Aromatic Amines in Organic Synthesis Part III; p-Aminocinnamic Acids and Their Methyl Esters

Abstract

Fifteen amine derivatives of cinnamic acid were synthesized by reaction of the corresponding benzaldehydes and malonic acid. The selected acids were then converted into methyl esters. Three esterification methods were tested with (1) thionyl chloride in methanol, (2) sulfuric acid in methanol, and (3) dimethyl sulfate in acetone. The latter method turned out to be the best, both in terms of reaction efficiency and product purity. The chemical structure and purity of all the synthesized compounds were verified by elemental analysis, ¹H and ¹³C NMR, and IR spectroscopy. The cinnamic acids and their esters, thanks to an extensive system of conjugated double bonds compared to analogous benzoic acids, can be used to obtain dyes for various applications, including non-linear optics and optoelectronics. Therefore, their basic spectroscopic properties are presented as well.

1. Introduction

Chemical compounds containing an amino group (or its derivative) are widely used in many fields of science. They are intensively studied because the electron-donating properties of the amino group influence their biological activity and spectroscopic and electrochemical properties [1]. Particular attention is paid to organic compounds with amino groups in the preparation of dyes [2], spectroscopic probes [3,4,5], solar cells [6], polymerization photoinitiators [7,8], and drugs [9].

Our research group and others have investigated the influence of different amino groups on the spectroscopic and electron-donating properties of various classes of dyes, such as azomethine [10], stilbazolium [11], hemicyanine [12,13,14], styrylquinolinium [15], oxazolone [16,17], biphenyl [18], BODIPY [19,20,21,22,23], chalcone [24,25], phenothiazine [26,27], anthracene [28,29], and pyrazolone [30,31], for many years. The application of dyes in modern technologies based on multi-photon absorption, spectroscopic probes, and sensors requires compounds with an extended system of conjugated double bonds. Such compounds are synthesized using either p-aminocinnamic aldehydes or their esters instead of p-aminobenzaldehydes (or their esters).

The esters of p-aminocinnamic acid derivatives can be synthesized by applying several methods. The most common procedure is based on the general Knoevenagel reaction. In the first step, the corresponding cinnamic acid is prepared, which is then esterified [32,33]. The esters can also be obtained from the appropriate benzaldehydes [34] or benzoyl bromides [35] and iodides [36] using the Wittig or Heck reactions, respectively.

In this paper, we describe the synthesis of fifteen derivatives of p-aminocinnamic acids and show the optimal way to transform them into methyl esters. The esters of p-aminocinnamic acids were synthesized in a two-step reaction, starting with easily accessible benzaldehydes and applying the first of the above-mentioned methods. The aldehydes were synthesized according to the procedure given by Gawinecki et al. [37]. Only p-(dibutylamino)benzaldehyde was prepared by applying the methodology given by van den Berg and co-workers [38]. The basic spectral properties of the synthesized compounds in two solvents of different polarities (methanol and ethyl acetate) are also described.

2. Results and Discussion

The amine methyl esters of cinnamic acid derivatives were prepared according to the synthetic routes outlined in Figure 1.

In the first method (Method A), the Knoevenagel condensation was used, and the corresponding cinnamic acids were obtained from amine derivatives of benzoic or naphthoic aldehydes. Six of these acids (2c, 2d, 2e, 2f, 2g, and 2k) have not been described in the literature yet. The reaction yield of cinnamic acid synthesis varies between 58% and 88%. A significantly lower yield of 29% was obtained for the 4-(dimethylamino)-2,6-dimethylcinnamic acid, probably due to steric hindrance. The purity of the synthesized acids was very high, and they were used in the next stage without further purification.

In the second stage, selected cinnamic acids were converted into methyl esters. Four of these compounds (3h, 3i, 3l, 3n) are new and have not been described in the literature so far. In search of the most effective esterification method, the process was carried out using three different reagents, i.e., thionyl chloride in methanol (Method B), sulfuric acid in methanol (Method C), and dimethyl sulfate in acetone (Method D). All these methods proved to be relatively effective, with reaction yields of 65–93% and high-purity products. The best, both in terms of the ease of carrying out the reaction (room temperature, no volatile by-products) and the highest yield, was the reaction with dimethyl sulfate (Method D). The yields of the reaction were greater than 75%, and the obtained esters were of high purity.

The structure and purity of all the synthesized acids and esters were confirmed by spectroscopic methods, namely ¹H NMR, ¹³C NMR, and IR (see spectra in Supplementary File), and by elemental analysis. The NMR spectra showed all signals from protons and carbons present in the product molecules, as described in the Experimental part. As can be seen, the proton signal of the carboxyl group occurs at about 12 ppm, whereas the characteristic signals from the protons of the methine group are in the range of 8.3 to 5.9 ppm. The coupling constant of these protons (³J_H,H = 15.8 Hz) indicates the trans configuration. The signal from the carbon of the carboxyl group is in the range of 168–169 ppm and is shifted by about 1–2 ppm in relation to the analogous benzoic acids [39]. After the esterification, the proton signal of the carboxyl group disappears, while the methyl signal appears at approx. 3.7 ppm. The analysis of the IR spectra confirms the presence of the carboxyl group in the tested acids thanks to the characteristic, strong signal at frequencies from 1653 cm⁻¹ (2m) to 1704 cm⁻¹ (2j). In most cases, these values are 5–20 cm⁻¹ higher than for analogous benzoic acids [39]. After esterification, this signal shifts to the range of 1702–1716 cm⁻¹.

Figure 2 shows the ¹H NMR spectra of 3-(1-methyl-1,2,3,4-tetrahydroquinolin-6-yl)acrylic acid (2l) in DMSO-d₆, its methyl ester, and the analogous benzoic acid to illustrate the shift in signal position associated with the elongation of the π electron system. Comparing these three spectra, the position of the proton signal of the carboxyl group changes little. For both the benzoic acid and the cinnamic acid derivative, it is at approx. 11.9 ppm. The protons of the methine group give signals at 7.41 and 6.15 ppm, with a coupling constant of approximately 16 Hz, indicating a trans configuration. Moreover, there is a visible shift in the signals coming from the protons of the aromatic ring, located in the ortho position in relation to the methine group, by about 0.2–0.4 ppm towards lower δ values (they are shielded more strongly than in benzoic acid). 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. Absorption maxima wavelength (λ_{max abs}; nm), molar absorption coefficient (ε_max; 10³ M⁻¹cm⁻¹), fluorescence maxima wavelength (λ_{max fl}; nm), fluorescence quantum yield (ϕ_fl; %), and Stokes shift (Δν^St.; cm⁻¹) in ethyl acetate and methanol.

Abbr.	1,4-Dioxane				Ethyl Acetate					Methanol
	λmax abs	λmax fl	ϕfl	Δν	λmax abs	εmax	λmax fl	ϕfl	Δν	λmax abs	εmax	λmax fl	ϕfl	Δν
2a	353	414	0.84	4174	355	28.9	426	0.61	4695	356	26.9	461	0.69	6398
2b	363	418	0.96	3626	360	34.3	426	0.62	4304	366	31.2	462	1.29	5677
2c	366	418	1.15	3399	364	36.5	427	0.93	4053	367	29.7	465	1.75	5743
2d	359	438	0.52	5024	358	41.8	433	0.45	4838	365	33.9	463	0.67	5799
2e	352	413	0.16	4196	352	23.7	432	0.30	5261	357	22.3	437	0.17	5128
2f	332	438	3.48	7289	329	20.5	439	0.31	7616	330	18.4	454	0.64	8277
2g	338	443	1.58	7012	336	19.3	445	1.80	7290	336	16.5	459	0.84	7975
2h	362	420	0.89	3815	361	33.2	429	0.66	4391	361	32.4	463	1.07	6103
2i	351	420	1.47	4681	348	25.5	430	1.28	5480	352	24.9	467	1.62	6996
2j	341	416	1.21	5287	341	22.8	425	1.06	5796	341	27.5	466	1.67	7866
2k	360	444	1.69	5255	357	27.2	443	1.63	5438	365	22.5	467	1.22	5984
2l	365	439	1.44	4618	365	31.4	434	1.29	4356	371	25.6	465	1.36	5449
2m	375	445	2.14	4195	374	27.2	444	2.38	4215	382	31.3	469	2.04	4856
2n	365	464	0.57	5845	362	20.4	478	0.39	6704	365	16.8	508	0.67	7712
2o	366	450	59.9	5100	370	17.2	471	73.8	5796	366	18.5	489	61.2	6873
3a	357	417	0.81	4030	356	31.2	426	0.74	4616	364	29.9	467	1.27	6059
3h	365	419	0.83	3531	364	31.4	429	0.81	4163	371	34.2	470	1.51	5678
3i	354	424	1.56	4664	352	19.5	432	1.18	5261	358	32.2	473	2.06	6791
3l	367	439	1.31	4469	367	32.2	434	1.29	4206	375	27.2	477	1.78	5702
3n	366	465	0.61	5817	365	24.1	478	0.48	6477	366	15.6	511	1.00	7753

. 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 π→π* 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⁻¹cm⁻¹ and 16,500 M⁻¹cm⁻¹ to 41,800 M⁻¹cm⁻¹ and 32,400 M⁻¹cm⁻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-aminocinnamic 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.

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 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.

The fluorescence quantum yield of the synthesized acids and esters is low, and it does not exceed 3.5% in non-polar (1,4-Dioxane), polar aprotic (EtOAc), and protic (MeOH) solvents, which proves that the deactivation of the S₁ excited state occurs mainly by non-radiative processes. The only exception is compound 2o, for which the fluorescence quantum yield is 59.9% in 1,4-dioxane, 74% in ethyl acetate, 70.8% in DMF, and 61% in methanol. The results suggest that the modification of the amino group does not significantly affect the fluorescence quantum yield of the cinnamic acids. However, the slightly higher fluorescence intensity in polar solvents, both protic and aprotic (see Table 1 and

Table 2. Photophysical data ^a for compounds 2a and 2o.

No.	Solvent	λmax abs	λmax fl	Δν	ϕfl	${\mathsf{\tau }}_1^{\mathbf{f}\mathbf{l}}$	${\mathsf{\tau }}_2^{\mathbf{f}\mathbf{l}}$	${\mathsf{\tau }}_{\mathbf{a}\mathbf{v}}^{\mathbf{f}\mathbf{l}}$	χ2	kr	knr	kr/knr	fos
		εmax				α1	α2
2a	1,4-Dx	353	414	4174	0.84	0.074	0.521	0.08	2.194	1.07	12.6	0.0085	0.619
		27.4				98.95	1.05
	EtOAc	355	426	4695	0.61	0.08	0.896	0.09	2.419	0.673	11.0	0.0061	0.625
		28.9				98.7	1.3
	DMF	357	448	5690	2.06	0.127	2.455	0.14	1.700	1.48	7.04	0.021	0.646
		27.4				99.48	0.52
2o	1,4-Dx	366	450	5100	59.9	1.684	2.506	1.95	1.096	3.06	0.205	1.49	0.466
		20.9				67.07	32.93
	EtOAc	370	471	5796	73.8	1.777	2.454	2.09	1.002	3.52	0.125	2.82	0.401
		17.2				53.05	46.95
	DMF	373	499	6842	70.8	1.836	2.673	2.53	1.110	2.80	0.115	2.42	0.539
		22.4				16.93	83.07

^a Absorption (λ_{max abs}; nm), fluorescence maxima (λ_{max fl}; nm), shift (Δν; cm⁻¹), maximum extinction coefficient (ε_max; 10³ M⁻¹cm⁻¹), fluorescence quantum yield (ϕ_fl; %), fluorescence lifetime (τ; ns), component amplitudes (α; %) and correlation coefficients (χ²), radiative (k_r; 10⁸ s⁻¹) and nonradiative (k_nr; 10⁹ s⁻¹) rate constants, and f_os oscillator strength.

), may be attributed to the protic nature of the solvent and/or high polarization character (dielectric constant of 32.66 for MeOH and 36.71 for DMF). This feature contributes to the stabilization of the excited states and the charge or dipole of the molecule [41,42] and molecular symmetry, preventing non-radiative decay [43,44,45]. Compound 2e showed the lowest fluorescence intensity regardless of the solvent. Probably, the presence of a methyl group in the ortho position in relation to the central double bond (meta position in relation to the amino group) causes the molecule to twist and leads to a coplanar conformation, which reduces the probability of radiative transitions. On the other hand, the ϕ_Fl values for compound 2o in 1,4-dioxane, EtOAc, DMF, and MeOH with respect to 2a are over 71-, 120-, 34- and 89-fold higher, despite the same electron-donating (NMe₂) and -withdrawing (COOH) groups. In fact, the electronic character of the N,N-dimethylamino group in 2o is significantly altered by the elongation of the conjugated π-system by introducing an additional aromatic ring into the molecule, which stiffens the molecule and lowers the HOMO-LUMO energy gap [46] compared to 2a. As a consequence, the fluorescence intensity increases.

The literature data for other naphthalene derivatives with a dimethylamine group: 6-(dimethylamino)-2-naphtoic acid (ϕ_fl = 0.53) [47], 6-(dimethylamino)-2-naphthaldehyde (ϕ_fl = 0.84) [46], and 2-(dimethylamino)-naphthalene (ϕ_fl = 0.89) [48] indicate that high fluorescence intensity is a feature of naphthalene derivatives substituted into position 2 with a dimethylamine group. In the case of derivatives substituted into position 1, the intensity is much lower and does not exceed several percent [48,49].

According to Lewis et al. [49] and Suzuki et al. [48] the structure of the compounds based on 2-substituted naphthalene is flattened with a low torsion angle for C-NMe₂ due to the lack of unbonded interaction with the H8 hydrogen atom. In the case of 1-dialkylamino-substituted naphthalene, the optimized conformation of the amino group corresponds to a structure having a twisted lone pair orbital relative to the carbon 2pπ orbital in the naphthalene ring. The lack of repulsion between the alkyl substituents on the nitrogen atom and the perihydrogen atom in the naphthalene ring counteracts the rotation of the amino group to the plane of the naphthalene ring, while simultaneously shifting the charge from the alkylamino group towards the electron-withdrawing substituent, which results in a redshift of the 2o absorption band. This is consistent with the MO calculation results [46].

Figure S19 shows the calculated molecular structure and electron distribution of the HOMO and LUMO energy levels of the 2a and 2o acids. A comparison of the electron distribution in the frontier molecular orbitals (MOs) reveals that HOMO–LUMO excitation moved the electron distribution from the dimethylamine group towards the withdrawing substituent via the aromatic π system, indicating a strong migration of the intramolecular charge transfer character of the acids.

The HOMO and LUMO energies for compound 2a were computed to be −5.552 and −1.771 eV, respectively. Similarly, the energies of the HOMO and LUMO of 2o are −5.410 and −2.079 eV, respectively. 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 (Table 1 and Table 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 (τ^fl) and fluorescence quantum yield (ϕ_fl) of 3-[6-(dimethylamino)naphthalen-2-yl]acrylic acid (2o) and 4-dimethylaminocinnamic acid (2a) are listed in Table 2. The values of τ^fl and ϕ_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 τ^fl values than those of 2a. In particular, decreases in average τ^fl can be recognized for them in the nonpolar solvent (1,4-Dx), and the ϕ_f values of these compounds become smaller. Since both compounds have the same substituents, the remarkable increase in τ^fl and ϕ_f is limited to the aromatic π 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 = ϕ_fl τ⁻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. The measured oxidation potential of the tested compounds.

Abbr.	Eox (mV)	Abbr.	Eox (mV)	Abbr.	Eox (mV)	Abbr.	Eox (mV)
2a	1132	2f	1285	2k	1072	3a	1183
2b	1213	2g	1264	2l	1150	3h	1306
2c	1261	2h	993	2m	1171	3i	1210
2d	1192	2i	1084	2n	1147	3l	1240
2e	1177	2j	963	2o	1066	3n	1171

.

The analysis of cyclic voltammograms of the tested compounds indicates that their electrochemical oxidation is irreversible. The position of the oxidation peaks depends on the amino substituents. For example, the values of the oxidation potential increase with the elongation of the aliphatic substituents at the amino group (2a is methyl groups, 2b is ethyl groups, and 2c is butyl groups). The lowest value is observed for compound 2j, which is a morpholine derivative. This means that 2j has the greatest tendency to lose electrons. The oxidation potentials of all the tested acids range from 963 to 1285 mV, with the highest value of 1285 mV for compound 2f with a methyl substituent in the ortho position to the N,N-dimethylamino group responsible for its coplanar conformation and the decoupling effect between the dimethylamino group and the electron-accepting part of the molecule [50]. Moreover, the oxidation potentials of cinnamic acids containing cyclic terminal groups, i.e., pyrrolidine, piperidine, and morpholine, depend on the change in aromatic ring–nitrogen resonance interaction as the saturated heterocycle is varied. In general, the extent of overlap of the nitrogen lone-pair electrons with the π-electrons of the aromatic ring in N-phenyl-substituted saturated heterocycles may depend upon the molecular conformation, the nature of the heterocycle, the angle of rotation of the aromatic ring about the N-C(phenyl) bond, and any p-substituent present [51]. The transformation of cinnamic acids into the corresponding methyl esters leads to an increase in oxidation potentials, e.g., from 993 and 1084 mV for 2h and 2i to 1306 and 1210 mV for 3h and 3i, respectively. This effect is probably related to the decrease in the dipole moment when the COOH group is replaced with COOMe (Δμ_g = 0.58 D for 2h→3h and Δμ_g = 1.07 D for 2i→3i). Furthermore, alkane bridges between C_ortho and N_amino, e.g., in julolidine derivatives, cause the nitrogen atom to reveal weaker electron-donor properties than that in the 1-pyrrolidino group (E_OX = 1171 mV vs. 993 mV). This is consistent with the σ_R⁰ substituent constants of the 1-N{2,6-[(CH₂)₃]₂} (−0.59) and N(CH₂)₄ (−0.63) amino groups [52]. The modification of cinnamic acids with stiffened electron-donating substituents leads to a planar conformation of the molecules, which increases the probability of charge separation and leads to a decrease in oxidation potentials. The importance of the electronic effect of amino groups is also confirmed by the linear correlation of E_OX values with the corresponding σ_R⁰ substituent constants [52,53,54] (see Figure S13 in Supplementary File). Cinnamic acids, which bear a stronger electron-releasing substituent, have a lower oxidation potential.

3. Materials and Methods

All starting reagents and solvents were purchased from Aldrich Chemical Co. (Milwaukee, MI, USA). The melting points were determined with the Büchi melting point apparatus MP-1. The ¹H (400 MHz) and ¹³C (100 MHz) NMR spectra were recorded on a Bruker Ascent^TM 400 NMR spectrometer (Bruker, Billerica, MA, USA). Dimethylsulfoxide (DMSO-d₆) was used as the solvent and tetramethylsilane as the internal standard. Elemental analysis was carried out by an Elementar Vario MACRO apparatus (Elementar Americas Inc., Ronkonkoma, NY, USA). The IR spectra were recorded on a Bruker spectrophotometer Vector 22 in the range of 400–4500 cm⁻¹ by the KBr pellet technique. The UV-Vis absorption and emission spectra were recorded on a Shimadzu UV-Vis Multispec-1501 spectrophotometer (Spectralab Scientific Inc., Markham, ON, Canada) and a Hitachi F-7100 spectrofluorometer (Hitachi High-Tech Corporation, Tokyo, Japan), respectively. The fluorescence quantum yield (FQY; ϕ) was calculated according to Equation (1) using coumarin I in ethanol as a reference [55].

$${\mathsf{\varphi }}_{\mathrm{s}}={\mathsf{\varphi }}_{\mathrm{r}\mathrm{e}\mathrm{f}}\frac{I_s{A}_{ref}}{I_{ref}{A}_s}\cdot \frac{n_s^2}{n_{ref}^2}$$

(1)

where I is the integrated intensity (area) (in units of photons) and n is the refractive index. The absorbances (A) of both the sample (s) and reference (ref) solution at an excitation wavelength (350 nm) was ca. 0.1.

The fluorescence lifetimes were measured using an Edinburgh Instruments single-photon counting system (FLS920P Spectrometers (StellarNet Inc., Keystone, FL, USA)). The apparatus utilizes a picosecond diode laser for the excitation-generating pulses of about 55 ps at 375 nm. The dyes were studied at dilute solution (~0.2 in a 10 mm cell). The fluorescence decays were fitted to double-exponential functions. The average lifetime, τ_av, is calculated as

$${\mathsf{\tau }}_{\mathrm{a}\mathrm{v}}=\frac{\mathsf{\Sigma }{\mathsf{\tau }}_{\mathrm{i}}{\mathsf{\alpha }}_{\mathrm{i}}}{\mathsf{\Sigma }{\mathsf{\alpha }}_{\mathrm{i}}}$$

(2)

where α_i and τ_i are the amplitudes and lifetimes.

An Electrochemical Analyzer EA9C MTM Cracow (Wieliczka, Poland) was used to determine the oxidation potential (E_ox) of the synthesized compounds. Measurements were made by cyclic voltammetry using anhydrous acetonitrile with 0.1 M of tetrabutylammonium perchlorate as a supporting electrolyte. The scan rate was 300 mV/s. A typical three-electrode setup, containing a platinum 1 mm electrode as the working electrode and platinum and Ag/AgCl as the auxiliary and reference electrodes, respectively, was applied for the measurements.

DFT calculations were carried out using the B3LYP/6-311+G(2d,2p) method. Calculations were carried out with the Gaussian 03 program [56].

3.1. Synthesis

The routes leading to the preparation of derivatives of p-aminocinnamic esters are shown in Figure 1.

3.1.1. Synthesis of the Amine Derivatives of Cinnamic Acid

For Method A, pyridine (15 mL), malonic acid (25 mmol), and the appropriate benzaldehyde (20 mmol) were placed in a flask. Then piperidine (20 drops) was added, and the solution was heated at 80 °C for 20 h under stirring. The resulting mixture was poured into 120 mL of ice water, and 5 mL of saturated NaHCO₃ was added. The precipitate was filtered off. Then, the crude precipitate was added to 200 mL of cold 1% NaOH, and after 1 min of mixing, it was filtered. The filtrate was neutralized to pH = 7 with 10% hydrochloric acid. The product of the reaction was filtered off and dried.

3.1.2. Synthesis of the Methyl p-(Dimethylamino)cinnamate

For Method B, thionyl chloride (21 mmol) was added dropwise to a solution of p-(dimethylamino)cinnamic acid (15 mmol) in anhydrous methanol within 2 h under vigorous stirring. After 5 h, most of the methanol was distilled off, and the residue was poured onto 70 mL of ice water. Saturated NaHCO₃ was added while stirring until the CO₂ bubbles were stopped to volatilize. The precipitate was filtered and recrystallized from methanol.

For Method C, concentrated H₂SO₄ (4 g) was added dropwise to anhydrous methanol (30 mL). Then p-(dimethylamino)cinnamic acid (15 mmol) was put into the flask. The mixture was heated and held at gentle reflux for 20 h. Most of the methanol was distilled off, and the residue was poured onto 80 mL of ice water. Next, KHCO₃ was added in portions until the CO₂ bubbles were stopped to volatilize. The precipitate was filtered and recrystallized from methanol.

For Method D, a total of 40 mL of acetone, 15 mmol of p-(dimethylamino)cinnamic acid, 20 mmol of dimethyl sulfate, and 21 mmol of potassium carbonate were put into a flask and stirred at r.t. for 24 h. The resulting mixture was filtered, and the filtrate was concentrated by evaporation to 15 mL and poured onto 100 mL of ice water. The precipitate was filtered off and dried. The crude product was crystallized from methanol.

4-Dimethylaminocinnamic acid (2a)

The compound was obtained as a brown solid; yield: 71%; mp 221 °C.

¹H NMR (400 MHz, DMSO-d₆) σ (ppm): 11.96 (s, 1H, COOH), 7.50-7.46 (m, 3H), 6.70 (d, J = 8.9 Hz, 2H), 6.22 (d, J = 15.8 Hz, 1H, -CH =), 2.97 (s, 6H, N(CH₃)₂).

¹³C NMR (100 MHz, DMSO-d₆) σ (ppm): 168.3 (COOH), 151.6 (C), 144.7 (CH), 129.8 (CH), 121.6 (C), 113.0 (CH), 111.8 (CH), 39.8 (N(CH₃)₂).

IR (KBr): 1679, 1605, 1529, 1372, 1231, 1203, 1191, 816.

Anal. Calcd for C₁₁H₁₃NO₂: C, 69.09; H, 6.85; N, 7.33. Found: C, 69.01; H, 6.74; N, 7.28.

3-[4-(Diethylamino)phenyl]acrylic acid (2b)

The compound was obtained as a yellow solid; yield: 80%; mp 184 °C.

¹H NMR (400 MHz, DMSO-d₆) σ (ppm): 11.91 (s, 1H, COOH), 7.47–7.43 (m, 3H), 6.65 (d, J = 9.0 Hz, 2H), 6.17 (d, J = 15.8 Hz, 1H, -CH =), 3.38 (q, J = 6.9 Hz, 4H, NCH₂), 1.10 (t, J = 7.0 Hz, 6H, CH₃).

¹³C NMR (100 MHz, DMSO-d₆) σ (ppm): 168.8 (COOH), 149.4 (C), 145.2 (CH), 130.5 (CH), 121.1 (C), 112.7 (CH), 111.5 (CH), 44.2 (NCH₂), 12.9 (CH₃).

IR (KBr): 1670, 1658, 1598, 1356, 1261, 1208, 1184, 818.

Anal. Calcd for C₁₃H₁₇NO₂: C, 71.21; H, 7.82; N, 6.39. Found: C, 71.15; H, 7.70; N, 6.32.

3-[4-(Dibuthylamino)phenyl]acrylic acid (2c)

The compound was obtained as a pale-yellow solid; yield: 68%; mp 138.5–140.5 °C.

¹H NMR (400 MHz, DMSO-d₆) σ (ppm): 11.91 (s, 1H, COOH), 7.47–7.43 (m, 3H), 6.62 (d, J = 8.9 Hz, 2H), 6.16 (d, J = 15.8 Hz, 1H, -CH=), 3.31 (q, J = 7.6 Hz, 4H, NCH₂), 1.50 (m, 4H), 1.35 (m, 4H), 0.92 (t, J = 7.3 Hz, 6H, CH₃).

¹³C NMR (100 MHz, DMSO-d₆) σ (ppm): 168.7 (COOH), 149.8 (C), 145.1 (CH), 130.4 (CH), 121.1 (C), 112.7 (CH), 111.6 (CH), 50.3 (NCH₂), 29.5 (CH₂), 20.1 (CH₂), 14.3 (CH₃).

IR (KBr): 1670, 1578, 1525, 1403, 1204, 1185, 819.

Anal. Calcd for C₁₇H₂₅NO₂: C, 74.15; H, 9.15; N, 5.09. Found: C, 74.10; H, 9.06; N, 5.11.

3-[4-(Dimethylamino)-2-methylphenyl]acrylic acid (2d)

The compound was obtained as a pale-yellow solid; yield: 63%; mp 176 °C.

¹H NMR (400 MHz, DMSO-d₆) σ (ppm): 12.00 (s, 1H, COOH), 7.71 (d, J = 15.8 Hz, 1H, -CH=), 7.56 (d, J = 8.6 Hz, 1H), 6.59–6.54 (m, 3H), 6.18 (d, J = 15.7 Hz, 1H, -CH=), 2.95 (s, 6H, N(CH₃)₂), 2.34 (s, 3H, ArCH₃).

¹³C NMR (100 MHz, DMSO-d₆) σ (ppm): 168.8 (CHO), 151.8 (C), 141.7 (CH), 139.1 (C), 128.2 (CH), 120.7 (C), 114.3 (CH), 113.7 (CH), 110.6 (CH), 40.1 (N(CH₃)₂), 20.3 (ArCH₃).

IR (KBr): 1680, 1586, 1514, 1366, 1290, 1099, 827.

Anal. Calcd for C₁₂H₁₅NO₂: C, 70.23; H, 7.37; N, 6.83. Found: C, 70.15; H, 7.27; N, 6.79.

3-[4-(Dimethylamino)-2,6-dimethylphenyl]acrylic acid (2e)

The compound was obtained as a pale-yellow solid; yield: 29%; mp 176 °C.

¹H NMR (400 MHz, DMSO-d₆) σ (ppm): 12.12 (s, 1H, COOH), 7.75 (d, J = 16.2 Hz, 1H, -CH=), 6.47 (s, 1H), 5.93 (d, J = 16.2 Hz, 1H, -CH=), 2.93 (s, 6H, N(CH₃)₂), 2.32 (s, 6H, ArCH₃).

¹³C NMR (100 MHz, DMSO-d₆) σ (ppm): 168.7 (COOH), 150.6 (C), 142.3 (CH), 139.2 (C), 120.8 (C), 119.3 (CH), 112.7 (CH), 40.1 (N(CH₃)₂), 22.5 (ArCH₃).

IR (KBr): 1672, 1581, 1364, 1313, 1219, 1201, 1144, 822.

Anal. Calcd for C₁₃H₁₇NO₂: C, 71.21; H, 7.82; N, 6.39. Found: C, 71.25; H, 7.78; N, 6.45.

3-[4-(Dimethylamino)-3-methylphenyl]acrylic acid (2f)

The compound was obtained as a light-gray solid; yield: 69%; mp 129 °C.

¹H NMR (400 MHz, DMSO-d₆) σ (ppm): 12.21 (s, 1H, COOH), 7.51–7.44 (m, 3H), 7.04 (d, J = 5.1 Hz, 1H), 6.37 (d, J = 15.9 Hz, 1H, -CH=), 2.72 (s, 6H, N(CH₃)₂), 2.29 (s, 3H, ArCH₃).

¹³C NMR (100 MHz, DMSO-d₆) σ (ppm): 168.3 (COOH), 154.5 (C), 144.4 (CH), 131.4 (CH), 131.3 (C), 128.3 (C), 127.5 (CH), 118.7 (CH), 117.0 (CH), 43.9 (N(CH₃)₂), 19.0 (ArCH₃).

IR (KBr): 1684, 1621, 1602, 1506, 1420, 1279, 1232, 1109, 982, 827.

Anal. Calcd for C₁₂H₁₅NO₂: C, 70.23; H, 7.37; N, 6.83. Found: C, 70.15; H, 7.31; N, 6.81.

3-[4-(Dimethylamino)-2,5-dimethylphenyl]acrylic acid (2g)

The compound was obtained as a brown solid; yield: 61%; mp 151 °C.

¹H NMR (400 MHz, DMSO-d₆) σ (ppm): 12.21 (s, 1H, COOH), 7.73 (d, J = 15.8 Hz, 1H, -CH=), 7.50 (s, 1H), 6.82 (s, 1H), 6.28 (d, J = 15.8 Hz, 1H, -CH=), 2.67 (s, 6H, N(CH₃)₂), 2.33 (s, 3H, ArCH₃), 2.23 (s, 3H, ArCH₃).

¹³C NMR (100 MHz, DMSO-d₆) σ (ppm): 168.3 (COOH), 154.6 (C), 141.5 (CH), 136.2 (C), 129.7 (CH), 128.8 (C), 126.3 (C), 120.2 (CH), 117.4 (CH), 43.7 (N(CH₃)₂), 19.5 (CH₃), 18.7 (CH₃).

IR (KBr): 1682, 1599, 1508, 1318, 1236, 1211, 1075, 984, 859.

Anal. Calcd for C₁₃H₁₇NO₂: C, 71.21; H, 7.82; N, 6.39. Found: C, 71.14; H, 7.73; N, 6.30.

3-[4-(Pyrrolidin-1-yl)phenyl]acrylic acid (2h)

The compound was obtained as a light-brown solid; yield: 73%; mp 261 °C.

¹H NMR (400 MHz, DMSO-d₆) σ (ppm): 11.90 (s, 1H, COOH), 7.49 − 7.45 (m, 3H), 6.53 (d, J = 8.8 Hz, 2H), 6.18 (d, J = 15.8 Hz, 1H, -CH=), 3.28 (t, J = 6.5 Hz, 4H, NCH₂), 1.95 (m, 4H, CH₂).

¹³C NMR (100 MHz, DMSO-d₆) σ (ppm): 168.7 (COOH), 149.5 (C), 145.4 (CH), 130.3 (CH), 121.4 (C), 112.7 (CH), 112.1 (CH), 47.7 (NCH₂), 25.4 (CH₂).

IR (KBr): 1673, 1588, 1526, 1391, 1261, 1179, 815.

Anal. Calcd for C₁₃H₁₅NO₂: C, 71.87; H, 6.96; N, 6.45. Found: C, 71.88; H, 6.87; N, 6.40.

3-[4-(Piperidin-1-yl)phenyl]acrylic acid (2i)

The compound was obtained as a pale-yellow solid; yield: 72%; mp 223 °C.

¹H NMR (400 MHz, DMSO-d₆) σ (ppm): 12.03 (s, 1H, COOH), 7.50–7.45 (m, 3H), 6.91 (d, J = 9.0 Hz, 2H), 6.25 (d, J = 15.8 Hz, 1H, -CH=), 3.26 (t, J = 5.0 Hz, 4H, NCH₂), 1.58 (m, 6H).

¹³C NMR (100 MHz, DMSO-d₆) σ (ppm): 168.6 (COOH), 152.8 (C), 144.7 (CH), 130.1 (CH), 123.7 (C), 114.9 (CH), 114.4 (CH), 48.7 (NCH₂), 25.4 (CH₂), 24.4 (CH₂).

IR (KBr): 1666, 1602, 1519, 1241, 1188, 1126, 817.

Anal. Calcd for C₁₄H₁₇NO₂: C, 72.71; H, 7.41; N, 6.06. Found: C, 72.72; H, 7.30; N, 6.03.

3-[4-(Morpholin-4-yl)phenyl]acrylic acid (2j)

The compound was obtained as a pink solid; yield: 88%; mp 247 °C.

¹H NMR (400 MHz, DMSO-d₆) σ (ppm): 12.09 (s, 1H, COOH), 7.54-7.38 (m, 3H), 6.95 (d, J = 8.9 Hz, 2H), 6.30 (d, J = 15.9 Hz, 1H, -CH=), 3.74 (t, J = 4.9 Hz, 4H, NCH₂), 3.21 (t, J = 4.9 Hz, 4H, OCH₂).

¹³C NMR (100 MHz, DMSO-d₆) σ (ppm): 168.5 (COOH), 152.8 (C), 144.6 (CH), 130.0 (CH), 124.9 (C), 115.2 (CH), 114.7 (CH), 66.4 (OCH₂), 47.8 (NCH₂).

IR (KBr): 1704, 1604, 1517, 1174, 1112, 820, 618.

Anal. Calcd for C₁₃H₁₅NO₃ C, 66.94; H, 6.48; N, 6.01. Found: C, 67.02; H, 6.36; N, 6.09.

3-(1-Methyl-2,3-dihydro-1H-indol-5-yl)acrylic acid (2k)

The compound was obtained as a brown solid; yield: 58%; mp 151 °C.

¹H NMR (400 MHz, DMSO-d₆) σ (ppm): 11.93 (s, 1H, COOH), 7.45 (d, J = 15.8 Hz, 1H, -CH=), 7.37 (s, 1H), 7.29 (d, J = 8.1 Hz, 1H), 6.46 (d, J = 8.1 Hz, 1H), 6.17 (d, J = 15.8 Hz, 1H, -CH=), 3.38 (t, J = 8.2 Hz, 2H, NCH₂), 2.91 (t, J = 8.3 Hz, 2H ArCH₂), 2.77 (s, 3H, NCH₃).

¹³C NMR (100 MHz, DMSO-d₆) σ (ppm): 168.8 (COOH), 155.6 (C), 145.4 (CH), 131.1 (C), 130.5 (CH), 123.7 (CH), 123.6 (C), 113.2 (CH), 106.3 (CH), 55.3 (NCH₂), 35.1 (NCH₃), 27.9 (ArCH₂).

IR (KBr): 1676, 1592, 1517, 1294, 1230, 1201.

Anal. Calcd for C₁₂H₁₃NO₂: C, 70.92; H, 6.45; N, 6.89. Found: C, 70.84; H, 6.41; N, 6.82.

3-(1-Methyl-1,2,3,4- tetrahydroquinolin-6-yl)acrylic acid (2l)

The compound was obtained as a brown solid; yield: 65%; mp 158 °C.

¹H NMR (400 MHz, DMSO-d₆) σ (ppm): 11.92 (s, 1H, COOH), 7.41 (d, J = 15.8 Hz, 1H, -CH=), 7.28 (d, J = 8.5 Hz, 1H), 7.22 (s, 1H), 6.54 (d, J = 8.6 Hz, 1H), 6.15 (d, J = 16.1 Hz, 1H, -CH=), 3.28 (t, J = 5.7 Hz, 2H, NCH₂), 2.90 (s, 3H, NCH₃), 2.69 (t, J = 6.2 Hz, 2H, ArCH₂), 1.87 (m, 2H).

¹³C NMR (100 MHz, DMSO-d₆) σ (ppm): 168.7 (COOH), 148.6 (C), 145.3 (CH), 129.0 (CH), 128.8 (CH), 122.5 (CH), 121.7 (C), 112.7 (CH), 110.6 (CH), 50.9 (NCH₂), 38.9 (NCH₃), 27.6 (ArCH₂), 21.9 (CH₂).

IR (KBr): 1672, 1658, 1599, 1522, 1321, 1269, 1206.

Anal. Calcd for C₁₃H₁₅NO₂: C, 71.87; H, 6.96; N, 6.45. Found: C, 71.81; H, 6.87; N, 6.38.

3-(9-Julolidyl)acrylic acid (2m)

The compound was obtained as a light-brown solid; yield: 64%; mp 178 °C.

¹H NMR (400 MHz, DMSO-d₆) σ (ppm): 11.86 (s, 1H, COOH), 7.34 (d, J = 15.8 Hz, 1H, -CH=), 7.01 (s, 2H), 6.08 (d, J = 15.7 Hz, 1H, -CH=), 3.19 (t, J = 5.7 Hz, 4H, NCH₂), 2.66 (t, J = 6.3 Hz, 4H, ArCH₂), 1.85 (m, 4H, CH₂).

¹³C NMR (100 MHz, DMSO-d₆) σ (ppm): 168.8 (COOH), 145.4 (CH), 144.9 (C), 127.7 (CH), 120.94 (C), 120.90 (C), 112.2 (CH), 49.6 (CH₂), 27.5 (CH₂), 21.6 (CH₂).

IR (KBr): 1653, 1592, 1513, 1312, 1256, 1160.

Anal. Calcd for C₁₅H₁₇NO₂: C, 74.05; H, 7.04; N, 5.76. Found: C, 73.98; H, 6.97; N, 5.72.

3-[4-(Dimethylamino)naphthalen-1-yl]acrylic acid (2n)

The compound was obtained as a yellow solid; yield: 61%; mp 166 °C.

¹H NMR (400 MHz, DMSO-d₆) σ (ppm): 12.42 (s, 1H, COOH), 8.33 (d, J = 15.7 Hz, 1H, -CH=), 8.20–8.17 (m, 2H), 7.90 (d, J = 8 Hz, 1H), 7.63–7.55 (m, 2H), 7.12 (d, J = 8Hz, 1H), 6.50 (d, J = 15.6 Hz, 1H, -CH=), 2.88 (s, 6H, N(CH₃)₂).

¹³C NMR (100 MHz, DMSO-d₆) σ (ppm): 168.2 (COOH), 153.2 (C), 140.7 (CH), 132.7 (C), 128.1 (C), 127.4 (CH), 126.3 (CH), 125.7 (CH), 125.3 (CH), 125.2 (C), 123.8 (CH), 119.7 (CH), 114.0 (C), 45.0 (N(CH₃)₂).

IR (KBr): 1676, 1611, 1570, 1417, 1338, 1212, 770.

Anal. Calcd for C₁₅H₁₅NO₂: C, 74.67; H, 6.27; N, 5.81. Found: C, 74.51; H, 6.32; N, 5.88.

3-[6-(Dimethylamino)naphthalen-2-yl]acrylic acid (2o)

The compound was obtained as a yellow solid; yield: 63%; mp 251 °C.

¹H NMR (400 MHz, DMSO-d₆) σ (ppm): 12.16 (s, 1H, COOH), 7.94 (s, 1H), 7.76 (d, J = 9.1 Hz, 1H), 7.69-7.62 (m, 3H), 7.24 (d, J = 9.1Hz, 1H), 6.95 (s, 1H), 6.50 (d, J = 15.9 Hz, 1H, -CH=), 3.04 (s, 6H, N(CH₃)₂).

¹³C NMR (100 MHz, DMSO-d₆) σ (ppm): 168.5 (COOH), 149.7 (C), 144.7 (CH), 136.2 (C), 130.2 (CH), 129.9 (CH), 128.0 (C), 127.0 (CH), 126.0 (C), 124.6 (CH), 117.5 (CH), 116.9 (CH), 105.8 (CH), 40.5 (N(CH₃)₂).

IR (KBr): 1683, 1612, 1599, 1428, 1312, 1202, 1176, 842.

Anal. Calcd for C₁₅H₁₅NO₂: C, 74.67; H, 6.27; N, 5.81. Found: C, 74.58; H, 6.39; N, 5.84.

Methyl 3-[4-(dimethylamino)phenyl]acrylate (3a)

The compound was obtained as a pale-yellow solid; yield: 73% (method B), 76% (method C), 82% (method D); mp 137 °C.

¹H NMR (400 MHz, DMSO-d₆) σ (ppm): 7.57–7.52 (m, 3H), 6.71 (d, J = 8.9 Hz, 2H), 6.32 (d, J = 15.9 Hz, 1H, -CH=), 3.68 (s, 3H, OCH₃), 2.98 (s, 6H, N(CH₃)₂).

¹³C NMR (100 MHz, DMSO-d₆) σ (ppm): 167.8 (COO), 152.2 (C), 145.7 (CH), 130.4 (CH), 121.8 (C), 112.2 (CH), 111.9 (CH), 51.5 (OCH₃), 40.1 (N(CH₃)₂).

IR (KBr): 1702, 1605, 1529, 1187, 1170, 815.

Anal. Calcd for C₁₂H₁₅NO₂: C, 70.23; H, 7.37; N, 6.83. Found: C, 70.16; H, 7.31; N, 6.77.

Methyl 3-[4-(pyrrolidin-1-yl)phenyl]acrylate (3h)

The compound was obtained as a yellow solid; yield: 71% (method B), 72% (method C), 83% (method D); mp 146 °C.

¹H NMR (400 MHz, DMSO-d₆) σ (ppm): 7.56–7.50 (m, 3H), 6.54 (d, J = 8.8 Hz, 2H), 6.28 (d, J = 15.8 Hz, 1H, -CH=), 3.67 (s, 3H, OCH₃), 3.28 (t, J = 6.5 Hz, 4H, NCH₂), 1.96 (m, 4H, CH₂).

¹³C NMR (100 MHz, DMSO-d₆) σ (ppm): 167.8 (COO), 149.7 (C), 145.9 (CH), 130.6 (CH), 121.2 (C), 112.1 (CH), 111.2 (CH), 51.5 (OCH₃), 47.7 (NCH₂), 25.4 (CH₂).

IR (KBr): 1708, 1701, 1605, 1527, 1393, 1184, 1164, 985, 814.

Anal. Calcd for C₁₄H₁₇NO₂: C, 72.71; H, 7.41; N, 6.06. Found: C, 72.65; H, 7.38; N, 6.01.

Methyl 3-[4-(piperidin-1-yl)phenyl]acrylate (3i)

The compound was obtained as a light-pink–yellow solid; yield: 84% (method B), 81% (method C), 93% (method D); mp 101 °C.

¹H NMR (400 MHz, DMSO-d₆) σ (ppm): 7.56–7.51 (m, 3H), 6.91 (d, J = 8.9 Hz, 2H), 6.35 (d, J = 15.9 Hz, 1H, -CH=), 3.68 (s, 3H, OCH₃), 3.29 (t, J = 5.0 Hz, 4H, NCH₂), 1.57 (m, 6H).

¹³C NMR (100 MHz, DMSO-d₆) σ (ppm): 167.7 (COO), 153.0 (C), 145.2 (CH), 130.4 (CH), 123.4 (C), 114.8 (CH), 112.9 (CH), 51.6 (OCH₃), 48.7 (NCH₂), 25.4 (CH₂), 24.4 (CH₂);

IR (KBr): 1708, 1625, 1604, 1518, 1190, 1170, 1127, 985, 816.

Anal. Calcd for C₁₅H₁₉NO₂: C, 73.45; H, 7.81; N, 5.71. Found: C, 73.37; H, 7.75; N, 5.77.

Methyl 3-(1-methyl-1,2,3,4-tetrahydroquinolin-6-yl)acrylate (3l)

The compound was obtained as a brown solid; yield: 73% (method B), 71% (method C), 88% (method D); mp 72 °C.

¹H NMR (400 MHz, DMSO-d₆) σ (ppm): 7.48 (d, J = 15.8 Hz, 1H, -CH=), 7.30 (d, J = 8.5 Hz, 1H), 7.25 (s, 1H), 6.54 (d, J = 8.6 Hz, 1H), 6.25 (d, J = 15.8 Hz, 1H, -CH=), 3.67 (s, 3H, OCH₃), 3.28 (t, J = 5.7 Hz, 2H, NCH₂), 2.91 (s, 3H, NCH₃), 2.68 (t, J = 6.1 Hz, 2H, ArCH₂), 1.87 (m, 2H).

¹³C NMR (100 MHz, DMSO-d₆) σ (ppm): 167.8 (COO), 148.8 (C), 145.9 (CH), 129.2 (CH), 128.9 (CH), 122.6 (C), 121.5 (C), 111.2 (CH), 110.6 (CH), 51.4 (OCH₃), 50.8 (NCH₂), 38.8 (NCH₃), 27.6 (ArCH₂), 21.9 (CH₂).

IR (KBr): 1708, 1597, 1524, 1324, 1210, 1157, 1004, 845, 799.

Anal. Calcd for C₁₄H₁₇NO₂: C, 72.71; H, 7.41; N, 6.06. Found: C, 72.60; H, 7.32; N, 6.10.

Methyl 3-[4-(Dimethylamino)naphthalen-1-yl]acrylate (3n)

The compound was obtained as a dark yellow oil; yield: 67% (method B), 65% (method C), 75% (method D).

¹H NMR (400 MHz, DMSO-d₆) σ (ppm): 8.40 (d, J = 15.7 Hz, 1H, -CH=), 8.19–8.17 (m, 2H), 7.92 (d, J = 8.2 Hz, 1H), 7.63–7.54 (m, 2H), 7.10 (d, J = 8 Hz, 1H), 6.59 (d, J = 15.6 Hz, 1H, -CH=), 3.65 (s, 3H, OCH₃), 2.87 (s, 6H, N(CH₃)₂).

¹³C NMR (100 MHz, DMSO-d₆) σ (ppm): 167.3 (COO), 153.5 (C), 141.3 (CH), 132.7 (C), 128.0 (C), 127.4 (CH), 126.5 (CH), 125.7 (CH), 125.3 (CH), 124.8 (C), 123.8 (CH), 118.1 (CH), 113.9 (CH), 51.9 (OCH₃), 44.9 (N(CH₃)₂).

IR (KBr): 1716, 1626, 1573, 1435, 1391, 1310, 1192, 1157, 1043, 765.

Anal. Calcd for C₁₆H₁₇NO₂: C, 75.28; H, 6.71; N, 5.49. Found: C, 75.31; H, 6.73; N, 5.37.

4. Conclusions

Fifteen p-aminocinnamic acids were prepared by a reaction of the corresponding p-aminobenzaldehydes with malonic acid (Method A). In the second step, the selected acids were converted into methyl esters. In search of the most effective synthesis route, three methods were tested, with thionyl chloride in methanol (Method B), with sulfuric acid in methanol (Method C), and with dimethyl sulfate in acetone (Method D). The desired products were obtained in a moderate-to-high yield of 65–93% and as high-purity products. The simplest reaction route for the preparation of the esters, which gave the highest yields, was Method D. It should also be noted that six of the synthesized acids (2c, 2d, 2e, 2f, 2g, and 2k) and four esters (3h, 3i, 3l, and 3n) have not been described in the literature so far.

It is clear from the electronic absorption spectra that the introduction of an additional double bond to the acid molecule redshifts the band maximum by approx. 50 nm relative to the corresponding benzoic acids. In general, the spectroscopic properties of the synthesized compounds depend on the amine substituent present in the phenyl ring and the solvent used. The amine group also influences the oxidation potentials of the tested cinnamic acids. The values of the oxidation potentials, determined by cyclic voltammetry, vary from 963 mV to 1306 mV depending on the acid structure.

The best photophysical properties have compound 2o with a naphthalene ring substituted in position 2 with an N,N-dimethylamino group. It is characterized by a high molar absorption coefficient, large Stokes shift, high fluorescence quantum yield, and relatively long fluorescence lifetime compared to other acids.