Suppression of Structural Change upon S₁–T₁ Conversion Assists the Thermally Activated Delayed Fluorescence Process in Carbazole-Benzonitrile Derivatives

Abstract

Thermally activated delayed fluorescence (TADF) molecules are gathering attention for their potential to boost the efficiency of organic light-emitting diodes without precious metals. Minimizing the energy difference between the S 1 and T 1 states (ΔE ST ) is a fundamental strategy to accelerate reverse intersystem crossing (RISC). However, the lack of microscopic understanding of the process prevents adequate design strategies for efficient TADF materials. Here, we focused on four carbazole-benzonitrile (Cz-BN) derivatives that possess identical ΔE ST but distinct TADF activities. We systematically compared their geometrical dynamics upon photoexcitation using time-resolved infrared (TR-IR) vibrational spectroscopy in conjunction with quantum chemical calculations. We found that the most TADF-active molecule, 4CzBN, shows little structural change after photoexcitation, while the TADF-inactive molecules show relatively large deformation upon S 1 −T 1 conversion. This implies that the suppression of structural deformation is critical for minimizing the activation energy barrier for RISC in cases of the Cz-BN derivatives.

Tetradentate Gold(III) Complexes as Thermally Activated Delayed Fluorescence (TADF) Emitters: Microwave‐Assisted Synthesis and High‐Performance OLEDs with Long Operational Lifetime

“…In this regard, OLEDs fabricated with Au I and Au III ‐TADF emitters ( τ <2 μs) have achieved small roll‐off (<10 %) . A recent study suggested that improving the rigidity of emitters could suppress the structural deformation upon S 1 –T 1 transformation thereby reducing the activation energy barrier for reverse intersystem crossing (RISC) and leading to efficient TADF . It is thus conceived that Au III complexes supported by trianionic tetradentate ligands (e.g.…”

Solvent Dependence of Structural Dynamics and Spin-flip Processes in 3,4,5-tri(9H-carbazole-9-yl)benzonitrile (ortho-3CzBN)

“…The spectral changes are similar to the trend in the THF solution reported in our previous report. 11 Because TR-IR in the region is sensitive to intramolecular charge transfer and distortion of molecular geometry 11 , this spectral change suggests a structural change associated with the ISC from S1 to T1. Although infrared spectral peaks of a specific functional group often change depending on solvents 18,19 , the IR stretching bands of o-3CzBN in the 1100-1700 cm −1 region were independent of the solvent polarity, indicating investigated in a previous report and correlated well to the experimentally observed spectra.…”

“…Although infrared spectral peaks of a specific functional group often change depending on solvents 18,19 , the IR stretching bands of o-3CzBN in the 1100-1700 cm −1 region were independent of the solvent polarity, indicating investigated in a previous report and correlated well to the experimentally observed spectra. 11 Here, we examined the S1 state spectra of o-3CzBN in toluene and MeCN solvents, by conducting similar calculations to simulate vibrational spectra ( Figure S5).…”

“…We employed the 6-31G(d,p) basis set and the B3LYP functionals following previous research. 11 The solvent effect was examined using the polarizable continuum model of toluene solution (dielectric constant: 2.379) and…”

“…Although all Cz units of o-3CzBN in the ground state are in the same plane, the location of one Cz unit shifts towards an out-of-plane direction in the T1 state, as revealed by TR-IR and quantum chemical calculations. 11 Intriguingly, o-3CzBN shows no TADF activity in toluene solution although it becomes TADF active in acetonitrile (MeCN) solution. 5 Therefore, the o-3CzBN is an ideal system to explore structural changes in higher-lying excited states by comparing the solvent dependence of TR-IR in toluene and MeCN.…”