Excited complex

In the above scheme A represents the excited polymer, Q the quenching agent and [A.Q] an excited complex. 

Although this mechanism could explain the inertness of di-t-butyl sulphide towards oxidation due to the absence of a-hydrogen atoms, it was later ruled out by Tezuka and coworkers They found that diphenyl sulphoxide was also formed when diphenyl sulphide was photolyzed in the presence of oxygen in methylene chloride or in benzene as a solvent. This implies that a-hydrogen is not necessary for the formation of the sulphoxide. It was proposed that a possible reactive intermediate arising from the excited complex 64 would be either a singlet oxygen, a pair of superoxide anion radical and the cation radical of sulphide 68 or zwitterionic and/or biradical species such as 69 or 70 (equation 35). 

Exchange reactions, hydrogen isotope, of organic compounds in liquid ammonia, 1, 156 Exchange reactions, oxygen isotope, of organic compounds, 2, 123 Excited complexes, chemistry of, 19,1 Excited molecules, structure of electronically, 3, 365... 

Pt2(P205H2) - (d8-d8), and Mo6Clft ( )6. Two- electron oxidations of Re2Cl and Pt2(P205H2)it have been achieved by one-electron acceptor quenching of the excited complexes in the presence of Cl, followed by one-electron oxidation of the Cl -trapped mixed-valence species. Two-electron photochemical oxidation-reduction reactions also could occur by excited-state atom transfer pathways, and some encouraging preliminary observations along those lines are reported. 

Exciplexesn6,nl) can be formed if the excitation energy B - B is higher than the one for A -> A in (1.8). Such an excited complex is associative in the excited state only, the corresponding ground state complex between A and B being dissociative (Fig. 9). Such exciplexes are important intermediates in e.g. cycloaddition reactions as precursors of diradicals 118) which are themselves precursors of the cyclized photoproducts. 

Exciplexes are excited-state complexes (the term exciplex comes from excited complex ). They are formed by collision of an excited molecule (electron donor or acceptor) with an unlike unexcited molecule (electron acceptor or donor) ... 

The Dq value of the complex between Cr and 1-butanol (2.6 0.2 kcal mol ) well conforms to the approximate value of 2-3 kcal moP indirectly estimated for the dissociation energy of the complex between F/ = (l )-(- -)-2-naphthyl-1-ethanol and methanol. ° Concerning the diastereomeric complexes, the homochiral adducts are invariably more stable than the heterochiral ones. This trend extends to the corresponding Si excited complexes as well. This observation, coupled with the appreciable deviation from linearity of the corresponding Ap values (Fig. 9), corroborates the view that the interaction forces in these complexes are affected by steric congestion to a different extent. Their sensitivity to steric factors is demonstrated by the diverging observations that (Table 4) (i) in the diastereomeric... 

In this Section, two types of photo-electron transfer processes with the MLCT state of complexes will be successively discussed. We will first introduce the direct photo-electron transfer from a DNA base to the excited complex Sect. 4.3.1. Afterwards we will coiKider the electron transfer between an excited... 

Photo-Electron Transfer from a DNA Base to the Excited Complex... 

The formation of the transient reduced complex can also be observed by flash photolysis in the presence of CT-DNA [1(X)]. This shows clearly the existence of a photoinduced electron transfer from a base of the polynucleotide to the excited complex. However, the relative amount of reduced complex which is photoproduced, is smaller in the presence of CT-DNA than in the presence of GMP this may be attributed to a more important back electron transfer process in the ion pair produced on the polynucleotide compared to that in solution with the mononucleotide. 

Photo-Electron Transfer from or to an Excited Complex Interacting with DNA, and a Quencher... 

In the presence of DNA reactions (17) and (18) that generate the excited complex directly or indirectly via reaction (19), become much slower or do not take place, and therefore the ECL disappears. This is due to the fact that the Ru(II) and Ru(III) complexes, physically bound to DNA, are protected by the negatively charged phosphate backbone from the reduction by C02. Thus the ECL titration of the metal complex in the presence of DNA has allowed the determination of the equilibrium constant and binding-site size for association of Ru(phen)3 to DNA [82]. 

The overall picture certainly became no simpler when quite a few examples were encountered where the nature of the nucleophile is decisive for the position at which the photosubstitution occurs. Just one recent example, taken from the work of Lammers (1974) is given in equation (8). To account for this influence of the nucleophile, the formation of an excited complex between aromatic compound and nucleophile has been suggested as the primary intermediate (de Vries, 1970). 

S = Sensitizer S = singlet or triplet excited S (SA), (S02) = electronically excited complexes. 

Dealing with type II processes, the question arises whether the substrate or the oxygen is activated when it enters the termination reaction which leads to the final produces) AOa. Therefore, we may distinguish an acceptor-activation mechanism from an oxygen-activation mechanism. Furthermore, in each of these mechanisms the activated species may be either the electronically excited A or 02, respectively, or the electronically excited complexes of the primarily excited species with A or 02, respectively (Chart I). 

While most reactions with which we deal in atmospheric chemistry increase in rate as the temperature increases, there are several notable exceptions. The first is the case of termolecular reactions, which generally slow down as the temperature increases. This can be rationalized qualitatively on the basis that the lifetime of the excited bimolecular complex formed by two of the reactants with respect to decomposition back to reactants decreases as the temperature increases, so that the probability of the excited complex being stabilized by a collision with a third body falls with increasing temperature. 

An example of the equivalent (photoaddition) reaction following hetero-molecular photoassociation is provided by the photochemical addition of maleic anhydride to anthracene." Livingston and coworkers100 have shown that the anthracene triplet state is not involved in this reaction and that, in terms of Eq. (47) in the appropriate form, q%. = 0.03. However, if the excited complex XMQ formed directly by light absorption in the charge-transfer band is the reactive intermediate, this produces the adduct with a computed efficiency of 347 . 

Stephenson monitored the quenching of aromatic hydrocarbon singlet states by observing a decrease in the sensitizer fluorescence intensity with added diene.158 A marked effect of the structure of both the sensitizer and the quencher was noted, with values of kq ranging from 4 x 109 to 8.7 x 10s however, no quantitative correlation between k and several factors that might influence the stability of the proposed excited complex could be obtained (see below). The important observation that singlet quenching led to... 

The confusion generated in the initial report185 on photoracemization of sulfoxides has recently been removed with the postulate that naphthalene singlet forms an excited complex with sulfoxides.186 Thus, despite the fact that the singlet state of 36 lies at 113 kcal, some 23 kcal above that of naphthalene, 36 quenches the fluorescence of this hydrocarbon with kes = 3.2 x 107Af -1 sec-1. From the dependence of the quantum yield of racemization on sulfoxide concentration (Eq. 34), a value of kes = 2.3 x 107Af-1 sec-1 was deduced. Since these values are the same within experimental error, it follows that the singlet state of naphthalene is responsible for photoracemization. 

The only identified reaction product of laser-ablated Mg atoms and acetylene under matrix isolation conditions is MgC=CH It was suggested that this reaction involves the insertion of excited-state Mg into the H—C bond to form an excited complex (equation 7), which then decomposes via H atom loss (equation 8). 

Cyano-substituted ethylenes react in a different way with aliphatic ketones. The orientation of photochemical cycloaddition (4.661 is the opposite of that found for electron-rich alkenes, and the reaction is highly stereoselective (4.69) in the early stages. These processes involve the formation and subsequent decay of an excited complex (exciplex) from the (n,n ) singlet state of the ketone and the alkene. Aryl ketones undergo intersystem crossing so efficiently that such a singlet-state reaction is rarely observed, but the reaction of a benzoate ester with an electron-rich alkene 14.70 rnay well be of this type, with the ester acting as electron-acceptor rather than electron-donor. 

Excimer formation in certain gases and vapor mixtures is known from modern UV lasers typical examples are excited complexes between rare gas... 

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