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Bull. Soc. Photogr. Imag. Japan. (2018) Vol. 28 No. 2: 18–22

LetterInfluence of Photo-illumination on Greenish Metallic Luster of Safflower RedPigment Film
Hitoshi Yajima, Maiko Sasaki, Keiko Takahashi, Masato Oshima, Kazuyuki Hiraoka, Morio Yashiro,
Katsumi Yamada

Abstract:

 visible photo-illumination on the carthamin red pigment solid film was carried out in the study. It was found that the greenish metallic
Aluster of the film was weakened by the photo-illumination and the absorption of the film was also weakened at the same time. Because thedeterioration of the carthamin chromophores was induced by the photo-illumination, the absorption in the green light band was probably an
important factor of the metallic luster generation.

Key words:

Carthamin solid film, Greenish metallic luster, Photo-fading, Carbonyl group

Pigments obtained from safflower (Carthamus tinctorius L.) has
been used as herbal medicine, food, colorant and cosmetics. Thereare some difficulties (such as thermal decomposition and photo-fading) in the extraction of carthamin (red pigment) from petals. It isdifficult to separate the water-soluble yellow pigment and the water-insoluble red pigment. Therefore, the red pigment is quite rare,
and it is hard to obtain the pure commercial products. Kuroda reported in her paper that the red pigment solid with a high purity
gave a greenish metallic luster. 1) Although the optical properties
and the photo-fading characteristics of the red pigment were evaluated with a mixture sample such as the dyed textiles or liquid phasesof low concentration, there is almost no discussion on its solid phaseat higher purity. 2,3) In our previous study, a highly pure red pigment
having a metallic luster was successfully obtained by a modificationof the traditional extraction method. Our specular reflection measurements, showed that the angle dependence on the wavelength at
the reflection maximum (λmax : 550 nm) was not found for the red
pigment film. Therefore, it was concluded that the greenish metallicluster of the pigment film was due to scattering by the bonding electrons not to interference color at multilayer structures.4) In thisstudy, photo-illumination for the solid film of the safflower red pigment was carried out to discuss the relationship between the absorption of the chromophores and the reflection.
The safflower red pigment was extracted by a traditional methodwith some modifications. 4) The pigment film was immobilized onto
the quartz crystal plate by casting from the concentrated aqueous
solution of the extracted pigment. These films were dried under dark
and ambient atmosphere at room temperature. The photo-illumination for the pigment film was carried out with a metal-halide lamp
(incident power: 30 mW) under humidified condition. Specularreflectance and transmittance spectra were obtained by a CCD array
Received 4th December, 2018; Accepted 14th December, 2018

spectrometer connected with fiber optics. Changes in the chemicalstructures of the red pigment by the photo-illumination were evaluated with a Fourier transform infrared spectrometer (FT-IR) and a
Raman micro-spectrometer.
Figure 1 shows the photograph of the safflower red pigment film
(thickness : 0.4 μm) on the quartz crystal plate after photo-illumination through a multi-color filter for 30 min. After the photo-illumination, the greenish metallic luster turned red color. The biggest
changing was found in the none filter part, and the secondary big
changing was found in the green filtered part. On the other hand,
the metallic luster was left as before under the red filtered part. Figure 2 shows the specular reflectance spectra of the pigment film after
photo-illumination through color filters. The reflectance values at
the wavelength of 550 nm decreased in the order of none filter, greenfilter, orange filter, red filter and unexposed part. Figure 3 shows theabsorption spectra of these filters and the red pigment film. Thisfigure indicates that the red filter can cut the light over the whole

Tokyo Polytechnic University, 1583, Iiyama, Atsugi, Kanagawa, 243-0297 JAPANFig. 1 P
 hotograph of the safflower red pigment film (thickness : 0.4μm) on the quartz crystal plate demonstrating the effect ofphoto-illumination through multi-color filter for 30 minutes.

Hitoshi Yajima, Maiko Sasaki, Keiko Takahashi, Masato Oshima, Kazuyuki Hiraoka, Morio Yashiro, Katsumi Yamada

Influence of Photo-illumination on Greenish Metallic Luster of Safflower Red Pigment Film

1910Unexposed
None filter
Red filter
Orange filter
Green filter

Reflectance / %86420480500520540560

Wavelength / nm
Fig. 2 Specular reflectance spectra of the pigment film (thickness : 0.4 μm) after the photo-illumination via color filters.

Transmittance / %8060

Red filter
Orange filter
Green filter
Film abs. (au.)

40200400450

500

550600650

Wavelength / nm
Fig. 3 Absorption spectra of color filters and the red pigment film (thickness : 0.4 μm).

absorption band of the red pigment, and the orange filter can transmit the light in the longer wavelength region of the red pigment.
Therefore, it is confirmed that the absence of the metallic luster wasinduced by the photo-illumination with the visible light on the absorption band of the red pigment. On the other hand, the change in
the absorption spectrum of the red pigment film caused by the exposure for 30 min. was very slight. These observations suggest thatthe deterioration of the pigment associated with the reduce metallic
luster probably took place only in uppermost surface layer of thefilm.
Then, the film thickness of the pigment was reduced to about 80

nm and the photo-illumination time was extended to 139 min. Figure 4 shows the photograph of the pigment film after the long photo-illumination. With the black background (for reflection evaluation), the greenish metallic luster was observed in the unexposedpart, and the color change from green to reddish orange was observed in the exposed part. With the white background (for transmission evaluation), the color of the pigment film turned orange inthe exposed part. In this way, considerable color changes wereachieved by the film thinning and the extended photo-illumination
time. Figure 5 shows the specular reflectance spectra of the pigment
film (80 nm) before and after the photo-illumination (139 min.). In

20Bull. Soc. Photogr. Imag. Japan. Vol. 28 No. 2 (2018)

Unexposed part

Exposed PartFig. 4 Photograph of the pigment film (thickness : 80 nm) after the long (139 min.) photo-illumination.

20Before
After

Reflectance / %1612840360

400

440

480

520

560600

Wavelength / nm
Fig. 5 Specular reflectance spectra of the pigment film (thickness : 80 nm) before and after the photo-illumination.

comparison with the rate of decrease in the reflectance at the 550
nm (-40%) for the thick film (0.4 μm) illuminated for 30 min.
(Fig.2), the reflectance value at the wavelength of 550 nm remarkably decreased (-70%) by the photo-illumination. The spectrum
shape after the photo-illumination had a curved part convex downward covering the green light band. Figure 6 shows the transmittance spectra of the pigment film before and after the photo-illumination. The absorption of the red pigment around the wavelength of550 nm was remarkably weakened by the photo-illumination, andthe absorption maximum shifted to 480 nm was found on the figure.
The orange state (λmax : 480 nm) is different from a yellow pigment
having the absorption maximum around 410 nm. As a general understanding, the red color of carthamin was obtained by expansionof the π-conjugation due to coupling with the yellow molecules. It
was suggested that this orange state is not simple recovery of yellow
pigment, but to a different type of decomposition products of redpigment.
In order to study the change in the chemical structures of the redpigment, the measurement of the rotation and vibration spectra
were carried out with a FT-IR and Raman spectrometer. As theobtained spectrum from the FT-IR measurement, some signals at1515, 1585, 1624 cm-1 due to conjugated aromatic C=O (carbonylgroup) stretching remarkably decreased after the photo-illumination. The carbonyl group is a part of the chromophore of the carthamin (red pigment). 5,6,7,8) Figure 7 shows the Raman spectra of thepigment film before and after the photo-illumination. The clear

Hitoshi Yajima, Maiko Sasaki, Keiko Takahashi, Masato Oshima, Kazuyuki Hiraoka, Morio Yashiro, Katsumi Yamada

Influence of Photo-illumination on Greenish Metallic Luster of Safflower Red Pigment Film

21Transmittance / %80
6040
Before
After

20
0
400

450

500

550

600

Wavelength / nm
Fig. 6 Transmittance spectra of the pigment film (thickness : 80 nm) before and after the photo-illumination.

Intensity / a.u.

Before
After

1600

1500

1400

1300

1200

1100

1000

Wave numbers / cm-1Fig. 7 Raman spectra of the pigment film (thickness : 80 nm) before and after the photo-illumination.

changes were found between the two spectra, especially, the absenceof the strong signals 1176 cm-1 (substituted aromatic) and 1600 /
1620 cm-1 (C=C ring stretch doublet) was found between before andafter the photo-illumination. 9,10)With these results, it was confirmedthat the deterioration of the carbonyl group and /or the shorteningof the π-conjugation were induced in the chemical structures of thered pigment by the photo-illumination. These discussions probably
contribute to make sure that the greenish metallic luster of the redpigment film was directly associated with the light absorption of thechromophores.
The visible photo-illumination to the safflower red pigment solid
film was carried out to discuss the optical characteristics in the study.

The greenish metallic luster of the pigment film was influenced andweakened by visible photo-illumination. It was confirmed by themeasurement of the rotation and vibration spectra that the deterioration of the carbonyl group and / or the shortening of the π-conjugation were induced in the chemical structures of the red pigment by
the photo-illumination. We are just started the approach for therelationship between the metallic luster of the pigment film and thebonding electrons in the pigment. By the discussion with the basicmodels such as Drude and Lorentz theories, the generation mechanism of the metallic luster will be elucidated in more detail in near
future.11)

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Bull. Soc. Photogr. Imag. Japan. Vol. 28 No. 2 (2018)

Acknowledgment
This work was supported by "FY2016 MEXT Private University
Research Branding Project". The authors wish to thank Prof. Rika
Matsumoto, Tokyo Polytechnic University, for measuring Raman
spectrum.

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