The 𝛽 value ranges from 0 to 1, and 𝛽/2 equals the 𝛷 ((# . Solid red curves in Figure The estimated 𝑘 ((# s of both samples were on the order of 10 9 (M - 1 s -1 ). These values are of the same order of magnitude as those of previously reported intermolecular TTA systems in solution. From the blue plots in Figure , the 𝑘 ((# value of TAB was 1.2 times higher than that of DPA. The 𝑘 ((# value can be dependent of the molecular diffusion rate and of the TTA reaction distance, i.e., the spatial extent of the electron density distribution in the triplet state. The diffusion-controlled reaction rate constant (𝑘 F ) can be described as depending solely on the solvent viscosity (𝜂), as shown by Eq. ( ): 𝑘 G = HIJ :K (5) where 𝑅 is the gas constant, 𝑇 is the temperature and 𝜂 is the viscosity. From this, 𝑘 G in toluene is estimated to be 10 10 M -1 s -1 using viscosity of 𝜂 = 0.56 mPa•s at 298 K and is five times larger than the 𝑘 ((# values. However, considering the low spin statistical factor (𝑓 = 0.4) in DPA, it can be said that 𝑘 ((# = 1.64(±0.02) ×10 9 M -1 s - 1 is essentially limited by the diffusion. Therefore, it is difficult to determine the intrinsic TTA-rate of the triplet pairs. That the TTA rate for the TAB system is larger than that of the DPA system however suggests that the effective delocalization of the triplet-exciton in the trimer increases the 𝑘 ((# . We also determined the 𝑘 ((# values in a viscous solvent of a mixture of toluene(T): paraffin(P) = 1:1 (v/v) (Figure , Table ). 𝑘 ((# TAB was slightly lower or unchanged from that of DPA, as shown by the red plots in Figure . This reduction of 𝑘 ((# for TAB in the viscous solvent would be caused by prolonging the pseudorotational time of the triplet-exciton. We note that this reversal is not explained by the Franck-Condon factor for the TTA (vide infra).