# Aromatic Amines in Organic Synthesis Part III; \(p\)-Aminocinnamic Acids and Their Methyl Esters
## 2 Results and Discussion
These results show that the naphthalene ring separating the electron-donating and the electron-withdrawing groups caused a decrease in the energy level of the highest occupied molecular orbital (HOMO) and an increase in the lowest unoccupied molecular orbital (LUMO). Thus, the HOMO-LUMO energy gap of 2a is higher than that of 2o. As a result, the electron transfer from HOMO to LUMO in 2o is relatively easier than in 2a. Therefore, compared to 2a, a bathochromic shift is observed in the electronic absorption spectrum of 2o. Figure 4 shows the absorption and fluorescence spectra of 2a and 2o in 1,4-dioxane and DMF at 293 K. It can be seen that changing the solvent from nonpolar 1,4-dioxane to polar DMF almost does not affect the position and shape of the absorption spectra. In contrast, the fluorescence maxima of the compounds in DMF significantly redshift compared to those in 1,4-Dx (Figure 4), resulting in larger Stokes shifts in the polar solvent (Tables 1 and 2). This effect is greater for the naphthalene derivative than for the benzene one. Based on the literature data, the redshifts in the fluorescence maxima of the aminocinnamic acids in polar solvents may result from a bulk relaxation of the solvent dipoles about the solute's excited-state dipole moment with some contribution of an intramolecular reorganization of the amino group [48]. The excited state lifetimes were determined by deconvolution of the fluorescence decay curves recorded using a single photon counting method, which are shown in Figure 5.  
The fluorescence lifetime (\(\tau^{\text{fl}}\)) and fluorescence quantum yield (\(\phi_{\text{fl}}\)) of 3-[6-(dimethylamino) naphthalene naphthalene-2 yl]acrylic acid (2o) and 4-dimethylaminocinnamic acid (2a) are listed in Table 2. The values of \(\tau^{\text{fl}}\) and \(\phi_{\text{f}}\) are found to be affected by the number of aromatic rings separating the electron-donating and electron-withdrawing substituents and their position, as well as solvent polarity. In the three solvents tested (DMF, EtOAc, and 1,4-Dx), 2a gives smaller \(\tau^{\text{fl}}\) values than those of 2a. In particular, decreases in average \(\tau^{\text{fl}}\) can be recognized for them in the nonpolar solvent (1,4-Dx), and the \(\phi_{\text{f}}\) values of these compounds become smaller. Since both compounds have the same substituents, the remarkable increase in \(\tau^{\text{fl}}\) and \(\phi_{\text{f}}\) is limited to the aromatic \(\pi\) system, i.e., the presence of a naphthalene ring substituted by a dimethylamino group in the two position, which is responsible for the planar structure of compound 2o even in the ground state. This plays an important role in the promotion of the radiative deactivation processes of its excited state. The radiative rate constants (\(k_{r}=\phi_{\rm fl}\ \tau^{-1}\)) for the two compounds in three solvents of different polarities are collected in Table 2. The values of \(k_{r}\) for 2o are higher than the values for 2a. The dominance of radiative processes in the deactivation of the 2o excited state results in an increase in the fluorescence quantum yield. Moreover, the increase in fluorescence lifetime is mainly due to the decrease in non-radiative rate constants, internal conversion, and intersystem crossing in case 2o compared to 2a,. for which non-emissive processes dominate [49]. The oxidation potentials of the tested aminocinnamic acids were determined based on cyclic voltammetry. The measurements were performed to show the influence of the acid structure on the possibility of electron loss. Examples of cyclic voltammograms are presented in Figure 6, whereas the oxidation potentials for all compounds tested are summarized in Table 3.  
Figure 4: Electronic absorption (**A**) and normalized emission spectra (**B**) of 2a and 2o aminocinnamic acids in 1,4-dioxane and DMF. Figure 1:0 The fluorescence decay curves of compounds 2a and 2o recorded for 1,4-dioxane and DMF, respectively; \(\lambda_{\rm EX}\) = 735 nm, IRF—instrument response function.