Mechanism of Enhanced Triplet-Triplet Upconversion in Organic Molecules.

We use time-dependent density functional theory (TDDFT) to investigate the mechanism of efficient triplet-triplet upconversion (TTU) in certain organic materials. In particular, we focus on materials where some singlets are generated in a two-step spin-nonconserving process (T + T → T → S). For this mechanism to contribute significantly, the intersystem crossing (ISC) from the high-lying triplet to the singlet (T → S) must outcompete the internal conversion (IC) to the low-lying triplet (T → T). By considering multiple families of materials, we show that the T → S ISC can be enhanced in a number of ways: the substitution of electron-donating (ED) and electron-withdrawing (EW) groups at appropriate positions; the substitution of bulky groups that distort the molecular geometry; and the substitution of heavy atoms that enhance the spin-orbit coupling (SOC). In the first two cases, the enhancements are consistent with El-Sayed's rule in that rapid T → S ISC requires significant differences in the characters of the S and the T wavefunctions. Together, these effects enable a wide tunability of T → S ISC rates over at least 5 orders of magnitude. Meanwhile, the T → T IC is inhibited in these systems due to the large T - T energy gap >0.5 eV, which entails a high energy barrier to the T → T IC and the prediction of a slow rate regardless of the substituents or the presence of heavy atoms. In this way, tuning the T → S ISC appears to provide an effective strategy to achieve systematic improvement of TTU materials.