Wide temperature range azobenzene nematic and smectic LC materials

Article abstract of DOI:10.1080/15421400600655733

We have developed a series of azobenzene liquid crystals (azo LC) compounds, both multicomponent as well as single component, that possess room temperature nematic and smectic phases. Some of these materials have a wide mesophase temperature range which m

Full text of DOI:10.1080/15421400600655733

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Wide Temperature Range
Azobenzene Nematic and
Smectic LC Materials
U. Hrozhyk a , S. Serak a , N. Tabiryan a & T. J.
Bunning b
a Beam Engineering for Advance Measurements Co.,
Winter Park, Florida, USA
b Air Force Research Laboratory, Wright Patterson Air
Force Base, Ohio, USA
Published online: 22 Sep 2006.
To cite this article: U. Hrozhyk , S. Serak , N. Tabiryan & T. J. Bunning (2006) Wide
Temperature Range Azobenzene Nematic and Smectic LC Materials, Molecular Crystals
and Liquid Crystals, 454:1, 235/[637]-245/[647], DOI: 10.1080/15421400600655733
To link to this article: http://dx.doi.org/10.1080/15421400600655733
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Mol. Cryst. Liq. Cryst., Vol. 454, pp. 235=[637]–245=[647], 2006
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400600655733
Wide Temperature Range Azobenzene Nematic and
Smectic LC Materials
U. Hrozhyk
S. Serak
N. Tabiryan
Beam Engineering for Advance Measurements Co.,
Winter Park, Florida, USA
T. J. Bunning
Air Force Research Laboratory, Wright Patterson Air Force Base,
Ohio, USA
We have developed a series of azobenzene liquid crystals (azo LC) compounds, both
multicomponent as well as single component, that possess room temperature
nematic and smectic phases. Some of these materials have a wide mesophase tem-
perature range which makes them useful for a variety of practical applications.
Two main strategies were used for controlling the temperature range of these
mesogenic azo LC materials: 1) chemical modification of azobenzene molecules
with different terminal pendants; and 2) development of multicomponent compo-
sitions. These materials were highly photosensitive exhibiting nonlinear refraction
and transmission at nanowatt=microwatt power levels. Physical, electro- and
nonlinear-optical parameters of many of these materials have been characterized
in detail.
Keywords: azobenzene; liquid crystals; multicomponent compositions; thermal phase
transitions
INTRODUCTION
Azobenzene based molecules are among the most significant chemical
structures for developing optically controlled devices and photonic sys-
tems. The interest on azobenzene based materials has been sustained
This work was supported by DoD SBIR programs.
Address correspondence to N. Tabiryan, Beam Engineering for Advance Measure-
ments Co., 809 S. Orlando Ave, Suite I, Winter Park, FL 32789, USA. E-mail:nelson@
beamco.com
235=[637]

236=[638]
U. Hrozhyk et al.
at a remarkably high level during the last three decades [1]. The
enormous variety of such materials relies on a simple structural unit,
=
the nitrogen–nitrogen double bond ꢀN Nꢀ.
Many azobenzene based materials are mesogenic, most of them, how-
ever, in a temperature range considerably exceeding room temperature.
Development of a wide assortment of azo LCs possessing different meso-
phases including room temperature liquid crystallinity is a viable task
for modern chemistry using new advanced techniques in the organic
synthesis field. We have undertaken that task focused on low-molecular
liquid crystalline azocompounds and their compositions.
Two main strategies can be used for controlling the mesophase tem-
perature range of azo LC materials: 1) chemical modification of the
azobenzene molecules with different lateral and terminal pendants;
and 2) development of multicomponent compositions. The first sub-
stantial work on the synthesis of an ‘‘interesting new class of nematic
liquid crystals’’ was published in 1971 by R. Steinstrasser and L. Pohl
[2]. They synthesized 43 azocompounds with alkyl terminal substitu-
ents CnH2nþ1 belonging to 6 homologous series (n ¼ 1 to 6)
H2nþ1CnꢀC6H4ꢀNBNꢀC6H4ꢀR
where alkyloxy (CmH2m þ1ꢀOꢀ) or acyloxy (CmH2m þ1ꢀCOOꢀ) pen-
dants were used as opposite terminal R.
Many of those compounds possessed an enantiotropic nematic
phase and the melting points of some were close to room temperature.
The most known of these azo LCs, 4-butyl-40-methoxyazobenzene
(BMAB, or BAAB-1)
H9C4ꢀC6H4ꢀNBNꢀC6H4ꢀOCH3
has a nematic phase in the temperature range 32 to 47ꢁC and can exist
in a supercooled nematic phase at room temperature.
The phase behavior of those newly obtained azo LCs and their
mixtures were not studied in details. However, it was demonstrated
that low melting point azo LCs can be created on the basis of simple
combinations of alkyl- and alkoxy-terminal pendants. Analogous
nematic azocompounds were synthesized also by H. Kelker [3] and
K. Murase [4].
J. van der Veen et al. in 1972 [5] and M. Moriyama et al. in 2001 [6]
synthesized a homologous series of symmetrical dialkylazobenzenes
H
2nþ1CnꢀC6H4ꢀNBNꢀC6H4ꢀCnH2nþ1
with n up to 16. Most of those compounds had no mesophase or possess
only a monotropic nematic or smectic phase only.

Wide Temperature Range Azobenzene LC Materials
237=[639]
Even though publications on the synthesis of azo LCs are scant,
including isolated works on the synthesis of low molecular azo LC
structures for special applications [7,8], these can be used to conclude
that the presence of two polar terminal pendants like the alkoxy group
leads to azocompounds with high melting temperatures whereas azo-
compounds with symmetrical non-polar terminal pendants have a
poor ability to form nematic phases. Hence, azobenzene molecules
with one non-polar alkyl or two alkyl, but nonsymmetrical, terminal
pendants are the most promising candidates to exhibit mesophases
at room temperature.
Both directions discussed above have been utilized in this work
including: 1) the study of two and multicomponent compositions
based on 4-n-alkyl-40-n-alkoxy(acyloxy)azobenzenes; and 2) synthesis
of new potentially mesogenic azocompounds based on nonsymmetrical
4,40-di-n-alkylazobenzenes
H2nþ1CnꢀC6H4ꢀNBNꢀC6H4ꢀCmH2mþ1
and 4-n-alkyl(alkoxy)-40 -cyanoazobenzenes
H2nþ1CnðOÞꢀC6H4ꢀNBNꢀC6H4ꢀCꢂN
Nonsymmetrical 4,40-di-n-alkylazobenzenes are interesting due to
the anticipated low melting temperatures whereas 4-n-alkyl(alkoxy)-
40-cyanoazobenzenes are expected to possess a large positive dielectric
anisotropy. In addition, the presence of the strong electron acceptor
ꢀCꢂN group could lead to acceleration of the dark reverse cis-trans
transition.
PREPARATIVE ROUTES
The syntheses of the 4-n-alkyl-40-n-alkoxy(acyloxy)azobenzenes and
4-n-alkoxy-40-cyanoazobenzenes were performed by alkylation of the
appropriate azophenols (obtained by azo coupling of diazonium salts
of 4-alkylanilines or 4-aminobenzonitrile with phenol) with the alkyl
halides by the Williamson reaction
R1ðCNÞꢀC6H4ꢀNBNꢀC6H4ꢀOH þ R2Hal
! R1ðCNÞꢀC6H4ꢀNBNꢀC6H4ꢀOR2
where R1 ¼ ꢀCnH2nþ1, R2 ¼ ꢀCmH2mþ1, or by the acylation of the
potassium salts of the appropriate azophenols with acyl chlorides:
R1ꢀC6H4ꢀNBNꢀC6H4ꢀOK þ R2ꢀCOCl
! R1ꢀC6H4ꢀNBNꢀC6H4ꢀOOCꢀR2
with R1 ¼ ꢀCnH2nþ1 and R2 ¼ ꢀCmH2mþ1
.

238=[640]
U. Hrozhyk et al.
The syntheses of nonsymmetrical 4-n-alkyl(R1)-40-n-alkyl(R2)
azobenzenes and 4-n-alkyl-40-cyanoazobenzenes were performed by
coupling 4-alkyl or 4-cyano nitroso benzene with 4-alkylanilines in
glacial acetic acid:
R1ðCNÞꢀC6H4ꢀNBNꢀC6H4ꢀNO þ H2NꢀC6H4ꢀR2
! R1ðCNÞꢀC6H4ꢀNBNꢀC6H4ꢀR2
with R1 ¼ ꢀCnH2nþ1 and R2 ¼ ꢀCmH2mþ1
.
Phase transition temperatures were measured using polarizing
microscope equipped with a hot stage although in cases of low melting
points, the obtained data were corrected by DSC measurements.
ROOM TEMPERATURE COMPOSITIONS
4-n-alkyl-40-alkoxy(acyloxy)azobenzenes with the lowest mesophase
range have, as a rule, even numbers of carbon atoms (n ¼ 2, 4, 6)
in the alkyl terminal substituent R1. However, all of them are in
the mesophase above room temperature. We chose compounds of
4-n-butyl-40-alkoxyazobenzene series (BAABs, n ¼ 4) as the main
components for developing room temperature compositions.
Increasing the number m of the carbons in the alkoxy terminal
chains of these homologoues, generally increases the melting tempera-
tures. In addition, they become more smectogenic with elongation of
the alkoxy tails. Smectogenic ability in the BAAB series starts from
the sixth carbon in the alkoxy chain leading, first, to appearance of
monotropic smectic A and then to an enantiotropic smectic A phase
for the eighth homolog.
In order to check this tendency we synthesized homologs with
m ¼ 11ꢀ16. BAAB-11 and BAAB-12 proved to be pure smectogens with
no nematic phase at all and with smectic ranges 62 to 68ꢁC and 54 to
64ꢁC, respectively. Further elongation of the terminal alkoxy- pendant
decreases the extent of participation of the highly polarizable aromatic
azobenzene core. The mesogenicity is lost after the 12-th carbon atom
since the physical properties of BAABs start to be defined by the pre-
dominantly isotropic intermolecular interactions of the hydrocarbon
tails. The highest obtained homologs appeared as wax-like substances
with their melting points very close to each other: 53–55ꢁC for BAAB-
13; 56–58ꢁC for BAAB-14; and 55–57ꢁC for BAAB-16.
In summary, the development of wide temperature range azo LC
compositions was accomplished using BAAB homologs with 10 or less
carbon atoms in the alkoxy pendant. Several materials consisting of
two to four components proved practically useful possessing room
temperature liquid crystalline phases. Some of the compositions

Wide Temperature Range Azobenzene LC Materials
239=[641]
include other 4-n-alkyl-40-alkoxy(acyloxy)azobenzenes with R1 ¼ C2H5-
and R1 ¼ C6H13- alkyl pendants as well as nonmesogenic azodopants.
Mesophase ranges of 12 compositions possessing nematic and smectic
A phases are presented in Figure 1. The melting point of the lowest
melting quaternary composition 4915 is as low as ꢀ7ꢁC. DSC curves
of this composition at heating and cooling of the sample are shown in
Figure 2 as an example.
The main physical, electro- and nonlinear optical properties of sev-
eral compositions are presented in Table 1. Brief definitions of the main
parameters characterized in Table 1 are summarized below, see also [9].
The ‘‘Constant of nonlinear refraction n2’’ determines the change in
the refractive index n ꢀ n0 of the material under the influence of a
light beam of power density I according to the formula n ꢀ n0 ¼ n2I.
It was measured for planar oriented 10 mm thick layer of the material
with a laser beam of k ¼ 532 nm wavelength and I ¼ 4.4 ꢃ 10ꢀ7 W=cm2
power density, polarized parallel to the orientation of the LC. The
‘‘incubation energy Einc’’ determines the amount of light energy that
FIGURE 1 Mesophase ranges of several room temperature azo LC
compositions. Numbers near the bars identify the compositions.

240=[642]
U. Hrozhyk et al.
FIGURE 2 DSC curves of composition 4915.
the LC has to be exposed to in order to start the photoinduced meso-
phase-isotropic phase transition. The ‘‘transition energy Eiso’’ deter-
mines the amount of light energy that the LC has to be exposed to
in order for the material to be transformed into an isotropic phase.
Both the transition and incubation energies are measured for a laser
beam of k ¼ 409 nm wavelength and I ¼ 6.2 ꢃ 10ꢀ3 W=cm2 power den-
sity, polarized parallel to the planar orientation of 10 mm thick LC
layers. The ‘‘optical anisotropy Dn’’ is defined as the difference
between the principal values of the refractive indices of LC. It was
measured for the wavelength k ¼ 633 nm at T ¼ 23ꢁC. The constant
of ‘‘orientation diffusion D’’ allows an evaluation of the free relaxation
time s of the LC director reorientation in cells of thickness L with hard
anchoring boundary conditions using the formula s ¼ L2=D. This is
related to the orientational viscosity c and the elastic constant K1 of
the liquid crystal by the expression D ¼ p2K1=c.
The smectic A phases of the compositions 8621 and 8721 are mono-
tropic, however, they are stable at the room temperature forming
giant (several millimeter thick), highly transparent, smectic monodo-
mains. A photo of such a monodomain is shown in Figure 3.
SINGLE COMPONENT ROOM TEMPERATURE AZO LCS
As mentioned above, symmetrical 4,40-dialkyl azobenzenes have
no mesophase or show monotropic nematic or smectic phases only.
We synthesized nonsymmetrical 4,40-di-n-alkylazobenzenes with the

241=[643]

242=[644]
U. Hrozhyk et al.
FIGURE 3 Highly transparent monodomain of smectic 8621 in a 4 mm thick
cell.
number of carbon atoms (n) 1 to 5 on one end and (m) 2 to 8 on the
other end of the azobenzene molecule. Insertion of two different alkyl
pendants into the molecule of azobenzene drastically changes its meso-
genic properties compared with the combination of alkyl-alkoxy sub-
stituents or combination of two symmetrical alkyl pendants. Phase
transition temperature of these synthesized nonsymmetrical 4,40-di-
n-alkylazobenzenes are listed in Table 2. A sharp decrease in the
yields of intermediate nitroso compounds obtained from 4-alkylani-
lines did not allow us to study compounds with the number of carbons
atoms in the alkyl chain larger than 6.
The appearance of the nematic phase in nonsymmetrical 4,40-di-n-
alkylazobenzenes is observed starting from the second carbon atom
in the shorter of the two pendants. Increasing the number of carbon
atoms lowers the phase transition temperature driving the melting
points of some individual compounds below 0ꢁC. The widest enantio-
tropic nematic phase, 3 to 34ꢁC, is obtained for 4-n-propyl-40-n-hepty-
lazobenzene (n ¼ 3, m ¼ 7 in Table 2). The nematic-smectic phase
transition point is ꢀ16ꢁC for 4-n-propyl-40-n octylazobenzene (n ¼ 3,
m ¼ 8 in Table 2); the smectic phase is monotropic in this case but its
existence means that the lifetime of the supercooled nematic phase of
this compound should be very long (the nematic range in this case
should be ꢀ16 to 25ꢁC).
Very weak but still anisotropic interactions of practically nonpolar
molecules of nonsymmetrical 4,40-di-n-alkylazobenzenes lead to
unique mesogenic properties of these azo compounds. Some of these
compounds, for example 4-n-butyl-40-n-hexylazobenzene (n ¼ 4,
m ¼ 6 in Table 2), are isotropic liquids having very low melting tem-
peratures. They could serve as highly effective dopants for depression
of the clearing temperature in the azo LC compositions. From a

Wide Temperature Range Azobenzene LC Materials
243=[645]
TABLE 2 Phase Transitions Temperatures of Nonsymmetric
Dialkyl Azobenzenes
ꢀ
Compound
Phase transitions, C
n ¼ 1
m ¼ 2
m ¼ 3
m ¼ 4
m ¼ 5
m ¼ 6
m ¼ 7
m ¼ 8
n ¼ 2
m ¼ 3
m ¼ 4
m ¼ 5
m ¼ 6
m ¼ 7
m ¼ 8
n ¼ 3
m ¼ 4
m ¼ 5
m ¼ 6
m ¼ 7
m ¼ 8
n ¼ 4
m ¼ 5
m ¼ 6
m ¼ 7
m ¼ 8
n ¼ 5
m ¼ 6
m ¼ 7
m ¼ 8
Cr 75 I
Cr 59 I
Cr 52 I
Cr 64 I
Cr 58.5 I
Cr 51 I
Cr 52 I
Cr 42.5 I
Cr 6 N 20 I
Cr 25 I [3] N monotr
Cr 19 I
Cr 14 I [13] N monotr
Cr 16 I [11.5] N monotr
Cr 11.5 I [3] N monotr
Cr 11 N 27 I
Cr ꢁ22 I [11] N monotr
Cr 3 N 34 I
SmA monotr [ꢁ16] Cr12 N 25.5 I
Cr 3 N 13.5 I
Cr ꢁ2 N 7 I
Cr 1 N 24 I
SmA monotr [ꢁ11] Cr 11 N 21 I
Cr 20 N 26 I
Cr 10 N 40 I
SmA monotr [4] Cr 15 N 35.5 I
practical point of view, in the case of nonsymmetrical 4,40-di-n-
alkylazobenzenes, we encountered the opposite problem, too low
clearing temperatures. Their mixtures with the higher melting
4-n-alkyl-40-n alkoxyazobenzenes may combine advantages of both
series of homologous making feasible development of materials with
even wider mesophase range.
CYANO-AZOBENZENES (PUSH-PULL STRUCTURES)
Inserting alkoxy or alkyl chains in the para-position of highly polar
structure of 4-cyanoazobenzene leads to compounds with high melting
points. Phase transition temperatures of 11 cyano azocompounds we

244=[646]
U. Hrozhyk et al.
TABLE 3 Phase Transition Temperatures of Several
Cyano Azocompounds
ꢁ
Phase transitions, C
Compounds
4-n-alkoxy-40-cyanoazobenzenes
hexyloxy
heptyloxy
octyloxy
4-n-alkyl-40-cyanoazobenzenes
Cr 99 N 116.5 I
Cr 91 N 110 I
Cr 102 N 111 I
methyl
ethyl
Cr 169 I
Cr 118 I
propyl
butyl
pentyl
hexyl
heptyl
octyl
Cr 90 N 91.5 I
Cr 100 I [76] N monotr
Cr 91 I [86] N monotr
Cr 89 I [72.5] N monotr
Cr 76 N 80 I
Cr 85 I [74] N monotr
have synthesized are given in Table 3. Individual compounds of the
series 4-n-alkoxy-40-cyanoazobenzenes do not show practical nematic
ranges. Although replacement of alkoxy groups by alkyl pendants
leads to some decrease of melting points (10 to 17ꢁC), the results are
still far from being as impressive as in case of nonsymmetrical 4,40-
dialkylazobenzenes. Most of 4-n-alkyl-40-cyanoazobenzenes have high
transition temperatures dominated by monotropic nematic phases.
Apparently, the high dipole moment associated with the cyano group
enhances the tendency for crystallization. In principle, lowering of
the melting temperatures of these, so-called ‘‘push-pull,’’ azo struc-
tures could be achieved by inserting lateral substituents into their
molecules in order to enhance molecular asymmetry.
SUMMARY
We have developed a large series of azo compounds, both multicompo-
nent as well as single component, that possess room temperature
nematic and smectic phases. Some of those materials have a wide
mesophase temperature range, which makes them useful for a variety
of practical applications. In addition, we have synthesized a series
of high temperature azo LCs and isotropic azo compounds with
very low melting points. Combination of those materials will allow
the development of azo LC materials with custom phase transition
temperatures. Physical, electro- and nonlinear-optical parameters of
many of those materials have been characterized in detail. These
materials are highly photosensitive exhibiting nonlinear refraction

Wide Temperature Range Azobenzene LC Materials
245=[647]
and transmission at nanowatt=microwatt power levels. They can be
used for developing optically active complex material systems such
as polymer dispersed liquid crystals, holographic PDLCs [10], and
POLICRYPs [11], as well as for developing optically controlled choles-
teric liquid crystals [12].
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Photochemistry and Photobiology, Horspool, W. & Lenci, F. (Eds.), 2nd Ed., CRC
Press: Boca Raton, 89-1 – 89-16.
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[2] Steinstrasser, R. & Pohl, L. (1971). Zeit. Fu¨r Naturforsh., 26b, 577.
[3] Kelker, H., Scheurle, B., Hatz, R., & Bartsch, W. (1970). Angew. Chem., 82, 984.
[4] Murase, K. & Watanabe, H. (1973). Bull. Chem. Soc. Jpn., 46, 3142.
[5] Van der Veen, J., De Jeu, W. H., Grobben, A. H., & Boven, J. (1972). Mol. Cryst. Liq.
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[6] Moriyama, M., Song, S., Matsuda, H., & Tamaoki, N. (2001). J. Mater. Chem., 11,
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& Neubert, M. (2001). Liq. Cryst., 28, 1483.
[9] Tabiryan, N. V., Serak, S. V., & Grozhik. V. A. (2003). J. Opt. Soc. Am. B, 20, 1.
[10] Sutherland, R. L., Tondiglia, V. P., Natarajan, L. V., Bunning, T. J., & Adams, W. W.
(1994). Appl. Phys. Lett., 64, 1074.
[11] Caputo, R., Sukhov, A. V., Tabirian, N. V., Umeton, C., & Ushakov, R. F. (2001).
Mol. Cryst. Liquid Cryst., 372, 263.
[12] Hrozhyk, U., Serak, S., & Tabiryan, N. Phototunable Photonic Bandgap Structures,
in Book of Abstracts of the 11th International Topical Meeting on Optics of Liquid
Crystals, October 2–7, 2005, Sand Key, Clearwater Beach, Florida. p. 119. See also
BEAM Co.’s catalog of room temperature azobenzene liquid crystal materials at
www.beamco.com.