Competition between Singlet Fission and Spin‐Orbit‐Induced Intersystem Crossing in Anthanthrene and Anthanthrone Derivatives

Abstract Singlet and triplet excited‐state dynamics of anthanthrene and anthanthrone derivatives in solution are studied. Triisopropylsilyl‐ (TIPS) or H‐terminated ethynyl groups are used to tune the singlet and triplet energies to enable their potential applications in singlet fission and triplet fusion processes. Time‐resolved optical and electron paramagnetic resonance (EPR) spectroscopies are used to obtain a mechanistic understanding of triplet formation. The anthanthrene derivatives form triplet states efficiently at a rate (ca. 10 7 s −1 ) comparable to radiative singlet fluorescence processes with approximately 30 % triplet yields, despite their large S 1 ‐T 1 energy gap (>1 eV) and the lack of carbonyl groups. In contrast, anthanthrone has a higher triplet yield (50±10 %) with a faster intersystem crossing rate (2.7 10 8 s −1 ) because of the n‐π* character of the S 1 ←S 0 transition. Analysis of time‐resolved spin‐polarized EPR spectra of these compounds reveals that the triplet states are primarily generated by the spin‐orbit‐induced intersystem crossing mechanism. However, at high concentrations, the EPR spectrum of the 4,6,10,14‐tetrakis(TIPS‐ethynyl)anthanthrene triplet state shows a significant contribution from a non‐Boltzmann population of the m s =0 spin sublevel, which is characteristic of triplet formation by singlet fission.