## 1 Introduction  
Organic light-emitting diodes (OLEDs) have become an interesting component in research and commercial markets. OLEDs show prominent advantages over traditional displays, namely their high contrast, high brightness, wide view angle, lack of back light requirements, light weight and thin films. OLEDs continue to be used for next-generation displays in mobile phones, televisions and other lighting resources, where they help to reduce the energy consumption [1; 2; 3; 4; 5; 6]. First-generation OLEDs are known as fluorescence OLEDs, and these were able to contribute only 25% of internal quantum efficiency while losing 75% in a non-radiative manner [7; 8; 9]. Later, phosphorescent OLEDs with metal and ligand complexes were identified as second-generation emitters in OLEDs. Within these, iridium and platinum-based heavy metal complexes were utilized to obtain triplet and singlet energies, and they showed an achievable internal quantum efficiency of 100% [10; 11; 12; 13; 14; 15]. Such maximal internal quantum efficiency was able to be reach through the intersystem crossing mechanism (ISC). This process requires the heavy metal-based phosphorescent emitter to be doped with a suitable host material with an appropriate doping concentration [16; 17; 18; 19; 20; 21; 22; 23; 24]. This helps to reduce the quenching process, such as aggregation-caused quenching (ACQ) and triplet-triplet annihilation (TTA) [25; 26; 27; 28; 29]. Third-generation OLEDs are metal-free fluorescent emitters that use suitable donor and acceptor molecules as building blocks. These type of emitters are known as thermally activated delayed fluorescence (TADF) emitters, and they work based on the reverse intersystem crossing (RISC) mechanism [30; 31; 32; 33; 34]. Both phosphorescent and TADF OLEDs need an efficient host material to enhance device efficiencies [35; 36]. There are three types of host materials that have been reported, namely hole transport (HT) type, electron transport (ET) type, and bipolar host materials. HT type host materials are made of electron-donating triphenylamine, carbazole and acridine derivatives. 4,4'-bis(\(N\)-carbazolyl)-1,1'-biphenyl (CBP), 1,3-bis(N-carbazolyl)benzene (mCP), and 4,4'-cyclohexyl denebis[\(N\),\(N\)-bis(4-methylphenyl)benzene] (TAPC) are well-known HT type host materials, but they do not show stable thermal properties. Tris(8-hydroxyquinoline)aluminum (Alq) and Bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) are ET type host materials that are used in phosphorescent and TADF OLEDs [37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49]. Host materials are essential energy suppliers to the dopant material that also prevent energy flow back from the dopant. Host materials should possess a higher triplet energy than that of the dopant material. At the same time, host materials should have suitable frontier molecular orbital energies (FMOs) to ensure effective carrier transportation from the adjacent layers [50; 51; 52]. Bipolar host materials have received much attention from research communities due to their bipolar nature. Bipolar host materials contain electron-donating and electron-accepting units in a single molecule. Moreover, bipolar host materials can provide proper carrier balance while supporting an effective carrier recombination at the emission layer [53; 54; 55; 56; 57]. Mostly, bipolar materials can be utilized for host materials as well as fluorescent emitters. In the present study, we designed and synthesized two new types of bipolar materials, namely \(N^{1}\)-(9,9-diphenyl-9\(H\)-fluoren-2-yl)-\(N^{1}\)-(4,6-diphenylpyrimidine-2-yl)-\(N^{4}\)-\(N^{4}\)-diphenylbenzene-1,4-diami ne (FLU-TPA/YR) and \(N^{1}\)-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-\(N^{1}\)-(9,9-diphenyl-9\(H\)-fluoren-2-yl) -\(N^{4}\),\(N^{4}\) diphenylbenzene-1,4-diamine (FLU-TPA/TRZ). These two new materials have an \(N^{1}\)-(9,9-diphenyl-9\(H\)-fluoren-2-yl)-\(N^{4}\),\(N^{4}\)-diphenylbenzene-1,4-diamine donor and a triazine or pyrimidine acceptor incorporated into them.