Acid–base properties of bis-pyrazolopyridine derivatives in nonaqueous solutions

Abstract Acid–base properties of four 3,5-dimethyl-1,7-diphenyl-bis-pyrazolo-[3,4-b; 4′,3′-e]-pyridine derivatives with different substituents in position 4, i.e. N,N -dimethylaminophenyl, phenyl, nitrophenyl, and methoxyphenyl (DMA-DMPP, H-DMPP, NO 2 -DMPP, and CH 3 O-DMPP) were investigated in nonaqueous solutions. Absorption and stationary fluorescence as well as time-resolved spectroscopy were applied as methods. The acidity of the amino group in DMA-DMPP which is preferentially protonated in the ground state, does practically not change in the first excited state. The second protonation step of DMA-DMPP and the protonation of the other compounds occurs at the heterocyclic subunit, in the ground state most probably on the pyridine nitrogen. The basicity of this nitrogen atom increases remarkably in the first excited state. Experimental results were compared with those of semiempirical calculations. The amino group as the preferential site of protonation before the pyridine nitrogen in the ground state was confirmed. According to quantum mechanical computations, one pyrazolo nitrogen is preferentially protonated in the first excited state. The protonation is accompanied by a geometrical relaxation leading to a planarized conformer with a lower total energy.