Research Note

Abstract

Energies and lifetimes of triplet states of a series of 2-cyclohexenones have been measured in solution under ambient conditions by time-resolved photoacoustic calorimetry (PAC). The PAC triplet lifetimes are in excellent agreement with lifetimes measured by kinetic absorption spectrophotometry (KAS) using nanosecond flash techniques, indicating that the present data indeed pertain to the enone triplets previously studied using KAS. The data demonstrate that, as previously proposed, the pi pi triplet energies in these systems are indeed correlated with triplet lifetimes, and vary sensibly with the anticipated extent of conformational flexibility of the C = C bond. No evidence supporting formation of metastable trans-cyclohexenones was obtained. The data also demonstrate greater precision and, for lifetimes, greater accuracy than has hitherto been claimed for the PAC technique for study of transients with lifetimes in the 25 ns range.

Formal Giese addition of C(sp3)–H nucleophiles enabled by visible light mediated Ni catalysis of triplet enone diradicals

“…To determine the possibility of photocatalytic energy transfer, the same reaction was conducted under UV-C (254 nm) irradiation in the presence of UV triplet sensitizers ( Scheme 2a ). 2 , 19 Although the desired products were obtained in lower yields, photosensitizers exhibiting higher triplet energies gave higher product yields, strongly suggesting that an energy transfer mechanism is indeed involved. 20 In addition, Mn powder ( E o = –1.42 V vs. SCE (saturated calomel electrode)), 21 which was used as an external reductant (comparable to E 1/2 (Ir( iii )/Ir( ii )) = –1.37 V vs. SCE), 17 also failed to promote the desired reaction, again indicating that a photoredox-mediated pathway does not prevail.…”

“…1 In addition to their well-recognized classical reactivities, enones have been independently applied in transition metal catalysis and photochemistry to achieve salient chemical transformations. However, the application of the triplet diradical species in transition metal catalysis remains elusive due to the extremely short lifetime (25–35 ns for 2-cyclohexen-1-one) 2 of the triplet diradical ( Scheme 1a ). 3 We therefore postulated that such a new reactivity could potentially be realized by adopting recent advances in visible light-mediated nickel catalysis.…”

Photocycloaddition of 4,4-Dimethylcyclohexenone to 1,1-Diphenylethylene. Evidence for a Triplet Exciplex Intermediate

“…Laser flash photolysis of DMC (70 mM) in cyclohexane yields the expected enone triplet; λ max 280 nm, τ 24 ns. − The enone triplet is quenched by oxygen with a rate constant k q ox = 4.6 × 10 9 L mol -1 s -1 . This value is slightly lower than a previously reported rate constant .…”

Mechanism of acid-catalyzed photoaddition of methanol to 3-alkyl2-cyclohexenones

“…Laser flash photolysis -kinetic absorption spectroscopy Using both nanosecond kinetic absorption spectroscopy (KAS) (8, 1 1) and time-resolved photoacoustic calorimetry (PAC) (9), the triplet lifetime of isophorone (la) in acetonitrile was found to be 79 ns, much higher than previously assumed (1). Thus, direct protonation of triplet excited states of l a by acids is more likely than previously concluded (1).…”

“…As far as the intermediacy of trans-cyclohexenones is concerned, we have repeatedly failed to detect such species on laser flash photolysis of a large number of cyclohexenones (8). Furthermore, PAC data (7,9) for isophorone, 3-methyl-2-cyclohexenone, and other 2-cyclohexenones rigorously exclude the formation of a high-energy ground state intermediate with a lifetime in the microsecond range and measurable (>0.05) quantum efficiency. That is, intersystem crossing and radiationless decay of the enone triplet accounts for essentially all (>98%) of the energy of the absorbed photon.…”