
242 (63.66), 237 (0.42), 225 (1.11), 197 (0.40), 185 (0.21), 168
3.2.2 Synthesis of compound 2
2a was prepared by heating and stirring a mixture of 1a (0.60), 149 (1.98), 125 (8.89), 121 (83.05), 115 (3.51), 104
(0.81 g, 5 mmol) and SOCl2 (0.59 g, 5 mmol) and several (3.87), 97 (67.34), 93 (100), 81 (1.65), 76 (4.38), 65 (77.63), 55
drops of DMF in toluene (30 mL) at ambient temperature (25.12), 51 (4.13), 43 (0.95), 39 (11.34), 36 (0.04), 29 (0.38);
for 5 h, then the solvent was removed under vacuum. 1H NMR: 0.914 (t, 3H, J = 6.4 Hz, CH3), 1.039−2.613 (m, 10H,
A brown liquid (2a) was obtained and used in the next the protons of cyclohexane), 7.370, 7.403, 8.058, 8.142 (d,
step without further purification.
J = 8.0, 8.4, 8.0, 8.4 Hz, 2H of each), 11.113 (s,1H, CHO).
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Photo & Electrochemical Properties of Azoaldehyde
ꢀ1077
(A): texture afer 10 min irradiation; (B): texture afer 20 min irradiation;
(C): texture afer 30 min irradiation; (D): texture afer 40 min irradiation;
(E): texture afer 50 min irradiation; (F): texture afer 60 min irradiation;
(G): texture afer 70 min irradiation.
Figure 4: Texture changes during radiation of UV-light of 365 nm of 3a taken at 152°C, ×300.
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1078ꢀ
ꢀ
Minyan Zheng, Yongsheng Wei, Wei Geng
4-(4-(4-ethycyclolhexylacyloxy)phenyldiazenyl) 236 (0.03), 226 (0.34), 211 (0.08), 197 (0.02), 183 (0.02), 169
benzaldehyde (3b), Relative molecular mass: 364.44, the (0.04), 152 (0.03), 141 (0.17), 133 (0.09), 121 (1.13), 119 (100),
mass fraction detected by LC is 98.8%, yield: 84%, orange 105 (0.61), 93 (0.95), 91 (11.76), 84 (0.21), 77 (0.50), 71 (0.18),
powder, m.p 85−86°C; Anal. Calcd for C22H24N2O3: C 72.50, 65 (1.13), 57 (0.13), 49 (0.15), 44 (0.09), 39 (0.09), 32 (0.30);
H 6.64, N 7.69, found: C 72.34, H 6.69, N 7.62; IR (KBr)ν: 1H NMR: 2.475 (s, 3H, CH3), 7.263, 7.340, 7.419, 8.119 (d,
2959, 2930 (s, C-H), 2852, 2744 (s, C-H of aldehyde), 2361, J = 8.4, 8.0, 8.4, 8.0 Hz, 2H of each), 8.066−8.084 (m, 4H),
2343, 1749 (vs, C=O), 1694 (vs, C=O), 1654, 1637 (m, N=N), 10.117 (s, 1H, CHO).
1559, 1495, 1458 (m, ArH), 1448, 1378, 1318, 1297, 1255, 1202,
4- ( 4- ( 4- e t h yl b e n z o yl ox y ) p h e nyl d i a z e nyl )
1156, 1124, 1099, 1007 (vs, C-O-C), 995, 902, 855 (s, trans benzaldehyde (3e), relative molecular mass: 358.39, mass
-R-N=N-R), 841, 817 (w, 1,4-Ar), 760, 725, 662 (w, (CH2)n) cm-1; fraction detected by LC is 98.7%, yield: 83%, orange
MS (70 ev) m/z (%): 364 (M+, 3.79), 360 (0.26), 351 (0.03), powder, m.p.129−130°C; Anal. Calcd for C22H18N2O3: C 73.73,
269 (0.08), 257 (0.30), 242 (33.81), 224 (1.09), 215 (0.25), H 5.06, N 7.82; found C 73.69, H 5.11, N 7.76; IR (KBr)ν: 3067,
197 (0.22), 185 (0.26), 168 (0.17), 157 (0.19), 147 (1.60), 2969, 2931, 2875 (s, C-H), 2815, 2726 (C-H of aldehyde), 1731
139 (12.54), 121 (31.37), 111 (55.06), 93 (8.83), 91 (2.09), 84 (vs, C=O), 1698 (vs, C=O), 1639 (m, N=N), 1609, 1599, 1507,
(0.11), 69 (100), 65 (21.13), 61 (0.30), 57 (2.34), 55 (19.26), 1496 (m, ArH), 1416, 1309, 1271, 1202, 1140, 1078, 1011 (vs,
1
51 (1.35), 41 (11.53); H NMR: 0.912 (t, 3H, J = 7.2 Hz, CH3), C-O-C), 929, 886 (s, trans-R-N=N-R), 848, 760, 725, 696,
0.979−1.039 (m, 2H, CH2), 1.212−2.528 (m, 10H, the protons 661(w, (CH2)n) cm-1; MS (70 ev) m/z (%): 358 (M+, 1.28),
of cyclohexane), 7.249 (m, 4H), 8.000, 8.028 (d, J = 8.4, 257 (0.37), 241 (0.06), 226 (0.37), 211 (0.02), 167 (0.10), 150
8.6 Hz, 2H of each), 10.108 (s,1H, CHO).
(0.04), 141 (0.17), 132 (0.94), 133 (100), 121 (1.58), 105 (7.29),
4-(4-(4-n-propycyclolhexylacyloxy)phenyldiazenyl) 93 (2.15), 79 (2.77), 65 (0.98), 57 (0.03), 51 (0.11), 44 (0.14),
1
benzldehyde (3c), relative molecular mass: 378.46, the 39 (0.12), 32 (0.12); H NMR: 1.300 (t, 3H, J = 7.2 Hz, CH3),
mass fraction detected by LC is 98.7%, yield: 80%, orange 2.765 (q, 2H, J = 7.2 Hz, CH2), 7.364, 7.414, 8.145 (d, J = 8.0,
powder, m.p. 99−100°C Anal. Calcd for C23H26N2O3: C 72.99, 8.4, 8.0 Hz, 2H of each ), 8.069-8.086 (m, 6H, H of benzene
H 6.92 N 7.40; found C 72.87, H 7.01, N 7.35 IR(KBr)ν: 2954, ring), 10.114 (s, 1H, CHO).
2929 (s, C-H), 2858, 2747 (s, C-H of aldehyde), 2679, 2361,
4-(4-(4-n-propylbenzoyloxy)phenyldiazenyl)
2343, 1748 (vs, C=O), 1698 (vs, C=O), 1671, 1648 (m, N=N), benzaldehyde (3f), relative molecular mass: 372.42, mass
1599, 1559, 1508, 1496 (m, ArH), 1376, 1315, 1202, 1164, 1123 fraction detected by LC is 98.8%, yield: 80%, orange
(vs, C-O-C), 977, 869 (s, trans-R-N=N-R), 814 (w, 1, 4-Ar), powder, m.p. 123−124°C; Anal. Calcd for C23H20N2O3: C 74.18,
797, 762, 727, 669 (w, (CH2)n); MS (70 ev) m/z (%): 378 (M+, H 5.41, N 7.52; found C 74.03, H 5.52, N 7.46; IR(KBr)ν: 3065,
9.61), 364 (0.14), 350 (0.08), 335 (0.02), 313 (0.02), 273 (0.07), 2960, 2930 (s, C-H), 2872, 2814, 2723 (C-H of aldehyde),
261 (0.05), 245 (0.21), 229 (0.11), 226 (50.73), 212 (0.79), 197 2361, 2342, 1732(vs, C=O), 1698 (vs, C=O), 1653 (m, N=N),
(0.08), 181 (0.01), 161 (0.17), 153 (11.91), 152 (3.13), 133 (0.70), 1610, 1590, 1559, 1496 (m, ArH), 1457 (m, ArH), 1417, 1310,
125 (61.48), 121 (28.24), 120 (0.73), 105 (3.30), 93 (12.91), 92 1270, 1196, 1179, 1143, 1100, 1079, 1011 (vs, C-O-C), 885 (s,
(0.81), 86 (0.85), 83 (45.90), 77 (1.35), 69 (100), 67 (5.16), 57 trans-R-N=N-R), 846 (w, 1, 4-Ar), 756, 696 (w, (CH2)n) cm-1;
(21.68), 51 (0.28), 41 (4.35), 29 (0.44); 1H NMR: 0.902 (t, 3H, MS (70 ev) m/z (%): 372 (M+, 1.50), 317 (0.02), 255 (0.05),
J = 1.6 Hz, CH3), 0.954−1.049 (m, 2H, CH2), 1.182-1.236 (m, 239 (0.02), 225 (0.22), 209 (1.88), 181 (0.23), 165 (0.07), 149
2H, CH2), 1.525-2.555 (m, 10H, the proton of cyclohexane), (0.74), 147 (100), 131 (0.60), 121 (0.67), 119 (5.07), 105 (0.71),
7.256 (d, J = 8.5 Hz, 2H of each ), 7.986−8.033 (m, 6H), 11.105 93 (0.78), 91(7.17), 77 (0.56), 65 (0.39), 51 (0.07), 41 (0.53), 32
(s, 1H, CHO).
(0.31); 1H NMR: 0.975 (t, 3H, J = 7.2 Hz, CH3), 1.676−1.732 (m,
4-(4-(4-n-methylbenzoyloxy)phenyldiazenyl) 2H, CH2), 2.699 (t, 2H, J = 7.6 Hz, CH2), 7.340, 7.411, 8.048 (d,
benzaldehyde (3d), relative molecular mass: 344.36; the J = 8.0, 8.4, 8.0 Hz, 2H of each), 8.138−8.156 (m, 6H), 10.111
mass fraction detected by LC is 98.7%, yield: 82%, orange (s, 1H, CHO).
powder, m.p. 137−138°C; Anal. Calcd for C21H16N2O3: C
4-(4-(4-n-butylbenzoyloxy)phenyldiazenyl)
73.24, H 4.68, N 8.13; found C 73.10, H 4.71, N 8.04; IR (KBr) benzaldehyde (3g), relative molecular mass: 386.44; the
ν: 3096, 3053, 2955, 2921 (s, C-H), 2841, 2822, 2733 (s, C-H of mass fraction detected by LC is 98.9%, yield: 81%, orange
aldehyde), 2361, 2342, 1733 (vs, C=O), 1700 (vs, C=O), 1637 powder, m.p. 106−107°C; Anal. Calcd for C24H22N2O3: C
(m, N=N), 1613, 1600, 1498 (m, ArH), 1411, 1377, 1313, 1298, 74.59, H 5.74, N 7.25; found C 74.48, H 5.87, N 7.16; IR(KBr)
1268, 1204, 1180, 1139, 1100, 1067, 1022 (vs, C-O-C), 873 (s, ν: 2959, 2932, 2858, 2814, 2723, 2361, 2342, 1732 (vs, C=O),
trans-R-N=N-R), 847, 831, 818 (w, 1, 4-Ar), 776, 742, 684 (w, 1698 (vs, C=O), 1637 (m, N=N), 1612, 1559, 1542, 1509, 1458
(CH2)n) cm-1; MS (70 ev) m/z (%): 344 (M+, 1.30), 330 (0.06), (m, ArH), 1417, 1397, 1380, 1309, 1298, 1270, 1201, 1142, 1073,
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Photo & Electrochemical Properties of Azoaldehyde
ꢀ1079
1010 (vs, C-O-C), 886 (s, trans-R-N=N-R), 846 (w, 1, 4-Ar), in spectroscopy grade methanol and maintained in dark
777, 751, 695, 670, 658, 634 (w, (CH2)n) cm-1; MS (70 ev) m/z for 2 days before the spectra were recorded. A ZF-7A UV
(%): 386 (M+, 1.84), 372 (0.04), 343 (0.03), 281 (0.04), 269 lamp was used for photoisomerization analysis (Shanghai
(0.07), 251 (0.10), 240 (0.04), 226 (1.10), 211 (0.11), 195 (0.07), Jiapeng Technology Co. Ltd.). The excitation band used
175 (0.06), 161 (100), 160 (0.65), 147 (0.21), 133 (2.48), 121 had a maximum at 365 nm. All the optical analyses
(1.58), 105 (2.11), 93 (2.39), 91 (9.24), 86 (0.12), 77 (0.78), 65 were performed in solution at room temperature. The
1
(1.37), 55 (0.44), 39 (0.23), 32 (0.49); H NMR: 0.957 (t, 3H, photoisomerization in mesophase was also carried out
J = 7.2 Hz, CH3), 1.328−1.418 (m, 2H, CH2), 1.601−1.697 (m, using the same UV-lamp for irradiation.
2H, CH2), 2.725 (t, 2H, J = 7.2 Hz, CH2), 7.345, 7.415, 8.137 (d,
J = 8.0, 8.4, 8.0 Hz, 2H of each), 8.052−8.070 (m, 6H), 10.116
(s,1H, CHO).
3.5 Calculation of the ratios of the length to
width of target molecules
4-(4-(4-n-pentylbenzoyloxy)phenyldiazenyl)
benzaldehyde (3h), Relative molecular mass: 400.47; the
mass fraction detected by LC is 98.9%, yield: 85%, orange The length and width of the target molecules were
powder, m.p. 105−106°C; Anal. Calcd for C25H24N2O3: C calculated by Chem Bio3D Ultra 12.0.2 (Combridgesof).
74.98, H 6.04, N 7.00; found C 74.82, H 6.21, N 6.87; IR(KBr) During the procedure the stereostructure of any molecule
ν: 3066, 2929, 2857 (s, C-H), 2809, 2717 (s, C-H of aldehyde), wasfirstdrawnbythesofware.Theoptimumconfiguration
2361, 2343, 1732 (vs, C=O), 1700 (vs, C=O), 1648, 1637 (m, of the molecule was then worked out by minimizing the
N=N), 1610, 1599, 1522, 1508 (m, ArH), 1417, 1380, 1310, molecular energy. At last, the farthest distance calculated
1298, 1270, 1200, 1179, 1144, 1073, 1012 (vs, C-O-C), 959, between any two atoms in the horizontal direction was
885 (s, trans-R-N=N-R), 846 (w, 1, 4-Ar), 760, 725, 697, 669, considered to be the molecular length, while the farthest
634 (w, (CH2)n) cm-1; MS (70 ev) m/z (%): 400 (M+, 1.27), distance calculated between any two atoms in the vertical
337 (0.34), 257 (0.43), 229 (28.82), 227 (1.83), 209 (5.15), 177 direction the width, so that the ratios of the molecular
(1.29), 175 (100), 165 (0.46), 147 (1.22), 131 (2.11), 121 (5.19), length to the width could be obtained.
109 (0.95), 105 (5.13), 93 (3.09), 91 (14.45), 77 (1.53), 71 (0.89),
65 (2.84), 55 (0.89), 41 (0.85), 32 (2.61); 1H NMR: 0.911 (t, 3H,
J = 6.8 Hz, CH3), 1.320−1.372 (m, 2H, CH2), 1.589-1.709 (m, 4 Conclusions
4H, CH2CH2), 2.714 (t, 2H, CH2), 7.314, 7.411, 8.135 (d, J = 8.0,
8.4, 8.0 Hz, 2H of each), 8.049−8.068 (m, 6H), 10.113 (s,1H, We have reported here eight new rod-like azo liquid crystal
CHO).
molecules with two different terminal rings. The molecular
structure allows all of them, even these with the terminal
methyl, to exhibit a mesophase behavior that belongs to
the nematic phase of schlieren type. All these compounds
can, in a methanol solution, gradually change their shape
3.3 Thermal analysis
Thermal transitions and enthalpies were determined by from trans to cis isomer under irradiation of UV light
DSC measurements carried out using TA equipment with (365 nm). Sunlight can recover their original structures by
a MDSC Q100 module using a heating rate 10°C min-1 transforming the cis isomer to the trans one with a recovery
and a nitrogen flow of 50 mL min-1. TGA measurements rate of 99%. An interesting phenomenon, involved the
were carried out using TA equipment with a Q500 odd-even carbon electronic effect on the compounds with
module using a heating rate of 10°C min-1. Mesomorphic a terminal benzene ring, was observed in the UV spectra
textures were determined using a LV100POL Polarizing of these target compounds. We performed photo-induced
Optical Microscopy (Nikon) with a LTS E350 hot stage studies on mesophase of these target compounds, which
(Linkam).
has been seldom reported until now. Results showed that
under irradiation of UV light, the molecules can also
change their shapes in mesophase, which was observed
by gradually disappearing of the mesogenic textures
under POM. The HOMO, LUMO and Eg obtained from the
cyclic voltammetric method showed that the electronic
3.4 UV-vis Spectroscopic Measurement and
photoisomerization
The UV-vis absorption spectra were recorded using an transition between the frontier molecular orbital (Eg) is
Agilent 8453 Spectrometer. The sample were dissolved related to the second maximum absorption peak of each
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Download Date | 3/21/16 12:30 AM

1080ꢀ
ꢀ
Minyan Zheng, Yongsheng Wei, Wei Geng
target molecule in the UV spectra. The effect of carbon [14] Furumi S., Akiyama H., Morino S., Ichimura K., A convenient
preparation of photoactive monolayers for liquid
number on Eg is in excellent agreement with that on the
crystal photoalignment by the surface adsorption of an
second maximum absorption wavelength in their UV
aminoalkylated, J. Mater. Chem., 1998, 8, 65-70
[15] Wang J.H., Liu L., Liu G.F., Guo J.X., Jia D.Z., Properties of novel
Spectra. The Eg and the absorption wavelengths can be
modified by changing the terminal alkyl group of the
target molecules; the subject requires further study in
order to change the photo-sensitive property of molecules
in a certain range.
azodyes containing powerful acceptor groups and thiophene
moiety, Synth. Met., 2000, 115, 213 217
[16] Zhang Y.Q., Ortega J., Baumeister U., Folcia C. L., Sanz-
Enguita G., Walker C., et al., Activated carbon modified with
4-(8-hydroxyquinoline-azo)benzamidine for selective solid-
phase extraction and preconcentration of trace lead from
environmental samples, Microchim Acta, 2010, 171, 225-232
[17] Nicolescu F.A., Jerca V.V., Dancus I., Petris A., Nicolescu T.V.,
Rau I.B., et al. Synthesis and characterization of side-chain
maleimide-styrene copolymers with new pendant azobenzene
moieties, J. Polym. Res., 2011, 18, 1009-1016
Acknowledgements: We gratefully acknowledge the
financial support from the National Natural Science
Foundation of China (No. 21102121), Natural Science
Foundation of Shaanxi Province (No. 2014JM2-2014).
[18] Liu J.Q., Zhang Q.Z., Zhang J.Z., Hou, W.G., Photochemical
behaviours of star-like liquid crystal with azobenzene terminal
groups, J. Mater. Sci., 2005, 40, 4517-4521
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