New V-shaped push-pull systems based on 4,5-di(hetero)aryl substituted pyrimidines: their synthesis and application to the detection of nitroaromatic explosives

A number of D– π –A– π – D type dyes based on pyrimidines, bearing various electron-donating carbazole and triphenylamine fragments, have been studied as sensing fluorophores. Fluorescence studies demonstrated that the emission of all derivatives in acetonitrile is sensitive to the presence of a number of nitroaromatic benzenoids, including explosives such as 2,4-dinitroanisole, picric acid, styphnic acid, 1,3,5-triethoxy-2,4,6-trinitrobenzene, 2,4-dinitrotoluene and 2,4,6-trinitrotoluene. Detection limits of fluorophores for the explosive compounds were in the range from 2 mM to 29 μ M. A selective fluorescence quenching response, including a sharp color change under UV, especially for the trinitrophenolics, makes these fluorophores promising fluorescence sensory materials for nitroaromatic explosives.


Absorption and emission properties
A characteristic feature of this class of compound is that their absorption and emission wavelengths can be easily adjusted by suitable design of their structures. To evaluate the effect of triphenylamine and carbazole moieties on photophysical properties, the fluorescence excitation and emission spectra of 6a-c were recorded. The spectra are shown in Figure 1 and the main optical properties are presented in Table 1.
The pyrimidine fluorophores 6a-c show broad absorption maxima in the region of 340-405 nm (ε 17700-23700 M −1 ·сm −1 ), which can be attributed to intramolecular charge-transfer excitation from the carbazole or triphenylamine moiety to the pyrimidine ring (Table 1, Fig. 1a). The second and third absorption maxima were seen at 294-361 and 237-299 nm, respectively. The effects of molecular structure on the absorption properties are seen in the hypsochromic shift of the long-wavelength absorption bands, of approximately 63 nm from the triphenylamine derivative 6a to the 9-phenyl-9H-carbazole compound 6c (Table 1). Since the ionization potential of triphenylamine is lower than that of carbazole, it is reasonable to suppose that enhancement of the electron-donating ability of the D-fragment of the dye is responsible for the hypsochromic shift. Such dependence is inherent in the absorption bands which are due to intramolecular charge transfer. 48 Effective channel deactivation of electronic excitation energy for dyes 6a-c is the source of the fluorescence (Table 1, Figure 1b). The excitation spectra coincided with the absorption bands ( Table 1). The influence of the structure of dyes 6a-c on the fluorescence spectra is similar to that found for the absorption spectra.
The Stokes shift is an important parameter for a fluorophore, since it provides directly a measure of the energy gap between the ground and the first excited state of the fluorophore. 34 For the fluorophores under consideration, their Stokes shifts to vary from 6497 (6b) to 9881 cm -1 (6c) (see Table 1). High values of the Stokes shifts may be due to a change in the dipole moments of dyes in the excited state, resulting from charge transfer from donor to acceptor, which is accompanied by a compensatory relaxation of solvent molecules. 23 (a)

Fluorescence quenching studies in acetonitrile solution
To evaluate the utility of the D-π-A-π-D type dyes 6a-c based on the pyrimidine scaffold for the detection of nitroaromatic explosives, the corresponding fluorescence measurements of the fluorophores 6a-c were carried out in acetonitrile solutions containing measured quantities of the compounds shown in Figure 2, which serve as quenchers of the fluorescence.        (5): before irradiation (a -no emission) and during irradiation (b -emission, λ ex = 375 nm) at room temperature.

OMe
The quenching efficiency was calculated by using the formula (I 0 -I)/ I 0 × 100%, where I 0 and I are the fluorescence intensities of fluorophores 6a, 6b or 6c before and after addition of the analyte, respectively. Figure 5 shows the quenching efficiency of nitroaromatics and related compounds.  We have tried to suggest quenching mechanism for example of interaction DNAN with a different fluorophores 6a-c. The linear response of the Stern-Volmer plot (in a range of concentrations from 0 to 5×10 -5 M) and the lifetime measurements upon addition of DNAN suggest the high role of static type of quenching process ( Figures S5-10 and Table S1 in Supplementary Material). In other words, formation of a complex between DNAN and 6a-c may be the origin of the quenching, a result similar to those reported by others in the studies of sensing behaviors of conjugated oligothiophenes. 49

Conclusions
In summary, new V-shaped push-pull systems based on 4,5-di(hetero)aryl substituted pyrimidines have been studied as sensing fluorophores. These dyes show remarkable sensitivity towards the presence of various nitroaromatic explosives. Selective fluorescence quenching response, including a sharp color change under UV lamp, especially for PA and SA, makes these fluorophores to be promising fluorescence sensory materials for nitro-containing explosives with a detection limit of 10 −5 mol/L. The present study may be a significant step towards the development of pyrimidine dyes with thiophene linkers, as a new series of sensing fluorophores.

Experimental Section
General. All reagents and solvents were obtained from commercial sources and dried by using standard procedures before use. Nitro-containing explosives, including 2,4-dinitroanisole (DNAN), picric acid (PA), styphnic acid (SA), 1,3,5-triethoxy-2,4,6-trinitrobenzene (TETNB), 2,4-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT) were of analytical grade and used directly without further purification. (Caution: Most nitro-containing compounds used in the present study are high explosives and should be handled only in small quantities.) 4,5-Bis[5bromo(thien-2-yl)]pyrimidine (4) was prepared according to the earlier reported method. 47 The solvents (1,4-dioxane and H 2 O) for the microwave-assisted Suzuki cross-coupling reaction were degassed by bubbling argon for 1 h. 1 H and 13 C NMR spectra were recorded on a AVANCE-500 instruments using Me 4 Si as an internal standard. Elemental analysis was carried on a Eurovector EA 3000 automated analyzer. Melting points were determined on Boetius combined heating stages. Flash column chromatography was carried out using Alfa Aesar silica gel 0.040-0.063 mm (230-400 mesh), eluting with ethyl acetate-hexane. The progress of reactions and the purity of compounds were checked by TLC on Sorbfil plates (Russia), in which the spots were visualized with UV light (λ 254 or 365 nm). Microwave heating were carried out in a Discover unimodal microwave system (CEM, USA) with a working frequency of 2.45 GHz and the power of microwave radiation ranged from 0 to 300 W. The reactions were carried out in 10 mL reaction tubes with hermetic Teflon seal. The temperature of the reaction was monitored using an inserted IR sensor by the external surface of the reaction vessel. UV-vis spectra were recorded for a 1×10 -5 M acetonitrile solution with Shimadzu UV-2401PC spectrophotometer. Fluorescence spectra measurements were performed on a Hitachi F-7000 fluorescence spectrophotometer at room temperature. Quantum yields (Ф) were estimated with 1N H 2 SO 4 solution of quinine bisulfate (Ф= 0.55) as a reference. 50 The fluorescence quenching studies were conducted in acetonitrile. For each analyte, the typical test procedure was as follows: a 2.5 mL of the acetonitrile solution of one of the fluorophores (1.0 × 10 −5 mol/L) was drawn and placed in a quartz cell with a standard size, and then the measurement conducted. In the absence of analyte, the fluorescence spectrum of pure fluorophore was first recorded. Subsequently, different amounts of analyte were respectively added into the above cell. Each time after fully mixing the analyte with the fluophore, the fluorescence spectrum was collected at once. [9-ethyl-9H-carbazole-3-boronic (5b) or 9H-carbazole-9-(4-phenyl)boronic acids (5c)] acid (1.25 mmol) and Pd(PPh 3 ) 4 (58 mg, 10 mol %) in 1,4-dioxane (4 mL). The resulting mixture was irradiated in a microwave apparatus at 165 °C (200 W) for 20 min. After that solvent was distilled off in vacuo, and the residue was purified by flash column chromatography (hexane/ethyl acetate, 1:3) to afford the desired cross-coupling products (6a, 6b and 6c).