Highly Fluorescent Dyes Containing Conformationally Restrained Pyrazolylpyrene (Pyrazoolympicene) Chromophore

The triflic-acid-promoted cyclization of 1-phenyl-3-(pyren-1-yl)-1H-pyrazole-4-carbaldehyde afforded a mixture of 9-phenyl-7,9-dihydropyreno (10,1-fg)indazole and 9-phenylpyreno(10,1-fg)indazole-7(9H)-one, readily separable by column chromatography. Both products contained a rigid six-ringed pyrazoolympicene backbone and exhibited bright fluorescence in chloroform solution and a weak fluorescence in the solid state. DFT and TD DFT calculations revealed that the lowest excited state (S1) of these compounds is populated via HOMO →LUMO π-π * transition. Furthermore, the synthesized compounds behaved as weak bases and their emission spectra showed substantial changes upon protonation. Therefore, they may be of interest for sensing of strongly acidic fluorophore environments.

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

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.