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 (S-1) of these compounds is populated via HOMO -> LUMO pi-pi * 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.

Automated summary

The triflic acid-promoted cyclization reaction resulted in a mixture of compounds with pyrazole backbone, exhibiting bright fluorescence and potential for sensing strongly acidic environments.