Effect of solvent on absorption and fluorescence spectra of a typical fluorinated azo dye for its acidic and basic structures

The effect of 15 polar solvents on absorption and fluorescence energies of a typical fluorinated azo dye, 4-(2,3,5,6-tetrafluoro-pyridin-4-yl azo)-phenol, was reported for its acidic, MH, and basic, M, structures.For MH, the absorption energy is described on the basis of multi-linear equation with Taft's π* (solvent polarity) and β (hydrogen bond acceptor) parameters while the fluorescence energy varies rectilinearly with free energy of transferring the proton to the surrounding solvent, ΔGt°.For M, the hydrogen bonding donor ability of protic solvent, α, is a predominant factor which affects the absorption energy while in aprotic solvents, the absorption energy correlates linearly with Kirkwood function. As the ability of the solvent for hydrogen bonding increases, the absorption band width will increase in parallel with the transition energy.

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► The effect of fluorine atoms on the spectral properties of the dye was studied.
► For the protonated form, MH, multi-parameter linear correlation was obtained for absorption energies.
► For MH, a good linear correlation was found between fluorescence energy and free energy for transferring the proton to the surrounding solvent.
► In aprotic solvents, a good linear correlation was found between absorption energies of M and Kirkwood function.
► For M, a good linear correlation was obtained between the extinction coefficient, ɛmax, value and the hydrogen bonding donor ability of the solvent, with increasing the band width in parallel with the transition energy.