Reactive excited states

Assigning reactive excited states in inorganic photochemistry. A. W. Adamson, Comments Inorg. 

Consequently, the antioxidant activity of GA in biological systems is still an unresolved issue, and therefore it requires a more direct knowledge of the antioxidant capacity of GA that can be obtained by in vitro experiments against different types of oxidant species. The total antioxidant activity of a compound or substance is associated with several processes that include the scavenging of free radical species (eg. HO, ROO ), ability to quench reactive excited states (triplet excited states and/ or oxygen singlet molecular 1O2), and/or sequester of metal ions (Fe2+, Cu2+) to avoid the formation of HO by Fenton type reactions. In the following sections, we will discuss the in vitro antioxidant capacity of GA for some of these processes. 

Illuminated chlorophyll molecules can form excitation states which are able to transfer energy on to the oxygen molecule, raising it from its ground state to the more reactive excited state known as singlet oxygen. 

Photocycloaddition of Alkenes and Dienes. Photochemical cycloadditions provide a method that is often complementary to thermal cycloadditions with regard to the types of compounds that can be prepared. The theoretical basis for this complementary relationship between thermal and photochemical modes of reaction lies in orbital symmetry relationships, as discussed in Chapter 10 of Part A. The reaction types permitted by photochemical excitation that are particularly useful for synthesis are [2 + 2] additions between two carbon-carbon double bonds and [2+2] additions of alkenes and carbonyl groups to form oxetanes. Photochemical cycloadditions are often not concerted processes because in many cases the reactive excited state is a triplet. The initial adduct is a triplet 1,4-diradical that must undergo spin inversion before product formation is complete. Stereospecificity is lost if the intermediate 1,4-diradical undergoes bond rotation faster than ring closure. 

The reaction is stereospecific for at least some aliphatic ketones but not for aromatic carbonyls.197 This result suggests that the reactive excited state is a singlet for aliphatics and a triplets for aromatics. With aromatic ketones, the regioselectivity of addition can usually be predicted on the basis of formation of the more stable of the two possible diradical intermediates obtained by bond formation between oxygen and the alkene.198... 

A photoaffinity label is a molecule that forms a highly reactive excited state when illuminated with light of an appropriate wavelength. While in this excited state the photoaffinity label can covalently modify groups on the enzyme molecule that are in close proximity to the label. Hence one can mix the compound and enzyme... 

The question of the identity of the reactive excited state was left open, although n-n excited state was considered to be more probable.140 The interaction diagram, Fig. 7, shows that a n-n state would have an additional stabilizing interaction Ri(tt) ->-R2(7r) with orbital coefficients in phase for an all-suprafacial concerted reaction. The dominant reaction would theoretically depend upon the relative placements of the several levels, and since no experimental information for maleic and fumarate esters is presently available, a clear choice cannot be made. It is interesting that the reactive excited state could be inferred if both stereochemistry and a good molecular diagram were available. 

Interestingly, after reaching the maximum at the 6-membered cycle, the yields drop again. This decrease in efficiency occurs despite the appreciable reduction in the distance between the terminal acetylenic carbons relative to the 6-membered analogue. Here, the efficiency may simply be a function of how photochemical excitation is distributed in the reactive excited state. Calculated enediyne geometries suggest the cyclization is more efficient for those enediynes where the terminal phenyl groups are rotated outside of the enediyne plane (Table 3). 

The reactive excited state in a photochemical reaction is usually either the Si state or the Ti state of the reactant molecule. These states can be characterised by reference to the absorption and emission spectra of the reactant. 

If the products formed from the direct and sensitised reactions differ then the triplet state formed on sensitisation is not the reactive excited state formed by direct irradiation. This situation is illustrated in Sections 8.2 and 8.3, where direct and photosensitised cis-trans isomerisation of alkenes was considered. 

Figure 10.4 Determining the reactive excited state of Cr(CN) + by sensitisation Adapted from F. Scandola and V. Balzani, Energy-Transfer Processes of Excited States of Coordination Compounds , Journal of Chemical Education, Volume 60 (10), 1983. American Chemical Society...

What is the lifetime of the reactive excited state, and the rate constant for the primary step leading from it to product in a concerted mechanism) or to a reactive intermediate ... 

The di-ji-methane reaction results in a 1,2-shift in a 1,4-diene unit, but such shifts sometimes occur in monoalkenes (2.44, and the mechanism must be different. The substrates are usually tetra-substituted ethylenes, and it is suggested that the reactive excited state is a Rydberg singlet state, which rearranges initially to give a carbene. Support for such a mechanism comes from the structures of products obtained from 1,2-dimethy(cyclohexene (2.45), which are... 

Similarities between [Ru(bpy),]2+ (discussed in Chapter 13) and [Pt,(pop)J4 are apparent. Reactive excited states are produced in each when it is subjected to visible light. The excited state ruthenium cation, [Ru(bpy)3]" +, can catalytically convert water to hydrogen and oxygen. The excited slate platinum anion, [Pt,(pop)J 4-, can catalytically convert secondary alcohols to hydrogen and ketones. An important difference, however, is that the ruthenium excited stale species results from (he transfer of an electron from the metal to a bpy ligand, while in the platinum excited state species the two unpaired electrons are metal centered. As a consequence, platinum reactions can occur by inner sphere mechanisms (an axial coordination site is available), a mode of reaction rot readily available to the 18-clectron ruthenium complex.-03... 

Reaction Cavities of Alkanophenones in Neat Solid and Liquid-Crystalline Phases. As mentioned previously, solid-state studies on the Norrish II processes of alkyl aryl ketones are unambiguous with respect to the triplet multiplicity of the reactive excited state. On the other hand, a bulky aryl auxochrome can create complications during the transformation of the excited triplet states to photoproducts in neat anisotropic phases. 

One of the most controversial questions encountered in studies of Cr111 photochemistry concerns the identity of the reactive excited state(s).71 As illustrated in Figure 6, uncertainty can arise because the Franck-Condon state initially populated during irradiation in the quartet absorption bands generally undergoes rapid equilibration to the lowest thexi quartet state, Q°u and/or intersystem crossing to the lowest doublet, D°. Reaction from either or both of the latter states must therefore be considered. 

At present, then, the situation with regard to the identity of the reactive excited state(s) in Crm substitutional photochemistry can be summarized as follows. The lowest quartet excited state is the sole reactive state in [Cr(CN)6]3- and is at least partly responsible for the photochemistry of... 

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