Exciplexes electronic factors

The loss of stereochemical control in the oxygenation of the isodicyclopentadiene derivative 29, even compared with typical carbon dienophiles, cannot be due to steric factors by the norbornane ring, since remote, and has been rationalized in terms of electronically facilitated reversal of the exciplex on that Ji-face which is most conducive to bonding in the first instance (Sch. 13) [27]. 

Aromatic substrates are by far the most commonly used substrates in the rapidly expanding area of photoinduced electron transfer [1,2]. This is obviously due to the favourable location of the frontier molecular orbitals in such compounds. The same factor facilitates the formation of electron transfer donor-acceptor (EDA) complexes both in the ground state (these possibly are intermediates in some thermal reactions, e.g. selected electrophilic substitutions), and in the excited state (exciplexes). 

As discussed earlier, deprotonation of a-carbon forms a major reaction pathway for the disappearance of the amine radical cation. Studies of photoinduced electron-transfer reactions of tertiary amines by Lewis [7, 11] and by Mariano [5, 10] have contributed significantly towards our understanding of the factors that control this process. Lewis and coworkers used product-distribution ratios of stilbene-amine adducts to elucidate the stereoelectronic effects involved in the deprotonation process [5, 10, 121, 122]. In non-polar solvents, the singlet excited state of tran -stilbene forms non-reactive but fluorescent exciplexes with simple trialkylamines. Increasing solvent polarity brings about a decrease in the fluorescence intensity and an increase in adduct formation. For non-symmetrically substituted tertiary amines two types of stilbene-amine adduct can be formed, as is shown in Scheme 9, depending on whether the aminoalkyl radical adding to the stilbene radical is formed by de-... 

The instability of exciplexes has been attributed to charge transfer Interactions in many cases (70), but other factors may also play Important roles (252). Possible exciplex decay pathways other than emission include dissociation into the products of an electron transfer process (230,231), as well as various... 

Apparently it is too ambitious to try to understand all factors at the same time. Here we confine the polymer systems to these having EDA pairs which form exciplex via the singlet excited state of an electron acceptor. The mesure-ments at room temperature are focussed on the molecular weight and solvent effects. These studies will shed light on the interpretation of the enhanced molecular interactions in polymer systems. 

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