High-Level Reverse Intersystem Crossing and Molecular Rigidity Improve Spin Statistics for Triplet-Triplet Annihilation Upconversion

The structural factors affecting triplet-tripletannihilation(TTA) at the molecular level are not well-understood. Here, our steady-statephotoluminescence and transient absorption results demonstrate thatthe spin statistical factor, & eta;, decreases from 0.60 to 0.46and 0.14 going from 9,10-diphenylanthracene (DPA) to the 1,5-DPA and2,6-DPA isomers, respectively, during photon upconversion with a platinumoctaethylporphyrin sensitizer. Density functional theory (DFT) showsthat & eta; depends on the energetics of hot triplet states and molecularrigidity. The significantly high conical intersection energy betweenthe S-0 and T-1 states for 9,10-DPA gives itslonger triplet lifetime. Time-dependent DFT calculations show that9,10-DPA and 1,5-DPA can undergo high-level reverse intersystem crossingfrom their T-2 and T-3 states, respectively, tothe bright S-1 state, increasing the limit of the spin statisticalfactor. Both factors ultimately serve to enhance the TTA efficiency.This work provides insight into designing molecules for efficientlight-emitting and photon upconversion applications.