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.

9-Aryl-phenalenones: Bioinspired thermally reversible photochromic compounds for photoswitching applications in the pico-to milliseconds range

“…A major difference is the superior stability of the natural counterparts conferred by the rearomatization of the NX form by hydroxy-oxo tautomerization. In the course of our investigations on bioinspired phenalenone derivatives, 38,39 we discovered that it is possible to alter the stability of the closed NX form by modifying the substituents in the 9-phenyl group. In this work, we show that replacing the phenyl substituent by heteroaryl groups (Scheme 1 middle) results in a dramatic variation of the NX lifetime over nine orders of magnitude.…”

7th Heron Island Conference on Reactive Intermediates and Unusual Molecules

“…meeting covering a wide range of topics, including phenol-arene cross-coupling (König et al), [1] oxaphosphirane complex (Streubel et al), [2] HNC mediated Mukaiyama reactions (Lupton and Nguyen), [3] hydroxyl radicals (Williams et al), [4] triplet excited states of ketones (Bucher), [5] aziridination of dicarbonyl compounds (Chan et al), [6] copper iodide complexes of macrocycles (Baranová et al), [7] electric field effects on radical stability (Coote and Gryn'ova), [8] photoreactivity of azirine (Gudmundsdottir et al), [9] carbocation cyclization/rearrangement cascades (Tantillo and Hong), [10] cleavage of electron- deficient acetylenes (Alabugin et al), [11] and a review of the HERON reaction (HEteroatom Rearrangement On Nitrogen) (Glover). [12] The next Heron Island Conference will take place in early July 2019.…”