X-ray-Triggered Thermoluminescence and Density Functional Theory Characterization of a gem-Diphenyltrimethylenemethane Biradical

Thermoluminescence (TL) from the excited triplet state of a gem-diphenyltrimethylenemethane biradical (34••*) is triggered by X-irradiation at 77 K followed by annealing to ~140 K. The new X-irradiation method reported here is simpler and more convenient than the previously employed γ-irradiation method. The TL spectrum of 34••* is similar to the photoluminescence spectrum of the 1,1-diphenylethyl radical (5•). The results of density functional theory (DFT) and time-dependent-DFT calculations of the ground state biradical 34•• suggest that no significant electronic interaction takes place between the diphenylmethyl and allyl radical moieties owing to its twisted geometry. Accordingly, the results also suggest that the excited state biradical 34••* has a similar molecular geometry and electronic structure as the triplet ground state. Both the experimental and computational results obtained for 34•• and 5• confirm that the main fluorophore of 34••* is the diphenylmethyl radical moiety.