Proceedings of 1st International Symposium for Color Science and Art 2019

Molecular Robots Use the Change of Colors as a Power Source:
Development of an Educational Tool for Young Students byUtilizing the Structural Color Materials
Toshihiro Hiejima, Hitoshi Yajima, Rika Matsumoto, Noritoshi Nambu,
Mitsutoshi Okano and Takayuki Uchida
Tokyo Polytechnic Univ., 1583 Iiyama, Atsugi, Kanagawa 243-0297, Japan
exhibits the helix-coil transition. This transition is triggeredby the photochromism from terminal SP units tomerocyanine (MC) units, as shown in Figure 1 [1]. In
general, the poly(L-glutamate) was found to attend withthe volume change relevant to the helix-coil transition,
which is the first order transition. In other word, PSPLG
can be a candidate of the photomechanical materials. In
this study, we report the bidirectional photo-mobility of
PSPLG gel films cross-linked with the diamine derivatives
having the different chemical properties.

ABSTRACTIn this project, we progress the development of themolecular robots by using the volume expansion and/orcontraction with the change of color and the educational
tool for young students by utilizing the structural color
materials. Accordingly, this article constitute of the two
independent themes related with the structural color. At
first, we found the first evidence for the bidirectional
photo-mobility revealed by the use of difference ofchemical properties in the diamine derivatives. Secondary,
we improve the fabrication method of HPC solution, which
can maintain the structural color for three months.

2. Experimental
PSPLG was synthesized by dehydration-condensation
reaction of poly(L-glutamic acid) and N-hydroxyethyl-SP
by using DCC/HOBt method. The typical SP substitution
ratio was estimated to be ca. 90 mol% from molarextinction coefficient at 355 nm in the observed absorption
spectra. Liquid crystalline gel film was prepared by crosslinking 25 wt% solution of PSPLG in dimethylacetamide,
where reveal the cholesteric lyotorpic liquid crystal. Here,
we use the pentaethylenehexamine (PEHA) and
1,11-Diamino-3,6,9-trioxaundecane (DATD) as diamine
derivatives (cross-linker). UV irradiation to the gel films
were carried out using an 8 W handy type UV lamp
(wavelength 365 nm). Photomechanical motions wererecord with a digital microscope.

1. INTRODUCTION
Structural colors are responsible for many of the brilliant
colors we see in nature. The blue of the sky, the rainbow ofcolors in an oil slick, the bright colors of peacock feathers,
the brilliant blue of a Blue Morpho butterfly, the metallic
colors of certain beetles, and the glimmering colors ofsome fish, are all due to structural color. In this project, weprogress the development of (I) molecular robots and (II)the educational tool by using the structural color.
Accordingly, this article constitute of the two independent
themes related with the structural color.
I. Molecular Robots Use the Change of Colors as aPower Source
1 Introduction
The study on the photomechanical effects taken inpolymer containing photochromic moieties is currently inprogress, in which the incident light energy (photons) isconverted into the mechanical work by photoisomerizationof azobenzen and diarylethene group. In the pioneering
works of Ikeda et al, the irradiation of the linearly polarized
ultraviolet (UV) light on these materials results in large
magnitude, unidirectional bending of cantilevers. Recently,
it was revealed that the photochromic poly(L-glutamate)with the spiropyran (SP) units in the side chain (PSPLG)

3. Results and Discussion
Figure 2 (a) and (b) show the photomechanicalresponses of PSPLG gel films containing the 8 mol%
PEHA and DATD to the irradiation of UV light (2 mW/cm2)
at 365 nm, respectively. Upon irradiation, PEHA gel bent
forward in the direction of irradiation of light, while DATD

Figure 2 Variation of shape in PSPLG gel linked with 8mol%-(a) PEHA and (b) DATD as a function ofirradiation time of UV light.

Figure 1. Photochromic behaviors of PSPLG.

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Proceedings of 1st International Symposium for Color Science and Art 2019
the interference and diffraction of light for Japanese highschool students. We focused on the hydroxypropyl
cellulose (HPC) as aORROstructural color materials
nO(See Figure 4). HPC was
found to reveal the threeOR
primary colors the by
R=H
orCHchanging the concentration
2CH(OH)CH3
in the aqueous dispersion.
Fig. 4 Chemical structure
Moreover, HPC is theof HPC.
medicalandpharmaceutical products, being extremely safety and easy tomanipulate for beginners. In this study, we report thefabrication method maintaining the structural color in thelong term.

gel bents toward the light. This result indicate that thevolume of surface irradiating light was expanded in PEHA
gel whereas it was contracted in DATD gel. To the best of
our knowledge, this is the first evidence for thebidirectional photo-mobility revealed by the use ofdifference of chemical properties in the diamine
derivatives. The mobility could be considered to beattributed to the aggregate resulted from the dipole-dipole
interaction between the merocyanes with zwitterioniccharacter, which is compound produced by thephotoisomerization of SP unit.
Figure 3 shows the variation of the displacement angle(θ) as a function of the exposure time of UV light (365 nm).
PEHA gels exhibit the different photo-bending behaviors
due to the concentration of cross-linker while the DATD
gels were identical behavior independent of theconcentration of cross-linker. Interestingly, DATD gels
exhibit the two-step photo-bending behaviors having aninflection point at 7 minutes. These results suggested thatchemical properties and the concentration of cross-linker
strongly correlated with the mechanism of thephoto-bending behaviors for PSPLG gel.

Displacement angle (θ) / degree

1003. Result and Discussion
Figure 5 shows the photographs of structural colors ofthe HCP solution. These HPC solutions can maintain thestructural colors for three months, though small air bubble
cannot be removed sufficiently with hand pomp.

PEHA

6 mol%
50

2. Experimental
HPC 5.0g (Nippon Soda Co. Ltd.) was dissolved
carefully in water (2.65g) to avoid the lumps, and evacuate
the vacuum storage bag with the hand pomp. After one
day, the transparent HPC solution change to the brilliant
red color. The detailed fabrication method will be upload
as a video on YouTube.

8 mol%

0DATD

6 mol%-50
8 mol%-100051015
Irradiation time of UV light / min20

Figure 5 Photographs of Structural colors obtained fromHPC solution.

Figure 3 Variation of the displacement angle θ of (a)PEHA and (b) DATD gel actuators as a function of 365
nm light exposure time.

As listed in Table 1, the structural colors change in orderof Blue → Green → Red with decreasing the concentrationof HPC solution. This result indicates to increase thecholesteric pitch in a manner of inversely proportional tothe concentration.

II Development of an Educational Tool for Young
Students by Utilizing the Structural Color Materials
1. Introduction
Structural color is color that
results from the way light interacts
withnanoor
micro-scale
structures in a material. It is notdue to dyes or pigments.
Interference colors are an exampleof structural color. These colors areproduced when two or more lightwaves interact. The interference between the waves
cancels some of the colors that make up white light, butnot others. The result is that a normally colorless material,
like soap solution, can appear colored. Structural color
could be an extremely effective probe not only inexperiencing scientific interest but also on understanding

Table 1. The concentration of HPC solution and thecorresponding structural colors.
Structural Color
Water / g [a]Concentration [b]
Blue
2.65
65.4
Green
2.90
63.3
Red
3.15
61.3
[a]: the weight of water for HPC 5g, [b]: weight percent
We will also discuss the temperature dependence ofreflectance spectra for HPC solutions.
4. REFERENCES[1] A. Fissi et al. Biopolymer, 1993, 33, 1505-1517.

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