Synthesis, Photophysical Properties, and Metal-Ion Recognition Studies of Fluoroionophores Based on 1-(2-Pyridyl)-4-Styrylpyrazoles

Abstract

A convenient access toward novel fluoroionophores based on 1-(2-pyridyl)-4-styrylpyrazoles (PSPs) substituted at position 3 with donor or acceptor aryl groups is reported. The synthesis proceeds in two steps: the first one via Wittig olefination of the appropriate 4-formylpyrazole and then Mizoroki–Heck coupling to yield the desired products in an overall yield of up to 69%. Photophysical properties of products (4-styryl) and their intermediates (4-vinyl) were explored, finding that they have strong blue-light emission with high quantum yields (up to 66%) due to ICT phenomena. The 3-phenyl PSP was studied as a turn-off fluorescent probe in metal ion sensing, finding a high selectivity to Hg2+ (LOD = 3.1 × 10–7 M) in a process that could be reversed with ethylenediamine. The sensing mechanism and binding mode of the ligand to Hg2+ were established by HRMS analysis and 1H NMR titration tests.

Styrylpyrazoles: Properties, Synthesis and Transformations

“…By performing steady state and time resolved spectroscopic measurements, competitive binding studies, circular dichroism and a DNA helix melting study they unequivocally demonstrated that DSDP binds with the ctDNA through groove In addition to pharmacological activities, 4-styrylpyrazoles have shown interesting photophysical properties. In particular, 1-(2-pyridyl)-4-styrylpyrazoles (PSPs) substituted at position 3 with donor or acceptor aryl groups have shown strong blue-light emissions with high quantum yields (up to 66%), due to intramolecular charge transfer (ICT) phenomena [45]. The 3-phenyl-1-(2-pyridyl)-4-styrylpyrazole 14 ( Figure 6) was studied as a turn-off fluorescent probe in metal ion sensing, showing a high selectivity to Hg 2+ (Limit of detection (LOD) = 3.1 × 10 −7 M) in a process that could be reversed with ethylenediamine.…”

“…The Wittig reaction of 4-formylpyrazole 80 with benzyltriphenylphosphonium bromide (81) in BuLi-THF afforded the corresponding (E)-and (Z)-4-styrylpyrazoles 82 with an E:Z ratio of approximately 1:3 (Scheme 25) [45].…”

“…Furthermore, 4-bromo-1-(4-nitrophenyl)-1 H -pyrazole did not react under the same conditions [ 71 ]. On the other hand, the Mizoroki–Heck coupling of previously prepared 4-vinylpyrazole 65 with halobenzenes 66 , using Pd(AcO) 2 (10 mol%) as a catalyst in the presence of a ligand ( o -MePh) 3 P (20 mol%), afforded the desired ( E )-1,3-disubstituted-4-styryl-1 H -pyrazoles 67 [ 45 ] ( Scheme 19 ). This reaction was performed under microwave irradiation to obtain the compound in a shorter reaction time.…”

“… Styrylpyrazoles 14 – 16 presenting relevant photophysical properties [ 45 , 48 , 49 , 50 , 51 , 52 ]. …”

“… Mizoroki–Heck coupling of 4-vinylpyrazole 65 with halobenzenes 66 to prepare ( E )-1,3-disubstituted-4-styryl-1 H -pyrazoles 67 [ 45 ]. …”