Development of an Organic Emitter Exhibiting Reverse Intersystem Crossing Faster than Intersystem Crossing

Area of Science:

• Materials Science
• Organic Electronics
• Photophysics

Background:

• Thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) require accelerated reverse intersystem crossing (RISC) and suppressed intersystem crossing (ISC) to reduce triplet exciton lifetimes.
• Inverted singlet-triplet (iST) systems with a negative energy difference (ΔE_ST) between the lowest excited singlet and triplet states are crucial for achieving efficient RISC.

Purpose of the Study:

• To explore the potential of an asymmetric hexa-azaphenalene (A6AP) core for developing novel iST materials.
• To design and synthesize a new A6AP derivative, A6AP-Cz, and investigate its photophysical properties for OLED applications.

Main Methods:

• Computational calculation of ΔE_ST for the designed A6AP-Cz molecule.
• Experimental synthesis and characterization of A6AP-Cz.
• Measurement of delayed fluorescence lifetime (τ_DF), RISC rate constant (k_RISC), and ISC rate constant (k_ISC).
• Temperature-dependent studies of τ_DF and analysis of fluorescence and phosphorescence spectra at 77 K.

Main Results:

• A6AP-Cz was successfully synthesized with a calculated ΔE_ST of -44 meV.
• The synthesized A6AP-Cz exhibited an exceptionally short τ_DF of 54 ns, the shortest reported for organic materials.
• Experimental confirmation of negative ΔE_ST through k_RISC (1.9×10⁷ s^-1) > k_ISC (1.0×10⁷ s^-1) and spectral analysis (-45 meV and -73 meV).

Conclusions:

• The asymmetric hexa-azaphenalene (A6AP) core shows significant potential for creating effective inverted singlet-triplet (iST) materials.
• A6AP-Cz demonstrates superior performance in accelerating RISC, leading to ultra-fast delayed fluorescence, a key advancement for OLED technology.