New conjugated bis(AZO) mesogens with "push-pull"substttuents

Article abstract of DOI:10.1080/15421400490487353

Two series of conjugated bis(azo) compounds with "push" (atkoxy, acyloxy) and "putt" (nitro) substitueras at the end positions were synthesized and their thermal behavior investigated. Smectic A and nematic mesophases were observed over long temperature intervals. Surprisingly, photoirradiation of these compounds does not give detectable amounts of the expected photoisomerized isomers.

Full text of DOI:10.1080/15421400490487353

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New Conjugated Bis(AZO)
Mesogens with “Push-PULL”
Substituents
Riccardo Labruto a , Giovanni Gottarelli a , Stefano
Masiero a , Gian Spada a & Jacques Malthete b
a Dipartimento di Chimica Organica “A. Mangini,” ,
Alma Mater Studiorum—Università di Bologna , Via
San Donato 15, Bologna, 40127, Italy
b Institut Curie, Section de Recherche , CNRS-UMR
168, 26, rue d'Ulm, Pairs Cedex 05, 75248, France
Published online: 18 Oct 2010.
To cite this article: Riccardo Labruto , Giovanni Gottarelli , Stefano Masiero ,
Gian Spada & Jacques Malthete (2004) New Conjugated Bis(AZO) Mesogens with
“Push-PULL” Substituents, Molecular Crystals and Liquid Crystals, 420:1, 1-8, DOI:
10.1080/15421400490487353
To link to this article: http://dx.doi.org/10.1080/15421400490487353
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Mol. Cryst. Liq. Cryst., Vol. 420, pp. 1–8, 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490487353
NEW CONJUGATED BIS(AZO) MESOGENS WITH
‘‘PUSH-PULL’’ SUBSTITUENTS
Riccardo Labruto, Giovanni Gottarelli, Stefano Masiero,
and Gian Piero Spadaꢀ
`
Alma Mater Studiorum—Universita di Bologna,
Dipartimento di Chimica Organica ‘‘A. Mangini,’’
Via San Donato 15, 40127 Bologna, Italy
Jacques Malthete
Institut Curie, Section de Recherche, CNRS-UMR 168, 26, rue d’Ulm,
75248, Pairs Cedex 05, France
Two series of conjugated bis(azo) compounds with ‘‘push’’ (alkoxy, acyloxy)
and ‘‘pull’’ (nitro) substituents at the end positions were synthesized and
their thermal behavior investigated. Smectic A and nematic mesophases were
observed over long temperature intervals. Surprisingly, photoirradiation of
these compounds does not give detectable amounts of the expected photoisome-
rized isomers.
Keywords: azo compounds; push-pull; photoisomerization
INTRODUCTION
Azo compounds were discovered around 1850 by W. H. Perkin in an
attempt to prepare synthetic quinine. The new compounds were soon
patented, and the era of azo dyes began: it soon flourished, but then started
to decline almost 100 years later [1].
In the present period, there is a revival of interest in azo compounds
as potentially useful for several modern applications, for example, in host-
guest systems for full-color reflective displays [2]; for photocontrolled
alignment exploiting their E-Z photoisomerization [3]; and for obtaining
photochemical switches for liquid crystalline systems [4], both chiral [5,6]
This work has been supported by MIUR (through a PRIN Project) and the University of
Bologna.
ꢀCorresponding author. E-mail: GianPiero.Spada@unibo.it
1

2
R. Labruto et al.
and achiral [7], molecular shuttles [8], and photoresponsive gels [9]. Azo
compounds have also been recently employed to obtain single-molecule
light-powered machines [10]. Several mono(azo) mesogens are described
in the literature [11], some [12,13] very recently; also, V-shaped mesogens
containing two nonconjugated azo groups have been described recently [14].
In connection with our research on molecular rectifiers [15,16], we were
interested in preparing conjugated bis(azo) compounds with ‘‘push’’ (–OR)
and ‘‘pull’’ (–NO2) substituents at the para end positions; we noticed that
some compounds were mesogenic.
Although the effect of nitro substitution in calamitic liquid crystals has
been the subject of a recent review [17], only a few nitro-containing mono
(azo) mesogens (see, for example, compounds 1a–c [11,18] have been
reported (Scheme 1).
Considering that these mesogens are new, have interesting colors, and
when used as dopants in host–guest systems should not disorganize the
liquid crystalline hosts [12,13]; and considering also that similar structures
are known to generate second harmonics [19], we thought of carrying out
an investigation on this class of compounds. In particular, the ester-type
2a–f and ether-type 2g–1 derivatives were synthesized. All of the deriva-
tives displayed liquid crystalline properties (Scheme 2).
RESULTS AND DISCUSSION
Synthesis
Derivative 3 (‘‘disperse orange 3’’) was diazotized with NaNO2=HCL and
copulated with phenol in aqueous NaOH. The resulting bis(azo) phenol 4
was then either O-acylated at room temperature (r.t.) with an acyl chloride
SCHEME 1

‘‘Push-pull’’ Bis(azo) Mesogens
3
SCHEME 2
in tetrahydrofurane (THF) in the presence of triethylamine to give 2a–f or
O-alkylated with an alkyl halide in the presence of CsCO3 in refluxing N,N-
dimethylformamide (DMF) to give 2g–l (Scheme 3).
Thermal Behavior
The thermal behavior of compounds 2a–l was investigated by differential
scanning calorimetry and optical polarizing microscopy; all the compounds
show mesogenic behavior. The phase assignment based on the optical
textures (Figure 1 shows a representative example of photomicrographs
obtained for 2j) and the phase transition temperatures are reported in
Table I.
In the ester-type derivatives (2a–f ), smectic A and nematic phases
are present in a wide range of temperatures. By increasing the length
of the R substituent, while the Cr-SmA and N-I transitions are essentially
SCHEME 3

4
R. Labruto et al.
FIGURE 1 SmA optical texture of 2j at 190ꢁC (crossed polars, magnification ꢂ 100).
unaffected as expected, the SmA-N transition temperature increases, and
the range of existence of the nematic phase is reduced from 120ꢁC for
2a to ca. 40ꢁC for 2f.
TABLE I Transition Temperatures for Derivatives 2a–l (all N-I transitions occur
with partial decomposition)
Compound
Phase
Temp(ꢁC)
Phase
Temp(ꢁC)
Phase
Temp(ꢁC)
Phase
2a
2b
2c
2d
2e
2f
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
149
150
148
146
144
145.5
160
160
158
158
158
158
SmA
SmA
SmA
SmA
SmA
SmA
SmA
SmA
SmA
SmA
SmA
SmA
200
198
205
230
240
261
245
254
255
267
272
263
N
N
N
N
N
N
N
N
N
N
320
300
260
260
280
300
270
280
270
270
I
I
I
I
I
I
I
I
I
I
I
I
2g
2h
2i
2j
2k
2l

‘‘Push-pull’’ Bis(azo) Mesogens
5
A similar trend is observed for the ether-type compounds (2g–l). In the
case of 2g, the derivative with the shortest substituent, the nematic phase
exists only in a relatively short interval (ca. 25ꢁC): this interval reduces on
increasing the methylene number and finally disappears for 2k (and 2l).
Photochemical Behavior
It is well known that azobenzene-type compounds undergo an efficient and
fully reversible E-Z photoisomerization reaction. Mesogens incorporating
the azobenzene moiety or doped with azobenzene derivatives are therefore
promising materials for optical switching as a consequence of the reversible
shape transformation of the photoactive molecules and the consequent
effect on phase transitions [4–7].
Absorption spectra of 2f and 2j in the 220–600 nm range are reported in
Figure 2. As expected, the main absorption band, corresponding to a ppꢀ
transition, is red shifted for the either-type derivative 2j. Chloroform solu-
tions of derivatives 2f and 2j (E,E form) were irradiated at different wave-
lengths (marked with arrows in Figure 2), and the photochemical
transformations were followed by UV-V is absorption. Surprisingly, in all
the experiments we did not observe any phototransformation under the
experimental conditions used.
FIGURE 2 Absorption spectra of 2f (full line) and 2j (dashed line) in chloroform.
Full line and dashed arrows indicate the wavelength used for the photoirradiation
of 2f and 2j, respectively.

6
R. Labruto et al.
The reason for the absence of a detectable amount of photoisomerized
isomers from the para-bis(azo) derivatives 2f and 2j in the same con-
ditions in which azobenzene (and even ‘‘push-pull’’ substituted (mono)azo-
benzenes) [20] shows the E-Z phototransformation reaction is under
investigation. A possible explanation for such a reduced photoreactivity,
which emerges from preliminary results on similar compounds [21], is
related to the extensive electronic conjugation of the two azo groups in
the para-bis(azo) compounds: this conjugation renders the two azo groups
photochemically nonindependent. On the other hand, analogous meta-
bis(azo) derivatives, in which the conjugation between the two azo units
is substantially lower, show the usual photoisomerization.
EXPERIMENTAL PART
General Information
The optical texture observation was performed with a Zeiss Standard polar-
izing optical microscope equipped with a Linkam hot stage or a Leitz Ortho-
lux polarizing optical microscope carrying a Mettler FP 82 heating stage
equipped with a Mettler FP 80 temperature controller. The thermal transi-
tions were determined with both optical microscopy and differential
scanning calorimetry (Perkin-Elmer DSC-7). Light excitation in the photo-
chemical experiments was performed by a mercury arc lamp (interference
filters were used to select the spectral range) or by the Xe lamp of a JASCO
J710 spectrometer (wavelengths were selected with the instrument mono-
chromator). 1H NMR and ESI-MS spectra were measured on Varian Gemini
200 and ZMD Micromass spectrometers, respectively.
Synthesis
4-[40-(400-nitrophenylazo)phenylazo]phenol 4
4-(40-nitrophenylazo)aniline 3 from Aldrich was purified by column
chromatography. 3 (2.06 mmol) and 1.3 ml HCl conc. were stirred in 2 ml
H2O at 0ꢁC. A cooled solution of NaNO2 (2.06 mmol) in 1 ml H2O was slowly
added, keeping the temperature below 4ꢁC. The excess of NaNO2 was elimi-
nated, adding several drops of urea in water solution. The brown diazonium
salt solution thus obtained was added to a solution of phenol (2.74 mmol)
and NaOH (17.5 mmol) in 3 ml water at 0ꢁC. The resulting solution was
acidified with HCl conc. and then filtered and washed with water. After
purification by column chromatography (petroleum benzine:Et2O 50:50),
dark red, amorphous dust of 4-[40-(400-nitrophenylazo)phenylazo]phenol 4
1
was separated in a 60% yield. H NMR d (200 MHz, CDCl3: 8.46 (d, 2H);
8.16 (d, 2H); 8.14 (d, 2H); 7.98 (d, 2H). MS-ESI: 346 (M-H)ꢃ .

‘‘Push-pull’’ Bis(azo) Mesogens
7
4-[40-(400-nitrophenylazo)phenylazo]phenyl alkanoates 2a–f
4-[40-(400-nitrophenylazo)phenylazo]phenol 4 (1 eq) was stirred over-
night with the appropriate acyl halides and Et3N (5 eq ) in anhydrous
THF at room temperature under argon. The mixtures were filtered, washed
with a little methanol, and dried under vacuum. The compounds were
purified with column chromatography in about 50% yields. 1H NMR d
(200 MHz, CDCl3): all the compounds gave similar values of d, differing only
in the number of alkyl chain CH2 protons; d 8.34 (d, 2H), 8.00–8.10 (m,
4H), 7.95 (d, 2H), 7.24 (d, 2H), 2.59–2.62 (t, 2H), 1.78–1.80 (q, 2H),
1.20–1.52 (m, alkyl chain CH2), 0.85–0.93 (t, 3H) MS-ESI: m=z; 2a 501.3
(M) ꢀ , 524 (M þ Na)þ ; 2b 487.5 (M) ꢀ , 510.5 (M þ Na)þ ; 2c 473.5
(M) ꢀ , 497 (M þ Na)þ ; 2d 459.5 (M) ꢀ , 482.5 (M þ Na)þ ; 2e 445.4
(M) ꢀ , 468.5 (M þ Na)þ ; 2f 431.4 (M) ꢀ , 454.5 (M þ Na)þ .
4-[40-(400-nitrophenylazo)phenylazo]alkoxybenzene 2g–l
4-[40-(400-nitrophenylazo)phenylazo]phenol 4 (1 eq ) with the appropri-
ate alkyl halides (1.5 eq) and Cs2CO3 (1.5 eq) in N,N-dimethylformamide
were stirred under argon atmosphere and heated at reflux. After 24 h,
the solution was concentrated under vacuum, dissolved in ether, and then
filtered. The ether solution was extracted with water, and the combined
organic layers were dried (MgSO4). The solid was purified by column chro-
1
matography (petroleum benzine: CH2Cl2) and gave about 55% yield. H
NMR (200 MHz, CDCl3): all the compounds gave similar spectra, differing
only in the number of alkyl chain CH2 protons; d 8.40 (d, 2H), 7.99–8.18
(m, 4H), 7.97 (d, 2H), 7.02 (d, 2H), 4.04–4.08 (t, 2H), 1.82–1.85 (q, 2H),
1.20–1.55 (m, alkyl chain CH2), 0.88–0.90 (t, 3H).
REFERENCES
[1] (1959). The Story of Chemical Industry in Basle, (Olten and Lausanne: Urs Graf
Publisher), pp. 118–154.
[2] (a) Broer, D. J. & Lub, J. (1996). U.S. patent 5506704.
(b) Taira, K., Iwanaga, H., Hotta, A., Nakai, Y., Ohtake, T., & Sunohara, K. (1996).
proceedings of AM-LCD ’96, p. 333.
[3] Ichimura, K. (2000). Chem. Rev., 100, 1874.
[4] Feringa, B. (Ed.). (2002). Molecular Switches Weinheim: (Wiley-VCH).
[5] Ruslim, C. & Ichimura, K. (2001). Adv. Mater., 13, 37.
[6] Pieraccini, S., Masiero, S., Spada, G. P., & Gottarelli, G. (2003). Chem. Commun., 598.
[7] Prasad, S. K. & Nair, G. G. (2001). Adv. Mater., 13, 40.
[8] Murakami, H., Kawabuchi, H., Kotoo, K., Kunitake, M., & Nakashima, N. (1997). J. Am.
Chem. Soc., 119, 7605.
[9] Murata, M., Aoki, M., Nishi, T., Ikeda, A., & Shinkai, S. (1991). J. Chem. Soc., Chem.
Commun., 1715.

8
R. Labruto et al.
[10] Huge, T., Holland, N. B., Cattani, A., Moroder, L., Seitz, M., & Gaub, H. E. (2002). Science,
296, 1103.
[11] (a) Demus, D., Demus, H., & Zaschke, H. (1974). Flussige Kristalle in Tabellen
(Leipzig: VEB, Deutscher Verlag fur Grundstoffindustrie).
(b) Demus, D. & Zaschke, H. (1984). Flussige Kristalle in Tabellen (Leipzig: VEB,
Deutscher Verlag fur Grudstoffindustrie), and references therein.
[12] Lai, L. L., Wang, E., Lee, L. J., Chen, J. J., & Yang, D. W. (2001). Liq. Cryst., 28, 157.
[13] Lai, L. L., Lee, L. J., Lee, G. H., Wang, Y., Lu, K. L., & Lee, S. J. (2001). Liq. Cryst., 28,
1513.
[14] Prasad, V. (2001). Liq. Cryst., 28, 145.
[15] Rinaldi, R., Branca, E., Cingolani, R., Masiero, S., Spada, G. P., & Gottarelli, G. (2001).
Appl. Phys. Lett., 78, 3541.
[16] Rinaldi, R., Maruccio, G., Biasco, A., Arima, V., Cingolani, R., Giorgi, T., Masiero, S., Spada,
G. P., & Gottarelli, G. (2002). Nanotechnology, 13, 398.
[17] Petrov, V. F. & Shimizu, Y. (2001). Liq Cryst., 28, 1627.
[18] (a) Imrie, C. T., Karasz, F. E., & Attard, G. S. (1993). Macromolecules, 26, 545.
(b) Li, M., Luckhurst, G. R., Timini, B. A., Qiu, H.-J., & Chen, X.-F. (2001). Chem. Res.
Chin. Univ., 17, 108.
(c) Blokhina, S. V., Ol’khovich, M. V., Lokhanova, A. V., & Trostin, A. N. (1998). Z. Fizich.
Khimii, 72, 399.
[19] Nalwa, H. S. & Miyata, S. (Eds.). (1997). Non Linear Optics of Organic Molecules and
Polymers (Boca Raton, FL: CRC Press).
[20] Park, N.-H., Park, S.-L., & Suh, K.-D. (2002). Liq. Cryst., 29, 1253.
[21] Cisnetti, F., Ballardini, R., Credi, A., Gandolfi, M. T., Masiero, S., Negri, F., Pieraccini, S.,
Spada, G. P. (2004). Chem. Eur. J., 10, 2001.