## 3 Results and Discussion
### Absorption and Fluorescent Properties of Synthesized Dyes
All of the \(N\)-substituted dyes exhibit larger Stokes' shifts in the range of 94 nm-146 nm compared to that of BBS (56 nm); thus, all the fluorescent dyes prepared emitted either a neutral blue or a greenish-blue fluorescence, as shown in Figure 3. The smallest Stokes shift among the set of dyes was that of **SB2 Bu**, which contains 2-butylamino substituents providing a comparatively higher energy gap between S\({}_{1}\) and S\({}_{0}\) states after a vibrational relaxation which may be caused by its highly twisted bond angle between phenyl ring and ethylene group, as shown in Scheme 2. In contrast, **SBnPy** containing pyrrolinyl groups gave rise to the largest Stokes's shift of 146 nm that was unusually larger than that of analogue **SB2 Bu**. The resulting shift is presumably a consequence of the greater twist bond angle (35\({}^{\circ}\)) between benzoxazole and phenyl systems, leading to enhanced vibration relaxation from S\({}_{1}\) state. The other three dyes **SBnBu**, **SBnHe**, and **SBnBe** showed very similar values in Stokes' shift, 108 nm, 108 nm and 105 nm, respectively, caused by their greater planarity, as seen in Scheme 2. It is of note that the unsubstituted dye **BBS** exhibits the smallest Stokes' shift of 56 nm in this series, which may be attributable to its planar structure. Five of the six synthesized dyes exhibit lower quantum yields than that of the unsubstituted dye (BBS) in dilute solution, as shown in Table 1 and Figure 3. **SBuAc**, however, does not. This observation may be associated with the alkyl or cyclic chains exerting a greater \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Dyes** & \(\lambda_{\text{max}}\)**\(\mathbf{{}^{ab}}\)** & \(\lambda_{\text{max}}\)**\(\mathbf{{}^{em}}\)** & **Stokes’ Shift** & \(\Phi_{\text{f}}\)**\(\mathbf{{}^{a}}\)** & \(\Phi_{\text{f}}\)**\(\mathbf{{}^{b}}\)** \\  & **(nm)** & **(nm)** & **(nm)** & **(nm)** & **\\ \hline
**BBS** & 375 & 431 & 56 & 0.86 & 0.17 \\
**Intermediate 1** & 410 & 513 & 103 & 0.26 & 0.26 \\
**Intermediate 2** & 353 & 472 & 119 & 0.28 & 0.11 \\
**SBnBu** & 350 & 458 & 108 & 0.68 & 0.66 \\
**SBnHe** & 352 & 460 & 108 & 0.36 & 0.30 \\
**SBnBe** & 360 & 465 & 105 & 0.66 & 0.61 \\
**SBnPy** & 325 & 471 & 146 & 0.18 & 0.15 \\
**SB2 Bu** & 351 & 447 & 94 & 0.72 & 0.71 \\
**SBnAc** & 320 & 435 & 115 & 0.93 & 0.90 \\ \hline \hline \end{tabular} \(\lambda_{\text{max}}\)**\(\mathbf{{}^{ab}}\)**, \(\lambda_{\text{max}}\)**\(\mathbf{{}^{em}}\)**: determined in toluene (1.0 \(\times\) 10\({}^{-5}\) mol/L). Quantum yield reference: Anthracene (\(\Phi\) = 0.27). \(\Phi_{\text{f}}\)**\(\mathbf{{}^{a}}\)** =quantum yield in dilute solution (1.0 \(\times\) 10\({}^{-5}\)mol/L). \(\Phi_{\text{f}}\)**\(\mathbf{{}^{b}}\)** = quantum yield in concentrated solution (1.0 \(\times\) 10\({}^{-3}\)mol /L).  
\end{table} 
Table 1: Absorption and fluorescent properties of the synthesized dyes. effect on non-radiative channels than on radiative emission. It also may be that the longer alkylamino chains introduced to the dye **SBnHe** enhanced the probability of non-radiative processes, such as a vibrational relaxation and an intersystem crossing, which may be due to the greater molecular motions and interactions with the surrounding environments. As a result of these processes, the efficiency of fluorescent emission decreased, leading to a reduction in the quantum yields of the dyes prepared. The lowest fluorescent efficiency (quantum yield 0.18 or 0.15) was detected with the dye **SBnPy** containing pyrrolinyl groups, which can be associated with the increase in the non-radiative channel caused by the flexible alicyclic rings interacting with the rest of the dye molecule. Furthermore, it was observed that the dyes substituted by a shorter butyl group (**SBnBu**, **SB2 Bu**) or aromatic substituent (**SBnBe**) exhibited much higher quantum yields.