Design of Conformationally Distorted Donor–Acceptor Dyads Showing Efficient Thermally Activated Delayed Fluorescence

A highly potent  donor-acceptor biaryl TADF dye is accessible by a concise two-step sequence, employing twofold Ullmann arylation and a sequentially Pd-catalyzed Masuda borylation-Suzuki arylation (MBSA). Photophysical investigations show efficient TADF at ambient temperature due to the sterical hindrance between the donor and acceptor moieties. The photoluminescence quantum yield amounts to F_PL = 80 % in toluene and 90 % in PMMA arising from prompt and delayed fluorescence with decay times of 21 ns and 30 µs, respectively. From an Arrhenius plot, the energy gap DE(S₁-T₁) between the lowest excited singlet S₁ and triplet T₁ state was determined to 980 cm^-1 (120 meV). A new procedure is proposed that allows us to estimate the intersystem crossing time to ≈ 10² ns.