Bull. Soc. Photogr. Imag. Japan. (2022) Vol. 32 No. 2: 21–23

Letters

Mixing Metallic Luster with Organic Dyes
Katsumi YAMADA, Taketo ODA

Abstract: oth crystal violet and a cyanine dye were incorporated into Nafion sheets by electrostatic co-adsorption method. A dye-mixed Nafion sheet
Bobtained under certain conditions exhibited a deep reddish-purple metallic luster. This color tone was not a simple color mixture of metallic
luster by each single dye, suggesting that the aggregate state of each dye was modified by the co-adsorption.

Key words:

Metallic luster, Crystal violet, Cyanine dye, Co-adsorption, Color mixing

In recent years, we have focused on metallic luster from solid filmsof organic dyes that do not contain metallic elements. We have already reported on the properties and mechanism of their metallic
lusters from safflower dyes extracted by traditional methods, crystal
violet and cyanine dyes. 1,2,3) General reflected light does not include
the light of the wavelengths that are absorbed, because the component of the reflected light is the light that the dye did not absorb.
However, the above-mentioned dyes and some solid films of dyes
reflect the same wavelengths of light that they absorb. The mechanism by which these lusters are generated is commonly explained byinterface bronzes.4) However, some organic dyes and graphite that
do not contain metal have a small amount of polarized light components in the reflected light, and it is necessary to investigate the generation mechanism of the reflection.
It is possible to disperse these organic dyes in a transparent polymer film or sheet. Therefore, these dyes can be used not only singly
but also mixture. We succeeded in extracting metallic luster withdifferent tones from a single dye by changing the conditions for immobilizing the dye to the transparent polymer sheet. There are cases
where the dye exists as a monomer in the solid film, and there arecases where it exists as a dimer or an aggregate. In the two cases, thewavelength of the resonating (absorbing) light is different, so thewavelength of the reflected light is also different.
In the case of color tone due to transmitted light mixing dyes, thelight removed from the incident light by their absorptions is simplyadded to achieve color mixture. In the actual transmitted light, scattered and emission light such as fluorescence are included. Whenconsidering only transmitted light, the dye excitation process is important. In dye-sensitized solar cells or a Z-scheme photochemicalsystem, multiple dyes may be used in one system to extend the lightabsorbing band. In these cases, the excitation process is an important factor, but the subsequent relaxation (deactivation) process iseven more important. The concern for their system is whether thedifferent dyes can play their roles independently. In such a dye cock-tail system, it is necessary to consider association of dyes, quenching,electron transfer (undesired electron transfer), energy transfer, andthe other interactions. 5)
In this research, two dyes were immobilized on a transparent
polymer sheet by electrostatic co-adsorption method, and it wasconfirmed whether the reflected light of these dyes was simply added and mixed individual reflection. It is a goal of the present study
to make it possible to design the color tone of metallic luster bymixing dyes with such a technique.
Crystal violet (CV)was dissolved in a good solvent, chloroform,
to prepare a 50 mmol/l solution. 3,3'-Dipropylthiadicarbocyanine
Iodide (CY)was dissolved in a good solvent ethanol to prepare a10 mmol/l solution. Mixed solutions were prepared by changing theratio of these solutions. A Nafion 117 as the transparent basement
sheet cut to 13 mm x 30 mm was immersed in this mixed solution
for 48 hours to electrostatically co-adsorb the two dyes. After that,the dye on the surface of the sheet was washed off, and the resulting
sheet was air-dried.
The reflectance spectrum of the dye-immobilized sheet was measured at the incident angle and detection angle of 30 degrees using a
diffuse transmission/reflection measurement system (Ocean Photonics OP-TF/RF-GONIO-MN)and a colorimeter (Konica Minolta Cd-26dG).
Figure 1 shows the reflectance spectra of Nafion 117 sheet (Nafion+CV and Nafion+CY) with immobilized CV (100 hours immersion) and CY (100 hours immersion). For the sheet of CV witha golden luster, reflection shoulder and maximum were observed atwavelengths of 550 nm and 615 nm, respectively, based on the absorption of H-aggregates and monomers of CV. 6) For the sheet ofCY with a vermilion gloss, reflection maximum was observed at awavelength of 640 nm due to the absorption of the CY monomer. 7,8)
Next, a mixed solution of CV and CY was prepared by changing

Received 2nd December; 2022, Accepted 20th December, 2022; Published 28th December, 2022.
Department of Engineering, Tokyo Polytechnic University,
5-45-1, Iiyamaminami, Atsugi, Kanagawa, 243-0297, Japan

22

Bull. Soc. Photogr. Imag. Japan. Vol. 32 No. 2 (2022)6Reflectance / %Reflectance / %CV:CY=7:3
CV:CY=3:7
CV:CY=1:9

3420

J-aggregate CY

Monomer CV

Nafion+CVNafion+CY

450500550

600

650700750

Wavelength / nm
Figure1. Reflectance spectra of the Nafion+CV and the Nafion+CY.

the ratio in the total volume of 10 ml, and the dyes were introduced
into the Nafion sheet by co-adsorption. Metallic luster was observed in all sheets obtained (Nafion+CV+CY). By increasing theratio of CY in the mixed solution, these color tones changed from acolor tone close to that with only crystal violet to a deep reddish
purple. (Fig. 2) Figure 3 shows the result of reflectance spectrum
measurement of Nafion+CV+CY. In Nafion+CV+CY with thehighest CV ratio CV:CY= 7:3, clear reflection maximum of CV wasobserved at wavelength of 615 nm. However, no CY monomer reflection appeared at wavelength 640 nm, and a new reflection maximum at the wavelength of 690 nm was appeared to be due to the CY
J-aggregate absorption.8) As the CV ratio in the mixed solution
decreases, the reflection maximum at 615 nm seems to be graduallyweakened and shifted to longer wavelengths. Assuming that thereflection phenomenon occurs only in a very shallow layer on thesurface of the material, there is a limit to the number of dyes that canFigure 2. Photographs of the obtained Nafion sheets.

210450

500

550

600

650

700750

Wavelength / nm
Figure 3. R eflectance spectra of the Nafion+CV+CY.

electrostatically bind to the negatively charged sulfonic acid groups
present on the surface of the Nafion sheet. Therefore, the number ofCVs will decrease when other dyes are incorporated. By using amixed solution with a high CY ratio, if the reflection maximum of
the incorporated CY monomer (λ=640 nm)and the reflection maximum of CV (λ=615 nm)overlap, the reflection maximum in thisband will be shifted to longer wavelengths. In order to confirm theaggregation state of CY in the concentrated solution, the transmission spectrum of the mixed solution was carried out. As a result,
J-aggregate of CY was not confirmed in the solution of single CY
solution and mixed solution with CV. These results suggest that CYin Nafion is unlikely to exist as a monomer state.
Figure 4 schematically shows the result of color mixture of reflected light in this study. As a result, it was not a simple summation ofthe reflected light of the single dyes. For CV alone, green and orange light with wavelengths of 550 nm and 615 nm, respectively, arereflected to incident white light. With CY alone, reddish-orange

Figure 4. Schematic diagrams for differences in reflection between Nafion+CV, Nafion+CY, and Nafion+CV+CY.

23Katsumi YAMADA, Taketo ODA Mixing Metallic Luster with Organic Dyes

light with a wavelength of 640 nm is reflected. Due to the mixing ofthe dyes, the reflected light with a wavelength of 640 nm by the CY
monomer is present but weakened. On the other hand, light with awavelength of 690 nm from the J-aggregate of CY was newly appeared, besides the reflection of CV. Finally, the color tone of theNafion+CV+CY (CV:CY=1:9)was deep reddish purple.
In this study, we succeeded in mixing colors with a metallic luster
through the co-adsorption of multiple dyes. Although the color
tone was not obtained by simply adding the reflected light of theindividual dyes, it is considered that this is because the state of theimmobilized dye (monomer or aggregate) is different from that ofthe single dye. In the presence of CV and CY, the reason why CY
does not remain monomeric in Nafion should also be investigated indetail from the viewpoint of the charging state of the dye.

Acknowledgement

This work was supported by "FY2022 Tokyo Polytechnic University International Research Center for Color Science and Art
Grant".

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