Observation of a novel emission from an exciplex of triplet 4-phenylbenzophenone with triethylamine

Direct Photocatalyzed Hydrogen Atom Transfer (HAT) for Aliphatic C-H Bonds Elaboration

Direct photocatalyzed hydrogen atom transfer (d-HAT) can be considered a method of choice for the elaboration of aliphatic C–H bonds. In this manifold, a photocatalyst (PC_HAT) exploits the energy of a photon to trigger the homolytic cleavage of such bonds in organic compounds. Selective C–H bond elaboration may be achieved by a judicious choice of the hydrogen abstractor (key parameters are the electronic character and the molecular structure), as well as reaction additives. Different are the classes of PCs_HAT available, including aromatic ketones, xanthene dyes (Eosin Y), polyoxometalates, uranyl salts, a metal-oxo porphyrin and a tris(amino)cyclopropenium radical dication. The processes (mainly C–C bond formation) are in most cases carried out under mild conditions with the help of visible light. The aim of this review is to offer a comprehensive survey of the synthetic applications of photocatalyzed d-HAT.

Interaction of Triplet Excited States of Ketones with Nucleophilic Groups: (π,π*) and (n,π*) versus (σ*,π*) States. Substituent-Induced State Switching in Triplet Ketones

The intramolecular interaction of ketone triplet excited states with nucleophilic substituents is investigated by studying the electronic properties of phenalenone and a range of phenalenones functionalized in position 9 as a model system. In accordance with the literature, a (π,π*) triplet excited state is predicted for phenalenone. Similarly, 9-fluoro-, 9-chloro-, and 9-methoxyphenalenone are calculated to have (π,π*) lowest triplet excited states, whereas the lowest triplet states of 9-bromo-, 9-iodo, 9-methylthio, and 9-dimethylaminophenalenone are predicted to have (σ*,π*) character. As a result of the interaction between halogen and oxygen lone pairs increasing with increasing orbital size, the antibonding linear combination of substituent lone pairs with oxygen lone pairs sufficiently rises in energy to change the character of the lowest triplet excited state of the 9-substituted phenalenones from (π,π*) to (σ*,π*). These unusual triplet excited states or exciplexes should essentially behave like (n,π*) triplets states, but will differ from pure (n,π*) states by showing significant spin densities at the substituent heteroatoms, predicted to reach values of 0.25 for 9-iodophenalenone, and ~0.5 for 9-dimethylaminophenalenone. Vertical T1–T2 excitation energies calculated indicate that the stabilization of the (σ*,π*) relative to the (π,π*) state can reach 1 eV. Preliminary calculations on the triplet excited states of 2-iodobenzophenone, 4-iodo-2-butanone, and iodoacetone indicate that intramolecular triplet exciplex formation should be a general phenomenon, as long as the ring being formed is at least a five-membered ring.