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

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. Open sesame! The difference between the major components of the thermoluminescence exhibited by 1‐ and 2‐naphthyl‐substituted methylenecyclopropanes is a consequence of dissimilarities in stereoelectronically governed rates of ring opening of radical cation intermediates.