Role of Structural Changes in the Triplet−Triplet Energy Transfer Process to Oxime Derivatives

Abstract

The triplet energy accepting properties of bridged hydroxy and methyl derivatives of acetophenone oxime were examined. Sandros and Agmon−Levine−Balzani (ALB) models were used to deduce the oxime triplet energies, , and the reorganizational intrinsic barriers, ΔG #(0), associated with the reaction. It is found for the first time that these compounds exhibit different degrees of nonvertical energy transfer (NVET) behavior, depending on their chemical structure. These structural properties have been investigated at both the AM1 and the B3LYP/6-31+G(d) levels of theory. The flexibility of the ground-state molecule and the structural changes occurring during the S0 → T1 sensitized transition seem to play crucial roles. It appears that two important rearrangements are involved, namely, rotation about the C−N double bond and a π system flattening associated with a torsion about the formal C−C single bond. To relate the NVET character to these structural changes, the energy variations associated with these latter were evaluated and successfully related to ΔG #(0). Moreover, the role of the thermal activation of the molecular coordinates is evidenced through the calculations of the ground and triplet potential energy surfaces (PESs) at DFT and CIS levels. A simple model, based on the Sandros equation, was derived to include the influence of such torsional modes on the energy-transfer behavior. Finally, the experimental quenching data were fitted to this model, leading to the conclusion that, although both C−N double bond and C−C single bond torsions occur during the sensitization, the thermal activation of C−C single bond torsion plays the major role in the NVET character of oxime derivatives.

Scope and Limitations of Baird's Theory on Triplet State Aromaticity: Application to the Tuning of Singlet–Triplet Energy Gaps in Fulvenes

“…However, it was previously found that a discrepancy between T 1 energy from experiments employing the Sandros equation (or extensions thereof) and the calculated adiabatic triplet energy can be as much as 25 %, depending on the structural flexibility of the triplet donor and acceptor. [21] In our studies no special care was taken to use only nonflexible sensitizers, so a measured T 1 energy 15 % higher than the calculated value is reasonable.…”

The Application of the Schmidt Reaction and Beckmann Rearrangement to the Synthesis of Bicyclic Lactams: Some Mechanistic Considerations

“…Hence, the ketoxime intermediates are reported as either non-specific or are assumed to be in the sterically favoured anti-arrangement. Papers reporting exclusive syn-ketoxime formation [30][31][32][33] are spectroscopically indistinguishable from the anti-ketoxime 10, [34][35][36] adding a significant amount of uncertainty as far as the geometry is concerned. No paper identified the orientation of the oxime group for compounds 11-14.…”

The Silane‐ene and Silane‐Acrylate Polymerization Process: A New Promising Chemistry?

“…Quenching of the ITX ketone triplet state (observed at 600 nm) by the silane in benzene occurs with a rate constant of 1 × 10 6 M −1 · s −1 and forms a silyl radical by a hydrogen transfer reaction as supported by the observation of the ketyl radical of ITX at 480 nm. Silyl radicals usually absorb in the UV range (300–330 nm) 14–16. As a consequence, the strong absorption of the ITX ketyl radical in this wavelength range prevents any direct observation of Si • in an ITX/silane system.…”

“…Silyl radicals can also be generated by an interaction between the silane and the t ‐butoxyl radical t ‐BuO • , this radical being formed14–16 by the photochemical decomposition of di‐ tert ‐butylperoxide. The hydrogen abstraction rate constant (5.5 × 10 7 M −1 · s −1 ) was determined by a classical Stern‐Volmer plot using the increasing time of the silyl radical as a function of the different silane concentrations.…”