Organic materials exhibiting thermally activated delayed fluorescence (TADF) have attracted much attention due to their enhanced efficiency in organic light emitting diodes (OLEDs) which is possible due to harvesting triplet excitons [1]. Red emitting materials are always the short slab among the materials exhibiting emission of three-primary colours, due to the anabatic internal conversion (IC) coefficient managed by the energy gap law [2]. Red phosphor materials are widely used in the industry at present owing to the 100% exciton utilization efficiency (EUE). The introduction of precious metals, such as Ir, Pt, etc., increase the costs and phosphorescent emitters are not eco-friendly [3]. OLEDs based on metal-free TADF materials can also achieve 100% internal quantum efficiency by reverse intersystem crossing (RISC) process of triplet excitons. High photoluminescence (PL) quantum yield (PLQY) and small energy splitting ($\Delta E_{st}$) between the first excited singlet state (S1) and excited triplet state (T1) are crucial factors for high-performance TADF materials [3]. New efficient orange-red TADF emitters are highly demanded to fill in the gap. The slow development of efficient orange-to-red TADF emitters is associated with the numerous strict molecular design considerations and corresponding difficulties. Naphthalimide acceptor-donor molecular design is suitable for the development of orange-red TADF emitters.
In this work we synthesized four emitters, which were obtained by the reactions of diphenylamine phenyl boronic acid and vinyl phenyl diphenylamine with 6,7-dibromo2-(2,6-dimethylphenyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione and 5,8-dibromo-2-(2,6-dimethylphenyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione. The yields of the compounds ranged from 52 to 70%. Glass transition temperatures of the synthesized glass-forming compounds were observed in the range of 132-156 °C. Ionization potential values of the compounds ranged from 5.38 to 5.69 eV (Fig. 1a) The PL spectra of THF solutions and films showed single peaks located at wavelengths varying from 600 to 650 nm (Fig. 1b). The positive solvatochromism indicates the intramolecular charge transfer character of the emission. The PL decay curves showed the presence of a long-lived emission components which were almost not influenced by the interaction with oxygen. The emission of the compounds underwent aggregation caused quenching. PLQY values of their dilute solutions ranged from 43 to 75% while those of the solid films ranged from 1 to 25%.
{{FIGURE_1a & 1b}}