Photochemistry mechanism determination

In this chapter we will endeavor to answer the following questions What does the photochemist do in preparing to investigate the photochemical behavior of a molecule What equipment does he use to carry out his experiments Once he has determined the results of the reaction, how can he develop a mechanism to account for these results In answering these questions we will be concerned mainly with the photochemistry of anthracene and related compounds. 

Recently Fouassier and Chesneau [219] studied the photochemistry of the system Eosin-PDO-MDEA in aqueous acetonitrile using steady-state irradiation and laser flash photolysis. The photopolymerization of methyl methacrylate (MMA) sensitized by the photoreduction of Eosin is investigated in acetronitrile to understand the mechanism of initiation and the enhancement in the rate of polymerization caused by the presence of PDO, 3. Rates, quantum yields of photopolymerization, and number average molecular weights of the polymer are determined with MMA (7 M), Eosin (3 x 10 5 M), and MDEA (0.1 M) in the presence and in the absence of 2 x 10-3 M PDO. 

Spectroscopy is also extensively applied to determination of reaction mechanisms and transient intermediates in homogeneous systems (34-37) and at interfaces (38). Spectroscopic theory and methods are integral to the very definition of photochemical reactions, i.e. chemical reactions occurring via molecular excited states (39-42). Photochemical reactions are different in rate, product yield and distribution from thermally induced reactions, even in solution. Surface mediated photochemistry (43) represents a potential resource for the direction of reactions which is multifaceted and barely tapped. One such facet, that of solar-excited electrochemical reactions, has been extensively, but by no means, exhaustively studied under the rubric photoelectrochemistry (PEC) (44-48). 

A double-potential-step chronoamperometry (DPSC) experiment consists of two CA experiments. The potential of the second step is normally adjusted so that the R molecules formed upon reduction of O in the first step is reoxidized to O in a diffusion-controlled process, but it might also be adjusted to other values with the purpose of detecting other species formed [7]. In contrast to the CA technique, DPSC is a reversal technique, where the intermediates/products formed during the first step are probed directly in the second step. In this sense, it corresponds to a pump/probe experiment in photochemistry. While CA, in general, provides little if any information about follow-up chemistry, DPSC is a very strong tool for distinguishing between different mechanisms such as for example E, ECj, and DIMl. It is also a good tool for the determination of the relevant rate eonstants. 

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