Evidence and mechanism of efficient thermally activated delayed fluorescence promoted by delocalized excited states

The Importance of Excited-State Energy Alignment for Efficient Exciplex Systems Based on a Study of Phenylpyridinato Boron Derivatives

Understanding excited-state dynamics is critical for improving the photoluminescence (PL) efficiency of exciplexes. A series of exciplexes based on conventional hole-transporting materials as donor and newly developed phenylpyridinato boron derivatives as acceptor were investigated. High PL efficiencies were achieved in only some combinations, and a large difference in performance among combinations provided insight into nonradiative processes in exciplex systems. Furthermore, the triplet local excited states (³ LE) of each donor and acceptor were found play an important role in triplet exciplex harvesting. Significant contributions from triplets were clearly observed when the charge-transfer excited states (¹ CT and ³ CT) and ³ LE were ideally aligned. We also demonstrated fine control of relative energy alignment via the concentration to improve the PL efficiency.

Vibrationally Assisted Intersystem Crossing in Benchmark Thermally Activated Delayed Fluorescence Molecules

Electrically injected charge carriers in organic light-emitting devices (OLEDs) undergo recombination events to form singlet and triplet states in a 1:3 ratio, representing a fundamental hurdle for achieving high quantum efficiency. Dopants based on thermally activated delayed fluorescence (TADF) have emerged as promising candidates for addressing the spin statistics issue in OLEDs. In these materials, reverse singlet-triplet intersystem crossing (rISC) becomes efficient, thereby activating luminescence pathways for weakly emissive triplet states. However, despite a growing consensus that torsional vibrations facilitate spin-orbit-coupling- (SOC-) driven ISC in these molecules, there is a shortage of experimental evidence. We use transient electron spin resonance and theory to show unambiguously that SOC interactions drive spin conversion and that ISC is a dynamic process gated by conformational fluctuations for benchmark carbazolyl-dicyanobenzene TADF emitters.

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 Φ_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 Δ E(S₁ - T₁) between the lowest excited singlet S₁ and triplet T₁ 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 ∼10² ns.

Excited state engineering for efficient reverse intersystem crossing

Reverse intersystem crossing (RISC) from the triplet to singlet excited state is an attractive route to harvesting electrically generated triplet excitons as light, leading to highly efficient organic light-emitting diodes (OLEDs). An ideal electroluminescence efficiency of 100% can be achieved using RISC, but device lifetime and suppression of efficiency roll-off still need further improvement. We establish molecular design rules to enhance not only the RISC rate constant but also operational stability under electrical excitation. We show that the introduction of a second type of electron-donating unit in an initially donor-acceptor system induces effective mixing between charge transfer and locally excited triplet states, resulting in acceleration of the RISC rate while maintaining high photoluminescence quantum yield. OLEDs using our designed sky-blue emitter achieved a nearly 100% exciton production efficiency and exhibited not only low efficiency roll-off but also a marked improvement in operational stability.

QM and ONIOM studies on thermally activated delayed fluorescence of copper(i) complexes in gas phase, solution, and crystal

QM and ONIOM studies reveal the thermally activated delayed fluorescence mechanism of two Cu(i) complexes.

Structure-Property Correlation in Luminescent Indolo[3,2-b]indole (IDID) Derivatives: Unraveling the Mechanism of High Efficiency Thermally Activated Delayed Fluorescence (TADF)

A series of indolo[3,2-b]indole (IDID) derivatives are designed as a novel structural platform for thermally activated delayed fluorescence (TADF) emitters. Intramolecular charge transfer (ICT)-type molecules consisting of IDID donor (D) and various acceptor (A) moieties are synthesized and characterized in the protocol of the systematical structure-property correlation. IDID derivatives exhibit high efficiency, prompt fluorescence as well as TADF with emission ranges tuned by the chemical structure of the acceptor units. Interestingly, almost all of the IDID derivatives show an identical energy level of the lowest triplet excited state (T₁) attributed to the locally excited triplet state of the IDID backbone (³LE_ID), while that of their lowest singlet excited state (S₁) is largely tuned by varying the acceptor units. Thus, we demonstrate the underlying mechanism in terms of the molecular engineering. Among the compounds, Tria-phIDID and BP-phIDID generate efficient delayed fluorescence based on the small energy gap between the lowest singlet and triplet excited states (ΔE_ST) and mediation of the ³LE_ID state. Organic light-emitting diodes with these Tria-phIDID and BP-phIDID as a dopant in the emitting layer show highly efficient electroluminescence with maximum external quantum efficiencies of 20.8% and 13.9%, respectively.