Engineering Fast Reverse Intersystem Crossing and Radiative Decay in [1,2,4]Triazolo[1,5-a]pyridine-Based TADF...

Area of Science:

• Materials Science
• Organic Electronics
• Photochemistry

Background:

• Thermally activated delayed-fluorescence (TADF) emitters are crucial for efficient organic light-emitting diodes (OLEDs).
• Balancing radiative decay rate (k_r) and reverse intersystem crossing (rISC) rate (k_rISC) is a key challenge due to opposing dependencies on molecular orbital overlap.
• The performance of TADF emitters is governed by excited-state dynamics influenced by molecular geometry, energy levels, and spin-orbit coupling.

Purpose of the Study:

• To computationally design and explore novel [1,2,4]triazolo[1,5-a]pyridine (TP)-based donor-π-acceptor emitters for TADF applications.
• To systematically analyze the relationships between molecular structure, excited-state dynamics, and TADF performance.
• To establish a design framework for high-performance TP-based TADF emitters.

Main Methods:

• Computational design and exploration of 30 TP-based donor-π-acceptor emitters across two molecular series (2,6TP and 2,7TP).
• Systematic analysis of parameters including donor-acceptor dihedral angle (θ_DA), singlet-triplet energy gap (ΔE_ST), spin-orbit coupling (SOC), radiative decay rate (k_r), reverse intersystem crossing rates (k_rISC), reorganization energy (λ), and activation energy for rISC (ΔG_rISC).
• Investigation of excited-state dynamics governing TADF performance.

Main Results:

• Optimized dihedral angles (∼77°-78°) in *ortho*-substituted dimethylacridine-based emitters led to low reorganization energies (λ ≈ 0.22-0.30 eV) and small ΔE_ST values (∼0.05 eV).
• These emitters exhibited moderate SOC (∼0.3-0.4 cm^-1), enhanced k_r (>10⁵ s^-1), and k_rISC exceeding 10⁶ s^-1, with low activation barriers for rISC (ΔG_rISC ≈ 0.08-0.10 eV).
• The study revealed that ΔG_rISC is strongly influenced by λ in addition to ΔE_ST, highlighting that minimizing ΔE_ST alone is insufficient for efficient TADF.

Conclusions:

• A comprehensive design framework for high-performance TP-based TADF emitters was developed.
• Synergistic optimization of molecular geometry, reorganization energy, and activation energy is crucial for efficient TADF performance.
• The findings provide valuable insights for the development of advanced materials for organic light-emitting diode applications.