Enhanced Intersystem Crossing and Transient Electron Spin Polarization in a Photoexcited Pentacene–Trityl Radical
Claudia E. Avalos, Sabine Richert, Etienne Socie, Ganesan Karthikeyan, Gilles Casano, Gabriele Stevanato, Dominik J. Kubicki, Jacques‐E. Moser, Christiane R. Timmel, Moreno Lelli, Aaron J. Rossini, Olivier Ouari, Lyndon Emsley
Abstract
-selective intersystem crossing rates, can be transferred to the tethered stable radical. The efficiency of this spin polarization transfer depends on many factors: local magnetic and electric fields, excited-state energetics, molecular geometry, and spin-spin coupling. Here, we present transient electron paramagnetic resonance (EPR) measurements on three pentacene derivatives tethered to Finland trityl, BDPA, or TEMPO radicals to explore the influence of the nature of the radical on the spin polarization transfer. We observe efficient polarization transfer between the pentacene excited triplet and the trityl radical but do not observe the same for the BDPA and TEMPO derivatives. The polarization transfer behavior in the pentacene-trityl system is also investigated in different glassy matrices and is found to depend markedly on the solvent used. The EPR results are rationalized with the help of femtosecond and nanosecond transient absorption measurements, yielding complementary information on the excited-state dynamics of the three pentacene derivatives. Notably, we observe a 2 orders of magnitude difference in the time scale of triplet formation between the pentacene-trityl system and the pentacene systems tethered with the BDPA and TEMPO radicals.