3.2. Design and Feasibility Analysis of Dyes

As shown in Figure 5, the hemciayanine dyes could be prepared by 2 steps; the chemical structures of raw materials, intermediates, and dyes are shown in Figure S1, while the geometry structures of the designed dyes are shown in Figure S2. Z1 (DYE-BD) could be easily synthesized in two steps following the references [15,16,17,18], and the route of the synthesis can serve as an example to illustrate the designed routes. The nitrogen center of pyridine features a basic lone pair of electrons; consequently, pyridine is a strong nucleophile. Thus, pyridine could react with 1-bromoethane to form pyridinium, while the second reaction is a typical Knoevenagel condensation reaction between the methyl group on the pyridinium and the carbonyl group in 4-diethylaminobenzaldehyde.

General method for prepare designed dyes. (a) formation of pyridinium; (b) preparation of designed dye.

Although the Z1 in Figure 5 has been reported and investigated by researchers [15,16,17,18], the properties of other chemicals could be changed by different substituent groups, and the reactivity of intermediates would be changed as well. Thus, analysis and design of proper synthesis routes are important and necessary. The feasibilities of the routes were analyzed by using the computational methods.

The first step of the preparation route of designed dyes in Figure 5 is a formation of quaternary pyridinium salts on pyridine derivatives. Due to the existence of unconjugated lone pair electrons on N in pyridine rings, all six derivatives of pyridine can form the corresponding pyridinium salts easily (see Figures S3–S5). The second step reaction, a nucleophilic reaction, is between the pyridinium salts and 4-diethylaminobenzaldehyde, while the reactive sites on the pyridinium salts are carbons bearing more electrons and more negative charges.

Figure 6 displays the charge distributions of the pyridinium intermediates. As a result of calculation, for the pyridinium salt (g) in Figure 6, the carbon, with an electron density of −0.391, in the methyl group should be the reactive site when it reacts with 4-diethylaminobenzaldehyde, while the methyl group in compound B1 is more reactive than methyl group in compound A1, which can be confirmed in the references [55,56,57,58,59,60,61]. Thus, the more negative charges of carbon in methyl group, the more reactive the methyl group will be. Sulfonic acid and carboxyl groups have a complex influence on the charge of the atoms in pyridinium salts. The carboxyl group (2 position) increases the electronegativity of the carbon in methyl group slightly, but the carboxyl (3 position) decreases the charge on the carbon of the methyl group. Thus, the methyl group of compounds B2, B3, B4, and B6 should have the similar reactivity to that of the compound B1, while the reactivity of methyl group in compound B5 should be lower than that of the compound B1. Thus, compounds B2, B3, B4, and B6 could react with 4-diethylaminobenzaldehyde, but the compound B5 may be difficult to or may not react with it.

Electronegativity of raw materials and intermediates calculated by Gaussian 09. (A1) 4-Picoline; (A2) 4-Methylpyridine-3-sulfonic acid; (A3) 4-Methylpyridine-2-sulfonic acid; (A4) 5-Methyl-3-pyridinesulfonic acid; (A5) 4-Methylnicotinic acid; (A6) 4-Methyl-pyridine- 2-carboxyl acid; (B1) 1-ethyl-4-methylpyridin- 1-ium bromide; (B2) 1-ethyl-4-methyl-3-sulfopyridin-1-ium bromide; (B3) 1-ethyl-4-methyl-2-sulfopyridin-1-ium bromide; (B4) 1-ethyl-3-methyl-5-sulfopyridin-1-ium bromide; (B5) 3-carboxy-1-ethyl-4-methylpyridin-1-ium bromide; (B6) 2-carboxy-1-ethyl-4-methylpyridin-1-ium bromide.

The enthalpy of a thermodynamic system is defined in Equation (2) [62,63].

H is the enthalpy (SI unit: Joule); U is the internal energy (SI unit: Joule); p is pressure (SI unit: Pascal); V is volume (SI unit: m3).

The standard enthalpy of formation (Δ_fH) of a compound is the change of enthalpy during the formation of 1 mole of the substance from its constituent elements in their standard states. The standard enthalpy change (Δ_rH) of any reaction can be calculated from the standard enthalpies of formation of reactants and products using Hess’s law. It could be illustrated in Equation (3):

The Gibbs free energy is defined in Equation (4):

T is the temperature (SI unit: Kelvin); S is the entropy (SI unit: Joule per kelvin).

The Gibbs free energy change (ΔrG) of any reaction can be calculated based on Equation (5):

According to the first step reaction between derivatives of pyridine and 1-bromoethane (Figure 5a), the reaction could be illustrated as A + D→B, so the ΔrH could be calculated by Δ_rH = Δ_fH(B) − Δ_fH(A) − Δ_fH(D), while the ΔG could be illustrated as Δ_rG = Δ_fG(B) − Δ_fG(A) − Δ_fG(D). The changes of enthalpy and Gibbs free energies in the first step reactions are calculated by Gaussian 09 in ethanol at 80 °C and shown in Table 6; similarly, the ΔrH values of the second reaction in Figure 5b could be illustrated as Δ_rH = Δ_fH(Z) + Δ_fH(F) − Δ_fH(B) − Δ_fH(E), while the ΔG values in Table 7 could be calculated by Δ_rG = Δ_fG(Z) + Δ_fG(F) − Δ_fG(B) − Δ_fG(E). Meanwhile, the bromide ion is not involved in the second step reactions. It seems all designed fluorescent dyes could be produced according to the thermodynamic analysis.

The enthalpy and Gibbs free energy values of chemicals in first step and changes of them in ethanol under 353K based on Gaussian 09.

The enthalpy and Gibbs free energy values of chemicals in second step and changes of them in ethanol under 353K based on Gaussian 09.

¹Hartree = 627.509 kcal mol⁻¹ = 27.2116 eV.