## 3 Results and Discussion
### Absorption and Fluorescent Properties of Synthesized Dyes
This effect has been reported in the literature and highlights the importance of considering the impact of the length and isomerism of the alkyl substituents on the fluorescence properties of the dyes [17; 18; 19; 20; 21; 22; 23]. In terms of the torsion angles, as illustrated in Figure 1, **SBnBe** and **SB2 Bu** are more distorted with respect to the stilbene moiety and exhibit higher fluorescent efficiency than the planar analogue **SBnHe**. The most striking results were obtained in the dye **SBnAc** containing \(N\)-acetyl groups. It possesses greater luminescence efficiency than that of corresponding **BBS**, both in dilute and concentrated solution. In dilute solution, **SBnAc** shows a quantum yield of 0.93 compared to 0.86 for BBS. In concentrated solution, the difference in efficiency becomes much larger (0.90 for **SBnAc**, 0.17 for BBS). The enhanced luminous efficiency of **SBnAc** could be attributable to the highly twisted structure throughout the molecule, as previously described in the discussion part of geometry optimization. As was mentioned in the introduction, the large decrease in quantum yield of BBS in the concentrated solution is likely caused by self-quenching, accompanied by red-shifted emission, as shown in Figure 4 and Table 1[24], where the quantum yield of BBS was reduced from 0.86 in dilute solution to 0.17 in concentrated solution. It can be assumed that BBS readily undergoes ACQ process leading to significant self-quenching and red shifting of fluorescence owing to energy being transferred to the other dye molecules rather than being emitted as a luminescence. Extent and type of aggregation can significantly change the luminescence behavior of molecules, as shown in Figure 4. BBS molecules exhibit a red-shifted emission spectrum in the aggregated state with head-to-tail stacking with slip stack of C-H-\(\cdots\); as has been quantum-simulated in the literature, it seems to be a form of J-aggregation type [5]. A similar red shift was reported in the PVDF film with the higher wt % of BBS [25]. It was also reported that the aggregated dye molecules generate a new excitonic band by energy splitting, leading to a red shift in both absorption and emission spectra with respect to the monomeric analogue [26; 27]. In contrast, aggregation-induced self-quenching occurred marginally for the synthesized dyes depending on the dye concentrations, thus very small reductions in quantum yields for the six dyes were observed (0.01-0.06) in the concentrated solution in comparison with that in the dilute solution. Therefore, it can be  
Figure 3: Photographs of synthesized dyes in dilute solution (toluene) under a UV light of 365 nm. emphasized that the inherent drawback of BBS molecule can be overcome sufficiently by the introduction of further substituents in the phenyl rings, particularly for application areas requiring high concentration of BBS-based fluorescent dyes. The normalized emission spectra of the six fluorescent dyes are illustrated in Figure 5.  
Figure 4: Red-shifted emission wavelength of the BBS in (A) dilute solution (1.0 \(\times\) 10\({}^{-5}\) mol/L). (B) In concentrated solution (1.0 \(\times\) 10\({}^{-3}\) mol/L). Figure 5: Normalized emission spectra of the synthesized dyes.