To increase 𝑘 ((# in the solution-based TTA-UC, it is necessary to either increase the diffusion rate of the emitter molecule or to enhance the effective TTA reaction zone between the triplet-excitons, i.e., the spatial extent of unpaired orbitals in the triplet-exciton. Solution TTA-UC materials using low-viscosity solvents such as toluene and tetrahydrofuran have been reported in numerous studies. When the molecular diffusion rate is larger than 𝑘 ((# , it is important to enhance the reactivity between the triplet excitons for improving 𝑘 ((# . When 𝑘 ((# is the diffusion-controlled rate constant, it might be better to use a lower viscosity solvent, but will be difficult to find a solvent that can dramatically improve 𝑘 ((# . Enhancing the TTA reaction distance would be plausible when tripletexcitons possesses highly delocalized unpaired orbitals. However, this simple expansion of the orbitals generally lowers the triplet energy level, causing difficulty in obtaining the radiative excited singlet state S1 emi from the two of the first excited triplet states (T1) as in the case of triplet-pair by pentacene, which is well known as a singletfission material.