## 1 Introduction  
Organic light emitting diodes (OLEDs) have given dramatic development in display technology and printed flexible electronics because of their efficiency enhancement while showing advantages of lower power consumption, high brightness, and contrast. The general OLED structure consists of two electrodes, namely, an anode and cathode at both ends. There is a hole-injecting layer (HIL), hole-transporting layer (HTL), emission layer (EML), electron-transporting layer (ETL), electron-injecting layer (EIL) embedded between two electrodes, the anode and cathode, which is known as a multilayer OLED structure. Multilayer OLED devices have improved the quantum efficiency when compared to single-layer devices [1; 2; 3; 4]. Hole-transporting materials (HTM) play a major role to transporting holes from the anode to the emission layer and providing charge balance to prevent efficiency roll-off, since the hole transportation process faces critical issues, like thermal instability, lower hole mobility, and hole injection barrier [5; 6; 7]. Small molecular hole-transporting materials expressed crystallization features during long operation times, but they showed shallow HOMO energy levels as an advantage. To overcome those issues, there are many hole-transporting materials synthesized with different structural modifications. Arylamine, spiro derivatives, and carbazole-based molecular structures have been extensively used as hole-transporting materials for their excellent electron-donating capability [8; 9; 10; 11; 12; 13]. The most abundantly using HTMs, 4,4,4-tris(\(N\)-carbazolyl) triphenylamine (TCTA), \(N\),\(N^{\prime}\)-Di(1-naphthyl)-\(N\),\(N^{\prime}\)-diphenyl-(1,1\({}^{\prime}\)-biphenyl)-4,\(A^{\prime}\)-diamine (NPB) and \(N\),\(N^{\prime}\)-Bis(3-methylphenyl)-\(N\),\(N^{\prime}\)-diphenylbenzidine (TPD), have been communicated with higher quantum efficiencies. However, NPB and TPD materials revealed poor thermal and morphological stabilities, and TCTA expressed inferior hole mobility [14; 15; 16; 17]. The OLED device degradation mechanism is not fully studied, but several reports indicate that major morphological changes of the amorphous hole-transporting layer (HTL) [18; 19]. One of the largest obstacles in the development of highly efficient and stable phosphorescent organic light emitting diode (PhOLED) is the design and synthesis of effective hole-transporting materials, which should possess higher hole mobility with lower injection barrier and stable morphological properties. Especially, triphenylamine (TPA) and its derivatives received an astonishing achievement in many optoelectronic studies, like perovskite solar cells (PSCs), OLEDs, and organic field effect transistors (OFET), in the form of small molecules and oligomers. There are many advantages associated with TPA moieties, which are electron-donating, low ionization potential, high hole mobility, and have a stable morphology and thermal properties. Predominantly, electron contribution depends on nitrogen atoms in the TPA and TPA-based moieties in which the N atom can be oxidized and transport positive charges (holes) adequately. Moreover, TPA groups can further modify with different substituent groups to extend the conjugation length which enhances the electronic properties additionally [20; 21; 22; 23; 24; 25; 26; 27; 28; 29]. Acridine (ACR)-based molecules have been extensively used in thermally-activated delayed-fluorescence (TADF) emitters and host materials, as electron-donating units and hole-transporting materials in solar cells and OLED applications. Although ACR can be utilized as hole-transporting material in OLED applications due to the presence of electron-donating nitrogen atoms at the 10 th position in between two phenyl rings, a structure that resembles the TPA molecule, but with two phenyl rings of TPA connected via a methylene bridge to form a hexagonal ACR molecule. The ACR molecule is more rigid than the TPA molecule, which will be useful in HTM applications in OLEDs.