Photoinduced electron transfer from synthetic chlorophyll analogue to fullerene C60 on carbon paste electrode: Preparation of a novel solar cell

https://doi.org/10.1016/S0302-4598(98)00221-9Get rights and content

Abstract

Upon a carbon paste electrode, fullerene C60 and successively methyl pyropheophorbide-a (chlorin) were casted to prepare a chlorin–fullerene modified carbon paste electrode (CFE). Photocurrents on the CFE were produced by irradiating of visible lights (>510 nm) in an aqueous solution of 0.05 M ethylenediaminetetraacetic acid and 0.1 M Na2SO4 at pH 6.7. Larger anodic photocurrent was induced by the CFE than by the carbon paste electrodes modified with either the fullerene or the chlorin. In addition, the photocurrent of the CFE was dependent upon the amount of fullerene casted. The photocurrent action spectra of the CFE (at 300 mV vs. Ag/Ag+) showed that photoinduced electron transfer occurred from the excited state of the chlorin to the fullerene and/or from the chlorin to the photoexcited fullerene, and the electron of the fullerene anion radical produced was then shifted to the carbon paste. Upon irradiation of >375 nm lights, the anodic photocurrent of the CFE was enhanced by increase in the illuminated light power and reached 0.03 mA cm−2 in the present system.

Introduction

Mimicking the electron transfer system in the primary process of photosynthesis is an object of interesting study [1]. A large number of biomimetic photoinduced electron transfer systems have been reported and are of two types: (1) homogeneous and (2) heterogeneous. For examples of the former, covalently linked donor–acceptor molecules were synthesized and photoinduced electron transfer in solution was observed by various spectroscopies 1, 2. In the latter, typically a variety of electrodes modified with donor and acceptor were irradiated and the photocurrents were measured 3, 4, 5, 6. The electrodes were primarily indium tin oxide (ITO) and gold (Au) and special techniques were usually necessary for modification: preparation of multi Langmuir Blodgett (LB) films containing donor and acceptor molecules, and fixation of covalently-linked donor–acceptor molecules on the electrodes 4, 5, 6. On the other hand, few studies using carbon paste electrodes for the measurement of photocurrent are available [7], although the carbon paste electrode has several advantages over ITO and Au electrodes: simple preparation, wide scale application, and generation of large photocurrents with no reflection of light on the carbon surface. Therefore, we selected the donor–acceptor modified carbon paste electrode to mimic the electron transfer system.

Studies on the generation of photocurrents on the ITO and Au electrodes modified with the donor–acceptor were carried out with porphyrins as the photosensitizer 3, 4, 5, 6. There are no reports that chlorin derivatives (=dihydroporphyrin), which have a similar structure to natural chlorophylls and a large absorption band at longer wavelengths, were used as the photosensitizer on the modified electrodes.

Here we report on preparation of chlorin (methyl pyropheophorbide-a 1, Fig. 1)–fullerene C60 modified electrodes (CFE) by a simple casting procedure and generation of anodic photocurrent by photoinduced electron transfer from the chlorin to the fullerene on the electrode.

Section snippets

Materials

Methyl pyropheophorbide-a 1 was synthesized from chlorophyll-a by the method in the literature [8]. Fullerene C60 (>99%) was purchased from Tokyo Chemical Industry. Dichloromethane and benzene (analytical grade) were purchased from Wako, and used without further purification. Tetrahydrofuran was distilled before use. Graphite powder (particle size=10 μm), tetra-n-butylammonium perchlorate (TBAP, specially prepared reagent), disodium dihydrogen ethylenediaminetetraacetate dihydrate (EDTA), and Na

Results and discussion

A benzene solution of fullerene C60 and a dichloromethane solution of a synthetic chlorophyll analogue (chlorin 1) were dropped and dried on the surface of a carbon paste electrode to prepare a chlorin–fullerene electrode (CFE). Measurements of photocurrents on the modified carbon paste electrodes were performed with the conventional three electrodes method by irradiating a deaerated aqueous solution of 0.05 M EDTA–0.1 M Na2SO4 (Fig. 2).

Little anodic current (ca. 0.1 nA cm−2) was observed on

Conclusion

Our investigation of carbon paste electrodes modified with fullerene C60 and synthetic chlorophyll analogue, chlorin 1 (CFE) determined CFE to be more easily prepared than electrodes modified with multilayer assembly and self-assembly of electron donor–acceptor. The anodic photocurrents were induced by irradiating the CFE with light around the Qy band of the chlorin (>650 nm) at 300 mV (vs. Ag/Ag+). Irradiation of the CFE with light around the Soret band of the chlorin (>375 nm) resulted in

Acknowledgements

We thank Prof. Dr. Hideyuki Takakura, Ritsumeikan University, for measurement of photocurrent action spectra. This work was partially supported by Grants-in-Aid for Scientific Research (Nos. 07454192 and 10146252) from the Ministry of Education, Science, Sports and Culture, Japan.

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