Effects of photo-irradiation on the optical properties and electronic structures of an azo-containing bent-core liquid crystal

Article abstract of DOI:10.1080/15421400903057884

With the ultraviolet-visible spectrophotometry and electronic structure calculations, we have investigated the effects of photo-irradiation on the optical properties and electronic structures of an azo-containing banana-shaped liquid crystal. The synthesized compound exhibits two optical absorptions at about 275 and 360nm, respectively. Our electronic structure calculations reveal that the two absorptions can be attributed to the π→σ* and π→π* transitions in the azo-chromophores of the banana-shaped liquid crystal. Our results have demonstrated that photo-dissociation takes place predominantly when dilute tetrahydrofuran solution of the azo-containing liquid crystal is exposed to the 365nm irradiation despite tran-to-cis photochemical isomerization is observed.

Full text of DOI:10.1080/15421400903057884

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Effects of Photo-Irradiation
on the Optical Properties and
Electronic Structures of an
Azo-Containing Bent-Core
Liquid Crystal
a
a
b
Yuan Ming Huang , Bao-Gai Zhai & Fu-Fang Zhou
a
College of Physics and Electronic Information,
Yunnan Normal University , Kunming, Yunnan, China
b
Modern Educational Technology Center, Yunnan
Normal University , Kunming, Yunnan, China
Published online: 05 Oct 2009.
To cite this article: Yuan Ming Huang , Bao-Gai Zhai & Fu-Fang Zhou (2009) Effects
of Photo-Irradiation on the Optical Properties and Electronic Structures of an Azo-
Containing Bent-Core Liquid Crystal, Molecular Crystals and Liquid Crystals, 510:1,
34/[1168]-42/[1176], DOI: 10.1080/15421400903057884
To link to this article: http://dx.doi.org/10.1080/15421400903057884
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Mol. Cryst. Liq. Cryst., Vol. 510, pp. 34=[1168]–42=[1176], 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903057884
Effects of Photo-Irradiation on the Optical Properties
and Electronic Structures of an Azo-Containing
Bent-Core Liquid Crystal
1
2
1
Yuan Ming Huang , Bao-Gai Zhai , and Fu-Fang Zhou
1
College of Physics and Electronic Information, Yunnan Normal
University, Kunming, Yunnan, China
2
Modern Educational Technology Center, Yunnan Normal University,
Kunming, Yunnan, China
With the ultraviolet-visible spectrophotometry and electronic structure calculations,
we have investigated the effects of photo-irradiation on the optical properties and
electronic structures of an azo-containing banana-shaped liquid crystal. The
synthesized compound exhibits two optical absorptions at about 275 and 360 nm,
respectively. Our electronic structure calculations reveal that the two absorptions
ꢀ
ꢀ
can be attributed to the p!r and p!p transitions in the azo-chromophores of the
banana-shaped liquid crystal. Our results have demonstrated that photo-
dissociation takes place predominantly when dilute tetrahydrofuran solution of
the azo-containing liquid crystal is exposed to the 365 nm irradiation despite
tran-to-cis photochemical isomerization is observed.
Keywords: azo-compound; banana-shaped liquid crystal; cis-trans isomerization;
photo-dissociation; trans-cis isomerization
1. INTRODUCTION
Being one novel class of banana-shaped liquid crystals (LCs),
azo-containing banana-shaped LCs bear significance in developing
photo-switchable molecular electronic devices because of the
Financial supports from Natural Science Foundation of China (No. 10674091),
the Scientific Research Fund for Returned Overseas Chinese Scholar from Education
Ministry of China (2005-546), and the Key Research Project of the Education Ministry
of China (No. 206110) are gratefully acknowledged. We thank Professor Gion Calzaferri
and Dr. Stephan Glause (University of Berne) for their kind assistances in the electronic
structure calculation.
Address correspondence to Yuan Ming Huang, College of Physics and Electronic
Information, Yunnan Normal University, Kunming, Yunnan 650092, China. E-mail:
dongshanisland@126.com
3
4=[1168]

Effects of Photo-Irradiation on the Optical Properties
35/[1169]
photoactive double bond –N¼N– in those substances [1–4]. The trans-
to-cis and cis-to-trans photochemical isomerizations of the double bond
–
N¼N– in azo-compounds play a critical role in the photo-switchable
molecular electronic devices [5]. As documented in literature, it is a
general belief that the irradiation of 365-nm ultraviolet may convert
trans azo-containing LC to its cis form while the irradiation of
4
36-nm blue light takes cis- azo-containing LC back to its trans form
[
1–4,6]. For an example, the spontaneous polarization of a ferroelectric
banana-shaped LC was decreased when the 365-nm ultraviolet was
turned on after the LC had been doped with a photoactive azo-
containing compound 4-(4-ethoxyphenylazo)phenyl hexanoate [7].
Although both the trans-to-cis and cis-to-trans photochemical
isomerizations are well documented for azo-compounds, photo-
dissociation may also take place when those azo-compounds are
exposed to the irradiation of energetic ultraviolet photons. Whether
the process of photo-dissociation takes place in azo-containing LCs is
not only critical to the performance of the photo-switchable molecular
electronic devices but also important to the understanding of the phy-
sics in the azo-containing banana-shaped LCs. The purpose of our
work is to investigate the effects of photo-irradiation on the optical
and electronic properties of an azo-containing banana-shaped LC.
Our results demonstrate that the photo-dissociation is the predomi-
nant process in the azo-containing LC when it is exposed to the
3
65-nm irradiation.
2. EXPERIMENTAL DETAILS
Following the procedures described in Ref. [1], we synthesized an
azo-containing banana-shaped compound. The synthesis route was
schematically shown in Figure 1. All analytical reagents were pur-
chased either from Alfa Aesar or from Guanghua Chemical Reagents
Co. Ltd (Guangdong, China). All reagents were used as received.
Intermediate compounds 4-(4-hydroxy-phenylazo)-benzoic acid ethyl
ester (intermediate A), 4-(4-hexyloxy-phenylazo)-benzoic acid ethyl
ester (intermediate B), 4-(4-hexyloxy-phenylazo)-benzoic acid (inter-
mediate C) and 4-(3-hydroxy-benzylidene-amino)-phenol (intermedi-
ate D) were purified by the techniques of recrystallization and the
silica gel chromatography, and then were characterized with the tech-
niques of nuclear magnetic resonance (AVANCE 400 MHz, Brucker),
Fourier transformed infrared spectroscopy, and mass spectroscopy,
respectively. Following the general route [8–10], we synthesized the
target compound, an azo-containing banana-shaped LC (E), by con-
densing the intermediates C and D in the solvent of dichloromethane

3
6/[1170]
Y. M. Huang et al.
FIGURE 1 Synthesis route of the azo-containing banana-shaped LC. Inter-
mediates A, B, C and D stand for 4-(4-hydroxy-phenylazo)-benzoic acid
ethyl ester, 4-(4-hexyloxy-phenylazo)-benzoic acid ethyl ester, 4-(4-hexyloxy-
phenylazo)-benzoic acid, and 4-(3-hydroxy-benzylidene-amino)-phenol,
respectively.
with the addition of catalysts 4-dimethylaminopyridine (DMAP) and
0
N,N -dicyclohexyl carbodiimide (DCC).
Analytical reagent tetrahydrofuran was used as the solvent to
prepare dilute solutions of the azo-containing banana-shaped LC.
A 10-mm-thick quartz cell was used as a cuvette of the solutions
for optical absorption measurements. The optical absorption spectra
of our synthesized target compound E were measured with an
ultraviolet-visible spectrometer UV-3501S (Tianjin Gangdong Ltd.,
China). A commercially available ultraviolet lamp was used to provide
a beam of 325 nm ultraviolet light for the ultraviolet irradiation.
The electronic structures of the target compound E and the two
intermediates C and D were calculated with the extended-H u¨ ckel
tight-binding method. BICON-CEDiT, an extended H u¨ ckel band struc-
ture and oscillator strength calculation package, was developed by the
group of Prof. Gion Calzaferri at University of Berne, Switzerland [11].

Effects of Photo-Irradiation on the Optical Properties
37/[1171]
It is a set of FORTRAN programs that allow extended-H u¨ ckel tight-
binding calculations for 1D-3D compounds to get their band structures
and density of states. In order to obtain the optimal molecular struc-
ture for the electronic structure calculation, we performed a geometry
optimisation of an individual molecule at AM1 level and then
extracted the geometry of the molecule. The AM1 program was imple-
mented in the MOPAC package (Quantum Chemistry Program
Exchange, Indiana University, USA). The standard Slater parameters
for C, H, O and N atoms were adopted from Ref. [12].
3. RESULTS AND DISCUSSION
Figure 2 shows the photo-irradiation time dependent optical absorp-
tion spectra of dilute solutions of the synthesized azo-containing
banana-shaped LC. The solvent was tetrahydrofuran while the con-
centration of the solution and the wavelength of the photo-irradiation
were 90 mg=L and 365 nm, respectively. Before the sample is exposed
to the 365-nm irradiation, the azo-containing banana-shaped LC can
FIGURE 2 Photo-irradiation time dependent optical absorption spectra of
dilute solutions of the synthesized azo-containing banana-shaped LC. The sol-
vent was tetrahydrofuran while the concentration of the solution and the
wavelength of the photo-irradiation were 90 mg=L and 365 nm, respectively.

3
8/[1172]
Y. M. Huang et al.
absorb light intensively at 360 nm but weakly at 275 and 440 nm, as
shown by the curve a in Figure 2. The broad but weak absorption at
about 440 nm suggest that our synthesized azo-containing banana-
shaped LC is a mixture of trans and cis azo-compounds even before
the 365 nm photo-irradiation. This might be caused by the room light
illumination in our lab. As the photo-irradiation time increases, the
absorption at 360 nm decreases quickly while the absorption at
2
75 nm increases correspondingly; in the meanwhile the broad absorp-
tion at 440 nm experiences a slight increases at first with a an obvious
decrease followed. As documented in literature for azo-compounds,
upon ultraviolet irradiation of around 360 nm (corresponding to the
ꢀ
p–p band of the azo-group) the elongated rod-like and energetically
more stable trans configuration changes into a bent cis-configuration.
The reverse transformation can be brought back by illuminating the
molecules with visible light of about 420 nm (corresponding to the
ꢀ
n–p band). If the above statement holds for banana-shaped azo-
compounds, the 360 nm absorption in Figure 2 can be assigned to
the electronic transition in trans banana-shaped LC while the 440 nm
absorption in Figure 2 can be assigned to the electronic transition in
cis banana-shaped LC. If so, the slight increase in the absorption
at 440 nm suggests that the 365-nm photo-irradiation can generate
trans-to-cis photochemical isomerization for our azo-containing
banana-shaped LC.
The electronic structures are now critically important to under-
stand the effects of the 365-nm photo-irradiation on the optical absorp-
tion of the banana-shaped LC because the optical properties and the
electronic structures of the banana-shaped LC are closely related.
Our previous work has demonstrated that the electronic structures
calculated with extended H u¨ ckel tight-binding program are helpful
to understand the optical properties of a series of LC polymers [13–
1
7]. Before we can understand the electronic structures of our target
compound, i.e., the azo-containing banana-shaped LC, the knowledge
on the electronic structures of intermediate compounds C and D will
be helpful. Figure 3 depicts the energy levels (left panel) and density
ꢀ
ꢀ
of states (right panel) of intermediate C. The p, n, p and r orbitals
ꢀ
are labelled in Figure 3. Figure 3 shows clearly that the p!p and
ꢀ
p!r transitions will generate optical absorptions at 3.58 eV
(
346 nm) and at 4.45 eV (279 nm), respectively. Although the non-
ꢀ
bonding orbit is also labelled, the n!p transition is prohibited due
to the selection rules. The results in Figures 2 and 3 reveals a close
correlation between the electronic transitions of the azo-containing
intermediate C and the optical absorptions of the azo-containing
banana-shaped LC.

Effects of Photo-Irradiation on the Optical Properties
39/[1173]
FIGURE 3 Energy levels (left panel) and density of states (right panel) of
intermediate C: 4-(4-hexyloxy-phenylazo)-benzoic acid.
Figure 4 represents the energy levels (left panel) and density of
ꢀ
states (right panel) of intermediate D. Similarly, the p, n and p orbi-
tals are labelled in Figure 4 for the intermediate D. From Figure 4,
FIGURE 4 Energy levels (left panel) and density of states (right panel) of
intermediate D: 4-(3-hydroxy-benzylidene-amino)-phenol.

4
0/[1174]
Y. M. Huang et al.
ꢀ
optical absorption at 4.3 eV (288 nm) will be resulted from the p!p
transition. In our experiment, we measured the optical absorption of
the dilute solutions of intermediate D, and our optical measurement
confirmed that the absorption of intermediate D took place at
2
90 nm (4.3 eV).
Figure 5 shows the energy levels (left panel) and density of states
(
right panel) of the azo-containing banana-shaped compound. It is
obvious that the electronic structures of the azo-containing banana-
shaped compound E are the simple summation of two independent
sets of electronic structures. The first set of electronic structures is
contributed by intermediate C while the second set of electronic struc-
tures is contributed by intermediate D. It is understandable because
there is no overlapping in the conjugations of the two kinds of chromo-
phores in E. As a result, the absorptions of our banana-shaped mole-
cule E will be a simple summation of the absorption of intermediate
C and the absorption of intermediate D.
On the basis of the calculated electronic structures, we can inter-
pret the optical absorptions of the azo-containing banana-shaped com-
pound E. The features of the electronic structures of E suggest that
the optical absorptions of E are contributed by intermediates C and
D independently. Therefore, the 360-nm absorption of E in Figure 2
ꢀ
originates from the p!p transitions in the azo-containing chromo-
phores. As shown in Figure 1, the two azo-containing chromophores
FIGURE 5 Energy levels (left panel) and density of states (right panel) of the
azo-containing banana-shaped compound E.

Effects of Photo-Irradiation on the Optical Properties
41/[1175]
are incorporated into the two branched legs of E. The photo-
irradiation induced decrease in the absorption at 360 nm suggests that
the photo-irradiation has broken some p conjugations in the azo-
linkages of the banana-shaped molecule E. Similarly, the 275-nm
ꢀ
absorption of E originates from the p!r transitions in the azo-
containing chromophores, and the photo-irradiation induced increase
in the absorption at 275 nm suggests that the photo-irradiation has
ꢀ
increased r bonding in the azo-linkages of the banana-shaped mole-
cule E. The photo-induced decrease in the p conjugations and a simul-
ꢀ
taneous increase in the r bonding indicate that photo-dissociation
process has taken place in the azo-containing banana-shaped LC.
4. CONCLUSION
With the ultraviolet-visible spectrophotometry and electronic structure
calculations, we have investigated the effects of photo-irradiation on
the optical properties and electronic structures of an azo-containing
banana-shaped LC. The synthesized compound exhibits two optical
absorptions at about 275 and 360 nm, respectively. Our electronic
structure calculations indicate that the two absorptions can be attrib-
ꢀ
ꢀ
uted to the p!r and p!p transitions in the azo-chromophores of
the banana-shaped LC. Our results have demonstrated that photo-
dissociation takes place predominantly when the dilute tetrahydro-
furan solution of the azo-containing LC is exposed to the 365 nm
irradiation despite tran-to-cis photochemical isomerization is observed.
The photo-irradiation induced photo-dissociation and the tran-to-cis
photochemical isomerization will be helpful to understand the photo-
physics and photochemistry in azo-containing banana-shaped LCs.
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