Quenching of excited states by stable free radicals. Effect of di-tert-butyl nitroxide on stilbene and naphthalene triplets

Abstract

Full experimental and kinetic details will be published in the full paper.(10) Obtained from Diaprep, Inc., in purity satisfactory for direct use.

Excited‐State Dynamics of Non‐Luminescent and Luminescent π‐Radicals

“…Paramagnetic species such as nitroxide radicals promote the exchange‐induced EISC of chromophores from their singlet to the triplet excited state . A rapid quenching occurs when triplet molecules collide with paramagnetic species like oxygen or nitroxide . In 1969, Hoytink first proposed two possible quenching mechanisms, that is, a borrowing mechanism of the transition intensity from the allowed transition and an electron‐exchange mechanism, namely EISC.…”

“…Stable radicals such as nitroxides have been known as good quenchers of excited singlet and triplet states since 1957 and the 1970s, respectively . Dynamic electron polarization (DEP) is a non‐equilibrium condition (non‐Boltzmann population) among electron spin sublevels.…”

“…Most studies have focused on their unique open‐shell nature in their electronic ground states, as organic radicals usually exhibit strong non‐radiative energy relaxation pathways, thus being weakly or non‐luminescent. Even when a luminescent chromophore is attached to a radical, its intrinsic nature leads to weakly emissive or emissionless low energy states owing to enhanced intersystem crossing (EISC) . Herein, we use the term “π‐radical/π‐radical moiety” to denote a spin system in which unpaired π‐electron(s) is delocalized in the molecule/moiety through π‐conjugation.…”

α-Methylstilbene and the Duality of Mechanism in the Quenching of Stilbene Triplets by Molecular Oxygen†

“…We obtained ([c]/[t]) s = 1.27 and 1.24 for the degassed and N 2 ‐saturated solution, respectively, in the absence of quenchers in good agreement with 1.24, the value reported in (2). As in the case of unsubstituted stilbene (11, 12) the presence of 2 DTBN increases the cis content of photostationary states. The percentage of cis starts at 55.7 in the absence of 2 DTBN and levels off at 58.6 at the higher 2 DTBN concentrations.…”

“…The above interpretation, although satisfactory in many respects, does not account for two important observations: (i) The quenching of stilbene triplets by the stable free radical di‐ tert ‐butyl nitroxide, 2 DTBN, also postulated to involve spin‐exchange at 3 p*, leads to an increase in cis isomer content for the benzophenone‐sensitized photostationary state from 59.5% to 63.5% (11, 12). (ii) Oxygen‐quenching interactions with stilbene triplets in the presence of air (13) or an oxygen atmosphere give O 2 ( 1 Δg) with 8 and 18 ± 5% efficiency, respectively (14).…”