**2. Antenna systems**

Absorbing photos by an antenna pigment is the first stage in photosynthesis. Pigments can be a chlorophyll, xanthophylls, phycocyanin, carotenes, xanthophylls, phycoerythrin and fucoxanthin depending on the type of organism and a wide variety of different antenna complexes are found in different photosynthetic systems. Each year, the energy of 1024 Joule reaches the planet's surface through solar radiation. It is interesting that this is three orders of magnitude more than what is projected for the future global anthropogenic energy demand of

1021 Joule (Moore, 2005). The development of artificial antenna for collecting and harvesting solar energy efficiently is an active and complex field as the distance between the pigments to be used, their respective angle, and electronic coupling, must be engineered carefully. Sakata et al. (2001) have designed and synthesized a well-dened, rigid-sheet-structured oligoporphyrin (Fig. 1) with 21 porphyrin chromophores. The compound is a model for light harvesting compounds in *Nature* and showed promising properties for collecting and harvesting solar energy.

<sup>\*</sup> Corresponding Author

Gathering Light: Artificial Photosynthesis 5

Synthetic porphyrin derivatives have been widely used to mimic the natural chlorophyll pigments to convert light energy into chemical energy. [Ru(bpy)3]2+ presents an absorption band in the region around 450 nm corresponding to a metal to ligand charge transfer (MLCT) band with an extinction coefficient of about 13,000M−1cm−1. Upon irradiation in the MLCT band, the input light energy is converted into a (d6) → (d5π\*) excited state, which in turn relaxes to form the lowest triplet state (3MLCT) in less than a picosecond. The oxidation potential of [Ru(bpy)3]3+ is similar to oxidized photosystem II primary donor in natural photosynthesis, therefore making it a suitable candidate to reproduce the oxidation

The hydrogen production from water splitting is an appealing solution for the future energy as discussed by Bockris (Bockris, 1977). A strategy is to employ solar, wind, ocean currents, tides or waves energy to water splitting. However, to evolve hydrogen efficiently in a sustainable manner, it is necessary first to synthesize a stable, low cost, and efficient, environmentally friendly and easy to use, synthesis, manufacture catalyst for water oxidation, which is the more challenging half reaction of water splitting. In past few years, there has been a tremendous surge in research on the synthesis of various metal compounds aimed at simulating water oxidizing complex (WOC) of photosystem II. Particular attention has been given to the manganese compounds aimed at simulating the water oxidizing complex of photosystem II (Umena et al., 2011)) not only because it has been used by *Nature*

to oxidize water but also because manganese is cheap and environmentally friendly.

Photovoltaic is a method of generating electricity by converting light into electricity. Photovoltaic devices work base on the concept of charge separation. A new family of devices, the dye-sensitzed solar cell, is shown in Fig. 2. In the system, there is an oxide layer (for example TiO2) which to allow for electronic conduction to take place. The material oxide layer is attached to a monolayer of the charge transfer dye. Photo excitation of the dye results in the injection of an electron into the conduction band of the oxide. Usually the iodide/triiodide couple restors the original state of the dye. The dye-sensitized solar cell made of low-cost materials, robust, does not require elaborate apparatus to manufacture, can be engineered into flexible sheets, requiring no protection from minor events like hail or tree strikes. Thus, there are technically attractive. In this devise, light is absorbed by a dye, the sensitizer is grafted onto the TiO2 surface and then light induced electron injection from

In Fig. 3 comparing between the spectral response of the photocurrent observed with the two sensitizers and TiO2 is shown (Grätzel, 2003). The incident photon to current conversion efficiency of the dye-sensitized solar cell is plotted as a function of excitation wavelength. Both chromophores show very high incident photon to current conversion efficiency values in the visible range. Some companies work to develop dye-sensitized solar cells for

**3. Photoactive chromophore** 

reactions performed by the natural system.

**5. The dye-sensitized solar cell approach** 

the adsorbed dye into the TiO2 conductive (Grätzel, 2003).

applications in cars and homes.

**4. Water oxidizing complex** 

Fig. 1. An artificial antenna for collecting and harvesting solar energy

Fig. 1. An artificial antenna for collecting and harvesting solar energy
