Photophysics primary excited state processes

Once a molecule is excited into an electronically excited state by absorption of a photon, it can undergo a number of different primary processes. Photochemical processes are those in which the excited species dissociates, isomerizes, rearranges, or reacts with another molecule. Photophysical processes include radiative transitions in which the excited molecule emits light in the form of fluorescence or phosphorescence and returns to the ground state and nonradiative transitions in which some or all of the energy of the absorbed photon is ultimately converted to heat. 

For a photoexcited molecule, the time allowed for a reaction to occur is of the order of the lifetime of the particular excited state, or less when the reaction step must compete with other photophysical processes. The photoreaction can be unimolecular such as photodissociation and photo isomerization or may need another molecule, usually unexcited, of the same or different kind and hence bimolectdar. If the primary processes generate free radicals, they may lead to secondary processes in the dark. 

In conclusion, photogeneration is one of the several photophysical processes in competition with one another in which the excited state may be involved. In this view, the primary photogeneration quantum yield (), defined as the number of charge pairs that are formed per absorbed light... 

Primary photochemical processes are processes initiated from an electronically excited state that yield a primary photoproduct that is chemically different from the original reactant. Photochemical processes are always in competition with photophysical processes that eventually restore the reactant in the ground state. Photoreactions leading to new products can be efficient only if they are faster than the competing photophysical processes. Therefore, it is essential to have a feeling for the time scales of the latter (Table 2.1, Figure 2.2). Photophysical processes of molecules in solution usually obey a... 

The mechanism of a photoreaction should ideally include a detailed characterization of the primary events as outlined by the classification of photochemical reaction pathways in Section 2.3 the lifetimes of the excited states that are involved in the reaction path, the quantum yields and hence the rate constants of all relevant photophysical and photochemical processes, in addition to the information about the structure and fate of any reactive intermediates, their lifetimes and reactivities. 

The characterization and utilization of photochemical processes are rapidly developing into one of the major areas of activity in modern inorganic and physical chemistry. In the past, the photochemistry of classical metal coordination complexes has received the greatest amount of attention, but recently the photochemistry of organometallic compounds has attracted notice In particular, the photochemistry and photophysics of uranyl compounds have been investigated for more than four decades and a great deal has been learned about the primary photoprocesses and the photo-induced reaction mechanisms displayed by these complexes (.3,4). The popularity of uranyl compounds in photochemical studies is derived from their ready availability and stability, their facile redox chemistry and photosensitivity and their rich excited state chemistry. Since current reviews of uranyl photochemistry are expected to appear in the near future, vide infra, further discussion of this topic here will be limited. 

Interestingly, imides possess a photochemical behavior that is very similar to that of ketones. Especially, phthalimides behave like phenyl ketones with respect to some of their photophysical properties. Despite many similarities there are at least three important differences. First, phthalimides are more prone to photoelectron transfer (PET) processes than ketones. This property was very successfully applied in the synthesis of a variety of amino acid derivatives (see Section 6.2.3.2). Second, the cyclization of imides often affords 0,7V-acetals as primary products, and this obviously has some consequences for the stability and the follow-up reactions of these products. Third, in contrast to aryl ketones, phthalimides are not quantitatively converted into the triplet state, and thus they may react both from the singlet and the triplet excited state. 

The efficiency of any photophysical or photochemical process is a function of both the properties of the reaction environment and the character of the excited state species. The fundamental quantity which is used to describe the efficiency of any photo process is the quantum yield (0) it is useful in both quantifying the process and in elucidating the reaction mechanism. Quantum yield has the general definition of the number of events occurring divided by the number of photons absorbed. Therefore, for a chemical process 0 is defined as the number of moles of reactant consumed or product formed divided by the number of einsteins (an einstein is equal to 6.02 X 10 photons) absorbed. Since the absorption of light by a molecule is a one-quantum process, then the sum of the quantum yields for all primary processes occurring must be one. Where secondary reactions are involved, however, the overall quantum yield can exceed unity and for chain reactions reach values in the thousands. When values of 0 are known or can be measured for a specific photochemical reaction the rate can be determined from ... 

The photochemistry of phenyl azide and its simple derivatives have received the most attention in the literature. The results of early studies were summarized in a number of reviews. " Over the last decade, modem time-resolved spectroscopic techniques and high level ab initio calculations have been successfully applied and reveal the detailed description of aryl azide photochemistry. This progress was analyzed in recent reviews. Femtosecond time resolved methods have been recently employed to study the primary photophysical and photochemical processes upon excitation of aryl azides. The precise details by which aryl azide excited states decompose to produce singlet arylnitrenes and how rapidly the seminal nitrenes lose heat to solvent and undergo unimolecular transformations were detailed. As a result of the application of modem experimental and theoretical techniques, phenylnitrene (PhN) - the primary intermediate of phenyl azide photolysis, is now one of the best characterized of all known organic nitrenes. " 5 "-2° - ... 

The primary photophysical processes occuring in a conjugated molecule can be represented most easily in the Jablonski diagram (Fig. 1). Absorption of a photon by the singlet state So produces an excited singlet state S . In condensed media a very fast relaxation occurs and within several picoseconds the first excited singlet state Si is reached, having a thermal population of its vibrational levels. The radiative lifetime of Si is in the order of nanoseconds. Three main routes are open for deactivation ... 

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