Design, synthesis and photoisomerization behavior of novel azobenzene-based dyes containing different alkyl chains and isolation groups

Article abstract of DOI:10.1016/j.tetlet.2016.10.101

A series of reactive azo dyes based on 4-phenylazophenol, containing hexenyloxy-, allyloxy-ethoxy-propoxy- and tetramethyl-disiloxanyl-propoxy substituents were synthesized. Additionally, allyl-functionalized azo-based chromophores with changeable isolati

Full text of DOI:10.1016/j.tetlet.2016.10.101

Accepted Manuscript
Design, synthesis and photoisomerization behavior of novel azobenzene-based
dyes containing different alkyl chains and isolation groups
Ihor M. Tkachenko, Yaroslav L. Kobzar, Olha G. Purikova, Alexander L.
Tolstov, Oleg V. Shekera, Valery V. Shevchenko
PII:
S0040-4039(16)31433-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.10.101
TETL 48271
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
3 September 2016
17 October 2016
26 October 2016
Please cite this article as: Tkachenko, I.M., Kobzar, Y.L., Purikova, O.G., Tolstov, A.L., Shekera, O.V., Shevchenko,
V.V., Design, synthesis and photoisomerization behavior of novel azobenzene-based dyes containing different alkyl
chains and isolation groups, Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.10.101
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Design, synthesis and photoisomerization behavior of novel azobenzene-based dyes
containing different alkyl chains and isolation groups
Ihor M. Tkachenko, Yaroslav L. Kobzar, Olha G. Purikova, Alexander L. Tolstov,
Oleg V. Shekera, Valery V. Shevchenko*
Institute of Macromolecular Chemistry of the National Academy of Sciences of Ukraine, 48
Kharkivske shosse, Kiev, 02160, Ukraine
*Corresponding author. Tel.: +38-044-559-5500; fax: +38-044-292-4064.
E-mail address: valshevchenko@yandex.ru
Abstract
A series of reactive azo dyes based on 4-phenylazophenol, containing hexenyloxy-, allyloxy-
ethoxy-propoxy- and tetramethyl-disiloxanyl-propoxy substituents were synthesised. Additionally,
allyl-functionalized azo-based chromophores with changeable isolation groups (hydroxymethylene-,
trimethylsiloxymethylene- and pentafluorophenoxymethylene-) as well as with an additional
azobenzene unit were successfully obtained. The photoisomerization behaviors of the synthesized azo
dyes were investigated in solution as well as in films. Importantly, the occurrence of
photoisomerization (at least near the surface after UV treatment) was confirmed in the crystalline dye
films. The resulting compounds can be employed for the synthesis of a wide variety of azo-based
materials which may be suitable for optical and electrooptical applications.
Keywords: azo dyes, photoisomerization, isolation group, chromophore-chromophore aggregation

Introduction
Organic dyes containing an azobenzene fragment are extensively used as tunable and light-
responsive chromophores due to reversible cis-trans photoisomerization.1-10 This isomerization is
reversible by photochemical and thermal pathways. However, the low thermal stability of azobenzene
dyes and high ability to aggregate in the solid phase have limited their applications in devices.11-14 One
approach to solve these problems is to incorporate the organic azo chromophores into a polymer
backbone (generally, polymers with azo units in the side chains) as well as the polyhedral oligomeric
silsesquioxane (POSS) core.13-20 Additionally, such azo-containing systems are able to form uniform
and ultrathin films. It is known that organic films which are smooth and homogeneous over large areas
possess advanced optic and optoelectronic properties.21 Many potential applications have been
proposed for azo-containing polymers or star-shaped molecules with the POSS core and azobenzene
dye arms, such as optical data storage, liquid crystal displays, molecular switches, nonlinear optical
devices, surface relief gratings, and as a stable active medium to induce reversible plasmonic
modulations of embedded metal nanostructures. Therefore, the development of new azo-containing
dyes bearing reactive substituents is of significant importance.
Literature reports have clearly demonstrated that the flexible spacer chain length of azobenzene
dyes plays an important role in regulation of the optical and liquid crystalline properties of materials
based on such dyes.17, 20, 22-26 Thus, it is crucial to control the distance between the azobenzene unit and
the terminal reactive group, that is, to regulate the spacer chain length of dyes. At the same time the
influence of the chemical nature of flexible alkyl chains on the photoisomerization behavior of azo-
containing materials has been poorly studied. On the other hand, incorporation of bulky groups into the
azo chromophore with constant spacer lengths between the azo dyes and medium (polymer, POSS)
represents another effective way to regulate the photooptical properties of azo-containing materials.
More importantly, these chemical bulky groups can suppress crystallization of azo-containing
materials (crystallinity leads to poor quality film surfaces) as well as serve as so-called suitable

isolation groups which are introduced to decrease the intermolecular dipole-dipole interaction of
chromophore fragments.27-29
Thus, the current work represents an approach to the design of reactive azobenzene dyes
possessing flexible alkyl chains of different chemical natures and lengths as well as azo dyes
containing allyl tails and isolation groups of different sizes. Additionally, the photoisomerization
behavior of the synthesized dyes were thoroughly investigated in solution as well as in films.
Results and discussions
For the synthesis of dyes 3, 5 and 7 containing flexible and reactive hexenyloxy-, allyloxy-
ethoxy-propoxy-, and tetramethyl-disiloxanyl-propoxy fragments, respectively, commercially
available 4-phenylazophenol (1) was chosen as the initial precursor (Scheme 1). The obtained dye 2,
synthesised according to a literature method,22 was readily transformed into the corresponding
hexenyloxy-substituted dye 3 by dehydrohalogenation using the KOH-ethanol system. It should be
noted that the synthesis of azo dye 3 has previously been reported from dye 1 and the prohibitively
expensive 6-bromo-1-hexene.20 However, our approach to the synthesis of dye 2 provides an easy and
inexpensive synthetic method. Dye 5 with the allyloxy-ethoxy-propoxy substituent was prepared via
etherification of compound 1 with excess 1,3-dibromopropane,31 and subsequent reaction of dye 4 with
2-allyloxyethanol in the presence of NaH.
Several recent studies demonstrated that inserting a flexible disiloxane segment into the lateral
alkyl chain of azo dyes can improve their photo-responsive properties.32,33 Therefore, 1,1,3,3-
tetramethyldisiloxane was used for the synthesis of dye 7 containing the tetramethyl-disiloxanyl-
propoxy substituent, and consequently, the terminal hydride functional group. Thus, dye 6 was
synthesized from the reaction of dye 1 with allyl bromide19 and subsequently converted into the azo
chromophore 7 via a hydrosilylation reaction. A 8-fold excess of 1,1,3,3-tetramethyldisiloxane was
used to ensure formation of the mono-substitution product.

Scheme 1. Synthetic pathways for the synthesis of dyes 3, 5 and 7.
Hence, the synthesized dyes 3, 5 and 7 consist of the following structural units: azobenzene
fragments, flexible spacers of different lengths and a terminal reactive vinyl group (3 and 5) or Si-H
group (7).
The introduction of isolation groups to the chromophore moieties should be an efficient approach
to minimize azobenzene aggregation and the crystallinity of materials based on such dyes. Thus, we
chose 5-[2-phenyldiazen-1-yl]-2-(prop-2-en-1-yloxy)benzaldehyde (8),34 carrying the reactive
aldehyde group at the ortho-position to the allyloxy-fragment of the aromatic ring, as a scaffold for the
attachment of a variety of isolation groups, including an additional azobenzene unit (Scheme 2).

Scheme 2. Synthesis of dyes 9-12.
The synthesis of azo dye 9 containing a hydroxymethylene group on the aromatic ring was
accomplished by reduction of the aldehyde group of dye 8 with NaBH4. The obtained dye 9, due to the
presence of the vinyl group, can be potentially used for hydrosilylation reactions with Si-H containing
compounds. In that context, protection of the hydroxyl groups of dye 9 is essential because they can
easily react with Si-H bonds (O-silylation).35,36 Thus, we obtained the trimethylsilyl-protected dye 10
(Scheme 2), and hence, OH-functionalized materials can be easily prepared where necessary. Notably,
dye 10 was easily hydrolyzed by atmospheric moisture. As a result, the photoisomerization behavior of
dye 10 was not studied. Although dye 10 is easily hydrolyzed, it may provide a useful starting material
for subsequent reactions, and due to the presence of the hydroxyl group, it is capable of further
functionalization.
It is known that perfluoroaromatic rings are electropositive, which could lead to reversible self-
assembly between the non-fluorinated aromatic units and the perfluoroaromatic units (Ar-ArF
interactions).37,38 Thus, we obtained the pentafluorophenyl-bearing dye 11 by the reaction of dye 9
with excess hexafluorobenzene (Scheme 2).

Furthermore, a new azo-chromophore 12 was synthesized via etherification of compound 8 with
dye 2. The additional azobenzene unit could act as an isolation group, and eventually to prevent
crystallization of the azo-containing materials based on dye 12. Additionally, the synthesis of dyes
with two azobenzene groups is an approach to increase chromophore concentration. Due to the
presence of reactive functional groups, the resulting vinyl-containing azo dyes (3, 5, 9, 11 and 12) and
dye 7 with the tetramethyl-disiloxanyl-propoxy fragment can be attached to the structure of hydride
(e.g. octakis-(dimethylsilyloxy)silsesquioxane) or vinyl-containing (e.g. octavinyl polyhedral
oligomeric silsesquioxane) medium via hydrosilylation reactions.
All of the synthesized dyes belong to the azobenzene type molecules according to Rau
1
classification.39,40 The structures of the synthesized compounds were determined by IR, UV-Vis, H,
13C, 19F NMR, and mass spectrometry techniques, and were in full agreement with the proposed
structures (ESI, Fig. S1-S8). The electronic absorption spectra of the prepared dyes exhibit two
characteristic absorption bands. The high intensity band at 343–347 nm is related to -* transition of
the trans form of the azobenzene moiety. The weak band in the range of 427–443 nm originates from
typical n-* transition.
The alkyl tail lengths of azo compounds 3 (0.86 nm), 5 (1.31 nm) and 7 (0.92) were calculated
using quantum-chemical calculations (ESI, Fig. S9). Although the alkyl chain lengths of azo dye 3 and
7 were comparable, they have different chemical structures.
Figure 1 represents the electronic absorption spectra of dyes 5, 11 and 12 and the kinetics of the
cis-trans photoisomerization of dyes 3, 5, 7, 9, 11 and 12 in CHCl3. The photoisomerization spectra of
dyes 3, 7 and 9 are reported in the ESI (Fig. S10). It was found that photoisomerization reached a
photostationary state within 35 s for solutions of dyes 3, 5 and 7, within 50 s for solutions of dyes 9
and 11, and within 40 s for solutions of dye 12. As seen in Figure 1 the intensity of the absorption band
at 343−347 nm (-* transition) decreased progressively, while the absorption band corresponding to
the n-* transition in the cis isomer at 427−443 nm increased with irradiation time, suggesting that

isomerization of the azobenzene chromophore from the trans to cis form occurs until a photostationary
state is reached.
Figure 1. UV-vis spectra of dyes (a) 5, (b) 11 and (c) 12 under UV irradiation at 365 nm.
(d) Kinetics of the cis-trans isomerization of dyes 3 (line a), 5 (line b), 7 (line c), 9 (line d), 11
(line e) and 12 (line f) in CHCl3 solution.
Compounds 3, 5 and 7 had similar observed photoisomerization rate constants: 0.131±0.031 s-1,
0.118±0.027 s-1 and 0.126±0.026 s-1, respectively, which indicate that the length of aliphatic fragments
has no significant effect on the photoisomerization. The photoisomerization rates of dyes 9 and 11 with
lateral substituents in the ortho position to the ether linkage of the azobenzene fragment were

relatively slower compared to dyes 3, 5 and 7. Evidently, the reduced rate of photoisomerization may
be attributed to the change of the electronic nature of the azobenzene fragment. The values of
photoisomerization rate constants were 0.055±0.013 s-1 for dye 9 and 0.058±0.013 s-1 for dye 11. The
chemical structure of azo dye 12 can be defined as a hybrid structure of the two dye types: (i) dyes
without the isolation group (3, 5 and 7) and (ii) dyes with isolation groups (9 and 11). Evidently, the
photoisomerization rate of the azobenzene unit without the lateral substituent of dye 12 is faster
compared to the azobenzene unit with lateral substituents in the ortho position to the ether linkage. For
this reason, the rate constant of dye 12 was 0.096 ± 0.021 s-1 which was higher than those for dyes 9
and 11, but lower than dyes 3, 5 and 7. It should be noted that all obtained azo dyes have shown a
similar reverse isomerization ability induced by irradiation with visible light (ESI, Fig. S11).
Next, we investigated the formation of films from the azo dyes by multi-step drop-casting
toluene solution onto glass substrates treated with poly(1-vinylpyrrolidone-co-styrene). All azo dyes
gave discontinuous and inhomogeneous films due to crystallization and aggregation. Thus,
transmission and reflection optical microscopy (TOM and ROM, respectively) revealed the crystalline
structure of azo dye 3, 9 and 11 (ESI, Fig. S12). A very rough film surface was observed for hybrid
azo dye 12. In general, it appears that the longer alkyl tails of azobenzenes 5 and especially 7 have a
positive effect on film-forming properties. Although azo-dye molecules 5, 7 and 12 are arranged in an
ensemble of clumps of different sizes in their films, these dyes gave somewhat more uniform and
amorphous films with a decreased microroughness. Evidently, the covalent linkage of the obtained
dyes to the polymer backbone or POSS molecules will facilitate the formation of uniform ultrathin
films.
UV-vis absorption spectra are sensitive to the interchromophore distance and orientation, and
UV-vis spectroscopy is a useful tool to evaluate the aggregation effect of organic dyes.41 The
absorption band corresponding to -* transition in the trans isomer of all dye films (especially for
crystalline films of azo dyes 3, 9 and 11) was broad (Fig. 2 and ESI, Fig. S13). This may be due to the
partial chromophore-chromophore aggregation of the azo dyes. Thus, the H- and J-aggregation of

azobenzene normally will result in the blue and red shift of maximum absorption band.42 The
absorption band (-* transition) of dye 7 is only slightly broader compared to its solution absorption
(Fig. 2a and ESI). Apparently, this is associated with the optimal quality of azo dye 7 film.
Figure 2. (a) Photoisomerization of dye 7 in film. (b, c) UV-vis spectra of (b) dye 9 and (c) dye
11 films after irradiation with UV light (365 nm). (d) Photoisomerization of dye 11 in film after UV
light annealing at 365 nm.
Importantly, the trans-cis photoisomerization of azobenzene still occurs in films based on dyes 5
and 7, containing flexible allyloxy-ethoxy-propoxy- and tetramethyl-disiloxanyl-propoxy fragments
respectively, as well as “hybrid” dye 12. It is worth noting that the photoisomerization of these dyes in
films was much slower than in solution. For example, a photostationary state for dye 5 was observed

after 20 min UV irradiation. At the same time it was observed that the photoisomerization rate and the
isomerization degree at the photostationary state significantly depend on the film thickness. Thus, the
photoisomerization rate constants of the investigated azo dye films were not calculated since the drop-
coated films from each dye were too varied in thickness for reliable measurement.
Interestingly, when solid films of azo dyes 3, 9 and 11 were UV irradiated up to 12 min, the
maximum of n-* electronic transitions of azobenzene chromophores (~440 nm) continuously
increased and the maximum of -* electronic transitions also increased (gradually) (Fig. 2b, c and
ESI, Fig. S13). Clearly, the intensity of the maximum cis absorption band increases as a result of
trans-to-cis photoisomerization. The formation of cis azobenzene isomers results in the destruction of
the face-to-face interactions of planar the trans-azobenzene moieties that lead to the increase of
intensity of the trans isomer band. Further UV irradiation, up to 35 min, resulted in a further increase
of absorption intensity of the trans band and absorption suppression at ~440 nm (cis absorption).
Further UV irradiation of the azo dye 11 film to 85 min led to a decrease of both the maximum of -*
and n-* electronic transitions (Fig. 2c). The most likely interpretation for this is that recrystallization
occurred with the formation of more stable trans-azobenzene structures and subsequent generation of
their aggregates.
In general, UV annealing can significantly improve the different properties of the films and
promote recrystallization so as to form preferred structures.43 Thus, the photoisomerization behavior of
dye 11 film after irradiation with UV light (365 nm, 1 h) and white light (>450 nm, 5 min) at room
temperature was examined. As seen from Figure 2d, surprisingly, the annealed sample film showed
typical photoisomerization behavior of the azobenzene moieties upon UV excitation and reaches a
photostationary state after about 10 min. Normally, trans-cis photoisomerization of azobenzene
chromophores is not observed in the bulk crystalline state because of the large geometric changes that
would be required in such densely packed crystal lattices.44 However, it was shown that
photoisomerization near the surface of crystals can occur.44 This aspect of the photoisomerization
behavior will be investigated in greater detail in due course.

In summary, a series of reactive azo dyes based on 4-phenylazophenol and containing
hexenyloxy-, allyloxy-ethoxy-propoxy- and tetramethyl-disiloxanyl-propoxy substituents were
synthesized. Additionally, we developed synthetic routes for several new azo-based chromophores
with
changeable
isolation
groups
(hydroxymethylene-,
trimethylsiloxymethylene-,
pentafluorophenoxymethylene- and azobenzene groups) as well as with an allyl tail. Typical
photoisomerization behavior was observed for all synthesized dyes in solution. The results of
photoisomerization experiments in solution show that introduction of side isolation substituents in the
ortho-position to the ether linkage of the azobenzene fragment of the dyes results in a decrease of
photoisomerization rate constants. More importantly, the occurrence of photoisomerization (at least
near the surface of crystalline films) was confirmed in the solid state of the azo dyes films. It is
reasonable to suggest that the obtained dyes can be considered for the synthesis of different azo-
containing materials based on POSS compounds or polymers and have good prospects for usage as
nanometer sized photoswitches, active layer materials with optical-limiting properties, and a medium
with photoinduced anisotropy for optical storage.
Acknowledgements
This work was partially supported by grant of the National Academy of Sciences of Ukraine No
0115U005242.
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Highlights
 A synthetic route for a group of new azo dyes is described.
 The design consists of using flexible alkyl chains and isolation groups.
 The photoisomerization behavior of the obtained dyes was thoroughly investigated.
 Prepared dyes have the potential to be used in stimuli responsive optical materials.

Graphical Abstract
Design, synthesis and photoisomerization behavior
Leave this area blank for abstract info.
of novel azobenzene-based dyes containing different
alkyl chains and isolation groups
Ihor M. Tkachenko, Yaroslav L. Kobzar, Olha G. Purikova, Alexander L. Tolstov, Oleg V. Shekera,
Valery V. Shevchenko