Exergonic Intramolecular Singlet Fission of an Adamantane-Linked Tetracene Dyad via Twin Quintet Multiexcitons

An adamantane-linked tetracene dyad (Tc-Ad-Tc) undergoes exergonic intramolecular singlet fission (SF), producing long-lived (τ = 175 μs) and high-energy (2 × 1.03 eV) multiexciton. Time-resolved absorption, fluorescence decay, and electron paramagnetic resonance (EPR) spectroscopic analysis revealed that the long-lived triplet species is generated in this system via a correlated triplet pair having singlet and quintet characteristics. Time-resolved EPR analysis revealed formation of syn and anti conformers in the quintet, i.e., 5(3Tc-Ad-3Tc)*. The quintet generation requires small conformational motion to induce singlet–quintet spin relaxation. The presence of aliphatic linkages, like the rigid adamantane group, may enable effective conservation of intrinsic high S1 and T1 levels of the original monomers, moderate bridge-mediated σ–π interaction leading to exergonic intramolecular SF involving 1Tc*-Ad-Tc → 1(3Tc-Ad-3Tc)*, and prevention of undesirable triplet–triplet annihilation, finally result in long-lived and high-energy multiexciton.