113

Original Research Paper

J. Jpn. Soc. Colour Mater., 96〔3〕，113–117（2023）

Relationship between Solvent Composition Shifts of Plant-Derived Photochromic Solutions
and their Photochromic Performance
Miki A katsuka＊, Kumiko Tasaki＊, Yoshiumi Kohno＊＊ and Masashi Shibata＊,†＊School of Bioscience and Biotechnology, Tokyo University of Technology, 1404-1 Katakura-machi, Hachioji, Tokyo 192-0982, Japan＊＊Department of Applied Chemistry and Biochemical Engineering, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8561, Japan†Corresponding Author, E-mail: mshibata@stf.teu.ac.jp(Received November 29, 2022; Accepted February 13, 2023 )Abstract
A solution composed of 3-deoxyanthocyanidin extracted from sorghum shell, a weakly acidic buffer solution, and dipropylene glycol showedphotochromism. For the industrial use of this natural photochromic dye, solidification or pigmentation is necessary. However, there is aconcern that the photochromic performance may deteriorate due to the change in solution composition through the process.
In this study, we investigated the effect of composition shifts of the dye solution, namely solvent volatilization, water loading, and acid, baseor polyvalent cation contamination on the photochromic performance.
Even when most of the water evaporated from the dye solution, neither the pH nor the photochromic properties of the solution changed.
In contrast, when a large amount of water, acid, or base was mixed into the solution, the pH of the solution shifted, and deterioration in thephotochromic performance was observed. Additionally, a small amount of polyvalent cations contaminating the dye solution shifted thesolution to a more acidic solution, and the photochromic performance decreased. In this case, it was possible to recover the photochromic
performance by altering the buffer solution used for the solution preparation to a slightly more basic solution.
Key-words: Photochromic pigment, Sorghum, 3-deoxyanthocyanidins, Composition shifts, pH

1．Introduction
Although most industrially used photochromic dyes arebanned for use in cosmetics or foods 1-3 ), the natural dye
3-deoxyanthocyanidin extracted from sorghum shells can beused in these fields 4-7 ).
The 3-deoxyanthocyanin molecule is known to undergo
structural change with / without UV irradiation as shown inFig. 1.
Owing to this chemical structural change, the solution ofthe dye turns red when irradiated with UV light and returns
to colorless under dark conditions.
UV-A+H＋,-H₂O-H＋,+H₂O
light-shielded
Decolorizing type（t-C form）
Fig. 1

Colorizing type
(FV+ form)

Chemical structural change by UV irradiation andshading of 3-deoxyanthocyanidin.

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The photochromism of the solution can be estimated by thecolor difference value between the two conditions.
The value varies drastically with the composition of thedye solution 8 ). We have previously reported that the typicalcomposition of the dye solution for excellent photochromic
performance is dipropylene glycol: pH 4 aqueous buffer =80:20 in volume.
This composition offers the most appropriate pH for thedye solution, namely 5.7; more acidic conditions inhibit
the decoloration process of the solution, and more basic
conditions inhibit the coloration process.
Although this dye is desired for use as a pigment (solidparticle) in industrial fields, it exhibits photochromism onlyin the above solution state, not in the solid state. Hence,
we have been trying to solidify the dye solution (powder inappearance) by holding the solution in pores in the porous
silica or by gelling the solution 9-10 ). It was found that theprocesses for reforming solutions may shift the compositionof the solution.
In this study, we considered several shifts from theappropriate composition, such as solvent volatilization,water increase due to moisture absorption, acid or basecontamination, and multivalent cation contamination, andexamined whether these compositional changes affected thephotochromic performance of the solution.

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