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Three States Involving Vibronic Resonance is a Key to Enhancing Reverse Intersystem Crossing Dynamics of an
Inkoo Kim1, Kwang Hyun Cho2, Soon Ok Jeon3
1Data and Information Technology Center, Samsung Electronics, Hwaseong 18448, Republic of Korea.
JACS Au
|September 1, 2021
Summary
Organoboron chromophores enable efficient organic light-emitting diodes (OLEDs) through a unique vibronic resonance mechanism. This discovery enhances understanding of thermally activated delayed fluorescence (TADF) for designing better blue-emitting molecules.
Area of Science:
- Materials Science
- Organic Electronics
- Quantum Chemistry
Background:
- Narrow-band blue-emitting organoboron chromophores utilizing the multiple-resonance (MR) effect are crucial for efficient organic light-emitting diodes (OLEDs).
- These chromophores exhibit thermally activated delayed fluorescence (TADF) despite a large singlet-triplet gap (ΔEST), with the underlying reverse intersystem crossing (RISC) mechanism not fully understood.
- The efficiency of RISC, especially with small spin-orbit coupling, remains a key question for molecular design in OLEDs.
Purpose of the Study:
- To elucidate the mechanism behind efficient reverse intersystem crossing (RISC) in organoboron emitters exhibiting thermally activated delayed fluorescence (TADF).
- To investigate the role of vibronic resonance in enhancing RISC dynamics in multiple-resonance (MR) organoboron chromophores.
- To provide insights for designing improved blue-emitting molecules for organic light-emitting diodes (OLEDs).
Main Methods:
- Semiclassical quantum dynamics simulations were employed to model the electronic and vibronic interactions.
- Analysis focused on the frequency matching condition between molecular vibrations and the electronic energy gap.
- Investigated geometry-dependent non-Condon coupling to an upper triplet state.
Main Results:
- A vibronic resonance, driven by frequency matching between vibrations and electronic energy gaps, was identified as a key factor enhancing RISC.
- This resonance involves three electronic states, with a high-energy mediating state playing a crucial role despite its low thermal population.
- Geometry-dependent non-Condon coupling to the upper triplet state, oscillating with the energy gap frequency, significantly boosts RISC.
Conclusions:
- Vibronic resonance is a critical mechanism for achieving efficient RISC in organoboron emitters, enabling TADF even with large ΔEST.
- The identified mechanism allows for efficient RISC across various ΔEST values, defying conventional energy gap laws.
- This study offers a new design strategy for developing high-performance blue-emitting molecules for advanced OLED applications.
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