Photo-Induced Reduction Reaction of Methylene Blue in an Ionic Liquid

doi:10.4236/ijoc.2011.14023

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International Journal of Organic Chemistry, 2011, 1, 158-161

doi:10.4236/ijoc.2011.14023 Published Online December 2011 (http://www.SciRP.org/journal/ijoc)

Photo-Induced Reduction Reaction of Methylene

Blue in an Ionic Liquid

Jun-ichi Kadokawa, Hironori Izawa, Tomoya Ohta, Satoshi Wakizono, Kazuya Yamamoto

Graduate School of Science and Engineering, Kagoshima University, Kagoshima, Japan

E-mail: [email protected]

Received September 24, 2011; revised October 31, 2011; accepted November 9, 2011

Abstract

Reduction of methylene blue (MB) occurred by photo irradiation at 280 - 370 nm wavelengths to a solution

of MB in an ionic liquid, 1-butyl-3-methylimidazolium chloride (BMIMCl), which was confirmed by color

change and UV-Vis measurement of the solution. Furthermore, the reduced MB was oxidized again by

standing the solution under the conditions of light shielding at 50˚C. The fluorescence spectra of the solution

excited at 350 nm suggested that the photo-induced reduction probably took place via electron-transfer from

BMIMCl to MB.

Keywords: Methylene Blue, Ionic Liquid, Reduction, Photo Irradiation

1. Introduction

Redox dyes where color depends on oxidation state have

been useful in studies in conjugation with electrochemi-

cal applications such as a redox indicator [1]. It can be ex-

pected that development of new redox system using re-

dox dyes leads to further application of the dyes. Methy-

lene blue (MB) is one of the representative redox dyes,

which possesses good electrochemical properties via its

redox reactions [2-6]. Therefore, it has been widely used

for basic electrochemical studies and applications, for ex-

ample, electrocatalysis, solar cells, and biosensors [2,7,8].

Redox reactions of MB reversibly occurred according to

Scheme 1. An oxidized MB exhibits blue color, whereas

a reduced MB (leucomethylene blue) has colorless na-

ture.

On the other hand, ionic liquids, which are low-melt-

ing salts and form liquids at the temperature below the boi-

ling point of water, have continued to receive a great deal

of attention for a wide variety of possible applications,

such as the solvent system for a large number of organic

and inorganic reactions, gel components, catalysis, sepa-

CH3

H3C

CH3

H3C

CH3

Oxidized MB

(

blue

)

Reduced MB

(

colo

less

)

Scheme 1. Redox reactions of MB.

ration, and electrochemical studies [9-17]. The utility of

the ionic liquids as media for various photophysical and

photochemical studies has also been explored [18-20].

For example, photo-induced electron-transfer reactions

in ionic liquids have been reported [21-23]. In this paper,

we report photo-induced reduction reaction of MB in an

imidazolium-type ionic liquid, 1-butyl-3-methylimidazo-

lium chloride (BMIMCl) in the absence of additional

reductant. Furthermore, the reduced MB could be oxi-

dized again by molecular oxygen under the conditions of

light shielding. Previously, redox reactions of some sub-

strates by pulse radiolysis in ionic liquids have also been

reported [24,25].

2. Results and Discussion

When the photo irradiation at 280 - 370 nm wavelengths

to a solution of MB in BMIMCl (0.125 μmol/g) was per-

formed at room temperature for 12 h, decolorization of

the solution occurred. We assumed that this decolori-

zation was owing to the reduction of MB, because it has

been well-known that the similar decolorization happens

by the general reduction reaction of MB in the presence

of some reductants such as glucose. To confirm occur-

rence of the reduction of MB in BMIMCl by the photo

irradiation, the UV-Vis measurement of the resulting so-

lution after the irradiation for 12 h was conducted. In

Figure 1, the UV-Vis spectra of the solution of MB in

BMIMCl before and after the photo irradiation are shown.

159

J.-I. KADOKAWA ET AL.

The absorption at around 660 nm due to MB obviously

decreased by the photo irradiation to largely disappear

after 12 h. When the decolorized solution was left stand-

ing under the conditions of light shielding at 50˚C with

stirring, the solution was colorized to exhibit blue again.

Indeed, the absorptions due to MB in the UV-Vis spectra

gradually increased as shown in Figure 1, but the inten-

sity was lower than that in the initial solution even after

light shielding for 6 h. The colorization was reasonably

explained by the oxidation of MB caused by molecular

oxygen. Furthermore, we confirmed that the change of

the UV-Vis absorptions in the above experiments was

similar as that observed in well-known redox reactions of

MB by glucose and molecular oxygen in alkaline aque-

ous solution, which we had carried out as the compara-

tive experiment according to the literature procedure

[26].

Because it had been considered that the aforemen-

tioned reduction of MB in BMIMCl was probably owing

to electron-transfer from BMIMCl to MB by the photo

irradiation, evidence for occurrence of the electron-

transfer was attempted to be provided by the fluorescen-

ce measurement of the solution of MB in BMIMCl. Fig-

ure 2 shows the fluorescence spectra of the solutions of

MB in BMIMCl in various concentrations excited at 350

nm, which is in the wavelength areas of the photo irra-

diation for the aforementioned reduction of MB. The

emissions at around 450 nm due to BMIMCl decreased

with increasing the concentrations of MB. The similar

decrease of the emissions in the fluorescence spectra

excited at other wavelengths such as 280, 300, and 330

nm was also observed with increasing the concentra- tions

of MB in BMIMCl. Because MB had not exhibited the

absorption at around 450 nm as shown in Figure 1, where

the emission of BMIMCl was observed, however, it was

evaluated that the decrease of the emissions in Figure 2

Figure 1. UV-Vis spectra of a solution of MB in BMIMCl

before and after photo irradiation (12 h), and spectra of the

solution which was left further standing under the con-

ditions of light shielding at 50˚C for 2, 4, and 6 h.

Figure 2. Fluorescence spectra of solutions of MB in

BMIMCl in various concentrations excited at 350 nm.

was not attributed to energy-transfer from BMIMCl to

MB by fluorescence resonance. Therefore, the result of

the fluorescence measurement in Figure 2 can be taken

to suggest that the electron-transfer has happened in the

solution of MB in BMIMCl by the photo irradiation to

cause the reduction of MB [27]. The similar excited state

of an imidazolium-type ionic liquid was previously re-

ported for the mechanism of photo-induced polymeriza-

tion of aniline [28]. The aforementioned redox reactions

of MB could be repeated by the cycle of the photo irra-

diation and light shielding experiments although the in-

tensity of absorption at around 660 nm due to the oxi-

dized state of MB in the UV-Vis spectra decreased ac-

cording to the repeated numbers.

3. Conclusions

In this paper, we reported the photo-induced reduction of

MB in BMIMCl. When photo irradiation at 280 - 370 nm

wavelengths to the solution of MB in BMIMCl was car-

ried out, decolorization of the solution occurred. The UV-

Vis analysis of the solution suggested occurrence of the

reduction of MB by the photo irradiation. Furthermore,

the oxidation of MB by molecular oxygen took place by

standing the solution under the conditions of light shi-

elding at 50˚C. The fluorescence measurement of the

solution excited at 350 nm indicated that the photo-in-

duced reduction of MB probably happened via the elec-

tron-transfer from BMIMCl to MB. The detailed studies

on the mechanism of the present reduction as well as the

photo-induced reductions using other redox dyes in the

ionic liquid are now in progress in our research group.

4. Experimental

All reagents were commercially available and used as

received. Redox reaction of MB by glucose and molecu-

J.-I. KADOKAWA ET AL.

160

lar oxygen in alkaline aqueous solution was conducted

according to the literature procedure [26].

As a typical procedure for redox reactions of MB in

BMIMCl, a solution of MB in BMIMCl (0.125 μmol/g)

was placed in a quartz cuvette (thickness 1 cm). After

photo irradiation at 280 - 370 nm wavelengths (USHIO

SX-UI 250HQ with 250W Hg lamp and UG11 filter) to

the cuvette was performed at room temperature for 12 h

for progress of the reduction, it was subjected to the UV-

Vis measurement. Then, oxidation of MB was carried

out by standing the solution under the conditions of light

shielding at 50˚C for 6 h with stirring, which was ana-

lyzed by the UV-Vis measurement. The UV-Vis and

fluorescence spectra were recorded on Jasco V-650 spec-

trometer and FP-6300 fluorometer, respectively.

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