Exciplex hydrogen bonding

These results clearly show that the stereoselectivity in the photocycloaddition of 1-CN and 2-CN with 2- and/or 3-furylmethanol is reasonably explained by the hydrogen-bonding in the excited states. Probably, a partial electron transfer from furylmethanols (F-OH) to the excited singlet states of cyanonaph-thalenes (ArCN) contributes the formation of polar exciplexes [ArCN8 F-OH8+], where ArCN8 interacts with more acidic OH group of F-OH8 to produce hydrogen bonds in the excited states. 

The diastereoselectivity dropped drastically in presence of protic methanol and totally disappeared for the corresponding silyl ethers. These data are in agreement with the presence of a hydroxy directing effect in the Patemo-Bilchi reaction. Threo stereoisomer can be favored through the formation of an hydrogen bond between triplet excited benzophenone and the substrate in the exciplex, while the formation of the erythro stereoisomer would be less favored due to allylic strain (Scheme 3.41). 

Adam and coworkers reported the regioselective and diastereoselective formation of oxetanes during the PB reaction of allylic alcohols (Scheme 7.27) [43, 44]. This group proposed that hydrogen-bond interactions in the exciplex played an important role in controlling the selectivity. D Auria and coworkers also observed a site-selective and diastereoselective formation of oxetanes in the PB reaction of 2-furylmethanol derivatives (Scheme 7.27) [45]. 

Triplet, radical anion, and radical cation spectra of furocoumarins have been determined by flash photolysis and pulse radiolysis. Triplet spectra are modified by hydrogen bonding and in water the triplets may be regarded as exciplexes. " The triplet states, semi-oxidized and semi-reduced radicals of haematoporphyrin, and three O-acyl derivatives have also been characterized. Triplet-state participation of several 4-hydroxy- and 4-methoxybenzothiazoles has been studied by flash photolysis. " These compounds are models for pheomelonin. 

An assessment of experimental observations concerning the influence of butyl methacrylate on the primary photochemical processes occurring in the reduction of benzophenone by triethylamine suggests that they may point to the participation of ternary exciplexes. Such species may be of the form [donor + acceptor + alkene] and may play an important role in the electron transfer processes. Studies have been reported which characterise the triplet states of m-, and p-hydroxybenzophenones. In non-hydrogen bond... 

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