Chemical reactions from the excited state are not the only—and not necessarily the most important—way in which a reaction can be initiated by light absorption. A completely different approach involves the generation of some activated, but ground state, intermediate through the photochemical step, thus leading to a photoinitiated reaction, even if (some of) the chemical steps are actually thermal reactions. The first group, perhaps scientifically trivial, but quite useful in the laboratory and industry, involves the generation of an inorganic radical, for example, chlorine atoms from Cl2.3 Besides this, new methods are increasingly being developed for the generation of the reactive intermediates from organic molecules. These methods are based on a sensitization process. This process may involve atom abstraction, based on the fact that several excited states have a strong diradicalic character, and thus may abstract an atom from a substrate, generating a pair of radical as under mild conditions. Hydrogen abstraction (from alcohols, acetals, even alkanes) is typical. The thus-formed radicals can be used for the alkylation of electrophilic alkenes.
Conversely, while, as has been mentioned already, each excited function has its own chemistry, there are two properties that all excited states share: easy reduction (because these states can accept an electron in the half-filled highest occupied molecular orbit (HOMO) and oxidation (because they can donate an electron from the half-filled lowest unoccupied molecular orbit (LUMO). Thus, redox chemistry, practically nonexistent in ground-state organic chemistry, is ubiquitous in organic photochemistry.4 Choosing a sensitizer that has no photochemistry of its own and easily enters redox processes, typically an aromatic, allows electron transfer sensitization to be carried out. The thus-formed radical ions have an interesting chemistry, which currently is being explored, including addition reactions and fragmentation reactions. The latter process is a further entry to radicals, which, along with the atom abstraction method just given, allows unconventional radical precursors (e.g., alkyl aromatics, silanes, ethers) to be used rather than the usual precursors, highly toxic and waste-producing stannanes.5
A key difference between photochemical and thermal reactions is that electronically excited states are situated at a much higher energy than the corresponding ground states (Figure 3.1). Conversely, excited states have a short lifetime (from microseconds to lower than nanoseconds for organic molecules), due to competition with physical deactivation. Thus, photochemical reactions take place on a high-lying potential surface, but can only overcome very small (a few kcal/mol) activation barriers, as opposed to the large activation energy of thermal reactions.
Figure 3.1 Energy profiles for photochemical and thermal reactions.
The other key difference is that the electronic structure is deeply changed in the excited states, and the energy of such states is 50 to 90 kcal M_1 for organic molecules, often making it possible that they react at rates typical of reactive intermediates, not saturated molecules. As an example, ketones are electrophiles (at the carbon atom) in the ground state. Actually these are weak electrophiles, and the use of a strong nucleophile or activation by a catalyst (an acid) is required in order to conveniently carry out a nucleophilic addition. Conversely, in the excited state ketones display a completely different reactivity, which can be likened to that of an alkoxy radical, both qualitatively (they undergo radical reactions at the oxygen atom) and quantitatively (e.g., hydrogen abstraction, from an alcohol, occurs with a rate constant around 106 M_1s" _1)2, 3 (Figure 3.2).
The different electron distribution in the excited state also may lead to other types of reactions. As an example, alkenes and polyenes display a low intermolecular reactivity, but undergo extremely fast rearrangements, since the p bonding character dramatically diminishes in the excited state. Thus, free rotation becomes feasible and, where appropriate, electrocyclic and sigmatropic processes take place6 (Figure 3.3).
This is not to be taken as an indication that the p bonding character necessarily decreases in the excited state. Actually, there are molecules in which a p bond becomes stronger upon excitation. Indeed, oxygen has a weak diradicalic character in the (triplet) ground state, while it is a very strong p electrophile in the singlet excited state. It reacts in a way similar to electrophilic alkenes, but many orders of magnitude faster7 (Figure 3.4).
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