Isomeric Bright Sky-Blue TADF Emitters Based on Bisacridine Decorated DBNA: Impact of Donor Locations on Luminescent and Electroluminescent Properties

Three isomeric boron-containing thermally activated delayed fluorescent (TADF) emitters, namely m-AC-DBNA, p-AC-DBNA, and m′-AC-DBNA, are constructed by incorporating an electron-donor acridine (AC) moiety into meta-, para-, or meta′-positions of an electron-accepting boron-embedded rigid framework. The substitutional positions are found to dramatically affect thermal, photophysical, and electroluminescent (EL) properties. The experimental results show that the para-substituted compound (p-AC-DBNA) exhibits higher decomposition temperature, higher photoluminescence (PL) quantum efficiencies, smaller singlet–triplet energy splitting, shorter delayed fluorescence lifetimes as well as a fast reverse intersystem crossing rate of over 10⁶ s⁻¹, compared to the meta-isomers (m-AC-DBNA and m′-AC-DBNA). Bright and highly efficient organic light-emitting diodes (OLEDs) with external quantum efficiencies (EQEs) up to 20.5% and 14.1% are achieved by employing p-AC-DBNA as doped and nondoped emitters in sky-blue OLEDs, respectively. Moreover, excellent doping-concentration independent EL properties and very low efficiency roll-off at a high luminance are achieved. This isomeric strategy provides a simple method to extend structural diversity of highly efficient TADF emitters, optimize optoelectronic properties, and demonstrate the relationship of delayed fluorescence lifetime and efficiency roll-off of the TADF devices. The three isomers also display distinct temperature-dependent emission and mechanochromism.