Accelerating PLQY and RISC rates in deep-blue TADF materials with the acridin-9(10H)-one acceptor by tuning the peripheral groups on carbazole donors

Abstract

Blue thermally activated delayed fluorescence (TADF) emitters usually suffer from poor color purity and low efficiencies, especially deep-blue emitters. Here, acridin-9(10H)-one (acridone, AD), featuring an orthogonal and highly rigid conformation, was used as an acceptor to construct a series of deep-blue TADF emitters (3,6-DCz-AD, 3,6-DPhCz-AD, 3,6-DtBuPhCz-AD, 3,6-DDPhCz-AD and 3,6-DDtBuPhCz-AD), which effectively restricted intramolecular relaxation and produced narrow full widths at half maximum of ∼55 nm. By extending the π-skeleton of the carbazole donor by tuning the peripheral groups on the carbazole ring to slightly increase the donor strength, both the energy splittings between the S₁ (¹CT) and T₁ (³LE) states and the T₁ and T₂ (³CT) states were gradually reduced, which facilitated the multichannel reverse intersystem crossing (RISC) and realized high k_RISC values of 10⁵ s⁻¹ for 3,6-DDPhCz-AD and 3,6-DDtBuPhCz-AD. At the same time, the extended transition dipole moment along with high molecular rigidity led to an extremely high radiative transition rate constant k_R of 10⁸ s⁻¹. 3,6-DDPhCz-AD and 3,6-DDtBuPhCz-AD exhibited external quantum efficiencies of 17.4% and 17.3% in doped organic light-emitting diodes (OLEDs) with CIE coordinates of (0.15, 0.11) and (0.15, 0.13), respectively. Tuning the peripheral groups on carbazole, even without changing donor distortion, proved to be a practical strategy for enhancing TADF efficiencies while maintaining color purity.