Symmetry-Based Design Strategy for Unprecedentedly Fast Decaying Thermally Activated Delayed Fluorescence (TADF). Application to Dinuclear Cu(I) Compounds

Inspired by molecular crystal theory of coupling symmetry-related transition dipole moments, we develop a model for rational design of Cu(I) complexes to achieve short TADF (thermally activated delayed fluorescence) decay times. This is, for example, important to reduce OLED stability problems and roll-off effects. Guided by the model, we design a new class of Cu(I) dimers focusing on Cu-2(tppb)(PPh3)(2)Cl-2 2 (tppb(PPh3)(2) = 1,2,4,5-tetrakis(diphenylphosphino)benzene). Indeed, this class of compounds shows particularly short TADF decay times as evidenced by luminescence studies over a temperature range of 1.5 K <= T <= 300 K and, thus, supports the proposed design strategy. The model is further supported by TD-DFT calculations. A key property of the strategy is that the new dimer(s) exhibit a drastically faster radiative rate of the transition between the lowest excited singlet state and the ground state than the related monomer, Cu(dppb)(PPh3)Cl 1 (dppb = 1,2-bis(diphenylphosphino)benzene). This is even valid at a small singlet-triplet energy gap of Delta E(S-1-T-1) = 390 cm(-1) (48 meV). Accordingly, we find a benchmark TADF decay time for the Cu(I) dimer 2 of only 1.2 mu s (radiative decay: 1.5 mu s). This is a factor of about three times shorter than found so far for any other Cu(I) complex with a similarly small energy gap. The presented design strategy seems to be of general validity.