Movement of molecules are always affected by the surrounding solvent molecules, so solvents have an essential effect in triplet-triplet annihilation (TTA) upconversion. In this article, the TTA upconversion system with Diiodo-BODIPY (MBDP-2I) as sensitizer and perylene as acceptor in six solvents of differing viscosity were investigated (toluene, 1,4-dioxane and four high viscosity phthalate ester solvents DMP, DBP, DNOP and DOP). The upconversion efficiency was positively correlated with the solvent viscosity. Among the six solvents, the highest upconversion quantum efficiency of 9.59% was observed in high viscosity DOP (η = 72.4 mPa s), almost 1.5 times higher than that in 1,4-dioxane (η = 3.2 mPa s, ΦUC = 6.36%). The efficient upconversion was attributed to the long-lived triplet excited states of MBDP-2I, efficient fluorescence quantum yield of perylene and efficient TTA in high viscosity phthalate esters solvents. The mechanism underlying the highly efficient TTA upconversion in the high viscosity phthalate esters solvents was investigated by solvent cage effect. In addition, significant room temperature phosphorescence of MBDP-2I at 765 nm was observed in the viscous phthalate esters solvents.
