Investigating Vibronic Coupling Effects in Multiple-Resonance Thermally Activated Delayed Fluorescence Molecules

Area of Science:

• Materials Science
• Organic Electronics
• Photophysics

Background:

• Multiple resonance thermally activated delayed fluorescence (MR-TADF) materials offer superior color purity over conventional TADF emitters.
• A key limitation of MR-TADF materials is their typically slow reverse intersystem crossing (rISC) rates, hindering practical applications.

Purpose of the Study:

• To investigate the influence of vibronic coupling on the rISC mechanism in MR-TADF emitters.
• To explore the effect of introducing heavy adamantyl substituents to modulate vibrational modes and rISC rates.

Main Methods:

• Synthesis of MR-TADF molecules functionalized with heavy adamantyl substituents.
• Spectroscopic and photophysical characterization to analyze excited-state dynamics and rISC processes.
• Computational modeling to understand the role of vibronic coupling and substituent effects.

Main Results:

• Addition of adamantyl groups reduced vibronic coupling in a specific excited-state transition.
• Surprisingly, adamantyl substituents facilitated rISC through an alternative pathway, increasing the overall rISC rate.
• The vibrationally damped MR-TADF molecule exhibited a higher rISC rate than anticipated.

Conclusions:

• Vibronic coupling plays a complex role in the rISC mechanism of MR-TADF emitters.
• Heavy substituents can unexpectedly enhance rISC rates by opening alternative pathways.
• Efficient rISC in MR-TADF may require less stringent vibrational damping than previously thought for donor-acceptor systems.