α‐Carbonyl Substituent Effect on the Lifetimes of Triplet 1,4‐Biradicals from Norrish‐Type‐II Reactions

Abstract

Triplet 1,4-biradicals were generated by Norrish-Type-II hydrogen transfer from alpha-heteroatom-substituted beta-branched butyrophenones 1-6 and detected by laser flash absorption measurements. For three oxy-substituted compounds 2-4 (R(alpha)=OH, OCOMe, OCOOEt) comparable lifetimes were determined in acetonitrile (roughly 1.5 micros). In benzene, divergent trends were observed: for the hydroxy compound 2 a lower lifetime of 790 ns was determined, whereas for 3 and 4 the lifetimes increased to 4.9 micros. Photolyses of the alpha-amino-substituted compounds 1 and 6 resulted in transient species with significant lower lifetimes (for 1 160 ns in benzene and 450 ns in acetonitrile; for 6 <100 ns in both solvents). The mesyloxy substrate 5 undergoes rapid C-O bond cleavage upon photolysis and no transient triplet species were detected. Computational (UB3 LYP/6-31G* and natural don orbital (NBO) analyses) results supported the assumption of a negative hyperconjugative interaction strongly stabilizing alpha-oxy-substituted over alpha-amino-substituted radicals.

From Fundamental Science to Product: A Bottom-up Approach to Sunscreen Development

“…TR-IY studies on gas-phase EF by Rodrigues et al revealed a tri-exponential decay involving a long-lived, > 900 ps component, which was assigned to long-lived triplet states 59 . Apart from excited triplet state reactions such as singlet oxygen sensitising, the existence of such triplet states could be particularly problematic in sunscreen formulation since aromatic carbonyl compounds are known to undergo fragmentation reactions from their triplet states, rather than phosphorescence to the ground state 67,68 . However, as previously discussed and as observed in the studies by Horbury et al on EF in solution, solvent effects -even those of the weakly perturbing cyclohexane -allow for photoisomerisation relaxation pathways to become accessible in EF, justifying its use in sunscreen formulations 36 .…”

Photocaged Hydrocarbons, Aldehydes, Ketones, Enones, and Carboxylic Acids and Esters that Release by the Norrish II Cleavage Protocol and Beyond: Controlled Photoinduced Fragrance Release

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“…The three golden routes and three examples of photocages(33,31,36) that realize these concepts Cet + Br -: 35A. G. Griesbeck et al…”

Diastereomeric Discrimination in the Lifetimes of Norrish Type II Triplet 1,4‐Biradicals and Stereocontrolled Partitioning of Their Reactivity (Yang Cyclization versus Type II Fragmentation)

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