Eliminating the Reverse ISC Bottleneck of TADF Through Excited State Engineering and Environment‐Tuning Toward State Resonance Leading to Mono‐Exponential Sub‐µs Decay. High OLED External Quantum Efficiency Confirms Efficient Exciton Harvesting

The electronic structure and photophysics of the recently designed organic direct singlet harvesting (DSH) molecule are explored, in which donor (D) and acceptor (A) are held at distance by two bridges. One of the bridges is functionalized with fluorene. This structure leads to an ultrasmall singlet–triplet energy gap of ∆E (S1−T1) ≈ 10 cm−1 (≈1 meV) between the charge transfer states 1,3CT and shows an energetically close‐lying 3ππ* state localized on fluorene. Dielectric constant variation of the environment leads to state crossing of 3ππ* and 1,3CT near ε = 2.38 (toluene), as confirmed through time‐dependent density functional theory (DFT) and state‐specific DFT/polarizable continuum model excited‐state calculations. Transient absorption (TA) and time‐resolved luminescence in the femtosecond to microsecond regimes show rates of intersystem crossing (ISC) and reverse ISC (rISC) of >109 s–1. Thus, a strictly mono‐exponential short‐lived photo‐luminescence decay (431 ns) is observed, revealing that rISC is no longer the bottleneck responsible for long thermally activated delayed fluorescence. Ultrafast TA displays a time constant of ≈700 fs, representing the relaxation time of DSH and its solvent environment to the relaxed 1CT state with a molecular dipole moment of ≈40 D. Importantly, OLED devices, emitting sky‐blue light and showing high external quantum efficiency of 19%, confirm that singlet and triplet excitons are harvested efficiently. Experimental and theoretical studies of the recently designed organic donor–acceptor molecule show an ultra‐small gap of ∆E (1CT(S1)‐3CT(T1)) ≈ 10 cm–1 (≈1 meV) and ultra‐fast reverse intersystem crossing (RISC) of >109 s–1, realized by polarity tuning of the 1,3CT and 3ππ* states to near resonance and evidenced by fs to µs time‐resolved spectroscopy. OLED devices showing external quantum efficiency of 19% confirm efficient singlet and triplet exciton harvesting.