## 3 Results and Discussion
### Absorption and Fluorescent Properties of Synthesized Dyes  
The six synthesized dyes are consistently hypsochromically shifted compared to the starting molecule (BBS) which contains no substituent. The introduction of two electron-donating groups, such as \(N\)-alkyl, anilino or pyrrolidine, readily destabilizes the LUMO by increasing the electron density in the LUMO, leading to an increased energy gap (\(\Delta\)E) between HOMO and LUMO. For instance, as in the Figure 2, the presence of \(n\)-hexyl substituents gives rise to a lower energy LUMO level (\(-2.2\) eV for **BBS**, \(-1.97\) eV for **SBnHe**); therefore, the calculated \(\Delta\)E increased from 3.45 eV to 3.71 eV, leading to the shorter wavelength absorption maximum of **SBnHe**. Table 1 shows that the introduction of an electron-withdrawing group (-NO\({}_{2}\)) onto phenyl ring affords a bathochromic shift in absorption maximum compared to BBS, whereas a hypsochromic shift is caused by the presence of an electron-donating group (-NH\({}_{2}\)). Therefore, the **intermediate 1** absorbs maximally at 410 nm, which constitutes a red shift of 35 nm from the absorption maximum of BBS, whereas **intermediate 2** exhibits an absorption maximum of 353 nm and a blue shift of 22 nm. As it was described previously, the absorption maxima were greatly affected by the electronic properties of additional substituents that could be introduced to the phenyl rings. Absorption maximum is thus greatly affected by the electronic properties of substituents attached to the phenyl rings of BBS. The three analogous dyes **SBnBu**, **SBnHe**, and **SB2 Bu** which contain \(N\)-alkyl groups exhibited very similar absorption maxima in the range of 350 nm and 352 nm; thus, it can be concluded that the carbon length or atom arrangement of \(N\)-alkyl substituent at the _ortho_-position of the carbon substituted by a benzoxazole ring did not affect the energy difference between HOMO-LUMO states. In contrast, the dyes containing cyclic groups, such as \(N\)-phenyl or pyrrolinyl groups, showed different characteristics. **SBnBe**  
Figure 2: Frontier orbital models of the 4,4\({}^{\prime}\)-bis(2-benzoxazolyl)stilbene and **SBnHe**. substituted by \(N\)-phenyl group exhibits an absorption maximum of 360 nm, a 15 nm hypysochromic shift with respect to BBS, whereas **SBnPy** substituted by pyrrolinyl groups absorbs maximally at 325 nm and is thus hypysochromically shifted by 50 nm compared to BBS. In the case of **SBnBe**, the increased \(\pi\)-conjugation between two phenyl rings via an -NH- linkage could minimize the hypysochromic shift leading to the compensation of the hypysochromic effect exerted by electron-donating groups [14]. For **SBnPy**, the combination of bulk and a lack of aromaticity of its pyrrolindo substituents therefore afforded a dramatic blue shift which is presumably a consequence of the highly twisted bond angle (35\({}^{\circ}\)) between benzoxazole and phenyl systems causing decreased \(\pi\)-conjugation leading to an additional energy difference between HOMO and LUMO. The absorption maxima of the two intermediates and six fluorescent dyes are summarized in Table 1. In the case of dye **SBnAc**, the absorption maximum was largely blue-shifted (33 nm) compared to that of corresponding **intermediate 2**. It can be assumed that the introduction of acetyl groups to the amino substituents should provide more electron withdrawing effect than the intermediate 2. However, the large twist angles (48\({}^{\circ}\)), as shown in Figure 1, may counteract the electronic effect to result in a bathochromic shift instead. In the six synthesized dyes, the longest emitter is **SBnPy** at 471 nm, whereas analogue dye **SB2 Bu** emitted at shortest wavelength of 447 nm, as shown in Table 1. Generally, the alkylamino-substituted dyes provide large Stokes shifts, because the presence of electron-donating alkylamino groups stabilizes the excited state of the molecule. The same properties were reported in case of pyrrolinyl or benzylamino groups substituted dyes [15; 16].