## 1 Introduction  
Triarylmethanes (TRAMs), diarylalkanes (DIAAs), diphenylmethane (DPM), and their derivatives have been widely used as key intermediates in synthesis, medicine, pharmaceutical, fine chemicals, and industrial chemistry. TRAMs and DIAAs are also used as photochromic agents, dyes, and building blocks for dendrimers. More interestingly, ring-hydroxylated TRAMs have been also reported to exhibit significant antioxidant, antitumor, antitubercular, and anti-inflammatory activities [1; 2; 3]. Due to their interesting chemical properties, both TRAM and DIAAs synthesis processes have been receiving increasing attention. Traditionally, they are industrially prepared via a Friedel-Crafts alkylation reaction of electron-rich aren with benzyl halide using conventional homogeneous Lewis acids such as AlCl\({}_{3}\)[4], FeCl\({}_{3}\)[5], ZnCl\({}_{2}\)[6], BF\({}_{3}\)[7], H\({}_{2}\)SO\({}_{4}\)[8], or their derivatives [9; 10] as catalysts in the liquid phase. However, these homogeneous acidic catalysts have a number of inherent shortcomings, including causing equipment corrosion and separation difficulties, being non-reusable and having low regioselectivity, as well as being environmentally hazardous and producing a large amount of acidic liquid waste after the reaction [11; 12]. Most acidic catalysts are also expensive and must be used with volatile organic solvents. Therefore, it is very important to develop efficient, green, cost-effective, non-volatile catalysts with high application feasibility. Ionic liquids (ILs) have been shown to be very promising green solvents with a melting point below 100 \({}^{\circ}\)C and excellent properties including extremely low volatility, a wide liquid temperature range, high thermal stability, and extraordinary solubility with many organic compounds [13; 14]. Due to these excellent properties, ILs have been applied as solvents and catalysts for organic reactions such as Friedel-Crafts reactions. In 1998, the chloroaluminate IL, [EMIM]Cl-AlCl\({}_{3}\), was firstly employed to catalyze the Friedel-Crafts reactions, and showed good yields close to 90% at a reaction temperature of 20 \({}^{\circ}\)C [15]. Although chloroaluminate IL with high acidic catalytic activity successfully played a good solvent-catalyst role, it has some serious limitations such as its oxyphilic property, which easily results in the formation of adducts with C-O functionalities and forming HCl and Al oxides irreversibly with traces of water [16]. To avoid this disadvantage, ILs were used as solvent only for immobilizing the metal triflate as Lewis acidic catalysts to catalyze Friedel-Crafts reactions, and these studies showed good product yields close to 90% under reaction temperatures higher than 80 \({}^{\circ}\)C [17; 18; 19]. Although metal triflate/ILs catalysts exhibited good catalytic activity, they gave low product yields in a recyclable test due to the non-uniform dispersion of metal triflate immobilized to ILs. Therefore, designing ILs without chloroaluminate systems and with solvent-catalyst properties to improve the above disadvantages is important. Bronsted acidic ionic liquids (BAILs) previously reported in the literature also exhibit the above-mentioned advantages of ILs when compared with traditional Bronsted acidic catalysts [20; 21]. Recently, many organic synthesis studies have been focused on applying strong BAILs as a dual solvent-catalyst for reactions such as esterification [22; 23], Fisher indole synthesis [24; 25], Mannich-type reactions [26], dehydration [27], and Beckmann rearrangement [28]. Moreover, the Friedel-Crafts alkylation that typically requires Lewis acidic catalysts has been also catalyzed by BAILs in recent years [29; 30; 31; 32; 33; 34].