Synthesis , characterization and evaluation of fluorimetric chemosensors for ions based on diphenylimidazole derivatives

The new 2,4,5-tri-substituted imidazole derivatives 2 and 3 were obtained in good yields through a Radziszewski reaction followed by alkylation. The new derivatives were characterized by the usual spectroscopic techniques and a detailed photophysical study was undertaken. The evaluation of the compounds as fluorimetric chemosensors was carried out by performing spectrofluorimetric titrations in acetonitrile in the presence of relevant organic and inorganic anions, and of alkaline, alkaline-earth and transition metal cations.


Introduction
Chemosensors are a subject of great interest and in recent years great efforts have been made to develop fluorescent probes that are capable of detecting ions with high sensitivity and selectivity.The ideal fluorophore for chemosensory application should have high fluorescence quantum yield, high molar extinction coefficients, longwavelengths of excitation and emission, long service life and high photostability. 1e imidazole nucleus plays an important role in medicinal chemistry and in biochemical processes as structural component of several biomolecules and has varied pharmacological activities. 2 Earlier studies reported by us showed that the optical and thermal properties of imidazole derivatives could be tuned by substitution of aryl groups at positions 2, 4 and 5 by five-membered heterocycles giving rise to innovative applications of these -conjugated systems in nonlinear optics, chemosensors and DNA intercalators. 3 now report the synthesis of new diphenylimidazoles in order to evaluate their photophysical properties and chemosensory ability.Therefore, imidazole derivative 2 bearing an heterocyclic π-bridge was obtained in good yield through a Radziszewski reaction.Direct alkylation of 2 gave the corresponding N-alkylated compound 3.The new derivatives were characterized by the usual techniques and a detailed photophysical study was undertaken.The evaluation of the compounds as fluorimetric chemosensors was carried out by performing spectrofluorimetric titrations in acetonitrile in the presence of relevant organic and inorganic anions, and of alkaline, alkaline-earth and transition metal cations.

Synthesis and characterization
The new imidazoles 2 and 3 bearing an arylthiophene bridge, were synthesized in moderate to good yields (58-80 %), by Radziszewski reaction between aldehyde 1 and benzil followed by alkylation (Scheme).The new compounds were completely characterized by the usual spectroscopic techniques (Table 1).
Scheme.Synthesis of 2,4,5-tri-substituted imidazole derivatives 2 and 3.The absorption and emission spectra of imidazoles 2 and 3 were measured in acetonitrile solutions (Table 1).Imidazole derivatives 2 and 3 have the same heterocyclic π-bridge, and differ only in the substituent on the nitrogen of the imidazole ring.
The relative fluorescence quantum yields were determined by using 10 -6 M solutions of DPA in ethanol as standard (Ф F = 0.95). 4For the Ф F determination, the fluorescence standard was excited at the wavelengths of maximum absorption found for each of the compounds to be tested and in all fluorimetric measurements the absorbance of the solution did not exceed 0.1.Diphenyl-imidazoles 2 and 3 exhibited excellent fluorescence quantum yields in acetonitrile (Ф F = 0.42 and 0.44) (Table 1).

Spectrophotometric/spectrofluorimetric titrations and chemosensing studies of 2 and 3 with metallic ions
Compounds 2 and 3 (10 -5 M) were evaluated as chemosensors in the presence of several relevant ions (AcO -, F -, Cl -, Br ).Spectrophotometric and spectrofluorimetric titrations were carried out in ACN solutions, in order to evaluate their chemosensory ability.
Spectrofluorimetric titrations of compound 2 revealed its ability to act as fluorimetric chemosensor for certain anions and cations.On the other hand, for compound 3, the introduction of an alkyl chain in the NH group of the imidazole heterocycle lead to a selective fluorimetric chemosensor for Fe 3+ and Hg 2+ (Table 2).For the remaining ions, no changes in the fluorescence intensity were seen upon titration.As a preliminary test, the differences in fluorescence intensity of compound 2 alone and in the presence of 100 equivalents of several ions in acetonitrile solution can be seen in Figure 1.
Spectrophotometric titrations did not reveal interaction with any of the cations and anions tested.
Table 2. Results obtained in spectrofluorimetric titrations in acetonitrile in the presence of several ion solutions for imidazoles 2 and 3.In the spectrofluorimetric titrations with Fe 3+ , a strong decrease of the fluorescence intensity (a chelation enhancement of quenching, CHEQ effect) was observed for the two derivatives, with a complete fluorescence quenching (2, 165 eq.; 3, 171 eq.) (Figure 2).A total quenching in the fluorescence was also achieved for compound 3 in the presence of Hg 2+ .Spectrofluorimetric titrations of compound 2 in presence of CNand Fions revealed that the plateau was achieved after the addition of 33 equivalents of Fion whereas 85 equivalents of CNion were needed.In both titrations, the intensity of the emission band was reduced (CHEQ effect), accompanied by a red-shift, and a new band was formed, with an iso-emissive point at 513 nm with CNion and 518 nm with Fion, indicating the presence of two emissive species in the solution.

Conclusions
The synthesis of new imidazole derivatives 2 and 3 was achieved in moderate to good yields by simple experimental procedures.The photophysical properties were evaluated

Figure 1 :
Figure 1: Acetonitrile solutions of compound 2 and compound 2 in the presence of 100 equivalents of several ions, visualized in a UV chamber under a 365 nm lamp.

Table 1 .
Yields, UV-visible absorption and fluorescence data for imidazoles 2 and 3 in acetonitrile solution.