Evidence for comparable reactivity of alkanone excited singlet and triplet states toward hydrogen donors

Abstract

Studies of solvent effects on the relative fluorescence quantum yield (<#>trel) and fluorescence lifetime (ts) of 2-adamantanone are presented. The ability of a given solvent to quench adamantanone fluorescence correlates well with the facility of the solvent as a hydrogen donor, suggesting that the quenching process involves at least partial intermolecular hydrogen abstraction by the adamantanone excited singlet state. Kinetic studies indicate that the alkanone , * singlet is just as reactive as the , * triplet toward quenching by hydrogen donors.

Reactivity and Efficiency of Singlet- and Triplet-Excited States in Intermolecular Hydrogen Abstraction Reactions

“…We find that singlet-excited acetone is 2−10 times more reactive than the triplet state. It is concluded, in agreement with the expectations expressed above but in variance with the earlier studies, ,,, that the singlet state is significantly more reactive toward hydrogen donors. This finding is also supported by the reactivity of a singlet-excited cyclic azoalkane toward the same set of hydrogen donors, which is 30−150 times higher than for the corresponding triplet state.…”

“…In contrast to the reports that acetone fluorescence is not quenched by tributyltin hydride, the fluorescence lifetime of 2-adamantanone was shortened by the same hydrogen donor with a quenching rate constant of 5 × 10 9 M -1 s -1 , which is only slightly lower than that assumed for triplet quenching of acetone (8 × 10 9 M -1 s -1 ); unfortunately, no data for the adamantanone triplet state are available to allow a direct comparison and a firm conclusion to be drawn …”

“…This study, which again employed piperylene as quencher to eliminate triplet contributions, was in contrast to the assumed lower singlet reactivity toward the more reactive stannane. The data for the fluorescence quenching of adamantanone by alcohols indicated also a similar reactivity as for quenching of triplet acetone, but the order of rate constants correlated better with the ionization potentials of the alcohols than with the strength of their C−H bonds, indicating a possible participation of charge-transfer interactions. , The rate constants for fluorescence quenching of chloropentafluoroacetone by alkanes, however, followed the usual order of C−H bond strengths, and not their ionization potentials …”

Mechanoluminescent Imaging of Osmotic Stress-Induced Damage in a Glassy Polymer Network

“…We were able to determine that the length of the individual bursts was of the order of a few milliseconds, by measuring with a sampling rate of 1000 Hz. The time scale of the emission does not arise from the relaxation of the excited states: the mechanochemically generated adamantanone transfers its excitation energy via Förster resonance energy transfer (FRET) to 9,10-diphenylanthracene acceptors that are incorporated in the network, which relax from their first singlet excited (S 1 ) state with a lifetime of 7.6 ns in cyclohexane 49 (even in the absence of FRET, the S 1 state of adamantanone has a lifetime of approximately 9 ns 50 ). The time scale of the bursts, or events, must therefore be a consequence of the bond-breaking processes.…”

Dioxetane Scission Products Unchanged by Mechanical Force

“…Although {Eu} has previously been employed to detect triplet ketones through triplet–triplet energy transfer, excited adamantanone has a number of features that result in predominantly singlet energy transfer in the absence of naphthalene as a mediator. First, the triplet lifetime of this ketone is very short, having been claimed to be as low as 0.15 ns15 (by comparison, acetone has a triplet lifetime of 5 μs25 and the singlet lifetime of adamantanone is 9.12 ns26), so the triplet‐energy acceptor does not compete effectively with the triplet ketone deactivation process at concentrations at which {Eu} can be dissolved in toluene (<2 m M ). Second, the singlet lifetime of adamantanone is much longer than the triplet lifetime and last, in contrast with most other dioxetanes, the singlet excitation yield is relatively high compared with the triplet excitation yield.…”