Influence of fluorine on the photophysical, electrochemical properties and basicity of 1,3-diphenylpyrazolo[3,4-b]quinoline derivatives

Abstract

The manuscript comprises the analysis of several photophysical (absorption and emission spectra, quantum efficiency, fluorescence life times) and electrochemical (oxidation potentials) properties of four fluorine-substituted 1,3-diphenyl-1H-pyrazolo[3,4-b]quinoline (PQ) fluorescent dyes. The experimental results are compared with those obtained by DFT calculations. Introduction of fluorine atoms in 1H-pyrazolo[3,4-b]quinoline molecule modifies some of its properties, like fluorescence quantum efficiency, HOMO and LUMO levels, absorption band position. The basicity of PQ molecule has been tuned by distance dependent electron-withdrawing effect of fluorine. This leads to the increased resistance of PQ fluorine derivatives to proton donors. The high fluorescence yield of 8-fluoro-1,3-diphenyl-1H-pyrazolo[3,4-b]quinoline is preserved for 0.1M TFA concentration, which is enough to quench the emission of unsubstituted 1,3-diphenyl-1H-pyrazolo[3,4-b]quinoline.