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

Design of a New Mechanism beyond Thermally Activated Delayed Fluorescence toward Fourth Generation Organic Light Emitting Diodes

Usually, development of organic molecules with efficient thermally activated delayed fluorescence (TADF) focuses on minimizing the energy gap between the lowest singlet and triplet state. However, although this is crucial, it is not sufficient for optimizing the emitter's molecular and electronic structure for OLED use. Here, we present a design strategy that leads us not only to a new type of emitter but also to a new exciton harvesting mechanism. This concept is realized (i) by drastically reducing the energy gap between the lowest singlet and triplet energy states, (ii) by rigidifying the molecular structure to reduce inhomogeneity effects that usually induce long emission decay tails lying even in the millisecond time range, (iii) by maximizing the Franck-Condon factors that govern intersystem crossing (ISC), (iv) by shifting the charge transfer states, (CT)-C-1 and (CT)-C-3, to become the lowest energy states applying polarity tuning, and (v) by providing energetically nearby lying states for spin-orbit coupling (SOC) and configuration interaction (CI) paths to speed up ISC. Using this concept, we design an "almost zero-gap" compound showing Delta E((CT)-C-1-(CT)-C-3) approximate to 16 cm(-1) (2 meV). Thus, thermal activation is no longer a time delaying key problem at T = 300 K. Moreover, if the emitter is applied in an OLED, fast ISC will allow us to harvest all singlet and triplet excitons through emission from the lowest excited CT singlet state. This benchmark mechanism, the direct singlet harvesting (DSH) mechanism, offers the great advantage of a significant reduction of the overall emission decay time to the submicrosecond range. This is a shorter decay than found for TADF emission so far. Accordingly, this mechanism leads us to beyond TADF toward a new era in the design of OLED emitters and opens the way for reducing stability problems and roll-off effects

Design strategies for materials showing thermally activated delayed fluorescence and beyond: Towards the fourth-generation OLED mechanism

Design strategies for molecules showing thermally activated delayed fluorescence (TADF) are discussed, and a new emitter concept based on an almost zero-energy-gap is developed. Thermal activation is not substantial. Applied in an organic light emitting diode, all singlet and triplet excitons are harvested directly in the lowest singlet state without time-delaying TADF. This landmarking mechanism, being beyond TADF, leads to emission decay times in the sub-s range

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

A highly potent donor-acceptor biaryl thermally activated delayed fluorescence (TADF) dye is accessible by a concise two-step sequence employing two-fold 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 Phi(PL) = 80% in toluene and 90% in PMMA arising from prompt and delayed fluorescence with decay times of 21 ns and 30 mu s, respectively. From an Arrhenius plot, the energy gap Delta E(S-1 - T-1) between the lowest excited singlet S(1 )and triplet T-1 state was determined to be 980 cm(-1) (120 meV). A new procedure is proposed that allows us to estimate the intersystem crossing time to similar to 10(2) ns