Triarylmethane Dyes for Artificial Repellent Cotton Fibers

Article abstract of DOI:10.1002/chem.201605572

Families of new hydrophobic and/or oleophobic triarylmethane dyes possessing long hydrocarbon or polyfluorinated chains have been prepared. When covalently grafted on to cotton fabric, these dyes give rise to a new type of colored superhydrophobic fibers.

Full text of DOI:10.1002/chem.201605572

DOI: 10.1002/chem.201605572
Communication
&
Surface Chemistry
Triarylmethane Dyes for Artificial Repellent Cotton Fibers
Ana Maria Montagut,[a] Erik Gꢀlvez,[a] Alexandr Shafir,[b] Rosa Marꢁa Sebastiꢀn,*[a] and
Adelina Vallribera*[a]
tensity of color and their brilliant shades of red, blue, and
green. It has also been reported that they possess diverse bio-
Abstract: Families of new hydrophobic and/or oleophobic
triarylmethane dyes possessing long hydrocarbon or poly-
logical properties such as antifungal, antiproliferative, antiviral,
cytotoxicity, phototoxicity toward tumor cells, and activity
fluorinated chains have been prepared. When covalently
toward the central nervous system.[13] Important dyes belong-
grafted on to cotton fabric, these dyes give rise to a new
ing to this class include the well-known Malachite Green and
type of colored superhydrophobic fibers.
Crystal Violet (Scheme 1). Triarylmethanes are leuco dyes that,
through chemical oxidation or photochemical irradiation, can
be converted to the chromatic form (Scheme 1).[14]
The interest in highly water- and oil-repellent surfaces has
grown in recent years, in part due to the promise of creating
self-cleaning surfaces.[1] A superhydrophobic surface is one
that achieves a water contact angle of 1508 or greater.[2] Water
coming into contact with a superhydrophobic surface forms
spherical bead-like drops, minimizing the solid–liquid contact
area. Self-cleaning relies on the spherical droplets rolling off
the surface and picking up particulate dirt in their path. Super-
hydrophobic textiles can be used as highly protective clothing
Scheme 1. Structure of leuco and chromatic triarylmethanes.
textiles and outdoor clothing materials. Furthermore, such tex-
tiles require less washes due to their self-cleaning property. To
form a surface with superhydrophobic properties, a low-
energy surface must be combined with high surface rough-
ness, and different preparation approaches have been report-
ed.[3] Textiles such as cotton already have some surface rough-
ness, and different treatments to render them superhydropho-
bic have been performed.[4] Some examples include coating
the fiber with silica nanoparticles followed by hydrophobiza-
tion,[5] with organosilane-modified silica nanoparticles,[6] with
fluoroalkylsilanes,[7] or with fluorocarbon polymers;[8] direct
functionalization of the textile has also been achieved with
polyalkylsiloxane[9] or carbon nanotubes.[10] Given that fibers
almost inevitably undergo a dyeing process, it may be envis-
aged that the process of dyeing might also be used to impart
hydrophobicity.[11]
The first goal of the present work was the synthesis of hy-
drophobic leuco triarylmethane derivatives of type 5 possess-
ing long hydrocarbon or polyfluorinated chains on one of the
aryl moieties (indicated in 5, Scheme 2). We addressed the syn-
thesis starting from the commercial 3,5-dibromobenzaldehyde,
1, through a palladium-catalyzed cross-coupling reaction for
the formation of C(sp2)ÀS bonds. Although frequently conduct-
ed under strongly basic reaction conditions,[15] a convenient
method for Pd-catalyzed CÀS cross-coupling of arylbromides
with various thiols was developed by Okauchi and co-work-
Triarylmethanes and their analogues are an important class
of synthetic dyes used to stain silk, wool, jute, leather, cotton,
and paper.[12] These dyes are known for their outstanding in-
[a] A. M. Montagut, Dr. E. Gꢀlvez, Dr. R. M. Sebastiꢀn, Dr. A. Vallribera
Department of Chemistry and
Centro de Innovaciꢁn en Quꢂmica Avanzada (ORFEO-CINQA)
Universitat Autꢃnoma de Barcelona, Campus UAB
08193-Cerdanyola del Vallꢄs, Barcelona (Spain)
E-mail: rosamaria.sebastian@uab.es
Adelina.vallribera@uab.es
[b] Dr. A. Shafir
Institut of Chemical Research of Catalonia (ICIQ)
Avda. Pa¨ısos Catalans 16, 43007 Tarragona (Spain)
Supporting information and the ORCID number(s) for the author(s) of this
article can be found under http://dx.doi.org/10.1002/chem.201605572.
Scheme 2. Synthesis of hydrophobic triarylmethane derivatives 5 and 6
(anh.=anhydrous).
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ers[16] using 1,1’-bis(diphenylphosphino)ferrocene (DPPF) as
ligand and N,N-diisopropylethylamine (DIPEA) as a base. The
mild reaction conditions make the process compatible with
the aldehyde groups. Thus, when 1 was treated under these
conditions with aliphatic or polyfluorinated thiols (2a or b, re-
spectively, 2 equiv) in anhydrous toluene under nitrogen at-
mosphere at reflux for 12 h, products 3a and b were obtained
in 89 and 93% yield, respectively (Scheme 2). At lower reaction
times, mixtures of mono- and dithiolated compounds were ob-
tained that could not be separated by column chromatogra-
phy.
Among the existing methods, the acid-catalyzed Baeyer con-
densation of aromatic aldehydes and N,N-dialkylanilines is one
of the most simple and straightforward approaches for the
synthesis of diaminotriarylmethanes.[17] It was recently discov-
ered that NbCl5 acts as a useful and mild Lewis acid catalyst in
the Baeyer condensation.[13a] Thus, benzaldehyde 3a reacted
with 2-(methyl(phenyl)amino)ethanol, 4a, under NbCl5 catalysis
(10 mol%) and solvent-free conditions to give leuco 5aa in
51% yield (Scheme 2). The same reaction starting with the
fluorinated 3b gave compound 5ba in 46% yield (Scheme 2).
Finally, compound 5bb was obtained in 44% yield when the
reaction was carried out using 3b and N,N-dimethylaniline, 4b
(Scheme 2). The mechanism of this reaction can be attributed
to a double regioselective electrophilic aromatic substitution
reaction. A qualitative way to establish the formation of the tri-
arylmethane derivative is the appearance of a blue-green color
due to partial photooxidation of compound 5 leuco to its
chromatic form 6. The existence of the chromatic form was
also confirmed by the presence of a small absorption band
(636 nm) in the visible region of the UV/Vis spectrum. The
chromatic form 6 was separated from the leuco 5 form by
column chromatography, due to the high affinity of the cation-
ic form to silica. The leuco form was confirmed by the appear-
ance of a characteristic “central” benzylic CÀH proton in the
1H NMR spectrum (5.28–5.45 ppm depending on the substitu-
ents in the aryl groups). The chromatic blue form 6 could be
obtained quantitatively by oxidation of leuco 5bb with 2,3-di-
chloro-5,6-dicyanobenzoquinone (DDQ) (Scheme 2). Com-
pounds 5aa and ba were not oxidized at this stage due to the
presence of the alcohol functionality and will be used later.
The cationic molecule 6 was difficult to purify and manipulate.
Our next objective was the synthesis of new hydrophobic
triarylmethane derivatives of type 11, possessing a different hy-
drophobic moiety (Scheme 3). Bisarylation of p-nitrobenzylal-
dehyde 7 was carried out in an excellent 81% yield. The subse-
quent reduction of the nitro (8) to amino group (9) took place
quantitatively. The aniline 9 reacted smoothly with cyanuric
chloride derivatives 10a and b in 60 and 65% yield, respective-
ly, giving rise to the leuco triarylmethane derivatives 11 a and
b. The corresponding chromatic blue forms 12a and b could
be obtained in excellent yields by oxidation with DDQ in THF
(Scheme 3).
Scheme 3. Synthesis of hydrophobic triarylmethane derivatives 11 and 12.
ing functional groups that can react with natural or synthetic
fibers to form covalent bonds. To ensure a good fixation ability,
we envisioned bifunctional dyes containing two reactive
groups on the molecule (Scheme 4).
Scheme 4. Synthesis of bifunctional dyes 13.
Treatment of dyes 5aa and ba, both possessing a primary al-
cohol, with butyllithium allowed the deprotonation of both
terminal primary alcohols, which then reacted with an excess
of cyanuric chloride through a nucleophilic aromatic substitu-
tion. An excess of cyanuric chloride was used to avoid di- and
tri-substitutions, and/or the formation of undesirable polymeric
derivatives. Moreover, light oxidation of the leuco to chromatic
form takes place throughout the reaction. During the purifica-
tion of the crude reaction mixture by silica-gel column chroma-
tography, separation of leuco and chromatic form is achieved.
Compounds leuco 13a and b were obtained in 63 and 66%
yields, respectively, under these conditions (Scheme 4).
With the synthesis completed, we proceeded to study the
hydrophobic and the oleophobic properties of the newly syn-
thesized dyes. These dyes were first deposited on a glass sur-
face using a spin-coating technique (addition of 0.1 mL of
a 2.5ꢃ10À3 m solution in acetone of each compound indicated
in Table 1). Measurement of the contact angle was performed
Taking into account that the leaching stability of dyed coat-
ing is essential for long-term applications and that wash-fast-
ness is a particular requirement for textiles, we then designed
hydrophobic triarylmethane-based reactive dyes by introduc-
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days, DDQ (2 mg, 7.9ꢃ10À6 mol) and 3 drops of HCl were intro-
duced into the reactor. For practical reasons, we decided to
carry out the oxidation as the last step once the dye was cova-
lently anchored on the cotton fabric. Afterwards, the stained
fabrics were rinsed with THF and acetone. The obtained organ-
ic solvents were colorless despite the high solubility of dyes
13a and b in acetone and THF, proving stability of the dye
through the covalent link to the cotton surface.
Table 1. Obtained contact angles of a droplet of different solvents on
a modified glass surface (previously coated with the indicated com-
pounds).
Entry
Compound[a]
Water
Olive oil
Hexadecane
[8]
[8]
[8]
1
2
3
4
5
6
7
8
Glass
5aa
5ba
5bb
11 a
11 b[b]
13a
13b
41
77
35
38
104
85
42
–
15
17
105
78
19
–
108
116
108
140
106
107
Dyed cotton fabrics are hereby referred to as A (for 13a-
modified fabric) and B (for 13b-modified fabric).The surface
morphologies of the original and modified cotton fabric B
were compared by SEM. No substantial differences were
found, which means that the surface feature is preserved de-
spite the dye grafting. Analysis by energy-dispersive X-ray
spectroscopy (EDS) established the presence of fluorine, sulfur,
and chlorine atoms at the surface, which is indicative of the
presence of dye 13b (Figure 1).
45
89
15
77
[a] 2.5ꢃ10À3 m solution in acetone. [b] Due to solubility problems, CHCl3
was used.
by the addition of a droplet of water (4 mL) on top of the previ-
ously modified glass surface. The highest contact angles corre-
spond to compounds 5bb (1168, entry 4) and 11 b (1408,
entry 6), which is in accordance with the presence of two per-
fluoroalkylic ponytail chains and dimethylamino groups. In
contrast, changing to polar ethanolmethylamino substituents,
5ba, the contact angle decreases from 1168 to 1088. The re-
sults also indicate that triarylmethane derivatives possessing
long hydrocarbon tails [5aa (778), 11 a (1088) and 13a (1068)]
have in general lower contact angles than their fluorinated an-
alogues [5ba (1088), 11 b (1408) and 13b (1078)]. Thus, fluori-
nated triarylmethane analogues were considered to be more
hydrophobic than the hydrocarbon ones.
Hydrophobicity measurements of the stained cotton fabrics
A and B (Figure 2) were carried out by the measure of the con-
tact angle of a droplet of water on the surface of the cellulose
For the oleophobicity measurements (Table 1), droplets of
olive oil and hexadecane were added on top of the treated
glass surface previously coated with the triarylmethane deriva-
tives indicated in Table 1 (addition of 0.1 mL of a 2.5ꢃ10À3
m
solution of the dye in acetone). Fluorinated 5ba, possessing
a polar hydroxyl functional group, gave the best results
(entry 5). In the case of olive oil, comparing both 5ba (fluori-
nated chains) and 5aa (alkylic chains), the results indicate a dif-
ference of 668 in the contact angle, the fluorinated compound
being more oleophobic. Comparing compounds 13a and b,
the contact angle is nearly doubled for the fluorinated 13b
(entries 7 and 8). In the experiments with hexadecane, the dif-
ferences are even more prominent. Again, fluorinated ana-
logues were more oleophobic than the hydrocarbon ones.
Compound 5ba possessing a polar hydroxyl functional group
and a fluorinated chain presented the best amphiphobic prop-
erties to both water and oil fluids.[18]
Then, we took some pieces of commercial cotton fabric
from a 100% cotton (knitted) T-shirt, which were initially
washed with liquid soap, rinsed with distilled water, and dried
at 1058 for three days. Subsequently, the activation of the
cotton pieces was carried out by sinking them in a 0.1m of
NaOH aqueous solution for 30 min and then rinsing with ace-
tone (4ꢃ5 mL). Afterwards, the staining process was carried
out in a screw-top 60 mL tube, by soaking of swatches of the
activated cotton fabrics (1ꢃ3 cm) in a solution of 7ꢃ10À3 g of
dyes 13a and b (0.23% weight) in 4.5 mL of THF. After three
Figure 1. Top image: Scanning electron microscope (SEM) image of stained
cotton fabric B (dye 13b). Bottom image: Energy dispersive analysis (EDS)
graph of the treated cotton fabric B.
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properties and confirms the stability of the covalent link of the
dye to the cellulose.
Finally, we carried out some color-fastness tests. The colori-
metric measurements were made on a reflectance spectropho-
tometer. The results indicate excellent color stability two
months after the dyeing process, which is important for indus-
trial applications (as can be observed in Figure 4 for a stained
cotton fabric B).
Figure 2. Schematic representation of stained fiber B (dye 13b). Photograph
of a droplet of water deposited on fiber B with a contact angle of 1528
(average of three measures, max. deviation Æ48).
Figure 4. Fastness test of stained cotton fabric B over time.
(1308 for fabric A and 1528 for fabric B). These results indicate
that stained cotton fabric B is a superhydrophic surface. In
fact, when placed in a beaker of water, swatches of the “un-
treated” cotton and of the dyed cotton fabric A sank to the
bottom, whereas the dyed cotton fabric B remained floating
on the surface, indicating higher hydrophobicity when the
fluorinated dye 13b was used (Figure 3). In addition, a 3ꢃ1 cm
piece of dyed fabric B was placed in a tube under mechanical
In summary, new triarylmethane dyes have been prepared.
The covalent link of reactive dyes to cotton fabrics has resulted
in new hydrophobic colored fibers. When dye 13b was used,
superhydrophobicity was achieved. These modified cotton
fiber have potential applications in sportswear and outdoor
textiles with water- or oil-repellent properties and excellent
color-fastness. Fluorinated dyes were, in general, more hydro-
phobic and oleophobic than their hydrocarbon analogues.
Acknowledgements
We thank financial support from Spain’s MICINN (CTQ2011-
22649, CTQ2013-46705-R, SEV-2013-0319 and CTQ2014-53662-
P), as well as MEC (Consolider Ingenio CSD2007-00006 and
CTQ2014-51912-REDC) and DURSI-Generalitat de Catalunya
(2014SGR1105 and 2014SGR1192).
Figure 3. 1) Untreated cotton; 2) stained cotton fabric B; 3) stained cotton
fabric A. Only cotton fabric B keeps floating in water.
Conflict of interest
stirring with 5 mL of a washing bath at 308C for 30 min. After
the washing, the fabric was rinsed with water and the process
was repeated three consecutive times. Another piece of dyed
fabric B was submitted to six consecutive washings following
the same protocol. We evaluated the contact angle again;
after three washings, dyed fabric suffered a slight decline of
108 (max. deviation Æ68), whereas after six washings, the de-
cline was 118 (max. deviation Æ28). Thus, dyed fabric B main-
tains the hydrophobic properties after repeated laundry pro-
cesses, which is in accordance with its excellent color-fastness
The authors declare no conflict of interest.
Keywords: cotton
fabric
·
oleophobicity
·
superhydrophobicity · surface modification · triarylmethane
dyes
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Manuscript received: November 29, 2016
Accepted Article published: January 12, 2017
Final Article published: && &&, 0000
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COMMUNICATION
Hydrophobic and/or oleophobic triaryl-
methane dyes possessing long hydro-
carbon or polyfluorinated chains were
prepared. When covalently grafted on
to cotton fabric, these dyes give rise to
colored superhydrophobic fibers.
&
Surface Chemistry
A. M. Montagut, E. Gꢀlvez, A. Shafir,
R. M. Sebastiꢀn,* A. Vallribera*
&& – &&
Triarylmethane Dyes for Artificial
Repellent Cotton Fibers
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ÝÝ These are not the final page numbers!