Rates of Ring Opening of Radical Cation Intermediates Govern Differences in Thermoluminescence between 1‐ and 2‐Naphthyl‐Substituted Methylenecyclopropanes

Abstract

In this study, we observed that methylenecyclopropane 1 c, possessing geminal phenyl and 1‐naphthyl groups at C‐2 of the three membered ring, emits green thermoluminescence (TL) in a low temperature matrix, associated with two sets of bands in the 480–600 and 600–700 nm regions. The bands are assigned to phosphorescence from 31 c* and triplet‐triplet (T−T) fluorescence (FL) from the corresponding trimethylenemethane biradical 32 c..*, respectively. In contrast, as previously reported, the analogous 2‐naphthyl derivative 1 b emits red TL associated with two bands in the 350–450 and 600–700 nm regions, which are assigned to FL from 11 b* and T−T FL from 32 b..*. The results of spectroscopic studies and density functional theory calculations show that the observed dissimilarity in the TL properties of 1 b and 1 c is a consequence of differences in the rate of methylenecyclopropane ring opening of 1 b.+ and 1 c.+ associated with stereoelectronic effects of the naphthyl groups.

Luminescent Radicals

“…These radical cation and anion are generally generated via simultaneous oxidation and reduction of a substrate that can be promoted by various methods such as γ-irradiation and X-irradiation. Energy-sufficient charge recombination allows the formation of the excited-state biradical. − Ikeda and co-workers demonstrated TL from excited-state biradicals that are transiently formed via successive photoirradiation, bond cleavage/formation, and charge recombination triggered by thermal activation in glassy methylcyclohexane (MCH) matrix at low temperature. A MCH matrix containing Ph 2 HD showed yellow TL originating from fluorescence of a Ph 2 CHD biradical based on its singlet manifold, while a MCH matrix with Ph 2 MCP showed green TL assignable to the fluorescence of a Ph 2 CTMM biradical based on its triplet manifold (Chart ).…”