Synthesis and optical properties of bispyrazolopyridine derivatives

A series of bispyrazolopyridine (BPP) dyes with electron deficient or electron rich aromatic rings were synthesized, and their optical properties were evaluated. Dyes 3a-c connect the BPP unite with electron deficient moieties (pyridine ( 3a ), nitrobenzene ( 3b ), benzonitrile ( 3c )) by a single bond; dyes 3d-e consist of BPP and electron rich moieties (anisole ( 3d ), N,N -diethylaniline ( 3e )). Their optical properties in five organic solvents and water solution were investigated. They show absorption at 310–375 nm and emission at 400–580 nm in organic solvents. Meanwhile, dyes 3a and 3c-d have high fluorescence quantum yield in organic solvents (40%–84%) and aqueous solution (69%–89%), and they also exhibit excellent light stability. The pH-influenced optical properties tests indicate dyes 3a-e show different fluorescence intensities from alkaline (pH = 10.0) to acid condition (pH = 1.0). The protonation of BPP units and aromatic rings was used to explain their emission property, while (TD)DFT calculations were developed to understand the differences in the optical properties.