# Aromatic Amines in Organic Synthesis Part III; \(p\)-Aminocinnamic Acids and Their Methyl Esters
## 2 Results and Discussion
On the other hand, methylation of acid 2l has no obvious effect on the NMR spectrum, except for the disappearance of the proton signal of the carboxyl group (11.9 ppm) and the appearance of the proton signal of the methyl group (3.7 ppm). Since the synthesized compounds can be used to obtain different groups of dyes with extended conjugated double-bond systems, their basic spectroscopic properties were investigated. The physicochemical data are presented in Table 1. The measurements were carried out in three solvents with significantly different properties, i.e., in 1,4-dioxane (1,4-Dx), methyl acetate (EtOAc), and methanol (MeOH). The UV-Vis spectra of the tested \(p\)-aminocinnamic acids indicate the presence of the main absorption band in the range 332-375 nm in 1,4-dioxane, 329-374 nm in ethyl acetate, and 330-382 nm in methanol. This band is related to the \(\pi\)\(\rightarrow\)\(\pi^{4}\) transition. The compound 2m shows the highest bathochromic shift with an absorption maximum of 374 nm for ethyl acetate and 382 nm for methanol. This is attributed to the stiffening of the alkylamino group, which facilitates the delocalization of electrons throughout the molecule. The opposite effect is observed for compound 2f. It has the most blue-shifted absorption band among the analyzed acids (absorption maximum at 329 nm in ethyl acetate and 330 nm in methanol). In this case, the presence of the methyl group in the _ortho_ position with respect to the amino group forces the twisting of the amino group with respect to the phenyl ring, which makes it difficult to delocalize the electrons of the amino group. The molar extinction coefficients of the \(p\)-aminocinnamic acids are in the range from 17,200 M\({}^{-1}\)cm\({}^{-1}\) and 16,500 M\({}^{-1}\)cm\({}^{-1}\) to 41,800 M\({}^{-1}\)cm\({}^{-1}\) and 32,400 M\({}^{-1}\)cm\({}^{-1}\) in EtOAc and MeOH, respectively. These values are similar to analogous benzoic acids [39] but, at the same time, are about 30% lower than cinnamaldehydes [40]. Figure 3 shows an example of normalized electronic absorption and emission spectra in methanol for selected \(p\)-aminocinanamic acids (2a, 2f, 2n). The absorption and fluorescence spectra of all synthesized acids and esters in 1,4-dioxane, ethyl acetate, and methanol are presented in the Supplementary File. Figure 2: \({}^{1}\)H NMR spectra of 1-methyl-1,2,3,4-tetrahydrodroquinoline-6-carboxylic acid, 3-(1-methyl-1,2,3,4-tetrahydrodroquinoline-6-yl)acrylic acid (2l), and methyl 3-(1-methyl-1,2,3,4-tetrahydrodroquinoline-6-yl)acrylate (3l) in DMSO-\(d_{6}\). The fluorescence spectra of the tested acids show a sharp band in the range of 413-464 nm in 1,4-dioxane, 426-478 nm in EtOAc, and 437-508 nm in MeOH (see Figures S10-S12 in Supplementary File). While the bathochromic shift of the absorption spectra with increasing solvent polarity is insignificant and amounts to a maximum of several nm, the shift of the fluorescence spectra is even more than 40 nm. This indicates a better stabilization of the excited state of the compounds in a more polar environment and suggests an increase in the dipole moment upon excitation. By comparing the spectral properties of the cinnamic acids (Table 1) with the corresponding benzoic acids [39], we can assess the effect of an additional vinyl group on the  
Figure 3: Normalized electronic absorption and emission spectra of selected aminocinnamic acids in methanol. absorption properties of these compounds. The electronic absorption spectra of the tested cinnamic acids, compared to the corresponding benzoic acids, are redshifted by about 40-55 nm. Only in the case of acids 2f and 2o is the shift smaller and equals about 30 nm regardless of the solvent polarity. Comparing the spectra of cinnamic acids to analogous cinnamic aldehydes [40], a shift towards shorter wavelengths by 16-23 nm in ethyl acetate and 27-36 nm in methanol is observed. The fluorescence spectra are also blue-shifted by 11-21 nm in both solvents used.