Bull. Soc. Photogr. Imag. Japan. (2021) Vol. 31 No. 2: 5–8

Original Paper

Metallic Luster from Two Organic Pigments without Metallic Elements-Relationship between Molecular Orientation by Stretching and Polarization
Reflection Takuma KAKI and Katsumi YAMADA

Abstract:A golden metallic luster was observed from crystal violet immobilized inside a Nafion sheet. By stretching this sheet, the degree of polarization of the reflected light was reduced. As a result of comparing the change in the degree of polarization due to stretching using different pigments, it was concluded that the reflected polarization characteristic of the crystal violet is related to the randomness of the direction of thetransition dipole moment.

Key words:

Metallic luster, Crystal violet, Cyanine pigment, Transition dipole moment

Introduction
In solid films of some natural or synthetic pigments or polymers,
a metallic luster is observed even though the molecular structure
does not contain any metallic elements. 1)2)3)4) However, only a few ofthese studies have described in detail the mechanism of the metal-like luster generation. We have studied the mechanisms of thegreen metallic luster generating from the solid film of a carthamin
pigment extracted from safflower by traditional methods. First, thereflection characteristics of the green metal-like luster were discussed, and it was found that the wavelength of the reflected light
does not depend on the incident / detection angle. 5) It was also
found that the solid film of this pigment was forcibly faded by anillumination with white light, and the green metal-like luster was
also weakened at the same time. 6) These facts suggested that thereflected color of this pigment film was not due to the structural
color and was based on the light absorption of the pigment. The
chemical synthesis was difficult7) and the required amount could not
be obtained by the traditional extraction method. Therefore, anotherpigment was explored. Since the carthamin is not industrially available, alternative pigments are required and fuchsine (fuchsin) has
been used. Fuchsine is a triphenylmethane-based synthetic pigment.
A metallic luster of almost the same color as the carthamin can beobtained from the solid of this pigment. Crystal violet (CV), which
is often used in dyeing and medical treatment, is also triphenylmethane-based, and the solid has a luster with a color tone close togold.8)
We decided to study the reflective properties of CV, which has ametallic luster with a different color tone than that of the carthamin.
In discussing the model of the metal reflection, the density of freeelectrons is important in the Drude model. It is unclear whether it isdirectly related in this CV case, but in order to control the density of
the CV, the pigment was dispersed in the polymer support at different ratios, and whether or not a metallic luster appeared was determined. As a result, it was found that a metallic luster appears even if
it is not a solid film composed only of a pigment, and that a metallic
luster begins to appear when the pigment is 30% to 40% by weight.
By using this pigment dispersion film, it became possible to adjust
the density of the pigment by stretching. The luster of a metal is
known to have a small amount of polarized light. The reflection ofthese materials is electromagnetic resonance by free electrons followed by a secondary light emission, and the polarization characteristics are due to the random vibration direction of the free electrons.
The reflected polarization characteristics of the CV's metallic luster
were evaluated, and the effect of stretching the pigment dispersion
polymer sheet on these characteristics was investigated in this study.

Experimental
Crystal violet chloride (CV), cyanine pigment, and solvent wereused in the purchased state without any additional purification. The
Nafion117 (thickness: 200 microns) sheet was purchased from avendor. A ethanol solution (50 mM) of CV was prepared as a pigment solution. In addition, a Nafion sheet of 12 mm x 30 mm wassoaked in the solution for 103 hours. After the sheet was taken outof the solution, the pigment on the surface of the Nafion sheet waswashed off with ethanol. Finally, the sample that had been air-dried
was used as a measurement sample (Nafion+CV). 3,3'-Dipropylthia
dicarbocyanine iodide was used as the cyanine pigment. A ethanol
solution (10 mM) of this cyanine pigment (CY) was prepared as a

Received 5th November, 2021; Accepted 5th December, 2021; Published 9th December, 2021
Department of Chemistry/Material, Tokyo Polytechnic University; 1583, Iiyama, Atsugi, Kanagawa, 243-0297, Japankyamada@chem.t-kougei.ac.jp

6Bull. Soc. Photogr. Imag. Japan. Vol. 31 No. 2 (2021)

(a)

(b)

Fig. 1 Chemical structures of crystal violet chloride(a)and 3,3'-dipropylthiadicarbocyanine iodide(b).

Fig. 3 Photograph of Nafion+CV.

Fig. 2 Schematic diagram of reflectance measurement.

pigment solution. In addition, a Nafion sheet of 12 mm x 30 mm
was soaked for 167 hours. After the sheet was taken out of the solution, the pigment on the surface of the Nafion sheet was then washed
off with ethanol. Finally, the sample that had been air-dried wasused as a measurement sample (Nafion+CY). The chemical structures of the two pigments are shown in Fig. 1.
The apparatus shown in Fig. 2 was used to measure the reflection
spectrum. The light source was a tungsten light. The sample was illuminated with incident light from a collimator lens (+polarizer) via
an optical fiber, and the reflected light was condensed with a (+analyzer) collimator lens via an optical fiber, and finally the reflected
light was introduced into the photodiode array spectroscopic photometer. A home-made jig was used to stretch the test sample.

Results and discussion
Because the Nafion sheet is an ion exchange resin, the CV is considered to be electrostatically immobilized in Nafion.9) In order toconfirm the color tone of the obtained sample, a photograph of the
Nafion+CV is shown in Fig. 3. Because the thickness of the Nafion
sheet is 200 μm, the transmitted light from the Nafion sheet (Nafion+CV) is hardly observed by the utilized light source. As shownin Fig. 3, Nafion+CV showed a metallic luster with a color tone
similar to gold when illuminated with a white light source. Therefore, it is considered that this color tone is due to reflection, which is
not affected by the transmitted light.
Fig. 4 shows the unpolarized reflection spectrum for each incident
angle and detection angle. The 100% reflectance was corrected by theincidence beam through the direct optical path into the collimatorlens on the detector side. The reflectance value of the reflection spectrum increased several times as the angle increased from 10 degrees
to 60 degrees. A shoulder at the around a wavelength of 540 nm andReflectance / %

12
60°

9

55°
50°

6

40°

3
0
400

30° 20° 10°

450

500

550

600

650

700

Wavelength / nm
Fig. 4 S pecular reflectance spectra of Nafion+CV at each angle.

a maximum around at the wavelength of 615 nm were observed ineach spectrum. It is considered that the former corresponds to the
H-dimer, which is the aggregated state of CV, and the latter corresponds to the absorption of the molecularly dispersed state. 10)11) In
addition, an increase in reflectance was observed in the wavelength
range shorter than 490 nm. It is considered that these reflected
lights are mixed and become gold. After that, we focused on the reflection with a wavelength of 540 nm to which human vision ishighly sensitive.
A polarizer and an analyzer were introduced into the optical pathof the measurement system, and the reflected polarization characteristics were studied. These reflectance of the unpolarized, S and P
polarized states at a wavelength of 540 nm were measured and compared for each angle. The result is shown in Fig. 5 as Fresnel reflectance. Except for the reflectance of P-polarized light, the reflectanceincreased with the increasing angle. With P- polarized light, thereflectance began to decrease as the angle increased from 10 degrees,
reaching the minimum value (about 1%) near the angle of 55 degrees. There was no Brewster angle in this sample due to the slight
residual P-polarized reflection. In the case of a colored sample, it is
known that the extinction coefficient is not 0 even in the dielectric
sample and there is no Brewster angle.12)
Based on the result of Fig. 5, the degree of polarization (DOP)
was calculated from the following formula using the reflectance (Rs,
Rp) in the S and P polarized states of 55 degrees, which had thelowest reflectance.
DOP%=((Rs - Rp)/(Rs + Rp)) x 100

7

Metallic Luster from Two Organic Pigments without Metallic Elements-Relationship between Molecular Orientation by Stretching and Polarization Reflection -The results are shown in Fig. 6. Up to 55 degrees, the DOP increased with the increase in the angle. The maximum value of Nafion+CV is 88% around 55 degrees. And, the maximum value is almost 100% for the pigment-free Nafion sheet, that is a complete
dielectric in the dry state. In this state, the DOP of the Nafion+CV
is very higher than the value of a real metal (ex: DOP is within 6%
for aluminum).
Then, the sample was stretched to change the density of the pigment in the Nafion sheet, and the change in the DOP was discussed
in the section. In this study, the stretching ratio is defined as(stretched film length / original length) x 100%, and the length of
1.8 times the original length is a 180% stretching. The relationshipbetween the stretching ratio and the DOP is shown in Fig. 7. The
DOP in the stretching direction decreased as the stretching ratio
increased. This result indicated that the DOP decreased due tostretching, and the polarization state became closer to that of thereal metal.
There are some reports that cyanine pigments generate a metallic
luster in their solid state.13)Next, the result of the cyanine pigment isshown for comparison of the molecular shapes. In Nafion+CY, areddish purple metallic luster was observed as shown in Fig. 8. Fig. 9shows the reflection spectrum in the unpolarized state at each angle.
It is considered that the difference in the color tone from Na-

fion+CV is caused by the difference in the wavelength of the reflection minimum between two pigments. The reflectances of Nafion+CY in the unpolarized, S and P polarized states at a wavelength of
600 nm were measured and compared for each angle. The results areshown in Fig. 10. Unlike Fig. 5 (Nafion+CV), the reflectance in theP-polarized state was almost 0% near 55 degrees. At any stretchingratio, the reflectance of the P-polarized light was almost 0% and the
DOP was almost 100% near 55 degrees. Using this angle, it wasdifficult to compare the influence in the DOP on the stretching ratios. Therefore, in Nafion+CY, the DOP by stretching was evaluatedat an angle of 40 degrees at which the reflectance remained to someextent.
The results are shown in Fig. 11. It was observed that the degreeof polarization tended to increase as the stretching ratio increased.

85

DOP / %

Takuma Kaki, Katsumi Yamada

Stretched direction
Unstretched direction

80

75

Reflectance / %

18

100

Unpolarized reflectance
S-polarized ref.
P-polarized ref.

15

120

140

160

180

200

Stretching ratio / %Fig. 7.Degree of polarization of Nafion+CV at each stretching ratio.

12
9
6
3
0

10

20

30

40

50

60

70

Angle / °Fig. 5 Fresnel reflectance of Nafion+CV at the wavelength of 540 nm foreach angle.
Fig. 8 Photograph of Nafion+CY.

100

8

Reflectance / %DOP / %

80
60
40
Nafion117
CV+Nafion

20

70°

6
4

60°
55°
50°
40°

2
0
10

20

30

40

50

60

70

80

90

Angle / °Fig. 6.Degree of polarization of Nafion+CV and Nafion117 sheet at eachangle.

0400

30° 20° 10°

450

500

550

600

650

700

Wavelength / nm
Fig. 9 S pecular reflectance spectra of Nafion+CY at each angle.

8Bull. Soc. Photogr. Imag. Japan. Vol. 31 No. 2 (2021)

14

Reflectance / %12Unpolarized reflectance
S-polarized ref.
P-polarized ref.

10
8
6
4
2
0
10

20

30

40

50

60

70

Angle / °Fig. 10 Fresnel reflectance of Nafion+CY at the wavelength of 600 nm foreach angle.

DOP / %

70

Stretched direction
Unstretched direction

65

60

100

120

140

160

180

Stretching ratio / %direction of the transition moment of one molecule and the averagemolecular image in the sheet are random and always changing. In
this state, it is considered that the reflected light is not very polarized. It is considered that the stretching of the Nafion sheet, whichis the support, aligns the directions of the CV molecules, so that thepolarization component is reduced and the DOP is also lowered. On
the other hand, for the CY in the Nafion+CY, the molecular structure related to coloration almost linearly exists, and therefore, thedirection of the transition moment can be determined in one direction. It is considered that the DOP of a molecule having such atransition moment increases when the molecule is oriented due tothe stretching of the support.

Summary
In this study, we evaluated the phenomena that a metallic luster
was observed from the solid film of a pigment having a molecular
structure that did not contain metal elements, and the mechanismsthat generated the metallic luster. The polarization reflection characteristics of the polymer sheet were compared using two pigmentswith different molecular structures. As a result, it was found that,
unlike the general understanding, the DOP of CV decreases due tothe stretching of the support. Since the direction of the transition
dipole moment of the CV molecule is random and always changing,
it is possible that the polarization component of the reflected lightwill decrease as the CV is oriented in a certain direction.

Fig. 11 Degree of polarization of Nafion+CY at each stretching ratio.

Generally, the stretching of the sample is performed to induce theorientation of the molecules. Therefore, it is expected that the polarization will become stronger when the sample is oriented by stretching. Unlike the general tendency, the result of the Nafion+CV
showed that the polarization was weakened by stretching, so thecause was considered below.
The relationship between the light absorption (and reflection) of
the pigment and the direction of the transition dipole moment of
the pigment was investigated in the study by Mizuguchi et al. 14)
Among their papers, it is proposed that light can be absorbed withthe same vibration as the direction of the transition moment of thepigment molecule. That is, it is considered that the light vibratingin the same direction as the vibration direction of the bonded electrons of the molecular structure related to the coloration of the pigment resonates, and if the secondary light is emitted, it becomes thereflected light. In such Rayleigh scattering or Thomson scattering,
the wavelengths of the resonant light and the reflected light are
maintained. In addition, when the properties of the material (ex.
pigment) are close to those of metal, the polarized light of the reflected light becomes less polarized like Thomson scattering. The
transition moment of the CV is caused by the conjugate of the central methane with the outer phenyl group, but this structure repeatedly shifts to another position at the next moment. Therefore, theReferences
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