The optical properties of triphenylmethane dye molecules and chromogens

Article abstract of DOI:10.1134/S003602440809032X

Industrial dye monomers, including malachite green, crystal violet, brilliant green, and methyl violet, were isolated by extraction with the use of heptane. UV light absorption bands characteristic of pure molecules were determined. The molecules of the dyes studied, which were ion pairs (formed by dye cations and oxalate or chlorine anions), did not absorb light in the visible range; that is, they were not chromogens. The conclusion was drawn that chromogen particles responsible for chromaticity were supramolecular dimers of nonchromogenic triphenylmethane series molecules. This conclusion was substantiated by trends in spectral transformations with the participation of immonium hydroxides obtained from dyes and side products of the synthesis of industrial dyes with quinoid molecular structures.

Full text of DOI:10.1134/S003602440809032X

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2008, Vol. 82, No. 9, pp. 1580–1588. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © Yu.A. Mikheev, L.N. Guseva, Yu.A. Ershov, 2008, published in Zhurnal Fizicheskoi Khimii, 2008, Vol. 82, No. 9, pp. 1770–1779.
OTHER PROBLEMS
OF PHYSICAL CHEMISTRY
The Optical Properties of Triphenylmethane Dye Molecules
and Chromogens
Yu. A. Mikheeva, L. N. Gusevaa, and Yu. A. Ershovb
a Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow, 117977 Russia
b Bauman State Technical University, Vtoraya Baumanskaya ul. 5, Moscow, 107005 Russia
e-mail: mik@sky.chph.ras.ru
Received May 8, 2007
Abstract—Industrial dye monomers, including malachite green, crystal violet, brilliant green, and methyl vio-
let, were isolated by extraction with the use of heptane. UV light absorption bands characteristic of pure mol-
ecules were determined. The molecules of the dyes studied, which were ion pairs (formed by dye cations and
oxalate or chlorine anions), did not absorb light in the visible range; that is, they were not chromogens. The
conclusion was drawn that chromogen particles responsible for chromaticity were supramolecular dimers of
nonchromogenic triphenylmethane series molecules. This conclusion was substantiated by trends in spectral
transformations with the participation of immonium hydroxides obtained from dyes and side products of the
synthesis of industrial dyes with quinoid molecular structures.
DOI: 10.1134/S003602440809032X
INTRODUCTION
The structural formula of immonium cations in the
dyes has the form
Triphenylmethane cationic dyes (TPMDs) are rep-
resentatives of the oldest class of synthetic dyes. They
have much in common with a large number of cationic
dyes of other classes, in particular, heterocyclic and
carbocyanine dyes. During many years, they have been
used as model objects for studies of the reason for col-
oration of organic substances. It has been believed that
the chromogens of the dyes specified are separate mol-
ecules containing certain chromophoric and auxochro-
mic groups [1–8]. Studies of the fractional composition
of industrial samples of nonionic copper phthalocya-
nine (CuPc) pigment powder, however, showed [9] that
a small amount of the amorphous phase it contained
dissolved in heptane, dioxane, and even water. The
solutions of individual CuPc molecules formed did not
absorb visible light, and their optical spectra were sub-
stantially different from the known spectrum of the dye.
It was therefore considered necessary to experimentally
check whether or not dyes of other classes behaved
similarly.
..
+
R2N
NR2
C
R'
Here, R = ëç3 and R' = H (MG), R = ëç3 and R' =
N(CH3)2 (CV), R = ë2H5 and R' = H (BG), or R = CH3
and R' = NH(CH3) (MV).
The molecular components of industrial dyes were
isolated by extraction and studied spectroscopically.
EXPERIMENTAL
We used MG, CV, and MV powders (ch. d. a. (pure
for analysis) grade, Shostka Plant of Chemical
Reagents) and a medical alcoholic solution of BG (1%)
as initial materials. The extraction of the MG, CV, and
MV molecules from powders was performed using
spectroscopically pure heptane on a water bath (≈
99°C); BG was extracted from alcoholic solutions at
≈25°ë. Chromogenic dye particles are insoluble in
heptane. In addition, we absorbed molecules from
aqueous sols of dyes by immersing cellulose triacetate
films into them. Aqueous sols of dyes were prepared by
mixing their alcoholic solutions (~10 mg in 10 ml of
In this work, we studied the most characteristic rep-
resentatives of triphenylmethane dyes, including mala-
chite green (MG), crystal violet (CV), brilliant green
(BG), and methyl violet (MV). These molecules carry
positive charges on imine group nitrogen atoms (immo-
nium cations) and can carry positive charges on amine
group nitrogen atoms (anilinium cations) [3]. Cation
charges are compensated by Coulomb interactions with
oxalate (in MG and BG) and chlorine (in CV and MV)
anions.
1580

THE OPTICAL PROPERTIES OF TRIPHENYLMETHANE DYE MOLECULES
1581
D
1.0
1
(‡)
4
2
0.5
3
1
2
3
4
2
3
40
32
24
16
ν × 10–3, Òm–1
1.0
0.5
(b)
4
3
1
2
40
32
24
ν × 10–3, Òm–1
Fig. 1. Changes in the optical spectra of solutions of commercial MG oxalate in (a) ethanol and (b) heptane in the presence of
(a) KOH and (b) ethanol. See text for explanations.
ethanol) with distilled water (~10 ml). Solutions of dye alkali is added to the solution. Curves 2 and 3 charac-
immonium hydroxides were prepared by mixing pow- terize changes of spectrum 1 with time (in 40 and
ders or their alcoholic solutions with aqueous solutions 80 min, respectively) at a KOH concentration of 8 ×
of KOH alkali (2–5%). The optical spectra of solutions 10–5 mol/l, and spectrum 4 corresponds to a KOH con-
and films were recorded on a Specord UV-VIS instru- centration of 3 × 10–3 mol/l.
ment.
Note that, in the absence of alkali in solution of MG
oxalate in ethanol (Fig. 1a, curve 1), there is no indica-
tion of an increase in the intensity of the carbinol band
at ν = 38000 cm–1 with time. This is evidence of oxalate
stability in alcoholic solution. The hydrolysis of oxalate
occurs in alkaline media. The character of the transfor-
mation of the spectrum reveals the step character of the
process. For instance, the intensity of the chromogen
band νmax = 16200 cm–1 of the dye decreases already at
very low alkali concentrations. Simultaneously, the
intensity of the broad band at νmax = 38000 cm–1
increases.
RESULTS AND DISCUSSION
The Spectra of Solutions of Initial Malachite Green
and Extracted Compounds
The optical spectrum of a solution of MG oxalate in
ethanol (1.2 × 10–5 mol/l) is shown in Fig. 1a, curve 1.
The spectral positions of absorption band maxima of
MG oxalate solutions are (ν, cm–1 (λ, nm) 16200 (617),
23500 (425), and 31600 (316). These data correspond
to the literature spectra [5, 10]. The apparent extinction
coefficient value for the main absorption band (617 nm)
Curves 1–3 contain the isobestic point at ν =
of MG chromogenic particles dissolved in alcohol cal- 35500 cm–1. This is evidence that either one intermedi-
culated per unit concentration of individual aromatic ate product or several products with equal spectro-
fragments (Ar) is ε617 = 4.4 × 104 l/(mol cm). Curves 2–4 scopic properties are formed at the intermediate reac-
in Fig. 1a show how spectrum 1 transforms into the tion stage. When the concentration of alkali increases to
spectrum of the carbinol leuco form of MG when KOH 3 × 10–3 mol/l, the isobestic point disappears, and the
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 9 2008

1582
MIKHEEV et al.
UV band of the final product appears (Fig. 1a, curve 4). carbinol MG form is, however, absent. This band only
It retains the shape of the bands of the precursors but appears in the presence of alkali. For instance, spec-
has a higher intensity.
trum 2 (Fig. 1b) transforms into spectrum 4 (Fig. 1b)
upon the addition of KOH (20 mg) into solution. We see
that the addition of alkali does not influence the shape
and intensity of the band of compound X at 28250 cm–1
but causes the appearance of a band at 33000 cm–1
characteristic of carbinol.
The final product of the hydrolysis of MG oxalate is
leucocarbinol. The maximum of the UV band of
carbinol in alkaline alcohol is situated at 38000 cm–1
(263 nm). The extinction coefficient value, ε263 = 2.8 ×
104 l/(mol cm), calculated for spectrum 4 (Fig. 1a)
almost coincides with that reported in [11]. The same
value is obtained from the ratio D617/D263 = ε617/ε263,
which corresponds to the complete transformation of
the aryl Ar fragments of MG into the carbinol form. An
inflection at 33000 cm–1 (~300 nm) is observed on the
low-frequency slope of the 38000 cm–1 band, exactly
where a weak additional carbinol absorption band is sit-
uated [10, 11].
Note that spectrum 4 (Fig. 1b) is distorted somewhat
at ν > 33000 cm–1 because of the superposition of the
turbidity spectrum of light scattering by KOH colloidal
particles, and a broadened band at 40000 cm–1 distorted
by light scattering is the sum of the band of carbinol
formed and, possibly, a high-frequency band of com-
pound X.
Note that the spectrum of the heptane extract
(Fig. 1b, spectrum 1) contains a well-defined UV band
at νmax = 41200 cm–1 (243 nm). A considerable contri-
bution to this band is made by individual MG mole-
cules transferred to heptane; these molecules do not
have absorption bands in the visible range. The pres-
ence of individual MG molecules in the heptane extract
follows from the appearance of the carbinol band at
33000 cm–1 under the action of alkali (Fig. 1b, curve 4)
in the absence of MG chromogenic particles. The only
source of carbinol can then be individual MG mole-
cules.
Spectrum 4 (Fig. 1a) contains a weak band at
28000 cm–1, which was not described in the literature.
This is the band of a side product (we denote it by X) of
dye synthesis rather than a band of MG cations. Com-
pound X is present in the initial dye sample also in the
form of oxalate, like MG. This follows from the appear-
ance of its band at ~28 000 cm–1 at a fairly large content
of alkali (Fig. 1a, curve 4) in the spectral interval where
the spectral curve of the dye (Fig. 1a, curve 1) has a
minimum. Oxalate of X is not very stable, and the
oxalate group splits off not only under the action of
alkali but also when the compound is heated in heptane.
As concerns initial MG oxalate, note that the manu-
The spectrum of compounds extracted with hep- facturer (Shostka chemical plant) ascribed the chemical
tane (10 ml) at ~100°ë from a sample of initial MG formula from book [1] to its product, namely,
(~100 mg) is shown in Fig. 1b (curve 1). The extract 2C23H24N23C2H2O4. This formula does not correspond
was filtered through a paper filter and diluted 2.5 times. to either the conditions of molecule stabilization or the
In this spectrum, the UV band of compound X at νmax
=
chemical synthesis characteristics. Indeed, according to
30300 cm–1 (~330 nm) is fairly intense and shifts chemical technology, MG oxalate is obtained from
bathochromically to 28000 cm–1 when heptane is immonium chloride by substituting oxalic acid anions
replaced with ethanol (Fig. 1b, curve 3). In heptane sat- for chlorine anions. This is ignored by the formula
urated with alcohol, the maximum of this band is situ- given, which contains three oxalic acid molecules in the
ated at 30000 cm–1, and in alcohol saturated with hep- electrically neutral form. This cannot ensure stable con-
tane, at 28250 cm–1 (350 nm) (Fig. 1b, curve 2). Hep- nection between two immonium cations in one com-
tane and alcohol have limited mutual miscibility and plex. The complex stability condition requires that all
form two layers.
positive charges be balanced by the equivalent number
of anions. A correct formula should be written as
2C23H24N2C2O4 · 2C2H2O4 or 2[C23H24N2OCOCOOH] ·
C2H2O4. Only then can the stabilization of two ammo-
nium-oxalate and two immonium-oxalate groups in one
molecule be explained.
Note that compounds of type X with similar UV
bands were observed in all the dyes studied. This is evi-
dence that impurity compound molecules in these dyes
have identical chromophore groups. Importantly, the
spectra of solutions (Fig. 1b, curves 1–3) obtained by
extraction with heptane contain no indications of the
presence of the absorption bands of the initial dye,
which is almost insoluble in heptane. Nor do we
observe the MG carbinol form band (38000 cm–1, see
Fig. 1a, curve 4).
As mentioned, MG leucocarbinol is formed in hep-
tane extracts containing alcohol under the action of
alkali from nonchromogenic (colorless) particles,
which can only be individual MG molecules containing
two aromatic fragments. A dye can be formed from
Spectrum 3 (Fig. 1b) characterizes an ethanolic nonchromogenic (colorless) MG molecules as a result
solution obtained after the evaporation of heptane from of self-assembly of supramolecular aggregates. Indeed,
the heptane extract and subsequent solution of the dry blue-green dye layers are formed on glass surfaces of
residue in ethanol. We see that, after transfer into etha- vessels during extract storage and also as a result of the
nol, the UV band of extracted compound X shifts evaporation of colorless heptane solutions with spec-
bathochromically with respect to the band in a solution trum 1 (Fig. 1b). The synthesis of chromogenic MG
in heptane. The band at 33000 cm–1 characteristic of the particles from colorless MG molecules can be observed
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THE OPTICAL PROPERTIES OF TRIPHENYLMETHANE DYE MOLECULES
1583
using a chromatographic paper strip absorbing a solu- natures is fairly large. Such effects should likely be
expected for fairly small aggregates, such as dimers.
tion in heptane. Paper turns colored as the solvent
moves over it and vaporizes.
The Spectra of Extracts
The role of MG chromogens can be played not only
by crystalline oxalate particles but also by supramolec-
ular dimers formed from nonchromogenic individual
MG molecules because of fairly strong intermolecular
interactions. The suggestion that the λmax = 617 nm
band is the band of supramolecular oxalate molecule
dimers is substantiated by changes in the intensity of
this band observed when a solution of MG in alcohol is
mixed with glacial acetic acid in a 2 : 1 volume ratio. In
a solution in alcohol, we have ε617 = 4.4 × 104 l/(mol cm),
whereas ε617 = 5.5 × 104 l/(mol cm) in a mixed solution.
In spite of this increase, the absorption coefficient of
MG oxalate in an alcohol–acetic acid mixture is lower
than that of MG chloride in 98% acetic acid, where it is
1.04 × 105 l/(mol cm) [7]. The observed discrepancy
from Aqueous-Alkaline Solutions of Malachite Green
Experiments with heptane extracts of the products
of the aqueous-alkaline hydrolysis of initial MG
oxalate give an independent substantiation of the
absence of chromogenic properties of individual MG
cations and the formation of chromogenic particles
only as a result of the aggregation of molecules that
form ion pairs with balanced electric charges.
As mentioned, the alkaline hydrolysis of MG
oxalate proceeds in at least two well-defined macro-
scopic kinetic stages, which can be observed by record-
ing UV spectra. At the same time, the kinetics of this
reaction includes three sequential stages. The primary
alkaline hydrolysis event is a fast reaction of exchange
between oxalate and HO– ions, whose activation energy
between the characteristics caused by different solvent should not be large,
(Ox–N+H(CH3) –ph'–C(ph)=pꢀ=N+–OCO–) + 2HO–
2
2
(I)
2Ox–N+H(CH3) –ph'–C(ph)=pꢀ=N+(CH3) OH + –OCOCOO–.
–
2
2
(II)
Here, Ox– is the oxalate anion, ph and ph' are the phenyl position, whose molecule is simultaneously immonium
hydroxide and ammonium oxalate.
and phenylene radicals, pꢀ is the cyclohexadiene
quinoid radical fragment, I is the formula of MG in the
The next stage is the neutralization of ammonium
compact form, and II is the product of alkaline decom- ions,
–
HO– + II
H2O + Ox– + N(CH3) ph'–C(ph)=pꢀ=N+(CH3) OH,
2
2
(III)
producing the nonsalt immonium hydroxide form.
absorption band of the carbinol molecule should be two
times higher if the quinoid groups of compounds II and
III do not absorb noticeably in this UV spectrum
region. This is what can explain the strengthening of the
ν = 38000 cm–1 band under step hydrolysis conditions
observed in Fig. 1a in passing from curves 1–3 (with
the isobestic point) to curve 4. The absence of absorp-
tion bands of compound II and III quinoid groups at
32000–42000 cm–1 is substantiated by the properties of
compound X considered below. The UV bands of
immonium hydroxides (II, III) and carbinol at
33000 cm–1 also have different intensities.
The third final stage of the process in all probability
involves the reaction of immonium hydroxide with the
HO– anion, which attacks the central carbon atom,
HO– + III
N(CH3)2ph'–C(OH)(ph)–ph'–N(CH3)2
(carbinol) + –OH.
Another reaction path with reaction rate limited by
the isomerization of compound III with monomolecu-
lar splitting off of the HO– anion and its subsequent
shift to the central carbon atom is less probable. The
process should then encounter obstacles because of the
low permittivity of ethanol (which hinders the dissoci-
ation of immonium hydroxide).
The spectrum of a solution in heptane, in which
immonium hydroxide forms II and III predominate, is
shown in Fig. 2a, curve 1. This solution was prepared
by the extraction with heptane (~10 ml) of compounds
Molecules II and III carry one dimethylaniline formed immediately after mixing an alcoholic solution
group each, whereas the carbinol molecule contains of MG oxalate (~50 mg in 20 ml of ethanol) with an
two such groups. Accordingly, the intensity of the UV aqueous solution of alkali (up to 4% KOH) and then
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 9 2008

1584
MIKHEEV et al.
band is smeared in this spectrum; the maximum trans-
(‡)
forms into a shoulder, and the spectrum closely repeats
the shape of the spectrum given in [10]. Also note that
a weak band of compound X present in spectra 1–3
(Fig. 2a) of heptane solutions in the form of an inflec-
tion bathochromically shifts in the spectrum of the
alcoholic solution saturated with heptane and is
observed as a well-defined maximum at 28000 cm–1
(350 nm).
D
1
1.0
0.5
2
4
1
3
There is one more important feature: colorless
freshly prepared heptane extracts of MG oxalate decol-
orized with alkali form dye layers on ground-glass
joints of glass ampules and spectroscopic cells. During
aging of aqueous-alkaline solutions, compounds II and
III gradually transform into carbinol, and heptane
extracts from them give less and less dye precipitate.
Carbinol itself does not form colored precipitates.
4
44
40
36
32
28
24
ν × 10–3, cm–1
(b)
Note that MG and CV carbinols were for the first
time obtained in [12] by treating dye chlorides with
dilute aqueous solutions of alkalis. They formed pre-
cipitates poorly soluble in water. Pure carbinols pre-
pared by recrystallization from ligroin (heptane) or
ether are stable and, contrary to what is stated in [10],
do not dissociate in alcoholic solutions into Ar cations
and hydroxyl ions [11, 12]. The idea of the dissociation
of carbinols [10] could emerge from the observation
that, at intermediate transformation stages, individual
nonchromogenic molecules II and III present in alka-
line solutions were capable of transforming not only
into carbinol but also into chromogenic dimers.
4
1.0
0.5
4
3
2
1
3
40
36
32
28
24
20
16
ν × 10–3, cm–1
The formation of colored precipitates from II and
III occurs very easily at interphase boundaries, because
individual molecules assume favorable mutual orienta-
tion in adsorption. For this purpose, chromatographic
paper strips immersed into a solution of II and III in
heptane can be used instead of a silicate surface. The
dye formed on a paper strip during heptane motion and
vaporization is easily washed out with alcohol. The
spectrum of such an eluate is shown in Fig. 2b, curve 1.
It contains not only the dye band at 16100 cm–1 (621 nm)
but also a comparatively intense carbinol band at
37900 cm–1 (264 nm); carbinol is also adsorbed by
chromatographic paper.
Fig. 2. Optical spectra of heptane extracts of the products of
alkaline hydrolysis of (a) MG oxalate and (b) alcoholic elu-
ates of compounds formed in the adsorption of colorless
MG hydrolysis products on filter paper and polyethylene
oxide particles. See text for explanations.
diluted 100 times. This spectrum contains a poorly
defined (inflection) band at 33000 cm–1. The subsequent
extraction of compounds accumulated in the same alka-
line solution showed that the band at 33000 cm–1 became
more intense as the solution was aged.
An increase in extraction temperature causes similar
effects. The spectra of the heptane extract (Fig. 2a,
spectra 2 and 3) obtained by heating (20 min) of MG
oxalate (54 mg) in 10 ml of aqueous alkali (5%) are
shown in Fig. 2a (spectra 2 and 3). The extract was
obtained by shaking with heptane (10 ml) and diluted
40 (spectrum 2) and 320 (spectrum 3) times. Both spec-
tra contain a well-defined maximum at 33200 cm–1
characteristic of the low-frequency MG carbinol band.
The adsorption formation of chromogenic species
also occurs on a nonpolar polyethylene oxide powder
insoluble in both heptane and alcohol. Spectra 2–4
shown in Fig. 2b were recorded for a heptane extract
prepared using a fairly concentrated aqueous-alkaline
solution of MG oxalate after mixing it with ethanol (in
a 1 : 1 ratio) and the addition of polyethylene oxide
(2 mg) to the mixture. It follows from these data that the
long-wave absorption bands observed immediately
after mixing and in 4 and 26 h, respectively, are almost
indistinguishable from the bands of MG oxalate cations
Mixing a heptane extract with ethanol causes the
formation of two layers, one of which is alcohol satu-
rated with heptane. The spectrum of such a layer is in alcoholic solutions and MG cations in 98% acetic
shown in Fig. 2a, curve 4. The 33000 cm–1 carbinol acid [7].
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THE OPTICAL PROPERTIES OF TRIPHENYLMETHANE DYE MOLECULES
1585
The Spectra of Solutions of Initial Crystal Violet
D
(‡)
and Extracted Compounds
1
The spectrum of an alcoholic solution of CV shown
in Fig. 3a (curve 1) has a visible light absorption band
at 17380 cm–1 (575 nm), ε575 = 9.5 × 104 l/(mol cm).
The dye band has a maximum at 17000 cm–1 (588 nm,
ε588 = 8.8 × 104 l/(mol cm)) in an aqueous solution
(Fig. 3a, curve 2), which agrees with the data obtained
in [5, 13]. Spectra 1 and 2 also contain a band at
29000 cm–1, which was not mentioned in the literature.
This band originates from impurity compound XCV,
whose molecules contain a chromophore group simi-
lar to X.
The spectral bands of CV are fairly stable in time in
both alcoholic and aqueous solutions, but their intensity
decreases in the presence of even very small amounts of
alkali. The addition of KOH up to a concentration of
10−3 mol/l into an alcoholic solution with spectrum 1
(Fig. 3a) causes its decolorization in 1–2 s. Simulta-
neously, a band at 38000 cm–1 with a low-intensity
shoulder at 33000 cm–1 appears (Fig. 3a, curve 3). This
transformation results from the formation of the
carbinol CV form and actually repeats the situation
with the formation of MG carbinol (Fig. 1, curve 4).
1.0
3
4
2
0.5
1
2
2
3, 4
4
40
32
(b)
24
16
ν × 10–3, cm–1
2
4
5
1.0
0.5
1
3
The band at 28000 cm–1 observed in an alcoholic
solution (Fig. 3a, curve 1) and corresponding to com-
pound XCV does not change in the presence of alkali.
The dilution of alcohol with heptane (alcohol was evap-
orated to 0.3 ml and heptane was added to 3 ml) causes
a hypsochromic shift of this band to 29000 cm–1
(Fig. 3a, curve 4).
The initial CV samples contain not only XCV impu-
rities but also unreacted leuco base (triphenylmethane
derivative) residues used to synthesize CV. Both com-
pounds are extracted with heptane under the conditions
of heating on a water bath, but residual triphenyl meth-
ane better dissolves in heptane at 25°ë than XCV. The
spectrum of a freshly prepared heptane extract of both
compounds is shown in Fig. 3b (curve 1). Curves 2 and
3 (Fig. 3b) are the spectra of a solution in heptane after
the precipitation (in 20 h) of colorless XCV (the spectra
were recorded after the initial heptane extract was fil-
tered and diluted 140 (1), 100 (2), and 280 times (3)).
The spectral transformation observed during hep-
tane extract aging is evidence that the XCV compound is
largely transferred into the precipitate (it has a band at
νmax = 30100 cm–1 (333 nm) in heptane). Triphenyl-
methane with an intense band at νmax = 37800 cm–1
(264.5 nm) and a weak band at 32600 cm–1 (306 nm)
remains in solution.
40
32
24
ν × 10–3, cm–1
Fig. 3. Optical spectra of (a) solutions of crystal violet in
various media and (b) extracts of residual triphenylmethane
and XCV dye synthesis side product. See text for explana-
tions.
at 25°ë than XCV, were absent. Note that the UV
absorption spectrum of a solution of triphenylmethane
in heptane is closely similar to the absorption spectrum
of an alcoholic solution of CV carbinol [10]; it is, how-
ever, known that carbinol is poorly soluble in heptane at
25°ë [11, 12]. The high solubility of the compound
with νmax = 37800 and 32600 cm–1 in heptane allows it
to be identified with triphenylmethane. The conclusion
that this impurity is not carbinol is substantiated by
experiments with treating its alcoholic solution (Fig. 3b,
spectrum 5) with hydrogen chloride. This treatment
does not result in the appearance of the spectral bands
of the dye, which is formed in the reaction between
carbinol and HCl. At the same time, treatment of the
same solution with a dilute solution of potassium
Part of the precipitate formed at 25°ë was repeat-
edly held in heptane. This gave a solution of XCV
close to saturation with spectrum 4 (Fig. 3b), νmax
=
30100 cm–1. The solution of another part of the precip-
itate in alcohol gave spectrum 5 (Fig. 3b) with a band
shifted to 27200 cm–1 (367 nm). In both spectra,
absorption bands of triphenylmethane (νmax = 37800
and 32600 cm–1), which was better soluble in heptane dichromate in sulfuric acid causes its coloration, it
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 9 2008

1586
MIKHEEV et al.
becomes colored like CV, which corresponds to the dimers were the reason for formulating the idea of the
mechanism of the synthesis of this dye [1–3, 6, 7].
dissociation of carbinols to dye cations and HO– anions
in the adsorption of carbinols on solid surfaces [5, 10].
It was assumed [5] that such dissociation occurred
especially easily in the presence of ionized centers on
solid surfaces. At the same time, the results of this work
are evidence that, first, dye cations are not chromogens,
and, secondly, the formation of dye layers is indepen-
dent of the presence of ionized centers on adsorbent
surfaces. Indeed, the surface of a chromatographic
paper contains OH groups only, whose polarity is not
higher than that of ethanol OH groups, and CV and MG
carbinols do not form chromogenic structures in etha-
nol by themselves, without the action of acids. At the
same time, immonium hydroxides easily form dye lay-
ers even on the surface of nonpolar polyethylene oxide
particles, especially rapidly if the particles are prelimi-
narily wetted with ethanol for plasticization, for
increasing molecular-segmentary mobility.
Note that the spectra of solutions of the precipitate
formed from a heptane extract in heptane and alcohol
contain one more UV band at νmax ≈ 41000 cm–1
(Fig. 3b, curves 4, 5). Its position corresponds to that of
the absorption band of individual MG oxalate mole-
cules (Fig. 1b, spectra 1–3), which allows us to suggest
the presence of individual CV chloride molecules.
Nonchromogenic CV molecules are likely trapped by
XCV crystals during precipitation.
The presence of nonchromogenic CV molecules in
heptane extracts also follows from gradual precipita-
tion from heptane of violet residues on spectroscopic
cell walls. As mentioned, this process also occurs in ini-
tially colorless solutions of nonchromogenic MG
oxalate in heptane. Colored layers especially rapidly
cover ground-glass joints of ampules containing such
extracts under the conditions of heating them on a water
bath, when CV and MG molecules can vaporize
together with heptane and are then adsorbed on ground-
glass joints. As with MG, the presence of nonchro-
mogenic CV molecules in heptane is easily determined
using a chromatographic paper strip immersed into a
colorless extract. As the front of the solution moves and
heptane is evaporated, violet coloration appears.
Note that the properties described above are also
characteristic of heptane extracts of the brilliant green
(an alcoholic medical preparation) and methyl violet
samples studied. The spectra of their alcoholic solu-
tions qualitatively coincide with the spectra of alco-
holic solutions of MG and CV.
The Spectra and Properties of Compounds
with Quinoid Structures of Molecules
The Properties of Alkaline Solutions of Crystal Violet
As mentioned, all the commercial dyes studied in
this work contain impurities that are extracted with
heptane. Impurity molecules possess chromophore
groups responsible for the appearance of similar UV
absorption bands at 30000–31000 cm–1 (333–320 nm)
in spectra of solutions in heptane. These bands shift
similarly when heptane is replaced with ethanol (to
28000 cm–1 (357 nm)). Bathochromic shifts of UV
bands caused by the transfer of molecules with a conju-
gated π-electronic system from a hydrocarbon medium
into a medium containing hydroxyls is known to be evi-
dence of an increase in the polarity of molecules caused
by electronic excitation and, accordingly, an increase in
the energy of interaction with the medium compared
with unexcited molecules [5].
The presence of a small amount of impurity mole-
cules with similar spectra in all the dye samples studied
can be explained if we consider the mechanism of oxi-
dative treatment of triphenylmethanes (MG, BG, and
CV) used in the production of dyes and dimethyl-
aniline, which is the initial compound for the prepara-
tion of MV [1–3].
The spectral picture of the interaction of CV with
alkali qualitatively reproduces the situation with MG.
For instance, an alcoholic solution of CV is rapidly
decolorized when mixed with a solution of KOH. The
intermediate products are immonium hydroxides,
whose pure molecules (of the II and III types) are not
colored. They are also intermediate products of the
interaction of aqueous alkali with crystalline CV pow-
ders. The final product of the alkaline hydrolysis of CV
is the corresponding carbinol. At intermediate reaction
stages, CV immonium hydroxides can be extracted
with heptane. Heptane extracts per se do not have dye
coloration, but immonium hydroxides present in them
can, as those present in MG, transform into violet
supramolecular dimers and larger aggregates (in con-
junction with the formation of carbinols). Unlike
immonium hydroxides, colorless CV carbinol, like MG
carbinol, does not give colored products without special
acid treatment.
As with MG, the formation of dye layers easily
occurs as a result of the adsorption of CV immonium
hydroxide molecules on walls and ground-glass joints
of glass ampules; these molecules vaporize simulta-
neously with heptane during heating. Tests with chro-
matographic paper strips absorbing the heptane extract
of immonium hydroxide also give visual results.
Note that the close similarity of the spectra of CV
and MG immonium hydroxide to the spectra of the cor-
responding carbinols and their ability to form colored
The oxidation of triphenylmethane (its general for-
mula is Ph1(Ph2)CH–ph'–N(CH3)2, where Ph1 and Ph2
are the phenyl groups with substituents of the corre-
sponding dyes in the para position) is an initiated chain
free-radical process (usually, initiated by lead dioxide),
whose final product is carbinol (Kb),
Ph1(Ph2)CH–ph'–N(CH3)2 + r•
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 9 2008

THE OPTICAL PROPERTIES OF TRIPHENYLMETHANE DYE MOLECULES
1587
Ph1(Ph2)C•–ph'–N(CH3)2(R•) + rH,
D
1.0
2
R• + O2
ROO•,
ROO• + Ph1(Ph2)CH–ph'–N(CH3)2
ROOH + R•,
ROOH
RO• + HO•,
RO• + Ph1(Ph2)CH–ph'–N(CH3)2
HO• + Ph1(Ph2)CH–ph'–N(CH3)2
Kb + R•;
0.5
H2O + R•;
4
2 ROO•
termination.
1
3
Here, r• is the initiator radical. In parallel with the for-
mation of carbinol, the chain oxidation of ëç3–
(CH3CH2–) groups occurs,
40
36
32
28
24
ν × 10–3, cm–1
Ph1(Ph2)CH–ph'–N(CH3)2 + r•
rH + Ph1(Ph2)CH–ph'–N(CH3)CH•2 ,
(R•1)
Fig. 4. Spectra of cellulose triacetate film 40 µm thick (1) in
the initial state, (2) after the absorption of X , (3) after
åG
holding in hydrochloric acid vapor for 30 min, and (4) after
holding in air for a day. See text for explanations.
R•1 + O2
R1OO•,
phN(CH3)CH2OO• + phN(CH3)2
R1OO• + Ph1(Ph2)CH–ph'–N(CH3)2
Ph1(Ph2)CH–ph'–N(CH3)CH2OOH + R•1 ,
phN(CH3)CH2OOH + phN(CH3)CH•2 ,
phN(CH3)CH2OOH
phN(CH3)CH2O• + HO•,
Ph1(Ph2)CH–ph'–N(CH3)CH2OOH
HO• + phN(CH3)2
phN(CH3)CH2O•
phN•(CH3) + HOph
2•Oph
H2O + phN(CH3)CH•2 ,
CH2O + phN•(CH3),
phNH(CH3) + •Oph,
termination.
Ph1(Ph2)CH–ph'–N(CH3)CH2O• + HO•,
HO• + Ph1(Ph2)CH–ph'–N(CH3)2
H2O + R•1 ,
Ph1(Ph2)CH–ph'–N(CH3)CH2O•
CH2O + Ph1(Ph2)CH–ph'–N•(CH3)
The condensation of formaldehyde with initial dim-
ethylaniline and monomethylaniline formed in the
reaction gives a mixture of carbinols containing various
numbers of methyl groups in their aniline fragments [1,
3]. This mixture inevitably contains oxidation side
products with quinoid molecular structures. The subse-
quent treatment of carbinols with hydrogen chloride or
oxalic acid results in the formation of dyes containing
the corresponding X compound salts as impurities, for
instance,
(R•2).
The nitrogen radical R•2 formed participates in the dis-
proportionation reaction with ROO• to produce quinoid
compounds,
ROO•(R1OO•) + R•2
ROOH(R1OOH) + Ph1(Ph2)ë = pꢀ = NCH3(X).
Ph1(Ph2)ë=pꢀ=NCH3 + HCl
The technological synthesis of MV involves the oxi-
dation of dimethylaniline phN(CH3)2 with the forma-
tion of formaldehyde. This is followed by sequential
condensation reactions between formaldehyde and ini-
tial dimethylaniline molecules and monomethylaniline
formed in the system. To produce monomethylaniline,
phenol phOH is introduced into the system [1]. Phenol
plays the role of a hydrogen donor for the nitrogen rad-
ical formed during oxidation and characterized by low
reactivity,
Ph1(Ph2)ë=pꢀ=N+H(CH3)Cl– (X+HCl–).
Experiments show that compound X chlorides and
oxalates have low stability and lose acid fragments
when extracted with heated heptane.
The reversible character of the addition of HCl to X
clearly manifests itself in experiments with cellulose
triacetate films containing XåG and XCV absorbed from
aqueous dye dispersions. The spectrum of such a film
with absorbed XåG is shown in Fig. 4, spectrum 2
(spectrum 1 corresponds to the pure film). Absorption
was performed by mixing an alcoholic solution of MG
oxalate (1.6%, 0.5 ml) with water (20 ml) and immers-
ing the film (40 µm thick) into this solution for 20 days.
phN(CH3)2 + r•
phN(CH3)CH•2 ,
phN(CH3)CH•2 + O2
phN(CH3)CH2OO•,
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 9 2008

1588
MIKHEEV et al.
Spectrum 3 (Fig. 4) was obtained after holding the film nic particles of these dyes are formed as a result of the
with spectrum 2 in HCl vapor over hydrochloric acid self-assembly of fairly strong supramolecular dimers
for several minutes. We see that the intensity of the and larger aggregates with the participation of intermo-
28600 cm–1 (350 nm) band of compound XåG lecular forces.
decreases sharply under the action of HCl. Simulta-
neously, the intensity of the UV absorption band at
REFERENCES
40000 cm–1 (X+H cation) increases. Importantly, there
1. I. M. Kogan, Chemistry of Synthetic Dyes (Otd.
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Moscow, 1938) [in Russian].
is no indication of visible light absorption.
Subsequent holding of the hydrochlorinated film in
air for a day almost completely restores the 28600 cm–1
band (Fig. 4, spectrum 4). The easy spontaneous dehy-
drochlorination of the X+HCl– salt is evidence that
quinoid compound X is a weak nucleophile, which can
be explained by its developed aromatic system of con-
jugated π-electrons.
2. I. M. Kogan, Chemistry of Dyes (Goskhimizdat, Mos-
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Reversible hydrochlorination was also observed for
an alcoholic solution of XCV. The intensity of its UV
band at νmax = 27800 cm–1 decreased substantially
under the action of HCl and increased when acid was
neutralized with alkali. Experiments with reversible
hydrochlorination of XåG and XCV are important
because they show that both the initial quinoid com-
pounds and quinoid cations do not have chromogenic
properties; that is, neither quinoid cations nor electro-
neutral quinoid structures impart dye properties to indi-
vidual molecules. The observation that XåG and XCV
lose their UV bands in the cationic form is directly
related to the property known for aniline compounds,
when the appearance of positive charges on nitrogen
atoms substantially hinders the participation of their
electrons in aromatic π-system photoexcitation
events [14].
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(1912).
To summarize, like electroneutral phthalocyanine
molecules, individual triphenylmethane dye molecules,
which are aromatic immonium cations with neutral
aniline (or cationic anilinium) groups do not absorb vis-
ible light. Exactly as with phthalocyanines, chromoge-
13. A. T. Vartanyan, Zh. Fiz. Khim. 30 (5), 1028 (1956).
14. E. S. Stern and C. J. Timmons, Gillam and Stern’s Intro-
duction to Electronic Spectroscopy in Organic Chemis-
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1974).
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