## 3 Results and Discussions  
The thermal properties of FLU-TPA/PYR and FLU-TPA/TRZ were studied with thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) under a nitrogen atmosphere. The glass transition temperatures of FLU-TPA/PYR and FLU-TPA/TRZ were 153 and 147 \({}^{\circ}\)C, respectively. The thermal decompositions of our FLU-TPA/PYR and FLU-TPA/TRZ were recorded at approximately 426 and 478 \({}^{\circ}\)C, respectively, with a 5% weight reduction. Both of our materials exhibited excellent thermal stabilities, which could improve the morphological stabilities during device operation. Thermal properties are shown in Figure 1 and summarized in Table 1.  
The UV-Vis absorption and photoluminescence (PL) spectra of FLU-TPA/PYR and FLU-TPA/TRZ are depicted in Figure 2. FLU-TPA/PYR and FLU-TPA/TRZ showed similar absorption at 410 and 412 nm, respectively. The band gap energy values of FLU-TPA/PYR and FLU-TPA/TRZ were recorded at 2.41 and 2.49 eV, and they were associated with onset absorption wavelengths of 515 and 498 nm, respectively. FLU-TPA/PYR showed an extended absorption when compared to FLU-TPA/TRZ. The photoluminescence spectra's maxima were found at 548 and 606 nm, respectively, in the toluene solvent for FLU-TPA/PYR and FLU-TPA/TRZ. Triazine acceptor-based FLU-TPA/TRZ expressed a longer emission peak at room temperature. The triplet energies of FLU-TPA/PYR and FLU-TPA/TRZ were obtained as 2.57 and 2.77 eV, respectively, and this is highly important for a host material to \begin{table}
\begin{tabular}{c c c} \hline \hline
**Bipolar Materials** & **FLU-TPA/PYR** & **FLU-TPA/TRZ** \\ \hline T\({}_{\mathrm{g}}\)(\({}^{\circ}\)C) & 153 & 147 \\ T\({}_{\mathrm{d}}\)(\({}^{\circ}\)C) & 426 & 478 \\ UV-Vis (nm) & 410 & 412 \\ PL (nm) & 548 & 606 \\ Band gap (eV) & 2.41 & 2.49 \\ Triplet energy (eV) & 2.57 & 2.77 \\ Highest occupied molecular orbital (HOMO) (eV) & 5.27 & 5.35 \\ Lowest occupied molecular orbital (LUMO) (eV) & 2.86 & 2.86 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Physical properties of FLU-TPA/PYR and FLU-TPA/TRZ. Figure 1: Differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) of FLU-TPA/PYR and FLU-TPA/TRZ. prevent energy flow back from the dopant material. From this information, we believed that our materials would reveal good efficiency enhancement with yellow phosphorescent OLEDs. The HOMO (highest occupied molecular orbital) energy ere were calculated, and they were found to be \(-5.27\) and \(5.35\) eV for FLU-TPA/PYR and FLU-TPA/TRZ, respectively. The LUMO (lowest unoccupied molecular orbital) energy values were obtained by adding the bang gap energy to the HOMO energy, and they were \(-2.86\) eV for both FLU-TPA/PYR and FLU-TPA/TRZ. The frontier molecular orbital energies (FMOs) facilitated an effective charge transportation from both the electrodes. To understand the molecular orbital distribution, we used basic density functional theory (DFT) with a basic set of 6-31 G while using a Gaussian 09. The frontier molecular orbital distributions are displayed in Figure 3. The LUMO distribution was noticed over the electron-withdrawing triazine and pyrimidine moieties, while the HOMO distribution was noticed over the electron-donating triphenylamine unit, but the HOMO distribution showed considerable overlapping. To establish the electroluminescent properties of our bipolar host materials FLU-TPA/PYR and FLU-TPA/TRZ, yellow phosphorescent OLED devices were fabricated with a PO-O1 yellow dopant (Figure 4). The yellow phosphorescent device structure was as follows: indium-tin oxide (ITO) (150 nm)/1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HATCN) (7 nm)/4,4'-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzeneamine](TAPC) (43 nm)/FLU-TPA/PYR or FLU-TPA/TRZ: 10 wt% iridium(III) bis(4-(4-_tert_-butylphenyl)thieno[3,2-c]pyridinato-N,C2') acetylacetate (PO-O1) (20 nm)/1,3,5-tri(m-pyridin-3-ylphenyl)benzene (TmPyPB) (35 nm)/8-quinolinolatto lithium (Liq) (1.5 nm)/aluminum (Al) (100 nm).