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 kRISC 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 kR 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.
- This article is part of the themed collection: Photofunctional Materials and Transformations