Theoretical Characterizations of TADF Materials: Roles of ΔG and the Singlet–Triplet Excited States Interconversion

Abstract

The thermally activated delayed fluorescence (TADF) phenomenon has attracted increasing attention because it can harvest 100% of the electro-pumped carriers to form singlet bound excited state for fluorescence. It is generally believed that the small energy gap between S 1 and T 1 (ΔE ST ) is essential for TADF to facilitate the reverse intersystem crossing (rISC). However, for a few donor−acceptor (D−A) organic compounds with small ΔE ST , the TADF phenomenon is absent, indicating that ΔE ST might not be a good molecular descriptor. Here, using our self-developed thermal vibration correlation function (TVCF) formalism in combination with quantum chemistry calculations, we revisit the key factors that dominate the TADF property for 11 D− A systems with small ΔE ST . Based on our theoretical results in comparison to experiments, we conclude that the activation energy ΔG is a good molecular descriptor to characterize the TADF performance because a significantly better linear relationship is observed between ΔG and the rISC rate constant (k rISC ) compared to that between ΔE ST and k rISC . These findings provide deeper understanding of the TADF mechanism, shedding light on the molecular design of high-performance TADF materials.

Theoretically elucidating high photoluminescence performance of dimethylacridan-based blue-color thermally activated delayed fluorescent materials

“…For both molecules, the calculated DE ST and reorganization energies between S 1 /T 1 (l S 1 T 1 ) are quite small; theoretically the ISC and RISC rates should be close to each other according to the Marcus theory. 64 Our calculations verify these analyzations. We recalculate the ISC/RISC rates with the experimental DE ST as shown in Table S1 in the ESI, † and compare the qualitative trend for both molecules with the experimental results.…”

“…According to the Marcus-Levich equation 64 (eqn (S1) in the ESI †), we can conclude that the nonradiative rates k nr exponentially decrease with the reorganization energy l S 1 S 0 decreasing, due to the quite large adiabatic DE S 1 S 0 . 34 For the ISC/RISC rates, DE ST is typically small and l S 1 T 1 is also quite small for both molecules.…”

“…5(b, d) and 6(b). Our calculations indicate that the twisting vibration of the donor DMAC and acceptor moiety significantly affect the nonradiative rates of S 1 -S 0 and reverse intersystem crossing rates, 64 in both molecules.…”

A Hybrid Materials Approach for Fabricating Efficient WLEDs Based on Di‐Ureasils Doped with Carbon Dots and a Europium Complex

“…Those rates even in better energy conditions with ΔES-T (S1 and T1 energy difference) around 2000 cm -1 were estimated to be in the order of WISC~10 7 s −1 for organic compounds. [86] However, in our case, the ΔES-T=6500 cm −1 is much higher and the WISC (considered to be 10 6 s −1 ) still is 1500 times less than the energy transfer rates from the bpyO2 to the host and 480 times from the host to the ttaligand. This unique effect, never reported before, is only possible due to the intrinsic characteristics of the di-ureasil host, highlighting the pivotal interest of the hybrid approach to produce efficient WLEDs.…”

Diazine-based thermally activated delayed fluorescence chromophores

“…21,22 Although more complex firstprinciple approaches, based on (spin)-vibronic models, have been recently proposed to explore the TADF mechanism, 10,23,24 the semi-classical theory remains a popular tool to rationalize and predict the impact of chemical substitution and establish structure-TADF properties relationships. 25,26,27,28 Noteworthy, even so DFT and TD-DFT are first-principles approaches, they still face challenges related to the unknown exact expression of the exchange-correlation kernel, 29,30 that might for instance lead to low-lying spurious CT states, and even state-of-the art TD-DFT solvation models may not reach chemical accuracy. 31 The diazine derivatives are six-membered aromatic heterocycles comprising of two nitrogen atoms in their structure.…”