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

Abstract

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.