Article
Extended Applicability of Classical Phenolphthalein: Color Changing
Polymeric Materials Derived from pH-Sensitive Acrylated
Phenolphthalein Derivatives
Carolin Fleischmann, Jia Cheng, Monir Tabatabai, and Helmut Ritter*
Institut fur Organische Chemie und Makromolekulare Chemie, Lehrstuhl fur praparative Polymerchemie,
̈
̈
̈
Heinrich-Heine-Universitat Dusseldorf, Universitatsstraße 1, D-40225 Dusseldorf
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: Polymerizable phenolphthalein derivatives with pH-sensitive proper-
ties were synthesized in the course of electrophilic aromatic substitution reactions.
N-(2-hydroxy-5-(1-(4-hydroxyphenyl)-3-oxo-1,3-dihydroisobenzofuran-1-yl)-
benzyl)acrylamide (3) and N,N′-(((3-oxo-1,3-dihydroisobenzofuran-1,1-diyl)bis(2-
hydroxy-5,1-phenylene))bis(methylene))diacrylamide (4) were obtained and
characterized. Copolymerization of 3 and 4 with acrylic acid (5a), N-(isopropyl)-
acrylamide (5b), and N,N-dimethylacrylamide (5c) enabled the synthesis of color
switchable materials. Thereby, water-soluble pH sensitive linear polymers 6 and 7
were prepared using monomer 3. Additionally, cross-linked polymers 8 were
prepared successfully by copolymerization of 4 and 5c. These pH-sensitive polymers
form hydrogels in aqueous solution. Linear viscoelastic behavior depending on the
concentration of the cross-linker was observed.
Nowadays, it is known that phenolphthalein possesses some
INTRODUCTION
■
carcinogenic effects.9 Thus, it is not used for medical applica-
tions anymore, but it is still an important indicator dye.
Principally, a polymer-bound phenolphthalein can be consid-
ered being nontoxic or at least less toxic than free phenolphthalein.
Thus, the development of polymerizable phenolphthalein deriv-
atives is of great interest. In addition to that, cross-linked phenol-
phthalein-containing polymers can be reused for many times.
Accordingly, they represent favorable alternatives for phenolph-
thalein in many applications, e.g., in industrial processes and in
chemistry classes in schools. In order to conserve the molecule’s
indicative properties the modification with polymerizable groups
has to proceed at the aromatic rings and cannot take place at the
phenolic hydroxyl groups. Recently, the synthesis of microporous
pH-switchable networks based on phenolphthalein-derivatives was
published by Antonietti et al. For this purpose, 3′,3″,5′,5″-tetrabro-
mophenolphthalein and 1,4-diethynylbenzene were subjected to a
Sonogashira-cross-coupling reaction. The resulting polymers can
be used for the adsorption of gases or as photoinitiators in order to
start free radical polymerizations.10 But due to the fact that those
polymers are not water-soluble they cannot be used as indicative
devices in aqueous solution.
Triphenylmethane dyes are important substances for analytical pur-
poses since many decades. For example, thymol blue, bromphenol
blue, phenol red, and bromocresol blue are used as indicators in
titrimetic determinations of vitamin B1.1 Some transition metal
complexes formed with ethyl violet can be used to detect traces of
arsenic ions in water.2 Malachit green exhibits fungicidal effects, but
due to its cytotoxic properties, it is not used in pharmaceuticals.
The compound is applied as an acid−base indicator like many other
triphenylmethane dyes.3 Some of the most important triphenyl-
methane dyes are represented by crystal violet and phenolph-
thalein.4
Phenolphthalein was discovered by Adolph von Baeyer et al. in
1871 while heating a mixture of phthalic anhydride, phenol and
sulfuric acid. Baeyer also observed the molecule’s indicative pro-
perties.5 It appears colorless in neutral solution whereas it is
colored pink in basic medium. The perceived color change is
caused by a structural change in basic medium. Namely, this
structural change is the opening of the lactone ring which is
induced by the deprotonation of the phenolic hydroxyl groups.
The formation of this quinoid structure of the molecule requires
a change in the hybridization of the central carbon atom. This
results in an increasing delocalization of the π-system although
the molecule is still not completely planar but propeller-like.6
Because of its indicative properties, phenolphthalein was
added to wines in Hungary in the 1930s in order to indicate
contaminations with basic additives. This enabled the detection
of laxative effects caused by phenolphthalein. During succeed-
ing years, the dye became one of the most important synthetic
laxatives, which can be considered to be due to its low toxicity
toward the kidneys and its low price.7,8
In the past, some dyes were covalently emplaced into copoly-
mers but to the best of our knowledge, pH-sensitive polymers
bearing covalently attached phenolphthalein derivatives have
not been described so far.11,12
Received: April 2, 2012
Revised: May 15, 2012
Published: June 26, 2012
© 2012 American Chemical Society
5343
dx.doi.org/10.1021/ma300670x | Macromolecules 2012, 45, 5343−5346