Photosynthesis photosensitization reactions

In plants, the photosynthesis reaction takes place in specialized organelles termed chloroplasts. The chloroplasts are bounded in a two-membrane envelope with an additional third internal membrane called thylakoid membrane. This thylakoid membrane is a highly folded structure, which encloses a distinct compartment called thylakoid lumen. The chlorophyll found in chloroplasts is bound to the protein in the thylakoid membrane. The major photosensitive molecules in plants are the chlorophylls chlorophyll a and chlorophyll b. They are coupled through electron transfer chains to other molecules that act as electron carriers. Structures of chlorophyll a, chlorophyll b, and pheophytin a are shown in Figure 7.9. 

In natural photosynthesis the quinones are widely used as electron carriers. Unfortunately, the low values of cpc in the reaction of quinones with 3Ru(bpy) + make the direct use of these important electron carriers rather inefficient. However, introduction of the electron carrier Rh(bpy) + into the inner volume of the vesicle in addition to photosensitizer Ru(bpy) +, provides much more efficient electron transfer from 3Ru(bpy) + to a quinone embedded into the membrane. This was found for System 25 of Table 1. 

Porphyrins are an important class of -> electron-transfer ligands. Photosynthesis is primarily driven by chromophores (light-harvesting antenna and reaction centers) which consist of special assemblies of porphyrins. Porphyrins have been intensively studied for their possible applications, including their use as photonic materials, catalysts, photosensitizers for photodynamic therapy, receptor models in molecular recognition, and components of -> electrochemical sensors [v]. 

To obtain as complete a picture as possible of the kinetics, it is important to obtain a maximum time resolution. Many biological reactions occur in a very short time (in fact, down to picoseconds in vision and photosynthesis) so that slower, more traditional techniques such as stopped-flow often lack adequate time resolution. Under fortunate circumstances where the system studied is photosensitive, or where the free-energy change between reactants and products is sufficiently small, then the concentrated energy impulse from a laser is a marvelous tool that can provide a wealth of information concerning the system. The intent of the review is not to offer an exhaustive review of the literature, which would be an overwhelming task. Rather we hope to delineate areas where lasers have played an important role, to discuss the importance of these investigations, and to point to areas where their potential is still untapped. 

There are several approaches to mimic functions of photosynthesis, and each has been reviewed extensively. These include the design of molecular assemblies duplicating functions of the photosynthetic reaction center [13,14], development of photosensitive materials capable of light harvesting and inducing electron transfer processes [15-17] and tailoring multi-component systems where light-induced electron transfer processes drive... 

One very interesting class of ET reactions at soft interfaces are those that are photoini-tiated. Following the pioneering studies of the Russian school, including those of Volkov [6] and Kuzmin [7], it has been shown that photosensitizers soluble in one phase are often adsorbed at the interface and can be quenched by electron donor or acceptors. This class of reaction offers interesting perspectives to design biomimetic approaches to artificial photosynthesis. Photoelectrochemistry at the interfaces between two immiscible electrolyte solutions (ITIES) is rather analogous to photoelectrochemistry at a semi-conductor electrode, where the potential drop within the semi-conductor should be considered. 

---