## 3 Results and Discussion  
### Design Concept of Dye Molecule and Synthesis of Dyes  
Although 4_A_\({}^{\prime}\)-bis(2-benzoxazolyl)stilbene (BBS) molecule has good thermal stability and exhibits remarkable optical properties, the compound possesses poor solubility in nonpolar organic solvents, owing to its high degree of symmetry and extensive conjugated system, leading to ready molecular stacking. For the same reason, quenching in the dye's solid state occurs readily. Therefore, all the novel dyes of this study were designed to introduce \(N\)-substituents that confer high solubility in organic solvents as well as to reduce the red-shift behavior that derived from the excimer formation in the solid state of BBS. In addition, the effects of a strong electron-withdrawing group (-NO\({}_{2}\)) and a strong electron-donating group (-NH\({}_{2}\)) on the spectral properties of dye molecules were compared. One of main purposes of this study was to improve quantum efficiency of fluorescence by introducing steric hindrance through \(N\)-substitution with bulky functions, such as an isobutyl group. In similar respect, to compare the effects of carbon length of \(N\)-alkyl groups, \(n\)-butyl and \(n\)-hexyl groups were introduced. In addition, the effect of cyclic functions, such as phenyl or pyrrolidine groups, on the spectral properties was investigated from the perspective of aromatic/non-aromatic substituents and torsional strain. Also studied was the impact of an \(N\)-acetyl group, introduced from the precursor containing amino groups to afford **SBnAc** dye with different hydrogen-bonding characteristics.  
In the preparation of dye **intermediate 1**, a nitration reaction was carried out under low temperature of 0-5 \({}^{\circ}\)C, resulting in reaction yield of 86.4%. As is the tendency with nitration reactions, higher temperatures caused the formation of larger amounts of side products, thereby dramatically decreasing yield.  
For \(N\)-alkylation reaction, the use of polar aprotic solvent, such as DMF, could enhance the reactivity of typical S\({}_{\text{N}}\)2 reaction [12]. In S\({}_{\text{N}}\)2 reactions between primary amines and alkyl1 halides, the formation of mono-alkylated products is more favorable than that of di-alkylated side-products; however, di-alkylation is inevitable. Thus, to increase selectivity toward generation of mono-alkylated compounds, the control of reaction rate was necessary in the reaction stage, resulting in reaction yield range of 48-56%. Two alkylated analogues (**SBnBu** and **SB2 Bu**) were selectively prepared from corresponding alkyl halide (1-bromobutane, 2-bromobutane), whereas the cyclic group substituted analogues (**SBnBe** and **SBnPy**) were prepared from benzyl bromide and 1,4-dibromobutane.  
The **SBnBu**, **SBnHe**, and **SB2 Bu** dyes contain hydrophobic \(N\)-alkyl groups located symmetrically on the central phenyl rings of the stilbene motif, thus providing solubility. In addition, it was intended that these substituents introduce steric strain to the ring of the stilbene and benzoxazole group with the aim of reducing self-quenching as well as improving solubility in organic solvents. Of special interest, in the case of **SB2 Bu** which contains bulky groups compared to that of **SBnBu**, were the effects of different bulkiness with the same carbon number.