Acid durability and photoisomerization experiment of chiral Schiff base azosalene

Short commentary

Acid durability and photoisomerization experiment of chiral Schiff base azosalene

Corresponding author: Akitsu T, Department of Chemistry, Faculty of Science, Tokyo University of Science,1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan, Tel: 81-3-5228-8271, Fax: 81-3-5261-4631, E-mail: akitsu@rs.kagu.tus.ac.jp

Abstract

Recently, due to fear of depletion of fossil fuels, photovoltaic cells are attracting attention as new energy generation technology. In order to solve the problem, we aim to increase battery efficiency by broadening the range of absorption wavelength using chiral Schiff base azosalen (CSBAZS) complex and other complexes. Therefore, in order to obtain a wider absorption region, zinc phthalocyanine having an absorption width near 300 nm and around 800 nm was used. In the process, when the CSBAZS complex was combined with zinc phthalocyanine to measure UV spectra, the absorption wavelengths of the two dyes overlapped and wide range of absorption spectra were observed. For utilizing the CSBAZS complex in a photovoltaic cell, a durability experiment under acidic conditions was conducted. When 1-5 drops of conc. HCl were added, the spectra showed blue shift. Moreover, azo group isomerization experiment by UV-vis was conducted under the same conditions, showing no change. These would be due to protonation of the azo group under acidic conditions. These absorption intensities showed almost no change, indicating that they work sufficiently as dyes for photovoltaic cells.

Introduction

To extend the light absorption of the dye to the longer wavelength side for photofuel cell [1], we investigated whether it is an appropriate molecule in the photofuel cell using chiral-Schiff base azosalen complex (hereafter abbreviated as CSBAZS). The photofuel cell is both inexpensive and sustainable as compared with current dye-sensitized solar cells with photofuel titanium dioxide (TiO2) and silver-supported titanium oxide (Ag / TiO2) as both poles, whereas the amount of current obtained is about 0.1 mA.

Therefore, if we efficiently absorb CSBAZS complex developed by our laboratory up to longer wavelength sunlight, it promotes the movement of more electrons to the conduction band, We thought about a strategy to improve.

In this context, to promote the movement of more electrons to the conduction band and develop up to longer wavelength sunlight, we synthesized CSBAZS complex (Figure 1).

In this context, to promote the movement of more electrons to the conduction band and develop up to longer wavelength sunlight, we synthesized CSBAZS complex (Fig.1). We thought about a strategy to improve the quantity. Characteristics of CSBAZS complex include red shift of absorption wavelength and increase of absorption by a π conjugated system of azo group, shift of absorption band caused by photoisomerization of azo group and orientation control by irradiation with polarized ultraviolet light.

In addition, complexes having azo dyes and azo groups generally have a weak point to decompose by ultraviolet irradiation for a long time, and it is known that they are hydrogenated by protons and electrons under strongly acidic conditions. Besides, there is a problem that the function of the dye must be retained by strong acidification when a pigment is appointed to a photofuel cell.

In order to investigate whether it can be used for the above-mentioned photo-fuel cell, we conducted a co-sensitization model experiment of CSBAZS complex and zinc phthalocyanine, a durability test in acid conditioning of CSBAZS complexes (FeL, CuL, ZnL) and photoisomerization experiments were conducted to increase the knowledge of the selection conditions of suitable dyes [2,3].

Experimental Section

CSBAZS complexes (FeL, CuL, ZnL) were synthesized according to the previous study. [4] UV was measured JASCO V-570 UV–vis–NIR spectrophotometer in the range 1500–200 nm at 298 K. Concentration of FeL solution (DMSO) in durability test under acidic condition is 5.19×10-6 mM for FeL, 2.91×10-6 mM for ZnL, 2.57×10-6 mM for CuL and conc. HCl was added to the solution to reproduce acidic conditioning. The amount of conc. HCl was 1 dropwise with a Pasteur pipette and a total of 5 drops were added. Colorimetric method with pH test paper was used for pH measurement.

Concentration of zinc phthalocyanine in co-sensitization model experiment was 5.26×10-7 mM, ZnL’s one was 3.05×10-6 mM.  FeL concentration in the isomerization experiment was 5.58 × 10-6 mM.

Results & Discussion

(Figure 2) depicts co-sensitization model of ZnL and Zinc Phthalocyanine. Absorption of azo group and Soret band of Zinc Phthalocyanine in the near 400 nm, Q band of Zinc Phthalocyanine near 700 nm are present. As these absorptions overlap each other, it becomes possible to absorb a wide range of wavelengths. This phenomenon can be said to be a very useful result when it is used for a photofuel cell.

   Figure 2: UV-vis spectra of ZnL and/or Zn Phthalocyanine.

As shown in (Figure 3,4& 5) (for FeL, ZnL, and CuL, respectively), when one drop of hydrochloric acid was added, the peak near 400 nm was largely blue-shifted in all the complexes compared to the state where conc.HCl wasn’t added. No change was observed even when adding from 1 drop to 5 drops of conc. HCl. It was thought that the azo group of CSBAZS complex was added with proton under acidic condition [4].

 Figure 3: UV-vis spectral changes of dropwise addition of HCL to FeL.

Figure 4: UV-vis spectral changes of dropwise addition of HCL to ZnL.

Figure 5: UV-vis spectral changes of dropwise addition of HCL to CuL.

1 drop of conc.HCl was added to FeL and then we conducted photo-isomerization experiment for azo-group of FeL (Figure  6). No change was observed even when irradiated with ultraviolet light(362nm) or visible light. UV and visible rays were irradiated alternately for 3 minutes each. Irradiated for a total of 15 minutes each.

Figure 6: UV-vis spectral changes of UV or visible light irradiation to HCL added FeL.

It was considered the azo group is no longer isomerized due to proton addition to the azo group. The absorption intensity does not change so much as compared with that before adding hydrochloric acid, however, it seems one functioned sufficiently as a dye.

From these facts, the protonation of the azo group occurs suddenly only by adding a small amount of conc. HCl, but the near 400nm peak shows the same absorption amount. Therefore, when combined with Zinc Phthalocyanine, it can absorb a wide range of wavelengths, it will be able to be used as a photo fuel cell.

Conclusion

In co-sensitization model experiments, it was possible to absorb a wide range of wavelengths useful for photofuel cells with two dyes which are FeL and Zinc Phthalocyanine. In the durability experiment under the acidic condition about FeL, the wavelength near 400 nm was largely blue-shifted by just adding 1 drop of conc. HCl. We were thought to be due to the addition of protons to the azo group of FeL. When an isomerization experiment was conducted with the azo group of FeL protonated, no result of isomerization was found. The absorption intensity around 400 nm does not change so much, however, it is useful as a dye. We think that supplementing each other’s weak points with multiple dyes is also one way to increase the efficiency of photofuel cells.

Reference

1 Motoharu M .Catal. Sci. Technol 2014 ;(4):1644.

2 Malgorzata Bayda . Dalton Trans .2017;46:1914.

3 Jean-Michel Savéant .Cédric Tard, J. Am. Chem. Soc 2014; 136: 8907−8910.

4 Shinnosuke Tanaka et al, Journal of the Korean Chemical Society, 2018 :62:4.

 

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