## 3 Results and Discussion  
### Synthesis, Spectral, and Structural Analysis of Styryl Quinolinium (SQ)  
N-alkyl quaternary ammonium compounds are commonly used as precursors for photochromic spiropyrans [33], various cyanine dyes [34], and RNA antagonists [35]. Here, we describe the two-step synthesis of SQ as follows: (1) 4-methylquinoline (lepidine) (**1**) is alkylated with methyl toluenesulfonate (**2**) to get the quaternary salt (**3**); and (2) the resulting lepidinium salt (**3**) is then applied in Knoevenagel condensation with 2-hydroxy naphthaldehyde (**4**) (Scheme 1). Dry toluene was used to alkylate equimolar quantities of lepidine and methyl 4-methylbenzenesulfonate. The solvent was chosen due to the easiness of crystallization and separation of the product. Lepidine has a very good solubility in toluene, but lepidinium methyl tosylate as a salt does not dissolve in toluene. Thus, the quaternary salt was obtained in 84% yield. The obtained salt (**3**) then was directly applied in Knoevenagel condensation with 2-hydroxy naphthaldehyde (**4**) to the SQ (**5**) formation. The SQ salt was obtained in high purity and yield.  
The obtained SQ (**5**) was slightly soluble in non-polar and well soluble in polar organic solvents. The emission spectra of SQ in four solvents--water, methanol, acetonitrile, and chloroform--were measured. The comparative ultraviolet-visible and fluorescent spectra of the SQ (**5**) under study at the same concentration of the solutions are shown in Figure S1 (Supplementary Materials Figure S1).  
The fluorescence spectra in the solvents chloroform, acetonitrile, and methanol were normal with a maximum in the range of 595-615 nm (Figure S1B) and were a mirror image of their absorption spectra (Figure S1A). This normality implies that the molecule was behaving consistently across these solvents, and there were no unexpected solvent-dependent changes or anomalies. In the polar water solvent, the emission spectrum had a discrete structure. When the emission spectrum has a discrete structure, it displays distinct, sharp lines or peaks instead of a wide, continuous band. This happens because the polar environment influences the electronic states of the dissolved molecules. Therefore, the solvent molecules stabilize certain electronic states of the solute. The calculated Stokes offset according to the type of solvent used was from 126 nm to 156 nm. The infrared spectra of SQ (**5**) offer the opportunity to study solid-phase effects, such as Evans' holes and Davydov's splitting of salt-like bands [36].  
It is characteristic of the IR spectra of charge-transfer compounds that almost all oscillations are mixed. The reason for this phenomenon is that the charge in the cationic part of the SQ is strongly delocalized, as a result of which an electronic and vibrational interaction occurs. The result of this interaction is a significant shift of the bands belonging to the corresponding normal oscillations to the lower wavenumbers and an increase in the intensity of some of them [37; 38]. The IR and Raman spectra of SQ (**5**) are shown in Figure S2 (Supplementary Materials Figure S2). The NMR data of **5** are presented in Figures S3-S8 (Supplementary Materials Figures S3-S8). The methine protons of the styryl double bond ofthe compound resonate as a singlet for 2H at 8.59 ppm among the aromatic proton signals. The probable reason is the deviation of the two aromatic cores from the plane of the double bond. The MS spectra are presented in Figure S9 (Supplementary Materials Figure S9).