Because of 𝑘 G > 𝑘 ((# for TAB, the Franck-Condon factors between the triplet-pair and the S2S0 pair prior to the S2-S1 internal conversion could be small at the collision distances due to the large reorganization energies in Eq. ( ) for the TTA-UC processes to yield S1. This is however excluded, because the free energy change (Δ𝐺 ((# ) for the TTA is evaluated to be -0.42 eV using Δ𝐺 ((# = E(S2) -2E(T1) in Figure . The reorganization energy of 𝜆 (= 2𝜆 ( ) for T1 + T1 -> S0 + S2 is estimated to be 0.42 eV by which conformations of two of the triplet excitons are rearranged to those of the two singlets. From Δ𝐺 ((# + 𝜆 ≈0 eV, a large Franck-Condon factor is conclusive, as detailed in the Supporting Information. Thus, in the present exergonic TTA systems, the rate constant of 𝑘 ((# is limited both by the molecular diffusion and by spin statistics, especially in the PtOEP/TAB system. This suggests that the larger 𝑘 ((# in TAB compared to DPA may originate from enhancement of the electron exchange between the triplet excitons due to the entropy-driven activation to induce TTA (TTA-ON in Figure ). Contrary to this, TTA rate constant is expected to be reduced when the activated hopping is frozen because the triplet spin-density distributions are localized to cause mismatches in the electron exchange at the T-T collision (TTA-OFF in Figure ). In toluene, 1/𝑘 LMC of TAB is estimated to be 1.7 ps (Figure ) at 298 K. Then, we have calculated a time duration (𝑡 VM++W%WMX ) for two 3 TAB* molecules being in contact on a single collision in toluene. It is known that the rate of TTA decreases drastically as the center-to-center distance between the TTA emitters becomes 1 Å longer. Based on this, given that the spin-spin exchange integral decays by exp(-ar) with a = 10 nm -1 , one can assume that the TTA reaction occurs while the two TAB molecules are within a 0.1 nm variation (Dr) at the distance of the T-T collision. Therefore 𝑡 VM++W%WMX can be estimated as the residence time required for a 3 TAB* molecule to diffuse at Δ𝑟 = 0.1 nm displacement. Since TAB molecules undergo Brownian motion in three-dimensional space, the mean square displacement (𝑑) can be expressed as follows: 60 𝑑 = √6𝐷𝜏