Long-lived Triplet Excitons Formed by Exergonic Intramolecular Singlet Fission of an Adamantane-linked Tetracene Dyad

An adamantane-linked tetracene dyad (Tc–Ad–Tc) undergoes exergonic intramolecular singlet fission (SF), producing longlived (τ = 175 μs) and high-energy (2 x 1.03 eV) multiexcitons. Timeresolved absorption, fluorescence decay, and electron paramagnetic resonance (EPR) spectroscopic analysis revealed that the long-lived triplet species is generated in this system via correlated triplet pair having singlet and quintet characteristics. Time-resolved EPR analysis revealed that conversion of ¹(³Tc–Ad–³Tc)* -> ⁵(³Tc–Ad–³Tc)* requires small conformational dynamics accompanied by molecular motion. Analysis of the geometries of the quintet states shows that formation of the long-lived multiexciton is enabled by precise and close alignment of the tetracene moieties, which leads to their moderate interaction in the singlet excited state, while triplet–triplet annihilation is prevented by quintet generation. The presence of aliphatic linkages, like the rigid adamantane group, might enable effective conservation of intrinsic S₁ and T₁ levels of the original monomers, and moderate bridge-mediated σ–π interaction leading to exergonic intramolecular SF involving ¹Tc*–Ad–Tc -> ¹(³Tc–Ad–³Tc)*.